Method and apparatus for recovery and conversion of halocarbon fumigants

Abstract

A method and apparatus (10) for capturing and converting ozone-depleting haloalkane fumigants from a mixed fumigant / air stream (14) to produce a fumigant-free air stream (28) by absorbing the ozone-depleting haloalkane fumigants into a metal hydroxide-alcohol buffer solution (26) in an absorber / scrubber (12) is disclosed. The haloalkane captured in the aqueous alcohol solution can react actively with the metal hydroxide in the alcohol solution to produce value-added products (such as precipitated metal halides) and another alcohol that further enhances absorption. The absorption solution is thoroughly mixed with a make-up alcohol and base stream to maintain the concentration of the metal hydroxide within the desired buffer solution range. The solid precipitated metal halide (52) is separated from the liquid stream, and the mixed alcohol stream (26) containing the metal hydroxide is recycled to the absorber / scrubber (12).

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Provisional Patent Application Serial No. 62 / 563,976, filed on September 27, 2017, the entire disclosure of which is incorporated herein by reference. Technical Field

[0003] The present invention relates to cost-effective methods and apparatus for treating fumigation waste / exhaust gases and more particularly to the recovery of halogenated hydrocarbons from air streams containing halogenated hydrocarbons and the concomitant conversion of the recovered halogenated hydrocarbons into useful compounds. Background Art

[0004] Fumigants are used to treat agricultural products such as fruit, grain or logs in enclosures before allowing them to be exported overseas, imported locally or sold locally. After fumigation, sufficient time has passed for the fumigant to be absorbed by the product. Subsequently, the enclosure is aerated so that the remaining fumigant is released into the atmosphere, which can be up to 50% of the initial amount applied. Halons, especially methyl bromide (MeBr), are commonly used fumigants.

[0005] Due to its toxicity, methyl bromide is considered an effective quarantine / phyto-sanitary fumigant for controlling insects and other pests. However, studies in humans have demonstrated acute and chronic health effects associated with methyl bromide, and the EPA has classified methyl bromide as a Group D compound. In addition to its toxicity, another serious problem with the use of methyl bromide is the damage it causes to the stratospheric ozone layer. In fact, methyl bromide is considered an ozone depleting chemical, and has therefore been classified as a Class I ozone depleting substance by the Montreal Protocol, while also being regulated by the Clean Air Act due to its 0.2% ozone depletion potential. Therefore, due to the harmful effects of methyl bromide on the environment, international, federal, and state regulations require strict control of methyl bromide emissions.

[0006] Due to the lack of effective alternatives, the use of methyl bromide as a fumigant has not been completely discontinued, but such use is only permitted when its emissions can be sufficiently reduced. This has adverse economic effects on, for example, logging companies, which are limited by federal and state regulations on the amount of fumigant they can release into the atmosphere, thereby limiting their operations.

[0007] At present, there are methods for capturing fumigants that are implemented on a large scale; however, they usually destroy the fumigants, produce byproducts and accompanying disposal problems. In addition, they are also expensive. Most existing methods rely on the use of activated carbon beds, and the absorption solution is non-recyclable. Other disadvantages associated with these prior arts include, for example, the following disadvantages. They involve complex systems including multiple stages, and therefore require high capital investment and operating costs. These technologies usually require changes to existing fumigation methods, which may affect the effectiveness and reliability of those methods, for example, each container requires a longer processing time. Existing emission control methods / processes mainly focus on using different adsorbents / absorbents to capture halogenated fumigants, which usually require further processing methods to safely dispose of halogen-containing reagents / absorbents, which increases the additional cost of the method and makes it less economically advantageous. Current processing systems are usually large, which makes them difficult to transport for on-site gas processing. Existing methods also usually involve extreme processing conditions of high temperature or energy input. Summary of the invention

