Effect of some operating variables on citrate recovery from model solutions by electrodialysis
The effect of pH and temperature (straight theta) on the overall performance indicators (i.e., solute recovery, rho, and Faraday, eta, efficiencies; specific energy consumption, epsilon, solute, JS, and water, JW, fluxes) of batch electrodialytic recovery of citric acid from model solutions was assessed at different values of feed solute concentration (cSf) and electric current density (j). Regardless of the initial feed concentration used, rho and JS were found to be independent of straight theta; eta and JW exhibited a positive trend with respect to straight theta, while epsilon a negative one. At the maximum temperature tested (33 degrees C), as the pH of the feed solution was varied from 3 to 7, rho increased from 0.90 +/- 0.08 to 0.97 +/- 0.02, eta grew from 0.09 +/- 0.02 to 0.50 +/- 0.01, JS practically doubled, epsilon reduced about 8 times, but JW increased from 3 to 4 times. So, the optimal conditions for this technique are to be determined by balancing the savings in the investment and maintenance costs against the energy costs. Copyright John Wiley & Sons, Inc.
Modeling of sodium acetate recovery from aqueous solutions by electrodialysis.
The main engineering parameters (i.e., ion transport numbers in solution and electro-membranes; effective solute and water transport numbers; effective membrane surface area, membrane surface resistances, and limiting current intensity) affecting the recovery of sodium acetate from model solutions by electrodialysis (ED) were determined in accordance with a sequential experimental procedure. Such parameters allowed a satisfactory simulation of a few validation tests carried out under constant or step-wisely variable current intensity. The performance of this ED process was characterized in terms of a current efficiency (omega) of about 93% in the constant-current region, a water transport number (t(W)) of about 15, and a specific energy consumption (epsilon) increasing from 0.14 to 0.31 kWh/kg for a solute recovery yield of 95% as the current density (j) was increased from 112 to 337 A/m2. The specific resistance of the anion- or cation-exchange membranes were found to be three or two times greater than those measured in aqueous NaCl solutions and are to be used to design and/or optimize ED stacks involved in the downstream processing of acetic acid fermentation broths.
Growth of Procaryotic Cells in Hollow‐Fiber Reactors
Hollow-fiber reactors, in the form of ultrafiltration modules, have been used for immobilizing enzymes and for culturing both plant and mammalian cells. Enzymes may be immobilized in hollow-fiber reactors without chemical modification. Consequently the disadvantages of chemical immobilization, such as loss of activity, are avoided. The kinetic data obtained in such systems reflect the kinetics of the native enzyme rather than that of a modified In the case of mammalian cells, the advantages of this type of immobilization are due primarily to the low-shear environment in which the cell is placed, coupled with a ready supply of nutrients, subject only to diffusion through the cell mass. These types of reactors have been mainly operated with cells grown outside hollow fibers, so that the cell mass occupies the shell side, with nutrient supplied through the lumen of the fibers. This is of some advantage for mammalian cells as the resulting growth resembles normal tissue growth: This type of operation has been shown to increase the per cell productivity of biologicals over that of cell surface culture technique^.^ Plant cells have also been immobilized in hollow-fiber devices and the low-shear environment is of obvious advantage in maintaining cell viability and in achieving high cell concentrations.6.’ An alternative to immobilization of enzymes is the immobilization of whole cells which, in nonviable form, are able to catalyze the desired reaction. This has been examined in hollow-fiber systems for a number of cases, and acceptable reaction rates and stability has been ~ b t a i n e d . ~ . ~ The hollow-fiber reactor offers several advantages over more “conventional” types of cell immobilization, which primarily rely on entrapping cells in various polymer matrices. By selection of fibers with appropriate molecular weight cut-offs, cells and some high-molecular-weight products are separated in the reactor itself. Thus the usual filtration step that follows fermentation is avoided, and in principle, selective membranes could be used to provide a partial purification of product from other extracellular metabolites. Contamination of the feed stream does not contaminate the cell culture; however, the use of nonsoluble raw materials is limited to those that would pass through the ultrafiltration membrane, that is, the substrates must practically be restricted to soluble, prefiltered materials. The growth of viable cells in hollow-fiber reactors offers advantages in that extremely high cell densities may be obtained, and consequently reactor productivities can be greatly enhanced. One of the problems associated with the high cell density is the necessity to ultimately restrict growth so that large pressures do not build up in the reactor and cause collapse of the fibers. As a model system, the present work will examine the production of lactic acid from glucose, as this reaction does not involve gas uptake or evolution. The question of transport of oxygen to a densely packed cell mass in these type of reactors has not been satisfactorily resolved, although gas-carrying