BACKGROUND OF THE INVENTION The invention relates to a process for recovering HF from systems comprising ammonium bifluoride, hydrogen fluoride and water, and, more particularly, for recovering HF in the manufacture of fluoroaromatics by the addition of sulfur trioxide or a solution of sulfur trioxide in sulfuric acid (hereinafter referred to as "oleum") to a spent reaction mass. U.S. Patent No. 4,912,268, discloses a process for the preparation of a fluoroaromatic compound which comprises feeding an aromatic amine in the presence of hydrogen fluoride to a reaction zone simultaneously with a diazotizing agent so as to effect diazotization of the aromatic amine, thermally decomposing the resulting diazonium salt substantially as it is formed, and removing the resulting fluoroaromatic from the reaction zone substantially as it is formed. While this method has distinct advantages over the prior art further improvements are desirable. These are centered in the work-up of the spent reaction mass from the reaction zone. The spent reaction mass, which contains substantially no fluoroaromatic product, comprises hydrogen fluoride, most of the water produced in the reaction, by-product fluorobiaryls, ammonium bifluoride and tarry non-volatile organic by-products. The water and hydrogen fluoride form an azeotrope with boiling point 111°C. When work-up of the spent reaction mass is attempted by distillation only about 65% of the hydrogen fluoride is recovered substantially free of water, by-product fluorobiaryls tend to steam distill and clog the condensers, and the still bottoms are viscous tarry non-volatile organic by-products and thus difficult to handle.

It is known in the art that sulfur trioxide or oleum can be used to recover hydrogen fluoride from its azeotrope with water. U.S. Patent 2,939,766 relates to a method for the recovery of hydrogen fluoride from a mixture of approximately one molecular proportion of alkali metal bifluoride, specifically sodium bifluoride, approximately one molecular proportion of hydrogen fluoride- water azeotrope and up to twenty-five molecular proportions of hydrogen fluoride by adding two molecular proportions of sulfur trioxide and thereafter distilling the mixture to recover the hydrogen fluoride. U.S. Patent 5,032,371 discloses a continuous process for the recovery of anhydrous hydrogen fluoride by contacting an aqueous solution of an alkali metal fluoride in hydrogen fluoride with a sulfur

trioxide-containing dehydrating stream. The process was said to be useful for the recovery of hydrogen fluoride from the alkali metal-containing heels produced in the processes for the production of aromatic fluorides. The use of sulfuric acid is said to be "advantageous as many of the possible contaminants and impurities regain dissolved in the sulfuric acid throughout the process" and because "sulfuric acid also contributes to the fluidity of the waste". There is no mention of the use of this process for the recovery of hydrogen fluoride from aqueous ammonium bifluoride solutions in these two patents.

SUMMARY OF THE INVENTION It has now been found that in the presence of SO3 or oleum, HF is efficiently recovered from an aqueous waste stream which is generated in manufacturing fluoroaromatics, for example, fluorobenzene from aniline. The process allows for numerous processing advantages such as greatly reducing the build-up of fluorobiphenyls in the HF recovery system and reducing build-up of viscous tarry non-volatile organic by-products in the still bottoms. Generally, the invention relates to HF recovery from systems comprising ammonium bifluoride, hydrogen fluoride and water.

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement to the process for the manufacture of fluoroaromatics described in U.S. Patent 4,912,268, the teachings of which are incorporated herein by reference. It relates particularly to the improved processing of fluorobenzene from aniline. The improvement of this invention comprises addition of sufficient sulfur trioxide, or a solution of sulfur trioxide in sulfuric acid to a spent reaction mass, i.e., to the substantially product-free liquid from the reactor, to convert the water content of the spent reaction mass to sulfuric acid and the ammonium bifluoride content to ammonium bisulfate and hydrogen fluoride, and of recovering the hydrogen fluoride contained therein by distillation. The amount of sulfur trioxide or oleum required to complete the conversion may be readily calculated to those skilled in the art, and is further exemplified hereinafter. More generally, the invention relates to recovery of hydrogen fluoride from systems comprising ammonium bifluoride, hydrogen fluoride, and water by the addition of sufficient sulfur trioxide or oleum to convert its ammonium bifluoride content to ammonium bisulfate and hydrogen fluoride, to convert its water content to sulfuric acid, and of recovering the hydrogen fluoride contained therein by distillation.

