Sciencemadness Discussion Board

Haber process via modified pressure washers

Fusionfire - 1-9-2012 at 09:16

Hi folks,

Some diesel/petrol powered high pressure washers can reach pressures of 250 bar. The Haber process is conducted at pressures from 150 - 250 bar.

Hot water pressure washers exist which can eject water at 140°C. With external heating (e.g. electric heating coils), it should be possible to heat a N2/H2 stream to 300 - 550°C.

So the basic idea is to modify an existing pressure washer to take in N2/H2 at the intakes rather than water. It is pressurised and as it runs over the pressure hose to the nozzle, you wrap some electric heating elements over it to raise the temperature. You then pass that through a catalyst bed, and recycle the output back into the intakes (so it is effectively a closed loop system).

Any plastic components within it would have to be replaced with ceramic/metal ones for high temperature resistance.

What do you all think? Will it work?

plante1999 - 1-9-2012 at 09:27

I find Cyanamide process better suited for ammmonia production in an amateur production, but If you know a machinist you might make a great improvement in home chemistry.

Original cyanamide process used nitrogen absorption in calcium carbide but This is quite hard to do (similar to the haber process in therm of the apparatus requirement difficulty), urea can be reacted with calcium oxide at red heat to make calcium cyanamide. Then the cyanamide can be turned into ammonia by hydrolysis.

Rogeryermaw - 1-9-2012 at 10:11

it's a good theory but it would depend on the type of pump used by the washer. many systems designed to pump water use impellers or pumps that can only produce pressure when acting on a fluid and lose pressure under cavitation. impellers, gear pumps, turbine pumps and many other types aren't capable of producing appreciable pressure in gaseous media. a modified air compressor may be more suited to the task.

kristofvagyok - 1-9-2012 at 10:42

Quote: Originally posted by Fusionfire  
Some diesel/petrol powered high pressure washers can reach pressures of 250 bar. The Haber process is conducted at pressures from 150 - 250 bar.

Hot water pressure washers exist which can eject water at 140°C. With external heating (e.g. electric heating coils), it should be possible to heat a N2/H2 stream to 300 - 550°C.

What do you all think? Will it work?


This stuff will cause a really large explosion.

If you want to use water pressure washer as a pressure pump for the reaction, then I thing that it will cause a really big explosion, especially when the hydrogen gas will get heated up to the reaction temperature. Why?

First: at these pressures and temperature perfectly sealed tubing is needed (we have an autoclave what is good till 60 atm and it is made from perfectly casted steel with 0mm gaps and perfect sealing) and I thing that it is impossible to make it at home.

Second: if the system would be heated on one side, it would be passed through a catalyst bed made from platinium, palladium ect, then the ammonia should be condensed down and the not reacted hydrogen/nitrogen should be passed back to the beginning of this circle. Question: if on one side there is 150 atm pressure and a few hundred celsius, then how will the ammonia condense down? Simply cool it down? Then the reheating will cost a lot, but elseway the nearly supercritical ammonia (did anyone checked a phase diagram for that?) won't separate.

Third: will a hi pressure water washer pump survive if it will used to work with gases? I think that it will get overheated in a minute.

Fourth: Pure hydrogen and nitrogen is required for this method what is air free (elseway it will explode), are these gases cheaper than buying a cylinder of ammonia from Linde, Messer, ect? We have got a big cylinder of ammonia in the lab for 10 years now and it is still not empty. The renting of the cylinder (monthly 5 euro) have costed much more than the gas in it, so is it worth to make an insane project like this?

unionised - 1-9-2012 at 11:58

In the fairly unlikely event that I need to produce (rather than simply buy) ammonia, I will piss in a bucket and leave it to ferment for a while.

On a more serious note, the pressure washer may well be built to take 250bar at 140C, but that doesn't mean it will still take 250 bar at 550C

Most things get a lot weaker when they are hot.

Siggebo - 1-9-2012 at 13:13

As already mentioned, a pressure washer (and most other water pumps) can not successfully compress gaseous media.

Compare the size of your average pressure washer - they deliver maybe 100-150 bar. A small workshop compressor is about the same size, and they give what? 8-10 bar?

Fusionfire - 1-9-2012 at 14:51

Hi again,

In response to the questions:

1) Why bother making ammonia at home?
For the fun of amateur science, retracing what it was like being a chemist before large factories in a distant land made our chemicals for us :)

2) Explosion hazard.
The N2 + H2 reaction is endothermic, so there is no risk of thermal runaway. The main explosion hazard comes from the high pressure, and that is easier to engineer against.

