1.1 Introduction
1.1.1 Distillation column anatomy (internal structure)
Regarding the equipment used to promote intimate contact of ascending vapor and
descending liquid in a distillation column or tower, a distinction is generally made
between plate or tray columns and packed columns, and in the case of the latter, between randomly and orderly packed columns, that is, random and structured
packings.
Figure 1.1 shows schematically a distillation column shell containing all relevant
equipment as employed in packed and/or trayed columns. From the top to the bottom: (1) a demister, i.e. a device for removal of entrained droplets from the vapor
leaving the column through the nozzle at the top of the column; a liquid distributor
(2) irrigating a randomly packed bed (3) laying on a V-shaped support plate (4). One
should note that normally a hold-down plate (hard to see here) is used to prevent bed
expansion in an upward direction due to pressure upsets during operation.
The liquid leaving the randomly packed bed is collected in a chevron (vane)-type
liquid collector (5), where it is mixed with the liquid feed entering through a nozzle
at the side of the column. The mixed liquid enters via a down pipe the liquid distributor (2), irrigating a bed consisting of two different sizes of a corrugated sheet structured packing (6), supported by a support plate (4). The liquid leaving the structured
packing bed is collected using a liquid collector (7) with holes in the bottom. This or
similar so-called chimney tray liquid collectors (9) are often used in combination
with the packed column vapor inlet device (8) shown here, to ensure a good initial
distribution of vapor entering the packed bed above. This particular device, known
as “schoepentoeter”, handles with ease two-phase (biphasic) feeds, as delivered by
falling film and thermosyphon type reboilers.
In the present case, a chimney tray collector (7) collects and mixes the liquid
leaving the packed bed above, and the another one (9) collects the liquid coming
from the vapor inlet device (schoepentoeter) (8), and delivers it via a central downcomer to the top tray of a section containing three fixed-valve, two-pass trays (10).
The liquid leaving the last tray falls down and is collected in the sump of the column.
Above the liquid level there is a vapor inlet nozzle. Usually, no vapor inlet devices
are used in tray columns, because the bottom tray generates a large enough pressure
drop to ensure a good initial distribution of vapor.
In addition to two vapor and two liquid feed lines, Figure 1.1 shows also two
manholes, that is, entrances to the inside of the column for installation during
construction and inspection during shutdown. In the case of packed columns, as
indicated here, manholes are always placed at the level of liquid distributors. The
liquid distributor (2) shown schematically in Figure 1.1 is a narrow trough type
with baffles on both sides that receive and spread the liquid jets coming from the
equidistantly arranged orifices in the side walls of the troughs. Detailed drawings
and photographs and related performance characteristics of devices shown schematically in Figure 1.1 can be found together with all other devices belonging to SulzerChemtech separations column portfolio on the website (www.sulzer.com). Similar
information and company-specific or proprietary designs can be found on the
3
4
CHAPTER 1 Types of Distillation Column Internals
(1)
(2)
(3)
(4)
(5)
(2)
(6)
(4)
(7)
(8)
(9)
(10)
FIGURE 1.1 A Computer-Aided Drawing Indicating the Main Types of VaporeLiquid Contacting
Devices and Auxiliary Equipment Encountered in Distillation
(For color version of this figure, the reader is referred to the online version of this book.)
(Copyright Sulzer Technical Review, Sulzer Management Ltd, Winterthur, Switzerland)
1.1 Introduction
websites of other well-established distillation equipment manufacturers, i.e. KochGlitsch (www.koch-glitsch.com), Julius Montz (www.montz.de), and Raschig
(www.raschig.com).
As indicated above, the trays as well as random packings and structured packings
with auxiliary equipment represent three essential types of vaporeliquid contacting
devices used in distillation operations. Typically distillation columns are equipped
either with trays, random packings, or structured packings, but if appropriate, a
distillation column can contain each combination of these or even all three of them.
1.1.2 Methods and main features of distillation equipment operation
In a distillation column, the liquid enters at the top, as reflux, and somewhere along
the column as feed, and descends driven by gravity, experiencing more or less
pronounced resistance caused by ascending vapor. The main purpose of distillation
equipment is to establish an intimate contact between ascending vapor and
descending liquid, by creating a large, intensively refreshed interfacial area. The
flow pattern and relative orientation of phases during contact depend on the type
of the vaporeliquid contactor.
In a tray column the ascending vapor and descending liquid are contacted stagewise, that is, by repeatedly contacting and disengaging two countercurrently flowing
phases over and over again under cross-current conditions. Namely, the liquid coming from inlet downcomer flows horizontally across perforated plates (tray decks)
and pours over the outlet weir into the outlet downcomer, moving to the tray below,
while the vapor flowing upwardly is forced to pass through perforations in the tray
deck and a shallow pool of moving liquid a number of times. The interface, being the
surface area of bubbles and/or droplets, is created to an extent depending mainly on
operating conditions, that is, the governing flow regime and surface tension in
conjunction with chosen tray design. Considering the fact that vapor and liquid
brought into an intimate contact on a tray usually leave the tray before reaching equilibrium, the number of actual trays should normally exceed the number of equilibrium stages or theoretical plates required to achieve specified separation at given
reflux ratio. Therefore, it is a common practice to consider performance of an actual
stage or plate or tray in the terms of the deviation from ideal, e.g. to define the “overall tray efficiency” per column section as the ratio of the number of equilibrium
stages to the number of actual stages (trays or plates) contained in that section.
The resulting column height will depend on the tray spacing chosen, which can
differ per section if appropriate. Namely, feed point or stage separates a conventional
distillation column into an upper (rectification) section and a lower (stripping) section, which, depending on thermal conditions and/or the nature of feed mixture, can
differ significantly in operating conditions and consequently in performance of the
internals involved.
In a packed column, mass transfer occurs continuously by countercurrent flow of
the liquid and vapor, and the efficiency is commonly expressed as the bed height
generating composition change equivalent to that of a theoretical plate (HETP). If
5
6
CHAPTER 1 Types of Distillation Column Internals
the HETP is known, then packed bed height required is simply the product of HETP
and the number of theoretical plates (equilibrium stages) required for separation at
given conditions.
Nevertheless this is not as simple as it appears. Namely, packing serves as carrier
for liquid, and the surface of the liquid film provides interfacial area for mass transfer. Complete wetting is a prerequisite for successful operation. To assure this, a
well-performing liquid distributor is required to provide adequate quality of initial
liquid distribution, while the quality of liquid distribution within the packed bed
will depend on type and size of packing, layout of a layer consisting of segments,
operating conditions, and surface tension.
However, due to the efficiency-deteriorating effect of liquid maldistribution,
which tends to develop with increasing bed depth, the height of a packed bed is usually limited to a height equivalent to a certain number of theoretical plates, 15 being
a typical number. The liquid leaving a bed of given height needs to be collected and
thoroughly mixed before being delivered to the distributor of the bed below. The column height required to install a liquid redistribution section is up to 2 m, which
means that in case of multistage separations ancillary equipment will contribute
significantly to total column height and cost. This is in practice considered to be a
good reason to avoid a packed column in a new design, while the manufacturers
have often been forced in revamp situations to install taller than usual beds, to ensure
that liquid redistribution sections can be accessed through existing manholes. It is
well known that this was done with success in numerous cases and that in industrial
columns equipped with standard-size packings (220e250 m2/m3) there are wellperforming beds of exceptional height (more than 20 stages and in excess of
10 m). With this, the outgoing assumption that “maximum bed height” extended
by 2 to 3 m can add, say, three to four stages as well as the height necessary to
compensate for anticipated loss in efficiency, was justified. However, this should
not be done at the cost of mechanical integrity of the bed; it will depend on the
type and size of packing as well as the construction material.
The extent of liquid maldistribution and consequently the allowable bed height
will depend on the structure of the packed bed, which may be randomly or orderly
packed. In other words, if considering a packed column, a clear distinction is made
between random and structured packings. The former, often called “dumped packings”, are small, open bodies of various shapes and sizes that are dumped into the
column (dry or wet, depending on situation) and settle randomly. Their liquid
spreading extent is rather limited, that is, it does not exceed the dimension of the
body. The structured packings are fabricated from corrugated sheets tightly packed
to each other with corrugations of neighboring sheets oriented in opposite direction,
forming a multiplicity of triangular flow channels crossing each other at an angle
depending on the corrugation inclination angle (usually 45 or 60 with respect to
horizontal axes). The height of a sheet metal packing element or packing layer is
usually about 0.20 m (European manufacturers) or 0.25 m or more (American manufacturers), and overall size and weight of individual segments is intended to comply
with requirements for installation through manholes. All segments put together form
1.2 Performance characteristics of vaporeliquid contactors
a layer of packing, and each layer is usually rotated by 90 with respect to the previous one. Therefore a packed bed consisting of structured packings represents a
highly ordered structure that allows radial spreading and mixing of phases at each
transition between packing layers. The extent of liquid and vapor spreading depends
on the corrugation inclination angle and is therefore larger in case of common 45
than in case of 60 packings.
