Pervaporation study for different binary mixtures in the esterification system of lactic acid with ethanol

Separation and Purification Technology 64 (2008) 78–87

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Pervaporation study for different binary mixtures in the esterification system of lactic acid with ethanol Patricia Delgado, María Teresa Sanz ∗ , Sagrario Beltrán Department of Chemical Engineering, University of Burgos, 09001 Burgos, Spain

a r t i c l e

i n f o

Article history: Received 10 January 2008 Received in revised form 16 July 2008 Accepted 4 August 2008 Keywords: Pervaporation PERVAP® Esterification Lactic acid Ethyl lactate

a b s t r a c t Pervaporation experiments were performed for some binary mixtures involved in the esterification of lactic acid with ethanol: water/ethanol, water/ethyl lactate and water/lactic acid. The effects of feed composition and operating temperature on the membrane performance were analyzed. Two different commercial hydrophilic membranes PERVAP® 2216 and PERVAP® 2201 were tested for the system water/ethanol. PERVAP® 2201 membrane was chosen for further experiments due to its better selectivity towards water. The permeation flux, as well as permeances, was found to increase with water feed concentration. The influence of temperature on the permeation flux can be described by an Arrhenius-type expression. However, not a great influence of temperature on the permeance was found. Separation factor and selectivity were found to depend on water feed concentration but not a clear influence of temperature was found on these two parameters. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Lactic acid esters are powerful high-boiling solvents that are biodegradable and non-toxic. Lactate esters can also be used in pharmaceutical preparations, food additives and fragrances. Current production of lactate esters by esterification of lactic acid with the corresponding alcohol suffers from low conversion and purity [1,2]. Esterification reactions are characterized by thermodynamic limitations on conversion. Higher ester yields can be obtained by shifting the reaction towards products formation by hybrid processes such as reactive distillation and pervaporation-aided reactor instead of using a large excess of one of the reactants, usually the alcohol. Pervaporation has gained increasing attention in many chemical processes as an effective energy-saving membrane technique. In this regard, the integration of a pervaporation process into conventional esterification processes is attractive because the separation is based on the transport of the components through the membrane, which is determined by the solubility and diffusivity of the components to be separated and it is not limited by the relative volatility of the components as in distillation processes [3]. In combination with a reactor, pervaporation is used to continuously remove one of the reaction products to shift the equilibrium reaction to higher yields; in most cases the removed product is water.

∗ Corresponding author. Tel.: +34 947 258810; fax: +34 947 258831. E-mail address: [email protected] (M.T. Sanz). 1383-5866/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2008.08.002

For a correct synthesis and design of reaction-pervaporation hybrid processes, reliable information on kinetic reaction and permeation performance is needed. In a previous paper [2] a detailed kinetic study was carried out for the heterogeneous ion exchange (Amberlyst 15) catalyzed synthesis of ethyl lactate: CH3 CHOHCOOH + CH3 CH2 OH  CH3 CHOHCOOCH2 CH3 + H2 O Additionally, vapor liquid equilibrium for the quaternary mixture involved in the process was determined in a previous work [4] to obtain reliable UNIQUAC interaction parameters to take into account the non-ideality of the liquid mixtures. The main aim of this work was to study systematically the effect of feed temperature and water feed composition on the permeation characteristics for different binary mixtures involved in the synthesis of ethyl lactate. The mixtures considered in this work were the following: water/ethanol, water/ethyl lactate and water/lactic acid. The results obtained have been compared in terms of the permeation flux, permeance, separation factor and selectivity obtained for the different mixtures. Two commercial membranes, PERVAP® 2216 and PERVAP® 2201, were tested for water/ethanol mixtures. Although both membranes were water selective, higher fluxes and separation factors were obtained for PERVAP® 2201. Consequently, this membrane was chosen to perform pervaporation experiments for the rest of the binary mixtures. Additionally the swelling behavior of the membrane PERVAP® 2201 was studied for the different components at room temperature.

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side can be calculated from its concentration in the liquid feed: Nomenclature

vapor

apparent activation energy of permeation (kJ mol−1 )

EP J Jo p P MW R T w W x y

permeation flux (kg h−1 m−2 ) pre-exponential Arrhenius factor (kg h−1 m−2 ) pressure (kPa) permeance (kg/h m2 kPa) molecular weight gas constant (kJ mol−1 K−1 ) absolute temperature (K) mass fraction weight of the membrane (g) mole fraction in the feed side mole fraction in the permeate side

where xi is the mole fraction of component i in the feed,  i the activity coefficient and psi the saturation vapor pressure at the temperature of the feed. Combining Eqs. (2) and (3) the permeation flux of a component through the membrane can be expressed as follows: (4)

