Ion Exchange Apparatus
Instruction Manual W9 ISSUE 22 Au g u s t 2011
Table Table of Contents Copyright and Trademarks............................ Trademarks ............................ .............................. ............................ 1 General Overview ............................. ............................. .............................. ............... 2 Equipment Diagrams............................................ .............................. ......................... 3 Important Safety Information..................... Information................................................... .............................. ............................. ....... 4 Introduction............................. ............................. .............................. ...................... 4 Electrical Safety............. ............................. .............................. ............................. .. 4 Wet Environment............................... Environment.. ............................. .............................. ............................. ........... 4 High Pressure................... .............................. ............................. ............................ 5 Heavy Equipment ............................ .............................. ............................... ........... 5 Chemical Safety ............................. .............................. ............................. .............. 5 Water Borne Hazards........................... Hazards ........................... .............................. ............................. ........ 5 Description ............................ ............................. .............................. ........................... 7 Overview............................................ ............................. .............................. ........... 7 Installation................................ Installation.. .............................. ............................. .............................. ........................ 9 Advisory.................................................. Advisory..................... ............................. .............................. ............................. ...... 9 Electrical Supply........................... Supply ........................... .............................. ............................. ................ 9 Installing the Equipment ......................... .............................. ............................. .... 10 Commissioning Commissioning ............................. ............................. .............................. .............. 11 Operation ........................... .............................. ............................. ............................ 13 Operating the Equipment............................................... ............................. ........... 13 Operation of the Conductivity Meter ............................. .............................. ........... 13 Equipment Specifications. Specifications.............................. ............................. .............................. ............................. . 14 Overall Dimensions Dimensions .......................... ............................. .............................. .......... 14 Electromagnetic Electromagnetic Compatibility ........................... ............................. ....................... 14 Equipment Location..................... Location................................................... .............................. ............................. ............... 14 Environmental Environmental Conditions.................................................. .............................. ...... 14 Routine Maintenance ........................... .............................. ............................. .......... 15 Responsibility Responsibility ........................... .............................. ............................. .................. 15 General................................................... General...................... ............................. .............................. ............................. .... 15 ii
Table of Contents Laboratory Teaching Exercises........................................... ............................. ......... 16 Index to Exercises .......................... ............................. .............................. ............ 16 Water Softening Theory................................... .............................. ........................ 16 Regeneration Regeneration Theory.............. .............................. .............................. ................... 16 Demineralisation Demineralisation Theory........................................ .............................. .................. 17 Resin Volume and Density .............................. ............................. ......................... 18 Exchange Capacity.......................................... ............................ .......................... 18 Data Sheet I......................... .............................. ............................. ....................... 18 Data Sheet II........................ .............................. ............................. ....................... 20 Data Sheet III....................... .............................. ............................. ....................... 23 Data Sheet IV ........................... .............................. ............................. .................. 23 Exercise A .......................... .............................. .............................. ........................... 25 Exercise B .......................... .............................. .............................. ........................... 27 Exercise C.................................... C....... ............................. .............................. ............................. .................. 29 Exercise D.................................... D....... ............................. .............................. ............................. .................. 32 Contact Details for Further Information.......................... Information .......................... .............................. ............. 33
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Disclaimer This document and all the information contained within it is proprietary to Armfield Limited. This document must not be used for any purpose other than that for which it is supplied and its contents must not be reproduced, modified, adapted, published, translated or disclosed to any third party, in whole or in part, without the prior written permission of Armfield Limited. Should you have any queries or comments, please contact the Armfield Customer Support helpdesk (Monday to Thursday: 0830 – 1730 and Friday: 0830 - 1300 UK time). Contact details are as follows: United Kingdom
International
(0) 1425 478781 (calls charged at local rate)
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Email:
[email protected] Fax: +44 (0) 1425 470916
Copyrigh t and Trademarks Copyright © 2011 Armfield Limited. All rights reserved. Any technical documentation made available by Armfield Limited is the copyright work of Armfield Limited and wholly owned by Armfield Limited. Brands and product names mentioned in this manual may be trademarks or registered trademarks of their respective companies and are hereby acknowledged.
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General Overview Ion exchange is a natural process in which ions held on the surface of a solid displace other ions, of similar and equivalent electrical charge, from a solution in contact with the solid. The displaced ions become attached (ie. held by electrostatic attraction) to the surface, while those originally on the surface go into solution. This process of exchange continues until the relative concentration of the two types of ions, on the surface and in solution, reach equilibrium. The process is reversible, the direction of the exchange depending upon these relative concentrations. The simplest example of practical ion exchange is in the softening of water, when Ca2+ ions in the water (causing hardness) are exchanged for Na+ ions on the exchange material. When equilibrium is reached, i.e. when the exchange capacity of the material is exhausted, it can be regenerated by applying a concentrated solution of a sodium salt, usually sodium chloride, to restore Na+ ions on the surface. By the use of suitable ion exchange materials in two or more stages it is possible to remove all dissolved salts from solution - the process of demineralisation. The ion exchange apparatus described in this manual enables both softening and demineralisation to be studied. Besides these uses in the treatment of water supplies, ion exchange processes are also widely employed in industry.