[0008] The present invention relates to a method and apparatus for capturing and converting fumigants using a reactive absorber / scrubber equipped with an aqueous buffer solution of circulating alcohol and metal hydroxide. Air containing a halocarbon fumigant (i.e., a fumigant / air mixture) is then passed through the absorber / scrubber apparatus so that the fumigant / air mixture is contacted with the buffer solution so that the fumigant is absorbed by the alcohol and the air of the fumigant is thereby cleaned. The captured halocarbon is then reacted with the metal hydroxide to produce a metal halide. A purge stream containing a concentrated metal halide solution or a metal halide slurry (slurry) is continuously discharged to avoid accumulation of solids and contaminants in the system. When discharged from the absorber / scrubber apparatus, the solid metal halide will precipitate and separate from the residual aqueous alcohol and metal hydroxide solution. After the salt (e.g., metal halide) recovery process, the solution is fed back to the absorber as an absorption solution. In an embodiment, the fumigant / air mixture is treated in a continuous mode, and the buffer solution is treated in a continuous mode while in a recirculation mode. In embodiments, the fumigant / air mixture is processed in a continuous mode and the buffer solution is processed in a batch mode. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] For a more complete understanding of the present invention, reference is made to the following detailed description of exemplary embodiments considered in conjunction with the accompanying drawings, in which:

[0010] Figure 1 is a schematic diagram of a method / apparatus for recovering and converting halogenated alkanes according to the present invention, which method / apparatus can be operated in batch or recycle mode;

[0011] Figure 2is a schematic diagram of a batch process / apparatus for recovering and converting halogenated alkanes according to the present invention;

[0012] Figure 3 is a schematic diagram of a recycling method / apparatus for recovering and converting halogenated alkanes according to the present invention;

[0013] Figure 4 is a graph showing the following research results: for scenes with and without packing, Figure 2 Effect of filler on halide absorption in batch mode depicted in , where the x-axis corresponds to time-on-stream and the y-axis represents the MeBr concentration in the liquid product;

[0014] Figure 5 is a graph depicting the results of a study of the absorption capacity of various solvents, which plots the MeBr concentration of the liquid product of various alcohol solvents (see y-axis) as a function of time (see x-axis);

[0015] Figure 6 is a graph illustrating the effect of the concentration of two commonly used metal hydroxides on the absorption capacity of various solvents, which plots the concentration of produced MeBr in the gas product (see y-axis) as a function of time (see x-axis) for various hydroxides and concentrations;

[0016] Figure 7 is a graph depicting, for the numbers associated with the solutions specified in Table 2 below (see x-axis), the steady-state value of MeBr removal from the gas phase (in percent) (see left y-axis) and the MeBr concentration in the MeBr-depleted gas (see right y-axis);

[0017] Figure 8 It is shown in Figure 3 A graph of the results of a study of the effect of gas-liquid (GL) contact time on MeBr absorption in a recirculation mode depicted in , which plots the MeBr concentration in the gas product (see y-axis) as a function of time (see x-axis) for different gas-liquid contact times;

[0018] Fig. 9 It is explained in Figure 3 A graph of the effect of superficial gas velocity (or residence time) on absorption in a recirculation mode depicted in , which plots the MeBr concentration in the gas product (see y-axis) as a function of time (see x-axis) for various flow rates;

[0019] Fig.10is a graph showing the effect of different gas / liquid ratios and flow rates on absorption for a 10 g NaOH / 100 ml ethanol solution, which plots the MeBr concentration of the gas product (see y-axis) against time (see x-axis) for different gas / liquid ratios and flow rates; and

[0020] Fig.11 is a graph depicting the effect of different gas / liquid ratios and flow rates on absorption for a 10 g KOH / 100 ml ethanol solution, for which the MeBr concentration of the gas product (see y-axis) is plotted against time (see x-axis). DETAILED DESCRIPTION

[0021] The exemplary embodiment disclosed below, which is only one of many exemplary embodiments of the present invention, is developed for the recovery of significant amounts of methyl bromide (which is used as a fumigant gas for fumigating wood logs) and its subsequent conversion into value-added products. Fumigation exhaust gases treated according to the present invention better comply with emission regulations. Although the methods and apparatus described below relate to the recovery and conversion of methyl bromide, it should be understood by those of ordinary skill in the art that they are also effective for the recovery and conversion of other halocarbon fumigants.

[0022] In the method of the present invention, the scrubbing solution is an alcohol-based buffer solution that is not discarded, but is separated from the generated suspended solids (i.e., metal halides) and recycled, thereby further improving the economic benefits and feasibility of the method. All reactants used in the proposed method are inexpensive compared to existing methods, and metal halides (such as sodium bromide) can be high-value commodities. Another by-product of the method disclosed herein is methanol, a solvent that is also recycled in the absorption solution to aid further absorption of the fumigant.

