Process for the production of at least one alkyl tertiobutyl ether from natural gas

The invention concerns a process for the synthesis of at least one alkyl tertiobutyl ether, preferably respectively MTBE or ETBE, from at least one alcohol and from isobutene, each synthesized at least partially from natural gas. The alcohol, preferably methanol or ethanol respectively, is synthesized at least partially from synthesis gas, a portion of said synthesis gas being prepared in natural gas steam prereforming zone. The isobutene is synthesized in a series of processes which includes direct transformation of natural gas to ethylene, known as oxidative coupling of methane (OCM), dimerization of ethylene to normal butene, isomerization of n-butene to isobutene, and the separation units associated therewith.

BACKGROUND OF THE INVENTION 
The present invention concerns a process for the production of at least one 
alkyl tertiobutyl ether at least partially from natural gas. 
Natural gas is an abundant raw fossil material mainly constituted by 
methane, whose current known reserves, of the order of 10.sup.14 m.sup.3, 
represent about 50 years of world consumption. The gas fields frequently 
contain large quantities of ethane, propane, other high alkanes 
(C.sub.3.sup.+ hydrocarbons, i.e., hydrocarbons containing at least 3 
carbon atoms per molecule), as well as other constituents such as H.sub.2 
O, CO.sub.2, N.sub.2, H.sub.2 S and He. The majority of the propane and 
other high alkanes associated with natural gas are liquefied as LPG 
(liquid petroleum gas). In helium rich fields (generally more than 0.3% by 
volume), helium is separated out because of its high commercial value. 
Hydrogen sulphide is also separated out because of its corrosive nature, 
and also water to prevent the formation of hydrates which make the 
transport of natural gas difficult. Natural gas can be purified by 
dehydration and desulphurisation, preferably to a sulphur compound content 
of less than 10 ppm. It is then termed uncondensed gas and contains mainly 
methane (for example, 55-99% by volume), also ethane, usually propane and 
may contain small amounts of nitrogen and/or carbon dioxide. 
However, the term natural gas is indiscriminately applied to purified gas 
obtained by dehydration and desulphurisation, and to unpurified gas 
obtained directly from a field and not subjected to any purification 
whatsoever. 
Most natural gas is used for private and industrial heating. However, some 
processes exist for transforming natural gas into higher hydrocarbons. 
Transformation of natural gas into alkyl tertiobutyl ether(s), such as 
methyl tertiobutyl ether (MTBE) or ethyl tertiobutyl ether (ETBE) 
respectively, is highly desirable. Alkyl tertiobutyl ethers are currently 
produced by reacting isobutene with an alcohol, such as methanol or 
ethanol respectively, and while said alcohols are readily synthesized from 
natural gas via synthesis gas (defined as a mixture containing carbon 
monoxide, hydrogen and, optionally, carbon dioxide), this is not the case 
for isobutene which is normally extracted from a C.sub.4 (hydrocarbons 
containing 4 carbon atoms per molecule) cut, produced from a catalytic 
cracking unit for petroleum cuts, for example. This source of olefins is 
now far too limited to fulfil the increasing demand for alkyl tertiobutyl 
ether(s) such as MTBE or ETBE. For this reason, a method of producing 
alkyl tertiobutyl ethers from natural gas would be of interest. 
SUMMARY OF THE INVENTION 
An object of the invention is to synthesize at least one alkyl tertiobutyl 
ether, preferably respectively MTBE or ETBE, from at least one alcohol and 
from isobutene, each synthesized at least partially from natural gas. In 
the process of the invention, the alcohol, preferably respectively 
methanol or ethanol, is synthesized from synthesis gas, said synthesis gas 
being itself partially prepared in a natural gas steam prereforming zone. 
The isobutene is synthesized in a series of processes which includes 
direct transformation of natural gas to ethylene, known as oxidative 
coupling of methane (OCM), dimerisation of ethylene to normal butene, 
isomerisation of n-butene to isobutene, and the separation units 
associated therewith. The process for the production of at least one alkyl 
tertiobutyl ether from natural gas is termed the oxyetherification 
process. 
