co2 capture process by amit chakma 1997

8/4/2019 Co2 Capture Process by Amit Chakma 1997

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~ Pergamon

Energy Convers. Mgmt Vol. 38, Suppl., pp. $51-$56, 1997© 1997 ElsevierScience Ltd. All rights reserved

Printed in Great BritainPIh S0196-8904(96)00245-2 o196-8904/97 $17.00 + o.o0

C O 2 C A P T U R E P R O C E S S E S -O P P O R T U N I T I E S F O R I M P R O V E D E N E R G Y E F F IC I E N C I E S

A m i t C h a k m a

Env i ronm en ta l S ys t em s Eng inee r ing , F acu l ty o f Eng inee r ing

The Un ive rs i t y o f Reg ina , Reg ina , S aska tchewan , Canada

A B S T R A C T

CO 2 c a p t u r e f r o m f l u e g a s s t r e a m s a n d d i s p o s a l i n t o g e o l o g i c a l f o r m a t i o n s h a s b e e n

cons ide red a s a t echn ica l ly feas ib l e bu t a cos t ly op tion fo r t he reduc t ion o f CO2 em iss ion in to

the a tm osphere . CO2 cap tu re i s the m a jo r cos t com ponen t . There fo re , the re i s cons ide rab le

incen t ive i n f i nd ing en e rgy e f f i c i en t and thus l e ss cos t ly p rocesses fo r t he cap tu re o f CO2 as

com pared to t he conven t iona l m onoe thano lam ine (ME A) based p rocesses . In t h is pape r , som e

s t ra teg i es fo r red uced ene rgy consum pt ion in a chem ica l so lven t based sepa ra t ion p rocess a re

ident i f ied and thei r impacts on the overal l process are d iscussed . © 1997 Elsevier Science Ltd

K E Y W O R D S

CO2 C ap tu re , Energ y Ef f i c iency , Mixed S o lvent , Enha nced S t ripp ing

I N T R O D U C T I O N

A m o n g t h e s e v e r a l o p t i o n s a v a i a l a b l e f o r t h e r e m o v a l o f C O 2 f r o m f l u e g a s s t r e a m s

abso rp t ion by chem ica l solven ts i s by fa r the m os t popu la r one . How ever , o the r op t ions m ay

a l so p rov ide som e a t t rac tive a l t e rna t ives t o t he chem ica l abso rp t ion p rocess . F o r exam ple , a

s tudy by M IT (1989) found tha t 02 com bus t ion com bined wi th f l ue gas recyc l ing m ay be a

cheap er op t ion . S o fa r nea r ly a l l t he feas ib il i ty s tud ies have c ons ide red " o f f - the -she l f" ME A

b a s e d t e c h n o l o g i e s f o r C O 2 r e m o v a l f r o m f l u e ga s e s a n d c o n c l u d e d t h a t C O 2 r e m o v a l a n d

d i sposa l w ou ld be ve ry expens ive (S m el se r e t . al , 1991; P adam sy and Ra i l t on , 1993). T he

im pl i ca t ion o f t hese s tud ies a re t ha t CO2 rem o va l i s no t a cos t e f fec t ive op t ion . Ho we ver ,

conc lus ions based on the adap ta t ion o f " o f f - the -she l f" t echno logy su i t ab l e fo r na tu ra l gas

p rocess ing fo r f l ue gas sepa ra t ion w i th l i t tl e m od i f i ca tion shou ld no t be accep ted a s t he " f ina l

word" i n t h i s m a t t e r . A num ber o f p rocess m od i f ica t ions can be m ade w h ich cou ld reduce the

CO 2 sepa ra t ion cos t . S om e o f t hese op t ions a re desc r ibed in the fo l low ing sec t ions o f t h is

paper .

$51



$ 5 2 C H A K M A : C O ,_C A P T U R E P R O C E S S E S

O P T I M I Z E D P R O C E S S D E S I G N

A s impl i f ied f lowshee t o f a chem ica l so lven t based CO2 separa t ion un i t i s shown in F igure 1 .

The f low shee t i s a s l igh t ly modi f i ed ver s ion o f a typ ica l M EA un i t fo r the t r ea tmen t o f f lue

gas s t r eams. However , t he r e a r e a f ew excep t ions . F i r s t , i s t he p r esence o f a p r e - con tac to r

be tw een the b low er and the absorber , and second , t he re a r e two s t eam in jec t ion l ines in the

str ipper.

