780
I n d . Eng. Chem. Res. 1993,32,180-I81
KINETICS, CATALYSIS, AND REACTION ENGINEERING Homogeneous Gas-Phase Oxidation of Methane Using Oxygen as Oxidant in an Annular Reactor Gary A. Foulds,'*+Brian F. Gray,*Sarah A. Miller,+and G. Stewart Walker+ Division of Coal and Energy Technology, CSIRO, Lucas Heights Research Laboratories, Priuate Mail Bag 7, Menai, NSW 2234, Australia, and Department of Applied Mathematics and Statistics, The University of Sydney, Sydney, NSW, Australia
The gas-phase partial oxidation of methane with oxygen has been investigated in a high-pressure quartz-lined annular reactor. The work undertaken consists of a systematic investigation of the effects of reactor tube wall temperature, pressure, feed oxygen concentration, and gas flow rate on methane conversion and methanol yield and selectivity. Methanol yields in the range of 1.5-2.3 mol 7% and selectivities in the range of 23-47 mol 7% have been observed, depending on the process parameters used. Increasing the oxygen concentration in the feed is found to decrease methanol selectivity dramatically, while yield exhibits a trade-off between decreasing selectivity and increasing conversion. The influence of pressure is most noticeable between 1.5 and 3.0 MPa, where substantially more methanol is produced at the higher pressure. The effect is less pronounced as the pressure is increased further. The most significant outcome of this study is the recognition of the importance of the interaction of the chemistry of the system and the heat-transfer properties of the reactor system. The system is very sensitive to heat release rate and exhibits a discontinuity in methane conversion, with hysteresis being observed under process conditions employing high feed oxygen concentrations and total gas flow rates. More importantly, highest methanol yields are observed on the downward sweep of reactor wall temperature, reinforcing the concept that the reaction is most sensitive to temperature and that low temperatures favor methanol production.
Introduction Natural gas utilization strategies either have relied on simple combustion or have involved the intermediate production of synthesis gas by steam reforming. Commonly, the synthesis gas production step accounts for 6070% of the total cost of all gas conversion processes, and it is generally recognized (Dautzenberg, 1990; Brown and Parkyns, 1991; Yarlagadda et al., 1988; Edwards and Foster, 1986; Morton et al., 1990; Michel, 1989; Haggin, 1988; Danen et al., 1991) that it is this area that needs to be targeted if a less capital cost-intensivetechnology is to be developed. The challenge to convert remote natural gas into a much more easily transportable commodity such as liquid product is of great industrial importance. Many natural gas deposita are geographically located so as to preclude conveying the gas by pipeline, which can be considerably more costly than transporting liquid product to suitable processing facilities (Parkyns, 1990; Leibsen et al., 1987). This creates an ideal opportunity for on-site conversion to a suitable liquid product. The direct conversion of methane to methanol by partial oxidation would satisfy both of the above requirements provided that necessary conversion and selectivity criteria are met, and as a result has recently attracted much interest. This is manifested by the number of technoeconomic evaluations (Gesser and Hunter, 1992; Kuo et
* To whom correspondence should be addressed. Present address: Department of Chemistry and Biochemistry, Division of Chemical Engineering and lndustrial Chemistry, James Cook University, Townsville, Queensland 481 1, Australia. t CSIRO. The University of Sydney.
*
0888-5885/93/2632-0180$04.00/0
al., 1989; Geerts et al., 1990; Edwards and Foster, 1986; Fox et al., 1990) and reviews (Mackie, 1991; Brown and Parkyns, 1991;Pitchai and Klier, 1986;Foster, 1985; Gesser et al., 1985; Garcia and Loffler, 1984) published. The partial oxidation of methane to methanol is usually accompanied by partial oxidation to carbon monoxide and formaldehyde and complete oxidation to carbon dioxide. The relative quantitites of these highly exothermic reactions are determined by the process parameters such as reaction temperature, pressure, feed composition, and space velocity (Walsh et al., 1992; Rytz and Baiker, 1991; Baldwin et al., 1991;Gesseret al., 1986,1987,1991;Onsager et al., 1989; Burch et al., 1989; Fukuoka et al., 1989; Yarlagadda et al., 1988; Brockhaus and Franke, 1977; Baurle et al., 1974; Lott and Sliepcevich, 1967; Pichler and Reder, 1933;Newitt, 1937). While yield and methanol selectivityare industriallyimportant, it is generally agreed that, in any chemical plant, major investment costa are normally associated with heat- and mass-transfer operations (Kuo et al., 1989; Geerts et al., 1990; Edwards and Foster, 1986). However, the homogeneous gas-phase partial oxidation reaction has been investigated previously under conditions which avoid heat generation by (i) operating at low reactant flow rate (Yarlagadda et al., 1988; Gesser et al., 1987),(ii) using inert diluent in the feed (Burch et al., 1989;Fukuoka et al., 1989; Brockhaus and Franke, 19771, and (iii) operating at low conversion (Brockhausand Franke, 1977), all of which are not industrially desirable. These different experimental approaches to the work have led to some disagreement in the results obtained, with methanol selectivities varying from less than 10% (Fukuoka et al., 1989) to over 80% (Yarlagadda et al., 1988; Gesser et al., 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 781 TO VENT
I
I
I
I TO VENT
TO VENT
I
TO VENT
TO VENT.