History The phenomenon of ion exchange was discovered in the middle of the nineteenth century, when H.S.M. Thompson observed that ammonium sulphate fertilizer applied to soil emerged as a solution of calcium sulphate. In 1850-54 the process was closely studied by J.T. Way, Consulting Chemist to the Royal Agricultural Society in England. He found that many forms of ion exchange occurred in various soils, and that the materials involved were complex hydrated aluminosilicates, known as zeolites. Way was able to prepare artificial aluminosilicates with ion exchange properties. The first practical use of ion exchange in water treatment was by the German R. Gans in 1905, when he used a synthetic material to soften water, regenerating it with sodium chloride. Since then, the range of synthetic materials has been greatly extended, with the introduction of sulphonated coals by Liebknicht in 1934, phenol-formaldehyde resins by Adams and Holmes in 1935 and polymerization resins based on styrene, by d’Alelio in 1944. The properties sought in the newer synthetic materials include physical and chemical stability as well as greatly increased exchange capacities. They can be made as either cation- or anion-exchangers and by using both types in series demineralization is possible. The first commercial ion-exchange demineralization plant was installed in 1937 at a brewery in Guildford, England.
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Equipment Diagrams
Figure 1: W9 Ion Exchange App aratus
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Important Safety Inform ation Introduction All practical work areas and laboratories should be covered by local safety regulations which m ust be follow ed at all times. It is the responsibility of the owner to ensure that all users are made aware of relevant local regulations, and that the apparatus is operated in accordance with those regulations. If requested then Armfield can supply a typical set of standard laboratory safety rules, but these are guidelines only and should be modified as required. Supervision of users should be provided whenever appropriate. Your W9 Ion Exchange Apparatus has been designed to be safe in use when installed, operated and maintained in accordance with the instructions in this manual. As with any piece of sophisticated equipment, dangers exist if the equipment is misused, mishandled or badly maintained.
Electrical Safety The equipment described in this Instruction Manual operates from a mains voltage electrical supply. It must be connected to a supply of the same frequency and voltage as marked on the equipment or the mains lead. If in doubt, consult a qualified electrician or contact Armfield. The equipment must not be operated with any of the panels removed. To give increased operator protection, the unit incorporates a Residual Current Device (RCD), alternatively called an Earth Leakage Circuit Breaker, as an integral part of this equipment. If through misuse or accident the equipment becomes electrically dangerous, the RCD will switch off the electrical supply and reduce the severity of any electric shock received by an operator to a level which, under normal circumstances, will not cause injury to that person. At least once each month, check that the RCD is operating correctly by pressing the TEST button. The circuit breaker MUST trip when the button is pressed. Failure to trip means that the operator is not protected and the equipment must be checked and repaired by a competent electrician before it is used.
Wet Enviro nment The storage tanks on the equipment require filling and draining in use. During use it is possible that there will be some spillage and splashing.
All users should be made aware that they may be splashed while operating the equipment, and should wear appropriate clothing and non-slip footwear. ‘Wet Floor’ warnings should be displayed where appropriate. Electrical devices in the vicinity of the equipment must be suitable for use in wet environments or be properly protected from wetting.
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Important Safety Information
High Pressure This apparatus is designed to operate with internal pressures greater than that of the surrounding atmosphere.
The feed pump can produce more flow that is required by the process so it is important to adjust the bypass on the feed pump to allow liquid to return to the feed tank and avoid excessive pressure in t he pipework. The setting of the clip on the bypass tubing is described in the Commissioning section. Ensure that the selector valves are set correctly to configure the flow through the apparatus before switching on the feed pump. This will avoid excessive pressure in the system.
Heavy Equipment This apparatus is heavy.
The apparatus should be placed in a location that is sufficiently strong to support its weight, as described in the Installation section of the manual.
Chemical Safety Details of the chemicals intended for use with this equipment are given in the Operational Procedures section. Chemicals purchased by the user are normally supplied with a COSHH data sheet which provides information on safe handling, health and safety and other issues. It is important that these guidelines are adhered to.
It is the user’s responsibility to handle chemicals safely.
Prepare chemicals and operate the equipment in well ventilated areas.
Only use chemicals specified in the equipment manuals and in the concentrations recommended. Follow local regulations regarding chemical storage and disposal.
Water B orn e Hazards The equipment described in this instruction manual involves the use of water, which under certain conditions can create a health hazard due to infection by harmful micro-organisms. For example, the microscopic bacterium called Legionella pneumophila will feed on any scale, rust, algae or sludge in water and will breed rapidly if the temperature of water is between 20 and 45°C. Any water containing this bacterium which is sprayed or splashed creating air-borne droplets can produce a form of pneumonia called Legionnaires Disease which is potentially fatal. Legionella is not the only harmful micro-organism which can infect water, but it serves as a useful example of the need for cleanliness. Under the COSHH regulations, the following precautions must be observed:
Any water contained within the product must not be allowed to stagnate, ie. the water must be changed regularly.
