Cyclic separation process

To develop a cyclic separation process, our next step v as to find a suitable desorbent. A suitable desorbent must allov selective adsorption and yet must desorb at a reasonable rate. Three aromatic desorbents v ere evaluated benzene, toluene, and o-xylene. 

The three main modes of chromatographic operation are elution chromatography, selective adsorption/desorption, and simulated countercurrent chromatography. Of these, elution chromatography, used as a cyclic batch process, was the first to be developed for large-scale separations. 

A gas mixture consisting of two components is separated using one or more adsorption beds The beds are used in a cyclic process composed of steps involving column pressurization, depressurization (blowdown) and flow through the columns at constant pressure Analysis of such separation processes in terms of a cell model proceeds as follows ... 

The primary requirement for an economic adsorption separation process is an adsorbent with sufficient selectivity, capacity, and life. Adsorption selectivity may depend either on a difference in adsorption equilibrium or, less commonly, on a difference in kinetics. Kinetic selectivity is generally possible only with microporous adsorbents such as zeolites or carbon molecular sieves. One can consider processes such as the separation of linear from branched hydrocarbons on a 5A zeolite sieve to be an extreme example of a kinetic separation. The critical molecular diameter of a branched or cyclic hydrocarbon is too large to allow penetration of the 5A zeolite crystal, whereas the linear species are just small enough to enter. The ratio of intracrystalline diffusivities is therefore effectively infinite, and a very clean separation is possible. 

FIGURE 8 Schematic diagram showing the two basic modes of operating an adsorption separation process (a) cyclic batch two-bed system (b) continuous countercurrent system with adsorbent recirculation. Concentration profiles through the adsorbent bed are indicated. Component A is more strongly adsorbed than B. (Reprinted with permission from Ruthven, D. M. (1984). Principles of Adsorption and Adsorption Processes, copyright John Wiley Sons, New York.)... 

TABLE II Examples of Cyclic Adsorption Separation Processes... 

The selection of a stationary phase is extremely important in GC since it is the major controllable variable of selectivity in the separation process. Stationary phases can be non-polar, polar, or of intermediate polarity. Cyclodextrins, cyclic oligosaccharides composed of varying numbers of glucopyranose units, were found recently to be extremely useful for the separation of chiral compounds. Three types of derivatives, 5-hydroxypropyl (hydrophilic), dialkyl (hydrophobic), and trifiuoroacetyl (intermediate) have been used, each of these phases having a selected area of specificity. 

Because snsnXioseparation proved to be impossible for DIBOA and DIMBOA attempts to accomplish at least an cnantiodifferentiation by means of the chiral solvating agent (CSA) NMR technique have been undertaken because NMR is based upon a rapid measuring and differentiation process in comparison to the chromatographic separation processes. The NMR discrimination of enantiomeric cyclic hemiacetals and methyl acetals was not described yet. On principle, a pair of... 

The SMB model consists of partial differential equations (PDFs) for the concentrations of chemical components, restrictions for the connections between different columns and cyclic steady-state constraints (Kawajiri and Biegler, 2006b). Previously, the SMB processes have been usually optimized with respect to one objective only. Recently, multi-objective optimization has been applied in periodic separation processes (Ko and Moon, 2002), in gas separation and in SMB processes (Subramani et al., 2003). Ko and Moon used a modified sum of weighted objective functions to obtain a representation of the Pareto optimal set. Their approach is valid for two objective functions only. On the other hand, Subramani et al. applied EMO to a problem where they had two or three objective functions. 

P. C. Wankai, Cyclic Separation Techniques, in A. E, Rodrigues and D. Tondeur (Eds.), Percolation Processes, Theory and Applications, pp. 443-516, Sijtholf and Noondholf, Alphen aan den Rijn, Netherlands. 1981. 

FIGURE 9.13. Experimental concentration and temperature at the bed outlet for thermal swing cyclic separation of n and isopentane on 5A molecular sieve. The process operates between two points on opposite sides of (such as B" and A in Figure 9.1 lo) and the trajectory during the... 

Large-scale adsorptive separation processes may be conveniently divided into two broad classes cyclic batch systems, in which the adsorbent bed is alternately saturated and regenerated in a cyclic manner, and continuous flow systems, generally involving continuous countercurrent contact between feed and adsorbent. The distinction between these two basic modes of operation is shown schematically in Figure 11.1. The present chapter is restricted to processes which operate in the cyclic mode while continuous contacting systems are discussed in Chapter 12. 

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