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High quality biodiesel obtained through membrane technology
Journal of Membrane Science 421-422 (2012) 154–164
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High quality biodiesel obtained through membrane technology I.M. Atadashi, M.K. Aroua n, A.R Abdul Aziz, N.M.N Sulaiman Chemical Engineering Department, Faculty of Engineering, University Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 April 2012 Received in revised form 3 July 2012 Accepted 4 July 2012 Available online 24 July 2012
In this study, a ceramic membrane with a pore size of 0.02 mm was used to purify crude biodiesel to achieve biodiesel product that meet both ASTM D6751 and EN 14241 standards specifications. The membrane system was successfully developed and used for the purification process. Process operating parameters such as transmembrane pressure, flow rate and temperature were investigated. Application of central composite design (CCD) coupled with Response Surface Methodology (RSM) was found to provide clear understanding of the interaction between various process parameters. Thus, the process operating parameters were then optimized. The optimum conditions obtained were transmembrane pressure, 2 bar, temperature, 40 1C and flow rate, 150 L/min with corresponding permeate flux of 9.08 (kg/m2 h). At these optimum conditions, the values of free glycerol (0.007 wt%) and potassium (0.297 mg/L) were all below ASTM standard specifications for biodiesel fuel. In addition the physical properties of biodiesel at the optimum conditions met both ASTM D6751 and EN 14214. This work showed that with ceramic membrane of pore size 0.02 mm, biodiesel with high qualities that meet the stringent standards specifications more than those currently in application can be achieved. & 2012 Elsevier B.V. All rights reserved.
Keywords: Ceramic membrane Palm oil Biodiesel Permeate flux Optimization
1. Introduction Biodiesel is a clean-burning fuel derived from vegetable oils, animal fats, or grease. The chemical structure of commercial biodiesel is fatty acid alkyl esters (FAAE) [1]. Due to renewability, low gaseous emissions and biodegradability, biodiesel is becoming very popular in the European Union (EU) which has set an objective to secure for motor biofuels a market share of 20% of the total motor fuel consumption by 2020 [2]. Because biodiesel is fully prepared from biomass material, it has minimal crude oil residues or metals, aromatic hydrocarbons, and sulfur. Like petrodiesel, biodiesel operates in diesel engines such as those used in private and commercial vehicles and farm equipment. Basically modifications of the engine are not required, and biodiesel maintains the payload capacity and range of petro-diesel. As biodiesel is oxygenated, it is more completely combusted. Besides it has better lubricity than petro-diesel, hence increasing the life times of the diesel engines. Further the higher flash points of biodiesel make it a safer fuel to store, handle and use [1,3]. Also, moderately lower emission profile of esters make them an ideal fuel for application in sensitive environments, such as heavily polluted cities, national parks and forests and marine areas [3]. Biodiesel is most commonly prepared through alkali-catalyzed transesterification. Transesterification is a chemical reaction between
n
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triglycerides and alcohol in the presence of catalyst (acid, alkali, or enzyme) as shown in Fig. 1 [2]. This reaction produces biodiesel as a primary product and glycerol as a secondary product. Commercially, biodiesel is produced from refined oils through one-step or two-step alkali-catalyzed transesterification. The schematic diagram of the conventional alkali-catalyzed transesterification is presented in Fig. 2 [3]. After transesterification reaction is completed, removal of glycerol is the first step to be carried out. And because of the difference in polarities and larger density difference between glycerol and esters, separation of glycerol is usually quick. Separation of glycerol from biodiesel mixture is usually achieved through gravitational settling or centrifugation. After glycerol is separated, crude biodiesel is subjected to either distillation or rotary evaporation in order to remove the residual alcohol. Conventionally biodiesel is purified via water washing and dry washing methods. Water washing is used to remove the remaining glycerol, soap, catalyst, methanol or salts from the alkyl esters [4]. After water washing is completed, the remaining water in biodiesel is removed; vacuum flash process, anhydrous Na2SO4, magnesol, amberlite, purolite etc., can be used as a final step to dry biodiesel. The process of biodiesel water washing usually provides final biodiesel product that satisfies the stringent biodiesel standards such as EN 14214 and ASTM D6751. However, separation of hot wash water and the acid from biodiesel in some cases requires application of two centrifuges. Besides, water washing process generates wastewater containing impurities such as free glycerol, residual catalyst and soap which must be carefully handled before being discharged. Otherwise disposal of
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biodiesel wastewater could negatively affect the environment [5]. The problems associated with water washing have led to the use of dry washing process to purify crude biodiesel. Although the process of dry washing with magnesol, or ion exchange resins, provides biodiesel with low impurities and good physicochemical properties, the lack of adsorbents reusability and little knowledge about the chemistry of the adsorbents discourage their use. Presently, membrane technology is being explored and exploited for the purification of biodiesel. Generally membrane processes play a critical role in purifying biotechnology products [6]. Membrane based separations are well-established technologies in protein separations, water purification, and gas separations. The use of membrane separation for the treatment of non-aqueous fluids is an emerging field in membrane technologies [2,7]. Membranes can be inorganic or organic (polymeric) in nature. The inorganic membranes particularly ceramic membranes are more appropriate for use with organic solvents because of their excellent thermal and chemical stability and their ability to withstand higher temperatures and pressures [1,2,8]. Further, the exceptional physical and chemical stability of ceramic membranes permits them to offer reproducible performance over long service lifetimes, which is well proven in many industrial installations. Also the ability of ceramic membranes to recover valuable products, concentrate process streams, and increase yields makes them a preferred and cost-effective method of filtration [9]. Several biodiesel membrane purification processes were evaluated with high-quality biodiesel being reported, in some cases biodiesel purity above 99% was achieved [4,10,11,12]. In a search of the literature, the smallest membrane pore size used by previous researchers was 0.05 mm. In addition most of the studies focused only on single impurity retention. To the best of our knowledge there is no research conducted so far on the purification of crude biodiesel using membrane with pore size of 0.02 mm. Therefore in this study, a ceramic membrane with a pore size of 0.02 mm was used to purify crude biodiesel considering simultaneous retention of glycerol and catalyst. The current work
155
also employed Central Composite Design (CCD) coupled with Response Surface Methodology (RSM) to optimize the process parameters such as transmembrane pressure, flow rate and temperature, therefore permitting the analysis of interaction between them in order to find out the optimum operating conditions.
2. Experimental 2.1. Transesterification reaction The transesterification of palm oil to biodiesel was carried out in a fume hood using a 5-liter batch reactor coupled with a thermometer, and a mechanical stirrer. The temperature of the reactor was maintained during the entire reaction time using a thermostatic water bath. In each run, 2.5 l of palm oil was charged into the reactor and agitated until the required reaction temperature was achieved. Subsequently, a mixture of potassium hydroxide and methanol, earlier mixed until total dissolution, was charged into the reactor. With regard to mass of the palm oil, 1 wt% of potassium hydroxide was used for the transesterification reaction. In addition the transesterification process was performed at a reaction temperature of 60 1C, reaction time of 1 h and at an ambient pressure. The reacting mixture was continuously mixed at an agitation speed of 645 rpm. After the transesterification process was completed, the byproduct, glycerol was separated overnight through gravitational settling. Although separation of glycerol from biodiesel via gravitational is time consuming, the process is less costly compared to centrifugation process [4]. Afterward, the biodiesel sample was submitted to rotary evaporation at a temperature of 65 1C under 600 mmHg vacuum for 45 min for the recovery of the residual methanol. Furthermore, a good number of experimental runs were performed to generate sufficient amount of biodiesel samples for the slated number of membrane purification experiments. The samples of the biodiesel were then analyzed to determine the initial concentrations of the contaminants present in the biodiesel samples. The biodiesel samples produced were then properly stored for membrane purification process. 2.2. Purification of biodiesel using membrane technology
Fig. 1. Transesterification process for the conversion triglycerides to biodiesel.
2.2.1. Membrane type A multi-channel tubular-type Al2O3/TiO2 ceramic membrane consisting of filtration area of 0.031 m2 was used for the experimental runs. The membrane with pore size of 0.02 mm was obtained from Jiangsu Jiuwu Hitech Co., China.
Dryer
Fat and oil
Alcohol
Transesterification reactor
Mixer
Settler (Centrifuge/ Decanter)
Biodiesel
Removal of excess methanol
Refined biodiesel
Neutralization /Distilled water washing
Acid (H3PO4)
Wastewater for treatment
Catalyst Distilled water Fig. 2. Process flow schematic diagram for biodiesel production and purification process.
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2.2.2. Permeate fluxes Distilled water was used to obtained the initial permeate fluxes. The initial permeate flux values were determined using distilled water at a temperature of 50 1C, flow rate of 150 L/min and transmembrane pressures of 1, 2 and 3 bar. The performance and efficiency of the membrane cleaning process were monitored using these operational conditions. Eq. 1 was used to determine the permeate fluxes: J¼
Q At
ð1Þ
where J ¼flux, Q¼mass (kg), A¼area (m2) and t ¼time (h). For the membrane ultrafiltration of crude biodiesel, the permeate flux was presented as a function of time. Using experimental conditions in Table 1, different permeate fluxes were generated. Furthermore, the permeate fluxes obtained with membrane of pore size 0.02 mm were also compared with permeate fluxes generated using membrane of pore size 0.05 mm.
Table 1 Operating conditions for the biodiesel membrane separation process. Run
TMP (bar)
Temperature (1C)
Flow rate (L/min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1.00( 1) 3.00(þ 1) 1.00( 1) 3.00(þ 1) 1.00( 1) 3.00(þ 1) 1.00( 1) 3.00(þ 1) 1.00( 1) 3.00(þ 1) 2.00(0) 2.00(0) 2.00(0) 2.00(0) 2.00(0) 2.00(0) 2.00(0) 2.00(0) 2.00(0) 2.00(0)
30.00( 1) 30.00( 1) 50.00( þ1) 50.00( þ1) 30.00( 1) 30.00( 1) 50.00( þ1) 50.00( þ1) 40.00(0) 40.00(0) 30.00( 1) 50.00( þ1) 40.00(0) 40.00(0) 40.00(0) 40.00(0) 40.00(0) 40.00(0) 40.00(0) 40.00(0)
60.00( 1) 60.00( 1) 60.00( 1) 60.00( 1) 150.00(þ 1) 150.00(þ 1) 150.00(þ 1) 150.00(þ 1) 105.00(0) 105.00(0) 105.00(0) 105.00(0) 60.00( 1) 150.00(þ 1) 105.00(0) 105.00(0) 105.00(0) 105.00(0) 105.00(0) 105.00(0)
2.3. Membrane ultra-filtration process 2.3.1. Process parameters The main process operating parameters investigated for the membrane ultrafiltration process are temperature, transmembrane pressure and flow rate. These process parameters were varied: temperature (30–50 1C), transmembrane pressure (TMP) (1–3 bar), and flow rate (60–150 L/min). The permeate flux was expressed as kg/m2 hr and the free glycerol and potassium contents as percentage. In another study conducted by Gomes et al. [11] to purify crude biodiesel using membrane system, transmembrane pressure of 1–3 bar and temperature of 50 1C were used. The authors noted significant removal of biodiesel impurities such as glycerol. Furthermore, Gomes et al. [4] used transmembrane pressure of 1–3 bar and a temperature of 60 1C during membrane biodiesel separation process and achieved considerable glycerol reduction in biodiesel. Also, Wang et al. [2] adopted flow rate of 50–150 L/min and temperature of 30– 70 1C for the purification of crude biodiesel and obtained reasonable glycerol removal with glycerol content in biodiesel below the ASTM D6751 and EN 14214 standards specification.
