Adsorption cyclic batch

Most methods of separating molecules in solution use direct contact of immiscible fluids or a sohd and a fluid. These methods are helped by dispersion of one phase in the other, fluid phase, but they are hindered by the necessity for separating the dispersed phase. Fixed-bed adsorption processes overcome the hindrance by immobilizing the solid adsorbent, but at the cost of cyclic batch operation. Membrane processes trade direct contact for permanent separation of the two phases and offer possibilities for high selectivity. 

Cyclic aromatic disulfides, polymerization reactions of, 23 706 Cyclic (arylene) disulfides, 23 712 Cyclic batch adsorption processes, 1-613 Cyclic bis(arylene tetrasulfide)s, 23 712 Cyclic carbon, polymer materials with, 15 177... 

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. 

Adsorption Separation and Purification Processes. Adsorption processes can be classified according to the flow system (cyclic batch or continuous countercurrent) and the method by which the adsorbent is regenerated. The Iwo basic flow schemes arc illustrated in Figure 3 The cyclic batch scheme is simpler but less efficient. It is generally used where selectivity is relatively high. Countercurrent or simulated countercurrent schemes arc more expensive in initial cost and arc generally used only for difficult separations in which selectivity is limited or mass-transfer resistance is high. 

Industrial-scale adsorption processes can be classified as batch or continuous. In a batch process, die adsorbent bed is saturated and regenerated in a cyclic, operation. In a continuous process, a countercurrent staged contact between lire adsorbent and die feed and desorbent is established by cidier a true or a simulated recirculation of die adsorbent. The efficiency of an adsorption process is significantly higher in a eoiuinuous mode of operation than in a cyclic batch mode. For difficult separations, batch operation may require 25 times more adsorbent inventory and twice die desorbent circulation rate than does a continuous operation. In addition, in a batch mode, the four functions of adsorption, purification, desorption, and displacement of the desorbent from the adsorbent are inflexibly linked, wtiereas a continuous mode allows mure degrees of freedom with respect to these functions, and thus a better overall operation. 

The general mode of operation of a cyclic batch adsorption process is illustrated in Fig. 8. In its simplest form such a process employs two adsorbent beds, each of which is alternately saturated and regenerated. During the saturation or adsorption cycle, adsorption is continued until the mass transfer zone has almost reached the bed outlet. At this point the beds are switched so that the spent bed is replaced by a freshly regenerated bed, while the more strongly absorbed species is removed from the spent bed in... 

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.)... 

Fig. 17. The two basic modes of operation for an adsorption process (a) cyclic batch system (b) continuous countercurrent system with adsorbent...

A fundamental understanding of sorption processes requires a detailed mechanistic knowledge of the equilibria, kinetics, and dynamics of the sorption process. The FSR is a cyclic batch process for which adsorption is carried out at a relatively higher pressure and desorption (regeneration) is accomplished at a lower pressure, generally using part of the product from the adsorption... 

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. 

Cyclic batch adsorption processes differ from each other mainly in the methods by which the adsorbent is regenerated during the desorption cycle (see Table 11.1). Four basic methods are in common use although combinations of two or more methods may also be used with advantage in particular situations. 

Steady-state operation of a continuous countercurrent adsorption system may be conveniently discussed in terms of the simple McCabe-Thiele analysis. The simplest type of continuous countercurrent process is illustrated in Figure 12.2, and it is evident that any of the separation processes discussed in Chapter 11 could, in principle, be carried out in such a system, rather than in a cyclic batch process. 

Periodic countercurrent systems are widely used in ion exchange and water purification systems for the removal of trace concentrations of organic components with beds of activated carbon.At low sorbate concentrations the adsorption isotherms for many organic pollutants are linear or only slightly favorable so that the mass transfer zone is wide, and in a simple two-bed cyclic batch system the LUB would therefore be uneconomically large. For such systems, in order to obtain an economic process, some form of counter-current operation is necessary in order to reduce the adsorbent inventory. 

