A High-Efficiency Reactor for the Pulsed Plasma.pdf

1、Plasma Chemistry and Plasma Processing,Vol.22,No.2,June 2002(2002)A High-Efficiency Reactor for the Pulsed PlasmaConversion of MethaneS.L.Yao1,2E.Suzuki,1N.Meng,1andA.Nakayama1Receied December 12,2000;accepted August 10,2001The authors recently deeloped a high-frequency pulsed plasma process for met

2、haneconersion to acetylene and hydrogen using a co-axial cylindrical(CAC)type ofreactor.The energy efficiency represented by methane conersion rate per unit inputenergy has been improed so that such a pulsed plasma has potential for commercialacetylene production.A pulsed plasma consists of a pulsed

3、 corona discharge and apulsed spark discharge.Most of energy is injected oer the duration of the pulsedspark discharge.Methane conersion using this kind of pulsed plasma is a kind ofpyrolysis enhanced by the pulsed spark discharge.In this study,a point-to-point(PTP)type of reactor that can produce a

4、 discharge channel oer the duration ofa pulse discharge was used for the pulsed plasma conersion of methane.The energyefficiency and carbon formation on electrodes hae been improed.The influencesof pulse frequency and pulse oltage on methane conersion rate and product selec-tiity were inestigated.Th

5、e features of methane conersion using PTP and CACreactors were discussed.KEY WORDS:Pulsed plasma;methane;acetylene;point-to-point reactor;energy efficiency.1.INTRODUCTIONCatalytic methane conversion directly to more valuable chemicals andliquid fuels has attracted much more attention for several yea

6、rs as theinvestment and operation cost can be possibly reduced at an estimate incomparison with a conventional methanol production.(16)This type ofstudy has some challenges since the product methanol is oxidized more read-ily than methane over a catalyst,even one of the most inert materials SiO2.(7)

7、Plasma conversion of methane has been studied as an emerging method to1Contribution from Catalysis Science Laboratory,Research Institute of Innovative Technologyfor the Earth,Kyoto 6190292,Japan.2To whom all correspondence should be addressed.2250272-4324?02?0600-0225?0 2002 Plenum Publishing Corpor

8、ationYao,Suzuki,Meng,and Nakayama226convert methane to acetylene,ethylene,hydrogen,methanol,and otherliquid products.(821)It has been found that a non-thermal pulsed plasma of a high frequency(8kHz)can be used to convert methane to acetylene and hydrogen,at ahigh acetylene selectivity of 92%and a hi

9、gh methane conversion rate of0.93B106molJ1at room temperature and atmospheric pressure using aco-axial cylindrical(CAC)type of reactor,indicating that this process ispotentially competitive with commercial acetylene production processes.(22)The highly endothermic reaction of methane to acetylene req

10、uires tempera-tures above 1500K.(8)The commercial processes include:(1)an arc dis-charge process developed by Huels,in which the reaction energy is supplieddirectly by an arc discharge;(8)(2)a partial combustion process developedby BASF,in which the reaction energy is supplied directly by incomplete

11、combustion of methane;(11)(3)the high-temperature hydrocarbon pyrolysisdeveloped by Wulff,in which the reaction energy is supplied using a cyclicregenerative technique of supplying fuel combustion heat to a refractorymass in one step and in a second step releasing the heat for pyrolysis ofhydrocarbo

12、ns to acetylene,(12)and(4)the calcium carbide process using areaction of calcium carbide with water.(23)A pulsed plasma consists of a pulsed corona discharge and a pulsedspark discharge.Ionization of the background gas(the gas in the wholeplasma space)occurs in the pulsed corona discharge,which yiel

13、ds a dis-charge channel of positive ions.(24)The pulsed spark discharge occurs afterions reach the electrode(s)due to the force exerted by the electric field.Mostof the energy is injected over the duration of the pulsed spark discharge andis used to heat the methane in the discharge channels to temp

14、eratures(25)atwhich methane pyrolysis occurs.(26,27)In the case of the CAC reactor used,the gas temperature downstreamof the plasma space of the reactor was estimated to be higher than thatupstream due to the energy injection.This temperature gradient in theplasma space indicated that the discharge

15、was also not uniform,since thedischarge at a higher temperature occurred more easily than that at a lowertemperature,the discharge tended to occur more frequently in the down-stream region of the reactor.These findings also implied that the space ofthe CAC reactor cannot be used effectively.Furtherm

16、ore,discharges in theplasma space with high background gas temperatures caused carbon forma-tion on the surfaces of the electrodes and finally the discharge cannot beoperated regularly.In this study,a point-to-point(PTP)type of reactor was used to giveonly a single discharge channel with a high reac

17、tion temperature.The influ-ences of pulse frequency and voltage on methane conversion were investi-gated,in comparison with those using the CAC reactor.A High-Efficiency Reactor for the Pulsed Plasma Conversion of Methane2272.EXPERIMENTAL PROCEDURESFigure 1 shows a pulse power generator(DP-15K35,Pul

