Separation process ii_2014__module_i
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Transcript of Separation process ii_2014__module_i
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
2
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%
3
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
4
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
6
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
8
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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
12
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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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18
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
21