Purolite Ion Exchange Design Calculation ProgramContents of help:

 

20  Water Treatment

21  Softening

22  Dealkalisation

23  Demineralization

24  Working Mixed Beds

25  Polishing Mixed Beds

26  Nitrate Removal

29  Program Difficulty

51  Influent Water Data

55  Design Calculation

56  IX Process Options

57  Treated Water Specifications

58  Neutralization of regenerants

59  Dealkalisation Options

61  Mixed Bed Options

62  Nitrate Removal

63  Design Calculation – Mixed Beds

64  Mixed Beds

65  Water Analysis

66  Extra Analytical Data

67  RAW WATER origin and pretreatment

68  Cycle Time and Flow Rate

69  Choice of Resin

70  Regeneration

71  Plant Design

72  Treated Water Quality

73  Pressure Drop Calculation

74  Operating Conditions – Working Mixed Beds

75  Operating Conditions – Polishing

76  Design Objectives

77  Overrun

20 Water Treatment

 Select the type of process to be carried out in your plant.

 SOFTENING: Exchange of hardness ions for sodium ions.

 DEALKALISATION: Removal of hardness associated with bicarbonates (alkalinity) using a weak acid resin. The program also includes permanent hardness removal by use of a strong acid cation resin, in the softening mode which is sometimes used in breweries.

 DEMINERALISATION: Water can be demineralised by means of cation and anion resins in separate vessels. Weakly functional resins can achieve partial demineralization with economical use of regenerants. More complete demineralization requires the use of strongly functional resins and higher regeneration levels.

  MIXED BEDS : Water can be purified to higher standards using a Working Mixed Bed or further purified after standard demineralization using Polishing Mixed Bed to remove any leakage remaining. Where the inlet load is negligible, this process is termed "polishing". If the ion exchange load is high enough to utilize a substantial proportion of the available operating capacity, such as water treatment directly after demineralization, this is termed a working mixed bed.

 NITRATE REMOVAL: There is a recommended limit for nitrate in potable water published by the World Health Organization. Consequently many countries have placed their own limits to cover the quality of the potable water available. Nitrate is removed by strong base anion resins. Where the water to be treated contains sulfate, this is removed preferentially, and nitrate capacity is reduced because the resin is loaded with sulfate. Purolite 520E is selective for nitrate over sulfate and all other common anions, thus all the capacity is available for nitrate removal.

 21 Softening

 1) The standard choice is 1 – Purolite C-100 or C-100E. Only a change to very special conditions, (high osmotic shock, very high TDS, presence of oxidizing agents etc.) would create a need to select another resin. The Purofine grade offers advantages of a smaller plant and use of less regenerants.

 2) Select one of the options shown in scroll box. Option 1 (Co-flow), used by default, is simpler to construct and operate. However salt utilization and hardness leakage are both high. Option 2 (Counter-flow) offers the lowest leakage.

 3) Input the Regeneration level grams per liter of resin:

·Co-flow (Option 1): 90–300  g/l

·Counter-flow: 40–150  g/l

 4) Standard concentration is 10% . Other concentrations will reduce operating capacity especially for Co-flow operation.

 5) Bed depth is important for counter-flow operation. Deeper beds above 0.7 m give higher capacity and lower leakage.

 6) The design program calculates capacity and leakage. If results are unsatisfactory, changes in regenerant dosage, mode, and flow rate can offer improvements.

 22 Dealkalisation

 Purolite weak acid cation resins are capable of removing cations associated with bicarbonate anions (or carbonates/hydroxides) to reduce total solids, and especially temporary hardness. Its main advantage is that regeneration can be achieved with practically a stoichiometric quantity of regenerant.

 The capacity for temporary hardness ions is particularly high, especially at low flow rates. (Removal of temporary hardness can reduce precipitates and scale which normally occur on the boiling of water.) Where bicarbonates are associated with sodium rather than hardness ions the operating capacity is significantly reduced. It should also be noted that lower feed water temperatures reduce operating capacity significantly. Also low regeneration temperatures increase the risk of calcium sulfate precipitation. Flow rates should be maintained to ensure regenerant is removed from the resin before precipitation commences.

Dealkalisation can also form part of a demineralization process, the salts of mineral acids are treated on the strong acid cation filter which follows the weak acid cation filter. Depending on the proportional load on each filter it can also be advantageous to allow some or all of the sodium alkalinity to over-run to the strong acid filter. The redistribution of load can produce a better balance in the size of filters and can have the added advantage that the operating capacity of the weakly functional resin improves if the hardness to alkalinity ratio being treated approaches 1.

  

23 Demineralization

Demineralization works by exchanging all cations of salts present in the water to be treated to hydrogen, thus converting the salts to acids. Passing the water through a following strong base resin in the hydroxide form will exchange the anions for hydroxide by acid neutralization to produce demineralised water of reasonably good quality. To obtain purer water a polishing stage should be added. This will form part of a separate design program. Unfortunately it is quite difficult to regenerate resins with strongly functional active groups, especially strong base anion resins which have high selectivity for the mineral anions, sulfate, nitrate, and chloride. A large excess of sodium hydroxide is therefore necessary to achieve a good regeneration. The Type-I strong base anion resins Purolite A-400, A-600, A-500, A-505, are more thermally stable than Type-II resins, Purolite A-200, A-300, A-510 and they are also more selective for weak acids. However they are the most difficult to regenerate. Acrylic Type-I resin Purolite A-850 can offer good silica removal and reasonable regenerability, however this type is also less thermally stable. Type-II resins are also easily regenerated, but silica leakage is often significantly higher.

Of the resins mentioned, Purolite A-500, A-505, and A-510 are macroporous. This more open structure also offers significant improvement in terms of resistance to organic fouling compared with the gel counterparts. However the acrylic resins Purolite A-850 and A-870 offer an even more effective solution to this particular problem. These resins also have superior resistance to osmotic shock compared to gel-type polystyrenic resins. Substantial savings can be made to regeneration costs by introduction of resins with weak functionality before their strong resin counterparts. Weak base resins are frequently used in front of strong base resins. These effectively remove mineral acids which can be regenerated with alkali using only an excess of 20% over theory in many cases. On the other hand strong base resins often require over 50% stoichiometric excess alkali for effective regeneration. The strong base resins may then be used to remove the weak acids such as silica and carbon dioxide.

 Hence there are a large number of process options to choose from. Purolite technical sales staffs are knowledgeable in making the required choices. Purofine variations of these resins may be chosen. They may be used at higher flow rates, in shallower beds, at lower levels of regeneration, offering considerable savings in running costs and producing better treated water quality. These differences necessitate different operating conditions from those used for standard resins so default values for standard resins are not appropriate. If the Purofine option is chosen at the Design Option stage the correct default values may be applied making for a more rapid solution. Of course only Purofine Grades may be used here.

