[PDF] Sucrose Inversion: An Experiment on Heterogeneous Catalysis*

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Sucrose Inversion: An Experiment on

Heterogeneous Catalysis*

ADEÂLIO MENDES, FERNAÄO D. MAGALHAÄES and LUIS M. MADEIRA

Departamento de Engenharia Quõ

Âmica, Faculdade de Engenharia, Universidade do Porto, Rua Roberto Frias, 4200-465 Porto, Portugal. E-mail: mendes@fe.up.pt Illustration of heterogeneous catalysis concepts in laboratory courses is not usually simple or economical. For our undergraduate senior lab course we have developed an environmentally friendly experiment dealing with several aspects of heterogeneous catalysis, having in mind the use of readily available and relatively inexpensive equipment and chemicals on a compact setup, which students can safely operate. The experiment deals with the acid-catalyzed sucrose inversion, performed in packed bed chemical reactors, where the catalyst is a cation-exchange resin in the H form. An additional reactor is included for illustrating an enzyme-catalyzed system. The conversion achieved is determined using the Flow Injection Analysis technique.

NOMENCLATURE

C sucr sucrose concentration, within the catalyst pellet (mol/m 3 C ssucr sucrose concentration at the catalyst surface conditions (mol/m 3 D e effective diffusivity (m 2 /s) d p particle diameter (m)

E...t†residence time distribution function

fdimensionless sucrose concentration profile

Ffeed flow rate (m

3 /s) kintrinsic rate constant (s ±1 k obs observed rate constant (s ±1

Lbed length (m)

PePeclet number

rradial distance (m) r 0 particle radius (m) ttime (s) usuperficial velocity (m/s)

Vreactor volume (m

3 X sucr sucrose conversion zdimensionless radius direction

Greek Symbols

"bed porosity

Thiele modulus

internal effectiveness factor fluid viscosity (Ns/m 2 fluid density (kg/m 3 space-time (s)

INTRODUCTION

HETEROGENEOUS CATALYSIS is a topic of

great industrial importance in Chemical Engi- neering and consequently plays a significant role in theoretical undergraduate courses. In fact,heterogeneous catalysis is present in most of chemical reaction engineering (CRE) handbooks, e.g. [1±4], and is an essential part of the majority of the undergraduate CRE courses [5±9]. This paper describes an experimental set-up on heterogeneous catalysis, running at a chemical engineering laboratory discipline, taught during the 1st seme- ster, 4th curricular year of the 5 years long

Chemical Engineering course at the University of

Porto, Portugal.

All the experiments now available at this

laboratory were designed having in mind: high safety, low investment andoperation costs,reduced environmental impact and high didactic content.

The sucrose inversion (hydrolysis) is acid-

catalyzed and can be conducted in a fixed bed, packed with a cation-exchange resin in the proto- nic form. This is the kind of experiment that fits perfectly the above mentioned principles and succeeded to capture the attention of an interna- tional company dedicated to the manufacture and commercialization of this kind of equipment [10].

The resin's three-dimensional network forms a

macroporous structure in which the ionically linked H cations are the acid sites. Sucrose under- goes acid hydrolysis into glucose and fructose (stereo-isomers) according to a pseudo-first order reaction [11, 12]: C 12 H 22
O 11 ‡H 2

Oÿ!

H C 6 H 12 O 6 ‡C 6 H 12 O 6 ...1†

The optical rotation of sucrose is positive

20D

ˆ‡66.58) and its hydrolysis with an acid

yields a 1:1 ratio of D-(‡) glucose and D-(ÿ) fructose, with positive and negative optical rotations, respectively. Since fructose has a greater optical rotation than glucose (ÿ92.48versus 52.78), * Accepted 19 January 2003. 893
Int. J. Engng Ed.Vol. 19, No. 6, pp. 893±901, 20030949-149X/91 $3.00+0.00 Printed in Great Britain.#2003 TEMPUS Publications. tionproceedstocompletion.Thisiswhyitis usuallycalledsucroseinversionreaction.

Sucroseinversiononcationicexchangeresins

wasstudiedbyReedandDranoff[11],using

AmberliteIR120,andbyGillilandetal.[12],

usingDowex50W-X8.Theseresearchersfound wasnotedastheparticleReynoldsnumbers (ud p /)variedfrom0.14to4.8,indicating

Anexperimentalset-upwasbuilttoconduct

transportconditionsandthenitcanbeusedto nessfactor'andtheThielemodulus,apartfrom otherissuessuchasflowpatterninapackedbed modernCREcurricula[5,6,8,13].Emphasisis (dataacquisition)andtotheuseofcomputer

Tomakethisexperimentevenmoreinteresting,

canbeusedasthebiocatalyst.Inthiscase,the equation.

