The present invention relates to the formation and subsequent lateral migration of vaporous clouds. More particularly, the invention relates to a method for increasing rainout from a hydrofluoric acid- containing cloud formed from the escape of hydrofluoric acid from its containment facility.

Industrial chemical plants, including petroleum refineries, often store and process hazardous fluids at elevated pressures. The petroleum refining industry in particular has compiled a record of safe, reliable unit operation through careful attention to maintenance and operator training. By minimizing accidents, the industry has successfully maintained a first line of protection against the hazards associated with the accidental release of corrosive and toxic fluids. The second line of protection is necessarily tailored to the characteristics of the individual fluids. For example, the principal hazard associated with releasing a light aliphatic hydrocarbon (e.g., propane) would be ignition and explosion of the dense gas cloud. In contrast, releasing a toxic but non-flammable gas would pose various safety and environmental hazards other than explosion.

With the expansion of population centers near the battery limits of numerous petroleum refineries, concerns have arisen regarding the ability of presently used containment methods to prevent migration of aerosol or vapor clouds containing toxic substances (e.g., hydrofluoric acid) outside the refinery in the unlikely event of an accidental release.

Further, the importance of hydrofluoric acid (HF) catalyzed isoparaffin-olefin al ylation continues to grow as gasolines are reformulated to meet increasingly stringent air quality regulations. Specifically, the

addition of HF alkylate to the gasoline pool can help compensate for the octane lost by removing alkyl-substituted aro atics.

Alkylation is a reaction in which an alkyl group is added to an organic molecule. Thus an isoparaffin can be reacted with an olefin to provide an isoparaffin of higher molecular weight. Industrially, the concept depends on the reaction of a C_ to C _g olefin with isobutane in the presence of an acidic catalyst producing a so-called alkylate. This alkylate is a valuable blending component in the manufacture of gasolines due not only to its high octane rating but also to its sensitivity to octane-enhancing additives. Industrial alkylation processes have historically used concentrated hydrofluoric or sulfuric acid catalysts under relatively low temperature conditions. Acid strength is preferably maintained at 88 to 94 weight percent by the continuous addition of fresh acid and the continuous withdrawal of spent acid. As used herein, the term "concentrated hydrofluoric acid" refers to a liquid containing at least about 85 weight percent HF.

Hydrofluoric and sulfuric acid alkylation processes share inherent drawbacks including environmental and safety concerns, acid consumption, and sludge disposal. For a general discussion of sulfuric acid alkylation, see the series of three articles by L.F. Albright et al., "Alkylation of

Isobutane with C. Olefins", 27 Ind. Encr. Chem. Res. ,

381-397, (1988). For a survey of hydrofluoric acid catalyzed alkylation, see 1 Handbook of Petroleum Refining Processes 23-28 (R.A. Meyers, ed. , 1986).

Hydrogen fluoride, or hydrofluoric acid (HF) is highly toxic and corrosive. However, it is used as a catalyst in isomerization, condensation, polymerization and hydrolysis reactions. The petroleum industry used anhydrous hydrogen fluoride primarily as a liquid

catalyst for alkylation of olefinic hydrocarbons to produce alkylate for increasing the octane number of gasoline. Years of experience in its manufacture and use have shown that HF can be handled safely, provided the hazards are recognized and precautions taken.

Though many safety precautions are taken to prevent leaks, massive or catastrophic leaks are feared primarily because the anhydrous acid will fume on escape creating a vapor cloud that can be spread for some distance. Previous workers in this field approached this problem from the standpoint of containing or neutralizing the HF cloud in the event of an accidental release.

U.S. Patents 4,938,935 and 4,985,220 as well as U.S. Patent 4,938,936 to Yan teach various methods for containing and/or neutralizing HF acid clouds following accidental releases.

But it would be particularly desirable to provide an additive which decreases the cloud forming tendency of HF without compromising its activity as an isoparaffin-olefin alkylation catalyst. Solvents and complexing agents for hydrofluoric acid have, in the past, been disclosed for various purposes as noted in the following references. U.S. Patent 2,615,908 teaches thioether-HF-copper complex compounds and a method for preparing the same. Potential uses for the thioether-HF-copper composition compounds are listed from column 6, line 55 through column 8 at line 3. The method is said to be useful for purifying HF-containing vent gases from an industrial HF alkylation plant. See column 7, lines 10-24.