United States patent U.S. Pat. No. 5,159,125 claims a process for aldol 
condensation of methanol or other alcohols with an olefin containing at 
least two carbon atoms, to synthesize an isobutanol-rich mixture of higher 
molecular weight alcohols. The isobutanol can then be separated and 
dehydrated to form isobutene and said isobutene is condensed with methanol 
to form MTBE. These processes are known to the skilled person. 
A further object of the process of the invention is to utilise the 
by-products of non selective oxidation of natural gas in the OCM reactor, 
i.e., carbon monoxide and carbon dioxide, as well as the hydrogen which is 
produced at the same time, as synthesis gas which at least partially 
supplies the unit for synthesizing at least one alcohol. 
Ethylene and other hydrocarbons can be produced by oxidative coupling of 
methane or a cut containing mainly methane in either sequential or 
simultaneous mode. 
Sequential mode oxidative coupling reaction consists in oxidising methane 
with a reducible agent followed by separate reoxidation of said agent by 
the oxygen in air. A number of metal oxides, in particular Mn, Ce, Pr, Sn, 
In, Ge, Pb, Sb, Bi, and Tb have been cited for use as reducible agents for 
this reaction in a number of United States patents, for example U.S. Pat. 
Nos. 4,499,323, 4,523,049, 4,547,611 and 4,567,307. 
The simultaneous mode oxidative coupling reaction (passing a mixture of 
methane and oxygen over a contact mass) can be written qualitatively as 
follows: 
EQU CH.sub.4 +O.sub.2 .fwdarw.C.sub.2 H.sub.6 +C.sub.2 H.sub.4 + other 
hydrocarbons+CO+CO.sub.2 +H.sub.2 +H.sub.2 O 
A number of patent documents (for example European patent applications 
EP-A2-210 383, EP-A1-189 079, EP-A1-206 044 and PCT application WO 86 
07351) mention the use of rare earth oxides, alkali and alkaline-earth 
oxides and titanium, zirconium, hafnium and zinc oxides either alone or 
mixed together as catalysts for oxidative coupling reaction of methane in 
simultaneous mode. 
Because of the occasionally appreciable presence of propane and sometimes 
of other high alkanes in natural gas, and occasionally the presence of 
light hydrocarbons from other units located, for example, close to the 
natural gas oxidative coupling unit, it is very advantageous to utilise 
propane and occasionally other high alkanes as well as methane and ethane. 
U.S. Pat. No. 5,025,108 thus claims a process for the oxypyrolysis of 
natural gas in which the natural gas is separated into two fractions, a 
first methane-rich fraction which is selectively oxidised by molecular 
oxygen in the presence of a contact mass (oxidative coupling of methane, 
OCM) to form an effluent which contains C.sub.2 hydrocarbons and water as 
the principal products, the effluent also containing small quantities of 
CO, CO.sub.2 and H.sub.2, and a second fraction of C.sub.2.sup.+ 
hydrocarbon enriched gas (i.e. containing at least 2 carbon atoms per 
molecule) which is mixed with said effluent when at least about 80% by 
volume of the molecular oxygen has been consumed in the methane oxidation 
step, the resulting mixture then being pyrolysed. 
A further possibility (U.S. Pat. No. 5,113,032) consists of separating the 
natural gas into three fractions in an oxypyrolysis process, i.e., a first 
methane-rich fraction, a second C.sub.2 -rich fraction and a third 
C.sub.3.sup.+ hydrocarbon-rich fraction, and to separately pyrolyse said 
second and third fractions to increase the ethylene yield, or to pyrolyse 
said second fraction and independently utilise the C.sub.3.sup.+ cut using 
any other process which is known to the skilled person. 
The present invention thus also concerns a process for the production of at 
least one alkyl tertiobutyl ether at least partially from natural gas 
wherein the ethylene required for isobutene production is produced by the 
oxypyrolysis process claimed in United States patents U.S. Pat. Nos. 
5,025,108 and 5,113,032. 
