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Note: The source of the technical material in this volume is the Professional

Engineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for Saudi

Aramco and is intended for the exclusive use of Saudi Aramco’s

employees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,

or disclosed to third parties, or otherwise used in whole, or in part,

without the written permission of the Vice President, Engineering

Services, Saudi Aramco.

Chapter : Process For additional information on this subject, contact

File Reference: CHE10402 R.A. Al-Husseini on 874-2792

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Distillation Hardware

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Engineering Encyclopedia Process


Saudi Aramco DeskTop Standards

CONTENT PAGE

DISTILLATION HARDWARE .............................................................................. 1

NOMENCLATURE.............................. ................................................................... 1

SELECTING TOWER CONTACTING DEVICES................................................. 3

Sieve Trays ....................................................................................... 5

Valve Trays....................................................................................... 5

Bubble-Cap Trays ............................................................................. 6

Packing ........................................................................................... 10

Figure 10 ......................................................................................... 12

Grids ............................................................................................... 13

Baffle Sections ................................................................................ 14

FACTORS AFFECTING TRAY PERFORMANCE ............................................. 17

Maximum Vapor Rate Considerations............................................ 27

Minimum Vapor Rate Considerations ............................................ 27

Maximum Liquid Rate Considerations ........................................... 27

Minimum Liquid Rate Considerations............................................ 27

MAIN TRAY DESIGN PARAMETERS............................................................... 28

Hardware Definitions ................................................................................. 29

Tower Diameter and Tray Spacing ................................................. 29

Downcomer Area ............................................................................ 30

Downcomer Clearance.................................................................... 30

Outlet Weir Height and Weir Length.............................................. 30

Multipass Trays............................................................................... 31

Contacting Area Definitions ........................................................... 31

Tray Pressure Balance ................................................................................ 32

Valve Tray Design Options ........................................................................ 37

Tower Internals........................................................................................... 39

Tray Transitions .............................................................................. 39

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Saudi Aramco DeskTop Standards

Downcomer Seal ............................................................................. 40

Seal Pan .......................................................................................... 41

Antijump Baffle .............................................................................. 42

Wire Mesh Entrainment Screens (Demisters)................................. 43

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Saudi Aramco DeskTop Standards 1

Nomenclature

Ad Clearance area between downcomer and tray below, in.2, cm2

A1 Area of one hole, ft2, m2Af Tray free area, ft2, m2

Ap Hole area, ft2, m2

At Active area (available for holes, valves, caps), ft2, m2

Aw Waste area, ft2, m2

b Notch width, in., mm

Co Orifice discharge coefficient

CSB Tray capacity parameter, ft/s, m/s

c Downcomer clearance, in., mm

D Nozzle diameter, in., mm

D Vapor-free liquid in downcomer height, in., mm

DT Tower diameter, ft, m

d Depth of V notch, in., mm

Fh Factor in calculation of effective liquid head

f Aeration factor

FP Flow parameter, (L/VL)(r V/r L)0.5

H Tray spacing, in., mm

hd Downcomer contraction pressure loss, in. liquid, mm liquid

he Effective liquid head over weir, in.

ho Head of liquid over weir, in., mm

hon Liquid height on notch, in., mm

hr Residual pressure drop, in. liquid, mm liquid

ht Total tray pressure drop, in. liquid, mm liquid

hv Hydrostatic head of vapor, in. liquid, mm liquid

hw Weir height, in., mm

hwi

Inlet weir height, in., mm

hwo Outlet weir height, in., mm

K 1 Viscosity/pressure correction factor of Va

L Liquid volumetric rate, ft3/s, m3/h

La Average static liquid head, in. liquid, mm liquid

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lfp Flow path length, in., mm

lud Length of clearance under downcomer, in., mm

lw Effective weir length, in., mm

lwi Inlet weir length, in., mm

LLH High Liquid Level

LLL Low Liquid Level

N p Number of tray passes

n Number of notches in weir

Pd Dry-tray vapor pressure drop, in. liquid, mm liquid

Pw Wet tray pressure drop, in. liquid, mm liquid

Q Liquid flow rate, gpm, L/sr Downcomer rise, in., mm

Sd Downcomer area, ft2, m2

Sdi Downcomer inlet area, ft2, m2

Sdo Downcomer outlet area, ft2, m2

St Column cross sectional area, ft2, m2

TT For towers, refers to the height of the tower measured Tangent-to-Tangent

U Minimum vapor velocity trough hole, ft/s, m/s

Ui Liquid velocity at downcomer entrance, ft/s, m/s

Uo Linear vapor velocity trough holes, ft/s, m/s

V Superficial vapor velocity, ft/s, m/s

Va Allowable superficial vapor velocity, ft/s, m/s

VL Total tray vapor load, ft3/s, m3/s

V N Vapor velocity based on the net tray area available for liquid disengagement, ft/s,

m/s

b Ratio of hole area to tray area available for holes

mL Liquid viscosity, cP, cP

r L Liquid density at operating conditions, lb/ft3, kg/m3r V Vapor density at operating conditions, lb/ft3, kg/m3

s Surface tension, dynes/cm, dynes/cm

* Distinguishes values associated with inboard downcomer trays

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Selecting Tower Contacting Devices

A contacting device must have good liquid and vapor handling capacities, good contacting

efficiency, reasonable pressure drop, and predictable turndown characteristics, and it must

also be economical. The devices available fall into two broad categories: cross-flow and

countercurrent. They are shown conceptually in Figure 1.

CROSS-FLOW VERSUS COUNTERCURRENT DEVICES

FIGURE 1

With cross-flow devices, the liquid flows horizontally across a flat plate, called a tray, that

contains a contacting device that thoroughly disperses the vapor into the liquid. The

dispersion process must produce sufficient interfacial area and maintain the phases in contact

with each other long enough to promote adequate mass transfer between the phases.

As the liquid flows across the tray, it is contacted by the rising vapor. At the far side of the

tray, the liquid enters a downcomer, which carries it to the tray below where the contacting process is repeated. The contacting area must be large enough to handle the required liquid

and vapor rates while promoting the desired mass transfer. Likewise, the downcomer must be

large enough to handle the liquid being processed.

