Fixed-bed sorption

D. Basmadjian and P. Coroyannakis, Equilibrium-theory revisited -Isothermal fixed-bed sorption of binary systems. 1. Solutes obeying the Langmuir isotherm. Chem. 

Casey, J.T. and A.I. Liapis (1985). Fixed bed sorption with recycle. Part III Consecutive reversible reactions. Chem. Eng. Res. Des. 63, 398. 

D. O, Cconey and . N. Light foot, Multicomponent Fixed-Bed Sorption of Interfering Solntes. 

Define the concepts of mass-transfer zone (MTZ) and breakthrough curve in fixed-bed sorption. 

In fixed-bed sorption, the concentration of the fluid phase and of the solid phase change with time as well as with position in the bed. At first, most of the mass transfer takes place near the inlet of the bed, where the fluid contacts the sorbent first. If the solid contains no solute at the start, the concentration of the fluid drops exponentially with distance essentially to zero before the end of the bed is reached. This concentration profile is shown by curve t, in Figure 9.8a, where c/cF is the concentration in the fluid relative to the feed concentration. After some time, the solid near the inlet becomes nearly saturated, and most of the mass transfer takes place farther from the inlet. The concentration gradient becomes S-shaped, as shown by curve tT The region where most of the change in concentration occurs is called the mass-transfer zone, MTZ, and its limits are arbitrarily chosen, often taken as the region in the bed where 0.05 < c/cF < 0.95. 

Fixed-Bed Sorption. Normally in practical cases, the concept of fixed-bed adsorbers is usually expressed in graphical terms by the breakthrough curve concept. The dynamic adsorption system is represented in Fig. 15.20. Effluent enters the column at solute concentration Cq, and a concentration gradient or profile is established within a finite zone (break-... 

FIGURE 15.22 Fixed-bed sorption of phenol using activated carbon. Breakthrough curves at four bed heights are shown. 

Fixed-bed sorption eolumns can be operated as single units or multiple units. The columns can operate by upflow or downflow, and multiple-column systems can operate in series or parallel. Figure 15.23 shows eolumns in series operating downflow. Each bed can be replaced as a eomplete, separate unit. When breakthrough occurs in the last column, the first column will be in equUibrium with the influent, thus achieving maximum adsorbent sorption capacity and utilization. This may be as a result of economic necessity. This first column will then beeome the last eolumn in the system. 

FIGURE 15.23 Fixed-bed sorption columns in series and parallel operating downflow. 

Cooney. D.O., and Strusi, F.P., Analytical description of fixed-bed sorption of two Langmuir solutes under nonequilibrium conditions, Ind. Eng. Chem. Fund.. 11(1). 123-126 (1972). 

Garg, D.R., and Ruthven, D.M.. Fixed-bed sorption behavior of gases with nonlinear equilibria, AIChE J., 19(4). 852-853 (1973). 

Bhatia, S.K., Combined surface and pore volume diffusion in porous media. AIChE J., 34(7). 1094-1105 (1988). Biyani, P.. and Goochee, C.F., Nonlinear fixed-bed sorption when mass transfer and sorption are controlling. AIChE J.. 34(10), 1747-1751 (1988). 

Tan, H.K.S., General solutions of second-order kinetics for fixed-bed sorption processes, Chem. Eng. Sci.. 44(11), 2756-2760(1989). 

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