Some of the advantages of the oxyetherification process for natural gas of 
the present invention are: 
a) the synthesis of at least one alkyl tertiobutyl ether is carried out 
using isobutene and alcohol(s) which are at least partially prepared from 
natural gas; 
b) the preparation of the synthesis gas necessary for the synthesis of at 
least one alcohol does not necessitate the installation of an expensive 
steam reforming unit for methane or a partial oxidation unit. A simple 
prereforming unit can produce most of the synthesis gas, the remainder 
being supplied by the OCM unit; 
c) the use of carbon oxides (CO, CO.sub.2) and hydrogen from the OCM 
reaction as synthesis gas for the production of at least one alcohol means 
that the by-products of this reaction can be better utilised. 
The oxyetherification process of the invention, for synthesizing at least 
one alkyl tertiobutyl ether, preferably respectively methyl tertiobutyl 
ether or ethyl tertiobutyl ether, at least partially from natural gas, 
preferably purified, comprises the following steps: 
(a) Steam prereforming a feedstock containing mainly natural gas, 
preferably practically free of CO.sub.2, and more preferably purified, in 
a prereforming zone in the presence of a contact mass to partially convert 
this fraction to an effluent containing mainly hydrogen and carbon oxides 
(CO, CO.sub.2). Said feedstock is preferably methane-rich and generally 
contains methane, ethane, propane, other hydrocarbons which are associated 
with natural gas and, possibly, non hydrocarbon gases. Prior to 
prereforming, said feedstock may be separated into a first fraction 
containing mainly methane, preferably substantially free of ethane, 
propane and other hydrocarbons which are associated with natural gas, said 
fraction then constituting the actual feedstock for the prereforming zone, 
and into a second fraction containing mainly hydrocarbons containing at 
least two carbon atoms per molecule (in particular mainly ethane and 
propane, and may or may not also contain non hydrocarbon gases), which can 
either be utilised in the process, for example by mixing it with the fifth 
fraction described in step (c) below and injecting said mixture into the 
section of the oxypyrolysis zone in which pyrolysis is effected, or 
purged. This optional prior separation step can be carried out in a 
specific fractionation zone. However, it is preferably carried out in the 
separation zone described in step (c) below in order to minimise 
investment costs. 
The prereforming process has been described, for example, by British Gas 
(Hydrocarbon Processing, p34, December 1990). All the C.sub.2.sup.+ 
hydrocarbons present in the feedstock are decomposed to methane. This 
latter establishes reforming equilibrium with hydrogen, carbon oxides and 
water vapour. 
(b) A major portion of the effluent from prereforming step (a) is mixed 
with a major portion of the second effluent from the OCM zone described in 
step (d) below, which has been dried and compressed. 
(c) The mixture formed in step (b) is then sent to a separation zone to 
produce the following effluents: 
a first fraction containing mainly carbon 
dioxide, 
a second fraction containing mainly methane, 
a third fraction containing mainly ethylene, 
a fourth fraction containing mainly hydrogen 
and carbon monoxide, 
a fifth fraction containing mainly ethane and C.sub.3.sup.+ hydrocarbons. 
(d) Oxidative coupling of methane (OCM) is carried out in an OCM zone. When 
the natural gas is to be oxypyrolysed, the oxypyrolysis zone comprises two 
successive sections: one section of the oxypyrolysis zone where OCM is 
effected and a further section of the oxypyrolysis zone where pyrolysis is 
effected; thus said OCM zone is in fact the section of the oxypyrolysis 
zone in which OCM occurs. The OCM zone is supplied with at least one gas 
fraction, preferably enriched in methane, preferably by at least a portion 
of the second fraction described in step (c). The OCM zone provides a 
first effluent containing mainly reaction product, i.e., ethylene, and a 
second effluent containing mainly by-products of the OCM reaction, i.e., 
hydrogen, carbon monoxide and carbon dioxide. 
When an oxypyrolysis zone is present, the supply for the section of the 
oxypyrolysis zone where pyrolysis is effected is preferably constituted by 
a major portion of the fifth fraction described in step (c) as described 
in United States patent U.S. Pat. No. 5,025,108 cited above. More 
preferably, said fifth fraction is separated into a first stream 
containing mainly ethane and a second stream containing mainly 
C.sub.3.sup.+ hydrocarbons, either to supply the section of the 
oxypyrolysis zone where pyrolysis is effected with at least a portion of 
said two streams which will be separately pyrolysed, as described in 
United States patent U.S. Pat. No. 5,113,032 cited above, or to supply the 
section of the oxypyrolysis zone where pyrolysis is effected with a major 
portion of said first stream and a major portion of said second stream is 
purged. 
