Moving bed operation

Difficulties of Moving-Bed Operation. The use of a moving bed iatroduces the problem of mechanical erosion of the adsorbent. Obtaining uniform flow of both soHd and Hquid ia beds of large diameter is also difficult. The performance of this type of operation can be gready impaired by nonuniform flow of either phase. 

To complete the simulation, the Hquid-flow rate relative to the soHd must be the same in both the moving-bed and simulated moving-bed operations. Because the soHd is physically stationary in the simulated moving-bed operation, the Hquid velocity relative to the vessel wall must be higher than in an actual moving-bed operation. 

The theoretical performance of the commercial simulated moving-bed operation is practically identical to that of a system ia which soHds dow continuously as a dense bed countercurrent to Hquid. A model ia which the dows of soHd and Hquid are continuous, as shown ia Figure 7, is therefore adequate. 

The question of whether adsorption should be done ia the gas or Hquid phase is an interesting one. Often the choice is clear. Eor example, ia the separation of nitrogen from oxygen, Hquid-phase separation is not practical because of low temperature requirements. In C q—olefin separation, a gas-phase operation is not feasible because of reactivity of feed components at high temperatures. Also, ia the case of substituted aromatics separation, such as xylene from other Cg aromatics, the inherent selectivities of iadividual components are so close to one another that a simulated moving-bed operation ia hquid phase is the only practical choice. 

The simulated moving bed operational mode involves four distinct functional zones, the adsorption, purification, desorption and buffer zones. These zones are described in detail in other parts of this book. We now examine the function of each zone as it applies to p-xylene adsorption and which can be extrapolated to the other aromatics separations. 

The Molex process developed by U.O.P. is unique not only in its liquid-phase operation but also in its adsorption system (1-8). Its adsorption system consists of a single adsorption tower with multiple inlet-outlet points and a special rotary valve. The adsorption tower has many smaller adsorption chambers interconnected in series, and it operates under the so-called simulated moving bed operation. Instead of moving the adsorbent bed, the simulated moving bed operates by simultaneously advancing inlet-outlet points periodically. At any time, the adsorber has four zones—viz., adsorption, primary rectification, desorption, and secondary rectification zones, and these zones advance simultaneously as the rotary valve turns periodically. Desorption of n-paraffins is achieved by displacement. 

Olefin Separation. U.O.P. s Olex Process. U.O.P. s other hydrocarbon separation process developed recently—i.e., the Olex process—is used to separate olefins from a feedstock containing olefins and paraffins. The zeolite adsorbent used, according to patent literature 29, 30), is a synthetic faujasite with 1-40 wt % of at least one cation selected from groups I A, IIA, IB, and IIB. The Olex process is also believed to use the same simulated moving-bed operation in liquid phase as U.O.P. s other hydrocarbon separation processes—i.e., the Molex and Parex processes. 

Fig. 6.7. Simulated moving-bed operating region for complete separation.

All moving-bed operations are compressed into the single line for the abscissa of e0 = 0.4 in the constant-n chart shown in Fig. 11. This onedimensional representation in the diagram needs to be developed into two dimensions for more lucid exposition. 

Moving-bed operations in which the particles are poised by hydrodynamic drag against gravity to the point of incipient fluidization is represented by Eq. (3.5) when 0 is substituted for e ... 

Area AOB in Fig. 12 shows the counter-down moving-bed operation described above, with the value of Q0 = 0 for no fluid flow to Q0 = 1 at incipient fluidization. 

To summarize, the deviation factor Q0 possesses the following range of values for different modes of moving bed operations ... 

Fig. 51. Effect of recycling in fixed and moving-bed operations. [Ardern, Dart, and Lassiat, Advances in Chem. Ser. No. 6, 13 (1951). Reprinted by permission.]...

Upper fixed-bed operation Lower moving-bed operation... 

Space velocity. Space velocity is defined as the hourly feed rate per unit amount of catalyst in the reactor. In fixed-bed and moving-bed operations, it may be reported in either volume or weight units. 

In moving beds operation, the resin is contacted countercurrently with the exhausting stream and regenerated stream. The operation and result are similar to a fixed bed. The advantage of operation is that there is a continuous product of uniform quality at less space, capital, and labor. The problem is a complexity of the design problem for an operating system. 

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