Process for continuous preparation of diester of oxalic acid

There is provided a process for the continuous preparation of a diester of oxalic acid, which comprises a first step of reacting carbon monoxide with an ester of nitrous acid in the gaseous phase; a second step of condensing the gaseous reaction mixture to separate a non-condensed gas from a condensed liquid containing the diester of oxalic acid; a third step of introducing the non-condensed gas of the second step to a regeneration column, thereby contacting it with a gas containing molecular oxygen and an alcohol, and recycling the resulting gas containing an ester of nitrous acid to the reactor of the first step; a fourth step of distilling out an alcohol containing a diester of carbonic acid formed as a by-product of the first step and obtaining a liquid diester of oxalic acid; and a fifth step of introducing the distillate of the fourth step to a hydrolysis column thereby hydrolyzing the diester of carbonic acid in the gas and recycling the alcohol as an alcohol source for the third step.

The present invention relates to a novel process for preparing a diester of 
oxalic acid, and particularly to a novel process whereby the production of 
a diester of oxalic acid is industrially advantageously carried out by a 
gaseous phase reaction with use of carbon monoxide and an ester of nitrous 
acid as the starting materials in the presence of a solid catalyst of 
platinum group metal series. 
Diesters of oxalic acid have been used as important starting materials for 
the syntheses of oxalic acid, oxamide, glycols, intermediates for dyes and 
pharmaceuticals. 
There has hitherto been known a process for preparing a diester of oxalic 
acid by contacting carbon monoxide and an ester of nitrous acid with a 
solid catalyst of platinum group metal series in the gaseous phase. This 
reaction itself is an extremely good reaction for the preparation of 
diesters of oxalic acid. However, in order to employ this reaction 
industrially, it is necessary to have a process whereby the reaction can 
be conducted continuously as is the case for other chemical reactions. 
The present inventors have conducted extensive researches with an aim to 
establish an industrially advantageous continuous process for the 
production of diesters of oxalic acid by contacting carbon monoxide and an 
ester of nitrous acid with a solid catalyst of platinum group metal series 
in the gaseous phase. As a result, it has been found that it is possible 
to obtain diesters of oxalic acid industrially extremely advantageously by 
employing a process which comprises; 
(1) a first step of introducing a gas containing carbon monoxide and an 
ester of nitrous acid into a reactor packed with a solid catalyst carrying 
a platinum group metal or its salt, and thereby conducting a catalytic 
reaction in the gaseous phase to obtain a product containing a diester of 
oxalic acid; 
(2) a second step of introducing the product of the first step to a 
condenser thereby to separate a non-condensed gas containing nitrogen 
monoxide formed by the catalytic reaction of the first step from a 
condensed liquid containing the diester of oxalic acid; 
(3) a third step of introducing the non-condensed gas of the second step to 
a regeneration column, thereby contacting it with a gas containing 
molecular oxygen and an alcohol, and recycling the resulting gas 
containing an ester of nitrous acid to the reactor of the first step; 
(4) a fourth step of introducing the condensed liquid of the second step to 
a distillation column and thereby distilling out an alcohol containing a 
diester of carbonic acid formed as a by-product by the catalytic reaction 
of the first step, and obtaining a liquid diester of oxalic acid; and 
(5) a fifth step of introducing the distillate of the fourth step to a 
hydrolysis column thereby hydrolyzing the diester of carbonic acid in the 
gas and recycling the alcohol thereby obtained, as an alcohol source for 
the third step. 
Now, each step of the present invention will be described. 
FIRST STEP 
A gaseous starting material containing carbon monoxide and an ester of 
nitrous acid, is introduced into a reactor packed with a solid catalyst of 
platinum group metal series, and thereby a catalytic reaction is carried 
out in the gaseous phase. 
As the reactor, a single tubular or multi-tubular column packed with a 
catalyst is useful. The contact time of the gaseous starting material with 
the solid catalyst of platinum metal series is set to be at most 10 
seconds, preferably from 0.2 to 5 seconds. 
As the solid catalyst of platinum group metal series, palladium is most 
useful, but platinum, rhodium, ruthenium, and iridium are also useful. 