The nominal size of random packing is usually given in numbers representing
characteristic dimension like the diameter in the case of rings (1e4 inch, i.e.
25e100 mm, for packings used in industrial columns), and the size of structured
packings manufactured in Europe is usually given in numbers representing the
amount of specific geometric area.
Capacity and efficiency of trays, random and structured packings are directly
related to pressure drop. Namely, the upward flow of vapor phase is associated
with a certain amount of pressure drop across the column internals, that of trays
being much higher than that exhibited under the same conditions by random and
particularly structured packings. A larger pressure drop creates a larger pressure
and, consequently, a larger temperature at the bottom of the column. If excessive,
it can detrimentally affect the separation (reduced relative volatility, bottom product
degradation by decomposition or polymerization, and/or need for a hotter heating
medium). A detailed account on effects of the operating pressure and the pressure
drop on distillation operation is given in Chapter 9.
One should note that with going deeper into vacuum the amount of tolerable
pressure drop decreases, and in extreme cases (high boiling, thermally unstable
chemicals) it may be so low that column internals need to be avoided. In such cases,
special, falling film type or spray type columns are considered.
1.2 Performance characteristics of vaporeliquid
contactors
As mentioned above, upon establishing the reflux and stage requirement of a
separation by detailed (rigorous) calculations/simulation, the designer must choose
between trays and packings and subsequently find the most appropriate one.
Namely, trays, random packings, and structured packings come in numerous
versions, which differ considerably in their performance characteristics, and in the
range of applications where both could be applied a thorough consideration of all
relevant factors is required. A recently published paper written by two authors representing a major distillation equipment manufacturer and a major user company,
gives a concise account on relevant performance characteristics, providing guidelines when to choose trays and when packings [3]. Therefore, as well as due to
the fact that a detailed account of trays, random packings, and structured packings
can be found in the Chapters 2, 3, and 4, respectively, there is no need in this chapter
to go into too much detail on these topics. In addition, various aspects of performance characteristics of trays and random and structured packings are addressed
7
8
CHAPTER 1 Types of Distillation Column Internals
to certain extent in Chapter 9 (high and low pressure distillation) and Chapter 10
(scale up), as well as in chapters addressing equipment testing and various applications in the book Distillation Operation and Applications [4].
The present, equipment-related introductory chapter addresses and qualitatively
compares performance characteristics of standard, well-established tray-type contactors, e.g. sieve, valve, and bubble cup trays, with random and structured packings.
1.2.1 Trays
Standard trays considered here are cross flow trays with active or bubbling area
situated in between inlet and outlet downcomers. Common nonproprietary devices
used to facilitate contact of vapor passing through the liquid flowing over the active
area are known generally as sieve, valve, and bubble cup trays or decks. Basic
geometric features of sieve, valve, and bubble cup trays are shown schematically
in Figure 2.7 in Chapter 2. Being essentially perforated thin plates, sieve trays are
simplest and cheapest to manufacture, and most importantly, exhibit good efficiency
over a wide range of operating conditions. However, if more flexibility is required,
then valve trays will be considered. Floating or movable valve trays are preferred in
refinery applications where more or less pronounced fluctuations in feed supply
occur from time to time. In general, floating-valve trays are more expensive and
prone to fouling and mechanical damage. The latter is not the case with so-called
fixed-valve trays, which combine advantages of a fully open floating valve tray
and a sieve tray at a price not exceeding that of common sieve trays.
Owing to the fact that in the case of sieve and floating or moving valve trays as
well as fixed-valve trays the liquid is maintained on the deck by kinetic energy of the
vapor, these trays are limited at low vapor flow rates, where weeping, if excessive,
leads to significant loss of efficiency. If the lower operating limit is a concern, bubble
caps, which employ a physical seal that prevents liquid from leaving the tray deck,
are considered. Their major drawback is constructional complexity and an appreciably higher price. However, more practical modifications of bubble cap trays
such as “tunnel trays” and “Thormann trays” (Figure 1.2) allow cost-effective implementation of these devices where appropriate (www.montz.de). A distinguishing
feature of a Thormann tray is that it can handle the lowest specific liquid flows,
down to 40 l/m2h. This, in combination with a rather low pressure drop, makes it
also suitable for vacuum applications.
Other well-established types of trays are: multidowncomer trays and various
high-capacity trays, as well as the trays without downcomers, such as dual flow trays
and baffle trays. The main feature that distinguishes dual flow trays is the fact that
both liquid and vapor are expected to use the same hole or neighboring holes, which
means that unlike with other trays, the mass transfer occurs under countercurrent
flow conditions. However, we will here consider mainly the characteristics of standard sieve, valve, and bubble cap cross flow trays, and other tray types (see Chapter
2) will be mentioned where appropriate, just to indicate their specific benefit with
respect to standard cross flow trays.
1.2 Performance characteristics of vaporeliquid contactors
FIGURE 1.2 Photographs of the Top View of a 3.4 m Diameter Thormann Tray and of a Cap
Containing Vapor Directing Openings That Push the Liquid across the Tray Following the Pattern
Shown in the Attached Drawing on the Left Side
(For color version of this figure, the reader is referred to the online version of this book.)
Courtesy of J. Montz GmbH.
Regarding the operation of a cross flow tray, one of the important parameters is
flow path length, because it affects efficiency. In case of a short distance between
downcomers, as encountered in small-diameter columns or on multi-downcomer
trays, the liquid is well mixed, that is, its concentration is homogeneous, and thus
the driving force for mass transfer is constant and the resulting efficiency is the
lowest one encountered in normally operating distillation columns. Longer flow
paths allow a uniform liquid flow pattern to develop approaching plug flow condition, which in turn ensures the largest driving force, leading to the highest achievable
tray efficiency. However, at large-diameter trays with large liquid flow paths, flow
anomalies like stagnant or recirculation zones can be induced by the geometry of
the tray, allowing a certain amount of backmixing to occur, leading to the loss
of efficiency. Here we speak in particular about the efficiency-deteriorating effect
of the liquid maldistribution on a tray. On the other hand, if the depth of froth layer
varies along the flow path, this can lead to vapor bypassing, which is a common form
of vapor maldistribution observed in practice [2].
9
10
CHAPTER 1 Types of Distillation Column Internals
The clear liquid height and consequently froth height on a tray are dictated by the
liquid flow rate and weir height, and an increase in weir height leads to increased
efficiency, which however comes at the cost of increased pressure drop. This is a
typical trade-off situation, while the weir height is dictated mainly by the amount
of affordable pressure drop. This means lower than normal (50 mm) weir height in
vacuum and higher in high-pressure applications. Another important design parameter
is the weir length, that is, the related maximum liquid load per unit weir length. If
excessive it will affect efficiency adversely. To accommodate larger liquid loads,
weir length needs to be extended, which can be done in different ways, but if this
proves to be insufficient a tray with the two or more downcomers is chosen.
If in excess of tolerated values, weeping on the low end and entrainment on the
high end represent respectively lower and upper limits of tray operating range. The
latter is a primary concern because it is related to the achievable approach to the
upper operating limit, and it hurts efficiency more then weeping that is more gradual
in appearance, and can be tolerated to a greater extent than entrainment. It is interesting to mention here that weeping at the end of the flow path is a minor problem,
but that occurring at the beginning is dangerous, because the liquid bypasses the tray
and falls on the liquid pouring into the downcomer of the tray below, thus effectively
bypassing two trays. This as well as many other peculiarities related to the design of
trays needs to be identified and handled accordingly during conceptual tray design,
and should get proper consideration during detailed design.
1.2.2 Random packings
Figure 1.3 shows a photograph of random packings that represent milestones in the
development of modern packed column technology. The first generation is the wellknown Raschig ring, patented in 1911; followed by the second-generation Pall ring
introduced in the 1960s; the third-generation IMTP saddle-type ring, introduced in
late 1970s; and after nearly 100 years of development, the fourth-generation Raschig
Super Ring [5]. There are also other well-established random packings of the third
generation (see Chapter 3), like the well-known Nutter ring, that belongs to the
FIGURE 1.3 Photograph of the Representatives of the First (Raschig Ring), Second (Pall Ring),
Third (IMTP Saddle), and Fourth (Raschig Super Ring) Generations of Random (Dumped)
Packings
(For color version of this figure, the reader is referred to the online version of this book.)
