International Conference on Global Trends in Engineering, Technology and Management (ICGTETM-2016)
Operational and Economic Assessment of Distillation Column from the
Performance of Tray
Nilesh P. Patil*1, Vilas S. Patil2
1
Assistant Professor, *2Professor Department of Chemical Engineering,
University Institute of Chemical Technology,
North Maharashtra University,
Umavinagar, Post Box No. 80, Jalgaon - 425 001, India
Abstract:
Currently distillation is the most commonly used
separation / purification technique in many industrial
applications. Irrespective of its higher energy
consumption and limited separation efficiency,
distillation remains the first choice of process
engineer. Therefore the study of distillation column
has unparalleled importance in process design. This
study comprises of column internals like trays and
packing’s. The paper rigorously reviewed the
literature published on current operational
phenomena and economic feasibilities of trays in
distillation columns. The operational features of the
column like pressure drop, energy consumption,
liquid holdup etc are discussed in brief. The
assessment of column based on the challenges in
operation like weeping, foaming, entrainment etc is
carried out individually. Moreover the collective
economical footprint of these problems on the
operational performance of whole bioethanol plant is
discussed critically. This paper summarizes the
operational and economic analysis of state-of-the art
column trays. The matrices includes sieve tray, valve
tray and bubble cap tray. In most of the cases
industrial results are unavailable. Further, the
research in this field is too limited to be of value for
industrial application.
Keywords: Trays; Thermally coupled column;
Energy saving; Optimization; Distillation; Twoenthalpy feed; Retrofit.
1. Introduction
Regarding distillation, De Koeijer, et. al (1) reported
that distillation offers a low thermodynamic
efficiency of about 5−20%. Additionally, it has large
negative environmental impact of carbon dioxide
since the heat is generated through the burning of
fossil fuels. In order to bring the significant increase
in its thermodynamic efficiency, economy, and
decrease in its
environmental impact, various
technologies like configurations with inter reboiler
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and Inter condensers, heat pump systems (2-4), multieffect columns (5-7), thermally coupled sequences
(8), and recently the internally heat-integrated
distillation columns HIDiC (9-13) have been
developed. Specially, HIDiC configurations offers a
great potential in energy savings as compared to
other technologies, particularly in splitting close
boiling mixtures by dividing the column in its
corresponding rectifying and stripping sections. It
results in the generation of temperature feasible
driving forces in the two sections of column.
Till date, beside its high energy consumption;
distillation remains the most widely used separation
technique. Because of its distinct advantages,
practically no other separation techniques can
compete with distillation. Distillation is the only
technique that has proved its applicability on large
scale. Olujic et. al. (11) claimed that ever growing
application of distillation results in the increase
energy consumption since it uses heat in
thermodynamically inefficient way. He further
quoted that high energy consumption is practically
the only weakness of distillation. Therefore there is
strong reason for the motivating the research towards
improving the energy efficiency of distillation all
over the world. Unfortunately, the research interests
get shifted largely towards the development of
alternative separation techniques with relatively
lower energy consumption. These facts collectively
lead to the operational and economic assessment of
distillation columns. As the energy consumption of
the column depends on performance of column
internals; assessment of tray performances is selected
as one of the major footprint in this direction.
The paper in detail summaries the comparative study
of column performances based on the different types
of trays. The matrices covers sieve tray, valve tray
and bubble cap tray etc. Also all the parameters and
problems associated with the performance of tray was
assessed. Finally the rigorous economic assessment
of impacts caused by the various parameters
associated with trays is carried out. Particularly the
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performance of the tray is assessed in terms of their
application for ethanol water system in distillery.
Also the other options for increasing the energy
efficiency of column like direct vapour
recompression, lowering compression ratio are
checked for their viability.
2. Types of Trays
Currently the trays of different geometries are
available in the market. However all this tray are
classified and belongs to two groups namely sieve
trays and valve trays. The types of tray used in
distillation columns are as follows:
2.1. Sieve Tray
Sieve trays are simply a metal plate having drilled
holes known as perforations (sieves) all over the plate
through which the vapour emerges and pass into the
bulk of liquid flowing over the tray. Typical sieve
layout over the tray follows either a square
or equilateral triangular hole pitch. Each tray are
provided with downcomer which guides the excess
liquid over the tray, to its lower tray and outlet weir
which maintains the pool of liquid over the tray,
thereby facilitating highest vapour liquid contact.
