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Thermodynamics of M u l t i - S t e p

Water

Decomposition Processes

r

James

E.

Funk

\

i

College of Engineer ing

Un ive rsi ty of Kentucky

Le xin gto n, Kentucky 40506

Processes which con ve r t water i n t o hydrogen and oxygen are of i n t e r e s t

f o r many re as on s,

inc lud ing th e many advantages accru ing t o t he

t r a n s -

p o r t of en ergy a s hydrogen. Hydrogen may be used a s

a

sourc e of th er -

m a l or e l e c t r i c a l ene rgy , depending on w hethe r

it

i s burned or used i n

an e lec t roch emic al dev ic e such

as

a f u e l

c e l l .

Hydrogen i s a l s o a

key

raw

material

i n t h e c he m ic al pr o c e s s i n d u s t r i e s

and i n pe tr ol eum r e f in i ng . It i s e s ti m a te d t h a t by 1975 t h e t o t a l

consumption of hydrogen i n t h e

U . S . w i l l

b e

a t

t h e r a t e of f o u r

t r i l -

l i o n c u b i c

f e e t pe r yea r and growing. Gaseous and l i q u id hydrocarbons

are

now th e pr i nc ip a l r a w m a t e r i a l s f o r p ro du ci ng l a r g e q u a n t i t i e s o f

hydrogen by means of e i t h e r c a t a l y t i c steam reforming or p a r t i a l oxi-

d a t i o n .

a r t i f i c i a l n a t u ra l g as ,

w i l l

increase the demand for hydrogen even

more. I t would c l e a r l y b e i n t h e i n t e r e s t o f c o n s e r v a ti o n of n a t u r a l

reso urc es t o develop an economical proce ss t o produce hydrogen f rom

w a t e r .

A

comprehensive stud y of therm al pro ce sse s t o produce hydrogen from

water was performed and re p or te d by Gene ral Motors ( 1 , 2 ) . A t h r e e

s t e p p r oc e s s

i n v o l vi n g e i t h e r t a n ta lu m c h l o r i d e o r bi sm u th c h l o r i d e

and a f ou r s t e p p r oces s u s ing e i t h e r mercury ch l o r id e or vanadium

c h l o r i d e

w e r e

desc r ibed . A ge ne ra l d i sc us s io n of energy requirements

f o r th e decomposition of wa ter was pu bl ish ed by Funk and Reinstrom 3 )

and , more recen t ly , a f o u r s t e p t he rm a l p r o c e s s w a s descr ibed by

deBeni and Marchetti

4 ) . A

re vie w o f t h e c u r r e n t s t a t u s

of

e l e c t r o -

l y t i c hyd rogen as a f u e l has been pub li shed by G rego ry , e t . a l . ( 5 ) .

The p r e s s u r e f o r in e xp e ns iv e a nd p l e n t i f u l p i p e l i n e g a s ,

\

Second L aw L i m i t a t i o n s

I f one gram mole of wa ter of l i q u id wa te r

a t

25OC and 1 atm i s conver-

t e d i n t o one gram mole of hydrogen and one ha lf gram mole of oxygen

a t

25OC and

1

atm t h e g i b b s f u n c t i o n f o r t h e sys tem in cr ea se s by 56.7

kc al , th e en tha lpy inc rea ses by 68.3 kc a l and the en t ropy inc re as es by

39 cal/OK. I f t h e decom position i s done re ve rs ib ly a t 25OC an d1 atm--

4

s ay i n a n e l e c t r o l y s i s c e ll -- 56 .7 k c a l , t h e cha nge i n t h e g i b b s func-

t ion , mus t be supp l i ed

as

u s e fu l work and

11.6 k c a l

t h e d i f f e r e n c e

1

between t he ent hal py change and gibbs fu nc t i on change, m ust be sup-

p l i e d

as

h e a t .

The amount of us e fu l work re qu ir ed may be decreased by ope ra t in g the

si ng le s te p decomposit ion a t some hig her temp erature. The amount

of

energy requi red a s h e a t

w i l l

i n c r e a s e by t h e

same

amount th a t

the

work

r equ i r ed i s decreased under th e b e s t c ase assumpt ions of e qua l spec i -

f i c h e a t s a nd p e r f e c t t h er m al r e g e n e r a t i o n of p r o d u c ts a nd r e a c t a n t s .

