Device and method for on-line cleaning of graphite heat exchanger for phosphoric acid by using fluosilicic acid

Abstract

The application discloses a device and method for on-line cleaning of a graphite heat exchanger of wet-process phosphoric acid by using fluosilicic acid, and belongs to the field of cleaning in the wet-process phosphoric acid concentration process. 3 The application adopts a solution with about 25% of omega (H2SiF6) for on-line circulation cleaning of the graphite heat exchanger of the wet-process phosphoric acid, the circulation flow rate of the solution is kept at 25 m / s, and the temperature is in the range of 80-83 DEG C; after 48 hours of circulation cleaning, the heat exchanger inner wall can be dredged by using a cleaning machine. The cleaning effect of the method can guarantee stable production of the device for 6 months, obviously shorten the cleaning time, and improve the device capacity by 2%; in addition, the method can not only avoid the safety risks existing in off-line cleaning, but also save cleaning investment costs.

Description

Technical Field

[0001] This invention relates to the field of cleaning in the wet phosphoric acid concentration process, and specifically to an apparatus and method for online cleaning of wet phosphoric acid graphite heat exchangers using fluorosilicic acid. Background Technology

[0002] Wet-process phosphoric acid is a core basic product of the phosphorus chemical industry, widely used in fertilizers, pesticides, food additives, pharmaceuticals, and other fields. Phosphoric acid concentration is a key process that determines product quality and production efficiency. Graphite heat exchangers, with their excellent corrosion resistance and high heat transfer performance, can be adapted to harsh acidic conditions and are the core heat exchange equipment in wet-process phosphoric acid concentration systems. They mainly complete the evaporation and concentration of dilute phosphoric acid, ensuring continuous and stable production operation.

[0003] Phosphate rock, the raw material for wet-process phosphoric acid production, contains impurities such as calcium, magnesium, and iron. During the concentration process, as the phosphoric acid concentration and temperature increase, the solubility of substances such as fluorosilicates, anhydrous calcium sulfate, and silica gel drops sharply. These substances precipitate out in large quantities and adhere tightly to the inner wall of the graphite heat exchanger tubes, forming a hard, dense scale layer. This scale layer becomes a key bottleneck restricting production efficiency. The scale layer significantly reduces the heat transfer coefficient of the heat exchanger, increases heat transfer resistance, and raises energy consumption. Simultaneously, it blocks the tube channels, reduces equipment processing capacity, shortens production cycles, and in severe cases, requires shutdown for cleaning, resulting in significant economic losses. Industry data shows that after scaling, the heat exchange efficiency of graphite heat exchangers decreases by more than 30%, and the production load decreases by 20%-40%, seriously affecting production continuity and economic benefits.

[0004] Currently, the cleaning of graphite heat exchangers in the industry is mainly divided into physical cleaning and chemical cleaning. Physical cleaning, such as high-pressure water flushing and mechanical scraping, is simple to operate and poses no risk of chemical corrosion. However, it can only remove loose surface scale and is ineffective at removing hard scale such as fluorosilicates and silica gel. It can also easily damage the graphite tubes and shorten the equipment's lifespan. Chemical cleaning is the mainstream method, commonly using single or compound reagents such as dilute sulfuric acid, sodium carbonate, and ammonium fluoride. Dilute sulfuric acid is effective against calcium sulfate scale but has weak dissolving ability against fluorosilicates and silica gel scale, resulting in a low cleaning rate. Ammonium fluoride can dissolve silica-based scale, but it is costly, and its decomposition products are highly corrosive, easily damaging auxiliary equipment such as circulating pumps. Some compound cleaning technologies have problems such as complex formulas, long cleaning cycles, difficult wastewater treatment, and high environmental pressure, making it difficult to meet the requirements of efficient, energy-saving, and environmentally friendly industrial production.

[0005] Fluorosilicic acid is a byproduct of wet-process phosphoric acid production. It is widely available, low in cost, and possesses excellent etching and dissolving properties. It can specifically react with fluorosilicates and silica gel scale to generate water-soluble substances, achieving highly efficient scale removal. Furthermore, it has extremely low corrosivity to graphite materials and does not damage the tube structure, thus becoming a research direction for cleaning technologies. Although existing related patents have attempted to apply fluorosilicic acid for cleaning, significant shortcomings still exist.