Pi =

Ji (xi i psi − yi pp )

(5)

The membrane selectivity is defined as the ratio of the permeances. Mole-based permeances have been used in this work to define the membrane selectivity: ˛memb =

Pi /MW,i

(6)

Pj /MW,j

The use of permeance and membrane selectivity is recommended [7] to compare the separation performance of the membranes. These two parameters allow distinguishing the effect of the nature of the membrane and the operating conditions. Wijmans and Baker [8] consider the overall separation factor, ˇpervap , achieved by a pervaporation process as the product of an evaporation separation step, ˇevap , and a membrane separation step ˇmembrane : ˇpervap = ˇevap ˇmemb

(7)

Although the permeating components become vapor after passing the membrane, this equation helps to understand the effect of the partial vapor pressure of the liquid phase on the pervaporation performance [8]. When the permeate partial pressures are much smaller than the feed partial vapor pressures, ˇmemb = ˛memb [8] and Eq. (7) becomes: ˇpervap = ˇevap ˛memb

(8)

The Antoine equation was used in this work to calculate the vapor pressure of each component:

2. Theory The separation performance of a pervaporation membrane can be described in terms of the total permeation flux through the membrane per unit of area and time and the separation factor of the membrane defined as [5]: wi,p wj,f wi,f wj,p

(1)

where wi,p , wj,p are the weight fractions of components i and j on the permeate side, and wi,f and wj,f the weight fractions of components i and j on the feed side. On the basis on the solution/diffusion model the flux of component i through the membrane is proportional to its partial vapor pressures on either sides of the membrane [6]: vapor

(3)

From Eq. (4) the permeance of a membrane is defined as the permeation flux divided by the permeant driving force. In pervaporation processes, the driving force is expressed in terms of the difference in the partial pressure of the component on the feed and on the permeate side:

Subscripts calc calculated value d dry membrane exp experimental value EtOH ethanol EL ethyl lactate f feed HL lactic acid i, j components p permeate w wet membrane W water

Ji = Pi (pi,f

= xi i psi

Ji = Pi (xi i psi − yi pp )

Greek symbols ˛memb membrane selectivity ˇpervap separation factor for the pervaporation process ˇevap evaporation separation factor ˇmemb membrane separation factor ısp Hansen solubility parameter (MPa0.5 ) dispersion Hansen solubility parameter (MPa0.5 ) ıd ıp polar Hansen solubility parameter (MPa0.5 ) ıh hydrogen bonding Hansen solubility parameter (MPa0.5 ) ε dielectric constant  activity coefficient  parameter of Eq. (13)

ˇpervap =

pi,f

− yi,p pp )





log psi (kPa) = Ai −

Bi Ci + T (◦ C)

(9)

where psi is the saturation vapor pressure in kPa and T the temperature in ◦ C. The constants, Ai , Bi , and Ci of the Antoine equation are listed in Table 1 together with the van der Waals properties ri and qi . In this work the activity coefficients of the components in the liquid phase were calculated using the UNIQUAC equation. The UNIQUAC binary interaction parameters have been already reported elsewhere [4]. 3. Experimental 3.1. Materials

(2)

where Ji is the partial permeation flux, Pi the permeance of the vapor membrane, pi,f the equilibrium partial vapor pressure on the feed side, yi,p the mole fraction in the permeate and pp the permeate pressure. The partial vapor pressure of each component on the feed

The membranes used in this work were two commercial hydrophilic membranes, PERVAP® 2216 and PERVAP® 2201 supplied by Sulzer Chemtech® . These membranes have a cross-linked PVA selective layer and a supporting layer of non-woven porous polyester. The PERVAP® 2216 membrane can stand a maximum feed

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Table 1 Pure components parameters: van der Waals properties, ri and qi , and Antoine equation constants Ai , Bi and Ci Compound

Water Ethanol Ethyl lactate Lactic acid

ri

qi

0.9200 2.1055 4.4555 3.1648

Antoine constants

1.4000 1.9720 3.9280 2.8800

Literature for the Antoine constants

A

B

C

7.0436 7.1688 7.8269 7.2471

1636.91 1552.60 2489.7 1968.21

224.92 222.42 273.15 158.94

[9] [9] [9] [10]

Table 2 Composition of lactic acid aqueous solutions used in this work Commercial lactic acid 50 wt.% 79–81 wt.%

