Biocatalyst particle

The internal mass-transfer effects can be reduced, however, by decreasing the particle dimensions of the porous support containing the biocatalyst. Particle-diameter decrease results in a reduction of the distance from the outer support surface that the substrate must cross and, consequently, also results in a decrease of the substrate concentration gradient. 

Biocatalyst particle shape and size characterization before and after treatment in the two selected media was examined in order to ascertain the role of the solvent on enzyme structure. The cmde biocatalyst preparation consisted of spherical micro-granules with a mean particle size of about 500 i.m. No particle size modifications were observed by SEM when the biocatalyst was treated by SC-CO2. After treatment in [bmim][PF6] a lower mean particle size of about 430 gm was observed, probably due to partial carrier dissolution. Conversely, by exposing the biocatalyst... 

The kinetic term, v, is a function of the kinetic parameters vector P and the particle substrate and product concentrations, cs and cP, respectively. Ds and DP are the corresponding effective diffusion coefficients and r is the particle coordinate (in the case of spherical geometry it is the radial distance). Parameter n depends on the geometry of the biocatalyst particle and is 0,1,2 for a plate, a cylinder and a sphere, respectively. Since concentrations on the particle surface are assumed to be identical with bulk concentrations, boundary conditions do not include the influence of external mass transfer. Solving the above differential equations, the observed reaction rate in the packed bed is evaluated from the rate of substrate flux to the particle or of product flux from the particle... 

It follows from the form of the model equations used, the temperature profile in the particle is not considered in the calculation of the observed reaction rate, because - under steady-state conditions - no heat accumulation occurs in the biocatalyst particle. Consequently, the variation of reaction rate with temperature change can be neglected, in view of the low temperature differences typical for enzyme flow micro calorimetry. 

Many immobilization techniques provide biocatalysts in which the enzyme is immobilized in the porous structure of the biocatalyst particle. In such cases, the... 

Similar results were obtained for the immobilization of glutaryl-7-ACA-acy-lase (own laboratory experiments). Figure 6 demonstrates the decrease of activity in the supernatant of the coupling reaction mixture and the concomitant increase in carrier-bound activity. Maximum activity was measured after only 6 h, leaving about 20 % of the initial activity in solution. During the next 14 h the remaining soluble enzyme was immobilized. However, an increase in activity could not be measured under standard conditions due to diffusional limitation and internal pH-shifts in the biocatalyst particles. According to these data. 

If the aim is to make a fair comparison of the effect of other factors (e. g. different solvents), then it is desirable to produce reaction mixtures of defined water activity. For this purpose, it is best if the two phases mentioned are separately preequilibrated to the target water activity before eventually combining them to start the reaction. In principle it is possible for the water activity to change somewhat from the pre-equilibrated value as components redistribute between the two phases. However, in practice such changes are small if the two phases noted are chosen. Another option is to pre-equilibrate the biocatalyst particles suspended in a non-aqueous fluid, and to add one final reactant at time zero. This reactant should be one added at fairly low concentration to prevent significant changes in water activity. These two options are illustrated in Fig. 8-4. 

The process of polymer bead formation can be divided into five steps (i) preparation of polyelectrolyte solution in its sodium salt form, (ii) addition of cell mass to (i) and dispersion, (iii) dropping of this suspension through a capillary tube into an Al3+ solution, (iv) hardening of beads in the Al3+ solution and (v) separation of biocatalyst particles and washing. 

The analysis will be done in three steps. In the fist step, differential equations will be developed by combining enzyme kinetics and mass transfer to obtain the substrate (and product) profile within the biocatalyst particle in the second step, local effectiveness factor profiles will be obtained from the previous step in the third step a global effectiveness factor will be obtained by adequately averaging that distribution. This global effectiveness factor describes the behavior of the biocatalyst particle as a whole and will be obtained in terms of measured and calculated parameters, being a useful way of incorporating IDR into enzyme reactor design and performance evaluation, as considered in section 5.3. 

Boundary condition I) assumes that EDR is negligible with respect to IDR, as may frequently occur for enzymes immobilized inside solid supports. If not, it is wrong since at the surface of the biocatalyst particle s = ss so and boundary condition I) should be replaced by an equation of continuity at the medium-particle interface. This situation will be analyzed afterwards. 

The final step is the determination of a global effectiveness factor from the profile of local effectiveness factors that adequately describe the behavior of the biocatalyst particle (membrane in this case) as a whole. Since the distribution of r values is... 

To assess the impact of IDR on enzyme kinetics, the value of intrinsic kinetics (V" and K) and mass transfer (Deff) parameters must be evaluated. Several strategies have been proposed to approximate the value of the intrinsic kinetic parameters. A reliable experimental procedure is the one proposed by Benaiges et al. (1986) which is basically based on comminuting the support to obtain particles so small than IDR becomes negligible (very low Osp see Eq. 4.54). Kinetic parameters can be determined then with that comminuted biocatalyst to have an estimate on the intrinsic values. Effectiveness factor can be approached then to the ratio of initial rates for the intact and comminuted biocatalyst (Kobayashi and Laidler 1973). An obvious drawback of this approach is that not always a biocatalyst particle small enough can be obtained to be free of IDR (effectiveness factor = 1). If a smooth correlation exists between effectiveness factor and particle size, extrapolation to size zero could give an approximate value and intrinsic kinetic parameters can be... 

Besides intrinsic kinetic parameters, mass transfer parameter, this is, the effective diffusion coefficient for substrate within the biocatalyst particle should be determined. Values of diffusion coefficients for a large number of substances can be found in chemical or biochemical handbooks of properties. However, these values correspond to diffusion in water, usually at a reference temperature. Some empirical correlations, like Eq. 4.64, have been proposed to determine Detr within porous matrices from the corresponding values in water ... 

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