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3. How Trays Work: Flooding

3.1. History of Distillation

The ancient Egyptians produced beer from barley that had a few percent of

alcohol. Next, wine was produced by fermenting grape juice, which had a

greater sugar content than barley. This brought the content of alcohol up to

about 14 percent. Next, fortified wines (like sherry or port) were made by

adding extra sugar and yeast to the fermenting grape juice. This increased

the alcohol up to about 17 percent. Much above this point, alcohol kills off the

yeast.

Next, distillation was used. I visited a primitive distillation plant in Peru. It

was a single-stage evaporation process. The fermented grape juice is

partially vaporized and the alcoholwater vapors are totally condensed. The

resulting condensate is 40 to 44 percent alcohol. The higher alcohol content

is obtained by vaporizing less of the still's content.

To go much beyond the 44 percent alcohol, one needs to introduce modern

process engineering technology:

Partial condensation

Reflux

Reboiler

I've shown a sketch of such a facility in Fig. 1.3. The idea is to generate reflux

to improve the separation between water and ethanol. To generate reflux, a

partial (rather than a total) condenser is required. Also, a way of adding

How Trays Work: Flooding
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more heat to the still, to match up with the capacity of the partial condenser,

is needed. I've now introduced a complex control loop into my plant. Alcohol

levels of 60-plus percent can be obtained with this two-stage evaporator.

Finally, we have the Patent Still, introduced by the Scots in the 1830s. Now,

distillation trays equipped with bubble caps and feed preheat are used. I

visited an apple orchard in England, which used the original design of the

Patent Still to produce apple brandy. A sketch of this apparatus, whose

design has not been altered in 180 years, is shown in Fig. 3.1. The sketch has

been copied from the original patent application filed in London in the 1830s.

With the multitrayed distillation column, ethanol concentrations (as limited

by the alcoholwater azeotrope) of 90-plus percent can be obtained, if

enough reflux and enough trays are used.

3.2. Tray Types

Distillation towers are the heart of a process plant, and the working

component of a distillation column is the tray. A tray consists of the following

components, as shown in Fig. 3.2:

Overflow, or outlet weir

Downcomer

Figure 3.1. The very first distillation tower was the Patent Still. This

drawing was filed with the original patent application submitted in

1835. Column on the right is a bubble-cap trayed tower. Column on

the left is a feed preheater vs. an overhead vapor condenser.
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Tray deck

There are two types of tray decks: perforated trays and bubble-cap trays. In

this chapter, we describe only perforated trays, examples of which are

Valves or flutter caps

V grid, or extruded-valve caps

Sieve decks

Jet trays

Possibly 90 percent of the trays seen in the plant are of these types.

Perforated tray decks all have one feature in common; they depend on the

flow of vapor through the tray deck perforations, to prevent liquid from

leaking through the tray deck. As we will see later, if liquid bypasses the

outlet weir and leaks through the tray deck onto the tray below, tray

separation efficiency will suffer.

3.3. Tray Efficiency

Distillation trays in a fractionator operate between 10 and 90 percent

efficiency. It is the process person's job to make trays operate as close to 90-

Figure 3.2. Perforated trays.


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percent efficiency as possible. Calculating tray efficiency is sometimes

simple. Compare the vapor temperature leaving a tray to the liquid

temperature leaving the trays. For example, the efficiency of the tray shown

in Fig. 3.3 is 100 percent. The efficiency of the tray in Fig. 3.4 is 0 percent.

How about the 10 trays shown in Fig. 3.5? Calculate their average efficiency

(the answer is 10 percent). As the vapor temperature rising from the top tray

equals the liquid temperature draining from the bottom tray, the 10 trays are

behavin as a sin le erfect tra with 100- ercent efficienc . But as there are

Figure 3.3. Hundred-percent tray efficiency.

Figure 3.4. Zero-percent tray efficiency.
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10 trays, each tray, on average, acts like one-tenth of a perfect tray.

Poor tray efficiency is caused by one of two factors:

Flooding

Dumping

In this chapter, we discuss problems that contribute to tray deck flooding.

Figure 3.5. Average tray efficiency = 10 percent.


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3.4. Downcomer Backup

Liquid flows across a tray deck toward the outlet weir. The liquid over-flows

the weir, and drains through the downcomer to the tray below.