3) Not enough pressure.
250 bar pressure washers exist. They tend to be top of the range ones, at least USD$2-3k.

3) Pressure washer pump not working against gases.
3 possible solutions:
i) Mix N2/H2 with a carrier liquid - e.g. water or CO2 (both are supercritical fluids under these conditions). Note partial pressure of N2/H2 will drop accordingly, so you'd have to use a really powerful pressure washer (250 bar) to compensate.

ii) Hydraulic coupling. Let the pressure washer continue to pump cold water. Have two tanks, both initially filled with a N2/H2 mix. Pump water into one at high pressure to displace the volume and pressurise a second N2/H2 tank. Basically if you have an outlet of water at 250 bar you can pressurise a gaseous mix using tanks, on a batch basis, to 250 bar without having to resort to any mechanical advantage.

iii) Mechanical coupling. Again let the pressure washer continue to pump cold water. Use that to provide torque to drive a separate high temp/high pressure gas compressor in a N2/H2 closed loop system. Under the circumstances though you might as well power a gas compressor off mains AC or a small engine :)

No doubt some backwards modifications will be needed to make the system capable of withstanding the high temps + pressures in the Haber process.

Can someone check the pressure exiting a pressure washer if the feed tank runs dry and it is pumping air?

Rogeryermaw - 1-9-2012 at 19:55

with some heated, heavy wall steel tubing at the exit, i would think that the best route would be one of the nitrogen compressors used at paintball fields and feed nitrogen and hydrogen to the inlet. they operate in excess of 3,000-3,500 psi, some even as high as 5,000 psi. they are by no means cheap to buy, but they are robust and would be more likely to survive such harsh conditions.
<a href="http://www.paintballcompressor.com/bauer.html">here are some examples</a>

watson.fawkes - 2-9-2012 at 07:00

Quote: Originally posted by Rogeryermaw  
they are by no means cheap to buy, but they are robust and would be more likely to survive such harsh conditions.
It's a better suggestion, but one that could be dangerous. Any time you've got significant H2 at high temperatures, there's an issue with hydrogen embrittlement. So the not-so-obvious risk is explosive failure of the compressor block. The most common solution is to use high-nickel alloys. I've looked at this a couple of times, and each time come away with the feeling that there's no reasonable way to miniaturize the Haber process short of a home machine shop and some expensive materials. Well, that's only mostly true. I suspect a MEMS-based microfluidic reactor could be made to work (and then run in parallel), but that's entirely unrealistic as an amateur prospect.
Quote: Originally posted by Fusionfire  
Mix N2/H2 with a carrier liquid - e.g. water or CO2 (both are supercritical fluids under these conditions). Note partial pressure of N2/H2 will drop accordingly, so you'd have to use a really powerful pressure washer (250 bar) to compensate.
The problem with a carrier liquid is finding an inert one. Both water and CO2 will react at those conditions. I wouldn't want to predict exactly what you'd get, but it won't be much ammonia.
Quote: Originally posted by Fusionfire  
Hydraulic coupling. Let the pressure washer continue to pump cold water. Have two tanks, both initially filled with a N2/H2 mix. Pump water into one at high pressure to displace the volume and pressurise a second N2/H2 tank.
This is either the same problem as the previous one, namely reactivity, or an instance the general problem, which is making an adequate seal at high pressure and temperature to keep the two fluids separated.
Quote: Originally posted by Fusionfire  
Again let the pressure washer continue to pump cold water. Use that to provide torque to drive a separate high temp/high pressure gas compressor in a N2/H2 closed loop system. Under the circumstances though you might as well power a gas compressor off mains AC or a small engine
And this is essentially the original Haber process as first prototyped by Haber, which, if you read the history deeply enough, was enabled by a machinist who figured out how to fabricate the appropriate seal.

EDIT: Grammar edit.

[Edited on 2-9-2012 by watson.fawkes]

bquirky - 2-9-2012 at 08:07

I read a interesting history book called "The Alchemy of Air" that go's into detail about the history of the Bosch Haber process and how each of these chalanges where overcome particularly h2 embrittlement.

apparently the first actual demonstration was in a machined block of solid quartz !

unionised - 2-9-2012 at 08:14

" Mix N2/H2 with a carrier liquid - e.g. water or CO2 (both are supercritical fluids under these conditions)."

