Indian Journal of Chemical Technology Vol. 10, November 2003, pp. 694-700

Articles

Studies on ionic mass transfer with insert helical tape promoters in batch fluidized beds

V Sujatha*, P Rajendra Prasad, C Bhaskara Sarma & G 1 V Jagannadha Raju

Department of Chemical Engineering, Andhra University, Yisakhapatnam 530 003, India

Received 27 May 2002; revised received 13 May 2003; accepted 10 Jun e 2003

Augmentation of mass transfer coefficients, due to the presence of helical tape promoters in batch fluidized beds has been presented. Mass transfer coefficients have been evaluated from the measured limiting current densities and the con­centrations of the reacting ions. The variables studied are the geometric parameters of the promoter-tape width, pitch and diameter of the promoter rod, the flow rate of the electrolyte, void fraction and particle diameter. It is found that kL in­creased with increased flow rate and particle diameter. The increase in tape width and diameter of the promoter rod resulted in slightly increased mass transfer coefficients. Pitch of the promoter showed negligible effect on kL.

In recent years, several theoretical and experimental studi es have been carried out to identify an appropri­ate augmentation technique suitable for a given sys­tem. The various techniques adopted include (a) the use of insert promoters, (b) converging and diverging sections , (c) the vibration or rotation of the fluid or the transfer surface, (d) the location of cross-flow elements and (e) the use of fluidizing solids . Re­circulating flow patterns are developed in a flowing fluid by employing the above techniques that generate intense turbulence. More turbulence scours away the boundary layer at the reacting surface, thus, providing more surface of contact for heat or mass transfer.

The introduction of insert promoter in fluidized bed (compou nd augmentation) makes the transport proc­ess more rapid . Yenkateswarlu et al. 1 obtained mass transfer data with coaxially pl aced discs on a central rod and reported about 12-fold augmentation. A criti­cal value els=0 .26 for increasing and decreasing coef­ficients was reported. Sitaraman2 studied ionic mass transfer with string of spheres in fluidized beds . The limiting currents were measured on the surface of the sphere and the confining wall of the fluidized bed. The variables studied are pitch, di ameter of the sphere, diameter of the particle and solids concentra­tion. Wall and surface mass transfer coefficients have been reported, with the string of spheres placed axi­all y. The wall mass transfer coefficients decreased

*For correspondence (E-mail: jn_iyer@yahoo.com; Fax: 91 22 24145614)

with increase in spacing. Rao and Raju3 employed co­axially placed cones on a rod (CPCR) as internal in fluidized beds, for the studies in mass transfer. The effects of cone diameter, cone spacing, partic le size and void fraction on mass transfer were studied. Twenty-fold improvement in mass transfer coeffi­cients over the smooth tube data were reported. The cone spacing and cone angle showed a definite change in the flow patterns of the flowing fluid past the reacting surface. Maximum coefficients were ob­served at the maximum cross-section of the cone. Mass transfer data in presence of ring promoter4 and cylindrical and mesh type promoters5 revealed signifi­cant increase in mass transfer coefficients.

The use of swi rl flow generating promoters as in­sert promoters, was found to be a good augmentative technique and easy to be incorporated in the existi ng system. Bergles6 made an exhaustive survey on evaluation of techniques to enhance heat and mass transfer. Several experimental investigations on aug­mentation techniques were summarized and a selec­tion criterion for a specific promoter was given in terms of pumping power performance. Studies on ionic mass transfer using helical tapes 7 as promoter in forced convection flow of the electrolyte showed sig­nificant increase in coefficients with increase in tape width and diameter of the promoter rod . Five to ten­fold augmentation in mass transfer coefficient was observed. Heat transfer studies employing the swirl flow generating promoters has been reported8

-12 in the

literature.

Sujatha et al. : Studies on ionic mass transfer with insert helical tape promoters in batch fluidized beds Articles

H T

p v,

Fig. !-Schematic diagram of the equipment [A- Entrance calming section, 8 - Test section, C- Ex it calming section, F- Flanges, G" Gz, G3, G4 - Gland nuts, H - Coiled copper tubes, P - Centrifugal pump, R1, R2 - Rota meters, T- Recirculating tank, V 1 to V 6 - Valves, E" E2 - Thermowell s, D - Grid, L- Solids in let port.)

Ravi et a/. 13 reported a maximum of 25-fold im­provement in mass transfer coefficient due to the ef­fect of coaxially placed discs on a rod as internal in fluidized beds . Raju et a/. 14 reported negligible effect of coil diameter and pitch on mass transfer, for the studies on effect of wires wound on the central rod as promoter in fluidized beds. Employing compound augmentation technique to obtain higher heat and mass transfer coefficients has been reported in the literature 15

-18

• Based on the literature review, the pres­ent study is undertaken to investigate the effect of helical tape in fluidized bed on mass transfer.

