3 replies to this topic

[TonyCO]

Hello,

 

I am working on a chemical system design for a brackish water desalination plant that requires large volumes of HCl for pH adjustment (~80,000 gal in poly atmospheric tanks) and the client is specifying an indoor facility. To minimize corrosion potential inside of the facility, we are considering a blanket system to flush acid vapors to a designated exhaust/scrubbing system. The firm that I am at does not have many other engineers that have worked on systems larger than small water treatment plant storage designs to evaluate this concept, so I thought it would be beneficial to inquire here for general advice as well. 

 

This is the general idea:

- Roof intake piping brings fresh air down to vent inlet header on storage tanks

- Air sweeps through head space of storage tanks (although not sure of reasonable velocities yet)

- Slightly acidic air exits through exhaust vent and joins header connecting other tank exhausts (sized large for pneumatic tank filling, estimated air flow with API2000 7th App.D.7)

- Exhaust header goes to inlet for either small venturi or absorber column to scrub acid emissions

- Roof mounted dispersion exhaust blower; suction from blower drives air flow throughout the system (haven't done any real calc's yet)

 

Does a system like this seem reasonable? From an emission standpoint, I'm not sure how to best estimate the HCl transferred to the sweeping air from convection. Is it worth going through more traditional mass transfer estimates? Is it reasonable to assume--from a mass loading standpoint--that a worst case scenario approximation would be vapor-liquid-equilibrium? This second option is my initial inclination, even though it seems like overkill for the estimate. 

 

Overall priority on this design is to minimize fugitive emissions escaping into the facility, from both a corrosion and operator safety standpoint, and provide an integrated scrubbing system. I've searched around quite a bit for a similar design concept and have found a lot on acid scrubbers and blanket gas systems, but have not seen very much similar to this overall design idea. General feedback or suggestions would be greatly appreciated.

 

Thank you,

 

Tony

[Pilesar]

Indoor facilities can also have some outside storage. The HCl storage should be outdoors, in my opinion. If the client has good reason for indoor storage, then the storage should have its own room with no doors or windows or vents connecting to the main facility and preferably in a separate building. But what advantage is a roof and walls for the storage tanks? Why not just put the storage to the outside of the facility wall to improve safety and cost?

[TonyCO]

Pilesar, thank you for the response. The plant will have its own chemical storage facility separate from process equipment, and inside of that the acid will be in its own masonry room as well. One of the client's previous plants has outdoor acid storage and has decided that despite the additional costs, they would like to go inside (ease of operation during inclement weather/seasonal conditions, among other reasons... the client is very friendly to operator preferences). Without this driving preference and the supporting budget, we would be going outside.

[Technical Bard]

I would purge the HCl tanks with nitrogen instead of air - oxygen and chlorine together can be highly corrosive to metals that may exist in the system.

A scrubber washed with Ca(OH)2 or KOH can capture the HCl from the vapours.  There are a number of firms who do this design specifically at various scales.

 

I would be loathe to put the tanks indoors - if a tank was to fail or leak (there are examples of failed valves/nozzles), you would face an HCl cloud in the building.  Better to put it outdoors and fill the bund area with crushed limestone.