1. HYDROGEN RECOVERY
Being from a background that is related to production of ammonia from
hydrocarbons, I am trying to be as elaborate as possible to explain the recovery of
hydrogen gas from the purge gas that has been trickled out to maintain the inert
(unwanted for ammonia production) levels in the ammonia synthesis loop. The
separation of mixture of gases is possible.
Here hydrogen recovery has to be done keeping in mind the continuous
supply of feed and not in batches. The mentioned purge gas has following
contents- hydrogen, argon, methane, nitrogen, water and ammonia. Keeping in
mind the different gases involved in the purge gas, the reasonable way of
separation of gases is combination of pressure swing adsorption process and
Joule-Thomson process, in tandem.
The pressure swing adsorption (PSA unit) is a process that separates a
specific gas from a mixture of gases under pressure based on its peculiar
molecular features that has affinity towards an adsorbent. The adsorbents used
here, do get saturated over a period of time and have to be regenerated. It is
always a good option to have couple of adsorbent beds that work in parallel. In
this case one bed will engage in adsorption of particular gases while the other will
be in regeneration process. I will let you know about the gas which is being used
for the regeneration process
Ammonia and water are the gases that are separated over here. It is a
much better to pre-treat the mixture of gas before feeding it to the PSA unit.
Bring the temperature of the feed gas to a point that helps in condensation of
particular gas in liquid form so that a knockout vessel will do most of the
separation. Keep in mind; we are not that anxious enough to go for the solid state
by recommending condensation.
Now that the gas is free from ammonia and water, let us move on to the
next step of throttling process or Joule- Thomson effect or cryogenic process. Here
the process has to be adiabatic hence a very better quality of insulation has to be

2. looked into to achieve the desired result. The throttling process helps us achieve
cryogenic temperatures that allows the liquefaction of gases and hence
separation. The heat exchange equipment is a much better option to pre- cool the
incoming gas with outgoing gas being heated up. The throttling is done inside a
vessel that allows us to achieve the cryogenic temperatures which is a result of
huge pressure drop across the throttling device.
Following are the boiling points of gases involved-
• Methane- “-161.5 deg C”
• Argon- “-185.9 deg C”
• Nitrogen- “-195.8 deg C”
• Hydrogen- “-252.9 deg C”
The minimum temperature that can be achieved to get the desired quality
of product gas depends on the pressure drop at throttling point and the insulation
which has been opted for. The separation process involves throttling the gas that
allows the condensation of mixture of unwanted gases and allowing us to keep
the desired hydrogen product in gaseous state.
The condensed mixture of unwanted gases can be used for the purpose of
regeneration for the PSA unit. The regeneration would be effective if the gas
flowing through the adsorbent beds is heated up to improve the removal rate of
ammonia and water. The recovered hydrogen gas is mixed with raw material for
recycling. The efficiency of the unit has to be monitored on regular basis by
tracking the gas analysis on regular basis.
Hope this was informative for the professionals and helpful enough for
educational purpose.