WO1995014636A1 - Manufacture of low-chlorides hypochlorous acid - Google Patents

Abstract

A continuous process of preparing hypochlorous acid comprising steps of: (a) contacting droplets (103) of an aqueous solution of a metal hypochlorite (101) having a volume median diameter of less than 500 micrometers with chlorine gas (102) to produce hypochlorous acid; (b) vaporizing at least 30 mole percent of the hypochlorous acid produced in step (a) into a vapor phase containing chlorine, water vapor, hypochlorous acid, and dichlorine monoxide and leaving a liquid phase aqueous hypochlorous acid solution; and (c) distilling the liquid phase hypochlorous acid solution using a stripping gas containing at least 20 mole percent chlorine to separate gaseous hypochlorous acid and dichlorine monoxide (120) from an aqueous brine (108). The process optionally additionally comprises steps of: (d) optionally absorbing the gaseous hypochlorous acid and dichlorine monoxide into water (122) to produce an aqeous solution of hypochlorous acid (123); and (e) optionally admixing the aqueous brine from step (c) with acid to convert chlorates therein to chlorine. The process is particularly useful in preparing hypochlorous acid for use in producing alkylene oxides because of the low chlorides content.

Description

MANUFACTURE OF LOW-CHLORIDES HYPOCHLOROUS ACID

There are several methods of making hypochlorous acids. Especially when the acids are subsequently to be used to react with organic compounds, for instance with olefins to make such compounds as halohydrins, it is desirable that the hypochlorous acids have a low content of halide, particularly that hypochlorous acid have a low chloride content to avoid formation of organic chlorides as by¬ products.

Low-chloride aqueous solutions of hypochlorous acid (H0C1) are known to be made by reacting aqueous alkali metal hydroxides or alkaline earth metal hydroxides, for example sodium hydroxide, in a reactor dryer with chlorine gas to make hypochlorous acid gas and solid metal chloride. The hypochlorous acid gas is condensed along with the water vapor to produce the desired aqueous solutions of hypochlorous acid. This process is disclosed in U.S. Patents

5,116,593 (May 26, 1992) and 5,037,627 (August 6, 1991) (both Melton et al.). Hilliard et al. discloses in U.S. Patent 5,213,771 (May 25, 1993) an apparatus to produce the hypochlorous acid solutions. In the disclosed process(es) an aqueous alkali metal or alkaline earth metal hydroxide solution is sprayed into a reactor with a chlorine atmosphere with a chlorine to hydroxide mole ratio of at least 22:1, such that hypochlorous acid is believed to be produced on the liquid drops with subsequent vaporization of the HOCl and water while the resulting chloride salt falls downward as a dry solid as disclosed in U.S. Patent 5,037,627. Temperatures of from 75° C to 153° C , preferably 90-140° C (both ranges as disclosed in U.S. 5,037,627), are used. Maintaining these temperatures requires heating of the feed alkali metal hydroxide solution to 80-110° C and heating of any recycled chlorine to 140° C (both as disclosed in U.S. 5,116,593 wherein the desired reaction temperature is 80-100°C) . The solid salt product of the disclosed process requires special handling equipment such as that described in U.S. patents 5,116,594 and 5,106,591 (Hilliard et al.) . To recover a low-chlorides HOCl solution, the disclosed process involves condensing the water vapor and part of the HOCl vapor from the reactor using temperatures of -5° to 20° C (as disclosed in U.S. 5,037,627), requiring refrigeration equipment. The product is a concentrated hypochlorous acid solution of 35 to 60 percent by weight hypochlorous acid (U.S. 5,037,627). The disclosed process suffers from several disadvantages. First, there is difficulty in handling solid salt product which includes separation of solid salt particles from gas and removal from the reactor. Second, the reactor must operate at high temperatures (75-150° C as disclosed in U.S. 5,037,627) to vaporize all of the HOCl and water from the salt. Finally, the disclosed process is energy inefficient, requiring large temperature swings on the large recycle gas stream (at least 22 moles of chlorine per mole of hydroxide as disclosed in U.S. Patent 5,037,627). This gas is cooled from the reaction/drying temperature of 75-150° C to the HOCl/water condensation temperature of -5° to 20° C and then reheated to 140° C for recycle to the reactor.

In U.S. Patent 4,146,578 (March 27, 1979), Brennan et al. disclose a process for making aqueous hypochlorous acid that is similar to the above described process in that a solution of alkali metal hydroxide is sprayed into a chlorine atmosphere resulting in HOCl vaporization and a dry solid salt. The primary difference is that the aqueous HOCl solution is produced by absorption of the HOCl in water as opposed to the condensation of the HOCl and water vapor. The difficulties of handling the solid salt and energy inefficiency are the same.

In U.S. Patent 3,578,400 (May 11, 1971), Wojtowicz et al. disclose the use of an organic solvent to extract HOCl from a brine solution. This process suffers from a need to further remove the HOCl from the organic solvent to produce an aqueous HOCl solution, a need to remove residual solvent from the brine solution, and undesirable reactions of HOCl with the organic solvent.

It would be desirable to have a process for preparing aqueous solutions of low chlorides hypochlorous acids which is continuous and produces high yields; operates at lower reaction temperatures than the process disclosed by Melton et al. ; does not require handling of solid salt by-products; and is more energy efficient (that is, does not require large heating/cooling cycles as disclosed by Brennan et al. , Melton et al. , and Hilliard et al. ) .

The invention includes a continuous process of preparing hypochlorous acid comprising steps of:

(a) contacting droplets of an aqueous solution of a metal hypochlorite having a volume median diameter of less than 500 micrometers with chlorine gas to produce hypochlorous acid; (b) vaporizing at least 30 mole percent of the hypochlorous acid produced in Step (a) into a vapor phase containing chlorine, water vapor, hypochlorous acid, and dichlorine monoxide and leaving a liquid phase aqueous hypochlorous acid solution; and

(c) distilling the liquid phase hypochlorous acid solution using a stripping gas containing at least 20 mole percent chlorine to separate gaseous hypochlorous acid and dichlorine monoxide from an aqueous brine.

The process optionally additionally comprises steps of:

(d) Optionally absorbing the gaseous hypochlorous acid and dichlorine monoxide into water to produce an aqueous solution of hypochlorous acid; and

(e) Optionally admixing the aqueous brine from Step (c) with acid to convert chlorates therein to chlorine;

Steps (a) , (b) , and (c) preferably occur simultaneously in an apparatus referred to herein as a reactor/distillation column. The invention additionally includes an apparatus for the preparation of hypochlorous acid comprising: (a) means for forming droplets of an aqueous solution of a metal hypochlorite having a volume median diameter of less than 500 micrometers; and (b) means for distilling liquid aqueous hypochlorous acid solution using a stripping gas. In another aspect, the invention is an apparatus for the preparation of hypochlorous acid comprising: (a) an elongated, generally vertically extending reactor vessel having a top and an opposing bottom, and a central axis therebetween, said reactor vessel having an upper reaction/HOCl desorption zone and a lower distillation zone; (b) means for removing vapor connected to the top of the reactor vessel; (c) means for forming droplets of an aqueous solution of a metal hypochlorite having a volume median diameter of less than 500 micrometers connected to the reactor vessel below the means for removing vapor and within or near an upper boundary of the reaction/HOCl desorption zone; ^" (d) outlet means for aqueous brine connected to a lower portion of the distillation zone; and (e) a chlorine infeed line connected to the distillation zone above the outlet means for aqueous brine.