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Armfield Instruction Manual
Any rust, sludge, scale or algae on which micro-organisms can feed must be removed regularly, i.e. the equipment must be cleaned regularly. Where practicable the water should be maintained at a temperature below 20°C. If this is not practicable then the water should be disinfected if it is safe and appropriate to do so. Note that other hazards may exist in the handling of biocides used to disinfect the water. A scheme should be prepared for preventing or controlling the risk incorporating all of the actions listed above.
Further details on preventing infection are contained in the publication “The Control of Legionellosis including Legionnaires Disease” - Health and Safety Series booklet HS (G) 70.
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Description Where necessary, refer to the drawings in the Equipment Diagrams section.
Overview All numerical references relate to Figure 1. Also refer to the schematic diagram showing the valve positions in Data Sheet III. The apparatus, which is designed for experiments on both water softening and demineralisation, is supplied split into two major components; a backboard (1) incorporating the main process components and a sump tank arrangement (14) for storing and pumping the associated liquids. Ion exchange takes place inside two vertical transparent columns (5 & 6), of approximately 16mm internal diameter, mounted on the backboard via manifolds at the top (7) and bottom (2). In use the left hand column (5) contains Cation exchange resin (golden coloured granules) and the right hand column (6) contains Anion exchange resin (white coloured granules). The manifolds at the top (7) and bottom (2) of the columns are fitted with lever operated isolating valves which allow the flow to be directed through one or both columns, in either direction, to suit the process requirements. Screwed connectors fitted with ‘O’ ring seals (3) allow the columns to be removed for cleaning or changing the type of exchange resin. The liquids to be passed through the exchange columns are stored in the sump tank arrangement (14) to the left of the apparatus and supplied via a pump (12) and flowmeter (9). The liquids are selected by lifting and traversing the sliding tube arrangement (13) at the front of the sump tank. The pump is operated using the electrical switch (4) at the right hand side of the process backboard and is connected via an in-line electrical connector (16). The flexible tube from the pump outlet to the selector assembly returns excess liquid to the f eed tank for reuse. An adjustable pinch valve (19) on the bypass tube is adjusted to give the correct flow conditions. This valve must not be fully closed to avoid excess pressure in the system. A flow control valve (10) at the base of the flowmeter allows the flow of water, regenerating solution etc. to be adjusted as required. A distribution manifold (8) above the flowmeter allows the pumped liquid to be supplied to the top of the Anion column, to the top of the Cation column or to the base of both columns as required by the process, simply by opening the appropriate lever operated valve. After passing through the columns the treated water, exhausted regenerating solution or wash water is fed to an effluent storage tank at the rear of the sump tank arrangement via flexible tube (11) from the top of the columns or flexible tube (17) from the bottom of the columns. This tank incorporates a lever operated valve (15) to facilitate draining. Lever operated valves (V4, V10 or V16) allow samples of water to be collected for analysis if required. The equipment includes a battery operated digital conductivity meter (19) that is connected to an inline sensor (18) in the return line (17) to the effluent tank. This allows the quality of the water emerging from the ion exchange column(s) to be monitored.
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Armfield Instruction Manual The various processes involved in the experiments are as follows: a. Water to be softened, which will pass downwards through the Cation exchanger only. b. Water to be demineralised, which will pass downwards through the Cation exchanger and then upwards through the Anion exchanger. c. Regeneration solutions (followed by distilled or demineralised water for flushing), which are stored in separate tanks, and will pass downwards through either the Cation or the Anion exchange column. d. Water (preferably distilled or demineralised) which will pass upwards through either column to flush out any sediment and to release any air trapped in the resin.
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Installation Adv is or y Before operating the equipment, it must be unpacked, assembled and installed as described in the steps that follow. Safe use of the equipment depends on following the correct installation procedure. Installation may be completed using a basic tool kit.
Electri cal Supply Electrical su pply for version W9-A The equipment requires connection to a single phase, fused electrical supply. The standard electrical supply for this equipment is 220-240V, 50Hz. Check that the voltage and frequency of the electrical supply agree with the label attached to the supply cable on the equipment. Connection should be made to the supply cable as follows: GREEN/YELLOW
-
EARTH
BROWN
-
LIVE (HOT)
BLUE
-
NEUTRAL
Maximum Current
- 1 AMP
Electrical su pply for version W9-B The equipment requires connection to a single phase, fused electrical supply. The standard electrical supply for this equipment is 120V, 60Hz. Check that the voltage and frequency of the electrical supply agree with the label attached to the supply cable on the equipment. Connection should be made to t he supply cable as follows: GREEN/YELLOW
-
EARTH
BROWN
-
LIVE (HOT)
BLUE
-
NEUTRAL
Maximum Current
- 1 AMP
Note: Version W9-B consists of a W9-G with a loose transformer to step-up the 120V supply to 220V to suit the equipment. The transformer should be sited adjacent to the 120V mains outlet socket in the laboratory, in a dry location. The mains lead from the W9-G is simply plugged into the 220V outlet socket on the front of the transformer. Where a 220V electrical supply is available in the laboratory, the mains lead from the W9-G can be connected directly to the 220V electrical socket in the laboratory (stepup transformer not used).
