Design of fixed-bed adsorption columns

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

A widely used method for adsorption of solutes from liquid or gases employs a fixed bed of granular particles. The fluid to be treated is usually passed down through the packed bed at a constant flow rate. The situation is more complex than that for a simple stirred-tank batch process which reaches equilibrium. Mass-transfer resistances are important in the fixed-bed process and the process is unsteady state. The overall dynjimics of the system determines the efliciency of the process rather than just the equilibrium considerations. 

The concentrations of the solute in the fluid phase and of the solid adsorbent phase change with time and also with position in the fixed bed as adsorption proceeds. At the inlet to the bed the solid is assumed to contain no solute at the start of the process. As the fluid first contacts the inlet of the bed, most of the mass transfer and adsorption takes place here. As the fluid passes through the bed, the concentration in this fluid drops very rapidly with distance in the bed to zero way before the end of the bed is reached. The concentration profile at the start at time / is shown in Fig. 12.3-la, where the concentration ratio cicg is plotted versus bed length. The fluid concentration c is the feed concentration and c is the fluid concentration at a point in the bed. 

After a short time, the solid near the entrance to the tower is almost saturated, and most of the mass transfer and adsorption now takes place at a point slightly farther from the inlet. At a later time t2 the profile or mass-transfer zone where most of the 

As seen in Fig. 12.3-la, the major part of the adsorption at any time takes place in a relatively narrow adsorption or mass-transfer zone. As the solution continues to flow, this mass-transfer zone, which is S-shaped, moves down the column. At a given time /3 in Fig. 12.3-la when almost half of the bed is saturated with solute, the outlet concentration is still approximately zero, as shown in Fig. 12.3-lb. This outlet concentration remains near zero until the mass-transfer zone starts to reach the tower outlet at time t. Then the outlet concentration starts to rise and at the outlet concentration has risen to Cj, which is called the break point. 

Two macroscopic methods to design adsorption columns are the scale-up and kinetic approaches. Both methods rely on breakthrough data obtained from pilot columns. The scale-up method is very simple, but the kinetic method takes into account the rate of adsorption (determined by the kinetics of surface diffusion to the inside of the adsorbent pore). The scale-up approach is useful for determining the breakthrough time and volume (time elapsed and volume treated before the maximum allowable effluent concentration is achieved) of an existing column, while the kinetic approach will determine the size requirements of a column based on a known breakthrough volume. 

The (plant-scale) design column should operate such that  

1 Huid residence time in the pilot and design column are the same. 

2 The total volume of fluid processed until breakthrough per mass of sorbent in the column is the same for both columns. 

Bed volume of design column, where Q is the fluid volumetric flowrate of the design column 

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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. 

In most adsorption processes the adsorbent is contacted by the fluid phase in a packed column. Such variables as the particle size, fluid velocity, and bed dimensions determine the pressure drop and have an important impact on the economics of the process since they determine the pumping cost as well as the extent of axial mixing and the heat transfer properties. The hydrodynamics of flow through packed beds have been extensively studied, and detailed accounts may be found in many chemical engineering textbooks. The present review is therefore limited to a brief summary of the principal features of the flow behavior which are important in the design of fixed bed absorbers. 

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. 

Janson and Hedman (1) recently published an excellent review of large-scale chromatography. Many of the broad process design and operation considerations are the same for affinity chromatography as they are for ion exchange or gel filtration. Most chromatography models, however, are based on the assumption of small feed pulses with linear equilibria (such as the widely-used plate theories (2)) and are not directly useful for affinity separations. In this paper we discuss and compare experimental results with two fixed-bed adsorption models that can be used to predict the performance of affinity columns. These two models differ only in the form of the rate-... 

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. 

Adsorption is a separation process in which certain components of a fluid phase are transferred to the surface of a solid adsorbent. Usually the small particles of adsorbent are held in a fixed bed, and fluid is passed continuously through the bed until the solid is nearly saturated and the desired separation can no longer be achieved. The flow is then switched to a second bed until the saturated adsorbent can be replaced or regenerated. Ion exhange is another process that is usually carried out in this semibatch fashion in a fixed bed. Water that is to be softened or deionized is passed over beads of ion-exchange resin in a column until the resin becomes nearly saturated. The removal of trace impurities by reaction with solids can also be carried out in fixed beds, and the removal of H2S from synthesis gas with ZnO pellets is a well-known example. For all these processes, the performance depends on solid-fluid equilibria and on mass-transfer rates. In this chapter the emphasis is on adsorption, but the general methods of analysis and design are applicable to other fixed-bed processes. 

An alternative approach is the technique of suspended bed chromatography. Here, ion exchangers and columns designed for packed bed operations may be used in alternative protocols. Briefly, adsorption is carried out in batch mode, and the resulting adsorbent suspension is filter collected/clarified in a conventional fixed bed contactor for washing and elution. This approach is enabled by the availability of pump-packed column chromatography systems. The technique was first demonstrated with a clarified... 

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