+7
r
i
D
A
CONTROL V A L V E
rp METERING V A L V E W ON AND OFF V A L V E
y
3 VAY V A L V E
@
PRESSURE CONTROL
1
RUPTURE
8
PRESSURE INDICATOR
@
F L O V CONTROL
SURE 0 0 FILTER
@
TEMPERATURE INDICATOR TEMPERATURE CONTROL
@
F L O V IINDICATOR
8 iiauio PRODUCT
Figure 1. Basic equipment layout of high-pressure (100 MPa) annular reactor rig.
1987),with other resulta falling between these two extremes (Foral, 1992; Rytz and Baiker, 1991; Burch et al., 1989). In addition, some reports have stated that selectivities are independent of oxygen concentration in the feed gas (Burch et al., 1989), while others have indicated that there is a strong dependence (Rytz and Baiker, 1991;Yarlagadda et al., 1988). Another area of potential discrepancy is the measurement of reaction temperature, with temperatures having been measured (i) outside the reactor wall (Burch et al., 1989), (ii) in the reactant gas (Yarlagadda et al., 1988, 1990; Gesser et al., 1987), while in some cases (iii) the position of measurement has not been specified (Foral, 1992;Onsager et al., 1989;Fukuoka et al., 1989;Brockhaus and Franke, 1977). This may be critical, since reaction yield and selectivity to methanol have been shown to be significantly affected by reaction temperature (Foulds et al., 1992b; Foral, 1992; Onsager et al., 1989; Burch et al., 1989). Considering the above, and the fact that the process economics are very sensitive to conversion and particularly methanol selectivity, it was decided to reinvestigate the gas-phase partial oxidation of methane in an annular flow reactor, as has been suggested in the literature (Fox et al., 1990; Kuo et al., 1989; Parkyns, 1990). The only control parameters available to the experimentalist are the equivalence ratio of CH4/02, the surface to volume ratio, the total pressure, the ambient (reactor tube wall) temperature, and the flow rate. With systems which are very nonlinear, such as this one, the system can be in two different states at the same parameter values depending on ita history. In such circumstances, clear distinction between system variables (reaction mixture temperature, concentrations) and control parameters (ambient temperature, flow rate, etc.) is absolutely es-
sential, and has not always been present in previous work, particularly in the high-pressure region. The approach used in this work consista of a systematic investigation of the behavior of the system as one or more of the control parameters are varied. Four control parameters, viz. total pressure, mixture strength (oxygen concentration in the feed), total gas flow, and ambient (tube wall) temperature, were varied. Tube wall temperature has been chosen as the preferred or bifurcation parameter, Le., the one which is varied at fixed values of the others. Part of this work has been presented at the Symposium of Natural Gas Upgrading, Division of Petroleum Chemistry, Inc., American Chemical Society (Foulds, 1992a,b). Experimental Section Reactor studies were carried out using a high-pressure tubular reactor with on-line analysis by gas chromatography. The equipment layout is shown in Figure 1. Methane P99.5 % ) and oxygen (S99.5 % ) were metered from their respective cylinders, using Brooks 58503 series mass flow controllers, to give the desired CH4/02 composition. The reactant gases were well mixed before entering the top of the vertical reactor. The gases flow down the reactor through heating zones 1-4 (see Figure 2) The gaseous products exit the reactor and pass through a cold condenser, followed by a back-pressure regulator, where the exit flow rate was measured using a wet gas meter or soap bubble meter depending on the flow rate. A small measured volume of the exit gas leaving the reactor was diverted via a needle valve to the gas chromatograph system for analysis. The reactor was constructed of stainless steel, lined with a close-fitting quartz glass insert (see Figure 2). The
782 Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993
-I l h
OUARTZ
i4
00
0 '
A
mvimi r~nwcmpti
MATrnG LOUE t
FURNACE TEWERATURE HDUXTORICONTROLLER
HEATIG LONE 2
FURNACE TEMPERATURE fflDICATOR/CONlROLLER
HEATHG ZONE 1
FURNACE TEMPERATURE fflDICATOR/CONTROLLER
HEATIG ZONE 1
FURNACE TEMPERATURE fflDICATOR/CONTROLLER
Oxygen and methane conversion were calculated as follows:
7
i
4
4 'i
SECTION A A
X = conversion, F = molar flow rate (NTP), i = in, o = out, and C, = j t h carbon-containing product. Methanol selectivity (S)was calculated as follows:
(3) Methanol yield was calculated on a carbon product basis (eq 5), as well as on a total product basis (Le., including H20, eq 4) as follows: (4) L
(5) I
01
W A R T 2 TUBE DO 0 t h
1
1
PRODUCT GASES
TO TRAP AND
VENT
Figure 2. Mechanical design of high-pressure (100MPa) annular reactor.
