Continuously operated stirred tank

The continuously operated stirred tank reaction in which the composition of the reaction mixture is assumed uniform and equal to the composition at the outlet. 

Biocatalysts in nature tend to be optimized to perform best in aqueous environments, at neutral pH, temperatures below 40 °C, and at low osmotic pressure. These conditions are sometimes in conflict with the need of the chemist or process engineer to optimize a reaction with respect to space-time yield or high product concentration in order to facilitate downstream processing. Furthermore, enzymes and whole cells are often inhibited by products or substrates. This might be overcome by the use of continuously operated stirred tank reactors, fed-batch reactors, or reactors with in situ product removal [14, 15]. The addition of organic solvents to increase the solubility of substrates and/or products is a common practice [16]. 

Figure 2.33. Temperature control, TC, of a continuously operated, stirred tank with an electric heater (Q).

The continuously operated stirred-tank reactor with continuous extraction of the unconverted enantiomer yields an enantiomeric excess of 95%. Afterwards, the unconverted enantiomer is racemized and reused in the synthesis process carried out by Coca Cola. 

Due to the behavior of the membrane reactor as a continuously operated stirred tank reactor (CSTR) [2], it can be used effectively to suppress side-reactions, for example the noncatalyzed reduction yielding the racemate in the oxazaborolidine reaction [11]. 

CSTR continuously operated stirred tank reactor... 

Any reaction exhibiting substrate inhibition should not be carried out in batch since it results in a longer residence time the high concentration of the substrate at the beginning lowers the reaction rate. A continuously operated stirred tank is preferred. At laboratory scale, fed-batch operation enables a low substrate concentration in the reactor and a higher reaction rate. 

The continuously operated stirred tank reactor is fed with reactants at the same time as the products are removed by an overflow or a level control system (Figure 8.1). This ensures a constant volume and, consequently with a constant volume flow rate of the feed, a constant space hme. We further assume the reactor contents... 

The contents of a continuously operated stirred tank are assumed to be perfectly mixed, so that the properties (e.g., concentration, temperature) of the reaction mixture are uniform in all parts of the system. Therefore, the conditions throughout the tank are the same and equal to the conditions at the outlet. This means that the volume element can be taken as the volume, VR, of the entire contents. Additionally, the composition and temperature at which the reaction occurs are the same as the composition and temperature of any exit stream. A continuous flow stirred tank reactor as shown in Figure 5-21 assumes that the fluid is perfectly well mixed. 

The use of membrane reactors is favorable not only with respect to an increase in the total turnover number. In certain cases the selectivity can also be increased by applying high concentrations of the soluble catalyst together with making use of the behavior of a continuously operated stirred-tank reactor. Basically, this is also possible with a catalyst coupled to an insoluble support, but here the maximum volumetric activity is limited by the number of active sites per mass unit of the catalyst. This has been shown for the enantioselective reduction of ketones (eq. (2)) such as acetophenone 5 with borane 6 in the presence of polymer-enlarged oxazaborolidines 8 and 9 [65-67]. 

Continuously operated stirred tanks can only attain homogenization if q > q (where q is the liquid throughput through the vessel). The mixing time is then 0 oc V/q (where V is the liquid volume of the vessel). Knowledge of q, in the case of propeller stirrers, enables the flow velocity along the heat exchanging surfaces of one or more concentric cylindrical coils round the stirrer (and consequently the heat transfer rates) to be calculated. Such an installation may be indispensable in case of extremely exothermic reactions. 

Schindler and Treybal [484] investigated the mass transfer in the L/L system in continuously operated stirred tanks with and without baffles. They established, that kc increased according to k oc and that the stirrer for the same power... 

In the past the population-equilibrium relationship (PER) was utilized with success for the mathematical description of the dispersed phase distribution. The complexity of the numerical treatment of the integro-differential PER can be circumvented by simulation techniques, with which the continuously operated stirred tank can be simulated with different models [17, 18, 87, 88]. 

The N-acetyl-D,L-amino acid precursors are conveniently accessible through either acetylation of D,L-amino acids with acetyl chloride or acetic anhydride in a Schotten-Baumann reaction or via amidocarbonylation I801. For the acylase reaction, Co2+ as metal effector is added to yield an increased operational stability of the enzyme. The unconverted acetyl-D-methionine is racemized by acetic anhydride in alkali, and the racemic acetyl-D,L-methionine is reused. The racemization can also be carried out in a molten bath or by an acetyl amino acid racemase. Product recovery of L-methionine is achieved by crystallization, because L-methionine is much less soluble than the acetyl substrate. The production is carried out in a continuously operated stirred tank reactor. A polyamide ultrafiltration membrane with a cutoff of 10 kDa retains the enzyme, thus decoupling the residence times of catalyst and reactants. L-methionine is produced with an ee > 99.5 % and a yield of 80% with a capacity of > 3001 a-1. At Degussa, several proteinogenic and non-proteinogenic amino acids are produced in the same way e.g. L-alanine, L-phenylalanine, a-amino butyric acid, L-valine, l-norvaline and L-homophenylalanine. 

A continuously operated stirred tank reactor (CSTR) for use up to 3 kbar and 300"C is shown in Fig. 4.10. The reactor is equipped with a fairly large sapphire window (visual observation and spectroscopic analysis. Spectroscopic studies may be conducted using a reflectance technique developed by Franck and Roth probing light enters the cell, passes through a sample layer with a precisely known thickness, and is reflected from a mirror which is positioned inside the fluid under investiga-... 

The calculation of a continuously operating stirred tank reactor used for polymerisation is based on the solution of the equation of material balance for the monomer [1] ... 

Ideal Continuously Operated Stirred Tank Reactor (CSTR)... 

In Figures 3.4 and 3.5, the RTDs of ideal reactors are presented together with the RTD of a real reactor. The ideal, continuously operated stirred tank reactor (CSTR) has the broadest RTD between all reactor types. The most probable residence time for an entering volume element is t = 0. After a mean residence time t = t), 37% of the tracer injected at time t = 0 is still present in the reactor. After five mean residence times, a residue of about 1% still remains in the reactor. This means that at least five mean residence times must pass after a change in the inlet conditions before the CSTR effectively reaches its new stationary state. 

The cascade consists of a series of ideal continuously operated stirred tank reactors, CSTR, connected one after the other. The outlet function of one CSTR is... 

In order to quantify the influence of the gas flow rate on the residence time of the particles a simple model can be used that represents the horizontal apparatus by a series of continuously operated stirred tank reactors (CSTRs). The principle of this model is illustrated in Fig. 7.40. The size (length) and number of the tanks express the intensity of back-mixing (mixing in the direction of solids transport). They are flctitious for an open process chamber (as in Fig. 7.38), but may correspond to... 

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