Experimental Procedure The schematic diagram of the experimental unit is

shown in Fig. 1. The equipment essentially consists of a re-circulating tank, a centrifugal pump, two ro­tameters, an entrance calming section, a test section, an exit calming section and thermo wells . The test

P- PITCH W - TAPE WIDTH

d - TAPE THICKNESS dR- DIAMETER OF THE PROMOTER ROD

Fig. 2-Detai ls of helical tape promoter.

section is provided with a number of micro electrodes fixed flush with its surface at equal spacing (0.01 m).

The details of the helical tape promoter are shown in Fig. 2. A 0.0016 m thick copper strip is bent to the helical shape and welded to the promoter rod. This assembly forms the helical tape promoter. The pro­moter is mounted in the test section coaxially by means of gland nuts and positioned by supporting grid. The equipment and the helical tape promoter are the same as that used in earlier studies 7. The electro­lyte consisted of equi molal solutions of potassium

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ferricyanide and potassium ferrocyanide with an ex­cess of indifferent electrolyte (0.5N)-sodium hydrox­ide. 100 L of the electrolyte is prepared in the storage tank. After inserting the promoter in the column, known volume of the solids is introduced in to the test section and the required flow rate of the electrolyte is adjusted by operating the control and by pass valves. The bed is expanded to the desired height and the flow rate and bed heights are noted. Limiting current data are measured at point copper electrodes for the reduction of potassium ferricyanide ion and for the oxidation of potassium ferrocyanide ion.

The limiting current is indicated by a small in­crease in voltage for a sharp increase in current. The experiments are repeated by varying the volume of the solids CVs), diameter of the particle (dp) , pitch of the helical tape (p), tape width (w) and diameter of the promoter rod (dR). Mass transfer coefficients are cal­culated by the expression,

... (1)

The viscosity ().!) and density (p) of the electrolyte are obtained using Oswald viscometer and specific gravity bottle respectively. The temperature is noted for each run to an accuracy of ±0.1 oc. Since the con­centration of the electrolyte in the present study is the same as that used in the studies of Lin et al. 19

, diffu­sivity (DL) values are taken from their data and are corrected according to the temperature maintained in the present experiment. The void fraction (E) is cal­culated using the following equation:

. " (2)

The range of variables covered is given in Table 1.

Results and Discussion

Literature indicates, that, the augmentation of kL due to the presence of helical tape promoter in homo­geneous flow can be attributed to (i) increase in ve­locity consequent to increase in the obstruction area and (ii) swirl flow generated by the promoter and its interaction with axial flow. In case of fluidized beds, the interstitial velocity (being very high compared to superficial velocity) and the convective movement of the solids generate intense turbulence and contribute to the improvement in mass transfer coefficient. It is an interesting case to study the compound effect that

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Indian J. Chern. Technol., November 2003

Table !-Range of variables covered

Variable Minimum Maximum

Flow rate of the electrolyte, (Qxl06)m3/s

Pitch of the helical tape, (px l02)m

49

Width of the helical tape, 0.3 (wxl02)m

Diameter of the promoter rod, 0.95 (dRx l02)m

Particle diameter, (dpx 102)m 0.29

Void fraction, (e) 0.383

Schmidt number, (Sc) 719

707

15

1.2

1.9

0.684

0.97

1331

Modified Rey nolds number, 142 1,00,923 ( (Repl(l -e))

10r-----------------------------------,

.. e .,. 0

2

..... .....---.----.- --........ .......... ~

o---~---- -o... .._., A -f. C ""0,0

10

' oB

de= o.osm

P,m W,m dR,m dp,m Sym 0 .15 0.003 0.0095 - + C 0 ·1 5 0.003 0 .0095 0.00684 • A

0.00684 0 8

- 0

u x 1ol,rrvs 70

Fig. 3-Augmentation of mass transfer coefficient due to the presence of fluidizing solids and promoter.

is the presence of helical tape promoters in fluidized beds on mass transfer coefficients.

Augmentation of mass transfer coefficients with the helical tape promoter is illustrated in Fig. 3 by plotting mass transfer coefficient against superficial velocity. Plot A in Fig. 3 represents the experimental data for the case of promoter in a fluidized bed. Plot D represents smooth tube data that is the predicted data from the equation of Lin et al. 19 for the case of homogeneous flow of electrolyte and plot C is the

Sujatha et al.: Studies on ionic mass transfer wi th insert he lical tape promoters in batch fluidi zed beds Articles

10~--------------------------------~

"' e LO

0

><_, .... w x 10~m Sym

0-6 v 0 ·8 0

1-0 0

0 -{}-

de= 0.05m p = 0.02m

dR = 0-0125m dp = 0.0049m

U)C102:10 m;s

\~.~3--~--L--L-J--~-1~-0~--------2~-~0~

Tap~ widlh(w),m

Fig. 4-Variation of mass transfer coefficient with tape width.

lor--------------------------.

