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Investigation and Performance Improvement of the Propane Pre-cooling Cycle in the PPMRC Liquefaction Process Mohamed Fahmy, Hoda Nabih, and Mohamed Abd El-Aziz Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04249 • Publication Date (Web): 23 Feb 2016 Downloaded from http://pubs.acs.org on February 27, 2016
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Investigation and Performance Improvement of the Propane Pre-cooling Cycle in the PPMRC Liquefaction Process M.F.M. FAHMY; Department of Chemical Engineering, Faculty of Engineering, Cairo University, Cairo, Egypt. * H.I. NABIH; Department of Chemical Engineering, Faculty of Engineering, Cairo University, Cairo, Egypt. Tel: 00202-27364657 E-mail:
[email protected] M.R. ABD EL-AZIZ, Cairo Oil Refining Company (CORC), Cairo, Egypt. ABSTRACT This study aims to improve the performance of the propane pre-cooling cycle used for pre-cooling of both natural gas and mixed refrigerant in the Propane Pre-cooled Mixed Refrigerant Cycle (PPMRC) in an LNG plant. The unit is simulated by Aspen HYSYS version 7.3 for optimization of the propane pre-cooling cycle to provide the minimum energy consumption of the propane compressors and the two air-coolers. The effect of sub-cooling temperature in the propane aircooler, the number of compression stages in the propane cycle and the quality of natural gas feed is investigated. Results reveal the extent of reduction in power consumption on decreasing the sub-cooling temperature of liquid propane. The optimal sub-cooling temperature of liquid propane and number of compression stages in the propane cycle is determined. The positive impact attained in the performance of propane cycle is higher for lean feed gas than for rich feed gas. The decrease of liquid propane sub-cooling temperature has a dominating influence on all performance criteria and hence, can be considered as the key contributor affecting the propane pre-cooling cycle in the PPMRC liquefaction process. Keywords: Natural gas – Liquefaction process - Propane 1. Introduction Natural gas is one of the cleanest, safest and most useful of all energy sources and helps to meet the world’s rising demand for cleaner energy1. It is one of the most important primary energy sources for the 21st century and it is expected that by 2020 the natural gas would account for about 30 percent of the total electricity generation2, 3. Natural gas is desirable because it is clean, efficient, safe, abundant and economical4, 5. The industrial and electric power sectors together account for 77 percent of the total projected world increase in natural gas consumption6. The natural gas composition varies depending on the gas field and the gas is commonly classified according to their liquids content as either lean or rich gas where the more the C2+ in the gas, the ‘richer’ the gas7, 8. When natural gas transportation by pipeline is overly expensive, gas must be either liquefied or converted to high value liquid products. Liquefaction of natural gas has advantages over chemical conversion in that LNG (Liquefied Natural Gas) has a heating value about 40% greater than liquid from chemical fuels derived from conversion of natural gas. Transportation of natural gas as LNG over large distances or across water bodies is economical or cost effective9, 10. The objective of producing LNG is the huge reduction in volume, a factor of (*) Author to whom correspondence should be sent. E-mail:
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640, decreasing the size and cost of the storage and transportation containers. LNG offers greater trade flexibility than pipeline transport, allowing cargoes of natural gas to be delivered where the need is greatest and the commercial terms are most competitive11. Liquefied natural gas can be made on low use days and vaporized on peak days to meet the demand12. The Asia Pacific region is by far the leading market for LNG, accounting for 61% of total imports in 2013 and Europe is closely followed by the Asia region where these regions cover 88% of total LNG imports13. To meet the increasing demand for natural gas, China by the year 2020 needs to import LNG at a level of more than 20 billion cubic meters per year14. The global demand of liquefied natural gas (LNG) has risen rapidly in recent years for the reasons of energy security. LNG operations are highly capitalized where upfront costs are large for construction of liquefaction facilities, purchasing specially designed LNG ships and building regasification facilities12. The largest component of the total cost of the LNG value chain is usually the liquefaction plant while the production, shipping and regasification components account for nearly equal portions of the remainder15. The natural gas liquefaction process is energy intensive due to its cryogenic condition which is below -162°C at atmosphere. Therefore, minimizing energy consumption is the major concern in natural gas liquefaction process design1618 . One