Hackney 1982 Aquacultural-Engineering

7/28/2019 Hackney 1982 Aquacultural-Engineering

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Aquacultural Engineering 1 (1982) 27 5-295

THE PERFORMANCE AND DESIGN OF PACKED COLUMNAERATION SYSTEMS FOR AQUACULTURE

G A R Y E . H A C K N E YJohn Carollo Engineers, 10840 WarnerAvenue, Suite 100, Fountain Valley, California, USA

an dJ O H N E . C O L T

Department of Civil Engineering, University of California, Davis, California 95616, USA

A BS TRA CTThe packed column aerator (PCA) is a highly ef ficient aerator that can be used fo roxygen and nitrogen + argon removal. Standard transfer efficiencies (No) for oxygenrange from 1.5 to 2.0 kg 02~kWh. If 1-2 m o f hydraulic head is available, No can be ashigh as 80 kg 02/kWh. A mass transfer model was developed for design purposes.Recommended design parameters and procedures are presented for full-scale PCA.

N O M E N C L A T U R EAA sC *CCtC iCoEHKK'KLaM

a r e a o f g a s - l iq u i d i n t e r f a c e ( m 2 )s p e c i fi c s u r f a c e a r e a ( m - a )s a t u r a t i o n c o n c e n t r a t i o n o f g a s i n l iq u i d ( m g l i t e r - a )e x i s ti n g c o n c e n t r a t i o n o f g a s in l i q u i d ( m g l i te r - a )o x y g e n c o n c e n t r a t i o n i n li q u i d a t t i m e t ( ra g l it e r - 1 )i n it i a l o r i n l e t o x y g e n c o n c e n t r a t i o n i n li q u i d ( m g l i te r - a )o u t l e t o x y g e n c o n c e n t r a t i o n o f w a t e r ( m g l it er -a ) .o v e ra l l e f f ic i e n c y o f p u m p a n d m o t o rh y d r a u l i c h e a d ( m )c o e f f i c i e n t o f d i f f u s i o n f o r g a s ( m h - 1 )c o e f f ic i e n t d e p e n d e n t o n s u r f a ce a r e a ( m )o v e r a l l m a s s t ra n s f e r c o e f f i c i e n t ( h - l )o x y g e n t r a n s f e r r a t e ( k g O z h - 1 )

2 7 5Aquacultural Engineering 0144-8609]82/0001-0275[$02.75 Applied Science Publishers Ltd,England, 1982Printed in Great Britain


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2 7 6 G . E . H A C K N E Y , J . E . C O L TNNoOROTEPai~P p u m pQQwQ'R =TT w =ZdC/dtdm/dtek =mrhn t

PlP2ttdPa~, =

m e a s u r e d t r a n s fe r e f f i c ie n c y ( k g 0 2 k w h - 1)s t a n d a r d t r a n s f e r e f f i c i e n c y ( k g 0 2 k w h -1 )o x y g e n r e q u i r e m e n t ( k g 0 2 h - 1)o x y g e n t r a n s f e r e f f i c ie n c yb l o w e r p o w e r ( k W )p u m p i n g p o w e r ( kW )h y d r a u l i c l o a d i n g r a t e ( m 3 m -2 s 1 )w a t e r f l o w r a t e ( 1 0 6 X l i te r h - 1 )v o l u m e t r i c f l o w r a t e ( m 3 s 1 )g a s c o n s t a n t = 0 . 2 8 7i n l e t t e m p e r a t u r e ( K )w a t e r t e m p e r a t u r e ( C )d e p t h o f c o lu m n ( m )r a t e o f c h a n g e i n g a s c o n c e n t r a t i o n i n t h e l iq u i d ( m g l it e r - 1 h - 1 )r a t e o f m a s s t r a n s f e r o f g a s in t o l iq u i d ( m g h - I )e f f ic i e n c y o f a ir b lo w e r a n d m o t o r1 - 4 ( f o r a i r )a n e x p o n e n t d e p e n d e n t o n A sm a s s f l o w r a t e a i r ( k g h - 1 )a n e x p o n e n t d e p e n d e n t o n A s(k- -1) /k , e q u a l t o 0 - 2 8 3i n le t p r e s s u r e ( k P a )o u t l e t p r e s s u r e ( k P a )a e r a t io n t im e ( h )m e a n d e t e n t i o n t i m e ( s)d e n s i t y o f a i r ( k g m - 3 )9 - 7 9 k N m - 3 a t 2 0 C

I N T R O D U C T I O N

T h e m a i n t e n a n c e o f a d e q u a t e d i s s o l v ed o x y g e n l ev e l s i n a q u a t i c c u l t u r e s y s t e m s isa s e ri o u s p r o b l e m . M a n y t y p e s o f a e r a t i o n d e v i c e s h a v e b e e n u s e d t o i n c re a s e p r o d u c -t i o n o r d e c r e a s e w a t e r re q u i r e m e n t s . T h e t r a n s f e r e f fi c i e n c y o f c o n v e n t i o n a l a e r a t i o nd e v i c e s s u c h a s d i f f u s e d a i r o r m e c h a n i c a l s u r f a c e a e r a t o r s i s l o w d u e t o t h e s h a l l o wd e p t h o f m o s t s y s t e m s a n d t h e r e q u i r e m e n t t o m a i n t a in h i g h d is s ol v ed o x y g e n l e v el s( C o l t a n d T c h o b a n o g l o u s , 1 9 8 1 ) . P r o b l e m s w i t h g as s u p e r s a t u r a t i o n w i ll l i m i t t h e u s e-f u ln e s s o f t h e h i g h l y e f f i c i e n t s u b m e r g e d a e r a t o r s ( C o l t a n d W e s te r s, 1 9 8 2 ) .

T h e p a c k e d c o l u m n a e r a t o r (P C A ) c o n s i s ts o f a c o l u m n f i ll ed w i t h a h i g h s u rf a c ea r e a p a c k i n g . W a t e r f lo w s d o w n o v e r t h e m e d i a i n a t h i n f i lm . B e c a u s e o f th e l a r geg a s - l iq u i d s u r f a c e a r e a , t h i s s y s t e m i s h i g h l y e f f ic i e n t f o r o x y g e n a n d n i t ro g e n


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P E R F O R M A N C E A N D D E S I G N O F P A C K E D C O L U M N A E R A T I O N S Y S T E M S 2 7 7t r a n s f e r , b u t w i ll n o t p r o d u c e d i s s o l v e d g a s s u p e r s a t u r a t i o n . D u e t o t h e l a c k o fi n f o r m a t i o n o n t h e d e s ig n a n d o p e r a t i o n o f t h e P C A , t h is d e v i ce h a s n o t b e e n w i d e l yu s e d i n a q u a c u l t u r e . T h e w o r k r e p o r t e d h e r e w a s u n d e r t a k e n t o d e v e lo p p e r f o r m a n c ea n d d e s i g n d a t a f o r a f u ll s c al e P C A . T h i s r e s e a r c h m a y b e u s e d a s a b a s i s f o r t h e d e s i g no f f u l l - s c a l e i n s t a l l a t i o n i n e x i s t i n g a n d p l a n n e d h a t c h e r i e s .

