Separation method and separation system of propylene epoxidation product

Abstract

The invention discloses a separation method and a separation system for propylene epoxidation products. The separation method comprises the following steps: carrying out first rectification on a propylene epoxidation product to remove propylene; carrying out second rectification on the material flow after propylene is removed, carrying out reaction on the obtained material flow containing formaldehyde and epoxypropane in the presence of a catalyst to generate 4-methyl-1, 3-dioxolane, and returning the material flow after reaction to the second rectification step to obtain 4-methyl-1, 3-dioxolane. And carrying out third rectification on the material flow containing the epoxypropane, the cumene and the 2-phenyl-2-propanol obtained by the second rectification, cleaning the obtained material flow containing the epoxypropane and the cumene, returning to the second rectification step, and finally obtaining a crude epoxypropane material flow from the second rectification step. According to the method, the concentration of formaldehyde in the crude epoxypropane material flow can be effectively reduced, the risk of formaldehyde blocking in a subsequent refining unit is relieved, and the content of formaldehyde in the crude epoxypropane material flow does not exceed 200mg / kg.

Description

Technical Field

[0001] The present invention relates to the field of propylene epoxidation. Specifically, it relates to a method for separating propylene epoxidation products that can reduce the formaldehyde content and a separation system for propylene epoxidation products, and is particularly applicable to propylene epoxidation products obtained by a co-oxidation process using cumene or ethylbenzene as a solvent. Background Art

[0002] In recent years, the co-oxidation method has become the mainstream technical choice for industrial production of propylene oxide. The co-oxidation method uses an organic peroxide as an oxidant to epoxidize an olefin to prepare the corresponding epoxide, and the organic peroxide is converted into the corresponding alcohol. A typical co-oxidation method such as the PO / SM process using ethylbenzene hydroperoxide can co-produce propylene oxide and styrene.

[0003] During the propylene oxidation process, the formation of aldehyde impurities, especially small-molecule aldehydes, is a common phenomenon. Propylene oxide products usually have strict requirements for aldehyde impurities. Therefore, a variety of technologies for refining and removing aldehydes from propylene oxide have been disclosed in the art. In the prior art, the removal of aldehydes from propylene oxide mainly targets acetaldehyde and propionaldehyde because these two aldehyde impurities are relatively more due to the process characteristics of the co-oxidation method. Acetaldehyde mainly comes from the decomposition of ethylbenzene hydroperoxide, while propionaldehyde mainly comes from the isomerization of propylene oxide. Currently, the extraction distillation method is widely adopted in industry. Through the extraction effect of solvents such as ethylbenzene and octane, the relative volatility difference between aldehydes and propylene oxide is increased, so as to effectively separate aldehydes from propylene oxide to obtain a high-purity product.

[0004] There is also a modified co-oxidation method that uses cumene hydroperoxide as an oxidant. This process can hydrogenolyze and recycle the co-produced alcohols, so there are no co-products other than propylene oxide. In addition, cumene hydroperoxide as an oxidant has the characteristics of being relatively stable and safe, and also has certain advantages in reaction selectivity.

[0005] In the operation practice of industrial plants, especially cumene hydroperoxide process plants, it has been found that different from other co-oxidation process plants, the content of formaldehyde impurities in the oxidation reaction products using cumene hydroperoxide has increased significantly and even exceeded the content of acetaldehyde and propionaldehyde. According to the prior art, after separating the crude propylene oxide stream from the epoxidation products, the formaldehyde content therein will reach 500 - 2000 mg / kg. When such crude propylene oxide is sent to the refining unit, although formaldehyde can still be completely removed from the product, blockages frequently occur at some process pipelines, seriously affecting the stable operation of the plant. Through systematic analysis, the blockage substances are attributed to polyoxymethylene, and it can be further explained that the formaldehyde impurities in the epoxidation reaction products gradually accumulate to reach the polymerization concentration limit during the separation and refining processes, resulting in local blockages.

[0006] To avoid the problem of formaldehyde polymerization, it is hoped that the content of formaldehyde in the crude propylene oxide stream separated from the epoxidation product and entering the refining unit can be reduced.

[0007] Patent CN113429368A describes that when separating crude propylene oxide from the product of ethylbenzene peroxidation, the side stream of the distillation column, the stream rich in aldehydes, acids, and water is treated by a combination of alkali washing and extraction, which can effectively reduce the impurities in the crude propylene oxide. This method can be used for reference to reduce the formaldehyde in the crude propylene oxide. However, when separating the crude propylene oxide, especially for the oxidation product using cumene hydroperoxide, since the water content is low, formaldehyde is concentrated at the top of the column and mixed with propylene oxide. When this stream is drawn out for alkali washing, even with an extraction and recovery link, the hydrolysis loss of propylene oxide cannot be underestimated.

