This preview shows page 1. Sign up to view the full content.
Unformatted text preview: Distillation Ensure Proper Design
and Operation of
To function properly at optimum capacity
and efficiency, multi-pass trays must be
designed so that the liquid and vapor
flows are properly balanced. This article
provides guidance on how to achieve that. Mark Pilling
Sulzer Chemtech USA, Inc. S tandard crossflow trays in distillation columns operate
with gravity-driven liquid flowing downward in a serpentine pattern while contacting the vapors that are
flowing upward. As the liquid flowrates increase, the frictional resistance and corresponding liquid head increase on the
tray deck and in the downcomers. Once the liquid resistance
becomes substantial, the tray configuration is split into multiple streams or passes.
Two-pass trays are quite common and generally don’t
pose significant design problems, since the flow paths and
tray designs are symmetric. Tray designs with more than two
liquid passes are considerably more challenging because of
their lack of symmetry, and need to be designed with care.
Three-pass trays are rare because of their inherent asymmetric design and difficulty to balance. Four-pass trays are more
common. Trays with more than four passes are rare as well.
Unless noted otherwise, the multi-pass trays discussed in this
article are four-pass trays. Hydraulic considerations
One of the most critical operational aspects is the balance
of the vapor and liquid streams on the tray deck. It is imperative that the vapor and liquid streams contact each other and
flow evenly across the tray.
When the vapor flow is disproportional to the liquid flow
in different areas of the tray, a compositional pinch can occur
and limit the efficiency. From a hydraulic standpoint, when
the flows are imbalanced, the side that is more heavily loaded
will prematurely limit the tray’s capacity.
The effect of an imbalance varies depending upon the tray
22 www.cepmagazine.org June 2005 CEP design and the application. For example, in applications that
are heavily vapor-loaded, excessive vapor flow to one section
will cause a flood in that section; in highly liquid-loaded
applications, excessive liquid flow to a section may cause a
localized flood there as well.
A common term used to evaluate the vapor and liquid
flows is V/L, the vapor/liquid ratio. When measured on a
molar basis, this value is generally unity for a column operating in a total reflux mode. Correspondingly, the V/L ratio
should also be unity for a perfectly balanced tray operating at
total reflux. Thus, V/L is the defining measure of balance
Since most columns do not operate at total reflux, the V/L
ratio may vary widely from unity. The absolute value of the
number is really not important, but the consistency of this number between the various passes of a multi-pass design is critical. Tray geometry and nomenclature
In its simplest form, a standard crossflow tray consists
of a tray deck and a downcomer. The tray deck is perforated with either sieve holes or valves that allow the vapor to
travel vertically upward through the deck to contact the liquid layer that is flowing horizontally across it. After contact, the vapor disengages from the liquid and travels to the
next tray immediately above. The mixing of the vapor and
liquid generally forms a froth or spray on or above the
deck. As the frothy liquid mixture leaves the deck, it travels into the top portion of the downcomer. The downcomer
is designed to disengage any additional vapor that is in this
mixture. The disengaged vapor leaves out the top of the Center
Panel D Panel C W3 W4 Panel A
C1 C2 Panel B OCDC
Side ■ Figure 1. Four-pass tray features. downcomer, while the clarified liquid travels downward
through the downcomer and is fed to the tray below.
Other important mechanical features on the tray are the
downcomer clearance (C) and the tray outlet weir (W). The
downcomer clearance is the vertical gap formed between the
lower edge of the downcomer wall and the tray deck. It
serves as a flow orifice that meters the liquid onto the tray
and also maintains a liquid seal on the downcomer.
The outlet weir is a vertical metal piece that is placed at
the interface between the end of the tray deck and the inlet of
the downcomer. Typically, it extends 25–75 mm (1–3 in.)
above the tray deck. It serves to maintain the desired liquid
level on the tray deck, and in many designs is used to ensure
a hydraulic seal above the downcomer clearance.
These features will be discussed later in more detail, since
they are important in balancing the vapor and liquid flows.
Multi-pass trays have the following geometric features:
Downcomers. All four-pass trays have three types of
downcomers (Figure 1). Side downcomers are formed by the
column wall and a downcomer panel. They are the most
common type of downcomer, and are found in all single- and
multi-pass tray designs.