(e) A major portion of the first effluent obtained in step (d), to which a 
major portion of the third fraction described in step (c) has preferably 
been added, is dimerised in a dimerisation zone to produce an effluent 
containing mainly n-butene. 
(f) A major portion of the effluent obtained in step (e) is isomerised in 
an isomerisation zone to produce an effluent containing mainly isobutene. 
(g) Synthesis of at least one alcohol, preferably respectively methanol or 
ethanol, is carried out in an alcohol synthesis zone, from a portion, 
preferably a major portion, of the fourth fraction obtained from step (c) 
and a portion, preferably a major portion, of the first fraction obtained 
from step (c) and optionally a portion, preferably a major portion, of the 
second effluent obtained in step (d), said portions of the fractions or 
optionally the second effluent being compressed and preferably completely 
mixed together before introduction into the alcohol synthesis zone to form 
at least a portion of the synthesis gas required for alcohol manufacture 
in the alcohol synthesis zone. A further portion of the synthesis gas 
required for the alcohol manufacture may optionally be introduced to the 
entrance to the alcohol synthesis zone from a further onsite or even 
offsite unit. 
In the particular case of methanol synthesis, which may be an industrial 
synthesis, a feature of the process of the present invention is that all 
the streams supplying the alcohol synthesis zone are combined so that the 
gas supplying said zone has a H.sub.2 /(2CO+3CO.sub.2) ratio generally 
equal to about 1, i.e., generally between 0.8 and 1.2, preferably between 
0.9 and 1.1. 
In the particular case of ethanol synthesis, two steps can be used: 
methanol is synthesized in a first step then, in a second step, a major 
portion of the methanol synthesized in the first step is transformed into 
ethanol, for example by homologation. A further portion of the ethanol may 
also be synthesized by hydration of ethylene. 
The alcohol synthesis zone can also be an onsite zone which is partially 
used for step (g) of the process of the invention. 
Finally, an effluent is obtained from the alcohol synthesis zone which 
mainly contains at least one alcohol, preferably methanol or ethanol 
respectively. 
(h) A major portion of the effluent obtained from step (f) is combined with 
at least a part, preferably a major part, of the effluent obtained from 
step (g) which is optionally completed by importing a further effluent 
mainly containing at least one alcohol supplied from a further onsite or 
an offsite unit, to synthesize at least one alkyl tertiobutyl ether, 
preferably respectively MTBE or ETBE, in an alkyl tertiobutyl ether 
synthesis zone. 
Regarding the contact mass in the prereforming zone (step (a) in the 
process of the invention), any contact mass which is known for this 
application can be used, for example a contact mass based on nickel 
dispersed on an oxide and preferably with a large specific surface area 
such as an alumina or stabilised alumina. 
Regarding the contact mass used in the oxidative coupling of methane zone, 
i.e., selective oxidation of methane to higher hydrocarbons (step (d) of 
the process of the invention), it is preferable to use a contact mass 
which satisfies the three following conditions: 
1) it must be capable of operating under normal oxidative coupling 
conditions, for example at a temperature generally between 650.degree. C. 
and 1200.degree. C., preferably between 650.degree. C. and 950.degree. C., 
2) it must produce C.sub.2.sup.+ hydrocarbons with a selectivity of 
generally at least 65% for conversion of 15% methane, and 
3) it must maintain its activity and selectivity over many hours of 
operation. 
Contact masses which satisfy the above conditions and which are thus 
preferred for use in the process of the invention are generally those 
which contain oxides and/or carbonates of alkali metals (such as lithium, 
sodium, potassium, rubidium and caesium), alkaline-earth metals (such as 
berylium, magnesium, calcium, strontium and barium) and rare earth metals 
(such as yttrium, lanthanum, praseodymium, neodymium, samarium, 
gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, 
europium and lutecium), either alone (as in the case of rare earths and 
alkaline-earths) or as a mixture (as in the case of alkaline-earth metals 
doped with alkali metals and rare earth metals doped with alkali and/or 
alkaline-earth metals). Other contact masses which satisfy the above 
conditions are those which contain oxides and/or carbonates of titanium, 
zirconium, hafnium, manganese, cerium, tin, indium, germanium, lead, 
antimony, zinc and bismuth, preferably with one or more oxides and/or 
carbonates of alkali, alkaline-earth or rare earth metals and silica. The 
contact masses described above are effective on their own or doped with 
halides, phosphorous oxides or sulphur. The contact masses are not, 
however, limited to the formulae indicated above: more generally, any 
contact mass which is suitable for this application may be used. 
