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1.2: Representation of Chemical Processes

  • Page ID
    18141
  • Chemical processes are inherently complex. In a continuous chemical plant (Shreve’s Chemical Process Industries, 1998) such as we have illustrated in Figure \(\PageIndex{1}\), raw materials are prepared, heated or cooled, and reacted with other raw materials.

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    Figure \(\PageIndex{1}\): Simplified flowsheet for the manufacture of ethyl alcohol from ethylene

    The products are heated or cooled and separated according to specifications. A chemical plant, including its utilities, has many components such as chemical reactors, distillation towers, heat exchangers, compressors and pumps. These components are connected to each other by pipelines or other means of transportation for carrying gases, liquids, and solids. To describe these complex systems chemical engineers use two fundamental elements:

    1. Structure: This is the manner in which the components of a plant are connected to each other with pipelines or other means of transportation. The structure is unique to a plant. Two components connected in different sequences can completely alter the nature of the products. Structure is represented using flowsheets. A complete version of a flowsheet, including all utilities, control, and safety devices is known as the Piping and Instrumentation Diagram (P+ID). Figure 1‐5 is a pictorial representation of a simple flowsheet.
    2. Performance: This is the duty or basic operating specifications of the individual units. The duty is described using Specification Sheets for all units of the process and by listing the properties of the streams connecting the units. The properties of the streams include flow rates, compositions, pressures and temperatures. In relatively simple systems, a single document includes the flowsheet and the properties of the streams. To describe complex systems, one needs several flowsheets as well as a collection of specification sheets.

    To perform material balances for complex systems, one uses information about the structure of the flowsheet and the performance of the units to determine the properties of the connecting streams. The processes illustrated in Figures 1‐1 through 1‐5 appear to be dramatically different; however, the fundamental concepts used to analyze these systems are the same. Hidden behind the complexity of these processes is a simplicity that we will describe in subsequent chapters. To make this point very clear, we consider the complex system illustrated in Figure \(\PageIndex{1}\) and we identify the scrubber as the object of a separate analysis as illustrated in Figure \(\PageIndex{2}\). In this text, most of our effort will be directed toward the analysis of single units such as scrubbers, distillation columns, chemical and biological reactors, in addition to systems such as Mono Lake. After establishing the framework for the analysis of single units, we will move on to a study of more complex systems in Chapter 7. Transient processes are examined briefly in Chapter 8, and an introduction to reaction kinetics is provided in Chapter 9.

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    Figure \(\PageIndex{2}\): Analysis of an individual unit

    The concept of analyzing small parts of a problem and then assembling the small parts into a comprehensive representation of the whole problem is extremely important. In addition, the concept of studying the whole problem and then breaking it apart into smaller problems is also extremely important. For example, a chemical complex, such as the one shown in Figure 1‐7, can be seen in terms of a sequence of progressively smaller elements. The entire production system consists of a natural gas plant that provides feed for an ethylene plant which, in turn, produces feedstocks for a vinyl chloride plant. Within the vinyl chloride plant there are various units such as reactors, distillation columns, etc., in addition to feed preparation units, secondary distillation units, utilities, and emergency systems such as a flare and the vessels associated with it. Similar units exist in the natural gas plant and the ethylene plant, and we have not shown those details in Figure 1‐7.

    However, we have shown the details of the mass transfer unit that must be analyzed as part of the design of the vinyl chloride plant. In addition, we have illustrated the details of the gas‐liquid mass transfer process that lies at the foundation of the design of the mass transfer unit. Clearly there is a series of length scales associated with the production of vinyl chloride and there is important analysis to be done at each length scale.

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    Figure \(\PageIndex{3}\): Hierarchy of length scales associated with the production of vinyl chloride

    The circles illustrated in Figure \(\PageIndex{3}\) represent control volumes that we use for accounting purposes, i.e., we want to know what goes in, what goes out, and what is accumulated or depleted. In some cases, we do not need to know what is happening inside the control volume and we are only concerned with the inputs and outputs of the control volume. This situation is suggested by Figure 1‐8 where we have shown only the inputs and outputs for the vinyl chloride plant. If both the steady state and dynamic behavior of the systems associated with the natural gas plant, the ethylene plant, and the vinyl chloride plant are known, the behavior of the vinyl chloride production system is also known. However, to learn how those systems behave, we need to move down the length scales to determine the details of the various processes. This is illustrated in Figure \(\PageIndex{4}\) where we have shown a mass transfer unit that is one element of the vinyl chloride plant, and we have shown a bubble at which mass transfer takes place within the mass transfer unit. In Figure \(\PageIndex{4}\) we have illustrated the concept that we must be able to keep track of molecular forms at a variety of length scales.

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    Figure \(\PageIndex{4}\): Control volume representation of the vinyl chloride plant 

    As another example of the importance of keeping track of molecular species in both large and small regions, we consider the problem of lead contamination in California (see Figure 1‐9). Stedling et al. (2000) suggest that we should keep track of lead in the San Francisco Bay estuary system; however, the lead that appears in the estuary comes from several sources. Endless weathering of granite in the Sierra Nevada mountains releases lead that is transported by streams and rivers and eventually arrives in the bay. Other lead comes from hydraulic mine sediments transported across the Central Valley and into the bay during the nineteenth century.

    Finally, the lead generated from the earlier use of leaded gasoline has made its way into the estuary by a variety of paths. Within the estuary itself, the impact of lead contamination varies. In the shallow salt marshes, seasonal floods and daily tidal flows have a small effect on the transport of lead, and the local bio‐reactors are confronted with an unhealthy diet. Clearly the study of lead contamination in the San Francisco Bay estuary requires keeping track of lead over a variety of length scales as we have indicated in Figure 1‐9. The analysis of this lead contamination process in Northern California has some of the same characteristics as the analysis of water conservation in Mono Lake, of stack gas scrubbing in a coal‐fired power plant, of cell growth in a chemostat, and of vinyl chloride production. In this text we will develop a framework that allows us to analyze all of these systems from a single perspective based on the axioms for the mass of multicomponent systems. This will allow us to solve mass balance problems associated with a wide range of phenomena; however, chemical engineers must remember that in addition to these physical problems, there are economic, environmental, and safety concerns associated with every process and these concerns must be addressed.

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    Figure \(\PageIndex{4}\): Multiple regions associated with lead contamination