[0008] Patent CN107686469A describes that by using one or more reaction entrainers selected from aniline, phenylhydrazine, semicarbazide, etc., aldehydes can be converted into high-boiling substances such as imines and hydrazones, and 99% of the formaldehyde in the crude propylene oxide can be removed by reactive distillation. This type of method is suitable for the refining process of crude propylene oxide. If it is used for reference during the separation of crude propylene oxide from the epoxidation product, nitrogen-containing impurities will inevitably be introduced into the system. For the system of cumene hydroperoxide oxidizing propylene, after separating the crude propylene oxide, the remaining reaction by-products need to hydrogenolyze benzyl alcohol to recover cumene, and for most hydrogenolysis catalysts, nitrogen-containing impurities are not welcome. Summary of the Invention

[0009] In view of the problems existing in the above prior art, the present application proposes a separation method and a separation system for propylene epoxidation products.

[0010] The inventors found in practice that under the action of certain acidic catalysts, propylene oxide and formaldehyde can react to form acetal-like 4-methyl-1,3-dioxolane, and under certain conditions, this cyclic acetal can exist stably. On this basis, the present invention proposes a separation process for propylene epoxidation products to obtain a crude propylene oxide stream with reduced formaldehyde content to facilitate the operation of the subsequent refining unit.

[0011] One of the objectives of the present invention is to provide a method for separating propylene epoxidation products, which includes subjecting the propylene epoxidation products to a first distillation to remove propylene; subjecting the stream after removing propylene to a second distillation, reacting the obtained stream containing formaldehyde and propylene oxide in the presence of a catalyst to form 4-methyl-1,3-dioxolane, returning the reacted stream to the second distillation step, and further subjecting the stream containing propylene oxide, cumene, and 2-phenyl-2-propanol obtained from the second distillation to a third distillation, washing the obtained stream containing propylene oxide and cumene and returning it to the second distillation step, and finally obtaining a crude propylene oxide stream from the second distillation step.

[0012] According to a preferred embodiment of the present invention, the method for separating propylene epoxidation products may include the following steps:

[0013] 1) The propylene epoxidation products enter a first distillation column to remove excess propylene;

[0014] 2) The stream after removing propylene enters a second distillation column, and a stream containing formaldehyde and propylene oxide is withdrawn from the top of the column. The withdrawn stream is introduced into an acetalization reactor equipped with a heterogeneous acidic catalyst, so that formaldehyde and propylene oxide react to form 4-methyl-1,3-dioxolane to obtain an acetalization reaction stream;

[0015] 3) Return the acetalization reaction stream to the second distillation column, and at the same time control the conditions at the bottom of the second distillation column to avoid the re-decomposition of 4-methyl-1,3-dioxolane; send the stream containing propylene oxide, cumene, and 2-phenyl-2-propanol obtained at the bottom of the second distillation column to a third distillation column;

[0016] 4) A stream mainly containing propylene oxide and cumene is distilled out from the top of the third distillation column, and this stream is returned to the second distillation column after being washed with water to recover propylene oxide;

[0017] 5) A crude propylene oxide stream is withdrawn from the top or near the top side line of the second distillation column, with a formaldehyde content not exceeding 200 mg / kg, and then it can be sent to a subsequent refining unit.

[0018] The separation process proposed by the present invention is mainly applicable to the system of epoxidizing propylene with cumene hydroperoxide. This is not only because the content of formaldehyde impurities in this system is relatively high, but more importantly, this system uses a non-polar solvent, mainly cumene, which avoids the presence of water, enabling 1. formaldehyde to be distilled out from the top of the second distillation column in the form of a low-boiling component. 2. reducing the loss of propylene oxide hydrolysis.

[0019] In step 1) of the separation method, the propylene epoxidation products are derived from the propylene epoxidation reaction using cumene hydroperoxide as the oxidant, and in the propylene epoxidation reaction, 10-95% of the total mass of the reaction system is used as the solvent.

[0020] In step 1) of the separation method, the excess propylene in the product is separated by the first distillation column. The propylene is withdrawn from the top or near-top side stream of the column and can be optionally returned directly or after treatment to the epoxidation reaction. The propylene content in the bottom stream of the first distillation column does not exceed 0.1 wt%, preferably does not exceed 0.01 wt%.

