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Professor Xijun Hu CENG 5210 Advanced Separation Processes
1
CBME 5210 Advanced Separation Processes
Instructor: Prof. Xijun Hu (Room 4559; Tel: 2358 7134;
Email: kexhu; web: http://kexhu.people.ust.hk)
TA: Mr. Xuanhao Mei, Lab 7104, email:
Aims:
This subject is to enable students to understand the principles and
processes of adsorption, chromatography and membrane
separation; to design an adsorber, a membrane unit to achieve a
specified separation; to optimize a chromatographic system.
Textbook:
D.D. Do, “Adsorption Analysis: Equilibria and Kinetics”,
Imperial College Press, 1998.
References: R.T. Yang, “Gas Separation by Adsorption Processes”, Butterworths,
Boston, 1987.
D.M. Ruthven, “Principles of Adsorption and Adsorption
Processes”, John Wiley & Sons, New York, 1984. S.D. Faust and O.M. Aly, “Adsorption Processes for Water
Treatment”, Butterworths, Boston, 1987.
R.L. Grob, “Modern Practice of Gas Chromatography”, 3rd ed., John
Wiley & Sons, Inc., New York, 1995.
C.J. Geankoplis, “Transport Processes and Unit Operations”, 3rd ed.,
Prentice Hall, Englewood Cliffs, New Jersey, 1993.
Assessments:
Assignments: 20% Final Exam: 80%
Professor Xijun Hu CENG 5210 Advanced Separation Processes
2
What to do if you have questions/problems:
Email me: [email protected]
Visit me at my office: Room 4559
Contact the TA
Encourages:
Ask & answer questions in the class
Discuss the course materials & homework after class
Preview the course materials before the class
Disciplines:
Turn off all mobile phones in the class
No talks between students in the class
Do not copy other’s homework (both people copied &
being copied will be penalized)
Do not cheat in the examinations
Professor Xijun Hu CENG 5210 Advanced Separation Processes
3
Lecture Outlines Week Lecture Content
2 Introduction Adsorption processes - why?
how?
Practical adsorbents;
Forces of adsorption
2,3 Adsorption
Equilibrium
(single
component)
Ideal Langmuir and BET
models;
Gibbs adsorption isotherm and
related models;
Dubinin - Polanyi theory
4 Practical
Approaches of
Pure Component
Adsorption
equilibrium
Energy distribution;
Pore size distribution
5 Adsorption
Equilibrium
(multi-
component)
Extended multicomponent
Langmuir;
Ideal adsorbed solution theory
(IAST)
6 Adsorption
kinetics
- fundamentals
Continuum diffusion;
Knudsen diffusion;
Surface diffusion
7 Adsorption
Kinetics
in a single
particle
Resistances to mass transfer in
practical systems;
Principles of diffusion in
porous media;
Uptake rate in batch systems
Professor Xijun Hu CENG 5210 Advanced Separation Processes
4
Experimental measurement of
intraparticle diffusivities
8 Adsorption
dynamics:
bed profiles and
breakthrough
curves
Equilibrium theory: analogy
with kinematic wave equation
Breakthrough curve;
Bohart-Adams model (Bed
Depth-Service Time, BDST)
9 Cyclic Gas
Separation
Processes by
Adsorption
Regeneration methods;
Thermal swing, pressure swing
and displacement systems;
Skarstrom Cycle
10 Membrane
Processes
Introduction, classification of
membrane processes;
Dialysis, gas permeation;
Flow patterns
11 Membrane
Processes
Reverse osmosis, ultrafiltration
12 Chromatographic
Separation
Basic principles
Optimizing separations in gas
chromatography
13 Final
Examination
7 May
Professor Xijun Hu CENG 5210 Advanced Separation Processes
5
Adsorption
In adsorption processes one or more components of a gas
or liquid stream (adsorbate or solute) are adsorbed on the
surface of a solid (adsorbent) and a separation is
accomplished.