[0023] Compared to currently available methods, the one-step, highly integrated wet reactive absorption method developed herein requires reduced capital and operating costs and mild process conditions. Through enhanced mass transfer characteristics, efficient waste gas treatment can be achieved with reduced equipment size, making it feasible for on-site treatment. In addition, the value-added byproducts from the method improve its cost-effectiveness and make the fumigant treatment economically close to self-sustaining.

[0024] Taking into account the previous preface comments, ref. Figure 1, wherein the apparatus 10 comprises an absorber / scrubber 12, such as a model PPS-24 steam scrubber from Vapor Tech, which is equipped to contact the fumigant gas and the dissolved metal hydroxide and thereby react them. It should be understood that any absorber or scrubber apparatus known to those of ordinary skill in the art is suitable for use as the absorber / scrubber 12 of the apparatus 10. For example, in embodiments, the absorber can be a metal cylindrical column or tower. In some embodiments, the column can be a packed column, while in other embodiments, it can be a tray tower. Various packings and trays are available and known, and their selection is well within the knowledge of those of ordinary skill in the art.

[0025] Continue to refer Figure 1 , a mixed air stream 14 comprising air and a halogenated alkyl such as methyl bromide originates from a fumigation process 16 in which a halogenated alkyl stream 18 has been used to fumigate agricultural products, such as logs 20 or fruits or grains (not shown) that have been stored in a fumigation container 22. Air 24 is continuously pumped through the fumigation container 22 to ventilate the fumigated agricultural products (i.e., logs 20) and the container 22 itself. In other embodiments, the mixed air stream 14 may originate from other sources and processes other than the fumigation process. The mixed air stream 14 leaving the container 22 carries the removed halogenated alkyl fumigant (e.g., methyl bromide) when it enters the absorber / scrubber 12 from the bottom. During its upward travel, the mixed air stream 14 contacts a countercurrent flow stream of an absorption solution 26 that is fed from the top of the absorber / scrubber 12 and travels downward through the absorber / scrubber 12. In another embodiment, the streams of the mixed air stream 14 and the absorption solution 26 may flow through the absorber / scrubber in the same direction.

[0026] The stream of the absorption solution 26 is a metal hydroxide-alcohol buffer solution, which is used to enhance the external mass transfer of the haloalkanes from the mixed air stream 14 to the absorption solution, and to provide a stable haloalkanes conversion medium. The haloalkanes (e.g., methyl bromide) are highly soluble in the absorption alcohol, thereby facilitating their transfer from the mixed air stream 14 to the absorption solution. A lean air stream 28 containing air and a reduced amount of haloalkanes is discharged from the top of the absorber / scrubber 12 and passes through an optional de-entrainment / demisting device 30 and an online haloalkanes detection / quantification (i.e., measurement) instrument 32 before being discharged to the atmosphere A.

[0027] Since the alkyl halide is super reactive to react with the metal hydroxide in the absorption solution, the bimolecular nucleophilic substitution reaction (SN2) produces two reaction products, wherein the metal is an alkali metal (i.e., sodium, potassium, or any one of the elements of Group 1A (1) of the Periodic Table). One reaction product is a product alcohol, which corresponds to the alkyl group of the alkyl halide and is a liquid miscible with the absorption alcohol. The other reaction product is a metal halide, which corresponds to the metal of the metal hydroxide and the halide of the alkyl halide, and which subsequently precipitates as a solid. The following reaction is the general reaction that occurs:

[0028]

[0029] More specifically, for example, when the alkyl halide is methyl bromide and the metal hydroxide is sodium hydroxide, the reaction will proceed as follows to produce methanol as the product alcohol and sodium bromide as the metal halide:

[0030]

