TR DISS 1525(1)

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    L IQUID-L IQUID SEPARATION IND I S C - S T A C K C E N T R I F U GE S

    J.P. VAN DER L INDEN

    TRdiss ^1525

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    LIQUID-LIQUID SEPARATION INDISC-STACK CENTRIFUGES

    PROEFSCHRIFT

    Ter verkrijging van de graad van doctor aan de TechnischeUniversiteit Delft, op gezag van de Rector Magnificus,Prof. dr. J.M. Dirken ten overstaan van een commissieaangewezen door het College van Dekanen, te verdedigenop Donderdag 5 februari 1987 te 14.00 uur

    door

    JOHANNES PETRUS VAN DER LINDENwerktuigkundig ingenieurgeboren te 's-Gravenhage

    TR diss]1525

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    Dit proefschrift is goedgekeurd door depromotor Prof. ir. E.J. de Jong

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    Stellingenbehorende bij het proefschrift

    "Liquid-liquid separationin disc-stack centrifuges"

    De algemene opvatting dat het olie-water grensvlak in schotelcentrifuges zich bij voorkeurop de distributiekanalen moet bevinden dient, uit oogpunt van het risico op extreem niet-uniforme doorstroming van het schotelpakket, herroepen te worden.

    Het dispergeren van de voeding van een schotelcentrifuge, dat met de acceleratie daarvangepaard gaat, heeft een niet te verwaarlozen invloed op het scheidingsrendement.

    De bewering van Schmitz, als zou maldistributie in het schotelpakket geen invlo eduitoefenen op het scheidingsrendement van een schotelcentrifuge, is onwaarschijnlijk.

    F.J. Schmitz; Milchwissenschaft 12 (1950) p- 4l8 - 425

    De wijze waarop Fumoto en Kiyose het door hen in een centrifugaalextractor gemetenweireffect correleren, is onjuist.

    H. Fumoto, R. Kiyose; J. Nucl. Sc. Techn. 1 (1980) 9, P- 694 - 699

    Voor wat betreft de hoogfrekwente permittiviteit van water-in-olie dispersies berust debewering van Kurkova e.a., als zou de voorspelling van Hanai nauwkeuriger zijn dan die vanWagner, op rekenfouten.

    Z.E. Kurkova e.a.; Zhurnal Prikl. Khim. ^6 (1983) 5, p. 1034 - 1037T. Hanai ; Kolloid Zeitschrift VJ1_ (I960) 1, p. 23 - 31K.W. Wagner ; Arch. Elektrotechn. 2 (1914) 9. P- 371 - 387

    Als argument voor de toepassing van een capacitieve meetmethode ter bepaling van dewaterconcentratie in olie, wordt vaak de aandacht gevestigd op de permittiviteit van water(" 80), olie (~ 2) en lucht (~ 1) . Het op deze wijze insinueren, dat de permittiviteit vaneen dispersie van twee of drie der bovenvermelde componenten een gewogen gemiddelde is vangenoemde waarden, is misleidend.

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    7De wettelijke bijdrage "pooling voorheen vrijwillig verzekerden", voortvloeiend uit de"Wet op de toegang tot ziektekostenverzekerden" (WTZ), is in flagrante strijd met hetverzekeringsprincipe.

    Artikel 246, Wetboek van Koophandel.

    8Het verdient aanbeveling om, in navolging van de ve rpl ich te pe ri odi eke keuring voorautomobielen (APK), ook bestuurders aan een periodieke keuring te onderwerpen.

    9

    Met het eerste lustrum van de eenmalige uitkering achter de rug, kangesteld worden dateenmalig net zo nmalig is als tijdelijke BTW-verhogingen tijdelijk zijn.

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    De tekst op de kraag van de pedel van de Technische Universiteit Delft is niet up to date.

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    Het verdient aanbeveling om de hondenbe las ting t e ver vangen door een ac c i jns ophondenvoer.

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    In navolging van Newtonse inzichten zou de benaming "centrifuge" plaats moeten maken voor"centripetuge".

    13Objectieve waarnemingen bestaan niet.Quidquid recipitur per modum recipiendis recipitur.

    Thomas van Aquino, 1225 - 1274

    Delft, 5 februari 1987J.P. van der Linden

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    To Ine, Joop and Stan

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    CONTENTSpage

    Acknowledgements 9

    Summary and conclusions 11

    Summary and conclusions (Dutch version) 13

    List of symbols 15

    Introduction 21

    1. Scope of present study 272. Separation efficiency 35

    2.1 Division of chapter 2 372.2 Objectives 372.3 Background theory 372.3.1 Sigma concept 372.3.2 Dispersion effect 42

    2.3-2.1 Dispersion effect in separators 422.3.2.2 Dispersion effect in agitated vessels 43

    2.4 Experiments 462.4.1 Experimental approach 462.4.2 Description 'of apparatuses 49

    2.4.2.1 Centrifuge pilot plant 492.4.2.2 Bypass distributor and dummy stack 53

    2.4.3 Measurement techniques 5**2.4.3-1 Dispersion experiments 542.4.3-2 Separation experiments 55

    2.5 Results and observations 572.5-1 Dispersion experiments 572.5-2 Separation experiments 63

    2.6 Conclusions 67

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    CONTENTS (continued)page

    Hydrodynamics 693.1 Division of chapter 3 713-2 Objectives 713.3 Background theory 71

    3-3.1 Hydrostatic pressure gradient 713.3.2 Interface position 743.3.3 Hydraulic capacity 763.3.4 Disc-stack 77

    3.3.4.1 Symmetrical flow solution 773-3.4.2 Asymmetrical case 803.3.4.3 Channel feed 823.3.4.4 Stability 83

    3.3.5 Weir 873.3.6 Hydraulic gradient 97

    3.4 Experimental 993.4.1 Description of separator 993.4.2 Measurement techniques and experiment procedures 103

    3.4.2.1 Interface experiments 1033.4.2.2 Weir head experiments 1053.4.2.3 Hydraulic capacity experiments 1093.4.2.4 Endoscope experiments 110

    3.5 Results and observations 1133-5-1 Interface experiments 1133-5.2 Weir head experiments 1233.5.3 Hydraulic capacity experiments 1343.5.4 Endoscope experiments 137

    3.6 Prediction of the interface position and hydrauliccapacity 1393.6.1 Approach 1393.6.2 Interface position 1393.6.3 Hydraulic capacity 149

    3.7 Conclusions 153

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    CONTENTS (continued)page

    In-line separation-monitoring 1554.1 Introduction 1574.2 Objectives 1574.3 Interface-monitoring, pulse-echo-method 158

    4.3-1 Basic principle 1584.3-2 Description of monitoring system 158

    4.3.2.1 Transducer 1584.3-2.2 Transducer position 1594.3-2.3 Constructional aspects l604.3.2.4 Signal interpretation 162

    4.3-3 Experience gained with the interface-monitoringsystem I63

    4.4 Water-content monitoring, capacitance technique 1644.4.1 Background theory 164

    4.4.1.1 Dielectric behaviour of dispersions 1644.4.1.2 Dual cell principle 1674.4.1.3 Sensor design 169

    4.4.2 Description of apparatuses 1714.4.3 Experiments 1724.4.4 Results and observations 1724.4.5 Experience gained with the capacitance technique 173

    4.5 Conclusions 175

    References 179

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    CONTENTS (continued)page

    Appendix A Physical and chemical properties and measurementprocedures 185

    Appendix B Hydraulic gradient model for the cylindrical partof the top disc 189

    Appendix C Interface calibration formula 193Appendix D Interface experimental conditions 195

    Appendix E Hydraulic capacity experimental conditions 198

    Appendix F Short cut model (channel feed) 199

    Appendix G Distribution model 203

    Curriculum vitae 207

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    ACKNOWLEDGEMENTS

    The assistance given by my promotor Prof. ir. E.J. de Jong, and hisenthusiasm, had a very stimulating influence on the progress of the project.Especially the great amount of freedom allowed by him, offered me theopportunity to learn from my own mistakes, which I consider a privilege.Thanks are due to the mechanical and chemical engineering students whocarried out the majority of the reported experiments and considerablycontributed to the interpretation of the results.I would like to thank Alfa-Laval AB for both the financial and technicalsupport and for their permission to carry out this wor k. I wouldparticularly like to thank Dr. C.G. Carlsson for his helpful advice andconstructive criticism during the course of this work.

    I would also like to express a word of thanks to all employees of theLaboratory for Process Equipment for their valuable help and assistance inthe realization of this work. Further I am indebted to Ir. J.A. Vogel (TPD)for providing reliable transducers, Mr. B. Jager (Wolf Endoscopie) who madethe endoscope system available, Dr. ir. W.Chr. Heerens (TUD) for hisassistance, Drs. W.J.P. Vink (TUD) for the pulse-echo-detector and Mr.L.G. Bonsen and his colleagues (Alfa-Laval Industrie B.V.) for providing ahermetic separator.For the preparation of the text and figures in this work, I would like toexpress my gratitude to Mrs. M.A. Engelsma for her critical comments on theEnglish text, to Mr. B.N. Sodderland and Mr. J.A. de Vries for theexcellent drawings and to Mrs. M.J.J. Tetteroo-La Croix and Mrs. P.W.M. vanHagen for their patience and skill during the endless hours of typing thisthesis.

    Thanks also to the management of Unilever Research Laboratory for theircontribution to this thesis. In particular for the time made available by

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    Mr. J. van der Meer, whose expert knowledge of the edi toria l side of thiswork proved invaluable.

    Gratitude is also due to my parents for their unflagging enthusiasm for theproject, for the way they always stimulated me and for the possibilitiesthey have offered in many fields, thus enabling me to reach this milestone.

    Last but not least, I wish to thank my wife for her sustained encouragementand loving tolerance during the many hours I spent working on this thesis athome.

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    SUMMARY AND CONCLUSIONS

    S e p a r a t i o n e f f i c i e n c y o f i n d u s t r i a l d i s c - s t a c k c e n t r i f u g e s r e s u l t from a ni n t e n s i f i e d s e d i m e n t a t i o n p r o c e s s , an d b y t h e m an ne r i n w h ic h m a c h i n e / l i q u i dc o n t a c t i n g a n d t r a n s p o r t t h r o u g h t h e c e n t r i f u g e a r e e s t a b l i s h e d . T het h e o r e t i c a l an d e x p e r i m e n t a l w o rk , d e s c r i b e d i n t h i s t h e s i s , i s i n t e n d e d t oe l u c i d a t e t h e c o n t r o l l i n g m e ch an is m s u n d e r r e a l i s t i c c o n d i t i o n s a nd t o g a i na n u n d e r s t a n d i n g o f t h e i r i n t e r a c t i o n s . T h i s s t u d y h a s d e a l t w i t h s e p a r a t i o no f l i q u i d - l i q u i d s y s t e m s a n d i n p a r t i c u l a r a w a t e r - i n - o i l d i s p e r s i o n .

