Abstract:
This invention relates to the removal from gaseous mixtures comprising hydrogen chloride or sulfuryl fluoride and hydrogen chloride of hydrogen fluoride impurity by solubilization of the latter with sulfur dioxide.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a process for removal of hydrogen fluoride contaminant from a mixture comprising gaseous hydrogen chloride. In accordance with another aspect, this invention relates to the removal of undesired hydrogen fluoride contaminant from a gaseous mixture comprising hydrogen chloride and sulfuryl fluoride. 
     It is known in the art that the hydrogen fluoride contaminant of a gaseous hydrogen chloride mixture can be removed therefrom by intimate contact with silica gel or a liquid composition consisting of water and sulfur trioxide. However, the resulting products, silicon tetrafluoride and chlorosulphonic acid, respectively, are volatile and pass out of the system with the hydrogen chloride, thus requiring additional expense for equipment and procedures to remove these by-products. See, for example, U.S. Pat. No. 3,074,779. Other prior art processes, for example, U.S. Pat. No. 3,140,916, require multiple step processes and equipment to achieve the removal of hydrogen fluoride. 
     The production of sulfuryl fluoride from gaseous mixtures of sulfur dioxide, chlorine and hydrogen fluoride (see U.S. Pat. Nos. 2,875,127 (use as a fumigant) and 2,772,144, 3,092,458 and 3,320,030 (processes) also has attendant problems of removing undesired excess hydrogen fluoride from product mixture containing sulfuryl fluoride. Such hydrogen fluoride apparently forms an azeotrope mixture with by-product hydrogen chloride and the azeotrope mixture cannot be successfully distilled and separated from the sulfuryl fluoride product. While the hydrogen fluoride contaminant can be removed according to prior art methods mentioned above, such methods suffer disadvantages in that additional equipment and procedures, etc., necessary to remove resulting hydrogen fluoride by-products or treat the same prior to waste disposal are encountered at additional expense. 
     Accordingly, an object of this invention is to provide a process for removing undesired hydrogen fluoride contaminant from gaseous mixtures comprising hydrogen chloride or sulfuryl fluoride and hydrogen chloride. Other objects and aspects, as well as the several advantages of the invention, will become apparent upon consideration of the accompanying disclosure and appended claims. 
     SUMMARY OF THE INVENTION 
     It has now surprisingly been found that if a gaseous mixture comprising hydrogen chloride (HCl) and hydrogen fluoride (HF), or comprising HCl, HF and sulfuryl fluoride (SO 2  F 2 ), is brought into intimate contact with liquid sulfur dioxide (SO 2 ), the HF contaminant can be solubilized and removed from the HCl or HCl and SO 2  F 2  components. In the latter instance, the HCl and SO 2  F 2  product mixture can be further distilled to remove the HCl and obtain the desired SO 2  F 2  product. 
     Accordingly, one embodiment of the present invention is directed to a process for the removal from gaseous mixtures comprising HCl and HF of the HF impurity or contaminant which comprises bringing said gaseous mixture into intimate contact with liquid SO 2 . In another embodiment of the present invention, a process for the removal of HF from a gaseous mixture comprising HCl and SO 2  F 2  is brought into intimate contact with liquid SO 2 . In each embodiment, the HF is solubilized in the SO 2  and removed, thereby providing HCl or HCl and SO 2  F 2  products substantially free of HF. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     As applied illustratively from a procedural standpoint practice of the present invention comprises subjecting or bringing the gaseous mixture to be purified into intimate contact with liquid SO 2  and maintaining contact therewith for a sufficient period of time to solubilize and substantially remove the HF component therefrom. The contact of the gaseous mixture to be purified can be done in batch or continuous operations. Preferably, continuous operations are employed. 
     The apparatus and related accessories necessary to carry out the process of the present invention are conventional and may be along the lines described in the appended examples. Typically, stainless steel sieve tray distillation columns having downcomers, reboiler and tube reflux condenser may be employed. Such apparatus and modifications thereof for carrying out the claimed process will be recognized by those skilled in the art. 
     In carrying out the process of the present invention, it is only necessary that the gaseous mixture from which HF is to be removed be contacted for a sufficient period of time with the liquid SO 2  to solubilize and remove substantially all of the HF component. Preferably, a highly saturated zone of liquid SO 2  is maintained. Generally, the gaseous mixture to be purified is introduced into a zone of liquid SO 2  in a manner such that the exit gas, e.g., HCl or HCl and SO 2  F 2 , as well as the SO 2  -HF by-product can conveniently be recovered. Thus, vertically-disposed distillation sieve tray columns, wherein the gaseous mixture can be fed at a point below the SO 2  zone and move upwardly therethrough, are highly preferred for use in the present invention. The temperature of the gaseous mixture-SO 2  contact zone must be such that the SO 2 , which has a boiling point of -10° C., is maintained in the liquid state. Usually, a temperature range in the column of from about 90° C. to about -75° C. or less is employed, depending, as will be recognized by those skilled in the art, upon the pressures employed. Those skilled in the art will also recognize that at higher temperatures SO 2  is corrosive. Pressures from atmospheric to about 300 psi can be employed, although pressures from about 10 to about 60 psi are preferably employed. For operations where gaseous mixtures comprising SO 2  F 2  are treated, temperature ranges of from about 20° to -75° C. in the column are preferred. 
     The degree of HF removal from the gaseous mixture to be purified can be determined by known analytical procedures. Amounts of HF down to about 50 ppm can be detected by present procedures. With the typical apparatus described, various samples can be drawn from different parts of the column and from the gaseous mixture exiting the column to determine the degree of HF removal. Where HF is to be removed from gaseous mixtures comprising HCl, a measured volume of the exit gas is bubbled into a known volume of water and the fluoride content measured using an Orion specific ion electrode. The total acid (HCl) can be obtained by titration to a phenolphthalein end point with caustic. 
     Where the gaseous mixture comprises HCl and SO 2  F 2 , sample portions of the exit gas can be absorbed in measured volumes of sodium acetate-acetic acid buffer solution and the samples analyzed according to conventional technique for sulfate and fluoride ion concentrations. Since, in the hydrolysis of SO 2  F 2  in the solution, two moles of fluorine are formed for every mole of sulfate, any excess fluorine present in sample above that obtained from the hydrolysis of a known quantity of SO 2  F 2  can readily be determined. Thus, the degree of HF removal by the SO 2  can readily be determined and those skilled in the art will appreciate that the contact or residence time of the gaseous mixture with the SO 2  can be increased where necessary to effect substantially complete removal of HF. It will also be recognized that vapor phase chromatograms of the exit gas can be employed to determine if the SO 2  used to remove the HF is vaporizing off with the exit gas, indicating a need to lower the temperature and maintain the SO 2  in a liquid state. 
     Where the gaseous mixture from which HF is to be removed comprises HCl, the HCl component may be directly recovered as the exit gas. It is to be noted, however, that some amount of HCl may also be dissolved in the liquid SO 2 . Where the gaseous mixture comprises HCl and SO 2  F 2 , the exit gas can be further distilled according to conventional techniques to recover each of the valuable HCl and SO 2  F 2  products. Additionally, the liquid SO 2  containing the solubilized HF can be recycled for utilization as initial reactants in the preparation of SO 2  F 2  or distilled to recover the SO 2  and HF. 
     The following examples illustrate the practice and advantages of the present invention. 
    
