2422
Ind. Eng. Chem. Res. 1992,31, 2422-2425
Pressure, Temperature, and Product Yield Relationships in Direct Oxidative Methane Conversion at Elevated Pressures and Moderate Temperatures Dennis E. Walsh,* Daniel J. Martenak, Scott Han, Robert E. Palerrno,+ James N. Michaels, and David L. Stern Central Research Laboratory, Mobil Research and Development Corporation, P.O. Box 1025, Princeton, New Jersey 08543-1025
Extending previous work on high-pressure, noncatalytic direct oxidative methane conversion, the relationship between operating pressure, reaction temperature, and CHI conversion/product selectivities has been examined. Experiments at < w 1 s contact time show that, as pressure increases from 3.0 to 10.0 MPa, the temperature required for complete consumption of feed oxygen declines by more than 100 "C (from 630 to 515 "C). Both CHI conversion and Cz+selectivity increase with increasing total pressure (up to -40% selectivity at 15% CHI conversion). Similar selectivity and conversion trends result as oxygen partial pressure is increased. Increasing CH30H selectivity and decreasing total yield of desired products at longer contact time may be plausibly explained by secondary reactions between Cz+products and CO,. The decreasing influence of a known oxidative coupling catalyst (Sm203)with increasing pressure is consistent with the view that the catalyst is primarily a radical generator in conventional catalytic oxidative coupling.
Introduction Direct oxidative conversionof CHI has typically involved partial oxidation to CH30H or conversion to C2+hydrocarbon products (primarily ethane and ethylene). Oxidative coupling (OC) to C2+is normally carried out below O.bMPa pressure and above -700 O C , while direct partial oxidation (DPO) to CH30H is usually performed at elevated pressures (typically 5-8 MPa) and temperatures at or below -475 "C. The overwhelming majority of oxidative upgrading work in the literature resides in these two regimes (Hutchingset al., 1989;Amenomiya et al., 1990). Studies involving relatively moderate excursions outside the usual proceesing boundaries in OC have been reported by Aaami et al. (1987),Baerns (19871,Hutchings et al. (1988),Onsager et al. (19891,and Ekstrom et al. (1990). A greater excursion outeide OC processing boundaries was recently reported by Walsh et al. (1992);the results of oxidative CHI upgrading studies at conditions atypical of either DPO or OC, viz. 55U-600 OC and 6.2 MPa, were reported. That study showed that, by using high pressure, significant amounts of OC products could be obtained without a catalyst at temperatures much lower than those of conventional catalytic OC operation. Furthermore, the presence of a conventional oxidative coupling catalyst (Sm203)produced only modest conversion and selectivity benefits under these conditions. Extending those initial findings, the present study was undertaken to examine the pressure dependency of operating temperature, conversion, and product selectivities in noncatalytic operation (cf. Figure 1). In addition, experimenta were performed to elucidate further the previously proposed parallel pathway for noncatalytic CHI conversion to CH30H or to C2+,and to examine how catalytic benefits vary with operating pressure. Experimental Procedure The reactor system, run procedures, and analytical methods were described previously (Walsh et al., 1992). Conversions and selectivities are both defined on a carbon basis. As before, "gas hourly space velocities" in noncaCurrent address: Department of Physical Chemistry, Hoffmann-LaRoche, 340 Kingsland St., Nutley, NJ 07110. 0888-5885/92/2631-2422$03.#/0
talytic runs were based on the total NTP (normal temperature and pressure) feed flows and on the reaction zone volume. Rssidence time was estimated based on flow rates calculated at operating conditions and the void fraction (w 30-35 %) of the packed reaction zone. Ultra-high-purity CHI Pnd C. P. grade 02, both supplied by Matheson, were used in this study. Samarium oxide, used as an oxidative coupling catalyst, was supplied by Aldrich and calcined in N2at 850 OC for 8 h.
