Applications in biorefineries

3.4.3 Issues with current extractors

Some difficulties for LLE equipment to overcome are noted here. For static extraction columns, the process offers the advantages of a large diameter, simple construction and operation, only a single operating interface, and small footprint compared to mixer-settler equipment. However, some parameters still need to be improved, such as the interfacial area, drop size, and drop velocity. Another issue of concern is mass-transfer efficiency, which should be improved. An important development in static column extractor is redistribution in the column. The minimization of packing size and drop size increases the interface of solvent. An agitated extraction column is an improvement over the static column. Several kinds of agitation were reported recently, such as the SCHEIBEL column, the Kuhni column, and the KARR reciprocating column [19]. Centrifugal extractors also improve the LLE process because they reduce diffusion path lengths and increase the driving force for separation. The typical application of this design is the one-stage or multistage centrifugal extractor applied for penicillin extraction [20].

Distribution coefficients and selectivities of a number of mixed solvent systems have been determined in order to assess their suitability for preferentially extracting ethanol from aqueous solution [24]. Consider-ing that the composite of an ethanol water mixture is not consistent in industrial operation, the screenConsider-ing method for measuring equilibrium distribution coefficients to minimize the variation is necessary for the LLE application. The measures include entrainment, incomplete equilibration, impurities, and temperature.

Several solvents were tested to obtain an efficient solvent [25]. The solvent was analyzed systematically by Offemanet al. [26] based on distribution coefficients. This study focused on varying chemical the struc-ture of 57 alcohol solvents, including chain length, hydroxyl position for x-alcohols, and branch strucstruc-ture.

For the unbranched alcohols, the separation factors increases as the hydroxyl group moved toward the middle of the chain, and with increasing molecular weight. For the branched alcohols with same molecular weight, the separation factor increases as the molecular weight increases. Factors such as temperature and solvent-to-feed ratio were optimized by Koullas [27]. Other solvents were also explored to extract ethanol from an aqueous solution. For instance, three ternary liquid-liquid systems containing water and ethanol were investigated, including amyl acetate, benzyl alcohol, and methyl isobutyl ketone, at 298.15 K and amyl acetate was found to be a better solvent than methyl isobutyl ketone and benzyl alcohol [28].

Another example includes three solvents considered as promising extractants: isoamyl acetate, isooctyl alcohol, and n-butyl acetate. In isoamyl acetate or isooctyl alcohol, the ethanol distribution coefficients were higher than 1 and the separation factors in Bancroft’s coordinates of the order of 70 and 2000. At room temperature (30^◦C), the solubility of ethanol in these systems increases with decreasing number of carbon atoms in the chain of solvents, giving higher values of the distribution coefficient and consequently lowering the separation factor. The distribution coefficients are greater than 1 and the separation factors are considerably greater than 2 for all the solvents studied (ethanol-water-I-butanol, ethanol-water-1-pentanol, and ethanol-water-1-hexanol ternary systems) [29].

Ionic liquids (ILs) are also applied to extract ethanol with much of the research focusing on imidazolium-based ILs as solvents. Recent studies have shown phosphonium-imidazolium-based ILs to have many advantages over imidazioum-based ILs for ethanol separation. Relative to imidazioum-based ILs, phosphonium-based ILs are less expensive, more thermodynamically stable, available on a multi-ton scale, and have already been used in industrial processes [12]. Phosphonium-based ILs also differ from most ILs because they have densities less than that of water. Since these ILs are less dense than water, conventional units used for the decantation of aqueous phases may be used. A 2011 study showed phosphonium-based ILs to be significantly more effective in ethanol extraction [12], with a reported maximum ethanol concentration range of 65% to approximately 90% using nine different phosphonium-based ILs [12].

To describe the effect of temperature on ethanol extraction by LLE, the bimodal curves were determined by the cloud-point method at 298.15, 308.15, and 318.15 K. The universal functional activity coefficient (UNIFAC) method proved to be more accurate than the non-random two-liquid (NRTL) and universal quasichemical (UNIQUAC) equations fitted to the experimental data. Under the experimental conditions used, ethanol extraction by 1,2-dichloroethane appears to be independent of the temperature [30].

Different chemicals or processes are added to the LLE process as needed in order to improve the efficiency of ethanol extraction. The process can be facilitated by technologies such as gas stripping, in which ethanol is dissolved into a solvent and then stripped by gas to improve the ethanol recovery ratio [31]. The addition of sodium chloride to the LLE process, using 2-ethylhexanol as the solvent, offers limited advantages because of the high cost [32]. However, ethanol solubility is significantly improved by adding a secondary solvent (capric acid, 1-hexanol and 2-ethyl hexanol) to the primary solvent (m-xylene);

a small amount of 2-ethyl hexanol can enhance the distribution coefficient and maintain the separation selectivity constant [33].

