Design and Rating of Trayed Distillation Columns

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-MAS-611

Design and Rating of Trayed
Distillation Columns

Information contained in this publication or as otherwise supplied to Users is
believed to be accurate and correct at time of going to press, and is given in
good faith, but it is for the User to satisfy itself of the suitability of the information
for its own particular purpose. GBHE gives no warranty as to the fitness of this
information for any particular purpose and any implied warranty or condition
(statutory or otherwise) is excluded except to the extent that exclusion is
prevented by law. GBHE accepts no liability resulting from reliance on this
information. Freedom under Patent, Copyright and Designs cannot be assumed.

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Process Engineering Guide:

Design and Rating of Trayed
Distillation Columns

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

3

1

SCOPE

4

2

FIELD OF APPLICATION

4

3

DEFINITIONS

4

4

DESIGN PHILOSOPHY

4

5

PERFORMANCE GUARANTEES

5

6

TRAY DESCRIPTION

6

7

DESIGN CALCULATIONS

9

7.1
7.2
7.3

Rough Design
Preliminary Design
Detailed Design and Rating

10
10
11

8

PROGRAM USE – DESIGN CONSIDERATIONS

12

8.1
8.2

General Considerations
Effects of Design Variables on Performance Parameters

12
13


9

PRACTICAL ASPECTS OF TRAY DESIGN

15

9.1
9.2
9.3
9.4
9.5
9.6
9.7

Liquid Feeds
Vapor Feeds
Two Phase or Flashing Feeds
Reboiler Returns
Liquid Drawoffs
Vapor Drawoffs
Reboiler Circuits

15
16
16
16
16
16
17

10

TRAY VIBRATION

23

10.1
10.2
10.3

Unstable Flow
Oscillation Frequencies
Analysis of Tray Vibration and Action to be Taken

23
26
26

11

REFERENCES

27

APPENDICES

A

TRAY DEFINITIONS

44


FIGURES

1

TYPICAL TRAY LAYOUT

7

2

LIQUID RECIRCULATION ON A LARGE ONE-PASS TRAY

7

3

BASIC CROSS FLOW TRAY LAYOUTS

8

4

TRAY DESIGN DEFINIOTIONS

9

5

LIQUID FEEDS

18

6

REBOIL RETURN NOZZEL

19

7

PARTIAL LIQUID DRAWOFF FROM BOTTOM OF
DOWNCOMER

20

8

CHIMMNEY TRAY

20

9

VAPOR DRAWOFF – INTERMEDIATE TRAY

21

10

TOWER INTERNALS FOR REBOILER CIRCUITS

22

11

IDEALIZED PRESSURE DROP CHARACTERISTIC

25

12

TYPICAL SIEVE TRAY PRESSURE DROP CHARACTERISTIC

25

DOCUMENTS REFERRED TO IN THIS PROCESS
ENGINEERING GUIDE

54


0

INTRODUCTION/PURPOSE

GBHE does not manufacture distillation trays - they are purchased from
specialist tray manufacturers. The detailed design of the trays IS generally
undertaken by the manufacturers. Competitive bids will generally be sought from
various manufacturers.
The role of the Process Engineer is to:
(a)

Specify the process requirements. In the form of Data Sheets.

(b)

Ensure that what is offered by the bidding manufacturers will meet these
requirements.

©

Compare the designs offered on technical merit; outstanding features of a
particular design may outweigh any additional cost which may be Incurred.

On existing plant there is often the need to assess the performance of trays for
several reasons:
(1)

To assess the reasons for any shortfall in the performance of the trays,
compared with design or earlier operation.

(2)

To assess up-rating capability of the existing trays from high rate plant
trials.

(3)

To explore modifications to the trays for operation at higher or lower rates.

The design of proprietary trays depends on the design methods supplied by their
manufacturers. These design methods are sometimes unreliable and expert
advice should be sought when considering proprietary trays.
This Guide has been prepared for GBH Enterprises.

4

SCCPE

This Guide deals with the design and rating of trays for distillation columns. It
covers neither guidance on the selection of trays and packings nor some aspects
of their performance characteristics: advice on both of these is given In GBHEPEG-MAS-610 (Reference 6).


Particular emphasis is placed on sieve and valve trays since these are the types
usually bought. Bubble cap and dualflow trays are mentioned. Proprietary trays
are not included.
The scope of the Guide is summarized In Its Clause headings:
2

Field of Application

3

Definitions

4

Design Philosophy

5

Performance Guarantees

6

Tray Description

7

Design Calculations

8

Program Use - Design Considerations

9

Practical Aspects of Tray Design

10

Tray Vibration

In addition, Appendices provide definitions of terminology used with distillation
trays.

2

FIELD OF APPLICATION

This Guide applies to the design of trayed distillation columns by Process
Engineers In GBH Enterprises worldwide.


3

DEFINITIONS

For the purposes of this Guide. the following definitions apply:
Fractionation Research Inc. (FRI) -

A cooperative research company. Many
of the definitions FRI use for tray design
are given In Appendix A.

With the exception of proper nouns. terms with initial capital letters which appear
In this Guide and are not defined above, are defined In the Glossary of
Engineering Terms (Reference 7).

