Methane activation at elevated pressures in quartz

Catdysia Today, 13 (1992) 609-611

609

Elsevier Science Publishers B.V.. Amsterdam

METHANE ACTIVATION AT ELEVATED PRESSURESIN QUARTZ Alvin R Weiss’, Jeng-shiangTsaih‘, Nadka Davidovaband Maria Tm+vab Worcutes Polytechic Institute, Worceskr, MA 01609, USA Gsitute of Khetics and Catalysis,Bu@arianAcademy of Sciences, 1040 Sofia, Bulgaria

The cxmsivc

curmt activity in the catalytic oxidative coupling of methane to ethane 2a + l/2 02 ca,a3 + w (1) hasraisedthequestionofwhattraespiresduringtheheat-uppaiodandinthe~~of~ ~withoutcatalyst.Inaddition,preJsuree~issignificantinthesensethata commerchl “atmosphericpressure” reactor may actually operate at about ten atmospheres to accommodatepressuredrops.

Figun 1. RcactantflowwasthNnlghthe . ammlus ofa1.4 cm IDx45.7 cm long quark tube 40 containing a 0.61 cm OD quanx tube usedasathermowell.Thereactor wasinsideaheavy-walledstables a steeltubeinafumace. cHJQ/Hemix~wereused.

P-M&l

P=ddobn

,o-+-btO~

_‘I

CI&foedratewasfkedat43Std. 8s cc/min(SCCM). The total i&t flow 5 rate was constant at 149 SCCM. ‘E s GC analyses establishedpIrniIlct .

Tempemtmepro&sweresteepin”~ therractor,andtheisokzmalzonef was5cminlengkHigherumvers-” ionswerehadintheemptyreactor thanwtlenitwaspackdwith4-20 mesh quartz chips, 14% vs 2.2% at 8OOC,1 atm, and 2.2 C.&/Q ratio. WeatIributethistotheshorter reshhcetimeinthein&sticesof the quartz chips.

P-3.4&m

g

Tempm

CC)

Figun1showsthatmethane~intheemptyarmularquartzreaaor’lightJoff~

1992-

Elsevier Science Publishers B.V.

Tempemtura (‘c)

610 Iowa tempemtwes when pressws are above 1 atm. Oxygen conversions are virtually complete at 550 C at ekvated pressures.Table 1 shows the dry gas analysis of products at 2.2 C&/O, ratio.Selectivity is poor without catalyst.

Tab&1. Dry Gu ii* 01 co CO?

CA Cl&

CR4 (96) TotdCy ml. (%)

A&b.

Cl&/&/b

0.33 0.69 4.15 11.53 11.00 3.10 0.00 0.67 3.09 0.30 0.40 1.19

= O.26/0.12/0.6 0.27 3.31 5.57 1.24

1.98 0.01 7.81 2.08 0.00 0.05 0.88 0.00 0.09 0.00 0.13 0.24 0.M 0.24 28.00 26.94 21.63 2!L33 23.98 1.07 5.11 23.19 21.20 30.52 0.00 25.69 34.38 0.00 6.18

2.62 0.02 7.68 2.47 0.35 0.10 21.59 33.88 8.26

3.19 1.43 0.01 0.01 7.73 7.33 2.55 243 0.43 0.00 0.04 0.09 22.90 22.28 32.92 30.86 8.52 1.86

1.61 2.20 2.78 0.01 0.01 0.01 6.92 6.79 6.54 2.64 2.93 3.09 0.07 0.30 0.33 0.24 0.14 0.04 2259 20.84 20.20 31.10 33.69 33.92 6.26 8.14 7.14

Thenwereiargeamountsofhydrogeninthe~,arenforthose~inwhich~ conversion was complete. Therefore, hydrogen must have been produced in the empty reactor after the oxygen had been converted, and & was not cornbusted. It is possible to calcub& equilibrium constants for the dehydrogenationreaction . l cTH4+ H2 (2) !kom the expaimental data. These are shown on. Figure 2. We believe that the longer residence times at high- pressures enabledthe ethane dehydrogenationreaction to approach equilibrium in the qktxireactor.

4 Flgme 2.

P -

lb

1.0 abn

P - 5.4abn

I

12

1

*

(l/T)"x 10'

Ouranalysesalsoshowedthatthewatergasshiftreaction Ct& + H2 CO + H20 4

I

12

I

10

3

I

12

(3)

approached equilibrium at high tempemture (850 C). These three reactions, plus two to accommodatethe non-sektive oxidation of methane -2C0 + 4H,O (4) 2cH,+ 34 C& + 2 Hz0 (5) CH4+ 202 area~offivemathematicallyindependentreactionswhichareadequatetoaccountfarthe productionratesofeachofthec&htspeciesinthesystem. ~em~importaatpointtobederivedfromthisw~isthatelevatedpreJsuresprovide sufflcimt~timetopermithighmethaneconvuknsandcomple&oxygen conversions at temperatures of about 55OC.The uncatalyzedreaction is not selective. Acknowledgement

This mh

v/as funded by NSF GrantINT-8&10539.

Table 1. Dry Gas Analysis. CHJOz/He 1 atm. Species(Mol%) 700C 750C 850C 0.69 4.15 Hz 0.33 3.10 02 11.53 11.00 0.67 3.09 CO 0.00 0.40 1.19 co2 0.30 0.05 0.88 c2H4 0.00 0.13 0.24 C2Hs 0.00 21.63 CH4 28.00 26.94 5.11 23.19 Conversion (%) 1.07 25.89 34.38 0.00 Total C2 sel. (%)

= O.?B/O.lS/O.S 5.4 atm. 500C 600C 750C 2.62 0.27 1.98 0.02 3.31 0.01 7.68 5.57 7.81 2.47 1.24 2.08 0.35 0.00 0.09 0.10 0.00 0.24 21.59 25.33 23.98 21.20 30.52 33.88 8.26 0.00 6.18 850C 3.19 0.01 7.73 2.55 0.43 0.04 22.90 32.92 8.52

5ooc 1.43 0.01 7.33 2.43 0.00 0.09 22.28 30.86 1.86

8.8 atm. 600C 750C 1.61 2.20 0.01 0.01 6.92 6.79 2.64 2.93 0.07 0.30 0.24 0.14 22.59 20.84 31.10 33.69 6.26 8.14 850C 2.78 0.01 6.54 3.09 0.33 0.04 20.20 33.92 7.14