    T he a c c e l e r a t o r o f t h e c e n t r i f u g e , d e no m i n a te d d i s t r i b u t o r , i n t o w h i c h t h ef e e d l i q u i d i s d i s c h a r g e d , c o n s i s t s o f an a x i a l t u b e p r o v i d e d w i t h r a d i a lg u i d i n g f i n s . I n i t i a l c o n t a c t b e t w e e n t h e f e e d l i q u i d a n d t h e s e p a r a t o ro c c u r s a t t h e t i p s o f t h e s e g u i d i n g f i n s . T h i s s h o c k w i s e c o n t a c t i s f o l lo w e db y a t r a n s p o r t a l o n g t h e g u i d i n g f i n s t o w a r d s t h e s u r f a c e l e v e l , w h i c hr e s u l t s i n s e v e r e s t r a t i f i c a t i o n . E x p e r i m e n t a l w o r k , p e r f o r m e d w i t hc e n t r i f u g e i n t e r n a l s , d e s i g n e d e s p e c i a l l y f o r t h i s p u r p o s e , c l e a r l yi n d i c a t e s t h a t t h e a b o v e - d e s c r i b e d p hen om en on i s t h e p r e d o m i n a t i n g s t a g e i nt h e d i s p e r s i o n p r o c e s s , r a t h e r t h a n t h e w a l l s h e a r a s s o c i a t e d w i t h t h es u b s e q u e n t a x i a l t r a n s p o r t . N ot o n l y c an t h i s d e t e r m i n a t i o n b e o f a s s i s t a n c ea s f a r a s d i r e c t e d d e s i g n i m p r o v e m e n t i s c o n c e r n e d , b u t , m o r e o v e r ,p h e n o m e n o n - o r i e n t a t e d e s t i m a t i o n s o f d i s p e r s i o n c h a r a c t e r i s t i c s w i l l beco m ep o s s i b l e . T h e a b o v e -m e n t i o n e d e f f e c t r e d u c e s s e p a r a t i o n e f f i c i e n c y i n t h ec a s e o f b o t h l o w f e e d f l ow r a t e s a nd h i g h n u m be rs o f r e v o l u t i o n s , w h ic h i se x p l a i n e d , a mong o t h e r t h i n g s , b y t h e p o s i t i o n o f t h e f e e d l i q u i d i n t e r f a c ew i t h i n t h e d i s t r i b u t o r .S e p a r a t i o n e x p e r i m e n t s p e r fo r m e d u n d e r r e a l i s t i c c o n d i t i o n s a f f i r m e d t h ei m p o r t a n t e f f e c t o f t h e w a t e r - o i l i n t e r f a c e p o s i t i o n o n t h e o v e r - a l ls e p a r a t i o n r e s u l t . W h e ne v er t h i s i n t e r f a c e w o u l d b e p o s i t i o n e d w i t h i n t h ef e e d p o i n t ( e i t h e r t h e d i s t r i b u t i o n c h a n n e l o r t h e p e r i p h e r y ) t h e r a t h e ru n d e s i r a b l e s h o r t - c u t e f f e c t w i l l t a k e p l a c e . I n t h i s c a s e , o n l y p a r t o f th ed i s c - s t a c k w i l l b e f lo w n w i t h l i q u i d , h e n c e r e s u l t i n g i n a d r a s t i c d r o p i ns e p a r a t i p n e f f i c i e n c y . M o r e o v e r , t h e i n t e r f a c e p o s i t i o n i n f l u e n c e sm a l d i s t r i b u t i o n w h i c h i s c a u s e d b y o c c u r r i n g a x i a l p r e s s u r e e f f e c t s . Thel a t t e r e f f e c t e x p l a i n s m e a su r ed o ptim um s e p a r a t i o n e f f i c i e n c y a s f u n c t i o n o fi n t e r f a c e p o s i t i o n .

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    Owing to i t s important e f fe c t on sepa ra tor per form anc e , much a t t e n t i o n i sp a i d t o t h e f a c t o r s d e t e r m i n i n g t h e i n t e r f a c e p o s i t i o n . A p a r t from t hedensi t ies of both phases and the radi i of both out letports , pressure drop isto be included in order to improve the interface formula. This was affirmedby e x p e r i m e n t a l w ork , p erf or m ed w i th an a c o u s t i c p u l s e - e c h o - m e t h o d ,successfu l ly incorpora ted wi th in the cent r i fug al sep ara to r .Connected wi th the impor tance of in- l in e sep ara t ion -m on i tor i ng , th e above-m e n ti on ed p u l s e - e c h o - t e c h n i q u e i s d e s c r i b e d i n d e t a i l , t o g e t h e r w ith acap acita nce tec hniq ue, w ith the aid of which low water con tent in o i l co uldb e m e a su re d . P h y s i c a l p r i n c i p l e s , a s w e l l a s e x i s t i n g l i m i t a t i o n s , ar edisc usse d and ve rif ie d through experim ental work.F rom th i s work i t can be conc luded tha t ex i s t ing theor i e s on d i e l ec t r i cproperties of dispersions show adequate agreement with experiments. Presenceo f a i r a f f e c t s t h e d i e l e c t r i c p r o p e r t ie s s u b s t a n t i a l l y , th u s l i m i t i n g t hemeasuring range.

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    SUMMARY AND CONCLUSIONS (D utc h v e r s i o n )V l o e i s t o f - v l o e i s t o f s c h e i d in g m et s c h o t e l c e n t r i f u g e sDe f a s e n s c h e i d i n g i n i n d u s t r i l e s c h o t e l c e n t r i f u g e s kom t t o t s t a n d d o o r e e nv e r s t e r k t s e d i m e n t a t i e p r o c e s e n i s m ede a f h a n k e l i j k v a n h e t a p p a r a a t /v l o e i s t o f k o n t a k t e n t r a n s p o r t d o or d e c e n t r i f u g e . H e t t h e o r e t i s c h e e ne x p e r i m e n t e l e w e r k , b e s c h r e v e n i n d i t p r o e f s c h r i f t , i s b e d o e l d t e rv e r d u i d e l i j k i n g va n d e k o n t r o l e r e n d e m e c h a n i sm e n en h u n o n d e r l i n g ei n t e r a k t i e s o n d e r r e a l i s t i s c h e o m s t a n d ig h e d e n . D ez e s t u d i e b e ha n de lt des c h e i d i n g v an v l o e i s t o f - v l o e i s t o f s ys te m en e n i n h e t b i j z o n d e r d i e v a n e e nw a t e r - i n - o l i e d i s p e r s i e .De a c c e l e r a t o r v an d e c e n t r i f u g e , d e z o ge n aa m d e d i s t r i b u t e u r , w a a r a an d ev o e d i n g w o r d t t o e g e v o e r d , b e s t a a t u i t ee n a x i a l e b u i s v o o r z i e n v an r a d i a l es c h o e p e n . I n i t i e e l k o n t a k t t u s s e n de v o e d i n g e n d e s e p a r a t o r v i n d t p l a a t s opd e s c h o e p t i p p e n . D i t s c h o k s g e w i j z e k o n t a k t w o rd t o p ge vo lg d d o or t r a n s p o r tl a n g s d e s ch oe pe n n a a r h e t v l o e i s t o f o p p e r v l a k , h e t g e e n r e s u l t e e r t i n e e na a n z i e n l i j k e s t r a t i f i k a t i e . E x p e r im e n t ee l w e rk , u i t g e v o e r d m et s p e c i a a l v o ord i t d o e l o ntw orp en c e n t r i f u g e - o n d e r d e l e n , t o o n t d u i d e l i j k a a n d a t b o v e n v e r m e l d fe no m ee n d e b e p a l e n d e s t a p i s i n h e t d i s p e r g e r i n g s p r o c e s , i n p l a a t sv an w a n d s c h u i f s p a n n i n g e n t e n g e v o l g e v a n h e t d a a r o p v o l g e n d e a x i a l et r a n s p o r t . D ez e p l a a t s b e p a l i n g k an v a n d i e n s t z i j n b i j g e r i c h t eo n t w e r p v e r b e t e r i n g e n e n b o v e nd i en f e n o m e e n g e r ic h t e s c h a t t i n g e n v an d i s p e r s i ek a r a k t e r i s t i e k e n m o g e l i j k m a k e n . B o v en v e rm e l d e f f e k t r e d u c e e r t h e ts c h e i d i n g s re n d e m e n t i n h e t g e v a l v a n l a g e v o e d i n g s d e b i e t e n , a l s m e d e h o g et o e r e n t a l l e n , h e t g e e n t e v e r k l a r e n i s m e t, o n d e r a n d e r e , h e t v l o e i s t o f n i v e a ui n d e d i s t r i b u t e u r .S c h e i d i n g s e x p e r i m e n t e n , u i t g e v o e r d o n de r r e a l i s t i s c h e o m s ta nd ig h ed e n h eb bende b e l a n g r i j k e i n v l o e d v an h e t w a t e r - o l i e g r e n s v l a k op h e t s c h e i d i n g s r e nd e m e n t a a n g e t o o n d . I n d i e n d i t g r e n s v l a k d i c h t e r b i j de c e n t r i f u g e - a s kom tt e l i g g e n dan h e t v o e d i n g s p u n t (o f d e d i s t r i b u t i e k a n a l e n o f de o m t r e k ) ,v i n d t e e n o n g e w e n s t k o r t s l u i t e f f e k t p l a a t s . I n d a t g e v a l w o rd t s l e c h t s e eng e d e e l t e v an d e s c h o t e l s t a p e l m et v l o e i s t o f d o o r s t r o o m d , h e t g e e n e e nd r a s t i s c h e r e n d e m e n t s d a l i n g t o t g e v o lg h e e f t . B o v e n d ie n b e n v l o e d t de

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    g r e n s v l a k p o s i ti e de m ate van m a l d i s t r i b u t i e d i e d o o r a x i a l e d r u k e f f e k t e nw o r d t v e r o o r z a a k t . D i t l a a t s t e v e r k l a a r t d e g em ete n o pt im a i n h e tscheidingsrendement als funkt ie van de grensvlakposi t ie .In verband met de be langr i jke invloed op he t separa t ie - rendement i s vee laandach t be s t ee d aan de i n v l oe ds fa k t o r en van he t g r en sv l a k . Beha lve dedichtheden van de twee fasen en de radi i van de ui t laatpoorten moet drukvalve rdisk on teerd worden om de grensvlakform ule te ve rb ete re n. Dit is bev estig ddoor e xp er i m en tee l werk , u i tg evo erd met een akoes t i sche puls -echo-methodedie met succes in de centrifuge is ingebouwd.In verband met het belang van in-l ine scheidingskontrole wordt bovenvermeldepu ls -echo-me thode g ed e t a i l l e e r d bes ch rev en , t ezamen met een c a p ac i t i ev et e c h n i e k , m e t b e h u l p w a a r v a n l a g e w a t e r g e h a l t e s in olie gemeten kunnenworden. Zowel de fysische pr in ci p es , a lsmede bes taa nd e be pe rk ing en wordenbe spr ok en en met expe r imenteel werk ge ve r i f ie er d. De kon klusie die ui t d i twerk getrokken kan worden, is dat de bestaande theorien op het gebied vandilektr ische eigenschappen van dispers ies adequate overeenstemming ver tonenm et de m e t in g e n . A a nw e zig he id v an l u c h t b e n v l o e d t d e d i l e k t r i s c h eeigenschappen aanmerkel i jk, hetgeen het meet t rajekt beperkt .

