PACKED COLUMNS

DEFINTION:

A vertical column or tower, usually cylinder placed inside which some solid

packing material (used to distribute liquid over it) for continuous intimate contact

between two fluids.

Usually in a packed columns two fluids are immiscible or partially

Miscible and are a gas and a liquid. For the close contact usually liquid is dropped

from the top under the influence of gravity and it is distributed over the solid mass in

the form of thin films and does not go straight down but follows a tortuous route. A

large surface area is thus exposed which is the heart in contact with the gas, i.e. the

solid mass is effectively irrigated in order to have an intimate contact with the gas.

The gas is blown from the bottom under pressure. It passes (counter-currently)

through the free space between the wetted particles of the packing.

PARTS OF PACKED COLUMN:

The basic unit of packed column consists essentially of following parts,

1) Shell

2) Packing

3) Packing support

4) Liquid Distributors

There are many other parts, e.g. Hold down grids, mist eliminators etc.

1) SHELL:

Shell is the main body of the unit, all the packing, distributors etc. is placed

inside it. In its construction, there is no mechanical system, it is simply a cylindrical (

may be rectangular) shell having the diameter smaller as compared to the height of the

tower and placed as erect as possible to have a uniform liquid distribution.

SIZE OF TOWERS:

There are several towers of different heights and diameters depending upon

the operation. However the height and the width (diameter) of a tower may exceed as

80 ft and 30 ft respectively

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2) PACKING:

We have already described that in a packed column in order to have an intimate contact

between a gas and liquid. A solid mass is necessary; this inert (to both gas and liquid) solid mass

is called the packing or fill.

Packing is the heart of the performance of a packed column because on these liquid is

distributed uniformly in the form of films in order to have a large surface area for gas contact,

which otherwise not possible

CLASSIFICATION OF PACKING:

Although many packing designs of many different materials are available and are used in

packed column operation, yet we are able to classify these packing as below.

One way of classification may be done as:

i) Broken mass

ii) Well shaped packing

iii) Grids (also well shaped but different)

Along with these types there are different methods use to install packing, however these way

may also be used to classify the packing.

i) Dumped or Random packing

ii) Stacked or Regular packing

Combining above two we may classify packing as,

i) Dumped or Random packing

ii) Stacked or Regular packing

iii) Grids

i) DUMPED OR RANDOM PACKINGS:

Random packing is also called irregular packing. These usually small pieces of

specific geometrical shape (may or may not ) and are so called because they are packed

randomly in the tower that is they are thrown at random in the hollow shell, however care is

there in throwing. Usually in order to avoid packing breakage, the shell is first filled with liquid

(water), and packing is dropped in this liquid. For detail see p.245, ref. 3.

They can be further classified as,

a) Broken mass

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b) Well shaped

a) BROKEN MASS:

These are the earliest one and were ceramic spheres, broken stones (rocks), gravel, or lumps of

coke screened to ½ - 4 inches size to eliminate small particles which might plug gas phase.

They have the advantages of very low cost and good corrosion resisting characteristics, yet they

are discarded due to the fact of small surface area and small free space between the particles

which restrict the gas flow, thus higher pressure drop may result. More high density of packing

imparts heavy weight to the tower and thus on its foundation. More although they are screened,

but during operation or installment, the weak edges may be broken and thus may clog the

voidage.

b) WELL SHAPED:

Due to the above difficulties in using broken mass packing, lighter, well shaped packing were

introduced. Although expansive but give uniform liquid distribution in the form of thin films,

thus greater surface areas, enough voidage for gas to flow with lesser pressure drops etc.

There is a long list of well shaped random packing which may be used now days.

RASCHIG RINGS:

Raschig rings are the oldest and cheapest one. These were so called because these were patented

by Dr. Raschig in Germany in 1907. Usually they are nothing but small pieces of a hallow

cylinder cut from a pipe or rolled from metal sheet having their height equal to their diameter.

