Study on Pressure Swing Adsorption Removing C+2 from Natural Gas as Raw Material for Thermal Chlorin

Jounlo NaturalCao CheityJournal of Natural Gas Chemistry 13(2004)53 -57SCIENCE PRESSStudy on Pressure Swing Adsorption Removing C2 from NaturalGas as Raw Material for Thermal ChlorinationSulan Xial*,Jiahua Zhu'，Xiaobin Zeng2，Zhaohua Feng'，Xinyu Cheng21. School of Chemical Engineering, Sichruan University, Chengdu 610065, China2. Honghe Chemical INC, Zigong 643000, China(Manuscript received October 02, 2003; revised December 22, 2003]Abstract: The experimental investigation dermonstrates that a satisfactory result can be expected forpressure swing adsorption (PSA) purification of natural gas as raw material for thermal chlorinationprocess. Using hh-4 molecular sieve as adsorbent for removing C2 components, the suitable adsorptionpressure is 0.4-0.45 MPa, desorption vacuum is 0.08- 0.09 MPa and circulation time is 20- 21 min.Key words: chloride methane, natural gas purification, pressure swing adsorption1. Introductionrination reaction will be influenced, ie. there will bemore perchlorides produced. In addition, the qual-Chloride methane (adbreviated as CMS) is theity of trichloro methane product will also be seriouslygeneral designation of monochloro, dichloro, trichloroinfluenced. The separating efficiency of the productand tetrachloro methanes. They all have impor-will be reduced and the dichloro methane in the prod-tant industrial application. Monochloro methane anduct due to side reaction will exceed the specified limit.trichloro methane are raw materials for organic siliconIn the natural gas provided in the Sichuan area, theand organic fluorin respectively. Dichloro methane iscontent of ethane and other C2 components is aboutan excellent organic solvent. And tetrachloro methane1% (V%). It can only be used as the raw material foris now largely used in the production of refrigerantproducing chloride methane when the C7 content isand fire-extinguishing agent.made less than 0.01% through purification.Concerning the production method of chlorideAccording to the principle of PSA purificationmethane, it can be divided into methanol method andof gaseous mixture, the different adsorbates will bemethane method according to the route of raw mate-separated by selective adsorption, mainly due to therial. And it can be divided into thermal-chlorinationdifferences in the adsorbing capacity of the adsorbatemethod, photo-chlorination method and catalytic-along with the change of pressure. Under high pres-chlorination method according to the way of chlori-sure, the impurity components containing C2 will benation. In China, the thermal-chlorination methodadsorbed when the natural gas is flowing through theis mainly adopted. Using natural gas and chlorine aspacked bed of adsorbent. Thus the purified naturalraw materials, the displacement reaction is conductedgas will be discharged from the outlet of adsorptionin a temperature range of 420 -440 °C, and the fourower.中国煤化工rbed impurity willchlorides are produced simultaneously [1]. Experiencebe remHCN M H Gegerate the ed-in the production indicates that if the C$ content insorbing capauioy.1 ie 1alll pal uuueters determiningthe natural gas raw material exceeds 0.01%, the chlo-the performances of the whole circulation operation* Corresponding author. E-mail: jhzhu@scu.edu.cn54Sulan Xia et al./ Journal of Natural Gas Chemistry Vol. 13 No. 1 2004process are: adsorbent capability, the cycle of oper-strength and flow resistance in packed bed, the com-ation pressure and the circulation time of adsorptionposition and volume quantity of desorption gas alongand desorption [2].with the change of desorption vacuum are particularlytested. The desorption volumes of methane and C22. Selection of adsorbent and PSA operationas well as separation coefficients (the ratio betweencondition for removing ethane and other C2the desorption volume of C2 and CH4) of four ad-components from natural gassorbents at different conditions of desorption vacuumare listed in Table 3.2.1. Determination of adsorbentTable 1 and Table 2 respectively give the typ-Table 3. The desorption volumes of methane and C士as well as separation coefficientsical composition of raw material natural gas andof four adsorbentsthe current purification requirement in the thermal-VacuumDesorption volume (ml/g)chlorination method for producing chloride methane.Adsorbent(MPa) CH4 C咕 Separation coefficientsTable 1. The composition of raw material natural gas0.05.2 101.9ComponentComposition (%)0.07.4 17.02.3CH495.570.0910.349.54.8CO22.02hh-26.2 10.51.7C2H60.808.2 20.12.5C3H80.1817.347.12.He0.005hh-35.6 18.03.20.0079.0 23.32.6N1.3413.9 46.53.4Ct0.078hh-4.6.3 21.03.3p=0.5 MPa10.0 26.00.09 15.9 53.2Table 2. The composition of product gas anddesorption gas to meet the demand forthe thermal chlorinationThe following conclusions can be drawn from theComposition inabove experimental data:product gas (%)desorption gas (%)(1) With the increase of desorption vacuum the92.792.5desorption volumes of the four adsorbents all increase.