Method for preparing dimethyl ketene by cracking isobutyric anhydride

Abstract

The invention provides a method for preparing dimethyl ketene by cracking isobutyric anhydride, which comprises the following steps of: cracking isobutyric anhydride serving as a raw material at high temperature to generate a cracking product containing dimethyl ketene and isobutyric acid, and then condensing and carrying out gas-liquid separation to obtain the dimethyl ketene. The content of the dimethyl ketene-beta-lactone dimer in a liquid phase product output by gas-liquid separation is controlled to be 5 to 5000 ppm, preferably 50 to 1000 ppm, and more preferably 100 to 500 ppm. By controlling the content of key impurities in the cracking product, the yield of the dimethyl ketene is higher, the coking condition of the condenser is obviously relieved, the operation cycle can be prolonged, and the production cost is reduced.

Description

Technical Field

[0001] This invention belongs to the field of chemical synthesis technology, specifically relating to a method for preparing dimethyl ketone by cracking isobutyric anhydride. Background Technology

[0002] Dimethyl ketone (DMK) is an important chemical raw material and a crucial intermediate in the preparation of 2,2,4,4-tetramethyl-1,3-cyclobutanediol (CBDO). CBDO is a key monomer for producing high-performance polyester PCCT. Adding CBDO to traditional polyesters can significantly improve their glass transition temperature, weather resistance, transparency, and impact strength, thus making it widely used in bottle flakes, high-grade films, sheets, molded products, coatings, and other fields.

[0003] Currently, the industrial production of dimethyl ketene (DMK) mainly utilizes the thermal cracking of isobutyric anhydride, which yields DMK at 400-500℃ and 10-20 kPaA. Isobutyric anhydride cracking is a second-level reversible reaction, with reported conversion rates of up to 80%. However, the cracking products contain a large amount of unreacted isobutyric anhydride, requiring separation and recycling. Furthermore, DMK is highly reactive and prone to polymerization and decarbonization reactions. Polymerization impurities can coke during condensation, causing blockages in the condenser tubes. Simultaneously, these polymerization impurities are difficult to separate from isobutyric anhydride and can enter the cracking reactor during recycling, further contributing to coking. Therefore, improving the condensation and separation efficiency of DMK is crucial for increasing its yield and mitigating condenser coking.

[0004] Patent CN 112174797 also discloses a method for separating dimethyl ketene. This patent uses isobutyric anhydride as a thermal cracking feedstock and adds inert gas at the reactor outlet to cool the cracked gas, achieving a dimethyl ketene yield of 20%. However, using inert gas for cooling reduces the partial pressure of the cracked gas, resulting in poor condensation. At the same time, a large amount of inert gas causes isobutyric acid entrainment, leading to a reverse reaction between isobutyric acid and dimethyl ketene in the gas phase, which affects the dimethyl ketene yield.

[0005] In summary, there is a need to find an efficient preparation method to solve the problems of condenser coking and low yield of dimethyl ketene in the existing technology. Summary of the Invention

[0006] To address the aforementioned problems in the prior art, the present invention aims to provide a method for preparing dimethyl ketene by cracking isobutyric anhydride, which can ensure product yield while reducing coking in the condenser.

[0007] The current method for producing dimethyl ketene by cracking isobutyric anhydride involves a high-temperature cracking reaction of isobutyric anhydride. During the condensation process, the cracking products undergo a reverse cracking reaction, as well as side reactions such as polymerization and decarbonization. The inventors of this application have discovered that a four-membered cyclic impurity A generated during the reaction is one of the important factors leading to coking in the condenser. This four-membered cyclic impurity is a dimethyl ketene-β-lactone dimer (CAS: 3173-79-3), with the structure shown in formula (1):

[0008] The impurity originates from the high-temperature polymerization of dimethyl ketene and is difficult to separate from isobutyric anhydride in subsequent separation processes. Its presence induces coking in the condenser and, by accumulating in the recycled isobutyric anhydride, affects the cracking reaction, leading to decreased selectivity. The study also found that the content of dimethyl ketene-β-lactone dimer needs to be controlled within a certain range; below a certain level, coking in the condenser is exacerbated. The presence of a certain amount of dimethyl ketene-β-lactone dimer in the reaction system can form complexes with nickel and iron ions in the condenser material, altering the surface properties of the metal and rendering it inactive in catalyzing coking, thus mitigating condenser coking.