reactor was mounted vertically and had an internal diameter of 10 mm and a heated length of 500 mm, which was heated by four independently controlled heaters (100, 150, 150, 100 mm in length, respectively). Gas-phase reaction temperatures were measured using a thermocouple which could be moved up and down the center of a stainless steel tube which was housed in a quartz glass tube (4-mm 0.d.) located in the center of the reactor. Reactor wall temperatures were measured using an external thermocouple which could be moved up and down the center of a stainless steel tubelocated against the outer wall of the reactor. For each of the runs, the tube wall temperatuk was increased (upward traverse) and decreased (downward traverse). All valves and lines from the base of the reactor to the gas chromatograph were heated to prevent product condensation. Product gases were taken directly to a Shimadzu GC8A gas chromatograph capable of analyzing for H20, CH3OH, and HCHO (2-m Porapak packed column with TCD) and via a dry ice/acetone trap (to remove H20, CH30H, and HCHO) to a Shimadzu GC8A gas chromatograph capable of analyzing for H2, 02,CO, C02, C2H4, and C2H6 (2-m Carbosphere packed column with TCD). Data were collected at steady state, as verified by at least four reproducible seta of GC data. The time taken for steady state to be reached varied from approximately 3 h at 1.5 MPa to 5 h at 5.0 MPa, due to the lower linear gas velocity at the higher pressure. The reaction was held at a particular temperature until a t least four reproducible seta of GC data were obtained. This was approximately 2 h and corresponds to the time taken for each data point to be collected. For the purposes of determining conversions, selectivities, and yields, the total exit gas flow was constantly measured. Volumetric flow rates were determined at ambient temperature and pressure.
CO, C02, HCHO, C2H4, and CzHc selectivities and yields were calculated on a similar basis. Mean values quoted reflect d greater than 95% confidence limit over at least four readings. Reactor runs were performed using a range of process conditions. Reactor tube wall temperatures employed ranged from 300 "C to a maximum of 450 "C, above which it has been shown (Burch et al., 1989) that methanol decomposes in the presence of methane and oxygen. Pressures were varied from 1.5 to 5.0 MPa, while oxygen concentration in the feed gas was varied from 2.6% to 9.5%. NOTE: The high oxygen concentrations used at high pressure are approaching the explosive limit and the reactor should be located in an isolated explosion containment area. Mass balances with respect to C, H, and 0 were determined for each of the runs and were found to be within 5%.
Results Effect of Tube Wall Temperature on Reaction Temperature. Under the conditions employed in this study, the major products of reaction are CH30H, CO, and C02, with trace amounts of HCHO, C2H4, and C2Hs (less than 0.1 mol%) being formed. The overall reactions which yield these products are all highly exothermic and generate temperature excursions, particularly at high oxygen conversions. As a result, the maximum temperature generated by the gas-phase reactions (reaction mixture temperature = T ) was often found to be considerably higher than the reactor tube outer wall temperature (Twd), controlled by the furnace set temperature. This is illustrated in Figure 3 where the difference between the reaction mixture temperature and the tube wall temperature (AT= T - Twd is shown as a function of tube wall temperature for the runs carried out at 5.0% and 9.5% oxygen in the feed at a total flow rate of lo00 mL min-' (NTP) and a total pressure of 3.0 MPa, and for
Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 783 70 I
12
I
I
rn
M
I
I
I
1
360
380
400
420
440
460
480
0
500
'
360
380
400
I
I
420
440
1
460
480
500
Twall CC)
Twall eC)
Figure 3. Plots of T - Twallfor 9.5% oxygen in the feed at a flow 0,downward sweep rate of lo00 mL min-l(., upward sweep of Twan; of Twall), 5.0%oxygen in the feed at a flow rate of lo00 mL min-l (A, upward sweep of Twall; A,downward sweep of Twdl),and 9.5%oxygen in the feed at a flow rate of 200 mL min-l(M, upward sweep of All runs at 3.0 MPa. 0,downward sweep of TWali).
Figure 5. Plots of methane conversion vs Twallfor 9.5% oxygen in the feed at a flow rate of loo0 mL min-l (0,upward sweep of Twall; 0,downward sweep of Twall)and 5.0% oxygen in the feed at a flow rate of 1000mL min-l (m, upward sweep of T,,II; 0,downward sweep of TWan).All runs at'3.0 MPa. IO
12
t
I
e
8 h
5
8
- 6
.-6
v1
ti
6 4
8
g2 0 0
1 360
380
400
I
1
420
440
I
460
480
500
Twall (T)
Figure 4. Plots of methane conversion vs Twallfor 9.5% oxygen in the feed at a flow rate of lo00 mL min-l ( 0 ,upward sweep of Twall; 0,downward sweep of Twall)and 9.5% oxygen in the feed at a flow rate of 200 mL min-l (m, upward sweep of Twdl;0,downward sweep of Twall).All runs at 3.0 MPa.
the run carried out with 9.5 9% oxygen in the feed at a total flow rate of 200 mL min-l and a total pressure of 3.0 MPa. A number of features are significant. In each case, there is a discontinuity in the maximum temperature attained, with the magnitude of the discontinuity increasing as the feed oxygen concentration is increased and as the total flow rate is increased. In addition, a hysteresis effect is associated with the run using 9.5% oxygen a t 1000 mL min-l, i.e., the discontinuity occurs at a higher temperature on the upward sweep than it does on the downward sweep. This, together with the appearance of a negative temperature coefficient of reaction rate at higher tube wall temperatures, indicates that with 9.5% oxygen and at a flow rate of 1000 mL min-l the system jumps to an intermediate thermal steady state, which may or may not be oscillatory as has recently been observed in a similar reactor system (Yarlagadda et al., 1990). CHo Conversion. The effect of flow rate on methane conversion as a function of reactor wall temperature is shown in Figure 4. Runs were carried out at 200 and 1000 mL min-l, using 9.5 % oxygen in the feed a t a total pressure of 3.0 MPa. Both runs exhibit the discontinuity with the major differencebeing the hysteresis effect associated with the run at higher flow rate. The slightly higher methane conversion observed a t the higher flow rate is a function of the heat release rate of the reaction and ita interaction with the heat-transfer properties of the reactor system.