Further, salts of these metals such as nitrates, sulfates, phosphates, 
halides, acetates, oxalates or benzoates, may be used. These materials are 
used as carried by an inert carrier such as active carbon, alumina, 
silica, diatom earth, pumice, zeolite, or Molecular Sieve. The amount to 
be used, in terms of the platinum group metal, is within a range of from 
0.01 to 10% by weight, usually from 0.2 to 2% by weight, relative to the 
carrier. 
The gaseous starting material, i.e., a gas containing carbon monoxide and 
an ester of nitrous acid may usually be used in a form diluted with an 
inert gas such as nitrogen or carbon dioxide. 
The ester of nitrous acid may preferably be an ester of a saturated 
monohydric aliphatic or alicyclic alcohol having from 1 to 8 carbon atoms 
with nitrous acid. As the alcohol component, there may be mentioned, for 
instance, an aliphatic alcohol such as methanol, ethanol, n- (and 
iso-)propanol, n- (iso-, sec- and tert-)butanol, n- (and iso-)amyl 
alcohol, hexanol, or octanol, and an alicyclic alcohol such as 
cyclohexanol, or methylcyclohexanol. These alcohols may contain a 
substituent, such as an alkoxy group, which does not hinder the reaction. 
Among these, methyl nitrite is most preferably used. 
It is necessary to carry out this reaction under such conditions that there 
is no formation of a liquid phase in the reaction zone. The conditions for 
no formation of a liquid phase in the reaction zone vary depending upon 
the reaction temperature, the reaction pressure and the kind and 
concentration of the ester of nitrous acid used, and therefore can not 
simply be determined. 
However, with respect to the reaction temperature, the reaction proceeds in 
a sufficiently high speed even at a low temperature, and the lower the 
reaction temperature is, the less side reactions occur. Accordingly, so 
long as the desired time yield can be maintained, the reaction is carried 
out at a relatively low temperature, i.e. usually from 50.degree. to 
200.degree. C., preferably from 80.degree. to 150.degree. C. Further, with 
respect to the reaction pressure, the reaction is carried out usually 
under a pressure from ambient pressure to 10 kg/cm.sup.2 (gauge pressure), 
preferably from ambient pressure to 5 kg/cm.sup.2 (gauge pressure). 
However, in some cases, the reaction pressure may be slightly lower than 
ambient pressure. 
The concentration of the ester of nitrous acid in the gaseous starting 
material may be varied over a wide range. However, in order to attain a 
satisfactory reaction rate, it is necessary to adjust the concentration to 
be at least 1% by volume, usually from 5 to 30% by volume. 
The concentration of carbon monoxide in the gaseous starting material may 
be varied over a wide range, and is usually selected within a range of 
from 10 to 90% by volume. 
SECOND STEP 
The product of the first step is led to a condenser, cooled to a 
temperature at which the diester of oxalic acid in the product is 
condensed, and separated into a condensed liquid and a non-condensed gas. 
The condensed liquid thus separated, contains small amounts of by-products 
such as a diester of carbonic acid, and an ester of formic acid, in 
addition to the intended diester of oxalic acid. On the other hand, the 
non-condensed gas contains non-reacted carbon monoxide, an ester of 
nitrous acid and the like, in addition to the nitrogen monoxide formed by 
the catalytic reaction of the first step. 
Further, during this step, a part of the intended diester of oxalic acid is 
carried by the non-condensed gas, and then hydrolized by water formed 
during the regeneration of nitrogen monoxide in the subsequent third step, 
and it is possible that the resulting oxalic acid accumulates within the 
gas recycling system. Furthermore, when the intended product is the one 
having a relatively high melting point, such as dimethyl oxalate, it is 
possible that the intended product solidifies and deposits on the wall of 
the condenser and finally plugs off the condenser. 
In order to solve these problems, it is possible to employ a method wherein 
the product of the first step is cooled for condensation at a temperature 
of at most the boiling point of an alcohol while contacting it with an 
alcohol, preferably an alcohol having 1 to 4 carbon atoms. For instance, 
when the intended product is dimethyl oxalate, it is preferred that the 
cooling and condensation are carried out at a temperature of from 
30.degree. to 60.degree. C. while supplying from 0.01 to 0.1 part by 
volume of methanol, relative to 100 parts by volume of the product to be 
treated. 
