A system for purifying sodium pyrophosphate containing metal impurities to produce disodium hydrogen phosphate

Abstract

This invention discloses a system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate, belonging to the field of waste treatment technology. The system includes a dissolving vessel equipped with a crude sodium pyrophosphate inlet pipe, a first water inlet pipe, and a first discharge pipe. The first discharge pipe is connected to a photocatalytic reaction device, which is connected to an electrochemical device. The electrochemical device is connected to a hydrolysis vessel via a reaction liquid outlet pipe. The hydrolysis vessel is equipped with a second water inlet pipe, a catalyst inlet pipe, and a second discharge pipe. The second discharge pipe is connected to a crystallizer, which is connected to a first filter, which is connected to an oven. This invention can obtain high-purity disodium hydrogen phosphate.

Description

Technical Field

[0001] This utility model belongs to the field of waste treatment technology, specifically relating to a system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate. Background Technology

[0002] Glyphosate mother liquor treatment is an industry-wide challenge. Whether produced via the glycine process or the IDA process, the mother liquor is characterized by high COD, high TP, high salt content, complex composition, and difficulty in degradation, making treatment very difficult and costly. Currently, the mainstream mother liquor treatment process uses oxidation methods to degrade the mother liquor and recover phosphorus, including incineration, high-temperature oxidation, medium-temperature oxidation, and low-temperature oxidation. Membrane separation technology or evaporation concentration is also used during the mother liquor treatment process to recover sodium chloride and phosphate. For example: membrane separation recovers sodium chloride, glyphosate, and other substances; evaporation concentration recovers glyphosate and sodium chloride; concentrated mother liquor undergoes targeted conversion to recover sodium pyrophosphate; catalytic oxidation of the mother liquor recovers phosphate, and so on. Each method has its advantages and disadvantages, and major glyphosate manufacturers employ different treatment processes, with most companies combining several of these processes to treat the glyphosate mother liquor. Currently, companies such as Xingfa Group, Xin'an Chemical, and Shandong Runfeng all have facilities for the targeted conversion of concentrated mother liquor to produce sodium pyrophosphate. The crude sodium pyrophosphate obtained has a main content of 40-70%, and the sodium chloride content ranges from 3-35% depending on the processing technology of each company. It also contains impurities such as sodium carbonate, sodium bicarbonate, sodium sulfite, sodium bisulfite, and organophosphates. For this route, whether membrane treatment desalination or evaporation concentration desalination process, the final crude sodium pyrophosphate contains high levels of sodium chloride. The product has many impurities, poor color, off-odor, low main content, and poor quality, resulting in low market price and limited application. Further purification is needed to improve the main content and quality.

[0003] Some companies refine crude sodium pyrophosphate to obtain sodium pyrophosphate, but the production process suffers from problems such as slow dissolution of crude sodium pyrophosphate, high viscosity of the solution, small processing capacity, large wastewater volume, high cost, and poor product quality. The phosphate product also exhibits high levels of residual sodium chloride and organophosphates, resulting in poor quality. For example, CN206915770U describes a system for purifying phosphorus resources after deep treatment of glyphosate mother liquor. CN1068098llA describes a method for preparing sodium pyrophosphate using crude sodium pyrophosphate, and CN106882781A describes a decolorization method for directly refining and extracting sodium pyrophosphate during the purification of crude sodium pyrophosphate to produce sodium pyrophosphate. These methods also suffer from the same problems in processing crude sodium pyrophosphate, including slow dissolution, high viscosity of the solution, small processing capacity, and high sodium chloride content in the sodium pyrophosphate product. Therefore, it is necessary to develop new process methods tailored to the characteristics of the materials.