340
360
380
400
420
449
460
480
Twall ('C)
Figure 6. Plots of methane conversion vs Twallfor 9.5% oxygen in the feed at a flow rate of loo0 mL min-l and pressure of 1.5 MPa (0, upward sweep of Twdl;0,downward sweep of TWal1) and 9.5%oxygen in the feed at a flow rate of lo00 mL min-l and a pressure of 5.0 MPa (m, upward sweep of TWa11 0;, downward sweep of T,.II).
Methane conversion as a function of reactor wall temperature and feed oxygen concentration is shown in Figure 5 (runswere carried out using 5 % and 9.5% oxygen in the feed a t a total flow rate of 1000mL min-l and a total pressure of 3.0 MPa). Apart from both runs exhibiting the discontinuity,two distinct features are evident. Firstly, increasing the oxygen concentration results in higher methane conversion at temperatures above the discontinuity, and secondly, a hysteresis effect occurs when 9.5 % oxygen is used, which is absent when 5 7% oxygen is used. In fact, hysteresis is still present when 7.5% oxygen is used (run not shown in Figure 5), which is indicative of the sensitivity of the system to oxygen concentration. The first trend is unaffected by decreasing the flow rate to 200 mL min-l, but the hysteresis observed for 9.5% oxygen disappears at the lower flow rate. The effect of pressure on methane conversion as a function of reactor wall temperature is shown in Figure 6. These runs were carried out at 5.0 and 1.5 MPa using 9.5% oxygen and 90.5 % methane in the feed a t a total gas flow rate of lo00mL min-l. In both cases, the discontinuity with ita associated hysteresis effect is observed. The discontinuity is pressure dependent and moves to lower reactor wall temperature as the pressure is increased. The discontinuity and associated hysteresis observed at 3.0 MPa (not shown in Figure 6) falls between those observed at 1.5 and 5.0 MPa in accordance with this trend. The same trend is also observed at 200 mL/min-l, but without
784 Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 8
t
0
- 1
B
6
380
4w
I
,
,
I
1
420
440
469
480
Twall ("c)
Figure 7. Plot of CH30H yield (e),CO yield (A),COZyield (m), and H20 yield (0) va for 5.0% oxygen in the feed at a flow rate of lo00 mL min-*. All runs at 3.0 MPa.
hysteresis. This is in agreement with results reported by Burch et al. (1989),which were obtained at low flow rates in a similar reactor system. CH30H Yield/Selectivity. Typical product yields of CH30H, CO, C02, and HzO as a function of reactor wall temperature using 5% oxygen in the feed at 3.0 MPa pressure and a flow rate of 1000 mL/min-' are shown in Figure 7. Small amounts of C02 and H2O are formed at temperatures below the discontinuity. Methanol yield passes through a maximum of 1.9mol % which occurs at TWd1= 404 "C (corresponding to a reaction temperature of 435 "C, 5.7 mol % methane conversion, and a methanol selectivity of 36.3 mol %) and then decreases with increasing temperature, while CO yield increases with increasing temperature, apparently a t the expense of methanol production. Table I lists the maximum CH30H yields, both on a total product basis (H20 included) as well as on a carbon product basis, with associated product yields and the process conditions used. CH30H selectivities,at maximum CH30H yield, are also included for comparison with previous work. CH30H selectivity exhibits the same trend as methanol yield, but the maxima do not necessarily coincide, as the latter is a function of CHI conversion as well as CH30H selectivity. For this reason it was decided to consider trends in yield (total product basis) primarily and, where significant, to examine selectivity as well. The yield of CH30H as a function of reactor wall temperature using 9.5% oxygen at a flow rate of lo00 mL min-l and a total pressure of 3.0 MPa is shown in Figure 8. The other products are omitted for clarity, while the yield using a flow rate of 200 mL min-l is included for comparison. As expected, the CH30H yield mirrors the hysteresis and passes through a maximum of 1.9 mol % at a reactor wall temperature of 395 "C (corresponding to a reaction temperature of 447 "C, 9.0 mol % methane conversion, and a methanol selectivity of 23.5% ) on the downward traverse. Of particular importance is the fact that maximum CH30H yield is inaccessible on the upward temperature traverse. Decreasing the flow rate to 200mL min-l results in the disappearance of the hysteresis effect, but the other trends are similar with CH30H yield passing through a maximum of 1.8 mol % at a reactor wall temperature of 397 "C (corresponding to a reaction temperature of 427 "C, 8.5 mol % methane conversion, and a methanol selectivity of 23%). The effect of feed oxygen concentration on CH30H yield is depicted in Figure 9, where the yields for the runs containing 5.0% and 9.5% oxygen at a flow rate of lo00 mL min-l and pressure of 3.0 MPa are shown. While the