G A S T R A N S F E R T H E O R Y I N T H E P C A

T h e r a t e a t w h i c h a s l i g h tl y s o l u b l e g a s , s u c h a s o x y g e n , n i t r o g e n o r a r g o n , is t r a n s -f e r r e d i n t o a l i q u id is p r o p o r t i o n a l t o t h e a r e a o f th e g a s - l i q u id i n t e r f a c e a n d t h eg r a d i e n t b e t w e e n t h e e x i s t i n g a n d s a t u r a t i o n c o n c e n t r a t i o n o f t h e g a s i n t h e s o l u t io n( L e w i s a n d W h i t m a n , 1 9 2 4 ) . I n e q u a t i o n f o r m t h i s c a n b e w r i t t e n a s :

d m- - = K A ( C * - - C ) ( 1 )d t

E q u a t i o n ( 1 ) c a n b e r e w r i t t e n a s :d C- - = K L a ( C * - - C ) ( 2 )d t

T h e n e w p r o p o r t i o n a l i t y c o n s t a n t , K L a , n o w c o n t a i n s t h e g a s d if f u s i o n c o n s t a n t , t h ea r e a o f t h e g a s - l iq u i d i n t e r f a c e a n d t h e v o l u m e o f t h e l iq u i d f il m . T h e r e f o r e , i t w o u l db e e x p e c t e d t h a t K L a w i ll v a r y w i t h t h e t y p e o f g a s b e i n g t r a n s f e r r e d , t h e g a s d i f f u s i o na r e a a v a il a b le , t h e f l o w r a t e o f w a t e r p a s s in g t h r o u g h t h e P C A a n d t h e t y p e o f p a c k i n g .

I n t e g r a t in g e q n ( 2 ) b e t w e e n t h e l i m i t s o f C i a n d C t a n d 0 a n d t g i v e s :C * - - C t _ e - ( K L a ) t ( 3 )C * - - C i

o r

C * - C 1In = ( K L a ) t ( 4 )\C * - - C t l

T h e e q u a t i o n p r o v id e s a s im p l e m e t h o d o f d e t e r m i n i n g t h e r e q u i re d a e r a t io n t i m e f o r ad e s i re d d i s so l v e d o x y g e n c o n c e n t r a t i o n w h e n t h e i n it ia l c o n c e n t r a t i o n a n d K L a a r ek n o w n . T h e l e f t- h a n d t e r m o f e q n ( 4 ) i s r e f e r re d t o a s th e l o g d e f ic i t o f t h e d i ss o l v edo x y g e n c o n c e n t r a t i o n .

I n p a c k e d c o l u m n s , t h e K L a v a l u e f o r o x y g e n is p r o p o r t i o n a l t o Q O . T S a n di n c re a s e s u p t o t h e p o i n t w h e r e t h e c o l u m n fl o o d s ( C o u l s o n e t a l . , 1 9 7 8 ) . F l o o d i n g o fa p a c k e d c o l u m n i s d e f i n e d a s a n i n v e r s io n o f p h a s e s s o t h a t t h e l iq u i d p h a s e b e c o m e sc o n t i n u o u s w i t h in t h e v o i d s p a c e b e t w e e n t h e m e d i a ( N o r t o n C o . , 1 9 77 ) . T h e m a s st r a n s f e r c h a r a c t e r i s ti c s o f P C A c h a n g e d r a s t ic a l l y w h e n f l o o d e d ( i. e. t h e P C A b e c o m e s


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2 7 8 G.E. HACKNEY, J. E. COLTa d i f f u s e d a i r a e r a t o r ) . L a r g e -s c a le p a c k e d c o l u m n s a r e g e n e r a l l y n o t d e s i g n e d f r o mi n d i v i d u a l m a s s tr a n s f e r c o e f f i c i e n t s (KLa) b e c a u s e o f d i f f i c u l t y i n e s ti m a t i n g t h ee f f e c t i v e w e t t e d a r e a .

Aerat ion t ime in the PCAI n t ri c k l in g f il te r s, E c k e n f e l d e r a n d B a r n h a r t ( 1 9 6 3 ) f o u n d t h a t t h e d e t e n t i o n t i m e

t h r o u g h t e st c o l u m n s w a s r e l a te d t o t h e d e p t h o f t h e c o l u m n , th e h y d r a u l i c lo a d i n gr a t e , a n d t h e s p e c i fi c s u r fa c e a r e a o f t h e m e d i u m i n t h e c o l u m n . T h e i r f in d i n g s a r es u m m a r iz e d b y e q n ( 5 ) :

K ~ m zt d - - Qn ' ( 5 )

F o r g la s s s p h e r e s ( 1 . 9 - 2 . 8 c m i n d i a m e t e r ) , K ' = 0 - 7 0 , m = 0 - 7 5 a n d n v a r i e d f r o m0 -7 5 t o 0 . 8 2 . T h e m a x i m u m l o a d in g r a t e u se d b y E c k e n f e l d e r a n d B a r n h a r t ( 1 9 6 3 )w a s 5 0 m s m - 2 h -1 . T h e s u r f a c e l o a d i n g r a t e s f o r p a c k e d c o l u m n s u s e d i n a q u a c u l t u r em a y r a n g e f r o m 1 0 0 t o 2 5 0 m 3 m - 2 h - l ( O w s l e y, 1 9 8 1 ) .

S u b s t i t u t i n g e q n ( 5 ) i n t o e q n ( 4 ) g i v e s :( C*--C i t - K 'A mZl n \ c - ~ _ C o l - ( K L a ) Q ~ ( 6 )

w h e r e t h e t e r m C o re p r e s e n ts t h e d i ss o lv e d o x y g e n c o n c e n t r a t i o n i n t h e w a t e r l e av i n gt h e P C A . I f KLa is a b s o r b e d i n t o t h e d e t e n t i o n t i m e c o e f f i c i e n t ( K ' ) , e q n ( 6 ) c a n b ew r i t t e n a s : ( C * - - C i t = K A r ~ ZIn Q n' ( 7 )\ C* - - Co lw h e r e K is n o w a s y s t e m c o e f f i c i e n t w h i c h re f l e c ts t h e o x y g e n t r a n s f e r c h a r a c t e r i s t i c so f a p a r t ic u l a r p a c k e d c o l u m n a e r a t i o n d e v i c e . T h e v a l u e o f C * i n e q n ( 7 ) is a f u n c t i o no f t e m p e r a t u r e , p r e ss u r e, s a l in i ty a n d t h e g a s c o m p o s i t i o n i n t h e c o l u m n . T h e d e s i g no f p a c k e d c o l u m n s f o r a q u a c u l t u r e w i ll d e p e n d o n t h e e v a l u a t i o n o f th e s y s t e mp a r a m e t e r s i n e q n ( 7 ) u n d e r t y p i c a l a q u a c u l t u r e c o n d i t i o n s .A ir f lo w requirements

A s o x y g e n is t r a n s f e r r e d i n t o t h e w a t e r , t h e o x y g e n le v e l i n t h e g a s p h a s e w i t h i n t h ec o l u m n w i ll d e c re a s e a n d t h e r e f o r e t h e o x y g e n t r a n s f e r r a t e a ls o d e c re a s es .

