Development of a New High efficiency Dual cycle Natural Gas Liquefaction Process

Transcription

1 Gastech 2017 Conference Development of a New High efficiency Dual cycle Natural Gas Liquefaction Process Chang LIN, Hong WANG, Gailing BAI, Di WU China Huanqiu Contracting & Engineering Corporation Limited. National Energy LNG Technology R&D Center. Beijing , P.R. China Abstract: A new natural gas liquefaction process is developed and named HQC-DMR. It is a dual mixed refrigerant liquefaction process, consisting of two separated closed cycles, pre-cooling cycle and liquefaction cycle, involving three kinds of flow structures. The pre-cooling and liquefaction cycles are configured with one or two pressure levels, and liquefaction cycle refrigerant is separated gas and liquid after pre-cooling and flows back to compressor after decompression to provide cold for natural gas liquefaction. The whole process of pre-cooling and liquefaction can be implemented within the same multi-streamed heat exchanger. Mixed refrigerant is adopted in each cycle, and composed by nitrogen and light hydrocarbons. HQC-DMR has been applied in engineering, and the built facilities operate steadily. It is a proven process with good availability and reliability and operability, and has a simple and compact structure. It is suitable for power-efficient, onshore and offshore floating, middle- and large- scaled natural gas liquefaction plants. Keywords: LNG, natural gas, liquefaction, process, DMR, mixed refrigerant 1. Background China Huanqiu Contracting & Engineering Corporation Ltd. (HQC), affiliated with China National Petroleum Corporation (CNPC), is a technology-oriented state-owned enterprise. It engages in such diversified business as consultation, R&D, engineering, procurement, construction, construction management and commissioning guidance. It has fulfilled the tasks of consultation, engineering, construction and EPC contracting for over 2,000 cross-industry large- and medium-scale domestic and overseas projects in more than 60 years, including 14 categories of plants. HQC has abundant experience in engineering and cryogenic technology, involving air separation and liquid nitrogen wash and ethylene complex. From the end of last century, HQC launched liquefied hydrocarbon businesses, to engineering design and construct Liquefied Natural Gas (LNG) receiving terminals and Liquefied Ethylene Gas (LEG) receiving terminals. During that period, HQC-owned intellectual property technology packaged in cryogenic storage and receiving & re-gasification of liquefied hydrocarbons 1

2 has been formed. Furthermore, in 2009, according to CNPC group business development strategy planning, HQC undertook the responsibility of technology development of natural gas liquefaction. After 5 years of research, HQC-DMR, a new liquefaction process, was proposed and put into industrial applications. 2. Process Flow Introduction The proposed process of HQC-DMR is a dual mixed refrigerant liquefaction process, consisting of two closed cycles, pre-cooling cycle and liquefaction cycle, involving three kinds of flow structures, DMR1 (figure 1), DMR2 (figure 2) and DMR3 (figure 3). Figure 1 shows the schematic diagram of the basic type (DMR1). It has two refrigerant cycles, and has one loop in each cycle. The two refrigerants are mixed refrigerants that consist of two or more components of nitrogen, methane, ethane, ethylene, propane, i-butane and i-pentane. The vapor and liquid of each refrigerant flows through the heat exchanger together without separation. In the process, one cold box (core component is plate-fin heat exchange, PFHE for short) can be used for the two cycles. The structure is simple and compact. The refrigerant of pre-cooling cycle (MR1 cycle) is compressed to 2.0~4.0MPaG, after heat transfer, it decompresses via J-T valve to provide cold energy to pre-cool natural gas, the refrigerant of liquefaction cycle (MR2 cycle) and high-pressure MR1 to the temperature of -20~-70 o C. MR2 cycle is compressed to 3.0~5.0MPaG, after heat transfer, it decompresses via J-T valve to provide cold energy to further cool natural gas and high-pressure MR2 to the temperature of -140~-160 o C. In the process, both MR1 and MR2 outflow from the warm end of cold box and then run into suctions of the compressors separately. The suction pressure is higher than atmosphere for operating safety, and could be changed for optimization. Natural gas feedstock could separate the heavier hydrocarbons after the pre-cooling. It is benefit on one hand to prevent the heavier hydrocarbons frozen at the following liquefaction cycle and on the other hand to improve the thermal efficiency. Figure 2 shows the schematic diagram of the DMR2. The difference on structure with DMR1 is that MR2 compressor operates with cold suction. That is, after decompression via J-T valve or expender, the low-pressure MR2 outflows from the middle session instead of the warm end of cold box, and then flows back to MR2 compressor with relatively low temperature (-20~-70 o C). 2

