Integration of reactive extraction biodiesel production

ReviewIntegration of reactive extraction with supercritical fluids for process intensi fication of biodiesel production:Prospects and recent advancesKeat Teong Lee a ,*,Steven Lim b ,Yean Ling Pang b ,Hwai Chyuan Ong b ,Wen Tong Chong ba School of Chemical Engineering,Engineering Campus,Universiti Sains Malaysia,14300Nibong Tebal,Seberang Perai Selatan,Pulau Pinang,Malaysia bDepartment of Mechanical Engineering,Faculty of Engineering,University of Malaya,50603Kuala Lumpur,Malaysiaa r t i c l e i n f oArticle history:Received 16April 2014Accepted 6May 2014Available online 12August 2014Keywords:Supercritical reactive extraction Biodiesel Bio-re fineryProcess intensi fication Product utilization Biofuelsa b s t r a c tCurrent world energy usage is trying to gradually shift away from fossil fuels due to the concerns for the climate change and environmental pollutions.Liquid energy from renewable biomass is widely regarded as one of the greener alternatives to partially ful fil the ever-growing energy demand.Contemporary research and technology has been focussing on transforming these bio-resources into ef ficient liquid and gaseous fuels which are compatible with existing petrochemical energy infra-structure.Due to the wide range of properties and compositions from different types of biomass,there are ample of processing routes available to cater for different demands and requirements.In addition,they can produce multi-component products which can be further upgraded into higher value products.This conceives the idea of bio-re finery where different biomass conversion processes are incorporated and proceed simultaneously at one location.However,the underlying complexity in integrating different processes with varying process conditions will undoubtly incurs prohibitive cost.Consequently,process intensi fication plays an important role in minimizing both the capital and operating costs associated with process integration in bio-re fineries.Recently,process intensi fication for biodiesel production has been developing rigorously due to increasing demand for cost-cutting measures.Supercritical fluid process allows biodiesel production to be performed without any addi-tion of catalyst.Meanwhile,catalytic in situ or reactive extraction process for biodiesel production successfully combines the extraction and reaction phase together in a single processing unit.In this review,the important characteristics and recent progress on both of the intensi fication processes for biodiesel production will be critically analyzed.The prospects and recent advances of supercritical reactive extraction (SRE)process which integrates both of the processes will also be discussed.This review will also scrutinize on the methods for these processes to compliment future bio-re finery setup and more ef ficient utilizing of all of the products generated.©2014Elsevier Ltd.All rights reserved.Contents 1.Introduction .......................................................................................................................552.Latest development of biodiesel production ...........................................................................................562.1.Current status ................................................................................................................562.2.Feedstocks ..................................................................................................................562.3.Production processes .........................................................................................................582.4.By-product utilization .........................................................................................................592.5.Challenges in biodiesel production .............................................................................................603.Supercritical biodiesel production ....................................................................................................603.1.Background ..................................................................................................................603.2.Recent development . (61)*Corresponding author.Tel.:þ6045996467;fax:þ6045941013.E-mail address:ktlee@usm.my (K.T.Lee).Contents lists available at ScienceDirectProgress in Energy and Combustion Sciencejournal h omepage:ww w.el sevier.co m/locate/pecs/10.1016/j.pecs.2014.07.0010360-1285/©2014Elsevier Ltd.All rights reserved.Progress in Energy and Combustion Science 45(2014)54e 783.2.1.Supercritical solvents (61)3.2.2.Process conditions (62)3.2.3.Reactor configurations (63)4.Catalytic reactive extraction for biodiesel production (63)4.1.Background (63)4.2.Recent development (63)4.2.1.Catalytic processes (63)4.2.2.Co-solvents (64)4.2.3.Enhanced reactive extraction (65)5.Supercritical reactive extraction(SRE) (65)5.1.Background (65)5.2.Recent advances (65)5.3.Process characteristics (66)5.3.1.Feedstocks (66)5.3.2.Pre-treatments (66)5.3.3.Supercritical solvents (66)5.3.4.Process variables (67)5.3.5.Agitation effect and heat sources (69)5.3.6.Product separation and characterization (69)5.3.7.Thermal stability (70)5.3.8.Mechanism (71)6.Prospects (72)6.1.Integration in bio-refinery (72)6.2.Product utilization (73)7.Critical issues and recommendations (73)7.1.Impact to biodiesel production (73)7.2.Energy and cost assessment (73)7.3.Future research needs (74)8.Conclusions (75)Acknowledgements (75)References (75)1.IntroductionEnergy supply and security is one of the most pressing issues shrouding our civilization development which remained to be tackled.For the past century,we have become over-reliant on fossil fuels to generate the energy we required for our technological