Fixed-Bed Adsorption Columns

Johnston, W.A. Designing Fixed-Bed Adsorption Columns, Chemical Engineering, Nov. 27, 1972, p. 87. 

Tphe breakthrough curve for a fixed-bed adsorption column may be pre-dieted theoretically from the solution of the appropriate mass-transfer rate equation subject to the boundary conditions imposed by the differential fluid phase mass balance for an element of the column. For molecular sieve adsorbents this problem is complicated by the nonlinearity of the equilibrium isotherm which leads to nonlinearities both in the differential equations and in the boundary conditions. This paper summarizes the principal conclusions reached from a recent numerical solution of this problem (1). The approximations involved in the analysis are realistic for many practical systems, and the validity of the theory is confirmed by comparison with experiment. 

Chemically activated carbon fibers (CACF) manufactured KOH impregnated to ACF and confirmed selective adsorption conduct of NO and NO in fixed bed adsorption column. NOx desorption studied on He at Stl/min up to SOOC using temperature programmed desorption (TPD), and observed surface characteristic at absorption-desorption. CACF was increased adsorptivity with offers selective adsorptivity by KOH in NOx adsorption, and CACF (KOH ACF == I 3) had an adsorptivity that was four times higher than that of ACF. NOx desorption on ACF was mostly occurred within 200 C. The results of surface characterization were found that NOx was produced as a KNOx(x=2,3) after adsorption and potassium ions were distributed without loss after desorption. 

Kaczmarski, K., Mazzotti, M., Storti, G., Morbi-delli, M. Modeling fixed-bed adsorption columns through orthogonal collocation on moving finite elements, Comp. Chem. Eng., 1997, 21, 641-660. 

A fixed-bed adsorption has several advantages over batch and continuous stirred tank reactor (CSTR) because the rates of adsorption depend on the concentration of viruses in solution. This point is especially important for virus removal because of the low concentration of viral contaminants. The design of a fixed-bed adsorption column involves estimation of the shape of the breakthrough curve and the appearance of the breakpoint. Computer simulation studies were done here to demonstrate the performance of a virus adsorber using the surface-bonded QAC beads which have a higher binding affinity for viruses over other proteins. 

A fixed-bed adsorption column is a vertical column packed with granular adsorption particles. A liquid stream to be purified is fed above the packing and flows down through the granules. If the feed stream is a gas or vapor, it may be sent to the bottom of the column and flows upward through the packing. 

Use the scale-up approach and the kinetic approach to design fixed-bed adsorption columns based on laboratory or pilot column data. 

Fig. 1. Schematic diagrams for flow through a fixed-bed adsorption column. Courtesy of Industrial and Engineering Chemistry.

DESIGN OF FIXED-BED ADSORPTION COLUMNS 12.3A Introduction and Concentration Profiles... 

Breakthrough Curve-Bed Depth Service Time (BUST) Model. In the operation of a fixed-bed adsorption column, the service time, t, of the bed can be related to the bed depth, Z, for a given set of conditions by a model and equation called the bed depth service time model (BDST). The BDST offers a rapid method of designing fixed-bed columns. The influent solute concentration, Cq, is fed to the column, and it is desired to reduce the solute concentration in the effluent to a value not exceeding Cj. At the beginning of the operation, when the adsorbent is still fresh, the effluent concentration is actually lower than the allowable concentration, Cj, but, as the operation proceeds and the sorbent reaches saturation, the effluent concentration reaches Cj. This condition is called the break point. 

Raghavan, N.S., and Ruthven, D.M., Numerical simulation of a fixed-bed adsorption column by orthogonal collocation method, AIChE J 29(6), 922-925 (1983). 

Zanker, A., Space rates for fixed-bed adsorption columns. Chem. Eng. (N.Y.), 26 November, 102 (1973). 

Ikeda, K., Performance of nonisothermal fixed-bed adsorption column with nonlinear isotherms, Chem. Eng. Sci., 34(7), 941-950 (1979). 

A laboratory fixed-bed adsorption column filled with a synthetic sulfonic acid cation-exchange resin in the acid form is to be used to remove Na" ions from an aqueous solution of sodium chloride. The bed depth is 33.5 cm, and the solution to be percolated through the bed contains 0.120 meq Na /cm At saturation, the resin contains 2.02 meq Na /cm resin. The solution will be passed through the bed at a superficial Lnear velocity of 0.31 cm/s. For this resin, Michaels [70] reports that the overall liquid mass-transfer rate 0.86t>2 , where is the superficial liquid velocity, cm/s, and is expressed as meq Na /cm s (meq/cm ). The relative adsorptivity of Na" " with respect to for this resin is a 1,20, and this is constant for the prevailing concentration level. Define the breakpoint concentration as 5% of the initial solution concentration, and assume that practical bed exhaustion occurs when the effluent concentration is 95% of the iniital. Estimate the volume of effluent at the breakpoint, per unit bed cross section. 

In this book a combination of the principles of separation processes, process modelling, process control and numerical methods is used to describe the dynamic behaviour of separation processes. The text is largely mathematical and analytical in nature. Adsorption processes are commonly operated in a cyclic manner involving complex sequences of individual steps which are dynamic in nature and three chapters in this book specifically address this separation process. Chapter 11 covers the fundamentals of adsorption processes and includes physical adsorption of pure gases and mixtures, mass transfer by convective transport and the roles of pore and surface diffusion in the adsorption process. Chapter 12 addresses the separation of multicomponent mixtures by the use of adsorption columns and includes the Gleuckauf, film resistance and diffusion models and adiabatic operation of a fixed bed adsorption column together with periodic operation. Chapter 14 addresses the thermodynamics of the physical adsorption of pure gases and multicomponent gas mixtures. 

The fixed bed adsorber is the most commonly used arrangement for adsorption and is discussed here. In this vessel the adsorbent is fixed whilst the inlet and outlet positions for process and regenerating streams are moved when the adsorbent is saturated. If continuous operation is required, the unit must consist of at least two beds, one of which is online whilst the other is being regenerated[3]. To understand the dynamics of a fixed bed adsorption column, a mass balance is performed around a disk of cross-sectional area to that of the column A) but with differential thickness dZ as shown in Figure 8.8. 

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