Giammarco-Vetrocoke of Italy solution :problems encountered in CO2 absorption section -NFCL EXPERIENCE

CO2 REMOVAL SECTION in ammonia plant in all over world it  has been more concerned  to save the ENERGY , EQUIPMENT FROM CORROSION ,
 Problems encountered in CO2 absorption section
Higher CO2 slip 
}         Bad regeneration 
}         Foaming 
}         GV carryover 
}         Maintaining vanadium ratio 
}         Pump back rotation 
}         ES-302 carryover 
}         Valve strucking 
}         EE-311 chokng 
}         EE-301 leaking 
 

CO2 REMOVAL SECTION : 
  
            This unit provides process gas free of CO2 (limit 1000 ppm) for the production of ammonia and necessary CO2 for Urea  production.  In this unit, CO2 in the process gas is absorbed by the GV solution in the Absorber, C-301 thus providing process gas with less than 1000 ppm of CO2.  Stripping of the absorbed CO2 is done in the two regenerators and CO2 stripped is supplied to Urea Plant.  CO2 removal section know how is by Giammarco-Vetrocoke of Italy.  The Vetrocoke solution consists of K2 CO3, Vanadium Pentoxide, Glycine  and DEA where V2O5 (Vanadium Pentoxide) is the corrosion inhibitor and glycine/DEA are the activators.  The chemistry involved in this unit is chemisorption and is explained as follows : 
  
  
                    CO2  +  H2O                                       HCO3-  +  H+          (1) 
  
                    K2CO3  +  HCO3-  +  H+                    2KH  CO3               (2) 
                    -------------------------------------------------------------------------------- 
                    K2CO3  +  CO2  +  H2O                     2KH  CO3               (3) 
  
            The reaction rate of (3) depends on the reaction rates of (1) and (2).  Reaction rate of (1) is slow and the activator activates this reaction by quickly introducing the gaseous CO2 in the liquid phase.  The activator glycine reacts with CO2 and forms glycine carbonate according to the reaction. 
  
         NH2 CH2 COO-  +  CO2                              COO-NH CH2 COO-  +  H+     (4) 
  
         COO-NH CH2 COO-  +  H2O                       NH2 CH2 COO-  +  HCO3-      (5) 
  
     The sum of (4) and (5) gives (1). 
  
            In solution regeneration, reaction (3) is reversed by application of heat and pressure reduction and the lean and semilean K2 CO3 solution is recirculated for further absorption of CO2.  The process gas from V-208 enters the CO2 removal section at 27.5 Kg/cm2g and 165 deg.C and passes through the reboilers and LP Boiler E-302 and then to E- 306A/B (DM Water Heater) getting cooled down to 113.5 deg. C and condensate is seprated in V-301 before entering the Absorber. 
  
            The process gas enters the tube side of E-301A/B giving its heat energy to the GV solution at the shell side of E-301A/B.  The solution from the bottom tray of C-302 (Regenerator under pressure) circulates through the reboiler by thermal siphoning.  The CO2 and H2O vapour along with solution enters C-302 bottom below the bottom tray and serves as stripping medium.  The heat energy released in E-302 shell is used to produce LS steam which is boosted into C-302 through the ejectors L-301A/B.  The outlet gas temperature of E-302 is 126.5 deg.C.  The gas outlet from E-302 is further cooled in DM water preheaters E-306A/B.  The gas is cooled down to 113.5 deg. C.  The resulting condensate in the process gas is separated in V-301 before entering the CO2 absorber.  
  
In the CO2 Absorber C-301 process gas flows upwards counter current to the solution flow (the solution is the regenerated  GV solution from C-303).  Semi-lean solution pumps P-302A/B/C takes suction from the take off tray below the packing of C-303 and pumps the solution to the middle of Absorber as semilean solution at 106 deg.C .  Lean solution pumps P-301A/B takes suction from the bottom of C-303 through the cooler E-303 and pumps the solution to the top of the absorber as lean solution.  E-303 cools the solution from 109 deg.C to 65 deg.C and in turn heats DM water from 40 deg.C to 104 deg.C.  The make up condensate to CO2 removal system is added at the suction of P-301A/B pumps at 59 deg.C to maintain the water balance in the system. 
  
            At the bottom of absorbers C-301 where the bulk of CO2 is absorbed, the high temperature improves the reaction rate for reaction No. (3) and for reaction No. (5) according to which the CO2 is absorbed by K2 CO3.  In the top part of the absorber, the lower temperature reduces the CO2 vapour pressure in the solution thereby minimising the CO2 content in the process gas.  This is made possible by keeping the reaction rate (5) sufficiently high even at this lower temperature by the OH concentrations in the lean solution fed at the top.  
  
