Vapor-Liquid Equilibrium of a Ternary System containing Dimethyl ether (DME) and Light Hydrocarbons

Transcription

1 Vapor-Lqud Equlbrum of a Ternary System contanng Dmethyl ether (DME) and Lght Hydrocarbons Tomoya Tsu,), Toshhko Hak ), Mtsuo Namba ) )Department of Appled Molecular Chemstry, College of Industral Technology, Nhon Unversty, Narashno, Japan )LPG Safety Research Laboratory, The Hgh Pressure Gas Safety Insttute of Japan, Machda, 94-5, Japan )Correspondng author (tsu@dc.ct.nhon-u.ac.p / FAX: ) INTRODUCTION Recently, dmethyl ether (DME) has been pad much attenton as a clean fuel. DME has many advantages as a substtute for lquefed petroleum gas (LPG). Sulfur oxdes (SOx), and partculate matters (PM) are not produced at the combuston. Infrastructures and equpments for LPG can be used wth slght mprovements, because physcal propertes of DME are smlar to those of lght hydrocarbons, propane and butane. Now, the ndustral producton of DME s about, ton/year n Japan, and the DME has been manly used as an aerosol propellant. In ths last decade, DME can be drectly syntheszed from water gas, carbon monoxde and hydrogen, by use of a new type catalyst dspersed n fluds, and a plot plant has already produced DME wth a rate of ton/day. So, a huge amount of DME wll be suppled n the future of Japan. Though the expermental data of vapor-lqud equlbrum (VLE) are essental for the practcal usage of fuel mxture, there are few data contanng DME and lght hydrocarbons. In ths study, a recrculate type apparatus was desgned to measure VLE, and the expermental VLE data were obtaned for the three bnary, propane + DME, DME + butane, propane + butane, and a ternary, propane + DME + butane, systems at.5 K. The bnary data were correlated wth Benedct-Webb-Rubn (BWR) equaton of state and a conventonal mxng rule. Usng the optmzed bnary nteracton parameters, VLE was estmated for the ternary system. EXPERIMANTAL Fgure shows a schematc dagram of a recrculate type apparatus. The apparatus was specfcally desgned for ths study, and based on a recrculete method. The equlbrum cell was made of stanless steel, and ts nner volume was about cm. The maxmum workng pressure of the cell was estmated up to MPa. Two pressure resstance wndows were equpped wth the cell, and phase

2 behavor can be observed by a fber scope and a CCD camera. The cell was held n a water bath wth volume about 45 L. Two magnetc drven recrcuratng pumps were also equpped n the cell. One was for vapor phase, the other for lqud phase. In the recrculatng lne, two oscllaton U-tube densmeters (Anton Paar 5P, Gratz, Austra) were also equpped. The densmeters were calbrated wth ntrogen, carbon doxde, propane and butane at.5 K. The precson of the densmeter was wthn. kg/m. The expermental temperature was measured by a thermster thermometer (Technoseven D46, Yokohama, Japan) wth a precson of. K. The pressure was measured by two absolute pressure sensors. One was Kyowa PHS-A-5KP, Tokyo wth a capacty of 5 kpa and a resoluton of. kpa. Other was Kyowa PHS-A-KP, Tokyo wth a capacty of MPa and a resoluton of. MPa. The procedure of the measurements was descrbed below: Frst, the lquefed sample was fed nto the evacuated cell by two syrnge pumps (ISCO 6D, Lncoln, U. S.). Usng the automatc pressure regulator (STEC UR-74MO-B, Tokyo), the mxture, wth dfferent composton, can be successvely loaded wthout changng temperature and pressure. After attanng a constant temperature and pressure, re recrcuraton was stopped to measure the densty, and successvely small porton of vapor and lqud phase was ndvdually pcked up by usng sx-way valve. For the determnaton of the composton, the sample of vapor : Sample Gas Cylnder : Syrnge Pump : Vacuum Pump 4: Automatc Pressure Regulator 5: Safety Valve 6: Thermstor Thermometer 7: Agtator 8: Pressure Sensor 9: Equlbrum Cell : Crculaton Pump : Oscllaton U-tube Densmeter : Sx-way Valve : Surge Tank 4: Samplng Bomb 5 : FID Gas Chromatograph 6: Water Bath Fgure Schematc Dagram of a Recrculate type Apparatus

