Hybrid Cryogenic Liquefaction Processes

Transcription

1 Hybrid Cryogenic Liquefaction Processes ICR0390 Rakesh Agrawal, Adam A. Brostow, D. Michael Herron, and Mark J. Roberts Air Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, PA , USA ABSTRACT Liquid nitrogen is used as a refrigeration source in a large number of applications. It is also shipped and vaporized to supply gaseous nitrogen for applications requiring small quantities of nitrogen. Liquid natural gas is produced at the source then transported to other locations for use as fuel. Liquefaction of either nitrogen or natural gas is a highly energy intensive process. It is essential to design liquefaction processes that are energy efficient and cost effective. The objective of this paper will be to first present a brief overview of the well-known gas liquefaction processes. This will be followed by the description of some state of the art processes. Finally, the latest results for newly developed hybrid liquefaction processes will be presented. These processes combine refrigeration methods commonly used in liquid nitrogen and liquid natural gas production. These results will highlight the optimum refrigeration methods for different temperature zones as gases are cooled from ambient temperature, liquefied, and subcooled. INTRODUCTION Liquid nitrogen (LIN) is used as refrigeration source in a large number of applications. It is used for food freezing. Rubber tires and scrap metal from old cars are reclaimed using cryogenic cooling techniques to make them brittle for easier fracturing and component separation. Biological materials such as bone marrow, blood, animal semen, tissue cultures, tumor cells and skin are preserved by cryogenic freezing and storage [1]. LIN is also shipped and vaporized to supply gaseous nitrogen for applications requiring small quantities of nitrogen. Large quantities of liquid oxygen are generally produced by feeding LIN to air distillation columns while withdrawing liquid oxygen. Liquefaction of nitrogen is a highly energy intensive process. It is essential to design liquefaction processes that are energy efficient and cost effective. The choice of a liquefaction cycle depends on the size of a plant. For small size plants, capital dominates the overall cost and energy efficiency is less important, and for larger size plants the reverse is true. While it is common to find LIN plants in the size range of tonnes per day (t/d); plants that are capable of producing in excess of 1000 t/d are now in operation. Large quantities of natural gas exist in remote areas far from major areas of natural gas consumption. Liquefied natural gas (LNG) enables the use of these remote gas supplies in locations of high energy consumption. LNG is produced in areas such as the Middle East, equatorial Africa and Southeast Asia in very large capacity or Baseload LNG plants. It is then transported long distances as liquid by ship to the consuming countries, and vaporized under pressure into pipeline networks to supply domestic and industrial consumption. LNG is expected to play an increasingly important role in the worldwide energy supply [2]. While LNG consumption has historically been limited to energy starved countries such as Japan and South Korea, increasing demand for natural gas and a decreasing cost of supply have made LNG cost competitive with pipeline supplied natural gas in regions where LNG imports have been limited, particularly North America [2,3]. The decreased cost of production is in part due to improvements in liquefaction technology. Typical LNG facilities comprise multiple parallel plants or trains. Significantly improved economies of scale have been achieved by rapidly growing LNG train sizes. Current LNG train sizes are approaching 5 mtpa (million tonnes per annum) or about 14,000 tonnes per day. 1

2 Like LIN production, LNG production is energy intensive. Power consumption for a state-of-the-art LNG train approaches 180 Megawatts. Energy efficiency is not directly a key driver in the design of liquefaction trains however, since power is generally generated onsite from Natural Gas, and LNG is produced at locations where natural gas supply far exceeds demand. Process efficiency can nonetheless become important indirectly, when it results in improved capital efficiency by increasing the LNG produced per unit investment. In this paper, first a brief historical development for liquefaction of gases is presented. This is followed by a discussion of current nitrogen and natural gas liquefaction processes and finally some of the latest developments are described. BRIEF HISTORY OF LIQUEFACTION [4] In 1895, Carl von Linde built the first industrial scale air liquefier. His liquefier utilized the Joule-Thompson effect with an adiabatic valve to provide refrigeration. His