CN106167451B - The method of meta-xylene and ethylbenzene multistep cooxidation production aromatic carboxylic acids - Google Patents

Abstract

The invention discloses a kind of methods of meta-xylene and ethylbenzene multistep cooxidation production aromatic carboxylic acids, the following steps are included: the paraxylene raffinate containing paraxylene, meta-xylene, ortho-xylene and ethylbenzene is carried out rectifying by (1) removes ortho-xylene, the mixture A containing meta-xylene and ethylbenzene is obtained；(2) mixture A is subjected to multistep oxidation and generates the mixture B containing M-phthalic acid and benzoic acid, M-phthalic acid crude product is gone out by Crystallization Separation, crystalline mother solution obtains benzoic acid by being concentrated by evaporation；(3) acetic acid and distilled water is respectively adopted to wash M-phthalic acid crude product, smart M-phthalic acid is obtained after dry.Each step and each unit operation all simple possibles of production process of the present invention, while being discharged without organic wastewater, it is a kind of economic and environment-friendly new process.

Description

Method for producing aromatic carboxylic acid by multi-step co-oxidation of m-xylene and ethylbenzene

technical field

The invention relates to a method for producing aromatic carboxylic acid, in particular to a method for producing aromatic carboxylic acid through multi-step co-oxidation of m-xylene and ethylbenzene.

Background technique

Purified isophthalic acid (PIA) is the raw material of polyester resin, mainly used in the preparation of polyester bottle flakes, fibers, unsaturated resins, and low melting point polyester products. The production method of PIA is to use m-xylene (MX) as raw material, obtain crude isophthalic acid (CIA) through liquid-phase oxidation, and then remove trace impurities in the product through hydrogenation refining to obtain polymer grade PIA. Various oxidation and refining methods are described in detail in the review literature (Process Economics Program Report 9E, Terephthalic Acid and DimethylTerephthalate, SRI Consulting, Menlo Park, California, 94025, January 1997.), and the literature (He Zuoyun, m-xylene and m-benzene Diformic acid production technology, synthetic fiber industry, volume 23, phase 2, 41-45, 2000.) further supplemented the domestic research progress. Because the production method of PIA is similar to the process of purified terephthalic acid (PTA) in many aspects, and the production scale is smaller than that of PTA, some companies at home and abroad usually convert small PTA devices to PIA after transformation.

Meta-xylene is the raw material of PIA, and most of the current methods of obtaining MX use mixed xylene as raw material. At present, the production of mixed xylenes from petroleum is the main production method of mixed xylenes. Crude oil is often distilled under reduced pressure to obtain naphtha fraction, which is sent to the reforming unit to generate a large amount of aromatics, and the reformed oil is then sent to the aromatics extraction unit to separate aromatics and non-aromatics, and then benzene, toluene and aromatics in aromatics are separated by rectification For heavy aromatics with carbon 9 or more, the final mixture of carbon 8 aromatics contains four components: m-xylene (MX), p-xylene (PX), o-xylene (OX), and ethylbenzene (EB). The approximate content and The physical properties are shown in Table 1.

Table 1 Contents and physical properties of each component of C8 aromatics produced from reformed oil

components MX PX OX EB content(%) 45% 20% 20% 15% Boiling point (°C) 139.1 138.4 144.4 136.2 Melting point (°C) -47.8 13.2 -25.2 -95.0

It can be seen that the boiling points of the four components are very close, and except o-xylene, the other three substances are difficult to separate by rectification. Although the melting points of the four aromatics are significantly different, they can theoretically be separated by cryogenic crystallization, but the system has multiple multi-component eutectic points, making it difficult to separate them by crystallization. This has been reported in many monographs and literature. Therefore, at present, the industry mainly adopts adsorption separation to obtain MX raw materials.

The adsorption separation method is used to adsorb MX with high selectivity, and then the MX is eluted with a desorbent. This method can obtain MX with high purity (MX>99.5%). Patents CN200980125280.6 and CN200610164101.8 respectively disclosed the special adsorbent composition and adsorption conditions of MX, and patent CN200810100400.4 disclosed the structure and adsorption process of MX adsorption device. Patent CN98102372.X and literature (Zhao Yuzhang, Yang Jian, xylene adsorption separation-isomerization combined process to produce high-purity m-xylene, petrochemical industry, volume 29, No. 1, 32-36, 2000.) disclose a A process for the adsorption and separation of MX by gas phase method. The adsorption method is a production method of m-xylene that has been implemented industrially at present. Its advantages are that the obtained MX product has high purity and wide adaptability to raw materials. However, the operation process of the adsorption method is complicated, requiring the use of a large amount of expensive adsorbent, and at the same time, a special desorbent is required to desorb MX, and the desorbed mixture must go through multiple rectification steps to recover the desorbent and purify the MX product. Therefore, the cost of preparing MX by adsorption is high, and the resulting MX is expensive, which limits its market application.

