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Department of Chemical and Biomolecular Engineering
Semester 5
EV 3108 Environmental Pollution Assessment
and Treatment Laboratory
Experiment 5
Ion Exchange Experiment
Name: Luo Jinjing (U026667H)
Lian Juan (U026682R)
Ng Wenfa (U026654B)
Group: EV 6
Date of Experiment: 31/08/04
Date of Submission: 07/09/04
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Abstract
High hardness concentration in areas where groundwater is extensively used is a
significant problem due to its tendency to form scale on the heating elements of industrial
boilers and the clogging of pipes due to the deposition of calcium carbonate. It is also a
major problem in the water that is to be used by the semi-conductor industry. Ion
exchange has been extensively used in the water treatment industry to soften the hard
water. Ion exchange works by using a polystyrene-divinylbenzene sulphonated strong
acid cation resin presaturated with sodium as the counter ion to remove the calcium ions
from the hard water by exchanging it with the sodium counter ion at the sulphonic (-SO3-)
acid exchange sites. Most ion exchange resins in the industry are operated in the
downflow packed mode. As the treatment process progresses, the number of available ion
exchange sites decreases. Thus there will be a point where the number of ion exchange
sites available is unable to ensure that the treated water meets a predetermined water
quality and this is the breakthrough point. The resin would then be regenerated, rinsed
and backwashed and then re-entered into service. Due to cost and time considerations,
there is a compromise between regeneration efficiency and column utilization where 50%
is the target set for both processes. A critical parameter in the determination of
regeneration efficiency is the theoretical exchange capacity.
This experiment is designed to illustrate the major parts of the industrial ion
exchange service cycle and to introduce the EDTA-Hardness titration method to
determine hardness. The objective of the experiment is to determine the exchange
capacity of the cationic exchange resin used. The theoretical exchange capacity of the
resin is 74176.08 mg as CaCO3 / l resin which concurs with that reported in the literature
for polystyrene-divinylbenzene resin. The regeneration efficiency is determined to be
53.6% which also agrees with that in the literature. In conclusion, the objective of the
experiment is successfully accomplished.
Keywords: Ion exchange, hardness, breakthrough, exchange capacity, regeneration
efficiency, polysulphonated polystyrene resin
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Table of Contents
I Introduction p.g.2
II Literature Review p.g.3
III Experimental Methods and Procedures p.g.12
Materials-Description of Apparatus p.g.12
Methods p.g.13
Experimental Design p.g.15
Comment on the experimental setup p.g.16
Comment on the experimental procedures p.g.16
Recommendations to enhance the experiment p.g.16
IV Results and Discussion p.g.17
Sources of errors in experiment p.g.25
V Conclusions p.g.26
Experimental Precautions p.g.26
Safety Precautions p.g.26
Acknowledgement p.g. 26
VI References p.g.27
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Introduction
High hardness concentration in the water source for a drinking water supply is a
major problem in many countries that utilizes groundwater as their main source of
drinking water such as the United States. Typical concentration of hardness in the water
is around 300-400 mg/l as CaCO3 (Snoeyink et al, 1980) Hardness is defined as the
concentration of multivalent metallic ions in solution (Peavy et al, 1985) There are two
types of hardness, carbonate hardness and non-carbonate hardness. The major focus of
this experiment is on carbonate hardness (in association with Ca2+ and Mg2+ ions) (Peavy
et al, 1985) Major problems associated with high hardness concentration in the water are
the precipitation of CaCO3 and Mg(OH)2 onto the heating elements of industrial boilers
and the inner part of pipes which will reduce the internal diameter of the pipe which will
eventually cause pipe clogging. The removal of hardness in the water used by the
semiconductor industry is also very important as the semiconductor industry uses
ultrapure water in their processes.
Carbonate hardness is very sensitive to temperature as its solubility decreases as
the temperature increases and thus it precipitates as a solid and contribute to the problems
listed above. (Peavy et al, 1985)
Ca(HCO3)2 CaCO3 + CO2 + H2O (1)
Mg(HCO3)2 Mg(OH)2 + 2CO2 (2)
In addition, under supersaturated conditions, the cations (Mg2+ , Ca2+ ) would react with
the anions in the water to form a solid precipitate as the solubility product is exceeded.
Currently, there are several methods available to remove the carbonate hardness
ion the water. (usually expressed as mg/l as CaCO3 ). The major methods include lime
softening and ion exchange. Ion exchange is the most dominant method in the market
because it is fast, the ion exchange resin can be regenerated and there is no need to handle
the precipitation sludge that is produced as in the case of lime softening. (Thompson et al,
1998) Lime softening requires chemicals to be added to the water that is to be treated in
order to remove the carbonate hardness as precipitates. After sedimentation, there will be
a low density voluminous sludge produced. Thus in lime softening, there is the extra cost
associated with the handling and disposal of the chemical sludge produced.
This experiment attempts to determine the exchange capacity of a cationic
exchange resin used in water softening. It also serves to illustrate the use of a mature
separation technology (ion exchange) can be applied to mitigate an environmental
problem (high hardness concentration in the water). In addition, theory and principles
behind the key elements of an industrial ion exchange service cycle are conveyed through
the actual operation of a cation ion exchange column following the steps in the service
cycle (backwash, exhaustion, regeneration, slow and fast rinse). Finally, the need for a
compromise between regeneration efficiency and column utilization is strongly
emphasized ion the experiment.
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Literature Review
Ion exchange is a process whereby an undesirable ion in the solution is exchanged
with a counter ion on the surface of the resin at the ion exchange sites. The major type of
resin used in ion exchange softening is the synthetic sulphonated polystyrene-
divinylbenzene copolymer resins saturated with the sodium counter ions. Assuming that
R is the surface groups, the ion exchange reaction can be represented as follows,
2RNa+ + Ca2+ or Mg2+ (aq) R2 (Ca2+ or Mg2+ ) (aq) + 2Na+ (3)
One Ca2+ ion would be attracted to two adjacent ion exchange sites which as the Na+ as
the counter ion. Thus the key requirement for the water softening ion exchange resin is
that the exchange sites must be closely situated with one another in order to allow the
divalent Ca2+ ions to bind effectively. As the ion exchange sites have a negatively
charged functional group (-SO3- in this case), the Ca2+ ion would be attracted to the ion
exchange sites by electrostatic attraction. Since the electrostatic force of attraction
between the -SO3- group and Ca2+ ions is stronger than that between the -SO3
- and Na+
ions, the Ca2+ would be preferentially attracted to the ion exchange sites while the Na+
ions would be displaced into the solution. The undesirable Ca2+ ions are now on the resin
surface. Thus the ion exchange resin used can be said to be selective towards Ca2+ .