1.2 Performance characteristics of vaporeliquid contactors
Sulzer portfolio (www.sulzer.com). Most recently, Koch-Glitsch introduced a
fourth-generation random packing known as Intalox Ultra packing, and performance
characteristics as observed in FRI tests make it suitable to improve throughput or
product purity in the case of a revamp of a column equipped with thirdgeneration random packings [6].
Indeed, regarding capacity the above-mentioned packings represent different
generations, and a Pall ring really was a big step forward compared to a Raschig
ring; and third-generation packings outperformed the Pall ring significantly; while
this cannot be claimed for the fourth generation compared to the third generation.
Anyhow a shift towards a better overall performance was achieved and this justifies
their position as the fourth generation of random packings.
In distillation applications it is a common practice to use stainless steel packings
or another alloy with sufficient corrosion resistance. However, in some applications
involving corrosive aqueous systems at low enough temperatures (<373 K or
100 C), packings made of much cheaper polypropylene or other suitable plastic
material may be considered. Ceramic packings find application in some highly
corrosive applications, but appear to be sensitive to damage, i.e. breakage due to
operation upsets that can lead to plugging, i.e. reduction of bed void or free area
(porosity) and consequently increased pressure drop. To avoid breakage during
installation, ceramic packings are dumped under wet conditions, i.e. into a column
filled with water.
1.2.3 Structured packings
Figure 1.4 shows three layers of a conventional corrugated sheet structured packing,
assembled from segments of the size that allows installation through column
manholes.
Corrugated sheet structured packings were introduced in early 1960s, by Sulzer,
in the form of so called BX packing. This packing was and still is made of perforated
wire gauze material, with corrugations inclined 60 to horizontal. At that time it was
considered as the proper internal for the very demanding heavy water separation, and
FIGURE 1.4 Photograph Illustrating the Layout of a Large-Diameter Corrugated Sheet
Structured Packing Bed
(For color version of this figure, the reader is referred to the online version of this book.)
Courtesy of J. Montz GmbH.
11
12
CHAPTER 1 Types of Distillation Column Internals
later on found thousands of applications in fine chemicals separation/purification.
A real breakthrough of structured packings occurred in the late 1970s, with the introduction of the Sulzer Mellapak series, made of much cheaper metal sheets. This as
well as similar packings developed by other manufacturers proved their value first as
replacements for various trays in vacuum applications, allowing large capacity
increases, often accompanied by significant efficiency increase. From that moment
on it became obvious that structured packings, due to the lowest pressure drop per
stage, are the best option for vacuum applications. With time it appeared that structured packings cannot perform satisfactorily at high operating pressures, where
trays, particularly those handling large liquid loads with ease, remain the preferred
choice. Random packings can perform well on both ends and in the middle of the
range of operating pressures encountered in distillation, but cannot outperform
structured packings or trays in their primary fields of application.
Nowadays the main suppliers and innovators in this field, in addition to the
pioneer Sulzer, are Koch-Glitsch, Montz, and Raschig. Presently manufacturers
from China and India are attempting to enter the global market by offering various
packings often at significantly lower prices. While Sulzer Mellapak, Koch-Glitsch
Flexipac, and Montz Montz-Pak rely on the established corrugated sheet structure,
with textured, perforated, or imperforated corrugated sheets that allow continuous
flow of liquid and force vapor to follow the channels, the corrugated sheets of
Raschig Superpac are open (see Figure 1.5), enabling both liquid and vapor to easily
switch to the other side. For vapor this means an effective flow angle that is steeper
than that of conventional corrugations. The reduced pressure drop brings a capacity
advantage that is equivalent to that of advanced designs of corrugated sheet structured packings, those employing a smooth bend towards the vertical on lower (see
Figure 1.6) or both ends of corrugations (see Figure 4.6 in Chapter 4).
The high performance packings of Koch-Glitsch, Montz, and Sulzer are
known as Flexipac HC, Montz-Pak M (imperforated) or MN (with holes), and
FIGURE 1.5 Photograph of a Sheet of Raschig Super-Pak
(For color version of this figure, the reader is referred to the online version of this book.)
Reprinted with permission from reference [7]
1.2 Performance characteristics of vaporeliquid contactors
FIGURE 1.6 Photograph Showing the Orientation of Subsequent Elements of a High-Capacity,
B1-250M Packing, with the Bottom of Corrugations Bent to Vertical
(For color version of this figure, the reader is referred to the online version of this book.)
Courtesy of J. Montz GmbH.
MellapakPlus, respectively. The favorable performance characteristics of the
Montz-Pak MN series packings have been demonstrated in tests conducted most
recently using semi- and industrial-scale installations [8,9].
Figure 1.7 shows details of corrugated sheet geometry, including all relevant
parameters that allow estimation of specific geometric area, ageo,p (m2/m3), hydraulic diameter for gas or vapor flow, dh (m), and packing void fraction or porosity,
ε (m3/m3) using geometry-based expressions given in Table 1.1.
b
h
s
β
δm
α
b′
FIGURE 1.7 Basic Geometry and Dimensions of a Corrugated Sheet
13
14
CHAPTER 1 Types of Distillation Column Internals
Table 1.1 Expressions for Estimation of Characteristic Parameters
of a Corrugated Sheet Structured Packing
ageo,p ¼ 4s/bh z 5.657/b
b ¼ 2h
b0 ¼ b/sina
h ¼ b/2
s ¼ [(b2/4)þh2]0.5
dh ¼ 2bh/(bþ2s)
ε ¼ 1 (ageo,pdm/2)
Specific geometric area
Corrugation base
Corrugation base (horizontal)
Corrugation height
Corrugation side
Hydraulic gas diameter
Void fraction (porosity)
Basic corrugation dimensions are corrugation height, h (m), corrugation side
length s (m), corrugation base length, b (m), the thickness of material, dm (m),
and corrugation inclination angle, a ( ). For a standard corrugation crimp or fold
(apex) angle, b ¼ 90 , only one parameter is needed to get all other dimensions.
It is the size of corrugation base length, b (m), which can easily be measured
when a packing is prepared for installation, directly or indirectly via the base length
at horizontal plane, b0 (m). These two are related through the corrugation inclination
angle, which also should be checked upon delivery of packing. Namely the corrugation inclination angle influences significantly the efficiency, capacity, and in particular the pressure drop. Table 1.2 shows the main packing dimensions for a number of
typical specific geometric areas.
Standard sizes according to specific geometric area are available in the range
from 100 to 750 m2/m3. Below 100 m2/m3 the preferred choices are various grids,
a robust structure used in fouling, mainly heat transfer services where separation
is not demanding. The standard, most widely employed size of structured packing
is 220e250 m2/m3, while applications of high-performance packings with a specific
geometric area of 350 m2/m3 packings have been gaining momentum during recent
years. Well-established 500 m2/m3 is considered as a good candidate for fine chemical separations (if pressure drop is a concern, then equal size, much more expensive
Table 1.2 Characteristic Dimensions of a Corrugated Sheet Structured Packing
with Corrugation Crimp (Fold) Angle of 90 , According to Specific Geometric Area
b0
ageo,p
(m2/m3)
100
250
350
500
750
ε
b
(mm)
a [ 45
h
(mm)
s
(mm)
dh
(mm)
dm [ 0.1 mm
(m3/m3)
56.6
22.6
16.2
11.3
7.5
80.1
32.0
22.9
16.0
10.7
28.3
11.3
8.1
5.7
3.8
40.0
16.0
11.4
8.0
5.3
23.4
9.4
6.7
4.7
3.1
0.9950
0.9875
0.9825
0.9750
0.9625
(mm)
1.2 Performance characteristics of vaporeliquid contactors
wire gauze packing is preferred), while 750 m2/m3 dominates in air distilling applications where minimizing column height while maintaining a low pressure drop is a
main design demand.
These considerations are valid for conventional designs with a corrugation inclination angle of 45 . Much less pressure drop and considerably higher capacity can
be achieved using the same packings with a steeper corrugation inclination angle,
which is usually 60 with respect to horizontal. However, the gain on the capacity
side is accompanied by a loss in efficiency [10]. Therefore this angle is not used
in demanding separations. Increasing surface area to compensate for efficiency
loss is not an option, because it results in linear increase in packing costs. One should
note that nearly the same capacity gain at the same efficiency can be obtained with
the before-mentioned high-performance versions of the same packing, without additional costs.
