İzmir Institute of Technology
Chemical Engineering Department
2008-2009 Spring Semester
CHE 310
CHEMICAL ENGINEERING LABORATORY I
Ion Exchange ın Packed Column
ION EXCHANGE IN PACKED COLUMN
Objectives:
-
to provide knowledge of ion exchange process ,
to investigate the dynamics of ion exchamge process in a packed bed,
to investigate the rate of ion exchange process .
Background:
Ion exchange is a reversible chemical reaction
wherein an ion from solution is exchanged for a
similarly charged ion attached to a solid particle. These
solid particles are ion exchange resins which contain
loosely held ions which are able to be exchanged with
other ions in solutions which come in contact with
them.
Conventional ion exchange resins consist of a
cross-linked polymer matrix with a relatively uniform
distribution of ion-active sites throughout the structure.
Ion exchange resins are classified as
 cation exchangers (exchangeable ions are cations),
 anion exchangers (exchangeable ions are anions).
A cation exchange resin with a negatively charged
matrix and exchangeable cations is shown in Figure 1.
Figure 1. Cation exchange resin with negatively
carged matrix and exchangeable cations
The size of the resin particles plays a role in ion
exchange processes. Smaller particles usually are more effective because of increased surface area, but
cause large head losses that drive up pump equipment and energy costs. Synthetic ion exchange resins are
usually cast as porous beads with considerable external and pore surface where ions can attach.
Temperature and pH also affect the effectiveness of ion exchange process, since pH is inherently tied to
the number of ions available for exchange, and temperature governs the kinetics of the process. Other
factors are the charge and hydrated radius of the ion. Generally ions with higher valency exchange for
those of lower valency. For example, Al3+ > Ca2+ > Mg2+ > K+  NH4+ > Na+. For ions of same charge, the
cation with smaller hydrated radius is selectively attached because it can move close to the site of the
exchangeable ion.
Ion exchange process can be accomplished by either a batch method or a column method. In the
batch method, the resin and solution are mixed in a tank, the exchange is allowed to come to equilibrium,
then the resin is separated from solution. The degree to which the exchange takes place is limited by the
preference the resin exhibits for the ion in solution. Because batch regeneration of the resin is chemically
inefficient, batch processing by ion exchange has limited potential for application.
Resin capacity is usually expressed in terms of equivalents per liter (eq/L) of resin. An equivalent
is the molecular or atomic weight of the molecule or ion divided by its electrical charge. For example, a
resin with an exchange capacity of 1 eq/L can remove 37.5 g of divalent zinc (Zn 2+, molecular weight of
65) from solution.
The applications of ion-exchange process vary from water purification to separation of various
antibiotics from fermentation broths. Ion exchange is applied for softening, deionization and desalination
of water. The food industry uses ion exchange in wine and sugar manufacture. Ion exchange also takes
place in living systems because cell walls, cell membranes, and other structures have charges.
In this experiment, NaCl slution will be passed through a column containing Dowex 50WX4 cation
resin in the form of spherical beads. This resin can take Na + ions from solution and return in the same
number of H+ ions, so that the solution maintains electrical neutrality. The reaction is as follows:
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Na L  H R  Na R  H L
(1)
where the subscripts L and R denote liquid and resin, respectively.
During ion exchange process in fixed bed, the concentration of the ions in the fluid and solid
phases change with time and position in the bed. The change in the effluent ion concentration with time
is defined as “breakthrough curve”.
As the NaCl solution passes through the column, it becomes more diluted in Na + and more
concentrated in H+, a change can be followed by sampling the H + concentration in the exit stream with a
pH electrode. Eventually, the H+ ion concentration in the effluent will be equal to that in the influent, i.
e. the resin becomes saturated with Na + ions and no further exchange occurs.
The exchange of a Na+ ion for a H+ ion is not instantaneous, since it takes time for a Na+ ion to
diffuse from the bulk of the liquid to the surface of the resin and then to some position inside the resin
where it can exchange for a H+ ion (and conversely for the H+ ion, which has to migrate back to the bulk
solution). The rate of exchange is expected to become slower as the Na+ ion concentration in the resin
approaches saturation. The present experiment attempts to find a correlation for the rate at which this
exchange process occurs.
The following analyses assume perfect mixing that is the ion concentrations in the liquid phase
and in the resin are uniform throughout the column. Also note that the rate of detachment of H + equals
the rate of attachment of Na+, namely dq/dt. Recognizing the rates of detachment from the resin, outflow
from the column, and accumulation in the liquid phase, a transient hydrogen-ion balance gives,
vho  V R
dq
dh
 vh  V L
dt
dt
(2)
Similarly, a transient sodium-ion balance gives,
vco  V R
dq
dc
 vc  V L
dt
dt
(3)
Differentiation of h plotted against t will give dh/dt. The rate of attachment, dq/dt, can then be found at
various times from equation (2). Next, equation (3) is rearranged to give the following differential
equation for c,
dc 1 
dq 

vco  c   VR 

dt VL 
dt 
(4)
which can be integrated numerically by Euler's method to give c as a function of time.