The center downcomer is located at the tower centerline,
and is formed between two downcomer panels offset from
the centerline. The center downcomer is symmetrical, and is
also used in two-pass designs. Two side downcomers and one
center downcomer are located on alternating trays, either all
the odd-numbered or all the even-numbered trays.
The third type of downcomer is an intermediate, or offcenter, downcomer (OCDC). The OCDC is located between
the side and center downcomers on the alternating trays
between those containing the side and center downcomers.
They are used for multi-pass tray designs that have more than two passes. The OCDC itself is asymmetric. The side closer
to the centerline of the tower (referred to as the inboard side)
has a longer chordal length than the side farther away from
the centerline (the outboard side).
Active area panels. Four-pass trays consist of two
mechanically different trays, each of which works in conjunction with the trays immediately above and below it. There are
four different types of active area panels on four-pass trays,
designated as A, B, C and D (Figure 1):
• Panel A: between the OCDC outboard clearance and
the side downcomer inlet
• Panel B: between the OCDC inboard clearance and the
center downcomer inlet
• Panel C: between the side downcomer clearance and
the OCDC outboard inlet
• Panel D: between the center downcomer clearance and
the OCDC inboard inlet.
The tray containing side and center downcomers will have
one A panel and one B panel on each half of the tray. The
tray containing two OCDCs will have one C panel and one D
panel on each half. These panels are described further below. Liquid flows
For the sake of simplicity, the fluid flow through the
downcomers and across the decks will be referred to as a liquid. This fluid is more likely to be a two-phase froth (especially in high-pressure applications), but the term liquid will
suit the purposes of the discussion. Figure 2 shows the directions of the liquid flows with respect to the downcomers
through a four-pass tray, which are as follows:
Liquid leaving the OCDC is split into two portions. One side
feeds panel A en route to the side downcomer (blue). The other
portion feeds panel B en route to the center downcomer (green).
If the tray is designed correctly, the liquid stream flowrates will
be proportional to the active area to which they are being fed.
Liquid leaving the center downcomer should always be
split into equal parts. These feed the D panels immediately on
either side of the center downcomer (violet).
The side downcomers are rather isolated and can only
transfer liquid to and from the OCDC outboard side. Liquid
to the side downcomer travels from the OCDC outboard
clearance across panel A and into the side downcomer (blue).
Liquid leaving the side downcomer travels across panel C
and then flows into the OCDC outboard side (red).
Balancing the liquid flows
Liquid flows are balanced by modifying the liquid restrictions in the downcomer and on the tray decks, namely, the
downcomer clearance and the tray outlet weir.
Downcomer clearances. All downcomer clearances need
to be sized to obtain the appropriate pressure drop. This is
CEP www.cepmagazine.org June 2005 23 Distillation done to ensure the proper capacity, sealing of the downcomer,
and characteristics of the flow from the downcomer. However,
since the side and center downcomers have only one possible
destination, the OCDC, no balancing mechanism needs to be
applied to them. The balancing of liquid with the downcomer
clearances is necessary only with the OCDC.
The downcomer clearance is calculated as a simple open
area arranged in the vertical plane. The orifice coefficient for
this opening is generally assumed to be constant. Therefore,
to adjust the downcomer clearance, either the clearance
height or the length (perpendicular to the flow) of the clearance needs to be decreased. Since the clearances are formed
by the downcomer chordal length, they cannot be made
wider, only narrower.
It is important to note that the effect of increasing the
height of a downcomer clearance varies depending upon the
liquid head in the downcomer. Because of this, it is never recommended to balance the liquid distribution from the two
sides of the OCDC by using different clearance heights.
Changing the length of the clearance alone is the recommended method of adjustment. Otherwise, the partitioning of
the liquid will vary with process rates.
The effective downcomer clearance area is decreased by
blocking off portions of the downcomer. This is typically
done with bracing, which both blocks the desired amount of
the clearance width and supports the downcomer wall. Care
must be taken to make the blocked sections small enough and
well enough dispersed to avoid flow maldistribution.