The structure and characteristics of the ion exchange resin plays a very important
role in the determination of the efficiency of the ion exchange process. For any ion
exchange resin, there are 4 key important components associated with it, they are the
existence of a 3-dimensional polymeric framework, ionic functional groups attached to
the framework, suitable counter ions and a solvent to hydrate the resin matrix. (Seader et
al, 1998)
In this experiment, a strong acid cation ion exchange resin which is fully ionized
over the entire pH range is used. (Seader et al, 1998) The resin is made from a copolymer
of styrene and divinylbenzene as can be seen from Figure 1. The main purpose of the
divinylbenzene is to form the cross linking between the polystyrene in order to create the
3-dimensional framework. With the 3-dimesional framework, pores would be present
which would allow the ions to diffuse to the surface ion exchange sites.
Figure 1: Resin made from styrene and divinylbenzene
Source: Seader et al, 1998
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However, without the presence of ionic functional groups on the surface of the
resin, ion exchange reactions would not take place. Thus as can be seen from Figure 2,
the copolymer is sulfonated and these functional groups can be used as the ion exchange
sites of the resin. The H atom associated with the -SO3- groups interact by means of
electrostatic interactions and the H+ can be easily replaced by another cation such as Ca2+
if the electrostatic force of attraction between the Ca2+ and -SO3- group is stronger than
that between the H+ and -SO3-. Thus the presence of ionic functional groups on the
surface of the resin is imperative to the process. In Figure 2, there is also an illustration of
a 3-dimensional polymeric framework structure of the interior of the ion exchange resins.
It shows that many ion exchange sites are present deep in the interior of the bead. In order
for these sites to be accessible, there must be a medium to carry the ions to be removed to
the ion exchange sites in order for ion exchange to occur.
Figure 2: Sulfonation of the styrene-divinylbenzene copolymer in a) and the 3-
dimensional structure of the resin bead b). Source: Seader et al, 1998
Even with the presence of ionic functional groups in the structure of the polymer
that make up the ion exchange resin, the ion exchange reaction may not take place readily
because the ions that is to be removed from the solution may not be preferentially
attracted to the ion exchange groups due to the strong electrostatic force of attraction
between the ion exchange group and the presaturated counter ion. Hence, the type of
counter ion that is to be used is very important. In principle, the ionic functional group
must have a high selectivity coefficient for the ion that is to be removed and a low
selectivity coefficient for the ion that is the original counter ion on the resin such as Na+ .
Data from the literature corroborated this principle. The selectivity coefficient of a SAC
for Ca2+ is 1.9 and that for Mg2+ is 1.67 while that for Na+ is only 1.0 (Clifford, 1999).
The selectivity coefficient is defined as follow for Ca2+ and Mg2+ binary ion exchange.
yCa2+ (1- x Ca
2+ )
KCa2+, Na
+ = ------------------------------------- (4)
x Ca2+ (1- y Ca
2+ )
where x Ca2+ is the equivalent fraction of Ca2+ in the solution and y Ca
2+ is the equivalent
fraction of Ca2+ in the ion exchange resin.
7
Finally, there must be a solvent which can bring the Ca2+ ions to the ion exchange
sites in the resin. For the polystyrene-divinylbenzene ion exchange resin, organic solvent
can’t be used because it will cause the resin to swell and no ion exchange can occur.
(Seader et al, 1998) However, if water is the solvent that hydrates the resin, the Ca2+
would be able to diffuse to the ion exchange sites in the interior of the resin bead and
effect ion exchange. Although the ion exchange resin would also swell if water is the
solvent used. Typically, the water account for about 40 -65 wt% of the resin beads which
would cause the exchange capacity (eq/kg) (wet) to be lower than that when the resin is
dry. (Seader et al, 1998) This is due to the swelling and the weight of the water which has
no exchange capacity being taken into account in the calculation. Thus it is very
important to differentiate the two exchange capacity during calculations.
Kinetics and Transport in Ion exchange
In general, the ion exchange reaction between the two different counter ions at the
exchange sites is very rapid due to the electrostatic nature of the attraction between the
ions and the surface ionic groups. Mass transfer may be the controlling factor in ion
exchange in some cases.
The mass transfer limitation to ion exchange has been widely reported in the
literature (Seader et al, 1998). They are external and intraparticle mass transfer limitation.
External mass transfer resistance is predominant when the bulk solution concentration of
the target Ca2+ ions is low and thus there is not enough driving force (concentration
gradient) to allow transfer of Ca2+ across the thin stagnant water film around the
individual resin beads to maintain a high concentration of Ca2+ at the surface ion
exchange sites. This can be mitigated by increasing the mixing in the bulk solution to
reduce the thickness of the water film around the resin beads if a batch process is used
but this is not the concern of the current experiment.
The second mass transfer resistance is that of intraparticle mass transfer limitation
where the bulk solution has a very high target [Ca2+] and there is difficulty in maintaining
a high [Ca2+] in the vicinity of the ion exchange sites in the interior of the resin beads due
to pore diffusion resistance. Pore diffusion resistance arises mainly due to the branching
of the pores in the resin and the small diameter of the pores. If the pores are highly
fractured and branched, then the Ca2+ ions would collide with the walls of the pores very
frequently which would result in very slow diffusion to the ion exchange sites. If the
diameter of the pores is very small (such as the size of the pore is only slightly larger than
the ionic radius of Ca2+ ), then Ca2+ would also collide with the walls of the pores very
frequently and hence this hindered its diffusion to the ion exchange sites. This is
analogous to Knudsen diffusion (Welty et al, 2001). It must be noted that the higher the
degree of cross linking in the resin, the lower the exchange capacity of the resin (Seader
et al, 1998) and the smaller the diameter of the pores in the resin.
