Stirred-tank reactor blending

The technical feasibility of a relatively low-pressure (less than 1000 psig) and low-temperature (less than 100°C) process for the hydrogenation of depolymerized (ammonolysis) Nylon-6,6 and/or a blend of Nylon-6 and -6,6 products has been described. While Raney Co 2724 showed little or no sign of deactivation during the semi-batch hydrogenation of the ammonolysis products, before and after C02 and NH3 removal, Raney Ni 2400 showed signs of deactivation even in the presence of caustic. Raney Co 2724 proved to be an effective and robust catalyst in a continuous stirred tank reactor study. 

Stirred tank geometries, for mixing and blending, 16 669-671 Stirred tank reactor(s) (STR), 9 660, 15 697-698... 

Caprolactam is also manufactured from toluene (Fig. 2) by oxidation of toluene (with air) to benzoic acid at 160°C and 10 atm pressure. The benzoic acid is then hydrogenated under pressure (16 atm) and 170°C in a series of continuous stirred tank reactors. The cyclohexane carboxylic acid is blended with oleum and fed to a multistage reactor, where it is converted to caprolactam by reaction with nitrosyl sulfuric acid. 

The objective of the following model is to investigate the extent to which Computational Fluid Mixing (CFM) models can be used as a tool in the design of industrial reactors. The commercially available program, Fluent , is used to calculate the flow pattern and the transport and reaction of chemical species in stirred tanks. The blend time predictions are compared with a literature correlation for blend time. The product distribution for a pair of competing chemical reactions is compared with experimental data from the literature. 

A stirred tank reactor 3 ft in diameter with a 12-in. flat-blade turbine has been used for a batch reaction in which the blending time of added reagents is considered critical. Satisfactory results were obtained with a stirrer speed of 400 r/min. The same reaction is to be carried out in a tank 7 ft in diameter, for which a 3-ft standard turbine is available, (a) What conditions would give the same blending time in the larger tank ... 

The ideal continuous stirred tank reactor is a reactor with well-stirred and back-mixed contents. As a result, instant blending of the feed with the reactor contents is assumed to occur. The composition of the contents of the reactor is uniform throughout the reactor. Consequently, the exit stream from the reactor has the same composition and temperature as the reactor contents. 

Now that the fundamental principles have been reviewed, consider the apphca-tion of mixing miscible liquids. Related topics Stirred Tank Reactors, Sections 6.27-6.29. For the blending of miscible liquids the main characteristic is to create flow or pumping. However, there is a tradeoff between pumping and power. 

The polymerization section consists of a series of continuously stirred tank reactors (24,25). The solvent (25) (usually cyclohexane) and monomers are fed into the reactor and an initiator as well as a randomizer [usually tetrahydrofuran (THF)] is added. The function of the randomizer is to make sure that no blocks are formed from a single monomer (Hall, Oxolanyl Cyclic Acetals as Anionic Polymerization Modifiers 68). This is because the reactivity ratios of the monomers are not ideal. Usually the reaction temperature is kept less than 110°C to prevent deactivation of the growing chain ends (25). Once polsrmerization is complete, alcohol is added to terminate the polymerization reaction and the polymer solution is transferred to holding tanks to be blended to increase imiformity. Subsequent steps of washing and filtering remove all the unreacted monomers, THF, and other chemicals. 

An important question for the design of continuous flow systems is When can the classic perfectly mixed assumption (ideal CSTR) be used in a continnons flow stirred tank reactor The blend time concept can be used here. If the blend time is small compared to the residence time in the reactor, the reactor can be considered to be well mixed. That is because the residence time is proportional to the characteristic chemical reaction time. A 1 10 ratio of blend time to reaction time is often used, but often, larger values result because the mixer must do other jobs, which lead to even smaller blend times. Frequently, residence time distributions are used to determine whether a reactor is well-mixed. It is usually easy to achieve well-mixed conditions in continuous flow, turbulent stirred vessels unless the reactions are very fast, such as acid-base neutralizations. Even in laminar systems the blend time can be made much less than the required residence time for the chemical reaction mainly because required residence times are so long for high viscosity reactants. For discussions of residence time distribution analysis, see Chapter 1, Levenspiel (1972), and Nauman (1982). 

In a stirred reactor, it takes time for liquid added at the surface or at any point in the tank to become blended with the bulk of the liquid. The mixing process can be followed by observing the color change after a basic solution with an indicator is neutralized by suddenly adding a slight excess of acid. If this test is done in a 2-liter vessel, most of the solution will appear free of base in less than a second, but wisps of color may persist for 2-3 seconds until the mixing is complete. If the same test is carried out in a 5000-liter... 

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