Reducing flux decline and fouling of direct contact membrane distillation by utilizing thermal brine from MSF desalination plant

Desalination 379 (2016) 172–181

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Reducing flux decline and fouling of direct contact membrane distillation by utilizing thermal brine from MSF desalination plant Ahmad Kayvani Fard a,b,⁎, Tarik Rhadfi a, Majeda Khraisheh c, Muataz A. Atieh a,b, Marwan Khraisheh a, Nidal Hilal a,d,⁎⁎ a

Qatar Environmental and Energy Research Institute (QEERI), Hamad Bin Khalifa University, Qatar Foundation, PO Box 5825, Doha, Qatar College of Science and Engineering, Hamad Bin Khalifa University, Qatar Foundation, PO Box 5825, Doha, Qatar Department of Chemical Engineering, Qatar University, P.O. Box 2713, Doha, Qatar d Centre for Water Advanced Technologies and Environmental Research, (CWATER), College of Engineering, Swansea University, Swansea SA2 8PP, United Kingdom b c

H I G H L I G H T S • • • •

The scaling on membrane for DCMD system is studied in depth. Rejected brine from MSF plant and seawater are investigated for scaling and fouling on membrane. Flux reduction by 8% is observed for system using rejected brine compared to 12–20% when the feed to the DCMD was seawater. DCMD is capable of producing highly pure water at temperature as low as 70 °C.

a r t i c l e

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Article history: Received 23 August 2015 Received in revised form 12 October 2015 Accepted 6 November 2015 Keywords: Membrane distillation Fouling Direct contact MD Seawater Scaling Flux

a b s t r a c t Fouling and scaling is one of the major challenges in membrane distillation process. This study investigates the utilizing of thermal rejected brine produced from multi-stage flash distillation (MSF) to minimize fouling and scaling on membranes in direct contact membrane distillation (DCMD) settings. The effect of operating parameters on permeate flux and the extent of scaling during long time operation was considered and compared when real seawater and reject thermal brine were used. The deposit morphology of scaling was observed using different analytical methods, including scanning electron microscopy and contact angle measurement. Thermal brine showed enhanced performance in terms of flux and scaling compared to seawater. It was found that flux is reduced by 8% compared to 12–20% when the feed to the DCMD was brine was used as compared to seawater. In addition, thermal brine feed showed better anti-fouling behavior compared to the fresh seawater which contains different organic and inorganic contaminants. Furthermore, utilizing waste heat contained in the thermal brine to raise the temperature of the feed was advantageous in increasing the energy efficiency of DCMD process. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Membrane distillation (MD) is a thermally driven technology developed over the last 60 years pioneered by Weyl and Findley [1–3]. In it hydrophobic membrane sheets are used to separate hot and cold stream of water and allows water to evaporate due to temperature difference.

⁎ Correspondence to: A.K. Fard, Qatar Environmental and Energy Research Institute (QEERI), Hamad Bin Khalifa University, Qatar Foundation, PO Box 5825, Doha, Qatar. ⁎⁎ Correspondence to: N. Hilal, Centre for Water Advanced Technologies and Environmental Research (CWATER), College of Engineering, Swansea University, Swansea SA2 8PP, United Kingdom. E-mail addresses: [email protected] (A. Kayvani Fard), [email protected] (N. Hilal).

http://dx.doi.org/10.1016/j.desal.2015.11.004 0011-9164/© 2015 Elsevier B.V. All rights reserved.

The very nature of the hydrophobic membrane allows water vapor to pass and rejects the liquid water. Difference in temperature (and consequently vapor pressure difference) in the two sides of the membrane is the driving force which causes water to vaporize and condense on the cold surface (or cold stream). The result of this physical–chemical operation is distillate with almost 99.99% salt rejection [4]. Unlike conventional desalination technology such as MSF and RO, MD does not suffer from salt entrainment which is non-volatile [5]. The MD process starts with passing the saline or concentrated solution on one side of the membrane at an elevated temperature, for example 70–80 °C. At the other side of the membrane, a lower temperature water at around 30–40 °C, creates a water vapor partial pressure difference between the two sides of the membrane and allows the