            Solution regeneration is carried out at two pressure levels, one at 1.04 Kg/cm2g and other at 0.1 Kg/cm2g for better utilization of stripping steam compared to the usual technique in which great part of the stripping steam exits the regenerator top  as unused excess.  The pressure in regenerator C-302 is regulated to obtain a temperature increase between the solution inlet and outlet of the regenerator in order to condense the above mentioned excess steam.  The heat stored in the rich GV solution exit the regenerator C-302, is recovered as flash steam which has been experimentally verified to be practically pure steam.  
  
            From C-302 top is taken off a rich solution stream at 106.5 deg.C that feeds Regenerator at low pressure C-303.  In C-303 the flashed steam regenerates the rich solution stream taken off from C-302 top.  The liquid levels at the bottom of C-303 and at the take off tray are maintained by controlling the flow of lean and semilean solution from C-302.  The lean solution from the bottom of C-303 at 109 deg.C gets cooled in E-303 and is pumped by lean solution pumps P-301A/B at 65 deg.C to the top of C-301.  From the take off tray of C-303 the solution goes to the Semilean pumps P-302A/B/C at 106 deg.C to be pumped to middle of C-301. 
  
The acid gas stream from the top of the Regenerator C-302 is cooled in the DM water preheater E-307 from 102 deg.C to 96 deg.C at 1.04 Kg/cm2g pressure.  C-302 pressure is maintained by PIC-015.  The vapour condensed is removed in V-304 (OH condensate separator).  The acid gas stream outletting the Regenerator C-303 at 94 deg.C and 0.1 Kg/cm2g is cooled in the O/H DM water heater E-308 to 91 deg.C and the vapour condensed is removed in the C-303 1st O/H condensate separator V-305.  C-303 pressure is maintained by PIC-001.  Again the acid gas is cooled in the condensers E-304A/B to 40 deg.C by cooling water.  The vapour condensed is separated in the C-303 2nd OH condensate separator V-302.  The CO2 is fed to the Booster compressor K-301 or it can be vented to atmosphere through PIC-026.  K-301 boosts the pressure of CO2 from 0.1 Kg/cm2g to 0.96 Kg/cm2g at 96 deg.C.  The discharge of Booster compressor joins the stream of CO2 from C-302 at the outlet of V-304 and the mixed stream gets cooled in the final OH condensers E-305A/B from 102 deg.C to 40 deg.C by cooling water.  The water vapour condensed is removed in final OH condensate separator V-303 and the CO2 saturated with water flows to Urea Plant.  The ammonia Plant battery limit conditions for the CO2 sent to Urea Plant are 0.6 Kg/cm2g and 40 deg.C. 
  
            The use of compressor K-301 on a very limited acid gas stream allows to utilise in the most advantageous way, the two pressure levels regeneration technique, since it allows to keep C-303 pressure at a lower level, thereby increasing the flashing steam of the solution coming from C-302 with evident energy saving. At the same time it allows to obtain all CO2 for Urea production at higher pressure. 
  
            The condensate separated out at V-304 and V-303 flows to V-305 and V-302 respectively under pressure where as condensate from V-305 and V-302 are pumped out by P-304 and P-305 condensate pumps respectively as make up to CO2 removal section and balance as process condensate to stripping unit.  There are two numbers lean solution pumps (P-301A/B) one steam turbine driven and the other motor driven.  Out of three semilean solution pumps (P-302A/B/C), two are steam turbine driven and the other motor driven. 
  
Two hydraulic turbines (DPTP-302 A/B) are connected to the turbine driven semilean solution pumps P-302A/B through auto clutch.  The letdown turbines sends the rich solution from Absorber bottom which is at a pressure of 27.5 Kg/cm2g to the Regenerator C-302 which is at a pressure of 1.04 Kg/cm2g.  The discharge side pressure of hydraulic turbine will be about 9 Kg/cm2g.  The differential pressure 18.5 Kg/cm2g is utilised to drive the semilean solution pumps.  This pressure energy approximately amounts to a power of 215 KW in each hydraulic turbine thus energy on steam driven turbines DSTP-302A/B is conserved to an extent of 215 KW on each turbine, by clutching Hydraulic turbine to the Semilean solution pumps. 
 Problems encountered in CO2 absorption section

