Design CalculationFor Softener Unit

Client :- Techpro

By Samar Seth

About the Paper

I ,as an intern, calculated the ion exchange softener design for a small sized powerplant of TechPro Inc. in May 2015, and would like to share the calculations in the form of this research paper . I have presented the various entities in the form of an excel sheet, which can be adjusted and understood with ease. I have presented the logic lucidly, and would recommend all readers to go through my articles before attempting to read this paper . I hope you will gain from this ….

RegardsSamar Seth

Why Ion Exchange ?

As discussed in my articles, there are essentially two methods of removing hardness , namely1. ion exchange and2.Chemical precpitation methods ( lime and lime soda ash softening ).

So why use ion exchange ? This is because, the level of hardness content of water required by the client in its power station is commercial zero i.e. below 5 ppm . After chemical precipitaiton methods, the general hardness content in the obtained water is around 100 ppm . Whereas for ion exchange , the soft water has hardness <5 ppm , which matches with the desired ppm .Additionally, in ion exchange softener the resin can be regenerated using chemicals NaCl, which can be obtained cheaply. Whereas in chemical precipitation methods, lime and soda ash becomes less feasible due to handling of extra sludge produced in the system , hence not used for softening. the soft water has hardness

The CalculationsThe client gave a stipulated water flow of 130 m3/hour, requiring 2 softeners , and gave an estimate of 75 ppm as CaCO3 of the feed total hardness.

As the softeners required are similar, equal flow of water will enter the two softeners. So the amount of water flow in each individual will be 130 / 2 =65 m3/hour . Hence, the net flow per softener is 65 m3/hour.

Allowing the plant to run for 22 hours, and leaving 2 spare hours for regeneration of the cation exchange resin, the output between regeneration for one softener is the product of the operating hours and the operating flow, ie 22*65 = 1430 m3 of water .

If the feed water sodium present is 42.36 ppm as CaCO3 , then the ratio of Na to TC is given by (feed water sodium)/(Feed total hardness + Feed water sodium ) , which gives (42.36)/(42.36 + 75 ) = .360941, i.e. 36 % approx.

Regeneration LogisitcsFor this plant, the resin selected is DUOLITE C-20 Na and the mode of regeneration is co-flow (C/F), which means that the backwash flow and operating flow will have the same direction.

The regenerant used in this plant is NaCl , due to its easy availability and low price .

The regeneration level of salt selected is 150 kg NaCl/m3 based on which the capacity of resin is evaluated as per the characteristics of the resin used .

Exchange CapacityThe excgange capacity of the regenerant is 69 Kg as CaCO3/M3, and can be corrected using the five tables given below.

According to this table, the correction factor 1 turns out to be 1, correction factor 2 due to the feed hardness is equal to 1, similarly comparing the values of the correponding items with the above table the correction factor 3 also turns out to be 1.

Whereas for 4 the value is .96, and .9 for correction factor 5(After comparing with the excel sheet)

The net exchange capacity can be obtained by simply multiplying the standard exchange value by all five correction factorsi.e. 69*1*1*1*.96*.9= 59.62 Kg as CaCO3

Work DoneThe work done is equal to the product of the feed total hardness and output between regeneration for one softener upon 1000, i.e. 1430*75/1000=107.3 Kg as CaCO3

During design calculations it is important to consider some extra margin, as a lot of water is wasted during regeneration and hence the waste water for this design calculation is assumed to be 20 m3.

As there shall be some extra load due to waste water that is being assumed, the work done by the waste water needs to be added to the previous value. The load due to the waste water is again the product of the feed total hardness and output between regeneration ( assumed volume of waste water ) , i.e. 20*75/1000=1.5Adding this waste water work value to the previously work value gives 107.3+1.5=108.8 Kg as CaCO3.

Resin VolumeThe amount of total resin volume can be calculated by dividing the work done ( total, including waste water) by the net exchange capacity, which gives108.8/59.62= 1.82489, i.e. 1.824 m3 approx.

