Fixed-bed adsorption process

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. 

The isotherms given by Equations 11.1, 11.3, and 11.4, or other types of isotherms, can be used to calculate the equilibrium concentrations of adsorbates in fluid and solid phases in batch and fixed-bed adsorption processes discussed below. 

The pellet is saturated with CO2 throughout a test run, but C 02 is replaced gradually by C 02 by diffusion, first through the pores, or interstices between the crystals and binder, and then through the crystals themselves. This is analogous to the situation in a fixed-bed adsorption process, where the crystals are analogous to the porous pellets, except that the transfer process in the interstices is a diffusive flow rather than a bulk gas flow. Smith (5) considered complex detailed models for the various flow paths in a fixed-bed process, but as there is inevitably some approximation involved in describing the structure of the interstices, a much simpler approach has been used here. 

Wang, N.-H. Versatile model for simulation of reaction and nonequilibrium dynamics in multicomponent fixed-bed adsorption processes, Comput. Chem. Eng., 1991, 15(11), 749-768. 

The performance of fixed-bed adsorbers is governed by equilibrium, kinetics of mass transfer and hydrodynamics. The objective of this part of the work is the prediction of breakthrough curves of mixtures of n/iso-paraffins and propane /propylene in a fixed-bed adsorber including parameters independently measured. The mathematical model for the fixed bed adsorption process is based on the following assumptions ... 

Figure 3.3.46 Typical design of a fixed-bed adsorption process.

Before focusing on some of the simpler solutions of such a system to develop an understanding of the single solute separation capabilities of a fixed-bed adsorption process, we will provide a few more definitions for a packed adsorbent bed in general. The density of the solid material of the adsorbent particles is p. The bulk density of the adsorbent bed pi, is related to the void volume e of the packed bed, solid material density p and the porosity of the particle Ep via... 

A number of analyses have been developed to account for such features in real fixed-bed adsorption processes. Two such analyses and their major results will be briefly identified below. A linear equilibrium model with dispersion is considered first. Lapidus and Amundson (1952) solved equation (7.1.4) which included the axial dispersion term under the following assumptions ... 

The analysis of elution chromatography with a liquid eluent that was presented earlier was based on a constant eluent velocity, v. Unlike simple fixed-bed adsorption processes with low pressure drops, analytical-scale chromatographic techniques for a mobile gas phase employ long packed columns, where the gas undergoes a considerable pressure drop. As a result, the gas velocity changes with column location. But the gas velocity is proportional to the molar gas volume at every location. Therefore, by Boyle s law, the gas pressure P and velocity Vz at any location are related to those at the inlet (subscript, in) and outlet (subscript, out) by... 

We make three assumptions now we have isothermal, nondispersive and equilibrium operation (these are exactly the same assumptions used to develop the De Vault equation (7.1.8) for fixed-bed adsorption processes). The assumption of equilibrium everywhere between phases 7=1 and 7" = 2 suggests... 

It is fortunate that for many fixed bed adsorption processes of commercial interest the shape of the mass transfer zone remains unaltered as it progresses through the majority of the bed because this leads to substantial simplifications in design. For a favourable isotherm, particularly one of Type I, the mass transfer wave spreads from a shock front as it progresses through the initial region of the bed. As explained earlier in this chapter the... 

Xylene Isomerization. After separation of the preferred xylenes, ie, PX or OX, using the adsorption or crystallization processes discussed herein, the remaining raffinate stream, which tends to be rich in MX, is typically fed to a xylenes isomerization unit in order to further produce the preferred xylenes. Isomerization units are fixed-bed catalytic processes that are used to produce a close-to-equiUbrium mixture of the xylenes. To prevent the buildup of EB in the recycle loop, the catalysts are also designed to convert EB to either xylenes, benzene and lights, or benzene and diethylbenzene. 

Many different forms of the energy balance have been used in fixed-bed adsorption studies. The form chosen for a particular study depends on the process considered (e.g., temperature swing adsorption or pressure swing adsorption) and on the degree of approximation that is appropriate. 

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. 

Adsorption in expanded or fluidised beds is now widely adopted for the direct recovery of protein products from particulate feedstocks. As an integrative protein recovery operation it circumvents process bottlenecks encountered with the solid liquid separation required upstream of fixed bed adsorption, while achieving considerable concentration and primary... 

Dehydration is by far the largest industrial separation of interest here. Removal of water was the first commercial application of molecular sieves. Dehydration and related fixed-bed adsorptive separations in the process industries account for more than half of the commercial molecular sieve business volume. 

An alternative approach is taken in the production of monosodium glutamate (MSG) which, unlike interferon, is secreted into the fermentation broth. The stages of downstream processing for MSG are shown in Figure 14.3. Again, a variety of unit operations, including centrifugation, crystallization, vaporization, and fixed-bed adsorption, are used in this process. 

Union Carbide s OlefinSiv Process. Union Carbide s OlefinSiv process is used mainly to separate n-butylenes from isobutylene 31). The basic hardware is the same as for the IsoSiv process for n-paraffin separation, and the process uses a rapid cycle, fixed-bed adsorption. Since this process separates straight-chain olefins from branched-chain olefins, it is reasonable to assume that a 5A molecular sieve is used as the adsorbent. Product purities are claimed to be above 99% for both n-butylene and isobutylene streams. 

The plug-flow model indicates that the fluid velocity profile is plug shaped, that is, is uniform at all radial positions, fact which normally involves turbulent flow conditions, such that the fluid constituents are well-mixed [99], Additionally, it is considered that the fixed-bed adsorption reactor is packed randomly with adsorbent particles that are fresh or have just been regenerated [103], Moreover, in this adsorption separation process, a rate process and a thermodynamic equilibrium take place, where individual parts of the system react so fast that for practical purposes local equilibrium can be assumed [99], Clearly, the adsorption process is supposed to be very fast relative to the convection and diffusion effects consequently, local equilibrium will exist close to the adsorbent beads [2,103], Further assumptions are that no chemical reactions takes place in the column and that only mass transfer by convection is important. 

When we started selling molecular sieves, little was known about heat and mass transfer in fixed bed adsorption/desorption. Design was an art. G.J. Griesmer, J.J. Collins, F.W.Leavitt, W.F. Avery, and K. Kiyonaga made it a science, a unit process we understand and can optimize. Griesmer led much of this activity and was primarily responsible for Union Carbide s Iso Sieve process for separating normal from iso paraffins [37]. 

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