Therefore, in order to allow the efficient ion exchange to occur, the structure of
the pores and anionic framework of the resin must be such that the pore size are large
enough for the target counter ion to easily pass through. This must also be the case for the
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counter ion originally on the resin. Although the higher the extent of cross linking in the
resin, the better the resin is able to withstand the pressure exerted on it by the flowing
water, a compromise must be reached in order to keep the anionic framework of the resin
to be sufficiently “open”.
Mass transfer into the beads can also be severely affected if the pores of the beads
are clogged up by suspended solids and natural organic materials (NOM) in the water.
Thus the water must be treated with coagulation and filtered to remove the suspended
solids and NOM before being sent to the ion exchange unit in order to increase the
service cycle life of the resin. (Seader et al, 1998)
In reality, ion exchange is free of kinetic and mass transfer limitation due to the
natural fast rate of reaction at the surface of the ion exchange sites and the judicious
choice of resins with the suitable presaturated counter ion and pore size which would not
hinder the diffusion of the target counter ion to the ion exchange sites.
Ion exchange service cycle
There are generally 5 components to the ion exchange service cycle; backwashing,
exhaustion, regeneration, slow rinse and fast rinse. (Clifford, 1999) The entire ion
exchange service cycle is shown in Figure 3.
Figure 3: Ion exchange service cycle
Soruce: Clifford, 1999
Backwash
During the backwash process the deionized water is pumped up through the ion
exchange resin column in order to fluidize the column. This phase usually takes about 5
to 15 minutes and the flow rate of the water is about 50 to 70 ml / min (Clifford, 1999).
During backwashing, the column beads are agitated and any regenerant that has been left
behind by the slow and fast rinse water would be removed. Typically, the regenerant is
trapped in the area between two beads that is not washed by the rinse water in the down
9
flow packed bed mode. In addition, the rinse water used to wash the regenerant away
may contain a certain amount of particulates which may be trapped in the interstitial
space between the beads. If these particles are not removed, they would increase the
pressure drop required to push the water through the column and this would result in
extra energy cost. Thus the backwash also helps to remove the particles trapped between
the resin beads as their density is much smaller than that of the resin beads and they
would be removed by the backwash water. Finally, the backwash may remove the air
bubbles that may be formed in the pores of the resin in the process of regeneration and
slow and fast rinse. If these small bubbles are not removed, there would block the
passage of the Ca2+ ions to the ion exchange sites in the interior of the beads. The back
wash process would agitate the beads and the surface tension of the water layer on the air
bubble would be disrupted and the bubble would burst. In order to ensure that the column
would be sufficiently clean after the back wash process, the volume of back wash water
needed would be about 5 column volumes.
Exhaustion
After the backwash, the ion exchange column is out into service to treat water
with high hardness concentration. Assuming that the column is at full exchange capacity
after the full regeneration and the [Ca2+] in the bulk solution is high, pore diffusion and
surface diffusion would be more important. The Ca2+ would diffuse into the anionic
framework of the resin by electrostatic force of attraction and it would be preferentially
attracted to the -SO3- surface groups due to the stronger electrostatic force of attraction
and displace the Na+ in the process. Initially, in the downflow packed bed mode, the area
of active ion exchange would be right at the inlet end of the resin column. This is because
there are many more ion exchange sites than there are Ca2+ so all the Ca2+ ions would be
retained on the resin ion exchange sites and thus very few Ca2+ ions would travel further
down with the water and perform ion exchange at exchange sites further down the length
of the column.
After almost all the exchange sites at the inlet of the column has been binded by
Ca2+ , the zone of active ion exchange would move further down due to the saturated
exchange sites at the inlet not being able to perform ion exchange anymore. This process
of the movement of the zone of active ion exchange is illustrated in Figure 4. The zone of
active ion exchange is the section of the column where Ca2+ is exchanging with the Na+
presaturated counter ion. Below the zone of active ion exchange is the zone where very
few ion exchange reactions take place and the exchange capacity of that zone is close to
100%.
10
Figure 4: Definition sketch of the zone of active ion exchange and the movement
of the zone down the length of the column with time. Source: Clifford, 1999
Thus the zone of active ion exchange would move down the length of the column
until it almost reaches the end of the column. At this point, breakthrough may start to
occur, as the effluent water from the column shows increasing amount of hardness.
Breakthrough is a concept related to the effluent concentration of a particular
contaminant exceeding some predetermined value. The predetermined value is usually in
accordance with the current environmental standards and regulations. Breakthrough
would occur because there are not enough exchange sites available to perform ion
exchange reactions with the Ca2+ before they flow out with the effluent water.
The flow rate of the water in the exhaustion period is in the range of 50 to 70
ml/min in order to provide enough driving force to push the water through the packed
resin bed where there is energy loss in the process. In addition, a higher flow rate would
also allow the Ca2+ to have a higher velocity in diffusing to the ion exchange sites via the
pores of the resins. Thus ion exchange is usually very fast and would require a smaller
column due to the short residence time required. The breakthrough of the ion exchange
resin is characterized by a very steep line due to the rapid increase in the effluent
concentration of hardness. This is shown in Figure 5. This is in sharp contrasts to that in
adsorption using activated carbon as the adsorbent where the slope of the breakthrough
curve is more gradual.
Figure 5: Breakthrough curve of an ion exchange resin bed.
Source: Clifford, 1999
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Regeneration
After the breakthrough has occurred, the ion exchange column must be
regenerated with a strong concentrated NaCl solution at low flow rate. The process
usually takes about 30 to 60 minutes. (Clifford, 1999).
The flow rate of the regenerant is kept low because of the need to increase the
contact time between the NaCl and the resin beads so that reverse ion exchange would
occur which would cause the Na+ ions to be preferentially bound to the ion exchange sites
and the Ca2+ ions to be displaced. Typical flow rate used in the industry is around 10ml /
min.
The fundamental principle behind regeneration is the fact that at high solution
concentration of Na+ , the selectivity of the ion exchange group (-SO3-) for Ca2+ and Na+
are reversed. Now, Na+ would preferentially bind to the ion exchange resin instead of
Ca2+ .
