Removal of Hydrogen Fluoride From Gaseous Mixture by Absorption on Alkaline Earth Metal Fluoride

This invention relates to the removal of hydrogen fluoride (HF) gas from gaseous mixtures containing up to about 20% of HF, based on the weight of the gaseous mixture, and other gases inert to alkaline earth metal fluorides, in the absence of water. The gaseous mixture is passed in contact with particulate, anhydrous alkaline earth metal chloride in the absence of water. Preferably, the anhydrous alkaline earth metal chloride is fluorinated by contact with a dry gaseous mixture inert to alkaline earth metal chloride except for a minor proportion of HF therein. Alternatively, a dry gas containing at least a major proportion of HF or a dry fluorine gas can be used to obtain the particulate, anhydrous alkaline earth metal fluoride employed in the process of this invention.

After absorbing HF gas in an amount ranging up to the absorption capacity of the specified anhydrous alkaline earth metal fluoride, the flow of the gaseous mixture is terminated or diverted and the absorbed HF is removed from the alkaline earth metal fluoride, preferably by desorption with heat. The regenerated alkaline earth metal fluoride is then reused in the HF removal process. Liquids, other than aqueous liquids, can be tolerated in the gaseous mixture as long as they do not collect in the process vessels in amounts sufficient to obscure significant amounts of the surface area of the absorption material.

The term "absorption" is used herein to include chemisorption, adsorption and absorption although the term "absorption" will be used herein to describe the process.

The alkaline earth metal compounds employed for this invention are the calcium, barium or strontium chlorides and fluorides in their anhydrous form. Calcium is the preferred alkaline earth metal and will be primarily used hereinafter to further demonstrate the invention. The anhydrous forms of alkaline earth metal chlorides are prepared by drying the hydrates at temperatures suitable to remove water of hydration.

The gaseous mixtures included herein which contain HF in amounts up to about 20%, based on the weight of the gaseous mixture, and other gases inert to alkaline earth metal fluorides include mixtures of HF, hydrogen chloride (HCl) and volatile organic gases obtained from various manufacturing processes; HF and HCl mixtures substantially free of other gases; HF and fluorine gas with and without other inert gases; dry air containing traces of HF; waste gases from petroleum alkylation processes involving anhydrous HF; waste gases in the aluminum metal and ceramic industries containing traces of HF; etc.

The process of this invention is especially useful for the treatment of HF -- contaminated gaseous hydrogen chloride encountered as a by-product in fluorinated hydrocarbon synthesis. More specifically, in commerical processes for the production of fluorinated hydrocarbons (such as CCl.sub.3 F, CCl.sub.2 F.sub.2, CClF.sub.3, CHCl.sub.2 F, CHClF.sub.2, CHF.sub.3, CH.sub.3 CClF.sub.2, CH.sub.3 CCl.sub.2 F, CH.sub.3 CF.sub.3, CCl.sub.2 FCClF.sub.2, CClF.sub.2 CClF.sub.2, and the like which products are useful as refrigerants, blowing agents and aerosol propellants) involving the fluorination with HF of chlorinated hydrocarbons, such as CCl.sub.4, CHCl.sub.3, CH.sub.3 CCl.sub.3, and CCl.sub.2 =CCl.sub.2, a large amount of hydrogen chloride containing varying but significant proportions of hydrogen fluoride is obtained as a by-product, which hydrogen chloride by-product cannot be discarded for obvious economic and ecological reasons. However, this contaminated hydrogen chloride must be purified so that the HCl can be used in industrial applications, for example, in oxychlorination reactions, food processing, pickling of steel, and in the treatment of brine used in electrolytic chlorine production, the presence of HF in such systems being intolerable.

F. R. Lowdermilk, U.S. Pat. No. 3,140,916, has suggested one method for removing HF contamination wherein a gaseous mixture of hydrogen chloride and hydrogen fluoride is passed through an aqueous solution of calcium chloride, whereby the hydrogen fluoride reacts with the calcium chloride in solution to form calcium fluoride as a precipitated solid that is recoverable by filtration. This multi-step aqueous technique has several serious disadvantages stemming from the problems in dealing with an aqueous system comprised of HCl, CaCl.sub.2, and CaF.sub.2. This mixture is highly corrosive and the precipitated CaF.sub.2 -in-water mixture forms an abrasive slurry, both conditions being very injurious to plant equipment. Lowdermilk also mentions that aluminum oxide has been used as an absorption agent for removal of HF from HCl.