U.S. Patent 3,531,546 discloses a HF-CO catalyst composition which is said to be useful for alkylation as well as olefin isomerization.

U.S. Patent 3,795,712 relates to acid catalysts comprising a Lewis acid, _. Bronsted acid, and a sulfone

of the formula R-S0 ₂ -R' , where R and R' are each separately a monovalent radical containing from 1 to 8 carbon atoms or form together a divalent radical having from 3 to 12 carbon atoms. U.S. Patent 3,856,764 teaches an olefin polymerization catalyst comprising (1) at least one organoaluminum compound, (2) at least one nickel compound selected from the class consisting of nickel salts of carboxylic acids, organic complex compounds of nickel, or nickel tetracarbonyl and (3) at least one hydrogen fluoride complex prepared by complexing hydrogen fluoride with a member of the class consisting of ketones, ethers, esters, alcohols, nitriles, and water. U.S. Patent 4,025,577 and 4,094,924 report the use of alkylation catalyst compositions containing HF, a metal halide, and sulfolane.

Promoters such as alcohols, thiols, water, ethers, thioethers, sulfonic acids, and carboxylic acids are disclosed in combination with strong Bronsted acids such as HF, fluorosulfonic and trihalomethanesulfonic acids in U.S. Patent 3,778,489. The promoters are said to modify the activity of the strong Bronsted acids for alkylation. U.S. Patent 5,073,674 relates to a an alkylation process which uses a liquid oniu polyhydrogen fluoride complex as reaction medium and catalyst. The Olah patent states that the disclosed complexing method reduces the volatility of the hydrofluoric acid. T e present invention is a method for increasing rainout from an autorefrigerated vaporous cloud containing hydrofluoric acid formed from the escape of hydrofluoric acid from its containment facility comprising admixing from about 50 to about 80 weight percent hydrofluoric acid with from about 20 to about 50 weight percent of at least one selected from the group consisting of a sulfone having the formula

R-SO -R', wherein R and R ¹ are the same or are different, and are selected from the group consisting of halides, alkyl groups, aromatic groups, alkylhalide groups, or arylhalide groups containing from about 1 to about 20 carbon atoms and a sulfone having the formula R-S0_, wherein R is a bidentate alkyl group, an aromatic group, an alkylhalide group, or an arylhalide group containing from about 1 to about 20 carbon atoms. Figure la is a plot of TMP/DMH ratio (trimethypentane/ dimethylhexane) as a function of weight percent sulfolane in HF for isoparaffin-olefin alkylation at 80°F, 175 psig, 10/1 isobutane/butene ratio and olefin WHSV of 0.3 hr -l

Figure lb is a plot of C _g + byproduct in the alkylate product as a function of weight percent sulfolane in HF under the conditions set forth with reference to Figure la.

Figure lc is a plot of the MON (Motor Octane Number) of the C_+ portion of the alkylate product produced by the isoparaffin-olefin reaction (described above with reference to Figure la) as a function of weight percent sulfolane in HF.

Figure 2 is a plot of vapor pressure at 76°F as a function of weight percent sulfolane in HF. Figure 3 is a plot of percent rainout (weight) as a function of vapor pressure for neat sulfolane, neat HF, and mixtures of HF and sulfolane containing from 20 to 80 percent sulfolane (weight) .

The present invention provides a method for increasing the rainout from an autorefrigerated vaporous cloud containing hydrofluoric acid formed from the escape of hydrofluoric acid from its containment facility. The method of the invention employs a sulfone additive which is compatible with hydrofluoric acid for use in a commercial isoparaffin-olefin alkylation process. Accordingly, the method of the invention may comprise admixing the sulfone with the

hydrofluoric acid and circulating the admixture as a liquid catalyst in a commercial isoparaffin-olefin alkylation process unit. Alternatively, the sulfone may be added to the hydrofluoric acid inventory in the unlikely event of a hydrofluoric acid escape from the containment facility.

