r/ChemicalEngineering • u/FullSignificance7258 • 1d ago
Design Exothermal reaction
I don’t understand: I have a flow of 40 m³/h going into the wrong tank. The reaction is exothermic (NH₄OH + HNO₃ → NH₄NO₃ + H₂O) with −51 kJ/mol. How do I calculate the evaporation flow for PSV sizing?
Someone told me the tank volume is not important can anyone explaine.
Me i will look for mass in the tank then obtain the flow by dvinding by /deltaH vap!
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u/Oddelbo 1d ago
You need to know how much heat is released by the reaction then divide this by deltaH vap, like you said. If the heat release is not a function of the tank size, just the flow into the tank, then the tank size is not important.
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u/FullSignificance7258 1d ago
Yes, the question is why it’s not a function of tank size. Why does the evaporation rate not depend on whether the tank contains 5 or 10 m³ of liquid? To me, that doesn’t seem logical.
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u/tabasco_lover 1d ago edited 1d ago
Reactive relief can be incredibly complex, but given the lack of details here, I am assuming you’re expected to make several simplifying conservative assumptions, most significantly:
The tank has a sufficient amount of the intended material to react with all of the unintended material for the time period you’re interested in (you did not clarify which is which, so let’s say you accidentally put NH4OH in an HNO3 tank).
The reaction is basically instantaneous, such that you are reacting the 40 m3/hr of NH4OH at essentially the rate it’s being added.
Given the above assumptions, the max heat release can only be the heat of reaction multiplied by the rate at which you’re adding the NH4OH.
If the tank were very small relative to the reagent addition rate, or the rate of reaction were very low, then the rate of heat release could certainly be less than this (and would by no means be constant), but it can’t be more just by increasing the amount of stuff in the tank. You can only react as fast as you can add the reagent.
As far as calculating the flow rate of material through the relief device required to remove heat at a certain rate: this wouldn’t depend on mass in the tank either. (Q/hr)/(deltaHvap) = mass/hr
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u/FullSignificance7258 1d ago
They told me that you can’t analyze using a flow rate (volume per hour) with the volume in the tank.
At first, I used n = m/M to find the limiting reactant.
So there was a flaw in my analysis
And the volume in tank it does not matter ( taht what doesn't make sens) cuz for me more weight in the tank more reaction so more mass flow evaporated !
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u/tabasco_lover 23h ago edited 23h ago
Imagine A + B -> C + D with a -10 kJ/mol heat of reaction. You know nothing about the kinetics.
A tank contains 1,000,000 mols of B. You start adding 1 mol/hr of A. What’s the absolute maximum rate at which heat will be generated? 10 kJ/hr (constrained by the rate at which you’re adding A).
If your reaction mixture has a constant latent heat of vaporization of 10 kJ/mol, how many mols/hr must your relief device pass to relieve the 10 kJ/hr heat of reaction? 1 mol/hr.
Do you see how the 1,000,000 mol B did not enter the equation, and that all that mattered was the rate of addition of A? This does ignore several real-world effects but is a worst case and is I suspect the simplified way your (homework?) problem is wanting you to approach this.
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u/Oeyoelala 10h ago
It depends a bit on how conservative your approach should be. I guess the products you have are diluted in water. So first all that volume needs to be heated to its boiling point before you get pressure build up because of water evaporation. But personally i would also go for the conservative approach, saying that every product added leads to heat release, leads to water evaporation. That evaporated water needs to be relieved
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u/Life-Is-Good27 1d ago
From a mechanical standpoint, a bigger tank likely allows for higher flow rates into it (physical limits on pipes to accept high flow rates and the reaction needs a certain residence time to occur).
So in real life, the tank size does have an impact.
Also, mechanically, larger tanks tend to have lower design pressures (a large tank needs to be thicker/have thicker walls to account for the hoop stress of pressure).
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u/Always_at_a_loss 1d ago
If this a work related project and not for school, then you shouldn’t be performing PSV sizing for reactive systems unless you have been appropriately trained in DIERS methodologies or something comparable by your employer. It’s a huge liability, can become very complicated, and it often requires access to bench scale experiments for the necessary data.