Use of forced circulation reboiler for hydrofluoric acid generation

Abstract

The present disclosure relates to an improved method to produce hydrofluoric acid from an aqueous sulfuric acid–hydrofluoric acid solution. Aspects of the disclosure are directed to a method where forced circulation under pressure is used to generate a hot liquid stream that is vaporized to produce a hydrofluoric acid vapor–water vapor stream comprising essentially only water and HF.

Description

USE OF FORCED CIRCULATION REBOILER FOR HYDROFLUORIC ACID GENERATIONTechnical Field

[0001] The present invention generally relates to an improved method to produce hydrofluoric acid from a fluorosilicic acid mixture. In particular, and without limitation, the present invention relates to a method where forced circulation under pressure can be used to vaporize a solution comprising hydrofluoric acid, sulfuric acid and water, which is produced from a fluorosilicic acid mixture. The present disclosure seeks to reduce the extent to which physical damage, corrosion and / or erosion which may occur in the heat exchangers used in the hydrofluoric acid reaction process.Background

[0002] Previous methods for producing hydrofluoric acid from a fluorosilicic acid mixture used heat exchangers to heat and vaporize a solution comprising hydrofluoric acid, sulfuric acid and water, which is produced from a fluorosilicic acid mixture. Additionally, small amounts of hydrochloric acid, silicon tetrafluoride and other low boilers may be present. In these prior methods, the heat exchangers used were costly and had a high rate of failure because the reactant stream was unstable, hot and highly corrosive. Specifically, excessive and unintended boiling is known to cause short equipment life in the heat exchangers.

[0003] A focus of the present disclosure is forced circulation under pressure vaporizing the hydrofluoric acid, sulfuric acid, and water solution across a pressure let-down device or pressure drop. As a result, boiling in the heat exchanger system is suppressed by evaporation under pressure, allowing increased lifetime for the heat exchanger components.Summary

[0004] The inventors have discovered an improved method of producing hydrofluoric acid from a solution comprising hydrofluoric acid, sulfuric acid, and water, which is produced from a fluorosilicic acid mixture. In one embodiment, the present disclosure is directed to a method for producing hydrofluoric acid from a solution comprising hydrofluoric acid, sulfuric acid, and water, which is produced from a fluorosilicic acid mixture, the method comprising: providing an aqueous stream comprising hydrofluoric acid and sulfuric acid to a flash tank comprising a gas outlet and a liquid outlet; heating the hydrofluoric acid-sulfuric acid mixture to a temperature ranging from 140°C to 220°C under enough pressure to maintain the mixture in an essentially all liquid form; reducing the pressure of the aqueous hydrofluoric acid-sulfuric acid stream across a pressure reducer with the resulting stream going to a flash tank; vaporizing the hydrofluoric acid-sulfuric acid in the flash tank; and sending the hydrofluoric acid vapor-water vapor stream to an absorber.

[0005] As used herein, the singular fonns “a,” “an,” and “the” include plural reference unless the context clearly dictates otherwise.Brief Description of the Drawings

[0006] The accompanying drawings, which comprise a part of this specification, illustrate several embodiments and, together with the description, serve to explain the principles disclosed herein.

[0007] FIGURE 1 shows a diagram of a heat exchanger according to one embodiment.

[0008] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate disclosed embodiments and, together with the description, serve to explain the disclosed embodiments. The drawings are not necessarily to scale. Certain dimensions, for example, may be exaggerated for purposes of clearer illustration.Detailed Description of the Illustrative Embodiments

[0009] Generally to begin the process, an aqueous fluorosilicic acid mixture, for example, a solution or an at least partially miscible mixture, enters a first reactor and is mixed with sulfuric acid. The sulfuric acid-fluorosilicic acid mixture can occur at a temperature ranging from 70°C to 150°C, such as, for example, from 80°C to 140°C, from 80°C to 130°C, from 90°C to 130°C, from 90°C to 120°C, or from 90°C to 110°C. In certain embodiments, the mixture occurs at a temperature ranging from 80°C to 130°C, or at a temperature ranging from 90°C to 110°C. Infurther embodiments, the mixture occurs at about 100°C.