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With countercurrent devices, the liquid flow is truly countercurrent to the vapor flow. The

efficiency of contact depends on the area available for mass transfer. In trays, this is

provided by bubbling vapor through the liquid, thereby producing sufficient interfacial area

for mass transfer. With packing, the interfacial area for mass transfer is provided by the

surface area of the packing. With baffle trays, the interfacial area is created by forcing the

vapor to flow through descending curtains of liquid, which breaks the liquid curtains into

droplets.

Generally, as the surface area of a device increases, the efficiency increases. However, as the

surface area increases, capacity decreases while cost rises. Thus, the final design involves

optimizing the capacity, efficiency, cost, and other process considerations for the variety of

possible internal designs.

Most Saudi Aramco units use valve trays; however, sieve trays are very common in the petroleum industry. There are very few Saudi Aramco towers with packing, although

potential applications exist, especially in vacuum crude distillation and in debottlenecking

existing units.

Cross-Flow Devices

Figure 2 illustrates a typical arrangement and key components for a one-pass tray and a two-

pass tray.

CROSS-FLOW CONTACTING DEVICES

FIGURE 2

The most common types of trays in use today are sieve, valve, and bubble cap trays.

Following is a review of these types of trays. Figure 6, after the section on bubble caps,

summarizes their characteristics.

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Sieve Trays

The contacting area consists of flat plates containing perforations, usually 1/2 in. (13 mm) in

diameter (Figure 3). They are the simplest trays to fabricate and are therefore the cheapest.

They also exhibit good capacity, excellent efficiency, and good turndown characteristics

(about 3/1). Their flat surface facilitates maintenance. Thus, they may be used in fouling

services, provided the hole size is large enough.

SIEVE TRAY DECK

FIGURE 3

Valve Trays

For most Saudi Aramco fractionation services, valve trays are the first choice. Valve trays

contain proprietary devices manufactured by Glitsch Inc., Koch Engineering, and Nutter

Engineering. The valve size, shape, weight, and other parameters vary from vendor to

vendor. Turndown is excellent, reaching 5/1. The valve tray capacity and efficiency are

about equal to those of a sieve tray, but cost is roughly 10% higher.

Valve trays are not recommended for severely fouling service, because deposits may interfere

with the valve movement. Valves specified with a dimple have a lower probability to stick to

the tray deck in the closed position. Valves can also be specified with an anti-rotation device

that will prevent rotation of the valve and wear of the valve legs.

Figure 4 illustrates valves from the three main vendors.

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VALVE TRAYS

FIGURE 4

Bubble-Cap Trays

Figure 5 illustrates bubble cap trays, which were the first type of tray developed for

continuous distillation. Although they provide excellent vapor-liquid contact over a wide

range of throughputs, they are relatively expensive to fabricate, install, and maintain. As

distillation hardware evolved, bubble-cap trays were largely displaced first by sieve trays and

later by valve trays. Despite their expense, bubble cap trays are sometimes specified in

fouling, low pressure drop, and high turndown services.

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BUBBLE-CAP TRAYS

FIGURE 5

TRAYS -- A SUMMARY OF CHARACTERISTICS

Tray

Type Capacity Efficiency

Cost per

Unit Area Flexibility Remarks

Sieve Medium to high. High. Equal to

or better than

other tray types.

Lowest of all

trays with

downcomers.

Medium.

3/1 can usually

be achieved.

Alternative to valve

trays when high

turndown is not

required.

Valve Medium to high;

as good as sieve

trays.

High. As good

as sieve trays.

Medium. About

10% greater than

Sieve Trays.

High.

Possibly up to

5/1.

First choice for most

applications. Not

recommended for

moderate to severe

fouling services.

BubbleCap

Medium to high,except low to

medium at high

liquid rate.

Medium to high. High. At leasttwice the cost of

sieve trays.

High.5/1 or slightly

higher.

Use for highflexibility where

fouling of valve trays

may be a problem.

FIGURE 6

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Downcomer Configurations

The standard type of downcomer is the straight, or chordal, downcomer shown in Figures 7

and 8. For a given tower diameter, a certain amount of the available tray cross-sectional area,

the downcomer area, is needed for liquid handling with the remainder, the bubble area,

available for vapor flow. Therefore, any changes that reduce the tray area used by

downcomers, increases the area available for vapor flow. Such a goal can be achieved by

using stepped or sloped downcomers as shown in Figure 7. The process performance

characteristics of sloped and stepped downcomers containing the same inlet and outlet areas

are identical. They can therefore be used interchangeably.

The required downcomer cross-sectional area is greater at the top of the downcomer where

most of the vapor disengagement takes place. Sloped or stepped downcomers provide the

required area at the top of the downcomer, and at the same time, they reduce the tray area

taken by the downcomers at the bottom. As a result, the tray area available for vapor-liquidcontact and vapor disengagement with stepped or sloped downcomers is higher than for

straight downcomers.

Sloped or stepped downcomers are most effective when used in trays with moderate-to-high

liquid rates to increase their vapor capacity (existing units) or to reduce the required tray

diameter (new units).

STEPPED VERSUS SLOPED DOWNCOMERS

FIGURE 7

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Downcomer Configurations (Cont'd)

For uniform liquid flow distribution onto a tray, the chord at the bottom of the downcomer

must have certain minimum length, often expressed as a percentage of the tower diameter. In

some services where very low liquid rates must be handled, this minimum chord length

provides a downcomer whose area is too large for the liquid flow rate being handled (that is,

the chord is about 6.8% of the tower cross-sectional area). A modified arc (also known as

segmental) downcomer can be specified (Figure 8) to overcome this limitation while still

meeting the minimum requirement. The modified arc downcomer has an area less than the

6.8% provided by the minimum (65% of tower diameter) chordal downcomer, but has a

projected weir length at least equal to the minimum. Some older towers may contain a full

arc-type downcomer. This style of downcomer functions in the same manner as a modified

arc but is more expensive to build and thus is no longer used in new towers.

STRAIGHT, MODIFIED ARC, AND ARC-TYPE DOWNCOMERS

FIGURE 8

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Countercurrent Devices

Packing, grids, and baffle sections are the three types of countercurrent devices reviewed in

this section. Figure 13 at the end of the section, summarizes their characteristics.

