Evaporation rate and ventilation calculation 2

[m2e]

For a chemical storage room, let's say you have a tank of chemical for a certain process, for example chlorine or fluoride for water treatment processes. How do you calculate the evaporation rate of such chemical, and how much ventilation is required to remove the vapor in order to meet OSHA or similar requirements?

 

[TiCl4]

Theoretical solution (i.e. a lot of work for not much gain):

For ideal solutions, you may first think of Henry's law to get the partial pressure above the liquid. However, if the solution is non-ideal, you will need to apply an activity coefficient for that particular solute/solvent mixture. To further complicate this, the gases you reference are also reactive with water, so you will need to develop an equilibrium expression (i.e. Cl2 + H2O <-> HCl + HOCL) and consider the relative volatilities of each component. Lastly, you will need to apply Fick's law of diffusion for this mixture to generate an estimate of the diffusion rate from the liquid surface. The controlling case for Fick's law will be how you set your boundary - will it be diffusion into an infinite space, or will you sweep the room with air to maintain a certain concentration?

A brief Google search revealed some data may exist for Henry's laws constants for "free" chlorine, so that data may relieve you of the need to develop an equilibrium and activity coefficient model.

Practical solution (what I would personally do):
1. If this is an existing installation, grab a cheap Cl2 monitor and walk out there in the middle of summer to measure ppms. Develop an abatement plan based on that reading. Pulling from electrical classification methods, a good starting point for ventilation calculations would be 7 changes of the room volume per hour if you cannot seal the tank and vent outside. If you cannot seal the tanks, then you'll still need Fick's law to calculate required sweep rate to maintain required ppms in the airspace.
2. If this will be a new installation, seal the tank and vent outside.

Personally, I would try to seal the tanks and vent outside if possible. Introducing an air sweep would be expensive due to both the installation cost and ongoing cost of energy/lost chemical.

 

[Snickster]

You probably could calculate but it would be a very complex calculation. However ventilation rates for spaces with sealed hazardous chemicals stored are set by building codes such as the IBC (International Building Code) and its accompanying code IMC (International Mechanical Code) and IFC (International Fire Code). In these codes you will find specific ventilation rates, allowed volumes of storage, and any requirements for sprinkler systems. It is all based on volume/mass stored. Also if it is open container then you would need to likely have local exhaust hoods. Ventilation rates for different types of special spaces storing chemicals or laboratories, etc. may be per IBC and ASHRAE 62.1 but the local building code would govern. The design of ventilation systems gets somewhat complex and usually a negative pressure is maintained in the room with chemicals relative to adjacent spaces.

A base rate for sealed chemical storage per IBC/IMC/IFC would be approximately 1 CFM/FT of storage room floor area but this may required to be increased based on what you really are storing and how much as explained in the Codes.

 

[shvet]

How do you calculate the evaporation rate of such chemical, and how much ventilation is required to remove the vapor in order to meet OSHA or similar requirements?

In theory evaporation rate is negligible as vapor and liquid inside of a tank are in equilibrium and vapor-to-air diffusion at the very end of a vent pipe is the only force affecting emission of vapors to surrounding area.

Clarify your question - do you mean breathing caused by liquid level changing, not evaporation?

 

[TiCl4]

Shvet,

OP's question implies to me that the "tank" is not sealed and is open to atmosphere inside a closed chemical storage room. Thus, the concentration of the chemical inside the room, if not ventilated, will rise to equilibrium, which is likely above the PEL for that chemical. Ideally, the "tank" would be sealed and vented outside, but that doesn't appear to be the starting point of this discussion.

 

[shvet]

@TiCl4

OP posted "a chemical storage room" what means for me that a chemical is stored. It means liquid level is not constant as a tank is not used as a charge vessel. It means that vapors emission during outbreath is much much more than evaporation rate. It means there is no sense to consider evacuation of vaporized vapors by ventilation. Only breathing makes sense for storage.

The only case that is able to take place that a tank is movable and is stored in storage room prefilled. But such case relates to containers and drums, not tanks.

Anyway OP seems lost interest to this topic so interpretations have a little sense.

 

[m2e]

Thanks for the responses. Sorry I have missed the notifications for the replies. The question is regarding a generic chemical storage room where the chemicals are stored. Alternatively, the situation could be that the chemicals are also stored in the pump/piping room where the chemicals are also injected into the piping. The chemicals are stored in tanks that are not air tight because they're being pumped out. If it's a pump room, then people could enter and service the pumps or other equipment. Hence, the room air should not be saturated with the chemical, but ventilated with fresh air.

 

[shvet]

Such case is unable to be predicted as airflow is uncontrolled so it is usually solved another way. Ventilation is able to keep average concentration in room but it is unable to guarantee that in the every workplace/walkway in room. One should install an exhaust hood to guarantee air inflow along all perimeter of this "tank", an expensive option I should say. Check an applicable local HVAC code for a proper design. Also note that outbreath is still a governing case for toxic vapors release.

A toxic releases in a confined space is not the best way to organize a workplace even that is ventilated. Such vapors should be treated or dispersed at controlled conditions.

 

[horacio Torres]

Chlorine and fluorine tank will never be open to the atmosphere-- Chlorine is storage in Chloroalkyl plants.in horizontals tanks, Transport is done in special tanks.


Check the chlorine institute web for the USA rules.

https://www.chlorineinstitute.org/

https://bookstore.chlorineinstitute.org/

Horacio

https://www.linkedin.com/company/lagotuy/