General Gas Vapor Treatment Systems

Batch and continuous ClO2 gas treatment systems have been developed and used for decontamination of fruits and vegetables [31-34,36-38,40-42]. Typically, the batch system (Figure 18.4) includes a ClO2 gas generator (such as a bench-scale CDG Technology or ClorDiSys Solutions generator), a treatment chamber, a diaphragm vacuum pump, and a thermo-hydro recorder. Produce samples are placed on expanded stainless steel shelves inside the chamber. ClO2 gas is generated from the generator and is stored in a Teflon storage bag before being injected into the chamber using a gas-sampling syringe. The injected volume of ClO2 gas (known concentration) is determined based on required ClO2 gas concentration for the treatment chamber volume. The injected ClO2 gas is distributed and circulated in the chamber by a diaphragm vacuum pump. Relative humidity (RH) and temperature inside the chamber are monitored using a thermo-hydro recorder. To measure ClO2 gas concentration, a certain amount (5 to 60 ml) of ClO2 gas-air mixture is taken out of the chamber using a gas-sampling syringe and dissolved in

Treatment cylinder

Treatment cylinder

CI02 gas storage

Injection

Pump

FIGURE 18.4 Schematic of a batch laboratory ClO2 gas treatment system for fruits and vegetables. (Patent pending, Purdue University.)

CI02 gas storage

Injection

Pump

FIGURE 18.4 Schematic of a batch laboratory ClO2 gas treatment system for fruits and vegetables. (Patent pending, Purdue University.)

deionized water, followed by analysis using a DPD (N,N-diethyl-p-phenylene-diamine) colorimetric analysis kit and a VVR photometer (CHEMetrics, Inc., Calverton, VA).

A continuous laboratory-scale ClO2 gas treatment system mainly consists of a bench-scale generator, a treatment chamber, a ClO2 gas dilution panel, a diaphragm vacuum pump, an ultrasonic humidifier, a wireless thermo-hygrometer, and a continuous ClO2 gas monitor. ClO2 gas from the generator is diluted with filtered air and passes through the treatment chamber. Relative humidity in the chamber is controlled using an ultrasonic humidifier. During the treatment, ClO2 gas concentration is continuously monitored using the continuous ClO2 monitor (Interscan Corp., Chatsworth, CA; http://www. gasdetection.com). An automated continuous pilot-scale ClO2 gas treatment system has also been developed (Figure 18.5). ClO2 gas is generated using a Mindox-M generator from ClorDiSys Solutions, Inc. Fruit and vegetables are placed on two movable shelves in a 4001 stainless steel chamber. After preconditioning (humidifying) the chamber using a humidifier (50 to 95%), the ClO2 gas is fed in and circulated by a gas blower. The gas concentration, RH, and pressure relief are continuously monitored and controlled by the Mindox-M generator. Each treatment cycle is programmed and run automatically. After the treatment, the products may be washed by spraying filtrated water inside the chamber for 5 to 10 minutes to reduce residual ClO2, if any residuals remain on the product and/or in the chamber.

Concentrations of ClO2 are reported either as ppm in volume or mg/l. When using ClO2 as a solution, 1 ppm is equivalent to approximately 1 mg/l. However, when ClO2 is used as a gas, 1 mg/l ClO2 is equivalent to approximately 332 ppm in volume under standard conditions (0°C, 1 atm) and 362 ppm under normal conditions (25°C, 1 atm). These conversions are based on the ideal gas law. In this chapter, the conversion factor of 1 mg/l = 362 ppm is used.

Treatment InterScan CIO; chamber gas monitor

Ultrasonic ClorDisys CI02 gas humidifier generator and monitor

FIGURE 18.5 Continuous pilot-scale ClO2 gas treatment system for fruits and vegetables. (Patent pending Purdue University.)

Treatment InterScan CIO; chamber gas monitor

Ultrasonic ClorDisys CI02 gas humidifier generator and monitor

FIGURE 18.5 Continuous pilot-scale ClO2 gas treatment system for fruits and vegetables. (Patent pending Purdue University.)

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