Recleaning Sodium Heat Transfer Systems

Recleaning Sodium Heat Transfer Systems W. H. BRUGGEMAN, F. C. HANNY, and H. F. KARNES

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: January 1, 1957 | doi: 10.1021/ba-1957-0019.ch008

General Electric Co., Schenectady, N.Y.

Occasional removal of residual metal from components is necessary to all users of sodium. In more complicated components, such recleaning can be time-consuming and hazardous. The relative merits and techniques of using alcohol, steam, and water for this purpose are discussed, as well as test data and evaluation of a new recleaning technique — the moist gas procedure. In this technique, the cleaning is accomplished by a mixture of water vapor and inert gas. The moist gas procedure minimizes the high-temperature excursions prevalent in steam cleaning and is thus applicable to sodium removal from large complex-system geometries.

T h e desirable properties of s o d i u m as a heat-transfer fluid f o r n u c l e a r p o w e r application have resulted i n the development of various phases of s o d i u m t e c h nology. O n e phase of the p r o g r a m was the e v o l u t i o n of c l e a n i n g techniques f o r the r e m o v a l of r e s i d u a l n o n d r a i n a b l e s o d i u m f r o m systems or components. B e cause of the c h e m i c a l v i g o r of the s o d i u m - w a t e r reaction, water cannot n o r m a l l y be used for this purpose. T h e paper summarizes the use of other r e c l e a n i n g agents a n d describes techniques e m p l o y e d i n u s i n g these agents. I n addition to their a p p l i c a b i l i t y to s o d i u m - c o o l e d n u c l e a r plants, these techniques are also useful i n a n y process system f o r the p r o d u c t i o n o r u t i l i z a t i o n of b u l k quantities of this alkali metal.

Recleaning Techniques T h e most obvious reason for the r e m o v a l of u n d r a i n e d s o d i u m films stems f r o m the c h e m i c a l a c t i v i t y of the m e t a l . S c r a p piles of s o d i u m - s y s t e m components, if left uncleaned, present the possibility of h y d r o g e n fires a n d the added h a z a r d of caustic b u r n s to personnel. T h i s is p a r t i c u l a r l y true i f the scrap is exposed to r a i n or h u m i d atmospheric conditions. S i m i l a r l y , i n m a k i n g extensive repairs to s o d i u m systems or components, the r e m o v a l of s o d i u m facilitates w o r k i n g c o n ditions, since it eliminates the possibility of the a b o v e - m e n t i o n e d complications. A f t e r s o d i u m recleaning, maintenance can be p e r f o r m e d b y personnel w h o need not be t r a i n e d i n the spécifie disciplines of s o d i u m technology. In the n u c l e a r field, a n a d d i t i o n a l reason f o r r e c l e a n i n g results f r o m the possibility of radioactive contamination of that p a r t of the system e x t e r n a l to the n u c l e a r reactor. S u c h contamination can occur either b y fission-product release or b y corrosion of the reactor a n d subsequent redeposition of the corrosion p r o d ucts i n the p u m p s , heat exchangers, or other parts of the r e a c t o r - c o o l i n g system. 67

In HANDLING AND USES OF THE ALKALI METALS; Advances in Chemistry; American Chemical Society: Washington, DC, 1957.