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evaporation through the membrane. The water vapor goes through the pores of membrane and condenses on the low-temperature side and distillate is formed. The most common types reported in literature are [6–8] namely: direct contact (DCMD), air gap (AGMD), sweeping gas (SGMD) and vacuum (VMD). In direct contact MD, the feed solution is in direct contact with the membrane on one side and the cold distillate (permeate) contacts the other side of the membrane; a configuration first introduced by Lawson and Lloyd [9], Martinez-Diez and Florido-Diez [10], and Phattarawik and Jiraratanon [11]. The major advantages of MD over other conventional processes such as thermal process are its lower operating temperature which directly affects the energy consumption [12]. Since MD is a hybrid thermal and membrane process, its low temperature operation and, on the other hand, low water production made it an energy inefficient process compared to other desalination techniques as reported by number of studies [13,14]. For a process to have a good application in the industry, high flux with moderate energy consumption is highly required. It is worth mentioning that although the flux is low, using renewable source of energy or waste heat energy from thermal process, such as rejected waste from the likes of petrochemical process can make the process more efficient and environment friendly. Ultimately, careful integration of MD in petrochemical plants and power plants/desalination could make it efficient and convenient. In practice, the use of MD for seawater desalination is restricted to pilot and small scale lab units due to few technical challenges. Technical challenges such as membrane pore wetting, fouling and scaling are areas where more research is needed to develop the process for industrial applications. In general, fouling is defined as the buildup of undesirable material on solid surfaces with an associated detriment of function such as decline in flux in the case of membrane processes. The types of fouling potentially found in MD systems can be divided into four categories: inorganic salt scaling or precipitation fouling, particulate fouling, biological fouling, and chemical membrane degradation [15,16]. MD due to its nature is more fouling resistant compared to RO as it is not a pressure driven process and only vapor is allowed to cross through the membrane [17]. Fouling in MD is one of major components of interest as an increase in fouling cause increases in costs of energy consumption, downtime, cleaning, required membrane area, and required membrane replacement, and creates problems with product water contamination from pore wetting [18,19]. While both RO and MD technologies include mass transfer through membranes, significant differences related to fouling does exist such as higher operating temperatures of MD, hydrophobic properties of MD membranes, the presence of temperature gradients in MD, and the larger pore sizes in MD [9]. In addition, MD operates at a much lower pressure than RO which are generally believed to aid the formation of compacted cake scales. Annually, huge quantities of brine from thermal desalination plant are rejected back to sea causing significant loss of water resources and create disposal challenges. The primary advantages of such brine are its higher heat content compared to seawater and also dosage of some chemicals such as antifouling and antiscalant which could be used in MD to reduce fouling and scaling. To mitigate the fouling and scaling on membrane for MD process, thermal brine is used and compared to fresh seawater to study the effect of operating parameters on flux decline and fouling. In addition, the heat from thermal rejected brine coming from the MSF desalination plant and rejected brine from the same source is used to augment fresh water production by DCMD. Accordingly, the aim of this study is to investigate the effect of the actual seawater MSF concentrate (thermal brine) in DCMD performance and compare the result with fresh untreated seawater. The permeate flux reduction caused by the deposition of organic and inorganic foulants and deposits is examined at a feed temperature ranging from 60 to 70 °C while varying the permeate temperatures between 30 to 40 °C. Detailed investigations on scaling and fouling of membranes are made using analytical techniques such as SEM and contact angle measurement.

