Patent Description:
Chlorine is a byproduct in many chemical reactions. Chlorine is toxic and manufacturers are required by laws and regulations to limit or eliminate chlorine emissions.

In wet scrubber systems, aqueous sodium hydroxide is used to increase the pH of the scrubbing liquid and to drive chlorine into solution. The sodium hydroxide also reacts with chlorine in gas streams to produce sodium hypochlorite and salt according to the following reaction:.

Document <CIT> discloses a method in which a chlorine-containing gas is introduced into a gas treating tank to remove the contained chlorine; a rotary fine bubble generator is provided on the gas treating tank and the gas is brought into contact with an absorption liquid containing a sparingly watersoluble solid alkali compound and the chlorine-containing gas is treated. The treated gas by this treating method is then introduced into a second gas treating tank to perform an alkali treatment.

Document <CIT> discloses a method for absorbing chlorine from a gas containing chlorine and carbon dioxide, and more particularly to a method for scrubbing small amounts of chlorine from a flue gas stream containing a large excess of carbon dioxide, with the scrubbed flue gas being discharged directly to the atmosphere.

Document <CIT> discloses a harm-removing apparatus comprising a circulation-water tank in which water having a pH of <NUM>-<NUM> is circulated to dissolve a halogen-containing discharge gas in the water and from which the residual discharge gas remaining undissolved is discharged, and a neutralization tank in which the water containing the discharge gas dissolved therein and discharged from the circulation-water tank is treated with a neutralizing agent under pressure and from which the water treated is returned to the circulation-water tank.

Document <CIT> discloses a regenerable flue gas desulfurization system whose end product is a commercial quality gypsum or other product easily landfilled. Sulfur containing flue gas is passed through an absorber where it is sprayed with a solution so as to initiate certain chemical reactions thereby stripping it of sulfur. Any alkali in this sulfur-containing solution is recovered and returned to the absorber with the remainder of this solution being oxidized in an oxidation/recirculation tank. The slurry from this tank may be recycled to a pre-absorber for chlorine removal if desired, otherwise, a portion of this slurry is sent for the recovery of gypsum.

Document <CIT> discloses a method for inhibiting scale deposits in an aqueous system. The method may include at least one of adding and forming anti-scalant in the aqueous system such that an amount of anti-scalant in the aqueous system is up to about <NUM> ppm, wherein the anti-scalant comprises at least one or polyvalent metal silicate and polyvalent metal carbonate, wherein the aqueous system has a pH of at least about <NUM>, and wherein a mean particle size of the anti-scalant is less than about <NUM> microns. The method includes at least one of adding and forming anti-scalant in the aqueous system, wherein the anti-scalant comprises at least one of polyvalent metal silicate and polyvalent metal carbonate, and adding copolymer of maleic anhydride and isobutylene.

Document <CIT> discloses a method for treating smoke and dust in zinc oxide by alkaline washing elimination, comprising the following steps: smoke and dust from a furnace are dedusted firstly by a cyclone deduster, and then enter an adjustable venturi to be washed by a sodium carbonate solution; solid phase dust, comprising dust collected in a boiler, dust eliminated by the cyclone deduster and dust washed down by the adjustable venturi, is mixed and then delivered to an alkaline tank for alkaline washing; clear liquid is returned to the system after the alkaline washing liquid is thickened, the clear liquid after the bottom flow passes through a vertical filter press is returned to the system, and the filter residue (zinc oxide) is mixed and then delivered for smelting and leaching; and the waste of the system is discharged by a pit.

It has been discovered that conventional chemistries used in chlorine scrubber systems accelerate the degradation of equipment vessels and piping, particularly those that come into contact with high temperature solutions (e.g., <NUM>-<NUM>) that include one or more of hypochlorite, "wet" chlorine, and hydrochloric acid. For example, these solutions are sprayed in wet scrubbing systems onto brick/mortar walls which degrades the mortar and in turn can cause the bricks to shift and break rubber lining on the equipment, thereby exposing and corroding a carbon steel casing of the equipment. The high temperature solutions can also can degrade fiber reinforced plastics that are sometimes used in the basin, spray piping, and spray headers of wet scrubbing systems. It is believed that the current practice of feeding sodium hydroxide to maintain a high pH in the scrubber water contributes to the problem.

