Process Engineering for Pollution Control and Waste Minimization_10

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Process Engineering for Pollution Control and Waste Minimization_10 pollution prevention follows the axiom, “an ounce of prevention is worth a pound of cure.” The U.S. Pollution Prevention Act of 1990 and pollution prevention experts conclude that it makes far more sense for a waste generator not to produce waste in the first place, rather than developing extensive, never-ending treatment schemes (1). For industrial pollution prevention, two general approaches are used to characterize processes and waste generation. The first approach involves gather- ing information on releases to all media (air, water, and land) by looking at the output end of each process, then backtracking the material flows to determine the various waste sources. The other approach tracks materials from the point where they enter a facility, or plant, until they exit as wastes or products. Both approaches provide a baseline for understanding where and why wastes are generated, as well as a basis for measuring waste reduction progress. The steps involved in these characterizations are similar and include gathering background information, defining a production unit, general process characterization, under- standing unit processes, and completing a material balance. These steps, when performed systematically, provide the basis for a pollu- tion prevention opportunity assessment. It begins with a complete understanding of the various unit processes and points in these processes where waste is being generated and ends with the implementation of the most economically and technically viable options. It may be necessary to gather information to demon- strate that pollution prevention opportunities exist and should be explored. Often, an assessment team is established to perform the steps along the way (2). A preliminary assessment of a facility is conducted before beginning a more detailed assessment. The preliminary assessment consists of a review of data that are already available in order to establish priorities and procedures. The goal of this exercise is to target the more important waste problems, moving on to lower-priority problems as resources permit. The preliminary assessment phase provides information that is needed to accomplish this prioritization and to assemble the appropriate assessment team (3). A subsequent detailed assessment focuses on the specific areas targeted by the preliminary assessment. Analyzing process information involves preparing a material and energy balance as a means of analyzing pollution sources and opportunities for eliminating them. Such a balance is an organized system of accounting for flow, generation, consumption, and accumulation of mass and energy in a process. In its simplest form, a material balance is drawn up according to the mass conservation principle: Mass in = mass out – (generation + consumption + accumulation) If no chemical or nuclear reactions occur and the process progresses in a steady state, the material balance for any specific compound or constituent is as follows: Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. Mass out = mass in A process flow diagram may be helpful by providing a visual means of organizing data on the material and energy flows and on the composition of streams entering and leaving the system (see Figure 1). Such a diagram shows the system boundaries, all stream flows, and points where wastes are generated. Boundaries should be selected according to the factors that are important for measuring the type and quantity of pollution prevented, the quality of the product, and the economics of the process. The amount of material input should equal the amount exiting, corrected for accumulation and creation or destruction. FIGURE 1 Example flow diagram (3). Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. A material balance should be calculated for each component entering and leaving the process, or other system being studied. A suggested approach for making ...