JPH11315053A - Recovery of methyl nitrite - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for recovering methyl nitrite, capable of solving recovery and safety problems accompanied by the recoveries of the methyl nitrite and nitrogen monoxide contained in a purged gas recovered from a zone for producing the methyl nitrite and capable of efficiently and safely recovering the methyl nitrite and the nitrogen monoxide as useful components contained in the purged gas. SOLUTION: This method for recovering methyl nitrite comprises bringing a purged gas recovered from a zone for producing the methyl nitrite into contact with methanol in a methyl nitrite-recovering tower to allow the methanol to absorb the methyl nitrite contained in the purged gas, recovering the obtained methanol solution of the methyl nitrite, and further bringing the nitrogen monoxide remaining in the methyl nitrite-separated purged gas into contact with molecular oxygen and methanol in a tower for recovering the nitrogen monoxide to produce methyl nitrite, and subsequently recovering the produced methyl nitrite methanol solution.

Description

DETAILED DESCRIPTION OF THE INVENTION TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a purge gas recovered from a methyl nitrite generation zone, and more particularly to a purge gas extracted from a gas derived from a zone where methyl nitrite is generated by contacting nitric oxide with molecular oxygen and methanol. The present invention relates to a method for efficiently and safely recovering methyl nitrite and nitric oxide, which are the active ingredients of the above.

[0002]

2. Description of the Related Art Gas derived from a methyl nitrite production zone, in particular, a zone in which nitric oxide is brought into contact with molecular oxygen and methanol to produce methyl nitrite, is formed by converting carbon monoxide and methyl nitrite into a solid catalyst. It is circulated and used in a method of industrially producing dimethyl carbonate by a gas phase contact reaction in the presence.

The method for producing dimethyl carbonate is, for example, as follows:
As disclosed in Japanese Patent Application No. 3-269950 (JP-A-6-2510), dimethyl carbonate is reacted by reacting carbon monoxide and methyl nitrite in a gas phase in the presence of a solid catalyst. A first step of generating, a second step of contacting a reaction gas containing dimethyl carbonate generated in the first step with dimethyl oxalate as an absorption solvent in a dimethyl carbonate absorption tower (absorption tower) to separate into an absorption liquid and a non-condensable gas The oxalic acid in the third step in which nitric oxide in the non-condensable gas in the second step is brought into contact with molecular oxygen and methanol in a methyl nitrite regeneration tower (regeneration tower) to produce methyl nitrite, and It comprises a fourth step in which dimethyl carbonate absorbed and separated into dimethyl is separated by distillation in an extraction distillation column and a dimethyl carbonate distillation column.

In the above method, the gas containing carbon monoxide and methyl nitrite is circulated between the first step, the second step, and the third step.
Is NO supplied to the regeneration tower for synthesizing gas such as carbon dioxide by-produced in the gas phase contact reaction and methyl nitrite.
Contains inert gas such as nitrogen accompanying _x gas.
For this reason, in the above-mentioned method, in order to prevent these gases from being accumulated in the circulating gas at a high concentration, a part of the circulating gas (a part of the gas derived from the regeneration tower) is removed from the gas circulation system. Purged continuously.

[0005] A high concentration of methyl nitrite is contained in the purge gas (purge gas extracted from a gas derived from a regeneration tower for regenerating methyl nitrite by bringing nitric oxide into contact with molecular oxygen and methanol). And nitric oxide, methyl nitrite and nitric oxide are recovered from the purge gas.

As a method for recovering methyl nitrite and nitric oxide from the purge gas, for example, Japanese Patent Laid-Open No.
As disclosed in Japanese Patent Publication No. 121250, molecular oxygen and methanol are brought into contact with a purge gas recovered from a methyl nitrite production zone to regenerate nitric oxide contained in the purge gas while producing methyl nitrite. A method is known in which the methyl nitrite thus obtained is absorbed in methanol together with methyl nitrite contained in the purge gas and returned to a methyl nitrite producing zone.

However, in this method, in order to regenerate and collect all the nitric oxide to methyl nitrite,
It is necessary to add a stoichiometric amount or more of a molecular oxygen-containing gas to nitric oxide, and molecular oxygen remains in the purge gas. Therefore, the presence of molecular oxygen in a gas containing a high concentration of methyl nitrite poses a danger of producing an explosive gas. In addition, since methyl nitrite is generated in a state where a high concentration of methyl nitrite is already contained in the purge gas, the concentration of methyl nitrite in the gas is further increased, and the methyl nitrite is not sufficiently absorbed by methanol. There is also a problem.

