Measurements of H2 /edible oil interfacial area in an agitated hydrogenator using a Ziegler-Natta catalyst

MEASUREMENTS OF H 2 /EDIBLE OIL INTERFACIAL AREA

IN AN AGITATED HYDROGENATOR USING A

MEASUREMENTS OF H2/EDIBLE OIL INTERFACIAL AREA IN AN

AGITATED HYDROGENATOR USING A ZIEGLER-NATTA CATALYST

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PROEFSCHRIFT

TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAPPEN AAN DE TECHNISCHE HOGESCHOOL DELFT, OP GEZAG VAN DE RECTOR MAGNIFICUS IR H.B. BOEREMA, HOOGLERAAR

IN DE AFDELING DER ELEKTROTECHNIEK, VOOR EEN COMMISSIE AANGEWEZEN DOOR HFT COLLEGE

VAN DEKANEN, TE VERDEDIGEN OP WOENSDAG 26 FEBRUARI 1975 TE 14.00 UUR

DOOR

KESHAB LAL GANGULI

SCHEIKUNDIG INGENIEUR

GEBOREN TE CALCUTTA

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BIBLIOTHEEK TU Delft P 1814 4202

DIT PROEFSCHRIFT IS GOIDGEKFLRD DOOR Dl PROMOTOR PROF DR IR N Vi FKOSSEN

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ACKNOWLEDGEMENT

I wish to express my gratitude to the Management of Unilever Research Labora-tory Vlaardingen and of Unilever N.V. for their permission to publish the results of the investigation in this form.

1 wish to acknowledge with deep appreciation the generous collaboration of all those who have contributed m any form to this work.

I am grateful to Ir.H.J.van den Berg for his many valuable comments during the period of this work.

Thanks are due toMrN.A.M.Lansbergenfor carrying out the experiments and Mr J.H.Smit for sketching the figures and the flow sheets.

Finally my thanks a r e due to Mrs Lida van Heteien for hei excellence, c a r e and speed in typing.

C O N T E N T S

Contents 3 1. Introduction 7

1.1 Purpose of this investigation 7 1.2 Literature on gas-liquid reaction and mterfacial area 9

2. Gas-liquid mterfacial area 12 2.1 Interpretation of gas-liquid mterfacial area 12

2.2 Measurements of mterfacial area: physical and chemical methods 13 3. Theoretical discussion and the apparatus for measurements of kinetic

data 17 3.1 Hydrodynamics of a falling film leactor 18

3.2 Theory on gas-liquid absori^tion kinetics 20

3.3 Description of the apparatus 28 3.3.1 Gas storage and purifier - Section A 29

3.3.2 Primary 'securing unit' and gas flow control 31

3.3.3 Reactor (Section C) 32 3.3.4 Substrate-catalyst-solvent mixer and dosing system (Section D) 33

3.3.5 Secondary securing unit and solvent purifiei (Section E). 34

3.4 Lxjierimeiital 35 3.4.1 PreiJaration of the catalyst 35

3.4.2 Pui ification ol substrates 35 3.4.3 Purification oi the solvent 36 3.4.4 Experimental procedure 36

4. Reaction kinetics and the catalyst 39 4.1 Physical absor{)tion lata 39 4.2 Hydrogenation reaction 40 4.2.1 Hydrogenation of cyclohexene 40 4.2.2 Hydrogenation of BO in iso-octane 43 4.2.3 Hydrogenation of pure BO 44

4.3 Hydrogenation kinetics 46 4.3.1 H2 diffusivity (ID ) and solubility (c* ) in BO 47

4.3.2 Reaction kinetic data 49 4.3.2.1 Ha as a function of catalyst concentration and temperature 49

4.3.2.2 Order of hydrogenation m hydrogen (n) 51

4.3.2.3 Energy of activation 52 4.3.2.4 Order of hydrogenation in the unsaturated triglycerides (m) 54

4.3.2.5 Determination of m 58 4.3.2.6 Preliminary conclusion 62 4.4 The surface depletion effect 64 4.5 The homogeneous cataljst 71 4.5.1 The catalyst activity 78 4.5.2 The catalyst selectivity 78 4.5.3 The catalyst stability 80

4.6 Conclusions 81

Measurements of H„/oil mterfacial a r e a 83 5.1 Description of the apparatus 83 5.1.1 Purification of reactans 84 5.1.2 The s t i r r e d tank reactor 84 5.2 ExiDerimental procedure 86 5.2.1 Testing the kinetic data 87 5.2.2 Gas hold-up measurements 88 5.2.3 Hg/oil mterfacial area 89 5.2.4 Determmatu n of k, S9 i j 90 91 100 106 112 114 118 122 124 124 5,3 5,4 5.5 5.6 5.7

Test results <A the kinetic data Gas hold-up

H„/oil mterfacial area Determination of k, Conclusion Appendix I Appendix II Appp . Appendi ' I IV Appendix V

Appendix VI 125 Appendix VII 127 Appendix VIII 128 Appendix IX 129 List of symbols 131 List of abbrevations 135 List of references 136 Summary 138 Samenvdtting 141

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

1.1 P u r p o s e of t h i s i n v e s t i g a t i o n

Gas-liquid systems are common and important features of industrial chemical p r o c e s s e s . There a r e varieties of gas-liquid contactors currently in use,

In general, the design, the scaling up and the modification of the contactors require the following data:

1. The gds-liquid mterfacial area

2. The gas hold-up and the maximum allowable superficial gas velocity 3. The equilibiium gas solubility in the liquid as a function of jiressuie and

tem-perature

4. The reaction kinetics when the gas reacts with the liquid 5. The heat of reactions and the heat transfer coefficients 6. The mass transfer coefficients

7. The power dissipation of the s t i r r e r

8. The residence time distributions of the gas and the liquid

9. The influence of solid phase (e.g. catalyst particle) in the liquid on the gas hold-up and the mterfacial area

The purpose of this investigation is to measure the gas-liquid mterfacial area and the gas hold-up man agitated edible oil hydrogenator. In a commercial hydro-genator, an edible oil is hydrogenated in the presence of a finely divided solid catalyst. Hence, the measurements have also been performed to determine the influence of a finely divided solid phase in the oil on the inteifacial area and the gas hold-up.

The gas hold-up m the reactor is determined by measuring visually the r i s e of the free liquid level in the vessel as a function of the agitator speed and its lo-cation, the superficial gas velocity and the solid (Kieselgulm) phase concentration m the oil.

Measuring the gas-liquid mterfacial area by visual teclmiques has not been leliable and reproducible. The alternative technique is an indirect one and has been in practice during the past decade. This is known as a 'chemical method'. The chemical method needs agas-liquid chemical reaction. The reaction has to be

fast and itskineticsare to be known. What is a fast reaction and how fast it should

be ha\e been quantitatively described in section 3.2. From the known leaction ki-netics the gas absorption l a t e s per unit intei facial area can be predicted fiom the theoiy of gas absoiption when accompanied bj a fast chemical leaction in the liquid phase. Theiefoie, the effective gas-Iiquid mteifacial a i e a c a n be calculated fi om the avei age gas absoiplion rates measui ed m a gas-Iiquid contacting system.

So far no work has been reported in litei ature on the detei mination of a g a s -liquid mterfacial a i e a by the chemical method fiom the measui ements of gaseous hydiogen absorption i ate s into an oil oi mtoanj othei liquid. All the known inves-tigations m liter atui e ha\e been centred on aqueous-electrolj tic solutions and low viscous hydrocarbon liquids. The data obtained by the chemical method in one system cannot be applied to another gas-liquid system having diffeienl physical properties, such as, mterfacial tension, viscosity etc. Because the method is only valid at a fast gasliquid reaction legime, it does not take any notice of the g a s -liquid mterfacial hydrodynamics.

In this investigation, hydrogen has been absorbed into - cyclohexene

- soyabean and safflower oils.

The absoiption has been perfoimed in these substiates both with and without a solvent.

F u r t h e r m o i e , the absorption has also been carried out m the substiates m the presence of a Ziegler-Natta type homogeneous catalyst. The gas absorption data into cyclohexene is used to test the suitability of the a])])aiatus foi the handling of the catalj st. The iirfornidtion on the hydrogen absoiption into soyabean and saf-flower oils a r e analysed to Imd the conditions of a fast edible oil hydi ogenation. The only hydrocaibon solvent used m this investigation is iso-octane. The Zieg-ler-Natta type catalj st is nickel di-isopiop\l salicylate Ni(dips)„ in combination with alununium ti i-isobutyl, Al(iBu) 1 he catalyst dissolves completely m all the hydrogenable substiates used in this imestigation.