The above phenomena is called selectivity reversal where as the ionic strength of
the water increases, the selectivity of the ion exchange resin for Ca2+ decreases and Ca2+
would be released into the bulk solution and the Na+ would be preferentially uptake.
(Clifford, 1999) The selectivity reversal can be explained using thermodynamic
equilibrium considerations. With a higher ionic strength water in contact with the ion
exchange site, there is increased tendency for the bound Ca2+ to escape from the exchange
site and move into the bulk solution. On the other hand, the high concentration of Na+
ions in the water would increase the tendency for the Na+ ions to be bound by the ion
exchange sites even though the exchange sites has a lower selectivity for Na+ compared
with Ca2+ . This phenomenon is able to occur due to the reversible nature of the
electrostatic force of attraction between the exchange site and bound ion.
If the [NaCl] regenerant is low, then the driving force for the uptake of Na+ by the
exchange site by selective reversal would be low due to lower ionic strength in the
solution and a lower selectivity reversal. Driving force is defined to be concentration
difference between the bulk solution and the surface of the resin. Thus the contact time
required to achieve full regeneration would be unacceptably long. Thus a high
concentration NaCl solution (10%) would be required in order to reduce the contact time
required. The concentration of NaCl used in the experiment is typical of that used in the
industry (5-20% NaCl). (Peavy et al, 1985)
In the industry, regeneration efficiency is usually targeted at 50% because it
would require a lot of high concentration of NaCl regenerant and a lot of time if full
regeneration is to be achieved. In addition, the cost of the NaCl regenerant and that of the
disposal of the concentrated waste solution after regeneration must also be considered.
Since the regeneration efficiency is set at 50%, then only 50% of the total
exchange sites available in the column would be used for exchange reaction in the next
round of softening. Hence, the column utilization would be reduced due to the fewer
12
number of exchange sites present. Therefore there must be a compromise by setting the
column utilization at 50% in order to compensate for the drop in regeneration efficiency.
This also explains why there is a need to regenerate the column soon after breakthrough
as there aren’t many ion exchange sites left in the column that could perform ion
exchange and this is also due to the fact that the breakthrough point would roughly
indicate that 50% of the column has been used.
Slow rinse and fast rinse
After the regeneration process, a small amount of regenerant solution may still be
present in the column, thus there is a need to wash them out using slow and fast rinse of
deionised water through the column. The typical time required for slow rinse is 10 to 30
minutes and that for fast rinse is around 5 to 15 minutes. (Clifford, 1999) If the
regenerant solution is not removed from the column, it may affect the quality of the
subsequent treated water because the high concentration of ions in the regenerant would
increase the Total Dissolved Solids (TDS) content of the treated water drastically. The
slow rinse is used to ensure maximum contact with all the exposed surfaces of the ion
exchange resin so as to remove the regenerant that may be left behind. The fast rinse is
used to flush out the small amount of regenerant solution from the ion exchange resin.
Potential drawbacks of ion exchange
The ion exchange process actually replace the Ca2+ ions in the water with Na+ ions
which may add to the salinity of the treated water if it is not properly controlled.
The concentrated waste solution created after the regeneration process must also
be properly treated and handled as it may otherwise cause secondary pollution.
The ion exchange resins are prone to damage by attrition of the flow rate of water
through them is too high, there is a maximum flow rate that can be used in any ion
exchange treatment system which may affect the configuration of the system
Theory behind the EDTA-hardness titration method to determine hardness
The method used to determine hardness is that of titration of the hardness solution
with ethylenediaminetetraacetic acid (EDTA).The kit used for the determination was
supplied by Hanna Instruments (HI 4812 Hardness Test Kit). The pH of the sample
solution is first raised to pH 10 by the addition of 5 drops of buffer solution.
Subsequently, a drop of the Eriochrome Black T (EBT) indicator is added. The EBT
which is blue in colour will react with the Ca2+ ion by chelating it in a complex formation
reaction to give a violet red compound as illustrated in Figure 6. It must be noted that the
stability of the complex formed between EBT and Ca2+ is lower than that formed between
EDTA and Ca2+ . Thus as EDTA is added, it would destroy EBT- Ca complex and form
EDTA- Ca complex instead. (Snoeyink et al, 1980) Hence as more and more EDTA is
added, less EBT- Ca remains and the solution turns more and more bluish in colour until
at the endpoint of the titration, all the EBT- Ca complex have been destroyed by EDTA
and the colour of the solution turns blue due to the presence of the uncomplexed EBT.
13
Figure 6: The formation of the EBT-Mg or EBT-Ca complex from EBT with the
resulting colour change. Source: Snoeyink et al, 1980
Other methods can also be used to test for the [Ca2+] in the mixture. They are the
conductivity method and the atomic absorption spectrophotometry method.
In the conductivity method, several solutions of known Ca2+ concentrations over
the expected concentration range of the experiment are prepared and are used to calibrate
the conductivity meter to get a calibration curve. After the calibration, the conductivity
meter can be used to measure conductivity of an unknown sample and use the calibration
curve to obtain the [Ca2+] in the sample.
The other method is that of atomic absorption spectrophotometry which is the
quantitative measurement of the amount of light absorbed by a sample with a certain
[Ca2+] at a particular wavelength. A calibration curve must also be prepared according to
that described in the conductivity method. The amount of light absorbed is directly
proportional to the quantity of Ca2+ ions present in the sample.
Industrial use of ion exchange
In the industry, ion exchange system is usually built in the shape of a cylindrical
pressure vessel and they are usually configured in sets of three. Two of the pressure
vessel would be under treatment run at any one time while the third would be on standby
or under regeneration. The most preferred mode of operating the ion exchange system in
the industry is the downflow packed bed mode.
14
Experimental Methods and Procedures
Materials
Figure 7: A schematic flow diagram of the ion exchange column used in the experiment
is shown below.
15
Bench-mounted unit was used in this experiment. This equipment is designed to
demonstrate the use of ion-exchange resins for either continuous water softening or
demineralization. It emulates the industrial operation of such units. The unit is supplied
by Armsfield.
The Hanna Instruments HI 4812 Hardness Test Kit is used to determine the hardness of
the samples collected from the effluent end of the ion exchange column.
