Method for synthesizing alkyl carbonate through gas-phase carbonylation

Abstract

The invention discloses a method for synthesizing alkyl carbonate through gas-phase carbonylation. Comprising a dechlorination method and a chlorine circulation process in the process of preparing alkyl carbonate through a gas-phase oxidative carbonylation reaction of carbon monoxide and alkyl nitrite. The content of water and alkyl alcohol in circulating gas is controlled through solvent absorption, alkyl chloride is synthesized with a chlorine supplementing agent HCl and alkyl chloroformate under the action of a dechlorinating agent, and the alkyl chloride is separated from a product through gas-liquid separation. When alkyl carbonate is produced by adopting the dechlorination method and the dechlorination process, the problem of equipment corrosion caused by chlorides can be avoided, the dechlorination process is a low-cost and environment-friendly chlorine circulation process, and stable operation of an industrial device is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of alkyl carbonate production, and particularly relates to a dechlorination method and a chlorine recycling process in the synthesis process of alkyl carbonate, and more particularly to a dechlorination method and a chlorine recycling process in the process of preparing alkyl carbonate by gas-phase oxidative carbonylation of carbon monoxide and methyl nitrite. Background Art

[0002] Alkyl carbonates are a class of environmentally friendly chemical products with promising development prospects. They are widely used in fields such as beauty and skincare, lithium-ion battery electrolytes, pharmaceutical products, pesticides, fertilizers, and detergents. They can replace highly toxic or carcinogenic substances such as phosgene and dimethyl sulfate in reactions to produce important chemical products, and are known as "new building blocks in the field of organic synthesis in the 21st century".

[0003] In the early days, alkyl carbonates were synthesized by the phosgene route. However, due to the high toxicity of the raw material gas and the corrosiveness of chloride ions, this process route has been gradually phased out. Taking dimethyl carbonate (DMC) as an example, the current process synthesis routes of DMC mainly include the transesterification method, the methanol oxidative carbonylation method, the direct synthesis method of methanol and CO2, and the urea alcoholysis method. Among them, the method of synthesizing DMC by gas-phase oxidative carbonylation of CO has the advantages of simple raw material preparation and low process cost, and is currently a relatively advanced DMC synthesis method.

[0004] Taking the synthesis of DMC as an example, its process route mainly includes two-step reactions, and the reaction equations are as follows:

[0005] 2CH3OH + 2NO + 1 / 2O2 → 2CH3ONO + H2O (1)

[0006] CO + 2CH3ONO → (CH3O)2CO + 2NO (2)

[0007] Among them, reaction (2) is a gas-solid phase catalytic reaction. Most of the existing industrial applications use palladium-based noble metal catalysts. In order to improve the stability of the catalyst, the most common measure is to add a certain amount of chlorine-containing compounds, such as HCl or methyl chloroformate, etc., to the raw material gas. Under the action of the chlorine replenishing agent, the active components of the catalyst can maintain a high valence state and improve the dispersion degree of the active components. In the carbonyl synthesis reaction, under the combined action of a high-temperature reaction environment and the catalyst, methyl chloroformate by-products are often generated simultaneously. Corrosive chlorides such as hydrogen chloride and methyl chloroformate cause serious corrosion of subsequent equipment and an increase in the chlorine content of the product dimethyl carbonate. At the same time, when hydrogen chloride, water, and hydrogen sulfide coexist in the system, it is easy to form an HCl-H2S-H2O system, increasing the corrosion risk of the system. A large number of corrosion-resistant materials, such as glass-lined, Hastelloy, and graphite, are used in the dimethyl carbonate separation process. This not only increases the equipment investment cost, but also requires higher equipment maintenance and management.

[0008] In the synthesis reaction system of alkyl carbonate, inorganic chlorides such as hydrogen chloride and organic chlorides such as methyl chloroformate will be produced, which are the main corrosive substances in the system. Reducing the content of corrosive chlorides will greatly reduce the equipment material grade. Ordinary stainless steel materials can be used in subsequent alkali treatment and distillation processes, which can greatly save the project construction investment cost and shorten the equipment procurement and construction period of the project.

[0009] As a method for eliminating methyl chloroformate and hydrogen chloride corrosion products, US5869729A patent reports using activated carbon conversion to convert methyl chloroformate. However, the patent reports that activated carbon cannot fully convert methyl chloroformate, resulting in free chlorine in the product dimethyl carbonate. Increasing the amount of activated carbon helps to improve the conversion rate of methyl chloroformate, but the increase in the amount of activated carbon causes an increase in equipment investment and energy consumption.

[0010] CN112457194A patent reports using an adsorbent to treat the crude dimethyl carbonate. Through adsorption, the chloride ion concentration in the crude dimethyl carbonate is lower than 2.0 ppm, achieving the purpose of dechlorination of the crude product, thereby effectively reducing the chlorine corrosion of the crude product and reducing the material requirements for subsequent treatment sections. However, the adsorbent used needs to be continuously regenerated.

[0011] According to the literature reports, at present, the removal of hydrogen chloride mostly uses chemical and physical adsorption with hydrogen chloride to achieve the effect of removing hydrogen chloride. In industrial applications, alkaline metal oxides such as CaO, Na2O, and ZnO are mostly used as the main components, but there are problems such as low chlorine capacity and poor mass transfer effect. The low service life of the dechlorination agent affects the stable operation of downstream equipment of the device. At the same time, the dechlorination agent used as an adsorbent also needs to be replaced frequently, increasing the operation cost and operation difficulty of the device. At the same time, the adsorption and removal of hydrogen chloride often need to be carried out at an appropriate temperature. The above dechlorination agents are mainly used for dechlorination of ultra-high temperature flue gas, while the temperature of the materials containing chlorides in the carbonate synthesis process is lower than 150 °C. The gas-phase mass transfer effect of the alkali metal oxide dechlorination agent is poor in this temperature range, resulting in a low dechlorination conversion rate.

[0012] Among the currently known methods for removing methyl chloroformate and hydrogen chloride, there is a lack of a method that can economically be used industrially to remove methyl chloroformate and hydrogen chloride corrosion products in the above-mentioned alkyl carbonate production process. Summary of the Invention

[0013] The purpose of the embodiments of the present invention is to provide a method for gas-phase carbonylation to synthesize alkyl carbonates, particularly a production method for gas-phase oxidative carbonylation of carbon monoxide and methyl nitrite to prepare dimethyl carbonate, so as to solve the problem of equipment corrosion caused by chlorides in the existing process for gas-phase carbonylation of CO to synthesize dimethyl carbonate, and develop a low-cost dechlorination method and chlorine recycling process.

[0014] The present invention provides a dechlorination method and a chlorine recycling process in the process of gas-phase carbonylation to synthesize alkyl carbonates, comprising the following steps:

[0015] (1) Using an alkyl alcohol as an absorbent, washing and removing the moisture in the recycled feed gas containing alkyl chlorides in a recycled feed gas washing and absorption tower;

[0016] (2) After mixing the dehydrated feed gas obtained in step (1) with a CO feed and a fresh chlorine supplement agent HCl, introducing them into a carbonylation reactor to carry out a gas-solid phase catalytic reaction, generating a mixed gas containing dimethyl carbonate, HCl, and alkyl chloroformate;

[0017] (3) Making the mixed gas obtained in step (2) contact with a supported dechlorination agent, and under the action of the dechlorination agent, HCl and alkyl chloroformate are converted into alkyl chlorides;

[0018] (4) Introducing the mixed gas containing alkyl chlorides obtained in step (3) into an absorption tower to contact with an ester absorbent. After gas-liquid separation in the absorption tower, the gas phase at the top of the absorption tower is discharged from the upper part and enters the recycled feed gas washing and absorption tower, and the mixed liquid containing alkyl carbonate and the ester absorbent is taken out from the bottom of the absorption tower;

[0019] (5) A part of the gas phase at the top of the recycled feed gas washing and absorption tower is discharged in the form of purge gas, and the other part is recycled as the dehydrated feed gas to step (2).

