TWI391372B - Method for recovering hydrogen from exhaust gas in refined phthalic acid plant - Google Patents

Description

Method for recovering hydrogen in a gas stream discharged from a phthalic acid plant

The invention relates to a method for effectively treating and recovering a process gas discharged from a phthalic acid production process, in particular to a method for removing harmful impurities of exhaust gas and recovering hydrogen in a gas discharged from a phthalic acid plant. .

Here is a brief introduction to the typical pure terephthalic acid production process to facilitate the description of this patent. The typical steps are as follows:

1. In the case of stepwise temperature rise and pressure increase, the impure crude terephthalic acid is completely dissolved in water.

2. The solution finally reaches a certain temperature and pressure. In the presence of the Group VIII metal and excess hydrogen, the hydrogen is subjected to a hydrogenation reduction reaction on the impurities present in the impure terephthalic acid, and the impurities such as 4-carboxybenzaldehyde are reduced to an easy A substance such as methylbenzoic acid separated by terephthalic acid.

3. By reducing the pressure of the treated solution, the terephthalic acid crystals are crystallized in one step or more steps, and the water and dissolved inert gas or other organic impurities are evaporated to form water vapor, inert gas, An exhaust gas stream of unreacted excess hydrogen and volatile impurities.

4. In order to effectively utilize the heat energy generated by crystallization, this part of the exhaust gas stream is often used as a preheating solution for the dissolution of impure terephthalic acid, so that most of the water vapor is condensed, but still contains a large amount of water vapor. Hydrogen, carbon monoxide and other volatile gases.

Most of the existing processes directly discharge these remaining exhaust gases into the atmosphere. There is no recycling of hydrogen, which causes waste of resources. The production process of purified phthalic acid or purified isophthalic acid mentioned in this patent is basically similar to that of purified terephthalic acid, and will not be described in detail herein.

China's CN1264360A patent proposes a method for treating and recovering hydrogen in the exhaust gas stream of a purified terephthalic acid plant, converting carbon monoxide and water vapor in the exhaust gas stream into carbon dioxide and hydrogen to improve the purity of hydrogen, but the patent is in use. The reformer or pressure swing adsorption method does not take effective measures to remove the organic substances present in the exhaust gas stream, including benzoic acid, p-toluic acid, phthalic acid, etc. before the hydrogen purification process, which will be cooled in the subsequent Condensation crystallization in the process, and thus has a serious impact on the bed blockage caused by the reformer and the pressure swing adsorption system; at the same time, the acidic organic matter present in the exhaust gas stream will have a serious impact on the catalyst composed of copper oxide; therefore, The process of direct conversion by treatment with harmful substances is not feasible. In addition, the specification of the patent also indicates that the methanation reaction can be used to convert carbon monoxide to methane gas to remove carbon monoxide. However, in the actual operation, when the newly generated methane gas is recovered synchronously with hydrogen, Pd in the hydrogenation reaction will also be caused. The -C catalyst is temporarily deactivated, affecting the normal reaction of the hydrogenation reactor. In addition, the concentration of hydrogen recovered cannot be guaranteed to meet the requirements of subsequent processes.

It is an object of the present invention to provide a method for recovering hydrogen in a vent gas stream of a phthalic acid plant to solve the above problems in the prior art. The invention removes the acidic substance and the high-boiling and easily crystallized substance in the process gas discharged during the crystallization of the phthalic acid by the rinsing treatment, thereby making it safe The whole place is applied to the subsequent process.

The technical solution provided by the present invention is as follows: a method for recovering hydrogen in a gas stream discharged from a phthalic acid plant, comprising the following steps:

A1. The exhaust gas directly obtained when the phthalic acid is recrystallized is kept at 200 ° C to 300 ° C and rinsed with a small amount of an alkali solution.

A2. The exhaust gas directly obtained when the phthalic acid is recrystallized is kept at 200 ° C to 300 ° C and alkali treated with a small amount of alkali solution, and the alkali washed gas is introduced into the reformer; the temperature of the reformer is controlled at 200 ° C to 300 ° C. The pressure is 0 to 4.6 MPa, and is catalyzed by a methanol cracking catalyst to convert carbon monoxide in the gas with water vapor to produce hydrogen.

A3. The exhaust gas directly obtained when the phthalic acid is recrystallized is kept at 200 ° C to 300 ° C with a small amount of alkali solution alkali washing, the alkali washed gas is introduced into the reformer; the temperature of the reformer is controlled at 200 ° C to 300 ° C The pressure is 0 to 4.6 MPa, methanol vapor is introduced into the reformer, and catalyzed by a methanol cracking catalyst, carbon monoxide and methanol vapor in the gas are reacted with water vapor to generate hydrogen.

b. introducing the exhaust gas treated by step a1, a2 or a3 into a heat exchanger or a cooling tower to condense the condensable gas; and introducing the obtained hydrogen rich stream into the pressure swing adsorption device to remove the hydrogen rich stream Other impurities, giving a hydrogen concentration of 99.99 wt% or more, carbon monoxide concentration 5 ppm wt of pure hydrogen recovery gas stream.

Among them, the carbon monoxide in the exhaust gas directly obtained during the recrystallization of phthalic acid accounts for the total amount of gas other than water. At 3000 ppm wt, the process proceeds to step a1 or a3; after detecting, the carbon monoxide in the exhaust gas directly obtained in the recrystallization of the phthalic acid accounts for >3000 ppm wt of the total amount of gas other than water, and proceeds to step a2 or a3; Wherein a1, a2 and a3 are optional ones, and the exhaust gas either passes through steps a1 to b; or through steps a2 to b; or through steps a3 to b.

The alkaline washing treatment of the exhaust gas ensures that the subsequent steps are carried out normally, preferably by hot alkali treatment, and most of the acidic substances and high-boiling crystallizable substances are removed in this step. This step can enter the pressure swing adsorption system while maintaining a high pressure, or enter the pressure swing adsorption system at a lower pressure to remove impurities such as carbon monoxide and carbon dioxide, and purify the hydrogen. Finally, the hydrogen purified by the pressure swing adsorption system is pressurized and added to the fresh hydrogen gas, and then enters the hydrogenation reactor for reaction. In order to ensure the quality of the hydrogenation reaction, the purity of the recovered hydrogen stream should be strictly monitored, and the unqualified gas should be selected for discharge or re-adsorption treatment.

The reaction formula of the reaction of carbon monoxide with steam to be converted into hydrogen is as follows: CO + H ₂ O = CO ₂ + H ₂ , and the conversion rate of carbon monoxide is up to 99%.

The reaction of methanol to steam reaction to hydrogen is as follows: CH ₃ OH + H ₂ O = CO ₂ + 3H ₂ .

In the above method for recovering hydrogen from a gas stream in a phthalic acid plant, the phthalic acid is at least one of meta-dibenzoic acid, p-xylene acid or phthalic acid.

In the foregoing method for recovering hydrogen from a gas stream in a phthalic acid plant, the alkali The alkali solution for washing treatment is 1 wt% to 45 wt% of NaOH or KOH solution, the pressure of the alkali solution is slightly higher than the pressure of the exhaust gas, and the temperature of the alkali solution should be less than or equal to the temperature of the exhaust gas. The pressure of the lye is slightly higher than the pressure of the exhaust gas to ensure that the exhaust gas can be sufficiently washed.

In the foregoing method for recovering hydrogen from a gas stream of a phthalic acid plant, the alkali washing treatment step may be carried out in a spray tower, a packed column or a tray column.

In the above method for recovering hydrogen in the gas stream of the phthalic acid plant, the water washing treatment step may be included before or after the alkali washing treatment step to pre-wash or prevent the gas from entraining the metal ions such as sodium and potassium of the alkaline liquid. The alkali washing treatment may be a multi-stage treatment to more thoroughly remove the acidic substances and the high boiling point crystallizable substances therein.

In the foregoing method for recovering hydrogen in a gas stream from a phthalic acid plant, the pressure swing adsorption unit in step b has a pressure operation range of 0 to 4 MPa and a temperature range of 55 °C.

In the foregoing method for recovering hydrogen in a gas stream from a phthalic acid plant, the exhaust gas is H ₂ , carbon monoxide, water vapor, carbon dioxide, acetic acid, benzoic acid, p-methylbenzoic acid or trace amount of phthalic acid entrained in the phthalic acid. At least one of them.

In the above method for recovering hydrogen in a gas stream from a terephthalic acid plant, the condensable gas is at least one of water vapor, acetic acid, p-toluic acid, benzoic acid or phthalic acid.

In the method for recovering hydrogen in a gas stream discharged from the terephthalic acid plant, the acidic substance is at least one of organic substances such as carbon dioxide, benzoic acid, acetic acid, methylbenzoic acid or phthalic acid.

In the foregoing method for recovering hydrogen from a gas stream of a phthalic acid plant, the methanol cracking catalyst is a copper-based catalyst and other usable catalysts.

The production process of the purified phthalic acid or the purified isophthalic acid mentioned in the present invention is basically similar to that of the purified terephthalic acid, and will not be described in detail herein.

In the present invention, the term "slightly" refers to an interval around the value in question. "Slightly lower than X" as used in the present application means an interval of 10% from X to X minus X, preferably a range of 5% from X to X minus X; and as used in the present invention Above X" represents a 10% interval from X to X plus X, preferably a range of 5% from X to X plus X.

Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Also, all publications, patent applications, patents, and other references are herein incorporated by reference.

The invention can recover the hydrogen in the process gas discharged during the crystallization of the phthalic acid to a certain concentration, and at the same time reduce the carbon monoxide emission, so that it can be safely applied to the subsequent process, and is suitable for various kinds of benzene. The formic acid production process, and can be applied to the process gas for the recovery of all hydrogen-containing crystallization processes, is a highly economical and environmentally valuable hydrogen recovery method. Since the exhaust gas of the present invention is subjected to alkali washing before entering the reforming furnace and the pressure swing adsorption system, the organic substances such as acetic acid, methylbenzoic acid and phthalic acid contained in the gas are eluted in the alkali washing step, and will not The reforming furnace and the pressure swing adsorption system form a blockage, and at the same time, it does not affect the catalytic effect of the catalyst composed of copper oxide or the like.

The invention will be further understood by the examples of the invention given below, but the scope of the invention is not limited thereto.

First embodiment:

The amount of materials in this embodiment is based on the parts by weight, and in the process gas discharged during the crystallization of the purified terephthalic acid to be treated, the supply amount per unit time contains 1000 parts of hydrogen and 1 part of CO. 20,000 parts of water vapor, 4 parts of CO ₂ , 0.5 parts of benzoic acid, 0.5 parts of p-toluic acid and 2 parts of other impurities, a pressure of 35 kg/cm ² g (gauge pressure), and a temperature of 240 ° C.

The exhaust gas is sent to the spray tower, and is alkali-washed with 35.5 kg/cm ² g, 40 ° C, and 7 wt% NaOH solution. The pressure of the sodium hydroxide [NaOH] solution is slightly higher than the pressure of the exhaust gas to ensure sufficient washing. Remove acid from the exhaust gas. After sufficiently washing with the alkali solution, 1000 parts of hydrogen, 1 part of carbon monoxide, 8000 parts of water vapor, and 2 parts of other impurities are obtained.

The alkali-washed exhaust gas is sent to a cooling tower or a heat exchanger, the gas is cooled to 35 ° C, water vapor is condensed therein, and then 1000 parts of hydrogen, 1 part of carbon monoxide, 2 parts of other impurities, and a small amount of water are contained. The hydrogen rich stream of steam is introduced into the pressure swing adsorption unit to obtain 1000 parts of pure recovered hydrogen containing a very small amount of impurities. The pure recovered hydrogen is introduced into the hydrogenation reactor for recycling.

Second embodiment:

The amount of materials in the present embodiment is based on the parts by weight, and in the process gas discharged during the crystallization of the purified terephthalic acid to be treated, the supply amount per unit time contains 100 parts of hydrogen and 15 parts of carbon monoxide. 10000 parts of water vapor, 4 parts of CO ₂ , 0.5 parts of benzoic acid, 0.8 parts of p-toluic acid and 1 part of other impurities, a pressure of 35 kg/cm ² g (gauge pressure), and a temperature of 240 ° C.