[0021] According to the requirements of the bottom stream withdrawal, the operating pressure of the first distillation column is 0 - 2.5 MPaG, and the bottom temperature is 80 - 180 °C. For convenience, the first distillation column can be split into several sections, and it is also a practical and feasible operation to carry out distillation separation at different pressures.

[0022] In step 2) of the separation method, the stream withdrawn from the bottom of the first distillation column and having the propylene removed is injected into the second distillation column. Under a preferred scheme, before the stream after the propylene removal enters the second distillation column, a step of adjusting the acidity and alkalinity is carried out. Preferably, the acid value of the injected stream after adjusting the acidity and alkalinity does not exceed 0.005 mg KOH / g. The acidity of the stream mainly comes from components such as formic acid generated in the propylene epoxidation reaction. Controlling the acid value can adopt common practices in the field such as injecting alkali. The injected alkali includes but is not limited to oxides, hydroxides, and basic salts such as carbonates of alkali metals or alkaline earth metals.

[0023] The second distillation column corresponds to the so-called crude propylene oxide separation column in the prior art. Usually, a crude propylene oxide stream is withdrawn from the top of the column. In the present invention, the operating pressure of the second distillation column is -0.05 - 0.1 MPaG, preferably, the operating pressure is 0 - 40 kPaG; the top temperature of the second distillation column is 15 - 60 °C, preferably, the top temperature is 30 - 45 °C. The mixture withdrawn from the top of the column contains formaldehyde and propylene oxide, wherein the content of propylene oxide is ≥90 wt%, and the formaldehyde content is 10 - 2000 mg / kg, preferably 50 - 500 mg / kg. The way to control the formaldehyde content is the flow rate of the stream withdrawn from the top of the column.

[0024] The stream withdrawn from the top of the second distillation column is introduced into an acetalization reactor so that formaldehyde and propylene oxide react to form 4-methyl-1,3-dioxolane. This reaction is essentially an acetalization reaction. The heterogeneous acidic catalyst loaded in the reactor is not particularly limited. Preferably, it can be one or a mixture of acidic resins, acidic molecular sieves, and heteropolyacids.

[0025] The operating temperature of the acetalization reactor is 40 - 100 °C, the operating pressure is 0.1 - 0.6 MPaG, and the formaldehyde content at the outlet is reduced to not more than 100 mg / kg.

[0026] Return the product after the reaction to the second rectification column. The 4-methyl-1,3-dioxolane component has a relatively high boiling point and will descend to the bottom of the column and flow out. The acid value of the material in the bottom of the second rectification column should be controlled not to exceed 0.01 mg KOH / g, and the bottom temperature of the second rectification column should be controlled not to exceed 170 °C. More preferably, the bottom temperature is controlled in the range of 120-150 °C. Under this condition, 1 wt% - 10 wt% of the propylene oxide component may remain in the bottom stream.

[0027] The operating pressure of the second rectification column is -0.05 to 0.1 MPaG. Preferably, the operating pressure is 0 to 40 kPaG.

[0028] In step 3) of the separation process, the stream containing part of propylene oxide, cumene, 2-phenyl-2-propanol, and the acetalization product 4-methyl-1,3-dioxolane obtained from the bottom of the second rectification column is sent to the third rectification column to recover propylene oxide.

[0029] The designed operating pressure of the third rectification column is 20 - 100 kPaA. Preferably, the operating pressure is 30 - 50 kPaG. The mixture of propylene oxide and cumene is steamed to the top of the column. The content of propylene oxide in the overhead stream is preferably not more than 20 wt%, and more preferably in the range of 2 wt% - 10 wt%.

[0030] The overhead temperature of the third rectification column is 95 - 150 °C. Preferably, the overhead temperature is 100 - 130 °C.

[0031] The bottom temperature of the third rectification column is 120 - 170 °C. Preferably, the bottom temperature is 130 - 150 °C.

[0032] The overhead stream of the third rectification column is returned to the second rectification after being washed with water. The acidity or alkalinity of the remaining water after washing satisfies pH ≥ 6. The washing can be carried out by water washing. An appropriate amount of alkali can be added to the washing water, or a multi-stage alkali-water washing method can be used to ensure that the stream returned to the second rectification column carries as little acidic components as possible. After the washed stream undergoes oil-water separation, it is separated into an oil-phase stream containing propylene oxide and an aqueous-phase stream. The oil-phase stream is returned to the second rectification column, and the aqueous-phase stream is discharged as wastewater. The oil-water ratio is appropriately controlled so that the pH value of the aqueous-phase stream is 9.0 - 10.0.

