Patent Application
20080003172
Preferred embodiments of the present invention provide a continuous process for separating hydrogen fluoride from an admixture feed stream comprising the steps of (a) providing an aqueous admixture comprising hydrogen fluoride and hexafluoroarsenic acid or a salt thereof; (b) causing said aqueous mixture to flow substantially along a first flow path in a reaction vessel while exposing said aqueous mixture to conditions effective to convert at least a portion of said hexafluoroarsenic acid or a salt thereof to arsenic acid and a salt thereof and hydrogen fluoride; and (c) intimately contacting the aqueous mixture with a flow of inert gas, preferably steam, moving in a generally counter-current direction relative to said first flow path. The counter-current flow of inert gas in accordance with the present invention preferably is highly effective at carrying hydrogen fluoride away from the reaction mixture and from the reaction vessel. Preferably, the counter-current flow of inert gas comprises forcing the inert gas to flow through the liquid in a direction that is generally opposite to the direction of flow of the reaction mixture feed components. As used herein, the term “direction of flow” generally refers to the flow of the material being described in a bulk or average sense and contemplates that at any given point along the flow path one or more molecules of the material in the flow may be moving in a direction that is not along the overall bulk flow path.
According to another aspect of the present invention, provided is a continuous process for converting hexafluoroarsenic acid, or salts thereof, into arsenic acid, or salts thereof, having the steps of (a) providing an aqueous admixture stream comprising hexafluoroarsenic acid or salt thereof, water, hydrogen fluoride, and an acid catalyst; (b) contacting the admixture stream with a counter-current stream of steam; and (c) converting the hexafluoroarsenic acid, or salt thereof, to arsenic acid, or salt thereof.
In preferred embodiments, the present invention treats an aqueous feed stream containing an admixture of hexafluoroarsenic acid, or any salt thereof, and hydrogen fluoride, by contacting the feed stream with a counter-current stream of steam to remove at least about 75% by weight, more preferably at least about 90% by weigh, and even more preferably substantially all of the hydrogen fluoride from the feed stream, and optionally to heat the feed stream. As used herein, the phrase “hexafluoroarsenic acid or salt thereof” refers to the nonvolatile pentavalent arsenic compounds, such as, for example, those described in commonly assigned U.S. Pat. No. 4,756,899 and U.S. Pat. No. 4,929,435. Examples of salts of hexafluoroarsenic acid which may be present in the starting aqueous mixture include, but are not limited to, potassium hexafluoroarsenate (KAsF6), sodium hexafluoroarsenate (NaAsF6), ammonium hexafluoroarsenate (NH4AsF6), calcium hexafluoroarsenate (Ca(AsF6)2) and magnesium hexafluoroarsenate (Mg(AsF6)2). Aqueous starting streams according to the present invention may also contain other components, including for example other acids such as sulfuric, phosphoric, fluosilicic, or fluosulfonic; bases such as sodium or potassium hydroxide; or suitable solvents for hexafluoroarsenic acid or salts thereof such as alcohols.
In certain embodiments, the aqueous feed stream comprises at least a portion of an effluent stream from a unit operation such as distillation, preferably the bottom stream from a distillation column, such as the bottoms that results from practicing the process of commonly assigned U.S. Pat. No. 4,756,899 or U.S. Pat. No. 4,929,435, or an aqueous mixture derived from such bottoms, or the like. In general, these streams comprise from about 75 to about 95 percent by weight hydrogen fluoride, from about 2 to about 20 percent by weight water, up to about 5 percent by weight sulfuric acid, and up to about 5 percent by weight hexafluoroarsenic acid or any salt thereof.