The exhaust gas is sent to the tray tray and rinsed with 35.5 kg/cm ² g, 50 ° C, and 15 wt% KaOH solution. The pressure of the NaOH solution is slightly higher than the pressure of the exhaust gas to ensure sufficient washing and removal of the exhaust gas. The acidic substance in the medium is sufficiently washed with an alkali solution to obtain an exhaust gas containing 100 parts of hydrogen, 15 parts of carbon monoxide, 7500 parts of water vapor, and 1 part of other impurities.

The alkali-washed exhaust gas is sent to a reforming furnace having a pressure of 30 kg/cm ² g, and the temperature of the reforming furnace is controlled at 200 ° C to 220 ° C, and a copper-based catalyst is used to catalyze the reaction to obtain 114 parts of hydrogen and 1 part of carbon monoxide. 14 parts of carbon dioxide, 1 part of other impurities and a large amount of water vapor.

The converted exhaust gas is sent to a cooling tower, the gas is cooled to 50 ° C, water vapor is condensed therein, and then 114 parts of hydrogen, 14 parts of carbon dioxide, 1 part of carbon monoxide and 1 part of other impurities and a small amount of water vapor are contained. The hydrogen rich stream was introduced into the pressure swing adsorption unit to obtain 114 parts of pure recovered hydrogen containing a very small amount of impurities. The pure recovered hydrogen is introduced into the hydrogenation reactor for recycling.

Third embodiment:

The amount of materials in this embodiment is based on the parts by weight, and in the process gas discharged during the crystallization of the purified terephthalic acid to be treated, 50 parts of hydrogen and 15 parts of carbon monoxide are contained in the supply amount per unit time. 10000 parts of water vapor, 9 parts of carbon dioxide, 0.5 parts of benzoic acid, 0.2 parts of p-methylbenzoic acid, 0.5 parts of other impurities, a pressure of 35 kg/cm ² g (gauge pressure), and a temperature of 240 ° C.

The exhaust gas is sent to the tray tray and rinsed with 35.5 kg/cm ² g, 40 ° C, and 15 wt% NaOH solution. The pressure of the NaOH solution is slightly higher than the pressure of the exhaust gas to ensure sufficient washing and removal of the exhaust gas. Acidic substance. After washing with sufficient alkali washing, an exhaust gas containing 50 parts of hydrogen, 15 parts of carbon monoxide, 7500 parts of water vapor, and 0.5 part of other impurities was obtained.

The alkali-washed gas is introduced into a reforming furnace having a pressure of 30 kg/cm ² g, and the temperature of the reforming furnace is controlled at 280 ° C to 300 ° C, and 50 parts of methanol vapor is added thereto, and catalyzed by a copper-based catalyst. Carbon monoxide, methanol and steam are converted into hydrogen to obtain an exhaust gas containing 211.5 parts of hydrogen, 0.5 part of CO, 7435.5 parts of water vapor, 63.5 parts of carbon dioxide, 1 part of methanol, and 0.5 parts of other impurities.

The converted exhaust gas is sent to a cooling tower, the gas is cooled to 35 ° C, water vapor and methanol are condensed therein, and then hydrogen-rich 211.5 parts of hydrogen, 0.5 parts of carbon monoxide, 63.5 parts of carbon dioxide, and a small amount of water vapor are added. The flow was introduced into a pressure swing adsorption unit to obtain 211.5 parts of pure recovered hydrogen gas containing a very small amount of other impurities. The pure recovered hydrogen is then introduced into the hydrogenation reactor for use.

The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and combinations thereof may be made without departing from the spirit and scope of the invention. Simplifications should all be equivalent replacements and are included in the scope of the present invention.