[0009] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows:

[0010] A method for preparing dimethyl ketene by cleavage of isobutyric anhydride, the method comprising the following steps:

[0011] Isobutyric anhydride is used as a raw material and is cracked at high temperature to generate cracking products containing dimethyl ketene and isobutyric acid. Then, the products are condensed and separated by gas-liquid separation. The content of dimethyl ketene-β-lactone dimer in the liquid phase product output by gas-liquid separation is controlled to be 5-5000 ppm, preferably 50-1000 ppm, and more preferably 100-500 ppm.

[0012] Specifically, the content of impurity A, represented by dimethyl ketene-β-lactone dimer (1), in the liquid phase product of the gas-liquid separation output is 5-5000 ppm, including but not limited to 5 ppm, 10 ppm, 50 ppm, 100 ppm, 500 ppm, 1000 ppm, 2000 ppm, 3000 ppm, 4000 ppm, 5000 ppm, or any combination thereof.

[0013] Since the dimethyl ketene-β-lactone dimer impurity is generated by the polymerization of dimethyl ketene at high temperatures during the condensation process of the pyrolysis products, the relevant operations and process conditions during the condensation process of the pyrolysis products are particularly important for the formation of this impurity. Through experiments, the inventors discovered that multi-stage condensation (preferably two-stage) combined with low-temperature solvent spraying can control the amount of dimethyl ketene-β-lactone dimer impurity, achieving the goal of controlling the content of dimethyl ketene-β-lactone dimer impurity in the liquid phase product output from gas-liquid separation to 5-5000 ppm, effectively solving the coking problem in the condenser.

[0014] In one embodiment, the condensation employs a multi-stage condenser combined with a low-temperature solvent spray condensation method.

[0015] Optionally, the multi-stage condenser has 1 to 3 stages, preferably a 2-stage condenser.

[0016] Optionally, the low-temperature solvent is at least one of acid anhydride, organic acid, etc., preferably acid anhydride, more preferably at least one of isobutyric anhydride and acetic anhydride.

[0017] The condenser used in the condensation described in this invention has no special requirements and can be a conventional selection in the field. For example, the condenser can be a tube heat exchanger, a plate heat exchanger, a plate-fin heat exchanger, etc., with tube heat exchangers being preferred. The material of the condenser can be 316L, 317L, 321H, etc., with 316L being preferred.

[0018] In one specific example, the condensation is performed using the following method:

[0019] S1: The pyrolysis products are fed into the first-stage condenser, and the pyrolysis products are sprayed with a first low-temperature solvent. The temperature of the first low-temperature solvent is -20-40℃, and the outlet temperature of the first-stage condenser is controlled at 50-200℃ to obtain the first-stage condensed products.

[0020] S2: Input the primary condensate into the secondary condenser, and spray the primary condensate with a second low-temperature solvent. The temperature of the second low-temperature solvent is -20-40℃, and the outlet temperature of the secondary condenser is controlled to be -10-50℃ to obtain the secondary condensate.

[0021] Optionally, the first low-temperature solvent and the second low-temperature solvent are selected from one or more of acid anhydrides and organic acids, preferably one or more of isobutyric anhydride, acetic anhydride, and isobutyric acid, and more preferably isobutyric anhydride; the first low-temperature solvent and the second low-temperature solvent may be the same or different.

[0022] Optionally, in step S1, the feed mass ratio of the first low-temperature solvent to the pyrolysis product is 0.1-2:1, including but not limited to 0.1:1, 0.3:1, 0.5:1, 0.7:1, 1:1, 1.3:1, 1.5:1, 1.8:1, 2:1 or any combination thereof, preferably 0.1-1:1.

[0023] Optionally, in step S1, the temperature of the first low-temperature solvent is -20 to 40°C, including but not limited to -20°C, -10°C, 0°C, 10°C, 20°C, 30°C, 40°C, or any combination thereof, preferably -10 to 0°C.

[0024] Optionally, the residence time of the pyrolysis products in the primary condenser in step S1 is 0.1-5s, including but not limited to 0.1s, 0.5s, 1s, 2s, 3s, 4s, 5s or any combination thereof, preferably 0.1-1s.

[0025] Optionally, the outlet temperature of the first-stage condenser in step S1 is 50-200℃, including but not limited to 50℃, 80℃, 100℃, 130℃, 150℃, 180℃, 200℃ or any combination thereof, preferably 50-150℃.