1.2.4 Grids
As mentioned above, there are a number of specific applications, like wash-zones
and pump-around sections in crude oil and vacuum columns in refineries, where
insensitivity to fouling and corrosion, low pressure drop, and heat transfer are
dominating concerns, and rather low separation results are required. For such
applications, robust, smooth-surface structured packings with specific geometric
areas between 50 and 100 m2/m3 like Mellagrid are used, or more frequently, robust
ordered structures known as grids (see Figure 1.8).
Being highly open structures, grids can handle easily situations where high vapor
and low liquid loads are involved. They allow high capacities at rather low pressure
drop and are usually used in combination with spray distributors. The turndown of
the latter is a limiting factor and their liquid distribution quality is poor [11], but
more than sufficient to handle typical heat transfer requirements. Another difficulty
with spray distributors is that a significant fraction of small droplets, prone to
entrainment, is generated. However this is accounted for in designs and the
FIGURE 1.8 Photograph of three elements of the Nutter grid
(For color version of this figure, the reader is referred to the online version of this book.)
Courtesy of Sulzer Chemtech Ltd.
15
16
CHAPTER 1 Types of Distillation Column Internals
state-of-the-art grids perform as inertial demisters with good de-entraining efficiency, provided vapor velocity is not too low.
Because a grid is a mostly open structure with lowest pressure drop it is highly
prone to vapor maldistribution. Therefore it is important to consider the section below,
that is, the flow pattern of ascending vapor, and if considered severely maldistributed,
a chimney tray is usually placed below the grid. To facilitate vapor distribution and
mixing within a grid bed, each neighboring layer is usually rotated by 45 .
An overview of variations in grid designs, made to fit specific applications, can
be obtained from brochures posted on the websites of packing manufacturers, e.g.
koch-glitsch.com, sulzer.com.
1.2.5 Ancillary equipment (internals) for packed columns
Figure 1.9 shows a drawing of a typical liquid redistribution section as employed in
packed columns, containing a chevron-type liquid collector placed above a narrowtrough liquid distributor. Usually liquid redistribution sections require 1.5e2 m column height for installation. One should note that conventional, chevron and chimney
tray type liquid collectors or catchers have limited free area for vapor (30%), while
FIGURE 1.9 Drawing of a Typical Liquid Redistribution Section
(For color version of this figure, the reader is referred to the online version of this book.)
Reprinted with permission from references [7] and [12]. Courtesy of J. Montz GmbH.
1.2 Performance characteristics of vaporeliquid contactors
narrow-trough distributors are usually designed with free areas in between 40% and
50%. In both cases the free area for vapor tends to decrease with increasing specific
liquid load, which means that pressure drop caused by a liquid redistribution section
may become substantial, and should be accounted for [12]. Packing support plates
are usually very open, and the related pressure drop is negligible. However their
design should be checked, particularly if random packings are considered, to see
whether enough free area is provided to avoid initiation of premature flooding at
the bottom of the bed.
The prerequisite for the proper functioning of packed beds is an adequate quality
of the initial liquid distribution. This means employing a distributor with a sufficient
density of uniformly distributed irrigation (pour or drip) points. The density of
irrigation points, expressed usually as the number of drip or pour points per unit
cross-sectional area, depends on the packing size, i.e. specific geometric area of
the packing considered, random or structured. Namely the smaller the dimensions
of the packing, the lower the ability of packing to spread the liquid radially, and
this is more pronounced in the case of random than structured packings.
If we take a 250 m2/m3 structured packing as a benchmark and 2 inch or 50 mm
as a random packing equivalent in this respect, then a common recommendation is to
use a distributor with 100 drip points per m2 of cross-sectional area. Although this
number could be halved in practice, it is considered to be a good design practice and
should be kept as such. Packings with larger specific geometric areas require distributors with increased density of drip points, and as a conservative guide, the number
of drip points should increase according to the ratio of specific geometric areas of the
packing size considered and the base case. In any case, the outer (periphery), that is,
circumferential ring of drip points, needs to be close to the column walls.
From an operating point of view, uniform initial liquid distribution means that
each irrigation point needs to deliver the same flow rate. Practically this means
that deviations with respect to a mean value should be below 5%, over the whole
operating range, which is within the design tolerances of state-of-the-art distributors
and can easily be checked during functionality test with water prior to shipment or
installation.
In general, as mentioned in the introduction of this chapter, the uniformity of
liquid distribution as established at the top section of a bed will degrade to a certain
extent compared to the initial profile, depending on the type and size of packings as
well as operating conditions. Liquid distribution within a randomly packed bed will
be supported and maintained by vapor load. However, with increasing bed depth, the
quality of liquid distribution tends to deteriorate mainly due to channeling. In a bed
of structured packings a natural distribution pattern will establish after one or two
packing layers. It will also tend to deteriorate with increasing bed depth, and this
is in both cases more pronounced with larger specific geometric area packings. In
order to avoid deterioration in efficiency beyond acceptable limits, design guidelines
usually limit maximum bed height/depth to the height equivalent of 15 theoretical
plates. If probable, the potential effect of liquid maldistribution can be estimated using a simple, parallel column method described in Chapter 4.
17
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CHAPTER 1 Types of Distillation Column Internals
1.3 Criteria for selection of vaporeliquid contactors
Various trays, and random and structured packings, differ considerably in their performance characteristics, and to arrive at the proper choice for an application
numerous parameters and factors need to be thoroughly evaluated. In the first
instance, this means detailed consideration of mass transfer efficiency, capacity,
and pressure drop, and the operating range of the devices considered suitable for
the given application, keeping in mind related costs. Liquid holdup that dictates total
liquid volume and residence time becomes a concern if thermally sensitive or reacting materials or highly explosive materials are distilled.
However, in industrial practice, the potential application-related constraints are
considered first, and this means that usually fouling and/or corrosion sensitivity or
tendency is considered first. The latter implies consideration of the construction material choice. There are also a number of other, more practical properties such as
foaming sensitivity, scalability, installation, ease of inspection and maintenance,
and experience that also need to be evaluated properly before a final choice is
made and a design specified. If different devices allow a reliable design, the cost becomes a decisive factor. For large-scale users of distillation, the price of internals is
often the dominant factor in decision-making.
In order to get a fair impression of the relative advantages and disadvantages of
devices being compared, Table 1.3 contains a summary in the form of a “customer
guide”, with numbers 1 to 5 standing for poor, fair, good, very good, and best or
excellent, respectively. The two extremes (1 and 5) represent the reason to avoid
or choose, respectively. Obviously, the reference device, i.e. sieve tray, represents
an average with respect to positive and negative extremes, and is a good standard
Table 1.3 Comparison of Performance Characteristic of Common Tray Types,
Random Packings and Structured Packings, Which Can Serve as a Selection Guide
Property
Sieve/Valve/
Bubble Cup Trays
Random
Packings
Structured
Packings
Capacity
Efficiency
Pressure drop
Turndown
Fouling sensitivity
Corrosion sensitivity
Construction material
Scalability
Access for inspection
Experience/know-how
Cost
3/3/2
3/3/3
3/3/3
3/4/5
3/2/3
3/2/3
3/3/3
3/3/2
3/3/3
5/5/4
4/4/2
3
4
4
4
2
1
5
4
2
5
2/3
5
5
5
4
2
1
4
4
1
5
1/2
(1 ¼ Poor, 2 ¼ Fair, 3 ¼ Good, 4 ¼ Very Good, and 5 ¼ Excellent)
1.3 Criteria for selection of vaporeliquid contactors
(reference) design. Other devices show some quite favorable properties that, however, are usually accompanied by certain losses in other properties. Certainly, this is
most strongly pronounced in the case of structured packings.
One should note that in the case of two numbers as related to cost and inspectability, the first number represents the situation from the 1990s, indicating that in the
meantime some improvements have occurred in this respect. Namely, sheet metal
packings as well as gamma-ray based diagnostics have become affordable, and
this gave a certain boost to wider application of structured packings, where appropriate. This and other relative performance related aspects and factors are discussed
in greater detail in the following subsections.
1.3.1 Capacity
The “capacity” here means volumetric vapor throughput, and operating capacity is
usually set by designers at 0.75 to 0.85 of the maximum capacity, i.e. point of onset
of flooding. A higher capacity implies a lower column diameter in new designs,
which in the case of tray columns depends on the tray spacing. Increasing tray
spacing reduces column diameter but increases the column height.
Sieve and valve trays can be considered equal in this respect in the whole range
of operating conditions, and bubble cap trays can also handle high vapor loads, but
not high liquid loads. In high-pressure applications, with very high liquid loads the
multiple downcomer (counterflow) trays exhibit capacities significantly exceeding
those achievable with simple one-pass crossflow sieve trays. The high-capacity trays
(see Chapter 2), utilizing in a proprietary way the area below the downcomers,
enable capacity increases equivalent roughly to the percent of increase in total
bubbling area.