3 phase was drectly sent to a gas chromatograph (GL Scence GC-5B, Tokyo) equpped wth a capllary column (Varant Molseve 5A Plot,. d..5 mm, length 5 m). Otherwse, the sample of lqud phase was expanded n a surge tank, and completely vaporzed. The vaporzed sample was sent to the gas chromatograph. The precson of composton analyss was estmated to be wthn. mole fracton. RESULT AND DISCUSSION To ensure the relablty of the expermental data, VLE was measured for propane + DME. Fgure shows VLE for propane + DME at.5 K. In the fgure, the data of Horstmann et al. were also llustrated. As shown n the fgure, the azeotropc pont can be seen n the vcnty of pure propane. Though the azeototopc pont was not clearly observed, the expermental data agreed well wth those of lterature. Fgures and 4 show VLE for DME + butane and propane + butane at.5 K, respectvely. In the fgure, the data of Fernandez et al. 4 was llustrated. In the two systems, there was no azeotropc pont. The data for DME + butane showed a smlar tendency wth the data of lterature. Otherwse, VLE for propane + butane can be regarded as an deal mxture. Fgure 5 shows VLE for propane + DME + butane at.88 MPa and.5 K. The expermental pressure was almost the same as that of pure DME. As shown n the fgure, the composton of vapor phase became close to that of lqud phase around pure DME..5.5 Dmethyl ether() + Butane().5 K Pressure P [MPa].5 exp. Ths work lt. Horstmann et al.(.9 K) calc. BWR eq. (m =.95) Propane() + Dmethyl ether() Pressure P [MPa].5 exp. lt. Ths work Fernandez et al. (.98 K) calc. BWR eq. (m =.949).5 K.5 Mole fracton of Propane x, y [-] Fgure VLE for Propane() + DME() at.5 K.5 Mole fracton of DME x, y [-] Fgure VLE for DME() + Butane() at.5 K

4 Fgure 6 shows the summary for the expermental result of lqud densty measurement. The corner of trangular prsm ndcates pure propane, DME and butane, respectvely. The heght corresponds to the densty. As shown n the fgure, the lqud densty tends to ncrease wth ncreasng DME concentraton. The expermental data were correlated wth equaton of state. The equaton employed was Benedct-Webb-Rubn (BWR) eq. 5 C P = RTρ + ( B RT A ) ρ T + 6 ( brt a) ρ + aαρ cρ + ( + γρ )exp( γρ ) T () Pressure P [MPa].5.5 Propane() + Butane().5 K exp. Ths work calc. BWR eq. (m =.8).5 Mole fracton of Propane x, y [-] Fgure 4 VLE for Propane() + Butane() at.5 K where A, B, C, a, b, c, α and γ are so called BWR constants. The values for propane and butane can be obtaned from the lterature 6, and those for DME have been already reported n our prevous work. The mxng rule employed was Stotler-Benedct eq. 7 A ( A A ) / = xx m () B = B + B x x () = ( C C ) C x x (4) / / a = x a (5) / b = x b (6) c / = x c (7) / = xα α (8) / γ = xγ (9) where m s a bnary nteracton parameter. In ths study, the bnary parameters were optmzed by usng the expermental data. In Fgs. -4, the optmzed results were also llustrated. Usng the bnary parameter, the calculaton showed good reproducblty for the three bnary systems. Table lsted the optmzed bnary nteracton parameters. The value for propane + DME was smlar to that for DME + butane, and the value for propane + butane was almost unty. Usng the optmzed bnary nteracton parameters, VLE and lqud densty were estmated for propane + DME + butane. Fgures 5 and 6 show the calculaton results of VLE and lqud densty for propane + DME + butane at.88 MPa and.5 K. As shown n the fgures,

5 calculatons well predcted VLE and lqud densty wth a hgh accuracy. Fgure 8 shows the calculaton of the ternary VLE n the whole range of composton. Vapor-lqud-lqud equlbrum (VLLE) was not observed n the calculaton, and VLE was mantaned n the whole range of composton. Azeotropc pont was calculated Fgure 5 VLE for Propane() + DME() + Butane() at.88 MPa and.5 K Fgure 6 Lqud Densty for Propane() + DME() + Butane() at.88 MPa and.5 K

6 ust n the bnary, propane + DME. Table Optmzed Bnary Interacton Parameters Propane() + DME() DME() + Butane() Propane() + Butane() m [-] Fgure 7 Calculated VLE for Propane() + DME() + Butane() at.5 K CONCLUSION Relable data of VLE and lqud densty can be obtaned for bnary and ternary systems contanng propane, DME and butane by use of apparatus specally desgned n ths study. Usng optmzed bnary nteracton parameters n the mxng rule of BWR eq., the VLE can be well predcted for mult component system contanng DME and LPG. ACKNOWLEDGEMENT We thank Mnstry of Economy, Trade and Industry, Japan, for fnancal supports and many helpful comments.

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