 

 24 Working Mixed Beds

 For operation, please obtain the separate disc from Purolite. Working mixed beds are used to directly ionize a feed water, typically with low total dissolved solids. They may also be considered when the residual leakage after conventional 2–4 stage [demineralization] processes is high. The section on Water Treatment explains their use in more detail. Influent Water Data describes the water to be treated and explains the impact of the water analysis on the process. Ionic loads are lower than those treated by demineralization, so specific flow rates and linear velocities may be higher than those used for demineralization.

 Beds should be sized to optimize flow rates. However constraints to meet ionic loads and resin operating capacity can apply.

 

 

25 Polishing Mixed Beds

For operation, please obtain the separate disc from Purolite. Polishing is the term applied to the removal of the last traces of ionic impurities in treated water. For further information see Influent Water Data. Efficient polishing is normally achieved using highly regenerated mixed beds of strong acid cation resins and strong base anion resins. Because the ionic loads are very low, the flow rates used are usually much higher than those used for demineralization. The bed size is designed to optimize flow rate. The choice of IX process options enables selection of three polishing systems, preferred resin ratio, internal or external regeneration, use of Trilite, and, if chosen the volume of inert resin required. The Design Calculation enables sizes of the anion and cation resin components to be calculated. 

26 Nitrate Removal

Nitrate removal works similarly to water softening. The resin is used in the chloride form, and the nitrate ion is exchanged for chloride. The regeneration is made with sodium chloride (usually at a concentration of 10%). Counter flow regeneration is generally recommended. This gives a lower nitrate leakage. If a higher leakage is acceptable it is generally more economic to blend back raw water than to use Co-flow regeneration. If co-flow regeneration has to be used for any reason, it is often advisable to give the resin a mix after the regeneration rinse. This will disperse the bank of nitrate form resin left at the bottom of the bed and produce a lower nitrate leakage initially and a more consistent leakage through the run. In order to further improve the quality of the treated water up to 25% of the sodium chloride, may be replaced with a sodium carbonate wash at a concentration of 5–10%. For each reduction of 10g/L of sodium chloride a replacement with at least 20 g/L of the sodium carbonate is needed. In any case significant losses in performance can often be expected if the sodium chloride level falls below 90g of NaCl/Litre of resin.

The choice of resin will depend upon the feed water. In particular the ratio of nitrate/nitrate + sulfate will determine the choice of a conventional resin or a nitrate selective resin, Purolite A520E. If in doubt, the latter is recommended. In any case Purolite A520E is recommended when the above ratio is less than 0.6. In fact advantages in operating capacity are not usually noticed unless the ratio is less than 0.5, however there are other advantages obtained from using Purolite A520E. Firstly, if there are sulfates in the water, over-run of the cycle, can produce water which is higher in nitrate than the original feed solution. This is because the sulfate displaces and concentrates the nitrate in the ion exchange resin. When the Purolite A520E is used the worst scenario is that the water remains as if there were no nitrate removal treatment. Thus when using a standard resin more careful and expensive monitoring is recommended. Secondly the nitrate selective resin does not, on average, substantially remove the sulfate from the feed water by exchange of this ion for chloride. Hence there is less risk to exceed the limits of chloride in potable water. (WHO limit is 450 mg/L).

The problem of exceeding the WHO limits on chloride and sulfate (250 mg/L) can be lessened by the use of a bicarbonate wash after the regeneration. This means that during a portion of the run, chloride is exchanged for bicarbonate, while sulfate is also retained in the resin. Thus the average leakage of the anions of mineral acids is reduced.

Where the waters to be treated are high in hardness ions there is a possibility of precipitation of these ions in the resin. The use of a small softener to treat the water used to dilute the sodium chloride regenerant and for the water used for the displacement rinse is required to avoid this problem. The Puredesign softening program can be used if necessary. It is also possible to combine nitrate removal and softening. The SAC and SBA resins may be combined as layers in one vessel with the SAC resin as the lower layer.

The Puredesign programs for nitrate removal and softening are used to find the solution for each layer in the vessel. When working out the vessel geometry enough room should be left to accommodate the partner

resin. The recommended aspect ratio (height/diameter) for each layer should be less than 1 and the bed depth of each layer greater than 750 mm.

When conventional resins are chosen, the choice will depend on a number of factors. Type I resin will in general maintain a slightly higher capacity than the Type II resin, and give a slightly lower leakage in counter-flow operation. This is so small, it is not shown in the program. If there is any risk of high pH in any part of the cycle, Type-II resins are preferred. When operating at higher flow rates, or in vessels of higher aspect ratios (near or above 2) macroporous resins are preferred.

The nitrate removal process is rarely if ever affected by organic fouling. The need for quite high levels of sodium chloride for regeneration to displace the nitrate avoids this problem. Like all resins, high levels of iron in the feed water should be avoided, see the warning on water analysis. When ion exchange resins are used for potable applications, the control of bacteria is of great importance. This is particularly true of nitrate removal. Nitrate is a nutrient, and if allowed to remain on exhausted resin is can support the rapid growth of bacteria throughout the ion exchange plant. Once this happens, it can be difficult to remove. For more information on resin storage and disinfection, please refer to the Purolite bulletins "The storage and transportation of ion exchange resins" and "The fouling of ion exchange resins and methods of cleaning". Briefly if the plant has to be shut down, the resin should be backwashed, treated preferably with alkaline brine or regenerated, and stored in salt solution.

29 Program Difficulty

 Was the complete water analysis entered? If not, please obtain more data. Otherwise please contact your local sales office with details of resin type in operation at the time of warning.

 

 51 Influent Water Data

For further information select one of the options below.

RAW WATER origin and pretreatment

WATER ANALYSIS

EXTRA ANALYTICAL DATA

MIXED BEDS

55 Design Calculation

This help screen covers several applications, so do not be surprised to find comments not specifically relevant to any particular treatment.

First screen has been provided for the last filter in the process option design. Each screen provides the full design data for one resin filter. Screens sequenced from the last filter to the first.

It may be useful to choose the resin you require at the outset, if you are familiar with your final requirements. Otherwise a choice can be made to enter any two from cycle time, flow rate, and net run. When these are added the third value and full design data will be calculated for a set of default values. Any parameters highlighted with a yellow letter can be altered to provide the required design conditions. When all values are acceptable "CTRL+ENTER" will provide the next screen so that details of this filter can be entered and adjusted. In many cases there will be an interaction between the filters. Hence you will be

returned to the first screen displayed, so that further changes can be made as necessary. You are invited to proceed through the design until design data for the process is complete. The Home key provides return to the first screen (SBA or WBA) if further changes are required. At this stage there is an option to neutralize regenerants.

For further information select one of the options below.

CYCLETIMEANDFLOWRATE

CHOICEOFRESIN

REGENERATION

PLANTDESIGN

TREATEDWATERQUALITY

PRESSUREDROPCALCULATION

OVERRUN

 

56 IX Process Options

 The choice of process options will depend on several factors. The following comments are offered for guidance. Where the plant is small (less than 1–1.5 m3 of resin per filter) a primary objective is to minimize capital cost, by reduction in the number of units and by not using a degassing tower. Degassing towers are optional in all process layouts.