[0020] Preferably, in step (2), a supported Pd-based catalyst is filled in the carbonylation reactor. Preferably, the alkyl alcohol in step 1) is a mixture of one or more selected from methanol and ethanol. By mass, the dosage ratio of the alkyl alcohol to the mass of the recycled feed gas is 4-10. The feeding temperature of the alkyl alcohol is lower than the temperature of the recycled feed gas. Preferably, the feeding temperature of the alkyl alcohol is 5-30 °C, and more preferably the feeding temperature is 10-20 °C. Taking methanol as an example, the alkyl alcohol feeding can be fresh methanol feeding or a mixture of recycled methanol in the system and fresh methanol. Preferably, a mixture of recycled methanol in the system and fresh methanol is used as the feed.

[0021] Preferably, in step 1), an on-line monitoring unit is provided on the gas-phase pipeline at the top of the recycled feed gas washing and absorption tower ( Figure 2) Monitor the moisture in the recycle gas, adjust the ratio of fresh methanol S8 to recycled methanol S9 according to the moisture content, and control the moisture content in the mixed methanol to be 50 - 300 ppm. When the moisture content is high, increase the proportion of fresh methanol. When the moisture content in the dehydrated recycle feed gas increases by 5 ppm each time, control the moisture content in the material after mixing fresh methanol S8 and recycled methanol S9 to decrease by 50 ppm by increasing the feed ratio of fresh methanol S8.

[0022] The recycle gas and the alkyl alcohol in step 1) can be absorbed through a gas-liquid contact device to reduce the water content in the recycle gas. Preferably, the gas-liquid contact device includes a packed absorption tower, a plate absorption tower, and a bubble column. More preferably, the gas-liquid contact device uses a plate absorption tower. Preferably, the number of theoretical plates of the gas-liquid contact device is 5 - 30.

[0023] The water content in the dehydrated feed gas obtained in step 1) is 10 - 500 ppm. Preferably, the water content in the dehydrated feed gas is 10 - 150 ppm. Preferably, the dehydrated feed gas contains no less than 200 ppm of methanol content. More preferably, the methanol in the feed gas is in a gaseous state, and its content is 0.02 - 5 wt%. In some embodiments, the water content in the gas can be analyzed online by an online chromatograph or a moisture analyzer, and the addition amount and feed temperature of the alkyl alcohol absorbent are adjusted through the detection of the moisture content.

[0024] In step 2), the dehydrated feed gas reacts with HCl under the action of a supported Pd-based catalyst to generate an alkyl carbonate through a gas-solid catalytic reaction. Preferably, the temperature of the gas-solid catalytic reaction is 80 - 130 °C, and more preferably the reaction temperature is 100 - 130 °C. The product is a mixed gas containing an alkyl carbonate, HCl, and an alkyl chloroformate. Preferably, the content of HCl therein is 1 - 100 ppm, and the content of the alkyl chloroformate is 10 - 100 ppm.

[0025] Preferably, based on the total weight of the supported Pd-based catalyst, the loading amount of Pd is 0.1% - 2 wt%, and more preferably 0.5 - 2 wt%. Preferably, the carrier includes one or a mixture of alumina, silica, hydrotalcite, and spinel.

[0026] Preferably, for the supported dechlorination agent in step 3), the carrier includes one or several of carbon nanotubes, activated carbon, and carbon fibers. Preferably, the carrier is activated carbon, including but not limited to one or several of coconut shell charcoal, wood charcoal, and coal-based charcoal. The active component contains a divalent metal. Preferably, the divalent metal active component is Zn and / or Cu. Based on the total mass of the dechlorination agent, preferably, the loaded metal content is 0.1 wt% - 40 wt%, and more preferably the content of the loaded metal is 0.1 wt% - 10 wt%.

[0027] Preferably, the dechlorination agent described in step 3) can be loaded below the supported Pd-based catalyst, or a separate dechlorination reactor can be set up at the rear of the hydroformylation reactor filled with the supported Pd-based catalyst. More preferably, a fixed-bed reactor is set up at the rear of the hydroformylation reactor filled with the supported Pd-based catalyst, and the dechlorination agent is randomly filled in the fixed-bed reactor. The mixed gas containing HCl and alkyl chloroformate undergoes a gas-solid catalytic reaction in the dechlorination reactor to generate alkyl chloride. Taking the synthesis of dimethyl carbonate as an example, HCl and alkyl chloroformate react to generate chloromethane, and the reaction equation is as follows:

[0028] HCl + CH3OH → CH3Cl + H2O

[0029] ClCOOCH3 → CH3Cl + CO2

[0030] Preferably, in step 4), the ester absorbent is alkyl oxalate, specifically, dimethyl oxalate, diethyl oxalate. By mass, the mass ratio of the ester absorbent to dimethyl carbonate in the mixed gas is 1.0∶1 to 3∶1, and more preferably the mass ratio is 1.0∶1 to 2∶1.

