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Accurate system design is critical to getting the most from a water treatment system. Since dozens of factors can impact the productivity of the system, the CADIX system design software program from Dow offers precise design recommendations to optimize system performance.

 

The goal of the designer of an ion exchange system is to ensure that the correct water quality and quantity is delivered with optimized regenerant consumption and capital cost. The optimum design depends on the relative importance of these parameters. Each are described in the following sections.

 

How to Design an Ion Exchange Resin System:

 

Step 1: Regeneration System Selection

 

There are a number of different ion exchange regeneration technologies that can be used, from the basic co-current regenerated systems to counter-flow block systems and through to packed bed technology, including the Dow UPCORE™ System.

 

Selection of regeneration system:

Counter-Current Regeneration Systems:in these systems, the regenerant is applied in the opposite direction to the service flow, resulting in reduced chemical consumption, improved water quality and less waste volumes compared to traditional co-current regenerated systems. Counter-current regeneration systems should provide a water quality of better than 2 µS/cm (0.5 MW.cm) and residual silica of 0.020 to 0.050 mg/l as SiO2. Depending upon water composition and regeneration conditions, the specific conductivity could be as low as 0.2 µS/cm (5 MW.cm). The normal counter-current endpoint is 4 µS/cm conductivity. A maximum endpoint value of 0.3 mg/l SiO2 above the average leakage should not be exceeded in order to avoid a high contamination of the polishing resin layer and unacceptably high silica leakage during subsequent cycles. Silica leakage can be minimized by operating the plant at silica break rather than conductivity end point. This secures the lowest silica leakage, but at the expense of a 5 -10 % throughput reduction.

 

There are two main types of counter-current systems:

  • Blocked Systems, including air hold down, water hold down and inert mass blocked. The service flow is downwards and regeneration upflow. To avoid disturbance of the resin polishing zone at the bottom of the vessel, the resin bed is held down (blocked) during regeneration by air pressure, water flow or an inert mass in the top part of the vessel. The regenerant passes up through the resin and out of a collector system in the middle part of the vessel. Such systems have similar high cylindrical height as co-current systems to allow resin backwash within the vessel.
  • Packed Bed Systems, these may be up-flow service with down-flow regeneration or down-flow service with up-flow regeneration, such as the Dow UPCORE system.

 

Co-Current Regeneration Systems:these are the simplest systems, where the resin is regenerated in the same direction as the service flow (downwards). The vessel has a large freeboard to allow expansion of the resin bed when backwashing is carried out to remove suspended solids and resin fines. Co-current regeneration single bed systems will generally produce water of much lower quality than counter-current systems, with typical leakage values ~10 times higher. Such quality will also be even more affected by the water composition, the type of regenerant chemical and dosage being used.

 

Step 2: Selection of Layout and Resin Types (Configuration)

 

The plant configuration will depend on the feed water composition, the water quality required and the economics of operation. The following general guidelines are given to help in configuration and resin selection. Due to their improved performance, the uniform DOWEX™ MARATHON™ resins are recommended over standard (polydispersed) resins. The uniform DOWEX UPCORE™ Mono resins are designed for UPCORE systems.

 

Selection of Resin Types:

Strong Acid Cation Resin

Strong Acid Cation Resin is used for water softening in the Na cycle and for demineralization when the temporary hardness in the feed is < 40%. For small plants and with HCl as regenerant, a strong acid cation also offers a simple effective solution on waters with > 40% temporary hardness. DOWEX MARATHON Cis the resin of choice for most applications and DOWEX UPCORE Mono C-600resin for UPCORE systems.

 

Weak Acid Cation Resin

Weak Acid Cation Resin is used as a single resin for dealkalization in the H cycle and for brackish water softening in Na cycle. In demineralization, the use of a weak acid cation in front of a strong cation is preferred with feed waters containing a high proportion of temporary hardness (>40%) and low FMA. This configuration has advantages in terms of regeneration efficiency and operating capacity.