THIRD STEP 
The non-condensed gas separated in the second step is led to a regeneration 
column and contacted with a gas containing molecular oxygen and an alcohol 
thereby to regenerate nitrogen monoxide in the gas into an ester of 
nitrous acid. 
As the regeneration column for this step, a usual gas-liquid contact 
apparatus such as packed column, a bubble column, a spray column, or a 
multi-staged column, may be employed. The alcohol to be used, is selected 
from alcohol components which may constitute said ester of nitrous acid. 
The non-condensed gas to be contacted with the alcohol and the gas 
containing molecular oxygen, may be introduced into the regeneration 
column individually or in a mixed state. 
In the regeneration column, a part of nitrogen monoxide is oxidized to 
nitrogen dioxide and at the same time, these substances are allowed to be 
absorbed and react with an alcohol and thereby to be regenerated as an 
ester of nitrous acid. 
In this step, it is preferred to control the concentration of nitrogen 
monoxide in the gas withdrawn from the regeneration column to be within a 
range of from 2 to 7% by volume, and to maintain the gas to contain as 
little nitrogen as possible, most preferably with substantially no 
nitrogen dioxide and oxygen. Namely, if the concentration of nitrogen 
monoxide in the regenerated gas is greater than the above mentioned upper 
limit, the reaction rate for the formation of the diester of oxalic acid 
is decreased and the yield is lowered, when said gas is recycled for use 
in the reactor of the first step. On the other hand, if said concentration 
is lower than the above-mentioned lower limit, the amounts of nitrogen 
dioxide and oxygen in the regenerated gas will be increased, and they will 
be a factor for substantial degradation of the activity of the platinum 
group metal catalyst of the first step. 
Accordingly, it is preferred that from 0.08 to 0.2 mole, in terms of 
oxygen, of the gas containing molecular oxygen, relative to one mole of 
nitrogen monoxide in the gas introduced to the regeneration column, is 
supplied and these gases are contacted with the alcohol at a temperature 
of at most the boiling point of the alcohol thus used. The contact time is 
preferably from 0.5 to 20 seconds. Further, the alcohol is used in such an 
amount as to be sufficient for completely absorbing and reacting the 
resulting nitrogen dioxide and an almost equimolar amount of nitrogen 
monoxide, and usually, from 2 to 5 parts by volume of the alcohol is 
preferably used relative to one part by volume of nitrogen monoxide in the 
gas introduced into the regeneration column. 
Further, since this invention is a continuous process, a loss of a nitrogen 
component is unavoidable, and its supplementation may be made by supplying 
the ester of nitrous acid to the reactor of the first step, or by 
introducing a nitrogen oxide such as nitrogen monoxide, nitrogen dioxide, 
dinitrogen trioxide or dinitrogen tetroxide, or nitric acid into the 
regeneration column of the third step. 
Further, in case the content of nitrogen monoxide in the non-condensed gas 
in the second step is great, and if the ester of nitrous acid is 
obtainable in an excess amount during the regeneration of the nitrogen 
monoxide into the ester of nitrous acid in the third step, the entire 
amount of the non-condensed gas needs not be led to the regeneration 
column and a part thereof may directly be recycled to the reactor of the 
first step. 
The gas containing the ester of nitrous acid and withdrawn from the 
regeneration column, is recycled to the reactor of the first step. 
Further, this regenerated gas may be mixed with another starting material 
i.e. carbon monoxide, and then the mixture may be supplied to the reactor. 
When the regenerated ester of nitrous acid is an ester of an alcohol having 
at least 4 carbon atoms, such as n-butyl nitrite, or n-amyl nitrite, it 
forms an azeotropic mixture with water formed as a by-product by the 
regeneration reaction and consequently, water is contained in the 
regenerated gas. Accordingly, if this gas is supplied in that state to the 
reactor of the first step, the water hinders the reaction for the 
formation of the diester of oxalic acid. Therefore, it is desirable that 
water in the gas is removed by an operation such as distillation before 
the gas is recycled to the reactor. On the other hand, when the 
regenerated ester of nitrous acid is methyl nitrite, ethyl nitrite, 
n-propyl nitrite, or i-propyl nitrite, it does not form an azeotropic 
mixture with water formed as a by-product by the regeneration reaction, 
and accordingly, the regenerated gas contains no water and may therefore 
be recycled to the reactor as it is. 