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Armfield Instruction Manual
Electrical supply for version W9-G The equipment requires connection to a single phase, fused electrical supply. The standard electrical supply for this equipment is 220V, 60Hz. Check that the voltage and frequency of the electrical supply agree with the label attached to the supply cable on the equipment. Connection should be made to t he supply cable as follows: GREEN/YELLOW
-
EARTH
BROWN
-
LIVE (HOT)
BLUE
-
NEUTRAL
Maximum Current
-
1 AMP
Installing the Equipment All numerical references relate to Figure 1. Position the equipment in the desired location on a firm level bench, with regard to the service connections listed in Electrical Supply above. Locate the sump tank arrangement to the left hand side of the process backboard. The diaphragm pump (12) is removed to avoid damage during transit and should be attached to the mounting plate on top of the sump tank assembly (14) using the fixings provided. The easiest method may be to insert the screws from underneath, with the nuts positioned on the outside of the tank assembly. Connect the sliding tube arrangement (13) at the front of the tank to the pump (12) using the flexible tubing supplied. The tube which projects furthest into the tank should be connected to the side connection on the pump inlet (see Connection 1). The tube with the shortest projection should be connected to one side of the Tconnector on the pump outlet (see Connection 2).
Connection 1
Connection 2
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Installation Connect the inlet of the control valve (10) at the bottom of the flowmeter (9) to the outlet of the pump using the flexible tubing supplied.
Connect the two flexible effluent return tubes (11) from the top manifold and (17) from the inline conductivity electrode to the tappings on the side of the tank assembly. Connect the two parts of the inline electrical connector (16) between the pump (12) and the rear of electrical switch (4) on the process backboard. Install the battery in the conductivity meter following the instructions supplied with the conductivity meter. Connect the lead from the conductivity electrode to the socket at the top of the conductivity meter. The equipment is ready for commissioning.
Commissioning All numerical references refer to Figure 1 in the Equipment Diagrams. Also refer to the schematic diagram showing the valve positions in Data Sheet III. It is suggested that clean tap water is used for initial testing of the equipment. 1. Ensure the equipment has been assembled in accordance with the instructions in the Assembly section above. 2. Fill the four supply tanks at the front of the sump tank arrangement (14) with clean tap water (l litre approximately). 3. Ensure that the drain valve (15) on the effluent tank is closed. 4. Lift the selector tube (13) and confirm that it will traverse along to each of the four compartments. 5. Connect the equipment to the electrical supply and confirm satisfactory operation of the RCD by pressing the TEST button (Refer to the notes on Electrical Safety). Reset the RCD. 6. Check all flexible connections are secure and all valves are closed. 7. Open the flow control valve at the base of the flowmeter and set the valve to approximately mid-position.
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Armfield Instruction Manual 8. Move the selector (13) to tank C (Test Water), open valves V3 and V6 then switch on the pump and allow the pump to prime. Water will flow through the bypass and return to tank C. Gradually close the Hoffman clip on the flexible bypass tube until the flow through the flowmeter becomes steady, indicated by a steady reading on the float. If necessary adjust the flow control valve to give a reading on the flowmeter while adjusting the bypass. 9. When the bypass is correctly adjusted, excess water will be returned to tank C when the flow control valve is closed avoiding over pressurisation of the pump and pipework. If the bypass is too far open then oscillation of the flow will be visible in the flowmeter. If the bypass is closed too far then the system may become over pressurised which could result in damage to the system and leakage of fluid. 10. Open valves V3 and V9 then check that water returns to the sump tank after flowing upwards through the Cation column (6). Close both valves. 11. Open valves V2 and V12 and check that water returns to the sump tank after flowing downwards through the Cation column (6). Close both valves. 12. Connect a flexible tube to sample valve V10 or place a container beneath the valve. Open valves V2 and V10. Check that water flows from valve V10. Close both valves. 13. Open valves V3 and V9, check that water returns to the sump tank after flowing upwards through the Anion column (5). Close both valves. 14. Open valves V1 and V15, check that water returns to the sump tank after flowing downwards through the Anion column (5). Close both valves. 15. Open valves V2, V13 and V15, check that water returns to the sump tank after flowing downwards through the Cation column (6) then downwards through Anion column (5). Close all three valves. 16. Connect a flexible tube to sample valve V16 or place a container beneath the valve. Open valves V2, V13 and V16. Check that water flows from valve V16. Close all three valves. 17. Open the drain valve (15) on the effluent tank and ensure that it operates correctly. Close the valve. 18. Connect the lead from the inline conductivity sensor to the socket marked INPUT at the top of the conductivity meter. Switch on the conductivity meter (9) by pressing the Power button. Check that the meter indicates the conductivity and temperature of the water (a series of dashes in the display indicates that the selected range is not correct – move the range selector switch until the conductivity of the water is indicated). 19. Check the equipment for any leaks.
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Operation Operating the Equipment Refer to the Laboratory Teaching Exercises for details on operating the equipment.
