Separation of Methane from Hydrogen and Carbon Monoxide by an

12

Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: June 16, 1983 | doi: 10.1021/bk-1983-0223.ch012

Separation of Methane from Hydrogen and Carbon Monoxide by an Absorption/Stripping Process VI-DUONG DANG The Catholic University of America, Department of Chemical Engineering and Materials Science, Washington, DC 20064

An absorption/stripping process calculation using propane as the absorption solvent for separation of methane from hydrogen and carbon monoxide has been performed. Detailed material and energy balances for the process as well as the dimensions of the absorber and the stripper are reported. Other major pieces of equipment such as heat exchangers pumps, and compressors were evaluated in order to determine the equivalent electrical energy of the process as approximately 13550 cal(e)/gm-mole methane produced. The purity of methane in the final stream is 96% by volume at 100°F and 1000 psia. The present process appears to be a potential working process for methane separation in large quantity. In c o a l g a s i f i c a t i o n processes such as the f l a s h hydropyr o l y s i s o r t h e E x x o n c a t a l y t i c p r o c e s s , methane i s t h e m a j o r d e s i r e d end p r o d u c t . H o w e v e r , methane i s u s u a l l y p r o d u c e d a l o n g w i t h hydrogen and/or carbon monoxide. To o b t a i n p i p e l i n e g r a d e m e t h a n e , i t i s n e c e s s a r y t o d e v e l o p e c o n o m i c a l methods o f s e p a r a t i n g methane f r o m h y d r o g e n a n d / o r c a r b o n m o n o x i d e m i x t u r e s . The hydrogen and c a r b o n monoxide a r e t h e n r e c y c l e d i n t h e p r o c e s s . S e v e r a l s e p a r a t i o n t e c h n o l o g i e s such as a b s o r p t i o n / s t r i p p i n g , c r y o g e n i c , c l a t h r a t e f o r m a t i o n h a v e been e x a m i n e d ( 1 ) . In t h i s r e p o r t , a method o f a b s o r p t i o n / s t r i p p i n g u s i n g p r o p a n e a s t h e s o l vent i s presented. A b s o r p t i o n / s t r i p p i n g i s a well-known chemical e n g i n e e r i n g o p e r a t i o n and c a n be r e a d i l y d e s i g n e d and c o n s t r u c t e d on a l a r g e s c a l e . T h e r e f o r e , t h e p r e s e n t method c a n be c o n s i d e r e d as a p o t e n t i a l l y u s e f u l method. I n t h e p r e s e n t w o r k , a f e e d g a s m i x t u r e o f 40% CH4, 45% H 2 , and 15% CO a t 1 0 0 ° F and 500 p s i a i s t o be s e p a r a t e d . A production c a p a c i t y o f 250 χ 10^ s c f / d a y o f CH4 i s a s s u m e d . The m e t h a n e i s d e l i v e r e d t o t h e p i p e l i n e a t 1 0 0 ° F and 1000 p s i a . For the a p p l i c a t i o n of a b s o r p t i o n / s t r i p p i n g , i t i s d e s i r a b l e to u t i l i z e a 0097-6156/83/0223-0235$06.00/0 © 1983 American Chemical Society In Industrial Gas Separations; Whyte, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

I N D U S T R I A L GAS


236

SEPARATIONS

s o l v e n t w h i c h h a s a h i g h s o l u b i l i t y f o r methane. T a b l e I shows some s o l u b i l i t y d a t a f o r methane i n w a t e r a n d i n v a r i o u s o r g a n i c solvents. I t i s s e e n t h a t methane h a s a h i g h e r s o l u b i l i t y i n propane than i n any o f t h e o t h e r s o l v e n t s l i s t e d . Therefore, pro pane was c h o s e n a s t h e s o l v e n t f o r t h e a b s o r p t i o n / s t r i p p i n g s e p a r a t i o n process investigated i n t h i s report. O p e r a t i n g c o n d i t i o n s f o r t h e a b s o r p t i o n and s t r i p p i n g towers a r e i m p o r t a n t d e s i g n p a r a m e t e r s f o r t h e p r o c e s s . Due t o v a p o r p r e s s u r e a n d e n t r a i n m e n t , p r o p a n e w i l l be p r e s e n t i n t h e e f f l u e n t gas s t r e a m s f r o m b o t h t h e a b s o r b e r a n d s t r i p p e r . Usually this q u a n t i t y o f propane i s n o t r e c o v e r e d and i s c o n s i d e r e d an economic loss. The amount o f p r o p a n e i n t h e g a s p h a s e i s m a i n l y d e p e n d e n t on t h e o p e r a t i n g t e m p e r a t u r e a n d p r e s s u r e o f t h e t o w e r s . F i g u r e 1 shows t h e v a p o r p r e s s u r e o f some o f t h e r e l e v a n t compounds a s a f u n c t i o n o f t e m p e r a t u r e . A t -230.8°F a n d -184°F, t h e v a p o r p r e s s u r e o f p r o p a n e i s a b o u t 0.1 mm Hg a n d 1 mm Hg respectively. Absorption/stripping process i s a conventionally p r a c t i c a l p r o c e s s so i t i s d e c i d e d t o e v a l u a t e t h e s e p a r a t i o n o f methane f r o m h y d r o g e n a n d c a r b o n m o n o x i d e by t h i s p r o c e s s . Since the o p e r a t i n g p r e s s u r e s o f t h e a b s o r b e r and s t r i p p e r a r e about 500 p s i a a n d 487 p s i a r e s p e c t i v e l y , t h e m o l e f r a c t i o n s o f p r o p a n e i n t h e o u t l e t gas streams o f t h e a b s o r b e r and s t r i p p e r a r e about 6.75 χ 1 0 " a n d 1 χ 10~4 a t -230.8°F a n d -184°F r e s p e c t i v e l y . When t h e a b s o r b e r o p e r a t e s a t h i g h e r t e m p e r a t u r e a s shown i n T a b l e I I , p r o p a n e l o s s e s i n c r e a s e . When t h e a b s o r b e r a n d s t r i p p e r o p e r a t e a t -230.8°F a n d -184°F r e s p e c t i v e l y , t h e p r o p a n e l o s s i s e q u i v a l e n t t o a b o u t 0.0850% and 3.28% r e s p e c t i v e l y o f t h e c o s t o f methane a s s u m i n g t h e c o s t o f methane a n d p r o p a n e i s t h e same. T h i s l o w p e r c e n t a g e o f p r o p a n e l o s s was c o n s i d e r e d a c c e p t a b l e . Hence i t was d e t e r m i n e d t o o p e r a t e t h e a b s o r b e r a t -230.8°F a n d t h e s t r i p p e r a t -184°F. 6

Process Description A s c h e m a t i c p r o c e s s f l o w s h e e t i s shown i n F i g u r e 2. I n l e t g a s , a m i x t u r e o f m e t h a n e , h y d r o g e n a n d c a r b o n m o n o x i d e a t 100°F and 500 p s i a ( s t r e a m 1) i s s u c c e s s i v e l y c o o l e d t o -140°C b y t h e o u t l e t g a s s t r e a m f r o m t h e a b s o r b e r a n d some r e c y c l e g a s . The a b s o r b e r i s a packed column o f 1 - i n . b e r l s a d d l e s w i t h 50% v o i d fraction. The r i c h l i q u i d f r o m t h e b o t t o m o f t h e a b s o r b e r i s h e a t exchanged w i t h t h e bottom l i q u i d from t h e s t r i p p e r . The s t r i p p e r i s a l s o a p a c k e d c o l u m n w i t h 1 - i n . b e r l s a d d l e s . The d i s s o l v e d methane, hydrogen, and c a r b o n monoxide i s s t r i p p e d o u t by h e a t i n g a t t h e bottom o f t h e s t r i p p e r . The o u t l e t g a s s t r e a m f r o m t h e s t r i p p e r i s h e a t e d i n a h e a t e x c h a n g e r by a r e c y c l e g a s s t r e a m a n d i s f u r t h e r c o m p r e s s e d t o p r o d u c e t h e f i n a l methane p r o d u c t a t 100°F a n d 1000 p s i a . To p e r f o r m a f u r t h e r d e t a i l e d p r o c e s s c a l c u l a t i o n o n t h i s multi-component a b s o r p t i o n and s t r i p p i n g p r o c e s s , vapor l i q u i d e q u i l i b r i u m d a t a f o r methane, hydrogen, and c a r b o n monoxide i s

In Industrial Gas Separations; Whyte, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

In Industrial Gas Separations; Whyte, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983. 2.0 41.0 29.5 174 144 129 120 110 83 59 43 32

1.8 29.5 21.5 125 107 97 89 80 56 41 31 23

1.2 19.0 14.5 83 71 63 59 51 33 24 20 16

0.9 9.0 7.5 41 37 33 30 24 15 11 10 8

Water Methanol^ Ethanol Propane Butane Pentane Hexane Octane Cyclohexane Benzene Heavy N a p h t h a Gas O i l 164

— — 140 — 80 — 41

2.6 55.0 38.0 200 ( 9 0 atm)

100

48.0

— — — — — — — — — 62

3.3 ( 1 2 0 atm)

140

i n 1.0 c c s o l v e n t a t p r e s s u r e

(1) The s o l u b i l i t y o f methane i n n - p r o p a n o l , i s o p r o p a n o l , n - b u t a n o l a n d i s o - b u t a n o l i s , w i t h i n e x p e r i m e n t a l e r r o r , t h e same a s i n m e t h a n o l .

80

60

40

77°F a n d 760 mm) d i s s o l v e d

20

ce CH. ( a t 4

The S o l u b i l i t y o f Methane i n W a t e r a n d i n O r g a n i c S o l v e n t s a t 77°F a n d a t P r e s s u r e s up t o 140 A t m o s p h e r e s

P r e s s u r e , atm

Table I .



SEPARATIONS


238

l/T F i g u r e 1.

(K-*)

V a p o r p r e s s u r e o f s e l e c t e d compounds v s .

l/T.