18、se ElectronicEngineering)and the PTP reactor.The pulse voltage was generated withtwo plasma-forming capacitors(C1(2B1010F)and C2(1B1010F)with ahydrogen thyraton used as a switch S2.The plasma forming capacitors werenegatively charged with a charging system consisting of a capacitor C0(8.5B103F),a gr

19、oup of FET switches S1and their drivers,a coil L0(1.4B106H),a booster transformer T(1:45),and four diodes D.A dcpower supply was used to charge the capacitor C0.Values of other parts L1,L2,R0,R1,R2,and R3were 1B104H,2B104H,200,1300,110,and 100,respectively.The voltages of capacitors C1and C2and the

20、dis-charge voltage were measured using two voltage probes V-1 and V-2(EP-50K,PulseElectronicEngineering),respectively.Dischargecurrentsthrough the anode and cathode were measured using two current trans-formers CTaand CTc(CTa:mode 21.0,CTc:mode 0.51.0,StrangenesIndustries),respectively.The signals f

21、rom the voltage probes and currenttransformers were recorded with a digital oscilloscope(TDS754D,Tek-tronix)having an analog bandwidth of 500MHz.The PTP reactor consisted mainly of a Pyrextube(1.5B102(o.d.)B1.2B102(i.d.)B0.8(length)m3),a quartz tube(1.2B102(o.d.)B8.0B102(i.d.)B0.4(length)mm3),andtwo

22、stainlesssteelwireelectrodes(1.5B103B0.18(length)m2)with sharp-pointed discharge terminals.Thegap distance between the two electrodes was 5.0B103m.Figure 2 shows the configuration of the CAC reactor,which its meth-ane conversion property was partly given for comparison with the PTP reac-tor.The CAC

23、reactor was comprised mainly of a Pyrextube(1.5B102Fig.1.A plasma system for methane conversion.Yao,Suzuki,Meng,and Nakayama228Fig.2.Configuration of a co-axial cylindrical(CAC)reactor for methane conversion.(o.d.)B1.2B102(i.d.)B0.8(length)m3),two quartz tubes(1.2B102(o.d.)B1.0B102(i.d.)B5.0B102(len

24、gth)m3),a stainless steel tube(1.2B102(o.d.)B1.0B102(i.d.)B0.15(length)m3),and a stainless steelwire(5B103B0.8(length)m2).A quartz tube and two acrylic resin hold-ers were used to keep the wire straight and on the axial of the Pyrextube.As has been described elsewhere,(22)methane conversion rate and

25、 acetyleneselectivity could be obviously improved using a pulse frequency higher than2kHz.Since the discharge using the CAC reactor occurred between the centralwire and the inside surface of the stainless steel tube,this type of dischargeA High-Efficiency Reactor for the Pulsed Plasma Conversion of

26、Methane229Fig.3.Configuration of a wire-to-plate(WTP)reactor for methane imaging analysis.could be imitated with a wire-to-plate(WTP)reactor.Here,the pulsedplasma of methane using a WTP reactor(Fig.3)was imaged using an imag-ing system consisting of a digital high speed camera(Memrecam Ci,NacImage T

27、echnology)equipped with a 2.54B102m color CMD sensor witha full pixel format of 572B434 pixels,a DRAM recording system having arecording time of 19s,an AF Nikkor ED 180mm f?2.8D IF lens(Nikon),a bellows attachment(PB-6,Nikon),and imaging transfer and storagedevices to view and convert captured image

28、s to TIFF format.The lumi-nescence from the plasma space was recorded with the imaging system at ashutter speed of 1?500s and a recording rate of 500 photos per second anda pulse power of a peak voltage of 12.3kV and a frequency of 250Hz.TheWTP reactor consisted mainly of a stainless steel wire(5.0B

29、103m)anda stainless steel plate(2.0B102B5.0B102m2).The distance between thestainless steel wire and plate was 4.5B103m.Methane was supplied at aflow rate of 5.0B107m3s1and a residence time of 84s in the WTP reactor.Methane was introduced into the upper part of the vertically orientedPTP reactor at a

30、 flow rate of 5B106m3s1.Hydrocarbons from the lowerpart of the reactor during discharge operation were analyzed with an onlinechromatograph(FID,GC103,Ohkura)equipped with a 2-m Porapak Ncolumn.All experiments were carried out over an interval of 120 to 180sdischarge operation with about 2400s restin

31、g time for analysis at atmos-pheric pressure and without external heating.The energy injection rate P in watts into the background methane gaswas calculated using Eq.(1)from the discharge voltage Viin volts,pulsefrequency in Hertz and anode current Iiin amperes at discharge time tiinseconds.These da

32、ta of Vi,Ii,and tiover a time range of 8B107s wereYao,Suzuki,Meng,and Nakayama230collected from the waveforms of voltage and anode current using a stopfunction of the oscilloscope while the product gas was sampled for analysis.PGFiViCViC12IiCIiC12(tiC1Ati)(1)The electric energy efficiency of this ki

33、nd of pulsed plasma was repre-sented by a methane conversion rate R in mol J1that was defined as:RGmoles of methane converted per secondP(2)3.RESULTS AND DISCUSSIONSince the pulse frequency has the strongest effect on the methane con-version rate,its influence using the PTP reactor was measured firs