The gas fraction, which is preferably methane enriched and oxidised (step 
(d) of the process of the invention) can be used without a diluant or can 
be diluted with at least one inert gas such as nitrogen, carbon dioxide or 
steam. For safety reasons and to avoid too great an increase in 
temperature, which would adversely affect the selectivity of the 
operation, the oxygen content in the methane should not in general exceed 
40 mole %; it is thus generally between 0.1 and 40 mole %, preferably 
between 5 and 25 mole %. 
The temperature of the oxidative coupling reaction (OCM) (step (d) of the 
process of the invention) is generally between 650.degree. C. and 
1200.degree. C., preferably between 650.degree. C. and 950.degree. C. 
The total pressure in the OCM zone (step (d) of the process of the 
invention) is generally between 1 and 100 bar (1 bar=10.sup.5 Pa), 
preferably between 1 and 20 bar. The contact time (i.e., the time required 
to consume at least 80% of the molecular oxygen in the feedstock) is 
normally between 10.sup.-6 and 1 second, preferably between 10.sup.-6 and 
10.sup.-1 second. 
The OCM reaction may be carried out in any type of reactor, in particular 
in a fixed bed, mobile bed, circulating bed or fluidised bed reactor, 
preferably a fixed bed reactor. 
Regarding the contact mass used for the ethylene dimerisation reaction 
(step (e) in the process of the invention), any contact mass or any 
soluble catalyst which is known for this application may be used, such as 
a nickel salt deposited on an oxide support such as alumina or, for 
example, a mixture of soluble nickel-containing complexes and a Lewis acid 
such as a mixture of a nickel carboxylate and a trialkylaluminium. A 
mixture of an alkyl titanate and an alkyl hydrocarbyl aluminium may also 
be used to selectively dimerise ethylene to but-1-ene as indicated in 
United States patents U.S. Pat. Nos. 4,532,370, 4,615,998 and 4,721,762. 
Regarding the contact mass used for the isomerisation of n-butene to 
isobutene (step (f) of the process of the invention), any contact mass 
which is known for this application can be used, in particular any contact 
mass with a very low acidity such as aluminas, which may be doped, 
silica-aluminas, silica-zirconias, or zeolites. It is particularly 
possible and advantageous to use the contact masses described in French 
patent application FR-A-2.695.636. 
Synthesis of at least one alcohol (step (g) of the process of the 
invention) is carried out using any process which is known to the skilled 
person. Thus, methanol synthesis from synthesis gas is an industrial 
process which is generally carried out in the presence of copper based 
catalysts and has been described in, for example, United States patents 
U.S. Pat. Nos. 3,388,972 and 3,546,140 from CCI and U.S. Pat. No. 
3,923,694 from ICI. Any contact mass which is known for this application 
can be used, such as copper, aluminium and zinc based catalysts described 
in United States patent U.S. Pat. No. 5,112,591. Synthesis of mixtures of 
alcohols have been described by us, for example in United States patents 
U.S. Pat. Nos. 4,780,481 and 4,791,141. Any contact mass which is known 
for this application may be used in this process. Catalysts based on group 
VIII metals such as nickel, iron and molybdenum have been described in the 
literature, and copper and cobalt based catalysts as described in United 
States patent U.S. Pat. No. 4,791,141 may be used. 
Regarding the contact mass used for the reaction synthesizing at least one 
alkyl tertiobutyl ether (step (h) of the process of the invention), any 
contact mass which is known for this application may be used, for example 
acid resins such as sulphonic resins or any acidic oxide or zeolite with 
an adequate acidity. This synthesis is described, for example, in United 
States patents U.S. Pat. Nos. 4,847,431, 5,013,407 and 4,847,430. 
