Fractionated designs chemical reaction processes

Question (b) is a matter of chemical kinetics and reduces to the need to know the rate equation and the rate constants (customarily designated k) for the various steps involved in the reaction mechanism. Note that the rate equation for a particular reaction is not necessarily obtainable by inspection of the stoichiometry of the reaction, unless the mechanism is a one-step process—and this is something that usually has to be determined by experiment. Chemical reaction time scales range from fractions of a nanosecond to millions of years or more. Thus, even if the answer to question (a) is that the reaction is expected to go to essential completion, the reaction may be so slow as to be totally impractical in engineering terms. A brief review of some basic principles of chemical kinetics is given in Section 2.5. 

The favorable working conditions of the thin-film evaporator enabled the equally successful application of the thin-film principle for fractionation (5), absorption, chemical reactions (6), and drying (7, 8). In these processes and applications, the thin-film apparatus is used mainly to treat heat sensitive and lower-viscosity products that flow on the influence of gravity alone. This particular type of apparatus in a special design can also be used to process highly viscous products to... 

This case study presents the design of a biochemical process for NO removal from flue gases, where an absorber and a bioreactor are the main units. Based on a rough estimation of Hatta numbers, it was concluded that a spray tower offering a large G/L interfacial and a small liquid fraction is the best type of equipment, favoring the main chemical reaction. The bioreactor was chosen as a CSTR. 

Chemical reactions do not take place instantaneously, and indeed often proceed rather slowly. In such cases, it is not practical to design the reactor for complete conversion of the limiting reactant instead, the reactor effluent emerges with some of the limiting reactant still present and is then usually subjected to a separation process to remove the unconverted reactant from the product. The separated reactant is then recycled to the reactor inlet. The fractional conversion of a reactant is the ratio... 

For production volumes that lie below about 10001 a , distUlative separation is preferably performed batchwise. For small product quantities, batch distillation has the advantage of lower investment costs, since the individual fractions can be separated one by one in the same plant. It is highly flexible, since it can easily be combined with other process steps. If the distillation vessel is designed as a stirred tank, then other reaction steps such as dissolution of sohds, chemical reactions, distUlative solvent change, liquid-liquid extraction, evaporative crystallization, crystallization with cooling, and precipitation can be carried out in the distillation appartus. 

One would obtain the pseudo-stationary state concentrations for each of the intermediate chemical species from the thermodynamics of irreversible processes if it were supposed that the relaxation time for the production or destruction of each of these species is very small compared to the half-time for the overall chemical reaction of the fuel to go to the product molecules. In the pseudo-stationary states approximation, the net rate of formation, Kp of each of the intermediate chemical species by chemical reactions is set equal to zero. This provides exactly the right number of simultaneous algebraic equations to express the concentration of each of the chemical intermediates in terms of powers of the concentrations of the fuel and product molecules. For example, in the hypothetical chain system given by Eqs. (130), (131), and (132), the pseudo-stationary mole fraction of B (which we shall designate as x ) is the solution of the equation ... 

It is often desirable to investigate the intermediates without appreciably disturbing the reaction system. Methods such as optical spectroscopy, or, to a lesser degree, mass spectrometry, which affect only a small fraction of the total number of species present, satisfy this requirement. On the other hand, the chemical methods of Section 5 rely directly on the participation of intermediates in processes not concerned with the original reaction. In general, then, allowance must be made for the perturbation caused by the foreign substance, or the experiment designed so that the results are applicable to the system free from additive. 

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