A simple and economical process for removing HF impurity from a gaseous mixture of hydrogen chloride has been disclosed in D. F. Merchant's copending application Ser. No. 354,715, Apr. 26, 1973, a continuation-in-part of Ser. No. 153,839, filed June 16, 1971. This copending application concerns the method of passing gaseous HCl containing HF in contact with substantially anhydrous, solid, particulate calcium chloride. It was noted, during the operation of this process, that the calcium fluoride, obtained in situ therein via the conversion of CaCl.sub.2, is endowed with the capability to absorb up to somewhat over 50% of its own weight of HF. Moreover, this calcium fluoride maintains the size, shape, appearance, free-flowing and other easy-handling characteristics of the precursor calcium chloride.

W. L. Colvin, in AEC Report K 1117, discloses the use of calcium fluoride as a sorbent for hydrogen fluoride, the said calcium fluoride being prepared by direct fluorination of calcium sulfate. However, he only observed a very low loading for the level breakthrough of HF. With the present invention, an average loading of about 20% of HF on CaF.sub.2 under normal operation is obtained.

Colvin likewise reports a surface area of 129 square meters/gram for his material. This calcium fluoride material employed herein has surface area of about 12-17 square meters/gram. Thus, the process of this invention provides the unobvious and unexpected result of removing more HF per unit weight of absorbent with considerably less surface area.

Laboratory tests showed that naturally occurring calcium fluoride does not have the ability to absorb a significant amount of HF. Naturally occurring calcium fluoride could absorb only about 5 to 6% by weight HF before breakthrough while the calcium fluoride made by reacting calcium chloride and HF can absorb about 20% HF by weight before breakthrough under the same conditions of flow, temperature and pressure.

The term "breakthrough", as used herein, is explained as follows:

When HF is being absorbed by passing a gas containing it through an absorption medium, the outlet concentration of HF in the gas is generally fairly uniform or increasing at a slow rate. When the outlet concentration of HF in the gas begins to increase at a rapid rate (as compared to previous rates), breakthrough has occurred.

Table I below summarizes measurement made on the surface area and absorption of HF on calcium fluoride (CaF.sub.2) prepared by passing anhydrous HF through a bed of anhydrous calcium chloride (CaCl.sub.2) in the absence of water until the CaCl.sub.2 was converted to CaF.sub.2 as determined by analysis of the solids. The absorbed HF was then desorbed by heat and air purge.

These results show a marked change in physical structure of the compound CaCl.sub.2 when reacted with HF to form CaF.sub.2. Also shown in Sample III is natural fluorspar, CaF.sub.2. Its surface area is on the order of that of the CaCl.sub.2. It absorbed only 10% HF under the same conditions as Samples I and II.

The HF absorbed by the "synthetic" calcium fluoride produced as above described can be stripped off by suitable heating means, and the thus regenerated CaF.sub.2 will again serve as an absorbent for additional HF present in the gaseous mixture.

The HF containing gaseous mixture treated in accordance with this invention to effect HF removal may contain from 0.3 to about 20 weight percent HF, based on total weight of the gaseous mixture. The regenerative-recycle system embodied herein is particularly suited, however, to those mixtures containing moderately large amounts of HF, e.g., from about 0.5 to about 6.0 weight percent. In a more useful application of this process, there may be present in admixture with an HF-containing gaseous hydrogen chloride mixture up to about 70 but preferably up to about 60 weight percent, based on the total weight of the gaseous mixture, of one or more volatile halogenated hydrocarbons, i.e., chlorofluorohydrocarbons, chlorohydrocarbons, and fluorohydrocarbons for example, CCl.sub.4, CCl.sub.3 F, CCl.sub.2 F.sub.2, CClF.sub.3, CHCl.sub.2 F, CHClF.sub.2, CHF.sub.3, CHCl.sub.3, CH.sub.3 CClF.sub.2, CH.sub.3 CCl.sub.2 F, CCl.sub.2 FCClF.sub.2, CClF.sub.2 CClF.sub.2, CCl.sub.2 CCl.sub.2, and the like. Such volatile halogenated hydrocarbons do not interfere with the HF absorption and undergo no discernible change from contact with the CaF.sub.2. The product HCl can be separated from these organic constitutents either by distillation to provide an anhydrous HCl gas or by absorption in water to provide an aqueous HCl solution. In the latter technique, the small amount of organic material that is absorbed in the aqueous acid may be easily removed by blowing with air or other gas or by absorption on activated carbon or molecular sieves.