The process of the invention can also be used to simultaneously prepare at 
least one alkyl tertiobutyl ether as well as a liquid fuel (for example 
petrol and/or gas oil) by dimerisation and/or oligomerisation of a portion 
of the olefins produced during oxidative coupling of methane (step (d) of 
the process of the invention), as has been described, for example, in 
United States patents U.S. Pat. Nos. 5,025,108 and 5,113,032. In this 
case, prereforming step (a) of the process of the invention can be deleted 
and only synthesis gas from step (d) (i.e., the second effluent from the 
OCM zone) is then used for alcohol production. The alcohol(s) is (are) 
advantageously then transformed into alkyl tertiobutyl ether(s) by 
reaction, in step (h) of the process of the invention, with the isobutene 
produced by dimerisation and isomerisation (steps (e) and (f) of the 
process of the invention) of the remaining ethylene fraction which has not 
been transformed into liquid fuel. The process of the invention can thus 
be used to produce a high performance liquid fuel base such as a high 
octane petrol, by mixing the fuel produced with at least a portion of the 
alkyl tertiobutyl ether(s) simultaneously produced. 
Finally, the invention also concerns a process for the simultaneous 
production of at least one alkyl tertiobutyl ether and a liquid fuel from 
natural gas, said process comprising the following steps: 
(1) a major portion of the second effluent from the OCM zone described in 
step (2) is dried and compressed then sent to a separation zone to produce 
the following effluents: 
a first fraction containing mainly carbon dioxide, 
a second fraction containing mainly methane, 
a third fraction containing mainly ethylene, 
a fourth fraction containing mainly hydrogen and carbon monoxide, 
a fifth fraction containing mainly ethane and C.sub.3.sup.+ hydrocarbons, 
(2) oxidative coupling of methane is carried out in an OCM zone to produce 
a first effluent containing mainly ethylene and a second effluent 
containing mainly hydrogen, carbon monoxide and carbon dioxide, 
(3) a portion of the first effluent obtained from step (2) is dimerised in 
a dimerisation zone to produce an effluent containing mainly n-butene, 
(4) a major portion of the effluent obtained from step (3) is isomerised in 
an isolmerisation zone to produce an effluent containing mainly isobutene, 
(5) synthesis of at least one alcohol is carried out in an alcohol 
synthesis zone, using a portion of the fourth fraction obtained from step 
(1) and a portion of the first fraction obtained from step (1), said 
portions being compressed before introduction into the alcohol synthesis 
zone to form an effluent containing mainly at least one alcohol, 
(6) a further portion of the first effluent obtained from step (2) 
undergoes at least one of the following reactions: dimerisation or 
oligomerisation in a reaction zone, to produce an effluent containing 
mainly liquid fuel, 
(7) a major portion of the effluent obtained from step (4) is combined with 
at least a portion of the effluent obtained from step (5) to synthesize at 
least one alkyl tertiobutyl ether in an alkyl tertiobutyl ether synthesis 
zone, 
(8) a major portion of the effluent obtained from step (6) is mixed with a 
portion of the alkyl tertiobutyl ether(s) to produce a liquid fuel base.

DETAILED DESCRIPTION OF THE DRAWING 
The following non limiting example represents an embodiment of the process 
of the invention which is illustrated in the Figure and can be used to 
carry out oxyetherification of natural gas in accordance with the 
invention. 
EXAMPLE (in accordance with the invention) 
In order to produce 100 000 T/year of MTBE, 860 kmoles/h of natural gas 
with the following composition (in mole %) were used: 
______________________________________ 
CH.sub.4 
88.7 
C.sub.2 
5.6 
C.sub.3.sup.+ 
2.6 
CO.sub.2 
2.3 
H.sub.2 S 
0.8 
100.00 
______________________________________ 
After purification and recovery of the LPG, 810 kmoles/h of a gas 
containing almost exclusively hydrocarbons containing one or two carbon 
atoms per molecule was sent via conduit (1) and decarbonated in unit (10). 
The product arriving via conduit (12) from unit (10) was separated into 
three fractions in unit (13), the "cold box". 