3 Figure 1 Schematic diagram of HQC-DMR1 (CN X) Figure 2 Schematic diagram of HQC-DMR2 (CN ) Figure 3 shows the schematic diagram of the DMR3, it is the most energy-efficient and power-saving process. In the process, the pre-cooling and liquefaction cycles are configured with two pressure levels. With respect to pre-cooling cycle (MR1 cycle), there is no vapor and liquid separation, part of the MR1 decompresses via J-T valve to a relatively high pressure, after to provide cold in the cold box, flows back the second stage suction of MR1 compressor. The rest of MR1 decompresses to a low pressure and then flows back the first stage suction of MR1 compressor. For the liquefaction cycle (MR2 3

4 cycle), the refrigerant MR2 is separated into vapor and liquid refrigerants after pre-cooling. The liquid refrigerant consists of the composition that is relatively heavier than the vapor refrigerant. The two refrigerant streams flow into PFHE dedicated channels, and then decompressed separately. Finally the two streams mixed into one flow and run back to MR2 compressor suction. The MR2 compressor operates with cold suction either, it is similar to that of DMR2 process. It is worth mentioning that the whole process of pre-cooling and liquefaction can be implemented within the same multi-streamed heat exchanger (cold box), and two parallel cold boxes can be used in the process for wide operation flexibility. After pre-cooling, natural gas is cooled to the temperature in the range of -20 o C and -70 o C, and then further cooled by liquefaction cycle to the temperature range of -140 o C and -160 o C to produce liquefied natural gas (LNG). After pre-cooling, the heavy hydrocarbons (C5+) will be separated from the NG feed generally. Feed gas NG MR1 Cycle HHC MR2 Cycle LNG Figure 3 Schematic diagram of HQC-DMR3 (CN ) 3. Simulation and Optimization Generally, the energy consumption of liquefaction process is accounting for 70%~80% of the total energy consumption of LNG plant. The simulation and optimization of liquefaction process is very important to achieve high energy efficiency in the design and 4

5 operation of refrigerant cycles, leading to low energy consumption. In the process, there are a lot of factors impacting on the energy consumption, including the composition of refrigerant, the suction and discharge pressures of compressor, cryogenic temperature of refrigerant, the flow rate of each loop and so on. Regarding to SMR (single mixed refrigerant) process, there are as many as about ten of optimization variables, more optimization variables will be involved in DMR process. Aspen HYSYS has been used to simulate the process and made the process optimization, usually. The Peng-Robinson equation of state is used both for the natural gas and the refrigerants. The principle of optimization algorithm of Aspen HYSYS Optimizer is mainly based on gradient information. First of all, to initialize the process model according to the experience, and then use the trend and gradient of the function to produce a series of points of convergence to an optimal solution. Yet the variables are hard to be optimized synchronously. For example, it fixes the refrigerant pressure to optimize refrigerant composition, and fixes the refrigerant composition to optimize pressure. Thus the optimal solution may be not the really best due to the limitation of the method. HQC has developed an optimization method, which is operated based on HYSYS model and genetic algorithm (GA). Aspen HYSYS is used to simulate the process at a potential optimal point, and the GA running on MATLAB is used to evaluate the solutions and produce new potential optimal points. That is, the mixed refrigerant cycle optimization model is established in HYSYS, and MATLAB code is used to create a HYSYS component object via data object interface. When the data are passed though into HYSYS program, the simulation results will be produced instantly and return to MATLAB for evaluation [1]. The introduction of genetic algorithm realizes the multi-variables synchronous optimization and global search. This method provides a more efficient and speed optimization strategy for the synthesis of mixed refrigerant cycles, and it is especially suitable for nonlinear and highly discrete optimization. For the projects with different conditions of feed gas and environmental, using the optimization method based on genetic algorithm, a tailor-made appropriate component and composition of mixed refrigerant and optimized process parameters can be provided for high efficiency and low energy consumption. It is well known that, in order to reduce the power input, it is crucial to minimize entropy generation due to the temperature difference between hot and cold streams [1-3]. During the simulation and optimization, usually, the object function is the total compression power of the liquefaction system, besides that, we determined the constraints of minimum approach temperature of no less than 2 degree C between hot and cold streams and LMTD (logarithmic mean temperature difference) of no less than 4 degree C. Through the optimization, the refrigerant 5