and social development until neglecting the devastating effects they might bring to our ecosystem.However,the quest to replace fossil fuels to more sustainable energy sources remains sluggish espe-cially in developing countries which account for more than two thirds of the world population.The slow transition from fossil fuels to alternative energy sources can be attributed to various factors such as low accessibility,high cost,insufficient infrastructure, inadequate technology and sub-par efficiency[1].Among the renewable energy sources,biofuels from biomass such as biodiesel are currently recognized as one of the best alternatives to partially displace the usage of fossil fuels in the energy sector[2].Biodiesel, which is usually derived from plant oils or animal fats,can be blended with mineral diesel up to20%w/w(B20)and applied to existing combustion ignition engine without any modifications. Apart from that,it is also known to be biodegradable,low toxicity, lower emissions of harmful pollutants(CO,SO x and unburned hy-drocarbons),easy handling and distribution[3].Despite these advantages,biodiesel advocates and developers stillfind it difficult to break into the energy market conventionally dominated by fossil fuels.Traditionally,biodiesel is produced using homogeneous basic catalysts such as sodium hydroxide(NaOH) and potassium hydroxide(KOH)[2].This production route de-mands a high purity oil feedstock which will otherwise reduce the process yield due to side-reactions such as saponification.In addition,homogeneous catalysts are usually difficult to be removed from the product stream and this will incur extra purification cost.In lieu with the shift from edible feedstocks to non-edible or waste feedstocks to avoid the food versus fuel ethical issue,other advanced biodiesel production methods have been explored intensively.In general,they can be categorized into three primary processes;the heterogeneous catalytic process,biological enzy-matic process and supercriticalfluids non-catalytic process.Each process has its own advantages and disadvantages while ample of research studies have been performed to further improve the processes in terms of the esters yield and cost-competitiveness.In this context,process intensification has been lauded as having huge potential to improve biodiesel production process tremen-dously through various cost-effective measures.Process intensifi-cation can be generally defined as any engineering development of novel apparatus or technique which resulted in a substantially smaller,cleaner and more energy-efficient production technology [4].Several process intensification measures proposed for biodiesel production include the novel oscillatory baffled reactor,hetero-genization of the catalysis,supercritical non-catalytic reactions, reactive extraction process and ultrasound/microwave assisted process[5,6].Reactive extraction or also known as in situ extraction combines the extraction and reaction processes together in a single unit operation.There are usually two routes for this to be done. Conventionally,biodiesel production from edible oils starts with the extraction of oil from the lipid-bearing solid material either through mechanical pressing or chemical extraction.The extracted liquid oil will then undergo several purification stages before sub-jected to transesterification process with short-chain alcohol to produce esters which are equivalent to biodiesel.The imple-mentation of reactive extraction allows the elimination of pre-extraction step which can potentially reduce the operating cost and time[7].The second type of reactive extraction deals with simultaneous removal of the glycerol from the ester phase duringK.T.Lee et al./Progress in Energy and Combustion Science45(2014)54e7855the reaction in an extraction column [8].This review focuses pri-marily on the former method and reactive extraction mention hereafter is referred to the former method unless speci fied other-wise.However,the usage of homogeneous base/acid catalysts in reactive extraction still resulted in several challenging issues similar to the conventional two-step homogeneous trans-esteri fication process.In order to avoid falling into the same quandary,supercritical reactive extraction (SRE)process is proposed for biodiesel pro-duction which enables the extraction and reaction processes to occur at a fast rate even without addition of any catalyst.Currently,no review has been done on the potential and challenges of SRE application on biodiesel production.From the past research works done on SRE process for biodiesel production [9e 15],it is believed that SRE process can become another sustainable production method for biodiesel especially on non-edible feedstocks such as Jatropha curcas L.(JCL)and algae.Therefore,in this review,the most recent and signi ficant advancement of technology in biodiesel production will be discussed.More in-depth discussions will be placed on supercritical fluids technology and catalytic reactive extraction process which act as the fundamental study for SRE process.The highlight of this review will be focussing on explaining the concept of SRE process,its current related research,in fluences of process parameters,advantages and challenges pertaining to the biodiesel production and st but not least,recom-mendations on future scienti fic studies are proposed for this novel process to move forward and to complement existing biodiesel production and future bio-re finery scheme with a more sustainable approach.test development of biodiesel production 2.1.Current statusGenerally,biodiesel can be regarded as a liquid fuel comprises of alkyl esters derived from transesteri fication of triglycerides or esteri fication of fatty acids