Volumetric activity

The direct way to improve the volumetric activity of monolithic catalysts is to increase its cell density, by which the volumetric surface is tremendously enlarged [15,34], Figure 9.8 shows that an increase in cell density from 300 to 400 cpsi is more effective on the DeNO performance than an increase of the vanadia concentration from 1.9 to 3%, which is, moreover, accompanied by the above mentioned problems. 

ZnS04.7H20, 1.45 MnCl24H20, 1.00 CUSO4.5H2O, 0.25 and H3BOJ, 0.10. The cell-free culture broth was concentrated 200-fold to a volumetric activity of 40 Phadebas units per milliliter. 

In most cases a value for the overall activity of enzyme is not very interesting. Much more important are both the specific activity, scaled to the mass of catalyst, and the volumetric activity, based on the activity per unit volume ... 

The volumetric activity of a (bio)catalyst can be enhanced by simple addition of more catalyst to a system. Specific activity, however, must be improved through optimization of the reaction conditions, or through variation of the structure of the carrier (Chapter 5) or even of the enzyme (Chapters 10 and 11). 

Enzyme Strain Specific activity (U/g wet cells) Volumetric activity (U/liter of broth) Productivity (U/liter/week)... 

Radionuclide release to the Central Hall (CH) could lead to contamination of air and surfaces. If Cs deposition were not taken into account, its volumetric activity in CH air would reach the maximum permissible value of 2-10 Bq/m [6]. 

Using the Methodic Guides [5] trough an assigned value a, for volumetric activity of / radionuclide in water and a specified human exposure time tj via path j the radiation dose Dij through this path due to water contamination by the indicated nuclide can be calculated via the following formulae ... 

For comparison purposes Table 2 also comprises generalized-over-RF-territoiy radiation situation data. Comparison of data on volumetric activities of radioactive substances in air, freshwater bodies and sea water between the above regions and averaged RF data over 1999-2003 shows that concentrations of radioactive substances... 

As an active sampling method, the volumetric air sampler aspirates a known volume of process air, capturing microorganisms into or onto a nutrient agar medium, a liquid, or a filter. Microorganisms are developed and quantified as an estimate of CPUs present in the sampled environment per cubic foot of air (or other volumetric measurement). The quantitative principles of volumetric (active) air sampling may be expressed by... 

The use of membrane reactors is favorable not only with respect to an increase in the total turnover number. In certain cases the selectivity can also be increased by applying high concentrations of the soluble catalyst together with making use of the behavior of a continuously operated stirred-tank reactor. Basically, this is also possible with a catalyst coupled to an insoluble support, but here the maximum volumetric activity is limited by the number of active sites per mass unit of the catalyst. This has been shown for the enantioselective reduction of ketones (eq. (2)) such as acetophenone 5 with borane 6 in the presence of polymer-enlarged oxazaborolidines 8 and 9 [65-67]. 

Biocatalyst costs are often an important factor in the overall cost of the product. One of the most important reasons to consider the immobilization of a biocatalyst is therefore the possibility of facilitated reuse or continuous utilization. This enables prolonged use of the biocatalyst and can thus significantly reduce the process costs. Furthermore, the stability of biocatalysts, in particular of enzymes and recombinant cells, can be improved dramatically in many cases by immobilization. Immobilization also creates the possibility of using the biocatalyst in a packed-bed or fluidized-bed reactor. Due to easy retention of the immobilized biocatalyst in the reactor, high volumetric activities can be realized and, in case of immobilized growing cells, the operation can be under wash-out conditions with respect to free cells. Obviously, the extra costs associated with the immobilization must be earned back by the possibility of a more efficient use. Many techniques are available for immobilizing biocatalysts [3] some of the more useful are discussed in Chapter 9. 

To address both the green chemistry and cost issues, a novel biocatalytic process was recently developed involving a lipolase-catalyzed resolution of a CNDE to produce the desired (S)-mono acid enantiomer, which was subsequently converted to pregabalin upon decarboxylation, hydrolysis, and hydrogenation (Scheme 8.11) [60]. The enzymatic step has an excellent volumetric activity of >500 gl-1 per day. Since the undesired R-enantiomer could be readily racemized to CNDE, the yield of... 

Cross-linking High volumetric activity Chemical modification of... 

Surface area >50 m2/g Binding capacity, volumetric activity... 

Immobilization by chemical cross-linking without the addition of an inert carrier or matrix can provide the means to stabilize and reuse a biocatalyst without dilution of volumetric activity. A major deficiency in all of the aforementioned immobilization methods is that a substantial amount of a catalytically inert carrier or matrix is used to bind or contain the biocatalyst. In many cases, the amount of carrier is two orders of magnitude higher than the protein catalyst. Unfortunately, direct cross-linking of the enzyme, followed by precipitation of an amorphous solid often results in low activity and poor mechanically properties and so this method is not often used. Recently, however, cross-linked enzyme crystals have been reported to give many of the desirable properties of immobilized enzymes without the need for a support material (Sect. 6.4.1). 

The enzyme ratio represents the ratio of the initial volumetric activities of the enzymes (dimensions U mL 1). For process conditions, enzyme activity under the actual steady-state reactor conditions is significant, and differs from initial rate conditions as determined by enzyme kinetics. Therefore, the optimum enzyme ratio, implying maximum conversion within minimum residence time, is not 0.5. 

Proton donors, such as phenols and alcohols, may not be used, which leads to formation of less sterically volumetric active centers. 

The new generation of fixed bed catalysts made fi om activated base metals (Metalyst ) is best described as an extremely durable collection of empty eggshells composed of high surface area activated metal. This technology offers an increase in volumetric activity at lower catalyst bulk densities without the presence of expensive, but not utilized, materials. In other words, the amount of metal in the reactor is reduced by as much as 40 to 77% leading to a more effective use of metal (1). In this paper, the effectiveness of this catalyst technology will be compared to others for the hydrogenation of various compounds (2, 3,4). 

In the past, there was no need for small-scale (< 1 MW) distributed hydrogen production from hydrocarbons where such reactors would likely be first employed. In any case, unsupported palladium membranes have required too much expensive metal to be economically feasible for all but special purposes. The potential availability of cost effective, high hydrogen permeance, palladium membranes supported on porous, stainless steel, tubular substrates combined with the advent of stable, high volumetric activity WGS catalysts is the driving force behind the system modeling... 

Specific activity of biocatalyst molar concentration of substrate B (alternatively coefficient in Eq. 5.3) initial molar concentration of substrate B coefficient in Eq. 5.3 concentration of biocatalyst time of a cycle of reactor operation enzyme activity initial enzyme activity molar concentration of enzyme species Eij volumetric activity of enzyme species Ey enzyme volumetric activity initial enzyme volumetric activity bioreactor feed flow-rate total flow-rate to downstream operations initial feed flow-rate to bioreactor i number of half-lives of biocatalyst use film volumetric mass transfer coefficient for substrate Michaelis-Menten constant catalytic rate constant first-order inactivation rate constant transition rate constants... 

Enzyme Specific activity (u/g wet cell) Volumetric activity (u/L of broth) Productivity [u/(L-wk)]... 

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