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FIG. 3.8.3 Ways of dealing with feed impurities. (Reprinted, with permission, from Smith and Patela, 1992.)

they should be removed before processing. No heuristic seems to be available in enough quantity to handle the amount of the inerts. If the impurities do not undergo reactions, they can be separated out after the reaction (see parts b and c in Figure 3.8.3).

If the impurities do not undergo reactions, a purge can be used (see part d in Figure 3.8.3). This way saves the cost of a separator but wastes useful feed material in the purge stream.

Of the preceding options, the greatest source of waste occurs when a purge is used. Impurities build up in recycling and building up a high concentration minimizes the waste of feed material and product in the purge. However, two factors limit the extent to which feed impurities can be allowed to build up:

High concentrations of inert material can have an adverse effect on the reactor performance. As more and more feed impurities are recycled, the recycling cost increases (e.g., through increased recycling gas compression costs) to the point where that increase outweighs the savings in raw material lost in the purge.

In general, the best way to deal with a feed impurity is to purify the feed before it enters the process. In the iso-propyl alcohol process (see Figure 3.8.2), the propane, an impurity in propylene, is removed from the process via a purge. This removal wastes some propylene together with a small amount of isopropyl alcohol. The purge can be virtually eliminated if the propylene is purified by distillation before entering the process.

Many processes are based on an oxidation step for which air is the first obvious source of oxygen. Clearly, because the nitrogen in air is not required by the reaction, it must be separated at some point. Because gaseous separations are difficult, nitrogen is normally separated using a purge, or the reactor is forced to as high a conversion as possible to avoid recycling. If a purge is used, the nitrogen carries process materials with it and probably requires treatment before the final discharge. When pure oxygen is used for oxidation, at worst, the purge is much smaller; at best, it can be eliminated altogether.

In the oxychlorination reaction in vinyl chloride production, ethylene, hydrogen chloride, and oxygen react to form dichloroethane as follows:

If air is used, a single pass for each feedstock is used, and nothing is recycled to the reactor (see Figure 3.8.4). The process operates at near stoichiometric feedrates to reach high conversions. Typically, 0.7 to 1.0 kg of vent gases are emitted per kilogram of dichloroethane produced.

When pure oxygen is used, the problem of the large flow of inert gas is eliminated (see Figure 3.8.5). Unreacted gases can be recycled to the reactor. This recycling allows oxygen-based processes to operate with an excess of eth-ylene thereby enhancing the hydrogen chloride conversion without sacrificing the ethylene yield. Unfortunately, this

Refrigerated Condenser

Cooling Water Condenser

Ethylene

Hydrogen Chlorid^

Cooling Water Condenser

Hydrogen Chlorid^

Reactor

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