The absorption tower used in carrying out the process of the invention can be initially charged with a suitable bed of particulate CaF.sub.2 prepared from the substantially anhydrous calcium chloride as described above, or in the preferred embodiment, is charged with the anhydrous CaCl.sub.2 which is then normally converted to the CaF.sub.2 when the feed stream of HF-containing gaseous HCl is passed in contact therethrough. The particulate calcium fluoride can range from powdery to granular type to pellets, that is, having an average particle size over the broad range of 0.025 to 0.375 inch (0.0625 - 0.937 cm.). In the preferred embodiment, however, the calcium fluoride is in pellet form, e.g., having a particle size of from about 0.25 to about 0.375 inch (0.625- 0.937 cm.).

The temperature at which the gaseous hydrogen chloride mixture is contacted with the particulate CaF.sub.2 may be in the range of about 60.degree.F (15.5.degree.C) to about 130.degree.F (54.5.degree.C), preferably from about 70.degree.F (21.degree.C) to 115.degree.F (45.degree.C). It is desirable to maintain the temperature of contact above the dew point of the gaseous mixture to prevent any organics present therein from condensing in the reactor column. Pressures may vary from essentially atmospheric up to about 250 psig (18 atm.), depending on other process conditions. Because the system is essentially anhydrous, exotic materials of construction are not required and the absorption tower and auxiliary piping and equipment can be of ordinary carbon steel, corrosion thereof not being an abnormal problem.

The term anhydrous as used herein means a substance containing less than 1000 and preferably less than 200 parts by weight of water per million (p.p.m.) parts of substance. Likewise, the term in the absence of water signifies a process carried out with a gas containing less than 1000 p.p.m. of water.

An advantage provided by this invention, in addition to simplicity of operation and low operating costs, is the general efficiency of absorption of HF, even at relatively high through-put rates. For example, after processing an HCl-organic gaseous mixture originally containing about 18% HF in accordance with this invention, the product gas will usually contain less than 0.05 weight percent HF, although flow rates of the feed gas may vary over the range of from about 1.0 to about 100 pounds per hour per square foot of particulate CaF.sub.2, measured at standard conditions of 15 psia pressure and 70.degree.F temperature. Stated in different terms, the foregoing results of HF absorption are obtained using periods of contact of the gaseous HCl mixture with the particulate calcium fluoride (also referred to as retention time) of from about 20 to about 600 seconds. The quality of the product gas (amount of HF contamination) is, of course, influenced to some extent by these flow rates.

The feed of the dry gaseous mixture to the particulate CaF.sub.2 bed is continued until an appropriate amount of HF is absorbed therein. This amount may be on the order of about 50% of the weight of the CaF.sub.2 at the maximum; however, it is apparent that one factor in absorption efficiency is the amount of free surface remaining in the CaF.sub.2 solids; therefore, the operator who desires a comparatively more pure HCl product can terminate the HCl gas feed, or divert the feed to a fresh tower before the CaF.sub.2 has absorbed the maximum amount. The so-called "spent" CaF.sub.2 solids are then regenerated by desorbing, that is, stripping the solids of absorbed HF. This is conveniently accomplished by any of a number of conventional methods: (1) external heating of the absorption column to around 225.degree. to 350.degree.F; (2) heating the column to 200.degree.F or higher and passing an inert gas, such as nitrogen, air or anhydrous HF through the solids; or (3) passing an inert gas, heating to 350.degree. F or higher, through the solids. The desorbed HF is recovered and can be used in hydrofluorination processes.