 Ion Exchange Process Option 1: SAC–WBA

 This option is not frequently used, particularly on a large industrial scale. Regeneration is very efficient, but water produced is of a comparatively high conductivity, and pH can be less than 7 but variable for a major part of the treatment cycle. Silica and carbon dioxide are not removed.

 Ion Exchange Process Option 2: SAC–SBA

 Advantages: Reasonable quality treated water used in any design where savings of regenerants are of lesser importance. CF (Co-flow) exhaustion/regeneration can be used where a conductivity of < 20 micro Siemens is required. CTF (Counter-flow) – where < 4 µS is required.

 Ion exchange Process Option 3: SAC–WBA–SBA

 This is recommended where EMA/Total Anions is substantial. Advantages: Where a degasser is not justified (low alkalinity in the feed water). Ideal choice where regenerant savings are important. Counter-flow operation is used where low leakage of ions is required and where use of regenerates is minimized. The user should remember to choose correct bead size grades as applicable.

 Ion exchange Process Option 4: WAC–SAC–WBA

 This is rarely used. It would only be chosen to treat water high in bicarbonates where treated water quality is not critical, and regenerant savings are of first priority.

 Ion exchange Process Option 5: WAC–SAC–SBA

 For treating waters high in bicarbonates, a degasser is useful for larger plants. The user should remember to choose correct bead size grades as applicable.

 Ion exchange Process Option 6: WAC–SAC–WBA–SBA

 This is the most complicated option and the one in which the design program can save the operator the most time. It is used to treat waters with substantial concentrations of bicarbonates, and equivalent mineral acidity. Large plants would be operated with a degasser. The higher capital cost offers compensation in low running costs, low effluent release, and ease of neutralization. Layered beds in the same vessels can reduce capital costs. If there are no separate compartments in the vessels, the user must remember to choose correct bead size grades as applicable.

 Ion exchange Process Option 7: SAC–SAC–WBA.

 This is rarely used as it produces water of low overall quality, with good removal of cations. It is however used with SAC–>SBA to follow as a polisher. This can produce a quality almost as good as counter-flow quality, when used in the co-flow mode. The design program can be used for the full design by operating in two separate stages. The treated quality from Option 7 is used as influent for Option 2.

 Ion Exchange Process Option 8: SAC–SAC–SBA

 This is a useful option when co-flow is chosen, offering some of the advantages of counter-flow operation, both in terms of regenerant utilization and in terms of ion leakage in comparison to a single co-flow system.

 Ion Exchange Option 9: SAC–SAC–WBA–SBA

 This is used in place of Option 8 where there is substantial equivalent mineral acidity and offers savings in regenerant costs. Where bicarbonates are significant and plant is large, use of a degasser is recommended. The user should remember to choose correct bead size grades as applicable.

 Having chosen the required process option, the degasser option if selected offers a choice of positions. This is only important where a WBA resin has been selected. The presence of CO2 improves the performance of polystyrenic resins, A100, A105, A103 and A106; hence the degasser should be placed in position 2. Acrylic resins on the other hand can give problems if loaded with CO2 so unless it is certain that performance requirements will be met, for A845 position 1 is recommended.

 Options for regeneration offer Co-flow and Co-flow Purofine as separate options. It is preferable to choose Purofine at this stage if this option is preferred. PUROFINE RESINS can offer a wide range of advantages depending on the conditions of operation. In softening it can offer higher capacity with consequently smaller vessels, it can also offer better regeneration efficiency and reduced running costs. Also, if required, superior kinetics offer the option to work at higher flow rates and smaller bed depths, offering greater flexibility of design. These changes can be achieved with minimal increase in pressure drop in carefully engineered systems. Early choice of PUROFINE will set default values so that the program will run more smoothly to the required design. Five counter-flow options are included. PB denotes packed bed systems with down-flow service, regeneration up-flow; FB denotes up-flow service with regeneration down-flow. Std. covers the more traditional air, water, or resin hold down systems and split-flow regeneration. The performance of any individual engineering design is as much a function of the detailed and sophisticated engineering as it is of the resin per se. It therefore makes it very difficult to distinguish performance differences which are caused from regeneration mode rather than from particular engineering virtues. Thus a standard set of performance data has been chosen for all three main types of counter-flow regeneration mode. Of course choice of the recommended particle size ranges can offer differences in pressure loss, and

this data is included. The choice of LB denotes the use of a layered bed of resins with weak functionality as an upper layer and strongly functional resin below. Such systems are operated with up flow counter-flow regeneration, and can offer both savings in the number of vessels and economical regeneration. Counter flow Purofine is also a choice. This may offer a useful combination of high quality of treated water together with minimum rinse volumes and some small gain in operating capacity and /or regenerant savings. It may also be possible to achieve mixed bed quality by specific counter-flow design using Purofine grades.

57 Treated Water Specifications

 The program provides the option to work to specifications for both conductivity and silica levels. In any ion exchange process, the ion leakage may be relatively high after regeneration, depending upon the level of regeneration, but will rapidly fall to a base permanent leakage. This value will be maintained during most of the run; hence in general the average leakage will be close to the permanent leakage. At the end of the cycle, the conductivity will rise, as will the silica concentration, as the resin becomes exhausted. Hence it is necessary to set acceptable end point limits for both conductivity and silica concentration, as requested. Accordingly it is important that the end point levels are sufficiently higher than the average levels specified, in order that the end point can be clearly identified by the measuring equipment. This is done automatically when average levels are entered. However options are available to set end-points closer to or further from average levels. The default levels chosen are typical ones usable in a general case. They are chosen to allow for a general ease of running of the program across the range of designs. Conversely it is not the intention to suggest that any particular design option should be ideal to treat the default water analysis provided. In addition leakage and end-point default values are given a wider default spread than that which can be achieved in specific operation. Hence the user should not be surprised if, when a higher average leakage value is entered, a lower end point than the default is suggested.

58 Neutralization of regenerants

 Regeneration neutralization is often an essential requirement. To achieve this it is necessary to increase the regenerant on either the cation system or the anion system. Generally the minimum regenerant levels have been set to achieve the specification of the treated water quality as economically as possible. It follows therefore that the regeneration level with the higher excess cannot be reduced. Usually this is the value which should be fixed.

 In some cases the discrepancy is so wide that the computer cannot achieve neutralization. Unless a change of process option can be made, the only alternative is to neutralize externally.

 It can pay dividends to look carefully at changes to leakage levels as a result of the neutralization exercise. It is possible that resin interaction has brought water quality well within specification and both regenerants can now be reduced without loss in performance. Also changes in overrun from weak to strong resins may offer advantages at the higher regeneration level. Moving through the screens several times before finally optimizing the neutralization may be worthwhile. If it is necessary to reduce regenerants after neutralization has been invoked, it is recommended to store the file, recover and modify the input data. In some cases reinspection using "Home" will provide a lower regeneration level to work with. If there is a substantial load of bicarbonate on the resin, the regeneration will produce salts including sodium carbonate. These salts will give a pH over 8, even though the excess hydroxide has been correctly neutralized. It is therefore necessary to increase the acid needed to neutralize these salts. In such cases the excess of acid can appear to be too high to match the anion excess, even when the regenerant mixture is correctly neutralized.

