Ventilation rate, H-release, batch reactor w/ 32% HCI +H2o at 150F, precious metal recovery

[DASHENG4551]

A heated 800 gallon batch reactor contains HCI at 32% concentration and city water, receives precious metals waste. The HCI oxygenates the process to precipitate the precious metals. Hydrogen is released into the headspace. the batch reactor diameter is 72 inches headspace is 28 cu.ft. The reactor is heated to 150F for 2 hours. Then cooled to 70F. The mass of the precious metal waste is unknown. At a 4% LEL concentration of hydrogen the exhaust fan will be ramped up to dilute the H concentration to less than 4% in compliance with NFPA 91. What is the minimum/maximum ventilation rate CFM to maintain less than 4% LEL?

 

[LittleInch]

How much hydrogen is being produced?
At what max rate?

Do you mean <4%of the LEL.

LEL of hydrogen is about 4 to 5 % in air?

But a lot of air is needed.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EdStainless]

Why not just vent it through a pilot burner?
You need to work the chemistry to figure out how much H2 will be generated.
What on earth is the reactor made out of?
Keeping it from getting hot may be biggest problem.

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P.E. Metallurgy, consulting work welcomed

 

[georgeverghese]

First, you need to know the range of metals concentration in this metal waste being charged per hour. And the breakdown by weight of each metal. Then write out the chemical reaction for each metal's conversion to chloride, which will also tell you how much H2 is released. There may also be decomposition of HCl to chlorine also. And water vapor in the wet exhaust. Some metals may exhibit 2 or more valencies, so that complicates matters further.

 

[DASHENG4551]

Thank you for your questions and thoughts.

There are at least 30 reactors. The reactors are manufactured of stainless steel I do not know the exact type, e.g., 304, 316, etc.

The precious metals are gold, silver. I make-up of the 800 gallon mix is 256 gallons of HCI and 544 gallons H20 of city water. The specific density is 0.9. As mentioned the mass containing the precious metal is unknown. Presently a 12 inch dia FRP duct at each reactor exhausts a maximum of 1,000 CFM per tank. A 28 sq.ft surface area 1-ft for headspace provides 28 cu.ft. The air flow rate and/ACH is way too high. If the LEL as measured in the 12 inch exhaust duct exceeds 4% the exhaust fan VFD ramps up to increase the OSA to dilute the exhaust air flow. Some typical rules of thumb for a similar type of precious metal recovery operation would be appreciated. Some best estimates towards, H2 release volume before equilibrium (2 hour process cycle) is reached. The cool down cycle is initiated thereafter to recover the precious metal coated anode.

 

[EdStainless]

So you are allowed to just dump the HCl and Cl vapors?
That explains why you don't burn this off.
If you are running this why don't you measure the H2 evolution?
By knowing the current H2 levels in the exhaust you can infer the amount that you need to handle and adjust the air accordingly.
How many cycles do your reactors last? I'll wager that they are in bad condition from the HCl.

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P.E. Metallurgy, consulting work welcomed

 

[DASHENG4551]

The exhaust vapors are conveyed to a wet fume scrubber. No burn offs.

The recoverable precious metal mass is a propriety "Client" information. There are other chemicals added to the Batch Reactor(s) to facilitate the precipitation and deposition process. These "other" chemicals are unknown, e.g., trade secret.

With too many unknowns it has been suggested that the more practical way to determine the volume of H2 release during the 'Heating" process cycle is to Monitor the H2 levels in the exhaust duct via the Drager H2 sensor and adjust the Exhaust Fan Speed through the VFD to9 maintain less than the 4% H2 LEL.

We will know what the fan SP, the RPM and with the respective fan curve determine the rate of flow, i.e., CFM. I will post the results of the investigation.

Thank you for your comments. Be well

 

[pierreick]

Hi,
To me you should consult local EPB about the technology suitable for your application . Not knowing where your facility is located , it's difficult to support you.
I believe that a thermal oxidizer is a suitable technology for this purpose, especially with the lack of accuracy of H2 quantity released .

For your current process I hope that your local instrument (H2 sensor ) is well calibrated and all your motors and electrical equipment are rated EXE .
Note : Probably good to have two sensors in service to validate the measurement .

Good luck
Pierre

 

[DASHENG4551]

All vapors are exhausted to a wet fume scrubber before discharging to atmosphere. The exhaust fan motors are external, belt driven. Where motors are in the airstream they are explosion proof. the VFDs are external to the Process Production/Reactor(s) room. Once startup is initiated the monitoring of the Reactors exhaust vapors will be conducted. A NIST rated and calibrated H2 sensor will be inserted into the exhaust duct for each 12-inch FRP duct upstream of the main exhaust duct to measure the actual H2 concentration. Will have to determine the air flow velocity and Static Pressure at the exhaust fan upstream of the fume scrubber. The test results and findings will be posted. Thank you all for your comments and interest.