Oxidation of methane to methanol

A marked improvement in yield, in selectivity or in both is obtained in the synthesis of methanol by the homogeneous direct partial oxidation of natural gas or other source of methane when the reactor space is filled with inert, refractory inorganic particles.

FIELD OF THE INVENTION 
This invention pertains to the direct partial oxidation of a gaseous feed 
comprising a source of methane to normally liquid products. More 
particularly, it pertains to converting a gaseous feed comprising natural 
gas admixed with gaseous oxygen to methanol and other liquid oxygenated 
organic products. 
BACKGROUND OF THE INVENTION 
Natural gas is an abundant fossil fuel resource. Recent estimates places 
worldwide natural gas reserves at about 35.times.10.sup.14 standard cubic 
feet, corresponding to the energy equivalent of about 637 billion barrels 
of oil. 
The composition of natural gas at the wellhead varies but the major 
hydrocarbon present is methane. For example the methane content of natural 
gas may vary within the range of from about 40 to 95 vol. %. Other 
constituents of natural gas may include ethane, propane, butanes, pentane 
(and heavier hydrocarbons), hydrogen sulfide, carbon dioxide, helium and 
nitrogen. 
Natural gas is classified as dry or wet depending upon the amount of 
condensable hydrocarbons contained in it. Condensable hydrocarbons 
generally comprise C.sub.3 + hydrocarbons although some ethane may be 
included. Gas conditioning is required to alter the composition of 
wellhead gas, processing facilities usually being located in or near the 
production fields. Conventional processing of wellhead natural gas yields 
processed natural gas containing at least a major amount of methane. 
Processed natural gas, consisting essentially of methane, (typically 85-95 
volume percent) may be directly used as clean burning gaseous fuel for 
industrial heat and power plants, for production of electricity, and to 
fire kilns in the cement and steel industries. It is also useful as a 
chemicals feedstock, but large-scale use for this purpose is largely 
limited to conversion to synthesis gas which in turn is used for the 
manufacture of methanol and ammonia. It is notable that for the foregoing 
uses, no significant refining is required except for those instances in 
which the wellhead-produced gas is sour, i.e., it contains excessive 
amounts of hydrogen sulfide. Natural gas, however, has essentially no 
value as a portable fuel at the present time. In liquid form, it has a 
density of 0.415 and a boiling point of minus 162.degree. C. Thus, it is 
not readily adaptable to transport as a liquid except for marine transport 
in very large tanks with a low surface to volume ratio, in which unique 
instance the cargo itself acts as refrigerant, and the volatilized methane 
serves as fuel to power the transport vessel. Large-scale use of natural 
gas often requires a sophisticated and extensive pipeline system. 
A significant portion of the known natural gas reserves is associated with 
fields found in remote, difficultly accessible regions. For many of these 
remote fields, pipelining to bring the gas to potential users is not 
economically feasible. 
Indirectly converting methane to methanol by steam-reforming to produce 
synthesis gas as a first step, followed by catalytic synthesis of methanol 
is a well-known process. Aside from the technical complexity and the high 
cost of this two-step, indirect synthesis, the methanol product has a very 
limited market and does not appear to offer a practical way to utilize 
natural gas from remote fields. The Mobil Oil Process, developed in the 
last decade provides an effective means for catalytically converting 
methanol to gasoline, e.g. as described in U.S. Pat. No. 3,894,107 to 
Butter et al. Although the market for gasoline is huge compared with the 
market for methanol, and although this process is currently used in New 
Zealand, it is complex and its viability appears to be limited to 
situations in which the cost for supplying an alternate source of gasoline 
is exceptionally high. There evidently remains a need for better ways to 
convert natural gas to higher valued and/or more readily transportable 
products. 
A reaction which has been extensively studied for many years is the direct 
partial oxidation of methane to methanol. This route, involving 
essentially the reaction of methane and gaseous oxygen according to the 
simple equation 
EQU CH.sub.4 +1/2O.sub.2 .fwdarw.CH.sub.3 OH 
could theoretically produce methanol with no by-product. The homogeneous 
reaction of methane with oxygen occurs most favorably under high pressure 
(10 to 200 atm.), moderate temperatures, (350.degree.-500.degree. C.), and 
at relatively low oxygen concentration. Oxidation to formaldehyde and deep 
oxidation reactions are minimized under these conditions. The mechanism of 
methanol formation is believed to involve the methylperoxy radical 
(CH.sub.3 OO.) which abstracts hydrogen from methane. Unfortunately, up 
until now the per pass yields have been limited. This limited yield has 
been rationalized as resulting from the low reactivity of the C-H bonds in 
methane vis-a-vis the higher reactivity of the primary oxygenated product, 
methanol, which results in selective formation of the deep oxidation 
products CO and CO.sub.2 when attempts are made to increase conversion. 
U.S. Pat. No. 4,618,732 to Gesser et al. describes an improved homogeneous 
process for converting natural gas to methanol. The selectivity for 
methanol is ascribed by the inventors to careful premixing of methane and 
oxygen and to eliminating reactor wall effects by use of glass-lined 
reactors. 