[0008]

SUMMARY OF THE INVENTION The present invention solves the problems of recovery and safety associated with the recovery of methyl nitrite and nitric oxide in the purge gas recovered from the above-mentioned methyl nitrite production zone. It is another object of the present invention to provide a method for recovering methyl nitrite, which can efficiently and safely recover methyl nitrite and nitric oxide as active components in a purge gas.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to contact a purge gas recovered from a methyl nitrite production zone with methanol in a methyl nitrite recovery column to absorb the methyl nitrite in the purge gas into methanol. The obtained methanol solution of methyl nitrite was recovered, and the nitric oxide remaining in the purge gas from which the methyl nitrite was absorbed and separated was brought into contact with molecular oxygen and methanol in a nitric oxide recovery tower to form a nitrous oxide. The problem is solved by a method for recovering methyl nitrite, which comprises generating methyl nitrate and recovering a methanol solution of the obtained methyl nitrite.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. As a methyl nitrite generation zone, a zone in which nitric oxide is brought into contact with molecular oxygen and methanol to generate methyl nitrite (for example, nitric oxide is brought into contact with molecular oxygen and methanol to regenerate methyl nitrite Methyl nitrite regeneration tower). The purge gas recovered from the methyl nitrite generation zone is a purge gas (for example, nitric oxide) extracted from a gas derived from a zone in which nitric oxide is brought into contact with molecular oxygen and methanol to generate methyl nitrite. And nitric acid and molecular oxygen and methanol to regenerate methyl nitrite.

As a process having the above-mentioned methyl nitrite production zone, there are a method of industrially producing dimethyl carbonate by a gas phase contact reaction between carbon monoxide and methyl nitrite in the presence of a solid catalyst, JP-A-1-121250
And a method of industrially producing dimethyl oxalate by subjecting carbon monoxide and methyl nitrite to a gas phase contact reaction in the presence of a solid catalyst.

The above-mentioned method for producing dimethyl carbonate includes the following four steps.
Process. That is, in the first step, a gas containing carbon monoxide and methyl nitrite is introduced as a raw material gas into a reactor filled with a platinum group metal and / or a compound thereof and a solid catalyst supporting a cocatalyst. This is a dimethyl carbonate synthesis step in which dimethyl carbonate is generated by performing a gas phase contact reaction to obtain a reaction gas containing dimethyl carbonate.

In the second step, the reaction gas in the first step is led to a dimethyl carbonate absorption tower (hereinafter referred to as an absorption tower) and brought into contact with dimethyl oxalate added as an absorption solvent (preferably with methanol and Contacting) and separating the liquid into an absorption liquid that has absorbed the dimethyl carbonate generated by the gas phase contact reaction in the first step and a non-condensable gas containing nitric oxide.

In the third step, the non-condensed gas in the second step is led to a methyl nitrite regenerating tower (hereinafter, referred to as a regenerating tower), and brought into contact with the supplied molecular oxygen and methanol to form a mixture in the non-condensed gas. Is a nitrite regenerating step in which nitric oxide is regenerated to methyl nitrite and circulated and supplied to the reactor of the first step. The fourth step is a dimethyl carbonate purification step in which methanol is removed from the absorbent in the second step by extractive distillation, and then dimethyl carbonate is separated by distillation.

The gas containing carbon monoxide and methyl nitrite is led out of the reactor of the first step as a gas (reaction gas) containing dimethyl carbonate from the reactor of the first step, and is then discharged from the reactor of the first step. Is extracted as a non-condensable gas from which dimethyl carbonate has been absorbed and separated, and finally from the regeneration tower in the third step as methyl nitrite is regenerated as a regenerated gas (regeneration gas). Then, for example, the gas is pressurized by a gas circulating machine installed in a conduit between the third step and the first step, is circulated and supplied again to the reactor of the first step, and is reused as a source gas. Here, the reactor in the first step, in which the gas containing carbon monoxide and methyl nitrite circulates,
The system consisting of the absorption tower in the step and the regeneration tower in the third step is called a gas circulation system.

At this time, the gas circulating in the system consisting of the first step, the second step and the third step contains a carbon dioxide gas by-produced in the gas phase contact reaction and a regeneration tower for regenerating methyl nitrite. because it contains an inert gas such as nitrogen with the NO _x gas is replenished in order to prevent the accumulation of these by-product gases and inert gases, between the gas circulation device and the regeneration tower in the third step A part of the circulating gas (a part of the gas (regeneration gas) derived from the regeneration tower) is purged as a purge gas by purging from a conduit of the above.

