Conversion of Single
Stage GV System to 2-Stage GV System for CO2 Removal In Ammonia Plant-IIFFCO AONLA
Ammonia is manufactured by steam reforming of natural gas. During
the process, CO2 is formed in the gaseous mixture and the same is
removed from the gaseous mixture in the
CO2 Removal Section designed by M/s. Giammarco Vetrocoke (GV) of Italy.
Original system
In the original system, the process gas containing CO2
enters at the bottom of GV absorber. GV solution enters the Absorber at the top
and absorbs CO2 from the process gas while flowing down though the packed beds.
CO2 free gas comes out from the top of Absorber and sent down to Methanator for
further processing. Rich GV solution containing CO2 is sent to Regenerator
operating at around 1.0 kg/cm2g. Here dissolved CO2 is separated from the
solution by flashing. CO2 is further removed from the cascading solution, by
stripping action of rising steam/vapors. The lean GV solution collects at the
bottom of Regenerator. From the Regenerator bottom lean GV solution goes to a
Flash Drum operating at 0.6 kg/cm2g. Here again, any dissolved CO2 in the
solution is separated. The separated CO2 is fed to Regenerator from the Flash
Drum with the help of LP steam ejectors. The lean GV solution is pumped from
Flash Drum to GV Absorber. The separated CO2 comes out from Regenerator top and
is cooled in coolers. After cooling and separation of condensate, CO2 is sent
to Urea Plant. The schematic diagram of
original system is as given below:
Modified System
The main features of the modified 2-stage GV process are (1)
Absorption by lean & semi lean solutions (2) High pressure &
low-pressure stripping. The features result in better absorption of CO2 in
Absorber and lower energy consumption for regeneration of the solution in
Regenerators.
In the modified system, the process gas containing CO2
enters at the bottom of GV Absorber. Lean GV solution enters the Absorber at
the top and absorbs CO2 from the process gas while flowing down though packed
beds. CO2 free gas comes out from the top of Absorber and is sent down to
Methanator for further processing. Semi lean solution enters at the middle of
Absorber above the second bed and absorbs CO2 from the process gas. When the GV
solution reaches the bottom of Absorber, it becomes rich in absorbed CO2.
from HP Regenerator semi lean draw off pan (located below
two packed beds) to LP Regenerator semi lean solution sump. Balance GV solution
in HP Regenerator is reboiled in GV Reboilers. Vapors from Reboilers again
enter the HP Regenerator bottom for stripping the GV solution. Live LP steam is
also injected into HP Regenerator for stripping the GV solution. Solution from
Reboilers is sent to HP Regenerator bottom. Lean solution from HP Regenerator
bottom is sent to LP Regenerator bottom.
Semi lean solution from LP Regenerator draw off pan, is
pumped to GV Absorber and enters above the top of second bed for absorption of
CO2. Lean solution from the bottom of LP regenerator is cooled in a heat exchanger
with DM Water and pumped to GV Absorber top.
CO2 from LP regenerator is boosted upto HP regenerator
pressure with a CO2 Blower and sent to Urea plant alongwith CO2 from HP
regenerator. Schematic diagram of the modofied system is given below:
Benefits of the
modification
Major benefits of the modification are:
1. Reduction
of CO2 slip through Absorber by around 600 ppm, which has resulted in:
·
Higher availability of CO2 for urea
production
·
Decrease in hydrogen consumption in
Methanation Section
2. Decrease
in LP steam consumption in CO2 Removal System from 38 MT/hr to 15 MT/hr.
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