Part II
Evaluating and Improving Environmental Performance of Chemical Processes

Overview

Evaluating the environmental impacts of chemical processes and improving the environmental performance of chemical process designs are complex tasks involving a wide variety of analysis tools. To systematically present these tools, the group of chapters that follows is organized into a framework commonly employed in the design of chemical processes. The key steps in this framework are listed below.

1. Specify the product to be manufactured and evaluate potential environmental fate (Chapter 5), releases, and exposures (Chapter 6).

2. Establish the input/ouput structure of the chemical process, including chemical synthesis pathways and potential byproducts (Chapter 7).

3. Evaluate potential emissions and environmental impacts associated with the conceptual process (Chapter 8).

4. Specify the unit operations and process flows and identify pollution prevention opportunities (Chapter 9).

5. Systematically examine the flowsheet to identify opportunities for environmental improvements and identify opportunities for energy and mass integration (Chapter 10).

6. Evaluate the environmental performance of the detailed process flowsheet (Chapter 11).

7. Evaluate the environmental costs associated with the process (Chapter 12).

The first step in the analysis is to identify and evaluate potential environmental impacts of the chemicals to be manufactured. Evaluating environmental impacts requires knowledge of emission rates, the environmental fate of those emissions, and the potential human health and ecosystem impacts associated with the chemical’s environmental fate. Estimates of emissions, fate, exposures and impacts, in turn, rely on a host of other chemical and physical properties and data for these properties may or may not be available. Thus, the task of evaluating potential environmental impacts of new chemicals is formidable. Nevertheless, such evaluations are prudent because they can identify key environmental issues at the earliest stages of the design process. Chapters 5 and 6 describe qualitative and quantitative tools that have been developed to identify and evaluate potential environmental fates, exposures, and impacts.

The next step in the design process involves selection of raw materials and chemical synthesis pathways. For many chemicals, a variety of synthesis pathways are possible, and each pathway will require slightly different raw materials and reaction conditions and may generate different byproducts. Chapter 7 describes some of the emerging tools available for identifying and evaluating alternative chemical synthesis pathways. These tools can be used in conjunction with the environmental assessment tools described in Chapters 5 and 6 to select preferred raw materials for chemical processes.

Once the basic input/output structure of the flowsheet has been established, it is prudent to perform a preliminary evaluation of environmental impacts. Chapter 8 presents methods for assessing the environmental performance of a process when only limited, conceptual information on the process design is available.

The next major opportunity for risk reduction occurs when the process flowsheet has been established. At this stage, pollution prevention opportunities should be considered for each of the unit operations in the process. Chapter 9 presents pollution prevention methods for common unit operations.

While Chapter 9 focuses on qualitative and semi-quantitative flowsheet evaluation tools, Chapter 10 examines more quantitative approaches. The goals of these quantitative analyses will be to improve the energy and mass efficiencies of the flowsheets. Pinch analysis methods will be presented and demonstrated through case studies.

Once the structure of the flowsheet has been established, a comprehensive evaluation of potential environmental impacts can be performed. Detailed estimates of process emissions can be performed. Environmental fates of the emissions and wastes can be estimated. Potential environmental impacts can be assessed. Chapter 11 presents the analysis tools that are available for evaluating the environmental performance of a chemical process. These tools are analogous to those presented in Chapters 5 and 6, but incorporate much more detail in estimating releases and assessing environmental fates.

Finally, Chapter 12 examines the integration of information about the environmental performance of a chemical process into more traditional economic evaluations. Hidden environmental costs, potential liability costs, and less tangible costs and benefits are described. Case studies are used to illustrate the environmental accounting principles.