results are different in that the run at lo00 mL min-I exhibits a hysteresis effect, the actual maximum yields are not very different (it should be noted that the maximum CH30H yield for the run using 9.5% oxygen is only accessible on the downward traverse). This may be somewhat misleading as yield is a function of both methane conversion and methanol selectivity, and the increase in methane conversion associated with increasing the oxygen concentration appears to mitigate the drop in methanol selectivity. The drop in methanol selectivity as the feed oxygen concentration is increased is very effectivelyshown in Table I, where it drops from 36.3 to 23.5 mol 9% as the oxygen concentration is increased from 5.0% to 9.5% (Table I; runs 7-9). This trend is more striking at a flow rate of 200 mL min-l where, while the hysteresis effect is absent, the selectivity passes through a maximum before steadily decreasing as the temperature is increased, with the maximum selectivity dropping from 47.1 to 23.0 mol % as the feed oxygen is increased from 2.5% to 9.5% (Table I; runs 1-4). The effect of pressure on methanol yield is shown in Table I. When a flow rate of lo00 mL min-l and 9.5% oxygen are used, the maximum methanol yield increases as the pressure is increased from 1.5 (1.3mol 7%;run 10) to 3.0 MPa (1.9mol % ;run 9). Hysteresis is present and both maxima are only accessible on the downward traverse a t reactor wall temperatures of 405 "C (reaction temperature = 476 "C) for 1.5 MPa and 395 "C (reaction temperature = 447 "C) for 3.0 MPa, respectively. Increasing the pressure to 5.0 MPa (run 11) results in no further increase in maximum methanol yield (1.5mol %), but does result in an increase in COn yield. Similar results are obtained for the runs carried out at these pressures using 5.0% oxygen in the feed and a flow rate of 200 mL min-l (see Table I; runs 2, 5, and 6), with the main difference being the absence of the hysteresis effect. The lack of a smooth trend in these results is in agreement with results reported earlier (Burch et al., 1989)and can be attributed in part to yield being a function of both methane conversion and methanol selectivity (Rytzand Baiker, 1991). Discussion The gas-phase partial oxidation of methane with oxygen, carried out in a quartz-lined annular reactor, yields a product spectrum consisting of CHsOH, CO, COz and HzO, with trace amounts of HCHO and CzHa. The relative quantities of these products can be varied depending on the process conditions used, such as reactor wall temperature, feed oxygen concentration, pressure, and total gas flow rate. The system is most sensitive to reactor wall temperature, although, as noted in earlier work (Rytzand Baiker, 1991),there is considerable interaction between the process variables. A discontinuity in methane conversion is observed as the reactor wall temperature is increased, with conversion increasing rapidly over a small range of temperature. The magnitude of the discontinuity increases as the feed oxygen concentration and total gas flow rates are increased, reflecting an increase in the heat release rate of the system. At higher oxygen concentrations and flow rates, the system is bistable and a hysteresis effect is associated with the discontinuity. In addition, methane conversion decreases gradually after the discontinuity, probably as a result of the negative temperature coefficient of the heat generation rate observed a t higher reactor wall temperatures (Foulds et al., 1992b). The discontinuity also exhibits a pressure dependence, moving to lower reactor wall temperatures
Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 785 Table I. Maximum Methanol Yields with Associated Selectivities, Conversions, and Process Parameters flow feed 02 rate run (mL/min) (mol %) 2.5 1 200 5.0 2 200 7.5 3 200 9.5 4 200 5.0 5 200 5.0 6 200 7 lo00 5.0 7.5 8 lo00 9.5 9 lo00 9.5 10 lo00 9.5 11 lo00
Conversion (mol % Tb ("C) 92.7 3.1 401 96.5 4.5 407 421 96.6 6.4 427 97.0 8.5 95.3 4.1 438 384 94.5 4.6 435 92.0 5.7 440' 95.3 6.5 447e 98.3 9.0 476e 92.0 7.7 435e 78.0 6.7
selectivity (mol %) yieldf (mol %) CH30H CO COz CH3OH' CH30Hd H20' CO' COz' 47.0 44.4 8.4 1.3 1.5 3.5 1.2 0.2 37.7 47.5 14.8 1.5 1.7 4.0 2.2 0.6 28.1 55.3 16.6 1.8 6.9 3.4 1.0 1.7 23.0 55.1 21.9 1.8 2.0 7.3 4.4 1.7 27.7 60.3 12.0 1.1 1.1 3.5 2.7 0.5 33.7 50.1 16.1 1.5 1.6 4.3 2.2 0.7 36.3 46.6 17.2 2.0 2.1 6.4 2.6 0.9 33.3 48.6 18.1 2.2 2.3 6.8 3.2 1.2 23.5 57.5 19.0 1.9 2.1 7.9 4.8 1.6 17.9 63.9 18.2 1.3 1.4 9.8 4.6 1.3 23.6 49.9 26.6 1.8 4.8 3.2 1.7 1.5 Reator wall temperature at which maximum methanol yield occurs. Reaction temperature at which maximum methanol yield occurs. Yield on total product basis; eq 4. Yield on carbon product basis; eq 5. e Hysteresis present with maximum methanol yield on downward traverse. f The yield of HCHO and CzHe was less than 0.1 % in all cases. press. TwdP (MPa) ("C) 3.0 390 3.0 395 400 3.0 400 3.0 420 1.5 375 5.0 3.0 404 402' 3.0 395e 3.0 405e 1.5 385' 5.0
-
2.5
I
0 360
380
400
,
,
,
420
440
460
480
Twall (T)
Figure 8. Plots of CH30Ijyield vs Twa1lfor 9.5 % oxygen in the feed ataflowrateof1OOOmLmin-1 (.,upwardsweepof T,&;O,downward sweep of TW,11) and 9.5% oxygen in the feed at a flow rate of 200 mL 0,downward sweep of T,+.,~I). All min-l (M, upward sweep of TWal1; runs at 3.0 MPa. 2.5
t
360
I
380
400
I
I
I
,
I
420
440
460
480
500
Twall eC)
Figure 9. Plots of CH30H yield vs Twallfor 9.5% oxygen in the feed at a flow rate of 1OOOmL min-l(O, upward sweepof Tw&;0,downward and 5.0% oxygen in the feed at a flow rate of lo00 sweep of TWal1) downward sweep of Tw,1l). All mL min-l (M, upward sweep of Twa1~, runs at 3.0 MPa.
