Trapping/Destroying Methyl Bromide on Activated Carbon
Following Commodity Fumigation

James G. Leesch, research entomologist, USDA–ARS, Horticultural Crops Research Laboratory, Fresno, CA 93727 and Gerhard F. Knapp, chemical engineer and President of GFK Consulting Ltd., San Clemente, CA

Methyl bromide is an extremely useful fumigant for the disinfestation of perishable and durable commodities prior to sale either in the domestic market or abroad. For years it has formed the backbone of the U.S. fight against the introduction of unwanted pests into this country or exporting unwanted pests to other countries on our commodities. It forms the basis for lifting export bans against quarantine insects in many countries around the world and is largely responsible for our ability to export commodities to partner nations. As it stands now, methyl bromide will be withdrawn from use and production on January 1, 2001 in the United States, as mandated by the Clean Air Act. USDA's Agricultural Research Service (ARS) has been heavily involved since 1994 in finding alternatives to methyl bromide because of its importance in U. S. agriculture. It has been estimated that the loss of methyl bromide without alternatives would amount to a loss in California alone of between $300 and $400 million in exports.

With this in mind, in 1995, ARS, along with a private cooperator, GFK Consulting Ltd., began conducting research on capturing methyl bromide on activated carbon so that, following commodity fumigation, little or no methyl bromide gas would escape to the atmosphere. Of course, the use of this system is based on the possibility that some legislative adjustments will be made to the Clean Air Act so that methyl bromide will be available for use in commodity pretreatment or quarantine situations beyond the 2001 deadline now imposed by the Act. In 1997, Great Lakes Chemical Corporation joined the cooperative research and development agreement (CRADA) in order to transfer the technology into practical uses within the agricultural community.

Research to date has identified the most favorable type of carbon for the sorption of methyl bromide as well as the best conditions for the sorption. Activated carbon derived from coconut shells has proven to sorb more methyl bromide than that derived from either coal or peat. The sorption of methyl bromide has been found to be inversely proportional to both the temperature and humidity of the gas stream containing the methyl bromide. Typical loading of the carbon with methyl bromide runs from 8 percent to 14 percent (grams of methyl bromide per 100 grams activated carbon) under the concentration and flow conditions commonly found in agricultural commodity fumigations. Recent research has also shown that carbon used to adsorb methyl bromide following desorption of previously sorbed methyl bromide, is most effective when the desorption, or regeneration, process is carried out at high rather than low temperature.

In conducting the research, we paid particular attention to designing the system to fit procedures presently used in typical commodity fumigations. When a fresh commodity, such as plums, grapes or cherries, is fumigated, it is very important to remove the methyl bromide quickly following the fumigation because many such fresh commodities are damaged by over-exposure to the fumigant. Typically, aeration fans remove 10 percent to 15 percent of the volume under fumigation per minute. Therefore, concentrations of methyl bromide go from very high (i.e., 15,000 ppm) to low concentrations (i.e., 500 ppm) in about 30 minutes. The adsorption unit has been tested both in the laboratory and in pilot versions using an initial concentration of 64 milligrams per liter (about 16,000 ppm) and an air flow through the carbon bed that was based on a 10,000 cubic feet per minute ventilation rate taken from a 72,000 cubic foot chamber.

To date, testing has shown that from 8 percent to 14 percent loading (grams MB per 100 grams activated carbon) of the activated carbon is common over a wide variety of temperatures and humidities. In addition, repeated use of carbon for capturing methyl bromide showed that volatiles from oranges had no effect on the amount of methyl bromide sorbed during each trapping cycle.

The desorption of the methyl bromide after each trapping cycle and the reactivation of the carbon can be carried out at a centrally located reclamation facility to which the carbon container with the methyl bromide on carbon can be shipped. The methyl bromide is desorbed from the spent carbon and thermally oxidized. The process yields sodium bromide which can then be recycled back into the chemical industry. In the process of desorbing the methyl bromide, the carbon is reactivated and returned to a container for reuse in capturing more methyl bromide.

Recently, a pilot unit was built to conduct tests on actual commodity fumigations. The pilot unit was first tested in 1997 at Valley Fig Growers in Fresno, California, and subsequently on Kiwi fruit in the Port of San Pedro as well as several sites on the eastern seaboard. In most of these tests, only a portion of the effluent stream of fumigant-ladened gas from the chamber was diverted to the pilot unit. However, at two sites, the entire fumigant stream from the chamber was diverted through the unit. During a tarpaulin fumigation of yams from Costa Rica, 83 percent of the methyl bromide used was trapped on the carbon. The reduction of methyl bromide in the vent stream for this type of fumigation will exceed 95 percent.

Today, methyl costs about $2 per pound. We estimate about $15 per pound would cover the methyl bromide, the transport of the container with carbon to the fumigation site, and removal of the container when the carbon is expended. The fumigation facility would be responsible for providing a fan to pull the exhaust gas through the ducting and carbon container and for the ducting to link the container to the fumigation chamber. The cost is a seven fold increase based on methyl bromide. However, based on a 72,000 cubic foot chamber that holds 51,000 flats of grapes, for example, that cost represents only an increase of seven cents in a flat of grapes.

The activated carbon removes all the methyl bromide from the exhaust stream until the carbon becomes saturated. The capture system is capable of removing 95 percent of the available methyl bromide from the fumigation facility. Evacuating more fumigant is possible but not practical because so much activated carbon is wasted collecting concentrations below 500 ppm. The capture unit could trap methyl bromide from any commodity fumigation being conducted either in a chamber, indoors under a tarpaulin, or in any sealed structure. The first commercial units for methyl bromide capture should be available in early 1999.

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Last Updated: October 6, 1998