Starting method for preparing benzoic acid through toluene liquid phase continuous oxidation

Abstract

The invention relates to the technical field of chemical industry, and provides a start-up method for preparing benzoic acid by continuously oxidizing a toluene liquid phase, which comprises the following steps: placing a mixture containing a catalyst and a toluene raw material in a reactor, continuously introducing air at a certain reaction temperature and reaction pressure, and reacting for 2-6 hours in advance in an intermittent or cyclic operation mode, and after the toluene conversion rate reaches a certain value, introducing a mixture containing the catalyst and toluene raw materials, and carrying out continuous oxidation reaction. According to the start-up method provided by the invention, a stable production state or a relatively high conversion rate can be achieved within a relatively short time, the concentration change of reactants and products in the start-up process is controllable and small in fluctuation, and the start-up method is suitable for continuous large-scale production.

Description

TECHNICAL FIELD

[0001] The present application relates to the chemical technology field, and in particular to a start-up method for preparing benzoic acid by continuous liquid-phase oxidation of toluene. BACKGROUND

[0002] Benzoic acid, also known as anisic acid, is an important chemical raw material and chemical reaction intermediate, mainly used for preparing sodium benzoate preservative, and for synthesizing drugs, dyes, plasticizers, mordants, bactericides, and fragrances. There are three main industrial production processes for benzoic acid: decarboxylation of phthalic anhydride, hydrolysis of trichlorotoluene, and liquid-phase air oxidation of toluene. In the decarboxylation of phthalic anhydride, decarboxylation reaction is accompanied by the generation of a small amount of biphenyl, phthalic acid, and benzophenone, and the operation process is complicated and costly. The hydrolysis of trichlorotoluene is a photochlorination reaction of toluene and chlorine under certain temperature and pressure conditions, followed by reaction with water to obtain crude benzoic acid. This method has many steps, many by-products, and difficult separation, and consumes chlorine, and the emission of HCl aqueous solution and other environmental pollution is serious, so this method is generally only a complementary method for the recovery of by-products of toluene chlorohydrolysis to prepare benzaldehyde or benzyl alcohol. The liquid-phase air oxidation of toluene has mild reaction conditions, and the unreacted toluene is recycled by condensation and returned to the system, so the utilization rate of toluene is high. In addition to the main reaction, there are some side reactions, and the by-products generated mainly include benzaldehyde, benzyl alcohol, and benzyl benzoate, but the price of these by-products is generally more than 4 times that of benzoic acid, which is economically good. Due to the above advantages, the liquid-phase air oxidation of toluene has become the main method for producing benzoic acid.

[0003] Currently, the industrial mature process for producing benzoic acid by catalytic oxidation of toluene in liquid phase mainly includes SNIA method and Mid-century (MC) method. The SNIA process is to catalyze the liquid phase oxidation of toluene at 140-155℃ and 0.6-0.8 MPa in a bubble stirred tank reactor with cobalt acetate as catalyst. The MC method is to catalyze the liquid phase oxidation of toluene with Co / Mn / Br ternary catalyst in acetic acid solvent. Chinese patent CN103601638A discloses a continuous production process and device for benzoic acid, which uses transition metal (including cobalt, manganese and nickel) octanoate as catalyst, and air is introduced from the bottom of the reaction tower to react with toluene liquid under the action of the catalyst. After a certain time, the reaction material completes the oxidation process in the reaction tower, and the benzoic acid is discharged and subjected to subsequent treatment. The gas containing toluene is discharged from the top of the tower and subjected to subsequent treatment. At the same time, the feed inlet of the reaction tower is continuously fed to realize the continuity of the reaction process. The continuous production process does not use bromide as an initiator, so there is no serious corrosion problem, and the halogen content in the product is also very low. However, the continuous production process has an induction period, and the reaction time needs to be 6-7 h. Although a lot of research has been done on the liquid phase oxidation of toluene, there is little report on the start-up method of the production. The literature (Modeling and automatic start-up control system design of toluene oxidation reactor start-up process, China University of Petroleum, 2021) discloses a start-up process of toluene oxidation reaction: first, open the feed valve, and raise the toluene liquid level to the set value, then start heating by feeding heating medium; observe the temperature of toluene in the reactor, and stop feeding the heating medium when the temperature rises to about 435 K; open the gas phase feed valve and the catalyst feed valve of the reactor to induce the reaction; if the temperature continues to rise, it means that the induction reaction is successful; then gradually open the gas-liquid two-phase feed valve of the toluene oxidation reactor to start continuous feeding. Since the liquid phase oxidation of toluene is a free radical reaction, there is an obvious induction period, so such start-up method may have problems of long start-up period, instability, and large material and energy consumption. SUMMARY

