1. ORGANICS How do we catch and kill them ? By François de Dardel
Technical Manager
Rohm and Haas
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2. ORGANICS AND ION EXCHANGE RESINS
Where do organics come from ?
What are they ?
How to measure them ?
What do they do to a demin water plant ?
How is resin fouling affecting plant operation ?
What resins to use ?
How to prevent resin fouling ?
How to restore fouled resins ?
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3. ORIGIN OF ORGANIC MATTER
Organics are mainly found in surface water (rivers, lakes)
Vegetals Animals
Domestic waste Industrial waste
While all water types used to feed an ion exchange system may contain organics, surface water is most likely to have higher concentration of organic compounds originating from the decomposition of vegetals, from animal faeces, from domestic waste and from industrial rejects.
These organic substances are likely to cause trouble in a water purification system.
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4. MEASUREMENT (KMnO4 method)
Principle
Oxidise with potassium permanganate
Boil in acidic conditions
Titrate excess KMnO4
Procedure
See notes
TOC vs COD
TOC measured as mg of Carbon
COD measured as Potassium Permanganate consumption
Very rough thumb rule:
1 mg/L TOC ~ 5.5 mg/L as KMnO4
There is no direct relation between KMnO4 and TOC measurement
KMnO4 Analytical Procedure
100ml water + 2 ml 5 N H2SO4
Add 20 ml of N KMnO4
Boil for 10 minutes
Add 20 ml of N Mohr’s salt {(NH4)2Fe(SO4)2•6H2O}
Titrate the excess of Mohr’s salt with N KMnO4
Read volume required for titration = y ml
Organic matter = 4 x y in mg/L as KMnO4 or y in mg/L as O2
TOC vs COD
TOC (total Organic Carbon) is now popular, as sensitive analytical instruments have been developed to measure it.
COD (Chemical Oxygen Demand) is measured with different chemical methods, one of those being the potassium permanganate method described above. The permanganate method “catches” only the oxidisable organic compounds, whilst the TOC analysis covers all organics, so there is no direct relation between results obtained with the two methods.
Experimental measurements as well as theoretical considerations have shown however that the “conversion factor” from TOC to the KMnO4 value usually ranges from 4 to 7.
It is believed that the old-fashioned permanganate value has a closer relation to organic fouling problems of ion exchange resins than the TOC number.
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5. REMOVAL FROM WATER We don’t guarantee a given degree of removal
Resins are there to exchange ions, not organics
(in industrial water treatment)
Usual degree of organic reduction
WBA 50 to 90 %
SBA polystyrenic 40 to 90 %
SBA polyacrylic 50 to 90 %
Ion exchange resins do remove organic compounds from water by a combination of two mechanisms:
Ionised organics (e.g. organic acids) are removed as ions
Other organic compounds can be removed by adsorption
However, very big organic ions may not penetrate the resin matrix, particularly if it is not very porous (gel type resins). On the other hand, the adsorption mechanism relies on a certain “affinity” of the resin for the organic molecules, which depends essentially on both the chemical structure of the organic compound in the water and the structure of the resin matrix.
Non-ionised, non polar organic compounds, such as low molecular weight hydrocarbons or alcohols, are not picked up by the resin.
Strongly basic resins in the OH form are more likely to remove weak organic acids from water than weakly basic resins. However, SBA resins are often of the gel type, and thus cannot pick up big molecules.
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6. ELUTION OF ORGANICS (from resin)
WBA 90 to 100 %
SBA type 1 50 to 90 %
SBA type 2 60 to 95 %
SBA Acryl 90 to 100 %
When elution (during regeneration) is less than 100 %, organics accumulate on resin
This is Resin Fouling
If organic compounds picked up by the resin during the service run are not completely eluted during regeneration, they slowly accumulate in the resin matrix cycle after cycle. After some time, they may block the access of inorganic ions or silica into the resin bead and occupy many of the available ion exchange sites in the bead structure. This is called organic fouling of the resin.
Long organic chains may get entangled in the resin polymer, which makes them difficult to remove during the short regeneration time. The high basicity of type 1 SBA resins also makes them more difficult to regenerate.
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7. Example of fouling Water with 5 mg/L as KMnO4 SBA resin type 2
Run length 100 Bed volumes
(e.g m3, 100 m3/h)
Organic load 100 x 5
= 500 mg KMnO4 per L resin
Organic removal 50 %
5 mg/L in ——> 2.5 mg/L out
250 mg organics are picked up by resin in each run
Elution 80 %
Only 0.8 x 250 = 200 mg are removed from resin
50 mg/L accumulate on resin in each run
After 200 cycles, resin contains 10 g/L of organics
5 mg/L
The slide illustrates how organics are slowly accumulating in the resin.
