An online desalination and scale prevention method for sulfuric acid concentration

Abstract

The present invention discloses a method for online desalination and scale prevention in sulfuric acid concentration, which comprises the following steps: 1) Pre-cool the high-temperature dilute sulfuric acid and then feed it into a crystallizer with a stirring paddle for cooling crystallization to precipitate salts and filter, so as to obtain desalted dilute sulfuric acid; 2) Preheat and heat up the desalted dilute sulfuric acid and then feed it into a boiler for concentration. During the concentration process, pulse oscillation is adopted to form eddy currents and disturbances to avoid the precipitation of residual salts in the boiler, thus completing online desalination and scale prevention. The present invention removes most of the salts in the dilute sulfuric acid by pre-cooling crystallization in advance, and adds an enhanced external field in the boiler to form eddy currents and strong disturbances, avoiding the deposition of residual salts in the boiler, playing a role in prolonging the operation time and cleaning cycle of the device, and having important significance for ensuring the safety and stable operation of the device.

Description

Technical Field

[0001] The present invention relates to a sulfuric acid concentration method, and particularly to a method for online desalination and scale prevention in sulfuric acid concentration. Background Art

[0002] At present, for the concentration of low-concentration sulfuric acid, a sulfuric acid concentration device is used to perform gradient concentration on sulfuric acid. Its advantages are simple equipment and mature technology. However, during the sulfuric acid concentration process, due to problems such as introduction of upstream impurities and equipment corrosion, a large amount of metal impurities are contained in the sulfuric acid. These impurities gradually increase with the increase of sulfuric acid concentration during the high-temperature concentration process, and the water content gradually decreases. Their solubility in sulfuric acid will gradually decrease, forming salt mud with strong adhesion and then depositing on heat exchange tubes or boilers and other parts, resulting in equipment scaling, heat exchange tube blockage or reduction of heat transfer efficiency; at the same time, most current acid concentration devices use glass-lined equipment. The salt mud accumulates and adheres into a tight solid in the equipment, which will seriously affect local heat transfer, easily cause porcelain explosion of the glass lining, and then lead to equipment corrosion and leakage, etc., affecting the stable operation of the device. Therefore, it is necessary to desalt the sulfuric acid concentration process to avoid sedimentation and accumulation of salts in the equipment.

[0003] Patent CN105152229A discloses a method for purifying and recycling waste sulfuric acid. This method uses cooling crystallization + membrane separation means to purify sulfuric acid containing salts, but the separation membrane can only withstand sulfuric acid with a concentration of less than 40wt%, and cannot be used under high-concentration sulfuric acid conditions. Moreover, the membrane separation device is expensive and has low industrial feasibility. In addition, the currently used sulfuric acid desalination method is mainly washing salts during shutdown, using an aqueous solution of low-concentration ferrous sulfate or other salts or clean water to wash the equipment. This method solves the sulfuric acid salting problem from the back end. When the salts are deposited tightly in the equipment, it is difficult to remove the caked salts only by washing. Moreover, some salts have poor water solubility and the aqueous solution cannot dissolve and carry them out. In addition, washing salts for high-temperature equipment requires continuous shutdown and cooling, and then reheating after the washing is completed, which not only affects the normal operation of the device, but also causes certain damage to the equipment and affects the operation life of the device. Summary of the Invention

[0004] In order to solve the above technical problems, the present invention proposes a method for online desalination and scale prevention in sulfuric acid concentration. The present invention removes most of the salts in dilute sulfuric acid by pre-cooling crystallization, and strengthens the external field in the boiler to form eddy currents and strong disturbances, avoiding the deposition of residual salts in the boiler, playing a role in extending the operation time and cleaning cycle of the device, and having important significance for ensuring the safety and stable operation of the device.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A method for online desalination and scale prevention in sulfuric acid concentration, comprising the following steps:

[0007] 1) Pre-cool the high-temperature dilute sulfuric acid and then feed it into a crystallizer with a stirring paddle for cooling crystallization to precipitate salts and filter, obtaining desalted dilute sulfuric acid.