genius was the realization that for the same pressure ratio across a Joule-Thompson valve, the amount of cooling (drop in temperature) increases rapidly as the absolute pressure of air is increased. Therefore, such a liquefier operated at about 125 bar while the pressure across the Joule-Thompson (JT) valve was dropped to approximately 5 bar. In 1902, Georges Claude demonstrated that it was possible to lubricate a piston expander with petroleum ether at cryogenic temperatures. He then built an air liquefier using his piston expander. Since this liquefier did not rely on a JT valve to supply all the refrigeration, it was much more efficient than the liquefier built seven years earlier by Linde. In 1935, Kapitza built a piston expansion engine with gas lubrication and in 1939 he built an air liquefier with an expansion turbine. Most liquefiers now use expansion turbines. For the liquefaction of natural gas, it was shown that cascade cycles are more efficient than the cycles using only expansion turbines. A cascade cycle uses multiple refrigeration loops. In a given refrigeration loop, a vaporized and warmed refrigerant is compressed, condensed, cooled, reduced in pressure to create a low temperature, then vaporized at the low temperature and warmed for compression. Cascade cycles for the liquefaction of natural gas generally use three refrigeration loops; the warmest loop employs propane as the working fluid; the intermediate temperature loop uses either ethane or ethylene as the working fluid and the coldest loop employs methane as the working fluid. Propane after compression is liquefied at ambient temperatures, this loop then liquefies compressed ethane or ethylene, which in return liquefies compressed methane. All three loops also provide refrigeration to natural gas. A cascade cycle requires multiple compressors and heat exchangers and can be complex to operate. In 1959, Kleemenko suggested the use of a multicomponent mixed refrigerant using a single compressor [6]. Air Products and Chemicals, Inc. later introduced a propane-precooled mixed refrigerant cycle that has become the work horse of baseload natural gas liquefaction plants [7]. With the introduction of propane precooling to the mixed refrigerant cycle, efficiency was greatly improved and a substantial increase in capacity per train became possible. Details of other refrigeration methods such as Stirling cycle, pulse tube refrigerators, etc. can be found elsewhere [8]. These cycles are generally more amenable when refrigeration is needed on a small scale due to inherent equipment size requirements that prevent economical scale-up of these processes. CONVENTIONAL NITROGEN LIQUEFACTION CYCLES Almost all the LIN plants in the size range greater than 5 t/d use some kind of nitrogen liquefaction cycle using expanders (expansion turbines). A typical two-expander liquefaction cycle is shown in Figure 1. In Figure 1, make-up nitrogen from an air distillation cold box is compressed to about 6 bar in a make-up compressor and is further compressed to a pressure of about 30 to 35 bar in a recycle compressor. A portion of the medium pressure nitrogen leaving the recycle compressor is further boosted to a pressure in the range of 60 to 85 bar in compressor-1 and compressor-2 then fed to a heat exchanger for cooling. A medium pressure nitrogen stream is withdrawn near the warm end of the heat exchanger and expanded in a warm expander to provide a portion of the 2

3 refrigeration needed for the liquefaction. A high pressure nitrogen stream is withdrawn from an intermediate location of the heat exchanger and expanded in a cold expander to provide the refrigeration in the cold part of the heat exchanger. The remaining portion of the high pressure nitrogen stream exits the cold end of the heat exchanger at a temperature below -170 C and is sent to an optional dense fluid expander. The pressure drop across this dense fluid expander is maximized subject to the constraint that very little vapor forms in the exhaust. The pressure of this stream is further reduced to about 6 bar in a JT valve and the resulting two-phase stream is separated in separator-1. The vapor from this separator and the exhaust streams from the cold and warm expanders are mixed at appropriate temperatures, warmed and returned to the recycle compressor. The liquid from separator-1 is further cooled and reduced to near atmospheric pressure through another JT valve. The resulting liquid is collected as liquid nitrogen from separator-2 and the vapor is recycled to the make-up compressor. s are used for providing refrigeration by extracting work from an expanding fluid. In the expansion process, the temperature of the expanded fluid is reduced. For liquefiers, it is essential that the work energy from an expander be recovered to increase