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2. Materials and methods 2.1. Laboratory-scale DCMD setup and operation A bench scale single stage DCMD module with 2 flow modes (co-current/counter current) was custom designed and tested as described by the authors in a previous study [20]. The automatic DCMD system includes feed flow and coolant recirculation flow metering plus connected ancillary equipment with data acquisition system (Fig. 1). The feed flow rate in both sides of the membrane were controlled using peristaltic pumps (Thermo Scientific model: FH100X, US). Temperature and pressure of the inlet and outlet streams of the membrane module were monitored using thermo resistance RTDs (model: RTDNPT-72-E, Omega Engineering, UK) and pressure transducers (model: PX309-030GI, Omega Engineering, UK). To monitor feed and distillate flow rate, temperature, and pressure the digital data display system (model: DP25B-E-230-A, Omega Engineering, UK) was used. To maintain constant feed concentration, ultra-pure DI water was added to the feed tank at regular times to replace water lost as distillate. Distillate conductivity was monitored using conductivity indicator (model: 3433E8A, 10 Cell Constant, Hatch, USA). The data was saved using a National Instruments data acquisition hardware (Chassis Model cDAQ9188; Module Model: NI-9219, National Instruments, US). The weight from the balance was acquired using a serial server (model: NI ENET 232, National Instruments, USA). Data storage and processing were developed using LAB View data acquisition software. The temperature of the feed liquid and distillate side was varied and controlled using heating and cooling circulators (model: F32-MA, Julabo, Germany). Deionized water with conductivity b2 TS/cm (Milli-Q PF Plus water) was used as the cold flow to the MD unit. The flux (J) of distillate was measured by the difference in the weight of distillate tank over certain time under given experimental conditions using a weighing balance (model: VWR# 97035-640, Mettler, Toledo) through active membrane area and time. Distillate flux is measured as kg/m2 h and reported as LMH (L/m2 h) and mathematically calculated as:

Distillate flux ¼

Difference in weight of distillate tank ðkgÞ : Membrane area ðm2 Þ  time ðhÞ

All the flow lines used in the DCMD system were insulated with foam to minimize the heat loss. Temperature of the distillate (Tp) was kept constant at 30 °C when effect of feed water temperature (Tf) was tested and inversely feed temperature was kept at 70 °C when various permeate temperatures were examined. Tests were run for around 12 days (300 h) when evaluating the performance of the DCMD system. To promote turbulence and enhance heat transfer, a mesh-like spacer is used. At the completion of each experiment, the membranes were removed from the DCMD cell; air dried, and kept in a desiccator until surface analysis. Each test was repeated 3 times to ensure correct reproducibility and average values were reported in this study. Error bars were removed to avoid confusion in the graphs. 2.2. Membrane The membrane used in this study is made of Poly Tetra Fluoro Ethylene (PTFE) with the support of a Poly Propylene (PP) sheet (Sterlitech Corporation, US) to provide the active layer with strength and a degree of rigidity during the tests. Table 1 summarizes the properties of the membrane used in this study. 2.3. Feed water Two different feed solutions to the DCMD system were used in this study namely; thermal brine and seawater. Seawater was collected from open sea in Arabian Gulf and thermal brine was collected from

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Fig. 1. Schematic diagram of the DCMD system.

local MSF desalination plant. Chemical analysis of each feed is given in Table 2. 2.4. Characterization tools To observe membrane's surface characteristic, cross section, pores, and fouling on the membrane, pristine membrane and used membranes are analyzed using a FEI Quanta 200 Environmental Scanning Electron Microscope (ESEM) with a resolution of 5 nm and a magnification ×200K was used. For that, a sample of membrane was frozen in liquid nitrogen and then fractured. Cross section and surface of the membrane were sputtered with gold and then transferred to the microscope for imaging. To measure pore size distribution, 225 μm random membrane samples were dried by liquid nitrogen and analyzed by SEM and by utilizing image analysis software, pore diameters and their distributions are calculated. To measure the contact angle of the membrane, a Contact Angle Measuring Instrument (DSA30, KRUSS GmbH, Germany) was used using

Table 1 Properties of PTFE membrane. Description

Specification

Pore size Thickness Pore size range Sterilization Active area

0.22 μm 175 μm thick 0.2 to 1.0 μm Auto-cleavable up to 130 °C 0.14 m2

the sessile drop method. To measure the contact angle, liquid droplet (DI water) is deposited onto the membrane surface using an I-shaped needle. The angle of the drop with the membrane is measured using the Young equation, assuming that the surface is smooth and homogeneous. Five readings were measured and an average was obtained from the results. The chemical analysis of the feed water and permeate Table 2 Chemical analysis of thermal brine and seawater used as feed to DCMD. Properties pH Conductivity Total organic carbon Fluoride Chloride Bromide Phosphate Nitrate Nitrite Sulfate Thiosulfate Lithium Sodium Potassium Magnesium Calcium Ammonium (NH4 +) meq anions meq cations meq ratio (anions/cations)

