Method for reducing salt-containing wastewater discharge amount in DMO synthesis process

Abstract

The invention discloses a method for reducing the discharge amount of salt-containing wastewater in a DMO synthesis process. The process method comprises the following steps: recovering nitric acid, supplementing total nitrogen of a system and reducing the salt content of the wastewater by additionally arranging a nitric acid reducer and utilizing gas-phase bubbling reverse contact, so that the salt-containing wastewater can be better decomposed and treated on the premise of not using a reduction catalyst, and the controllability and no harmfulness of the system are ensured; meanwhile, the nitric acid waste liquid treated by the nitric acid reducer is neutralized again by adopting an alkaline substance, so that the mass of the nitric acid finally discharged into a reclaimed water recycling system can be reduced to be below 43kg / h, and the concentration of the nitric acid is reduced to be below 0.1 wt%. The method not only greatly reduces the generation of salt-containing wastewater, but also reutilizes HNO3, and the methyl nitrite (MN) generated by the reaction can be used as a reaction raw material to return to a previous system for reaction, so that the consumption of the reaction raw material is reduced, and the system energy consumption is reduced while the wastewater discharge is reduced.

Description

Technical Field

[0001] The invention relates to the field of dimethyl oxalate (DMO), and in particular to a method for reducing the discharge amount of saline wastewater in a DMO synthesis process. Background Art

[0002] The main synthesis process of dimethyl oxalate (DMO) is that methanol reacts with nitrogen oxides to form methyl nitrite (MN), and methyl nitrite is coupled with CO on a catalyst of active components such as Pd to form methyl oxalate (DMO); NO in the carbonylation reaction product is reoxidized to N2O3, and methyl nitrite (MN) is produced in a cycle. The reactions occurring in the MN regeneration tower system and the nitric acid reduction reactor are shown in the following equations (1) and (2), respectively:

[0003] 2NO+1 / 2O2+2MeOH→2MN+H2O (1)

[0004] HNO3+2NO+3MeOH→3MN+2H2O (2)

[0005] During startup, the NO required by the MN regeneration tower system and the nitric acid reducer system is generated by the reaction of sodium nitrite and nitric acid.

[0006] At present, the by-product nitric acid in the dimethyl oxalate synthesis process is mainly treated in two ways at home and abroad: one is to neutralize it with sodium hydroxide and then send it directly to the wastewater treatment device, which has high requirements for the subsequent wastewater device and a large processing volume, resulting in the loss of nitrogen and imbalance of the total nitrogen in the system; the second is to use a catalyst to convert the by-product nitric acid into methyl nitrite (MN), and then recycle it. Although this method solves the problem of salt content in wastewater and the problem of total nitrogen in the system, the catalyst will be brought to every corner of the DMO synthesis system with the system, increasing the uncertainty and harmfulness of the system (Anyang Yongjin compressor inlet buffer tank burning accident, Xinhang Energy esterification tower inlet elbow burning accident, etc.).

[0007] The existing technology does not effectively recycle the HNO3 solution in the MN regeneration tower: on the one hand, a large amount of NaOH is consumed to neutralize the HNO3 solution, and a large amount of saline wastewater is generated after neutralization. The saline wastewater needs to be discharged into the sewage treatment system for treatment. The evaporation and crystallization through the evaporation and crystallization unit greatly increases the processing load of the post-system; on the other hand, the use of the reduction catalyst also increases the operating energy consumption and cost of the device, seriously affecting the long-term stable operation of the device. To this end, we provide a method for reducing the discharge of saline wastewater in the DMO synthesis process. Summary of the invention

[0008] The purpose of the present invention is to provide a method for reducing the discharge of saline wastewater in the DMO synthesis process in view of a series of problems such as large alkali consumption, difficult treatment of saline wastewater, and high operating cost of the device caused by HNO3 produced by the side reaction of the MN regeneration tower.

