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I was asked for evaluation of doing the leak test and pressurization step in the Hydrocracking unit start up with Nitrogen instead using Hydrogen. We have 1 reciprocant compressor for make up and one centrifugal compressor for recycle gas, I would like to know what do I have to consider to make this evaluation, what I know by now is that my recycle gas molecular weight is 4 and N2 is 28, so my centrifugal compresor could not be able to increase the pressure more than 250 psi (aprox). what should I take in account?. Is there any gain doing this?
 
Answers
10/09/2015 A: keith bowers, B and B Consulting, kebowers47@gmail.com
A disadvantage is that hydrogen will easily leak through a crack that nitrogen will not. Use nitrogen for a 'rough check 'for gross leaks, but hydrogen for 'proof test' before admitting hydrocarbons.
26/04/2015 A: Narendra Naidu, NWR, nbpnnaidu@gmail.com
The pressuring step with gaseous N2 eliminates the risk of having H2 leak and fire after a turnaround or new Unit.
The things to consider are:
What is the MPT of the Reactor and other thick walled Vessels in unit, can they be pressurised to full pressure at your ambient temperature. if not then the N2 has to ciruclated at lower pressure and heated ot bring all theystem above MPT.
Is your RGC designed to run on N2, if not you may have to stop your RGC during N2 pressuiring.
your may need a contractor who can supply truck gaseous N2 and your system pressure to check for leaks.
Do you have a hook up to connect this N2 to your system.
24/04/2015 A: Prabhakar Reddy Seerapu, SATORP, redi_nfcl@yahoo.co.in
We used liquid nitrogen + helium mixture from external supplier and vaporized it for leak test in our Hydrotreater units.With nitrogen header pressure limited in units,we cannot pressurise to such high operating pressure of Hydrocrackers because of limited suction pressure and higher molecular weight.even with nitrogen, compressor discharge temperatures will be another limitation which can be checked in vendor data sheet as they normally specify allowable discharge temperature in Nitrogen case which is higher than normal recycle gas case.
24/04/2015 A: Kumar Kurapaty, KNPC, kumar83che@gmail.com
You can opt for Liquid Nitrogen as a replacement of Hydrogen, There will be no requirement of Compressor if you use Liquid Nitrogen. Inject Liquid Nitrogen as required and heat the same using Process heater until the required Pressure is achieved.