Symbols, process distillation

Figure 4.3 Zosimos teacher, Maria, points at a symbol of distillation, the process she is said to have invented. (Michael Maier, Symbola Aurea Mensae [1617])...

Distillation is a process that separates the components in a mixture by boiling point. At the heart of a distillation system is the column. Distillation columns come in two basic designs plate and packed. Flow arrangements vary from process to process. The symbols allow the technician to identify primary and secondary flow paths. The two standard symbols for distillation columns are shown in Figure 7-13. A distillation system is a complex arrangement of equipment and instruments. In most cases, all of the equipment covered in this text could be found in service within a distillation system. 

Bums and Hazzan demonstrated tlie use of event tree and fault tree analysis in tlie study of a potential accident sequence leading to a toxic vapor release at an industrial chemical process plant. The initiator of tlie accident sequence studied is event P, the failure of a plant programmable automatic controller. Tliis event, in conjunction willi the success or failure of a process water system (a glycol cooling system) mid an operator-manual shutdown of tlie distillation system produced minor, moderate, or major release of toxic material as indicated in Fig. 21.4.1. The symbols W, G, O represent tlie events listed ... 

In the early alchemical texts (and in particular the Rosarium philosophorum ), this picture of soul alchemy is developed in parallel with descriptions of what was a sore physical process. Thus it was that the soul development of the alchemist went hand in hand with an actual physical operation, and this operation, the details of which have not been wholly lost, involved colour and form changes within a sealed flask, isomorphic to the inner changes of soul alchemy, described by these bird symbols. Thus we had a physical process which involved a blackening, a whitening a rapid iridescence of colours, a circular distillation stage, and a final sublimation. Part of the task of modern alchemy must be to rediscover this physical process, and explore its further ramifications. 

The philosophy of process intensification has been traditionally characterized by four words smaller, cheaper, safer, slicker. And indeed, equipment size, land use costs, and process safety are among the most important PI incentives. But process intensification can (and should) also be placed in a broader context—the context of sustainable technological development. Several years ago DSM published a picture symbolizing its own vision of process intensification (32), in which skyscraping distillation towers of the naphtha-cracking unit are replaced by a compact, clean, and tidy indoor plant (see Figure 3). The importance of PI for sustainable development and its role in the company s responsible business has been further stressed in a recent publication by the company s CEO, Peter Elverding (33). Here,... 

In professionally drawn flowcharts, special sj mbols are used to denote different types of process units such as distillation columns and heat exchangers. We will not generally use these symbols in this text, since our main purpose is to show you how to perfonn material and energy balance calculations. Simple boxes are perfectly adequate to represent process units on the flowcharts you draw for these calculations. 

In the description of this process, the following symbols are used in addition to those explained above. The mole fraction of the most volatile component in the feed is represented by X, in the distillate by XD, and in the bottoms by xB. The subscript j is used as the counting integer for the number of the stages. Since the distillate is withdrawn from the accumulator (j = 1) and the bottoms is withdrawn from the reboiler (j = N the mole fractions in the distillate and bottoms have double representation that is, X Di = xu (for a column having a total condenser) and xBi = xNi. For the case where the column has a partial condenser (D is withdrawn as a vapor), X , = yu. 

Symbols and diagrams have been developed for most pieces of industrial equipment, process flows, and instrumentation. The symbols covered in this chapter include those typically used with valves, piping, tanks, pumps, compressors, steam turbines, motors, heat exchangers, cooling towers, furnaces, boilers, distillation columns, and reactors. Figure 7-6 shows many of the basic symbols for valves. 

On a typical flow diagram, distillation columns, reactors, boilers, and furnaces are drawn as they visually appear in the plant. If a proprietary process includes several types of equipment not typically found on a standard symbol file, the designer will draw the device as it visually appears in the unit. 

The symbol used on a diagram for a plate column should indicate the type of tray used in the system bubble-cap, valve, or sieve. The first distillation column was invented in 1917. Today, a number of modifications allow modern process technicians to operate much more efficiently. The design, however, still includes the original still-on-top-of-a-still approach. The basic components of a plate distillation column are a feed line feed tray stripping section below the feed line enriching or rectifying section above the feed line overhead vapor outlet, side-stream outlet, and bottom outlet reboiler instrumentation for level, temperature, flow, pressure, and composition control outer shell and a top reflux line. 

As we saw in the ammonia process, the reactor is usually the key unit. The reactor will often dictate whether a chemical process is possible. Separators are secondary only to the reactor in most processes the performance of the separator(s) will often determine if a process is profitable. Examples of separators include dryers, filters, absorbers, adsorbers, and centrifuges. The method used to separate a mixture - distillation, condensation, or filtering - is usually indicated by a specific symbol on the flowsheet. We will introduce these specific units later, as needed. 

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