Thermodynamics of Aqueous Systems with Industrial Applications

30 Flow Calorimetry of Aqueous Solutions at Temperatures up to 325° C R. H . WOOD and D. SMITH-MAGOWAN

Downloaded by COLUMBIA UNIV on May 29, 2014 | http://pubs.acs.org Publication Date: October 29, 1980 | doi: 10.1021/bk-1980-0133.ch030

University of Delaware, Newark, D E 19711

1) Need The fact that we are gathered together at a conference on "Thermodynamics on Aqueous Systems with Industrial Application" indicates the importance of thermodynamic data on aqueous solu tions. In particular, there is a great need for data on high temperature aqueous systems. Because of the experimental diffi culties, there are relatively few measurements on these systems and yet they are of very great industrial importance. As the temperature of water approaches the critical tempera ture, 374°C, it becomes a low dielectric constant solvent with the dielectric constant approaching one tenth its value at 25°C. This is about the same dielectric constant as 1,1 dichloroethane at 25°C. Because of this large change in solvent properties there are corresponding very large changes in the properties of electrolytes dissolved in water. The few data available indicate(1-4) that the heat capacities of dilute aqueous solution be come very large and negative as the temperature approaches the critical temperature and the data presented below will show that the relative apparent molal enthalpy, L , becomes very large and positive at higher temperatures. These very large changes in thermodynamic properties make it very difficult to predict high temperature properties of solutions from low temperature measure ments and increase the need for accurate measurements. Φ

2)

Information

Obtained

C a l o r i m e t r i c m e a s u r e m e n t s , when combined w i t h t h e n o r m a l l y a v a i l a b l e room t e m p e r a t u r e t h e r m o d y n a m i c p r o p e r t i e s , g i v e v a l u e s f o r f r e e e n e r g y , e n t h a l p y , h e a t c a p a c i t y and even volume a t h i g h temperatures. We have been a c t i v e l y d e v e l o p i n g two t y p e s o f c a l o r i m e t e r s w h i c h w i l l o p e r a t e a t e l e v a t e d t e m p e r a t u r e s and p r e s s u r e s . One t y p e i s a h e a t o f m i x i n g c a l o r i m e t e r t o measure e n t h a l p i e s o f d i l u t i o n i n order to obtain d i f f e r e n c e s i n p a r t i a l molal enthalpy 0-8412-0569-8/80/47-133-569$05.00/0 © 1980 American Chemical Society

In Thermodynamics of Aqueous Systems with Industrial Applications; Newman, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

570

T H E R M O D Y N A M I C S OF AQUEOUS SYSTEMS W I T H INDUSTRIAL

APPLICATIONS

or heat c o n t e n t s :

This property simply considered i s the f i r s t temperature d e r i v a t i v e o f t h e f r e e e n e r g y o r a c t i v i t y and c a n be u s e d t o o b t a i n o s m o t i c c o e f f i c i e n t s and a c t i v i t y c o e f f i c i e n t s by t h e r e l a t i o n s h i p s : T Φ(ϋΙ,Ι) - 4>(m,T ) = {mV2vR}/


2

3L (-£)d(i)

2

and m £ny(m,T) = [(m,T)-l] + / ((m' ,T)-1 Jd^nm ο

1

where ï i s a r e f e r e n c e t e m p e r a t u r e . We have a l s o d e v e l o p e d a h e a t c a p a c i t y c a l o r i m e t e r f o r these extreme c o n d i t i o n s . The h e a t c a p a c i t y i s t h e s e c o n d t e m p e r a t u r e d e r i v a t i v e o f t h e f r e e e n e r g y and c a n be u s e d t o c a l c u l a t e t h e t e m p e r a t u r e d e p e n d ence o f e q u i l i b r i a by t h e r e l a t i o n s h i p :

AGiLPi . ΔβίΙ,Ρΐ

+

Δ

Η

(

ΐ

>

ρ

)