A 500 ml measuring cylinder is used to measure the volume of solution collected in the
600 ml glass beakers for each 5 minute interval
14 600ml Pyrex borosilicate glass beakers were used in the experiment to collect solution
samples from the effluent end of the ion exchange column.
Methods
A. Water softening
In order to determine the exchange capacity of a cationic resin in the softening of
water, the following procedures were taken.
1. Backwashing
Refer to Figure 7. Fill cation exchanger column with cation resin to a depth of about
10 cm. Open valves 3 and 6, and backwash using deionised water for five minutes.
Gradually turn off the flow of water and measure the final depth.
2. Softening
Refer to Figure 7. Put selector tube into test water, open valves 2 and 10. Set flow rate
to between 50 and 70 ml/min. Samples were collected continuously for an interval of
five minutes using the 600 ml glass beakers.
Measure the exact volume of the sample using the 500 ml measuring cylinder. Pour
the sample back into the beaker and take 50 ml of the sample (for low hardness
concentration) or an appropriate volume (if the hardness concentration is high)to
determine the hardness.
The procedures for the determination of the hardness concentration using the test kit
is described below
16
Hardness test:
5 drops of Reagent 1 (the buffer solution) was added to the appropriate volume of
the sample and mix carefully swirling the vessel in the same direction.
1 drop of Reagent 2 (the EBT indicator) was then added and mix as described
above. The solution became a red-violet color.
The plunger of the titration syringe was completely pushed into the syringe. Insert
tip into Reagent 3 (the EDTA titration solution) and pull the plunger out until the
lower edge of the seal is on the 0 mark of the syringe.
Slowly add the titration solution dropwise, swirling to mix after each drop.
Continue adding the titration solution until the solution becomes purple, then mix
for 15 seconds after each additional drop until the solution turns blue.
Read off the millimeters of titration solution from the syringe scale and multiply
by 300 (for high concentration range of CaCO3, namely 0-300 mg/L) to obtain
mg/L (ppm) CaCO3. If the result is lower than 30 mg/L (low concentration range
of CaCO3), multiply the reading by 30 to obtain mg/L (ppm) as CaCO3.
Appropriate dilution was done to reduce the time required for titration and the
final concentration of the hardness was corrected for the dilution factor.
Continue collecting samples until hardness rises above 100 mg/L as CaCO3.
Wash out the remaining Ca2+ in the column using deionised water until the hardness
is below 5 mg/L as CaCO3. Measure the hardness and volume of the solution
collected.
3. Regenerate
Refer to Figure 7. Put selector tube into sodium chloride solution (10 %NaCl), open
valves 2 and 10. The flow rate was set to 10 ml/min and regeneration was carried out
for 30 minutes. After regeneration, deionized water is passed through the bed to wash
out any remaining regenerant until the hardness of the effluent water is below 5 mg/L
as CaCO3.
17
Experimental Design
This experiment was designed to give the students a feel of the operation of an
actual ion exchange column in the industry. The experiment illustrates the use of ion
exchange technology to solve the environmental problem of excessive hardness in the
water. This is a realistic experiment as ion exchange is widely used in the industry to
soften the water especially in the semiconductor industry.
The values of flow rates, duration of operation, breakthrough point, and
regenerant solution concentration are all typical values used in the industry. This is due to
the need to let the students have a feel of the values used in the industry for the various
parameters.
The water to be treated has an abnormally high hardness concentration which is
normally not found in the environment due to the need to reduce the time required to
achieve breakthrough during the stipulated 5 hours laboratory period. A high influent
hardness concentration would rapidly use up the ion exchange sites in the resin to the
point where breakthrough occurs.
The experiment walks the student through the process of backwashing, exhaustion,
regeneration and rinse which are the major parts of the ion exchange service cycle in the
industry. Through this experiment, the students would have a better understanding of the
actual operation of ion exchange in the water treatment industry as well as the theory and
principles behind it.
The students are also introduced to the concept of continuous monitoring of
process parameters such as effluent hardness concentration in this case. Continuous
monitoring of these parameters is very common in the industry.
In addition, the students were introduced to the determination of hardness using a
commercial test kit which is frequently used in the industry because of the convenience
of the test kit and as a basis of comparison of process data from different plants around
the world. However, the students are also reminded of the limitations and inherent
inaccuracy of such test kits which they must have knowledge of in order to properly
assess the data that they collected.
Taken holistically, Experiments 5 and 6 forms important sections of a water
treatment process train in the semi-conductor industry for example. The coagulation-
flocculation principles illustrated in Experiment 6 is used to remove the colloidal
particles, suspended particles and the natural organic matter (NOM) that may clog the ion
exchange resin. After coagulation and flocculation, sedimentation and filtration are
usually carried out first before the water is passed to the downstream process of ion
exchange as illustrated in Experiment 5 to remove the Ca2+ ions which contribute to
hardness in the water.
18
Comments on the experimental setup
The flow meter used in this experiment was unable to give a constant flow rate
over the length of the experiment such as the flow rate would fluctuate wildly between
the values of 50 to 70 ml / min during the course of the experiment. Although the flow
rate need not be known exactly as it is not one of the parameters in the calculation, but
the variation of the flow rate would create pressure surge effects through the ion
exchange resin column. The effect of this is two fold. Firstly, the pressure surge may
cause channeling effect in the column where a certain portion of the resin is completely
not used in ion exchange. This would reduce the effective exchange capacity of the resin
and would lead to process inefficiency. Secondly, the pressure surge would increase the
rate of attrition of the resin beads in the resin column such that the resin beads must be
replaced more frequently due to destruction of the interior anionic framework caused by
the pressure surge effect.
Comments on the experimental procedures
In general, the procedures that were provided for this experiment was sufficient
for the safe execution of the experiment but they were not so detailed that the students
lost the incentives to think through the whole experiment.
However, the flow diagrams of the ion exchange system and the procedures for
using the Hanna Instruments Test Kit provided at the back of the lab manual are not very
legible.
Recommendations to enhance the experiment
The flow rate fluctuations were due to the accumulation of air bubbles in the ion
exchange system and the experiment group felt that this must be rectified by the
laboratory technicians in order to reduce the damage to the resin beads and the possible
negative influence on the experimental data collected by the students.