Outlet weirs. Weir loading is measured as a volumetric
flow across a unit length of weir. The crest of the liquid over
a weir is essentially a function of the weir loading to the 2/3
power. As weir loading increases, the crest increases, the liq- uid height on the upstream tray panel increases, and finally
the hydraulic resistance for the vapor increases.
In order to properly balance the weir loadings on a multipass tray, the weir loading for the side and center downcomers needs to be constant, and the weir loadings on either side
of the OCDC need to be constant. As with the clearances,
changing the vertical height of different weirs to balance
passes is not an acceptable method, since the crest over the
weir is a function of both the weir height and the weir load. If
this were to be done, the crest over the weir would vary
under different process conditions and would not allow proper balancing of the tray under all conditions. Therefore, the
only acceptable way to balance passes with the outlet weir is
to make the effective weir length either longer (swept-back
weirs) or shorter (picketing).
Swept-back weirs. Side downcomers tend to have a relatively short weir length because the downcomer chord is far
from the tower centerline. This is especially important in
equal-bubbling-area designs, since the side panel A will have
the same liquid flow as the center panel B, but the side downcomer weir is much shorter than the center downcomer weir.
Figure 3 compares a swept-back weir design (on the right)
with a standard weir design. The swept-back weir creates a
longer effective side downcomer and makes it easier to balance the weir loading on the two panels.
An effect of using a swept-back weir is that the weir occupies some additional bubbling area on the A panel. This
reduction of area is generally minimal, but it must be
accounted for in the tray balancing process.
Picketed weirs. Picketed weirs (Figure 4) are used to block
off a portion of the weir evenly over the entire weir length.
They effectively shorten the flowing length of the weir.
Swept-Back Weir Liquid
Area Area Behind Weir Standard Weir
Layout ■ Figure 2. Four-pass tray liquid flow paths. 24 www.cepmagazine.org June 2005 CEP Swept-Back Weir
Layout ■ Figure 3. Standard and swept-back weir layouts. 3 in. (75 mm) 3 in. (75 mm) Weir
Weir Length Tray Deck ■ Figure 4. Picket fence weir with an effective weir length of 50%. When designing picketed weirs, it is important to place
the pickets uniformly over the length so as to not block off
large sections of the weir and create localized maldistribution. The height of the pickets is also important — they
should be tall enough that the froth height on the tray deck
will not exceed the height of the pickets at normal operating conditions. Pickets are typically designed for 50% of
the tray spacing. Vapor flows
Vapor flows upward through the trays following the path
of least resistance. Assuming that no vapor tunnels are used,
some vapor flows are trapped on one side of a downcomer
and have no choice but to flow to the panel directly above.
Other vapor flows are split and then can flow to either of two
panels. The distribution of this split is determined by the
hydraulic resistance from each of the panels.
For instance, vapor leaving panel A is trapped on the
outboard side of the intermediate downcomer, with no
option but to flow upward through panel C. However,
once it proceeds through panel C, it can rise above the
OCDC and flow to either panel A or panel B, depending
on the pressure balance.
Vapor leaving panel B is trapped between the inboard
sides of the two OCDCs. This vapor can travel to either of
the D panels above, but since the tray is symmetrical, the
flow between these two panels should be essentially even,
unless there was some initial maldistribution. This specific
transition allows the equalization of vapor between the two
halves of the tower.
Vapors leaving panels C and D are isolated to one half of
the column by the center downcomer. These vapors are free
to travel to either panel A or panel B, but cannot travel to the
other half of the tower. The distribution of the vapor above
the OCDC is critical to balancing irregularities in the tray
design or operation. Layout and balancing of four-pass trays
There are generally two schools of thought when it comes
to balancing four-pass trays: equal bubbling area and equal
Equal bubbling area means that the active area panels
A–D will be essentially the same size with the same amount
of orifice perforation (open) area. This design method
assumes that the vapor will be split into quarters and will
flow equally to each of the panels. In order to balance the
tray, the liquid will then need to be split into quarters as well
and proportioned equally to each of the panels.
In an equal-flow-path-length design, the downcomers are
arranged to create an equal flow-path length along each of the
panels A–D. This design, by definition, cannot have equal
bubbling area on the panels since the tower cross-section is
round. Therefore, the side panels (A and C) will have considerably less bubbling area than the more central panels (B and
D). The proper design technique here assumes that these panels will have a perforated area that is proportional to their
active area and that the vapor will flow to these panels proportional to their active area. The liquid will need to be proportioned to the tray panels relative to their active area as well.