The liquid withdrawn from the regeneration tower is an alcohol solution 
containing water formed as by-product by the regeneration reaction. This 
may be refined by an operation such as distillation to such an extent that 
the water content in the alcohol becomes to be at most 5% by volume, 
preferably at most 2% by volume, and may then be reused as an alcohol 
source for the third step, and in a proper case, as an alcohol source for 
the second step. 
FOURTH STEP 
The condensed liquid separated in the second step is led to a distillation 
column and distilled by a usual operation, whereby the intended product of 
the diester of oxalic acid is obtained as the distillation residue. 
In the distillate, there are contained, in addition to the alcohol, a 
diester of carbonic acid formed as a by-product by the catalytic reaction 
in the first step, and a small amount of an ester of formic acid. 
FIFTH STEP 
The distillate of the fourth step is led to a hydrolysis column and 
contacted with steam, whereby the diester is carbonic acid in the 
distillate is hydrolized to the alcohol and carbon dioxide. 
This hydrolysis can readily be carried out by a gas phase reaction in the 
presence of an alumina catalyst such as, e.g., Neobead P (trade name) made 
by Mizusawa Kagaku Co., at a temperature of from 150.degree. to 
250.degree. C. Further, in this step, the ester of formic acid present, in 
a small amount, in the distillate, will likewise be hydrolized and 
converted to an alcohol. 
The gaseous alcohol withdrawn from the hydrolysis column is condensed and 
then recycled as a part of the alcohol source for the regeneration column 
of the third step. Further, in the case where in the second step, the 
condensation is carried out while contacting the non-condensed gas with an 
alcohol, a part of the alcohol obtained in the fifth step may be supplied 
as the alcohol source. 
Further, the distillation column and the hydrolysis column used in the 
fourth and fifth steps, may be usual apparatus such as a packed column, a 
multi-stage column, a forced agitation type thin film column.

Now, the process of the present invention will be described in detail in 
accordance with the flow sheet diagram (FIG. 1) illustrating an embodiment 
of the invention. In the drawing, 1 designates a reactor, 2 designates a 
condenser, 3 designates a regeneration column 4 designates a distillation 
column, 5 designates a hydrolysis column, 6 designates a heat exchanger, 
and 11 to 29 represent conduits (pipe lines). 
A gas containing carbon monoxide, an ester of nitrous acid, nitrogen 
monoxide and so on is compressed by a gas-recycling device (not shown) and 
introduced into the top of a multitubular reactor 1 having reaction tubes 
packed with a platinum group metal catalyst, via a conduit 21. A catalytic 
reaction is carried out in the gaseous phase in the reactor 1. The gas 
formed by the reaction upon passing through the catalyst layer, is 
withdrawn from the bottom and introduced to a condenser 2 via a conduit 
11. 
In the condenser 2, while being contacted with an alcohol supplied from a 
conduit 13, the reaction-formed gas is condensed, and the condensed liquid 
containing mainly the diester of oxalic acid is led from the bottom via a 
conduit 14 to a distillation column 4. On the other hand, a non-condensed 
gas containing non-reacted carbon monoxide and the ester of nitrous acid, 
nitrogen monoxide formed as a by-product and so on, is introduced from the 
top of the bottom of the regneration column 3 via a conduit 12. 
In the regeneration column 3, the non-condensed gas is countercurrently 
contacted and reacted with a gas containing molecular oxygen and supplied 
to the bottom via a conduit 16 and an alcohol supplied to the top via a 
conduit 18, whereupon an ester of nitrous acid is formed. 
In the regeneration column 3, the oxidation reaction of nitrogen monoxide 
to nitrogen dioxide is followed by the absorption reaction thereof to the 
alcohol. If the nitrogen source for the formation of the ester of nitrous 
acid is inadequate, a nitrogen oxide may be supplied via conduit 15. The 
gas containing the ester of nitrous acid formed in the regeneration column 
3 is recycled to the reactor 1 via conduits 19 and 21 together with carbon 
monoxide supplied anew from a conduit 20. On the other hand, the water 
formed as a by-product in the regeneration column 3 is withdrawn in a form 
of an aqueous alcohol solution from the bottom via a conduit 17. This 
aqueous alcohol solution is subjected to an operation such as distillation 
to remove the water in the liquid, and thereafter may be reused as an 
alcohol source to be supplied to the regeneration column 3 or the 
condenser 2 via said conduit 18 or 13. 