6 composition and operation parameters are adjusted to match the cooling curve of the natural gas. 3 Industry Application HQC-DMR process was applied industrially as early as The first case is An sai LNG project, which is located at Shan xi province of China and with the capacity of 0.5 million tonnes per annum (mtpa). In August 2012, the LNG plant was built and started up successfully. At that time, it was the largest LNG plant of China on capacity. The overview of the plant is as shown in Figure 4. It is configured with one LNG train, one LNG storage tank and ten truck loading arms, as well as the utilities and auxiliary facilities. The plant is designed and constructed based on HQC-DMR1 process and the imported key equipments (e.g. cold box and refrigerant compressors). In the project, HQC had provided all engineering and technical services, including the liquefaction process, process design package (PDP), preliminary and detailed engineering design, procurement, construction, commissioning and plant running protection for half year. In the second half of 2011, the second proven project, Shan dong Tai an LNG project, was begun to engineering design based on HQC-DMR3. The design capacity of LNG production was 0.6mtpa. For the project, HQC provided all engineering and technical services, either. The Plant completed the mechanical completion at late of 2013, started up successfully and passed the performance test in August The plant photos are shown in Figure 5. The project used the localized materials and equipments in the maximum. Except that the in-tank LNG pump are imported, all the other equipments and materials are localized, including cold box, mixed refrigerant compressors, boil off gas (BOG) compressor with cold suction, distributed control system (DCS), and so forth. The two above mentioned LNG plants operated steadily and achieved the design specifications. It is robustly proven the good availability and reliability and operability of HQC-DMR process. Besides those, two PDPs (process design package) for large scale LNG projects had also developed utilizing HQC-DMR (DMR3), which consisted of PFDs, PIDs, equipment process data sheets, instrument data sheets, plot plan etc. One PDP is for a certain project with the capacity of 2.6mtpa and proposed to construct in tropical desert climate of Middle East, the other is for a certain project with the capacity of 5.5mtpa and proposed to construct in the polar cold climate. Both of them had passed the technical review by expert committee of CNPC group, and the first developed large scale PDP (2.6mtpa) had also passed the HAZOP & SIL analysis by US ERM and QRA analysis by Norway GEXCON. Furthermore, a set of Pre-FEED materials based on HQC-DMR process had been produced and provided for a Djibouti LNG Plant with the capacity of 2.7mtpa and passed the review by the client. 6

7 View of overall plant (a) Process facilities (b) Figure 4 Photos of Shan xi An sai LNG plant 7

8 Purification unit (gas treating) (a) Liquefaction unit (b) Pipe rack and LNG storage tank (c) Figure 5 Photos of Shan dong Tai an LNG Plant 8

9 4 Specific Power The specific power is a benchmark to evaluate the thermodynamic effectiveness of a liquefaction design to produce LNG and is also an important index for process evaluation. It is expressed as kw shaft power for liquefaction only, per ton per day of LNG rundown to the storage tank [2]. The specific powers of the above mentioned projects are listed in the table below (Table 1). Table 1 Conditions and specific power of the projects Projects An Sai LNG Tai An LNG PDP-1 PDP-2 Environmental Northern Northern Tropical North Pole temperate temperate desert Process HQC-DMR1 HQC-DMR3 HQC-DMR3 HQC-DMR3 Capacity, mtpa Feed gas composition, mol% Methane: Nitrogen: Others: 97% 0.5% 3.5% 95% 1% 4% 88% 4% 8% 94% 1% 5% Feed gas input pressure, MPaG Feed gas input temperature, degree C Cooling medium Air+Water Water Air Air Specific power, kw/tpd Note: HQC-DMR3 is the improved process based on HQC-DMR1. Both the process structure and the components of mixed refrigerant are improved. The projects with large capacity have lower specific powers. It is because the high pressure of feed gas and the adoption of more efficient compressor. It also can be seen from the table, the low ambient temperature is benefit for the liquefaction, which indicates that the mixed refrigerant cycle can effectively utilize the low temperature of environment. Based on the same project conditions of PDP-1, the other existing mature industrial application liquefaction technologies are also used to liquefy the natural gas for process research and specific power comparison. The results of home study are as shown in Table 2. From the comparison, it shows that HQC-DMR has a lower specific power. It is a high efficiency process. Table 2 Comparison of specific power for various liquefaction processes Process type HQC-DMR SMR C3MR Cascade Specific power kw/tpd 13.6~ ~ ~12.9 > 13.0 Note: There is a range of specific power for each process instead of only one value. It is because each type of process has several flow structures be studied. 9

