Membran in Biodiesel

mbrane. In addition, the potential of functionalised carbon nanotubes to serve

Contents

. . .sel convs in bio. . .l produ. . .tion bas

5.2. Possible combinations of membrane and catalyst in biodiesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1369

Biotechnology Advances 30 (2012) 13641380

Contents lists available at SciVerse ScienceDirect

Biotechnology Advances5.2.1. Membrane without incorporated catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13695.2.2. Membrane with incorporated catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1370

6. Effect of process parameters in biodiesel production by membrane reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13716.1. Effect of reaction temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13716.2. Effect of methanol to oil ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13726.3. Effect of catalyst concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13726.4. Effect of reactant ow rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13726.5. Effect of trans-membrane pressure (TMP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13736.6. Effect of membrane pore size and thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13737. Advantages of catalytic membrane reactor in7.1. Environmentally friendly process7.2. Lower investment cost . . . . .

Corresponding author. Tel.: +60 4 5996475; fax: +E-mail address: [email protected] (S.H. Tan).

0734-9750/$ see front matter 2012 Elsevier Inc. Alldoi:10.1016/j.biotechadv.2012.02.009ed on oil droplet size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1368ed on catalytic membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1368ed on pervaporation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13685.1.2. Membrane separation bas5.1.3. Membrane separation bas5.1. Concepts and principles .5.1.1. Membrane separa1. Introduction . . . . . . . . . .2. Limitations in conventional biodie3. Process intensication technologie4. Concept of membrane reactor . .5. Membrane technology in biodiesethis paper will also discuss the effects of process parameters for transesterication in a membrane reactorand the advantages offered by membrane reactors for biodiesel production. This discussion is followed bysome limitations faced inmembrane technology. Nevertheless, based on the ndings presented in this review,it is clear that the membrane reactor has the potential to be a breakthrough technology for the biodieselindustry.

2012 Elsevier Inc. All rights reserved.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1365ersion technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1366diesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1367. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1367ction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1368. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1368as catalysts while being incorporated into the membrane for transesterication is discussed. Furthermore,

catalyst in the catalytic meaim of this paper is to reviewmental concepts of the memResearch review paper

Membrane technology as a promising alternative in biodiesel production: A review

Siew Hoong Shuit, Yit Thai Ong, Keat Teong Lee, Bhatia Subhash, Soon Huat Tan School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia

a b s t r a c ta r t i c l e i n f o

Available online 16 February 2012

Keywords:BiodieselMembrane reactorCatalytically inert membraneCatalytically active membrane

In recent years, environmental problems caused by the use of fossil fuels and the depletion of petroleumreserves have driven the world to adopt biodiesel as an alternative energy source to replace conventionalpetroleum-derived fuels because of biodiesel's clean and renewable nature. Biodiesel is conventionallyproduced in homogeneous, heterogeneous, and enzymatic catalysed processes, as well as by supercriticaltechnology. All of these processes have their own limitations, such as wastewater generation and high energyconsumption. In this context, the membrane reactor appears to be the perfect candidate to produce biodieselbecause of its ability to overcome the limitations encountered by conventional production methods. Thus, the

the production of biodiesel with a membrane reactor by examining the funda-brane reactor, its operating principles and the combination of membrane and

j ourna l homepage: www.e lsev ie r .com/ locate /b iotechadvbiodiesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1373. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1373. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1375

60 4 5941013.

rights reserved.

7.3. Overcoming the limitation caused by chemical equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13767.4. High process exibility of feedstock conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13767.5. Complying with international standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1377

8. Membrane life-time and fouling in biodiesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13779. Limitations in membrane technology for biodiesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1378

10. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1378Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1378References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1378

1. Introduction

Human civilisation has always relied on the utilisation of energy.As illustrated in Fig. 1, the industrial sector, consisting of diverse

it has established its commercial value in the automobile markets ofEurope, the US, Japan, Brazil and India (Janaun and Ellis, 2010).Moreover, the implementation of the directive on the promotion ofthe use of biofuels for transport in the EU (Directives 2003/30/EC)mandated the increased use of biofuels to power transportationfrom 2% to 5.75% between 2005 and 2010, triggering a huge demandfor biodiesel (Mabee, 2007). Unlike conventional diesel fuel, biodieseloffers several advantages, including renewability, higher combustion

1365S.H. Shuit et al. / Biotechnology Advances 30 (2012) 13641380industrial groups that include manufacturing, agriculture, miningand construction, accounted for 52% of global energy used in 2007;the residential sector for household activities comprise 14% of thetotal; the transportation sector, providing services, such as movingpeople and goods by road, rail, air, water and pipeline, uses 27%;and the commercial sector, which consists of businesses, institutions,and organisations that provide services, comprises 7%. Although theglobal economic recession drove a reduction of energy consumptionby 1.1% in 2009, the International Energy outlook 2010 (IEO, 2010)projections still predicted an increase of global energy consumptionby 49%, or 1.4% every year until 2035 (EIA, 2010). This predictedincrease forecasts increasing demand of resources for energy produc-tion. According to the statistical review conducted by British Petroleum(BP) (BP, 2009), global energy production depends heavily on oil (35%),coal (29%) and natural gas (24%) to satisfy the global energy demand, asshown in Fig. 2. Fossil fuels are the world's slowest-growing source ofenergy, and their supplies are decreasing daily. The increasing demandfor energy production throughout the projection period will lead to anincrease in the price of these resources. In addition, the growing emis-sion of carbon dioxide, sulphur dioxide, hydrocarbons and volatileorganic compounds (VOCs) from the combustion of fossil fuels couldresult in air pollution, globalwarming and climate change. These negativeimpacts on the environment are the target of current energy policies thatemphasise cleaner, more efcient and environmentally friendly technol-ogies to increase the supply and usage of energy (Hammond et al., 2008;Hoekman, 2008; Monni and Raes, 2008; Sawyer, 2009). Thus, develop-ments in alternative renewable energy sources have become indispens-able for sustainable environmental and economic growth. Among theexplored alternative energy sources, considerable attention has beenfocused on biodiesel because it is widely available from inexhaustiblefeedstocks that can effectively reduce its production cost.

Biodiesel, which is also known as fatty acid methyl ester (FAME),is a mixture of monoalkyl esters of long-chain fatty acids derivedfrom renewable lipid feedstocks, such as vegetable oil and animalfats. Because biodiesel has similar physical properties to diesel fuels,Fig. 1. Global energy consumption in 2007 (EIA, 2010).efciency (Fazal et al., 2011), cleaner emission (Janaun and Ellis,2010), higher cetane number, higher ash point, better lubrication(Lin et al., 2011) and biodegradability (Wardle, 2003). Dependingon the climate, local soil conditions and availability, various biolipidshave beenused in different countries as feedstocks to produce biodiesel.Biolipid feedstocks can be divided into four categories: virgin vegetableoils, waste vegetable oils, animal fats and non-edible oils. Virgin vegeta-ble oil feedstock refers to rapeseed, soybean, sunower and palm oil(Demirbas, 2008), while waste vegetable oil refers to these oils thathave been used in cooking and are no longer suitable for humanconsumption (Conservation ADoE, 2011; Lam et al., 2010). Animal fatsinclude tallow, lard and yellow grease (Atadashi et al., 2010) while thenon-edible oils include Jatropha (Shuit et al., 2010b; Yee et al., 2009),neem oil, castor oil, tall oil (Demirbas, 2008) and microalgae (Ahmadet al., 2011).

Several modication techniques, such as dilution, microemulsion,pyrolysis and transesterication have been used to reduce the viscosityof vegetable oil (Andrade et al., 2011). Of these processes, transesteri-cation is the most widely used; this method involves the alcoholysis ofvegetable oil to produce alkyl ester. Generally, the mechanism consistsof three consecutive reversible reaction steps. The rst step involves theconversion of triglycerides (TG) to diglycerides (DG) and later tomono-glycerides (MG). Subsequently, the monoglycerides are converted toglycerol. Each reaction step produces an alkyl ester. Thus, a total ofthree alkyl esters are produced in the transesterication process(Sharma and Singh, 2008). The overall reaction that occurs in transes-terication is simplied in Fig. 3 (Lim and Teong, 2010).Fig. 2. World energy production in 2009 (BP, 2009).

des

1366 S.H. Shuit et al. / Biotechnology Advances 30 (2012) 13641380Common transesterication reactions that are used to producebiodiesel include homogeneous catalysed transesterication, hetero-geneous catalysed transesterication, enzymatic catalysed transester-ication and supercritical technology (Marchetti et al., 2007). Each ofthese methods has its own disadvantages that eventually limit theeconomic feasibility and low environmental impact of the entirebiodiesel production process. The limitations of conventional biodieselproduction technology will be discussed in this paper. In addition,certain process intensication technologies in biodiesel production willalso be discussed. Because of its ability to overcome those limitations,membrane technology has the potential to be a promising alternativefor biodiesel production. Therefore, the aim of this paper is to presentthe production of biodiesel via membrane reactor, which includes theoperation principles, possible combinations of membrane and catalystand the effect of the process parameters of the transestericationreaction on the membrane reactor. Finally, the advantages offered bymembrane technology in biodiesel production will be discussed. Thediscussion will also focus on the ability of the membrane reactor toproduce biodiesel in a more economical and environmentally friendlymanner.

2. Limitations in conventional biodiesel conversion technology

In the conversion of vegetable oil by the transesterication process,the reversible reaction between the reactant and product indicatesthat the formation of biodiesel is highly dependent on the proportionof the reactant and the conditions of the transesterication process.According to Le Chatelier's principle, large quantities of alcohol areneeded to shift the equilibrium of the reaction to the product sideand increase the yield of biodiesel (Othman et al., 2010). Unfortunately,high consumption of alcohol is associated with higher production cost.