In the distillation column 4, the alcohol and diester of carbonic acid as a 
by-product are distilled and the intended product of the diester of oxalic 
acid in a form of a liquid is withdrawn via a conduit 22. 
The distillate is passed through a conduit 23, heated by a heat exchanger 
6, passed through a conduit 24, mixed with steam supplied from a conduit 
25 and led to a hydrolysis column 5. 
In the hydrolysis column 5, the diester of carbonic acid and the ester of 
formic acid in the gas, will be hydrolyzed in the gaseous phase into the 
alcohol and carbon dioxide by the action of an alumina catalyst. The 
formed gaseous alcohol is passed through a conduit 26, cooled by a heat 
exchanger 6, and then freed from the carbon dioxide in the gas and at the 
same time condensed, in a condenser (not shown). Then, the liquid alcohol 
is passed through conduits 27, 28, and recycled as an alcohol source to be 
supplied to the regeneration column 3 via a conduit 18. 
Further, a part of this alcohol may also be reused as an alcohol source to 
be supplied to the condenser 2 via conduits 29 and 13, as the case 
requires. 
Now, the invention will be described in detail with reference to the 
following Examples. 
EXAMPLE 1 
In the tubes of a stainless multi-tubular reactor, which has 6 tubes having 
an inside diameter of 36.7 mm and a height of 550 mm, there was packed 3 
kg (3 liters) of a .gamma.-alumina catalyst in a form of pellets having a 
diameter of 5 mm and a height of 3 mm and carrying 0.5% by weight of 
palladium. 
A gaseous mixture of carbon monoxide and the regenerated gas from the 
regeneration column mentioned below [pressure: 0.2 kg/cm.sup.2 (gauge 
pressure), composition: 22.0% by volume of carbon monoxide, 9.1% by volume 
of methyl nitrite, 3.1% by volume of nitrogen monoxide, 9.4% by volume of 
methanol, 8.5% by volume of carbon dioxide and 47.0% by volume of 
nitrogen] was preheated to about 90.degree. C. by a heat exchanger, and 
then introduced from the top of this catalyst layer at a rate of 12.0 
Nm.sup.3 /hr. by a diaphragm gas-recycling pump, and the temperature of 
the catalyst layer was maintained at 104.degree. to 117.degree. C. by 
circulating hot water to the shell side of the reactor. 
The gas passed through the catalyst layer was led to the bottom of a 
Rasching ring packed condenser of gas-liquid contact type having an inside 
diameter of 158 mm and a height of 1,400 mm, and from the top of the 
condenser, methanol was introduced at a rate of 5.6 liters/hr., whereby 
the countercurrent contact was carried out at a temperature of about 
35.degree. C. (i.e. 30.degree. C. at the top of the condenser and 
40.degree. C. at the bottom of the condenser). From the bottom of the 
condenser, there was obtained 2.8 kg/hr. of a condensed liquid 
(composition: 46.6% by weight of dimethyl oxalate, 4.9% by weight of 
dimethyl carbonate, 0.03% by weight of methyl formate and 48.0% by weight 
of methanol). On the other hand, from the top of the condenser 13.6 
Nm.sup.3 /hr. of a non-condensed gas (composition: 15.4% by volume of 
carbon monoxide, 3.9% by volume of methyl nitrite, 6.8% by volume of 
nitrogen monoxide, 24.2% by volume of methanol, 7.6% by volume of carbon 
dioxide and 41.4% by volume of nitrogen) was obtained. 
To this non-condensed gas, 140 liters/hr. of oxygen and 9 liters/hr. of 
nitrogen monoxide were mixed (the molar ratio of oxygen to nitrogen 
monoxide in the gas being 0.15) and the mixture was led to the bottom of 
the gas-liquid contact type regeneration column having an inner diameter 
of 158 mm and a height of 1,400 mm. From the top of the column, methanol 
(including the methanol recycled from the regeneration column), was 
supplied at a rate of 40 liters/hr. (1.77 liters/hr. of which was the one 
supplied from the hydrolysis column mentioned below). The countercurrent 
contact was carried out at a temperature of about 35.degree. C. (i.e. 
