Optimisation of post combustion carbon dioxide capture by use of a - PowerPoint PPT Presentation

Optimisation of post combustion carbon dioxide capture by use of a facilitated carrier membrane Natsayi Chiwaye, Thokozani Majozi and Michael Daramola , * School of Chemical and Metallurgical Engineering, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, Johannesburg, 2000, South Africa * Corresponding author: michael.daramola@wits.ac.za; Tel +27117177536

Outline Background and Motivation Problem Statement Model Development Case Study Conclusion 1 1

Background and Motivation  Post combustion capture Electricity Generation Fuel Steam Boiler N 2 Gas separation problem CO 2 capture unit Flue gas Air CO 2 70% N 2 4-15% CO 2 1 bar Membranes: Advantages  Draw backs of chemical absorption by amines  Less energy intensive  Huge energy demand during regeneration of amine  No moving parts hence low maintenance  Corrosive to equipment  Relatively more environmentally friendly  The solvent degrades in the presence of common Membranes: Challenges flue gas  Driving force  Other technologies Low CO 2 concentration in flue gas, low feed pressure  Adsorbents  Need for membranes with high CO2 permeance  Membranes  And selectivity 3 2

Background and Motivation  Fixed site carrier facilitated membrane  Transport of CO 2 across the membrane is due to diffusion and the reversible reaction of CO 2 and NH 2 groups in the presence of H 2 O.  FSC membranes enhanced permeance and increased CO 2 selectivity  Therefore results in lower cost of CO 2 capture  FSC membrane application considerations  Permeance highly dependent on relative humidity  Water vapour as sweep is suitable  Water highly permeable 3 4

Background and Motivation Hussain & Hagg He & Hagg He et al., (2015) Current Study 2010 (2014) Process flow Predetermined Predetermined Predetermined Superstructure based model Membrane stages 2 2 2 Multi Components 4 4 2 4 Pressure ratio fixed fixed fixed Variable Relative humidity - fixed - variable  Recycle stream - - - Permeate pressure Vacuum & sweep vacuum vacuum Vacuum & sweep generation gas CO 2 /H 2 O selectivity 4.4e8 1 - 1 4 4

Aim & Objectives  Aim  To develop a mathematical model for the optimal design of FSC process flow system minimising the total annualized cost in order to further reduce the cost of CO 2 capture by FSC membrane.  Objectives  To develop a comprehensive FSC superstructure  To determine the effect of varying pressure ratio on the total cost of CO 2 capture  To investigate the effect of permeate pressure generation by vacuum and, or sweep gas  The feasibility of this proposed system is evaluated by optimizing the process based on the minimum total annualised cost of capturing CO 2 . 5 5

Problem Statement  Given:  Flue gas of known flowrate, components, temperature and pressure  Desired permeate purity and desired capture ratio  Permeance and selectivity of the membrane  Determine:  The membrane process system that minimises the total annualised costs for the carbon capture for target separation factor.  The optimum operating and design conditions of the membrane units:  flowrate of streams,  area of the membrane,  permeate and retentate pressure,  Relative humidity  sweep gas flow rate and  compressor and vacuum pumps power consumption. 6 6

Model Development Model Development  Major assumptions  Concentration polarisation on the membrane is negligible  The pressure drop along the membrane is negligible.  The overall permeance of component is not affected by pressure nor by concentration variation  Counter-current flow is considered.  Constraints  Gas permeation  Mass balances  Energy consumption of compressors, vacuum pumps and energy recovered by expanders  Heat transfer area  Separation targets- capture ratio and product purity  Objective function 8

Flue gas Flue gas Residue N 2 H 2 O Final residue Retantate recycle Flue gas Retantate recycle Retantate recycle Residue Retantate Retantate Retantate Feed Retantate Feed  Superstructure Membrane 1 Membrane 2 Membrane 1 Sweep Sweep Permeate Permeate Water Water Permeate Permeate vapour vapour sweep sweep Permeate Permeate regenetation regenetation H 2 O H 2 O Permeate Permeate H 2 O H 2 O Permeate Permeate Permeate Permeate Permeate recycle Permeate recycle Permeate recycle Product CO 2 7 Permeate recycle Final product Product H 2 O