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    LIST OF SYMBOLS 4b

    bCCvCcCoCCLdDDdd

    gr

    o u t

    EE bEkmff...f IFFgHhhjKK

    d i s

    w i d t h , c h a n n e l w i d t hw e i r c o n s t a n t , c o n s t a n tw e i r c o n s t a n t f o r t h e v e l o c i t yw e i r c o n s t a n t f o r t h e c o n t r a c t i o n e f f e c te m pt y c e l l c a p a c i t a n c ec a p a c i t a n c el e n g h t o f c y l i n d r i c a l p a r t o f t o p d i s cc a p a c i t o r p l a t e d i s t a n c eg r a v i t y d i s c d i a m e t e ra g i t a t o r d i a m e t e rd e p t h , d r o p l e t d i a m e t e rc r i t i c a l d e p t hb r i n k d e p t hmaximum d ro p le t d iam ete rd i s t a n c e from t r a n s d u c e r to t r i g g e r s u r f a c ed i s t a n c e fro m t r i g g e r s u r f a c e t o b a ck o f m i r r o ri n n e r d i a m e t e r o f s e p a r a t o r b o w ls p e c i f i c e n e r g yb u r s t r a t i oEkman numberf u n c t i o nwater fraction in the feed, by weight

    dimensionm

    FFmmmmmmmmmmmm

    kg/kgwater fraction of the oil stream leaving the bowl kg/kg

    disc

    separator feedforceg r a v i t a t i o n a l a c c e l e r a t i o nh e i g h t a b o v e t h e w e i rc a u l k t h i c k n e s sh e i g h tj J = - 1c o r r e c t i o n f a c t o r f o r t h e d i s t r i b u t o r p r e s s u r ed r o pc o r r e c t i o n f a c t o r t o b e i n s e r t e d in d i s c - s t a c k

    k g / sNm /s 2m

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    Kweirgrad

    kLL.1

    MMMM(A)mnNNN(A)NNN sNhOPPPPQqQwqr

    caulk

    pressure formulacorrection factor to be inserted in weir pressureformulacorrection factor to be inserted in gradientpressure formulaunit vectordistance from mirror 'edge to opposite bowl wallmeridian co-ordinatesound path from transducer to interface and backsound path from transducer to trigger surfaceand backsensor length, bowl length, scale of the energycontaining eddiestangential velocity profile in between discsmolecular weightlengthintegral of Mempirical coefficient in pressure equationnumber of revolutionsnumber of revolutionsmeridian velocity profile in between discsintegral of NAvogadro's numbernumber of caulks per discnumber of disc spacesnumber of holes per discoil flowhydrostatic pressuretransformed radiusreduced pressureweir heightvolumetric flow ratespecific flowseparated waterfraction of the flow going outwardsdistance across which velocity difference is taken

    kgm

    rpm1/8

    1/mole

    kg/sPa

    Pamm3 /sm 2/skg/s

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    r radius m1? position vector mR. inner radius disc-stack, excluding flanges mR_ outer radius disc-stack, excluding flanges mR_ inner radius disc-stack, including flanges mR;. outer radius disc-stack, including flanges mR. interface radius ml

    channel radius mliquid surface radius in the distributor moil outlet radius mgravity disc radius mReynolds numberRossby numberscalarwall shear Nslopecritical slopetemperature Cfluid particle velocity vector m/sflowmeter voltage Vcharacteristic velocity scale m/svelocity at point 1 m/s

    v_ velocity at point 2 m/sv stokes sedimentation velocity in the earth's

    gravitational field m/sv critical velocity m/sv1. tangential fluid velocity m/sv . surface velocity m/sov. meridian fluid velocity m/sv speed of sound in water m/ssww weir length, width mW water flow kg/sWe Weber numberx wall distance co-ordinate mz axial co-ordinate, height co-ordinate m

    ch

    wReRoSSSScTuU

    I

    Uv

    out

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    G R E E K

    a r e s i s t a n c e f a c t o r N s / m 5a p r e s s u r e r a t i oa p o l a r i z a b i l i t y m3f $ r e c o v e r y k g / k gf } r e c o v e r y b y p a s s d i s t r i b u t o r k g / k g$ r e c o v e r y o r i g i n a l d i s t r i b u t o r k g / k g pg N/m 3T e n t r a n c e a n g l e w i t h d i s c g e n e r a t r i c e s rad< 5 d e p t h , E k m a n l a y e r t h i c k n e s s mA d i f f e r e n c ee d i s s i p a t e d p o w e r W/kgc p e r m i t t i v i t y of v a c u u m F/mE r e l a t i v e d i e l e c t r i c c o n s t a n tE * c o m p l e x r e l a t i v e d i e l e c t r i c c o n s t a n tn d y n a m i c v i s c o s i t y Pa.si i l e n g t h s c a l e of e n e r g y d i s s i p a t i n g e d d i e s m9 h a l f c o n e a n g l e / l a t i t u d eA s t r e a m p a r a m e t e rA w a v e l e n g t h of s o u n d mv k i n e m a t i c v i s c o s i t y m 2/ s d i m e n s i o n l e s s w a l l d i s t a n c e c o - o r d i n a t en 3.1415...n m o l a r r e f r a c t i o n m 3p d e n s i t y k g / m

    3

    a d i e l e c t r i c c o n d u c t i v i t y 1/Omo * c o m p l e x d i e l e c t r i c c o n d u c t i v i t y 1/Omc r interfacial tension N/mI Ambler's equivalent area m2T time interval, relaxation time sT shear tension Paw* c i r c u m f e r e n t i a l a n g l e* v o l u m e f r a c t i o n d i s p e r s e d p h a s et^ f r a c t i o n of d i s c s l o a d e d w i t h l i q u i dI D a n g u l a r v e l o c i t y r a d / s18

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    INDICES

    aBDccapchcolddisFhiid1noodrefsymw

    actualblind disccontinuous phasecapacitydistribution channelcollecting channeldispersed phasedistributordistributor foothorizontal, hypothetic,interfaceinner disclow frequencynormaloilouter discreferencesymmetricalwater

    high frequency

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    INTRODUCTION

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    Centrifugal separation

    Centrifugation is an old technique which is characterized by application ofcentrifugal instead of gravitational acceleration with intent to increasesedimentation velocities of particles to be separated from a mixture. Withinan extremely diversified range of industries centrifugal separators are usedfor a multitude of operations including recovery of chemicals, sol vents andca ta ly st s, clar ification of liquids, e.g. beer and wine, classification ofpigments and purification. The history and working principles of the discstack centrifuge, a specific type of centrifugal separator, and subject ofthis thesis, are presented below.

    History of the disc-stack centrifuge

    In June 1878 the Swedish inventor Dr. Gustaf de Laval was granted a patentfor a mac hin e that was to revol utio nize the dairy industry, and, in duecourse, other industries as well. With this machine separation of cream frommilk by means of settling and skimming became superfluous. Now milk could beskimmed more efficiently and with up to 20 % higher capacities. Tow ard s I89Oa Germ an named Cle men s von Be chto lshe im inserted discs in the separatorbowl. By mea ns of these so-called Al pha discs the sepa rat ion a rea, andth er ef or e the capacity, could be improved considerably. Since the separatorwas also suitable for separati ng s oli ds from a liq ui d, it wa s s oonint rodu ced in other ind ust rie s. Centrifu gal separators quickly found newapplications during and after the war: first to purify lube oil for turbinesand ot her ma ch in es , and after 1920, with the growing engineering industry,to purify cutting oils, hardening oils and many more workshop liquids.Dur ing this time , the se parat or also began to be increasingly used in theprocess industries for other products. In the nea r future the centrifug alse pa ra to r might be used for various separations (e.g. condensate-water) inthe offshore industry for reasons of environmental re gul ati ons, sti pul ate dby the government, which become stricter every day.

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    Working principlesDisc-stack centrifuges exist in more designs, which differ in the way theseparated phases are discharged. With respect to the liquid phase dischargeopen and hermetic separators can be distinguished. Open separators dischargethe separated liquid(s) through overflow ports. Hermetic separators areprovided with rotary seals to avoid foaming and contact with air. Otherhermetic designs contain a centripetal pump or "paring ring" with a vanedpump impeller mounted on the stationary feed pipe, in which the kineticenergy of the rotating liquid is largely converted into pressure energyenabling pressurized discharge. With respect to the solid phase discharge,the solids retaining type, and the peripheral discharge separator can bedistinguished. In the solids retaining type, the accumulated solids whichare deposited against the bowl wall must be removed periodically by hand,which requires stopping and disassembling of the bowl, and removal of thedisc-stack. More or less automatic solids discharge is accomplished in theperipheral discharge separator provided with nozzles, which continuouslyremove solids along with a portion of the liquid phase. In other designsintermittent discharge is realised by a hydraulic mechanism, which opensvalves or uncovers a peripheral slit. The use of the latter type is limitedto solids having the degree of fluidity or plasticity required to movethrough the exposed peripheral openings. Within this thesis attention isrestricted to liquid- liquid separation for which reason there is chosen forthe solids retaining open separator, which will be described below. Besides,the open separator was selected for its accessibility which, in view of theexperimental work to be performed, would be advantageous. The solidsretaining bowl consists of a cylindrical body and conical hood held togetherby a threaded lock ring (see figure 1.1). Standard interior fittingscomprise the distributor, an axial ribbed tube with a flared lower endcarrying the intermediate discs, which matches the slope of the bowl hood toform the heavy phase outlet channel. Secured to the neck of the bowl by asmaller lock ring is the gravity disc, through which the heavy phaseoverflows into the collecting cover of the bowl hood.A stationary oil feed pipe, attached to the frame hood, projects down intothe neck of the bowl. In the purifier bowl heavy phase is introduced throughthe feed pipe at the start of the run. By the action of the centrifugal

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    force a rotating ring is formed at the bowl wall with a vertical cylindricalsurface inside the outer edge of the top disc. When the liquid seal has beenes ta bl is he d in t his way, the bowl is feeded and separation commences. Thefeed rises into the disc-stack through aligned holes in the distributor andin te rm ed ia te discs. The function of the intermediate discs is to divide theseparation zone into thin layers, so that the separated phases only have totravel a very short distance to free themselves from each other. The denserpha se is thrown radially out ward al ong the unde rsid es of the discs byce nt ri fu ga l force. The lighter phase, by virtue of its lower density, takesup a position nearest the axis of rotation and is removed. The mutual"border", of both the light and heavy phase is called the interface.