[0031] Still reference Figure 1 , a stream 34 containing metal halides and alcohol is supplied from the absorber / scrubber 12 to an absorption solution drum 36. A small make-up stream 38 containing absorption alcohol is also supplied to the absorption solution drum 36 to compensate for any absorption alcohol that may be lost due to reaction with the product alcohol or evaporation into the air stream or other losses. Similarly, a make-up alkali stream 40 containing a certain concentration of metal hydroxide and alcohol is supplied to the absorption solution drum 36. The exhaust stream 42 leaves the absorption solution drum 36 via a pump 44. After leaving the pump 44, the stream 42 is divided into an absorption solution stream 26 and a purge stream 46. The purge stream 46 passes through a solid-liquid separation device 48 such as a Russell Eco separator, which produces (i) a stream 50 containing a negligible amount of dissolved salts (e.g., metal halides), which is returned to the absorption solution drum 36, and (ii) collected solids 52, i.e., metal halides, which can be packaged for sale.

[0032] The transfer of the halogenated alkane fumigant from the mixed air stream 14 to the liquid absorption solution in the absorber / scrubber 12 occurs primarily on the column packing or tray (not shown) of the absorber / scrubber 12. The absorption solution typically comprises an alkali-alcohol solution and may be, for example, methanol, ethanol, butanol, isopropanol, or a combination thereof. Typically, the mass fraction of alcohol in the absorption solvent will vary, but in some embodiments, the mass fraction will be from about 0 to about 1. Therefore, the water content (i.e., aqueous component) of the absorption solvent ranges from about 0 to about 1. Typically, the concentration of the metal hydroxide in the absorption solution will vary, but in some embodiments, the concentration will be from about 0 g / 100 ml solvent to about 50 g / 100 ml solvent. The flow rate at which the absorption solution stream 26 flows into the absorber / scrubber 12 will depend on the following variations: (i) the concentration of the fumigant in the gas stream, (ii) the gas stream flow rate, (iii) the desired fumigant concentration in the outlet gas stream, and (iv) the solubility of the fumigant in the absorption solution, etc.

[0033] The flow rate can be determined by one of ordinary skill in the art and is typically determined as part of the process design. It is expected that the concentration of the fumigant in the lean air stream 28 will be from trace amounts (i.e., in the range of parts per billion) to undetectable amounts. The temperature of the absorption column in the absorber / scrubber 12 will be maintained at a selected value to increase the solubility of the fumigant in the alcohol and also inhibit the evaporation of the solvent into the gas stream. In some embodiments, the temperature will be maintained at a value of about room temperature (e.g., about 20°C to about 26°C) to about 50°C. The operating pressure in the absorption column will depend on the pressure of the mixed air stream 14 and may be higher than atmospheric pressure. In some embodiments, the operating pressure may be at least about atmospheric pressure (e.g., 1 atmosphere, or 14.7 pounds per square inch).

[0034] The product alcohol formed will generally be miscible with the absorption solution and the metal halide will precipitate, facilitating its separation from the product stream of the reactor. For any absorption solution stream, the concentration of the metal hydroxide should be controlled to meet any TSS and TDS specified by the manufacturer of the pump 44 and any relevant pipe design specifications.

[0035] As described above, the method of the present invention can be implemented in conjunction with the fumigation of agricultural products in various containers or packages. In some embodiments, the container can be a sealed shipping container, a trailer, a rail car, a factory (mill) and a warehouse. The fumigated product can be an agricultural product, including wood products such as logs and fruits or grains. The concentration of the fumigant in the mixed air stream 14 leaving the container 22 during ventilation will change over time. In certain embodiments, the concentration will change from about 0.25g fumigant / g mixture to about 0.0g fumigant / g mixture. The flow rate of the air stream 24 will vary according to the size of the halocarbon fumigant and the container 22. In the fumigation of wood products in a container, the air flow rate of the air stream 24 at room temperature can be as high as 1200ft 3 / min.

[0036] After the liquid level in the absorber / scrubber 12 remains constant for a period of time, the gas flow is turned on and the run time measurement begins. The countercurrent flow of the absorption solution 26 and the gas stream to be cleaned provides a large contact area and creates a relatively high concentration difference between the two phases, which is the driving force for mass transfer.