[0010] In the first reactor the reaction: H2SiF62HF + SiF4can occur. A gas outlet from the first reactor can be sent to an absorption tower wherein water and hydrofluoric acid (HF) are scrubbed by sulfuric acid, which go back to reactor 1, and the gas phase containing SiF4(silicon tetrafluoride or STF) can be sent back to hydrolysis reactors, wherein the SiF4is converted to fluorosilicate acid by the reaction: 3SiF4+ 2H2O -> 2H2SiF6+ SiO2. The SiO2can be removed by filtration and the fluorosilicate mixture can be sent back to reactor 1. In the hydrolysis reactors, fresh fluorosilicate mixture can be fed as a feed stock.

[0011] The first reactor also comprises a liquid outlet, which can pump the resulting liquid H2SO4, H2O and HF mixture to a reboiler system. At this point the liquid is 100% liquid as it is the liquid stream from the reactor. It may contain small amounts of low boiling components, for example but not limited to STF, HC1 or air.

[0012] In the reboiler system, steam or other heating media can be applied using heat exchangers to raise the temperature of the aqueous H2SO4and HF mixture above its vaporization temperature. In one embodiment the stream is heated in one heat exchanger to temperatures between 135°C and 180°C, such as, for example, between 140°C and 175°C, between 145°C and 170°C, and between 150°C and 165°C. The temperature at which the operation occurs is dependent on the FSAconcentration that was employed to reactor 1 .

[0013] In certain embodiments, the aqueous H2SO4 and HF mixture can be heated in one or more heat exchangers. The first heat exchanger can heat to a temperature ranging from 100°C to 250°C such as, for example, from 100°C to 220°C, from 110°C to 200°C, from 110°C to 190°C, from 115°C to 180°C, from 120°C to 180°C, from 120°C to 170 C, from 120°C to 160°C, from 125°C to 180°C, from 125°C to 170°C, from 125°C to 160°C, from 130°C to 180°C, or from 130°C to 175°C. If present, a second heat exchanger can heat to a temperature ranging from 120°C to 200°C such as, for example, from 130°C to 200°C, from 130°C to 180°C, or from 140°C to 180°C.

[0014] In certain embodiments, the heat exchanger heats to a temperature ranging from 120°C to 180°C. In other embodiments, the heat exchanger heats to a temperature ranging from 130°C to 175°C or from 140°C to 180°C. The required temperature will depend on the concentration of FSA send to the first reactor and the pressure of the process stream in the heat exchangers.

[0015] In certain embodiments of the reboiler system, the heating can be done under pressure, such as, for example, a pressure ranging from 0 bar to 10 bar. Nonlimiting examples include pressure ranging from 0 barg to 10 barg, 0 barg to 9 barg, 0 barg to 8 barg, 0 barg to 6 barg, 0 barg to 4 barg, 0.5 barg to 10 barg, 0.5 barg to 9 barg, 0.5 barg to 8 barg, 0.5 barg to 6 barg, 0.5 barg to 4 barg, 1 barg to 10barg, 1 barg to 9 barg, 1 barg to 8 barg, 1 barg to 6 barg, 1 barg to 4 barg, 1.5 barg to 10 barg, 1.5 barg to 9 barg, 1.5 barg to 8 barg, 1.5 barg to 6 barg, 1.5 barg to 4 barg 2 barg to 10 barg, 2 barg to 9 barg, 2 barg to 8 barg, 2 barg to 6 barg, 2 barg to 4 barg, 3 barg to 10 barg, 3 barg to 9 barg, 3 barg to 8 barg, 3 barg to 6 barg, 4 barg to 10 barg, 4 barg to 9 barg, 4 barg to 8 barg, and 4 bar to 6 bar. In certain embodiments, the pressure ranges from 0.5 barg to 10 barg, and in further embodiments, the pressure ranges from 1 barg to 9 barg or from 1.5 barg to 8 barg. In all cases the solution is heated under pressure to ensure the liquid stream is essentially all liquid, meaning the stream is at least 99% liquid, such as, for example, at least 99.5%, at least 99.7% and at least 99.9% liquid, based on the mass of the stream.

