Market-Driven Evolution of Gas Processing Technologies for NGLs by        Yuv R. Mehra, P.E.,                 formerly President & COO, Advanced Extraction Technologies, Inc. Natural gas liquids (NGL's) are liquid hydrocarbons that are recovered from natural gases in gas processing plants, and in some cases, from field processing facilities. They include ethane, propane, butanes, pentanes and heavier components. These hydrocarbons exist as gases in the reservoir. The heaviest hydrocarbons (pentanes and heavier) that condense when they reach the surface separators due to reduction in temperature are termed condensate or natural gasoline. It is sometimes necessary to extract these hydrocarbons to prevent their forming hydrates. With a few exceptions, these NGL's are generally more valuable as petrochemical feedstock than as produced gas, and can be recovered from natural gas streams as a liquid product and sold separately from the pipeline gas. The NGL's are separated from the produced gas by a variety of techniques that have evolved over the past fifty years, the most common of which are discussed here. Absorption Process Lean oil absorption is a process in which the NGL's are removed by contacting the natural gas with a liquid hydrocarbon solvent (oil). After this lean oil absorbs the NGL's in an absorber column, the resulting rich oil is subjected to a distillation process to separate NGL's for sale and recycling the regenerated lean oil to the absorber column. Although first developed in 1911 (Cannon, 1997), the basic absorption process has been modified and improved in response to the market forces and technological advances. In the ambient lean oil absorption process the natural gas is contacted with the lean oil (molecular weight of about 150) in an absorber column at the ambient temperature of about 100°F. The rich oil exiting the bottom of the absorber flows into a rich oil depropanizer (ROD) which separates the propane and lighter components and returns them to the gas stream. The rich oil is then fractionated in a still, where the NGL's (C4+) are recovered as an overhead product and the lean oil is recycled to the absorber column. Typically, 75 percent of butanes and 85-90 percent of pentanes and heavier components are recovered. In the refrigerated lean oil absorption process, the lean oil is chilled against propane refrigerant to improve the recovery of propane to the 90 percent level, and depending upon the gas composition, up to 40 percent of ethane may be recovered (Elliot, 1997). Since reducing the molecular weight of lean oil enhances the lighter component absorption and an external refrigerant is used to chill the lean oil, 100 to 110 molecular weight lean oils are generally used in this process. Refrigeration Process Refrigeration plants are the simplest of any gas processing facility options. In the straight refrigeration process, the natural gas stream is chilled to about -30°F with an external propane refrigeration system. The condensed liquids are separated in a low temperature separator and stabilized in a deethanizer column. In most cases, the overhead from the deethanizer column is compressed, cooled and recycled to the inlet gas stream. The bottoms product comprising the NGL's (C3+ components) are typically trucked. The gases leaving the low temperature separator are cross-exchanged with the inlet gases prior to flowing into the sales gas pipeline. Depending upon the gas composition and pressure, propane recoveries range from 30 to 50 percent (Tannehill et al., 1991). Cryogenic Processes When it is desirable to recover ethane from the natural gas streams, it becomes necessary to chill the feed gas to significantly lower temperatures on the order of -120°F. To achieve this temperature, ethane or ethylene refrigerant system is cascaded into a propane refrigeration system. The lower temperature chilling in this cascaded refrigeration process significantly improves the propane recoveries into the 90+ percent level while recovering about 70 percent of the contained ethane. The C2+ NGL's leave the plant via a pipeline. With increasing demand for ethane, an alternative to the expensive cascaded refrigeration process that simplifies the equipment requirements evolved. The chilling of gas by reducing the gas pressure across a control valve is also known as the Joule-Thomson (J-T) effect. In particular, colder temperatures on the order of -100°F could be achieved by reducing the pressure of the chilled gas/liquid mixtures through an expansion device. Due to this chilling, the J-T process  achieves high ethane recoveries, typical ethane recoveries are about 70 percent. Since the primary source of chilling is provided by the reduction of gas pressure, this process does not require ethane refrigeration and its economics are most favorable if it is not necessary to recompress the sales gas, i.e. the high gas side pressure drop is free. Since for most gas processing facilities gas side pressure drop has a significant cost penalty, the first turbo-expander process was introduced in 1964 (Elliot, 1997) to reduce the cost of gas recompression. By isentropically expanding the chilled gases, through a high speed centrifugal turbine to produce cryogenic temperatures on the order of -150°F (Tannehill,1993), about 75 