Operation of the Conductiv ity Meter The conductivity meter is supplied separately and is designed to sit on the bench top alongside the equipment. The conductivity meter is powered by an internal 9 volt PP3 Alkaline battery. An inline conductivity sensor is installed at the outlet from the bottom manifold so that it can monitor the conductivity of the water following the ion exchange process. The lead from the inline sensor is connected to the socket marked INPUT at the top of the conductivity meter. To operate the conductivity meter, press the POWER button and adjust the position of the range switch until the meter indicates the Conductivity and Temperature. Dashes in the display indicate that the range switch is in the wrong position and should be adjusted to suit. The meter gives a direct reading of conductivity, corrected for temperature, in units of micro Siemens ( μS) or milli Siemens (mS) depending on the position of the range switch. For further information on the conductivity meter refer to the instruction leaflet supplied with the equipment.
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Equipment Specifi cations Overall Dimensions Height - 0.9m Width - 1.1m Depth - 0.45m
Electromagnetic Compatibility This apparatus is classified as Education and Training Equipment under the Electromagnetic Compatibility (Amendment) Regulations 1994. Use of the apparatus outside the classroom, laboratory or similar such place invalidates conformity with the protection requirements of the Electromagnetic Compatibility Directive (89/336/EEC) and could lead to prosecution.
Equipment Location The equipment is fully self-contained and is designed to be bench mounted. The equipment requires connection to a single phase, fused electrical supply. Four metres of supply cable are provided with the equipment. A source of clean water will be required for filling the sump tank and a suitable drain for disposing of effluent from the equipment (involving dilute hydrochloric acid, dilute sodium hydroxide and dilute sodium chloride). A mains electrical supply is required to operate this product. Refer to Electrical Supply in the Installation section.
Environmental Conditions This equipment has been designed for operation in the following environmental conditions. Operation outside of these conditions may result reduced performance, damage to the equipment or hazard to the operator. a. Indoor use; b. Altitude up to 2000m; c. Temperature 5°C to 40°C; d. Maximum relative humidity 80% for temperatures up to 31°C, decreasing linearly to 50% relative humidity at 40°C; e. Mains supply voltage fluctuations up to ±10% of the nominal voltage; f.
Transient over-voltages typically present on the MAINS supply; Note: The normal level of transient over-voltages is impulse withstand (overvoltage) category II of IEC 60364-4-443;
g. Pollution degree 2. Normally only nonconductive pollution occurs. Temporary conductivity caused by condensation is to be expected. Typical of an office or laboratory environment.
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Routine Maint enance Responsibility To preserve the life and efficient operation of the equipment it is important that the equipment is properly maintained. Regular maintenance of the equipment is the responsibility of the end user and must be performed by qualified personnel who understand the operation of the equipment.
General Disconnect the equipment from the electrical supply when not in use. Clean the storage tanks with distilled or deionised water and flush both columns if different solutions and resins are to be used. Drain any effluent contained in the sump tank after every experiment to a suitable laboratory drain. The conductivity sensor is usually cleaned adequately by passing clean water through the system after use. However, the sensor can be removed from the inline housing if manual cleaning of the electrodes becomes necessary. To remove the sensor, unscrew the large sealing plug on t he side of the housing (opposite the lead from the sensor) then push the sensor through the opening. The sensor can be totally removed by passing the plug through the opening. The sensor body is sealed into the housing using an ‘O’ ring. This should be lubricated with soapy water before reinserting the sensor into the inline housing after cleaning. Push the sensor fully into the housing then replace the large sealing plug.
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Laboratory Teaching Exercises Index to Exercises Exercise A Exercise B Exercise C Exercise D
Water Softening Theory The most usual ion exchange material employed in water softening is a sulphonated styrene-based resin, supplied by the makers in the sodium form. This resin has a strong affinity for calcium and magnesium ions, and will also remove ferrous ions after the more or less complete removal of calcium and magnesium. Softening can be carried out as a batch process by stirring a suspension of the resin in the water for a period until equilibrium, or an acceptable level of hardness, is reached. However, it is more convenient to operate a continuous flow process by passing the water slowly downwards through a column of resin beads. The exchange reaction takes place rapidly enough for the upper layers of the bed to approach exhaustion before the lower layers being able to exchange ions. There is thus, a zone of active exchange which moves down the column until the resin at all depths becomes exhausted. The position at an intermediate stage can be illustrated as shown below.
When the zone of active exchange reaches the bottom of the column, the emerging water begins to show an increasing hardness. This is the breakthrough point, when it becomes necessary to regenerate the resin with a strong sodium chloride solution.
Regeneration Theory Theoretically, for every millequivalent (meq) of hardness as CaCO 3 removed from the water under treatment, one millequivalent of NaCl is required for regeneration, ie. 1g of hardness as CaCO 3 removed requires 1.17g NaCl for regeneration (equivalent weights: CaCO3 50.0; NaCl 58.5).