Dimensions of TankThe required bed height has been assumed as 1.5 metres

The area can be calculated by dividing the resin volume by the above heighti.e. 1.824/1.5=1.216m2From this the diameter of the tank can be calculated by Area= pi*r^2 as the resin vessel is vertical cylindrical type.

Putting the value of area in the above equation and solving for 'r' gives .624, and as diameter is twice the radiusd=.624*2=1.249 m

Considering some extra margin the diameter of the tank in the power plant is taken as 1.3 m for convenience.

So, the bed height of the resin will be calculated using the diameter as 1.3, so it will be pi*[d/2]^2*h= 1.824/(3.14*0.65*0.65)=1.375 m

With the required extra free board space as 80 %, the Height on straight (HOS) = Bed height*(1+.8), i.e. 1.375*(1+0.8)=2.475 m

Leaving some extra space, the HOS provided is 2.6metres

Salt For RegenerationAs mentioned earlier in this paper, salt is used for regenerating the cation ion exchange resin.The amount of salt that will be utilized for regeneration depends on the resin volume and regeneration leveland is given by

Provided Resin Volume*Regeneration Level=150*1.824=273.6 Kg

The salt saturation level in the brine tank is given to be 24%, so the suggested capacity of brine tank can be calculated as (Salt Required/Salt concentration)/1.2=950L where 1.2 is the specific gravity of 24% solution

Thus, the suggested capacity of Brine tank is 950 L.

The provided capacity is 1500 L, to ensure extra margin.

As 1000L=1m3, so 1500L is equal to 1.5 m3

HOS of Brine tank is 1.5 m, so the area of the brine tank is Volume/HOS of brine tank=1m^2

The diameter is {(1/pi)^1/2}*2=1.19mand the selected diameter is 1.2 meters

Regeneration Sequence OfSoftener Unit

Client-Techpro

For one VesselMode of Regneration- Co-flow(C/F)

Area of the Vessel-1.32m2

Regenerant Quantity- 273.6 Kg , 950L of 24% Brine SolutionSalt Concentration-10%

BackwashThe backwash flow is given as 12.5 m3/(m2*Hour)=12.5 m/hour, and

the flow rate is defined as the backwash flow* Area of vessel=1.32*12.5=16.478m3/Hour

If the operation time is decided as 10 minutes, then the volume of water needed for backwash is 16.478*10/60=16.478/6=2.746

Note- The water that is utilized in backwash needs to be filtered

Salt InjectionThe volume required for salt injection is defined as the regeneration quantity/Salt concentration/1000/1.05=2.825 m3 after substituting appropriate values, the design basis is taken to be 10-12% at 2-8 Bed Volume/Hour (BV/Hour)

The flow rate considered is the bed volume per hour (2 BV/Hour)*Volume of salt injection=2.825*2=5.65 m3/Hourand the operation is carried out for 30 minutes...

Note – Filtered water is used in salt injection

Slow RinseThe slow rinse process is generally carried out to drive out the left over salt solution in the resin vessel

The volume required is defined as the designated Bed Volume (2 BV/Hour)*Resin Volume provided= 1.824*2=3.648 m3

The flow rate of slow rinse is 3.599 m3/hour, and the time can be calculated as Slow rinse Volume/Flow rate = 3.648/3.599=1.0136 hours

Multiplying by 60 to get the answer in minutes gives 60.8 minutes

Note- The water used in slow rinse should be filtered

Final RinseThe final rinse for one softener is equal to the service flow of water entering one softener which is equal to 65 m3/hourIf it is operated for ten minutes then the volume of water that will be produced is equal to 65*10/60=10.833 m3

Note- Only filtered water should be used for final rinse

In summation for regeneration sequence the total time is the sum of the time taken for backwash, salt injection, slow rinse and final rinse, i.e. 10+30+60.8+10=110.8 minutes and the waste water volume will be equal to 2.746+2.825+3.955+10.833=20.359 m3

Please refer to the following link for the excel spreadsheet-https://docs.google.com/spreadsheets/d/1vu6XDV_79sDsq9te6FzvPaOZ2WF6iFkqY-eUCyFUF4A/edit?pli=1#gid=1767627426