Steady-state reactor design

As a consequence of the complete mixing, a continuous flow stirred tank reactor also operates isothermally. Therefore, in the steady state it is not necessary to consider the mass and energy balances simultaneously. Optimum conditions may be computed on the basis of the material balance alone, and then afterwards the energy balance is used, in principle (see Sec. 10.4), to determine the external conditions required to maintain the desired temperature. 

When two perfectly mixed react Hs are connected in series, the mass balance for the second reactor is  

Note that V is the volume of one reactor. For n reactors in series 

These formulas may be used for the study of the kinetics of a first-order reaction by measuring x, C o. Fand V and then determining k. Alternately, for a given reaction, they can be used for determining the volume required to achieve a certain production. 

With the irreversible reaction A + B when equimolar quantities of A and B are fed to the reactor, the following equation is obtained  

When xai is eliminated by means of (10.2.2-3), so that the final conversion is written solely in terms of the conditions at the inlet (a o = 0), the following 

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Example 4.2 used the method of false transients to solve a steady-state reactor design problem. The method can also be used to find the equilibrium concentrations resulting from a set of batch chemical reactions. To do this, formulate the ODEs for a batch reactor and integrate until the concentrations stop changing. This is illustrated in Problem 4.6(b). Section 11.1.1 shows how the method of false transients can be used to determine physical or chemical equilibria in multiphase systems. 

Since we are dealing with the product of the two reactant concentrations, making them approximately equal is the best way to minimize reactor holdup. Thus steady-state reactor design favors compositions that are somewhat similar. From a dynamic viewpoint, the system can handle disturbances more easily if the concentrations of the two reactants are very different (very small zA and large zs). We saw an indication of this in the ternary process considered earlier. Control structure CS2 worked when the concentration of the limiting reactant wras very low, but failed when the concentration of the limiting reactant was in the 0.15 mole fraction region. 

Catalyst deactivation in large-pore slab catalysts, where intrapaiticle convection, diffusion and first order reaction are the competing processes, is analyzed by uniform and shell-progressive models. Analytical solutions arc provid as well as plots of effectiveness factors as a function of model parameters as a basis for steady-state reactor design. 

The important quantity for steady state reactor design is the effectiveness factor As an... 

The general material balance of Section 1.1 contains an accumulation term that enables its use for unsteady-state reactors. This term is used to solve steady-state design problems by the method of false transients. We turn now to solving real transients. The great majority of chemical reactors are designed for steady-state operation. However, even steady-state reactors must occasionally start up and shut down. Also, an understanding of process dynamics is necessary to design the control systems needed to handle upsets and to enable operation at steady states that would otherwise be unstable. 

Some of the alternative TOF instrument designs involve replacing the beryllium filter with either a crystal or a mechanical chopper to monochromate the incident beam. With this change, the spectrometer can be used with a higher incident neutron energy (typically E 50 meV) so that a smaller momentum transfer Q is possible for 5 the same energy transfer (21,22). With a monochromatic incident beam, a beryllium filter is sometimes substituted for the chopper after the sample in order to increase the scattered intensity but with a sacrifice in the,minimum Q attainable. Energy transfers up to 100 meV (800 cm" ) can be achieved with TOF spectrometers at steady state reactors before the incident neutron flux is limited by the thermal spectrum of the reactor. (With hot moderators such as at the Institut Laue-... 

The steady-state economic design of a process with this type of reaction requires consideration of the effects of reactor size and temperature on the entire plant. High recycle flowrates of A and a large reactor operating at a low temperature will suppress the production of D. But this will require a large capital investment in the reactor and separation sections of the plant and consume significant energy. 

The optimum steady-state economic design was determined with these new kinetic parameters, and the parameters are given in Table 7.4. The FS2 flowsheet is used with a ratio (2p,/2totai = 0.1. The impact of the kinetic parameters on the optimum design is striking. The hotter reaction requires a much larger recycle flowrate and a higher reactor inlet temperature for the same reactor exit temperature 7 ollt = 500 K. These lead... 

The main objective of this section is to provide a quantitative example in which the besf process is not the optimal steady-state economic design. In this example reactor system, temperature control is the measure of product quality. For this type of system, dynamic controllability is improved by increasing the heat transfer area in the reactor. 

The safety philosophy adopted for the SRS restart power limits is to operate at steady-state reactor power levels that will result in no significant core damage on a statistical basis, in the very unlikely event of a design basis event (DBE). 

The equations that have been developed for design using these pseudo constants are based on steady-state mass balances of the biomass and the waste components around both the reactor of the system and the device used to separate and recycle microorganisms. Thus, the equations that can be derived will be dependent upon the characteristics of the reactor and the separator. It is impossible here to... 

In the last part of Chapter 7.4 (Transient Studies) the experimental work on ethylene oxidation was shown. There the interest was to investigate what occurs and how fast, after a thermal runaway started. The previous chapter discussed the criteria of how to design reactors for steady-state operation so that runaways can be avoided. One more subject that needs discussion is what transient changes can cause thermal runaways. 

The design equations for a CSTR do not require that the reacting mixture has constant physical properties or that operating conditions such as temperature and pressure be the same for the inlet and outlet environments. It is required, however, that these variables be known. Pressure in a CSTR is usually determined or controlled independently of the extent of reaction. Temperatures can also be set arbitrarily in small, laboratory equipment because of excellent heat transfer at the small scale. It is sometimes possible to predetermine the temperature in industrial-scale reactors for example, if the heat of reaction is small or if the contents are boiling. This chapter considers the case where both Pout and Tout are known. Density and Q ut wiU not be known if they depend on composition. A steady-state material balance gives... 

Solution With Z>, = 0, a reaction wiU never start in a PFR, but a steady-state reaction is possible in a CSTR if the reactor is initially spiked with component B. An anal5dical solution can be found for this problem and is requested in Problem 4.12, but a numerical solution is easier. The design equations in a form suitable for the method of false transients are... 

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