CO2 Removal - Process Technology for Ammonia Plants

1. CO2 Removal - Process Technology
for Ammonia Plants
By:
Gerard B. Hawkins
Managing Director, CEO

2. Why Remove CO2 ?
 Poisons Synthesis catalyst
 Can be used downstream in Urea production
 If allowed into the loop would
 Also act as inert in loop
 Increase the compression costs
 Used as a product in its own right
 Carbonated drinks
 Refrigeration units
 Can be exported to methanol plant

3. Systems for Removing CO2
 Started with Monoethanolamine
 Chemical absorption
 Highly corrosive
 High energy consumption (14% of total
plant)
 Modern plants use MDEA
 Other options include physical solvents
 Selexol (UOP)
 Benefield (hot potassium carbonate)
 Vetrocoke

4. MDEA
 Uses diethanolamine with an amine
activator
 No corrosion inhibitors required
 No solvent degradation is observed
 Acts as a combination of physical and
chemical solvent
 CO2 can be recovered by flashing only

5. Typical System for MDEA
Treated
Gas
Feed
Gas
CO2

6. Advantages of MDEA
 Some stated above
 Can use MDEA to retrofit plants using MEA
 Can tailor balance between physical and
chemical
 Allows for versatility especially in retrofit
cases

7. Benfield System
Treated
Gas
Feed
Gas
CO2 and H2O
Semi
Lean 120°C
Lean 75°C
Reboilers
Absorber
Regenerator

8. CO2 Removal issues
 Benfield Problems
• Dirty solution
• Foaming due to contaminants or
degradation products
• DEA degradation products
• Difficulty of oxidizing V4+ to V5+
 Results
• Low DEA concentration
• Poor corrosion protection
• Reduced CO2 removal capability

9. Vanadation
 Method of protecting stable magnetite layer
from corrosion
 Must reduce V5+ and deposition on surface
 Steady state is reached when V5+ : V2O5 is
0.4
 If ratio less than 0.3 - rapid break down and
corrosion
 High levels can lead to degradation of DEA
 Most plants have a slow reduction of V5+ to
V4+
 Can counter by addition of KNO2

10. Benefield - Water Balance
 Large volumes of solution being circulated
 Must keep water balance right to ensure
strength of solution is between 27 and 30%
 >30% - crystallisation and vessel
corrosion
 <27% - poor scrubbing

11. Benfield Water Balance
Benfield
System
Process Gas
Inlet Absorber
Pump Gland Seals
Purges to levels
34 te/hr
2 te/hr
21 te/hr
1 te/hr
52 te/hr
6 te/hr
Absorber Inlet Catchpot
PG Exit Absorber
CO2 Catchpot Drain
CO2 Export Main

12. Common Problems - Benfield
 High DP on Absorber
• High level in base - flooding inlet gas line
• Foaming due to contaminants
• Fouled packing causing restriction to flow
• High gas flow causing solution to be held up
• High circulation rates leading to flooding
• Temperature variations

13. Common Problems - Benfield
 Consequences
• Solution help up creating low levels in regen
tower
• Make gas supply to syn gas machine
reduced
• Vibrations/Surging of machine
• Benfield solution carried forward from
absorber and deposited on methanator
catalyst

14. Common Problems - Benfield
 High PD in Regenerator
• High vapour rates leading to hold up of
solution from low level in base of regen
column
• High make gas temperatures or excessive
steam usage in reboilers
• Foaming
• Fouled/Choked packing
• Mechanical failure of trays or distributors

15. Common Problems - Benfield
 Consequences
• Levels fall in base of regen column
• Reduction in suction head available to
pumps - cavitation
• Reduced flows to absorber - high CO2 in exit
gas
• Trips methanator on high temperature
• Carry over of benfield solution into CO2
condensate recovery system
• Excess boiling of solution - changes in
concentration and potential for corrosion

16. Amine Promotion Mechanism
Slow reaction
of CO2 and
hydroxylation
Fast reaction
of CO2 and amine
Hydrolysis of
carbamate to amine
Free amine diffusion
Carbamate diffusion
HCO3
-
Promotion mechanism of organic Amine on CO2 Absorption
into Potassium Carbonate solution
interfaceGas phase Liquid phase
CO2
CO2
HCO3
-

17. Benfield CO2 Removal Improvements
 LRS 10 Benfield Promoter
• British Gas Technology
• New amine mixture
• Completely compatible with DEA
• Replaces DEA in hot potash solutions
• Improves absorption and desorption of
CO2
• Reduces CO2 removal energy

18. Effect of 3% LRS 10 to Benfield
Time (hours)
%CO
Conc
CO2 concentration in absorber overheads

19. Benefits of LRS 10 Promotion
 Retrofit easy, just add 3% LRS solution
 CO2 slip reduces immediately, giving scope
to:-
• Increase plant throughput, or
• reduce steam/carbon ratio, or
• reduce methane to synloop, and
• plant becomes more controllable

20. UOP Lo-Heat Process Schematic