[

| . V,

+

/

ζ

(

Δ

£

ρ

d T 1 1 )

d (

|

( )

S i m i l a r l y the temperature dependencies o f the r e l a t i v e apparent m o l a l h e a t c o n t e n t c a n be d e t e r m i n e d f r o m t h e h e a t c a p a c i t y b y : 1

1

V" ' ^

88

111

V »*)

/ (^(m,T)

+

2

- C °(0,T))dT p

These c a l o r i m e t e r s c a n be used t o d e t e r m i n e t h e s e t h e r m a l p r o p e r t i e s t h r o u g h o u t a w i d e r a n g e o f p r e s s u r e s . The p r e s s u r e dependence c a n be used t o c a l c u l a t e v o l u m e t r i c p r o p e r t i e s by means o f t h e r e l a t i o n s h i p s : τ

Φ - "(1τ> τ

+v

ρ

Φ* =- 4) T

3 P

3)

Τ

3Γ Ρ

Advantages In a f l o w c a l o r i m e t e r t h e t h e r m o d y n a m i c p r o p e r t i e s a r e mea-



30.

WOOD AND SMITH-MAGOWAN

Calorimetry of Aqueous Solutions

571

sured i n a f l o w i n g stream contained i n a small diameter t u b i n g . F l o w c a l o r i m e t r i c t e c h n i q u e s have been u s e d f o r many y e a r s a t room t e m p e r a t u r e b e c a u s e o f t h e i r s p e e d a n d c o n v e n i e n c e ^ : ^ ' . For o p e r a t i o n s a t h i g h temperatures o r w i t h a h i g h vapor p r e s s u r e s o l v e n t , t h e advantages o f u s i n g f l o w c a l o r i m e t r i c techniques a r e overwhelming. I n t h e f i r s t p l a c e t h e r e i s no v a p o r s p a c e s o t h e r e i s no n e c e s s i t y f o r c o r r e c t i o n s f o r t h e change i n t h e v a p o r space c o m p o s i t i o n . Because o f t h e s m a l l d i a m e t e r t u b i n g u s e d , r e l a t i v e l y t h i n w a l l e d t u b i n g i s s t r o n g enough t o c o n t a i n t h e h i g h p r e s s u r e s and a s a r e s u l t t h e c a l o r i m e t e r c a n be c o n s t r u c t e d f o r very r a p i d thermal response. A t h i r d advantage i s t h a t e x p e r i m e n t s c a n be r u n c o n s e c u t i v e l y w i t h o u t c o o l i n g a n d r e l o a d i n g the c a l o r i m e t e r . A l l t h a t i s n e c e s s a r y i s t o s t a r t pumping i n t o t h e c a l o r i m e t e r f r o m room t e m p e r a t u r e t h e f l u i d s f o r t h e n e x t measurement. I n t h e f o l l o w i n g s e c t i o n we w i l l show t h a t t h e s e a d v a n t a g e s o f f l o w c a l o r i m e t r i c t e c h n i q u e s c a n be r e a l i z e d i n p r a c t i c e f o r measurements on h i g h t e m p e r a t u r e aqueous s o l u t i o n s by d i s c u s s i n g t h e o p e r a t i o n o f s e v e r a l i n s t r u m e n t s t h a t have been constructed i n our laboratory. 4)