[0004] Chinese patent CN109399593A, published on March 1, 2019, discloses a method for purifying crude sodium pyrophosphate to produce disodium hydrogen phosphate and sodium chloride. The method involves mixing crude sodium pyrophosphate and water in a hydrolysis reactor at a mass ratio of 1:0.5. Phosphoric acid is added dropwise to control the pH of the mixture within the range of 1.0-8.8. After adding an oxidizing agent, the mixture is stirred at 70-120°C for 5-60 minutes to dissolve and hydrolyze the crude sodium pyrophosphate. The crude sodium pyrophosphate is a crude product obtained by incinerating glyphosate mother liquor and diglyphosate mother liquor wastewater, with sodium pyrophosphate and sodium chloride as its main components. The main components are sodium pyrophosphate and sodium chloride, with a sodium pyrophosphate content of 40-85%, a sodium chloride content of 3-35%, and a total organic carbon content of 0.1-0. 3%, also contains sodium carbonate, sodium bicarbonate, sodium sulfite, sodium bisulfite, and organophosphorus impurities, with an aqueous solution pH of 9.5-10.55; Sodium chloride product recovery: The slurry in the hydrolysis vessel described in step (1) is discharged hot and filtered to obtain sodium chloride product and filtrate; Disodium hydrogen phosphate recovery: The filtrate described in step (2) is cooled and mixed with alkali in a mixer, wherein the alkali is one or a combination of sodium hydroxide, sodium carbonate, and sodium bicarbonate, and the pH of the mixed solution is controlled in the range of 8-10 to generate disodium hydrogen phosphate. After filtering out impurities, it is sent to the crystallization vessel. After the slurry is cooled to 70-85℃, water and disodium hydrogen phosphate crystals are added to the crystallization vessel, and the temperature is controlled at 0-30℃ for 2-8 hours to precipitate disodium hydrogen phosphate dodecahydrate. The fully crystallized slurry is filtered and separated to obtain disodium hydrogen phosphate dodecahydrate product. The disodium hydrogen phosphate dodecahydrate product obtained by this patent has low purity and is ineffective in removing the metal impurities. Utility Model Content

[0005] The purpose of this invention is to solve the problems of existing technologies and provide a system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate. Crude sodium pyrophosphate is added to a dissolving vessel through a crude sodium pyrophosphate inlet pipe, and demineralized water is added to the dissolving vessel through a first inlet pipe to dissolve the sodium pyrophosphate solution. The sodium pyrophosphate solution is then fed into a photocatalytic reaction device through a first feed pipe. The photocatalyst absorbs photon energy, generating electron-hole pairs. Holes have strong oxidizing properties and can oxidize low-valence iron and manganese ions in the solution to high-valence iron and manganese ions. The solution after photocatalytic oxidation enters an electrochemical device, where an oxidation reaction occurs at the anode under the action of an electric field, further promoting the oxidation state transformation of metal ions and making them more readily reacted with... A complexing agent forms a complex, while a reduction reaction occurs at the cathode, producing hydrogen gas or other reduction products. The complexing agent forms a stable complex with the metal ions. Under the influence of an electric field, the complex moves towards the cathode and is separated from the solution by electrodeposition or other separation methods. After electrochemical treatment, the sodium pyrophosphate solution, free of metal impurities, is transported to a hydrolysis reactor. A catalyst is introduced into the hydrolysis reactor through a catalyst inlet pipe, and the sodium pyrophosphate is decomposed under the action of the catalyst to obtain disodium hydrogen phosphate. After the hydrolysis reaction is completed, the solution is transferred to a crystallizer through a second feed pipe, where disodium hydrogen phosphate crystallizes out. A centrifugal separator separates the crystals from the solution. The crystals are dried in an oven to obtain the final disodium hydrogen phosphate product. This invention can obtain high-purity disodium hydrogen phosphate.

[0006] This utility model is achieved through the following technical solution: A system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate includes a dissolving vessel. The dissolving vessel is equipped with a crude sodium pyrophosphate inlet pipe, a first water inlet pipe, and a first feed pipe. The first feed pipe is connected to a photocatalytic reaction device, which is connected to an electrochemical device. The electrochemical device is connected to a hydrolysis vessel via a reaction liquid outlet pipe. The hydrolysis vessel is equipped with a second water inlet pipe, a catalyst inlet pipe, and a second feed pipe. The second feed pipe is connected to a crystallizer, which is connected to a first filter, which is connected to an oven.