Units mS/cm mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L

Formula

TOC F− Cl− Br− PO3− 4 NO− 3 NO− 2 2− SO4 S2O2− 3 Li+ Na+ K+ Mg2+ Ca2+ NH4+

Brine

Seawater

8.34 76.8 2.113 b2.5 32,127 46 b2.5 b2.5 b2.5 4025 b2.5 b2.5 18,434 491 1738 521 b2.5 991 983 1.01

7.6 65 5.477 b2.5 19,661 32 b2.5 b2.5 b2.5 3013 b2.5 b2.5 12,858 343 1147 459 b2.5 618 685 0.90

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are analyzed by Ion Chromatography (IC) (Thermo Scientific Dionex IonPac®). 3. Results and discussion 3.1. Effect of operating parameter on membrane fouling Fig. 2 shows the effect of feed temperature on flux and consequently fouling of the membrane used in the experimental system. Flux is presented with respect to different feeds at fixed permeate temperature and various feed temperatures. It was observed that increasing feed temperature, increases the flux of permeate produced due to increase in driving force (vapor pressure) at higher temperature; mathematically it is related to vapor pressure and temperature using the Antoine equation; a trend well reported in literature. On the other hand, fouling and the decline in the flux were more dominant when seawater was used compared to the cases where thermal brine was used as a feed. The presence of antifouling material in the brine may have played an important role in this reduction as seawater undergoes different pre-treatment stages before entering the MSF plant rendering it cleaner with less foulant. Furthermore, as the temperature increases, it was observed that the extent of fouling increased due to wetting phenomena (at higher temperature, chance of membrane wetting gets higher [20,21]). The extent of fouling effect can be also observed with increasing temperature. For brine, when feed temperature is 70 °C, the flux decline is estimated to be around 11% of its original value while for feed at 60 °C, the decline is only 8%. With an increase in feed inlet temperature, the saturation index of the feed bulk is increased and solubility of some salts reduces. At membrane surface concentration of ions are more compared to bulk solution and increasing temperature makes ion concentration even more prominent which makes the fouling rate higher at higher temperature [22,23]. However, increasing the permeate temperature while keeping the feed temperature constant, decreased the flux (Fig. 3). Higher vapor pressure can be obtained, in principal, by decreasing permeate temperature while keeping the feed temperature where higher temperature difference could increase vapor pressure. In general three factors help in increasing flux when increasing feed temperature or decreasing permeate temperature which are: increasing vapor pressure due to feed temperature, increasing driving force due to higher ΔT due to reduction of permeate temperature, and increasing temperature polarization due to increase of feed temperature [24]. Hence, the highest flux can be obtained at the highest and the lowest feed and permeate temperatures, respectively. Moreover, the experimental tests at different temperatures showed that the effect of feed temperature on the distillate flux

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was higher than the effect of the permeate temperature. This was observed when the flux at the 70–30 °C system was compared with that at 60–20 °C and showed a greater flux with the 70–30 °C system even though the temperature difference in the two systems was the same (ΔT = 40 °C). This due to the fact that vapor pressure and temperature are co-related exponentially and increasing temperature, increases vapor pressure. Due to increase in flux at lower permeate temperature, chance of wetting is higher and fouling might increase at lower permeate temperature [25]. Increasing temperature in general can affect the solubility and crystal formation of salts. Solubility of alkaline salts increases with increasing the temperature while other salts such as calcium carbonate and magnesium hydroxide have inverse relation with temperature. Effect of temperature on bio-foulant also is an important factor due to the lack of tolerance at higher temperature for some microorganisms. Similarly, temperature polarization may have a substantial effect on scaling and fouling since the solubility of common foulants and biological foulants are highly temperature dependent. Inversely, decreasing in temperature at membrane surface due to temperature polarization might stop precipitation of CaCO3 and CaSO4 which are less soluble at higher temperatures, but may make non-alkaline salts scale more readily. Due to the inverse solubility of the salt and increased flux contributing to higher concentration polarization in the feed stream, there is a significant increase in fouling rate with change in feed temperature. As depicted in Figs. 2 and 3, at higher temperatures, permeate flux increases which leads to increase in higher concentration of organic compounds and some salts at the membrane–water interface due to the concentration polarization. This effect is more dominant in seawater compared to thermal brine due to the presence of more organic matter. Thermal brine prior to entering the system was dosed with anti-fouling and foaming agents which helps in organic foulants and therefore causing less scaling and fouling on membrane surface [20,26]. Effect of flow rate on fouling and flux decline were also investigated as depicted in Fig. 4. Trends in Fig. 4 suggest that as the system's flow rate increases, the flux increases as a result. Similarly, an increase in the feed flow rate results in feed velocity increases, which in turn causes both heat and mass transfer coefficients to increase. Increasing the heat and mass transfer coefficients changes the flow regime from laminar to turbulent flow (or from transition regime to turbulent flow regime). The heat transfer coefficient of the feed side was estimated to be around 1190; however, when the feed flow rate of the system increased, the heat transfer coefficient increased to 2163. Consequently, the Reynolds number was found to increase from 1105 (laminar flow regime) to 6379 (turbulent regime). As a result, with an increase in flow rate, flux increases as a