In Industrial Gas Separations; Whyte, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983. 5

4

1.19xl0 1.319xl0 8892

8415 8415 8415

6

5

5

3

2

1.58xl0

4.81xl0

l.llxlO

4.77xl0

4.77xl0

- 76

-112

-148

-200

-230.8

Absorber + Stripper

6

Stripper

2.79xl0

Absorber

lb/day l o s s o f Propane

6

0.085

0.126

1.133

% o f Propane Cost L o s s Assuming Methane and Propane C o s t t o be t h e Same

Propane Losses a t D i f f e r e n t Absorber Temperatures ( S t r i p p e r T e m p e r a t u r e = -184°F) Plant Capacity: 2 5 0 x l 0 SCF/Day

- 58

T(°F)

Table I I .



240

F i g u r e 2. method.


SEPARATIONS

F l o w d i a g r a m f o r methane s e p a r a t i o n b y a b s o r p t i o n / s t r i p p i n g



12.

DANG

Methane

Separation

from Hydrogen

and

Carbon

241

Monoxide

n e e d e d . E q u i l i b r i u m K - v a l u e s o f methane a n d p r o p a n e a r e o b t a i n e d f r o m D e P r i e s t e r (2). S o l u b i l i t y d a t a f o r hydrogen i n propane i s o b t a i n e d f r o m T r u s t a n d K u r a t a (3)· S o l u b i l i t y o f carbon monoxide i n propane i s o b t a i n e d from P r a u s n i t z and S h a i r ( 4 ) . The a b s o r b e r c a l c u l a t i o n i s b a s e d o n 1 l b - m o l e o f g a s s t r e a m e n t e r i n g i n t o t h e b o t t o m o f t h e a b s o r b e r . The e n t e r i n g l i q u i d i s f e d a t a r a t e o f 1.5 l b - m o l e / l b m o l e o f g a s . F o r 9 5 % methane removed, a p r e l i m i n a r y e s t i m a t e o f methane a b s o r b e d i s 0.38 l b m o l e , c a r b o n m o n o x i d e a b s o r b e d i s 0.13 l b - m o l e , h y d r o g e n a b s o r b e d i s 0.1 l b - m o l e . The a b s o r b e r i s o p e r a t e d u n d e r i s o t h e r m a l c o n d i t i o n s o f -146°C. The l i q u i d t o g a s mass r a t i o a t t h e t o p o f t h e t o w e r i s 3.85 w h i l e a t t h e b o t t o m i t i s 2.11 w i t h a n a v e r a g e v a l u e o f 2.98. The k e y component i s m e t h a n e . U s i n g a method o f a v e r a g e a b s o r p t i o n f a c t o r w h i c h f o r methane i s 6.478 a n d 9 5 % methane r e m o v a l , i t i s p o s s i b l e t o d e t e r m i n e t h e number o f t h e o r e t i c a l t r a n s f e r u n i t s t o be 2. W i t h t h e s e c o n d i t i o n s , methane i n t h e o u t l e t g a s s t r e a m o f t h e a b s o r b e r i s 0.002 l b - m o l e , c a r b o n m o n o x i d e i s 0.13 l b - m o l e and h y d r o g e n 0.44 l b - m o l e . The m o l a r c o n c e n t r a t i o n s o f t h e a b s o r b e r e f f l u e n t g a s a r e CH^, 3.5 χ 1 0 ~ ; CO, 0.227; H , 0.769; C3H3, 6.75 χ 1 0 ~ . The m o l a r c o n c e n t r a t i o n s i n t h e l i q u i d p h a s e a r e CH4, 0.101; CO, 0.005; H , 0.003; C H , 0.891. Absorber design r e q u i r e s the c a l c u l a t i o n of the q u a n t i t y 3

2

6

2

L

_G

3

8

w h i c h i s 0.79 f o r t h e p r e s e n t c a s e w h e r e L a n d G a r e mass

P

v e l o c i t i e s o f l i q u i d and g a s , L a n d and g a s . A t f l o o d i n g c o n d i t i o n s

G are densities of l i q u i d

where ap i s s u r f a c e a r e a o f p a c k i n g p e r u n i t tower volume, ε i s f r a c t i o n a l v o i d v o l u m e o f d r y p a c k i n g y£ i s l i q u i d v i s c o s i t y , g i s l o c a l g r a v i t a t i o n a l c o n s t a n t . T h e r e f o r e assuming 60% o f f l o o d i n g c o n d i t i o n s , G i s 2920 I b / h r f t . Since i t i s desired to p r o d u c e 250 χ 1 0 ^ s c f / d a y methane, t h e i n l e t g a s f l o w i n t o t h e a b s o r b e r i s 2.35 χ 10° l b / h r . From t h i s a n d t h e g a s mass v e l o c i t y , i t i s d e t e r m i n e d t h a t 3 t o w e r s a r e n e e d e d , e a c h one w i t h a d i a m e t e r o f 18.5 f t . The t o w e r h e i g h t i s d e t e r m i n e d by t h e h e i g h t o f a t r a n s f e r u n i t w h i c h i s 20.7 f t . (_5) . T h e r e f o r e , t h e h e i g h t o f t h e p a c k e d t o w e r i s 41.4 f t . O t h e r d e t a i l e d t o w e r c h a r a c t e r i s t i c s a r e g i v e n i n Table I I I . The s t r i p p i n g t o w e r i s o p e r a t e d a t -120°C a n d 487 p s i a a n d t h e m o l a r r a t i o o f l i q u i d t o g a s i s 9.7. A g a i n t a k i n g methane a s t h e k e y component, and 9 5 % methane s t r i p p e d , a b o u t 0.0962 l b m o l e c

2




242

Table I I I .