[0009] The present invention is achieved through the following technical solutions:

[0010] A method for reducing the discharge of saline wastewater in a DMO synthesis process comprises the following steps:

[0011] Step 1: Pd / Al2O3 is used as a catalyst to catalyze the reaction of carbon monoxide (CO) and methyl nitrite (MN) in a fixed bed reactor to synthesize dimethyl oxalate (DMO), while generating nitric oxide (NO) as a byproduct. NO is then oxidized to dinitrogen trioxide (N2O3), and MN is cyclically produced for the synthesis of DMO;

[0012] Step 2: Nitric oxide, oxygen and C1-C4 alcohol are used to synthesize methyl nitrite in a MN regeneration tower, and nitric acid is produced as a by-product. The synthesized methyl nitrite is used as an intermediate product to synthesize DMO;

[0013] Step 3: In the DMO synthesis reaction, water vapor and oxygen will reduce the selectivity of DMO and the activity of the catalyst. Separating the DMO synthesis and methyl nitrite synthesis can avoid introducing water vapor and oxygen into the DMO synthesis reaction;

[0014] Step 4: In the DMO synthesis system, fresh CO and circulating gas containing methyl nitrite pressurized by a compressor are mixed at a pressure of 0.423 MPa, preheated to 106°C and then enter a shell-and-tube reactor (DMO reactor) filled with a Pd / Al2O3 spherical catalyst;

[0015] Step 5: The reaction products in the DMO reactor are sent to the DMO washing system, and DMO, DMC and other organic matter are cooled and washed with methanol;

[0016] Step 6: The methanol solution is sent to the DMO purification system, the top circulating gas pressure is 0.305MPa, after being cooled to 40°C, it is mixed with 0.5MPa oxygen and then enters the methyl nitrite regeneration tower system;

[0017] Step 7: The reaction gas is mixed with O2 and enters from the bottom of the methyl nitrite regeneration tower, and MeOH enters from the top of the regeneration tower. The gas pressure at the top of the tower is 0.26MPa and the temperature is 40°C. Most of the synthesis gas enters the DMO circulating gas compressor for compression, and a small amount of gas is sent to the methyl nitrite recovery system for treatment and then discharged;

[0018] Step 8: The nitric acid solution at the bottom of the MN regeneration tower enters the nitric acid reducer (C-42104A / B) for treatment, and the nitric acid is recovered by gas bubbling reverse contact without using a reduction catalyst;

[0019] Step 9: One path of gas from the DMO gas removal tower top separation tank flows (bubbles) through the nitric acid reduction booster and is divided into four paths and enters the first, second, third and fourth reaction zones of the nitric acid reducer through flow metering for liquid phase bubbling, and another path of gas flow (gas phase) is measured by a flow meter and enters the first reaction zone of the nitric acid reducer for gas phase;

[0020] Step 10: Nitric acid, methanol and bubbling gas react in the 1st, 2nd, 3rd and 4th reaction zones in the nitric acid reducer to generate MN which enters the middle of the MN regeneration tower along the process gas pipeline;

[0021] Step 11: The alcohol-containing waste liquid at the bottom of the nitric acid reducer is cooled to 60°C and then enters the alcohol-containing waste liquid flash tank (operating pressure 0.05MPa) for flash evaporation. The flash gas is sent to the methyl nitrite recovery system. The alcohol-containing waste liquid after flash evaporation is sent to the methanol dehydration tower feed tank. The remaining nitric acid solution is then neutralized by alkaline substances to form sodium nitrate solution, which is sent to the sewage and reclaimed water reuse device for treatment.

[0022] As a preferred technical solution of the present invention, the nitric acid is generated by a side reaction of nitrogen dioxide and water during the MN synthesis process.

[0023] As a preferred technical solution of the present invention, the water vapor and oxygen need to be separated and collected.

[0024] As a preferred technical solution of the present invention, the DMO, DMC and other organic substances need to be cooled and washed with methanol.

[0025] As a preferred technical solution of the present invention, the pressure range of the solution containing nitric acid at the bottom of the MN regeneration tower is 0.26-0.29 MPa, and the temperature range is 45°C-50°C.

[0026] As a preferred technical solution of the present invention, the ratio of the gas inflow (bubbling) to the gas flow (gas phase) is 1.2-1.3:1, preferably 1.25:1.

[0027] As a preferred technical solution of the present invention, the reaction temperature range of the nitric acid reducer is 60°C to 85°C.

[0028] As a preferred technical solution of the present invention, the reaction pressure of the nitric acid reducer is ≤0.305MPa.

