Foundations of Biochemical Engineering - American Chemical Society

13

Downloaded by UNIV OF MISSOURI COLUMBIA on April 15, 2013 | http://pubs.acs.org Publication Date: January 18, 1983 | doi: 10.1021/bk-1983-0207.ch013

Macroscopic Thermodynamics and the Description of Growth and Product Formation in Microorganisms J. A. ROELS Delft University of Technology, Netherlands Central Organization for Applied Scientific Research, T.N.O., P.O. Box 108, 3700 A C ZEIST, The Netherlands

From the point of view of macroscopic thermodynamics living organisms are energy transducers c o n v e r t i n g a source of energy,e.g. chemical substances or photons, i n t o other forms o f energy. As such they are subject to the c o n s t r a i n t s posed by the first and second laws o f thermodynamics. As micro organisms are open systems and as such e x i s t i n a s t a t e o u t s i d e e q u i l i b r i u m , n o n - e q u i l i b r i u m thermo dynamics provide the p e r f e c t v e h i c l e f o r a first approach to the d e s c r i p t i o n of t h e i r behaviour. The concept o f the thermodynamic e f f i c i e n c y of growth i s developed and it is shown t h a t , as a r u l e of thumb, the maximum observed e f f i c i e n c i e s are about 0.65 i r r e s p e c t i v e the nature of the energy supplying process. A number of notable exceptions are shown to be most probably caused by l i m i t a t i o n s other than a v a i l a b l e energy. The nature o f growth and product formation i s discussed i n terms o f the c o u p l i n g o f the transformation of a given amount of substrate energy i n t o biomass energy to the energy obtained from a flow of e l e c t r o n s to a l e v e l of high to a l e v e l of low energy. The treatment i s shown to r e s u l t i n a r e l i a b l e r u l e of thumb f o r a first estimate o f the order of magnitude of the growth y i e l d o f an organism feeding on a given energy supplying t r a n s f o r m a t i o n process.

The b i o s p h e r e o n e a r t h i s , t h e r m o d y n a m i c a l l y s p e a k i n g , i n a c e r t a i n s e n s e an o p e n s y s t e m . I t r e c e i v e s e n e r g y f r o m t h e s u n i n t h e f o r m o f r a d i a t i o n . The e n e r g y o f t h e p h o t o n s r e a c h i n g e a r t h i s , i n p a r t , converted t o chemical energy i n a process c a l l e d 0097-6156/83/0207Ό295$08.00/0 © 1983 American Chemical Society

In Foundations of Biochemical Engineering; Blanch, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


296

BIOCHEMICAL

ENGINEERING

p h o t o s y n t h e s i s . F o r the l a r g e r p a r t t h i s energy takes the form o f c a r b o h y d r a t e s , e.g. s u g a r s , s t a r c h e s and c e l l u l o s i c s . The components o f t h e b i o m a s s o f t h e p r i m a r y p r o d u c e r s a r e t h e s t a r t i n g p o i n t o f a wide v a r i e t y o f t r a n s f o r m a t i o n s , which takes p l a c e under c a t a l y t i c a c t i o n o f l i v i n g organisms. As a f i n a l r e s u l t t h e s o l a r e n e r g y i s c o n v e r t e d t o l e s s energy r i c h forms o f r a d i a t i o n w h i c h t r a n s p o r t energy t o o u t e r space o r i s used t o d e c r e a s e t h e e n t r o p y o f t h e b i o s p h e r e . The r e a s o n i n g d e v e l o p e d above shows t h a t t h e b i o s p h e r e i s a s y s t e m , w h i c h i s s u b j e c t t o a f l o w o f e n e r g y . The e n e r g y e n t e r s t h e s y s t e m a t a l o w e n t r o p y l e v e l and l e a v e s i t a t a s u b s t a n t i a l l y h i g h e r e n t r o p y l e v e l . As s u c h t h e b i o s p h e r e c a n m a i n t a i n a s t a t e w i t h a n e n t r o p y l o w e r t h a n t h e maximum c o r r e s p o n d i n g t o t h e r m o d y n a m i c e q u i l i b r i u m and p r o c e s s e s known as " l i f e " r e s u l t (J_). A s i n g l e organism o r a s p e c i e s f e e d i n g on a g i v e n energy s u p p l y i n g p r o c e s s e x i s t s i n much t h e same p o s i t i o n as t h e b i o s p h e r e as a w h o l e . I t i s an open s y s t e m t h r o u g h w h i c h e n e r g y f l o w s f r o m a low e n t r o p y s t a t e , e.g. c h e m i c a l e n e r g y s t o r e d i n compounds more r e d u c e d t h a n C 0 , t o a h i g h e n t r o p y s t a t e , e.g. h e a t a t a l o w t e m p e r a t u r e l e v e l . As a r e s u l t t h e o r g a n i s m s c a n m a i n t a i n a s t a t e o u t s i d e t h e r m o d y n a m i c e q u i l i b r i u m and c a n continue performing the processes c h a r a c t e r i s t i c f o r t h e i r " l i f e " . As o r g a n i s m s a r e s y s t e m s w h i c h e x i s t o u t s i d e t h e r m o d y n a m i c e q u i l i b r i u m and i r r e v e r s i b l e p r o c e s s e s a r e t a k i n g p l a c e , t h e f o r m a l i s m o f thermodynamics of i r r e v e r s i b l e p r o c e s s e s c o n s t i t u t e s the l o g i c a l v e h i c l e t o t r e a t t h e i r b e h a v i o u r . I n the p r e s e n t a r t i c l e t h e f o r m a l i s m w i l l be b r i e f l y s u m m a r i z e d f o r t h e p u r p o s e o f i t s a p p l i c a t i o n t o m i c r o o r g a n i s m s engaged i n g r o w t h and p r o d u c t f o r m a t i o n . F o r a more t h o r o u g h t r e a t m e n t o f t h e b a s i c f o r m a l i s m t h e r e a d e r i s r e f e r r e d t o t h e s t a n d a r d t e x t s ( 2 - 4 ) and e a r l i e r w o r k o f t h e p r e s e n t a u t h o r ( 5 , 6^). 2

Macroscopic

t h e r m o d y n a m i c s and p r o c e s s e s

i n open s y s t e m s .

