Adsorptive processes, design

The basic adsorption process design. Sub-tasks within that include the adsorbent selection, made in view of aU of the requirements imposed on the dehydration process. The adsorption step time, regeneration and cooHng step times all need to be settled and these in view of mechanical details. The overall vessel configuration, for example, the vessel ID and length, which quantities are typically sized based on pressure drop. Finally we need to make some estimate of the expected service Hfetime for the adsorbent product. 

Mechanical design of the adsorber then takes up the remainder of the engineering effort to produce a workable adsorption process design. Once a vessel is sized to provide the required inventory of adsorbenf we need to provide the mechanical details, which include flow distribution devices, bed supports and the required vessel wall thickness to withstand the working pressure and added stresses encountered during regeneration and repeated de-pressurization and re-pressurization. 

It is worth noting than adsorption process design is a mature science and there is seldom a need to employ detailed numerical models to solve the pdes described in Section 9.4. There are some spedahzed circumstances that may fall outside the norms for which many of the today s design tools have been formulated. Only in these circumstances is a more rigorous process simulation required for obtaining a design. 

The range of applications for gas and liquid separation and purification by adsorption is large and growing. The strong research and development activity in this area is facilitated by the flexibility of practical adsorptive process designs such as pressure and thermal swing adsorption, and SMB adsorption, as well as by the availability of a large spectrum of new and old micro-and mesoporous adsorbents. 

Clearly, the factors determining the isosteric heats of adsorption of gas mixtures are complex. Many more binary experimental data have to be gathered and analyzed before any acceptable theory to explain the observed behavior emerges. There is also an urgent need for generating a comprehensive and systematic database for multicomponent gas (/ > 2) isosteric heats of adsorption for aiding practical adsorptive process designs. 

Kinetics studies and dynamic continuous-flow investigations offering information on the rate of adsorption, together with hydrodynamic parameters, are very important for adsorption process design. 

K.E.Guhbins, in Preprints of the Topical Conference on Separation Science and Technologies, Part II, Section 6 New Development in Adsorption Process Design and Simulations, AIChE Annual Meeting, Los Angeles, California, November, 1997, 1266. 

Methodology of gas adsorption process design. Separation of propane/propylene and n/iso-paraffins mixtures... 

The Aromax process was developed in the early 1970s by Toray Industries, Inc. in Japan (95—98). The adsorption column consists of a horizontal series of independent chambers containing fixed beds of adsorbent. Instead of a rotary valve, a sequence of specially designed on—off valves under computer control is used to move inlet and withdrawal ports around the bed. Adsorption is carried out in the Hquid phase at 140°C, 785—980 kPA, and 5—13 L/h. PX yields per pass is reported to exceed 90% with a typical purity of 99.5%. The first Aromax unit was installed at Toray s Kawasaki plant in March 1973. In 1994, IFP introduced the Eluxyl adsorption process (59,99). The proprietary adsorbent used is designated SPX 3000. Individual on-off valves controlled by a microprocessor are used. Raman spectroscopy to used to measure concentration profiles in the column. A 10,000 t/yr demonstration plant was started and successfully operated at Chevron s Pascagoula plant from 1995—96. IFP has Hcensed two hybrid units. 

Liquid adsorption processes hold a prominent position ia several appHcations for the production of high purity chemicals on a commodity scale. Many of these processes were attractive when they were first iatroduced to the iadustry and continue to iacrease ia value as improvements ia adsorbents, desorbents, and process designs are made. The UOP Parex process alone has seen three generations of adsorbent and four generations of desorbent. Similarly, Hquid adsorption processes can be applied to a much more diverse range of problems than those presented ia Table 3. 

U.S. Enviionmental Piotection Agency, Process Design Manualfor Carbon Adsorption, SwiadeU-Diesslei Co., Pittsbuigh, Pa., 1971, pp. 3—68. 

FIG. 16-46 Pressurized adsorber vessel. (Reptinted with peirrussion of EPA. Reference EPA, Process Design Manual for Carbon Adsorption, U.S. Envir. Protect. Agency., Cincinnati, 1973.)... 

Adsorption The design of gas-adsorption equipment is in many ways analogous to the design of gas-absorption equipment, with a solid adsorbent replacing the liqiiid solvent (see Secs. 16 and 19). Similarity is evident in the material- and energy-balance equations as well as in the methods employed to determine the column height. The final choice, as one would expect, rests with the overall process economics. 

Mass Transfer Rale Consideralions - As discussed previously, the mass transfer mechanism involved in industrial adsorption processes is complex. Generally, basic physical data on the materials involved are insufficient for design. Experimental mass transfer rate data for the specific adsorbate-adsorbent system are usually required for good design. 

The adsorption process generally is of an exothermal nature. With increasing temperature and decreasing adsorbate concentration the adsorption capacity decreases. For the design of adsorption processes it is important to know the adsorption capacity at constant temperature in relation to the adsorbate concentration. Figure 11 shows the adsorption isotherms for several common solvents. 

Adsorption is influenced by the surface area of the adsorbent, the nature of the solvent being adsorbed, the pH of the operating system, and the temperature of operation. These are important parameters to be aware of when designing or evaluating an adsorption process. 

The adsorption action of activated carbon may be explained in terms of the surface tension (or energy per unit surface area) exhibited by the activated particles whose specific surface area is very large. The molecules on the surface of the particles are subjected to unbalanced forces due to unsatisfied bonds and this is responsible for the attachment of other molecules to the surface. The attractive forces are, however, relatively weak and short range, and are called Van der Waals forces, and the adsorption process under these conditions is termed as a physical adsorption (physisorption) process. In this case, the adsorbed molecules are readily desorbed from the surface. Adsorption resulting from chemical interaction with surface molecules is termed as chemisorption. In contrast to the physical process described for the adsorption on carbon, the chemisorption process is characterized by stronger forces and irreversibility. It may, however, be mentioned that many adsorption phenomena involve both physical and chemical processes. They are, therefore, not easily classified, and the general term, sorption, is used to designate the mechanism of the process. 

Evaluation of design options. Costs are required to evaluate process design options for example, should a membrane or an adsorption process be used for purification ... 

Data used for the design of adsorption processes are normally derived from experimental measurements. The capacity of an adsorbent to adsorb an adsorbate depends on the compound being adsorbed, the type and preparation of the adsorbate, inlet concentration, temperature and pressure. In addition, adsorption can be a competitive process in which different molecules can compete for the adsorption sites. For example, if a mixture of toluene and acetone vapor is being adsorbed from a gas stream onto activated carbon, then toluene will adsorbed preferentially, relative to acetone and will displace the acetone that has already been adsorbed. 

The absorption and adsorption processes must be designed specifically for each waste-gas system. Most are relatively new processes, and generalized cost data are not available. 

EPA, Process Design Manual for Carbon Adsorption, U.S. Envir. Protect. Agency., Cincinnati, 1973. 

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