[0029] As a preferred technical solution of the present invention, the alkaline substance is sodium hydroxide, calcium hydroxide, sodium carbonate, sodium bicarbonate, ammonia water, etc., and the concentration range is 10% to 30%.

[0030] As a preferred technical solution of the present invention, the mass of nitric acid in the sodium nitrate solution is reduced to below 43 kg / h, and the concentration of nitric acid is reduced to below 0.1 wt%.

[0031] Compared with the prior art, the present invention provides a method for reducing the discharge of saline wastewater in the DMO synthesis process, which has the following beneficial effects:

[0032] 1. The present invention adds a nitric acid reducer, which utilizes gas-phase bubbling reverse contact to recover nitric acid, replenish total nitrogen in the system, and reduce the salt content of wastewater. Without using a reduction catalyst, it can better decompose and treat saline wastewater and ensure the controllability and harmlessness of the system.

[0033] 2. The present invention not only greatly reduces the generation of salt-containing wastewater, but also makes secondary use of HNO3. The methyl nitrite (MN) produced by the reaction can be returned to the previous system reaction as a reaction raw material, which reduces the consumption of reaction raw materials and reduces the system energy consumption while reducing the wastewater discharge. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a flow chart of the present invention. DETAILED DESCRIPTION

[0035] 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.

[0036] Embodiment 1

[0037] See also Figure 1 , In this embodiment: A method for reducing the discharge of salt-containing wastewater in the DMO synthesis process, comprising step 1: Step 1: Pd / Al2O3 is used as a catalyst, and carbon monoxide (CO) and methyl nitrite (MN) are used to catalyze the reaction in a fixed bed reactor to synthesize dimethyl oxalate (DMO), while generating nitric oxide (NO) as a byproduct, and NO is further oxidized to dinitrogen trioxide (N2O3), and MN is cyclically produced for synthesizing DMO;

[0038] Step 2: Nitric oxide, oxygen and C1-C4 alcohol are used to synthesize methyl nitrite in a MN regeneration tower, and nitric acid is produced as a by-product. The synthesized methyl nitrite is used as an intermediate product to synthesize DMO;

[0039] Step 3: In the DMO synthesis reaction, water vapor and oxygen will reduce the selectivity of DMO and the activity of the catalyst. Separating the DMO synthesis and methyl nitrite synthesis can avoid introducing water vapor and oxygen into the DMO synthesis reaction;

[0040] Step 4: In the DMO synthesis system, fresh CO and circulating gas containing methyl nitrite pressurized by a compressor are mixed at a pressure of 0.423 MPa, preheated to 106°C and then enter a shell-and-tube reactor (DMO reactor) filled with a Pd / Al2O3 spherical catalyst;

[0041] Step 5: The reaction products in the DMO reactor are sent to the DMO washing system, and DMO, DMC and other organic matter are cooled and washed with methanol;

[0042] Step 6: The methanol solution is sent to the DMO purification system, the top circulating gas pressure is 0.305MPa, after being cooled to 40°C, it is mixed with 0.5MPa oxygen and then enters the methyl nitrite regeneration tower system;

[0043] Step 7: The reaction gas is mixed with O2 and enters from the bottom of the methyl nitrite regeneration tower, and MeOH enters from the top of the regeneration tower. The gas pressure at the top of the tower is 0.26MPa and the temperature is 40°C. Most of the synthesis gas enters the DMO circulating gas compressor for compression, and a small amount of gas is sent to the methyl nitrite recovery system for treatment and then discharged;

[0044] Step 8: The nitric acid solution at the bottom of the MN regeneration tower enters the nitric acid reducer (C-42104A / B) for treatment, and the nitric acid is recovered by gas bubbling reverse contact without using a reduction catalyst;

[0045] Step 9: One path of gas from the DMO gas removal tower top separation tank flows (bubbles) through the nitric acid reduction booster and is divided into four paths and enters the first, second, third and fourth reaction zones of the nitric acid reducer through flow metering for liquid phase bubbling, and another path of gas flow (gas phase) is measured by a flow meter and enters the first reaction zone of the nitric acid reducer for gas phase;

[0046] Step 10: Nitric acid, methanol and bubbling gas react in the 1st, 2nd, 3rd and 4th reaction zones in the nitric acid reducer to generate MN which enters the middle of the MN regeneration tower along the process gas pipeline;

[0047] Step 11: The alcohol-containing waste liquid at the bottom of the nitric acid reducer is cooled to 60°C and then enters the alcohol-containing waste liquid flash tank (operating pressure 0.05MPa) for flash evaporation. The flash gas is sent to the methyl nitrite recovery system. The alcohol-containing waste liquid after flash evaporation is sent to the methanol dehydration tower feed tank. The remaining nitric acid solution is then neutralized by alkaline substances to form sodium nitrate solution, which is sent to the sewage and reclaimed water reuse device for treatment.