Vapor bubbles up through the sieve holes, or valve caps, on the tray deck,

where the vapor comes into intimate contact with the liquid. More precisely,the fluid on the tray is a froth or foamthat is, a mixture of vapor and liquid.

In this sense, the function of a tray is to mix the vapor and liquid together to

form a foam. This foam should separate back into a vapor and a liquid in the

downcomer. If the foam cannot drain quickly from a downcomer onto the tray

below, then the foamy liquid or froth will back up onto the tray above. This is

called flooding.

3.5. Downcomer Clearance

Referring to Fig. 3.6, note that the downcomer B is flooding. The cause is loss

of the downcomer seal. The height of the outlet weir is below the bottom

edge of the downcomer from the tray above. This permits vapor to flow up

downcomer B. The up-flowing vapor displaces the downflowing liquid. That

is, the vapor pushes the liquid up onto the tray abovewhich is a cause of

flooding. On the other hand, Fig. 3.7 shows what happens if the bottom edge

of the downcomer is too close to the tray below. The high pressure drop

needed for the liquid to escape from downcomer B onto tray deck 1 causes

the liquid level in downcomer B to back up onto tray deck 2. Tray 2 then

floods. Once tray 2 floods, downcomer C (shown in Fig. 3.7) will also back up

and flood. This process will continue until all the tray decks and downcomers

above downcomer B are flooded.
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The weir height on many trays is adjustable. We usually adjust the weir

height to between 2 and 3 inches. This produces a reasonable depth of liquid

on the tray to promote good vapor-liquid contact.

The crest height is similar to the height of water overflowing a dam. It is

calculated from

where crest height = inches of clear liquid overflowing the weir

GPM = gallons (U.S.) per minute of liquid leaving the tray

The sum of the crest height plus the weir height equals the depth of liquid

on the tray deck. One might now ask, "Is not the liquid level on the inlet side

of the tray higher than the liquid level near the outlet weir?" While the

answer is "Yes, water does flow downhill," we design the tray to make this

factor small enough to neglect.

3.6. Vapor-Flow Pressure Drop

We have yet to discuss the most important factor in determining the height of

liquid in the downcomer. This is the pressure drop of the vapor flowing

through the tray deck. Typically, 50 percent of the level in the downcomer is

due to the flow of vapor through the trays.

When vapor flows through a tray deck, the vapor velocity increases as the

vapor flows through the small openings provided by the valve caps, or sieve

holes. The energy to increase the vapor velocity comes from the pressure of

the flowing vapor. A common example of this is the pressure drop we

measure across an orifice plate. If we have a pipeline velocity of 2 ft/s and an

orifice plate hole velocity of 40 ft/s, then the energy needed to accelerate the

vapor as it flows through the orifice plate comes from the pressure drop of

the vapor itself.

Let us assume that vapor flowing through a tray deck undergoes a pressure

drop of 1 psi (lb/in ). Figure 3.8 shows that the pressure below tray deck 2 is

10 psig and the pressure above tray deck 2 is 9 psig. How can the liquid in

downcomer B flow from an area of low pressure (9 psig) to an area of high

pressure (10 psig)? The answer is gravity, or liquid head pressure.

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The height of water needed to exert a liquid head pressure of 1 psi is equal

to 28 inches. of water. If we were working with gasoline, which has a specific

gravity of 0.70, then the height of gasoline needed to exert a liquid head

pressure of 1 psi would be 28 inches/0.70 = 40 inches of clear liquid.

3.6.1. Total Height of Liquid in the Downcomer

To summarize, the total height of clear liquid in the downcomer is the sum of

four factors:

Liquid escape velocity from the downcomer onto the tray below.

Weir height.

Crest height of liquid overflowing the outlet weir.

The pressure drop of the vapor flowing through the tray above the

downcomer. (Calculating this pressure drop is discussed in Chap. 4.)

Unfortunately, we do not have clear liquid, either in the downcomer, on the

tray itself, or overflowing the weir. We actually have a froth or foam called

aerated liquid. While the effect of this aeration on the specific gravity of the

liquid is largely unknown and is a function of many complex factors (surface

tension, dirt, tray design, etc.), an aeration factor of 50 percent is often used

for many hydrocarbon services.

