Optimum Reaction Volume

The reaction volume appeared to be critical in the acylation reaction as it governs the interaction between the amine, phenylacetic acid, and the enzyme-active site. 

Keeping the amounts of the reactants as well as other variables constant, the reaction volume was varied. The results (Fig. 12) indicate that the smaller reaction volumes give higher conversion of the amine to the corresponding amide [14]. 

---

Moderate Reactor Productivity. The rhodium catalyst is continuously recycled, but the catalyst is inherently unstable at low CO partial pressures, for example in the post-reactor flash tank. Under these conditions the catalyst may lose CO and eventually form insoluble Rhl3 resulting in an unacceptable loss of expensive catalyst. This reaction is also more likely to occur at low water concentrations, hence in order to run the process satisfactorily catalyst concentrations are kept low and water concentrations relatively high. Hence through a combination of lower than optimum reaction rate (because of low catalyst concentrations) and water taking up valuable reactor volume the overall reactor utilization is less than optimum. 

Activity measurements. Activity and selectivity measurements were performed at 10 psig in a 14-mm internal diameter glass fluid bed reactor using 25 grams of 90 to 38 micron catalyst particles. A reactant mixture of approximately 18 volume % 02 7 volume X NH3 and 7 volume % CH3OH and the balance of helium was fed to the catalyst, and temperature and contact time were varied to find the optimum yield of HCN. Optimum reaction temperatures were found to range from 425° to 475°C with contact times of 3 to 5 seconds (calculated at STP>. Fixed bed reactor studies produced similar results. The yields reported in this paper are based on carbon fed, unless otherwise noted. More details on catalyst performance can be found in our patents (7,8). 

Using this reaction block in a domestic microwave oven, the optimum conditions for the reaction were found to be four cycles of 2 min irradiation at 180 W followed by 2 min of rest. The cyclic approach was forced by the expandable vessels reaching their full capacity (20 times the reaction volume) after 2 min of irradiation, thereby requiring a rest period to allow the gas to contract before continuing the irradiation. These issues could obviously have been avoided by utilising reaction temperature control. In the above approach, there is no way of assessing the time spent at reflux thereby making reaction optimisation a difficult task due to reproducibility issues. 

As noted above, enzymatic hydrocyanations are preferably performed at pH < 5, to suppress the non-enzymatic bacl ound reaction whereas the pH optimum of the common nitrilases is 7. A compromise pH is obviously required and we accordingly assessed the effects of the pH on the MeHnL-mediated hydrocyanation of benzaldehyde (2a, see Figure 16.3) in a biphasic aqueous-diisopropyl ether (DIPE) medium. We found that enantioselectivity was maintained at pH 5.5, which we adopted as a compromise pH for the bienzymatic reactions, provided that the aqueous buffer phase accounted for <10% of the reaction volume. PfNLase was the obvious choice for the second step as it stayed active at pH 5.5 and converted (S)- and (R)-la at comparable rates. 

A crude cell-free extract contained 20 mg of protein per milliliter. Ten microliters of this extract in a standard total reaction volume of 0.5 ml catalyzed the formation of 30 nmoles of product in 1 min under optimum assay conditions (optimum pH and ionic strength, saturating concentrations of all substrates, coenzymes, activators, and the like), (a) Express v in terms of nmoles/assay, nmoles x mT x min", nmoles X liter" x min", moles x liter" X min", M X min", (b) What would v be if the same 10 p,l of extract... 

For a rapid conversion of lab-scale results into an economically viable reaction-pervaporation system, an optimum value can be determined for each parameter. Based on experimental results as well as a model describing the kinetics of the system, it has been found that the temperature has the strongest influence on the performance of the system as it affects both the kinetics of esterification and of pervaporation. The rate of reaction increases with temperature according to an Arrhenius law, whereas the pervaporation is accelerated by an increased temperature also. Consequently, the water content fluctuates much faster at a higher temperature. The second important parameter is the initial molar ratio. It has to be noted, however, that a deviation in the initial molar ratio from the stoichiometric value requires a rather expensive separation step to recover the unreacted component afterwards. The third factor is the ratio of membrane area to reaction volume, at least in the case of a batch reactor. For continuous operation, the flow rate should be considered as the determining factor for the contact time of the mixture with the membrane and subsequently the permeation... 