Usually their diameters ranging from ¼ - 4 inches or more up to 6 inches. They are usually

randomly packed, however bigger sizes that is 4 - 6 inches sizes may be hand stacked.

They have the advantages of low cost, sound structure, availability in widest variety of material,

very much efficient work than the broken mass but they are not as good as the modern packing

as give more internal liquid channeling and direct more liquid towards the walls of the tower.

LESSING RINGS:

Similar to Rashig rings, with a slight modification as a partition in the Rashig rings as shown.

No much data is available about their features, but generally stronger than the Raschig rings and

have an improvement in efficiency due to larger area; however, this improvement is minor.

CROSS - PARTITION RINGS:

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These are again a modification to the Raschig rings, but this time a cross-partition is there. This

gives more strength and higher efficiency. Usually, they are stacked and thus have low pressure

drop and no side wall thrust.

SPLINED RINGS:

These are Raschig rings but are modified in a way that they are splined on the inside of the rings

(may also outside) in order to increase the surface area.

SPIRAL RINGS:

Usually stacked, Raschig rings, with the specialization of helical passage inside. These take the

advantage of internal whirl of gas liquid and offer extra contact surface over Raschig Rings,

Lessing rings and cross-partition rings, but give higher pressure drop.

BERL SADDLES:

Like Raschig rings, one of the two which are base of many modern packing is the original

saddles and so called because they resemble in shape with saddle (of horse). They have the

disadvantages of more cost and easy breakage (than Raschig rings) but they are more efficient

than Raschig rings in most of the applications.

They create tight spots in the bed and produce channeling; however, not as produced as Raschig

rings. Although they have smaller free gas space than Raschig rings and Lessing rings but their

aero-dynamic shape is better, thus give a lower pressure drop and little side thrust. They are

usually made in size of ½ - 3, and are ordinarily made of chemical stoneware.

INTALOX SADDLES:

They may start the second generation with Pall rings. They are nothing but the modified form of

the Berl saddles. They are modified, so that, the adjacent elements do not blank off any

significant portion of wetting liquid, to avoid stagnant pools of liquids, trapping of gas bubbles

and violent changes in the direction of the gas. Thus they are more efficient and have lower

pressure drop along with more capacity than the original saddles, however more costly and have

tendency to break in bed. Equivalent packing to intalox is also called Flexi saddles and Novalox

saddles. They are usually made of ceramics, can be made of other materials.

SUPER INTALOX:

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Again a modification to the intalox saddles. In these the smooth edges of the saddles are

scalloped, more holes are inserted.

These modifications are helpful to promote drainage of liquid, thus eliminate any possibility of

stagnant pockets, also provide more operating space to gas (vapor) to rise. Thus they have higher

capacity and higher efficiency than the equivalent Intalox saddles. They are available in ceramic

and plastic materials.

PALL RINGS:

These are derived from the second basic form Raschig rings, and are nothing but the Raschig

rings with windows in them, more the bending of window tongues inwardly. They are more

difficult to manufacture than Raschig rings and costly, but give lower (half) pressure drop than

Raschig rings. They have higher capacity and higher efficiency and lower pressure drop than all

the packing described, having considerable side thrust on column wall.

They are usually made in metal, plastic and ceramic material, however, the ceramic Pall rings are

not popular having inferior performance to that of ceramic Intalox saddles.

HY-PAK TOWER PACKING:

These are similar to the Pall rings but have more internal tongues to improve the surface area.

They may give equivalent efficiency in larger size. They are available in metals only.

INTALOX METAL TOWER PACKING (IMTP):

They may start the third generation of the packing. They are made to have high void fraction and

well distributed surface area of Pall rings with low aero-dynamic drag of the saddle shape. They

give more open shape than Pall rings and give more improvements to liquid spread, more give

adequate mechanical strength.