05, 6)

For t h e purpose o f the present a n a l y s i s o f m i c r o b i a l metabolism, a g i v e n amount o f m i c r o o r g a n i s m s i s c o n s i d e r e d t o be an e n e r g y t r a n s d u c e r . I t i s s c h e m a t i c a l l y r e p r e s e n t e d i n f i g . 1. The s y s t e m e x c h a n g e s c h e m i c a l e n e r g y and h e a t w i t h t h e e n v i r o n m e n t . F o r s i m p l i c i t y ' s s a k e t h e c a s e o f p r o c e s s e s i n v o l v i n g radiâtional e n e r g y i s e x c l u d e d . The b a s i c f o r m a l i s m , h o w e v e r , c a n be e a s i l y extended t o i n c l u d e these s i t u a t i o n s . The s t a t e o f t h e s y s t e m c a n be c h a r a c t e r i z e d by a number o f e x t e n s i v e q u a n t i t i e s ; t h e s e s p e c i f y t h e amount o f t h e v a r i o u s c h e m i c a l s u b s t a n c e s and t h e amount o f e n e r g y p r e s e n t i n t h e s y s t e m . F o r e a c h e x t e n s i v e q u a n t i t y , w h i c h c a n be a t t r i b u t e d t o t h e s y s t e m , a b a l a n c e e q u a t i o n c a n be f o r m u l a t e d ; i t e x p r e s s e s t h e a c c u m u l a t i o n o f t h e q u a n t i t y i n s i d e t h e s y s t e m as t h e sum o f t h e c h a n g e s o f i t s amount due t o t r a n s f o r m a t i o n and t r a n s p o r t p r o c e s s e s r e s p e c t i v e l y . M a t h e m a t i c a l l y t h i s c a n be e x p r e s s e d a s :



13.

ROELS

Macroscopic

Thermodynamics

and

Growth

\ HEAT

Figure 1. An open system for macroscopic analysis. It exchanges chemical sub stances and heat with the environment. The flow of chemical substances, Φι, is characterized by the elemental composition and the partial enthalpy of the chemical substances.


297

298

BIOCHEMICAL

ENGINEERING

(1) V

V

s


I n t h e f o r m u l a t i o n o f eqn. (1) i t i s assumed t h a t t h e volume and t h e s u r f a c e a r e a o f t h e s y s t e m do n o t change. I n t h e p r e s e n t a r t i c l e t h e d i s c u s s i o n w i l l be r e s t r i c t e d t o s y s t e m s i n a s t a t i o n a r y s t a t e , i . e . systems f o r which the time d e r i v a t i v e a p p e a r i n g a t t h e l e f t hand s i d e o f eqn. (1) has become z e r o . I n s u c h a c a s e eqn. (1) c a n be s i m p l i f i e d t o : (2) V W i t h r e s p e c t t o the t r a n s f o r m a t i o n p r o c e s s e s open to a g i v e n n o n - e q u i l i b r i u m system the e x t e n s i v e q u a n t i t i e s c h a r a c t e r i z i n g t h e s y s t e m can be d i s t i n g u i s h e d i n t o two g r o u p s : c o n s e r v e d and non-conserved q u a n t i t i e s . S o - c a l l e d conserved q u a n t i t i e s cannot be p r o d u c e d o r consumed i n t h e t r a n s f o r m a t i o n p r o c e s s e s o p e n t o a g i v e n s y s t e m . T h e r e f o r e , t h e f i r s t t e r m a t t h e r i g h t hand s i d e of eqn. (1) i s n e c e s s a r i l y z e r o and eqn. (2) c a n be s i m p l y w r i t t e n as : (3)

E q u a t i o n (2) e x p r e s s e s t h e f a c t t h a t f o r e a c h c o n s e r v e d q u a n t i t y t h e t r a n s p o r t t o a s y s t e m i n s t a t i o n a r y s t a t e must e x a c t l y match t r a n s p o r t from t h a t system. For a non-conserved q u a n t i t y such s i m p l i f i c a t i o n i s not p o s s i b l e . The a p p l i c a t i o n o f t h e f o r m a l m a c r o s c o p i c t h e o r y t o t r a n s f o r m a t i o n p r o c e s s e s i n o p e n s y s t e m s i s b a s e d on t h e f o r m u l a t i o n o f b a l a n c e e q u a t i o n s f o r a number o f c o n s e r v e d q u a n t i t i e s and an a d d i t i o n a l t h e r m o d y n a m i c c o n s t r a i n t a l l o w i n g t h e f o r m u l a t i o n of a u s e f u l e f f i c i e n c y measure. The e l e m e n t a l b a l a n c e e q u a t i o n s . In m i c r o b i a l conversion p r o c e s s e s t h e amounts o f t h e v a r i o u s a t o m i c s p e c i e s a r e c o n s e r v e d . T h i s o b s e r v a t i o n r e s u l t s i n the f o r m u l a t i o n of e l e m e n t a l balance e q u a t i o n s . U s i n g eqn. (3) t h e e l e m e n t a l b a l a n c e e q u a t i o n f o r a t o m i c s p e c i e s j c a n , a g a i n a s s u m i n g a s t a t i o n a r y s t a t e , be e x p r e s s e d as ( 5 , 6 ) : η Σ Φ.«*. .= 0 i-i 1

(4)

1 J


13.

Macroscopic

ROELS

Thermodynamics

and

Growth

299


I n e q n . ( 4 ) Φ. s t a n d s f o r t h e n e t m o l a r f l o w o f compound i t o t h e s y s t e m , e. . s t a n d s f o r t h e number o f m o l e s o f a t o m i c s p e c i e s j i n one m J l e o f compound i . E q u a t i o n s o f t h e t y p e o f eqn. ( 4 ) p r o v i d e one c o n s t r a i n t t o t h e n e t exchange f l o w s f o r e a c h atomic species considered. The thermodynamic c o n s t r a i n t s . The a p p l i c a t i o n o f n o n - e q u i l i b r i u m thermodynamics t o t r a n s f o r m a t i o n p r o c e s s e s i s based on t h e f o r m u l a t i o n o f two b a s i c b a l a n c e e q u a t i o n s . The f i r s t o n e , a b a l a n c e e q u a t i o n f o r e n e r g y , c a n , by v i r t u e o f t h e f a c t t h a t the f i r s t l a w o f t h e r m o d y n a m i c s a s s u r e s e n e r g y t o be a c o n s e r v e d q u a n t i t y i n any s y s t e m , f o r a s y s t e m i n s t a t i o n a r y s t a t e be expressed as: Φ

Ε

= 0

(5)

i n w h i c h Φ^ i s t h e n e t f l o w o f e n e r g y t o w a r d s t h e s y s t e m . Thermodynamics show t h a t , f o r a n open s y s t e m o n w h i c h no work i s p e r f o r m e d by e x t e r n a l f o r c e f i e l d s , t h e e n e r g y f l o w t o w a r d s t h a t s y s t e m c a n be e x p r e s s e d as f o l l o w s : Φ

Φ

Ε - Η

+

Σ

Φ

Α

(

6

)

1

I n t h i s e q u a t i o n Φ^ i s t h e s o - c a l l e d h e a t f l o w o f P r i g o g i n e ( 2 ) and t h e h. a r e t h e p a r t i a l m o l a r e n t h a l p i e s o f t h e compounds exchanged w i t h t h e environment. I f eqns. ( 5 ) and ( 6 ) a r e combined t h e f a m i l i a r b a l a n c e equation f o r enthalpy i s obtained. I t allows the c a l c u l a t i o n of the heat exchanged w i t h t h e environment from t h e f o l l o w i n g equation: Φ

Η

= - Σ Φ h.