4

DESIGN PHILOSOPHY

It is assumed that GBHE-PEG-MAS-610 (Reference 6) has been consulted and
that even If a firm decision to use trays has been made, the selection criteria
have been checked to ensure that important factors have not been overlooked.
GBHE does not manufacture distillation trays; they are purchased from specialist
tray manufacturers. Generally speaking, the tray manufacturers have more
experience In tray design than GBH Enterprises Process Engineers. It is
therefore preferable for the manufacturer to take responsibility for the tray design.
The role of a GBHE Process Engineer then becomes one of:
(a)

Comparing the technical merits of designs proposed by tray
manufacturers in a competitive bid situation.

(b)

Ensuring that what is proposed will work.

(c)

Seeking modifications where proposals appear unsatisfactory.

If modifications are required the aim should be to agree changes with the
manufacturer which do not diminish his design contingencies, while removing the
risk of poor performance. This is generally achievable since we will usually be
seeking extra contingency in the design.
Tray manufacturers' expertise is usually in hydraulic design (flooding, weeping,
pressure drop, etc). Few are prepared to commit themselves to determining the
efficiency. This has been normal practice until recent years, but is now changing.


Most manufacturers will enter into discussions, and may be prepared to share
their experience on efficiency as It relates to your application and occasionally
they will commit themselves.
It is Important to distinguish between tray design and column design. Almost
certainly, the vessel will be on a much longer delivery than the trays, typically
about 1 year compared with 12 weeks. Process Engineers often need to decide
the column diameter and approximate height long before they want to talk to tray
manufacturers. This can be done using In-house design methods and ensuring
sufficient contingency for the manufacturer's final design.
The recommended practice for tray design is dependent on the purpose for
which the design is required:
(a)

A rough column sizing is usually all that is needed for initial flowsheeting
studies.

(b)

Approximate designs are required for pre-sanction flowsheeting, where
cost estimating is the main requirement.

(c)

Detailed designs are required for:
(1)

(2)

Assessment of existing column capacity, either with the existing
trays or by considering modified tray designs.

(3)

(d)

In-house column Sizing, prior to enquiry on tray manufacturers, to
determine diameter and approximate height.

Checking manufacturers' proposals, either for a new column or for
debottlenecking an existing column.

For a sanction estimate, enquiry on tray manufacturers is the preferred
approach, but detailed designs can be produced in-house If project
confidentiality dictates.


5

PERFORMANCE GUARANTEES

The question arises of whether to seek guarantees from tray manufacturers. This
needs to be decided by each Project Team, because there are 'pros and 'cons. If
a guarantee is obtained, the manufacturer is bound to do all he can to resolve
any shortcoming in his tray design, up to the limit of his liability (which will be
stated In the guarantee). In practice, It may not be clear whether a particular
problem has been caused by the tray design, the tray installation, or the
subsequent operation. Since the manufacturer is bound by a guarantee, he will
be concerned not to say or do anything which could be construed as an
admission of liability. Experience shows that, even where there is no
guarantee, most manufacturers are very willing to help resolve the problem,
Since they stand to learn from a detailed knowledge of what went wrong, no
matter whose fault It is.
Guarantees do not cover consequential losses. If a failure occurs and the plant is
shut down, the consequential loss will far exceed any sum in the manufacturer's
guarantee - In cash terms a guarantee is of little value. Furthermore, the
guarantee will generally contain clauses defining requirements of access for
testing, dismantling and examination In the event of failure. The guarantee may
become void if we do not meet these requirements.
Before seeking a guarantee, It is Important to be clear that It will be helpful to us
when a problem arises and that It can be enforced. Tray efficiency and foaming
are two areas of uncertainty which can make a guarantee difficult to enforce. For
example, If we have specified the number of trays on the basis of our own vaporliquid equilibrium model and the manufacturer has specified tray efficiency, he
may claim that a failure to meet design is due to an error In our VLE model.
Such a claim may be difficult to refute.

6

TRAY DESCRIPTION

While there are exceptions, the vast majority of column and tray designs are
essentially similar and their features are described below. In describing trays,
customary terminology (largely based on FRI definitions) is introduced. A
comprehensive listing of definitions is given In Appendix A. The definitions are
those used by Fractionation Research Inc and are understood by the majority of
tray manufacturers. Also, other terminology in common use is Included.


Distillation columns are always cylindrical vessels and the trays are essentially
circular. Each tray consists of two main functional parts -the bubbling area
(sometimes called active area) and the downcomer, each functioning as its name
suggests. The bubbling area achieves the contact between vapor and liquid; the
liquid flows across the tray while the vapor passes through holes, in the tray floor.
The downcomer transfers liquid leaving a tray down to the tray below.
The Simplest design is a one pass tray (Figures 1 & 3) in which the downcomer
is of segmental shape. One pass trays are used mostly in small and medium
sized columns (up to 3 m diameter). As diameter Increases, the liquid load on a
one pass tray tends to limit its capacity and higher capacity can be achieved with
a two pass tray. Large one pass trays also tend to suffer loss of efficiency
through recirculation eddies at the sides of the trays as shown in Figure 2. Three
and four pass trays (Figure 3) are not recommended because it is difficult to
ensure the proper distribution of liquid and vapor to each pass; failure to do so
results In the passes operating at different L V ratios With a consequent risk of
poor efficiency.
A column sectional sketch is shown In Figure 4, which shows the various types of
downcomer used: straight (as In Figures 1 & 3), sloped and stepped.
Also shown are the recessed seal pan and the Inlet weir. Either of these can be
used, when the liquid load is low, to ensure a positive seal at the bottom of the
downcomer, this is to avoid the possibility of vapor flowing up the downcomer
and causing premature flooding of the downcomer. Most trays have an outlet
weir to ensure that there is an adequate liquid holdup to achieve good efficiency.


