Non-isothermal operation

With exothermic reactions, it may be necessary for reasons of safety to control the temperature rise in the reactor and in this case coils of tubing are normally provided through which cold fluid is circulated (see Fig. 1). On the other hand, with endothermic reactions, it may be necessary to heat the reactants to ensure a reasonable rate of reaction. Heating may also be required to enable a viscous mixture of reactants to be stirred without excessive power consumption. 

If the reactant temperature changes, then it is no longer possible to treat the rate coefficient fe as a constant during integration of the design equation and the energy balance must be considered as well as the mass balance. 

Since batch reactions usually involve liquids and solids, the differences between AH and AU and Cp and Cy are normally very small. 

If the mean specific heat capacity does not change appreciably during reaction, rearranging eqn. (34) and integrating leads to the result 

Equation (35) must now be solved simultaneously with the design equation (5). 

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Equation 5-16 is the time required to aehieve a eonversion Xa for either isothermal or non-isothermal operation. Equation 5-16 ean also be expressed in terms of eoneentration at eonstant fluid density as... 

The most comprehensive simulation of a free radical polymerization process in a CSTR is that of Konopnicki and Kuester (15). For a mechanism which includes transfer to both monomer and solvent as well as termination by combination and disproportionation they examined the influence of non-isothermal operation, viscosity effects as well as induced sinuoidal and square-wave forcing functions on initiator feed and jacket temperature on the MWD of the polymer produced. 

Despite the aforementioned shortcomings the experimental data obtained using commercial autoclaves can be kinetically inteipreted in spite of non-isothermal operation, as shown further on in this chapter. Kinetic expressions obtained in this manner will rather be interpolation equations than equations reflecting real reaction mechanisms. 

The importance of dispersion and its influence on flow pattern and conversion in homogeneous reactors has already been studied in Chapter 2. The role of dispersion, both axial and radial, in packed bed reactors will now be considered. A general account of the nature of dispersion in packed beds, together with details of experimental results and their correlation, has already been given in Volume 2, Chapter 4. Those features which have a significant effect on the behaviour of packed bed reactors will now be summarised. The equation for the material balance in a reactor will then be obtained for the case where plug flow conditions are modified by the effects of axial dispersion. Following this, the effect of simultaneous axial and radial dispersion on the non-isothermal operation of a packed bed reactor will be discussed. 

In addition, reactor operations are also classified by the way their temperature (or heat transfer) is controlled. Three operational conditions are commonly used (i) isothermal operation—the same temperatures exist throughout the reactor, (ii) adiabatic operation—no heat is transferred into or out of the reactor, and (iii) non-isothermal operation— the operation is neither isothermal nor adiabatic. 

Figure 5.8. Cyclohexane conversion as function of Da and Pe numbers at 493 K. The sweep gas/feed gas ratio is 4.23. (a) Isothermal operation, (b) Non-isothermal operation, with a parabolic temperature profile between 463 K (reactor s ends) and 493 K (center), (c) different regions observed. The selectivity considered was ten times the Knudsen. From Moon and Park [5.33], with permission of Elsevier Science.

For non-isothermal operation the heat balance equation must be considered. It is easy to demonstrate that the equation giving the temperature profile may be written as ... 

Ching, C.B. and Ruthven, D.M. (1986) Experimental study of a simulated counter-current adsorption system - IV. Non-Isothermal operation. Chem. Eng. Sci., 41, 3063. 

Free-radical polymerizations are highly exothermic. A typical adiabatic temperature rise for bulk (mass) polymerization of 200-500°C may not be uncommon. The overall activation energy for polymerization is in the order of 80 15 kj mol . The dramatic increase in the heat load during the gel-effect period can result in loss of temperature control, non-isothermal reactor operation and potential rimaways. Non-isothermal operation, aside from safety concerns, can also adversely affect product quality. 

Batch adsorber easy to set up and collection of non-isothermal operation... 

The cooling duties required for the PSR are comparable to those for the PSA unit in the PSA PFR option and are much lower than those required for both PFR units (both isolated and in series with a PFR). The cooling strategy is important to guarantee reactor stability and prevent runaways. For a PFR, reaction rates can only be controlled over the catalyst temperature. In contrast, in the PSR a kind of self control is exerted by the fact that reactants need to desorb prior to being able to react. This renders the PSR far less sensible to temperature rises than the PFR. As a result the non-isothermally operated PSR can attain superior selectivity over the PFR for a wider operating regime. 

New windows of opportunity for perfusive structured catalysts (not discussed in this chapter). Flow-through monoliths where the internal mass/heat transfer rates are enhanced due to the presence of convection were also considered. Non-isothermal operation [144] and design rules for maximum transport enhancement [17,145] are among the most relevant results. 

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