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    CHAPTER 1

    SCOPE OF PRESENT STUDY

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    6 --

    s e p a r a t o r

    F ig .1 .2 .B lock d iagram o f t he cen t r i f uga l separa to r1. f eed , d i s pe rs i on 5 .2. acce le ra te d feed 6 .3 . p re s ep a ra t ed f eed 7.k. byp ass , f eed in excess 8 .of hydra ul ic capa c i ty 9.

    p r e s e p a r a t e d h e a v y p h a s es epa ra t ed heav y phas es epa ra t ed l i gh t phas ed ischarged heavy phased ischarged l i gh t phase

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    I n o r d e r t o a v o i d m i s u n d e r s t a n d i n g w i t h r e s p e c t t o t h e d i f f e r e n ti d e n t i f i c a t i o n s of t y p i c a l m ass f lo w s , f ig u r e 1.2 p r e s e n t s t h e s t r u c t u r e o ft h e c e n t r i f u g a l s e p a r a t o r u se d i n t h i s s t u d y , t o g e t h e r w i t h t h e te r m i n o lo g yu s e d i n t h i s t h e s i s 1 . B e s i d e s , t h i s f i g u r e w i l l b e r e f e r r e d t o , i n o r d e r t oe x p l a i n so m e of t h e e x p e r i m e n t a l r e s u l t s . T he f ee d l i q u i d i s p a r t l y h a n d le db y t h e b o w l up t o t h e amo u n t w h i ch can b e " s w a l l o w ed " b y t h e b o w l . T h er e m a i n i n g p a r t o f th e f e e d , b y w h ic h t h e h y d r a u l i c c a p a c i t y i s e x c e e d e d , i sd i s c h a r g e d t o g e t h e r w i t h t h e s e p a r a t e d l i g h t p h a s e , w h ic h i s t y p i c a l o f t h eo pe n s e p a r a t o r a p p l i e d w i t h i n t h i s s t u d y ( s e e a l s o p a r ag r a ph 3 - 4 - 1 f o r ad e t a i l e d d e s c r i p t i o n ) . T he p h y s ic s b eh in d t h i s h y d r a u l i c c a p a c i t y w i l l b ep a i d a t t e n t i o n t o b e l o w . P r e s e n c e o f a p r e - s e p a r a t i o n e f f e c t d ep en ds uponb o t h t h e p o s i t i o n o f t h e i n t e r f a c e and t h e f e e d p o i n t p o s i t i o n . F o ri n s t a n c e , i n a c re am s e p a r a t o r , w h ic h s e p a r a t e s b u t t e r f a t from m i lk ( m ilk =h ea vy p h a s e ) , t h e i n t e r f a c e a s w e l l a s t h e d i s t r i b u t i o n c h a n n e l a r ep o s i t i o n e d n e a r t h e r o t a t i n g a x i s , h e n c e m a x i m i zi n g t h e skim m ed m i l kr e s i d e n c e t im e w i th i n t e n t t o a c h i e v e l o w e s t p o s s i b l e f a t c o n t e n t i n t h em i l k p h a s e . I n t h i s a n d s i m i l a r c a s e s t h e s e p a r a t o r i s d e s i g n a t e d ac o n c e n t r a t o r , l ac k in g a p r e - s e p a r a t i o n e f f e c t . T he o p p o s i t e s i t u a t i o n i sf o u n d i n a l u b r i c a t i o n o i l s e p a r a t o r w hic h s e p a r a t e s d e n s e r s o l i d s( c l a s s i f i e r ) a nd o r w a t e r ( p u r i f i e r ) f o r w hic h t h e i n t e r f a c e an d f e e dp o s i t i o n s a r e l o c a t e d n e a r t h e o u t s i d e of t he d i s c - s t a c k , th u s p r o vi d in gmaximum re s i d e n c e t i me fo r t h e o i l p h as e w i t h i n t e n t t o m i n i m i ze w a t e r an do r s o l i d s c o n t e n t i n t h e d i s c h a r g e d o i l p h a s e . I n t h i s s i t u a t i o n p r e -s e p a r a t i o n o u t s i d e t h e d i s c - s t a c k i s f o ll ow e d by f i n e s r e m o v a l w i t h i n t h ed i s c - s t a c k . T h i s i m p l i e s t h a t th e d i s c - s t a c k i s n o t l oa d e d w i th t h e e n t i r ef e e d c o n c e n t r a t i o n .S e p a r a t i o n e f f i c i e n c yIn an i n d u s t r i a l s e p a r a t o r , t h e p r o d u c t p a r t i c l e s i z e d i s t r i b u t i o n andc o n c e n t r a t i o n r e s u l t f ro m p a r t i c u l a t e p r o c e s s e s c o n t r o l l e d by b o t hs e d i m e n t a t i o n f o r c e , a nd b y t h e m an ne r i n w h ic h b o t h th e , f e e d a c c e l e r a t i o n ;d i s t r i b u t i o n a nd t r a n s p o r t t h ro u g h t h e s e p a r a t o r a r e e s t a b l i s h e d . G en era lp r i n c i p l e s o f d e s i g n a n d t h e o p e r a t i n g r a n g e s o f d i s c - s t a c k c e n t r i f u g e s h a v e

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    e x t e n s iv e l y b e en d e s c r ib e d i n t erms of i n p u t - o u tp u t c h a r a c t e r i s t i c s u nd ercon di t ions which are of ten overs im pli f ied and ve ry o f te n ve ry rem ote fromi n d u s t r i a l r e l e v a n c e . C e n t r i f u g a l f o r ce s , t o g et he r w ith d ra g and c o r i o l i sf o rc e s t o w hich a s e d im e n t in g p a r t i c l e i s s u b j e c t e d i n t h e c o n i c a l d i s cs p a c e w i th i t s r a th e r c o mp le x f l u id f l o w f i e ld , d e t e r min e t h e p a r t i c l et ra j ec t o ry from which the s tep to sep ar a t io n e f f i c i en cy i s q u i ck ly made.T h is m ethod was dev elo pe d by Ambler [ 1 , 2, 3 . ' t] and i s known as the so -ca l led s igma concept , which expresse s the se pa ra to r 's p erfor m anc e in termso f e q u i v a l e n t a r e a o f a s e t t l i n g t a n k , w hich i s t h e o r e t i c a l l y ca p ab le ofdo ing the same amount of work in a un i t g r a v i t a t i o n a l f i e l d . More d e t a i l sabout this concept can be found in paragraph 2 . 3 - 1 .I n t h e c o u r s e o f t h i s s t u d y much e f f o r t h a s b ee n p u t i n t o d i s p e r s i o ncharac te r i s t i cs under wel l -def ined condi t ions : Coue t te exper iments , a s wel las sedimentation experiments. This work, which unambigously pointed out theimportance of dispers ion mechanisms in s t i r red vessels , was not included int h i s t h e s i s .Measurement of drop s izes in the f inely dis t r ibuted dispers ions of water inoi l , especia l ly encountered in the present s tudy, caused much brain-racking.P h o t o g r a p h i c a n a l y s i s , i n s p i t e of i t s r e l i a b i l i t y , a pp ea re d a l ab o r io u sp i ec e o f w ork, w he re as t h e o c c u r r i n g d i s t r i b u t i o n s w e re t y p i c a l l y w i d e,hence demanding la rge samples . Commerc ia l ly ava i lab le sys tems , wi th theFraunhofer di f f ract ion as basic pr inciple , produced resul ts dependent on theg r a d e o f d i l u t i o n , w h i l e th e lo we r end of t h e d i s t r i b u t i o n ( d r o p l e td i ame ter s s ma ll er th a n t h e wave l e n g th of l i g h t ) s u b s t a n t i a l l y c o n t r i b u t e dt o t h e o u t r e a d i n g . B o t h t h e a s s u m p t i o n t h a t t h e p a r t i c l e s a r e p e r f e c t l yopaque and the assumpt ion th a t th e Fraun hofer d i f f r a c t i o n app rox im at ionh o ld s , a re no t f u l l y met and may exp lain the observed phenomenon. Anothersystem was te s t e d , in which p a r t i c l e s f rom a d i l u te d sample were f lownth r o u g h a c a p i l l a r y e q u ip p e d w i th a l i g h t s o u r c e a nd a p h o to d io d e . Ap a r t i c l e p a s sin g t h e l i g h t way w ou ld p r o d u c e a d ip i n t r a n s m i t t e d l i g h ti n t e n s i t y b e in g p r o p o r t i o n a l t o i t s p r o j e c t e d a r e a . A p a r t f r o m d i s p e r s e dphase d r op le t s , suspended so l id pa r t i c le s were a l so measured , r e s u l t i n g intoo h igh base s igna ls .T hese a s p e c t s , t o g e th e r w i th f i n a n c i a l c o n s e q u e n c e s , a s s o c i a t e d w i th t h ea b o ve -m e n ti on e d p a r t i c l e a n a l y s e r s , s u p p o r t e d th e d e c i s i o n t o e x p r e sssepara to r per formance in t e rms of mass - re la ted recovery which cou ld be30

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    m e a s u r e d b o t h b y a c h e m i c a l a n a l y s i s ( K a r l F i s c h e r ) a nd b y a m as s b a l a n c em etho d a s d e s c r i b e d i n p a r a g ra p h s 2 . 4 . 3 - 1 a n d 2 . 4 . 3 - 2 . A n o t h e r i m p o r t a n tc o n s i d e r a t i o n h a s b e e n t h a t a d r op s i z e d i s t r i b u t i o n o f b o th t h e f ee d andd i s c h a rg e d p h a s e s w o u ld o n l y p r o v i d e f o r a d d i t i o n a l , b u t n o t c o m p l e t e ,i n f o r m a t i o n .The p r e s e n t s t u d y h a s d e l i b e r a t e l y b e e n f o c u s e d o n o t h e r p h en o m e n a : t h ee f f e c t o f b o t h d i s p e r s i o n p h en om e na a nd i n t e r f a c e p o s i t i o n on s e p a r a t i o ne f f i c i e n c y . I t c an b e p re su m ed t h a t w i th k no w le dg e o f t h e s e p a r a t o r ' s i n p u tc h a r a c t e r i s t i c s ( p a r t i c l e s i z e d i s t r i b u t i o n a nd v olu me c o n c e n t r a t io n ) ,c o m b in ed w i t h t h e s e p a r a t i o n c h a r a c t e r i s t i c ( c o l l e c t i n g e f f i c i e n c y a sf u n c t i o n o f p a r t i c l e s i z e ) , t h e o u t p u t c h a r a c t e r i s t i c s ca n b e c a l c u l a t e d . I nan i n d u s t r i a l s e p a r a t o r , h ow ev er , a c c e l e r a t i o n , d i s t r i b u t i o n an d t r a n s p o r to f l i q u i d w i l l , o n l y i n a few c a s e s , be e s t a b l i s h e d w i t h o u t s i m u l ta n e o u so c cu r re n ce o f p o p u l a t i on e v e n t s , w h ic h make i n p u t - o u t p u t c h a r a c t e r i s t i c sb a s e d u p o n p o p u l a t i o n b a l a n c e c o n c e p t s d o u b t f u l . E s p e c i a l l y i n t h e c a s e o fm e c h a n ic a ll y u n s t a b l e s y s te m s , s uc h a s f l o c c u l a t e d s o l i d s o r l i q u i d - l i q u i ds y s t e m s p a r t i c l e d i s r u p t i o n / d r o p l e t b r ea k -u p s t r o n g l y i n f l u e n c e s s e p a r a t i o nc h a r a c t e r i s t i c s .T h e r e i s b u t l i t t l e l i t e r a t u r e a v a i l a b l e on t h e se p o p u l a t i o n e v e n t s u nd err e a l i s t i c c o n d i t i o n s a nd e s p e c i a l l y on t h e s e e v e n t s i n i n d u s t r i a l s c a l es e p a r a t o r s . I t i s t h e o b j e c t of t h i s w ork t o p r o v i d e k no w le dg e on b o t h t h ee x a c t c a u s e s a nd l o c a t i o n s w h ere t h e s e e v e n t s t a k e p l a c e , i n o r d e r t o a s s i s tt h e f o r m u l a t i o n o f d e s i g n a n d s c a l e - u p s t r a t e g i e s , r a t h e r t h a nq u a n t i f i c a t i o n i n t h e form o f c o r r e l a t i o n s on d i s p e r s i o n d a t a , w h ic h s o v e r yo f t e n d i s r e g a r d t h e c o n t r o l l i n g m e ch an is m s.I n t h i s s t u d y t h e i n t e r f a c e p o s i t i o n w h ic h , a p a r t from b e i n g t h e p a r a m e t e ro f i n t e r e s t , p r o v i d e s i n d i r e c t i n f o r m a t i o n on p r e s s u r e d ro p i n t h es e p a r a t o r . P r e s s u r e d r o p a s p e c t s w i l l b e p r e s e n t e d i n c h a p t e r 3t w h er ea s t h ee f f e c t o f i n t e r f a c e p o s i t i o n on e f f i c i e n c y w i l l be e x p l a i n e d i n c h a p t e r 2 .HydrodynamicsA t a n e a r l y s t a g e o f t h e p r o j e c t a t t e n t i o n was p a i d t o t h e m o d e l l i n g o f t h ei n t e r d i s c flo w w i th t h e f i n i t e el em e n t m eth od a p p l y i n g a s o f t w a r e p a c k a g eb e i n g d e v e l o p e d a t t h e D e l f t U n i v e r s i t y o f T e c h n o l o g y . B o t h t h e e x i s t e n c e o fc o m m e r c i a l l y a v a i l a b l e s o f t w a r e p a c k a g e s , c a p a b l e o f s o l v i n g t h e f u l l N a v i e r

