NZ763627B2 - Process and apparatus for alkyl halide fumigant recovery and conversion - Google Patents

Abstract

Process and apparatus (10) are disclosed for capturing and converting an ozone-depleting alkyl halide fumigant from a fumigant/air mixed stream (14) by absorbing it into a metal hydroxide-alcohol buffer solution (26) in an absorber/scrubber (12) to produce a fumigant-free air stream (28). The captured alkyl halide in aqueous alcohol solution can actively react with the metal hydroxide in alcohol solution to produce a value-added product, such as a precipitate metal halide, and another alcohol that further enhances absorption. The absorbing solution is well-mixed with make-up alcohol and alkali streams to maintain the concentration of the metal hydroxide in the desired buffer solution range. The solid precipitate metal halide (52) is separated from the liquid stream, and the metal hydroxide-containing mixed alcohol stream (26) is recycled to the absorber/scrubber (12).

Description

PROCESS AND APPARATUS FOR ALKYL HALIDE FUMIGANT RECOVERY AND CONVERSION Cross-Reference to Related Application This application claims priority to US. Provisional Patent Application Serial No. 62/563,976 ?led September 27, 2017, the entire disclosure of which is incorporated herein by reference.

Field of The Invention The present invention relates to ffective ses and apparatus for the treatment of fumigation exhaust gas, and, more particularly, to the ry of alkyl halide from an air stream containing same and the concomitant sion of the recovered alkyl halide into useful compounds.

Background of The Invention Fumigants are used to treat agricultural products, such as fruits, grains or logs, in enclosures before these products are allowed to be exported as, imported or distributed locally. After fumigation, suf?cient time elapses for the fumigant to be absorbed by the products. Subsequently, the enclosures are aerated, causing the release of residual fumigant, which can be as high as 50% of the original amount applied, into the atmosphere. Alkyl halides, especially methyl e (MeBr), are commonly used fumigants.

Due to its toxicity, methyl e is considered to be an effective quarantine/phyto-sanitary fumigant in the control of insects and other pests. However, studies in humans have con?rmed acute and chronic health effects associated with methyl bromide, and the EPA has classi?ed methyl bromide as a Group D compound. In addition to its toxicity, another serious concern about the use of methyl bromide is the damage it causes to the stratospheric ozone layer. In fact, methyl e is considered an ozone-depleting chemical and thus it has been classi?ed as a Class I ozone-depleting substance by the Montreal protocol, while also falling under the Clean Air Act with an ozone depletion ial of 0.2%. As a result, international, federal and state regulations require strict control of methyl bromide emissions because of their hazardous effect on the environment.

Due to the lack of effective alternatives, the use of methyl e as a fumigant has not been completely discontinued, but such usage is only permitted if its emissions can be suf?ciently reduced. This has an adverse economic impact on, for example, g companies, which are limited by federal and state regulations on how much of the fumigant they can release into the here, thus limiting their operations.

Currently, there are methods being ented on a large scale for capturing fumigants, however, they typically destroy the fumigants, creating ducts and attendant disposal problems. Moreover, they are also ive. Most of the current methods rely on the use of activated carbon beds, and the absorbing solution is not recoverable. Other omings associated with these current technologies include the following, for example. They involve cated systems comprising multiple stages, thus requiring high capital investment and operating expense. These logies often require the modification of existing fumigation processes which may affect the effectiveness and reliability of the processes by, for example, requiring longer sing time per container. The existing emission control methods/processes mainly focus on the capture of the halogenated fumigants using different adsorbents/absorbents, whereby a further treatment process is usually required for the safe disposal of the halogen- containing agents/absorbents, which adds extra costs to the process and makes it less economically favorable. Current treatment systems are typically large which makes their transportation for onsite gas ent dif?cult. Existing processes also typically involve extreme treatment conditions of high temperature or energy input.

