Refinements In Adsorption Processing

Abstract

Publication Rights Reserved This paper is to be presented at the 36th Annual Fall Meeting of the Society of Petroleum Engineers of AIME in Dallas October 8–11, 1961, and is considered the property of the Society of Petroleum Engineers. Permission to publish is hereby restricted to an abstract of not more than 300 words, with no illustrations, unless the paper is specifically released to the press by the Editor of JOURNAL OF PETROLEUM TECHNOLOGY or the Executive Secretary. Such abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in JOURNAL OF PETROLEUM TECHNOLOGY or SOCIETY OF PETROLEUM ENGINEERS JOURNAL is granted on request, providing proper credit is given that publication and the original presentation of the paper. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and considered for publication in one of the two SPE magazines with the paper. 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.