Reactor concentration

In the preceding section, the choice of reactor type was made on the basis of which gave the most appropriate concentration profile as the reaction progressed in order to minimize volume for single reactions or maximize selectivity for multiple reactions for a given conversion. However, after making the decision to choose one type of reactor or another, there are still important concentration effects to be considered. 

When more than one reactant is used, it is often desirable to use an excess of one of the reactants. It is sometimes desirable to feed an inert material to the reactor or to separate the product partway through the reaction before carrying out further reaction. Sometimes it is desirable to recycle unwanted byproducts to the reactor. Let us now examine these cases. 

Single irreversible reactions. An excess of one feed component can force another component toward complete conversion. As an 

An excess of ethylene is used to ensure essentially complete conversion of the chlorine, which is thereby eliminated as a problem for the downstream separation system. 

In a single reaction (where selectivity is not a problem), the usual choice of excess reactant is to eliminate the component which is more difficult to separate in the downstream separation system. Alternatively, if one of the components is more hazardous (as is chlorine in this example), again we try to ensure complete conversion. 

Single reversible reactions. The maximum conversion in reversible reactions is limited by the equilibrium conversion, and conditions in the reactor are usually chosen to increase the equilibrium conversion  

Example 6.6 Ethyl acetate can be produced by the esterification of acetic acid with ethanol in the presence of a catalyst such as sulfuric acid or an ion-exchange resin according to the reaction  

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In a catalytic reactor, concentrations and temperature change along the flow path of the reactants, and in some cases also normal to the flow. The sum of all these changes over the catalyst-filled volume in time will give the production of the reactor. There are several methods to account for all these changes, illustrated on Figure 8.1.1. 

Simulation of a semibatach reactor concentrations versus time of the type A + B —kl—> C... 

Simulritiun of a nonisothermal batch reactor Concentration versus time... 

In contrast to the first two reactors, concentrations within the tubular flow reactor are... 

Program REFRIG2 calculates the dynamic behaviour and generates a phase-plane plot for a range of reactor concentrations and temperatures. 

ILLUSTRATION 12.5 MASS TRANSFER IN A FIXED BED REACTOR—CONCENTRATION GRADIENTS BETWEEN THE BULK FLUID AND THE EXTERNAL CATALYST SURFACE... 

Fig. 6. Dynamical phase diagram of the ascorbic acid/copper(II)/oxygen system in a CSTR in the kf — [Cu2+]0 plane. Fixed reactor concentrations [H2Asc]0 = 5.0x10 4M [H2SO4]0 = 6.0 x 10-5 M [Na2SO4]0 = 0.04M. Symbols O, steady state , oscillations , bistability. The asterisk ( ) marks the Takens-Bogdanov point. Strizhak, P. E. Basylchuk, A. B. Demjanchyk, I. Fecher, F. Shcneider, F. W. Munster, A. F. Phys. Chem. Chem. Phys. 2000, 2, 4721. Reproduced by permission of The Royal Society of Chemistry on behalf of the PCCP Owner Societies.

Chlorination of oleic acid dissolved in carbon chloride was tested in a flow reactor at 12.8 C with the tabulated results (Roper, Chem Eng Sci 2 27, 1953). Chlorine (A) and oleic acid (B) were dissolved separately in CC14 and mixed in the liquid phase at the inlet to the reactor. Concentrations are gmol/liter and time is in seconds. Check a second order mechanism. 

Fig. 4 The reactor concentrations for the conditions of Fig. 3. Note that the inlet concentrations depend on inlet temperature.

The CSTR model, on the other hand, is based on a stirred vessel with continuous inflow and outflow (see Fig. 1.2). The principal assumption made when deriving the model is that the vessel is stirred vigorously enough to eliminate all concentration gradients inside the reactor (i.e., the assumption of well stirred). The outlet concentrations will then be identical to the reactor concentrations, and a simple mole balance yields the CSTR model equation ... 

Let us choose a feedforward control system that holds both reactor temperature T and reactor concentration Cj conslant at their steadystate values, f and. The feed flow rate F and the jacket temperature Tj are the manipulated variables. Disturbances are feed concentration C o and feed temperature 7. ... 

Concentration of component / in a fed-batch reactor Concentration of component / in the feed stream Concentration of component/ in compartment i of a continuous reactor... 

Equation (100) applies also to a continuous-flow reactor in which the contents experience no back-mixing (equiveilent to plug flow) time has been eliminated from the expressions. When complete back-mixing is achieved in a continuous-flow reactor, concentration gradients are absent and we have... 

Starting with a sucrose concentration AO = 1.0 millimol/liter and an enzyme concentration Ceo = 0.01 millimol/liter, the following kinetic data are obtained in a batch reactor (concentrations calculated from optical rotation measurements) ... 

A complete mix reactor is one with a high level of turbulence, such that the fluid is immediately and completely mixed into the reactor. The outflow concentration and the reactor concentration are equal, and the diffusion term is zero due to the gradient being zero. Figure 6.1 shows an illustration of the concept. If we make the entire reactor into our control volume, then a mass balance on the reactor gives Rate of = Flux rate - Flux rate - - Source — sink... 

Influent, effluent, and reactor concentrations - of ozone in the gas phase ... 

In the present discussion, emphasis will be placed on the control of continuous reactors, concentrating on the several examples of Figure 3.19 in the order of the letter designations of individual figures used there. 

Fig. 1.25. Reaction in series—batch or tubular plug-flow reactor. Concentration Cr of intermediate product P for consecutive first order reactions, A -> P -> Q...

Figure 2.62 gives results over a range of reactor volumes. The reactor volume that minimizes TAC ( 1,117,000 per year) is 20 m3, giving a recycle flowrate D of 0.1241 kmol/s. Figure 2.63 gives the values of variables and parameters for the 10 m3 reactor process. The reactor concentration is Ca = 2.099 kmol/m3 (z = 0.262 mole fraction A). The column has 16 trays. The distillate composition is 0.3304 mole fraction A. Energy cost is 427,400 per year. The capital cost of the column is 123,000. The capital cost of the... 

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