2.3.2. Separation process Fig. 3 illustrates experimental setup of biodiesel ceramic membrane separation unit. The setup consists of crude biodiesel feed tank, water bath, circulating pump (digital Masterflex L/S peristaltic) membrane module, digital weighing balance etc. Also the experimental setup was provided with pump tubing (ChemDurance chemical resistant) with a size of 16 (ID ¼44 mm, OD¼2.36 mm). The pressure and temperature were monitored using pressure gauges and temperature indicator. The crude biodiesel was charged into feed tank with a capacity of 5 l and circulated using a pump through the membrane tube at operating conditions presented in Table 1. The inlet and outlet pressures were adjusted using the valves at the end of the membrane tubes to determine the proper operating transmembrane pressure. Thermostatic water bath was used to monitor the system temperature. In this study, 20 experimental runs were carried out. Further acidified water was added to improve the retention of biodiesel contaminants in the membrane system. The experimental module was operated by recycling the biodiesel concentrate. The mass permeate fluxes were automatically recorded by a
Retentate
Feed tank Inlet pressure
Outlet pressure
P
P Ceramic membrane
Valve Valve
Heating equipment
Valve
Circulating pump Permeate Digital balance
Fig. 3. Experimental setup of biodiesel ceramic membrane separation unit.
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digital balance every 12.5 min during the filtration process. All the runs were halted after 1.15 h filtration time. The initial samples, numbered as original samples and the permeates, were collected after the experiments and analyzed. The separation of impurities using membranes reduces the number of steps required in the conventional treatment, besides less water is consumed [11]. 2.3.3. Experimental design The experiments were designed using Design of experiment software Version 8.0.0 (Stat-Ease Inc., USA). The software was used to optimize the effects of process parameters such as transmembrane pressure, flow rate, and temperature for the purification of crude biodiesel. It is noteworthy to mention that several optimization methods such as Full Factorial Design, Central Composite Design, and D-optimal Designs etc. are used to optimize a process. In this study, Central Composite Design (CCD) coupled with Response Surface Method (RSM) was selected. The free glycerol and potassium contents are the main responses. To provide a true measure of error due to the natural variations, six replicated center points were chosen and conducted in randomized order. The number of replicates is selected to offer a wide region where the standard error of prediction remains reasonably stable. Table 2 presents coded and actual levels of the process parameters. The process parameters are transmembrane pressure 1–3 bar, flow rate 60–150 L/min, and temperature 30–50 1C. The coded values are designated as 1 (low), 0 (medium), þ1 (high), a and þ a. Alpha (a) is a distance from the center point which might either be inside or outside the range, with the high value of 2k/4(where k is the number of factors). The value of alpha was set at 1 to generate a face-centered central composite design. This is required because it is only a three-level design, which can ensure that the axial runs will not be anymore extreme values than the factorial portion [13]. Compared to the conventional approach, which utilizes one parameter at a time, this method can determine the interaction between effects of process operating parameters, besides it can provide good estimations of errors. Zabeti et al. [14] reported that the experimental cost and time could be reduced since the total number of trials is minimized. The levels of each process operating parameter were chosen based on the works conducted [2,4,11]. The relationship between dependent and independent parameters in this study is explained by the following second-order polynomial model: Y ¼ bo þ
k X
biXi þ
i¼1
k X
biiXi2 þ
i¼1
k X k X
bijXiXj
ð2Þ
i¼1j¼1
where xi are the input variables, which impact the response variable Y, and b0, bi, bii and bij are the regression coefficients. 2.4. Membrane cleaning process In membrane cleaning, components are physically, chemically or hydraulically removed. When the membrane cleaning process is conducted, a membrane module is temporarily out of order. In practice, it is always good to apply the lowest possible cleaning time and make filtration time last as long as possible [15]. The cleaning process is more effective when using transmembrane Table 2 Process parameters levels in actual and (coded) forms. Parameters
Unit
Low level
High level
TMP Temp Flow rate
bar 1C L/min
1( 1) 30( 1) 60( 1)
3( þ1) 50( þ) 150( þ)
157
pressure of 0.45 bar. In this experiment, the membrane module was thoroughly cleaned to restore or maintain the performance of the membrane in terms of its permeability and protect the equipment. The cleaning process of the membrane was carried out using water and detergent until the biodiesel was completely removed. For 45 min, a solution of 1% NaOH at a temperature of 70 1C was circulated. The module was cleaned with water and then rinsed with warm distilled water [4]. The process of membrane cleaning was quite efficient and fast. To ascertain the reproducibility of the experiments, the permeate flux using distilled water was recorded after each cleaning process. It was observed that the permeate flux values obtained after membrane cleaning were almost identical to the initial permeate flux values. 2.5. Characterization of produced biodiesel To establish the efficiency of the membrane separation process, the initial concentrations of biodiesel contaminants (free glycerol, and catalyst (potassium)) were first determined. The level of the contaminants were measured based on the methods provided by biofuels standard specifications for related oils such as modified version of the AOCS technique for the analysis of free glycerol in oils and fats (Ca 14–56), and ASTM D6751-03 for potassium determinations. Furthermore, the physical properties of the biodiesel produced were determined at the optimum operating conditions. 2.5.1. Measurement of free glycerol content in biodiesel To determine the concentration of glycerol in the biodiesel samples, the modified version of the AOCS technique for the analysis of free glycerol in oils and fats (Ca 14–56) was used. The technique involved reaction of glycerol in aqueous medium with excess sodium periodate to form formaldehyde, formic acid, and iodic acid, and then addition of potassium iodate to react with the formed sodium periodate and the iodic acid. In comparison to gas chromatography, the determination of free glycerol using periodates titration gives low cost. Moreover, the method is simple and straightforward, fast and sufficiently reliable. The method involving periodate as an oxidant reagent for the determination of free glycerol was carefully evaluated and it was found that the method is very successful in relation to precision and accuracy [4]. Further investigation on the determination of glycerides in oils and esterified glycerol using periodate method after transesterification showed similar results. The method is straightforward and easily reproducible with great precision [16]. The glycerol content in both the feed and permeate were examined after each run. 2.5.2. Determination of potassium content in biodiesel The content of potassium in both crude and refined biodiesel was determined using Inductively Coupled Plasma (ICP). The analysis of the biodiesel samples was based on the techniques prescribed by international standard specification (European: EN 14109 and EN 14108). In these techniques, the sample dilution is conducted using organic solvents (xylene, petroleum ether or cyclohexane). In this study, the dilution of the samples and organometallic standards for the calibration was carried out using xylene as an organic solvent. The analytical standards were prepared from a stock solution of 100 mg kg 1, pipetting the volume to give concentrations in the range of 0.0–2.0 mg L 1 in 10 mL volumetric flasks. The samples were prepared weighing 0.4 g and 1.0 g of sample for the determination of potassium, in a volumetric flask of 10 mL, and then diluted with xylene. The standards and samples were analyzed immediately after the dilution. The fuel/oxidant ratio and aspiration rate were optimized prior to starting the analysis. Moreover, organometallic
I.M. Atadashi et al. / Journal of Membrane Science 421-422 (2012) 154–164
standards were used to spike both the biodiesel and oil samples, then dilution with xylene was performed [17]. The coefficients of retention (%R) of free glycerol and potassium contents were calculated using Eq. 3: %R ¼
½ðC f -CperÞ 100 Cf
ð3Þ
where Cf and Cper are the mass fractions in the feed and the permeate for free glycerol and potassium, respectively. 2.5.3. Determination of physical properties of biodiesel The physical properties of the biodiesel produced were investigated in accordance to the techniques prescribed by the American Society for Testing and Materials such as viscosity at 40 1C (ASTM D445-06) flash point (ASTM D93-07), pour point (ASTM D97-93), cloud point (ASTM D2500), and density at 15 1C (ASTM D4052-96).
3. Results and discussions 3.1. The biodiesel separation process The application of ceramic membrane with a pore size of 0.02 mm has presented high retention of free glycerol and potassium during biodiesel purification as shown in Table 3. Both ASTM D6751 and EN 14214 standards specified glycerol content in biodiesel to be 0.02 wt%. Thus, achievement of low free glycerol content in biodiesel provides numerous advantages such as reduced settling problems and low aldehydes and acrolein emissions. Furthermore low catalyst concentration in biodiesel prevents deposits in the injectors (carbon residue), reduces filter blockage (sulphated ashes) and decreases possibilities for engine weakening [18]. The separation of biodiesel from glycerol and potassium was achieved using membrane ultrafiltration process. Before starting the ultrafiltration process, crude biodiesel samples were subjected to rotary evaporation to completely remove the residual methanol. Afterward acidified water was added to the biodiesel samples. Results of the tests conducted showed that addition of acidified water improved the separation process. The addition of acidified water led to the formation of aqueous phase containing catalyst, salt, glycerol, and other related water-soluble substances. This has reduced the solubility of biodiesel in glycerol, Table 3 Concentrations of free glycerol and potassium in permeate (final biodiesel). Run
Free glycerol (wt%)
Potassium (mg/L)
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
0.023 0.019 0.025 0.0.22 0.018 0.024 0.016 0.023 0.025 0.016 0.014 0.008 0.018 0.006 0.021 0.017 0.019 0.007 0.009 0.017
0.561 0.523 0.495 0.532 0.451 0.632 0.343 0.491 0.389 0.456 0.367 0.294 0.332 0.276 0.435 0.391 0.534 0.436 0.393 0.285
producing agglomeration of larger droplets of glycerol which are too big to pass through the membrane pores [11]. Besides, addition of acidified water helps in breaking down the soap into water soluble salt and free fatty acids, with the free fatty acids remaining in the biodiesel. Acid neutralizes the catalyst as well, forming salt and water. Thus the ability of the membrane to retain the aqueous phase containing the contaminants led to the generation of permeates (biodiesel) with low concentrations of contaminants whose values were in some cases below those prescribed by ASTM D6751 and EN 14214 standards as presented in Table 3. The significant retention of free glycerol mass content lower than 0.02% and significant reduction of potassium mass content demonstrates the effectiveness of the methodology used. Improvement in the performance of membrane for the separation of biodiesel and free glycerol following water addition was investigated by Saleh et al. as well [19]. After free glycerol and biodiesel were separated through decantation process for about 8–12 h, polymeric membrane was used to purify the biodiesel obtained. The addition of water to the crude biodiesel samples ranging from 0.06 to 0.2 wt% aided the retention of glycerol by the formation of two immiscible phases: a water and glycerol phase, and a biodiesel phase which suggests that the principle of separation of free glycerol from biodiesel is that of the retention of a finely dispersed water and glycerol phase by the membrane.