[0031] Preferably, measured by a volume flowmeter, in step 5), the purge gas accounts for 0.05% - 1% of the recycle gas, and more preferably the purge gas accounts for 0.05% - 0.5% of the recycle gas.

[0032] Compared with the prior art, the beneficial effects of the present invention are:

[0033] (1) The present invention uses a supported carbon material as the dechlorination agent to convert HCl and alkyl chloroformate chlorides in the product mixed gas into alkyl chloride through a gas-solid catalytic reaction, which can be economically used in industry to remove methyl chloroformate and hydrogen chloride corrosion products in the above-mentioned production process of alkyl carbonate, and avoid the corrosion of downstream equipment.

[0034] (2) For the catalyst for hydroformylation to prepare dimethyl carbonate, supplementing the HCl chlorine supplement agent is beneficial to maintaining the long-term stability of the catalyst. The present invention realizes the chlorine circulation process in the process of gas-phase carbonylation to prepare alkyl carbonate through steps such as absorption to control the water and methanol content in the gas-phase circulation system, dechlorination conversion reaction, and gas-liquid separation, realizes the stability of the process during long-term operation, and avoids the frequency of plant shutdown for maintenance. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic diagram of an embodiment of the chlorine circulation process in the production process of dimethyl carbonate;

[0036] Figure 2 It is a schematic diagram of the on-line test of the water content of the recycled raw material gas in the chlorine circulation process during the production process of dimethyl carbonate;

[0037] Figure 3 It is a schematic diagram of the production process of dimethyl carbonate without a dechlorination step;

[0038] In the figure: 1 is a carbonylation reactor, 2 is a preheater, 3 is a compressor, 4 is a recycle raw gas scrubbing and absorption tower, 5 is a methanol dehydration tower, 6 is an absorption tower, 7 is an absorbent recovery tower, 8 is a product purification tower, 9 is a chloride neutralization kettle, and 10 is a dechlorination reactor.

[0039] S1 is the CO gas raw material, S2 is the chlorine supplement agent raw material, S3 is the dehydrated raw gas, S4 is the carbonylation reaction raw gas containing CO and alkyl nitrite, S5 is the mixed gas containing dimethyl carbonate, HCl, and alkyl chloroformate, S19 is the mixed gas containing alkyl chloride, S20 is the oxygen raw material, S6 is the overhead gas of the recycle raw gas scrubbing and absorption tower, S7 is the bottom liquid phase of the recycle raw gas scrubbing and absorption tower containing methanol, water, etc., S8 is the fresh methanol raw material, S9 is the recycled methanol recovered from the top of the methanol dehydration tower, S10 is the wastewater at the bottom of the methanol dehydration tower, S11 is the gas phase at the top of the absorption tower, S12 is the mixed liquid containing alkyl carbonate and ester absorbent, S13 is the crude reaction liquid of alkyl carbonate, S14 is the ester absorbent, S15 is the light component liquid containing alkyl chloroformate, S16 is the alkyl carbonate product, S17 is the raw material of the chloride neutralization kettle, and S18 is the waste liquid. Specific embodiments

[0040] The present invention will be further described below with reference to embodiments. However, the present invention is not limited to the listed embodiments, and should also include any other known changes within the scope of the rights required by the present invention. The analysis methods and analysis devices involved in the embodiments are as described below.

[0041] The content analysis of organic compound substances (including carbonates, methanol, nitrites, oxalates, chloroformates, etc.) in the gas phase and liquid phase is carried out using a gas chromatograph 8090 manufactured by Agilent Technologies, equipped with a capillary column of HP-VOC manufactured by Agilent Technologies. The determination of Cl content is carried out by fluorescence method.

[0042] Example 1

[0043] Figure 1 It is a schematic diagram of an embodiment of the chlorine circulation process in the production process of dimethyl carbonate.

