Refinements In Adsorption Processing
Authors: Landes S.H. 
Published: December 2007

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Abstract
The solid adsorbent process for recovery of hydrocarbons from natural gas is a cyclic process that has found favor in recent years. Although it is not a new process, there is a lack of design data available for the process man to use in evaluating dry bed systems. Some activity coefficients for the commonly used adsorbents are presented herein. They are only a portion of the data desired on adsorbents. The need for a standard method of testing new adsorbents is discussed.
A calculation method is presented for design of adsorption units as well as for evaluating proposed or existing equipment. A method of testing the unit to determine it's recovery efficiency is also outlined.
It is hoped that the gas industry will adopt standard methods for the adsorption systems similar to those developed for the absorption systems.

Introduction
People concerned with the selection and purchase of gas processing equipment frequently have problems in evaluating competing proposals. The evaluation is not a simple one. Final equipment selection may require answers to all of the following: adequacy of the process design; equality of the brands proposed; field service; price; company history and quality of previously furnished equipment. We would like to present information that will aid the layman in evaluating the process design of an adsorption unit. The information consists of activity coefficients for commonly used desiccants, a design calculation method and a method of testing the performance of field units.

TYPES OF ABSORBENTS
An adsorbing solid is generally an extremely porous "solid foam" with a large internal surface area. Many different types of adsorbents, each with a particular affinity for certain vapors, have been developed for industrial use. Fuller's earth, bauxite, acid-treated clays, bone char and synthetic resins are widely used in petroleum oil refining, sugar refining, water purification and recovery of-toxic poisons. These adsorbents are not applicable to the hydrocarbon recovery process. The adsorbents which can be used in light hydrocarbon processing are activated carbon, silica gel, activated alumina, activated gel-alumina mixture, and molecular sieves.

CHARACTERISTICS OF GAS ADSORBENT SOLIDS
The amount of vapor adsorbed per unit weight of an adsorbent under equilibrium conditions is dependent upon the temperature, the pressure, the nature of the adsorbent, and the nature of the adsorbed vapor. Amorphous solids in general are more adsorbent than crystalline materials. Characteristics of the adsorbing solid which play essential roles are the extent of the surface area, the structure of the surface, the size and distribution of the pores, contamination of the surface, and the activation processes employed in the preparation of the adsorbent.
We are primarily interested in adsorbing hydrocarbons such as propane, butanes, pentanes and hexanes plus. We are also interested in the adsorption of water vapor in order to dehydrate the natural gas stream. In general, activated carbons have the greatest adsorption capacities for the hydrocarbon vapors and are vastly superior for the recovery of propane and the butanes. Silica gel and the activated gel-alumina mixtures have excellent adsorptive capacity for the heavier hydrocarbons such as hexanes and heptanes plus, being nearly as good as activated carbon for adsorbing these heavier components. The gel and activated mixtures all have excellent water adsorption characteristics. Activated alumina is an excellent dehydration medium but has relatively poor adsorptive capacity for the hydrocarbon vapors. The activated carbons are not good water adsorbers and are not used for dehydration.
Figure 1 shows the relative dehydration characteristics of the various adsorbents. Activated alumina has the advantage of being able to withstand rougher treatment and shocks of liquid water and heat without disintegration. Silica gel tends to break into smaller particles when subjected to shocks of liquid water.