Читать книгу Introduction to Desalination - Louis Theodore - Страница 45

The complete design specification for a medium-sized chemical process would cover several hundred pages. It would include diagrams, tables, and discussion of all aspects of the plant, including chemical, environmental, mechanical, electrical, metallurgical, and civil engineering considerations. To understand a complete design, an engineer or scientist must have training and experience in this area.

Any plant whether for the production of canned fruit, vacuum cleaners, sulfuric acid, or potable water may be visualized as a box into which raw materials and energy are fed and from which useful products, waste, and energy emerge. Ordinarily the manufacturing process involves a number of consecutive operations or steps through which the materials in the process pass.

Figure 3.1 is a schematic representation of materials passing into and out of an unidentified process. In this hypothetical three-step process, raw materials A and B are fed into Step 1; material C is drawn off while material D is passed to Step 2 for further processing. One cannot tell whether C is a useful product or waste from the sketch. In Step 2, it is necessary to combine raw material E with D in order to produce F. In Step 3, material F is separated into G, H, and I, ending the process. Again, the figure does not show which of these last three items are useful products. The arrows lettered A, B, C, etc., represent material streams, and the sketch is known as a flow diagram.

Figure 3.1 Flow diagram for a three-step manufacturing process.

Although nothing has been said about the specific nature of the process, this flow diagram nevertheless conveys a great deal of information; namely, that in this three-step process, three raw materials A, B, and E, are required to produce four products C, G, H, and I, and that these seven materials enter and leave the manufacturing process at the points shown on the diagram (Reynolds 1992).

As one might expect, a process flow diagram for a chemical or environmental plant is usually significantly more complex than that for a simple facility. For the latter case, the flow sequence and determinations often reduce to an approach that employs a “railroad” or sequential type of calculation that does not require iterative calculations (Reynolds 1992).

To the environmental engineer, but particularly the chemical engineer, the proceeding flowchart is the key instrument for defining, refining, and documenting a process. The process flow diagram is the authorized process blueprint, the framework for specifications used in equipment designation and design; it is the single, authoritative document employed to define, construct, and operate the process (Kauffman 1992).

There are several essential constituents to a detailed process flowchart beyond equipment symbols and process stream flow lines. These include equipment identification numbers and names; temperature and pressure designations; utility designations; mass, molar, and volumetric flow rates for each process stream; and a material balance table pertaining to process flow lines. The process flow diagram may also contain additional information such as energy requirements, major instrumentation, environmental equipment (and concerns), and physical properties of the process streams. When properly assembled and employed, a process schematic provides a coherent picture of the overall process; it can pinpoint some deficiencies in the process that may have been overlooked earlier in a study, e.g. instrumentation overkill, byproducts (undesirable or otherwise), and recycle needs. Basically, the flowchart symbolically and pictorially represents the interrelation among the various flow streams and equipment, and permits easy calculation of material and energy balances.

Various symbols are universally employed to represent equipment, parts, valves, piping, etc. Some of these are depicted in the schematic in Figure 3.2. Although a significant number of these symbols are used to describe chemical and petrochemical processes, only a few are needed for simpler facilities. These symbols obviously reduce, and in some instances replace, detailed written descriptions of the process. Note that many of the symbols are pictorial, which helps in better describing process components, units, and equipment (Theodore 2014).

Figure 3.2 Common process flow diagram symbols.

The degree of sophistication and details of a process flow diagram usually vary with both the preparer and time. It may initially consist of a simple freehand block diagram with limited information on the equipment; later versions may include line drawings with pertinent process data such as overall and componential flow rates, utility and energy requirements, environmental equipment, and instrumentation. During the later stages of a project, the process flow diagram will be a highly detailed P&I (piping and instrumentation) diagram. For information on P&I diagrams, the reader is referred to the literature (Theodore 2014).

In a sense, process flow diagrams are the international language of the engineer, particularly the practicing engineer. Chemical engineers conceptually view a plant as consisting of a series of interrelated building blocks that are defined as units or unit operations (Abulencia and Theodore 2007; Theodore 2011; Theodore and Ricci 2010). A plant ties together the various pieces of equipment that make up the process. Process flow diagrams follow the successive steps of a process by indicating where the pieces of equipment are located, and the material streams entering and leaving each unit (Felder and Rousseau 1986; Green and Perry 2008; McCabe et al. 1993).

Before attempting to calculate the material or energy requirements of a process, it is desirable to attain a clear picture of the process. The best way to do this is to draw the aforementioned process flow diagram, i.e. a line diagram showing the successive steps of a process. As mentioned, flow diagrams are very important for saving time and eliminating mistakes. The beginner should learn how to draw them properly and cultivate the habit of sketching them at the slightest excuse. Dupont et al. (2016) provide details.

In many cases, simplified flow diagrams are prepared to illustrate the process. These are often for a special purpose and do not show all the details of the process. A common type of process flow diagram shows the major unit operations and chemical reactors with their interconnecting piping and an identification of the materials being processed. The units are not shown to scale, but the drawing may resemble the equipment used for the operation. Such a diagram would be called a graphic process flow diagram. For a small process of a few units, a pictorial process flow diagram with drawings of equipment approximately to scale may be used. Photographs are usually unsatisfactory because the arrangement and connections are seldom clear. Equipment is arranged logically on a process flow diagram to show the flow of materials through the process, but a photograph shows the final physical arrangement determined by structural requirements, without regard to the process flow. When being constructed, all tall absorber columns may be grouped together for structural support, and large heat exchangers may be grouped together for ease of maintenance. The successful individual must learn to read process flow diagrams and relate them to the actual plant layout.

If sufficient data are available, a quantitative process flow diagram may be prepared showing the flow rates, compositions, temperatures, and pressures throughout the process. Such flow sheets may be very complicated. Flow sheets showing instrumentation and controls are also occasionally prepared.