[0007] Preferably, the photocatalytic reaction device includes a light-transmitting reaction body, a light source disposed outside the reaction body, and a catalyst layer disposed inside the reaction body.

[0008] Preferably, the light source is an ultraviolet lamp or an LED lamp.

[0009] Preferably, the electrochemical device includes an electrolytic cell and a power source. The electrolytic cell is provided with an anode and a cathode, which are connected to the power source. The electrolytic cell is provided with a complexing agent inlet pipe and a reaction liquid outlet pipe.

[0010] Preferably, the dissolving vessel is equipped with a first stirring device.

[0011] Preferably, the hydrolysis vessel is equipped with a second stirring device.

[0012] Preferably, the crystallizer is equipped with a third stirring device.

[0013] Preferably, the dissolving vessel is provided with a first temperature control unit, which includes a first temperature sensor and a first heat exchange jacket on the dissolving vessel. The first heat exchange jacket is provided with a first heat exchange inlet pipe and a first heat exchange outlet pipe, and the first heat exchange inlet pipe is provided with a first heat exchange valve.

[0014] Preferably, the hydrolysis vessel is provided with a second temperature control unit, the second temperature control unit includes a second temperature sensor and a second heat exchange jacket provided on the hydrolysis vessel, the second heat exchange jacket is provided with a second heat exchange inlet pipe and a second heat exchange outlet pipe, and the second heat exchange inlet pipe is provided with a second heat exchange valve.

[0015] Preferably, the crystallizer is provided with a third temperature control unit, which includes a third temperature sensor and a third heat exchange jacket on the crystallizer. The third heat exchange jacket is provided with a third heat exchange inlet pipe and a third heat exchange outlet pipe, and the third heat exchange inlet pipe is provided with a third heat exchange valve.

[0016] Compared with the prior art, this utility model has the following advantages and beneficial effects: I. This utility model provides a system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate. Crude sodium pyrophosphate is added to a dissolving vessel through a crude sodium pyrophosphate inlet pipe, and demineralized water is added to the dissolving vessel through a first inlet pipe to dissolve the sodium pyrophosphate solution. The sodium pyrophosphate solution is then fed into a photocatalytic reaction device through a first feed pipe. The photocatalyst absorbs photon energy, generating electron-hole pairs. Holes have strong oxidizing properties and can oxidize low-valence iron and manganese ions in the solution to high-valence iron and manganese ions. The solution after photocatalytic oxidation enters an electrochemical device. Under the action of an electric field, an oxidation reaction occurs at the anode, further promoting the oxidation state transformation of metal ions, making them more easily complexed with a complexing agent. Simultaneously, a reduction reaction occurs at the cathode, producing hydrogen gas or other reduction products. A complexing agent forms a stable complex with the metal ions. Under the influence of an electric field, the complex moves towards the cathode and is separated from the solution through electrodeposition or other separation methods. After electrochemical treatment, the sodium pyrophosphate solution, free of metal impurities, is transported to a hydrolysis reactor. A catalyst is introduced into the hydrolysis reactor through a catalyst inlet pipe, and the sodium pyrophosphate decomposes under the action of the catalyst to obtain disodium hydrogen phosphate. After the hydrolysis reaction is complete, the solution is transferred to a crystallizer through a second feed pipe, where disodium hydrogen phosphate crystallizes out. A centrifugal separator separates the crystals from the solution. The crystals are dried in an oven to obtain the final disodium hydrogen phosphate product. This invention can obtain high-purity disodium hydrogen phosphate.

[0017] II. The present invention provides a system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate. In the photocatalytic reaction device, the photocatalyst absorbs photon energy and generates electron-hole pairs. Holes have strong oxidizing properties and can oxidize low-valence iron ions and manganese ions in the solution to high-valence states, effectively oxidizing low-valence iron ions and manganese ions in the sodium pyrophosphate solution to high-valence iron ions and manganese ions.