When chlorine is the halogen and calcium oxide or hydroxide is the base used for preparation of the hypochlorite, the instant process advantageously generates low-chlorides hypochlorous acid solutions (HOCl) and features high yields (greater than 70 mole percent, preferably greater than 80 mole percent based on hypochlorite) of HOCl from the readily available raw materials of chlorine and calcium hypochlorite (from lime) , low energy consumption (preferably less than 3 lb steam/lb HOCl (3 kg steam/kg HOCl) ) , and mild operating temperatures (30-80° C) . The process is particularly advantageous for the production of hypochlorite solutions by reaction of chlorine with an alkaline earth hydroxide slurry such as that of calcium hydroxide in a manner that permits use of such readily available, but only partially soluble bases by converting them to hypochlorite to produce HOCl. The hypochlorite solution is fed to a stripping column through an atomizing nozzle, where the hypochlorite reacts with chlorine gas to produce HOCl which is then stripped from the liquid. Chlorine is advantageously present in the stripper both from fresh chlorine feed and from a recycle gas stream. The HOCl gas in equilibrium with dichlorine monoxide gas (the anhydride of HOCl) is then absorbed in fresh water in an absorption column to produce the low-chlorides HOCl solutions. When the process of preparing HOCl is combined with the additional steps to prepare alkylene oxides, this process advantageously provides a method to reduce by-product halogenated organic compounds, preferably by at least half as compared with, for example propylene dichloride (PDC) formation from reaction of gaseous propylene with gaseous chlorine in the presence of water in a continuous reactor.

Brief description of the drawings

Figure 1 is a diagrammatic representation of one embodiment of the invention.

Figure 2 is a sectional view of an embodiment of an atomization nozzle useful in the practice of the invention.

Detailed Description of the Invention

In Figure 1, a preferred embodiment of the overall process is illustrated diagrammatically. .An aqueous solution of metal hypochlorite enters reactor/distillation column 100 through line 101 and spray means 103 from which droplets of the solution enter reaction/HOCl desorption zone 105 of reactor/distillation column 100. The hypochlorite solution is reacted with chlorine which enters through line 102 and recycle line 132 to form HOCl which is vaporized and goes into headspace 104, which roughly corresponds to a vapor removal zone, and out line 120; while liquid falls to distillation zone 106 which is preferably packed for further vaporization of HOCl. Stripping gas including chlorine enters line 102 and through recycle line 132 while steam enters line 133 and enters column 100 through line 132. Liquid remaining after stripping and distillation falls into liquid removal zone 150 and exits via line 108 through pump 109 and out line 110. The vapor proceeds through line 120 to absorber 121 where it flows upward against a counter current of water which enters at line 122. Product HOCl solution exits via line 123 through pump 124 through heat exchanger 125 from which a portion is recycled through line 126 while the remainder exits through line 127. Remaining vapor exits through line 128 through blower 129 from which a portion exits through line 130 while a portion is recycled through line 131 back through line 132 to join incoming steam which enters through line 133.

In the following description, each step of the invention is explained beginning with chemical reactions illustrating the step with the use of lime as base, calcium hypochlorite as hypohalite, and chlorine as halogen. The invention includes the use of bases, such as sodium hydroxide, calcium hydroxide, magnesium hydroxide, sodium carbonate, calcium carbonate, magnesium oxide, or calcium oxide; halogens including chlorine, bromine, or iodine, but preferably chlorine.

a) Contacting droplets of an aqueous solution of a metal hypochlorite having a volume median diameter of less than 500 micrometers with chlorine gas to produce hypochlorous acid;

Ca(OCl)₂ (1)+ 2H₂0 (1) + 2C1₂ (g) ---> CaCl₂ (1)+ 4HOC1 (1)

EQUATION 1

Achieving high yields of hypochlorous acid requires minimizing the main side reaction which is chlorate formation:

0C1" + 2HOC1 > C10₃ ^~ + 2C1^" + 2H+

EQUATION 2

The rate at which this main side reaction occurs has been studied and reported by many authors (for example, Kokoulina, D. V., and L. I. Krishtalik, "The Volume Reaction Forming Sodium Chlorate in the Anolyte of the Chlorate Electrolyzer", Elektrokhymiyia. vol. 7, No. 3, pp. 346-52, March 1971) . These studies show chlorate formation to be a function of HOCl concentration and hypochlorite concentration as shown in Equation 2. Kokoulina also shows this chlorate formation rate to be maximum at pH values between 6 and 7 and to increase with temperature. To maximize the desired reaction and minimize the side reaction, the temperature is preferably kept low (less than 70° C) , the HOCl concentration kept low (for example less than 5 moles/liter) , and the chlorine partial pressure kept high (for example greater than 300 mm Hg (40 kPa) ) .

Additionally, to minimize this side reaction, residence times are advantageously kept low and a chlorine or other non-reacting gas, for instance nitrogen or steam, atmosphere is preferably maintained.

Another side reaction involves the explosive decomposition of dichlorine monoxide in the vapor space when concentrations exceed 23 mole percent and an ignition source is present:

C1₂0 (g) ---> Cl₂ (g) + 1/2 0₂ (g)

EQUATION 3

Dilution with a non-reacting gas, such as nitrogen, steam, chlorine, or mixtures thereof, is effective in avoiding the explosive concentration. Avoiding a source of ignition is also advantageous. The vapor pressure of C1₂0 above water so? αtions dictates that an HOCl concentration of greater than 4.5 molar is required for concentrations of C1₂0 greater than 23 mole percent at atmospheric pressure (101.3 kPa) ; therefore, concentrations are preferably below 4.5 molar HOCl.

Contact of hypohalite with halogen preferably takes place in a reactor/distillation column, also referred to herein as stripper. The reactor distillation column includes at least a means for forming droplets of an aqueous solution of a metal hypochlorite having a volume median diameter of less than 500 micrometers and means for distilling liquid aqueous hypochlorous acid solution using a stripping gas. The means for these two functions are conveniently thought o as two zones of the apparatus: a reaction/spray zone and a distillati..α zone. In more detail, the vertical reactor/distillation column (stripper) is conveniently described as comprising four sections or zones, which, listed from top to bottom, are: A vapor removal zone, a reaction/HOCl desorption zone, a distillation (stripping) zone, and a liquid removal zone. Overall, the column is operated with vapor and liquid moving countercurrent to each other, with liquid entering near the top of the column and flowing down and the vapor entering near the bottom, flowing up. In the vapor removal zone, product HOCl vapor in equilibrium with dichlorine monoxide (C1 0) , water vapor, unreacted chlorine gas, and any inert gases such as air are taken off of the top of the column for transfer to the absorber (Step d) . In the reaction/HOCl desorption zone, liquid aqueous calcium hypochlorite solution (or optionally alkali metal hypochlorite solution) is sprayed (preferably through a nozzle which is preferably located near the boundary between the vapor removal and reaction/HOCl desorption zone) into an atmosphere containing chlorine gas where the chlorine reacts with the hypochlorite of the liquid drops producing hypochlorous acid (HOCl) which then quickly desorbs from the liquid into the vapor space (Equation 4) and establishes an equilibrium with C1₂0 (Equation 5) . The liquid passes down through the vapor to the stripping zone where the remaining HOCl from the reaction zone is stripped from the liquid by the upflowing gas preferably utilizing distillation enhancing means such as packing or trays to enhance the gas/liquid contact. In the liquid removal section both the liquid from the stripping zone is removed and the stripping gas is introduced to the column. In the liquid removal section there is a liquid level which is a reservoir of liquid at the bottom of the column that acts as a seal to prevent gas from being removed from the bottom of the column. Liquid continuously flows into this reservoir from the distillation section and is continuously removed from the column to maintain a constant volume of liquid. The stripping gas, comprising chlorine, water vapor, and inert gases such as air enters the column above the liquid level in the bottom of the column and below the distillation section, preferably below the distillation enhancing means. The column (all 4 sections) typically has a cylindrical shape, although a polyhedral shape is optionally used, and is mounted vertically. Those skilled in the art will recognize that these zones are not necessarily physically discrete. Rather, the zones are described for convenience in describing the actions that occur within the reactor/distillation column. Optionally, there is some physical overlap or separation between the zones.