Summary of the Invention The present invention relates to processes and apparatus for capturing and ting fumigants using a reactive absorber/scrubber, which is equipped to circulate an aqueous buffer solution of alcohol and metal hydroxide. Air containing alkyl halide fumigant (i.e., fumigant/air mixture) is then passed through the absorber/scrubber apparatus such that the nt/air mixture and the buffer solution come into contact, causing the nt to be absorbed by the l and thereby cleaning the air of the fumigant. The captured alkyl halide then reacts with the metal ide to yield a metal halide. A purge stream containing trated metal halide solution, or metal halide slurry, is continuously discharged to avoid buildup of solids and contaminants in the system. Upon discharge from the absorber/scrubber apparatus, the solid metal halide will be precipitated and separated from the aqueous alcohol and metal hydroxide solution that s.

After the salt (e.g., metal halide) recovery process, the solution is fed back to the absorber as absorbing solution. In an embodiment, the fumigant/air mixture is processed in continuous mode and the buffer solution is processed in continuous mode but recirculating mode. In an embodiment, the fumigant/air mixture is processed in continuous mode and the buffer on is processed in batch mode.

Description of the Drawings For a more complete understanding of the present invention, reference is made to the ing detailed description of exemplary embodiments considered in conjunction with the accompanying drawings, in which: Figure l is a schematic diagram of a process/apparatus; which can be operated in either batch or recirculation mode for recovering and converting an alkyl halide in accordance with the present invention; Figure 2 is a schematic diagram of a batch process/apparatus for recovering and converting an alkyl halide in accordance with the present invention; Figure 3 is a schematic diagram of a recirculation process/apparatus for recovering and converting an alkyl halide in accordance with the present invention; Figure 4 is a graph showing the results of a study on the effect of g on halide absorption in the batch mode ed in Figure 2; wherein the x-axis ponds to the time-on-stream and the y axis represents MeBr concentration in the liquid product; for ios both with and without packing; Figure 5 is a graph depicting the results of a study on the absorbing capacity of various solvents; by plotting the MeBr concentration of the liquid product (see the y-axis) of various alcohol solvents as a function of time (see the x-axis); Figure 6 is a graph illustrating the impact of the trations of two common metal hydroxides on the absorbing ty of various solvents; by plotting the resultant MeBr concentration in the gas product (see the y-aXis) as a function of time (see the X-aXis) for various hydroxides and concentrations; Figure 7 is a graph depicting the stabilized values of MeBr removal (in percentage) from the gas phase (see the left-hand y-aXis) and the MeBr concentration in MeBr-depleted gas (see the right- hand y-aXis) for the s (see the X-aXis) associated with the solutions speci?ed in Table 2 below; Figure 8 is a graph g the results of a study on the effects of gas-liquid (G-L) contact time on MeBr absorption by ng MeBr concentration in the gas product (see the y-aXis) as a function of time (see the X-aXis) for varying gas-liquid contact times, in the recirculation mode depicted in Figure 3; Figure 9 is a graph illustrating the effect of superficial gas velocity (or residence time) on absorption in the recirculation mode depicted in Figure 3 by plotting MeBr concentration in the gas product (see the y-aXis) as a function of time (see the X-aXis) for multiple ?ow rates; Figure 10 is a graph g the effect of varying gas/liquid ratios and ?ow rates on absorption for 10g NaOH/lOO ml Ethanol solution by plotting MeBr concentration of the gas product (see the y-aXis) against time (see the X-aXis) for varying gas/liquid ratios and ?ow rates; and Figure 11 is a graph depicting the effect of g gas/liquid ratios and ?ow rates on tion for 10g O ml ethanol on by plotting MeBr concentration of the gas product (see the y-aXis) against time (see the X-aXis) for varying gas/liquid ratios and ?ow rates.

Detailed Description of Exemplary Embodiments The exemplary embodiment disclosed hereinafter, which is only one of many exemplary embodiments of the present invention, was developed for application in the recovery of a substantial amount of the methyl bromide used as the fumigation gas for fumigating logs of wood, followed by its sion to a value-added product. The waste fumigation gas treated in accordance with the present invention better complies with emission regulations. Although the process and apparatus bed below are directed to the recovery and conversion of methyl bromide, it should be understood by a person of ordinary skill in the art that they are also effective for recovering and converting other alkyl halide fumigants.