[0008] 2) Preheat and heat up the desalted dilute sulfuric acid and then feed it into a boiler for concentration. During the concentration process, pulse oscillation is used to form eddy currents and disturbances to avoid precipitation of residual salts in the boiler, completing online desalination and scale prevention.

[0009] As a preferred embodiment of the present invention, in step 1), the high-temperature dilute sulfuric acid is sulfuric acid with a mass concentration of 5 - 95%, preferably 50 - 95%, which contains any one or more of a total of 200 - 500 ppm of metals Al, Cr, Fe, Mg, Mn, Mo, Ni, Zn.

[0010] Preferably, the high-temperature dilute sulfuric acid can be, for example, the dilute sulfuric acid recovered from the nitration reaction unit of the DNT (2,4-dinitrotoluene) process, or the dilute sulfuric acid recovered from the nitration reaction unit of the nitrophenol device, etc.

[0011] As a preferred embodiment of the present invention, in step 1), the temperature for cooling crystallization is -5°C to 10°C, preferably -5°C to 5°C; the time for cooling crystallization is 0.5 - 2 h;

[0012] Preferably, the rotation speed of the stirring paddle in the crystallizer is 100 - 200 rpm.

[0013] As a preferred embodiment of the present invention, in step 1), seed crystals are added to the crystallizer to promote the crystallization of salts;

[0014] Preferably, the seed crystals are porous silica microspheres;

[0015] Preferably, the addition amount of the seed crystals is 0.001 - 0.05% of the total metal content in the dilute sulfuric acid, preferably 0.005 - 0.04%.

[0016] As a preferred embodiment of the present invention, during the concentration process in step 2), the sulfuric acid concentration temperature is controlled at 150 - 200°C.

[0017] As a preferred embodiment of the present invention, a pulse pump is installed on the outer wall of the boiler. The pulse pump is fixedly connected to a plurality of annular baffles through a connecting rod extending into the boiler interior. The annular baffles are non-fixedly sleeved on the heat exchange tubes in the boiler and are evenly distributed along the heat exchange tubes.

[0018] As a preferred embodiment of the present invention, the connecting rod and the annular baffle form up-and-down oscillation and left-and-right reciprocating oscillation under the action of the pulse pump, causing the fluid in the boiler to generate eddy currents and strong disturbances, avoiding the precipitation of salts on the surface of the heat exchange tubes and the inner wall of the boiler, and simultaneously scraping off a small amount of precipitates on the surface of the heat exchange tubes.

[0019] As a preferred embodiment of the present invention, the oscillation frequency of the pulse pump is 1 - 10 Hz, preferably 3 - 8 Hz.

[0020] As a preferred embodiment of the present invention, before the desalted dilute sulfuric acid obtained in step 1) enters the boiler for concentration, it first enters a heat exchanger to conduct heat exchange with the high-temperature dilute sulfuric acid, pre-cooling and reducing the heat of the high-temperature dilute sulfuric acid, and simultaneously pre-heating and raising the temperature of the desalted dilute sulfuric acid to achieve energy recovery and utilization.

[0021] As a preferred embodiment of the present invention, in step 1), the salts precipitated by cooling and crystallization are filtered through a plate filter, and the filtration pore size is 0.8 - 2 μm.

[0022] As a preferred embodiment of the present invention, after heat exchange, the desalted dilute sulfuric acid is heated to 80 - 150 °C, preferably 100 - 140 °C; the high-temperature dilute sulfuric acid is cooled to 10 - 60 °C, preferably 20 - 50 °C.

[0023] The method for online desalination and scale prevention in the present invention does not limit the sulfuric acid concentration process, and an intermittent process or a continuous process can be optionally adopted, which is easy to adjust for those skilled in the art based on the methods disclosed above.