FIGURE 1

TYPICAL TRAY LAYOUT

FIGURE 2

LIQUID RECIRCULATION ON A LARGE ONE-PASS TRAY


FIGURE 3

BASIC CROSS FLOW TRAY LAYOUTS


FIGURE 4

TRAY DESIGN DEFINITIONS


7

DESIGN CALCULATIONS

Three categories of design can be identified which fall roughly in line with the
stages of estimate in a Project for a new plant:
(a)

Class D - budget - rough design.

(b)

Class C - pre-sanction - preliminary design, still mainly for costing.

(c)

Class B - sanction - detail design.

For the assessment of existing trays or for plant modifications (except where a
new column is required) all calculations will be in the detailed design category.
The following Clauses are concerned primarily with Sieve and valve trays, which
are the commonest types of tray. Bubble cap and dualflow trays are also briefly
considered; for these and other types. Seek expert advice.

7.1

Rough Design

The equation given In the FRI Design Handbook (Reference 1), Section 5.1, P4,
converted to metric units, is:


To use this It is necessary to choose a tray spacing. Values in the range 400 600 mm will generally be satisfactory. For high pressure duties (above l0 bar) a
600 mm spacing should be chosen, because downcomer backup will be the
capacity limitation and a higher spacing will give more scope for tray design.

7.2

Preliminary Design

Commercially available tray design programs are suitable for preliminary tray
design.
These programs typically use correlations which may or may not be up-to-date,
and the results should not be used for detailed design. Nevertheless, it is
convenient to use and provides a good preliminary design, with little effort.
When the preliminary design stage has been reached It is strongly recommended
to gather the design data on GBHE Data Sheet pro-forma’s, allowing this to be
done In a structured and systematic manner which should avoid the confusion
which can so easily arise if the data are transferred manually from, other
commercially available programs.
Depending on the data supplied, the program will design or check the
performance of valve, sieve and bubble cap trays with t, 2, 3 or 4 flow paths. The
design/performance alternatives are available In three aspects of the design:
(a)

Tower diameter:
Specify the diameter, or the program will calculate the diameter from the
specified maximum fraction of flood.

(b)

Downcomer design:
Specify downcomer width(s), or the program will balance jet flood and
downcomer loading.

(c )

Tray flexibility:
Specify the number of units (valve or bubble cap) or fractional hole area
(Sieve), or the program will design to achieve the specified turndown ratio
and maximum pressure drop.


Also for sieve trays, the program gives an assessment of tray vibration; this is
similar to, but not the same as the GBHE procedure (Clause 8). For valve trays
the number of valves should fall in the range 50 to 120 per square meter of active
area.

7.3

Detailed Design and Rating

Detailed design and rating calculations are required in the following
circumstances:
(a)

Column sizing, to determine diameter and height, prior to enquiry on tray
manufacturers.

(b)

Performance assessment of existing trays.

(c)

Exploring tray design options for uprating an existing column or changing
its duty.

(d)

Comparing tray manufacturer's proposals to establish their suitability and
rank them on technical merit.

The first of these starts with an approximate sizing as described In 5.2. Then, as
in the other cases, the starting point for detailed calculations is an established
design; Data Sheets and tray drawings should be available for existing trays, and
manufacturers' quotations should contain enough detail for assessment. If
quotations do not contain enough data we should request the missing
Information. The tray manufacturing business is a highly competitive one and we
can usually choose to go to another supplier, so we usually get what we ask for,
provided our request is reasonable.
No design correlation is totally accurate: It may have a built-In contingency
(aiming to be safe for all circumstances) or it may be fitted to available data and a
safety factor applied in the design process. The latter approach is generally taken
with distillation tray correlations. As a general rule, flooding correlations fit the
data used to produce them with a scatter of about ± 20%. It is therefore usual to
design for a safety factor of about 1.2, equivalent to 82% flood.
Plant data may show that a given system with a given tray design floods at, say,
105% of the predicted flood point. Care is needed In using this Information In the
design of new trays or the assessment of existing trays for a new duty:


(I)

How accurate were the plant data? It should be possible to obtain
plant data which show a mass balance accurate to within 5% and a
heat balance within 10%, but this is not often achieved In practice.

(2)

The same system may not perform so well on a different tray
design or column diameter. It may flood at a lower predicted %
flood If the correlation does not properly account for the effects of
mechanical features In the tray design.

(3)

A different system may not perform so well on the same tray
design. It may flood at a lower predicted % flood if the correlation
does not properly account for the effects of system physical
properties.

For sieve trays, commercial programs are available. They are typically capable
only of rating tray designs; design has to be done iteratively with repeated rating
runs.
For valve trays, commercial programs are available. Although it was said in 7.1
they are suitable for preliminary design only, they can be used for detailed design
in two circumstances:
(i)

For uprating studies, when data are available on the existing trays at
conditions close to flood, since this allows Judgment to be taken on the
accuracy of the program's correlations for the system and tray design
being studied.

(ii)

For comparison of manufacturer's quotations, to show which design gives
us the best design margins.