CASCADE MINI RINGS (CMR):

These are similar to the Pall rings, but have ratio 1:3 to (height to diameter) as compared to 1:1

Pall ring. The lower ratio orients the particles with their open side facing the vapor flow, thus

reducing fraction and exposing more surface to mass transfer. They may available in plastic,

metal and ceramics.

CMR-TORBO:

This is a variation in the CMR. The difference is that the walls and the tongues are perforated as

compared to the normal CMR, thus are more efficient.

CHEMPAK:

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These are very similar to the Pall rings. Thus may make by cutting the Pall rings vertical from

the centre, to make two out of one. This promotes vapor-liquid contacts, mixing and may liquid

spread. They are also called as Levapak (LVK).

HcKp:

Pall ring with a more open structure and an enhanced arrangement of internal drip tabs. They are

stated as ideal for use in high liquid rate system. They are usually made metallic.

JAEGER TRI-PACK:

The cylindrical shape of Pall rings are replaced by spherical one thus provides more void space

and better distribution. They are usually made of plastic (called Hackette) and metal.

NOR-PAC RING:

In these types of packing we replace the solid walls of the Pall rings by wide openings. Although

less surface area is available, but reduces the friction and good drainage of the liquid is possible.

The material of construction is plastic.

FLEXIMAX:

Just like saddles but with well spread (wider) surface area. They are usually metallic

FLEXIMAX.

LANPAC:

The polyhedron shape, composed of many small porous (ribs, filaments, rods, struts and pointed

fingers) in a complex (cross-linked and uniformly spaced thoroughly) way (an open structural

framework).

This complex structure gives an open structure with a high surface area, nesting and interlocking.

They are made of plastic material.

IMPAC:

Another complex packing like LANPAC, but gives higher and better distributed surface area.

They are made of plastic and metal material.

TELLER ROSETTE:

Also known as Tellerettes and usually stacked. They have high interstitial hold up and give high

efficiency, as low side thrust and low pressure drop. More they are low unit weight

packing. They are made of plastics (polyethylene) and are not used where solubility and

reactivity is a problem.

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PROTRUDED PACKING:

These are partially rounded sheets with perforations. They are made in small sizes. They have

high efficiency and are used widely in laboratory columns.

DIXON PACKING:

Dixon packing are special packing and expensive. These are Lessing rings made from wire

mesh. These give great interfacial area and very low pressure drop.

More KNIT MESH and McMahon packing are available in wire mesh; McMahon packing are

steel wire mesh. McMohan packing are steel wire mesh formed into a Berl saddle shape.

OTHERS:

Many other random packing is available which are not described here; however they are shown

(Nutter ring, Hiflow ring, Intalox Snowflake packing, etc.).

ii) STACKED PACKING:

These came later in existence; however they are since near 1940s. Unlike random packing

stacked packing are those which are not thrown randomly in the hollow shell, but they are

arranged systematically i.e. they are stacked in the tower.

CLASSIFICATION:

Such packing may be classified as,

a) Random packing that can be stacked

b) Wire mesh structured packing

c) Corrugated - sheet packing

a) RANDOM PACKING THAT CAN BE STACKED:

During the study of random packing we have studied such types of packing. These are the

random packing which can be stacked e.g. Raschig rings, Tellerets ,etc., however when hand

stacked the usual size is not as randomly packed sized may have but some what larger e.g.

Raschig rings which are stacked, usually have the diameter 4 - 6 in.

More in a randomly packed towers, sometimes we have to stack two or more layers of packing

above the packing supports, we will study this later.

TYPES OF STACKING:

There are usual two ways of stacking the random packings, which may be

i) S (square) - shaped

ii) D (diamond) - shaped

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However this classification is not general, only

Applicable to those packing which based on the Ranching ring structure.

b) WIRE MESH STRUCTURED PACKINGS:

SULZER WIRE GAUZE :

This packing is made from the fine diameter wire. The packing elements consist of parallel,

perforated sheets of wire mesh. These packing elements (7 in. tall) are stacked in the shell to the

required height. These are usually available in 316 stainless steel wire- mesh, also in other

corrosion resistant metals.