The appropriate manipulation and integration of equation (3) yields,
q
v
q   dq 
VR
0
t
 c
0
0
 c dt 
VL
c
VR
(5)
Thus, the concentration of Na+ ions in the resin (q) at any time t is v/V R times the shaded area in
the plot of c versus t (see Figure 1), less the amount retained in the liquid V Lc. In particular, the total
exchange capacity qo can be found by this method.
The ion exchange process is reversible, so that the resin can be regenerated by allowing it to
come in contact with a strong acid (HCl).
c
co
h
: Concentration of Na+ ions in solution leaving the column, meq/ml.
: Concentration of Na+ ions in solution entering the column, meq/ml.
: H+ concentration in the solution leaving the column, meq/ml.
2
ho
q
qo
t
v
VR
VL
: H+ concentration in the solution entering the column, meq/ml.
: Concentration of ions in the resin, meq/ml of resin.
: Total exchange capacity of the resin.
: Time, s.
: Volumetric flow rate of solution, ml/s.
: Volume occupied by the resin beads in the column, ml.
: Volume occupied by the liquid in the column, ml.
Figure 1. Breakthrough curve.
Materials:
A typical strong acid cation exchange resin, Dowex 50WX4 (Fluka) will be used in this experiment.
The resin is based on a styrene/divinylbenzene copolymer, “X4” denotes the degree of crosslinking, i. e.
4%. The functional groups are sulfonic acid that is bound to a porous polymer resin bead.
Figure 2. Experimental Set-Up
A schematic diagram of the experimental set up is shown in Figure 2. The set up consists of the
following parts:
- A peristaltic pump with fine adjustment 0-100 ml/min for water flow control and with fitting
suitable to be connected to the column
- Ion exchange resin, 20-50 mesh (300-840 m)
- A polypropylene column (1.47 cm internal diameter)
- A pH-meter at the column outlet
- A beaker for NaCl solution
- A beaker for HCl solution
- Waste beakers.
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Experimental procedure
-
-
Clamp the ion-exchange column to a buret support and place waste beaker underneath the
column.
Weight the resin to be packed into the column (wresin)
Make slurry of the resin and pour into the column until the resin settles.
Measure the bed height (hbed)
Calculate the volume of the packed bed (Vbed)
Measure the pH of the inlet NaCl solution (ho)
Using deionized water, adjust the speed-control setting on the peristaltic pump against the flow
rate (ml/sec) at which the ion exchange process will be carried out.
Try to keep solution over the resin at all times to prevent "channeling" in the column, a process
that creates paths for the solution to pass through the column without equilibrating with the resin
phase.
Switch off the pump.
Switch on the pump to pass the NaCl solution down through the bed.
Take the first effluent sample into a beaker and record the time.
Take the effluent samples into beakers at certain time intervals.
Measure the pH values.
Stop passing the NaCl solution until the pH of the exit stream shows negligible change.
Pour the resin on a vacuum filter and measure the volume of liquid contained in the bed (V L).
Then repacked the resin and regenerate it by passing the HCl solution until the pH of the exit
stream shows negligible change.
Pass deionized water until the free hydrochloric acid solution has been purged from the column
(rinsing).
Empty the regenerated resin from the column into a small beaker for storage.
Data Analysis:
-
Convert all pH values into concentrations h of H+ ion.
-
Plot h against t and comment on the shape of the curve.
-
Calculate the amount detached from resin for each run and discuss your results
-
By following the theory, determine values for dq/dt, c, and q, at several intermediate values of
time.
-
Calculate “breakthrough capacity” which is defined as the necessary bed volume of water passed
through the column until the influent and effluent H + ion concentrations are equal.
-
Compare the qo evaluated with the value given by the supplier of the resin.
Question for Consideration:
-
What kind of information can we get from breakthrough curve and how can we use the
information in operation of industrial ion exchange processes. Give an example.
-
Discuss the effects of thefollowing parameters on the shape of the breakthrough curve.
– resin bed height
– resin particle size
– effcolumn cross-sectional area
-
Can we use experimental results to design a column 8 m in diameter? Explain.
-
Propose the alternative set-up to use the lower amount of NaCl solution for this experiment.
Acknowledgment: Author thanks Dr. Bayraktar for his helps in preparing this manual.
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