[0024] By desalinating before sulfuric acid concentration, the present invention can find an outlet for the salts in sulfuric acid, prevent sulfuric acid containing a large amount of salts from entering the special equipment of the sulfuric acid concentration boiler, and avoid a large amount of salts from depositing on the boiler and the heat exchange tubes inside it to form a tight and difficult-to-remove solid; meanwhile, a pulse oscillation device is added in the sulfuric acid concentration boiler, especially at the heat exchange tubes where solids are prone to precipitate, and eddy currents and disturbances are generated in the fluid through the pulse oscillation effect to avoid the deposition of residual salts in the boiler. In addition, the present invention couples the cooling of high-temperature dilute sulfuric acid and the heating of desalted dilute sulfuric acid, with a mature process flow, simple equipment structure, low energy consumption and high industrial feasibility. It can remove salts in sulfuric acid online and prevent boiler scaling, solving the problems of easy blockage of sulfuric acid concentration devices and frequent stop for salt washing. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic partial cross-sectional structure diagram of a sulfuric acid concentration boiler.

[0026] In the figure, 1, boiler; 2, heat exchange tube; 3, connecting rod; 4, annular baffle. DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention will be further described below through specific embodiments. The embodiments of the present invention are only for the illustration of the present invention and do not limit the scope of the present invention.

[0028] The raw material information adopted in the following embodiments of the present invention is as follows:

[0029] Dilute sulfuric acid A: Dilute sulfuric acid recovered from the nitration reaction unit of the DNT process of the TDI plant of Wanhua Chemical. The main component is 85% sulfuric acid, which contains 18 ppm of metal Al, 37 ppm of metal Cr, 238 ppm of metal Fe, 5 ppm of metal Mg, 15 ppm of metal Mn, 10 ppm of metal Mo, 26 ppm of metal Ni, and <1 ppm of metal Zn.

[0030] Dilute sulfuric acid B: Dilute sulfuric acid recovered from the nitration reaction unit of the nitrobenzene plant of Wanhua Chemical. The main component is 52% sulfuric acid, which contains 17 ppm of metal Al, 67 ppm of metal Cr, 316 ppm of metal Fe, 9 ppm of metal Mg, 18 ppm of metal Mn, 12 ppm of metal Mo, 39 ppm of metal Ni, and <1 ppm of metal Zn.

[0031]

Example 1

[0032] A method for online desalination and scale prevention in sulfuric acid concentration, comprising the following steps:

[0033] 1) Pre-cool 50 t of high-temperature dilute sulfuric acid A at 176 °C to 50 °C and then feed it into a crystallizer with a stirring paddle. Perform cooling crystallization at 0 °C. After stirring at a speed of 100 rpm for 1.5 h, filter out the precipitated salts with a plate filter with a pore size of 1.2 μm to obtain desalted dilute sulfuric acid, and its composition is shown in Table 1;

[0034] During the above crystallization process, porous silica microspheres with a mass of 0.008% of the total metal content in the dilute sulfuric acid are added as crystal seeds to promote the crystallization of salts.

[0035] 2) Preheat and heat up the desalted dilute sulfuric acid to 120 °C and then feed it into a 180 °C boiler for concentration. As Figure 1 shown, a pulse pump is installed on the outer wall of the boiler 1. The pulse pump is fixedly connected to a plurality of annular baffles 4 through a connecting rod 3 extending into the boiler interior. The annular baffles 4 are non-fixedly sleeved on the heat exchange tubes 2 in the boiler and are evenly distributed along the heat exchange tubes 2. The annular baffles 4 form up-and-down oscillation and left-and-right reciprocating oscillation under the action of the pulse pump, causing the fluid in the boiler to generate eddy currents and strong disturbances, avoiding the precipitation of salts on the surface of the heat exchange tubes and the inner wall of the boiler, and at the same time scraping off a small amount of precipitates on the surface of the heat exchange tubes during the concentration process to complete online desalination and scale prevention. The oscillation frequency of the pulse pump is 5 Hz. The composition of the concentrated sulfuric acid is shown in Table 1.