3.2. Permeate fluxes The initial permeate fluxes were determined using distilled water. The initial permeate flux values obtained using distilled water at a temperature of 50 1C, flow rate of 150 L/min and transmembrane pressures of 1, 2 and 3 bar were 57 kg/m2 h, 64 kg/m2 h and 76 kg/m2 h respectively. These operational conditions were used as a reference to monitor the performance and efficiency of the membrane cleaning process. Some of the permeate fluxes derived during biodiesel membrane purification process are presented in Fig. 4. The fluxes determined differ and were dependent on the combination of the process operating parameters. In cross flow filtration process, the gel layer continuously builds up until it reaches a certain thickness, then the hydrophilic compounds mount up on the surface of the membrane and form larger droplets that are removed from the layer. A steady state thickness of the gel layer can be seen in Fig. 4. This figure reveals a decrease in the permeate flux in the first (37.5 min), which during course of the runs stabilized. The permeate fluxes presented in Fig. 4 for the membrane with a pore size of 0.02 mm are lower 35 30 Permeate Flux (kg/m2hr)
158
25 20 15 10 5 0 0
20
40 Time (min)
60
Fig. 4. Permeate flux vs. time for membrane of pore size 0.02 mm.
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Permeate flux (kg/m2hr)
100
159
Table 4 Retention coefficients (%R1 and %R2) of glycerol and potassium.
80
Run order
TMP (bar)
Temp. (1C)
Flow rate (L/min)
%R1 (free glycerol)
%R2 (potassium)
60
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1.00 3.00 1.00 3.00 1.00 3.00 1.00 3.00 1.00 3.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00
30.00 30.00 50.00 50.00 30.00 30.00 50.00 50.00 40.00 40.00 30.00 50.00 40.00 40.00 40.00 40.00 40.00 40.00 40.00 40.00
60.00 60.00 60.00 60.00 150.00 150.00 150.00 150.00 105.00 105.00 105.00 105.00 60.00 150.00 105.00 105.00 105.00 105.00 105.00 105.00
95.86 96.25 97.50 95.62 97.27 95.76 99.14 94.82 97.15 96.01 98.15 99.25 97.65 99.21 98.52 99.07 98.56 99.05 98.15 99.20
90.45 87.34 94.14 90.89 94.67 91.25 95.15 90.29 94.92 92.21 95.48 94.83 91.15 95.13 95.37 96.21 95.52 94.72 94.21 96.15
40
20
0 0
10
20
30
40
50
60
70
Time (min) Fig. 5. Permeate flux vs. time for membrane of pore size 0.05 mm.
compared to those generated from membrane with a pore size of 0.05 mm as shown in Fig. 5 [20]. However better quality biodiesel product were achieved via membrane with pore size of 0.02 mm. The lower permeate fluxes generated in Fig. 4 using membrane with a pore size of 0.02 mm could be attributed to the membrane matrix of smaller pore size. Also, as expected, very high permeate flux was reported when membrane with a pore size of 0.1 mm was used for the purification of biodiesel [2]. The larger pore size of the microfiltration (MF) membrane provides a slightly larger flux at the expense of the retention of the glycerol droplets [21]. Choi et al. [22] remarked that continuous flux reduction of ultrafiltration membrane with time is as a result of other ‘‘incrustation’’ phenomena, such as concentration polarization, pore blocking or molecule adsorption followed by the formation of fouling layer. However, back flushing of the membrane module could lead to high permeate flux [23]. In this work, the optimum experimental conditions for membrane with a pore size 0.02 mm is attained at TMP of 2 bar, temperature of 40 1C and flow rate of 150 L/min which provided permeate flux of 9.08 (kg/m2 h). 3.3. Optimization of process parameters To generate high-quality biodiesel, this study focused on the optimization of process parameters such as flow rate, transmembrane pressure and temperature using Response Surface Methodology. Response Surface Methodology was used to improve biodiesel membrane purification which allows low cost biodiesel purification process in an industrial scale. Besides, RSM is selected for the optimization process as it is sufficient enough to analyze the effects of process parameters on membrane performance for the purification of crude biodiesel. The process parameters selected were used to identify the optimum conditions that have effects on the membrane purification of biodiesel. For each experimental run, the coefficients of retention (%R) of free glycerol and potassium contents were recorded. With regard to the results achieved, it was observed that purification of biodiesel is dependent on all the process parameters. Table 4 shows the coefficients of retention for the free glycerol and potassium which varies in the ranges of 94.82– 99.25% and 87.34–96.21% respectively. A major advantage of RSM is that the interactions of factors are considered in the experimental design and easily observed based on the resulting regression equations. Diagnostics of the residuals, the difference between predicted and actual free glycerol and potassium content, showed that transformation is not required to enhance both retentions of free glycerol and potassium models. Thus to
measure the curvature effects with the help of Design Expert software, the experimental data generated were fitted to the polynomials of higher degree equations such as quadratic, and two factor interactions (2FI) models etc. Based on the ANOVA analysis, quadratic models best fitted the experimental data. The designed experiments were analyzed using analysis of variance at 95% level of confidence. Further the quadratic models to predict the retentions of free glycerol and potassium (K) in terms of coded factors are presented in Eqs. 4 and 5. R1 ðFree glycerol Þ ¼ þ 98:710:85A þ 0:30B þ 0:33C0:63AB 0:54A1:000E002BC -2:06A2 þ 0:063B2 0:21C2
ð4Þ
R2 ðKÞ ¼ þ 95:271:74A þ 0:61B þ 1:25C0:20AB0:24AC 0:96BC0:57A2 þ 0:024B2 1:99C2
ð5Þ
where %R is a function of transmembrane pressure (A), temperature (B) and flow rate (C). The positive sign in front of the terms shows synergistic effect while the negative sign indicates antagonistic effect [24]. To assess the goodness of fit, the results generated were then analyzed by the ANOVA as presented in Table 5. The ANOVA showed the effects of single parameters and the interaction between the parameters on the responses. The p value is the probability value used to ascertain the significance of each of the coefficient which may invariably indicate the pattern of relationship between the parameters. The linear terms were all statistically significant as can be seen from the ANOVA table. All the coefficients were considered in the design to minimize error. The closeness of the p-value to zero (0.00) is used to judge the significance of the results. The confidence level should be 95% for the effect to be statistically significant. This indicates that the p-value should be less than or equal to 0.05 [25]. The significance means that the approximate value of the variable coefficient is bigger than a value that would be achieved from the experimental noise. Further the statistical analysis of variance showed overall models’ p-values (probability of error value) to be lower than 0.0001, which indicates that the second-order polynomial regression model is highly significant. Alternatively, the lack of fit is above 0.01 which also indicates that the data are well fitted to the models. In addition, the goodness of fit was determined by evaluating the coefficients of determination (R2) to validate the models. Therefore to establish the reliability of the models the data
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Table 5 ANOVA for the response surface models. Source
Model A-TM B-Temp C-Flow rate AB AC BC A2 B2 C2 Residual Lack of Fit
Retention of free glycerol
Retention of potassium
F-value
p-value
Remarks
F-value
p-value
Remarks
23.73 40.39 5.21 6.22 18.20 13.29 4.514E-003 65.65 0.062 0.66 10 1.10
o0.0001 o0.0001 0.0455 0.0318 0.0016 0.0045 0.9478 o0.0001 0.8085 0.4342 0.18 0.4592
significant
16.2 40.64 5.04 21.16 0.42 0.62 10.06 9.10 2.155E-003 14.72 10 0.86
o 0.0001 o 0.0001 0.0486 0.0010 0.5309 0.4485 0.0100 0.0130 0.9639 0.0033 0.74 1.39
significant
not significant
not significant
obtained should be predicted with sound accuracy by the model in comparison to the experimental data. Fig. 6(a) and (b) presents predicted and experimental values for the retention of free glycerol (a) and K (potassium) (b) using the developed model equations. Tamunaidu and Bhatia [25] reported that values greater than 80% must be obtained for a good agreement between predicted values and experimental values. In this study, the membrane provided coefficients of determination (R2 ¼96%) for free glycerol retention and (R2 ¼ 94%) for potassium. This showed that the accuracy and general availability of the polynomial models are very good, and the response trends can be well analyzed by the model [26]. Besides, the predicted R-square, which is a measure of goodness of the model predicted values for the responses, was above 80% for all values in the experiments conducted. This indicated that the agreement between the predicted values by the models and experimental values is good [25]. Moreover, the adequacy of all the models generated were tested through lack of fit F-test [27]. The lack of fit of the quadratic models were not statistically significant since the probability values were all more than 0.05. And also the model equations have almost accounted all the variations that might have existed. Further adequate precision of the ‘‘model’’ measures the signal to noise ratio. Consequently a ratio greater than 4 is desirable. The models ratios of 13.596 and 13.628 were obtained for the retention of free glycerol and potassium respectively. These values are much higher than 4, this indicated adequacy of the models. Further it was observed that the values were desirable for all the models. Also, lower values of the coefficient of variation (CV) ranging from 0.43% to 1.59% suggested good precision and reliability of the experiments [25]. 3.3.1. Effect of process parameters on the retention of biodiesel impurities The influences of the process parameters investigated over the responses were discussed by means of statistical analysis carried out (main effects and interaction). The sign of the interaction obtained is associated to the type of the interaction. A positive interaction implies that increasing one factor improves the effect of the other factor and vice versa [28]. 3.3.1.1. Effects of process parameters on free glycerol retention The ANOVA Table 5 showed that the linear terms, transmembrane pressure (A), temperature (B) and flow rate (C) as well as the interactions of transmembrane pressure–temperature (AB) and transmembrane pressure–flow rate (AC), and the quadratic term of A2 were statistically significant to the retention of free glycerol during the separation and purification process, with
Fig. 6. (a) Predicted and experimental values for %R1 (free glycerol) and (b) predicted and experimental values for %R2 (potassium).
