Substrate residence time

In one of the early studies Cho and Hwang [3.54] studied the integration of continuous fermentation and membrane separation using ethanol selective silicone rubber hollow-fiber membranes. Relative to conventional continuous fermentation, the performance of PVMBR resulted in high yeast cell densities, reduction of ethanol inhibition, longer substrate residence time of, more glucose consumption, and recovery of clean and concentrated ethanol. A 10-20 % increase in ethanol productivity was achieved. Kaseno et al. 

Figures 7.15-7.18 show how reactor capacity (i.e., maximum productivity, g product/g enzyme/min) is affected by operational parameters. Enzyme and substrate concentration appear to affect reactor capacity to a large extent (Figures 7.15 and 7.16) especially at lower enzyme concentrations. Effects of permeate flux and reactor volume on conversion are related to each other by means of substrate residence time in the reaction vessel. Ingreasing residence time, that is increasing reactor volume or lowering fluxes, results in higher conversions at the expense of lower capacities (Figures 7.17 and 7.18).

Catalyst activity does the catalyst provide a high enough TOF for the given substrate residence time in the reactor. ... 

Substrate size- and shape-selective reactions can also be explained in terms of effective concentration and transition state stabilization. 1) In a mixture of substrates a higher concentration can be achieved for those having a complementary size and shape to the nanoreactor portals and cavity (and thus can enter the nanoreactor) compared to those substrates that can not easily enter or do not fit within the cavity. In some cases, when the substrate encapsulation is a slow process (slow diffusion), substrate encapsulation can become the rate-determining step. 2) One can also imagine that substrates of identical size and shape can both enter the cavity, but the transition state of one of the reaction pathways is stabilized to a greater extent than the other. Importantly, for all encapsulated reactions one should keep in mind that the substrate residence time within the nanoreactor and the kinetic rates of the encapsulated reaction should at least have a comparable magnitude. ... 

Reliable mechanistic conclusions require high intrazeolite yields that account for the majority of the substrate mass balance. This can be a challenge because of the small-scale reactions often conducted for mechanistic studies. In addition, rapid removal of the products from the zeolite, and/or low conversions to decrease residence time, is occasionally necessary because of the sensitivity of the reaction products to the zeolite environment.44,45 Intrazeolite products are generally recovered by extractive techniques from either the intact zeolite, or from a mixture formed after mild digestion of the zeolite. Polar solvents such as tetrahydrofuran or acetonitrile coupled with a continuous extraction technique is in particular an effective means to remove polar products with an affinity for the interior of the zeolite.44 Zeolite digestion with mineral acids, in order to liberate the products, must be conducted with care in order to prevent acid catalyzed product decomposition or reaction.46,47... 

The biodegradability within a period of time corresponding to the residence time in a sewer network (typically between a few hours and one day) must be reasonably detailed. Therefore, the fast biodegradable fractions considered as Ss and fast hydrolyzable substrate must be included as separate fractions. On the contrary, what is not biodegradable within 1-2 days is of minor interest. As a consequence, there is no need to distinguish between a rather slowly biodegradable, particulate fraction of a substrate and a fraction that is inert, whatever it is — soluble or particulate. 

In an electrochemical enzyme membrane reactor an electrochemical flow-through cell using a carbon-felt anode is combined with an enzyme-membrane reactor. The residence time is adjusted by the flow of the added substrate solution. The off-flow of the enzyme membrane reactor only contains the products p-hydroxy benzaldehyde and p-hydroxy benzylalcohol. By proper adjustment of the residence time and the potential, total turnover of the p-hydroxy toluene, which is introduced into the reactor in 13 mM concentration, can be obtained. In a 10-day run, the enzyme underwent 400000 cycles and the polymer-bound mediator, which was present in a higher concentration than the enzyme, underwent more than 500 cycles. At the end, the system was still active. By proper selection of the residence time, one can either... 

Add a calculation of the operating parameters substrate consumption rate to micro-organisms ratio (Load), solids residence time (0m) and sludge age ((),) to the program. Experiment with variations in the values of these parameters under various conditions of flow and feed concentration to test their value as process control parameters. 

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