In the process of the present ion, the scrubbing solution is an alcohol-based buffer solution, which is not discarded, but rather is separated from the produced ded solid, namely metal halide, and recycled, thereby further enhancing the ic ef?ciency and viability of the process. All of the reactants for the proposed process are inexpensive compared to existing processes, and metal halides, such as sodium bromide, can be highly valuable ity ts. Another by-product of the process disclosed herein is methanol, a solvent that is also recycled in the absorbing solution to aid further absorption of the fumigant.

The ep highly integrated wet reactive tion process as developed herein requires reduced capital and operating cost and mild process conditions in comparison to currently available methods. With intensi?ed mass transfer characteristics, highly nt waste gas treatment can be achieved with reduction in equipment size, which makes it viable for onsite processing. In addition, the added-value uct from this process improves its cost- effectiveness, and makes the fumigant treatment close to being self-sustaining economically.

While bearing in mind the foregoing prefatory comments, reference is made to Figure 1, in which apparatus 10 comprises an absorber/scrubber 12, such as Model No. PPS-24 Vapor Scrubber from Vapor Tech, equipped to bring the fumigant gas and dissolved metal hydroxide into contact and thereby enabling their on. It should be understood that any absorber or scrubber apparatus known to persons having ry skill in the art is suitable for use as the absorber/scrubber 12 of the apparatus 10. For example, in an embodiment, the absorber could be a ic cylindrical column or tower. In some embodiments, the column could be a packed column, while in others it could be a tray tower. Various gs and trays are available and known and their selection is well within the knowledge of persons of ordinary skill in the art.

With continued reference to Figure 1, a mixed air stream 14 sing air and an alkyl halide, such as methyl bromide, is derived from a fumigation s 16 in which an alkyl halide stream 18 has been used to fumigate an agricultural product, such as wooden logs 20, or fruit or grain (not shown), which has been stored in a fumigation container 22. Air 24 is pumped continuously through the fumigation container 22 to aerate the fumigated agricultural products (i.e., the logs 20), as well as the ner 22 itself. In other embodiments, the mixed air stream 14 may be derived from other sources and processes besides a fumigation process. The mixed air stream 14 exiting the container 22 carries with it the removed alkyl halide nt (e.g., methyl bromide) as it enters the absorber/scrubber 12 from the bottom. During its upward travel, the mixed air stream 14 comes into contact with a counter-current ?owing stream of absorbing solution 26 that is fed from the top of the absorber/scrubber 12 and travels downwardly h the absorber/scrubber 12. In another embodiment, the mixed air stream 14 and the stream of absorbing solution 26 can ?ow through the absorber/scrubber in the same direction.

The stream of absorbing on 26 is a metal hydroxide-alcohol buffer solution, which functions to enhance the external mass transfer of alkyl halide from the mixed air stream 14 to the absorbing solution, as well as to provide a stable alkyl halide conversion media. The alkyl halide (e.g., methyl bromide) is highly soluble in the ing alcohol, thereby facilitating its transfer from the mixed air stream 14 to the absorbing solution. A lean air stream 28 comprising air and a d amount of alkyl halide is discharged from the top of the absorber/scrubber 12 and passes through an optional de-entrainment/demisting device 30 and an in-line alkyl halide detecting/quantification (i.e., measuring) instrument 32 before venting to the atmosphere A.

As the alkyl halide is super active to react with the metal hydroxide in the absorbing solution, two reaction products result from a bimolecular nucleophilic substitution reaction (SN2), wherein the metal is an alkali metal (i.e., sodium, potassium or any of the Group 1A (1) elements of the ic . One reaction product is a product alcohol which corresponds to the alkyl group of the alkyl halide and which is a liquid that is miscible with the absorbing l. The other reaction t 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 which occurs: CnH2n+1X + YOH —’ CnH2n+1OH + YX Alkyl Metal Alcohol Metal Halide Hydroxide Halide More particularly, where, for example, 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: CHsBr + NaOH —> CHsOH + NaBr Methyl Sodium Methanol Sodium Bromide Hydroxide Bromide Referring still to Figure l, stream 34, which contains metal halide and alcohols, is supplied to absorbing solution drum 36 from the absorber/scrubber 12. A small make-up stream 38, which contains absorbing alcohol, is also supplied to the absorbing solution drum 36 to sate for any absorbing l that may have been lost due to reaction with the product alcohol or evaporation into the air stream or other losses. Similarly, a make-up alkali stream 40, which contains metal hydroxide and alcohol at a certain tration, is supplied to the absorbing solution drum 36. A discharge stream 42 exits the absorbing solution drum 36 via pump 44.