34、t.Methaneconversion rate increased with increasing pulse frequency below 9.92kHzandpeakedat9.92kHzwithamethaneconversionrateof2.73B106molJ1,which was higher than that obtained using the CACreactor(Fig.4).in order to make clear where such a difference arose,theimaging system was used to image the dis

35、charge in the WTP reactor.Theresult indicated that the pulsed discharge occurred in only one dischargechannel with a luminous filament over the duration of a pulse dischargeFig.4.Influence of pulse frequency on methane conversion rate using PTP and CAC reactors.Methane flow rates were 5.0B106and 2.5

36、B106m3s1for PTP and CAC reactors,respect-ively.Capacitor voltages(V-1)were 11.1(?)and 14.7kV(?)for PTP reactor and 11.1kV forCAC reactor(G).A High-Efficiency Reactor for the Pulsed Plasma Conversion of Methane231Fig.5.Image of the pulsed plasma of methane using the WTP reactor.Only a filamentaryspar

37、k channel between the discharge gap was found over a pulse discharge duration.(Fig.5).The discharge channel completely bridged the discharge gap ran-domly in the whole gap space.The pulsed plasma using the PTP reactor alsooccurred in one discharge channel over the duration of a pulse discharge.(25)T

38、herefore,the difference between the PTP reactor and CAC reactor wasthat the discharge channel was fixed using the former but not fixed usingthe latter.This finding implied that not only methane but also acetylene andhydrogen products would be still in the plasma space of the CAC reactorand could fur

39、ther react to yield other products,such as C2H4,via the reac-tions given in Eqs.(3)(6).The low methane conversion rate using the CACreactor was possibly due to the above reverse reactions of acetylene toethylene.CH4CeGCH3CHCe(3)H2CeG2HCe(4)C2H2CHGC2H3(5)C2H3CHGC2H4(6)The conversion of methane to ace

40、tylene and to ultimate products was gener-ally believed to follow a path:CH4C2H6C2H4C2H2polymerscarbonAt temperatures above 1270K,all reactions could be very rapid.(9)At apulse frequency higher than 10kHz,methane conversion rate decreased withincreasing pulse frequency and carbon formation was obser

41、ved downstreamof the electrodes of the PTP reactor.These findings simplified that the aboveacetylene further reactions to carbon took place.There was an obvious car-bon formation on the inside surface of the stainless steel tube of the CACreactor but not on the electrodes of the PTP reactor.This was

42、 due to thehigh gas flow rate(higher than 5B106m3s1(methane input flow rate)andlow residence time(less than 0.015s in the plasma space(discharge channelvolume:about 7.5B109m3)when using the PTP reactor.The gas flow rateYao,Suzuki,Meng,and Nakayama232and residence time using the CAC reactor were high

43、er than 2.5B106m3s1(methane input flow rate)and 4.8s,respectively.The selectivity of acetylene production increased,but those of ethyleneand ethane decreased with the increase in pulse frequency(Fig.6).Theselectivity of acetylene achieved a level of 85.1%at methane conversion of23.5%.At a pulse freq

44、uency lower than 6kHz,acetylene selectivity usingthe PTP reactor was higher than that using the CAC reactor.This suggestedthat the above reverse reactions of acetylene occurred in the CAC reactor,however,such reactions did not take place in the PTP reactor.At a pulsefrequency higher than 6kHz,acetyl

45、ene selectivity of the PTP reactor waslower than that of the CAC reactor.This was possibly due to the abovepolymerization and reverse reactions of acetylene occurring downstream ofthe plasma space at high temperatures caused by high-energy inputs to thesmall plasma space of the PTP reactor at high p

46、ulse frequencies.Thesefindings also suggested that the rapid cooling of the products would beimportant to inhibit further reactions of the C2H2to carbon andpolymerization.The influence of capacitor voltage on selectivity of each product isshown in Fig.7.The selectivity of acetylene increased with in

47、creasingcapacitor voltage and tended to a level of around 85%at a voltage above11kV.Another factor that affected the energy efficiency was the pulse voltage.Figure 8 shows the relationship between methane conversion rate and thecharge voltage of the plasma forming capacitors.Methane conversion rateF

48、ig.6.Selectivity of each product using PTP and CAC reactors.Methane flow rates were asper Fig.4.Capacitor voltages(V-1)were 11.1(filled)and 14.7(empty)kV for PTP reactorand 11.1kV for CAC reactor.A High-Efficiency Reactor for the Pulsed Plasma Conversion of Methane233Fig.7.Selectivity of each produc

49、t at various capacitor voltages(V-1).Methane flow rate andpulse frequency were 5.0B106m3s1and 8.13kHz,respectively.increased with increasing capacitor voltage.This was due to the high energyinput at higher voltage since the energy input is proportional to voltage.The high energy input could yield a

50、discharge channel of a high temperatureand a high methane conversion rate.(27)Methane conversion and acetylene yield are shown in Fig.9.Methanecould be converted at a rate of 40%with an acetylene yield of 32%at volt-ages above 11kV.Fig.8.Methane conversion rate at various capacitor voltages(V-1).Exp