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Laboratory Teaching Exercises In practice it is not possible to achieve complete regeneration with this quantity of NaCl, since this would require an unacceptable long contact period. Larger quantities of NaCl are therefore used, generally twice or more the theoretical amount. The regeneration efficiency is thus around 50%. A high level of regeneration gives a resin with a high exchange capacity approaching its theoretical, but it is uneconomic to operate at such a rate that this capacity is fully used in softening. In other words, a high regeneration efficiency is associated with a low degree of column utilisation, and vice versa. The practical operation of an ionexchange bed is therefore a compromise in which the regeneration efficiency and the column utilisation are both in the region of 50%. After regeneration, distilled or demineralised water is passed through the bed to wash out any remaining regenerant. Water to be treated by ion exchange must be free of suspended solids which would block the passage-ways, reduce flow rates and interfere with the exchange process. To remove fine solids which may get into the bed, and to release any air pockets, the column is backwashed periodically by an upward flow of water which fluidises the bed and agitates the resin beads. The rate of flow of water through the bed in softening is usually not more than 40ml/min per cm2 of surface area of bed. Regeneration rates are about one tenth of this.
Demineralisation Theory The removal of all dissolved salts from water can be achieved by using a two-stage ion exchange process. The water is first passed through a strong cation exchanger working on the hydrogen ion cycle, when cations in the water are replaced by H + ions, giving a solution of acids. This is then passed through an anion exchanger in the hydroxyl ion form, when the acid ions are replaced by OH - ions, which with the H+ ions, produce water. It is often sufficient to use a weakly basic anion exchanger, which will remove all anions except HCO3- (due to dissolved carbon dioxide) and H3SiO4- (due to dissolved silica). For a higher quality product water, a strongly basic anion exchanger must be used as the final stage, but it is generally more economical to precede this with a weakly basic anion exchanger of high exchange capacity to remove the bulk of the anions, and a degassing tower to release CO 2 from solution. The strongly basic resin is then required only to remove silica and any residuals of other anions which may still be present. This process can reduce total dissolved solids to below 1mg/l. Demineralisation can also be performed in a single stage by using a mixed bed of strong cation and anion exchangers. The water repeatedly comes in contact with the two resins alternately, and is ultimately of very high purity. To enable the two resins to be regenerated with sulphuric acid and sodium hydroxide respectively, they are first stratified with an upward flow of water, the anion resin being of lower density and therefore carried to the top. After regeneration, the two resins are re-mixed by compressed air.
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Armfield Instruction Manual
Resin Volume and Densit y When dry ion-exchange resins are immersed in water the beads swell as a result of hydration of the fixed and counter-ions (i.e. the attraction of water molecules to the ions) and the repulsion between the fixed ions. A distinction must therefore be made between the dry and wet volumes and densities of a resin. It is also important to wet a resin thoroughly, before placing in a test column, in order to avoid damage as a result of the swelling. The density of a resin can also be given as t he true density, i.e. mass per unit volume of the beads alone, or as the apparent density, i.e. mass per unit volume of the bed, including the voids. For typical resins the true wet density is usually between 1.1 and 1.3 g/cm3, and the apparent wet density between 0.7 and 0.8 g/cm 3. The inside diameter of both exchange columns is approximately 16mm. For accurate results it is suggested that the actual inside diameter of both columns is measured and recorded before filling with exchange resin.
Exchange Capacity The exchange capacity of a resin is a measure of the quantity of ions which can be exchanged per unit mass of volume of the resin. The theoretical exchange capacity may be defined as the number of exchangeable ions which it contains per unit mass or volume. In practice it is not feasible to provide a long enough contact period for complete equilibrium to be attained, nor is it economic to regenerate fully. The practical exchange capacity is therefore rather less than the theoretical. It is expressed in various units, of which the most useful are probably millequivalents (meq) of exchanged ions per gram of dry resin, and meq/l of wet resin bed. In the softening of water it is also common practice to express the exchange capacity in terms of mass of CaCO 3 rather than millequivalents, the usual unit being kg CaCO 3 /m3 of wet resin bed.
Data Sheet I Backwash
Figure 2
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Laboratory Teaching Exercises
Regenerate
Figure 3
Softening
Figure 4
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Armfield Instruction Manual
Data Sheet II Backwash
Figure 5
Figure 6
20
Laboratory Teaching Exercises
Regenerate
Figure 7
Figure 8
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Armfield Instruction Manual
Demineralise
Figure 9
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Laboratory Teaching Exercises
Data Sheet III
Schematic Diagram of UOP7 showing valve positions
Data Sheet IV To determi ne the hardness of a sample of water usin g Wankl yn soap solution Equipment required (not supplied by Armfield Ltd): Burette, typically 100ml capacity Stand for burette Measuring cylinder, typically 250ml Stoppered flask, typically 250ml capacity Wanklyn soap solution, typically 1 litre Procedure: Install the burette in its stand and fill the burette with Wanklyn soap solution. 23
Armfield Instruction Manual Measure the volume of the sample of water using the measuring cylinder. Either adjust the sample of water to a known volume eg. 50 or 100ml or note the actual volume of the sample. Transfer the sample of water from the measuring cylinder to the stoppered flask. Titrate 1ml of soap solution into the stoppered flask. Insert the stopper and shake the contents. Observe if a lather forms on the surface of t he sample. If no lather forms, titrate another 1ml of soap solution into the sample and shake the contents. Repeat the addition of soap solution until a lather forms. Note the amount of soap solution added to the sample of water. Calculation of Hardness: Using Wanklyn soap solution, 1ml of soap solution = 1mg of CaCO 3. Hardness of water =
mg of caCO3 of water Volume of water sample (ml) For example, if 10ml of soap solution is titrated into a 100ml sample of water before a lather forms, then the hardness of the water is:
= 200mg of caCO3 per litre of water
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Exercise A Objective To determine the exchange capacity of a cationic resin in the softening of water.