    
     EXAMPLE 1 
     A 2-inch internal diameter stainless steel sieve tray insulated column having a total of 15 sieve trays with downcomers (e.g., trays with openings so that liquid flowing across the trays downwardly through the column is subjected to vapors rising up the column), and equipped with a thermosyphon reboiler, a stainless steel shell and tube reflux condensor was charged with a chilled (about -55° C.) mixture of SO 2  (466 grams), HF (25 grams) and HCl (113 grams). The reflux condensor was refrigerated by a chilled methylene brine and heat was applied to the reboiler by means of an electrical heating coil. Sample ports were located in the vapor space below the first tray (0) and above every third tray from which samples could be drawn for analysis. Thermocouples were employed to measure the temperature at each sample point as well as the column bottoms liquid, bottom vapor, and condensor vent. A Bourdon type pressure gauge equipped with a diaphragm measured the pressure at the top of the column and U-tube filled with fluorocarbon oil was employed to measure the pressure drop across the column. 
     The charged column was operated at temperatures ranging from about 15° C. at the bottom to about -60° C. at the top under reflux conditions at 15 psi for a period of 5 hours in order to achieve equilibrium. During such time, the reboiler was set at 546 Btu/hr. Following the attainment of equilibrium, vapor samples were drawn and analyzed to determine the HF--HCl contents at various levels and the exit gas (about 10-12 ml dissolved in 10-20 ml water) analyzed for fluoride content using an Orion specific ion electrode and the total HCl determined by titration to a phenolphthalein end point with caustic. 
     As a result of such operations, the HCl exit gas was found to be substantially free of HF. 
     EXAMPLE 2 
     Utilizing substantially the same equipment and procedures as set forth in Example 1, a charge of HF (2.5 grams), SO 2  (1 lb.) and HCl (0.25 lb.) were charged into the 15 tray column and the column operated at reflux under 21 psi. The reboiler was set at 160° C. Various measurements of temperature and mole percentages of components at different tray levels were taken after the charge had reached equilibrium. The following results obtained from the zone below the first tray (0) and above every third tray to the top (tray 15) were obtained: 
     
         ______________________________________          Mole %Tray No.  Temp. °C.                HF       SO.sub.2                                HCl______________________________________15        -68.5      0.0126   --     99.9812        -68.5      0.0125   --     &#34;9         -68.5      0.0129   --     &#34;6         -59.5      0.033    20.3   --3           8.5      1.18     98.82  00          10.0      9.39     90.61  0______________________________________ 
    
     The data obtained illustrates the effective removal of HF from the gaseous mixture (vapors) flowing upwardly through the SO 2  zone (e.g., trays 0 through 6), with only minute amounts of HF being present in the HCl vapor in zones 9-15. Other similar operations were carried out, with HF concentrations as high as 60 mole percent at tray 0 and 32% at tray 3 being reduced to 0.2% at tray 15, thus indicating substantially complete removal of HF from the column exit gas (HCl). 
     EXAMPLE 3 
     In operations utilizing equipment and procedures as set forth in Examples 1 and 2 above, a chilled mixture (about -55° C.) comprising SO 2  F 2 , HCl, SO 2  and about 10-15% by weight HF was charged into the column. After equilibrium of the components had been reached, samples of the column exit gas were absorbed in a given volume of sodium acetate-acetic acid buffer solution and analyzed for fluorine with an Orion specific ion electrode and sulfate ions according to standard tests. Since SO 2  F 2  hydrolyzes in water to give two moles of fluorine for every mole of sulfate ion formed, the ratio of fluorine/sulfate should theoretically be 2.0 if no additional fluorine from the HF component is present. In such operations with different trials, fluorine to sulfate ion ratios of 1.87, 1.93, 2.19 and 2.14 were obtained, indicating substantial removal of HF by the liquid SO 2  zone in the column. 
     The above described embodiments of the invention are presented for the purpose of illustration. It is to be understood that the invention can be practiced by different embodiments without departing from the spirit and scope thereof and that the invention is not intended to be limited by the above specific embodiments.