A hypochlorite solution is advantageously injected into the reaction/HOCl desorption section of the column above any trays or the packing. The hypochlorite solution is an aqueous solution of sufficient concentration of metal hypochlorite not to require excessive water for later disposal, but insufficient concentration to enhance chlorate formation or precipitate metal hypochlorite especially when an alkaline earth hypochlorite is used. Preferably, the concentration of calcium hypochlorite, for example, is at least 0.5 molar, more preferably at least 1.0 molar, but preferably less than 2.5, more preferably less than 2.0 molar. This solution is advantageously prepared from commercially available hypochlorite or is, pref:^{- "}ily, formed by reacczon of chlorine and an aqueous solution or slurr_ £ hydroxide, preferably an alkaline earth hydroxide or oxide or alkali metal hydroxide, more preferably an alkaline earth hydroxide or oxide, for example lime, in a device separate from reaction/distillation device of the present invention. Such reaction is within the skill in the art

Temperatures of the hypochlorite feed solution are sufficient to avoid the need for refrigeration equipment to cool the feed, preferably at least 40° C, more preferably at least 50° C, but not sufficient to resu^lt in decomposition of the hypochlorite to chlorate, preferably less than 100° C, more preferably less than 90° C. The temperature of the hypochlorite feed is used to partially control the temperature of the stripping column. Any required heating of the feed to the desired temperature is accomplished in a number of ways, such as use of heat exchangers, direct injection of steam, or use of steam in a spray atomization means such as that described in Figure 2. In addition to or separate from heating of the feed liquid, the temperature of the stripper is achieved by direct injection of steam to the stripping column as described hereinafter. The pressure of the feed is advantageously sufficient to produce droplet sizes with a volume median diameter of less than 500 micrometers from the atomizing means.

In a preferred embodiment, the aqueous hypochlorite solution is atomized, that is reduced to small droplets, so that the HOCl is easily separated from the water. .Any atomizing means within the skill in the art is suitably used. The volume median diameter of the droplet out of the atomizing means is preferably less than 500 micrometers, more preferably less than 200 micrometers. The smaller diameter is important to maximize surface area of the droplets to enhance mass transfer of chlorine into the droplet and HOCl desorption from the droplet. Because of the potential for liquid drop entrainment by the countercurrent gas flow, the minimum droplet diameter is preferably greater than 10 micrometers, more preferably greater than 20 micrometers. The means used for atomization is preferably any spray nozzle within the skill in the art that achieves the preferred droplet size, for instance a single phase liquid spray nozzle, commercially available for example from Spraying Systems Co., or one that operates on the principle of gas expansion to break up the liquid into fine droplets as illustrated in Figure 2. A two phase nozzle advantageously employs steam, chlorine, or any other gas or combination thereof that does not interfere undesirably with the reaction and distillation.

The two phase nozzle represented by Figure 2 is of a type referred to in the art as a Y-jet nozzle. In the description steam is used as illustrative of the gas which is alternately chlorine gas or a mixture of chlorine and steam. The nozzle has a housing 201 with steam inlet port 211, liquid inlet port 212, steam connecting chamber 221, liquid connecting chamber 222, steam acceleration chamber 231, liquid acceleration chamber 232, steam expansion chamber 241, steam and liquid mixing chamber 242, and an outlet 251. Steam inlet port 211 allows connection of steam piping (not shown) to nozzle housing 201. Steam connecting chamber 221 is generally cylindrical with an inside diameter of 4 times the inside diameter of steam acceleration chamber 231 and a length sufficient to connect steam port 211 to steam acceleration chamber 231. Steam acceleration chamber 231 is generally cylindrical with an inside diameter and a length such that the steam achieves sonic velocity prior to steam expansion chamber 241 with the steam flow rate limited to the minimum amount sufficient to achieve a volume median droplet size of less than 500 micrometers from the discharge of outlet 251. Steam expansion chamber 241 has a length to diameter ratio of 5 with a diameter equal to 1-1.5 times the diameter of steam acceleration chamber 231. Steam and liquid mixing chamber 242 is an extension of steam expansion chamber 241 between outlet 251 and intersection 261 of steam expansion chamber with liquid acceleration chamber 232. Steam and liquid mixing chamber 232 has the same diameter as steam expansion chamber 231 and a length to diameter ratio sufficient for uniform steam and liquid mixing and liquid droplet formation, for example 12. Steam inlet port 211 is connected to steam connecting chamber 221, steam connecting chamber 221 to steam acceleration chamber 231, steam acceleration chamber 231 to steam expansion chamber 241, and steam expansion chamber 241 to steam and liquid mixing chamber 242 with center lines of each chamber being generally aligned. Liquid inlet port 212 allows connection of liquid feed piping (not shown) to nozzle housing 201. Liquid connecting chamber 222 is generally cylindrical with an inside diameter of 4 times the inside diameter of liquid acceleration chamber 232 and a length sufficient to connect liquid inlet port 212 to liquid acceleration chamber 232. Liquid acceleration chamber 232 is generally cylindrical with a length to diameter ratio of 5 and a length equal to the length of steam acceleration chamber 231. Liquid inlet pc 212 is connected to liquid connecting chamber 222, and liςpαid c __necting chamber 222 to liquid acceleration chamber 232 with center lines of each chamber being generally aligned. Liquid acceleration chamber 232 and steam expansion chamber 241 intersect at an angle of 45 degrees,) at intersection 261 where steam expansion chamber 241 joins steam and liquid mixing chamber 242.

One or more nozzles are used to atomize the aqueous hypochlorite liquid feed into fin., droplets as described. The number and orientation of nozzles is sufficient to provide a desirable droplet size while handling a predetermined volume of liquid. If more than one nozzle is required to handle the volume of liquid flow and meet the drop size requirements, the orientation of the multiple nozzles is such that a uniform spray of droplets over the cross-sectional area of the column is achieved while still avoiding spray onto the column walls. The atomization nozzle(s) is mounted inside the stripping column preferably near the top of the reaction/HOCl desorption section of the column, more preferably sufficiently above the trays or packing to advantageously result in reaction of the hypochlorite with chlorine (believed to take place on the liquid drop surface) followed by an equilibration of HOCl between the vapor and liquid phases (also referred to herein as HOCl desorption from the droplet) prior to entering the packing or trays of the distillation section for final stripping of the hypochlorous acid. At least two seconds of free- fall time before contacting the packing or trays in the stripping section is sufficient time for the reaction of chlorine and hypochlorite to HOCl and for the subsequent equilibration of HOCl. The nozzle is oriented to spray the hypochlorite solution downward along the centerline of the vertical distillation column to avoid contact of the liquid with the column walls. In addition, the nozzle is preferably designed with a full cone spray with cone angle of 5-30 degrees, preferably 15 degrees, and oriented such that the exit spray does not contact the walls of the column before contacting the packing of the distillation zone. Designs and orientations effective for avoiding wall contact include positioning the nozzle(s) a sufficient distance from the wall such that the cone angle of the spray and the distance to the top of the packing or tray cause the spray to impinge on the packing or tray rather than the wall of the column. A plurality of such nozzles are optionally, for instance, incorporated into a compound nozzle. Each of the multiple nozzles advantageously fed from a common header and oriented to give a broader spray pattern. These compound nozzles are advantageously then arranged in a, for example grid, pattern over the entire column such that the spray uniformly covers the cross-sectional area of the column without contacting the walls of the column. The design of the reaction/HOCl desorption section in terms of diameter and height is easily done by someone skilled in the art based on the drop size, spray cone angle, and gas volumetric flowrate.