Results and Discussion Influence of Pressure on Temperature, CHI Conversion, and Product Selectivities. Table I presents the data obtained in runs performed between 3.0 and 10.0 MPa These experiments employed the same feed oxygen concentration ( 14%) used in the previous study of Walsh et al. (1992). Flow rates were adjusted so that residence time was approximately constant (0.6-0.8s) for all pressures investigated. The temperature/pressure data, plotted in Figure 2,indicate the dramatic decline in reactor temperature required for complete oxygen consumption (from 630 to 515 "C) as pressure is increased (from 3.1 to 10.2MPa). Because residence time was maintained constant, accompanying the temperature decrease over this pressure range was a 4-5-fold increase in reactor throughput leading to increased production rates of useful producta. The selectivity and conversion data from Table I are illustrated in Figure 3. The carbon selectivity to non-COX species triples, increasing from 10% to 32%, as pressure increases from 3.1 to 5.1 MPa. Further increases in pressure also improve selectivity,through to a lesser extent (from 32% at 6.1 MPa to 40% at 10.2 MPa). Accompanying the increased selectivity is a moderate, approximately linear increase in CHI conversion such that, at the highest pressure, conversion is -60% higher than at 3.1 MPa (14.8% vs 9.3%). Increasing selectivity with increasing CHI conversion is contrary to conventional experience in both DPO and OC of CHI (Rytz et al., 1991; Hutchings et al., 1989). Since O2consumption is complete in all runs,increasing CHI conversion with increasing preeeure results from more selective oxygen utilization via decreased COXproduction. This is illustrated in Figure 4,which also shows increasing
-
0 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 11,1992 2423 Table I. Oxidative CHI Conversion at 3.0-10.0 MPa, -14 vol % O,, and with Inert Reactor Packing run no. 1 2 3 4 Dressure. MPa 3.1 4.1 5.1 6.2 ieed flow rate, cm3/min NTP 274 531 667 760 30800 GHSV 12650 24 500 35 100 temp at 100% O2 conv, OC 630 585 565 550 0.6 t,(s) at reactor T and P 0.8 0.6 0.6 14.5 12.6 CHI conversion, % 9.3 9.8 product carbon selectivities, % co 68.5 65.2 59.6 56.7 21.5 11.5 8.8 9.6 COP 2.4 1.5 CH30H 1.8 4.0 19.5 C2H8 6.4 14.0 16.3 7.6 12.2 1.8 3.7 CZH4 2.7 C2H2 0.7 1.1 C3Hs 0.9 1.0 1.0 29.2 8.2 19.3 32.2 total cz+ 10.0 23.3 31.6 33.7 total non-CO, 2.6 1.3 3.6 3.8 C2HdC2H4 4.2 yield (per pass) of non-COXproducts, % 0.9 2.3 4.6 relative productivity of non-COXproductsa 1.0 5.0 12 13
-
a
5 8.2 1052 48 600 535 0.6 13.5
6 10.2 1225 56600 515 0.7 14.8
56.4 6.4 3.0 21.1 8.7 2.6 1.8 34.2 37.2 2.4 5.0 21
54.1 6.1 2.2 20.4 12.6 2.6 2.0 37.6 39.8 1.6 5.9 29
~~
(Feed rate x yield per pass)/(run 1 feed rate x run 1 yield per pass). 900
'
'1
800 p 0
e- Oxidative
Coupling
Non-catalytic oxidative coupling complete oxygen consumption 0.6.0.8 s contact time
I
700 -1
c
0
co
co2 CH,OH A C;
80
n
U
Selectivity, %
5
JUU
I-
-
I
t 300 b
400
J----~
I
0
Direct Partial Oxidation I
2.5
7.5
5.0
2.0
4.0
6.0
8.0
10.0
1 0
Pressure, MPa
I
10.0
Figure 4. Selectivity vs pressure.
Pressure, MPa
Figure 1. Operating regimes for the oxidative upgrading of methane.