After leaving the pump 44, the stream 42 is divided into the absorbing solution stream 26 and a purge stream 46. The purge stream 46 passes h a solid-liquid separation apparatus 48, such as a l Eco Separator, resulting in (i) a stream 50 containing a negligible amount of dissolved salt (e.g., metal halide), which stream 50 is returned to the absorbing solution drum 36, and (ii) ted solids 52, namely metal halide, that can be packed for sale.

The transfer of the alkyl halide fumigant from the mixed air stream 14 to the liquid absorbing solution in the absorber/scrubber 12 takes place primarily on the column g or trays (not shown) of the absorber/scrubber 12. The ing solution typically comprises an alkali- alcohol solution and may, fo r example, be methanol, ethanol, butanol, isopropanol, or combinations thereof. The mass fraction of the alcohol in the absorbing solvent will vary, in l, but in some ments the mass fraction will be from about Oto about 1. Accordingly, the water content (i.e., aqueous component) of the absorbing solvent ranges fr om about O to about 1. The concentration of metal hydroxide in the absorbing solution will vary, in general, but in some embodiments the tration will be fr om about O g/100 ml solvent to about 50 g/100 ml of solvent. The fl ow rate of the absorbing solution stream 26 as it flows into the absorber/scrubber 12 will vary depending on, amongst other things, (i) the concentration of the fumigant in the gas stream, (ii) gas stream flo w rate, (iii) the desired fum igant concentration in the outlet gas , and (iv) the solubility of the fum igant in the absorbing solution.

The fl ow rate is determinable by persons of ordinary skill in the art and is typically determined as part of the process . It is desired that the concentration of fum igant in the lean air stream 28 will be fr om a trace amount (i.e., in the parts per billion range) to an undetectable amount. The temperature of the absorption column within the absorber/scrubber 12 will be maintained at a value selected to enhance the solubility of the fu migant in the alcohol, and also inhibit evaporation of the solvent into the gas stream. In some embodiments, this ature will be kept at a value fr om about room ature (e.g., about 20°C to about 26°C) to about 50°C The operating pressure in the absorption column will depend on the re of 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 (101.33 kPa)).

The fo rmed product alcohol will, in general, be miscible with the absorbing solution, and the metal halide will precipitate, thereby tating its separation fr om the product stream of the reactor. For any absorbing solution stream, the concentration of metal hydroxide should be controlled to meet any TSS and TDS ied by the manufacturer of the pump 44, and any associated pipe design specifi cation.

As mentioned, the process of the present invention may be practiced in conjunction with fu migation of agricultural products in various containers or ures. In some embodiments, the containers may be sealed shipping containers, trailers, railway cars, mills, and warehouses.

The fu migated product may be agricultural ts, including wood products such as logs, as well as fruits or grains. The concentration of the fumig ant in the mixed air stream 14 exiting the container 22 during aeration will vary with time. In certain embodiments, the concentration will vary from about 0.25 g fumigant/g e to about 0.0 g fu migant/g mixture. The flow rate of the air stream 24 will vary depending on the alkyl halide fu migant, and the size of the container 22. In the fumigation of wood products in shipping containers, the air flow rate of the air stream 24 may be as high as 1200 ft3/min (0.567 m3/s) at room ature.

After the liquid level in the absorber/scrubber 12 remains unchanged for some time, gas flow is turned on, and the measuring of time-on-stream commences. The countercurrent flowing stream of absorbing on 26 and the gas stream to be d provides large contact area and creates a relatively higher concentration difference n the two phases, which is the g forc e for mass transfer.

