Characteristics of Cascade and C3MR Cycle on Natural Gas Liquefaction Process

Transcription

1 Characterstcs of Cascade and C3MR Cycle on atural Gas Lquefacton Process Jung-n Yoon, Ho-saeng Lee, Seung-taek Oh, Sang-gyu Lee and Keun-hyung Cho Abstract In ths paper, several dfferent types of natural gas lquefacton cycle. Frst, two processes are a cascade process wth two staged compresson were desgned and smulated. These nclude Inter-cooler whch s conssted to Propane, Ethylene and Methane cycle, and also, lqud-gas heat exchanger s appled to between of methane and ethylene cycles (process) and between of ethylene and propane (process). Also, these cycles are compared wth two staged cascade process usng only a Inter-cooler (process1). The COP of process and process3 showed about 13.99% and 6.95% hgher than process1, respectvely. Also, the yeld effcency of LG mproved comparng wth process1 by 13.99% lower specfc power. Addtonally, C3MR process are smulated and compared wth Process. Keywords Cascade. C3MR. LG. Inter-cooler. I. ITRODUCTIO ATURAL gas s the mxture wth methane, ethane, propane, butane, etc., and methane accounts for about 8% of these components, and normal bolng pont s about -16 o C.[1] atural gas s beng preferred as the green energy whch s colorlessness, odorless and non-toxcty. Furthermore consumpton rate of natural gas s ncreasng accordng to ncrement of nternatonal ol prces.[] atural gas transportaton systems are dvded nto PG (Ppelne atural Gas) system and LG (Lquefed atural Gas) system. LG system has an advantage whch s easer to transfer wth handlng smaller volume about 1/6 than PG system. Therefore, natural gas lquefacton ndustry has been n the spotlght recently as a hgher value-added ndustry.[3] However, some developed companes monopolze lquefacton plant market. The researches and developments are started n 196s. D. L. Andress of Phllps company descrbed about development of Optmzed cascade process,[4] Kkkawa et al. smulated mxed refrgerant lquefacton process usng pre-coong loop and expander wth CHEM CAD software,[5] Terry et al. analyzed Jung-n Yoon s wth Pukyong atonal Unversty, Busan, , Korea, (correspondng author to provde phone: ; fax: ; e-mal: Yoonj@ pknu.ac.kr). Ho-saeng Lee, wth Pukyong atonal Unversty, Busan, , Korea, (e-mal: purger77@pknu.ac.kr). Seung-taek Oh s wth Pukyong atonal Unversty, Busan, , Korea, (e-mal: tmdxorl@pknu.ac.kr). Sang-gyu Lee s wth Korea Gas Corporaton, Inchon, 46-13, Korea, (e-mal: lsg@kogas.or.kr) Keun-hyung Cho s wth Korea Gas Corporaton, Inchon, 46-13, Korea, (e-mal: kevn@kogas.or.kr) and compared representatve lquefacton process wth Hysys software,[6] Wen-Sheng Cao et al. smulated lquefacton process usng refrgerant whch mxed ntrogen and methane wth Hysys software, and then compared performances wth mxed refrgerant lquefacton process.[7] In the Korea, Yoon et al. smulated cascade process wth Hysys software, and then offered basc data to ths research.[8]-[1] In ths study, we wll offer basc data those are analyzed Characterstcs of Performance of Cascade and C3MR processes through smulaton wth Hysys software to secure to secure a compettveness n the ndustry of natural gas lquefacton plant. II. LIQUEFACTIO PROCESS A. Basc Cascade Process Ths process conssts of three pure refrgerants whch have dfferent bolng temperature, such as methane, ethylene and propane. Frst of all, natural gas s cooled to -35 o C n the propane cycle, then t s cooled to -9 o C n the ethylene cycle, fnally t s lquefed to -155 o C n the methane cycle. B. Cascade Process wth Two Staged Inter-cooler Ths process s that two staged compresson wth ntercooler type s appled on the basc cascade process (process1). Lquefed refrgerant from condenser s bypassed and evaporate n the ntercooler after expanded. Therefore man refrgerant to evaporator s sub-cooled. C. Cascade Process wth Two Staged Inter-cooler These processes are those lqud-gas heat exchangers are appled to between of two cycles. One of these processes s that sub-cooled lqud refrgerant whch s bypassed from nter-cooler n the ethylene cycle and hot gaseous refrgerant from outlet of hgh pressure compressor n the methane cycle are exchanged the heat n the lqud-gas heat exchanger to lquefy the hot gaseous methane (process). The other process s that one more lqud-gas heat exchanger s appled to between of propane and ethylene cycle (process3) on the process. Fg. 1 shows about process. D. C3MR(Propane Mxed Refrgerant) Process Ths process are formed two cycles such as MR (Mxed Refrgerant) cycle and C3 (Propane) cycle. atural gas s pre-cooled to about -35 o C through C3 cooler, than lquefed at -16 o C n the MR heat exchanger. Ths process s shown n the Fg. 185