as the pressure is increased. These phenomena, when considered in conjunction with preliminary kinetic modeling (Fouldset al., 1992b),strongly indicate the occurrence of a cool flame mode, which may be oscillatory at these concentrations, supporting the findings of Yarlagadda et al. (1990). These results also highlight the interaction of the chemistry of the highly nonlinear reaction system with the heat-transfer properties of the reactor system, since the above phenomena are associated with a tangential interaction between the heat release curve and the heat loss line due to Newtonian cooling (Gray and Jones, 1984).
Methanol forms at the start of the discontinuity and passes through a maximum as the reactor wall temperature is increased. Further increasein the wall temperature results in a decrease in methanol yield and an increase in other carbon oxides, predominantly CO. Increasing the pressure from 1.5 to 3.0 MPa results in a greater yield of methanol. However, this trend is not sustained, and a further increase in pressure to 5.0 MPa does not result in a noticeable increase in methanol yield. While there is some influence of pressure on conversion, it appears that this effect is predominantly due to the loweringof the wall temperature at which the discontinuity occurs. The magnitude of the decrease in wall temperature at which the discontinuity occurs diminishes at higher pressures, e.g., it drops from 420 to 380 O C as thepressure is increased from 1.5 to 3.0 MPa, but only drops to 370 O C as the pressure is increased to 5.0 MPa (allruns at 5% 02, 200 mL min-l). In addition, competing formation of COz also occurs at higher pressure. Increasing the feed oxygen concentration results in a dramatic decrease in methanol selectivity, but increases the methane conversion substantially. Methanol yield, which is a function of both conversion and selectivity, increases as the oxygen concentration is increased, but the increase diminishes at higher oxygen concentrations as the decrease in selectivity dampens the effect of increased conversion. The effective residence time (time taken for 02 to be consumed) as shown in previous work (Fouldset al., 1992a), is a function of both the totalgas flow rate and the reactor wall temperature. It has been shown (Foulds et al., 1992b) that, for similar induction periods, higher reactor wall temperatures are required when greater gas flow rates are used. Once it has been ensured that reaction takes place in the reactor, increasing the total gas flow rate results in an increase in the rate of heat generation. For the same conversion, keeping the other process parameters fixed, this results in a higher reaction temperature being attained. When higher gas flow rates are employed in conjunction with high feed oxygen concentrations, hysteresis is observed (Figure 8). There is little difference between the maximum methanol yields obtained, but it is interesting to note that the maximum methanol yield occurs on the downward sweep of the hysteresis. Conclusions
To conclude,it is worthwhile comparing our results with those reported on the gas-phase partial oxidation of methane using tubular flow systems. We observe methanol yields in the range of 1.5-2.3 mol % and selectivities in
786 Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993
the range of 23-47 mol %, depending on the process parameters used. These values agree well with the results reported by Foral (1992), Rytz and Baiker (1992), and Burch et al. (1989),but are considerably lower than those reported by Gesser et al. (1988). In agreement with Gesser et al. (1988), Rytz and Baiker (1991), and Ford (1992), increasing the oxygen concentration in the feed is found to decrease methanol selectivity dramatically, while yield exhibits a trade-off between decreasing selectivity and increasing conversion. The influence of pressure is most noticeable between 1.5 and 3.0 MPa, where substantially more methanol is produced at the higher pressure, supporting results reported by Gesser et al. (19881, Rytz and Baiker (1991), and Foral(l992). However, at higher pressures the effect is less pronounced, which is in more agreement with the findings of Burch et al. (1990). However, perhaps the most important result that has emanated from this work is the recognition of the importance of the interaction of the chemistry of the system and the heat-transfer properties of the reactor system. Preliminary modeling studies have shown that the system is very sensitive to small changes in overall heat-transfer coefficient (Foulds et al., 1992b). This has been manifested experimentally by hysteresis being observed under process conditions giving high heat release rates. More importantly highest methanol yields are observed on the downward sweep of reactor wall temperature, reinforcing the concept that the system is most sensitive to temperature and that low temperatures favor methanol production. Thus, future work should involve research strategies that lower the temperature at which the discontinuity occurs. Methods that have been used to date involve the use of hydrocarbons that generate methyl radicals more easily than methane (Hunter et al., 1990;Burch et al., 1989) or the use of solids to catalyze the formation of methyl radicals (Omata et al., 1992;Kastanas et al., 1989;Durante et al., 1989,1990,Helton and Anthony, 1989;Fukuoka, 1989. In addition, the fact that the region of high methanol yield is only accessible from the downward traverse has important implications with respect to varying all control parameters in searching for windows of high methanol yield.