Water

Lactic acid monomeric

Dimer

Trimer

Higher oligomers

50.91 19.13

45.92 59.86

3.05 17.71

0.12 2.82

– ≤0.48

water concentration of 40 wt.%, while PERVAP® 2201 can stand a maximum feed water concentration of 90 wt.%. Ethyl lactate was supplied by Flucka with a reported purity of 99 wt.%. Ethanol of 99.9 wt.% purity was purchased from Merck. Water was nanopure. Two different aqueous lactic acid solutions (50 and 79–81 wt.%) were obtained from Flucka. The amount of polymerized lactic acid was determined by HPLC (see Section 3.3). The composition of the lactic acid commercial solutions is reported in Table 2. The purity of the rest of the chemicals was checked by gas chromatography. 3.2. Apparatus and procedure The experimental set-up used in this work is shown in Fig. 1. Pervaporation experiments were performed using a stirred tank reactor of 5 L capacity. The membrane was installed in a stainless steel permeation cell (Sulzer Chemtech® ) with an effective membrane area in contact with the feed mixture of 170 cm2 . The temperature of the feed liquid mixture was kept constant by using a thermostat (±0.5 ◦ C). The temperature was monitored and registered just before the entrance of the pervaporation module with a Pt-100 probe. In pervaporation processes, concentration polarization is generally assumed to be of minor importance. The feed flow rate across the membrane was chosen high enough to avoid mass transfer resistance from the bulk liquid phase to the feed-

membrane interface. This effect was studied in the membrane PERVAP® 2201 for water/ethanol mixtures at low water concentration in the feed [5] by varying the feed flow rate. Experiments were performed at flow rates from 0.12 to 1.22 L/min showing that 0.2 L/min was enough to avoid the effect of concentration polarization. The rest of the pervaporation experiments were carried out at 0.6 L/min. On the downstream side, the permeate was evaporated and condensed on two parallel glass cold traps filled with liquid nitrogen to ensure that all permeates were fully collected. This phase change was achieved by lowering the partial pressure on the permeate side with the help of a vacuum pump. The downstream pressure was maintained around 1 mbar (±1 mbar). The system was allowed to reach steady state before samples were collected. To reach the steady state faster, the membrane was kept in the membrane module overnight together with the feed mixture and a slight vacuum on the permeate side to reduce the risk of building folds. The total flux was determined gravimetrically at fixed time intervals by weighing the mass of the permeate collected. The values reported are an average of three experiments. In each run the feed concentration was considered constant due to the small amounts of permeate in comparison to the amount in the reservoir. This was verified by analyzing feed samples at the end of each experiment. For water/lactic acid mixtures, a heating element was placed around the entrance of the permeate vessels to avoid freezing of

Fig. 1. Experimental set-up for the pervaporation experiments.

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the permeate that would block the entrance of the permeate vessels due to extreme low vapor pressure of lactic acid. 3.3. Sample analysis Water, ethyl lactate and ethanol were analyzed using a Hewlett Packard (6890) gas chromatograph (GC) equipped with series connected thermal conductivity (TCD) and flame ionization (FID) detectors. Helium, 99.999% pure, was used as carrier gas. The GC column was a 30 m × 0.25 mm bonded phase fused silica capillary column. The injector and detectors were at 523.15 and 533.15 K respectively. The oven was operated at programmed temperature, from 363 to 473 K. 1,2-Propanediol was used as internal standard for analysis of the samples. Lactic acid and its oligomers were analyzed by HPLC using a Hewlett Packard 1100 series liquid chromatograph. Quantification by UV detection was made at a wavelength of 210 nm. Samples were analyzed using a mobile phase of water + acetonitrile in gradient concentration at a flow rate of 1 mL/min. Water was acidified using 2 mL of 85% phosphoric acid in 1 L of solvent. The HPLC column was an ACE3C18 (4 mm × 150 mm). The column oven was maintained at 40 ◦ C. Based on titration of a lactic acid dilute solution (around 8 wt.%) a response factor for lactic acid monomer was obtained. The response factor for the dimer was obtained by back titration of a lactic acid solution (around 50 wt.%) containing only the monomer (L1 ) and dimer (L2 ). The response factor for L2 was found to be 10% larger than the L1 response factor. For trimer (L3 ) a response factor 10% larger than that of L2 was assumed and for L4 a value 10% larger than that of L3 was assigned. A similar quantitative analysis of lactic acid aqueous solution was followed by Asthana et al. [11]. 4. Results and discussion 4.1. Sorption experiments The separation performance of a pervaporation membrane depends on the degree of swelling of the membrane due to the different interactions of the membrane and the liquid feed components. The degree of swelling is defined as: Degree of swelling (%) =