Using the conditions of the fast leaction, the hydrogenation kinetics of soya-bean oil have been determined on a film l e a c t o i . This is done bj measuring the hjdiogen alisoiption rates as a function of the p i e s s u i e the tempciatuie the catalyst activit} and the unsatuiated t r i g h c e i i d e concentiation m an oil the io-dine value (I,V,) of an oil.

The known hydrogenation kinetics and the conditions of the fast leaction have been utilised to deteimme the hydrogen-oil mteifacial area m an agitated tank.

The mterfacial area is calculated from the measured gas absorption r a t e s m the agitated tank. The absorption rates a r e performed as a function of the agitator speed and its distance from the free liquid surface, the superficial gas velocity and the percentage of Kieselguhr in the liquid.

1.2 L i t e r a t u r e on g a s - I i q u i d r e a c t i o n a n d m t e r f a c i a l a r e a The determination of leaction kinetics and of the mterfacial area are inter-related for a gas-liquid reacting system.

The volume of literature published on gas-liquid reactions since the early days of Hatta (1) is abundant. The purpose of this section is to outline a short review on the literature of gas-liquid reactions. Some of the relevant literature will be briefly discussed to illustrate the nature of typical work already available.

V/esterterp et al, (2) used the reaction of oxygen with sodium sulphite m wa-ter in the presence of copper (Cu ) ions catalyst to dewa-termine the mwa-terfacial area by the chemical method. He did not, however, verify the kinetic data and

took them from various literature (2) which are:

a. At sulphite concentrations below 0,02 molar, the conversion rate is of first order in sulphite and independent of oxygen concentration.

b. At sulphite concentrations above 0.01 molar the conversion rate is of the first order lu oxygen and independent of the sulphite concentration.

De Waal (3) determined the kinetics of the same reaction on a falling film r e -actor ill the presence of Cobalt (Co ) ions. His conclusion was that the behaviour of copper as catalyst was irregular. Therefore, copper should never be applied when the reaction is used to determine the mterfacial a r e a ,

He also accepted that the reactionwas first order in oxygen and he measured all his gas absorption rates at atmospheric p r e s s u r e . He found the order of oxi-dation in a sodium sulijhite concentration to be zero above 0.4 kmol sulphite per

3

m water, De Waal used this reaction kinetics to measure the mterfacial area under a fast reactron regime.

Literature evidence (4) suggests that the oxidation of sodium sulphite in the liquid phase is a second order reaction in oxygen. This evidence is supported by Astarita (5). Reith (6) determined the order of the same reaction m a falling film reactor by varying the oxygen p r e s s u r e under afast reaction regime and

confir-med the order to be two in oxygen. Reith used this model reaction to measure the gas-liquid mterfacial area m agitated tanks and bubble column r e a c t o r s . He also measured k, S for the same systems using the same leaction under a slow l e a c t

-i j ion regime.

The oxidation of sodium sulphite has attracted a large number of workers to use it as a model reactioninelucidatmggas-liquid systems. Sodium sulphite is a cheap raw mater ral and an can be used as a source of oxygen. The reaction can be used to establish an order of magnitude of mterfacial area m comnieicial equipments,

1 he absorption of gaseous oxygen with or without a dilutent into an

electro-lyte in the presence or the absence of a dissolved catalyst has been considered by a numbei of w o r k e r s , Jhaverretal (7) determmed the kmetics of the oxidation of aqueous solutions of cuprous chloi ide. The detei mination was carried out by measui ing oxygen absoiption r a t e s into the liquid medium at a fast leaction r e -gime. The measurements were peiformed m a s t u r e d cell of known gas-lrquid mteifacial a i e a . The reaction is first ordei m oxygen and second m cuprous

chlo-1 ide

J haven et al. (8) also investigated the absoiption of oxygen into solutions of sodium dithionite (Na„&„0,). 1 he leaction speed isfastenough to study the kine-tics in a laminar jet and m a continuous s t i r i e d cell apparatus. The inlet and out-let liquid samples weie analysed chemically foi sodium dithiomte and fiom this the rate of absoijition of oxygen was olitamed at a given liquid flow rate and at a piedeteinuned pai tial jiressure of oxygen. The reaction is zeio oidei m oxygen at all dithionite conceiiti ations, 1 he oulei of leaction with lespect to the dithio-mte concentiation depends on the molai concenti at ion of sodium dithionite in wa-ter. It IS f u s t Older at less than 0.08 kmol/m and second ordei above 0.08 kniol/m .

The absorption ol chloi me m wdtei has been studied Ijj Bi lan et al. (9) m a shoit wetted wallcoluimi. The leaction is reversible and f u s t order m chlorine. OiUiland ct al. (10) studied the absoiption lates of puie chloi ine and chlorine in nitiogcn into aqueous hydiochloiic acid solutions of fei i ous chloi ide. 1 he al)-soiption l a t e s foi experiments with a dilute gasphase agieed well with a second oidei kinetics. But the puie chloi me absoiption l a t e s did not fit when a second oidci reaction kmetic was assumed.

A num)»i of peojjle looked into the absoiption of JKU c and dilute CO^ into vdi lous tyjies of alkiiie solutions. Nijsmgetal (11) studied the late of absoiption

of pure CO, into solutions of NagCOg/NaHCOg and of KgCO /KHCO in a wetted wall column. The absorption rates into the drlute solutions agreed well with an i r -reversible first o r d e r reaction kinetic. Danckwerts (12) reported the absorption r a t e s of dilute COp into aqueous solutions of Caustic soda. These kinetic data were used by Desai (13) et al. to determine the mterfacial area in a reactor m which he studied the gas-liquid kinetics of the reaction between gaseous acetylene and chlorine dissolved in tetrachloroethane.

The preceding discussion throws some light on how equipment of known mter-facial area can be used to determine various tyijes of gas-liquid reaction kinetics. The cross-section of literature outlined above is afair representation of the tyj^e of work available. The work, so far, is concentrated on gas absorption by aqueous and electrolytic solutions.

The mterfacial physical pi operties are few of the determining features of the gas dispersion c h a r a c t e r i s t i c s into a liquid. Thev a r e partly responsible for con-tioiling the coalescence rate of gas bubbles m a liquid medium, Reith mentions that the gas bubble dispeisions in dilute aqueous solutions of inorganic salts a r e very different from the dispersions m pure water. The physical pi operties e.g. viscocilies densities surface tension etc. of these solutions nearly do not differ. Therefoie, if the bubble dispersion charactei istics influence the gas-lrqurd rn-terfacial area, the electrolytes and the non-electrolytes ai e expected to have two different types of mterfacial a r e a s .

The prime justification of this investigation is to measure an mterfacial area of a non-aqueous system by a new tyjje of gas-hquid reaction. The gas is hjdrogen and the liquid is non-aqueous, viscous, oily and starkly non-electrolytic. The type of the reaction and the nature of the gas-liquid interface have not, so far, been considered,

2. GAS-LIQUID INTERFACIAL AREA

2.1 I n t e r p r e t a t i o n of g a s - l i q u i d m t e r f a c i a l a r e a

To interpret the physical meaning of a gas-liquid mterfacial a r e a , the influ-ence of the age distribution function (14) on the physical mass transfer coeffi-cient controlling the effectiveness of the area r e q u i r e s to be analysed.

A gas molecule can diffuse m a gas medium nearly ten thousand times faster than in a liquid phase. The resistance to mass transfer, therefore, lies to the liquid side. When a gas reacts with a liquid, the absorption rate m addition to the mterfacial hydrodynamics will also depend on the reaction kinetics. Hence two situations can be identrfied:

1. A gas absorption into a liquid without a chemical reaction or with a slow che-mical reaction.

2. A gas absorption into a liquid with a fast or an instantaneous chemical reaction. In case 1 the absorption rate of a pure gas into a liquid m the absence of a chemical reaction is described by:

i> - k- S A c (2.1)

If the gas r e a c t s very slowly, the chemical reaction will not influence the liquid side r e s i s t a n c e . The gas absorption rate will depend on the age distribut-ions of the liquid elements on the interface. Hence Lq. (2.1) can be written:

4) = S v'lD S' • Ac (2,2)

The physical meaning of S' is the i ate of surface renewal and l / S ' is, in fact, the average life of the liquid elements on the surface. According to Eq. (2,2), the longer the age of the surface elements, the slower will be the average gas ab-sorption r a t e . Thus the effective gas-liquid mterfacial area will have very little to do with the geonietr ic area of the gas-liquid interface. In fact, the effective a r e a will be s m a l l e r than the geometric area. In case 2, the gas absorption r a t e IS accompanied by a fast or an instantaneous chemical reaction. The physical implication of such a reaction is that the depletion of the reactants takes place close to the interface. Thus, the gas molecules diffusing into the liquid medium a r e consumed rapidly and a steep concentration gradient is set up. Consequently,

the absorption rate is chemically enhanced. The gas absorption rate is then d e s -cribed b y

<^g *g • F^. (2.3)

No general c o n elation for F , that is, the enhancement in the gas absorption due to a chemical reaction is possible to derive theoretically. F is very high (>3) for a fast or instantaneous gas-liquid chemical reaction. When F , is so high, the absorption rate per unit surface will be independent of the age of the surface elements. The effective area will then be the same as the geometric mterfacial area which is m contrast to the case of physical absoiption,

The preceding discussion shows that the geometric area which is effective for gas absorption depends not only on the mterfacial hydrodynamics but also on the reaction time. Furthermore the physical and the chemical gas absorptions have different mterfacial a r e a s . This conclusion is in agreement with the evidence produced by the recent work of Joosten et al. (15,,

2,2 M e a s u i e m e n t s of m t e r f a c i a l a r e a : p h y s i c a l a n d c h e m i c a l m e t h o d s

Vermeulen (16) iiitioduced the light transmission method to measure a g a s -liquid mterfacial area. He measured the disiiersion of light in a gas--liquid mix-ture. Fig. 2.1 illustrates his measurements.

The light transmission probe was located at the wall of the agitated tank. All the measurements are local. The method fails at a higher gas hold-up which is at a high agitating speed.

Caldeibank (17) used alight scattei ingteclmique to determine the local m t e i -facial area m a gas-liquid agitated tank. He demonstrated that the area varied considerably both axially and radially along the tank. One serious drawback with this technique is that, m an optically dense system occurring at a high agitating speed and superficial gas velocity, it fails completely due to multiple scattering. There is evidence in the literature (18) that the gas bubbles ai-e disintegrated m the neighbourhood of the agitator and coalesce near the wall. It can be conclu-ded from the literature that the ultimate value of the mterfacial area will depend on the coalescence and the disintegration r a t e s . And the physical mterfacial p r o -perty IS one of the contributing factors of the coalescence r a t e .

Mean drop diameter db

rio [rev / s ]

I ig. 2.1. Influence of agitatoi s])eecl on mean drop diai .elei (after \einieulen, 1955).

H T - 10 inch 8.44 vol gas m ccl, • helium o ail run 1 A ail lun 2 V an 1 un 3

Reith (6) also measures the intei facial a r e a photographically. His l e s u l t s , togethei with others, are shown m Fig. 2.2. The plot shows that the mterfacial area in the ionic sulphite solution is higher than that m the water. This can only be due to the fact that ionic solutions produce smaller gas bubbles. Reith's work also shows that the a r e a s m e a s u r e s by the chemical method are larger by a factor of nearly 2 than those measured b) the photographic teclmique.

Perhaps the physical method in an agitated gasliquid system does not p r o duce a true picture. It can at the most measure local values. Provided the g a s -liquid reaction kinetics a r e known aind the reaction is fast, the mterfacial a r e a measuiements obtained b} the chemical method probably present the correct o r -der of magnitude. This conviction is also shaiedby Astarita (5). To measure the gas-liquid mterfacial area by the chemical method, the following conditions have to be

satisfied-1, Gas-phase resistance to mass tiansfer is negligible. 2, Gas absorption takes place under the fast reaction legime,

S/i;[m2/m3] 3000

1000-500

" o [ r e v ' s ]

Fig. 2.2. Interfacial area See ref. 6. Keith Kawecki Calderbank * W e s t e r t e r p + Keith measurement in Vg.lO"^ (ni/s) 0 8.05 1 D 2.23 ' A 1.41 1.50 1.17 • 1.32-4. a i 7 agitated tank D.IO"^ (m) 19.1

"

"

reactor (after Reith, 1968).

liq.

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))

100 g Na2S02 per litre H2O

J J "

* Incorrect reaction kinetics + Correct reaction kinetics

The measurement of a gas-liquid interfacial area by the chemical method will have the following advantages.

1. It determines an effective area which is integral over the whole agitated g a s -liquid reactor as opposed to the local values obtained by the physical methods. It includes bubble coalescence and dead zone of the agitated r e a c t o r .

2. It is not too difficult to measure since it only involves measuring gas absorp-tion rates at various values of agitator speed and superficial gas velocity. It is also easy to measure the effect of the reaction speed constant on the

inter-facial area by varying the reaction temperature and the catalyst concentration when the reaction is catalytic.

3. It IS not dependent on the reactor hydrodynamics, that is, the measurements are independent of the reactor geometry.

4. It measures the area under conditions at which mass transfer is actually taking place.

3. THEORETICAL DISCUSSION AND THE APPARATUS FOR MEASUREMENTS OF KINETIC DATA

It has been mentioned m the preceding section that the kinetic data of a model reaction must be known to determine a gas-liquid interfacial area by the chemical method. In this investigation the hydrogenationofatriglyceride oil with or without the presence of a solvent is intended to be a model reaction. Since the hydrogenat-ion IS a catalytic reacthydrogenat-ion, a homogeneous catalyst is necessary to determine the right type of reaction kinetics m a n apparatus of known gas-liquid mterfacial a r e a . In section 1.2, it has been indicated that the common types of apparatus used for determining a gas-liquid reaction kinetic are:

- falling film reactor - laminar jet apparatus - stirred cell.

The selection of an apparatus isdictatedby a preconceived idea of the consi-dered gas-liquid reaction speed and a quantitative knowledge on the hydrodynamics of a reactor. The reaction speed of the hydrogenation of a triglyceride oil is d e -pendent on the reaction temperature and the catalyst concentration. Thus, the speed can be varied between a moderately fast to a very fast reaction. A stable laminar jet has a residence time of the order of 0.01 second. To achieve a mea-surable gas absorption rate m that apparatus, a large quantity of the catalyst would be required. To establish the absorption rate at a low catalyst concentrat-ion will be a serious measuring problem. Hence, the apparatus is considered to be unsuitable for the present investigation. The hydrodynamics of a s t i r r e d cell IS not adequately quantified. Thus, it would be difficult to establish the physical absorption rate m the apparatus. The correct physical absorption rate must be known in this investigation to determine the molecular diffusion of hydrogen m the oil accurately. Thus, a s t i r r e d cell is not considered to be a measuring apparatus for this investigation,

The hydrodynamicsof a falling film reactor is quantified and its velocity profile has been worked out theoretically (19). The residence time of a falling film can be achieved in the order of 1 second. This residence time would be suit-able for the hydrogenation reaction and because of the small conversion during

the short period, the reaction can be maintained in the pseudo n order regime with respect to hydrogen concentration in the oil.

In the subsequent sections of this chapter, the theories are discussed which have been used m this investigation for a falling film reactor. An apparatus in-corporating a falling film reactor is also described.

3.1 H y d r o d y n a m i c s of a f a l l i n g f i l m r e a c t o r

In this investigation, a falling film reactor has been used to measure the hydrogenation r a t e . Hence, it is essential that the hydrodynamics of such a r e -actor are clearly understood.

It has been shown that gas absorption rate m a liquid without a chemical reaction in such an apparatus can be predrcted confidently from the penetiation theory (20) coupled with the s t r e a m l m e flow mathematical equation (21). The velocity profile of a newtonian fallingfilmhas been sketched in Fig. 3.1, The profile is semiparabolic. The falling film is kept thin and free of p e r t u r -bation by controllmg the volumetr re Irqurd flow r a t e . The film thickness is maintained below 5% of the diameter of the rod over whrch rt flows.

