An onstream spectrophotometer for mixing studies in a channel reactor model

Shorter Commumcatlons

618 Lkpartment of Chemrcal Engrneenng Unrverslty of FIonda Gamesvrlle, FL. 3261 I, V S A

HONG H LEE

NOTATION concentration concentration at time zero activation energy rate constant frequency factor preexponential factor m eqn (13) proportlonahty constant In eqn (1) order of a reaction constant in eqn (1)

constant in eqn (13) R’ gas constant t time T temperature TO temperature at time zero REFERENCES

[II Anthony D B andHoward J B, AICkEl

197622625 121 Levensplel 0 Chemrcal Reactron Engrneenng p 28 Wiley New York I%2 [31 Badzmch S and Hawksley P G W , Ind Engng Chem Proc Des &a 19709521 r41 Kramers H and Westerterp K R , Elements of Chemtcal Reactor Desrgn and Operatron, p 12 Academic Press, New York 1963

The average shape of a mixture of particles in a packed bed (Recerved 17 May 1977. accepted 26 September 1977) The calculauon of packed bed performance mvmably reqmres knowledge of the packing size and shape If the packing IS of untform size and shape the values are used due&y If the packmg 1s a mixture of different sme particles of the same shape then a mean size IS evaluated by one of a number of appropriate formulae a&able III the hterature If the packmg ISa mixture of partmles of dtfIerent shapes then a problem seems to exist as no formula appears to be avatlable m the hterature for calculatmg the average shape of such a mixture The apparent absence of this information IS most surpnstng as one would have expected that a knowledge of the average shape of a muture of particles would be also uuportant m a wide variety of applications mvolvmg rmxtures of partmulate solids In the present case mqmry was prompted by a recent statement[l]. based on theoretical conmderattons, that “a vanabon of from 0 8 to 09 in the coke shape factor increases maxuntun production of a blast furnace by about 696” As an Increase m productton of this magnitude m the steel mdustry represents a considerable economic gam indeed, the question was followed up only to show that no mformatlon was avauable as to how m&t these shape factors be calculated from the knowledge of coke shapes which are known to vary with the coke au[2] IncIdentally, the question of averagmg shapes IS also pertment

addltlon

for tbe blast furnace burden as a whole smce m to the coke It also mcludes the metalbcs (ore, smter,

pelets)-all of which have dtierent shapes Although blast furnaces have been h&h&ted here It IS obvious that the question of the averagmg of shapes IS qmte general and of wide apphcabthty The purpose of this not2 is, therefore, to draw attennon to the existence of an important problem and to present some average shape results obtamed with mixtures of spheres and cylinders of the same size The apparatus used consisted of a glass tube 56mm rad packed to a he& of 1000mm with nuxtures of 13 mmdla spheres and correspondmg band-made cylinders Au was passed through the column and the resultant pressure drops were measured for a range of flow rates m the usual way Reynold’s numbers used were in the range 60-850 As the average partmle shape was calculated from the Ergun equation great care was taken to ensure that all other bed parameters remamed constant throughout Except for bed poroslty this was not d&cult To ensure that the bed porosity (g) was the same in each case the same volume of the column was always packed with the same solid volume of the packmgs After some

expenence this could be achieved with the muumum dmturbance of the uutmlly formed bed For each bed 20 runs were made at varying au rates The values of average shape factor, whmh m this case were equal to sphencuy (#), were obtamed from the measured pressure drops (AP) by solvmg the quadratic form of the Ergun equatlon[4] These are gwen in Table 1 which shows excellent agreement the theoretical and the measured values for the two prune components and, therefore, mdlcates that confidence may also be placed m the results obtamed for the mixtures between

Table 1 Average shape ($) results PaCklllg Spheres 20% Cyhnders 40% Cyhnders 50% Cyhnders 6096 Cyhnders 8(w6 Cylinders Cyhnders

4j theoretical

$ experimental

SD

IO 7 7 7 7 9 0870

1004 0991 0982 0976 0967 0925 0 873

0003 0005 0004 0005 0004 0003 0004

The results are also presented graphlcally m Fig 1 together with two solid hnes calculated on the assumphon that the two commonly used average size formulae as Indicated on the graph also apply for particle shapes By tnspectlon thm IS not the case here It must be stressed that the average shape factors presented here were obtained from pressure drop results in a packed bed with only the smgle fluid flowmg and they may be qmte different for other systems and applications In the Bow through packed beds pressure loss depends on the size and shape of the flow channels and it IS these that are changed by the mtroductlon of a second component-the changes betng charactensed by the changes m the bed porosity (c) and the shape (9) In the case of partrcles of ddferent size the tmportant effect IS not m the change of the average parttcle sme of the mature but m the change of the bed porosity As IS well known the progresWC addition of smaller srze particles to a bed of larger particles decreases the porosity of the nuually unsaturated mixture until saturation IS reached-the porostty tncreasmg thereafter as the mature becomes supersaturated The pressure drop, of course, exhibits an inverse relationship