[0044] 1) Carbonylation reaction: Recycle gas (10.0% (volume) carbon monoxide, 15.0% (volume) methyl nitrite, 4.0% (volume) nitric oxide, 5.0% (volume) methanol, 1.7% (volume) carbon dioxide, 63.1% (volume) nitrogen, 0.02% (volume) hydrogen chloride, 0.18% (volume) methyl chloride)

[0045] 6000 Nm3 / h, preheat it using a preheater, and after preheating to a temperature of 100 °C, add it to a shell-and-tube hydroformylation reactor. The shell-and-tube hydroformylation reactor consists of 5000 tubes with a diameter of 30.5 mm and a length of 8000 mm. In the upper two-thirds of the length of the reaction tubes, a hydroformylation palladium catalyst (Pd loading is 0.5 wt%, with alumina as the carrier) is filled. Under the conditions of a reaction temperature of 115 °C and a reaction pressure of 0.5 MPaG, a hydroformylation reaction occurs to produce dimethyl carbonate. The product is a mixed gas containing dimethyl carbonate, HCl, and alkyl chloroformate, where the HCl content is 10 ppm and the alkyl chloroformate content is 100 ppm.

[0046] 2) Dechlorination reaction: Fill a dechlorination reactor with a dechlorination agent. The dechlorination agent uses coconut shell activated carbon as the carrier, and based on the total weight of the dechlorination agent, the Zn loading is 0.1 wt%. HCl and methyl chloroformate in the mixed gas containing dimethyl carbonate, HCl, and alkyl chloroformate react on the above dechlorination agent to produce methyl chloride, obtaining a mixed gas containing alkyl chlorides (4.0% (volume) carbon monoxide, 3.0% (volume) methyl nitrite, 16.0% (volume) nitric oxide, 5.2% (volume) methanol, 1.7% (volume) carbon dioxide, 63.9% (volume) nitrogen, 0.1999% (volume) methyl chloride, 6% (volume) dimethyl carbonate).

[0047] 3) Gas-liquid separation: The mixed gas containing alkyl chlorides is added to the lower part of an absorption tower with an inner diameter of 1600 mm, a height of 8000 mm, and filled with Pall rings under the condition of heat tracing in the form of a gas. Dimethyl oxalate is added from the upper part of the absorption tower through a pump at a rate of 1500 kg / h. The liquid component flow rate at the bottom of the absorption tower is 3392 kg / h, which is a condensate liquid fraction consisting of 44.6 wt% dimethyl oxalate, 42.6 wt% dimethyl carbonate, and 3.0 wt% methanol. The gas phase at the top of the absorption tower is discharged at a flow rate of 5328 Nm 3 / h, and its composition (by volume) includes an uncondensed gas fraction of 4.5% carbon monoxide, 3.4% methyl nitrite, 18.0% nitric oxide, 4.5% methanol, 1.9% carbon dioxide, 0.22% methyl chloride, and 67.7% nitrogen.

[0048] 4) Recycle gas scrubbing: The gas phase at the top of the above absorption tower is mixed with oxygen and then added to the bottom of a recycle feed gas scrubbing and absorption tower with a diameter of 6300 mm and a height of 10000 mm filled with 350Y packing. Methanol is added to the top of the tower at a flow rate of 26000 kg / h for countercurrent contact. A methanol solution containing 36 wt% water is discharged from the bottom of the recycle feed gas scrubbing and absorption tower. This solution is fed into a methanol dehydration tower, and dehydrated methanol obtained at the top can be used as one of the methanol raw materials for the top of the recycle gas scrubbing tower. The gas phase at the top of the recycle feed gas scrubbing and absorption tower is discharged at 5 Nm3 The flow rate of the vent gas discharged is [X] m³ / h, the water content in the gas phase is 80 ppm, and the methanol content is 5 wt%.

