Cation & Anion Exchanger Design Capacity

[BRENT511]

Hello all,

I am trying to develop a spreadsheet that will calculate the service time of both cation and anion exchangers based on the feed water quality. I have done a calculation based on the design feed water quality and capacity of the ion resin, however I am not getting the 12 hr service time that was calculated from the design conditions.

The documentation I have on the exchangers states that the service duration is 12 hours based on a 1100 gpm feed flow with the following cationic and anionic amounts:
Cations- 92.5 ppm CaCO3
Anions- 136.4 ppm CaCO3

The Cation Resin documentation gives the following
Total Capacity(min.) - 2000 meq/L(43.7 Kgr/cubic ft) (Na form)
Moisture Retention - 44 - 48% (Na form)
Reversible Swelling Na to H(max.)- 8%
Total Cation Resin Volume - 282.5 cubic ft(8000 L)


The Anion Resin documentation gives the following
Total Capacity(min.)- 1300 meq/L (28.4 Kgr/cubic feet) (Cl form)
Moisture Retention - 48 - 54% (Cl form)
Reversible Swelling Cl to OH(max.) - 20%
Total Anion Resin Volume - 494.5 cubic feet(14000 L)


Calculation:
Since the Cation and Anion resin capacities are in Na and Cl meq/l respectively, I converted the feed water anionic and cationic concentrations from ppm CaCO3 to meq/L.

92.5 ppm CaCO3 - 4.02 meq/L as Na
136.4 ppm CaCO3 - 3.85 meq/L as Cl


I then converted the incoming water flow rate of 1100 gpms to Liters/hr
1100 gpm = 249810 Liters/hr

I then multiplied the water flow rate by the cation and anion concentrations
Cation Exchanger: 1005249.9 meq/hr
Anion Exchanger : 961199.8 meq/hr

Total Resin Capacity = Resin Volume* Resin Capacity
Cation Exchanger = (8000)*(2000) = 16000000 meq
Anion Exchanger = (14000)*(1300) = 18200000 meq


To determine the cycle time I then divided the resin capacity by the cation and ion meq. flow rate. The service times I calculated for the cation and anion exchangers are as follows
Cation Exchanger: 15.9 hrs
Anion Exchanger : 18.9 hrs


Could someone please help me understand why I am not getting the 12 hr design cycle time?

Thanks

 

[bimr]

Some observations:

1. Ion exchange resin capacities are usually listed in H+ and OH- for demineralizers, and Na+ form for water softeners.

2. Anion resin in the Cl- form will only remove nitrates.

3. Resin capacities are variable based on the strength of regenerant. A higher dosage of acid (in lbs/cubic feet of resin) results in greater ion exchange capacity and leakage.

4. It is generally more economical to use a reverse osmosis system as pretreatment.

Try the Lenntech calculator:

http://www.lenntech.com/calculators/exchange/ion-exchange.htm

http://www.soci.org/~/media/Files/C...urse Sept 2012/Brian_Windsor_calculation.ashx

 

[shahyar]

Hi,
First of all, the water analysis is not very accurate. In an ideally accurate water analysis:
ppm cations in CaCO3 = ppm anions in CaCO3

I din't bother to check all your calc but your conversion from ppm CaCO3 to meq/L seems wrong.


92.5 ppm CaCO3/ 50 = 1.85 meq/(L as Na)

16000000/(249810*1.85)= 35 hr

If you have two cation exchanger vessels, 35 /2 = 18, every 18 hours you have one regeneration operation.


136.4 ppm CaCO3 /50 = 2.73 meq/(L as Cl)

18200000/(249810*2.73)= 27 hr

If you have two Anion exchanger vessels, 27 /2 = 14, every 14 hours you have one regeneration operation.
Hope this help

 

[BRENT511]

Hi shahyar,

Thanks for getting back with me.

The water quality that was used for design basis of the ion exchangers was 92.5 ppm as caco3 cationics and 136.4 ppm caco3 as anionics.

As far as the conversion from ppm caco3 to meq/L as Na, I divided the ppm caco3 by the molecular weight of sodium which is how I got 4.02 for the cationics... I thought you divided by 50 to get meq/L as caco3?

 

[shahyar]

Hi,

1. The water analysis is a way off:
(136.4-92.5)/(136.4+92.5)*100 = 19% This should be less than 5 or 10% (I don't remember off top of my head)
This is what I always check at the beginning of a design.
2. Such writing is wrong:
92.5 ppm CaCO3 - 4.02 meq/L as Na
ppm as CaCO3 or meq/L does NOT refer to any specific ion. I can explain it easier for meq/L.
"meq" means "milli EQUVALENT", it means total of all ions in a equivalent unit.
Always to convert ppm CaCO3 to meq/L divide it to 50.

 

[bimr]

Agree that the total cations as CaCO3 should equal the total anions. If not equal, it is customary to assume the difference is alkalinity.