[jacarandaspoon]

I am looking at designing a reactor for a reaction which contains 0.6 mL 1M HCl, 3 mL H2O and 4 mL 30% ammonia.  It's been a long time since I did any undergrad chemistry, can someone please remind be how to calculate the pressures that we're going to generate here and does it depend on the vessel size?

[Enthalpy]

30%w/w ammonia is about the solubility limit at room temperature, meaning 1atm ammonia partial pressure. If air is present before the volume is constrained, already 1+1atm absolute pressure is expected.

But this depends already on the presence of water before or after the volume is constrained. Dilution would reduce the ammonia partial pressure. By the way, I wouldn't trust a linear formula in this case, because the strong proportion of ammonia molecules in the solution lets them interact. This needs an experimental curve.

What can still happen is that the heat from the reaction with the acid reduces the solubility more efficiently than it reduces the proportion of ammonia. Then, the pressure may increase. This needs a numerical verification, based on the enthalpies of formation of HCl, NH₃ and NH₄Cl, all in water solution. The big ammonia concentration changes its heat of formation in solution, but I would just start with the heat of formation in dilute solution, hoping that the result is already clear enough.

[MNIO]

I don't understand.  (1) You're trying to design a reactor for reaction 0.6 mL HCl, 3mL of H2O and 5mL of 30% NH3(in water?), 8.6mL in total?  (2) you want to know if pressure depends on volume?, (3) you haven't taken undergrad chemistry for a long time?

I'm sorry but this makes no sense.  What exactly are you trying to make?

[jacarandaspoon]

Sorry I left out a couple of important points:

There are a range of ionic salts in the reaction mixture as well.  None of which will contribute to the pressure. This is the current scale of the reaction which is done in a pressure vessel which we have no information on.  We are wanting to scale this to a 1L reaction.  The reaction is performed at 250 degC. 

[Borek]

How/when do you mix the reactants? HCl/NH₃ react very fast, you will be mostly limited by the mixing speed.

Is there a headspace over the mixture?

[Enthalpy]

30% is about the solubility limit of NH₃. Depending on when the acid, the salts and the water are added, the salts might expel the NH₃ from the solution, and that would make a BIG volume and possibly pressure. The acid consumes about half the ammonia.

To my feeling, this question should be addressed, and possibly the headspace or automatic safety checks should prevent the burst. But I can't provide any figures nor meaningful opinion about whether the salts expel ammonia from water.

[MNIO]

Sorry I left out a couple of important points:

There are a range of ionic salts in the reaction mixture as well.  None of which will contribute to the pressure. This is the current scale of the reaction which is done in a pressure vessel which we have no information on.  We are wanting to scale this to a 1L reaction.  The reaction is performed at 250 degC. 

*******
you just wrote
  (1) your 7.6 mL of reactants contains variety of ionic salts
  (2) you have no information about the reactor other than its a pressure vessel
  (3) you want to scale the reactor up to handle 1 L of your reactants
  (4) the reaction temp is 250°C

in addition to your previous statements of
  (5) reactants are 0.6mL 1M HCl + 3mL H2O + 4 mL  of 30% HCl
  (6) you have no chemistry background
  (7) is pressure and volume related.

********
my general comments are
  (1) Your reaction doesn't make sense.  What exactly are you trying to make?
  (2) if you don't have a background in chemistry or chemical engineering,
       you should not be attempting this.
  (3) to properly scale up a reactor involves identifying reactor and reaction
       parameters and defining dimensionless scaleable variables.  That can't
       be done without more information than you've given here. 
     
My suggestion to you is this.  If this is for an industrial application, hire an outside consultant.