Thermochemical process for producing methane and oxygen

The invention comprises a recirculatory process for producing methane and oxygen in which iodine and an oxide in a lower valency stage are reacted with methanol, dimethylether or a mixture thereof at an elevated temperature to form the corresponding oxide having a higher valency stage and methyl iodide, the methyl iodide is reacted with water to form hydrogen iodide and reform the methanol and/or dimethylether, the hydrogen iodide is reacted with carbon dioxide to form methane and water and the oxide in the higher valency state is decomposed to release oxygen and reform the oxide having a lower valency stage, the methane and released oxygen are removed and the remaining components are recirculated. PAC SPECIFICATION

This invention relates to a multi-stage thermochemical circulatory process 
for producing methane from carbon dioxide and water. 
It is known to decompose water into its components hydrogen and oxygen by 
using heat and chemical compounds carried in a circuit, the hydrogen 
obtained subsequently being catalytically reacted with carbon dioxide to 
form methanol (e.g., see U.S. Pat. No. 1,875,714) and then converting the 
methanol to methane (e.g., see U.S. Pat. No. 3,928,716. Methane is 
suitable for the production of energy, since there is no need to modify 
the distribution network nor the consumers' technical installations. Since 
carbon dioxide can be extracted from the flue gas of coal-fired power 
plants and power plants which use natural gas and can be reduced with 
hydrogen to form methane, a process which utilizes the hydrogen obtained 
from water undoubtedly constitutes a simple form of recycling carbon. 
The manufacturing process used for this purpose should have as its object, 
inter alia, the following desirable characteristics: firstly, in order to 
facilitate passage, only liquid or gaseous components should be conveyed 
in the circuit; secondly, highly corrosive components should only be used 
at temperatures below approximately 500.degree. C; thirdly, the 
endothermic reaction, which must take place at very high temperatures in 
order to obtain high efficiency, should serve to separate oxygen. This 
renders it possible to form a corrosion-resistant oxide coating in the 
parts of the plant which transfer heat. Furthermore, a further object 
should be to minimize the total number of processing steps and to utilize, 
for the process, the exothermic heat of the reaction of carbon dioxide 
with hydrogen so that the reduction of carbon dioxide to form methane 
should not be effected as an additional or subsequent processing step of a 
water decomposing plant, but should be incorporated as an integral part of 
a recirculatory process. It is a further object of the invention that pure 
hydrogen not be produced and reacted separately and subsequently, but that 
a hydrogen carrier component of the circuit which derives its hydrogen 
from reaction of a precursor with water be used to react with carbon 
dioxide to form methane. 
These and other objects are achieved according to the present invention by 
providing a recirculatory process for producing methane and oxygen which 
comprises: 
A. Reacting iodine and an oxide in a lower valency stage with a reactant 
selected from the group consisting of methanol, dimethylether and a 
mixture of methanol and dimethylether at an elevated temperature to form 
the corresponding oxide having a higher valency stage and methyl iodide; 
b. Hydrolysing the so formed methyl iodide to form hydrogen iodide and 
re-form the dimethylether and/or methanol; 
c. Reacting the so formed hydrogen iodide with carbon dioxide to form 
methane and re-form iodine and water; 
d. Decomposing the oxide in a higher valence stage into the corresponding 
oxide in a lower valence stage and releasing oxygen, 
and in which the oxygen released in (d) and the methane formed in (c) are 
removed from the system whilst the remaining components are re-utilised in 
reactions (a) to (d). 
In the method of the invention reaction in (a) produces a precursor in 
methyl iodide in which on reaction with water (b) forms hydrogen iodide 
which is the hydrogen carrier component which reacts with carbon dioxide 
to form methane (c). 
The oxides having a low valence, i.e. having a low oxygen content, thereby 
act as oxygen acceptors. Such oxides may be an oxide of sulphur, antimony, 
vanadium, arsenic, uranium, tellurium, bismuth or selenium. The oxides of 
vanadium, antimony and arsenic can be used as acceptors in the presence of 
water. Owing to their higher solubility, the use of oxides of vanadium, 
antimony, arsenic, uranium, tellurium, bismuth and selenium in the form of 
alkali salts thereof is preferable to the use of pure or aqueous oxides. 