Assuming the anion exchange resin has a moisture of 55% and a shipping weight of 720 g/L, the dry matter of the clean resin is 325 g/L. The adsorbed organic compoounds will increase this dry matter to 335 g/L, i.e. an increase of 3%. Even if the density of carboxylic groups in the organic compound is not very high, these carboxylic groups will cause the retention of sodium hydroxide in the resin, and result in high rinse volumes and increeased conductivity in service (see slide 15).
2.5 mg/L
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8. Humic acid Humic acid structure proposed by Dragunov
Most natural organic compounds found in surface water are long chain carboxylic acids, called humic and fulvic acids. The fulvic acids are soluble in inorganic acid solutions, whereas the humic acids are insoluble in acids and soluble in alkaline solutions. These acids also contain amine and amide groups.
All of these natural acids have a partially aromatic structure, shown here with benzene rings, although quinone and indole rings are also found.
Their molecular mass can range from a few hundred to several thousand Dalton.
Another category of organic compound are polysaccharides, which are high molecular mass hydrophilic substances ( to more than Dalton) resulting from the decay of algae and bacteria.
A typical decay product or building block is gallic acid:
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9. NATURAL ORGANIC ACID (partial structure)
Molecular mass
This is a model showing the aromatic structure, carboxylic groups and other possible side chains (e.g. quinone and amine groups).
Naturally occurring organic compounds being globally acidic, they are rejected by cation exchange resins and picked up by anion resins.
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10. STRONGLY BASIC RESINS (partial structure)
Polystyrenic anion resins have a completely aromatic structure, whereas polyacrylic resins are essentially aliphatic, except for the cross-links which are usually made of divinylbenzene.
Polystyrenic
Polyacrylic
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11. ELECTROVALENT BONDING
Attraction between
cation & anion
Anion resin
The picture shows the normal ion exchange reaction between a functional group of the anion exchange resin and an acidic group of the organic acid in the water.
Organic acid
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12. VAN DER WAALS BONDING Attraction between aromatic rings Anion resin
This is the second mechanism, which we called “affinity” in a former slide: there is an attraction between the benzene rings in the resin and those in the organic compound from the water.
The van der Waals or dispersion forces arise through synchronisation of the motion of the electrons in the separate atoms as they approach each other. Here, the so-called p-electrons of the benzene rings are shared.
Note the anthraquinone group on the left is also capable of “coupling” with the aromatic rings in the resin.
Organic acid
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13. ADSORPTION OF THE ORGANIC ACID
Polystyrene
anion resin
Organic
acid
Polystyrene
anion resin
Adsorption of organic acid is a combination of both mechanisms.
Combination of ionic and Van der Waals attraction
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14. FOULING OF RESINS Regeneration breaks the ionic bonds
It doesn't break the Van der Waals bonds
Acrylic resins are not aromatic !
Van der Waals forces are therefore weak
Acrylic resins don't get fouled
Use Amberlite IRA458 or IRA478
Resin fouling occurs when the adsorbed organic compounds are not completely eluted during regeneration.
The caustic soda used for regeneration neutralises the acidic organic compound, which should now leave the resin, but the compound is retained by the van der Waals forces, which are not weakened by the caustic soda.
In addition, long chain organics entangled in the resin matrix are not easy to extract anyway.
Because acrylic ion exchange resins are essentially aliphatic, i.e. contain very few aromatic rings, the van der Waals forces are much weaker than with styrenic resins, and the elution of organic compounds is thus greatly improved.
It is important to note that while acrylic anion resin are much less prone to fouling than styrenic resins, this does not mean that the adsorption of organic compounds is higher: only their elution is more effective.
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15. HOW DOES FOULING GET NOTICED ?
Problems in plant operation
NaOH is absorbed inside resin
(on – COOH groups)
Rinse is long
Na leakage increases
SiO2 leakage increases
Capacity decreases
Moisture content decreases
Long rinse
At the time of regeneration, the carboxylic groups of the organic acid in the resin are neutralised with caustic soda, and convert to sodium carboxylate groups. If the compound is not eluted during regeneration, the sodium ions will remain in the resin structure and will be slowly hydrolysed by water during the rinse period. These sodium ions will cause a conductivity increase. Some, but not all, will eventually disappear in the rinse effluent.
Those Na ions remaining in the resin will cause sodium leakage during the service run. They will also create an additional problem, which is the elution of some of the silica molecules already sitting in the resin. This is a kind of “self regeneration effect”, because the sodium ions will combine with the OH ions released by the resin during the normal anion exchange to create a very dilute NaOH concentration, which can displace the silica in the resin.
Additionally, the organic compounds accumulated in the resin occupy valuable ion exchange sites, and block the resin pores. This is why capacity and moisture content decrease.