\\

t

If the

pr oces s

i s

depicted on a temp erature e ntropy diagram, t h e work

r e d u c t i o n i s equa l t o t h e a r e a enc losed when t he p r oces s l oop i s

cl os ed by allowing t h e c oo le r hydrogen and oxygen

t o

form water

,

I

I

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81

The total work and heat requirements

4) over the I reactions to obtain

I

1

\

i=I

w

=

AG, -

AS(^)

i=l

are obtainedby summing

3)

and

[T i)

-

To] 5 )

Another important feature becomes apparent if the process is divided

into J reactions which have positive entropy changes and L reactions

which have negative entropy changes.

To

minimize the required work,

the first group of reactions

should be operated at some high tempera-

ture,

TH,

and the second group operated at To.

In this case,

j=

As is evident from Eqn. (-71, the required

-

To]

work is zero when

There is no reason why

( 8 ) cannot be satisfied along with

7 )

i=I i=J

l=L

This result cannot be obtained for a single step process, in which

case the zero work requirement must be accomplished by a temperature

manipulation rather than the selection

of

a suitable sequence

of

reactions.

Work of Separation

The theoretical work of separation, AGs, required to separate a mix-

ture of ideal gases into its components is given by

AGs

= -

R T . ~k In Xk 10)

is the number of moles of the kth component and xk is the

whereole fr ction of that camponent.

Fig. 1 shows a reaction process which accomplishes the reaction

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/

82

The amount of

material

e n t e r i n g t h e s e p a r a t o r dep end s o n

E ,

t h e f r a c -

t i o n a l mola r c onve r s i on o f R1, w hich i n t u rn de pe nds on t h e s t a nda rd

f r e e ene rgy change fo r

t h e reaction,

AGR.

t h e s t a n da r d f r e e e n e rg y ch an ge f o r a r e a c t i n g m i x tu r e

of

i d e a l g a s e s

with Eqn. (10) y i e l d s

Combining t h e de f i n i t i o n of

A G ~ A G ~ - R T [ ~

-1 I n

s1 r2

In

s2 --- - C p - C r )

In p*]

(12)

where

and p* i s t h e o p e r a t i n g p r e s s u r e

4 x 1 .

(12)

shows t h a t t h e t h e o r e t i c a l work of s e p a r a t i o n

i s

g r e a t e r

t h a n s t a n d ar d f r e e e ne r gy ch an ge f o r t h e r e a c t i o n . I

An example of th e t h e o re t i c a l work of se pa ra t io n f o r the vanadium

c h l o r i d e p r o c e s s i s shown i n F igs . 2 and 3. Fig. 2 i s

a

schematic of

t h e f i r s t s t a g e i n which t h e r e

i s

a g a s ph as e r e a c t i o n o f c h l o r i n e

w i t h w a t e r

a t

1000K a t

1

atm. P ig . 3 shows th e th eo re t i ca l w o r k of

s e pa ra t i on a nd it may b e noted t h a t th e sep ara t io n work

i s

i nc re a s e d

i f the mi x t u re l e a ve s the r e a c t i o n cham ber a t less t ha n e qu i l i b r i um

c ond i t i ons . F i g. 3 i s

f o r t h e s e p a r a t i o n of a l l four components and

the minimum work requirement i s 9.2 k c a l pe r gram mole of hydrogen

produced.

A

s imilar

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 of o n ly t h e

HC1

and O 2 y i e l d s a work requirement

of

7.1 kca l .

i

The Vanadium Chloride Process

I

Thi s p roc e s s

w a s

s t u d ie d i n c o n si d er a bl e d e t a i l . A p l a n t l a you t was /

The en t i r e vanadium c hl or id e process

i s

shown i n F ig . 4. The

sums

of

t he e n t ha lpy , e n t ropy ,

and f r e e energy changes o n o t e x a c t l y equal

t h o s e f o r w a t e r com pos it ion becau se of q ue st io na bl e thermochemical-data.

made assuming a h el iu m c o o l e d n u c l e a r r e a c t o r a s t h e h e a t s ou rc e.

Estimates

w e r e

made f o r pumping, he a t re gen era t ion , etc. The r e s u l t s

are shown

i n T ab le 1 and, as can b e s e e n , t h i s p r o c e ss

i s

no t as

e f f i c i e n t

as

a

w a t e r

e l e c t r o l y s i s p l an t .

