Distillation Sequences, Complex Columns and Heat Integration

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-MAS-604

Distillation Sequences, Complex
Columns and Heat Integration

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Process Engineering Guide:

Distillation Sequences,
Complex Columns and Heat
Integration

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

2

1

SCOPE

2

2

FIELD OF APPLICATION

2

3

DEFINITIONS

2

4

SEQUENCING OF SIMPLE COLUMNS

3

4.1
4.2

Sidestream Columns
Multi-Feed Columns

4
5

5

SIMPLE COLUMN SEQUENCING AND HEAT
INTEGRATION INTERACTIONS

5

5.1
5.2

Energy Quantity and Quality
Heat Integration within the Total Flowsheet

5
6

6

COMPLEX COLUMN ARRANGEMENTS

8

6.1
6.2
6.3

Indirect Sequence with Vapor Link
Sidestream Systems
Pre-Fractionator Systems

8
9
10


7

COMPLEX COLUMNS AND HEAT INTEGRATION
INTERACTIONS

11

FIGURES

1

DIRECT AND INDIRECT SEQUENCES

3

2

A SINGLE SIDESTREAM COLUMN REPLACING 2
SIMPLE COLUMNS

4

3

A TYPICAL MULTI-FEED COLUMN

5

4

TYPICAL GRAND COMPOSITION CURVE

6

5

TYPICAL INDIRECT SEQUENCE WITH VAPOUR LINK

8

6

SIDESTREAM STRIPPER AND SIDESTREAM
RECTIFIER

9

7

SIMPLEST PRE-FRACTIONATOR SYSTEM

9

8

SIMPLEST PRE-FRACTIONATOR SYSTEM

10

9

PETLYUK COLUMN

10


0

INTRODUCTION/PURPOSE

Capital and operating costs depend on the sequence of columns within a
continuous Distillation Train as well as on heat integration with the rest of the
process. The optimum sequence can result in significant savings. Complex
Columns, especially those with sidestreams, partial condensers or PreFractionators, can also reduce costs.

1

SCOPE

The aim is to give the reader an appreciation of the factors involved in the
sequencing of simple columns and in Complex Column arrangements. The
guide is not intended as a step-by-step procedure to arrive at an optimal solution.
Complex Columns and heat integration are also dealt with at an appreciation
level.
The occurrence of azeotropes, multiple liquid phases, solids formation or
chemical reactions may make certain column sequences impossible and may
invalidate some of the heuristics put forward.
The guide gives the reader the background necessary to seek advice from
experts in the process synthesis techniques used to arrive at optimal solutions
and to interpret their recommendations.

2

FIELD OF APPLICATION

This Guide applies to the process engineering community in GBH Enterprises
worldwide.


3

DEFINITIONS

For the purposes of this Guide, the following definitions apply:

Complex Columns

Distillation columns with more than one of any of the
following: feeds, vapor outlets, liquid outlets.
Examples are columns with a sidestream off-take, as
well as those with sidestream strippers or rectifiers,
direct or indirect. Systems involving a PreFractionator or a partial condenser are also included.

Distillation Train

A sequence of two or more columns to achieve the
desired split into streams of specified compositions.

Pre-Fractionator

A distillation column which achieves a "sloppy" split of
the middle component(s). The split top and bottom
products are then fed to separate conventional
columns.

With the exception of terms used as proper nouns or titles, those terms with initial
capital letters which appear in this document and are not defined above are
defined in the.

4

SEQUENCING OF SIMPLE COLUMNS

With a three product system the choice is between the direct and the indirect
sequence. In the direct sequence the lowest boiling product is removed first,
while in the indirect sequence the highest boiling product is separated first,
requiring a further stage of separation to obtain the lowest boiling component as
a product. FIGURE 1 DIRECT AND INDIRECT SEQUENCES


FIGURE 1

DIRECT AND INDIRECT SEQUENCES

It is important to establish a good system; typically there will be 25% to 50% cost
difference (capital plus operating) between the best and worst sequences.
As more products are involved the problem expands.


Thus with multi-component systems there is a need to identify the good
sequences without costing all the possibilities.
It has been shown that the total number of trays is approximately constant for all
sequences. Capital cost differences therefore result from changes in column
diameter and reboiler and condenser requirements. These are all functions of the
vapor rate within the column. The vapor rate is also a major influence in terms of
operating cost (or energy requirement).
Good sequences will therefore have a low total vapor rate compared to bad
sequences;
Two heuristics, both of which reduce vapor rate, can be applied in deciding
between possible column sequences:
(a)

Favor the split that removes 25% to 50% of the feed in the distillate.

(b)

Do difficult separations last.