Packing

Although a packed tower design may result in a smaller tower diameter, the total cost of the

installation with packing, packing supports, distributors, and redistributors is generally higher

than that of a trayed tower.

The most common uses of packing in distillation services are:

• Applications where pressure drop across the internals is critical, such as in vacuum

distillation.

• Revamps, especially where downcomers consume a large percentage of the tower's

cross-sectional area or where downcomer filling is high; examples are, heavily liquid

loaded towers such as debutanizers and depropanizers.

• Corrosive services where ceramic packings are more economical than alloy trays.

• In towers less than 2 ft in diameter.

Saudi Aramco units using packing are the crude vacuum distillation columns, ADIP

extractors (2-in. polypropylene Intalox Saddles), and Merox oxidizers (1.5-in. carbon

Raschig rings).

Random Packings (Also Called Dumped Packings)

Random packings are the most frequently used countercurrent devices (Figure 9). Their name

derives from the fact that they are dumped into the column and orient randomly.

The most widely used packing today is the Pall ring. It comes in a number of sizes and

materials of construction. As the ring size increases, the capacity increases while the pressure

drop, cost, and efficiency decrease. Thus, for a given design, there is an optimum economic

combination of ring size, tower diameter, and tower height.

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COUNTERCURRENT DEVICES - RANDOM PACKING

FIGURE 9

Since 1978, several other packings have come on the market that provide improved

performance characteristics. These include Norton Company's Intalox Metal Tower Packing

(IMTP) also known as Metal Intalox, and Nutter Engineering's Nutter Ring. Raschig ringsare used infrequently, while Intalox saddles are generally preferred for applications requiring

ceramic packing.

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Structured (Ordered) Packings

Structured packing devices are fabricated in bundles from crimped sheet metal and installed

in the tower in layers having a fixed orientation. Since they provide more surface area per

unit volume than random packings, they are more efficient. However, they cost two-to-four

times as much.

Of the contacting devices available, structured packings provide the lowest pressure drop per

theoretical stage of contacting as well as the best capacity/efficiency combination. This

feature makes them especially attractive in vacuum towers.

There are several brands and suppliers, including Flexipac by Koch Engineering, Gempak by

Glitsch, Intalox Structured by Norton, Montz by Nutter Engineering, and Mellapak by Sulzer.

One of these devices, by Koch Engineering, is shown in Figure 10.

STRUCTURED PACKING BY KOCH ENGINEERING

FRONT VIEW ISOMETRIC VIEW

PASTEUP NEED TO BRING IN SCAN

SCAN REMOVED BECAUSE OF DIFFICULTY PUTTING ON DISC

Figure 10

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Grids

Grids are similar to structured packing in that they are fabricated in panels and installed in an

ordered manner. However, their efficiency characteristics are much poorer due to their high

open area and low surface area per unit of volume. The first grid to appear on the market,

circa 1961, was the Glitsch grid. It was intended for use in services where entrainment

removal was critical but where fouling was too severe to use crinkled wire mesh screens.

In recent years, several new grids have come on the market. They are Flexigrid by Koch

Engineering and Snapgrid by Nutter Engineering. Pictures of these major grids are shown in

Figure 11.

VARIOUS TYPES OF GRIDS

FIGURE 11

Because of their high capacity and low pressure drop, grids have also been used in heat

transfer sections (pumparounds) of vacuum crude distillation and other heavy hydrocarbonfractionators. The liquid is introduced on the top layer of grid via spray nozzles.

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Baffle Sections

There are two basic types of baffle sections. The first type is sheds; the second type is disc

and donuts (Figure 12). These devices operate differently from grids or packing. In baffle

sections, the liquid cascades from baffle to baffle in the form of liquid curtains. As the vapor

flows through these curtains, the liquid is broken up into droplets and mass transfer occurs.

However, this is a very inefficient liquid/vapor contacting mechanism.

For severe fouling services, baffle sections are about the only internal available if long run

lengths are required. Because of their high open area, they have high capacity but very poor

efficiency. Thus, baffle sections require a disproportionate amount of tower height for the

functions they perform.

SHEDS/DISC AND DONUTS

FIGURE 12

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COUNTERCURRENT DEVICES -- A SUMMARY OF CHARACTERISTICS

Device Capacity Efficiency

Cost per

Unit Area Flexibilit

y

Remarks

Packing (Pall

Rings, Metal

Intalox, Nutter

Rings, etc.)

Medium. Medium to

High.

Medium to

low, depending

on material of

construction.

> 3/1. Good for DP

service.

Mainly used in

vacuum

pipestills and in

various high

liquid rateabsorbers.

Structured

Packing

Flexipac;

Montz

Gempak;

Mellapak

Intalox-

Structured

Medium to

very high

depending

on size

used.

Medium to

very high

depending on

size used.

High - at least

two times

dumped

packing cost.

> 3/1. Best efficiency

per unit of DP.

Glitsch Grid

Flexigrid

Snapgrid

Very high. Poor as

fractionation

device. Good

for entrainment

removal and

heat transfer.

Medium to

high.

Low:

less than

2/1.

Good for high

vapor-low

liquid service

to minimize

effect of

entrainment.

Used in wash

zones of heavy

hydrocarbon

fractionators

where

moderate

coking occurs.

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Sheds and Disc

and Donuts

Very high. Poor as

fractionation

device.

Medium. Low.

< 1.5/1

Used in severe

fouling service;

e.g., slurry

pumparound incat fractionator.

FIGURE 13

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Factors Affecting Tray Performance

Flow Regimes - Spray and Froth

Movies taken during operations in various towers have indicated that different flow regimes

can exist on a tray. The first is the froth regime. In this regime, vapor passes through the

liquid on the tray as discrete bubbles of irregular shape. As the vapor rate increases, jets and

bubbles of rapidly changing shape are observed. If the vapor rate is raised still further, a gas

jet issues from the orifice and some of the liquid is shattered into droplets in a regime called

the spray regime. In the spray regime, the vapor phase is continuous, whereas in the froth

regime, the liquid phase is continuous (Figure 14). Spray regime operation occurs primarily

at high vapor velocities and low liquid rates. The froth regime in high pressure systems is

also referred to as the emulsion regime.