Initially, we will focus on the mesoscopic description associated with the radiative transfer equation. Then, we will introduce the single-scattering approximation and two macroscopic approximations the PI approximation and two-flux approximation. AH of these discussions are based on the configuration shown in Fig. 6. Collimated emission and Lambertian emission wiU also be considered in the discussion later they correspond to the direct component and the diffuse component of solar radiation, respectively. Throughout our study, the biomass concentration Cx is homogeneous in the reaction volume V (assumption of perfect mixing), and the emission phenomena in V are negligible. The concentration Cx is selected close to the optimum for the operation of the photobioreactor the local photon absorption rate. 4 at the rear of the photobioreactor is close to the compensation point A.C (see Section 5 and chapter Industrial Photobioreactors and Scale-up Concepts by Pruvost et al.). 

In the separate molecules, the M-L and N-L vibration will, for small displacements, be approximately harmonic it follows (cf. Section 8.2.3.2) that the profiles of the free-energy changes as functions of bond distance can be treated as parabolic. It may be shown that in the reaction complex (when the bond distances have taken up their optimum-compromise volume) the resultant potential-energy curve is also parabolic. We shall presume that the free-energy changes can be identified with potential energy changes. 

This plot shows the equilibrium line for ammonia synthesis, adiabatic reaction lines, lines with the same reaction rate (there is a difference of one order of magnitude between the rate lines) and the optimum reaction path (i.e. the temperature at which the reaction rate is maximum at each ammonia concentration). It is clear that if this optimum reaction path were followed from inlet to outlet concentration in the reactor system, then the minimum catalyst volume would be required. 

It is seen that the operating lines for the internally cooled reactor is closest to the optimum reaction path and that the operating line for the quench reactor is the poorest approach to the optimum line- As a consequence, the required catalyst volume is normally smallest for the internally cooled reactor and largest for the quench cooled reactor. 

Zn-Al-MCM-41 and Al-MCM-41 were used as catalysts for the synthesis of DABCQ (Scheme 41) [157]. The most suitable Si/(Zn+Al) ratio of the catalyst for the cyclization of ethanolamine to highly selective synthesis of DABCQ was 75. When the Si/(Zn+Al) and Si/Al molar ratios were increased, the yield and selectivity of DABCQ decreased. The optimum reaction conditions for the highly selective synthesis of DABCQ were foxmd at 563 K with 0.5 h (weight hourly space velocity), 4h TQS (time on stream) and 1 3 volume ratio of ethanolamine and water. 

We have used 1.5-5 jig (19-50 ng/p,l) protein (monomer) per 80-100 p,l reaction volume, probed for 1 -2 h at room temperature in a humid chamber. The following buffer is recommended for initial experiments 12 mM HEPES (pH 7.8), 80 mM KCl, 1 mM EDTA, 1 mM EGTA, 12% glycerol, 0.5 mg/ml poly(dIdC)-poly(dIdC), and 4% Marvel milk protein [29]. (HEPES is effective as a buffer at pH 6.8 to 8.2 the optimum for NF-kB protein binding is 7.5 to 8.0.) This involved application of protein directly onto the slide siuTace over a 26 X 20-mm area, with volume scaled up accordingly when required. At least 40 mM NaCl or KCl should be used. Experiments should be performed in a humid chamber. 

Reactants must diffuse through the network of pores of a catalyst particle to reach the internal area, and the products must diffuse back. The optimum porosity of a catalyst particle is deterrnined by tradeoffs making the pores smaller increases the surface area and thereby increases the activity of the catalyst, but this gain is offset by the increased resistance to transport in the smaller pores increasing the pore volume to create larger pores for faster transport is compensated by a loss of physical strength. A simple quantitative development (46—48) follows for a first-order, isothermal, irreversible catalytic reaction in a spherical, porous catalyst particle. 

Consider the reversible first order reaction A R. It is possible to determine the minimum reactor volume at the optimum temperature Tgp( that is required to obtain a fractional conversion X, if the feed is pure A with a volumetric flowrate of u. A material balance for a CESTR is... 

There is an interior optimum. For this particular numerical example, it occurs when 40% of the reactor volume is in the initial CSTR and 60% is in the downstream PFR. The model reaction is chemically unrealistic but illustrates behavior that can arise with real reactions. An excellent process for the bulk polymerization of styrene consists of a CSTR followed by a tubular post-reactor. The model reaction also demonstrates a phenomenon known as washout which is important in continuous cell culture. If kt is too small, a steady-state reaction cannot be sustained even with initial spiking of component B. A continuous fermentation process will have a maximum flow rate beyond which the initial inoculum of cells will be washed out of the system. At lower flow rates, the cells reproduce fast enough to achieve and hold a steady state. 

---