68

A D V A N C E S IN

CHEMISTRY SERIES


If excessive, this c o n t a m i n a t i o n must be r e m o v e d b y a q u e o u s - a c i d d e c o n t a m i n a t i o n p r i o r to equipment maintenance. N e u t r a l i z i n g the c h e m i c a l a c t i v i t y of the s o d i u m is the first step i n such decontamination. I n r e c l e a n i n g of d r a i n e d s o d i u m systems, the s o d i u m combines w i t h a h y d r o x y ! r a d i c a l to f o r m a s o d i u m c o m p o u n d a n d free h y d r o g e n . [ A n exception to this is the use of l i q u i d a m m o n i a w h i c h is a solvent for s o d i u m (1, 2 ) . T h e expense of e q u i p m e n t a n d c o m p l e x i t y of operation p r e c l u d e its use o n large systems a n d , hence, it is not covered i n this paper.] T h e p r i n c i p a l agents used are water, steam, or the alcohols. M a n y of the p r o b l e m s encountered are independent of these materials but are d i r e c t l y influenced b y the design of the system or component b e i n g cleaned. T h e importance of p r o p e r design i n facilitating ultimate r e c l e a n i n g cannot be overemphasized. A s such, p r i o r to discussing details of r e c l e a n i n g t e c h niques, generalizations are m a d e of those considerations w h i c h s h o u l d be factored into the o r i g i n a l design of s o d i u m equipment. S o d i u m systems s h o u l d be completely d r a i n a b l e . I n a d d i t i o n to drains o n a l l vessels, d r a i n lines s h o u l d be p r o v i d e d at a l l pockets or traps i n the p i p i n g system. P i p i n g s h o u l d be sloped to a m a x i m u m degree, at least 3% or greater. Since s o d i u m is nonvolatile a n d , thus, cannot be r e a d i l y r e m o v e d b y evaporation, the attention p a i d to drainage s h o u l d be an o r d e r of m a g n i t u d e greater t h a n that a p p l i e d to a conventional l i q u i d system. A s the r e c l e a n i n g reaction releases h y d r o g e n gas, the system s h o u l d be e q u i p p e d w i t h large v e n t i n g lines or openings. T h e m a j o r h a z a r d i n the r e c l e a n i n g of s o d i u m systems is due to the r a p i d e v o l u t i o n of h y d r o g e n a n d the subsequent overpressure a n d b u r s t i n g of components not p r o v i d e d w i t h adequate relief capacity. G e n e r a l i z a t i o n of adequate vent size is impossible. H o w e v e r , i t m u s t be r e m e m b e r e d that for each p o u n d - m o l e (23 pounds) of s o d i u m that reacts, 0.5 p o u n d - m o l e (180 cubic feet S . T . P . ) of h y d r o g e n gas w i l l be f o r m e d . It is difficult to determine i n a c o m p l e x geometry w h e n the r e c l e a n i n g reaction is complete. It is f a i r l y certain, h o w e v e r , that a s t r a i g h t - t h r o u g h system is cleaned i f the reactant flows t h r o u g h for a sufficient l e n g t h of time. T h i s is not t r u e i n a p a r a l l e l - p a t h system. I n this latter case, undetected s o d i u m plugs c a n r e m a i n t h r o u g h the i n i t i a l phases of the r e c l e a n i n g o n l y to react v i g o r o u s l y d u r i n g the final water rinse. A c c o r d i n g l y , p a r a l l e l paths s h o u l d be v a l v e d w h e n e v e r possible. V a l v i n g permits assurance that each i n d i v i d u a l p a t h is open. F o r p a r a l l e l paths that cannot be v a l v e d , as i n heat exchangers, utmost caution s h o u l d be exercised. It is a desirable objective to c l e a n the least c o m p l e x arrangement possible, a n d therefore, the system should be kept simple. F o r a c o m p l e x arrangement, components or sections of systems s h o u l d be disassembled a n d cleaned i n d i v i d u a l l y . I n the r e c l e a n i n g of s o d i u m films, either alcohol or steam is u s e d i n the i n i t i a l phase, f o l l o w e d b y a final water rinse. B o t h of the i n i t i a l agents h a v e advantages a n d disadvantages for the r e c l e a n i n g a p p l i c a t i o n a n d , thus, the choice of technique is influenced b y the application. ALCOHOL RECLEANING. U n l i k e water, the alcohols react s l o w l y w i t h s o d i u m . T h e a b i l i t y to conduct controllable reactions i n the l i q u i d state is the p r i n c i p a l advantage of alcohol r e c l e a n i n g . T h e h e a t - a b s o r b i n g capacity of the b o i l i n g alcohol prevents overtemperature of the reaction a n d , hence, tends to p r o v i d e selfr e g u l a t i o n . Because of the r e f l u x i n g a n d condensing action of the alcohol, it is often used i n r e m o v i n g a s o d i u m h e e l f r o m vessels. T h e p r i n c i p a l disadvantage of this technique is the fire h a z a r d attendant w i t h b o i l i n g h y d r o c a r b o n s . T h i s , together w i t h m a t e r i a l cost, p r i m a r i l y limits the use of alcohol r e c l e a n i n g to s m a l l , delicate components. T h e technique is also u s e f u l i f the r e c l e a n i n g is p e r f o r m e d b y someone f a m i l i a r w i t h chemicals but not necessarily w i t h s o d i u m technology. A l c o h o l s h o u l d n e v e r be used i n r e c l e a n i n g s o d i u m - p o t a s s i u m o r potassium systems i n o r d e r to a v o i d the potentially violent potassium super o x i d e - h y d r o c a r b o n reaction. T h e rate of the s o d i u m - a l c o h o l reaction is a function of water content, m o l e c u l a r weight, a n d n u m b e r of h y d r o x y radicals of the alcohol. T h e lowest alcohol, m e t h a n o l , reacts steadily w h e n at r o o m temperature b u t w i t h o u t u n d u e v i g o r . A l c o h o l s above b u t a n o l react so s l o w l y as to be ineffective. P r a c t i c a l aspects


BRUGGEMAN,

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69

dictate the use of either m e t h a n o l or ethanol. I n a d d i t i o n to cost, the h i g h e r m o n o h y d r i c alcohols are not completely miscible w i t h water.