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16. FOULING INDEX Example 1 Example 2 Example 3
Degassed water, 6 meq/L Tot Anions
12 mg/L KMnO4
N = 12/6 = 2 Moderate
Example 2
Undegassed, thin water, 1.5 meq/L
N= 12/1.5 = 8 Highly fouling
Example 3
High TDS water, 15 meq/L, 3 mg/L KMnO4
N= 3/15 = 0.2 Not fouling
OM (mg/L as KMnO4)
Total Anions (meq/L)
N =
The fouling index as defined above is a characteristic of the water, not one of the resin.
With the same concentration of organics, a water with high salinity will be less “fouling” than one with low salinity. This is because the low salinity water will cause a longer cycle, so that the quantity of organics picked up on the resin will be higher.
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17. ORGANIC LOAD Example 1 Example 2 6 meq/L Tot Anions, 12 mg/L KMnO4
Assume a capacity of 0.6 eq/L for SBA alone
BV treated = 1000 x 0.6/6 = 100
Organic load = 100 x 12 mg = 1.2 g/LR as KMnO4
Example 2
Thin water, 1.5 meq/L, 12 mg/L KMnO4
BV treated = 1000 x 0.6/1.5 = 400
Organic load = 400 x 12 mg = 4.8 g/LR as KMnO4
The definition of Organic Load:
Quantity of organics going through the resin
Not quantity removed by the resin
Water in example 2, with the same organic content as water in example 1, has four times less salinity, and is four times more fouling than water 1.
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18. ORGANIC LOAD (cont.) New Example
High TDS water, 15 meq/L, 30 mg/L KMnO4
Combination WBA / SBA
WBA cap 1.0 eq/L, ionic load 10 meq/L
——> 100 BV treated
SBA cap 0.5 eq/L, ionic load 5 meq/L
Each litre of WBA gets
24 mg x 100 BV = 2.4 g as KMnO4
WBA picks up about 60% of organics
lets through 40%
SBA gets 0.4 x 30 = 12 mg/LH2O
Each litre of SBA gets 12 mg x 100 BV = 1.2 g as KMnO4
Org 30 mg/L
Ions 15 meq/L
2.5 mg/L
WBA
SBA
Cap 1.0 eq/L
Cap 0.5 eq/L
In WBA / SBA combinations, the fouling index and the organic load after WBA must be re-calculated.
The SBA resin will see only the organics that have not been picked up by the WBA resin.
Org 12 mg/L
Ions 5 meq/L
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19. RESISTANCE TO FOULING Resin N max max load g/LR as KMnO4
Amberjet 4200 Cl
Amberjet 4400 Cl
Amberjet 4600 Cl
Amberlite IRA402 Cl
Amberlite IRA405 Cl
Amberlite IRA410 Cl
Amberlite IRA458 Cl
Amberlite IRA478RF Cl
Amberlite IRA900 Cl
Amberlite IRA910 Cl
Amberlite IRA
Amberlite IRA
Amberlite IRA70RF
The above table is only approximate, but can be used for a basic selection of the anion exchange resins based on the organics in the feed water. The table reflects the fact that acrylic anion resins (IRA67, IRA458, IRA478) have a much better resistance to fouling than styrenic resins.
Type 2 SBA resins (Amberjet 4600, Amberlite IRA410) are more resistant to organic fouling than type 1. Macroporous SBA resins (IRA900, IRA910) are also better than their gel countertypes.
Comparing resins of the same type but with different porosity, it can be seen that Amberjet 4200 is less affected than Amberjet 4400 (low moisture i.e. low porosity).
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20. Organic SCAVENGER (organic trap)
Is used to protect demin plant
Reduction of organic load on WBA and SBA
Today, not much used
Acrylic resins used instead in the demin line
Scavenger resins, also called organic traps, are SBA resins operated in the chloride cycle, i.e. regenerated with NaCl, placed in front of a demineralisation train to protect the anion exchange resins (operated there in the OH cycle) from fouling.
For good results, the organic trap must contain a macroporous resin. Rohm and Haas recommends Amberlite IRA900 or IRA958 for this purpose.
Efficiency of a scavenger resin is only limited: the reduction of organics is rarely greater than 75%. Usual efficiency is 50 to 70%.
It must be noted that the organic trap will, at least partially, exchange its Cl ions for SO4 and HCO3 ions from the water. The water composition is thus likely to be changed at the inlet of the anion exchange resin column.
Now that low-fouling acrylic resins are available, less scavenger resins are used industrially.
