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Part (1)

Generally, if the absorption factor A = L/mV is greater than 1 for a component, then any degree

of separation can be achieved. The larger A is the fewer stages (or trays) are required to

achieve a given level of separation, although the absorbent flow rate may become too large.

Absorption The removal of one or more selected components (absorbate) from a mixture of gases by absorption into a suitable liquid (absorbent) through a gas-liquid interface. Stripping Reverse of absorption.

Liquid Out

A= 15 mole

Gas In

Pure Liquid In

A= 0 mole

Gas Out A= 5 mole

A= 20 mole

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Gas Treating in Major Industrial Processes

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Liquid in

Vapor in

Vapor out

Liquid out

1

2

N–1

N

Trayed Tower

Liquid in

Vapor in

Vapor out

Liquid out

Spray Tower

Liquid in

Vapor in

Vapor out

Liquid out

Packed Column

Countercurrent contact of

liquid and vapor on a

series of metal trays or

plates

A vessel containing one or

more sections of a packing

material over whose

surface the liquid flows

downward by gravity, as a

film or as droplets

between packing elements

Used when one or two

theoretical stages are

required, where a very low

pressure drop is allowed

and the solute is very

soluble in the liquid phase

Liquid in Vapor out

Vapor in

Liquid out

Centrifugal Contactor

Liquid in

Vapor in

Vapor out

Bubble Column

Liquid out

A column which is partially filled with liquid

into which the vapor is bubbled

• Used when one or two stages are required

• Vapor pressure drop is high

• Used when the solute has a very low

solubility in the liquid

Consists of stationary ringed housing

intermeshed with a ringed rotating section

• Liquid is fed near the centre of the packing,

from which it is caused to flow outward by

centrifugal force.

• This result in a very high mass transfer

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Liquid in

Vapor in

Vapor out

Liquid out

1

2

N–1

N

Trayed Tower

The function of a tray is to facilitate contact

between the vapour phase and liquid phase so

that mass transfer between the 2 phases can

take place.

Weir

Plate

Foam

Phase Contact on a Contacting Tray

Each tray can be considered as being made

up of 3 sections: weir, bubbling area, and

downcomer.

The bubbling area is the place where vapour-

liquid contact takes place.

The function of a weir is to maintain a

desired liquid level on the tray.

Downcomers are used to guide liquid flow

from an upper tray to a lower tray.

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slots cap

riser plate

plate

cap

leg

plate

Three type of tray openings: perforation,

valve cap and bubble cap.

Sieve tray:

simplest

• 1/8 to ½ in in diameter

• called sieve tray or

perforated tray

Valve tray:

• has much large opening

• 1.5 to 2 in in diameter

• As vapor rate increased, the

valve rises, providing a larger

peripheral opening for vapor

to flow into the liquid to

create froth

Bubble-cap tray:

• consists of bubble caps 3-6

in in diameter

Sieve Tray

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Bubble-Cap Tray

The bubble-cap consists of a slotted cap on a central riser.

Valve Tray

A valve tray is a flat perforated plate, with each

perforation fitted with a movable disk (the

"valve").

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Flow weeps through

the holes at low vapor

velocity

Weeping

Liquid cannot get

down the column

at high vapor velocity

Flooding

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Used for continuous countercurrent

contacting of gas and liquid in absorption

as well as vapor-liquid contacting in

distillation

Consists of:

1. Cylindrical column

2. Distributing space at the bottom

3. Distributing device at the top

4. Gas outlet at the top

5. Liquid outlet at the bottom

6. Packing material

Operation:

Why packing? To provide large area of intimate contact between liquid and gas

Gas enters the distributing space at the

bottom below the packed section and rises

upward to contact the descending liquid

Clean gas out

Dirty gas in

Mist Eliminator

Liquid Sprays

Packing

Liquid outlet

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Packing Materials

Random packings are dumped into a column

during installation and allowed to fall in

random

Various types of packings made of different types of materials of construction are

available, and both random and structured packings are commonly used

Structured packings are considerably more

expensive per unit volume than random

packings. They come with different sizes and

are neatly stacked in the column. Structure

packings usually offer less pressure drop and

have higher efficiency and capacity than

random packings.

Packing Materials

Many of them are available, but most common ones are used in the industry

(a) Raschig ring (b) Berl saddle

(c) Pall ring (d) Intalox metal Jaeger metal Tri-Pack

Older types and seldom used Second generation

Made of plastic or metal

Void space 0.9-0.96

Area 100-200 m2/m3

Third generation

Combination of Pall ring and Berl saddle

Porosity: 0.95-0.98

Only slightly more efficient than Pall ring

Commercial packing are available in sizes of 3 – 75 mm

Mostly made of clay, porcelain, metal, or plastic.