Ill ...... E

10 • 0 de= 0-05m dR.m Sym x....J ..¥ p = 0 -02m 0-0095 0

w=0.006m 0.0125 • dp = 0-0049m Q.Q19 0-

2 uxlO :10 mts

2 2 3

Fig. 5-Variation of mass transfer coefficient with diameter of the promoter rod .

lOr--------------------------------------,

0

4::J'K 0 t; 01!. ~ ~ 0

D D D D l!.D D D l!. 0 00

0 t;

~ de= 0 ·05m dp.m ~- Sym

0

~ p = 0-02 m 0-00684 0

X w = O.OOJm 0-004 9 6

dR = 0-0095m 0 -0029 0

2 2 10 60

u x1o2,m;s

Fig. 6-Variat ion of mass transfer coefficiem with velocity 111

flu idized bed for different particle diameters.

~-0

lOr-------------------------------------~

P = 0 .02m

w = O.O OJm

dR = O.OQ9Sm

dp,m Sym

0-00684

0 .0049

0 -0029 ~

2~--~~--~~~~L-------~--~--~~~ 0.02 0 -1 0·6

1 - E

Fig. ?-Variation of mass transfer coefficient with solids fraction ( I -E)

experimental data with promoter having the dimen­sions: p = 0.15 m, w = 0.003 m and dR = 0.0095 min homogeneous flow . The mass transfer data in fluid­ized beds20 in the absence of the promoter is shown by the plot B.

The comparison of plots A and D shows the fol ­lowing augmentation in coefficients . At a velocity of 0.075 rnls, 18.1-fold augmentation in coefficient is noted due to the presence of the promoter in fluidized beds over the smooth tube coefficients and the im­provements decrease as the velocity is increased. kL values in the present study are 2.66-fold higher than those obtained due to the presence of the promoter and the present mass transfer data are 31.4 percent higher over the kL values obtained in a fluidized bed without promoter. The reason for improvement can be attributed to the swirl flow generated by the promoter, the increased interstitial veloci ty and convective movement of solids.

The effects of the geometric variables of the pro­moter in fluidized beds on mass transfer coefficients are shown in Figs 4 & 5. The increase in the value of the pitch has negligible effect on mass transfer coeffi­cients (figure not shown). However, the increase in tape width slightly increased the mass transfer coeffi­cients as shown in Fig. 4. The increase in the diameter of the promoter rod marginally increased the coeffi­cients as represented in Fig. 5. The effect of the parti­cle diameter is shown in Fig. 6 by plotting the mass transfer coefficient against the velocity. In the present study, the particle diameter has been varied from 0.0029 to 0.00684 m. The plots in Fig. 6 indicate, that, the coefficients increase with increase in the par­ticle size.

In Fig. 7, the coefficients are plotted against the solids fraction (1- c:) . The plots show that the coeffi­cients steadily increase with increase in the solids fraction till a value of 0.1 to 0.2 is attai ned and there-

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after the coefficients remain nearly constant. The kL obtained at any time in a batch fluidized bed can be assumed to be due to the turbulence created by parti­cle movement and that of fluid velocity. As the fluid velocity increases, the concentration of solids de­creases. The contribution of the fluid turbulence in­creases and that of particle movement decreases in such a way that they are self-compensatory. So the mass transfer coefficient remains more or less same. As the fluid velocity decreases, exactly the reversal happens, till the solid concentration ( 1-E) reaches a value of about 0.2, below which, the turbulence cre­ated to increase the mass transfer coefficient is mostly because of the fluid flow and very small contribution is from particle movement. Similar observations have been noted with regard to trends in coefficient studies in batch fluidized beds20

, insert promoter in fluidized beds3 and cross-flow elements in fluidized beds21

.

The experimental data using Richardson and Zaki22

relationship reveals that the superficial velocity bears a functional relationship with voidage as,

. .. (3)

Present mass transfer data obtained for the different helical tape promoters are shown in Fig. 8 with c: as a function of Rep at dp = 0.0049 m. The mass transfer data in a fluidized bed for dP = 0.0049 m is shown in

M <:' 0

<D 0 6 I-

a..l-ou

"" U> .... p

Indian J. Chern. Techno!., November 2003

3x1o3r--------------------------------,

I~

2

P,m w,m dR,m Sym

0 .15 0 .003 0.0095 0

0-02 0 .006 0 .019 A

I I

I I

I

A

I I

dc=0.05m

dp=0 ·0049m

10 o~.~~----~~--~-J€--L-~~~1~.o~----~

Fig. 8- Plot of experimental data in accordance with Richardson and Zaki correlation.

the same figure . It is apparent from the plots that the presence of promoter has not affected the nature of fluidization and the slope of the line is same as that reported by Richardson and Zaki22

. The data are correlated following the method of development of generalized correlations as presented in earlier

Fig. 9-Correlation plot of experimental data.