[0018] III. The present invention provides a system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate. The first stirring device enables more thorough dissolution of sodium pyrophosphate; the second stirring device enables more thorough hydrolysis; and the third stirring device allows for convenient adjustment of crystal growth rate and particle size distribution.

[0019] IV. The present invention provides a system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate. The first temperature control unit, the second temperature control unit and the third temperature control unit respectively enable sodium pyrophosphate to dissolve more thoroughly, hydrolyze more thoroughly and facilitate the adjustment of crystal growth rate and particle size distribution. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the structure of this utility model; Figure 2This is a schematic diagram of the structure of Embodiment 2 of this utility model; The components are as follows: 1. Dissolving vessel; 2. Crude sodium pyrophosphate inlet pipe; 3. First water inlet pipe; 4. First feed pipe; 5. Photocatalytic reaction device; 6. Electrochemical device; 61. Reaction liquid outlet pipe; 7. Hydrolysis vessel; 8. Second water inlet pipe; 9. Catalyst inlet pipe; 10. Second feed pipe; 11. Crystallizer; 12. First filter; 13. Drying oven; 14. First stirring device; 15. Second stirring device; 16. Third stirring device; 17. First temperature sensor; 18. First heat exchange jacket; 19. First heat exchange inlet pipe; 20. First heat exchange outlet pipe; 21. First heat exchange valve; 22. Second heat exchange jacket; 23. Second heat exchange inlet pipe; 24. Second heat exchange outlet pipe; 25. Second heat exchange valve; 26. Third temperature sensor; 27. Third heat exchange jacket; 28. Third heat exchange inlet pipe; 29. ​​Third heat exchange outlet pipe; 30. Third heat exchange valve; 31. Second temperature sensor. Detailed Implementation

[0021] The present invention will be further described in detail below with reference to the embodiments, but the implementation of the present invention is not limited thereto.

[0022] Example 1 like Figure 1 As shown, this embodiment provides a system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate. The system includes a dissolving vessel 1, which is equipped with a crude sodium pyrophosphate inlet pipe 2, a first water inlet pipe 3, and a first feed pipe 4. The first feed pipe 4 is connected to a photocatalytic reaction device 5, which is connected to an electrochemical device 6. The electrochemical device 6 is connected to a hydrolysis vessel 7 via a reaction liquid outlet pipe 61. The hydrolysis vessel 7 is equipped with a second water inlet pipe 8, a catalyst inlet pipe 9, and a second feed pipe 10. The second feed pipe 10 is connected to a crystallizer 11, which is connected to a first filter 12, and the first filter 12 is connected to an oven 13.

[0023] Example 2 like Figure 2 As shown, this embodiment provides a system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate. The system includes a dissolving vessel 1, which is equipped with a crude sodium pyrophosphate inlet pipe 2, a first water inlet pipe 3, and a first feed pipe 4. The first feed pipe 4 is connected to a photocatalytic reaction device 5, which is connected to an electrochemical device 6. The electrochemical device 6 is connected to a hydrolysis vessel 7 via a reaction liquid outlet pipe 61. The hydrolysis vessel 7 is equipped with a second water inlet pipe 8, a catalyst inlet pipe 9, and a second feed pipe 10. The second feed pipe 10 is connected to a crystallizer 11, which is connected to a first filter 12, and the first filter 12 is connected to an oven 13.

[0024] The photocatalytic reaction device 5 includes a light-transmitting reaction body, a light source disposed on the outside of the reaction body, and a catalyst layer disposed inside the reaction body.

[0025] The light source is either an ultraviolet lamp or an LED lamp.

[0026] The electrochemical device 6 includes an electrolytic cell and a power source. The electrolytic cell is equipped with an anode and a cathode, which are connected to the power source. The electrolytic cell is equipped with a complexing agent inlet pipe and a reaction liquid outlet pipe 61.

[0027] The dissolving vessel 1 is equipped with a first stirring device 14.