PIA, which is a polyester raw material, is used together with purified terephthalic acid (PTA) as a copolyester component in most cases. For example, as a bottle-grade copolyester, PIA only accounts for 2-3% of PTA, and PIA only accounts for 20-40% of low melting point polyester. Therefore, in order to avoid the difficult problem of separating MX, some patents adopt the method of mixed xylene co-oxidation to obtain the mixed dicarboxylic acid product of PIA and PTA to reduce the cost of preparing PIA. Patent CN200610154821.6 first proposes to use mixed xylene as raw material for liquid-phase co-oxidation to obtain a mixture of terephthalic acid, isophthalic acid, and phthalic acid, and then use phthalic acid and p- and m-carboxylic acids Significant difference in solubility, a mixture of PTA and PIA was obtained by crystallization separation, as a raw material for copolyester. Similar schemes are mentioned many times in subsequent patents. For example, patent CN201210014597.6 proposes to use a mixture of p-xylene (PX) and MX as a raw material, add a part of PX to adjust the composition into a specific ratio, and then oxidize and refine it to produce special raw materials for various copolyesters. Patent CN200980109527.5 proposes to use C8 aromatics containing mixed xylene and ethylbenzene as raw materials, firstly remove ethylbenzene in C8 aromatics through appropriate separation methods, and then remove o-xylene by rectification, and the remaining PX and MX mixture Then liquid-phase oxidation is carried out to obtain the mixture product of PTA and PIA. But this patent does not propose a definite ethylbenzene separation method, nor does it provide specific co-oxidation conditions. The boiling points of ethylbenzene and p-xylene differ only by 1.8°C, and they are also very close in nature, so it is difficult to separate them. In view of this, the patent CN200810022473.6 proposes a scheme of co-oxidizing C8 aromatics containing ethylbenzene to obtain a mixture of phenylacetic acid and three dicarboxylic acids. However, in fact, benzoic acid is generally generated after ethylbenzene is oxidized, and it is difficult to generate phenylacetic acid. The proposed scheme is difficult to implement, and the use of the mixed polycarboxylic acid generated is also very limited. Therefore, in order to obtain MX and PIA at low cost, we must find another way and adopt new ideas and methods.

Contents of the invention

The invention provides a method for producing aromatic carboxylic acid through multi-step co-oxidation of m-xylene and ethylbenzene. The method can produce m-xylene at low cost while recovering ortho-xylene (OX), and obtain m-xylene through oxidation of m-xylene. Phthalic acid.

A method for producing para-xylene and co-producing meta-xylene, comprising: containing a carbon 8 aromatic hydrocarbon mixture containing para-xylene (PX), meta-xylene (MX), ortho-xylene (OX) and ethylbenzene (EB) Separation of p-xylene by adsorption to obtain a p-xylene raffinate containing p-xylene, m-xylene, o-xylene and ethylbenzene, and part of the p-xylene raffinate is subjected to isomerization reaction and combined with the C 8 aromatic hydrocarbon mixture ; Another part of the xylene raffinate is rectified to remove o-xylene to obtain a mixture containing m-xylene and ethylbenzene.

The isomerization reaction is to convert m-xylene, o-xylene and part of ethylbenzene into mixed xylene containing p-xylene, and at the same time deethylate and remove part of ethylbenzene that is difficult to convert.

A method for the production of aromatic carboxylic acids by multi-step co-oxidation of m-xylene and ethylbenzene, comprising the following steps:

(1) The p-xylene raffinate containing p-xylene, m-xylene, o-xylene and ethylbenzene is rectified to remove o-xylene to obtain a mixture A containing m-xylene and ethylbenzene;

(2) The mixture A is subjected to multi-step oxidation to generate a mixture B containing isophthalic acid and benzoic acid, the crude product of isophthalic acid is separated by crystallization, and the crystalline mother liquor is concentrated by evaporation to obtain benzoic acid;

(3) Wash the crude product of isophthalic acid with acetic acid and distilled water respectively, and obtain refined isophthalic acid after drying.

In step (1):

The p-xylene raffinate is a C8 aromatic hydrocarbon mixture containing m-xylene, o-xylene, ethylbenzene and a small amount of p-xylene after p-xylene is separated by adsorption.

In terms of mass percent concentration, the p-xylene content in the p-xylene raffinate is less than 1%, the content of m-xylene is 65-70%, the content of o-xylene is 20-30%, and the content of ethylbenzene is 3-10%. Further, the content of p-xylene in the p-xylene raffinate is less than 1%, the content of m-xylene is 65-69%, the content of o-xylene is 25-28%, and the content of ethylbenzene is 5-8%.

The boiling point of OX differs from that of MX by 5.3°C. As a preference, a rectification tower is used to separate o-xylene. The operating conditions are: feed from the middle and lower part of the rectification tower, the number of theoretical plates is 70-120, and the top reflux ratio is 10. ~20, the temperature at the top of the tower is 139~141°C, and the temperature at the bottom of the tower is 155~170°C.

Under the above-mentioned operating conditions, obtain the mixture A of MX and EB that OX content is less than 0.3% from the rectifying column top, wherein MX content is greater than 90%, and EB content is less than 10%, as further producing refined isophthalic acid (PIA ) and benzoic acid; the bottom of the rectifying tower obtains the heavy fraction of OX content > 85%, and the heavy fraction can be sent to the isomerization unit to be converted into mixed xylene and C 8 aromatic hydrocarbon mixture and be used for adsorption to prepare PX, and can also Further rectification to produce high-purity OX products.