In the present invention, a purge gas recovered from a methyl nitrite production zone (a zone in which nitric oxide is brought into contact with molecular oxygen and methanol to produce methyl nitrite);
That is, the sub-gas contained in the purge gas from such a gas circulation system (the purge gas extracted from the gas derived from the regeneration tower that regenerates methyl nitrite by bringing nitric oxide into contact with molecular oxygen and methanol). In order to efficiently recover methyl nitrate, first, the purge gas is brought into contact with methanol in a methyl nitrite recovery tower to absorb the methyl nitrite in the purge gas into methanol, and the resulting methanol solution of methyl nitrite is removed. Collection is performed.

That is, in order to recover methyl nitrite and nitric oxide contained in the purge gas from the gas circulation system,
Conventionally, for example, as shown in FIG. 3, methanol has been directly fed to a regeneration tower through a conduit 19 (see FIG. 3). However, in the present invention, methanol is brought into contact with a purge gas before being fed to the regeneration tower, and is thus prepared in advance. After the methyl nitrite in the purge gas is absorbed in methanol, it is fed to the regeneration tower (see FIGS. 1 and 2). As a result, there is no danger of locally producing explosive gases when mixing the purge gas with the molecular oxygen-containing gas.

Further, in order to recover the nitrogen monoxide remaining in the purge gas from which the methyl nitrite has been absorbed and separated, the purge gas is brought into contact with molecular oxygen and methanol in a nitrogen monoxide recovery tower, and the regeneration tower is recovered. As in the above, methyl nitrite is generated (nitrogen monoxide contained in the gas is regenerated into methyl nitrite), and the obtained methanol solution of methyl nitrite is collected. As a result, the efficiency of absorption of methyl nitrite in the step of recovering methyl nitrite in gas (in the present invention, the column for recovering nitric oxide) increases, and the loss of methyl nitrite can be reduced.

Next, the first to third steps of the production of dimethyl carbonate and the fifth step including the recovery of methyl nitrite of the present invention will be described in detail. "First step of dimethyl carbonate production" In the first step of dimethyl carbonate production, dimethyl carbonate is synthesized by adding carbon monoxide to a reactor filled with a platinum group metal and / or a compound thereof and a solid catalyst supporting a promoter. The reaction is carried out by introducing a raw material gas containing methyl nitrite and performing a gas phase contact reaction.

As the above-mentioned solid catalyst, for example, those in which a platinum group metal and / or a compound thereof and a cocatalyst described in JP-A-3-141243 are supported are effective. With these solid catalysts, platinum group metals and / or
Alternatively, examples of the platinum group metal supported on a carrier as a compound thereof include palladium, platinum, iridium, ruthenium, and rhodium, with palladium being most preferred. Further, in addition to the platinum group metal, a compound of a metal such as copper, iron, bismuth and cerium may be used as a co-catalyst at least one compound.
Seeds may be supported. As the carrier, activated carbon, alumina, silica, diatomaceous earth, zeolite, viscous mineral, and the like can be used.

Carbon monoxide and methyl nitrite are usually diluted with a gas inert to the reaction, such as nitrogen or carbon dioxide,
The raw material gas is fed to the reactor so that the contact time with the solid catalyst is usually 10 seconds or less, preferably 0.2 to 5 seconds. In addition, as a reactor filled with a solid catalyst, a single-tube or multi-tube reactor is preferable.

The concentration of methyl nitrite in the raw material gas is determined from the viewpoint of reaction rate and safety. That is, in order to obtain a satisfactory reaction rate, the concentration of methyl nitrite is preferably 1
It is necessary that the concentration be not less than 3% by volume, but since methyl nitrite is an explosive compound, it is not preferable that the concentration of methyl nitrite is too high, and usually 3 to 25% by volume.
Is preferred. In addition, the concentration of carbon monoxide in the raw material gas can be changed over a wide range. However, in a continuous process, part of the circulating gas is purged as described above. Is not preferred. Therefore, a suitable concentration of carbon monoxide is industrially usually 1 to 50% by volume, and more preferably 5 to 30% by volume.

The reaction temperature is preferably relatively low as long as a desired reaction rate can be obtained, and is usually 50 to 200 ° C, preferably 80 to 150 ° C. The reaction pressure is usually from normal pressure to 10 kg / cm ² (gauge pressure), preferably from 1 to 10 kg / cm ² .
6 kg / cm ² (gauge pressure).