Packing should have the following properties:

should be inert must be strong should provide good contact

high porosity to avoid high pressure drop low cost

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Channeling: It is flow of liquid in streams within empty space

Occurs at low liquid flow rates and when the gas or liquid flow is much greater at

some points than at others

In here, the packing is not very well wetted and dry spots appear on the packing

surface and the rest of liquid flows through the openings which will collect as a

stream

Results in low efficiency of the tower

Effect of channeling can be minimized by making:

tower diameter 8 × packing diameter

Entrainment:

It is the drag of liquid by gas to the upper part

More likely occur at large gas velocity

Mesh, or mist collector, is placed above the liquid inlet

Packed Columns vs. Plate Columns

1. Conditions Favoring Packed Columns:

Small-diameter columns (less than 0.6m)

More choices in materials of construction

for packings especially in corrosive service

(e.g. plastic, ceramic, metal alloys)

Lower pressure drop (important in vacuum

distillation)

Less liquid entrainment · low liquid hold-

up, especially suitable for thermally

sensitive material

Foaming liquids can be handled more

readily (less agitation of liquid by the

vapour)

2. Conditions Favouring Plate columns:

Columns of large diameters and towers

that have more than 20 to 30 stages.

Variable liquid and/or vapour loads

Low liquid rates and high liquid

residence time

Dirty service (plate columns are easier

to clean)

Presence of thermal or mechanical stress

due to large temperature changes which

might lead to cracked packings

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Columns Process Design: Four Basic Principles Extent of separation

# of equilibrium stages, predicted with high confidence

Graphical methods

Approximate methods - Kremser equation

Rigorous methods - computer simulation

Time of phase contact

Rate = (Driving force)/Resistance

Driving force = departure from equilibrium

Resistance (difficult to predict)

operating conditions: T, P, composition

physical properties of both phases

velocity, flow regime

Columns Process Design: Four Basic Principles

Permissible pressure drop

Fixes diameter of column

Energy requirements

Heat

producing temperature changes

creating a new phase

overcoming heat of solution effects

Mechanical/Electrical

moving fluids from one location to another

dispersing liquids and gases

operate moving parts of machinery

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1. Entering gas or liquid flow rate, composition, temperature and pressure

2. Degree of separation desired

3. Choice of absorbent (or stripping) agent

4. Operating pressure and temperature and allowable pressure drop

5. Minimum absorbent (or stripping) flow rate

6. Number of equilibrium stages

7. Heat effects and need for cooling

8. Type of absorber (stripper) equipment

9. Height of absorber (stripper)

10. Diameter of absorber (stripper)

General Design Considerations …

For an absorber (in general)

Operating pressure should be high and temperature low

to minimize stage requirements and/or absorbent flow rate and

to lower the equipment volume required to accommodate the gas flow.

However:

both compression and refrigeration of a gas are expensive.

Then:

the most absorbers are operated at feed-gas pressure, which may be greater

than ambient pressure and ambient temperature.

which can be achieved by cooling the feed gas and absorbent with cooling

water, unless one or both streams already exists at a sub-ambient temperature.

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General Design Considerations …

For a stripper (in general)

Operating pressure should be low and temperature high

to minimize stage requirements and/or stripper agent flow rate.

However:

Maintenance of a vacuum is expensive.

Then:

Commonly stripper are operated at a pressure just above ambient pressure

and ambient temperature

A high temperature can be used, but it should be not so high as to cause

undesirable chemical reactions.

Operating temperature and pressure must be compatible with the

necessary phase conditions of the streams being contacted.

Flooding velocity: it the upper limit of the gas flow where the liquid flows over at a given

flow of liquid

At normal conditions, with low gas velocity, the liquid flows downward through the

packing without influencing the upward gas flow.

Loading point: it is the gas flow rate when the gas starts to hinder the liquid downflow

Above this limit, tower is not functioning

local accumulation or pools of liquid start to appear in the packing

Pressure drop of the gas starts to rise at a faster rate

At the loading point, the liquid can no longer flow down through the packing and is

blown out with the gas

In actual operation: gas velocity is well below flooding, half flooding velocity

Loading: the liquid flow is reduced due to the increased gas flow; liquid is held in the

void space between packing

Flooding: the liquid stops flowing altogether and collects in the top of the column due to

very high gas flow