Chlorine gas is fed into the bottom vapor space of the column above the liquid level and below the optionally packed bed. The chlorine gas is advantageously fresh chlorine (100 percent chlorine) or a mixture of fresh chlorine and recycled gas (20-90 percent by volume chlorine in the recycled gas, with the balance being water vapor and inerts (materials inert to HOCl and chlorine) such as air or nitrogen) . The fresh chlorine is advantageously provided in an amount at least stoichiometric to react the calcium hypochlorite to HOCl according to Equation 1. It is preferable to avoid having unused calcium hypochlorite because of the enhanced formation of chlorates in the presence of HOCl and hypochlorite together as shown in Equation 2. Thus, preferably, at least an amount of fresh and recycle chlorine in excess of stoichiometric to the amount of hypochlorite is used to help lower the pH of the reacted solution to less than 5.5, preferably a pH of less than 4.5, to avoid chlorate formation, more preferably the fresh and recycle chlorine is used in an amount of at least 10 times stoichiometric for reaction of chlorine with hypochlorite (that is, at least 10 moles chlorine per mole of OCl^") . The araotm^* of recycle gas is advantageously sufficient to provide a total gas to liquid feed mole ratio (that is, total moles of fresh and recycle gas divided by total moles of liquid feed, which includes moles of water, hypochlorite, and salt) of at least 0.25, preferably at least 0.7 and less than 3.0, preferably less than 2.0. This range of gas to liquid provides sufficient gas to strip the HOCl from solution, while minimizing the size of the equipment required to handle the gas. The chlorine composition of the total gas stream, including the fresh chlorine, recycle gas, and steam, is advantageously at least sufficient to provide the necessary chlorine for reaction with the hypochlorite and provide the excess chlorine needed to minimize chlorate formation while also keeping the vapor to liquid ratio in the desired ranges, that is preferably at least 20 mole percent chlorine. The maximum chlorine composition in the total gas stream is determined by the water vapor content at the desired stripper temperature, that is less than 95 mole percent, preferably less than 90 mole percent chlorine. Chlorine gas compositions in this range also provide sufficient total gas for the stripping of HOCl from the liquid.

The rising chlorine, optionally with other stripping gases, is introduced at a rate to attain a velocity in the vessel sufficient to enhance the stripping of HOCl in the distillation zone (that is, 1-8 ft/sec (0.3-2.4 m/sec) ) , but insufficient to cause flooding of the distillation zone or entrainment of liquid drops greater than 20 micrometers in diameter in the reaction/HOCl desorption zone.

Conditions of chlorination and distillation are not critical, but to enhance vaporization of HOCl, the temperature is advantageously at least 30° C, preferably at least 40° C and to avoid rapid decomposition of HOCl to chlorates, the temperature is advantageously less than 80° C, preferably less than 70° C. This temperature is partially controlled by the liquid hypochlorite solution feed temperature.

Steam is preferably added to the gas feed or recycle gas to assist in achieving the preferred temperatures. The steam is preferably used in an amount less than 3 kg steam/kg of HOCl product on a 100 percent or dry HOCl basis, more preferably less than 2.5 kg steam/kg of HOCl to minimize subsequent heat removal from the steam condensation for example in Step (d) . Steam (water vapor) introduced below the packing has two functions: to provide energy to vaporize the remaining HOCl and to provide the stripping gas for the required mass transfer. The steam advantageously is fresh steam or recycled steam, for instance from the top of the absorber column (Step d) or from other parts of the process. Sufficient steam, along with the fresh and recycle chlorine gas, is used to provide a stripping factor of 1.0-3.0, preferably 1.1-1.5. Stripping factor is defined as the relative volatility of HOCl times the mole ratio of gas to liquid. Sufficient chlorine is preferably used to maintain the liquid in the stripper at a pH of less than 5.5, more preferably less than 4.5. This amount corresponds to 0.8-3.5 kg chlorine/ kg steam, preferably 2 kg chlorine/ kg steam. This chlorine advantageously is fresh gaseous chlorine or recycled chlorine, for instance from the top of the absorber column (Step d) or from other parts of the process such as off-gas from the chlorate removal reactor (Step e) .

b) Vaporizing at least 30 mole percent of the hypochlorous acid from a liquid phase into a vapor phase containing chlorine, water vapor, hypochlorous acid, and dichlorine monoxide;

HOCl (1) ---> HOCl (g)

EQUATION 4

2 HOCl (g) ---> C1₂0 (g) + H₂0 (g)

EQUATION 5

In the practice of the invention according to the preferred embodiment which includes the nozzle(s) or atomizer means, an equilibration of HOCl between the vapor and liquid droplets (that is, the HOCl concentration in the vapor space is in equilibrium with the HOCl concentration in the liquid, also referred to herein as HOCl desorption) advantageously occurs in the reaction/HOCl desorption zone of the stripper column. The amount of HOCl desorption from the liquid to the vapor is primarily a function of temperature, pressure, HOCl concentration in the liquid, and the ratio of total moles of gas to total moles of liquid. In the practice of this invention, the maximum HOCl concentration in the liquid is determined by the concentration of hypochlorite in the feed according to the reaction stoichiometry of Equation 1. The vapor/liquid equilibrium data for HOCl partitioning between the liquid and vapor phases is well described in the literature, for example C. H. Secoy and G. H. Cady, "The Effect of Temperature and Pressure on the Solubility of Chlorine Monoxide in Water", Journal of the American Chemical Society, vol. 63, pp. 2504-8, (1941) ; H. Imagawa, "Chemical Reactions in the Chlorate Manufacturing Electrolytic Cell. Part 1: The Vapor Pressure of Hypochlorous Acid on its Aqueous Solution", Journal of Electrochemical Society of Japan, vol. 18, pp. 382-5, (1950); and H. Imagawa, "Studies on Chemical Reactions of the Chlorate Cell. Part 2 : The Vapor Pressure of Hypochlorous Acid on its Mixed Aqueous Solution with Sodium Chlorate", Journal of Electrochemical Society of Japan, vol. 19, pp. 271-4, (1951) . In addition, the vapor phase equilibrium of HOCl and dichlorine monoxide is described by H. D. Knauth et al. in "Equilibrium Constant of the Gas Reaction Cl₂0 + H₂0 = 2HOC1 and the Ultraviolet Spectrum of HOCl", Journal of Physical Chemistry, vol. 83, pp. 1604-1612, (1979) . The amount of HOCl desorption into the vapor space and prior to the packed stripping section over the preferred ranges of variables is, therefore, at least 30 mole percent, preferably at least 50 mole percent. Higher desorption percentages are advantageous to minimize HOCl decomposition to chlorates. However, the conditions of temperature and concentration necessary to achieve higher desorption percentages also lead to HOCl decomposition. Thus, in balancing these considerations to maximize total HOCl recovery while minimizing chlorate formation one generally observes less than 70 mole percent HOCl desorption before the liquid falls into the distillation section of the column. The HOCl in the vapor space is in equilibrium with dichlorine monoxide according to Equation 5. With calcium hypochlorite solution feed concentrations below 0.25 mole/liter, HOCl is dominant in the vapor phase; dichlorine monoxide dominates (that is, the dichlorine monoxide represents greater than 50 mole percent of the HOCl that was desorbed) in the vapor phase with calcium hypochlorite feed concentrations above 0.25 molar. For example a 0.9 molar calcium hypochlorite solution feed concentration reacted with chlorine gas after atomization of the liquid desorbs 67 mole percent of the product HOCl at 38° C, 500 mm Hg (66.6 kPa) pressure absolute, and a gas/liquid mole ratio of 1.5.

