Reaclors

A useful classification of lands of reaclors is in terms of their concentration distributions. The concentration profiles of certain limiting cases are illustrated in Fig. 7-3 namely, of batch reactors, continuously stirred tanks, and tubular flow reactors. Basic types of flow reactors are illustrated in Fig. 7-4. Many others, employing granular catalysts and for multiphase reactions, are illustratea throughout Sec. 23. The present material deals with the sizes, performances and heat effects of these ideal types. They afford standards of comparison. 

In an ideal continuously stirred tank reaclor (CSTR), the conditions are uniform throughout and the condition of the effluent is the same as the condition in the tank. When a batteiy of such vessels is employed in series, the concentration profile is step-shaped if the abscissa is the total residence time or the stage number. The residence time of individual molecules varies exponentially from zero to infinity, as illustrated in Fig. 7-2>e. 

Batch reaclors are tanks, usually provided with agitation and some mode of heat transfer to maintain temperature within a desirable range. They are primarily employed for relatively slow reactions of several hours duration, since the downtime for filling and emptying large equipment may be an hour or so. Agitation maintains uniformity and improves heat transfer. Modes of heat transfer are illustrated in Figs. 23-1 and 23-2. 

Except in the laboratoiy, batch reactors are mostly liquid phase. In semibatch operation, a gas of limited solubility may be fed in gradually as it is used up. Batch reaclors are popular in practice because of their flexibility with respect to reaction time and to the lands and quantities of reactions that they can process. 

Selectivity A significant respect in which CSTRs may differ from batch (or PFR) reaclors is in the product distribution of complex reactions. However, each particular set of reactions must be treated individually to find the superiority. For the consecutive reactions A B C, Fig. 7-5b shows that a higher peak value of B is reached in batch reactors than in CSTRs as the number of stages increases the batch performance is approached. 

Tubular flow reaclors operate at nearly constant pressure. How the differential material balance is integrated for a number of second-order reactions will be explained. When n is the molal flow rate of reactant A the flow reactor equation is... 

Frictional Pressure Drop Usually this does not have a significant effect on the reaclor size, except perhaps when the flow is two-phase. Some approximate relations fbe cited that are adequate for pressure-drop calculations of homogeneous flow reactions in pipelines. The pressure drop is given by... 

When the RTD of a vessel is known, its performance as a reaclor for a first-order reaction, and the range within which its performance will fall for other orders, can be predicted. 

Dispersion In tubes, and particiilarly in packed beds, the flow pattern is disturbed by eddies diose effect is taken into account by a dispersion coefficient in Fick s diffusion law. A PFR has a dispersion coefficient of 0 and a CSTR of oo. Some rough correlations of the Peclet number uL/D in terms of Reynolds and Schmidt numbers are Eqs. (23-47) to (23-49). There is also a relation between the Peclet number and the value of n of the RTD equation, Eq. (7-111). The dispersion model is sometimes said to be an adequate representation of a reaclor with a small deviation from phig ffow, without specifying the magnitude ol small. As a point of superiority to the RTD model, the dispersion model does have the empirical correlations that have been cited and can therefore be used for design purposes within the limits of those correlations. 

Simulations of the lands of reaclor intended for the pilot or plant scale. Howto do the scale-up to the plant size, however, is a sizable problem in itself. 

Flow Reactors Fast reactions and those in the gas phase are generally done in tubular flow reaclors, just as they are often done on the commercial scale. Some heterogeneous reactors are shown in Fig. 23-29 the item in Fig. 23-29g is suited to liquid/liquid as well as gas/liquid. Stirred tanks, bubble and packed towers, and other commercial types are also used. The operadon of such units can sometimes be predicted from independent data of chemical and mass transfer rates, correlations of interfacial areas, droplet sizes, and other data. 

Phthalic anhydride. Naphthalene is oxidized by air to phthalic anhydride in a Bubbling flmdized reaclor. Even though the naphthalene feed is in liquid form, the reaction is highly exothermic. Temperature control is achieved by removing heat through vertical tubes in the bed to raise steam [Graham and Way, Chem. Eng. Prog., 58, 96 (Januaiy 1962)]. 

Each group of molecules reacts independently of any other group, that is, as a batch reaclor. Batch conversion equations for power law rate equations are ... 

Numerical solutions of the maximum mixedness and segregated flow equations for the Erlang model have been obtained by Novosad and Thyn (Coll Czech. Chem. Comm., 31,3,710-3,720 [1966]). A few comparisons are made in Fig. 23-14. In some ranges of the parameters n or fte ihe differences in conversion or reaclor sizes for the same conversions are substantial. On the basis of only an RTD for the flow pattern, perhaps only an average of the two calculated extreme performances is justifiable. 

Eroducts of reaction, the membrane reaclor can make conversion eyond thermodynamic equilibrium in the absence of separation. 

The properties of the zeolite play a significant role in the overall performance of the catalyst. Understanding these properties increases our ability to predict catalyst response to changes in unit operation. From its inception in the catalyst plant, the zeolite must retain its catalytic properties under the hostile conditions of the FCC operation. The reaclor/regenerator environment can cause significant changes in chemical and structural composition of the zeolite. In the regenerator, for instance, the zeolite is subjected to thermal and hydrothermal treatments. In the reactor, it is exposed to feedstock contaminants such as vanadium and sodium. 

FIG. 70. Cross-sectional view of a expanding ihermal plasma deposilion reaclor. (Courtesy of M. C. M. van de Sanden. Eindhoven University of Technoloav. Eindhoven. The Netherlands.)... 

Film imperfections are one of the more serious quality problems for both the producer and the film converter, Film imperfections can arise from many sources. Most commonly, they arc the result of contamination in the reaction system, m posl-reaclor handling, during shipping and unloading, or in the end users equipment. 

The analysis of this problem led to the solution illustrated below. reactor was constructed as a semibatch reaclor tsolvent initially chargfl into the reactor and reactant fed over time) and a heating and cooling cycle emphiyed (see Figure 9.3.4). The reactor u is operated and die results are shown in Figiirc- 9.3.5-9.3.7. 

Figure 13-19 Real reaclor modeled as a CSTR and PFR in series.

As discussed previously, because the reaction is first order, the conversion calculated in Example 13-5 would be valid for a reaclor with complete... 

Figure 14-18 Combinaiions of ideal reactors used to model real tubular reaclors, (a) two ideal PFRs in parallel (bi ideal PFR and ideal CSTR in parallel,...

Reaclor type Stirred tank/ Bubble column Tubular Fixed bed fricklc bed... 

From the reaclor coil, the solution passes into a (low-through photometer equipped with a 480-nm interference filler,... 

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