[0016] The heated aqueous H2SO4 and HF mixture can then be flashed to a downstream pressure across one or more of the following: a flashing valve, an orifice plate, a restriction in piping, a spray nozzle, and / or other such device to cause a decrease in pressure in order to release pressure. The pressure differential between the pressure in the heating system and the downstream pressure can range from 0.1 to 50 psi, such as, for example, from 0.1 to 40 psi, from 0.1 to 30 psi, from 0.1 to 5 psi, from 0.5 to 20 psi, from 0.5 to 15 psi, from 0.5 to 10 psi, from 0.5 to 5 psi, from 1 to 20 psi, from 1 to 15 psi, from 1 to 10 psi, from 1 to 5 psi, from 2 to 20 psi, from 2 to 15 psi, from 2 to 10 psi, from 2 to 5 psi, from 3 to 20 psi, from 3to 15 psi, from 3 to 10 psi, from 5 to 20 psi, from 5 to 15 psi, and from 5 to 10 psi.

[0017] The gas stream from the flash can be sent to a flash vessel and then the hydrofluoric acid vapor-water vapor stream is sent to an absorber, wherein sulfuric acid can be used to remove the water, and some HF, which flows or is pumped back to the first reactor. The gaseous HF resulting from the absorber can be sent for final purification. The amount of sulfuric acid in the hydrofluoric acid vapor-water vapor stream is less than 0.1% by weight, such as, for example, less than 0.01% by weight.

[0018] The liquid stream resulting from the second reactor can be sent to a stripper column to remove the HF from this stream. The tails of the stripper column can be sent through a cooler, and then can be sent to optionally three different places: back to the stripper column to cool the stripper column receiver, send to the drying column to remove water from the HF rich gas stream, and sent for disposal or to another process that can use the diluted sulfuric acid, e.g., a phosphoric acid production unit.

[0019] The stripper column uses both steam and air to remove residual HF from the liquid stream that came from the second reactor. The gaseous stream from the stripper, containing HF, water and air, can be sent to a drying column which used sulfuric acid, optionally sent from the cooled tails of the stripper column, to remove water. The water-sulfuric acid stream can be sent back to the first reactor.

[0020] The gaseous stream from the drying column can be sent to an absorber wherein sulfuric acid absorbs the HF with a stream of mostly air and other noncondensables resulting from the heads of the absorber. This stream can be sent directly or indirectly to the scrubber. The sulfuric acid stream, containing HF, can be sent to the HF generator absorber as the sulfuric acid inlet stream.

[0021] Figure 1 depicts a heat exchanger in accordance with the present disclosure. This may be one or a plurality of heat exchangers. The stream is pumped through this heat exchanger, which uses steam to heat the stream. The exiting stream is essentially all liquid and is sent to a pressure reducing device. In this descriptive figure, the pressure reducing device is a pressure control valve. The pressure of the stream is reduced across this valve and the stream is sent to the flash tank.Prophetic Example

[0022] The following description is directed to a potential embodiment of the present disclosure. The flows and concentrations are not limiting and are representative of the general concepts of the present disclosure. All percentages given below are weight percents.

[0023] A stream containing -42% fluorosilicic acid in water flows to reactor 1. Sulfuric acid also flows to reactor 1 by the tails of the STF generation column (also contains the water that is vaporized in reactor 1), and the tails of the HF generatorcolumn (also contains HF and water). The streams are mixed in reactor 1 at about 100°C, which allows the following reaction to take place: H2SiF6-> 2HF + SiF4.

[0024] The vapor from reactor 1 is sent to the STF generation column, which removes any water from the gaseous stream, allowing essentially pure STF gas to be sent to the concentration section of the reactor for hydrolysis to convert the STF recycle to H2SiF6.