2 III. SIMULATIO CODITIO A. Condton of smulaton In ths smulaton, a feed gas composton s assumed wth treated natural gas whch s removed acd gas, water, mercury and heavy hydrocarbons from gera LG plant. Table 1 shows feed gas and MR composton. Assumed condtons of smulaton of are shown n Table. Feed gas mass flow s set based on trane capacty 5MTPA (Mllon Ton Per Annum, a bypass flow rate of cascade process s set 15% of mass flow of each cycle, a mddle pressure s set 5% of hgh pressure and outlet pressure of expanson valve of the ntercooler nlet sde s assumed same as mddle pressure. Component TABLE I COMPOSITIO OF ATURAL GAS & MR Mole fracton of Cascade[%] Mole fracton of MR[%] trogen.7. Methane.8.44 Ethane Propane.4.15 so-butane.1 - n-butane.9 - Total 1 1 B. Equatons TABLE II ASSUMED CODITIOS Parameter Cascade C3MR Refrgerant [-] Methane, Ethylene, Propane MR Bypass flow rate [%] 5 - Mddle pressure [%] 5 - Feed gas mass flow rate [kg/s] Feed gas temperature [ ] 3 Feed gas pressure [kpa] 5 nd flud temperature [ ] 4 Two knds of man equatons are used for the lquefacton smulaton.[13] The Peng- Robnson equaton of state apples functonalty to some specfc component-component nteracton parameters, whch can be used n the calculaton of phase equlbrum. It s wrtten by RT P V b V a b 1 j 1 1 a b a a V b bv b j ab a a 1 k j a j. (1) Where P [Pa] s a pressure, R [ m/kg K] s a gas constant, T [K] s a temperature, V [m3/kg] s a specfc volume, a and b are the constants relatng to the gas speces, x s mole fracton of a certan component, k s bnary nteracton coeffcent. It s rewrtten as 3 P Z ap A 3 1 BZ A B 3B Z AB B B bp B RT RT () Fg.1 Schematc dagram of cascade process usng lqud-gas heat exchanger 186

3 Where Z s a constrngent factor, A and B are the coeffcents relatng to the gas state parameters. The Lee-Kesler-Plocker equaton s an accurate general method for non-polar substances and mxtures, whch can be used n the calculaton of enthalpy and entropy of mxed components. It s gven by P Z r o r Z Z (3) Mass flow rate [kg/s] Process1 Process Process3 where w s an acentrc factor, o and r denote the relevant parameters of smple and reference lquds. C3 C C1 IV. RESULTS AD COSIDERATIOS Refrgerants [ - ] A. Cascade process Fg. 4 Comparson of refrgerant mass flow rate Fg. 4 shows comparson of varaton of each refrgerant mass flow rate. Only mass flow rate of propane of Process decreased about.4%, but that of ethylene and methane ncreased about 8.% and 4.4% than those of process1. Mass flow rate of all refrgerants of Process 3 are ncreased about.3 ~ 9.4%. Accordng to supplement of lqud-gas heat exchanger, bypass mass flow rate ncreased and mass flow rate of refrgerant to each cycle s evaporator also decreased then that of whole cycles ncreased. The other hand, temperature of nlet of compressor s drop owng to ncrement of sub-coolng and then the compresson work s decreased. Therefore, we can conclude that COP s mproved. Performances of each process n a same condtons such as lquefacton rate and temperature are shown n Fg. 5. Refrgeraton capacty of process s.9% hgher and that of process 3 s lower than that of process 1. Snce a state of hgh temperature of methane s cooled wth bypassed ethylene Capacty [MW] R W COP Performance [ - ] Fg. 5 Comparson of performance Process1 Process Process COP [ - ] Fg. Schematc dagram of C3MR 187