[0036] After the above process establishes a cycle, before the desalted dilute sulfuric acid obtained in step 1) enters the boiler for concentration, it first enters a heat exchanger to exchange heat with the high-temperature dilute sulfuric acid, pre-cooling and reducing the heat of the high-temperature dilute sulfuric acid, and at the same time pre-heating and raising the temperature of the desalted dilute sulfuric acid to achieve energy recovery and utilization.

[0037] Table 1. Changes in the composition of sulfuric acid before and after crystallization and concentration

[0038]

[0039]

[0040] When the method in this embodiment is used for sulfuric acid concentration, the overall heat exchange efficiency of the boiler device decreases by about 5% within the overhaul cycle (1 year), and the salt cleaning cycle > 2 years / time.

[0041]

Example 2

[0042] A method for online desalting and scale prevention in sulfuric acid concentration, comprising the following steps:

[0043] 1) Pre-cool 50t of high-temperature dilute sulfuric acid B at 154 °C to 30 °C and then send it into a crystallizer with a stirring paddle, carry out cooling crystallization at -5 °C, after stirring at a speed of 150 rpm for 1 h, filter out the precipitated salts with a plate filter with a pore size of 0.8 μm to obtain desalted dilute sulfuric acid, and its composition is shown in Table 2;

[0044] In the above crystallization process, porous silica microspheres with a mass of 0.01% of the total metal content in the dilute sulfuric acid are added as crystal seeds to promote the crystallization of salts.

[0045] 2) Pre-heat and raise the temperature of the desalted dilute sulfuric acid to 100 °C and then send it into a 160 °C boiler for concentration. As Figure 1 shown, a pulse pump is installed on the outer wall of the boiler 1, and the pulse pump is fixedly connected to a plurality of annular baffles 4 through a connecting rod 3 extending into the boiler interior. The annular baffles 4 are non-fixedly sleeved on the heat exchange tubes 2 in the boiler and are evenly distributed along the heat exchange tubes 2. The annular baffles 4 form up-and-down oscillation and left-and-right reciprocating oscillation under the action of the pulse pump, causing the fluid in the boiler to generate eddy currents and strong disturbances, avoiding the precipitation of salts on the surface of the heat exchange tubes and the inner wall of the boiler, and at the same time scraping off a small amount of precipitates on the surface of the heat exchange tubes during the concentration process to complete online desalting and scale prevention. The oscillation frequency of the pulse pump is 6 Hz. The composition of the concentrated sulfuric acid is shown in Table 1.

[0046] After the above process establishes a cycle, before the desalted dilute sulfuric acid obtained in step 1) enters the boiler for concentration, it first enters a heat exchanger to exchange heat with the high-temperature dilute sulfuric acid, pre-cooling and reducing the heat of the high-temperature dilute sulfuric acid, and at the same time pre-heating and raising the temperature of the desalted dilute sulfuric acid to achieve energy recovery and utilization.

[0047] Table 2. Changes in sulfuric acid composition before and after crystallization and concentration

[0048]

[0049] When the sulfuric acid concentration method in this example is adopted, the overall heat exchange efficiency of the boiler device decreases by about 7% within the major overhaul cycle (1 year), and the salt cleaning cycle is > 2 years / time.

[0050]

Example 3

[0051] A method for online desalination and scale prevention in sulfuric acid concentration, comprising the following steps:

[0052] 1) Pre-cool the high-temperature dilute sulfuric acid A at 186 °C to 48 °C and then send it into a crystallizer with a stirring paddle. Perform cooling crystallization at -2 °C. After stirring at a speed of 100 rpm for 2 h, filter out the precipitated salts with a plate filter with a pore size of 1 μm to obtain desalted dilute sulfuric acid, and its composition is shown in Table 3;

[0053] During the above crystallization process, porous silica microspheres with a mass of 0.02% of the total metal content in the dilute sulfuric acid are added as crystal seeds to promote the crystallization of salts.