The synthesis reaction of dimethyl carbonate is carried out as described above, and dimethyl carbonate, dimethyl oxalate, nitric oxide, carbon dioxide, unreacted carbon monoxide and methyl nitrite,
Further, a reaction gas containing an inert gas or the like is led out of the reactor. The target dimethyl carbonate is separated by introducing this reaction gas into the absorption tower in the second step and absorbing it into dimethyl oxalate fed from the top of the absorption tower.

"Second Step of Production of Dimethyl Carbonate" The absorption of dimethyl carbonate in the second step of production of dimethyl carbonate is carried out by bringing the above reaction gas into contact with dimethyl oxalate in an absorption tower as follows. The feed amount of dimethyl oxalate in the absorption tower depends on the amount of dimethyl carbonate in the reaction gas introduced into the absorption tower, but is usually 3 to 10 times by weight, preferably 4 to 10 times the weight of dimethyl carbonate in the reaction gas.
6 times the weight. The operating temperature of the absorption tower is preferably low in order to efficiently absorb dimethyl carbonate. However, if the temperature is too low, dimethyl oxalate solidifies and the energy is disadvantageous. ~ 100 ° C,
Preferably 30-80 degreeC is good.

A small amount of dimethyl carbonate and dimethyl oxalate accompany the non-condensed gas separated in the absorption tower.
Since these are completely lost when they are brought to the next step, it is preferable to feed a small amount of methanol from the top of the absorption tower to recover the accompanying dimethyl carbonate and dimethyl oxalate. At this time, the feed amount of methanol is
Usually, 5 to 30 dimethyl carbonate in the reaction gas is used.
% By weight, preferably 10 to 20% by weight. Since the non-condensed gas contains a large amount of nitric oxide generated in the first step in addition to unreacted carbon monoxide and methyl nitrite, the non-condensed gas contains Nitric oxide is regenerated to methyl nitrite.

The dimethyl carbonate thus absorbed by the dimethyl oxalate is converted into low-boiling compounds such as methanol and by-produced traces of methyl formate in a known manner in the fourth step of the above-mentioned dimethyl carbonate production. After being separated by extractive distillation with dimethyl acid, it is further separated by distillation.

[Third Step of Production of Dimethyl Carbonate] In the regeneration of methyl nitrite in the third step of production of dimethyl carbonate, the above-mentioned non-condensed gas is converted into molecular oxygen (molecular oxygen-containing gas) in a regeneration tower as follows. And methanol. At this time, as the regeneration tower, a usual gas-liquid contacting device such as a packed tower, a bubble tower, a spray tower, and a step tower is used.

As the molecular oxygen-containing gas, pure oxygen gas, oxygen gas diluted with an inert gas such as nitrogen, or air is used. In the regeneration tower, a molecular oxygen-containing gas is usually supplied in an amount of 0.08 to 0.2 mol based on oxygen with respect to 1 mol of nitrogen monoxide in the gas introduced into the regeneration tower. This gas is preferably contacted with methanol at a temperature of 60 ° C. or less, and the contact time is preferably 0.5 to 2 seconds.

Methanol is used in an amount not less than that required for completely absorbing and reacting nitrogen dioxide generated from nitric oxide and molecular oxygen and almost equimolar thereto. Usually, the amount is preferably 2 to 5 mol per 1 mol of nitric oxide in the gas introduced into the regeneration tower. As described in the fifth step of the production of dimethyl carbonate, the methanol used in the regeneration tower is prepared by bringing methyl nitrite in the purge gas into contact with the purge gas in the methyl nitrite recovery tower in advance and absorbing the methyl nitrite in the purge gas. You.

In the production of dimethyl carbonate and the like, methyl nitrite and nitric oxide are lost by dissolving in the absorbing solution of the absorption tower or the can solution of the regeneration tower or purging the circulating gas. methyl source (nitrogen monoxide, nitrogen dioxide, dinitrogen trioxide, NO _x gases such as dinitrogen tetroxide, or nitric acid), but is supplied, typically, NO _x gas is supplied to the regeneration tower of the third step You.

Since the liquid discharged from the regeneration tower is a methanol solution containing water generated by the regeneration reaction of methyl nitrite, the water content in the methanol solution is usually 2% by volume or less, preferably 0.1% by distillation or the like. It is industrially advantageous to reuse it in the second step or the third step after purifying it to be 2% by volume or less.

