Anhydrous hydrofluoric acid rectification device

Abstract

The utility model discloses an anhydrous hydrofluoric acid rectification device which comprises a spray tower and a material storage tank, a liquid inlet pipe is arranged on one side wall of the spray tower, two groups of rectification assemblies are arranged on the side wall, far away from the liquid inlet pipe, of the spray tower through liquid outlet pipes, the rectification assemblies are connected with the material storage tank, and the spray tower is provided with a spray assembly through a rotary cleaning mechanism. A collecting box is arranged at the bottom end of the spraying tower, a filtering assembly is arranged in the collecting box through a mounting mechanism, and a liquid discharging pipe is arranged at the bottom end of the collecting box. According to the utility model, produced anhydrous hydrofluoric acid can be firstly sprayed and washed to ensure full and uniform spraying, so that the washing effect is ensured, a small amount of impurities are removed, then rectification and condensation operation is carried out, the purity of the anhydrous hydrofluoric acid can be effectively ensured, and the inner wall of the spray tower can be scraped when the inner wall needs to be cleaned and scraped, so that the inner wall of the spray tower can be cleaned and scraped. And the inner cleanliness is ensured, the situation that the service life and the spraying and washing effect are affected due to corrosion is avoided, and the spraying device is connected with the spraying connecting pipe after cleaning is completed, so that follow-up spraying and washing are facilitated.

Description

Technical Field

[0001] The utility model relates to the technical field of anhydrous hydrofluoric acid, in particular to an anhydrous hydrofluoric acid distillation device. Background Art

[0002] Anhydrous hydrofluoric acid is anhydrous hydrogen fluoride, a widely used chemical product. It is a colorless fuming liquid with a concentration of more than 99% hydrofluoric acid, which is easily gasified under reduced pressure or high temperature. It is mainly used to prepare fluoride salts, halogenated alkanes, fluorine refrigerants, corrode glass, impregnate wood, electrolyze elemental fluorine, etc. Anhydrous hydrofluoric acid is extremely active in chemical properties and has strong corrosiveness. It can react with alkalis, metals, oxides and silicates.

[0003] The production process of anhydrous hydrofluoric acid is to obtain crude hydrogen fluoride gas by reacting sulfuric acid and fluorite, which contains a lot of impurities such as dust, silicon fluoride, sulfur dioxide, sulfuric acid, and water. In order to ensure the purity of anhydrous hydrofluoric acid, it is necessary to wash, distill and condense the anhydrous hydrofluoric acid.

[0004] However, the existing anhydrous hydrofluoric acid distillation device has certain defects in actual use. Before distillation, the anhydrous hydrofluoric acid is washed and impurities are removed by sulfuric acid. However, after spray washing, its impurities are easily attached to the inner wall of the tower, which is not convenient for cleaning. Over time, it is easy to cause corrosion of the inner wall, which affects the service life and the effect of subsequent washing and impurity removal. Utility Model Content

[0005] The purpose of the utility model is to provide an anhydrous hydrofluoric acid distillation device to solve the problems raised in the above background technology.

[0006] To achieve the above-mentioned purpose, the utility model provides the following technical solutions: an anhydrous hydrofluoric acid distillation device, comprising a spray tower and a storage tank, a liquid inlet pipe is provided on one side wall of the spray tower, two groups of distillation components are provided on the side wall of the spray tower away from the liquid inlet pipe through a liquid outlet pipe, the distillation component is connected to the storage tank, the spray tower is provided with a spray component through a rotating cleaning mechanism, the top of the spray tower is symmetrically provided with spray connecting pipes, the spray connecting pipes are connected to the spray component through a plug-in component, the bottom end of the spray tower is provided with a collecting box, a filtering component is provided in the collecting box through an installation mechanism, and a drain pipe is provided at the bottom of the collecting box.

[0007] Preferably, in order to distill and purify anhydrous hydrofluoric acid and improve the purity of the product, two groups are provided to ensure the distillation effect. The distillation assembly includes a distillation tank connected to a liquid outlet pipe, the outlet of the distillation tank is connected to a condensation reflux pipe, a water inlet pipe is provided at the bottom end of the condensation reflux pipe, and a water return pipe is provided at the top end. The two groups of distillation assemblies have the same structure, and the condensation reflux pipe of one group of distillation assemblies away from the spray tower is connected to a storage tank.