With sulfuric acid regeneration, two separate cation columns should be used in order to allow acid dilution at the weak acid resin inlet. For counter-flow regeneration, a double compartment layered bed cation including a facility for acid dilution at the weak acid cation inlet can be used, but is more complex to operate. Selected resins are DOWEX MAC-3or DOWEX UPCORE MAC-3for UPCORE systems.

 

Strong Base Anion Resin Type I

Strong Base Anion Resin Type I as a single resin is particularly recommended for treating low FMA (Free Mineral Acid) water with high silica and where low silica leakage is required (~20 ppb in counter-current operation). The resin can be regenerated up to 50°C (122°F) for more effective silica removal. DOWEX MARATHON Aresin is designed for general demineralization and for UPCORE plants, the resins are DOWEX UPCORE Mono A-500or DOWEX UPCORE Mono A-625resin.

 

Strong Base Anion Resin Type II

Strong Base Anion Resin Type II is well suited for small plants, owing to its excellent regeneration efficiencies for water compositions where CO2 and SiO2 are < 30% of the total feed anions. Type II anions have a much better operating capacity and regeneration efficiency compared to Type I, but are limited to lower temperature operation (<35°C/95°F caustic treatment) and have a higher SiO2 leakage (~50 ppb in counter-current operation). DOWEX MARATHON A2resin is the resin of choice for general demineralization and DOWEX UPCORE Mono A2-500for the UPCORE system.

 

Weak Base Anion Resin

Weak Base Anion Resin is used as a single resin to obtain partially deionized water without removal of CO2 and SiO2. For complete demineralization, the combination of weak base and strong base anion is an excellent choice for larger plants, as it provides optimum operation costs. The weak base has very high regeneration efficiency and provides additional capacity to the system. The weak and strong anion combination is well suited to treat waters with low alkalinity or degassed feed, when the FMA (Cl + NO3 + SO4) is typically > 60% of the total anions. DOWEX MARATHON WBA resin is the resin of choice for general demineralization.

Weak base anions are particularly effective in handling natural organics, which are usually high molecular weight weakly acidic compounds that affect both weak base and strong base anion resins. In a weak base - strong base anion configuration, some of the organics will pass through the weak to the strong base. The design should therefore account for SBA organic loading at the end of the cycle, as the resin will require additional NaOH to desorb the organics. There are important differences in loading capacity or reversibility to organics between different anion types.

 

The weak and strong anion resins can either be designed in two separate vessels or for counter-flow regeneration in one vessel with or without a separation nozzle plate. For separated anions, DOWEX MARATHON WBAand DOWEX MARATHON Aresins are recommended. DOWEX MARATHON WBAand DOWEX MARATHON A LBare designed to be used together as a layered bed in a single column without a nozzle plate. For UPCORE plants, the resins are DOWEX UPCORE Mono WB-500and DOWEX UPCORE Mono A-500or DOWEX UPCORE Mono A-625.

 

Step 3: Chemical Efficiencies for Different Resin Configurations

 

Due to differences in the regenerability of strong and weak functionalized resins, the configurations described in "Selection of layout and resin types (configuration)" will have different chemical efficiencies. The chemical efficiency of regeneration (also known as stoichiometry) for an IX resin is defined 

***

 

 

As the resin usage of the regenerant chemical is non-ideal, the chemical efficiency is always >100%. The efficiency therefore becomes worse as the value increases. The following table gives typical regeneration efficiencies for different resin types and combinations in co-current and counter-current regeneration systems.

