Using a New Diffusivity Model to Acceleratethe Drying of Biopolymer FilmsWilliam Brennan,1 Lauren Briens,1* Cedric Briens1 and John Zanin2

1. Western Fluidization Group, Faculty of Engineering, The University of Western Ontario, London, Ontario, Canada N6A 5B9

2. Accucaps Industries Limited, Strathroy, Ontario, Canada N7G 3H8

A model was developed to study the drying rate of biopolymer films. The diffusivity of water in this biopolymer film is an exponential function ofthe water concentration. This creates a situation where simply increasing gas velocity can decrease the actual drying rate.

The model revealed that the main factor limiting the drying of the biopolymer film was a significant and rapid decrease in the diffusivity ofwater through the film as the film dried. To avoid this a pulsing scheme was proposed where the velocity of the drying gas would be varied duringthe experiment; this variation allowed for a compromise between a high overall drying rate and maintaining a high diffusivity within the film. Itwas found that the optimum combination of gas velocities was 0.8 and 0.6 m/s with a duration of 10 min at the high and low gas velocities.

On a elabore un modele pour etudier le taux de sechage des pellicules de biopolymere. Le coefficient de diffusion de l’eau dans cette pellicule debiopolymere est une fonction exponentielle de la concentration d’eau. Cela cree une situation ou une simple augmentation de la vitesse des gazpeut diminuer concretement le taux de sechage. Le modele a revele que le facteur principal limitant le sechage de la pellicule de biopolymere etaitune diminution importante et rapide du coefficient de diffusion de l’eau au travers de la pellicule pendant que cette derniere sechait. Pour evitercette situation, on a propose un mecanisme de pulsion selon lequel on varierait la vitesse du gaz deshydratant durant l’experience; cette variationa permis d’etablir un compromis entre un taux de sechage generalement eleve et le maintien d’un coefficient de diffusion eleve a l’interieur de lapellicule On a constate que la combinaison optimale des vitesses des gaz etait de 0,8◦m/s et de 0,6◦m/s avec une duree de 10 minutes a la vitessemaximale et minimale des gaz.

Keywords: drying, mass transfer, mathematical modelling, optimization, pharmaceuticals

INTRODUCTION

Drying biopolymer films is a critical stage in many indus-trial processes. Pharmaceutical, nutraceutical, biomedi-cal, and the food industries all use biopolymer films in

their products. The drying step in these processes can be the slow-est step due to limitations in biopolymer film drying, stemmingfrom temperature sensitivity of the biopolymer film or productscombined with a generally poor knowledge of the drying processof biopolymer films (Perry and Green, 1997).

Knowledge of biopolymer drying is limited. The complex natureof biopolymer films, combined with the difficulties in directlyanalysing the film has restricted research in this important field.There have been attempts at modeling the behaviour of biopoly-mer films. However, most have had limited accuracy and successin application (Bonazzi et al., 1997).

The rate of mass transfer of water from a biopolymer film thatis being dried decreases significantly with time. This is due to thedependence of the diffusivity on the local water content in the

film. It was found that the diffusivity could be modelled as anexponential function of the water content in the film. The modeldeveloped by Briens et al. (2009) accurately predicts the behaviourof a biopolymer film during drying.

The objective of this research was to apply the model describedin Briens et al. (2009) to the optimization of the conditions fordrying biopolymer films.

EXPERIMENTAL EQUIPMENT AND METHODSThe biopolymer film contained 37 wt% water, 21 wt% plasti-cizer, and 42 wt% 200 bloom strength porkskin gelatin. Aftermixing the components, the film was prepared by hot rolling the

∗Author to whom correspondence may be addressed.E-mail address: lbriens@uwo.caCan. J. Chem. Eng. 87:761–765, 2009© 2009 Canadian Society for Chemical EngineeringDOI 10.1002/cjce.20207

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Figure 1. Experimental vial.

mixture to a thickness of 4 mm and then allowing the film to coolfor 40 min. The cooled film was placed on vials (Figure 1) thatprovided constant contact with water on one side and exposure tocontrolled experimental conditions on the other side. The biopoly-mer film seals the vial, ensuring that all water lost from the vialmust diffuse through the film.

A drying chamber was used to achieve precise control of theexperimental conditions such as air velocity, humidity, and tem-perature. For these experiments, the humidity and temperature ofthe drying air were kept constant at 10% relative humidity and19◦C; only the gas velocity was varied. The experimental setupis shown in Figure 2. The superficial gas velocity was controlledwith a pressure regulator and calibrated sonic nozzles. The airentered the closed end of the drying chamber. Wet and dry bulbtemperature measurements were taken during the experiments toverify the air temperature and humidity within the drying cham-ber. The vials were orientated so that the biopolymer film wasdirectly exposed to the oncoming air (Figure 2).

RESULTS AND DISCUSSIONAn earlier study (Briens et al., 2009) showed that the diffusivityof water in the biopolymer film is an exponential function of thelocal moisture content:

(Dab)i = a × exp(ˇ × Ci) (1)

with ˛ = 3 × 10−10 and ˇ = 2.6 × 10−4

The values of ˛ and ˇ were determined empirically and arespecific to the biopolymer film used in this research; the valuesdepend on the composition of the biopolymer film.

