Troubleshooting in Distillation Columns

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-MAS-609

Troubleshooting in Distillation
Columns
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Process Engineering Guide:

Troubleshooting in Distillation
Columns

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

2

1

SCOPE

2

2

FIELD OF APPLICATION

2

3

DEFINITIONS

2

4

FLOW DIAGRAM FOR TROUBLESHOOTING

2

5

GENERAL APPRAISAL OF PROBLEM

4

5.1
5.2
5.3

Is the Problem Real?
What Is the Magnitude of the Problem?
Is it the Column or the Associated Equipment which is
Causing the Problem?

4
4

6

PROBLEMS IN THE COLUMN

4

6.1
6.2

Capacity Problems
Efficiency Problems

4
5

7

PROBLEMS OUTSIDE THE COLUMN

6

7.1
7.2
7.3
7.4

Effect of Other Units on Column Performance
Column Control System
Improper Operating Conditions
Auxiliary Equipment

6
6
6
7

4


8

USEFUL BACKGROUND READING

9

BIBLIOGRAPHY

7

8

FIGURES

1


3

2

DETERMINATION OF COLUMN CAPACITY

5

DOCUMENTS REFERRED TO IN THIS PROCESS
ENGINEERING GUIDE

9


0

INTRODUCTION/PURPOSE

The old adage - prevention is better than cure - applies to the design of all
process equipment. The majority of distillation column designs meet their defined
requirements, the designer having assessed and made suitable allowances for
uncertainties in equilibria data or tray/packing efficiency. Occasionally a new
column will not give the required performance because of a mistake made at the
design stage - maybe as a result of poor distribution in a packed tower.
More often problems in distillation columns are associated with changed
circumstances - changed operating conditions or changed product specifications
- or in the extreme a changed duty to effect a different separation.
Whatever gave rise to the problem the distillation column is not achieving the
required separation and/or capacity. This is the realm of the troubleshooter.
Troubleshooting is not an exact science. A diversity of approaches can be
successfully adopted - one such approach is outlined in this guide.
1

SCOPE

This Guide offers advice on how to approach troubleshooting in distillation
columns and refers readers to useful published information.

2

FIELD OF APPLICATION

This Guide applies to GBHE process engineering community worldwide.

3

DEFINITIONS

For the purposes of this Guide, no special definitions apply.
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 Glossary.


4


A general guideline in the form of a flow diagram for approaching troubleshooting
problems is given in Figure 1 [Ref 1].
Such a diagram cannot cover all problem areas but it does serve as a useful
starting point. However the most important attribute the chemical engineer can
bring to a plant problem is the ability to think and apply logic. Do not be rushed
into taking ill considered actions. Discount the inevitable suggestion that this is
one case that does not follow universally proven laws. FIGURE 1 FLOW
DIAGRAM FOR TROUBLESHOOTING


FIGURE 1



5

GENERAL APPRAISAL OF PROBLEM

The loss of production resulting from poor column performance produces an
atmosphere which demands that actions have to be taken. However actions
should not be taken just to demonstrate that things are being done. Accept the
flak, the problem is more likely to be quickly resolved by following a systematic
procedure. All aspects should be investigated before attempting to determine the
solution.
5.1

Is the Problem Real?

The first action is to establish that there really is a problem.
Collect and analyze all the data available. Check that instruments, metering
devices, analyses etc are all measuring what they should be i.e. instruments
have not been transposed, chart ranges are correct, agreed analytical
procedures are being followed.
Carry out a component and an overall mass balance and an overall heat
balance. The former can point to analyzer error, the latter to incorrect flow rate
measurement.
5.2

What Is the Magnitude of the Problem?

A realistic estimate of the loss of profit associated with poor column performance
helps decide the effort which should be expended to solve the problem.
The cost of lost production also helps in setting technical targets. A less than
perfect technical solution may be appropriate to minimize production losses in
the short term. The long term solution may involve considerable capital outlay
and/or have to await a planned plant shut down.
5.3

Is it the Column or the Associated Equipment which is Causing the
Problem?