Temperature, OC
Non-catalytic oxidative coupling complete oxygen consumption 0.6 0.8 s contact time
-
500
2.0
0
4.0
6.0
8.0
10.0
12.0
Pressure, MPa
Figure 2. Operating temperature vs pressure.
'1
Non-catalytic oxidative coupling complete oxygen consumption s contact time
4o
0.6.0.8
Selectlvity or CH4 conversion 20
I
0
2.0
4.0
6.0
6.0
10.0
12.0
Preaaure, MPa
Figure 3. Selectivity and conversion va pressure.
C2+selectivity and the constant CHBOHselectivity with increasing pressure. Additionally, although C2Hs is the
Table 11. Oxidative CH4 Conversion at -0.6 s (t,) and Comparable Total Pressure and Variable 0, Partial Pressure run no. 7 8 5 pressure, MPa 8.2 8.2 a2 O2 in feed, vol % 10.8 14.0 6.9 oxygen partial pressure, MPa 0.88 1.14 0.56 temp at 100% O2 conv, OC 565 565 535 CHI conversion, % 5.6 8.2 13.5 product carbon selectivities, % co 59.4 56.8 56.4 10.7 7.1 6.4 COP CH30H 12.7 7.5 3.0 20.3 21.1 16.0 CPH, 0.7 4.8 8.7 CZH4 1.2 2.6 C2H2 0.5 2.2 1.8 C3Hs total cz+ 17.2 28.5 34.2 total non-CO, 29.9 36.0 37.2 22.8 4.2 2.4 C2H6/C2H4
dominant Czt product in all the runs,a substantialincrease in the preferred C2H4 product results as pressure and C2+ selectivity increase (Table I). Because the feed O2concentration was constant in the experiments summarized in Table I, increasing the total pressure also increased oxygen partial pressure. For comparison, Table I1 illustrates the effect of varying oxygen partial pressure at constant total pressure and residence time. Trends similar to those described above are evident. As expected, increasing O2leads to increased conversion. Again,C2+and total non-CO, product selectivities increase with increasing conversion. Also, increased C2H4 in the
2424 Ind. Eng. Chem. Res., Vol. 31, No. 11, 1992 Table 111. Catalytic Advantages as a Function of Operating Pmssum, -14 vol % O2 run no. 4 9 1 10 pressure, MPa 6.2 6.2 3.1 3.1 catalyst none Sm2O3 none Sm2O3 catalyst wt, g 0.8 0.8 feedflowrate,cm3/minNTP 760 814 274 274 temp at 100% O2 conv, "C 550 550 630 630 -t,(s) at reactor T and P 0.6 0.6 0.8 0.8 CHI conversion, % 12.6 13.6 9.3 13.8 product carbon selectivities, % co 56.7 55.9 68.5 72.4 9.6 6.1 21.5 11.1 COZ 1.5 1.7 1.8 3.2 CH30H C2H6 16.3 20.9 6.4 10.7 CZH4 12.2 11.1 1.8 2.5 C2H2 2.7 2.1 0.1 C3H8 1.0 2.2 32.2 36.3 8.2 13.3 total c2+ total non-COX 33.7 38.0 10.0 16.5 1.3 1.9 3.6 4.3 CZHB(C2Hl 4.0 4.9 0.8 1.8 Cz+ veld 5.2 0.9 2.3 total non-CO, yield 4.2
C2+product accompanies increased conversion. The temperature required for complete O2consumption declines moderately at the highest O2 concentration. The ability of a known OC catalyst (SmzO3) to enhance yields under moderate-temperature, high-pressure conditions was examined in the earlier studies of Walsh et al. (1992). Those results at 6.2 MPa are presented again in the first two columns of Table 111. They indicate only modest selectivity and conversion benefits. Thus, the catalyst's role is much leas signiscant at high pressure than at conventional low-pressure, high-temperature OC conditions where the achievement of significant conversions and C2+ selectivities depends strongly on its presence. It might be expected, therefore, that the importance of a catalyst would increase as preasure is reduced. This was examined by performing similar contact time experiments at 3.1 MPa with and without the Smz03catalyst, the results of which are shown in the last two columns of Table