FROTH REGIME VERSUS SPRAY REGIME OPERATION

FIGURE 14

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Flow Regimes - Spray and Froth (Cont'd)

Operation in the spray regime can be very detrimental to good tower performance, causing

tray efficiency to drop sharply because the liquid and vapor residence times are reduced.

While spray regime operations have been observed on all the widely used trays discussed

earlier, the spray regime has been investigated primarily with sieve trays.

Under spray regime conditions, the vapor rate is sufficient to "blow through" the liquid,

thereby making the vapor phase continuous. In fact, the term blowing is often used to

describe the spray regime. Because the liquid rate is usually set by the process and cannot be

increased, the most effective way to suppress the spray regime is to dissipate the jet leaving

the orifice as quickly as possible. The most obvious way to dissipate the jet is to increase the

open area on the tray, thereby reducing jet velocity. A second way is to use smaller orifices;

for example 1/8-in. holes versus 1/2- or 1/4-in. holes used on sieve trays. Because the

distance to dissipate a jet is a function of the orifice diameter, the smaller the orifice the faster the jet will dissipate. A third way is to use valve trays. Because the vapor leaves the valve

element almost horizontally, its vertical velocity component is greatly reduced and its jet is

more quickly dissipated.

Entrainment

Entrainment is defined as the liquid carried by the vapor from a given tray to the tray above.

As the vapor rate in the contacting area is increased, the amount of energy being dissipated

also increases. This energy creates the interfacial area needed for good contacting between

the liquid and the vapor. It also expands the froth or spray height on the tray, therebydecreasing the distance between the top of the spray and the tray above. As this disengaging

distance decreases further, some of the liquid is carried, or entrained, to the tray above as

droplets (Figure 15). The smallest drops will be entrained to the tray above while the largest

drops will fall back to the entrainment generation tray. As the quantity of entrainment

increases, the tray above becomes overloaded and floods, and the tray efficiency drops

sharply. Flooding must be avoided to maintain good tower control and design fractionation.

The quantity of entrainment generated is dependent on vapor rate, liquid rate, system physical

properties, and certain hardware parameters.

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GENERATING ENTRAINMENT

FIGURE 15

Jet Flooding

Jet flooding sets the vapor-handling capacity of almost all cross-flow trays. In jet flooding,

the liquid is projected or jetted to the tray above by the vapor leaving the tray's orifice. If

sufficient liquid is entrained to the tray above, the liquid will overload the downcomers, and

the tray will flood. When flooding occurs, the liquid begins to back up on the tray until the

inter-tray space is filled with a dense froth (Figure 16). This causes the next higher tray to

flood, and flooding moves up the tower until the liquid is carried out the top of the tower.

When flooded, the tower fractionates poorly and is very difficult to control.

The approach to flooding conditions is quantified as % jet flood or % flood. This is the ratio,

expressed as percent, of the vapor velocity between the trays, V, divided by the maximum

vapor velocity that will not cause flooding. The maximum velocity is called allowable vapor velocity, Va.

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Jet Flooding (Cont'd)

Because jet flooding sets the maximum capacity of the tower, it must not be exceeded.

Furthermore, as the percent of the jet flood velocity moves from 90% to 100%, the

entrainment rate increases exponentially and the tray efficiency falls off sharply.

Perry's Chemical Engineers' Handbook and vendor literature provide correlations for

determining jet flooding.

PERCENT JET FLOOD VERSUS EFFICIENCY

FIGURE 16

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Downcomer Inlet Velocity

As the liquid leaves the contacting area on a tray, it enters the downcomer. Since it enters as

a froth (20-50% liquid by volume), it must be disengaged before it flows to the tray below.

The downcomer provides residence time for disengaging and acts as a conduit for liquid flow

to the tray below.

If the entrance area is too small and the froth cannot readily enter the downcomer, the froth

height will increase in the contacting area. This height will continue to increase until there is

sufficient head to force the froth into the downcomer or until the froth reaches the tray above,

causing flooding.

Downcomer Residence Time

The difference between the liquid and vapor densities, ρL-ρV, is one measure of the difficultyof separation in the downcomer. Thus, based on buoyancy considerations, as the difference

of ρL-ρV gets smaller, disengaging becomes more difficult. For this reason, the downcomer

sizing criteria allow lower velocities (higher residence time) for high-pressure systems, whereρL-ρV is low.

Downcomer Filling

The liquid height in the downcomer is called downcomer filling, expressed in inches of clear

liquid or as a percent of the tray spacing. Since the liquid enters the downcomer as a froth, the

actual fluid level in the downcomer will be higher than the filling calculated as clear liquid

(Figure 17). The exact height depends on the average froth density in the downcomer. As theliquid travels downward in the downcomer, the vapor disengages and escapes from the top of

the downcomer. If the downcomer is sized properly, the liquid leaving should be essentially

clear liquid. Thus, there is a froth density gradient down the downcomer that ranges from the

froth density on the tray (at the top) to clear liquid at the bottom.

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DOWNCOMER FILLING

FIGURE 17

Downcomer Sizing Criteria

Downcomer inlet velocity, based on vapor-free liquid, normally should not exceed 0.4 ft/s to

assure an adequate area for vapor disengaging. For foamy liquids, the inlet velocity is limited

to 0.2 ft/s. However, based on Saudi Aramco experience with crude stabilizer columns,

downcomer inlet velocity can be higher without downcomer flooding limitation.

Allowable downcomer filling is 50% for normal systems and 40% for foaming systems.Further, the downcomer shall be sized to allow 5 seconds minimum residence time for low-

pressure columns and 7 to 8 seconds for high-pressure columns (greater than 400 psi) and

systems with high foam stability.

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Weeping

At low vapor velocities, the dry-tray pressure drop of the tray is insufficient to support the

liquid head on the tray; as a result, some liquid begins to flow intermittently through the vapor

openings. This liquid bypassing begins at the "weep point." As the vapor rate decreases

further, more liquid pours through the holes and weeping becomes continuous.

Although the total quantity of liquid that weeps is constant at a given vapor rate, the weep rate

per hole fluctuates. That is, some holes are weeping while others are in the vapor bubbling

mode. At any instant, a given hole may be bubbling, weeping, or doing neither, in a

random distribution across the contacting area of the tray.

