Recovery step

Precipitation. The precipitation of aluminum tribydroxide ia the recovery step of the Bayer process is achieved either by loweting the temperature or by diluting the pregnant Hquor and reduciag its pH. Both methods reverse the direction of equation 35, but seeding with previously precipitated crystals is required ia order to initiate nucleation. 

Recovery. The principal purpose of recovery is to remove nonproteinaceous material from the enzyme preparation. Enzyme yields vary, sometimes exceeding 75%. Most industrial enzymes are secreted by a microorganism, and the first recovery step is often the removal of whole cells and other particulate matter (19) by centrifugation (20) or filtration (21). In the case of ceU-bound enzymes, the harvested cells can be used as is or dismpted by physical (eg, bead mills, high pressure homogenizer) and/or chemical (eg, solvent, detergent, lysozyme [9001 -63-2] or other lytic enzyme) techniques (22). Enzymes can be extracted from dismpted microbial cells, and ground animal (trypsin) or plant (papain) material by dilute salt solutions or aqueous two-phase systems (23). 

Techniques used in bioseparations depend on the nature of the product (i.e., the unique properties and characteristics which provide a handle for the separation), and on its state (i.e., whether soluble or insoluble, intra- or extracellular, etc.). All early isolation and recovery steps remove whole cells, cellular debris, suspended solids, and colloidal particles, concentrate the product, and, in many cases, achieve some degree of purification, all the while maintaining high yield. For intracellular compounds, the initial harvesting of the cells is important... 

Such isolated enzyme approaches for deracemization have a clear disadvantage in that they require two operational manipulations with an intermediate recovery step. A one-pot strategy is offered by employing whole-cell biotransformations with strains containing set(s) of complementary dehydrogenases operating in both biooxidative and bioreductive modes. Trace amounts of the intermediate ketone species can be isolated in several cases. In order to lead to an efficient deracemization... 

In fairness, processes do exist where the cost of the recovery step has little influence on the reactor design, but these are the exceptions. 

Fig. 1. General overview of a PHA production process. All PHA production processes consist of a fermentation and a recovery step, followed by polymer processing for specific applications. In many cases the fermentation is divided into two stages, a biomass production and a PHA accumulation stage (for further detail see text). Several methods for recovery of the material have been described, of which solvent-based and non-solvent-based recovery protocols are illustrated...

Fig. 13.1 Chemistry as it is carried out in the laboratory or manufacturing plant traditionally involves a recovery step after each conversion step. Dotted circles are compounds in the reaction medium, closed circles are isolated, pure compounds [3, 4].

As mentioned above, combined catalytic conversions through combinations of chemocatalytic conversions are not well represented. Although the organic chemist has an extensive synthetic and catalytic toolkit, large differences in reaction conditions often hinder the combined use of tools in one-pot conversions or in a cascade mode without recovery steps. 

The concepts and various examples given show that we may foresee a renaissance in synthesis methods by the integration of bio- and organic syntheses. Fine chemicals of the future will be produced by cascade multi-step catalytic procedures without intermediate recovery steps. 

To obtain the initial equilibrium concentrations of the various ions, the solution is taken to contain Fe2(S04)g, FeSO, H2SO4 and a small amount of CuSO. Leach liquor is recycled after the recovery step so traces of CuSO are always present. Analytical concentrations of these substances and the equilibrium constants for each equilibrium reaction must be known. Mass balances for Fe(III), Fe(II), Cu(II) and SO 2- and a charge balance supplement the mass action equations. This nonlinear set of equations can be solved by the well-known Newton-Raphson method (6). 

Although biotechnological processes often have been stated to be energy efficient in that the reaction temperature is low, it is important to realize that the product concentrations are low and that the product recovery step is often the most energy consuming. 

Holmium is obtained from monazite, bastnasite and other rare-earth minerals as a by-product during recovery of dysprosium, thulium and other rare-earth metals. The recovery steps in production of all lanthanide elements are very similar. These involve breaking up ores by treatment with hot concentrated sulfuric acid or by caustic fusion separation of rare-earths by ion-exchange processes conversion to halide salts and reduction of the hahde(s) to metal (See Dysprosium, Gadolinium and Erbium). 

Holmium chloride is obtained from rare-earth minerals. Recovery steps are discussed above (see Holmium). The rare-earth mineral is cracked by acid attack by heating with hydrochloric acid. The water-soluble chloride salt is filtered and separated from insoluble residues. The hydrated chloride salt is heated at 350°C in a current of hydrogen chloride to yield anhydrous H0CI3. Heating in air in the absence of hydrogen chloride yields holmium oxychloride, HoOCl. Hohnium chloride may be purified by distdlation or vacuum sublimation. 

The fermentation step to produce penicillin GA is the major cost element in the overall process to produce 6-APA. This is substantially due to the high cost of sterile engineering (Table 4.6 and 4.7). Clarification, extraction and solvent recovery steps are also significant, a reflection of the dilute and impure composition of fermentation broths. The concentration of 6-APA in the final broth has a big effect on total process costs. Thus increasing final 6-APA concentrations from 1.2-6.0% have been calculated to reduce production costs by over 50% (Table 4.8). By contrast the 6-APA production step cost is quite small, and is less that half the cost of the solvent recovery process (Table 4.6). The costs of the immobilized enzyme is not insignificant in a recent calculation it was estimated at 2.5 /kg 6-APA (Rasor and Tischer, 1998). 

The porous structure is of an open-cell type so that water can readily penetrate the pores. During adsorption, the hydrophobic portion of the molecule is preferentially adsorbed on the hydrophobic polystyrene surface of the adsorbent through van der Waals attraction. The compounds being adsorbed do not penetrate substantially into the microsphere but remain adsorbed at the surface thus allowing the adsorbate to be rapidly eluted during the recovery step. 

---