Alabama Gas Revamps Its Distribution Liquefier

Pipeline & Gas Journal, August, 2001 by Bruce Painter, Javid H. Talib, Randal J. Winkler, Brian C. Price

Alabama Gas Corp. has operated its liquefied natural gas facility (LNG) in Pinson, AL since 1965. The facility provides peakshaving capacity to Alabama Gas Corp.'s distribution system, which now serves almost 473,000 customers in north and central Alabama. As one of the oldest facilities of its kind in the United States, it has provided reliable service for many years, but has reached the point where system requirements, technology advancements, efficiency, and maintenance requirements necessitated a change in the facility. Alabama Gas is in the final phases of constructing a replacement LNG production facility, designed and constructed by Black & Veatch Pritchard Inc., which will increase liquefaction capacity from 3.6 MMscf/d to 12 MMscf/d. This article discusses the development of,the project from initial inception to current status.

Alabama Gas is a subsidiary of Energen Corp. and has provided natural gas service to its customers since 1852. Its customer base has grown to include residential, commercial, industrial, and power generation facilities throughout north and central Alabama. The supply requirements for this wide range of growing customers requires a diverse gas supply portfolio, one which uses LNG storage to supplement system requirements and increase reliability. To meet this requirement, Alabama Gas commissioned its fast LNG plant in October 1965.

The original liquefier is a "cascade" process, utilizing propane, ethylene, nitrogen, and methane. The system includes a 65-foot vertical cold box which houses over 20 stainless and aluminum, spiral wound exchangers insulated with rock wool, a 2,000-hp natural gas-fired, integral engine/compressor package, and a multitude of connecting piping, electrical, and instrumentation equipment. Gas pre-treatment was originally handled by an MEA (methyl ethyl amine) system, which was replaced in 1987 with a, three-bed molecular sieve system. Heat rejection includes a cooling tower connected to numerous shell and tube exchangers and an evaporative system for propane condensation. Operation of this equipment is by pneumatic and manual controls. Replacement parts and years of temperature cycling within the cold box have caused maintenance costs to continue to rise. Change was inevitable.

Project Development

The decision to replace the liquefaction unit quickly evolved to include reviews of numerous process options, drivers, capacity requirements, long-range predictions of supply, and even consideration of obtaining and relocating an existing facility. Numerous studies were conducted by Alabama Gas and Black & Veatch to determine the capacity of the new facility and the type of driver to be used for the refrigeration compression. Capacities of 5, 8, 10 and 12 MMsfc/d were studied and corresponding cost estimates developed. Gas turbines and electric motor drivers were considered for the refrigeration compressor.

A 12 MMsfc/d plant utilizing an electric motor drive was chosen as the most economically attractive option. Concerns involving system demand, useful life of equipment at Alabama Gas Corp.'s other liquefaction facility in Montgomery, AL, energy costs, operating restrictions, and system accessibility were calculated into the equation. Final bids for the work resulted in a design build contract with Black & Veatch in Overland Park, KS, utilizing its patented PRICO Process.

PRICO Process

Many of the early LNG peak shavers utilized a variety of cascade processes. In order to simplify the liquefaction process, Black & Veatch developed a single mixed refrigerant loop called the PRICO Process. This employs a single mixed refrigerant loop to accomplish the gas liquefaction. This refrigerant is a mixture of nitrogen and hydrocarbons ranging from methane to isopentane. The refrigeration compressor can be a single case compressor without intercooling or can be intercooled to reduce the power requirements. Utilization of a single refrigeration loop greatly simplifies the piping, controls, and equipment arrangements for the liquefaction unit.

The process cools the natural gas feed from ambient conditions to gas liquefaction temperatures and then further cools the gas to minimize vaporization when sending the LNG to atmospheric storage tanks. A simplified flow diagram of the process is shown.

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The new process employs proven process equipment such as brazed aluminum plate fin heat exchangers and a refrigerant compressor driven by a gas turbine or electric motor which provides the desired long-term operational reliability. The process employs only one mixed refrigerant circuit with a two-stage compressor. This simple refrigeration system has proven operating experience in numerous peakshaving plants and base-load plants.

The use of a single refrigeration system is the key aspect of a simple and reliable LNG plant. The old style cascade systems suffered from complex operation and the maintenance cost of two or more refrigeration systems. The use of a single system not only reduces equipment count (and cost) but the number of instruments involved in the control systems is reduced by 50 percent or more.