S u c h p a r t i a l m i s c i b i l i t y prevents subsequent d i l u t i o n of the alcohol w i t h water d u r i n g the final phases of the recleaning. I n addition, the h i g h e r m o n o h y d r i c alcohols, glycols, a n d glycerols tend to p o l y m e r i z e a n d t h e r m a l l y decompose, p a r t i c u l a r l y at temperatures of their higher b o i l i n g points a n d i n the presence of sodium. T h e extent of such p o l y m e r i z a t i o n a n d decomposition was v i v i d l y illustrated i n one e x p e r i m e n t p e r f o r m e d b y the authors. I n this case, ethylene g l y c o l was used to reclean a 4 - i n c h p i p i n g system. P l u g g i n g of the 4 - i n c h l i n e o c c u r r e d w h i c h , u p o n examination, was f o u n d to have resulted f r o m deposition of siltlike products. A t y p i c a l a l c o h o l - r e c l e a n i n g operation c a n best be described b y considering a n actual c l e a n i n g operation p e r f o r m e d at the K n o l l s A t o m i c P o w e r L a b o r a t o r y (operated f o r the U . S. A t o m i c E n e r g y C o m m i s s i o n , b y the G e n e r a l E l e c t r i c C o . , Schenectady, Ν . Y . ) . I n the case covered, the i t e m recleaned was a 3000-gallonsp e r - m i n u t e l i n e a r - i n d u c t i o n electromagnetic ( E M ) p u m p . T h e p u m p r e c l e a n i n g provides a good e x a m p l e of the use of the alcohol technique, since the t h i n - d u c t sections a n d t h i n - i n t e r n a l expansion bellows are susceptible to s o d i u m h o l d u p a n d subsequent t h e r m a l distortion a n d damage, i f h i g h reaction temperatures prevail. F i g u r e 1 represents a flowsheet of the cleaning setup. T h e d r a i n e d p u m p is m o u n t e d to r e v o l v e o n rollers i n order to assure e v e n heating a n d complete r e action of s o d i u m h e l d u p i n the complex bellows configurations. T h e alcohol, nitrogen, a n d flame arrester connections are made t h r o u g h s w i v e l joints. A f t e r c h e c k i n g joints for tightness, the procedure is as follows: O p e n l o w - p r e s s u r e ( N a O H ( s )

+

= total moles of gas entering i n time θ = mole fraction of water v a p o r i n this gas = moles of water reacting i n time Θ = time i n t e r v a l i n hours balance based on a 1 2 6 - g r a m - m o l e s o d i u m charge is:

Component Nitrogen Water Sodium Sodium hydroxide Hydrogen

Initial M (l—y) My 126 0 0

I n a second e q u a l time i n t e r v a l , the m a t e r i a l balance w i l l b e : Component Nitrogen Water Sodium Sodium hydroxide Hydrogen

Initial M (1—y) My 126—Wi Wi 0

Final M (1—y) My—Wi 126—Wi Wi YïWi

Final M (1—y) My—W 126— (W, + Wt ) Wi + Ws VSWi 2

S i n c e the rate of reaction varies d u r i n g the r u n due to temperature a n d c o n centration changes, the moles of caustic f o r m e d per u n i t time w i l l also v a r y . H e n c e , W i does not necessarily e q u a l W . A t the e n d of the test, the total amount of caustic f o r m e d w i l l e q u a l a s u m m a t i o n of the moles p r o d u c e d ( S W ) i n each time i n t e r v a l . T h e u n r e a c t e d s o d i u m at the end of the r u n w i l l therefore be equal to 126 -2W. 2

T h e e n t h a l p y of each component above 77 °F. is calculated f r o m the t e m p e r a ture data. A n arithmetic average of the gas temperatures at the start and end of the time i n t e r v a l was used i n the calculations of the gas enthalpies. T h e t e m p e r a tures of the nonvolatile components are defined as: U to Ti T

average inlet gas temperature, °F. average outlet gas temperature, °F. i n i t i a l reactor temperature, °F. final reactor temperature, °F. cal. C = m o l a r specific heat, g. mole °F. Reactor mass = 324 g. moles f

= = = =


A D V A N C E S IN CHEMISTRY SERIES

74


T h e enthalpy of each component is therefore: Nitrogen.

(M)

( C m ) (fcr-ti)

Water.

(M„) (C

Sodium.

( 1 2 6 - 2 W ) (Cx.) ( T - T * ) - ( W )

Sodium hydroxide.

( s W ) (Cx. ) ( T - T )

Hydrogen.