Measurements

o f Δ^Η

The f i r s t a t t e m p t i n o u r l a b o r a t o r y t o a p p l y f l o w t e c h n i q u e s t o h i g h t e m p e r a t u r e o p e r a t i o n was t h e c o n s t r u c t i o n by D r . E . E . Messikomer o f a f l o w , h e a t - o f - m i x i n g calorimeter(12). Unfortu n a t e l y , because t h e t h e r m o p i l e s used i n t h i s i n s t r u m e n t d i d n o t work above 100°C t h e i n s t r u m e n t was l i m i t e d t o t h i s t e m p e r a t u r e . H o w e v e r , t h e r e s u l t s were e n c o u r a g i n g b e c a u s e t h e y showed t h a t v e r y r a p i d and a c c u r a t e t h e r m o d y n a m i c d a t a c o u l d be o b t a i n e d a n d t h a t t h e o p e r a t i o n o f t h e c a l o r i m e t e r was as e a s y a t 100°C a s i t was a t room t e m p e r a t u r e . Because o f t h e encouraging r e s u l t s o b t a i n e d w i t h t h e f i r s t c a l o r i m e t e r , D r . James M a y r a t h b u i l d a new v e r s i o n w h i c h was s u c c e s s f u l l y o p e r a t e d up t o A schematic o f t h i s c a l o r i m e t e r i s g i v e n i n f i g u r e 1. B a s i c a l l y i t i s a h e a t - f l o w c a l o r i m e t e r i n w h i c h t h e two l i q u i d s t o be m i x e d a r e pumped a t room temperature i n t o a counter c u r r e n t heat exchanger, a f t e r which t h e y a r e e q u i l i b r a t e d w i t h an aluminum c a l o r i m e t r i c b l o c k . Next t h e two l i q u i d s a r e m i x e d , a n d t h e h e a t g e n e r a t e d by t h e m i x i n g p r o c e s s i s e x t r a c t e d f r o m t h e f l o w i n g s t r e a m by a s e r i e s o f t h e r m o p i l e s w h i c h c a n measure t h e h e a t e x t r a c t e d . Using several heat e x t r a c t o r s g u a r a n t e e d t h a t most o f t h e h e a t g e n e r a t e d i s m e a s u r e d by t h e t h e r m o p i l e s a n d t h e sum o f t h e v o l t a g e on t h e t h e r m o p i l e s i s t h e n a measure o f t h e r a t e o f h e a t p r o d u c t i o n . A d i a g r a m o f a t y p i c a l r u n i s g i v e n i n f i g u r e 2 w h i c h shows t h e power g e n e r a t e d by m i x i n g o f magnesium c h l o r i d e w i t h w a t e r a t 200°C. I n t h i s c a l o r i m e t e r a h e a t o f d i l u t i o n t a k e s 30 m i n u t e s a n d f r o m an i n i t i a l b a s e l i n e i t t a k e s a b o u t 15 m i n u t e s f o r the c a l o r i m e t e r t o reach a steady s t a t e . The s e n s i t i v i t y o f t h i s c a l o r i m e t e r was e q u i v a l e n t t o b e i n g a b l e t o d e t e c t a 2 X 1 0 ~ K

200°c(LU.

4



572


APPLICATIONS

Figure 1. Flow heat of mixing calorimeter: (a and b) solutions to be mixed; (c) calorimetric block; (d) thermopiles for detecting heatflow;(e) exit for mixture


30.



τ

0

10

1

20

573


1

30 T/mln

1

40

ι

SO

Γ

60

Figure 2. Results of an enthalpy of dilution run on aqueous MgCl at 473 Κ (thermopile voltage vs. time—maximum 1.23 zt 0.002; Q = 1.129 watts; Δ Τ = 12.8 K) 2


574


APPLICATIONS

t e m p e r a t u r e r i s e on m i x i n g t h e c a l o r i m e t r i c f l u i d s . Some t y p i c a l r e s u l t s f r o m t h i s c a l o r i m e t e r a r e shown i n f i g u r e s 3 and 4 . It s h o u l d be n o t e d t h a t t h e h e a t o f d i l u t i o n o f magnesium c h l o r i d e a t 200°C i s 40 kJ m o l " and t h a t t h i s i s c l o s e t o t h e h e a t o f r e a c t i o n o f H w i t h OH*" a t room t e m p e r a t u r e . Thus, the heat e f f e c t s i n w a t e r a t 200°C a r e e x t r e m e l y l a r g e and c h a n g i n g r a p i d l y with temperature. This i s a f u r t h e r reminder o f the d i f f i c u l t y o f p r e d i c t i n g t h e p r o p e r t i e s o f h i g h t e m p e r a t u r e aqueous s o l u t i o n s f r o m t h e i r p r o p e r t i e s a t room t e m p e r a t u r e s . 1