In addition, the flow diagrams of the ion exchange system and the procedures for
using the Hanna Instruments Test Kit provided at the back of the lab manual can be made
to be clearer so that further batches of students would have less difficulty in reading the
instructions.
19
Results and Discussions
Part A. Water softening
Determination of influent hardness
Volume of tested water used: 1ml
Volume of titration solution (EDTA) used: 0.49 ml
Hardness of influent: 0.49*5*300 =735 mg/L as CaCO3
The high range test kit is used since the prepared influent has very high hardness
concentration. 1 ml of influent was diluted with 4 ml of deionised water and then titrated
with EDTA solution.
2.Softening
Table 1: Tabulation of results of the samples collected:
Sample Time(min) Depth of
column(cm)
Volume of
water
treated(ml)
Wet
Volume
(ml)
EDTA
used(ml)
Hardness(mg/L
as CaCO3)
1 0 15.1 310 26.68 0.10 3.0
2 5 15.2 290 26.86 0.10 3.0
3 10 15.1 285 26.68 0.13 3.9
4 15 15.0 285 26.50 0.16 4.8
5 20 15.0 285 26.50 0.21 6.3
6 25 14.9 280 26.33 0.32 9.6
7 30 14.9 280 26.33 0.49 14.7
8 35 14.8 280 26.15 0.73 21.0
9 40 14.7 280 25.98 1.16 35.0
10 45 14.7 280 25.98 1.66 49.8
11 50 14.5 280 25.62 2.50 75.0
12 55 14.4 275 25.45 0.73 109.5
13 60 14.3 275 25.27 1.03 154.5
14 65 14.3 275 25.27 1.42 213.0
15 70 14.3 275 25.27 1.79 268.5
16 75 14.2 275 25.09 2.47 370.5
17 80 14.1 275 24.92 0.36 540.0
18 85 14.1 275 24.92 0.41 615.0
20
Samples 1 to 11,
The low range test kit was used i.e. 50 ml of test water was used,
Hardness (mg/l as CaCO3) = volume of titration solution (EDTA) * 30
For example,
Sample 1
Hardness (mg/l as CaCO3) = volume of titration solution (EDTA) * 30
= 0.10 * 30
= 3.0
Samples 12 to 16
The high range test kit was used, i.e. 10 ml of test water was used,
Hardness (mg/l as CaCO3) = volume of titration solution (EDTA) * 30 *5
For example,
Sample 12
Hardness (mg/l as CaCO3) = volume of titration solution (EDTA) * 30 *5
= 0.73 * 30 * 5
= 109.5
Sample 17 & 18
The high range test kit was used, i.e. 1 ml of test water was diluted with 4 ml of deionised
water.
Hardness (mg/l as CaCO3) = volume of titration solution (EDTA) * 5 *300
For example,
Sample 17
Hardness (mg/l as CaCO3) = volume of titration solution (EDTA) * 5 *300
= 0.36 * 5* 300
= 540.0
From the table,
Average hardness = 2497.1/18
= 138.72 mg/l as CaCO3
21
Determination of the hardness removed till the breakthrough point (rough estimate)
Summation of (hardness concentration * Volume of solution treated till breakthrough
point)
= 93616 mg as CaCO3
Softening
Total volume of solution collected: 3100ml
For 10ml of solution tested,
EDTA used: 0.49ml
Hardness of the solution: 0.49*5*30 = 73.5 mg/l as CaCO3
Determination of wet volume of resin bed:
Final depth (cm) of resin from the last sample = 14.1cm
Depth Final4
1.5)( bedresin of Wet volume
2
= [ x 1.52 / 4] x 14.1
= 24.92ml
Influent hardness = 735 mg/l CaCO3
Average hardness of samples = 138.72 mg/l as CaCO3
Substitute the above values into the equation,
bedresin of Wet volume
collectedsolution of Volume Hardness) Average-Hardness(Influent capacity Exchange
= [(735 – 138.72 )mg/l x3100mL] / 24.92mL
= 74176.08 mg as CaCO3 / l resin
22
Graph of Hardness of water against volume of water treated:
Hardness(mg/L
as CaCO3)
Cumulative
volume of
water
treated(ml)
3.0 310
3.0 600
3.9 885
4.8 1170
6.3 1455
9.6 1735
14.7 2015
21.0 2295
35.0 2575
49.8 2855
75.0 3135
109.5 3415
154.5 3690
213.0 3965
268.5 4240
370.5 4515
540.0 4790
615.0 5065
Hardness(mg/L as CaCO3) vs volume of water
treated (mL)
y = 1.3497e0.0012x
0.0100.0200.0300.0400.0500.0600.0700.0800.0
0 2000 4000 6000
Volume of water treated(mL)
Hardness(mg/L as
CaCO3)
hardness(mg/Las CaCO3)
Expon.(hardness(mg/Las CaCO3))
Graph 1: Plot of Hardness in the effluent versus volume of water treated
23
Determination of milligrams of hardness as CaCO3 removed up to breakthrough point
The milligrams of hardness as CaCO3 removed from the water up to the breakthrough
point are given by the area between the curve plotted, and the horizontal line which
represented the original hardness of the water.
From the graph plotted,
The breakthrough point was defined as the hardness of 12th sample taken which is 109.5
mg/L as CaCO3.