(7)

1

Equation ( 7 ) i n t r o d u c e s one new unknown f l o w , t h e h e a t f l o w Φ^, and one a d d i t i o n a l c o n s t r a i n t , hence t h e t o t a l number o f unknown f l o w s does n o t change by t h e a p p l i c a t i o n o f t h e f i r s t l a w o f thermodynamics. A second r e s t r i c t i v e e q u a t i o n r e s u l t s from t h e b a l a n c e e q u a t i o n f o r e n t r o p y . F o r a system i n s t a t i o n a r y s t a t e the b a l a n c e equation f o r entropy r e s u l t s i n the f o l l o w i n g expression:

i n w h i c h Tig i s t h e t o t a l e n t r o p y p r o d u c t i o n i n t h e s y s t e m , Φ 0

δ

ENGINEERING

(9)

Furthermore, be w r i t t e n as :

t h e r m o d y n a m i c s show t h a t t h e e n t r o p y f l o w c a n

0 i

(12)

ί

The p a r t i a l m o l a r g

i

=

h

i "

T

s

f r e e e n t h a l p y i s d e f i n e d by: ( 1 3 )

i

E q u a t i o n (12) w i l l be shown t o a l l o w t h e f o r m u l a t i o n o f an e f f i c i e n c y m e a s u r e , w h i c h c a n be used t o a n a l y s e g r o w t h and p r o d u c t f o r m a t i o n i n m i c r o o r g a n i s m s , i t s d e v e l o p m e n t w i l l be undertaken i n the next s e c t i o n . The t h e r m o d y n a m i c

efficiency.

The e n e r g y and e n t r o p y c o n t e n t o f c h e m i c a l s u b s t a n c e s . The t h e r m o d y n a m i c t h e o r y o u t l i n e d above c a n , i n p r i n c i p l e , be s t r a i g h t f o r w a r d l y a p p l i e d t o t h e d e s c r i p t i o n o f m i c r o b i a l growth and p r o d u c t f o r m a t i o n . I n o r d e r t o p e r f o r m s u c h an a n a l y s i s , t h e r m o d y n a m i c d a t a a r e needed r e g a r d i n g t h e compounds w h i c h a r e exchanged w i t h t h e environment, i . e . t h e p a r t i a l molar e n t h a l p i e s



13.

ROELS

Macroscopic

Thermodynamics

and

Growth

301

and e n t r o p i e s o f t h e compounds i n v o l v e d i n t h e p r o c e s s e s i n t h e s y s t e m need t o be known. I n p r i n c i p l e t h e p a r t i a l m o l a r q u a n t i t i e s , e n t h a l p y a s w e l l as e n t r o p y , a r e f u n c t i o n s o f t h e c o n c e n t r a t i o n s o f e a c h and e v e r y c h e m i c a l compound p r e s e n t , i . e . d e f i n i t e v a l u e s c a n n o t be a t t r i b u t e d t o a s i n g l e compound. I n the approximation t o the e n e r g e t i c s o f m i c r o b i a l growth presented h e r e , i d e a l i t y o f t h e m i x t u r e o f compounds i n v o l v e d w i l l be assumed. T h i s i m p l i e s t h e p a r t i a l m o l a r t h e r m o d y n a m i c q u a n t i t i e s to be e q u a l t o t h e s p e c i f i c m o l a r q u a n t i t i e s , t h e s e l a t t e r q u a n t i t i e s depend o n i n t e n s i v e v a r i a b l e s l i k e t e m p e r a t u r e and p r e s s u r e and t h e c o n c e n t r a t i o n o f t h e compound c o n s i d e r e d o n l y . To a good d e g r e e o f a p p r o x i m a t i o n , e n t h a l p y c a n , a t a g i v e n t e m p e r a t u r e and p r e s s u r e , be assumed i n d e p e n d e n t o f t h e c o n c e n t r a t i o n o f t h e compound u n d e r c o n s i d e r a t i o n . The f r e e e n t h a l p y i s , h o w e v e r , by v i r t u e o f t h e e n t r o p y c o n t r i b u t i o n t o t h a t q u a n t i t y ( e q n . 1 3 ) , d e f i n i t e l y c o n c e n t r a t i o n d e p e n d e n t . The f o l l o w i n g relationship holds: g

L

= g? + R T l n C

(14)

i

i n w h i c h g? i s t h e f r e e e n t h a l p y a t a g i v e n t e m p e r a t u r e and p r e s s u r e and u n i t c o n c e n t r a t i o n o f t h e compound, w h i c h i s c a l l e d standard free enthalpy. C. i s t h e c o n c e n t r a t i o n o f compound i . As a f i r s t a p p r o a c h s t a n d a r d q u a n t i t i e s , i . e . a s s u m i n g u n i t c o n c e n t r a t i o n s , w i l l be u s e d i n t h e p r e s e n t e v a l u a t i o n o f g r o w t h and product formation. One f u r t h e r c o n v e n i e n t c o n v e n t i o n needs t o be d i s c u s s e d . E n e r g y , and hence a l s o d e r i v e d q u a n t i t i e s l i k e e n t h a l p y and f r e e e n t h a l p y , c a n n o t be a t t r i b u t e d a d e f i n i t e v a l u e ; i t s m a g n i t u d e c a n o n l y be d e f i n e d w i t h r e s p e c t t o a g i v e n r e f e r e n c e s t a t e w h i c h i s a t t r i b u t e d a zero energy l e v e l . A convenient r e f e r e n c e s t a t e f o r t h e e v a l u a t i o n o f g r o w t h and p r o d u c t f o r m a t i o n i n m i c r o o r g a n i s m s i s o b t a i n e d i f C 0 , H 0 , 0 and N a r e a s s i g n e d a z e r o e n e r g y l e v e l . The e n e r g y o f a compound t h u s becomes e q u a l t o i t s e n e r g y o f c o m b u s t i o n t o C 0 , H 0 and N . T h i s c o n v e n t i o n c a n be m o t i v a t e d by t h e f a c t t h a t m i c r o o r g a n i s m s c a n u n d e r no c i r c u m stances d e r i v e u s e f u l energy from processes i n which o n l y C 0 , H 0 , 0 and N a r e i n v o l v e d . The m o l a r f r e e e n t h a l p i e s and e n t h a l p i e s _ o f c o m b u s t i o n a t s t a n d a r d c o n d i t i o n s w i l l be termed Ag°. and A h , r e s p e c t i v e l y . ^ From e q n . (7) i t i s c l e a r t h a t t h e e n f n a l p y o f c o m b u s t i o n , A h ° ^ , equals t h e heat o f combustion. The f r e e e n t h a l p y o f c o m b u s t i o n , A g . , i s m a r k e d l y d e p e n d e n t on t h e c o n c e n t r a t i o n s o f t h e r e a c t a n t s i n v o l v e d ( e q n . ( 1 4 ) ) and most b i o l o g i c a l p r o c e s s e s t a k e p l a c e i n aquous s o l u t i o n s a t a h y d r o g e n i o n c o n c e n t r a t i o n c o r r e s p o n d i n g t o a pH o f 7 r a t h e r t h a n a t u n i t c o n c e n t r a t i o n o f t h e H - i o n , c o r r e s p o n d i n g t o a pH o f z e r o . The f r e e e n t h a l p i e s o f c o m b u s t i o n t o l i q u i d w a t e r , t h e HC0 i o n (the p r e d o m i n a n t f o r m i n w h i c h C 0 e x i s t s a t a pH o f 7) and N a t a pH o f 7 c a n t h u s be c o n s i d e r e d more r e l e v a n t t o b i o l o g i c a l 9

