Day29 - CE 561 Lecture Notes Fall 2009 Day 29 The fixed bed...

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CE 561 Lecture Notes Fall 2009 p. 1 of 10 Day 29: The fixed bed catalytic reactor The fixed bed catalytic reactor is one of the most widely used reactor types in the refining and petrochemicals industry. In its simplest form, it is just a tube filled with solid catalyst, through which gaseous (or, less frequently, liquid) reactants flow and are converted into products. When a new process is to be developed, this is often the first type of reactor to be considered. Froment and Bischoff (p. 392) present a list of large-scale refining and petrochemical processes that are carried out in fixed bed reactors. On the pages following that list, they list key factors that have led to improvements in these processes. Note that many of these are not due to the kind of reactor engineering that we learn in this course, but on ‘common sense’ or infrastructure improvements. An example of such an advance is the use of larger multitube reactors, which was made possible by improved welding techniques for making them and by increased shipping clearance for moving them to the plant site. Nevertheless, we will focus on the kinetics and reactor modeling in such systems. Advances in these areas have also made important contributions. Froment and Bischoff’s chapter on fixed bed reactors is quite good, and we will follow their book more closely than usual in this section. One of the first questions that we must address in designing a fixed bed reactor is if and how we will add or remove heat to and from the reactor. The simplest choice is to use an adiabatic reactor. Because heat transfer is not an issue in this case, the reactor can be a single vessel of relatively large diameter that will require no utilities during steady state operation, and only a single catalyst bed will be required. Unfortunately, many reactions of interest cannot be successfully carried out in a single adiabatic reactor. If the reaction is sufficiently endothermic, then the temperature in the reactor will drop as the reaction proceeds, and the reaction may become unacceptably slow before the desired amount of reaction has occurred. For the case of exothermic reactions, the adiabatic reaction temperature may be higher than economically acceptable reactor materials can withstand, or high temperatures may lead to unfavorable equilibria or production of unwanted byproducts. The next-simplest choice is to use a series of adiabatic reactors with interstage heating or cooling. Examples of such reactors are given on pages 395 and 396 of Froment and Bischoff. This allows us to add heat to an endothermic reaction or remove it from an exothermic reaction while still having separate heat exchangers and reactors, or separate heat exchange and reaction sections within the same reactor. Finally, we could have continuous heat addition or removal through the wall of the fixed bed reactor.
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Day29 - CE 561 Lecture Notes Fall 2009 Day 29 The fixed bed...

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