[0033] The stream flowing out from the bottom of the third rectification column contains 2-phenyl-2-propanol and 4-methyl-1,3-dioxolane.

[0034] Under the stable operating state, a crude propylene oxide stream with a formaldehyde content not exceeding 200 mg / kg can be withdrawn from the top or a side line near the top of the second rectification column and sent to the subsequent refining unit.

[0035] A second object of the present invention is to provide a separation system for propylene epoxidation products for carrying out the separation method described above, including:

[0036] A first distillation column: configured to receive propylene epoxidation products, discharge a propylene stream and a stream after removing propylene;

[0037] A second distillation column: configured to receive the stream after removing propylene, an acetalization reactant, and a stream returned from a third distillation column, and discharge a stream containing formaldehyde and propylene oxide, a crude propylene oxide stream, and a stream containing propylene oxide, cumene, and 2-phenyl-2-propanol;

[0038] An acetalization reactor: configured to receive the stream containing formaldehyde and propylene oxide and discharge an acetalization reactant;

[0039] A third distillation column: configured to receive the stream containing propylene oxide, cumene, and 2-phenyl-2-propanol, and discharge a stream containing propylene oxide and cumene, and a stream containing 2-phenyl-2-propanol.

[0040] Preferably, the separation system further includes:

[0041] A first mixer: configured between the first distillation column and the second distillation column to receive the stream after removing propylene from the first distillation column and an alkali injection adjustment stream, and discharge the stream with adjusted acidity and alkalinity into the second distillation column;

[0042] A second mixer: configured to be connected to the third distillation column, receive the stream containing propylene oxide and cumene, and a cleaning stream, and discharge a mixture stream;

[0043] An oil-water separator: configured to be connected to the second mixer, receive the mixture stream, and discharge an oil phase stream to return to the second distillation column and an aqueous phase stream.

[0044] The present invention can effectively reduce the concentration of formaldehyde in the crude propylene oxide stream, alleviate the risk of formaldehyde polymerization blockage in the subsequent refining unit, and the formaldehyde content in the crude propylene oxide stream does not exceed 200 mg / kg. The crude propylene oxide or the stream with a high content of propylene oxide is basically not in contact with water, and the hydrolysis loss is small. The reaction does not require the introduction of other components, has little interference with other units of the process, removes small molecule aldehydes at a low cost, and is beneficial to alleviating the pipeline blockage phenomenon in the subsequent refining unit. Description of the Drawings

[0045] Figure 1 It is a process flow diagram of a separation method for propylene epoxidation products of the present invention.

[0046] Figure 1 Marking Explanation:

[0047] 1 - Propylene epoxidation product stream;

[0048] 2 - propylene stream;

[0049] Stream after propylene removal;

[0050] Stream for alkali injection adjustment;

[0051] Stream after acid - base adjustment;

[0052] Stream containing formaldehyde and propylene oxide;

[0053] Stream for acetalization reaction;

[0054] Stream containing propylene oxide, cumene and 2 - phenyl - 2 - propanol;

[0055] Crude propylene oxide stream;

[0056] Stream containing propylene oxide and cumene;

[0057] Cleaning stream;

[0058] Aqueous phase stream;

[0059] Oil phase stream;

[0060] Stream containing 2 - phenyl - 2 - propanol;

[0061] T1 - First distillation column;

[0062] T2 - Second distillation column;

[0063] T3 - Third distillation column;

[0064] R1 - Acetalization reactor;

[0065] M1 - First mixer;

[0066] M2 - Second mixer;

[0067] Y1 - Oil - water separator.

