Ho

Steam

Condenser

Heater

Feed

Steam

Steam

Steam

Product A

Furnace

Product B

Next, the focus is on the nonuseful streams. Checklist B (see Table 3.9.6) should be completed for each nonuseful stream. Each nonuseful stream should be analyzed as follows:

Can it be eliminated or minimized at the source? If not, can the need for waste treatment be avoided or minimized via reuse, recycling, or co-product sale? If not, the stream must be treated or rendered as nonhaz-ardous to the environment. (Waste treatment is discussed in Step 8.)

The first two routes often result in attractive economic returns in addition to the environmental benefits. Treatment, while having environmental benefits, seldom has an economic return. For example, separating a gaseous raw material from a reactor purge stream for recycling or reuse may be advantageous. Removing VOCs from the air leaving the dryer involves passing the air through a carbon bed and recirculating it to the heater and dryer.

Reactors can be redesigned to minimize the buildup of latex with a polished interior surface.

Vapor-liquid and liquid-liquid separators can reduce pollution in a plant. Figure 3.9.1 shows a process flowsheet before the detailed engineering phase. Table 3.9.7 summarizes the points in Figure 3.9.1 where separation devices need to be considered. Since the quantities of the recovered material are small, high-value products are the most likely targets for separators, especially from liquid-liquid separators. However, for vapor-liquid separators, the cost is usually low since only the cost of a separator pad is normally involved, and moderately valued products can be targets for these separators (Woinsky 1994).

A major problem that interferes with the ability of operating plants to minimize their waste is a lack of adequate process measurements, such as flows and pH. Installing measurement devices where the waste is generated allows individual streams with waste-reduction potential to be identified and controlled (Jacobs 1991).

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