Sufficient head space is preferably provided above the atomization nozzle (in the vapor removal zone, also referred to herein as overheads) to allow disengagement of entrained liquid droplets from the overheads vapor containing the HOCl and dichlorine monoxide along with chlorine and water vapor; that is the gas velocity, length of column, and droplet size are controlled to allow the liquid droplets to fall downward to the stripping section of the column. To aid this disengagement, the droplet size out of the nozzle is preferably greater than 20 micrometers. The design parameters and requirements for preventing and/or eliminating entrained liquid drops from a gas are well known in the art and are described in many references, such as Chemical Engineers' Handbook. Fourth Edition, Robert H. Perry et al. , ed. , 1963, pp. 18-82 to 18-88. Liquid entrainment in the overheads gas is preferably minimized because the liquid contains salt which raises the chloride concentration of the absorbed HOCl in the next step (Step d) . Demisting devices are optionally and preferably used in the overheads vapor to remove any entrained liquid drops from the gas. Such demisting devices include chevron horizontal demisters or vertical wire mesh demisters or others as described in the above refer ce. .Any liquid removed by the demister advantageously is returned to the bottom of the stripping column for discharge with the brine.

The length of the column, especially the distance between the nozzle outlet and packing is sufficient to provide at least 2 seconds of settling time for the liquid drops from the nozzle before contacting the column walls or the packing, but less than 120 seconds. The distance between the nozzle outlet and the packing is preferably sufficient to allow reaction of the hypochlorite in the liquid with chlorine to produce HOCl with subsequent equilibration of the HOCl between the vapor and liquid phases but not great enougr. to permit decomposition of the HOCl remaining in the liquid before additional stripping can occur in the packed section of the column.

c) Distilling the remaining liquid phase hypochlorous acid solution using a stripping gas containing at least 20 mole percent chlorine to separate gaseous hypochlorous acid and dichlorine monoxide from an aqueous brine; Below the reaction/HOCl desorption zone in which the liquid calcium hypochlorite solution is introduced, is the distillation zone containing either packing or trays designed to enhance the contact of the downward liquid flow from the reaction/HOCl desorption zone with the upward gas flow introduced below the distillation section. The distillation section, also referred to herein as a stripping section, is preferably packed. Although a trayed column is suitably used for the gas/liquid contacting for the stripping, trays often have higher liquid residence times than packing which leads to additional decomposition of HOCl to chlorates and are therefore not preferred in the practice of this invention. The packing, in a preferred embodiment, is suitably any commercially available material such as random packing (saddles, pall rings, etc.) or structured packing (Goodloe column packing, a wire mesh packing commercially available from Metex Corporation^. Sulzer packing, a corrugated sheet packing commercially available from Koch Engineering Company, Inc.; and the like) and is preferably of a material resistant to corrosive chemicals, particularly HOCl and C1₂0 (such as plastic, ceramic, titanium, and the like) . The preferred packing material is that which achieves the maximum mass transfer with the minimum of liquid residence time such as structured packings. At least 1 theoretical transfer unit in addition to the initial HOCl desorption from the reaction/HOCl desorption zone is used to effect an overall HOCl recovery (combination of HOCl from the reaction/desorption zone and from the distillation zone) of at least 70 mole percent based on hypochlorite in the feed, preferably at least 80 mole percent.

In the practice of the invention, distillation is accomplished at least partially through use of a stripping gas. Conveniently, the stripping gas is or includes the chlorine used to react with the hypochlorite. It is introduced as explained in Step (a) of contacting. While the stripping gas optionally contains other than chlorine, such as water vapor, nitrogen or air, it contains at least 20 mole percent chlorine to react with the incoming hypochlorite, more preferably at least 30 mole percent chlorine. While the pressure at which the distillation takes place is not critical, it is preferably minimized to allow easy distillation of the HOCl and yet maximized to keep the size of the distillation equipment small. The pressure of the column is, therefore, advantageously greater than 150 torr (20 kPa) , preferably greater than 300 torr (40 kPa) and less than 1250 torr (167 kPa) , preferably less than 1000 torr (133 kPa) .

Overhead vapor (HOCl and C1₂0) with excess chlorine, water vapor and inert gases preferably goes to produce an aqueous solution of HOCl (Step d) while the chloride brine exits the bottom of the distillation apparatus in the liquid removal zone and is preferably sent forward to brine treatment for chlorate removal (Step e) . The brine contains some residual, unstripped HOCl and dissolved chlorine in an amount less than 5000 ppm by weight, preferably less than 2000 ppm, more preferably less than 1000 ppm by weight. A reactor/distillation apparatus designed and operated as described advantageously provides HOCl recovery yields based on the value of the initial hypochlorite used of greater than 80 mole percent (which represents the combined recovery from the reaction/desorption section and the distillation section) .

d) Optionally absorbing the gaseous hypochlorous acid and dichlorine monoxide into water to produce an aqueous solution of hypochlorous acid

The reactions in this step (d) are the inverse of those mentioned for the reactor/distillation column (Steps a-c) (Equations 4 and 5) ; HOCl and Cl₂0 vapors are absorbed into water in this Step (d) . Decomposition of HOCl to chlorate in the final HOCl solution is minimal because the HOCl solution is substantially free of chloride ions, that is preferably less than 1000 ppm chlorides, more preferably less;than 500 ppm chlorides, and most preferably less than 200 ppm chlorides. To produce a low chlorides HOCl solution substantially free of chloride ions, which is highly preferred in the practice of the invention, the water used for absorption is also very low in chloride concentration preferably less than 1000 ppm, more preferably less than 500 ppm chlorides, and most preferably less than 200 ppm and the liquid entrainment from the distillation step (Steps a-c) is minimized as previously discussed. The term "low-chloride hypochlorous acid" or "HOCl solution" is used to refer to a solution of hypochlorous acid in water having a hypochlorous acid concentration of at least 1 weight percent, preferably at least 3 weight percent and less than 10 weight percent, preferably less than 7 weight percent, which is substantially free of chloride ion. Chloride ions are preferably in low concentrations because they contribute to the undesirable production of chlorinated organic compounds when the hypochlorous acid solution is used, for example, to react with olefins to produce chlorohydrins and they accelerate decomposition of HOCl to chlorates.

To form the aqueous HOCl solution, HOCl and C1₂0 vapor from the reactor/distillation column are advantageously fed countercurrent to fresh (meaning water with low chlorides as defined) water in a column (hereinafter absorber) . The water enters the top of the absorber above any packing or trays, preferably at a temperature in the range of from 10° C to 60° C, more preferably at 40° C. The HOCl solution in the bottom of the column is preferably cooled by means within the skill in the art such as by use of a heat exchanger, to temperatures of from 30° C to 60° C, preferably to 40° C. This cooled HOCl solution is partially recycled to the column, preferably in the center portion of the column to help remove the heat of absorption of the HOCl and water vapor in order to maintain the column temperature below 60° C, preferably below 50° C. These temperatures are maintained to minimize the decomposition rate of HOCl to chlorates and to enhance the absorption of the HOCl into the water.