Adsorption vs Distillation
Distillation has advantages of simplicity and scalability
so it is a standard against which other processes are
measured. However, distillation is an energy-intensive
process. The ease of separation by distillation is
determined by the relative volatility, which for an ideal
binary mixture is simply the ratio between the vapor
pressures. If the relative volatility becomes close to
unity, the number of stages and reflux ratio required to
achieve the specified separation will be much higher, and
so the costs of equipment & energy.
Professor Xijun Hu CENG 5210 Advanced Separation Processes
6
The analogous parameter for adsorption is the separation
factor defined by:
iji i
j j
X Y
X Y
/
/
where Xi and Yi are the equilibrium mole fractions of
component i in the adsorbed phase and fluid phase,
respectively.
The analogy is purely formal, there is NO quantitative
relationship between the separation factor and relative
volatility. For two given components the relative
volatility is fixed whereas the separation factor varies
widely depending on the adsorbent.
When will adsorption be favored?
Relative volatility is less than 1.5, such as the
separation of isomers, where the separation factor
for adsorption is infinity by using zeolites.
The bulk of feed is a low-value, more volatile
component, the product is in low concentration so
a large reflux ratio is required.
The two groups of components have overlapping
boiling ranges.
A low temperature and a high pressure are required
for liquefaction.
For small to medium throughput (<50,000 m3/h).
Professor Xijun Hu CENG 5210 Advanced Separation Processes
7
Categorizations of adsorptive separation processes
Based on method of adsorbent regeneration
- TSA (Temperature Swing Adsorption):
regenerated by heating
- PSA (Pressure Swing Adsorption):
regenerated by lowering the pressure
Based on feed composition
- Bulk separation: 10 weight percent or more of
the mixture is adsorbed.
- Purification: almost all impurities, which are
usually less than 10%, are adsorbed, pure gas at
purities higher than 99.999% can be obtained.
Based on mechanism of separation
- Steric effect: by using molecular sieve, only
small and properly shaped molecules can
diffuse into the adsorbent, other molecules are
totally excluded.
- Kinetic effect: by the differences in diffusion
rates of different molecules.
- Equilibrium effect: by the equilibrium
adsorption of the mixture.
Professor Xijun Hu CENG 5210 Advanced Separation Processes
8
Representative commercial gas adsorption
separation
Separation Adsorbent I. Gas bulk separation
Normal paraffins/iso-paraffins,
aromatics
N2/ O2
O2/ N2
CO, CH4, CO2, N2, A, NH3/H2
Acetone/vent streams
C2H4/vent streams
Zeolite
Zeolite
Carbon molecular sieve
Zeolite, activated carbon
Activated carbon
Activated carbon
II. Gas purification
H2O/olefin-containing cracked
gas, natural gas, air, synthesis
gas, etc.
CO2/C2H4, natural gas, etc.
Organics/vent streams
Sulfur compounds/natural gas,
hydrogen, liquefied petroleum
gas (LPG), etc.
Solvents/air
Odors/air
NOx/N2
SO2/vent streams
Hg/chlor-alkali cell gas effluent
Silica, alumina, zeolite
Zeolite
Activated carbon, others
Zeolite
Activated carbon
Activated carbon
Zeolite
Zeolite
Zeolite
Professor Xijun Hu CENG 5210 Advanced Separation Processes
9
Adsorbents
Good adsorbents should have
high surface area or micropore volume (for large
adsorption capacity)
large pore network for the transport of molecules to
the interior (for fast kinetics)
The porous solid must have small pore size (micropore)
with a reasonable porosity to satisfy the first
requirement and have a network of large pore size
(macropore) for the second requirement.
Micropore: d < 2 nm
Mesopore: 2 < d < 50 nm
Macropore: d > 50 nm
Professor Xijun Hu CENG 5210 Advanced Separation Processes
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Adsorbent Manufacturing
method
Surface
area
(m2/g)
Pore
diamete
r (Ao
)
Usage
Activated
carbon
by thermal
decomposition
of coal, wood,
vegetable shells,
etc.
