Rate-based modelling and simulation of CO 2 absorption and desorption columns using

Transcription

1 Rate-based modelling and simulation of CO 2 absorption and desorption columns using piperazine promoted potassium carbonate Henrik Lund Nielsen Jozsef Gaspar Philip Loldrup Fosbøl Center for Energy Resources Engineering (CERE) Technical University of Denmark (DTU)

2 Introduction ti Challenging to find solvent systems with high capacities, fast absorption rates and low energy requirements Aqueous solutions of potassium carbonate (K 2 CO 3 ) promoted by piperazine pp (PZ) is a promising solvent Pilot experiments show better absorption rates than 7m MEA Risk of precipitation at high solute concentrations Potentially causing operational challenges Causing modelling challenges as well 2

3 Purpose Develop a rate-based model for CO 2 absorption and desorption of columns using aqueous mixtures of PZ and K 2 CO 3 Analyse the effect of different solvent compositions on the column performance variables Absorber: L/G ratio, column height, rich loading Desorber: reboiler energy consumption, lean loading 3

4 Process description Absorption and desorption processes are simulated independently Heat integration ti not analysed 4

5 Modelling 5

6 Physical properties Density and viscosity are correlated and validated against experimental data Surface tension, diffusivities and kinetic rate constants are calculated from correlations in literature Mixture properties are validated against data or estimated using linear mixing-rules 6

7 Rate-based model validation Red dotted lines showing +/- 15% Spec. reboiler duty CO2 flux Average deviation 14.3 % 13.3 % 7

8 Process specifications Variable Value Unit Desorber rich solvent 100 C temp. Desorber rich solvent rate kmol/s Desorber diameter 7 m Desorber height 10 m Desorber pressure kpa Absorber lean temp. 40 C Absorber pressure kpa Flue gas rate kmol/s Flue gas temp. 40 C Flue gas CO2 mole fr Flue gas H2O mole fr Column packing Mellapak - 250Y 8

9 Heat of reaction between CO 2 and solvent Basic thermodynamic analysis of energy requirements 2m capacity means thatt the solvent has a total capacity of 2 molal (mol/kg H 2 O) free CO 2 Small or moderate amounts (kj/molco O2) of K CO 3 tends to increase heat of absorption compared to pure PZ Large amounts may, however, result in a lower heat of absorption Heat of abs m K2CO3/molal 0m PZ 2m capacity 4m capacity 6m capacity 10m capacity 9

10 Energy consumption constant tcapacity Minimum spec. Reboiler duty 10

11 Energy increasingi solvent concentration ti 11

12 Absorber b capture rate behaviour Drop in capture rate observed for some precipitating high-concentration solvents Fixing the capture rate can result in more than one solution at different L/G ratios 12

13 Solid precipitation it ti effect on heat evolvement Drop in capture rate caused by drop in heat of absorption Loading (mol CO2/(mol K2CO3 + 2 mol PZ)) At increasing L/G ratio, the CO 2-10 loading gets lower and solids -20 (mainly KHCO 3 ) dissapear Causes a sudden jump in heat of -30 absorption -40 Column temperature gets lower -50 and reaction rates are slowing down -60 Expected phenomenon: like heat -70 evolved on ice formation. Now -80 seen for precipitation in CO 2 solvent -90 Heat of Abs (kj J/mol CO2) Point of precipitate formation L/G ratio 13

14 Absorber b solvent flow and column height ht 4m capacity solvents capture rate fixed at 90 % 35 2m PZ 3m K2CO3-0.5m PZ 1m K2CO3-1.5m PZ 3.5m K2CO3-0.25m PZ 35 2m PZ 3m K2CO3-0.5m PZ 1m K2CO3-1.5m PZ 3.5m K2CO3-0.25m PZ Column heigh ht (m) Comulm heig ght (m) L/G ratio (mole/mole) Relative rich loading 14

15 Absorber b solvent flow and column height ht 10m capacity solvents capture rate fixed at 90 % Due to jump in capture rate, the graphs for 5m and 7m K 2 CO 3 solvents are not continuous 35 5m PZ 2m K2CO3-4m PZ 5m K2CO3-25m 2.5m PZ 7m K2CO3-15m 1.5m PZ 35 5m PZ 5m K2CO3-2.5m PZ 2m K2CO3-4m PZ 7m K2CO3-1.5m PZ Colu umn height (m m) umn height (m m) Col L/G-ratio (mole/mole) Relative rich loading 15

16 Conclusion A rate-based column model for CO2 absorption into K2CO2/PZ solvents is succesfully developed and tested Desorption analysis The lowest reboiler duties are reached with high K 2CO 3 / low PZ solvents The calculated minimum reboiler duties generally follows the trend of heat of absorption (Figure, slide 9) Addition of K 2 CO 3 results in lower reboiler duties Addition of PZ results in higher reboiler duties 16

17 Conclusion Absorbtion analysis The lowest column height ht requirements and the highesth rich loadings are obtained with low K 2 CO 3 / high PZ solvents Jumps in capture rates are caused by precipitation starting to form In general, the absorber performance seems to be favoured by a high amount of PZ in the solvent, whereas the desorber performance seems to be favoured by high K 2 CO 3 content and a low PZ content Further work Dynamic modelling 17