The sieve and bubble cap tray options in most commercially available programs
should not be used for detailed design work.
The FRI Tray Design Handbook also contains design methods for dualflow trays
(sieve trays with no downcomers), bubble cap trays and baffle trays. Computer
programs are available for the first two of these, but they are rating programs and
design has to be done by repeated rating. Bubble cap tray programs are difficult
to use and require a large number of data items to describe the tray design.
Dualflow tray programs are simpler.


8

PROGRAM USE - DESIGN CONSIDERATIONS

The aim is to explain the significance of the data and how it is affected by
changes in the tray design. This is to help the user not only to Judge whether a
given design is a good one, but also to change the design to improve its
performance. For tray types other than sieve and valve trays, expert help should
be sought.

8.1

General Considerations

The art of tray design is in balancing the design so that the risks of poor
performance from various causes are nicely balanced, resulting in a design
which will work well but is not overly conservative in a way that makes it too
expensive. Since the GBHE Process Engineer will generally be evaluating
existing or proposed designs, the major concerns are:
(a)

Will it work for the operating conditions required (I.e. over the specified
operating range).

(b)

Are the risks to performance (flooding, entrainment, weeping, etc) well
balanced in the sense that none of them stands out as being much closer
to its limit than the others.

An Important consideration is the likelihood of future uprating, because this could
put more emphasis on having higher margins on the flooding parameters and
less on turndown.
The main performance parameters which need to be considered are:
(1)

At maximum design rates: system limit, Jet flooding, entrainment,
downcomer loading, pressure drop and downcomer backup.

(2)

At minimum design rates: weeping, dumping and downcomer sealing.


8.2

Effects of Design Variables on Performance Parameters

This Clause concentrates on the main performance parameters and how a tray
design can be modified to improve them. Generally, the same considerations
apply to both valve and sieve trays. Extra comments are given in Appendix E.
Design criteria for commercially available programs are expressed differently:
some uses the concept of Safety Factor, while others use % flood. % flood is 100
times the reciprocal of the safety factor. For weeping and dumping, the Safety
Factor may be the same as the Turndown in other programs.
For flooding criteria, a safety factor of 1.2 is generally recommended (equivalent
to 82% flood). Values closer to flood (I.e. lower safety factor or higher % flood)
are often obtainable, but should not be used without seeking expert advice since
there is an increased risk of loss of efficiency or flooding.
(a)

System limit - In concept the system limit represents the ultimate capacity
of a column. No matter what tray design you choose, you cannot do better
than the system limit correlation suggests. Where the Jet flood
approaches the system limit (which often happens with high pressure
systems), there is little hope of increasing column capacity by changes in
tray design, or even by changing to packing.

(b)

Jet flooding safety factor may be increased by:
(1)

Increasing column diameter (a major change which could have far
reaching consequences).

(2)

Increasing tray spacing (also a major change. but easier to
accommodate In most cases).

(3)

Reducing downcomer Size, e.g. by changing from straight to sloped
downcomers: this will generally be feasible.

(4)

Increasing hole area and or reducing hole size, but these will
reduce the weeping

Note that two values of Jet flood are given by, one at constant
vapor liquid ratio and the other at constant liquid rate. The first of these is
usually the one relevant to distillation duties and always gives a lower
safety factor. Some manufacturers quote the constant liquid rate value,
which is more optimistic.

(C)

Entrainment - In vacuum duties loss of efficiency due to entrainment may
be the capacity limitation; no loss is expected if the rate of entrainment is
less than 10% of the liquid rate, but some loss is expected above 20%.
Entrainment rate may be reduced by the same modifications to the design
as Increase Jet flood safety factor.
Some programs do not estimate entrainment rates. A conservative
estimate of valve tray entrainment can be made using other methods. The
% hole area is calculated from the number of valves, assuming 0.012 m2
area for each valve, and the hole diameter is taken as 12.7 mm. This
applies to conventional round valves only.

(d)

Maximum Downcomer Liquid Velocity at Top
This is based on the separation of vapor from liquid In the downcomer.
The downcomer will probably be able to operate at considerably
higher rates, recycling vapor to the tray below. This will cause little loss of
efficiency, but the froth density in the downcomer will be reduced and may
lead to downcomer backup flooding. This is unlikely to be a problem
provided the downcomer backup (as clear liquid) does not exceed about
45% of the (tray spacing + outlet weir height) and the system pressure
does not exceed about 10 bar. The safety factor may be increased by
increasing the downcomer Inlet area - using a sloped downcomer will
minimize the loss of bubbling area.

(e)

Downcomer Backup flooding
Some programs also list a froth height, but this should be disregarded
because the correlation used for froth density is not reliable. For nonfoaming low pressure (less than 10 bar) systems, the clear liquid
backup should not exceed 55% of the tray spacing plus the outlet weir
height. For foaming or high pressure systems (greater than 10 bar, a
backup less than 25% should be satisfactory, but will often be too
conservative – seek expert advice. The % downcomer backup may be
reduced by increasing tray spacing, reducing pressure drop, or Increasing
the downcomer escape area.

(f)

Downcomer seal - It is important that the clear liquid height on the tray
should not be lower than the bottom edge of the downcomer. If it is, there
is a risk that vapor will enter the downcomer and cause flooding by
aerating the froth In the downcomer, similar to a foaming or high pressure
system.


