By Marta J. Bourke and Anthony F. Mazzoni
For presentation at the Gas Conditioning Conference March 6-8, 1989
Activated carbon has been successfully used in numerous gas processing applications for many years. Activated carbon's highly porous nature and large surface area make it an ideal adsorbent for removing trace contaminants from liquid and gaseous streams. For some inorganic contaminants encountered in gas processing operations, removal can be enhanced by impregnation of the activated carbon to promote both physical adsorption and chemisorption.
This paper will review uses for activated carbon in gas conditioning in the following areas:
- Purification of re-circulating amines and glycols
- Desulfurization of natural gas
- Mercury removal from natural gas
- Equipment and catalyst protection
Specific examples will be reviewed to provide basic guidelines and understanding of the proper utilization, selection, and design of activated carbon and related systems. Operating parameters, performance results, and benefits for a number of actual cases will be discussed.
Activated Carbon and Granular Activated Carbon Selection
Granular activated carbon can be manufactured from any carbonaceous material (coal, wood, peat, coconut shell, etc.). Granular activated carbon is generally produced by grinding the raw material, adding a suitable binder for hardness, re-compacting, and crushing to the correct mesh size. The carbon-based material is converted to activated carbon by thermal decomposition of the organic materials present in a controlled non-oxygen atmosphere. The resultant product has an incredibly large surface area per unit volume and a network of submicroscopic pores where adsorption occurs. One pound of activated carbon provides a surface area equivalent to 125 acres.
Because granular activated carbons can be made from various raw materials, differences will exist in the finished product. Domestically, most granular activated carbons are manufactured from various grades of coal. Some grades of coal need a chemical pretreatment prior to thermal activation to develop proper pore structure. Table I shows some of the properties of activated carbon and how these vary with the raw material. These differences in activated carbon properties make some products better suited than others for specific applications.
Iodine Number is the most common standard for indicating total surface area available for adsorption. It is defined' as the milligrams of iodine adsorbed by one gram of activated carbon and it approximates the internal surface area (micropores) in square meters per gram. Table I shows how Iodine Number can vary for activated carbons produced from different materials.
Densities of the activated carbon can vary drastically; fewer pounds of a activated carbon with a low density will fill the same volume as a higher density product. This is significant because the contaminant removal capacity of this volume of low density activated carbon is severely reduced. The concept of volume activity then becomes important in selecting activated carbons. A simple calculation for determining the volume activity of activated carbon is to multiply the bulk density by the Iodine Number. Thus, two containers having the same volume of activated carbon will have different total surface areas if the densities are different.
Abrasion resistance refers to a granular activated carbon's ability to withstand degradation during handling and is expressed in terms of an Abrasion Number; the higher the Number, the more resistant a activated carbon is to abrasion. This characteristic is especially important for high pressure applications, where granular activated carbon fines in the effluent can cause process problems and where activated carbon is on line for extended periods.
Ash content of granular activated carbon can be significant as some activated carbons have a high total ash content which contribute nothing for adsorption. These inorganic materials can leach from the granular activated carbon and cause operational problems such as foaming.
Typical Properties of Activated Carbons
Produced from Different Raw Materials
|Bulk Density as packed
in column lbs/ft3