10 5 Scale A technology can be applied to build how large scale facilities that depends on the equipment manufacturing capacity and marketing demands. Mass production is conducive to saving investment, so large-scale facility is welcome to vigorous market demand of LNG product. In our technology research and engineering project implementation procedures, the manufacturing capacity of key equipments of LNG train had also been investigated throughout China domestic and overseas. Furthermore, HQC had also joint domestic well-known equipment manufacturers to develop equipments with larger capacity than the existing. So far, completely adopting China localized equipments, the proposed HQC-DMR process is suitable to construct the LNG train with the capacity of no more than 3.0mtpa, which is configured with single compressor and driver string for each refrigerant cycle. To enlarge the capacity of LNG train, parallel equipment or international purchased equipment (especially the driver), will be adopted. Based on the current international manufacturing capacity of equipments, HQC-DMR process could be used to build the LNG train as large as 6.0mtpa. 6 Summary HQC has developed a new dual mixed refrigerant liquefaction process, HQC-DMR. Due to easy adjustment of the mixed refrigerant formula, the process features a good flexibility to variation in the feed gas conditions and ambient conditions. The process began to put into industrial application in So far, two LNG plants with the capacity of more than 0.5mtpa have been constructed in China and operated for more than one year. The successful industrial application verified the performance and operability of HQC-DMR process well. Two PDPs for large scaled LNG plant have also developed using HQC-DMR, one is for a certain project with the capacity of 2.6mtpa and proposed to construct in tropical desert climate, the other is for a certain project with the capacity of 5.5mtpa and proposed to construct in the polar cold climate. As well, a Pre-FEED had completed for Djibouti LNG plant with the capacity of 2.7mtpa. In addition, optimization method based on genetic algorithm was developed. It realizes the multi-variables synchronous optimization and global search to optimize process parameters and provide tailor-made appropriate composition of mixed refrigerant for high energy efficiency of different projects. Compared with the existing mature industrial application liquefaction technology, the liquefaction power consumptions of HQC-DMR process are lower than those of SMR process and Optimized Cascade process, as well as slightly lower than that of C3MR process. Besides high efficiency, the process has a simple and compact structure. It is 10

11 suitable for medium and large scaled LNG plant. Besides onshore LNG facility, HQC-DMR applied to a floating LNG project is now in process of research and design. BRIEFLY INTRODUCTION OF AUTHOR Madam Hong Wang and Gailing Bai are responsible to the key technology development project, and also are the main contributors for the new developed liquefaction technology of HQC-DMR. Di Wu and I are the technical backbones of the research. ACKNOWLEDGEMENT The new liquefaction technology of HQC-DMR is a result of our research team, which consists of more than 10 technologists. The authors are grateful to the research team and the colleagues contributed to the development and industrial application of HQC-DMR. The authors also wish to acknowledge the cooperating organizations and the industry peers contributed to the development and industrial application of the technology. REFERENCES [1] Xue-feng Zheng, Hong Wang, Chang Lin, Di Wu, optimization of process parameters of natural gas liquefaction based on HYSYS model and genetic algorithm. Chemical Engineering, Jul, 2014 vol. 42: [2] Wim Dam, Siem-Mung Ho, Engineering Design Challenges for the Sakhalin LNG Project.GPA Annual Convention Proceedings, 2001: 11p. [3] Sanggua Lee and Yong-myuan Yang, Koreans develop efficient liquefaction process for LNG called KSMR and with simple structure. LNG Journal, Oct. 2014: