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Analysis of the drying process of a biopolymer (poly-hydroxybutyrate) in rotating-pulsed fluidized bed
Chemical Engineering and Processing 50 (2011) 623–629
Contents lists available at ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Analysis of the drying process of a biopolymer (poly-hydroxybutyrate) in rotating-pulsed fluidized bed F.C. Godoi a,∗ , N.R. Pereira b , S.C.S. Rocha a a b
Dept of Thermofluidynamics (DTF), School of Chemical Eng. (FEQ), State University of Campinas, Avenue Albert Einstein 500, 13083-852 Campinas, São Paulo, Brazil LTA/CCTA/UENF, R. Alberto Lamego, 2000, CEP 28013-600, Campos dos Goytacazes, RJ, Brazil
a r t i c l e
i n f o
Article history: Received 6 July 2010 Received in revised form 14 March 2011 Accepted 26 March 2011 Available online 1 April 2011 Keywords: PHB Rotating-pulsed fluidized bed Drying kinetics Fluid dynamic behavior
a b s t r a c t A rotating-pulsed fluidized bed (RPFB) dryer was employed to conduct the drying of polyhydroxybutyrate (PHB) cohesive granules. Along the experiments, it was possible to identify, visually, 3 different dynamic regimes that were related with the temperature profile, the drying kinetics and the fluid dynamic behavior. The drying kinetics of PHB showed a short constant drying rate period followed by a decreasing drying rate period. The constant drying rate (Nc ) and final moisture content (dry basis) were related to the rotation frequency (responsible for the pulsation effect), temperature and velocity of the inlet air. Furthermore, measurements of molecular mass (gel permeation chromatography analysis) and Carr Index (flowability test) on PHB samples were done before and after the drying. The RPFB dryer showed to be appropriate to dry the PHB granules, resulting in an excellent fluid dynamic behavior that provided uniform drying of the solid. The best conditions of drying were identified at 7 Hz of rotation frequency, 90 ◦ C and 0.55 m/s of inlet air temperature and velocity. At these conditions the dried PHB reached final moisture content of 0.56% (wet basis) after 2 h of drying. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The environmental impact caused by the indiscriminate use of petroleum-based plastics has reached extremely high levels. One way to reduce the amount of plastic residue is the use of biodegradable materials. Biodegradable polymers are obtained from renewable sources and contribute to the geochemical cycle. Generally, biodegradable polymers have a degradation time in the range of six to twelve months against forty to fifty years of synthetic polymers [1,2]. Poly-hydroxybutyrate (PHB) is a biodegradable thermoplastic polyester that can be considered analogous to many conventional petroleum-derived plastics currently in use. Furthermore, it has some additional advantages such as being biocompatible and can be produced from a renewable raw material in a sustainable technology from economical to ecological point of view [2,3]. In Brazil, PHB is produced by bacterial fermentation of the sugar cane in a process integrated to sugar and ethanol production. Therefore, it can be considered a green polymeric material. The process to obtain PHB granules consists of fermentation, crystallization, solvent extraction and drying [4]. After extraction, the granules of PHB present water in excess (around 30%, on wet basis), which results
∗ Corresponding author. Tel.: +55 19 3521 3882. E-mail address: [email protected] (F.C. Godoi). 0255-2701/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2011.03.005
in cohesiveness of the material [5]. The drying is an important step of the PHB production because an adequate post-processing is achieved only at moisture contents around 0.50% (on wet basis). Even though, few studies have been done about the drying of PHB [5,6]. The cohesive nature of wet PHB hampers its fluidization in the conventional system. Some works can be found that have employed the air pulsation to improve fluidization of cohesive particles [7–12]. In the rotating-pulsed fluidized bed the air pulsation is caused by the rotation of a flat holed disk installed bellow a multiorifice gas distributor. When the rotating disk frequency is fixed, one or more spouts are formed – depending on the number of holes – and the process behaves as a spouted bed. When the rotation frequency of the disk is low, the spouts rotate and regions of fixed bed exist between them. At high rotation velocity of the disk, the total system is fluidized as in a conventional fluidized bed [13]. The aim of this work was to study the drying of PHB in a RPFB dryer. The drying experiments were performed for three different rotation frequencies of the disk (responsible for the pulsation effect), inlet air temperatures and velocities. During the drying tests, the air temperature profiles at the inlet, outlet and inside the dryer, the fluid dynamic behavior and the particle moisture content were evaluated. Furthermore, measurements of the molecular mass (gel permeation chromatography analysis) and the Carr Index on PHB samples before and after the drying were done.
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Table 1 Properties of the dried PHB particles. dp (m)
s (g/cm3 )
800
1.24
2. Materials and methods 2.1. Poly-hydroxybutyrate (PHB) PHB (moisture content = 30–32%, on wet basis) was provided by Biocycle® (Serrana, Brazil). In Table 1, the solids properties that were important for this work are shown. In spite of belonging to Group B, according to Geldart’s classification for dry powders [14], preliminary fluidization tests showed that this solid presents slugging (piston shape) in conventional fluidized bed (CFB) due to its cohesiveness when wet. Therefore, the rotating-pulsed fluidized bed dryer was chosen as alternative to perform the experiments. Fig. 2. Rotating disk installed below the drying chamber.
2.2. The rotating-pulsed fluidized bed dryer (RPFB dryer) Fig. 1 illustrates the experimental set-up for the drying experiments that consists basically of the drying equipment, pressure meters and blower. One holed disk installed below the distributor plate of the drying chamber improves the distribution of the air (Fig. 2). The gas flows through an opening that causes the pulsation effect when the disk rotates. The frequency of rotation, f (Hz), is given by the rotational velocity of the disk. One electrical device (12) is responsible for rotating the disk. The control of the rotating velocity of the disk was done by a frequency inverter. The pressure meters are compounded by the manometers 9, 10 and 11 (Fig. 1). They were used to determine the bed pressure drop (P, cm of H2 O), the pressure drop of the orifice plate (P1 , cm of H2 O) and the static pressure before the orifice plate (Pestat,1 , cm of Hg). With the last two measurements, it was possible to calculate the air velocity (Vair ) at the entrance of the bed [15]. For the drying experiments, 600 g of wet PHB (at ambient temperature) were fed through the top of the drying chamber. According to Fig. 1, the air was supplied by a blower (6) and was heated by an electrical heater (2). A silica gel bed (5) was installed at the pipeline in order to guarantee low humidity of the inlet air. A cyclone (4) was installed at the bed outlet to collect the fines elutriated from the dryer column. All the experiments were conducted in batch regime.
Table 2 Experimental design matrix. Tair (◦ C)
f (Hz)
Vair (m/s)
−1 −1 −1 −1 1 1 1 1 0 0 0
−1 −1 1 1 −1 −1 1 1 0 0 0
−1 1 −1 1 −1 1 −1 1 0 0 0
2.3. Drying experiments The PHB drying process was analyzed through the combinations of experimental conditions shown in Table 2. That makes a 2-level factorial design [16] with 3 repetitions at the center point. Table 3 shows the real values of the independent variables. The levels of the independent variables were fixed according to preliminary fluid dynamics and drying tests [17]. Each run of the experimental design was carried out continuously during 2 h. Along the experiments, visual observations were done and related with the fluid dynamic behavior of the particles, the temperature profile and the drying kinetics. It was possible to obtain characteristics of the fluid dynamic behavior because the drying chamber was made from a transparent material (acrylic). Notes about the particles behavior inside the bed were taken every time that a sample was getting out for moisture measurement. Details of how the visual observations were done will be made clear in Section 3.1. The fluid dynamics during the drying tests was evaluated by measurements of the bed pressure drop (P = |P2 − P1 |) versus the drying time. The temperature profiles were obtained by meaTable 3 Coded and real values of the independent variables.
Fig. 1. Experimental set-up for the drying experiments.
Independent variables
Lower level (−1)
Center point (0)
Upper level (+1)
Tair (◦ C) f (Hz) Vair (m/s)
70 7 0.40
80 10 0.55
90 13 0.70
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work was 40 times. The Carr Index is calculated from Eq. (1), where b is the powder bulk density, and t is the powder tapped density [23]. These tests were repeated three times for each sample.
CI = 100 × 1 −
b t
(1)
3. Results and discussion 3.1. Drying experiments Visual observations during the drying experiments enabled the identification of three different regimes, as described below and illustrated in Fig. 4:
Fig. 3. Thermocouples and pressure meters positions.
surements of the inlet (T1 ), the bulk (T2 ) and the outlet (T3 ) air temperatures every 5 min. Fig. 3 illustrates on detail the thermocouples and pressure meters positions. For the drying kinetics determination ((X − Xeq )/(X0 − Xeq ) as function of time), samples of approximately 3.0 g of PHB were taken for moisture measurements during the drying tests. Moisture contents were determined by the static gravimetric method, in an oven at atmospheric pressure and 105 ◦ C for 24 h. The equilibrium moisture content (Xeq ) was determined from the drying curve as the dynamic equilibrium moisture content, when the drying rate was practically null at each drying condition.
- Regime 1 – observed at the beginning of the drying process, with formation of channels, dead zones and elutriation of fine particles. For the experiments performed at 90 ◦ C and Vair = 0.70 m/s this regime could be almost ignored. - Regime 2 – formation of small agglomerates of particles with difficult fluidization. High rate of elutriation (until approximately 10 min) and big explosions of air bubbles at the bed surface. This regime occurred from 10 to 25 min of drying (depending on the drying conditions). - Regime 3 – Vigorous fluidization with excellent mixing inside the bed. Channeling and explosions of big bubbles were not identified. This behavior stayed stable until the end of the tests. 3.2. Temperature profiles during the drying tests The inlet air temperature (T1 ) stayed constant with stable plateaus of 70, 80 and 90 ± 1 ◦ C along the 2 h of experiment. Figs. 5 and 6 illustrate temperature profiles inside the bed of particles, which was named bulk temperature (T2 ) and of the outlet air (T3 ), measured at the region just above the particles bed (for the exact positions of these measurements refer to Fig. 3). It could be observed that the initial lower temperatures indicate that the heat supply is limiting the drying rate. Also, it can be seen that the bulk and outlet air temperatures increased from the beginning of
2.4. Characterization of PHB before and after the drying In order to evaluate the effects caused by the drying conditions on the material characteristics, measurements of the molar mass and Carr Index before and after drying were performed. 2.5. Gel permeation chromatography (GPC) The molar mass distribution is an important characteristic of a polymer that affects the mechanical properties and thermoplastic behavior of the material at high temperatures. In order to evaluate damages caused by drying, measurements of PHB molar mass before and after drying were done [18]. The molar mass of the samples was determined at 30 ◦ C using a GPC system (model 410, Waters Corp., Milford, MA, USA), fitted with Styragel HR column (Waters Corp., Milford, MA, USA). Samples were dissolved in chloroform at a 0.2% concentration, the injection volume was 200 L and the flow rate was 1 mL/min. Monodisperse polystyrene standards were used in the calibration curve determination. 2.6. Carr Index The Carr Index is frequently used as an indication of the flowability of a powder [19–22]. In the flowability test the powder is gently loaded through a funnel into a 250 mL cylinder and weighed to calculate its bulk density. Next, the cylinder is tapped on a vibrated platform until the volume stops changing, which in the case of this
Fig. 4. Visual observations during the drying experiments: (a) Regime 1, (b) Regime 2 and (c) Regime 3.
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Fig. 5. Temperature profiles of the bulk air. Numbers in the figure correspond to the dynamic regimes.
Fig. 7. Temperature profiles of the inlet and outlet air during the drying. Center point of the factorial design (0,0,0).
Fig. 8. Pressure drop versus drying time at 70 ◦ C. Fig. 6. Temperature profiles of the outlet air. Numbers in the figure correspond to the dynamic regimes.
the process followed by a stable plateau after an interval from 20 to 40 min of drying, depending on the conditions of drying air temperature and velocity. What means that the temperature reached steady state after those times (from 20 to 40 min), when the surface water had already been evaporated and the drying behavior was characterized by Regime 3. Fig. 7 also illustrates that the inlet air temperature control was effective. 3.3. Fluid-dynamic behavior during the drying tests Curves of bed pressure drop versus drying time obtained for all experiments, excepted for (0,0,0), are shown in Figs. 8 and 9. The bed pressure drops become constant for drying times above 20 min. This finding indicates that the stability is reached after 20 min, observed in Figs. 5 and 6. Figs. 8 and 9 show that the air velocity presented the highest effect on the bed pressure drop, followed by the frequency of pulsation. The air temperature did not show any effect on particles fluid dynamic behavior. It can also be seen that the influence of
Fig. 9. Pressure drop versus drying time at 90 ◦ C.
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Fig. 10. Influence of inlet air temperature on the drying kinetic curves.
Fig. 12. Influence of inlet air velocity on the drying kinetic curves.
the frequency of rotation gains importance when high air velocity (0.70 m/s) is employed. At this air velocity, the pressure drop is reduced with increasing f. Based on these results, it may be suggested that at higher frequencies the bed has no time to settle (due to the reduction of channeling and bubbling), and the particles can easily flow inside the bed.
decreasing drying rate period predominated on the PHB kinetics of drying (Fig. 13). The confidence of the drying kinetics data can be demonstrated by the curves plotted for the three tests conducted at the center point of the experimental design (Fig. 13). Reproducibility of the data is evident. Table 4 shows the measurements of the final moisture contents of the dried PHB granules. It is possible to verify that the PHB drying in the RPFB was efficient resulting in low values of the final moisture content. The moisture content specified for adequate post processing of PHB granules is 0.5%. Experiments (+1,−1,−1) and (+1,+1,+1) of the experimental design reached final moisture contents of 0.56 and 0.55 after 2 h of drying, respectively, which are very close to the recommended value.
3.4. Influences of Tair , f and Vair on the drying kinetics The experimental design described in Table 2 enabled to identify the effects of the independent variables (Tair , f and Vair ) on the drying kinetics, which are shown in Figs. 10–12 for the experiments (−1,−1,+1), (+1,−1,+1) and (+1,+1,+1), respectively. Overall drying kinetics curves obtained for the PHB at different drying conditions (data not shown) presented the same aspects, in terms of process variables influence. As expected, the air temperature and air velocity influenced positively the process. The frequency of rotation did not show observable influence on the drying kinetics for the range tested. It is seen in the drying curves that a constant drying rate appears at the beginning (which explains the PHB cohesiveness) and the
Fig. 11. Influence of rotating disk frequency on the drying kinetic curves.
3.5. Factorial design Even though decreasing drying rate period have been predominated on the PHB kinetics of drying, the coefficient of constant drying rate period Nc was chosen as a dependent variable in the fac-
Fig. 13. Drying kinetics of the PHB in RPFB—center point of the factorial design (0,0,0).
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Table 4 Final moisture content of PHB granules (wet basis). Run
Final moisture [%]
(−1,−1,−1) (−1,−1,+1) (−1,+1,−1) (−1,+1,+1) (+1,−1,−1) (+1,−1,+1) (+1,+1,−1) (+1,+1,+1) (0,0,0) (0,0,0) (0,0,0)
1.22 0.95 1.16 0.93 0.56 0.69 0.64 0.55 0.74 0.73 0.74
Fig. 15. Pareto chart for Xf /X0 (significant at 95.0% level).
Fig. 14. Pareto chart for Nc (significant at 95.0% level).
torial design of this work. The reason of this choice of response was to evaluate how the process variables affect the drying when a large amount of surface water exists and affects the dynamic regime. The other dependent variable chosen was the final moisture content Xf /X0 after 2 h of drying. The statistical results presented in Table 5 and Figs. 14 and 15 show that the temperature (p-value = 0.004) and the velocity (pvalue = 0.02) of the drying air affect positively the constant drying rate period coefficient (Nc ) at a confidence level of 95%. The short constant drying rate period is more affected by temperature than by velocity of the inlet air, Fig. 14. This is an expected result when drying is controlled by external conditions and it can be explained by the fact that surface water is abundant at the beginning of the process. Consequently, at higher temperatures this water evapo-
rates quickly. In a lesser extent, air velocity also accelerates this process by increasing the convection near the surface. As can be seen in Figs. 14 and 15 the rotation frequency does not present a significant effect on the constant drying rate period. This phenomenon can be explained by the fact that for the whole range of pulsation frequency used (7–13 Hz), the particles inside the bed have a behavior similar to that of a conventional fluidized bed. It means that the RPFB dryer was efficient in providing good mixing of the particles inside the bed and only the external conditions of the drying air exerted significant influence on the constant drying rate. For the dependent variable Xf /X0 , the pareto chart shows that only the air temperature (p-value = 0.03) is significant. With the increase of the air temperature, the final moisture content decreased, Fig. 15. As shown before in the drying kinetics analysis, at the end of the process, all surface moisture had been evaporated and only internal moisture on the structure of the material is still present. In this case, only the temperature is influential for diffusion and evaporation of the internal water. According to the results previously discussed it is possible to affirm that the best condition applied for the PHB drying was at the one denominated by (+1,−1,−1), having higher air temperature and lower air velocity and air pulsation. Although the frequency of pulsation is not statistically significant for the responses analyzed, its lowest value was chosen as it results in lower energy consumption. At operating condition (+1,−1,−1), the final moisture content was close to the recommended (0.50%) and statistically equal to the value reached by the extreme condition (+1,+1,+1), see Table 4. 3.6. Molar mass distribution
Table 5 Nc and Xf /X0 for each drying experiment. Run
Nc [min−1 ]
Xf /X0 [kg/kg]
(−1,−1,−1) (−1,−1,+1) (−1,+1,−1) (−1,+1,+1) (+1,−1,−1) (+1,−1,+1) (+1,+1,−1) (+1,+1,+1) (0,0,0) (0,0,0) (0,0,0)
0.020 0.028 0.018 0.025 0.030 0.043 0.035 0.044 0.029 0.032 0.037
0.026 0.022 0.024 0.021 0.012 0.015 0.014 0.011 0.014 0.016 0.016
GPC analyses were done for samples of wet PHB and PHB dried at the following conditions of the factorial design: (−1,−1,−1), (+1,−1,−1) and (+1,+1,+1). These conditions express the less severe, the optimum and the more severe applied in the drying tests. Table 6 shows the weight-average molecular mass (Mw ) and polydispersity of the analyzed samples. As can be seen, a considerable difference on the molecular weight distribution did not occur after the drying in RPFB dryer. So, it is possible to affirm that the operating conditions applied are secure to maintain these PHB characteristics and as a consequence, no damages were caused on the mechanical properties of the PHB due to the drying process.
F.C. Godoi et al. / Chemical Engineering and Processing 50 (2011) 623–629 Table 6 Weight-average molar mass (Mw ) and polydispersity of the PHB samples by GPC. Samples
Mw [Da]
Polydispersity
Wet PHB Dried at (−1,−1,−1) Dried at (+1,−1,−1) Dried at (+1,+1,+1)
690 685 691 631
2.4 2.2 2.4 2.5
Table 7 Carr Index (CI) for wet, dried at condition (+1,−1,−1) and lyophilized PHB. Samples
b [g/cm3 ]
t [g/cm3 ]
CI [%]
Wet PHB Dried at (+1,−1,−1) Lyophilized PHB
0.49 0.49 0.46
0.71 0.52 0.48
31 5 4
r2 Tair Vair X Xeq X0 Wair
629
correlation coefficient air temperature [◦ C] air velocity [m/s] average moisture [kg water/kg dry solid] equilibrium moisture content [kg water/kg dry solid] initial moisture content [kg water/kg dry solid] air flow rate [kg/min]
Greek letters P1 pressure drop of the orifice plate [cm H2 O] s solid density [g/cm3 ] powder bulk density [g/cm3 ] b t powder tapped density [g/cm3 ] References
3.7. Flowability evaluation Table 7 demonstrates that the wet PHB can be classified as cohesive material, with CI > 28% [23]. Cohesiveness results from the high moisture content that forms water bridges between the contact points of the solid particles. This result corroborates with the fluid dynamic behavior of wet PHB in a conventional fluidized bed (CFB), where poor fluidization with formation of slugs is reported. In order to stress that PHB drying in RPFB gives a free-flowing material, the flowability was compared to lyophilized PHB particles. For the PHB dried in RPFB dryer or lyophilized, the CI fell abruptly resulting in a powder classified as a free-flowing material (see Table 7). 4. Conclusions The RPFB dryer was adequate to dry the cohesive particles of PHB, resulting in a stable fluid dynamics of the bed and providing uniform drying of the solid. The final moisture content obtained at 90 ◦ C, 7 Hz and 0.55 m/s was very close to the recommended value of the 0.50%. The rotation frequency exerted considerable influence on the fluid dynamic behavior only at high air velocity of 0.70 m/s. The drying kinetics of PHB showed a short constant drying rate period followed by a decreasing drying rate period. Moreover, on the range tested, the Nc was affected by temperature and velocity of the inlet air and was not affected by the rotation frequency. The final moisture content was affected mainly by the air temperature. The drying conditions did not modify significantly the molar mass of the dried PHB, which demonstrates that the quality of the material was maintained. Furthermore, after drying, the PHB particles flowability changed from cohesive to a free-flowing material. Acknowledgements We acknowledge the financial support of FAPESP and CAPES for this work. We also thank the support of the School of Chemical Engineering of the University of Campinas – UNICAMP, and BYOCICLE® by the samples of PHB supplied. Appendix A. Nomenclature
CI dp f Nc Pestat,l
Carr Index [%] Sauter diameter [m] rotation frequency [Hz] coefficient of constant drying rate period [min−1 ] static pressure before the orifice [mmHg]
[1] A.K. Mohanty, M. Misra, G. Hinrichsen, Biofibres, biodegradable polymers and biocomposites: an overview, Macromolecular Materials and Engineering 276 (3–4) (2000) 1–24. [2] J. Asrar, J.C. Hill, Biosynthetic processes for linear polymers, Journal of Applied Polymer Science 83 (3) (2002) 457–483. [3] P.A. Holmes, Applications of PHB—a microbially produced biodegradable thermoplastic, Physics in Technology 16 (1) (1985) 32–36. [4] Manelatto PE, Nazareno DS, inventors, PHB Industrial SA, assignee. Process for extracting and recovering polyhydroxyalkanoates (PHAS) from cellular biomass, Brazil; 2008. [5] F.C. Godoi, E.A.F.S. Boin, N.R. Pereira, S.C.S. Rocha, O.P. Taranto, Fluid dynamic and drying studies of poly-hydroxybutyrate (PHB) in rotating-pulsed fluidized bed, in: Proceedings of the 4th inter-American drying conference: challenges for efficient drying processes, Montreal, Canada, 2009. [6] O. Lima, Drying study of poly-hydroxybutyrate in fluidized bed, COPPE/UFRJ, Rio de Janeiro, 2004. [7] M.C.B. Ambrosio-Ugri, O.P. Taranto, Drying in the rotating-pulsed fluidized bed, Brazilian Journal of Chemical Engineering 24 (2007) 95–100. [8] X.S. Wang, M.J. Rhodes, Pulsed fluidization—a DEM study of a fascinating phenomenon, Powder Technology 159 (3) (2005) 142–149. [9] L.F.G. Souza, M. Nitz, P.S. Cancella, O.P. Taranto, Drying of sodium acetate in a pulsed-fluid bed dryer—influences of temperature and pulsation frequency on the drying rate, in: Proceedings of the 16th International Drying Symposium (IDS 2008), Hyderabad, India, 2008. [10] O.P. Taranto, M.W. Silva, Experimental study of microcrystalline cellulose coating process in rotative-pulsed fluidized bed, in: Proceedings of the 15th International Drying Symposium (IDS 2006), Budapest, Hungry, 2006, pp. 672–677. [11] V.R. Elenkov, T.G. Djurkov, Rotating-pulsed fluidized bed dryer for highmoisture content bioproducts, in: Proceedings of the 8th International Drying Symposium (IDS 1992), 1992, p. 1636–1641. [12] M. Nitz, O.P. Taranto, Film coating of theophylline pellets in a pulsed fluid bed coater, Chemical Engineering and Processing: Process Intensification 47 (8) (2008) 1412–1419. [13] Z. Gawrzynski, R. Glaser, J. Zgorzalewicz, Drying of granular material in pulsofluidized bed, Hungarian Journal of Industrial Chemistry 17 (1989) 245–255. [14] T. Geldart (Ed.), Gas fluidization technology, John Wiley & Sons, Chichester, 1986. [15] E. Ower, R.C. Pankhurst, The measurement of air, Pergamon Press, New York, 1977. [16] G.E.P. Box, J.H. Hunter, Statistics for experimenters—an introduction to design data analysis and model building, John Wiley & Sons ed., New York, 1978. [17] F.C. Godoi, Fluid-dynamics and drying of poly-hydroxybutyrate (PHB) in rotating-pulsed fluidized bed, State University of Campinas, Campinas, 2009. [18] M. Chanda, Advanced polymer chemistry: a problem solving guide, Marcel Dekker, New York, 2000. [19] A.P. Tinke, R. Govoreanu, I. Weuts, K. Vanhoutte, D. De Smaele, A review of underlying fundamentals in a wet dispersion size analysis of powders, Powder Technology 196 (2) (2009) 102–114. [20] N. Harnby, A.E. Hawkins, D. Vandame, The use of bulk density determination as a means of typifying the flow characteristics of loosely compacted powders under conditions of variable relative humidity, Chemical Engineering Science 42 (4) (1987) 879–888. [21] E. Emery, J. Oliver, T. Pugsley, J. Sharma, J. Zhou, Flowability of moist pharmaceutical powders, Powder Technology 189 (3) (2009) 409–415. [22] K. Thalberg, D. Lindholm, A. Axelsson, Comparison of different flowability tests for powders for inhalation, Powder Technology 146 (3) (2004) 206–213. [23] R. Carr, Evaluating flow of solids, Chemical Engineering 72 (1965) 163.
Contents lists available at ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Analysis of the drying process of a biopolymer (poly-hydroxybutyrate) in rotating-pulsed fluidized bed F.C. Godoi a,∗ , N.R. Pereira b , S.C.S. Rocha a a b
Dept of Thermofluidynamics (DTF), School of Chemical Eng. (FEQ), State University of Campinas, Avenue Albert Einstein 500, 13083-852 Campinas, São Paulo, Brazil LTA/CCTA/UENF, R. Alberto Lamego, 2000, CEP 28013-600, Campos dos Goytacazes, RJ, Brazil
a r t i c l e
i n f o
Article history: Received 6 July 2010 Received in revised form 14 March 2011 Accepted 26 March 2011 Available online 1 April 2011 Keywords: PHB Rotating-pulsed fluidized bed Drying kinetics Fluid dynamic behavior
a b s t r a c t A rotating-pulsed fluidized bed (RPFB) dryer was employed to conduct the drying of polyhydroxybutyrate (PHB) cohesive granules. Along the experiments, it was possible to identify, visually, 3 different dynamic regimes that were related with the temperature profile, the drying kinetics and the fluid dynamic behavior. The drying kinetics of PHB showed a short constant drying rate period followed by a decreasing drying rate period. The constant drying rate (Nc ) and final moisture content (dry basis) were related to the rotation frequency (responsible for the pulsation effect), temperature and velocity of the inlet air. Furthermore, measurements of molecular mass (gel permeation chromatography analysis) and Carr Index (flowability test) on PHB samples were done before and after the drying. The RPFB dryer showed to be appropriate to dry the PHB granules, resulting in an excellent fluid dynamic behavior that provided uniform drying of the solid. The best conditions of drying were identified at 7 Hz of rotation frequency, 90 ◦ C and 0.55 m/s of inlet air temperature and velocity. At these conditions the dried PHB reached final moisture content of 0.56% (wet basis) after 2 h of drying. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The environmental impact caused by the indiscriminate use of petroleum-based plastics has reached extremely high levels. One way to reduce the amount of plastic residue is the use of biodegradable materials. Biodegradable polymers are obtained from renewable sources and contribute to the geochemical cycle. Generally, biodegradable polymers have a degradation time in the range of six to twelve months against forty to fifty years of synthetic polymers [1,2]. Poly-hydroxybutyrate (PHB) is a biodegradable thermoplastic polyester that can be considered analogous to many conventional petroleum-derived plastics currently in use. Furthermore, it has some additional advantages such as being biocompatible and can be produced from a renewable raw material in a sustainable technology from economical to ecological point of view [2,3]. In Brazil, PHB is produced by bacterial fermentation of the sugar cane in a process integrated to sugar and ethanol production. Therefore, it can be considered a green polymeric material. The process to obtain PHB granules consists of fermentation, crystallization, solvent extraction and drying [4]. After extraction, the granules of PHB present water in excess (around 30%, on wet basis), which results
∗ Corresponding author. Tel.: +55 19 3521 3882. E-mail address: [email protected] (F.C. Godoi). 0255-2701/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2011.03.005
in cohesiveness of the material [5]. The drying is an important step of the PHB production because an adequate post-processing is achieved only at moisture contents around 0.50% (on wet basis). Even though, few studies have been done about the drying of PHB [5,6]. The cohesive nature of wet PHB hampers its fluidization in the conventional system. Some works can be found that have employed the air pulsation to improve fluidization of cohesive particles [7–12]. In the rotating-pulsed fluidized bed the air pulsation is caused by the rotation of a flat holed disk installed bellow a multiorifice gas distributor. When the rotating disk frequency is fixed, one or more spouts are formed – depending on the number of holes – and the process behaves as a spouted bed. When the rotation frequency of the disk is low, the spouts rotate and regions of fixed bed exist between them. At high rotation velocity of the disk, the total system is fluidized as in a conventional fluidized bed [13]. The aim of this work was to study the drying of PHB in a RPFB dryer. The drying experiments were performed for three different rotation frequencies of the disk (responsible for the pulsation effect), inlet air temperatures and velocities. During the drying tests, the air temperature profiles at the inlet, outlet and inside the dryer, the fluid dynamic behavior and the particle moisture content were evaluated. Furthermore, measurements of the molecular mass (gel permeation chromatography analysis) and the Carr Index on PHB samples before and after the drying were done.
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Table 1 Properties of the dried PHB particles. dp (m)
s (g/cm3 )
800
1.24
2. Materials and methods 2.1. Poly-hydroxybutyrate (PHB) PHB (moisture content = 30–32%, on wet basis) was provided by Biocycle® (Serrana, Brazil). In Table 1, the solids properties that were important for this work are shown. In spite of belonging to Group B, according to Geldart’s classification for dry powders [14], preliminary fluidization tests showed that this solid presents slugging (piston shape) in conventional fluidized bed (CFB) due to its cohesiveness when wet. Therefore, the rotating-pulsed fluidized bed dryer was chosen as alternative to perform the experiments. Fig. 2. Rotating disk installed below the drying chamber.
2.2. The rotating-pulsed fluidized bed dryer (RPFB dryer) Fig. 1 illustrates the experimental set-up for the drying experiments that consists basically of the drying equipment, pressure meters and blower. One holed disk installed below the distributor plate of the drying chamber improves the distribution of the air (Fig. 2). The gas flows through an opening that causes the pulsation effect when the disk rotates. The frequency of rotation, f (Hz), is given by the rotational velocity of the disk. One electrical device (12) is responsible for rotating the disk. The control of the rotating velocity of the disk was done by a frequency inverter. The pressure meters are compounded by the manometers 9, 10 and 11 (Fig. 1). They were used to determine the bed pressure drop (P, cm of H2 O), the pressure drop of the orifice plate (P1 , cm of H2 O) and the static pressure before the orifice plate (Pestat,1 , cm of Hg). With the last two measurements, it was possible to calculate the air velocity (Vair ) at the entrance of the bed [15]. For the drying experiments, 600 g of wet PHB (at ambient temperature) were fed through the top of the drying chamber. According to Fig. 1, the air was supplied by a blower (6) and was heated by an electrical heater (2). A silica gel bed (5) was installed at the pipeline in order to guarantee low humidity of the inlet air. A cyclone (4) was installed at the bed outlet to collect the fines elutriated from the dryer column. All the experiments were conducted in batch regime.
Table 2 Experimental design matrix. Tair (◦ C)
f (Hz)
Vair (m/s)
−1 −1 −1 −1 1 1 1 1 0 0 0
−1 −1 1 1 −1 −1 1 1 0 0 0
−1 1 −1 1 −1 1 −1 1 0 0 0
2.3. Drying experiments The PHB drying process was analyzed through the combinations of experimental conditions shown in Table 2. That makes a 2-level factorial design [16] with 3 repetitions at the center point. Table 3 shows the real values of the independent variables. The levels of the independent variables were fixed according to preliminary fluid dynamics and drying tests [17]. Each run of the experimental design was carried out continuously during 2 h. Along the experiments, visual observations were done and related with the fluid dynamic behavior of the particles, the temperature profile and the drying kinetics. It was possible to obtain characteristics of the fluid dynamic behavior because the drying chamber was made from a transparent material (acrylic). Notes about the particles behavior inside the bed were taken every time that a sample was getting out for moisture measurement. Details of how the visual observations were done will be made clear in Section 3.1. The fluid dynamics during the drying tests was evaluated by measurements of the bed pressure drop (P = |P2 − P1 |) versus the drying time. The temperature profiles were obtained by meaTable 3 Coded and real values of the independent variables.
Fig. 1. Experimental set-up for the drying experiments.
Independent variables
Lower level (−1)
Center point (0)
Upper level (+1)
Tair (◦ C) f (Hz) Vair (m/s)
70 7 0.40
80 10 0.55
90 13 0.70
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work was 40 times. The Carr Index is calculated from Eq. (1), where b is the powder bulk density, and t is the powder tapped density [23]. These tests were repeated three times for each sample.
CI = 100 × 1 −
b t
(1)
3. Results and discussion 3.1. Drying experiments Visual observations during the drying experiments enabled the identification of three different regimes, as described below and illustrated in Fig. 4:
Fig. 3. Thermocouples and pressure meters positions.
surements of the inlet (T1 ), the bulk (T2 ) and the outlet (T3 ) air temperatures every 5 min. Fig. 3 illustrates on detail the thermocouples and pressure meters positions. For the drying kinetics determination ((X − Xeq )/(X0 − Xeq ) as function of time), samples of approximately 3.0 g of PHB were taken for moisture measurements during the drying tests. Moisture contents were determined by the static gravimetric method, in an oven at atmospheric pressure and 105 ◦ C for 24 h. The equilibrium moisture content (Xeq ) was determined from the drying curve as the dynamic equilibrium moisture content, when the drying rate was practically null at each drying condition.
- Regime 1 – observed at the beginning of the drying process, with formation of channels, dead zones and elutriation of fine particles. For the experiments performed at 90 ◦ C and Vair = 0.70 m/s this regime could be almost ignored. - Regime 2 – formation of small agglomerates of particles with difficult fluidization. High rate of elutriation (until approximately 10 min) and big explosions of air bubbles at the bed surface. This regime occurred from 10 to 25 min of drying (depending on the drying conditions). - Regime 3 – Vigorous fluidization with excellent mixing inside the bed. Channeling and explosions of big bubbles were not identified. This behavior stayed stable until the end of the tests. 3.2. Temperature profiles during the drying tests The inlet air temperature (T1 ) stayed constant with stable plateaus of 70, 80 and 90 ± 1 ◦ C along the 2 h of experiment. Figs. 5 and 6 illustrate temperature profiles inside the bed of particles, which was named bulk temperature (T2 ) and of the outlet air (T3 ), measured at the region just above the particles bed (for the exact positions of these measurements refer to Fig. 3). It could be observed that the initial lower temperatures indicate that the heat supply is limiting the drying rate. Also, it can be seen that the bulk and outlet air temperatures increased from the beginning of
2.4. Characterization of PHB before and after the drying In order to evaluate the effects caused by the drying conditions on the material characteristics, measurements of the molar mass and Carr Index before and after drying were performed. 2.5. Gel permeation chromatography (GPC) The molar mass distribution is an important characteristic of a polymer that affects the mechanical properties and thermoplastic behavior of the material at high temperatures. In order to evaluate damages caused by drying, measurements of PHB molar mass before and after drying were done [18]. The molar mass of the samples was determined at 30 ◦ C using a GPC system (model 410, Waters Corp., Milford, MA, USA), fitted with Styragel HR column (Waters Corp., Milford, MA, USA). Samples were dissolved in chloroform at a 0.2% concentration, the injection volume was 200 L and the flow rate was 1 mL/min. Monodisperse polystyrene standards were used in the calibration curve determination. 2.6. Carr Index The Carr Index is frequently used as an indication of the flowability of a powder [19–22]. In the flowability test the powder is gently loaded through a funnel into a 250 mL cylinder and weighed to calculate its bulk density. Next, the cylinder is tapped on a vibrated platform until the volume stops changing, which in the case of this
Fig. 4. Visual observations during the drying experiments: (a) Regime 1, (b) Regime 2 and (c) Regime 3.
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Fig. 5. Temperature profiles of the bulk air. Numbers in the figure correspond to the dynamic regimes.
Fig. 7. Temperature profiles of the inlet and outlet air during the drying. Center point of the factorial design (0,0,0).
Fig. 8. Pressure drop versus drying time at 70 ◦ C. Fig. 6. Temperature profiles of the outlet air. Numbers in the figure correspond to the dynamic regimes.
the process followed by a stable plateau after an interval from 20 to 40 min of drying, depending on the conditions of drying air temperature and velocity. What means that the temperature reached steady state after those times (from 20 to 40 min), when the surface water had already been evaporated and the drying behavior was characterized by Regime 3. Fig. 7 also illustrates that the inlet air temperature control was effective. 3.3. Fluid-dynamic behavior during the drying tests Curves of bed pressure drop versus drying time obtained for all experiments, excepted for (0,0,0), are shown in Figs. 8 and 9. The bed pressure drops become constant for drying times above 20 min. This finding indicates that the stability is reached after 20 min, observed in Figs. 5 and 6. Figs. 8 and 9 show that the air velocity presented the highest effect on the bed pressure drop, followed by the frequency of pulsation. The air temperature did not show any effect on particles fluid dynamic behavior. It can also be seen that the influence of
Fig. 9. Pressure drop versus drying time at 90 ◦ C.
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Fig. 10. Influence of inlet air temperature on the drying kinetic curves.
Fig. 12. Influence of inlet air velocity on the drying kinetic curves.
the frequency of rotation gains importance when high air velocity (0.70 m/s) is employed. At this air velocity, the pressure drop is reduced with increasing f. Based on these results, it may be suggested that at higher frequencies the bed has no time to settle (due to the reduction of channeling and bubbling), and the particles can easily flow inside the bed.
decreasing drying rate period predominated on the PHB kinetics of drying (Fig. 13). The confidence of the drying kinetics data can be demonstrated by the curves plotted for the three tests conducted at the center point of the experimental design (Fig. 13). Reproducibility of the data is evident. Table 4 shows the measurements of the final moisture contents of the dried PHB granules. It is possible to verify that the PHB drying in the RPFB was efficient resulting in low values of the final moisture content. The moisture content specified for adequate post processing of PHB granules is 0.5%. Experiments (+1,−1,−1) and (+1,+1,+1) of the experimental design reached final moisture contents of 0.56 and 0.55 after 2 h of drying, respectively, which are very close to the recommended value.
3.4. Influences of Tair , f and Vair on the drying kinetics The experimental design described in Table 2 enabled to identify the effects of the independent variables (Tair , f and Vair ) on the drying kinetics, which are shown in Figs. 10–12 for the experiments (−1,−1,+1), (+1,−1,+1) and (+1,+1,+1), respectively. Overall drying kinetics curves obtained for the PHB at different drying conditions (data not shown) presented the same aspects, in terms of process variables influence. As expected, the air temperature and air velocity influenced positively the process. The frequency of rotation did not show observable influence on the drying kinetics for the range tested. It is seen in the drying curves that a constant drying rate appears at the beginning (which explains the PHB cohesiveness) and the
Fig. 11. Influence of rotating disk frequency on the drying kinetic curves.
3.5. Factorial design Even though decreasing drying rate period have been predominated on the PHB kinetics of drying, the coefficient of constant drying rate period Nc was chosen as a dependent variable in the fac-
Fig. 13. Drying kinetics of the PHB in RPFB—center point of the factorial design (0,0,0).
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Table 4 Final moisture content of PHB granules (wet basis). Run
Final moisture [%]
(−1,−1,−1) (−1,−1,+1) (−1,+1,−1) (−1,+1,+1) (+1,−1,−1) (+1,−1,+1) (+1,+1,−1) (+1,+1,+1) (0,0,0) (0,0,0) (0,0,0)
1.22 0.95 1.16 0.93 0.56 0.69 0.64 0.55 0.74 0.73 0.74
Fig. 15. Pareto chart for Xf /X0 (significant at 95.0% level).