300-
2000
10-60 Removal of
organic
vapors;
H2
purification
Silica gel by acid
treatment of
sodium silicate
solution & then
dried
340-
800
20-140 dehydrate
gases &
liquids;
fractionate
hydrocarbon
Activated
alumina
hydrated
aluminum oxide
is activated by
heating to drive
off the water
200-
500
20-140 drying;
gas chromatograph
y
Molecular
sieve
zeolites
porous
crystalline
aluminosilicate
containing
precisely
uniform pores
300-
1200
3-10 drying,
separations
based on
molecular
size &
shape
Professor Xijun Hu CENG 5210 Advanced Separation Processes
11
Properties to characterize adsorbents:
Specific pore volume (cm3/g).
Specific surface area (m2/g).
Particle density(g/cm-3).
Average pore diameter (Ao ).
Pore size distribution
Professor Xijun Hu CENG 5210 Advanced Separation Processes
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Physical Adsorption Chemisorption
Low heat of adsorption
(<2 or 3 times latent heat
of evaporation)
High heat of adsorption
(>2 or 3 times latent heat
of evaporation)
Non specific Highly specific
Monolayer or multilayer.
No dissociation of
adsorbed species.
Only significant at
relatively low
temperatures.
Monolayer only
May involve dissociation.
Possible over a wide range
of temperature.
Rapid, non-activated,
reversible.
No electron transfer
although polarization of
sorbate may occur.
Activated, may be slow
and irreversible.
Electron transfer leading to
bond formation between
sorbate and surface.
Professor Xijun Hu CENG 5210 Advanced Separation Processes
13
Forces and Energies of Adsorption
1. Van der Waals forces (Dispersion-Repulsion).
This is always present. By combining the attraction
potential and the repulsion between two isolated
molecules, we have the Lennard-Jones potential
function:
4
12 6
r r
which is sketched in the following figure. The force
constants and are characteristics of the particular
molecular species and available in the literature.
2. Electrostatic interactions (polarization, dipole, and
quadrupole).
This is significant only for adsorbents having ionic
structure.
Professor Xijun Hu CENG 5210 Advanced Separation Processes
14
Entropy changes
Physical adsorption from the gas phase is
exothermic, why? Let us look at the thermodynamic
argument.
Since the adsorbed molecules is more regular,
so the disorder degree is lower
entropy is lower 0 gasads SSS
For the adsorption to happen, the free energy
change, G, should be negative, hence
0 STHG
H < 0
adsorption is exothermic
Professor Xijun Hu CENG 5210 Advanced Separation Processes
15
Adsorption equilibrium isotherm
(Single component)
This is the equilibrium relationship between the
concentration of a solute in the fluid phase and its
concentration on the solid phase at a given
temperature. Most isotherms can be classified
into five types, as shown below.
Professor Xijun Hu CENG 5210 Advanced Separation Processes
16
The numerous isotherm models are based on three
different approaches:
1. The Langmuir approach: This was given by
Langmuir in 1918, assuming the adsorption system is
in dynamic equilibrium, where the rate of evaporation
is equal to that of condensation. It is the most useful
for data correlation in separation processes.
2. The Gibbs approach: This one employs the Gibbs
adsorption isotherm:
0 dnAd s (1)
where is the spreading pressure, A is the surface
area, ns is the number of moles, and is the chemical
potential. An integration of the Gibbs equation results
in the desired isotherm. By assuming one equation of
state (like the ideal gas law), a corresponding isotherm
can be obtained.
3. The potential theory: First formalized by Polanyi in
1914, the adsorption system is viewed as a gradual
concentration of gas molecules toward the solid
surface due to a potential field.