Although a distillation column may be said to be not giving the desired
performance it is important to establish if the problems are with the column itself
or a result of other factors outside the column.
This distinction may be difficult to define because of limited and often
inconsistent data. The troubleshooter must gather all the information available
(charts, log books, design information) and listen to and assess the views of plant
operators.

6

PROBLEMS IN THE COLUMN

Problems in the column relate to loss of capacity or efficiency. In some cases,
usually if the column performance has suddenly deteriorated rather than declined
gradually, the root cause of the problem can be easily identified. In other cases
there may be a number of possible causes and the information collected will
have to be sifted and analyzed to support or eliminate possibilities. Is the column
being asked to do a duty beyond what it was designed for? Is it being operated in
a manner very different from the design intent?
6.1

Capacity Problems

The maximum hydraulic capacity of a column is the highest throughput at which it
will operate without flooding. The capacity is a function of the vapor and liquid
rates, column geometry and system properties.
At the flood point liquid accumulates on trays or in packing because of excessive
vapor flow, i.e. pressure drop, or maybe mechanical restriction.
Flooding in a column could be indicated by:
(a)

surges of liquid overhead;

(b)

erratic or high pressure drop across the column;

(c)

fluctuating level in column bottom;

(d)

a high temperature profile;

(e)

falling base level or reduced bottoms flow.

Flooding usually continues until the operator takes action.
The primary tool for investigating capacity problems is a differential pressure
instrument, or preferably instruments, to measure pressure drop across various
tray or packing sections. Static pressure gauges with pressure drop readings
obtained by difference should not be considered, as they will rarely be accurate
enough.
The actual column capacity can be determined by plotting the pressure drop
against, say, the total overhead vapor rate, as outlined in Figure 2 below.


FIGURE 2

DETERMINATION OF COLUMN CAPACITY

The use of radioisotope scanning is a useful method of determining the cause of
flooding, e.g. downcomer back-up.
6.2

Efficiency Problems

Poor efficiency usually results from inadequate liquid and vapor contact or
insufficient disengagement of liquid from vapor.
The key indicators of efficiency problems are:
(a) off specification product;
(b) excessive reflux requirement;
(c) high boil-up rate.
Poor efficiency can also be confused with analytical errors or improper
operating conditions - these causes should be eliminated first.


If low efficiency is established and if the column is operating stably with no
hydraulic problems there is little chance of increasing efficiency in a trayed
column without recourse to the redesign of a more efficient contacting device.
The same holds for packed columns if the distributor design and installation is
good.

7

PROBLEMS OUTSIDE THE COLUMN

There are many ways in which problems outside the column can effect column
performance. A few instances are given below.
7.1

Effect of Other Units on Column Performance

It is difficult to evaluate the performance of a column if upstream and sometimes
downstream units are not operating steadily. For example, pressure changes
may limit pump capacity or there may be surges of low boiling components in the
feed to the column which can lead to problems in the condenser or vacuum
system.
Even with other units operating steadily the performance may not be as required.
This can be as a result of changes in feed composition, rate or conditions
compared to those for which the column was originally designed. Such changes
should be carefully evaluated. For example a cold rather than a flashing feed can
upset the heat balance of the column. Also a feed rate above design may not be
compatible with reboiler/condenser capabilities in effecting the required
separation.
7.2

Column Control System

The control systems employed can cause, or contribute to, operational problems
in distillation columns. A few typical examples are given below:
(a)

A temperature sensor for composition control should be situated at a point
in the column that will provide sensitivity to composition changes - failure
to do so can lead to sluggish control and products being off specification
for unacceptable periods of time.

(b)

The wrong setting of a temperature sensor/control can lead to the situation
where a required tops or bottoms composition cannot be achieved.


(c)

An incorrectly calibrated instrument for gauging the base level in a reboiler
can initiate flooding. If the instrument was calibrated for a liquid of higher
density than is practically the case, the actual level would be higher than
the indicated level - maybe even into the column.