Procedure Make up 10 litres of water with a hardness of between 600 and 700 mg/l as CaCO 3 by dissolving an appropriate amount of calcium chloride in tap water, after allowing for the hardness already in the tap water. Determine the hardness of this solution using Wanklyn soap solution (described in Data Sheet IV) or other method, and place it in the test water reservoir. (This is much harder than waters normally encountered, but is used here in order to keep the duration of the softening experiment within reasonable limits). Make up 500ml of 10% w/v NaCl solution by dissolving 50g NaCl in distilled water. Place this solution in regenerant tank B. Fill tank D with distilled or deionised water (clean tap water can be used if more convenient). Backwashing See Data Sheet I, Figure 2 (Upward flow of water through the bed). Backwashing removes any sediment from the bed, ensures that the resin beads are fully wetted and swollen and removes any air pockets which would interfere with the ion-exchange process. Fill the left hand cation exchanger column with cation resin (golden coloured granules) to a depth of 300mm. Select tank D, open valves V3 and V6, and backwash for five minutes at a flow sufficient to expand the bed by not more than 50% (typically 100 ml/min). Gradually turn off the flow and measure the final depth of the resin. Do not drain the bed as this would allow air to enter. Regenerate See Data Sheet I, Figure 3 (Downward flow of salt solution through the bed). Select tank B, open valves V2, V12 (and V10 if a sample is required). Set flowmeter to not more that 50ml/min. Before the salt solution is fully used up, add distilled water to the regenerant tank, continue the flow through the bed until the effluent no longer tastes salty. Softening See Data Sheet I, Figure 4 (Downward flow of hard water through the bed). Select tank C, open valves V2 and V10. Set flowmeter to between 50 and 70ml/min. Collect samples of 500ml at five minute intervals. Determine hardness of each sample. Continue the softening until the hardness of the effluent rises above 100mg/l as CaCO3.
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Armfield Instruction Manual
Results and Calculations
Plot the hardness readings against the volume of water treated and note the breakthrough point at which the increase in hardness starts. Calculate the milligrams of hardness as CaCO 3 removed from the water up to the breakthrough point. Graphically, this is given by the area between the curve plotted and the horizontal line, representing the original hardness of the water. Knowing the wet volume of the resin bed, calculate the exchange capacity of t he resin as meq/ml of wet volume.
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Exercise B Objective To determine the regeneration efficiency of an ion-exchange softening system.
Procedure After completion of the experiment 'SOFTENING', ie. when the hardness exchange capacity of the resin has been used up to just beyond the breakthrough point, carry out regeneration with 500ml of 10% w/v salt solution but this time collect the whole of this solution after it has passed through the bed (via valve no. V10), draining the bed in doing so. (Note that having drained the bed in this experiment it will be necessary to backwash it (Data Sheet I, Figure 2) to expel air before carrying out any further experiments.) Determine the sodium ion concentration in the collected regenerant by measuring Na+ (after dilution) by flame photometry or other means. Knowing the volume of solution collected calculate the meq of NaCl which has passed through the bed. Hence by subtraction from the original quantity of NaCl applied (20g or 342 meq), determine the meq of NaCl actually used in regeneration. Compare this with the theoretical quantity of NaCl equivalent to the amount of hardness removed in the experiment 'SOFTENING' and hence, calculate the efficiency of regeneration as a percentage. The efficiency so calculated is based on the NaCl actually used in regeneration. However, in operation it is not possible to apply this quantity precisely, and an excess has to be applied. Further experiments may therefore be carried out using different quantities of NaCl for regeneration in solution from 5 to 10% in strength, in order to determine practical regeneration efficiencies. In these experiments the procedure should be as in the first experiment 'SOFTENING', ie. the used regenerant solution should not be collected, and the distilled water should be used to flush out the last of the regenerant from the bed. The efficiency will then be calculated by comparing the quantity of NaCl applied with the equivalent amount of hardness removed in experiment 'SOFTENING'. These regeneration experiments must, of course, be alternated with softening experiments so that softening capacity can be correlated with regeneration efficiency.
Results and Calculations Final resin depth (mm) = Sodium ion concentration (meq) =
Original quantity of NaCl
= 342 meq (20g).
Amount of NaCl collected
= 27
Armfield Instruction Manual Actual exchange capacity
28
= original quantity - volume collected
Exercise C Objective To study the demineralisation of water and to determine the exchange capacities of a hydrogen ion cation exchanger and an anion exchanger.
