Mass Transfer in a Small Scale Flue Gas Absorber Experimental and Modeling

Transcription

1 Mass Transfer in a Small Scale Flue Gas Absorber Experimental and Modeling P.J.G. Huttenhuis, E.P. van Elk, S. Van Loo, G.F. Versteeg Procede Gas Treating B.V., The Netherlands 11 th MEETING of the INTERNATIONAL POST- COMBUSTION CO 2 CAPTURE NETWORK 20 May 2008, Vienna, Austria

2 Content Introduction; Shell ASAP unit; Rate based model; Results and Discussion; Conclusions & Future work. 11 th CO 2 capture meeting; Vienna; Austria 20 May

3 Introduction Typical flowsheet: 11 th CO 2 capture meeting; Vienna; Austria 20 May

4 Introduction Main problems with this technology are: - Degradation due to the presence of oxygen; - Corrosion; - High absorber costs; - High regeneration costs ± 70 % of operational costs (4 GJ / tonne CO 2 ). Better solvents required. This Work: Study of performance of aqueous MDEA and comparison with pilot plant data of an absorber desorber. 11 th CO 2 capture meeting; Vienna; Austria 20 May

5 Introduction Process design: Hydrodynamics (a); Mass transfer parameters (k L and k G ); VLE (physical and chemical); Kinetics (enhancement). Film model Accurate prediction of fundamentals required; Rate based model required because absorber is not operating at equilibrium conditions. 11 th CO 2 capture meeting; Vienna; Austria 20 May

6 Introduction Kinetics tertiary amines: k CO 2(g) + R3N (l) + H2O (l) HCO k 3 (l) + R3NH (l) (1) 2 Reaction with tertiary amine is slow and has a low regeneration energy; R =k [CO ][R N] mole.m.s CO (second order reaction) Kinetics primary-secondary amines: Reaction with primary / secondary amine (carbamate formation) is fast and has higher regeneration energy; Reaction is complex and can be described with zwitterion mechanism: k CO 2 (g) + R1R 2NH (l) R1R 2NH COO (l) (2a) k2 R R NH COO (l) + B (l) R R NCOO (l) + BH (l) (2b) + - k b k 1 2 b [CO ][R R NH] R CO = mole.m.s 2 1 k 1 + k k k [ B] 2 2 b 11 th CO 2 capture meeting; Vienna; Austria 20 May

7 Introduction Mass transfer: J CO 2 C 2 CCO 2, G = m k mk E CO, L G L CO 2 Pseudo first order regime: E CO2 =Ha if 2 < Ha << E CO2, Ha = E CO, 2 k D ov k L CO D = 1+ ν D 2 am am [ Am] [ CO ] CO 2 2 Diffusion limited regime: E CO2 =E CO2, if 2 < E CO2, << Ha 11 th CO 2 capture meeting; Vienna; Austria 20 May

8 Shell ASAP unit ASAP Unit (Amine Screening APparatus) is developed by Shell Global Solutions - Amsterdam; Designed to test the performance of different solvents; Gas flow up to 1.3 Nm 3 /hr; Liquid flow up to 6 kg/h; D=2.5 cm, L=1.45 m; Sulzer EX laboratory packing; area is 1735 m 2 /m 3. Following solvent have been tested and simulated: - 50 wt.% MDEA solvent circulation is 80 m 3 solvent / tonne removed CO th CO 2 capture meeting; Vienna; Austria 20 May

9 Shell ASAP unit PEFS Absorption Unit GC Gas samples cooling coil dpi cooling coil Treated gas H 2 S / CO bar 1-5 bar Mix gas Absorption column Regeneration column H 2 S / CO 2 N 2 CO 2 Air Feed gas Catch pot Up to 1200 nl/hr Liquid samples Fat solvent cooling coil N 2 H 2 S/N 2 Lean solvent pump kg/hr 11 th CO 2 capture meeting; Vienna; Austria 20 May

10 Shell ASAP unit 11 th CO 2 capture meeting; Vienna; Austria 20 May

11 Rate Based Model Two (rate based) models are under development at Procede: 1. Top down model (SimCol): Unit operations; Mass transfer + kinetics; 2. Bottom up model (Pro2Sim): Fundamentals (equilibrium, flash and rate models); Unit operations; In this work SimCol is used to describe the absorber of the ASAP unit located at the Shell laboratories in Amsterdam. 11 th CO 2 capture meeting; Vienna; Austria 20 May

12 Rate Based Model - Flux model for mass transfer; - Ideal vapor / liquid phase (fugacity = 1); - Tray to tray procedure (Blauwhof 1985); - Series of ideal CISTR s; - No pressure drop and back mixing; - Constant heat and mass transfer parameters (k L, k G, a, h L, C P ); - Physical parameters only function of temperature (ρ, μ, D, K, m); - Limited to blend of two amines. 11 th CO 2 capture meeting; Vienna; Austria 20 May

13 Rate Based Model Following input data are required: - Solvent type; - Gas and liquid temperatures; - In- and outlet CO 2 concentrations; - Lean solvent concentration; - Mass transfer parameters: k G, k L and a; - Physical properties (density and heat capacity); - Reaction and absorption enthalpy; Output: - Required absorber dimensions; - Temperature profile in absorber; - Concentration profile in column; - Speciation of the liquid. 11 th CO 2 capture meeting; Vienna; Austria 20 May

14 Model results and Discussion Influence number of trays : Column length [m] number of trays [-] Minimum number of calculated trays should be at least 30 to assure plugflow in the column. 11 th CO 2 capture meeting; Vienna; Austria 20 May

15 Model Results and Discussion Influence k L : Influence k G : Conclusion: Mass-transfer is generally k L limited and not k G. 11 th CO 2 capture meeting; Vienna; Austria 20 May

16 Model Results and Discussion Influence lean loading: Conclusion: Mass-transfer is highly depended on lean loading. 11 th CO 2 capture meeting; Vienna; Austria 20 May

17 Model Results and Discussion Driving force in absorber: 11 th CO 2 capture meeting; Vienna; Austria 20 May

18 Model Results and Discussion Calculated activity coefficient in CO 2 loaded 50 wt.% MDEA at 40 C as calculated by electrolyte NRTL model: Ref: PhD thesis Posey (1996) University of Texas. 11 th CO 2 capture meeting; Vienna; Austria 20 May

19 Model Results and Discussion Calculated column length: Calculated length; good agreement with experiments (1.45 m). 11 th CO 2 capture meeting; Vienna; Austria 20 May

20 Model Results and Discussion Flux and chemical enhancement: 11 th CO 2 capture meeting; Vienna; Austria 20 May

21 Model Results and Discussion Temperature profile: 11 th CO 2 capture meeting; Vienna; Austria 20 May

22 Model Results and Discussion Speciation in liquid phase: 11 th CO 2 capture meeting; Vienna; Austria 20 May

23 Conclusions & Future work - SimCol can be used for qualitatively absorber calculations; - Simple thermodynamic and enthalpy model not sufficient for accurate calculations Extension to more accurate thermodynamic models, like E-EOS models; - Good prediction for MDEA experiments; other solvents will be studied. 11 th CO 2 capture meeting; Vienna; Austria 20 May

24 Acknowledgement - Shell Global Solutions for the experimental part of this work; - This work has been carried out as part of the CAPTECH project; - CAPTECH is sponsored by the Dutch Ministry of Economic Affairs and is part of the EOS Long Term (Energy Research Strategy) research program. 11 th CO 2 capture meeting; Vienna; Austria 20 May

25 Thank you! 11 th CO 2 capture meeting; Vienna; Austria 20 May