In accordance with one aspect of this invention, it has been discovered that the degradation of the equipment and conduits can be mitigated by modifying the chemistry used in the scrubbing operation.

In one aspect, this disclosure provides a method for removing chlorine from a chlorine-containing gas stream comprising the steps defined at claim <NUM>. The dependent claims outline advantageous ways of carrying out the method.

Various exemplary embodiments of this disclosure will be described in detail with reference to the following figures, wherein like numerals reference like elements, and wherein:.

The scrubbing system and methods described herein can be used to effectively remove or reduce chlorine from any chlorine-containing gas stream while preventing deterioration of equipment and conduits in the system. The scrubbing systems can be wet scrubbing systems that are used in industrial processes that generate chlorine containing gas. As one example, in steel mills, steel is pickled with HCl, which requires the acid to be regenerated in a spray roasting process that produces a waste gas stream containing chlorine gas and hydrochloric acid. The chlorine needs to be removed before the gas is vented to the environment.

In embodiments of the scrubbing system according to the present invention, the scrubbing liquid that strips the chlorine from the gas includes potassium carbonate.

The treatment composition further includes a scale inhibitor, which inhibits deposition in scale-forming waters of the scrubbing system. The scale inhibitor is an organophosphonate such as <NUM>-phosphono-<NUM>,<NUM>,<NUM>,-butane tricarboxylic acid (PBTC).

The potassium carbonate salt and the scale inhibitor are added to the scrubbing liquid in a weight ratio that is in the range of from <NUM>:<NUM> to <NUM>:<NUM>, from <NUM>:<NUM> to <NUM>:<NUM>, from <NUM>:<NUM> to <NUM>:<NUM>, or from <NUM>:<NUM> to <NUM>:<NUM>, for example. The potassium carbonate salt can be added to the scrubbing liquid in amounts of, for example, <NUM>/hr to <NUM>/hr, <NUM>/hr to <NUM>/hr, or <NUM>/hr or <NUM>/hr depending on operating conditions. The scale inhibitor can be added to the scrubbing liquid in amounts of, for example, <NUM>/hr to <NUM>/hr, from <NUM>/hr to <NUM>/hr, or from <NUM>/hr to <NUM>/hr, depending on operating conditions. The specific amount that is needed will depend on a variety of factors that are unique to each process such as the number of spray booms, burner temperatures, dust scrubber packing age, water scrubbing packing age, waste acid profile, roaster oxide screw degradation or unavailability, etc..

The treatment composition can be added to the scrubbing liquid of the scrubbing system as an aqueous treatment composition that can include, for example, from <NUM> to <NUM> wt. % or <NUM> to <NUM> wt. % scale inhibitor, and from <NUM> to <NUM> wt. %, <NUM> to <NUM> wt. %, <NUM> to <NUM> wt. % of potassium carbonate salt, or from <NUM> to <NUM> wt. % of potassium carbonate salt. About <NUM>-<NUM> (<NUM>-<NUM> gallons) of such a treatment composition could typically be added to the scrubbing liquid each day, depending on operating conditions.

The use of potassium carbonate salt provides for an increase in pH of the scrubbing liquid, which facilities driving the chlorine gas into solution and also provides buffering capacity by increasing the carbonate alkalinity. This can provide for more consistent pH control in the scrubber, which means fewer sodium hydroxide totes and less operator burden to maintain the pH within limits. The scale inhibitor allows the treatment composition to achieve the goal of maintaining a high pH and removing chlorine while preventing the spray equipment from malfunctioning due to scaling caused by the potassium carbonate salt or its reaction products with chlorine. Likewise, the scale inhibitor allows the pH to be increased higher than would otherwise be possible, which further facilitates driving chlorine into solution to effect its removal. The treatment composition also reduces the blowdown required for the sump liquid since higher concentrations of the potassium carbonate salt and/or its reaction products can be recirculated without significant scaling.