In step (2):

The separation of EB from C8 aromatics has always been a technical difficulty. The boiling point difference between EB and MX is only 3°C, so it is difficult to cut by rectification. Although the melting points of the two are quite different, cryogenic crystallization is theoretically possible, but the crystallization separation is characterized by a low yield, and a considerable part of MX is still difficult to recover in the mother liquor, and the cost of cryogenic separation is relatively high.

The present invention carries out multi-step co-oxidation to the mixture A containing MX and EB. As preferred, when the mixture A is oxidized, acetic acid is used as a solvent, cobalt salt, manganese salt, and hydrogen bromide are used as catalysts, and the oxidizing agent is an oxygen-containing gas.

Preferably, the cobalt salt is cobalt acetate, and the manganese salt is manganese acetate.

Preferably, the oxygen-containing gas is air.

As preferably, the operation condition of main oxidation is: temperature is 180～210 ℃, pressure is 1.0～1.5MPa, the reaction time is 45～90 minutes with the average residence time of solvent, the weight ratio of feed acetic acid and m-xylene is 2-4:1, the catalyst concentration in terms of atomic concentration is: Co is 200-500ppm, Mn is 200-500ppm, and Br is 400-1000ppm.

Under the above-mentioned oxidation conditions, the two methyl groups on MX are oxidized to corresponding alcohols, aldehydes, acids in turn, and finally converted to isophthalic acid (IA). The conversion rate of MX is greater than 98%, and the yield of crude isophthalic acid Greater than 94%.

Studies have proved that the ethyl group on ethylbenzene is relatively active. Under the above oxidation conditions, the ethyl group will be gradually oxidized into a monocarboxyl group, and ethylbenzene will be converted into benzoic acid (BA). The EB conversion rate is greater than 99%, and the BA yield Greater than 96%.

The generated benzoic acid has a great solubility in acetic acid solvent, and the two constitute a dicarboxylic acid co-solvent, which can promote the oxidation of alkyl aromatic hydrocarbons and can accelerate the oxidation reaction. Therefore, the formation of benzoic acid is a favorable factor for the oxidation of m-xylene and ethylbenzene. On the other hand, since the solubility of benzoic acid in the solvent acetic acid and water is much greater than that of isophthalic acid, the two oxidation products are easily separated by crystallization, so that isophthalic acid is enriched in the solid phase and benzoic acid is enriched in the liquid phase. phase, thereby obtaining two carboxylic acid products respectively.

Subsequent multi-step oxidation is carried out on the reaction solution after the main oxidation, and a small amount of reactants and intermediate products that are not completely converted are deeply oxidized into products.

After the multi-step oxidation, all the reactants and intermediate products in the reaction liquid after the main oxidation can be converted into the products of isophthalic acid and benzoic acid, so that no further hydrogenation is required. After research, this is feasible, because the solubility of isophthalic acid generated by oxidation in acetic acid solvent is also relatively large. Under appropriate oxidation conditions, the reactants, intermediates and products are all dissolved in the solvent, and there is no or very little There is solid precipitation, which allows the unreacted substances dissolved in the liquid phase to be almost completely converted into final products through multi-step oxidation without being wrapped in solids as impurities.

It should be noted that the multi-step oxidation process has also been proposed in the past, and is used in the process of producing terephthalic acid by oxidation of p-xylene. For example, patents US4877900, US 4772748, and JP 59-93029 proposed the use of 3~ 4-step oxidation converts PX into polymer grade terephthalic acid without hydrofining step; patent US2002/0183546A1, CN02810929.5 also disclosed its two-step oxidation scheme to prepare medium-purity terephthalic acid more than ten years ago , the present inventor also proposed a step-by-step temperature rise oxidation method in the patent CN201210238659.1 several years ago. However, a common feature of these multi-step or multi-stage oxidation processes is that the oxidation conditions of the subsequent steps are more severe than the oxidation conditions of the first step. ℃, the pressure is 1~3MP higher. This is because some intermediates generated in the oxidation process such as p-carboxybenzaldehyde (4-CBA) and colored impurities and product terephthalic acid have very little solubility in acetic acid, and co-crystallization is separated out as solid immediately after generation. Encapsulated impurities are only able to leach out of the solid into the liquid phase and be oxidized at higher temperatures. However, the situation is obviously different for the oxidation of m-xylene. The intermediates and products of the reaction have high solubility, and there are few solids in the liquid phase of the main oxidation and secondary oxidation steps. Therefore, it is easy to remove all intermediates in the liquid phase by multi-step oxidation. transform. Therefore, the multi-step oxidation method adopted in the present invention is significantly different from the scheme proposed in the past, that is, the secondary or tertiary oxidation steps after the main oxidation all adopt milder temperature and pressure conditions than the main oxidation, which is more convenient for implementation and reduces equipment simultaneously. Investment and consumption of raw materials.