Ww − Wd × 100 Wd

(10)

where Ww and Wd are the weight of wet and dry membranes respectively. Some sorption experiments were carried out by removing the separation layer of the membrane from the supporting layer. Dry membranes of exactly known mass were immersed in a closed bottle containing the different components. After equilibrium, the swollen membrane was wiped out with tissue paper to remove the superfluous liquid and weighed again. The weight increment corresponds to the liquid sorbed by the membrane. The degree of swelling for PERVAP® 2201 membrane for the different species were around: 250 for water, 176 for ethanol, 200 for ethyl lactate and 345 for lactic acid (50 wt.%) aqueous solution. It can be recognized that the degree of swelling is higher for water compared to ethanol and ethyl lactate. The highest degree of swelling corresponds to lactic acid aqueous solution. This can be due to the fact that lactic acid contains a hydroxyl and a carboxylic group which can form hydrogen bonds with the hydrogen in the hydroxyl group of the PVA in the active layer. 4.2. Pervaporation experiments The effects of pervaporation temperature and feed composition on permeation flux, permeance, separation factor and selectivity

Fig. 2. Influence of feed water concentration at 335.45 K for PERVAP® 2201 (solid symbols) and PERVAP® 2216 (open symbols) on partial permeation flux: water (, ), ethanol (, ) and on permeate composition (䊉, ).

are presented in this section for water/ethanol, water/ethyl lactate and water/lactic acid feed mixtures. The results obtained for the investigated systems are presented in four different types of figures: (a) total permeation flux and water permeate composition as a function of feed water composition (b) Arrhenius plot (c) permeance as a function of feed water composition and (d) separation factor and membrane selectivity as a function of feed water composition together with the distillation separation factor. 4.2.1. System water + ethanol 4.2.1.1. Comparison of PERVAP® 2201 and PERVAP® 2216. For water/ethanol feed mixtures, the effects of pervaporation temperature (327.75, 335.45 and 341.65 K) and feed composition on pervaporation separation performance were compared for two commercial membranes PERVAP® 2201 and PERVAP® 2216. Fig. 2 shows water and ethanol permeation fluxes at 335.45 K as a function of feed water concentration for both membranes. As the water concentration increases the partial permeation fluxes for water and ethanol increase probably due to the higher swelling of the hydrophilic membrane in contact with water. In this figure, the water weight fraction in the permeate is also presented. It can be observed that it is almost independent (around 0.93–0.94) of feed water concentration for PERVAP® 2201 membrane. Similar results were found by Van Baelen et al. [12] for the same membrane (PERVAP® 2201) for binary mixtures of water/ethanol or isopropanol. Water permeance has been plotted in Fig. 3 as a function of feed water composition at the three investigated temperatures for both membranes. The dependence of water permeance with feed water composition follows the same behaviour for both membranes; water permeance increases with an increase in feed water concentration. Although some scatter can be observed with temperature, not a strong dependence of water permeance on temperature can be assumed. Fig. 3 shows higher water permeances for PERVAP® 2201 than for PERVAP® 2216 in the operating conditions of this work. Based on these results, PERVAP® 2201 was chosen for further experiments. 4.2.1.2. Separation performance of PERVAP® 2201. The measured total permeation flux is plotted in Fig. 4a as a function of water weight fraction in the feed at the three different operating temperatures (327.75, 335.45 and 341.65 K). For all temperatures investigated permeation flux increases with feed water content. As it has been previously explained, this increase can be explained by the stronger swelling of the hydrophilic membrane in the presence

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motion of the polymer chains improving the diffusion of the permeant molecules [13]. The temperature dependence of the total permeation flux, Jtotal , can be expressed by an Arrhenius-type relation [5]:

 E  P

Jtotal = Jo exp −

Fig. 3. Influence of feed water concentration on water permeance for PERVAP® 2201 (solid symbols) and PERVAP® 2216 (open symbols) at different operating temperatures (,♦ 327.75 K; ,  335.45 K; ,  341.65 K).

of water. Consequently, the polymer chains become more flexible, reducing the energy required for the transport through the membrane [5]. In Fig. 4a, the permeate composition is also presented as a function of feed water composition for the different temperatures studied in this work. The dependence for both parameters is minimal in the range studied in this work. Fig. 4a shows that the total permeation flux increases with the feed temperature. With increasing temperature, the driving force increases because of the increasing vapor pressure, and therefore the permeation flux also increases (see Eq. (4)). Additionally, an increase in the operating temperature causes an increase in the