Acknowledgment We thank K. Wong and D. Chivers for technical assistance and BHP Co. Ltd. for financial support. Nomenclature Cj = carbon-containing product j (e.g., CH4, COZ) F = molar flow rate into the reactor (mol min-l) Fo = molar flow rate out of the reactor (mol min-1) NTP = normal temperature and pressure 0.d. = outer diameter (mm) Sj = selectivity to carbon-containing product j (mol % ) T = reaction temperature ("C) Twall= reactor wall temperature ("C) A T = difference between reaction temperature and reactor wall temperature (T= T,~I)("C) TCD = thermal conductivity detector X, = conversion of component j (mol 7%) yield, = yield of component j (mol %) yieldj* = yield of component j on carbon product basis (mol %)
Literature Cited Baldwin, T. R.; Burch, R.; Squire, G. D.; Tsang, S. C. Influence of Homogeneous Gas Phase Reactions in the Partial Oxidation of Methane to Methanol and Formaldehyde in the presence of Oxide Catalysts. Appl. Catal. 1991, 74, 137-152.
Bauerle, G. L.; Lott, J. L.; Sliepcevich, C. M. Oxidation of Methane at Elevated Pressures 11. A Reaction Mechanism. J . Fire Flammability 1974,5, 190-202. Brockhaus, R.; Franke, J. Process for the Manufacture of Formaldehyde and Methanol by Partial Oxidation of Methane. UK Patent Application GB 2 006 757,Sept. 22,1977. Brown, M. J.; Parkyns, N. D. Progress in the Partial Oxidation of Methane to Methanol and Formaldehyde. Catal. Today 1991,8, 305-335. Burch, R.; Squire, G. D.; Tsang, S. C. Direct Conversion of Methane toMethanol. J.Chem.SOC.,Faraday Trans.1 1989,85 (lo),35613568. Danen, W. C.; Ferris, M. J.; Lyman, J. L.; Oldenborg, R. C.; Rofer, C. K.; Steit, G. E. Methane to Methanol Conversion by Direct Partial Oxidation. Symposiumon Methane Upgrading, Presented before the Division of Petroleum Chemistry, Inc., American Chemical Society, Atlanta Meeting, April 14-19, 1991;pp 166171. Dautzenberg, F. M. The Characteristics and Challenges of New Natural Gas Technologies. Eurogas'SO, Proceedings from the European Applied Research Conference on Natural Gas, Trondheim, Norway, May 28-30,1990;pp 179-194. Durante, V. A.; Walker, D. W.; Seitzer, W. H.; Lyons, J. E. Vapor Phase Hydroxylation of Methane. Pacifichem '89, Abstract, Conference Proceedings; 1989;Vol. 48,pp 23-26. Durante, V. A.; Walker, D. W.; Gussow, S. M.; Lyons, J. E. Silicometallate Molecular Sieves and their use as Catalysts in Oxidation of Alkanes. United States Patent 4,918,249,April 17, 1990. Edwards, J. H.; Foster, N. R. The Potential for Methanol Production from Natural Gas by Direct Catalytic Partial Oxidation. Fuel Sci. Technol. Znt. 1986,4 (4),365-390. Foral, M. J. The Noncatalytic Partial Oxidation of Natural Gas to Methanol. Symposium on Natural Gas Upgrading 11, Presented before the Division of Petroleum Chemistry, Inc., American Chemical Society, San Francisco Meeting, April 5-10, 1992;pp 34-40. Foster, N. R. Direct Catalytic Oxidation of Methane to Methanol-A Review. Appl. Catal. 1985, 19,l-11. Foulds, G. A.; Miller, S.A.; Walker, G. S. Gas Phase Partial Oxidation of Methane. Symposium on Natural Gas Upgrading 11,Presented before the Division of Petroleum Chemistry, Inc., American Chemical Society, San Francisco Meeting, April 5-10,1992a;pp 26-33. Foulds, G. A.; Gray, B. F.; Griffiths, F.; Walker, G. S. Theoretical Modelling of the Gas Phase Partial Oxidation of Methane. Symposium on Natural Gas Upgrading 11, Presented before the Divisionof Petroleum Chemistry, Inc., American Chemical Society, San Francisco Meeting, April 5-10, 199213;pp 51-60. Fox, J. M.; Chen, T.-P., Degen,B. D. An Evaluation of Direct Methane Conversion Processes. Chem. Eng. Prog. 1990, April, 42-50. Fukuoka, N.; Omata, K.; Fujimoto, K. Effect of Additives on Partial Oxidation of Methane. Pacifichem '89, Abstract, Conference Proceedings, 1989;Vol. 135,pp 106-107. Garcia, E. Y.; Loffler, D. G. Partial Oxidation of Methane. Lat. Am. Chem. Eng. Appl. Chem. 1984,14, 267-284. Geerta, J. W. M. H.; Hoebink, J. H. B. J.; Van der Wiele, K. Methanol from Natural Gas. Proven and New Technologies. Catal. Today 1990,6, 613-620. Gesser, H. D.; Hunter, N. R. Methane Conversion by Oxidative Processes. In Methane Conversion by Oxidative Processes; Wolf, E. E., Ed.; Van Nostrand Reinhold New York, 1992;Chapter 12, pp 403-425. Gesser, H. D.; Hunter, N. R.; Prakash, C. B. The Direct Conversion of Methane to Methanol by Controlled Oxidation. Chem. Rev. 1985,85, (4),235-244. Gesser, H. D.; Hunter, N. R.; Morton, L. Direct Conversionof Natural Gas to Methanol by Controlled Oxidation. United States Patent 4,618,732,Oct 21,1986. Gesser, H.D.;Hunter, N. R.; Morton, L. A.; Yarlagadda, P. S.; Fung, D. P. C. The Direct Conversionof Methane to Methanol by a High Pressure Partial Oxidation Reaction. Prepr.-Am. Chem. SOC., Diu. Fuel Chem. 1987, 32 (3),255-259. Gesser, H. D.; Hunter, N. R.; Morton, L. The Direct Conversion of Methane to Methanol. Symposium on Methane Upgrading, Presented before the Division of Petroleum Chemistry, Inc., American Chemical Society, Atlanta Meeting, April 14-19,1991; pp 160-165.