[0006] For example, Chinese invention patent CN100573027C discloses a descaling method for graphite heat exchangers used in phosphoric acid concentration. It employs a compound cleaning solution of dilute fluorosilicic acid, organophosphonic acid corrosion inhibitors, film-forming substances, and surfactants. The process involves pretreatment with high-pressure water for 12 hours, followed by 12 hours of circulating cleaning at 60°C, totaling nearly 40 hours. This method is cumbersome and involves complex formulations. Chinese invention patent CN102506608A discloses a method for cleaning wet-process phosphoric acid concentration heat exchangers. This method uses a compound cleaning agent of fluorosilicic acid and concentrated phosphoric acid, circulating at 70-75°C for 8-10 hours, achieving a high cleaning rate. However, it requires the addition of concentrated phosphoric acid, increasing reagent costs and wastewater acidity. Furthermore, the process parameters are not suitable for graphite materials, resulting in poor cleaning uniformity. Another method uses a compound of fluorosilicic acid and hydrochloric acid, which, while having a fast dissolution rate, involves complex acid-base processes and cumbersome operations. Hydrochloric acid is highly corrosive and can easily damage metal components, hindering long-term stable operation of the equipment.

[0007] Overall, existing fluorosilicic acid cleaning technologies generally suffer from problems such as unreasonable concentration control, non-optimized process parameters, cumbersome processes, complex formulas, high costs, and insufficient environmental friendliness. They fail to fully utilize the advantages of fluorosilicic acid byproducts, easily leading to resource waste and secondary pollution, and making it difficult to balance cleaning effectiveness and economic efficiency.

[0008] Therefore, developing a cleaning method that uses fluorosilicic acid, a byproduct of wet-process phosphoric acid, as the core, with a simple process, high cleaning efficiency, low cost, environmental friendliness without secondary pollution, and is suitable for graphite heat exchanger operating conditions, can solve the shortcomings of existing technologies, ensure the long-term efficient and stable operation of equipment, and has important practical significance and industrial application value for promoting energy conservation, emission reduction, quality improvement and efficiency enhancement in the wet-process phosphoric acid industry. Summary of the Invention

[0009] The purpose of this invention is to provide an apparatus and method for cleaning wet-process phosphoric acid graphite heat exchangers using fluorosilicic acid, which effectively simplifies the cleaning process, reduces hoisting safety risks, saves cleaning costs, reduces cleaning time, extends the operating time of the apparatus, and improves phosphoric acid yield.

[0010] The technical solution of the present invention: a device for online cleaning of wet phosphoric acid graphite heat exchangers using fluorosilicic acid, comprising a graphite heat exchanger with a manhole and an acid inlet respectively provided on the side and bottom, a perforated blind plate detachably installed at the acid inlet, and further comprising a circulation loop and a replenishment branch. The circulation loop is used to circulate the cleaning fluid inside the graphite heat exchanger. It includes a circulation pipe and a fluorosilicic acid pump. One end of the circulation pipe extends into the graphite heat exchanger through a manhole, and the other end is connected to the acid inlet. The fluorosilicic acid pump is connected in series with the circulation pipe. The replenishment branch is used to replenish the cleaning fluid to the graphite heat exchanger. It includes a hose and a fluorosilicic acid tank. One end of the hose is connected to the fluorosilicic acid tank, and the other end extends into the interior of the graphite heat exchanger through a manhole.

[0011] Furthermore, the graphite heat exchanger includes a cylindrical body, a reducing pipe connected to the upper end of the cylindrical body, a flange provided at the lower part of the cylindrical body, the acid inlet located in the middle of the flange, and the manhole opened at the upper side end of the cylindrical body.

[0012] Furthermore, the perforated blind flange is installed at the acid inlet of the flange, and the circulation pipe is sealed to the acid inlet through the hole of the perforated blind flange.

[0013] Furthermore, the perforated blind plate is disc-shaped, with a through hole of DN100 at its center.

[0014] Furthermore, an acid outlet valve is provided at the outlet of the fluorosilicic acid tank, and the acid outlet valve is connected to a hose.

[0015] Furthermore, the circulation pipe is a steel-lined rubber pipe, and the fluorosilicic acid pump is made of carbon fiber.

[0016] A method for online cleaning of wet-process phosphoric acid graphite heat exchangers, which eliminates the need to remove the graphite heat exchanger from the production line and directly utilizes fluorosilicic acid, a byproduct of wet-process phosphoric acid production, as the cleaning solution, includes the following steps. Step 1: Modify the pipeline and connect the device. Specifically, use a blind flange to seal the phosphoric acid inlet, remove the lower reducer of the graphite heat exchanger, install a perforated blind flange at the lower flange of the graphite heat exchanger, and connect the circulation pipe through the central hole of the perforated blind flange to form an online cleaning device. Step 2: Start the device for circulation cleaning. Extract fluorosilicic acid solution from the fluorosilicic acid tank and inject it into the graphite heat exchanger through a hose. The mass concentration of the fluorosilicic acid solution ω(H2SiF6) is 25%. Start the fluorosilicic acid pump to make the fluorosilicic acid solution circulate and clean online between the circulation pipe (11) and the graphite heat exchanger for 48 hours. Step 3: After the circulation cleaning is completed, drain the cleaning solution and then use a cleaning machine to unclog the inner wall of the graphite heat exchanger.

[0017] Furthermore, the cleaning fluid circulation rate is 25m. 3 / h, temperature is 80℃-83℃.

[0018] Furthermore, in step 2, during the cyclic cleaning process, fluorosilicic acid solution is continuously or intermittently replenished from the fluorosilicic acid tank (15) through a hose to maintain the concentration and level of the cleaning solution.

[0019] The beneficial effects of this invention are as follows: the heat exchanger does not need to be disassembled offline during the cleaning process, which avoids the safety risks introduced by the use of cranes and reduces costs; the cleaning process can be completed in only 2 steps, which significantly shortens the cleaning time and increases the equipment capacity by 2%; the cleaning solution used is a by-product of the concentrated process, which does not require the addition of other agents, ensuring the use of fluorosilicic acid in downstream processes while also having the advantage of being environmentally friendly and free from secondary pollution. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of specific embodiments of the present invention, the accompanying drawings involved in the embodiments are briefly described below. Obviously, the following drawings are only some embodiments of the present invention, and those skilled in the art can obtain other related drawings based on these drawings without creative effort.

[0021] Figure 1 This is a process diagram of the wet-process phosphoric acid concentration procedure. Figure 2 This is a process diagram of a device for circulating cleaning of graphite heat exchangers; Figure 3 It is a design with perforated blind plates. Figure 1 ; Figure 4 It is a design with perforated blind plates. Figure 2 ; Figure reference numerals: 1-Phosphoric acid inlet, 2-Graphite heat exchanger outlet, 3-Manhole, 4-Graphite heat exchanger, 5-Graphite heat exchanger inlet flange, 6-Reducing pipe, 7-Expansion joint, 8-Axial flow pump, 9-Acid circulation tank, 10-Flash chamber, 11-Circulation pipe, 12-Fluorosilicic acid pump, 13-Hose, 14-Outlet valve, 15-Fluorosilicic acid tank, 16-Blind flange with perforation. Detailed Implementation

[0022] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. However, it should not be construed that the scope of the subject matter of the present invention is limited to the following embodiments. All modifications, substitutions and alterations made based on ordinary technical knowledge and common practices in the art without departing from the above-described technical concept of the present invention are included within the scope of the present invention.

[0023] This invention provides a method for online high-efficiency cleaning of wet phosphoric acid graphite heat exchangers using fluorosilicic acid with ω(H2SiF6)25%, thereby ensuring stable system operation and improving yield.

[0024] Reference Figure 1 The diagram shows the process flow of the wet phosphoric acid concentration process. The specific flow is as follows: the phosphoric acid solution enters the flash chamber 10 from the inlet 1, and then enters the acid circulation tank 9. The axial flow pump 8 pumps the acid into the graphite heat exchanger 4 for heating and concentration. To avoid thermal expansion and contraction of the equipment, the heat exchanger 4 is connected to two ends with reducing pipes 6. One end of the lower reducing pipe 6 is connected to the heat exchanger 4 through the flange 5, and the other end is connected to the pipeline through the expansion joint 7. The axial flow pump 8 is connected to the pipeline.

[0025] Example 1: Based on the original production line, an online cleaning device was temporarily modified to form (e.g., Figure 2 (As shown), it includes the following components: Graphite heat exchanger 4: The object to be cleaned, keep it in its original position on the production line.

[0026] Blind plate with holes 16: Disk-shaped, with a through hole of DN100 at the center.

[0027] Circulation pipe 11: Steel-lined rubber pipe, used to form a cleaning fluid circulation loop. One end extends into the graphite heat exchanger 4 through manhole 3, and the other end is sealed to the acid inlet at flange 5 through the central hole of perforated blind plate 16.

[0028] Fluorosilicic acid pump 12: The pump body is made of carbon fiber and is installed in series on the circulation pipe 11 to drive the circulation of cleaning fluid.

[0029] Hose 13: Acid-resistant hose used to replenish cleaning fluid to the graphite heat exchanger 4. One end is connected to the acid outlet valve 14 of the fluorosilicic acid tank 15, and the other end extends into the interior of the graphite heat exchanger 4 through the manhole 3.

[0030] Fluorosilicic acid tank 15: Used to store fluorosilicic acid cleaning solution, which is a byproduct of wet phosphoric acid production.

[0031] Acid outlet valve 14: Located at the outlet of the fluorosilicic acid tank 15, used to control the outflow of cleaning solution.

[0032] Blind flange: Used to seal phosphoric acid inlet 1 to prevent backflow of fluorine-containing gas.

[0033] Example 2: During the circulating cleaning process, the specific operation is as follows: First, use a blind flange to block the inlet 1, and keep the acid outlet 2 of the graphite heat exchanger unobstructed to prevent the backflow of fluorine-containing gas, allowing it to flow to the flash chamber 10 and the tail gas treatment device; second, remove the reducer 6 at the bottom of the graphite heat exchanger 4, and add a perforated blind flange 16 at the acid inlet flange 5 of the graphite heat exchanger. The diameter of the middle hole of this blind flange is DN100, and its design drawing is as follows. Figure 3 and Figure 4 As shown, one end of the circulation pipe 11 is welded to the perforated blind plate 16, and the other end of the circulation pipe 11 is connected to the fluorosilicic acid pump 12 via a rubber hose. Finally, the manhole 3 of the graphite heat exchanger is opened, and the circulation pipe 11 is connected from the outlet end of the fluorosilicic acid pump 12 into the heat exchanger 4 to ensure the circulation of fluorosilicic acid. Additionally, an acid-resistant hose 13 needs to be connected from the acid outlet valve 14 of the fluorosilicic acid tank 15 into the heat exchanger 4. Its function is to initially introduce acid and replenish the acid level (fluorosilicic acid will decompose due to heat during the heating and cleaning process, causing the level to drop). During the cleaning process, the fluorosilicic acid concentration is maintained at approximately 25%, and the acid flow rate is 25 m³ / s. 3 The acid temperature is kept constant between 80-83℃ for 48 hours. After 48 hours of circulating cleaning, the blockage can be cleared in time using a cleaning machine. The whole process takes 3 days.

[0034] Comparative example: Traditional cleaning methods require offline cleaning of graphite heat exchangers. The operation process includes disassembling, hoisting and installing the upper and lower components of the graphite heat exchanger, totaling 40 sections. The components are first soaked in 15-18% fluorosilicic acid and then circulated for 120 hours. After the acid is drained, the blockage is cleared. The entire process takes 7 days to complete.

[0035] Nine sets of graphite heat exchangers were cleaned, with each set being cleaned every six months. By adopting the improved device and cleaning method of this invention, a total of 2.7 million yuan in hoisting costs and 324 people's labor costs can be saved annually. In addition, the method shortens the cleaning time from 7 days to 3 days, significantly reducing the cleaning time and increasing the uptime. Furthermore, it reduces hoisting operations and lowers safety risks.