619

Shorter Commumcattons

tba cotnpos~aon IS 5% The maxuruun change m the pwos~ty expected for a mtxture of spheres wttb dtatneter ratto equal to the shape rabo used here, t e 0 87 IS 5 5% which agrees well witi the proposed expectattons Although anaMgres between tlm effects of dtierent suzes on tire one hand and that of ddferent shapes on the other on the srxe and shape of the flow channels are expected from geomemcal constderattons It IS not quote clear as to what mtght the analogy of a satnratton[5] m a mixture of different sure partrcks be tn a muture of different shape partrcles It IS hoped that thts question wtll be answered by future research and untd then It would be wtse not to extrapolate beyong the present knowledge coNcwgIoN The resufts presented here show that the effect of mixing of parttck shapes on the Bow resistance m a packed bed IS analogous to that of mumg parttcles of dtfferent stxe which grves nse to the well-establtshed vartatton of the bed porosity The results have also demonstrated the need for further research to obtam formulae for averagtn8 particle shapes to complement those avadable for averagtng parttcles stxes Fmally, the Importance of shape m packed beds shown here supports the concluston by Haughey and Bevertdge[3] that “Parttcle shape and stze dtstrtbutton are the two factors most hkely to affect packmg structure and properttes”

D-

“OL

1

03 FRACTION

05

.

.

07

.

.

.

09

I

CYLINOERS

~artmnf

of Mefallugy

The Unrversty of Wollongong Wollongong. N S W 2500 Austraita

N STANDISH

G MCGREGOR

Ftg 1 Shape factor results for mtxtures of spheres and cyhnders of the same stae

PI Palella S , Gtult A4 , Bamaba P , Sacerdote R and Tatnmaro When parttcles of dtierent shape are added to the bed COIISISF, Symp Bfast Fgmoce Aerodynamrcs. p 76 Austr lnst tmg of stngk shape partrcles the srxe and &ape of the flow Mm Met, Wollongong 1975 channels are changed agam so tt rs not surprtsmg that these PI Standrsh N and Dnnkwater J P ,Chem Engng Ser. 1970 28 changes should ~ntlucnce the resultant pressure drop From the 1619 focegomg it would also be expected that the effect of mtxurg P and Bevertdge G S G , Can J Chem Engng I31 ym&y; d&rent shapes would also be surular. vtx the maxunum change of comparable extent occurmg at some mtermechate composttton M Szekely J and Themehs N J , Rate Phenomena In Process Metallugy, p 641 Wday, New York 1971 Ftgure 1 shows the maxtmum change tn the present result occors at 45% cyhnders tn the mixture and the extent of the change at r51 Ftmtas C C , US Bur Mm Bull No 307 1929

An onstream spectrophotometer

for mixing studies in a channel reactor model

(Recerved 6 September 1977, accepted I3 September 1977) A channel reactor was the basts of contmuous steel-making pilot plants used by both Rudskt et a1 [l] and Worner et al [2] Molten uon flowing along a narrow shallow channel IS re8ned by a sertes of oxygen lets and the tmpunttes are dtssolved m a slag phase In the case of Rudskt the slag flowed co-currentty whereas m the WORCRA pdot plant the flow IS countercurrent and better removal of phosphorus and sulphur IS achieved. together with tncreased recovery of tron However the effecttveness of countercurrent flow depends on the extent of axral mtxmg The aim of the study was to find the type of flow pattern induced m the metal phase by each let, to study the mteractron of flow patterns of adjacent Jets. and to relate tlus to the axtal mlxmg -AL A water model of the channel reactor was budt conststmg of a IOcm wrde perspex channel along which water flowed Imtml tnvesttgatrons wrth dye mJectlon mdtcated that a smgle Jet produced mtxrng zones wntt well defined hmtts Three technques were considered for studying the mixing zones tn more detad CES Vol 33 No M