Fig. 2. Effect of various feed temperatures on flux decline for thermal brine and seawater (Tp; 30 °C, feed flow rate: 1.5 L/min).

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Fig. 3. Effect of various permeate temperatures on flux decline for thermal brine and seawater (Tf; 70 °C, feed flow rate: 1.5 L/min).

result of reduced temperature and concentration polarization. Therefore, ion concentration decreases and temperature at membrane surface increases. Therefore, increasing flow rate leads to a reduction in the rate of flux decline. This can be seen in Fig. 4 where flux decline when flow rate is 2.5 L/min is 9% while for the system with a flow rate of 0.5 L/min flux decline is 12%. When scaling increases due to the temperature and concentration polarization, it produces a hydrodynamically slow moving layer of water at the water–membrane interface. Lower flow rate increases

temperature polarization as the water residence time is extended. The fouling layer creates additional thermal resistance boundary causing reduction in the heat transfer coefficient from the feed bulk to the evaporation and condensation interfaces. Similarly, the mass transfer coefficient close to the membrane surface is also often reduced due to the presence of scaling. This leads to an increase in the concentration of dissolved ions close to the membrane interface, reducing the local vapor pressure and thereby reducing flux, in addition to increasing fouling [27]. Moreover, at higher flow rates, fast movement of water on the surface of membrane creates a shearing effect which helps in removing deposits and reducing participation. This can reduce scale formation on membrane and lower the fouling deposition rate. The deposition in thermal brine is even lower than that of seawater due to the presence of antiscalant and also pre-treatment stage (Fig. 4). As seen in Fig. 4, the flux decline using brine is less than seawater due to the presence of such scaling. The flux decline for the highest flow rate (2.5 L/min) is 5% for thermal brine while 10% for the system using seawater. Comparing the data from temperature and flow rate, effect of flow rate on permeate flux is more dominant compared to the temperature. This can be seen when results in Fig. 2 and Fig. 4a are compared. At any point in

Fig. 4. Effect of feed inlet flow rate on flux decline for seawater (a) and thermal brine (b) (Tf: 70 °C Tp: 30 °C).

Fig. 5. Effect of flow pattern and feed direction on flux decline for thermal brine and seawater (Tf; 70 °C Tp; 30 °C, feed flow rate: 1.5 L/min (baseline test)).

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Fig. 4a the flux of the system with flow rate of 2.5 LMH is 2.5 times higher than the flux of system with flow rate 0.5 LMH while temperature effect for systems at 70 °C and 60 °C at the same point in graph is only 1.75 times the increase in permeate flux. In general increasing circulation velocity, increases the permeate flux until it reaches an asymptotic value where the flux stay constant even with increasing circulation velocity [28–30]. This is due to the fact that at low flow rates, “relatively thick” thermal boundary layer is present at the membrane–water interface. As the circulation rate increases, the thermal boundary layer gets thinner and thinner (hence, higher fluxes) till it reaches a value that no matter how high the circulation velocity is, there has to be a thermal boundary layer and the fluxes can be increased beyond that.