of the primary challenges in the LNG industry is to improve the efficiency of the current natural gas liquefaction processes together with cost savings19. Different energy recovery process configurations in the natural gas liquefaction processes and their potential improvements of energy savings were studied while considering the capital costs20. Nitrogen based single and dual expander processes were analyzed for efficiency improvement considering compression energy minimization as an objective21. Multi-stage expander refrigeration cycles were proposed and analyzed for the development of an efficient LNG process22. The three key elements in liquefaction are compression needed in the refrigeration cycles, the power to drive those cycles and the heat exchanger technology and the past five decades has met major technical developments in these three areas23. Liquefaction has used more mature technologies and simpler process equipment24 and research interests has led to recent advances in LNG value chain25, 26. Various combinations of refrigeration cycles are used in licensed LNG production processes, but most employ gas turbine-driven compressors to achieve the necessary cryogenic temperatures27. Pre-cooling process and adding an expander in the liquefaction cycle is an effective way to increase liquefaction efficiency for various liquefaction cycles28. Previous works were limited to the calculation of the thermodynamic efficiency of different natural gas liquefaction processes29-31. Natural gas liquefaction step consumes the highest energy because of the energy consumed by compressor units in the cryogenic liquefaction process and thus, attempts have been made to minimize energy consumption32. A combined process integrating natural gas liquefaction and liquid recovery was proposed33. A new natural gas cascade liquefaction cycle that utilized propane, nitrogen monoxide (N2O), and nitrogen gas (N2) cycles was designed with staged compression where a three-stage compression cycle showed a higher efficiency than the single-stage process. The new liquefaction cycle required less specific power (16% reduction) and reduced compressor work by 15 % for the same amount of liquefied natural gas (LNG) as compared to the optimized cascade cycle34. There are three main types of liquefaction cycles: cascade, mixed refrigerant and expansion cycles35. Air Products and Chemicals Incorporation (APCI) has launched the APX process (C3/MR/N2 cycles), SHELL a DMR process, LINDE a process with three mixed refrigerant cycles and IFP/Axens another DMR process with plate-fin heat exchangers. Deciding which of the process to be used for a given project is now much more difficult and many factors must be considered to make a proper comparison36-38. The performances of natural gas 2 ACS Paragon Plus Environment
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liquefaction cycles were compared and the study included a cascade cycle with a two-stage intercooler consisting of propane, ethylene and methane cycles and also, a modified staged compression process. It was shown that the coefficient of performance of the cascade cycle with a two-stage intercooler was higher than the basic cycle by 13.7% while the modified staged compression process was higher by 29.7% and an improvement of 28.5%. in the yield of liquefied natural gas was obtained39. The LNG process; Propane Pre-cooled Mixed Refrigerant Cycle (PPMRC), so far the most common process used, has been applied in LNG plants producing from 1 to 5 MMTPA of LNG per train using gas turbine or electrical drivers. The process has proven to be efficient, flexible, reliable and cost-competitive40. A base-load plant typically has a production capacity above 3 MMTPA (million tons per annum) of LNG and the main world-wide LNG production capacity comes from this type of plant applying the PPMRC process41. The versatility of the propane pre-cooled mixed refrigerant cycle makes it well-suited to accommodate this ever changing industry and is considered as the dominant liquefaction cycle42. Large-scale liquefaction of natural gas in the Propane Pre-cooled Mixed Refrigerant Cycle (PPMRC) consumes a large amount of energy because of high power consumption in the compressors and the two air coolers thus, optimization is necessary. Studies for improvement of the propane pre-cooling mixed refrigerant cycle requested modifications of the existing LNG plant configurations43, 44. One of the most important challenges in the natural gas liquefaction plants is to improve the plant energy efficiency20. A good understanding of design and operational requirements and efficiencies of natural gas liquefaction systems is essential for the success of the gas liquefaction plant. The real keys in developing a successful liquefaction plant are equipment selection and it’s configurations as well as operating parameters determination which meet the plant’s capacity goals45 and reduce cost and increase project feasibility46. In terms of energy efficiency, configuration