SEPARATIONS

Summary o f D i m e n s i o n s a n d C h a r a c t e r i s t i c s of A b s o r b e r s and S t r i p p e r s

Absorbers Diameter o f Absorber Number o f A b s o r b e r H e i g h t o f P a c k e d Tower P r e s s u r e Drop i n P a c k e d W a l l T h i c k n e s s o f Tower

Tower

= = = = =

18.5 f t 3 41.4 f t 1.94 p s i a 2.98 i n . Strippers

Diameter o f S t r i p p e r Number o f S t r i p p e r H e i g h t o f P a c k e d Tower P r e s s u r e Drop i n Packed W a l l T h i c k n e s s o f Tower

Tower

= = = =

17.6 f t 3 22.8 f t 0.41 p s i a 2.35 i n .


12.

DANG

Methane

Separation

from Hydrogen

and

Carbon

243

Monoxide

o f m e t h a n e , 0.0045 l b - m o l e o f c a r b o n m o n o x i d e , 0.002 l b - m o l e o f hydrogen a r e s t r i p p e d . The p r e l i m i n a r y e s t i m a t e s o f t h e l i q u i d t o gas r a t i o i s 26.7 a t t h e t o p and 24.0 a t t h e b o t t o m w i t h a n a v e r a g e o f 25.4. The a v e r a g e s t r i p p i n g f a c t o r f o r methane i s 21.2 and two t r a n s f e r u n i t s a r e needed. Under t h e s e c o n d i t i o n s 0.0046 l b - m o l e o f m e t h a n e , 0.0032 l b - m o l e o f c a r b o n m o n o x i d e , and 0.00036 l b - m o l e of hydrogen remain i n t h e bottom l i q u i d phase. I n the bottom o u t l e t stream o f t h e l i q u i d phase, mole f r a c t i o n s o f t h e c h e m i c a l s p e c i e s a r e CH4, 0.0051; CO, 0.0036; H , 0.0004; C H g , 0.991. The o u t l e t m o l e f r a c t i o n o f t h e g a s p h a s e f r o m t h e s t r i p p e r i s CH4, 0.960; CO, 0.019; H , 0.021; C H , 0.0001. To d e s i g n t h e s t r i p p e r , a g a i n methane i s c h o s e n a s t h e k e y component. 2


2

P

L {G

3

i s t a k e n t o be 7.81.

3

g

At

flooding

^0.2 0.0018 g

P

P

c g L

2 T h e r e f o r e , t h e g a s mass v e l o c i t y i s 1372 l b / h r f t f o r 60% f l o o d i n g c o n d i t i o n w i t h a gas mass f l o w o f 1.28 χ 1 0 l b / h r . It is d e t e r m i n e d t h a t 3 t o w e r s a r e r e q u i r e d e a c h one 17.6 f t i n d i a m eter. The h e i g h t o f a t r a n s f e r u n i t i s a 11.4 f t and t h e t o t a l h e i g h t o f a p a c k e d t o w e r i s 22.8 f t . P r e s s u r e drop i n the tower i s 0.41 l b / i n . A summary o f t h e c o m p o s i t i o n o f v a r i o u s s t r e a m s i s g i v e n i n T a b l e I V and s t r i p p e r c h a r a c t e r i s t i c s a r e g i v e n i n Table I I I . A thermal energy b a l a n c e of the p r o c e s s i s performed f o r t h e c o o l e r s , f o r s t r e a m s 1 and 2 i n F i g u r e 2 and t h e h e a t e x c h a n g e r between s t r e a m s 5 and 7. The h e a t i n p u t f o r t h e h e a t e x c h a n g e r between t h e a b s o r b e r and t h e s t r i p p e r i s c o n s i d e r e d t o be n e g l i gible. The e l e c t r i c a l power i n p u t t o t h e c o m p r e s s o r i s 5135 horsepower. The t h e r m a l e n e r g y l o a d f o r t h e h e a t e r b e t w e e n s t r e a m s 13 and 9 i n F i g u r e 2 i s 2.2 χ 1 0 c a l / m i n and f o r t h e c o o l e r a t t h e o u t l e t gas s t r e a m f r o m t h e a b s o r b e r i s 5.06 χ 1 0 c a l / m i n . A f u r t h e r breakdown o f t h e t h e r m a l energy r e q u i r e m e n t o f t h e p r o c e s s i s g i v e n i n T a b l e V. Detailed calculations are given i n the Appendix. C o m b i n i n g t h e t h e r m a l e n e r g y and t h e pumping and c o m p r e s s i n g power r e q u i r e d f o r t h e p r o c e s s a s shown i n F i g u r e 2, t h e t o t a l t h e r m a l e n e r g y r e q u i r e d i s 30955 c a l / g - m o l e w h i c h i s a b o u t 11.8 t i m e s h i g h e r t h a n t h e minimum i d e a l s e p a r a t i o n e n e r g y r e q u i r e d w h i c h i s c a l c u l a t e d t o be 2621 c a l / g - m o l e CH^. 6