[0008] Preferably, in order to facilitate the rotation with the fixing ring and the cleaning ring, and then to clean and scrape the inner wall of the spray tower after use, the rotating cleaning mechanism includes a motor arranged in the middle of the top of the spray tower, a screw is fixedly provided at the output end of the motor, the screw passes through the spray tower and extends to the interior to be engaged with a movable ring through a thread, a support plate is fixedly provided at the bottom end of the interior of the spray tower, and the bottom end of the screw is connected to the support plate through a bearing.

[0009] Preferably, in order to ensure the effect of spray washing and ensure sufficient contact with anhydrous hydrofluoric acid, the spray assembly includes connecting fixed pipes equidistantly arranged on the side of the movable ring, a nozzle is provided at the end of the connecting fixed pipe, and nozzles are equidistantly arranged under the nozzle.

[0010] Preferably, in order to facilitate the cleaning and scraping operations on the inner wall of the spray tower, ensure cleanliness, avoid corrosion of the inner wall, etc., the rotating cleaning mechanism also includes a fixed ring arranged on the outside of the nozzle, and a cleaning ring matching the inner wall of the spray tower is provided on the outside of the fixed ring, and guide blocks are symmetrically provided on the side surfaces of the cleaning ring, and the inner wall of the spray tower is provided with a guide groove matching the guide block.

[0011] Preferably, in order to be able to be plugged into the spray connecting pipe during use without affecting the use of spraying, and to be able to be temporarily separated during cleaning without affecting the cleaning use, the plug-in assembly includes a plug-in tube symmetrically arranged on two of the connecting fixed tubes, and the plug-in tube is provided with a plug ring matching the spray connecting pipe, and sealing rings are equidistantly provided on the side surfaces of the plug ring.

[0012] Preferably, in order to separate the washed liquid and cleaned impurities and facilitate the separate collection and treatment of solid and liquid, the filter assembly includes a card frame arranged inside the collection box, and a filter net is embedded in the card frame.

[0013] Preferably, in order to facilitate the installation and disassembly of the filter assembly and the subsequent cleaning and replacement operations of the filter assembly, the installation mechanism includes mounting blocks symmetrically arranged on both sides of the card frame, and the inner wall of the collection box is provided with a mounting groove matching the mounting block, a pull plate is fixedly provided at one end of the card frame, and an empty groove matching the pull plate is provided on the side wall of the collection box, and the two sides of the pull plate are connected to the collection box through a limit assembly.

[0014] Preferably, in order to ensure stability during assembly and use, ensure the filtering effect, and facilitate subsequent pulling out for cleaning or replacement operations, the limiting assembly includes an adjustment plate connected to the pull plate through a sliding structure, and limiting columns are symmetrically arranged on the side walls of the adjustment plate. An adjustment groove matching the adjustment plate is provided on the collection box, and a limiting hole connected to the limiting column is provided in the adjustment groove.

[0015] Preferably, in order to ensure stability during assembly and use, ensure the filtering effect, and facilitate subsequent pulling out for cleaning or replacement operations, the sliding structure includes sliding cavities symmetrically opened inside the two sides of the pull plate, and a movable plate is connected to the sliding cavity through a spring. A connecting column is fixed at the end of the movable plate, and the connecting column slides out of the sliding cavity and is fixedly connected to the adjusting plate.

[0016] Compared with the prior art, the beneficial effects of the utility model are:

[0017] (1) Through the spray tower and distillation components, the produced anhydrous hydrofluoric acid can be sprayed and washed to remove a small amount of impurities contained therein, and then distilled and condensed, thereby effectively ensuring its purity;

[0018] (2) The rotating cleaning mechanism, spray assembly and plug-in assembly can ensure that the normal spray washing operation of sulfuric acid is not affected during spray washing, and the spraying is fully uniform, thereby ensuring the washing effect. When the inner wall needs to be cleaned and scraped, the inner wall of the spray tower can be scraped to ensure the cleanliness of the interior and avoid corrosion, which affects the service life and spray washing effect. After the cleaning is completed, it can be connected to the spray connection pipe to facilitate the subsequent spray washing.