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    acr oss the fluid layer . This problem has been thoroughly described inliterature by the civil engineers for whom fluid height over a dam is anin di re ct flow measurement technique. In the latter situation typical fluidheights are in the range from a few inches to some feet, whereas the weirhea d in the ligh t phase outlet of a centrifugal separator amounts to a fewmillimetres, resulting in extremely different Re yn ol ds num be rs (seeparagraph 3-3-5) Finally, liquid gradients can be expected to play a role caused by theexistence of viscous friction effects in the open channel upstream the lightphase outlet, due to its specific geometry (see paragraph 3-3-6).The equations governing the oil water interface, as well as the feed liquidinterface, will be derived without too many details in paragraphs 3-3-2 and3-3-3- Wi th res pec t to the three pre ssure drop effects outlined above, ashort literature review is presented. Further in the experimental study, theinterface and hydraulic capacity experiments are presented, and the variouspressure effects are quantified as function of geometrical parameters, suchas caulk thick ness and gravi ty disc di ame ter , type of feed: channel orperipheral, operational parameters: rotation speed and flow and phys icalconstants: viscosity and densities.

    Choice of test fluids

    In this par tic ula r expe rim enta l study a solid bowl separator was chosen,which is specifically used as a purifier, separating water from mineral orvegetable oil. Mapping out the programme of work belonging to the centrifugeproject, typical questions were taken into account, which are reflected in apre -ar ran ged rese arc h pr ogra mme of an unspeci fied company active in thefield of vegeta ble oil ref ining. It is experienced that vegetable oilsexposed to higher temperatures quickly oxidize before they ultimately becomerancid. Turbine lubrication oil (Shell Turbo T3 2) was the refore used as atest f lu id , bec ause of its excell ent stability properties, together withion-exchanged water as the dispersed phase. More details, with respect tophysical properties, can be found in Appendix A.

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    J o i n t r e s e a r c h p r o g r a m m e

    M any q u e s t i o n s a bo u t h y d r o d y n a m i cs , s e p a r a t o r e f f i c i e n c y and p r o c e s sm o n i t or in g and c o n t r o l , h av e u l t i m a t e l y l e d t o t h e i n i t i a t i o n o f a j o i n tr e s e a r c h p r o g r a m m e o f A l f a L a v a l R e s e a r c h ( T um b a, S w ed en ) a n d D e l f tU n i v e r s i t y o f T e c h n o l o g y ( L a b o r a t o r y f o r P r o c e s s E q u i p m e n t ) .SummaryI n c o r r e s p o n d e n c e w i t h t h e t h r e e f i e l d s o f i n t e r e s t : e f f i c i e n c y ,h y d r o d y n a m i c s a nd m o n i t o r i n g t e c h n i q u e s , t h i s t h e s i s i s s u b d iv i d ed i n t ot h r e e m a i n c h a p t e r s .C h a p t e r 2 w i l l d e a l w i t h e f f i c i e n c y an d i s m a i nl y f o cu s ed u po n d i s p e r s i o ne f f e c t s i n t h e s e p a r a t o r by w h ic h t h e u p p e r l i m i t o f t h e s e p a r a t i o ne f f i c i e n c y i s d e t e r m i n e d .Ch a p te r 3 w i l l de a l wi th hydrodyna mic a s p e c t s wh ic h a r e o f imp or t a n c e t o t hes e p a r a t o r : t h e i n t e r f a c e p o s i t i o n and h y d r a u l i c c a p a c i t y a r e b o t h i n f l u e n c e dby p r e s s u r e dr op e f f e c t s .I n c h a p t e r 4 a t t e n t i o n w i l l be p a i d t o s e p a r a t o r m o n i t o r i n g t ec h ni qu e sd e v e l o p e d w i t h i n t h i s r e s e a r c h p r o g r a m m e .

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    CHAPTER 2SEPARATION EFFICIENCY

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    2.1 Division of chapter 2

    The theoretical part of this chapter c ompr ises bas ic equa tion s gov ern ingseparation efficiency as function of throughput, particle sedimentation dataand separator characteristics. Atten tion is paid to the assum pti ons fromwhi ch these eq uat ions are derived . The experimental part of this chaptercomprises experiments performed with a handicapped separator with i ntent tolocate the dispersion effect. Separation experiments are stated showing theeffect of int erf ace po sit ion , feed type and caulk type upon s epar atio nefficiency in a realistic situation.

    2.2 Objectives

    In this chapter the dispersion effect, occurring within the separator, willbe located. Further, the importance of various geometrical and ope ratio nalfactors will be determined and explained.

    2.3 Background theory

    2.3.1 Sigma concept

    In analysing the performance of a cent rifu ge, wh en a hi gh de gr ee ofsep arat ion is i mpo rta nt , the beh avio ur of the smallest particles in thesystem is usually the controlling factor. For a variety of centrifuge typesAmbler [2, 3] developed the so-called sigma concept. With the sigma conceptthe particle diameter, d, can be calculated of which $ 0% will be separated.The sigma value I is an index of the size of a centrifuge and is in fact thecalculated equivalent area of a sett lin g t ank, theor etica lly capabl e o fdoing the same amount of work in a unit gravitational field:

    Q = 2 v 2 (2.1)gwith v the Stokes' velocity in the earth's gravitational field:

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    Ap g dz18 n (2.2)

    and the sigma value for the disc-stack centrifuge (see figure 2.1A):2 n N io2 ( R | - Rp

    3 g tan 6a nd f o r a t u b u l a r - b o w l c e n t r i f u g e w i t h b ow l l e n g t h L ( s e e f i g u r e 2 . I B ) :

    ( 2 . 3 )

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    a centrifuge, when a high degree of separation is important, the behaviourof the smallest particles in the system is usually the controlling factor.Viscous resistance may therefore be considered to be of prime importance.Second, it contains the negligence of the acceleration effect.A. Brunner in his thesis [7] quantifies this effect for a spherical particlein the gravitational field.Third, it does not take into account hindered settling which has a negativeeffect on the effective settling velocity. The effect, caused by mutualinterference of the sedimenting particles, can be quantified by inserting acorrection factor being a function of volume fraction, Reynolds number andshape such as reported among others by Richardson.& Zaki [53]. Besides,corrections must be made for the effects of flow in narrow channels , wherethe particle may be a considerable proportion of the flow gap, resulting inparticle-wall interactions.2. No particle disruptionConcerning particle disruption an example is given by Ambler [3] whomentions the precipitate of ferric hydrate, the stability of which isinfluenced to a great degree by the electrolyte concentration of the fluidphase.In a quiescent field such a precipitate shows a high value of v . Whenintroduced into the rotating bowl of a centrifuge the abrupt change inangular velocity substantially changes the diameter of the individualparticles. If the retention time in the centrifuge is short, even though theagglomerates do reform in time, there is an apparent shift of Q/Z to asmaller value for the same degree of clarification as the rotational speedof the centrifuge, and its shearing effect on the feed increases.Empirical data show that the exponents of the disc radius and angularvelocity may vary slightly for various systems. Ambler [1] presents thiscorrection which is not dimensionally correct, but is sometimes used bycentrifuge suppliers in predicting performance of disc-stack centrifugesfrom small scale tests:

    2 n Ng J'S (R/-75 . Ri2.75,

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    This correction considers and places a value on the effect of the initialrotative energy on the size of dispersed particles by reducing the exponentof (o, the angular velocity and/or the increasing lag of the sedimentingparticles behind the rotational speed of the rotor with increasing rotorspeed. It further considers the difficulty of securing proper distributionthrough discs as they become larger in size by adjusting the exponent of R.More details on this item are stated in paragraphs 2.3-2 and 2.5-1.3. Uniform distribution of flow over the disc spacesThis assumption implies that a disc-stack can be considered to be built upas parallel connected settling chambers receiving equal portions of flow.A. Brunner [8] reports that this assumption is violated in practice.Separating water from fuel oil, a non-evenly distributed solids layer, wasobserved on the inner bowl wall, increasing in thickness and ash content inthe direction of the bowl bottom. Schmitz [56] compared the separationresults of a modified cream separator (increased maldistributionaccomplished by the insertion of an extended top disc) with the separationresults of the original separator, but could not determine a difference.Contrary to Brunner [8], Schmitz [56] is of the opinion that the lower discspaces, contributing for the greater part to clarification, should be rathertrivial, and that separation should not be affected by maldistribution. Inhis opinion the latter is caused by an improved separation of the upper discspaces which compensates for the worsened separation performance of thelower disc spaces, being loaded to a greater extent compared with the upperdisc spaces. This point of view seems rather dubious: the extreme situationcan be compared to a leaky filter.In practice one sometimes encounters the application of disc-stacks with asomewhat unusual composition. The upper half of the disc-stack containsdiscs with wider spaces compared with the discs in the lower part. Apartfrom fouling aspects the maldistribution might in this way be reduced.However, this option has to be "paid" for with a reduction of the totalnumber of disc spaces being N in the I-equation.

