Cells space-time yield

Several definitions are often used to define the performance of an electrode or electrochemical reactor. These are the current efficiency, process efficiency and electrode/cell space-time yield. In some cases, particularly with high surface electrodes an... 

Biocatalysts in nature tend to be optimized to perform best in aqueous environments, at neutral pH, temperatures below 40 °C, and at low osmotic pressure. These conditions are sometimes in conflict with the need of the chemist or process engineer to optimize a reaction with respect to space-time yield or high product concentration in order to facilitate downstream processing. Furthermore, enzymes and whole cells are often inhibited by products or substrates. This might be overcome by the use of continuously operated stirred tank reactors, fed-batch reactors, or reactors with in situ product removal [14, 15]. The addition of organic solvents to increase the solubility of substrates and/or products is a common practice [16]. 

The design optimization of an electrolytic cell aims at a high throughput with a low energy consumption at the lowest feasible cost. The throughput of an electrochemical reactor is measured in terms of the space time yield, Yt, defined as the volumetric quantity of the metal produced per unit time per unit volume of the process reactor. This quantity is expressed as ... 

The space time yield, therefore, depends on the current density and the specific cathode area. In an electrolytic cell, the specific cathode area is generally very small thus the space time yield is low as compared to most of the pyrometallurgical reactors. A good cell design should aim at obtaining a high value of the space time yield. 

The high space-time yields are the result of a doubling time of only 30 min and its applicability for high cell-density cultures. However, it is hardly possible to excrete overexpressed proteins into cultivation media. In addition, accumulation of pyrogenic lipopoly-saccharides in its outer membrane (a distinctive feature of Gram-negative bacteria) make additional purification steps necessary if pharmaceutical proteins are produced by E. coli [29]. 

In spite of several drawbacks (i.e. expensive and laborious handling procedures, low space-time yields (Table 2.1), high demand on biosafety, potential contaminations, limited applicability for continuous fermentations [129], and problems obtaining the same glycosyla-tion profile from batch to batch [130]), mammalian cell cultures are widely used for small-scale expression and more recently even on a multi-cubic-meter scale. The system works like insect... 

Similarly, whole-cell Lactobacillus kefir DSM 20587, which possesses two alcohol dehydrogenases for both asymmetric reduction steps, was applied in the reduction of tert-butyl 6-chloro-3,5-dioxohexanoate for asymmetric synthesis of ft rf-butyl-(31 ,5S)-6-chloro-dihydroxyhexanoate (Figure 7.5), a chiral building block for the HMG-CoA reductase inhibitor [ 17]. A final product concentration of 120 him and a specific product capacity of 2.4 mmol per gram dry cell were achieved in an optimized fed-batch process. Ado 99% was obtained for (3R,5S)- and (3.S, 55)-te/ f-butyl-6-chloro-dihydroxyhexanoate with the space-time yield being 4.7 mmolL-1 h-1. 

Tan et al. [29] demonstrated the use of a plug flow reactor of immobilized Lactobacillus kefiri cells for the synthesis of the intermediate (5I )-hydroxyhexane-2-one. This immobilized-cell reactor operated at a maximum conversion yield of 100% and a selectivity of 95%. The production of (5/ )-hydroxyhexane-2-one was extended to an operation time of 6 days. During this time (91 residence times), a space-time yield of 87gL xday 1 and a productivity of 07 8 gwet cell weight 1 were obtained. 

To achieve a large production rate, the current density should be as high as possible. Particularly, industrial cells need a satisfactory current density and space-time yield , that is, production per time and cell volume, because the investment costs and consequently the production costs are enlarged with increasing electrode area and cell volume. But, naturally, the current density is limited by different reasons that have to be considered. 

In short, microbial cells can be employed as very effective reactors for the conversion of substrates to products, operating in mixed aqueous-apolar systems, optimized for the best space-time yields attainable at lowest cost. 

With this system we converted 135 mM styrene (relative to the total liquid volume) to styrene oxide in 10 h at a cell dry weight of around lOg/L aqueous phase, with an average activity of 152 U/L total liquid volume. This corresponds to a space-time yield of 1.1 g (5)-styrene oxide per liter and hour. These are the highest specific activities reported thus far for a microbial epoxidation process. ... 

The heterogeneously catalyzed Mn02-mediated oxidation of diacetone-L sorbose to diacetone-2keto-L sorbic acid, the latter being a precursor to vitamin C, at nickel anodes and based on the chemical oxidation of the substrate by NiOOH is of technical relevance. The limiting current density in 1 M KOH solution is under operation conditions only 10 A/cm2 leading to relatively poor space-time yields. Robertson and Ibl showed that acceptable space-time yields can by obtained by using thin layer cells of Swiss roll type (193, 194), which leads to an efficient compression of the cell width to fractions of a millimeter. 

For laboratory cells, minimizing the energy consumption and optimizing the space-time yield are not as important. It is more important that the different reaction parameters like electrode materials, diaphragms, and the working potentials can be varied easily. For electrolyses under potential control, a three-electrode construction has to be used, which is schematically shown in Figure 22.8. 

If a large space-time yield is also required in the laboratory application, a capillary gap cell (disc-stack) is a good choice (Fig. 22.11). Only the upper-... 