with short-chain alcohols as acyl ac-ceptors.A typical transesteri fication and esteri fication reactions to produce methyl esters are shown in Fig.1(a)and (b)respectively.Biodiesel shares a lot of similar physical and chemical properties with mineral diesel which makes it an ideal replacement in compression ignition engines.However,its higher viscosity and lower energy density renders pure biodiesel not suitable to be applied in the engines directly.Instead,it has to be blended together with mineral diesel according to a fixed proportion.Currently,most vehicle and engine manufacturers worldwide have approved the usage of B5biodiesel blend (5%biodiesel and 95%diesel by volume)in their engines with a large part of them have even raised the maximum limit up to bustion of biodiesel blends in the engines has been proven to contribute to several encouraging effects such as lubricity enhancement,engine wear reduction and better combustion pro files [16,17].In order to encourage the usage of biodiesel to replace conventional diesel,many countries have mandated a fixed percentage of biodiesel volume (ranging from 1%up to 10%volume)in their diesel supply mix [18].Several countries have also introduced financial in-centives such as carbon credit or tax exemption to lower the price of biodiesel blends to become more economically competitive and also encouraging more investors to develop the industry.Research and development for biodiesel production is still being performed vigorously by researchers from all over the world to overcome the underlying challenges and to fully realizing its potentials as a sus-tainable energy source.Generally,the research works for biodiesel production are focused on three primary areas which are the feedstocks,the process and the by-products.2.2.FeedstocksOne of the advantages of biodiesel is that it can be produced from a variety of biomass sources and thus not limited to any geographical region unlike fossil fuels.Established biodiesel pro-duction usually employs feedstock derived from edible sources such as rapeseed,soybean and palm oil to produce biodiesel which are widely regarded as first generation biofuels [19e 21].Major producing countries for first generation edible feedstocks with their respective yields are summarized in Table 1.First generation feedstocks are readily available since commercial plantations have begun a long time ago and their supply chains are firmly estab-lished.However,ethical issues such as food shortage and forest encroachment result in a call to shift to more sustainable alterna-tive feedstocks which are not fit for human consumption [3].The second generation biofuels are then developed primarily from non-edible feedstocks derived from plants such as JCL,Calophyllum inophyllum ,Linseed ,Cerbera odallam and from waste materials such as palm oil mill ef fluent,waste cooking oil and municipal waste.Non-edible plants for second generation biodiesel production can often be planted in semi or non-arable lands and thus avoiding the land shortage issue while generating higher revenues for under-utilized lands.These feedstocks together with those from waste materials are relatively cheap to obtain which can help to reduce the feedstock cost for biodiesel production.Unfortunately,they still suffer from a lot of technical challenges since they are relatively wild and scienti fic knowledge pertaining to these feedstocks is still insuf ficient [22].The taxonomy of these wild plants has not been explored in details unlike their edible counterparts which resultsinFig.1.Schematic diagram for typical (a)transesteri fication and (b)esteri fication process in biodiesel production.Table 1Major edible feedstocks for biodiesel production and their major producing countries.Feedstocks Major producing countries [25]Oil content (%)[26]Oil yield(kg/ha/yr)[26]Rapeseed EU,China,Canada 35e 50600e 1000Soybean China,US,Brazil 15e 21300e 450Palm Indonesia,Malaysia,Thailand20e 502500e 4000Sun flower Ukraine,Russia,EU 30e 51280e 700Cottonseed China,India,Pakistan 18e 25n/aPeanut China,India 36e 56340e 440CoconutPhilippines,Indonesia,India63e 65600e 1500K.T.Lee et al./Progress in Energy and Combustion Science 45(2014)54e 7856inconsistent yield and volatile market price.In addition,these feedstocks often contain higher amount of impurities in the form of moisture and free fatty acids (FFA).Consequently,they are not suitable to undergo conventional homogeneous basic catalytic process and additional puri fication steps will be required.This will incur extra expenses to the biodiesel production process and thus they are generally not favoured among biodiesel developers.However,most of the recent commercial biodiesel productions were already being designed to accommodate second-generation feedstocks to complement existing first generation biodiesel pro-duction [23,24].Once a cost-effective production method has being established,it will undoubtedly encourage more biodiesel de-velopers to follow suit and set the trend for the future.Besides second generation non-edible feedstocks,biodiesel production utilizing macroalgae and microalgae is also under intensive development in recent years.Biodiesel produced from these algae is collectively known as third generation biodiesel.The main difference of using algae for biodiesel production compared to previous generations is their higher photosynthesis capability which enables them to provide higher product yield per cultivation area while at the same time sequesters larger amount of CO 2from the atmosphere [2].Furthermore,they do not compete with land or fresh water resources if cultivated off-shore in contrast to other oleaginous oil crops.However,much like the second generation feedstocks,the cultivation