ΓROCESS FOR REMOVAL OF IODINE FROM A GASEOUS STREAM This invention relates to a process for removal of molecular iodine from a gaseous stream. Iodine is one of the most useful halogens for the production of high value chemicals. For example, processes for the production of acetic acid and acetic anhydride use organic and inorganic iodine—containing compounds as intermediates, promoters and catalysts. Also, oxyiodination processes producing iodoaromatic derivatives are known and these iodoaromatic derivatives can be converted to useful carboxylic acid derivatives or aryl sulfide derivatives. In all of these processes iodine—containing gas stream are produced and often the iodine is present as molecular iodine.

The gas containing the molecular iodine often contains oxygen in addition to molecular iodine. For example, vent gas streams from an oxyiodination process or gas streams resulting from the combustion of organic iodine—containing species such as methyl iodide or acetic acid or anhydride tars often contain both oxygen and molecular iodine. Removal and recovery of iodine from these streams is desirable because of the detrimental effects of iodine vapor on the environment and because of the high cost of iodine.

U.S. 3,943,229 describes the use of certain cross- linked anion exchange resins for the removal of iodine and iodine—containing compounds from the gas phase.

U.S. 3,838,554 describes the use of metal salt- impregnated sorption agents to remove iodine from gas streams.

U.S. 3,992,510 describes the use of a basic aqueous sodium thiosulfate solution for the removal of iodine from gas streams.

U.S. 4,434,241 describes a process that contacts the iodine—containing gas stream with aqueous alkali hydroxide. U.S. 3,762,133 describes the use of fluorocarbon absorbents as scrubber fluids for the removal of iodine and its volatile compounds from gas streams.

We have discovered a process which not only provides for facile removal of molecular iodine from gas streams but also provides for a facile and safe recovery of the iodine in a form suitable for reuse. The process produces no salts and does not use organic solvents. The process is self—contained and thus does not present pollution considerations. One object of this invention is to provide a process to remove and recover iodine from gas streams, particularly those which contain molecular oxygen. Another object of this invention is to provide a process to remove and recovering iodine from gas streams without the use of organic reagents or solvents and thus minimize the risk of contamination and fire. A further

object of this invention is to provide a process for the removal and recovery of molecular iodine from gas streams which does not consume chemical reagents or produce unwanted chemical byproducts and, therefore, is economical and has a minimal effect on the environment. In summary, the process of this invention can be thought of as a process comprising

(A) contacting, within a contacting zone, a gaseous stream containing molecular iodine and a water stream containing ionic iodide,

(B) removing from the contacting zone a gaseous stream containing substantially no molecular iodine or ionic iodide and a water stream which contains iodide, and (C) removing molecular iodine from the water stream which contains iodide by heating. The first step of the process of this invention involves contacting a gaseous stream containing molecular iodine and a water stream containing ionic iodide.

The gaseous stream containing molecular iodine can comprise any gaseous stream suitable for practice of this invention which contains molecular iodine. The gaseous stream may contain other compounds including those present in air or air that has been depleted of or enriched in oxygen. Although the invention has

particular utility because it does not use organic materials, some organic materials can also be present in the gaseous stream. Excessive quantities of some oxides of nitrogen are detrimental due to oxidation of the ionic iodide to iodine. Acidic species, such as HCL,

HI, or H S, may be present in the iodine—containing gas stream. The gas may also contain entrained solids such as those found in smoke, and these are often removed from the gas to some extent when the gas is contacted with the aqueous ionic iodide solution. Unlike many processes of the prior art, water vapor may be present in the gas stream. Preferably the gaseous stream is composed of air or air that has been depleted in oxygen containing combustion products such as carbon dioxide and water along with the molecular iodine.

The amount of molecular iodine in the gaseous stream can range from less that 10 ppm up to the saturation level of the gas, which is determined by the temperature and pressure of the gas. The temperature of the gaseous stream can range from 0°C to 35°C. In a preferred embodiment a heated gas stream is first cooled before it is contacted with the ionic iodide—containing aqueous solution. If the gas is prechilled below its dew point, then some of the iodine can be removed and recovered at this stage. The prechilling and partial recovery can be accomplished by

a cold solid surface or by a water scrubber or similar means.