2

2

2

2

2

2

2

2

2

2

2

0

C 1

0

3

2

2


302

BIOCHEMICAL

ENGINEERING

t r a n s f o r m a t i o n s . The f r e e e n t h a l p y o f c o m b u s t i o n u n d e r t h e s e c o n d i t i o n s w i l l be i n d i c a t e d A g ! . The e n t h a l p y o f c o m b u s t i o n i s h a r d l y a f f e c t e d by t h e pH. T^e f r e e e n t h a l p i e s and e n t h a l p i e s o f c o m b u s t i o n a r e known t o obey r e g u l a r i t i e s ( 6 , 8 ) . T h e s e c a n be t r e a t e d u s i n g t h e c o n c e p t o f t h e d e g r e e o f r e d u c t i o n a s i n t r o d u c e d by M i n k e v i c h and E r o s h i n (_7) and e x t e n d e d and g e n e r a l i z e d by t h e p r e s e n t a u t h o r ( 5 , 6 ) . The g e n e r a l i z e d d e g r e e o f r e d u c t i o n , γ., o f a compound w i t h r e s p e c t t o m o l e c u l a r n i t r o g e n i s d e f i n e d by: Downloaded by UNIV OF MISSOURI COLUMBIA on April 15, 2013 | http://pubs.acs.org Publication Date: January 18, 1983 | doi: 10.1021/bk-1983-0207.ch013

0

Ύ

=

ί

4

+

a

i"

2

b

( 1 5 )

i

I n w h i c h a. and b. a r e t h e number o f m o l e s o f Η and 0 p r e s e n t i n one C-mole ( b e i n g t h e amount c o n t a i n i n g 12 grams o f c a r b o n ) o f coumpound i . F o r a component i t h e e n t h a l p i e s and f r e e e n t h a l p i e s o f c o m b u s t i o n p e r C-mole, t o be i n d i c a t e d A h ^ and Ag°. r e s p e c t i v e l y , a r e , t o a f i r s t a p p r o x i m a t i o n , a u n i q u e f u n c t i o n o f t h e d e g r e e o f r e d u c t i o n , γ., as i n t r o d u c e d i n e q n . ( 1 5 ) . A s t a t i s t i c a l a n a l y s i s o f d a t a f o r some 60 o r g a n i c compounds o f b i o l o g i c a l s i g n i f i c a n c e revealed the existence of the f o l l o w i n g r e g u l a r i t i e s : 0

0

Ah .

= 115γ.

CL

Ag°.

(16)

' 1

= 94.4γ. + 86.6

(17)

The r e s i d u a l e r r o r o f t h e e s t i m a t e i s 18 k J f o r b o t h e q n s . ( 1 6 ) and ( 1 7 ) . E q u a t i o n (16) s t a t e s t h a t t h e h e a t o f c o m b u s t i o n p e r C-mole i s more o r l e s s d i r e c t l y p r o p o r t i o n a l t o t h e d e g r e e o f r e d u c t i o n . As i s a p p a r e n t f r o m eqn. (17) s u c h a s i m p l e p r o p o r t i o n a l i t y r e l a t i o n does n o t a p p l y t o f r e e e n t h a l p i e s o f combustion. E q u a t i o n s (16) and (17) show t h a t s y s t e m a t i c d e v i a t i o n s b e t w e e n f r e e e n t h a l p i e s and h e a t s o f c o m b u s t i o n must e x i s t . F o r s u b s t r a t e s o f a low degree o f r e d u c t i o n the f r e e e n t h a l p i e s o f combustion exceed t h e heats o f combustion, f o r s u b s t r a t e s of a h i g h d e g r e e o f r e d u c t i o n t h e r e v e r s e a p p l i e s . T h i s phenomenon c a n be i l l u s t r a t e d i f t h e e n t r o p y c o n t r i b u t i o n t o t h e f r e e enthalpy o f combustion, T A s . , i s c a l c u l a t e d . I t i s obtained from the equation: 0

Ag°. 6

C1

0

0

CI

CI

= Ah . - TAs .

(18)

I n f i g . 2 t h e r e l a t i o n i s shown b e t w e e n t h e s a i d e n t r o p y c o n t r i b u t i o n and t h e d e g r e e o f r e d u c t i o n u s i n g d a t a f o r a v a r i e t y o f o r g a n i c compounds. A d e f i n i t e t r e n d c a n i n d e e d be shown t o e x i s t ( a p a r t f r o m an i n c i d e n t a l o u t l y e r ) : The e n t r o p y c o n t r i b u t i o n i n c r e a s e s w i t h i n c r e a s i n g degree o f r e d u c t i o n . I n v i e w o f the o b s e r v a t i o n t h a t f r e e e n t h a l p y changes a t a pH o f 7 may w e l l be more r e l e v a n t i n m i c r o b i o l o g i c a l p r o c e s s e s , t h e e n t r o p y c o n t r i b u -



13.

Macroscopic

ROELS

501 ο

Thermodynamics

ι 2.5

and

303

Growth

ι 5

ί 7.5

Γ Figure 2. The entropy contribution, T A s ° (kJ/C-mole), to the free enthalpy of combustion at standard conditions, as a function of the degree of reduction, y, of the compounds considered, for acids (%), carbohydrates (A), alkanes (Ο), ethene and ethyne (O), alcohols (%), acetone (|Λ aldehydes (A), and amino acids (*). c


304

BIOCHEMICAL

ENGINEERING

0 f

t i o n t o t h e f r e e e n t h a l p y o f c o m b u s t i o n a t a pH o f 7, T A s , was a l s o c a l c u l a t e d . T h e s e v a l u e s a r e p l o t t e d as a f u n c t i o n o? t h e d e g r e e o f r e d u c t i o n i n f i g . 3. The f e a t u r e s o f f i g s . 2 and 3 a r e s e e n t o be v e r y much a l i k e , e x c e p t f o r t h e a c i d s , w h i c h have a m a r k e d l y h i g h e r T A s a t a pH o f 7, i . e . t h e i r f r e e e n t h a l p y o f c o m b u s t i o n i s îower a t t h a t pH. T h i s d i f f e r e n c e b e t w e e n s t a n d a r d c o n d i t i o n s and a s i t u a t i o n i n w h i c h t h e c o n c e n t r a t i o n s d i f f e r f r o m u n i t y , e.g. a t a pH o f 7, i s c h a r a c t e r i s t i c f o r a l i m i t a t i o n o f t h e use o f s t a n d a r d f r e e e n t h a l p y changes i n t h e a n a l y s i s o f p r o c e s s e s , where t h e c o n c e n t r a t i o n s a t t h e l o c a l e o f t h e p r o c e s s may d i f f e r f r o m u n i t y : t h e Ag° v a l u e s c a n o n l y be a p p l i e d t o an a p p r o x i m a t e a n a l y s i s . F o r d e t a i l e d c o n s i d e r a t i o n s the A g v a l u e s at the c o n c e n t r a t i o n s at the l o c a l e o f e n e r g y g e n e r a t i o n a r e n e e d e d . S t i l l , as w i l l be shown, t h e approximate analyses g r e a t l y c o n t r i b u t e to the understanding of m i c r o b i a l energetics.