process efficiency. This is done by either loading an expander with an electric generator or a compressor for some other process fluid. When an expander is directly coupled to a compressor, the arrangement is called a compander. As seen from Figure 1, companders are widely used in liquefiers. s used in the liquefaction industry typically have isentropic efficiencies in the range of 85 to 90%. The dense fluid expanders are essentially reverse-running liquid pumps. Generally the make up and recycle compressors are high efficiency, multistage centrifugal machines. The liquefier of Figure 1 is quite efficient. The use of a dense fluid expander contributes to increased efficiency but its use is optional. The working pressure range of the modern brazed plate and fin aluminum heat exchangers now approaches 100 bar. For increased efficiency, the pressure of the high pressure nitrogen steam is increased to maximum feasible values. Some equipment details about the heat exchangers and the machinery can be found elsewhere [9]. There are several variations of the two-expander cycles. Some of these variations are described in other references [1,4,10]. Two-expander cycles are quite energy efficient and are generally economical for LIN plants greater than 50 t/d. There are several operating plants in the size range of 100 to 500 t/d using two-expander cycles. In Figure 1, if none of the expanders are used then the liquefaction process reduces to the one proposed by Carl von Linde. Such a cycle would be uneconomical except in rare cases when extremely small quantities of LIN need to be produced. In Figure 1, if the warm expander and the dense fluid expander are removed, then the resulting oneexpander process is similar to the one used by George Claude. Another one-expander liquefier obtainable from Figure 1 is when the cold expander and the dense fluid expander are eliminated from Figure 1. In this case, all the high pressure nitrogen is sent to separator-1 and warm expander spans a much lower temperature. Single-expander cycles are cost effective up to plant sizes in the neighborhood of 50 t/d. Stirling cycle has been used for really small size plants (less than 5 t/d). A NEW NITROGEN LIQUEFIER Past attempts to improve the efficiency of a two-expander nitrogen liquefier have included the use of an ammonia or freon chiller to precool the high pressure and medium pressure nitrogen streams to about -40 C prior to their processing in the heat exchanger of Figure 1. However, improvements in turbo-expander efficiencies and environmental restrictions on the use of certain refrigerants have reduced the applicability of such precooling approaches. On the other hand, attempts to liquefy nitrogen by using the closed loop refrigerant cycles similar to the ones used to liquefy natural gas have resulted in lower efficiencies due to the low temperatures that must be achieved with the refrigerant. In this study we propose nitrogen liquefaction cycles that use either a cascade cycle with multiple refrigeration loops or a mixed refrigerant (MR) loop to provide refrigeration from near ambient temperature down to about -100 to C. The colder refrigeration is then provided by an expander. A MR-expander LIN cycle is shown in Figure 2. In the proposed MR-expander nitrogen liquefaction cycle of Figure 2, the nitrogen expander loop is similar to the one in Figure 1 except that no warm expander is used. Instead a MR loop is used to provide refrigeration at warm 3

4 temperatures. The MR is typically composed of methane and ethane, along with other hydrocarbon compounds. The MR vapor exits at the warm end of the heat exchanger at about two to three atmospheres. Its molar flowrate is similar to the nitrogen molar flowrate through make up compressor separator-2 which in return is about 20% more than the LIN production from separator-2. The MR vapor is compressed in the neighborhood of 20 bar in the MR compressor and results in a two-phase stream after cooling to near ambient temperature in the ambient cooler. Cooling water or ambient air may be used for this cooling duty. The vapor and liquid fractions are separated in a MR-separator. The liquid fraction is partially cooled and then reduced in pressure across JT valve-1 and fed back to the heat exchanger for evaporation. The vapor fraction is cooled, liquefied and reduced in pressure across JT valve- 2 and returned to the heat exchanger for evaporation. Similar to the two-expander cycle, the nitrogen through recycle compressor and the compressor mounted to the expander is compressed to a high pressure in the neighborhood of 70 bar. A fraction of this high pressure nitrogen after cooling is sent to the expander while the other fraction exits the cold end of the heat exchanger. The cooled high pressure portion is let down in pressure across