The first is generally the more important consideration.
These heuristics can be used with confidence to eliminate the bad sequences
(those which contradict both heuristics) for relatively ideal systems. Great care
must be taken if this approach is adopted for non-ideal systems, the local
physical chemistry expert should be consulted.
Having identified the good sequences these can be compared in more detail
using available distillation computer programs, e.g. CHEMCAD.
Two further column types, although strictly speaking complex columns, are
considered under the simple column heading:
(c)

sidestream columns;

(d)

multi-feed columns.

These column types should be considered immediately after the best simple
sequences have been identified.


4.1 Sidestream Columns
A sidestream column is worth considering if the middle product does not have a
high purity requirement. A typical application of one sidestream column replacing
two simple columns is shown below.
FIGURE 2

A SINGLE SIDESTREAM COLUMN REPLACING 2 SIMPLE
COLUMNS

In addition to eliminating a column the sidestream column arrangement requires
a lower total vapor rate and energy savings of up to 40% can be achieved.
For maximum middle product purity liquid sidestreams should be used above the
feed (main impurity the more volatile component), vapor sidestreams below the
feed (main impurity the less volatile component).


4.2

Multi-Feed Columns

If there is more than one feed to a column the most suitable arrangement is a
system in which feeds go to the optimum feed trays, for example:
FIGURE 3

A TYPICAL MULTI-FEED COLUMN

This is a more efficient arrangement than either mixing the feeds and feeding to
one feed tray in the column or using two columns to achieve the separation. To a
first approximation, a feed should go to a tray where the ratio of the key
components in the liquid (or vapor) is the same as their ratio in the liquid (or
vapor) phase of the feed.


5

SIMPLE COLUMN SEQUENCING AND HEAT INTEGRATION
INTERACTIONS

Having established good simple column sequences attention should be given to
heat integration interactions, both within the sequence and in the total flowsheet.
5.1

Energy Quantity and Quality

The optimum design of a distillation system necessarily includes a consideration
of heat integration to minimize operating cost. The latter is dependent on both
energy load (the quantity of heat required) and energy level (the quality of heat
required).
The criterion of determining a minimum total vapor rate to identify good
sequences in a simple column system satisfies the requirement of minimizing the
quantity of heat.
The temperature difference across a column, i.e. temperature in reboiler minus
temperature in condenser, is related to the quality of heat. Columns with small
temperature differences will be easier to fit into any heat integration scheme.
Therefore the sum of all the column temperature differences (ΣΔT) within a
sequence is a measure of the integratability or energy quality of that sequence.
Operating pressure fixes reboiler and condenser temperatures. For initial studies
this should be assumed to be atmospheric or just above unless extenuating
circumstances are known (e.g. thermolabile substances or low boilers involved).
Good sequences have been shown to have low ΣΔT, indeed when considering
heat integration good sequences get better, although by pressure shifting "poor"
sequences can be improved.
The strategy should therefore be:
(a)

Identify the good simple column sequences, and then

(b)

Consider heat integration, shifting column pressure as required/possible.

5.2

Heat Integration within the Total Flowsheet

The next stage is to consider heat integration of the simple column sequence
within the total process flowsheet.


The Heat Exchanger Network (HEN) provides a ready means of calculating a
target utility requirement of a flowsheet where all conditions, including column
pressures, are considered fixed. However the target can be reduced further by
systematically identifying appropriate changes to process operating conditions
which enhance integration possibilities.
A flowsheet including distillation can be conveniently divided into a "distillation
system" and a "background process". The distillation system is taken to contain
only the reboiler and condenser heat loads. The background process accounts
for everything else including any feed preheating and/or product rundown heat
loads.
5.2.1 Grand Composite Curves
The heat interactions between the distillation system and the background
process are explored using a thermodynamic approach. This involves a
consideration of total flowsheet temperature (T) versus enthalpy (H) profiles, or
grand composite curves. The grand composite curve is normally obtained using a
computer program, i.e., CHEMCAD.
The T versus H profile shows the net requirement for heating and/or the net
requirement for cooling of that process at all temperatures after process
interchange has been taken into account. A typical profile is given below.
FIGURE 4

TYPICAL GRAND COMPOSITION CURVE


The pinch divides the process into two distinct parts. above the pinch the process
is a net sink (one requiring heat, although individual parts such as ABC may be
self sufficient in heat) and below the pinch the process is a net source (one
having a surplus of heat, although again individual parts such as EFG may be
self satisfying).
Grand composite curves are constructed for the distillation system and the
background process. The T versus H profile for the distillation system can be
simplified by linking the reboiler and condenser as clearly identifiable column
units. Drawing the two profiles on the same diagram clearly shows if the columns
are "appropriately" or "inappropriately" placed with respect to the background
pinch temperature. (In theory, heat integration with the background process can
be used to operate a column for "free" if the temperatures of the reboiler and
condenser both lie above or below the pinch.)
The T versus H profile can also be manipulated manually or via CHEMCAD to
study the effect of changes in column pressure, or feed temperature, or the
applicability of intermediate reboilers and/or condensers etc.. Readers are
referred to the text of the original report for a more detailed appreciation of this
topic. This also covers cost and design implications of heat integration, suffice to
state here that the methodology proposed consists of 5 stages:
(a)

The flowsheet is split into two parts, namely the distillation system and the
background process.