[0068] Figure 1In the process, the propylene epoxidation product stream 1 enters the first distillation column T1 to separate propylene therein. The propylene stream 2 is withdrawn from the top or near-top side line of the first distillation column and can be directly returned or returned to the epoxidation reaction after treatment. The stream 3 after removing propylene withdrawn from the bottom of the first distillation column enters the first mixer M1, and the acidity and alkalinity of the stream 3 after removing propylene are adjusted by injecting an alkali-adjusting stream 4. The stream 5 after adjusting the acidity and alkalinity enters the second distillation column T2. The stream 6 containing formaldehyde and propylene oxide is withdrawn from the top of the second distillation column and enters an acetalization reactor R1 equipped with a heterogeneous acidic catalyst. The acetalization reaction stream 7 containing 4-methyl-1,3-dioxolane generated by the reaction returns to the second distillation column T2 and descends to the bottom of the column. The stream 8 containing propylene oxide, cumene, and 2-phenyl-2-propanol obtained at the bottom of the second distillation column enters the third distillation column T3. The stream 10 containing propylene oxide and cumene is withdrawn from the top of the third distillation column and enters the second mixer M2. After being fully mixed with the cleaning stream 11 in the second mixer M2, the mixture enters an oil-water separator Y1. The separated oil-phase stream 13 returns to the second distillation column T2, and the water-phase stream 12 is discharged as wastewater. The stream 14 containing 2-phenyl-2-propanol flows out from the bottom of the third distillation column. Under the condition of stable operation of the system, the crude propylene oxide stream 9 is withdrawn from the top or near-top side line of the second distillation column. Specific Embodiment

[0069] The present invention will be specifically described below in conjunction with specific drawings and embodiments. It is necessary to point out here that the following embodiments are only for further illustration of the present invention and should not be construed as limiting the protection scope of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention still fall within the protection scope of the present invention.

[0070] In addition, it should be noted that the various specific technical features described in the following specific embodiments can be combined in any suitable manner without conflict. To avoid unnecessary repetition, the present invention will not separately describe various possible combination methods.

[0071] Furthermore, any combination can be made between various different embodiments of the present invention as long as it does not violate the idea of the present invention. The technical solutions formed thereby belong to a part of the original disclosure content of this specification and also fall within the protection scope of the present invention.

[0072] If there is no special limitation on the raw materials used in the examples and comparative examples, they are all disclosed in the prior art. For example, they can be directly purchased or prepared according to the preparation methods disclosed in the prior art.

[0073]

Example 1

[0074] Example 1 adopts the separation process as Figure 1 shown:

[0075] The propylene epoxidation product stream 1 enters the first distillation column T1 to separate propylene therein. The propylene stream 2 is withdrawn from the top or near-top side line of the first distillation column and can be optionally returned directly or after treatment to the epoxidation reaction. The stream 3 after removing propylene withdrawn from the bottom of the first distillation column enters the first mixer M1, and the acidity and alkalinity of the stream 3 after removing propylene are adjusted by injecting an alkali to adjust the stream 4. The stream 5 after adjusting the acidity and alkalinity enters the second distillation column T2. The stream 6 containing formaldehyde and propylene oxide is withdrawn from the top of the second distillation column and enters the acetalization reactor R1 equipped with a heterogeneous acid catalyst. The acetalization reaction stream 7 formed by the reaction of formaldehyde and propylene oxide returns to the second distillation column T2 and descends to the bottom of the column. The stream 8 containing propylene oxide, cumene, and 2-phenyl-2-propanol obtained at the bottom of the second distillation column enters the third distillation column T3. The stream 10 containing propylene oxide and cumene is withdrawn from the top of the third distillation column and enters the second mixer M2. After being fully mixed with the cleaning stream 11 in the second mixer M2, the mixture enters the oil-water separator Y1. The separated oil-phase stream 13 returns to the second distillation column T2, and the water-phase stream 12 is discharged as wastewater. The stream 14 containing 2-phenyl-2-propanol flows out from the bottom of the third distillation column. Under the condition of stable operation of the system, the crude propylene oxide stream 9 is withdrawn from the top or near-top side line of the second distillation column.

[0076] The separation object refers to the epoxidation reaction outlet stream of a certain CHP-PO industrial device, and its main components are as shown in Table 1 below:

[0077] Table 1

[0078] propylene propylene oxide cumene 2-phenyl-2-propanol 42.6％ 9.6％ 19.9％ 23.9％

[0079] The impurity content including small molecule aldehydes is as shown in Table 2 below:

[0080] Table 2

[0081] formaldehyde acetaldehyde propionaldehyde methanol acetone 92 ppm 11 ppm 63 ppm 23 ppm 211 ppm

[0082] The propylene epoxidation product stream 1 with the above composition enters the first distillation column T1 to remove propylene. The pressure of the distillation column T1 is 0.22 MPaG, the number of trays is 60, the bottom temperature is 119 °C, and the top temperature is -21 °C.

