Analysis and Design of a Distillation Column: Chemical Engineering

This report delves into the design of a distillation column, a crucial process in chemical engineering for separating components based on their boiling points. The report begins with an introduction to distillation principles, explaining how a continuous column can separate a feed mixture into two product streams. It outlines the steps involved in designing a distillation column, including defining product specifications, selecting operating pressure, choosing appropriate vapor-liquid equilibrium (VLE) data, determining the number of theoretical stages using the Fenske equation, and selecting tray efficiency and appropriate tower internals. The report also covers tower sizing, tray hydraulics, and process control schemes. Furthermore, it includes a detailed analysis of a specific distillation problem, providing calculations for feed temperature, minimum reflux ratio, bubble and dew point temperatures, minimum and actual number of stages, feed stage position, column diameter, and column height, along with tray design considerations. The assignment utilizes shortcut calculation methods to approximate the solution for multicomponent distillation based on given feed composition and flow rate data.

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Introduction
Distillation is a process used in the separation of various components in a feed mixture
depending on their relative boiling points. A simple and continuous column is able to make the
separation of any two components be two product streams.
Principles of Distillation
A distillation column defines a series of equilibrium flashes that has two feeds as well as two
product streams. The exiting liquid is often at bubble point while the exiting vapors are normally
at dew point. The compositions abide by the equation yi=Ki*xi
A liquid “A” which is sub cooled is heated and the concentration is maintained until it gets to the
bubble point when it begins to boil (B). The vapors that is generated during boiling is having the
equilibrium position that is given by “C” which is about 50% richer is components A as

compared to the initial liquid. The basis of distillation is the difference between the compositions
of vapour and liquid.
Steps to the design of Distillation Column
1. Define the specification of the product: The heavy and key components are chosen in
which three main ways are used inn the specification of desired product:
A composition of one of the components in any of the products
A certain physical feature for example vapours pressure for any of the product

All the aspects of column design are impacts by the operating pressure and a few of the
considerations as the pressure rises include
 Reduction in the needed exchanger sizes
 More mechanical cost of the column
 Increase in the minimum number of stages
2. Select a suitable operating pressure

3. Select appropriate VLE pressure: The correct VLE as well as enthalpy data ranges should
be chosen for property calculations.
The correct methods of calculation should also be picked as inappropriate methods, ranges of
data or even models may result in poor results. The close boiling systems are often finicky when
it comes to converging and hence notice should be taken for non-ideality
4. Determine the number of theoretical values: The Fenske equation is used in the
calculation of the minimum number of the theoretical trays as shown:
For more adequate tray count approximation, a reflux ration and run cases are assumed in Aspen
Plus make use of DISTWU model while HYSYS model Shortcut Column model. All the work of
design of the column is carried out using theoretical trays
5. Choose tray efficiency: Overall= No. of theoretical trays/No. of actual trays

Point= (yout-yin)/ (yeqn-yin)
Murphree=point efficiency even though over the whole tray
Among the lessons learned from tray efficiency include:
 The estimates of efficiency in established processes are free from trouble with
conventional internals
 Caution is needed as there is often a learning curve that has new enhanced internals

 Conservation with efficiency should be considered with high capacity internals
6. Choose appropriate tower internals: The appropriate packing or tray type is selected
depending on the application and among the considerations include turndown
requirements, fouling pressure, allowable pressure drop. The Kister’s table is used in
making the selection. Column zoning may aid capacity
7. Conduct tower sizing and tray hydraulics: The thumb rules Heat factor of Q/d2=350,000,
Duty in BTU/hr and diameter in feet
General Factor of (R+F)/d2=250 which defines the flow in diameter in BPD in feet are used
Software tools such as SULCOL and KGTower are used in refining the estimates of the diameter
The table below is used as a guide for the initial estimates

The typical design values include
 Jet and downcomer flood is less than 85%
 The downcomer backup of the clear fluid is less than 40% of the spacing of the tray plus
weir height
 The downcomer exist velocity is less than 1.5 ft. /sec
 The dry trap drop is less than 2 inches of the liquid or less than 15% of the spacing of tray
 Weird load is less than 80gpm/ft. for one pass and less than 120gpm/ft. for two or even
more passes
 Height of weight of between 2 and 3 inches is used
 0.06 to 1 inch is the head loss under the downcomer
8. Select a process control scheme: The top or down [roducts are the only ones that may be
controlled not unless to some extent coupled. temperature control automatically results in
composition control except for cases where some kind of analysis is made available

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