strategies in liquefaction cycles change the number and association of equipment that make-up the liquefaction cycle within a feasible range47. It was indicated that in case of threestage compression systems, additional power savings was obtained and was energetically efficient, even better than a two stage mixed refrigerant (C2/C3) process48, 49. The application of C3MR to a floating natural gas liquefaction plant was studied where the pre-cooling cycle had triple compressions while the liquefaction cycle had only one compression50. Approaches for the optimal design of either single mixed refrigerant cycles or to systems consisting of two of these in cascade was presented indicating the importance of considering multistage compression and capital costs during optimization51, 52. A mixed refrigerant (MR) cycle was optimized while applying two compression stages in the cycle53. The single mixed refrigerant (SMR) process has been optimized for NG liquefaction and four compression stages and four intercoolers have been used54. A Modified Single Mixed Refrigerant (MSMR) technology was proposed for offshore natural gas liquefaction and the refrigerant required three compression stages55. The C3MR Cascade cycle was studied while applying one, two and three compression stages56. A practical method was proposed for determining the optimum mixed refrigerant composition and the task of compression was performed in four stages57, 58. A study handling the dual mixed refrigerant (DMR) process was operated using two mixed refrigerants having different compositions and the warm mixed refrigerant was compressed in two compression stages59. Also, a dual mixed refrigerant (DMR) encountering three compressors has been examined for possible application to liquefied natural gas floating, production, storage, and offloading (LNG FPSO)60. The Propane Pre-cooled Mixed Refrigerant (PPMR) Cycle accounts for a very significant proportion of the world’s base load LNG production capacity and its performance improvement is essential. In the PPMR process, there are two main refrigerant cycles; the pre-cooling cycle which 3 ACS Paragon Plus Environment
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uses a pure component; propane, and the liquefaction cycle which uses a mixed refrigerant (MR). An optimal LNG plant is categorized by having low initial cost as well as low energy consumption46, 53. Previous studies showed that the liquefaction energy efficiency can be improved by optimizing the mixed refrigerant composition and mass flow rate53. A simulation of the propane pre-cooled mixed refrigerant (C3-MR) liquefaction plant planned to be built with three pressure levels of propane cooling indicated that the specific horse power for the C3-MR process depends on natural gas supply temperature and pressure61. The MR composition abiding with the variation in ambient conditions has been determined for the small and mid-scale LNG plants62, 63 and methods for the selection of the refrigerant composition were proposed64-66. About 77 % of base-load natural gas liquefaction plants employ propane pre-cooled mixed refrigerant cycle (APCI) where the propane cycle and the multi-component refrigerant (MCR) cycle are involved in the cooling and liquefaction of natural gas67. Thus, the majority of the liquefaction plants use the APCI liquefaction technology mainly because of its advantages of which are the flexibility, ease of refrigerant make up, better specific power compared to other technologies operating at the same conditions. Also, the use of mixed refrigerants in addition to utilizing the spiral wound heat exchanger increases the thermal efficiency of the cycle68, 69. Further, it was indicate that more than 95 % of the installed LNG facilities use a pre-cooling cycle as an initial stage of the liquefaction process and a clear idea of the technical advantages/disadvantages of the pre-cooling cycle is essential for future project developments70. Hence, propane pre-cooling cycle is a main part of the liquefaction technique and it was specified that it represents about 20 % of the heat load of the liquefaction process67. Thus, the main objective of this study is to investigate and analyze the performance of the propane pre-cooling cycle in the PPMRC liquefaction process in an attempt to reduce the high power consumption which would finally allow savings in total cost of the process. 