reference to the p-value less than 0.05. Even though the linear terms are significant but the transmembrane pressure (A) is more significant and has a larger effect due to high F-value of 40.30. The interaction term AB (F-value 18.20) and the quadratic term A2 (F-value 65.65) plays a vital role in the retention of free glycerol. Response surfaces are viewed as two- and three-dimensional plots that present the response as a function of one or two factors. Contour plots obtained using quadratic equations are a powerful tool for optimization process [28]. Fig. 7(a–c) presents 3D and 2D plots for the removal of free glycerol. Fig. 7(a) and (b) shows that
I.M. Atadashi et al. / Journal of Membrane Science 421-422 (2012) 154–164
161
Fig. 7. (a) Plots for Response surface and contour presenting the effects of flow rate (L/min) and TMP (bar) on the retention of free glycerol by biodiesel membrane separation: (a) response surface 3D and (b) contour plot (2D). (b) Plots for Response surface and contour presenting the effects of temperature (1C) and TMP (bar) on the retention of free glycerol by biodiesel membrane separation: (a) response surface 3D and (b) contour plot (2D). (c) Plots for Response surface and contour presenting the effects of flow rate (L/min) and temperature (1C) on the retention of free glycerol by biodiesel membrane separation: (a) response surface 3D and (b) contour plot (2D).
at low TMP, the retention of free glycerol increase with increase in flow rate and temperature. However, at higher TMP, a reduction in the retention of free glycerol can be observed due to fact that the quadratic term of TMP is more significant with a negative effect (Eq. (4)). In both Fig. 7(b) and (c) the effect of temperature
is significant in the retention of glycerol. At higher temperature, there is a considerable increase in the retention of free glycerol, because the quadratic term of temperature is more significant with positive term (Eq. (4)). Thus temperatures above 40 1C could result in achievement of over 99% free glycerol retention. However much
162
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higher temperatures could favor solubility of glycerol in biodiesel, therefore these need to be avoided. In addition, gradual increase in flow rate created an avenue for the permeation of the glycerol through the membrane pores. The surface response plots showed good favourability of high flow rate in the retention of glycerol.
3.3.1.2. Effects of process parameters on the retention of potassium The ANOVA Table 5 revealed that the terms A, B, C, BC, A2 and B2 were statistically significant during the process of retaining the potassium in the membrane module. The significance of the terms is confirmed by their p-values which are all below 0.05. Fig. 8(a–c)
Fig. 8. (a) Plots for Response surface and contour presenting the effects of temperature (1C) and TMP (bar) on the retention of potassium by biodiesel membrane separation: (a) response surface 3D and (b) contour plot (2D). (b) Plots for Response surface and contour presenting the effects of flow rate (L/min) and TMP (bar) on the retention of potassium by biodiesel membrane separation: (a) response surface 3D and (b) contour plot (2D). (c) Plots for Response surface and contour presenting the effects of flow rate (L/min) and temperature (1C) on the retention of potassium by biodiesel membrane separation: (a) response surface 3D and (b) contour plot (2D).
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illustrates surface response and the contour plots of the predicted K retention. The figures also illustrated that at higher TMP, the K retention coefficient is low. Further the removal of K is more favoured with the high temperature used. However the highest temperature (50 1C) used in this experiment is still below the temperature used by Wang et al. [2]. The favourability of K retention at high temperatures could possibly indicate that the temperatures used could not influence the solubility of the polar compounds to the level of allowing the compounds to permeate through the membrane. As a result more K was retained in the membrane module. It can be seen that, high flow rate favoured retention of K, but higher TMP shows negative effects on the retention of K. 3.4. Optimization There are quite a number of methods available to optimize a process. Baroutian et al. [13] reported that based on the model predicted, which was statistically validated, numerical hill-climbing algorithms was used to investigate the most desirable outcome. Further Tamunaidu and Bhatia [25] remarked that numerical optimization provides up-to-date and a comprehensive description of the most effective methods in continuous optimization. It responds to the growing interest in optimization in business, science, and engineering by focusing on the techniques that are most suited to practical problems. The process parameters and responses (retention of free glycerol and potassium) with respect to low and high limits satisfy the criterion defined for the optimal conditions are presented in Table 6. The optimization was carried based on the limits of process parameters and responses generated for membrane with pore sizes of 0.02 mm; thus the optimum conditions are presented in Table 7. At these optimum conditions, the values of free glycerol (0.007 wt%) and potassium (0.297 mg/L) were all below ASTM standard specifications for
biodiesel fuel. Also the physical properties of biodiesel at the optimum conditions met both ASTM D6751 and EN 14214 as presented in Table 8. Good agreement between the results obtained for experimental and predicted values verified the validity of the models and the existence of the optimum conditions. The accuracy of the models can be further justified by p-values presented in the ANOVA tables. The results obtained confirmed that RSM with appropriate design of experiment can be successfully used for the optimization of the process parameters in a separation process. Thus, this study is focused on the application of RSM to optimize biodiesel membrane separation process. The optimization process may offer useful information pertaining to the development of efficient and economic processes for the purification of crude biodiesel using membrane systems. 3.5. Membrane cleaning process Membrane cleaning is as important as the ultrafiltration process itself, since it is necessary in the determination of the economic and technical practicability of the process on a commercial scale, where efficiency and repeatability are quite important. Fig. 9 presents some permeate fluxes after membrane cleaning process. As can be seen in Fig. 9, the permeate fluxes are comparable to the initial permeate fluxes generated as mentioned in Section 3.2. This indicated that the performance of the membrane is maintained during the course of the experimental runs.
4. Conclusion Based on the investigations carried out, the following conclusions can be made: 100
Ultimate goal
Experimental region
TMP (bar) Temp (1C) Flow rate (L/min) Membrane pore size (0.02 lm) Free glycerol retention (%) Potassium retention (%)
In range In range In range
13 30 50 60 150
In range In range
94.82 99.25 87.34 96.21
Permeate fulx (kg/m2hr)
Table 6 Constrains for the parameters and responses in numerical optimization. Parameters
163
TMP 1 bar TMP 2 bar TMP 3 bar
80
60
40
20
Table 7 Optimization results and model evaluation. TMP (bar)
Temp (1C)
%R (free glycerol) %R (potassium) Flow rate (L/min) Predicted Experimental Predicted Experimental
2
40
150
98.84
98.32
94.53
0 0
10
20 30 Time (min)
Properties
Test method
Unit
ASTM Standard
EN 14214
Membrane process
Flash point
ASTM D93-07 ASTM D445-06 ASTM D4052-96 ASTM D2500 ASTM D97-93
1C
130 min
4101
179
mm2/s
1.9–6.0
3.5 5.0
4.91
kg/m3 1C 1C
Report
860 900
878 14 5
Density Cloud point Pour point
50
Fig. 9. Permeate flux vs. time using distilled water at a flow rate of 150 (L/min) and a temperature of 50 1C.
94.79
Table 8 Comparison of physical properties of biodiesel produced.
Viscosity
40
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1. Application of ceramic membrane consisting of 0.02 mm pore size has permitted efficient separation of crude biodiesel from different contaminants. 2. Application of acidified water was a key factor to the successful retention of glycerol and catalyst (potassium) during the course of membrane purification of crude biodiesel. 3. The membrane provided better permeate (less impurities biodiesel). And at optimum conditions, the membrane provided biodiesel with physical properties that met both ASTM D6751 and EN 14214. 4. The application of central composite design (CCD) coupled with Response Surface Methodology was found to provide clear understanding of the interaction between various process parameters for the purification of biodiesel. 5. The results obtained showed significant effects of the process parameters (TMP, temperature and flow rate). However overall assessment showed that the membrane performed better at moderate transmembrane pressures than at higher temperature. 6. The optimum condition obtained were TMP, 2 bar, temperature, 40 1C and flow rate, (150 L/min) with corresponding permeate flux of 9.08 (kg/m2 h). 7. Compared to the membrane with a pore size of 0.05 mm, membrane with a pore size of 0.02 mm presented lower permeate fluxes but high-quality biodiesel was achieved. This is a demonstration of the versatility of membrane technology. Even if in the future the standards of biodiesel quality become more stringent, ceramic membranes with pore size of 0.02 mm or lower could be used. However a compromise has to be made between permeate flux and biodiesel quality when selecting the membranes. 8. It was found that after several months of application, the ceramic membrane used retained its performance capabilities. 9. Although membrane processes are more environmentally friendly compared to biodiesel water washing, further studies need to be carried out to assess the amount of wastewater discharged due to membrane cleaning process.
References [1] M.A. Dube´, A.Y. Tremblay, J. Liu, Biodiesel production using a membrane reactor, Bioresour. Technol. 98 (2007) 639–647. [2] Y. Wang, X. Wang, Y. Liu, S. Ou, Y. Tan, S. Tang, Refining of biodiesel by ceramic membrane separation, Fuel Process. Technol. 90 (2009) 422–427. [3] I.M. Atadashi, M.K. Aroua, A.A. Abdul Aziz, High quality biodiesel and its diesel engine application: a review, Renew. Sustainable Energy Rev. 14 (2010) 1999–2008. [4] M.C.S. Gomes, N.C. Pereira, S.T.D. Barros, Separation of biodiesel and glycerol using ceramic membranes, J. Membr. Sci. 352 (2010) 271–276.