(a) Restdence time dlstrtbutton measurements These gave dtrect measurement of axial mtxmg and can indicate gross effects of recycle flow, dead volume or bypass flow However, because of practical dtfficulttes m achtevrng correct Inlet and exit condtttons for the tracer[3] and the relatively large errors associated wtth measunng the tad of a dtstrtbutton, the technique lacks the resolution requtred for detatled study or for selection of an appropriate flow model (b) Measurement of velocity profiles l&s techmque IS sutted only to a steady flow pattern If a measunng probe ts used tt IS hkely to dtsturb the flow pattern Time lapse photography can be used (c) Analysis of tracer concentratton wtthm the model Several tracers and methods of analysts can be used The method chosen was to analyse spectrophotometrtcally for methylene blue using the IOcm wide perspex channel as the absorption cell Ths technique has the advantage that no probes are Inserted to take samples or measurements so tbe 8utd flow IS undisturbed except for the tracer mtector The analysts taken is an average across

Shorter Commumcatlons

620

the channel width which from the pomt of vtew of studymg axtal mtxing ISan appropriate average to take ONSl.REAMSPECTROPBOTOMgTER The detads of the spectrophotometer are shown m Ag 1 An Incandescent hght source was operated at 170 V from a stabdrsed voltage source A collunatmg lens was used to gave a parallel beam and a ground glass screen dispersed the Image of the filament The lirst baWe set the diameter of the bght beam (4 mm) with the second baffle (5 mm) and photocell (6 mm) being shghtly larger III order to grve some latttude tn altgnment The perspex channel was located between the first and second batlles Close baffling was necessary as it was intended to use the unit without darkening the surroundmgs Problems were encountered with low angle reflectton m the photocell tube but these were overcome with the msertton of a tube of 400 grade sdtcon carbtde abrasive paper The Instrument was then found to be unaffected by external light conditaons Maxtmum absorption of light by methylene blue solutions m water occurs at 6600A soan IIford Spectrum Red 608 filter was selected This filter was placed at the aperture end of the photocell tube rather than at the source m order to ftlter any stray light and so reduce Its intensity The photocell was a cadmium sulphtde photoreststor used as one reststor m a Wheatstone brtdge Changes m hght tntenstty were recorded as an off-balance current through a microammeter connected between tbe two legs of the bridge A stable supply voltage for the bridge was achieved usmg a 6V dry cell and allowmg 30 mm warm up Irntlal problems with cahbratlon usmg standards made with tap water were overcome by using dtstdled water However su#ictent dtstdled water supplies were not available so a paper element filter was used on the water supply, and this was found to gwe reproducible readings On a plot of meter reading versus the logartthm of relattve concentratton. the cabbratton IS linear wtthm a tolerance of 5% over a concentra0on range of 100 Such good sensitivity at low concentrattons requued that parttcular care be taken to avold contammatlon of low concentratton standards The stabdlty of methylene blue solutions was checked by taking optical density readings on a commercial spectrophotometer before and after mtense aeration for 1 hr There was a sbght increase m optical density (less than 2%) probably caused by evaporation, but this error IS well wtthm the ltmtts of the spectrophotometer used with residence times of a few mmutes Response time of the umt was less than one second, provided the photocell was not flooded with light between tests As an onstream unit, the spectrophotometer was mounted on a lathe bed so that it could be moved along the length of the channel using the channel as the absorption cell A shdewlre was arranged to give voltage output proportional to distance along the channel A typical experimental layout and spectrophotometer output IS shown in Fig 2 Methylene blue dye was injected contmuously through two 1 mm bore stainless steel caplanes The traverses along the line X-X were made at 5 mm intervals and an average concentratton profile esttmated In several cases, time intervals up to 1 hr were allowed between readings but no further drift of dye upstream was observed The Fig 2 shows the

SECOND BAFFLE

Smm@

INCIDENT

BEAM 8mm@

I I FIRST BAFFLE 4mm@

RI RED FILTER

Rg I Spectrophotometer

PHOTOCELL 6mm #

design

tPresent address Department of Metallurgy, Imperml College, London. England

IJ LANCE RUN

Al INJECTORS

(a)

Distance

alang channel,

cm

(b)