Claims

1. A device for online cleaning of wet-process phosphoric acid graphite heat exchangers using fluorosilicic acid, characterized in that: The graphite heat exchanger (4) is provided with a manhole (3) on the side and an acid inlet on the bottom. A perforated blind plate (16) is detachably installed at the acid inlet. It also includes a circulation loop and a liquid replenishment branch. The circulation loop is used to circulate the cleaning fluid inside the graphite heat exchanger (4). It includes a circulation pipe (11) and a fluorosilicic acid pump (12). One end of the circulation pipe (11) extends into the graphite heat exchanger (4) through a manhole (3), and the other end is connected to the acid inlet. The fluorosilicic acid pump (12) is connected in series with the circulation pipe (11). The replenishment branch is used to replenish cleaning fluid to the graphite heat exchanger (4). It includes a hose (13) and a fluorosilicic acid tank (15). One end of the hose (13) is connected to the fluorosilicic acid tank (15), and the other end extends into the graphite heat exchanger (4) through the manhole (3).

2. The apparatus for online cleaning of wet-process phosphoric acid graphite heat exchangers using fluorosilicic acid according to claim 1, characterized in that: The graphite heat exchanger (4) includes a cylinder, a reducing pipe (6) is connected to the upper end of the cylinder, a flange (5) is provided at the lower part of the cylinder, the acid inlet is located in the middle of the flange (5), and the manhole (3) is opened at the upper side of the cylinder.

3. The apparatus for online cleaning of wet phosphoric acid graphite heat exchangers using fluorosilicic acid according to claim 1, characterized in that: The perforated blind flange (16) is installed at the acid inlet of the flange (5), and the circulation pipe (11) is sealed to the acid inlet through the hole of the perforated blind flange (16).

4. The apparatus for online cleaning of wet phosphoric acid graphite heat exchangers using fluorosilicic acid according to claim 3, characterized in that: The perforated blind plate (16) is disc-shaped, with a through hole of DN100 at its center.

5. The apparatus for online cleaning of wet phosphoric acid graphite heat exchangers using fluorosilicic acid according to claim 1, characterized in that: An acid outlet valve (14) is provided at the outlet of the fluorosilicic acid tank (15), and the acid outlet valve (14) is connected to the hose (13).

6. The apparatus for online cleaning of wet phosphoric acid graphite heat exchangers using fluorosilicic acid according to claim 1, characterized in that: The circulation pipe (11) is a steel-lined rubber pipe, and the fluorosilicic acid pump (12) is made of carbon fiber.

7. A method for online cleaning of a wet-process phosphoric acid graphite heat exchanger using the apparatus described in any one of claims 1-6, characterized in that: The method does not require removing the graphite heat exchanger (4) from the production line, and directly uses fluorosilicic acid, a byproduct of wet-process phosphoric acid production, as the cleaning solution. It includes the following steps: Step 1: Modify the pipeline and connect the device, specifically: use a blind flange to seal the phosphoric acid inlet (1), remove the lower reducer (6) of the graphite heat exchanger (4), install a perforated blind flange (16) at the lower flange (5) of the graphite heat exchanger (4), and connect the circulation pipe (11) through the central hole of the perforated blind flange (16) to form the online cleaning device as described in any one of claims 1-6; Step 2: Start the device for circulation cleaning. Extract fluorosilicic acid solution from the fluorosilicic acid tank (15) and inject it into the graphite heat exchanger (4) through the hose (13). The mass concentration ω(H2SiF6) of the fluorosilicic acid solution is 25%. Start the fluorosilicic acid pump (12) to make the fluorosilicic acid solution circulate and clean online between the circulation pipe (11) and the graphite heat exchanger (4) for 48 hours. Step 3: After the circulation cleaning is completed, drain the cleaning fluid and then use a cleaning machine to unclog the inner wall of the graphite heat exchanger (4).

8. The method for online cleaning of wet phosphoric acid graphite heat exchangers using fluorosilicic acid solution according to claim 7, characterized in that: The cleaning fluid circulation rate is 25m. 3 / h, temperature is 80℃-83℃.

9. The method for online cleaning of wet phosphoric acid graphite heat exchangers using fluorosilicic acid solution according to claim 7, characterized in that: In step 2, during the cyclic cleaning process, fluorosilicic acid solution is continuously or intermittently replenished from the fluorosilicic acid tank (15) through a hose (13) to maintain the concentration and level of the cleaning solution.

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