[0004] In order to overcome the shortcomings of the prior art, the present application provides a start-up method for producing benzoic acid by continuous liquid phase oxidation of toluene, which shortens the reaction induction period and start-up time, reduces the waste of materials and energy, and makes the start-up and production process more stable.

[0005] In order to achieve the above-mentioned purpose, the technical scheme of the present application is as follows: A start-up method for producing benzoic acid by continuous liquid phase oxidation of toluene, which first places a mixture containing catalyst and toluene raw material in a reactor, then continuously introduces air at a certain reaction temperature and reaction pressure, and pre-reacts for 2-6 h in an intermittent or cyclic operation mode. After the toluene conversion rate reaches a certain value, the mixture containing catalyst and toluene raw material is introduced to carry out continuous oxidation reaction.

[0006] Further, the concentration of the catalyst in the mixture is 0.15-0.20 wt.%, further preferably 0.18 wt.%.

[0007] Further, in the circulation mode, the benzyl alcohol is removed from the circulating material before the circulation.

[0008] Further, the toluene raw material is pure toluene or a solution formed by dissolving pure toluene in an organic solvent.

[0009] Further, the toluene is refined toluene or toluene by-produced in other processes.

[0010] Further, the organic solvent is one or more than two of aliphatic carboxylic acid, acetonitrile, benzyl benzoate. Further, the aliphatic carboxylic acid is acetic acid, propionic acid or butyric acid.

[0011] Further preferably, the organic solvent is acetic acid.

[0012] Further, the catalyst is one or both of cobalt salt and manganese salt.

[0013] Further, the cobalt salt is one or more than two of cobalt acetate, cobalt naphthenate, cobalt iso-octoate, cobalt acetylacetonate. The manganese salt is one or more than two of manganese acetate, manganese naphthenate, manganese iso-octoate, manganese acetylacetonate. Further preferably, the catalyst is cobalt iso-octoate.

[0014] Further, the pre-reaction time is 3-4.5 h.

[0015] Further, the reaction temperature is 100-180 ℃, and the reaction pressure is 0.3-2 MPa. Further preferably, when the toluene raw material is pure toluene, the reaction temperature is 165 ℃, and the reaction pressure is 0.65 MPa; when the toluene raw material is acetic acid solution of toluene, the reaction temperature is 110 ℃, and the reaction pressure is 0.3 MPa.

[0016] Further, the delayed feeding in the acetic acid solution system of toluene can improve the conversion rate of toluene.

[0017] Further, in the specific implementation, the start-up method of the continuous liquid phase oxidation of toluene to produce benzoic acid according to the present application comprises the following steps in the batch operation: (1) placing the mixture containing the catalyst and the toluene raw material in the reactor, and setting the temperature and pressure of the reactor; (2) continuously feeding air, and the liquid volume / air flow ratio in the reactor is 0.4-2; (3) After pre-reaction for 2-6 h, continuously feed the raw material containing toluene with dissolved catalyst and withdraw the oxidation liquid at a certain flow rate to complete the start-up.

[0018] Further, in the specific implementation, the circulation operation mode in the start-up method for the continuous liquid phase oxidation of toluene to produce benzoic acid according to the present application mainly includes the following steps: (1) Put the mixture containing catalyst and toluene raw material into the reactor and set the reactor temperature and pressure; (2) Continuously feed air, and the liquid volume / air flow rate in the reactor is 0.4-2; (3) Continuously withdraw the reaction liquid and remove benzyl alcohol through the subsequent rectification system, and then recycle the withdrawn liquid from which benzyl alcohol has been removed to the reactor; (4) After pre-reaction for 2-6 h, stop the circulation and switch to the normal process to complete the start-up.

