Exercise 5-1 Water Deionization EXERCISE OBJECTIVE Familiarize yourself with conductivity probes and learn how to calibrate and use them for conductivity measurement. Also, learn how ion-exchange resins work and use them to remove ions from the process water. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Introduction An ions story Ion-exchange resins Cation-exchange resin. Anion-exchange resin. Ion-exchange resins applications Water softening. Deionization. Regeneration Water softening resin regeneration. Deionizing resin regeneration. DISCUSSION Introduction In 1935, two English scientists, Basil Albert Adams and Eric Leighton Holmes, discovered a new type of resin that would revolutionize the world of water treatment. This resin had the capacity to capture ions in water and exchange them for another type of ion from the resin lattice. For this reason, the type of resin they discovered is called ion-exchange resin. Water softening, purification and deionization, as well as juice softening, sugar manufacturing, antibiotic extraction, separation of metals, and uranium recovery are some examples of ion-exchange resins applications. Below is a description of some of the mechanisms associated with the use of ionexchange resins and a description of ion-exchange resins work. An ions story Many chemicals such as acids, bases, and salts dissolve in water and, hereafter, exist as ions. That is, most electrically neutral molecules dissolve into a positive ion (i.e., a cation) and a negative ion (i.e., an anion). Ions are free to move in water and the more ions there is, the more conductive the solution is. An ion can either be a big chunk of a molecule or a single ionized atom. The diameter of an ion is usually below 10-10 m, which prevents filtration using traditional filters since they cannot filter particles with a diameter under 10-7 m. Fortunately, ions are electrically charged and this characteristic can be used to discriminate them from water molecules using an ion-exchange resin. Festo Didactic 86013-00 125
Ex. 5-1 Water Deionization Discussion Ion-exchange resins The basic idea behind ion-exchange resins is to remove undesirable ions from water and replace them with ions having no influence the process. The replacement of an ion with one (or more) ion(s) having the same total electric charge allows keeping the system electrically neutral. The resins allowing ion exchange usually come in the form of beads with a diameter around 0.5 mm. The beads are made of long chains of polystyrene, a polymer. The polystyrene chains are cross-linked by divinylbenzene (DVB) molecules. The DVB glues the strands of polystyrene together and allow the beads to preserve their integrity by making them insoluble in most solvents. The polystyrene chains joined by DVB are similar to a spaghetti ball; there is a lot of space between the polystyrene strands. The beads are therefore porous and, because of the hydrophilic nature of some of the molecules composing it, water is drawn into the ion-exchange resin beads. The water inside the resin is called chemical moisture. To the structure of the resin, charged chemical groups are permanently attached at regular intervals. Keeping the whole resin neutral, ions of opposite electrical charge, called counterions, are associated to the permanently attached groups. The counterions are mobile and, if displaced from the vicinity of a fixed group by another charged ion, they can move through the space between the polystyrene strands. Therefore, when process water comes into contact with an ion-exchange resin bead, the ions in the process water found their way into the structure of the resin. Moving from one permanently attached group to another, they drive the counterions into the process water. Hence, the counterions replace the ions from the process water, which are now trapped in the resin bead. The ion-exchange resin beads come in two main types: cation-exchange resin and anion-exchange resin beads. Cation-exchange resin In a cation-exchange resin, negatively charged groups, such as, are permanently attached to the polystyrene structure. To compensate for these negatively charged groups, there are positive counterions. Depending on the application for which the resin is designed, the counterions can be sodium ions ( ), hydrogen ions ( ), or any other small compatible ions. Figure 5-12 shows a representation of a cation-exchange resin bead. 126 Festo Didactic 86013-00
Ex. 5-1 Water Deionization Discussion Permanently attached group Counterions Polystyrene chain Divinylbenzene cross-link Chemical moisture Figure 5-12. Cation-exchange resin. Anion-exchange resin Anion-exchange resins use the same principle as cation-exchange resins, but their permanently attached groups are positively charged (e.g., ). The counterions for the positive groups are frequently chlorine ions ( ) or hydroxide ( ). Ion-exchange resins applications As mentioned in the introduction, applications using ion-exchange resins are numerous. Below, we focus on two of these applications: water softening and water deionization. Water softening Water containing high levels of minerals, such as calcium and magnesium ions ( and ), is said to be hard water. In some circumstances, the minerals in hard water can precipitate and clog pipes. The result of pipe clogging can be disastrous if it occurs in crucial pieces of equipment, such as boilers or cooling towers. To reduce the hardness of water, plants often use ion-exchange resins to remove minerals from water and replace them with more soluble ions, such as sodium ( ), that do not precipitate readily and obstruct pipes. To remove calcium and magnesium ions, hard water is run through an ionexchange resin filter using sodium as counterions. Each calcium or magnesium ion that enters the resin is replaced with two sodium ions. Calcium and magnesium ions are multivalent cations; they have an electric charge of 2+. Replacing them with two sodium ions keeps the whole reaction neutral. Equation (5-7) details the reaction for calcium ions removal. In this equation, stands for the resin. This reaction takes place easily in the resin because calcium Festo Didactic 86013-00 127