RT

(11)

where Ep is the apparent activation energy of permeation, Jo the pre-exponential factor, and T the absolute temperature. The activation energy can be calculated from the slope of the logarithm of overall permeation fluxes versus the inverse absolute temperature at different feed compositions (Fig. 4b). An apparent activation energy of total permeation of 58 kJ mol−1 was found from the average value of the slopes at different water concentrations in the feed (range of variation: 52 −69 kJ mol−1 ). Water and alcohol permeances at the three investigated temperatures are presented in Fig. 4c as a function of water concentration in the feed. Permeances increase with feed water concentration because of swelling of the hydrophilic membrane, but do not show strong dependence on temperature in the range studied in this work. This fact can be mathematically explained taking into account that the temperature dependence of the permeance is minimized by including the partial vapor pressure on the feed side in the denominator (see Eq. (5)). Additionally, based on the solutiondiffusion model permeance is related to the solubility (Si ) and diffusivity (Di ). An increase in the operating temperature reduces the sorption but is compensated with an increase in the diffusion. Similar behavior was observed by Sanz and Gmheling [14] with the same membrane and Qiao et al. [15] with PERVAP® 2510 in the pervaporation of water/isopropanol mixtures. In Fig. 4d, separation factor and membrane selectivity at the three different temperatures have been plotted as a function of

Fig. 4. Pervaporation performance of PERVAP® 2201 for water/ethanol mixtures. (a) Influence of feed water concentration on total permeation flux (solid symbols) and permeate composition (open symbols) at different operating temperatures. (b) Temperature dependence of total permeation flux at various feed water concentrations. The continuous lines represent the results of the Arrhenius fit. (c) Influence of feed water concentration on water permeance (solid symbols) and ethanol permeance (open symbols) at different operating temperatures. The continuous lines are obtained with Eq. (13) and parameters from Table 4. (d) Influence of feed water concentration on separation factor (open symbols) and membrane selectivity (solid symbols) at different operating temperatures. The continuous line represents the distillation separation factor at 341.65 K. Symbols for (a, c and d) (,♦ 327.75 K; ,  335.45 K; ,  341.65 K).

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Fig. 5. Pervaporation performance of PERVAP® 2201 for water/ethyl lactate mixtures. (a) Influence of feed water concentration on total permeation flux (solid symbols) and permeate composition (open symbols) at different operating temperatures. (b) Temperature dependence of total permeation flux at various water concentrations. The continuous lines represent the results of the Arrhenius fit. (c) Influence of feed water concentration on water permeance (solid symbols) and ethyl lactate permeance (open symbols) at different operating temperatures. The continuous lines are obtained with Eq. (13) and parameters from Table 4. (d) Influence of feed water concentration on separation factor (open symbols) and membrane selectivity (solid symbols) at different operating temperatures. The continuous line represents the distillation separation factor at 327.15 K. Symbols for (a, c and d) (, 318.95 K; , ♦ 327.15 K).

Fig. 6. Pervaporation performance of PERVAP® 2201 for water/lactic acid mixtures. (a) Influence of feed water concentration on total permeation flux (solid symbols) and permeate composition (open symbols) at different operating temperatures. (b) Temperature dependence of total permeation flux at various water concentrations. The continuous lines represent the results of the Arrhenius fit. (c) Influence of feed water concentration on water permeance (solid symbols) and lactic acid permeance (open symbols) at different operating temperatures. The continuous lines are obtained with Eq. (13) and parameters from Table 4. (d) Influence of feed water concentration on separation factor (open symbols) and membrane selectivity (solid symbols) at different operating temperatures. The continuous line represents the distillation separation factor at 348.15 K. Symbols for (a, c and d) (, ♦ 327.75 K; ,  335.45 K; ,  341.65 K; 䊉,  348.15 K).

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water weight fraction in the feed. Both, separation factor and membrane selectivity, are strongly dependent on the feed composition. But, opposite to flux, separation factor and membrane selectivity, decrease with increasing water concentration in the feed. The higher swelling of the membrane in contact with water facilitates the permeation of the different components leading to a decrease in the separation factor and in the membrane selectivity. Regarding the effect of temperature, not a great influence on the separation factor and membrane selectivity was observed. Therefore, the best pervaporation performance will be obtained at the maximum operating temperature of the membrane. The distillation separation factor, ˇevap , at 341.65 K has been also plotted in Fig. 4d. The distillation separation factor (also called relative volatility) at a given temperature and pressure is a measure of the differences in volatility between two components, and it is defined as: ˇevap =

yi /xi yj /xj

(12)