Resin Type/Configuration Regeneration System Typical Regeneration Efficiency (%)
Strong Acid Cation Co Current HCl 200-250
  Counter Current HCl 120-150
  Co Current H2SO4 250-300
  Counter Current H2SO4 150-200
Weak Acid Cation   105-115
Weak Acid + Strong Acid Cation   105-115
Strong Base Anion Type I Co Current 250-300
  Counter Current 140-220
Strong Base Anion Type II Co Current 150-200
  Counter Current 125-140
Weak Base Anion   120-150
Layered Base Anion   120-130

 

Step 4: Atmospheric Degassifier

 

The decision to install an atmospheric degassifier is principally economic. Removing carbon dioxide before it reaches the anion resins will reduce NaOH chemical consumption and this should be balanced against the cost of the degassifier. Generally the economical balance is not in favor of a degassifier for small plants (up to about 10 m3/h or 45 gpm). For larger plants, if the total CO2 is greater than 50-100 mg/l (ppm), the pay-back time for a degassifier should be short.

Atmospheric degassifiers usually reduce residual CO2 down to 5 mg/l. In order to have a safety margin for design, a residual value of 10 mg/l CO2 is recommended.

For systems requiring very low levels of residual CO2, a vacuum degassifier is used. This reduces the CO2 to below 1 mg/L.

 

Step 5: Resin Operating Capacities and Regenerant Levels

 

In co-current operation, the product water quality requirements will define the minimum levels of acid and caustic regenerant to be used. The regenerant levels and the feed water composition will then define the resin operating capacity. Although high regenerant levels result in increased capacity and lower ionic leakages, the chemical efficiency of the system becomes worse.

More precise determinations of resin operating capacities and ionic leakages can be calculated using the CADIXdesign program or from the engineering curves given in the DOWEX™ Resin Engineering Brochures. These Brochures provide complete engineering information for individual DOWEX resins, including operating capacity and leakage curves as well as backwash and pressure drop data.

 

Guidelines for Typical Regeneration Level and Corresponding Resin Operating Capacity:

 

Regeneration System Regenerant Level Typical Operating Capacity
(g/l) (lbs/ft³) (g/l) (lbs/ft³)
Co Current Regeneration        
HCl 80-120 5-7.5 0.8-1.2 17.5-26
H2SO4 150-200 9.5-12.5 0.5-0.8 11-17.5
NaOH 80-120 5-7.5 0.4-0.6 8.5-12
Counter Current Regeneration        
HCl 40-55 2.5-3.5 0.8-1.2 17.5-26
H2SO4 60-80 3.75-5 0.5-0.8 11-17.5
NaOH 30-45 2-2.8 0.4-0.6 8.5-13

 

The choice between hydrochloric acid and sulfuric acid is principally economic. HCl is a trouble-free regenerant with high efficiency. H2SO4 is less efficient and has lower operating capacity, particularly if stepwise regeneration is required in high hardness waters to avoid calcium sulfate precipitation.

 

Guidelines for amounts and concentrations of H2SO4 in stepwise regeneration:

Calcium in Feed Water (%) Amount and Concentration of H2SO4
Ca < 15 3%
15 < Ca < 50 1/3 at 1.5% and 2/3 at 3%
50 < Ca < 70 1/2 at 1.5% and 1/2 at 3%
Ca > 70 1% or use HCl

 

The specifications on the purity of the regeneration chemicals have to assure a trouble-free operation of the ion exchange resins after regeneration. Recommendations on the quality of regeneration chemicals (12KB PDF) are given.

In order to compensate for non-ideal operating conditions and resin aging on a working plant, it is recommended to apply a safety factor to operating capacity figures. Typical safety margins are 5% for cations and 10% for anions. Once the operating capacity has been determined, the required resin volume can be calculated from the throughput (volume treated per cycle) as follows:

 

 

 

FORMULA

 

Step 6: Vessel Sizing

 

The vessels should be made from typical, well-known materials of construction such as rubber-lined carbon steel or fiberglass. The vessel should have distribution / collector systems that give a good distribution of fluids during all phases of the operation. For this reason, a maximum vessel diameter of 3.5m (11.5 feet) is recommended. It is advisable to install sight-glasses in order to check resin levels and separation in the case of layered beds and mixed beds.