[5] P. Jaruwat, S. Kongjao, M. Hunsom, Management of biodiesel wastewater by the combined processes of chemical recovery and electrochemical treatment, Energy Convers. Manage. 51 (2010) 531–537. [6] R. van Reis, A. Zydney, Bioprocess membrane technology, J. Membr. Sci. 297 (2007) 16–50. [7] D.Y.C. Leung, X. Wu, M.K.H. Leung, A review on biodiesel production using catalyzed transesterification, Appl. Energy 87 (2010) 1083–1095. [8] I.M. Atadashi, M.K. Aroua, A.R. Abdul Aziz, N.M.N. Sulaiman, Membrane biodiesel production and refining technology: a critical review, Renew. Sustainable Energy Rev. 15 (2011) 5051–5062. [9] R. Sondhi, R. Bhave, G. Jung, Applications and benefits of ceramic membranes, Membr. Technol. 2003 (2003) 5–8. [10] H.Y. He, X. Guo, S.L. Zhu, Comparison of membrane extraction with tradition extraction methods for biodiesel production, JAOCS 83 (2006) 457–460. [11] M.C.S. Gomes, A.A. Pedro, C.P. Nehemias, Biodiesel production from degummed soybean oil and glycerol removal using ceramic membrane, J. Membr. Sci. 378 (2011) 453–461. [12] J. Saleh, M. Dube, A. Tremblay, Effect of soap, methanol, and water on glycerol particle size in biodiesel purification, Energy Fuels (2010), http://dx.doi.org/ 10.1021/ef1011353, xxx:000–000. [13] S. Baroutian, M.K. Aroua, A.A.A. Raman, N.M.N. Sulaiman, A packed bed membrane reactor for production of biodiesel using activated carbon supported catalyst, Bioresour. Technol. 102 (2011) 1095–1102. [14] M. Zabeti, W.M.A. Wan Daud, M.K. Aroua, Activity of solid catalysts for biodiesel production: a review, Fuel Process. Technol. 90 (2009) 770–777. [15] R.C. Basso, L.A. Viotto, L.A. Guaraldo Gonc- alves, Cleaning process in ceramic membrane used for the ultrafiltration of crude soybean oil, Desalination 200 (2006) 85–86. [16] D. Naviglio, R. Romano, F. Pizzolongo, A. Santini, A. de Vito, L. Schiavo, G. Nota, S.S. Musso, Rapid determination of esterified glycerol and glycerides in triglyceridefats and oils by means of periodate method after transesterification, Food Chem. 102 (2007) 399–405. [17] A. de Jesus, M.M. Silva, M.G.R. Vale, The use of microemulsion for determination of sodium and potassium in biodiesel by flame atomic absorption spectrometry, Talanta 74 (2008) 1378–1384. [18] M. Berrios, R.L. Skelton, Comparison of purification methods for biodiesel, Chem. Eng. J. 144 (2008) 459–465. [19] J. Saleh, A.Y. Tremblay, M.A. Dube´, Glycerol removal from biodiesel using membrane separation technology, Fuel 89 (2010) 2260–2266. [20] I.M. Atadashi, M.K. Aroua, A.R. Abdul Aziz, N.M.N. Sulaiman, Optimization of membrane ultrafiltration process for the removal of free glycerol and soap from crude palm biodiesel, J. Sep. Purif. Technol., submitted for publication. [21] J. Saleh, M.A. Dube´, A.Y. Tremblay, Separation of glycerol from FAME using ceramic membranes, Fuel Process. Technol. 92 (2011) 1305–1310. [22] H. Choi, Z. Kai, D.D. Dionysios, B.O. Daniel, A.S. George, Influence of cross-flow velocity on membrane performance during filtration of biological suspension, J. Membr. Sci. 248 (2005) 189–199. [23] C. Almandoz, C. Pagliero, A. Ochoa, J. Marchese, Corn syrup clarification by microfiltration with ceramic membranes, J. Membrane Sci. 363 (2010) 87–95. [24] N. Taufiqurrahmi, A.R. Mohamed, S. Bhatia, Production of biofuel from waste cooking palm oil using nanocrystalline zeolite as catalyst: process optimization studies, Bioresour. Technol. 102 (2011) 10686–10694. [25] P. Tamunaidu, S. Bhatia, Catalytic cracking of palm oil for the production of biofuels: optimization studies, Bioresour. Technol. 98 (2007) 3593–3601. [26] S. Hu, Y. Wang, H. Han, Utilization of waste freshwater mussel shell as an economic catalyst for biodiesel production, Biomass Bioenergy 35 (2011) 3627–3635. [27] D.C. Montgomery, Design and Analysis of Experiment, fifth ed, Wiley, Inc., New York, USA, 1997. [28] A. Casas, C.M. Ferna´ndez, M.J. Ramos, A´. Pe´rez, J.F. Rodrı´guez, Optimization of the reaction parameters for fast pseudo single-phase transesterification of sunflower oil, Fuel 89 (2010) 650–658.
Contents lists available at SciVerse ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
High quality biodiesel obtained through membrane technology I.M. Atadashi, M.K. Aroua n, A.R Abdul Aziz, N.M.N Sulaiman Chemical Engineering Department, Faculty of Engineering, University Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 April 2012 Received in revised form 3 July 2012 Accepted 4 July 2012 Available online 24 July 2012
In this study, a ceramic membrane with a pore size of 0.02 mm was used to purify crude biodiesel to achieve biodiesel product that meet both ASTM D6751 and EN 14241 standards specifications. The membrane system was successfully developed and used for the purification process. Process operating parameters such as transmembrane pressure, flow rate and temperature were investigated. Application of central composite design (CCD) coupled with Response Surface Methodology (RSM) was found to provide clear understanding of the interaction between various process parameters. Thus, the process operating parameters were then optimized. The optimum conditions obtained were transmembrane pressure, 2 bar, temperature, 40 1C and flow rate, 150 L/min with corresponding permeate flux of 9.08 (kg/m2 h). At these optimum conditions, the values of free glycerol (0.007 wt%) and potassium (0.297 mg/L) were all below ASTM standard specifications for biodiesel fuel. In addition the physical properties of biodiesel at the optimum conditions met both ASTM D6751 and EN 14214. This work showed that with ceramic membrane of pore size 0.02 mm, biodiesel with high qualities that meet the stringent standards specifications more than those currently in application can be achieved. & 2012 Elsevier B.V. All rights reserved.
Keywords: Ceramic membrane Palm oil Biodiesel Permeate flux Optimization
1. Introduction Biodiesel is a clean-burning fuel derived from vegetable oils, animal fats, or grease. The chemical structure of commercial biodiesel is fatty acid alkyl esters (FAAE) [1]. Due to renewability, low gaseous emissions and biodegradability, biodiesel is becoming very popular in the European Union (EU) which has set an objective to secure for motor biofuels a market share of 20% of the total motor fuel consumption by 2020 [2]. Because biodiesel is fully prepared from biomass material, it has minimal crude oil residues or metals, aromatic hydrocarbons, and sulfur. Like petrodiesel, biodiesel operates in diesel engines such as those used in private and commercial vehicles and farm equipment. Basically modifications of the engine are not required, and biodiesel maintains the payload capacity and range of petro-diesel. As biodiesel is oxygenated, it is more completely combusted. Besides it has better lubricity than petro-diesel, hence increasing the life times of the diesel engines. Further the higher flash points of biodiesel make it a safer fuel to store, handle and use [1,3]. Also, moderately lower emission profile of esters make them an ideal fuel for application in sensitive environments, such as heavily polluted cities, national parks and forests and marine areas [3]. Biodiesel is most commonly prepared through alkali-catalyzed transesterification. Transesterification is a chemical reaction between
n
Corresponding author. Tel.: þ603 796 746 15; fax: þ603 796 753 19. E-mail address: [email protected] (M.K. Aroua).
0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.07.006
triglycerides and alcohol in the presence of catalyst (acid, alkali, or enzyme) as shown in Fig. 1 [2]. This reaction produces biodiesel as a primary product and glycerol as a secondary product. Commercially, biodiesel is produced from refined oils through one-step or two-step alkali-catalyzed transesterification. The schematic diagram of the conventional alkali-catalyzed transesterification is presented in Fig. 2 [3]. After transesterification reaction is completed, removal of glycerol is the first step to be carried out. And because of the difference in polarities and larger density difference between glycerol and esters, separation of glycerol is usually quick. Separation of glycerol from biodiesel mixture is usually achieved through gravitational settling or centrifugation. After glycerol is separated, crude biodiesel is subjected to either distillation or rotary evaporation in order to remove the residual alcohol. Conventionally biodiesel is purified via water washing and dry washing methods. Water washing is used to remove the remaining glycerol, soap, catalyst, methanol or salts from the alkyl esters [4]. After water washing is completed, the remaining water in biodiesel is removed; vacuum flash process, anhydrous Na2SO4, magnesol, amberlite, purolite etc., can be used as a final step to dry biodiesel. The process of biodiesel water washing usually provides final biodiesel product that satisfies the stringent biodiesel standards such as EN 14214 and ASTM D6751. However, separation of hot wash water and the acid from biodiesel in some cases requires application of two centrifuges. Besides, water washing process generates wastewater containing impurities such as free glycerol, residual catalyst and soap which must be carefully handled before being discharged. Otherwise disposal of
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biodiesel wastewater could negatively affect the environment [5]. The problems associated with water washing have led to the use of dry washing process to purify crude biodiesel. Although the process of dry washing with magnesol, or ion exchange resins, provides biodiesel with low impurities and good physicochemical properties, the lack of adsorbents reusability and little knowledge about the chemistry of the adsorbents discourage their use. Presently, membrane technology is being explored and exploited for the purification of biodiesel. Generally membrane processes play a critical role in purifying biotechnology products [6]. Membrane based separations are well-established technologies in protein separations, water purification, and gas separations. The use of membrane separation for the treatment of non-aqueous fluids is an emerging field in membrane technologies [2,7]. Membranes can be inorganic or organic (polymeric) in nature. The inorganic membranes particularly ceramic membranes are more appropriate for use with organic solvents because of their excellent thermal and chemical stability and their ability to withstand higher temperatures and pressures [1,2,8]. Further, the exceptional physical and chemical stability of ceramic membranes permits them to offer reproducible performance over long service lifetimes, which is well proven in many industrial installations. Also the ability of ceramic membranes to recover valuable products, concentrate process streams, and increase yields makes them a preferred and cost-effective method of filtration [9]. Several biodiesel membrane purification processes were evaluated with high-quality biodiesel being reported, in some cases biodiesel purity above 99% was achieved [4,10,11,12]. In a search of the literature, the smallest membrane pore size used by previous researchers was 0.05 mm. In addition most of the studies focused only on single impurity retention. To the best of our knowledge there is no research conducted so far on the purification of crude biodiesel using membrane with pore size of 0.02 mm. Therefore in this study, a ceramic membrane with a pore size of 0.02 mm was used to purify crude biodiesel considering simultaneous retention of glycerol and catalyst. The current work
155
also employed Central Composite Design (CCD) coupled with Response Surface Methodology (RSM) to optimize the process parameters such as transmembrane pressure, flow rate and temperature, therefore permitting the analysis of interaction between them in order to find out the optimum operating conditions.
2. Experimental 2.1. Transesterification reaction The transesterification of palm oil to biodiesel was carried out in a fume hood using a 5-liter batch reactor coupled with a thermometer, and a mechanical stirrer. The temperature of the reactor was maintained during the entire reaction time using a thermostatic water bath. In each run, 2.5 l of palm oil was charged into the reactor and agitated until the required reaction temperature was achieved. Subsequently, a mixture of potassium hydroxide and methanol, earlier mixed until total dissolution, was charged into the reactor. With regard to mass of the palm oil, 1 wt% of potassium hydroxide was used for the transesterification reaction. In addition the transesterification process was performed at a reaction temperature of 60 1C, reaction time of 1 h and at an ambient pressure. The reacting mixture was continuously mixed at an agitation speed of 645 rpm. After the transesterification process was completed, the byproduct, glycerol was separated overnight through gravitational settling. Although separation of glycerol from biodiesel via gravitational is time consuming, the process is less costly compared to centrifugation process [4]. Afterward, the biodiesel sample was submitted to rotary evaporation at a temperature of 65 1C under 600 mmHg vacuum for 45 min for the recovery of the residual methanol. Furthermore, a good number of experimental runs were performed to generate sufficient amount of biodiesel samples for the slated number of membrane purification experiments. The samples of the biodiesel were then analyzed to determine the initial concentrations of the contaminants present in the biodiesel samples. The biodiesel samples produced were then properly stored for membrane purification process. 2.2. Purification of biodiesel using membrane technology
Fig. 1. Transesterification process for the conversion triglycerides to biodiesel.