In general, syntheses with isolated enzymes can be performed with higher selectivity and space-time yield than with whole cells, but they require in any case the coupling of coenzyme regenerating reactions. 

As an example, it has been estimated that for the industrial production of fine chemicals, biotransformations should accomplish a minimum space-time yield of 0.1 g l-1 h-1 and a minimum final product concentration of 1 g l-1, while for pharmaceuticals, the minimum requirements are 0.001 g l-1 h-1 and 0.1 g l1 for volumetric productivity and product concentration, respectively [6]. Analysis of enzymes with recognized industrial potential, such as cytochrome P450, showed that some of the parameters are already within industrially relevant ranges [7]. The improvements achieved with these biocatalysts through protein, cell, and process engineering are based on the understanding of their molecular arrangement and catalytic mechanisms. 

Capillary gap cell — The undivided capillary gap (or disc-stack) cell design is frequently used in industrial-scale electroorganic syntheses, but is also applicable for laboratory-scale experiments when a large space-time yield is required. Only the top and bottom electrodes of c.g.c. (see Figure) are electrically connected to - anode and cathode, respectively, whereas the other electrodes are polarized in the electrical field and act as -> bipolar electrodes. This makes c.g.c. s appropriate for dual electrosynthesis, i.e., pro duct-generating on both electrodes. 

The cell construction ensures a high electrode area per volume unit together with small electrode distances, which, even with poorly conductive electrolytes and/or at low current densities, results in good space-time yields. A high turbulence of the flowing electrolyte is due to the mesh structure and ensures high conversion rates. 

Another example in which a biocatalytic transformation has replaced a chemo-catalytic one, in a very simple reaction, is the Mitsubishi Rayon process for the production of acrylamide by hydration of acrylonitrile (Fig. 1.42). Whole cells of Rhodococcus rhodocrous, containing a nitrile hydratase, produced acrylamide in >99.9% purity at >99.9% conversion, and in high volumetric and space time yields [121]. The process (Fig. 1.42) currently accounts for more than 100000 tons annual production of acrylamide and replaced an existing process which employed a copper catalyst. A major advantage of the biocatalytic process is the high product purity, which is important for the main application of acrylamide as a specialty monomer. 

Hydantoinase-Carbamoylase System for t-Amino Acid Synthesis Despite a number of reports of strains with L-selechve hydantoin-hydrolyzing enzymes [38] the commercial application of the hydantoinase process is stiU restricted to the production of D-amino acids. Processes for the production of L-amino acids are Umited by low space-time yields and high biocatalyst costs. Recently, a new generation of an L-hydantoinase process was developed based on a tailor-made recombinant whole cell biocatalyst. Further reduction of biocatalyst cost by use of recombinant Escherichia coli cells overexpressing hydantoinase, carbamoylase, and hydantoin racemase from Arthrohacter sp. DSM 9771 were achieved. To improve the hydan-toin-converting pathway, the level of expression of the different genes was balanced on the basis of their specific activities. The system has been appUed to the preparation of L-methionine the space-time yield is however still Umited [39]. Improvements in the deracemization process from rac-5-substituted hydantoins to L-amino acids still requires a more selective L-hydantoinase. 

Cells with three-dimensional electrodes have bipolar electrodes such electrodes are characterized by the feature that one part of the electrode is anode and another part cathode. This can be realized in different ways, such as the pile capillary gap cell (Beck/Guthke cell. Chapter 32) [9,80], the Swiss roll cell [81], and packed- and fluidized-bell cells (Chapter 32) [82-84]. These cells are developed to meet economic demands, such as high space-time yield and simplicity in construction they are discussed in Chapter 31. 

Porous electrodes may be used to achieve a high space-time yield, as they possess a large internal surface. A consequence of using such electrodes is, however, that interior mass transfer and ohmic resistance effects may lead to a nonuniform potential distribution, which affects the selectivity of the reaction. The cell design and the adjustable parameters must thus be optimized for each reaction [64-67,120]. 

The phenomenon of charge transport, which is unique to all electrochemical processes, must be considered along with mass, heat, and momentum transport. The charge transport determines the current distribution in an electrochemical cell, and has far-reaching implications on the current efficiency, space-time yield, specific energy consumption, and the scale-up of electrochemical reactors. 

This whole-cell biotransformation is still a subject of research for several reasons Besides fhe desired product (1 )-PAC, several by-products are formed through enzymatic reduction of fhe product or fhe substrate benzaldehyde, resulting in the formation of l-phenylpropan-2,3-diol and benzylalcohol, respectively. Further byproducts are acetoin, butane-2,3-dione, l-phenyl-propane-2,3-dione, benzoic acid and 2-hydroxypropiophenone, leading to a reduced yield of the desired product and difficult product isolation. To circumvent fhis problem strain improvement and reaction engineering have been used [15, 16]. Application of an isolated enzyme in a two-phase system resulted in improved space-time yield and high product purity [17]. 

Equation (2) is the base relationship electrochemical engineers utilize to describe and optimize the cells and make compromises among the competing factors such ass space-time yield, energy consumption, product quality and materials of construction. 

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