( W ) (C ) 2

Reactor mass.

(324) (C .)

H20

)

(fcr-if) -

(W) (C r

0H

H2

F

r

4

+

H20

)

(t,-77) (C )

(W) ( C

Na

N a 0 H

)

(T -77) r

(Τ -77) Γ

0,-77) (T,-T ) 4

T h e s u m m a t i o n of the above enthalpies represents the total e n t h a l p y change of the reactants, the products of reaction, a n d the system. T h e heat of reaction between water a n d s o d i u m then equals this e n t h a l p y change plus the heat loss. T h e latter is a f u n c t i o n of the reactor temperature a n d c a n be calculated f r o m the e x p e r i m e n t a l l y d e t e r m i n e d relationship of heat loss vs. temperature. T h i s heat balance of the system is expressed as: H

R

where

=

enthalpy change +

Q

L

HR =

heat of reaction at 77° F .

Q

heat loss

L

=

Γ" kcal. H I I 45.7 J L g.-mole J

T h e total heat e v o l v e d f r o m the reaction i n a g i v e n i n t e r v a l therefore equals (45.7) ( W ) . Since the equation n o w contains o n l y one u n k n o w n , a solution c a n be obtained for the total moles of s o d i u m reacting per u n i t time. RESULTS OF HUMID GAS STUDIES. T h e test data were obtained i n a configuration w i t h m o r e s o d i u m h o l d u p t h a n w o u l d be present i n the p i p i n g of a p r o p e r l y d r a i n e d system. T h e y do represent, however, the m a g n i t u d e of possible h o l d u p i n certain components a n d sections of p i p i n g configurations. T h e results of the study indicate the comparative effects of several operational variables. These data are m o r e useful for this c o m p a r i s o n t h a n for estimating exact temperature excursions i n specific system geometries. A s a n objective of the cleaning procedure is to r e m o v e s o d i u m f r o m a system w i t h as l o w a temperature rise as possible, the heat of reaction between s o d i u m a n d water v a p o r is a p p r o x i m a t e l y 45 k c a l . per g r a m - m o l e , the adiabatic temperature rise of the stoichiometric reaction masses w o u l d be more t h a n 2000 °F. T h e h u m i d gas cleaning technique attempts to l i m i t this temperature rise b y c o n t r o l l i n g the rate of reaction a n d b y a i d i n g i n r e m o v a l of the e v o l v e d heat. T h e r e are several factors w h i c h influence the rate of reaction i n a g i v e n system. S o m e d e p e n d o n system characteristics w h i c h cannot be controlled d u r i n g the cleaning operation; o p e r a t i o n a l - p r o c e d u r e factors are controllable. T h e first category includes s o d i u m quantity, s o d i u m s u r f a c e - t o - v o l u m e ratio, a n d presence of u n d r a i n a b l e s o d i u m pockets. T h e second category includes water vapor concentration i n h u m i d gas, h u m i d gas feed rate or residence time, h u m i d gas turbulence, a n d h u m i d gas a n d o r i g i n a l system temperatures. T h e study was e x p l o r a t o r y i n nature, a n d o n l y five runs were made. C o n s e quently, o n l y a f e w of the c o n t r o l l e d test factors were e x a m i n e d . O p e r a t i o n a l conditions of the r u n s made are listed i n T a b l e I. T h e effectiveness of each c a n be m e a s u r e d b y the s y s t e m - t e m p e r a t u r e rise a n d also b y the fraction of water v a p o r reacted (utilization f a c t o r ) .


BRUGGEMAN, H A N N Y A N D

KARNES—RECLEANING SODIUM HEAT TRANSFER SYSTEMS

75

Table I. Operational Conditions Initial s o d i u m quantity Initial g a s - l i q u i d interface area Initial s o d i u m temperature Gas inlet temperature

6.4 l b . (126 gram-moles) 0.97 sq. feet 350 ° F . 350 ° F .