As shown in Figure 3, the water content of the biopolymerfilm near the exposed surface decreased during drying. Equation(1) shows that the diffusivity also decreased, which resulted in alower mass transfer rate of water through the film near the sur-face. The overall rate of water transferred through and lost fromthe film therefore gradually dropped, slowing down the drying.Both experimental and modelling studies indicated that there weretwo significant resistances to the mass transfer of water from thevial to the drying air: diffusion through the biopolymer film andmass transfer from the external surface of the biopolymer film tothe bulk of the drying air.

To maintain a high mass transfer rate of water throughout theentire biopolymer film, the exposed surface of the film must notbe allowed to become very dry. A pulsing scheme was proposedto provide a compromise between conditions that promote highexternal transfer from the film to the drying air stream and condi-tions that provide a high rate of mass transfer through the film bymaintaining a high moisture content of the film near the surface.

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Figure 2. Schematic diagram of the drying chamber.

Figure 3. Predicted water content within the biopolymer film at asuperficial gas velocity of 1 m/s.

Figure 4 shows the predicted water concentration profilethrough the biopolymer film during the proposed pulsing scheme.Initially (Figure 4a), the water content was high and uniformthroughout the entire film. However, after a drying phase, thewater concentration decreased throughout the film. Figure 4bshows that the water concentration through the film significantly

changed, “recovering” back towards the initial water concentra-tion during a rest phase.

During the rest phase, the air velocity on the biopolymer filmwas reduced allowing the water concentration to recover andtherefore a higher mass transfer through the biopolymer film wasmaintained. After the rest phase, the pulsing regime resumed witha new drying phase (Figure 4c), followed by another rest phase(Figure 4d). The length of time of a rest phase did not allow thewater concentration to completely recover back to the initial con-ditions. A high rate of mass transfer through the biopolymer filmis important, but the overall objective is still to dry the biopolymerfilm.

The optimum pulsing scheme to promote continuous high masstransfer through and exiting the film is a combination of gas veloc-ities and pulse cycle. Previously, it was found that the optimum,steady superficial gas velocity for drying was ∼0.8 m/s with theair at 10% RH and 19◦C (Briens et al., 2009). This value wasassumed for the “high pulse phase.” Figure 5 shows the pre-dicted optimum gas velocity for the “low pulse phase” when using0.8 m/s for the high pulse phase and equal pulse cycles of 10 min.The predicted optimum gas velocity for the low pulse phase is0.6 m/s.

Ten minutes was considered the shortest duration for the highpulse phase due to practical equipment limitations; through amodified design of a tumble dryer, it would be possible to havealternating zones of high and low drying rates as the biopolymerproducts travel through the dryer. The duration of the short low

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Figure 4. a: Concentration profile at the end of drying pulse 1. b: Concentration profile at the end of rest pulse 1. c: Concentration profile at the end ofdrying pulse 2. d: Concentration profile at the end of rest pulse 2.

Figure 5. Model prediction for optimum low gas velocity with high gasvelocity of 0.8 m/s, 10–10 pulse.

pulse phase must be long enough to allow the water content ofthe outer film layer to be replenished. If, however, this phase istoo long, the overall drying rate decreases. Figure 6 predicts thatthe optimum duration of the low pulse phase should be between∼10 and 15 min.

Figure 6. Model prediction of overall mass loss at 0.8 m/s high and0.6 m/s low gas velocity at multiple relaxation lengths.

Figure 7 compares experimental results to model predictionsfor drying under constant gas flow and drying using a pulsingscheme. Optimum conditions were used for both constant andpulsed gas flow, allowing direct comparison between the perfor-mance under the two drying schemes. The experimental results

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Figure 7. Comparison between optimum constant gas velocity andpulsed gas experimental runs.

closely match the model predictions for both pulsed and constantgas flow. The pulsing scheme improved the amount of overallmass loss from the biopolymer film by over 20% during a 180 minrun at optimum pulsed drying conditions, when compared tooptimum constant drying conditions.

CONCLUSIONSThe diffusivity of water through a biopolymer film is affected bythe water content of the film: as the water content decreases, thediffusivity decreases. The optimum drying conditions use a pulsescheme of 0.8 m/s for 10 min during the “high pulse phase” with0.6 m/s for the 15 min “low pulse phase.” This allows for a com-promise between external and internal mass transfer limitations.Overall, this optimization improved the drying performance byover 20% over the nonpulsed flow. This is a significant change inthe drying performance, using a relatively simple technique. Vari-able drying conditions compensate for the changing properties ofthe biopolymer film as it dries. This type of scheme could alsobe applied to other materials that undergo significant behaviourchanges during drying.

ACKNOWLEDGEMENTSThe authors would like to acknowledge the financial support ofthe Ontario Centres of Excellence and the Natural Sciences andEngineering Research Council of Canada.

NOMENCLATURECi water concentration of slice (i) of the biopolymer film

(mol/m3)Dabi diffusivity of water in biopolymer film in slice (i) (m2/s)˛ coefficient for diffusivity functionˇ coefficient for diffusivity function

REFERENCESBonazzi, C., A. Ripoche and C. Michon, “Moisture Diffusion in

Gelatine Slabs by Modeling Drying Kinetics,” Drying Tech.15(6–8), 2045–2059 (1997).

Briens, L., W. Brennan, C. Briens and J. Zanin, “Modelling MassTransfer Through a Biopolymer film,” (submitted) DryingTech. (2009).

Perry, R. H. and D. W. Green, “Perry’s Chemical Engineers’Handbook,” 7th ed., McGraw-Hill, New York, NY (1997).

Manuscript received February 20, 2008; revised manuscriptreceived February 26, 2009; accepted for publication March 10,2009.

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