Revamping out-of-date “hot pot” units: Better efficiency, less energy(L.Tomasi, Giammarco-Vetrocoke s.r.l., Venice, Italy)

Abstract

The CO2 removal system is a key area of an Ammonia plant in which is still possible to make worth-whileimprovements. Six Indian plants using the best available Hot Potassium Carbonate technology of yesteryear have beenconverted to GV low-energy process with Dual-pressure regeneration in the course of a normal plant turnaround.

Foreword

The new scenario makes imperative for the older Ammonia plant a drastic reduction on the specific energy consumptioncombined with an enhanced flexibility for capacity increase in order to make them competitive with the new designedAmmonia plants.The CO2 removal section is one of the major consumers of energy in Ammonia plant and has been identified as themost appealing area of improvements to attain said objectives.Giammarco-Vetrocoke (GV) is a well-known licensor of a leading CO2 Removal Process based on hot potassiumcarbonate activated solution, and since many years is very active in the field of revamping with extensive experience indesigning and implementation of revamping projects for out-of-date “hot pot” CO2 Removal units based on the GVprocess and/or on the competitors ones like Benfield, Carsol and Catacarb.The GV revamp strategy has always been the implementation of well proven and advanced technologies in order toachieve the best improvements at the lowest cost with the maximum utilization of the existing equipment.The annual turnaround of a plant is strictly kept as the maximum required time to implement the hook-up up of therevamped schemes without affecting the normal operation and the full capacity production.

Giammarco-Vetrocoke profile

Giammarco-Vetrocoke, established in Venice (Italy), is active in the field of CO2 Removal systems since 1950 for theindustrial application and commercialisation of the inventions and discoveries of Dr. Giuseppe Giammarco who, first inthe world, introduced the “activators” on the “hot pot” solution drastically enhancing the CO2 absorption efficiency.

Starting from 1980 GV devoted great part of its activities to the introduction on the market of the GV low-energyprocess with Dual-pressure regeneration which allowed a reduction on the specific energy consumption by 35-45%.

The GV Dual-activated solution widely applied starting from 1990 is the last achievement of GV and is based on twoselected activators which, added to the “hot pot” solutions, work in synergy allowing a drastically reduction on the CO2slip coupled with a lower sizing required for the columns.

The GV Processes have been licensed in over 340 plants built all over the world out of which 60 units utilise the GVlow-energy process, 40 the GV Dual-activated solution and 78 are revamped units.

The main strength of GV lies in the licensing of its proprietary technologies which are supported by a detailed ProcessDesign Package fully developed in house by a team of specialized and skilled people. In addition to the technology, GVshall provide site assistance for plant supervision, pre-commissioning, commissioning and start-up up to full capacityoperation.

State of Art

The absorption of the CO2 by a “hot pot” solution is a chemical reaction where the CO2 reacts with the dissolvedPotassium Carbonate forming Potassium Bicarbonate, according to the reaction (A) proceeding left to right, while byheating, the Potassium Bicarbonate is decomposed releasing the CO2 and restoring the Potassium Carbonate as perreaction (A) proceeding right to left.

CO2 + H2O + K2CO3 D 2KHCO3 (A)

Since the formation and the decomposition of the Potassium Bicarbonate is controlled by the law of the chemicalequilibrium, the Potassium Carbonate/Bicarbonate are co-existing inside the solution and it is a common rule to expressas Fractional Conversion (FC) the content of Bicarbonate present as Carbonate.

The CO2 absorption/regeneration process, in the simplest industrial configuration (Fig. 1 – One Stage ConventionalScheme), consists in one absorption column (absorber), where the CO2 is washed out of a synthesis gas under pressure,by a counter-current flow of the aqueous solution containing in the main Potassium Carbonate (lean solution at low FC)and one regeneration column (stripper), in which the aqueous solution containing in the main Potassium Bicarbonate(rich solution at high FC) is depressurised and heated to the boiling point producing steam which is used to strip-out incounter-current the CO2 from the solution exit from the absorber.

The lean solution is circulated by a pump from the bottom of the stripper to the top of the absorber while the richsolution is fed back again to the top of the stripper by let-down of the pressure prevailing at the absorber bottom.The lean solution feeding the absorber is split into two streams: the main portion to the mid point and the balanceportion, deeply cooled in order to reduce the CO2 partial pressure required to achieve a CO2 slip of 1000 ppmv at least,to the top.