Inlet G a s T o t a l gas feed rate, g.-mole/hr. 94 540 44 540 550


Run No. 1 2 3 4 5

H 0 content, mole % 49 9 100 51 100 2

H2O feed rate, g.-mole/hr. 46 47 44 290 550

System Pressure, Atm. 1 1 1 1 0.1

T h e reaction-vessel temperature as a f u n c t i o n of time is presented i n F i g u r e 5. O n l y i n the first two runs were results obtained w h i c h w o u l d be considered satis factory for c l e a n i n g components — i.e., the temperature rises were v e r y moderate. In the other runs, the temperature excursions were excessive. B a s e d o n these temperatures a n d h e a t - b a l a n c e calculations, the instantaneous heat of reaction for each r u n was d e t e r m i n e d . These calculations, presented i n F i g u r e 6, r e a d i l y e x p l a i n the temperature excursions experienced. F i g u r e 7 is a plot of the u t i l i z a t i o n factor vs. time. T h e first two r u n s indicate that a n average of o n l y 8% of the water v a p o r f e d to the reaction vessel reacted w i t h the sodium. T h e fraction of unreacted water increased the heat r e m o v a l capacity of the n i t r o g e n diluent, thereby m i n i m i z i n g the temperature rise of the system. Conditions w h i c h y i e l d e d greater u t i l i z a t i o n factors u s u a l l y resulted i n greater temperature excursions. T h e r a p i d rate of s o d i u m conversion i n r u n s 4 a n d 5 is u n d o u b t e d l y a result of the effect of temperature o n reaction rate. A comparison of F i g u r e s 5 a n d 7 reveals that the rate of reaction o n s o d i u m c o n v e r sion changed as the reaction temperature d r o p p e d . I n r u n s 1 a n d 2 the rate d i d not appear to be m a t e r i a l l y affected because the reaction temperature r e m a i n e d f a i r l y constant.

I600h

200h

n u

I 0

ι 20

I 40

Figure 5.

ι ι I 60 80 100 TIME-MINUTE?

I 120

I 140

1 160

Reaction temperature vs. time




TIME - MINUTES Figure 6.

Heat generation vs. time


BRUGGEMAN, H A N N Y A N D KARNES—RECLEANING

SODIUM HEAT TRANSFER SYSTEMS

77

A s u m m a t i o n of the instantaneous u t i l i z a t i o n factors represents the total amount of s o d i u m reacted d u r i n g the r u n . T h i s s u m m a t i o n is presented i n F i g u r e 8. C o m p l e t e conversion of a l l of the s o d i u m was not r e a l i z e d i n the r e l a t i v e l y short p e r i o d of the r u n s .


A s a n i l l u s t r a t i o n of the effect of h u m i d - g a s - w a t e r concentration, a plot of m a x i m u m temperature vs. water content was m a d e for r u n s 1 t h r o u g h 3. T h e plot of F i g u r e 9 shows that the temperature excursions were effectively r e d u c e d as the water concentration decreased. W i t h o u t i n e r t - g a s diluent, the temperature rise exceeded 750°F. A 1 to 1 d i l u t i o n r e d u c e d the temperature excursions to 100°F. A f u r t h e r d i l u t i o n of 9 to 1 decreased the temperature rise to less t h a n 70 ° F . T h e temperature rise results, of course, f r o m the rate of heat e v o l u t i o n d u e to the c h e m i c a l reaction. T h e presence of the inert gas not o n l y serves to r e m o v e some of this heat b u t also decreases the quantity of water reacting. T h u s , w h e n m i x e d w i t h inert gas, o n l y 8% of the water v a p o r reacted w i t h the s o d i u m . W h e n u n d i l u t e d steam was used, a p p r o x i m a t e l y 6 0 % of the available w a t e r reacted. T h e inert gas p r o b a b l y contributes to the diffusion resistance of the water i n m i g r a t i n g to the s o d i u m interface. I n addition, the increased v o l u m e of the gas due to d i l u t i o n decreases the residence time a n d , hence, decreases the o p p o r t u n i t y for the water v a p o r to react. H o w e v e r , e v e n u n d e r e q u a l residence times, as noted i n c o m p a r i n g r u n s 2 a n d 4, the temperature rise i n the reaction vessel is greater at the h i g h e r water concentration. T u r b u l e n c e of the gas i n the reaction vessel, as expected, affects the t e m p e r a ture rise of the s o d i u m . U n d e r c o m p a r a b l e w a t e r - v a p o r concentrations, increased t u r b u l e n c e resulted i n greater c h e m i c a l activity. I n c o m p a r i n g r u n s 4 a n d 1, it is noted that the gas rate was increased a p p r o x i m a t e l y s i x f o l d w h i l e the v a p o r c o n centration was m a i n t a i n e d constant. T h e r e s u l t i n g increase i n gas v e l o c i t y c h a n g e d the R e y n o l d s n u m b e r (based o n cross flow i n the 6 - i n c h pipe) f r o m l a m i n a r c o n d i t i o n to one of t u r b u l e n c e . T h e m a x i m u m temperature rise increased f r o m 100°F. to m o r e t h a n 900°F., a n d the u t i l i z a t i o n factor increased threefold. A l t h o u g h the greater t u r b u l e n c e increases the cooling rate to the i n e r t - g a s carrier, it also results 0.8,

,

0.7h

TIME · MINUTES Figure 8.