This risk is particularly serious if the downcomer backup is less than 100
mm or the liquid velocity through the orifice at the bottom of the
downcomer is less than 0.3 m/s - these are "experience" factors. A
downcomer seal can be achieved in one of three ways:
(1)

Provide an inlet weir higher than the bottom edge of the
downcomer; Inlet weir is are not often used except In low liquid rate
designs (less than 1.2 m 3 hr m2 of tower area).

(2)

Provide a recessed seal pan, which generally allows the bottom
edge of the downcomer to be at the tray floor level.

(3)

Ensure that the outlet weir height is at least 15 mm above the
bottom edge of the downcomer.

In considering the downcomer seal, use the liquid height on the tray, HL,
rather than the inlet liquid height on the tray panel below. HIN, which is
believed to be an overestimate.
(g)

Pressure drop - there are several reasons why It may be desirable to limit
the pressure drop of a tray design, such as:
(1)

To minimize degradation of heat-sensitive materials, especially in
the column sump and reboiler.

(2)

To avoid the reduction in relative volatility which normally occurs
with increasing pressure.

(3)

To avoid downcomer flooding due to backup.

For the first two, packing may be a better choice than trays.
There are two main components to tray pressure drop: the pressure drop
through the holes In the tray floor and the liquid head on the tray. Although the
Correlation for the pressure drop through the holes Includes an allowance for
the presence of liquid on the tray. It is usually referred to as the dry tray
pressure drop.


The dry tray pressure drop can be reduced by:
(i)

Increasing the hole area.

(II)

Increasing the number of holes.

(iii)

Reducing the hole size.

All of these will reduce the turndown available, by increasing the vapor load
at which weeping starts.
The liquid head on the tray can be reduced by reducing the outlet weir height.
The normal height is 50 mm; this can be reduced to 25 mm with little or no
loss in efficiency, but below 25 mm the efficiency can be expected to suffer.
Reduced outlet weir height should increase the turndown but only marginally.
With low outlet weir heights it is necessary to pay attention to the downcomer
seal (see (f) above).

9

PRACTICAL ASPECTS OF TRAY DESIGN

Recommendations on some of the more common Issues are outlined below.
9.1

Liquid Feeds

The most common arrangement is to introduce the liquid at the inlet side of the
tray (see Figure 5). The design should not produce a non-uniform flow pattern on
the tray. Feeds should not be introduced Into a down comer because of the risk
of causing premature flooding due to boiling In the down comer through heat
transfer either from a hotter feed, or to a cooler more volatile feed.

9.2

Vapor Feeds

Intermediate vapor feeds are often introduced through a perforated feed pipe
similar to that for liquid feeds. Care is needed to avoid disturbance of the liquid
on the tray below. Typically the feed branch is centrally mounted with horizontally
oriented slots for the vapor distribution. This helps mixing with the vapor coming
up the column. Drain holes should be provided.


9.3

Two Phase or Flashing Feeds

Good distribution is difficult if not impossible to achieve. Commonly, a pipe
distributor of the type shown In Figure 5 is used, but with much larger orifices. A
problem which needs to be considered is the possibility of slug flow either in the
distributor or In the pipework to it. If slug flow exists at bends or orifices,
equipment will be damaged due to severe vibration. Slug flow may be avoided by
reducing the pipe diameter.

9.4

Reboiler Returns

Most reboilers return a two phase mixture to the column. The flow regime will
generally be mist flow. If this is simply fed through the Inlet branch, impingement
of the liquid on the column wall opposite will probably cause erosion and may
lead to failure. The most common design of internal distributor is shown in Figure
6. This directs the vapor/liquid mixture downwards, the liquid falling to the column
base and the vapor turning round, losing much of its velocity and flowing up to
the bottom tray.

9.5

Liquid Drawoffs

Liquid removal at an intermediate position in a column may be either from a
chimney tray or a modified downcomer sump. It is recommended that a chimney
tray should always be used for total liquid removal. For partial removal, a
chimney tray is recommended, when most of the liquid is being removed, but
when only a small proportion is removed one of the designs shown In Figure 7
may be used.
Figure 8 shows a chimney tray for partial liquid removal. The overflow weir is
omitted for total removal. The following points should be noted:
(a)

For large columns, several chimneys should be used. Chimneys may be
round or rectangular.

(b)

Total chimney area is normally 15 - 25% of the tower area.

(c)

Seal welding is recommended to avoid leakage.

(d)

The annular area between the top of the chimney and the hat should be
equal to or greater than 1.25 limes the chimney area.


(e)

The sump at the liquid drawoff nozzle is not essential, but provides extra
liquid head Without increasing the weight of liquid on the tray.

(f)

The overflow weir height is set by the residence time required. This is
typically 3 - 5 min. based on the liquid drawoff rate and subject to sufficient
provision for vapor disengagement.

9.6

Vapor Drawoff

Vapor drawoff from an Intermediate position in the column may be done by one
of the options shown In Figure 9. Vapor removed by any of these must be
assumed to contain some liquid from the spray In the Inter-tray space. Provision
may be needed for removing this liquid and returning it to the column.

9.7

Reboiler Circuits

Figure 10 shows the most common column internal arrangements for reboiler
Circuits. These provide for the recirculation of liquid through the reboiler which
will normally operate with much less than 100% vaporization. Design B is
preferred for thermosyphon reboilers since it provides a constant liquid head.
Vortex breakers should generally be used on downward flowing nozzles.