咕0.012.98When the increase of desorption vacuum is from 0.050.76MPa to 0.07 MPa, the increase of desorption volumeof Cz is comparatively small; while the increase ofN27.11.77desorption vacuum is from 0.07 MPa to 0.09 MPa,He,Ar0.17the increase of desorption volume of C2 increases byH2Oabout 2.2 times as much on an average.H2S<50 mg/m3(2) Among the above-mentioned four adsorbents,hh-4 has the largest desorption volume of C2, indi-The selection of adsorbent is vially important forcating its largest adsorbing capacity of C去.the purification task. In industry, the adsorbents used(3) Desorption vacuum is in direct proportion toin PSA for removing C2 from natural gas are mainlydesorption volume. Too low a vacuum is unfavor-activated carbon, molecular sieve, alumina and silica中国煤化工e is a sudden changegel. According to the characteristics of purifying C2ent for the four adsor-from natural gas, four different types of molecularMHc N M H Gracum should not besieve are chosen to examine in our experiments. Be-lower than 0.07 MPa, it is best controlled in the rangesides apparent physical properties, such as mechanicalof 0.08 -0.09 MPa.Journal of Natural Gas Chemistry Vol. 13 No.1 20045hh-4 has relatively better adsorption capabilitydetermined under the conditions of process pressureamong the four adsorbents when the above mentionedand period (shown in Table 4.).factors are taken into account.2.2. Determination of operation conditionhh-4Based on the above-mentioned experiment, hh-4 was selected as the main adsorbent, together withCsome other adsorbents in certain proportions (Figure1). For the adsorption and vacuum desorption of an 8-Al2O2Al2O,tower system (llustrated in Figure 2), the key opera-tion parameters influencing the yield rate, purity andAdsorptionDesorptionimpurity of gas product, such as the change of desorp-Figure 1. Sketch of adsorbent loading and gastion pressure and circulation period, were tested andflowing888$→器__88士Figure 2. Flow chart of eight-tower adsorption and vacuum desorptionTable 4. Process pressure and setting period for2.2.1. Effect of adsorption pressure on C2 con-every stages in the operationtent in purified gasProcess pressureSettingStageThe relationship between the measured adsorp-(MPa)period (s)tion pressure and the C2 content in purified gas using0.5~300hh-4 as adsorbent, is shown in Figure 3.It can be seenEID0.5→0.33(from Figure 3 that the CI content of the purified gas0.3- + 0.113decreases rapidly with the increase of adsorption pres-Adverse discharging0.11→0.02~70sure. While in plant operation, too high adsorptionVacuum pump0.02→-0.08~200pressure will result in comparatively high energy con-E2R-0.08- + 0.11E1R0.11- + 0.3sumpt中国煤化工. should not exceed: control adsorptionFR0.3-0.5~40pressuYHCNMHGIPa.56Sulan Xia et al./ Journal of Natural Gas Chemistry Vol. 13 No.1 2004300The fllowing can be concluded from the abovedata: .2(1) Through adjusting adsorption time, the prod-区200uct gas of different purity can be obtained. Butthe lower the C2 content, the lower the productivity.150 EFor industrial operation, not only the requirement ofJ 100product purity should be met, but also the capabil-ity of equipment should be brought into full play. It0Eis better to control the productivity in the range of50%-52% and the C2 content less than 0.01%.0.250.300.350.400.450.50(2) Along with the extension of adsorption time,Adsorption pessure (MPa)the productivity tends to rise slowly while the C2Figure 3. The influence of adsorption pressure onC2 content in purified gascontent climbs rapidly. So, it is better to control ad-sorption time in the range of 20- 21 min.2.2.2. The influence of circulation time on the2.2.3. Effect of degree of desorption vacuumproductivity and the C2 content in purifiedon the content of C2 in gas productgasThe degree of desorption vacuumn influences theGiven the conditions that the composition of nat-quality of the gas product directly. This is a keyural gas is basically steady in the commodity pipeparameter to the variable pressure adsorption. Thenetwork, the productivity and purity of the productexperimental relation of the change of Cz content in(according to C2 content) are closely related to thegas product with the degree of desorption vacuum iscirculation time of adsorption-desorption. The exper-shown in Table 5.imental result is shown in Figure 4.Table 5. Relation between the degree of desorptionvacuum and content of C7 in gas productDegree ofContent of8FVacuum (MPa)c古(x10-6)(x10-6)0.0799171122135663712195 10660.0857120110814042 3521 63.2554E0.09462012550001:0[As shown in Table 5, with the increase of the de-gree of desorption vacuum, the content of C2 in the48 4152021222324gas product will decrease. When the degree of vac-Adsorption time (min)uum is equal to 0.08 -0.09 MPa, the C2 content canbe less than 0.006325%. In order to ensure the con-tent of C2 in the gas product less than 0.01%, thevacuum degree should be controlled in the range of200上0.08- -0.09 MPa.50 E3. Conclusions心100Through experiments the authors have obtained中国煤化工purification for remov-21;ural gas as raw mate-H. C N M H Gufactured by thermal.Figure 4. The influence of adsorption time on thechlorination. The hh-4 molecular sieve is selected asproductivity and the Ct content in pu-the preferred adsorbent for tests. The data obtainedrified gasJournal of Natural Gas Chernistry Vol. 13 No.1 200457in the 8-tower adsorption and vacuum desorption sys-terial has improved the thermal- chlorination processtem with a flow rate of 4300 Nm' /h of product gasand stabilized the product quality very effectively.indicate that the C2 components can be efficiently re-moved from the raw material natural gas to a contentReferencesless than 0.01% under adsorption pressure not higherthan 0.5 MPa and adsorption time not more than 20[1] Cheng Zh Q. Industry of Chlorine and Alkali, 1999,min. The suggested desorption vacuum is in the range(8): 22.of 0.08- 0.09 MPa. It has been verified in practice[2] Ye Zh H. Chemical Adsorption and Separation Pro-that the PSA purification of natural gas as raw ma-ess, Bejing: China Petrochemical Press, 1992中国煤化工MYHCNMHG