According to an aspect of the present invention, provided is process for separating hydrogen fluoride from an aqueous admixture feed stream comprising hydrogen fluoride and hexafluoroarsenic acid, or a salt thereof, by contacting the admixture stream with a steam stream flowing substantially counter-currently with respect to the admixture stream, to separate at least a portion, preferably a substantial portion and even more preferably substantially all of the hydrogen fluoride from the admixture stream. In certain embodiments, the anhydrous hydrogen fluoride stripped from the admixture is formed into a product stream. This product stream may also contain water vapor. The hydrogen fluoride can be either recovered as a product or recycled to a hydrogen fluoride manufacturing process or a combination of these. Recycled hydrogen fluoride subsequently can be recovered and the residual water can be reacted with oleum to form sulfuric acid, which is useful in certain hydrogen fluoride manufacturing processes.
Referring to
Preferably, the steam is passed through the vessel in a quantity, at a rate and under conditions sufficient to remove, preferably on a substantially continuous basis, at least about 75% by weight, more preferably at least about 95% by weight, and even more preferably substantially all of the hydrogen fluoride present in the feed stream such that relatively little of the hydrogen fluoride exits the vessel with reaction product stream 60. The product stream 50 exiting the column 10 preferably comprises anhydrous hydrogen fluoride stripped from the admixture, and may also include water vapor. The stripped admixture, or products derived therefrom, exits the column 10 as stream 60. As described in more detail below, this by-product stream 60 preferably comprises arsenic acid derived from the hexafluoroarsenic acid of the admixture feed stream.
The amount of steam to be used in any particular application may vary widely within the scope of the present invention, and it is contemplated that in view of the teachings contained herein those skilled in the art will be able to readily determine the desired amount for any particular application. One important characteristic influencing the selection of the amount of inert gas to be used is the temperature of the inert gas to be used and the desired contact temperature. In certain preferred embodiments, for example with a reaction temperature of from about 140 to about 170° C., it is preferred to use from about 0.5 to about 2.5 pounds of inert gas, preferably steam, per pound of reactants fed to the vessel. In many embodiments, including the particular embodiments described in this paragraph, it is also generally preferred to use from about 10 to about 100, more preferably from about 20 to about 90, pounds of inert gas, preferably steam, per pound of HF contained in the feed to the vessel. In certain embodiments, a process is conducted at a temperature of about 150° C., wherein a minimum of about 8 moles of steam gas, per mole of hydrogen fluoride in the admixture prior to steam addition, is utilized in order to substantially remove all of the hydrogen fluoride. Because the partial pressure of hydrogen fluoride increases with temperature, higher contact temperatures require less steam while lower contact temperatures require more steam. Also, the degree and uniformity of mixing of the steam with the liquid admixture are factors in determining the amount of steam to be used.
In certain preferred embodiments, the steam also supplies heat to the admixture, raising the temperature of components not contained in the steam stream as they pass counter-currently through the vessel. The feed components admixture may also be partially preheated prior to contacting the steam. Preferably, the steam raises the temperature of the admixture to a degree sufficient to aid removal of the preferred amounts of hydrogen fluoride, and preferably substantially all of the hydrogen fluoride, from the feed stream. In certain preferred embodiments, the temperature of the reactant stream components introduced as part of the feed admixture, after contacting the steam and exiting the vessel, is from about 75° C. to about 200° C., and even more preferably from about 130° C. to about 175° C. Applicants have found that it is preferred in many embodiments to use an average process temperature greater than about 75° C. to ensure adequate removal of hydrogen fluoride from the feed stream. On the other hand, applicants believe that the use of average process temperatures of greater than about 200° C. are not preferred since in excess of about this temperature conventional materials of construction for the relevant processing equipment may be damaged or become unfit for their intended purpose.
According to another aspect of the present invention, provided is a continuous process for converting hexafluoroarsenic acid, or salts thereof, into arsenic acid, or salts thereof, having the steps of: (a) providing an aqueous admixture feed stream containing hexafluoroarsenic acid or salt thereof, water, hydrogen fluoride, and an acid catalyst; and (b) contacting said admixture stream with a substantially counter-current flow of inert gas, such as a stream of steam, to produce a product stream comprising a substantial amount of the hydrogen fluoride and a by-product stream comprising arsenic acid or salt thereof.