FIGURE 6

REBOIL RETURN NOZZLE


FIGURE 9

VAPOR DRAWOFF - INTERMEDIATE TRAY


NOTES
1.

Nozzle A is usually sufficient for small diameter towers.

2.

Nozzle B should be used for larger columns whenever vapor withdrawn is
a significant portion of the total vapor flow and or there is concern that
horizontal vapor flow could interfere with tray action.

3.

Nozzle C is likely to contain less liquid than Nozzle A at the same height.

4.

A Shield above Nozzle B may be desirable to protect from weepage from
the tray above.

5.

A shield is desirable over Nozzle A or B to prevent liquid draining down
column wall from entering the nozzle


FIGURE 10 TOWER INTERNALS FOR REBOILER CIRCUITS
RECIRCULATING TYPES


NOTES
1.

Refer to Reference 5 which describes the advantages and disadvantages
of various reboiler arrangements.

2.

See Figure 6 for arrangement below bottom tray for return nozzle.

3.

In design A. the effective surge volume in the base of the column may be
constrained by the high and low liquid level limits requited by the
thermosyphon reboiler for efficient operation.

4.

Vortex breakers should be used on downward flowing nozzles.

10

TRAY VIBRATION

Tray vibration is a problem which afflicts perhaps 1% of trayed distillation
columns. We have a method for dealing with tray vibration, but unfortunately the
method predicts that many tray designs will suffer from tray vibration. The
procedure to avoid the risk of vibration generally constrains the design, is likely to
make it more expensive and could well constrain the operation of the column.
Since such a small proportion of columns actually suffer. It is not general practice
to consider vibration in tray design. However, when a column is being designed
for a service with a history of tray vibration, the possibility of its recurrence in the
new design must be considered carefully.
Tray vibration occurs at relatively low rates of operation and has been observed
mostly in valve trays but also in sieve trays and one instance in a proprietary type
of dualflow tray. When It occurs, tray vibration causes damage to the tray
structure. Experience has shown failures from fatigue cracking of trays, tray
support beams, tie beams and tray-to-column supports. Extensive cracking has
occurred within hours of operation at the damaging vapor rates and at least one
case of total internal collapse is known as well as a case of shell cracking due to
vibration transmission through the tray supports.
Most, but not all, of the columns have been large (range: 1.5 to 9 m 10) and the
most severe vibrations have generally occurred at rates close to the weep point.
The processes have ranged from high vacuum to high pressure though the
majority; have been aqueous systems in the vacuum range.


Under normal operating conditions, bubble formation at the holes in the tray floor
is an apparently random process, holes bubbling at different times across the
tray. At the conditions of tray vibration, the behavior is different - all holes bubble
together, exactly in time (synchronous bubbling). Where damage to the trays
results, it has been found that the frequency of the synchronous bubbling
coincides with the natural frequency of some part of the tray structure - usually
one or more of the major support beams. The result is exactly like soldiers on a
bridge who don't break step. If their step frequency matches the natural
frequency of the bridge structure, lives are at risk.
The analysis of the tray vibration process rests on the consideration of three
phenomena:
(a)

Unstable flow conditions of the vapor passing through the trays.

(b)

The existence of a process determining the frequency of vibration for a
given system.

(c)

The coincidence of this frequency with the natural frequency of some part
of the tray structure.

10.1

Unstable Flow

The pressure drop across a tray can be considered as the sum of two terms:
(a)

The pressure drop across the holes in the tray floor (dry tray pressure
drop)

(b)

The static liquid head at the tray floor (clear liquid height).

Figure 11 shows how these two components would vary with vapor rate in an
idealized situation. The dry tray pressure drop Increases with vapor rate, while
the clear liquid height falls due to increased aeration of the liquid. The total
pressure drop passes through a minimum and there is therefore a range of vapor
rates where the pressure drop is essentially Independent of vapor rate and the
flow is thereby unstable.


Figure 12 shows the calculated pressure drop of an actual sieve tray. The
pressure drop falls off at low rates due to weeping, but there is a region where
the pressure drop changes little with rate and this was where tray vibration was
observed (Reference 3).
The mathematical treatment of this concept is given In Reference 2, resulting in
the following equation to predict the critical vapor velocity through the orifices in
the tray floor:

where:
C

Dry tray pressure drop expressed as the number of velocity
heads

hc

Clear liquid height at operating conditions (m)

K

(bubbling areal/(hole area). For conventional valve trays.
hole area = 0.0 12 m2 per valve

ρl ρv

liquid and vapor densities (kg. m3)

Vc

critical vapor velocity (m/s).

The coefficient C is calculated as follows:
For sieve trays: C = 5.4 x PHI1 x S5

(10.2)

where: PHI1 and S5 may be read from a computer output.
For valve trays: C = 2.7 for valves with a venturi orifice in the tray floor. For the
usual valve type with a plain orifice C is a function of tray thickness:


FIGURE 11 IDEALIZED PRESSURE DROP CHARACTERISTIC

FIGURE 12 TYPICAL SIEVE TRAY PRESSURE DROP CHARACTERISTIC
(Ref.1)


10.2

Oscillation Frequencies

As noted above there are two types of oscillation:
(a)

The bubbling process at the orifices in the tray floor.

(b)

Oscillation of the tray structure.