30.degree. C. at the top of the column and 40.degree. C. at the bottom of 
the column), whereby nitrogen monoxide in the gas was regenerated into 
methyl nitrite. To 14.2 Nm.sup.3 /hr. of the regenerated gas from the 
regeneration column (composition: 15.4% by volume of carbon monoxide, 8.0% 
by volume of methyl nitrite, 2.8% by volume of nitrogen monoxide, 24.2% by 
volume of methanol, 7.6% by volume of carbon dioxide and 41.3% by volume 
of nitrogen), there was added 550 liters/hr. of carbon monoxide, and the 
mixture was supplied to and compressed by said gas recycling pump. The 
discharged gas was cooled to 20.degree. C. to remove condensed methanol, 
and then led to the reactor. 
On the other hand, 1.2 liters/hr. of an aqueous methanol solution 
containing 20.0% by weight of water, withdrawn from the regeneration 
column, was subjected to distillation to remove water and then reused as a 
methanol source of said column. To a distillation column having an inside 
diameter of 30 mm and a height of 3,000 mm, 2.8 kg/hr. of the condensed 
liquid withdrawn from said condenser was introduced and distilled at a 
temperature of 63.degree. C. at the top and 166.degree. C. at the bottom. 
From the bottom, 1.32 kg/hr. of a dimethyl oxalate liquid having a purity 
of 98.0% by weight was obtained. On the other hand, 0.96 Nm.sup.3 /hr. of 
a gaseous distillate composed of 96.7% by volume of methanol, 3.2% by 
volume of dimethyl carbonate and 0.02% by volume of methyl formate, was 
obtained. 
This gaseous distillate was led to a hydrolysis column having an inside 
diameter of 28.4 mm and a height of 1,000 mm [packed with 500 ml of 
Neobead P (trade name) made by Mizusawa Kagaku Co.] and contacted with 50 
g/hr. of steam at about 200.degree. C., whereby dimethyl carbonate and 
methyl formate in the gas were hydrolized. The methanol thereby obtained 
was recycled as a methanol source for said regeneration column at a rate 
of 1.77 liters/hr. 
The initial space time yield of dimethyl oxalate in this Example was 432 
g/l.hr. and no decrease in the space time yield was observed even after 
480 hours of this continuous reaction. 
EXAMPLE 2 
In the tubes of a stainless multi-tubular reactor, which has 6 tubes having 
an inside diameter of 36.7 mm and a height of 550 mm, there was packed 2.5 
kg (2.5 liters) of a .gamma.-alumina catalyst in a form of pellets having 
a diameter of 5 mm and a height of 3 mm and carrying 0.5% by weight of 
palladium. 
A gaseous starting material compressed to a pressure of 1.8 kg/cm.sup.2 
(gauge pressure) (composition: 20.0% by volume of carbon monoxide, 15.1% 
by volume of methyl nitrite, 3.1% by volume of nitrogen monoxide, 13.2% by 
volume of methanol, 2.0% by volume of carbon dioxide and 46.9% by volume 
of nitrogen) was preheated to about 90.degree. C. by a heat exchanger, and 
then introduced from the top of this catalyst layer at a rate of 5.4 
Nm.sup.3 /hr. by a diaphragm gas-recycling pump, and the temperature of 
the central portion of the catalyst layer was maintained at about 
110.degree. C. by circulating hot water to the shell side of the reactor. 
The gas passed through the catalyst layer was led to the bottom of a 
Rasching ring packed condenser of gas-liquid contact type having an inside 
diameter of 158 mm and a height of 1,400 mm, and from the top of the 
condenser, methanol was introduced at a rate of 1.3 liters/hr., whereby 
the countercurrent contact was carried out at a temperature of 40.degree. 
C. at the top of the condenser and 43.degree. C. at the bottom of the 
condenser. From the bottom of the condenser, there was obtained 2.2 kg/hr. 
of a condensed liquid (composition: 48.0% by weight of dimethyl oxalate, 
1.5% by weight of dimethyl carbonate, 0.3% by weight of methyl formate and 
48.0% by weight of methanol). On the other hand, from the top of the 
condenser, 5.0 Nm.sup.3 /hr. of a non-condensed gas (composition: 13.3% by 
volume of carbon monoxide, 7.4% by volume of methyl nitrite, 11.9% by 
volume of nitrogen monoxide, 14.2% by volume of methanol, 2.4% by volume 
of carbon dioxide and 50.9% by volume of nitrogen) was obtained. 