(b)

Using HEN techniques the T versus H profile of the background process is
obtained.

(c)

A T versus H profile, including certain simplifying assumptions, is
constructed for the distillation system with each reboiler and condenser
linked together as recognizable column units.

(d)

The two profiles are then put together and manipulated to identify design
options.

(e)

A range of reducing utility targets, along with associated capital cost
implications, is identified. From these options the final topology, with the
correct balance between capital and operating costs, is selected.


6

COMPLEX COLUMN ARRANGEMENTS

The simplest of the complex column arrangements has been described in 4.1, a
single sidestream column. In this situation the sidestream column replaced two
adjacent simple columns. Other complex column configurations are described
below.
To be worthwhile the complex column should obviously use less energy and
have a similar, or lower capital cost than the simple columns arrangement.
6.1

Indirect Sequence with Vapor Link

The indirect sequence with a vapor link uses a partial condenser rather than a
total condenser at the top of the first column.
FIGURE 5

TYPICAL INDIRECT SEQUENCE WITH VAPOR LINK

The feed to the second column is saturated vapor instead of saturated liquid. The
additional heat in the vapor feed replaces part of the reboiler requirement of the
second column.
Typically this arrangement may save about 10% on energy. Column pressures
must be suitable to allow operation.

6.2

Sidestream Systems

Sidestream systems, as opposed to a simple sidestream column, allow
production of three pure products with up to 25% saving in energy. The two best
known configurations are the Sidestream Stripper and the Sidestream Rectifier.
(See Figure 6).
FIGURE 6

SIDESTREAM STRIPPER AND SIDESTREAM RECTIFIER

The columns are linked by liquid and vapor lines and must operate at the same
pressure.
Vapor links can be avoided if another complete column is added to the first
sidestream column - the two columns do not then have to operate at the same
pressure. The capital cost is higher.


An example is illustrated in Figure 7.
FIGURE 7 EXAMPLE OF A SIDESTREAM COLUMN

6.3

Pre-Fractionator Systems

Pre-Fractionator systems only achieve a sloppy split of the middle component(s).
The simplest system consists of three columns with conventional reboilers and
condensers and liquid links.


FIGURE 8 SIMPLEST PRE-FRACTIONATOR SYSTEM

The energy savings from such a system are rarely greater than 10% and the
extra column, reboiler and condenser increase the capital cost substantially.
Such a pre-fractionating system is unlikely to be an attractive proposition unless
spare columns are available.
More complex systems have been proposed, for example the PETLYUK column.
(See Figure 9).


FIGURE 9

PETLYUK COLUMN

Such systems realize energy savings of up to 50%.
However, because of doubts about controllability the PETLYUK column is rarely
used in practice (investigations are in hand to resolve this problem).
Heuristics based on feed composition offer a useful guide in deciding when to
consider complex column arrangements in a sequence of simple columns.
Thus with three products, A (the lightest), B, C (the heaviest) and based on feed
composition consider:
(a) Sidestream columns : When B large with respect to A or C and a slack
specification on B with respect to A or C
(b)

Indirect with vapor link

: When C large, B small, easy separations.

(c)

Sidestream systems

: B small.

(d)

Pre-Fractionator systems : B large.

If consideration of the feed heuristics identify a promising complex column
opportunity this should be assessed using an available distillation computer
program.

To summarize Clauses 4 and 6:
(1)

In simple column sequences heuristics based on vapor rate within the
column will identify bad sequences for ideal systems. Care must be taken
if this approach is adopted for non-ideal systems.

(2)

The good column sequences should be compared in more details using
an available distillation program, e.g. CHEMCAD.

(3)

Having established a good simple sequence, consideration should be
given to the introduction of complex column arrangements to further
reduce energy requirements and/or capital costs.

(4)

Heuristics based on feed composition are a useful guide in deciding when
to consider complex column arrangements.

(5)

Assess promising complex column arrangements using an available
distillation program.

7

COMPLEX COLUMNS AND HEAT INTEGRATION INTERACTIONS

Complexing increases the temperature difference across columns and makes
heat integration more difficult. However both complexing and heat integration
have the same effect – they decrease load but increase level.
Consideration should be given to the benefits to be gained from complexing
followed by heat integration and vice-versa. The effects of complex columns can
be studied using T versus H profiles. Quite often heat integration will achieve
larger savings. In general complexes that destroy heat integration links are not
worth considering.
Maximum savings are likely if use can be made of both complexing and heat
integration. Complexing and heat integration may give rise to problems in startup and/or operational flexibility. These should be assessed to ensure that the
potential savings are realistic.


Distillation Sequences, Complex Columns and Heat Integration

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