How to avoid CO2 entering Deionized water tank 1

[22854]

We have an atmospheric API storage tank, venting to atmosphere that receives deionized water and goes from it to mixed beds.
It has no protection against CO2 entering the tank with the air during pump out.
Which options do we have to avoid it ?

1) We tried N2 blanketing but don't want to loose it. In that case we should install a system to control inside pressure and N2 flow in-out and a PVRV to protect the tank if the N2 control system fails.
1a)Which should be the pressure inside de the tank ?.
1b)How can I calculate flow of N2 ?
1c)Which should be the set point for the PVRV.

2) Has anybody tried to filter the air through Sodium Carbonate?.
2a) How to calculate the filter and the required volume of Sodium Carbonate ?
2b) How can I determine when to change the Carbonate ?
2c) During pumping in, will the air pass through the filter bed counter currently with no problem?

Answers to my questions and any other options will be very much appreciated.
Thanks in advance

 

[TD2K]

I'm not sure what you mean by "We tried N2 blanketing but don't want to loose it" and then you go on to ask questions about N2 blanketing. First, I suggest going through a copy of API 2000 (tank pressure protection) and get a copy of Protectoseal's or Shand and Jur's catalogues on tank vacuum/breakers.

With an N2 blanket, your tank would have a pressure vacuum valve installed on it to vent N2 on high pressure (as the tank fills) or allow air into the tank on low pressure (as the tank empties) to avoid collapsing the tank. The N2 would be admitted to the tank with a regulator valve (Anderson greenwood makes a tank blanketing valve as do others) to maintain just above an atmospheric pressure so as long as the regulator works properly, the atmospheric vent should never let air into the tank.

As far as the set point, you need to look at the tank's design for pressure and vacuum. Tank pressure vacuum valves need a reasonable margin between the set point and the actual pressure to achieve significant flow. If the tank is designed for 6 inches of water column pressure, you might need to set the pressure valve at 4" of water column or less to avoid the tank's pressure from exceeding its 6" design pressure (you do not have an allowable 10% overpressure like pressure vessels) at the maximum outflow rate of gas. Similar arguement with vacuum. Shand and Jurs and Protectoseal all make tank vent/vacuum valves, suggest you get a copy of their catalogue, they go through this also as does API 2000.

The N2 flow rate is set by the change in water level. As the water level in the tank rises, you vent N2 gas out due to displacement. As the water level falls, you have to bring in gas to prevent a vacuum. Basically, 1 volume of gas per volume of water. You can size the N2 valve for the maximum pump out rate or for a more typical outflow rate if you are willing to occassionally let air into the tank through the vacuum breaker. Again, I suggest looking at API 2000, they go through this quite well.

As far as using sodium carbonate, unless your tank can take a 'reasonable' value for vacuum, you are likely going to have problems staying within its vacuum rating when bubbling the air through a sodium carbonate solution (if that is what you imply by your last section).

 

[22854]

Thank you for your comments.

Regarding the N2 "loose" I tried to mean that we simply put a N2 hose into the top of the tank to mainain a N2 atmosphere without controlling the amount of N2 that was vented out from the tank.

With sodium carbonate I meant a filter with a bed of solid Sodium carbonate. Is this normally used for this purpose ?

 

[TD2K]

Make sure you do it properly and think through the what-ifs so that you are covered in the design. If you want to simply connect an N2 hose, you need to ensure you have sufficient relief capacity on the tank for the maximum flow rate you can get through the hose.

You can continually purge the tank with a small amount of N2 to keep air swept out or at least minimized. Unless this flow rate is big enough to handle the maximum pump out rate from this tank, you will draw air back into the tank during those periods. As a starting point, look at the typical level changes in the tank with time, you can select a flow rate of N2 from that information to handle typical level changes to avoid sucking air back into the tank.

 

[DeltaCascade]

Isn't it overkill to keep CO2 from air out of the demin water tank before it goes to mixed bed demin polishers? Perhaps this is why the tank does not have such protection in the first place (talk with the original designers if possible?) Minimizing aeration and splashing in the tank should be enough?? The mixed beds will remove any CO2 (H2CO3) ... (remember, the decarbonator that you likely have between the upstream demin plant cation and anion exchangers removes relatively high concentrations of CO2 originally associated with carbonate alkalinity, NaHCO3, etc)

The demin water pH may fluctuate due to CO2 absorption but dont let this confuse you as it is unbufferred water (it doenst take much to change pH .. indeed, if you can even measure it as it's condictivity is so low). A test of the demin storage tank water carbonate alkalinity should show that CO2 content (from air) is actually quite small. (just avoid high CO2 concentration gas streams, such as flue gases or decarbonator vents from venting around the tank)...

Good luck

 

[cynwrig]

Why do you need to keep out CO2? As has been ststed, normal demin storage does not have elaborate purges, etc. I once saw a water tank go sherical when someone tirned up an air flow without first checking the relief capacity!!!!

 

[nbucska]

By the way, Na2Co3 won't adsorb CO2, you guys must think at NaOH. <nbucska@pcperipherals.com>