and production technology for algae are not yet mature.This resulted in less than optimal yield and higher energy consumption especially during harvesting and drying.Moreover,the requirement for advanced bio-reactor for ef ficient microalgae production is still very prohibitive and troublesome to maintain while open pond system is susceptible to be polluted by other microorganisms.Even though a sustainable and economically competitive third generation biodiesel production is still in intensive studies,several researches have started to develop theories and preparing relevant technology for the next generation of biodiesel feedstocks.Theo-retically,fourth generation biodiesel feedstocks will take advantage of the advancement in biotechnology,metabolic engineering and genome research in order to improve cellular metabolism and characteristics of oxygenic photosynthesis plants or microor-ganism.Through manipulation of the genome and recombinant DNA techniques,it is possible to increase the photosynthesis ef fi-ciency by several folds and thus greatly enhance the output of lipids for biodiesel conversion [27].This enables the realization of the cell factory concept where the continuous transformation of energy from sunlight to biofuels using biomass can be more direct,clean and cost-effective.In addition,the enhancement of CO 2consump-tion by biomass allows the carbon cycle of the relevant biofuels production to shift from neutral to negative.In other words,the superior carbon sequestration ability will allow more CO 2to be absorbed compared to its total emissions during the complete life cycle of biofuels production.Preliminary laboratory research works have already managed to produce volatile biofuels such as short-chain alcohols or aldehydes from metabolic engineering of cyn-obacteria [28].While there are a lot of potentials and bene fits which can be derived from fourth generation biofuels,several technical risks still persist due to the lack of fundamental study and knowledge base on the relevant engineering techniques and tech-nology.Furthermore,it will consume additional time in order to locate the most optimum production method and cost-effective equipment to ensure fourth generation biofuels to be economi-cally competitive with other energy sources in the market.A summary on the progression of biodiesel feedstocks has been depicted in Fig.2.It is believed that first generation feedstock will remain to be the dominant raw materials for biofuels production in the next decade due to the established supply chains.Second generation feedstock can help to complement the existing biofuels production by increasing its supply security and lower the feed-stock cost due to volatile market.They are expected to play a bigger role especially after improvement in their productivity and culti-vation techniques.The ultimate objective is undoubtedly to move progressively towards utilizing carbon negative feedstock for sus-tainable biofuelsproduction.Fig.2.Progression of biodiesel feedstocks and their important characteristics.K.T.Lee et al./Progress in Energy and Combustion Science 45(2014)54e 78572.3.Production processesDue to the wide variation of feedstocks for biodiesel production, it will be impossible to have only one-size-fits-all production pro-cess.Depending on the physical and chemical properties of the feedstocks,each biodiesel production process will have its own advantages and disadvantages.The most common and commer-cially established production process for biodiesel fromfirst gen-eration edible feedstock is homogeneous basic catalytic process using sodium hydroxide(NaOH)or potassium hydroxide(KOH) [29].The process is relatively simple and easy to maintain since high yield can be achieved close to normal room temperature and pressure.This can be attributed to the low mass transfer resistance since the catalysts exist in the same liquid phase as the reactants. However,they are not suitable for feedstocks which contain high amount of impurities(water and FFA)commonly found in non-edible feedstocks and algae.These impurities are capable of deac-tivating the basic catalysts through side-reactions such as saponi-fication which form soaps and reduce the desirable esters yield [30].Since homogeneous catalysts will usually being retained in the same phase as the products post-reaction,additional purifica-tion and separation steps need to be introduced in order to fulfil minimum fuel standards.On the other hand,homogeneous acid catalysts such as hydrochloric acid(HCl)and hydrosulphuric acid (H2SO4)can withstand higher content of impurities in their feed-stocks.However,their basic reaction rates are slower by approxi-mately10order of magnitudes compared to basic catalyst counterparts in transesterification process of triglycerides.Conse-quently,they are usually employed as the esterification reagents of FFA in a two-step process prior to basic transesterification[31].Heterogeneous solid catalysts are subsequently popularized to counter the inherent weaknesses in homogeneous catalytic system. The advantages of using solid catalysts are elimination of washing step[32],easier separation of catalyst from the product stream, lower product contamination levels,easy catalyst recycling and reduction of corrosion problems since acid sites are chemically bounded with the solid catalyst[33].These can render biodiesel production process to become more economically viable and able to compete with established petroleum-based diesel fuel.The ideal solid catalyst for transesterification and esterification reactions should have characteristics such as an interconnected system of large pores,a moderate to high concentration of strong acid or basic sites and a hydrophobic surface[34].However,most