2. Process Description The present study investigates the propane pre-cooling cycle used for cooling of both treated natural gas (NG) and mixed refrigerant (MR) in the PPMRC process aiming to enhance and attain a better performance of the natural gas liquefaction process. The propane pre-cooled mixed refrigerant cycle (PPMRC) licensed by APCI is the technology applied in the LNG plant located at Damietta, Egypt (SEGAS). The propane refrigeration system operates in a closed loop utilizing propane evaporating at different pressure levels to supply refrigeration to the NG feed circuit and the MR circuit. In this process, the evaporated propane is compressed in a multi-stage centrifugal compressor driven by a frame 7 gas turbine driver to 18 bara. Further, propane from discharge of propane compressor is de-superheated and condensed by ambient air in the propane cooler condenser to provide a saturated liquid refrigerant at a temperature of 50 oC. The condensed propane at 50 oC is collected in the propane accumulator. The propane liquid is then sub-cooled in the propane sub-cooler before being supplied to the evaporators. The cooling is achieved in kettletype exchangers with propane refrigerant boiling and evaporating in a pool on the shell side, and with the process stream flowing in impressed tube passes. Propane vapors generated by pressure reduction and heat loads on each level of propane evaporators are returned to propane compressor via suction drums. Vapor propane fed to the first stage is compressed and blended inside the propane compressor with the propane vapor from the second stage evaporators to supply the second stage suction. This is repeated for each stage until the final stage discharge. 3. Methodology The study was conducted to detect the propane pre-cooling cycle configuration and operating parameters which provide the minimum energy consumption of propane compressors and two 4 ACS Paragon Plus Environment
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air–coolers used for cooling and sub-cooling of both gas and liquid propane The configuration and the operating parameters which maximize energy savings in the process and achieve minimum total cost were to be determined for the enhancement of the performance of the propane pre-cooling cycle. Three different configurations of the propane cycle; three, four (Figure 1) and five compression stages have been examined for different sub-cooling temperatures of the liquid propane in the second air cooler and all calculations were conducted for the different qualities of natural gas feed; lean and rich gas. The quested configuration is that which provides the minimum energy consumption of the propane compressors and the two air coolers used for cooling and subcooling of both gas and liquid propane. Thus, the procedure followed studied the influence of the following parameters: - Sub-cooling temperature of liquid propane in the second air cooler (AC2 propane sub-cooler). - Number of compression stages in the propane cycle; three, four and five stages. - Quality of natural gas feed (lean and rich gas). The maximum sub-cooling temperature; 35 oC is the temperature at which the fraction of liquid propane obtained at a given pressure P is equal to unity reached after the expansion step. At this point, the propane is a saturated liquid and represents the maximum obtainable flow rate of liquid propane in the propane refrigeration cycle. Any further sub-cooling is just more cost expenses without any benefits of higher fractions of liquid propane. Further, the minimum sub-cooling temperature; 50 oC is the temperature obtained at the operating pressure of the propane cycle indicating the saturated liquid propane temperature. The modeling and simulation of the propane cycle was conducted by the process simulator Aspen HYSYS version 7.3 to investigate the effect of variation of operating and design parameters on the performance characteristics of the propane pre-cooling cycle such as: - Power consumption in propane compressors. - Power consumption in the fin fans air cooler propane condenser (AC1). - Power consumption in the fin fans air cooler propane sub-cooler (AC2). - Propane flow rate in the propane closed cycle. - Duties of the two air-coolers used for cooling and sub-cooling of both gas and liquid propane. - Total refrigeration duty. - Total fixed and operating costs for each of the investigated configurations. Aspen HYSYS process simulator was selected as the simulation tool because it has features that accommodate many of the special requirements involved in the NG liquefaction processes and also due to its dynamic modeling capabilities71-73. HYSYS extensive thermodynamic libraries include a wide range of property calculation methods and thus convey robustness to the property calculations21. The simulation was conducted for the three different configurations using the Peng-Robinson Stryjek-Vera (PRSV) equation of state (EOS) 74-77. Table (S1) shows the mass flow rates (capacity) and conditions of the natural gas feed and the mixed refrigerant in a PPMRC of a large capacity C3-MR LNG train with design MR/NG mass ratio of 2.116 used at the SEGAS (Spanish Egyptian Gas Company) and the average molecular weight of MR = 24.907 kg/kg mol. The compositions of the natural gas feed (lean and rich) and the mixed refrigerant are summarized in table (S2). The condensation pressure is 18 in the propane condenser. The temperature and evaporation pressure values of each compression stage in the propane cycle for the different cases of three, four and five compression stages are summarized in table (S3). Figure (1) shows a simulation process of the liquefaction cycle using Aspen HYSYS. The specifications and data corresponding to the operational units used in the simulation of the PPMRC are summarized in table (S4). 5 ACS Paragon Plus Environment