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    4. Separation criterion

    In the der iva tio n the Z-equation for a disc- stack cen tri fug e (equation(2.3)) the particle trajectories within the conical disc space form a basis.Com ple te sep ara tio n of a p art icl e is assumed to be accomplished when theheavier particle reaches the upper disc. In practice, however, this need notbe a guarantee for separation while the particle is subjected to drag forcesdue to the inwards directed fluid velocity which for the higher A-values, isan i mportant fac tor highe r than the mean velocity. De Paz [52] points outthe significance of this effect which raises R in the I-equation dependentupon ge om et rica l dat a of the disc-stack, physical properties of the liquidand solid contaminant and feed rate. Experimental evidence for this model ispresented by Carlsson [12] who finds a reasonable agreement.

    5- Effect of caulk type

    K.H. Br unn er [9] presen ts exp eri men tal data obta ined with a single discseparator with a suspension of polyvinyl acetate particles in water.Separation results obtained with 8 long caulks are significantly better thanthose obtained without caulks. Thi s is expl ain ed by a supp ressi on of theci rcumfe rential fluid ve lo ci ty , hence stabilising the interdisc flow (seeparagraph 3-34.4). Besides, the effects of viscosity and density differenceare found to play an important role. Secondary flow increasing on decreasingviscosity affects separation of pa rt ic le s, es pe ci al ly when den sitydi ff er ence is low. No difference is observed in between the performance ofthe free disc space and the disc space with 24 point caulks.

    6. Effect of feed type

    Apart from the interdisc flow, choice of feed type determines whether cro ssflow occurs, located at the disc periphery. In the peripherally feeded discstack the particles separated will, after sliding down the discs, par tia llymove towards the outer disc space, while the rest of them is being carriedalong with the feed for the higher discs. Because the subsequent separationis incomp let e, it is expected that the overall efficienc y decreases onincreasing fluid velocity caused by an intensified backmixing. This effect

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    can be e l i m in a te d , for the gre a ter pa r t , by feeding the d is c-s tac k v ia thedis t r ibut ion channels . Exper imenta l ev idence i s presented by K.H. Brunner[ 9 ] .7. Effect of interface position (purifier)

    Ambler [2 ] s ta te s th at for the maximum eff icie nc y the p o si ti o n of the holesin the disc-stack, through which the feed is dis tr ibuted, must correspond toth e po s i t io n of th e in te r f ac e . I f th i s correspondence does not e x is t , thena t l e a s t som e of th e X - v a l u e of t h e f r a c t i o n o f t h e d i s c - s t a c k , l y i n gbetween the in ter face and the feed holes , i s not ava i lab le for pur i f ica t ionof the l i g h t ( i f the in te rfa ce is outside the holes) or of th e heavy phase( i f t h e i n t e r f a c e i s i n s i d e t h e h o l e s ) . P u r i f y i n g o i l A. B r un n er [8 ]m en tio ns t h a t t h e i n t e r f a c e s h o u ld b e p o s i t i o n e d j u s t o u t s i d e t h e d i s c s ta c k . Svensson & Von Schultz [60] rep ort tha t i t should l i e as clo sel y asposs ib l e t o the d i sc pe r ip he ry , whereas Trowbr idge [64 ] c l a im s th a t i tshould never be positioned within the stack because of fouling problems.2.3.2 Dispersion effect

    2.3-2.1 Dispersion effect in separators

    From the foregoing it became clear that kn ow le dg e of the di sp er si onphenomena within the distributor is of utmost importance in order to gain anunderstanding of the overall separation characteristics of the ce nt ri fu ge .In the open literature not much is published about dispersion effects withincentrifugal separators. Bell and Brun ner [5] studi ed floe br eak- up in adecanter centrifuge. Experiments with ultrasonically dispersed PVAC floesproved that break-up primarily depends on the bowl speed and the feed pipeposit ion. The greatest extent of floe break-up occurred when the feed pipeposition corresponded to the most shallow pond depth, i.e., feeding in theconical region of the bowl and at the maximum speed.In the latter case effici ency shows an optimum with res pec t to rotat ingsp eed. Be ll and Brunner suggest that larger pond depths may result in lowervalues of dissipated power per unit volume and hence less break-up.

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    F u m oto an d K i y o s e [ 2 4 ] i n v e s t i g a t e d d r o p f o r m a t i o n i n a h i g h s p e e dc e n t r i f u g a l e x t r a c t o r . From t h e i r r e s u l t s , t h e a v e r a g e d r op d i a m e t e r i sp r o p o r t i o n a l to th e Weber number to the power of - O.58, when the con tac tt ime i s long. However, the a g i t a t i o n v e lo c i t y d id not a f fe c t the av erag edrop s i z e , when the co nta ct t ime is sh ort {approximately 10 s ) . During the seexperiments the aqueous-organic flow ra te ra t i o was f ixe d at 1 .0. R o ta t i o nspeeds in between 600 and 2000 rpm were chosen. Datar [13] reports thatshea r b reak-up in a cen t r i fuga l s epa ra to r can be r educed by the use o fhermetic feed systems and by incorporat ion of a conical shaped inlet device( f igure 2 .2) which acc e ler a te s the feed to bowl spe ed , wi tho ut in c u r r in gsudden pressure drops. Apart f rom shear reduct ion this conical inlet , whichwas patented by Alfa-Laval [63], reduces foaming which is a typical problemoccurr ing wi th in c ream separa tors .

    Fig . 2 . 2 New c e n t r i f u g e f e e d g e o m e t r y d e s i g n e dto reduce e f fec ts o f shear [63J

    2.3-2.2 Dispersion effect in agitated vessels

    Turbulence

    D rop s i z e d i s t r i b u t i o n s d ep en d on t u r b u l e n c e i n t e n s i t y , d r op b r e a k - u pmechanism and th e o cc ur ren ce of c oa les ce nc e . The complexity of turbu lenttwo-phase dispersed f lows demands analysis in s tat is t ical terms. The primary43

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    c o n t r i b u t i o n s to th e s tud y of such flows are given by Kolmogorov [40] andHinze [35]- These authors, independently, suggested that the maximum size ofdrople t s table against break-up by turbulence can be est imated by means ofd i m e n s i o n a l a n a l y s i s b as ed on t h e a s s u m p t io n t h a t t h e r a t e o f e n e r g ydissipation in the f low is the key parameter characteriz ing the structure oft u rb u le n c e f l u c t u a t i o n s . The s m a l l - s c a l e v e l o c i t y f l u c t u a t i o n s a r edetermined by the local ra te of energy dissipation per unit mass of f lu id eand the kinematic vi sc os i ty v . Kolmogorov d ef in es th e le n g th s c a l e of theenergy d iss ipa t ing edd ies asnK = (v'/e)*" (2.6)

    For local isotropy to exist, the linear scale L of the energy containingeddies must be large compared to the Kolmogorov microscale n . If thiscondition is met in any small volume of characteristic dimension r (r>>r\ , u 2(r) is

    Kind epe nd ent o f v i s c o s i t y and a fu nc t io n of e on ly . In th i s case , and fo rvery small values of r as w ell , the form of t he u n iv e r s a l fu nc ti on can beobtained from dimensional analysis:u 2 ( r) = C^ (E r ) s for L>>r>>nK (2 .7)u 2 (r ) = C2 c r 2 / v fo r L>>n >>r (2 .8 )

    L oc al i s o t r o p y i s p o s s i b l e i n a s t i r r e d v e s s e l e q u ip p e d w i t h a t u r b i n ea g i t a t o r . The c o n d i t i o n s o f t u r b u l e n c e i n su ch a m i x i n g v e s s e l c an bepr ed ict ed from a mo dified Rey nolds number NDJ/v w h er e D i s t h e a g i t a t o rdi am ete r and N i t s speed i n r ev olu tion s per second. Shinnar [58] suggeststhat ful ly developed turbulence exists i f th is modified Reynolds number isabove 10' .In case of no n- iso tro pi c turb ulence i t has been shown by Konno e t a l . [ 4 l ]t h a t b reak -up i s governed by sp a t ia l d i s t r i bu t io n of average ve lo c i t ie s of44

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    flow or by turbulent eddies that have the largest velocity component in thedirection of the flow. The u2 (d ) may be expressed as follows:

    maxu2 (d ) = C, (N d )2 (2.9)max 3 max -"

    B r e a k - u p c r i t e r i aB r e a k - u p of d r o p l e t s may be c a u s e d e i t h e r by t u r b u l e n t p r e s s u r e f l u c t u a t i o n so r by v i s c o u s s h e a r f o r c e s in c a s e d r o p l e t s are very smal l (d>n ),drop size relations can bederived based upon the dimensionless Weber number which compares kineticenergy of the oscillating droplet to surface energy:

    u J(d) p dWe = (2.11)

    Drop size formulae

    Combination of velocity functions with break-up cr i t e r i a r esu l t s inequations predict ing drop sizes in agitated dispersions as a function ofmixing parameters and physical constants . Val idi ty of such a drop s ize

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    e q u a t i o n c a n o n l y e x i s t i f t h e c o n d i t i o n s a r e m et u po n w h i c h t h e v e l o c i t yf u n c t i o n a nd t h e b r e a k - u p c r i t e r i o n a r e b a s e d . I f , h o w e v e r , d r o p s i z e s i nt h e d i s p e r s i o n a nd K o lm o g or ov m i c r o s c a l e a r e o f t h e s am e o r d e r o f m a g n i t u d e ,i t i s n o t p o s s i b l e t o d i s c r i m i n a t e b e t w ee n t h e c o n d i t i o n s m e n t i o n e d a b o v e( d < < n a n d d > > n ) . T h e d r o p s i z e e q u a t i o n i n t h e i s o t r o p i c c a s e w i t hi n e r t i a l b r e a k - u p i s o b t a i n e d b y c o m b i n i n g o f ( 2 . 7 ) a nd ( 2 . 1 1 ) :

    d = C,, f 2 - ) ^ e~ ' ( 2 . 1 2 )max t l p 'cF o r t h e n o n - i s o t r o p i c c a s e , w i t h i n e r t i a l b r e a k - u p , e q u a t i o n s ( 2 . 9 ) a n d( 2 . 1 1 ) a r e c o m b i n e d :

    d = C c o 2 p ~ 3 N~ 3 ( 2 . 1 3 )max 5 c '

    2.h E x p e r i m e n t s2 . 4 . 1 E x p e r i m e n t a l a p p r o a c hD i s p e r s i o n e x p e r i m e n t s

    P r e l i m a r y e x p e r i m e n t s s ho w ed t h a t s e p a r a t i o n o f a n u nm i xe d f e e d a p p e a r e d t ob e i n c o m p l e t e , w h ic h w as d e d i c a t e d t o t h e e x i s t e n c e o f a d i s p e r s i o n e f f e c t .A c o m p l i c a t i o n i n t h e s t u d y o f t h i s d i s p e r s i o n e f f e c t h a s b e e n t h ec o e x i s t e n c e o f d i s p e r s i o n a n d s e p a r a t i o n w h ic h m ade i t i m p o s s i b l e t o m e a s u ret h e s e p he n om en a i n d e p e n d e n t l y . B e c a u s e p r o p e r s a m p l i n g o f t h e a c c e l e r a t e da n d d i s p e r s e d f e e d f ro m a r o t a t i n g s y s t e m w as a l s o i m p o s s i b l e , an i n d i r e c tm e a s u r e m e n t w as c o n s i d e r e d t o b e t h e o n e a n d o n l y s o l u t i o n f o r t h i s p r o b l e m .T h e m e a s u r e m e n t t e c h n i q u e w h i c h a c t u a l l y h a s b ee n a p p l i e d , c o n s i s t e d o f as e d i m e n t a t i o n a n a l y s i s p e r fo rm e d d i r e c t l y a f t e r t h e d i s p e r s i o n p r o c e s s h a st a k e n p l a c e . T h i s h a s b e e n a c c o m p l i s h e d b y h a n d i c a p p i n g t h e s e p a r a t i o na c t i o n i n s t a l l e d i n t h e d i s c - s t a c k c e n t r i f u g e b y r e p l a c e m e n t o f t h e d i s c s t a c k by a dummy s t a c k . T h e l a t t e r d e v i c e c o n s i s t s o f a m a s s i v e p i e c e o fb r a s s , b e i n g g e o m e t r i c a l l y i d e n t i c a l t o t h e e x t e r i o r o f t h e d i s c - s t a c k . Ad e s c r i p t i o n o f t h e dummy s t a c k c a n b e f o un d i n p a r a g r a p h 2 . 4 . 2 . 2 .