Total utilization vs. time



78

1200,

1000

800


I* 6001

INITIAL SYSTEM TEMR

400

200

CONSTANT: WATER FEED RATE 44-47 6» -MOL 20 40 60 80 W\TER VAPOR CONCENTRATION - % VOL. Figure 9.

100

Maximum temperature vs. water vapor concentration

i n increased mass transfer w h i c h is a c c o m p a n i e d b y a n accelerated reaction rate. O n the basis of these data it appears that the increased rate of reaction m o r e t h a n offsets the g a i n i n heat transfer. In order to determine i f the advantages gained b y the h u m i d - g a s technique c o u l d be d u p l i c a t e d b y using l o w temperature superheated steam, r u n 5 was made. I n this r u n , 300 ° F . steam was f e d to the reaction vessel m a i n t a i n e d at a pressure of 0.1 a t m . T h e reaction was v e r y vigorous, a n d the temperature rose f r o m 350 °F. to 1440°F. w i t h i n 10 minutes. It is b e l i e v e d that the excessive temperature rise was d u e to the extreme gas turbulence o c c u r r i n g at subatmospheric conditions. T h i s rise is also attributed to the greater w a t e r - f e e d rate of this r u n . T h e h i g h temperatures e x p e r i e n c e d resulted i n the f o r m a t i o n of s o d i u m m o n o x i d e f r o m the c a u s t i c - s o d i u m reaction. These oxides, p r o d u c e d as a fine powder, were c a r r i e d i n the gas stream a n d caused p l u g f o r m a t i o n i n the discharge p i p i n g . A s the s o d i u m is depleted b y its reaction w i t h the water v a p o r , the caustic concentration is increased. S i n c e s o d i u m h y d r o x i d e is p r a c t i c a l l y insoluble i n s o d i u m , the reaction rate w i l l b e unaffected b y the presence of the caustic solids, p r o v i d i n g that adequate agitation exists. T h e i m p o r t a n c e of this factor becomes greater as the fraction of the s o d i u m decreases, f o r unless fresh l i q u i d m e t a l is exposed to the water vapor, the reaction rate w i l l decrease. T h e c h e m i c a l c o m b i n a t i o n of the water a n d the s o d i u m must t h e n depend u p o n the diffusion of the water molecules t h r o u g h the caustic b a r r i e r . S i n c e s o d i u m h y d r o x i d e is m o r e t h a n twice as dense as s o d i u m , i t w o u l d appear that its f o r m a t i o n w o u l d have no effect on the reaction rate — i.e., it w o u l d sink to the bottom of the pocket, thereby exposing fresh s o d i u m . P r e v i o u s observations b y the authors i n d i c a t e d that caustic f o r m e d d u r i n g a reaction r e m a i n e d as a film over the s o d i u m . T h e support of this film is p r e s u m e d to be d u e to a surface tension p h e n o m e n o n . G a s - f l o w conditions c o n d u c i v e to c a u s t i c - f i l m f o r m a t i o n were p r o b a b l y a factor i n the e x p e r i m e n t a l r u n s . F o r example, i n r u n 1, it is p r o b a b l e that after


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a film was f o r m e d i n i t i a l l y , a l l subsequent s o d i u m - w a t e r reactions necessitated the diffusion of water molecules t h r o u g h the caustic " s k i n " over the unreacted sodium. I n order to obtain some order of magnitude estimate of this diffusional process, the data of runs 1 a n d 2 were a n a l y z e d . T h e analysis assumed the presence of a continuous a n d u n b r o k e n , u n i f o r m l y distributed film. B a s e d o n the analysis, a diffusivity coefficient for the conventional diffusion equation ( F i c k ' s l a w r e p r e sented b y dc/dt = D d c/dX ) was obtained. T h e v a l u e calculated w a s : 5 χ 10~ sq. c m . p e r second at 400°F. 2