SWEET GAS

ABSORBER

L E A N S O L V E N T ~ 1

FLUE

G A S : L . ., .J " " - ] , , , , r ',

BLOWER PRE 1CONTACTOR

STEAM

_ 1n

m

- ' ~ S T R A F E R

Figure 1: A s impl i f ied f lowshee t o f an op t imized CO2 separa t ion un i t.

P re -Con tac to r

The p re - con tac to r i s an on l ine mixer where the f lue gas s t r eam i s mixed wi th l ean so lven t

so lu t ion p r io r to Rs en t r ance in to the bo t tom sec t ion o f the absorber . I ts m ain ro l e i s t o r educe

the he igh t o f t he absorber by absorb ing the bu lk o f the CO2 f rom the f lue gas s t r eam. Le an

so lven t i s i n j ec t ed in to the f low ing gas s t r eam ups t r eam o f the p r e - con tac to r th rough a h igh

e f f i c i e n c y n o z z l e i n t h e f o r m o f f i n e d r o p le t s. B o t h t h e f l u e g a s s t r e a m a n d t h e s o l v e n t

d rop le t s f low co-cur r en t ly . I n t imate mix ing o f the amine d rop le t s and the f lowing f lue gass t r e a m o c c u r d u r i n g t h e i r p a s s a g e t h r o u g h t h e p r e - c o n t a c t o r . A p r o p e r l y d e s i g n e d p r e -

con tac t ing s ys t em c an eas i ly p rov ide separa t ion equ iva len t to ha l f an equ i l ib r ium s t age to a

fu l l equ i l ib r ium s t age. Th i s imp l i es tha t t he num ber o f ac tua l s tages r equ i r ed can be r e duce d

by 5 to 10 stages.

S i n c e t h e f i n e a m i n e d r o p l e t s a r e n o w l o a d e d w i t h C O 2 , t h e i r c a r r y - o v e r i n t o t h e m a i n

con tac t ing sec t ion o f the absorber co lum n can have se r ious nega t ive e f fec t s on the separa t ion

process . There i s a need to p l ace a mis t e l imina t ion dev ice a t t he bo t tom o f the absorber . The

mis t e l imina to r adds to the overa l l p r essu re d rop and thus inc reases the wo rk load o f the

b lower .



CHAK MA: CO2 CAPTURE PROCESSES $53

Live Steam Iniection

The flowsheet shown in Figure 1 also indicates external steam injection at two points, one at

the bottom, and one at the top of the regenerator tower. The purpose of live steam injection is

two fold. First, it enhances the stripping process by providing additional heat and stripping

vapor further to those generated in the reboiler, and secondly, it reduces the solvent

degradation rate by reducing the acid gas loading of the solvent before it enters the reboiler.

The net effect is that the cyclic capacity of the solvent, expressed as the difference in the acid

gas loading of the lean and the rich solvent increases, thus reducing the solvent circulation

rate for a given separation duty. Typical cyclic capacity of MEA for a standard design is about

0.2 mole of CO2/mole of MEA. With live steam injection this can be increased to over 0.3

mole of CO2/mole of MEA, which represents a reduction of solvent circulation rate in excess

of 50%.

MIXED SOLVENTS

The use of mixed solvents in the natural gas processing industry is now very common. Many

of the older plants using conventional solvents have been converted to mixed solvents either

to increase the throughput of the plant or to solve operations problems such as corrosion.

However, in many cases the switch over to mixed solvents has also led to considerable energy

savings. Most of the proprietary solvents marketed by the major solvent manufacturers are

based on mixed amines. By judicious choice of an amine mixture, process ef ficiency of the

gas separation plants can be enhanced significantly.

Regeneration Energy Reouirements:

The main source of energy consumption in a chemical solvent based CO 2 separation process

is the regenerat ion process. As much as 80% of the total energy is consumed during solvent

regeneration.

The total energy required to regenerate a CO2 loaded solvent can be expressed as follows:

Total Energy = Heat of Reaction + Sensible Heat + Latent Heat of Vaporization of Water

+ Latent Heat of Vaporization of the Solvent (Partial).

In the regeneration step, first the rich solvent temperature must be raised to the stripper

temperature by sensible heat transfer. The amount of heat required for this process is dictated

by the specific heat capacity of the solvent which does not vary much among the various

solvents. In addition, the water component of the solvent must also be vaporized to generate

the stripping vapor. While the specific heat capacity and the latent heat of vaporization of

water remains the same for all solvents, the energy required for this step depends on the

proportion of water present in a given solvent. The higher the water content, the greater the

energy requirement for this step. For example, the energy required for the vaporization of

water would be greater for a 30 wt% MEA solution than that of a 50 wt% TEA solution.



