demin tank vent filter- scrub CO2 ? 4

[davefitz]

Is anyone aware of a filter for use on a vent for a large demin storage tank, where the purpose of the filter is to capture any CO2 which may be suctioned into the vent during tank drawdown.

We have a large demin tank , and the CO2 absorbed during drawdown is causing the water CO2 concentration to exceed allowable limits for the boiler feedwater.

 

[bimr]

What is usually done for this application is to add a nitrogen blanket to the tank.

The nitrogen blanket is maintained by a pressure system controlled by a regulator. As the tank is filled with water, the pressure builds and the gas is vented to the atmosphere. And consequently, as the water is pumped from the tank, the pressure drops and replacement gas enters the tank. It is common practice to equip storage tanks with vacuum relief valves or rupture discs that prevent a vacuum and subsequent collapse of the tank. Nitrogen blanketing can be applied to multi-tank systems and has the advantage of optimizing nitrogen consumption, since, as one tank is emptied the gas can be displaced to another. When water is transferred from tank to tank, the inert gas is merely exchanged. During filling or emptying the pressures vary, however relief valves are set to release at pressures above design pressure.

Nitrogen blanket storage tanks will not support life and should be thoroughly purged before being entered by maintenance personnel. Signs warning of this hazard are normally displayed on nitrogen blanketed tanks.

 

[davefitz]

Thanks , BIMR. I had forgot about the N2 purge option.

Another useful design detail ( for large tanks) is to have 2 independent vents/ vacuum breakers; I've seen photos of tanks that imploded due to birdnests or other debris plugging the single vent.

 

[Techcitizen]

We have what is called a CO2 absorber connected to the DM tank vent. It is a pot filled with KCL solution located on the ground. Pipe from the vent is immersed into this solution. The immersed portion of the pipe has small holes through which the air gets sucked in, passes through the KCL solution whereby the CO2 gets absorbed and then enters the tank. There is another simple design in which the absorber pot filled with KOH flakes is installed on the vent. If you need further information please put your e-mail address in this thread and we can then send you the details / drawing. N2 blanketing is usually a costlier alternative.

 

[davefitz]

Thanks Gajjar. My e-mail address is <david.fitzgerald@pgnmail.com>-

 

[bimr]

You do have some other options. If you are a power plant, the CO2 absorber may not be the complete answer because you are also concerned about absorbing oxygen as well. A condensate tank is a major source of oxygen in the cycle.

If you are a power plant, you may also look at:
1. Use of a floating cover, similar to that used in the oil industry in the condensate tank.
2. Or, you can use a vacuum degasifier

 

[Techcitizen]

davefitz, I am unable to send message on your e-mail address. You may give an alternate address or write your fax no.

 

[davefitz]

gajjar:

I am very interested in the CO2 scrubber you described. The following e-mail addrersses should work, and a fax :

<david.fitzgerald@pgnmail.com>
<davefitz7501@yahoo.com>
919-235-6129

Thanks again

 

[kaicy]

dear mr gajjar,

i am on the look out for the constructional details of a co2 absorber for dm tank. it will of great help to me if you can pass on more information and a sketch. my e-mail id: kaicyem@ieee.org

 

[Martijnpeters]

Dear Mr gajjar,
Your solutions to the CO2 problem, both the KCl-scrubber and the KOH-bed, sound simple and reliable to me. could you pass me the details and drawing of both?
my email: martijn_pfh@yahoo.co.uk

 

[SafariMike]

Hello,

we´re trying to solve this problem by using a 3 stage strainer with silicagel (to absorb moisture) and Sodium Hydroxide.
Unfortunately the construction is very bad, so the chemical collapsed after a few days. Now we are trying to modify the filter construction. This is still ongoing. I will keep you informed about this.



safarimike@web.de

 

[cub3bead]

N2 blanketing is very effective but you have to make vacuum relief provisions to prevent the tank from collapsing on itself. Also, most of these tanks are not designed for much in the way of positive pressure. You usually design for about +2&quot; W.C. Fisher used to make a neat combo PRV/Vacuum Relief valve for this service. Your N2 supply is likely from 3000 psig bottles so you will need multiple pressure step down regulaors to contol the N2 pressure in the tank.

Basic question however, how is your DM water being added to the cycle? If introduced into the condenser hotwell properly, the air removal system should take care of the problem. Are you spraying the DM water in over the tube bundle or just letting it flow into the hotwell?