Hardness removed up to breakthrough point = 735*109.5 – integration (1.3497e^0.0012x )
from 0 to 109.5
= 79191.5 mg as CaCO3
B. Regeneration of an ion-exchange softening system
Total solution collected: 1650ml
Titration solution (EDTA) used: 0.40 ml
Hardness of the solution = 0.4*5*300
= 600 mg/l as CaCO3
Theoretical exchange capacity (meq/l) = Exchange capacity /50
= 74176.08 / 50
= 1483.52 meq / l
(Exchange capacity can be found in “Calculation” of Experiment A)
Final depth = 14.1 cm (from part A)
Depth Final4
1.5)( bedresin of Wet volume
2
= [ x 1.52 / 4] x 14.1
= 24.916 ml
= 0.024916L
24
Actual exchange capacity (meq/L) =bedresin of Wet volume
on)/50regeneratiin collected CaCO of(Amount 3
= (600*1.65/ 50) / 0.024916
=794.67
Regeneration efficiency = capacity exchange ltheoretica
capacity exchange actual * 100%
= 794.67 / 1483.52 * 100%
= 53.6%
As can be seen from Table 1, the depth of the ion exchange column decreases
with time. This is due to the progressive compaction of the ion exchange column due to
the pressure exerted by the flow of water through the column. The origin of the void
space between the resin beads is due to the separation of the resin beads (which were
sticking together) during the upflow countercurrent fluidized bed backwashing of the
column. After the backwashing, there would be an increase in the extent of void space
due the resin beads not being so densely packed together in the column. During the
service run of the resin column, the pressure exerted by the flow of water at high velocity
would serve to compact the resin column and reduce the void space that may exist
between the individual beads. This observation is very important in the industry because
as the reins beads are packed more closely together, there may be increased attrition
between the resin beads due to the increased contact between the individual resin beads.
The attrition may arise due to the rotational motion between the individual resin beads.
The major effect of attrition is that the structural integrity of the porous resin beads may
be affected and this may ultimately lead to the collapse of sections of the pores in the
resin thus reducing the number of effective exchange sites available due to the blocking
of the pores which reduce access to the ion exchange sites. The most serious case would
be the complete destruction of the resin beads and this would increase the operating cost
of the ion exchange due to the need to frequently change the resin beads.
Thus the flow rate used in the service time of the resin should be around 50 to 70
ml / min in order to achieve a compromise between desired treated water flow rate
(productivity) and resin attrition (cost). Higher flow rate is not desirable as the resin
attrition problem would be more severe leading to higher operating cost due to the need
to frequently change the resin beads.
Secondly, on the plot of hardness versus volume of water treated, it is evident that
the hardness of the water is very low at the start of the treatment process and it gradually
increases as more water is treated. Significant rise in the hardness of water occurs at
around 2500 ml of treated water where the corresponding hardness is about 20 mg/l as
CaCO3 Thereafter, the hardness of the effluent water increases very rapidly till it passes
through the breakthrough point which was set at 100 mg/l as CaCO3 . The slope of the
breakthrough portion of the curve is very steep as characteristic of typical ion exchange
25
breakthrough curves. This is in sharp contrast to that found in adsorption on activated
carbon where the slope of the breakthrough curve is more gradual.
The hardness of the effluent at the initial stages of the treatment is low because
the ion exchange column has a large number of ion exchange sites compared to the
number of Ca2+ ions in the influent flow. Thus there is very rapid uptake of the Ca2+ by
the ion exchange sites on the resin at the influent end of the column. This is due to the
rapid kinetics of the ion exchange reaction and low mass transfer resistance in the pores
of the resin beads as the type of resin beads has been chosen to optimize the rate of
uptake of Ca2+ ions from the solution. Hence, the effluent concentration of hardness is
low because most of the Ca2+ ions present in the influent would have been removed by
the ion exchange resin. It is to be noted that with the removal of Ca2+ onto the ion
exchange resin, there would be a 2 equivalent release of Na+ ions into the effluent due to
the exchange of 2 Na+ ions for 1 Ca2+ ion in the exchange process. This can be lead to a
drastic increase in the [Na+] in the treated water which may violate drinking water quality
guidelines if the process is not controlled properly.
However, there would always be some degree of hardness remaining in the
effluent water because the removal efficiency of the ion exchange resin is not 100%. The
reason behind this is that some of the Ca2+ may be energetic enough to diffuse to the ion
exchange sites in the interior of the ion exchange resin beads. Thus the Ca2+ ions would
be swept by the flow of water away and would appear in the effluent of the system. At
the start of the treatment process, the zone of active ion exchange would move down the
column due to the exhaustion of almost all the ion exchange sites at the initial portion of
the resin. Exhaustion is a relative term as there may be still some number of ion exchange
sites in the “exhausted” zone which can still bind Ca2+ . However, the number is small
compared to the total number of exchange sites in the “exhausted” zone. In addition, the
ion exchange sites that are available can be separated into two categories, those on the
surface of the resin beads and those in the interior of the resin beads which are only
accessible by pores. The ion exchange sites on the surface are more accessible and they
are used to uptake the Ca2+ early in the treatment process. The ion exchange sites in the
interior of the resin beads are only accessible through the fragmented and tortuous pores
leading to them and are in general more inaccessible to the Ca2+ ions. Thus these sites are
only utilized when the surface sites are almost completely bound with Ca2+ ions.
The concept of zone of active ion exchange gives the wrong impression that all
the ion exchange reactions at a particular time would all take place within the zone of
active ion exchange at that time. However, this is not true. The surface exchange sites at
the outlet end of the column may have already been used up to a certain extent (though
not completely) even though the zone of active ion exchange may be still in the middle of
the column. Thus the zone of active ion exchange may refer to the zone of the column
where most of the ion exchange reactions are taking place. Thus “exhausted” zone of the
column is defined here to be the zone of the column where almost all of the surface and
interior ion exchange sites have been bound by Ca2+ ions.
26
As the ion exchange sites in the column becomes increasingly bound by Ca2+ ions,
it is increasingly difficult for the Ca2+ ions to be able to bind at a free unexchanged
exchange site. Thus the Ca2+ ions may not be able to bind to any sites as it passes through
the column and it will escape into the effluent which would cause the hardness of the
effluent to increase. Thus the rise in the hardness of the effluent water is an indication of
the deceasing number of ion exchange sites available in the resin. With the depletion of
the ion exchange sites in the column, the zone of active ion exchange moves down the
column.