"Fifth Step of Producing Dimethyl Carbonate (Recovery of Methyl Nitrite)" The recovery of methyl nitrite in the fifth step of producing dimethyl carbonate including the recovery of methyl nitrite of the present invention is carried out as follows. Before the methanol used in the tower is fed to the regeneration tower, the methanol is brought into contact with the purge gas in the methyl nitrite recovery tower in advance. Then, the methyl nitrite in the purge gas absorbed in methanol is fed as a methanol solution to the upper part (preferably the top) of the regeneration tower in the third step, while the purge gas from which methyl nitrite has been absorbed and separated is The fifth step is fed to the nitric oxide recovery tower.

The apparatus for bringing the purge gas into contact with methanol (methyl nitrite recovery tower) does not need to be special, and is a gas-liquid contact type absorption apparatus such as a packed tower, a tray tower, and a bubble bell tower. I just need. The position at which the absorber is installed may be between the point where the circulating gas is purged and the point where the oxygen-containing gas is mixed with the purge gas. When purging from the top of the regeneration tower, this absorber may be directly installed at the top of the regeneration tower.

The temperature at which the purge gas is brought into contact with methanol is preferably as low as possible because this contact is an operation for absorbing methyl nitrite, but it is not necessary to make the temperature extremely low because the amount of methanol used is usually sufficient. No, -5
Preferably it is in the range of 30 ° C. As described above, the amount of methanol used in the methyl nitrite recovery tower is the amount used in the regeneration tower because the one that has absorbed methyl nitrite in the purge gas in this step is fed to the regeneration tower for use. Usually, it is preferably 2 to 5 mol per 1 mol of nitric oxide in the gas introduced into the regeneration tower.

The purge gas is obtained by extracting a part of the gas (regeneration gas) derived from the regeneration tower in the third step and feeding it to the fifth step. The amount of the purge gas depends on the degree of accumulation of the by-product gas and the inert gas in the circulation gas.
_x The amount of inert gas such as nitrogen introduced into the circulating gas _by gas replenishment is equal to or greater than the volume of the gas phase portion of the gas circulation system consisting of three devices, usually a reactor, an absorption tower and a regeneration tower. 1 to 30% / hr.

"Fifth Step of Production of Dimethyl Carbonate (Recovery of Nitric Oxide)" In the fifth step of the production of dimethyl carbonate including the recovery of methyl nitrite of the present invention, the nitric oxide is recovered by absorption and separation of methyl nitrite. The purge gas thus obtained is brought into contact with molecular oxygen (molecular oxygen-containing gas) and methanol in the nitrogen monoxide recovery tower in the same manner as in the regeneration tower. Then, the methyl nitrite regenerated from the nitric oxide remaining in the purge gas is absorbed by the methanol and circulated and supplied to the middle stage of the regenerator as a methanol solution. The apparatus and operating conditions in this case are that the feed amount of the molecular oxygen-containing gas is 0.2 mol or more based on oxygen with respect to 1 mol of nitrogen monoxide in the purge gas, and that NO _x gas is not supplied. , And methanol are fed directly without passing through the methyl nitrite recovery column, as in the regeneration column described above.

Next, the present invention will be described with reference to a flow sheet (FIG. 2) of a dimethyl carbonate production process including one embodiment of the present invention. One embodiment of the present invention is shown in FIG. A raw material gas containing carbon monoxide, methyl nitrite and nitrogen monoxide is placed in a conduit 20 at the top of a multitubular reactor 1 in which a platinum group metal-based solid catalyst is filled in a reaction tube. (Not shown) and introduced through conduit 22. Then, a catalytic reaction is performed in the gas phase in the reactor 1, and the reaction gas that has passed through the catalyst layer is taken out from the lower part of the reactor and introduced into the absorption tower 2 through the conduit 11.

In the absorption tower 2, the reaction gas and the conduit 13,
Due to the countercurrent contact with methanol and dimethyl oxalate respectively introduced from 14, dimethyl carbonate in the reaction gas is absorbed by dimethyl oxalate and separated. A liquid composed of dimethyl carbonate, dimethyl oxalate and methanol is taken out from the lower part through a conduit 15 and separated and purified in a known purification step (not shown). On the other hand, a non-condensable gas containing unreacted carbon monoxide, methyl nitrite, and nitric oxide generated by the gas phase contact reaction is supplied from the upper portion to the lower portion of the regeneration tower 3 through the conduit 12.

In the regeneration tower 3, countercurrent contact between the non-condensable gas and the molecular oxygen-containing gas introduced through the conduit 16 and methanol introduced from the top through the conduit 19 is performed to regenerate methyl nitrite. . When the nitrogen source necessary for the regeneration of methyl nitrite is insufficient, NO _x gas is introduced through the conduit 17.