[0026] Optionally, in step S2, the feed mass ratio of the second low-temperature solvent to the pyrolysis product is 0.1-1:1, including but not limited to 0.1:1, 0.3:1, 0.5:1, 0.7:1, 0.9:1, 1:1 or any combination thereof, preferably 0.1-0.5:1.

[0027] Optionally, in step S2, the temperature of the second low-temperature solvent is -20-40°C, including but not limited to -20°C, -10°C, 0°C, 10°C, 20°C, 30°C, 40°C or any combination thereof, preferably 0-20°C.

[0028] Optionally, the residence time of the pyrolysis products in the secondary condenser in step S2 is 0.1-5s, including but not limited to 0.1s, 0.5s, 1s, 2s, 3s, 4s, 5s or any combination thereof, preferably 0.1-2s.

[0029] Optionally, the outlet temperature of the secondary condenser in step S2 is -10-50℃, including but not limited to -10℃, 0℃, 10℃, 20℃, 30℃, 40℃, 50℃ or any combination thereof, preferably 0-20℃.

[0030] In the method described in this invention, after the above-mentioned condensation operation is completed, a gas-liquid separation operation is also included. Conventional operating methods in the field can be adopted. In practical applications, the following method can be used:

[0031] The condensate output from the condenser (such as the secondary condensate mentioned above) enters the gas-liquid separator to obtain liquid and gas products respectively.

[0032] The gas-liquid separator operates under the following conditions: temperature of 0-20℃, pressure of 10-30 kPaA, and residence time of 0.2-2 s.

[0033] The liquid product mainly comprises isobutyric acid, isobutyric anhydride, dimethyl ketone-β-lactone dimer 5-5000 ppm, and may also contain a low-temperature solvent introduced by the condensation process.

[0034] The gaseous product mainly comprises dimethyl ketone, approximately 97-98 wt%, and also contains small amounts of CO, CO2, etc.

[0035] The condensation process adopts the above method. The secondary condensation product obtained from the secondary condensation outlet in step S2 is separated into liquid phase by gas-liquid separation. The content of dimethyl ketene-β-lactone dimer in the liquid phase product can be controlled within the range of 5-5000 ppm.

[0036] In one embodiment, the process of using isobutyric anhydride as a raw material to pyrolyze at high temperature to generate pyrolysis products containing dimethyl ketene and isobutyric acid is a known conventional reaction type. The relevant operations and process conditions of this pyrolysis process, as well as the equipment used, can all be carried out using the corresponding conventional selections in the art. There are no particular restrictions on this. Those skilled in the art can optimize the process based on existing technology and known processes according to actual needs.

[0037] Optionally, the pyrolysis reaction is carried out at a temperature of 300-650℃, a pressure of 10-30 kPaA, and a residence time of 0.2-2 s.

[0038] In detail, the temperature of the pyrolysis reaction includes, but is not limited to, 300°C, 350°C, 400°C, 450°C, 500°C, 550°C, 600°C, 650°C or any combination thereof; the pressure includes, but is not limited to, 10 kPaA, 15 kPaA, 20 kPaA, 25 kPaA, 30 kPaA or any combination thereof; and the residence time includes, but is not limited to, 0.2 s, 0.5 s, 0.8 s, 1 s, 1.3 s, 1.5 s, 1.8 s, 2 s or any combination thereof.

[0039] Optionally, the pyrolysis reaction uses a conventional type of pyrolysis reactor, such as a tubular reactor.

[0040] In some specific examples, the pyrolysis products comprise 18-37 wt% dimethyl ketene, 23-40 wt% isobutyric acid, and 15-50 wt% isobutyric anhydride.

[0041] Optionally, the pyrolysis products output from the pyrolysis reactor are at a temperature of 300-650°C, including but not limited to a range of 300°C, 400°C, 500°C, 600°C, 650°C, or any combination thereof.

[0042] In the method described in this invention, after the above-mentioned gas-liquid separation process is completed, a solvent absorption operation is also included; this is a conventional post-processing method used in the field. In practical applications, the following method can be used:

[0043] The gaseous product from the gas-liquid separation is fed into an absorption tower, where dimethyl ketone is absorbed by a solvent to obtain a dimethyl ketone solution.

[0044] The operating conditions of the absorption tower include: temperature of 0-20℃, pressure of 10-100KPaG, and residence time of 2-10s.