As mentioned earlier, upper capacity limit is reached at the point of onset of the
so-called entrainment flooding. That is, in low and medium liquid load situations,
increase in vapor velocity leads to excessive entrainment, i.e. a fluidized bed of
droplets that can expand until reaching the tray above. There a portion of entrained
droplets gets aspirated and a major portion is deflected back onto the tray from
where they originated and into the downcomer, eventually causing the tray above
or the original tray to flood. This can be effectively suppressed or delayed by
installing below each tray a droplet removal device, like a conventional demister.
Structured packings have proven to be of added value in such situations, by compensating effectively as mass transfer device for loss of tray efficiency due to reduced
contact time (practically a spray or jet regime operation).
In high liquid load situations usually the liquid handling limitation of downcomers is a common cause of flooding. There are practical and proprietary ways
to delay this considerably by arranging downcomer design accordingly, for instance
by using sloped downcomers. Also a swept-back weir can prove useful, but when the
specific weir liquid load reaches an unbearable value two, or more tray passes are the
only option, with the number of passes or downcomers increasing with increasing
column diameter.
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CHAPTER 1 Types of Distillation Column Internals
The ultimate vapor throughput limit is the so-called system limit, i.e. a vapor load
that exhibits upward force preventing dispersed droplets from falling down. However,
this is not limiting if one chooses to utilize proprietary designs made to deal properly
with such a situation. A typical example is the well-established Shell ConSep tray
[13], where above a sieve tray an axial cyclone device is placed to mechanically
separate entrained droplets. In this way the capacity of a distillation column can be
pushed to an extreme, i.e. well beyond the system limit. Owing to extremely short
contact times and the fact that mass transfer occurs under co-current conditions, a
loss of efficiency is inevitable. This, however, is compensated for in practice (in revamps) by installing additional ConSep trays at tray spacings lower than original
tray spacings. There are also other similar devices available [7], but it must be stressed
here that pressure drop associated with operation of these devices is very high, and can
be afforded only in a limited number of non-sharp high-pressure separations.
In the case of packings, the capacity is directly related to packing size, i.e. specific geometric area. The larger the random packing size or corrugation dimensions
(smaller specific geometric area) the larger the capacity. Structured packings with
specific geometric areas below 100 m2/m3 and their more robust equivalents, so
called grid packings, find frequent use in high-capacity low efficiency applications,
such as those encountered in pump-around sections of refinery vacuum columns. In
multistage separations, the structured packings with areas from 200 to 750 m2/m3
are commonly utilized, depending on the nature of the separation. More area means
more efficiency, but also more pressure drop and consequently less capacity. This is
the best example of the interdependence of the most important operating parameters:
capacity, pressure drop, and efficiency. In new designs more capacity implies
smaller diameter but a taller column, and vice versa. This means that in each situation a trade-off needs to be addressed to arrive at a cost-effective solution.
Regarding capacity, random packings are not exciting, i.e. they do not outperform sieve and valve trays. In general, structured packings are by far the most favorable device in this respect. Indeed, at equal efficiencies, structured packings exhibit
significantly larger capacities than random packings. However, the potential gain,
which is highest under vacuum conditions, tends to reduce gradually with increasing
operating pressure, i.e. the increasing specific liquid load.
Indeed, the specific liquid load is an important parameter, because practitioners
are used to use it as the criterion for when and when not to consider structured packing. In general this is directly related to operating pressure. Since increased pressure
leads to increased vapor density, with liquid density remaining unaffected, the result
is a decreased diameter and consequently an increased specific liquid load. Structured packing failures experienced at high pressures in the past have been related
to insufficient efficiency, mainly because of using structured packings with low
area and inadequate corrugation inclination angle, both considered favorable in
revamp situations to enable desired capacity gains.
Structured packings can provide expected capacity and efficiency only if
ascending vapor does not interfere with descending liquid. This often occurs at transitions between packing layers. The liquid needs to drain from the packing layer and
1.3 Criteria for selection of vaporeliquid contactors
fall down on the top of the layer below. But beyond a certain point, ascending vapor
starts to interfere with the liquid draining from the packing elements and the liquid
starts to build up at the transitions between packing layers. Further increase in vapor
load forces a large fraction of dispersed liquid to go up instead of going down and the
column becomes hydraulically flooded, i.e. inoperable. With increasing specific
liquid load, this all becomes more pronounced, and the onset of loading and subsequently of flooding shifts to lower absolute values of the vapor load factor.
One should note that the bend at the bottom of corrugations of a highperformance structured packing ensures a smooth drainage of liquid, leading to a
delay in the onset of loading, which allows reaching a significantly higher capacity
compared to conventional unbent corrugations.
1.3.2 Pressure drop
The basis for comparison is neutral, i.e. the pressure drop per theoretical plate
(stage). If we take that of trays as average, then random packing may be considered
better and structured packing is, by far, the best choice. In fact this is the distinguishing characteristic and advantage of structured packings compared to random packings and particularly to trays, which makes structured packings to be the first option
for multistage separations under vacuum. However, as mentioned before, structured
packings should be avoided in high-pressures operation, characterized by extremely
high specific liquid loads, low surface tensions, and rather small differences in liquid
and vapor densities. This is not so with random packings, which can perform satisfactorily in high-pressure distillation columns, at a lower pressure drop than a tray,
but may be considered less practical or cost-effective than trays. A typical reason is
the need for frequent redistribution of liquid, i.e. additional height needed to accommodate liquid redistribution sections.
One should note that conventional, chevron, and chimney-tray type liquid collectors as well as liquid distributors have limited free area for vapor flow, which tends to
decrease with increasing specific liquid load. In such situations, the pressure drop
caused by a liquid redistribution section may become substantial, and should be
accounted for. A paper by Rix and Olujic [12] introduced a simple and reliable
method for the estimation of the pressure drop of common liquid collectors and distributors as a function of vapor load and free area. Packing support plates are usually
a very open area, and the related pressure drop is practically negligible.
1.3.3 Efficiency
In order to be able to compare trays and packings in this respect, separation efficiency
is considered here on the basis of the column unit volume. This is straightforward for
packings, but for trays it means considering the tray efficiency in conjunction with
column height corresponding to tray spacing. On average, there is no difference
among basic tray types in this respect, while random and structured packing appear
more favorable, with structured packing being the best in this respect.
21
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CHAPTER 1 Types of Distillation Column Internals
Interesting to mention here is that sieve trays, with hole diameters down to 1 mm
as encountered in air distilling columns, ensure the highest efficiency and capacities
and in conjunction with very low spacing enable building most compact tray distillation columns. However, large specific area structured packings can compete with
this, and owing to their much lower pressure drop per stage, dominate in lowpressure air distilling columns (see the chapter on cryogenic distillation in [4]).
Tray efficiency depends on the flow regime established under given operating
conditions on the tray. Extremes in this respect are, as mentioned before, the spray
regime and bubble regime. The former prevails in vacuum services where high vapor
loads occur in combination with low liquid loads, while the bubble region, often
called emulsion regime, occurs when high liquid load is combined with low vapor
load, which is a situation typically encountered in high-pressure distillations. In
the spray regime, the main part of the liquid is dispersed in droplets and the vapor
phase is continuous. In the bubble or emulsion regime, the liquid phase is continuous
and the vapor is dispersed as small bubbles. The region in between these two extremes is the so-called mixed or froth regime, with liquid as continuous phase and
vapor passing through liquid in the form of bubbles or jets of different sizes. Here
a relatively large interfacial area is combined with moderate vapor and liquid velocities, i.e. residence times, leading to favorable efficiencies. Under the spray regime,
both the interfacial area (surface area of dispersed drops) and residence times of vapor are on the low side, causing poor efficiencies in general. In the emulsion regime,
the vapor is dispersed as small bubbles that are carried by liquid and therefore
remain longer in intimate contact, generating high efficiencies. This depends also
on surface tension, which tends to decrease with increasing pressure, i.e. temperature. Under such conditions, hydrocarbons tend to generate high interfacial areas,
resulting in significantly higher efficiencies than achievable with aqueous systems.
Those of common chemicals are somewhere in between.
In general, high liquid loads cause liquid to flow faster across the tray. Therefore
the small bubbles as encountered in emulsions formed under high-pressure conditions may not have a chance to escape and a fraction of them can be taken with
the liquid into the downcomer and to the tray below, causing some backmixing
that can compromise efficiency gains due to increased interfacial area. Therefore
the best guidance regarding the expected level of efficiency in high-pressure distillations and in general can come from the same or similar applications.