GOODLOE:

This packing is made of multifilaments of fine diameter wires. These wires are knitted together

to form a tube. They have high efficiency and low pressure drop, however not much data is

available. They are available most commonly in 316 stainless steel, but also carbon steel,

aluminum, alloys, plastic; kynar and Teflon are the manufacturing materials.

SPRAY PAK PACKINGS:

Spraypak is made from layers of expanded metal screen fastened together and pressed into a

corrugated form, with the corrugation angle being 90o or less. The corrugated material is then

bolted together through the apexes of the corrugations into sections of 10 to 20 layers. These are

then trimmed to fit the circular-column section and place into the column, with each section

alternately at right angles (with respect to the corrugations) to the other. This packing has a good

contact area, low pressure drop, and provides a uniform flow pattern and is copared more with

the tray type performance than other packings. It is usually used in large diameter towers, and

available as about 24 in. in diameter, but smaller 10 in. are also available. The material of

construction is metal only.

OTHERS:

In spite of these many others are also available some of them may be Panapak, Stedman, and

Drippoint

c) CORRUGATED PACKINGS:

There is a no. of corrugated packing, such as Mellapak, Flexipac, Gempak, Monz B1, Montz

BSH, Flexeramic, etc. We are not going in detail of these; however for constructional features

figures of such are shown.

COMPARISON OF STACKED AND RANDOM PACKINGS:

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- Stacked packing offers a far greater specific surface area (thus greater efficiency) than

random packing.

- Stacked packing have lower packing factor, thus have greater capacity. However the

capacity and efficiency advantages are decreased rapidly at high liquid rates or pressures. A no.

of such cases is experienced in industry. The cause of this cause is not well understood.

- The spread of surface area (also affect efficiency) tries us to select a random packing.

- Structured packing have a much lower pressure drop than random packing, because in

case of random packing resistance to vapor flow is mostly due to expansion and contraction, thus

higher pressure drop may exist, but in the case of structured packing there are regular flow

channels which keep expansion and contraction to a minimum value, the friction loss however is

due to loss through bends, which is far low resistance to vapor flow. However the capacity and

this lower resistance permit in comparison more surface area in a bed of stacked packing.

- Channeling is more severe in stacked packing; this is why random packing is preferred.

However stacking reduces the wall thrust than the in comparison to random packing.

- The cost is more in stacked packing as compared to the random packing.

- Structured packing have the advantage of self wetting.

- The liquid inventory (the product of liquid hold up and the packing volume) for

structured packing is in term lower than the random packing.

iii) GRIDS:

They are also well shaped and are packed in a systematic manner as stacked packing. They are

usually used in square column section. They have relatively large spaces between them, and thus

give very low pressure drop. More they are easy to assemble as bigger, also have the advantage

of accepting fluids with suspended solids. The problem however is not obtaining of good liquid

distribution even at high rates, etc. In comparison their efficiency is lower than those of both

random and stacked packing. They are principally used in direct contact heat transfer scrubbing

and de- aerating services. Constructional features of some grid packing are shown in figures.

3) PACKING SUPPORT:

In order to place the packing inside the shell a solid carrier called packing support or support

plate is the necessary thing.

It must be enough strong to carry the weight of the wetted packing and re- distributors (if are

placed on packing not attached to the tower), etc.

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It should have enough free area in order to allow the liquid and gas with a minimum of

restriction i.e. the packing support should be designed in such a way that the pressure drop is not

excessive and flooding velocity is not lower than that of packing. Every effort is made to obtain a

large a support free surface area as possible with the surety of the strength of the support. If one

will say that good tower performance is definitely linked with packing support then it will not

wrong.