[0049] Example 2

[0050] 1) Carbonylation reaction: The recycle gas (8.8% (vol) carbon monoxide, 7.6% (vol) methyl nitrite, 4.4% (vol) nitric oxide, 2.0% (vol) methanol, 1.78% (vol) carbon dioxide, 75.2% (vol) nitrogen, 0.01% (vol) hydrogen chloride, 0.19% (vol) methyl chloride) has a volumetric flow rate of 3800 Nm 3 / h, and is preheated using a preheater to a temperature of 100 °C and then added to a shell-and-tube carbonylation reactor. The carbonylation reactor consists of 5000 tubes with a diameter of 32 mm and a length of 9000 mm. The same catalyst as in Example 1 is used for carbonylation. The reaction tubes are filled with a carbonylation palladium catalyst and an inert alumina ceramic ball diluent (the mass ratio of the carbonylation palladium catalyst to the inert alumina ceramic ball diluent is 1.6:1, and they are filled after being mixed evenly). The carbonylation reaction occurs at a reaction temperature of 118 °C and a reaction pressure of 0.4 MPaG to produce dimethyl carbonate. The product is a mixed gas containing dimethyl carbonate, HCl, and alkyl chloroformate, where the HCl content is 5 ppm and the alkyl chloroformate content is 60 ppm.

[0051] 2) Dechlorination reaction: A dechlorination agent is filled in the dechlorination reactor, and the filling amount of the dechlorination agent is 5.5 m 3 . The dechlorination agent uses coal-based activated carbon (specific surface area of 1503 m 2 / g) as the carrier, and the Zn loading amount is 5.0% based on the total weight of the dechlorination agent. A mixed gas containing alkyl chlorides is obtained (6.5% (vol) carbon monoxide, 3.0% (vol) methyl nitrite, 9.0% (vol) nitric oxide, 2.0% (vol) methanol, 1.78% (vol) carbon dioxide, 73.72% (vol) nitrogen, 0.20% (vol) methyl chloride, 3.8% (vol) dimethyl carbonate).

[0052] 3) Gas-liquid separation: The mixed gas containing alkyl chlorides is added in gas form to the lower part of an absorption tower with an inner diameter of 1600 mm, a height of 8000 mm, and filled with Pall rings at a condition of steam tracing at 115 °C. Dimethyl oxalate is added from the upper part of the absorption tower through a pump at a rate of 1730 kg / h. The liquid component flow rate at the bottom of the absorption tower is 2892 kg / h, which is a condensate liquid fraction consisting of 60.3 wt% dimethyl oxalate, 38.6 wt% dimethyl carbonate, and 1.0 wt% methanol. The gas phase at the top of the absorption tower has a flow rate of 3226 Nm 3The uncondensed gas fraction discharged from / h consists of 6.9% carbon monoxide, 3.3% methyl nitrite, 9.8% nitric oxide, 2.2% methanol, 1.9% carbon dioxide, 0.22% methyl chloride and 75.68% nitrogen by volume.

[0053] 4) Recycle gas scrubbing: The gas phase at the top of the above absorption tower is mixed with oxygen and then added to the bottom of the recycle raw gas scrubbing and absorption tower with a diameter of 6300 mm and a height of 7500 mm filled with 1000Y packing. Methanol is added to the top of the tower at a flow rate of 19000 kg / h for countercurrent contact. A methanol solution containing 32 wt% water is discharged from the bottom of the recycle raw gas scrubbing and absorption tower. This solution is fed into the methanol dehydration tower, and dehydrated methanol obtained at the top of the tower can be used as one of the methanol raw materials for the top of the recycle gas scrubbing tower. The recycle raw gas scrubbing and absorption tower discharges purge gas at a flow rate of 30 Nm 3 / h. The water content in the gas phase is 65 ppm and the methanol content is 2 wt%.

[0054] Example 3

[0055] 1) Carbonylation reaction: Recycle gas (11.3% (by volume) carbon monoxide, 14.6% (by volume) methyl nitrite, 3.5% (by volume) nitric oxide, 2.1% (by volume) methanol, 1.6% (by volume) carbon dioxide, 66.6% (by volume) nitrogen, 0.015% (by volume) hydrogen chloride, 0.23% (by volume) methyl chloride), with a volumetric flow rate of 5500 Nm 3 / h, is preheated using a preheater to a temperature of 100 °C and then added to a shell-and-tube carbonylation reactor. The carbonylation reactor consists of 5000 tubes with a diameter of 32 mm and a length of 9000 mm. The carbonylation catalyst uses silica as the carrier and the Pd loading is 2.0 wt%. The reaction tubes are all filled with carbonylation palladium catalyst and inert alumina ceramic ball packing (the mass ratio of carbonylation palladium catalyst to inert alumina ceramic ball diluent is 3.8:1, and they are mixed evenly before filling). The carbonylation reaction occurs at a reaction temperature of 123 °C and a reaction pressure of 0.6 MPaG to produce dimethyl carbonate. The product is a mixed gas containing dimethyl carbonate, HCl, and alkyl chloroformate, with the HCl content being 60 ppm and the alkyl chloroformate content being 50 ppm.