Both of these design methods are quite popular and have
their own advantages and disadvantages.
With any tray design, the ultimate goal is to have the vapor
and liquid contact evenly across the entire active area of the
tray. The critical division of fluids on a four-pass tray is at the
OCDC. This is because the liquid can move in either direction
from the OCDC depending on the pressure balance, and the
vapor will adjust above the OCDC as well. In order to balance
the fluid flows, adjustments must be made to make sure the
split is proportional to the bubbling areas that it is feeding.
Step 1: Adjusting downcomer sizes and locations for
capacity. Multi-pass trays should be designed so that the
downcomer sizes match the process requirements. The
desired result is that the downcomers have similar, or even
identical, velocities. The location of the outside wall of the
side downcomer is fixed, as is the centerline of the center
downcomer. The other dimensions and locations are available
for adjustment during tray layout.
The side and center downcomer widths must be adjusted
such that they will have the same velocities. With equal-bubbling-area designs, this is rather straightforward, since it is
assumed that the liquid will be split into quarters. With equalflow-path-length designs, the proportioning of liquid between
the side and center downcomers is an iterative procedure,
since changes to any of the downcomer dimensions will affect
the corresponding active areas and thus change the proportioning of the liquid flowing to the side and center downcomers.
The size of the OCDC will be set to match the velocities in
the side and center downcomers. However, the centerline of
CEP www.cepmagazine.org June 2005 25 Distillation the OCDC will need to be adjusted to make the panels either
equal bubbling area or equal flow-path length. The cross-sectional area of the OCDC varies as a function of the downcomer
width and the location of the centerline. Therefore, to set the
downcomer velocity in the OCDC, the designer must know its
required location and then calculate its necessary width.
This nested iterative process can be quite challenging.
When the downcomers are sloped to improve the performance
of the tray, the design process becomes even more challenging. To be successful at these layouts, the designer needs to be
very patient or have a layout optimization program.
Step 2a: Balancing flows for an equal-bubbling-area
design. A balanced design for an equal-bubbling-area tray
requires the following features:
Deck orifices. Deck orifices must be allocated equally to
all passes, with 25% of the orifices in each pass. Actual bubbling areas need to be approximately the same, but the number of orifices or slot areas should be exact. This can be difficult with larger valves or orifices on the tray deck, since the
outer panels (A and C) have more curvature and can make it
difficult to fit enough valves on these panels.
Outlet weirs. The weir on the top of the center downcomer, W2, must be picketed in order to have the same effective
length as the weir on the top of the side downcomer, W1.
This assures that the crest over the weir is equal on both passes regardless of the total liquid rate. Picketing is the only
acceptable way to achieve this. As discussed earlier, varying
the weir height is unacceptable because this type of design
produces even distribution for only one specified liquid rate.
The weir on the inboard side of the OCDC, W4, should also
be picketed to have the same effective length as the outboard
side weir of the OCDC, W3. This maintains the same crest over
the weir for both streams entering the intermediate downcomer
and thus helps to keep the liquid level equal on both the C and
D passes. Picketing of the OCDC weirs is mandatory if the
vapor is equalized across the OCDC (e.g., by vapor tunnels or
truncated downcomers) but is optional with convenitonal downcomers since the vapor has no alternative path.
Downcomer clearances. The clearance on the inboard
side of the OCDC, C2, should be blocked (or picketed) so
its effective release length is equal to the clearance on the
outboard side of the intermediate downcomer, C1. This
ensures an equal split of the liquid to the passes on either
side of the downcomer.
Step 2b: Balancing flows for an equal-flow-path-length
design. Balancing modifications for an equal-flow-pathlength design are basically similar to those used for equalbubbling-area designs, although there are some differences.
Deck orifices. The deck orifices must be allocated to the
individual passes so that their number is proportional to the
bubbling area of that pass.
