Reaction vessels, chemical engineering

The tasks of chemical engineers are the design and operation of chemical reactors for converting specific feed material or reactants into marketable products. They must have knowledge of the rates of chemical reactions involved, the nature of the physical processes interacting with the chemical reactions, and conditions which affect the process. The rates of the physical processes (mass and heat transfer) involved in commonly used chemical reactors can often be estimated adequately from the properties of the reactants, the flow characteristics, and the configuration of the reaction vessel. Chemical process rate data for most industrially important reactions cannot, however, be estimated reliably from theory and must be determined experimentally. 

Tipnis, S.K., Penney, W.R. and Fasano, J.B., 1994. An experimental investigation to detemiine a scale-up method for fast competitive parallel reactions in agitated vessels. American Institute of Chemical Engineers Symposium Series, 299, 78-91. 

In the majority of chemical processes heat is either given out or absorbed, and fluids must often be either heated or cooled in a wide range of plant, such as furnaces, evaporators, distillation units, dryers, and reaction vessels where one of the major problems is that of transferring heat at the desired rate. In addition, it may be necessary to prevent the loss of heat from a hot vessel or pipe system. The control of the flow of heat at the desired rate forms one of the most important areas of chemical engineering. Provided that a temperature difference exists between two parts of a system, heat transfer will take place in one or more of three different ways. 

The earth itself is the reaction vessel and chemical plant. The complicated reaction chemistry and thermodynantics involve ntixers, reactors, heat exchangers, separators, and flnid flow pathways that are a scrambled design by nature. Only the sketchiest of flowsheets can be drawn. The chemical reactor has complex and ill-defined geometry and must be operated in intrinsically transient modes by remote control. Overcoming these difficulties is a trae frontier for chemical engineering research. 

AIChemE (1992a) Emergency Relief Systems for Runaway Chemical Reactions and Storage Vessels (American Institute of Chemical Engineers, New York). 

Whenever chemical reactions occur these are the key to the process design. The engineer must be aware of what kinds of reactions are possible. He must also keep in mind that there are no such things as pure reactants, nor does the stream emerging from his reaction vessel ever contain just the desired product. Nearly always, a number of reactions occur and other products than those desired are produced. The engineer s purpose in investigating the reaction step is to increase the yields of desired products while reducing the quantity of unwanted substances. 

Le Chatelier s principle also predicts that the yield of ammonia is greater at higher pressures. High-pressure plants are expensive to huild and maintain, however. In fact, the first industrial plant that manufactured ammonia had its reaction vessel blow up. A German chemical engineer, Carl Bosch, solved this problem by designing a double-walled steel vessel that could operate at several hundred times atmospheric pressure. Modern plants operate at pressures in the range of 20 200 kPa to 30 400 kPa. 

In vitro multi-enzyme systems are set up by the combination of enzyme modules including pathway and even pathway-unrelated enzymes. Also, the synthesis of saccharides in combination with de novo enzymatic sugar synthesis can be accomplished. This so-called combinatorial biocatalysis can be performed in sequential reactors or in a one-pot reaction vessel which challenges further reaction engineering for optimization. Even the combination of an enzyme module with a chemical... 

The greatest safety hazard in chemical engineering operations is without question caused by uncontrolled chemical reactions, either within the chemical reactor or when flammable chemicals escape from storage vessels or pipes. Many undergraduate students are never exposed to the extremely nonlinear and potentially hazardous characteristics of exothermic free radical processes. 

This was developed at the Chemical Engineering Department of Valladolid University, Spain (see Fig. 9.4-8). In this type of reactor the temperature and pressure effects are isolated. This is achieved by using a cooled wall vessel, which is maintained near 400 C, and a reaction chamber where the reactants are mixed and reaction takes place. This reaction chamber is made of a special material to withstand the oxidizing effect of the reactants at a maximum temperature of 800 °C and a pressure of 25 MPa. It is enclosed in the main vessel, which is pressurized with the feed-stream before entering the reaction chamber, so that it works at about 400 °C and does not suffer from the oxidizing atmosphere. It is made of relatively thin stainless steel [15]. 

The design of chemical reactors encompasses at least three fields of chemical engineering thermodynamics, kinetics, and heat transfer. For example, if a reaction is run in a typical batch reactor, a simple mixing vessel, what is the maximum conversion expected This is a thermodynamic question answered with knowledge of chemical equilibrium. Also, we might like to know how long the reaction should proceed to achieve a desired conversion. This is a kinetic question. We must know not only the stoichiometry of the reaction but also the rates of the forward and the reverse reactions. We might also wish to know how much heat must be transferred to or from the reactor to maintain isothermal conditions. This is a heat transfer problem in combination with a thermodynamic problem. We must know whether the reaction is endothermic or exothermic. 

The mathematical definition of a chemical reaction rate has been a source of confusion in chemical and chemical engineering literature for many years. The origin of this confusion stems from laboratory bench-scale experiments that were carried out to obtain chettiical reaction rate data. These eai ly experiments were batch-type, in which the reaction vessel was closed and rigid consequently, the ensuing reaction took place at constant volume. The reactants were mixed together at time t - 0 and the concentration of one of the reactants, was measured at various times f. The rate of reaction was determined from the slope of a plot of as a function of time. Letting be the rate of formation of A per unit volume (e.g., g mol/s dm ), the investigators then defined and reported the chemical reaction rate as... 

Multiphase Reactors Reactions between gas-liquid, liquid-liquid, and gas-liquid-solid phases are often tested in CSTRs. Other laboratory types are suggested by the commercial units depicted in appropriate sketches in Sec. 19 and in Fig. 7-17 [Charpentier, Mass Transfer Rates in Gas-Liquid Absorbers and Reactors, in Drew et al. (eds.), Advances in Chemical Engineering, vol. 11, Academic Press, 1981]. Liquids can be reacted with gases of low solubilities in stirred vessels, with the liquid charged first and the gas fed continuously at the rate of reaction or dissolution. Some of these reactors are designed to have known interfacial areas. Most equipment for gas absorption without reaction is adaptable to absorption with reaction. The many types of equipment for liquid-liquid extraction also are adaptable to reactions of immiscible liquid phases. 

Achievements made wifhin fhe field of reaction engineering will increase fhe applicability of biocatalysts even more. For example, the use of microreactors is still in its infancy. Cascade catalysis and multi step conversions [81], a common domain of biocatalysis, will boost the application of biocatalysis for the transformation of highly reactive compounds or intermediates. Moreover, this might diminish operating time and costs as well as consumption of auxiliary chemicals and use of energy. For example, Bacher et al. published fhe six-step synthesis of labelled riboflavin using eight different enzymes in one reaction vessel [82]. 

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