It is an object of this invention to selectively convert a gaseous feed 
comprising methane to higher-valued liquid oxygenates. 
It is a further object to provide a selective method for the direct 
homogeneous partial oxidation of a gaseous hydrocarbon feed comprising 
methane to methanol. 
It is a still further object of this invention to provide a method for 
converting a mixture of natural gas and oxygen to methanol and other 
high-valued oxygenates with increased selectivity and/or yield. 
These and other objects will become evident to one skilled in the art on 
reading this entire specification and appended claims.

DESCRIPTION OF THE INVENTION INCLUDING BEST MODE 
Surprisingly, we have now found that both the yield and the selectivity in 
the direct homogeneous partial oxidation of a gaseous feed comprising 
methane and gaseous oxygen are improved when the empty reactor is packed 
with a low surface area solid such as sand. Inasmuch as sand is not 
recognized as having an effect on organic chemical reactions, its effect 
in the present instance is not understood. 
It is contemplated that any inert inorganic solid having a low surface area 
can be used in the present invention. The term "inert" as used herein 
means that the solid is unaffected physically or chemically by exposure to 
the feed at reaction temperature, and that it is devoid of patent 
catalytic activity for the direct partial oxidation of methane. 
Non-limiting examples of refractory solids contemplated as useful in the 
present invention include sand, crushed quartz, Vycor, and non-hydrous 
aluminas such as corundum (alpha alumina). In general, the surface area of 
the particulate solid is not more than 50 M.sup.2 /g (square meters/gm), 
preferably not more than 10 M.sup.2 /g and most preferably not more than 1 
M.sup.2 /g, as determined by the B.E.T. method using nitrogen adsorption. 
Although it is contemplated that in some instances the packing material 
may be provided as a monolith, the more generally useful, less expensive 
preferred form is as a particulate solid. 
In the practice of the present invention, it is preferred to use a dual 
flow system, i.e., a system in which the natural gas and the oxygen or air 
are kept separate until mixed just prior to being introduced into the 
reactor. However, if desired, the oxygen and natural gas may be premixed 
and stored together prior to the reaction. The preferred dual flow system 
minimizes the risk of fire or explosion. In the dual flow system, the 
amount of oxygen flow is controlled so as to prepare a reaction mixture 
that contains 2 to 20 percent by volume, more preferably 2 to 10 percent 
of oxygen. Air may be used instead of oxygen. The residence time of the 
gaseous feed in the reactor, computed in all cases herein on an empty 
reactor basis, is within the range of about 0.1 to 100 minutes, preferably 
about 2 to 10 minutes, and most preferably about 4 to 8 minutes. 
The temperature in the reaction zone is from about 300.degree. C. to 
500.degree. C., and preferably about 350.degree. C. to 450.degree. C. In 
the preferred mode of operation, the reactor temperature is at least 
sufficient to insure conversion of substantially all of the oxygen, i.e., 
more than 90% of the oxygen in the feed, and preferably more than 95%. 
The apparatus shown in FIG. 1 of the drawing, which was used in generating 
the examples which follow, will now be described. 
A methane source such as natural gas is fed via line 1 through valve 2 and 
passes via line 3 to a mass flow controller 4. It is then passed through 
check valve 5 and via line 6 passed to one arm of a mixing cross 7, 
another arm of which is fitted with a pressure gauge 14. The gaseous 
oxygen source is passed through line 8, through valve 9 and line 10 to 
mass flow controller 11, check valve 12 and then via line 13 to a third 
arm of the mixing cross 7. The mixed gasses exit the mixing cross 7 via 
the fourth arm, and pass through line 15 to mixing coil 16. The mixing 
coil consists of 40 feet of 1/8 inch O.D. tubing. The gas exits from the 
mixing coil and flows via line 7 which is fitted with a sand-filled filter 
18, a T-fitting 19, a mass flow controller 20 and check valve 21. The gas 
mixture exiting from check valve 21 passes via line 22 to the inlet 
section of reactor 23 which is mounted in furnace 24. To maintain a 
constant pressure at the inlet to the reactor, one arm of the T-fitting 19 
is connected to a Grove loader back pressure regulator 25 fitted with 
pressure gauge 26 and a bleed-off vent line 27. The pressure in line 22 
and at the inlet to the reactor is indicated by pressure gauge 28. After 
passing through the Pyrex-lined reactor 23 mounted in furnace 24, hot 
gasses exiting the reactor pass via line 29 to a Grove loader 
back-pressure regulator 30 fitted with pressure gauge 31. The Grove loader 
is fitted with auxiliary line 32 and valves 34 and 35 connected to line 32 
by a T-fitting. During start up of the run, valve 35 is closed and valve 
34 is open, and the gasses exiting from the Grove loader are vented from 
valve 34 via line 33. After the temperature of the reactor has been 
adjusted, valve 34 is closed and the gasses passing from the Grove loader 
back-pressure regulator pass via line 37 to serially arranged cold trap 
38, 39 and 40 via lines 41 and 42. The cold gas is then passed via line 43 
to gas sample bomb 44, and then via line 45 to wet test meter 46 from 
whence they are vented via line 47. 
