Shortcut Methods of Distillation Design

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-MAS-603

Shortcut Methods of Distillation
Design

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Process Engineering Guide:

Shortcut Methods of Distillation
Design

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

2

1

SCOPE

2

2

ESTIMATIONOF PLATEAGE AND REFLUX
REQUIREMENTS

3

2.1
2.2

Generalized Procedure for Nmin and Rmin
Equation based Procedure for Nmin and Rmin

4
6

3

PREDICTION OF OVERALL PLATE EFFICIENCY

7

4

SIZING OF MAIN PLANT ITEMS

7

4.1
4.2

Column Diameter
Surface Area of Condensers and Reboilers

8
8


FIGURES
1

NON-IDEAL EQUILIBRIUM CURVE

3

2

AT A GLANCE CHART BASED ON FENSKE,
UNDERWOOD

5

PLATE EFFICIENCY CORRELATION OF O’CONNEL

7

3

DOCUMENTS REFERRED TO IN THIS PROCESS
ENGINEERING GUIDE

10


0

INTRODUCTION/PURPOSE

Short cut methods of distillation design (including means of sizing main plant
items) have an important role to play because they:
(a)

Give a feel for the ease or difficulty of separation and the sensitivity to
design data.

(b)

Assist in defining alternative process schemes for budget costing and
process selection.

(c)

Assist in the planning and design of semi-technical distillation trials (e.g.
Oldershaw column simulation).

(d)

Assist in the setting up of rigorous calculations using computer methods.

(e)

Provide a means of checking that results from rigorous computer methods
are sensible.

The last of these is MOST IMPORTANT.
Conversely, there are situations where short cut methods should be avoided, or
at most used only for an initial evaluation:
(1)

Where the system is highly non-ideal (risk of two liquid phases).

(2)

Where VLE data are not available (it is often found that such systems are
highly nonideal).

(3)

For the simulation of existing columns for purposes of optimization,
uprating or other process changes.

In (3) short cut methods can be most unreliable, failing to give both single point
solutions and trends with any predictable accuracy.


1

SCOPE

The areas in which the use of short cut methods of design are recommended
require only a very simplified approach - a 'back of the envelope' calculation. This
guide therefore presents procedures to define plateage and reflux requirements
based on Fenske-Underwood-Gilliland. This is not necessarily the most accurate
method, but is simple and quick to use. Similarly, a simple means of predicting
plate efficiency is presented.
For budget costs, the equation given in the FRI Design Handbook for trial tower
diameter may be used for trayed and packed columns. Also, simple equations
are presented for quick sizing of condensers and reboilers.

2

ESTIMATION OF PLATEAGE AND REFLUX REQUIREMENTS

Two approaches are presented for the estimation of plateage and reflux
requirements.
Both are based on the Fenske-Underwood method:
(a)

a simple graphical approach,
and

(b)

a method based on the equations written for a binary system.

Note that the calculation of Nmin is usually more accurate than that of Rmin. This is
because the latter depends on the intersection of the operating and equilibrium
lines which is usually not accurately located when the equilibrium line is
represented as a mean constant relative volatility. Figure 1 shows a non-ideal
system where the equilibrium looks reasonably well represented by a mean
constant relative volatility. However, for the example given Rmin is 40% greater
than Underwood predicts while Nmin is only 10% greater than Fenske predicts.


FIGURE 1

NON-IDEAL EQUILIBRIUM CURVE


2.1

Generalized Procedure for Nmin and Rmin

(a)

Use the key component concept, i.e. reduce the separation of a multicomponent system to a binary problem.
To do this, establish the two key components, i.e. the two components
which it is desired to separate. These are designated the light key
component (LK) and the heavy key component (HK). All material more
volatile than LK is treated as this component and all material less volatile
than HK is considered to be HK. Work on a molar basis. The relative
volatility, assuming ideality, is given by:

(b)

A rough guide to the relationship between relative volatility and boiling
point difference is as follows:
B.Pt difference °C
2
5
10
20
30
50

(c)

α
1.05
1.11
1.25
1.6
2.0
3.1

Short cut methods of design should only be used when α > 1.2.
When α < 1.2, small changes in activity coefficient (non-ideality) can be
very important.

(d)

Knowing a use Figure 2 as an aid to setting up or checking computations
or simply for obtaining a feel for the difficulty of separation.
The data in Figure 2 were calculated using Fenske-Underwood. To a first
approximation the Gilliland correlation predicts optimum still performance
at:


Nmin x 2 = Noperating
Rmin x 1.3 = Roperating
The boil-up rate per 100 te/h feed rate plotted in Figure 2 was calculated
at Rmin x 1.3 value.
FIGURE 2

AT A GLANCE CHART BASED ON FENSKE, UNDERWOOD


2.2

Equation Based Procedure for Nmin and Rmin

The recommended Fenske-Underwood equations to calculate Nmin and Rmin
assume that relative volatility and molal overflows are constant (heats of mixing
and heat losses are also assumed negligible).
(a)

Fenske Equation

where:
XD = mole fraction of more volatile component in distillate.
XB = mole fraction of more volatile component in bottoms.
(b)

Underwood Equation

When the feed is liquid at its boiling point:

where:
XF = mole fraction of more volatile component in feed.
If the distillate is of high purity XD ® 1 and the equation simplifies to:


(c)

Gilliland Correlation

Gilliland derived a correlation between the number of theoretical plates, a finite
reflux ratio, Nmin and Rmin. For the accuracy required in this procedure, the
economic optimum can be taken as:
2 x Nmin

= Number of theoretical plates required

1.3 x Rmin

= Operating reflux ratio

3

PREDICTION OF OVERALL PLATE EFFICIENCY

For cost purposes the number of theoretical trays needs to be translated into the
number of actual trays required for a particular separation.
O'Connell's correlation is the suggested method for predicting plate efficiency
(see Figure 3). This correlation gives a reasonable estimation of plate efficiency
bearing in mind the scant input data required. Only a knowledge of the relative
volatility of key components and the viscosity of the feed at average column
conditions is required.


FIGURE 3

PLATE EFFICIENCY CORRELATION OF O'CONNEL
(Trans.Amer.Inst.Chem.Engrs., Vol 42, p741, 1946)


4.1

Column Diameter

The equation given in the FRI Design Handbook to determine column diameters
for sieve plate systems can be used generally to give an approximate sizing, viz.:


4.2

Surface Area of Condensers and Reboilers

The rate of heat flow, q = UATm

For a particular still, with liquid feed and no heat loss, q can be assumed to be
the same for condenser and reboiler calculations.


DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE
This Process Engineering Guide makes reference to the following document:
Fractionation Research Inc. Design Handbook (referred to in Clause 1 & 4.1).


Shortcut Methods of Distillation Design

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