Although weeping can occur on all tray types, it is less of a problem in valve trays, which are

the most widely used tray in Saudi Aramco plants. Since weeping occurs only at reduced

rates, it is the major factor in determining tray turndown, the range of vapor loadings over which acceptable fractionation is achieved. (See Tray Turndown discussion.) For sieve trays,

turndown ratio is usually between 2/1 to 3/1; for valve trays, 3/1 to 5/1.

As Figure 18 shows, when vapor rate decreases, weeping increases very rapidly and tray

efficiency begins to decrease sharply. Weepage up to 20% of the liquid rate has small effect

on efficiency and is acceptable.

EFFECT OF WEEPING ON EFFICIENCY

FIGURE 18

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Dumping

When all the liquid flows through the holes on a tray, that is, no liquid flows over the weir,

dumping is said to occur.

When dumping takes place, tray efficiency is extremely poor and the products will be off-

spec. Trays should not be operated in the dumping region.

Tray Turndown

Turndown is a measure of the hydraulic flexibility of the tray. It is defined as the ratio of

maximum to minimum loadings in a range over which acceptable tray performance is

achieved. This usually is the range over which the tray efficiency stays at or above the design

value (Figure 18).

As Figure 18 shows, there is a relatively flat portion of the efficiency curve where design (or

better) efficiency is obtained. At low vapor rates, however, excessive weeping decreases

efficiency; at high vapor rates (above 90% of flood), excessive entrainment decreases

efficiency.

Entrainment-Weeping -- Tray Operating Window

Figure 19a shows the fractional weepage and entrainment curves for a typical sieve tray with

a moderate to high liquid rate. Using Figures 19a and b, note the difference in operating

ranges for 20% fractional weepage and entrainment. The moderate to high liquid rate

provides a good turndown ratio. The low liquid rate provides a poor turndown ratio. Sieve

trays can usually be designed to provide a turndown ratio of 2/1 to 3/1; valve trays, up to 5/1.

If the liquid rate on a tray is low (say below 1.5 gpm/in. of weir per pass), the operating

window on the tray is extremely small or nonexistent. This is shown in Figure 19b.

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EFFECT OF LIQUID RATE ON SIEVE TRAY TURNDOWN

(a) (b)

FIGURE 19

Tray Efficiency

The vapor and liquid phases must be dispersed thoroughly and remain in contact long enough

for mass transfer to occur and to achieve good efficiency. The vapor residence time is the

time for the vapor to flow through the volume of froth on the tray. Likewise, the liquid

residence time is the time for the liquid to flow through the volume of froth on the tray. Both

of these variables depend on liquid and gas rates as well as the weir height and bubble area on

the tray.

The efficiency is also affected by the vapor and liquid diffusivities. Since these values are

fixed for a given system, there is no way to change them through tray hardware changes.

To achieve good efficiency, a designer must optimize the weir height, open area, bubble area,number of liquid passes, and other variables. Excessive weeping, entrainment, and operation

in the spray regime must be avoided.

Figure 20 illustrates the effect of tower loading on valve and sieve tray efficiencies. The

operating range for the valve tray is wider, reaching to very low turndown. Sieve tray

efficiency may be somewhat higher near design loadings.

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EFFECT OF TOWER LOADING ON TRAY EFFICIENCY

VALVE TRAY VERSUS SIEVE TRAY

FIGURE 20

Tray Performance Diagram

A fractionating tray must be operated within a certain range of vapor and liquid rates to give

optimum performance and an economical design. Outside this range, efficiency drops off or

the tower becomes inoperable. The effects of vapor and liquid rates on tray performance are

depicted schematically on Figure 21 and are summarized in the text that follows.

TYPICAL TRAY PERFORMANCE DIAGRAM

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Maximum Vapor Rate Considerations

A very high vapor rate may cause:

• Jet flooding, excessive entrainment, or spray regime operation.

• High pressure drop across the tray, resulting in excessive downcomer filling and

subsequent tray flooding.

Minimum Vapor Rate Considerations

A very low vapor rate may cause:

• Weeping or dumping.

• Poor contacting and tray efficiency because of inadequate vapor and liquid mixing.

These conditions can result from insufficient vapor loading or from excessive open area on

the tray, both of which produce a low vapor velocity through the tray openings.

Maximum Liquid Rate Considerations

High liquid rates may cause:

• Tray flooding because of insufficient disengaging in the downcomers, excessive tray

pressure drop, and excessive downcomer filling.

• Tray flooding because of excessive downcomer entrance or exit velocity and

downcomer bridging.

Minimum Liquid Rate Considerations

Low liquid rates may cause:

• Spray regime operation at high vapor rates.

• Vapor bypassing up the downcomers, if the downcomer is not sealed.

• Poor contacting and low tray efficiency, because of inadequate liquid residence time on

the tray due to operation in the spray regime.

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Main Tray Design Parameters

The development of a design for a new tray generally follows the steps listed below:

1. Define the design vapor and liquid loadings.

2. Determine tray spacing, diameter, and layout.

3. Calculate hydraulics, pressure drop, and downcomer (DC) filling.

4. Evaluate flexibility.

5. Produce a balanced tray design.

The tray design procedure is iterative and involves repeating several of these steps, even whena tray design computer program is used. In this section we will focus on:

• Hardware definitions.

• Tray pressure balance.

• Main tray design variables that the engineer needs to determine performance and key

parameters that the tray design must satisfy.

• Valve tray design options

The Saudi Aramco Design Practice, ADP-C-001, contains information and criteria for tray

design. A Glitsch valve tray design manual is provided as a separate class handout.

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Hardware Definitions

Figures 22 and 23 are typical layouts of single-pass trays, illustrating the tray characteristics

discussed in the following sections.

Tower Diameter and Tray Spacing

These two parameters set the capacity of the tower. As the distance between trays (the tray

spacing, H) increases, tower capacity increases. For most services, the most economical

spacing falls between 18-24 in. Spacings above 36 in. provide little capacity advantage and

are not usually recommended. Likewise, tray spacings as low as 12 in. can be used, but this

increases the tower diameter (DT) required to handle a given set of vapor and liquid loadings.

In addition, low spacings make maintenance much more difficult. The Saudi Aramco

Engineering Standards specify minimum tray spacing requirement at various tower diameters.

