Joumul of Food Engineering 35 ( 1YYX) ?SY--266 0 199X Elsevier Science Limited. All rights reserved

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Determination of Effective Mass ‘lI=ansfer Coefficient (Kc) of Patulin Adsorption on Activated Carbon Packed Bed

Columns with Recycling

Mehmet Mutlu” & Vural Giikmen

Department of Food Engineering, Hacettepe University, 06532 Ankara, Turkey

(Received 4 October 1997; accepted 7 January 1998)

ABSTRACT

In this study, a simple and effective technique for determining overall mass transfer coefficients in fixed bed sorption columns with recycling is presented. The overall mass transfer coejjicient (KC) of patulin for adsorption on activated carbon packed columns were determined at different flow rates. The K, values increased from 4.74 x IO ~’ to 12.96 x IOF’ cm s - ’ when the flow rate increased from 5 to 100 ml min ~ IS which is a very clear indication of turbulence created through the column to eliminate the film resistances on the activated carbon sugace. 0 1998 Elsevier Science Limited. All rights reserved

INTRODUCTION

In fixed-bed adsorption, the concentration in the fluid phase and the solid phase change with time as well as with position in the bed. A knowledge of mass transfer coefficients and of the equilibrium isotherm would greatly facilitate the design of a fixed bed adsorber and prediction of the length of the adsorption cycle between regenerations.

Theoretical

For a fixed bed column system, equations for mass transfer are obtained by making a solute material balance for a section dL of the bed as shown in Fig. 1.

*To whom correspondence should be addresed. Tel.: +90 312 299 21 23; fax: +90 312 209 21 24; e-mail: gmehmet@eti.cc.hun.edu.tr

259

260 M. Mutlu, K Gijkmen

of Particles

dL

X=0 X=L Fig. 1. Schematic representation of the shell balance for packed column.

The model equation for a packed column with axial dispersion is (Petersen, 1972; Mutlu & Piskin, 1990)

aw sdLc+(l-c)p,dL= -

at U,C - U,(C+dC)

where: C: W

E uO: t: The term F is the external void fraction of the bed, and solute dissolved in the pore fluid is included with the particle fraction 1 --E. The rate of accumulation in the fluid and in the solid is the difference between input and output flows. The change in superficial velocity through the column is neglected. For adsorption from a dilute solution, the first term in eqn (1) which describes the accumulation in the fluid is usually negligible compared to the accumulation on the solid. Then eqn (1) becomes;

concentration of adsorbate (mol) total mass adsorbed on the unit mass adsorbent surface (dimensionless) particle density (g cm-“) packed bed length (cm) superficial velocity (cm s- ‘) time (s)

aw (1 -&)Ppyg = -uog

The mechanism of transfer to the solid includes diffusion through the fluid film around the particle and diffusion through the pores to internal adsorption sites. The actual process of physical adsorption is practically instantaneous, and equilibrium is assumed to exist between the surface and the fluid at each point inside the particle. The transfer process is approximated using an overall coefficient (K,) and an overall driving force.

aw Cl--E)pp at - = K,a(C - c*>

Mass transfer of patulin adsorption 261

The strongly favourable adsorption equilibrium concentration in the fluid is practic- ally zero (C* = 0). If the accumulation term for the fluid is neglected eqn (2) and eqn (3) are combined to give eqn (4).

- I/, g = K,aC (4)

Equation (5) is obtained by integration of eqn (4).

K,aL \n$=-- (5)

0 I/,,

N is the so-called ‘number of transfer units’ and defined as (McCabe et al., 1985)

N= K,aL

UC,

and the concentration at the end of the packed bed is given by;

C = C,e N (6)

If the reservoir shown in Fig. 2 is a perfectly mixed tank, the total mass balance gives

Adsorption

I

1 Column Reservoir

Fig. 2. Schematic representation of recycled adsorption system.