The first fraction, evacuated via conduit (14) then conduit (15) following 
compression, was constituted by hydrogen, carbon monoxide and any 
incondensables present. This fraction served as the supply gas for 
methanol synthesis unit (34). The composition of this fraction did not 
correspond to the methanol synthesis stoichiometry. Thus enough CO.sub.2 
was drawn from the purge from decarbonation unit (10) and sent via conduit 
(11) and conduit (17) so that adding the gas in conduits (15) and (17) 
produced a gas supply via conduit (18) for unit (34) which had a H.sub.2 
/(2CO+3CO.sub.2) ratio of approximately 1. The remaining purged gas from 
unit (10) which was not taken by conduit (17) was evacuated via conduit 
(16). Thus, of the 187.5 kmoles/h of purge from unit (10) via conduit 
(11), 107.5 went via conduit (17) and 80 kmoles/h was purged to the 
atmosphere via conduit (16). 
The second fraction containing mainly methane left unit (13) via conduit 
(19) and a portion (5254 kmoles/h) was sent via conduit (24) to oxidative 
coupling unit (6) for oxypyrolysis. The other portion of said second 
fraction (764.5 kmoles/h) was sent via conduit (25) to prereforming unit 
(26) to produce the hydrogen required for methanol synthesis 
stoichiometry. 
The third fraction, corresponding to 484.5 kmoles/h of C.sub.2.sup.+, was 
sent to fractionation unit (21) via conduit (20). 
In unit (26), 764.5 kmoles/h of gas were transformed into 1038.5 kmoles/h 
of gas (dry state) by virtue of an injection of steam via conduit (27) to 
form a gas with the following composition: 
______________________________________ 
CO 0.51 
CO.sub.2 
6.12 
H.sub.2 
25.86 
C.sub.1 
67.51 
100.00 
______________________________________ 
Gas arriving from conduit (28) was mixed with the effluent gas from unit 
(6) in conduit (7) then conduit (8) following compression, and the mixture 
in conduit (9) was decarbonated in unit (10). 
Decarbonation was necessary to prevent formation of compressed solid 
CO.sub.2 in cold box (13). 
Unit (21) firstly separated out C.sub.3.sup.+ compounds which were returned 
via conduit (22) to the pyrolysis section of unit (6) installed downstream 
of the OCM reactor. The remaining fraction from conduit (20) was then 
distilled to give 296.5 kmoles/h of ethylene and 172 kmoles/h of ethane. 
The ethane was also sent to unit (6) via conduit (22) for pyrolysis to 
ethylene as described in United States patent U.S. Pat. No. 5,025,108. 
Unit (6) was supplied via conduit (5) with oxygen from air distillation 
unit (3) which was supplied with air via conduit (2) and producing 
nitrogen via conduit (4). Finally, the 296.5 kmoles/h of ethylene from 
unit (21) was sent to dimerisation unit (29) via conduit (23). 142 
kmoles/h of n-butene was thus produced at the same time as 620 kmoles/h of 
a heavy fraction C.sub.4.sup.+ which was purged via conduit (30). 
The linear butene was sent via conduit (31) to unit (32) which was an 
isomerisation unit for transforming n-butene into isobutene. The isobutene 
was sent via conduit (33) to methyl tertiobutyl ether (MTBE) synthesis 
unit (37) which also received methanol from methanol synthesis unit (34) 
via conduit (36). 
Finally, 12487 kmoles/h of ether, i.e., 100 000 T/year, was obtained from 
conduit (38). The purge was evacuated via conduit (35). In the present 
case, all the methanol required for the manufacture of the MTBE, namely 
4650 kg/year, had been manufactured from the effluent gas from 
oxypyrolysis unit (6). 
The entire disclosures of all applications, patents, and publications cited 
above and below and of corresponding French Application 93/12,404, filed 
Oct. 15, 1993, are hereby incorporated by reference. 
The preceding examples can be repeated with similar success by substituting 
the generically or specifically described reactants and/or operating 
conditions of this invention for those used in the preceding examples. 
From the foregoing description, one skilled in the art can easily ascertain 
the essential characteristics of this invention and, without departing 
from the spirit and scope thereof, can make various changes and 
modifications of the invention to adapt it to various usages and 
conditions.