two separators to yield final LIN product. Sample calculations were done to compare the MR-expander liquefier (Figure 2) with the two-expander liquefier of Figure 1 at an overall production rate of about 640 t/d. Dense fluid expander was not included for either of the liquefiers. The relative powers for various machines are listed in Table 1 The MR-expander is found to be about 8% better in total power consumption. It is also worth noting that the total installed machinery power, which includes power generated in the expander and the power consumed by the compressor mounted on this expander, is about 30% lower (107.5 vs ) for the MR-expander cycle! This helps to keep the total machinery cost for the MR-expander cycle comparable to that for the two-expander cycle. However, the use of a MR loop adds some cost and this process will generally be attractive for mid to big size LIN plants. There are many variations of the MR loop that can be employed. Some lead to machinery and process simplification while others lead to increased efficiency. Details of these modifications can be found in a U.S. patent [11]. One of the precautions with the MR-expander cycle is to insure that no hydrocarbon leaks into LIN product. This is particularly true when some LIN is to be fed to an air distillation cold box. This requires a careful design of the brazed plate and fin heat exchanger such that whenever a nitrogen stream is in passages that are adjacent to MR passages, its pressure be much higher than the MR pressure. In other words, only high pressure nitrogen passages should be adjacent to the MR passages. This will insure that in the event a leak were to develop, the leak flow will be from nitrogen to MR. As an added safety feature, LIN product could be continually analyzed for any hydrocarbon presence in a buffer tank and then transferred to the product storage tank. Air Products has safety experience for this technology through the design, construction and operation of LIN and liquid oxygen plants that use refrigeration from liquefied natural gas [12-14]. Table 1 Relative and Powers Conventional and Hybrid Nitrogen Liquefiers MR- Two Make-up Recycle MR Total Externally Supplied Power Warm 15.2 Cold Total Installed Machinery Power 107.5(*) 152.4(*) * The expanders are mechanically coupled to compressors of equal power 4

5 Figure 1 A Two- Nitrogen Liquefier Make-Up Recycle Makeup N2 Medium Pressure N2 High Pressure N2 Warm End Heat Exchanger Warm -1 Cold -2 Cold End Separator-2 JT Valve Dense Fluid LIN Separator-1 Figure 2 An MR- Nitrogen Liquefier Makeup N2 Ambient Cooler MR Make-Up Recycle Medium Pressure N2 High Pressure N2 MR Separator Warm End JT Valve-1 JT Valve-2 Heat Exchanger Cold N2 Cold End -2 Separator-2 Dense Fluid / JT LIN Separator-1 5

6 CONVENTIONAL NATURAL GAS LIQUEFACTION CYCLES Single MR Process The Single MR process was a substantial improvement over the Cascade Processes existing at the time, simplifying the process and minimizing the number of equipment items. The advantage resulted from the use of a mixed component refrigerant rather than pure fluids to efficiently provide refrigeration over the temperature range required. Natural gas at liquefaction pressures in the neighborhood of bar exhibits complex cooling behavior (Figure 3). Mixed refrigerants offered a way to provide the required refrigeration over the temperature range efficiently by tailoring the composition of the mixed fluid so as to boil over a temperature range closely matching the refrigeration demand. The single MR process proved to be simple and relatively efficient, and it represented a major step forward in baseload LNG technology. C3-MR Process Following on the successful introduction of Single MR for baseload applications, Air Products developed the C 3 -MR process. The C 3 -MR process improved upon the Single-MR process by using a simple propane refrigeration loop for pre-cooling (Figure 3) With the introduction of propane precooling to the mixed refrigerant cycle, efficiency was greatly improved and a substantial increase in capacity per train became possible. Whereas the first baseload Single MR train at Libya had a capacity of about 0.6 mtpa per train, the train capacity at the first C 3 -MR Plant (Brunei) is over 1.0 mtpa per train. This increase in capacity is largely due to the reduction in mixed refrigerant volumetric flow due to the use of propane precooling. The reduction effectively de-bottlenecked the design of both the heat exchange and the mixed refrigerant compression equipment. C 3 -MR LNG train sizes have continued to grow with the availability of larger drivers and compressors, larger heat exchanger manufacturing capability, and continued process development. Recent trains under construction utilizing this concept with two GE Frame 7 gas turbines directly coupled to process compressors have liquefaction capacities of 4.5 to 5 mtpa. Figure 3 C 3 -MR Natural Gas Cooling Curve Figure 4 AP-X TM Hybrid LNG Process Natural Gas Cooling Curve Propane Pre-Cooling 0 Propane Pre-Cooling Temp. C -50 Temp. C -50 N 2 Sub-Cooling Mixed Refrigerant Liquefaction -150 Mixed Refrigerant Liquefaction and Sub-Cooling Duty Duty 6