Higher CO2 slip }         Bad regeneration }         Foaming }         GV carryover }         Maintaining vanadium ratio }         Pump back rotation  }         ES-302 carryover }         Valve strucking }         EE-311 chokng }         EE-301 leaking  }Once GT-C trips and power supply to pumps got cut off resulting in EP-302 B C trips. }Gas got backup in EC-303 via EP-302 AS NRV not holding the backflow }As a result pump start back rotating with  full rpm without lube oil supply. Thereby shaft become red-hot and bearing seized. }Also EC-303 Bottom sump got collapsed }After that incident modification carried out for EP-302 NRV flapper (stoppers provided) and also valve close  logic provided for semi lean low flow nearly 250 m3 and lean low flow 50 m3. }In this condition semi lean FV-09  control valve close automatically and lean FV-08 closes automatically   Incase of struck up of valves i.e., 03-LV-04A/B or LV-002A/B the second valve has to be taken in line by closing the upstream of struck up valves by continuously increasing the valve output to get the desired level.  }  }GV solution carried away with process gas and get settled in EE-311(mainly bottom exchanger)  thereby developing dp between PV-71 and PV-74. it can be decreased by maintaining required lean solution flow (not in excess) or demister modification EE-301 leaking     Due to this regeneration rate got affected and hydrogen carry over with CO2 thereby affecting CO2 compressor(due to change in molecular weight)  }As the GV solution is corrosive, it may corrode/erode the vessels pipelines etc. at the operating temperatures and pressure.  Hence to avoid the same, the passivation of vessels, pipelines etc, is done. } The passivation forms a layer of oxide film over the surfaces by the addition of vanadium pent oxide in the circulating K2CO3 solution without process gas. }During this passivation period,  the activator, glycine is not added to the solution. }For the passivation operation the composition must be as follows :        K2CO3 = 200 g/l expressed as K2O (23.5% W/W as K2CO3)        Vanadium = 58/l expressed as V2 05 (0.4%  W/W as V2 05) }The solution is prepared as per above requirement and by means of sump preparation pump P-308, pumped to the regenerator C-302 bottom. }.  Passivation is subdivided into two types    (i) Static passivation     (ii)  Dynamic passivation.  During passivation G V  Solution  pumps are to be changed over.   }It is mainly for passivation of reboiler  }Regenerator C 302  bottom is filled up with solution.  Reboiler E 301 A/B  solution inlet and vapour outlet pipes also get completely filled.  } Pressurize C 302 with  N2  up to 2.7 kg/cm2, setting its PCV at above mentioned pressure.  }Solution is gradually heated by introducing steam through E 301 A/B so as to maintain the solution not less than 140 deg. C and without reaching the boiling point since it is preferable to avoid entering of steam into the regenerator.  Maintaining the temperature as high as possible, without reaching boiling point is important  for obtaining the best consistency of the passivation film.  }The above conditions are maintained  for 2 days, regularly checking column pressure and solution temperature.  During the operation regular analysis of K2CO3 and V2O5 will be carried  out in order that the solution concentration remains approximately constant and adding V2O5 if this falls below 4 g/l.  }It is mainly for passivation  of equipment and lines handling GV Solution    }Dynamic Passivation immediately follows static passivation.  }To start with Absorber is pressurised to about 18 kg/cm2g NG and solution circulation is established.  }Pressurize the regenerator  C 303 with N2 upto 2.2 kg/cm2; setting the vent valve (PIC 001) accordingly.  Keep C 302 pressurised at 2.2 Kg/cm2 with N2, controlling the pressure by PIC 015.  } It is important that C 302 & C 303 are at the same pressure for allowing the solution to flow from C 302 to C 303 by gravity.  } The passivation is carried out by circulating the hot solution maintaining a total solution flow rate of not less than 1400 M3/hr.  Process gas should not be admitted to the Absorber C 301.     }Adjust the steam flow rate so as to maintain the temperature of about 140 deg.C on C-302 bottom.  Continue with the operation for about 4 days regularly checking columns pressure and solution temperature.  }During the operation, regulr analysis of K2CO3 and V2O5 will be carried out in order to check that the solution concentration remains approximately constant and adding V2O5 if this falls below 4 g/l.  }At the end of the operation stop the steam to E 301 A/B and when solution cools below 100 deg C,  depressurize C 302 keeping a reduced solution circulation till the introducing of process gas.  Plant shut down, manholes opening or the columns shell drying is not permitted so as to avoid oxidation or alteration of the protective film, which has to be maintained till the process gas introduction.  } After completion of passivation partially solution is drained from C 302 to the preparation tank T 302 in order to add glycine and DEA are again pumped back to the system.  This operation is repeated to attain the required concentration before the process gas is introduced to the CO2 absorber.     }Short time shut down              Incase of a short time shut down during which the process gas is stopped maintaining the Absorber C 301 under pressure and the solution is maintained in circulation, the plant can be restarted without any re-passivation.     Long time shut down  } Incase of a long time shut down with opening of columns Manholes and consequent exposure of columns to air, the plant restart has to be preceded by a repassivation.  The process gas should not be admitted.  C 301 can be pressurised with NG.  }The GV solution, which has been stored in the storage tank T 301,is utilized for the repassivation at the standard concentration.     }Check the pentavalent vanadium (V2O5) content which should not be below  2.5 g/l expressed as V2O5.  The solution in the storage tank tends to oxidize itself but if necessary and in order to speed up oxidation air can be  injected to the storage tank T 301 to reach   the required V+5 concentration.  Some times it may be necessary to add V2O5 or to keep vanadium content 2.5 g/l.  }Solution is circulated as per normal procedure.  }Adjust the steam flow rate to E 301 A/B to maintain C 302 bottom temperature of  100-102 deg.C. Go on with the passivation operation for a minimum of 36 hours, better for 2-3 days, periodically checking the solution composition and temperature.     }At the end of the operation, steam is to be cut off to E 301 A/B, maintaining reduced circulation till the process gas is introduced.     }process Gas is being vented at PV-25A/B at EV-208 O/L.  } The absorber should be isolated with 03-HSV-004 and its bypass closed, 03-HV-002 and its bypass closed. 03-PV-071 closed.   }All EC-301 drains should be closed.  }Partially shift the venting to EV-301 O/L 03-HV-01. When gas starts venting at 03-HV-01, EC-302 bottom GV temperature picks up and reaches a temperature of 128 degc at 03-TI-11 and EC-302 pressure 03-PIC-15 is 1.1 kg/cm2g.  } Ensure 03-PIC-28 is in auto with 0.6 kg/cm2g set point and EC-303 column pressure is being maintained 750 mmwcg at 03-PIC-26 kept in auto mode. Also ensure lean solution pump EP-301A or B and one semi lean pump EP-302A or B or C are running with 200 and 650 m3/hr flows at 02-FRC-8 and 02-FRC-09 respectively.     }when ever condensate levels gets accumulated in the over head condensate system of EC-302 and EC-303 start EP-304A/B and EP-305A/B.  }When condensate in EV-301 gets accumulated send it to ET-351 seeing the level in EV-301.EC-301 level maintained through control valves 03-LV-039A/B or C.  }Slowly increase the pressure of EC-301 by opening 03-HSV-04 bypass and when pressure gets equalized slowly shift the venting to 03-PIC-071 and increase the back pressure and the corresponding set points of 02-PIC-023, 02-PIC-025.  }As the plant load increases increase the semi lean flow at 02-FRC-09 by starting another semi lean pump to maintain EC-301 O/L CO2 slip content below 1000 ppm.  } Now the plant is ready with CO2 for Urea plant start up. If required EK-301to be started as per start up procedures and shift the CO2 venting at 03-PIC-026 to 03-PIC-028.     }Before starting EK-301 ensure lube oil circulation is established and cooling water is i/s for lube oil cooler, seal air is lined up.  }Then thoroughly drain the condensate in the suction through suction drain, keep the casing drain open and drain condensate  } line up EK-301 suction I/V, discharge I/V also open the discharge line drain and drain the condensate.  } Keep re-cycle valve 03-FV-323 in full open condition.  } Reset IS-306 and start EK-301 and load the compressor after resetting 03-fy-323 in the SDS plessy.  }Initially close 03-HIC-323 slowly after some out put FIC-323 takes over 03-HIC-323 and gets operated in auto.  }  Then slowly open 03-PIC-027 until 03-PIC-26 gets closed.     }Low level in EC-301, E03-LSXL-040A/B/C          Incase of low level gas from EC-301 will escape and can lead to hydraulic turbine failure. Process gas can enter to regenerators and over pressurize regenerators. Contamination of co2 will lead to compressor surging.  }Simultaneous trip of semi lean pumps, IS-302A/B/C              If both the pumps trips co2 slip exit EC-301 will increase rapidly and will lead to methanator trip on high bed temperature  }Simultaneous trip of lean pump, IS-301A/B              If the pumps trips co2 slip exit EC-301 will increase rapidly and will lead to methanator trip on high bed temperature  } E03-MTS-06, Manual trip of IS-५    }Due to trip of absorber the automatic trip actuations are                             IS-5         IS-6                                  IS-7             IS-411                                   IS-306                                                  IS-550                                   E03-HV-6/7/9 close                                   E03-FV-8/9 close                                     on low flow                                     E03-UV-03 close                                                }Take process gas venting on E02-PV-025.  }Ensure supply of recycle hydrogen flow from A-1 to FDS through E04-PV-05. If recycle hydrogen from Ammonia-I is not available, START EK-204 with suction from the outlet of EV-208 and line up recycle hydrogen to the header