T h e r e q u i r e d a i r fl o w r a t e c a n b e d e t e r m i n e d b y p e r f o r m i n g a n o x y g e n m a s s b a l a n c eo n t h e P C A . T h e o n l y o x y g e n d e m a n d i n t h e a e r a t o r is t h e t r a n sf e r o f o x y g e n i n t o t h ew a t e r. T h e r e f o r e , th e m a x i m u m a m o u n t o f o x y g e n re q u ir e d ( O R ) c a n b e e s t im a t e d :

O R = Q w ( C * - - C i) X 1 0 - 6 ( 8 )


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P E R F O R M A N C E A N D D E S I G N O F P A C K E D C O L U M N A E R A T I O N S Y S T E M S 2 7 9a n d t h e r e f o r e , t h e m a s s a ir fl o w c a l c u l a t e d :

Q w ( C * - C i )r h - x 1 0 - 6 ( 9 )0.230TEs i n ce o n a m a s s b a s i s a i r i s 2 3 % o x y g e n .A erator performance

T h e p e r f o r m a n c e o f a e r a ti o n d e v ic e s is e x p re s s e d in t e r m s o f t h e m a s s o f o x y g e nt r a n s fe r r e d p e r u n i t o f e n e r g y e x p e n d e d ( k g 0 2 k W h - l ) . T h e r a t e a t w h i c h o x y g e nt r a n s f e r t a k e s p l a c e is s i m p l y t h e p r o d u c t o f t h e l iq u i d f lo w r a t e a n d t h e c h a n g e i no x y g e n c o n c e n t r a t i o n w h i c h o c c u r s w i t h in t h e c o l u m n :

M = Q w ( C o - C i ) 1 0 - 6 ( 1 0 )T h e r e q u i r e d p o w e r is th e s u m o f t h e p o w e r n e c e s s a ry t o p u m p w a t e r (P pu m p) u p tot h e a e r a t o r a n d t h e p o w e r u s e d t o s u p p l y a ir ( P a i r ) t o t h e d e v i c e .

I g n o r in g p i p e f r ic t i o n a l lo s s e s p u m p i n g p o w e r i s c a l c u l a te d f r o m t h e f o l lo w i n ge q u a t i o n :

0'711 ( 1 1 )P p u m p - ET h e p o w e r r e q u i r e m e n t f o r t h e a i r b l o w e r is c a l c u l a te d u s i n g th e f o r m u l a f o r

a d i a b a ti c c o m p r e s s io n ( Y u n t , 1 9 7 9 ) :

Pai r ne \ \ ~ - ~ ]T h e t r a n s f e r e f f i c ie n c y o f th e a e r a t o r i n k g 0 2 k W h 1 is e q u a l t o t h e m a s s o f o x y g e n

t r a n s f e r r e d ( e q n ( 1 0 ) ) d i v id e d b y t h e s u m o f t h e p o w e r i n p u t s ( e q n (1 1 ) + e q n (1 2 ) ) .T h e r e s u l t s o f o x y g e n t r a n s f e r t e s t s ar e r e p o r t e d a t s t a n d a r d c o n d i t i o n s o f 2 0 C , a n da z e ro i n i ti a l d is s o lv e d o x y g e n c o n c e n t r a t i o n . R e s u l ts c o n d u c t e d u n d e r o t h e r c o n -d i t i o n s a r e c o m m o n l y c o n v e r t e d t o s t a n d a r d c o n d i t i o n s u s i n g t h e f o l l o w i n g e q u a t i o n( M e t c a l f & E d d y I n c . , 1 9 7 9 ) :

N ( 9 " 0 7 7 1N o = \ C ~ C i ] ( 1 ' 02 4) 20 - -Vw ( 1 3)E q u a t io n ( 1 3 ) w a s d e v e lo p e d f r o m s u rf a ce a n d s u b m e r g e d a e r a ti o n s y s t e m s . T h ev a l i d i ty o f t h is e q u a t i o n f o r t h e P C A n e e d s to b e c o n f i r m e d . T h e m a s s t r a n s f e r c o e f f i-c i e n t ( K ) w h i c h a p p e a r s i n e q n ( 7 ) c a n b e c o r r e c t e d t o 2 0 C b y t h e f o l l o w i n ge q u a t i o n :

K 2 o = K r ( 1 . 0 2 4 ) 2 0 -T x 4 ( 1 4 )w h e r e K 2 o = t r a n s f e r c o e f f i c i e n t a t 2 0 C a n d K T = t r a n s f e r c o e f f ic i e n t a t T w .


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280 G.E. HACKNEY, J. E. COLTMATERIALS AND METHODS

T h e pa c k e d c o l u m n a e ra to rThe PCA used for laboratory testing is shown schematically in Fig. 1. It consists

of five major components: the column, the flow distribution plate, the high surfacearea media, the medium support and the flow collector.

L o w D O W a t er - -

A ir I A / / ~ A / ' r VentStackPerforatedDistrbutonlate~.~ ~ Thin ilm of w~

High Surface AreaP a c k i n g

' ~ A e r a t e d W a t e rI I

I - I

Fig. 1. Section throu gh a packed column aerator.

C o l u m n : The 0-28 m diameter Plexiglass column was 0-98 m long. TransparentPlexiglass was used in its construction to allow visual observations to be made duringtesting. The column contains two 4-5 cm diameter air vents (or inlets) at its base (seeFig. 1).

F l o w d i s t r i b u t i o n p l a t e . " Water was distributed over the top of the media using anorifice distributor (800 holes m-2). The plate contained two 2.9 cm diameter air ventstacks which terminated at the top of the flow distribution chamber. These stacksallowed air ventilation of the PCA.


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P E R F O R M A N C E A N D D E S I G N O F P A C K E D C O L U M N A E R A T I O N S Y S T E M S 281H i g h s u r f a c e a r e a p a c ki n g ." Norton Actifll plastic pall rings were used as the

col umn packing media. A phot ograp h of the four sizes of media tested is shown inFig. 2 and the pert ine nt characteristics are presente d in Table 1.

Fig. 2. Photograph of media used in testing.

TABLE 1Packing characteristicsaPr od uc t nu mber Diam eter (cm) Surface area (m 2/m 3)b Pieces (m 3)

02-0160 2.54 207 50 89402-0200 3-81 128 13 77402-0240 5.08 102 6 35702-0370 8.89 85 1 165a Eckert (1979).b Surface area of packing medium when randomly placed; values in Norton Co. product informationare sightly different.