59 Dealkalisation Options

 The dealkalisation option contains the choices of direct dealkalisation which uses solely a weak base resin. This option offers removal but allows the permanent hardness to remain in the treated water. A simple softening treatment to follow the dealkalisation affords the possibility of removing all the hardness whilst at the same time giving removal of TDS associated with the temporary hardness. Where possible it is recommended to place a degasser or some means to reduce the carbon dioxide generated. Where the CO2 is still present it is recommended to use a design factor of 0.75 for the softener.

61 Mixed Bed Options

 The option to choose working or polishing mixed beds is made at the outset (water treatment options). This provides a suitable set of default values in the analysis table. Please refer to Influent Water Data. The Flow Rate in m3/h is required. Working mixed beds usually follow two stage demineralization. For working mixed beds proceed to resin ratio options. If condensate polishing was chosen, further options apply. Make-up can be used to further purify treated water from a counter-flow plant or a working mixed bed. The Make-up Polishing operates in the same way as the mixed bed condensate polishing. The operator is required to choose the correct option according to his needs and design accordingly. The option using a separate cation filter may be chosen where the cation load is especially high, for instance where a high capacity for ammonia, amines, or iron is needed. The print-out naturally records that make-up or condensate polishing has been chosen. Options of resin ratio are chosen by the operator, having used the program to determine the volumes of each component required for the chemical treatment. In some cases the volume of one of the components may be lower than the minimum requirement to achieve suitable contact between resin and solution. 40% cation generally offers a slight excess of cation capacity, for most waters. However, condensate, for example dosed with ammonia may have an excess of cations requiring treatment, because the ammonia is not fully ionized. Likewise iron may be present as soluble oxides, and only the cation component is needed for their removal. Conversely waters high in silica may need an excess of anion resin to achieve the correct component balance. When treating condensate contains, an excess of cation resin can be preferable, one of the main objectives is to protect the condensate from contamination with sea water which arises from condenser leakage. If chlorides are not contained by the anion resin, this has the advantage of a more sensitive end-point when hydrogen chlorides are eluted, which gives a higher conductivity per ppm of chloride leakage. Higher ratios of cation/anion are designed to deal with metal hydroxides arising from corrosion, or for ammonia removal. If low silica leakage is the main priority, stoichiometric ratios should be chosen. Internal regeneration is of course simpler and more economic. However, it can give rise to regenerant hide-out in any joints or vessel irregularities, so reducing treated water purity. Also the risk of regenerant passing directly to the treated water distribution is eliminated. In general, better resin separation can be made in an external regeneration backwash vessel, than can be made in situ. For condensate polishing at the highest flow rates, external regeneration is essential. The regeneration distribution and collection system inside the polisher can increase the pressure-loss prohibitively. The Trilite Option should be chosen where an inert spacer is required. Care must be taken when making this choice. The presence of oil and grease can make the system unworkable by contamination of the inert phase. In such cases, it is often recommended to save the interface after backwashing, and reintroduce at the next backwash. This can be done using the two-phase conventional mixed bed design option, or by adding a notional inert to Trilite (say 1mm bed depth). If this option is used the inert data must be deleted from the print-out. For the Trilite option it is essential to select TL grade resins in order that the inert resin is properly positioned. Other grades may be selected for comparative purposes on pressure loss etc. For the two phase option, MB and PL grades are recommended for working mixed beds, MB, TL, PL or Purofine grades are recommended for polishing mixed beds. FL grades should not be used without consulting Purolite. Purofine grades are recommended for the production of Ultra-Pure Water.

 

62 Nitrate Removal

 Nitrate removal works similarly to water softening. The resin  is used in the chloride form, and the nitrate ion is exchanged for  chloride. The regeneration is made with sodium chloride (usually at  a concentration of 10%). Counter flow regeneration is generally  recommended. This gives a lower nitrate leakage. If a higher  leakage is acceptable it is generally more economic to blend  back raw water than to use Co-flow regeneration. If co-flow  regeneration has to be used for any reason, it is often advisable  to give the resin a mix after the regeneration rinse. This will  disperse the bank of nitrate form resin left at the bottom of  the bed and produce a lower nitrate leakage initially and a more  consistent leakage through the run.  In order to further improve  the quality of the treated water up to 25% of the sodium chloride,  may be replaced with a sodium carbonate wash at a concentration of  5–10%. For each reduction of 10g/L of sodium chloride a replacement  with at least 20 g/L of the sodium carbonate is needed. In any case  significant losses in performance can often be expected if the  sodium chloride level falls below 90g of NaCl/Litre of resin.

 The choice of resin will depend upon the feed water. In particular the ratio of nitrate/nitrate + sulfate will determine the choice of a conventional resin or a nitrate selective resin, Purolite A520E. If in doubt, the latter is recommended. In any case Purolite A520E is recommended when the above ratio is less than 0.6. In fact advantages in operating capacity are not usually noticed unless the ratio is less than 0.5, however there are other advantages obtained from using Purolite A520E. Firstly, if there are sulfates in the water, overrun of the cycle, can produce water which is higher in nitrate than the original feed solution. This is because the sulfate displaces and concentrates the nitrate in the ion exchange resin. When the Purolite A520E is used the worst scenario is that the water remains as if there were no nitrate removal treatment. Thus when using a standard resin more careful and expensive monitoring is recommended. Secondly the nitrate selective resin does not, on average, substantially remove the sulfate from the feed water by exchange of this ion for chloride. Hence there is less risk to exceed the limits of chloride in potable water. (WHO limit is 450 mg/L).

 The problem of exceeding the WHO limits on chloride and sulfate (250 mg/L) can be lessened by the use of a bicarbonate wash after the regeneration. This means that during a portion of the run, chloride is exchanged for bicarbonate, while sulfate is also retained in the resin. Thus the average leakage of the anions of mineral acids is reduced.

 Where the waters to be treated are high in hardness ions there is a possibility of precipitation of these ions in the resin. The use of a small softener to treat the water used to dilute the sodium chloride regenerant and for the water used for the displacement rinse is required to avoid this problem. The Puredesign softening program can be used if necessary. It is also possible to combine nitrate removal and softening. The SAC and SBA resins may be combined as layers in one vessel with the SAC resin as the lower layer.

 The Puredesign programs for nitrate removal and softening are used to find the solution for each layer in the vessel. When working out the vessel geometry enough room should be left to accommodate the partner resin. The recommended aspect ratio (height/diameter) for each layer should be less than 1 and the bed depth of each layer greater than 750 mm.

 When conventional resins are chosen, the choice will depend on a number of factors. Type I resin will in general maintain a slightly higher capacity than the Type II resin, and give a slightly lower leakage in counter-flow operation. This is so small, it is not shown in the program. If there is any risk of high pH in any part of the cycle, Type-II resins are preferred. When operating at higher flow rates, or in vessels of higher aspect ratios (near or above 2) macroporous resins are preferred.

