Continuous extractive ultrafiltration

Table II. Summary of Continuous Extractive-Ultrafiltration Process in First Continuous Run...

Ultrafiltration membranes can be very efficient at removing colloidal particles of an emulsion but normally will not stop dissolved oil from permeating. Since most membranes are polymeric, they are more stable in the presence of water, so they are best suited for aqueous systems. Since they produce only one well-clarified phase (the permeate), they should be applied to processes with stable micelles where clear continuous phase is required and where losses of continuous phase with the micellar phase can be tolerated. The use of ultrafiltration membranes in an extractive ultrafiltration process for recovery of carboxylic acids is discussed by Rodriguez et al. [/. Membrane Sci., 274(1-2), pp. 209-218 (2006)]. 

Table I. Pertinent Data for the Continuous Extraction and Ultrafiltration of a Toluene/Water Solution...

In this section, the flow vs. force configuration of a continuous stirred tank separator (CSTS) will be illustrated with a few examples. The examples cover crystallization, solvent extraction, ultrafiltration and gas permeation. 

Cell-free translation system, used for the identification of cloned genes and gene expression, has been investigated extensively as a preparative production system of commercially interesting proteins after the development of continuous-flow cell-free translation system. Many efforts have been devoted to improve the productivity of cell-free system [1], but the relatively low productivity of cell-free translation system still limits its potential as an alternative to the protein production using recombinant cells. One approach to enhance the translational efficiency is to use a condensed cell-free translation extract. However, simple addition of a condensed extract to a continuous-flow cell-free system equipped with an ultrafiltration membrane can cause fouling. Therefore, it needs to be developed a selective condensation of cell-free extract for the improvement of translational efficiency without fouling problem. 

A system based on microdialysis coupled with flow-injection chemiluminescence detection allows for direct sampling of unbound drug without extractive sample preparation [72], A similar approach based on continuous ultrafiltration has also been reported [73]. Modifications designed to overcome challenges of low solubility and high-non-specific binding in the ultrafiltration approach have also been described [74]. 

Continuous lactic acid production from whey permeate is carried out in a process that consists of three separate operations in (1) a bioreactor, (2) an ultrafiltered (UF) model, and (3) an ED cell. With the UF process, recycling of all or part of the biomass is achieved. It is also possible to separate low molecular weight metabolites, such as sodium lactate, resulting from lactose fermentation. This product can then be extracted and concentrated continuously by ED. A disadvantage of continuous lactic acid production is, however, that it tends to clog the ultrafiltration membranes, which restricts permeate flow (Bazinet, 2004). 

A membrane cell recycle reactor with continuous ethanol extraction by dibutyl phthalate increased the productivity fourfold with increased conversion of glucose from 45 to 91%.249 The ethanol was then removed from the dibutyl phthalate with water. It would be better to do this second step with a membrane. In another process, microencapsulated yeast converted glucose to ethanol, which was removed by an oleic acid phase containing a lipase that formed ethyl oleate.250 This could be used as biodiesel fuel. Continuous ultrafiltration has been used to separate the propionic acid produced from glycerol by a Propionibacterium.251 Whey proteins have been hydrolyzed enzymatically and continuously in an ultrafiltration reactor, with improved yields, productivity, and elimination of peptide coproducts.252 Continuous hydrolysis of a starch slurry has been carried out with a-amylase immobilized in a hollow fiber reactor.253 Oils have been hydrolyzed by a lipase immobilized on an aromatic polyamide ultrafiltration membrane with continuous separation of one product through the membrane to shift the equilibrium toward the desired products.254 Such a process could supplant the current energy-intensive industrial one that takes 3-24 h at 150-260X. Lipases have also been used to prepare esters. A lipase-surfactant complex in hexane was used to prepare a wax ester found in whale oil, by the esterification of 1 hexadecanol with palmitic acid in a membrane reactor.255 After 1 h, the yield was 96%. The current industrial process runs at 250°C for up to 20 h. 

One of the common side effects observed during extractive bioconversion is the accumulation of unwanted by-products in the system which may affect the productivity during continuous operation (14). The build up of glycerol and other non-volatile products was shown to decrease the ethanol yields during repeated fermentations in a two-phase system (12). The problem was, however, solved by dialysing the fermentation broth and also adding more yeast cells. It appears that the combination of ultrafiltration with the phase system may circumvent the problem of by-product inhibition in most of the cases. 

This part, on applications, covers the following unit operations 8. Evaporation 9. Drying of Process Materials 10. Stage and Continuous Gas-Liquid Separation Processes (humidification, absorption) 11. Vapor-Liquid Separation Processes (distillation) 12. Liquid—Liquid and Fluid-Solid Separation Processes (adsorption, ion exchange, extraction, leaching, crystallization) 13. Membrane Separation Processes (dialysis, gas separation, reverse osmosis, ultrafiltration) 14. Mechanical-Physical Separation Processes (filtration, settling, centrifugal separation, mechanical size reduction). 

A similar climate exists for development of adhesives from Pinus radiata bark in Australia and New Zealand. A 22 ton/day bark extraction plant was built by New Zealand Forest Products Ltd. at Kinleith, New Zealand. These extracts have proved to be more difficult to use than wattle tannin due to their comparatively high molecular weight, the high viscosity of most extract preparations, their rapid rate of reaction with formaldehyde, and the often higher proportion of carbohydrate impurities. Current information (L. J. Porter, 1987) is that production of tannin by New Zealand Forest Products Ltd. has now ceased. In an attempt to make more uniform extracts with lower proportions of carbohydrates, ultrafiltration (257, 258) fractionation on Amberlite XAD-B gel (239), and fermentation (220) purifications have been investigated. Various reactions such as sulfonation and either acid- or base-catalyzed cleavage have been employed to reduce the viscosity of these extracts. A number of adhesive formulations based on P radiata bark extracts have been developed. However, technical difficulties continue to inhibit the commercial use of Pinus radiata bark extracts in wood adhesives. 

A second approach to efficient synthesis of gluconic acid and sorbitol has been the use of cell-free GFOR from Z. mobilis. Using a crude extract of Z. mobilis in a continuous ultrafiltration membrane reactor, excellent substrate conversion and enzyme stability were maintained for about 10 days (Silva-Martinez et al. 1998). However, it has been concluded that the strict requirements for enzyme stability would be costly for commercial production and downstream processing, which is a strong argument favoring use of permeable cells (Nidetzky et al. 1997). 

Figure 6.4.1. Well-stirred separations (a) continuous well-stirred crystalUzer (MSMPR) (b) continuous well-stirred solvent extraction device (c) continuous well-stirred ultrafiltration cell (d) continuous well-stirred gps separation ceii.

Simultaneous and continuous product removal by physical or chemical means such as extraction or ultrafiltration. 

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