P a c k i n g s u p p o r t : Large diameter holes were drilled in the support plate to reduceresistance to water and air flow.

F l o w c o l le c to r : T h e flow collector was used to collect the aerated water fordissolved oxygen me asure ment prior to discharge.


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2 8 2 6 . E . H A C K N E Y , J . E . C O L TE q u i p m e n t

D i ss o lv e d o x y g e n a n d w a t e r t e m p e r a t u r e m e a s u r e m e n t s w e r e m a d e w i t h a Y e l l o wS p r in g s I n s t r u m e n t s C o . (Y S I ) M o d e l 5 4 A B P d i s so l ve d o x y g e n m e t e r . T h e m e t e r w a se q u i p p e d w i t h a Y S I M o d e l 5 7 3 9 p r e ss u re c o m p e n s a t i n g D O p r o b e a n d a M o d e l5 7 9 1 A s u b m e r s i b l e s ti r re r .

T h e d i ss o lv e d o x y g e n m e t e r w a s c a l ib r a t ed b e f o r e e a c h t e s t r u n u s i n g a Y S I 5 0 7 5c a l i b r a t i o n c h a m b e r a n d a ir s a t u r a t i o n t a b l e s (W e iss , 1 9 7 0 ) . A t m o s p h e r i c p r e ss u r ec o r r e c t i o n s w e r e i n c o r p o r a t e d i n t o al l c a l ib r a t io n s w i t h b a r o m e t e r r e a d i n g s t a k e n f r o ma st a n d a r d l a b o r a t o r y m e r c u r y b a r o m e t e r . W a t e r f l o w s w e r e m e a s u r e d w i t h a p o i n tg a u g e a n d V - n o t c h w e i r .

Data r ed u c t i o nT h e p o w e r re q u ir em e n t s w e r e c o m p u t e d f r o m e qn s ( 1 1 ) a n d ( 1 2 ) a s s u m i n g a n

o v e ra l l e f fi c i e n c y f o r p u m p i n g a n d c o m p r e s s in g e q u a l t o 8 0 a n d 7 0 % , r e s p e c ti v e ly . N oa n d K w e r e c o n v e r t e d t o s t a n d a r d c o n d i t i o n s u s in g e q n s (1 3 ) a n d ( 1 4 ) , r e s p ec t iv e l y .W a t e r t e m p e r a t u r e s r a n g e d f r o m 1 9 . 0 t o 2 0 -5 C .

Tes t pro ced u re sF o u r s e p a ra t e te s ts w e r e p e r f o r m e d t o d e t e r m i n e : 1 . m i n i m u m a ir f l o w r e q u ir e -

m e n t s ; 2 . o v e r a ll p e r f o r m a n c e o f t h e l a b o r a t o r y a p p a r a t u s ; 3 . t h e r e l a ti o n s h i p b e t w e e nt h e d r iv i ng f o r c e a n d o x y g e n t ra n s f e r r a te a n d 4 . t h e r e la t io n s h i p b e t w e e n o x y g e nt r a n sf e r a n d p a c k i n g d e p t h . E a c h o f t h e f o u r t e s t s a re d e s c r ib e d .

Minimu m a ir f l o w r eq u i r emen t s : A q u a l i t a t iv e t e s t w a s d e v e l o p e d t o d e t e r m i n e t h er e g i o n w h e r e a ir s u p p l y b e g a n t o l im i t o x y g e n tr a n s fe r . N o r t o n 2 . 5 4 c m p a c k i n g w a su s e d . T h e h y d r a u l i c l o a d i n g r a t e f o r t h e t e s t w a s a r b i t r a ri l y s e t a t 1 11 m 3 m - 2 h - 1.

Per f o rm ance o f l abo ra to ry appara tu s : T h e p e r f o r m a n c e t e s ts w e r e c o n d u c t e d o n al m P C A . A i r f lo w t h r o u g h t h e a e r a t o r w a s h e ld c o n s t a n t a t t h e m a x i m u m r a te o f t h eb l o w e r ( 0 - 4 m 3 m i n - l ) . T h e q u a n t i t y o f o x y g e n t r a n s fe r r e d b y e a c h p a c k i n g s iz e w a sm e a s u r e d a t h y d r a u l i c l o a d i n g r a t e s v a r y i n g f r o m 3 7 t o 2 2 2 m 3 m - 2 h - I .

Dr iv ing f o r ce e f f e c t : U s in g a 1 m c o l u m n , 3 -8 1 c m p a c k i n g a n d t h r e e d i f f e re n th y d r a u l i c l o a d i n g r a t e s ( 4 0 , 1 1 1 a n d 2 1 5 m 3 m - 2 h - l ) , t h e e f f e c t o f t h e d r iv i n g f o r c eo n o x y g e n t r a n s f e r w a s d e t e r m i n e d . I n i t i a l D O l e v e l s w e r e a l t e r e d b y a e r a t i n g t h ew a t e r i n t h e h o l d i n g t a n k p r i o r t o p a ss in g i t t h r o u g h t h e P C A . T h e i ni ti a l D O l e v e l si n th i s t e s t v a r i e d b e t w e e n 2 - 5 ra g / l it e r a n d 8 - 3 m g / l i t e r .

E f f e c t o f pa c ki ng d e p th : O x y g e n t ra n s f e r w a s d e t e r m i n e d a t d e p t h s o f 0 - 1 7 , 0 . 2 7 ,0 - 3 7 , 0 . 6 0 a n d 1 -0 0 m f o r t h e f o u r p a c k i n g siz es. H y d r a u l i c l o a d i n g v a ri e d b e t w e e n4 0 a n d 2 2 0 m 3 m - 2 h - 1.


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PERFORMANCE AND DESIGN OF PACKED COLUMN AERATION SYSTEMS 2 8 3RESULTS

Minimum air flow measurementsT h e r e s u lt s o f t h e q u a l i t a ti v e t e s t d e s i g n e d t o e x a m i n e t h e e f f e c t s o f ai r f l o w r a t eo n o x y g e n t r a n s f e r a r e r e p o r t e d i n F ig . 3 . A t l o w a i r f l o w s , o x y g e n t r a n s f e r is r e d u c e d .

A s t h e a i r f l o w r a t e is i n c r e a s e d , t h e o x y g e n t r a n s f e r r a t e i n c re a s e s s h a r p l y u p t o a g a s -l iq u i d r a ti o ( G / L ) o f 0 .9 a n d t h e n s l o w l y a p p r o a c h e s a n a s y m p t o t i c v a l u e .

V o l u m e t r i c G / L R o t i o0 0 . 9 I . B 2 . 7 3 . 6

B

I 1 I

- B

- - 7

I I I I 60 tO0 200 300 400A i r F l o w R o te t t i l e r / m ; n

Fig. 3. Effluent dissolved oxygen level as funct ion of air flow rate.