3.2. Effect of flow pattern on membrane fouling The effect of having a more efficient heat transfer in the system and hence better flux and less fouling was also investigated when two different flow patterns (counter current and co-current) were tested (Fig. 5). Also to investigate the effect of laminar flow and turbulent flow, spacer (mesh-like sheet) was removed from the DCMD setup and flux and fouling were studied as depicted in Fig. 5. Presence of a spacer was found to enhance the mass transfer and flux of the MD systems when compared with the spacer-free systems. The percentage of fouling was also observed to be less in the system with spacer compared to the spacer-free system. The flow dynamic and changing from laminar to turbulent mode increased the film heat transfer coefficient (h) and hence enhanced the mass flux of the MD system. Therefore introducing the spacer into the flow channels is responsible for destabilizing the laminar flow and promoting eddies; hence, the flow regime is no longer laminar and changes to turbulent regime. This in fact will enhance mass transfer and heat and momentum transfer. In the presence of spacer, the temperature on the feed side of the

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membrane surface increases, creating more vapor pressure difference and fluxes. Similarly fouling and flux reduction is less in system with spacer compared to the spacer-free system. In the presence of spacer, due to creation of eddies and non-uniform flow pattern, chance of deposition and scaling reduces; therefore reduction in flux is observed to be less than that of the spacer-free system. When brine is used in a system with spacer flux decline is around 8% while when spacer is removed, flux decline increases to 20%. For seawater these values are 37% and 55%, respectively. The system with counter-current mode has better heat and mass transfer compared to the co-current system. The overall permeate flux from the counter-current system was found to be higher than the co-current one which proves that the counter-current is more mass transfer-efficient mode when compared to co-current due to the enhancement of the difference in temperature and hence the driving force. Due to that, fouling is more predominant in the co-current mode compared to counter-current mode too.

3.3. Surface characterization of membrane To further verify the fouling pattern on the membrane by thermal brine and seawater, SEM analyses were carried out to observe the extent of fouling across the membrane surface as well as the penetration through the membrane pores. Fig. 6 shows the morphology and pore structure of virgin membrane before being used in MD. As shown in Fig. 6 the membrane's active side (PTFE) is bonded to the support layer (PP) thermally which enhance rigidly and temperature resistivity of the membrane. The active layer is more porous compared to the support layer and the pore size is bigger in PP side compared to the active layer. Fig. 7 shows the pore size distribution of the membrane's active side. 225 μm (15 μm × 15 μm) random membrane samples were analyzed for data in Fig. 7 and the results show 83% of pores to be higher than 0.22 μm, with a mean pore size of 0.36 μm and the maximum pore

Fig. 6. SEM image of cross section and surface morphology of the virgin PTFE membrane.

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Fig. 7. Random pore size distribution of virgin PTFE membrane.

diameter obtained from the SEM imaging was 0.96 μm. Sharp pore size distribution reduces the potential water leakage and pore wetting through the membrane.

Later after the test, membranes were analyzed with SEM to observe the salt and scaling deposition. As shown in Fig. 8 the membrane surface was covered by deposit, which reduced the membrane permeability.

Fig. 8. SEM image of virgin PTFE membrane (a), fouled membrane used with thermal brine as feed (b, c) and seawater as feed (d, e) after 300 h of operations.

A. Kayvani Fard et al. / Desalination 379 (2016) 172–181 Table 3 Change of contact angle of virgin membrane and fouled membranes at various conditions (Tf: 70 °C, Tp: 30 °C, feed flow rate: 1.5 L/min). Condition

Contact angle (deg.)

virgin membrane Fouled membrane with brine after 6 days Fouled membrane with brine after 300 h Fouled membrane with seawater after 6 days Fouled membrane with seawater after 300 h