The pressure of the gaseous stream can range from 0.1 atmosphere or less to 100 atmospheres or greater. Low pressure are not preferred because of evaporation of the water from the ionic iodide containing aqueous solution. Preferred pressure range from the iodine- containing gas is 1—100 atmospheres and most preferred is 1—10 atmospheres. One atmosphere operation is satisfactory in most instances.

The water stream containing ionic iodide is comprised of water and an ionic iodide. The ionic iodide is preferably one that has appreciable solubility in water. Ionic iodides from groups IA and IIA of the periodic table are preferred. Ammonium iodide and hydrogen iodide also can be used. Ionic halides reactive towards hydrolysis are not preferred. Alkali metal iodides are more preferred due to their excellent solubility in water, their stability toward hydrolysis and their nonvolatile nature. The most preferred ionic metal iodides are those of lithium, sodium or potassium.

The concentration of the ionic iodide can range from below 1% by weight to that approaching the saturation point of the solution at the use temperature of the process. More preferred concentrations of the lithium, sodium or potassium iodides range from about

30% by weight to 70% by weight depending on the metal ion and the temperature used, and the most preferred concentrations of these metal iodides ranges from 50% by weight to 60% by weight depending on the metal ion and the temperature used.

The gaseous stream containing molecular iodide and the water stream containing ionic iodide are contacted at a temperature in the range of 0°C to 100°C, preferably 0 to 50°C and most preferably 0 to 35°C. Generally lower temperatures are preferred when possible because one of the equilibria involved in the invention which can be represented:

I ₂ (aq) + I (aq) = I ₃ (aq)

and aqueous triiodide is favored at low temperature. Other practical considerations, such as salt solubility and energy requirements, may alter the optimum concentrations and temperatures outside of these ranges depending on the actual nature of the gas stream involved.

The gaseous stream containing molecular iodine and the water stream containing ionic iodide can be contacted within a contacting zone using any conventional means which results is adequate contact to enable practice of the invention. For example, contact

of the iodine—containing gas with the aqueous ionic iodide may be static or dynamic. Dynamic operation is preferred, and may involve situations where the gas is stationary and the liquid is moving, where the gas is moving and the liquid is stationary or, where both the gas and liquid are moving. The situation where both the gas and the liquid are moving is preferred, and the liquid should be dispersed to allow maximum feasible liquid surface to gas contact. This can be accomplished by a conventional scrubber. Several scrubbers or other gas—liquid contact devices may be used in sequence to insure thorough removal of trace amounts of iodine. The process may be operated in either the batch or continuous mode. The second step of the process of this invention involves removing from the contacting zone a gaseous stream containing substantially no molecular iodine or ionic iodide and a water stream which contains iodide. The method of removal depends upon the mode of original contact of the gaseous stream and the water stream. If the initial mode of contact is static, then, after equilibration, the gaseous stream may be displaced from the contact zone by another gas, and the contacting and removal processes can be repeated until the water stream becomes saturated beyond the desired degree at which time the water stream may be pumped or drained from the

contact zone. In the dynamic case where the gaseous stream is stationary and the water stream is moving, the gaseous stream is removed by the same method used in the static case and the water stream can be removed in the same way as in the static case or it can be pumped or drained from the zone with a portion being removed for iodine recovery while fresh solution is added. In the dynamic case where the water phase is stationary and the gaseous stream is moving, the gaseous stream is preferably continuously bubbled through the water phase and allowed to vent from the contact zone, and the water phase can be drained or pumped from the contact zone after the maximum desired degree of saturation has been reached. In the preferred case where both the gaseous stream and water stream are moving, the water stream moves countercurrent to the gaseous stream and is drained or pumped from the contact zone while the gaseous stream vents from the top of the contact zone. The water stream, once removed from the contacting zone, may be recycled to the contacting zone or processed for iodine recovery, or a portion removed for iodine recovery while fresh solution is added to the recycling liquid stream.