Figure 3.8. Vapor P causes downcomer backup.
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This means that if we calculated a clear liquid level of 12 inches in our

downcomer, then we would actually have a foam level in the downcomer of 12

inches/0.50 = 24 inches of foam.

If the height of the downcomer plus the height of the weir were 24 inches,

then a downcomer foam height of 24 inches would correspond to downcomer

flooding. This is sometimes called liquid flood.

This discussion assumes that the cross-sectional area of the downcomer is

adequate for reasonable vapor-liquid separation. If the downcomer loading

(GPM/ft of downcomer top area) is less than 150, this assumption is okay, at

least for most clean services. For dirty, foamy services a downcomer loading

of 100 GPM/ft would be safer.

3.7. Jet Flood

Figure 3.9 is a realistic picture of what we would see if our towers were made

of glass. In addition to the downcomers and tray decks containing froth or

foam, there is a quantity of spray, or entrained liquid, lifted above the froth

level on the tray deck. The force that generates this entrainment is the flow

of vapor through the tower. The spray height of this entrained liquid is afunction of two factors:

The foam height on the tray

The vapor velocity through the tray

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High vapor velocities, combined with high foam levels, will cause the spray

height to hit the underside of the tray above. This causes mixing of the liquid

from a lower tray with the liquid on the upper tray. This backmixing of liquid

reduces the separation, or tray efficiency, of a distillation tower.

When the vapor flow through a tray increases, the height of froth in the

downcomer draining the tray will also increase. This does not affect the foam

height on the tray deck until the downcomer fills with foam. Then a further

increase in vapor flow causes a noticeable increase in the foam height of the

tray deck, which then increases the spray height.

When the spray height from the tray below hits the tray above, this is called

the incipient flood point, or the initiation of jet flooding. Note, though, that jet

flood may be caused by excessive downcomer backup. It is simple to see in a

glass column separating colored water from clear methanol how tray

separation efficiency is reduced as soon as the spray height equals the tray

spacing. And while this observation of the onset of incipient flood is

straightforward in a transparent tower, how do we observe the incipient

flooding point in a commercial distillation tower?

The reason I can write with confidence on this subject is that I worked with a

4-inch demonstration transparent column at the Chevron Refinery in Port

Arthur, Texas, in 1989. I used the little distillation tower to explain to plant

Figure 3.9. Entrainment causes a jet flood.


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Figure 3.11 illustrates this point. Point A is called the incipient flood point,that point in the tower's operation at which either an increase or a decrease

in the reflux rate results in a loss of separation efficiency. You might call this

the optimum reflux rate; that would be an alternate description of the

incipient flood point.

Figure 3.10. A simple depropanizer.


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3.8.2. Bypassing Steam Trap Stops Flooding

I wake up early to answer email questions before breakfast. Here's today's

question from South Africa:

Hi Norman. We have a distillation tower that floods. Delta P on trays below

feed point is stable; delta P above feed (trays 1622), increase from 9 to 19

KPA. Condenser and reflux drum is internal in tower, and we cannot

measure the reflux rate. Yesterday, bypassed steam trap on reboiler outlet,

and flooding stopped. Conclude that flooding tower due to defective steam

trap. What's your opinion? Note tower fractionation also improved after

trap bypassed.

Regards,

Jon Sacha

Dear Jon: You're quite wrong. When you bypassed the steam trap, you blew

the condensate seal on the reboiler outlet. This permitted uncondensedsteam to blow through the reboiler, thus reducing the reboiler duty. The

reduction in the reboiler duty reduced the vapor flow up the tower and

hence the internal reflux rate. This unloaded the trays and stopped the

flooding. Your observation that the tower fractionation improved as a

consequence of bypassing the steam trap was a positive indication that

you had degraded tray efficiency due to entrainment. That is, you were

operating above the tower's incipient flood point. Certainly, there isnothing amiss with your reboiler steam trap. You should try to water wash

the trays above the feed point onstream, as the trays in this service are

typically subject to NH Cl salt sublimation. Hope this helps.

Figure 3.11. Definition of the incipient flood concept.

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One of the most frequent causes of flooding is the use of carbon steel trays.