Performance of laboratory apparatusT h e o x y g e n t r a n sf e r p e r f o r m a n c e o f t h e 1 m l a b o r a t o r y P C A a s a f u n c t i o n o f

h y d r a u l i c l o a d in g i s p r e s e n t e d g r a p h i c a ll y i n Fi g . 4 . I n t h e c o m p u t a t i o n o f p u m p i n gp o w e r 0 .3 m w a s a d d e d t o t h e 1 m c o l u m n h e i g h t t o a c c o u n t f o r in l e t a n d o u t l e tc o n d i t i o n s . T h e s t a n d a r d t r a n s f e r e f f i c i e n c y ( N o ) r an g e d f r o m 1 t o 2 k g 0 2 k W h 1 . O f


10/21

2 8 4o3 ,

G . E . H A C K N E Y , J . E . C O L TI 0 0 2 0 0 3 0 0 4 0 ( 2

J I I 3

/ -

00

~ 2 . 5 4 c m

I I I 0I 0 0 2 o 0 3 0 0 4 o 0Hydrau l ic Load/nO Ra te, O, rn3/rn 2 h

F i g . 4 . T h e e f f e c t o f h y d r a u l i c l o a d i n g r a t e o n N o

5 . 0 8 c mB . 8 9 c m5. 81 cm

t h e f o u r p a c k i n g s iz e s t e s t e d , t h e i n t e r m e d i a t e s iz e s ( 3 .8 1 c m a n d 5 . 0 8 c m ) h a d t h eb e s t o x y g e n t r a n s f e r c h a r a c t e r i s ti c s . T h e o x y g e n t r a n s f e r r a te o f t h e s m a l le s t p a c k i n gs iz e ( 2 - 5 4 c m ) i s t h e h i g h e s t a t l o w f l o w s b u t b e g i n s to d r o p o f f a t a h y d r a u l i c l o a d i n gr a t e o f a b o u t 1 0 0 m 3 m - 2 h - ~. T h e o x y g e n t r a n s f e r r a t e o f t h e l a r g e st p a c k i n g s iz er e m a i n s c o n s t a n t r e g a r d l e s s o f h y d r a u l i c l o a d i n g b u t t r a n s f e r s s u b s t a n t i a l l y l es so x y g e n t h a n t h e t w o i n t e r m e d i a t e siz es .T h e e f f e c t o f d r iv in g f o r c e o n p e r f o r m a n c e

T h e o x y g e n t r an s f e r r a te o f t h e P C A is d i r e c t ly p r o p o r t i o n a l t o t h e d r iv i n g f o r c e ,C * - - C i ( F i g . 5 ) . A t a g i ve n C * - - C i , t h e s m a l le r m e d i a h a d t h e h i g h e s t o x y g e nt r a n s f e r r a t e a t t h e h y d r a u l i c l o a d i n g r a t e t e s t e d ( 1 1 1 m ~ m - 2 h - l ) .M o d e l t e s t s

T h e o x y g e n t r an s f e r i s a fu n c t i o n o f p a c k i n g d e p t h a n d s u r f a c e a r e a , h y d r a u l i cl o a d i n g r a t e a n d s y s t e m c o e f f i c i e n t s (e q n ( 7 ) ) . T h e l o g d e f i c i t i s p r e s e n t e d a s af u n c t i o n Z , a n d Q f o r t h e 3 . 1 8 c m p a c k i n g s iz e i n F i g . 6 . O x y g e n t r a n s f e r i n t h e P C A


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PERFORMANCE AND DESIGN OF PACKED COLUMN AERATION SYSTEMS 2850 5 lO

0

/

O0 5 IO( C~- C ) , m g / l l t e r

Fig. 5. The effect of the driving force on N.

is found to be directly related to packing depth, but does not depend on Q. The samerelationship was found for the ot her packing sizes.

Because oxygen transfer did not depend on Q, eqn (7) was simplified to:( C * - C i IIn \ C ~ _ C o / = b + K Z (15)

where b = a dimensionless const ant that depends on incidenta l aeration that occursprior to water enterin g the PCA and K = a const ant representing the gas transfer char-acteristics of a pa rticular PCA and packing in m -1.Values of K for each media size are presented in Fig. 7. The values of K shown havebeen adjusted to 20C.

Except for the 2.54 cm packing, the value of K is cons tant. While b is a fun cti onof Q to a slight extent (Fig. 8) a value of 0.4 can be used for design purposes. K isa direct function of the specific surface area of the packing tested. The regressionequation is equal to:

K = 0.30 + 0.O109A s regression coeffi cient = 0.97 (16)where A s = specific surface area o f the medium in m 2 m -3.


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2 8 6 G. E . HACKNE Y, J . E . COLT

~ tN

~ s

E

I I

I

~5

~5

~5

E

.g=o


13/21

03

2 -

/ -

00

P E R F O R M A N C E A N D D E S IG N O F P A C K E D C O L U M N A E R A T I O N S Y S T E M S 28750 1


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288 G.E. HACKNEY, J. E. COLTSubstituting 0.4 for b and eqn (17) for K in eqn (16) yields:

l n ( C * - - C i t\ ~ - - ~ o t = 0-4 + (0.30 + 0.0109As)Z (17)Equation (17) is based on the packing media tested in this work and should not beapplied to other types of packing directly.

DISCUSSION

The PCA is a highly efficient aerator. The standard oxygen transfer efficiency (No)ranges from 1-5 to 2.0 kg 02 kWh-a (Fig. 4). If 1-2 m of head is available, No may beas high as 83 kg 02 kWh -1. In this case, the only power requirement would be for thefans.A i r f l o w

Air flow through the column has a significant effect on the performance of thePCA. Below a G/L ratio of 0.9 (Fig. 3), the PCA performs poorly. Maximum G/Lratios can be computed from eqns (9) and (10). Assuming that O T E = 0.01, Ci = 0and T = 20C, the G/L ratio is equal to 3.3. Because of the low head losses through thecolumn, fans can be used rather than blowers. In design of the air system, care is neces-sary to prevent short-circuiting. If an orifice system (Fig. 1) is used, exhaust fans canbe mounted on the upper air vent stack. A counter-current air flow is more desirablethan a co-current flow. A G/L ratio of 3 should be used for design purposes.H y d r a u l ic l o a d in g

As the hydraulic loading rate increases, the air head losses (Ap) increase until atsome point the column floods. The drop in performance of the 2.54 cm packing isprobably the result of air flow restrictions and flooding. For the packing tested, flood-ing occurs at a gas head loss of APf = 1.63 kPa m -I (2.0 in of water per foot of media)(Zenz, 1979). In the chemical industry, packed columns are commonly designed for amaximum loss equal to 0-75 APf. Using the specific pressure drop data presented byNorton Co. (1977) or generalized pressure drop correlations (Coulson e t a l . , 1978;Zenz, 1979), the maximum design loading is estimated to range from 150 m 3 m -2 h-1for the 2.54 cm packing to >340 m 3 m -2 h -1 for the 8-89 cm packing. These loadingrates are based on AP = 0-75 APf and G/L = 3. Hydraulic loading rates above 220 m 3m -2 h -1 were not tested because of pumping limitat ions.C o l u m n d i a m e t e r

