New NGL-recovery process provides viable alternative

Robert R. Huebel
Michael G. Malsam
Randall Gas Technologies
Houston

Operational scenarios for two uses of a new refrigeration process for recovering NGLs from natural gas have shown it to enhance operability and reduce capital and operating expenditures when compared with the two more traditional process choices—straight refrigeration and turboexpander.

Straight refrigeration units that most often use propane as refrigerant have proven to be economical and reliable. Their operating temperature, however, typically about –35° F., limits NGL extraction. For higher NGL recovery, today’s processor is left with a cryogenic turboexpander.

IPOR (IsoPressure Open Refrigeration) has been developed by Randall Gas Technologies, a division of Lummus Technology, a CB&I company, to bridge this gap. The advanced refrigeration process can economically achieve essentially total C₃+ recovery from most natural gas streams. Using conventional closed-loop mechanical refrigeration combined with an open-loop mixed refrigeration cycle, the new technology can achieve NGL recovery efficiencies comparable to that of advanced turboexpander cycles but for lower capital and operating expenditures.

This article reviews the fundamentals of the IPOR process, including process features, benefits, and applicability. It also presents case studies that compare process performance with both straight refrigeration and advanced turboexpander cycles and economic analysis.

Diverse environments

Natural gas conditioning and processing plants are somewhat unique in that the raw material feedstock is typically fed into the plant at the pressure, flow rate, and composition at which it is produced.

Consequently, natural gas processing plants have considerable variation in size, complexity, and configuration, depending upon specific reservoir production characteristics, geography, customer specifications, and market drivers. These range from simple dewpoint plants with capacities less than 5 MMscfd and minimal hydrocarbon recovery to large deep cut ethane extraction straddle plants which process in excess of 1 bcfd .

With such a diverse operating environment, it is a bit surprising that natural gas processors have had essentially only two process technology choices for extracting hydrocarbon liquids from natural gas: either straight refrigeration or turboexpander. Among more than 1,600 operating natural gas processing plants shown in Oil & Gas Journal’s Worldwide Gas Plant Survey, about 80% use either straight refrigeration or turboexpander technology (OGJ, June 6, 2011, p. 88).

With the last new lean oil plant built some 30 years ago, the estimated portion of new gas plants built today using these two technologies is greater than 95%.

Straight refrigeration units, which most often use propane or ammonia, can be built for essentially any capacity or feed-gas composition, are of mild steel construction, are relatively simple to construct and operate, and have proven to be economical and reliable. However, with their operating temperature typically limited to about –35° F., their capability for NGL extraction is limited.

For higher NGL recovery, today’s processor has but a single choice: cryogenic turboexpander. Since its inception in the late 1960s, turboexpander technology has evolved into the technology of choice for deep NGL-product recovery. As designs were refined, turboexpander technology essentially displaced lean-oil technology for high LPG or ethane-extraction applications.

Several variations of the technology are available, depending upon the targeted product recovery and feed-gas conditions, with proprietary designs offering even higher efficiencies. With operating temperatures as low as –200° F., NGL product recoveries approaching 98%+ are technically feasible.

With straight refrigeration technology, the benefits for the customer include low capital and operating expenditures (CAPEX and OPEX), a broad range of applicability, early production capabilities, but limited NGL recovery. Expander technology offers superior NGL-recovery potential but higher CAPEX and OPEX and a longer time to initial operation due to the long lead time of such specialty equipment as the turboexpander and brazed aluminum heat exchangers.

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