The consumption of alcohol could be reduced by using acid or

O

O C R1CH2

O

O C R2CH

O

O C R3CH2

+ 3 R OH

Triglyceride Alcohol

Fig. 3. Transesterication of triglycerialkaline catalysts, which could improve the reaction rate and biodieselyield. However, homogeneous acid solutions that catalyse transester-ication processes, such as sulphuric (Sahoo et al., 2007), hydrochloric(Boucher et al., 2008), or sulphonic acids (Guerreiro et al., 2006) havebeen largely ignored because they increase the time consumption ofthe process, require a higher reaction temperature and are corrosiveby nature. Although the use of homogeneous alkaline catalysts, suchas sodium (Rashid et al., 2008) and potassium hydroxide (Rashidand Anwar, 2008) could overcome these limitations, it has beenreported that the alkaline catalysed reaction is sensitive to the purityof the reactant. The presence of water and free fatty acids in the rawfeedstock could induce a saponication process in which the freefatty acid produced by the hydrolysis of triglycerides reacts with thealkaline catalyst to form soap. The dissolved soap in the glycerolphase would increase the solubility of methyl ester in the glyceroland complicate the subsequent separation process (Vicente et al.,2004). Also, the removal of either the homogeneous acidic or alkalinecatalyst using hot distilled water would eventually result in the needto dispose of wastewater (Xie and Li, 2006).

Heterogeneous catalyst has been viewed as an alternative solutionto replace the homogeneous catalyst because it is non-corrosive andenvironmentally benign.However, theheterogeneous catalytic reactionusually faces a mass transfer resistance problem because the constitu-tion of the three-phase system (triglycerides, alcohol and solid catalyst)in the reactionmixture limits the pore diffusion process and reduces theactive site availability for the catalytic reaction, thereby decreasing thereaction rate (Mbaraka and Shanks, 2006). Catalyst support can mini-mise themass transfer limitation, but the active species in the supportedcatalyst can easily be corroded by alcohol, shortening the catalyst life-cycle (Liu et al., 2008). The biodiesel obtained from the biocatalytictransesterication process that uses an enzyme as a catalyst seemsattractive and encouraging for because the product is easily separablewithout side reactions (Jegannathan et al., 2008), but biodiesel fromthis process is not yet commercially viable because of the requirementof longer reaction time and the unfavourable reaction yield in compari-son to the alkaline catalyst. It has been reported that the enzymatictransesterication process requires 24 h to achieve a biodiesel yield of90% (Oda et al., 2005). Most importantly, the major obstacle to thisprocess is the high cost of the enzyme. The enzyme also requires veryspecic reaction conditions because the denaturation of the enzymeand its deactivation as a result of feed impurity could decrease itsefciency (Dizge et al., 2009).

Supercritical alcohol transesterication provides a new path for theproduction of biodiesel without the aid of a catalyst. The supercriticalcondition could overcome the mass transfer limitation by enabling themixture of triglyceride and alcohol to become a homogeneous phase(Pinnarat and Savage, 2008). However, the major drawbacks of thisnon-catalytic process are its large energy requirement and its infeasibil-ity for large-scale industrial application because of the increased

O

O C R1R

O

O C R2R

O

O C R3R

OHCH2

OHCH

OHCH2

+

Fatty Acid Alkyl Ester Glycerol

with alcohol (Lim and Teong, 2010).production cost imposed by the high reaction temperature and pressure(Yin et al., 2008). Moreover, the supercritical process is potentiallyhazardous and requires attention to personal risk and safety.

Both the catalytic and non-catalytic transesterication down-stream processes will receive a mixture of product, biodiesel andglycerol, aswell as unreacted reactant and catalyst. Ineffective biodieselseparation and purication may cause severe diesel engine problems,such as plugging of lters, coking on injectors, carbon deposits, exces-sive engine wear, oil ring sticking, engine knocking, and thickeningand gelling of lubricant oil (Demirbas, 2007). In order to obtain high-purity biodiesel, the downstream of the transesterication processwill undergo various complementary separation stages, such as glycerolseparation, catalyst neutralisation and biodiesel purication. Themulti-ple downstream processes are time-consuming and require additionalcost. A recent report revealed that the current downstream processingalone constituted over 60-80% of the total cost of a transestericationprocess plant (Tai-Shung, 2007). In addition, the multiple separation

1367S.H. Shuit et al. / Biotechnology Advances 30 (2012) 13641380and purication stages could cause loss of the biodiesel, resulting in adecrease in the pure biodiesel yield.

3. Process intensication technologies in biodiesel production

The problems described above, including the wastewater issue(Xie and Li, 2006), limited mass transfer (Mbaraka and Shanks,2006), thermodynamic equilibrium (Cao et al., 2008b), high energyrequirement (Yin et al., 2008) and multiple downstream processingsteps (Tai-Shung, 2007) in biodiesel production can be improved byprocess intensication technologies. These technologies involve theuse of novel reactors or coupled reaction/separation processes toenhance the reaction rate and to reduce the residence time (Qiu etal., 2010).

Recently, novel reactors, such as themicro-channel reactor, oscillatoryow reactor, rotating/spinning tube reactor and cavitational reactor, havebeen developed and applied to improve themixing andmass/heat trans-fer between the oil and methanol in biodiesel production (Qiu et al.,2010). The micro-channel reactor can achieve a rapid reaction ratebecause it has a high volume/surface ratio, short diffusion distance andfast and efcient heat dissipation and mass transfer (Qiu et al., 2010;Wen et al., 2009). The micro-channel reactors used in biodiesel produc-tion include micro-channel reactors with T- or Y-ow structures, zigzagmicro-channel reactors (Wen et al., 2009) and slit channel reactor (Kaluet al., 2011). Because of the high heat transfer rate, it has been reportedthat the micro-channel reactor consumed less energy than the conven-tional stirrer reactor (Qiu et al., 2010). However, the micro-channel reac-tor suffers from the drawback of low production throughput, which isattributed to the limitations of the micro-fabrication technology that isused to produce the micro-channel. Furthermore, the high investmentcost of the micro-channel reactor prohibits the addition of more reactorsin parallel to amplify the production of biodiesel (Kalu et al., 2011).

The oscillatory ow reactor is a type of continuous plug ow reactor(PFR) (Phan et al., 2011) in which the orice plate bafes are equallyspaced, and a piston is used to produce oscillatory ow (Qiu et al.,2010). The combination of bafes and oscillatory motion intensiesthe radial mixing by the formation of periodic vortices in the bulkuid, causing an increase in mass and heat transfer while maintainingplug ow (Phan et al., 2011; Qiu et al., 2010). In addition, the oscillatoryow reactor can also improve the residence timedistribution (RTD) andmulti-phase suspension (Zheng et al., 2007). Because the oscillatoryow reactor can achieve long residence times, it can be designed witha smaller length to diameter ratio, which eventually helps to improvethe economy of biodiesel production because of the smaller foot-print, lower capital, reduced pumping cost and ease of control (Qiu etal., 2010).

The rotating/spinning tube reactor is a shear reactor containingtwo tubes. The inner tube rotates rapidly within the concentricstationary outer tube. Both tubes are separated by a narrow annulargap, which produces Couette ow when the reactants are introduced.Because of the high shear rate, the reactants are mixed and movethrough the gap as a coherent thin lm. This thin lm provides alarge interfacial contact area to enhance the reaction rate betweenthe oil and the methanol. As a result, less mixing power and reactiontime are required to produce biodiesel using a rotating/spinning reactorcompared to a conventional reactor. This type of reactor is suitable tohandle feedstocks with high FFA because the residence time is short(Qiu et al., 2010).

The cavitational reactor is another type of novel reactor that hasbeen used successfully in biodiesel production (Gogate and Kabadi,2009; Kelkar et al., 2008; Pal et al., 2010; Qiu et al., 2010). Cavitationis dened as the generation of cavities followed by their growth andviolent collapse, causing high local energy densities, temperaturesand pressures (Gogate and Kabadi, 2009; Qiu et al., 2010). Cavitationenhances the mass transfer rate of the reaction by creating conditions

of local intense turbulence and liquid micro-circulation currents inthe reactor (Gogate and Kabadi, 2009; Kelkar et al., 2008; Qiu et al.,2010). Cavitational reactors can be classied into two types: hydrody-namic cavitation and acoustic cavitation (Kelkar et al., 2008; Qiu et al.,2010). Hydrodynamic cavitation can be generated by using a restrictioncomponent, such as an orice plate, a throttling valve or a venture,placed in a liquid ow (Gogate and Kabadi, 2009; Kelkar et al., 2008).At the constriction area, the kinetic energy or velocity of the liquidincreases, but the local pressure decreases. (Gogate and Kabadi, 2009).A hydrodynamic cavitation reactor is more effective for mixing ofimmiscible liquids (Pal et al., 2010). Themixing efciency of a hydrody-namic cavitation reactor has been reported to be 160400 times higherthan that of the conventional mixing method (Qiu et al., 2010). There-fore, the hydrodynamic cavitation reactor consumes half of the energyrequired by conventionalmechanical stirring (Pal et al., 2010). A reactorthat generates cavitation by ultrasound is known as a sonochemicalreactor (Gogate and Kabadi, 2009) or an acoustic cavitation reactor(Qiu et al., 2010; Wu et al., 2009). Ultrasound causes a series ofcompression and rarefaction cycles by alternately compressing andstretching the molecular spacing of the medium (Colucci et al., 2005).Low-frequency ultrasound irradiation is useful for the emulsicationof immiscible liquids, such asmethanol and oil. Emulsication is a resultof the induced collapse of cavitation bubbles that disrupt the phaseboundary of methanol and oil (Rokhina et al., 2009). Emulsions withlarge interfacial areas providemore reaction sites for transestericationand eventually increase the reaction rate (Chand et al., 2010). It hasbeen reported that the operating parameters, such as temperature,pressure, reaction time and catalyst concentration, are signicantlyreduced in ultrasound-assisted transesterication (Deshmane et al.,2008; Kalva et al., 2008). However, sonochemical reactors suffer fromerosion and particle shedding at the delivery tip surface because ofthe high surface energy intensity (Gogate and Kabadi, 2009). Also, thescale-up of a sonochemical reactor is relatively more difcult than it isfor a hydrodynamic cavitation reactor because the former relies on asource of vibration (Qiu et al., 2010).