0 f

Q

T h e r e a l s o e x i s t compounds o f w h i c h t h e e n e r g y r e l e a s e d on t h e i r c o m b u s t i o n does d e f i n i t e l y n o t f o l l o w t h e t r e n d s i n d i c a t e d by e q n s . (16) and ( 1 7 ) . E x a m p l e s a r e o x y g e n and n i t r i c a c i d w h i c h have a " h e a t o f c o m b u s t i o n " w h i c h i s c l o s e t o z e r o and a d e g r e e o f r e d u c t i o n o f -4 and -5 r e s p e c t i v e l y . By v i r t u e o f t h i s f e a t u r e t h e s e compounds can s e r v e as v e r y e f f i c i e n t " e l e c t r o n a c c e p t o r s " . The t h e r m o d y n a m i c e f f i c i e n c y . The t h e r m o d y n a m i c t h e o r y d e v e l o p e d e a r l i e r was shown t o r e s u l t i n eqn. ( 1 2 ) , a r e s t r i c t i v e e q u a t i o n r e g a r d i n g the f l o w s o f m a t t e r exchanged w i t h the e n v i r o n ment by an open s y s t e m . On eqn. (12) a d e f i n i t i o n o f t h e t h e r m o d y n a m i c e f f i c i e n c y c a n be b a s e d i f t h e d i s s i p a t i o n , ΤΠ , i s compared t o t h e t o t a l o f t h e f l o w s o f f r e e e n t h a l p y e n t e r i n g t h e s y s t e m . However, a p r o b l e m w h i c h was a l r e a d y i n d i c a t e d above has t o be t a c k l e d . The amount o f e n e r g y c a n n o t be s p e c i f i e d i n a u n i q u e way and i t can o n l y be d e f i n e d w i t h r e f e r e n c e t o a b a s e l e v e l , w h i c h i s a r b i t r a r i l y a t t r i b u t e d z e r o e n e r g y c o n t e n t . As s o o n as s u c h a r e f e r e n c e s t a t e has been c h o s e n , t h e t h e r m o d y n a m i c e f f i c i e n c y i s e a s i l y c a l c u l a t e d . The p r o c e d u r e i s i l l u s t r a t e d i n f i g . 4. The t h e r m o d y n a m i c e f f i c i e n c y , fL,» i s d e f i n e d e q u a l t o t h e r a t i o o f t h e f r e e e n t h a l p y g a i n e d i f t h e compounds l e a v i n g t h e s y s t e m were t r a n s f o r m e d t o t h e r e f e r e n c e s t a t e , t o t h a t , w h i c h w o u l d be o b t a i n e d i f t h i s p r o c e d u r e were a p p l i e d t o t h e compounds e n t e r i n g t h e s y s t e m . I t i s e a s i l y u n d e r s t o o d t h a t η , i s c o n s t r a i n e d between z e r o , i f a l l t h e f r e e e n t h a l p y e n t e r i n g trie s y s t e m i s d i s s i p a t e d and u n i t y i f Ag!j e q u a l s Ag^ , i . e . i f t h e d i s s i p a t i o n equals zero. I t i s important to r e a l i z e t h a t the f o r m e r c o n s t r a i n t s t r o n g l y depends on t h e c o r r e c t c h o i c e o f t h e reference state. A p p l i c a t i o n s of the

theory

A e r o b i c g r o w t h w i t h o u t p r o d u c t f o r m a t i o n . The a p p l i c a t i o n of the theory to a e r o b i c growth without f o r m a t i o n of products



13.

Macroscopic

ROELS

Thermodynamics

and

305

Growth

k J/C-mole

100

50

Π Ο

Θ

Δ*

*

-50

*

*

*

** *

7.5

2.5 Υ

Figure 3. The entropy contribution, T A S (kJ/C-mole), to the free enthalpy of combustion at a pH of 7, as a function of the degree of reduction, y, of the com pounds considered; symbols as in Figure 2. C

0 /



306

BIOCHEMICAL

ENGINEERING

w i l l now be shown. F i g u r e 5 shows t h e s y s t e m and t h e exchange f l o w s f o r t h i s c a s e . The e l e m e n t a r y c o m p o s i t i o n s and t h e h e a t s o f c o m b u s t i o n o f t h e compounds exchanged w i t h t h e e n v i r o n m e n t a r e i n d i c a t e d . A e r o b i c g r o w t h i s f o r t h e p r e s e n t a n a l y s i s assumed t o i n v o l v e u p t a k e o f a n i t r o g e n s o u r c e , a c a r b o n s o u r c e and o x y g e n and t r a n s p o r t t o t h e e n v i r o n m e n t o f c a r b o n d i o x i d e , w a t e r and new b i o m a s s . The b i o m a s s i s assumed t o be one compound, w h i c h c a n be c h a r a c t e r i z e d f u l l y by i t s e l e m e n t a l c o m p o s i t i o n . As t h e e n t h a l p y as w e l l as t h e f r e e e n t h a l p y o f c o m b u s t i o n , i . e . t h e e n t h a l p y and f r e e e n t h a l p y c o n t e n t w i t h r e s p e c t t o t h e r e f e r e n c e s t a t e adopted i n t h e p r e s e n t a n a l y s i s , i s zero f o r oxygen, carbon d i o x i d e and w a t e r , i t i s e a s i l y u n d e r s t o o d t h a t t h e thermodynamic e f f i c i e n c y c a n be d e f i n e d a s f o l l o w s : Φ Ag° n'th . u

-

—

^

—

(19)

^

cN i n which Φ , and Φ^ a r e t h e f l o w s o f b i o m a s s ( C - m o l e / h r ) , s u b s t r a t e ^ C - m o l e / h r ) and n i t r o g e n s o u r c e ( m o l e / h r ) t o o r f r o m t h e s y s t e m ( a s i n d i c a t e d i n f i g u r e 5 ) . Ag° and A g ° a r e t h e f r e e e n t h a l p i e s _ o f c o m b u s t i o n o f a C-mole o i b i o m a s s and s u b s t r a t e r e s p e c t i v e l y . A g ^ i s t h e f r e e e n t h a l p y o f combustion o f a mole of t h e n i t r o g e n source. E q u a t i o n (19) c a n a l s o be w r i t t e n a s : g

η,.