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21. SCAVENGER RESINS Resin choice Regeneration Efficiency
Polystyrenic macroporous
Amberlite IRA900 Cl
Polyacrylic macroporous
Amberlite IRA958 Cl
Regeneration
2 BV of a 10% NaCl + 2% NaOH solution
Sizing :
So that organic load is about 10 g/L as KMnO4
10 to 30 BV/h
Efficiency
Organic removal 50 to 90%
Composition of feed water to demin plant is changed (more Cl, less HCO3 and SO4)
With the action of 10% brine, the resin beads contract like a sponge and expel the organics taken up during the exhaustion run. The addition of a little caustic soda helps neutralising the organic acids, and to some extend can hydrolyse them, cutting them into smaller pieces that are easier to remove from the resin. Regeneration is best carried out at 40 to 50°C. Contact time should be 40 to 60 minutes.
In some cases, the scavenger resin is regenerated only every 4 or 5 cycles of the demineralisation system.
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22. WBA resins help ! Combine WBA macroporous resin with SBA
WBA protects SBA
Combine styrenic and acrylic matrix
e.g. IRA96 + IRA458
3-compartment Amberpack® anion column
Compartment 1 designed as “integrated organic trap”.
Use reverse flow regeneration for WBA
Amberlite
SBA
IRA458RF
WBA 2
IRA96RF
WBA 1
IRA96RF
As we have seen previously, SBA resins are more sensitive to organic fouling than WBA. Those also reduce the organic load to the SBA resin. Therefore, a combination of WBA and SBA resins will ensure a better removal of the organics and a lower risk of fouling.
A combination of acrylic and styrenic structures will also enhance the efficiency of the system when a more complete removal of organics is required.
In Amberpack® systems, a separate organic trap can be included in the anion exchange resin column. Its resin volume would be designed to get an organic load of 10 to 15 g/L as KMnO4, and some additional caustic soda (10 to 20 g/LR) would be added to the regenerant to ensure good elution of the organics.
A separate resin compartment is recommended to prevent the resin loaded with organics to mix with the “good” WBA resin. Also, regeneration in reverse flow prevent the “good” resins from being contaminated with organics at the time of regeneration.
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23. DETERGENTS Detergents are particularly harmful to anion resins
They cause irreversible fouling
(they can practically not be removed)
Example : ABS (Alkylbenzene sulphonates)
Best bet : Acrylics (again !)
Synthetic detergents, particularly of the ABS type, are very likely to foul anion exchange resins. Typical affinity tables show the following data:
Anion Type 1 Type 2
OH 1 1 Cl
HSO
Benzene sulphonate
This means that a benzene sulphonate will be extremely difficult to regenerate off a styrenic type 1 resin, whilst it is very likely to be picked up during the exhaustion run.
Some phenolic resins can be used as organic traps for detergents (e.g. Duolite A7) but the best common way to reduce the risk of fouling is to use an acrylic resin in the anion exchange column.
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24. HOW TO LIMIT FOULING Good pre-treatment Limit loading
Flocculation
Chlorination
Scavenger resins
Membrane filtration
Ultrafiltration
Reverse osmosis
Good resin selection
Acrylic SBA
Use a WBA to protect SBA
Limit loading
Short runs
Use more resin (“dilution”)
Regenerate more efficiently
Increase NaOH level
Hot NaOH (type 1 only!)
Regular de-fouling treatment
(see next slide)
Prevention of organic fouling
The above recommendations will greatly reduce the risk of fouling.
Most of the recommended measures are explained in other slides. Here a few additional comments:
Flocculation removes humic substances of high molecular mass. The efficiency of removal ranges from 40 to 70 %.
Chlorination hydrolyses organic molecules to smaller compounds and to CO2. However, chlorination is efficient only with a minimum contact time, and any residual chlorine must be removed from the water before the ion exchange system, otherwise resins will be damaged by oxidation.
Ultrafiltration is particularly useful to remove high molecular mass compounds such as polysaccharides.
Limiting the run or increasing anion resin volume will prevent a high concentration of organics on the resin.
Using hot caustic ( °C) for the elution of organics is effective, but can only be done with styrenic type 1 SBA resins.
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25. RESTORING RESIN QUALITY
Best cure : alkaline brine treatment
Solution of 10 % NaCl + 2 % NaOH
See details in the notes
Acid treatment
(useful when iron is complexed with organics)
Treatment with 10% HCl
After treatment, check resin rinse
Alkaline brine treatment
Solution of 10 % NaCl (100 g/L) + 2 % NaOH (20 g/L)
Prepare 3 BV of that solution
Pass through resin in 1 hour
Air scour the resin for 45 to 60 minutes
If possible, soak overnight
Rinse with 3 BV of demineralised water
Regenerate with double quantity of NaOH
Acid treatment
(useful when iron is complexed with organics)
Use 10% HCl
Pass 1.5 BV in 30 minutes
Air scour
Pass another 1.5 BV
When the risk of organic fouling is serious, a regular brine treatment should be considered and the appropriate equipment made available on a permanent basis.
In all cases, prevention (see the previous slide) should have high priority.
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26. Thank you!
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