 The nitrate removal process is rarely if ever affected by organic fouling. The need for quite high levels of sodium chloride for regeneration to displace the nitrate avoids this problem. Like all resins, high levels of iron in the feed water should be avoided, see the warning on water analysis. When ion exchange resins are used for potable applications, the control of bacteria is of great importance. This is particularly true of nitrate removal. Nitrate is a nutrient, and if allowed to remain on exhausted resin is can support the rapid

growth of bacteria throughout the ion exchange plant. Once this happens, it can be difficult to remove. For more information on resin storage and disinfection, please refer to the Purolite bulletins "The storage and transportation of ion exchange resins" and "The fouling of ion exchange resins and methods of cleaning". Briefly if the plant has to be shut down, the resin should be backwashed, treated preferably with alkaline brine or regenerated, and stored in salt solution.

 

63 Design Calculation – Mixed Beds

 DESIGN PRINCIPLES:

 In principle the design calculation operates in an identical fashion. However in most cases the objectives are quite different. Optimizing capacity and ion exchange load are generally less important than optimizing linear flow rate and specific flow rate.

For further information select one of the options below.

OPERATING CONDITIONS – WORKING MIXED BEDS

OPERATING CONDITIONS – POLISHING

DESIGN OBJECTIVES

 

 

64 Mixed Beds

Two water treatment options are available. Working Mixed Beds should be chosen where the mixed bed is required to purify a raw water or from a two stage deionization, or indeed any water where the concentration of total cations or anions is roughly in the range of 5–30 ppm CaCO3 or 0.1–0.6 meq/l. For higher concentrations basic demineralization followed by working or make-up polishing mixed bed should be considered. For lower concentrations or where water conditioning, for example with ammonia, is being used, the polishing program is more suitable. Default values with appropriate ionic concentrations are available for all mixed bed applications. For condensate polishing the concept of default values is different. The default value given represents only a kind of average over a period when for example a small leak has remained undetected. More usually it is preferable to enter data for the pure condensate and if necessary the analysis of a simulated leak to investigate what happens to the performance. Care should be taken to avoid default values used for demineralization by choosing "New Project" from the File options. There is a facility to add ammonia or amines to both mixed bed options. These will be included in the load. To estimate the capacity if operations past the ammonia break is intended, the ammonia/amine should be omitted.

 IRON LOADING

 The behavior of iron can be complex. In particular for condensate polishing it is usual that a high proportion of the total iron is insoluble or particulate. For this reason the entered value is divided by five to calculate the soluble portion which is treated as ionic load. If it is known that the soluble load is higher, that soluble value should be increased by a factor of five to determine the correct ionic load from iron. This applies to all polishing programs, including make-up. Condensate can be passed through the make-up plant in certain situations, and in is convenient to treat all polishing in the same way. The objective of any design

should be to prevent any build-up of iron on the resin. It presence can affect resin kinetics of both anion and cation resins. It can cause fouling which eventually becomes irreversible. It acts as a catalyst for the oxidation of the resin structure. This in turn can increase the concentration of organic leachable into the condensate as well as causing slow degradation of the resin. Special care should be taken to avoid iron loading on resin together with residual oxygen when operating with oxygen rich condensate.

 

65 Water Analysis

 Concentrations may be entered either in meq/l or in ppm of calcium carbonate. Where ionic concentrations are very low, for example, when entering condensate data, more accuracy is obtained by operating in ppm rather than meq/L. The choice is made at the centre of the water analysis screen. Meq/l may be converted to ppm of calcium carbonate by multiplying by 50. Ions in ppm, "as is" should be converted to ppm of calcium carbonate by dividing the value by the equivalent weight of the ion, and multiplying by 50 (one can use calculator by pressing F8 button). French degrees can be converted to ppm of calcium carbonate by multiplying by 10. German degrees can be converted by multiplying by 17.85 (1 degree DH = 10 mg CaO per liter). If your water contains hydroxides there is a risk of precipitation within the resin phase. Hence the program should not be used before contacting your local sales office. Even quite low concentrations of iron can affect operating performance.

 In the case of water softening concentrations above 0.5 ppm in the inlet feed can build up in the resin. This iron is difficult to regenerate and can gradually lead to resin fouling. Regular cleaning with hydrochloric acid, either in situ, if materials of construction of the softener permit, or if not, after resin transfer is recommended. This is best done before a substantial quantity of the iron has the possibility to become permanently fixed to the resin. Hence whenever levels of over 200 mg/l become fixed to the resin immediate resin cleaning is recommended. This could occur in 2–3 months with an inlet concentration of 0.5 ppm. Where long cycles are used and substantial quantities of iron can be loaded before regeneration even 0.2 ppm can slowly give rise to fouling. Where it is not possible to use HCl for cleaning, proprietary cleaning agents which may contain citric acid or other suitable complexing agents are recommended. In certain parts of the world softener size is increased to allow for higher feed water iron concentrations. There appear to be no consistent guidelines to calculate this increase. It must be assumed that this calculation depends on the chemistry of the iron, the cleaning treatments recommended, the softener design and the operating conditions chosen. Clearly the facility is available to adjust the water analysis to produce a correction to the plant load by increasing either the hardness or the iron content to achieve the objectives of the operator. Both the hardness and the iron are calculated to load stoichiometrically at the maximum valency. (Hardness as divalent and iron as trivalent). Care should be taken when cations classified as others are included. If ions should be included in the softening load these should be added to the iron or the hardness values. Thus transitional metals and other alkaline earth metals should be added in this way. The rule is that others will be treated in the same way as sodium, potassium or ammonia. This principle also applies to dealkalisation and demineralization.

 Likewise there is a similar problem when completing the analysis of anions in water. Because the selectivity of anions can vary widely, it is even more important to make a correct classification. Anions classified as strong are those which behave as mineral acids, for example phosphate or bromide. The acids of these anions would be taken up by a weak base resin, or selectively held on the strong base resin, if there is no weak base in the line. Those classified as weak, for example boric acid are not held so strongly on weak base resins and hence are easily displaced to the strong base resins. Such anions are treated similarly to the bicarbonate ion.

 Returning to iron behavior, in the case of demineralization, higher concentrations of iron can be safely treated. However sulfuric acid does not remove iron as efficiently as hydrochloric acid. Here feed water iron levels above 1 ppm can give problems, and occasional regeneration with hydrochloric acid is recommended. Where iron is present in colloidal form, or where it is present together with organic matter,

it can pass directly through the cation filter, and may be taken up on the following anion resin. In such cases it is beneficial to carefully consider which anion resins to choose, and again to put in place regular cleaning treatments with alkaline brine, together with less frequent treatments with hydrochloric acid. As mentioned above iron is often present in colloidal or insoluble form. This is particularly important in the case of condensate polishing. It is assumed in any given analysis that 80% is insoluble, and is removed by filtration rather than by ion exchange. Thus it is known that all iron is soluble, the value entered should be five times higher. It should also be pointed out that a typical condensate analysis is a rarity. The polishing process is in place particularly to deal with excursions from the typical. The program is designed to help in the prediction of performance longer term, when such excursions occur. However it is not designed to predict kinetic performance in the short term.