Procedure Fill the left hand cation column to a depth of 300mm with a cation exchanger resin (golden coloured granules) in the hydrogen ion form. Fill the anion column to a depth of 300mm with an anion exchange resin (white coloured granules) in the hydroxyl form. Fill tank A with 500ml of a 10% hydrochloric acid solution. Fill tank B with 500ml of a 5% sodium hydroxide solution. Fill tank C with 10 litres of test water containing 800 to 1000mg/l of dissolved solids. Fill tank D with distilled or demineralised water. If tap water is used, the concentrations of the principal cations and anions, as well as the total dissolved solids, must be determined if not already known (eg. from the water undertaking's figures). From a knowledge of the concentrations of the main cations and anions in the water to be used, calculate the total strength in meq/litre. This will be used in calculating the exchange capacities of the two resins. The electrical conductivity should also be measured. Additional equipment required (Not supplied by Armfield): pH Meter Stop Clock Backwashing See Data Sheet II, Figures 5 and 6 (Upward flow of water through the bed). Each column should be separately backwashed in the manner described in experiment 'SOFTENING'. In each case, the rate of backwashing should be controlled to give not more than 50% expansion of the bed. Measure the final depths of the two beds. Regeneration (CATION) See Data Sheet II, Figure 7 (Downward flow of acid through the cation bed). Regenerate the cation exchanger. Select tank A, open valves V2 and V12. Follow the acid with distilled or demineralised water from tank D, to flush out any surplus acid. Check pH of effluent and continue flushing until pH has returned to above 5.0. Regeneration (ANION) See Data Sheet II, Figure 8 (Downward flow of sodium hydroxide through the anion bed). Regenerate the anion exchanger. Select tank B, open valves V1 and V15, followed by distilled or demineralised water from tank D until pH of the effluent has returned to below 9.0. Demineralisation See Data Sheet II, Figure 9 (Downward flow of test water through both columns in series). 29
Armfield Instruction Manual Select tank C, open valves V2, V13 and V15. Set flow rate to between 50 and 70ml/min. Note time at which flow is started and take conductivity readings at 5 minute intervals. At 20 minute intervals draw off samples from valve V10 and measure the pH value. Note the time when the conductivity of the demineralised water begins to rise, i.e. the breakthrough point at which one of the resins has become exhausted. As soon as possible after this point, take another small sample from valve no. V16 and measure its pH. If this pH is higher than the values previously recorded, it indicates that the cation exchanger has become exhausted. It is advisable to confirm this by drawing one or two further samples for pH determination. The experiment should be stopped at this point, and the exchange capacity of the cation exchanger calculated. It is then possible to determine the exchange capacity of the anion exchanger in this experiment. If, on the other hand, the pH of the cation exchanger effluent continues at a low value, the rising conductivity of the final effluent indicates that the anion exchanger is exhausted, and its capacity can be calculated. In the latter event, the exchange capacity of the cation exchanger can be determined by continuing the flow of water through the first column only, collecting the water which passes through it and measuring pH values until the breakthrough point, when the pH begins to rise.
Results and Calculations
In the demineralisation experiment, the breakthrough point is detected by readings of pH (for the cation exchanger) or conductivity (for the anion exchanger) instead of by direct measurement of concentrations as in the softening experiment. In order to calculate the exchange capacities in terms of millequivalents, it is necessary to convert pH or conductivity readings to meq/litre. a. To convert pH values to meq/l If pH reading is x Hydrogen ion concentration = 10 -x gram-moles/litre = 103-x meq/litre b. To convert conductivity values to meq/l For water with a given content of salts, the electrical conductivity is closely proportional to the concentration of total dissolved solids. Although these solids consist of several salts of varying electrolytic properties, it is sufficiently
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Exercise C accurate to assume that electrical conductivity is also proportional to the total concentration in terms of meq/litre. The constant of proportionality was established by determination of the electrical conductivity and the strength of meq/l of the raw water. Hence the electrical conductivity of the demineralised water can be converted to meq/l. In any event these figures should be very low. Final Depths: CATION = ANION =
Exchange capacities can now be calculated.
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Exercise D Objective To determine the regeneration efficiency of a cation resin and an anion resin.
Procedure CATION RESIN Fill tank A with 500ml of a 10% hydrochloric acid. Fill tank B with 500 ml of a 5% sodium hydroxide solution. Fill tank C with 10 litres of test water. Fill tank D with distilled or demineralised water. Backwash Select tank D, open valves V3 and V6. Regenerate Select tank A, open valves V2 and V10. Collect the whole of the solution. ANION RESIN To determine the regeneration efficiency of the anion resin it will be necessary to carry out the full demineralisation experiment 'DEMINERALISATION'. Note: Since the exchange capacities of cation resins are generally greater than those of anion resins, it is expected that the anion resin will be first to be exhausted.
Results and Calculations Final Depth (CATION)
=
Final Depth (ANION)
=
Sodium ion concentration (meq/ml) =
Original quantity of sodium hydroxide used = Amount of sodium hydroxide collected
=
Actual exchange capacity = Original quantity - amount used
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