Professor Xijun Hu CENG 5210 Advanced Separation Processes
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Adsorption at low coverage:
Linear isotherm (Henry's law)
q Kc or q = K' p (2)
where q is the adsorbed concentration, c is the
solute concentration and p is the gas pressure. K
is called the Henry constant. This linear isotherm
is not common, but many systems follow this
relationship in the dilute region.
From the ideal gas law, K=K’RT.
The temperature dependence of the Henry
constant follows the vant Hoff equation:
20
20 ln
;'ln
RT
U
dT
Kd
RT
H
dT
Kd
(3)
where H0 and U0 are the changes in enthalpy
and internal energy during adsorption.
Irreversible (rectangular) isotherm
The adsorbed amount is independent of the solute
concentration (q = constant). It happens when the
adsorption affinity is extremely high (large
molecules).
Professor Xijun Hu CENG 5210 Advanced Separation Processes
18
Isotherms based on the Langmuir approach
The Langmuir isotherm is the simplest and still
the most useful equilibrium equation for both
chemical and physical adsorption. It is based on
the following assumptions:
1. Adsorption is at fixed number of definite,
localized sites.
2. Each site can hold only one adsorbate molecule
(monolayer).
3. All sites are equivalent.
4. No interaction between adsorbed molecules,
even on adjacent sites.
Professor Xijun Hu CENG 5210 Advanced Separation Processes
19
The Langmuir equation can be derived by the
kinetics of condensation and evaporation of gas
molecules at a unit solid surface. If is the
fraction of the solid surface covered by a
monolayer of adsorbate, then the rate of
evaporation from the surface (desorption) is
proportional to (i.e., kd). Similarly, the rate of
condensation of gas molecules onto the surface
(adsorption) is proportional to the fraction of free
sites remaining, (1- ), and the absolute gas
pressure, p, (i.e., kap(1- )). Equilibrium is
established when these two rates are equal.
kap(1-)=kd (4)
where ka and kd are the rate constants for
condensation (adsorption) and evaporation
(desorption). By rearranging Eq. (4), the fraction
of surface covered, , is obtained as:
bP
bp
Pkk
pk
ad
a
1 (5)
Professor Xijun Hu CENG 5210 Advanced Separation Processes
20
The parameter, b=ka/kd, is called affinity constant
& related to the heat of adsorption (Q) by
)/exp(0 RTQbb (6)
where b0 is a constant, R is the gas constant and T
the temperature in Kelvin. Since adsorption is
exothermic (Q > 0) b should decrease with
increasing temperature. The higher value of b, the
stronger the adsorption.
Eq. (6) can be used to calculate the heat of
adsorption. From the assumptions of identical
sites and no interaction between adsorbed
molecules, the heat of adsorption should be
independent of coverage ().
In general, equation (5) can be written as:
q qbp
bpq q
bc
bcs s
1 1
, or (7)
where q is the adsorbed phase concentration, qs is
the maximum (saturation) adsorption capacity, so
Professor Xijun Hu CENG 5210 Advanced Separation Processes
21
=q/qs. The parameters qs and b are obtained by
fitting the Langmuir model to experimental data.
The Henry constant is
obtained by taking the
limit p 0:
sp
bqp
qK
lim
0' (8)
By rearrangement of
equation (7) we get
pbqqq ss
1111 (9)
Therefore if we plot
1/q vs. 1/p, a linear
line should be
obtained with a slope
of 1
bqs
, an intersect on y-axis of 1
qs
, and an
intersect on x-axis of -b.
q
p
low T
high T
1/p
1/q
-b
1/q slope=1
bqs s
0
Professor Xijun Hu CENG 5210 Advanced Separation Processes
22
Langmuir isotherm on nonuniform surfaces: Because Langmuir isotherm doesn’t well fit the data
for the whole pressure region, other more rigorous
models have been developed.
Freundlich isotherm If the adsorption sites are not identical, the total
adsorbed amount is summed over all types of sites.