[0019] (3) Through the provided installation mechanism and filter assembly, the washed liquid and cleaned impurities can be separated and filtered, and the solid and liquid can be collected separately, which is convenient for subsequent processing operations. The installation mechanism can easily remove the filter assembly, and then it can be convenient to regularly clean or replace the filter to ensure the filtering effect. At the same time, the stability of the filter assembly can be ensured during use to avoid sliding or offsetting, which may affect the filtering effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic structural diagram of an anhydrous hydrofluoric acid distillation device proposed by the utility model;

[0021] Figure 2 This is a schematic structural diagram of a distillation component in an anhydrous hydrofluoric acid distillation device proposed by the utility model;

[0022] Figure 3 This is a schematic diagram of the internal structure of a spray tower and a collecting box in an anhydrous hydrofluoric acid distillation device proposed by the utility model;

[0023] Figure 4 It is a partial structural schematic diagram of a rotating cleaning mechanism and a spray assembly in an anhydrous hydrofluoric acid distillation device proposed by the utility model;

[0024] Figure 5It is a partial structural schematic diagram of a plug-in assembly and a spray assembly in an anhydrous hydrofluoric acid distillation device proposed by the utility model;

[0025] Figure 6 This is a schematic diagram of the internal structure of a collecting box in an anhydrous hydrofluoric acid distillation device proposed by the utility model;

[0026] Figure 7 It is a partial structural schematic diagram of an installation mechanism in an anhydrous hydrofluoric acid distillation device proposed by the utility model;

[0027] Figure 8 The utility model provides a schematic structural diagram of an adjusting groove and a limiting hole in an anhydrous hydrofluoric acid distillation device.

[0028] In the figure: 1. spray tower; 2. liquid inlet pipe; 3. collecting box; 4. liquid outlet pipe; 5. motor; 6. distillation tank; 7. condensation reflux pipe; 8. return pipe; 9. water inlet pipe; 10. storage tank; 11. spray connecting pipe; 12. support plate; 13. screw; 14. guide groove; 15. pull plate; 16. moving ring; 17. connecting fixed pipe; 18. spray pipe; 19. fixing ring; 20. cleaning ring; 21. guide block; 22. plug-in pipe; 23. spray head; 24. plug-in ring; 25. sealing ring; 26. filter screen; 27. drain pipe; 28. mounting groove; 29. ​​mounting block; 30. card frame; 31. adjusting plate; 32. limiting column; 33. connecting column; 34. moving plate; 35. spring; 36. sliding cavity; 37. adjusting groove; 38. limiting hole. DETAILED DESCRIPTION

[0029] The following will be combined with the drawings in the embodiments of the utility model to clearly and completely describe the technical solutions in the embodiments of the utility model. Obviously, the described embodiments are only part of the embodiments of the utility model, not all of the embodiments. Based on the embodiments in the utility model, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the utility model.

[0030] See also Figure 1-Figure 8The utility model provides an embodiment: an anhydrous hydrofluoric acid distillation device, including a spray tower 1 for spraying and removing impurities from the produced hydrofluoric acid, wherein the preparation of anhydrous hydrofluoric acid can be achieved by reacting fluorite powder and sulfuric acid, which is a relatively mature technology and is not described in detail here. A liquid inlet pipe 2 is installed on one side wall of the spray tower 1 for conveniently introducing the generated anhydrous hydrofluoric acid product, and a liquid outlet pipe 4 is installed on the other side wall for conveniently discharging after spraying and washing. Two groups of distillation components are arranged at the end of the liquid outlet pipe 4 (the number of which can be set according to actual conditions to ensure the purity of the distillation), and the distillation components include a distillation tank 6 (with a reboiler, etc.) and The condensation reflux pipe 7 is connected by a connecting pipe. The temperature inside the distillation tank 6 is 25-30°C. The distilled hydrogen fluoride is discharged from the top of the distillation tower in a gaseous form and enters the condensation reflux pipe 7. A water inlet pipe 9 is provided at the bottom of the condensation reflux pipe 7, and a water return pipe 8 is provided at the top (the water inlet pipe 9 and the water return pipe 8 are only partially illustrated in the figure, which is a relatively mature technology and will not be described here. Condensation can also be performed in other ways). It is convenient to condense the hydrogen fluoride in the pipe, and then it can be distilled and condensed again, and then connected to the storage tank 10 for collection. A degassing tower can also be provided as needed to ensure the purity of the distillation. This is a relatively mature technology and will not be described here.