26 www.cepmagazine.org June 2005 CEP Weirs. The center downcomer weir, W2, should be picketed
so that its length divided by the length of the side downcomer
weir, W1, is proportional to the panel B bubbling area divided
by the panel A bubbling area. One way to confirm that the
design is balanced is to check that the weir loading is the same
for the side and center weirs. Since picketing of the center
downcomer tends to minimize the tray capacity, the amount of Vapor Tunnels
Vapor tunnels are tubes or channels that pass through either
center or off-center downcomers and allow vapor to travel from
bubbling areas on either side of the downcomers in order to
equalize pressures and flows on the tray (Figure 5). Tunnels
through the center downcomer allow equalization between the
opposite halves of the trays. Tunnels through the OCDC allow
equalization between the outboard passes (A and C) and the
inboard passes (B and D).
If a column is fed and drawn properly, tunnels through the
center downcomer are not needed, since the tray is symmetric.
Small irregularities can be equalized above the center downcomer between the B passes on either side.
Nor should vapor tunnels on the OCDC be needed if the trays
are balanced properly and are fed and drawn properly. With
equal-flow-path-length designs with sloped downcomers, the
ratio of vapor going between passes A and C will not be the
same as the ratio of vapor going between passes B and D.
Therefore, if vapor tunnels were to be used, there would be flow
through them. However, in order to balance the V/L ratio on
these trays as closely as possible, we prefer to keep the C and D
vapors isolated so that all of the vapor from pass C goes to pass
A and all the vapor from pass D goes to pass B. We prefer to not
use vapor tunnels on the OCDC.
Assuming the feed and draw streams are properly designed,
our preference is to not use vapor tunnels at all. A properly balanced tray should overcome small imbalances without vapor tunnels. Also, vapor tunnels may actually hurt performance when
used in an OCDC with an equal-flow-path-length design. Finally,
vapor tunnels add complexity and cost to the tray design. ■ Figure 5. Off-center downcomer (OCDC) vapor tunnel. picketing should be kept as low as possible. This can be done
by making the side downcomer weir effectively longer by
widening the side downcomer, by using a swept-back weir on
the side downcomer, or by a combination of the two.
For the equal-flow-path-length design, the liquid flowing
from the side downcomer to the OCDC is significantly less
than the liquid flowing from the center downcomer to the intermediate downcomer. Therefore, the weir on the outboard side
of the OCDC, W3, should also be picketed to have a length
equal to the length of the inboard weir, W4, multiplied by the
ratio of the panel C area to the panel D area (i.e., W3 = W4 ×
[Area C/Area D]). This maintains a weir length proportional to
the liquid flow to each pass and ensures the same crest over the
weir for both streams entering the intermediate downcomer,
and thus helps to keep the liquid level equal on both passes.
Varying weir heights between balancing passes is not
Downcomer clearances. The clearance on the outboard
side of the OCDC, C1, should be blocked so its effective
release length is equal to the clearance on the inboard side of
the intermediate downcomer, C2, multiplied by the area ratio (C1 = C2 × [Area A/Area B]). This ensures a proportional
split of the liquid to the passes on either side of the downcomer. (Note that the downcomer clearance is not varied for
the inboard and outboard sides of the OCDC.) Final thoughts
If a tray is properly sized, then a balanced design should
perform as expected. However, if a tray is not properly balanced, the effects can cause a tray to fail. Proper flow balancing cannot make a poorly sized tray work, but improper flow
balancing can certainly make a moderate or aggressively
sized tray fail miserably.
MARK PILLING is the manager of technology for Sulzer Chemtech USA, Inc.
(Tulsa, OK; Phone: (918) 447-7652; E-mail: firstname.lastname@example.org), where
he specializes in high-performance trays and various process technologies.
His responsibilities include new product development and testing,
development of equipment design correlations and programs, as well as
troubleshooting and conducting test runs on new equipment. He has
written various papers on mass transfer issues and product developments
and has two patents. He is a registered professional engineer and holds a
BS in chemical engineering from Univ. Oklahoma. He is a member of the
Technical Committee for Fractionation Research, Inc. www.cepmagazine.org or Circle No.121
CEP www.cepmagazine.org June 2005 27 ...
View Full Document
This note was uploaded on 12/29/2011 for the course CHE 128 taught by Professor Scott,s during the Fall '08 term at UCSB.
- Fall '08