2.2.1. Membrane type A multi-channel tubular-type Al2O3/TiO2 ceramic membrane consisting of filtration area of 0.031 m2 was used for the experimental runs. The membrane with pore size of 0.02 mm was obtained from Jiangsu Jiuwu Hitech Co., China.
Dryer
Fat and oil
Alcohol
Transesterification reactor
Mixer
Settler (Centrifuge/ Decanter)
Biodiesel
Removal of excess methanol
Refined biodiesel
Neutralization /Distilled water washing
Acid (H3PO4)
Wastewater for treatment
Catalyst Distilled water Fig. 2. Process flow schematic diagram for biodiesel production and purification process.
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2.2.2. Permeate fluxes Distilled water was used to obtained the initial permeate fluxes. The initial permeate flux values were determined using distilled water at a temperature of 50 1C, flow rate of 150 L/min and transmembrane pressures of 1, 2 and 3 bar. The performance and efficiency of the membrane cleaning process were monitored using these operational conditions. Eq. 1 was used to determine the permeate fluxes: J¼
Q At
ð1Þ
where J ¼flux, Q¼mass (kg), A¼area (m2) and t ¼time (h). For the membrane ultrafiltration of crude biodiesel, the permeate flux was presented as a function of time. Using experimental conditions in Table 1, different permeate fluxes were generated. Furthermore, the permeate fluxes obtained with membrane of pore size 0.02 mm were also compared with permeate fluxes generated using membrane of pore size 0.05 mm.
Table 1 Operating conditions for the biodiesel membrane separation process. Run
TMP (bar)
Temperature (1C)
Flow rate (L/min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1.00( 1) 3.00(þ 1) 1.00( 1) 3.00(þ 1) 1.00( 1) 3.00(þ 1) 1.00( 1) 3.00(þ 1) 1.00( 1) 3.00(þ 1) 2.00(0) 2.00(0) 2.00(0) 2.00(0) 2.00(0) 2.00(0) 2.00(0) 2.00(0) 2.00(0) 2.00(0)
30.00( 1) 30.00( 1) 50.00( þ1) 50.00( þ1) 30.00( 1) 30.00( 1) 50.00( þ1) 50.00( þ1) 40.00(0) 40.00(0) 30.00( 1) 50.00( þ1) 40.00(0) 40.00(0) 40.00(0) 40.00(0) 40.00(0) 40.00(0) 40.00(0) 40.00(0)
60.00( 1) 60.00( 1) 60.00( 1) 60.00( 1) 150.00(þ 1) 150.00(þ 1) 150.00(þ 1) 150.00(þ 1) 105.00(0) 105.00(0) 105.00(0) 105.00(0) 60.00( 1) 150.00(þ 1) 105.00(0) 105.00(0) 105.00(0) 105.00(0) 105.00(0) 105.00(0)
2.3. Membrane ultra-filtration process 2.3.1. Process parameters The main process operating parameters investigated for the membrane ultrafiltration process are temperature, transmembrane pressure and flow rate. These process parameters were varied: temperature (30–50 1C), transmembrane pressure (TMP) (1–3 bar), and flow rate (60–150 L/min). The permeate flux was expressed as kg/m2 hr and the free glycerol and potassium contents as percentage. In another study conducted by Gomes et al. [11] to purify crude biodiesel using membrane system, transmembrane pressure of 1–3 bar and temperature of 50 1C were used. The authors noted significant removal of biodiesel impurities such as glycerol. Furthermore, Gomes et al. [4] used transmembrane pressure of 1–3 bar and a temperature of 60 1C during membrane biodiesel separation process and achieved considerable glycerol reduction in biodiesel. Also, Wang et al. [2] adopted flow rate of 50–150 L/min and temperature of 30– 70 1C for the purification of crude biodiesel and obtained reasonable glycerol removal with glycerol content in biodiesel below the ASTM D6751 and EN 14214 standards specification.
2.3.2. Separation process Fig. 3 illustrates experimental setup of biodiesel ceramic membrane separation unit. The setup consists of crude biodiesel feed tank, water bath, circulating pump (digital Masterflex L/S peristaltic) membrane module, digital weighing balance etc. Also the experimental setup was provided with pump tubing (ChemDurance chemical resistant) with a size of 16 (ID ¼44 mm, OD¼2.36 mm). The pressure and temperature were monitored using pressure gauges and temperature indicator. The crude biodiesel was charged into feed tank with a capacity of 5 l and circulated using a pump through the membrane tube at operating conditions presented in Table 1. The inlet and outlet pressures were adjusted using the valves at the end of the membrane tubes to determine the proper operating transmembrane pressure. Thermostatic water bath was used to monitor the system temperature. In this study, 20 experimental runs were carried out. Further acidified water was added to improve the retention of biodiesel contaminants in the membrane system. The experimental module was operated by recycling the biodiesel concentrate. The mass permeate fluxes were automatically recorded by a
Retentate
Feed tank Inlet pressure
Outlet pressure
P
P Ceramic membrane
Valve Valve
Heating equipment
Valve
Circulating pump Permeate Digital balance
Fig. 3. Experimental setup of biodiesel ceramic membrane separation unit.
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digital balance every 12.5 min during the filtration process. All the runs were halted after 1.15 h filtration time. The initial samples, numbered as original samples and the permeates, were collected after the experiments and analyzed. The separation of impurities using membranes reduces the number of steps required in the conventional treatment, besides less water is consumed [11]. 2.3.3. Experimental design The experiments were designed using Design of experiment software Version 8.0.0 (Stat-Ease Inc., USA). The software was used to optimize the effects of process parameters such as transmembrane pressure, flow rate, and temperature for the purification of crude biodiesel. It is noteworthy to mention that several optimization methods such as Full Factorial Design, Central Composite Design, and D-optimal Designs etc. are used to optimize a process. In this study, Central Composite Design (CCD) coupled with Response Surface Method (RSM) was selected. The free glycerol and potassium contents are the main responses. To provide a true measure of error due to the natural variations, six replicated center points were chosen and conducted in randomized order. The number of replicates is selected to offer a wide region where the standard error of prediction remains reasonably stable. Table 2 presents coded and actual levels of the process parameters. The process parameters are transmembrane pressure 1–3 bar, flow rate 60–150 L/min, and temperature 30–50 1C. The coded values are designated as 1 (low), 0 (medium), þ1 (high), a and þ a. Alpha (a) is a distance from the center point which might either be inside or outside the range, with the high value of 2k/4(where k is the number of factors). The value of alpha was set at 1 to generate a face-centered central composite design. This is required because it is only a three-level design, which can ensure that the axial runs will not be anymore extreme values than the factorial portion [13]. Compared to the conventional approach, which utilizes one parameter at a time, this method can determine the interaction between effects of process operating parameters, besides it can provide good estimations of errors. Zabeti et al. [14] reported that the experimental cost and time could be reduced since the total number of trials is minimized. The levels of each process operating parameter were chosen based on the works conducted [2,4,11]. The relationship between dependent and independent parameters in this study is explained by the following second-order polynomial model: Y ¼ bo þ
k X
biXi þ
i¼1
k X
biiXi2 þ
i¼1
k X k X
bijXiXj
ð2Þ
i¼1j¼1
where xi are the input variables, which impact the response variable Y, and b0, bi, bii and bij are the regression coefficients. 2.4. Membrane cleaning process In membrane cleaning, components are physically, chemically or hydraulically removed. When the membrane cleaning process is conducted, a membrane module is temporarily out of order. In practice, it is always good to apply the lowest possible cleaning time and make filtration time last as long as possible [15]. The cleaning process is more effective when using transmembrane Table 2 Process parameters levels in actual and (coded) forms. Parameters
Unit
Low level
High level
TMP Temp Flow rate
bar 1C L/min
1( 1) 30( 1) 60( 1)
3( þ1) 50( þ) 150( þ)
157
pressure of 0.45 bar. In this experiment, the membrane module was thoroughly cleaned to restore or maintain the performance of the membrane in terms of its permeability and protect the equipment. The cleaning process of the membrane was carried out using water and detergent until the biodiesel was completely removed. For 45 min, a solution of 1% NaOH at a temperature of 70 1C was circulated. The module was cleaned with water and then rinsed with warm distilled water [4]. The process of membrane cleaning was quite efficient and fast. To ascertain the reproducibility of the experiments, the permeate flux using distilled water was recorded after each cleaning process. It was observed that the permeate flux values obtained after membrane cleaning were almost identical to the initial permeate flux values. 2.5. Characterization of produced biodiesel To establish the efficiency of the membrane separation process, the initial concentrations of biodiesel contaminants (free glycerol, and catalyst (potassium)) were first determined. The level of the contaminants were measured based on the methods provided by biofuels standard specifications for related oils such as modified version of the AOCS technique for the analysis of free glycerol in oils and fats (Ca 14–56), and ASTM D6751-03 for potassium determinations. Furthermore, the physical properties of the biodiesel produced were determined at the optimum operating conditions. 2.5.1. Measurement of free glycerol content in biodiesel To determine the concentration of glycerol in the biodiesel samples, the modified version of the AOCS technique for the analysis of free glycerol in oils and fats (Ca 14–56) was used. The technique involved reaction of glycerol in aqueous medium with excess sodium periodate to form formaldehyde, formic acid, and iodic acid, and then addition of potassium iodate to react with the formed sodium periodate and the iodic acid. In comparison to gas chromatography, the determination of free glycerol using periodates titration gives low cost. Moreover, the method is simple and straightforward, fast and sufficiently reliable. The method involving periodate as an oxidant reagent for the determination of free glycerol was carefully evaluated and it was found that the method is very successful in relation to precision and accuracy [4]. Further investigation on the determination of glycerides in oils and esterified glycerol using periodate method after transesterification showed similar results. The method is straightforward and easily reproducible with great precision [16]. The glycerol content in both the feed and permeate were examined after each run. 2.5.2. Determination of potassium content in biodiesel The content of potassium in both crude and refined biodiesel was determined using Inductively Coupled Plasma (ICP). The analysis of the biodiesel samples was based on the techniques prescribed by international standard specification (European: EN 14109 and EN 14108). In these techniques, the sample dilution is conducted using organic solvents (xylene, petroleum ether or cyclohexane). In this study, the dilution of the samples and organometallic standards for the calibration was carried out using xylene as an organic solvent. The analytical standards were prepared from a stock solution of 100 mg kg 1, pipetting the volume to give concentrations in the range of 0.0–2.0 mg L 1 in 10 mL volumetric flasks. The samples were prepared weighing 0.4 g and 1.0 g of sample for the determination of potassium, in a volumetric flask of 10 mL, and then diluted with xylene. The standards and samples were analyzed immediately after the dilution. The fuel/oxidant ratio and aspiration rate were optimized prior to starting the analysis. Moreover, organometallic
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standards were used to spike both the biodiesel and oil samples, then dilution with xylene was performed [17]. The coefficients of retention (%R) of free glycerol and potassium contents were calculated using Eq. 3: %R ¼
½ðC f -CperÞ 100 Cf
ð3Þ
where Cf and Cper are the mass fractions in the feed and the permeate for free glycerol and potassium, respectively. 2.5.3. Determination of physical properties of biodiesel The physical properties of the biodiesel produced were investigated in accordance to the techniques prescribed by the American Society for Testing and Materials such as viscosity at 40 1C (ASTM D445-06) flash point (ASTM D93-07), pour point (ASTM D97-93), cloud point (ASTM D2500), and density at 15 1C (ASTM D4052-96).