[0056] 2) Dechlorination reaction: A dechlorinating agent is filled in the dechlorination reactor, and the filling amount of the dechlorinating agent is 2.5 m 3 . The dechlorinating agent uses wood-based activated carbon (specific surface area of 1301 m 2 / g) as the carrier, based on the total weight of the dechlorinating agent, the Cu loading is 9.1%. A mixed gas containing alkyl chlorides is obtained (5.9% (volume) carbon monoxide, 3.9% (volume) methyl nitrite, 14.2% (volume) nitric oxide, 2.1% (volume) methanol, 1.6% (volume) carbon dioxide, 66.4% (volume) nitrogen, 0.25% (volume) methyl chloride, 5.6% (volume) dimethyl carbonate).

[0057] 3) Gas-liquid separation: The mixed gas containing alkyl chlorides is added in gaseous form to the lower part of an absorption tower with an inner diameter of 4000 mm, a height of 5800 mm, and filled with Pall rings at a temperature of 115 °C with steam tracing. Dimethyl oxalate is added from the upper part of the absorption tower through a pump at a rate of 1820 kg / h. The liquid composition flow rate at the bottom of the absorption tower is 2800 kg / h, which is a condensate liquid fraction consisting of 68.3 wt% dimethyl oxalate, 30.5 wt% dimethyl carbonate, and 0.5 wt% methanol. The gas phase at the top of the absorption tower is discharged at a flow rate of 3328 Nm 3 / h, and its composition (by volume) includes a non-condensed gas fraction of 6.2% carbon monoxide, 4.1% methyl nitrite, 15.1% nitric oxide, 2.2% methanol, 1.7% carbon dioxide, 0.22% methyl chloride, and 75.7% nitrogen.

[0058] 4) Recycle gas scrubbing: The gas phase at the top of the above absorption tower is mixed with oxygen and then added to the bottom of a recycle feed gas scrubbing and absorption tower with a diameter of 3000 mm and a height of 3800 mm filled with 1000Y packing. Methanol is added to the top of the tower at a flow rate of 24700 kg / h for countercurrent contact. A methanol solution containing 60 wt% water is discharged from the bottom of the recycle feed gas scrubbing and absorption tower. This solution is fed into a methanol dehydration tower, and the dehydrated methanol obtained at the top of the tower can be used as one of the methanol raw materials for the top of the recycle gas scrubbing tower. The water content of the dehydrated methanol is 300 ppm, and the water content of fresh methanol is 70 ppm. The mass flow rates of the dehydrated methanol and fresh methanol fed are controlled to be 1:1.8 based on the online moisture analysis test results. The recycle feed gas scrubbing and absorption tower discharges purge gas at a flow rate of 15 Nm 3 / h, and the water content in the gas phase is 150 ppm, and the methanol content is 0.5 wt%. When the water content in the recycle feed gas increases by 5 ppm each time, the water content in the material after mixing fresh methanol S8 and recycled methanol S9 is controlled to decrease by 50 ppm by increasing the feed ratio of fresh methanol S8. When the water content in the recycle feed gas increases to 155 ppm, the mass flow rate ratio of the dehydrated methanol to fresh methanol fed is adjusted to 1:6.8.

[0059] Comparative Example 1

[0060] The method of Example 1 was used to produce dimethyl carbonate, except that there was no dechlorination step and dechlorination reaction.Figure 3 It is a schematic diagram of the process.