The size of the column will depend on the packing size (Norton Co., 1977) and thetype of distribution system. Poor distribution of the liquid and flow down the side-walls decrease the performance of the PCA. These two effects are reduced in larger


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P E R F O R M A N C E A N D D E S IG N O F P A C K E D C O L U M N A E R A T I O N S Y S T EM S 2 8 9c o l u m n s . C o l u m n d i a m e t e r s f o r f u l l- sc a le u n i t s s h o u l d b e l a rg e r t h a n e i g h t t i m e s t h ep a c k i n g d i a m e t e r ( C o u l s o n e t a l . , 1 9 7 8 ) . I n t h e c h e m i c a l in d u s t r y , d i s t r i b u t o r s a r ed e s ig n e d t o a p p l y t h e l i q u i d t o t h e s u r f ac e o f t h e p a c k i n g a t a m i n i m u m o f 4 5 p o i n t sp e r m 2 ( E c k e r t , 1 9 7 9 ) . B e c a u s e o f s m a l le r c o l u m n d i a m e t e r s a n d c o l u m n h e i g h t s i na q u a c u l t u r e , m o r e d i s t r i b u t i o n p o i n t s a r e d e s ir a b le . T h e d i s t r ib u t i o n p l a t e u s e d i n t h i se x p e r i m e n t h a d 8 0 0 p o i n t s / m 2. A d d i t i o n a l i n f o r m a t i o n o n t h e d e s ig n o f d i s t r ib u t o ra n d m e d i a s u p p o r t p l a te s is p r e s e n t e d b y C o u l s o n e t a l . ( 1 9 7 8 ) , E c k e r t ( 1 9 7 9 ) a n dN o r t o n C o . ( 1 9 7 8 ) .V e r i fi e d o x y g e n t r an s f er r a t e e q u a t i o n

T h e d r i v i n g fo r c e t e s t s v e r if i e d t h a t t h e o x y g e n t r a n s f e r r a t e i s p r o p o r t i o n a l t o t h ed r iv i n g f o r c e , C * - - C i . T h e r e f o r e , t h e p e r f o r m a n c e u n d e r f ie ld c o n d i t i o n s c a n b ec o m p u t e d f r o m e q n ( 1 3 ) a f t e r so lv in g f o r N :

[ C * - - C i ~N = N O ~ ~ 1 " 0 2 4 ( T w - 2 0 ) ( I 8 )

T h e 1 - 0 2 4 ( T w - 2 ) c o r r e c t i o n f o r e q n s ( 1 3 ) , ( 1 4 ) a n d ( 1 8 ) w a s d e v e l o p e d f r o m w o r k o ns u r fa c e a n d s u b m e r g e d a e r a t o r s i n t h e w a s t e w a t e r f ie ld ( H u n t e r , 1 9 7 9 ) . T h e e f fe c t s o ft e m p e r a t u r e o n o x y g e n tr a n s f e r i n t h e P C A n e e d t o b e e v al u a te d .

F o r t h e t h r e e l ar g e st p a c k i n g s iz e s t e s te d , t h e f o l l o w i n g m a s s t r a n s f e r e q u a t i o n m a yb e u s e d :

( C * : C i l =l n \ c , _ C o ! 0 . 4 + K Z ( 1 9 )

T h e v a l u e s o f K a r e l i st e d i n T a b l e 2 . W h i le t h e v a l u e o f K d e c r e a s e s as t h e s iz e o f th ep a c k i n g i n c r e a s e s , t h e m a x i m u m h y d r a u l i c l o a d i n g r a te a l so i n c r e as e s . T h e r e la t iv e c o s to f t h e s m a l l e r p a c k i n g s iz e s i s g r e a t e r t h a n t h e i n c r e a s e in th e i r p e r f o r m a n c e ( T a b l e 2 ) .

T A B L E 2Recom mended des ign pa ramete rs fo r packed co lumn ae ra to r s

M e d i a K a t 2 0 C R e c o m m e n d e d M a x i m u m M i n i m u m R e l at iv esize (m- ~) loading loading colum n cost o f(cmJ (m3/m 2 h) (m3/m 2 hJ diam eter {m) pack2-54 2 .50 100 150 0.2 4 .03-81 1-71 220 a 300 0.3 2-45.03 1.58 220 a > 340 0-4 2.08.89 1.05 220 a > 340 0.7 1-0

a H ighes t loading ra te tes ted .Note: m3/m~ h 0.409 0 = gal lons /f l ~min .


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2 9 0 G . E . H A C K N E Y , J . E . C O L TE x c e p t f o r t h e 2 - 5 4 c m p a c k i n g , th e o x y g e n t r a n s fe r c h a r a c t e r i s t i c s o f t h e P C A

d o e s n o t d e p e n d o n Q , t h e h y d r a u l i c l o a d i n g r a t e . T h e l o g d e f i c i t t e r m ( e q n ( 4 ) ) ise q u a l t o ( K L a ) t . B e c a u s e K L a ~ Q O . VS ( C o u l s o n e t a l . , 1 9 7 8 ) a n d t ~ Q - O . v s ( E c k e n -f e l d e r a n d B a r n h a r t , 1 9 6 3 ) , t h e t r a n s f e r c h a r a c t e r i s t ic s o f t h e P C A s h o u l d b e i n d e -p e n d e n t o f Q u p t o th e p o i n t t h a t t h e c o l u m n f l o o d s .

T h e P C A h a s b e e n u s e d f o r a ir t r a n s f e r i n d is s o l v e d a i r f l o t a t i o n ( B r a t b y a n d M a r a is ,1 9 7 5 ) a n d r e m o v a l o f s u p e r s a t u r a t e d n i t r o g e n ga s i n h a t c h e r i e s ( O w s t ey , 1 9 8 1 ) . F o rn i t ro g e n t r a n s fe r t h e m a s s t r an s f e r m o d e l i s m o d i f i e d t o :

i n \ ~ 1 = 4 ( 0 . 4 + K Z ) ( 2 0 )w h e r e q5 = ( K L a ) N 2 / ( K L a ) o 2 .

E x p e r i m e n t a l v a l u e s f o r q5 r a n g e f r o m 0 . 8 9 t o 1 . 1 6 (B i l s t a d a n d L i g h t f o o t , 1 9 8 0 ;S p e e c e & H u m e n i c k , 1 9 7 3 ). T h e v a lu e s o f C * f o r n i t r o g e n + a r g o n c a n b e o b t a i n e df r o m W e i ss ( 1 9 7 0 ) . F o r n i t r o g e n + a r g o n r e m o v a l a G / L r a t i o o f 3 w i l l b e a d e q u a t e . I fi t is n e c e s s a r y t o r e m o v e n i t r o g e n + a r g o n a n d a d d o x y g e n , a G / L o f 5 sh o u l d b e u s e d .A n a l y t i c a l p r o c e d u r e s f o r d i s s o l v ed g as s u p e r s a t u r a t i o n h a v e b e e n r e v i e w e d b yD ' A o u s t a n d C l a rk ( 1 9 8 0 ) a n d W e i t k a m p a n d K a t z ( 1 9 8 0 ) . F o r d e g a ss in g a p p l i c a t io n s ,t h e P C A w o u l d h a v e t o b e d e s i g n e d o n a A P o r t o t a l g a s p r e s s u r e b a si s ( C o l t , 1 9 8 2 ) .