To this non-condensed gas, 119.0 liters/hr. of oxygen was mixed (the molar 
ratio of oxygen to nitrogen monoxide in the gas being 0.2) and the mixture 
was led to the bottom of the gas-liquid contact type regeneration column 
having an inner diameter of 158 mm and a height of 1,400 mm. From the top 
of the column, methanol was supplied at a rate of 5.0 liters/hr. (1.33 
liters/hr. of which was the one supplied from the hydrolysis column 
mentioned below). The countercurrent contact was carried out at a 
temperature of 40.degree. C. at the top of the column and 42.degree. C. at 
the bottom of the column, whereby nitrogen monoxide in the gas was 
regenerated into methyl nitrite. The regenerated gas from the regeneration 
column (composition: 13.0% by volume of carbon monoxide, 16.3% by volume 
of methyl nitrite, 3.4% by volume of nitrogen monoxide, 14.7% by volume of 
methanol, 2.3% by volume of carbon dioxide and 50.0% by volume of 
nitrogen), was supplied to and compressed by said gas recycling pump at a 
rate of 5.1 Nm.sup.3 /hr. To 4.7 Nm.sup.3 /hr. of the discharged gas, 
there was added 0.7 Nm.sup.3 /hr. of a gaseous mixture containing 66.8% by 
volume of carbon monoxide, 6.3% by volume of methyl nitrite, 1.3% by 
volume of methanol and 25.6% by volume of nitrogen, and the mixture was 
led to the reactor. 
On the other hand, 4.2 liters/hr. of a methanol solution containing 5.0% by 
weight of water, withdrawn from the regeneration column, was subjected to 
distillation to remove water and then reused as a methanol source for said 
column. 
To a distillation column having an inside diameter of 30 mm and a height of 
3,000 mm, 2.2 kg/hr. of the condensed liquid withdrawn from said condenser 
was introduced, and distilled at a temperature of 63.degree. C. at the top 
and 166.degree. C. at the bottom. From the bottom, 1.07 kg/hr. of a 
dimethyl oxalate liquid having a purity of 99.0% by weight was obtained. 
On the other hand, 0.74 Nm.sup.3 /hr. of a gaseous distillate composed of 
98.50% by volume of methanol, 1.13% by volume of dimethyl carbonate and 
0.29% by volume of methyl formate, was obtained. 
This gaseous distillate was led to a hydrolysis column having an inside 
diameter of 28.4 mm and a height of 1,000 mm [packed with 500 ml of 
Neobead P (trade name) made by Mizusawa Kagaku Co.] and contacted with 
17.0 g/hr. of steam at about 200.degree. C., whereby dimethyl carbonate 
and methyl formate in the gas were hydrolized. The methanol thereby 
obtained was recycled as a methanol source for said regeneration column at 
a rate of 1.33 liters/hr. 
The initial space time yield of dimethyl oxalate in this Example was 421 
g/l.hr. and no decrease in the space time yield was observed even after 
480 hours of this continuous reaction. 
EXAMPLE 3 
In the tubes of a stainless multi-tubular reactor, which has 8 tubes having 
an inside diameter of 28.0 mm and a height of 1,000 mm, there was packed 
3.85 kg (3.85 liters) of a .gamma.-alumina catalyst in a form of pellets 
having a diameter of 5 mm and a height of 3 mm and carrying 0.5% by weight 
of palladium. 
A gaseous starting material compressed under a pressure of 1.8 kg/cm.sup.2 
(gauge pressure) (composition: 20.0% by volume of carbon monoxide, 7.0% by 
volume of ethyl nitrite, 3.0% by volume of nitrogen monoxide, 6.0% by 
volume of ethanol, 3.2% by volume of carbon dioxide and 59.8% by volume of 
nitrogen) was preheated to about 90.degree. C. by a heat exchanger and 
then introduced from the top of the catalyst layer by a diaphragm gas 
recycling pump at a rate of 23.0 Nm.sup.3 /hr., and the temperature of the 
central portion of the catalyst layer was maintained to be about 
110.degree. C. by circulating hot water to the shell side of the reactor. 