Other physical properties like diffusivities of vapor and liquid affect efficiency
directly but these are associated with the system under consideration and change
with operating pressure accordingly (see Chapter 9).
Unfortunately, the term “tray efficiency” is general and one must know exactly
the definition of the efficiency, when designing or rating a distillation column.
Namely a distinction is generally made between point efficiency, Murphree tray
efficiency, and overall tray efficiency. Point and Murphree efficiency rely on the
same definition, i.e. they represent the ratio of actual to ideal changes in composition
of the vapor phase occurring during contact with the liquid on a tray. Change in
composition is the difference between the outlet and inlet values of the fraction of
1.3 Criteria for selection of vaporeliquid contactors
the given component in the vapor phase. The ideal one is that which is needed to
achieve equilibrium. The point efficiency is related to a specific point, for instance
a hole at a sieve tray, assuming that vapor and liquid upon leaving the contact zone,
corresponding approximately to hole diameter, are in equilibrium; while the
Murphree efficiency is based on the assumption that the liquid leaving the tray is
in equilibrium with the vapor leaving the same tray.
Owing to the nature of the cross flow contact of two phases, a liquid element
experiences on its way from inlet to outlet weir multiple contacts with vapor of
the same composition (vapor is usually well mixed directly upon disengagement
from liquid), so that the driving force for mass transfer tends to decrease along
the liquid flow path. In other words, due to a horizontal concentration gradient developing on cross flow trays with sufficient liquid path length, the tray efficiency
exceeds to a certain extent the point efficiency. With a liquid flow pattern resembling
that of plug flow, the highest efficiency would be achieved (to achieve the highest,
theoretically possible efficiency both phases should be moving countercurrently in
plug flow). If the liquid on a tray is well or perfectly mixed, i.e. both phases having
a homogeneous composition, then the Murphree tray efficiency will be equal to the
point efficiency. This is physically possible, or closely approached, if the liquid flow
path is as short as is encountered in multidowncomer trays. Being the lowest efficiency by definitiondwhich, unlike the Murphree tray efficiency, cannot exceed
1, i.e. become larger than the ideal onedthe point efficiency is usually considered
by designers as a safe design value.
Unfortunately, the liquid flow pattern on a cross flow tray deviates more or less
pronouncedly from plug flow, and this tends to increase with increasing column
diameter. Therefore the overall tray efficiency will be lower than the Murphree
tray efficiency. Predictive models account for the liquid maldistribution effect, as
well as for loss of efficiency due to excessive entrainment, if a column is operated
beyond the design point, say above 0.8 of flooding.
Overall tray efficiency is in fact the efficiency of a column or a column section,
resulting from the ratio of the theoretical and actual plates or trays contained. Operating conditions of rectification and stripping sections are usually different and
therefore efficiencies will also differ. Critical locations are the top and bottom of
each section, and proper averages need to be used for design or rating purposes.
The efficiency of both random and structured packings is directly related to their
specific geometric area. More area, i.e. smaller packing dimensions, means more
efficiency per unit bed height; however, this is accompanied by correspondingly
increasing pressure drop, reduced capacity, and an increased cost. Random packings
are cheaper but less efficient on the basis of the same capacity as structured packings. In the case of corrugated sheet structured packings, going from 45 to 60 inclination angle (with respect to horizontal) means for the same specific geometric area
a much lower pressure drop, resulting in a certain gain in capacity accompanied
however by an equivalent loss in efficiency. Indeed, interdependency of capacity
and efficiency creates typical trade-off situations that a column designer needs to
deal with when designing a new, or revamping an existing, column.
23
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CHAPTER 1 Types of Distillation Column Internals
As mentioned before, the newest generation of corrugated sheet structured packings, employing a smooth bend to vertical orientation on the bottom or both ends of
corrugations exhibits significantly higher useful capacities at equal or even somewhat better efficiencies then their conventional counterparts, at similar costs.
1.3.4 Turndown
Turndown or operating range is the ratio of maximum to minimum vapor loads at
which a tray or packed column operates satisfactorily, i.e. at acceptable efficiency.
If we take for trays 2/1 to 3/1 as standard, then floating, i.e. movable, valves can
be pushed higher, even above 4/1. This could also be expected from a bubble cap
tray, but being practically limitless on the low liquid load side, this device scores
best among trays. Fixed-valve trays and the trays with truncated downcomers, which
use the area below the downcomer as bubbling area, have the range of a welldesigned sieve tray in this respect. Some well-established trays like multidowncomer trays that also allow quite high capacities in rather large specific liquid
load situations, may have difficulties in reaching a 2/1 turndown. This is especially
true because multidowncomer trays are often used at close spacings. As usual there
is always a price to be paid for an exceptional gain made on one of the important
design and operating parameters.
Random and structured packings behave similarly, and their turndowns are well
above 3/1. However, one should note that random packings are limited on the low
side to certain minimum specific liquid load due to their poor liquid distribution
properties at low specific liquid loads. Structured packings have no limitation in
this respect. However, both random and structured packings can usually have
much larger turndown than a conventional liquid distributor. High turndown distributors require at least two levels of liquid pour points and are significantly costlier
than standard 2/1 designs. In any case, it is important that at the very beginning
of design activities it is specified which turndown needs to be ensured by the design
of the distributor. This, however, must be checked and confirmed experimentally
prior to installation, during a distributor functionality and accuracy test performed
using water.
One should note that turndown requirement is dictated by the application and
represents a design specification that directly affects the final choice of equipment.
1.3.5 Sensitivity to fouling
Various types of solid material get somehow into a closed system and move around
dispersed in liquid streams. These solids come with the feed and tend to deposit on
trays, packings, and other internals, and it is just a question of time as to when this
will lead to plugging of orifices and other types of openings, covering the heat transfer surfaces, and so forth. This causes efficiencies and capacities to decrease
gradually and leads eventually to inoperability of the devices affected and consequently to a premature shutdown of the column.
1.3 Criteria for selection of vaporeliquid contactors
In general, fouling is system dependent, and a distinction is made between
systems with no fouling tendency and systems with fouling tendency. The latter
are usually categorized and dealt with as moderate or severe fouling systems.
Sieve and bubble trays seem to be less sensitive in this respect than movable
valves, the latter being similar to random and structured packings. In all cases
sensitivity to fouling increases with decreasing characteristic dimensions, like
hole diameter on sieve trays or corrugation dimensions of structured packings.
The smaller the dimension the more sensitive the device to fouling. While under
ultra-pure conditions as are encountered in air distilling columns it is possible to
take an efficiency and capacity advantage of using small 1 mm holes, in foulingsensitive cases the specified hole size will be more like 12e15 mm.
Fixed-valve trays are generally considered to be less susceptible to fouling than
common floating-valve trays, provided no sticky materials are involved. Here also
the dimensions of openings play a role. In general, an advantage of trays is that trays
can be effectively cleaned while packings and the liquid distributors need to be
removed from the column to perform this.
Packings and liquid distributors are generally more sensitive to fouling by
various types of solid deposits than trays. Therefore, potential fouling, for instance
by rust, which is probable if the column shells and other upstream devices are made
of carbon steel, needs to be prevented. Since the debris is usually carried by liquid
streams entering the column, the distributor is first to be affected; and in the case of
an orifice pan distributor with the orifices at the bottom, this can within a short time
lead to catastrophic consequences for packing performance. This, however, is
usually anticipated, and nowadays the standard choice is a narrow trough distributor
with orifices placed 5 cm above the distributor bottom plate. For systems with
fouling tendencies, this in combination with external strainers installed on feed lines
is sufficient to ensure the smooth operation of packing in between two planned
column shutdowns.
However, if fouling is a real threat, i.e. the system under consideration is prone to
severe fouling, as encountered in well-known “dirty services” in refineries, only the
equipment capable of performing adequately is considered. This is the place where
various grid packings find application, and if trays are preferred, then different
designs of baffle trays or dual-flow trays may be proper choice. These devices can
have large capacities but are notorious for their low efficiencies. This, however, is
not a concern because most dirty services are easy separations. Spray distributors,
which are used frequently in conjunction with grids in some dirty services, are
extremely sensitive to plugging. Here, well-working and easy-to-clean external
strainers are an absolute necessity.
1.3.6 Sensitivity to corrosion
Here both random and structured packings score worst, because the thickness of
metal is well below that taken usually as the corrosion allowance during tray design.
This is particularly true with sheet metal corrugated structured packings
25
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CHAPTER 1 Types of Distillation Column Internals
manufactured from sheets with a thickness of 0.1 mm. So, the material chosen for
packings must be mechanically adequate. In such cases the cost of corrosionresistant material is a decisive factor.
