INNOVATION

4344 | Sulzer Technical Review 2/201114

Carbon capture and storage (CCS)involves the responsible utiliza-tion of fossil fuels without

endangering the planet. CO2 emissionsfrom fossil fuel combustion (particularlyfrom coal-fired and gas-fired powerplants) are considered to be the primary

contributor to this problem. Due to the significant interest in mitigating thisproblem, there has been an unprecedent-ed surge in activity to prove the techno-economic viability of CO2 capture tech-nologies from flue gas streams frompower plants 1. Capturing CO2 requires

a lot of energy (e.g., solvent regeneration);hence, the focus of every single tech -nology provider is to reduce this energycost wherever possible. Another challenge results from the

need to remove CO2 (typically between3.5 vol% and 14 vol%) from very large

The new MellapakCC™ structured packing for post-combustion CO2 capture

Pushing the boundaries in process intensificationSulzer has developed a new structured packing for absorbing carbon dioxide(CO2) more efficiently from the flue gas stream of fossil-fueled power plants.The new MellapakCC™ structured packing significantly reduces the columnsize and the pressure drop across the CO2 absorber — thus reducing capitaland operational expenses for the customer.

Carbon capture and storage (CCS)

pilot plant in Niederaussem,

Germany.

© RWE

volume gas streams that thereforerequire very large column sizes. Thepressure drop inside the absorber alsopresents a significant cost. If this expensecan be well managed, large savings canbe made. In this context, the right choiceof mass transfer equipment is of greatimportance, and structured packingoffers an excellent solution because itreduces the column dimensions (capitalexpenses, CAPEX) and provides lowpressure drop (operational expenses,OPEX) over the CO2 absorber. Specifically for post-combustion CO2

capture, Sulzer Chemtech has developeda new MellapakTM structured packing(Sulzer MellapakCC™), to fit the indi-vidual, process-specific requirements. Ittargets process intensification by signif-icantly reduced pressure drop to lowerOPEX and aims for maximum separationperformance to reduce CAPEX. Thisarticle presents a significant step infurther lowering the costs associatedwith CO2 capture in post-combustionplants.

State-of-the-art design of a CO2 absorberFigure 2 provides a schematic overviewof the amine-based scrubbing process forCO2 capture from a flue gas stream whichcould, e.g., come from a coal-fired powerplant. All three columns shown have thepotential for process intensification, butthis article refers exclusively to the largeCO2 absorber column. A typical 800 MWcoal-fired power plant can emit up to3000000 m3/h of flue gas, correspondingto some 6000000 t/yr of CO2 emitted.Assuming a superficial gas velocity of2.1 m/s, an indicative value for therequired cross-sectional area for theabsorber would be about 400 m2 (columninner diameter approx. 23 m). A previous study performed by Sulzer

Chemtech (Menon et al., 2009) showedthat Mellapak structured packing in the

CO2 absorber offers better mass transferperformance than random packing. Up to 15% savings in CAPEX (columnbed heights) can be achieved here. Inaddition, Mellapak structured packingoffers significantly lower pressure dropthan random packing (up to a factor of 2). This improvement allows engineers

either to reduce the column internaldiameter (up to 10% CAPEX savings) orto save significant OPEX (reduced elec-tricity demand due to reduced pressuredrop across the flue gas blower upstreamof the absorber). Another advantage ofusing Mellapak structured packing is thereduced amount of material required perunit geometrical area as compared torandom packing. This reduction posi-tively influences the total investmentcosts (CAPEX).Presented below is an analysis of

the impact of pressure drop on the eco-nomics of this process. The new SulzerMellapakCC structured packing has the potential to reduce the pressure

drop in the CO2 absorber by 20% to 35%over the already efficient conventionalMellapak structured packing. It is impor-tant to bear in mind that unlike thechemical and hydrocarbon processingplants, which are designed for a life-time of 10 to 15 years, power plants are typi cally designed for an operational lifespan of 30 to 40 years. A simple life-cycle cost analysis for

a typical 800MW power plant over a 30- to 40-year span clearly demonstratesthe potential savings in electrical costs(by reducing pressure drop). Each mbarof pressure drop saved represents con-siderable savings in operating cost. Infigure 3 the annual savings in electricitycosts are calculated, assum ing a reductionof 5mbar in pressure drop over the CO2absorber column. The savings calculated amount to

EUR 225000 per year in electricity costsfor the flue gas blower. Hence, a savingsin pressure drop of even 5 mbar over a30-year lifetime operation of the powerplant, could save up to EUR 6750000 in

15Sulzer Technical Review 2/2011 |

INNOVATION

1 Typical example of a CO2 capture unit fitted into an existing post-combustion power plant.

Generator

Steam-turbine

Cooling tower

Boiler

Fuel and air

Bottom ash

Particle removal

Fly ash

Sulphur removal

Sulphur Flow gas blower(Fan)

Cooler

Carbon dioxide absorber

CO2 rich absorbent CO2 lean absorbent

Low temperature

heat

Flue gasto

atmosphere

Low temperature

heat

Steam

Carbon dioxide stripper

Mechanical energyCarbon dioxide compressor

Carbon dioxideDetails see figure 2.

electricity costs. Such costs need to beevaluated during the absorber designand when choosing the type of packingand associated internals.

