Displacement desorption

Desorption Displacement desorption Gas separation adsorption Liquid separation adsorption Pressure swing adsorption Thermal swing adsorption asbestos fibers, 3 304 for bioremediation, 3 782 capillary condensation, 1 585, 591 dessicants and, 3 375-377 detergency and, 3 428 129 in dispersions, 3 707 filler measurement via, 22 571 general separation heuristics for,... 

Displacement damage function, 14 436 Displacement desorption, factors governing choice of method, l 614t Displacement plating, 24 141 Displacements per atom (dpa), 14 436 Displays... 

Displacement desorption Good for strongly held species avoids risk of cracking reactions during regeneration avoids thermal aging of adsorbent Product separation and recovery needed (choice of desorbent is crucial)... 

A more sophisticated development of the same general principle is the Sorbex process, developed by UOP, which is illustrated in Fig. 14. In this system a single fixed adsorbent bed is divided into a number of discrete sections, and the feed, desorbent, raffinate, and extract lines are switched through the bed by a rotary valve. The process operates essentially isothermally with regeneration of the adsorbent by displacement desorption. There are four distinct zones in the bed, with changes in liquid flow rate between zones. Each zone consists of several sections (Fig. 14). 

Displacement desorption, in which the adsorbate is driven off by a more strongly adsorbing species, entails an additional regeneration cycle to desorb the expelling agent and can thus only be justified for recovery of a high-value adsorbed intermediate. 

Displacement desorption. The method is essentially the same as for purge gas stripping but the regeneration stream is adsorbed and displaces the adsorbed species... 

Only displacement desorption is used for paraxylene separation processes, which is carried out in continuous countercurrent systems. 

Both adsorption and regeneration are operated periodically. The following methods can be considered for regeneration Thermal Swing Adsorption (TSA), Pressure Swing Adsorption (PSA), inert gas stripping, and displacement desorption. 

Liquid Adsorption. Adsorption takes place selectively because of (1) polarity differences between adsorbate and unadsorbed liquid, or (2) differences in size and structural characteristics of molecules. In the first category we may mention activated carbon, in the second one the molecular sieves. Liquid adsorption is suitable for both recovery of small amounts from dilute solutions, and for bulk separations. Purification is feasible with very selective adsorbent. Recovery of adsorbate and regeneration of the adsorbent may be performed by (1) Thermal Swing Adsorption, (2) Pressure Swing Adsorption, (3) inert-purge swing, or (4) displacement desorption. 

Displacement Desorption Here, an additional desorption medium, which is preferably adsorbed by the adsorbent displaces the adsorbed component. The desorption medium has to be chosen in such a way, that it is easily separated from the adsorbate. Figure 4-21 shows an adsorption plant for the separation of a mixture of normal and iso-... 

Displacement Desorption. The temperature and pressure are maintained essentially constant, as in purge gas stripping, but instead of an inert purge the adsorbed species are displaced by a stream containing a competitively adsorbed species, as in displacement chromatography. The method is applicable to both gas and liquid systems. 

Steam stripping, which is widely used in the regeneration of solvent recovery systems using an activated carbon adsorbent, can be considered as a combination of thermal swing and displacement desorption. Vacuum desorption, which is used in some versions of the Union Carbide IsoSiv process for separation of medium-chain linear paraffins as well as in some air separation systems can be considered as a special case of pressure swing. 

Displacement desorption is useful for the separation of strongly adsorbed species where conditions of adsorbent stability or reactivity of the sorbate... 

Displacement desorption requires a somewhat more complex process scheme than either the pressure swing or thermal swing system and it is therefore generally used only in situations where the simpler methods fail. As an example of this type of process we consider the widely used adsorption separation of medium molecular weight (C,o-C,g) linear paraffins from mixtures with branched chain and cyclic isomers. 

Several different adsorption processes for the separation of linear paraffins have been developed including Ensorb (Exxon), IsoSiv (Union Carbide), T. S. F. (Texaco), the Shell process, and the Leuna Werke process. The latter has been called Parex (paraffin extraction) but the choice of name is unfortunate because of possible confusion with the UOP Parex process for separation of xylene isomers. All these processes use a 5A molecular sieve, generally in binderless form to minimize nonselective adsorption. The C,o-C,g linear paraffins are strongly adsorbed even at temperatures as high as 350°C. Thermal swing desorption is not feasible since the temperature required for desorption is so high that coking would occur. The alternatives are therefore vacuum desorption, which is used in some versions of the IsoSiv process, or displacement desorption which is used in most if not all of the other processes. 

Steam, which is introduced at the bottom of the bed. The hot adsorbent is returned to the top of the column by a gas lift system, and it is then cooled and dehydrated in the top section by heat exchange and contact with the purge gas stream. The process can thus be considered as a combined thermal swing-displacement desorption cycle. 

Sorbex is the generic name used by UOP for their simulated countercurrent sorption process which has been successfully developed for a variety of large-scale commercial separations. All Sorbex processes in current operation operate in the liquid phase, but in principle the process could also be applied to a vapor phase system. In order to understand the Sorbex process it is simplest to consider a true isothermal countercurrent displacement desorption system, as sketched in Figure 12.11. Such a system is similar in its essential features to the Hypersorption system but without the additional complexity of the thermal swing. 

FIGURE 12.11, Schematic diagram showing the roles played by the four sections of a Sorbex countercurrent displacement desorption process. 

The Sorbex system is in effect similar to a periodic countercurrent adsorption system with displacement desorption. The individual sections of the column function as separate beds so that moving the feed and product points... 

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