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    If it would be possible to introduce an unmixed feed in the distribut ionchan nel obtaining a complete separation, it could then be concluded that inthe above-mentioned situation the distributor must have been entirelyre sp on si bl e for the dis per sio n e ff ec t. In ord er to create such anexperiment, a special distributor, the s o-calle d bypa ss dis trib utor (seeparagraph 2.4.2.2) was designed. This bypass distributor accelerates waterand oil, whereas both phases are physically separa ted in order to excludedispersion. By shortening the water discharge pipes, the position from wherephysical contact between the two phases would occur could be changed.Comparison of the results led to the designation of the exact location whichwould predominate the ultimate dispersion effect.

    The below-mentioned locations are expected to contribute more or less to thedispersion process.First loc ation where the primary contact between the separator and the feedliquid takes place is the top nut, whic h, bec ause of its co nica l sha pe,de fl ec ts the liquid flow, which is discharged through the feed pipe. Due tothe restricted contact time hardly any transfer of momentum is e xpe cted totake place in the angular direction.Second location comprises the fin tips in the distributor, where initialintensive contact between the separator and the feed liquid takes place.Third location is the traject in between the fin tip and the liquid sur facein the di st ri bu to r situated at the pressure side of the guiding fin. Alongthis traject the feed liquid will be stra tif ied . Prediction of the filmth ic kn es s seems a formidable task, whereas a free surface is encountered incombination with a free "channel width". The length of the traject isfunction of R only.eFourth location is the inner wall of the distributor, where axial shearbe come s more and more important when the feed liquid surface radius, R , ispositioned outwards, hence resulting in low liquid depths.Fifth lo ca ti on is the coni cal shaped lower end of the distributor, whereEkman layers and hence high shear rates may occur, as far as the flowstability criterion is satisfied.

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    T-01 , - 02~ -

    T-03

    .N-01 -O D-

    u m j K

    E-0," > '

    P-01

    -P-02

    J ! J H _P-03

    I '" 1P-04

    r."P-05

    " ' I P-06 O.S.C.

    T T . - " "

    T-Ot Fl-02 FL-OI E -03 E - O t Fl - Mlotad .rmogtn 19 kW

    vir

    TI ;

    T"

    i![JL

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    Separation experimentsEffect of int er fa ce p os it i on , feed and caulk typeAs discu ssed above, in a cen tr i fu ga l sep arato r the po si t io n of the w ater -oi lin terface is bel ieved to play an important role in the separat ion process .I n o r d e r t o v e r i f y t h e s i g n i f i c a n c e o f t h i s s t a t e m e n t e x p e r im e n t s w ereperform ed w ith the cen tr i fug e p i l o t p l an t , measuring sepa rat ion eff ic iencyand in te r fac e pos i t io n s imul taneous ly .2 .4 .2 Descr ipt ion of apparatuses2 .4 .2 .1 C e n t r i f u g e p i l o t p l a n tT h e p i l o t p l a n t , o f w h i c h t h e f l o w s h e e t i s d e p i c t e d i n f i g u r e 2 . 3 ,ba s ic a l l y co ns i s t s o f a d i spers ion produc ing sec t ion which d i sp e rs es wate rin o i l from the s to ra ge v e s se l s which w i l l be separa ted p ar t i a l l y by thece nt r i fu ge . A regen erat ion sec t ion co ns is t in g of a f lash evap orator removesthe u n se pa ra te d wa ter phase and de l iv er s the water free o i l to the s toragevessel , hence forming a closed oil loop.S torage vesse l sThe o i l a s w e ll a s t h e w a te r s t o r a g e v e s s e l a r e p r o v id e d w i th e c c e n t r i cp la c ed , p i tched -blade d a g i ta to rs and have ca pa ci t ies of 1500 and 500 l i t r e sr e s p e c t i v e l y . T e m p e r a tu r e i s c o n t r o l l e d by e x p a n s io n ty p e t h e r m o s t a t i cvalves which are si tuated in the steam supply to the heating coils .

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    Dosage systemO i l i s pum ped wi th a n e c c e n t r i c s c r e w pump wi th v a r i a b le spe e d d r i v e w i th ac a pa c i ty in the r a nge f r om 250- 2500 l i t r e s pe r hour . Wa te r i s pum pe d wi th ap i s t o n d o s i n g pum p w i t h s t r o k e a d j u s t m e n t , p r o v i d e d w i t h a p r e s s u r ea c c u m u l a t o r b e f o r e t h e s p r i n g l o a d e d b a c k p r e s s u r e v a l v e .

    Fig.2.U Knife mixer.

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    Dispersion sectionDispersion was produced in two 30 liter knife mixers with variable drive(see figure 2.4). The entrance was situated at the lowest point and theoutlet at the highest point, close to the mechanical seal, in order to avoidintroduction of air.One can choose to apply only one mixer or both, or to bypass both knifemixers, hence providing an unmixed feed. Rotation speed can be varied in arange from approximately 350-1400 rpm. Photographic analysis provided dropsize information on the produced dispersion. At a rotation speed of 700 rpm56 4 um was found for the maximum drop size, for a water fraction of 5-5%by weight. Experiments, not included within this thesis, indicated thevalidity of equation (2.13), as far as the effect of rotation speed andliquid viscosity were concerned. In spite of reasonable residence times inthe knife mixers, different separation results were obtained when applyingone instead of two identically adjusted mixers. This was observed mostclearly when separation was almost complete, indicating that the timedependent behaviour of the distribution was restricted to the smallerdroplets in the distribution.RegenerationA 3 in3 flash vessel provided with 4 whirl jet nozzles was installed. Vacuumwas maintained with a condenser and a vacuum pump. To provide for a steadyspray from the nozzles, partial recirculation was realised. The inlettemperature was maintained at 90 C, to provide for an adequate separationresult, with the aid of a plate heat exchanger and steam as a heatingmedium. The recycle stream is cooled down with a second plate heat exchangerbefore discharge into the storage vessel.Centrifugal separatorAn extensive description of the separator can be found in chapter 3-4.1.

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    2 .4 .2 .2 Bypass d is tr ib u to r and dummy stac kThe bypass d i s t r ib u t or s dep ic ted in f igure 2 .6 A-C ac ce le ra te bo th o i l andwater physically separated from one another. This is accomplished by feedingth e w a te r v i a a s e p a r a t e w a t e r fe e d p ip e i n to a s l i g h t l y c o n i c a l s h ap e dchamber , to the ou ts ide o f which two copper p ipes a re connec ted whichtransport the water beyond the location where dispersion is believed to takepl ac e. These pipes are connected to two of the s ix holes in the d is t r ib u to r ,whereas th e o i l f low is es ta bl is hed v ia the remaining four ho le s . The f i r s td e si g n , deno m inated type I (dep icted in f igu re 2 .6A) , showed a re s t r i c t edh y d r a u l i c c a p a c i t y f o r t h e o i l p h a s e . T he s e co n d d e s i g n , t y p e I I , w a simproved by placing the conical shaped chamber direct ly on the top nut ofthe sp in d le , hence e l imin a t ing the re s t r i c t io n fo r the o i l f low. The l a t t e rt y p e , d e p i c t e d i n f i g u r e 2 .6 B , was c ha ng e d i n s uc h a way t h a t p a r t i a lbypassing was obtained by shor tening the discharge pipes such as shown inf igure 2 .6C.The dummy st a c k , of which th e drawing i s shown in fi g u re 2 . 7 , i s a ma ss iv eb ra ss body prov ided wi th 4 rad ia l ly d i rec ted channe ls connec ting the ou te rdummy space with the co l le ct in g channels of the d is t r ib u to r .

    Fig .2.7 Dummy stack with four radia l holes .53

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    2.4.3 Measurement techniques

    2.4.3.1 Dispersion experiments

    Mass balance

    Measuring W, the water feed and 0, the oil feed and Q the discharged waterphase mass flow the inlet and outlet dispersed phase fraction can becalculated as well as the separation recovery p:

    W (2.14)in W + 0

    out

    p =

    W0

    1

    -+

    -

    QwW - Qwf .outf.i n

    (2.15)

    (2.16)

    The accuracy of the resul t for fj mainly depends upon the ac cu rac y of t hemeasurements of the mass flows W and Q . In spi te of the fact tha t a st rokewadjustable metering pump has been used, it was chosen to measure both W andQ with the stopwatch/bucket method. Typical sample times have been in therange of 20 minutes. For the mass measurements a precision balance (Mettlertype PN 11 N) was used with a capacity of 10 kg and an accuracy 0.1 g. Inall cases the accuracy of fJ has been better than 0.5%- Additional advantageis that the results found are time averaged.Experimental procedure

    The centrifuge pilot plant was started up and the desired conditions weremet within close limits.Before mea sur ing eff ici enc ies with this bypass distributor, the hydrauliccapacity of both phases was determined in order to be sure that hydrauliccapacity would never be exceeded.With the pulse-echo-method, described in paragraph 4 . 3 . t he i n t e r f ace wasmeasured.