The third step of the process of this invention involves removing molecular iodine from the water stream which contains iodide. Recovery of the molecular iodine

is accomplished by heating the aqueous mixture produced by the contact of the iodine—containing gas with the aqueous ionic iodide solution. While not wishing to be bound by theory, we believe that heating the solution changes the position of the above shown equilibrium to disfavor triiodide. The vapor pressures of the components existing are such that molecular iodine steam sublimes or distills, depending on the pressure, from the heated solution. The continuous removal of iodine in this manner continually displaces the equilibrium as the steam is condensed and optionally returned to the solution in part or entirely via a refluxing or distilling action while the molecular iodine is collected as a condensate that is not returned to the solution. The condensed water may be returned to the water stream in part or entirely, or it may be collected in part or entirely as a distillate. The iodine recovery process is faster if at least some of the water stream is collected as distillate. The condensed water then may be returned to the residual salts after the recovery process is completed. Typically, the water stream is heated to boiling temperature at one atmosphere pressure and the iodine is recovered as a crystalline sublimate. In one embodiment of the process the pressure of the reaction is raised to two to five atmospheres so temperatures may be achieved that allow

the iodine to be condensed as a liquid above its melting point of ca. 113°C while the condensed water is returned to the solution and/or collected as distillate. Preferably, the molecular iodine is removed from the water stream by heating to boiling at one atmosphere, condensing the iodine as crystalline sublimate and collecting a portion of the water as distillate.

Broadly, the temperature of the water stream which contains iodide ranges from 40 to 150 ^P C, preferably 100 to 150°C and the pressure ranges from 0.1 to 5 atmospheres, preferably 1 to 5 atmospheres.

The iodine recovered using the process of this invention can be used in many process without further purification or may be further refined by techniques known to those skilled in the art.

The following example is presented to illustrate practice of this invention but is not intended in any way to limit the scope of the invention.

A scrubber was constructed as follows. The top of a l inch O.D. by 14 inch long glass Vigreax column was fitted with a liquid inlet line extending 1 to 1/2 inches down into the column. A gas vent port was also provided at the top of the column. The other end of the tube was connected to the outlet port of a pump. Another length of tubing was attached to the inlet side of the pump, and the opposite end of this line was

inserted through one of the necks of a 5—liter 3—necked flask and extended to the bottom of the flask. The bottom end of the assembled column was fitted to one of the other necks of the 5—liter 3—necked flask. The 5—liter 3—necked flask was loaded with a solution prepared from 500 grams of potassium iodide and one liter of water. The scrubber pump was continuously operated at a pumping rate of 48 ml per minute. The scrubber solution circulated through the flask, pump inlet line, pump and pump outlet line to the top of the column, down through the column and back into the flask.

An air stream was saturated with iodine vapor by metering air at 65 standard cubic centimeters per minute (SCCM) through molten iodine at 122°C. The iodine- saturated air stream was transported through heated glass lines held at 275°C, mixed with an additional 400 SCCM air and introduced into the remaining neck of the 5—liter 3—necked flask. After more than 6 hours of operation the vent gas exiting the vent port at the top of the column was tested with wet starch—KI paper. The color of the paper did not change, indicating the near total absence of iodine in the vented gas stream. Introduction of the stream of air containing molecular iodine was continued for a total time of 24 hours. The used scrubber solution was dark red and weighed 77.7 g more than the starting scrubber solution.

Molecular iodine was recovered from this solution by placing the solution in a 500 ml capacity round bottom flask fitted with an ambient water temperature cooled reflux condenser and heated to reflux for 6 hours. During the reflux time iodine crystals deposited on the surface of the reflux condenser while the water vapor condensed and returned to the solution. The condenser contained 76 grams of molecular iodine. The solution recovered from the refluxing flask was analyzed for potassium and total iodine. The molar ratio of all forms of iodine to potassium was 1.02, indicating that potassium iodide was the principle remaining iodine- containing species in solution.