[0045] In detail, the absorption operation temperature includes, but is not limited to, a range of 0°C, 3°C, 5°C, 7°C, 10°C, 12°C, 15°C, 18°C, 20°C, or any combination thereof; the pressure includes, but is not limited to, a range of 10 kPaG, 30 kPaG, 50 kPaG, 80 kPaG, 100 kPaG, or any combination thereof; and the residence time includes, but is not limited to, a range of 2 s, 4 s, 6 s, 8 s, 10 s, or any combination thereof.

[0046] The solvent is selected from one or more alkanes and carboxylic acid esters, preferably one or more C4-C10 carboxylic acid esters; for example, butyl isobutyrate, ethyl acetate, isobutyl hexanoate, decane, isobutyl isobutyrate, etc.

[0047] The mass ratio of the solvent feed rate to the gas phase product feed rate is 2-15:1, including but not limited to 2:1, 4:1, 6:1, 8:1, 10:1, 12:1, 14:1, 15:1 or any combination thereof, preferably 4-9:1.

[0048] The dimethyl ketene solution output from the absorption tower has a concentration of approximately 6-30 wt%. This solution can be used directly as a product to synthesize 2,2,4,4-tetramethylcyclobutanedione and other organic synthesis intermediates.

[0049] Dimethyl ketone can also be obtained by further processing methods such as distillation.

[0050] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0051] In the method of the present invention, the content of dimethyl ketene-β-lactone dimer, an impurity, in the liquid phase product output by gas-liquid separation is controlled to be between 5-5000 ppm, thereby reducing coking in the condenser.

[0052] The method of this invention employs multi-stage condensation combined with low-temperature solvent spraying during the condensation process, enabling rapid condensation and cooling of the pyrolysis gas. Rapid separation of isobutyric acid and dimethyl ketene suppresses the reverse reaction, while rapid cooling of dimethyl ketene inhibits its polymerization, controlling the content of the impurity dimethyl ketene-β-lactone dimer. Ultimately, this reduces the loss of dimethyl ketene during the separation process, improves selectivity, extends the operating cycle, and lowers production costs. Attached Figure Description

[0053] Figure 1 This is a gas phase spectrum of the liquid phase products output from the condensation process in Embodiment 1 of the present invention. Detailed Implementation

[0054] To facilitate understanding of the present invention, the following description, in conjunction with embodiments, will further illustrate the invention. It should be understood that the following embodiments are merely for a better understanding of the invention and do not imply that the invention is limited to these embodiments.

[0055] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items. Locational terms such as top and bottom, mentioned or possibly used in this specification, are defined relative to the constructions shown in the accompanying drawings; they are relative concepts and may therefore vary depending on their location and usage.

[0056] It should be noted that the endpoints and any values ​​of the ranges disclosed in this specification are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0057] In this invention, unless specific experimental steps or conditions are specified in the embodiments, all are performed according to conventional experimental procedures or conditions in the art. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagents or instruments.

[0058] The main sources of raw materials used in the various embodiments and comparative examples of this invention are as follows:

[0059] Isobutyric anhydride: 98.5 wt%, Aladdin;

[0060] Acetic anhydride: 99 wt%, Aladdin;

[0061] Isobutyric acid: 99.9 wt%, Aladdin.

[0062] The main test methods used in the various embodiments and comparative examples of this invention are as follows:

[0063] Gas chromatography analysis: Agilent Technologies, 8890 gas chromatograph, FID detector, PH-5 capillary column (25.0m x 0.53mm x 0.02mm), chromatographic analysis was performed using temperature programmed method: 120℃ held for 1 min, ramped up to 260℃ at 15℃ / min and held for 10 min, injection port 260℃, split ratio 50:1, detector FID, 260℃.

[0064] The pyrolysis products were analyzed by online chromatography to calculate the reaction outlet conversion and selectivity;

[0065] The liquid phase product from gas-liquid separation was analyzed to determine the content of the impurity dimethyl ketene-β-lactone dimer, and the condenser outlet conversion rate and selectivity were calculated.

[0066] The yield of dimethyl ketene (DMK) is calculated based on the mass balance of the absorbent in the absorption tower. The increase in the mass of the absorbent is the mass of DMK obtained from absorption. There is a 2μm filter at the outlet of the condenser. The filter is weighed before and after each experiment to calculate the amount of carbon deposits generated.

[0067] Description of the main equipment used in the various embodiments and comparative examples of this invention:

[0068] Cracking reactor: Yantai Keli Chemical Equipment Co., Ltd., tubular cracking reactor.

