As the new millennium approaches, so does the proposed methyl bromide phaseout. The scientific community is delving deep to find possible alternatives to methyl bromide. But a single solution may not exist.
A Canadian company, Knowzone Solutions, Inc., Etobicoke, Ontario, believes containing, recapturing and recycling methyl bromide offers the potential to maintain its beneficial uses, yet dramatically reduce ozone-damaging emissions into the atmosphere.
Praxair Canada, Inc., granted Knowzone Solutions, Inc., the worldwide right to the patented Bromosorb™ process that contains, recaptures and recycles methyl bromide.
"The Bromosorb™ process provides a unique opportunity to capture approximately 95 percent of the methyl bromide currently being released to the atmosphere," says Errick Willis, president of Knowzone Solutions.
In chamber or space fumigation, most of the methyl bromide is vented into the atmosphere. Soil fumigation uses a predetermined amount of methyl bromide in a designated space for a specific residency period. Approximately 50 percent of the initial amount of methyl bromide is absorbed and the rest is vented to the atmosphere at the end of this period.
"Our tests on cherries and grapes in chamber fumigation have shown approximately 20 percent of the methyl bromide is either consumed or absorbed by the fruit or its containers, with the balance vented to the atmosphere. The amount of methyl bromide absorbed varies, depending on the commodity and how it is fumigated," says Willis.
As part of a joint collaboration to help the Chilean Horticultural Industry meet environmental obligations to reduce the emissions of ozone-depleting substances, Environment Canada is helping build a methyl bromide recovery plant. The plant has been installed on the premises of David del Curto SA in Kalinka, Chile, where grapes are fumigated.
As part of the project, Knowzone Solutions developed and supervised a testing program to ensure the plant met its intended performance specifications. Don Smith, a leading expert on recycling technology, observed the testing and provided a report for the Executive Committee of the Multilateral Fund. (Copies of the report are available from Errick Willis, Ph: 416-622-7920 or fax: 416-622-6723.)
The stainless steel methyl bromide recovery plant is a self-contained unit that sits on a concrete base adjacent to the fumigation chamber. It is connected by PVC ducts to one of the fumigation chambers. The solid-wall fumigation chamber is equipped with internal fans to ensure good fumigant mixing throughout the chamber.
Gas comes into the recovery plant from a low position on the back wall of the chamber, and the return line enters in the back near the top of the chamber, feeding into the internal circulation duct. Electrical controls and instruments are contained in a cabinet. Since the fumigation chamber was designed to operate automatically, an operator is only needed to start the various phases of the operation and to monitor the gas concentration from time to time.
To initiate the fumigation process, the chamber is filled with fruit and methyl bromide is introduced into the chamber in accordance with label instructions. Once the commodity, soil, or space has been successfully fumigated, the methyl bromide-containing air is circulated through the Bromosorb™ unit where methyl bromide is adsorbed on Halozite®, a type of zeolite, and the exhaust is recycled through the fumigation area. This cooled-loop circulation continues until the concentration of methyl bromide at the inlet and outlet of the Bromosorb™ unit are equalized. At this point, the exhaust stream containing less than 50 parts per million (ppm) of methyl bromide is released into the atmosphere to make the area safe for personnel.
"For the second cycle, we load the fumigation chamber while the adsorbent is heating," Willis explains.
Zeolite holds more methyl bromide when it is cold and much less when warm. This different adsorptive capacity is used to release methyl bromide from the zeolite so it can be returned to the fumigation chamber. The adsorbent material rejects water, so its efficiency is unaffected in high humidity. It also has greater adsorptive capacity than carbon at low inlet concentrations, so it can operate efficiently in concentrations of less than 500 ppm.
The heating cycle continues until the temperature of the internally circulating gas that leaves the zeolite reaches 285 oF (140.6 oC). Next, the internal circulation fan is started and methyl bromide is introduced from the recovery plant into the chamber. The desorption of methyl bromide into the chamber from the hot zeolite begins. At this point, additional methyl bromide can be added. The zeolite is then cooled and fumigation continues.
Recaptured methyl bromide is stored on the Halozite® adsorbent until it is needed again. If the Bromosorb™ unit is permanently attached to a fumigation chamber, methyl bromide can be regenerated while the fumigation chamber is emptied of the treated commodity and refilled. Regeneration occurs in a closed loop with air heated to 285 oF, which releases the captured methyl bromide from the Halozite® adsorbent.
While in the closed loop, the recycled methyl bromide is tested to ensure its chemical composition meets the profile of virgin methyl bromide. The gas chromatograph also measures the concentration of methyl bromide so a precise calculation can be made of the additional amounts that must be added and to ensure that fumigation is consistent with the label instructions. Once the fumigation chamber has been reloaded with the commodity, the recycled methyl bromide is reintroduced to the chamber along with the specified amount of virgin methyl bromide, and the process repeats itself.
"We put the Chilean plant through seven complete cycles before the performance tests," says Willis. "In all, the plant underwent 12 cycles in a week. The testing program included five runs. We conducted four empty-chamber fumigation runs and we did one fumigation on a pallet of fruit to see if there were any gross changes in plant performance or methyl bromide purity. The fumigation chamber was left sealed until the end of run 3, then vented in preparation for run 4, where we used fruit. The chamber was also vented after runs 4 and 5.
"We concluded that one pallet of fruit in a 283m3 chamber would not generate sufficient water vapor or volatile compounds to carry out definitive purity tests. We realized that testing methyl bromide residues on the fruit from only one pallet would not be realistic," says Willis.
The runs were performed at different times during the day and evening with a range of ambient temperatures. However, there was only a small difference in the rate of cooling. If the recovery plant is going to be used when ambient temperatures are much higher, attention should be given to the cooling rates and subsequent adsorption process to ensure the recycle process occurs within a 2-hour time limit. The performance specification called for a reduction in methyl bromide concentration in the chamber to 500 ppm within 45 minutes.
"There was a loss of approximately 1.6 Kg of methyl bromide in each run," notes Willis. "The most probable cause is leaks in the connection between the chamber and the recycling unit, because there was no pressure change during the heating and cooling stream. Circulating gases undergo a temperature change from approximately 100 oF during cooling to hotter than 300 oF during heating. A pressure increase of 0.35 bars (5.25 pounds per square inch) would have been expected from a closed system without zeolite.
"Another potential cause of the methyl bromide loss is a breakdown of the methyl bromide on the zeolite. There was no indication on the gas chromatograph of any other compound being formed and there was no odors detected that would be present with acids," says Willis.
These tests were carried out to achieve a minimal fumigant concentration of 48 g/m3, which is equivalent to a 95.5-percent reduction. The adsorption time was extended to 60 minutes for all runs. In the fifth run, a concentration of 616 ppm (94 percent) was achieved. Adsorption performance results are very repeatable.
Further tests with the fumigation chamber loaded with fruit will be necessary. These tests are required to see if the fruit and its packaging significantly affect the adsorption and desorption process and the amount of time needed. They will also show whether the recycled methyl bromide contains unacceptable impurities.
A portable unit, the Bromosorb™ is ideal for space or soil fumigation. The Halozite® adsorbent is maintained at ambient pressure and the methyl bromide remains stable indefinitely. This allows the portable Bromosorb™ unit to be moved to the next fumigation location where the adsorbent is regenerated and the methyl bromide can be used.
"If broadly applied, the Bromosorb™ technology can dramatically reduce the amount of damaging methyl bromide being released to the atmosphere, while preserving its use as an important fumigant," says Willis.