[0031] See also Figure 1-Figure 8In order to better wash and remove impurities from hydrofluoric acid, ensure its purity, and facilitate subsequent distillation operations, the interior of the spray tower 1 can also be cleaned after a period of use to ensure cleanliness and prevent impurities from adhering to the inner wall of the spray tower 1, which may easily cause corrosion, etc., affecting the service life and the effect of subsequent spray washing. A motor 5 for a rotating cleaning mechanism is installed at the top of the spray tower 1. A screw 13 is fixedly installed at the output end of the motor 5 through a coupling. The screw 13 penetrates the spray tower 1 and extends to the inside. A support plate 12 is provided at the bottom end through a bearing, and the support plate 12 is arranged at the bottom end of the spray tower 1. A moving ring 16 is meshed with a thread on the screw 13. A connecting and fixing pipe 17 of a spray assembly is welded to the circumferential side of the moving ring 16. The number of the connecting and fixing pipes 17 can be four groups, and the ends thereof are commonly connected to an annular nozzle 18. A plurality of groups of nozzles 23 (the number can be six or eight groups, etc., to ensure sufficient uniformity of spraying) are equidistantly installed below the nozzle 18, and the spray tower 1 Two groups of spray connecting pipes 11 are installed on the top of the spray tower 18 (which can be connected to the liquid storage tank through a machine pump, and the liquid inside the liquid storage tank can be sulfuric acid, which can be easily extracted for spray washing). In order to facilitate the connection with the spray pipe 18, two connecting fixed pipes 17 are symmetrically provided with plug-in pipes 22 of the plug-in assembly, and the top of the plug-in pipe 22 is welded with a plug ring 24 that is convenient for plugging with the spray connecting pipe 11. In order to ensure the sealing of the connection, two groups of sealing rings 25 are arranged on the surrounding side (made of acid-resistant material, relatively mature technology, and will not be repeated here). In order to facilitate the cleaning operation, a fixing ring 19 is arranged on the outside of the spray pipe 18, and a cleaning ring 20 is bonded to the outside of the fixing ring 19 for convenient cleaning and scraping of the inner wall of the spray tower 1. In order to ensure that when the motor 5 rotates with the screw 13, it can move up and down with the movable ring 16 and the spray pipe 18, two groups of guide blocks 21 are symmetrically arranged on the surrounding side of the cleaning ring 20, and the inner wall of the spray tower 1 is provided with a guide groove 14 for convenient sliding of the guide block 21 up and down.

[0032] See also Figure 1-Figure 8In order to collect the acid and impurities after spray washing and realize solid-liquid separation at the same time, and facilitate separate collection and treatment, the bottom end of the spray tower 1 is connected to a collection box 3, in which a card frame 30 of a filter assembly is arranged inside, and a filter screen 26 is embedded inside the card frame 30. A drain pipe 27 is installed at the bottom of the collection box 3 below the filter screen 26 to facilitate the discharge of the liquid after washing and filtering. In order to facilitate the installation and extraction of the card frame 30, mounting blocks 29 of the mounting mechanism are symmetrically welded on both sides of the card frame 30, and a mounting groove 28 for facilitating the sliding of the mounting block 29 is opened on the inner wall of the collection box 3. In order to facilitate pulling and pushing operations, a pull plate 15 is welded at one end of the card frame 30, and a pull plate 15 is opened on the side wall of the collection box 3. An empty slot corresponding to the pull plate 15 is provided to facilitate the opening and closing operation. In order to ensure that there is no movement during use and to ensure the filtering effect, sliding cavities 36 of the sliding structure in the limiting assembly are provided inside the pull plate 15 on both sides. A moving plate 34 is slidably connected to the inside through a spring 35 and can slide in the sliding cavity 36. A connecting column 33 is welded to the end of the moving plate 34 away from the spring 35. The connecting column 33 slides out of the sliding cavity 36 and is welded with an adjusting plate 31. Two groups of limiting columns 32 are welded on the side wall of the adjusting plate 31. Adjustment slots 37 adapted to the adjusting plate 31 are provided on both sides of the empty slot of the collection box 3, and limiting holes 38 for plugging and limiting with the limiting columns 32 are provided in the adjusting slots 37.