TRAY LAYOUT DEFINITIONS

FIGURE 22

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DOWNCOMER ARRANGEMENTS

FIGURE 23

Downcomer Area

This is the area in Figures 22 and 23 (Sdi, Sdo) that handles liquid as it flows from a giventray to the tray below. The edge of the downcomer is usually chordal in shape, and its

maximum width is called the downcomer rise (r).

Downcomer Clearance

This is the vertical clearance (c) between the tray floor and the bottom edge of the downcomer

apron (Figure 22).

Outlet Weir Height and Weir Length

As the liquid leaves the contacting area and enters the downcomer, it flows over the outlet

weir (Figure 22). The height of the weir (hw) is set by the designer to provide liquid holdup

on the tray and promote efficient liquid/vapor contacting. The weir length (lw) is the same asthe downcomer chord length.

Inlet weirs are discussed later in the downcomer seal section.

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Multipass Trays

As the liquid rate on a tray increases, the capacity of the tower can usually be increased if the

liquid flow is split into more than one path (Figure 24). Such split-flow trays are called

multipass trays. On multipass trays, the downcomers nearest the tower centerline are inboarddowncomers, while those farthest away are called outboard downcomers.

TRAY PASS ARRANGEMENTS

FIGURE 24

Contacting Area Definitions

During tray design, such terms as active area, hole area, waste area, and free area are used.

They are explained below (see also Figures 22 and 23).

Active or Bubble Area (At )

This is the area between the downcomers where vapor/liquid contacting occurs. It is used in

calculating tray efficiency, but does not set the tray's capacity.

Hole/valve/cap Area (A p )

This is the open area or hole area provided within the bubble area to permit vapor to enter,

contact, and pass through the liquid on the tray. For a sieve tray, it equals the total area of all

the holes on a given tray.

The hole area is usually expressed as a fraction of the active area (A p/At) and is determined

by various correlations for each tray type.

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Waste Area (Aw )

Waste area is any part of the active area that is farther than 3 in. from the edge of a contacting

device. Since vapor does not contact the liquid in this area, it is not included in the active

area. Waste area frequently occurs when tray blanking, inlet weirs, or recessed inlet boxeshave been specified.

Free Area (Af )

Test data have shown that as the vapor flows through and leaves the active area (A t) it

expands over the downcomer(s) and its velocity drops. Thus, an area greater than the bubble

area is available for vapor flow. This larger area is known as the free area (Af ). The free area

is what determines the tray's capacity. When multipass trays are designed, the tray (either

inboard or outboard) that has the smallest free area is used to set the tray's capacity. For trays

with sloped or stepped downcomers, the average free area is used.

Flow Path Length (lfp )The length of the contacting area in the direction of the liquid flow (see Figure 23).

Tray Pressure Balance

A tray pressure balance illustrates the factors that determine downcomer filling. Figure 25

illustrates the pressure balance for a two-pass sieve tray; the pressure balance for a one-pass

tray and for a valve tray is similar.

The components of the pressure balance are described below. Their values are generally

expressed in terms of clear liquid height at the tray conditions, for example, inches of clear liquid.

• Dry tray pressure drop (Pd). It is the pressure drop through the tray openings, sieve

holes in this case. It does not take into account any effects from the presence of the

liquid (dry tray).

• Average liquid static head (La).

• The sum of Pd and La is the pressure drop between the trays.

• Pressure loss under the downcomer or downcomer contraction pressure loss (hd) resultsfrom the flow of the liquid through the downcomer clearance.

• Downcomer liquid filling (D).

A pressure balance between the trays through two paths, through the tray openings and

through the downcomer, results in the following equations.

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Tray Pressure Balance (Cont'd)

Pressure balance for inboard downcomer filling: D* = hd* + Pd

* + La* + La

Pressure balance for outboard downcomer filling: D = hd + Pd + La +La

*

The * distinguishes inboard from outboard downcomer trays (Figure 25). It was assumed

that the vapor density, r V, is significantly lower than the liquid density, r L.

Pressure balance for single pass trays:

D = hd + Pd + 2La

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For light ends columns operating under high pressures, it is necessary to consider the effect of

vapor density in design calculations. The liquid height in the downcomer in such cases

should be determined by the equations available in ADP-C-001, 3.4.1.

PRESSURE BALANCE FOR A TWO-PASS SIEVE TRAY

Inboard Downcomer: D* = h*d + P*d + L*a + La for r V << r LOutboard Downcomer: D = hd + Pd + La + L*a for r V << r L

FIGURE 25

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Main Tray Design Variables and Performance Parameters

Below is a list of the main variables determined during tray design and a list of key

performance parameters affected by the tray design. A discussion relating the items in the

two lists follows.

Main Tray Design Variables

• Tray diameter.

• Tray spacing.

• Number of tray passes.

• Downcomer area.

• Downcomer type.

• Active or bubble area.

• Open or hole area.

• Weir height.• Downcomer clearance.

Key Performance Parameters

• Jet flooding.

• Downcomer filling (or downcomer flooding).

• Downcomer inlet velocity.

• Dry and total tray pressure drop.

• Pressure drop under the downcomer.

• gpm/in. of weir.

• Weeping and tray flexibility.

Tray diameter and tray spacing are the two most important variables in tray design because

they determine the diameter and height of the tower. They affect two important performance

parameters: jet flooding and downcomer filling (downcomer flooding). The tray diameter is

the main variable in determining the velocity of vapor between trays and, as a result, jet

flooding. Tray spacing also affects jet flooding. Higher tray spacing between trays allows

more liquid droplets to settle before they reach the tray above; thus it helps reduce

entrainment and the percent jet flood. Improvement for tray spacings above 36 in. is

marginal. Because of the trade-off between tower diameter (tray diameter) and height (tray

spacing), finding the most economical design may require the evaluation of alternative tray

spacings.

Tray diameter and tray spacing also affect downcomer filling. Vapor velocities determined

by the tray diameter affect the tray pressure drop of a tray and as a result the downcomer

filling. Tray spacing sets the height available for vapor disengagement in the downcomers.

Liquid height is the numerator and tray spacing the denominator in determining the percent

downcomer filling.

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Main Tray Design Variables and Performance Parameters (Cont'd)

The Saudi Aramco Engineering Standards, AES-C-001, 5.1.3, specify minimum tray spacing

requirements for tower access and service.