The absorber is advantageously either a packed bed or trayed column, preferably a packed bed column. The packing advantageously is a random packing such as pall rings or saddles or structured packing such as those described for the stripper in Step a-c. The packing is preferably made of an advantageous corrosion resistant material such as ceramic, Teflon™ polytetrafluoroethylene commercially available from E. I. du Pont de Nemours and Company, Kynar™ polyvinylidene fluoride commercially available from Pennwalt Chemicals Corp., or titanium. The packing is advantageously in at least two sections with liquid distributors on the top of each section to ensure the packing is fully wetted to provide optimum gas/liquid contacting. The top section distributes the fresh water into the column, while the bottom section distributes liquid from the top packed section and the recycle HOCl solution from the heat exchanger. Fresh water enters the top of the absorber, while gas from the stripper of Step a-c enters below the bottom packed section. Liquid HOCl solution in the bottom of the column is cooled in an external heat exchanger and partially recycled to the column as described above. Uncondensed gases including for example chlorine, water vapor, and inert materials such as air or nitrogen, exit the top of the absorber where it advantageously is partially recycled to the stripper of Step a-c and partially removed for scrubbing to remove chlorine prior to venting to the atmosphere.

The column is preferably operated at 50-1250 mm Hg (6.7-166.7 kPa) absolute pressure, more preferably 300-1000 mm Hg (40-133.3 kPa) , most preferably 500-760 mm Hg (66.7-101.3 kPa) using a vacuum source. Discharge gases from the vacuum source preferably are scrubbed for example with calcium hydroxide (lime) slurry or sodium hydroxide (caustic) or otherwise treated to remove any chlorine prior to venting. Liquid effluent from such a scrubber, containing an aqueous mixture of metal hypochlorite, unreacted metal hydroxide, and metal chloride is optionally recycled to a chlorination reaction such as with an hydroxide or oxide, for example lime for hypochlorite generation.

The product of this step (d) is preferably a solution of HOCl in water having a concentration of from 1 to 10 weight percent, more preferably from 3 to 7 weight percent HOCl, most preferably 4-5 weight percent HOCl, preferably having less than 1000 ppm chloride more preferably less than 500 ppm chloride, most preferably less than 200 ppm chloride at 40-60° C. Sufficient water is used to achieve these concentrations of HOCl. When the absorber column apparatus is as described for the preferred embodiment, the solution exits the bottom of the absorbing apparatus. The solution is optionally fed forward for example to be reacted with an olefin. Chlorine and water vapor are preferably removed overhead from the absorber column and preferably are recycled to the HOCl stripper (Step a-c) for instance via a mechanical blower or eductor. Alternatively, however, the water vapor and other materials are otherwise treated or recovered; for instance, the water vapor is optionally condensed, all by means within the skill in the art.

e) Optionally admixing the aqueous brine from Step (c) to convert chlorates therein to chlorine.

Chlorates in the chloride brine from the HOCl stripper (Steps a-c) are optionally reduced by a variety of processes within the skill in the art.

Preferably, however, chlorates are advantageously removed by reaction with HC1 or other acids to yield HOCl (Equation 6) with further reaction to chlorine (Equation 7) which is optionally stripped from the brine (Equation 10) . The reactions are illustrated by:

HC10₃ (1) + 2HC1 (1) > 3HOC1 (1) EQUATION 6

HOCl (1) + HC1 (1) ---> Cl₂ (1) + H₂0 (1)

EQUATION 7

The overall reaction is then:

HCIO3 (1) + 5HC1 (1) ---> 3C1₂ (1) + 3H₂0 (1)

EQUATION 8

R. Dotson reports in "Kinetics and Mechanism for the Thermal

Decomposition of Chlorate Ions in Brine Acidified with Hydrochloric Acid", Journal of Applied Chemical Biotechnology, vol. 25, 1975, pp. 461-464, that Equation 6 is the rate limiting step and that the kinetics are represented by the following empirical expression with units of moles/liter, °K, and minutes:

-dtCl0₃-]/dt = 1.83 x 10¹⁸ _e ⁽' ⁵⁰⁵⁶/^RT) [C103-] [H⁺]₂ e^[cl"l

EQUATION 9

The above expression is advantageously used to design an advantageous reactor for reduction of chlorates. For example, for 99 mole percent reduction in chlorates, 140° C, pH = 1, [Cl^~] = 4 moles/liter, the required residence time is 16.5 minutes. The reaction conditions are advantageously adjusted to meet the residence time requirements of the process. For example, higher tern- rature and lower pH lowers the residence time while lower temperature and higher pH lengthens the time required for reduction of the chlorates.

An advantage of reducing the chlorates to chlorine using HC1 is that the chlorine is optionally degassed from the solution and recovered for use, for example, in producing the hypochlorite from for example a metal hydroxide or oxide for feed to Step a or in the stripping column of Steps a-c of the process of the invention, thus increasing the overall yield from chlorine.

Cl₂ (1) ---> Cl₂ (g)

EQUATION 10

After the reaction is complete, the product acidic calcium chloride brine is advantageously neutralized, preferably with either calcium hydroxide slurry to a pH of 5-6.5 or alkali metal, especially sodium, hydroxide to a pH of 5-8 before discharge. The resulting brine is optionally cooled for energy recovery prior to discharge.

Those skilled in the art will recognize that in each step of the process of the invention there are chemicals which are often corrosive to steel and the like; therefore, apparatus in contact with such chemicals is advantageously made of or lined with materials resistant to the chemicals such as polytetrafluoroethylene, Kynar™ polymer (which is polyvinylidene fluoride, commercially available from Pennwalt Chemicals Corp.), titanium, or other resistant material.

Practice of the process of the invention advantageously results in the continuous preparation of aqueous solutions of low chlorides (less than 200 ppm by weight) hypochlorous acid in high yields (greater than 80 percent from hypochlorite) . The process of the invention improves on processes of prior art by operating at low reaction/distillation temperatures (40-70° C) ; operating at high HOCl absorber temperatures (40-60° C) ; producing an aqueous brine solution instead of a solid salt by-product; avoiding large heating/cooling cycles on the recycle gas; and providing for recovery of chlorine value from by-product chlorates.

The following examples are included to illustrate but not limit the invention. Examples of the invention (Ex.) are designated numerically. In these examples, all ratios, parts, and percentages are by weight unless otherwise designated.

EXAMPLE 1: DEMONSTRATION OF THE HIGH YIELD RECOVERY OF HOCl FROM HIGH CONCENTRATION CALCIUM HYPOCHLORITE BY USING A REACTIVE SEPARATION PROCESS

An aqueous calcium hypochlorite solution containing 10.6 weight percent calcium hypochlorite and 0.919 weight percent calcium chlorate was pumped continuously at 14.8 lb/hr (6.7 kg/hr) through a heat exchanger to heat the liquid to 82° C, and then to a titanium liquid spray nozzle (commercially available from Spraying Systems Co. under the trade designation, model 1/4 LN-2Ti-1.5 hydraulic atomizing nozzle) which was located along the axial centerline and 27 inches (686 mm) from the top of an insulated, vertically mounted, glass column (hereafter referred to as the stripper) . The stripper had three contiguous sections, a topmost section, a middle section and a bottom section. The topmost section was a 27 inch (686 mm) long, 6 inch (152 mm) inside diameter glass column which contains approximately 8 inches (203 mm) of 1/4 inch (6.35 mm) ceramic saddle packing (commercially available from Norton Company under the trade designation Intalox™ saddles) resting on a perforated glass support approximately 9 inches (23 mm) from the very top of the column. Excess chlorine gas along with an equilibrium mixture of HOCl, Cl₂0 and water vapor exit out the top of this section. The middle section of the stripper (hereafter referred to as the

"reaction/desorption zone") consists of a 35 inch (889 mm) long, 12 inch (305 mm) inside diameter glass column into which droplets of liquid exiting the previously mentioned atomization nozzle were sprayed. The bottom section of the stripper was a 58 inch (1473 mm) long, 6 inch (152 mm) inside diameter glass column which does not contain packing but does contain a perforated glass packing support ar roxiιr,ata±y J8 inches (33 mm) from the very bottom of the stripper. 1 botx.j.n section was equipped with: (a) one 1/2 inch (12.7 mm) c nipple located 9 inches (23 mm) above the packing support to i ,uce fresh chlorine gas into the system, (b) a second 1/2 inch ( mm) glass nipple located 13 inches (33 mm) below the packing si rt which was optionally used to introduce fresh condensate, (c) a tl...d 1/2 inch (12.7 mm) glass nipple located at the very bottom of the column for removing aqueous brine solution, and (d) a 2 inch (51 mm) nipple located 9 inches (23 mm) below the packing support which was used to introduce chlorine gas and small amounts of water vapor recycled from a HOCl absorber (described hereafter) .