The regeneration gas generated in the regeneration tower 3 is supplied to the conduit 2
0 and 22 are supplied to the reactor 1 together with freshly supplied carbon monoxide from the conduit 21. On the other hand, water by-produced in the regeneration tower 3 is taken out from the lower part through the conduit 18 in the form of a methanol solution. After the water in the liquid is removed by an operation such as distillation, the methanol
The methanol is supplied to the absorption tower 2, the methyl nitrite recovery tower 4, and the nitric oxide recovery tower 5 through the reactors 24 and 27, and is recycled.

A part of the regeneration gas generated in the regeneration tower 3 is purged from the gas circulation system and introduced into the methyl nitrite recovery tower 4. In the methyl nitrite recovery column 4, countercurrent contact between the purge gas and methanol supplied from the conduit 24 is performed,
Methyl nitrite is absorbed and recovered in methanol. This methanol solution is taken out from the lower part and supplied to the upper part of the regeneration tower 3 through the conduit 19.

The purge gas from which methyl nitrite has been absorbed and separated in the methyl nitrite recovery tower 4 is taken out from the upper part of the tower and introduced into the lower part of the nitric oxide recovery tower 5 through a conduit 25. In the nitric oxide recovery tower 5, as in the regeneration tower 3, countercurrent contact between the purge gas and the molecular oxygen-containing gas introduced through the conduit 26 and methanol introduced from above through the conduit 27 is performed. Methyl nitrite is regenerated.

The methyl nitrite generated in the nitric oxide recovery tower 5 is taken out from the lower part through a conduit 29 in the form of a methanol solution and supplied to the middle stage of the regeneration tower 3. Nitrogen gas and a small amount of by-product gas contained in the gas fed to the nitric oxide recovery tower 5 are discharged through a conduit 28.

[0046]

Next, the present invention will be described in detail with reference to Examples and Comparative Examples of the production of dimethyl carbonate including one embodiment of the present invention. The space time yield (STY) of dimethyl carbonate (STY) (kg / m ³ · hr) in Examples and Comparative Examples is based on the contact time between carbon monoxide and methyl nitrite θ (hr), and the dimethyl carbonate produced during that time. Is defined as a (kg) and the amount of the catalyst filled in the reaction tube is defined as b (m ³ ).

STY (kg / m ³ · hr) = a / (b × θ)

Example 1 In a tube of a stainless steel multi-tube reactor consisting of six tubes having an inner diameter of 27 mm and a height of 500 mm, a tube was disclosed in
Activated carbon (Shirasagi:
Takeda) was filled with 1.71 L (liter; hereinafter the same) of a solid catalyst (4 mmφ × 6 mm) supporting palladium chloride and cupric chloride. 4.02 kg / cm of the catalyst layer was applied to the catalyst layer from above using a diaphragm gas circulation pump.
² (Gauge pressure) Raw material gas (composition: carbon monoxide 20.0% by volume, methyl nitrite 10.0% by volume, nitric oxide 4.0% by volume, methanol 7.0% by volume, carbon dioxide gas 1.0% by volume and 58.0% by volume of nitrogen)
After preheating to 0 ° C., the mixture was fed at a rate of 6.80 Nm ³ / hr, and hot water was passed through the shell side of the reactor to carry out the reaction while maintaining the temperature at the center of the catalyst layer at about 120 ° C. . At this time, the space-time yield (STY) of dimethyl carbonate was 338 k
g / m ³ · hr.

Gas passing through the above catalyst layer (reaction gas)
To the bottom of a Raschig-filled gas-liquid contact absorption device (absorption tower) having an inner diameter of 100 mm and a height of 1300 mm, methanol 0.18 L / hr from the top and 20 from the top.
From 0mm below, dimethyl oxalate 2.50kg / hr
While contacting at a tower top temperature of 35 ° C. and a tower bottom temperature of 55 ° C. As a result, the absorption liquid obtained from the bottom of the column.
The composition of 28 kg / hr is 78.1% by weight of dimethyl oxalate, 16.8% by weight of dimethyl carbonate, 4.2% of methanol.
% By weight and 0.1% by weight of methyl formate.