When an exponentially decaying energy distribution is
assumed, the Freundlich isotherm is obtained, which
was originally an empirical equation:
mn KpqorKpq /1 (10)
where K and n (m) are constants determined
experimentally. The isotherm is favorable when n<1,
linear if n=1, and unfavorable if n>1.
q
p
n=1 n<1
n>1
0
lnp
lnq
lnK
0
slope=n
Professor Xijun Hu CENG 5210 Advanced Separation Processes
23
Taking logarithm on both sides of equation (10) to give
Kpnq lnlnln (11)
So by plotting lnq vs. lnp we should obtain a linear
line with the slope being the exponential constant n,
and the intersect on y-axis being lnK.
Sips (Langmuir-Freundlich) isotherm
This is the hybrid Langmuir-Freundlich isotherm, it
takes into account the fragmentation of the
molecule so one molecule occupies 1/t sites.
t
t
sbp
bpqq
1 (12)
It is a three parameter isotherm (qs, b, t).
Unilan isotherm (UNIform energy distribution
& local LANgmuir equation)
s
be p
be p
ss
s
2
1
1ln (13)
Three parameters are qs, b, s.
Toth isotherm
q q
b p
bps
t
t t
1
11
/
/ (14)
Three parameters are qs, b, t.
Professor Xijun Hu CENG 5210 Advanced Separation Processes
24
Multilayer adsorption:
BET (Brunauer-Emmett-Teller) isotherm
smonomonos p
p
cq
c
cqppq
p 11
)( (15)
where qmono is the monolayer saturation
concentration and ps is the saturation vapor
pressure. By using experimental data in the range
of p/ps = 0.05 to 0.35, the left-hand side of
equation (15) is plotted against the relative
pressure (p/ps), the values of qmono and k can be
obtained from the slope and intercept of the linear
line. Knowing the molecular area (eg., 16.2
Ao
2
/molecule for nitrogen at 77K), the value of the
surface area can be calculated directly from qmono.
0 p/p
p
q(p -p)
slope=mono
s
s
q1 c-1
monoqc c
Professor Xijun Hu CENG 5210 Advanced Separation Processes
25
Surface areas from the BET & Langmuir
equations
When qmono is obtained, we can calculate the
specific surface area of the adsorbent by the
following equation:
Sq g g solid)N A
M g molt
mono
w
(m / g solid)2 ( /
( / )
0 (16)
where Mw is the molecular weight, N0 is
Avogadro’s number (6.023x1023/mol), and A is
the surface area of one molecule, which is
16.2x10-20m2, for N2 at 77K.
The BET isotherm is only valid in the range of
p/ps = 0.05 to 0.35, beyond this region, BET is not
a good model because of the capillary
condensation (p/ps > 0.3) or the system fails to
form multilayer adsorption (p/ps < 0.05).
There is also a Langmuir surface area calculated
by substituting qmono with the Langmuir maximum
adsorption capacity parameter qs.
Professor Xijun Hu CENG 5210 Advanced Separation Processes
26
Isotherms based on the Gibbs approach
From thermodynamics, we have the Gibbs-Duhem
equation at equilibrium:
0 ndAdSdT (17)
where A is surface area, is spreading pressure,
and n is the number of moles of adsorbate per unit
volume of adsorbent.
The spreading pressure is a measure of the
tendency of a liquid phase
to spread (complete wetting) on a second, liquid
or solid phase. The spreading pressure S is the
difference between the work of
adhesion W1,2 between the phases and the work of
cohesion W1,1 of the phase under consideration:
S = W1,2 - W1,1
Equally, the spreading pressure can be expressed
as the difference between the surface
tensions σ1and σ2 and the interfacial tension σ1,2:
S = σ2 - σ1 - σ1,2
If the spreading pressure is positive, the phase
under consideration spreads; if it is negative,
wetting is not complete.