262 M. Mutlu, K Giikmen

dV - =o dt

(7)

and the component balance gives

-=-&,-C,) de, dt z

where r is the residence time in the reservoir (V/Q). The substitution of eqn (6) into eqn (8) gives

dc, -=- dt

i (CleCN-C,) z

The solution of eqn (9) according to the initial condition of

t=O, v=vo, c=c<,

gives the change of reservoir concentration with time

C=Coexp[-(e-N-l)tlr]

(9)

MATERIALS AND METHODS

Materials

Activated carbon granules (Nuchar, Covington, VA, USA) were used as adsorbent. Patulin working solution was prepared to a concentration of 2 mg 1-i in water (pH:4.0) using pure crystalline patulin (Sigma, USA) as described elsewhere (Gok- men & Acar, 1996). The pH of the solution which has vital importance on the stability of patulin was adjusted to the value of 4.0.

Adsorption rate studies

The determination of performance characteristics of the activated carbon packed column were obtained for pat&n. In these continuous experiments, the cylindrical Teflon@ column (length/diameter = l/2) containing 1.5 g of activated carbon was tested in a closed recirculation system, as shown in Fig. 2.

250 ml of aqueous patulin solutions (initial concentration = 2 mg 1-l) was used. The solutions were recirculated through the activated carbon granules by a peri- staltic pump (Watson-Marlow, 302s) at four different flow rates (5, 20, 50, 100 ml min-‘). Those values correspond to the residence times (z) of 50, 12.5, 5 and 2.5 min, respectively. Samples of 0.2 ml were drawn from the reservoir at 5 min intervals for the first 30 min and then at 15 min intervals to follow the decrease in the concentration of the test solute. Patulin concentration in the samples was determined by HPLC using the chromatographic conditions described elsewhere (Gokmen & Acar, 1996). All experiments were carried out at room temperature.

Mass transfer of patulin adsorption ‘63 _ .

Adsorption rates of patulin at different flow rates were obtained by plotting the solute concentration in the reservoir versus time.

RESULTS AND DISCUSSION

Figure 3 illustrates the predicted reservoir patulin concentration time profiles and experimental measurements for various flow rates. An almost perfect fit is obtained between the model output and the experimental measurements. As shown in the figure, the rate of adsorption increases while the flow rate increases. At all flow rates, the adsorption of patulin tends to reach to a very low equilibrium concentra- tion value of less than 0.001 mg ll’. So, the assumption of setting up the model equation (eqn (3)) which is describing the equilibrium concentration of the adsor- bate (patulin) in the effluent is practically zero (C* =0) is valid.

The overall mass transfer coefficients were determined by the above-mentioned matching procedure between the experimental measurements and the model output. Equation (10) describing the change of reservoir concentration with time was rearranged to get a linearized form (Fig. 4) with a slope term given below;

e N -1 slope= -

T

El 5 ml/min

0 20 ml/min

0 50 mllmin

A 100 mllmin

__ Model Fit

0 SO 100

time, min

150

Fig. 3. Comparison of the experimental measurements with the model output for rccircula- tion rates of 5. 20. SO, and 100 ml min ‘.

264 M. Mutlu, K Gtikmen

all flow rates.

at the zero intercept value. tions shown in Fig. 4 were

Slope values obtained using the linear regression equa- solved to calculate the number of transfer units, N, for

The calculated values of the number of transfer units, N, and the effective overall mass transfer coefficients, KC, are given in Table 1. The value (a) which is the external particle surface area per unit volume of particle (3/R%60 cm-‘) is rated by the manufacturer.

Table 1 clearly shows that, the effect of increase in the turbulence through the column positively affects the overall mass transfer coefficient. At the same time, the number of transfer units required to adsorb the same amount of patulin should decrease.