There are two basic types of support plates.

a) Counter-current

b) Separate flow passages for liquid and gas

a) COUNTER-CURRENT:

These are so called because the gas and liquid have the same opening to flow counter-currently.

These are shown in fig. They are usually the simple perforated plates (largely spaced bars), but

these are not adequate, because many of the holes become blocked by the rings and the gas and

liquid must pass counter-current through the same openings which thus contributes towards

liquid holdup and flooding of the plate. This is principally a problem in random packing.

Conditions can be improved by stacking (arranging) two are three layers of rings on the support

plate in order to escape opening blockages. In this way we may use larger diameter whole plates

which are less liable to flooding.

With such type support plates the free area for gas flow can be ranged up to 90% of the column

cross sectional area, but this type as mentioned is easily clogged by the packing pieces.

b) SEPARATE FLOW PASSAGES FOR LIQUID AND GAS:

Separate flow passages plates are preferred to the above because they can be used the free area

up to 90% with a very minimum blockage of holes due to constructional features. In these the

gas and the liquid passages are not the same holes, but different holes are bored for two different

phases. These are shown in figure. The figure ( ) is used to get 85 to 90 % free area, used

with various modifications and is made of many different materials such as metals, ceramics and

plastics.

Grid packing is usually supported on bars laid across the towers or on the vertical pillars resting

on the tower floor. Wood grids are supported in sections with gaps between each section to

allow for the swelling of the wetted wood. Usually gaps are ¼ in. per length.

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In large towers the weight of the packing is very much. Thus it may be supported on the floor of

the tower, which can be supported on a brick or concrete foundation.

4) LIQUID DISTRIBUTORS:

Dry packing is of-course completely in effective for mass transfer. It is principally due to the

uneven or non adequate distribution of the liquid from the top.

Actually requirement of good contact between liquid and gas is the harder thing to meet,

especially in the case of large towers. In ideal conditions, liquid dropped from the top should

distribute over the top of the packing and should flow in thin films overall the packing surface all

the way down the tower, but this ideality is not approached. In real the films tend to grow

thicker in some places and thinner in others, so that the liquid collects into small rivulets and

flows along localized paths through the packing, then much of the packing surface may be dry

(mostly in low liquid rates), or at best , covered by a stagnant film of liquid. This effect is

known as channeling. This channeling is the main cause of the poor performance of the packed

towers. More along with channeling, side- slip of liquid may be a problem. Side slip is nothing

but the liquid tendency to flow out of the packing and travel down along the walls of the column.

This is principally when the ratio of the tower diameter to packing diameter is less than 8: 1. In

order to increase the efficiency of the tower, something is to be done inside the tower.

Channeling is more pronounced in stacked packing, thus it is customary not to use stacked

packing, thus random packing is to be used, and that is why most of the installments are

randomly packed unless stacking is necessary. However in the random packing i.e. dumped

packing, the packing density (the no. of the packing pieces per cubic units) is less near the walls,

thus liquid gets the tendency to segregate towards the walls.

This effect can be reduced in stacked packing.

In order to remove the difficulty of dry packing i.e. channeling, initial distribution of the liquid is

very much necessary. The importance of this initial distribution is shown in figure.

In large towers the initial distribution of the liquid is not enough, but the liquid re-distribution is

to be done at regular intervals. Roughly a re-distributor should be installed at an interval 3 - 10

times the tower diameter, but at least after every 20 ft. These liquid re- distributors have made

possible to build a column of diameter of about 30 ft. working satisfactorily.

In most of the cases where stacked packing is present there is no / little need for liquid re-

distributors.

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The system of distribution varies according to the type of the packing used, e.g. the random

packing is self distributing, thus a simple distributor having 6 - 12 in. apart liquid feed points is

adequate. In spite to random, grid packing has poor self- distributing characteristics and thus it is

necessary to supply the liquid to a no. of points on each slat.

TYPES OF LIQUID DISTRIBUTORS:

Several types of liquid distributors may be used.