A very important application of ethanol LLE process is to integrate ethanol fermentation in order to reduce product inhibition. The plug flow reactor is used with the LLE process to removal dodecanol.

By this new method, the ethanol productivity is multiplied by 5 and a solution of 407 g/L of glucose is totally fermented with Saccharomyces cerevisiae, which cannot normally work with more than 200 g/L glucose [34]. To maximize LLE for ethanol production, operation conditions, such as temperature, time, feed concentration, and phase ratio, are optimized. The cooling temperature and time have been investigated with the finding that the maximum extraction was obtained at 95^◦C with 5 minutes of contact time [35].

Matsumura et al. [36] tested selectivity ratios of 25 types of solvents, including alcohol and ester. Tri-n-butyl phosphate, 2-ethyl-1-butanol, 3-phenyl-1-propanol, sec-octanol, and polypropylene glycol P1200 were considered to be good extractants, and 100 g/L ethanol was considered to be the right concentration for solvent extraction. Polypropylene glycol P-1200, 2-ethyl-1,3-hexanediol, and methyl crotonate had a relatively low toxicity to microbial growth. Modified chemicals are also used for better separation of ethanol. For example, 1-ethyl-3-methylimidazolium methanesulfonate leads to higher values of solute distribution ratios and selectivities than 1-ethyl-3-methylimidazolium trifluoromethanesulfonate, except for selectivity values at high solute concentrations [37].

The problem of LLE technology related to the fermentation ofS. cerevisiae or other microorganisms is that cells will be attracted to the liquid-liquid interface and then a yeast layer will build at the interface, which creates a mass transfer barrier to reduce the rate of ethanol extraction [38].

On an industrial scale, a continuous pilot plant has been constructed for fermentative production of ethanol, using LLE to remove the product, with recycling of the fermented broth raffinate. The plant was operated for up to 18 days with feed glucose concentrations in the range 10.0%– 45.8% (w/w). The solvent was n-dodecanol, and immobilized yeast was used to overcome the problem of emulsification without adverse effect on the ethanol production rate [39].

3.5.2 Biodiesel

Biodiesel production is another area where LLE can be used. Biodiesel, usually as fatty acid methyl ester (FAME), is generated from the esterification reaction of long-chain fatty acid or triglyceride with methanol. The methanol is not only the reactant, but the methanol can serve as a solvent to extract the oil from the feedstock for biodiesel production. This concept was studied on cottonseeds to generate cottonseed meal and biodiesel products. An extraction rate of 98.3% could be achieved for cottonseed oil, while the free fatty acid (FFA) and water content of cottonseed oil were reduced to 0.20% and 0.037%, respectively, meeting the requirement of alkali-catalyzed transesterification [40]. A similar concept was applied to rapeseed oil, with sodium hydroxide used as catalyst, and 98.2% triglyceride conversion ratio was reached at 9:1 methanol to oil [41].

Besides alcohol serving as the extractant, oil/fatty acid can also serve as the extractant for the alcohol, which is specifically applied in the ethanol recovery from aqueous fermentation broth. Different vegetable oils and their fatty alcohol and fatty ester derivatives were studied for the distribution coefficients and separation factors in the partitioning of ethanol and water from the aqueous mixtures. The results showed that castor oil, ricinoleyl alcohol, and methyl ricinoleate all had higher ethanol distribution coefficients and similar or reduced separation factors [42]. As ethanol can be utilized to generate fatty acid ethyl ester (FAEE), another form of biodiesel, the ethanol extraction process was integrated with biodiesel production from oleic acid by using lipase as the catalyst. This enzymatic esterification of ethanol and oleic acid resulted in higher than 50% conversion with simultaneous reduction of ethanol content in the broth [43].

Liquid-liquid extraction is also used to reduce the glycerol concentration in the biodiesel product in order to meet the ASTM D6751-02 standard [44]. It is possible to separate glycerol from methyl oleate by using different combinations of solvents (hexane, methanol, and water) [45].

Biodiesel was also proposed as the extractant for butanol production because butanol and biodiesel can dissolve each other. This strategy benefits the process in that biodiesel-based glycerol serves as a substitute

for butanol production. The production of biodiesel/butanol blend will be an integrated process without any cost of butanol separation. The overall economic value of butanol production is therefore improved [46].