The overall mass transfer coefficient KC basically includes the internal, ki”t, and external, keXt, mass transfer resistances. It is quite obvious that the increase in the turbulence would directly affect the film resistances located around the activated carbon granules and the increase in turbulence diminishes those resistances. How- ever, the transfer of the patulin within the, particle is actually based upon the mechanism of diffusion, where the solute molecules must penetrate further and

0.5

0

-0.5

-1

Q u 2 -1.5

n -L

-2.5

0 10 20 30 40

time, min

Fig. 4. The plot of ln(CIC,) vs. time to estimate the number of transfer units, N.

Mass transfer of patulin adsorption 265

TABLE 1 Change in Overall Effective Mass Transfer Coefficients (K,) with Flow Rate

Flow rate Q (ml min _ ‘)

Number of transfer units, N (dimensionless)

Overall mass transfer coejjticient, &.x1@ (ems-‘)

1.61 4.74 0.49 5.77

50 0.33 9.67 100 0.22 12.96

--

further into the particle to reach the adsorption sites. For this group of experiments, we know that the degree of adsorbate loading was low (Mutlu et al., 1997) so to a good approximation kint can be taken as the constant initial effective internal mass transfer coefficient.

Assuming that the external mass transfer coefficient, I&, but not the effective internal mass transfer coefficient, ki”t, is a function of the fluid turbulence (and therefore also the flow rate) at the adsorbate particle-solution interface, the estima- tion of the internal resistance is possible by extrapolating the flow rate versus overall mass transfer coefficient graph (Fig. 5) to a zero flow rate value (Ozdural, 1994). Under this condition, the phase boundary resistances are entirely dissipated to obtain the resistance offered by the particle alone. Figure 5 shows the overall

least sqllaEfit

y = 0.08% + 4.393 3 = 0.!277

I I I I 0 25 50 75 125

Fig. 5. The technique of estimation of internal mass transfer coefficient, kinl.

266 M. Mutlu, K Giikmen

resistance versus flow rate. The effective internal mass transfer coefficient kint was found to be 4.39 x lo-* cm s-‘.

CONCLUSION

Studies concerning the adsorption of patulin on the activated carbon in batch systems have been reported in our previous paper (Mutlu et al., 1997). In this study, a model is presented that successfully describes the continuous adsorption of patulin on an activated carbon packed column with recycling. The model is simple and it provides an effective tool for determining overall mass transfer coefficients in fixed bed adsorption systems.

The recycled adsorption system described here enables more than 99% of patulin initially present in the feed solution to be removed. The pH adjustment of the solution (pH 4.0) ensures that the reduction in patulin concentration is due only to adsorption on activated carbon. It is very well known that patulin contamination of apple juice is still an important problem for the juice industry. The recycled adsorp- tion process now offers an easy way to overcome this problem. So, future studies should be focused on the design and optimisation of fixed bed adsorption columns for the reduction of patulin in apple juice without any remarkable adverse effect on the astrigenic juice quality.

REFERENCES

Mutlu, M., Htzarctoglu, N. & Gokmen, V. (1997). Patulin adsorption kinetics on activated carbon, activation energy and heat of adsorption. Journal of Food Science, 62(l), 128-130.

Gokmen, V. & Acar, J. (1996). Rapid reversed phase liquid chromatographic determination of patulin in apple juice. Journal of Chromatography A, 730, 53-58.

Mutlu, M. & Piskin, E. (1990). Surface modification of polyurethane biomaterials by plasma glow discharge and investigation of their blood compatibilities by stimulus-response tech- nique and moment analysis. Journal of Medical and Biological Engineering and Computing, 28,232-236.

Petersen, E. E. (1972) Chemical Reaction Analysis. Prentice Hall, Englewood Cliffs, NJ. McCabe, W. L., Smith, J. C. & Herriot, P. (1985) Unit Operations of Chemical Engineering,

4th edn. McGraw-Hill, New York, pp. 699-701. Gzdural, A. R. (1994). Determination of overall mass transfer coefficients in fixed bed

sorption columns. Chemical Engineering and Technology, 17, 285-289.