# =

1^.

* »

Tig. 3.1. Velocit} profile of a laminai falling film. a. 1 ilni foi mation section

b. Transition zone

Literature evidence (22) shows that below a falling film Reynolds number of 20, the surface is expected to stay free of perturbation. Fig. 3.1 shows schema-tically that a falling film after it emerges out of the film formation section undergoes a transition period before the half parabolic velocity profile is fully developed. The transition jieriod is caused due to the change m the velocity p r o -files between the film for mation section and the free surface where the gas is

contacted. In the film formation section, the liquid descends with a parabolic v e -locity profile that is, the velocities at the walls are zero. The surface layer af-tei emerging fiom the section accelerates and reaches the velocity for a well-developed flow fuither down the wall. The duration of this period is a function of the momentum tiansport and caii be estimated from the Fourier numlier, which

is:

F„ - ^ ~ 0.3 (3.1)

O ^A

This entrance effect makes the residence time of the surface of the transition zone longer than it is used in the flow equations given at the end of this section. The entrance effect has been measured by various investigators, their data have been taken ovei and are shown m E'lg. 3.2. The entrance effect factor, f. is c o r -related according to:

, ' I D '

<t> 2 S c \ ^. f (3.1.1)

g \ TTt h

s

Another departure of the liquid flow pattern from the idealised one assumed m the equation is the presence of standing waves (23) near the bottom of the r e -actor wall. These waves are caused due to the tiansport of surface active

mate-1 lals to the suiface. The consequence of the standing waves is a dead zone for the jjhysical absorptron rates of a gas rnto a Irqurd. As stated m section 3.4,2 that the

liquids used in this investigation are extremely well purified. Furthermore, the presence of the standing waves has been observed only m a gas-aqueous solution system. In this investigation, the hydrogen absorption r a t e s into viscous trigly-cer ide oils have been measured. During the hydrogen absorption measurements into the oils, no standing waves have ever been obseived. Theiefoie, the effects of the standing waves are neglected.

The velocrty profile of a very thmfallmgfilni, that is, A < 0.05 U is indepen-dent of thegeometry of the surface ovei which it flows. 1 he simpler mathematical

0 95

The penetration theory equation

^g = 2SC',/^7* *h

0 100 200 300

Ig. 3.2. I nil ance effect of a falling film l e a c t o i . 1 lom ex]ierimentfa

1 .J.Cullen et al. Tianb. F a i . Soc. 52, 1956, 11J W.iX.J.Duken et al. Biochern./eitb. 219, 1930, 452 P.Kaimond et al. A.I.Ch.L.J. 5, 1959, bC

L.l .Scriven et al. A.l.Ch.I .J. 5, 1959, 397

3 5^2^5

equations which ai e dei lyed theoretically for afallmgfilm over a flat film can be used foi a vertical round lod. The equations (19) are:

j^ _ pg 7t D S V - 1.5 < v > ^ 1 - ^ s 2 ^ (Re) 4 .S<v>p 's^ V F lii L (3.2) (3,3) (3.4) (3.5) 3,2 T h e o r y on g a s - l i q u i d a b s o r p t i o n k m e t i c s

When a gas is absorbed in a liquid and does not react, the transport resistan-ces can be wi itten in the well-known form:

K„ ^ k + H ^ ^ <^-^> G B L

where

c^ - ^ (3.7)

In this investigation, pure hydrogen has been used to avoid the gas side r e s i s -tance. Hence, Eq. (3.6) reduces to:

KZ '- H^TF- (3.8)

Thus, the gas absorption rate is dependent only on the liquid side r e s i s t a n c e . There are a number of theories available to elucidate and predict the liquid side resistance, k. . In this investigation, the film (24) and the penetration (20) theo-r i e s have been applied.

Both the film and the penetration theories jjostulate that the interface is in-stantaneously saturated with the gas and the transport near the interface takes place by the molecular diffusion. The film theory idealises the hydrodynamic situation by assuming that the contact between a gas and a liquid is continuous. On each side of the interface, there is a stationaiy layer which is only a few microns deep. The concentration gi'adient of the gas molecules m the layer is very steep. The transport rate of the gas molecules through the layer is much larger than the accumulation r a t e . Therefore the concentration gradient in the layer can be considered to be independent of time. In general, in the bulk of the liquid, the gas molecules are so efficiently dispersed by the eddies that the con-centration IS uniform at all points. Thus, the theory states:

.2

ID — 1 = 0 (3.9)

The instantaneous gas absorption at the interface is then

I D ^ f x

xrO

X ( ° ' - °A> <'^-^°>

and

The hydrodynamic situation assumed by the penetration theory is slightly dif-ferent. These assumptions may well be more realistic. In general, the theory

states that the eddies in the bulk of the liquid are resjjonsrble for bringing the li-quid elements to the gas-lili-quid interface. The lili-quid elements at the interface stay for a while and then they return to the bulk. The dejjth of the liquid bulk is much greater compared with the depth of the elements at the interface, i he gas molecules, at the interface, penetrate into the stationar}' liquid elements by mo-lecular diffusion. The concentration of the gas molecules m the liquid elements is time dependent. Thus, the theory states:

2

ID - ~ ; ~Y (3.12)

c X

The following boundarj conditions apply:

c = c ^ x , ^ 0 t = 0 (3.13)

c - c * x = 0 t > 0 (3.14)

c c^ X ^ t > 0 (3.15)

The concentration of the gas molecules in the liquid elements is then des-cribed by: 00

2 { -i^

y/"^ x V4 IDt

The concentration gradient at the interface is:

re

c X

c - c^

0 V

(3.17)

The aver age gas absorption rate predicted by the peneti ation theory is:

g 2 S ( c * - c^)^ ^ (3.19)

k L - 2 x ^ . (3.20)

Foi a very shoit contact tiniiC, for instance, a short falling film reactor and the liquid containing no dissolved gas at the time of contact Eq. (3.19) reduces to:

*g - ^ S ^* \ 1 ^ ^ ^^'^^^

Gas absorption into a liquid medium is enhanced if the gas reacts with the liquid 01 with one of the liquid components. This enhancement factor is defined as

F -r^ (3.22) C (t)

F ^ can be shown (6) to be a function of the dimensionless parametei Ha:

F \ 1 + Ha (3.23)

The Hatta number (Ha - ~) is a parameter of a gas-liquid reaction speed. In this investigation, hydrogen is absoi Ijed into a hydrogenable substrate in a falling film r e a c t o r . When the substrate is mixed with the Ziegler-Natta tyj^e ho-mogeneous catalyst, hydrogenreacts with a doulile bond between two carbon atoms of the substrate m the liquid phase. When the reaction is i r i e v e i sible, the follow-ing power law can be assumed:

R^ ^ K c" b " ' (3.24)

When the following conditions are satisfied:

b > >c* and b ~ b (3.25) O 0 1 ^ '

Eq. (3.24) reduces to: R , - K c" (3.26) <^ n and K - K . b"^ (3.27) n ^ t q , (3.26) IS then a reaction rate of pseudo n order irreversible chemical

reaction. When the reaction is fast, that is. Ha > 3, both the film and the penetrat-ion theories predict the same gas absorptron rates (see ref. 30).

Thus taking the film theory correlation for the sake of simplicity, a mass balance for the component c becomes:

'2

ID, - ^ = K c" (3.28) 1 2 n

( X

To apply Eq. (3.28) to a falling film reactor, the following condition is to be satisfied:

Sp < 0.15 (3.29)

To solve Eq. (3.28) the following boundary conditions apply:

X = 0 c = c* (3.30)

X = S c = 0 (3.31)

The solution (5) of Eq. (3,28) when

t / - 2 , K ID, ( c ^ ) " " ^

Ha = -1^^±1 ° ^ > 3 . 0 IS

•^L

<P' = S • c H - ^ • K ID, ( c " ) " ^ (3,32)

g > n+l n 1 ^ ' \ • ) Thus, according to Eq. (3.32) at a fast reaction regime a log-log plot of the measured gas absorption rates as a function of the reaction pressure at constant

temperature and catalyst concentration should result in a straight line. The slope of the line is -p— , hence n can be calculated,

To determine m, Eq, (3,27) is substituted mEq. (3.32) and the following con-ditions are satisfied to make the resulting equation applicable for a falling film reactor;

b

Ha' > 3 and Ha' << q = - 2 - (3.33)

re*

b^ ~ b ~ b^ and - | ^ = 0 (3.34)

Thus, Eq. (3.32) becomes:

<^' = S c*\ -^ . K . ID, (c*)"'-^ b ^ (3.35)

g > n+l 1 ^ ' o \ • ' The log-log plot of the measured gas absorption r a t e s as a function of b at the given reaction conditions will yield a straight line. The slope of the line is -p-, hence m is known.

To determine n with m , n order kmetics a log-log plot of (|)'/b against the jiressure at the given reaction conditions should be made and the slope IS —g— according to Eq. (3.35).

When 111 = 0, Eq. (3.35) reduces to Eq. (3.32). It should be emphasized here that the equations do not contain any hydrodynamic p a r a m e t e r . The gas absorpt-ion rate should be fully insensitive to the interfacial and bulk hydrodynamics. The reaction takes place within a very short distance from the interface. Only under these conditions is a gas absorption rate the same as the gas-liquid chemical reaction rate and proportional to the total effective geometric gas-liquid inter-facial area at the given reaction conditions.

This unique property of the equation allows to quantify the hydrogenation rate from the measurements of the pure hydrogen absorption rate in the oil on a known interfacial area. The known kinetics of the reaction then allow to determine the gas-liquid interfacial area of any reactor from the gas absorptron measurements under a fast reaction regime.

Ithas so far been, tacitly assumed that one of the conditions, namely, b ~ b of Eqs (3.25) and (3.33) can always be satisfied. In fact it has been realised in this investigation that this condition is only satisfied for certain hydrogenation condit-ions. The maximum amount of unsaturated triglycerides present in an edible oil

IS fixed by 'mother nature' and none of the oils at the present moment available has an l.\ . of more than 150. Thus the non-volatile solute concentration of the oils cannot be increased at will beyond a certain value.

At a high reaction speed the interface suffers from a shortage of unsaturated triglycerides. Thus the reaction conditions should properly be selected to satisfy b ~ b . This also means that the concentrationprofilesof the unsaturated trigly-cerides near the interface must be known as a function of the reaction conditions. Only then can the conditions be selected when the surface concentration will not be sigmficantl} changed over the reactor length. 'I he hydrogenation of the edible oils also encountei s the two following conditions:

' ^ 2

r^ = ^ ^ 0.01 (3.36)

b A 1 A 2

q = — 2 . - 2.10 to 3.5- (3.37)

re'

Because of these two conditions and the fact that the reaction is not infinitely fast, see section 4.4, the known film and penetration theoiy correlations cannot be applied to workout the concentration profiles as a function of the reaction con-ditions. The appropiiate equations m dimensionless forms are:

2

r c ' r e ' ^.in m

, Z2 re c B (3.38)

r ^ B r B J_ n m

1 he following boundaiy conditions apjjly:

0 Z > 0 ^ ^ (3.40)

Z 0 0 > 0 "^ -^ (3.41) B

Z 0

Z — C O 0 ^ 0 ^ ' _ ^ (3.42)

No analytical solution IS known foi the Lqs (3.38) and (3.39). The two equations have been solved numerically by the finite difference technrque. The details of the

technique and the flow diagram of the program for a digital computer a r e given in Appendix I. T a b l e 3 , 1 , T h e o p e r a t i n g u n i t s , S E C T I O N A 1, 2 . 3. 3A. 4 , 5. 5A. 6. 7. 7A. 8, 9. 9A, 10. 1 1 . 12. 13. N, or Hg cylinder BTS column Mol. sieve p a r t Asbestos p a r t Mol. sieve Mol. sieve part Asbestos part PgO column Anti-surge Hg barometer Hg vapour trap Gas r o t a m e t e r Gage Hg. manometer Hg vapour trap Regulator Reducer Gas exit valve

Ng inlet valve to securing

S E C T I O N B 14. Valve for securing 15. P r i m a r y securing unit 16. Cold finger 17. Vacuum pump 18. Vacuum buffer 19. E^ buffer 20. Solenoid valve 21. Fine regulator 22. H„ m a s s flow transmitter 23. Mass flow meter

23A. Mass flow r e c o r d e r

24. Gage Hg manometer 25. Operation p r e s s u r e manometer 26. Differential p r e s s u r e manometer 26A. Isolator 27. Photoelectric cell S E C T I O N C 2 8 . 2 9 . 3 0 . 30A 3 1 . 3 2 . 32A 3 3 . 3 4 . 3 5 . 3 6 . 3 7 . 3 8 . 39. 4 0 . 4 1 , 4 2 , 42A. 4 3 . 4 4 . 4 5 , 4 6 . 4 7 . 4 8 . Reactor tank Film reactor Cap Indentation Holes Shaft Cylinder Hand wheel Threaded end Rod Teflon ring Housing Jacket Isolator Link U tube Jacket Hollow shaft Constant head Holes Shaft Hand wheel Threaded end Heating coil Bath 27

48A. Gas sampling unit 49, Isolator

49A, Thermocouple

S E C T I O N D

50, Tri-isobutyl aluminium 50A, Injection port

51. Ni.dips. 51A. Injection port 52. Oil cylinder 53. Solvent cylinder 54. Isolator

55, Liquid r o t a m e t e r 55A, Inlet valve to rotameter 56, Thermocouple

57, Constant level tank 58. Overflow pipe 59. Buffer mixer 59A. Degassing unit 59B. Flow meter 59C. Level controller 59D. Hg manometer 59E.Iso-octane condenser 59 F. Cryostat 59G. Regulating valve 59H, Solenoid valve 60. Magnetic s t i r r e r 61, Recirculation pump 62, Sample trap S E C T I O N E 63, 63A 64, 65. 66. 67. 68. 69. 70. 71. 71A 72. 73. 73A 74. 74A. 75, 76, 77, 78. 79. Iso-octane reboiler Iso-octane reboiler Condenser receiver Receiver drain Iso-octane feed Gas-flow indicator Condenser

Catalyst injection syringe N2 inlet valve

Secondary securing unit Vacuum pump P„0_ column A 0 Mol. sieve Asbestos Mol. sieve Asbestos BTS column Gage Hg manometer Hg vapour trap Gas r o t a m e t e r Regulator 80. Np cylmder 81. ALO, column 82. Iso-octane batch feed 83. Iso-octane receiver

3,3 D e s c r i p t i o n of t h e a p p a r a t u s

Al(iBu)„ IS extremely reactive to oxygen and moisture. Hence, one of the major functions of the apparatus used in determining the hydrogenation kinetics is to ensure that all the operating units, except the reactor, are supplied with oxygen and moisture free nitrogen atmosphere. The inert atmosphere p r o t e c t s

the Al(iBu)2 completely during the various phases of the operation. The r e a c -tant hydrogen is also purified m this apparatus before the reaction.

The apparatus has been sketched m Fig. 3.3, The names of the individual units and their coi responding numbers have been given in Table 3.1, This d e s -cription of the apparatus should be read in conjunction with Fig, 3,3. The opera-ting units which a r e sketched m Fig. 3.3 but not described in this section have

been mentioned elsewhere in this thesis. The assembly of the glass (Duran 50) apparatus can be divided into five sections:

1. section A: gas storage and pui ifier.

2. section B p r i m a r y 'securing unit' and gas flow control. 3. section C l e a c t o r .

4. section D: substrate-cdtalyst-solvent mixer and dosing system.

5. section E: catalyst handling, solvent purifier and secondary'securing unit'.

3.3.1 G a s s t o r a g e a n d p u r r f r e r - S e c t i o n A

The reactant hydi ogen and the inert introgen are both stored m standard m e -tal cylinders 1. Specifications of the gases are given in Table 3.2.

Both the hydrogen and nitrogen storages and the purification systems are identical.

T a b l e 3.2 - S p e c i f i c a t i o n of g a s e s Specification Nitrogen Hjdrogen

type (Belgian) Standard (Dutch) H2O content 5 PPM 10 PPM

O2 content 3 PPM 10 PPM H, content 2 PPM Rest Ng content Rest 900 PPM

Therefore, the numbers of the identical pieces of equipment in Fig. 3.3 a r e the same. Hence, specific reference to hydrogen and nitrogen is avoided and the general term ' g a s ' will be used instead in this particular section.