[0025] The liquid outlet of reactor 1, with optionally a sulfuric acid stream resulting from the drying column, is pumped to the reboiler system. There steam is indirectly applied using one or more heat exchangers. Depending on the flow rate, it may be needed to heat the H2SO4and HF mixture above its atmospheric vaporization temperature. Backpressure is applied via a flashing valve to ensure that essentially no boiling occurs in the reboiler system.

[0026] The steam increases the temperature of the aqueous sulfuric acid- hydrofluoric acid system to about 175°C. This gaseous stream is flashed across a flashing valve and is sent to a flash vessel for separation and the vapor phase to an HF generation absorption column. In the HF generation column sulfuric acid absorbs water from the HF allowing dry HF to be produced, which then goes to the purification operations.

[0027] The liquid stream from the reboiler vessel contains sulfuric acid, water and -3-5% HF. This HF is stripped by steam and air in a stripper column. The tailsof the stripper are cooled, some sent to the stripper receiver to cool it, some sent to the drying column and an amount equal to the sulfuric acid and water, from the fluorosilicic acid solution and steam, flows which came into the process are sent as waste to either waste treatment or to a customer, e.g., a phosphoric acid producer, which can use it.

[0028] The gaseous stream from the stripper is dried by sulfuric acid.Optionally, depending on the size and temperature of the purification operation, HF may be recycled to the drying column. The water in the gaseous stream is removed by the cooled sulfuric acid-water mix from the tails of the stripper and sent back to the reboiler system by the pumping system. The HF from the HF air stream from the heads of the drying column is sent to a final absorber which has sulfuric acid to recover the HF and allow essentially HF-free air and other noncondensables, to pass through the vent system to the scrubber. The HF-sulfuric acid stream is sent to the HF generator column to recover the HF and allow the sulfuric acid to flow to reactor 1.

Claims

CLAIMS:

1. A method for producing hydrofluoric acid comprising: providing an aqueous stream comprising hydrofluoric acid and sulfuric acid to a flash tank comprising a gas outlet and a liquid outlet; heating the hydrofluoric acid-sulfuric acid mixture to a temperature ranging from 140 C to 220 C under enough pressure to maintain the mixture in an essentially all liquid form; reducing the pressure of the aqueous hydrofluoric acid-sulfuric acid stream across a pressure reducer with the resulting stream going to a flash tank; vaporizing the hydrofluoric acid sulfuric acid in the flash tank; and sending the hydrofluoric acid vapor-water vapor stream to an absorber.

2. The method according to claim 1 , wherein the temperature of the aqueous sulfuric acid-hydrofluoric acid stream is heated under pressure from 110°C to 220°C.

3. The method according to claim 1 or claim 2, wherein the pressure of the aqueous sulfuric acid-hydrofluoric acid stream is maintained to ensure that the stream is essentially all liquid.

4. The method according to claim 3, wherein the pressure of the aqueous sulfuric acid-hydrofluoric acid stream is between 0.5 and 10 barg.

5. The method according to any one of claims 1-4, wherein the stream is essentially all liquid.

6. The method according to claim 5 wherein the stream is at least 99% liquid based on a total mass.

7. The method according to any one of claims 1-6, wherein the pressure drop between the pressurized liquid stream and the flash tank is greater than 1 bar.

8. The method according to claim 7, wherein the pressure drop between the pressurized liquid stream and the flash tank is greater than 1.5 bar.

9. The method according to any one of claims 1-8, wherein pressure drop is taken by one or more of a flashing valve, an orifice plate, a restriction in piping and a spray nozzle.

10. The method according to any one of claims 1-9, wherein the hydrofluoric acid vapor-water vapor stream comprises essentially all water and HF.

11. The method according to claim 10, wherein the hydrofluoric acid vaporwater vapor stream comprises less than 0.1% sulfuric acid based on weight.

12. The method according to claim 11 , wherein hydrofluoric acid vaporwater vapor stream comprises less than 0.01% sulfuric acid based on weight.

13. The method according to any one of claims 1-12, wherein the hydrofluoric acid vapor-water vapor stream comprises between 37% and 100%HF, with the balance being water by mass percentage.

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