The "regenerated" bed of CaF.sub.2 solids after cooling is used for another cycle of treatment of HF-containing gaseous mixture, and after HF absorption has reached the desired level, as discussed above, the regeneration cycle is repeated. The number of complete cycles for which a specific CaF.sub.2 charge may be employed is limited by the actual physical breakdown of the charge due to the heating and cooling cycles. Therefore, the number of cycles is limited by the allowable pressure drop of the system. At that point, the absorbent CaF.sub.2 should be replaced with a fresh bed of the like or, preferably, substantially anhydrous, particulate CaCl.sub.2. The spent CaF.sub.2 solids (which may have lost its good physical properties because of physical degradation) is recovered from the bottom of the tower. This can be used as a raw material for preparing hydrofluoric acid in an H.sub.2 SO.sub.4 -reactor kiln.

The process of this invention is clarified and illustrated by the following examples:

A by-product gaseous mixture obtained from the process of hydrofluorinating chloroform is comprised of hydrogen chloride containing about 60% organics (halogenated hydrocarbons, mostly CHClF.sub.2 with lesser amounts of CHCl.sub.2 F, CHCl.sub.3 and CHF.sub.3) and 10.3 % hydrogen fluoride, based on the weight of HCl present. The gas mixture is passed through a one inch diameter steel column filled with 3.3 feet of particulate calcium fluoride, at 15 psia and 90.degree.F, superficial gas velocities ranging from 0.15 to 1.5 ft/min, equivalent to retention times on the order of 110 to 330 seconds. The CaF.sub.2 has a particle size of 3/16 to 1/4 inch and is obtained from the process of contacting anhydrous CaCl.sub.2 of said particulate size range with a stream of substantially the same composition as noted above in the absence of water.

The HF content of the gaseous HCl mixture is reduced from 10.3% to 0.38%, based on HCl content, equivalent to an HF removal efficiency of 96.4%. A total of 0.36 cu. ft. of the feed gas is passed through the bed until the HF absorbed therein is 14% by weight. The calcium fluoride is regenerated (absorbed HF stripped off) by heating the column to 350.degree.F with electrical heating tape. The CaF.sub.2 is then ready to treat further quantities of the feed HCl gas mixture in like manner.

A by-product gaseous mixture obtained frm the hydrofluorination of methyl chloroform is comprised of HCl containing about 54% volatile halogenated hydrocarbons (mostly CH.sub.3 CClF.sub.2 with lesser amounts of CH.sub.3 CCl.sub.2 F and CH.sub.3 CF.sub.3) and 18% HF, based on the weight of the HCl. The gas mixture is passed through a two inch diameter steel column containing a 2.6 ft. bed of CaF.sub.2 as embodied in this invention. The CaF.sub.2 is made by passing the above described gaseous mixture through anhydrous CaCl.sub.2 of particle size 1/8 to 1/4 inch diameter pellets. After the CaCl.sub.2 is converted completely to CaF.sub.2, the absorbed HF is driven off by suitable means and the CaF.sub.2 is ready for reuse as an absorbent. Typical operating conditions are as follows: pressure is 2 psig and the temperature of contact about 100.degree.F. The system is subject to four cycles of operation, the CaF.sub.2 being regenerated between the cycles by the method of the previous example. The results are tabulated as follows:

At the end of the fourth cycle, the CaF.sub.2 is removed from the column as a free-flowing solid without any appreciable change in size, shape or appearance.

A gaseous HCl mixture similar to that of Example II is passed through a two ft. diameter column having a bed of 4.5 feet of particulate CaF.sub.2 prepared by passing the HCl mixture similar to that in Example II through anhydrous CaCl.sub.2 until the CaCl.sub.2 is fully converted to CaF.sub.2, then driving off the absorbed HF by suitable means, leaving CaF.sub.2. Operation is at 100.degree.F and 155 psig. Gas flow is 2 CFM with a velocity of about 0.6 ft./min. and a contact time of 450 seconds. Two cycles are run in the test with regeneration of the CaF.sub.2 between cycles being accomplished by heating the column externally to 400.degree.F and passing a hot 350.degree.F nitrogen purge through the CaF.sub.2 to drive off absorbed HF. The results are summarized in the following table:

The following table shows the results of a series of absorption runs using a 6 inch diameter column by 5.5 ft. bed depth of CaF.sub.2 operating on a mixture of HF, HCl, CH.sub.3 CClF.sub.2, and other chlorinated fluorocarons in minor amounts or a mixture of HF, HCl, CHClF.sub.2 and other chlorinated fluorocarbons in minor amounts. All absorption tests we made on the same charge of CaF.sub.2.