where y and x are the composition in the vapor and liquid phases respectively. It indicates how easy or difficult the separation by distillation will be. When the distillation separation factor is equal to 1 the separation by distillation of the two components would be impossible under the given conditions. In Eq. (12), component i is usually the more volatile component, but in this work component i was always water for a better comparison with the pervaporation separation factor and membrane selectivity, since the membrane is selective towards water. Fig. 4d shows the maximum pressure azeotrope at low water concentration for this particular binary system (distillation separation factor = 1). In the pervaporation of water/ethanol mixtures, the membrane contribution is opposite to the effect of the evaporation step, being selective for the less volatile component, water. Consequently, based on the expression for the separation factor, ˇpervap (see Eq. (7)) higher values will be obtained for the ˇmemb than for ˇpervap . In this system, the permeate partial pressure is much smaller than the feed partial vapor pressure and ˇmemb = ˛memb . According to the values of the distillation separation factor higher values for the membrane selectivity than for the separation factor are observed in Fig. 4d. At the azeotropic point ˇevap = 1 and ˇpervap = ˛memb . 4.2.2. System water + ethyl lactate The pervaporation temperatures studied in this system (318.95 and 327.15 K) for PERVAP® 2201 were low enough to avoid the hydrolysis reaction to take place in some extent. The ethanol content in the feed was determined after every pervaporation experiment and was maintained lower than 0.5 wt.%. In Fig. 5a, the total permeation flux and the permeate concentration at the two different temperatures are showed as a function of feed water concentration for water/ethyl lactate mixtures. High values of water permeate composition were obtained in the concentration and temperature range covered in this work. The values of the total permeation fluxes were of the same order to the values obtained for water/ethanol mixtures. As for water/ethanol mixtures total permeation flux increases with feed water concentration and temperature. Assuming Arrhenius dependence on temperature for the total permeation flux an average apparent activation energy of permeation of 44 kJ mol−1 was obtained (Fig. 5b) (range of variation: 30–56 kJ mol−1 ). Water and ethyl lactate permeances are presented in Fig. 5c as a function of feed water composition at the two different temperatures. Water permeance increases with feed water concentration but ethyl lactate permeance shows rather flat relationship with feed water concentration. As for the water/ethanol system permeances do not show a strong dependence on operating temperature in the water concentration and temperature range covered in this work.

In Fig. 5c, higher values of ethyl lactate permeances are shown than for ethanol permeances in the pervaporation of water/ethanol mixtures (see Fig. 4c). It should be kept in mind that the permeation driving force for ethyl lactate, i.e., feed partial ethyl lactate vapor pressure minus the partial permeate pressure, is smaller than the permeation driving force for ethanol. Values for the driving force of ethyl lactate were between 1.1 and 0.4 kPa at the lowest and the highest feed water concentration respectively; consequently higher values of ethyl lactate permeance will be obtained than for ethanol permeance (see Eq. (5)). The separation factor and the membrane selectivity have been plotted in Fig. 5d as a function of feed water concentration at the different temperatures. A kind of maximum for the separation factor around 0.2–0.3 water weight fraction can be observed. For higher feed water weight fractions the separation factor decreases with an increase of feed water concentration. From Fig. 5d can be also observed that membrane selectivity for water/ethyl lactate shows a different tendency from that of the separation factor, increasing with water feed concentration. The reason for this different behavior from separation factor could be that while selectivity is mainly dominated by the membrane transport properties, separation factor takes into account the membrane transport properties and thermophysical properties of the feed mixture [16]. This behavior agrees with the values of water and ethyl lactate permeances (see Fig. 5c) since ethyl lactate permeance remains more or less constant, water permeance increases with increasing feed water concentration, consequently the membrane selectivity increases with feed water concentration as defined as the ratio of permeances. From Fig. 5d can be also seen that temperature has not a great influence on separation factor and membrane selectivity. In this system, separation factors were higher than membrane selectivity at low feed water concentration, but differences become smaller with increasing water feed concentration. In the same figure, the distillation separation factor at 327.15 K has been also plotted. A maximum pressure azeotrope (distillation separation factor = 1) at high water concentration has been calculated by using the UNIQUAC binary interaction parameters reported in a previous work [4]. This separation problem can be solved by pervaporation as showed by the values of the separation factor higher than the unity. In Fig. 5d can be observed that at the azeotropic point, ˇevap = 1 and ˇpervap = ˇmemb  ˛memb . Comparing Figs. 4d and 5d lower values for the membrane selectivity can be observed for water/ethyl lactate mixtures than for water/ethanol mixtures. This result agrees with the values of the ethyl lactate and ethanol permeances since the selectivity is defined as the ratio of the permeances. 4.2.3. System water + lactic acid The effect of feed water concentration in the pervaporation performance of PERVAP® 2201 for water/lactic acid mixtures has been studied in the concentration range from 0.39 to 0.68 superficial water weight fraction at four different operating temperatures (327.75, 335.45, 341.65 and 348.15 K). For a good explanation on defining lactic acid concentration and its oligomers in solution the reader is referred to Vu et al. [17]. The PERVAP® 2201 membrane was not affected up to around 150 working hours in contact with lactic acid solution. After that, generation of bubbles from the PVA membrane was observed, leading to higher lactic acid concentration in the permeate. Similar behaviour was observed by Benedict et al. [18] with GFT-1005 membrane which remained unchanged after 150 h of exposure to lactic acid in the temperature range 80–95 ◦ C. Similar to the other two binary mixtures studied in this work, total permeation flux increases with water feed concentration at every operating temperature (Fig. 6a). But water/lactic acid mixtures permeate faster than water/alcohol and water/ester mixtures.

