BENEFITS OF INTEGRATING NGL EXTRACTION AND LNG LIQUEFACTION TECHNOLOGY Authors Doug Elliot Wesley R. Qualls IPSI LLC ConocoPhillips Company President, Chief Operating Officer Director, Technology Licensing P.O. Box 2166 3000 Post Oak Blvd. – Mail Stop 59 Houston, TX 77252-2166 Houston, TX 77056 713-235-2400 713-235-4521 [email protected] [email protected] Shawn Huang Jong Juh (Roger) Chen ConocoPhillips Company Sr. Vice President Process Engineer IPSI LLC 600 N. Dairy Ashford P.O. Box 2166 Houston, TX 77079 Houston, TX 77252-2166 281-293-4124 713-235-4151 [email protected] [email protected] Jame Yao R. J. Lee Vice President, LNG Technology (Formerly) IPSI LLC IPSI LLC P.O. Box 2166 P.O. Box 2166 Houston, TX 77252-2166 Houston, TX 77252-2166 713-235-5231 713-235-2546 [email protected] [email protected] Ying (Irene) Zhang Process Engineer IPSI LLC P.O. Box 2166 Houston, TX 77252-2166 713-235-3179 [email protected] Copyright ©– 2005, ConocoPhillips Company Unpublished Prepared for Presentation at AIChE 2005 Spring National Meeting 5th Topical Conference on Natural Gas Utilization (TI) Session 16c – Gas. AIChE Shall Not Be Responsible For Statements or Opinions Contained in Papers or Printed in its Publications. Abstract The combination of changing global markets for natural gas liquids (NGL) with the simultaneous increase in global demand for liquefied natural gas (LNG) has stimulated an interest in the integration of NGL recovery technology with LNG liquefaction technologies. Historically, the removal of “heavy” or high-freezing-point hydrocarbons from the feed to LNG plants has been characterized as “gas conditioning” and achieved using one or more distillation columns. While some attempts to provide reflux to the distillation columns marginally enhanced NGL recovery, little emphasis was placed on maximizing NGL recovery as a product from the LNG process. As such, the integration of the two processes was not a priority. Integrating state-of-the art NGL recovery technology within the CoP LNGSM Process1, formerly the Phillips Optimized Cascade LNG Process, results in a significant reduction in the specific power required to produce LNG, while maximizing NGL recovery. This corresponds to a production increase in both LNG and NGL for comparable compression schemes as compared to stand-alone LNG liquefaction and NGL extraction facilities. In addition, there are potential enhancements to the overall facility availability and project economics using the integrated concept. This integrated concept has been applied to three ongoing international NGL/LNG projects using the CoP LNG Process. In these cases, LNG production has increased by approximately 7%, while using the same process horsepower. 1.0 Introduction Due to clean burning characteristics and the ability to meet stringent environmental requirements, the demand for natural gas has increased considerably over the past few years. Projections reflect a continued increase for the next several years. However, it is a clean burning methane rich gas that is in demand as opposed to the typical raw gas that exists in nature, which often includes additional components such as heavier hydrocarbons and other impurities. The heavier hydrocarbons, once separated from natural gas, are referred to as Liquefied Petroleum Gas (LPG) and Natural Gas Liquids (NGL). Impurities may include carbon dioxide, hydrogen sulfide, mercaptans, nitrogen, helium, water, and even trace contaminants such as mercury and trimethylarsine. Natural gas must be “conditioned” prior to liquefaction to remove undesired components. This “conditioning” normally takes place in separate or stand- alone facilities and typically includes the extraction of heavier hydrocarbons such as LPG and NGL. The “conditioned” gas is then typically fed to pipelines for distribution. However, transportation to distant markets through gas pipelines is not always economically or technically feasible. As such, natural gas liquefaction has become a viable and widely accepted alternative. The economics of liquefying natural gas is feasible due mainly to the great volume reduction achieved upon liquefying, which creates the ability to store and transport large volumes. The demand for Liquefied Natural Gas (LNG) in North America has increased considerably as energy demands have increased at the same time wellhead gas supply to pipelines has decreased. LNG is a natural alternative to supplement gas pipelines as the infrastructure to process and burn the gas is largely in place. In addition, LNG is a highly reliable source of gas. For instance, ConocoPhillips’ Kenai, Alaska facility, which utilizes the CoP LNG Process, has supplied LNG to Japan for over thirty-five years without missing a single cargo. Figure 1 is an