2

7

7



1000

487

487

487

-230.8

-184

-184

100

100

110

-110

-100

90

90

4

5

6

7

8

9

10

11

12

13

500

487

487

485

487

485

500

487

-230.8

-230.8

500

2

100

1

Pressure psia

6

7.1xl0

2xl0

8

6

1.77xl0

7.1xl0

7.1xl0

6

3.06xl0

3.06xl0

8.12xl0

3.06xl0

8.48xl0

2.23xl0

5.64xl0

5.64xl0

a . day y day /

Rate

8

0.0035

0.0035

0.0035

0.0035

0.0035

0.019

0.021

0.96

(4.64xl0 )

0.019

0.021

0.96

7

0.769

0.769

0.769

0.769

0.769

0.227

0.227

0.227

0.227

0.227

0.0036

0.0004

0.0051

7

0.019

0.021

0.96

0.0051

0.0025

8

0.227

0.1013

0.769

0.15

0.15

CO

7

0.0035

0.45

0.4

2

0.45

H

0.4

CH, 4

Mole F r a c t i o n H

0

0

3 8

6

6.75xl0"

6.75xl0"

6.75xl0"

6.75xl0"

6.75xl0"

0.0001

0.0001

0.999

0.0001

0.891

6.75xl0"

C

of Different

8

6

(8.68xl0 )

6

(9.5 x l O )

6

7

7

7

7

(2.32xl0 )

Flow

T e m p e r a t u r e , P r e s s u r e , F l o w R a t e , and M o l e F r a c t i o n Numbered S t r e a m s i n t h e F l o w D i a g r a m

3

Temp °F

Stream Number

Table IV.


6

6

6

6

6


Energy Requirement

=

cal(t)/g-mole

T o t a l T h e r m a l and E q u i v a l e n t E l e c t r i c a l

Energy

= =

cal(t)/g-mole cal(t)/g-mole

= =

cal(e)/g-mole cal(t)/g-mole

T h e r m a l E n e r g y S u p p l i e d t o H e a t e r B e t w e e n S t r e a m s 13 and 9 Thermal Energy Load t o C o o l e r i n A b s o r b e r O u t l e t

Thermal Energy Requirement

T o t a l Pumping Power/g-mole methane p r o d u c e d E q u i v a l e n t T o t a l T h e r m a l Pumping P o w e r / g - m o l e CH^

= = = = =

(Horsepower) (Horsepower) (Horsepower) (Horsepower)

E n e r g y R e q u i r e m e n t f o r t h e S e p a r a t i o n o f Methane by A b s o r p t i o n / S t r i p p i n g U s i n g P r o p a n e

C o m p r e s s o r Power ( H o r s e p o w e r ) Pumping Power R e q u i r e d f r o m B o t t o m o f A b s o r b e r t o Top o f S t r i p p e r Pumping Power R e q u i r e d f r o m B o t t o m o f S t r i p p e r t o Top o f A b s o r b e r Pumping Power o f I n l e t Gas B l o w e r E s t i m a t e d Pumping Power f o r F l u i d T r a n s p o r t

Electrical

T a b l e V.


30955

36.5 84.1

12334 30834

5135 2.78x10^ 3.3x105 4973 1796

INDUSTRIAL

246

GAS

SEPARATIONS

Acknowledgment P a r t o f t h e w o r k was p e r f o r m e d a t B r o o k h a v e n N a t i o n a l o r a t o r y a n d was a c k n o w l e d g e d h e r e b y .

Lab-

Literature Cited 1.


2.

3. 4. 5.

Dravo Corporation "Hydrogasification Gas Processing Studies"; Contract CPD-7209, February 27, 1978. DePriester, C. L. "Light-Hydrocarbon Vapor-Liquid Distribution Coefficients, Pressure-Temperature-Composition Charts and Pressure-Temperature Nomographs"; Chem. Eng. Progress Symp. 1953, 7, 49. Trust, D. B . ; Kurata, F. "Vapor-Liquid Phase Behavior of the Hydrogen-Carbon Monoxide-Propane Systems"; AIChE J . 1971, 17, 86. Prausnitz, J . M.; Shair, F. H. "A Thermodynamic Correlation of Gas Solubilities"; AIChE J . 1961, 7, 682. Bennett, C. O.; Myers, J . E. "Momentum, Heat and Mass Trans fer"; 2nd ed., McGraw-Hill Co., 1974, pp. 571-574.


DANG

12.