[0019] Further preferably, the liquid volume / air flow rate is 0.5.

[0020] The method according to the present application is applicable to the radical oxidation reaction of toluene or alkyl aromatic hydrocarbons.

[0021] The present application has the following advantages and beneficial effects: Compared with the existing start-up method for the liquid phase air oxidation of toluene, the method according to the present application can reach a stable production state or a higher toluene conversion rate in a shorter time, the concentration changes of reactants and products are controllable and have small fluctuations during the start-up process, and the method is suitable for continuous large-scale production. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is the relationship between toluene conversion rate and reaction time when the continuous oxidation reaction of toluene is carried out after pre-reaction for 4.5 h in an intermittent manner in a bubble column reactor using a pure toluene reaction system.

[0023] Figure 2 is the relationship between toluene conversion rate and reaction time when the continuous reaction is directly carried out without pre-reaction in a bubble column reactor using a pure toluene reaction system.

[0024] Figure 3 is the relationship between toluene conversion rate and reaction time when the continuous oxidation reaction of toluene is carried out after pre-reaction for 6 h in an intermittent manner in a bubble column reactor using a pure toluene reaction system.

[0025] Figure 4 is the relationship between toluene conversion rate and reaction time when the continuous oxidation reaction of toluene is carried out after pre-reaction for 1.5 h in an intermittent manner in a bubble column reactor using a pure toluene reaction system.

[0026] Figure 5 is the relationship between the conversion of toluene and the reaction time when the toluene continuous oxidation reaction is preceded by 4.5 h of pre-reaction in a bubble column reactor in a circulating manner using a pure toluene reaction system, wherein the circulating material is not removed of benzyl alcohol.

[0027] Figure 6 is the relationship between the conversion of toluene and the reaction time when the toluene continuous oxidation reaction is preceded by 4.5 h of pre-reaction in a bubble column reactor in a circulating manner using a pure toluene reaction system, wherein the circulating material is not removed of benzyl alcohol.

[0028] Figure 7 is the relationship between the conversion of toluene and the reaction time when the toluene continuous oxidation reaction is preceded by 4.5 h of pre-reaction in a bubble column reactor in a circulating manner using a pure toluene reaction system, wherein the circulating material is not removed of benzyl alcohol.

[0029] Figure 8 is the relationship between the conversion of toluene and the reaction time when the toluene continuous oxidation reaction is preceded by 4.5 h of pre-reaction in a bubble column reactor in a circulating manner using a pure toluene reaction system, wherein the circulating material is not removed of benzyl alcohol.

[0030] Figure 9 is the relationship between the conversion of toluene and the reaction time when the toluene continuous oxidation reaction is preceded by 3 h of pre-reaction in a bubble column reactor in a batch manner using a pure toluene / acetic acid reaction system.

[0031] Figure 10 is the relationship between the conversion of toluene and the reaction time when the toluene continuous oxidation reaction is preceded by 3 h of pre-reaction in a bubble column reactor in a batch manner using a pure toluene / acetic acid reaction system.

[0032] Figure 11 is the relationship between the conversion of toluene and the reaction time when the toluene continuous oxidation reaction is preceded by 3 h of pre-reaction in a bubble column reactor in a batch manner using a pure toluene / acetic acid reaction system.

[0033] Figure 12 is the relationship between the conversion of toluene and the reaction time when the toluene continuous oxidation reaction is preceded by 3 h of pre-reaction in a bubble column reactor in a batch manner using a pure toluene / acetic acid reaction system. DETAILED DESCRIPTION

[0034] The application will be further described in the following specific examples. The following examples are only descriptive and are not limiting, and cannot limit the protection scope of the application.