 There always remains the possibility that some colloids and or complexes can pass directly through the ion exchange filters. If such situations occur, special resins and processes may be introduced to eliminate these problems. The program will warn you if different aspects of the analytical data are inconsistent or insufficient. In particular, if the analysis is not balanced. You are permitted to proceed if you wish. In some cases, particularly after reverse osmosis the water may be slightly unbalanced.

66 Extra Analytical Data

 The data entered at the top of the screen contains values for data which is extra to specific data for the individual ion concentrations. In particular Total Hardness may be provided instead of calcium plus magnesium values. If both are provided the calcium and magnesium values will be used. You will be alerted if the data is inconsistent. If only hardness is given, this will be treated as calcium to give safest design option (because calcium requires more resin, or higher regeneration levels for its removal). Likewise alkalinity will replace bicarbonates, and equivalent mineral acidity (EMA) will replace chlorides. If chlorides plus sulfates and nitrates do not match EMA, a warning will be displayed. If only a conductivity value is given a rough softening plant will be calculated by using a factor of 0.65 to obtain a TDS level in ppm of calcium carbonate. It is assumed that this is hardness giving a conservative plant.

 

67 RAW WATER origin and pretreatment

 The origin and pretreatment can be useful for recommending a particular process or cleaning treatment. They are not used for the design calculation itself. The level of organics after such pretreatment is important. The level of organics should be entered as Kubel 10 min boil either as KMnO4 or O2. Currently this is the most frequently used method. Though the test protocol varies slightly from country to country, it is sufficiently accurate for its purpose. Levels obtained by other techniques should be converted using the following factors:

Method Concentration

T.O.C. 1 ppm

KMnO4 – 10 min boil (Kubel)

3 ppm as O2

12 ppm as KMnO4

KMnO4 – 30 min at 100°C

2–2.5 ppm as O2

8–10 ppm as KMnO4

KMnO4 – 4 hr at 1 ppm as O2

27°C

4 ppm as KMnO4

Ultra Violet 3 ppm

Results using permanganate can also be expressed as O2. To convert from "as KMnO4" to "as O2" divide the result by 4. It is expected that TOC will become the norm within the next few years. Hence it is necessary to review this situation regularly.

 The temperature of raw water can be important in demineralization processes. Temperatures above 35°C can preclude the use of Type-II and acrylic strong base resins, and even acrylic weak base resins where a short rinse is essential (production of water at less than 50 micro Siemens). It will also affect the capacity of weak base resins and the pressure drop of resins generally. Temperatures above 60°C are not generally recommended for strong base anion resins where high conversion to the hydroxide form of any part of the bed is anticipated. However temperatures up to 70C or even 80C can sometimes be used successfully, provided some loss of resin lifetime is accepted.

 

68 Cycle Time and Flow Rate

 The program calculates automatically the gross throughput which includes the extra water needed to operate the regeneration and rinses. The net throughput is the water supply required to be produced by the design. This calculation is made from the flow rate requirement input in cubic meters per hour together with the required length of cycle. Alternatively if the total net throughput is known the flow rate will be calculated. In fact any two pieces will be used to complete the hydraulic design. If there is an existing plant the resin volume may be fixed and the other parameters altered to obtain the best option.

 CYCLE LENGTH:

 The increase in cycle length will decrease the specific flow rate (BV/h). Increase in the absolute flow rate in meters cubed per hour (m3/hr) will increase the volume of resin required. When commencing a design calculation, the information provided on the error message can be incorrect. It is recommended to maintain the desired cycle time initially. Completion of the initial calculation can often produce satisfactory results. For resins with strong functionality, provided that operation conditions are within the recommended limits, the increase in the flow rate will cause near pro-rata increase in resin volume. Hence the specific flow rate (BV/hr) will be approx. constant. An operator who is familiar with the program, or is experienced in plant design can therefore rapidly see which changes will produce the required performance. On the other hand resins with weak functionality are not so predictable because the operating capacity changes with flow rate. Hence it is useful to experiment with several options to see what happens. This technique provides the possibility to "Home in" on desired conditions more rapidly. It is important to remember that the program treats resin combinations which contain both weakly and strongly functional resins with like charge as an integrated system. The operator does not for instance have to add extra regenerant to meet the requirements of the preceding weakly functional resin. Attempts to do so could result in the operator "chasing his tail".

 CYCLE LOAD

 The plant needs water for backwash, dilution of regenerants, displacement rinse and fast rinse. These all contribute to the ionic load. Hence the design gives net and gross volumes of water to be treated and calculates the total load for operation. It is possible to reduce total volume of weakly functional resin by optimizing the load over-run. That is by allowing some of the load which could be treated by the resin of weak functionality to pass to the resin of strong functionality following. This is achieved by entering "1" in the overrun space. Although the screens are integrated the interaction produces different volumes of weak

and strong resins which ultimately affect the overrun. Hence for final optimization, it is recommended to carry out the overrun optimization in both "weak" and "strong" screens until the approximately the same overrun number is obtained for both weakly and strongly functional resins.

  

69 Choice of Resin

Options are provided to use the full range of commonly available Purolite resins. The choice of cation resins is relatively straight forward. Purolite C-100 is the standard choice for demineralization. This can be provided in the Hydrogen form (C-100H) and saves regeneration on the first cycle. C-100E (sodium form, normally used for softening) may be used for potable applications. Lower cross linked resins are used for fast kinetics in shallow beds, or for chromatographic separations. Macroporous resins such as Purolite C-150 and C-160 are used for special selectivity and where osmotic stress is high. Choice of anion resins is more complex and has been dealt with in the section on Demineralization.

  

70 Regeneration

The regeneration mode should be chosen according to water quality requirements, the need to either save regenerants, or reduce capital cost by increasing regeneration level. Co-flow plants are less sophisticated and are usually cheaper to design and build. Counter-flow plants are the converse but offer superior water quality, and regenerant savings. Combinations are possible and can be useful to improve regenerant neutralization. There is a wide choice of counter-flow designs. Typical values for the several types are generally similar except for changes in pressure drop according to the resin grading and optimum bed depth. Particular variations in these designs can offer marginally different capacities, quality, and (leakage), from those shown, and choice of rinse volumes should be referred to the design engineer. Operating capacity varies according to regeneration level, water analysis and other operating conditions according to resin type.

 IONIC LOAD

 The quantity of resin required is calculated from this value and the gross ionic load. The resin volume may be rounded to the nearest unit package volume as required. Of course the application of a design factor reduces the operating capacity. The design screen shows the theoretical capacity without the design factor in operation. The printout shows both the theoretical capacity and actual operating capacity calculated by applying the design factor.