Fig. 14. Pareto chart for Nc (significant at 95.0% level).
torial design of this work. The reason of this choice of response was to evaluate how the process variables affect the drying when a large amount of surface water exists and affects the dynamic regime. The other dependent variable chosen was the final moisture content Xf /X0 after 2 h of drying. The statistical results presented in Table 5 and Figs. 14 and 15 show that the temperature (p-value = 0.004) and the velocity (pvalue = 0.02) of the drying air affect positively the constant drying rate period coefficient (Nc ) at a confidence level of 95%. The short constant drying rate period is more affected by temperature than by velocity of the inlet air, Fig. 14. This is an expected result when drying is controlled by external conditions and it can be explained by the fact that surface water is abundant at the beginning of the process. Consequently, at higher temperatures this water evapo-
rates quickly. In a lesser extent, air velocity also accelerates this process by increasing the convection near the surface. As can be seen in Figs. 14 and 15 the rotation frequency does not present a significant effect on the constant drying rate period. This phenomenon can be explained by the fact that for the whole range of pulsation frequency used (7–13 Hz), the particles inside the bed have a behavior similar to that of a conventional fluidized bed. It means that the RPFB dryer was efficient in providing good mixing of the particles inside the bed and only the external conditions of the drying air exerted significant influence on the constant drying rate. For the dependent variable Xf /X0 , the pareto chart shows that only the air temperature (p-value = 0.03) is significant. With the increase of the air temperature, the final moisture content decreased, Fig. 15. As shown before in the drying kinetics analysis, at the end of the process, all surface moisture had been evaporated and only internal moisture on the structure of the material is still present. In this case, only the temperature is influential for diffusion and evaporation of the internal water. According to the results previously discussed it is possible to affirm that the best condition applied for the PHB drying was at the one denominated by (+1,−1,−1), having higher air temperature and lower air velocity and air pulsation. Although the frequency of pulsation is not statistically significant for the responses analyzed, its lowest value was chosen as it results in lower energy consumption. At operating condition (+1,−1,−1), the final moisture content was close to the recommended (0.50%) and statistically equal to the value reached by the extreme condition (+1,+1,+1), see Table 4. 3.6. Molar mass distribution
Table 5 Nc and Xf /X0 for each drying experiment. Run
Nc [min−1 ]
Xf /X0 [kg/kg]
(−1,−1,−1) (−1,−1,+1) (−1,+1,−1) (−1,+1,+1) (+1,−1,−1) (+1,−1,+1) (+1,+1,−1) (+1,+1,+1) (0,0,0) (0,0,0) (0,0,0)
0.020 0.028 0.018 0.025 0.030 0.043 0.035 0.044 0.029 0.032 0.037
0.026 0.022 0.024 0.021 0.012 0.015 0.014 0.011 0.014 0.016 0.016
GPC analyses were done for samples of wet PHB and PHB dried at the following conditions of the factorial design: (−1,−1,−1), (+1,−1,−1) and (+1,+1,+1). These conditions express the less severe, the optimum and the more severe applied in the drying tests. Table 6 shows the weight-average molecular mass (Mw ) and polydispersity of the analyzed samples. As can be seen, a considerable difference on the molecular weight distribution did not occur after the drying in RPFB dryer. So, it is possible to affirm that the operating conditions applied are secure to maintain these PHB characteristics and as a consequence, no damages were caused on the mechanical properties of the PHB due to the drying process.
F.C. Godoi et al. / Chemical Engineering and Processing 50 (2011) 623–629 Table 6 Weight-average molar mass (Mw ) and polydispersity of the PHB samples by GPC. Samples
Mw [Da]
Polydispersity
Wet PHB Dried at (−1,−1,−1) Dried at (+1,−1,−1) Dried at (+1,+1,+1)
690 685 691 631
2.4 2.2 2.4 2.5
Table 7 Carr Index (CI) for wet, dried at condition (+1,−1,−1) and lyophilized PHB. Samples
b [g/cm3 ]
t [g/cm3 ]
CI [%]
Wet PHB Dried at (+1,−1,−1) Lyophilized PHB
0.49 0.49 0.46
0.71 0.52 0.48
31 5 4
r2 Tair Vair X Xeq X0 Wair
629
correlation coefficient air temperature [◦ C] air velocity [m/s] average moisture [kg water/kg dry solid] equilibrium moisture content [kg water/kg dry solid] initial moisture content [kg water/kg dry solid] air flow rate [kg/min]
Greek letters P1 pressure drop of the orifice plate [cm H2 O] s solid density [g/cm3 ] powder bulk density [g/cm3 ] b t powder tapped density [g/cm3 ] References
3.7. Flowability evaluation Table 7 demonstrates that the wet PHB can be classified as cohesive material, with CI > 28% [23]. Cohesiveness results from the high moisture content that forms water bridges between the contact points of the solid particles. This result corroborates with the fluid dynamic behavior of wet PHB in a conventional fluidized bed (CFB), where poor fluidization with formation of slugs is reported. In order to stress that PHB drying in RPFB gives a free-flowing material, the flowability was compared to lyophilized PHB particles. For the PHB dried in RPFB dryer or lyophilized, the CI fell abruptly resulting in a powder classified as a free-flowing material (see Table 7). 4. Conclusions The RPFB dryer was adequate to dry the cohesive particles of PHB, resulting in a stable fluid dynamics of the bed and providing uniform drying of the solid. The final moisture content obtained at 90 ◦ C, 7 Hz and 0.55 m/s was very close to the recommended value of the 0.50%. The rotation frequency exerted considerable influence on the fluid dynamic behavior only at high air velocity of 0.70 m/s. The drying kinetics of PHB showed a short constant drying rate period followed by a decreasing drying rate period. Moreover, on the range tested, the Nc was affected by temperature and velocity of the inlet air and was not affected by the rotation frequency. The final moisture content was affected mainly by the air temperature. The drying conditions did not modify significantly the molar mass of the dried PHB, which demonstrates that the quality of the material was maintained. Furthermore, after drying, the PHB particles flowability changed from cohesive to a free-flowing material. Acknowledgements We acknowledge the financial support of FAPESP and CAPES for this work. We also thank the support of the School of Chemical Engineering of the University of Campinas – UNICAMP, and BYOCICLE® by the samples of PHB supplied. Appendix A. Nomenclature
CI dp f Nc Pestat,l
Carr Index [%] Sauter diameter [m] rotation frequency [Hz] coefficient of constant drying rate period [min−1 ] static pressure before the orifice [mmHg]
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