Improved fumigation process for stored foodstuffs by using phosphine in sealed chambers

In this paper we present an innovative device designed and constructed to improve the fumigation process for stored foodstuffs with the use of phosphine gas in sealed chambers. The device allowed a considerable reduction in phosphine production time (from about 5 to 7 days for traditional systems to 2 days for the equipment considered), maintaining the system below the inflammability threshold, and at the same time achieving the total exhaustion of aluminum (or magnesium) phosphide so as to avoid toxic residues at the end of the process. With the standard device currently available on the market, after the normal 5-7 day fumigating period, the powder residue contains as much as 1-2% (w/w) of phosphide. Thus the residues, according to current legislation, have to be considered toxic and harmful. To overcome this disadvantage, appropriate modifications were made to the cylindrical tray used for the fumigation process: a nebulizer was installed, which has the function of increasing the moisture of the air spreading around the phosphide pellets and allowing a more rapid reaction with phosphide. Moreover, the cylindrical tray was also heated by means of an electrical resistance, and temperature was checked by a thermostat, so as to always obtain the same efficiency, independently of outside temperature, for both hot and cold periods, since reaction speed depends on the system temperature considered. In addition, a control device for air saturation allows condensation processes to be avoided. Using the modified cylindrical tray we performed tests to determine the best values of humidity and temperature for the process concerned, avoiding phosphine concentrations that might result in a fire hazard, and the remixing of phosphide pellets inside the cylindrical tray. Our experimental data allowed us to obtain a mathematical model used to gain an insight into the process in question.