The potassium carbonate salt/scale inhibitor composition can be used in the scrubbing liquid alone or in addition to sodium hydroxide. When used in combination with the potassium carbonate salt/scale inhibitor, the sodium hydroxide can be used at a greatly reduced rate, which can lower the overall cost of treatment. Reducing the amount of sodium hydroxide also reduces the sodium hypochlorite that is in the scrubbing liquid, which is believed to be one cause of the deterioration of the fiber reinforced plastics (e.g., fiber reinforced epoxies, vinyl esters, etc.) that are used in the scrubbing equipment. In this regard, the reaction products of potassium carbonate salt are chlorite and/or chlorate salts (e.g., potassium chlorite and/or potassium chlorate), which are less corrosive than sodium hypochlorite. If sodium hydroxide is used to increase pH, it can be present in amounts of less than half of the potassium carbonate salt by weight, less than a quarter of the carbonate salt by weight, less than one tenth of the carbonate salt by weight, or less than one fiftieth of the potassium carbonate salt by weight.

The treatment composition also includes a scavenger compound that mitigates atmospheric discharge of chlorine gas in one or two ways: (<NUM>) will remove chlorine gas when sprayed into the gas path, and/or (<NUM>) it will scavenge sodium hypochlorite in the circulating water. Suitable compounds are sodium thiosulfate and sodium bisulfite. Sufficient amounts of these compounds can be added so that preferably the concentration of sodium hypochlorite in the circulating water is kept to below <NUM>,<NUM> ppm, and preferably below <NUM> ppm or below <NUM> ppm.

The methods and compositions described herein can employ various wet scrubbing equipment, including a venturi scrubber, for example. <FIG> is a schematic diagram of a venturi scrubber system <NUM>. The venturi scrubber system <NUM> includes venturi scrubber <NUM> having a gas inlet <NUM> that introduces the gas <NUM> which contains chlorine. In some embodiments, the gas <NUM> also includes HCl. Nozzle <NUM> sprays the aqueous scrubbing liquid <NUM> so that the liquid contacts the chlorine-containing gas. The scrubbing liquid that contacts the chlorine-containing gas includes primarily water (e.g., more than <NUM> wt. % or more than <NUM> wt. %), the treatment composition (potassium carbonate salt/scale inhibitor), and optionally sodium hydroxide and sodium thiosulfate. Packing material <NUM> increases the contact area between the gas and the liquid. Basin <NUM> receives the liquid containing the chlorine gas and/or reaction products which collect as sump <NUM>. Makeup liquid <NUM>, which is preferably water (e.g., at least <NUM>% water) can be added as needed through inlet <NUM>. Recycle loop <NUM> pumps the sump liquid <NUM> to nozzle <NUM>. Valve <NUM> can control the flow of sump liquid <NUM> to the nozzle. The sump liquid <NUM> can be removed through outlet <NUM>, e.g., as blowdown. The scrubbed gas <NUM> exits through gas outlet <NUM>.

The addition of the potassium carbonate salt, scale inhibitor, sodium hydroxide, and/or scavenger compound to the process water can be monitored and controlled.

Since maintaining an elevated pH (e.g., in the range of <NUM>-<NUM>, <NUM>-<NUM>, or <NUM>-<NUM>) facilitates driving the chlorine gas into solution to be removed from the gas path by the scrubbing system, the injection rate of potassium carbonate salt and optionally sodium hydroxide can be controlled by an online pH instrument whereby pH is measured and the amounts of these components added to the process water are controlled based on the measured pH, e.g., by using an automatic controller that includes a processor that calculates the desired addition amount of the treatment composition based on measured pH. The controller can include a PID loop, for example. In one embodiment, the pH is controlled to be within the range of <NUM>-<NUM>. Increasing the pH control range closer to <NUM>. could increase the removal of chlorine, but may contribute to nozzle and packing fouling in the scrubbing equipment. Thus, the amount of the potassium carbonate salt and optionally caustic that is added to the scrubbing liquid is variable depending on the operating conditions of the wet scrubber system, but should be sufficient to elevate the pH of the scrubbing system to the desired operating level.