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This fact is supported by the sorption results (see Section 4.1) which showed that the degree of swelling was higher for lactic acid aqueous solutions enhancing the permeation flux. From Fig. 6a can be also observed that total permeation flux increases by increasing the operating temperature. Arrhenius-type dependence with temperature can be assumed (Fig. 6b) obtaining and average apparent activation energy of 59 kJ mol−1 (variation range: 37–70 kJ mol−1 ). The water weight fraction in the permeate was also plotted in Fig. 6a as a function of feed water concentration. It was approximately constant over the concentration and temperature range covered in this work (around 97 wt.%). Water and lactic acid permeances have been plotted in Fig. 6c versus feed water content. It can be observed that permeances increase with feed water content but again not a strong dependence on temperature can be observed. While values of the same order for water permeance in water/ethanol and water/ethyl lactate mixtures were obtained, higher values of water permeance were reached for water/lactic acid mixtures. As it has been previously explained, water/lactic acid mixtures have higher permeation flux due to a better swelling of the membrane in contact with lactic acid solution. The values of lactic acid permeances must be carefully considered since its driving force is very low and consequently its pervaporation data are poorly suited to the calculation of permeances. The values for the partial vapor pressure of lactic acid on the feed side are similar to the partial vapor pressure on the permeate side which brings very small values of the permeation driving force for lactic acid. The maximum value for the driving force corresponding to lactic acid was only 0.025 kPa, which means high values of lactic acid permeance. Due to these small values of the permeation driving force, small errors in any experimental variable can drag errors in the calculation of the permeance of lactic acid [8]. Wijmans and Baker [8] suggested that for very small values of the driving force, the pervaporation data obtained are poorly suited to the calculation of permeances. The separation factor and the membrane selectivity achieved in the pervaporation process for water/lactic acid mixtures have been plotted in Fig. 6d together with the distillation separation factor at 348.15 K. Separation factor decreases with an increase of feed water concentration. However, not a clear influence of temperature on separation factor can be observed. The high values for the distillation separation factor are due to the extremely low vapor pressure of lactic acid (1 mmHg at 85 ◦ C) [19]. Fig. 6d shows that the values obtained for the membrane selectivity are extremely low. This is related to the small vapor pressure of lactic acid and the cor-

85

Fig. 7. Ethanol (), ethyl lactate () and lactic acid (䊉) permeances as a function of water permeance for all pervaporation experiments.

responding values of lactic acid permeances since the membrane selectivity is defined as the ratio of permeances. Anyway, the actual membrane selectivity is favourable still to water as can be observed in the high values of the water permeate composition (Fig. 6a). Huang and Yeom [20,21] studied the separation performance of PVA membrane for water/ethanol and water/acetic acid mixtures. These authors found lower separation factors and higher permeation fluxes for water/acetic acid mixtures compared with water/ethanol feed mixtures. Van Baelen et al. [12] studied the pervaporation of water/alcohol mixtures and water/acetic acid mixtures by using the same membrane as in this work (PERVAP® 2201). They observed that for water/acetic acid mixtures the total permeation flux was nearly constant with water feed concentration. This total permeation flux for water/acetic acid mixtures was higher than for water/ethanol or isopropanol mixtures, but not higher than for water/methanol mixtures in the water concentration range 40–80 wt.% at 60 ◦ C [12]. 4.2.4. Comparison between water/ethanol, water/ethyl lactate and water/lactic acid mixtures Ethanol, ethyl lactate and lactic acid permeances have been plotted in Fig. 7 versus water permeance for all the pervaporation experiments. Some authors [15] consider the slope of this relation-

Table 3 Physicochemical properties and solubility parameters of systems components Ethanol

Ethyl lactate

Lactic acid

Water

Molecular structure Molecular weight Boiling point (K) [10] Molar volumebp (cm3 mol−1 ) [10]

46.07 351.44 62.70

118.13 427.65 139.04

90.08 490 88.98

H2 O 18.02 373.15 18.83

Radius of gyration (A) [10] Dipole moment (Debyes) [10] Dielectric constant, ε

2.259 1.6908 22.420 ◦ C [22]

3.622 2.4000 13.120 ◦ C [9]

3.298 1.1392 2319 ◦ C [19]

0.615 1.8497 80.220 ◦ C [9]

Solubility parameters (MPa0.5 )

ıd

ıp

ıh

ısp

Ethanol [24] Ethyl lactate [24] Lactic acid Water [24] PVA [15]