2


7

B a s e d o n this coefficient a n d assuming the v a l i d i t y of F i c k ' s l a w , it is c a l c u lated that at 400 °F. a n d for h u m i d - g a s concentrations i n the 9 to 50% water range, a p p r o x i m a t e l y 400 hours w o u l d be r e q u i r e d for a 1-inch thick pocket of s o d i u m to react. T h i s order of magnitude calculation indicates the desirability of extreme turbulence i n recleaning. T o a v o i d h i g h temperatures, such turbulence should best be i n t r o d u c e d b y a h u m i d gas of l o w - w a t e r content or after the b u l k of the t h i n s o d i u m films of a system have been reacted. T h e diffusion rate also points out the u n d e s i r a b i l i t y of r e l a t i v e l y deep, stagnant pockets. T h e f o r m a t i o n of a caustic s k i n over l i q u i d s o d i u m presents a further u n d e s i r able condition. S o d i u m h y d r o x i d e w i l l react w i t h s o d i u m to f o r m s o d i u m m o n o x ide a n d h y d r o g e n . T h i s reaction, however, does not proceed at a significant rate u n t i l the temperature is above 700 °F. a n d then is o n l y moderately exothermic. T h e danger lies i n the water associated w i t h the s o d i u m h y d r o x i d e — i.e., the caustic tends to absorb and r e t a i n water even at elevated temperatures. T h i s is evident f r o m the e q u i l i b r i u m concentrations presented i n F i g u r e 10 (3). If, for example, the caustic s k i n is at e q u i l i b r i u m w i t h 500 °F. gas at 50% h u m i d i t y , it w i l l contain 12% water. Since the film is not tenacious, it c a n r e a d i l y b r e a k u p a n d m i x w i t h the l i q u i d sodium. T h e reaction of wet caustic a n d s o d i u m is v e r y vigorous and u s u a l l y results i n a r a p i d temperature excursion. A t the h i g h temperature, the c a u s t i c - s o d i u m reaction also occurs. A f u r t h e r test was r u n to demonstrate the m e t h o d b y w h i c h this caustic layer is i n i t i a l l y f o r m e d a n d subsequently b r o k e n . H u m i d gas was i n t r o d u c e d to the reaction vessel p a r a l l e l to the surface of the l i q u i d s o d i u m at l a m i n a r - f l o w c o n ditions. T h e temperature rise was v e r y moderate d u r i n g this operation, indicating the reaction of some of the s o d i u m . Inert gas was t h e n directed n o r m a l to the surface. A localized temperature rise of a p p r o x i m a t e l y 150°F. was i m m e d i a t e l y apparent, i n d i c a t i n g a r a p i d reaction of aqueous caustic a n d s o d i u m . O n e r u n was also c a r r i e d out to determine the effectiveness of air as a means of r e m o v i n g r e s i d u a l sodium. H e a t e d a i r was i n t r o d u c e d to the reaction vessel at a rate t h e r m o c h e m i c a l l y equivalent to nitrogen containing 5 0 % water v a p o r — i.e., the total heat e v o l v e d b y the s o d i u m - o x y g e n reaction i n the f o r m a t i o n of s o d i u m oxide was equivalent to that realized b y the s o d i u m - w a t e r reaction i n the p r o d u c t i o n of s o d i u m h y d r o x i d e . A vigorous reaction o c c u r r e d due to the o x y g e n - s o d i u m reaction, the t e m p e r a ture rise exceeding 1000°F. i n 15 minutes. T h e f o r m a t i o n of white fumes of s o d i u m oxide began to restrict the flow of gases. It was apparent that the a i r - s o d i u m reaction rate was m u c h greater than that obtained i n the h u m i d i f i e d nitrogen r e action and, hence, c o u l d not be r e c o m m e n d e d for effective cleaning. APPLICATION OF HUMID-GAS TECHNIQUE TO LARGE COMPONENTS.

I n a d d i t i o n to

the

the tests described above, the h u m i d - g a s technique has been applied to various larger scale recleaning tests. O n e series i n v o l v e d s o d i u m r e m o v a l f r o m a bellows sealed 8 - i n c h stop v a l v e . T h e second series i n v o l v e d the r e c l e a n i n g of a p a r a l l e l m u l t i t u b e heat exchanger. In both of these series of operations, the gaseous r e agent was fed at a temperature between 300° a n d 400 °F. T h e w a t e r - v a p o r content was v a r i e d stepwise to 5, 10, 20, 50, 70, a n d finally 100% steam. Because of the complexities of the systems, each concentration of h u m i d gas was m a i n t a i n e d for a 1-hour p e r i o d i n order to attain complete contact w i t h a l l parts of s o d i u m - w e t t e d surface. T h e final rinse i n a l l cases was b o i l i n g water.



80

10,0001 PTS.NoOH/100 PTS. H 0 e


100

350 250 300 TEMPERATURE- E

200

200 3 0 0

4 0 0

500

7 0 0

400 450 500 550

e

Figure 10.

