Heater

FIG. 16-55 Adsorbent wheels for gas separation: (a) horizontal with fixed beds; (b) vertical monolith. (Reprinted with permission of UOP.)

FIG. 16-56 Annular bed for liquid separation. [Reprinted with permission ofMarcel Dekker. Reference: Canon, Begovich, and Sisson, Sep. Sci. Technol., 15, 655-678 (1980).]

placed downward by opening a valve at the bottom and injecting a measured slug of carbon into the top of the vessel. The exhausted slug (a small fraction of the total charge in order to approximate fully coun-tercurrent operation very closely) is transferred to the sweeten-off tank, where residual product is displaced. It next is dewatered and fed to the regeneration furnace. The carbon is eventually returned to the adsorber where fluid flow is interrupted briefly to permit carbon transfer in, as in continuous ion exchanger systems.

Liquid chromatography has been used commercially to separate glucose from fructose and other sugar isomers, for recovery of nucleic acids, and other uses. A patent to Sanmatsu Kogyo Co., Ltd [Yoritomi, Kezuka, and Moriya, U.S. Patent number 4,267,054, 1981] presents an improved chromatographic process that is simpler to build and operate than simulated moving-bed processes. Figure 16-57 [Keller et al., gen. refs.] diagrams its use for a binary separation. It is a displacement-purge cycle where pure component cuts are recovered, while cuts that contain both components are recycled to the feed end of the column.

The UOP CyclesorbSM is another adsorptive separation process with semicontinuous recycle. It utilizes a series of chromatographic columns to separate fructose from glucose. A series of internal recycle streams of impure and dilute portions of the chromatograph are used to improve the efficiency [Gerhold, U.S. Patent numbers 4,402,832, 1983; and 4,478,721, 1984]. A schematic diagram of a six-vessel UOP Cyclesorb process is shown in Fig. 16-58 [Gembicki et al., gen. refs., p. 595]. The process has four external streams and four internal recycles: Dilute raffinate and impure extract are like displacement steps; and impure raffinate and dilute extract are recycled from the bottom of an adsorber to its top. Feed and desorbent are fed to the top of each column in a predetermined sequence. The switching of the feed and desorbent are accomplished by the same rotary valve used for Sorbex switching (see hereafter). A chromatographic profile is established in each column that is moving from top to bottom, and all portions of an adsorber are performing a useful function at any time.

The concept of a simulated moving bed (SMB) was originally used in a process developed and licensed under the name UOP Sorbex process [Broughton, Bieser, and Anderson, Pet. Int. (Milan), 23(3), p. 91 (1976); Broughton, Bieser, and Persak, Pet. Int. (Milan), 23(5), p. 36 (1976)]. The following discussion is based on that process, but the concepts can be generally applied. In a moving-bed system for continuous countercurrent effect, solids move continuously in a closed loop past fixed points of introduction and withdrawal of feed and regenerant. The same effect can be achieved by holding the bed stationary and periodically moving the points of introduction and withdrawal of the various streams. Shifting those points in the direction of fluid flow simulates the movement of solid in the opposite direction. Since moving the positions continuously is impractical, a finite number of access lines are provided to a limited series of adsorbent beds. The Sorbex commercial application of this concept is portrayed in Fig. 16-59, which shows the adsorbent as a stationary bed and the auxiliary distillation columns needed to separate the desor-bent from raffinate and extract so that it can be recycled to the process. In this example, feed flows through the three beds of Zone 1, where the most strongly adsorbed component A is adsorbed and depleted from the raffinate. Desorbent flows through the three beds of Zone 3 to displace component A into the extract. The less strongly adsorbed component B is adsorbed slightly in the two beds of Zone 4 between raffinate outlet and desorbent, inlet to prevent it from contaminating extract in Zone 3. Component B is desorbed in the four beds of Zone 2 between the extract outlet and the feed inlet and most leaves with the raffinate. A pump draws liquid from the bottom outlet to the top inlet of the adsorbent chamber. All flows in the beds are downward, simulating an upward flow of solid. The four active port positions are likewise moved downward by the selection of the rotary valve. The rotary valve functions in the same manner as a multiport stopcock or chromatography valve, sequencing the four streams on the right to the lines connected to the inlet/outlet nozzles on the adsorber. The next position of the rotary valve will direct desorbent to

FIG. 16-57 Sanmatsu Kogyo chromatographic process. (Reprinted with permission of Wiley. Reference: Keller, Anderson, and Yon, Chap. 12 in Rousseau, Handbook of Separation Process Technology, John Wiley & Sons, Inc., New York, 1987.)

line 3, extract from 6, and feed to 10; and raffinate moves to the top of the column at line 1. The liquid flow rate in each of the four zones is different because of the addition or withdrawal of the various streams and is controlled by the circulating pump. The circulating pump must be programmed to pump at four different rates in order to keep the flow in each zone constant. The variables available for fine tuning performance are the cycle time, measured as the time required for one rotation of the rotary valve, and the liquid flow rate in Zones 2, 3, and 4. Chemical analyses at the liquid circulating pump can trace the performance of the entire bed and are used for changing operating conditions. Other versions of the SMB system have been used commercially by Toray Industries, Illinois Water Treatment and Mitsubishi (see "Displacement Purge").

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