Towers with high liquid rates use more than one tray pass. The upper acceptable limit of

liquid rate per tray pass is about 15 gpm/in. of weir or according to the ADP criteria,

5000 gph/ft of diameter. Tray hydraulics at higher liquid rates become unpredictable.

The downcomer area determines the inlet velocity in the downcomer and along with the tray

spacing, the residence time of liquid in the downcomer. ADP specify maximum inlet velocity

and minimum residence time requirements. Vendors (see Glitsch valve tray design manual)

have similar criteria.

When the downcomer inlet velocity sets the size of the downcomer (usually at high liquidrates), it may be possible to increase the cross sectional space available for vapor flow by

using sloped downcomers. The liquid leaving the downcomer is relatively clear of vapor;

thus, higher velocities (on clear liquid basis) at the bottom of the downcomer are acceptable.

The active or bubble area of a tray normally is the area left after the downcomer area is

determined. Very small residence time and small flow path lengths along the bubble area

may result in low tray efficiencies.

The open or hole area of a tray affects tray performance parameters such as dry tray pressure

drop, weeping and tray flexibility, and the transition between the froth and spray regimes on

the tray. Lowering the open area increases the vapor velocity through the holes, the dry tray pressure drop, and as a result, the downcomer filling. High vapor velocities through the

holes, especially when the liquid rates are low, may result in a spray rather than froth vapor-

liquid contact on the tray. High open area reduces the flexibility of the tray and may result in

weeping and dumping at turndown conditions.

The weir height is a key factor in determining the liquid height on the tray. As such, it affects

tray performance parameters such as pressure drop, weeping, and tray flexibility. It also

affects the spray-froth transition and the tray efficiency. Along with the downcomer

clearance, it determines the downcomer sealing. The downcomer clearance also affects the

pressure loss under the downcomer (or downcomer contraction pressure loss hd) and

therefore, downcomer filling. For trays with high liquid rates, shaped lip downcomers helpreduce pressure drop.

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Valve Tray Design Options

Valve trays are normally specified for new column designs. They are relatively inexpensive

and provide good vapor/liquid contact over a wide throughput range.

Valve tray designs are normally developed by the various tray fabricators that submit

quotations. Precise design methods vary, but all fabricators follow the same general

procedures. Most major tray fabricators issue design manuals for general use which illustrate

these procedures. Sample design manuals include the following:

(a) Glitsch, Inc. (Dallas, Texas)

Ballast Tray Design Manual (Bulletin No. 4900)

5th Edition (December 1989)

(b) Koch Engineering Company, Inc. (Wichita, Kansas)Flexitray (R) Design Manual (Bulletin 960-1, 1982)

(c) Nutter Engineering Company (Tulsa, Oklahoma)

Float Valve Design Manual

April 1976

Such manuals can be used to:

• Determine the preliminary size of new columns and tray components.

• Check tray fabricators' designs for new columns and trays.

• Evaluate existing columns under operating conditions differing from original designs.

Tray fabricators' design manuals are updated periodically, so care must be taken to utilize the

latest manuals. Tray designs developed by tray vendors or via vendor manuals should be

checked using the Saudi Aramco design criteria (ADP-C-001). For final design, the tray

should be rated by the vendor. Figure 26 is typical of tray design results from valve tray

vendors.

PROCESS/PRO II can be used to develop new tray designs and rate existing ones.

PROCESS/PRO II uses the Glitsch valve tray design method. For sieve and bubble cap trays,it derates the valve tray results by 5 and 15% respectively. It is necessary to simulate the

tower to rate or design a tray with PROCESS or PRO II.

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BALLAST TRAYS DESIGNED BY GLITSCH INC.

FIGURE 26

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Tower Internals

The following is a list of common tower internals. The design of most of these internals is

covered in ADP-C-001. In addition to ADP, vendors can provide designs and design criteria.

• Tray support.

• Tray pass transitions.

• Downcomer seal.

• Antijump baffle.

• Wire mesh entrainment screens.

• Tower inlets.

• Tower drawoffs.

• Reboiler drawoffs.

An overview of tray pass transitions, downcomer seal, anti-jump baffle, and wire meshentrainment screens is below.

Tray Transitions

Figure 27 illustrates the tray transition arrangement for one-pass trays to two-pass trays.

TRAY TRANSITIONS

FIGURE 27

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Seal Pan

Seal pans provide seal for the bottom tray. A typical seal pan arrangement is shown in Figure

29.

SEAL PAN

FIGURE 29

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Antijump Baffle

Figure 30 illustrates a typical antijump baffle. Antijump baffles are used to prevent liquid

collision in the center of the inboard downcomer. The specific need for this baffle varies with

liquid rate and the type of tray used.

ANTIJUMP BAFFLE

FIGURE 30

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Wire Mesh Entrainment Screens (Demisters)

In some towers, entrainment of liquid can cause serious product contamination and

degradation. Crinkled wire mesh screens are installed to provide a surface upon which the

entrained liquid can coalesce to prevent this problem. These screens must be carefullydesigned. If the velocity through the screen is too low, maximum coalescence will not occur.

If the velocity is too high, coalesced liquid will be re-entrained from the screen.

Screen coking may also be a problem, depending on temperature, type of screen, and

feedstock quality. Each tower must be considered individually and past or similar experience

relied upon.

An example of wire mesh screen efficiency is provided in Figure 31. For optimum

performance, the kinetic energy factor F = V [r V/(r L-r V)]0.5 for the vapor entering the screen

should fall within the design range of the screen. If it falls below this range, the cross-

sectional area of the screen should be reduced somewhat by addition of a donut-shaped bafflearound the screen.

EXAMPLE OF WIRE MESH EFFICIENCY

Kinetic Energy Factor, F = VρV

ρL – ρV

0.5

FIGURE 31

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GLOSSARY

active area The tray deck area where the liquid-vapor contacts take place.

antijump baffle Tower internal device placed over the inlet of an inboard

downcomer in order to prevent liquid from one side from

jumping to the other side. See figure in the text.

arc downcomer A type of downcomer. See figure in Downcomer Configuration

section.

baffle sections Horizontal or low-angle contacting devices creating cascades of

liquid for contact with rising vapor. There are two basic types

of baffle sections: sheds, and disks and donuts. See the figures

in the text.

blank tray Tray used to collect liquid from higher trays or packing. Blank

trays do not provide vapor-liquid contact. A synonymous term

is chimney tray.

bubble cap tray A type of tray. The vapor goes through risers and inverted caps

making contact with the liquid when leaving the caps.