[0054] 2) Preheat and raise the temperature of the desalted dilute sulfuric acid to 136 °C and then send it into a 176 °C boiler for concentration. As Figure 1 shown, a pulse pump is installed on the outer wall of the boiler 1. The pulse pump is fixedly connected to a plurality of annular baffles 4 through a connecting rod 3 extending into the boiler interior. The annular baffles 4 are non-fixedly sleeved on the heat exchange tubes 2 in the boiler and are evenly distributed along the heat exchange tubes 2. The annular baffles 4 form up-and-down oscillation and left-and-right reciprocating oscillation under the action of the pulse pump, causing the fluid in the boiler to generate eddy currents and strong disturbances, avoiding the precipitation of salts on the surface of the heat exchange tubes and the inner wall of the boiler, and at the same time scraping off a small amount of precipitated substances on the surface of the heat exchange tubes during the concentration process to complete online desalination and scale prevention. The oscillation frequency of the pulse pump is 8 Hz. The composition of the concentrated sulfuric acid is shown in Table 1.

[0055] After the above process establishes a cycle, before the desalted dilute sulfuric acid obtained in step 1) enters the boiler for concentration, it first enters a heat exchanger to exchange heat with the high-temperature dilute sulfuric acid, pre-cooling and reducing the heat of the high-temperature dilute sulfuric acid, and at the same time preheating and raising the temperature of the desalted dilute sulfuric acid to achieve energy recovery and utilization.

[0056] Table 3. Changes in sulfuric acid composition before and after crystallization and concentration

[0057]

[0058]

Comparative Example 1

[0059] Sulfuric acid concentration was carried out in substantially the same manner as in Example 1, except that: in step 2), when concentrating dilute sulfuric acid, a conventional boiler equipped only with heat exchange tubes and without a pulse pump and an annular baffle was used for concentration. The changes in the composition of sulfuric acid before and after crystallization and concentration are shown in Table 4.

[0060] Table 4. Changes in the composition of sulfuric acid before and after crystallization and concentration

[0061]

[0062]

[0063] When sulfuric acid concentration was carried out using the method in this comparative example, the overall heat exchange efficiency of the boiler device decreased by 18% within 6 months, seriously affecting the energy consumption of the device and requiring shutdown for cleaning.

[0064]

Comparative Example 2

[0065] Sulfuric acid concentration was carried out in substantially the same manner as in Example 1, except that: the seed crystal in step 1) was replaced with the same mass of FeSO 4 .

[0066] When sulfuric acid concentration was carried out using the method in this comparative example, FeSO 4 used as a seed crystal formed acid sludge in sulfuric acid and adhered to the crystallizer, resulting in a decrease in the overall cooling efficiency of the crystallizer, incomplete cooling of dilute sulfuric acid, insufficient precipitation of salts, and the changes in the composition of sulfuric acid before and after crystallization and concentration are shown in Table 5; at the same time, the overall heat exchange efficiency of the boiler device decreased by 12% within 8 months and required shutdown for cleaning.

[0067] Table 5. Changes in the composition of sulfuric acid before and after crystallization and concentration

[0068]

[0069] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the method of the present invention, several improvements and supplements can be made, and these improvements and supplements should also be regarded as the protection scope of the present invention.

