Method for preparing acetic acid from PTA tail gas

Abstract

The invention discloses a method for preparing acetic acid from PTA tail gas, and belongs to the technical field of chemical energy conservation and consumption reduction, the method comprises the following steps: introducing the tail gas of a PTA tail gas washing tower into a tail gas drying system, removing moisture from the tail gas discharged from the PTA tail gas washing tower, then enabling the dried tail gas to pass through a CO2 pressure swing adsorption system, and separating CO2 through the specific adsorption effect of an adsorbent on CO2 molecules, cO2 adsorption and desorption are realized, and pure CO2 is collected and enters a storage tank. The device solves the problem of energy waste caused by tail gas emission to the air of the existing PTA device, and is mainly applied to the aspect of acetic acid recovery.

Description

Technical Field

[0001] The invention relates to the technical field of chemical energy conservation and consumption reduction, and in particular to a method for preparing acetic acid by utilizing PTA tail gas. Background Art

[0002] In today's industrial production field, environmental protection and efficient use of resources have become crucial issues. For PTA (purified terephthalic acid) devices, a large amount of tail gas is generated during the production process, which contains a certain amount of CO2.

[0003] With the intensification of global climate change, the issue of CO2 emissions has attracted widespread attention. Excessive CO2 emissions not only lead to the greenhouse effect and have a negative impact on the global climate, but also cause a waste of resources. Therefore, finding an effective way to treat and utilize CO2 in the tail gas of PTA units has great practical significance.

[0004] As an important chemical raw material, acetic acid is widely used in many fields. Traditional acetic acid production methods often rely on fossil fuels, which is not only costly but also further increases CO2 emissions. Extracting CO2 from the tail gas of PTA units and converting it into acetic acid provides a new sustainable way to produce acetic acid.

[0005] On the one hand, the large-scale production of PTA units has made the potential resource of CO2 in tail gas huge. If these CO2 can be effectively extracted and converted, it can not only reduce the pressure on the environment, but also realize the recycling of resources. On the other hand, the continuous development of chemical technology also provides technical support for the extraction and conversion of CO2. At present, there are some advanced technical means to achieve efficient extraction of CO2 in tail gas and convert it into acetic acid through specific chemical reactions. Summary of the invention

[0006] The object of the present invention is to provide a method for preparing acetic acid by utilizing PTA tail gas, so as to solve the problem of energy waste caused by exhaust gas of PTA device to be discharged into the air as mentioned in the above background technology.

[0007] The present invention provides a method for preparing acetic acid by utilizing PTA tail gas, which is characterized by:

[0008] Step 1, introduce the tail gas from the PTA tail gas washing tower into the tail gas drying system, remove moisture from the tail gas discharged from the PTA tail gas washing tower, and then pass the dried tail gas through the CO2 pressure swing adsorption system, separate CO2 through the specific adsorption of CO2 molecules by the adsorbent, realize the adsorption and desorption of CO2, collect pure CO2 and enter the storage tank, the CO2 pressure swing adsorption system is composed of an absorption tower A and an absorption tower B in parallel, when the absorption tower A is adsorbing, the other absorption tower B is adsorbing for vacuum desorption regeneration, and pressure swing absorption beds are set in the absorption towers A and B, the pressure swing absorption beds use molecular sieves as absorbents and are densely packed, the CO2 pressure swing adsorption system adopts sequential automatic control, the absorption tower adsorption pressure is controlled at 0.35MPa, and the desorption pressure is -0.07MPa;

[0009] Step 2: The biogas from the public is first pre-treated to remove hydrogen sulfide and water from the biogas, and then passes through the CH4 pressure swing adsorption system. The main components of the treated biogas are CH4 and CO2. The CH4 pressure swing adsorption system is the same as the CO2 pressure swing adsorption system in step 1, except that the CH4 is discharged into the CH4 storage tank instead of being discharged externally.

[0010] Step 3: CO2 and CH4 enter the catalytic reactor for reaction. The temperature of the catalytic reactor is controlled at 200-250°C and the pressure is controlled at 1.8-2.0MPa. The thermodynamic reaction occurring in the catalytic reactor is:

[0011] The first step of the reaction: CH4+M(metal)=CHx-M+1 / 2(4-x)H2(escape)

[0012] Reaction step 2: CHx-M+CO2+1 / 2(4-x)H2(supplement)=CH3COOH+M(metal)

[0013] That is, the overall reaction: CH4+CO2=CH3COOH

[0014] Wherein M (metal) is one or two or more of Co, Cu, Pd and Rh as active metals; according to experiments, when Pd is selected as the active metal, a higher conversion rate can be obtained, and in addition, the active metal M is catalyzed by Pd and Co as a combined reaction, and TiO2 is used as a carrier, wherein Pd is the main active component, accounting for 2% of the catalyst content;