FIGURE 12.10. Comparison of continuous countercurrent, periodic countercurrent, and cyclic batch adsorption systems showing effectiveness of adsorbent utilization as a function of bed length. (From ref. 15, reprinted with permission.)... 

In the Molex process the Sorbex system is used for the separation of linear and branched hydrocarbons, using as adsorbent 5A molecular sieve. This is the same adsorbent as is used in the cyclic batch processes such as Ensorb and Isosiv. In contrast to the situation with the Cg aromatics, the separation factor is very large and it seems unlikely that for such an easy separation the increased cost of a Sorbex unit is justified by the reduction in adsorbent inventory and/or desorbent circulation rate. Detailed cost comparisons do not appear to have been published, but it seems likely that any economic advantage which the Molex process may have arises more from the energy savings associated with liquid phase operation than from the intrinsic advantages of a countercurrent adsorption system. 

Large-scale adsorption processes can be divided into two broad classes. The first and most important is the cyclic batch system, in which the adsorption fixed bed is alternately saturated and then regenerated in a cyclic manner. The second is a continuous flow system which involves a continuous flow of adsorbent countercurrent to a flow of feed. 

There are four basic methods in common use for the cyclic batch adsorption system using fixed beds. These methods differ from each other mainly in the means used to regenerate the adsorbent after the adsorption cycle. In general, these four basic methods operate with two or sometimes three fixed beds in parallel, one in the adsorption cycle, and the other one or two in a desorbing cycle to provide continuity of flow. After a bed has completed the adsorption cycle, the flow is switched to the second newly regenerated bed for adsorption. The first bed is then regenerated by any of the following methods. 

The simulation reported here consists of the following sequential operating procedure applied to an initially evacuated adsorber (I) charge up to P = 10 bar, (II) constant-pressure feed, and (III) discharge down to F = 1.25 bar. This example encompasses the major steps of every cyclic batch adsorption process for gas separation, in which regeneration of the bed is accomplished by reducing the pressure at essentially constant temperature, as is the case in pressure swing adsorption. The input parameters and output variables for each phase are listed in Table 1. 

PSA and VSA use a column filled with a molecular sieve, typically activated carbon, silicagel, alumina, or zeolite, for differential adsorption of the gases CO2 and H2O, alloeing CH4 pass through [47, 49]. The molecules are adsorbed loosely in the cavities of the molecular sieve and not irreversible bound [46]. It is a cyclic batch process where adsorption is performed on a relatively higher pressure (around 800 kPa) and desorption (regeneration) at lower pressure [51]. H2S,... 

Countercurrent operation of an adsorption column in which gaseous or liquid feed is passed continuously through a bed of adsorbent countercurrent to a flow of solid adsorbent is, in principle, more efficient than the previous descriptions of cyclic batch operations because countercurrent flow maximizes the average driving force for mass transfer between fluid and adsorbent. The saturated spent adsorbent emerging from the adsorber... 

Both batch and continuous adsorption processes are used. In a batch process, the adsorbent bed is allowed to become saturated with adsorbed material and is subsequently regenerated in a cyclic manner. In a continuous process, usually the counter-current mode is adopted for adsorption and desorption, either in time form or in simulated mode. Continuous operation offers many advantages with respect to the efficiency of adsorbent utilization. Thus, for... 

Industrial-scale adsorption processes can be classified as batch or continuous (53,54). In a batch process, the adsorbent bed is saturated and regenerated in a cyclic operation. In a continuous process, a countercurrent staged contact between the adsorbent and the feed and desorbent is established by either a true or a simulated recirculation of the adsorbent. 

Adsorption is usually done in a batch mode in fixed, agitated, or expanded bed adsorption units. The process usually consists of cyclic operations of adsorption and desorption in two steps. Desorption or recovery of the adsorbed solutes from the adsorbents can be done by either changing the temperature or pH, or using a solvent, which also regenerates the adsorbents for use in the next operation cycle. 

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