Professor Xijun Hu CENG 5210 Advanced Separation Processes
27
At constant temperature, Eq. (17) yields the Gibbs
adsorption isotherm:
constant)=(T 0 ndAd (18)
At equilibrium, the chemical potential of the
adsorbed phase is equal to that of the gas phase:
pRTgas ln0 (19)
Tconstant at )(ln pRTdd (20)
RTA
n
pd
d
T
ln
(21)
This is the relationship between the adsorption
isotherm and the corresponding equation of state.
: Joule/m5; A: m2; n: mol/m3;
R: (Joule)/(mol K)
By integrating Eq. (21), the spreading pressure, ,
is found:
p dpp
q
A
RT0 (22)
Henry’s law:
For the ideal gas law, pV= nRT, so A= ns RT.
RT
A
n
pd
d s
Tln (23)
pKpdd
' );(ln
(24)
Professor Xijun Hu CENG 5210 Advanced Separation Processes
28
RT
AKKp
RT
pAK
RT
Anq s
'=K ;
'
(25)
which is the Henry’s law.
Volmer isotherm:
If the equation of state is taken as:
nRTA (26)
where is the area occupied by n molecules. This
is analogous to P(V-b)=nRT:
2
A
nRT
d
d
T
(27)
From the Gibbs isotherm [Eq. (21)], we have
2
A
Ad
p
dp (28)
22
)()(])[(
A
Ad
A
Ad
A
dA
p
dp (29)
Integrate to get
AAp )ln(ln or
AAp )](ln[
AAp exp)( (30)
Professor Xijun Hu CENG 5210 Advanced Separation Processes
29
The fractional loading, , is simply the area
occupied by n molecules divided by the total area:
A
(31)
Eq. (30) becomes
1exp
)1(p (32)
Let b=, we have
1exp
1bp (33)
This is the Volmer isotherm to describe the
adsorption on surfaces where the mobility of
adsorbed molecules is allowed, but no interaction
among the adsorbed molecules.
Professor Xijun Hu CENG 5210 Advanced Separation Processes
30
Isotherms based on the potential theory
Dubinin-Polanyi theory:
For the adsorbed fluid under the pressure of
adsorption forces, the free energy change is
G RTf
fRT
p
p
s
s
ln ln (34)
is a function of the volume of adsorbed fluid
(W). It is independent of temperature for
dispersion forces:
T W
0 (35)
W W 02exp (36)
where W0 is the specific micropore volume.
The rearrangement of Eq. (36) gives the Dubinin-
Radushkevich equation:
q q Dp
ps
s
exp ln
2
(37)
where qs, D are the model parameters, and ps is
the saturation vapor pressure which can be found
from literature or also as a parameter to be
optimized (extracted) from experimental data.
Professor Xijun Hu CENG 5210 Advanced Separation Processes
31
Heat of adsorption
The isosteric heat of adsorption (-H) is
determined from the Clausius-Clapeyron equation
applied to adsorption isotherm:
2
ln
RT
H
T
p
q
(38)
Integrate assuming H independent of T,
tconsRT
Hp tanln
(39)
A plot of lnp vs 1/T at constant q gives a linear
line with a slope of H/R.
For the Langmuir isotherm, we have
s
s
q
bp
dT
dq
RT
Q
RT
H
122
(40)
where we allow the maximum adsorbed capacity,
(qs) to vary with temperature.
s
s
q
bp
dT
dqRTQH
12
(41)
However, the isosteric heat of adsorption (-H) is
constant for the Langmuir equation if the
saturation capacity is independent of temperature,
but changes with the surface loading for other
isotherms.
Professor Xijun Hu CENG 5210 Advanced Separation Processes
32
Measurement of adsorption isotherm
(Batch adsorption)
The adsorption isotherm is often obtained by
using a batch reactor. In liquid adsorption, this is
conducted in a stirred tank. A certain volume of
liquid (V) with known concentration (c0) is fed
into the tank, then a known mass of adsorbent
particles (m) is added to the solution. After
sometime (usually overnight to a few days,
depending on the particle size) the system
becomes steady state. The final concentrations in
the liquid phase (cf) and in the solid phase (qf) are
in equilibrium. By taking a material balance we
have
q m c V q m c Vf f 0 0 (42)
So qf can be calculated as
qq m c V c V
mf
f 0 0 (43)
Now we get one isotherm point (cf, qf), more
isotherm data can be obtained by changing
relative amount of solution and solid, or the initial
liquid concentration and repeat the above
procedure.