III. As discussed above, at this pressure the noncatalytic C2+ selectivities (and, therefore, yields) are markedly poorer than those observed at or above 5.0 MPa. In the presence of SmzO3, the conversion increases by 50% over noncatalytic operation (13.8%VB 9.3%) in contrast to only an 8% increase at 6.2 MPa (13.6% vs 12.6%). Although the yield of desired products at 3.1 MPa in the presence of the catalyst is less than that observed with or without catalyst at 6.2 MPa, it is -2.6 times higher than its noncatalytic counterpart, the catalytic to noncatalytic ratio is only 1.2 at 6.2 MPa. This result is directionally consistent with the catalyst's increasing importance at the low pressures and elevated temperatures conventionally employed. Reaction Network: CH,OH vs C2+ Formation. Previous work (Walsh et al., 1992) suggested that direct oxidative CHI upgrading to either CH30H (via DPO) or C2+(via OC) proceeds via the parallel pathways indicated below:
cH4
- & c2* &
COX
path2
It was assumed that both pathways are zero order in CHI due to ita preaence in large excess or alternatively that the
Table IV. Influence of Contact Time in High-pressure, Noncatalytic CHI Conversion run no. 4 11 pressure, MPa 6.2 6.2 temperature, "C 550 550 O2 in feed, vol % 14 14 contact time, s 0.6 30 CH, conversion, % 12.6 11.4 O2 conversion, % 100 100 product carbon selectivities, % co 56.7 53.7 9.6 27.4 CO2 CHBOH 1.5 3.6 16.3 12.3 C2H6 12.2 1.4 C2H4 2.7 0.4 CzHz C3HB 1.0 1.2 total Cz+ 32.2 15.3 total non-COX 33.7 18.9
CHI dependency, if nonzero order, is the same for both pathways; then the relative rates of the first steps would be given by: r1
kPo:
- =r2 kZP0,' This expression was consistent with the data for y > x and k2having a higher activation energy than kl (i.e., increaeing faster than kl with increasing temperature). In the present study, the declining CH30Hselectvity with increasing O2 partial pressure (Table 11) observed at 8.2 MPa supplements and supports the limited data in the earlier study indicating y > x . The results shown in Table IV suggest that a pathway from C2+product to CH30H exists at extended reaction times. At short contact times, O2consumption is complete and C2+ dominates the non-CO, product selectivity. At identical temperature, preasure, and feed O2concentration, but substantially longer contact time, CH30H selectivity is increased and apparent CHI conversion declines modestly. Since O2consumption is complete at contact times below 1 s (run 4), the changing selectivities at longer contact time (run 11) appear to result from reactions of the products which do not involve molecular oxygen. Ekamples of such thermodynamically feasible reactions at the experimental conditions are given below: C2H4 + 2C02 4CO + 2H2 AG,,ooc(kcal) = -12.4 +
C2H4 + C02
-
C2H6 + 2C02
2CO + CH4
+
-
4CO + 3H2
(1) AG,,ec(kcal) = -18.7 (2) AG,,ec(kcal) = -4.7 (3)
2C2HB + CO CH30H + 2CH4 + 2C AG,,.,(kcal) = -10.1 (4) CHSOH + CO -w C02 + CH, AG,ec(kcal) -30.1 (5)
Reactions such as these would account for the loss of C2+ products and for the modest increase in CH30H yield at extended residence times. (Note: the impact of reactions such as (4) above on observed carbon balances would be small and within the error in carbon balance closure normally observed,viz. >-98%; supporting the occurrence of reactions like (4) was a weight loss upon air calcination of the inert packing amounting to