The ob je c t he re i s n o t to d e s c r i b e a n i n e f f i c i e n t p r o ce s s - any number

of such proce sses can be e a s i l y d e v i s e d . It is, ra ther

an a t t e m p t t o

qu i c k l y l o s e t h e i r a ppe a l when subjected t o sa ne what more p ra c t i c a l

co ns ide ra t ion s of work of sep ara t io n , the rmal reg ene ra t io n , pumping

F e r

etc .

Such

a

r e s u l t i s n o t e s p e c i a l l y s u r p r is i n g i n v i e w of the

ob l e c t i ve , w hich , i n i t s most fundamental

t e r m s ,

i s

an a t t em pt t o con-

v e r t h e a t t o u s e f u l work more e f f i c i e n t l y t h a n i n a s ta te of t h e a r t

power plant .

Refe rences

1.

r

i n d i c a t e t h a t p r o c es s e s which may be i n i t i a l l y a t t r a c t i v e

can

q u i t e

F i n a l

Technica l Repor t - Ammonia P rod uct i on F e a s ib i l i t y Study ,

Al l i so n Div i s ion of Genera l Motors ,

EDR

4200, November,

1965.

f

2.

System Study of Hydrogen Gen era tio n by Thermal Energy, A ll is o n

Div i s io n of Genera l Motors EDR 3714, vol . 11, Supplement A, 1964.

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3.

Funk, J.

E. ,

and Reinstrom,

R . M . ,

Energy Requi rements i n t h e

Product ion

of

Hydrogen From Water ,

I

EC Proc. D e s .

D e v . ,

v o l . 5, N o . 3, pp 366-342, Ju ly 1966 .

4 .

deBeni,

G .

and Marchet t i ,

C . ,

Hydrogen, Key

t o

t h e Energy Market ,

Eur o Spe c t r a , 9 ,

46-50,

June, 1970.

Parameter

Maximum helium t em -

p e r a t u r e

Minimum helium t em -

p e r a t u r e

H e l i u m

system pres-

sure

P r o c e s s h e a t i n p u t

T o t a l h e a t re-

5. Gregory, D. P., Ng D.Y.C. , and Long, G .

M.,

t o be pu bl is he d i n The

Elect rochemis t ry of Cleaner Environments, J . O ' M . Bockr is , Ed.,

New York, Plenum P re s s ,

1971.

U n it s Systems Remarks

F

2000

2000 1500 1500

F 37 37

67 67

atm

1 0

1

10

1 H e

p r e s s u r e

drop

e q u a l s

1 0

p s i i n

a l l c a s e s

k c a l 1 5 5

155

475 475

Adknowledgment

This work

w a s

done a t t h e A l l i s o n D i v is i on

of

General

Motors

and was

p a r t i a l l y s u pp o rt ed by

U.S.

Army Engineer

Reac to r s

Group,

F o r t B e l -

v o i r , V i r g i n i a . P e rm i ss io n

t o

p u b l i s h t h e s e

r e s u l t s

i s g r a t e f u l l y

acknowledged.

Tab le

1

Vanadium Chloride

Process

D a t a Tabu la t ion

I

j e c t e d

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85

L

l

n

c

0

L

0

s z

Z

.e,

u

0

Z

a

2

a

. .

m

a,

Lc

a

:

a,

a

Lc

d

JZ

U

4

.

a .

E

m

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8

100

-

0

2

0

N

.L

Temperature

=

IOOOOK

Pressure

=

atm

al l components separated)

0 . 2

0.4 0.6 0.8

1.0

Extent

of

reaction -e

F i g .

3

Th e o re ti c a l Work of Separat ion

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7

112 Q l)

Stage I

g]+Cl, g) -2HCl g )

+

1/2

O, g) a t

1340OF

+

-

77OF

Stage

III

4VCt3 s)+2VCl4

9 )

+OVCC 8 ) a t 1340OF

Stage

Ip:

@

c

2VCI4

I)e

V C J ,

(E) C12 s )

a t

77OF

Enthalpy, entropy, and f ree energy tab ulat ion

values in kilocalories)

. .

Fig Vanadi- Chloride Process

9

O r

-26.71 73.4

-46.0 48.3'

5 3 9

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