Reaction (a) is preferably conducted at 140.degree. to 240.degree. C, 
reaction (b) at from 120.degree. to 240.degree. C, reaction (c) at from 
25.degree. to 400.degree. C and decomposition (d) at from 850.degree. to 
950.degree. C. All the reactions (a) to (d) are preferably conducted at 
super-atmospheric pressure. Thus reaction (a) is preferably conducted at 
from 40 to 100 absolute atmospheres reaction (b) at from 20 to 80 absolute 
atmospheres reaction (c) at from 40 to 80 absolute atmospheres and 
decomposition (d) at from 20 to 30 absolute atmospheres. 
The following processes with aqueous oxides are given by way of example. 
##EQU1## 
Dimethylether can be replaced by methanol or a mixture of the two 
compounds. The antimony oxide can be replaced by, for example, 
vanadium-IV-oxide or arsenic-III-oxide. The oxides V.sub.6 O.sub.13 and 
As.sub.2 O.sub.5 formed therefrom can be thermally reconverted to 
thestarting oxides with the giving-off of oxygen. 
The following generally recirculatory process can be realised with the 
corresponding alkali compounds, for example with that of vanadium; 
##EQU2## 
Owing to the relatively simple transportability, it is advantageous to use 
sulphur dioxide as the oxygen acceptor. By way of example, the 
recirculatory system can then be as follows: 
##EQU3## 
The reintroduction of the sulphate-pyrosulphate melt necessary in this 
instance can be dispensed with if sulphur dioxide is used in a pure or 
aqueous form, thus resulting in a particularly advantageous process: 
##EQU4## 
By way of example, an approximately 70 to 75% sulphuric acid concentration 
can be obtained when the total solution is heated very rapidly, for 
example from 25.degree. to 240.degree. C, by, for example, dropping it 
onto a hot shaped substrate made from glass. The rate of vaporization of 
the methyl iodide is then rendered more rapid than that of the back 
reaction. Thus, for example, one can work with molar ratios of CH.sub.3 
OH/I.sub.2 = 2/1 to 3/1, initial concentrations in H.sub.2 SO.sub.4 of, 
for example, 60% to 65%, final concentrations of, for example 70% to 75%, 
a pressure of 1 to 75 absolute atmospheres, starting solutions being 
chosen which have been saturated with sulphur dioxide at, for example, 
20.degree. to 30.degree. C after introducing CH.sub.3 OH and iodine. 
The solution can also be stripped with sulphur dioxide at, for example 
50.degree. C and additionally acidified with hydrochloric acid. The 
equilibrium is thereby displaced to the right: 
EQU CH.sub.3 OH + H.sup.+ + I.sup.- .fwdarw. CH.sub.3 I + H.sub.2 O 
it is possible that sulphur might be formed as a product of an undesirable 
reversible reaction of sulphur dioxide and water. However, this is 
prevented with increasing temperature and sulphuric acid concentration 
with increased total pressures. Temperature, partial pressure of iodine 
and the dwell time of methanol and/or dimethyl either have to be 
correspondingly adjusted. In general, one works at approximately 40 to 100 
absolute atmospheres, 140.degree. to 240.degree. C and with 65 to 80% 
sulphuric acid. The dwell time in, for example, a column type reactor 
should be less than 1 minute with regard to methanol and/or dimethyl 
ether. 
If all the components are considered to be ideal gases, and if it is 
assumed that all the other components are substantially less soluble in 
sulphuric acid than in water at the specified pressures and temperatures, 
so that the activity of aqueous sulphuric acid is not affected, the 
following gas composition above sulphuric acid of 80% concentration 
obtains at, for example 500.degree. K, expressed in absolute atmospheres. 
I.sub.2 = 1.5; SO.sub.2 = 10; CH.sub.3 OH = 1; C.sub.2 H.sub.6 O = 14.54; 
CH.sub.3 I = 26.26; H.sub.2 O = 2.15; H.sub.2 SO.sub.4 = 5.66 .multidot. 
10.sup.-4 ; HI = 1.39 .multidot. 10.sup.-4. 