C O N C L U S I O NT h e P C A is a h ig h l y e f f i c ie n t a e r a to r t h a t h a s p o t e n t i a l in a q u a c u l t u r e f o r b o t h o x y g e nt r a n s f e r a n d n i t r o g e n r e m o v a l . H o w t h e d e v i c e s a r e u s e d i n a c tu a l i n s t a l l a t i o n s i s u p t ot h e i m a g i n a t i o n o f t h e d e s i g n e r . T w o a l t e r n a t i v e d e si g n c a l c u l a t i o n s a r e p r e s e n t e d i nt h e a p p e n d i x .

R E F E R E N C E S

B i ls ta d , T . & L i g h t fo o t , E . N . (1980) . P re d ic t ing oxy ge n a nd n i t roge n a dso rp t i on r a t e s f rom sub -m e rge d suspens ions o f ga s bub b l e s . Prog. Water Tech . , 12 (6 ) , 23 -35 .B ra t by , J . & Ma ra is , G . V . R . (1975). S a t u ra t o r pe r fo rm a nc e i n d i sso lve d -ai r (p re ssu re) f l o t a t i on .W a ter R es . , 9 , 9 2 9 - 3 6 .C o l t , J . E . (1982) . The c om pu t a t i on a nd r e po r t i ng o f d is so lve d ga s l e vel s. War. R es . , in press.C o l t , J . & W e s te r s, H . (1982). P rod uc t i on o f ga s bub b l e d is e a se by a e ra t i on . T rans . A m . F i sh . So c . ,1 1 1 , 3 4 2 - 6 0 .C o l t , J . E . & Tc ho ba no g l ous , G . (1981). De s ign o f a e r a t i on syst em s fo r a qua c u l t u re . I n : Pro ceed -ings o f t h e B io eng ineer ing Sym p o s iu m f o r F i sh Cu l tu r e , e ds L . J . A l i e ns & E . C . Ki nne y , F i shC u l t u r e S e c t i o n o f t h e A m e r i c a n F is h e r ie s S o c i e t y a n d t h e N o r t h e a s t S o c i e t y o f C o n se rv a ti onEng i ne er s , B e the sda, M a ry l a nd , pp . 138 -48 .C o u l son , J . M. , R i c ha rdson , J . F . , B a c khurs t, J . R . & Ha rke r , J . H . (1978 ) . Chemica lEngineer ing,3rd e dn , vo l. 2 , P e rga m on , O xfo rd .


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PERFORMANCE AND DESIGN OF PACKED COLUMN AERATION SYSTEMS 29 1D'Aoust , B. G. & Clark, M. J. R. (1980). Analysis of supersaturated air in natural water and reser-voirs. Trans. Am. Fish. Soc., 109, 708-24.Eckenfelder, Jr., W. W. & Barnhardt , E. L. (1963). Performance of a high rate trickling filter usingselected media../. Wat. Poll. Contr. Fed., 35, 1535-51.Eckert, J. S. (1979). Design of packed columns. In: tlandbook of Separation Techniques forChemical Engineers, ed. P. A. Schweitzer, McGraw-Hill, New York, pp. 1-221-1-253.Hunter, J. S. III (1979). Accounting for the effects of water temperature in aerator test proce-dures. In: Proceedings: Workshop Toward an Oxygen Transfer Standard, EPA-600/9-78-021,ed. W. C. Boyle, US Environmental Pro tecti on Agency, Cincinnati, Ohio, pp. 85-9.Lewis, W. K. & Whitman, W. C. (1924). Principles of gas adsorption. J. Ind. Eng. Chem., 16,1215-20.Metcalf & Eddy, Inc. (t979). Wastewater Engineering: Treatment, Disposal. Reuse, 2nd edn,McGraw-Hill, New York.Norton Co. (1977). Design informance for packed towers. Bulletin DC-11, Akron, Ohio.Norton Co. (1978). Packed tower internals. Bulletin TA-8OR, Akron, Ohio.Owsley, D. E. (1981). Nitrogen gas removal using packed columns. In: Proceedings of the Bio-engineering Symposium ]or Fish Culture, eds L. J. Allen & E. C. Kinney, Fish Culture Section

of the American Fisheries Society and the Northeast Society of Conservation Engineers,Bethesda, Maryland, pp. 71-82.Speece, R. E. & ttumenick, M. J. (1973). Carbon dioxide stripping from oxygen activated sludgesystems. J. Wat. Poll. Contr. Fed., 45,412-23.Weast, R. C. (ed.) (1974). Handbook of Chemistry and Physics, CRC Press, Cleveland, Ohio.Weiss, R. F. (1970). The solubility of nitrogen, oxygen, and argon in water and sea water. Deep-Sea Res., 17, 721-35.Weitkamp, D. E. & Katz, M. (1980). A review of dissolved gas supersaturat ion literature. Trans.Am. Fish. Sot., 109, 659-702.Yunt, F. W. (1979). Gas flows and power measurement. In: Proceedings of Workshop TowardanOxygen Transfer Standard, EPA-600/9-78-021, ed. W. C. Boyle, US Environmental ProtectionAgency, Cincinnati, Ohio, pp. 105-27.Zenz, F. A. (1979). Design of gas absorption towers. In: Handbook of Separation Techniques forChemical Engineers, ed. P. A. Schweitzer, McGraw-Hill, New York, pp. 3-49-3-108.

APPENDIX - DESIGN EXAMPLESExam ple s o f PCA in s ta llat io n

T w o p o s s i b le f u ll -s c a le P C A c o n f i g u r a t i o n s a r e s h o w n i n F i gs A 1 a n d A 2 . B o t he x a m p l e s a re i n t e n d e d f o r a r a c e w a y s y s t e m . W h e r e F i g . A 1 i l l u s t r a te s n o e l e v a t i o nb e t w e e n r a c e w a y , w h i l e F i g . A 2 s h o w s a 0 - 6 m d r o p b e t w e e n e a c h r ac e w a y .