3. Results and discussions 3.1. The biodiesel separation process The application of ceramic membrane with a pore size of 0.02 mm has presented high retention of free glycerol and potassium during biodiesel purification as shown in Table 3. Both ASTM D6751 and EN 14214 standards specified glycerol content in biodiesel to be 0.02 wt%. Thus, achievement of low free glycerol content in biodiesel provides numerous advantages such as reduced settling problems and low aldehydes and acrolein emissions. Furthermore low catalyst concentration in biodiesel prevents deposits in the injectors (carbon residue), reduces filter blockage (sulphated ashes) and decreases possibilities for engine weakening [18]. The separation of biodiesel from glycerol and potassium was achieved using membrane ultrafiltration process. Before starting the ultrafiltration process, crude biodiesel samples were subjected to rotary evaporation to completely remove the residual methanol. Afterward acidified water was added to the biodiesel samples. Results of the tests conducted showed that addition of acidified water improved the separation process. The addition of acidified water led to the formation of aqueous phase containing catalyst, salt, glycerol, and other related water-soluble substances. This has reduced the solubility of biodiesel in glycerol, Table 3 Concentrations of free glycerol and potassium in permeate (final biodiesel). Run
Free glycerol (wt%)
Potassium (mg/L)
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
0.023 0.019 0.025 0.0.22 0.018 0.024 0.016 0.023 0.025 0.016 0.014 0.008 0.018 0.006 0.021 0.017 0.019 0.007 0.009 0.017
0.561 0.523 0.495 0.532 0.451 0.632 0.343 0.491 0.389 0.456 0.367 0.294 0.332 0.276 0.435 0.391 0.534 0.436 0.393 0.285
producing agglomeration of larger droplets of glycerol which are too big to pass through the membrane pores [11]. Besides, addition of acidified water helps in breaking down the soap into water soluble salt and free fatty acids, with the free fatty acids remaining in the biodiesel. Acid neutralizes the catalyst as well, forming salt and water. Thus the ability of the membrane to retain the aqueous phase containing the contaminants led to the generation of permeates (biodiesel) with low concentrations of contaminants whose values were in some cases below those prescribed by ASTM D6751 and EN 14214 standards as presented in Table 3. The significant retention of free glycerol mass content lower than 0.02% and significant reduction of potassium mass content demonstrates the effectiveness of the methodology used. Improvement in the performance of membrane for the separation of biodiesel and free glycerol following water addition was investigated by Saleh et al. as well [19]. After free glycerol and biodiesel were separated through decantation process for about 8–12 h, polymeric membrane was used to purify the biodiesel obtained. The addition of water to the crude biodiesel samples ranging from 0.06 to 0.2 wt% aided the retention of glycerol by the formation of two immiscible phases: a water and glycerol phase, and a biodiesel phase which suggests that the principle of separation of free glycerol from biodiesel is that of the retention of a finely dispersed water and glycerol phase by the membrane.
3.2. Permeate fluxes The initial permeate fluxes were determined using distilled water. The initial permeate flux values obtained using distilled water at a temperature of 50 1C, flow rate of 150 L/min and transmembrane pressures of 1, 2 and 3 bar were 57 kg/m2 h, 64 kg/m2 h and 76 kg/m2 h respectively. These operational conditions were used as a reference to monitor the performance and efficiency of the membrane cleaning process. Some of the permeate fluxes derived during biodiesel membrane purification process are presented in Fig. 4. The fluxes determined differ and were dependent on the combination of the process operating parameters. In cross flow filtration process, the gel layer continuously builds up until it reaches a certain thickness, then the hydrophilic compounds mount up on the surface of the membrane and form larger droplets that are removed from the layer. A steady state thickness of the gel layer can be seen in Fig. 4. This figure reveals a decrease in the permeate flux in the first (37.5 min), which during course of the runs stabilized. The permeate fluxes presented in Fig. 4 for the membrane with a pore size of 0.02 mm are lower 35 30 Permeate Flux (kg/m2hr)
158
25 20 15 10 5 0 0
20
40 Time (min)
60
Fig. 4. Permeate flux vs. time for membrane of pore size 0.02 mm.
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Permeate flux (kg/m2hr)
100
159
Table 4 Retention coefficients (%R1 and %R2) of glycerol and potassium.
80
Run order
TMP (bar)
Temp. (1C)
Flow rate (L/min)
%R1 (free glycerol)
%R2 (potassium)
60
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1.00 3.00 1.00 3.00 1.00 3.00 1.00 3.00 1.00 3.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00
30.00 30.00 50.00 50.00 30.00 30.00 50.00 50.00 40.00 40.00 30.00 50.00 40.00 40.00 40.00 40.00 40.00 40.00 40.00 40.00
60.00 60.00 60.00 60.00 150.00 150.00 150.00 150.00 105.00 105.00 105.00 105.00 60.00 150.00 105.00 105.00 105.00 105.00 105.00 105.00
95.86 96.25 97.50 95.62 97.27 95.76 99.14 94.82 97.15 96.01 98.15 99.25 97.65 99.21 98.52 99.07 98.56 99.05 98.15 99.20
90.45 87.34 94.14 90.89 94.67 91.25 95.15 90.29 94.92 92.21 95.48 94.83 91.15 95.13 95.37 96.21 95.52 94.72 94.21 96.15
40
20
0 0
10
20
30
40
50
60
70
Time (min) Fig. 5. Permeate flux vs. time for membrane of pore size 0.05 mm.
compared to those generated from membrane with a pore size of 0.05 mm as shown in Fig. 5 [20]. However better quality biodiesel product were achieved via membrane with pore size of 0.02 mm. The lower permeate fluxes generated in Fig. 4 using membrane with a pore size of 0.02 mm could be attributed to the membrane matrix of smaller pore size. Also, as expected, very high permeate flux was reported when membrane with a pore size of 0.1 mm was used for the purification of biodiesel [2]. The larger pore size of the microfiltration (MF) membrane provides a slightly larger flux at the expense of the retention of the glycerol droplets [21]. Choi et al. [22] remarked that continuous flux reduction of ultrafiltration membrane with time is as a result of other ‘‘incrustation’’ phenomena, such as concentration polarization, pore blocking or molecule adsorption followed by the formation of fouling layer. However, back flushing of the membrane module could lead to high permeate flux [23]. In this work, the optimum experimental conditions for membrane with a pore size 0.02 mm is attained at TMP of 2 bar, temperature of 40 1C and flow rate of 150 L/min which provided permeate flux of 9.08 (kg/m2 h). 3.3. Optimization of process parameters To generate high-quality biodiesel, this study focused on the optimization of process parameters such as flow rate, transmembrane pressure and temperature using Response Surface Methodology. Response Surface Methodology was used to improve biodiesel membrane purification which allows low cost biodiesel purification process in an industrial scale. Besides, RSM is selected for the optimization process as it is sufficient enough to analyze the effects of process parameters on membrane performance for the purification of crude biodiesel. The process parameters selected were used to identify the optimum conditions that have effects on the membrane purification of biodiesel. For each experimental run, the coefficients of retention (%R) of free glycerol and potassium contents were recorded. With regard to the results achieved, it was observed that purification of biodiesel is dependent on all the process parameters. Table 4 shows the coefficients of retention for the free glycerol and potassium which varies in the ranges of 94.82– 99.25% and 87.34–96.21% respectively. A major advantage of RSM is that the interactions of factors are considered in the experimental design and easily observed based on the resulting regression equations. Diagnostics of the residuals, the difference between predicted and actual free glycerol and potassium content, showed that transformation is not required to enhance both retentions of free glycerol and potassium models. Thus to
measure the curvature effects with the help of Design Expert software, the experimental data generated were fitted to the polynomials of higher degree equations such as quadratic, and two factor interactions (2FI) models etc. Based on the ANOVA analysis, quadratic models best fitted the experimental data. The designed experiments were analyzed using analysis of variance at 95% level of confidence. Further the quadratic models to predict the retentions of free glycerol and potassium (K) in terms of coded factors are presented in Eqs. 4 and 5. R1 ðFree glycerol Þ ¼ þ 98:710:85A þ 0:30B þ 0:33C0:63AB 0:54A1:000E002BC -2:06A2 þ 0:063B2 0:21C2
ð4Þ
R2 ðKÞ ¼ þ 95:271:74A þ 0:61B þ 1:25C0:20AB0:24AC 0:96BC0:57A2 þ 0:024B2 1:99C2
ð5Þ
where %R is a function of transmembrane pressure (A), temperature (B) and flow rate (C). The positive sign in front of the terms shows synergistic effect while the negative sign indicates antagonistic effect [24]. To assess the goodness of fit, the results generated were then analyzed by the ANOVA as presented in Table 5. The ANOVA showed the effects of single parameters and the interaction between the parameters on the responses. The p value is the probability value used to ascertain the significance of each of the coefficient which may invariably indicate the pattern of relationship between the parameters. The linear terms were all statistically significant as can be seen from the ANOVA table. All the coefficients were considered in the design to minimize error. The closeness of the p-value to zero (0.00) is used to judge the significance of the results. The confidence level should be 95% for the effect to be statistically significant. This indicates that the p-value should be less than or equal to 0.05 [25]. The significance means that the approximate value of the variable coefficient is bigger than a value that would be achieved from the experimental noise. Further the statistical analysis of variance showed overall models’ p-values (probability of error value) to be lower than 0.0001, which indicates that the second-order polynomial regression model is highly significant. Alternatively, the lack of fit is above 0.01 which also indicates that the data are well fitted to the models. In addition, the goodness of fit was determined by evaluating the coefficients of determination (R2) to validate the models. Therefore to establish the reliability of the models the data
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Table 5 ANOVA for the response surface models. Source
Model A-TM B-Temp C-Flow rate AB AC BC A2 B2 C2 Residual Lack of Fit
Retention of free glycerol
Retention of potassium
F-value
p-value
Remarks
F-value
p-value
Remarks
23.73 40.39 5.21 6.22 18.20 13.29 4.514E-003 65.65 0.062 0.66 10 1.10
o0.0001 o0.0001 0.0455 0.0318 0.0016 0.0045 0.9478 o0.0001 0.8085 0.4342 0.18 0.4592
significant
16.2 40.64 5.04 21.16 0.42 0.62 10.06 9.10 2.155E-003 14.72 10 0.86
o 0.0001 o 0.0001 0.0486 0.0010 0.5309 0.4485 0.0100 0.0130 0.9639 0.0033 0.74 1.39
significant
not significant
not significant
obtained should be predicted with sound accuracy by the model in comparison to the experimental data. Fig. 6(a) and (b) presents predicted and experimental values for the retention of free glycerol (a) and K (potassium) (b) using the developed model equations. Tamunaidu and Bhatia [25] reported that values greater than 80% must be obtained for a good agreement between predicted values and experimental values. In this study, the membrane provided coefficients of determination (R2 ¼96%) for free glycerol retention and (R2 ¼ 94%) for potassium. This showed that the accuracy and general availability of the polynomial models are very good, and the response trends can be well analyzed by the model [26]. Besides, the predicted R-square, which is a measure of goodness of the model predicted values for the responses, was above 80% for all values in the experiments conducted. This indicated that the agreement between the predicted values by the models and experimental values is good [25]. Moreover, the adequacy of all the models generated were tested through lack of fit F-test [27]. The lack of fit of the quadratic models were not statistically significant since the probability values were all more than 0.05. And also the model equations have almost accounted all the variations that might have existed. Further adequate precision of the ‘‘model’’ measures the signal to noise ratio. Consequently a ratio greater than 4 is desirable. The models ratios of 13.596 and 13.628 were obtained for the retention of free glycerol and potassium respectively. These values are much higher than 4, this indicated adequacy of the models. Further it was observed that the values were desirable for all the models. Also, lower values of the coefficient of variation (CV) ranging from 0.43% to 1.59% suggested good precision and reliability of the experiments [25]. 3.3.1. Effect of process parameters on the retention of biodiesel impurities The influences of the process parameters investigated over the responses were discussed by means of statistical analysis carried out (main effects and interaction). The sign of the interaction obtained is associated to the type of the interaction. A positive interaction implies that increasing one factor improves the effect of the other factor and vice versa [28]. 3.3.1.1. Effects of process parameters on free glycerol retention The ANOVA Table 5 showed that the linear terms, transmembrane pressure (A), temperature (B) and flow rate (C) as well as the interactions of transmembrane pressure–temperature (AB) and transmembrane pressure–flow rate (AC), and the quadratic term of A2 were statistically significant to the retention of free glycerol during the separation and purification process, with
Fig. 6. (a) Predicted and experimental values for %R1 (free glycerol) and (b) predicted and experimental values for %R2 (potassium).