Professor Xijun Hu CENG 5210 Advanced Separation Processes
33
Example: A wastewater solution having a volume
of 1.0 m3 contains 0.21 kg phenol/ m3 of solution
(0.21 g/L). A total of 1.40 kg of fresh granular
activated carbon is added to the solution, which is
then mixed thoroughly to reach equilibrium.
Using the isotherm in the figure below, what are
the final equilibrium values, and what percent of
phenol is extracted?
Professor Xijun Hu CENG 5210 Advanced Separation Processes
34
Solution 1: The given values are c0=0.21 kg
phenol/ m3, V=1.0 m3, m=1.40 kg carbon, and q0
is assumed zero. Substituting into eq. (11)
qf(1.40)+ cf(1.0)=0(1.40)+0.21(1.0) This is a linear line, which is plotted in the figure
below together with the isotherm. The equilibrium
values are obtained from the intersection:
cf=0.062 kg phenol/ m3
qf=0.106 kg phenol/kg carbon
The percent of phenol extracted is
% extracted
c c
cf0
0
1000 210 0 062
0 210100 70 5( )
. .
.( ) .
Professor Xijun Hu CENG 5210 Advanced Separation Processes
35
Solution 2: From the mass balance equation (eq.
20), we have
qf(1.40)+ cf(1.0)=0(1.40)+0.21(1.0)
1.4qf+ cf=0.21
Fitting the Freundlich equation to the isotherm
data, we obtain
qf = 0.194cf1/4.4848
Solving the mass balance and isotherm equations
together, we get the solution.
cf=0.062 kg phenol/ m3
qf=0.106 kg phenol/kg carbon
The percent of phenol extracted is
% extracted
c c
cf0
0
1000 210 0 062
0 210100 70 5( )
. .
.( ) .
Professor Xijun Hu CENG 5210 Advanced Separation Processes
36
Volumetric measurement rig for gas
adsorption isotherm
It has two separate chambers. One has a supply
bomb and the other contains an adsorption cell.
Each chamber has a transducer & a thermocouple
to record the pressure & temperature. The volume
of each chamber is also known.
Professor Xijun Hu CENG 5210 Advanced Separation Processes
37
Before an equilibrium experiment starts, a
molecular drag pump is used to evacuate the
system to 10-5 mmHg. The operation procedure is
described below.
1. Activated carbon was crushed to about 0.1
mm and dried in an oven at 200oC for three hours
to remove excess moisture. The carbon was then
weighted and loaded into the adsorption cell.
2. The whole system was first evacuated to
vacuum using an Alcatel molecular drag pump and
the activated carbon particles in the adsorption cell
were heated up to 300oC and kept at this
temperature and under vacuum for overnight to
degas any possible adsorbed species.
3. Pure gas was then dosed into the supply
bomb and the pressure and temperature were
recorded when constant readings were reached.
Professor Xijun Hu CENG 5210 Advanced Separation Processes
38
4. A small amount of pure gas was dosed
from the supply bomb into the adsorption cell
which was kept isothermal by using a water bath.
After the pressure in the adsorption cell becomes
constant (typically about two hours), the pressures
and temperatures in both supply bomb and
adsorption cell were recorded.
5. The amount adsorbed which was in
equilibrium with the pressure in the gas phase of
the cell was calculated from the amount supplied
from the supply bomb via the ideal P-V-T
relationship.
6. Steps 4 and 5 were repeated to get the
equilibrium isotherm data at higher pressures until
a full isotherm curve was obtained.