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4. Results and Discussion 4.1. Effect of sub-cooler temperature on propane cycle performance (Lean feed gas) The propane pre-cooling cycle having four compression stages is presented in figure (1).The results of the influence of sub-cooling temperature on the propane flow rate in propane cycle are presented in figure (2) as well as in tables (1) and (S5). The maximum sub-cooling temperature for liquid propane is 35 oC at the operating pressure of the propane cycle (suction pressure to compressor is 1.5 bar). It is clear from figure (2) that as the sub-cooling temperature of liquid propane decreased from 50 °C to 35 °C there was a noticeable reduction in the propane flow rate in the propane cycle. The reduction in the propane flow rate is attributed to the increase in the propane liquid fraction obtained after throttling through the J/T valves. Table (S5) reflects the influence of sub-cooling temperature of liquid propane in sub-cooler on the propane vapor fraction obtained after the throttling valve located before the mixed refrigerant cooler (MR/HHP) as shown in figure (1). It is obvious that the lower the sub-cooling temperature of liquid propane, the lower was the propane vapor fraction (higher liquid fraction) after the throttling valve and thus, at the outlet of MR/HHP cooler which means that for a specific refrigeration duty, a reduction in the mass flow rate of cold liquid propane could be used in the cycle. The reduction in flow rate of liquid cold propane provided in the closed propane cycle resulted in reducing the power consumption of compressors. Figure (3) and table (2) show the decrease in total power consumption of compressors in the propane cycle obtained as the subcooling temperature of liquid propane decreases from 50 °C to 35 °C. Table (2) presents the power consumption in different compression stages of the propane cycle. Figures (4) and (5) show that the power consumption decreased in propane condenser while it increased in propane sub-cooler as propane sub-cooling temperature decreased from 50 °C to 35 °C. It is apparent from figures (6) and (7) that as the sub-cooling temperature of liquid propane decreases from 50°C to 35 °C, the total power consumption for propane compressors and fans of both propane condenser (AC1) and propane sub-cooler (AC2) decreased from 6.73*104 to 6.05*104 kW and a noticeable saving of the total power consumption of 10.12 % was achieved. This saving is attributed to the reduction of the power consumption of propane compressors since they almost play the dominating role in the total power consumption evaluation. 4.2. Effect of natural gas feed quality on propane pre-cooling cycle Figures (8), (9) and (10) represent the influence of sub-cooling temperature in propane sub-cooler on propane flow rate in the propane cycle and power consumption for both cases of lean and rich feed gas using the same number of compression stages (four stages) in the propane cycle. It is clear from figure 8 that both cases of lean and rich feed gas possessed the same trend and the propane flow rate was reduced with the decrease of sub-cooling temperature of liquid propane. But propane flow rate required for the case of rich feed gas was greater than the corresponding propane flow rate required in case of lean feed gas by about 1 % at 35 °C sub-cooling temperature. This is attributed to the increase of content of heavier hydrocarbons (C2+) in the rich feed gas causing a higher enthalpy difference for the rich gas and leading to a higher refrigeration duty being required thus, a higher propane flow rate was needed. The reduction in the liquid cold propane flow rate provided in the closed propane cycle results in reducing the power consumption of compressors for both cases of lean and rich feed gas. It is apparent from figure (9) that as the sub-cooling temperature of liquid propane decreased from 50 °C to 35 °C, the total power consumption of compressors in the propane cycle decreased 6 ACS Paragon Plus Environment
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for both cases of lean and rich feed gas. This benefit must be balanced against the installation cost and power consumption cost of the sub-cooler. Also, the power consumption of compressors for the case of rich feed gas was always slightly greater than the corresponding power consumption in the case of lean feed gas attributed to the presence of more C2+ in the case of rich gas. Figure (10) shows that the decrease in sub-cooling temperature of liquid propane leads to a considerable reduction in the power consumption of fans in the propane condenser (AC1) for both cases of lean and rich feed gas while the power consumption of fans in the propane condenser (AC1) for the case of rich gas feed was slightly greater than the