[0061] The oxo synthesis product is a mixed gas containing dimethyl carbonate, HCl, and alkyl chloroformate. The HCl content is 10 ppm, and the methyl chloroformate content is 100 ppm. The gas-phase composition (by volume) of the absorption tower includes 4.5% carbon monoxide, 3.4% methyl nitrite, 18.0% nitric oxide, 4.5% methanol, 1.9% carbon dioxide, 71.9% nitrogen, and a 9 ppm HCl uncondensed gas fraction. The methyl chloroformate content in the liquid phase at the bottom of the absorption tower is 870 ppm. To ensure that the Cl content in the product meets the standard, a dechlorination device of a chloride neutralization kettle is added after the product refining tower.

Claims

1. A method for synthesizing alkyl carbonate by gas-phase carbonylation, comprising the following steps: (1) Using an alkyl alcohol as an absorbent, washing and removing the moisture in the recycled feed gas containing alkyl chloride in a recycled feed gas scrubbing absorption tower; (2) After mixing the dehydrated feed gas obtained in step (1) with CO feed and fresh chlorine supplement agent HCl, introducing them into a hydroformylation reactor to carry out a gas-solid phase catalytic reaction to generate a mixed gas containing dimethyl carbonate, HCl, and alkyl chloroformate; (3) Making the mixed gas obtained in step (2) contact with a supported dechlorinating agent, and under the action of the dechlorinating agent, HCl and alkyl chloroformate are converted into alkyl chloride; (4) Introducing the mixed gas containing alkyl chloride obtained in step (3) into an absorption tower to contact with an ester absorbent. After gas-liquid separation in the absorption tower, the gas phase at the top of the absorption tower is discharged from the upper part and enters the recycled feed gas scrubbing absorption tower, and the mixed liquid containing alkyl carbonate and the ester absorbent is taken out from the bottom of the absorption tower; (5) A part of the gas phase at the top of the recycled feed gas scrubbing absorption tower is discharged in the form of purge gas, and the other part is recycled as the dehydrated feed gas to step (2).

2. The method according to claim 1, characterized in that The alkyl alcohol in step 1) is one or more selected from methanol and ethanol. By mass, the mass ratio of the amount of alkyl alcohol used to the mass of the recycled feed gas is 4-10.

3. The method according to claim 1, characterized in that The water content in the dehydrated feed gas obtained in step 1) is 10-500 ppm, preferably the water content in the dehydrated feed gas is 10-150 ppm.

4. The method according to claim 1, characterized in that, In step 2), the dehydrated feed gas and HCl undergo a gas-solid phase catalytic reaction under the action of a supported Pd-based catalyst to generate a mixed gas containing alkyl carbonate, HCl, and alkyl chloroformate, wherein the HCl content is 1-100 ppm and the alkyl chloroformate content is 10-100 ppm.

5. The method according to claim 1, wherein The supported dechlorinating agent in step 3), the carrier includes one or several of carbon nanotubes, activated carbon, and carbon fiber. Preferably, the carrier is activated carbon, including one or several of coconut shell charcoal, wood charcoal, and coal-based charcoal; the active component contains divalent metal. Preferably, the divalent metal active component is Zn and / or Cu. By the total mass of the dechlorinating agent, preferably the supported metal content is 0.1 wt% - 40 wt%, and more preferably the supported metal content is 0.1 wt% - 10 wt%.

6. The method according to claim 1, wherein In step 4), the ester absorbent is alkyl oxalate, including dimethyl oxalate and / or diethyl oxalate. By mass, the mass ratio of the ester absorbent to dimethyl carbonate in the mixed gas is 1.0:1 - 3:1, and more preferably the mass ratio is 1.0:1 - 2:

1.

7. The method according to claim 1, wherein In terms of volume flowmeter, the purge gas in step 5) accounts for 0.05% - 1% of the recycle gas, and more preferably the purge gas accounts for 0.05% - 0.5% of the recycle gas.

8. The method according to claim 1, wherein In step (2), the hydroformylation reactor is filled with a supported Pd-based catalyst.

9. The method according to claim 1, characterized in that The feeding temperature of the alkyl alcohol is lower than the temperature of the recycled feed gas. Preferably, the feeding temperature of the alkyl alcohol is 5-30 °C, and more preferably the feeding temperature is 10-20 °C.

10. The method according to claim 1, wherein The temperature of the gas-solid phase catalytic reaction is 80-130 °C, and more preferably the reaction temperature is 100-130 °C.

Images