It is known that commercially available sulfuric, arsenic, or perchloric acid or mixtures thereof can be added to an aqueous mixture comprising hexafluoroarsenic acid, or salt thereof, to catalyze the hydrolysis of the hexafluoroarsenic compound. Hydrolysis of the hexafluoroarsenic compound produces arsenic acid and hydrogen fluoride. Representative hydrolysis reactions according to this process may include:
HAsF6+4H2O→H3AsO4+6HF
MAsF6+4H2O→MH2AsO4+6HF
X(AsF6)2+8H2O→X(H2AsO4)2+12HF.
wherein M represents monovalent cations and X represents divalent cations and is analogous for multivalent cations.
Mixtures of acids in any proportions may be used and the mixture may be of two or more acids. The preferred acid for certain embodiments of the the present invention is sulfuric acid or arsenic acid.
In certain embodiments, the admixture feed stream is fully or partially preheated before contacting the steam. One reason for preheating the admixture feed stream is to concentrate the hexafluoroarsenic acid, or salt thereof, for a subsequent hydrolysis reaction. Without the preheating step, in certain embodiments the concentration of the hexafluoroarsenic acid or salt thereof could be so low that an undesirably large amount of acid would be required to effectively catalyze the hydrolysis reaction. The use of such additional acid would increase the cost of operation of the process. Preferably, the starting aqueous material has a hexafluoroarsenic acid or salt thereof concentration of about 20 to about 50 percent by weight. If the starting material contains less than this concentration, an evaporation step is preferably used.
For embodiments utilizing an evaporator, the aqueous admixture feed stream is subjected to evaporation so as to concentrate the hexafluoroarsenic acid or salt thereof by removing a portion of the hydrogen fluoride. Preferably, the starting aqueous mixture is heated to a temperature of from about 50° C. to about 150° C. More preferably, the starting aqueous mixture is heated to a temperature of from about 70° C. to about 105° C. The vaporized hydrogen fluoride optionally may be recovered as a product or may be recycled to the beginning of the hydrogen fluoride manufacturing process.
It is also known that hydrogen fluoride and water form an azeotrope when the weight percent of the hydrogen fluoride (based on the total weight of the hydrogen fluoride and water) is at least 38. Thus, for the present reaction, the acid catalyst serves to break this azeotrope, as well as to catalyze the hydrolysis of the hexafluoroarsenic acid or salt thereof.
The amount of acid catalyst present in the admixture is preferably sufficient to break the hydrogen fluoride and water azeotrope and catalyze the reaction, preferably at least about 45 weight percent based on the total weight of the aqueous mixture (including acid catalyst). More preferably, the amount of acid catalyst is about 45 to about 85 weight percent based on the total weight of the aqueous mixture. In certain embodiments, the amount of acid catalyst is present in approximately a 1:1 ratio with the amount of hexafluoroarsenic acid being reacted.
Although the catalytic hydrolysis of hexafluoroarsenic acid, or salt thereof, will partially proceed in the presence of hydrogen fluoride, in order to transform substantially all hexafluoroarsenic acid or salt to arsenic acid or salt, substantially all of the anhydrous hydrogen fluoride must be removed from the reaction mixture.
The contacting the admixture stream with a countercurrent flow of steam effectively removes not only substantially all of the hydrogen fluoride initially present in the admixture, but also removes substantially all of the hydrogen fluoride as it evolves from the reaction. That is, as a portion of the hexafluoroarsenic acid or a salt thereof undergoes hydrolysis in the packed column, hydrogen fluoride evolves. The continuous flow of steam, flowing counter-currently with respect to the flow of the admixture, strips substantially all of the evolving hydrogen fluoride from the admixture, thus allowing the remaining hexafluoroarsenic acid or a salt, which is now substantially free of hydrogen fluoride, to continue undergoing hydrolysis. In certain preferred embodiments, the amount of unconverted hexafluoroarsenic acid present in the by-product stream is less than about 100 ppm, more preferably less than about 50 ppm, and even more preferably less than 10 ppm.