15.8 16 18.8 15.5 15.96

8.8 7.6 7.6 16 14.11

19.4 12.5 18.8 42.3 23.93

26.5 21.7 27.6 47.8 39.07

o

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Table 4 Correlation parameters of Eq. (13) for the three investigated systems System

Water/ethanol

Water/ethyl lactate

Water/lactic acid

i

 W = 4.49  EtOH = 11.02

 W = 2.62  EL = 0.43

 W = 4.33  HL = 4.12

ship as an indicator of the degree of coupling transport. The slopes show that ethanol has the strongest coupling effect while ethyl lactate presents the lowest coupling effect. Table 3 summarizes some physical properties of ethanol, ethyl lactate, lactic acid and water. The highest coupling between ethanol and water could be related with the structure of the molecule (steric effects). Ethanol has lower molecular weight, molar volume and radius of gyration than ethyl lactate and lactic acid. Based on the roughly constant values of the permeances with temperature and the dependence of permeances with feed water concentration, a simple first order regression to the permeance respect to the water weight fraction in the feed can be assumed: ln(P) = ln(Pi,o ) + i wW

(13)

The slopes ( i ) for the different binary mixtures have been listed in Table 4. The slopes for ethanol, ethyl lactate and lactic acid follow the same order as the degree of coupling (see Fig. 7). The fraction of increase of water permeance ( W ) with water weight fraction in the feed has the lowest value for the system water/ethyl lactate (2.62). The slopes of water permeance have values of the same order for water/ethanol ( W = 4.49) and water/lactic acid mixtures ( W = 4.33). Although higher values of water permeance were found for water/lactic acid than for water/ethanol mixtures, the dependence on water feed concentration has been found similar for these two systems. The significance of the Hansen solubility parameters on the development of pervaporation has been pointed out in a review about polymeric membrane pervaporation [23]. Hansen solubility parameter consists of three components: the dispersion interaction, ıd ; the polar interaction, ıp ; and the hydrogen bonding interaction, ıh . Table 3 provides the solubility parameters of the different components systems. No solubility parameters were found in the literature for lactic acid and a group contribution procedure [24] was used to obtain partial solubility parameters. Therefore these values must be carefully considered. In the literature [23] values of ıh for C2 –C4 carboxylic acids have been reported in the range 9–13 (MPa)0.5 . The higher value calculated for lactic acid can be due to the

presence of a hydroxyl group, lactic acid is a hydroxycarboxylic acid. The high value of ıh for water agrees with the selective behaviour of PVA membranes for water [23]. Dispersion and polar interactions of ethanol, ethyl lactate and lactic acid are comparable. However, ıh presents the lowest value for ethyl lactate and for ethanol and lactic acid values of the same order. The values of the ıh have a good correlation with the fraction of increase of water permeance with feed water concentration (Table 4). Similar trend can be observed with the dielectric constant of the components of the systems. The partial permeation fluxes have been calculated by using the parameters reported in Table 4. The calculated ratio of the permeation fluxes (Ji /Jwater )calc for the three mixtures have been plotted in Fig. 8 versus the experimental ratio (Ji /Jwater )exp Eq. (13) can reproduce the experimental permeation fluxes without high errors. 5. Conclusions Pervaporation experiments were performed for three binary mixtures: water/ethanol, water/ethyl lactate and water/lactic acid by the membrane PERVAP® 2201. In particular, the effects of water content in the feed and temperature were investigated. This membrane was chosen due to the higher total permeation fluxes and separation factors obtained for water/ethanol mixtures compared with the values obtained with PERVAP® 2216. The results show that the total permeation flux increases in the order ethanol  ethyl lactate < lactic acid. This tendency can be related with the higher degree of swelling of the membrane in contact with lactic acid solutions. In all cases the actual selectivity of the membrane was found to be towards water. The total and partial permeation fluxes were found to increase with the water content in the feed and the operating temperature. Permeances also increase with increasing feed water content. Arrhenius dependence with temperature was found for the total permeation flux while little dependence was observed for the permeance on temperature. The highest values for water permeance were obtained for water/lactic acid mixtures. But similar dependence of water permeance on feed water concentration was found for water/ethanol and water/lactic acid mixtures. Acknowledgments Financial support from the “Junta de Castilla y León” through Grant BU019A/05 and “Consejería de Educación y Fondo Social Europeo” through predoctoral Grant EDU/1490/2003 is gratefully acknowledged. References

Fig. 8. Calculated permeation flux ratio (Ji /Jw )calc with Eq. (12) and parameters from Table 4 versus experimental permeation flux ratio (Ji /Jw )exp for water/ethanol (), water/ethyl lactate () and water/lactic acid (䊉) mixtures.

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