WO1996026893A1 - Method for recovering bromine from methyl bromide - Google Patents

Abstract

A process for recycling methyl bromide is provided including adsorbing the methyl bromide (currently vented to the atmosphere) with activated carbon or another suitable adsorbent, desorbing of the methyl bromide and other adsorbed compounds from the carbon, thermally oxidizing the desorbed methyl bromide under conditions designed to maximize formation of hydrogen bromide and to minimise formation bromine, and using the thus-formed hydrogen bromide as a raw material for making a bromine-containing chemical.

Description

METHOD FOR RECOVERING BROMINE FROM METHYL BROMIDE

The invention relates to recycling methyl bromide. Background of the Invention Methyl bromide (CH₃Br) is used world-wide as a fumigant, e.g. , for post-harvest commodity treatment. Commodities which are typically fumigated include dried fruits and nuts, fresh fruit and vegetables, grains and milled products and non-food products. Methyl bromide is also used for soil and dwelling fumigation.

After fumigation, methyl bromide is usually released into the atmosphere. Methyl bromide is a hazardous, toxic substance, and contributes to stratospheric ozone depletion. The EPA designated methyl bromide as a Class 1 ozone depleter on March 18, 1993; the U.S. Clean Air Act (Section 602) requires that any Class 1 ozone-depleting substance be withdrawn from production, importation and use in the United States seven years after listing. Thus, there is a need for methods of controlling emissions of methyl bromide.

Summary of the Invention The invention features a process for controlling the emissions of methyl bromide into the atmosphere by capturing the methyl bromide and then recycling the bromine content of the methyl bromide as a raw material for the bromine industry and for the manufacture of other bromine-containing chemicals. The process preferably includes adsorbing the methyl bromide onto activated carbon or another suitable adsorbent, then desorbing the methyl bromide and other adsorbed compounds from the adsorbent and thermally oxidizing the desorbed methyl bromide under conditions designed to maximize formation of hydrogen bromide and to minimize formation of bromine, absorbing the hydrogen bromide in water as hydrobromic acid or as a bromide salt through neutralization, and finally using the thus-formed hydrogen bromide as a raw material for making a bromine-containing chemical. Preferably, the oxidation of the methyl bromide to hydrogen bromide takes place in the absence of a metal catalyst, typically in the presence of excess oxygen, at sufficient temperature to assure greater than 99% destruction of the methyl bromide, and to convert greater than 98% of the bromine content of the methyl bromide to hydrogen bromide. In preferred embodiments, a sulfur source (e.g. sulfuric acid, sulfur dioxide, hydrogen sulfide, sulfur, etc.) is added during the thermal oxidation. By operating at sufficiently high temperature, there is virtually no limit to the degree of destruction efficiency that can be achieved with thermal oxidation. Performing the oxidation at high temperature and without a metal catalyst avoids catalyst contamination from the combustion of sulfur and sulfur compounds. Catalyst contamination often results in periodic shutdowns for catalyst replacement. The thermal oxidation chamber is not affected significantly by the combustion of sulfur and sulfur compounds.

In preferred embodiments, a sulfur source (e.g., sulfuric acid, sulfur dioxide, hydrogen sulfide, or sulfur) is added during oxidation; after desorption, the carbon is reused for adsorption; the hydrogen bromide is converted to hydrobromic acid, or, with the addition of the appropriate base, sodium, potassium, or other bromide salts. In a particularly preferred embodiment, the hydrogen bromide resulting from oxidation is recovered as hydrobromic acid, as an aqueous solution of sodium bromide, or as anhydrous sodium bromide, which can be used directly or subsequently utilized as a raw material for the manufacture of bromine. In other preferred embodiments, the hydrogen bromide resulting from the oxidation is recovered as an aqueous solution of sodium bromide and sodium sulfate, or as an anhydrous mixture of sodium bromide and sodium sulfate, which are subsequently used for the manufacture of bromine.

