Adsorption Design and Operating Practices

Adsorption of n-Hexane from a Natural Gas with Silica Gel 

Hexane is to be recovered from a natural gas with silica gel. Molecular weight of the gas is 17.85, the pressure is 55.4 atm, temperature is 94°F, and the content of n-hexane is 0.853 mol % or 0.0982 lb/cuft. The bed is 43 in. deep and the superficial velocity is 11.4ft/min. Other data are shown with the sketch  

The total amount adsorbed to the breakpoint, at C/C = 0.01, per sqft of bed cross section is 

Dp = 0.01 ft, particle diameter, a = 284 sqft/cuft, packing external surface, pb = 52 lb/cuft, bed density, e = 0.35 bed voidage. 

The calculated concentration profile is compared in the figure with experimental data, Run 117, of McLeod and Campbell, Soc. Pet. Eng. /., 166 (June 1966)  

From these and physical property data, the Schmidt and Reynolds numbers are calculated as 

Saturation content of adsorbate is 0.17 Ib/lb solid. Accordingly, the coefficient of the linear adsorption isotherm is 

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PAC typically has a diameter less than 0.15 mm, and can be applied at various locations in a treatment system. GAC, with diameters ranging from 0.5 to 2.5 mm, is employed in fixed-bed adsorbents such as granular media filters or post filters. Each of these factors must be properly evaluated in determining the use of activated carbon in a practical application. The primary treatment objective of activated carbon adsorption in a particular water treatment plant determines the process design and operation multiple objectives can not, in most cases, be simultaneously optimized [5-40]. 

Methane can be oxidatively coupled to ethylene with very high yield using the novel gas recycle electrocatalytic or catalytic reactor separator. The ethylene yield is up to 85% for batch operation and up to 50% for continuous flow operation. These promising results, which stem from the novel reactor design and from the adsorptive properties of the molecular sieve material, can be rationalized in terms of a simple macroscopic kinetic model. Such simplified models may be useful for scale up purposes. For practical applications it would be desirable to reduce the recycle ratio p to lower values (e.g. 5-8). This requires a single-pass C2 yield of the order of 15-20%. The Sr-doped La203... 

To design and implement systems for water treatment, it is not only the adsorption charaeteristics of the activated carbon that must be considered but also the effect that the carbon may have on the practical operation of the unit. In this context, the pressure drop generated across a GAC bed is one of the most important factors, as it also is for gas phase applications. The particle size distribution should be optimized to attain an acceptable pressure drop commensurate with the desired rate of adsorption. The carbon attrition resistance is another important parameter. Part of the operating cost of adsorbers is due to the loss of carbon fines during transport, handling, and regeneration. 

A common practice is to develop a specific model for the adsorptive process of interest and use simplistic descriptions (models or empirical) of pure and multicomponent gas adsorption equilibria and kinetics in order to describe the effects of various operating variables to obtain an optimum design. The effort is always closely tied to experimental verification and empirical fine-tuning using actual process data from pilot plants. A comprehensive set of data on pure and multicomponent adsorption equilibria of the components of SMROG on an activated carbon and a 5A zeolite is available in published literature.73... 

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