snoman701 said:
Lou said:
, or a superatmospheric pressure reactor, which happens to work quite well.
What are the advantages of pressurized reaction?
Can you slow the reaction by pressurizing it? Well controled with controlled titration of acid?
What kind of pressures?
I've likely got enough stainless pressure vessels that I could manage at least a two gallon 100 PSI reactor without issue.
Is the nitrogen dioxide reabsorbed by the water, or does it have to be vented off? (or, without looking up the phase diagram, it may never actively boil off, but immediately hydrolise a water molecule)
Once vented off, wouldn't you still need to scrub it, or is it then just much easier to vent off at a controlled rate?
My previous polymer experience had me doing a reasonable amount of pressurized work, some at supercritical pressures...so it's a set of tools (& and equipment) that I've already got a stubby handle on.
I wouldn't mess with this unless you have more than a stubby handle on working with pressurized systems and a very darn good idea of what ALL the reactions are. More than one accident has occurred with this technology, including one where a whole refinery had to be evacuated after a rupture disk blew and superheated NOx filled the facility. I've done a bit with SCF myself (CO2, H2O, C4H10, etc).
Many years back when I first ran the process, I was using a 75% Ag balance copper granulate charge and putting in a substoichiometric amount of nitrate to do the job (under ambient conditions). I was running it in a 316L (304L is better fyi) 4 liter Parr autoclave. My first experiment, if I recall correctly, I got up to some 100 psi or so pNO2/NO. As it was still climbing and my regulator only delivered to 200 psi g, I opened and quickly closed the valve to admit O2 into the chamber and noticed an immediate settling down of the pressure and a 20+Centigrade exotherm!
I have 40 some page paper I did on this process. It's in employ at several very large silver refineries where 100K ounce digestions are conducted. I used it for making silver nitrate for silver cells on much smaller scale.
The reaction runs under the conditions of extra oxygen and extra water. Water and oxygen oxidize formed NO2, much like in the latter part of the Ostwald process, to produce nitric acid. This happens at 50+ Centigrade and 50+ PSI O2 overpressure, which must be maintained at all times to prevent a loss of containment accident of the NO2.
4 NO2 (g) + O2 (g) + 2 H2O (l) → 4 HNO3 (aq) (Equation 1)
This then in turn reacts with more silver to generate more NOx, and at the end, heat is pumped into the system through the reactor jacket, whereas at first, cooling is required (both jacket and immersion coil). The end results is that almost 100% theoretical usage of nitrate can be had.
Now, there's one complicating fact that gets people into trouble. This reactions in equations 2 and 3.
3 Ag (s) + 4 H+(aq) + 4 NO3-(aq) → 3 Ag+(aq) + 3 NO3- (aq) + 2 H2O (l) + NO (g) (Equation 2)
which occurs when the nitric concentration in the system falls below about ~15.2 % (the order of the reaction changes, and NO is the product,
not NO2). Now that NO will happily react to produce NO2 with excess oxygen, but it takes a while. Ask anyone who knows anything about NOx scrubbing. NO2 is the easy one to control. NO is the pain in the side and needs residence time. This can lead to overpressurization.
2 NO (g) + O2 (g) → 2 NO2 (g) (Equation 3)
That NO will also make nitric acid (per Eq 3), but the rate of Eq 3 is less than the rate of Eq 1. Of course there's also other side reactions (like HNO2, nitrous acid production).
Ag (s)+ 2 H+(aq) + 2 NO3-(aq → Ag+(aq) + NO3- (aq) + H2O (l) + NO2 (g) (Equation 4)
Equation 4 only happens so long as the nitric is in excess, relative to its rate of consumption. This is what you want to have happen. The whole thing became a nasty set of related equations, with some very nasty partials that related to the shrinking sphere problem. Anyway, we had to balance out the increase in surface area of the spherical shot with the decrease in acid concentration, and acid cycling back into the system. All that to hold some sort of steady state.
The only easy way to address the control issue is to add the silver to the nitric acid, under pressure, as spherical granules. That I would feel completely cool with doing. But no one wants to do that. That's expensive. That's complicated. The media serves as a reflux center for nitric acid every time the feed hopper is opened, so wash downs are needed, which means a pressure pump.