Gas Adsorption Cycles

Purging of the adsorbate with an inert gas at much reduced pressure is feasible in high pressure adsorption plants. The adsorption of Example 15.2, for instance, is conducted at 55 atm, so that regeneration could be accomplished at a pressure of only a few atmospheres without heating. If the adsorbate is valuable, some provision must be made for recovering it from the desorbing gas. 

Ignition of adsorbents in external furnaces is practiced to remove some high molecular weight materials that are difficult to volatilize. This is done, for example, for reactivation of carbon from water treating for trace removal of impurities such as phenol. Caustic solution can convert the phenol into soluble sodium phe-nate in readily disposable concentrated form as an alternate process for regeneration. 

Displacement of the adsorbate with another substance that is in turn displaced in process is practiced, for instance, in liquid phase recovery of paraxylene from other Cg aromatics. In the Sorbex process, suitable desorbents are toluene and paradiethylbenzene. This process is described later. 

Commercial processes have been arbitrarily divided into two groups purifications and bulk separations. In purification processes, relatively dilute streams of adsorbate (e.g., 10 weight percent or less) are absorbed, whereas in bulk separations, 10 weight percent or more is adsorbed. Different adsorption cycles are used for each group. 

Adsorption of w-Hexane from a Natural Gas with Silica Gel 

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Pressure-Swing Cycle. In a gas-adsorption cycle, the partial pressure of an adsorbate can be reduced by reducing the total pressure of the gas This change can be used to effect a desorption. The lower the total pressure of the desorbing step, the greater the maximum delta loading will be ... 

Less propane and butanes are produced compared to natural gas Hquids by the adsorption process than are obtained normally for the same gas by the oil-absorption process. Because adsorption efficiency increases with a decrease in temperature, the adsorption cycle should operate at the lowest temperature that is economically feasible. 

Solid-Bed Dehydration. Sihca gel, bauxite, activated alurnina, or molecular sieves can be used for removing dissolved water to meet propane specifications. The soHd-bed dehydrators are used in a cycHc adsorption process. After an adsorption cycle has completed, the bed is heated with a purge gas or a vaporized Hquid-product stream for regeneration. If the latter is used, the Hquid product is condensed, separated from the free water, and returned to the process. After the beds are regenerated, they are cooled and returned to the adsorption cycle. 

Adsorption-Control Equipment If a gas stream must be treated for a short period, nsnally only one adsorption unit is necessary, provided, of course, that a sufficient time interval is available between adsorption cycles to permit regeneration. However, this is usually not the case. Since an nninternipted flow of treated gas is often required, it is necessary to employ one or more units capable of operating in this fashion. The units are designed to handle gas flows without interruption and are charac terized by their mode of contact, either staged or continuous. By far the most common type of adsorption system used to remove an objectionable pollutant from a gas stream consists of a number of fixed-bed units operating in such a sequence that the gas flow remains nninternipted. A two- or three-bed system is nsn ly... 

In the adsorption cycle, the wet inlet gas flows downward through the tower. The adsorbable components are adsorbed at rates dependent on their chemical nature, the size of their molecules, and the size of the pores. The water molecules are adsorbed first in the top layers of the desiccant bed. Dry hydrocarbon gases are adsorbed throughout the bed, As the upper layers of desiccant become saturated with water, the water in the wet gas stream begins displacing the previously adsorbed hydrocarbons in the lower desiccant layers. Liquid hydrocarbons will also be absorbed and will fill pore spaces that would otherwise be available for water molecules. 

Operating cycles for liquid contacting processes such as ion exchange are somewhat more complex than those for gas adsorption. They consist of these steps ... 

The earlier PSA units typically utilized 2,3, or 4 adsorber beds in a cyclic sequence in which one of the beds was always on an adsorption step while the other bed or beds were being regenerated. However, these systems were inherently inefficient for two reasons. First, the blowdown from adsorption pressure to the low waste pressure caused large losses of the desired product gas which was stored in the bed at high pressure. Second, some pure product gas was cycled to waste since it was used to purge a regenerating bed. 

Gas-adsorption processes Involve the selective concentration (adsorption) of one or more components (adsorbates) of a gas (or vapor) at the surface of a microporous solid (adsorbent) The attractive forces causing the adsorption are generally weaker than those of chemical bonds and are such that, by Increasing the temperature of the adsorbent or reducing an adsorbate s partial pressure, the adsorbate can be desorbed The desorption step Is quite Important in the overall process First, desorption allows recovery of adsorbates In those separations where they are valuable, and second, It permits reuse of the adsorbent for further cycles ... 

Commercial gas-adsorption processes (see Table I) can be divided into bulk separations, in which about 10 weight percent or more of a stream must be adsorbed, and purifications, in which usually considerably less than 10 weight percent of a stream must be adsorbed. Such a differentiation is desirable to make because in general different process cycles are used for the different categories, as will be discussed later. 

A sample data plot is shown in Figure 8. Feed gas is 40% N2 and 60% CH4. A mass balance is calculated for CH4 and N2 for each step in the process as well as for the complete adsorption cycle. The mass balance includes a Van der Waals calculation of the amount of gas stored in column voids and the volume of gas adsorbed on the zeolite. Typically, the independently calculated mass balances for CH4 and N2 were 100 3%. 

Fig. 4 shows experimental profiles of the mean temperature, the pressure and the flow rate during charges of the cycles 5, 100 and 700. Whatever the cycle, during the charge performed at high flow rate, the exothermic adsorption of the different components of natural gas entails an increase of the mean temperature of the vessel (Fig. 4a). The range in variation of the mean temperatures is the consequence of the coupling between the power delivered by the gas adsorption and the heat transfer inside the composite block. The decrease of this range during the charge as a function of the cycle number is a first indication of the evolution of the...

A purge-swing adsorption cycle is usually considered to be one in which desorption takes place at the same temperature and total pressure as adsorption. Desorption is accomplished either by partial-pressure reduction using an inert gas purge or by adsorbate displace-... 

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