Preferably, the present invention employs two-step oxidation, but is not limited to two-step oxidation.

As preferably, the operating conditions of the secondary oxidation are: the temperature is 170-207° C., the pressure is 0.8-1.4 MPa, the reaction time is 45-90 minutes based on the average residence time of the solvent, and the weight ratio of the feed acetic acid to m-xylene It is 2-4:1, and the catalyst concentration in terms of atomic concentration is: Co is 200-500ppm, Mn is 200-500ppm, and Br is 400-1000ppm.

The operating conditions of the secondary oxidation are relatively milder than those of the main oxidation, the temperature is 3-10°C lower, and the pressure is 0.1-0.2MPa lower.

After two-step oxidation, the solid content in the slurry is less than 5%, and the concentration of impurity m-carboxybenzaldehyde (3-CBA) is less than 40ppm.

After the above two steps of oxidation, MX and EB have been completely oxidized, and the concentration of its representative intermediate 3-CBA in the mother liquor has been reduced to a trace level.

The oxidation reactor can be a bubble column, a gas-liquid stirred tank and other types of gas-liquid contact and reaction devices.

The part of the reaction output containing the mixture B of isophthalic acid and benzoic acid is sent to the crystallization unit for decompression and temperature reduction to precipitate solid IA.

Preferably, multi-stage crystallizers are used during crystallization, and the crystallizers at each stage are gradually reduced in pressure and temperature to separate out isophthalic acid.

The purpose of using multi-stage crystallization is to control the supersaturation of crystallizers at all stages so as to facilitate the growth of crystals.

Preferably, oxygen-containing gas is fed into the first crystallizer to deeply oxidize the remaining reactants and intermediate products in the mixture B.

The oxygen-containing gas is fed into the first crystallizer to deeply oxidize the remaining reactants and intermediate products in the mixture B, so that the precipitated isophthalic acid meets the requirements of the polymerization grade.

The temperature of the last crystallizer determines the recovery rate of solid product. For the crystallization process of isophthalic acid, the pressure of the final crystallizer generally needs to be evacuated to form a negative pressure to reduce the temperature and maximize the crystallization and precipitation of IA products.

Preferably, the operating conditions of the last stage crystallizer are: the pressure is -0.05-0.08MPa, the temperature is 80-100°C, and the solid content in the obtained solid slurry is 36-40%.

The solid content rate is the ratio of the mass of solids in the solid slurry to the total mass of the solid slurry.

Because the solubility of benzoic acid in acetic acid is far greater than that of isophthalic acid, the product isophthalic acid is separated out as a solid, and the product benzoic acid is still enriched in the liquid phase, and these two products can be separated by liquid-solid separation.

The output slurry of the above-mentioned last-stage crystallizer is sent to the liquid-solid separation unit, and the isophthalic acid filter cake and the oxidation mother liquor containing benzoic acid are obtained by filtration, centrifugation or other liquid-solid separation methods. A part of the oxidation mother liquor is directly returned to the oxidation reactor, and another part of the oxidation mother liquor is sent to the benzoic acid separation unit to evaporate the solvent acetic acid to obtain the solid benzoic acid product. In the oxidation mother liquor, the mass ratio of stream A for separating benzoic acid to the total oxidation mother liquor is 15 to 95%. The higher the ratio of the stream A, the lower the concentration of benzoic acid in the oxidation mother liquor. The entrainment of benzoic acid in the formic acid filter cake solids was also lower. The purity of the crude benzoic acid obtained by evaporating the oxidation mother liquor is 95-98%, and a product with higher purity can be obtained through further vacuum distillation or recrystallization. The solvent acetic acid evaporated from the benzoic acid separation unit is all sent to the subsequent pickling unit as the pickling washing solution for the isophthalic acid filter cake.

The heat and water generated by the main oxidation reactor and the secondary oxidation reactor are removed from the reactor through the evaporation of the solvent, and carried by the oxidation tail gas into the solvent dehydration tower, whose main function is to remove the water generated by the reaction.

The solvent dehydration tower can be directly installed on the upper part of the main oxidation reactor and integrated with the reactor, or it can be installed separately.

The amount of oxidation tail gas in the secondary oxidation reactor is much smaller than that of the main oxidation reactor, and the pressure is lower. It can be boosted by a compressor and sent to the solvent dehydration tower, or it can be condensed separately and directly sent to the tail gas without condensing the tail gas. Processing unit purification treatment.

Part of the dehydrated acetic acid at the bottom of the solvent dehydration tower is refluxed into the main oxidation reactor, and part of it is taken out as the washing liquid for pickling in step (3).

The water vapor containing the inert tail gas at the top of the tower enters the condenser, and about 1/3 of the condensate is returned to the solvent dehydration tower as reflux water, and about 2/3 is taken out as the washing water for washing in step (3).

The non-condensable gas in the condenser is sent to the tail gas treatment unit for purification and then discharged.

The condenser generally adopts a multi-stage condensation method to generate different grades of steam to recover the heat of the tail gas.

