Separative stages

The most frequent application of phase-equilibrium calculations in chemical process design and analysis is probably in treatment of equilibrium separations. In these operations, often called flash processes, a feed stream (or several feed streams) enters a separation stage where it is split into two streams of different composition that are in equilibrium with each other. 

Figure 10.9 Incremental oil yield versus separator stages...

Capital and operating costs will increase as more separator stages are added to the process line, so a balance has to be struck between increased oil yield and cost. It is uncommon to find that economics support more than 3 stages of separation and one or two stage separation is more typical. The increased risk of separation shut down is also a contributing factor in limiting numbers. 

In the higher pressure sub-region, which may be extended to relative pressure up to 01 to 0-2, the enhancement of the interaction energy and of the enthalpy of adsorption is relatively small, and the increased adsorption is now the result of a cooperative effect. The nature of this secondary process may be appreciated from the simplified model of a slit in Fig. 4.33. Once a monolayer has been formed on the walls, then if molecules (1) and (2) happen to condense opposite one another, the probability that (3) will condense is increased. The increased residence time of (1), (2) and (3) will promote the condensation of (4) and of still further molecules. Because of the cooperative nature of the mechanism, the separate stages occur in such rapid succession that in effect they constitute a single process. The model is necessarily very crude and the details for any particular pore will depend on the pore geometry. 

Mobil MTG and MTO Process. Methanol from any source can be converted to gasoline range hydrocarbons using the Mobil MTG process. This process takes advantage of the shape selective activity of ZSM-5 zeoHte catalyst to limit the size of hydrocarbons in the product. The pore size and cavity dimensions favor the production of C-5—C-10 hydrocarbons. The first step in the conversion is the acid-catalyzed dehydration of methanol to form dimethyl ether. The ether subsequendy is converted to light olefins, then heavier olefins, paraffins, and aromatics. In practice the ether formation and hydrocarbon formation reactions may be performed in separate stages to faciHtate heat removal. 

Crystallization Kinetics. Crystallization of HDPE proceeds in two separate stages. During the first stage, HDPE crystaUizes rapidly. 

The Separation Stage. A fundamental quantity, a, exists in all stochastic separation processes, and is an index of the steady-state separation that can be attained in an element of the process equipment. The numerical value of a is developed for each process under consideration in the subsequent sections. The separation stage, which in a continuous separation process is called the transfer unit or equivalent theoretical plate, may be considered as a device separating a feed stream, or streams, into two product streams, often called heads and tails, or product and waste, such that the concentrations of the components in the two effluent streams are related by the quantity, d. For the case of the separation of a binary mixture this relationship is... 

For the case of separating a binary mixture, the following conventions are used. The concentrations of the streams are specified by the mol fraction of the desired component. The purpose of the separation process is usually to obtain one component of the mixture in an enriched form. If both components are desired, the choice of the desired component is an arbitrary one. The upflowing stream from the separation stage is the one in which the desired component is enriched, and by virtue of this convention, a is defined as a quantity the value of which is greater than unity. However, for the processes considered here, a exceeds unity by only a very small fraction, and the relationship between the concentrations leaving the stage can be written, without appreciable error, in the form... 

Fig. 1. The analogy between the separation stage and the transfer unit or equivalent theoretical plate (a) ia a stagewise process (b) ia a...

Fig. 2. Separation stages arranged to form a simple cascade. Terms are defined in text.

Agitated Line Mixer See Fig. 18-32. This device, which combines the features of orifice mixers and agitators, is used extensively in treating petroleum and vegetable oils. It is available in sizes to fit V2- to 10-in pipe. The device of Fig. 18-33, with two impellers in separate stages, is available in sizes to fit 4- to 20-in pipe. 

Conventional stoiehiometrie equations show the reaetants that take part and the produets formed in a ehemieal reaetion. However, there is no indieation about what takes plaee during this ehange. A detailed deseription of a ehemieal reaetion outlining eaeh separate stage is referred to as the meehanism. Meehanisms of reaetions are based on experimental data, whieh are seldom eomplete, eoneerning transition states and unstable intermediates. Therefore, they must to be eon-tinually audited and modified as more information is obtained. 

The efficiency of separation will obviously affect the purity of the liquid, and it may be necessary to provide a series of separate stages to meet the standard required by the specification. 

The syntheses of pyrido[3,2-d]pyrimidines from pyridines utilize a 2,3-disubstituted pjrridine and insert the C-2 and N-3 atoms, either previously linked (route i) or in separate stages (route ii). 

Multidimensional planar chromatographic separations, as we have seen, require not only a multiplicity of separation stages, but also that the integrity of separation achieved in one stage be transferred to the others. The process of separation on a two-dimensional plane is the clearest example of multidimensional separations. The greatest strength of MD-PC, when properly applied, is that compounds are distributed widely over two-dimensional space of high zone (peak) capacity. Another... 

The enantiomeric purity that can be obtained as a function of a for one, two, and three stages is given in Table 8-1. It is apparent that the higher the a value, the fewer the number of separations stages required to reach 99 % enantiomeric purity. For an a value of 5, the use of three stages allows one to obtain > 99 % purity. The required purity of the end-product defines the minimum performance requirement of the resin. 

Table 8-1. Enantiomeric purity obtained as a function of a values nonchromatographic systems. > 4 and separation stages for...

The vapor from the HPS flows to the primary absorber and the liquid is pumped to the stripper. The HPS is essentially a separation stage with an external cooler located between the primary stripper and absorber. In some units, they are a single tower. 

FIGURE 8.1 Schematic diagram of four interactive but separate stages of drug discovery and development. 

Telescope the Process by Combining Stages. This has been done successfully in the conversion of propylene to acrylonitrile by direct ammoxidation rather than oxidation to acrolein followed by reaction with ammonia in a separate stage, as was described in the earlier patent literature. The oxychlorination of ethylene and HC1 directly to vinyl chloride monomer is another good example of the telescoping of stages to yield an economic process. 

If a reaction may be instituted in separate stages, satisfying condition (a) or (b), then... 

A very important part of such an undertaking is to be clear about what stages of a chemical process generate the most waste. Often this is found to be the separation stage, after the transformation of reactants to products, where all the various components of the final mixture are separated and purified. Approaches to chemical reactions which help to simplify this step are particularly powerful. Such an approach is exemplified by heterogeneous catalysis. This is an area of chemistry where the catalysts used are typically solids, and the reactants are all in the hquid or gas phase. The catalyst can speed up the reaction, increase the selectivity of the reaction, and then be easily recovered by filtration from the liquid, and reused. 

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