[0015] Step 4: If the surface species generated by the release of H2 in step 3 are deficient in hydrogen, hydrogen is added during the reaction, and any one of the following four methods is used to add hydrogen;

[0016] Method 1: CH4 / CO2+H2 alternating feeding reaction;

[0017] Method 2: CH4+H2 / CO2 alternate feeding reaction;

[0018] Mode 3: CH4+CO2 / H2 alternating feeding reaction;

[0019] Mode 4: CH4+CO2+O2 are fed and reacted simultaneously;

[0020] Step 5: The product after the reaction enters a gas-liquid separation tank to separate non-condensable gases such as hydrogen and carbon monoxide. The liquid phase enters a distillation tower for distillation and purification. The bottom temperature of the tower is controlled at 100°C-120°C, so that acetic acid with a higher boiling point flows out from the bottom of the distillation tower and is pumped into the acetic acid tower through a bottom pump. The organic matter in the gas phase at the top of the distillation tower is sent to the HPCCU (high pressure catalytic combustion) unit for combustion.

[0021] Preferably, in step 4, method 3 is adopted: CH4+CO2 / H2 is fed alternately for hydrogen replenishment, the switching time of the alternating feed is 300s, and the proportion of hydrogenation is controlled at 40%-60%.

[0022] Preferably, the feature is that the adsorbent is any one of activated carbon and molecular sieve.

[0023] Compared with the prior art, the technical effects and advantages of the present invention are:

[0024] 1. The present invention uses tail gas CO2 and public biogas CH4 to produce acetic acid, which reduces carbon emissions and production costs.

[0025] 2. The present invention utilizes Pd and Co as a combined reaction catalyst, uses TiO2 as a carrier, wherein Pd is the main active component, to prepare acetic acid, has a high conversion rate and selectivity, reduces CO2 emissions by 13,400 tons per year, and generates 16,400 tons of acetic acid.

[0026] 3. The CO2 and CH4 adsorption separation of the present invention adopts the same CO2 pressure swing adsorption technology using molecular sieves, which can directly save costs.

[0027] 4. The present invention adopts CH4+CO2 / H2 alternating feeding and improves the reaction efficiency by controlling the time of cyclic feeding.

[0028] 5. Use pretreatment to remove hydrogen sulfide and moisture from biogas to prevent corrosion of pipelines and permanent poisoning of adsorbents. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a flow chart of the process processing system of the present invention. DETAILED DESCRIPTION

[0030] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] In one embodiment; a method for producing acetic acid using PTA tail gas, characterized in that:

[0032] Step 1, introduce the tail gas from the PTA tail gas washing tower into the tail gas drying system, remove moisture from the tail gas discharged from the PTA tail gas washing tower, and then pass the dried tail gas through the CO2 pressure swing adsorption system, separate CO2 through the specific adsorption of CO2 molecules by the adsorbent, realize the adsorption and desorption of CO2, collect pure CO2 and enter the storage tank, the CO2 pressure swing adsorption system is composed of an absorption tower A and an absorption tower B in parallel, when the absorption tower A is adsorbing, the other absorption tower B is adsorbing for vacuum desorption regeneration, and pressure swing absorption beds are set in the absorption towers A and B, the pressure swing absorption beds use molecular sieves as absorbents and are densely packed, the CO2 pressure swing adsorption system adopts sequential automatic control, the absorption tower adsorption pressure is controlled at 0.35MPa, and the desorption pressure is -0.07MPa;

[0033] The specific process of CO2 pressure swing adsorption system is as follows:

[0034] After the absorption tower A completes the adsorption, the first pressure equalization and pressure reduction is carried out to release the gas from the absorption tower A to the absorption tower B. This stage runs for 1500s.

[0035] After the pressure equalization of the absorption tower A is completed, the pressure is reduced to normal pressure in the opposite direction of adsorption, and the C02 is desorbed and sent to the storage tank. This stage runs for 900s;

[0036] After the CO2 inversion of the absorption tower A is completed, the tower is evacuated using a vacuum pump to regenerate the molecular sieve adsorbent. This stage runs for 600 seconds.

[0037] After the absorption tower A is evacuated, the pressure-equalizing and depressurizing gas from the absorption tower B is used for pre-pressurization. This stage runs for 1500s.