[0069] Primary / Secondary Condenser: Yantai Keli Chemical Equipment Co., Ltd., Tubular Heat Exchanger.

[0070] Example 1

[0071] The feed rate of isobutyric anhydride was adjusted to 5 g / min using a horizontal flow pump, and the temperature was raised to 200°C. The feed was then introduced into a pyrolysis reactor at 450°C for a residence time of 0.5 s and an outlet pressure of 10 kPa. The pyrolysis product, obtained at 450°C, consisted of 34.24 wt% dimethyl ketene, 43.94 wt% isobutyric acid, and 19.5 wt% isobutyric anhydride. The conversion rate was 80.5%, and the molar selectivity of DMK was 96%.

[0072] The pyrolysis products are fed into the primary condenser at a feed flow rate of 5 g / min. Isobutyric anhydride is sprayed onto the pyrolysis products at a flow rate of 2.5 g / min. The isobutyric anhydride temperature is 0℃ and the residence time is 1 s. The outlet temperature of the primary condenser is controlled at 50℃ to obtain the primary condensed products.

[0073] The primary condensate is fed into the secondary condenser, and isobutyric anhydride is sprayed onto the secondary condensate at a flow rate of 0.5 g / min, an isobutyric anhydride temperature of 0℃, a residence time of 1 s, and the outlet temperature of the secondary condenser is controlled at 20℃ to obtain the secondary condensate.

[0074] The secondary condensate output from the condensation process enters the gas-liquid separator, where the liquid phase product is collected and sampled for analysis. The contents of isobutyric acid are 35.45 wt%, isobutyric anhydride are 61.13 wt%, and dimethyl ketene-β-lactone dimer are 500 ppm.

[0075] The gaseous product was fed into the absorption tower, and butyl isobutyrate solvent was simultaneously introduced at a mass ratio of 1:4. The operating temperature was 20℃, the pressure was 10 kPaG, and the residence time was 10 s, yielding a 19.5 wt% dimethyl ketone solution. The molar yield of DMK was 76.54%, the DMK loss rate during condensation was 0.96%, the reaction was carried out continuously for 24 h, and the carbon deposit on the filter was 0.05 g.

[0076] Example 2

[0077] The feed rate of isobutyric anhydride was adjusted to 3.9 g / min using a horizontal flow pump, and the temperature was raised to 200°C. The resulting product was then fed into a pyrolysis reactor at 650°C for a residence time of 1 s and an outlet pressure of 20 kPa. The pyrolysis product, obtained at 650°C, consisted of 33.97 wt% dimethyl ketene, 43.66 wt% isobutyric acid, and 14.8 wt% isobutyric anhydride. The conversion rate was 85.2%, and the molar selectivity of DMK was 90%.

[0078] The pyrolysis products were fed into the primary condenser at a feed flow rate of 3.9 g / min. Isobutyric acid was sprayed onto the pyrolysis products at a flow rate of 0.39 g / min, an isobutyric acid temperature of -20℃, and a residence time of 5 s. The outlet temperature of the primary condenser was controlled at 150℃ to obtain the primary condensed products.

[0079] The primary condensate is fed into the secondary condenser, and isobutyric anhydride is sprayed onto the secondary condensate at a flow rate of 1.95 g / min, an isobutyric anhydride temperature of 10℃, a residence time of 2 s, and the outlet temperature of the secondary condenser is controlled at 50℃ to obtain the secondary condensate.

[0080] The secondary condensate output from the condensation process enters the gas-liquid separator, where the liquid phase product is collected and sampled for analysis. The contents of isobutyric acid are 42.84 wt%, isobutyric anhydride are 56.73 wt%, and dimethyl ketene-β-lactone dimer are 1000 ppm.

[0081] The gaseous product was fed into the absorption tower, and ethyl acetate solvent was simultaneously introduced at a mass ratio of 1:2. The operating temperature was 15℃, the pressure was 30 kPaG, and the residence time was 8 s, yielding a 30 wt% dimethyl ketone solution. The molar yield of DMK was 72.77%, the DMK loss rate during condensation was 5.1%, the reaction was carried out continuously for 24 h, and the carbon deposit on the filter was 0.79 g.