The microwave reactor is another intensication technology forbiodiesel production. The main function of a microwave reactor isnot to improve themixing of oil andmethanol but to use its irradiationto transfer energy directly into the reactants and thus accelerate thetransesterication. Because both polar and ionic components areavailable in the mixture of oil and methanol/alcohol, a microwavereactor plays an important role in themore efcient heating of reactantsto the desired temperature because of the energy interactions at themolecular level (Barnard et al., 2007). Compared to a conventionalthermal heating reactor, a microwave reactor is able to achieve similarbiodiesel conversion with a shorter reaction time and in a moreenergy-efcient manner (Qiu et al., 2010).

All of the above mentioned novel reactors intensify the transester-ication by either enhancing the mixing of oil and methanol orimproving the heat transfer between the two liquid phases. However,none of these novel reactors, except the membrane reactor, is able toovercome the limitation caused by chemical equilibrium in transes-terication. Therefore, themembrane reactor offers another interestingprocess intensication technology for biodiesel production that will bediscussed in detail in this paper.

4. Concept of membrane reactor

A membrane reactor is also known as a membrane-based reactiveseparator (Sanchez Marcano and Tsotsis, 2002). According to IUPAC, amembrane reactor is dened as a device that combines reaction andseparation in a single unit (Caro, 2008). Generally, the classicationof a membrane reactor is based on four concepts (Ertl et al., 2008):the reactor design (extractor, distributor or contactor), the membraneused in the reaction (organic, inorganic, porous or dense membrane),whether it is an inert or catalytic membrane reactor and the reaction

that occurs in membrane reactor (such as dehydrogenation (Caro,

2008), esterication (Buonomenna et al., 2010; Caro, 2008), waterdissociation (Caro, 2008) or wastewater treatment (Drioli et al.,2008)). In addition to providing the separation, a membrane reactoralso enhances the selectivity and yield of the reaction (SanchezMarcano and Tsotsis, 2002). As illustrated in Fig. 4, there are twobasic congurations of membrane reactor (Lipnizki et al., 1999a).Fig. 4A shows the layout of a membrane reactor system in which themembrane reactor appears as an external process unit. On the otherhand, the membrane reactor shown in Fig. 4B combines the reactorand membrane separator into a single unit. In comparison to theconventional biodiesel production process, the main advantageoffered by themembrane reactor, especially the integratedmembranesystem, is the reduction of the capital and operating costs because ofthe elimination of the intermediate processing steps (SanchezMarcano and Tsotsis, 2002). Recently, the membrane reactor has

Glycerol

Biodiesel

Methanol

Triglycerides

Fig. 5. Schematic diagram of membrane to remove glycerol from the product stream.

Glycerol

Biodiesel

Methanol

Triglycerides

Fig. 6. Schematic diagram of membrane to retain un-reacted triglycerides within the

1368 S.H. Shuit et al. / Biotechnology Advances 30 (2012) 13641380been applied as a promising technology in biodiesel production(Baroutian et al., 2010; Cao et al., 2008b; Dub et al., 2007).

5. Membrane technology in biodiesel production

5.1. Concepts and principles

Membrane separation involves the use of a selective barrier(membrane) to regulate the transport of substances, such as gases,vapours and liquids, at different mass transfer rates (Bddeker,2008; Sirkar and Ho, 1992). The mass transfer rates of differentsubstances are controlled by the permeability of the barrier towardthe feed components (Bddeker, 2008). In the production of biodiesel,the membrane plays an important role by removing glycerol from theproduct (biodiesel) stream (Guerreiro et al., 2006; Saleh et al., 2010)or retaining the un-reacted triglycerides within the membrane(Baroutian et al., 2011; Cao et al., 2008b; Dub et al., 2007) as shownin Figs. 5 and 6 respectively. There are two basic principles of operationin biodiesel production via membrane technology; separation based onoil droplet size (Cao et al., 2008a, 2008b) or catalytic membrane(Guerreiro et al., 2006, 2010; Shao and Huang, 2007). Pervaporationalso seems applicable to biodiesel production.

5.1.1. Membrane separation based on oil droplet sizeMembrane separation based on oil droplet size requires a micro-

porous membrane, which is typically a ceramic membrane (Baroutianet al., 2010, 2011; Cao et al., 2008a) or a carbon membrane (Dub etal., 2007). The operation principle of themembrane used in amembranereactor for biodiesel production is illustrated in Fig. 7 (Dub et al., 2007).Because of the difference in polarity, methanol is immiscible with oilsand lipids (Cao et al., 2008a; Shuit et al., 2010a). Therefore, a mixtureofmethanol and lipidwill exist in a two-phase systemor as an emulsionof lipid droplets suspended in a methanol rich phase (Cao et al., 2008a;Dub et al., 2007). The immiscibility of the lipid and themethanol is themain cause of the mass transfer limitation in the transesterication

Fig. 4. Basic layout of membrane reactor (A) a conventional membrane reactor system

(B) an integrated membrane reactor system (Lipnizki et al., 1999a).reaction, but this emulsied system is favoured in the operation of amembrane reactor (Dub et al., 2007). In the emulsied system, transes-terication is believed to occur at the interface between lipid dropletsand the continuous methanol phase in which they are dispersed(Ataya et al., 2006). It has been reported that biodiesel and glycerol, aswell as the catalysts (both acid and alkaline catalysts), are soluble inmethanol (Cao et al., 2008a; Zhou et al., 2006). Thus, the unreactedlipidswill be suspended and dispersed in amixture ofmethanol, biodie-sel, glycerol and catalyst on the membrane retentate side (Cao et al.,2008a). Because of its smaller molecular size, methanol and othersoluble components, such as biodiesel, glycerol and catalysts, are ableto pass through the microporous membrane into the permeate streamwhen the transmembrane pressure (TMP) is increased (Baroutian etal., 2010).Meanwhile, the emulsied lipid dropletswith largermolecularsize are trappedwithin themembrane to be continuously converted intobiodiesel (Baroutian et al., 2010; Cao et al., 2008a; Dub et al., 2007).

5.1.2. Membrane separation based on catalytic membraneMembrane separation based on the catalytic membrane involves a

non-porous dense polymer membrane, such as poly(vinyl alcohol)(PVA) (Guerreiro et al., 2006, 2010; Shi et al., 2010). The operation ofthis type of membrane is based on the interaction between the targetcomponent and the polymer functional groups of the membrane(Guerreiro et al., 2006). In biodiesel production via this type of catalyticmembrane, glycerol and methanol are able to form hydrogen bondswith the OH groups in the polymer membrane (Guerreiro et al.,2006). Therefore, the glycerol and methanol are continuously removedfrom themixture during the reaction (Guerreiro et al., 2006; Saleh et al.,2010). Meanwhile, the unreacted lipids and the produced biodiesel areretained within the membrane because of their difference in chemicalproperties with the polymer group of the membrane. In this case, theseparation is carried out under atmospheric pressure (Guerreiro et al.,2006).

5.1.3. Membrane separation based on pervaporationSeparation by pervaporation does not rely on the relative volatilities

of the components but on the relative rates of permeation through amembrane. Pervaporation is also performed with a non-porous densemembrane that is usually made from a polymer or zeolite (Shao andHuang, 2007; Sharma et al., 2004). Therefore, pervaporation has alwaysbeen hailed as clean technology to replace conventional energy-intensive separation processes, such as evaporation and distillation(Sae-Khow and Mitra, 2010). Pervaporation is most often applied tomembrane.

Fig. 7. Separation of oil and FAME by micro-porous membrane (Dub et al., 2007).

1369S.H. Shuit et al. / Biotechnology Advances 30 (2012) 13641380the dehydration of organic solvents, the removal of organic compoundsfrom aqueous solutions and the separation of organicorganic mixtures(Khayet et al., 2004). However, based on the concept of the catalyticmembrane, pervaporation is believed to be a possible operation principlein biodiesel production. The concept and applications of pervaporationhave been reviewed in detail in several articles (Lipnizki et al., 1999a,1999b; Pangarkar and Pal, 2008; Sae-Khow and Mitra, 2010; Shao andHuang, 2007).

The pervaporation process is distinct fromothermembraneprocessesbecause it combines permeation and evaporation in a single module.A phase change occurs for molecules that permeate through themembrane toward the downstream site (Pangarkar and Pal, 2008). Byapplying a lower pressure at the permeate side of the membrane, adriving force is created to remove target solutes from the solutionmixture (Sae-Khow and Mitra, 2010). Solution-diffusion is the well-recognised mechanism to describe mass transport through non-porous membranes (Lipnizki et al., 1999b; Sae-Khow and Mitra, 2010;Shao and Huang, 2007). The permeation of solute molecules throughthe membrane occurs in ve main steps, which are shown in Fig. 8(Sae-Khow and Mitra, 2010). First, the solutes in the reaction mixturediffuse through the liquid boundary layer of the membrane feed (PL1to PL2). At the membrane-liquid interface, specic solutes are selectivelypartitioned into themembrane (PL2 to PM1). Under a pressure difference,the solute molecules diffuse across the membrane (PM1 to PM2)(Sae-Khow and Mitra, 2010). Next, the desorption of solute moleculesinto the vapour phase occurs at the downstream surface of the lm(PM2 to PV1) (Pangarkar and Pal, 2008; Sae-Khow and Mitra, 2010).Lastly, the gas molecules of the solute diffuse away from the membranethrough the boundary layer on the permeate side (PV1 to PV2)(Sae-Khow andMitra, 2010). The sorption of solutes into the membranedepends on the interaction between the solutes and the polymer groupsFig. 8. Permeation of solute molecules through non-porous dense membrane (Sae-Khow and Mitra, 2010).in the membrane (Pangarkar and Pal, 2008). Therefore, glycerolmolecules have a high probability of being selectively partitioned bythemembrane because hydrogenbonds are formed between the glycerolmolecules and theOHgroups of the polymermembrane (Guerreiro et al.,2006).