Des ign d a ta A v e r a g e f l o w r a t e = 3 0 6 m 3 h -1I n f l u e n t D O l e v e l , C i = 6 m g / l i t e rW a t e r t e m p e r a t u r e = 1 0 CA i r t e m p e r a t u r e = 2 0 CA t m o s p h e r i c p r e s s u re = 1 a t m = 1 0 1 - 3 2 5 k P aD e s i r e d D O l e v e l , C o = 1 0 m g / l i t e rA i r s a t u r a t i o n D O l e v e l a t 1 0 C = 1 1 . 2 8 m g / l i t e rN o r t o n 3 . 8 1 c m m e d i a i s t o b e u s e dG / L = 3


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2 9 2 G.E. HACKNEY, J. E. COLT

/ I T m ,1.15m .75mT ~R A C E W A YPump.SUbmersibleVents 8 Fan sPerforated istribut/onlate

~ J ~ f - - ~ F ib * rg la s s C o lum n

" . J I ~ Pe r fo ra ted Suppor t P la teR A C E W A Y /

F ig . A 1 . P C A i n h o r i z o n t a l r a c e w a y .

0 5 5 m

, 1P u m p

Fig. A2.

~ A i r Vents Fans/ / sPsrforoted istributionlots~ 0 ~ Fiberglass olumn, , , ~ , ~ r o T , o~ . . e m j , II-Ja I ~ Perforated upport late

TR A C E W A Y

/P CA in d ropped r acew ay .

D e s i g n b a s i s1 . D e t e r m i n e t h e v a lu e o f K t o b e u s e d :

( a ) F r o m T a b l e 2 , K = 1 .7 1 a t 2 0 C ;( b ) A p p l y i n g e q n ( 1 4 ) , K l o = 1 -35 .

2 . D e t e r m i n e Z , t h e r e q u i r e d p a c k i n g m e d i u m d e p t h u s in g e q n ( 1 5 ) :

In \ ~ - ~ o / - 0 -4 + K Z


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gives:PERFORMANCE AND DESIGN OF PACKED COLUMN AERATION SYSTEMS 293

Z = 0-75m3. Determine the application area required based on a hydraulic loading rate of220 m 3 m -2 h -1.

3 0 6 m 3 h -lA r e a = = 1 . 3 9 m 2220 m 3 m -2 h -1Diameter ~> 1.3 m

4. Deter mine power requiremen ts for the device.(a) Hydraulic power:Note: the amount of lift required to transport water from one raceway toanoth er is different i n the two examples. (See Figs A1 and A2.)

(i) Hydraulic power requireme nt for horiz ontal raceways (Fig. A1).Q ' T Hp -

EH = Z + (inlet and outl et condit ion)

= 0.75m + 0.4m = 1-15mTherefore:

(ii)

0.0 85( m 3 s 1) x 9. 80(kN m -3) x 1-15(m)p = 0-8= 1.20 kW

Similarly for dropped raceways (Fig. A2).

where

(b) Air blower power:( i )

an d

P = 0-57 kW

H = 0-55 m

Air requireme nts using G/ L = 3 gives:G = 918 m 3 h -1

( rh ) = Q apa= 0.31 kg s 1


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2 9 4 G . E . H A C K N E Y , J . E . C O L T( ii ) A i r p r e ss u r e d r o p t h r o u g h t h e c o l u m n . A l l o w i n g fo r 0 . 1 2 4 k P a h e a d

l o s se s a t d i s t r i b u t i o n a n d p a c k i n g s u p p o r t p l a t e , a n d u si n g d a t a f r o mN o r t o n C o . (1 9 7 7 ) f o r p r es su r e d r o p p e r m e t r e d e p t h o f pa c k i ng .

T h e n

a n d t h e r e f o r eA p = 0 . 4 9 0 k P a /m

= 0 . 4 9 x 0 . 7 5= 0 . 3 6 7 k P ao v e r a l l A P = 0 - 3 6 7 k P a + 0 . 1 2 4 k P a = 0 - 4 9 1 k P a

( ii i) A d i a b a t i c c o m p r e s s i o n u s in g e q n ( 1 2 ) ; w i t h P l = 1 0 1 . 3 2 5 k P a a n dP 2 = 1 0 1 - 8 1 6 k P a , t h e n a i r b l o w e r p o w e r is :

P = 0 . 1 8 k W( c ) T o t a l p o w e r r e q u i re d

P t o t a l = P p u m p nt- P a i r( i) T o t a l p o w e r r e q u i re d f o r h o r i z o n t a l r a c e w a y s

P to ta l = 1 . 2 0 k W + 0 . 1 8 k W = 1 . 3 8 k W( ii ) T o t a l p o w e r re q u i r e d fo r d r o p p e d r a c e w a y s

P t ot al = 0 . 5 7 k W + 0 . 1 8 k W = 0 . 7 5 k W5 . U s i n g e q n ( 8 ) , o x y g e n t r a n s f e r r a te :

M = 1 -2 2 kg O 2 h -16 . D e t e r m i n e a e r a t o r f ie l d p e r f o r m a n c e , N

N = M/ P( a ) I n h o r i z o n t a l r a c e w a y s :

N = 0 . 8 8 k g 0 2 k w h -1( b ) I n d r o p p e d r a c e w a y s :

N = 1 . 6 3 k g O 2 k w h -17 . D e t e r m i n e a e r a t o r p e r f o r m a n c e a t s t a n d a r d c o n d i t i o n s , N o , u si n g e q n ( 1 4 )

( a ) I n h o r i z o n t a l r a c e w a y s :N o = 1 . 9 5 k g O z k w h -1

( b ) I n d r o p p e d r a c ew a y s :


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PERFORMANCE AND DESIGN OF PACKED COLUMN AERATION SYSTEMS 2 9 5N o = 3 -6 1 k g O2kWh -1

8 . C o m p a r e t h e P C A p e r f o r m a n c e w i t h a m o r e c o n v e n t i o n a l a e r a t o r t o a ss es s i tsf e a s i b i l i t y .

S t a n d a r d p e r f o r m a n c e r a ti n g s f o r s u r fa c e a e r a t o r s r an g e f r o m 1 .2 t o 2 . 4 k g0 2 kW h 1 (C o l t a n d T c h o b a n o g l o u s , 1 9 8 1 ) . T h e p e r f o r m a n c e o f t h e P C A i n t h eh o r i z o n t a l r a c e w a y s is n e a r t h e u p p e r p e r f o r m a n c e l im i ts o f s u rf a c e a e r a t o r s b u ti n s ta l le d i n t h e d r o p p e d r a c e w a y s , i t s p e r f o r m a n c e is a l m o s t t w i c e t h a t o f s u r f a c ea e r a t o r s . I f t h e r a c e w a y s w e r e c o n s t r u c t e d w i t h a 1 -2 m ( 4 ft ) d r o p , t h e t r a n s f e re f f ic i e n c y , N o , w o u l d b e 1 5 k g 0 2 k w h -1 . D e p e n d i n g o n t h e h y d r a u l ic l o a d i n gr a te ( a n d t h e r e f o r e A P ) , N o m a y r a ng e f r o m 1 0 t o 8 0 k g O z k W h - 1 w h e n a na d e q u a t e h y d r a u l i c h e a d is a v a i la b l e .

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Hackney 1982 Aquacultural-Engineering

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