The Path Diagram

Similarly, on the liquid path, the load of the pollutant in the uth liquid source is given by 

Similarly, one can develop the liquid path diagram as shown in Fig. 7.4, It involves three nodes, w = 1—3, which are related by the stripper and the biotreatment units. 

The path diagram provides the big picture for mass flow from a species viewpoint. This is a fundamentally different vision from the equipment-oriented description of a process (the flowsheet), in which the big picture is lost. The path diagram can also be used to determine the effect on the rest of the diagram of manipulating any node. In addition, as will be shown later, it provides a systematic way for identifying where to remove the pollutants and to what extent they should be removed. 

The path diagram for this process is given by Fig. 7.3. The first step in Fig. 7.3 is a stripping process in which the polltthuit is transferred from a liquk) stream (source w = 1) to a gaseous stream (stMirce v = 1). Because of the countercmrent contact of the two streams, die m e for each stream and 0V 

As mentioned before, the path diagram can be used to predict the effect of intercepting one stream on the rest of the streams. In order to quantify this relationship, let us consideF that all gaseous and liquid nodes on the path diagruns are intercepted. Upon interception, the composition and load of the vth gaseous node are altered 

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In order to demonstrate the construction of the path diagram, consider Fig. 7.2 (El-Halwagi et al., 19%), which illustrates a section of a process involving the pollutant laden streams. Figure 7.2 also shows the various loads and compositions of the pollutant throughout the process. For a given pollutant-laden stream v, the term is the flowrate of the stream, y is the composition of the pollutant, and d>i, is the load of the pollutant in the stream, defined as... 

Figure 7.6 Screening MSAs using the path diagram (from Ei-Hatwagi et al., 1996. Reproduced with permission of the American Institute of Chemical Engineers. Copyright 1996 AlChE. All rights reserved).

Figure 7.8 is a schematic representation of the path diagram for the liquid sources. 

Having pre-screened the interception strategies, we should integrate these strategies with the path diagram as illustrated by Fig. 7.5. First, the path-diagram equations (P7.1) should be revised to include potential interception of all nodes as follows ... 

The path diagram marks the correlations of all primary independent variables and their assumed causal influence on other independent variables and on dependent variables. In addition, all relationships between dependent variables are indicated. 

For a risk snapshot , the connection between cause and effect can be depicted with the aid of the path diagram for mutually independent elements (O, F, and C) in Fig. 5 ... 

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