1. SIMPLE ONES:

Simple one may be the spray nozzles, preferably the solid-cone type, may be in the form of

horizontal pan make excellent distribution especially in the self distributor random packing, but

they may clogged by the solid particles suspended in the solvent. More the splashing due to jet

produces fine drops which are carried away by the gas and thus a mist (spray) eliminator is

necessary at the exit. Also there target is the important thing, because if they will throw the

liquid towards wall, this liquid will never come in to the bulk. A perforated pipe (ring type)

distributor is an alternate to this may be used. As from its name this consists of perforated rings

as in garden fountains, and is used where high liquid rates or relatively smaller rates are present

and is suitable for low pressure loss. However with clean liquids it offers minimum restriction to

gas flow and can be used for high liquid flows.

Another simple method for stacked packing is to pack the top 2 - 3 ft. with random rings, using a

simple distributor at the top at a point 1 ft. apart.

2. ORIFICE DISTRIBUTORS :

Consists of flat tray equipped with a number of riser gas flow and perforations in tray floor for

discharge of liquid i.e. the inlet for liquid and outlet for the gas are different. Some times,

perforations are eliminated and a V-notch in each riser for passage of liquid is set. They are not

used where there is any risk of the plugging of holes.

3. NOTCHED CHIMNEY TYPE:

They are superior to orifice one as they do not have the tendency of plugging.

4. TROUGH TYPE DISTRIBUTORS:

These are often used in column of 4 to more diameters. These non splash distributors are a series

of parallel troughs laid on packing in a direction right angles to the top row of slats with notches

in the sides of the troughs directly above each slat. The distributor is not subject to plugging and

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do not restrict gas flow, and have wide operating range. In order to have an efficient work,

troughs must be leveled carefully.

Such distributions are effective both for stack packing and grid packing, although in the case

stacked packing it is very difficult (impossible) to space the notches directly in the rings top

layer.

PREVENTION OF WALL THRUST:

In order to prevent side slip (wall thrust) the diameter of tower should be at least 8 times the

packing diameter, however it is recommended that if possible the ratio of packing diameter to the

column should not exceed 1: 15. In other cases adequate distributors along with the side wipers

(wall wipers) are used. Side wipers are down slope rings which throws the liquid towards the

bulk of the packing. An example of these is a tagging ring shown in figure.

GAS DISTRIBUTION:

If the gas enters through simple horizontal nozzle, it will form a jet impinging on the opposite

wall and produce regions of high pressure under some parts of the packing. If the pressure drop

in the packing is of the same order of magnitude as these pressure fluctuations, the distribution of

the gas velocities in the packing is likely to vary considerably, whereas a high pressure drop in

the packing has the effect of neutralizing the pressure fluctuations and creating a uniform gas

distribution. It is therefore necessary to pay particular attention to the gas distribution in the

stacked packing which has comparatively low pressure drop. For gas distribution the gas should

enter the tower at a low velocity, preferably not more than twice its velocity in the packing.

However there are no special precautions required for random packing owing to the high

pressure drop in the packing.

HOLD DOWN GRIDS:

They are also known as packing restrainers. Some times in the process, a tower may be surged

due to high gas velocity, thus there is a danger to the breakage of the packing especially ceramic

packing. For this a hold down plate is used which is enough heavy to hold down about the

packing. It always rest on the packing, restrains the upward movement of the packing and saves

it from crushing.

PRESSURE DROP:

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Due to the resistance of packing in a packed column, there is a considerable pressure drop exists

for the gas flowing upwardly. This pressure drop is a function of both fluid flows, because one

occupies the same channel as the other and exists actually due to skin friction and form drag,

with form drag predominant at the higher velocities. It has been estimated that not over 10 % of

the pressure drop is the result of skin friction.