Model Development Model Development rto R i n , oa R  Major mass balance constraints i f f R x ft R i n i rto  Feed mixer R rtr R i n , i n rr , , f R   i n ,   rtr R f s n ff t r t rr t p m rm p R x Rx Rx R i n rr , , f x i , n n i n , , r r i n n , , p r i n rt R rtr n R bw i n , n n R i n rr , , i n , fsn fsn fsn R R fs R R rt R i n , i n , i n , i n , i n , Membrane 1 Membrane  Bubble column pmr R i n , sg R pms R i n ,     i n , f s f s n b w Water R R R , iI iH O pmr R vapour i , n i , n i , n 2 i n , pms sweep R regeneration i n , pm R i n , H O H O 2  Balance on permeate condenser / sweep 2 pm pm R R i n , i n , gas recovery pmr R pmr R i n pr , , i n ,        pos R p m p m s s g w o RRRRn n ; , iI iH O opw op i n , R R op pos R i i , n i , n i , n i , n 2 R i i i n ,       p m p m s w o RR R n n ; , iI iH O H O i , n i , n i , n 2 2 9 10

Model Development Model Development  Permeate pressure range for sweep  Permeate pressure range for vacuum  Allowable membrane area  Relative humidity  Sweep gas flow rate  Separation targets- capture ratio and product purity  Target capture ratio  Desired purity 11 12

Model Development  Objective function  Cost of electricity OPEX  Cost of labour  Purchase and installation cost of operational units CAPEX     m i n T A C O P E X C A P E X 12 13

Case Study  Case study (He & Hägg, (2014) ) Parameter Value  Techno economic feasibility study of CC by Flue gas flow rate (kmol/s) 26.6111 FSC membrane Flue gas temperature (°C) 50  Predetermined two membrane stage process Mole fractions of components CO 2 0.137 flow N 2 0.7289  Cascading process flow, no recycle streams H 2 O, 0.0365 O 2 0.0973 Parameter Value Membrane Temperature (°C) 35 Membrane permeance of CO 2 (kmol/m 2 bar.s) 2.48E-05 CO 2 /N 2 selectivity 135 Permeate pressure (bar) 0.25 CO 2 /H 2 O selectivity 1 Retentate pressure (bar) 2 CO 2 /O 2 selectivity 30 13 14

Results and Discussions Results and Discussion Scenario 1 Scenario 2 Scenario 3 Scenario 4 Process flow Predetermined Model determined Model Model determined determined Membrane stages 2 3 3 3 Pressure ratio Parameter Variable Variable Variable Relative humidity Parameter variable Variable variable Permeate pressure Vacuum Vacuum Combination Sweep gas    Recycle streams - 14 15

Results & Discussion Results and Discussion Scenario 1 2 3 4 Scenario 1 2 3 4 Number of mem stages 2 3 3 3 Specific membrane area 7708.1 3348.2 3526.8 3911.0 Capture ratio (%) 90 90 90 90 (m 2 /tCO 2 .h) CO 2 product purity (%) 95 95 95 95 Heat transfer area (m 2 ) 78605.9 112319.2 67405.9 34932.7 TAC (M $) 174,7 144.1 141.8 144.4 CO 2 capture rate (ton/h) 521 521 521.3 521.3 Operating costs, (M $) 46.5 44.8 50.3 52.6 Capital costs (M $) 128,2 99.6 91.5 91.7 Specific power 296 286 321 292 Total membrane (Mm 2 ) 4.05 1.75 1.83 2.04 consumption (kWh /ton) Total net power (MW) 154,6 149.0 167.2 176.1 Specific energy (GJ/tCO 2 ) 1.065 1.03 1.15 1.22 Total power (MW) 208 224 217.5 223.7 TLC ($/tCO 2 ) 44.7 36.8 36.3 36.9 Power recovered by 53.4 75.1 76.9 47.6 % saving on TLC - 17.6 18.7 17.4 expander (MW) 15 16

Results & Discussion  Conclusion  Integration and optimisation will help in making the CCS by FSC membranes more economical  Combination of sweep and vacuum give optimum flow  Membrane area decrease by 56.7%  Cost of capture is reduced by 17%. 16 20

Thank you Natsayi Chiwaye, Thokozani Majozi and Michael Daramola* School of Chemical and Metallurgical Engineering, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, Johannesburg, 2000, South Africa * Corresponding author: michael.daramola@wits.ac.za Tel +2711 717 7536