of the high efficiency heterogeneous catalysts involved expensive rare com-pounds such as zirconium dioxide(ZrO2),titanium dioxide(TiO2), zeolites and other alkaline earth metal oxides.Their preparation and synthesizing steps are also tedious,time-consuming and pro-hibitive[6].Recently,researches on carbon-based solid catalysts derived from low-value feedstock can help to minimize the exor-bitant catalyst cost and without the disposal problem since they are biodegradable[35].However,there is still in need of more scientific studies to enhance their catalytic activity and overcome catalyst leaching issue during the reactions.Bio-catalytic biodiesel production employing enzymes has also been studied intensively in the past decades as an alternative to chemical catalytic production.The most common enzymes used for biodiesel production are lipases derived from bacteria,yeast and filamentous fungi such as Burkholderia cepacia[36],Candida antarctica lipase B[37]and Thermomyces lanuginosus lipase[38]. Bio-catalysts are known to exhibit high activity and selectivity under room conditions which are suitable for feedstocks with high FFA and moisture content.They also produce fewer amounts of wastewater and less energy demanding compared to their chemical catalyst counterparts.However,the major stumbling blocks for their large-scale production are the associated expensive cost of enzyme procurement and rapid inactivation by short-chain alco-hols such as methanol[39].Nevertheless,the advancement of biotechnology engineering and immobilization techniques has the potential to improve the properties of enzymes and provide more cost-effective strains for biodiesel production in the future.Apart from conventional catalytic processes,non-catalytic super-critical process and catalytic reactive extraction have also been lauded as viable biodiesel production routes.Non-catalytic supercritical biodiesel synthesis wasfirst performed by Saka and Kusdiana[40] using rapeseed oil in a high-pressure batch reactor.Their results proved that biodiesel production could be carried out without the usage of catalyst as opposed to conventional studies.On the other hand,catalytic reactive extraction process was pioneered by Har-rington and D'Arcy-Evans[41]with sunflower oil seeds with the addition of H2SO4as catalyst.These two processes which are closely related to the SRE process in this review will be discussed in greater details in the subsequent sections.Over the years,other process in-tensifications have also been introduced to biodiesel processing in a bid to minimize the chemical equilibrium limitations and economic penalties.These include the introduction of ultrasound and micro-wave irradiation to the reaction as different agitation effects and heat sources.It is believed that these energy sources can improve the miscibility of the reactants and thus increase the product yields and shorten the processing time by as much as ten folds compared to conventional process[42].Reactive separation processes which integrate reaction and separation of products simultaneously in a single processing unit have also garnered huge interest from biodiesel researchers[43].Continuous removal of products from the liquid-phase reaction can theoretically improve the productivity and selec-tivity while minimizing energy usage for subsequent product sepa-ration[44].This can be achieved since the concentrations of each reactant are maintained in large excess majority of the time due to constant products removal.This can also prevent equilibrium limi-tation as products are prevented from accumulating together with the reactants.Reactive distillation process is one example of the reactive separation techniques in which the reaction and separation occur in a fractional distillation column.However,reactive distillation is only advantageous when the optimum conditions of the reaction are compatible with the distillation conditions.Otherwise,the product yield and quality can be greatly affected[45].The application of membrane technology in biodiesel production also enables the sep-aration of esters and glycerol from the un-reacted triglyceride mole-cules.They cannot pass through the pores of membrane due to their immiscibility with methanol which is used as the continuous phase. The removal of products from the reaction stream allows the reaction equilibrium to maintain at the product side while at the same time alleviate major product contamination at the downstream purifica-tion processes[46].The main challenges of membrane separation in biodiesel production are the relatively high cost of membrane syn-thesis,prevention of membrane fouling,low mechanical strength and high energy required to maintain the pressure across the membrane [47].Available technologies for biodiesel production through trans-esterification and esterification routes are summarized in Fig.3.It should be noted that while reactive extraction is technically a type of reactive separation process,the extraction route described in this review functions quite differently from other typical reactive separation processes in the context of biodiesel production.Reactive distillation or membrane separation works primarily on simulta-neous product removal from the reactant mixture during reaction to maintain the reaction equilibrium on the product side.On the other hand,reactive extraction in this context aims to transfer the extrac-tant from the solid biomass into the reactant mixture concurrently during reaction.This eliminates the requirement for separate extraction phase pre-reaction.Consequently,reactive extraction process does not compete directly with other reactive separationK.T.Lee et al./Progress in Energy and Combustion Science45(2014)54e78 58。