Finally, at breakthrough point, the zone of active ion exchange would have move
to near the effluent end of the ion exchange column where there is now a substantially
lower portion of ion exchange sites which are able to bind Ca2+ ions and thus the
probability of the Ca2+ ions to be bound is reduced and most of the Ca2+ ions in the
influent would not be bound and would exit in the effluent leading to the rise in the
hardness of the effluent water. There is this misconception about the breakthrough point
being the point at which the ion exchange column is completely exhausted. This is
certainly not true. The breakthrough point is set arbitrarily, normally it is set at the point
where the ratio of the concentration of the effluent to that of the influent is 0.05 but other
values of the ratio has also been used depending on the drinking water guidelines for that
particular chemical. In this case the Singapore drinking water guideline for total hardness
is 100 mg/l as CaCO3 . Thus the breakthrough point should be interpreted as the
exhaustion of the ion exchange resin to the point where the resin is no longer able to treat
the influent water to the required drinking water quality and the resin needs to be
regenerated. It does not indicate a complete exhaustion of the resin column ion exchange
capacity. In fact there is still some ion exchange capacity left in the resin at breakthrough
point because of the influent hardness concentration is higher than the effluent hardness
concentration for a period of time after breakthrough has occurred. Theoretically, the
complete exhaustion of the resin would occur when the influent and effluent hardness
concentration are the same indicating that there are no ion exchange sites available in the
resin to bind Ca2+ ions.
The slope of the ion exchange breakthrough curve is much steeper than that of
adsorption by activated carbon. This is due to the structure of the ion exchange resin. In
the ion exchange resin, although the interior ion exchange sites are only accessible by
fractured and tortuous pores but the anionic framework of the resin is still considerably
more “open” than that of the activated carbon. Thus the pores of the ion exchange resins
are usually big enough for the target ions to freely diffuse in and out of the pores with
little hindrance from the effect of “hindered diffusion in solvent filled pores” (Welty et al,
2001). Thus the ion exchange resin to be used for a specific application to remove a
particular ion must take the size of the ion into consideration so that the average pore size
in the resin is bigger than the ionic radius of the ion. However, this is not the case in
activated carbon as the pores are usually of widely different sizes. Thus once an ion
exchange resin column is near to breakthrough point, the fraction of the available ion
exchange sites would be significantly less than the fraction of available adsorption sites
in activated carbon. Hence, the activated carbon still has the ability to adsorb a significant
portion of the adsorbate present in the influent stream due to the present of adsorption
27
sites deep in the pores. Thus there is more removal of the influent species than in the case
of ion exchange and thus the breakthrough curve of activated carbon is more gradual.
Sources of errors in the experiment
At the beginning of this experiment, a 5-minute-backwashing was done before the
softening process. This backwashing process may not be sufficient in the presence of
contaminants in the resin.
The resin bed may have been denatured due to the usage for a long time. It would
reduce the effective number of sites where the ion-exchange processes can take place.
The volume of the samples collected in part A may not be accurate as lose of
samples may exist during the transfer from beakers to the measuring cylinder.
After the softening process in part A, deionised water was used to wash out the
remaining Ca2+ in the column. At the beginning of this process, a small bubble was found
in the tubes. This could have affected the total volume of the solution collected and the
average hardness calculated.
During the collection of the solution, a small amount of solution spilled out of the
beaker. The spillage of solution decreased the total volume and may affect the average
hardness.
There may be errors in the hardness test. The end point of the titration was
determined until the solution turns blue. Thus the end points for each sample may not be
the same.
Parallax error may occur. The syringe used in the hardness test has markings with
very small intervals. It was hard to determine the accurate readings.
In part A of this experiment, samples should be continuing collected until
hardness rises above 100 mg/L as CaCO3. However, six more samples were taken after
reaching the breakthrough point in our experiment. Calculation of average hardness
depending on all these samples may lead to a higher value.
28
Conclusion
This experiment has managed to determine the exchange capacity (actual and
theoretical) of a cationic exchange resin and the regeneration efficiency.
In addition, this experiment has also managed to use a very simple experimental
setup to illustrate the principles and theory behind the major parts of the industrial ion
exchange service cycle which are namely, backwash, exhaustion, regeneration and rinse.
It also exposed the students to the type of reasoning and thinking behind the
calculation of both theoretical and actual exchange capacity and regeneration efficiency
of the ion exchange resins which are important process control parameters in the water
treatment industry
The theoretical exchange capacity was determined to be 74176.08 mg as CaCO3
per liter of resin beads and the regeneration efficiency was 53.6%.
The objective of the experiment to determine the theoretical exchange capacity of
the cationic exchange resin has been successfully achieved.
Experimental Precautions
1. When took readings of the depth, eye must be placed on horizontally level with the
ruler to avoid parallax error.
2. For addition of titrating solution, the tips of pipette must be clean to avoid
contamination and thus affect the final reading.
3. Care must be taken when determine the end point of the titration.
4. Eye must be placed on horizontal level with the measuring cylinder when measure
the volume of sample solution collected to avoid parallax error.
Safety Precautions
1. Extra care must be taken when handling with glassware especially during the washing
of the glassware. In the event of breakage, clean the place immediately and inform the lab
demonstrator.
2. Wear gloves when preparing samples and cleaning glassware and always put on
goggles.
Acknowledgement
We would like to thank the lab demonstrator and Sandy for their help and
guidance during the experiment.
29
Supporting information
Supporting information can be found at the back of this report.
References:
Clifford, Dennis A (1999). “Ion Exchange and Inorganic Adsorption” Water Quality and
Treatment, 5th Ed., Letterman, Raymond.D, ed. McGraw Hill, New York, 9.1-9.35
Peavy H.S, Rowe, D.R.and Tchobanoglous, G. (1985). “Water Quality: Definitions,
Characteristics, and Perspectives” and “Engineered Systems for Water Purification”
Environmental Engineering, 1st Ed., McGraw Hill, New York, 35-36 and 162-165
respectively
Seader, J.D and Henley, Ernest J. (1998). “Adsorption, Ion Exchange and
Chromatography” Separation Process Principles, 1st Ed.,John Wiley & Sons, 778-870
Snoeyink, Vernon L. and Jenkins, David (1980). “Coordination Chemistry” Water
Chemistry, 1st Ed.,John Wiley & Sons, 203-204
Tchobanoglous, G, Burton, Franklin.L. and Stensel, H.David. (2003). “Advanced
Wastewater Treatment” Wastewater Engineering: Treatment and Reuse, 4th Ed., McGraw
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Thompson, Mark A and Azar, Badi(Steve) (1998). “Evaluation of conventional and
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No.6, 229-238
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