Process intensification: Development of MellapakCC™structured packingPressure drop was found to have aprofound impact on the plant OPEX.Therefore, Sulzer Chemtech started anR&D program to find the ‘sweet spot’with respect to mass transfer efficiencyand hydraulic performance of the struc-tured packing. The target was tomaximize or maintain the effective inter-facial area (or wetting), while simulta -neously minimizing the pressure drop.To further optimize the geometry ofMellapak structured packing, it was nec-essary to fix the process of operation and

select a benchmark Mellapak packingtype. The system applicable for post-com-

bustion capture lies predominantly in the liquid-side controlled regime,involving a fast chemical reaction.Mellapak types with a relatively lowspecific geometrical area (between 200and 250 m2/m3) were chosen as a bench-mark for further improvement. The rea-soning behind this choice was simple:packing types with low specific areasexhibit a more complete wetting charac-teristic than those with higher specificareas. This characteristic results in a better

use of the installed metal area in an opti-mization of the involved packing cost.Hence, Mellapak types Mellapak™ 2X(200m2/m3) and Mellapak™ 250.X(250 m2/m3) were the candidates selectedfor further optimization. A systematicparametric study looked into the influence of the microstructure, hole size, numbers of holes, and angle of cor-rugation. This study resulted in two new

packing types with special geometricalfeatures: MellapakCC™-2 and Mella-pakCC™-3, which deviated in specificsurface areas. These two packing typeswere developed by optimizing the

packing geometry in terms of thematerial requirement (lower CAPEX),the pressure drop (lower OPEX), and the high effective interfacial area (lower CAPEX and OPEX).

Performance of MellapakCC™-2and MellapakCC™-3Figures 4a and 4b show the hydraulicperformance of MellapakCC-2 and MellapakCC-3. The hydraulic perfor -mance was measured using an air-watersystem in a 1000mm column at variousliquid loads typically seen in post-com-bustion capture applications. First of all,the measurements confirmed the signif-icant reduction in pressure drop whencomparing the new MellapakCC struc-ture with conventional Mellapak struc-tured packing. Using MellapakCC-2instead of Mellapak 2X results in up to20% lower pressure drop (see figure 4a).This impact becomes even more signifi-cant (as seen in figure 4b) where up to35% reduction in pressure drop ispossible when using the MellapakCC-3instead of Mellapak 250.X.Additionally, the pressure drop curves

(see figure 4c)—at a constant liquidload of 25m3/(m2

•h) and varying gasflow rates were found to be similar for MellapakCC-2 and MellapakCC-3packings. The separation performance of the

new packing types was measured byexperiments wherein CO2 (from air) wasabsorbed in a NaOH solution. This is aliquid-side controlled system and fallsunder the fast-reaction regime. Since allrelevant physical properties are knownfor the CO2/NaOH system, it is possibleto back-calculate the effectively availablemass transfer area that determines themass transfer efficiency of the packing. The mass transfer performance (figure

4d) shows the efficiency (requiredpacking height) of the new packings interms of the effectively available mass

16 | Sulzer Technical Review 2/2011

INNOVATION

2 Schematic of CO2 capture plant based on amine-scrubbing process.

3 Electricity costs savings for the flue gasblower for a medium-sized 800MW Europeanpower plant.

Process parameter ValueFlue gas rate, G 3000000 m3/h

Pressure drop reduction, Δp 5 mbar

Fan efficiency, η 0.75

Operating time, t 8100 h/yr

Electrical costs1), c 0.05 EUR /kWh

Energy per year, E = G • Δp • t / η 4.5 • 106 kWh / year

Electrical costs, C = E • c EUR 225000 /year1) Average electrical cost in EU25 countries (Eurostat, 2007)

Direct contact cooler

Flue gas(from FGD)

Flue gas blower

Condensate

Flue gas(to atmosphere)

Absorber Make-up water

Regenerator

Lean solvent

Rich solvent

CO2 to compressor

transfer area at a constant gas flow rate and varying liquid loads. Mella-pakCC-2 and Mellapak 2X have nearlythe same mass transfer performance, asalso MellapakCC-3 and Mellapak 250.X.It is evident that MellapakCC-3 providesbetter separation efficiency than Mella-pakCC-2. This advantage is created bythe higher geometrical area of Mella-pakCC-3 over MellapakCC-2. This dif-ference also implies that compared withconventional Mellapak 2X, the newMella pakCC-3 will require approximately20% less packing height with up to 35%reduced pressure drop (due to reducedpacking height and due to reducedpressure drop per meter of packingheight). This would make Mella-pakCC-3 the packing of choice, becausethe advantage of higher separation effi-ciency (due to the higher geometricalarea) would complement the significantsavings in pressure drop.