For damaging tray vibration to occur me bubbling frequency must coincide with
the natural frequency of one or more elements of the tray structure. The
oscillations observed in columns suffering from tray vibration have been in the
range 15 to 50 Hz.
As noted In Reference 2, several models have been tried for predicting the
bubbling frequency, but none fitted the data particularly well and an empirical
correlation is used to predict the oscillation frequency:
f = 31 p0.3

(10.3)

where:
f = bubbling frequency (Hz)
ρ = column pressure (bar).
For estimates of the natural frequencies of the tray structure, methods are to be
found in standard mechanical engineering texts (e.g. Reference 4) but specialist
advice should be sought.

10.3

Analysis of Tray Vibration and Action to be Taken

As was noted In the Introduction to Clause 10, only about 1% of trays suffer from
tray vibration and it is not normally considered for new tray designs unless there
is a history of tray vibration in the service being considered.
In the analysis of tray vibration, two criteria are considered:
R1 = (actual vapor velocity) (Critical vapor velocity)
R2 = (bubbling frequency)/(structure natural frequency).


If both these lie in the range 0.8 - 1.2 resonant conditions are likely, resulting in
severe tray damage. To avoid this at least one of the criteria needs to be moved
outside the range 0.6 to 1.6. This is usually done by altering the tray design to
move R1.
K is the reciprocal of the fractional hole area. The actual hole velocity is thus
proportional to K. In the Equation shown In 10.1 the critical hole velocity is
proportional to 1 K. Thus the criterion R 1 IS proportional to K2. The fractional
hole area is thus a powerful means of moving a tray design out of the Vibration
damage region. It is important to consider not only the design point at which the
trays are expected to operate, but also the full range of operation from startup to
shutdown and including any steaming out which operates with refluxed water.
In almost all cases where vibration has occurred, the action taken has been to
reduce the number of holes or valves on the tray and this has been universally
successful. It does however affect the tray hydraulics: increasing pressure drop,
risking entrainment and bringing the tray closer to flooding.
An alternative approach is to change the natural frequency of the tray structure.
Since we do not fully understand the factors which determine the bubbling
frequency, there is less confidence in this approach. It is in principle possible to
move the natural frequency of the tray structure, but this can mean major
changes, requiring stiffer beams.


11

REFERENCES

1

FRI Fractionation Tray Design Handbook,
Five volumes as follows:
Vol 1 - Sieve, bubble cap and dualflow trays.
Vol 2 - Packings, proprietary tests, baffle trays, FRI report index.
Vol 3 - Computer program listings.
Vol 4 - FRI experimental data,
Vol 5 - Design practices.
(Volumes 1, 2 & 5 are generally the most useful).

2

Brierley R J P, P J M Whyman and J B Erskine; Flow induced vibration of
Distillation and absorption column trays; Distillation - 3rd International
Symposium; EFCE Publication Series 3: IChemE Symposium Series No
56; London, England, 1979.

3

Waddington W; Vibration excitation of sieve tray columns by bubbling; M
Eng Thesis, Sheffield University, England; 1973,

4

Den Hartog J P: Mechanical Vibration; McGraw-Hill, New York, USA;
1947.

5·

GBHE-PEG-MAS-616 Selection of Reboilers for Distillation Columns

6

GBHE-PEG-MAS-610 Selections of Internals for Distillation Columns

7

Glossary of Engineering Terms


APPENDIX A
A.1

TRAY DEFINITIONS

INTRODUCTION

The following definitions are those used by FRI these are understood by most of
the Tray Manufacturers (especially those who are FRI Members - see Appendix
F). Some alternative definitions are also included.
The definitions are given in two groups:
(a)

Those relating to the mechanical aspects of the tray.

(b)

Those relating to the tray hydraulic performance.

A.2

MECHANICAL DEFINITIONS

Most of the definitions described below are illustrated In Figures 3 and 4.
A.2.1 Superficial Area
Superficial area is the empty column cross sectional area

A.2.2 Bubbling Area (or Active Area)
Bubbling Area is the superficial area minus the sum of downcomer top,
downcomer seal and inactive areas

A.2.3 Inactive Area
Any section of the tray floor more than the nearest perforation, valve Unit
or bubble cap, and any area behind an envelope downcomer.

A.2.4 Free Area
Free area is the minimum cross-sectional area between trays available for
vapor flow, (It should be noted that Free Area is sometimes used for the
total area of the holes in the tray floor, especially in academic literature).


A.2.5 Hole Area
Hole area is the total hole area available for vapor flow through an
Installed tray.
% Hole area is defined as 100 x (hole area bubbling area).

A.2.6 Downcomer
Downcomer top area is the horizontal area at the downcomer entrance.
Downcomer bottom area is the minimum horizontal area at the bottom of
the downcomer conduit.
Downcomer seal area is the horizontal area below the bottom of the
downcomer used to seal the downcomer and distribute liquid to the tray. It
is defined by the column wall and:
(a)

the edge of the downcomer. Or

(b)

the edge of the seal pan (If used), or

(c)

the inlet we" (If used).

Area under the downcomer (or downcomer escape area) is the minimum
area through which the liquid must flow to leave the downcomer. It may be
horizontal or vertical. It constitutes a restriction orifice and contributes to
the liquid backup in the downcomer.
Downcomer clearance is the vertical distance from the tray floor, or the
floor of a seal pan, to the bottom of the downcomer. This is the definition
used in the FRI Manual Volume 1.
Downcomer width at the top (not an FRI definition) is the maximum
distance across a downcomer perpendicular to the downcomer panel at
the top.
Downcomer width at the bottom (not an FRI definition) is the maximum
distance across a downcomer perpendicular to the downcomer panel at
the bottom.