Feedstocks useful in the isoparaffin-olefin alkylation process include at least one isoparaffin and at least one olefin. The isoparaffin reactant used in the present alkylation process has from about 4 to about 8 carbon atoms. Representative examples of such isoparaffins include isobutane, isopentane, 3-methylhexane, 2-methylhexane, 2,3-dimethylbutane and 2,4-dimethylhexane. The olefin component of the feedstock includes at least one olefin having from 2 to 12 carbon atoms. Representative examples of such olefins include butene-2, isobutylene, butene-1, propylene, ethylene, hexene, octene, pentene, and heptene, merely to name a few. The preferred olefins include the C. olefins, for example, butene-1, butene-2, isobutylene, or a mixture of one or more of these C olefins, with butene-2 being the most preferred. Suitable feedstocks for the process of the present invention are described in U.S. Patent 3,862,258 to Huang et al. at column 3, lines 44-56, the disclosure of which is incorporated by reference as if set forth at length herein.

The molar ratio of isoparaffin to olefin is generally from about 1:1 to about 100:1, preferably from about 1:1 to about 50:1, and more preferably from about 5:1 to about 20:1.

The catalyst complex of the present invention comprises from about 50 to about 80 weight hydrofluoric acid and the halogenated sulfonic acids together with from about 20 to about 50 weight percent of a sulfone having the formula R-SO -R ¹ , wherein R and R ¹ are the same or are different, and are selected from the group

consisting of halides, alkyl groups, aromatic groups, alkylhalide groups, or arylhalide groups containing from about 1 to about 20 carbon atoms. The sulfone may alternatively have the formula R-S0-, wherein R is a bidentate alkyl group, an aromatic group, an alkylhalide group, or an arylhalide group containing from about 1 to about 20 carbon atoms.

Of these useful sulfones, sulfolane (tetramethylene sulfone) formula:

isparticularly preferred. More specifically, it has been found that a particular range of sulfone concentrations provide useful levels of isoparaffin- olefin alkylation activity while effectively suppressing the cloud forming tendency of the strong acid. In a preferred embodiment, the catalyst composition comprises from about 20 to about 40 weight percent sulfolane and from about 60 to about 80 weight percent HF. The most preferred catalyst composition comprises from about 20 to about 30 weight percent sulfolane together with from about 70 to about 80 weight percent HF.

The catalyst composition of the present invention may be readily substituted for the concentrated hydrofluoric acid catalyst in an existing hydrofluoric acid alkylation process unit, for example, a riser reactor alkylation process unit, without substantial equipment modifications. Accordingly, the conversion

conditions for the process of the present invention resemble those of typical commercial hydrofluoric acid alkylation processes.

The present alkylation process is suitably conducted at temperatures of from about 0 to about 150°F, preferably from about 50 to about 150°F, and more preferably from about 70 to about 110°F. Pressure is maintained to ensure a liquid phase in the alkylation reaction zone. Pressures typically range from about 20 to about 1200 psig, preferably from about 50 to about 500 psig. The reaction zone is preferably free from added hydrogen. Olefin feed rates generally range from about 0.01 to about 50 WHSV and more preferably from about 0.5 to about 20 hr ^" WHSV. The mixed isoparaffin-olefin reactants may be contacted with the catalyst composition of the invention in any suitable reaction vessel, examples of which include stirred-tank reactors as well as riser-type reactors. Contact time for the mixed isoparaffin-olefin feed and the catalyst composition of the invention typically are within the range of from about 0.1 second to about 50 seconds, and are preferably from about 8 seconds to about 25 seconds.

The relative amounts of catalyst and reactants are defined herein by the acid-to-oil ratio. The volumetric acid-to-oil ratio (as used herein) is the ratio of the sum of the volumes of ASO (acid soluble oil) , acid, and sulfone to the total isoparaffin and olefin reactor feed. The volumetric acid-to-oil ratio typically falls within the range of about 0.1:1 to about 10:1, preferably from about 0.1:1 to about 5:1. Contact time between the catalyst and the isoparaffin- olefin feedstock typically ranges from about 1 to about 50 seconds, preferably from about 8 to about 25 seconds.

The sulfone component of the alkylation catalyst composition may be added by injection directly into the

alkylation process unit, or may be mixed with the hydrocarbon charge, or may be mixed with the fresh and/or the circulating acid catalyst component, or with a stream of mixed acid/additive catalyst. Downstream from the alkylation reaction zone, the sulfone is preferably separated from the alkylate product stream, mixed with fresh and/or circulating acid and/or circulating acid/additive catalyst mixture, and recycled to the alkylation reaction zone. The particular separation technique selected, however, depends upon the characteristics of the selected sulfone.