1.3.7 Choice of construction material
While sieve and bubble cup trays can be made of corrosion-resistant materials,
which usually exhibit less mechanical strength, movable valve trays are less suitable
because the valve legs tend to break during operation. Packings are best in this
respect because they can be manufactured from all kinds of corrosion-resistant materials (plastics, stoneware, glass, etc.). In general, the manufacturing of random
packings is more flexible and less expensive than that of structured packings.
For packings in standard applications, stainless steel is a common choice. If this
is not sufficient, then more resistant and correspondingly more expensive alloys
need to be considered. However, the price of materials like tantalum is immense
compared to stainless steel, and only further decrease in thickness can make it
affordable. A common approach in cases of highly aggressive chemicals is to
consider ceramics or glass. Random packings may turn to be more cost-effective
then structured packings in this respect.
1.3.8 Access for inspection
An obvious advantage of standard cross flow trays is that they allow access to insides
of columns for inspection and cleaning, and repairs if needed. This, however, is
rarely possible with multi-downcomer trays, which have a mechanical structure
that practically does not allow access for inspections. In packed towers, provisions
are made to allow access to liquid distributors for inspections and maintenance, but a
packed bed cannot be inspected physically. In recent years, gamma-ray scanning has
become a well-developed and affordable technology used frequently for the inspection of trayed and packed columns failing to perform correctly. In cases of damage,
packings need to be removed, and this is more difficult and costlier with structured
than with random packings.
1.3.9 Scalability
Original bubble cup trays have been less suitable for installation in large-diameter
columns than sieve and valve trays; however, the more practical and effective tunnel
tray and Thormann tray designs can be considered equal to sieve and valve trays in
this respect. The largest column equipped with Thormann trays is 9 m in diameter.
Due to physical inaccessibility, trays are not suitable for installation in columns
with diameters below 0.8 m. However, if trays are required, then so-called cartridge
trays are considered; they are assembled in sections of about five trays, held together
by rods, and are pushed into a flanged column. Figure 1.10 shows a photograph of a
tunnel cartridge tray. Proprietary sealing devices, e.g. a metal coil spring placed into
1.3 Criteria for selection of vaporeliquid contactors
FIGURE 1.10 Photograph of a Section of a Cartridge Tunnel Tray
(For color version of this figure, the reader is referred to the online version of this book.)
Courtesy of J. Montz GmbH.
a circumferential groove at the periphery of each tray, can prevent any leakage of
liquid and/or vapor.
Random and structured packings are easier to scale up and down, and in cases of
large diameters the main concern is getting the liquid distributor installed and operated properly. To ensure sufficient gradient one or two gravity predistribution stages
are necessary, which need to be designed and installed with care and within certain
leveling tolerances to achieve required uniformity of liquid predistribution. From the
state-of-the-art narrow trough distributors all pour points are expected to be within
less than 5% of deviation from the mean value.
When considering laboratory and pilot scale packed columns with diameters
below 0.3 m, one should note that pressure drop may be significantly lower (random
packings) or larger (structured packing) than that encountered in industrial-scale
columns. In the former case, the reason is a larger porosity in the wall zone; while
in latter case, the reason is that wall wipers represent an impenetrable obstacle for
vapor and there are many short corrugation channels ending at the wall, forcing
vapor streams to change flow directions more frequently.
1.3.10 Know-how/experience
Some 20 to 30 years ago, the number of applications and the operational experience,
as well as the related design know-how for distillation columns equipped with structured packing, was limited. Nowadays, after many thousands of laboratory and
industrial-scale applications, both random and structured packings are similar in
27
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CHAPTER 1 Types of Distillation Column Internals
this respect to conventional sieve, valve, and bubble cup trays. Certainly, introduction of novel devices on an industrial scale is always accompanied by certain amount
of risk.
1.3.11 Cost
Conventional bubble cup trays are more demanding and expensive to manufacture
than sieve and valve trays, and therefore are considered to be in the same cost category as random packings.
This cost category also now includes sheet metal structured packings, which are
significantly cheaper than 20 years ago, when they were considered to be the worst
case regarding costs. Today, structured packings are still considered to be more expensive per unit volume than random packings. This, however, may not always be so. The
basic price of packings depends on the quantity of material used, and doubling the
specific geometric area means doubling the basic price. Due to a much higher thickness of basic material (0.3e0.4 mm), wire gauze packings are correspondingly more
expensive than common ones made of sheet metal (about 0.1 mm). Much thicker
material is required to manufacture random packing, and here, thickness depends
on the size of the packing elements. The manufacturing process in the case of structured packings is less dependent on specific geometric area than in the case of random
packings, where the number of pieces increases significantly with decreasing packing
element dimensions. Therefore, the doubling of specific geometric area may lead, in
the case of random packings, to more than doubling the price.
1.3.12 Operating pressure
As mentioned before under capacity and pressure drop considerations, structured
packings are often the first choice at low to moderate vacuum; random packings
could be considered and trays are generally not considered suitable for any
demanding vacuum application. At atmospheric and above-atmospheric pressures
(below 5 bar or 5$105 Pa) structured packings could be used, but both random packings and trays are expected to be more appropriate choices. In high-pressure applications, trays are the preferred choice; random packings will perform acceptably,
and structured packings should be avoided.
1.3.13 Liquid load
Since the specific liquid load in a distillation column increases proportionally with
pressure increase, trays are the first choice for high pressure, i.e. high specific liquid
load applications. With decreasing liquid load, trays tend to lose ground to random
packings that appear capable of operating well under high and moderate liquid loads.
The latter is a range where structured packings become interesting option and structured packing is definitely the most favorable device for very low specific liquid
loads. Here trays should generally be avoided, but as mentioned before, there are
1.3 Criteria for selection of vaporeliquid contactors
some tray designs, like bubble cap trays and in particular the Thormann trays that
perform acceptably under vacuum conditions, and therefore find application in cases
where other factors favor trayed columns.
1.3.14 Liquid holdup
Liquid holdup is implicitly considered in the pressure drop and capacity of
vaporeliquid contacting devices; however it needs to be considered explicitly when
thermally sensitive or explosive mixtures are distilled. Then, the amount of liquid contained in the operating column, as well as its residence time, becomes an important
consideration. In this respect, trays and structured packings are extremes and the
low holdups and residence times of structured packings can be further decreased
by increasing the corrugation inclination angle to a tolerable limit with respect to
the inevitable loss on the efficiency side.
On the other hand, in reactive distillation applications (see Chapter 8), large
liquid holdups and residence times are required, making trays the preferred choice.
To create similar conditions in packed columns, reactive sections need to be nearly
flooded, and this is something that could be arranged in a controlled way by using
so-called sandwich packings [7].
1.3.15 Foaming
The hydrodynamics of an operating tray create perfect conditions for the generation
of foams, which occur mainly due to the presence of surface-active materials. Also
aqueous systems are prone to foaming. Foaming is detrimental to capacity, and
therefore should be anticipated and accounted for during column design.
Unlike a froth, which is a biphasic condition most widely encountered on operating trays, a foam is stable and can persist much longer. In general, the time needed
for a foam to collapse is longer than the residence time of a liquid at certain locations, like on a tray deck or in a downcomer. Capacity limitations are related to
the fact that foaming systems tend to entrain earlier and heavier than no foaming systems. If anticipated, a typical antifoaming measure is to increase column diameter
accordingly, and for various tray designs there are some mechanical provisions
possible for reducing foaming tendency. If foam appearance or its causes cannot
be eliminated, the use of antifoam agents is considered, which usually brings relief
but is often costly and the antifoam agents should be removed in a subsequent separation step to avoid contamination of downstream products.
Packings appeared to be much less sensitive to foaming than trays and therefore
are sometimes considered to be the preferred choice for foaming systems.
1.3.16 Heat transfer
In refineries there are a number of operations where the heat transfer is the main
target for designers of distillation columns. These are usually primary columns
(crude oil columns, heavy crude vacuum columns, fluid catalytic cracker main
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CHAPTER 1 Types of Distillation Column Internals
fractionators) with fractions covering large spans of operating temperatures, which
utilize pump-around sections. Trays are preferred and sieve and valve trays are common choices; however, if liquid loads are excessive, special designs can be considered. If the pressure drop is critical, like in vacuum and near-atmospheric pressure
operations, then columns are equipped with fouling-resistant grids or (random or
structured) packings. In some cases only certain column sections appear to be critical, and so it is not surprising to see that striving for maximum effectiveness results
in columns combining trays and random and structured packings.