In the case of random packing it is greater, because expansion and contraction losses and

considerable turbulence are created by the flow of two fluids around the individual solid packing

elements. However in the case of structured packing the drop in pressure is very low as there are

regular flow channels which keep expansion and contraction to a minimum value, the friction

loss lies only due to the loss through bends, which is far low resistance to vapor flow. Grids

have least pressure drop.

PRESSURE DROP AS a FUNCTION OF GAS FLOW:

It is important to see the pressure drop variations with the variations in the gas flow. For this we

may draw three general curves i.e.

1. for dry conditions ( packing )

2. for wet-drained conditions

3. for irrigated conditions

1. FOR DRY CONDITIONS:

It is observed that for dry packing there is a direct relation between the flow rate of the gas and

the pressure drop through the packing. When the packing is dry, the line so obtained is straight

and has a slope of about 1.8 i.e. it makes an angle of 60.95 o with x-axis, as shown. Thus pressure

drop increases with the 1.8 the power of the velocity of the gas.

2. FOR WET DRAINED CONDITIONS :

The case with the wet drained packing is quite similar to the dry packing. Again straight line is

the result, but this time not with the same pressure drop but with result of larger pressure drop.

3. FOR IRRIGATED CONDITIONS :

In case when there is a constant flow rate of liquid coming from the top the graph line

characteristics are not as before. In this case the relationship between pressure drop and gas flow

rate initially follows a line parallel to that of dry packing, however the pressure drop is greater as

compared to the dry or wet packing because the liquid in the tower has reduced the space

available for gas to flow. After this, with further increase in mass velocity i.e. at moderate

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velocity, the line gradually becomes steeper and steeper because gas is forcing the liquid

upwardly and then liquid hold up is increased with gas rate. The point at which the liquid hold

up starts to increase i.e. where the slope of the line is deviating the value 1.8 is called as loading

point. At the loading point the curve present shows quicker rise in pressure drop which is

proportional to 2.5 Th of the gas flow.

More increase in gas velocity further cause’s rise in pressure drop more rapidly and at a point

line is about vertical (when the pressure drop is about 2 - 3 in. of water/ft. of packing). At this

point the flooding is reached thus this point is called as flooding point.

EXPLANATION VIA FIGURE:

In the figure we see up to point 1 on the curve, the pressure drop characteristics are similar to

curve A & B. The slope is same but with increased pressure drop. Observations have indicated

that the orderly trickling of liquid downward through the packing with no liquid build up is the

result. At point 1 change in slope occurs and indicates the pressure drop decrease which is

more rapidly with an increase in gas velocity. This point may not be distinguished enough to

allow observations of any change in the flow pattern characteristics. Perhaps it might be possible

to observe an increase in the quantity of the liquid retained in the packed section. This retained

liquid is referred as hold up, thus point 1 is called as loading point and the velocity of the gas is

named as loading velocity.

After this point a greater amount of liquid hold up exists. Observations show a layer at the top of

the packing and a gradual filling of the packing voids with liquid. The liquid now has filled a

large portion of the packing, and the gas must bubble through it. This condition is sometimes

called as visual flooding. More increased gas rate corresponding to visual flooding gives birth

to a second change in slope of the pressure drop line i.e. point 2. This point is known as

flooding point.

Gas velocity in an operating packed column must be lower than the flooding velocity. However

as flooding is approached, most or the entire packing surface is wetted, maximizing the constant

area between gas and liquid, but the pressure drop is too much. Thus the designer uses a velocity

far enough from the flowing velocity to ensure safe operation, also not as low as to require a

much larger column.

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Operation of packed column is not practically good above the loading point. For simplicity and

safety packed towers are designed using gas velocities of about 50 - 70 % of the flooding at the

expected liquid rate.

DROP AT CONSTANT GAS FLOW:

It is a general rule that each type of packing material has a fixed void volume for liquid passage

so that the liquid rate increases, the voids filled with liquid, so the cross-sectional area available

for gas flow is reduced, thus for constant gas velocity, it is observed that the pressure drop

increases with an increasing liquid rate, this is shown as a line D in the curves.