Flux and selectivity are the two most important parameters inpervaporation. Flux is expressed in terms of the partial pressuredifference across the two sides of themembrane, and the concentrationgradient or vapour pressure difference is maintained either by keepinga constant vacuum on the permeate side or by introducing a sweep gasto depress the partial pressure (Sae-Khow and Mitra, 2010). The selec-tivity of solutes is governed by sorption and diffusion, depending on thesolute. Sorption depends on the solubility parameter of the solutes andthe membrane material. Apart from the physical properties of thesolutes, such as the size, shape and molecular weight, the availabilityof inter/intramolecular free space in the polymer also affects thediffusion coefcient (Pangarkar and Pal, 2008). The last two steps inpervaporation, the desorption step (PM2 to PV1) and the diffusion ofgas phase from the membrane through the boundary layer (PV1 toPV2), are rapid and nonselective, offering the least resistance in theoverall transport process (Pangarkar and Pal, 2008).

5.2. Possible combinations of membrane and catalyst in biodieselproduction

Catalytic membranes in biodiesel production can be classied intotwo categories: membranes that do not incorporate catalyst andmembranes that do incorporate catalyst. In addition, the potentialapplication of mixed matrix membrane (MMM) with embeddedfunctionalised carbon nanotubes (CNTs) in biodiesel production willbe discussed. The role of themembrane in this particular congurationis as a medium to provide intimate contact between the oil and thealcohol (Buonomenna et al., 2010).

5.2.1. Membrane without incorporated catalystThis type of noncontact conguration between the membrane and

the catalyst is also known as the catalytically inert membrane(Buonomenna et al., 2010) in which the catalysts are added to thereactants but not embedded inside the membrane. The most commoncatalytically inert membranes in biodiesel production are the TiO2/Al2O3 in ceramic membrane (Baroutian et al., 2010, 2011), ltaniumceramic membrane (Cao et al., 2008a, 2008b) and carbon membrane(Dub et al., 2007) with the separation concept based on oil dropletsizes. The pore sizes of these membranes range from 0.02-0.05 m(Baroutian et al., 2010; Cao et al., 2008b; Dub et al., 2007). Thecatalysts used for catalytically inert membranes include sulphuricacid (H2SO4) (Dub et al., 2007) or potassium hydroxide/sodiumhydroxide (KOH/NaOH) (Baroutian et al., 2010; Cao et al., 2008a,2008b). The schematic diagram for the transesterication reactionvia catalytically inert membrane is shown in Fig. 9 (Baroutian et al.,2010; Cao et al., 2008a, 2008b). Initially, a pre-determined amountof oil and a homogeneous mixture of methanol/KOH are chargedinto a mixing vessel for pre-mixing. Next, the reaction mixture isheated to the desired reaction temperature before charging into themembrane reactor. The permeate stream consists of FAME (biodiesel),glycerol, methanol and catalysts (Baroutian et al., 2010; Cao et al.,2008a; Dub et al., 2007). Oil droplets with molecular size of 12 m(larger than the pore size of membrane) (Cao et al., 2008b;DeRoussel et al., 2001) are trapped on the retentate side and recycledback into themixing vessel (Cao et al., 2008b). The backpressure valveand cooler bring the permeate stream to atmospheric conditions (Caoet al., 2008b). The permeate stream can subsequently be separatedinto non-polar and polar phases (Cao et al., 2008a). The non-polarphase (collectively known as the FAME-rich phase) consists of morethan 85% FAME, and the remainder consists ofmethanol, trace amount

of DG and catalysts (Cao et al., 2008b). In order to comply with the

chan

M

ally

1370 S.H. Shuit et al. / Biotechnology Advances 30 (2012) 13641380American Society for Testing and Materials (ASTM) or EuropeanStandards (EN) standards for biodiesel, further purication of theFAME-rich phase is conducted to remove methanol, DG and catalysts(Cao et al., 2008a). Meanwhile, the polar phase, which is also knownas the glycerine-rich phase, contains a mixture of glycerol, methanol,catalysts and FAME (Cao et al., 2008b). Results have shown that thiscatalytic membrane reactor was capable of achieving a high oil-to-FAME conversion of more than 90% for both H2SO4 and KOH catalysts(Dub et al., 2007).

Methanol that permeates through the membrane is recycled backto the membrane reactor in order to reduce the overall methanol tooil molar ratio (Cao et al., 2006). There are two ways to recyclemethanol back to the reactor: recycling of the methanol from thedistillation of the FAME-rich phase (Baroutian et al., 2010) and directrecycling of the glycerine-rich phase (polar phase) (Cao et al., 2008b).

Feed pump

Safety and relief valves

Retentate stream

Permeate stream

Oil feed tank Methanol/catalyst feed tank

Mixing vessel

Heat ex

Purge and quench tank

Fig. 9. Schematic diagram of transesterication reaction via catalyticIt has been reported that more FAME is distributed into the FAME-rich phase by direct recycling of the glycerine-rich phase. Therefore,this process can facilitate the production of higher purity FAME andreduce the amount of water required for the FAME puricationprocess (Cao et al., 2008b).

A packed bed membrane reactor consisting of a catalyst supportedby activated carbon (Baroutian et al., 2011) was used to avoid thepermeation of catalysts through the membrane. The catalysts wereprepared by adding activated carbon ranging from 550810 m insize into a potassium hydroxide solution. The mixture was subse-quently agitated at a temperature of 25 C for 24 h. Next, the catalystswere packed inside the tubular TiO2/Al2O3 ceramic membrane reactor(Baroutian et al., 2011). The highest oil to FAME conversion for thispacked bed membrane reactor was 93.5% (Baroutian et al., 2011),which was comparable to the conversion achieved by the membranereactor with the addition of H2SO4 or KOH catalysts. Moreover, it hasbeen reported that high-quality biodiesel was produced from such areactor without washing or purication steps (Baroutian et al.,2011).

5.2.2. Membrane with incorporated catalystA membrane that incorporates catalyst in which the catalyst is

immobilised in the polymeric matrix is commonly known as a cata-lytically active membrane (Buonomenna et al., 2010). Polymericmembranes (Guerreiro et al., 2006, 2010; Zhu et al., 2010) are usuallyused as catalytically active membranes (Sarkar et al., 2010). Amembrane can be made catalytically active by heterogenisation ofhomogeneous catalysts or incorporation of heterogeneous catalystsinside the polymer matrix (Buonomenna et al., 2010). The catalyticallyactive membrane combines reaction and separation in a single step,realising the concept of reactive separation (Buonomenna et al.,2010); for this reason, the membrane is known as a separative reactor(Stankiewicz, 2003). Presently, poly(vinyl alcohol) (PVA) membranesare the only reported polymer membranes that have been tested inbiodiesel production (Guerreiro et al., 2006, 2010; Sarkar et al., 2010;Zhu et al., 2010) because of their high hydrophilicity, good thermalproperties and good chemical resistance (Guan et al., 2006).

A PVA membrane must be modied before it can be transformedinto a catalytically active membrane. There are two important stepsin preparing a catalytic PVA membrane: crosslinking of PVA followedby esterication of the free PVA-OH groups (Guerreiro et al., 2006,

Coriolis meter

Back pressure valve

Recycled back

Phase separation

Vacuum pump

ger

embrane module

CoolerFAME phase

Polar phase

Biodiesel

Purge

inert membrane (Baroutian et al., 2010; Cao et al., 2008a, 2008b).2010). Sulphosuccinic acid (Guerreiro et al., 2006), succinic acid(Castanheiro et al., 2006), fumaric acid (Guan et al., 2006), maleicacid (Figueiredo et al., 2008) and glutaraldehyde (GA) (Wang andHsieh, 2010) can be used as the membrane crosslinking agents.Higher degrees of crosslinking can enhance the thermal stability of themembrane (Guan et al., 2006), but they can also cause the membraneto be less hydrophilic and more brittle (Kim et al., 1994). In biodieselproduction, increased crosslinking can reduce the degree of membraneswelling in oil and methanol, thereby reducing the biodiesel yield be-cause oil and methanol are prohibited from diffusion into the mem-brane in the catalytic reaction (Guerreiro et al., 2006). Esterication ofthe free PVA-OH groups by 5-sulphosalicylic acid (SA) (Castanheiro etal., 2006; Guerreiro et al., 2006) is an important step in making themembrane catalytically active because sulphonic (SO3H) functionalgroups are introduced into the polymer matrix (Castanheiro et al.,2006). The modied PVA membrane shows higher catalytic activitywith larger amounts of SA because of the increased amount of SO3Hgroups in the polymer matrix (Castanheiro et al., 2006). In addition tofunctionalisation with SA, PVA can be transformed into a catalyticmembrane by blending with poly(styrene sulphonic acid) (PSSA),which contains strong acidic groups (Zhu et al., 2010). It has beenreported that 92% conversion of oil into FAME can be achieved by aPSSA/PVA membrane in 8 h of reaction time. In addition to functionali-sation with sulphonic groups, the annealing temperature is also ofcritical importance during the synthesis of the membrane because itcontrols the degree of crosslinking and the number of SO3H groups

1371S.H. Shuit et al. / Biotechnology Advances 30 (2012) 13641380available in the membrane. Such a blended membrane also showed astable conversion of 80% after 5 repeated runs (Zhu et al., 2010).