Typical sizes of sieves vary based on their
applications. Small sieves are not suitable for fouling
or corrosive liquids as there are chances of blockage,
leading to excessive pressure drop and premature
flooding. At the same time small sieves are beneficial
in reducing the weeping resulting in the increased
tray capacity. Ultimately small sieve trays offers
better turndown ratio.
As far as the manufacturing cost is concerned, trays
with larger sieves are cheaper since the holes can be
punched and because of large diameter holes are
fewer while the trays with smaller sieves are more
expensive since drilling is the only option. In case of
varying vapour and liquid loadings in the column,
there will be the requirement of varying hole area.
Therefore instead of manufacturing different trays to
meet the requirement, it is convenient to use the same
tray layout, and only the required hole area is
adjusted by blanking strips. These trays have an
advantage of low pressure drop, high efficiency,
throughput capacity, and simplified configuration.
Such trays are properly fits for large scale
rectification, separation at low pressure and are
successfully used in industry.
Vapour
Figure: 1 Schematic Diagram of Distillation Column with Sieve Trays
2.1.1. Characteristics of Sieve Trays
Sieve tray has simplest design
All parts are mechanically immovable
There is no liquid seal
Susceptible to weeping at low vapour flow
or high liquid rates
Offers low pressure drop
Comparatively cheaper in cost
Not Suitable for fouling or corrosive liquids
2.2. Valve Tray
Valve trays are quite similar to that of sieve trays
having drilled holes (perforations) all over the plate;
the only difference is that each hole is provided with
a liftable caps / flapper valve. During normal
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operation vapour flow lift the caps, thus self creating
a flow area for the passage of vapour. These caps
directs the vapour to flow horizontally into the liquid
over the tray, thus providing better mixing compared
to sieve trays. These lifted caps gets down when the
vapour velocity fall below it’s designed. Particularly
these trays find an application in a case where vapour
velocity is not constant. The main purpose of cap /
valve is to prevent the liquid from dumping through
the holes during the fluctuating gas velocities; since
low gas velocities are no longer able to sustain the
pressure exerted by the liquid over the tray. Each tray
is provided with downcomer and outlet weir for the
smooth operation of column. Tray layout is either
a square or triangular in structure. Sizes of the holes
and valves may vary based on their applications.
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These trays are not suitable for fouling or corrosive
liquids as there are chances of valve damage resulting
in tray efficiency reduction. Besides this these trays
offers a very good package of considerable
advantages over a sieve trays. The most remarkable
advantage is its capacity to prevent the dumping of
liquid at the time of low vapour velocity. This ability
empowers these trays in retaining the desired liquid
holdup over a tray deck which ultimately results in
stable column operation. More precisely the time
required for the column to stabilize gets considerably
reduced. At the same time reduction in weeping
results in increased tray capacity. For valve trays
turndown ratio is also better. As far as the
manufacturing cost is concerned, valve trays are
more expensive as compared to that of sieve trays
because of additional cost of flapper valves. In case
of requirement of varying hole area, the option of
blanking is available.
Vapour
Figure: 2 Schematic Diagram Distillation Column with Valve Trays
the liquid over a tray, thus enabling maximum vapour
liquid contact per tray. The level of liquid over a tray
2.2.1. Characteristics of Valve Trays
is always maintained below the top edge of the riser
to prevent dumping of liquid down the tower. Each
Simple design with flapper valve
tray is provided with downcomer and outlet weir.
Some parts are mechanically movable
Tray layout for mounting the bubble caps is either
Flapper valve offers a liquid seal at low
a square or triangular. The sizes of bubble caps vary
vapor velocity
based on their applications.