What is important to note is that therelative benefit gained from the Mella-pakCC packing compared with the con-ventional Mellapak packing will also beapplicable to any system that belongs tothe fast-reaction regime (like the CO2absorbers in post-combustion capture).In other words, if the packing heightusing a particular solvent is known forMellapak 250.X, then MellapakCC-3 willrequire the same packing height for masstransfer for that solvent, but will have35% less pressure drop.

Deployment of MellapakCC™ in pilot and demo plantsSulzer's new MellapakCC packing family,which has been specifically developedfor the post-combustion CO2 capturemarket, can significantly reduce pressuredrop in the CO2 absorber, while givingthe same separation performance asequivalent conventional Mellapak struc-

tured packing types. Consequently, Mel-lapakCC packings demonstrate signifi-cant potential in reducing CAPEX andOPEX demands in a post-combustionCO2 capture environment. Both Mella-pakCC-2 and MellapakCC-3 are nowready for deployment in pilot and demoplants in the near future.Although MellapakCC-3 looks like the

obvious candidate for optimizing theabsorber design, there might be goodreasons to go with the MellapakCC-2packing, for instance, if a processlicensor would like to retrofit an existing Mellapak 2X design using MellapakCC-2. Here, the packing heightwould remain the same, but the absorberwould gain the advantage of a 20%reduced pressure drop. The selection ofthe right packing is a trade-off betweencolumn dimensions, separation efficiency,and pressure drop.

Abhilash Menon Sulzer Chemtech Ltd.Sulzer-Allee 48P.O. Box 658404 WinterthurSwitzerlandPhone +41 52 262 61 84abhilash.menon@sulzer.com

Markus DussSulzer Chemtech Ltd.Sulzer-Allee 48P.O. Box 658404 WinterthurSwitzerlandPhone +41 52 262 67 14markus.duss@sulzer.com

17Sulzer Technical Review 2/2011 |

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4 Hydraulic and mass transfer performance of MellapakCC-2 and MellapakCC-3.

References• Menon, A., Duss, M., Bachmann, C.:“Post-combustion Capture of CO2(Case study of a generic amine-based absorption process),”Petroleum TechnologyQuarterly, (Quarter 2, 2009): pp. 115-121

• Duss, M., Menon, A.: “Optimized AbsorberDesign for Post-combustion CCS,”Proceedings of the 9th Distillation andAbsorption Conference, 12–15 September,Eindhoven, Netherlands.

• Eurostat (2007),http://epp.eurostat.ec.europa.eu/

1.5

1.0

0.5

0

F-factor (Pa0.5)

Pressure drop comparison: MellapakCC-2 vs. Mellapak 2X

1.5

1.0

0.5

0

F-factor (Pa0.5)

1.5

1.0

0.5

0

F-factor (Pa0.5)

∆p

/∆z (

mb

ar/

m)

∆p

/∆z (

mb

ar/

m)

∆p

/∆z (

mb

ar/

m)

∆p

/∆z (

mb

ar/

m)

Pressure drop of Mellapak

0 1.5 2 2.5 3

300

250

200

150

100

Liquid load (m3/m2h)

Effective interfacial area of Mellapak

0 10 20 30 40 50 60

b)a)

c) d)

0 0.5 1 1.5 2 2.5 3 0 0.5 1 1.5 2 2.5 3

Pressure drop comparison: MellapakCC-3 vs. Mellapak M250.X

System: Water-AirColumn ID: 1000 mmPacking height: 3 m 35% lower

System: Water-AirColumn ID: 1000 mmPacking height: 3 mLiquid load: 25 m3/m2h

System: NaOH/CO2/airF-factor: 2 Pa0.5

System: Water-AirColumn ID: 1000 mmPacking height: 3 m 20% lowerMellapakCC-2: 0 m3/m2hMellapakCC-2: 25 m3/m2hMellapakCC-2: 50 m3/m2hMellapak 2X: 25 m3/m2h

MellapakCC-3: 0 m3/m2hMellapakCC-3: 25 m3/m2hMellapakCC-3: 50 m3/m2hMellapak 250.X:25 m3/m2h

MellapakCC-3MellapakCC-2Mellapak 250.XMellapak 2X

MellapakCC-3MellapakCC-2Mellapak 250.XMellapak 2X