[0033] Working principle: When the utility model is in use, the produced anhydrous hydrofluoric acid is introduced into the spray tower 1 through the liquid inlet pipe 2, and sulfuric acid is introduced through the spray connecting pipe 11 at this time, and is introduced into the spray pipe 18 through the plug-in pipe 22 and the connecting fixed pipe 17, and is sprayed out through the nozzle 23, so as to wash the hydrofluoric acid and remove impurities, and then it passes through the distillation tank 6 and the condensation reflux pipe 7 in turn through the liquid outlet pipe 4 for distillation and condensation, and then the anhydrous hydrofluoric acid after distillation is collected and stored through the storage tank 10. When the spray tower 1 needs to be cleaned after use, the motor 5 rotates forward, driving the screw 13 to rotate, and then under the cooperation of the guide block 21 and the guide groove 14, the movable ring 16 moves downward on the screw 13, and the plug-in pipe 22 leaves the spray connecting pipe 11, and at this time, it moves downward with the fixed ring 19 and the cleaning ring 20, so that the inner wall of the spray tower 1 can be cleaned. The cleaning operation is performed, and the cleaned impurities fall onto the filter screen 26 in the collection box 3, and the liquid for spray washing is filtered by the filter screen 26 and discharged through the drain pipe 27 for collection. After the cleaning is completed, the motor 5 rotates in the opposite direction, and the whole is moved up and reset, and the plug ring 24 on the plug-in tube 22 enters the spray connecting pipe 11, and cooperates with the sealing ring 25 to complete the plug-in assembly, which is convenient for the next spray washing. When the filter screen 26 needs to be cleaned after using it for a period of time, the two adjusting plates 31 are moved, and the connecting column 33 and the movable plate 34 slide in the sliding cavity 36, and the spring 35 is forced to shrink and store potential energy, so that the limiting column 32 leaves the limiting hole 38, loses the limiting position of the pulling plate 15, and can be pulled out, and the mounting block 29 slides in the mounting groove 28 to move the card frame 30 out of the outside world, which is convenient for cleaning or replacing the filter screen 26.

[0034] In summary, by means of the spray tower 1 and the distillation assembly, the produced anhydrous hydrofluoric acid can be spray-washed first to remove a small amount of impurities contained therein, and then distilled and condensed, thereby effectively ensuring its purity; by means of the rotating cleaning mechanism, the spray assembly and the plug-in assembly, the normal spray washing operation of sulfuric acid can be not affected during the spray washing, thereby ensuring sufficient uniformity of the spray, and thus ensuring the washing effect; and when the inner wall needs to be cleaned and scraped, the inner wall of the spray tower 1 can be scraped to ensure the cleanliness of the interior and avoid corrosion thereof, thereby affecting the use. The service life and spray washing effect can be improved. It can also be connected to the spray connecting pipe 11 after cleaning is completed, which is convenient for subsequent spray washing. Through the provided installation mechanism and filter assembly, the washed liquid and the cleaned impurities can be separated and filtered, and the solid and liquid can be collected separately, which is convenient for subsequent processing operations. The installation mechanism can easily remove the filter assembly, and then it can be convenient to regularly clean or replace the filter net 26 to ensure the filtering effect. At the same time, the stability of the filter assembly can be ensured during use to avoid sliding or offset, which affects the filtering effect.

[0035] It is obvious to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, the embodiments should be regarded as exemplary and non-restrictive from any point of view, and the scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims be included in the present invention. Any reference numeral in a claim should not be regarded as limiting the claim to which it relates.

Claims

1. An anhydrous hydrofluoric acid distillation device, comprising a spray tower (1) and a storage tank (10), characterized in that: A liquid inlet pipe (2) is provided on one side wall of the spray tower (1); two groups of distillation components are provided on the side wall of the spray tower (1) away from the liquid inlet pipe (2) via a liquid outlet pipe (4); the distillation components are connected to the storage tank (10); the spray tower (1) is provided with a spray component via a rotating cleaning mechanism; the top of the spray tower (1) is symmetrically provided with a spray connecting pipe (11); the spray connecting pipe (11) is connected to the spray component via a plug-in component; the bottom of the spray tower (1) is provided with a collecting box (3); a filtering component is provided in the collecting box (3) via a mounting mechanism; and the bottom of the collecting box (3) is provided with a drain pipe (27).