No weeping at low vapour flow or high
liquid rates
These trays are not suitable for fouling or corrosive
Offers moderate pressure drop
liquids. Besides this these trays offers a very good
Comparatively expensive in cost
advantages over its rivals. These trays offer better
Not Suitable for fouling or corrosive liquids
efficiency in terms of prevention of liquid from
dumping resulting due to low vapour velocity. It
2.3. Bubble Cap Tray
enables them in retaining the desired liquid holdup
over a tray ultimately resulting in a stabilized column
Bubble cap trays are complex in structure. A tray
operation. More precisely the time required for to
consists of a metal plate having drilled or punched
stabilize the column gets reduced to its minimum. At
holes all over the plate. Each hole is provided with
the same time reduction in weeping results in
riser (small, short pipes set into the tray) or chimney
increased tray capacity. These trays offers better
with a cap over it. A cap is mounted over a riser in
turndown ratio as compared to other trays. As far as
such a way that, a proper space for the passage of
duty is concerned, these trays ensure the most
vapour should be maintained in between them. Cap
efficient separation, but at the same time offer a
directs the flow of rising vapour to turn through 180°
highest cost among its competitors.
and slots in the cap makes the gas to bubble through
Vapour
Figure: 3 Schematic Diagram of Distillation Column with Bubble Cap Trays
ISSN: 2231-5381
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2.3.1. Characteristics of Bubble Cap Trays
Complex design with bubble caps
All parts are mechanically immovable
Riser offers a liquid seal at low vapor velocity
No weeping at low vapour flow or high liquid rates
Offers higher pressure drop
Comparatively expensive in cost amongst all
Not Suitable for corrosive liquids
3. Operational and Economic Assessment of Trays
The table represents the comparative study of tray parameters and their effects on the performance of column
Parameters
Efficiency
Cost
Pressure Drop
Maintenance
Capacity
Turndown
Sieve Tray
High (≈ 0.9)
Low (X)
Low
Low
High
Types of Trays
Valve Tray
High (≈ 0.9)
Moderate (1.5X)
Moderate
Low to Moderate
Very High
Approx 2:1 (Unsuitable
for varying loads)
Approx 4-5:1 (Suitable for
varying loads)
Fouling Tendency
Low
Low to Moderate
Where X – Comparative Parameter
Table: 1 Comparative study of tray parameters
4. Parameters Affecting the Performance of
Column
In this section, the process variables known to have
effects on the tray performance, including pressure
drop, foaming, tray efficiency and weeping etc are
discussed in detail.
4.1. Active Area
Active area is the area of the tray either perforated or
fitted with valves or bubble caps and available for
vapour/liquid contacting. Larger the active area,
larger is the capacity of tray.
Bubble Cap Tray
Moderate (≈ 0.8)
High (2X to 3X)
High
High
Moderate
Very High
(Suitable for low liquid
loads)
High
4.3. Hole Area
Hole area is the area collectively available for
passage of vapors through perforations / valves /
bubble cap slots. This is the most critical factor in the
design of tray operating range since high vapour
velocity will cause heavy liquid entrainment and high
pressure drop, whereas low vapour velocity will
cause sevier weeping. The open area will also affects
the liquid back-up in a downcomer.
4.4. Tray Spacing
Tray spacing is a vertical distance between two
adjacent trays. It also effects the spray height along
with allowable liquid head in the downcomer.
4.2. Downcomer Area
4.5. Pressure Drop
Downcomer area is the area available for the transfer
of liquid from one tray to the next tray below it. The
main function of downcomer is to allow for vapour
disengagement from the liquid. Undersized
downcomer will always results in downcomer
flooding.
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Pressure drop is simply the sum of pressure drop
caused by the resistance to vapour flow through the
tray open area and the head of clear liquid on the tray
deck. In most of the cases pressure drop is the major
concern as it directly relates the steam consumption.
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Therefore the efforts will always be in order to
maintain the as low pressure drop as possible.
Increasing the number of flow paths in high liquid
rate services is also one of the way to reduce the
pressure drop.
products, particularly, for applications with a small
temperature difference between the bottom and top of
the column.
5.2. Internal Heat Integration in Distillation
Column
4.6. Entrainment
In this technique unlike a conventional vapour
recompression system, the vapour leaving the
stripping section is compressed to a desired level.
Compression of these vapors results in a two-pressure
operation in a column ensuring the operation of
rectification section at higher pressure. Continuous
condensation along the rectification section releases
the heat and transferred it to stripping section. In
stripping section this heat is utilized for continuous
evaporation. In this way the compression ratio is
minimized (9, 10, 17). Using this technique a
significant reduction in compression ratio can be
obtained; but practically this technology was never
implemented. It raises the big question on the
efficiency and actual viability of this technique.