[0035] Example 1 A start-up method for the continuous liquid-phase oxidation of toluene to benzoic acid involves first weighing 50 mL of toluene and placing it in a bubbling reactor. Then, cobalt isooctanoate is weighed and dissolved in toluene to prepare a mixture with a cobalt isooctanoate concentration of 0.18 wt.%. The temperature of the bubbling reactor is slowly increased to 165 °C, the air pressure is increased to 2 MPa, and the air flow rate is adjusted to a liquid volume / air flow rate ratio of 0.5 min. -1 The reaction was initiated. After a pre-reaction period of 4.5 h in an intermittent manner, a mixture (a toluene solution with a cobalt isooctanoate concentration of 0.18 wt.%) was continuously introduced, while the product was continuously collected, and the reaction began. Results are shown in [reference needed]. Figure 1 ,Depend on Figure 1 It is evident that by reacting intermittently for 4.5 hours before starting the reaction, the toluene conversion rate can quickly reach 30% after 1.4 hours, and stabilize after 2 hours, with the conversion rate remaining stable at around 35%.

[0036] Comparative Example 1 A start-up method for the continuous liquid-phase oxidation of toluene to benzoic acid involves first weighing 50 mL of toluene and placing it in a bubbling reactor. Then, cobalt isooctanoate is weighed and dissolved in toluene to prepare a mixture with a cobalt isooctanoate concentration of 0.18 wt.%. The temperature of the bubbling reactor is slowly increased to 165 °C, the air pressure is increased to 2 MPa, and the air flow rate is adjusted to a liquid volume / air flow rate ratio of 0.5 min. -1 Without prior reaction, a mixture (a toluene solution with a cobalt isooctanoate concentration of 0.18 wt.%) was directly and continuously fed into the reactor, while the product was continuously collected to initiate the reaction. Results are shown in [reference needed]. Figure 2 ,Depend on Figure 2 It is evident that without prior reaction, the toluene conversion rate increases slowly when the continuous liquid-phase oxidation reaction is carried out directly. It takes 8.3 hours for the toluene conversion rate to reach 28%, and it has not yet reached a steady state.

[0037] Comparative Example 2 A start-up method for the continuous liquid-phase oxidation of toluene to benzoic acid involves first weighing 50 mL of toluene and placing it in a bubbling reactor. Then, cobalt isooctanoate is weighed and dissolved in toluene to prepare a mixture with a cobalt isooctanoate concentration of 0.18 wt.%. The temperature of the bubbling reactor is slowly increased to 165 °C, the air pressure is increased to 2 MPa, and the air flow rate is adjusted to a liquid volume / air flow rate ratio of 0.5 min. -1 The reaction was initiated. After a 6-hour pre-reaction in an intermittent manner, a mixture (a 0.18 wt.% toluene solution of cobalt isooctanoate) was continuously introduced while samples were continuously collected. The results are shown in [reference needed]. Figure 3 .Depend on Figure 3It is evident that by reacting intermittently for 6 hours before starting the reaction, the toluene conversion rate quickly rises to 43%, then slowly decreases, and even after another 3 hours of reaction, it still has not reached stability. This shows that an excessively long pre-reaction time actually leads to large fluctuations in the system concentration during the start-up phase.

[0038] Comparative Example 3 The only difference from Example 1 is that the reaction was pre-treated intermittently for 1.5 hours; the results are shown in [link to example]. Figure 4 .Depend on Figure 4 It is evident that by pre-reacting in an intermittent manner for 1.5 hours before starting the reaction, the toluene conversion rate only reached 32% after 9 hours. The relatively low toluene conversion rate and the long time required are due to the early continuous feeding time, which resulted in insufficient accumulation of free radical concentration.

[0039] Example 2 A start-up method for the continuous liquid-phase oxidation of toluene to benzoic acid involves first weighing 50 mL of toluene and placing it in a bubbling reactor. Then, cobalt isooctanoate is weighed and dissolved in toluene to prepare a mixture with a cobalt isooctanoate concentration of 0.18 wt.%. The temperature of the bubbling reactor is slowly increased to 165 °C, the air pressure is increased to 2 MPa, and the air flow rate is adjusted to a liquid volume / air flow rate ratio of 0.5 min. -1 Simultaneously, the product was continuously collected, and the reaction began. After benzyl alcohol was removed by subsequent distillation, the product was pumped back to the reactor to continue the reaction. After a preliminary reaction in a circulating manner for 4.5 h, a mixture (a toluene solution with a cobalt isooctanoate concentration of 0.18 wt.%) was continuously introduced, and the product was continuously collected to begin the reaction. Results are shown in [reference needed]. Figure 5 .Depend on Figure 5 It can be seen that by pre-reacting with the benzyl alcohol removal material recycling method before starting the reaction, the toluene conversion rate can quickly reach 40% after 0.1 h, and stabilize after another 3.4 h of reaction, with the conversion rate remaining stable at around 35%.