The possibilities are manifold, sorting them out requires careful checking of the
calculated versus apparent conditions at as many points as possible and
analyzing any apparent differences.
7.3

Improper Operating Conditions

It is common practice to start-up plant at reduced design rates. If care is not
taken, for example operation at a reflux ratio higher than design, this can lead to
problems with columns operating below their minimum operating points.
Another frequent situation is that conditions are set which are incompatible with
the objectives. For example, setting a specific overhead product rate that is
insufficient to remove the required amount of the more volatile component(s) as
distillate. With a set amount of distillate removal a similar situation can exist if no
account is taken of a small change in feed composition.

7.4

Auxiliary Equipment

Auxiliary equipment may cause problems that seem to originate in the column
itself. It is easy to apportion a deterioration in column performance above a
certain feed rate as a fall off in tray efficiency rather than a reboiler or condenser
limitation. However if the energy flow does not remain proportional to the feed
rate a capacity limitation in auxiliary equipment should be suspected.
A possible short cut to pinpointing capacity limitations in auxiliary devices is to
ascertain if valves are consistently running wide open. This may be an indication
of control valve limitations, or of reboiler, condenser or pump inadequacy.
Numerous other factors may give the appearance of faulty column design:
(a)

corrosion effects;

(b)

a drift in instrument calibration;

(c)

degradation of materials;


(d)

accumulation of trace impurities in recycle streams;

(e)

leaking heat exchangers etc.

8

USEFUL BACKGROUND READING

As mentioned earlier, troubleshooting is a complex and diverse topic. Many
papers have been written on the subject. Four recent publications summarized
below, provide useful additional material.
(a) Harrison, France, Troubleshooting Distillation Columns [Ref 2]
This is a four part series dealing with trouble shooting in operational distillation
columns.
Part 1 Technique and Tools
The basic troubleshooting technique - identify the problems, determine the cause
and recommend a remedy - is discussed. This is followed by an outline of the
tools available to the troubleshooter - isotope scanning, video-camera viewing,
shut down inspection.
Part 2 Packed Columns
General characteristics of packed columns and types of packing are discussed.
The importance of liquid distribution and the design of distribution opposite good
column performance is emphasized.
Part 3 Trayed Columns
Different types of flooding are discussed together with the most common tray
types (sieve, valve and bubble-cap). A short treatment of common causes of
inefficiency is presented.


Part 4 Auxiliary Equipment
Heat exchangers, vacuum pumps or ejectors, process pumps and
instrumentation are recognized as critical components of distillation columns.
Reboilers, in particular, are considered in detail.
(b)

Tray Distillation Columns [Ref 3]

This booklet gives a useful overview as to planning tests (including pre-start up),
methods of measurement and sampling and computation and interpretation of
results. For detailed information on a specific topic reference is made to other
articles in the literature.
There is a clearly presented worked example illustrating how mass and enthalpy
balances can be checked and column capacity and efficiency calculated.
(c)

Packed Distillation Columns [Ref 4]

This booklet for the performance evaluation of packed distillation columns draws
heavily on that for tray distillation columns [Ref 3], suitably modified for the
different internals.
(d)

Henry Z Kister, Distillation Operation [Ref 5]

An excellent book dedicated to the distillation troubleshooter. Past experiences
with distillation columns are considered and used to suggest means of preventing
similar incidents in the future.
The whole gamut of distillation technology is covered - trayed and packed
columns, column commissioning and field testing, control philosophy, case
histories of columns that did not work etc.
An invaluable source for the troubleshooter.


9

BIBLIOGRAPHY

Ref

Source

[1]

McLaren, Upchurch, Guide to Trouble Free Distillation,
Chem. Eng, 139, 1 June, 1970.

[2]

Harrison, France, 'Troubleshooting Distillation Columns', Chemical
Engineering, March-June, 1989. Also available as Bulletin 389 from
Glitsch Ltd., 1990.

[3]

Tray Distillation Columns: A Guide to Performance Evaluation.
AIChE Equipment Testing Procedure, 1987, 2nd edition.

[4]

Packed Columns: A Guide to Performance Evaluation.
AIChE Equipment Testing Procedure, 1990, 2nd edition.

[5]

Henry Z Kister, Distillation Operation, McGraw-Hill Publishing Company,
1990.


Troubleshooting in Distillation Columns

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