The sulfone may partition between the acid and the alkylate-containing hydrocarbon reactor effluent, or may remain in either the hydrocarbon or the acid phase, or may form a third discrete phase, depending upon the characteristics of the selected sulfone. If the boiling point of the selected sulfone does not overlap major hydrocarbon products, distillation is preferred to separate and recycle the sulfone. Higher boiling (e.g. >200°C) sulfones may require extraction (for example, liquid-liquid solvent extraction) to be efficiently recovered from alkylation byproducts such as ASO (acid soluble oil) .

Examples

The following Examples demonstrate the effectiveness of the sulfone-containing catalyst composition of the invention for catalyzing isoparaffin-olefin alkylation. Example 1 demonstrates the well-known effectiveness of anhydrous HF as an isoparaffin-olefin alkylation catalyst and is presented for comparison to evaluate the effectiveness of the alkylation catalyst composition of the present invention (Examples 2-4) .

Example 1 - Comparative

Anhydrous HF (40 grams, obtained from Matheson Chemical Company of Bridgeport, New Jersey) was condensed into a clean, dry autoclave (1000 cc) . Isobutane (100 grams) was added, and the autoclave was stirred at 1500 rpm. The autoclave was brought to room temperature (22°C, 71°F) and pressurized to 100 psig. A pre-mixed 10:1 weight:weight mixture of isobutane:2-butene feed (obtained from Matheson Chemical Company) was added at a rate of 250 cc/hour for 2 hours under autogeneous pressure for a total isobutane:2-butene charge of 500 cc. A 10-15°F (5-8°C) temperature rise was observed during feed addition resulting in an average reaction temperature of 27-30"C (80-85°F). The autoclave was sampled (300 cc) immediately after feed addition was complete. The sample was flashed at room temperature and quenched with a chilled water trap. Samples of the liquid and gas products were analyzed by capillary GC (60m DB-1 column) . The results of Example 1 are shown in the Table A below.

Examples 2-4 The following procedure was followed for Examples 2-4. In a typical experiment, 10 grams of sulfolane (tetramethylene sulfone, Phillips Petroleum Co.) was loaded into a clean, dry 1000 cc autoclave. Sulfolane was stored in a vacuum desiccator over P-0_ prior to use. The autoclave was sealed and cooled with liquid nitrogen. The autoclave was evacuated and 40 grams of anhydrous HF (Matheson) were condensed into the autoclave. The HF/sulfolane mixture was warmed to room temperature (71°F) . Isobutane (100 grams) was added to the mixture, the autoclave was pressurized to 100 psig and stirred at 1500 rpm. A pre-mixed 10/l wt/wt isobutane/2-butene feed (Matheson) was then introduced at 250 cc/hr. A 5-10°F temperature rise was typically

observed during reaction. After two hours, feed addition was halted and a 300 cc liquid sample was obtained. The liquid sample was depressured through an ice cooled trap (filled with 50 cc of water) which was connected to a gas sampling bomb and wet test meter. The liquid alkylate product and gas sample were analyzed with a Varian 6000 gas chromatograph equipped with a 60 meter DB-1 capillary column.

Table A below lists the results with HF/sulfolane mixtures containing up to 50 wt% sulfolane in HF. With 20 wt% sulfolane in HF (125 moles HF per liter sulfolane) , performance was comparable to pure HF. The ratio of high octane trimethyl-pentanes to lower octane dimethylhexanes (TMP/DMH) was 9.4 with 80/20 HF/sulfolane compared to 9.2 with pure HF. Performance diminished slightly upon adding 50 wt% sulfolane to HF (63 moles HF per liter of sulfolane) . Alkylate with 50/50 HF/sulfolane catalyst had a TMP/DMH ratio of 6.5 and contained 11.8 wt% C _g +. A 40/60 HF/sulfolane catalyst (42 moles of HF per liter of sulfolane) showed no activity for alkylation. The only observed product was butyl fluoride formed by hydrofluorination of 2-butene feed. Thus, the useful concentration range for sulfolane in HF was shown to be below about 60 wt% in HF (greater than about 40 moles of HF per liter of sulfolane) .

