Discharge reactors, chemical processing

Industrial chemical processes rarely involve just one process unit. One or more chemical reactors are often present, as are units for mixing reactants, blending products, heating and cooling process streams, separating products from each other and from unconsumed reactants, and removing potentially hazardous pollutants from streams prior to discharging the streams to the plant environment. Before we analyze such processes, we must take a closer look at what we mean by a system. 

At the basic level CVD reactors fall into two classifications—open and closed reactor systems. In the closed CVD system the precursors are loaded into the CVD chamber together with the specimens to be treated. The system is then closed and the temperature increased to initiate the chemical process. The process continues for a time sufficient to produce the required effect. The temperature is then reduced to ambient so that the reactor may be opened and the specimens removed. This reactor design is frequently used for the purification of metals and chromizing parts. The dominant type of CVD reactor is the open type. Flowing precursors continuously enter this reactor and the gaseous by-products of the chemical process are continuously removed from the reactor (usually with a vacuum pump) and appropriately treated for discharge into the environment. 

The possibilities of extremely rapid processing and the production of useful new products stimulates interest in the development of plasma reactors for the chemical process industries. Categories for potential near-term application of the discharge reactor include (8) ... 

Galvanic cells in which stored chemicals can be reacted on demand to produce an electric current are termed primaiy cells. The discharging reac tion is irreversible and the contents, once exhausted, must be replaced or the cell discarded. Examples are the dry cells that activate small appliances. In some galvanic cells (called secondaiy cells), however, the reaction is reversible that is, application of an elec trical potential across the electrodes in the opposite direc tion will restore the reactants to their high-enthalpy state. Examples are rechargeable batteries for household appliances, automobiles, and many industrial applications. Electrolytic cells are the reactors upon which the electrochemical process, elec troplating, and electrowinning industries are based. 

In chemical laboratories, small flasks and beakers are used for liquid phase reactions. Here, a charge of reactants is added and brought to reaction temperature. The reaction may be held at this condition for a predetermined time before the product is discharged. This batch reactor is characterized by the varying extent of reaction and properties of the reaction mixture with time. In contrast to the flasks are large cylindrical tubes used in the petrochemical industry for the cracking of hydrocarbons. This process is continuous with reactants in the tubes and the products obtained from the exit. The extent of reaction and properties, such as composition and temperature, depend on the position along the tube and does not depend on the time. 

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