Thermally unstable salt, according
to the equilibrium reaction Giammarco-Vetrocoke of Italy solution
The
gas leaving the CO conversion section has a CO2 content of 21-22 mole% (dry
basis). After separation of process condensate in EV-208 the gas enters
the CO2 removal unit.
In the GV hot pottassium carbonate (HPC) dual activated solution, the CO2 is
chemically combined with the pottassium carbonate via formation of pottassium
bicarbonate, a thermally unstable salt, according to the equilibrium
reaction
CO2 + H2O + K2CO3 <----> 2
KHCO3
(1)
The CO2 absorption mechanism involves the following steps
CO2 +
H2O
<----> HCO3- + H+
(2)
CO3-- +
H2O
<----> HCO3- + OH-
(3)
------------------------------
CO2 + H2O +CO3--
<----> 2HCO3-
(4)
H2NCH2COO- + CO2 <----> -OOCHNCH2COO-
+ H+ (5)
-OOCHNCH2COO- + H2O <----> H2NCH2COO-
+ HCO3- (6)
------------------------------
CO2 + H2O <---->
HCO3- + H+
The reaction rate of (4) is determined by the sum of rates of (2) & (3)
where (2) represents the slow step of the absorption reaction localised in the
resistance for transfer of the CO2 molecules from the gas phase to the liquid
phase.
Glycine works as a CO2 carrier by rapidly introducing the CO2 into the liquid
phase via formation of glycine carbamate (5). At high temperature and in
the presence of OH- the glycine carbamate is hydrolysed and the
activator is restored with formation of bicarbonate ion according to reaction
(6).
Reactions (5) & (6) take place continuously and (7), the sum of two
reaction rates, represents reaction (2) which occurs very quickly due to the
enhanced mass transfer rate induced by glycine activation. The addition
of a secondary amine is effective to further enhance the rate of glycine to act
as CO2 carrier according to a synergistic effect in promoting the rate of
hydrolisis of carbamate (6) which represents the slower step of (5) & (6).
Basically a secondary amine has activation mechanism similar to that of
glycine. But glycine having a much higher reaction rate than amine, it
will prevail reacting much faster with CO2 via glycine carbamate formation
while amine will only partially do it resulting largely free to catalyse the
glycine hydrolisis.
A much reduced amine content is sufficient to effectively improve the overall
absorbing/desorbing efficiency, allowing in the meantime less glycine content
than the mono-activated solution. So, the chemicals makeup is drastically
reduced and the solution stability increased.
The absorption is carried out in two stage : at the first stage, the bulk
of CO2 is absorbed; at the second a reduced stream of strongly regenerated cold
solution is utilized to get very low CO2 slippages due to the very low CO2
vapour pressure of the dual activated solution.
At the regeneration phase, the dual activation improves the fractional
conversion of bicarbonate to carbonate (right to left in reaction
1) getting this way a lower CO2 vapour pressure in the solution compared to the
conventional mono-activated solutions allowing a reduction in the CO2 slip from
the absorption step and a reduction in the amount of stripping steam.
The process gas supplied by the CO2 removal unit to the methanator will contain
less than 0.1 mole% CO2.
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 =
K2CO3 + HCO3- + H+ = 2KHCO3 (2)
------------------------------
K2CO3 + CO2 + H2O = 2KH CO3
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- + 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.
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