(151c) Design of Units for Mercury Removal from Hydrocarbon Gas Streams

Design of Units for Mercury Removal from Hydrocarbon Gas Streams

Robert W. Soffel, Selective Adsorption Associates, Inc.

John Markovs, Adsorption Solutions, LLC

Abstract:

The hydrocarbon industry became aware of the detrimental effects of mercury some 40 years ago when the aluminum heat exchangers were damaged by mercury at the Skikda LNG plant in Algeria. Since mercury is widely present in natural gas, subsequent plants have chosen to install Mercury Removal Units (MRU’s) to protect heat exchangers and other plant operations. The predominant means for removing the mercury has been the use of fixed bed adsorption.

Review is made of the process design criteria for achieving dependable long-term operation of mercury removal units. Essentially total mercury removal is required, since even part per trillion concentrations of mercury in the gas can damage aluminum heat exchangers. The effects of gas residence time in the MRU as it affects the attainable purity are examined, as well as the effects of the adsorbent particle size. Examples of MRU designs are presented for a variety of feed gas conditions, including wet and dry feed gases. The physical and chemical properties of the four types of commercially-available mercury removal adsorbents are compared. The design of the MRU vessel and its relationship to the process gas pressure drop is examined. The importance of designing single-phase flow protection into the mercury removal process to attaining the desired design performance is discussed. Experience-based recommendations for achieving reliable mercury removal performance and predictable mercury adsorbent life are described.