Spray nozzle outlet flow (6.7 kg/hr at 82° C) was directed downward as a roughly cone-shaped pattern of atomized droplets averaging 100 microns (100 micrometers) in diameter (droplet size taken from Spaying Systems performance data at these flow conditions) that subsequently fall into the top of the reaction/desorption zone, which was maintained at 460 mm Hg (61.3 kPa) absolute pressure and 38° C. Fresh chlorine gas at 12.5 lb/hr (5.65 kg/hr); recycle gas (containing 90.9 volume percent chlorine, with the balance of the gas being 4 volume percent water vapor and 5 volume percent air) at 4 actual cubic feet per minute (0.0005 m³/sec at 0° C and 101.325 kPa) at 35° C; 0.0 lb/hr (0.0 g/sec) steam, and fresh condensate at 68 lb/hr (30.8 kg/hr) were continuously introduced near the bottom of the column via the connections described above. Aqueous calcium chloride brine containing small amounts of chlorates and HOCl was pumped from the bottom of the column at 83.2 lb/hr (37.8 kg/hr) . This brine was found by ion chromatography to contain 0.311 weight percent calcium chlorate and by iodometric titration to contain 0.311 weight percent HOCl.

Vapor containing chlorine, water, HOCl, chlorine monoxide, and air continuously passed out the top of the stripping column and flowed via a 2 inch (51 mm) pipe to a 2 inch (51 mm) glass nipple connection located 10 inches (254 mm) above the very bottom of an uninsulated, vertically mounted glass column (152 mm inside diameter, 3048 mm overall length and hereafter referred to as the absorber) operating at 418 mm Hg (55.7 kPa) pressure absolute and between 28-38° C.

The absorber contained two, vertically aligned packed sections separated by a distance of 457 mm. Each was filled with ceramic saddle-shaped packing (commercially available from Norton Company under the trade designation Intalox™ saddles) which was used to provide good contact between chilled water and the incoming vapor in order to enhance the absorption of HOCl into water. The lower packed section of the absorber which consisted of a 380 mm bed of 1/2 inch (12.7 mm) Intalox™ saddles resting on a perforated glass packing support approximately 8 inches (203 mm) above the vapor entrance to the absorber. The upper packed section consisted of a 1300 mm (height) bed of 1/4 inch (6.35 mm) saddles resting on a perforated glass packing support approximately 18 inches (457 mm) above the lower section of packing.

Essentially all of the incoming water vapor and greater than 90 percent of the HOCl and C1₂0 coming from the top of the stripper and supplied to the absorber below the lower packing were condensed in the lower packed section. This was achieved primarily by recirculating 90-95 percent of the absorber bottoms material (aqueous solution collected at the bottom of the absorber containing between 2 and 3 weight percent HOCl) through a shell and tube heat exchanger at 5-10 liters/min using a centrifugal pump. The absorber bottoms were chilled in the exchanger to approximately 15-20° C (using ethylene glycol at 5-10° C as a refrigerant) and then fed back onto the top of the 1/2 inch (12.7 mm) saddles. The remaining amount of the HOCl solution that was not recirculated (32.5 kg/hr) was recovered as product.

Fresh condensate water at 15-20° C was fed at a 8.81 g/sec rate to the top of the absorber column where it was then distributed across the top of the upper packed section. Remaining amounts of HOCl and/or C1₂0 vapor were recovered in this upper section of packing by . intercurrent contact with the chilled condensate, while the unabsorbed chlorine (which was saturated with water vapor at 34° C and 55.7 kPa absolute) and air was recycled back to the bottom of the stripper by using a combination barometric eductor/vapor-liquid separator operating at 2 psig (115 kPa) . The liquid leaving the bottom of this upper section of packing (which contained less than 0.5 weight percent HOCl) was redistributed across the top of the 1/2 inch (12.7 mm) saddles previously mentioned.

The absorber bottoms liquid that was sent to waste disposal (3.25 kg/hr) was sampled via a 1/8 inch (3.175 mm) diameter piece of tubing (made from fluorocarbon polymer, commercially available from E. I. du Pont de Nemours & Co. under the trade designation Teflon™ PFA) attached to the waste line just downstream of the bottoms pump discharge. After purging the sample line for 15-30 seconds, 5-10 g samples were caught in a chilled vial. These samples were found by iodometric titration to contain 2.175 weight percent HOCl (which includes trace amounts of dissolved free chlorine) and by ion chromatography to contain 0.003 weight percent calcium chlorate. The OCl" species material accountability (that was the combined total moles of OCl" leaving bottom of both the stripper and the absorber as HOCl and stoichiometric equivalent chlorates divided by the total moles of OCl^" entering the system as Ca(OCl)₂ fed to the reactor) for this example was 99.4 percent and the recovered HOCl yield (based on Ca(OCl)₂ fed to the reactor) was 67.8 percent.

This example shows that high recovered yields of HOCl are possible by reacting atomized droplets of aqueous calcium hypochlorite in a high chlorine concentration gaseous environment, while simultaneously desorbing the HOCl product away from the reacting droplets followed by recovering the HOCl in a packed absorption tower using chilled water. The recovery of hypochlorous acid from atomization into an unpacked column as in the procedure of this example was a measure of the hypochlorous acid vaporized from the droplets (Step (b) of the present invention) .

EXAMPLE 2: DEMONSTRATION OF THE EFFECT OF EXCESSIVELY HIGH HYPOCHLORITE FEED TEMPERATURE UPON RECOVERED HOCl YIELD

Feed, 10.1 weight percent aqueous calcium hypochlorite solution, was preheated to 119° C and was then fed at 14.0 lb/hr (6.3 kg/hr) to the separation system described in Example 1, subjected to similar conditions except that the stripper column reaction/desorption zone temperature was maintained at 52° C in part by the addition of 2 lb/hr (0.9 kg/hr) of 5 psig (136 kPa) saturated stripping steam. Calcium chloride brine was removed from the bottom of the stripper at 83.7 lb/hr (37.9 kg/hr) and was found to contain 0.343 weight percent calcium chlorate and 0.098 weight percent unstripped HOCl. Product exited from the absorber bottom at 72.5 lb/hr (32.9 kg/hr), and analyzed (by the same iodometric titration procedure used in Example 1) to contain 1.78 weight percent HOCl, representing a 62 mole percent yield.