Ag° — A o / r g

s

+

(20) ( /c )Ai» C )

4

N

In the formulation o f t h i s equation a balance f o r atomic n i t r o g e n i s u s e d t o r e l a t e t h e f l o w s Φ , and Φ . Y i sa yield factor • Ν χ sx · f o r b i o m a s s o n s u b s t r a t e o n a p e r C-mole b a s e ; C-moles o f b i o m a s s p r o d u c e d p e r C-mole o f s u b s t r a t e consumed, t h u s i t i s d e f i n e d by: f f

%

γ " = φ /φ SX X s

(21)

A n o t h e r u s e f u l e f f i c i e n c y measure i s t h e s o - c a l l e d e n t h a l p y e f f i c i e n c y o f g r o w t h , η^, i t i s by a n a l o g y o b t a i n e d i f t h e Ag°. i n eqn. (20) a r e r e p l a c e d by t h e r e s p e c t i v e A h . . As was alreaày i n d i c a t e d t h e e n t h a l p y e f f i c i e n c y does n o t have t h e f u n d a m e n t a l p r o p e r t i e s o f t h e thermodynamic e f f i c i e n c y a s t h e r e s t r i c t i o n f o l l o w i n g from t h e a p p l i c a t i o n o f t h e second law o f thermodynamics ( s e e e q n . ( 1 1 ) ) d o e s n o t pose an u p p e r l i m i t t o Φ^. H e n c e , p r o c e s s e s f o r w h i c h η„ e x c e e d s u n i t y c a n by no means be e x c l u d e d . I t c a n e a s i l y b e shown t h a t t h e e f f i c i e n c y m e a s u r e s d e v e l o p e d a b o v e , i . e . η^, and η^, a l l o w t h e f o r m u l a t i o n o f e x p r e s s i o n s f o r t h e d i s s i p a t i o n ΤΠ and t h e h e a t p r o d u c t i o n (-Φ„). The f o l l o w i n g S n 0


13.

ROELS

FREE

Macroscopic

Thermodynamics

and

307

Growth

ENTHALPY

ENTERING

SYSTEM

FREE

ENTHALPY


LEAVING

FREE

ENTHALPY

OF

REFERENCE

SYSTEM

STATE

Figure 4. The thermodynamic efficiency, ηα, of a process. A g / and A g / are the amounts of free enthalpy gained when the compounds entering and leaving the system, respectively, are transformed to the reference state.

c

_\h°-0

co

2

_H 0 2

cs

W

Figure 5.

W

CHa

b

Nc

i° i i

System and flows for thermodynamic analysis of aerobic growth formation of products.


without

BIOCHEMICAL

308 equations

ENGINEERING

hold: (22)


(23) I n eqn. (22) D e q u a l s t h e d i s s i p a t i o n ΤΠ . For a e r o b i c growth w i t h o u t product f o r m a t i o n the a b s o l u t e m a g n i t u d e second t e r m a p p e a r i n g a t t h e l e f t hand s i d e o f eqn. (11) c a n be shown t o be s m a l l as compared t o Φ^/Τ, i . e . t h e exchange o f h e a t l a r g e l y e x c e e d s t h e exchange o f " c h e m i c a l e n t r o p y " ( 5 , 6^. T h i s i m p l i e s t h a t D and a r e , t o a good a p p r o x i m a t i o n e q u a l and t h i s , o f c o u r s e , a l s o a p p l i e s t o η * Π^· I n t h i s c a s e an a n a l y s i s b a s e d on an e n t h a l p y e f f i c i e n c y o f g r o w t h i s more o r l e s s v a l i d and hence a l s o t h e i n t e r p r e t a t i o n of t h e s e c o n d law as t o f o r b i d p r o c e s s e s w i t h u p t a k e o f h e a t . As has been shown e a r l i e r (6) t h e v e r y f a c t t h a t eqn. (16) i s more o r l e s s v a l i d i m p l i e s t h a t h e a t p r o d u c t i o n and oxygen consumption, Φ , are p r o p o r t i o n a l a c c o r d i n g to the f o l l o w i n g equation : a n a

Φ„ = 460

Φ

(24)

ο

The a p p r o x i m a t e v a l i d i t y o f t h i s e q u a t i o n i s e a s i l y u n d e r s t o o d as f o l l o w s . The d e g r e e o f r e d u c t i o n γ. i n d i c a t e s t h e number o f moles o f e l e c t r o n s a v a i l a b l e f o r t r a n s f e r t o o x y g e n on c o m p l e t e c o m b u s t i o n o f a C-mole o f a compound t o C0 , H 0 and N . On t r a n s f e r t o oxygen t h e e n e r g y o f t h e e l e c t r o n s i s d i s s i p a t e d , r e s u l t i n g i n a h e a t p r o d u c t i o n o f 115 k J p e r mole o f e l e c t r o n s . On a e r o b i c g r o w t h p a r t o f t h e e n e r g y c o n t e n t o f t h e s u b s t r a t e and the n i t r o g e n source i s conserved i n the form o f newly s y n t h e s i z e d biomass, the e l e c t r o n s c o r r e s p o n d i n g to the remainder a r e t r a n s f e r r e d t o o x y g e n and p r o v i d e d i s s i p a t i o n . As f o u r moles o f e l e c t r o n s a r e a c c e p t e d by one m o l e o f o x y g e n eqn. (16) shows 460 k J t o be g e n e r a t e d f o r e a c h m o l e o f oxygen consumed. The v a l i d i t y o f eqn. (24) shows t h a t a t r e a t m e n t can a l s o be b a s e d on o x y g e n e f f i c i e n c y o f g r o w t h C5"\7) · A s u b s t a n t i a l body o f d a t a on a e r o b i c g r o w t h w i t h o u t p r o d u c t f o r m a t i o n s u p p o r t e d by ammonia as a n i t r o g e n s o u r c e has r e c e n t l y been r e v i e w e d ( 9 ) . From t h e s e d a t a t h e v a l u e s o f η (- η ) and t h e d i s s i p a t i o n p e r u n i t b i o m a s s p r o d u c e d (D/Φ , k J / C - m o l e ) were c a l c u l a t e d . The r e s u l t s were a v e r a g e d f o r e a c h o f t h e c a r b o n s o u r c e s c o n s i d e r e d and a r e shown i n f i g s . 6 and 7. A l t h o u g h , not u n e x p e c t e d l y , i t i s c l e a r t h a t c o n s i d e r a b l e s c a t t e r i s e x i s t e n t i n b o t h f i g s . 6 and 7,some g l o b a l r e g u l a r i t i e s seem t o be p r e s e n t , w h i c h a r e i n d i c a t e d i n t h e f i g u r e s . F o r s u b s t r a t e s w i t h a d e g r e e o f r e d u c t i o n l o w e r t h a n about 5, t h e thermo dynamic e f f i c i e n c y a v e r a g e s 0.58 ( t h e o n l y s i g n i f i c a n t o u t l y e r b e i n g t h e v e r y low e f f i c i e n c y o b s e r v e d f o r g r o w t h s u p p o r t e d by 2

2

2



13.