Advantageously, using the method of the invention, methyl bromide is desorbed and thermally oxidized without generating hazardous waste. The only emissions are carbon dioxide and water formed from the combustion of methyl bromide and auxiliary fuel. The desorbed methyl bromide generally is thermally oxidized in such a manner as to produce virtually 100% hydrogen bromide, thus allowing the product of oxidation to be readily (or easily) absorbed as an aqueous solution of hydrobromic acid or bromide salts, which can be used directly or which serve without further processing as a substitute for naturally occurring sources of bromine. Moreover, using the invention, methyl bromide emissions to the atmosphere can be controlled to any degree of efficiency required by regulatory agencies. Further, in use the invention does not disrupt typical fumigation processes. Other features and advantages of the invention will be apparent from the following description of the preferred embodiments and from the claims.

Brief Description of the Drawings Fig. 1 is a schematic diagram of a type of fumigation facility.

Fig. 2 is a schematic diagram of a desorption- thermal oxidation process.

Description of the Preferred Embodiments Referring to Fig. 1, the commodity or commodities are stacked on pallets in a convenient area known in the industry as a "grid". Tarps are draped over the stacked pallets in order to make a relatively impervious enclosure (Fig 1, T-101) . Tarped volumes may range up to 140,000 cubic feet. Gaseous methyl bromide is injected into the air space under the tarps to obtain a methyl bromide concentration specified by the appropriate regulatory agency.

After a specified length of time, the mixture of air and methyl bromide ("contaminated air") under the tarps is displaced by clean air through the action of ventilation fan K-101. As the contaminated air is removed with ventilation fan K-101, fresh air from the surrounding area enters under the tarped area and replaces the contaminated air. In another fumigation method (not shown) , the "tarped" grid is replaced by a fixed chamber for fumigation of commodities. The principle of operation of the fixed chamber is the same as that of the "tarped" grid. Gaseous methyl bromide is injected into the chamber to obtain the specified methyl bromide concentration. After a specified length of time, the "contaminated air" is removed with a ventilation fan, and fresh air is admitted to the chamber through special openings to replace the contaminated air. In yet another method (not shown) , the commodity or commodities to be fumigated are placed inside a vacuum chamber. After evacuation, methyl bromide is admitted under specified conditions. After the fumigation has been completed, the contaminated air is removed with a vacuum pump. The chamber is "flushed" several times with air and re-evacuated each time in order to reach the allowable residual methyl bromide concentration.

The contaminated air is directed into a carbon adsorber (Fig. 1, R-101) . The adsorptive capacity of some adsorbents, especially that of activated carbon, is reduced when the contaminated air stream has a high relative humidity. This may for instance be the case when the commodity undergoing fumigation is cold fruit, and the ambient air is relatively warm. Under such conditions, the relative humidity of the contaminated air is reduced before it reaches the carbon adsorber. The preferred method for reducing the relative humidity of the contaminated air is to heat it by various convenient means. When contaminated air at 45°F and 100% relative humidity is heated to 65°F, its relative humidity is reduced to less than 50%, which is the preferred maximum relative humidity for optimum adsorption on activated carbon. In the carbon adsorber, the majority of the methyl bromide is adsorbed by a bed of activated carbon. Clean air, which meets regulatory requirements, is discharged from the adsorber to the atmosphere via an exhaust stack (Fig. 1, point 4) . The contaminated air may pass through a plurality of adsorber beds which are arranged in parallel. Most of the methyl bromide will be in the first fumigation chamber air volumes that are exhausted into the adsorber.

It is well known that maximum adsorptive capacity of the activated carbon is achieved by switching the contaminated air stream between two or more parallel adsorbers as the methyl bromide concentration decreases in the contaminated air. At high methyl bromide concentrations, the contaminated air may for instance go to adsorber "A", which is nearing its maximum adsorptive capacity. As its methyl bromide concentration decreases, the contaminated air is switched to adsorber "B" which is still far from reaching its adsorptive capacity. When adsorber "A" has reached maximum adsorptive capacity, as indicated by the air exhausted from "A" no longer meeting the regulatory methyl bromide concentration limit, adsorber "A" is removed from service for regeneration or reactivation, and adsorber "B" takes its place. A freshly regenerated or reactivated adsorber is brought into service to take the place of adsorber "B".

One suitable activated carbon for use in the adsorber beds is BPL 4x10, available from Calgon Carbon Corporation. Other examples of suitable adsorbents include Norit R 1540, available from Norit Americas Corporation.