[0082] Example 3

[0083] The feed rate of isobutyric anhydride was adjusted to 5.4 g / min using a horizontal flow pump, and the temperature was raised to 200°C. The resulting product was then fed into a pyrolysis reactor at 400°C for pyrolysis. The residence time in the reactor was 1.5 s, and the outlet pressure was 30 kPa. The pyrolysis product, obtained at 400°C, consisted of 28.99 wt% dimethyl ketene, 37.21 wt% isobutyric acid, and 30.4 wt% isobutyric anhydride. The conversion rate was 69.6%, and the molar selectivity of DMK was 94%.

[0084] The pyrolysis products were fed into the primary condenser at a feed flow rate of 5.4 g / min. Acetic anhydride was used to spray the pyrolysis products at a flow rate of 5.4 g / min. The isobutyric anhydride temperature was -10℃ and the residence time was 0.6 s. The primary condenser outlet temperature was controlled at 50℃ to obtain the primary condensed products.

[0085] The primary condensate is fed into the secondary condenser, and acetic anhydride is sprayed onto the secondary condensate at a flow rate of 1.08 g / min, an isobutyric anhydride temperature of 40℃, a residence time of 5 s, and the outlet temperature of the secondary condenser is controlled at 10℃ to obtain the secondary condensate.

[0086] The secondary condensate output from the condensation process enters the gas-liquid separator, where the liquid phase product is collected and sampled for analysis. The contents of the liquid phase product are: isobutyric acid 19.78 wt%, isobutyric anhydride 16.16 wt%, acetic anhydride 63.8 wt%, and dimethyl ketene-β-lactone dimer 100 ppm.

[0087] The gaseous product was fed into the absorption tower, and isobutyl hexanoate solvent was simultaneously introduced at a mass ratio of 1:6. The operating temperature was 10℃, the pressure was 50 kPaG, and the residence time was 6 s, yielding a 13.6 wt% dimethyl ketene solution. The molar yield of DMK was 64.74%, the DMK loss rate during condensation was 1.05%, the reaction was carried out continuously for 24 h, and the carbon deposit on the filter was 0.02 g.

[0088] Example 4

[0089] The feed rate of isobutyric anhydride was adjusted to 2.2 g / min using a horizontal flow pump, and the temperature was raised to 200°C. The feed was then introduced into a pyrolysis reactor at 550°C for a residence time of 2 s and an outlet pressure of 20 kPa. The pyrolysis product, obtained at 550°C, consisted of 29.71 wt% dimethyl ketene, 38.17 wt% isobutyric acid, and 27.1 wt% isobutyric anhydride. The conversion rate was 72.9%, and the molar selectivity of DMK was 92%.

[0090] The pyrolysis products were fed into the primary condenser at a feed flow rate of 2.2 g / min. Acetic anhydride was used to spray the pyrolysis products at a flow rate of 4.4 g / min. The isobutyric anhydride temperature was 20°C and the residence time was 0.4 s. The primary condenser outlet temperature was controlled at 200°C to obtain the primary condensed products.

[0091] The primary condensate is fed into the secondary condenser, and acetic anhydride is sprayed onto the secondary condensate at a flow rate of 0.66 g / min, an isobutyric anhydride temperature of 30℃, a residence time of 0.5 s, and the outlet temperature of the secondary condenser is controlled at -10℃ to obtain the secondary condensate.

[0092] The secondary condensate output from the condensation process enters the gas-liquid separator, where the liquid phase product is collected and sampled for analysis. The contents of the liquid phase product are: isobutyric acid 12.91 wt%, isobutyric anhydride 76.78 wt%, acetic anhydride 10.14 wt%, and dimethyl ketene-β-lactone dimer 5000 ppm.

[0093] The gaseous product was fed into the absorption tower, and decane solvent was simultaneously introduced at a mass ratio of 1:9. The operating temperature was 5℃, the pressure was 70 kPaG, and the residence time was 4 s, yielding a 9.7 wt% dimethyl ketone solution. The molar yield of DMK was 66.49%, the DMK loss rate during condensation was 0.86%, the reaction was carried out continuously for 24 h, and the carbon deposit on the filter was 0.58 g.

[0094] Example 5

[0095] The feed rate of isobutyric anhydride was adjusted to 15 g / min using a horizontal flow pump, and the temperature was raised to 200°C. The feed was then introduced into a pyrolysis reactor at 300°C for a residence time of 0.2 s and an outlet pressure of 10 kPa. The pyrolysis product, obtained at 300°C, consisted of 21.53 wt% dimethyl ketene, 27.62 wt% isobutyric acid, and 49.9 wt% isobutyric anhydride. The conversion rate was 50.1%, and the molar selectivity of DMK was 97%.