[0038] After the pre-pressurization of the absorption tower A is completed, the feed tail gas is used to continuously increase the pressure to reach the required adsorption pressure. At this time, CO2 is absorbed by the bed layer and the remaining gas is discharged. This stage runs for 1500s;

[0039] After the pressure increase of the absorption tower A is completed, the pressure equalization and pressure reduction process of the absorption tower A is entered again. Through the alternating operation of the two towers AB, the tail gas is continuously treated;

[0040] In step two, the biogas from the public is first pretreated to remove hydrogen sulfide and water from the biogas, and then passes through the CH4 pressure swing adsorption system. The main components of the treated biogas are CH4 and CO2. The CH4 pressure swing adsorption system is consistent with the CO2 pressure swing adsorption system in step one, and also achieves gas separation by adsorbing CO2. However, since the raw gas entering the adsorption system is different, the reaction tail gas of the device enters the adsorption system in step one, so the unabsorbed gas is directly discharged to the outside in the fifth stage of the absorption tower. In step two, the treated biogas is used as the raw material, and the unabsorbed gas CH4 is the raw material required for the reaction, so the CH4 is discharged into the CH4 storage tank and not discharged to the outside. This is different from the process of step one;

[0041] Step 3: CO2 and CH4 enter the catalytic reactor for reaction. The temperature of the catalytic reactor is controlled at 200-250°C and the pressure is controlled at 1.8-2.0MPa. The thermodynamic reaction occurring in the catalytic reactor is:

[0042] The first step of the reaction: CH4+M(metal)=CHx-M+1 / 2(4-x)H2(escape)

[0043] Reaction step 2: CHx-M+CO2+1 / 2(4-x)H2(supplement)=CH3COOH+M(metal)

[0044] That is, the overall reaction: CH4+CO2=CH3COOH

[0045] Wherein M (metal) is one or two or more of Co, Cu, Pd and Rh as active metals; according to experiments, when Pd is selected as the active metal, a higher conversion rate can be obtained, and in addition, the active metal M is catalyzed by Pd and Co as a combined reaction, and TiO2 is used as a carrier, wherein Pd is the main active component, accounting for 2% of the catalyst content;

[0046] Step 4: The release of H2 in step 3 makes the generated surface species lack hydrogen, and hydrogen is added during the reaction. How to add hydrogen to the catalytic reactor becomes a factor affecting the catalytic reaction, because through the two steps of the thermodynamic reaction, it can be found that hydrogen is involved. Whether hydrogen is mixed with CH4 into the catalytic reactor, or mixed with CO2 into the catalytic reactor, or hydrogen enters the catalytic reactor alone, it may affect the result of the reaction, so we conducted a comparative experiment on the effect of four feeding methods on the reaction, including three hydrogen feeding methods and one oxygen feeding method.

[0047] Method 1: CH4 / CO2+H2 alternating feed reaction

[0048] Method 2: CH4+H2 / CO2 alternating feed reaction

[0049] Method 3: CH4+CO2 / H2 alternating feed reaction

[0050] Method 4: CH4+CO2+O2 simultaneous feeding reaction

[0051] In the feed method one, hydrogen and CO2 are mixed and then alternately fed into the catalytic reactor with CH4. The reason for choosing this method is that in the second step of the thermodynamic reaction, hydrogen is supplemented during the CO2 addition stage. In the feed method two, hydrogen and CH4 are mixed and then alternately fed into the catalytic reactor with CO2. The reason for choosing this method is that in the first step of the thermodynamic reaction, the dehydrogenation of CH4 is controlled, and a small amount of hydrogen can increase the conversion rate of C2+ (two-carbon and above organic matter). In the feed method three, CO2 and CH4 are mixed and then alternately fed into the catalytic reactor with hydrogen. This method takes into account that the dissociation and adsorption of CH4 can be promoted by CO2. In addition, the simultaneous feeding of the two gases can directly generate acetic acid on the catalyst surface, so hydrogen is used alone to supplement hydrogen. In the feed method four, CH4, CO2 and O2 are simultaneously fed into the catalytic reactor. The oxygen-supplementing feed is designed in the hope of changing the thermodynamic process through oxygen coupling and reducing the activation energy required for the reaction.

[0052]

[0053] Through experiments, the selectivity of acetic acid in the product of oxygen-supplemented feed in mode 4 is as high as 89%, but the lowest conversion rate results in less product. The acetic acid obtained by the three feeding methods of hydrogenation is higher than that of oxygen-supplemented feed, which is related to the catalyst used. For example, the use of oxygen-supplemented feed requires an additional suitable catalyst. The hydrogen-supplemented feed in mode 1 makes the selectivity of acetic acid as high as 94%, and the conversion rate also reaches 80%, but the reaction rate of this feeding method is the slowest and cannot meet the needs of the device. The hydrogen-supplemented feed in mode 2 can control the activation of CH4 and improve the conversion rate of CH4, but the selectivity of acetic acid is the lowest, only about 72%. At the same time, the product by-products are more complex, resulting in the difficulty and cost of subsequent separation process design. Relatively speaking, the feed in mode 3 can obtain a higher conversion rate and selectivity. The acetic acid content in the converted product accounts for 83%, and the remaining 17% of the by-products are organic substances such as methanol, ethanol and formaldehyde, whose boiling points are much lower than acetic acid for easy separation. At the same time, the reaction rate can also meet the needs. Therefore, the reaction adopts the feeding method of mode 3, that is, CH4 and CO2 are mixed and pressurized and then fed alternately with hydrogen alone. The switching time of the alternate feeding is 300s, and the proportion of hydrogenation is controlled at 40%-60%.