Fig. 2 shows a process for the desorption of methyl bromide from the activated carbon that has been used in the process shown in Fig. 1, the reactivation of the carbon, the thermal oxidation of the desorbed methyl bromide to hydrogen bromide, and the absorption of the hydrogen bromide as an aqueous sodium bromide solution. When the activated carbon reaches its methyl bromide adsorption limit, which is defined as the maximum weight methyl bromide adsorbed on the activated carbon which still results in meeting the regulatory emission requirements, the complete carbon adsorber including the spent activated carbon is shipped to a centrally located facility for regeneration or reactivation. At the same time, the desorbed methyl bromide is thermally oxidized to hydrogen bromide, carbon dioxide, and water. Alternatively, the spent activated carbon is removed from the carbon adsorber and shipped to a centrally located facility for regeneration or reactivation. Regenerated activated carbon by itself, or reloaded into a carbon adsorber bed, is then returned to the fumigation facility. As shown in Fig. 2, spent activated carbon is regenerated in a carbon regeneration furnace (Fig 2, F- 102) . The regeneration furnace is operated at high temperature (preferably at least 1,500°F in the lower oxidizing regeneration zone) with less than 1% oxygen in the upper desorption zone. Typical residence times for the carbon in the regeneration furnace is 10 to 30 minutes to assure complete desorption. The regeneration furnace renews the ability of the carbon to again adsorb methyl bromide. The desorbed methyl bromide is thermally oxidized in the afterburner (F-103) . Sufficient air, fuel and sulfur (or sulfur dioxide, sulfuric acid or other sulfur compounds) are injected into the afterburner to oxidize the methyl bromide and combustibles coming from the regeneration furnace to carbon dioxide and water and to convert a minimum of 98% of the bromine content of the methyl bromide to hydrogen bromide. The afterburner preferably operates at 1600°F to 2300°F, with at least 1 second residence time, and with at least 3% by volume oxygen. The amount of sulfur required to convert the bromine content of methyl bromide to hydrogen bromide is approximately 0.2 lbs sulfur for each lb of bromine. The introduction of sulfur into the combustion chamber assures that virtually no free bromine is generated.

Under typical afterburner oxidizing conditions (1400°F to 2200°F, 1% to 15% oxygen) , without the addition of sulfur, a substantial percentage (10% - 50%) of free bromine would be generated instead of the desired hydrogen bromide.

Under reducing conditions with excess combustibles and no oxygen, a shift to virtually 100% hydrogen bromide can be achieved. The hydrogen bromide must now be removed from the products of combustion before the excess combustibles are oxidized in a second afterburner, which operates under the usual oxidizing conditions.

The hot flue gas from the afterburner (F-103) consists mainly of nitrogen, oxygen, carbon dioxide, water vapor, hydrogen bromide and mainly sulfur trioxide. It is rapidly cooled in an adiabatic quench where it is deluged with sprays of water or sprays of recirculating scrubber solution. Alternatively, the hot flue gases are first cooled in a heat recovery device, such as a waste heat boiler, before undergoing the adiabatic quench process. Direct rapid quench cooling, without heat recovery, minimizes the possibility of the reformation of organic bromine compounds, some of which may be analogous to furans and dioxins, from the products of combustion. The quenched flue gas is then scrubbed to absorb the hydrogen bromide and sulfur trioxide. Sodium hydroxide is added to neutralize the absorbed hydrogen bromide and sulfur trioxide to their respective neutral salts sodium bromide and sodium sulfate. The aqueous scrubber solution consists of a neutral solution of water, sodium bromide and sodium sulfate.

Advantageously, the scrubber solution containing sodium bromide can then be used, without further treatment, as a raw material in the production of bromine containing chemicals. The sodium bromide produced by the oxidation process substitutes for equal quantities of sodium bromide found in natural brines which are typically used by bromine production plants as a raw material.