aerial view of the recently constructed Atlantic LNG facility located in Point Fortin, Trinidad, comprised of three trains and a fourth under construction, all utilizing a modernized version of the CoP LNG Process. •10 MTPA from Trains 1,2&3 •Integrated NGL Extraction •5 MTPA from Train 4 (Under Construction) Figure 1 – The Atlantic LNG Facility – Artist Rendition, Courtesy of Atlantic LNG Company of Trinidad & Tobago Pentanes and heavier hydrocarbons, including aromatics having a high freezing point, must be substantially removed to an extremely low level in order to prevent freezing and subsequent plugging of process equipment in the course of liquefaction. In addition, heavy components must also be removed in order to meet BTU requirements of the LNG product. The heavy hydrocarbons separated from LNG, may then be utilized as petrochemical sales or for gasoline blending. In fact, NGL and/or LPG liquids may command a greater value than LNG. Many efforts have focused on recovery of these heavy hydrocarbons. However, most of the effort has been placed on removal of the heavy hydrocarbons in a separate NGL plant, located upstream of the LNG liquefaction facility. Alternatively, since all components having a higher condensing temperature than methane will be liquefied in the liquefaction process, it becomes technically practical to integrate NGL recovery within LNG liquefaction. Duplication of processing equipment and refrigeration requirements are avoided with an integrated approach. In fact, a substantial cost savings may be achieved when NGL recovery is effectively integrated within the liquefaction process. There have been attempts for NGL recovery within the LNG facility.(1) For example, lighter NGL components are recovered in conjunction with the removal of C5+ hydrocarbons using a scrub column in a propane pre-cooled mixed refrigerant process, as disclosed in the literature.(2, 3) The NGL recovery column is often required to operate at a relatively high pressure (typically above 550 psig) in order to conserve refrigeration compression horsepower requirements. Although refrigeration horsepower is conserved utilizing a high-pressure column, separation efficiency is often significantly reduced due to less favorable operating conditions, i.e. lower relative volatility. With careful integration of the NGL recovery and LNG processes, the overall efficiency of the integrated process can be significantly improved, thereby increasing NGL recovery and reducing specific power consumption.(4) This paper provides examples of NGL recovery technology integrated within the CoP LNG Process, while also presenting recovery and efficiency performance. 2.0 Traditional Stand-Alone Gas Plant Upstream of Liquefaction Plant A number of NGL recovery processes have been developed for natural gas and other gas streams.(5, 6, 7) Among various NGL recovery processes, the cryogenic expansion process has become the preferred process for deep hydrocarbon liquid recovery from natural gas streams. Figure 2 depicts a typical cryogenic expansion process configuration. In the conventional turbo-expander process, feed gas at elevated pressure is pretreated for removal of acid gases, water, mercury and other contaminants to produce a purified gas suitable for cryogenic temperatures. The treated gas is typically partially condensed utilizing heat exchange with other process streams and/or external propane refrigeration, depending upon the gas composition. The resulting condensed liquid, containing the less volatile components, is then separated and fed to a medium or low-pressure fractionation column for recovery of the heavy hydrocarbon components. The remaining non-condensed vapor, containing the more volatile components, is expanded to the lower pressure of the column using a turbo- expander, resulting in further cooling and additional liquid condensation. With the expander discharge pressure essentially the same as the column pressure, the resulting two-phase stream is fed to the top section of the fractionation column. The cold liquid portion acts as reflux, enhancing recovery of heavier hydrocarbon components. The vapor portion combines with the gas in the overhead of the column. The combined gas exits the column overhead as a residue gas. After recovery of available refrigeration, the residue gas is then recompressed to a higher pressure, suitable for pipeline delivery or for LNG liquefaction. To Pipeline or LNG Plant Recompression Expander / Compressor Feed Gas H2S/CO2 Removal Dehydration (Optional) Propane Refrigeration Demethanizer H2O NGL Extraction Reboiler NGL (C2+ or C3+) Figure 2 - Block diagram for typical NGL extraction plant The fractionation column (as described) acts essentially as a stripping column since expander