 REGENERANTS

 The regeneration conditions are an important part of the design. Sodium chloride is generally chosen for water softening but potassium chloride can sometimes be preferred. If this choice is required the program should be operated with sodium chloride and the stoichiometric equivalent of potassium chloride used. The concentration of sodium chloride of 10% gives optimum performance. Concentrations of less than 5% are wasteful in water usage and full potential capacity will not be obtained. For higher concentrations the operator should satisfy himself that the volume of regenerant is at least equal to one bed volume, otherwise distribution of regenerant can be inefficient. Concentrations above 20% can be wasteful for this reason, and are not generally recommended. Although saturated brine has been used in certain designs, this is very wasteful and there is a risk of poor hydraulics and osmotic shock as the resin swells during rinse. Turning to demineralization: For cation resins there is a useful choice in regenerants. Hydrochloric acid is the preferred regenerant when considering the maintenance of consistent resin performance with limited risk. Iron fouling is reduced, there is less risk of precipitation, and regeneration is generally more efficient so

keeping the resin bed cleaner. On the other hand it is more corrosive, fumes can be hazardous, and it can be more expensive. Sulfuric acid therefore has its place. Nitric acid is a strong oxidizing agent which can de-crosslink resin (increasing the moisture retention) and can lead to explosive conditions when used incorrectly; hence it is only to be recommended after consultation with Purolite. Nevertheless hydrochloric acid is an efficient regenerant and the hydrochloric acid data can be used for the plant design. Sulfuric acid should be used at recommended concentrations according to the proportion of calcium to total cations. Where the ratio is high, calcium sulfate precipitation can occur within the resin bed. This can be avoided by operating at higher flow rates, so that waste regenerant leaves the bed before it has time to precipitate (operating in this way can cause a risk if the regeneration is interrupted) and by adjusting the acid concentration to give the best compromise. Please note an average value given in the design screen. The printout gives details for the two stages of the stepwise regeneration, on which this design program is based. If a single stage regeneration is to be used, then the first concentration should be chosen and the design factor of 0.85 should replace the standard one of 0.9. It is important that the flow rates are chosen according to the operator's recommendations, and should always give a resin contact time of greater than 20 minutes (preferably greater than 30 minutes, but not longer than 45 min). Shorter contact times reduce efficiency of regeneration, while longer results in precipitation. The flow rate should be carefully chosen, usually 8–15 BV/hr to achieve these objectives.

 RINSE VOLUMES

 Rinse volumes are chosen according to the resin and design. If the water is heavy in iron or organics a longer rinse volume or rinse recycle should be used.

 

71 Plant Design

 Vessel design options are controlled by recommendations within the program. Warnings are offered when the conditions are not generally suitable. The design engineer’s recommendations should be adhered to. It is important to use a bed depth in line with plant diameter to give a suitable aspect (height/diam) of 1–2 in most cases. Tall narrow vessels can result in premature resin breakdown. Very shallow beds can give poor ion exchange. A high linear velocity and deep bed can result in high pressure drop across the bed, high pumping costs, and possible resin breakage. A design rating of 0.9 ensures against shortfall in throughput as a result of changes in feed water quality, resin deterioration, and some plant design inefficiency. Where the plant is small and designed to a low budget, it may be useful to increase the safety by reduction of this number to 0.8 or less. In some instances, the plant may be flow rate or cycle time limited and a low design factor may be essential. In such cases there is no need for an extra reduction.

 

72 Treated Water Quality

 Treated Water quality data is given for the plant as the design proceeds. This cannot be truly assessed until all vessels have been evaluated. It should be emphasized that typical values for leakage are generally significantly lower than those quoted, for several reasons. The design safety factor ensures that there is excess resin and regenerant. If this is not fully needed because of good engineering design, a lower leakage will result. Plants are also often run to a constant safe throughput, effectively increasing the true regeneration level with the same effect. Any reduction in water demand can result in better performance of the flow rate sensitive weakly functional resins which results in a higher capacity. This in turn can reduce load on the strong resins ultimately improving performance which also can reflect in lower leakages. For these reasons typically obtained field data cannot be directly compared with design data. Normally the treated water quality will deteriorate towards the end of the run. The run will be continued until the water quality is no longer acceptable, or it is considered that further loading could prejudice the efficiency of the subsequent regeneration. This is particularly important when considering the loading and reversible

removal of organics. Leakage end-point is usually significantly higher than the average. This enables detection. The program only considers one parameter on one line as an end-point of the cycle, except for mixed beds which are designed separately. The end-point may be hardness leakage for softening alkalinity or hardness for dealkalisation, sodium for cation limited plants, which is linked to conductivity after the anion bed, and usually silica for anion limited plants. Historically, cation limited plants have been preferred because increase in conductivity arising from increase of sodium as the cation exhausts, is easy and cheap to measure, and the result is instantaneous. On the other hand anion limited plants, required to produce high quality water, will leak silica at exhaustion. This is expensive to monitor, and the analysis result may be so slow that silica has passed to the treated water supply, putting equipment at risk. This is less important when there is a mixed bed downstream, but the mixed bed may become prematurely exhausted. One way to avoid the silica release is to bury the silica probe in the anion bed. Alternatively the actual cycle time can be reduced to about 2/3 of the design.

 

73 Pressure Drop Calculation

 For Water Treatment, the viscosity, based on temperature, will be calculated. For other solutions it will be necessary to provide the viscosity of the solution (sugar syrups, organic solvents etc.) in cps units. Flow rate: to convert from volume flow rate (m3/h/m3 of resin, BV's/h) to linear flow rate, (m/h), multiply by the bed depth in meters. Pressure Drop is proportional to bed depth. Resin Grades Specific Process Systems often require specific resin particle size grades. The codes are as follows:

Grade Code

Detail Mm

ST Standard grade 0.3–1.2

MB Mixed Bed

C or A 0.42–1.2, 0.3–1.2

C High Flow 0.42–1.2

DLLayered bed

(WAC,WBA)0.3–0.63

DLLayered bed (SAC,SBA)

0.63–1.2

PL Polishing 0.42–1.0

TL Trilite C or A0.71–1.2, 0.42–

0.85CL Continuous 0.42–0.85

FL Fluidized bed 0.5–1.0

S Special clean 0.42–1.2

OOther customer

specPFC,FA Purofine 0.42–0.71

 

74 Operating Conditions – Working Mixed Beds

 For working mixed beds (all mixed beds on separate disc), the vessel diameter should be adjusted to give a linear velocity close to 40–60 m/h. For certain waters it may be necessary to operate more slowly. When

using the program to design any type of mixed bed, changing the cycle time, is a very important consideration to achieve the required objectives. This will change the resin volumes of the components and this helps to size the bed correctly. One of the main problems is that only a small volume of one of the resins is needed. This poses problems in vessel sizing. By ensuring that enough of each component is available for the computer to overcome the hydraulic constraints for each component allows for initial bed sizing. The desired resin ratio is then calculated manually, and if necessary the bed down-sized to meet the optimized requirement. It should be emphasized that a longer cycle time than the one desired is not a bad thing, provided the it achieves the objective of optimized bed sizing. The capacity of the resin bed can be affected considerably by the initial water analysis and the treated water quality requirements. Relatively small changes in the acceptable quality can make large differences in the optimum resin ratio, which should be recommended, and how to measure the end-point. It the bed is cation limited, an increase in sodium as the cation exhausts can cause a slow or a rapid increase in conductivity, depending on the flow rate of operation. If the average conductivity is quite low, say less than 1 micro Siemens per cm. then the end-point may be 10 times or more higher, say 1–2 micro Siemens. However if silica and carbon dioxide do not need to be removed the choice of a mixed bed with a high cation ratio may allow acceptable water of a higher conductivity, such as 5–20 micro Siemens to be produced more economically. On the other hand if low silica leakage is required, a silica meter is the safest way to ensure best water quality is produced. The analytical response to silica can be slow, so it can be advantageous to bury the silica probe a small distance up the bed to ensure there is some protection against silica passing to the treated water supply. Alternatively the cycle time can be shortened to say, 2/3 of the design to prevent silica leakage.