Although the design principles are the same, heat transfer may be associated with
specific details that need to be accounted for properly. For instance, short packed beds
have proved to be effective in condensing services, but due to progressive condensation the liquid load tends to increase towards the bottom of the bed at the cost of the
vapor load, leading to a situation with two extremes in loads, high vapor load in combination with low liquid load at the top and vice versa at the bottom. The average
design load needs to be chosen carefully to avoid exceeding hydraulic maximums
at the top or bottom that could eventually lead to premature flooding.
Regarding the fact that various grids are considered for heat transfer services,
particularly those associated with fouling resistance, one should note that their
heat transfer efficiency is lower than that of equivalent structured packings and their
required section height will be larger. If this brings some capacity gain, then consideration of a grid may be justified and the choice will probably depend on economics.
1.3.17 Installation
Regarding installation, both tray and structured packing columns are quite
demanding, while randomly packed columns can be considered to be rather easy
in this respect. Metal and plastic random packings are installed usually under dry
conditions, while ceramic packings must be wet packed to avoid breakage.
For tray and structured packing columns, all mechanical pieces need to be manufactured in sizes and weights that can be handled by a single person and that can
enter the column through the column manhole. Trays are clamped to the rings
welded to inside columns walls according to specified tray spacings, and if the
tray spacings are small, then installation becomes more difficult due to limited
area for workers to move. In any case, tray decks need to have openings (manways)
to allow inspectors and workers to climb through. Installation proceeds from bottom
up in each section. The same is with structured packings consisting of layers that
need to be assembled, block by block, taking care that layers are rotated with respect
to each by usually 90 (see Figure 1.6). Importantly, blocks making a layer need to
be tightly packed to each other and to the walls. Here, the wall wipers are essential,
to fill the space between the curved sides of the packing and the column walls, and
during the operation, to scrape the liquid from the wall and bring it back into the bed
again as well as to prevent vapor escaping liquid contact along the wall. Since the
installation crew walks above a packing layer, provisions have to be arranged to
avoid damaging and/or unleveling of the packing due to walking over it.
1.3 Criteria for selection of vaporeliquid contactors
Unlike common industrial distillation columns, the columns used for cryogenic
air separation purposes are fully assembled in shops, i.e. in horizontal position. The
reason for this and all the interesting details related to design and operation of trays
and packed columns used for obtaining technical and high-purity oxygen, nitrogen,
and argon as well as some other industrial gases, can be found elsewhere [4].
1.3.18 Revamping
The present chapter is written from the standpoint of the design of new columns. However, user companies are frequently confronted with the need to enhance performances
of existing columns to reach new goals without excessive investments. Looking back,
the 1980s and early 1990s were golden years of revamping, when trays from columns
used in vacuum services were replaced mainly by structured packings. Sometimes (e.g.
when diameters were based on vapor velocities well below 80% of flooding to control
the pressure drops, in combination with large tray spacings) it appeared possible to
achieve rather large capacity increases accompanied by separation improvements,
and in some multistage applications this was accompanied by further significant benefits. A typical example is the reduction in bottom product (bottoms) temperature to a
level eliminating the need to supplement and remove afterwards antipolymerization
additives. Separation improvements were a consequence of the fact that achieved pressure drop reductions were such that this resulted in significant increases in relative volatilities. Namely, with the larger tray spacings, the effective height available for the
installation of packings including redistribution sections was so large that it was
possible to accommodate more equilibrium stages than in the original design, leading
to significant reductions in reflux, i.e. specific energy requirements.
New distillation columns are now designed more tightly, which means that there
is no spare capacity in columns as was often the case in the past, particularly not to
such an extent as encountered in the previously mentioned trayed columns used in
vacuum services. Regarding the fact that columns are usually designed for 80% of
flooding, this allows an increase in the capacity of the column to an operable point
close to flooding limit, if the loss of efficiency due to increased entrainment as well
as increased pressure drop can be tolerated. To maximize capacity, operators are also
used to increase operating column pressures to design limits.
However, this is of limited extent and a substantial increase in capacity is
possible only by replacing existing internals with more effective ones. These can
be structured packings or random packings as well as different types of highcapacity trays, depending on the system and operating conditions. Under vacuum
conditions, structured packings are the best choice, and at above-atmospheric pressures they gradually lose ground to random packings and various high-capacity
trays. The latter dominate high-pressure distillation revamps.
Indeed the most common revamp objective in the processing industries is
increasing column capacities, while preserving efficiencies. Sometimes, the need
for maximizing the capacity is such that a certain loss of efficiency is tolerated.
Less frequently, the objective of a revamp is improving separation while preserving
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CHAPTER 1 Types of Distillation Column Internals
capacity. Presently, the choice of adequate equipment is such that all kinds of revamp
objectives can be achieved, including operation well beyond system limit [7].
1.3.19 Large column diameter
With increasing plant capacities, the size of distillation columns increases. However,
this is not a burden, and various trays, random packings, and structured packings can
be found in columns with diameters well above 10 m. The main challenge on the
process side is the increased potential for the maldstribution of both the liquid
and the vapor. This means that trays and layers of structured packings need to be leveled within given tolerances for deviation from horizontal. On the mechanical side,
larger support beams are needed and additional care should be exercised to avoid
interference with the flow pattern of the ascending vapor.
On the tray side, self-supporting multi-downcomer trays are most suitable for
large-diameter columns, and if cross flow trays or two-pass trays with long liquid
flow paths are considered, provisions must be made to increase insufficient hydraulic
gradient using vapor-push devices to move liquid steadily across the tray as close as
possible to a plug flow pattern.
In the case of packings, initial liquid distributions must be of adequate quality,
taking care that necessary predistribution stages do not introduce disturbances that
could lead to the appearance of segmental maldistributions, i.e. a situation where
one half of the bed receives more liquid that the other half.
One should note that in the case of large column diameters, assuring a good
initial distribution of vapor is an imperative, particularly in vacuum services where
necessary pressure drop cannot be afforded. Regarding the fact that the volume of
vapor in common vacuum applications is huge, large horn-like tangential inlets or
two or more streamlined vapor inlets are combined to direct vapor to spread uniformly across the column cross-section. It is interesting to note that even the largest
liquid distributors are assembled and water tested prior installation, which however
is impossible with vapor distributors. Fortunately, designers can rely on Computational Fluid Dynamics (CFD) simulation tools [14], and the resulting (proprietary)
designs generate pronounced but symmetrically arranged maldistributions, which
are smoothed out within the first two to three layers of the packing at the bottom
of a bed. As indicated in air/water tests performed using a 1.4 m diameter test column, only a highly asymmetrical, chordal type of initial gas maldistribution penetrates deep into a bed consisting of structured packings, and therefore could have
a detrimental effect on efficiency [15].
1.4 Closing remarks and outlook
There is no ideal vaporeliquid contactor. The main advantage of structured packings
is their relatively low pressure drop per theoretical plate or equilibrium stage, which
makes them the most effective vaporeliquid contactor for low-pressure, mainly
References
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increasing operating pressure, because this is accompanied by an increasing specific
liquid load, i.e. decreasing free area for vapor. At operating pressures above 5 bar
(5$105 Pa) there will be no capacity benefits anymore. Using a low specific geometric area packing and/or a steeper inclination angle to compensate for this is not an
option because large jets of surging liquid tend to chock the flow channels forcing
the vapor to flow through a limited number of free channels. In this way, both phases
get maldistributed and generate significant amounts of back-mixings, causing efficiencies to drop below acceptable values.
With increasing operating pressure, the capability to handle large specific liquid
loads without affecting capacity and efficiency adversely becomes imperative and
this provides a distinctive advantage for conventional and advanced trays. Also,
random packings can handle high specific liquid loads with ease, but are generally
considered more suitable than trays only for medium- and high-pressure applications, where their lower pressure drops could be a prevailing factor.
Atmospheric to 5 bar (5$105 Pa) pressure is an overlapping region where a thorough evaluation of the relative advantages and disadvantages of trays and packings
is needed to arrive at proper choices, which however are usually the most costeffective ones.
Regarding the fact that the sustainability, i.e. energy efficiency, of distillation
operations tends to increase with decreasing operating pressure and pressure drop,
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Although impressive improvements have been achieved in the recent past,
further enhancements in the overall performances of trays, random packings, and
structured packings are needed and are to be expected. It is probable that an additional push in this direction will come from a growing number of Chinese and Indian
distillation equipment manufacturers.
Acknowledgements
Julius Montz GmbH and Sulzer Chemtech Ltd are acknowledged for providing photographs
and other illustrations used in this chapter.
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