Methane

Separation

from Hydrogen

and Carbon

Monoxide

247

Appendix


Energy C a l c u l a t i o n f o r t h e S e p a r a t i o n o f Methane from P r o c e s s G a s i f i c a t i o n Stream U s i n g Propane A b s o r p t i o n / S t r i p p i n g The e n e r g y c a l c u l a t i o n s f o r t h e p r o c e s s a r e b a s e d o n e l e c t r i c a l and t h e r m a l energy. I t i s assumed t h a t t h e h e a t e x c h a n g e r b e t w e e n t h e a b s o r b e r and t h e s t r i p p e r o p e r a t e s i d e a l l y s o n o n e t heat l o s s o r g a i n w i l l be r e a l i z e d i n t h i s u n i t . The e l e c t r i c a l e n e r g y r e q u i r e m e n t s a r e f o r ( 1 ) t h e pumps b e t w e e n t h e a b s o r b e r a n d the s t r i p p e r , (2) t h e i n l e t gas blower, (3) t h e compressor a t t h e end f o r t h e p r o d u c t methane, a n d ( 4 ) pumping power f o r f l u i d transport. Thermal energy i s s u p p l i e d f o r (1) t h e h e a t e r between s t r e a m s 9 and 1 3 , a n d ( 2 ) t h e c o o l e r a t t h e o u t l e t o f t h e a b s o r b e r . The c a l c u l a t i o n s w i l l b e p e r f o r m e d s e p a r a t e l y i n t h e f o l l o w i n g sections. A.

Pumping Power R e q u i r e d t o T r a n s p o r t F l u i d f r o m B o t t o m o f A b s o r b e r t o Top o f S t r i p p e r

ο L i q u i d f l o w r a t e = 8.48 χ 10 l b / d a y 8.48xl0

m

8

l b / d a y χ 7.48 g a l / f t

36.52 l b / f t = 7.24xl0 D e n s i t y o f l i q u i d = 36.52

6

3

χ 24 h r / d a y

gal/hr = 1.21xl0 lb/ft

3

5

gal/min

3

H e i g h t o f s t r i p p i n g t o w e r p a c k i n g i s 22.8 f t . C o n s i d e r heading o f t h e tower i s 20% a d d i t i o n a l . Then t h e t o w e r h e i g h t ΔΖ i s 27.4 f t . The f l u i d i s t r a n s p o r t e d b y 30 p i p e s a n d t h e d i a m e t e r o f t h e p i p e i s a b o u t 0.25 f t . The e n e r g y b a l a n c e f o r each p i p e i s 2 ΔΖ + 0.8 W g

+

s

= 0 S

c

(A-l)

D C

8

. u 4 V e l o c i t y u i n each p i p e =

8.48xl0 x4 j 24x36.52 χ π χ 3 0 χ . 2 5

= 6.56xl0 Re = 6

=

Ρ

5

f t / h r = 182.4 f t / s e c

182.4 f t / s e c χ 0.25 f t χ 36.52 l b / f t «ι ο ί ο " 36.52 , - -4 1.2x10 χ ^ ^ χ 6.72x10

3 =

3

8 e

5

3

x

2

0n n

2

American Chemical Society Library t155 16th St. N. W.

In Industrial Gas Separations; Whyte, T., et al.; Washington, 0. C. Society: 20039 ACS Symposium Series; American Chemical Washington, DC, 1983.

l

0


248

SEPARATIONS

R e l a t i v e roughness f a c t o r and f r i c t i o n f a c t o r a r e t a k e n t o be 0.0006 a n d 0.004, r e s p e c t i v e l y . Therefore, the f r i c t i o n a l loss term i s 2

ι l

w

f

2x0.004x182.4 x37.4 32x0725

=

1

=

1

2

0 4

/ 4

_

/ f

t

l

b

-, f /

/ l

1

K

b

From E q . ( A - l ) , we c a n c a l c u l a t e t h e l o s s p e r s e t o f p i p e s ( 1 0 p i p e s ) c o n n e c t i n g each a b s o r b e r and s t r i p p e r i s w


s The

total

. - ( 1 2 4 4 χ 10 + 27.4) . O.o

5

5

8

4

f

t

l

/

l

b

r

=

8

l b / d a y χ 5.05χ1θ"

Τ

7

hr/ft-lb

f

24 h r / d a y = 2.78xl0

Β.

b

e l e c t r i c a l power r e q u i r e d f o r t h e 30 p i p e s i s

15584 f t - l b / l b χ 8 . 4 8 x l 0 F

1

Ρ

5

Pumping Power R e q u i r e d t o T r a n s p o r t F l u i d f r o m B o t t o m o f S t r i p p e r t o Top o f A b s o r b e r