ROELS

Macroscopic

Thermodynamics

and

Growth

309

0.25

Figure 6. Thermodynamic efficiency, η , of aerobic growth with NH as the nitrogen source, plotted as a function of the degree of reduction, γ, of the substrate. Theoretical limits due to the second law and C-limitation. Shown are averages of experimental data for methane (%), n-alkanes (A), methanol (J^), ethanol (*ψ), glycerol (®), mannitol (O), acetic acid (Δλ lactic acid glucose (*), formalde hyde (VA gluconic acid (S), succinic acid (@), citric acid (®), malic acid (®), formic acid (®), oxalic acid (A)ιη

S


310

BIOCHEMICAL

Î

1270

1240

I·


A

ENGINEERING

Figure 7. Dissipation of aerobic growth (kJ/C-mole of biomass produced) for aerobic growth with ΝΗ as a nitrogen source as a function of the degree of reduction of substrate. Theoretical limits due to second law and C-limitation. Average of experimental data for various substrates, symbols as in Figure 6. Ά



13.

ROELS

Macroscopic

Thermodynamics

and

311

Growth

o x a l i c a c i d ) , t h i s corresponds to a d i s s i p a t i o n of roughly 400 k J / m o l e o f b i o m a s s p r o d u c e d . F o r s u b s t r a t e s w i t h a d e g r e e o f r e d u c t i o n e x c e e d i n g 5 t h e thermodynamic e f f i c i e n c y p r o g r e s s i v e l y d e c r e a s e s and t h e d i s s i p a t i o n i n c r e a s e s up t o 1400 kJ/C-mole f o r t h e c a s e o f g r o w t h s u p p o r t e d by methane. I n f i g s . 6 and 7 t h e r e s t r i c t i o n s o f u n i t e f f i c i e n c y and z e r o d i s s i p a t i o n r e s p e c t i v e l y , a s imposed by t h e s e c o n d law a r e i n d i c a t e d as w e l l as a l i m i t imposed by a l i m i t a t i o n o f a d i f f e r e n t n a t u r e , i . e . c a r b o n l i m i t a t i o n . The l a t t e r l i m i t m e r i t s a more t h o r o u g h d i s c u s s i o n . The d e g r e e o f r e d u c t i o n , γ , o f b i o mass f r o m a v a r i e t y o f s o u r c e s a v e r a g e s 4.8 (5,6,10) and hence by v i r t u e o f eqn. (16) i t s e n e r g y c o n t e n t i s a b o u t 550 k J / C - m o l e . I f g r o w t h i s s u p p o r t e d by a s u b s t r a t e o f a d e g r e e o f r e d u c t i o n e x c e e d i n g 4.8 i t s e n e r g y c o n t e n t w i l l e x c e e d t h e s a i d 550 k J / C - m o l e and h e n c e , even i f a l l s u b s t r a t e c a r b o n were f i x e d i n b i o m a s s , a thermodynamic e f f i c i e n c y l o w e r t h a n u n i t y w o u l d be o b t a i n e d . A c o m p l e t e e x p l o i t a t i o n o f t h e e n e r g y p r e s e n t i n t h e s u b s t r a t e w o u l d r e q u i r e f i x a t i o n o f a d d i t i o n a l low e n e r g y c a r b o n e.g. f r o m C0 . T h i s i s , however, e x c l u d e d , as o n l y one c a r b o n s o u r c e i s assumed t o be s u p p l i e d . On o b s e r v a t i o n o f f i g . 6 i t becomes c l e a r t h a t t h e t r e n d s o b s e r v e d i n t h e e x p e r i m e n t a l v a l u e s o f t h e thermodynamic e f f i c i e n c y mimic t h e shape o f t h e t h e o r e t i c a l r e s t r i c t i o n s a t a l e v e l o f a b o u t 60%. The d e v i a t i o n between t h e t h e o r e t i c a l l i m i t and t h e v a l u e s a c t u a l l y o b s e r v e d i s q u i t e e a s i l y u n d e r s t o o d i n g e n e r a l terms i n t h e r e g i o n o f low d e g r e e s o f r e d u c t i o n , where the energy a v a i l a b l e i n the s u b s t r a t e r a t h e r than i t s carbon c o n t e n t l i m i t s the v a l u e s o f Y . The t h e o r e t i c a l l i m i t o f u n i t y sχ · c a n n e v e r be r e a c h e d as any p r o c e s s needs a n o n - z e r o d i s s i p a t i o n t o proceed at a non-zero r a t e (2-4). In f a c t the r a t e at which a p r o c e s s p r o c e e d s , e.g. t h e r a t e o f g r o w t h o f t h e amount o f biomass, i s to a c e r t a i n extent i n c r e a s i n g w i t h i n c r e a s i n g d i s s i p a t i o n . V a r i o u s o p t i m a l i t y p r i n c i p l e s ( 1 1 ) , may d i c t a t e an o p t i m a l thermodynamic e f f i c i e n c y o f r o u g h l y t h e m a g n i t u d e o b s e r v e d h e r e ( 1 1 , 1 2 ) . The b e h a v i o u r a t h i g h d e g r e e s o f r e d u c t i o n i s l e s s e a s i l y u n d e r s t o o d on f u n d a m e n t a l g r o u n d s . The t h e r m o dynamic e f f i c i e n c y c o u l d w e l l a p p r o a c h t h e t h e o r e t i c a l l i m i t c l o s e r with a s u f f i c i e n t l y l a r g e d i s s i p a t i o n . A p p a r e n t l y o t h e r phenomena t o w h i c h t h e f o r m a l m a c r o s c o p i c t r e a t m e n t p r o v i d e s no c l u e s , l i m i t t h e maximum c o n s e r v a t i o n o f s u b s t r a t e c a r b o n i n b i o m a s s t o a b o u t 2/3 o f t h e maximum. O b v i o u s l y one has t o r e s o r t t o b i o c h e m i c a l t h e o r y t o o b t a i n c l u e s t o the n a t u r e o f t h e mechanisms u n d e r l y i n g t h e phenomenon. T h e r e e x i s t s s t i l l a n o t h e r u s e f u l q u a n t i t y , w h i c h has been used t o s y s t e m a t i z e t h e e x p e r i m e n t a l d a t a on t h e e f f i c i e n c y o f g r o w t h s u p p o r t e d by v a r i o u s c a r b o n s o u r c e s ; i t i s P a y n e s y i e l d on a v a i l a b l e e l e c t r o n s , Υ , ( 1 3 ) . I t i s d e f i n e d as t h e amount o f b i o m a s s p r o d u c e d p e r mole o f e l e c t r o n s a v a i l a b l e f o r t r a n s f e r t o o x y g e n on c o m p l e t e c o m b u s t i o n . N u m e r i c a l l y t h e number o f m o l e s o f e l e c t r o n s a v a i l a b l e f o r t r a n s f e r t o o x y g e n on 2