The injection rate of scavenger compound can be controlled using an oxidation reduction potential (ORP) instrument, with a control range of <NUM>-<NUM> mV as a target.

The addition of the treatment compositions and/or scavenger compound to the scrubbing liquid can be controlled by pumps and valves that receive signals from a controller (e.g., CPU or other processor) that determine the desired amounts of components to add to the sump liquid. The controller may also control the valves and pumps in the system to control the quantity of sump liquid that is sprayed into the wet scrubber system, or to control the amount and/or composition of makeup water, or to control the amount of blowdown. The blowdown rate can be controlled to maintain absorber efficiency and prevent gas path carryover issues.

The temperature of the scrubbing liquid in the sump can be in the range of from <NUM>-<NUM>, from <NUM>-<NUM>, and from <NUM>-<NUM>.

The treatment compositions and methods described herein can prevent or slow down the degradation of equipment vessels and piping used in the scrubbing system as compared to conventional techniques that employ primarily sodium hydroxide to raise the pH of the scrubbing liquid. The equipment vessels or piping in the scrubber can be made of - or include components that are made of made of - plastic, fiber reinforced plastic, or rubber. Examples of materials used in the equipment vessels or piping can include epoxy resins such as aromatic amine cured epoxy resins and epoxy vinyl ester resins.

In embodiments, the treatment compositions and methods of treatment of the invention can also reduce the environmental impact of chlorine-producing operations by mitigating the discharge of chlorine gas to the atmosphere and also by making the operations more efficient.

A field trial was conducted on a venturi scrubber installed on an acid recovery roaster in a steel mill. The scrubber includes fiber reinforced plastic (FRP) piping, which are susceptible to degradation in the presence of sodium hypochlorite, "wet" chlorine gas, and hydrochloric acid solutions when operated at high temperatures. The scrubber equipment and piping had experienced degradation problems due to the practice of feeding only sodium hydroxide to the scrubbing liquid.

In this experiment, a treatment composition was added to the scrubbing liquid as needed based on a measured pH to maintain the scrubber solution pH within a defined operating range. The experimental treatment composition is an aqueous solution including potassium carbonate (about <NUM>-<NUM> wt. %) and PBTC (about <NUM> to <NUM> wt. A sodium thiosulfate solution (about <NUM>-<NUM> wt. %) is separately added.

PH OL is a pH measurement form online measurement instrumentation. The remaining measurements were taken with a calibrated portable instrument using a grab sample point taken at the dust scrubber recirculation pump discharge. In Table <NUM> below, PH OL is the measured pH from the online instrumentation, PH SU is the pH measured on the grab sample, COND is conductivity in milliSiemens/cm, and ORP is oxidation-reduction potential in millivolts.

<FIG> is a graph showing the conductance of the scrubbing water in this experiment at each measurement point together with the pH. <FIG> shows that the conductance of the scrubbing water is significantly reduced by the replacement of caustic with the treatment composition. This is directly correlated to the reduction of corrosive ions in the scrubbing water, such as hydroxide. The experiment thus demonstrates that the treatment composition can be added to maintain the operating pH of the scrubbing water while significantly reducing the capacity of the scrubbing water to support reactions that break down the scrubber system equipment.

Claim 1:
A method for removing chlorine from a chlorine-containing gas stream, the method comprising:
adding a potassium carbonate salt and a scale inhibitor to a scrubbing liquid in a wet scrubbing system, the potassium carbonate salt and scale inhibitor being added to the scrubbing liquid in a weight ratio that is in the range of from <NUM>:<NUM> to <NUM>:<NUM>;
contacting the chlorine-containing gas stream with the scrubbing liquid;
whereby the potassium carbonate salt, and optionally sodium hydroxide, is added in amounts that are sufficient so that the scrubbing liquid has an alkaline pH;
whereby the scale inhibitor is an organophosphonate compound; and
further comprising adding a scavenger compound comprising sodium thiosulfate or sodium bisulfite to the scrubbing liquid.