This invention allows recovery of over 90% of the hydrogen fluoride free of water. The recovered HF may then be recycled back to the reaction zone. It is a further advantage of this invention that by-product fluorobiaryls, which otherwise steam distill and cause severe conde _* ιser plugging problems, are reduced by up to 99%, greatly diminishing the condenser clogging problem. It is yet a further advantage of this invention that the addition of sulfur trioxide or oleum alters the physical properties of tarry non-volatile organic by-products which constitute the still bottoms, converting them from a sticky residue to freely suspended solids in sulfuric acid and greatly facilitating discharge and disposal of the still bottoms.

The process of this invention may be operated on spent reaction mass either batchwise or continuously.

A preferred embodiment of this invention includes the addition of sulfur trioxide to the reactor overflow material at a lower temperature followed by a higher temperature distillation for hydrogen fluoride removal. This allows the destruction of the fhiorobiphenyls at a temperature well below the temperature at which they are carried overhead by distillation. The effect of temperature on HF recovery is given in the tables below. When the sulfur trioxide addition is made at higher temperatures, a substantially reduced level of fluorobiphenyl destruction results since distillation strips the fhiorobiphenyls from the sulfur trioxide-sulfuric acid medium before they can react. If the fhiorobiphenyls are carried overhead, this leads ultimately to severe condenser plugging problems. While equipment plugging was not a problem on a laboratory scale because of the small amounts of materials processed, during commercial scale operations plugging from fhiorobiphenyls was encountered.

The following examples and tables are to be construed as merely illustrative and not limitative of the scope of the invention.

EXAMPLE 1 A 150 g sample of overflow material from a fluorobenzene reactor was charged to a stirred 500 ml vessel equipped with a mechanical agitator, distillation head and brine-cooled take-off condenser leading to a refrigerated receiver. The condenser was composed of silicon carbide and all other internal surfaces in the distillation and receiver were coated with Teflon ^® fluorocarbon resin to prevent corrosion. The composition of this material was 8.9% water, 11.6% ammonium bifluoride, 75.9% hydrogen fluoride, 0.3% mixed fhiorobiphenyls, and 3-4% of both suspended and dissolved non-volatile organic by-products or tars. To the chilled mixture was slowly added 59.5 g of sulfur trioxide, the calculated amount required

for complete reaction with the water in the sample. The sulfonated reaction mass was then heated; as hydrogen fluoride distilled off, the temperature slowly rose to 100°C, at which point the temperature was held constant and the distillation continued until no more hydrogen fluoride distilled. The distillate contained 119.0 g of hydrogen fluoride, 0.5 g of sulfate, and 0.7 g of water. This quantity of hydrogen fluoride conesponds to a 93% recovery of the total inorganic fluoride in the sample, including that charged as ammonium bifluoride. 97% of the fluorobiphenyls in the sample was destroyed. The non-volatile organic by-product tars in the sample were fluidized by the sulfur trioxide treatment, remaining suspended in the predominantly sulfuric acid heel at the end of the distillation.

EXAMPLE 2 Sulfur trioxide and reactor overflow material from the fluorobenzene process, in a ratio to assure that all of the water present would be reacted, were fed simultaneously and continuously to a distillation heel such as that produced in Example 1 and at a rate such that the boiling point of the mixture was maintained at 100°C. Hydrogen fluoride was continuously taken off overhead with the extent of recovery and composition similar to that obtained in Example 1. From 150 g of overflow material fed in this manner (at a rate of 2.0 g/min), 118.2 g of hydrogen fluoride containing 0.8% sulfate and 0.4% water was collected. The inorganic fluoride recovery as hydrogen fluoride was 92% About 65% of the fluorobiphenyls were destroyed.