 

75 Operating Conditions – Polishing

 For Make-up polishing, flow rate limitations are even more important than they are for working mixed beds. Optimum linear velocity should be set at 40–60m/h, and 30–40 BV/h. There should be less cause to operate more slowly because of water analysis constraints. It longer cycle times are preferred, this could result in flow rates lower than the optimum. As a general rule, both for make-up and condensate polishing, higher flow rates afford better filtration, especially for polishing condensate, where removal of suspended solids is an important consideration. The principles of cycle end point and its measurement also apply to make polishers as well as for Working Mixed Beds.

 Optimum linear velocity for condensate polishing can vary between 60–120 m/h depending on the system and the preferred cycle time. In general, because mixed beds designed on flow rate can be calculated quickly by hand, the advantages of the program are less apparent. However it is much easier to quantify all details of actual performance with no extra effort. It is expected that comparison of actual mixed bed performance with design data will allow for increased sophistication from feed back of practical data from the field.

 Currently predictions on quality are quite general, and can be related more to the effect of condenser leakage than to steady conditions of operation. The program objectives can be two fold. Firstly to size the plant according to flow-rate and possibly ammonia or amine load. Secondly to simulate the effects of a condenser leak. The design program can be used for both objectives. In the case of the plant design, a typical condensate analysis is required. When a significant condenser leak has occurred, the resin performance can be simulated by entering the condensate leak analysis. Having cured the leak, it is recommended to double regenerate the resin bed using at least 120g/L HCL, and 160g/L H2SO4 as appropriate, and 100g/L of NaOH per regeneration. It may be appropriate and convenient to employ the resin on resin technique between regenerations. Also it is useful to have a spare resin charge available to maintain continuous condenser treatment, (where this is necessary). When making a design it is important that sufficient time is allowed for the resin transfer and full separation and regeneration. In the absence of condenser leaks the limiting factor for cycle length is the pressure loss. This allows the possibility for the resin to be as fully regenerated as possible thus increasing the purity of the condensate and the kinetics of ion exchange. The specified performance is generally regarded as sufficient for most requirements. A number of subtle variations in the engineering and various trace impurities in the water analysis can have

significant effects on kinetics. In general water quality will be superior to the predicted specification. Problems in solving the polishing designs can occur where the cycle time is very short and at least one of the design factors is low. The principles of the design factor operation apply here as it does for Working Mixed Beds. It is even more important here to recommend that, where necessary, an attempt is made to solve for a longer cycle time, or to alter the operating conditions to increase basic driving design factors. It is possible that resin volumes are so small that bed depth, pressure loss and flow rate parameters cannot be solved for the cycle time required.

 There is no problem in operating a mixed bed for a shorter cycle than the prescribed design. For mixed beds, the choice of the correct resin grades may be critical according to the design. Quite often the cycle time for condensate polishing is limited by pressure loss rather than by ionic load. Macroporous and super gel resins should be taken off line when the pressure loss exceeds 2 bar/m, and gel types at 1.5 bar/m. This has the advantage of ensuring an even higher regeneration level than the design, further improving water quality in subsequent cycles, because of higher conversion to hydrogen and hydroxide forms respectively.

 

76 Design Objectives

 The objectives to be achieved are to minimize pressure drop at optimum flow-rate, while at the same time component grading must be chosen to achieve good resin separation for the regeneration process. For general mixed beds (using 2 components) MB grades are recommended, unless especially good water quality is required. This can require perfection in separation. Trilite grades with an inert spacer to prevent cross contamination of regenerants, or to provide superior separation on resin transfer is an excellent option in many cases. However, if there is any risk of contamination with oil or grease, an inert layer is not recommended. The interface may be diverted to a separate catch pot and reintroduced before the next backwash separation.

 Even if resin separation is near perfect after backwash, the use of in-situ regeneration can still allow for contact of the component resins with the wrong regenerant at the centre collector. Attempts to place the collector at the interface can fail if, for whatever reason, any changes in cation volume take place. Even if the collector is perfectly placed, the wrong regenerant can diffuse past the collector. From experience, it has been found that it is preferable for the cation resin to be converted to the sodium form, than for the anion resin to be converted to the acid form. The presence of organic matter and weakly basic groups can extend anion rinse times significantly. The recommended approach is to bury the collector in the cation resin, and to regenerate the anion first. This will convert the cation resin at the collector to the sodium form. However the following cation regeneration will remove most of it, if not all.

 For ultra-pure water treatment, excellent separation is achieved with Purofine Resins, and their superior regenerability and kinetics ensures up to 30% longer cycles of the highest purity water. Where high flow rate is a priority, PL grades are a useful alternative option, giving low pressure loss, and good cation kinetics.

 Typically most types of mixed bed will give a leakage of 0.1 micro Siemens or below, in other words a resistivity of over 10 meg-ohm. The capacity will be dependent on the leakage at the cycle end-point, as the leakage often rises gradually. The capacity in the program is based on an end-point approximately ten times the average or permanent leakage, around 1–2 micro-Siemens depending upon the application. Where the water requirement is intermittent, it is recommended to fit a recycle loop for water recirculation. This can save water on start-up and avoid the inconsistent quality which occurs on start-up after a shutdown.

 

77 Overrun

 The objective of the overrun is to allow ionic load to pass from the weakly functional resin to the following strongly functional one, in order to minimize total resin volumes in each unit. It should be pointed out that other concepts of optimization are possible. Some examples are: Optimization of use of regenerant quantity, of resin inventory cost, speed of regeneration, rinse minimization, running costs, and so on. The program allows the user to choose the overrun he feels is appropriate and to view the effects, and so move to the desired objective. The use of overrun has the advantage that it can more effectively load the weakly functional resins which also gives more time for loading the kinetically slower weakly functional resin. Also by allowing less selective ions to be displaced to the strongly functional resin the capacity of the weakly functional resin can be maximized while still allowing effective regeneration of the strongly functional resin. The procedures for overrun may take time. Care should be taken when operating in high overrun of weak base anion beds where influent water is high in organic load/Total Anions. The organic load can be displaced to the strong base resin with consequent risk of increased fouling. This is especially important in co-flow operation. If it is required to ensure that there is zero risk of this situation, a safety factor (negative over-run) can be applied to the weak base resin design.