Heterogeneous catalysts can also be embedded into the polymermatrix in place of homogeneous catalysts. Hydrotalcite, Mg6Al2(OH)16(CO32-).4H2O is a suitable solid base catalyst for biodieselproduction because of its large specic surface area (Bastiani et al.,2004) and strong Lewis basicity (Roelofs et al., 2000). This catalyticmembrane is prepared by dispersing 1 g of hydrotalcite into a 10%PVA solution. The membrane showed a promising yield of biodiesel(more than 95%) (Guerreiro et al., 2010). Polyacrylonitrile (PAN) isanother potential polymer membrane material that can be appliedto the biodiesel process. However, the only reported use of a PANmembrane has been the purication of the produced biodiesel byremoving free glycerol (Saleh et al., 2010). Because of the lack offunctionality in the PAN membrane, it must be modied either bycrosslinking with SA or blending with a polymer containing SO3Hgroups before it can be transformed into a catalytic membrane.

Polymeric membranes suffer from a lack of chemical and thermalstability (Ismail et al., 2009) and are easily broken (Guerreiro et al.,2006). These deciencies and the high fabrication cost of inorganicmembranes (Ismail et al., 2009) have encouraged the developmentof the more capable mixed matrix membrane (MMM). In comparisonto competing materials, MMM offers promising fabrication cost,mechanical strength and chemical and thermal stability. MMM is aheterogeneous membrane that incorporates an inorganic ller in apolymer matrix. MMM combines the superior permeability andselectivity of inorganic membranes with the economical processingcapabilities of polymeric membranes. The rigid, adsorptive andporous inorganic phase in theMMMoffers good separation properties,and the presence of the exible polymer makes membrane formingeasier, solving the fragility problems encountered by inorganicmembranes (Ismail et al., 2009). The most common inorganic llersfor MMMs include zeolites (Mahajan et al., 1999), carbon molecularsieve (CMSs) (Peng et al., 2006) and CNTs (Peng et al., 2007). Theunique properties of CNTs, such as their high aspect ratio and surfacearea, simple functionalisation and dispersion in organic polymer,enhance the mechanical strength of the MMM with minimal llercontent. The simplication of pore dimension control at the nanometrescale (Ismail et al., 2009) has made CNTs a suitable inorganic llermaterial in the polymer matrix. In addition to improving the physicalproperties of the membrane, the functionalised CNTs can also act as acatalyst for the transesterication reaction. Recent studies have shownthe capability of amino-functionalised CNTs to serve as a solid basecatalyst for transesterication reactions (Villa et al., 2009, 2010). Amino-functionalised CNTs are simply known as nitrogen-functionalised CNTs.Different amide groups, including triethylamine, ethylamine and pyrroli-dine, can be readily grafted into CNTs (Villa et al., 2009). Triethylaminehas higher basicity than other amide groups, making it the most activecatalyst, which is able to achieve almost complete conversion under cer-tain reaction conditions (Villa et al., 2009). These amino-functionalisedCNTs have shown extremely stable catalytic activity, which could obtaingreater than 90% conversion, even after 6 reaction cycles (Villa et al.,2009). Therefore, mixed matrix polymer membranes with embeddedfunctionalised CNTs could represent a breakthrough for applications inbiodiesel production.

Such properties as the membrane thickness, swelling capabilityand active site concentration play an important role in enhancingthe biodiesel yield. It has been observed that membranes that arecapable of swelling in oil give higher biodiesel yields (Guerreiro etal., 2006, 2010). This improvement is caused by the increased oilconcentration in the membrane, which leads to higher catalytic activity(Guerreiro et al., 2006). The concentration of active sites and the thick-ness of the catalytic polymermembranewere reported to fall within therange of 1.263.80 mmol/g and 0.040.13 mm, respectively (Guerreiroet al., 2006; Zhu et al., 2010). The reported basic site concentration of

the heterogeneous catalysts (CNTs and activated carbon) that havethe potential to be incorporated into the membranes were found to bebetween 1.00 (Villa et al., 2009) and 1.58 (Baroutian et al., 2011)mmol/g.

A schematic diagram of the transesterication reaction via catalyti-cally active membrane is shown in Fig. 10. A pre-determined amountof oil and methanol were mixed, heated and pumped into a membranereactor. Glycerol was continuously removed from the reaction mixtureonce it was produced. The permeate stream contained a binary mixtureof glycerol/methanol, which was recovered in cold trap immersed inliquid nitrogen. Meanwhile, the retentate that contained unreacted oilwas returned to the mixing vessel to be circulated back into themembrane reactor for further reaction (Figueiredo et al., 2008;Guerreiro et al., 2006).

Glycerol and methanol are able to permeate through the PVAmembrane because of the hydrogen bonds formedbetween the glyceroland methanol molecules and the OH groups in the polymer. It has beenreported that no oil or FAME were detected in the permeate stream,indicating that product loss can be avoided (Guerreiro et al., 2006). Ascompared to the catalytically inert membrane, the advantage of thiscatalytically active membrane is the elimination of the puricationprocess for the post-reaction permeate stream.

6. Effect ofprocess parameters inbiodiesel production bymembranereactor

In addition to the typical process parameters (reaction tempera-ture, methanol to oil ratio and catalyst concentration), other processvariables, such as the reactant ow rate, trans-membrane pressure,membrane thickness and pore size (for membrane separation basedon oil droplet selection), also have a great inuence on the biodieselyield and need to be taken into consideration when biodiesel isproduced by membrane technology. In order to produce biodiesel ina more sustainable and cost effective manner, the important processparameters that should be taken into consideration will be discussedin the following section.

6.1. Effect of reaction temperature

In order to reduce the total reaction time, a higher reactiontemperature (without evaporation of methanol) is required for trans-esterication (Cheng et al., 2010). Therefore, for transesterication totake place in a stirring batch reactor, the reaction temperatures forhomogeneous acid and base catalysed transesterication should be80 C and 25120 C, respectively (Marchetti et al., 2007). However,for a membrane reactor in which the separation is based on the oildroplet size, a lower reaction temperature between 5070 C is usedto synthesise biodiesel (Baroutian et al., 2011; Cao et al., 2008a;Dub et al., 2007). The reason that the reaction temperature is keptas low as possible in the membrane reactor is to encourage the exis-tence of a two-phase systembetween themethanol and the lipid. At el-evated temperatures, the system tends to be homogeneous. Asmentioned earlier, transesterication is believed to occur at the surfacesof the oil droplets that are suspended in themethanol stream; therefore,the resulting heterogeneous phases are needed for the operation of themembrane reactor. Moreover, the purity of FAME is reduced as thereaction temperature is increased because the solubility of oil andother intermediates in FAME increase with temperature (Cheng et al.,2010).

Transesterication with a minimum temperature of 60 C hasbeen reported with the use of a dense membrane (Guerreiro et al.,2006, 2010). A possible reason for this result is the thermal mobilityof the molecules inside the membrane, which increases at higherreaction temperatures and thus generates extra free volume space(Ong et al., 2011), enhancing the permeation of larger moleculessuch as glycerol. Moreover, glycerol demonstrates a signicant

decrease in viscosity at higher reaction temperatures. If the viscosity

stre

cha

nk

ap iuid

icall

1372 S.H. Shuit et al. / Biotechnology Advances 30 (2012) 13641380is reduced, the circulation of the mixture becomes easier (Gomes etal., 2010).

Irrespective of the type of membrane used to synthesise biodiesel,the conversion of oil to FAME was found to be positively affected byincreasing the reaction temperature (Dub et al., 2007). This increasecan be easily justied because transesterication is an endothermic pro-cess (Samart et al., 2009). Therefore, from Le Chatelier's principle, byincreasing the temperature, the equilibrium of the reaction is shiftedto the forward direction, which favours the conversion of oil into FAME.

6.2. Effect of methanol to oil ratio

It has been proven that at higher methanol to oil ratios, the timerequired for the reaction mixture to achieve a homogeneous liquidsystem increases. This relationship indicates that at higher methanol

Feed pump

Retentate

Heat ex

Oil feed tank Methanol feed ta

Mixing vessel

Cold trin liq

Fig. 10. Schematic diagram of transesterication reaction via catalytto oil ratios, the two-phase system can be maintained so that theoil-rich phase could not pass through the membrane (Cheng et al.,2010). Unlike the reaction temperature, a higher methanol to oilratio favours the production of biodiesel in a membrane reactor. Amethanol to oil ratio of 24:1(in molar ratio) or 1:1 (in volume ratio)is usually used for the production of biodiesel in a membrane reactorin which the separation is based on oil droplet size (Baroutian et al.,2011; Cao et al., 2008a; Cheng et al., 2010). The methanol to oilratio in a catalytic based membrane was reported to be 6:1 (Shi etal., 2010), 26:1 (Zhu et al., 2010), 106:1 (Guerreiro et al., 2006) and254:1 (Guerreiro et al., 2010). However, no clear explanation wasgiven of the need for such a large amount of methanol to producebiodiesel in the former method. Regardless of the operational principleof themembrane, the conversion increased in proportion to the ratio ofmethanol to oil because the reaction was driven towards biodieselproduction (Shi et al., 2010).