[0037] Figure 2 A batch method and apparatus 110 for recovering and converting halogenated alkanes according to the present invention is depicted. Hereinafter, the setup will be referred to as a batch mode setup. Raschig rings (PTFE, L×OD×thickness of 3mm×3mm×1mm) are filled into an absorber 112 to a height of 6″. After such filling, 50 ml of the absorption / reaction solution is first transferred into the reservoir of the batch mode setup, and the run time is measured from the time the gas flow is turned on. The MeBr-rich gas stream 114 flowing out of the gas source 116 is regulated by a mass flow controller 118 (MFC) and passes through a check valve 120 and a control valve 122 on the way to the inlet 124 of the absorber 112. The MeBr-rich gas stream 114 is supplied to the bottom 126 of the packed bed and bubbles through the flooded packed bed before leaving through the outlet 128, and then passes through the gas chromatography device 130 or the exhaust port 132 via valve 134.

[0038] Figure 3 A schematic diagram of an apparatus 210 for recovering and converting halogenated hydrocarbons according to the present invention is depicted, which apparatus 210 can be operated as a batch or recycle process. Hereinafter, the apparatus will be referred to as a recycle mode apparatus. Figure 3As shown, the effective flow path of the absorber / reactor portion 212 of the recirculation mode device has an ID of 1 / 4", a height of 21.26" (54 cm), and is filled with 3 mm Raschig rings (PTFE, L×OD×thickness of 3 mm×3 mm×1 mm). After such filling, 20 ml of the solution is first transferred into the recirculation mode device and then circulated at a selected flow rate for the absorption process. The MeBr-rich gas stream 214 flowing out of the gas source 216 is regulated by a mass flow controller (MFC) 218 ​​and reaches the inlet 224 of the absorber 212 through a check valve 220 and a control valve 222. A high pressure liquid chromatography pump 226 drives the fluid flow. After the liquid level at the bottom of the absorber 212 remains unchanged for 10 minutes, the gas flow is turned on and the run time measurement is started. The countercurrent flow of the absorption solution 228 and the gas stream to be cleaned provides a large contact area and produces a relatively high concentration difference between the two phases, which is the driving force for mass transfer. Gas 230 exits absorber 212 through outlet 232 and then passes through gas chromatography device 238 or exhaust 236 via valve 234 .

[0039] In order to describe the present invention in more detail, the following examples are given:

[0040] Example 1 Batch Mode Operation: Role of Filler

[0041] The filling ( Figure 4 ) on the halide absorption, and the MeBr concentration in the liquid phase was measured. In both cases, during the initial 2 hours TOS (operating time), it was found that the liquid phase MeBr concentration increased rapidly, but when the filler was used, it increased at a much faster rate. Through the filling, the MeBr concentration approaches a stable value from the second hour, and seems to reach the following value at about the third hour of the method, which is considered to be the maximum absorption capacity of the solvent. The filler provides a large interfacial contact area between the gas and the liquid by destroying the bubbles, which has the effect of enhancing interphase (inter-phase) mass transfer. Therefore, the packing material with a high specific surface area per unit volume can be used to improve the process efficiency of capturing MeBr by the liquid phase.

[0042] Example 2 Batch Mode Operation: Base Solvent Screening

[0043] From a set of experiments carried out in a batch mode apparatus, Figure 5 The data shown in . The higher the number of carbon atoms, molecular weight, or boiling point of a solvent, the higher its absorption capacity. Since butanol is much more expensive than the other three solvents based on cost considerations, IPA will probably be the most viable solvent.

[0044] Example 3 Batch Mode Operation: Metal Hydroxide Screening

[0045] The MeBr-rich liquid solution cannot be directly disposed of without significantly increasing costs. Therefore, the captured MeBr can be reacted with metal hydroxide (YOH) by mixing the MeBr-rich liquid solution with YOH in a separate vessel, or it can be directly reacted with YOH during absorption, which can even enhance absorption by simultaneously maintaining the mass transfer driving force.

[0046] The present invention adopts the latter approach. The two most commonly used YOHs (ie, NaOH and KOH) were used to study the effect of YOH addition on the capture and conversion processes in a batch mode unit.

[0047] According to Table 1, NaOH is generally more soluble in water than in alcohols. Therefore, pure DI-H2O and DI-H2O / alcohol mixtures were first investigated as base solvents. For the latter mixture, due to the formation of two phases, namely, an aqueous phase containing almost all NaOH and an alcohol phase containing almost no NaOH, this solvent system exhibited a much lower absorption capacity than the alcohol-based system, as shown in Table 1. Figure 6 shown.