reference to the p-value less than 0.05. Even though the linear terms are significant but the transmembrane pressure (A) is more significant and has a larger effect due to high F-value of 40.30. The interaction term AB (F-value 18.20) and the quadratic term A2 (F-value 65.65) plays a vital role in the retention of free glycerol. Response surfaces are viewed as two- and three-dimensional plots that present the response as a function of one or two factors. Contour plots obtained using quadratic equations are a powerful tool for optimization process [28]. Fig. 7(a–c) presents 3D and 2D plots for the removal of free glycerol. Fig. 7(a) and (b) shows that
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161
Fig. 7. (a) Plots for Response surface and contour presenting the effects of flow rate (L/min) and TMP (bar) on the retention of free glycerol by biodiesel membrane separation: (a) response surface 3D and (b) contour plot (2D). (b) Plots for Response surface and contour presenting the effects of temperature (1C) and TMP (bar) on the retention of free glycerol by biodiesel membrane separation: (a) response surface 3D and (b) contour plot (2D). (c) Plots for Response surface and contour presenting the effects of flow rate (L/min) and temperature (1C) on the retention of free glycerol by biodiesel membrane separation: (a) response surface 3D and (b) contour plot (2D).
at low TMP, the retention of free glycerol increase with increase in flow rate and temperature. However, at higher TMP, a reduction in the retention of free glycerol can be observed due to fact that the quadratic term of TMP is more significant with a negative effect (Eq. (4)). In both Fig. 7(b) and (c) the effect of temperature
is significant in the retention of glycerol. At higher temperature, there is a considerable increase in the retention of free glycerol, because the quadratic term of temperature is more significant with positive term (Eq. (4)). Thus temperatures above 40 1C could result in achievement of over 99% free glycerol retention. However much
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higher temperatures could favor solubility of glycerol in biodiesel, therefore these need to be avoided. In addition, gradual increase in flow rate created an avenue for the permeation of the glycerol through the membrane pores. The surface response plots showed good favourability of high flow rate in the retention of glycerol.
3.3.1.2. Effects of process parameters on the retention of potassium The ANOVA Table 5 revealed that the terms A, B, C, BC, A2 and B2 were statistically significant during the process of retaining the potassium in the membrane module. The significance of the terms is confirmed by their p-values which are all below 0.05. Fig. 8(a–c)
Fig. 8. (a) Plots for Response surface and contour presenting the effects of temperature (1C) and TMP (bar) on the retention of potassium by biodiesel membrane separation: (a) response surface 3D and (b) contour plot (2D). (b) Plots for Response surface and contour presenting the effects of flow rate (L/min) and TMP (bar) on the retention of potassium by biodiesel membrane separation: (a) response surface 3D and (b) contour plot (2D). (c) Plots for Response surface and contour presenting the effects of flow rate (L/min) and temperature (1C) on the retention of potassium by biodiesel membrane separation: (a) response surface 3D and (b) contour plot (2D).
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illustrates surface response and the contour plots of the predicted K retention. The figures also illustrated that at higher TMP, the K retention coefficient is low. Further the removal of K is more favoured with the high temperature used. However the highest temperature (50 1C) used in this experiment is still below the temperature used by Wang et al. [2]. The favourability of K retention at high temperatures could possibly indicate that the temperatures used could not influence the solubility of the polar compounds to the level of allowing the compounds to permeate through the membrane. As a result more K was retained in the membrane module. It can be seen that, high flow rate favoured retention of K, but higher TMP shows negative effects on the retention of K. 3.4. Optimization There are quite a number of methods available to optimize a process. Baroutian et al. [13] reported that based on the model predicted, which was statistically validated, numerical hill-climbing algorithms was used to investigate the most desirable outcome. Further Tamunaidu and Bhatia [25] remarked that numerical optimization provides up-to-date and a comprehensive description of the most effective methods in continuous optimization. It responds to the growing interest in optimization in business, science, and engineering by focusing on the techniques that are most suited to practical problems. The process parameters and responses (retention of free glycerol and potassium) with respect to low and high limits satisfy the criterion defined for the optimal conditions are presented in Table 6. The optimization was carried based on the limits of process parameters and responses generated for membrane with pore sizes of 0.02 mm; thus the optimum conditions are presented in Table 7. At these optimum conditions, the values of free glycerol (0.007 wt%) and potassium (0.297 mg/L) were all below ASTM standard specifications for
biodiesel fuel. Also the physical properties of biodiesel at the optimum conditions met both ASTM D6751 and EN 14214 as presented in Table 8. Good agreement between the results obtained for experimental and predicted values verified the validity of the models and the existence of the optimum conditions. The accuracy of the models can be further justified by p-values presented in the ANOVA tables. The results obtained confirmed that RSM with appropriate design of experiment can be successfully used for the optimization of the process parameters in a separation process. Thus, this study is focused on the application of RSM to optimize biodiesel membrane separation process. The optimization process may offer useful information pertaining to the development of efficient and economic processes for the purification of crude biodiesel using membrane systems. 3.5. Membrane cleaning process Membrane cleaning is as important as the ultrafiltration process itself, since it is necessary in the determination of the economic and technical practicability of the process on a commercial scale, where efficiency and repeatability are quite important. Fig. 9 presents some permeate fluxes after membrane cleaning process. As can be seen in Fig. 9, the permeate fluxes are comparable to the initial permeate fluxes generated as mentioned in Section 3.2. This indicated that the performance of the membrane is maintained during the course of the experimental runs.
4. Conclusion Based on the investigations carried out, the following conclusions can be made: 100
Ultimate goal
Experimental region
TMP (bar) Temp (1C) Flow rate (L/min) Membrane pore size (0.02 lm) Free glycerol retention (%) Potassium retention (%)
In range In range In range
13 30 50 60 150
In range In range
94.82 99.25 87.34 96.21
Permeate fulx (kg/m2hr)
Table 6 Constrains for the parameters and responses in numerical optimization. Parameters
163
TMP 1 bar TMP 2 bar TMP 3 bar
80
60
40
20
Table 7 Optimization results and model evaluation. TMP (bar)
Temp (1C)
%R (free glycerol) %R (potassium) Flow rate (L/min) Predicted Experimental Predicted Experimental
2
40
150
98.84
98.32
94.53
0 0
10
20 30 Time (min)
Properties
Test method
Unit
ASTM Standard
EN 14214
Membrane process
Flash point
ASTM D93-07 ASTM D445-06 ASTM D4052-96 ASTM D2500 ASTM D97-93
1C
130 min
4101
179
mm2/s
1.9–6.0
3.5 5.0
4.91
kg/m3 1C 1C
Report
860 900
878 14 5
Density Cloud point Pour point
50
Fig. 9. Permeate flux vs. time using distilled water at a flow rate of 150 (L/min) and a temperature of 50 1C.
94.79
Table 8 Comparison of physical properties of biodiesel produced.
Viscosity
40
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1. Application of ceramic membrane consisting of 0.02 mm pore size has permitted efficient separation of crude biodiesel from different contaminants. 2. Application of acidified water was a key factor to the successful retention of glycerol and catalyst (potassium) during the course of membrane purification of crude biodiesel. 3. The membrane provided better permeate (less impurities biodiesel). And at optimum conditions, the membrane provided biodiesel with physical properties that met both ASTM D6751 and EN 14214. 4. The application of central composite design (CCD) coupled with Response Surface Methodology was found to provide clear understanding of the interaction between various process parameters for the purification of biodiesel. 5. The results obtained showed significant effects of the process parameters (TMP, temperature and flow rate). However overall assessment showed that the membrane performed better at moderate transmembrane pressures than at higher temperature. 6. The optimum condition obtained were TMP, 2 bar, temperature, 40 1C and flow rate, (150 L/min) with corresponding permeate flux of 9.08 (kg/m2 h). 7. Compared to the membrane with a pore size of 0.05 mm, membrane with a pore size of 0.02 mm presented lower permeate fluxes but high-quality biodiesel was achieved. This is a demonstration of the versatility of membrane technology. Even if in the future the standards of biodiesel quality become more stringent, ceramic membranes with pore size of 0.02 mm or lower could be used. However a compromise has to be made between permeate flux and biodiesel quality when selecting the membranes. 8. It was found that after several months of application, the ceramic membrane used retained its performance capabilities. 9. Although membrane processes are more environmentally friendly compared to biodiesel water washing, further studies need to be carried out to assess the amount of wastewater discharged due to membrane cleaning process.
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