Enzyme reactors plug-flow

The alternative to batch mode operation is continuous operation. In the continuous mode there is a continuous flow of medium into the fermentor and of product stream out of the fermentor. Continuous bioprocesses often use homogenously mixed whole cell suspensions. However, immobilised cell or enzyme processes generally operate in continuous plug flow reactors, without mixing (see Figure 2.1, packed-bed reactors). 

High reaction rate in Equation 5.71 is favored by a high concentration of enzymes (CE ) and high concentration of feed (CA). This means that a plug-flow or ideal-batch reactor is favored if both the feed material and enzymes are to be fed to the reactor. 

An enzyme reaction has the M-M rate equation with rm = 13 mol/liter.min and K, = 0.03 mol/liter. Starting concentration is Cs = 10 mol/liter and the flow rate is 10 liter/hr. Find conversions in plug flow and stirred tank reactors. 

A specific enzyme acts as catalyst in the fermentation of reactant A. At a given enzyme concentration in the aqueous feed stream (25 liter/min) find the volume of plug flow reactor needed for 95% conversion of reactant A (Cao = 2 mol/liter). The kinetics of the fermentation at this enzyme concentration is given by... 

The modeling of real immobilized-enzyme column reactors, mainly the fluidized-bed type, has been described (Emeiy and Cardoso, 1978 Allen, Charles and Coughlin, 1979 Kobayashi and Moo-Young, 1971) by mathematical models based on the dispersion concept (Levenspiel, 1972), by incorporation of an additional term to account for back-mixing in the ideal plug-flow reactor. This term describes the non-ideal effects in terms of a dispersion coefficient. 

Cooking extruders have been studied for the liquefaction of starch, but the high temperature inactivation of the enzymes in the extruder demands doses 5—10 times higher than under conditions in a jet cooker (69). For example, continuous nonpressure cooking of wheat for the production of ethanol is carried out at 85°C in two continuous stirred tank reactors (CSTR) connected in series plug-flow tube reactors may be included if only one CSTR is used (70). 

In a plug-flow enzyme reactor (or tubular-flow enzyme reactor), the substrate enters one end of a cylindrical tube which is packed with... 

Immobilized Enzyme Reactor (Fixed-Bed Reactor with Plug-Flow)... 

The plug flow reactor has been mainly utilized for the removal of phenol in waste streams by HRP [76, 83] and Coprinus cinereus peroxidase [2]. According to Buchanan et al. [83], who modeled the kinetics of the HRP-aromatic substrate system and applied to PFR and CSTR, plug-flow configuration is recommended when working with low HRT, since considerably less enzyme would be required for equal phenol removal. However, for long HRTs, a multiple-stage CSTR would be more efficient than a PFR, due to the lower rate of enzyme inactivation. 

From a practical point of view, a prolonged operation of a reactor with enzyme in suspension is not feasible. Procedures to retain the enzyme in the tubular reactor have been developed in order to maintain high enzymatic activity and to avoid enzyme washout. A plug-flow reactor operated with immobilized enzyme is known either as a fixed bed reactor or a fluidized bed reactor, depending on the characteristics of the flow pattern and the immobilized enzyme. Since mechanical stirring is not required in plug flow reactors, the support material is not damaged by the impeller, which may be a drawback in CSTR with immobilized enzyme. 

Enzyme Assay Procedure. The catalytic potency of the immobilized g-galactosidase was determined in a plug flow reactor ( 9). Glucose liberated by the catalytic activity of 3-galactosi-dase on lactose was determined by the glucose oxidase-chromogen method (21 ) with some modifications. 

Chemical analysis for bound enzyme, following steady state plug flow reactor operation, was based on the tryptophan content of the complex. A modification of the method of Daiby et al.(22) was employed. 

Unfortunately, most enzymes do not obey simple Michaelis-Menten kinetics. Substrate and product inhibition, presence of more than one substrate and product, or coupled enzyme reactions in multi-enzyme systems require much more complicated rate equations. Gaseous or solid substrates or enzymes bound in immobilized cells need additional transport barriers to be taken into consideration. Instead of porous spherical particles, other geometries of catalyst particles can be apphed in stirred tanks, plug-flow reactors and others which need some modified treatment of diffusional restrictions and reaction technology. 

When cross-linked crystals of thermolysin were applied in peptide synthesis in ethyl acetate, they were stable for several hundred hours at amazingly low enzyme consiunption, whereas a soluble enzyme preparation became inactive within a short period of time. Again it is worthwhile to consider the quality of the soluble enzyme preparation. When soluble thermolysin was stored in mixed aqueous-organic solutions, it lost about 50% of its activity within the first day of incubation only to be then quite stable for the next 15 days. It is possible that the initial inactivation was caused by an unstable fraction of thermolysin and that crystals of thermolysin no longer contained this unstable fraction [118]. Productivity comparable to that of crystals was achieved with thermolysin adsorbed on Amberlite XAD-7 resin which was employed in continuous plug flow reactors with tert-amyl alcohol as solvent [119]. 

A plug flow reactor may be realized using immobilized enzymes within a column reactor or using soluble enzymes within a cascade of membrane reactors. A batch or a repetitive batch process with soluble enzymes (see below) has the same productivity as the plug flow reactor. 

The fixed bed reactor, behaving as a plug flow reactor, is most often used for immobilized enzyme reactions. Typically, the reactor is used with an upward direction of the flow to avoid compression of the bed and to release gas bubbles generated during the reaction. Reactor design may be done readily without knowing the detailed enzyme kinetics. Kinetic measurements are performed with a recirculation reactor and the data are plotted in the form 1/v = f(%) (see above). From this plot, the residence time necessary to reach a desired conversion x can be calculated as described. The different enzyme concentrations in the recirculation reactor and in the plug flow reactor have to be considered. 

---