Rate-based modelling and simulation of CO absorption and

Rate-based modelling and simulation of CO2absorption and desorption columns using piperazine promoted potassium carbonatepiperazine promoted potassium carbonate

Henrik Lund Nielsen – Jozsef Gaspar – Philip Loldrup Fosbøl

Center for Energy Resources Engineering (CERE) –Technical University of Denmark (DTU)

I t d tiIntroduction•Challenging to find solvent systems with high capacities, fast absorption rates and low energy requirementsabsorption rates and low energy requirements

•Aqueous solutions of potassium carbonate (K2CO3) promoted by piperazine (PZ) is a promising solventp p ( ) p g

–Pilot experiments show better absorption rates than 7m MEA•Risk of precipitation at high solute concentrations

i ll i i l h ll–Potentially causing operational challenges–Causing modelling challenges as well

DTU Chemical Engineering, Technical University of Denmark 07 August 20152

PPurpose•Develop a rate-based model for CO2 absorption and desorption of columns using aqueous mixtures of PZ and K COcolumns using aqueous mixtures of PZ and K2CO3

•Analyse the effect of different solvent compositions on the column performance variablesp

–Absorber: L/G ratio, column height, rich loading–Desorber: reboiler energy consumption, lean loading


P d i tiProcess description•Absorption and desorption processes are simulated independently

H t i t ti t l d–Heat integration not analysed


M d lliModelling


Ph i l tiPhysical properties•Density and viscosity are correlated and validated againstexperimental dataexperimental data

•Surface tension, diffusivities and kinetic rate constants are calculatedfrom correlations in literature

•Mixture properties are validated against data or estimated usinglinear mixing-rules


R t b d d l lid tiRate-based model validation

Spec. reboiler duty CO2 fluxRed dotted lines showing +/- 15%


Average deviation 14.3 % 13.3 %Red dotted lines showing +/ 15%

P ifi tiProcess specificationsVariable Value UnitDesorber rich solventtemp.

100 °Cp

Desorber rich solvent rate 12.90 kmol/sDesorber diameter 7 mDesorber height 10 mDesorber pressure 162.0 kPaDesorber pressure 162.0 kPaAbsorber lean temp. 40 °CAbsorber pressure 101.3 kPaFlue gas rate 3.228 kmol/sFl t 40 °CFlue gas temp. 40 °CFlue gas CO2 mole fr. 0.1325 -Flue gas H2O mole fr. 0.1211 -Column packing Mellapak -

250Y


H t f ti b t CO d l tHeat of reaction between CO2 and solvent•Basic thermodynamic analysis of energy requirements

2 it th t th–2m capacity means that thesolvent has a total capacity of 2 molal (mol/kg H2O) free -20

0

O2)

CO2

–Small or moderate amountsof K2CO3 tends to increase -60

-40

(kJ

/mol

CO

0m PZ

2m capacityof K2CO3 tends to increaseheat of absorption comparedto pure PZ

L t

-80

Hea

t of

abs

. p y

4m capacity

6m capacity

10m capacity

–Large amounts may,however, result in a lower heatof absorption -120

-100

0 2 4 6 8 10

H


m K2CO3/molal

E ti t t itEnergy consumption – constant capacity

Minimum spec. Reboiler duty


E i i l t t tiEnergy – increasing solvent concentration


Ab b t t b h iAbsorber – capture rate behaviour•Drop in capture rate observed for some precipitatinghigh concentration solventshigh-concentration solvents

–Fixing the capture rate can result in more than one solution at different L/G ratios


S lid i it ti ff t h t l tSolid precipitation effect on heat evolvement•Drop in capture rate caused by drop in heat of absorption 0 0 1 0 2 0 3 0 4 0 5 0 6

Loading (mol CO2/(mol K2CO3 + 2 mol PZ))

drop in heat of absorption–At increasing L/G ratio, the CO2

loading gets lower and solids( i l KHCO ) di 20

-10

00 0.1 0.2 0.3 0.4 0.5 0.6

(mainly KHCO3) dissapear–Causes a sudden jump in heat of

absorption-40

-30

-20

J/m

ol C

O2)

–Column temperature gets lowerand reaction rates are slowingdown -60

-50

Hea

t of

Abs

(kJ

Point of precipitateformation

–Expected phenomenon: like heat evolved on ice formation. Now seen for precipitation in CO2

l t

-80

-70

L/G ratio


solvent

13

-90

Ab b l t fl d l h i htAbsorber – solvent flow and column height•4m capacity solvents – capture rate fixed at 90 %

35

2m PZ 1m K2CO3 - 1.5m PZ

3m K2CO3 - 0.5m PZ 3.5m K2CO3 - 0.25m PZ35

2m PZ 1m K2CO3 - 1.5m PZ

3m K2CO3 - 0.5m PZ 3.5m K2CO3 - 0.25m PZ

20

25

30

ht

(m)

20

25

30

gh

t (m

)

10

15

20

Col

um

n h

eig

h

10

15

20

Com

ulm

hei

g

0

5

5 7 9 11 13 15 17 190

5

0.5 0.6 0.7 0.8 0.9 1


L/G ratio (mole/mole) Relative rich loading

Ab b l t fl d l h i htAbsorber – solvent flow and column height•10m capacity solvents – capture rate fixed at 90 %

D t ”j ” i t t th h f 5 d 7 K CO l t t –Due to ”jump” in capture rate, the graphs for 5m and 7m K2CO3 solvents are not continuous

5m PZ 2m K2CO3 - 4m PZ

5m K2CO3 2 5m PZ 7m K2CO3 1 5m PZ

5m PZ 2m K2CO3 - 4m PZ

5m K2CO3 - 2.5m PZ 7m K2CO3 - 1.5m PZ

25

30

35

m)

5m K2CO3 - 2.5m PZ 7m K2CO3 - 1.5m PZ

25

30

35

m)

15

20

25

um

n h

eig

ht

(m

15

20

um

n h

eig

ht

(m

0

5

10Col

u

0

5

10Col


0 5 10 15 20L/G-ratio (mole/mole)

0.2 0.4 0.6 0.8 1Relative rich loading

C l iConclusion•A rate-based column model for CO2 absorption into K2CO2/PZ solvents is succesfully developed and testedsolvents is succesfully developed and tested

•Desorption analysis–The lowest reboiler duties are reached with high K2CO3 / low PZ solventsg 2 3 /–The calculated minimum reboiler duties generally follows the trend of heat of

absorption (Figure, slide 9)–Addition of K2CO3 results in lower reboiler dutiesAddition of K2CO3 results in lower reboiler duties–Addition of PZ results in higher reboiler duties


C l iConclusion• Absorbtion analysis

Th l t l h i ht i t d th hi h t i h l di–The lowest column height requirements and the highest rich loadings areobtained with low K2CO3 / 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 highamount of PZ in the solvent, whereas the desorber performance seems to be favoured by high K2CO3 content and a low PZ contentseems to be favoured by high K2CO3 content and a low PZ content

•Further work–Dynamic modelling


Documents

Rate-based modelling and simulation of CO absorption and