A recessed seal pan (also known as a recessed sump) is used at the
bottom of a downcomer to provide a positive seal to prevent vapor
entering the down comer from below during operation (see Figure 4).

A.2.7 Flow Path
Average width of flow path is the arithmetic mean of the wall-to-wall distance at
the inlet, centre and outlet positions of the bubbling area for a single pass tray.
For a mufti-pass tray it is the sum of the average widths for each pass,
Length of flow path is the distance from the Inlet edge of the tray to the outlet
edge of the bubbling area.

A.3
A.3.1

HYDRAULIC DEFINITIONS
Flow Regime

On the bubbling area of a tray, the action of the two-phase mixture can take one
of three forms, described as flow regimes. Whichever flow regime obtains on a
tray depends mostly on operating pressure and the liquid to vapor flow ratio.
Three flow regimes are described below; a fourth regime (bubbly flow) is possible
but is rarely obtained in practice on commercial trays.
A.3.2

Spray Regime

Under vacuum conditions where the vapor density is low and hence vapor
velocities are high, the action is vapor phase continuous (droplets of liquid in the
vapor rather than bubbles of vapor in the liquid). Tray efficiency tends to be less
than in the other regimes. Foam tendencies will not affect the bubbling area since
the high vapor velocities will effectively tear any foam apart.

A.3.3

Mixed Froth Regime

This is the most common regime and occurs from around atmospheric up to
moderate pressure (say about 10 bar, depending on flow rates). Close to the tray
floor is froth (bubbles of vapor in the liquid). On top of the froth is a region in the
spray regime. Severe foaming tendencies may affect the bubbling area.


A.3.4

Emulsion Regime

This occurs at high pressure (above about 15 bar). Since vapor density is high,
the vapor volume rate is relatively low and the kinetic energy of the liquid flowing
across the tray breaks the vapor streams leaving the holes in the tray floor into
small bubbles, less than 5 mm. Disengagement of vapor from this froth is slow
and downcomer flooding is the usual limitation on tray capacity.

A.3.5

System Limit (or Ultimate Capacity)

A limiting combination of vapor and liquid loads which is a function of system
properties only. If exceeded, massive entrainment of liquid droplets will occur. It
can be overcome only by increasing the column cross sectional area available for
vapor flow.

A.3.6

Entrainment

Liquid picked up by vapor and carried through the floor of the tray above. This
causes back mixing of the liquid and can cause significant loss of efficiency,
especially in vacuum systems

A.3.7

Flooding

Inoperability due to excessive retention of liquid inside the column.

A.3.8

Jet Flooding

Flooding due to massive entrainment; downcomers are operable. Jet flooding is
the usual cause of cause of flooding in vacuum and low pressure duties (less
than about 5 bar).

A.3.9

Downcomer Backup Flooding

The downcomer is totally full of froth; liquid backs up onto the tray. Downcomer
backup is the usual cause of flooding in high pressure systems (above about 10
bar) and in foaming systems.


A.3.10

Downcomer Choking Flood

This is caused by a froth volume too great to pass through the downcomer
entrance. In normal operation froth leaving the tray enters the downcomer from
the active area. In the downcomer, the vapor in the froth disengages from the
liquid and rises back out of the top of the downcomer. A downcomer choking
flood IS caused by the interaction between this escaping vapor and the froth
leaving the active area.
A.3.11

Foam

Foam is a mass of bubbles stabilized by surface effects. Coalescence will be
slow.

A.3.12

Froth

Froth is a mass of bubbles that will begin to collapse as soon as agitation
ceases. Coalescence will be rapid· of the order of 1 s.

A.3.13

Weep Point

As the vapor rate through a tray is reduced from its maximum value, a rate is
reached where liquid starts to leak through holes in the tray floor; this is known
as the weep point.

A.3.14

Dump Point

As the vapor rate is further reduced the weeping rate increases. Eventually, all
the liquid on the tray leaks through the tray floor and none goes over the outlet
weir. The point at which this happens is known as the dump point.

A.3.15

Efficiency

The FRI efficiency correlation involves three efficiency definitions:
(a)

Point efficiency, which represents the approach to equilibrium at a point on
the active area of a tray.


(b)

Murphree efficiency, which represents the separation achieved on an
Individual tray in relation to an Idealized tray In which the liquid and vapor
are fully mixed and at equilibrium. On a real tray the Murphree efficiency
may exceed 100%.

(C)

Overall efficiency, which represents the efficiency of a group of trays in a
Column as the number of theoretical trays divided by the number of actual
trays required to achieve a given separation. The Murphree and overall
efficiencies are often nearly the same, but can be quite different.


DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE
This Process Engineering Guide makes reference to the following documents:
GBHE ENGINEERING DOCUMENTS

GBHE-PEG-HEA-507

Selection of Reboilers for Distillation Columns
(referred to In Clause 11)

GBHE-PEG-MAS-610

Selection of Internals for Distillation Columns
(referred to In Clauses 1, 4 and 11) .

GBH Enterprises

Glossary of Engineering Terms (referred to In
Clauses 3 and 11)


Design and Rating of Trayed Distillation Columns

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