[0054] The products after the reaction enter the gas-liquid separation tank to separate non-condensable gases such as hydrogen and carbon monoxide. The liquid phase enters the distillation tower for distillation and purification. The bottom temperature of the tower is controlled at about 100℃-120℃, so that acetic acid with a higher boiling point flows out from the bottom of the distillation tower and is pumped into the acetic acid tower through the bottom pump. Organic matter in the gas phase at the top of the distillation tower, such as alcohols and aldehydes, is sent to the HPCCU (high pressure catalytic combustion) unit for combustion.

[0055] In one embodiment, the adsorbent is any one of activated carbon and molecular sieve.

Claims

1. A method for producing acetic acid using PTA tail gas, characterized in that: Step 1, introduce the tail gas from the PTA tail gas washing tower into the tail gas drying system, remove moisture from the tail gas discharged from the PTA tail gas washing tower, and then pass the dried tail gas through the CO2 pressure swing adsorption system, separate CO2 through the specific adsorption of CO2 molecules by the adsorbent, realize the adsorption and desorption of CO2, collect pure CO2 and enter the storage tank, the CO2 pressure swing adsorption system is composed of an absorption tower A and an absorption tower B in parallel, when the absorption tower A is adsorbing, the other absorption tower B is adsorbing for vacuum desorption regeneration, and pressure swing absorption beds are set in the absorption towers A and B, the pressure swing absorption beds use molecular sieves as absorbents and are densely packed, the CO2 pressure swing adsorption system adopts sequential automatic control, the absorption tower adsorption pressure is controlled at 0.35MPa, and the desorption pressure is -0.07MPa; Step 2: The biogas from the public is first pre-treated to remove hydrogen sulfide and water from the biogas, and then passes through the CH4 pressure swing adsorption system. The main components of the treated biogas are CH4 and CO2. The CH4 pressure swing adsorption system is the same as the CO2 pressure swing adsorption system in step 1, except that the CH4 is discharged into the CH4 storage tank instead of being discharged externally. Step 3: CO2 and CH4 enter the catalytic reactor for reaction. The temperature of the catalytic reactor is controlled at 200-250°C and the pressure is controlled at 1.8-2.0MPa. The thermodynamic reaction occurring in the catalytic reactor is: The first step of the reaction: CH4+M(metal)=CHx-M+1 / 2(4-x)H2(escape) Reaction step 2: CHx-M+CO2+1 / 2(4-x)H2(supplement)=CH3COOH+M(metal) That is, the overall reaction: CH4+CO2=CH3COOH The active metal M uses Pd and Co as a combined reaction catalyst, using TiO2 as a carrier, in which Pd is the main active component, accounting for 2% of the catalyst content; Step 4: If the surface species generated by the release of H2 in step 3 are deficient in hydrogen, hydrogen is added during the reaction, and any one of the following four methods is used to add hydrogen; Method 1: CH4 / CO2+H2 alternating feeding reaction; Method 2: CH4+H2 / CO2 alternate feeding reaction; Mode 3: CH4+CO2 / H2 alternating feeding reaction; Mode 4: CH4+CO2+O2 are fed and reacted simultaneously; Step 5: The product after the reaction enters the gas-liquid separation tank to separate non-condensable gases such as hydrogen and carbon monoxide. The liquid phase enters the distillation tower for distillation and purification. The bottom temperature of the tower is controlled at 100°C-120°C, so that acetic acid with a higher boiling point flows out from the bottom of the distillation tower and is pumped into the acetic acid tower through the bottom pump. The organic matter in the gas phase at the top of the distillation tower is sent to the HPCCU unit for combustion.

2. A method for producing acetic acid using PTA tail gas according to claim 1, characterized in that: In step 4, method 3 is adopted: CH4+CO2 / H2 is fed alternately for hydrogen supplementation, the switching time of the alternating feed is 300s, and the proportion of hydrogenation is controlled at 40%-60%.

3. A method for producing acetic acid using PTA tail gas according to claim 1, characterized in that: The adsorbent is either activated carbon or molecular sieve.

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