The carryover of liquid droplets due to high vapour
velocity is known as entrainment. It is observed that
increase in vapour velocity causes a heavy
entrainment.
4.7. Weeping
The dumping of liquid over the tray due to low
vapour velocity is known as weeping. It is observed
that decrease in vapour velocity causes a heavy
weeping and can be reduced by increase in hole flow
factor.
5. Parallel Techniques for Increasing the Energy
Efficiency of Column
In this section various parallel techniques aiming at
the increasing energy efficiency of the column are
discussed. Li et. al (14) in his paper studied the
hydrodynamic and mass transfer performance of two
test flow-guided sieve trays and concluded that flowguided sieve tray, as compared to simple sieve trays
shows better characteristics. He further quoted that
increased hole size will result in decrease pressure
drop and increased efficiency making it suitable for
highly viscous mixtures.
5.1. Vapor recompression
According to Pribic et. al (15) vapor recompression
seems to be the best known arrangement for energy
savings in distillation. It consists of condensation of
overhead vapors of column to liquid, and using the
heat liberated through condensation to reboil the
bottom liquid from the same column. The
temperature driving force required to enable the heat
to flow from cooler top vapors to bottoms hot product
liquid is ascertained either by compressing the
overhead vapor and condensing them at a higher
temperature. The same can be accomplished by
reducing the pressure on the reboiler liquid to make it
boil at a low temperature followed by compression of
bottoms vapor back to column pressure. But from
operational point of view later one is not suitable,
rather it will be more energy intensive. Therefore the
earlier arrangement known as heat pump will be
preferred o easy (16). But vapor recompression is not
suitable for all separation applications. It is suitable
for applications involving close-boiling point
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5.3. Feed Conditions in Distillation Column
Since distillation is a most widely used separation
method in the chemical process industries and the
scale of operation is often a large, relatively small
improvements in capital cost or in energy use can
make a significant impact on overall production cost.
Retrofitting existing equipment will also contribute
significantly. Some of the most expensive distillation
systems comprise of a high-pressure columns
processing vapor feeds. These columns require
refrigeration. Wankat et. al. (18) introduced the 2enthalpy feed (2-F) system in which the feed is
divided into two portions that are at the same
composition but one part is vapor and the other is
liquid. This technique is proved to be most effective
for vapor feeds that require refrigeration. The only
disadvantage of the process is that columns with
vapor feeds typically have larger diameters than
columns with liquid feeds, but the same can be
countered in having less energy consumption in the
reboiler. Further Soave et. al. (19) developed a
modified 2-F process for refrigerated distillation
systems in which the cold distillate is use to condense
part of the feed. Wankat reported that in 2-F system
with vapor feed, part of the feed is condensed leaving
the remaining as a vapor at same composition.
Besides its several advantages, requirement of
constant heating and cooling diminishes the
usefulness of 2-F system for retrofitting to increase
capacity (20). The 2-F system for vapor feeds can be
applied to rectifiers, strippers and complete columns
with liquid feed (21-24).
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5.4. Heat Integration in Distillation Column
Trailing two techniques are applicable for increasing
the energy efficiency of distillation column, whereas
this technique is applicable for improvement in
energy efficiency of overall process.
In this
technique the columns are operated based on their
pressure difference. More precisely the column
operating at vacuum is driven by the top vapors of
column operating at pressure. If the columns are
operating at same temperature then intentionally a
pressure difference is created there in order to ensure
the smooth application of the technique. This
technique is also known as multi pressure distillation
technology.
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Configuration of Ideal Heat Integrated Distillation Columns.
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11. Olujic,
6. Conclusion
From the overall assessment of the various tray types
it is observed that every tray has their own merits and
demerits. But still movable valve trays are found to
be most suitable for various types of applications. A
valve tray with movable flapper offers better
operating characteristics amongst all. They perform
great when the domain of efficiency, capacity,
turndown, maintenance etc with a slight higher cost.
Various technologies like heat pump systems, multieffect columns, thermally coupled columns etc are
developed to bring the significant increase in
thermodynamic efficiency and economy of
distillation columns. The same techniques will also
help in bringing the significant reduction in
environmental impact of carbon dioxide emissions.
Particularly HIDiC configurations are found to be
beneficial in energy savings for splitting close boiling
mixtures.
7. Abbreviation
2-F
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