Tanks, stirred homogeneity

The uneasiness involved in basing the design of very large stirred tanks for homogenization on the expression n3 = const, had already been mentioned years ago by Kipke. In his paper [277] over outstanding problems in stirring, he pointed out, that the macro-scale of turbulence depended upon the size of the stirrer, whereas micro-scale of turbulence was merely a function of the dissipated stirrer power per unit volume and hence independent of scale (see Section 1.4.2). In scale-up of a stirred tank to industrial-scale (e.g. 12 0 x 26 m V = 3000 m ), if P/V = const the micro-scale of turbulence does not change (micro-eddies with dimensions A % 0.2 mm), whereas its macro-scale, which was characterized by the size of the primary eddy A, which is A w 0.08 D here, determines the primary eddy of ca. 0.7 m diameter, whose size would therefore far exceed the size of the experimental tank. 

Jahoda M., Machon V., Homogenization of Liquids in Tanks Stirred by multiple Impellers, Chem. Eng. Technol. 17 (1994), p. 95-101... 

One of the main tasks of the stirred tank is homogenization of the inflowing starting material and the reaction mixture with the aid of a suitable stirrer. To maintain a homogeneous reaction mixture in the vessel, the mixing time should be at most 10 % of the time constant of the reaction (initial concentration divided by the reaction rate). Apart from the use of stirrers, it is also possible to mix the reactor contents by using jet mixers (injection of the circulating liquid) and loop reactors. 

En2yme techniques are primarily developed for commercial reasons, and so information about immobilisation and process conditions is usually Limited. A commercially available immobilised penicillin V acylase is made by glutaraldehyde cross-linking of a cell homogenate. It can be used ia batch stirred tank or recycled packed-bed reactors with typical operating parameters as iadicated ia Table 2 (38). Further development may lead to the creation of acylases and processes that can also be used for attaching side chains by ensymatic synthesis. 

Much of the basic theory of reaction kinetics presented in Sec. 7 of this Handbook deals with homogeneous reaclions in batch and continuous equipment, and that material will not be repeated here. Material and energy balances and sizing procedures are developed for batch operations in ideal stirred tanks—during startup, continuation, and shutdown—and for continuous operation in ideal stirred tank batteries and plug flow tubulars and towers. 

The profiles of temperature and composition shown in Fig. 23-3 are not of homogeneous liqmd reactions, but are perhaps representative of all lands of reactions. Only in stirred tanks and some fluidized beds are nearly isothermal conditions practically attainable. 

The name continuous flow-stirred tank reactor is nicely descriptive of a type of reactor that frequently for both production and fundamental kinetic studies. Unfortunately, this name, abbreviated as CSTR, misses the essence of the idealization completely. The ideality arises from the assumption in the analysis that the reactor is perfectly mixed, and that it is homogeneous. A better name for this model might be continuous perfectly mixed reactor (CPMR). 

Homogeneous reactions are those in which the reactants, products, and any catalysts used form one continuous phase (gaseous or liquid). Homogeneous gas phase reactors are almost always operated continuously, whereas liquid phase reactors may be batch or continuous. Tubular (pipeline) reactors arc normally used for homogeneous gas phase reactions (e.g., in the thermal cracking of petroleum of dichloroethane lo vinyl chloride). Both tubular and stirred tank reactors are used for homogeneous liquid phase reactions. 

An Experimental Study Using Feed Perturbations for a Free-Radically Initiated Homogeneous Polymerization in a Continuous-Flow Stirred-Tank Reactor... 

Power input to a homogeneous reaction stirred tank is 0.5-1.5 HP/1000 gal, but three times this amount when heat is to be transferred. 

Ideal CSTR (continuous stirred tank reactor) behavior is approached when the mean residence time is 5-10 times the length of time needed to achieve homogeneity, which is accomplished with 500-2000 revolutions of a properly designed stirrer. 

Figure 5.3-4. Semibatch stirred-tank reactor with alternative feed lines homogeneous reaction.

The conditions under which the material is tested are crucial. The top of the exposure chamber must be sealed and the tank should contain no free air space. A stirring mechanism in the tank keeps the leachate mixture homogeneous and a heater block keeps it at an elevated temperature as required for the test. Stress conditions of the material in the field should also be simulated as closely as possible. The original U.S. EPA Method 9090 test included a rack to hold specimens under stress conditions but was revised when some materials shrank in the leachate. Due to the hazardous nature of the material, testing should be performed in a contained environment and safety procedures should be rigorously followed. 

In practice, it is often possible with stirred-tank reactors to come close to the idealized mixed-flow model, providing the fluid phase is not too viscous. For homogenous reactions, such reactors should be avoided for some types of parallel reaction systems (see Figure 5.6) and for all systems in which byproduct formation is via series reactions. 

The ideal continuous stirred tank reactor is the easiest type of continuous flow reactor to analyze in design calculations because the temperature and composition of the reactor contents are homogeneous throughout the reactor volume. Consequently, material and energy balances can be written over the entire reactor and the outlet composition and temperature can be taken as representative of the reactor contents. In general the temperatures of the feed and effluent streams will not be equal, and it will be necessary to use both material and energy balances and the temperature-dependent form of the reaction rate expression to determine the conditions at which the reactor operates. 

The physical situation in a fluidized bed reactor is obviously too complicated to be modeled by an ideal plug flow reactor or an ideal stirred tank reactor although, under certain conditions, either of these ideal models may provide a fair representation of the behavior of a fluidized bed reactor. In other cases, the behavior of the system can be characterized as plug flow modified by longitudinal dispersion, and the unidimensional pseudo homogeneous model (Section 12.7.2.1) can be employed to describe the fluidized bed reactor. As an alternative, a cascade of CSTR s (Section 11.1.3.2) may be used to model the fluidized bed reactor. Unfortunately, none of these models provides an adequate representation of reaction behavior in fluidized beds, particularly when there is appreciable bubble formation within the bed. This situation arises mainly because a knowledge of the residence time distribution of the gas in the bed is insuf-... 

An elegant example of a paired mediated reaction has been reported by Chaussard and Lahitte [69] EDF (Electricite de France), who use Cr(VI) generated at the anode of a divided cell to oxidize the methyl side chain of a nitro-aromatic and Ti(III) generated at the cathode to reduce the nitro group. The reduction step, due to the faster homogeneous rate can be performed within the cell, whereas the oxidation has to be performed in a stirred tank reactor. 

An excellent production figure for (R)-mandelonitrile (2400 g/1 per day) was achieved by Kragl et al. [105] using a continuously stirred tank reactor in which an ultrafiltration membrane enables continuous homogenous catalysis to occur from an enzyme (PaHnl) which is retained within the reaction vessel. In order to quench the reaction the outlet of this vessel was fed into a vessel containing a mixture of chloroform and hydrochloric acid, which allowed for accurate product analysis. 

In the continuous stirred tank reactor (CSTR) instant mixing to achieve a homogeneous reaction mixture is assumed so that the composition throughout the reactor is uniform. During the reaction, monomer is fed into the system at the same rate as polymer is withdrawn. The heat problem is somewhat diminished because of the constant removal of heated products and the addition of nonheated reactants. 

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