Liquid flow rate = 8.12xl0

8

8.12xl0

8

36.52 l b / f t

=

1.15xl0

5

lb/day l b / d a y χ 7.48 g a l / f t 3

χ 24 h r / d a y

3

χ 60 m i n / h r

gal/min

A g a i n we t a k e 2 0 % a d d i t i o n a l h e i g h t f o r t h e h e i g h t o f t h e a b s o r b e r w h i c h h a s a p a c k i n g h e i g h t o f 41.4 f t so t h e t o t a l h e i g h t o f t h e a b s o r b e r i s 50 f t . L e t ' s a p p l y t h e same m e c h a n i c a l e n e r g y b a l a n c e Eq. ( A - l ) t o c a l c u l a t e t h e pumping power h e r e . Velocity of the fluid i s 8.12xl0 x4 8

v

m

m

24x36.52 χ π χ 3 0 x . 2 5

R

e

m

uDp μ

m

6

β

2

9

χ

1

0

5

174.9x0.75x36 52 l b / f t 1.2x10

f

t

/

h

r

=

1

7

4

#

9

f

t

/

s

e

c

2

χ Ι'. 62.4

3 m

χ 6.72x10

U s i n g t h e r e l a t i v e r o u g h n e s s f a c t o r a n d f r i c t i o n f a c t o r t o be 0.0006 a n d 0.004 a g a i n . The f r i c t i o n a l l o s s i n t h e p i p e i s


12.

DANG

Methane

ι l

w

f

=

Separation from Hydrogen

2x0.004x174.9 x50 32.2x.25

.

=

1

5

2

c

o

and Carbon

_ -

n

0

f

t

l

b

/

f

l

b

So t h e t o t a l e n e r g y r e q u i r e d p e r s e t o f p i p e ( 1 0 p i p e s ) each s t r i p p e r and a b s o r b e r i s w

S

_ - ( 1 5 2 0 χ 10 + 50) U.ο

=

1

9

0

6

3

_

f t

l b

249

Monoxide

/

l

connecting

b

I

The t o t a l pumping power i s 19063 f t - l b / l b χ 8 . 1 2 x l 0

8

l b / d a y χ 5.05χ1θ"

f


P

=

24

f

hr/day

Ρ = 3.3xl0 C.

Ρ hr/ft-lb

7

5

I n l e t Gas B l o w e r 7 Incoming gas t o absorber

3

i s 2.32x10

f t /day. Main f l o w r a t e a t

2 i s 2920 l b / h r f t a t a d e n s i t y PM Ρ = „ L , Τ = 230°F = 127°K g Ζ RT* ° av

absorber

a

K

CH, : 4

Τ

CO :

T

H

2

Ζ M

av av

5

T

r

= τ ί τ = 0.665, Ρ = = 0.7426, Z„ = 0.56 191 ' r 14.7x45.8 c

r

- § 1 - 0.955, P

fl

r " 3 0

3

" -

8 1 4

'

P

=

f

= 0.986,

2

r « ïOxfes

= '

6 5 7

- 0.32

' \

' °-

9 8

= 0.4x0.56 + 0.15x0.32 + 0.45x0.98 = 0.71 = 0.4x16 + 0.15x28 + 0.45x2 = 11.50

g G_ p

g

m

0.71x10.73x230

2970 l b / h r f t 3.28

lb/ft

2 =

^

= 3.28

f

t

/

h

r

=

J

lb/ft

Q

2

5

f

t

/

s

e

c

3


INDUSTRIAL

250

GAS

SEPARATIONS

Number o f t u b e s i n a b s o r b e r = η _

2.32xl0

7

1.231x10

3

ft /day χ 4 2 2 24 h r / d a y χ π χ nD f t

R

e

uDp ~7T~

=

342

=

2

μ

nD

6 f t / h r =|

2

2.38x10 nD

Dx3.28 l b / f t co =4 0.007 x 6 . 7 2 x 1 0

X

8

P

nD

ft ft-lb lb 291 2 5 η D


sec 1

W

f

=

_ 2fLU g D c

2 = =

2x0.004x10 f t 32.2 D X

n D

342 2

f

2

l e t η = 3 0 , D = 0.25 f t F r i c t i o n a l l o s s / t u b e = 330.63

g c B

gas f l o w = 2 . 3 2 x l 0

7

ft-lb /lb f

(A-2)

ΔΖ + l w + nW =0 f s

3

ft /day

r

(=5.64xl0

7

lb/day)

P r e s s u r e d r o p i n t o w e r = 1.3 i n H 0 / f t d e p t h , ΔΖ - 41.4 f t 2

1.3x41.4 f t H 0 £_ 12 o

T o t a l p r e s s u r e drop

λ

= 4.49 f t - l b . / l b f

g

A p p l y i n g E q . (A-2) t o o n e s e t o f p i p e ( 1 0 p i p e s ) f o r i n c o m i n g g a s i n t o t h e a b s o r b e r , one g e t s (41.4+4.49) + 330.63x10 + nW

W

s

=

4.49+41.4+3306.3 0.8

4190.2

- 0

ft-lb lb f

lb 7 sec

5.64x10 E n e r g y r e q u i r e d = 4 hr/day7*3600 s e c / h r

4190.2 X

D.

4

550

2

9

7

3

Γ

C o m p r e s s o r a t End o f P r o d u c t M e t h a n e Between S t r e a m s 7 a n d 8 k-1 IP =

3.03x10 k-1

p

l

q

f

h.

k

" -x


(A-3)

12.

DANG

Methane

Separation

from Hydrogen

and

Carbon

251

Monoxide

C where

k =

= 1.31 f o r methane ν

For t h e present

ρ

case

.

