Maximize propane recovery and ethane rejection at cryogenic gas plants

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Maximize propane recovery and ethane rejec on at cryogenic gas plants The shale gas revolu on in the US has resulted in an abundance of NGL in the marketplace. The increase in output has also produced an excess of ethane, leading to a decrease in price for the chemical. In response to this dynamic, midstream processors have been exploring ways to increase NGL produc on while limi ng the amount of ethane content. A number of ways exist to recover NGL from gas streams. They range from simple Joule-Thompson (J- T) units and mechanical refrigera on to more complex NGL recovery designs, such as cryogenic turboexpander plants. One of the most commonly used technologies throughout the industry over the past 30 years has been the Gas Subcooled Process.a Gas-subcooled (GS) plants can be operated in both ethane-recovery and ethane-rejec on mode. This type of facility is o en referred to as a dual-mode process design, and it provides owner-operators with the flexibility to adjust plant opera ons to meet market demand. Ethane is a valuable hydrocarbon component that can be recovered as a liquid from natural gas streams. Typically, when the price of ethane is high, processors recover ethane as part of the NGL stream, where it is then frac onated and sold as a feedstock to the petrochemical industry to make ethylene. When the price of ethane is low, it is more profitable for processing facili es to reject it to the sales gas stream. Over the last several years, large increases in available US NGL volumes have been met with insufficient increases in industrial ethane demand. This scenario, coupled with regional transporta on bo lenecks, has caused ethane prices to be severely depressed and made it economical for NGL recovery plants to reject as much ethane as possible. For standard GS plants opera ng in ethane-rejec on mode, this means unavoidably rejec ng some of the s ll-valuable propane and butane to sales gas, which nega vely impacts plant profitability. As a result, significant economic value can be generated by implemen ng flexible processing solu ons that enable GS plants to operate in high-ethane-recovery or full- ethane-rejec on mode, without sacrificing propane and butane recovery. Here, a technical evalua on of solu ons for improving propane recovery and enhancing ethane rejec on at GS facili es is provided. Overview of standard GS technology. Compared to newer-genera on dual-mode process technologies, GS technology is economically limited. It typically requires excessive residue compression to achieve the same propane recovery levels that newer technologies are capable of providing in ethane-rejec on mode. Table 1 shows typical ethane and propane recovery rates for a GS plant opera ng in ethane-recovery and ethane-rejec on modes. Actual recovery rates will vary depending on a number of variables, including plant inlet condi ons, the composi on of the natural gas stream entering the plant, product specifica ons, product delivery condi ons and process design specifica ons. T. Miller, Audubon Engineering, Denver, Colorado D. Beck, Audubon Engineering, Houston, Texas TABLE 1. Typical GS plant recovery rates Component Ethane recovery Ethane rejec on Ethane 80%-93% < 2% Propane 98%-99% 85%-94% Butane 99+% 97%-99%

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