In addition to removing the hydrogen fluoride, the counter-current flow of steam may heat the admixture to maintain a specific reaction temperature. In certain preferred embodiments, the reaction temperature is maintained at from about 145° C. to about 170° C., and more preferably from about 150° C. to about 160° C. However, for certain embodiments, especially those that utilize arsenic acid as an acid catalyst, it is particularly preferably to maintain the reaction temperature at less than 165° C. so that the arsenic acid concentration in the by-product stream does not become too high (i.e., greater than about 80%). A by-product stream having a concentration of arsenic acid greater than about 80% by weight can potentially lead to higher viscosities and the formation of a arsenic pentoxide precipitate.
The resulting product stream of vaporized anhydrous hydrogen fluoride, which may also contain water vapor, and can be recovered as a product or recycled to a hydrogen fluoride manufacturing process.
Preferably, the resulting by-product stream comprises from about 5 to about 25 weight percent arsenic acid or salt thereof, from about 15 to about 40 weight percent water, and from about 45 to about 85 weight percent acid catalyst. More preferably, the resulting mixture comprises about 20 to about 25 weight percent arsenic acid or salt thereof, about 15 to about 25 weight percent water, and about 55 to about 65 weight percent acid catalyst. This mixture may be cooled in order to make the product less corrosive and easier to handle and may be transferred to storage tank for subsequent processing.
The arsenic acid in the by-product stream or from the storage tank may be recovered or this resulting mixture can be rendered nonhazardous by the use of known methods such as discussed by Nancy J. Sell, “Solidifiers for Hazardous Waste Disposal”, Pollution Engineering, 44 (August 1988) and Elio F. Arniella et al., “Solidifying Traps Hazardous Wastes”, Chemical Engineering, 92 (February 1990). Typically, the acid mixture will be converted to a water insoluble salt, such as calcium arsenate, which can then be rendered nonhazardous. For example, this mixture may be reacted with commercially available calcium oxide so as to form the water insoluble calcium arsenate, Ca3(AsO4)2. The reaction which occurs is:
2H3AsO4+3CaO→Ca3(AsO4)2+3H2O
Other calcium or magnesium compounds may also be suitable, provided that the compound supplies calcium or magnesium ions for a reaction with the arsenate. This includes but is not limited to calcium or magnesium hydroxides, chlorides, carbonates, and oxides or combinations such as dolomites including calcium and magnesium carbonate. The mixture may also be reacted with any suitable alkali for neutralization. Cement may then be added in various ratios to solidify and chemically stabilize the insoluble calcium arsenate. When subjected to the EPA Toxic Characteristics Leach Procedure Test, the cement encased calcium arsenate meets the criteria and is considered nonhazardous.
Certain aspects of the invention may be further understood with reference to the following non-limiting examples:
The batch process described in U.S. Pat. No. 5,089,241 is utilized to convert hexafluoroarsenic acid to arsenic acid using sulfuric acid.
The aqueous bottoms of a hydrogen fluoride distillation process, which comprise hexafluoroarsenic acid, water, and hydrogen fluoride, are collected and then processed through a flash evaporator to remove a substantial amount of the hydrogen fluoride present. The resulting admixture is then transferred into a 250-gallon, Teflon-lined reactor. Sulfuric acid is added to the reactor in a 1:1 weight ratio with the hexafluoroarsenic acid. The admixture heated to about 125° C. and mixed. This step removes more of the HF from the admixture. Steam at 150 psig is then injected directly into the admixture to heat, agitate, and purge HF from the admixture. The temperature of the reaction admixture is maintained at 150-160° C. The steam purging is continued for 8 hours to convert the hexafluoroarsenic acid to arsenic acid. To insure this specification level is met, the aqueous reaction product is analyzed before transferring the aqueous reaction product to a storage vessel. The analysis of the components of the aqueous reaction product includes the following:
total acid concentration by an acid-base titration;
HF concentration by specific ion electrode with suitable activity buffer;
arsenic acid concentration by iodometric titration;
sulfuric acid concentration by calculating the difference after applying the proper calculation factors (=Total acid−HF−arsenic acid);
water concentration by calculating the difference after applying the proper calculation factors; and
hexafluoroarsenic acid by obtaining sample of the mixture, adjusting the sample's pH, adding a complexing agent, then extracting away from the aqueous solution and spectrophotometrically comparing the extract to a standard curve.