3

^3xl0"

5 χ

4

8

5

6

l

b

/

i

n

2

χ

M

4

3


= 4108

I f t h e compressor i s 80% e f f i c i e n t , = 5135 P. E.

Pumping Power f o r F l u i d

2

/ f t

2

.31" .31 ( ΙΟΟοΥ

4.64xl0 f t / d a y 24 h r / d a y χ 60 m i n / h r Ρ

± n

- 1

Ρ

t h e n t h e power r e q u i r e d

Transport

Pumping power i s r e q u i r e d t o t r a n s p o r t f l u i d i n t h e s t r e a m s 9, 1 0 , and 1 3 . Gas f l o w r a t e and d e n s i t y i n t h e s e s t r e a m s a r e 7 . 1 x l 0 l b / d a y and 0.44 l b / f t , r e s p e c t i v e l y . U s i n g a 0.25 f t diameter pipe, v e l o c i t y i n the pipe i s 6

3

7-lxlO 30x0.44 l b / f t uDp Re = — - = μ

6

lb/day χ 4

=

χ 24x3600 χ π χ

1

2

?

f

t

/

g

e

c

.25

127x2.5x0.44

- 8 τ = 2.65x10 0

7T (.0183x.23+.009x.77) χ

χ

n

6.72x10

R e l a t i v e roughness f a c t o r and f r i c t i o n f a c t o r a r e a g a i n t a k e n t o be 0.0006 and 0.004, r e s p e c t i v e l y . Assuming t h e t o t a l t r a n s p o r t l i n e a s 600 f t , f r i c t i o n a l l o s s i n f l u i d t r a n s p o r t i s 2 2fu L "TD~ c

ι l

w

f

=

2 2x0.004x127 x 6 0 0 32.2x0.25

=

=

Q 9

6

1

,7

n

C4 f

t

_ "

W i t h 8 0 % e f f i c i e n c y o f t h e pump, t o t a l e n e r g y l b ^ / l b and t h e pumping power r e q u i r e d i s 1.2xl0 P

4

=

ft-lb /lb f

χ 7.1xl0

6

24 = 1796

1 U l

b

/ 1 U

f

/

l

b

4 l o s s i s 1.2x10 f t -

l b / d a y χ 5.05χ1θ"

7

Ρ

-hr/ft-lb

hr/day

Ρ


f

INDUSTRIAL

252

F.

Thermal Energy R e q u i r e d i n Heat Exchanger and 9

Q = 7.1xl0

6

lb/day χ 7 Btu/lb-mole 8

= 1.13xl0

Btu/day

= 2.2xl0

7

cal/min


Thermal energy 7

B e t w e e n S t r e a m s 13

° F / 7 . 8 9 l b / l b - m o l e (110-90) ° F

χ 252 c a l / B t u j24

8

2.2xl0

SEPARATIONS

Btu/day

= 1.3xl0

=

GAS

s u p p l i e d t o t h e heat

h r / d a y χ 60 m i n / h r

exchanger

c a l / m i n χ 24 h r / d a y χ 60 m i n / h r

χ 16 g/g-mole

7

3 . 0 6 x l 0 l b / d a y χ 454 g / l b =36.5 cal(t)/g-mole «

G.

Energy

Required t o t h e Cooler i nAbsorber O u t l e t

(-Q) = 2 . 2 3 x l 0

7

lb/day χ 7 Btu/lb-mole

Stream

°F /9.73 l b / l b - m o l e

(-156 + 146) χ 1.8 = 2.89xl0

8

= 2.89xl0

8

Btu/day

= 5.06xl0

7

cal/min

Energy

Btu/day χ 252 c a l / B t u J 24 h r / d a y χ 60 m i n / h r

supplied to the cooler

_ 5.06xl0

7

c a l / m i n χ 24 h r / d a y χ 60 m i n / h r 3.06xl0

7

χ 16 g/g-mole

l b / d a y χ 454 g / l b

I t i s a l s o i n t e r e s t i n g t o r e p o r t t h e t h e o r e t i c a l minimum e n e r g y requirement f o r t h e present process P

W

= RT Σ x . F I n min,Τ j j Λ

x

2 P

jF l

For t h e p r e s e n t p r o c e s s , c o m p o s i t i o n temperature and p r e s s u r e o f i n c o m i n g g a s a r e : C H , 0.4; H , 0.45; CO, 0.15; 100°F a n d 500 psia, respectively. Temperature and p r e s s u r e o f f i n a l p r o d u c t g a s e s a r e a t 100°F a n d 1000 p s i a . For complete s e p a r a t i o n 4

2


12.

W

DANG

Methane

Separation

from Hydrogen

and Carbon

Monoxide

0

. = 1 . 9 8 B t u / l b - m o l e °Rx560°R [0.4 I n J " ? ^ min,Τ 0.4x500 _ι_ η / «: ι + 0.45 1η

1000 .- .45x500

. -, c 1000 + 0.15 1η J .15x500 n

Ί

Ί

Ίc

g

n

Separation of Methane from Hydrogen and Carbon Monoxide by an

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