In step (3):

The isophthalic acid filter cake obtained from the liquid-solid separation unit contains 10-30% of the oxidation mother liquor, which contains benzoic acid and other trace impurities, which must be removed.

The isophthalic acid filter cake is sent to the pickling unit to remove the oxidation mother liquor entrained in the isophthalic acid filter cake.

Part of the pickling solution in the pickling unit is acetic acid, a regenerating solvent evaporated from the benzoic acid separation unit, and part of it is dehydrated acetic acid from the bottom of the solvent dehydration tower. The dry basis mass ratio of the pickling solution to the isophthalic acid filter cake is 1 ~3:1.

The pickling mother liquor after pickling is returned to the main oxidation reactor as a solvent, and the isophthalic acid solid after pickling is sent to the water washing unit.

The equipment of the pickling unit can adopt the current commonly used washing devices, such as beating tanks, filters, countercurrent washing towers, belt spray washing devices, etc., and can be single-stage washing or multi-stage countercurrent washing to improve efficiency. , wherein liquid-solid countercurrent washing is the most efficient washing method, see the countercurrent washing equipment and method introduced by the inventor in patent CN201010241238.5.

Most of the oxidation mother liquor entrained in the isophthalic acid solid after pickling (95-99.5%) has been removed, but the remaining traces of benzoic acid and acetic acid in the filter cake still need to be further removed to make the product meet the requirements of polymerization grade. For this, a water washing step is also required.

The isophthalic acid solid output of the pickling unit is sent to the water washing unit, and the distilled water produced by the solvent dehydration tower overhead condenser is used to wash again, and the dry basis mass ratio of the used distilled water to the isophthalic acid solid is 1～3: 1.

The equipment and process of the water washing unit can also adopt devices and processes similar to those of the pickling unit, and the description will not be repeated.

The washing mother liquor after water washing is all sent to the top of the solvent dehydration tower as the reflux water of the solvent dehydration tower.

If the acid-washed isophthalic acid solid is dried and then sent to the water-washing unit, the acid content of the washing mother liquor in the water-washing unit is extremely low, and can be combined with the reflux water of the condenser and then sent to the top of the solvent dehydration tower; The final isophthalic acid solid is not dried and sent to the washing unit in the form of filter cake, then the washing mother liquor contains a small amount of acetic acid, and the two reflux liquids should be sent to the solvent dehydration tower separately. In this case, the reflux of the washing mother liquor The inlet should be 2 to 5 trays lower than the reflux liquid of the condenser. The specific reflux position of the washing mother liquor is easily determined according to its acid concentration when designing the solvent dehydration tower.

After washing, the isophthalic acid solid is dried and becomes the refined isophthalic acid product. The main impurities in this product are 3-CBA content < 20ppm, benzoic acid content < 20ppm, and terephthalic acid content < 1%. Quality requirements for polyester products.

If further improving the ratio of the oxidized mother liquor extraction liquid for separating benzoic acid, increasing the consumption of pickling and washing solvents, and improving the efficiency of multistage countercurrent washing, the impurity content can be further reduced in the product.

Compared with prior art, the beneficial effect of the present invention is:

(1) the present invention has just obtained the oxidized raw material that is rich in m-xylene and contains a small amount of ethylbenzene only by rectifying separation OX from p-xylene (PX) raffinate, has avoided expensive MX adsorption separation method, and then Through two-step co-oxidation and two-step washing, the purified terephthalic acid product that meets the requirements of polyester production is obtained, and industrial-grade benzoic acid product is produced by-product, which saves the harsh hydrofining step and can be produced in a cheap way Large-scale production of isophthalic acid and benzoic acid products to realize high-value utilization of raffinate produced in the PX production process;

(2) adopt the method provided by the invention, every ton of raw material solution containing m-xylene 91.5%, ethylbenzene 7.5%, p-xylene 1.0% can obtain about 1.4 tons of refined isophthalic acid by two-step oxidation and two-step washing (PIA) and 0.085 tons of benzoic acid (BA), 3-CBA content＜20ppm in the gained PIA, benzoic acid content＜20ppm, about 1% of terephthalic acid, benzoic acid product purity＞98%, productive rate and purity of product are relatively high;

(3) Each step and each unit operation of the production process of the present invention are simple and feasible, and at the same time, there is no discharge of organic waste water, which is an economical and environmentally friendly new process.

Description of drawings

Fig. 1 is the flow chart that the present invention takes carbon 8 aromatics as raw material to produce m-xylene and ethylbenzene;

Fig. 2 is that the present invention takes m-xylene and ethylbenzene as the flow chart of raw material production refined isophthalic acid and benzoic acid.

Detailed ways

The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be noted that the following embodiments are intended to facilitate the understanding of the present invention, but do not limit it in any way.