[0048] Table 1: Solubility of YOH in different solvents

[0049]

[0050] The results for all tested solutions are presented in Figure 6 and obtained from a batch mode apparatus. For alcohol-based solutions, increasing the YOH concentration can greatly improve the absorption capacity. The results also show that KOH is more active than NaOH and that ethanol appears to be the best solvent among all the studied solvents when YOH is added.

[0051] The stable values ​​of % MeBr removed from the gas phase using different absorption solutions and the MeBr concentration in the MeBr-depleted gas were corrected to account for the effect of the solvent vapor pressure, and the results are summarized in Figure 7 and Table 2. The most effective absorption solution was 20 g KOH / 100 ml ethanol.

[0052] Table 2: Solvent ID Reference

[0053]

[0054]

[0055] Example 4 Recirculation Mode Operation: Formation of Suspended Solids

[0056] The main challenge of the recycling experiments comes from the precipitation of solids due to (i) the formation of lower soluble products (from the reaction of YOH and MeBr) and (ii) the loss of alcohol solvent at high gas flow rates when no make-up alcohol solution is added to the system. Experiments conducted with 10 g and 20 g KOH / 100 ml ethanol (without MeBr) at a recycle rate of 5 ml / min and an air flow rate of 40 sccm showed that there was no KOH or NaOH in the precipitate, indicating that the precipitate was a product of the reaction of YOH with MeBr.

[0057] Example 5 Recirculation Mode Operation: Effect of Gas-Liquid (GL) Contact Time

[0058] The effect of GL contact time in a recirculation mode unit was investigated experimentally by keeping the absorption solution in batch mode. The results are summarized in Figure 8 It should be noted that although there is hysteresis in the measurements of the 20 sccm and 30 sccm experiments, the reduction in the solution volume in the absorber is negligible because the solution concentration is far in excess and roughly constant (because there is solution above the filler and the filler is always immersed in the solution), and the gas composition at the top surface of the solution can remain constant during the 3-hour experiment. Therefore, the data reported in the second figure for the results of 20 sccm and 30 sccm can be considered as real performance data.

[0059] As expected, a decrease in the gas flow rate can effectively reduce the amount of MeBr in the gas outlet flow, and the best result is 0.3 mol% at 20 sccm. Figure 2 %, where the outlet gas composition is in the range of 1.05-1.07 mol %), the MeBr composition is reduced to 0.67-0.7 mol % at 40 sccm. This reduction can be attributed to the reduced ID and increased length of the absorber.

[0060] Example 6 Recirculation Mode Operation: NaOH / H2O Solution as Model Solvent

[0061] In order to investigate the performance of the method using the absorption solution in recirculation mode while avoiding pump clogging problems, a 20 g NaOH / 100 ml H2O solution was used.

[0062] like Fig. 9As shown in, at a flow rate of 40 sccm MeBr / air, the liquid (20g NaOH / 100ml H2O) flow rate was increased from 3.5ml / min to 10ml / min. The results show that a liquid flow rate of 5ml / min is sufficient to achieve complete wetting of the absorber filling in recirculation mode. At 10ml / min, the gas flow rate was reduced to 20sccm. As expected, the MeBr mol% in the gas phase decreased compared to the MeBr mol% at 40sccm. The experiment was also carried out using 35ml solution at 20sccm, but in non-recirculation (batch) mode. Compared to the operation of 20sccm / 10ml / min, the mixing caused by the recirculation of the liquid phase seems to be compensated by the gas flow. For the comparison of batch mode and recirculation mode, based on the water recirculation results, at a liquid flow rate of 10ml / min, there seems to be no difference between batch mode and recirculation mode, but more experimental runs will be needed to confirm this observation. Furthermore, although this may not apply to the YOH / ethanol system, it should be noted that the recirculation mode requires only 20 ml (or even less) of solution, whereas the batch mode requires 35 ml of solution, for capturing and converting a similar amount of MeBr. Furthermore, for practical operation, it is better to run in a recirculation mode, which would allow the stream leaving the slurry reservoir to enter a precipitator for removal of solid (salt) products, and to add a make-up ethanol (or solvent) stream to the process stream after it leaves the precipitator. The result of 3.78 mol% for 20 sccm / 35 ml is worse than the result of 3.69 mol% for 40 sccm / 50 ml (3 g NaOH / 100 ml H2O) ( Figure 2 The main reason is that the recirculation mode device ( Figure 3 ) is the surface residence time of the batch mode device ( Figure 2 )’s surface residence time is 1 / 3.2 of that.