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    2.4 .3- 2 Sep arat ion experimentsFor th e e f f ic ie n c y two measurement te chn ique s were used: f i r s t , the massbalance concept already discussed in paragraph 2.4.3-1 and second, the KarlFis che r an al y si s . With resp ect to the l a t t e r : a coulometr ic version was usedin which iod ine was generat ed e l ec t r o l y t i c wi th in the r e a c t i o n v e s se l . Theio d i n e to ge th e r wi th a no n- sp ec i f i ed chemica l subs t ance forms a complexwhich reacts with water. Via conductivity measurement in the reaction vesselthe end point determinat ion is automatical ly made.W i th t h e p u l s e - e c h o - m e t h o d , d e s c r i b e d i n p a r a g ra p h 4 . 3 , th e i n t e r f a c ep o s i t i o n was measured in t he o u te r d is c sp ace (80.5

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    v. . A V- A . ^ 000 v- - 000 V . - - Q -

    .3 -0 4 ^

    n,separatorrotat ion speed (rpm)

    " ,^ -. ^ v \ \ \ ? Jov\

    >\ \ \Condit ions:or iginal di s t r ibutordummy stackou tle t radius Ro=30.5mmgravity disc Dgr=83.0mmfeed tempera ture T = 50C

    \\ \\

    ; \ag i ta to rr o t a t i onspeed[rpm]

    0 * )375620870

    inletwaterf ract ion[%wl2.02.02.02.5

    typeofcurve\ \o \\?0

    \ST*) n o n mixed feed Fig.2.8 Recovery for var ious feed character ist ics

    i 10 i 14 r~153 12 1356

    ( m l / 2 s - 1 / 2 )

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    2.5 Results and observations

    2.5-1 Dispersion experiments

    The effects of inlet dispersion

    Th e r es ul ts of pre li mi na ry experiments performed with a handicappedseparator (dummy stack) fed with both a non-mixed feed and a real dispersioncan be observed in figure 2.8 in which the separation recovery by weight wasplotted versus the square root of Q/I. The equivalent separation a rea I wascalculated with equation (2.4) substituting the interface radius for Rp andthe outer dummy stack radius and height for R. and L respectively, accordingto figure 2.7- Under the assumption of no particle disruption, experimentaldata points with identical feed characteristics should form a curve, whichtypical form would depend only on the drop size distribution, as well as themathematical relationship in between efficiency and drop size. E. g. in thisplot of f$ vs /(Q/2) a straight line is expected whenever the cumulative masswould be linear to drop size and the sepa rati on ef fi ci en cy could berepresented by a step function of drop size.As was already mentioned in paragraph 2.3-1 a shift of Q/I to smaller valuesis exp ected whe n separator rotation speed increases. As can be observed infigure 2.8 this is not fully the case beca use cle ar m a x i m a o cc ur inseparation recovery, which position is function of feed characteristics andseparator rotation speed.

    Th is ph en om en on can be exp la in ed by the existence of more break-upmechanisms. Both the existence of stratification along the fins as well asaxial shear being a typical function of R can explain the trends of figure2.8.

    Bypass distributor (type I)

    The hydraulic capacity of the first design bypass distributor appeared to belimited with respect to the oil phase shown in figure 2.9- The capacity withrespect to the waterflow app eared to be 125 1/h i nde pen dent of rotat ionspeed. Within this limited area, six combined experiments were made. For six

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    Conditions:

    2000-

    \

    1000 -I

    dummy stackoil ou tl e t radius Ro = 30.5 mmpure oil feedfeed tem pe ra ture 50 CD typetype II

    i 1 1 1000 2000 3000 4000 5000 6000 7000- rotation speed (rpm)

    Fig.2.9. Hydraulic capacity of the bypass-distr ibutor.58

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    combinations of rotation speed and feed flow, recovery was measured withboth the original and the bypass distributor. The results are presented intable 2.1. Outlet concentration with bypass distributor appears to beroughly half of that in the case of the original distributor.

    Table 2.1 Data of the experiments with the type I bypass distributor and theoriginal distributor with dummy stack

    rotationspeedn

    [rpm]

    303030283750250035956031

    feed flow

    F[kg/h]

    513642641640511274

    recoveryoriginaldistributor*o[*w]

    96.2597.3293.5799.1094.7591.23

    recoverytype I bypassdistributor^b

    [?.w]

    98.3798.7197-4199-4198.2295-38

    *-h1 _ f i r >

    ["]

    0.4350.4810.403O.6560.3390.527

    Conditions: non-mixed feedinlet water fraction - 1.5% (w)inlet temperature 50 C 'gravity disc diameter 79-5mm.

    Bypass distributor (type II)

    Figure 2.10 shows the results for the type II bypass distributor withdischarge pipes being connected to two of the six holes in the lower end ofthe distributor (paragraph 2.4.2.2, figure 2.6 B).It is observed that even in the case of c o m p l e t e b y p a s s , i n c o m p l e t eseparation occurs. Most probably this is caused by a formation of a spray ofwaterdroplets, which was extensively studied by Schilp [55]- This was alsoobserved during the experiments with the type I bypass distributor (seetable 2.1).

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    1.00-

    .99

    .97 -

    .96

    .95

    .94.93 -

    .92

    .91.90

    .89

    i

    Cond i t i ons :b y p a s s d i s t r i b u t o r , t y p e I Idummy stacko i l ou t l e t rad ius Ro= 30 .5mmgrav i t y d i sc Dgr=83mmf e e d t e m p e r a t u r e T o = 5 0 Cf e e d r a t e [ k g / h ]

    5007501000126015101760

    n o t a t i o noDAV+X

    1000 2000 3000 4 00 0 500 0 6000 7000 s e p a r a t o r r o t a t i o n s p e e d . n [ r p m ]

    F ig .2 .10 Recovery wi th complete bypass.

    6 0

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    o

    ~ = Q = = = . ^0 ' ^A ^ - .^v - 1 ^ -C ^

    Cond i t i ons :bypass d i s t r i lo r i g i na l d i s t r idummy stacko i l ou t l e t radg r a v i t y d i s c Df e e d t e m p e r afeed ra te[kg/hl5007501000126015101760

    n o t a t i o n00v+X

    0 1000 2000 300 0 iOOO 500 0 6000 7000 s e p a r a t o r r o t a t i o n s p e e d , n [ r p m ]

    F ig.2 .11 Recove ry wi th ou t and wi th pa r t i a l by pas s.

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    Fo r feed r a t e s in th e ran ge from 500 - 1750 kg /h, and ro ta ti on speeds inbetween 2250 and 7300 rpm, identical experiments were performed. Figure 2.11s ho w s t h e r e s u l t s fo r t h e o r i g i n a l d i s t r i b u t o r as w e l l a s t h e b y p as sd i s t r i b u t o r w i th s h o r t d i sc h a rg e p ip es ( p a r a g r a p h 2 . 4 .2 .2 , f i g u r e 2 .6 C) .O u t l e t c o n c e n t r a t i o n w a ter i n o i l in t h e c a se of th e o r ig in a l d i s t r i b u t o r ,a p p e a r s t o b e r o u g h l y f i v e ti m e s a s h i g h a s c o m p a r e d t o t h e o u t l e tc o n c e n t r a t i o n w a t e r i n o i l , i n t h e c a s e o f t h e d i s t r i b u t o r w i t h s h o r td i s c h a r g e p i p e s . From t h i s o b s e r v a t i o n i t c a n be c o n c l u d e d t h a t t h es ig n i f i c a n c e o f e i t h e r l o c a t i o n 2 ( f i n t i p s ) , o r 3 ( s t r a t i f i c a t i o n ) o r b o th ,i s of p rime imp o r t a n c e . I n o r d e r t o v e r i f y t h e s t a t e m e n t t h a t t h e f e e dli q u id su rfac e rad ius w il l be of any importance, d isp er sio n experiments wereperformed with the standa rd di s tr ib u to r and dummy sta ck and both to p d i s c s .In this way a correct comparison could be made, whereas the other parameterswould be id en t ic a l . For ins tance the in te r f ac e p o s i t io n was m ain t a ine d insuc h a way th a t i t has been a unique function of f low, i rr es p e c ti v e of th eto p d i s c u s ed . T h is w as v e r i f i e d w i th t h e p u l s e - e c h o - m e th o d , i n d i c a t i n gd i f f e r e n c e s w i t h i n 1 m i l l i m e t r e . F i g u r e 2 .1 2 show s t h e r e s u l t s , w hic hclear ly indicate that the choice of a top disc , which is jus t able to handlethe feed l iquid f low, is advantageous with respect to recovery. This a lsofavours the conclusion that hermet ic separators wil l have a typical ly bet terperfo rm ance compared to open sep ara tors whereas in hermet ic sepa rato rs thed i s t r i b u t o r w i l l be c om p le te ly f i l l e d w i t h l i q u i d . The e x p e r i m e n t a l d a t ab el on g in g to th e abo ve-m entio ned experimen ts are reporte d by Van der Donk[ 1 5 ] .

    i.o -

    0 .95

    0 . 8 5 -

    S 0. i Romm.2630 .5

    Fbypasskq /h1 5 U2932

    Ogrmm.

    78 .08 3 . 0u62 5 6 7/ a / T l m " 2 s " " 2 l

    Fig.2.12. Effect of feed liquid surface radius.

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    2.5-2 Separat ion experimentsEffec t of in te r face pos i t ionThe resul ts of the separat ion experiments have been depicted in f igure 2.13in which recovery (5 i s p lot ted versus int er fa ce rad ius R. . Three im po rta ntradi i were indica ted wi th ver t ica l l ines : the channel rad ius , the outer d iscra di us and the top d is c r ad iu s. The experiment numbers are pr in te d cl o se tothe experimental points . In the plot the experiment t r ios can be recognisedbelonging to one and th e same g ra v i t y d is c d ia m et er . Optimum se p ar a t io nresu l t s a r e ob ta ined wi th the in t e r f ace pos i t i oned jus t ou t s ide the d i sc stack (4 - 5 mm from the periphery) .B ea rin g in m ind th a t the t r aj e c t (1 - [$) in f igure 2.13 is prop ort ion al tot he o u t l e t f r a c t i o n , o ne ca n v e r i f y t h a t , i n t h e optim um p o i n t , o u t l e tf r a c t i o n i s rou gh ly h a lf as bi g as for th e po ints away from the optimum.Moreover, the importance can be i l l u s t r a t e d compar ing the (3 | J /3n) D with

    1(3fJ /3 R. ) . Fo r the chan nel cu rv es , l e f t of the optimum, (3f l/3R.) i sest imated to be 0.6 % / mm and (3p73n)D to be 0.2 % / 1000 rpm. In other1words 1 mm i n t e r f a c e sh if t can compensate for a decrease in rot at io n speedwith1 3000 rpm!W i t h t h e d i s t r i b u t i o n m o d e l , o f w h i c h t h e d e r i v a t i o n i s p r e s e n t e d i nAppendix G, a computer simulation was made for the interdisc flows under thec i rcumstances of the separa t ion exper iments depic ted in f igure 2 .13 . F igure2 .14 shows the numer ica l re su l t s in the form of s o -c a l l e d m a ld is t r ib u t i o nnumbers and circulation numbers of which the definitions can be found in ther igh t-han d side upper corner of f igure 2.14. The in d ic e s o and i re fe r tot h e o u t e r a n d i n n e r d i s c s , a nd CH a nd PE t o t h e f e e d t y p e . T hem aldis t r ibu t ion number i s def ined to be the f ra c t i o n of flow r a t e , be in ghigher than the uniform flow rate, whereas the circulation number is definedto be the fract ion of the total f low rate , which is outwards directed. Thel a t t e r phenomenon was obs erv ed in the lower di sc s and appeared unique forthe channel feeded outer disc-stack. This f low reversal was in most casesr e s t r i c t e d t o s i x o r sev en d i s c s i n t h i s p a r t i c u l a r s i t u a t i o n , r e s u l t i n g i na ci rc u la t i o n f low along the periphe ry and through the ch an ne l . I t amountsto some 1% of th e fe ed flow when the in te rf a ce is far from the