1. Production of m-xylene

As shown in Figure 1, a typical PX production unit uses naphtha reforming to produce aromatics and adsorption to separate PX from C8 aromatics. The main process includes two modules: adsorption separation and isomerization. Containing the C 8 aromatics mixture S100 of the composition shown in Table 1 and the equilibrium stream S104 output by the isomerization unit, it is mixed and sent to the PX adsorption unit U101 together, and after adsorption and separation, the PX product with a purity of 99.5% is obtained, which is produced by the stream S101 The output system, the raffinate S102 is sent to the isomerization reactor U102 to convert the raw material S102 into the equilibrium composition S104 containing PX, and at the same time remove part of the ethylbenzene, and then mix with the raw material S100 to adsorb and separate PX, and so on. Complete conversion. In order to obtain MX, the present invention draws a part from the PX raffinate S102, marked as S105, and obtains a stream S107 rich in MX after removing OX through rectification.

The composition of the drawn PX raffinate S105 is as follows:

MX 67%, OX 27.5%, EB 5%, PX 0.5%.

S105 of above-mentioned composition is sent into rectifying tower U103, removes o-xylene OX, rectifying tower condition is:

The number of theoretical trays is 100, the position of the feed tray is the 65th (from top to bottom), the feed temperature is 158°C, the top reflux ratio is 16, the bottom temperature is 167.8°C, the top temperature is 139.2°C, and the top pressure is For normal pressure.

Under the above conditions, every ton of feed S105 can obtain 0.70 tons of overhead fraction S107, 0.3 tons of tower bottom fraction S106, and the compositions of the two fractions are respectively:

S107: MX 91.5%, EB 7.5%, PX 0.7%, OX 0.3%

S106: MX 10.6%, EB 0.01%, PX 0.1%, OX 89.3%

The obtained stream S107 is rich in MX and contains a small amount of EB and traces of PX and OX, which is sent to the oxidation unit as the raw material for co-oxidation.

2. Multi-step co-oxidation

The raw material liquid S107 obtained by the above method is fed into the reactor as the feed stream S200A of the oxidation reactor U201 in FIG. At the same time, air is blown in for liquid-phase oxidation. The main oxidation reactor was a non-stirred bubble column reactor whose conditions are listed in Table 2.

Table 2 Operating conditions of main oxidation reactor U201

In the table, MX/HAc represents the mass ratio of MX and acetic acid (HAc) in the feed stream S200, the pressure is the tower top gauge pressure, the solution residence time=mixture volume/solvent acetic acid flow rate, and the tail gas oxygen concentration refers to the dry basis of oxygen in S210 concentration.

Under the above conditions, the reaction results of reactor U201 are listed in Table 3.

Table 3 Reaction results of main oxidation reactor U201

In the table, 3-CBA refers to the main reaction intermediate m-carboxybenzaldehyde, which together with the colored impurities is the main impurity that needs to be removed. The output stream S201 is rich in IA and also has a small amount of unoxidized raw materials and intermediate products. It is sent to the secondary oxidation reactor U202, which is a gas-liquid stirred tank. The reaction conditions are listed in Table 4, and the oxidation results are listed in Table 5.

Table 4 Secondary oxidation reactor U202 operating conditions

Table 5 Secondary oxidation reactor U202 reaction results

The crystallization unit U203 uses three-stage crystallizers to evaporate the solvent step by step under reduced pressure, and simultaneously precipitate solid IA. A small amount of air is fed into the first crystallizer to supplement the oxidation, and the liquid-phase reactants that cannot be converted in the oxidation reactor U202 are deeply converted. The conditions and results of the three-stage crystallizer are listed in Table 6.

Table 6 U203 conditions and results of the three-stage crystallizer

The slurry output from the crystallizer U203 is sent to the liquid-solid separation unit U204, and the oxidation mother liquor is separated from the solid IA by vacuum filtration to obtain the isophthalic acid filter cake S204A, which contains 15% of the oxidation mother liquor and 4.5% of Benzoic acid and other trace impurities are sent to pickling unit U206 for washing.

The mother liquor output by the liquid-solid separation unit U204 is divided into two streams, 50% of the mother liquor S204B is directly recycled back to the oxidation reactor U201, and the other half of the mother liquor S204C is pumped out and sent to the benzoic acid separation unit U205. In this unit, the solvent in the mother liquor The acetic acid is evaporated, and the acetic acid S205 is sent to the pickling unit U206 as washing liquid, and the concentrated benzoic acid melt is further purified to a purity of 98% or higher by vacuum rectification, and a small amount of high-boiling residue at the bottom of the rectification tower is discharged after recovering the catalyst . Under the condition that 50% mother liquor is evaporated to separate benzoic acid, the concentration of liquid phase benzoic acid in the oxidation reactor can be controlled to be no more than 5%.

