Biocatalyst bed

Operation is continuous with a constant flow-rate of reaction medium fed to the reactor where the biocatalyst is packed forming a submerged bed. The reactor can be fed from the bottom or the top. At laboratory scale it is often preferred to use bottom feeding because it is easier to maintain the level of liquid above the biocatalyst bed it also precludes from bed compaction. At large scale top feeding is frequently used... 

Eq. 5.16 represents the model of steady-state operation of CPBR. It allows the determination of the steady-state X for any given combination of M at/F. This equation reduces in one the number of degrees of freedom of the system but there are still two degrees of freedom that allows flexibility of operation, since from the three operational variables Meat, F and Si, two can be established separately. A common situation is the use of Eq. 5.16 to determine the enzyme load required (Meat of the biocatalyst) to obtain the desired substrate conversion (X) for a certain mass flow of substrate (F Si). This equation can also be used for reactor design, since its dimensions are determined by the biocatalyst bed volume, which directly depends on biocatalyst mass, according to ... 

As seen, Eqs. 5.9 and 5.16 are formally equal, if it is considered that the residence time (t) in a continuous reactor corresponds to the operation time (t) in a batch reactor. Both type of reactors exhibit striking similarities and the substrate profiles that develop through time in the BSTR are analogous to the substrate profiles that develop through the biocatalyst bed in the CPBR. Actually a CPBR can be considered as an infinite number of BSTR connected in series. 

Once kii has been determined, the curve of reactor operation (X vs t) can be obtained from Eqs. 5.73 or 5.74. Values of Xi are obtained from Eqs. 5.16 or 5.24 for a certain enzyme load and feed flow-rate in the bioreactor. Eqs. 5.73 and 5.74 also allow bioreactor design (volume determination). In the case of CPBR, the volume of the catalytic bed can be directly determined from the amount of biocatalyst required, by dividing its mass by the apparent density of the biocatalyst bed, which is easily determined. In the case of CSTR, the volume of reaction can also be determined from the amount of biocatalyst required, by dividing its mass by the biocatalyst concentration, which is usually determined by hydrodynamic considerations. 

Usually, Nr -h 1 reactors will be required to absorb non-productive time (discharge, cleaning and filling of reactor). Solving the equation that represents enzyme inactivation under operation conditions (i.e. Eq. 5.76) and the equation that model conversion profiles within the biocatalyst bed in CPBR (Eq. 5.79), residual enzyme activity in each bioreactor after each time interval can be determined and feed flow-rate to each bioreactor during each interval calculated as ... 

Several scouting experiments were performed to find the best pH conditions. Figure 3 reports the ratio between the PG specific activity measured after the purification procedure (ASf) and the initial PG specific activity (ASi). At pH 3.5, the microspheres are able to remove from the broth the major part of the protein without PG activity, thus providing a four time increase of the enzyme specific activity. The purified PG from Kluyveromyces marxianus was immobilised following the above procedure. Batch reactions in the packed bed reactor were done to evaluate the biocatalyst stability. After an initial loss, due to enzyme release, the residual PG activity reaches a plateau value corresponding to about 40% of the initial activity. Probably, some broth component interfered during the immobilisation reaction weakening the protein-carrier interactions. 

Itoh, N., Nakamura, M., Inoue, K. and Makino, Y. (2007) Continuous production of chiral 1,3-butanediol using immobilized biocatalysts in a packed bed reactor promising biocatalysis method with an asymmetric hydrogen-transfer bioreduction. Applied Microbiology and Biotechnology, 75 (6), 1249-1256. 

A fixed bed or slurry bioreactor incorporates the biocatalyst immobilized on a solid support in an aqueous solution, mineral nutrients and an assimilable source of carbon. 

The liquid hydrocarbon stream to be treated may be a crude oil, heavy crude oil, bitumen, or a refined fraction of the crude oil. The hydrogen gas stream is added to the mixture of the hydrocarbon stream with the organic solvent. The reactor, which is fed upflow, is a packed bed of biocatalyst dispersed on a support and is operated at about 74°C. Alternatively, the reactor can also be a batch reactor under stirring conditions. 