Figure 2 depicts a batch process and apparatus 110 for recovering and converting an alkyl halide in accordance with the present invention. Hereinafter, this setup will be referred to as the Batch Mode Setup. Raschig rings (PTFE, L x O.D. x thickness of 3 mm x 3 mm x 1 mm) are packed into an absorber 112 to a height of 6" (152.4 mm). After such packing, 50 ml of absorbing/ reaction solution is fi rst transferred into the reservoir of the Batch Mode Setup, and the time-on-stream is measured fr om the time the gas flow is turned on. A stream of MeBr-rich gas 114 flowing from gas source 116 is regulated by a mass flow controller 118 (MFC) and passes through check valve 120 and control valve 122 on its way to an inlet 124 of the er 112. The MeBr-rich gas stream 114 is supplied to a bottom portion 126 of the packed bed, and bubbles through the flooded packed bed before exiting through outlet 128 and then passing through a gas chromatography setup 130 or vent 132 via valve 134.

Figure 3 depicts a schematic diagram of apparatus 210 which can be operated as a batch or recirculation process for recovering and converting an alkyl halide in accordance with the present invention. Hereinafter, this setup will be referred to as the ulating Mode Setup. As shown in Figure 3, the effec tive flo w path of the absorber/reactor section 212 of the ulatingMode Setup has an ID of¼" ( 6.35 mm), a height of 21.26" (54 cm) and is packed with 3 mm Raschig rings (PTFE, L x O.D. x thickness 3 mm x 3 mm x 1 mm). After such packing, 20 ml of solution is first transferred into the Recirculating Mode Setup and then circulated at a selected flow rate for the absorption s. A stream ofMeBr-rich gas 214 flowing from gas source 216 is regulated by mass fl ow controller (MFC) 218 and passes through check valve 220 and control valve 222 on its way to an inlet 224 of the absorber 212. essure liquid tography pump 226 drives the fluid flow. After the liquid level at the bottom of the absorber 212 remains unchanged for 10 min, gas flow is turned on, and the measuring of time-on-stream commences. The countercurrent flow of absorbing solution 228 and the gas stream to be cleaned provides large contact area and creates a relatively higher concentration difference between the two phases, which is the driving fo rce for mass transfer. Gas 230 exits the absorber 212 through outlet 232 and then passes through a gas chromatography setup 238 or vent 236 via valve 234.

In order to be the invention in more details, the fo llowing examples are set forth: Example 1 Batch Mode Operation: Effect of Packing The effect of packing (Figure 4) on halide absorption was d in the Batch Mode Setup using pure IPA opyl Alcohol), and the MeBr concentration in the liquid phase was measured.

The liquid phase MeBr concentration was found to rapidly increase during the ?rst 2 hours TOS (Time-On-Stream) for both cases, but at a much faster rate when using packing. With packing, the MeBr tration ches a stable value starting from the second hour and appears to reach that value, considered to be the maximum absorbing ty of the solvent, at about the third hour of the process. The packing provides large interfacial contact area between the gas and liquid, by breaking the gas bubbles, the effect of which is the enhancement of inter-phase mass transfer. Therefore, packing materials with high speci?c e area per unit volume can be used to improve the s ef?ciency for the capture of MeBr by the liquid phase.

Example 2 Batch Mode Operation: Base Solvent Screening The data shown in Figure 5 were collected from a set of experiments conducted in the Batch Mode Setup. The higher the number of carbon atoms, or molecular weight, or boiling point of the solvent, the higher is its absorbing capacity. Since Butanol is much more expensive than the other three solvents, based on cost considerations, IPA will likely be the most viable solvent.

Example 3 Batch Mode Operation: Metal Hydroxides Screening The MeBr-rich liquid solution cannot be directly disposed of without incurring signi?cant cost, effort was also made to develop a green process. Therefore, the captured MeBr can either react with the 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 may even enhance absorption because of the concomitant preservation of driving force for mass transfer.

The present invention adopts the latter approach. The two most ly used YOH (i.e., NaOH and KOH) were used to investigate the effect of the addition of YOH on the capture and conversion process in the Batch Mode Setup.

According to Table l, NaOH is generally much more soluble in water than in ls. Therefore, pure DI-HZO, and DI-HZO/alcohol mixture were first studied as the base solvent. For the latter mixture, because of the formation of two phases, an aqueous phase that contains almost all the NaOH, and the l phase that contains little NaOH, this solvent system exhibited much lower absorbing capacity than the alcohol-based systems, as shown in Figure 6.