[0028] The hydrolysis vessel 7 is equipped with a second stirring device 15.

[0029] The crystallizer 11 is equipped with a third stirring device 16.

[0030] The melting vessel 1 is provided with a first temperature control unit, which includes a first temperature sensor 17 and a first heat exchange jacket 18. The first heat exchange jacket 18 is provided with a first heat exchange inlet pipe 19 and a first heat exchange outlet pipe 20. The first heat exchange inlet pipe 19 is provided with a first heat exchange valve 21.

[0031] The hydrolysis vessel 7 is provided with a second temperature control unit, which includes a second temperature sensor 31 and a second heat exchange jacket 22. The second heat exchange jacket 22 is provided with a second heat exchange inlet pipe 23 and a second heat exchange outlet pipe 24. The second heat exchange inlet pipe 23 is provided with a second heat exchange valve 25.

[0032] The crystallizer 11 is provided with a third temperature control unit, which includes a third temperature sensor 26 and a third heat exchange jacket 27. The third heat exchange jacket 27 is provided with a third heat exchange inlet pipe 28 and a third heat exchange outlet pipe 29. The third heat exchange inlet pipe 28 is provided with a third heat exchange valve 30.

[0033] Among them, the photocatalytic reaction device: the catalyst layer adopts Photocatalyst and supporting method (fixed bed or fluidized bed); light source parameters: ultraviolet lamp wavelength 254nm, light intensity 100-300mW / cm², reaction time 10-30min, ensuring... Oxidation rate ≥ 95%. Electrochemical device: The complexing agent in the complexing agent inlet pipe is EDTA or citric acid; Electrolytic cell parameters: Anode uses... The electrodes and cathode are Pt / C electrodes, with an electrolysis voltage of 3-5V and a current density of 10-20mA / cm², which promotes the electrodeposition of metal complexes on the cathode surface.

[0034] Compared with the prior art, this utility model has the following advantages and beneficial effects: I. This utility model provides a system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate. Crude sodium pyrophosphate is added to a dissolving vessel 1 through a crude sodium pyrophosphate inlet pipe 2. Deionized water is added to the dissolving vessel 1 through a first water inlet pipe 3, dissolving to obtain a sodium pyrophosphate solution. The sodium pyrophosphate solution is then fed into a photocatalytic reaction device 5 through a first feed pipe 4. The photocatalyst absorbs photon energy, generating electron-hole pairs. Holes have strong oxidizing properties and can oxidize low-valence iron and manganese ions in the solution to high-valence iron and manganese ions. The solution after photocatalytic oxidation enters an electrochemical device 6. Under the action of an electric field, an oxidation reaction occurs at the anode, further promoting the oxidation state transformation of metal ions, making them more easily complexed with complexing agents. During the process, a reduction reaction occurs at the cathode, producing hydrogen gas or other reduction products. A complexing agent forms a stable complex with the metal ions. Under the influence of an electric field, the complex moves towards the cathode and is separated from the solution through electrodeposition or other separation methods. After electrochemical treatment, the sodium pyrophosphate solution, free of metal impurities, is transported to a hydrolysis reactor. A catalyst is introduced into the hydrolysis reactor 7 through a catalyst inlet pipe 9. The sodium pyrophosphate is decomposed under the action of the catalyst to obtain disodium hydrogen phosphate. After the hydrolysis reaction is complete, the solution is transferred to a crystallizer 11 through a second feed pipe 10. In the crystallizer 11, disodium hydrogen phosphate crystallizes and precipitates. A centrifugal separator separates the crystals from the solution. The crystals are dried in an oven 13 to obtain the final disodium hydrogen phosphate product. This invention can obtain high-purity disodium hydrogen phosphate.

[0035] II. The present invention provides a system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate. In the photocatalytic reaction device 5, the photocatalyst absorbs photon energy and generates electron-hole pairs. Holes have strong oxidizing properties and can oxidize low-valence iron ions and manganese ions in the solution to high-valence states, effectively oxidizing low-valence iron ions and manganese ions in the sodium pyrophosphate solution to high-valence iron ions and manganese ions.