$ 54 C H A K M A : C O_ , C A P T U R E P R O C E S S E S

F i n a l ly , p a r t o f t h e s o l v e n t i t s e lf w o u l d a l s o b e v a p o r i z e d . T h e a m o u n t o f e n e r g y r e q u i r e d b y

v a r i o u s s o l v e n t s f o r t h i s p a r t c a n b e c o m p a r e d t o t h e l a t e n t h e a t s o f v a p o r i z a ti o n . F i n a l l y ,

s u f fi c i en t h e a t m u s t b e p r o v i d e d t o b r e a k u p t h e C O 2 - s o l v e n t c o m p l e x f o r m e d d u r i n g t h e

a b s o r p t i o n p r o c e ss . T h i s c a n b e a c c o u n t e d f o r b y t h e h e a t o f r e a ct i on .

Hea t s o f r eac t ion o r en tha lp ies o f so lu t ion fo r va r ious so lven t s a r e p rov ided in T ab le 1 .

T a b l e 1 :

S o l v e n t

Hea t s o f r eac t ion ( en tha lp ies o f so lu t ion ) fo r va r ious so lve n t s

C o n c e n t r a t i o n ( M ) C O 2 L o a d i n g

M E A 5 ( 3 0 w t % ) 0 . 4

D E A 3 .5 ( 3 6 w t % ) 0 . 4 6 5

T E A 3 . 35 ( 5 0 w t % ) 0 . 5 6 2

4 . 2 8 ( 5 0 w t % )D E A 0.5

E n t h a l p y o f S o l u t i o n

( k J / m o l o f C O 2 )

7 2

53.2

A s c a n b e s e e n f r o m T a b l e 1 , M E A ' s h e a t o f r e a c t i o n ( e n t h a lp y o f s o l u t i o n ) i s h i g h e r

t h a n t h a t o f a l l t h e o t h e r s o l v e n t s l i s t e d . A s a r e s u l t m o r e e n e r g y m u s t b e p r o v i d e d t o

r e g e n e r a t e M E A t h a n t h e o t h e r l e s s r e a c ti v e s o l v e n ts .

I n a d d i t i o n t o t h e e n t h a l p y o f s o l u ti o n , M E A ' s l a t en t h e a t o f v a p o r i z a t i o n i s a l s o h i g h e r

t h a n t h a t o f o t h e r l e ss a c t i v e s o l v e n t s a s s h o w n i n t h e T a b l e 2 b e l o w .

T ab le 2 : L a ten t Hea t s o f Vapo r iza t ion fo r va r ious so lven t s .

S o l v e n t

M E A

D E A 6 7 0

T E A 5 3 5

M D E A

H e a t o f V a p o r i z a ti o n

( k J / k ~ )

8 26

5 50

B a s e d o n t h e h e a t s o f r e a c t i o n a n d t h e l a t e n t h e a t s o f v a p o r i z a t i o n , i t i s c l e a r t h a t M D E A

w o u l d o f f e r b e t t e r e n e r g y s a v i n g s c o m p a r e d to M E A . H o w e v e r , t h e p r o b l e m is t h a t th e m a s s

t r a n s fe r r a te o f M D E A i s l o w e r t h a n th a t o f M E A .

T a b l e 3 s h o w s t y p i c a l v a l u e s o f t h e f ir s t o r d e r r a te c o n s t a n t f o r t h e o v e r a l l r e a c t i o n b e t w e e n

v a r i o u s a m i n e s a n d C O 2 . A m o n g t h e c o n v e n t i o n a l a m i n e s , M D E A h a s t h e l o w e s t k l " o f a ll .

T he re fo re , i t s abso rp t ion r a te fo r CO2 i s a l so low er than fo r o ther am ines .



Table 3:

CHAKM A: CO., CAPT URE PROCESSES $55

Typical values of the overall forward rate constant for CO2-amine reactions.

AMINE TYPE

MEA

DEA 1500

DIPA 400

TEA

MDEA

kl

(mols/L.s)

7600

16.8

9.2

In general, the higher the rate of reaction, the greater the mass transfer rates. As can be seen

from Table 3, one can choose various amines and their blends to achieve a desired mass

transfer rate. For example, assuming the diffusional resistances to be the same, MEA will

provide the highest mass transfer rate while MDEA will result in the lowest rate among theamines listed in Table 3. This is why MEA is considered to be the best solvent for CO2

removal from flue gases. This higher reactivity also implies that more heat is required to

regenerate the MEA solvent.

At a first glance, one might expect that the overall reaction rate of CO2 with a blend of

different amines will depend primarily on the proportion of each amine present in the mixture.