Table 1: Solubility ofYOH in different solvents NaOH (g/100 ml solvent) KOH (g/100 ml solvent) Water 1 l l l 12 Methanol 23 .8 43 .4 Ethanol < IPA - l l The results for all the solutions tested are presented in Figure 6, and were obtained from the Batch Mode Setup. For alcohol-based solutions, the increase of YOH concentration can greatly enhance the absorbing capacity. The results also reveal that the KOH is more active than NaOH, and ethanol appears to be the best t among all studied solvents when YOH is added.

The stabilized values of % removal of MeBr from gas phase and the MeBr concentration in the MeBr-depleted gas using different absorbing solutions were corrected to account for the effect of solvent vapor pressure, and the results ized in Figure 7 and Table 2. The most ive absorbing solution is 20 g KOH/lOO ml Ethanol.

Table 2: Solvent ID reference MeBr Removal % MeBr_GC% Solvent ID Solution composition in Gas Phase: 1 1.5g NaOH+50 ml Water 12.67% 3.69% 2 50 ml IPA 13.35% 3.59% 3 3 g NaOH+45 ml IPA+5 ml H20 18-15% 3.43% 4 50 ml MeOH 11.06% 3.35% 16g NaOH+38 ml Water+12 ml Ethanol 30-76% 2.95% 6 1.5g NaOH+50 ml Ethanol 63.85% 1.51% 7 2.5 g 0 ml Ethanol 64.64% 1.48% 8 5 g NaOH+50 ml MeOH 63.65% 1.42% 9 10 g 0 ml MeOH 69.18% 1.22% 5 g NaOH+50 ml Ethanol 74.64% 1.07% 11 5 g KOH—-50 ml IPA 79.24% 0.89% 12 5 g KOH—-50 ml Ethanol 79.62% 0.86% 13 10 g KOH+50 ml Ethanol 86.12% 0.59% Example 4 Recirculating Mode Operation: ion of Suspended Solids The major challenges for the recirculation test were from the precipitation of solids due to (i) the formation of lower solubility products (from the reaction of YOH and MeBr) as well as (ii) the loss of alcohol solvents at high gas ?ow rates when no make-up alcohol solution was added to the system. Tests with 10g and 20g KOH/100 ml Ethanol at 5 ml/min recirculation rate and 40 sccm air ?ow rate (without MeBr) revealed that there was no KOH or NaOH in the precipitates which indicates that the precipitates are the products from the reaction of the YOH with MeBr.

Example 5 Recirculating Mode Operation: Effect of Gas-Liquid (G—L) Contact Time The effect of G—L contact time in the Recirculating Mode Setup was experimentally studied by keeping the absorbing solution in batch mode. The results are ized in Figure 8. It should be noted that, although there was a lag in the measurement of the 20 sccm and 30 sccm experiments, since the solution concentration was far in excess and approximately constant, the decrease of solution volume in the absorber was negligible (as there was solution above the packing, and the packing was always immersed in the solution), the gas composition at the top surface of the solution can remain constant during the 3-hour duration of the experiment. ore, the data reported in the second plot for the 20 sccm and 30 sccm results can be considered the real performance data.

As expected, the se in gas ?ow rate can effectively decrease the amount of MeBr in the gas exit stream, and the best result was 0.3 mol% at 20 sccm. Compared with the set of experiments ted in the Batch Mode Setup ted in Figure 2), where the exit gas composition was in the range of 1.05-1.07 mol%, the MeBr composition d to .7 mol% at 40 sccm. This reduction can be attributed to the reduced ID and increased length of the absorber.

Example 6 Recirculating Mode Operation: NaOH/H20 Solution as Model Solvent In order to study the performance of the s with the absorbing solution in recirculation mode while avoiding the pump blockage problem, 20 g NaOH/lOO ml H20 solution was used.

As shown in Figure 9, at 40 sccm MeBr/Air ?ow rate, liquid (20g NaOH/lOO ml H2O) ?ow rate was increased from 3.5 ml/min to 10 ml/min. The s show that the 5 ml/min liquid ?ow rate is suf?cient for achieving complete wetness of the absorber packing in the recirculation mode.