[0048] The following Examples 2 to 5 all use the device in Example 1 to carry out the production process.

[0049] Embodiment 2

[0050] Taking the addition of one nitric acid reducer as an example, when the gas flow rate is 4850Nm3 / h, the nitric acid inlet concentration in the nitric acid reducer is 5.8wt%, the reaction temperatures of the first, second, third and fourth chambers are 65°C, 70°C, 75°C and 80°C respectively, and the reaction pressure is 0.305MPaG. By adding alkaline substances for neutralization, the nitric acid concentration in the waste liquid can be reduced to 0.1wt%. The specific data are shown in Table 1:

[0051] Table 1 Comparison data table

[0052]

[0053] Embodiment 3

[0054] Taking the addition of one nitric acid reducer as an example, when the gas flow rate is 4850Nm3 / h, the nitric acid inlet concentration in the nitric acid reducer is 5.8wt%, and the reaction pressure is 0.305MPaG. By adjusting the reaction temperature of the first, second, third, and fourth chambers, and then adding alkaline substances for neutralization, the nitric acid concentration in the waste liquid can be reduced to 0.08wt%. The specific data are shown in Table 2:

[0055] Table 2 Reaction temperature control table

[0056]

[0057]

[0058] Embodiment 4

[0059] Taking the addition of one nitric acid reducer as an example, when the gas flow rate is 4850Nm3 / h, the reaction temperatures of the first, second, third and fourth chambers are 65°C, 75°C, 80°C and 85°C respectively, and the reaction pressure is 0.305MPaG. By adjusting the inlet concentration of nitric acid and then neutralizing it with alkaline substances, the nitric acid concentration in the waste liquid can be reduced to 0.07wt%. The specific data are shown in Table 3:

[0060] Table 3 Nitric acid inlet concentration control table

[0061]

[0062] Embodiment 5

[0063] Taking the addition of two nitric acid reducers (in parallel) as an example, when the gas flow rate is 4850Nm3 / h, the nitric acid inlet concentration in the nitric acid reducer is 5.8wt%, the reaction temperatures of the first, second, third and fourth chambers are 65°C, 75°C, 80°C and 85°C respectively, and the reaction pressure is 0.305MPaG. By adding alkaline substances for neutralization, the nitric acid concentration in the waste liquid can be reduced to 0.04wt%. The specific data are shown in Table 4:

[0064] Table 4 Nitric Acid Reducing Device Quantity Control Table

[0065]

[0066]

[0067] The present invention is described above by way of example. It should be noted that, without departing from the core of the present invention, any simple deformation, modification or other equivalent replacement that can be made by a person skilled in the art without inventive effort falls within the protection scope of the present invention.