 

The design of the vessels should give a maximum resin bed depth, while limiting the pressure drop across the resin bed to ~1 bar. The optimum column diameter must be a balance between the resin bed height, the ratio of resin height to diameter (H/D) and the linear velocity. H/D should be in the range 2/3 to 3/2.

 

Vessel sizing should be adjusted to allow for resin expansion if backwashing is performed (80-100% of the settled resin bed height), resin swelling during service, the minimum bed height requirements and the guidelines given for service and regenerant flow rates in the Design Guidelines table below. These values are for orientation and should not be regarded as exclusive. Some applications may function outside of the guidelines. Typical resin bed depth is 1.2 m (4 ft) for co-current and block regeneration systems and 2 m (6.5 ft) for counter-current packed bed systems.

 

Design Guidelines for Operating DOWEX™ Resins

Swelling:

 

Strong Acid Cation Na+ → H+

5-8%

Weak Acid Cation H+ → Ca+

15-20%

Strong Base Anion Cl- → OH-

15-25%

Weak Base Anion FB → HCl

15-25%

   

Bed depth, min.

 

Co-current single resin

800 mm (2.6 ft)

Counter-current single resin

1200 mm (4 ft)

Layered bed strong base anion

800 mm (2.6 ft)

Layered bed weak base anion

600 mm (2 ft)

   

Backwash flow rate:

 

Strong Acid Cation

10-25 m/h (4-10 gpm/ft2)

Weak Acid Cation

10-20 m/h (4-8 gpm/ft2)

Strong Base Anion

5-15 m/h (2-6 gpm/ft2)

Weak Base Anion

3-10 m/h (1.2-4 gpm/ft2)

   

Flow rates:

 

Service/fast rinse

5-60 m/h (2-24 gpm/ft2)

Service/condensate polishing

75-120 m/h (30-50 gpm/ft2)

Co-current regeneration/displacement rinse

1-10 m/h (0.4-4 gpm/ft2)

Counter-current regeneration/displacement rinse

5-20 m/h (2-8 gpm/ft2)

   

Total Rinse requirements:

 

Strong Acid Cation

2-6 Bed volumes

Weak Acid Cation

3-6 Bed volumes

Strong Base Anion

3-6 Bed volumes

Weak Base Anion

2-4 Bed volumes

 

Step 7: Number of Lines

Based on the flow rate and throughput required, the number of lines operating at the same time needs to be defined. The simplest layout with 2 lines (1 in operation, 1 in standby) can be used in most cases. With large plants (> 400 m3/hr or 1800 gpm), however, it may be more appropriate to have 3 lines (2 x 50% in parallel, 1 in standby) in order to reduce system redundancy, optimize flow conditions and reduce vessel sizing. In making a design, it is important to ensure that there is enough time for the standby lines to complete regeneration before they are required to go back on line. The optimum number of lines with minimum redundancy can be calculated using the following formula:

 

If the equation predicts a non-integer result, the number should be rounded down to obtain the optimum number of lines. For example, a 10 hour run length with a 3 hour regeneration time gives a ratio of 4.3, so the number of lines with minimum redundancy would be 4.

 

 

Step 8: Mixed Bed Design Considerations

A polishing mixed-bed will be required if the product water specification is below that achievable from the demineralization plant alone or if a higher degree of safety is required to ensure water quality. The mixed bed outlet water should be about 0.10 µS/cm at 25°C and 0.010 to 0.020 mg/l as SiO2 (10 to 20 ppb).

Below are some general guidelines for designing a working mixed bed downstream of a demineralization plant:

  • Resin volume ratio of cation to anion should be in the range 40:60 to 60:40.
  • Flow rate 20-40 bed volumes per hour.
  • Regenerant levels of 80-100 g HCl/L or 120-160 g H2SO4/L cation and 80-100 g NaOH /L anion.
  • Maximum service run length < 4 weeks.
  • Maximum silica loading lower than 1.0 g SiO2 /L anion resin at end-of-cycle.
  • Minimum cation resin bed depth of 450 mm (1.5 feet).

 

 

 


 

 
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