On the other hand, the concentration of methyl nitrite in 6.64 Nm ³ / hr of the non-condensable gas (gas introduced into the regeneration tower) obtained from the top of the column is determined by the fact that methyl nitrite is consumed in the reaction, Since it was lower than in the above, methyl nitrite was regenerated in the next regeneration tower. A part of methyl nitrite in the absorption tower to dissolve the absorption liquid, was performed at the same time replenishment of the NO _x gases. That is, after mixing 7.5 NL / hr of a gas containing 78 NL / hr of oxygen gas and 14.0% by volume of nitrogen monoxide into a non-condensable gas, the mixture is mixed with an inner diameter of 158 m.
m, a gas-liquid contact absorber (regeneration tower) having a height of 1400 mm, methanol 1.5 L / hr introduced from the top through a methyl nitrite recovery tower (described later), a top temperature of 30 ° C., and a bottom temperature of 40 ° C. To regenerate methyl nitrite.

Regeneration gas 6.66 N derived from the regeneration tower
The composition of m ³ / hr was as follows: 17.5% by volume of carbon monoxide, 10.1% by volume of methyl nitrite, 4.0% by volume of nitric oxide, 1.01% by volume of carbon dioxide, and 60.4% by volume of nitrogen. Therefore, 6.5 NL / hr was purged. The purge gas was led to a gas-liquid contactor (methyl nitrite recovery column) having an inner diameter of 18 mm and a height of 250 mm, and was brought into countercurrent contact with 1.5 L / hr of methanol cooled to 10 ° C. introduced from the top of the column to form a purge gas. The contained methyl nitrite was absorbed in methanol. The methanol solution having absorbed the methyl nitrite was withdrawn from the bottom of the column and fed to the top of the regeneration tower.
Further, since the gas led out from the top of the methyl nitrite recovery tower contains 0.2% by volume of methyl nitrite and 4.0% by volume of nitric oxide, 1.0 NL / hr of air is further mixed. Then, it was fed to the next nitric oxide recovery tower.

The above gas introduced into the bottom of a gas-liquid contactor (nitrogen monoxide recovery tower) having an inner diameter of 27 mm and a height of 300 mm was fed from the top of the column and methanol cooled to 5 ° C. 0.2
L / hr was brought into countercurrent contact to regenerate nitric oxide in the purge gas into methyl nitrite and absorbed in methanol. The obtained absorbing solution contained 0.4% by weight of methyl nitrite and was fed to the middle stage of the regeneration tower. At this time, methyl nitrite was contained in the gas discharged from the top of the nitric oxide recovery tower.
It contained 00 ppm by volume and 50 ppm by volume of nitric oxide.

The partially purged regeneration gas was 6.66.
After Nm ³ / hr is pressurized by the gas circulating pump, and additionally supplied carbon monoxide 0.15 Nm ³ / hr, carbon monoxide 20.0% by volume, methyl nitrite 10.0 volume%, carbon monoxide The reactor was led to a composition of 4.0% by volume of nitrogen, 7.0% by volume of methanol, 1.0% by volume of carbon dioxide and 58.0% by volume of nitrogen.

On the other hand, 1.7 L / hr of methanol containing 2.0% by weight of water derived from the regeneration tower was subjected to distillation to remove water, and then absorbed, a regeneration tower, a methyl nitrite recovery tower, and a nitric oxide recovery tower. Reused as a source of methanol in the column. In addition, dimethyl carbonate was distilled from 3.28 kg / hr of the absorption liquid derived from the absorption tower to obtain 0.540 kg / hr by distillation.
hr obtained continuously.

Comparative Example 1 In Example 1, the methyl nitrite recovery tower was not installed. That is, in the flow sheet shown in FIG. Dimethyl carbonate was produced in the same manner as in Example 1, except that absorption and regeneration of nitric oxide to methyl nitrite were performed in the same manner. Other operations in the flow sheet shown in FIG. 3 are the same as those in FIG.

In the same manner as in Example 1, dimethyl carbonate was generated in the reactor (STY: 338 kg / m ³ · hr), and dimethyl oxalate was absorbed by dimethyl oxalate in the absorption tower.
Methyl nitrite was regenerated in the regeneration tower, and a part of the regeneration gas derived from the regeneration tower was purged. However, methanol fed to the regeneration tower was directly fed to the regeneration tower through the conduit 19.

