Ethylene Treatment of Post-Harvest Agricultural Storage Using Cyclic Operation of Plasma/Adsorption-Catalytic Process

During the storage, agricultural products continue their metabolic processes and physiological responses before harvesting, resulting in quality changes related to afterripening. In particular, ethylene produced from the respiration process of agricultural products acts as a plant hormone that regulates the growth of plants such as seed germination and ripening. The increase in ethylene concentration in the storage facility significantly reduces the commercial value of agricultural products. The negative effects of ethylene include leaf yellowing, change in texture, and russet spotting. Even though there are several technologies available for controlling ethylene, practical applications are scarce. Non-thermal plasma process is a relatively new technology, in which ethylene is removed by a variety of plasma-induced reactive species. The plasma process has several advantages of high response speed and miniaturization of the system, while it has a poor selectivity to carbon dioxide and may generate harmful byproducts. Plasma-catalyst hybrid process has been actively studied as a method to overcome the disadvantages while maintaining the merits of the plasma treatment method. However, it is a great disadvantage to continuously supply electric energy for plasma generation. In this work, plasma/adsorption-catalytic process was investigated, which consists of repeated cycles of adsorption step to concentrate ethylene and plasma-catalyzed decomposition step. Since no electricity is used for the adsorption step, the operating cost can be greatly reduced and the adsorbed ethylene can be completely oxidized by the catalyst. The plasma reactor is filled with a catalytic adsorbent capable of highly concentrating ethylene. In the first step, ethylene is passed through the reactor to adsorb and concentrate ethylene, and in the next step, a high voltage is applied to the plasma reactor to rapidly decompose the previously adsorbed ethylene. The Ag/zeolite (or Pd/zeolite) catalytic adsorbent prepared by ion exchange method was found to be capable of efficiently adsorbing ethylene having high vapor pressure. When the catalytic adsorbent was directly exposed to the plasma discharge, plasma-induced reactive species accelerated the oxidation and increased the carbon dioxide yield in the decomposition step. The proposed ethylene treatment system was able to greatly reduce the ethylene oxidation time and ultimately the energy consumption. In addition, the cyclic treatment consisting of a long-time adsorption step and a short-time oxidation step led to a reduction in toxic by-products such as ozone and nitrogen-containing compounds as compared to typical continuous plasma treatment process.