3.5.3 Carboxylic acids

Liquid-liquid extraction technology has also been used to remove organic acids, such as tartaric acid and lactic acid, from agroindustrial wastewater. The recovery of carboxylic acids from the byproducts of sugar-cane treatments by the LLE technique was investigated for pollution control and food safety. This kind of separation can be achieved by single contact and multiple contacts in a continuous countercurrent MORRIS extractor [47]. The mixture of tributylphosphate and dodecane was optimized for appropriate partition ratio, pH to separate aconitic acid with purity of 98% w/w [48], which is possible to apply on an industrial scale. A high concentration of acrylic acid was obtained from sugar through LLE using di-isopropyl ether as extractant after selection and modeling of several solvents [49]. The LLE system includes the alamine series and alkane, which can separate lactic acid without any toxicity to microbes (Lactobacillus casei subsp. rhamnosus (ATCC 11443)) [50].

However, LLE is not widely used in separating carboxylic acids from the fermentation broth, even though much research has been focused on this area. The issue is the lack of an appropriate extractant that has favorable distribution coefficients for carboxylic acid extraction. Amine extractant, due to high alkalinity, can often react with organic acids in order to increase the extraction yield and the selectivity.

A simple mixer-settler setup can easily perform this type of reactive extraction, and amine solvent can be recycled after back extraction of the organic acid with trimethylamine or the pH-swing-, diluent-swing-, or temperature-swing regeneration methods [51]. Some examples of reactive extraction for carboxylic acids follow. Lactic acid was separated by alamine 336 solution (20%–40%) in toluene at 25^◦C–60^◦C [52].

Acetic acid was purified by LLE through equal volumes of alamine-336 and 2-ethyl-1-hexanol resulting in 85% extraction efficiency below pH 3.5. The esterification of ethyl and butyl alcohols was conducted at 3:1 molar ratio of alcohol to acetic acid. Water inhibited esterification to 5%–20%, although the theoretical value is 65%–75% [53].

Many researchers have been working on the succinic acid extraction from the fermentation broth, in which acetic acid is generated as a byproduct and strongly inhibits the fermentation process. When applying amine to extract succinic acid, applying emulsion to liquid membranes can help the separation ratio reach 98% [54]. Considering the potential environmental effects when applying organic solvent as extractants, ionic liquid was tested to extract organic acids, including L-lactic, L-malic, and succinic acids, with the result that phophonium-based ionic liquids are better extractants than the organic solvents traditionally used.

3.5.4 Other biorefinery processes

Many other biorefinery products have been investigated for extraction by LLE technology. Hydrocarbons can only be generated by a limited number of microbial species, and LLE technology can be integrated with cell cultivation for hydrocarbon production. Brief contact between concentrated algaeBotryococcus braunii and hexane can extract hydrocarbon generated by this species effectively; moreover, the algae’s cell growth and hydrocarbon production are not impaired during the repeated extractions by hexane [55]. Extraction of 1,3-propanediol from aqueous solution was fairly difficult due to the lack of a simple efficient extractant.

A solvent screening study revealed that no solvent could yield satisfactory separation results [56]. The cosolvent of ethyl acetate and ethanol was recently proposed by Boonsongsawat [57] for 1,3-propanediol (1,3-PD) extraction. Glycerol supplement can also increase the distribution coefficient of 1,3-PD. When applied in fermentation broth, the result was an increase in the distribution coefficient from 0.14 to 0.2.

Overall, the conventional LLE for 1,3-propanediol is not industrially feasible, considering that the extraction

process requires the handling of large quantities of solvents and, in particular, the 1,3-PD extraction and separation efficiency is too low [58]. An alternative method is to convert 1,3-propanediol with aldehyde to form highly hydrophobic acetals (2-methyl-1,3-dioxane [2MD]) through a cyclic reaction, then extract them using an organic solvent such as o-xylene, toluene, or ethylbenzene, and finally hydrolyze the acetals to 1,3-PD. Liquid-liquid extraction is also used as an alternative enantioseparation for the crystallization approach. Bisnaphthyl phosphoric acid was used as extractant for phenylglycinol separation at a laboratory and industrial scale. After six extraction stages, the purity and yield were 70% and 36%, respectively [59].

The chiral n-protected alpha-amino acid derivatives transfer from an aqueous solution to the organic phase for enantioselective separation by lipophilic carbamylated quinine as chiral selector and phase carrier. The enantiomeric purity of N-(3,5-dinitrobenzoyl)-leucine exceeded 95% enantiomeric excess with 70% overall yield with a single extraction and back-extraction step [60].