6.3. Effect of catalyst concentration

The emulsion of oil molecules in the alcohol stream may cause amass-transfer problem in a membrane reactor, especially at a highmethanol to oil ratio. Therefore, a higher catalyst concentration isrequired to achieve the complete conversion to oil. It has beenreported that for 20 min of reaction time, oil conversions of 61.1%and 100% were achieved for catalyst concentrations of 0.05 and0.5 wt.%, respectively. Despite the low catalyst concentration of 0.05%,the reaction was still capable of achieving complete oil conversion butrequired a longer residence time (1 h) (Cheng et al., 2010; Tremblayet al., 2008). For the acid catalyst (H2SO4), oil conversion was signi-cantly increased when H2SO4 concentration was increased from 0.5 to2%. However, oil conversion was not signicantly different (less than10%) when H2SO4 concentration was further increased to 4 and6 wt.%. This result implies that the high concentration of the acid cata-lyst was not necessary in the membrane reactor (Dub et al., 2007).For similar concentrations and reaction times, base catalysts providedmuch higher oil conversion than acid catalysts because of the faster re-action of the base catalyst (Dub et al., 2007). The catalytic membranealso exhibited the same results; themembrane showed higher catalyticactivity with an increased concentration of SO3H groups embedded inthe polymer matrix (Castanheiro et al., 2006). The amount of SO3Hgroups in the polymeric membrane depends on the degree of mem-

Three way valve

Permeate stream

am

Cold trap immersedin liquid nitrogen

Vacuum pump

nger

Membrane module

mmersednitrogen

y active membrane (Figueiredo et al., 2008; Guerreiro et al., 2006).brane crosslinking with succinic acid (Guerreiro et al., 2006) and thepercentage of free OH groups in the membrane to be esteried with5-sulphosalicylic acid (Castanheiro et al., 2006).

6.4. Effect of reactant ow rate

There has been no detailed study on the effect of the reactant owrate on biodiesel production in a membrane reactor. However, asignicant increase in the conversion of oil to FAME could be observedas the ow rate of reactants increases (Baroutian et al., 2011; Dub etal., 2007). This could be caused by an improvement of the mixingintensity at higher ow rate or greater ow circulating velocity(Baroutian et al., 2011) because the reactants (oil/methanol) andcatalyst will be owing in a turbulent ow regime (Vospernik et al.,2004). Mixing is a crucial factor for the increase of the reaction rate intransesterication because oil is immiscible with methanol. Withoutmixing, the reaction only occurs at the interface between the layers ofmethanol and oil (Kumar et al., 2010; Meher et al., 2006).

Concentration polarisation is a common problem for membraneseparation. Concentration polarisation is caused by the accumulationof retained solute at the membrane interface, forming a secondarylayer that restricts the transport of the permeating species (Porter,1972). Concentration polarisation reduces the permeation rate ofthe more permeable component but favours the permeation of theless permeable components (Bakhshi et al., 2006). This problem canbe easily solved by increasing the reactant ow rate. With the

1373S.H. Shuit et al. / Biotechnology Advances 30 (2012) 13641380increase of this rate, the resulting turbulent ow will reduce thethickness of the boundary layer caused by the retained solute. Therefore,the mass transfer resistance at the boundary layer on the upstream ofthemembrane can be reduced, leading to an increase in the total perme-ation ux (Peng et al., 2007).

6.5. Effect of trans-membrane pressure (TMP)

TMP is dened as the pressure difference between the feed sideand the permeate side of the membrane (Nishimoto et al., 2010).TMP is the driving force for membrane separation (TakhtRavanchiet al., 2008) and is normally kept constant throughout the process(Nishimoto et al., 2010). The TMP used in the production of biodieselranges between 37.9 and 173.4 kPa (Cao et al., 2008a, 2008b; Dub etal., 2007). The permeate ux increases with higher TMP because agreater driving force is applied for separation (Gomes et al., 2010).In dense polymeric pervaporation membranes, TMP affects thesorption and desorption rates across the membrane. At higher TMP,the driving force drawing the molecules across the membrane andsweeping them away from the permeate side is increased, resultingin higher mass transfer rates (Ong et al., 2011).

In biodiesel production via membrane reactor, TMP is possitivelyaffected by the concentration of unreacted oil in the emulsion withinthe reactor. If the residence time of the reaction is insufcient, theconcentration of theunreacted oil in the reactorwill increase. Therefore,it was hypothesized that TMP is dominated by the residence time of thetransesterication reaction. A residence time of 60 min for rened,bleached and degummed (RBD) oil and 80 min for waste cooking oil isrequired to produce a stable and constant TMP. Under these residencetimes, the resulted TMP generated for RBD oil and waste cooking oilwas 80 and 90 kPa respectively (Falahati and Tremblay, 2012).

Besides, TMP is correlated with the viscosity of the reactionmixture. This relation is clear when the glycerine-rich phase isrecycled back to the reactor for methanol recovery. In other words,an increased content of glycerol in the reaction mixture wouldrequire higher TMP. However, the increase of TMP from increasedglycerol content does not cause any negative effect in the membranereactor. Assuming complete oil conversion in the batch reaction, theglycerol contentwould be 8.5% in the FAME/glycerol/methanolmixture,and the corresponding TMP would only be 90 kPa. This TMP value is farbelow the recommended membrane operating pressure of 1000 kPa(Cao et al., 2008b).

The TMP proles in the membrane reactor can also act as anindicator to check the progress of the transesterication reaction. Asharp increase in the TMP indicates that transesterication is notoccurring, and the oil has become a continuous phase within themembrane reactor. On the other hand, a constant and stable TMPprole for all operating times reveals that a sufcient amount of oilhas been transesteried into FAME, allowing the continuous operationof the membrane reactor (Tremblay et al., 2008).

6.6. Effect of membrane pore size and thickness

For a membrane reactor with separation based on the molecularsize of the components, the selection of a membrane with a suitablepore size becomes extremely important in order to forbid oil dropletsfrom passing through the membrane. It has been proven that no oil isfound in the permeate stream when a membrane with a pore sizebetween 0.051.4 m is used to produce biodiesel. This separationoccurs because the reported average size of the oil droplets falls inthe range of 12400 m, which is much larger than the membranepore size (Cao et al., 2006).

The thickness of the membrane is an additional vital factor thatneeds to be taken into consideration for the use of catalytic membranesin biodiesel production. Zhu et al. (2010) reported that the membrane

thickness used in transesterication falls in the range of 0.040.26 mm. They also reported that the transesterication rate increasedat the initial stage of the reaction, as the membrane thicknessdecreased. However, the same oil conversion could be achieved bymembranes of any thickness by the end of the reaction because thecatalytic membrane had become swollen by reactants, and the activesites of the catalyst contained by the membrane that could be exposedto reactants for catalytic conversion (Zhu et al., 2010).

7. Advantages of catalytic membrane reactor in biodiesel production

The catalytic membrane reactor is a new technology for biodieselproduction. This technology can offer an alternative to overcome thecommon limitations arising from conventional biodiesel productionprocesses. The advantages of the catalytic membrane reactor forbiodiesel production will be discussed in the following section.

7.1. Environmentally friendly process

The production of biodiesel via catalytic membrane reactor isundeniably an environmentally friendly process because of its lowenergy consumption. Transesterication in a catalytic membranereactor is carried out under mild operating conditions. The highestreported reaction temperature in a membrane reactor was 70 C(Dub et al., 2007), which is quite similar to the conventional homo-geneous transesterication (65 C) (Berchmans and Hirata, 2008) butmuch lower than either heterogeneous or supercritical transesteri-cation. The reaction temperature for transesterication using a solidbasic catalyst, such as magnesium oxide (MgO), calcined hydrotalcite(CHT), zinc oxide (ZnO), KNO3/KL zeolite and KNO3/ZrO2, falls in therange of 180200 C (Di Serio et al., 2006; Jitputti et al., 2006). It hasbeen reported that at approximately, 100 C, and alkaline catalystsexhibited very low catalytic activity that only produced a FAMEyield of 20% (Di Serio et al., 2006). Additionally, transestericationby solid acid catalysts like tungstated zirconia (WO3/ZrO2), sulphatedtin oxide (SO4/SnO2), sulphated zirconiaalumina (SZA) and sulphatedzirconia (SO42/ZrO2) were carried out in the range of 200300 C(Chen et al., 2007; Furuta et al., 2004; Jitputti et al., 2006). In addition,there is more hidden energy required in the synthesis of heterogeneouscatalysts because most heterogeneous catalysts must be calcined athigh temperatures, ranging from 200500 C (Albuquerque et al.,2008; Lu et al., 2009). Unlike solid catalysts, catalytically active mem-branes (for example, functionalised PVA membranes with sulphonicgroups) are fabricated in a low temperature environment (Guerreiroet al., 2006). Of all of the reported biodiesel productionmethods, super-critical transesterication requires the most extreme reaction tempera-ture (240340 C) and reaction pressure (5.78.6 MPa) (Hawash et al.,2009). As compared to biodiesel produced in a catalytic membranereactor (70 C and 173.4 kPa), the reaction temperature and pressurerequired for the supercritical process are 5 and 50 times higher,respectively.

From the perspective of chemical requirements, the catalyticmembrane reactor could reduce the usage of solvents and chemicalsthat are harmful to the environment. For the conventional productionmethod, the reported concentration for the alkaline catalyst is in therange of 0.51% (NaOH) (Marchetti et al., 2007). The concentrationof the acid catalyst varied from 14%, depending on the FFA contentin the oil (Narasimharao et al., 2007; Wang et al., 2006b). Comparedto the catalyst concentration in the conventional methods, the useof catalysts in the catalytic membrane reactor is lower: 0.05% for thebasic catalyst (Tremblay et al., 2008) and 2% for the acid catalyst(Dub et al., 2007). The catalytically inert membrane reactor andsome catalytically active membranes also consume much less metha-nol than supercritical technology in which the methanol to oil ratio isnormally higher than 40 (Barnwal and Sharma, 2005; Sharma andSingh, 2009). Table 1 summarises the reaction conditions and perfor-

mances for various types of transesterication processes.

Table 1Comparison of reaction conditions and performances of various biodiesel production methods.