Reaction 1 constitutes as essential step in accordance with the invention 
since, in this instance, with the use of dimethyl ether/methanol, the 
decomposition products of water can be separated by means of 
gaseous/liquid components in accordance with 
EQU H.sub.2 O + I.sub.2 .fwdarw. 2 H.sup.+ + 10.sup.- + I 
with the secondary reaction 
EQU HSO.sub.'.sup.- = IO .fwdarw. HSO.sub.4.sup.- + I.sup.- 
the separation of sulphuric acid or sulphur trioxide is known as the 
reversal of the contact process with, for example, V.sub.2 O.sub.5 acting 
as a catalyst. Pressures of 20 to 30 absolute atmospheres and separating 
temperatures of 850 to 950.degree. C appear to be particularly suitable. 
The hydrolysis of methyl iodide with small concentrations and at 
temperatures of approximately 100.degree. C is known per se. Dimethyl 
ether, dissolved hydrogen iodide and methanol are formed in the presence 
of methanol in increased concentrations. In order to obtain as highly a 
concentrated hydrogen iodide solution as possible, it is advantageous to 
work at temperatures of 160.degree. to 240.degree. C and pressures of 20 
to 80 absolute atmospheres. However, it is also particularly advantageous 
to use dissolved compounds such as CdI.sub.2, ZnI.sub.2, HgI.sub.2 and 
CuI.sub.2 which shift the equilibrium of hydrolysis to the right by 
sequestering with hydrogen iodide, so that temperatures of 120.degree. to 
180.degree. C and pressures of from 10 to 50 absolute atmospheres are 
adequate. In order to carry out the fourth reaction, one can, for example 
vaporize a solution of hydrogen iodide and thus obtain hydrogen and iodine 
in the gas phase, in addition to water vapour and nonreacted hydrogen 
iodide. It may also be advantageous to heat the hydrogen iodide solution 
to approximately 250.degree. to 400.degree. C and to subject it to the 
direct action of carbon dioxide without vaporizing any substantial 
proportion of the water. The thermal consumption of this reaction can be 
largely met from the exothermic, superimposed reaction of hydrogen with 
carbon dioxide. This total reaction constitutes a further essential step 
in accordance with the invention. Ranges of from 40 to 80 absolute 
atmospheres and from 25.degree. to 400.degree. C are suitable for carrying 
out the reaction. It may be advantageous to use conventional hydrogenation 
catalysts such as platinum, nickel-and copper compounds such as CuI or, 
alternatively, compounds of the elements of the eighth group. Molecular 
sieves may also be used.

Referring to FIG. 1, the mass balance is given hereinafter in molar numbers 
per 0.5 mol of O.sub.2. 
1.0 of SO.sub.2, 2.36 of H.sub.2 O are fed to the reactor I by way of the 
pipe 18, and 2.1074 of C.sub.2 H.sub.6 O, 0.7616 of SO.sub.2, 0.1637 of 
H.sub.2 O and 1.1142 of I.sub.2 are fed to the reactor I by way of the 
pipe 24. At a temperature of approximately 227.degree. C and a pressure of 
approximately 56 absolute atmospheres, 1.0 of SO.sub.2 is reacted in the 
reactor I with 1.0 of C.sub.2 H.sub.6 O and 2.36 of H.sub.2 O and 1.0 of 
I.sub.2 in accordance with equation 1 to form 1.0 of SO.sub.3, 2.36 of 
H.sub.2 O and 2.0 of CH.sub.3 I. 1.0 of SO.sub.3 and 2.36 of H.sub.2 O are 
fed to a reactor II by way of a pipe 15. The SO.sub.3 is catalytically 
decomposed in the reactor II at a temperature of approximately 950.degree. 
C and a pressure of 30 absolute atmospheres, so that 1.0 of SO.sub.2, 2.36 
of H.sub.2 O and 0.05 of O.sub.2 leave the reactor II and are fed to an 
apparatus III by way of a pipe 16. The oxygen (0.5 of O.sub.2) is 
separated out in the apparatus III and is removed from the process by way 
of a pipe 17. The remaining 1.0 of SO.sub.2 and 2.36 of H.sub.2 O are 
returned to the reactor I by way of a pipe 18. 2.0 of CH.sub.3 I 1.1074 of 
C.sub.2 H.sub.6 O, 0.7616 of SO.sub.2, 0.1637 of H.sub.2 O and 0.1142 of 
I.sub.2 are removed from the reactor I by way of the second outlet pipe 19 
and are introduced into a separating apparatus V by way of a heat 
exchanger IV. The water is condensed in the separating apparatus V and, 
together with the iodine, is fed as the first liquid phase to a pipe 21 by 
way of a pipe 20 (0.1637 of H.sub.2 O, 0.1142 of I.sub.2). The only 
slightly soluble methyl iodide (2.0 of CH.sub.3 I) is obtained as a 
second, heavier liquid phase and is conducted into the reactor VI by way 
of a pipe 22. The other products, namely 0.7616 of SO.sub.2 and 1.1074 of 
C.sub.2 H.sub.6 O are also fed to the pipe 21 by way of a pipe 23. 