4 Specfc power [kj/kg] Process1 Process Process3 Type [ - ] Fg. 6 Comparson of specfc power condensng temperature s decreased as a result, refrgeraton capacty of process ncreased. Refrgeraton capacty of Process 3 decreased, even though condensng temperature s lower. Ths s because, mass flow rate of propane to nlet of propane evaporator whch has domnant nfluence on refrgerant capacty s decreased as bypassed. Compressor work of process and that of process 3 are 11.44% and 8.6% lower than that of process 1. In ths fgure, process 3 shows hgher compressor work than that of Process. Ths s because, requrement of refrgerant s ncreased wth decrement of mass flow rate of propane to nlet of propane evaporator as lqud-gas heat exchanger s added. COP means effcency of system and that s calculated by refrgeraton capacty per compressor work. COP of process shows the hghest ncrement rato about 13.9%. Fg. 6 and Fg. 7 show power consumpton per productvty of LG and productvty of LG per power requrement. Power consumpton per productvty of LG of Process s the largest decrement rato about 11.44% and productvty of LG per power requrement of Process s the hghest ncrement rato about 1.68%. As a result, performance and effcency of process s expected that s the best n these processes. Productvty [kg/h/kw] Process1 Process Process3 Type [ - ] Fg. 7 Comparson of productvty B. C3MR process Ths process has the hghest performance and effcencyt n exstng lquefacton plants and t has a poston up to 7% n lquefacton plant market n the world. For these reasons, t s smulated and analyzed. Performances of C3MR s shown n Table 3. TABLE III PERFORMACES OF C3MR Process Compressor Work [MW] Refrgerant Capacty [MW] COP [-] Specfc Power [kj/kg] C3MR Cascade ,968.5 Its performances show dfferece cuased by contang heat exhcang lne such as recovery of waste heat from cascade n prncpal prameter. Later on, research on C3MR process wll be proceeded and we wll develop more effcent C3MR process. V. COCLUSIOS In ths study, lquefacton processes are smulated and analyzed focused on decrement of power consumpton per productvty of LG and ncrement of productvty of LG per power requrement by redusng compressor work whch has a great nfluence on effcency of lquefacton process. Results are showed as follows. 1. Mass flow rate of only propane of process decreased about.4%, but that of the others ncreased about.3 ~ 9.4%.. Refrgeraton capacty of Process ncreasd, but that of process decreased compared wth process 1. Compressor works of Process and process 3 decreased and COP of process shows the hghest ncrement rato about 13.9%. 3. Power consumpton per productvty of LG of Process s the largest decrement rato about 11.44% and productvty of LG per power requrement of Process s the hghest ncrement rato about 1.68%. In ths research, process s the top ranked wth hgh Effcency n cascade processes and we wll use of these result to research on mprovement of performance and effcency n lquefacton process. ACKOWLEDGMET Ths research was supported by a grant from the Gas Plant R&D Center funded by the Mnstry of Land, Transportaton and Martme Affars (MLTM) of the Korean government. REFERECES [1] Sang gyu Lee, Kun hyung Choe, Young myung Yang, The state of art of LG Lquefacton Plant Technologes, The 3rd Korean Congress of Refrgeraton, vol. 3, pp [] H. S. Chang,, B.. Lee, B. S. Gu, A Rase Plan of compettveness of Internal Company n the overseas Plant market, Constructon & Economy Research Insttute of Korea, Vol. 19, pp. -3, 7 [3] Seung Taek Oh, Ho Saeng Lee, Jung In Yoon, Sang Gyu Lee, Development of LG Lquefacton Process, Journal of the SAREK, Vol. 38, o. 3, pp , 9 188

5 [4] D. L. Andress, The Phllps Optmzed Cascade LG Process a Quarter Century of Improvement, The Permsson of the Insttute of Gas Technology, 1996 [5] Yoshtug Kkkawa et. al, Development of Lquefacton Process for atural Gas, Journal of Chemcal Engneerng of Japan, Vol. 3, o. 4, pp , 1997 [6] L. Terry, Comparson of Lquefacton Process, LG Journal 1, o. 3, pp. 8-33, 1998 [7] Wen-sheng Cao et. al, Parameter Comparson of Two Small-scale atural Gas Lquefacton Process n Skd-mounted Packages, Apples Thermal Engneerng, o. 6, pp , 6 [8] Seung Taek Oh, Hyun Woo Km, Ho Saeng Lee, Gyeong Beom Y, Jung In Yoon, Sang Gyu Lee, Smulaton of LG lquefacton cycle usng [9] two stage ntercooler, Proceedngs of KIGAS Sprng conference, pp. 5-8, 9 [1] H. S. Lee, S. T. Oh, H. W. Km, J. I. Yoon, G. B. Y, S. G. Lee, Analyss of Cryogenc Refrgeraton Cycle usng Two Stage Intercooler, 5th Internatonal Conference on Dffuson n Solds and Lquds, pp. 4-41, 9 [11] H. S. Lee, S. T. Oh, H. W. Km, W. J. Cho, J. I. Yoon and S. G. Lee, Comparson of Performance of Cryogenc Lquefacton Process wth Expander, Proceedngs of the KSPSE Sprng Conference, pp. 5-54, 9 [1] Seung-taek Oh, Ho-saeng Lee, Gyeong-Beom Y, Jung-n Yoon, Sang-gyu Lee, Characterstcs of cryogenc lquefacton cycle usng two stage compresson type, Proceedngs of the SAREK Summer Annual Conference, pp. 19, 9 [13] C. R. Robert, The Propertes of Gases a Lqud, 4 th Edton, McGraw-Hll Book Company,