The flow of gas from tne cilmder 1, is controlled wrth a Loosco p r e s s u r e reducer, 11 and a fine regulator, Fairchildlliller model 16 vacuum legulator, 10. The flow IS indicated on a rotametei, 8 and its p r e s s u r e is measui ed on a gage

manometer, 9, The open end, 9A of the manometer to the atmosphere is filled with iodine on active coal from Hicol Chemicalien N,V, to safeguard against any possible escape of mercury vapour (Hg, vapour trap) to the environment. After the r o t a m e t e r , 8 the gas flows over an anti-surge Hg barometer, 7, also fitted with a vajoour trap, 7A, The function of the barometer is to protect the r e s t of the glass apparatus against a possible p r e s s u r e surge.

The gas after the barometer e n t e r s a battery of glass cylinders. The battery removes oxygen and moisture from the gas s t r e a m s completely. The function of each cylinder and the chemicals it contains are summarised in Table 3.3.

T a b l e 3,3 - D e s c r i p t i o n of c l e a n i n g b a t t e r y No. of cylinder (see Fig. 3.3) 2 3A 3 4 5A 5 6 Description of chemicals Cu on active coal (BTS pellets) Asbestob granules (fills the first tenth of tlie cylmdei)

Mol. sieve (rest of the cylinder)

Mol. sieve

Asbestos granules (fills the last tenth of the cjlindei) Mol. sieve ' ^ " 5 Function Kemoves 0„ Aljsorbs water Aljsoi bs water Absorbs water .Absorljs water Absorps watei Removes water P rocess occurs Removal is achieved by oxodizing Cu to CuO; in-dicated by the change in colour from bluish grey to black

Absorjjtion. The end point IS indicated by the change in colour from light brown to darker brown Absorption Absorption see 3A Absorption Hygroscopic binding

S/i;[m2/m3] 3000

1000-" o [ r e v ' s ]

Fig. 2.2. Interfacial area See ref. 6. Keith Kawecki Calderbank Westerterp Keith measurement m vg.10-2 (m/s) 0 ^.05 1 D 2.23 ' A 1.41 1.50 1.17 • 1.32-4. an 7 agitated D.IO"^ (m) 19.1 ' 5

))

"

"

tank reactor (after Reith, 1968)

liq.

H 2 O

ii

It

ii

100 g Na^SOg per litre H2( ,, )>

* Incorrect reaction kinetics + Correct reaction kinetics

The measurement of a gas-liquid interfacial area by the chemical method will have the following advantages.

1. It determines an effective a r e a which is integral over the whole agitated g a s -liquid reactor as opposed to the local values obtained by the physical methods. It includes bubble coalescence and dead zone of the agitated reactor.

2. It is not too difficult to measure since it only involves measuring gas absorp-tion r a t e s at various values of agitator speed and superficial gas velocity. It is also easy to measure the effect of the reaction speed constant on the

inter-facial a r e a by varying the reaction temperature and the catalyst concentration when the reaction is catalytic,

3. It is not dependent on the reactor hydrodynamics, that is, the measurements a r e independent of the reactor geometry,

4, It measures the area imder conditions at which mass transfer is actually taking place.

3. THEORETICAL DISCUSSION AND THE APPARATUS FOR MEASUREMENTS OF KINETIC DATA

It has been mentioned in the preceding section that the kinetic data of a model reaction must be known to determine a gas-liquid interfacial area by the chemical method. In this investigation the hydrogenation of a triglyceride oil with or without the presence of a solvent is intended to be a model reaction. Since the hydrogenat-ion IS a catalytic reacthydrogenat-ion, a homogeneous catalyst is necessary to determine the right type of reaction kinetics in an apparatus of known gas-Iiquid interfacial a r e a . In section 1.2, it has been indicated that the common types of apparatus used for determining a gas-Iiquid reaction kinetic are:

- falling film reactor - laminar jet apparatus - stirred cell.

The selection of an apparatus is dictated by a preconceived idea of the consi-dered gas-liquid reaction speed and a quantitative knowledge on the hydrodynamics of a r e a c t o i . The reaction speed of the hydrogenation of a triglyceride oil is d e -pendent on the reaction temperature and the catalyst concentration. Thus, the speed can be varied between a moderately fast to a very fast reactron. A stable laminar jet has a residence time of the order of 0.01 second. To achieve a mea-surable gas absorption rate in that apjjaratus, a large quantity of the catalyst would be required. To establish the absorption rate at a low catalyst concentrat-ion will be a serious measuring problem. Hence, the apparatus is considered to be unsuitable for the present investigation. The hydrodynamics of a s t i r r e d cell is not adequately quantified. Thus, it would be difficult to establish the physical absorption rate m the ajjparatus. The correct physical absorption rate must be known in this investigation to determine the molecular diffusion of hydrogen m the oil accurately. Thus, a s t i r r e d cell is not considered to be a measuring apparatus for thrs mvestigation.

The hydrodynamrcsof afallmgfilm reactor IS quantified and its velocity profile has been worked out theoretically (19). The residence time of a falling film can be achieved in t h e o r d e r of 1 second. This residence time would be suit-able for the hydrogenation reaction and because of the small conversion during

the short period, the reaction can be maintained in the pseudo n order regime with respect to hydrogen concentration in the oil.

In the subsequent sections of this chapter, the theories a r e discussed which have been used m this investigation for a falling film reactor. An apparatus in-corporating a falling film reactor is also described.

3.1 H y d r o d y n a m i c s of a f a l l i n g f i l m r e a c t o r

In this investigation, a falling film reactor has been used to measure the hydrogenation r a t e . Hence, it is essential that the hydrodynamics of such a r e -actor are clearly understood.

It has been shown that gas absorption rate in a liquid without a chemical reaction in such an apparatus can be predicted confidently from the penetration theory (20) coupled with the streamline flow mathematical equation (21). The velocity profile of a newtonian fallingfilmhas been sketched m Fig. 3.1. The profile is semiparabolic. The falling film is kept thin and free of p e r t u r -bation by controlling the volumetric liquid flow r a t e . The film thickness is maintained below 5% of the diameter of the rod over which it flows.

1

W

y

Tig. 3.1. Velocit; profile of a laminai falling film. a. Film for matron section

b. Transition zone

Literature evidence (22) shows that below a falling film Reynolds number of 20, the surface is expected to stay free of perturbation. Fig. 3.1 shows schema-trcally that a falling film after it emerges out of the film formation section undergoes a transition period before the half parabolic velocity profile is full> developed. The transition period is caused due to the change in the velocity p i o -files between the film formation section and the free surface where the gas is contacted. In the film formation section, the liquid descends with a parabolic v e -locity profile that is, the velocities at the walls are zero. The surface layer af-tei emerging from the section accelerates and reaches the velocity for a well-developed flow fuither down the wall. The duration of this period is a function of the momentum tiansport and can be estimated from the Fourier number, which

i s :

F - i i - 0.3 (3.1) o ^l

This enti ance effect makes the residence time of the suiface of the transition zone longer than it is used m the flow equations given at the end of this section. The entrance effect has been measured by various investigators, their data have been taken over and are shown in Fig. 3.2. The entrance effect factor, f, is c o r -related according to:

4) - 2 S c \ -• f (3.1.1) g \ Tt h \ • I

s

Another departure of the liquid flow j^attern from the idealised one assumed m the equation is the presence of standing waves (23) near the bottom of the r e -actor wall. These waves are caused due to the transport of surface active mate-r i a l s to the sumate-rface, mate-riieconsequenceof the standing waves is a dead zone fomate-r the physical alisoriitron lates of a gas into a Irqurd. As stated m section 3.4.2 that the liquids used m this investigation are extremely well purified. Furthermore, the presence of the standing waves has been observed only in a gas-aqueous solution system. In this investigation, the hydrogen absorption r a t e s into viscous trigly-ceride oils have been measui ed. Dui mg the hydrogen absoiption measurements into the oils, no standing waves have ever been observed. Iherefoie, the effects of the standing waves are neglected.