This Example shows that preheating a high concentration (greater than 9-10 weight percent) aqueous calcium hypochlorite feed to temperatures of 119° C before reaction can cause increased yield loss to chlorates. EXAMPLES 3-9: DEMONSTRATION OF THE EFFECT OF RECYCLE CHLORINE GAS FOR INCREASING THE RECOVERED YIELD OF HOCl

A series of Examples were run using a hypochlorite feed .. 15.0-15.5 lb/hr (6.8-7.0 kg/hr) at 10.6 weight percent calcium hypochlorite (specific gravity of 1.182) in the procedure of Example 1 wherein fresh chlorine was supplied at a rate of 81 mole/hr. Varying recycle gas rates from the absorber to the stripper were used, and were expressed as a dimensionless ratio of the molar gas flow rate (mole/hr) to the liquid feed molar flow rate (mole/hr) . The molar gas flow rate represented in the vapor/liquid mole ratio in Table 1 includes only the measured amount of chlorine gas and water vapor being recycled back to the stripper from the absorber. The results were shown in Table 1 below:

Table 1 The Effect of Chlorine Recycle on HOCl Yield

Example Calcium Vapor/Liquid Chlorine in Recovered No. Hypochlorite Mole Ratio Recycle Gas HOCl Yield

Feed Rate (mole percent) (percent)

(kg/hr)

3 6.8 0.40 61 51

4 6.8 0.40 61 53

5 7.0 0.39 61 50

6 7.0 0.58 88 56

7 6.8 0.61 85 58

8 6.9 0.96 95 67

9 7.0 1.09 91 68

The data from Table 1 show that increasing the ratio of chlorine recycle gas flow to hypochlorite feed rate increases the amount of recovered HOCl yield increasing the rate at which HOCl was stripped from the atomized droplets.

EXAMPLES 10-13: DEMONSTRATION OF THE IMPROVEMENT IN RECOVERED HOCl YIELD ACHIEVED BY REACTIVE, MULTISTAGE DISTILLATION OF Ca(OCl) OVER THAT ACHIEVED BY A SINGLE STAGE REACTIVE DESORPTION The separation and analysis procedures of Example 1 were repeated under similar conditions except that: (a) the bottom section of the stripper was modified to 4 inches (102 mm) inside diameter and filled to a depth of 40 inches (1016 mm) with 1/4-inch (6.35 mm) Intalox™ ceramic saddle packing, and (b) steam (0.315 - 0.327 g/sec) at 5 psig (34.5 kPa gauge) was introduced into the stripper along with the recycle vapor from the absorber via the 51 mm glass nipple just below the bottom of the packing (as described in Example 1) . This was done to achieve multiple equilibrium stages of separation between HOCl and the calcium chloride brine within the stripper; and thus to obtain a higher recovered yield of HOCl than what was possible by using a single stage reaction/desorption employing the spray atomization nozzle alone (as was done in Example 1) . The results are summarized in Table 2 below (Note: as in Examples 3-9, the vapor-to-liquid ratio is the dimensionless ratio of the measured molar flow rate of recycled absorber gas in mole/hr to the molar flow rate of the incoming hypochlorite feed to the atomization nozzle in mole/hr.) :

Table 2 Improvement In Recovered HOCl Yield Effected By The Use of

Multistage Reactive Separation

EXAMPLE Ca(OCl)₂ Ca(OCl) Steam Vapor/ Mole HOCl No. (weight Rate (g/sec) Liquid Percent Cl Yield percent) (g/sec) Mole in Recycle (percent

Ratio Gas )

10 11.55 1.95 0.315 1.35 >90 79.4

11 11.54 1.89 0.327 1.40 >90 80.0

12 11.70 1.95 0.327 1.35 >90 79.3

13 11.60 1.74 0.327 1.52 >90 83.8

Note: Operating pressure and temperature of the stripper column reaction/desorption zone for these runs were 87 kPa and 51 °C and > is used to represent "greater than . " The data for this example show that a multiple equilibrium stage reactive separation process (achieved by employing the use of packing and small amounts of steam in the stripper in addition to a atomization spray nozzle) can raise the recovered yield of HOCl by 10- 12 absolute percentage points above the yields obtainable by using a single equilibrium stage separation device (that is the atomization spray nozzle as employed in Example 1) alone.

Claims

We Claim :

1. A continuous process of preparing hypochlorous acid characterized in that droplets of an aqueous solution of a metal hypochlorite having a volume median diameter of less than 500 micrometers is contacted with chlorine gas to produce hypochlorous acid, at least 30 weight precent of which hypochlorous acid is vaporized into a vapor phase containing chlorine, water vapor, hypochlorous acid dichlorine monoxide leaving a liquid phase aqueous hypochlorous acid solution.

2. A continuous process of preparing hypochlorous acid characterized in the combination of steps:

(a) contacting droplets of an aqueous solution of a metal hypochlorite having a volume median diameter of less than 500 micrometers with chlorine gas to produce hypochlorous acid;

(b) vaporizing at least 30 weight percent of the hypochlorous acid into a vapor phase containing chlorine, water vapor, hypochlorous acid and dichlorine monoxide and leaving a liquid phase aqueous hypochlorous acid solution; and

(c) distilling the liquid phase hypochlorous acid solution using a stripping gas containing at least 20 mole percent chlorine to separate vapor phase hypochlorous acid and dichlorine monoxide from an aqueous brine.

3. The process of Claim 1 or 2 wherein Steps (a) , (b) , and (c) occur simultaneously in an apparatus.

4. The process of Claim 1 or 2 wherein at least 70 percent of the hypochlorous acid is vaporized and distilled into a vapor phase.

5. The process of Claims 1, 2, or 4 wherein at least 80 percent of the hypochlorous acid is vaporized into a vapor phase, the molar ratio of stripping gas to liquid feed is from 0.25 to 3.0.

6. The process of Claims 1, 2, 3 or 4 wherein the metal hypochlorite is an alkali metal or alkaline earth metal hypochlorite.

7. The process of Claim 6 wherein the metal hypochlorite is calcium hypochlorite.

8. The process of Claims 1, 2, 4, or 7 wherein the aqueous solution of metal hypochlorite has a concentration of at least 1 molar, the process takes place at a temperature of from 40° C to 80° C.

9. The process of Claims 1, 2, 3, 4, 5, 6, 7 or 8 wherein the droplets are produced using a two phase nozzle.

10. The process of Claim 9 wherein the droplets have a volume median diameter of less than 200 micrometers.

11. The process of Claim 10 wherein the droplets have a volume median diameter of greater than 20 micrometers.

12. The process of Claim 1, 2, 4 or 7 wherein the process additionally comprises a step (d) of absorbing the gaseous hypochlorous acid and dichlorine monoxide into water to produce an aqueous hypochlorous acid solution.

13. The process of Claim 12 wherein the process additionally comprises a step (e) of admixing the aqueous brine from Step (c) with water to convert chlorates therein to chlorine.

14. The process of Claims 1, 4, 7, 12 or 13 wherein the chlorine produced in Step (e) is recycled for use in production of hypochlorite or for use in any of Steps (a) , (b) , or (c) .

15. An apparatus suitable for the preparation of hypochlorous acid comprising:

(a) means for forming droplets of an aqueous solution of a metal hypochlorite having a volume median diameter of less than 500 micrometers; and

(b) means for distilling liquid aqueous hypochlorous acid solution using a stripping gas containing at least 20 mole percent chlorine.

16. The apparatus of Claim 23 wherein the means for forming droplets is a two phase nozzle.

17. An apparatus suitable for the preparation of hypochlorous acid by the process of Claim 1 or 2 comprising: (a) an elongated, generally vertically extending reactor vessel having a top and an opposing bottom, and a central axis therebetween, said reactor vessel having an upper reaction/desorption zone and a lower distillation zone;

(b) means for removing vapor connected to the top of the reactor vessel;

(c) means for forming droplets of an aqueous solution of a metal hypochlorite having a volume average diameter of less than 500 micrometers connected to the reactor vessel below the means for removing vapor and within or near an upper boundary of the reaction/desorption zone;

(d) outlet means for aqueous brine connected to a lower portion of the distillation zone;

(e) a chlorine infeed line connected to the distillation zone above the outlet means for aqueous brine.