Nitrogen and hydrogen are also supercritical fluids under those conditions.

Supercritical fluids are not a magical solution to all problems. The pumps will probably struggle with the mixture and,as has been pointed out, CO2 will react with ammonia (and possibly hydrogen).

Fusionfire - 2-9-2012 at 19:00

Touche H2 embrittlement and NH3 + CO2 <-> H2N-COONH4, I forgot about those.

So clearly if the Haber process is to be done at home you'd have to have tight control over the materials of the reactor vessel and no SC CO2 must be involved.

What about using the pressure washer as a source for high pressure water and the double-container approach I suggested?

1) Buy alumina ceramic, aluminium or stainless steel 316 containers and tubing. These materials are resistant to NH3 and hydrogen gas. Some contracted work to a workshop will probably be necessary to make the pressure vessels. E.g. buy thick tubes to the aforementioned materials and make end cap seals for them to turn them into pressure vessels.

2) Connect the two containers together with a shut-off valve between them.

3) Fill both containers with N2/H2. Have the catalyst bed in the dry vessel.

4) Fill one container with water at 250 bar, using the pressure washer, with the valve between the containers open.

5) Let the water displace the volume of N2/H2 and compress it to 150 - 250 bar. No pistons or hydraulic fluid is needed if you are not seeking to exceed the pressure output of the compressor in the reaction vessels.

6) Heat up the vessel containing the N2/H2 mix. Obviously, no heating of the water containing vessel is necessary.

Unfortunately this is a batch process that is subject to further optimisation, e.g. getting the ammonia to flow back into the water filled tank to dissolve in the water, so that you can drive the equilibrium forward.

We can also use a N2 compressor as suggested by rogeryermaw but we can probably only use this to compress N2 and not H2, due to the risk of embrittlement. And it is more expensive.

bbartlog - 2-9-2012 at 19:18

Quote: Originally posted by plante1999  
...urea can be reacted with calcium oxide at red heat to make calcium cyanamide. Then the cyanamide can be turned into ammonia by hydrolysis.


Uh, you can skip the red heat and the cyanamide here. Urea plus base plus water will give you ammonia as the urea is hydrolyzed; in this case the net reaction is
CaO + H2O + CO(NH2)2 -> CaCO3 + 2NH3

Rogeryermaw - 2-9-2012 at 21:52

in the vessels apart from the compressor, it should not be too difficult to reduce or eliminate the threat of hydrogen embrittlement through the use of coatings or linings of metals that are insusceptible. aluminum and copper and certain alloys of stainless steel should do the trick. perhaps electroplating the inner diameter of a steel or stainless steel pipe with copper would suffice. that leaves the internal parts of the compressor to deal with. unfortunately, these compressors are made with tight tolerances and would likely not take well to plating or coatings. that leaves either constructing and machining the necessary parts or checking that the internals are made of metals suitable for the task. either way you go, it would be very cost prohibitive.

personally, i will stick to ammonium salts and base. dry the gas and condense through liebig cooled with dry ice/acetone mixture.

vmelkon - 3-9-2012 at 14:46

If demonstrating the principle of NH3 production is what you are after, then do it at lower temperature and pressure.

Also, I think the apparatus needs to have a hot side and a cold side. The cold side is where the ammonia would liquify and you drain away the liquid to an outside tank.

watson.fawkes - 3-9-2012 at 17:25

Quote: Originally posted by Fusionfire  
5) Let the water displace the volume of N2/H2 and compress it to 150 - 250 bar. No pistons or hydraulic fluid is needed if you are not seeking to exceed the pressure output of the compressor in the reaction vessels.
You've got a boundary between water and N2/H2 here. You're assuming, implicitly, that there will be no mixing between these. At very least, you're going to get water evaporation and have something of a ternary reaction mixture. Who knows? Maybe you nitrite or nitrate out this mixture. I don't know.

In addition, the compression ratio is around 1:150-250, which has a couple of problems. The first is that the pressurizing container has to be 150-250 times the volume of the reaction vessel, and this is true about any "single piston stroke" reactor; true you have a water piston, but the volume ratio still applies. The other problem is amount of heating that this compression ratio induces. You'll have a difficult enough time rejecting all the heat, and an even harder time not turning the water into steam before the heat escapes.

