Shell and tube reactors

Five percent random error was added to the error-free dataset to make the simulation more realistic. Data for kinetic analysis are presented in Table 6.4.3 (Berty 1989), and were given to the participants to develop a kinetic model for design purposes. For a more practical comparison, participants were asked to simulate the performance of a well defined shell and tube reactor of industrial size at well defined process conditions. Participants came from 8 countries and a total of 19 working groups. Some submitted more than one model. The explicit models are listed in loc.cit. and here only those results that can be graphically presented are given. 

CSTRs, shell-and-tube reactors, and single-tube reactors, particularly a single adiabatic tube. Realistically, these different reactors may all scale similarly e.g., as but the dollar premultipliers will be different, with CSTRs being more expensive than sheU-and-tube reactors, which are more expensive than adiabatic single tubes. However, in what follows, the same capital cost will be used for all reactor types in order to emphasize inherent kinetic differences. This will bias the results toward CSTRs and toward shell-and-tube reactors over most single-tube designs. 

Why are the CSTRs worth considering at all They are more expensive per unit volume and less efficient as chemical reactors (except for autocatalysis). In fact, CSTRs are useful for some multiphase reactions, but that is not the situation here. Their potential justification in this example is temperature control. BoiUng (autorefrigerated) reactors can be kept precisely at the desired temperature. The shell-and-tube reactors cost less but offer less effective temperature control. Adiabatic reactors have no control at all, except that can be set. 

Design a shell-and-tube reactor that has a volume of 24 m and evaluate its performance as the reactor element in the process of Example 6.2. Use tubes with an i.d. of 0.0254m and a length of 5m. Assume components A, B, and C all have a specific heat of 1.9 kJ/(kg-K) and a thermal conductivity of 0.15W/(m-K). Assume 7 ,>, = 70°C. Run the reaction on the tube side and assume that the shell-side temperature is constant (e.g., use condensing steam). Do the consecutive, endothermic case. 

Extend Problem 6.12 to a two-zone shell-and-tube reactor with different shell-side temperatures in the zones. 

Shell-and-tube reactors may have dtldp = 3 or even smaller. A value of 3 is seen to decrease u dp/Dr by a factor of about 3. Reducing the tube diameter from Qdp to 3dp will increase Dr by a factor of about 10. Small tubes can thus have much better radial mixing than large tubes for two reasons R is lower and Dr is higher. 

All conventional reactors, tested before using the micro reactor (simply since micro reactors were hardly available at that time), only fulfilled the demands of one measure, at the expense of the other measures. For instance, a single-tube reactor can be operated nearly isothermally, but the performance of the oxidative dehydrogenation suffers from a too long residence time. A short shell-and-tube reactor provides much shorter residence times at improved heat transfer, which however is still not as good as in the micro reactor. 

Exothermic processes, with cooling through heat transfer surfaces or cold shots. In use are shell-and-tube reactors with smaii-diameter tubes, or towers with internal recirculation of gases, or muitipie stages with intercooiing. Chlorination of methane and other hydrocarbons results in a mixture of products whose relative amounts... 

Figure 10.1 Schematic diagram of a packed-bed membrane shell-and-tube reactor...

Dense palladium-based membranes. Shown in Table 10.1 are modeling studies of packed-bed dense membrane shell-and-tube reactors. All utilized Pd or Pd-alloy membranes except one [Itoh et al., 19931 which used yttria-stabilized zirconia membranes. As mentioned earlier, the permeation term used in Ihe governing equations for the tube and shell sides of the membrane is expressed by Equation (10-51b) with n equal to 0.5 [c.g., Itoh, 1987] or 0.76 [e.g., Uemiya et al., 1991]. 

Modeling studies of packed-bed dense membrane shell-and-tube reactors... 

Figure 10.6 compares the model and experimental results of direct thermal decomposition of CO2 using a dense yttria-stabilized zirconia membrane shell-and-tube reactor [Itoh et. al., 1993]. The agreement for the reactor conversion is very good. At a CO2 feed rate of less than 20 cm /min and with a membrane thickness of 2,(XX) pm the conversion is significantly enhanced by the use of a permselective membrane for oxygen. Beyond a feed rate of 20 cm /min., however, the difference in conversion between a membrane and a conventional reactor. 

Table 10.2 lists most of the published models of packed-bed (inert) porous membrane shell-and-tube reactors. The membrane materials used to validate the models are primarily porous Vycor glass or alumina membranes. 

Ethylbenzene dehydrogenation in a packed-bed shell-and-tube reactor using a porous membrane has been modeled by Mohan and Govind [1988b] and Wu and Liu [1992]. The kinetics for the reaction... 

Because the large heat of reaction must be removed, it is expected that a shell-and-tube reactor will be needed, with the catalyst packed inside the tubes. The heat duty for the reactor is... 

Small production units employ 4 inch diameter brass tubes 16 inches long, heated by electric resistor ribbons, while large production units require shell and tube reactors heated by a molten salt circuit. Organic heat exchange media are not recommended as at 250 °C they would undergo a fairly rapid thermal degradation. 

Figure 22.8 Sorption-reaction (SR) process for removal of trace organic contaminants (a) schematic drawing of a two column SR process, (b) shell and tube reactor configuration for the process, (c) isotherms for adsorption of trace vinyl chloride monomer (VCM) on an activated carbon.

Partial oxidation of hydrocarbons employing mixed metal oxides as catalysts comprises an economically important class of reactions for the upgrading of base feed stocks [3]. An illustrative example of it is the partial oxidation of o-xylene and/or naphthalene to phthalic anhydride (PA) with a world production of 3.2 million metric tons per year, industrially carried out in shell and tube reactors using air as the oxidizing agent [4]. 

The results in Table 6.5 show that isothermal piston flow is not always the best environment for consecutive reactions. The adiabatic reactor gives marginally better results, and its capital cost will probably be lower. However, both reactors as modeled here assume preheated feed. The adiabatic reactor would require a feed preheater. A shell-and-tube reactor might be used for both the preheating and the reaction, although the reactor would no longer be isothermal. This is a point for detailed design calculations. 

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