See the figures in the text.

cartridge tray Prefabricated tray and downcomer assembly. See figure in text.

chimney tray Tray used to collect liquid from higher trays or packing.

Chimney trays do not provide vapor-liquid contact. A

synonymous term is blank tray.

choking Accumulation of froth bridged over the inlet of a downcomer,

slowing down the transfer of liquid to the trays below.

chordal downcomer Vertical straight downcomer across a chord of the tower cross

section. Synonymous with straight downcomer. See Figure 7,

Downcomer Configuration section.

column A vertical vessel containing contacting devices such as trays or

packing, used to perform separations such as distillation or

extraction. A synonymous term is tower.

countercurrent devices Devices in which the liquid flow is truly countercurrent to the

vapor flow.

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cross-flow devices Devices in which liquid flows horizontally across a flat plate.

debottlenecking Removal of a process or equipment constraint.

demisting Elimination of entrained liquid droplets at the top of a packed

bed or a trayed tower.

disc and donuts A type of baffle section. See the figures in the text.

downcomer area The cross-sectional area of downcomers.

downcomer clearance The vertical distance between the bottom of the downcomer and

the tray deck.

downcomer contractionpressure drop

Pressure drop of the liquid passing under the downcomer.

downcomer filling Height of liquid in the downcomer. It is often expressed in

inches of clear liquid or a percent (clear liquid) of the tray

spacing.

downcomer flooding Overloading of the tray interspace with liquid, caused by high

downcomer filling.

downcomer rise The horizontal radial distance between the center of the chord of

a straight outboard downcomer and the vessel wall.

downcomer seal Hydraulic seal of the downcomer outlet. See figures in the text.

downcomers Tower internals that allow the tray liquid to pass to the tray

below.

dry-tray pressure drop Part of the pressure drop that is not related to the presence of the

liquid on the tray, that is, the pressure of the vapor through the

contacting device.

dumped packing Packing type, consisting of small (2-in. is typical) devices with

large open space, placed in the tower (dumped) in random

orientation. A synonymous term is random packing.

dumping Weeping of all the liquid, so that no liquid flows over the weir.

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entrainment Liquid carryover by the vapor to the tray above.

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flexibility Refers to capacity related flexibility. See Turndown.

flooding Overloading of the tray interspace with liquid. Frequently, the

term refers to jet flooding.

flow regimes The movement of liquid and vapor on a tray.

free area The tray cross-sectional area available for vapor flow.

froth A flow regime in which vapor passes through a liquid on the

tray as discrete bubbles of irregular shape.

grids Countercurrent contacting devices fabricated in panels and

installed in an ordered manner. In contrast to structured

packing, grids provide wide clearances. See the figures in thetext.

hole area The open area provided within the bubble area to permit vapor

to enter, contact, and pass through the liquid on the tray.

inboard downcomer Downcomer positioned by the vessel wall.

jet flooding Overloading of the tray interspace with liquid, caused by

excessive entrainment.

modified arc downcomer A type of downcomer. See Figure 8 in Downcomer Configuration section.

multiple downcomer tray Proprietary type of tray. See figure in Downcomer

Configuration section.

outboard downcomer Downcomer positioned by the vessel wall.

packing Devices that provide countercurrent vapor-liquid contact in

distillation columns.

percent jet flood (%

flood)

The ratio, expressed as a percent, of the vapor velocity between

the trays, V, divided by the maximum vapor velocity that will

not cause flooding.

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plates Contact points of all the vapor and liquid in a column, such as it

occurs on column trays. The term theoretical plates is used to

indicate that equilibrium is reached at the contact point between

all the vapor and all the liquid. The actual plates reflect theobtained tray efficiency. A synonymous term is stages.

pumparound Heat removal from a stream pumped from a tray to a higher

tray.

random packing Packing type, consisting of small (2-in. is typical) devices with

large open space, placed in the tower (dumped) in random

orientation. A synonymous term is dumped packing.

seal pan Tower internal device placed over the inlet of an inboard

downcomer in order to prevent liquid from one side from jumping to the other side. See figure in the text.

sheds A type of baffle section. See Figure 7 in the text.

sieve tray A perforated plate type of tray.

sloped downcomer A type of downcomer. See Figure 7 in Downcomer


spray A flow regime in which a gas jet issuing from the orifice

shatters some liquid into droplets.

stages Contact points of all the vapor and liquid in a column, such as

occurs on column trays. The term theoretical stages is used to

indicate that equilibrium is reached at the contact point between.

The actual stages reflect the obtained tray efficiency. A

synonymous term is plates.

stepped downcomer A type of downcomer. See Figure 7 in Downcomer


straight downcomer Vertical straight downcomer across a chord of the tower cross

section. Synonymous with chordal downcomer. See Figure 8

in Downcomer Configuration section.

structured packing Countercurrent contacting devices fabricated from thin crimped

sheets of metal and installed in layers having a fixed orientation.

See the figures in the text.

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superficial velocity Velocity based on the tower diameter rather than the cross-

sectional area available for flow.

support ring Horizontal ring welded to the tower walls that are used tosupport a tray.

tower See column.

tray loadings Tray vapor and liquid rates.

tray pass number The number of individual paths of liquid on a tray.

tray spacing The vertical distance between two trays.

tray turndown The ratio of maximum to minimum tray loadings in a range over which acceptable performance is achieved.

truss Tray support beam.

turndown Operation at reduced capacity.

ultimate capacity The largest vapor load a tower can handle, as predicted by the

Stokes law on droplet entrainment.

valve tray A type of tray with contacting devices that can be opened and

closed. See the figures in text.

waste area Any area in the active area that is farther than 3 in. from the

edge of a contacting device.

weeping Liquid flow through the tray openings.

weir A vertical strip at the inlet or outlet of a tray used to maintain

liquid height on the tray or a liquid seal at the outlet of the

downcomer. See figure in text.

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