Page 12 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Chapter

Distillation

Part 1: Distillation Process Performance

Efficient and economical performance of distillation sented. Nomenclature for (1) distillation performance
equipment is vital to many processes. Although the art and design is on page 102 (2) absorption and stripping on
and science of distillation has been practiced for many page 121 and (3) tray hydraulic design on page 221.
years, studies still continue to determine the best design
procedures for multicomponent, azeotropic, batch, mul- Equilibrium Basic Considerations
tidraw, multifeed and other types. Some shortcut proce-
dures are adequate for many systems, yet have limitations Distillation design is based on the theoretical consider-
in others; in fact the same might be said even for more ation that heat and mass transfer from stage to stage (the-
detailed procedures. oretical) are in equilibrium [225-2291. Actual columns
with actual trays are designed by establishing column tray
The methods outlined in this chapter are considered efficiencies, and applying these to the theoretical trays or
adequate for the stated conditions, yet some specific sys- stages determined by the calculation methods to be pre-
tems may be exceptions to these generalizations. The sented in later sections.
process engineer often “double checks” his results by
using a second method to verify the “ball-park” results, or Dechman [lo91 illustrates a modification to the usual
shortcut recognized as being inadequate for fine detail. McCabe-Thiele diagram that assumes constant molal over-
flow in a diagram that recognizes unequal molal overflow.
Current design techniques using computer programs
allow excellent prediction of performance for complicat- Distillation, extractive distillation, liquid-liquid extrac-
ed multicomponent systems such as azeotropic or high tion and absorption are all techniques used to separate
hydrogen hydrocarbon as well as extremely high purity of binary and multicomponent mixtures of liquids and
one or more product streams. Of course, the more vapors. Reference 121 examines approaches to determine
straightforward, uncomplicated systems are being predict- optimum process sequences for separating components
ed with excellent accuracy also. The use of computers pro- from a mixture, primarily by distillation.
vides capability to examine a useful array of variables, It is essential to calculate, predict or experimentally
which is invvaluable in selecting optimum or at least pre- determine vapor-liquid equilibrium &a in order to ade-
ferred modes or conditions of operation. quately perform distillation calculations. These data need
The expense of fabrication and erection of this equip to relate composition, temperature, and system pressure.
ment certainly warrants recognition of the quality of meth- Basically there are two types of systems: ideal and non-
ods as well as extra checking time prior to initiating fabri- ideal. These terms apply to the simpler binary or two
cation. The general process symbol diagram of Figure 8-1 component systems as well as to the often more complex
will be used as reference for the systems and methods pre- multicomponent systems.

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