Mixing Ammonia Cracked Gas with Nitrogen

Use of venturis might not be the ideal sulution. For your application I would suggest to use a gas mixer system, either consisting of suitable mass flow controllers or a system working with a gas mixing valve. Please refer to: http://www.wittgas.com/EN/witt_applications.html

Regards nudnik

Thanks Nuddiks.

Really we have actually a mixing station working well without any gas as a help, the unique difference is that H₂ & N₂ come from two separate tanks from a supplier. That station mixes both gases @70 psi, 30°C helped with a venturi and storing the mixing in a little vessel. After that, we reduce the pressure of the mixing at 1750 mm WAC and then, to the furnace. That is all.

The new project is to use a cracking equipment for generating by ourselves the H₂ supply, with the conditions shown above. The big difference is that we must mix at low pressure and we do not know if this works well with a venturi, or simply permiting that H₂/N₂ from cracking in a line + N₂ in another line, enter freely to the mixing tank simply equaling pressure of each (and probably putting a check valve en every flow).

That is the big issue.

Hi georgeh,

So if I understand you correctly: You will have a large tank at low pressure and want to feed the two gases into that tank at more or less the same low pressure?

Hm, the question is if you want a continuous feed of both gases supplied simultaneously or if you could allow the system to work alternatively with two tanks, one filling up while the mixed gas is drawn from the second tank and vice versa?

Option one, continuos simultaneous feed and withdrawel:
You could do this with so called "pressure equilibrating" pressure controller and proerly sized orifices or control valves (depending on whether you want only one pre-set mixture composion or if you might add varying quantities of N2). The tricky issue with this solution is that the pressure in both feed lines must be identical! thats done with the "pressure equilibrating" controller. There are mechanical solutions available. If you need an idea where to get them, please let me know. By the way, I guess you might also need a tank to ensure proper operating conditions.

Option two, batch process:
n this case you determind the required fractions of the gases to be mixed. Then you fill the tank with gas A up to a pressure that corresponds to the required fraction. shut off supply of gas A and fill in gas B until the end pressure has been reached. Then shut off supply of gas B, and draw your mixture from the tank until the minimum permissible pressure in the tank has been reached. Calculation of the required volume (or pressure) fractions could be done using the ideal gas law (low pressures).

Check valves should be installed in the feed lines for any solution you might select. Furthermore, I would strongly suggest to check proper mixture composition with a suitable gas analyser, taking sample gas from the mixed gas outlet line.

Hope this is of service to you, regards
nudnik

What does the - part of of "±150 m³/hr" mean?

A tank (i.e. a pressure vessel?) is not needed and neither is a venturi. The two services might simply be piped together at the metered flowrates. Provided the flow in the pipe is turbulent, i.e. the Reynolds Number is above 2300 (which it usually is in gas pipework), sufficient mixing will take place in the pipework on the way to the point-of-use.

I guess the problem is that the mixture needs to have a specific accuracy and the flows and pressures may vary during the day (that's at least my experience with similar applications). If that is the situation you will need a mor accurate control over the gas flows than you can achieve with simple flow restrictor.

Another way to get the desired gas mixture would, of course, consist in designing a more sophisitcated control concept including the use of online gas analysers and a master-slave control scheme for controlling gas flows and pressures. this is no witch-work, but far more expensive!

Regards
nudnik

Thanks Nudnik & PWSlack for your helpings.

Everyone has a bit about we might do.

First, our process is a continuos simultaneous feed and withdrawel.

For the actual one we have a downstream gas analyser only for the mixing proportion information.

Also our actual process has a panel station regulating every flow with a regulator valve & a flowmeter type rotameter, and two rapid close valves before and after.

Regulator might reduce pressure from 2000 to 1750 mm WC (it acts as a constant flow restrictor) & the rotameter indicates any flow variation, helped with the analyser reading. Putting in every line a similar panel station before a small acumulator might work. Turbulent flow? I will check, but I am pretty sure that it is.

Let´s work, and thankfullness for every opinion and helping.