Example Fumigation of imported fruit with methyl bromide is conducted by stacking the fruit containers approximately 7 feet high on top of a 9000 square foot "grid" marked on a warehouse floor. Sealed plastic tarps supported by a ceiling rack are then draped over the fruit to make a relatively leak proof eight foot high enclosure which serves as a fumigation chamber. The tarps are sealed against the warehouse floor by placing "sand sausages" on top of the plastic. The overall volume of the fumigation chamber is 72,000 cubic feet. To start the fumigation, 288 lbs gaseous methyl bromide

(CH₃Br) is injected under the tarps. By using electric window fans placed inside the fumigation chamber, the methyl bromide is mixed with the air in the fumigation chamber, and is distributed around and between the stacked fruit containers. Once the required fumigation time has expired, the fumigation chamber is ventilated by means of a fan capable of extracting up to 10,000 cubic feet per minute or more of air containing methyl bromide ("contaminated air") from the fumigation chamber. Some of the "sand- sausages" are removed from around the tarp perimeter to allow fresh air to enter into the fumigation chamber. The contaminated air withdrawn from the fumigation chamber is heated as required to reduce its relative humidity below 50%. If the contaminated air, for instance, is at 45°F and saturated with water vapor (i.e. 100% relative humidity) , it's relative humidity is reduced below 50% by heating it to 65°F.

Contaminated air, with less than 50% relative humidity, and with an initial concentration of up to 15,000 ppm v/v (parts per million by volume) methyl bromide, is directed into an activated carbon ("carbon") adsorber. The adsorber may be transportable, which means that the "spent" carbon is transported in the adsorber to the regeneration facility. The adsorber may also be permanently installed, in which case the "spent" carbon has to be removed from the adsorber and transported in separate containers to the regeneration facility.

As the contaminated air passes through the carbon adsorber, the methyl bromide and other adsorbable compounds that may have been given off by the fruit, are adsorbed on the active surfaces of the carbon. The higher the concentration of methyl bromide in the contaminated air, the more the methyl bromide is adsorbed onto the carbon. Starting at an initial contaminated air concentration of 15,000 ppm v/v methyl bromide, the concentration drops rapidly as fresh air is introduced into the fumigation chamber, and can reach 500 ppm v/v in less than one hour. It is assumed, for this example, that the contaminated air stream meets regulatory compliance limits when the methyl bromide concentration has been reduced to 500 ppm v/v. Under these conditions, a typical carbon can adsorb as much as 15% or more of its own weight in methyl bromide. An even higher "loading" can be achieved by switching to a second adsorber during the ventilation before the contaminated air concentration reaches 500 ppm v/v. The first adsorber loading will then be greater than 15% when it reaches its capacity, or is "spent". Carbon is "spent" when the clean air leaving the first adsorber no longer meets the regulatory compliance limit of 500 ppm v/v. At this point, the second adsorber takes the place of the first adsorber, and a new adsorber, filled with regenerated carbon, or with "virgin" carbon, takes the place of the second adsorber. There are other ways of optimizing the usage of carbon that are well known to those skilled in the art. This example shows only one practical optimization method.

The spent carbon is transported to a carbon regeneration facility. Regeneration is typically accomplished with hot steam or other gas at moderate temperatures (200°F to 300°F) or it is done with hot flue gas in rotary kilns, fluidized beds or rotary hearth furnaces at much higher temperatures (800°F to 1900°F) . The high temperature processes are used to desorb the adsorbed material and reactivate the active surfaces of the carbon, while the moderate temperature processes simply desorb the adsorbed material. If the materials desorbs readily and does not polymerize or hydrolyze, moderate temperature regeneration is often sufficient to reestablish adequate adsorption properties. If moderate temperature regeneration is not sufficient, the carbon has to undergo high temperature regeneration and reactivation. The desorbed methyl bromide, assumed to be 95% of that used for the fumigation, or 274 lbs, plus steam, other reactivation gas or flue gas, are thermally oxidized in an "afterburner". The afterburner is a typical thermal oxidizer, consisting of a combustion chamber, a gas or liquid fuel burner, a means of injecting the methyl bromide containing gases, and a means of injecting sulfuric acid, sulfur dioxide, hydrogen sulfide, sulfur or other sulfur compounds. The methyl bromide containing gases are injected into the thermal oxidizer so as to mix well with the hot combustion gases generated by the burner. 274 lbs methyl bromide contain about 231 lbs bromine (Br ) . At the same time, the appropriate amount of sulfuric acid, for instance, is also injected into the combustion chamber. 0.2 lb sulfur or 0.61 lb sulfuric acid is injected for each pound of bromine. The total amount of sulfuric acid injected is about 141 lbs. The combustion chamber is designed to provide approximately 1 second residence time. The oxygen concentration in the combustion chamber should be at least 3% to provide for high methyl bromide destruction efficiency. The injection of sulfur into the combustion chamber causes a drastic change in the hydrogen bromide (HBr) to bromine ratio. Without sulfur, considerable quantities of free bromine would leave the combustion chamber along with hydrogen bromide. The addition of sulfur brings about that a minimum of 98% of bromine leaves the combustion chamber as hydrogen bromide. The afterburner combustion gases are cooled and scrubbed with water and the appropriate base, such as sodium hydroxide. The following table shows the amounts of acids leaving the afterburner for this example, the amount of base required for neutralization, and the amount of salts generated: Acids formed:

Sulfuric acid 141 lbs Hydrogen bromide 234 lbs Base required: Sodium hydroxide 231 lbs

Salts generated:

Sodium sulfate 204 lbs Sodium bromide 298 lbs Total salts 502 lbs The salts are collected as an aqueous solution of up to 15 wt% dissolved salts, and sent to manufacturers of bromine and bromine containing compounds where they serve as a high quality source of bromine.

Other embodiments are within the claims. For example, it is also possible to produce dry sodium bromide, or other bromide salts, through an evaporation and crystallization process. All of the distillate from the evaporation process would be recycled to the hydrogen bromide scrubbing system. No hazardous waste would be generated through the production of dry bromide salts. Another embodiment allows for the separate recovery of the sulfur trioxide in the form of sulfuric acid. The sulfuric acid can be re-used in the afterburner to convert further bromine to hydrogen bromide. Sulfuric acid is recovered by operating two separate absorbers, where the first absorber contains sulfuric acid, which readily absorbs sulfur trioxide, while allowing hydrogen bromide, which is much more volatile, to pass on to the second absorber, where it in turn in scrubbed from the flue gas in the previously mentioned fashion. This embodiment virtually eliminates the on-going need for sulfur or sulfur compounds, except for make-up of inevitable losses that occur in any operation. This embodiment also results in the generation of pure sodium bromide solution, or dry sodium bromide, without the presence of other salts.

What is claimed is:

Claims

1. A process for recycling the bromine content of methyl bromide that has been used as a fumigant, comprising desorbing said methyl bromide that has previously been adsorbed, after being used as a fumigant, onto an adsorbent; thermally oxidizing said desorbed methyl bromide in the absence of a metal catalyst at sufficient temperature to assure greater than 99% destruction of the methyl bromide, and to convert greater than 98% of the bromine content of the methyl bromide to hydrogen bromide; and using said hydrogen bromide to synthesize bromine or a bromine-containing chemical.

2. The process of claim 1 wherein a sulfur source is added during the oxidation step.

3. The process of claim 2, wherein said sulfur source is selected from the group consisting of sulfur, hydrogen sulfide, sulfur dioxide, and sulfuric acid.

4. The process of claim 1 wherein the adsorbent is reused for adsorption after the desorption step.

5. The process of claim 1 wherein after generation said hydrogen bromide is absorbed as hydrobromic acid.

6. The process of claim 1 wherein after generation said hydrogen bromide is absorbed as a bromide salt.

7. The process of claim 1 wherein the hydrogen bromide resulting from the oxidation step is recovered as an aqueous solution of sodium bromide and is used as a raw material for the manufacture of bromine.

8. A process for recycling the bromine content of methyl bromide that has been used as a fumigant, comprising using methyl bromide to fumigate a commodity; adsorbing said methyl bromide onto an adsorbant; desorbing said adsorbed methyl bromide from said adsorbant; thermally oxidizing said desorbed methyl bromide in the absence of a metal catalyst at sufficient temperature to assure greater than 99% destruction of the methyl bromide, and to convert greater than 98% of the bromine content of the methyl bromide to hydrogen bromide; and using said hydrogen bromide to synthesize bromine or a bromine-containing chemical.

9. The process of claim 1 wherein the adsorbent comprises activated carbon.