[0083] The propylene stream 2 withdrawn from the top of the first distillation column is propylene with a purity of not less than 90 wt%, which can be returned to the epoxidation reaction for recycling. The composition of the stream 3 after removing propylene withdrawn from the bottom of the column is as shown in Table 3 below:

[0084] Table 3

[0085]

[0086] The actual logistics similar to logistics 3 in the industrial plant contains about 50 - 100 ppm of formic acid and other organic acids, and the titration acid value is 0.05 - 0.08 mg KOH / g. Inject a 20wt% NaOH solution into mixer M1 according to 0.1 g of NaOH per kilogram of material to adjust the acid value of the injected logistics not to exceed 0.005 mg KOH / g, and then enter the second distillation column T2.

[0087] The operating pressure of the second distillation column T2 is 20 kPaG, the number of trays is 65, the bottom temperature is 149 °C, the top temperature is 40 °C, and the acid value of the material in the bottom of the second distillation column is 0.003 mg KOH / g.

[0088] The overhead product of the second distillation column, logistics 6 containing formaldehyde and propylene oxide, enters the acetalization reactor R1, which is filled with 732 type acidic resin. The reaction temperature is 80 °C, the operating pressure of the reactor is 0.3 MPaG, the inlet formaldehyde content is 320 mg / kg, the outlet formaldehyde content is 81 mg / kg, and the content of 4-methyl-1,3-dioxolane is 689 mg / kg.

[0089] The bottom product of the second distillation column, logistics 8 containing propylene oxide, isopropylbenzene and 2-phenyl-2-propanol, is composed as shown in Table 4 below:

[0090] Table 4

[0091] propylene oxide cumene 2-phenyl-2-propanol formaldehyde 4-methyl-1,3-dioxolane 7.67％ 50.2％ 42.1％ 2 ppm 395 ppm

[0092] Logistics 8 containing propylene oxide, isopropylbenzene and 2-phenyl-2-propanol enters the third distillation column T3. The operating pressure of the third distillation column T3 is 30 kPaA, the number of trays is 55, the bottom temperature is 131 °C, and the top temperature is 109 °C.

[0093] The overhead product of the third distillation column, logistics 10 containing propylene oxide and isopropylbenzene, is fully mixed with 0.1% NaOH cleaning logistics 11 in mixer M2, and then the mixture is separated in the oil-water separator Y1 to obtain the oil-phase logistics 13 and return it to the second distillation column T2; the water-phase logistics 12 is discharged as wastewater, and the oil-water ratio is appropriately controlled to make the pH value of the water-phase logistics 12 be 9.0 - 10.0.

[0094] In the bottom product of the third distillation column, logistics 14 containing 2-phenyl-2-propanol, the isopropylbenzene content is about 45.4 wt%, and the 2-phenyl-2-propanol content is about 54.6 wt%, which is sent to the subsequent unit for utilization.

[0095] When this separation process operates stably, a crude propylene oxide stream 9 can be side-drawn from the 5th tray from the top of the second distillation column. In addition to propylene oxide, the formaldehyde content in the stream is 191 mg / kg, the acetaldehyde content is 32 ppm, and the propionaldehyde content is 344 ppm. The crude propylene oxide stream 9 can be sent to the refining unit to further refine the propylene oxide product.

[0096]

Example 2

[0097] The raw materials and process flow are the same as those in Example 1.

[0098] The difference is that in order to further reduce the bottom temperature of the second distillation column, the operating conditions are changed to an operating pressure of -50 kPaG, 65 trays, a bottom temperature of 131 °C, and a top temperature of 15 °C.

[0099] After stable operation, the formaldehyde content at the inlet of the acetalization reactor is 289 mg / kg, the formaldehyde content at the outlet is 71 mg / kg, and the content of 4-methyl-1,3-dioxolane is 604 mg / kg.

[0100] The formaldehyde content in the bottom of the second distillation column is 2 ppm, and the content of 4-methyl-1,3-dioxolane is 377 ppm.

[0101] In the crude propylene oxide stream side-drawn from the 5th tray from the top of the second distillation column, in addition to propylene oxide, the formaldehyde content is 187 mg / kg, the acetaldehyde content is 33 ppm, and the propionaldehyde content is 415 ppm.

[0102]

Comparative Example 1

[0103] The raw material composition is the same as that in Example 1, and the operating conditions and bottom composition of the first distillation column are the same. The bottom stream is injected into the second distillation column without adjusting the acid value with alkali.

[0104] The crude propylene oxide stream is directly drawn from the top of the second distillation column, without a corresponding acetalization reactor and the streams introduced into the acetalization reactor and returned to the second distillation column.