7 A NEW NATURAL GAS LIQUEFIER While the C 3 -MR process remains the preferred option in many cases, there exists substantial developing demand for larger train sizes than can easily be achieved with C 3 -MR. For example, trains using multiple GE Frame 7 or Frame 9 gas turbine drivers or large electric motors can be configured. While it remains feasible to further increase train capacity with a C 3 -MR process, new designs must be developed for several major equipment items at capacities exceeding 5.0 mtpa. For example, the propane and centrifugal MR compressors are approaching single casing flow limits at current world scale LNG plant production levels. In response to continuing demand for increased LNG train capacity and lower unit cost, Air Products has developed and patented the AP-X TM Hybrid LNG Process [15,16]. The new process cycle is an improvement to the C 3 -MR process in that the LNG is subcooled using a simple, efficient nitrogen expander loop instead of mixed refrigerant (figure 4). Other variations of this process include a dual MR version where nitrogen is likewise used for subcooling. In addition to improving the efficiency, the use of the nitrogen expander loop makes greatly increased capacity feasible. It does this by reducing the flow of both propane and mixed refrigerant. Volumetric flow of mixed refrigerant at the low-pressure compressor suction is about 60% of that required by the C 3 -MR process for the same production. Mass flow of propane is about 80% of that required by the C 3 -MR process (Table 2). With the new process, train capacities up to 8 mtpa are feasible in tropical climates, in existing compressor frame sizes, without duplicate/parallel compression equipment, and using a spool-wound heat exchanger of a size currently being manufactured. The AP-X TM process cycle is depicted in figure 5. As is the case with C 3 -MR process, propane is used to provide cooling to a temperature of about -30 C. The feed is then cooled and liquefied by mixed refrigerant, exiting the Main Cryogenic Heat Exchanger at a temperature of about -120 C. Final subcooling of the LNG to 160 C is done using cold gaseous nitrogen from the nitrogen expander. If the Nitrogen expander loop and Subcooler are removed from Figure 5 flowsheet, the process reverts to a C3-MR process, albeit one that produces relatively warm LNG with a high bubble point pressure. It is possible however, to operate a train in reduced capacity as a C3-MR process by changing the composition of the circulating mixed refrigerant; making it lighter. Figure 5 AP-X TM Hybrid Process MR JT Valve Separator LNG LNG Main Cryogenic Heat Exchanger Subcooler Natural Gas Feed C3 Fuel Gas N2 Coldbox N2 C3 MR N2 Recycle 7

8 Why Nitrogen for the Loop? Nitrogen is the preferred working fluid for the subcooling expander cycle primarily because of its physical properties. Nitrogen has a vapor pressure of 17 to 23 bar at the required natural gas liquefaction temperature and has several advantages over a fluid that has a lower vapor pressure (e.g. methane). For example, the elevated pressure results in a relatively small volumetric flow rate in the low-pressure nitrogen circuit. This decreases the size and therefore the cost of the associated equipment. To put it in perspective, the volumetric flow of nitrogen entering the nitrogen compressor is about 35% percent of the volumetric flow of low pressure mixed refrigerant at the compressor suction. In addition, elevated pressure improves the efficiency by reducing the effect of pressure drop losses. Pressure drop losses are proportional to the absolute pressure. As a result, a given pressure drop has 10 times the impact at 2 bar as the same P would have at 20 bar. Table 2 Relative Comparison of AP-X TM and C 3 -MR Processes C 3 -MR AP-X TM Capacity, mtpa Up to 5 Up to 8 MR volumetric flow per unit production (relative) Propane compressor mass flow per unit production (relative) Specific Power (relative) CONCLUSIONS The Hybrid cycles presented combine features of cycles used in Nitrogen and Natural Gas liquefaction. By combining mixed refrigerant and expander cycles in an optimal way, the efficiency and cost effectiveness of both Natural Gas and Nitrogen liquefaction can be improved. A brief review of the known nitrogen expander cycles for LIN production is presented. The two-expander cycle is the current work horse of the industry. A modification is proposed where most of the warm