+


5)

Heat C a p a c i t y

Measurements

The s u c c e s s o f t h e m i x i n g c a l o r i m e t e r s f u r t h e r e n c o u r a g e d us to c o n s t r u c t a f l o w , heat-capacity c a l o r i m e t e r using the basic design p r i n c i p a l s o f Patrick Picker et A schematic d i a gram o f o u r h e a t c a p a c i t y c a l o r i m e t e r i s g i v e n i n f i g u r e 5 . The o p e r a t i o n o f t h e i n s t r u m e n t i s as f o l l o w s : W a t e r i s pumped w i t h a h i g h p r e s s u r e l i q u i d c h r o m a t o g r a p h y pump t h r o u g h a 1.5 mm o u t s i d e d i a m e t e r h a s t a l l o y t u b e t h r o u g h a h e a t e x c h a n g e r and o n t o a c o p p e r c a l o r i m e t r i c b l o c k where i t i s e q u i l i b r a t e d a t t h e r e f e r ence t e m p e r a t u r e . The f l u i d i n t h e t u b e i s t h e n h e a t e d a b o u t 2 ° Κ and t h e r e s u l t i n g t e m p e r a t u r e r i s e d e t e c t e d by a t h e r m i s t o r . The stream then l e a v e s t h e c a l o r i m e t r i c b l o c k through t h e c o u n t e r c u r r e n t h e a t e x c h a n g e r and goes t o a s a m p l e i n j e c t i o n v a l v e w h i c h a l l o w s a s a m p l e l o o p ( c o n t a i n i n g 10 ml o f t h e s o l u t i o n t o be measured) t o be i n t e r j e c t e d i n t o t h e f l o w s t r e a m . The s t r e a m t h e n goes t h r o u g h a. s e c o n d c o u n t e r c u r r e n t h e a t e x c h a n g e r and i n t o an i d e n t i c a l Q l o r i m e t r i c u n i t w i t h h e a t e r and t h e r m i s t o r t o detect the temperature r i s e . A f t e r l e a v i n g the block through the counter c u r r e n t heat exchanger the s o l u t i o n e x i t s the system t h r o u g h a back p r e s s u r e r e g u l a t o r . I n o p e r a t i o n t h e pump and h e a t e r s a r e t u r n e d on w i t h w a t e r f l o w i n g t h r o u g h b o t h c a l o r i m e t r i c u n i t s and a w h e a t s t o n e b r i d g e c o n t a i n i n g t h e two t h e r m i s t o r s i n o p p o s i t e arms i s b a l a n c e d . A f t e r a steady s t a t e i s reached the s a m p l e l o o p v a l v e i s opened and t h e s a m p l e s o l u t i o n f l o w s t h r o u g h the second c a l o r i m e t r i c u n i t . When t h e sample h i t s t h e h e a t e r and t h e r m i s t o r on t h e s e c o n d u n i t t h e r e i s a change i n h e a t c a p a c i t y and a c o n s e q u e n t change i n t e m p e r a t u r e . The h e a t e r on t h i s u n i t i s a d j u s t e d t o r e b a l a n c e t h e t h e r m i s t o r b r i d g e and t h u s t o keep t h e t e m p e r a t u r e r i s e e x a c t l y t h e same. The r a t i o o f t h e power a p p l i e d t o t h e h e a t e r w i t h w a t e r f l o w i n g and w i t h t h e sam p l e f l o w i n g i s then the r a t i o o f t h e v o l u m e t r i c heat c a p a c i t i e s o f t h e two s o l u t i o n s . The r a t i o o f t h e mass f l o w s t h r o u g h t h e c a l o r i m e t r i c u n i t s i n t h e two c a s e s i s j u s t t h e r a t i o o f t h e d e n s i t i e s o f w a t e r and o f t h e s o l u t i o n t o be measured a t t h e t e m p e r a t u r e o f t h e sample l o o p . T h i s c a n be e a s i l y shown u s i n g t h e a s s u m p t i o n s t h a t 1) a t c o n s t a n t c o m p o s i t i o n , t h e mass f l o w i s i n d e p e n d e n t o f t e m p e r a t u r e 2) a t c o n s t a n t t e m p e r a t u r e , t h e v o l u m e t r i c f l o w i s i n d e p e n d e n t o f t h e c o m p o s i t i o n and 3) t h e h e a t and volume o f m i x i n g a t t h e i n t e r f a c e between t h e two s o l u t i o n s p r o -