M

1


312

BIOCHEMICAL

ENGINEERING

c o m b u s t i o n o f a C-mole o f a g i v e n s u b s t r a t e i s e q u a l t o t h e d e g r e e o f r e d u c t i o n , γ., as d e f i n e d by eqn. ( 1 5 ) . As a f i r s t a p p r o x i m a t i o n , t a k i n g i n t o account the f a c t t h a t biomass from a«-·• vVW a. rAi.^ eW tWy^ νo fΛ. I mUX i.V c.rLVUo b iJLaj. a l sIo7uWUr1c.eU^t7 s can beVJ t.w eW lI— lJ. J- r e p r e s e n t e d by t h e COU c a n be c o n s i d e r e d composition formula C H O . 5 N 0 . 2 » / p r o p o r t i o n a l t o X]^ ( o r r a t h e r n ) (§, 6 ) Y

l e 8

C

0

e

H


Υ

, av/e

= 5.85

η., th

(25)

F i g u r e 8 shows t h e d a t a d e p i c t e d i n f i g s . 6 and 7 i n a p l o t o f Y . v e r s u s t h e d e g r e e o f r e d u c t i o n γ. The t r e n d s and t h e o r e t i c a l l i m i t s a r e a l s o shown. Growth w i t h e l e c t r o n a c c e p t o r s o t h e r than oxygen. In the p r e c e e d i n g s e c t i o n a c a s e was t r e a t e d i n w h i c h o x y g e n was t h e e l e c t r o n a c c e p t i n g m o i e t y . Oxygen i s , h o w e v e r , by no means t h e o n l y compound, w h i c h c a n p e r f o r m s u c h f u n c t i o n . O t h e r examples a r e n i t r a t e and s u l p h a t e . The c h a r a c t e r i s t i c f e a t u r e o f an e l e c t r o n a c c e p t o r i s t h a t f r e e e n t h a l p y i s gained i n the o v e r a l l p r o c e s s o f t r a n s f e r o f an e l e c t r o n f r o m a g i v e n s o u r c e (a s u b s t r a t e ) to the acceptor. T h i s f r e e enthalpy i s p a r t l y d i s s i p a t e d t o p r o v i d e t h e n e c e s s a r y i r r e v e r s i b i l i t y and i t c a n be u s e d t o t r a n s f e r c a r b o n ( - d i o x i d e ) f r o m a low e n e r g y l e v e l t o a h i g h e n e r g y one. I n t a b l e I a summary i s p r o v i d e d o f t h e e n e r g y b e c o m i n g a v a i l a b l e on t r a n s f e r o f one m o l e o f e l e c t r o n s f r o m g l u c o s e t o a g i v e n e l e c t r o n a c c e p t o r engaged i n a g i v e n reduction process. Table

I.

F r e e e n t h a l p y g a i n e d a t pH = 7 and s t a n d a r d c o n d i t i o n s on t r a n s f e r o f one m o l e o f e l e c t r o n s f r o m g l u c o s e t o a g i v e n e l e c t r o n a c c e p t o r (14)

e l e c t r o n acceptor + * (NH ) N +

4

2

S

moles of e l e c t r o n s accepted 6

(HS~) 2-

SO, 4

(HS

)

0

1

Ag ^ ^ kJ 79.1

^Sai/^ kjfmole 13.2

2

27.7

13.9

8

150.7

18.8

6

171.9

28.7

9

S 0

3 N0 ~ 2

(HS~) +

6

435.5

72.6

+

598.4

74.8

(NH ) 4

N0 "

(NH )

8

N0 "

(N ) 2

5

559.5

111.9

(H 0)

4

473.9

118.5

3

3

°2 *

4

2

I n b r a c k e t s reduced

form of e l e c t r o n a c c e p t o r


Macroscopic


ROELS

Thermodynamics

and Growth

313

'av/e g DM/mole 6h

SECOND LAW

4H

2h

1h

Figure 8. Yield on electrons available for transfer to oxygen, Yav/e, (g DM/mole av/e) for aerobic growth with NH as a nitrogen source. Theoretical limits due to the second law and C-limitation. Average of experimental data for various substrates, symbols as in Figure 6. 3


BIOCHEMICAL

314

ENGINEERING


The v a l u e s o f t h e f r e e e n t h a l p y g a i n e d p e r m o l e o f e l e c t r o n s t r a n s f e r r e d a r e a r r a n g e d i n o r d e r o f i n c r e a s i n g m a g n i t u d e . The " e f f i c i e n c y " o f an e l e c t r o n a c c e p t o r t h u s i n c r e a s e s f r o m t h e t o p to t h e b o t t o m o f t h e t a b l e and f a i t h i n l i f e ' s u n i v e r s a l s t r i v i n g f o r more o p t i m a l g r o w t h w o u l d assume p r e f e r e n c e f o r t h e e l e c t r o n a c c e p t o r s a t t h e b o t t o m o f t h e t a b l e i f two o r more e l e c t r o n acceptors are simultaneously a v a i l a b l e . Growth w i t h o u t e x t e r n a l l y s u p p l i e d e l e c t r o n a c c e p t o r s . I n a number o f c a s e s g r o w t h i s o b s e r v e d t o t a k e p l a c e w i t h o u t an i d e n t i f i a b l e seperate e l e c t r o n a c c e p t o r b e i n g p r e s e n t . I n such case t h e s u b s t r a t e o r a s u b s t r a t e d e r i v e d moiety i s both donor and a c c e p t o r o f e l e c t r o n s . F o r s i m p l i c i t y ' s s a k e o n l y t h e c a s e o f a n a e r o b i c g r o w t h on a s i n g l e c a r b o n s o u r c e w i t h f o r m a t i o n o f a s i n g l e p r o d u c t and NH^ as t h e n i t r o g e n s o u r c e w i l l be t r e a t e d . I t i s e a s i l y u n d e r s t o o d t h a t an a n a l y s i s o f a n a e r o b i c g r o w t h c a n be b a s e d o n a b a l a n c e o n t h e d e g r e e o f r e d u c t i o n as d e f i n e d by eqn. ( 1 6 ) . T a b l e I I shows t h e f l o w s o f t h e v a r i o u s compounds and t h e i r c o n t e n t o f e l e c t r o n s a v a i l a b l e f o r t r a n s f e r t o oxygen on f o r m a t i o n o f Φ C-moles of b i o m a s s . χ Table I I .

A degree o f r e d u c t i o n balance f o r anaerobic growth w i t h o u t e x t e r n a l e l e c t r o n a c c e p t o r s

molecular species

degree of reduction

substrate N-source

Y

(NH^)

contribution to degree o f reduction balance

flow

γ Φ /Y" 's X SX 3ο.Φ 1 χ

Φ /Υ" χ

s

X

3

C φ

Y

Φ

SX

1 χ B i o m a s s (CH ,0, N -) a1 b1 c1 4

x

- γ Φ χ χ

χ

Product

- γ φ Ρ Ρ

φ Y

P

Ρ

2

0

Φ /Υ" --Φ -φ χ sx χ ρ

0

H 0

0

Φ

0

co 2

W

By v i r t u e o f t h e f a c t t h a t no e x t e r n a l e l e c t r o n a c c e p t o r s are present the c o n t r i b u t i o n s t o the degree o f r e d u c t i o n balance as shown i n t h e l a s t c o l u m n o f t a b l e 2 must add up t o z e r o , i t follows : Φ

Φ

Foundations of Biochemical Engineering - American Chemical Society

Recommend Documents