1.0 NL / hr of air was mixed with the purge gas, led to a gas-liquid contactor (nitrogen monoxide recovery tower) having an inner diameter of 27 mm and a height of 300 mm, and introduced at 5 ° C. from the top of the tower.
The solution was brought into countercurrent contact with 0.2 L / hr of cooled methanol to absorb the methyl nitrite contained in the purge gas into methanol, and nitric oxide in the purge gas was regenerated into methyl nitrite and absorbed into methanol. The obtained absorbing solution contained 1.3% by weight of methyl nitrite and was fed to the middle stage of the regeneration tower. At this time, the gas discharged from the top of the nitric oxide recovery tower contained 200 ppm by volume of methyl nitrite and 80 ppm by volume of nitric oxide. The other results were the same as in Example 1. Table 1 shows the behavior of methyl nitrite and nitric oxide in the purge gas in Examples and Comparative Examples.

[0058]

[Table 1]

[0059]

According to the present invention, the purge gas recovered from the methyl nitrite generation zone, in particular, the gas extracted from the zone where methyl nitrite is generated by contacting nitric oxide with molecular oxygen and methanol is extracted. Solving the recovery and safety issues associated with the recovery of methyl nitrite and nitric oxide in purge gas to efficiently and safely recover the active ingredients methyl nitrite and nitric oxide in purge gas Can be. That is, according to the present invention, methyl nitrite in a purge gas extracted from a gas (regeneration gas) derived from a regeneration tower that regenerates methyl nitrite by bringing nitric oxide into contact with molecular oxygen and methanol is efficiently used. Can be collected. Further, since the absorption efficiency of methyl nitrite with respect to methanol is increased by preliminarily absorbing only methyl nitrite in the purge gas with methanol to lower the concentration of methyl nitrite, the concentration of the methyl nitrite in the purge gas regenerated into methyl nitrite is increased. Most of the nitric oxide can be recovered as methyl nitrite. Further, in order to supply the oxygen-containing gas necessary for the regeneration of methyl nitrite from nitric oxide in a state where the concentration of methyl nitrite in the purge gas is reduced in advance,
There is no danger of generating explosive gas, and methyl nitrite can be recovered safely. If the method of the present invention is applied, in a method of producing dimethyl carbonate by performing a gas phase contact reaction between carbon monoxide and methyl nitrite in the presence of a solid catalyst, it is preferable that the subcomponent, which is an active ingredient in a purge gas from a gas circulation system, be used. Methyl nitrate and nitric oxide can be efficiently and safely recovered, and an industrially suitable continuous production method of dimethyl carbonate can be provided. The same excellent effect can also be obtained in a method of producing dimethyl oxalate by causing a gas phase contact reaction between carbon monoxide and methyl nitrite in the presence of a solid catalyst.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of the present invention.

FIG. 2 is a flow sheet showing an example of dimethyl carbonate production including one embodiment of the present invention.

FIG. 3 is a flow sheet showing a comparative example of dimethyl carbonate production including one embodiment of the present invention.

[Explanation of symbols]

1 is a reactor, 2 is an absorption tower, 3 is a regeneration tower, 4 is a methyl nitrite recovery tower, 5 is a nitric oxide recovery tower, and 11 to 29 are conduits.

Claims (5)

[Claims] 1. A purge gas recovered from a methyl nitrite production zone is brought into contact with methanol in a methyl nitrite recovery tower to absorb the methyl nitrite in the purge gas into methanol, and the resulting methanol solution of methyl nitrite is obtained. The nitric oxide remaining in the purge gas from which the methyl nitrite has been absorbed and separated is brought into contact with molecular oxygen and methanol in a nitric oxide recovery tower to produce methyl nitrite. A method for recovering methyl nitrite, comprising recovering a methanol solution of methyl nitrate. 2. A methanol solution of methyl nitrite recovered from a methyl nitrite recovery tower is supplied to a methyl nitrite generation zone, and a methanol solution of methyl nitrite recovered from a nitric oxide recovery tower is converted to methyl nitrite. The method for recovering methyl nitrite according to claim 1, which is supplied to a production zone. 3. The method for recovering methyl nitrite according to claim 1, wherein the methyl nitrite producing zone is a zone for producing methyl nitrite by bringing nitric oxide into contact with molecular oxygen and methanol. 4. A purge gas recovered from a methyl nitrite generation zone is a purge gas extracted from a gas derived from a zone in which nitric oxide is brought into contact with molecular oxygen and methanol to generate methyl nitrite. The method for recovering methyl nitrite according to claim 1 or 2. 5. A zone for producing methyl nitrite by bringing nitric oxide into contact with molecular oxygen and methanol, wherein methyl nitrite regeneration is carried out by bringing nitric oxide into contact with molecular oxygen and methanol to regenerate methyl nitrite. The method for recovering methyl nitrite according to claim 3 or 4, which is a tower.