HF/sulfolane catalyst performance is plotted as a function of sulfolane loading in Figures la, lb, and lc. Alkylate quality increased slightly upon adding 20 wt% sulfolane to HF (C_+ MON 97.5 vs 97 for pure HF) , then decreased with further sulfolane dilution. Activity for isoparaffin/olefin alkylation was not observed above about 50 wt% sulfolane in HF.

Vapor pressures (Torr) for HF/sulfolane mixtures are plotted as a function of sulfolane loading in Figure 2, showing that diluting HF with sulfolane dramatically reduces vapor pressure.

Examples 5-7 HF/sulfolane mixtures were evaluated in a 1.5 foot long riser reactor having total volume of about 120 cc. The reactor was first filled with liquid isobutane at approximately 130 psig. Approximately 100 cc of premixed HF/sulfolane catalyst (70/30 wt/wt for Example 5, 60/40 wt/wt for Examples 6 and 7) was pressured into the reactor from the bottom, displacing a portion of the isobutane. The isobutane flow was maintained to keep the injection nozzle clear. Hydrocarbon residence time in the riser was approximately 6 seconds with a hydrocarbon drop diameter of about 300 microns. The olefin feed was obtained from a commercial HF alkylation process unit and was characterized by the following average composition:

Conversion conditions and results for Example 5 are shown below in Table B. The yields reported in Table B are given as g C_.+ alkylate per g olefin converted.

Examples 8-15

Examples 8-15 examined the cloud-forming tendency of various mixtures of HF and sulfolane. The cloud-forming tendency was measured in terms of rainout, which is defined as the fraction of the released substance which falls to the ground within a defined area downwind from the point of release. For example, if 50% (weight) of a released substance falls to the ground within the defined area downwind from the point of release, the rainout is 50% (weight) . Rainout is inversely related to cloud-forming tendency. In the event of an accidental release, material which precipitates or rains out cannot pose a toxic hazard downwind from the site of the accidental release. The HF and sulfolane were mixed in a 2-liter stirred autoclave which was fabricated from 316 stainless steel and contained a cooling/heating coil. The autoclave was pressured with nitrogen, and the contents of the autoclave were then released through a nozzle into a flow chamber. The liquid rainout was collected in trays and the material which carried over was scrubbed with water in accordance with the procedure set forth below.

The sulfolane was weighed and placed into the 2- liter autoclave. Anhydrous HF from a cylinder was measured using a calibrated Kel-F lined sight glass and pressured into the autoclave. The amount of HF used per run varied from 109 to 434 grams. Once the material was loaded into the autoclave, the wind velocity in the flow chamber was adjusted to the desired setting, usually about 0.9 m/s to simulate a typical release wind speed. Ambient humidity was used. The autoclave mixer was also turned on for several minutes to mix the contents. Water was placed into each of three dropout trays to catch the precipitated HF and sulfolane as well as to minimize evaporative losses. The water-filled trays were weighed to

determine the pre-release base line weights.

The autoclave pressure was then set with nitrogen and the autoclave temperature was set by warm water or a Neslab cooler. The valve to the release orifice was then opened, and the material flowed into the chamber. The orifice diameter was 0.635 mm. The dimensions of the flow chamber were approximately 40 in. (length) by 12 in. (height) by 6 in. (depth) . Because the release material had sufficient inertia to travel a greater length than the flow chamber, an impingement plate was placed at the end of the third tray. This plate was covered with steel wool to minimize splashing and to produce rainout of 98 percent with the base case water runs. The liquid rainout was collected in the trays, with most of the material going into the tray closest to the impingement plate. The trays were then weighed to determine how much material rained-out. The data were then corrected for evaporation of water from the trays, liquid hold-up in the impingement plate pad, residual material in the autoclave, and material that adhered to the walls of the chamber.

The rainout data are shown below in Table C, and plotted in Figure 3. Table C also lists the corresponding C_+ alkylate TMP (trimethylpentane) content produced with each of the HF/sulfolane mixtures as isobutane:butene alkylation catalyst. These results show that the preferred catalyst composition provides excellent rainout (>70 wt.%) while also producing a high quality alkylate product.

Table C Conditions: about 50 psig, 90°F, 0.635 mm orifice diameter.

Changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention which is intended to be limited only by the scope of the appended claims.