[0096] The pyrolysis products are fed into the primary condenser at a feed flow rate of 15 g / min. Isobutyric anhydride is used to spray the pyrolysis products at a flow rate of 22.5 g / min, an isobutyric anhydride temperature of 40℃, and a residence time of 0.1 s. The primary condenser outlet temperature is controlled at 100℃ to obtain the primary condensed products.

[0097] The primary condensate is fed into the secondary condenser, and isobutyric anhydride is sprayed onto the secondary condensate at a flow rate of 1.5 g / min, an isobutyric anhydride temperature of -20℃, a residence time of 0.1 s, and the outlet temperature of the secondary condenser is controlled at 0℃ to obtain the secondary condensate.

[0098] The secondary condensate output from the condensation process enters the gas-liquid separator, where the liquid phase product is collected and sampled for analysis. The contents of isobutyric acid are 11.61 wt%, isobutyric anhydride are 88.22 wt%, and dimethyl ketene-β-lactone dimer are 5 ppm.

[0099] The gaseous product was fed into the absorption tower, and isobutyl isobutyrate solvent was simultaneously introduced at a mass ratio of 1:15. The operating temperature was 0℃, the pressure was 100 kPaG, and the residence time was 2 s, yielding a 6.1 wt% dimethyl ketone solution. The molar yield of DMK was 48.25%, the DMK loss rate during condensation was 0.7%, the reaction was carried out continuously for 24 h, and the carbon deposit on the filter was 0.19 g.

[0100] Comparative Example 1

[0101] The method is the same as in Example 1, except that the condensation process uses a large amount of non-low-temperature solvent spraying, resulting in the content of impurity A in the separation process being only 3 ppm.

[0102] The feed rate of isobutyric anhydride was adjusted to 5 g / min using a horizontal flow pump, and the temperature was raised to 200°C. The feed was then introduced into a pyrolysis reactor at 450°C for a residence time of 0.5 s and an outlet pressure of 10 kPa. The pyrolysis product, obtained at 450°C, consisted of 34.24 wt% dimethyl ketene, 43.94 wt% isobutyric acid, and 19.5 wt% isobutyric anhydride. The conversion rate was 80.5%, and the molar selectivity of DMK was 96%.

[0103] The pyrolysis products are fed into the primary condenser at a feed flow rate of 5 g / min. Isobutyric anhydride is used to spray the pyrolysis products at a flow rate of 25 g / min. The isobutyric anhydride temperature is 120℃ and the residence time is 0.1 s. The primary condenser outlet temperature is controlled at 240℃ to obtain the primary condensed products.

[0104] The primary condensate is fed into the secondary condenser, and isobutyric anhydride is sprayed onto the secondary condensate at a flow rate of 25 g / min, an isobutyric anhydride temperature of 120℃, a residence time of 0.1 s, and the outlet temperature of the secondary condenser is controlled at 70℃ to obtain the secondary condensate.

[0105] The secondary condensate output from the condensation process enters the gas-liquid separator, where the liquid phase product is collected and sampled for analysis. The contents of isobutyric acid are 2.91 wt%, isobutyric anhydride are 97.04 wt%, and dimethyl ketene-β-lactone dimer are 3 ppm.

[0106] Simultaneously, the gaseous product was fed into the absorption tower at an operating temperature of 20℃, a pressure of 10 kPaG, and a residence time of 10 s to obtain dimethyl ketene absorbent. The molar yield of DMK was 69.55%, the DMK loss rate during the condensation process was 10.06%, the reaction was carried out continuously for 24 hours, and the amount of carbon deposited on the filter was 10.3 g.

[0107] Comparative Example 2

[0108] Referring to the method in Example 1, the difference is that: the low-temperature solvent spray was not introduced during the condensation process, and natural cooling was used. Other operations remained unchanged. The product output from the condensation process entered the gas-liquid separator, and the liquid phase product was collected and sampled for analysis. The content of isobutyric acid was 54.76 wt%, the content of isobutyric anhydride was 43.85 wt%, and the content of dimethyl ketene-β-lactone dimer was 5500 ppm.