If you are making up to the DA, have you checked it for proper operation?

 

[davefitz]

cub3bead:
The original condensate makeup to condenser was sprayed over the tube bundle for dearation and degassing, but the unit is now 2 shift cycling, undergoing a cold startup every 2 days. The massive steam venting from the HRSG during cold startup translates to huge condensate makeup requirements, well in excess of the original design.

The original makeup nozzles were supplemented with added nozzles that discharge under hotwell water level, with zero dearation and zero degassing. Also, the hogger/ vaccuum pumps don't work worth a squat, so degassing wouldn't be effective during initial operation anyway.

A better solution would be an official vacuum dearator with degassing and dearation sized as by experience with cold startup makeup, and a suitably sized aux boiler.

 

[cub3bead]

Have you looked at the Liqui-Cel membrane for degassing the condenser makeup? Please see their products at:

http://www.liquicel.com/

Are you also having problems with high Fe in your condensate? What is the condenser tube material?

 

[DeltaCascade]

You likely need to do nothing at the tank. The boiler feed-water deaerator will scrub and take out the dissolved CO2 AND O2 before the water sees the pre-boiler systems (economizers) and the boiler proper. Demin tank is likely stainless steel or lined, likely also the boiler make-up water line to the deaerator scrubbing section. Demin water corrosivity is due mainly to low conductivity and dissolved oxygen content. Yet, as in a typical boiler system, you should not need to do anything to the tank.
What CO2 specs for boiler feed water do you refer to? CO2 can be a problem, but usually in steam condensate systems where CO2 generated in boiler from decomposition of carbonates causes problems: it &quot;flashes&quot; with steam and later dissolves into steam condensate to cause low and corrosive condensate pH, but then you typically feed volatile alkaline amines to control condensate pH...
Am I missing something? Good luck.
Cheers//

 

[SafariMike]

Your basically right, but our deaerator is working with 108 °C and 0,25 bar. Therefor a effective deaeriation does not happen.

To explain my problem, we have in our plant 2 Blocks, wich get the same water from a demin tank. Block 1 has a deareator whick operates at 165 °C and the newer Block 2 is working at 108 °C only. In Block 1 our steam conductivity is according VGB Guidelines, but Block 2 has since the beginning much problems with conductivity. From the beginning we also had a few problems with the Oxigen, but this could be solved by modifying the sparges with a smaller diameter. One more problem is also, that the makeup water is not going to the condenser as common, it is going directly to the deaerator.

I´ve already done a test to fill most of the make-up water into the hotwell. It shows that the Oxygen could be reduced by 60%, but unfortunately it has no effect to the conductivity, as I expected.

If somebody has another idea, I would be grateful to hear this.

Micha


safarimike@web.de

 

[davefitz]

In our case , there is no dearator per se. The only dearation and degassing occurs in the condenser as the makeup is sprayed over tubes and ejected thru the vacuum pumps. Also, during startup venting of the HP superheater, some degassing occurs .

True, some CO2 is evolved from breakdown of the organic actived oxygen scavenger in the LP drum at 300F. But most CO2 in our case is from saturation at atmospheric conditions in the demin water storage tank, and we have a huge makeup requirement during startup.

 

[DeltaCascade]

Davefitz, what do you think of cub3bead's comments? Is there a niche for this?
Seems there are these options...
- demin tank N2 blanketing?
- fixing the hog ejectors or vaccum pumps?
- retrofitting existing condenser for vaccuum deaeration?
- install new vaccuum deaeration system?
- CO2 absorber on demin tank (?)
- install membrane deaerator?
It seems there is no condensate tank, just the condensor with large liquid surge capacity?
Interesting problem. Cheers

 

[davefitz]

deltacascade:

I think you have the right summary. We are proceeding on the following plan:
a) provided more cooling water to the vacuum pumps to improve their performance.-done
b)relocate the emergency makeup line from hotwell injection to above tube bundle spray to improve degassing-someday
c)install new nozzles on makeup feed line to allow installationof a degassing membrane separator ( CO2 ) and demineralizer (Si)- being done
d)modify the control logic for the HRSG's HP bypasss to minimize need to vent HP vent ( loss of condensate) during morning restarts. ( 2-shift cycling combined cycle)