alW»





Figure 3.

Apparent molal enthalpy of aqueous NaCl as a function of molality and temperature



APPLICATIONS


576


30.




SAMPLE

LOOP

BACK . PRESSURE REGULATOR PUMP

VACUUM CHAMBER

ADIABATIC SHEILD THERMISTOR

HEATER

Figure 5.

Schematic of flow, heat-capacity calorimeter


577

578


APPLICATIONS

d u c e s a n e g l i g i b l e change i n v o l u m e . Experience with the Picker i n s t r u m e n t has shown t h a t t h i s l a t t e r a s s u m p t i o n i s q u i t e a c c u r ate. The r e s u l t i n g e q u a t i o n f o r t h e h e a t c a p a c i t i e s o f t h e two solutions is P

S,2 P'

1

2

.

l

p

1

Λ

.

Where P. and P a r e t h e powers i n t h e h e a t e r w i t h s o l u t i o n s 1 and 2 f l o w i n g and p-| and p a r e t h e d e n s i t i e s o f t h e s o l u t i o n s a t t h e temperature o f t h e sample l o o p . The r e s u l t i n g i n s t r u m e n t has a r e s p o n s e t i m e f o r changes o f e l e c t r i c a l power i n p u t o f 50 s e c o n d s f o r 99% r e s p o n s e , a s e n s i t i v i t y t o a change i n power o f a b o u t 0 . 0 0 5 % and a p r o v e n c a p a b i l i t y o f o p e r a t i n g w i t h t h i s s e n s i t i v i t y up t o t e m p e r a t u r e s as h i g h as 325°C. The r e s u l t s o f some measurements on s o d i u m c h l o r i d e s o l u t i o n s a r e g i v e n i n f i g u r e 6 . The d i f f i c u l t y o f p r e d i c t i n g p r o p e r t i e s o f s o l u t i o n s a t h i g h t e m p e r a t u r e s i s e m p h a s i z e d by t h e s e results. A t l o w c o n c e n t r a t i o n s t h e r e i s a v e r y l a r g e change i n h e a t c a p a c i t y as t e m p e r a t u r e i n c r e a s e s w h i c h i s n o t p r e s e n t a t higher concentrations. Indeed, the heat c a p a c i t y d i f f e r e n c e s a t 25°C seems a l m o s t n e g l i g i b l e compared w i t h t h e changes f o u n d w i t h c h a n g i n g t e m p e r a t u r e a t l o w m o l a l i t y and w i t h c h a n g i n g m o l a l i t y at high temperature. I t i s d i f f i c u l t t o compare t h e s e r e s u l t s w i t h t h e p r e v i o u s r e s u l t s o f o t h e r a u t h o r s s i n c e we c h o s e t o make measurements a l o n g an i s o b a r w h i c h was a c c e s s i b l e a t a l l t e m p e r a t u r e s . This a l l o w s c o n v e n i e n t d a t a r e d u c t i o n . O t h e r a u t h o r s have g e n e r a l l y measured a l o n g t h e s a t u r a t e d w a t e r v a p o r p r e s s u r e c u r v e w i t h c o n sequent c o m p l i c a t i o n s i n data r e d u c t i o n . While a t p r e s e n t , o n l y t h i s one i s o b a r has been s y s t e m a t i c a l l y i n v e s t i g a t e d , a few measurements o f t h e p r e s s u r e dependence a t 320K and 572K have been made. These r e s u l t s show t h a t a t a l l t e m p e r a t u r e s t h e p r e s s u r e dependence o f t h e h e a t c a p a c i t y i s a p p r e c i a b l e and needs t o be more f u l l y e v a l u a t e d , s i n c e t h e p r e s e n t body o f v o l u m e t r i c d a t a i s n o t s u f f i c i e n t l y p r e c i s e t o make t h e s e c o r r e c t i o n s a c curately. In f a c t , o u r v e r y l i m i t e d r e s u l t s a t d i f f e r e n t p r e s s u r e s s u g g e s t t h a t t h e p r e c i s i o n o f t h i s p r o c e d u r e may be s u f f i c i e n t t o supplant volumetric determinations a t high temperature and p r e s s u r e s . 2


