BY DR YUSUF R.O & DR O.A.A. ELETTA Department of Chemical
Engineering SEPARATION PROCESS II Slide 2 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 Slide 3 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 Quizzes30% Examination70% 3 Slide 4 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 Slide 5 I t 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) 5 Slide 6 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 Slide 7 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 7 Slide 8
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 Slide 9 9 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 Slide
10 10 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) Slide 11 11 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 F p which decreases with increasing packing size or
increasing void fraction. Eg A tower packed Slide 12 12 Slide 13 13
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 Slide 14 14 Slide
15 15 Slide 16 16 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. Slide 17 17 Slide 18 18 Material
Balance over control volume gives Slide 19 19 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 Slide 20 20 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 Slide
21 21