Especially when the valve caps are also carbon steel, the valves have a

tendency to stick in a partially closed position. This raises the pressure drop

of the vapor flowing through the valves, which, in turn, pushes up the liquid

level in the downcomer draining the tray. The liquid can then back up onto

the tray deck and promote jet flood due to entrainment.

Of course, any factor (dirt, polymers, gums, salts) that causes a reduction in

the open area of the tray deck will also promote jet flooding. Indeed, most

trays flood below their calculated flood point, because of these sorts of

problems. Trays, like people, rarely perform quite up to expectations.

The use of movable valve caps in any service where deposits can accumulate

on the tray decks will cause the caps to stick to the tray deck. It's best to

avoid this potential problem. Use of grid trays with fixed cap assemblies is

preferred for most services.

3.10. Optimizing Feed Tray Location

From the design perspective, the optimum feed tray, for a feed with only two

components, is that tray where the ratio of the two components matches theratio in the feed. If the feed is at its bubble point temperature, then the feed

temperature and the tray temperature will be the same, at the same

pressure.

But that's only for a binary feed composition. In multicomponent distillation,

the ratio of the key components in the feed will typically not coincide with the

ratio of the key components in the liquid on the tray, even though the tray

temperature is the same as the feed at its bubble point temperature.

So the question is, which of the following two criteria should be used to

determine the feed tray location:

Where the ratio of the key components in the liquid on a tray matches the

ratio of the key components in the feed?

Or where the feed temperature matches the temperature of the tray?

My practice is to identify both possible locations and then locate the feed

nozzle halfway between the two options. I would provide two alternate feed


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nozzles at each of the other above locations, except the operators would

likely never use them.

Incidentally, when I refer to "Key Components," I mean, for example:

DebutanizerNormal Butane and iso-Pentane

DepropanizerPropane and iso-Butane

De-EthanizerEthane and Propylene

Gasoline Splitteriso-Hexane and cyclo-Hexane

All of the above services have feeds with dozens of other non-key

components.

3.11. Catacarb CO Absorber Flooding

I've never told this story to anyone. Not even to Liz or my mom. It occurred in

Lithuania in 2006. I had been hired to expand the capacity of the hydrogen

plant that was limiting refinery capacity. The bottleneck was the absorber

that removed CO with catacarb solution from the hydrogen product. This

absorber was subject to flooding as the catacarb circulation rate increased inproportion to H production. That is, the solution was carried overhead with

the hydrogen product.

I studied the design of the tower, but could not see an explanation for the

flooding. Nevertheless, I decided to modify all the 40 trays in the absorber.

The materials were ordered, and the labor force organized. However, the

morning the absorber was opened, I received a call from my assistant, Joe.

"Hey, Norm, there's kind of a plate in front of the solution inlet nozzle (see

Fig. 3.12). It don't show on the drawings. What you want to do with that

plate?"

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"Joe," I answered, "I'll be there in 10 minutes."

I looked at the plate. Dimension "x" was only about inch. Evidently, the

plate was intended as an inlet solution distributor. I calculated that the delta

P, as the solution flowed underneath the plate, was about 15 inches:

Delta H (inches) = 0.6 (V)

Where V was the solution velocity through the -inch gap in feet per

second.

The plate was 12 inches high. As the solution rate increased, the liquid

would back up over the top edge of the plate and be blown out of the top

of the absorber. So, I told Joe to have the bottom 2 inches of the plate cut

off to increase "x" to 2 inches.

"And, Joe," I continued, "Also, close up the tower afterward."

Figure 3.12. Restriction of the inlet distributor causes entrainmentof the catacarb solution.

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"Norm, but what about all the tower tray changes?" Joe protested.

"Don't argue. I know what I'm doing."

When the tower started up a week later, the Hydrogen Plant bottleneck was

gone. The plant manager never found out what I did, or that I had wasted

$20,000 for unused tray materials. Perhaps, since I had achieved theobjective, he wouldn't have cared. Anyway, the alternate proposal to expand

H plant capacity, submitted by a major engineering contractor, would have

cost $3,000,000.

"All's well that ends well."

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Citation

Norman P. Lieberman; Elizabeth T. Lieberman: Working Guide to Process Equipment,

Fourth Edition. How Trays Work: Flooding, Chapter (McGraw-Hill Professional, 2014),

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