THEORETICALBACKGROUND

tantsreactfaster(ortheproductsareformed faster)thantheydiffuseinward(oroutward), reactant/productwithintheresinparticleto quantityisafunctionoftheThielemodulus,, whichforafirst-orderreactionisgivenas: ˆr 0 k D e 1=2 ...2†wherer 0 istheparticleradius,kisthereactionrate constantandD e istheeffectiveporediffusion coefficient.Therelationshipbetweenandis, conditions[2]: 3 coth...†ÿ1 ...3† ofthesystemisinthekineticregimeand!1. sufficientlylarge,and!3=.

Astraightforwardwaytoevaluatetheimpor-

clesizes,sayr 01 andr 02 .Iftheattainedcatalytic thetwoobservedkineticconstants,canbe obtainedfromEquation(3)andfromthedefini- tionoftheeffectivenessfactor(sincek obs

ˆk):

k obs1 k obs2 1 2 r 02 r 01 coth 1

ÿ11

coth 1 r 02 r 01 1 r 02 r 01 ÿ1 ...4† where 2 wasreplacedbyafunctionof 1 2 1 r 02 r 01 ...5† compute 1 fromthemeasuredapparentkinetic constants.Theothervariables... 2 1 2 †arethen obtainedfromEquations(5)and(3).

Inordertocomputek

obs1 andk obs2 ,onemustbe reaction'skineticparameter.Thisobviously describeitasanidealplugflowreactor,and thereforetheconversionisgivenby[1]: X sucr

ˆ1ÿe

ÿk obs ...6† whereisthespace-time,basedonthereactor

Whenaxialdispersioncannotbeneglected,a

totalsegregationmodel,whichassumesthat X sucr 1 0 X batch

E...t†dt...7†A.Mendesetal.894

[14]: X sucr 1 0 ...1ÿe ÿk obs t 2 3 pe ...ÿ...Pe...ÿt† 2 =4t†† dt ...8† wherePeisthePecletnumber,whichcanbe anumericaliterativeproceduremustbeusedto computek obs

Asexplained,aftercomputationofk

obs ,students candeterminetheThielemodulusandtheeffec- sionlimitationsandtoidentifywhichregime, size.

Itisalsoveryinterestingtoaskstudentsto

determinethesucroseconcentrationprofiles profile[1]: fˆ C sucr C ssucr 1 zsinh...z† sinh...9† where,C ssucr isthesucroseconcentrationatthe surfaceconditionsandzˆr/r 0 isthedimensionless

EXPERIMENTALSET-UP

Packedbedcatalyticreactors

Theexperimentalset-updevelopedcanbeseen

easyasA,BandC.Thereactorsconsistin withAmberliteIR120resinoftwodifferent diameters(8.810 ±4 mand3.110 ±4 m)were

Fig.1.Experimentalset-upsketch.

tion)isabout1.9meq/cm 3 ofresinbed[15]andits apparentdensityis1.2710 3 kg/m 3 .Thecalcium alginatebeadsweremadedropping,dropbydrop andwiththehelpofaperistalticpump,asodium alginateaqueoussolution(1%w/w)containing idesolution(0.2M).Theexchangeofsodiumby bead,wheretheinvertaseistrapped.

Thethreereactorsarekeptataconstant

mostaticbath.Thereactantsolution(aqueous onecanseehowtheuseoftwofour-wayvalves thecatalystisallowedtostoptheentranceof bubbles.Thereactors'conversionisanalyzed usingaFIA(FlowInjectionAnalysis)system, describedbelow.

Aftereachexperiment,watermustbeflown

througheachreactorforsomeminutes,inorder toremovetheremainingsucroseandavoidaccu- tubingthatcontactsthesucrosesolution.

Wehaveobservedthatresinregeneration(with

aconcentratedHClsolution)isnotnecessary duringonesemester.

Flowinjectionanalysis

FIAisarecentanalyticalmethodthatcanbe

metryorothermethods.Aremarkableadvantage smallamountsofsampleandspecificreactant.

FIAimplementation,sincewebelievethatits

relativelowcostandsimplicitymakeitquite suitableforstudentlaboratories.

Glucoseisthecomponenttobeanalyzedbythe

solutionoftwoenzymes,glucose-oxidaseand peroxidase(PeridochromGlucoseGOD-PAP fromBoehringerMannheim),givingacolored compoundthathasamaximumabsorbencyat

510nm.

Typically,aFIAsetupiscomposedoffour

sections[16±18]:

1.Pumpingsystem.Oneperistalticpumpisused

tion)andthecarrier(waterinourcase). allowsforpredeterminedamountsofsample andspecificreactanttobemixedintothe carrierstream.

3.Reactionsystem.That'swherethereaction

betweenthesampleandthespecificreactantquotesdbs_dbs17.pdfusesText_23

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