The data herein relating to HF content of gases and solids was obtained by standard analytical techniques. The bed samples were obtained by "coring" of the bed.

The following table shows the results of a series of absorption runs using a 2 ft. diameter column with a 4.5 ft. CaF.sub.2 bed depth operating on the gases described in Example 4 above. All absorption tests were made on the same charge of CaF.sub.2.

Regeneration of the CaF.sub.2 used in the series of Example V was performed as follows:

1. The 2 ft. diameter bed was heated to 350.degree.F by applying heat to the outside of the 2 ft. diameter column. They dry nitrogen, at the rate of 1 SCFM per sq. ft. of bed area, was heated to 400.degree.F by a tube heater and passed through the bed. The HF on the CaF.sub.2 was desorbed and collected in a chilled vessel. The CaF.sub.2 was completely desorbed with less than 0.5% HF on CaF.sub.2 remaining. This method was used for the regenerations of the majority of the runs.

Another regeneration procedures is as follows:

2. The 2 ft. diameter CaF.sub.2 bed was heated externally, by applying heat to the outside of the 2 ft. diameter column, to 350.degree.F. Then a vacuum of 25 inches of Hg was placed on the 2 ft. diameter bed. The HF was desorbed from the CaF.sub.2 and recovered in a chilled receiver. The residual HF on the CaF.sub.2 was less than 0.5% by weight. This method was used for the last two runs of Example V. In both these cases, the desorption was made in the same direction as the HF was originally adsorbed on the CaF.sub.2, that is, cocurrent with the absorption stream.

Regeneration of the CaF.sub.2 contained in the 6 inch diameter column was performed as follows:

The 6 inch diameter column was jacketed so that 150 psig steam could be applied to the vessel externally. The CaF.sub.2 bed containing absorbed HF was heated to about 350.degree.F, then air at about 150.degree.F was introduced at a rate varying from 0.25 to 1.0 SCFM per sq. ft. of CaF.sub.2 bed area. This air was passed counter current to the direction of the absorption of the HF, i.e., if the HF was absorbed from the organic stream by passing the organic stream from the bottom of the CaF.sub.2 bed through the top of the CaF.sub.2 bed, then the desorbing air was passed from top to bottom of the CaF.sub.2 bed. The CaF.sub.2 bed was desorbed to less than 0.5% HF based on CaF.sub.2 weight.

Barium chloride dihydrate (BaCl.sub.2 .sup.. 2H.sub.2 O) was mixed with calcium chloride dihydrate (CaCl.sub.2 .sup.. 2H.sub.2 O) in the ratio of 9 parts BaCl.sub.2 .sup.. 2H.sub.2 O and 1 part CaCl.sub.2 .sup.. 2H.sub.2 O. A wet cake of the above was pressed into a billet under 3000 psi hydraulic pressure. The billet was then dried at 300.degree.F for 16 hours. The billets were then crushed, screened, and fines less than 1/4 inch were discarded.

The 1/4 inch pieces were placed in a 2 I.D. .times. 24 inches long steel pipe modified to allow sampling at 8 inches length, 16 inches length, and the outlet. Six Hundred fifty-five grams of the above described mix were placed in the 2 inches I. D. .times. 24 inches long tube. The tube was placed in a chlorofluoro carbon gas stream containing approximately 3% by volume HF, 36% by volume HCl, and 61% by volume organics. After 45 hours, the tube was removed and analyzed. Residence time was about 200 seconds. Analysis after 45 hours showed 29.6% by weight HF absorbed on the solids. Analysis of the solids showed no residual chlorides in the solids (i.e., complete conversion).

The table below shows the percentage amount of hydrogen fluoride removed from the gas stream at various times beginning at 2 hours up to 45 hours.

The barium fluoride pellets as prepared above may be regenerated in accordance with one of the procedures of Example VI or Example VII. Thereafter, the anhydrous barium fluoride may be used in absorption runs as set forth in Examples IV or V.