EXAMPLES 
The following examples are intended to illustrate the present invention 
without limiting the scope thereof, which scope is defined by this entire 
specification including appended claims. All amounts, proportions and 
selectivities shown are on a weight basis unless explicitly stated to be 
otherwise. 
The following terms are defined. "Total Hydrocarbon Conversion" refers to 
the percentage of feed carbon converted when natural gas feedstock is 
used. "Selectivity" is defined as the percentage of carbon in a specific 
product, e.g. methanol, etc., formed from the converted feed carbon. 
"Yield" is defined as the product of conversion and selectivity. 
"Residence Time" in all cases refers to that calculated by dividing the 
volume of the empty reactor by the volume of feed at reaction temperature 
and pressure fed per minute to the reactor. 
Experiments were run in the following manner. When reactor materials were 
used, 8.0 cc of 20/40 mesh gamma-alumina, (Na)ZSM-5, or 30/80 mesh washed 
sand were mixed with 8.0 cc of 30/80 mesh washed sand and loaded into a 
16.5 mm i.d. Pyrex-lined reactor. The runs were performed using natural 
gas feed and the composition of the natural gas is given in Table I. 
(Na)ZSM-5 as used herein means sodium-exchanged ZSM-5. 
TABLE I 
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Composition of Natural Gas Feed 
Component Vol. % 
______________________________________ 
Methane 95.66 
Ethane 2.46 
Propane 0.33 
Butanes 0.12 
C.sub.5 's 0.01 
CO.sub.2 0.90 
H.sub.2 0.52 
100.00 
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6-7 volume percent of oxygen was co-fed and thoroughly premixed with the 
hydrocarbon feed prior to reaction in the apparatus shown in FIG. 1. 
Operating conditions for the experiment were 360.degree.-450.degree. C. 
(minimum temperature required for complete O.sub.2 consumption), 960 psig, 
and 400 cc/min feed flow (4 min. residence time based on empty tube). Gas 
products were determined by a Carle refinery gas analyzer and liquid 
products were analyzed by GC/MS. Liquid-phase oxygenates other than 
methanol were not quantified individually, but contained compounds such as 
formic acid, acetic acid, ethanol, dimethoxymethane, etc. 
EXAMPLE 1 (Control) 
This example is not within the scope of the present invention. It is 
included only for comparison. 
In this example, the apparatus described in FIG. 1 with no packing in the 
pyrex-lined reactor was used to convert a feed mixture of natural gas and 
oxygen. The run conditions and results for all four examples are 
summarized in Table II to provide convenient comparison. 
EXAMPLE 2 
This example illustrates the invention using a washed, meshed sand packing 
instead of the empty reactor. 
The results in Table II clearly demonstrate a marked improvement in both 
selectivity for methanol plus other liquid oxygenates and yield of 
methanol plus other oxygenates per pass. 
EXAMPLE 3 
This example is not within the scope of the present invention. In this 
example gamma alumina was substituted for the sand packing of Example 2. 
EXAMPLE 4 
This example, too, is not within the scope of the present invention and it 
is shown only for comparison. In this example (Na)ZSM-5 packing was 
substituted for the sand of Example 2. 
TABLE II 
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Example No. 1 2 3 4 
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Packing None Sand .gamma.-Alumina 
(Na)ZSM-5 
Surface Area, M.sup.2 /g 
NA &lt;5 244 208 
Alpha Value 0 &lt;1 ca 1 1-2 
Pressure, psig 
960 960 960 960 
Temperature, .degree.C..sup.1 
360 400 450 410 
Residence Time, min. 
4 4 4 4 
Feed Flow, cc/min. 
400 400 400 400 
O.sub.2 in Feed, % 
6.4 7.0 6.6 6.6 
CH.sub.4 Conv., % 
3.6 4.0 3.1 2.4 
C.sub.2 Conv., % 
28.4 27.2 23.0 25.1 
C.sub.3 Conv., % 
59.0 49.6 55.9 44.4 
C.sub.4 Conv., % 
79.7 64.2 76.5 57.6 
Total Hydrocarbon 
5.5 5.9 5.1 4.3 
Carbon Conv., % 
Product Selectivities 
CO, % 49.4 40.0 48.2 58.7 
CO.sub.2, % 21.8 21.7 24.0 20.8 
CH.sub.3 OH, % 
25.8 27.2 21.6 14.2 
Other Oxygenates, % 
3.0 11.1 6.3 6.4 
CH.sub.3 OH + Other 
28.8 38.3 27.9 20.6 
Oxygenates, % 
% Yield, CH.sub.3 OH + 
1.6 2.3 1.4 0.9 
Other Oxygenates 
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.sup.1 Minimum temperature required for complete O.sub.2 consumption.