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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.

[pawan]

Dear Rishi
ITs a common situation in Ammonia plants & during startups U shud / can use HTS exit gas directly.
Nothing very special in simple calculation ......

Calculate conversion of CO based on HTS exit composition & LT-guard volume.
Calculate possible temp rise.
Calculate LTS Guard exit Composition
Find the dew point & U r done.

[Pronab]

Operation principle of HTS / LTS reactor are superheated temperature at all the time and steam should not be allowed to condense.
During start-up / shut down it is difficult to maintain but extreme precautions should be taken to avoid condensation.
 To calculate condensation temperature at different pressure need a mass balance upstream of LTS reactor.
 Calculate LTS inlet steam mol%.
 Calculate partial pressure of steam using:
Partial pressure = Total pressure steam mol%
Find out saturation temperature of steam at this partial pressure from steam table. Using thumb rule is a great option as:
Saturation temperature (Deg C) = 100 (Pressure in Bar) ¼
Typical LTS reactor inlet steam to dry gas ratio is 0.45 ~ 0.50 mol/mol with a 3.5 steam to carbon ratio in primary reformer. I wonder to see a 4.0 ratio of steam to dry gas inlet of LTS. Is it normal or during start –up condition? Reactor bed temperature which you have (300 / 2500 / 170 Deg F) at 100 psig (6.89 barg) is already at subcooled condition. Temperature should be more than 162 Deg C at anywhere in the bed. LTS Reactor can be heated by using HTS effluent gas with a precaution to avoid excess temperature rise.
A typical ammonia plant HTS / LTS Reactor operating data:
(With a steam to carbon ratio of 3.5)
Reactor Inlet Temperature, Deg C Inlet pressure, Barg Steam to dry gas ratio, mol/mol Outlet Temperature, Deg C
HTS 370 35.5 0.59 482
LTS 209 35.2 0.45 233


Regards.
Pronab.