[0063] Example 7 Recirculation Mode Operation: YOH / Alcohol Solution

[0064] The YOH / alcohol solution could not be successfully processed in the recirculation mode for 3 hours due to the formation of precipitates that blocked the pump. For the 10 g NaOH / 100 ml ethanol solution, experiments were performed at different gas / liquid ratios while keeping the gas flow rate at 50 sccm. Fig.10 As expected, the MeBr concentration in the outlet gas stream decreases with increasing L / G ratio due to higher coverage of the packed bed cross section and increased liquid residence time.

[0065] Apart from Fig.11In addition to the data presented in , 20 g KOH / 100 ml ethanol at flow rates of 5 ml / min and 10 ml / min was also tested, but the experiment had to be aborted because the pump pressure was too high (>1600 psi), even after only 5 minutes of recirculation. The liquid flow rate was fixed at 5 ml / min based on the results of previous experiments using water. When the liquid flow rate was set to 10 ml / min, the pump pressure rose very quickly to a value of >1600 psi. Fig.11 The observations shown in confirm the previous conclusions: higher YOH concentrations enhance MeBr capture and ethanol exhibits better performance than IPA.

[0066] It should be understood that the embodiments described herein are merely exemplary, and those skilled in the art may make many changes and modifications without departing from the spirit and scope of the present invention. All these changes and modifications are included within the scope of the present invention as defined in the appended claims.

Claims

1. A method for capturing and converting fumigants using an absorber / scrubber apparatus, the method comprising the following steps: a) feeding an alcohol and a metal hydroxide buffer solution into the absorber / scrubber unit; b) continuously feeding a halocarbon fumigant / air mixture into the absorber / scrubber apparatus such that the halocarbon fumigant / air mixture contacts the buffer solution, wherein at least a portion of a fumigant component of the halocarbon fumigant / air mixture is transferred from the halocarbon fumigant / air mixture to the buffer solution, and the fumigant component reacts with a metal hydroxide component of the buffer solution to produce a second mixture comprising product alcohol, unreacted metal hydroxide, and metal halide; c) discharging the second mixture from the absorber / scrubber unit into an absorption solution tank and combining the second mixture with a small make-up stream containing absorption alcohol and a make-up alkali stream containing a certain concentration of metal hydroxide and alcohol as a discharge stream; d) dividing the effluent stream into a purge stream and a third mixture comprising an alcohol and a metal hydroxide solution; e) separating the metal halide from the purge stream to produce solid metal halide and a stream containing negligible amounts of dissolved metal halide, the stream containing negligible amounts of dissolved metal halide being returned to the absorption solution tank, wherein the steps d) and e) are performed continuously and combined with the steps a) to c); and f) recycling the third mixture comprising alcohol and metal hydroxide solution to the absorber / scrubber unit in step a), wherein the third mixture comprising alcohol and metal hydroxide solution is used as the alcohol and metal hydroxide buffer solution during repetition of step a).

2. The method of claim 1 further comprising the step of exhausting the treated lean air stream to post-processing equipment and measuring instruments for discharge to the atmosphere.

3. The method of claim 1, wherein the alcohol and the alcohol of the metal hydroxide buffer solution comprise two alcohols. The method according to claim 1 , wherein the alkyl halide is methyl bromide.

5. The method of claim 1, wherein the halocarbon fumigant / air mixture is fed into the absorber / scrubber unit in a first direction and the buffer solution is fed into the absorber / scrubber unit in a second direction opposite to the first direction.

6. The method of claim 1, wherein the halocarbon fumigant / air mixture is fed into the absorber / scrubber unit in a first direction and the buffer solution is fed into the absorber / scrubber unit in a second direction that is the same as the first direction.