Water

vapor

partial pressure over aqueous

solution

of sodium hydroxide

In r e c l e a n i n g the 8 - i n c h b e l l o w v a l v e , thermocouples were attached to the t h i n m e m b e r e d bellows for test purposes. In three c l e a n i n g operations of the v a l v e , there was n o evidence of temperature rise b e y o n d the gas feed t e m p e r a t u r e level. I n the recleaning of the heat exchanger, several of the p a r a l l e l , r e t u r n - b e n d U - t u b e s (1 i n c h i n diameter) were p u r p o s e l y p i t c h e d to assure a l o n g section ( a p p r o x i m a t e l y 10 feet) of t r a p p e d s o d i u m . A s w o u l d be expected f r o m the p r e vious estimate of the diffusion coefficient for water t h r o u g h concentrated caustic, the h u m i d gas was not effective i n r e m o v i n g these s o d i u m pockets. T h e h u m i d gas d i d successfully reclean the balance of the h e a t - e x c h a n g e r circuits. I n this phase of the operation, the m a x i m u m temperature rise d i d not exceed 130 ° F . A l t h o u g h the pockets of s o d i u m were not r e m o v e d f r o m several of the p i t c h e d U - t u b e s , the 100% steam phase was f o l l o w e d b y a b o i l i n g water rinse. T h e tubes were cleaned of the l o n g s o d i u m pocket a l t h o u g h a localized temperature rise of a p p r o x i m a t e l y 900 ° F . was experienced. T h i s , of course, f u r t h e r demonstrates the desirability of complete d r a i n a b i l i t y . It also indicates that w i t h the b u l k of the s o d i u m area r e m o v e d b y the i n i t i a l h u m i d - g a s procedure, b o i l i n g water c a n be added to a system c o n t a i n i n g a n unreacted pocket w i t h reasonable expectancy that serious overpressure w i l l not occur. H o w e v e r , this technique w o u l d not be r e c o m m e n d e d if the s o d i u m pocket was adjacent to t h e r m a l sensitive components. T h e concentrations of water a n d n i t r o g e n w e r e established b y use of a s t r e a m nitrogen m i x i n g m a n i f o l d . If a p p l i e d to large systems, some economy c a n result f r o m p r o d u c i n g the steam i n place. Specifically, a h i g h - t e m p e r a t u r e gas b l o w e r c o u l d be e m p l o y e d to recycle the inert gas i n the system to be recleaned. P i p e heaters w o u l d i n i t i a l l y be used to elevate the gas temperature to a p p r o x i m a t e l y 300°F. A t this time, a water spray c o u l d introduce moisture to the r e c y c l e gas stream. T h e system heaters c o u l d t h e n be secured, since the c h e m i c a l heat


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release is i n excess of the heat of v a p o r i z a t i o n of the water b e i n g injected. B y c o n t r o l l i n g the w a t e r - f e e d rate, continuous water concentration changes to the o r i g i n a l i n e r t gas c o u l d be made. A relief v a l v e w o u l d be necessary to v e n t off the o r i g i n a l inert gas a n d h y d r o g e n b u i l d - u p . D e p e n d i n g o n the amount of s o d i u m i n v o l v e d a n d system heat losses, a cooler m i g h t be r e q u i r e d i n the recycle system.


Conclusions B o t h steam a n d alcohol are useful techniques f o r the r e m o v a l of s o d i u m residues f r o m p i p i n g systems a n d components. T h e p r i n c i p a l disadvantage of steam r e c l e a n i n g is the possibility of h i g h - t e m p e r a t u r e excursions due to the l o w heat capacity of the steam. Tests h a v e b e e n conducted i n w h i c h these t e m perature rises a r e m i n i m i z e d b y d i l u t i o n of the steam w i t h a n inert gas. T h i s h u m i d - g a s technique is r e c o m m e n d e d for r e c l e a n i n g operations w h e r e the use of steam w o u l d b e preferable to alcohol a n d w h e r e l o c a l overtemperature c o u l d result i n system e q u i p m e n t damage.

Acknowledgment T h e authors w i s h to acknowledge the contributions of personnel i n the S I R C o o l a n t T e s t i n g U n i t at K n o l l s A t o m i c P o w e r L a b o r a t o r y . T h e y also w i s h to t h a n k W . W . K e n d a l l , R. W . W o o d , a n d W . J . W e a v e r f o r their contributions to the h u m i d - g a s technique studies. T h e assistance of G r a c e K . Peppas a n d Irene C . C h o u i n a r d i n p r e p a r i n g the m a n u s c r i p t is gratefully acknowledged.

Literature Cited (1)

K e n y o n , A. R., L a c y , P . M. C . , " U s e of L i q u i d A m m o n i a for R e m o v i n g S o d i u m Residues," AERE K e p t . C.E./R. 1139 (1953). (2) J a c k s o n , C . B . , "Liquid Metals H a n d b o o k — S o d i u m - N a K S u p p l e m e n t , " 3rd ed., p. 265, A t o m i c E n e r g y C o m m i s s i o n , D e p a r t m e n t of N a v y , Washington, D . C . , 1955. (3) P e r r y , J. H., " C h e m i c a l E n g i n e e r s ' H a n d b o o k , " 3rd e d . , p. 173, M c G r a w - H i l l , N e w Y o r k ,


Recleaning Sodium Heat Transfer Systems

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