At 10 , the gas ?ow rate was reduced to 20 sccm. As expected, the MeBr mol% in gas phase was reduced compared to that at 40 sccm. The experiment was also run at 20 sccm with 35 ml solution but in non-recirculating (batch) mode. Compared with the 20 sccm/lO ml/min run, it seems the mixing due to the recirculation of liquid phase can be compensated for by the gas ?ow.

For the comparison of batch mode and recirculation mode, based on the water ulation results, there appears to be no ence for batch mode and recirculating mode at a liquid ?ow rate of 10 ml/min, although more mental runs will be needed to con?rm this observation.

Also, although this may not be applicable for the hanol system, it should be noted that the recirculation mode requires only 20 ml (or even less) of solution, while the batch mode requires 35 ml solution, for capturing and converting similar amounts of MeBr. Furthermore, for practical ion, it is better to run in recirculation mode, which will enable the stream exiting the slurry reservoir to be passed into a precipitator for removal of the solid (salt) product, and add a make-up ethanol (or solvent) stream to the s stream after it exits the precipitator. The sccm/35 ml result of 3.78 mol% is worse than that from the 40 sccm/50 ml (3g NaOH/lOO ml H2O) result of 3.69 mol% (Figure 2 Batch Mode Setup), even with the NaOH concentration increased to 20 g OO ml H2O. The main reason is that the superficial residence time in the Recirculating Mode Setup (Figure 3) is 1/32 that of the Batch Mode Setup (Figure 2).

Example 7 Recirculating Mode Operation: YOH/Alcohol solution The YOH/alcohol solution cannot be successfully processed in recirculating mode for 3 hours due to the formation of precipitates which clog the pump. For the 10g NaOH/lOO ml Ethanol solution, experiments were run (Figure 10) at different quid ratios while fixing the gas flow rate at 50 seem. As expected, the MeBr concentration in the exit gas stream decreases with the increase of L/G ratio, due to a higher coverage of the cross-section of the packing bed and increased liquid residence time.

In addition to the data presented in Figure 11, 20g KOH/100 ml Ethanol at flow rates of 5 ml/min and 10 ml/min was also tested, but the experiment had to be suspended because the pump pressure was too high (>1600 psi (8274 kPa) ) even after only 5 minutes of recirculation.

The liquid flo w rate was fix ed at 5 ml/min based on the previous experimental results using water. When the liquid fl ow rate was set to 10 ml/min, the pump pressure rose to a value >1600 psi (8274 kPa) very quickly. The ations shown in Figure 11 confirm the previous conclusion that higher YOH concentration enhances the MeBr capture, and l ts better performancethan IP A It will be understood that the embodiments described herein are merely exemplary and that a person of ordinary skill in the art may make many ions and modifications without ing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the scope of the invention as definedin the followingclaims.

Claims (16)

1. A s for capturing and converting fumigants using an absorber/scrubber apparatus, the process comprising the steps of: (a) feeding an aqueous alcohol and metal hydroxide buffer solution into the absorber/scrubber apparatus; (b) continuously g an alkyl halide fumigant/air mixture into the er/scrubber apparatus such that the alkyl halide fumigant/air mixture contacts the alcohol and metal hydroxide buffer solution, wherein a fumigant component of the alkyl halide fumigant/air e transfers from the alkyl halide fumigant/air mixture to the alcohol and metal ide buffer solution to thereby create a reaction by which, the fumigant component reacts with a metal hydroxide component of the alcohol and metal hydroxide buffer solution to yield a second mixture comprising product l, unreacted metal hydroxide, and a metal ; (c) feeding the second mixture from the absorber/scrubber apparatus to an absorbing drum and ing it with a small make-up stream, which contains absorbing alcohol, and a make-up alkali stream, which contains metal hydroxide and alcohol at a certain concentration, to form a discharge stream; (d) dividing the discharge stream into a purge stream, and an absorbing solution stream; (e) separating the metal halide from the purge stream to yield solid metal halide and a stream comprising the unreacted metal hydroxide and the product alcohol, wherein the dividing and ting steps are med continuously; and (f) recirculating the ing solution stream into the absorber/scrubber apparatus, and recirculating the stream comprising the unreacted metal hydroxide and the product alcohol into the absorbing drum, wherein the alcohol and metal hydroxide buffer solution comprises an alcohol selected from the group consisting of methanol, ethanol, l and isopropanol; and wherein the alkyl halide is methyl bromide or methyl iodide.