Ex. 5-1 Water Deionization Discussion and magnesium ions have higher affinity with the resin negative groups than sodium ions. (5-7) Deionization Reducing the hardness of water does not change its ion content; the conductivity of the water remains the same. This can prove a problem for applications requiring low conductivity or deionized water. To completely remove positive and negative ions from water to lower the conductivity, a filter combining both cationexchange and anion-exchange resins is used. In such a deionization filter, the counterions of the cation-exchange and anion-exchange resins are hydrogen ions ( ) and hydroxide ions ( ) respectively. When released, the counterions from both resins recombine into water. Hence, undesired ions become trapped into the deionizing filter and are replaced with pure water. Below, Equation (5-8) to Equation (5-10) detail the deionization process for sodium chloride ( ). Equation (5-11) to Equation (5-13) detail the deionization process for calcium sulfate ( ). In these equations, stands for the cation-exchange resin and stands for the anion-exchange resin. (5-8) (5-9) Figure 5-13. Sodium chloride (2D). (5-10) (5-11) Figure 5-14. Sodium chloride (3D). (5-12) (5-13) Regeneration With time, the resin in a filter gets exhausted. That is, all counterions have been replaced with ions from the treated water. Fortunately, the ion exchange process is reversible. Restoring the resin is called regeneration. Regeneration can be done several thousand times without greatly influencing the resin capacity. The process for the regeneration of resin depends of the type of resin. The processes that are usually used to regenerate industrial water softening and water deionization resins are briefly described below. 128 Festo Didactic 86013-00
Outline Water softening resin regeneration A water softening resin can be regenerated using a concentrated sodium chloride solution (with a 10% concentration of salt). This solution is passed through the resin and, due to the high concentration of salt, the sodium ions gradually replace the ions trapped in the resin. Equation (5-14) shows the chemical reaction that occurs when a trapped calcium ion is removed from the resin. Once the restoration process is complete, the residual solution of sodium chloride and calcium chloride must be disposed of. (5-14) Deionizing resin regeneration Deionization filters combine cation-exchange and anion-exchange resins. These two resins must be restored using two different procedures. The cation-exchange resin is treated with a hydrochloric acid solution ( ) and the anion exchange resin is treated with a sodium hydroxide solution ( ). Equation (5-15) and Equation (5-16) resume the two regeneration processes. Both reactions produce a salty solution as a waste from the regeneration process. (5-15) (5-16) PROCEDURE OUTLINE The Procedure is divided into the following sections: Set up and connections Conductivity measurement Conductivity of acidic water Curve analysis PROCEDURE Set up and connections 1. Connect the equipment as the piping and instrumentation diagram (P&ID) in Figure 5-15 shows. Use Figure 5-16 and Figure 5-17 to position the equipment correctly on the frame of the training system from the 3532 series or use Figure 5-18 to position the equipment on the frame from the 3531 series. Use the basic setup presented in the Familiarization with the Training System manual. Table 5-1 lists the equipment you must add to the basic setup in order to set up your system for this exercise. Festo Didactic 86013-00 129
Table 5-1. Equipment required for this exercise. Name Part number Identification Volumetric flask 38406 Scopulla 38442 Graduated cylinder 38447 Precision scale 38485 Latex gloves 40331-54 Pipette 40331-65 Safety glasses 40348 ph transmitter 46945 AIT Metering pumps 46956 Paperless recorder 46972 UR Water analyzer 46989 Chemical tanks 46994 Acetic acid 5% (v/v)(vinegar) 76764 130 Festo Didactic 86013-00
24 V from the Electrical Unit Filter Figure 5-15. P&ID. Festo Didactic 86013-00 131
Figure 5-16. Front setup (series 3532). 132 Festo Didactic 86013-00
Figure 5-17. Back setup (series 3532). Festo Didactic 86013-00 133
Figure 5-18. Setup (series 3531). 2. Wire the emergency push-button so that you can cut power in case of an emergency. The Familiarization with the Training System manual covers the security issues related to the use of electricity with the system, as well as the wiring of the emergency push-button. 3. Wire the paperless recorder to record the output of both transmitters. 134 Festo Didactic 86013-00