to 85 percent of ethane could now be recovered. By achieving the cryogenic temperatures, ethane is condensed and the chilled stream enters near the top of the demethanizer column for removing excess methane from the C2+ NGL product. Energy recovered by the gas expander is utilized to compress the lower pressure gas to reduce the overall gas compression requirements of the facility. The basic design of the turbo-expander process is still in use today. AET Process NGL Technology Since the introduction of the turbo-expander process, ethane supply in the United States continued to steadily increase throughout the 1970's. In early 1980's, for the first time in the U.S., ethane supply exceeded ethane demand whereby the price of ethane as petrochemical feedstock was lower than the Btu equivalent price if it were to be left in the gas stream. Thus it became necessary for the gas processors to have the ability for not recovering ethane while maintaining the extraction of propane and heavier NGL's -- the term 'ethane rejection' was introduced. While the turbo-expander plants built since the mid-1980's have been equipped for the ethane rejection operation, the limited flexibility of the cryogenic process resulting in loss of propane and heavier components has remained a concern for the gas processor. For example, under the ethane recovery operation 80 percent of ethane and 98 percent of propane are typically recovered; whereas under the ethane rejection operation 6 percent of ethane, 60 percent of propane and 85 percent of the butanes are typically recovered, assuming of course that no sophisticated ethane rejection capabilities requiring substantial capital outlay have been installed (Tannehill,1991). The patented, proven AET Process technology uses non-cryogenic absorption to maximize NGL production and to provide the market-driven component flexibility. The basic processing scheme recovers from a natural gas stream up to 96 percent of ethane and 99 percent of propane. In addition, the AET Process configuration offers on-line flexibility of maintaining propane recovery at the 98 percent level without requiring the recovery of ethane to exceed 2 percent. Some key differences between the AET Process technology and a conventional refrigerated lean oil process are as follows: Lower Molecular Weight Solvents The AET Process technology makes use of hydrocarbon solvents having molecular weight in the range of 70-90. For equivalent equipment size (pumps, columns, piping, exchangers etc.), a higher molar flow rate of solvent enhances the absorption capacity resulting in a larger gas throughput or conversely in higher component recoveries. Recognizing the higher vapor pressure of lighter solvents, the preferred molecular weight range achieves a cost effective balance between capital investment and solvent losses. In most applications, the required solvent is present in the feed and there is no need for any external solvent make-up. Reboiled Absorber Column Contrary to a conventional refrigerated lean oil plant wherein an absorber column is followed by a Rich Oil Deethanizer or Rich Oil Demethanizer (ROD) and the overhead vapors from ROD flow into the sales gas pipeline, in the AET Process plant the absorption and stripping sections are combined into one column. This reboiled absorber column can be operated as a demethanizer or a deethanizer. Heavier Feed Components Bypassed In order to maintain a relatively low solvent molecular weight in the preferred range, the heavier components in the inlet gas (primarily C7+ if present) which are condensed by inlet gas chilling are sent to a front-end stabilizer. When required, the overhead vapors from the stabilizer are compressed and returned to the inlet. The stabilizer is designed to produce a bottoms product that meets the light end specification of the desired C5+NGL product. Solvent Pre-saturator To improve the efficiency of absorption, the lean solvent is pre-saturated with the gases leaving the absorber overhead. Removing the heat of absorption of lighter components in the lean solvent ensures that the residue gas leaves the unit at its coldest processing temperature. As shown in the basic processing scheme, the pre-saturated solvent enters the reboiled absorber column at the top. Simplified Metallurgy As for the absorption (ambient and refrigerated) and simple refrigeration processes, most all equipment and piping required for the AET Process plant use simpler carbon steel metallurgy. To achieve high ethane recoveries, the cascaded refrigeration, J-T and turbo-expander processes, due to the significantly colder temperatures, require expensive high alloy steel--such as stainless steel--metallurgy. References 1. Cannon, R., 1997. "75 Years of Service: A Brief History of the Gas Processors Association, 1921-1997", Oil & Gas Journal Supplement, Mar. 4, 5p. 2. Elliot, D.G., 1997. "Technical Committee Guides Plant Design", Oil & Gas Journal Supplement, Mar. 4, 37p. 3. Tannehill, C.C., and Gibbs, J.E., 1991. "Evaluation of Hydrocarbon Liquid Disposition", GRI Topical Report No. GRI-91/0231, 47p. 4. Tannehill, C.C., and Galvin, C., 1993. "Business Characteristics of the Natural Gas Conditioning Industry", GRI Topical Report No. GRI-93/0342, 202p.
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