Claims (10)

A method for recovering hydrogen in a gas stream discharged from a phthalic acid plant comprises the following steps: a1. maintaining an exhaust gas directly obtained by recrystallizing phthalic acid at 200 ° C to 300 ° C with a small amount of alkali solution, and removing the same Acidic substance and high boiling point easily crystallized substance; a2. The exhaust gas directly obtained when recrystallizing the phthalic acid is kept at 200 ° C to 300 ° C, and alkali-washed with a small amount of alkali solution to remove the acidic substance and the high boiling point easily crystallized substance. The alkali-washed gas is introduced into the reforming furnace; the temperature of the reforming furnace is controlled at 200 ° C to 300 ° C, the pressure is 0 to 4.6 MPa, and the carbon monoxide in the gas is reacted with steam to be converted into hydrogen by a methanol cracking catalyst; a3. The exhaust gas directly obtained when the phthalic acid is recrystallized is kept at 200 ° C to 300 ° C and washed with a small amount of an alkali solution to remove the acidic substance and the high boiling point easily crystallized substance, and the alkali washed gas is introduced into the reforming furnace; The temperature of the reformer is controlled at 200 ° C to 300 ° C, the pressure is 0 to 4.6 MPa, methanol vapor is introduced into the reformer, and the catalyst is catalyzed by a methanol cracking catalyst. The carbon monoxide and the methanol vapor are reacted with water vapor to be converted into hydrogen; b. the exhaust gas treated by the step a1, a2 or a3 is introduced into a heat exchanger or a cooling tower to condense the condensable gas; and the obtained hydrogen rich gas is further obtained. The flow is introduced into the pressure swing adsorption device to remove other impurities in the hydrogen-rich gas stream, and the hydrogen concentration is above 99.99 wt%, and the carbon monoxide concentration is obtained. 5 ppm wt of pure hydrogen recovery gas stream; wherein, in the exhaust gas obtained directly during the recrystallization of phthalic acid, carbon monoxide in the exhaust gas accounts for the total amount of gas other than water At 3000 ppm wt, the process proceeds to step a1 or a3; after detecting, the carbon monoxide in the exhaust gas directly obtained in the recrystallization of the phthalic acid accounts for >3000 ppm wt of the total amount of gas other than water, and proceeds to step a2 or a3; Wherein a1, a2 and a3 are optional ones, and the exhaust gas either passes through steps a1 to b; or through steps a2 to b; or through steps a3 to b. A method for recovering hydrogen in a gas stream discharged from a terephthalic acid plant as described in claim 1, wherein the phthalic acid is at least one of meta-dibenzoic acid, p-xylene acid or phthalic acid. A method for recovering hydrogen in a gas stream discharged from a phthalic acid plant according to claim 1 or 2, wherein the alkali solution for the alkali washing treatment is 1 wt% to 45 wt% of sodium hydroxide [NaOH] or potassium hydroxide. [KOH] solution, the pressure of the alkali solution is slightly higher than the pressure of the exhaust gas, and the temperature of the alkali solution should be less than or equal to the temperature of the exhaust gas. A method for recovering hydrogen in a gas stream of a phthalic acid plant as claimed in claim 3, wherein the alkali washing treatment step is carried out in a spray tower, a packed column or a tray column. A method for recovering hydrogen in a gas stream of a phthalic acid plant as claimed in claim 1 or 2, wherein the alkali washing treatment step further comprises a water washing treatment step before or after the alkali washing treatment step. A method for recovering hydrogen in a gas stream of a phthalic acid plant as claimed in claim 1 or 2, wherein the pressure swing adsorption device of the step b has a pressure operation range of 0 to 4 MPa, and a temperature range 55 ° C. A method for recovering hydrogen in a gas stream of a phthalic acid plant as claimed in claim 1 or 2, wherein the exhaust gas is hydrogen, mono-oxidation At least one of carbon, water vapor, carbon dioxide, acetic acid, benzoic acid, p-toluic acid or phthalic acid. A method for recovering hydrogen in a gas stream of a phthalic acid plant as claimed in claim 1 or 2, wherein the condensable gas is water vapor, acetic acid, p-toluic acid, benzoic acid or phthalic acid. At least one of them. A method for recovering hydrogen in a gas stream of a phthalic acid plant as claimed in claim 1 or 2, wherein the acid substance is at least one of carbon dioxide, benzoic acid, acetic acid, methylbenzoic acid or phthalic acid. . A method of recovering hydrogen in a gas stream of a phthalic acid plant as claimed in claim 1 or 2, wherein the methanol cracking catalyst comprises a copper-based catalyst.