7. The method of claim 1, wherein the metal hydroxide component comprises sodium hydroxide.

8. The method of claim 1, wherein the metal hydroxide component comprises potassium hydroxide.

9. The method of claim 1, wherein the buffer solution comprises an alcohol selected from the group consisting of methanol, ethanol, butanol, and isopropanol.

10. The method of claim 1, wherein the makeup base stream containing a concentration of metal hydroxide and alcohol comprises the same alcohol and metal hydroxide as the metal hydroxide component of the alcohol and metal hydroxide buffer solution.

11. The process of claim 1 wherein said small make-up stream containing absorption alcohol comprises the same alcohol as said alcohol of said alcohol and metal hydroxide buffer solution.

12. The method of claim 1, wherein the fumigant component of the Halon fumigant / air mixture is derived from a process other than a fumigation process.

13. The method of claim 1, wherein steps a)-e) are repeated using the third mixture as the buffer solution. The method according to claim 1 , wherein the buffer solution has an alcohol component with a mass fraction of greater than 0 up to 1.

15. The method according to claim 1, wherein the buffer solution has an aqueous component with a mass fraction of 0 to 1.

16. The method of claim 1, wherein the concentration of the metal hydroxide component is in the range of greater than 0 g / 100 ml of the buffer solution up to 50 g / 100 ml of the buffer solution.

17. The method of claim 1, wherein the concentration of the fumigant component ranges from greater than 0 g fumigant / g fumigant / air mixture up to 0.25 g fumigant / g fumigant / air mixture.

18. An apparatus for capturing and converting a fumigant, comprising: a first feed device configured and arranged to feed alcohol and a metal hydroxide buffer solution into the absorber / scrubber; a second feed device configured and arranged to continuously feed a halocarbon fumigant / air mixture into the absorber / scrubber such that the halocarbon fumigant / air mixture contacts the buffer solution, wherein a fumigant component of the halocarbon fumigant / air mixture is configured and arranged to transfer from the halocarbon fumigant / air mixture to the buffer solution, and at least a portion of the fumigant component is configured and arranged to react with a metal hydroxide component of the buffer solution to produce a second mixture comprising an alcohol and a metal halide; a discharge device configured and arranged to discharge the second mixture from the absorber / scrubber to an absorption solution tank and to combine the second mixture with a small make-up stream containing absorption alcohol and a make-up alkali stream containing a concentration of metal hydroxide and alcohol as a discharge stream; a dividing device configured and arranged to divide the effluent stream into a purge stream and a third mixture comprising an alcohol and a metal hydroxide solution; a return device configured and arranged to recycle said third mixture comprising alcohol and metal hydroxide solution to said absorber / scrubber unit as said alcohol and metal hydroxide buffer solution; and A separation device is configured and arranged to continuously separate the metal halide from the purge stream to produce solid metal halide and a stream containing negligible amounts of dissolved metal halide, the stream containing negligible amounts of dissolved metal halide being returned to the absorption solution tank.

19. The apparatus of claim 18 further comprising: a post-processing device configured and arranged to treat the lean air stream exiting the absorber / scrubber prior to discharge of the lean air stream to the atmosphere, the post-processing device including instrumentation suitable for measuring fumigant composition of the lean air stream.

20. The apparatus of claim 18, wherein the absorber / scrubber comprises a metal cylindrical column.

21. The apparatus of claim 18, wherein the absorber / scrubber comprises a tray tower.

22. The apparatus of claim 18, wherein the absorber / scrubber comprises a packed column.

23. The apparatus of claim 18, wherein the absorber / scrubber is configured and arranged to be maintained at a temperature of 20°C to 50°C.

24. The apparatus of claim 18, wherein the absorber / scrubber is configured and positioned to be maintained at an operating pressure of at least one atmosphere.

25. The apparatus of claim 18, wherein the Halon fumigant / air mixture is configured and arranged to process in a continuous mode and the buffer solution is configured and arranged to process in a continuous but recirculating mode.

26. The apparatus of claim 18, wherein the Halon fumigant / air mixture is configured and arranged to be processed in a continuous mode and the buffer solution is configured and arranged to be processed in a batch mode.

27. The method of claim 1, wherein the alcohol of the buffer solution comprises aqueous alcohol.

28. The apparatus of claim 18, wherein the alcohol of the buffer solution comprises aqueous alcohol.

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