Batch reactors enzymatic reactions

Aim of this work was to optimise enzymatic depolymerization of pectins to valuable oligomers using commercial mixtures of pectolytic enzymes. Results of experiments in continuous and batch reactor configurations are presented which give some preliminary indications helpful to process optimisation. The use of continuous reactors equipped with ultrafiltration membranes, which assure removal of the reaction products, allows to identify possible operation policy for the improvement of the reaction yield. 

Willeman et al. [26] modeled the enzyme-catalyzed cyanohydrin synthesis in a stirred batch tank reactor. Assumption of a mass transfer limitation (Figure 9.3b) is made, which results in a low concentration of substrate in the aqueous phase, thus suppressing the non-enzymatic reaction. In a well-stirred biphasic system the enzyme concentration was varied, keeping the phase ratio constant A maximum rate of conversion is reached at the concentration where mass transfer of the substrate becomes limiting. Further increase of enzyme concentration does not enhance the reaction rate [27]. The different results achieved by the two groups are explained by the different process strategies. No mass transfer limitation could be detected by Hickel et al. because the stirring rate in the aqueous phase was not varied [26]. 

Batch-stirred-tank reactors [12-21] are usually used for screening enzymatic reactions in dense gases. The design of the system is shown in Figure 9.2-1. Initially, the reaction mixture was pumped into the reactor and then the enzyme-preparation was added. Finally, dry gas was pumped into the reactor, up to the desired pressure. The initial concentration of the reactant never exceeded its solubility-limit in the gas. 

Recirculating batch reactors (Figure 9.2-2) [23,24] are mainly used for kinetic studies of enzymatic reactions performed in dense gases. 

Briggs and Haldane [8] proposed a general mathematical description of enzymatic kinetic reaction. Their model is based on the assumption that after a short initial startup period, the concentration of the enzyme-substrate complex is in a pseudo-steady state (PSS). For a constant volume batch reactor operated at constant temperature T, and pH, the rate expressions and material balances on S, E, ES, and P are... 

An attractive MR concept is called cascade MR. Two reactions of which the optimal reaction conditions are significantly different can be carried out in series. Two different lyases have thus been used for the production of 1-alanine from fumaric acid in two consecutive reactions. A UF membrane had to be used to retain both enzymes in their appropriate reactors. Each reactor could then be operated at the most optimal conditions of pH and temperature. In another example, GDP-mannose was prepared by linking two reactors through a UF membrane. The overall enzymatic consumption in the two-step eMR cascade was only 15% of that in the batch reactor and 45% of that in a single eMR. 

Enzymatic reactions have been monitored by several procedures. In the case of solid-phase enzymes, analysis is best achieved by periodically withdrawing small aliquots of fluid-phase reaction medium, after solid-fluid separation has occurred via gravity settling (e.g., disabling the agitator in batch reactors), filtration, or centrifugation. The aliquot can then be analyzed via chromatography or spectroscopy. Water content... 

Different types of reactors are applied in practice (Figure 1.14). Stirred tank reactors (STR), very often applied for homogeneous, enzymatic and multiphase heterogeneous catalytic reactions, can be operated batchwise (batch reactor, BR), semi-batchwise (semibatch reactor, SBR) or continuously (continuous strirred tank reactor, CSTR)... 

Batch, mechanically mixed pressure vessels are suitable for preliminary screening enzymatic reactions in supercritical fluids. They are cheaper and much more easily controlled than the various flow reactor types. However, to obtain suitable reaction rate data for up-scaling, it is necessary to run experiments in a flow reactor. 

Figure 4.9-2 Batch reactor set-up at VTT Chemical Technology for screening enzymatic reactions in SCFs.

Optically active amines are important intermediates and chiral auxiliaries in the technical synthesis of agrochemicals and pharmaceuticals. BASF, one of the world s leading producers of chiral amines, developed a process based on the enzymatic resolution of racemic amines 49 with Burkholderia plantarii lipase immobilized on polyacrylate (Scheme 16) [75,76]. Methoxyacetic acid estars are particularly well suited for the stereospecific enzymatic differentiation, giving both the free amine (S)-49 and the acylated product R)-50 in high ee. The reaction stops at 50% conversion and the selectivity factor was calculated to be as high as 500. A plug-flow or batch reactor can be used for the enzymatic reaction and the residence time is in the range of 5-7 h. The more important amine (R)-49 can be liberated with the aid of base and is subsequently purified by distil-... 

In kinetic smdies of enzymatic reactions, rate data are usually tested to determine if the reaction follows the Michaelis-Menten model of enzyme-substrate interactions. H. H. WeetaU and N. B. Havewala [Biotechnol. Bioeng. Symp., 3, 241 (1972) studied the production of dextrose from cornstarch using conventional glucoamylase and an immobihzed version thereof. Their goal was to obtain the data necessary to design a commercial facility for dextrose production. Their studies were carried out in a batch reactor at 60°C. Compare the data below with those predicted from a Michaelis-Menten model with a rate expression of the form... 

E14.5 An enzymatic reaction is carried ont in a batch reactor according to the reaction Urea + Enzyme ( ) -> Complex ES -> 2NH3 + CO2 + E The reaction is irreversible and represented by the following rate ... 

The two common modes of reactor operations are batch and continuous modes. Batch reactors are commonly nsed for kinetics studies, which are important for process optimization and scale-np. System design in this mode is simple, controllable, and cheap. Typically, reaction snbstrates are first added to the reactor, followed by the immobilized lipase. CO2 is then introduced to the reactor and pressurized to the desired level. Figure 6.3 shows a schematic diagram of a setup used for enzymatic transesteriflcation of lamb fats and microalgae lipids to biodiesel using Novozym 435 under SC-CO2 (Taher, 2009, 2014). 

The reports mentioned above provide a systematic coverage of the nonimmobi-lized enzymatic reactors used in biocatalytic reactions under continuous flow operation. Results from microreactor experiments were comparatively higher than conventionally mixed batch reactors in terms of conversion rate and improvement of product yield as demonstrated for hydrolysis [140], dehalogenation [141], oxidation [142], esteriflcation [143], synthesis of isoamyl acetate [144,145], synthesis of cyanohydrins [147,148], synthesis of chiral metabolites [153], reduction [151], and bioluminescent reaction [149]. The small volumes involved and the favorable mass transfer inherent to these devices make them particularly useful for the screening of biocatalysts and rapid characterization of bioconversion systems. The remarkable results of such studies revealed that the product yield could be enhanced significantly in comparison with the conventional batch runs. 

---