The methyl iodide is hydrolyzed in the reactor VI at approximately 
200.degree. C and approximately 60 absolute atmospheres. For this purpose, 
a total of 31.0 of H.sub.2 O are introduced into the reactor VI in 
addition to 2.0 of CH.sub.3 I, 0.5 of the H.sub.2 O being fed from the 
outside by way of a pipe 25, and 30.5 of the H.sub.2 O being fed from a 
reactor VII by way of a pipe 25. Dimethyl ether (1.0 of C.sub.2 H.sub.6 O) 
is predominantly produced in the reactor VI in addition to a small 
quantity of methanol. The dimethyl ether flows through a pipe 27 into a 
pipe 28 which is connected to the collecting pipe 21 leading to the 
reactor I. The hydrogen iodide solution (2.0 of HI and 30.0 of H.sub.2 O) 
further produced in the reactor VI is catalytically reacted with 0.25 of 
CO.sub.2 in the reactor VII at approximately 350.degree. C and 
approximately 60 absolute atmospheres. 0.25 of CH.sub.4 and 1.0 of I.sub.2 
are thereby produced and are fed by way of a pipe 29 to a separating 
apparatus VIII in which the methane is separated from the iodine and from 
the mixture of CO.sub.2 and HI which also exists. The separated methane is 
removed from the process by way of a pipe 30. The iodine (1.0 of I.sub.2) 
enters the pipe 28 and then, by way of the pipes 21 and 24, together with 
the other products flowing in these pipes, flows into the reactor I by way 
of the heat exchanger VI in which the mixture is preheated. 
A variant of the process in accordance with the invention is shown 
diagrammatically in FIG. 2. The reactors and apparatus I to VI are 
identical to those of the embodiment of FIG. 1 with respect to their 
correlation by way of the pipes 15 to 24 and with respect to their 
functions. 
The essential difference resides in the fact that the hydrolysis of the 
methyl iodide and the catalytic reaction of hydrogen iodide with carbon 
dioxide are carried out in a single column XII. The hydrolysis is effected 
in the region A at approximately 200.degree. C. 2.0 of C.sub.3 I and 0.5 
of H.sub.2 O are introduced into this region by way of pipes 31 and 36 
respectively. The two pipes 31 and 36 pass through respective heat 
exchangers 31a and 31b arranged within the column XII. The methyl iodide 
and the water are preheated in the heat exchangers to a temperature of 
approximately 60.degree. C. The heat required for this purpose is derived 
from the partially condensing products rising within the column XII. 
The aqueous solution of hydrogen iodide produced in region A flows through 
pipes 32 into region B in which it is vaporized at a temperature of 
approximately 280.degree. C. The heat required for this purpose is fed by 
way of a pipe 33 which is connected to an apparatus 34 by means of which 
the heat, produced in the region C located therebelow, is carried off. It 
will be seen from the drawing that a circuit between the regions B and C 
is closed by means of a pipe 35. The aqueous hydrogen iodide solution 
vaporized in region B is introduced, under preheating, into the region C 
by way of gravity pipes 37 and is reacted with carbon dioxide in catalyst 
beds 38 in the region C. This reaction is effected at a temperature of 
approximately 350.degree. C. The products produced in region C enter the 
upper portion of the column XII by way of uptake pipes 39. The upper 
components are condensed or dissolved by utilizing their heat. The gases 
dimethyl ether, methane and, if required carbon dioxide as well as 
non-condensed methyl iodide are separated from one another in the 
apparatus IX and X. The non-reacted methyl iodide is separated out in IX 
and is fed into the pipe 31 by way of the pipe 40. The methane is 
separated out in X and is conducted out of the process by way of a pipe 
41.