power consumption in the case of lean gas feed which is related to the higher content of C2+ in the rich gas feed. On the other hand, figure (11) reveals that this decrease in the sub-cooling temperature of liquid propane was accompanied by a remarkable increase in the power consumption of fans in the propane subcooler (AC2) for both lean and rich feed gas because of the increase in propane temperature difference required in the propane sub-cooler. It is also shown that the power consumption of fans in the propane sub-cooler (AC2) for the case of rich feed gas was slightly greater than the corresponding power consumption for the case of lean feed gas. Figure (12) shows that there was a considerable reduction in the total power consumption of fans with the decrease of the sub-cooling temperature of the liquid propane while its value for the case of rich gas feed was slightly greater than that for lean feed gas, a result attributed to the presence of higher contents of C2+ in the rich gas. It is clear from figure (13) that as the subcooling temperature of liquid propane decreased, a considerable reduction in the total power consumption in compressors and fans of the propane condenser and propane sub-cooler was obtained for lean and rich feed gas where the total power consumption for the case of rich feed gas was greater than that for the case of lean feed gas. Results reflected the dominating role of the power consumption of propane compressors as compared to the power requirements of fans of propane condenser and propane sub-cooler. 4.3. Effect of number of compression stages on propane cycle performance (Lean feed gas) Figure (14) and table (3) show that the decrease in the sub-cooling temperature was accompanied by a considerable reduction in the propane flow rate in the propane cycle for the different cases of three, four and five compression stages. Moreover, it is apparent that as the number of propane compression stages increased, a noticeable reduction in the propane flow rate in the propane cycle was obtained for any sub-cooling temperature. This is due to the lower enthalpy resulting for the liquid propane which in turn increases the enthalpy difference of propane in each evaporator (chiller) and thus, increases the propane refrigeration capacity. A saving in the propane flow rate of about 1.8 % was obtained when the number of propane compression stages was increased from three to five stages at a sub-cooling temperature of 35 ºC for liquid propane in the sub-cooler (AC2). Figure (15) shows that the power consumption in compressors in propane cycle is markedly reduced as the number of propane compression stages is increased from three to five stages resulting in 10.2 % saving at 35 ºC sub-cooling temperature of liquid propane. The achieved results in this multi-staging is due to that heat removal encountered from the propane stream at the inter-stage pressure, lowers discharge temperatures from the high-stage compressor than would be produced by a single-stage system at the same pressure differential between condensing and evaporating pressures of the cycle. Accordingly, an increase in refrigeration capacity of propane refrigerant is affected. Thus, the liquid propane refrigerant enters the evaporator at a lower enthalpy and thus, increases the resultant refrigeration effect. Consequently, a lower propane flow rate was required which reduced the power requirement in compressors. 7 ACS Paragon Plus Environment
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Moreover, the compression ratio of each stage in a multistage system was smaller than that in a single-stage unit so compressor efficiency was increased. Thus, energy consumption was reduced as the number of stages increased. The drawbacks of the multistage system are the higher initial cost and the more complicated system than that for a single-stage system. Figures (16) and (17) show that the individual power consumption of fans in propane condenser and propane sub-cooler were slightly reduced with the increase of number of compression stages from three to five stages at any sub-cooling temperature. It is apparent from figure (18) that the summation of total power consumption for the propane compressors and the two air cooler fans was considerably reduced with the decrease of sub-cooling temperature of liquid propane and also, with the increase of number of compression stages from three to five stages at any sub-cooling temperature. This is due to the resultant increase in refrigeration effect which in turn decreased the propane flow rate requirement resulting in a saving of 9.5 % in the total power consumption in the case of five stages. Figure (19) and table (S6) summarize the influence of the number of propane compression stages on the savings of propane flow rate in the propane cycle, total power consumption and total duty of both propane condenser and propane sub-cooler at the sub-cooling temperature of 35 ºC. 4.4. Cost evaluation = + $
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