Transestericationprocesses

Reaction conditions

Catalyst used Temperature,C

Methanolto oilmolar ratio

Catalystconcentration,wt.%

Freefatty acidlimitation, %

Puricationof FAME

Yield, % Reference

Homogeneousbase

NaOH 60 6:1 1 b1 Repeatedwashing(Atadashi et al.,2011)

97.1 (Rashid et al.,2008)

Homogeneous acidWaste cooking oil H2SO4 95 20:1 4 No

limitationRepeatedwashing(Atadashi et al.,2011)

Conversion>90% (Wang et al.,2006b)

Soybean oil H2SO4 65 30:1 1 Nolimitation

Repeatedwashing(Atadashi et al.,2011)

Conversion>99% (Narasimharaoet al., 2007)

Heterogeneousbase

MgO (III) 200 11:1 5 Unknown Simple washing(Atadashi et al.,2011)

>95 (Di Serio et al.,2006)

CHT 200 11:1 5 Unknown Simple washing(Atadashi et al.,2011)

>95 (Di Serio et al.,2006)

KNO3/KL zeolite 200 6:1 3 Unknown Simple washing(Atadashi et al.,2011)

77.2 (Jitputti et al.,2006)

KNO3/ZrO2 200 6:1 3 Unknown Simple washing(Atadashi et al.,2011)


ZnO 200 6:1 3 Unknown Simple washing(Atadashi et al.,2011)


Heterogeneousacid

WO3/ZrO2 250 40:1b 6.7b Nolimitation

Simple washing(Atadashi et al.,2011)

Conversion>90% (Furuta et al.,2004)

SO4/SnO2 200 6:1 3 Nolimitation



SZA 300 40:1b 6.7b Nolimitation


80 (Furuta et al.,2004)

SO42/ZrO2 230 12:1 2 Nolimitation


>90 (Chen et al.,2007)

Supercriticalmethanol

320 43:1 Nolimitation

No washing 100 (Hawash et al.,2009)

Catalytically inertmembrane

Base NaOH 65 24:1 0.5 b1 Simple washing(Saleh et al.,2010)

97.7 (Cao et al.,2008a)

A packed bed membranereactor with activatedcarbon supported catalyst

70 24:1 143.75 mg/cm3 (massof catalyst per unitvolume reactor)

unknown No washing Conversion=93.5% (Baroutian etal., 2011)

Acid H2SO4 70 Flow rate:6.1 ml/min

2 Nolimitation

Simple washing(Saleh et al.,2010)

Conversion=90% (Dub et al.,2007)

Catalyticallyactivemembrane

Zr(SO4)2 65 6:1 Zr(SO4)2:SPVAa =1:1(mass ratio)

Nolimitation

Unknown Conversion>90% (Shi et al.,2010)

PSSA 64 26:1 PSSA:PVA=1:2 (massratio)

Nolimitation

Unknown Conversion>90% (Zhu et al.,2010)

Succinic acid as crosslinkingagent

60 106:1 PVA membrane is 20%crosslinked withsuccinic acid

Nolimitation

No washingc 94.3b (Guerreiro etal., 2006)

a SPVA=sulphonated poly(vinyl alcohol).b Self-estimation.c No washing is performed because glycerol is removed during transesterication.

1374 S.H. Shuit et al. / Biotechnology Advances 30 (2012) 13641380

Even though the operating parameters are similar to those ofhomogeneous transesterication, the problem of wastewater genera-tion can be greatly reduced if biodiesel is produced through membranetechnology. In homogeneous transesterication, the crude biodieselproduced after separation from the glycerol phase still contain catalysts,unreacted alcohol, soaps and free glycerol (Leung et al., 2010). The com-mon approaches to the purication of the biodiesel include washingwith distilled water or ether or the use of a solid adsorbent. Adsorbentsuch as Magnesol can selectively adsorb those hydrophilic materialssuch as glycerol, MG and DG. Other solid absorbents like activatedclay, activated carbon and activated bre can also be used to purifybiodiesel (Atadashi et al., 2011). However, washing with hot distilledor deionised water is the best way to purify biodiesel because bothglycerol and methanol are highly soluble in water (Karaosmano lu etal., 1996; Leung et al., 2010). In both acid and base-catalysed transester-ication, the washing process consists of two steps: neutralisation andwater washing. Hot distilled/deionised water at 6080 C showedpromising performance for washing the FAME phase because of thehigher diffusivity of glycerol from FAME to the water phase at higherwashing temperature (Atadashi et al., 2011). In order to achieve less

Additionally, glycerol is separated from the membrane as it is formed,eliminating the need for the washing step to remove free glycerolcontent from the FAME phase.

7.2. Lower investment cost

Typical process ow diagrams of biodiesel production in theconventional process and in the catalytic membrane reactor areshown in Figs. 11 and 12, respectively. In the catalytic membranereactor, both the separation and catalysis processes are combined insingle unit operation (Vankelecom, 2002). The integration of theseprocesses into a catalytic reactor is able to reduce the number ofoperating units, as well as the number of processing steps, therebyis the leading to a reduction in the size and complexity of the plantand a consequent reduction of the investment cost (Dittmeyer et al.,2004). One of the main factors contributing to the high productioncost of biodiesel is the need for downstream processes, which includebiodiesel separation and purication (Hasheminejad et al., 2011). Thecatalytically inert membrane has the advantage of not requiring

b

Phas

G C

1375S.H. Shuit et al. / Biotechnology Advances 30 (2012) 13641380than 0.02% glycerol content in the biodiesel, seven consecutivewashingsteps have been reported by Gomes et al., 2010. In the acid-catalysedtransesterication, H2SO4 was neutralised by CaO, followed by a gravityseparation to remove the produced CaSO4. CaO was used because itsprice is lower than those of other alkali substances. After the removalof the acid catalyst, the FAME phase undergoes the same washingprocess as the material from the base-catalysed transesterication toremove glycerol, methanol and other contaminants (Zhang et al.,2003). In the production of biodiesel via catalytically inert membrane,washing would still be needed to remove the catalyst in the permeatestream, but fewer washing steps would be required because a lowercatalyst concentration (0.05%) could be used (Tremblay et al., 2008).In the conventional separation method, 10 L of water are consumed towash 1 L of biodiesel. In contrast, only 0.002 L of water per litrebiodiesel would be needed to purify biodiesel produced via themembrane method (Saleh et al., 2010). Assuming a biodiesel produc-tion of 20 million tonnes per year (Licht, 2007) and a biodiesel densityof 900 kg/m3 (Knothe et al., 2005), approximately 59 billion gallons ofwastewater are produced by the conventional separation method, andthis amount of wastewater could be signicantly reduced to only 12billion gallons by applyingmembrane separation to biodiesel productionand purication. The catalytically active membrane has the potential toeliminate the wastewater problem because the washing step is notrequired. As mentioned in Section 5.2.2 the catalyst is embedded in thepolymer matrix, thus, the neutralisation step is not required.

Catalyst mixing

Methanol

Catalyst

Transesterification

Pharmaceutical glycerin

Oil sources

Neutralising acid/alkaline

NeutralisationFig. 11. Process ow diagram of conventional homogeneous acid/alkaline-cCrude glycerin

Glycerin purificationdecantation to separate the two phases obtained after transesterica-tion (Gomes et al., 2011). The catalytically active membrane reactorhas the potential to simplify FAME and glycerol separation, catalystneutralisation.

Even though the phase separation between FAME andwater can beeasily carried out, the equilibrium solubility of water in FAME afterwashing is higher than the water content stated in the internationalstandard (Gomes et al., 2010) (500 ppm for both ASTM and ENstandard (Knothe et al., 2005). Therefore, vacuum drying is usually re-quired to remove water from the FAME before storage (Gomes et al.,2010). The neutralisation unit in a conventional production plant(Sdrula, 2010) could also be eliminated because the catalyst is embed-ded inside the polymer matrix and would not mix with the reactant.

A combination of centrifugation and water washing is used toenhance the separation of glycerol and impurities from the FAMEphase. For this method, sufcient residence time is required for theless dense oil to oat to the surface of the water, thereby resultingin the preferential separation of the heavy phase. Because glycerol isfully miscible with water and insoluble in the FAME phase, almostall of the glycerol is easily removed by this separation method. How-ever, there are several disadvantages to this method, such as its highinitial investment cost, high power consumption and the requirementfor considerable maintenance (Saleh et al., 2010). This separation stepis unnecessary if biodiesel is produced via catalytically activemembrane because glycerol can be simultaneously removed from

Washing

Vacuum drying FAME

Methanol Distillation

Upper layer

Bottom layer

Crude iodiesel

e separation

ravity settlingentrifuge

Neutralisation

Methanol Distillationatalysed transesterication reaction (Saleh et al., 2010; Sdrula, 2010).

FAME

Transesterification in catalytically active

membrane Methanol

Crude biodiesel


p

cat

1376 S.H. Shuit et al. / Biotechnology Advances 30 (2012) 13641380the reaction mixture, and the need for phase separation betweenFAME and glycerol is eliminated.

Furthermore, both the catalytically inert membrane and the cata-lytically active membrane eliminate the inter-stage temperature andpressure changes (Dittmeyer et al., 2004) required in biodiesel process,such as supercritical technology in which the mixture needs to becooled before separation of glycerol from the biodiesel can be carriedout (Demirbas, 2007). The elimination of this step indicates that boththe energy and the number of heat exchangers required for the processcould be reduced.

7.3. Overcoming the limitation caused by chemical equilibrium

Another attractive benet offered by catalytic membrane reactorsfor biodiesel production is the ability of the process to overcome thelimitation imposed by chemical equilibrium and achieve completeconversion. As noted, the transesterication reaction is a reversiblereaction that can never reach 100% completion (Cao et al., 2008b).The typical conversion for transesterication is 98% or lower(Knothe et al., 2005). The existence of chemical equilibrium in trans-esterication is proven by monitoring the progress of a transesteri-cation reaction by bre-optic near-IR spectroscopy with correlationto 1H nuclear magnetic resonance (NMR). The experiment showed thatin the rst 30 min, approximately 85% of the reaction was completed,while in the following 30 min, the reaction only proceeded by approxi-mately 7%, indicating that the reaction began to approach chemicalequilibrium (De Boni and Lima da Silva, 2011).