[0040] Comparative Example 4 A start-up method for the continuous liquid-phase oxidation of toluene to benzoic acid involves first weighing 50 mL of toluene and placing it in a bubbling reactor. Then, cobalt isooctanoate is weighed and dissolved in toluene to prepare a mixture (a toluene solution with a cobalt isooctanoate concentration of 0.18 wt.%). The temperature of the bubbling reactor is slowly increased to 165°C, the air pressure is increased to 2 MPa, and the air flow rate is adjusted to a liquid volume / air flow rate ratio of 0.5 min. -1 Simultaneously, the product was continuously collected, and the reaction began. The product, without removing benzyl alcohol, was directly pumped back to the reactor to continue the reaction. After a pre-reaction of 4.5 h in a circulating manner, a mixture (a toluene solution with a cobalt isooctanoate concentration of 0.18 wt.%) was continuously introduced, and the product was continuously collected to begin the reaction. Results are shown in [reference needed]. Figure 6 .Depend on Figure 6It is evident that the toluene conversion rate is extremely low, approximately 0%, when benzyl alcohol is not removed from the recycled material. A possible reason is that toluene is oxidized to a free radical reaction, and benzyl alcohol, being a free radical inhibitor, severely suppresses this reaction.

[0041] Comparative Example 5 The only difference from Example 2 is that the concentration of cobalt isooctanoate in the mixture is 0.14 wt.%, and the results are shown in [link to example]. Figure 7 .Depend on Figure 7 It is evident that when the catalyst concentration is too low, the toluene conversion rate is only 25%. Further reduction in catalyst concentration leads to a low accumulation of free radicals, resulting in a low toluene conversion rate.

[0042] Comparative Example 6 The only difference from Example 2 is that the concentration of cobalt isooctanoate in the mixture is 0.21 wt.%, and the results are shown in [link to example]. Figure 8 .Depend on Figure 8 It is evident that with an excessively high catalyst concentration, the toluene conversion rate is only 30%. This demonstrates that an excessively high catalyst concentration can also cause a slight decrease in the conversion rate.

[0043] Example 3 A start-up method for the continuous liquid-phase oxidation of toluene to benzoic acid involves first weighing 14 mL of toluene and 40 mL of acetic acid into a bubbling reactor. Then, cobalt acetate, manganese acetate, and sodium bromide (initiator) are weighed and added to the above solution to prepare a mixture. The concentrations of cobalt acetate, manganese acetate, and sodium bromide in the mixture are 0.2 wt.%, 0.4 wt.%, and 0.4 wt.%, respectively. A mixture of the same composition is prepared simultaneously as the raw material. The temperature of the bubbling reactor is then slowly increased to 110 °C, the air pressure is adjusted to 0.3 MPa, and the liquid volume / air flow rate ratio is 1.35 min. -1 After a 3-hour intermittent pre-reaction period, the prepared raw materials were continuously introduced while the product was continuously collected to initiate the reaction. Results are shown below. Figure 9 .Depend on Figure 9 It is evident that by reacting intermittently for 3 hours before starting the reaction, the toluene conversion rate can quickly reach 40% after 0.7 hours, and stabilize after 2 hours, with the conversion rate remaining stable at around 50%.

[0044] Comparative Example 7 A start-up method for the continuous liquid-phase oxidation of toluene to benzoic acid involves first weighing 14 mL of toluene and 40 mL of acetic acid into a bubbling reactor. Then, cobalt acetate, manganese acetate, and sodium bromide (initiator) are weighed and added to the above solution to prepare a mixture. The concentrations of cobalt acetate, manganese acetate, and sodium bromide in the mixture are 0.2 wt.%, 0.4 wt.%, and 0.4 wt.%, respectively. A mixture of the same composition is prepared simultaneously as the raw material. The temperature of the bubbling reactor is then slowly increased to 110 °C, the air pressure is adjusted to 0.3 MPa, and the liquid volume / air flow rate ratio is 1.35 min. -1 Without prior reaction, the prepared raw materials were directly and continuously fed in, while the product was continuously collected, and the reaction began. The results are shown in [reference needed]. Figure 10 ,Depend on Figure 10 It is evident that without prior reaction, starting the continuous reaction directly results in a low toluene conversion rate, which increases slowly over time.