2. The process of claim 1, further comprising the step of discharging a treated lean air stream to rocessing equipment and measuring instrument for emission to atmosphere.

3. The process of claim 1 or claim 2, wherein the alcohol of the alcohol and metal hydroxide buffer solution includes two ls.

4. The process of any one of claims 1-3, wherein the alkyl halide fumigant/air mixture is fed into the absorber/scrubber apparatus in a first direction and the alcohol and metal hydroxide buffer solution is fed into the absorber/scrubber apparatus in a second direction, which is opposite the first direction.

5. The process of any one of claims 1-3, wherein the alkyl halide fumigant/air mixture is fed into the absorber/scrubber apparatus in a first ion and the alcohol and metal hydroxide buffer solution is fed into the absorber/scrubber apparatus in a second direction, which is the same as the first direction.

6. The process of any one of claims 1-5, wherein the metal hydroxide component comprises sodium hydroxide or potassium hydroxide.

7. The process of any one of claims 1-6, wherein the alcohol and metal hydroxide buffer on has an alcohol ent with a mass fraction of greater than 0 up to about 1.

8. The process of any one of claims 1-7, wherein the alcohol and metal hydroxide buffer on has an aqueous component with a mass fraction of greater than 0 up to about 1.

9. The process of any one of the preceding claims, wherein the metal hydroxide component has a concentration in a range of greater than 0 g/100 ml of the l and metal hydroxide buffer solution up to about 50 g/100 ml of the alcohol and metal hydroxide buffer solution.

10. The s of any one of the preceding claims, wherein the fumigant component has a concentration in a range of greater than 0 g fumigant/g fumigant/air mixture up to about 0.25 g fumigant/g of fumigant/air mixture.

11. Apparatus for capturing and converting fumigants, comprising: a first g means configured and ed for feeding an aqueous alcohol and metal hydroxide buffer solution into an absorber/scrubber; a second feeding means configured and ed for continuously feeding an alkyl halide fumigant/air mixture into the er/scrubber such that the alkyl halide fumigant/air mixture contacts the alcohol and metal hydroxide buffer solution, wherein a nt component of the alkyl halide fumigant/air mixture transfers from the alkyl halide fumigant/air mixture to the alcohol and metal hydroxide buffer solution to thereby create a reaction by which, the nt component reacts with a metal hydroxide component of the alcohol and metal hydroxide buffer solution to yield a second mixture comprising product alcohol, ted metal hydroxide, and a metal halide; an absorbing drum configured for feeding the second mixture from the absorber/scrubber tus to the absorbing drum and combining it with a small make-up stream, which contains absorbing alcohol, and a make-up alkali stream, which contains metal hydroxide and alcohol at a certain concentration, to form a discharge stream; dividing means to divide the discharge stream into a purge stream, and an absorbing solution stream; separating means ured and arranged for continuously separating the metal halide from the purge stream to yield solid metal halide and a stream comprising the unreacted metal hydroxide and the product alcohol; and a return means configured and arranged to return the ing solution stream into the absorber/scrubber apparatus, and for recirculating the stream comprising the unreacted metal hydroxide and the product alcohol into the absorbing drum, wherein the alcohol and metal hydroxide buffer solution comprises an alcohol selected from the group consisting of methanol, ethanol, butanol and isopropanol; and wherein the alkyl halide is methyl bromide or methyl iodide.

12. The apparatus of claim 11, further comprising: post-processing ent means configured and arranged for treating a lean air stream rged from the absorber/scrubber prior to its g to atmosphere, the post-processing treatment means including an instrument adapted to measure a fumigant component of the lean air .

13. The apparatus of claim 11 or claim 12, n the absorber/scrubber comprises a ic cylindrical column, or a tray tower, or a packed column.

14. The apparatus of any one of claims 11-13, wherein the absorber/scrubber is configured and arranged to be kept at a temperature of from about 20° C to about 50°

15. The apparatus of any one of claims 11-14, wherein the absorber/scrubber is configured and arranged to be kept at an operating pressure of at least about one atmosphere.

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