2

6)

Conclusion

These p r e l i m i n a r y r e s u l t s show t h a t t h e p r o m i s e o f f l o w c a l o r i m e t r i c t e c h n i q u e s f o r i n v e s t i g a t i n g t h e thermodynamic p r o p e r t i e s o f h i g h t e m p e r a t u r e aqueous s o l u t i o n s has been r e a l i z e d . A l t h o u g h t h e r e a r e many e x p e r i m e n t a l d i f f i c u l t i e s i n a d a p t i n g




579


T/K

1 Figure 6.

Apparent molal heat capacity of aqueous NaCl at 177 bars as a function of molality and temperature


580

THERMODYNAMICS OF AQUEOUS SYSTEMS WITH INDUSTRIAL APPLICATIONS

these t e c h n i q u e s t o high temperature o p e r a t i o n , these problems have now been s o l v e d and we have a r a p i d and s e n s i t i v e m e a s u r i n g instrument. As a r e s u l t we c a n now g e t down t o t h e b u s i n e s s o f e x p l o r i n g aqueous s o l u t i o n c h e m i s t r y a t t e m p e r a t u r e s up t o 325°C.


Literature Cited

1. a) Gardner, W.L.; Mitchell, R.E.; Cobble, J.W. J. Phys. Chem., 1969, 73, 2025. b) Cobble, J.W. and Murray, Jr., R.C. Discussions Faraday Soc., 1977, 64. 2. C-Τ. Liu and W.T. Lindsay. J. Soln. Chem., 1972, 1, 45. 3. Likke, S. and Bromley, L.A. J. Chem. Eng. Data, 1973, 18, 189. 4. Puchkov, L.V.; Styazhkin, P.S.; Fedorova, M.K. J. Applied Chem. (Russ.), 1976, 49, 1268. 5. Monk, P. and Wadsö, I. Acta Chem. Scand. 1968, 22, 1842. 6. Picker, P.; Jolicoeur, C.; Desnoyers, J.E. J. Chem. Thermo dynamics, 1969, 1, 469. 7. Gill, S.J. and Chen, Y.-J. Rev. Sci. Instruments, 1972, 43, 774. 8. Picker, P.; Fortier, J.-L.; Philip, P.R; Desnoyers, J.E. J. Chem. Thermodyn., 1971, 3, 631. 9. Elliot, K.; Wormald, C.J. J. Chem. Thermodynamics, 1976, 8, 881. 10. Messikomer, E.E.; Wood, R.H. J. Chem. Thermodynamics, 1975, 7, 119. 11. Mayrath, J.E., "A Flow Microenthalpimetric Survey of Electro lyte Solution Thermodynamic Properties from 373K to 473K" Ph.D. Dissertation, University of Delaware, June, 1979. RECEIVED

January 30, 1980.


Thermodynamics of Aqueous Systems with Industrial Applications

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