Fig 2 The dye concentration profile measured by the spectrophotometer (a) Layout of experiment h = IOcm M = 36OOdyne (b) Spectrophotometer output existence of a well defined m~xmng zone In Ftg. 2. the sharp peak at A corresponds to a 10 cm mark scribed on the perspex. whdst surface damage to the penpex regtstered as small reproducible vanatrons m zero reading, the largest of which 1s at E. Zero concentration traverses were ma& with and w&out the lances operatmg and wtth low intensity blowing there was neglwble readmg as shown m Ftg 2 At htgh blowing rates there was some interference to the hght path due to small bubbles and to the depression formed by the Jet Undtspersed dye near the ttps of the mlectors was detected by the spectrophotometer C and tmmedtately downstream of the lance the dye concentration IS denoted by D The onstream spectrophotometer was used to measure concentration profiles and the size of the mixing zones It was also used to determine the extent of interaction of adjacent mixing zones between two lances for a range of Jet parameters and lance spacings coNcLusoNg It was concluded that a smgle Jet produced muting zones wtth well defined hmtts and that each Jet acts as a parttal banxr to axtal mtxmg The extent of mteractton between Jets More detmled results are described elsewbere[4] where it was concluded that a starred tanks m senes model with recycle flow components was an appropriate descnptton of flow Residence time dtstnbuuon measurements alone would be msuficrnt to choose between this model and a dispersion model However for opbmisatlon and control of this type of reactor the IWO models suggest tierent behavtour In this way the onstream spectrophotometer has been used to supplement the information obtained from resdence time distributions to establish the flow mechanisms leading to axial mixing in the channel type reactor Pepanfnent of Mrtallufgy UnruersIty of Melbourne

PadmIle Vztvno Austrah

3052

D S CONGCHIEt N B GRAY

621

Shorter Commurucatlons REFE.RENCEs

[3] Levensplel

Rudzkl E M , Gtlles H L , Pease 3 K and Wleland G E , J Ibfetals 1969 21 57 [Z] Worner H K and Baker F H , Trans Iron Steel last Japan 1971 11 277

1605

[I]

Cl and Turner

J’ C

[4] Conoclue D S and Gray N tractlve Metallurgy Symposium to be published

R

, Chem

B , Paper Melbourne

Engmg Set

1970 25

presented to ExNovember lw5.

Solid circulation studies in a gas solid fluid bed (Recerued

12 JuI\

1977

uccepted

It IS now well known that the malor cause of solid circulation m a fluid bed IS the presence of bubbles Two dimensional experiments have shown the carry over of solid in the bubble wake and cloud region at the scale of the bed[l] Local motions at the scale of a bubble diameter. were also detected and are known as the drlfl effect [2] Exp&RIMENTAL

WORK

An experlmental mvestigatlon was performed to precise the consequences of these facts for a freely Buldlsed bed In a I5 cm dla and 45 cm height gas sohd fluId bed of sand particles (100-200 ~1 u,,,, = 3 1 cmlsec), the bubbles were detected using a double hght probe and the three charactenstlc times method(41 Design details may be found elsewhere[S] The radial bubble flow profiles for a gas velocity of 7 47 cm/set are given m Rg I They are rn close agreement with the previous published data[3] the bubbie flow profiles present a maxlmum near the column wall at the bottom of the bed and m the column axls higher m the fluldlsed Iayer[3] In the same mstallatlon. and for the same expenmenlal condttlons sohd mlxmg was studaed by perturbmg the bed with a pulse of hot soltd put umformly at the upper part of the bed Further expertmental details are presented elsewhere(61 The responses m several pomts of the bed were detected usmg small hnearlzed

27 September

1977)

thermistors A multichannet analyser was used to smooth the data Some examples of responses are given m Fig 2. for dtferent locatlons m the bed Depending upon the locahsatlon of the measurement point the responses are more or less damped and a more or less Important dead time exists Thts allows to define the solld clrculatlon pattern quahtatlvely It IS tmposslble that the soltd goes from a very damped response pomt to an undamped one It IS impossible too that IS goes from a pomt presenting a long dead time to a pomt presenting a shorter one One may also define the size of well mlxed cells that may exgst rn certain parts of the bed the points inside such a cell must exhlblt the same response Fmalfy the drrections and magmtude of the sohd flow between such cells may be found The results show that In the upper part of the bed the sohd descends along the walls and at about 25 cm from the bottom It penetrates mslde the layer A dead zone exists near the wall at this level At 25cm from the bottom exists a dlstrlbutlon node from where the sohd IS dlstnbuted up and down in the axis of the column The solid descends slowly m the axls and comes up along the walls In the lower part of the bed The ascending velocity of the sohd from the node to the upper surface along the vertical axis of the column Is greater There

IC

I

2

Fig

I

I

R.cm

Bubble flow radial profiles

I

4

(h) +

6

IOcm

0

25 cm Q 45 cm