[0045] Comparative Example 8 The only difference from Example 3 is that the temperature of the bubbling reactor was slowly increased to 95 °C, and the results are shown in [link to example]. Figure 11 .Depend on Figure 11 As can be seen, the toluene conversion rate is 40%. The low reaction temperature and low free radical concentration caused a slight decrease in the toluene conversion rate.

[0046] Comparative Example 9 The only difference from Example 3 is that the temperature of the bubbling reactor was slowly increased to 185 °C, and the results are shown in [link to example]. Figure 12 .Depend on Figure 12 As can be seen, the toluene conversion rate is 45%. Excessively high reaction temperatures also cause a slight decrease in the toluene conversion rate and simultaneously lead to reactor corrosion.

[0047] The above description has clearly disclosed the start-up method for the continuous liquid-phase oxidation of toluene to benzoic acid according to the present invention. Those skilled in the art will readily understand that modifications and improvements can be made to the present invention; however, any modifications and improvements made to the present invention should be within the scope of the present invention as long as they do not depart from its spirit.

Claims

1. A start-up method for the continuous liquid phase oxidation of toluene to benzoic acid, characterized in that, First, the mixture containing the catalyst and toluene raw material is placed in the reactor, then air is continuously introduced at a certain reaction temperature and reaction pressure, and pre-reaction is carried out for 2-6 h in a batch operation or a cycle operation mode; after the toluene conversion rate reaches a certain value, the mixture containing the catalyst and toluene raw material is introduced again, and continuous oxidation reaction is carried out.

2. The start-up method of claim 1, wherein In the cycle operation mode, benzyl alcohol is removed before the circulating material participates in the cycle.

3. The start-up method of claim 1, wherein The toluene raw material is pure toluene or a solution formed after pure toluene is dissolved in an organic solvent.

4. The start-up method of claim 3, wherein The organic solvent is an aliphatic carboxylic acid, acetonitrile or benzyl benzoate.

5. The start-up method of claim 1, wherein The concentration of the catalyst in the mixture is 0.15-0.20wt.%, and the catalyst is one or both of a cobalt salt and a manganese salt.

6. The start-up method of claim 5, wherein The cobalt salt is one or more of cobalt acetate, cobalt naphthenate, cobalt iso-octoate and cobalt acetylacetonate; and the manganese salt is one or more of manganese acetate, manganese naphthenate, manganese iso-octoate and manganese acetylacetonate.

7. The start-up method of claim 1, wherein The pre-reaction is carried out for 3-4.5 h.

8. The start-up method of claim 1, wherein The reaction temperature is 100-180℃, and the reaction pressure is 0.3-2MPa.

9. The start-up method of claim 1, wherein The batch operation mode includes the following steps: (1) The mixture containing the catalyst and toluene raw material is placed in the reactor, and the reactor temperature and pressure are set; (2) Air is continuously introduced, and the liquid volume / air flow ratio in the reactor is 0.4-2; (3) After pre-reaction for 2-6 h, the toluene raw material containing the dissolved catalyst is continuously introduced, and the oxidation liquid is withdrawn at a certain flow rate to complete the start-up.

10. The start-up method of claim 1, wherein The cycle operation mode includes the following steps: (1) The mixture containing the catalyst and toluene raw material is placed in the reactor, and the reactor temperature and pressure are set; (2) Air is continuously introduced, and the liquid volume / air flow ratio in the reactor is 0.4-2; (3) The reaction liquid is continuously withdrawn and benzyl alcohol is removed through a subsequent rectification system, and the withdrawn liquid from which benzyl alcohol is removed is recycled to the reactor; (4) After pre-reaction for 2-6 h, the cycle is stopped, the normal process is switched on, and the start-up is completed.

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