4. Do not power up the instrumentation workstation yet. Do not turn the electrical panel on before your instructor has validated your setup that is not before step 9. 5. Connect the solenoid valve so that a voltage of 24 V dc actuates the solenoid when you turn the power on in step 9. 6. Fill a chemical tank with a solution of sodium chloride (i.e., table salt) containing 5 gram of salt per liter. Carefully follow the procedure of Ex. 3-1 to prepare this solution. 7. Before proceeding further, complete the following checklist to make sure you have set up the system properly. The points on this checklist are crucial elements for the proper completion of this exercise. This checklist is not exhaustive. Be sure to follow the instructions in the Familiarization with the Training System manual as well. f All unused male adapters on the column are capped and the flange is properly tightened. The hand valves are in the positions shown in the P&ID. The chemical tank is filled with the appropriate solution and is carefully labeled. You are wearing the appropriate PPE. The vent tube is properly installed. 8. Ask your instructor to check and approve your setup. 9. Power up the electrical unit. This starts all electrical devices. Conductivity measurement 10. Before making conductivity measurement you must perform an air calibration of the conductivity sensor as described in the Familiarization with the Training System manual. 11. Test your system for leaks. Use the drive to make the pump run at low speed in order to produce a small flow rate. Gradually increase the flow rate up to 50% of the maximum flow rate the pumping unit can deliver (i.e., set the drive speed to 30 Hz). Repair all leaks. 12. Start the pump and set the drive speed to 45 Hz. Festo Didactic 86013-00 135
13. Fill the column up to 25 cm of water. Then, close HV1 and open HV6 to put the process workstation into recirculation mode. 14. Once the process workstation is in recirculation mode, wait about 1 minute and read the conductivity of the process water on the transmitter. Record the conductivity of the process water below. 15. If the conductivity is smaller than 500 S/cm, use the metering pump connected to the solution of sodium chloride to inject salt into the process water in order to increase the conductivity. 16. On the conductivity transmitter, watch the conductivity of the process water increase up to 500 S/cm. Then, stop the metering pump. 17. Disconnect the solenoid valve so that all process water goes through the deionization filter. 18. On the conductivity transmitter, watch the conductivity of the process water decrease down to 20 S/cm. 19. Follow the procedure in the Familiarization with the Training System manual to transfer the data from the paperless recorder to a computer. Conductivity of acidic water 20. Make sure the ph probe is properly inserted into the connection port on the process workstation. 21. Connect the solenoid valve to a 24 V source. 22. Fill a chemical tank with a solution of 0.08 mol/l of acetic acid (i.e., vinegar). Carefully follow the procedure of Ex. 3-1 to prepare this solution. 23. If the pump is not running, start it and set it to 45 Hz. 24. Make sure the column is filled with water up to 25 cm and that the process workstation is in recirculation mode. 136 Festo Didactic 86013-00
25. Once the process workstation in recirculation mode, wait about 1 minute, and read the conductivity and the ph value of the process water on the transmitters. Record them below. 26. Using the metering pump connected to the chemical tank containing the solution of acetic acid, start injecting the solution into the process water. 27. On the paperless recorder, watch how the conductivity and ph of the process water change when the solution is injected into the process water. 28. Wait until the ph of the process water has decreased down to a value between 3.5 and 4.0, and stop the metering pump injecting the acetic acid solution. 29. Disconnect the solenoid valve so that all process water goes through the deionization filter. 30. On the paperless recorder, watch how the conductivity and ph of the process water change when the process water is filtered. 31. Follow the procedure in the Familiarization with the Training System manual to transfer the data from the paperless recorder to a computer. 32. Stop the system, turn off the power, and store the equipment. Do not forget to rinse the ph probe and store it in a storage solution as described in the Familiarization with the Training System manual. 33. Stop the system, turn off the power, and store the equipment. Curve analysis 34. Plot the data using spreadsheet software. 35. How does the ph of acidic water change when the water is filtered? Festo Didactic 86013-00 137
Ex. 5-1 Water Deionization Conclusion CONCLUSION You are now familiar with the different types of conductivity probes. You have learned how to configure, calibrate, and use an electrodeless conductivity probe. You also learned how to use a deionization filter to remove ions from water and reduce its conductivity. REVIEW QUESTIONS 1. Can sodium ions be removed from a solution using a 10 micrometer filter? Briefly explain why. 2. What is the main disadvantage of conductivity measurement using electrode cells? 3. Which electrical principle is used to measure conductivity using an electrodeless conductivity probe? 4. Which parameter determines the conductivity of water? 5. How does a cation-exchange resin work? 138 Festo Didactic 86013-00