System separation

5 SIZE EXCLUSION CHROMATOGRAPHY AND CONFINEMENT 2.5.1 Separation System 

Size exclusion chromatography (SEC) has been widely used since its introduction during the 1960s. It offers a simple yet unbiased method to characterize the molecular weight distribution of a polymer. Although it uses a flow system, the separation principle and the analysis are based on a static property of the polymer molecules in solution. We briefly look at the separation system here before learning the principle. 

The liquid that comes off the column is called the eluent. A detector with a flow cell is placed downstream to measure the concentration (mass/volume) of the polymer in the eluent. A differential refractometer is most commonly used to measure 

Fignre 2.56 shows the signal intensity of the detector plotted as a function of retention time (t ), the time measured from the injection of the polymer solution. The retention volume (Vr), the cumulative volume of the fluid out of the column since the injection, can also be used for the abscissa. The curve is called a retention curve or a chromatogram. The height of a point on the curve above the baseline is proportional to the concentration at a given retention time. The signal maximizes at the peak retention time (Ip). The integral of the curve is proportional to the total 

SEC has other names. When the mobile phase is an organic solvent, SEC is also called gel permeation chromatography (GPC). When it is aqueous, SEC is also called gel filtration chromatography (GEC) or aqueous GPC. 

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Figure 1.46 shows a flowsheet without any heat integration for the different reactor and separation system. As before, this is probably too inefficient in the use of energy, and heat integration schemes can be explored. Figure 1.5 shows two of the many possible flowsheets. 

The synthesis of the correct structure and the optimization of parameters in the design of the reaction and separation systems are often the single most important tasks of process design. Usually there are many options, and it is impossible to fully evaluate them unless a complete design is furnished for the outer layers of the onion. For example, it is not possible to assess which is better. 

Figure 1.5 A different reactor design leads not only to a different separation system but also to additional possibilities for heat integration. (From Smith and Linnhoff, Trans. IChemE, ChERD, 66 195, 1988 reproduced by permission of the Institution of Chemical Engineers.)...

An excess of ethylene is used to ensure essentially complete conversion of the chlorine, which is thereby eliminated as a problem for the downstream separation system. 

In a single reaction (where selectivity is not a problem), the usual choice of excess reactant is to eliminate the component which is more difficult to separate in the downstream separation system. Alternatively, if one of the components is more hazardous (as is chlorine in this example), again we try to ensure complete conversion. 

Despite these problems, a choice of separation system must be made and the design progressed further before it can be properly assessed. 

Assume initially that a phase split can separate the reactor effluent into a vapor stream containing only hydrogen and methane and a liquid stream containing only benzene, toluene, and diphenyl and that the liquid separation system can produce essentially pure products. 

Figure 4.9 shows a plot of Eq. (4.12). As the purge fraction a is increased, the flow rate of purge increases, but the concentration of methane in the purge and recycle decreases. This variation (along with reactor conversion) is an important degree of freedom in the optimization of reaction and separation systems, as we shall see later. 

Figure 4.12 The reaction-separation system for the production of butadiene sulfone.

Whether heat integration is restricted to the separation system or allowed with the rest of the process, integration always benefits from colder reboiler streams and hotter condenser streams. This point is dealt with in more general terms in Chap. 12. In addition, when column pressures are allowed to vary, columns with smaller temperature differences are easier to integrate, since smaller changes in pressure are required to achieve suitable integration. This second point is explained in more detail in Chap. 14. 

Westerberg, A. W., The Synthesis of Distillation-Based Separation Systems, Comp. Chem. Eng., 9 421, 1985. 

Also, if there are two separators, the order of separation can change. The tradeoffs for these two alternative flowsheets will be different. The choice between different separation sequences can be made using the methods described in Chap. 5. However, we should be on guard to the fact that as the reactor conversion changes, the most appropriate sequence also can change. In other words, different separation system structures become appropriate for different reactor conversions. 

We should be on guard for the fact that as the reactor conversion changes, the most appropriate separation sequence also can change. In other words, different separation system structures become appropriate for different reactor conversions. 

Once the process route has been chosen, it may be possible to synthesize flowsheets that do not require large inventories of materials in the process. The design of the reaction and separation system is particularly important in this respect, but heat transfer, storage, and pressure relief systems are also important. 

Waste also can be minimized if the separation system can be made more efiicient such that useful materials can be separated and recycled more effectively. 

It is often possible to use the energy system inherent in the process to drive the separation system for us by improved heat recovery and in so doing carry out the separation at little or no increase in operating costs. 

The synthesis of reaction-separation systems. The recycling of material is an essential feature of most chemical processes. The use of excess reactants, diluents, or heat carriers in the reactor design has a significant effect on the flowsheet recycle structure. Sometimes... 

An alternative way of deriving the BET equation is to express the problem in statistical-mechanical rather than kinetic terms. Adsorption is explicitly assumed to be localized the surface is regarded as an array of identical adsorption sites, and each of these sites is assumed to form the base of a stack of sites extending out from the surface each stack is treated as a separate system, i.e. the occupancy of any site is independent of the occupancy of sites in neighbouring stacks—a condition which corresponds to the neglect of lateral interactions in the BET model. The further postulate that in any stack the site in the ith layer can be occupied only if all the underlying sites are already occupied, corresponds to the BET picture in which condensation of molecules to form the ith layer can only take place on to molecules which are present in the (i — l)th layer. 

McCabe-Thiele diagrams for nonlinear and more practical systems with pertinent inequaUty constraints are illustrated in Figures 11 and 12. The convex isotherms are generally observed for 2eohtic adsorbents, particularly in hydrocarbon separation systems, whereas the concave isotherms are observed for ion-exchange resins used in sugar separations. 

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