PRESSURE DROP EXPRESSION:

Perhaps there is no real accurate expression to calculate drop in pressure through packed

columns, however there are several correlations that are useful for design purposes.

Pressure drop may be calculated using orifice equation with suitable correction for the presence

of liquid.

On these basis Leva developed the following correlation for pressure drop in irrigated bed,

P = C2 10 3 Ut g Ut 2

where,

P = drop in pressure, ( in H2O/ft )

g = gas density, ( Ib/ft3 )

Ut and ut = superficial velocities of gas and liquid respectively, ( ft/s )

C2 and C3 are constants e.g. for Rasching rings with nominal size ½ in.,

and 3/32 in. wall thickness with C2 = 3.50 and C3 = 0.0577.

This correlation was developed from the test data for the air - water system operating below

flooding point.

Morris and Jackson have arranged experimental data for a wide range of a solid rings and grids.

The graphs are shown by which the no. of velocity heads “ N ” lost per unit height of packing is

found for appropriate value of the velocity rate and N is used in

- P = ½ N g Ug 2 L

where,

- P = pressure drop

g = gas density

Ug = gas velocity ( based on empty column cross-sectional area )

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and L = height of the packing.

An empirical correlation of experimental data for pressure drop has been presented by Leva and

Eckert et al. for Pall rings.

When the data is available, the most accurate method of obtaining the pressure drop for flow

through a bed of packing is from the manufacturer’s own literature. This is usually available in

logarithmic plot of a gas rate against pressure drop, with a parameter of liquid flow-rate on the

graph. Typical curves for four packings are shown.

The methods described apply only to conditions at or below the loading point. If we wish to

apply to conditions above the loading point, the calculated pressure drop would be too low.

Thus it is to be cheked before applying that whether the column is operating at or below the

loading point.

LOADINGS AND FLOODING POINTS CORRELATIONS:

There is a no completely generalized expression for calculating the onset of the loading, semi-

empirical correlations may be used.

A useful graphical correlation for flooding rates was first presented by Sherwood et al. and later

by Lobe et al. For random dumped packings where ( U t2 AP g / g 3 PL ) ( L / W ) is plotted

against ( L / G ) .( g / L)1/2.

where,

Ut = superficial gas velocity, m/s

AP = total area of packing, m2 (per m3 bed )

= fractional voids in dry packing

g = gravitational constant, 9.8067 m/s2

L and g = liquid and gas densities, kg/m3

L = liquid mass rate, kg/m2-s

G = gas mass rate, kg/m2-s

L = liquid viscosity, m . pascal . s ( cp )

W = viscosity of water at 293 K, (1 cp)

Later work with air and liquids other than water l;ed to modifications of Sherwood correlation,

first by Leva and then by Eckert. The recent modifications by Eckert is shown in figure. The

ordinate group including Φ, the ratio of the density of the water to the density of the liquid, and

also that of ratio Ap/3, characteristics for a particular packing material has been replaced by

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packing factor Fp also a characteristic of given packing( m -1), it is determined experimentally,

not taken for the packing geometry.

EXPREESION FOR LIQUID HOLD UP:

Sometimes it is desirable to know the volumetric hold up of liquid phase in the column, e.g. if

the liquid is involved in a chemical reaction or if a constant system for column is to be designed.

For gas liquid systems the hold up of liquid ( H ) for conditions below the loading phase has

been to vary approximately as the 0.6 power of the liquid rate and for the ring and saddles

is given as

H = 0.143 ( L / / d ) 0.6

where,

L/ = liquid flow rate, kg / m2- s

d = equivalent diameter of the packing, mm

H = hold up in m3 liquid per m3 of the column

So when the 25 mm Rasching rigs are used with L/ = 1.0 kg /m2-s and d = 20 mm, then

H = 0.021 m3/ m3 of column.

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