[0109] Simultaneously, after the gaseous product is fed into the absorption tower, the resulting dimethyl ketone absorbent is obtained. The molar yield of DMK is 71.48%, the DMK loss rate during the condensation process is 7.5%, the continuous reaction time is 24 hours, and the amount of carbon deposited on the filter is 15.3 g.

Claims

1. A method for preparing dimethyl ketene by cracking isobutyric anhydride, characterized in that the steps include... include: Isobutyric anhydride is used as a raw material and is cracked at high temperature to generate cracking products containing dimethyl ketene and isobutyric acid. Then, the products are condensed and separated by gas-liquid separation. The content of dimethyl ketene-β-lactone dimer in the liquid phase product output by gas-liquid separation is controlled to be 5-5000 ppm, preferably 50-1000 ppm, and more preferably 100-500 ppm.

2. The method according to claim 1, characterized in that, The condensation process employs a multi-stage condenser combined with a low-temperature solvent spray condensation method. Optionally, the multi-stage condenser has 1-3 stages, preferably a 2-stage condenser; Optionally, the low-temperature solvent is at least one of acid anhydride and organic acid, preferably acid anhydride, and more preferably at least one of isobutyric anhydride and acetic anhydride.

3. The method according to claim 1 or 2, characterized in that, The condensation method is as follows: S1: The pyrolysis products are fed into the first-stage condenser, and the pyrolysis products are sprayed with a first low-temperature solvent. The temperature of the first low-temperature solvent is -20-40℃, and the outlet temperature of the first-stage condenser is controlled at 50-200℃ to obtain the first-stage condensed products. S2: Input the primary condensate into the secondary condenser, and spray the primary condensate with a second low-temperature solvent. The temperature of the second low-temperature solvent is -20-40℃, and the outlet temperature of the secondary condenser is controlled to be -10-50℃ to obtain the secondary condensate.

4. The method according to claim 3, characterized in that, The first low-temperature solvent and the second low-temperature solvent are selected from one or more of acid anhydrides and organic acids, preferably one or more of isobutyric anhydride, acetic anhydride, and isobutyric acid, and more preferably isobutyric anhydride; the first low-temperature solvent and the second low-temperature solvent may be the same or different.

5. The method according to claim 3 or 4, characterized in that, In step S1, the feed mass ratio of the first low-temperature solvent to the pyrolysis products is 0.1-2:1, preferably 0.1-1:1; and / or, Step S1: The temperature of the first cryogenic solvent is -10 to 0°C; and / or, The residence time of the pyrolysis products in the primary condenser in step S1 is 0.1-5 s, preferably 0.1-1 s; and / or, The outlet temperature of the first-stage condenser in step S1 is 50-150℃.

6. The method according to any one of claims 3-5, characterized in that, In step S2, the feed mass ratio of the second low-temperature solvent to the pyrolysis products is 0.1-1:1, preferably 0.1-0.5:1; and / or, Step S2: The temperature of the second low-temperature solvent is 0-20°C; and / or, The residence time of the pyrolysis products in the secondary condenser in step S2 is 0.1-5 s, preferably 0.1-2 s; and / or, The outlet temperature of the secondary condenser in step S2 is 0-20℃.

7. The method according to any one of claims 1-6, characterized in that, The method used for gas-liquid separation is as follows: The condensate output from the condenser enters the gas-liquid separator, where liquid and gaseous products are obtained respectively.

8. The method according to claim 7, characterized in that, The gas-liquid separator operates under the following conditions: temperature 0-20℃, pressure 10-30 kPaA, and residence time 0.2-2 s.

9. The method according to any one of claims 1-8, characterized in that, The pyrolysis reaction is carried out at a temperature of 300-650℃, a pressure of 10-30 kPaA, and a residence time of 0.2-2 s. Optionally, the pyrolysis products output from the pyrolysis reactor are at a temperature of 300-650°C.

10. The method according to any one of claims 1-9, characterized in that, It also includes a solvent absorption operation, using the following method: The gaseous product from the gas-liquid separation is fed into an absorption tower, where dimethyl ketene is absorbed by a solvent to obtain a dimethyl ketene solution. Optionally, the operating conditions of the absorption tower include: temperature of 0-20℃, pressure of 10-100KPaG, and residence time of 2-10s; Optionally, the solvent is selected from one or more alkanes and carboxylic acid esters, preferably one or more C4-C10 carboxylic acid esters; Optionally, the mass ratio of the solvent feed rate to the gaseous phase product feed rate is 2-15:1, preferably 4-9:1.

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