According to Le Chatelier's principle, the equilibrium of the trans-

Oil sources

Fig. 12. Proposed process ow diagram of transesteriesterication reaction can be shifted toward higher conversion byhaving one reactant in excess or by selectively removing of one ofthe products generated in the reaction (Castanheiro et al., 2006).Therefore, a higher methanol to oil ratio is needed to increase oilconversion (Shi et al., 2010). Unlike other conventional methods,the catalytic membrane reactor is capable of driving the transesteri-cation reaction further toward completion by simultaneously removingthe products from the reaction mixture. The separation depends on thetype of membrane used in the catalytic membrane reactor. With amicro-porous membrane, FAME and glycerol (Baroutian et al., 2010,2011; Cao et al., 2008a, 2008b; Dub et al., 2007) are separated from

R C

O

O CH3 H2O

Methyl ester Water

Fig. 13. Hydrolysis of methyl ester to form FFA (the reaction mixture, while for a dense polymeric pervaporationmembrane, glycerol and methanol are separated into the permeatestream (Guerreiro et al., 2006). The catalytic membrane reactor canenhance and increase the overall reaction ratewhen an enzyme (lipase)is used as the catalyst. In the conventional lipase-catalysed transesteri-cation, glycerol is easily adsorbed onto the surface of the lipase, reduc-ing the activity and operational stability of the lipase (Su et al., 2007).Therefore, the continuous removal of glycerol bymembrane technologycan decrease the inhibition of lipase, thereby increasing the overallreaction rate (Vankelecom, 2002).

7.4. High process exibility of feedstock conditions

Water and free fatty acid (FFA) found in oil sources can createsignicant problems in transesterication (Atadashi et al., 2011). Asshown in Fig. 13, the presence of water or moisture in the feedstockcan cause hydrolysis of the formed methyl esters back to FFA (VanGerpen and Knothe, 2005), resulting in reduced product. At thesame time, water will also hydrolyse triglyceride to diglyceride andFFA, especially at higher temperatures (as shown in Fig. 14)(Atadashi et al., 2011). It has been reported that 0.1% water in an oilsource is sufcient to reduce the conversion of oil to FAME during thetransesterication reaction (Demirbas, 2007). In short, the presence ofwater will result in the production of more FFA and reduce the FAMEyield.

FFA in the reaction mixture will react with water and an alkalinecatalyst, such as KOH or NaOH, to form a saponied product (soap),as shown in Fig. 15. The saponied product tends to be strengthened

Crude glycerin

Glycerin urification

Pharmaceutical glycerin


ion reaction in catalytically active membrane reactor.at ambient temperatures, forming a gel-like mixture that is difcult torecover. Excessive soap formation increases catalyst consumption,reduce its effectiveness, and causes difculties in glycerol separationand crude biodiesel purication (Atadashi et al., 2011). The recom-mended level of FFA in oil for homogeneous base-catalysed transes-terication is reported to be less than 1% (Lam et al., 2010).

The homogeneous two-step acidbase catalysed transesterica-tion reaction has been proposed as one of the biodiesel productionmethods for oil with high FFA content. First, the high FFA oil issubjected to acid esterication to remove the FFA from the oil. Theacid esterication is carried out at a temperature of 50 C for 1 h to

R C

O

OH 3HC OH

FFA Methanol

R=alkyl) (Van Gerpen and Knothe, 2005).

for 2 h using a NaOH catalyst. The reported FAME yield for this two-

R1 C

O

OHH2OHC

H2C

O C

O

R2

O C

O

R1

HC

H2C

O C

O

R2

OH

ceri

1377S.H. Shuit et al. / Biotechnology Advances 30 (2012) 13641380step transesterication is approximately 90%, which is much higherthan the one-step alkaline base transesterication (Berchmans andHirata, 2008). Although this process can achieve high FAME yield, itinvolves a greater number of processing steps and reagents. Afteracid esterication, the reaction mixture must be allowed to settle for2 h; next, the methanolwater fraction in the top layer is removed be-fore alkaline base transesterication. More NaOH is needed in two-step process because it not only serves as a catalyst but also neutralisesH2SO4 in the acid esterication. Although the heterogeneous acid cat-alyst and supercritical technology could also be used to produce bio-diesel from oil with high FFA content, those processes are energyintensive andwould increase the operating cost of biodiesel production,as mentioned in Section 7.1.

The catalytic membrane reactor, especially with a catalyticallyactive membrane, appears to be a suitable alternative to producebiodiesel from oil with high FFA content because it can be easilymodied into an acidicmembrane by introducing SO3H as a functionalacid group into the polymermatrix. Furthermore, thewater ormoisturecontent found in oil sources can be separated by polymer membranes,such as PVA and PAN, during the pervaporation process (Chapman etal., 2008), thereby preventing thewater fromhydrolysing the producedFAME back to FFA. Therefore, cheaper feedstocks such as non-edibleoils, waste cooking oils and even unrened crude oils with high FFAcontent can be used in biodiesel production (Hasheminejad et al.,2011).

7.5. Complying with international standards

Such impurities as glycerol, MG and DG are unfavourable forconvert FFA to esters using an acid catalyst (H2SO4, 1% w/w), therebyreducing the FFA concentration to below 2%. The second step is alka-line based catalysed transesterication, which is carried out at 65 C

H2C O C

O

R3

Triglyceride Water

Fig. 14. Hydrolysis of triglyceride to form diglyengine performance and have negative effects on the environment.High free glycerol content in biodiesel can cause gum formationaround injector tips and valve heads, causing problems in the fuelsystem. In addition, the burning of glycerine produces the toxic com-pound acrolein (Hasheminejad et al., 2011). Therefore, the producedbiodiesel should be separated from these impurities. Of all thereported biodiesel rening and purication methods, the membraneseparation technology has been proven to be a promising technology

(CH2)7CH CH(CH2)7CH3 KOHC

O

HO

Oleic acid Potassium hydroxid

Fig. 15. Soap formation by using oleic acithat can produce and purify high-quality biodiesel that meets inter-national standards (Sdrula, 2010).

Experimental results indicate thatmembrane separation technologyis able to reduce the free glycerol content in biodiesel to a level below0.02 mass percent, which fulls the international standards (Gomes etal., 2011; Saleh et al., 2010). In membrane separation, only 0.225% ofwater by mass was added to FAME to improve separation efciency(Saleh et al., 2010). Furthermore, compared to conventional biodieselpurication methods, membrane separation can produce biodieselwith higher purity and reduce the loss of ester during the reningprocess. Moreover, the water content, density at 20 C and kinematicviscosity of the biodiesel puried by the membrane technology werealso found to comply with the international standards (He et al., 2006).

8. Membrane life-time and fouling in biodiesel production

Because catalytically inert membranes come in contact withstrong acid or base catalysts during operation, it is vital to select amembrane with high resistance to degradation and corrosion. Carbonmembranes are able to resist the harsh environment in the productionof biodiesel when H2SO4 or NaOH is used as a catalyst. It has beenreported that no tangible evidence of degradation of the carbonmembrane is observed, even after 10 months of operation and contactwith acid or base solution (Dub et al., 2007). The blended PSSA/PVA(a kind of catalytically active membrane) showed a stable conversionof 80% after 5 repeated transesterication runs with 8 h of reactiontime (Zhu et al., 2010). The polyethersulphone used in the biocatalyticmembrane microreactor also showed good stability with no decay ofits catalytic activity for at least 12 days of continuous operation(Machsun et al., 2010).

Fouling is one of the major challenges in membrane processes.Fouling of membranes is attributed to the accumulation and depositionof solutes or particles in the feed onto the membrane surface and intothe membrane pores (Pagliero et al., 2007). In biodiesel production,

H2C O C

O

R3

Diglyceride FFA

de and FFA (R=alkyl) (Atadashi et al., 2011).the agglomeration size of glycerol is inuenced by the alcohol concen-tration in the emulsion. Increased alcohol concentration favours theformation of smaller glycerol agglomerates. Therefore, when a ceramicmembrane (catalytically inert membrane) is used to synthesise orpurify biodiesel, the presence of excess alcohol, soap and catalyst inthe reaction mixture favours membrane fouling and decreases thepermeate ux. This phenomenon probably occurs because the greateramount of alcohol used in the reaction enables the glycerol and other

K+O C

O

(CH2)7CH CH(CH2)7CH + H2O

Soape Water

d as example (Atadashi et al., 2011).

(Guerreiro et al., 2010; Shi et al., 2010; Zhu et al., 2010). Therefore,the ability of the synthesised polymeric membrane to separate glycer-

1378 S.H. Shuit et al. / Biotechnology Advances 30 (2012) 13641380ol from the product stream remains unstudied. Additionally, the engi-neering aspects of themembrane reactor have beenminimally studiedbecause most publications have only offered proofs of concepts. Bio-diesel production using membrane reactors is still running undernon-optimal conditions. Therefore, it is a challenge to choose thebest possible combination between catalyst andmembrane. Optimisa-tion studies and modelling will be needed to advance the membranereactor into commercial operation.

Although high biodiesel yield can be obtained via catalyticallyinert membrane (mainly the micro-porous ceramic and carbon mem-branes), a water-washing step is still needed to purify the producedbiodiesel. The purication problem can be reduced by using catalyti-cally active membranes (constructed from polymeric membranes)in the ow conguration studied by Guerreiro et al., 2006, 2010;Sarkar et al., 2010 and Zhu et al., 2010. However, the polymeric mem-branes face the problem of low mechanical strength. It has beenreported that the tested polymer membranes break before a highconversion of biodiesel could be achieved (Guerreiro et al., 2006).Therefore, more attention is needed to the selection of membranesand operating conditions to avoid membrane failure.

The success of functionalised CNTs as a catalyst in b

Membran in Biodiesel

Documents

Transcript of Membran in Biodiesel