refrigeration is provided by a MR cycle. The much colder refrigeration is provided through the use of a nitrogen expander. This results in the optimum refrigeration method being used at different temperature levels. Starting at ambient temperature down to about -100 C to -150 C, a mixed refrigerant loop is found to be more efficient. However, as temperature is lowered further, efficiency of the MR loop drops and an expander is found to be more efficient in providing refrigeration. This synergy results in new MR-expander cycles that are much more efficient than the currently used expander processes. Economies of scale continue to favor increasing train size for baseload LNG plants. The new process is a hybrid of a C 3 -MR cycle for pre-cooling and liquefying LNG and a nitrogen gas compressor/expander cycle for sub-cooling LNG. The process achieves high efficiency and low production cost by using both cycles to their best advantage. Table 2 presents a comparison of the C 3 -MR and AP-X TM processes. With the new process, an increase in single train capacities from 5 to 8 mtpa is feasible in tropical climates, using existing compressor frame sizes without split casing or duplicate compression equipment, and using a spool-wound heat exchange equipment of a size currently being manufactured. Acknowledgement The Authors are grateful for the continued support and encouragement provided by Mr. George A. Parrish and Mr. James C. Bronfenbrenner for the development of these new liquefaction processes. 8

9 English to French/Translation of Abstract Les Procédés de Liquéfaction Cryogènes Hybrides L' azote liquide se utilise comme réfrigérant dans une grande nombre d'applications. D ailleurs, on le transporte et le vaporise afin de fournir de l'azote gazeux pour des applications qui ne nécessitent que quantités d'azote très petites. Le gaz liquide naturel se produit à la source et ensuite se transporte à d autres endroits afin de l utiliser comme combustible. La liquéfaction soit de l'azote soit du gaz naturel est un procédé très intensif quant à l'énergie impliqué. C'est très important à concevoir des procédés de liquéfaction qui sont efficaces quant à l'energie qu à le coût. L'objectif de ce rapport est tout d'abord de présenter une revue brève globale des procédés de liquéfaction de gaz qui son bien connus. Ensuite on y présentera une description de quelques procédés très actuels. Enfin, on présentera les résultats les plus courants à propos des procédés de liquéfaction hybrides. Ces procédés réunient les méthodes de réfrigération qu on utilise le plus souvent en la production de l'azote liquide et du gaz naturel liquide. Ces résultats indiqueront les méthodes de réfrigeration optimales pour les zones de température différentes pendant qu on réduit la température des gaz de la température ambiante à celle de liquéfaction. References 1. Agrawal, R., Rowles, H. C., Kinard, G. E., Cryogenics, Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, John Wiley, Vol. 7, (1993) Byrne, L and Audi, C, Liquefied Natural Gas, Morgan Stanley Dean Witter Industry Report, September 22, Morris, R and Schmitz, M, Liquefied Natural Gas, SalomonSmithBarney Equity Research: Exploration and Production, April 19, Agrawal, R. and Herron, D. Michael, Air Liquefaction/Distillation, Encyclopedia of Separation Science, Academic Press, London, (2000) Timmerhaus, K. D. and Flynn, T. M., Cryogenic Process Engineering, Plenum Press, New York (1989), Chapter Kleemenko, A. P., One Flow Cascade Cycle, Proc. 10 th Int. Congress of Refrigeration (1959), 1, Gaumer, L.S. and Newton, C.L., Combined Cascade and Multicomponent Refrigeration System and Method, U.S. Patent 3,763,658 (1973). 8. Flynn, T. M., Cryogenic Engineering, Marcel Dekker, New York (1996), Ch. 6,. 9. McGuinness, R. M., In Oxygen-Enhanced Combustion, C. E. Baukal, Editor, CRC Press, Boca Raton (1998), Ch Thorogood, R. M., In Cryogenic Engineering, B. A Hands, Editor, Academic Press, London (1986), Ch Brostow, A. A., Agrawal, R., Herron, D. M., Roberts, M. J., Process For Nitrogen Liquefaction, U.S. Patent 6,298, Agrawal, R., Liquefied Natural Gas Refrigeration Transfer To A Cryogenic Air Separation Unit Using High Pressure Nitrogen Stream, U. S. Patent 5,137,558 (1992). 13. Agrawal, R. and Ayres, C. L., Production Of Liquid Nitrogen Using Liquefied Natural Gas As Sole Refrigerant, U. S. Patent 5,139,547 (1992). 14. Agrawal, R. and Cormier, T. E., Use of Liquefied Natural Gas (LNG) Coupled With A Cold To Produce Liquid Nitrogen, U. S. Patent 5,141,543 (1992). 15. Roberts, M.J. and Agrawal, R, Hybrid Cycle For The Production Of Liquefied Natural Gas, U. S. Patent 6,308,531 (2001). 16. Roberts, M.J., Petrowski, J.M.,Liu, Y.N., and Bronfenbrenner J.C., Large Capacity Single Train AP-X TM Hybrid LNG Process, Gastech 2002; Doha, Qatar 9