As mentioned throughout the discussion of preliminary process synthesis and the creation of the process flow diagram, the process simulator usually plays an important role, even if a simulation model is not prepared for the entire flowsheet. When parts of a simulation model exist, it is common for the design team to assemble a more comprehensive model, one that enables the team to exam- ine the effect of parametric changes on the entire process. In other cases, when the process simulators have not been used for design, a simulation model is often created for comparison with the pilot-plant data and for parametric studies.
High-speed PCs and laptop computers, which have excel- lent graphical user interfaces (GUIs), have replaced work- stations as the preferred vehicle for commercial simulators, and are now finding widespread use throughout the chemical process industries. The use of simulators, which is the subject of the next chapter, has become commonplace in assisting the design team during process creation.
4.6
SUMMARY
Having studied this chapter, the reader should
1. Be able to create a preliminary database for use in preliminary process synthesis—involving the manu- factures of basic chemical products.
2. Understand the steps in preliminary process synthesis and be able to use them to develop other flowsheets for the manufacture of vinyl chloride and tPA (correspond- ing to the other branches of the synthesis trees in Figures 4.9 and 4.17), as well as for the manufacture of other chemicals.
3. Understand the steps taken by the design team in preparing one or more base-case designs. For the manufacture of vinyl chloride, or another chemical, you should be able to create a detailed process flow diagram and understand the need to complete the task-integration step begun during preliminary pro- cess synthesis and carry out the process integration step. In addition, you should be able to determine whether continuous or batch operation is more suit- able.
REFERENCES
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EXERCISES
4.1 For an equimolar solution of n-pentane and n-hexane,
compute:
(a) The dew-point pressure at 1208F (b) The bubble-point temperature at 1 atm
(c) The vapor fraction, at 1208F and 0.9 atm, and the mole fractions of the vapor and liquid phases
4.2 For the manufacture of vinyl chloride, assemble a preliminary database. This should include thermophysical property data, MSDSs for each chemical giving toxicity and flammability data, and the current prices of the chemicals.
4.3 Consider the flowsheet for the manufacture of vinyl chloride in Figure 4.8.
(a) If the pyrolysis furnace and distillation towers are operated at low pressure (1.5 atm), what are the principal disadvantages? What alternative means of separation could be used?
(b) For the process shown, is it possible to use some of the heat of condensation from the C2H4Cl2to drive the reboiler of the first
distillation tower? Explain your response. If not, what process change would make this possible?
4. Know how to prepare for the detailed design step, that is, to expand the database to include important kinetics data and the like, and to seek data from a pilot plant
when necessary. You should also recognize the need for a model of the process, usually implemented by pro- cess simulators, to be covered in Chapter 5.
(c) Consider the first reaction step to make dichloroethane. Show the distribution of chemicals when ethylene is 20% in excess of the stoichiometric amount and the chlorine is entirely converted. Assume that 100,000 lb/hr of vinyl chloride are produced. (d) Consider the first distillation tower. What is the advantage of cooling the feed to its bubble point at 12 atm as compared with introducing the feed at its dew point?
(e) Why isn’t the feed to the pyrolysis furnace heated with the hot pyrolysis products?
(f) What is the function of the trays in the direct chlorination reactor?
(g) Suggest ways to reduce the need for fuel and hot utilities such as steam.
4.4 (a)To generate steam at 60 atm, two processes are proposed: (1) Vaporize water at 1 atm and compress the steam at 60 atm. (2) Pump water to 60 atm followed by vaporization.
Which process is preferred? Why?
(b) In a distillation tower, under what circumstances is it desirable to use a partial condenser?
4.5 Synthesize a flowsheet for the manufacture of vinyl chloride that corresponds to one of the other branches in the synthesis tree in Figure 4.9. It should begin with reaction path 4 or 5.
4.6 Using the chemical engineering literature, complete the detailed database for the detailed design of the base-case process in Figure 4.19. When appropriate, indicate the kind of data needed from a pilot plant and how this data should be regressed.