Fusionfire - 3-9-2012 at 19:51

Quote: Originally posted by watson.fawkes  
Quote: Originally posted by Fusionfire  
5) Let the water displace the volume of N2/H2 and compress it to 150 - 250 bar. No pistons or hydraulic fluid is needed if you are not seeking to exceed the pressure output of the compressor in the reaction vessels.
You've got a boundary between water and N2/H2 here. You're assuming, implicitly, that there will be no mixing between these. At very least, you're going to get water evaporation and have something of a ternary reaction mixture. Who knows? Maybe you nitrite or nitrate out this mixture. I don't know.

In addition, the compression ratio is around 1:150-250, which has a couple of problems. The first is that the pressurizing container has to be 150-250 times the volume of the reaction vessel, and this is true about any "single piston stroke" reactor; true you have a water piston, but the volume ratio still applies. The other problem is amount of heating that this compression ratio induces. You'll have a difficult enough time rejecting all the heat, and an even harder time not turning the water into steam before the heat escapes.


The pressurising container doesn't need to be 150 - 250 times the volume of the reaction vessel, because you can also use heat from the work done to compress it to raise the pressure in the reaction vessel.

Assuming the pressurisation process is adiabatic:

P<sub>1</sub>V<sub>1</sub><sup>γ</sup> = P<sub>2</sub>V<sub>2</sub><sup>γ</sup>

Using:
P<sub>1</sub> = 1 bar
V<sub>1</sub> = 51.61956 volume units
V<sub>2</sub> = 1 volume unit
γ for a diatomic gas as 1.4

Therefore P2 = 250 bar

The temperature from compressing 51.61956 volume units (say litres) to 1 litre adiabatically increases from 300K to 1179.938K.

Therefore we will have to vent some heat to bring the reaction temperature down to 773K, or we could rely on the endothermic nature of the reaction to absorb the heat for us.

There are other ways of achieving the desired reaction pressure/temperature. The method I am suggesting is adiabatic compression followed by heat dissipation. Another method is isochoric heating followed by compression, or anything in between.

watson.fawkes - 4-9-2012 at 05:08

Quote: Originally posted by Fusionfire  
The temperature from compressing 51.61956 volume units (say litres) to 1 litre adiabatically increases from 300K to 1179.938K.
OK, I was a little over-enthusiastic about a 200:1 ratio. But you've calculated a 50:1 ratio, and it will have all the same problems I mentioned if you want to exclude H2O from the reaction gas mixture. Assuming that vapor contamination at ambient temperature is acceptable (I'm still not sure of that), you'll need isothermal compression just to keep the vapor pressure down. That's slow, and you'd need to find a way to operate the compressor at less than full duty cycle. Isothermal compression yields a final compression ratio of about 100:1, relying on heat for the rest of the pressure.

zed - 8-9-2012 at 15:41

Ummm. Were I interested in producing ammonia, I might resort a pressurized system, wherein I force urine out of my pecker, and then hydrolyse it.

Conveniently, I already have all of the required equipment.

I do like the idea of using a pressure washer to produce industrial-type conditions. Likewise, an airless paint sprayer might be serviceable. Several not too expensive models, are built with stainless steel guts....A prerequisite for dealing with the corrosive mixtures, we long to pressurize.

Arsole - 8-9-2012 at 16:17

"Ummm. Were I interested in producing ammonia, I might resort a pressurized system, wherein I force urine out of my pecker, and then hydrolyse it."

One of the funniest comments I have read on this site.

macckone - 23-9-2020 at 15:54

Clearly the haber bosch process is a challenge for the home chemist.
The temperature range is high and the presure is also high.
But the required contact time is reasonably low (30 seconds).
With a 2.5 cm diameter tube with a length of 50 cm, the flow can be fairly slow (less than 2cm/sec).

The configuration in a commercial plant is an inner heated tube with a heating coil and insulation inside a pressurized out tube.
The gas has to be cooled below 123C for the ammonia to condense.
The outer shell is usually pressurized with straight nitrogen and the inner heated tube is at zero or slightly negative pressure relative to the outer tube.
In a commercial plant the vessels are huge and the flow rates are optimized.

Fyndium - 24-9-2020 at 09:15

Just for the record, urea + sodium hydroxide is proven to be an excellent and cheap method to produce ammonia in large quantities.

Now, let's return to Haber.