Department of Chemical and Biomolecular Engineering. Semester 5. EV 3108 Environmental Pollution Assessment and Treatment Laboratory

Transcription

1 Department of Chemical and Biomolecular Engineering Semester 5 EV 3108 Environmental Pollution Assessment and Treatment Laboratory Experiment 5 Ion Exchange Experiment Name: Luo Jinjing Lian Juan Ng Wenfa (U026667H) (U026682R) (U026654B) Group: EV 6 Date of Experiment: 31/08/04 Date of Submission: 07/09/04 1

2 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 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 2

3 Table of Contents I Introduction II Literature Review III Experimental Methods and Procedures Materials-Description of Apparatus Methods Experimental Design Comment on the experimental setup Comment on the experimental procedures Recommendations to enhance the experiment IV Results and Discussion Sources of errors in experiment p.g.2 p.g.3 p.g.12 p.g.12 p.g.13 p.g.15 p.g.16 p.g.16 p.g.16 p.g.17 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 3

4 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 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 Ca 2+ and Mg 2+ 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 (Mg 2+, Ca 2+ ) 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. 4

5 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 polystyrenedivinylbenzene 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 + + Ca 2+ or Mg 2+ (aq) R2 (Ca 2+ or Mg 2+ ) (aq) + 2Na + (3) One Ca 2+ 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 Ca 2+ ions to bind effectively. As the ion exchange sites have a negatively charged functional group (-SO3 - in this case), the Ca 2+ ion would be attracted to the ion exchange sites by electrostatic attraction. Since the electrostatic force of attraction between the -SO3 - group and Ca 2+ ions is stronger than that between the -SO3 - and Na + ions, the Ca 2+ would be preferentially attracted to the ion exchange sites while the Na + ions would be displaced into the solution. The undesirable Ca 2+ ions are now on the resin surface. Thus the ion exchange resin used can be said to be selective towards Ca 2+. 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,

6 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 Ca 2+ if the electrostatic force of attraction between the Ca 2+ 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 Ca 2+ is 1.9 and that for Mg 2+ is 1.67 while that for Na + is only 1.0 (Clifford, 1999). The selectivity coefficient is defined as follow for Ca 2+ and Mg 2+ binary ion exchange. yca 2+ (1- x Ca 2+ ) KCa 2+, Na + = (4) x Ca 2+ (1- y Ca 2+ ) where x Ca 2+ is the equivalent fraction of Ca 2+ in the solution and y Ca 2+ is the equivalent fraction of Ca 2+ in the ion exchange resin. 6

7 Finally, there must be a solvent which can bring the Ca 2+ 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 Ca 2+ 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 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 Ca 2+ ions is low and thus there is not enough driving force (concentration gradient) to allow transfer of Ca 2+ across the thin stagnant water film around the individual resin beads to maintain a high concentration of Ca 2+ 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 [Ca 2+ ] and there is difficulty in maintaining a high [Ca 2+ ] 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 Ca 2+ 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 Ca 2+ ), then Ca 2+ 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 7

8 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. Backwash Figure 3: Ion exchange service cycle Soruce: Clifford, 1999 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 8

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 Ca 2+ 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 [Ca 2+ ] in the bulk solution is high, pore diffusion and surface diffusion would be more important. The Ca 2+ 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 Ca 2+ so all the Ca 2+ ions would be retained on the resin ion exchange sites and thus very few Ca 2+ 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 Ca 2+, 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 Ca 2+ 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%. 9

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 Ca 2+ 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 Ca 2+ 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,

11 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 Ca 2+ 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 Ca 2+ and Na + are reversed. Now, Na + would preferentially bind to the ion exchange resin instead of Ca 2+. The above phenomena is called selectivity reversal where as the ionic strength of the water increases, the selectivity of the ion exchange resin for Ca 2+ decreases and Ca 2+ 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 Ca 2+ 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 Ca 2+. 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 11

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 Ca 2+ 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 Ca 2+ 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 Ca 2+ is lower than that formed between EDTA and Ca 2+. 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. 12

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 [Ca 2+ ] in the mixture. They are the conductivity method and the atomic absorption spectrophotometry method. In the conductivity method, several solutions of known Ca 2+ 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 [Ca 2+ ] 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 [Ca 2+ ] 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 Ca 2+ 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. 13

14 Experimental Methods and Procedures Materials Figure 7: A schematic flow diagram of the ion exchange column used in the experiment is shown below. 14

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 ml 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 15

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 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 Ca 2+ 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. 16

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 coagulationflocculation 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 Ca 2+ ions which contribute to hardness in the water. 17

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. 18

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)

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 = 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 = From the table, Average hardness = /18 = mg/l as CaCO3 20

21 Determination of the hardness removed till the breakthrough point (rough estimate) Summation of (hardness concentration * Volume of solution treated till breakthrough point) = 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 2 (1.5) Wet volume of resin bed Final Depth 4 = [ x / 4] x 14.1 = 24.92ml Influent hardness = 735 mg/l CaCO3 Average hardness of samples = mg/l as CaCO3 Substitute the above values into the equation, (Influent Hardness - Average Hardness) Volume of solution collected Exchange capacity Wet volume of resin bed = [( )mg/l x3100ml] / 24.92mL = mg as CaCO3 / l resin 21

22 Hardness(mg/L as CaCO3) Graph of Hardness of water against volume of water treated: Cumulative Hardness(mg/L as CaCO3) volume of water treated(ml) Hardness(mg/L as CaCO 3 ) vs volume of water treated (ml) Volume of water treated(ml) Graph 1: Plot of Hardness in the effluent versus volume of water treated y = e x hardness(mg/l as CaCO3) Expon. (hardness(mg/l as CaCO3)) 22

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 12 th sample taken which is mg/l as CaCO3. Hardness removed up to breakthrough point = 735*109.5 integration (1.3497e^0.0012x ) from 0 to = 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 = / 50 = meq / l (Exchange capacity can be found in Calculation of Experiment A) Final depth = 14.1 cm (from part A) 2 (1.5) Wet volume of resin bed Final Depth 4 = [ x / 4] x 14.1 = ml = L 23

24 (Amount of CaCO3 collected in regenerati on)/50 Actual exchange capacity (meq/l) = Wet volume of resin bed = (600*1.65/ 50) / = Regeneration efficiency = actual exchange theoretical exchange capacity capacity * 100% = / * 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 24

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 Ca 2+ ions in the influent flow. Thus there is very rapid uptake of the Ca 2+ 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 Ca 2+ ions from the solution. Hence, the effluent concentration of hardness is low because most of the Ca 2+ ions present in the influent would have been removed by the ion exchange resin. It is to be noted that with the removal of Ca 2+ 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 Ca 2+ 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 Ca 2+ may be energetic enough to diffuse to the ion exchange sites in the interior of the ion exchange resin beads. Thus the Ca 2+ 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 Ca 2+. 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 Ca 2+ 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 Ca 2+ ions. Thus these sites are only utilized when the surface sites are almost completely bound with Ca 2+ 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 Ca 2+ ions. 25

26 As the ion exchange sites in the column becomes increasingly bound by Ca 2+ ions, it is increasingly difficult for the Ca 2+ ions to be able to bind at a free unexchanged exchange site. Thus the Ca 2+ 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 Ca 2+ ions and thus the probability of the Ca 2+ ions to be bound is reduced and most of the Ca 2+ 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 Ca 2+ 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 26