The improvement made to the previous scheme was the adoption of two solution circulation pumps (Fig. 2 – Two

Stages Conventional Scheme), by which the main portion of the regenerated solution (semi-lean solution at intermediateFC), withdrawn from a mid point of the stripper, feeds separately the mid point of the absorber, while the balanceportion, after a stronger regeneration in the bottom section of the stripper (lean solution at a very low FC) and a finalcooling, feeds the top of the absorber, where, owing to the much lower CO2 partial pressure consequent to the lower FC,a CO2 slip of 500 ppmv is normally achieved.A saving of about 10% on the regeneration heat requirement is achieved by this scheme.

The further improvement made to the conventional schemes, in order to meet the growing demands of energy savings,was the implementation of a Low-Heat regeneration scheme (Fig. 3 and Fig.4. One Stage and Two Stages Low-HeatSchemes). The adopted technology is based on the installation of a flash tank operating at lower pressure of the stripperwhere the lean or the semi-lean solution, after depressurization, are developing flashing steam, which is recompressedto the stripper by steam ejectors and/or mechanical steam recompression.A saving of about 20% on the regeneration heat requirement is achieved by these schemes.

A milestone on the “hot pot” units was the introduction on the market of the GV Low-Energy process (Fig. 5 –GVLow-Energy Dual Pressure Regeneration Scheme) with a substantial innovative design to approach the energy saving.The concept of the GV Low-Energy process is a two stages scheme with two separate strippers working at differentpressure, HP (high pressure) stripper and LP (low pressure) stripper (2) (4).Part of the rich solution from the absorber (about 60% of the total circulation) is introduced into the top of the HPstripper while the remainder, let down in pressure by about 1 kg, into the LP stripper.

The heat for regeneration is supplied to HP stripper only by a process gas reboiler or/and by direct/indirect steam.The semi-lean solution withdrawn from mid point of the HP stripper feeds the mid point of LP from which, afterreleasing steam by flash, is pumped to the lower zone of the absorber. The lean solution is fed from the bottom of HPstripper to the bottom of LP stripper and thence, after releasing steam by flash, is cooled and pumped to the top of theabsorber.The pressure difference between the strippers HP and LP is such that sufficient flashed steam is produced to strip outthe CO2 from the rich solution fed to the top of the LP stripper achieving the same FC of the semi-lean solutionwithdrawn from the HP stripper.A saving of about 40% on the regeneration heat requirement is achieved by this innovative scheme.

The Table 1 summarise the performances of the different schemes

SCHEME Specific Regeneration Heat (Kcal/Nm3 CO2)typical (operating)

CO2 slip (ppmv)typical

Fig. 1 – One Stage Conventional 1200 (1300) 1000Fig. 2 – Two Stages Conventional 1080 (1200) 500Fig. 3 – One Stage Low-Heat 950 1000Fig. 4 – Two Stages Low-Heat 850 500Fig. 5 – GV Low-Energy 650 200-300

Table 1

GV Revamp – Benefits & Economics

The great experience and know-how acquired by GV in the field of revamping has been profitably applied to the out ofdate “hot pot” CO2 Removal Units. The revamping package proposed by GV can be implemented all of a sudden or stepby step according to the client requirement and/or internals constraints.

The first step is the conversion of the One Stage Conventional and/or One Stage Low-Heat Schemes (Fig. 1 and/or Fig.3) to the Two Stages Schemes (Fig. 2 and/or Fig. 4).

The main modification required to convert an existing stripper from one stage into a two stages is the insertion of atake-off tray (TOT) at appropriate elevation in order to have two separate solution streams exiting from the stripper thatis the semi-lean from the new TOT and the lean from the bottom. A new lean solution circulation pump shall beimplemented for the solution stream feeding the top of the absorber.GV has set up an in-house accurate simulation to calculate the mass transfer in the existing stripper internals,identifying the most efficient type/size of packing and its best sharing between lean and semi-lean solution required toobtain the right FC on the regenerated solutions. A high-efficiency solution distribution/re-distribution coupled to thenew packing configuration is a further benefit included on the proprietary GV know-how.The modification can be easily implemented inside the normal turnaround time.