[0036] III. The present invention provides a system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate. The first stirring device 14 makes the sodium pyrophosphate dissolve more thoroughly; the second stirring device 15 makes the hydrolysis more thorough; and the third stirring device 16 allows for convenient adjustment of the crystal growth rate and particle size distribution.

[0037] IV. The present invention provides a system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate. The first temperature control unit, the second temperature control unit and the third temperature control unit respectively enable sodium pyrophosphate to dissolve more thoroughly, hydrolyze more thoroughly and facilitate the adjustment of crystal growth rate and particle size distribution.

[0038] The above description is merely a preferred embodiment of the present utility model and is not intended to limit the present utility model in any way. Any simple modifications or equivalent changes made to the above embodiments based on the technical essence of the present utility model shall fall within the protection scope of the present utility model.

Claims

1. A system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate, characterized in that: The apparatus includes a dissolving vessel (1), which is equipped with a crude sodium pyrophosphate inlet pipe (2), a first water inlet pipe (3) and a first discharge pipe (4). The first discharge pipe (4) is connected to a photocatalytic reaction device (5), which is connected to an electrochemical device (6). The electrochemical device (6) is connected to a hydrolysis vessel (7) through a reaction liquid outlet pipe (61). The hydrolysis vessel (7) is equipped with a second water inlet pipe (8), a catalyst inlet pipe (9) and a second discharge pipe (10). The second discharge pipe (10) is connected to a crystallizer (11), which is connected to a first filter (12) and a drying oven (13).

2. The system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate according to claim 1, characterized in that: The photocatalytic reaction device (5) includes a light-transmitting reaction body, a light source disposed outside the reaction body, and a catalyst layer disposed inside the reaction body.

3. The system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate according to claim 2, characterized in that: The light source is an ultraviolet lamp or an LED lamp.

4. The system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate according to claim 1, characterized in that: The electrochemical device (6) includes an electrolytic cell and a power source. An anode and a cathode are provided in the electrolytic cell. The anode and cathode are connected to the power source. A complexing agent inlet pipe and a reaction liquid outlet pipe (61) are provided on the electrolytic cell.

5. The system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate according to claim 1, characterized in that: The dissolving vessel (1) is equipped with a first stirring device (14).

6. The system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate according to claim 1, characterized in that: The hydrolysis vessel (7) is equipped with a second stirring device (15).

7. The system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate according to claim 1, characterized in that: The crystallizer is equipped with a third stirring device (16).

8. The system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate according to claim 1, characterized in that: The dissolving vessel (1) is provided with a first temperature control unit. The first temperature control unit includes a first temperature sensor (17) provided on the dissolving vessel (1) and a first heat exchange jacket (18) provided on the dissolving vessel (1). The first heat exchange jacket (18) is provided with a first heat exchange inlet pipe (19) and a first heat exchange outlet pipe (20). The first heat exchange inlet pipe (19) is provided with a first heat exchange valve (21).

9. The system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate according to claim 1, characterized in that: The hydrolysis vessel (7) is provided with a second temperature control unit. The second temperature control unit includes a second temperature sensor (31) provided on the hydrolysis vessel (7) and a second heat exchange jacket (22) provided on the hydrolysis vessel (7). The second heat exchange jacket (22) is provided with a second heat exchange inlet pipe (23) and a second heat exchange outlet pipe (24). The second heat exchange inlet pipe (23) is provided with a second heat exchange valve (25).

10. The system for purifying sodium pyrophosphate containing metallic impurities to produce disodium hydrogen phosphate according to claim 1, characterized in that: The crystallizer (11) is provided with a third temperature control unit, which includes a third temperature sensor (26) and a third heat exchange jacket (27) on the crystallizer (11). The third heat exchange jacket (27) is provided with a third heat exchange inlet pipe (28) and a third heat exchange outlet pipe (29). The third heat exchange inlet pipe (28) is provided with a third heat exchange valve (30).

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