Claims

1. A method for reducing the discharge of saline wastewater in a DMO synthesis process, characterized in that: The method comprises the following steps: using Pd / Al2O3 as a catalyst, using carbon monoxide (CO) and methyl nitrite (MN) to catalyze the reaction in a fixed bed reactor to synthesize dimethyl oxalate (DMO), while generating nitric oxide (NO) as a byproduct, and further oxidizing NO to dinitrogen trioxide (N2O3), and cyclically producing MN for synthesizing DMO; Step 2: Nitric oxide, oxygen and C1-C4 alcohol are used to synthesize methyl nitrite in a MN regeneration tower, and nitric acid is produced as a by-product. The synthesized methyl nitrite is used as an intermediate product to synthesize DMO; Step 3: In the DMO synthesis reaction, water vapor and oxygen will reduce the selectivity of DMO and the activity of the catalyst. Separating the DMO synthesis and methyl nitrite synthesis can avoid introducing water vapor and oxygen into the DMO synthesis reaction; Step 4: In the DMO synthesis system, fresh CO and circulating gas containing methyl nitrite pressurized by a compressor are mixed at a pressure of 0.423 MPa, preheated to 106°C and then enter a shell-and-tube reactor (DMO reactor) filled with a Pd / Al2O3 spherical catalyst; Step 5: The reaction products in the DMO reactor are sent to the DMO washing system, and DMO, DMC and other organic matter are cooled and washed with methanol; Step 6: The methanol solution is sent to the DMO purification system, the top circulating gas pressure is 0.305MPa, after being cooled to 40°C, it is mixed with 0.5MPa oxygen and then enters the methyl nitrite regeneration tower system; Step 7: The reaction gas is mixed with O2 and enters from the bottom of the methyl nitrite regeneration tower, and MeOH enters from the top of the regeneration tower. The gas pressure at the top of the tower is 0.26MPa and the temperature is 40°C. Most of the synthesis gas enters the DMO circulating gas compressor for compression, and a small amount of gas is sent to the methyl nitrite recovery system for treatment and then discharged; Step 8: The nitric acid solution at the bottom of the MN regeneration tower enters the nitric acid reducer (C-42104A / B) for treatment, and the nitric acid is recovered by gas bubbling reverse contact without using a reduction catalyst; Step 9: One path of gas from the DMO gas removal tower top separation tank flows (bubbles) through the nitric acid reduction booster and is divided into four paths and enters the first, second, third and fourth reaction zones of the nitric acid reducer through flow metering for liquid phase bubbling, and another path of gas flow (gas phase) is measured by a flow meter and enters the first reaction zone of the nitric acid reducer for gas phase; Step 10: Nitric acid, methanol and bubbling gas react in the 1st, 2nd, 3rd and 4th reaction zones in the nitric acid reducer to generate MN which enters the middle of the MN regeneration tower along the process gas pipeline; Step 11: The alcohol-containing waste liquid at the bottom of the nitric acid reducer is cooled to 60°C and then enters the alcohol-containing waste liquid flash tank (operating pressure 0.05MPa) for flash evaporation. The flash gas is sent to the methyl nitrite recovery system. The alcohol-containing waste liquid after flash evaporation is sent to the methanol dehydration tower feed tank. The remaining nitric acid solution is then neutralized by alkaline substances to form sodium nitrate solution, which is sent to the sewage and reclaimed water reuse device for treatment.

2. A method for reducing the discharge of saline wastewater in a DMO synthesis process according to claim 1, characterized in that: The nitric acid is generated by a side reaction of nitrogen dioxide and water during the MN synthesis process.

3. A method for reducing the discharge of saline wastewater in a DMO synthesis process according to claim 1, characterized in that: The water vapor and oxygen need to be separated and collected.

4. The method for reducing the discharge of saline wastewater in a DMO synthesis process according to claim 1, characterized in that: The DMO, DMC and other organic substances need to be cooled and washed with methanol.

5. The method for reducing the discharge of saline wastewater in a DMO synthesis process according to claim 1, characterized in that: The pressure range of the solution containing nitric acid at the bottom of the MN regeneration tower is 0.26-0.29 MPa, and the temperature range is 45°C-50°C.

6. The method for reducing the discharge of saline wastewater in a DMO synthesis process according to claim 1, characterized in that: The ratio of the gas inflow (bubbling) to the gas flow (gas phase) is 1.2-1.3:

1.

7. The method for reducing the discharge of saline wastewater in a DMO synthesis process according to claim 1, characterized in that: The reaction temperature range of the nitric acid reducer is 60°C to 85°C.

8. The method for reducing the discharge of saline wastewater in a DMO synthesis process according to claim 1, characterized in that: The reaction pressure of the nitric acid reducer is ≤0.305MPa.

9. The method for reducing the discharge of saline wastewater in a DMO synthesis process according to claim 1, characterized in that: The alkaline substance is sodium hydroxide, calcium hydroxide, sodium carbonate, sodium bicarbonate, ammonia water, etc., and the concentration range is 10% to 30%.

10. The method for reducing the discharge of saline wastewater in a DMO synthesis process according to claim 1, characterized in that: The mass of nitric acid in the sodium nitrate solution is reduced to below 43 kg / h, and the concentration of nitric acid is reduced to below 0.1 wt %.

Images