Adsorption countercurrent adsorber

Alternatively, one can resort to the use of moving- or fluidized-bed systems in which the adsorbent, and sometimes also the catalyst, is continuously withdrawn from the reactor to undergo an external regeneration. Ideally, one tries to achieve countercurrent adsorbent flow pattern to optimize utilization of the adsorptive capacity. The major problems of such arrangements are those of solids handling (e.g., gas-tight... 

Nitrogen dioxide, N02, produced by a thermal process for fixation of nitrogen, is to be removed from a dilute mixture with air by adsorption on silica gel in a continuous countercurrent adsorber. The mass flow rate of the gas entering the adsorber is 0.50 kg/s it contains 1.5% N02 by volume, and 85% of the N02 is to be removed. Operation is to be isothermal at 298 K and 1 atm. The entering gel will be free of N02. If twice the minimum gel rate is to be used, calculate the gel mass flow rate and the composition of the gel leaving the process. The equilibrium adsorption data at this temperature are (Treybal, 1980) ... 

The problems posed by the motion of solid particles or a moving bed of particles in countercurrent adsorption processes have led to a number of different operational configurations. In a gas-fluidized multistage countercurrent adsorber, there are multiple sieve-tray plates in a vertical column. The adsorbent particles are fluidized by the gas stream to be cleaned entering through the perforations in the sieve tray. The fluidization level is not very vigorous. 

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

At the completion of adsorption, the less selectively adsorbed components have been recovered as product. However, a significant quantity of the weaMy adsorbed species are held up in the bed, especially in the void spaces. A cocurrent depressurization step reduces the bed pressure by allowing dow out of the bed cocurrendy to feed dow and thus reduces the amount of product retained in the voids (holdup), improving product recovery, and increases the concentration of the more strongly adsorbed components in the bed. The purity of the more selectively adsorbed species has been shown to depend strongly on the cocurrent depressurization step for some appHcations (66). A cocurrent depressurization step is optional because a countercurrent one always exists. Criteria have been developed to indicate when the use of both is justified (67). 

None of the selectively adsorbed components is removed from the adsorption vessel until the countercurrent depressurization (blowdown) step. During this step, the strongly adsorbed species are desorbed and recovered at the adsorption inlet of the bed. The reduction in pressure also reduces the amount of gas in the bed. By extending the blowdown with a vacuum (ie, VSA), the productivity of the cycle can be greatiy increased. 

The repressurization step returns the adsorber to feed pressure and completes the steps of a PSA cycle. Pressurization is carried out with product and/or feed. Pressurizing with product is done countercurrent to adsorption so that purging of the product end continues indeed it may be merely a continuation of the purge step but with the bed exit valve closed. Pressurizing with feed cocurrent to adsorption in effect begins adsorption without producing any product. 

Most adsorption systems use stationary-bed adsorbers. However, efforts have been made over the years to develop moving-bed adsorption processes in which the adsorbent is moved from an adsorption chamber to another chamber for regeneration, with countercurrent contacting of gases with the adsorbents in each chamber. Union Oil s Hypersorption Process (90) is an example. However, this process proved uneconomical, primarily because of excessive losses resulting from adsorbent attrition. 

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 cychc operation. In a continuous process, a countercurrent staged contact between the adsorbent and the feed and desorbent is estabhshed by either a tme or a simulated recirculation of the adsorbent. 

Additional stripping of the adsorbates from the adsorbent and purging of them from the voids can be accomplished by the addition or a purge step. The purge can begin toward the end of the depressurization or immediately afterward. Purging is accomphshed with a flow of produc t countercurrent to adsorption to provide a lower residual at the product effluent end of the bed. 

Continuous Countercurrent Systems Most adsorption systems use fixed-bed adsorbers. However, if the fluid to be separated and that used for desorption can be countercurrently contacted by a moving bed of the adsorbent, there are significant efficiencies to be realized. Because the adsorbent leaves the adsorption section essentially in equilibrium with the feed composition, the inefficiency of the... 

Although the continuous-countercurrent type of operation has found limited application in the removal of gaseous pollutants from process streams (Tor example, the removal of carbon dioxide and sulfur compounds such as hydrogen sulfide and carbonyl sulfide), by far the most common type of operation presently in use is the fixed-bed adsorber. The relatively high cost of continuously transporting solid particles as required in steady-state operations makes fixed-bed adsorption an attractive, economical alternative. If intermittent or batch operation is practical, a simple one-bed system, cycling alternately between the adsorption and regeneration phases, 1 suffice. 

Focusing discussions on carbon adsorption processes, in a pulsed-bed adsorber, the carbon moves countercurrent to the liquid. The effeet is of a number of staeked, fixed-bed eolumns operating in series. Spent earbon is removed from the bottom of... 

A countercurrent moving-bed adsorption column is used to remove benzene from a gaseous emission. Activated carbon is employed as the adsorbent. The flowrate of the gas is 1.2 kg/s and it contains 0.027 wt/wt% of benzene. It is desired to recover 99% of this pollutant. The activated carbon entering the column has 2 X 10 wt/wt% of benzene. Over the operating range, the adsorption isotherm (Yaws et al., 1995) is linearized to... 

Cocurrent depressurization, purge, and pressure-equalization steps are normally added to increase efficiency of separation and recovery of product. At the end of the adsorption step, the more weakly adsorbed species have been recovered as product, but there is still a significant amount held up in the bed in the inter- and intraparticle void spaces. A cocurrent depressurization step can be added before the blowdown step, which is countercurrent to adsorption. This increases the amount of product produced each cycle. In some applications, the purity of the more strongly adsorbed components has also been shown to be heavily dependent on the cocurrent depressurization step [Cen and Yang, Ina. Eng. Chem. Fundam., 25, 758-767 (1986)]. This cocurrent blowdown is optional because there is always a countercurrent one. Skarstrom developed criteria to determine when the use of both is justified [Skarstrom in Li, Recent Developments in Separation Science, vol. II, CRC Press, Boca Raton, pp. 95-106 (1975)]. 

Eluxyl A process for separating /7-xylene from its isomers, using an adsorbent-solvent technique. The process is based on simulated countercurrent adsorption where the selective adsorbent is held stationary in the adsorption column. The feed mixture to be separated is introduced at various levels in the middle of the column, as in the Sorbex process. The /r-xylene product can be more than 99.9 percent pure. Developed by IFP and Chevron Chemical. A large pilot plant was built in Chevron s site at Pascacougla, MS, in 1994 and a commercial plant on the site was announced in 1996, Since then, the process has been widely licensed. 

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