3. Pickling and water washing

The filter cake S204A with a moisture content of 15% output from the liquid-solid separation unit U204 is sent to the pickling unit U206, and washed with a clean acetic acid solvent to remove the mother liquor in the filter cake and reduce the impurity content in the solid product. There are two sources of washing liquid acetic acid: one part is the solvent S205 evaporated from the benzoic acid separation unit U205, plus the acetic acid evaporated from the top of the secondary oxidation reactor U202 and the third-stage crystallizer, the mass ratio of the amount of this part of acetic acid to the dry solid About 1:1; the second part is to draw a part of S208C from the reflux acetic acid S208B of the dehydration tower, and the mass ratio of the amount of this part of acetic acid to the dry solid is about 0.5:1. The two parts of acetic acid are heat-exchanged and cooled to recover energy, and then the temperature is adjusted to about 120°C, and then sent to the washing tower to contact and wash the filter cake in countercurrent. The washing tower is a vertical agitated liquid-solid countercurrent moving bed introduced by an inventor in the patent CN201010241238.5. The filter cake is added from the upper part of the tower and discharged from the bottom. The washing liquid is added from the lower part of the tower and discharged from the top, and is further cooled to 90 °C for filtration A small amount of precipitated solid is recovered and sent to the main oxidation reactor U201. After using 1.5 times the weight of acetic acid to wash the solid in countercurrent, more than 99.5% of the mother liquor entrained in the filter cake is replaced by fresh acetic acid, the filter cake is dried, and after most of the acetic acid is removed, the residual benzoic acid concentration in the solid is still about 300ppm , 60ppm of acetic acid remains, and further washing is required to remove residual benzoic acid and acetic acid.

Dry the solid S206A output from the pickling unit and send it to the water washing unit U207 to wash away the remaining traces of benzoic acid. The washing liquid of the water washing unit uses distilled water S209 produced by the condenser U209 at the top of the dehydration tower, and 2/3 of it is sent to the washing tower U207 Lower part, in countercurrent contact with solid S206A. The water washing unit U207 is also a liquid-solid countercurrent moving bed similar to the pickling unit U206. The mass ratio of washing water to solid is still 1.5:1. The feeding temperature of the washing liquid is adjusted to 100°C, and the upper output is added from the lower part of the tower, and then All sent to the dehydration tower as reflux liquid S207. After countercurrent washing the solid with 1.5 times the weight of distilled water, the filter cake is dried to obtain a solid product of refined isophthalic acid. At this time, the residual amount of benzoic acid on a dry basis is about 15 ppm, and the content of the remaining components in the solid is: PIA99.3%, PTA0.7%, 3-CBA20ppm.

The tail gas S201B output from the top of the main oxidation reactor is sent to the rectification tower U208 to remove the water generated by the reaction, and part of the dehydrated acetic acid S208B at the bottom of the rectification tower U208 is refluxed into the oxidation reactor, and part of it is taken out as pickling solution S208C and sent to the acid Washing unit; the water vapor containing inert tail gas at the top of the tower enters the condenser U209, part of the condensate S209 flows back to the dehydration tower U208 as reflux water S209C, and part of it is extracted as the washing water S209B of the washing unit and sent to the washing unit U207. The non-condensable gas in the condenser U209 is sent to the tail gas treatment unit U210 for purification and then discharged. The parameters of the solvent dehydration tower U208 are: 60 theoretical tower plates, the temperature at the bottom of the tower is 196°C, and the temperature at the top of the tower is 180°C. In the reflux liquid of the rectification tower, the ratio of the condensed reflux liquid S209C at the top of the tower to the washing mother liquor reflux liquid S207 is 1: 2. The content of acetic acid in the discharged non-condensable gas is less than 50ppm, which can be recovered by further condensation. The tail gas at the top of the secondary oxidation reactor U202 is less, so it is directly condensed by a condenser, the condensate is combined with S208C and sent to the pickling unit U206, and the non-condensable tail gas is directly sent to the tail gas purification unit U210.

In this embodiment, the solvent dehydration tower is divided into two sections: one section contains 5 to 15 theoretical trays and is directly installed on the upper part of the main oxidation reactor U201, and is integrated with the bubble column to jointly form the main oxidation unit U201, as shown in this As introduced by the inventor in previous patents CN200420107473.3 and CN200510048977.1; another section of rectification tower is externally installed separately, and contains 45-55 theoretical plates to form the dehydration tower unit U208. The tail gas and steam produced by the main oxidation reactor U201 pass through the upper dehydration section to separate part of the acetic acid and the liquid droplets or particles entrained in the tail gas, and then enter the dehydration tower U208 to separate acetic acid and water, and the water vapor and non-condensable gas enter the condensation from the top of the tower The distilled water S209 is obtained from the U209, the non-condensable tail gas is sent to the U210 to purify and recover the pressure energy, and then discharged, 1/3 of the condensed water U209 is refluxed to the U208, and 2/3 is pumped out and sent to the washing unit U207 as washing water, and the mother liquor S207 after washing is mixed with The reflux water S209C returns to the dehydration tower U208 together. The aqueous acetic acid S208B produced at the bottom of the dehydration tower extracts liquid S208C that is 0.5 times the weight of solid S204A, and sends it to the pickling unit U206 together with the acetic acid S205 produced by the benzoic acid separation device, and the mother liquor S206B after pickling is sent back to the main oxidation reactor. In this way, the distilled water and fresh acetic acid produced by the dehydration tower U208 are used to the maximum extent for the washing of the solid filter cake, thereby reducing the impurity content carried by the solid on the one hand, and greatly reducing the discharge of organic wastewater on the one hand.