698

Sujatha et al. : Studies on ionic mass transfer with insert helical tape promoters in batch fluidi zed beds Articles

w I ~0

de = 0 -05m

P :0 -0Sm A---- Coa ia lly placrd con rs on~ rod(Huidiud)brds(Rr())

w =0-0llm 8 -X - Prrse-nt Study

dR = 0.019m C-0 - String~ spt'le'rrs in ftuidfnd brds (R e-f. 2)

dp = Q.003m .... ... Wir n wound on a rod in tluidi:r:rd brds(Rrf. 14.)

0-1 ,,'no,---z--------;-,o:,,,------,L,o',--------,J..,os_J

Fig. I 0-Comparison plot.

studies3•13

'14 in terms of the modified J 0 factors (Jo£),

modified Reynolds numbers [Repf( 1-£)] and pertinent dimensionless groups of the promoter. The regression analysis of the data yielded the following equation with standard deviation of 9.31 percent and the aver­age deviation of 7.58 percent

lo £ = 0.36[Rep/(1-£)r025 (pldcr 006 (wldc)0072 (dR/dc)023

. ., (4)

The data in accordance with the Eq. (4) is shown in Fig. 9.

Comparison of the results of present study with the mass transfer data of earlier studies2

·3

·14 obtained with

turbulence promoters in fluidized beds are shown in Fig. I 0. Line A represents the mass transfer data ob­tained for the case of coaxially placed cones on a rod (CPCR) as internal in fluidized beds3 and the present data are indicated by the line B. The mass transfer factors obtained with the CPCR internals are consis­tently higher than those values obtained in the present study . The reason being the smooth flow produced by the helical tapes compared to the CPCR internal. Line C represents the data of Sitaraman2 with string of spheres as internals in fluidized beds. At low Rey­nolds numbers the mass transfer factors obtained in the present study are slightl y lower than the data of Sitaraman and at hi gh Rey nolds numbers the mass transfer factors obtained by helical tapes dominated the values obtained in case of string of spheres. Thi s may be due to the reason that the swirl flow generated by the tape and its interac tion with ax ial flow may have resulted in more turbulence. The present data are consistently higher than the data obtai ned by Raj u er al. 14 for the presence of wires wound on rod in fluid­ized beds.

Conclusions It may be concluded from the results reported here

that, (i) mass transfer coefficient increases with in­crease in diameter of the particle (dp), (ii) the increase in the value of width (w) and pitch (p) of the helical tape have marginal effect on the mass transfer coeffi­cient, (iii) increased diameter of the promoter rod re­sulted in s li ght increase in kt., (iv) mass transfer coef­ficients increased with increase in solids fraction (1-£) up to I 0 to 20 percent and remained nearly constant thereafter and (v) the kt. values are enhanced up to 18.1-fold over the smooth tube coefficients.

Nomenclature cross-sectional area of the e lectrode, m2

concentration of the react ing ions, Kg moles/m3

diameter of the conduit , m diameter of the particle, m diffus ivity of the e lectro lyte, m2/s diameter of promoter rod, m Faraday , 96500 coulombs/equivalent limiting curren t, A

kL mass transfer coefficient , m/s p pitch of the helical tape, m Q volumetric fl ow rate o f e lec tro lyte , m3/s

11 superfic ia l velocity based on conduit diameter, m/s Vr volume of the fluidized bed, m3

VP volume of promoter, m3

V, volume of solids, m3

w width of the he lical tape, m

Greek letters € void fracti on Jl vi scosity of the e lec troly te , Ns/m2

p density of the e lectrolyte, Kg/m3

o tape thickness, m

Dimensionless groups 10 mass transfer factor,(h/u) Sc213

Rev Reynolds number, dr u p!Jl Sc Schmidt number, Jll(pDL)

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Heat and Mass Tra11.1jer, 22 (2000) 195.

2 Sitaraman T S, Augmentation of mass transfer by coaxial string of spheres as intema/ in tubes and fluidi zed beds, Ph. D. Thesis. Un ivers ity of Madras, Madras, 1977 .

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4 Ramabrahmam G, Kalyana C hakravarthy K & Yenkateswarlu P, Studies 0 11 ionic mass transfer and pres­sure drop using turbulence promoters, paper presented at Nati onal Semi nar o n Application of Natural Resources­C HEMINAR-93 . Bhubaneswar, 24-25 Feb 1993.

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