[0105] The operating pressure of the second distillation column is 20 kPaG, 65 trays, a bottom temperature of 171 °C, a top temperature of 42 °C, and the acid value of the material in the bottom of the second distillation column is 0.11 mg KOH / g.

[0106] The material in the bottom of the second distillation column contains about 2.35% of propylene oxide and enters the third distillation column. The operating pressure of the third distillation column is 30 kPaA, 55 trays, a bottom temperature of 131 °C, and a top temperature of 110 °C.

[0107] For comparison, in the crude propylene oxide stream drawn from the top of the second distillation column, the formaldehyde content is 675 mg / kg, the acetaldehyde content is 217 mg / kg, and the propionaldehyde content is 552 mg / kg.

[0108]

Comparative Example 2

[0109] The raw materials, process flow, and conditions such as the pressure and temperature of the distillation column are the same as those in Example 1.

[0110] The difference is that for the stream after propylene removal, no alkali is injected in mixer M1, and it enters the second distillation column at an acid value level of 0.06 mg KOH / g, and the operation of taking the overhead stream to the acetalization reactor is carried out.

[0111] After stable operation, the formaldehyde content at the inlet of the acetalization reactor is 391 mg / kg, the formaldehyde content at the outlet is 67 mg / kg, and the content of 4-methyl-1,3-dioxolane is 912 mg / kg.

[0112] The content of propylene oxide in the bottom draw of the second distillation column is similar to that in Example 1, but the formaldehyde content increases to 29 ppm, and the content of 4-methyl-1,3-dioxolane decreases to 113 ppm.

[0113] For comparison, in the crude propylene oxide stream taken from the side draw at the 5th tray from the top of the second distillation column, in addition to propylene oxide, the formaldehyde content is 371 mg / kg, the acetaldehyde content is 171 ppm, and the propionaldehyde content is 462 ppm.

[0114]

Comparative Example 3

[0115] The raw materials and process flow are the same as those in Example 1. Alkali is injected into the stream after propylene removal in mixer M1 to reach an acid value level of 0.003 mg KOH / g.

[0116] The difference is that the operating conditions of the second distillation column are changed to an operating pressure of 20 kPaG, the number of trays is 65, the bottom temperature is 171 °C, and the top temperature is 42 °C.

[0117] After stable operation, the formaldehyde content at the inlet of the acetalization reactor is 370 mg / kg, the formaldehyde content at the outlet is 65 mg / kg, and the content of 4-methyl-1,3-dioxolane is 824 mg / kg.

[0118] The formaldehyde content in the bottom of the second distillation column is 21 ppm, and the content of 4-methyl-1,3-dioxolane is 163 ppm.

[0119] For comparison, in the crude propylene oxide stream taken from the side draw at the 5th tray from the top of the second distillation column, in addition to propylene oxide, the formaldehyde content is 352 mg / kg, the acetaldehyde content is 169 ppm, and the propionaldehyde content is 413 ppm.

[0120] It can be seen from the comparative examples that the acidic environment or high-temperature conditions in the second rectification column will exacerbate the re-decomposition of the acetalization product, generating formaldehyde again, thereby leading to an increase in the formaldehyde content in the crude propylene oxide stream.

Claims

1. A method for separating propylene epoxidation products, comprising subjecting the propylene epoxidation products to a first distillation to remove propylene; subjecting the stream after removing propylene to a second distillation, reacting the obtained stream containing formaldehyde and propylene oxide in the presence of a catalyst to form 4-methyl-1,3-dioxolane, returning the reacted stream to the second distillation step, subjecting the stream containing propylene oxide, cumene and 2-phenyl-2-propanol obtained from the second distillation to a third distillation, washing the obtained stream containing propylene oxide and cumene and returning it to the second distillation step, and finally obtaining a crude propylene oxide stream from the second distillation step.

2. The method for separating propylene epoxidation products according to claim 1, characterized in that it comprises the following steps: 1) The propylene epoxidation products enter a first distillation column to remove propylene; 2) The stream after removing propylene enters a second distillation column, and a stream containing formaldehyde and propylene oxide is withdrawn from the top of the column. The withdrawn stream is introduced into an acetalization reactor equipped with a heterogeneous acid catalyst, so that formaldehyde and propylene oxide react to form 4-methyl-1,3-dioxolane to obtain an acetalization reaction stream; 3) Return the acetalization reaction stream to the second distillation column, and send the stream containing propylene oxide, cumene and 2-phenyl-2-propanol obtained from the bottom of the second distillation column to a third distillation column; 4) Obtain a stream containing propylene oxide and cumene from the top of the third distillation column, and wash the stream obtained from the top and return it to the second distillation column; 5) Withdraw a crude propylene oxide stream from the top or near the top side line of the second distillation column.