The velocity piofile of a very thmfallmgfilm, that is, A < 0.05 D is indepen-dent of the geometry ol the surface ovei which it flows. The simpler mathematical

0 9 0

The penetration theory equation 't'g = 2Sc'./SP tf,

0 100 200 300

Pig. 3.2. Lntiance effect of a falling film l e a c t o i . Piom experiments

L.J.CuUeu et al. T i a n s . I a i . Soc. 52, 1956, 113 M.N.J.Diiken et al. Biochem.Zeits. 219, 1930, 452 P.Kaimond et al. A.l.Ch.I .J. 5, 1959, 86

L.L.Scr iven et al. A.l.Ch.I .J. 5, 1959, 397

3 5 A 2 V S

e q u a t i o n s w h i c h a r e d e m e d t h e o r e t i c a l l } for a f a l l i n g f i l m o v e r a flat film c a n b e u s e d for a v e r t i c a l r o u n d r o d . 1 he e q u a t i o n s (19) a r e : fj pg 71 D S V = 1.5 < v > - ^ S Afi ( R e ) , t = ^ s V 4 .S<v>P (3.2) (3.3) (3.4) (3.5) 3.2 t h e o r y o n g a s - l i q u i d a b s o r p t i o n k i n e t i c s

When a g a s is a b s o r b e d in a liquid and d o e s not r e a c t , the t r a n s p o r t r e s i s t a n -c e s -can be w r i t t e n m the w e l l - k n o w n f o r m :

1_

He-k, (3.6)

where

P-He

RT (3.7)

In this investigation, pure hydrogen has been used to avoid the gas side r e s i s -tance. Hence, Eq. (3.6) reduces to:

1 _

He-k, (3.8)

Thus, the gas absorption rate rs dependent only on the liquid side r e s i s t a n c e . There a i e a numbei of theories available to elucidate and predict the liquid side lesistdiice, k . In this investigation, the film (24) and the penetration (20)

theo-i j

I les have been applied,

Both the film and the penetration theoi les postulate that the interface is in-stantaneously saturated with the gas and the transport near the interface takes place by the molecular diffusion. The film theoiy idealises the hydrodynamic situation by assuming that the contact between a gas and a liquid is continuous, On each side of the interface, there is a stationary layer which is only a few microns deejD, The concentration gradient of the gas molecules m the layer is very steep. The transjrort rate of the gas molecules tlirough the layer rs much larger than the accumulation r a t e . Therefore the concentration gradient in the layer can be considered to be independent of time. In general, m the bulk of the liquid, the gas molecules a r e so efficiently dispersed by the eddies that the con-centration is uniform at all points. Thus, the theory states:

2

ID — 2 0 <?x

The instantaneous gas absorption at the mterface is then

(3.9) I D ^ r X xrO and

f '"• - «A>

The hydrodynamic situation assumed by the penetration theory is slightly dif-ferent. These assunrptrons may well be more realrstic. In general, the theory

states that the eddies m the bulk of the liquid a r e responsible for bringing the li-quid elements to the gas-lili-quid interface. The lili-quid elements at the interface stay for a while and then the> leturn to the bulk. The depth of the liquid bulk is much greater compared with the depth of the elements at the interface. The gas molecules, at the interface, penetrate into the stationary liquid elements by mo-lecular diffusion. The concentration of the gas molecules in the liquid elements IS time dependent. Thus, the theory states:

2

I D ^ - | = f 2 . (3.12) ( X

The following boundary conditions apply:

c = c ^ x > 0 t = 0 (3.13)

c - c * x - 0 t > 0 (3.14)

c = c ^ x = oo t > 0 (3.15) The concentration of the gas molecules in the liquid elements is then d e s

-cribed by:

00

c = (c^ - c j I / e"^ . d^ + c . (3.16) V^4 IDt

where

1 - erf I x/v/4IDt ) erfc [x/v'41Dt ] ? j e~^ • d^ \/^ X

V4 IDt I h e concentration gradient at the interface is:

c* - c , t c ( X "A

^

r e

- ID — _rx ^ (c* - c ) \ ^^ (3.18)

lx=0 A > ^t

The average gas absorption rate predicted by the penetration theory is:

4>g= 2 S ( c * - c ^ ) \ 2 - (3.19)

\ = 2 \ ^ . (3.20) Foi a very short contact time, for instance, a shoit falling film reactor and

the liquid containing no dissolved gas at the time of contact Eq. (3.19) reduces to:

*g = 2 S c* \ ^ (3.21) ° s

Gas absorption into a liquid medium is enhanced if the gas reacts with the liquid or with one of the liquid components. This enhancement factor is defined as

F - - x ^ (3.22) c d)

F ^ can be shown (6) to be a function of the dimensionless p a r a m e t e r Ha:

F - •^ 1 + Ha^ (3.23)

The Hatta number (Ha =-^) is a parameter of a gas-liquid i eaction speed. or

In thrs investrgation, hydrogen is alisorted into a hydrogenable substrate in a

falling film reactor. When the substrate is mixed with the Zieglei-Natta tyj^e

ho-mogeneous catalyst hydrogen l e a c t s with a double bond between two c a r t o n atoms of the substrate in the liquid phase. When the reaction is irreversrble, the follow-ing power law can be assumed:

R^ = K c" b " ' (3.24)

When the following conditions a r e satisfied:

b > >c* and b ~ b (3.25) O 0 1

Eq. (3.24) reduces to:

and

R„ = K c" (3.26)

K = K . b " ' (3.27) n

Eq. (3.26) IS then a reaction rate of pseudo n order irreversible chemrcal reactron. When the reactron rs fast, that rs, Ha > 3, both the frlm and the penetrat-ion theories predict the same gas absorptron rates (see ref. 30).

Thus takrng the film theory correlation for the sake of simplicity, a mass balance for the component c becomes:

.2

I D ^ - ^ = K^ c" (3.28)

c X

To apply Eq. (3.28) to a falling film reactor, the following condition is to be satisfied:

Sp < 0.1 S (3.29)

To solve Eq. (3,28) the following boundary conditions apply:

X = 0 c = c* (3.30)

X = S c = 0 (3.31)

The solution (5) of Eq. (3.28) when

J - L . . K ID, (c*)""^

H a = -^^-^ " ^ >3.0 IS

•^L

g

^ • "^'NiJl • ^n ^ 1 <'^**""^ <^-^^>

Thus, according to Eq. (3.32) at a fast reaction regime a log-log plot of the measured gas absorption rates as a function of the reaction p r e s s u r e at constant

temjDerature and catalyst concentration should result in a straight line. The slope of the line is —y— , hence n can be calculated.

To determine m, Eq. (3.27) is substituted mEq. (3.32) and the following con-ditions are satisfied to make the resultmg equatron applrcable for a fallrng frlm reactor;

b

Ha' > 3 and Ha' << q = - ^ (3.33) yc

dh

b ~ b ~ b and —i-- 0 (3.34)

O I I- t \ • I

Thus, Eq. (3.32) becomes:

4>' - S c * \ - ^ . K . ID, (c*)""-^ b ™ (3.35) g > n+l 1 ^ ' o \ • / The log-log plot of the measured gas absorption r a t e s as a function of b at the given reaction conditions will yield a straight line. The slope of the line is -p-, hence m is known,

To determine n with m , n order kinetics a log-log plot of (f)'/b against the p r e s s u r e at the given reaction conditions should be made and the slope rs —^ according to Eq, (3,35),

When m - 0, Eq, (3.35) reduces to Eq. (3.32). It should be emphasized here that the equations do not contain any hydrodynamic p a r a m e t e r . The gas absorpt-ron rate should be fully insensitive to the interfacial and bulk hydrodynamics. The reaction takes place within a very short distance from the interface. Only under these conditions is a gas absorption rate the same as the gas-liquid chemical reaction rate and proportional to the total effective geometric gas-liquid inter-facial area at the given reaction conditions.

This luiique property of the equation allows to quantify the hydrogenation rate from the measurements of the pure hydrogen absorption rate in the oil on a known mterfacial a r e a . The known kinetics of the reaction then allow to determine the gas-liquid interfacial area of any reactor from the gas absorption measurements under a fast reactioir regrme.

Ithas so far been, tacitly assumed that one of the conditions, namely, b ~ b of Eqs (3.25) and (3.33) canalways be satisfied. In fact it has been realised in this investigation that this condition is only satisfied for certain hydrogenation condit-ions. The maximum amount of unsaturated triglycerides present in an edible oil

IS fixed by 'mother nature' and none of the oils at the present moment available has an I.V. of more than 150. Thus the non-volatile solute concentration of the oils cannot be increased at will beyond a certain value.