As in Example 1, the non-volatile organic by-products were converted from a sticky residue that tended to adhere to vessel walls to freely suspended solids in the predominantly sulfuric acid heel was produced during the distillation.

EXAMPLE 2A Sulfur trioxide was fed continuously to a recirculating stream of reactor overflow material from the fluorobenzene process held at 50°C in a vessel with a residence time of at least 3 hours, in a ratio to insure that all of the water present would be reacted, and to which reactor overflow material was continuously fed. The material treated with sulfur trioxide was withdrawn from this vessel at a rate equivalent to the overflow feed rate and fed to a distillation heel such as that produced in Example 1, maintained at a boiling point of 100°C. Hydrogen fluoride was continuously taken off overhead with the extent of recovery and composition similar to that obtained in Example 1. From 150 g of overflow material fed in this fashion, 118.0 g of hydrogen fluoride containing 0.8% sulfate and 0.4% water was

collected; recovery of hydrogen fluoride was 92%. 90% of the fluorobiphenyls were destroyed.

As in Example 1, the non-volatile organic by-products were converted from a sticky residue that tended to adhere to the vessel walls to freely suspended solids in the predominantly sulfuric acid heel that was produced in the distillation.

COMPARATIVE EXAMPLE The apparatus, procedure, and feed was as described in Example 1, except that sulfur trioxide was not added to the reactor overflow material before distilling the hydrogen fluoride from it. Recovery of hydrogen fluoride was 64% under these circumstances; no significant reduction of the fluorobiphenyls initially present was observed. In the absence of added sulfur trioxide, the non-volatile organic by-product tar residues coated the internal Teflon ^® fluorocarbon resin- coated surfaces of the distillation vessel with a sticky black coating.

TABLES

The following tables show the effect of temperature on HF recovery. Although the recovery of HF is maximized at temperatures of 140°C or above, it will be appreciated by those skilled in the art that process operability factors may dictate operating below the temperature for maximum recovery.

HF Recovery-Effect of SO3 Treatment Distillation of Reactor Overflow

The composition comprises 67.5% HF, 13.5% NH4HF2 and 13.4% H ₂ 0, 3.6% TARS/DOC The total HF content (HF + NH4HF2) was 77.3g/100g O/F; SO3 requirement (1:1 SO3 - H2O) was 59.5g/100g O/F; and 65% oleum requirement (1:1 SO3 - H2O) was 91.7g/100g O/F. The following table shows HF recovery versus distillation temperature for SO3 using stoichiometric 1:1 SO3-H2O for all distillations.

Temperature °C Reactor O/F, g Distillation HF. g Total HF Recovery. %

80 100 62.6 81.0

90 100 63.8 82.5

100 100 68.5 88.6 110 100 70.2 90.8

120 100 71.5 92.5

The following table shows HF recovery versus distillation temperature for 65% oleum using stoichiometric 1:1 S03-H20 for all distillations.

Temperature °C Reactor O/F. ξ Distillation HF. g Total HF Recoverv- %

80 100 54.7 70.8

90 100 64.7, 60.8 83.7, 78.7

100 100 67.4 87.2

110 100 67.8 87.7

120 100 72.0 93.2

130 100 71.6 92.6

140 100 72.2, 73.2 93.4, 94.8

The following comparative table shows the HF recovery versus distillation temperature in the absence of SO3 or oleum.

Temperature °C Reactor O/F. g Distillation HF. g Total HF Recovery. % 90 100 35.3 45.7

100 100 40.3 52.1

110 100 45.9 59.4

Comparable results have beer, demonstrated using synthetic mixtures of a composition comprising 78.8% HF, 9.2% H20, 12.0% NH4HF2. The total HF content (HF + NH4HF2) was 87.2g/100g O/F; SO3 requirement (1:1 SO3 - H ₂ 0) was 40.8g/100g O/F; and 65% oleum requirement (1:1 SO3 - H2O) was 62.8g/100g O/F. That is, by adding an amount of SO3 or oleum equivalent to the water content of the mass, significant enhancement of HF recovery can be obtained.