However, this is not always the case. Due to the interactions between various amines and

their reacted species, the overall mass transfer rate can be enhanced significantly by the

addition of rate promoters or catalysts. Regeneration energy requirement for such a mixed

solvent can be reduced by as much as 30% compared to that of MEA (Chakma, 1995).

Mass Transfer Enhacement with Mixed Amines

The mass transfer enhancement with mixed amines may be explained by the so called

"shuttle mechanism". The shuttle mechanism for the case of CO2 absorption in aqueous

mixtures of primary and tertiary amines is illustrated in Figure 2. At the gas-liquid interface

both the amines react with CO2. However, due to its higher reactivity, more of the primary

amine is depleted at the interface than the tertiary amine. Figure 2 neglects the amount of

tertiary amine that reacts with CO2 at the interface for the sake of simplicity. Thus it depicts

only the reaction between CO2 and the primary amine RNH2 at the interface. The reacted

primary amine in its protonated (RNH3 +) and carbamate forms (RNCOO-) travels from the

interface to the bulk of the solution and then transfers the CO2 to the un-reacted tertiary amine

R3N. The primary amine is therefore regenerated and return to the interface to pick up more

CO2. This enhances the overall absorption rate.



$ 5 6 C H A K M A : C O , . C A P T U R E P R O C E S S E S

C O 2

G A S F I L M

:-2 -2 --2 .2 -2 --2 -2 --2 -2 -2 ~ -2 -2 -2 -2 --2 2 2 ~ 2 ~ 2 ~ 2 ~ 2 2 ~ 2 ~ 2 ~ 2 ~ 2 ~ 2 ~ S ~ 2 ~ 2 ~ 2 2 2 2 S ~ 2 ~ 2 ~ 2 ~ 2 2 2 ~ 2 ~ 2N T E R F A C E+

2 R N H + C O - . ~ R N H + R N C O O2 2 I ' ' ~ 3

~ L I Q U I D F I L M

I " IN H + R N C O O

3

+

R N

3 B U L K

2 R N H + H C O " + R N H + LIQUID2 3 3

. . . . . . . . . . . . . . . . . . . . . . . h . ° ° . . . . . o . . . . . . . • . . . . . . . ° ° . . . ° i . . . . . . . • . . . . . •° °° . ° o° • .

Figure 2 . A schemat i c r ep resen ta t ion o f the shu t tl e mec han i sm fo r CO2 absorp tionby an aqueous so lu tion con ta in ing a p r imary and secondary amine .

M E M B R A N E - S O L V E N T H Y B R I D PR O C E S S

The p re - con tac to r show n in F igure 1 can be r ep laced by a conven t iona l mem brane as has been

sugge s ted fo r the separa t ion o f CO2 f rom h igh p ressu re na tu ra l gas s t r eams. S ince o f f - the -

she l f mem brane b ased p rocesses a r e p r essu re d r iven , f o r a mem brane- so lven t hybr id p rocess

to be used in the separa t ion o f CO2 f rom f lue gas s t reams, p r essu re o f the f lue gas needs to be

increased s ign i f ican t ly . The co s t o f such an opera t ion ou tweighs any benef i t t o be ga ined f rom

the r educed so lven t c i r cu la t ion r a t e . As such , membrane- so lven t hybr id p rocesses a r e no t

sui table for CO2 separat iuon f rom f lue gas s t reams.

C O N C L U S I O N

A num ber o f op t ions fo r the r educ t ion o f r egenera t ion energy r equ i r ed in a chem ica l so lven t

based CO2 separa t ion p rocess have be en iden t i fi ed . The y inc lude the use o f a p r e - con tac to r ,

in j ec t ion o f ex te rna l s t eam and the u t i l i za tion o f mixed so lven ts . The co mb ined e f f ec t o f a l l

t hese op t ions can r educe the r egenera t ion energy r equ i r ement by as much as 30% com pared to

t h e c o n v e n t i o n a l M E A p r oc e s s.

R E F E R E N C E S

.

2.

3.4 .

A. Chak ma , Ene rgy Con vers. M gmt. , 36 (6-9) , 427-430 (1995).

M IT R epor t M IT-EL-89-003 , M IT , Cambr idge , M A (1989) .

R. Pa dam sey and J . Rai l ton, Ene rgy Convers. Mgm t. , 34 (9- I I ) , 1165-1175 (1993).C. Sm elser , S .C. Stock and G.J . McC leary, EP RI Rep or t IE-7365, Vol . 1 (1991).

co2 capture process by amit chakma 1997

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