Biological catalysts in the form of enzymes, cells, organelles, or synzymes that are tethered to a fixed bed, polymer, or other insoluble carrier or entrapped by a semi-impermeable membrane . Immobilization often confers added stability, permits reuse of the biocatalyst, and allows the development of flow reactors. The mode of immobilization may produce distinct populations of biocatalyst, each exhibiting different activities within the same sample. The study of immobilized enzymes can also provide insights into the chemical basis of enzyme latency, a well-known phenomenon characterized by the limited availability of active enzyme as a consequence of immobilization and/or encapsulization. 

Immobilized biocatalysts are enzymes, cells or organelles (or combinations of these) which are in a state that permits their rense (The Working Party of Immobilized Biocatalysts, 1983). Examples are insolnble enzymes, e.g. nsed in a fixed bed reactor or soluble enzymes, e.g. used in a semipermeable membrane reactor. This chapter will describe methods of industrial interest for making biocatalysts insoluble. 

When operating continuously at steady state each particle in a bed is subject to constant conditions but the concentration of reagents changes with the position in the column. When substrate is converted to product in a single pass the pattern of conversion down the bed resembles that seen when the same reaction is followed with respect to time in a batch reactor. This stems from the fact that distance travelled through the column is equivalent to processing with an equal concentration of biocatalyst in the batch reactor for the period of the column contact time. 

The fluidized bed reactor has been used for phenol removal instead of fixed bed as most of the products formed are insoluble. The operation in packed bed reactors would lead to clogging phenomena and undesirable pressure drop [47, 88]. When deactivation of biocatalysts occurs and regeneration is needed, the liquid-solid circulating fluidized bed is a worthy alternative, as demonstrated for phenol polymerization [89]. The continuous enzymatic polymerization was carried out in a riser section and a downcomer was used for the regeneration of the coated immobilized particles. 

One of the greatest hurdles for the application of biocatalysis is the need to operate processes under conditions that can differ dramatically from those in which the enzymes evolved. Many techniques are used in order to preserve catalytic activity and minimize the costs associated with the biocatalyst. In cases where the cost of the biocatalyst is a concern, an enzyme might be immobilized and used in a packed column or a fluidized bed reactor so as to enable reuse. Here also the enzyme must be stable for extended periods and may even be used under nonaqueous conditions and elevated temperatures. Recombinant technology has revolutionized the applications of biocat-... 

Packed bed High catalytic density Biocatalyst must be immobilized in... 

Immobilized forms of penicillin amidases and acylases have replaced whole-cell biocatalysts for the production of 6-APA and 7-ACA as they can be reused many times, in some cases for over 1000 cycles. Another major advantage is the purity of the enzyme, lacking the /3-lactamase contaminants often present in whole cells. The productivity of these biocatalysts exceeds 2000 kg prod-uct/kg catalyst. A typical process for the production of 6-APA employs immobilized penicillin G acylase covalently attached to a macroporous resin. The process can be run in either batch or continuous modes. The pH of the reaction must be maintained at a value between 7.5 and 8 and requires continuous adjustment to compensate for the drop caused by the phenylacetic acid generated during the course of the reaction. Recycle reactors have been used, as they allow both pH control and the use of packed bed reactors containing the immobilized catalyst. The enzymatic process is cheaper, although not... 

Previous studies of our work group demonstrated that isomaltose exhibits a distinct higher affinity towards certain dealuminated p-zeolites as opposed to other carbohydrates like fructose or glucose [94, 109]. Sucrose is not adsorbed at all. As a consequence, a process could be developed which directly removes the isomaltose from the reaction solution by adsorption onto zeolite. For this purpose a fluidized bed reactor has been utilized with a special focus on the separation of the two solid phases (Fig. 14). The biocatalyst containing entrapped dextransucrase is produced by the jet-cutter method [110] the alginate beads have a mean particle size of 0.5 mm. To accomplish an adequate high density of biocatalyst, silica flour (30% w/v) is included. The particle diameter of the second solid phase (zeolite) is adjusted to 10 pm. As a consequence, zeolite is loaded with isomaltose inside the reactor and can then freely exit the reactor together with the product solution, whereas the biocatalyst is retained inside the fluidized bed reactor [92, 94],... 

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