The benefit achieved with the first step is about 10% saving on the Specific Regeneration Heat with an halved CO2 slipof 500 ppm at least or even lower by utilising the GV Dual activated solution (4).

The second step is the final conversion to the GV Low-Energy process with Dual Pressure Regeneration (Fig. 5).

The modification required is a new LP stripper with eventually the related CO2 cooling section comprising a Condenser,a Separator and/or, depending on the Urea unit requirement, a Booster or an Ejector to partially or totally recompressthe lower pressure CO2 stream to required battery limit final pressure.The new equipment and most of the required connecting piping can be implemented with the plant in operationproviding in advance the tie-ins for the final assembly during the normal turnaround time.The final benefit achievable, depending on the existing plant configuration and on the operating efficiency, is a savingof about 40 to 50% on the Specific Regeneration Heat with a CO2 slip, based on the GV Dual activated solution, of 200-300 ppmv (4).The new added LP stripper coupled with the new lean solution circulation pump give a good extra margin to the plant,which can increase the production capacity from 30 to 50%.The flash tank of the Low-Heat schemes can be retained for the after revamp operation as semi-lean pump sumpconnected to the new LP stripper achieving a saving on the stripper internal space required for said service.All the existing equipment can be basically re-utilised for the after revamp operation, minimizing the procurementrequirement with the relating piping connection and making the pay back time very appealing.

Achievements

The proposed revamp has been already licensed by GV for 6 (six) Indian CO2 Removal plants, based on Benfield Low-Heat Process (1), out of which 5 at One Stage and 1 at Two Stages, all converted to the GV Low-Energy Process (Fig.5).The status of the revamping is the following:

- 3 plants in operation – (IndoGulf Fert. - Jagdishpur; IFFCO AI - Aonla; IFFCO AI – Phulpur)- 1 plant under construction (National Fert. Ltd. AI – Vijaipur)- 2 plants under engineering ( RCF AI/AII – Thal)

The operating results of IndoGulf Fert. unit after revamp are reported as example 1 (3):

“The revamped CO2 Removal plant was commissioned end July 2003 and plant start up was very smooth and withoutproblem. The steam consumption reduced from 32 t/h to around 15 t/h and CO2 slip reduced from 1000 ppm to about400 ppm. The project is successfully running since last three years and savings envisaged are being realized oncontinuous basis”

The operating results of IFFCO AI (Aonla) unit after revamp are reported as example 2 (Client communication)

- LP Steam consumption in GV system before the modification was 40 t/h which has been reduced to around 14-16t/h.

- CO2 slip from the GV absorber before the modification was 1000-1200 ppm which has been reduced to 300-400ppm.

Conclusion

The paper has presented a very appealing revamp for out-of-date “hot pot” CO2 Removal units in order to improve plantperformances and save energy. The GV approach is to customize the target of the revamp to the real needs of theClients, focusing its efforts on the energy saving and/or on the capacity increasing or on an efficient compromisebetween both of them.GV has acquired a great experience in the management of the revamps and can offer to its Clients a wide portfolio ofservices spreading from the feasibility study to the commissioning/start-up of the revamped units (5).The full re-utilisation of the existing equipment coupled to the short shutdown period required for the modification,compressed inside a normal plant turnaround, make very short the payback of the investment.

References

1. Tomasi, L.: “Revamp of CO2 removal one-stage low-heat process into Giammarco-Vetrocoke two stages low-energy process” - Fertiliser News, 63-65 (Dec 2004).

2. Tomasi, L.: “Revamp of CO2 removal units with twin parallel regenerators to the GV low-energy system with dual-pressure regenerators” - Nitrogen 2005 International Conference, Bucharest (Feb 2005).

3. Datta, C.K.; Gupta, A.K.; Agarwal, R. (IndoGulf Fert.): “Energy efficiency improvement in CO2 removal system –An innovative approach” - Ammonia Technical Manual, 219-223 (2006)

4. Tomasi, L.; Corò, M.: “ CO2 removal at the OMIFCO complex in Oman: Balancing energy efficiency and capitalcost” - Nitrogen 2007 International Conference, Bahrain (Feb 2007).

5. Tomasi, L.; Kruglov, I.N.: “Energy saving and capacity increase at the JSC Cherepovetsky Azot ammonia plant byconversion to the GV CO2 removal process” - Nitrogen 2008 International Conference, Moscow (Apr 2008).