3. The method for separating propylene epoxidation products according to claim 1 or 2, characterized in that: The propylene epoxidation products are derived from a propylene epoxidation reaction using cumene hydroperoxide as an oxidant, and in the propylene epoxidation reaction, 10-95% by mass of the total reaction system of cumene is used as a solvent.

4. The method for separating propylene epoxidation products according to claim 2, characterized in that in the step 1): The operating pressure of the first distillation column is 0-2.5 MPaG, and the bottom temperature is 80-180 °C; and / or, The propylene content in the stream withdrawn from the bottom of the first distillation column does not exceed 0.1 wt%, preferably does not exceed 0.01 wt%.

5. The method for separating propylene epoxidation products according to claim 2, characterized in that in the step 2): Before the stream after removing propylene enters the second distillation column, a step of adjusting the acidity and alkalinity is carried out; preferably, the acid value of the stream after adjusting the acidity and alkalinity does not exceed 0.005 mg KOH / g.

6. The method for separating propylene epoxidation products according to claim 2, characterized in that in the step 2): In the stream containing formaldehyde and propylene oxide, the content of propylene oxide is ≥90 wt%, and the formaldehyde content is 10-2000 mg / kg, preferably 50-500 mg / kg; and / or, The heterogeneous acid catalyst is selected from at least one of acidic resin, acidic molecular sieve, and heteropolyacid; and / or, The operating temperature of the acetalization reactor is 40-100 °C, and the operating pressure is 0.1-0.6 MPaG; and / or, The formaldehyde content at the outlet of the acetalization reactor does not exceed 100 mg / kg; and / or, The acid value of the material in the bottom of the second distillation column does not exceed 0.01 mg KOH / g; and / or, The operating pressure of the second distillation column is -0.05 to 0.1 MPaG, preferably, the operating pressure is 0 to 40 kPaG; and / or, The top temperature of the second distillation column is 15 to 60 °C, preferably, the top temperature is 30 to 45 °C; and / or, The bottom temperature of the second distillation column does not exceed 170 °C, preferably, the bottom temperature is 120 to 150 °C.

7. The separation method of propylene epoxidation product according to claim 2, characterized in that in the step 3): The operating pressure of the third distillation column is 20 to 100 kPaA; preferably, the operating pressure is 30 to 50 kPaG; and / or, The top temperature of the third distillation column is 95 to 150 °C, preferably, the top temperature is 100 to 130 °C; and / or, The bottom temperature of the third distillation column is 120 to 170 °C, preferably, the bottom temperature is 130 to 150 °C.

8. The separation method of propylene epoxidation product according to claim 2, characterized in that in the step 4): The acidity and alkalinity of the remaining water after water washing satisfy pH≥6.

9. The separation method of propylene epoxidation product according to claim 2, characterized in that in the step 5): The formaldehyde content in the crude propylene oxide stream does not exceed 200 mg / kg.

10. A separation system for propylene epoxidation product, which is used to perform the separation method according to any one of claims 1 to 9, comprising: The first distillation column: which is configured to receive the propylene epoxidation product, discharge the propylene stream and the stream after removing propylene; The second distillation column: which is configured to receive the stream after removing propylene, the acetalization reactant, and the stream returned from the third distillation column, and discharge the stream containing formaldehyde and propylene oxide, the crude propylene oxide stream, and the stream containing propylene oxide, cumene, and 2-phenyl-2-propanol; The acetalization reactor: which is configured to receive the stream containing formaldehyde and propylene oxide and discharge the acetalization reactant; The third distillation column: which is configured to receive the stream containing propylene oxide, cumene, and 2-phenyl-2-propanol and discharge the stream containing propylene oxide and cumene and the stream containing 2-phenyl-2-propanol; Preferably, the separation system includes: The first mixer: which is configured between the first distillation column and the second distillation column to receive the stream after removing propylene from the first distillation column and the alkali injection adjustment stream, and discharge the stream with adjusted acidity and alkalinity into the second distillation column; The second mixer: which is configured to receive the stream containing propylene oxide and cumene and the cleaning stream and discharge the mixture stream; The oil-water separator: which is configured to receive the mixture stream and discharge the oil phase stream to return to the second distillation column and the water phase stream.

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