Claims

1. A method for online desalination and scale prevention in sulfuric acid concentration, characterized in that, it comprises the following steps: 1) Pre-cool the high-temperature dilute sulfuric acid and then send it into a crystallizer with a stirring paddle for cooling crystallization, precipitate salts and filter to obtain desalted dilute sulfuric acid; 2) Preheat and raise the temperature of the desalted dilute sulfuric acid and then send it into a boiler for concentration. During the concentration process, pulsed oscillation is used to form eddy currents and disturbances to avoid the precipitation of residual salts in the boiler, and online desalination and scale prevention are completed; A pulsed pump is installed on the outer wall of the boiler. The pulsed pump is fixedly connected to a plurality of annular baffles through a connecting rod extending into the boiler. The annular baffles are sleeved on the heat exchange tubes in the boiler in a non-fixed manner and are evenly distributed along the heat exchange tubes; The connecting rod and the annular baffles form up-and-down oscillation and left-and-right reciprocating oscillation under the action of the pulsed pump, causing the fluid in the boiler to generate eddy currents and strong disturbances, avoiding the precipitation of salts on the surface of the heat exchange tubes and the inner wall of the boiler, and at the same time scraping off a small amount of precipitates on the surface of the heat exchange tubes; In step 1), seed crystals are added to the crystallizer to promote the crystallization of salts; The seed crystals are porous silica microspheres.

2. The method for online desalination and scale prevention in sulfuric acid concentration according to claim 1, characterized in that, in step 1), the high-temperature dilute sulfuric acid is sulfuric acid with a mass concentration of 5-95%, and it contains any one or more of a total of 200-500 ppm of metals Al, Cr, Fe, Mg, Mn, Mo, Ni, Zn.

3. The method for online desalination and scale prevention in sulfuric acid concentration according to claim 2, characterized in that, in step 1), the high-temperature dilute sulfuric acid is sulfuric acid with a mass concentration of 50-95%.

4. The method for online desalination and scale prevention in sulfuric acid concentration according to claim 2, characterized in that, in step 1), the temperature for cooling crystallization is -5°C to 10°C; the time for cooling crystallization is 0.5-2 h.

5. The method for online desalination and scale prevention in sulfuric acid concentration according to claim 4, characterized in that, in step 1), the temperature for cooling crystallization is -5°C to 5°C.

6. The method for online desalination and scale prevention in sulfuric acid concentration according to claim 4, characterized in that, the rotation speed of the stirring paddle in the crystallizer is 100-200 rpm.

7. The method for online desalination and scale prevention in sulfuric acid concentration according to claim 1, characterized in that, the addition amount of the seed crystals is 0.001-0.05% of the total metal content in the dilute sulfuric acid.

8. The method for online desalination and scale prevention in sulfuric acid concentration according to claim 7, characterized in that, the addition amount of the seed crystals is 0.005-0.04% of the total metal content in the dilute sulfuric acid.

9. The method for online desalination and scale prevention in sulfuric acid concentration according to claim 1, characterized in that, during the concentration process in step 2), the sulfuric acid concentration temperature is controlled at 150-200°C.

10. The method for online desalination and scale prevention in sulfuric acid concentration according to claim 1, characterized in that, the oscillation frequency of the pulsed pump is 1-10 Hz.

11. The method for online desalination and scale prevention in sulfuric acid concentration according to claim 10, characterized in that, the oscillation frequency of the pulsed pump is 3-8 Hz.

12. The method for online desalination and scale prevention of sulfuric acid concentration according to any one of claims 1-6, characterized in that, before the desalted dilute sulfuric acid obtained in step 1) enters the boiler for concentration, it first enters a heat exchanger to exchange heat with high-temperature dilute sulfuric acid, so that the high-temperature dilute sulfuric acid is precooled and cooled, and at the same time the desalted dilute sulfuric acid is preheated and heated up to realize energy recovery and utilization.

13. The method for online desalination and scale prevention of sulfuric acid concentration according to claim 12, characterized in that, after heat exchange, the desalted dilute sulfuric acid is heated up to 80-150 °C; the high-temperature dilute sulfuric acid is cooled down to 10-60 °C.

14. The method for online desalination and scale prevention of sulfuric acid concentration according to claim 13, characterized in that, after heat exchange, the desalted dilute sulfuric acid is heated up to 100-140 °C; the high-temperature dilute sulfuric acid is cooled down to 20-50 °C.

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