Reactor design stirred-tank

Information about special design considerations like stirrer drives, sealings, and also seU-aerating stirrers can be found in Sittig (1983). Of all reactors the stirred-tank reactor is the most versatile to carry out different jobs. 

The mixing device is an important element in designing stirred tank reactors. Perry s handbook or the monograph of Tatterson (1991) can be consulted for the selection of mixers. A good presentation can be found in McCabe et al. (1992). 

Basic Design ofEnv me Reactors Under Ideal Conditions. Batch Reactor Continuous Stirred Tank Reactor Under Complete Mixing Continuous Packed-Bed Reactor Under Plug Flow Regime... 

The simplest flow-sheet for the reaction Aj o Aj is the RD column sequence with an external recycling loop shown in Fig. 5.1. The system as a whole is fed with pure Aj. According to the assumed relative volatility of the two components a > 1, the reaction product A2 is enriched in the column distillate product whereas the bottom product contains non-converted reactant Aj, which is recycled back to the reactor (continuous stirred tank reactor, CSTR, or plug flow tube reactor, PFTR). The process has two important operational variables the recycling ratio cp = B/F, that is the ratio of recycling flow B to feed flow rate F, and the reflux ratio of the distillation column R = L/D. At steady-state conditions, D = F since the total number of moles is assumed to be constant for the reaction Aj A2. As principal design variables, the Damkohler number. 

Wet Oxidation Reactor Design. Several types of reactor designs have been employed for wet oxidation processes. Zimpro, the largest manufacturer of wet oxidation systems, typically uses a tower reactor system. The reactor is a bubble tower where air is introduced at the bottom to achieve plug flow with controlled back-mixing. Residence time is typically under one hour. A horizontal, stirred tank reactor system, known as the Wetox process, was initially developed by Barber-Cohnan, and is also offered by Zimpro. 

Continuous-flow stirred-tank reactors ia series are simpler and easier to design for isothermal operation than are tubular reactors. Reactions with narrow operating temperature ranges or those requiring close control of reactant concentrations for optimum selectivity benefit from series arrangements. 

Two complementai y reviews of this subject are by Shah et al. AIChE Journal, 28, 353-379 [1982]) and Deckwer (in de Lasa, ed.. Chemical Reactor Design andTechnology, Martinus Nijhoff, 1985, pp. 411-461). Useful comments are made by Doraiswamy and Sharma (Heterogeneous Reactions, Wiley, 1984). Charpentier (in Gianetto and Silveston, eds.. Multiphase Chemical Reactors, Hemisphere, 1986, pp. 104—151) emphasizes parameters of trickle bed and stirred tank reactors. Recommendations based on the literature are made for several design parameters namely, bubble diameter and velocity of rise, gas holdup, interfacial area, mass-transfer coefficients k a and /cl but not /cg, axial liquid-phase dispersion coefficient, and heat-transfer coefficient to the wall. The effect of vessel diameter on these parameters is insignificant when D > 0.15 m (0.49 ft), except for the dispersion coefficient. Application of these correlations is to (1) chlorination of toluene in the presence of FeCl,3 catalyst, (2) absorption of SO9 in aqueous potassium carbonate with arsenite catalyst, and (3) reaction of butene with sulfuric acid to butanol. 

There are a variety of ways of accomplishing a particular unit operation. Alternative types of process equipment have different inherently safer characteristics such as inventory, operating conditions, operating techniques, mechanical complexity, and forgiveness (i.e., the process/unit operation is inclined to move itself toward a safe region, rather than unsafe). For example, to complete a reaction step, the designer could select a continuous stirred tank reactor (CSTR), a small tubular reactor, or a distillation tower to process the reaction. 

Bourne, J.R. and Dell Ava, P., 1987. Micro- and macromixing in stirred tank reactors of different sizes. Chemical Engineering Research and Design, 65, 180-186. 

Mixing Models. The assumption of perfect or micro-mixing is frequently made for continuous stirred tank reactors and the ensuing reactor model used for design and optimization studies. For well-agitated reactors with moderate reaction rates and for reaction media which are not too viscous, this model is often justified. Micro-mixed reactors are characterized by uniform concentrations throughout the reactor and an exponential residence time distribution function. 

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. 

The reactor can have a number of designs depending on individual requirements but in general both stirred tank and packed column reactors are viable, glass is the normal material of construction. After leaving the reactor, the bromine is separated from the aqueous solution and purified by standard techniques to get reagent ready for recycle into the process. Having removed bromide from the effluent stream and only added water and a small amount of sulphate, the final effluent is much cleaner. 

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