BY

DR YUSUF R.O & DR O.A.A. ELETTA

Department of Chemical Engineering

SEPARATION PROCESS II

COURSE OUTLINE

Gas Absorption, Solvent Extraction, Multicomponent gas absorption, Extractive and azeotropic distribution, Evaporation, Multiple effect evaporator, Adsorption, Crystallization, Ion Exchange, Reverse Osmosis, Membrane separation Processes. Core; 3 Credits Pr CHE 442

Recommended Texts

Chemical Engineering Volume II by Richardson and Coulson

Unit Operations of Chemical Engineering by Mc Cabe and Smith

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THINGS TO NOTE

Note: Attendance is compulsory at lectures. Only students

with minimum of 75 % attendance will be allowed to sit for

exams

Assessment

CA comprising Tests and Quizzes 30%

Examination 70%

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GAS ABSORPTION

Introduction

It is a mass transfer operation in which a

soluble vapour is absorbed from its mixture

with an inert gas by means of a liquid in which

the solute gas is more or less soluble.

Desorption or gas stripping is the reverse of

gas absorption. Comom apparatus employed

is the packed column

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It consists of a cylyndrical column equipped with a

gas inlet and distributing space at the bottom; a liquid

Inlet and distributor at the top; gas outlet at top and

liq outlet at the bottom; and a supported mass of

inert packings

Working of the absorption tower

The solute containing gas, enters the column from

the base of the column and flows up tru the

interstices in the packing counter – current to the

flow of the liquid ( with intimate contact)

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The solute in the gas is absorbed by the fresh liquid

entering the column and the lean gas leaves the top

of the tower. The enriched liquid flows down the

tower and is discharged at the bottom of the column

TYPES OF PACKING

3 principal types

•Randomly dumped: these are made of cheap, inert

materials (clay, porcelain, plastics), thin – walled

metals rings or aluminium. The packing allows

high void spaces and large passages for the fluids are

achieved by making the packing units unit irregular

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Or hollow.eg ceramic Berl saddles and Rashing

rings

•Pressure drop and limiting flowrates

•Stacked by hand

•Structured or ordered packing:: has ordered

geometry evolved

Definition of terms

•Channeling

•Flooding

•Loading Point

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Pressure Drop and Limiting Flow Rates

The fig below shows a typical data for the pressure

drop in a packed tower. The pressure drop per unit

packing depth comes from the fluid friction which is

plotted on logarithmic coordinates against the gas

flow rate Gy which is expressed in mass of gas per

hr per unit of cross sectional area based on the

empty tower. The flowrate is related to the

superficial gas velocity by the eqn

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For dry packing, the line so obtained is straight and

has a slope of about 1.8 which implies that the

pressure drop increases with the one eighth power

of the velocity. For wet packing, ( ie packing is

irrigated by a constant flow of liquid), the pressure

drop is greater than that in dry packing cos the liq

in the tower reduces the space available for gas

flow. At moderate gas velocities, the line for

irrigated packing gradually becomes steeper as, the

gas now impedes the down flowing lq and the liq

hold uo increases with the gas rate. The pont at

which liq hold up increas is called the loading point

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Increasing the gas velocity further leads to a more

rapid increase in pressure drop, the lines become

almost vertical.

For an operating column, the gas velocity must be

lower than the flooding velocity. As flooding is

approached, most of the packing surface is wetted

The choice of velocity must be far enough from the

flooding velocity to ensure a safe operation but not

Too low as to warrant a much larger column.

(pg 713)

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Flooding velocity depends strongly on the type and

size of packing and the liquid mass velocity.

Packing characteristics are accounted for by a

packing factor Fp which decreases with increasing packing

size or increasing void fraction.

Eg

A tower packed

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An empirical equation for the limiting pressure

drop is

------1

where

Fp = Packing factor and, it is dimensionless

eqn 1 can be used for packing factors from 10 to

60. For higher values of Fp, the pressure drop at

flooding can be taken as 2.0

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15

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Principles of Absorption

The diameter of a packed absorption column

depends on the quantities of gas and liquid handled,

their properties and the ratio of one stream to the

other.

The height of the tower and hence the total volume

of packing, depends on the magnitude of the desired

concentration changes and on the rate of mass

transfer per unit of packed volume. Calculations of

the tower height, therefore, rest on material

balances, enthalpy balances, and estimates of driving

force and mass – transfer coefficients.

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Material Balance over control volume gives

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Operating line equation

x and y rep the bulk compositions of the liq and gas,

resp in contact with each other at any given section

through the column.

Assumption

•The composition at any given elevation are

independent of position in the packing

The absorption of a soluble component from a gas

mixture makes the total gas rate V decrease as the

gas passes tru the column, and the flow of L increase

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This results in the operating line slightly curved

Limiting gas – liq ratio

Eqn 6 shows that the avg slope of the operating line

is L/V, ie the ratio of the molal flows of liq and gas

For a given gas flow, a reduction in liq flow

decreases the slope of the operating line.

Consider the line ab in the figure below. Assume

that the gas rate and the terminal concs xa, ya and

yb are held fast and the liq flow L decreased. The

upper end of the operating line and xb, the conc of

the strong liquor increases. The max possible liquor

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