Ind. Eng. Chem. Res., Vol. 32, No.5, 1993 787 Gray, B. F.; Jones, J. C. The Heat Release Rates and Cool Flames of AcetaldehydeOxidation in a ContinuouslyStirred Tank Reactor. Combust. Flame 1984,57,3. Haggin, J. New Wave of Methane Conversion Technology Expected Soon. Chem. Eng. News 1988,May 9,45-47. Helton, T. E.; Anthony, R. G. Methanol and Formaldehyde Production From Natural Gas. Pacifichem '89, Abstract, Conference Proceedings, 1989;Vol. 154,pp 143-145. Hunter, N. R.; Gesser, H. D.; Morton, L. A,; Yarlagadda, P. S.; Fung, D. P. C. Methanol Formation at High Pressure by the Catalyzed Oxidation of Natural Gas and by the Sensitized Oxidation of Methane. Appl. Catal. 1990,57,45-54. Kastanas, G. N.; Tsigdinos, G. A.; Schwank, J. Methane Activation over Molybdenum and Tungsten Based Mixed Oxides. Symposium on Natural Gas Conversion, Presented at the 1989 Spring National A.1.Ch.E. Meeting, Texas, April 2-6, 1989;paper 52d. Kuo, J. C.W.; Kresge, C. T.; Palermo, R. E. Evaluation of Direct Methane Conversion to Higher Hydrocarbons and Oxygenates. Catal. Today, 1989,4,463-470. Leibson, I.; Davenport, S. T.; Muenzler, M. H. Costa to Transport Natural Gas. Hydrocarbon Process. 1987,April, 47-50. Lott, J. L.; Sliepecevich,C. M. Partial Oxidation of Methane at High Pressures. Znd. Eng. Chem. Process Des. Dev. 1967,6(l),67-74. Mackie, J. C. Partial Oxidation of Methane: The Role of Gas Phase Reactions. Catal. Rev.-Sci. Eng. 1991,33 ( 1 & 2), 169-240. Michel, S. Cost to make CO or HP + CO. Hydrocarbon Process. 1989,April, 37-39. Morton, L. A.; Hunter, N. R.; Gesser, H. D. Methanol, A Fuel for Today and Tomorrow. Chem. Znd. 1990,July, 457-462. Newitt, D. M. The Oxidation of Hydrocarbons at High Pressure. Chem. Rev. 1937,21,299-317.
Omata, K.; Fukuoka, N.; Fujimoto, K. Methane Partial Oxidation to Methanol-Solid Initiated Homogeneous Methane Oxidation. Catal. Lett. 1992,12,227-230. Onsager, 0.T.; Lodeng, R.; Soraker, P.; Anundskaaa, A.; Helleborg, B. The Homogeneous Gas Phase Oxidation of Methane and the Retarding Effect of Basic/Inert Surfaces. Catal. Today, 1989,4, 355-363. Parkyns, N.D. Methane Conversion-A Challenge to the Industrial Chemist. Chem. Br. 1990,Sept, 841-844. Pichler, H.; Reder, R. Uber die Bildung von Methylalkohol bie der Partiellen Oxydation von Methan und Athylen unter Hohem Druck. Angew. Chem. 1933,46,161-165. Pitchai, R.; Klier, K. Partial Oxidation of Methane. Catal.Rev.-Sci. Eng. 1986,B (l),13-88. Rytz, D. W.; Baiker, A. Partial Oxidation of Methane to Methanol in a Flow Reactor at Elevated Pressure. Znd. Eng. Chem. Res. 1990,30,2287-2292. Walsh, D. E.; Martenak, D. J.; Han, S.;Palermo, R. E. Direct Oxidative Methane Conversion at Elevated Pressures and Moderate Temperatures. Ind. Eng. Chem. Res. 1992,31,1259-1262. Yarlagadda, P. S.;Morton, L. A.; Hunter, N. R.; Gesser, H. D. Direct Conversion of Methane to Methanol in Flow Reactor. Znd. Eng. Chem. Res. 1988,27,252-256. Yarlagadda, P. S.;Morton, L. A.; Hunter, N. R.; Gesser, H. D. Temperature Oscillations During the High Pressure Partial Oxidation of Methane in a Tubular Flow Reactor. Combust. Flume 1990,79,216-218. Received for review September 16,1992 Revised manuscript received December 17,1992 Accepted January 11, 1993