In this embodiment, about 1.4 tons of refined isophthalic acid (PIA) can be obtained by two-step oxidation and two-step washing for every ton of raw material solution containing 91.5% m-xylene, 7.5% ethylbenzene, and 0.5% p-xylene. ) and 0.085 tons of benzoic acid (BA), 3-CBA content＜20ppm in the gained PIA, benzoic acid content＜20ppm, terephthalic acid＜1%, benzoic acid product purity＞98%. Each step and each unit operation of the production process of the invention are simple and feasible, and at the same time, there is no discharge of organic waste water, which is an economical and environmentally friendly new process.

The embodiments described above have described the technical solutions and beneficial effects of the present invention in detail. It should be understood that the above descriptions are only specific embodiments of the present invention, and are not intended to limit the present invention. All within the scope of the principles of the present invention Any modifications, supplements and equivalent replacements should be included within the protection scope of the present invention.

Claims (9)

1. a kind of method of meta-xylene and ethylbenzene multistep cooxidation production aromatic carboxylic acids, which comprises the following steps:

(1) the paraxylene raffinate containing paraxylene, meta-xylene, ortho-xylene and ethylbenzene is subjected to rectifying removal neighbour two Toluene obtains the mixture A containing meta-xylene and ethylbenzene；

In the mixture A, meta-xylene content is greater than 90%, and less than 0.3%, ethyl-benzene level is less than ortho-xylene content 10%；

(2) mixture A is subjected to multistep oxidation and generates the mixture B containing M-phthalic acid and benzoic acid, pass through Crystallization Separation M-phthalic acid crude product out, crystalline mother solution obtain benzoic acid by being concentrated by evaporation；

The operating condition of main oxidation are as follows: temperature is 180~210 DEG C, and pressure is 1.0~1.5MPa, when with the average stop of solvent Between the meter reaction time be 45~90 minutes, the weight ratio for feeding acetic acid and meta-xylene is 2~4:1, is catalyzed in terms of atomic concentration Agent concentration are as follows: Co is 200~500ppm, and Mn is 200~500ppm, and Br is 400~1000ppm；

Compared with main oxidation, the oxidation step after main oxidation is using the reaction condition more mitigated；

(3) acetic acid and distilled water is respectively adopted to wash M-phthalic acid crude product, smart M-phthalic acid is obtained after dry.

2. the method for production aromatic carboxylic acids according to claim 1, which is characterized in that right in terms of mass percent concentration Paraxylene content is less than 1% in dimethylbenzene raffinate, and meta-xylene content is 65~70%, and ortho-xylene content is 20~ 30%, ethyl-benzene level is 3~10%.

3. the method for production aromatic carboxylic acids according to claim 1, which is characterized in that in step (1), using rectifying column essence Fraction is from ortho-xylene, operating condition are as follows: feeds from rectifying column middle and lower part, theoretical cam curve is 70~120, and overhead reflux is than 10 ~20,139~141 DEG C of tower top temperature, 155~170 DEG C of column bottom temperature.

4. the method for production aromatic carboxylic acids according to claim 1, which is characterized in that when oxidation mixture A, be with acetic acid Solvent, cobalt salt, manganese salt, hydrogen bromide are catalyst, and oxidant is oxygen-containing gas.

5. it is according to claim 1 production aromatic carboxylic acids method, which is characterized in that in step (2), by mixture A into Row two-step oxidation generates the mixture B containing M-phthalic acid and benzoic acid.

6. the method for production aromatic carboxylic acids according to claim 5, which is characterized in that the operating condition of secondary oxidation are as follows: Temperature is 170~207 DEG C, and pressure is 0.8~1.4MPa, and the reaction time is 45~90 points in terms of the mean residence time of solvent Clock, the weight ratio for feeding acetic acid and meta-xylene is 2~4:1, the catalyst concn in terms of atomic concentration are as follows: Co is 200~ 500ppm, Mn are 200~500ppm, and Br is 400~1000ppm.

7. the method for production aromatic carboxylic acids according to claim 1, which is characterized in that in step (2), using more when crystallization Grade crystallizer, the operating condition of afterbody crystallizer are as follows: pressure is -0.05~-0.08MPa, and temperature is 80~100 DEG C, is obtained To slurry of solids in solid holdup be 36~40%.

8. the method for production aromatic carboxylic acids according to claim 1, which is characterized in that in step (2), oxidation reaction is produced Raw oxidized tail gas is sent into the water that solvent dehydration tower rectifying elimination reaction generates, and the dehydration acetate part of solvent dehydration tower tower bottom is returned Stream enters oxidation reactor, cleaning solution of the part extraction as pickling in step (3)；The vapor of tower top tail gas containing inertia enters Condenser, partial condensation liquid flow back into solvent dehydration tower as recirculation water, and another part condensate liquid is as washing in step (3) Washing water, washing mother liquor are back to solvent dehydration tower as recirculation water again.

9. the method for production aromatic carboxylic acids according to claim 1, which is characterized in that in step (2), crystalline mother solution evaporation When being concentrated and separated benzoic acid, cleaning solution of the recovered solvent acetic acid as pickling in step (3), pickling mother liquor reflux to oxygen are evaporated Change reactor as solvent.