Reactor variables

Reactor Variable Study. Assuming that the kinetic models are valid, we have a means to rapidly explore the effects of making certain changes in the catalyst or in the operating conditions. Fortunately, Wisseroth published the results for two runs at 100 C and two more runs at 20 atm in his Table 3 (1 ). 

Although both of these models provide a reasonable description of the precipitation polymerization process, they do not illustrate the relationship between the reactor variables and the polymer particle properties. 

Let us consider the steady-state behavior of the reactor in Example 1 under a constant input, [Foe,Fje]- The reactor variables will reach an equilibrium point, [C e, Te, Tje], vanishing the derivative terms in Eq.(15). In the following, incremental variables are considered ... 

Figure 6-12 shows plots of CA(f) and T(t) for different initial reactor variables. We choose CAi = 0 (starting the reactor with pure solvent) and also Ca = 2.0 (starting the... 

Figure 12-1 Sketch of two- and three-phase reactors. Variables must be specified in each phase a, and y.

Once the initiation has occurred and polymerization has begun, the MW of the product can be controlled by several reactor variables. For example, the effect of reactor temperatrue is shown in Fig. 6. Raising the temperature of polymerization greatly enhances the rate of termination, i.e., it makes the metal-polymer bond less stable and more inclined to undergo -elimination. But its effect on the propagation rate is minor in comparison, so the result is shorter chains and increased MI. 

In conclusion to this section, research in the RTD area is always active and the initial concepts of Danckwerts are gradually being completed and extended. The population balance approach provides a theoretical framework for this generalization. However, in spite of the efforts of several authors, simple procedures, easy to use by practitioners, would still be welcome in the field of unsteady state systems (variable volumes and flow rates), multiple inlet/outlet reactors, variable density mixtures, systems in which the mass-flowrate is not conserved, etc... On the other hand, the promising "generalized reaction time distribution" approach could be developed if suitable experimental methods were available for its determination. 

Additional considerations are necessary when the control structure of reactors is to be determined. Reactors are usually highly nonlinear and poorly modeled. Therefore, approaches like those discussed previously are rarely suitable. Exact, offset-free control of all relevant reactor variables is neither possible nor necessary. Usually, keeping the variables in a certain operating range is sufficient. This is often accomplished in an indirect manner—for example, by eliminating as many disturbances as possible at their source. Shinnar [22] has proposed a new philosophical definition of the reactor control problem, a definition that has yet to be put on a quantitative basis and in algorithmic form. 

Note that only temperature and partial pressures or concentrations have been mentioned as variables. For a catalytic reaction, these are the only variables of importance from a kinetics standpoint. Time variables like space-velocity, space-time, or residence-time, are reactor variables rather than reaction variables, and will be involved in the development of the reactor simulation model. 

Our first choice for setting production rate should be to alter one of these variables in the reactor. The variable we select must be dominant for the reactor. Dominant reactor variables always have significant effects on reactor performance. For example, temperature is often a... 

Fixing a flowrate in a recycle stream does not conflict with our discussion of picking a dominant reactor variable for production rate control in Step 4. Flow controlling a stream somewhere in all recycle loops is an important simple part of any plantwide control strategy. 

To control the economic objectives we must have measurements and manipulated variables. However, in the example reactors we have looked at so far it should be clear that tve have only a limited number of manipulated variables, especially after we have taken care of the heat management issues. How is it then possible to achieve any level of economic control of a reactor The answer lies in a concept introduced by Shinnar (1981) called partial control. In short it means that only a few dominant variables in the process (e.g., temperatures, key components) are identified, measured, and controlled by feedback controllers. Then, by varying the setpoints for the dominant variables, it becomes possible to position the process such that all the important economic variables stay within acceptable ranges. We will elaborate more on this important concept in the next section but first we introduce the classification of reactor variables used by Shinnar. 

Allowable Spread in Residence Time and Effect of Reactor Variables on Residence Time Distribution... 

Polymerization process control can benefit significantly from using online state estimation techniques. In general, online control of polymer properties such as molecular weight, MWD, copolymer composition, MI, density, etc. is difficult, mainly because of the lack of adequate online or in-process sensors. Therefore, many of these polymer property parameters are controlled indirectly by controlling first-level process variables such as temperature, pressure, and the flow rates of various reactants, solvents, and catalysts. When some deviations in polymer properties are detected through laboratory sample analysis, certain reactor variables need to be adjusted. Extensive plant experience might be required to make such process adjustments, or model-based online state estimator can be used. 

Click OK. The correspondence between the computer variables and the reactor variables is ... 

A few experiments have been carried out in the laboratory scale with a one litre hydrolysis vessel, connected to a small impeller pump and a Sartorius laboratory module fitted with DDS GR6-P membranes (0.2 m ). However, the flow resistance in this module was too large, and it was soon concluded that a resonably constant flux was unattainable. Despite these difficulties, the qualitative behaviour of the reactor variables could be predicted from the model and verified experimentally. For example, with decreasing flux DH increased, but the rate of the base consumption decreased, while the protein concentration in the permeate remained quite stable as predicted. The hydrolysate was evaluated and found comparable in quality to ISSPH produced in the batch process. These results have encouraged us to continue the work in pilot plant with the DDS-35 module, where we can expect considerably more favourable flow conditions. The first experiments carried out so far indicate that a reasonable flux in the order of 50 1/m /h (approx. 1 1/m /min.) can be attained but that foaming problems necessitate the construction of pressurized air free reactor. Future studies will therefore be needed to produce a complete experimental verification of the derived model. 

The space time is longer than the mean residence time dne to the gas expansion in the reactor (variable volnme system). 

This means that the aggregate size distribution mirrors the turbulence spectrum of the reactor. Variables that determine mixing are reactor design, number and design of baffles, impeller design, power input, feed concentration, feed rate, location and number of inlet tubes, and so on. The position of the inlet tube(s) and the conditions near these feed points are also important and generally the solutions should be introduced close to the agitator [23, 24]. 

Table 4.5 Common instrumentation required in fluidized-bed reactors. Variable measured Provision/instrument Comments...

The plug flow reactor (PFR) model is used to describe chemical reactions in continuous, flowing systems. The PFR model is used to predict the behaviour of chemical reactors, so that key reactor variables, such as the dimensions of the reactor, can be estimated. PFR s are also sometimes called Continuous Tubular Reactors (CTR s). 

Defenses against common cause failures all involve "diversity" of one kind or another. One form, called equipment diversity, involves use of instruments operating on different principles to measure the same reactor variable. Use of different kinds of components in the amplifying and scram logic systems leading from the... 

Figure 1.6 Schematic describing intimate interrelationships that exist among the reactor variables, rheological properties, processing variables, and physical/mechanical properties of polymer products.

Models of this type are sometimes useful in optimization when the real relations are complex such as in polymerization reactors. There it is often hard to express desired properties of the final product in terms of reactor variables (ik) ... 

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