At a high reaction speed the interface suffers from a shortage of unsaturated triglycerides. Thus the reaction conditions should properly be selected to satisfy b ~ b . This also means that the concentrationprofilesof the unsaturated trigly-cerides near the interface must be known as a function of the reaction conditions. Only then can the conditions be selected when the surface concentration will not be sigmficantlj changed over the reactor length. The hydrogenation of the edible oils also encountei s the two following conditions:

ID

""l ' ID ^•'^^ <^-^^>

b . 4 io2

2.10 to 3.5- (3.37) o

Because of these two conditions and the fact that the reaction is not infinitely fast, see section 4.4, the known film and penetration theory correlations cannot be applied to workout the concenti ationjirof lies as a function of the reaction con-ditions. The apjjropi late equations m dimensionless forms a r c

(3.38)

(3.39)

The following boundaiy conditions apjjly:

0 - 0 Z > 0 ^ j " (3.40) Z - 0 6 » > o ' ^ ^ - ' - (3.41) r\' r 0' , Z2 re ,. ^ ^ B r B 1 r Z^ ^ «

c'"

z

No analytical solution IS knownfor the Eqs (3.38) and (3.39). The two equations have been solved numerically by the finite difference technrque. The details of the

technique and the flow diagram of the program for a digital computer a r e given in Appendix I. T a b l e 3 . 1 . T h e o p e r a t i n g u n i t s . S E C T I O N A 1. 2 . 3 . 3A. 4 . 5 . 5A. 6. 7. 7A. 8. 9. 9A. 10. 1 1 . 1 2 . 1 3 . N2 or H cylinder BTS column Mol. sieve part Asbestos p a r t Mol. sieve Mol. sieve p a r t Asbestos p a r t PgO column Anti-surge Hg barometer Hg vapour trap Gas rotameter Gage Hg. manometer Hg vapour trap Regulator Reducer Gas exit valve

Ng inlet valve to securing

S E C T I O N B 14. Valve for securing 15. Primary securing unit 16. Cold finger 17. Vacuum pump Vacuum buffer 18. 19. 20. 21. 22.

23. Mass flow meter 23A. Mass flow r e c o r d e r

H2 buffer Solenoid valve Fine regulator

H„ mass flow transmitter

24. Gage Hg manometer 25. Operation p r e s s u r e manometer 26. Differential p r e s s u r e manometer 26A. Isolator 27. Photoelectric cell S E C T I O N C 2 8 . 2 9 . 30. 30A, 3 1 . 32. 32A, 3 3 . 3 4 . 3 5 . 3 6 . 3 7 . 3 8 . 3 9 . 4 0 . 4 1 . 4 2 . 42A. 4 3 . 4 4 . 4 5 . 4 6 . 4 7 . 4 8 . Reactor tank Film reactor Cap . Indentation Holes Shaft , Cylinder Hand wheel Threaded end Rod Teflon ring Housing Jacket Isolator Link U tube Jacket Hollow shaft , Constant head Holes Shaft Hand wheel Threaded end Heating coil Bath 27

48A. Gas sampling unit 49. Isolator

49A. Thermocouple

S E C T I O N D

50. Tri-isobutyl aluminium 50A. Injection port

51. Ni.dips. 51A. Injection port 52. Oil cylinder 53. Solvent cylinder 54. Isolator

55. Liquid rotameter 55A. Inlet valve to rotameter 56. Thermocouple

57. Constant level tank 58. Overflow pipe 59. Buffer mixer 59A. Degassing unit 59B. Flow meter 59C. Level controller 59D. Hg manometer 59E. Iso-octane condenser 59 F. Cryostat 59G. Regulating valve 59H, Solenoid valve 60. Magnetic s t i r r e r 61. Recirculation pump 62. Sample trap S E C T I O N E 63. 63A 64. 65. 66. 67. 68. 69. 70. 71. 71A 72. 73. 73A 74. 74A 75. 76, 77, 78, 79. 80. 81. 82. 83. Iso-octane reboiler Iso-octane reboiler Condenser receiver Receiver drain Iso-octane feed Gas-flow indicator Condenser

Catalyst injection syringe Ng inlet valve

Secondary securing unit Vacuum pump P„0- column A o Mol. sieve Asbestos Mol. sieve Asbestos BTS column Gage Hg manometer Hg vapour trap Gas rotameter Regulator N2 cylinder ALOn column Iso-octane batch feed Iso-octane receiver

3.3 D e s c r i p t i o n of t h e a p p a r a t u s

Al(iBu)„ IS extremely leactive to oxygen and moisture. Hence, one of the major functions of the apparatus used m determining the hydrogenation kinetics IS to ensure that all the operating units, except the r e a c t o i , a r e supphed with oxygen and moisture free nitrogen atmosphere. The inert atmosphere protects

the Al(iBu)2 completely during the various phases of the operation. The r e a c -tant hydrogen is also purified m this apparatus before the reaction.

The apparatus has been sketched m Fig. 3.3. The names of the individual units and their corresponding numbers have been given m Table 3.1. This d e s -cription of the apparatus should be read in conjunction with Fig. 3.3. The opera-ting units which a r e sketched in Fig. 3.3 but not described in this section have been mentioned elsewhere in this thesis. The assembly of the glass (Duran 50) apparatus can be divided into five sections:

1. section A: gas storage and purifier.

2. section B: p r i m a r y 'securing unit' and gas flow control. 3. section C: r e a c t o r .

4. section D: substrate-catalyst-solvent mixer and dosing system.

5. section L: catalyst handling, solvent purifier and secondary'securing unit'.

3.3.1 G a s s t o r a g e a n d p u r i f i e r - S e c t i o n A

The reactant hydrogen and the inert introgen are both stored m standard m e -tal cylinders 1. Specifications of the gases a r e given in Table 3.2.

Both the hydrogen and nitrogen storages and the purification systems a r e identical.

T a b l e 3.2 - S p e c i f i c a t i o n of g a s e s Specification Nitrogen Hjdrogen

type (Belgian) Standard (Dutch) HgO content 5 P^^M 10 PPM

Og content 3 PPM 10 PPM Hg content 2 PPM Rest Ng content Rest 900 PPM

Therefore, the numliers of the identical pieces of equipment in Fig. 3.3 a r e the same. Hence, specrfic reference to hydrogen and nitrogen is avoided and the general term ' g a s ' will be used instead in this particular section.

The flow of gas from tne cilinder 1, is controlled with a Loosco p r e s s u r e reducer, 11 and a fine regulator, FairchildHiIler model 16 vacuum regulator, 10, The flow is indicated on a rotameter, 8 and its p r e s s u r e is measured on a gage

manometer, 9. The open end, 9A of the manometer to the atmosphere is filled with iodine on active coal from Hicol Chemicalien N,V, to safeguard against any possible escape of mercury vapour (Hg. vapour trap) to the environment. After the rotameter, 8 the gas flows over an anti-surge Hg barometer, 7, also fitted with a vapour trap, 7A, The function of the barometer is to protect the r e s t of the glass apparatus against a possible p r e s s u r e surge.

The gas after the barometer e n t e r s a battery of glass cylinders. The battery removes oxygen and moisture from the gas s t r e a m s completely. The function of each cylinder and the chemicals it contains a r e summarised in Table 3.3.

T a b l e 3.3 - D e s c r i p t i o n of c l e a n i n g b a t t e r y No. of cylinder (see Fig. 3.3) 2 3A 3 4 5A 5 6 Description of chemicals Cu on active coal (BTS pellets) Asbestos granules (fills the first tenth of the cylinrler)

Mol. sieve (rest of the c_vUnder)

Mol. sieve

Asbestos granules (fills the last tentli of the c>lindei) Mol. s i e \ e P - , 0 . _{2 0} Function Removes ©„ Aljsorbs water Absorbs water Aljsorbs water Absorbs water Absorps water Kemoves water P r o c e s s occurs Removal is achieved by oxodizing Cu to CuO; in-dicated by the change in colour from bluish grey to black

Al)sori)lion. The end point IS indicated by the change in colour from light brown to darker brown Absorption Absorjition see 3A Absor])tion Hygroscopic binding