Reactor design constraints

Although the design of a reactor system for the MTG process involves classical chemical engineering principles, the unique catalyst and reaction mechanisms impose important design constraints. These include the highly exothermic nature of the reaction, the need for essentially complete methanol conversion, steam deactivation of the catalyst, the "band-aging phenomena, and durene formation. 

A key parameter in reactor design is the ratio of reactants at the inlet. Usually, this constraint, imposed by the chemist or by the technologist, should be respected by the designer within a tight tolerance. Because phenol must enter the reactor only as vapor at 150 °C, where the phenol partial pressure is 0.42 atm, the minimum H2/phenol ratio should be about 4 1 if the total gas pressure is 2 atm. This value could drop to 3 1 if the temperature might be raised to 160 °C, or by lowering the pressure, or by dilution with an inert. Hence, it is good to keep in mind that the ratio of reactants at the reactor inlet is in many situations very different than the ratio of fresh reactants. 

However, design constraints may limit our ability to exercise this strategy concerning fresh reactant makeup, An upstream process may establish the reactant feed flow sent to the plant. A downstream process may require on-demand production, which fixes the product flowrate from the plant. In these cases, the development of the control strategy becomes more complex because we must somehow adjust the setpoint of the dominant variable on the basis of the production rate that has been specified externally. We must balance production rate with what has been specified externally. This cannot be done in an open-loop sense, Feedback of information about actual internal plant conditions is required to determine the accumulation or depletion of the reactant components. This concept was nicely illustrated by the control strategy in Fig. 2.16, In that scheme we fixed externally the flow of fresh reactant A feed. Also, we used reactor residence time (via the effluent flowrate)... 

The production of copolymers leads to some additional constraints to reactor design beyond what is required for homopolymer. The most important of these is composition drift. The reactivity ratios of a monomer mixture define the composition of a copolymer that is instantaneously produced from a given monomer mixture. This is true in a plug flow reactor or a backmixed reactor. However, in the plug flow reactor, the copolymer composition drifts from that produced from the initial monomer composition to that produced by the monomer composition at the end of the polymerization. In contrast, in the backmixed reactor, all copolymer produced is of the same composition, which... 

Oxidation of organic and inorganic species in aqueous solutions can find applications in fine chemical processes and wastewater treatment. Here, the oxidant, often either air or pure oxygen, must undergo all the mass transfer steps mentioned above in order for the reaction to proceed. During the last decade, increased environmental constraints have resulted in the application of novel processes to the treatment of waste streams. An example of such a process is wet air oxidation. Here, the simplest reactor design is the cocurrent bubble column. However, the presence of suspended organic and inert solids makes the use of monolith reactors favorable. 

The first commercial application of the Mobil Methanol-to-Gasoline (MTG) process has now been in operation for over a year and a half in the Gas-to-Gasoline (GTG) plant in New Zealand. The catalyst used in the process and the reaction mechanism impose important design constraints. This paper discusses the approach used for the scale-up and design of the fixed-bed MTG reactor system. Design philosophy and selection of equipment to meet the stipulated process and operating objectives are also reviewed. 

The concept of a highly automated scale-up process is enticing. One vision of such a process for homogeneous (liquid phase, non-catalytic) reactions starts with little more than a list of reactants, solvents, and desired products. Given this information as well as constraints imposed by economic, environmental, safety, and practical factors, a highly automated system could include both software and hardware components to generate an optimal reactor design. The necessary software components would ... 

The reactor design equations in this book can be applied to all components in the system, even inerts. When the reaction rates are formulated using Equation 2.8, the solutions automatically account for the stoichiometry of the reaction. This is the simplest and preferred approach, but it has not always been followed in this book. Several examples have ignored product concentrations when they do not affect reactions rates and when they are easily found from the amount of reactants consumed. Also, some of the analytical solutions have used stoichiometry to ease the algebra. The present section formalizes the use of stoichiometric constraints. We begin with a matrix formulation for the reaction rates of the components in multiple reactions. The presentation is rather elegant from a mathematical viewpoint and does have some practical utility. 

You have been assigned to model the reactor and decide which is the best Choice, Some of the design constraints that you must satisfy indude the production rate, the mimmal final conversion, the maximal tube length (it is fine to use reactor length less than 100 cm, one simply leaves the end of the tube empty of catalyst), the maximal inlet pressure, and the minimal outlet pressure. 

Can you meet these design constraints with all three of the catalysts Once you have met the constraints, you can optimize the reactor operation over the remaining design decision variables. What is your final choice of catalyst size, and at what nominal Inlet pressure will you run the reactor Indude a plot of the pressure, conversion, and Thiele modulus versus reactor length for your final design. 

Figure 13.38 presents the results of the dynamic simulation. Large production increase or decrease can be easily achieved, while the product purity is held on specification. Reaction selectivity remains high, so that the production of heavies is minimised. The constraint related to hydrogen / toluene ratio is satisfied most of the time. Note that the reactor design is such that the operation point is on the upper stable branch in Fig. 13.35. In contrast, if the design is on the unstable branch, close to the turning point, then operability problems appears (Bildea et al., 2002).

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