2. An anhydrous hydrofluoric acid distillation device according to claim 1, characterized in that: The distillation assembly comprises a distillation tank (6) connected to a liquid outlet pipe (4); the outlet of the distillation tank (6) is connected to a condensation reflux pipe (7); a water inlet pipe (9) is provided at the bottom end of the condensation reflux pipe (7) and a water return pipe (8) is provided at the top end; the two groups of distillation assemblies have the same structure, and the condensation reflux pipe (7) of the distillation assembly away from the spray tower (1) is connected to the storage tank (10).

3. An anhydrous hydrofluoric acid distillation device according to claim 1, characterized in that: The rotary cleaning mechanism comprises a motor (5) arranged at the middle of the top end of the spray tower (1); a screw (13) is fixedly provided at the output end of the motor (5); the screw (13) passes through the spray tower (1) and extends to the inside where a movable ring (16) is engaged through a thread; a support plate (12) is fixedly provided at the bottom end of the inside of the spray tower (1); and the bottom end of the screw (13) is connected to the support plate (12) through a bearing.

4. An anhydrous hydrofluoric acid distillation device according to claim 3, characterized in that: The spray assembly comprises a connecting fixed pipe (17) which is equidistantly arranged on the circumferential side of the moving ring (16); a nozzle (18) is arranged at the end of the connecting fixed pipe (17); and a spray head (23) is equidistantly arranged below the nozzle (18).

5. An anhydrous hydrofluoric acid distillation device according to claim 4, characterized in that: The rotating cleaning mechanism also includes a fixing ring (19) arranged outside the spray pipe (18), a cleaning ring (20) matching the inner wall of the spray tower (1) is arranged outside the fixing ring (19), and guide blocks (21) are symmetrically arranged on the circumferential side of the cleaning ring (20), and a guide groove (14) matching the guide block (21) is opened on the inner wall of the spray tower (1).

6. An anhydrous hydrofluoric acid distillation device according to claim 5, characterized in that: The plug-in assembly comprises a plug-in tube (22) symmetrically arranged on two connecting fixed tubes (17), the plug-in tube (22) being provided with a plug-in ring (24) matching the spray connection tube (11), and sealing rings (25) being equidistantly arranged on the circumferential side of the plug-in ring (24).

7. An anhydrous hydrofluoric acid distillation device according to claim 1, characterized in that: The filter assembly comprises a card frame (30) arranged inside the collection box (3), and a filter net (26) is embedded inside the card frame (30).

8. An anhydrous hydrofluoric acid distillation device according to claim 7, characterized in that: The mounting mechanism comprises mounting blocks (29) symmetrically arranged on both sides of the card frame (30); the inner wall of the collection box (3) is provided with a mounting groove (28) matching the mounting block (29); a pull plate (15) is fixedly provided at one end of the card frame (30); a hollow groove matching the pull plate (15) is provided on the side wall of the collection box (3); and both sides of the pull plate (15) are connected to the collection box (3) via a limit assembly.

9. An anhydrous hydrofluoric acid distillation device according to claim 8, characterized in that: The limiting assembly comprises an adjusting plate (31) connected to the pull plate (15) via a sliding structure, limiting posts (32) are symmetrically arranged on the side wall of the adjusting plate (31), an adjusting groove (37) matching the adjusting plate (31) is provided on the collecting box (3), and a limiting hole (38) plugged into the limiting post (32) is provided in the adjusting groove (37).

10. An anhydrous hydrofluoric acid distillation device according to claim 9, characterized in that: The sliding structure comprises sliding cavities (36) symmetrically opened inside the two sides of the pulling plate (15), the sliding cavity (36) is connected to a moving plate (34) via a spring (35), a connecting column (33) is fixedly provided at the end of the moving plate (34), and the connecting column (33) slides out of the sliding cavity (36) and is fixedly connected to the adjusting plate (31).

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