126 ± 2.5° 110 ± 2.5° 108 ± 1.7° 104 ± 2.9° 65.3 ± 2.1°

The extent of deposition on the membrane tested with seawater is higher than membranes tested with thermal brine. In the case of thermal brine, deposits are less and fouling is not severe. On the other hand, seawater tested membrane is covered with big salts and a thick layer of deposits. A small cake layer had covered the membrane surface when thermal brine was used as feed water (Fig. 8b, c). However, a fluffy amorphous fouling layer with small crystals can be observed on the membrane surface after the DCMD of seawater is used (Fig. 8d, e). The formation of the small crystals or the amorphous fouling layer could be due to heterogeneous composition of the seawater which has not been treated before use in the system. Scaling and fouling on membrane surface not only limit the active surface area for mass transport but also reduce the membrane surface hydrophobicity. Increasing hydrophilicity (reducing hydrophobicity) can cause membrane wetting leading to the passage of liquid water to the membrane pores, which in turn delays the mass transfer of water vapor across the membrane. Contact angle measurement confirms the change of the membrane surface characteristic from hydrophobic to hydrophilic after testing with two saline solutions (Table 3). Contact angle is measured using DI water and for virgin membrane before usage, after 6 days, and at the end of experiments. All experimental condition is set to be similar at feed temperature being 70 °C, permeate temperature at 30 °C, and feed flow rate at 1.5 L/min. The result of the contact angle shows the following order: fouled membrane with seawater after 300 h (65.3 ± 2.1°), fouled membrane with seawater after 6 days (104 ± 2.9°), fouled membrane with brine after 300 h (108 ± 1.7°), fouled membrane with brine after 6 days (110 ± 2.5°), and virgin membrane (126 ± 2.5°) (Fig. 9). This suggests that the organic as well as inorganic foulants which adhered on the membrane surface led to the loss of membrane hydrophobicity. The extent of fouling is even higher when seawater is used as feed water where contact angle is reduced to around 65° where at same condition when thermal brine is used it only dropped to 110°. Thermal brine is dosed with anti-fouling and other chemicals in the pre-treatment stage when used in MSF plant. Therefore, thermal brine feed showed better anti-fouling behavior compared to the fresh seawater which contains different organic and inorganic contaminants. Antiscalants and anti-fouling generally prevent inorganic scaling, and can strongly fight carbonate scales in the feed. The extent of preventing other salts such as phosphate, sulfate, and fluoride, disperse colloids, and metal oxides is also reported in literature [31,32]. Antiscalants works in different mechanisms to delay scaling such delaying nucleation, reducing the precipitation rate, distorting crystal structure, and altering CO2 concentration [33]. Thermal brine used in this study was already dosed with such chemical which is a common technique in desalination pre-treatment due to its minimum cost as dosing needed is around 10 ppm [33]. However, dosage of anti-scalants should be controlled for use in membrane desalination and MD as high dosage and presence of some organic content in anti-scalants often reduce the surface tension of the water, which can promote membrane wetting [34]. The high contact angles of virgin membrane can be attributed to the Fig. 9. Contact angle measurement of virgin membrane (a) fouled membrane with brine after 6 days (b) fouled membrane with brine after 300 h (c), fouled membrane with seawater after 6 days (d), and fouled membrane with seawater after 300 h.

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Table 4 Chemical analysis of permeate produced with DCMD using different feeds. Properties

Unit

pH Conductivity Total organic carbon Fluoride Chloride Bromide Phosphate Nitrate Nitrite Sulfate Thiosulfate Lithium Sodium Potassium Magnesium Calcium Ammonium (NH+ 4 )

μS/cm mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L

Formula

TOC F− Cl– Br− PO43− NO− 3 NO− 2 2− SO4 S2O2− 3 Li+ + Na K+ Mg2+ Ca2+ NH4 +

Distillate from brine

Distillate from seawater

7.01 2.69 0.14 b0.1 0.9 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 0.8 b0.1 b0.1 b0.1 b0.1

6.50 2.94 0.4 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1

high hydrophobicity of the membrane. The hydrophobic nature of membrane permits only vapor to pass and rejects water, ensuring high selectivity in the process of MD. Higher contact angle in combination with other factors such as smaller pore size, lower surface energy and higher surface tension lead to higher liquid entry pressure be greater than the pressure difference at the membrane's liquid/vapor interface to prevent pore wetting [35]. 3.4. Permeate quality Water quality is one of the major parameters needed to be analyzed for any desalination technology. Quality of the distillate produced from DCMD bench scale unit is analyzed by ion chromatography. Analysis of permeate produced by MD using seawater and thermal brine are tabulated in Table 4. Both feed show good permeate quality with no major observation on membrane wetting which alters the water quality. Although the conductivity of the feed increases by time (Fig. 10), distillate quality showed no change in terms of conductivity and remained in the range of 3–4 μS/cm. A big change in terms of conductivity may be an indication of membrane wetting which will also cause a dramatic change in terms of permeate flux. During this study no such effect has been observed. Decline of flux discussed earlier was mainly due to fouling and scaling on membrane surface.

Fig. 10. Feed conductivity change by time for thermal brine and seawater.

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