Since the sulfuric acid and water concentrations were measured indirectly, their reported values are approximate. Thus, the aggregate relative concentrations of the individual components does not necessary equally exactly 100%, but instead is approximately 100%.
The total amount of steam used for purging is approximately twice the weight of the material in the reaction mixture and puts a large amount of water into the HF recovery section. It is found that this large volume of steam upsets in the water balance of a corresponding HF production process thereby leading to lower HF product quality.
These examples describe a continuous process for converting hexafluoroarsenic acid to arsenic acid using a counter-current flow of steam.
In each of these examples, a flanged 4″×8′ Teflon-lined carbon steel pipe is used as a reaction column. Dispersion hats were used at the top and bottom of the column to provide good contact between the steam and the reaction mixture and good liquid dispersion over the packing material. The packing material was Glitsch knit PFA (Goodlow) mesh hand packed. All temperature was measured by Teflon coated thermocouples. A differential pressure (dP) transducer was used to monitor/measure the pressure differetial between the top and bottom of the column. The feed heater was a ¼″ PFA tube inside a ¾″ copper tube with 150 pound steam in the annulus. The steam/HF vent cooler was a ¾″ PFA tube inside a 1½″ carbon steel pipe with cold water in the annulus. The product draw was a ¼″ PFA tube inside a ¾″ copper tube with cold water in the annulus. Steam was regulated by a pressure regulator and a manual control valve. Analysis of samples are preformed by procedures similar to those described in the comparative example.
The feed material for the reactor column was made by mixing the hexafluoroarsenic acid, HF and, water mixture with sulfuric acid. This mixture was preheated to 125° C. and then pumped into the top of the column. The steam was concurrently heating the column. The steam rate was measured by condensing the HF and water venting the top of the column and determining the column bottom draw rate for a period of time and then analyzing both samples. As the sulfuric acid and steam mixed, the temperature increased to 150-160° C. and the HF was removed allowing the hydrolysis to go to completion. The process parameters and process results are provided in Table A.
Examples 1 and 2 demonstrate the conversion of hexafluoroarsenic acid or salts to arsenic acid using sulfuric acid to break the HF and water azeotrope. Example 1 is a batch process currently in use. Example 2 is a continuous process with significant improvement in that the reaction time is much shorter and the amount of steam used is about one half of the previous process because of the more intimate mixing of the steam and reaction mixture.
This example demonstrates the use of arsenic acid to break the HF and water azeotrope.
The process of Example 2 is repeated, except that arsenic acid is used as an acid catalyst instead of sulfuric acid, and the feed into the column is maintained at 150-160° C. The process parameters and process results are provided in Table B.
Example 3 represents a further improvement with respect to the time, steam and sulfuric acid savings of Example 2, and has the added benefit of producing a more concentrated arsenic waste or a marketable product. The arsenic acid produced is suitable but not limited to the wood preservative industry. Runs A and B demonstrate the need for a longer residence time using arsenic acid but the conversion is substantially 100% complete.
This example demonstrates the new process in a continuous mode with recycle of the bottoms draw, after separation of precipitated salt, back through the heater to be mixed with the HF, water and hexafluoroarsenic acid stream (called FEB). Only half of the arsenic acid bottoms were recycled back and mixed with the feed for the continuous process. In the initial startup, however, there would need to be either a supply of previously made arsenic acid or mix sulfuric acid with the feed before recycling and subsequent elimination of the feed sulfuric acid.
The test was run for 1 week, 24 hours a day. These samples were taken over a 42 to 63 hour span and are representative of the whole test. Results are reported in Table C below.
Having thus described a several particular embodiments of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements, as are made obvious by this disclosure, are intended to be part of this description though not expressly stated herein, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only, and not limiting. The invention is limited only as defined in the following claims and equivalents thereto.