The gas passed through the catalyst layer was led to the bottom of a 
Rasching ring packed condenser of gas-liquid contact type having an inside 
diameter of 158 mm and a height of 1,400 mm, and from the top of the 
condenser, ethanol was introduced at a rate of 8.0 liters/hr., whereby the 
countercurrent contact was carried out at a temperature of 60.degree. C. 
at the top and 63.degree. C. at the bottom. From the bottom of the 
condenser, there was obtained 2.5 kg/hr. of a condensed liquid 
(composition: 54.7% by weight of diethyl oxalate, 1.8% by weight of 
diethyl carbonate, 0.3% by weight of ethyl formate and 41.6% by weight of 
ethanol). On the other hand, from the top of the condenser, 24.9 Nm.sup.3 
/hr. of a non-condensed gas (composition: 16.7% by volume of carbon 
monoxide, 4.6% by volume of ethyl nitrite, 4.6% by volume of nitrogen 
monoxide, 16.0% by volume of ethanol, 3.0% by volume of carbon dioxide and 
54.0% by volume of nitrogen) was obtained. 
To this non-condensed gas, 118.5 Nl/hr. of oxygen was mixed (the molar 
ratio of oxygen to nitrogen monoxide in the gaseous mixture being 0.104) 
and the mixture was led to the bottom of the gas-liquid contact type 
regeneration column having an inner diameter of 158 mm and a height of 
1,400 mm. From the top of the column, ethanol is supplied at a rate of 2.3 
liters/hr. (1.33 liters/hr. of which was supplied from the hydrolysis 
column mentioned below). The countercurrent contact was carried out at a 
temperature of 40.degree. C. at the top of the column and 42.degree. C. at 
the bottom of the column, whereby nitrogen monoxide in the gas was 
regenerated into ethyl nitrite. The regenerated gas from the regeneration 
column (composition: 18.4% by volume of carbon monoxide, 7.1% by volume of 
ethyl nitrite, 3.1% by volume of nitrogen monoxide, 6.2% by volume of 
ethanol, 3.3% by volume of carbon dioxide and 60.9% by volume of 
nitrogen), was supplied to and compressed by said gas recycling pump at a 
rate of 22.6 Nm.sup.3 /hr. To 22.3 Nm.sup.3 /hr. of the discharged gas, 
there was added 0.7 Nm.sup.3 /hr. of a gaseous mixture containing 71.5% by 
volume of carbon monoxide, 4.4% by volume of ethyl nitrite, 0.6% by volume 
of ethanol, and 23.6% by volume of nitrogen, and the mixture was led to 
the reactor. 
On the other hand, 8.9 liters/hr. of an ethanol solution containing 4.3% by 
weight of water, withdrawn from the regeneration column, was subjected to 
dehydration and then reused as an ethanol source for said column. 
To a distillation column having an inside diameter of 30 mm and a height of 
3,000 mm, 2.5 kg/hr. of the condensed liquid withdrawn from said condenser 
was introduced and distilled at a temperature of 78.degree. C. at the top 
and 185.degree. C. at the bottom. From the bottom, 1.38 kg/hr. of a 
diethyl oxalate liquid having a purity of 98.9% by weight was obtained. On 
the other hand, 0.52 Nm.sup.3 /hr. of a gaseous distillate composed of 
97.8% by volume of ethanol, 1.7% by volume of diethyl carbonate and 0.5% 
by volume of ethyl formate, was obtained. 
This gaseous distillate was led to a hydrolysis column having an inside 
diameter of 28.4 mm and a height of 1,000 mm [packed with 500 ml of 
Neobead P (trade name) made by Mizusawa Kagaku Co.] and contacted with 
18.0 g/hr. of steam at about 200.degree. C., whereby diethyl carbonate and 
ethyl formate in the gas were hydrolized. The ethanol thereby obtained was 
recycled as a ethanol source for said regeneration column at a rate of 
1.33 liters/hr. 
The initial space time yield of diethyl oxalate in this Example was 355 
g/l.hr. and no decrease in the space time yield was observed even after 
480 hours of this continuous reaction.