Recleaning Sodium Heat Transfer Systems - Advances in Chemistry

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 - Advances in Chemistry

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