Condensation on LTS Catalyst

[rishi079]

Yesterday,night While working first time on the ammonia plant startup.I was confronted with a question.LTS guard bed was cold and we were warming up with hot natural gas stream from front end of the plant.We realised it was taking painstakingly long.We were into it 2 days already.Bed profile looked like this 300 deg F at the top 250 deg F at the middle and 170 deg F at the bottom.I was asked if we could use a slip stream of main process gas to warm up the LTS catalyst.Process gas here being HTS effluent.I would like to know how can I calculate at what temperature the steam will start to condense at different pressures and different steam to gas ratios.The back pressure on the bed was 100 psig .Normally the bed operates at 400 psig and steam to gas ratio is 4.

 

[RPilch]

You probably have gotten the plant up and running by now but I'll offer my advice anyway.

This is a pretty basic ChE calc. Calculate the partial pressure of steam at each pressure and steam to gas ratio. Then, look up the equilibrium temperature of water at each partial pressure. That'll be the dew point (within a few degrees). You can make a table of dewpoints for the operators with pressure on one axis and steam to gas ratio on the other.

Important: Ask your catalyst supplier for how far above the dewpoint you should stay. Add this margin to your table of dewpoints so the operators can't make a mistake. A wet LTS bed is a mess and the catalyst pellets will lose strength and activity.

Also, be careful to use desulfurized natural gas so you don't prematurely deactivte the catalyst.

I also think you should confirm with your catalyst supplier if using HTS gas as a heating medium is acceptable.

Good luck. Let me know how you made out.