Drying Kinetics of Rice Straw under Isothermal and Nonisothermal

Publication Date (Web): May 30, 2012 ... drying kinetics due to its online weight recording and minimal material requirements as well as its capacity ...
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Drying Kinetics of Rice Straw under Isothermal and Nonisothermal Conditions: A Comparative Study by Thermogravimetric Analysis Dengyu Chen, Yi Zhang, and Xifeng Zhu* Key Laboratory for Biomass Clean Energy of Anhui Province, University of Science and Technology of China, Hefei 230026, China ABSTRACT: A thermogravimetric approach was applied to study the drying of powdered rice straw particles to determine the drying kinetics under isothermal and nonisothermal drying conditions. The isothermal drying experiments were performed at 50, 60, 70, and 80 °C, and the nonisothermal drying experiments were performed at a heating rate of 10 °C/min from 30 to 140 °C. The thermogravimetric approach was suitable for determining the effective moisture diffusivity and drying kinetics due to its online weight recording and minimal material requirements as well as its capacity for precise temperature control. The drying process under both conditions mainly occurred during the falling rate period. However, the trend of water loss in rice straw under nonisothermal conditions was different from that under isothermal conditions, especially in terms of the drying rate and the final moisture content. Four widely used models were chosen to describe the isothermal drying process, and their corresponding nonisothermal models were obtained for describing the observed nonisothermal drying behavior. The results of model fitting showed that the logarithmic model was the best model for describing isothermal drying, whereas the nonisothermal Henderson model had the best fit for results obtained under nonisothermal conditions. The effective moisture diffusivity varied from 4.35 × 10−8 to 5.13 × 10−8 m2/s, and the drying activation energy values were 5.3 and 6.5 kJ/mol under isothermal drying conditions and nonisothermal drying conditions, respectively.

1. INTRODUCTION As an important renewable energy source, biomass has great potential for solving the shortage problems of conventional fossil fuels.1 Pyrolysis is a very important technology in char and liquid fuel production, both of which require small and dry particles of biomass feedstock.2,3 Unfortunately, biomass, including those from agricultural residues, often has high moisture content.4 The excessive water in biomass is detrimental to the energy density of the raw materials and the reliability of the pyrolysis system as well as the quality of bio-oil produced.5 Therefore, drying pretreatment is essential to improve the utilization efficiency of biomass. Kinetics analysis of drying is a very important tool for better understanding of the water transfer mechanisms. It also provides useful information for designing and improving a dryer.6,7 The activation energy is a key drying parameter for describing the energy level of water molecules in the drying process, and effective moisture diffusivity (Deff) is the other key parameter for describing the moisture diffusion mechanism of biomass.8 Thermogravimetric analysis (TGA) has been widely used for the kinetics analysis of the biomass. Its application for biomass drying has many advantages, such as ease of operation, minimal material requirement, precise temperature control, and online recording of experimental data.9,10 In recent years, the drying characteristics of biological materials have been extensively studied by numerical simulation methods or empirical mathematical models under isothermal conditions.11−15 However, these existing methods may not be suitable for the determination of the drying kinetics and effective moisture diffusivity of powdered biomass particles. On one hand, achieving isothermal drying conditions is usually difficult and in some cases impossible. Isothermal conditions not only imply that the temperature gradient in the sample can © 2012 American Chemical Society

be ignored; they also require that the sample temperature immediately reaches the desired drying temperature and does not change with time.16,17 Drying equipment, such as ovens, dryers, or other self-fabricated equipment, cannot precisely control the drying temperature and the air flow rate. Thus, the experimental error is sometimes relatively high. Previous studies have shown that the prediction values fail to fit the experimental data when a temperature gradient is present during drying.8,18 On the other hand, drying studies and methods mainly focus on the massive, cylindrical, and spherical materials, such as wood, potato, rice.8,11,14,19 However, scarce attention has been paid to powdered biomass. Therefore, a rapid and simple approach for determining the drying kinetics and effective moisture diffusivity of powdered biomass particles is an urgent necessity. The nonisothermal drying of biomass is gaining increasing attention because it is not only an important drying technique but also the first step in biomass pyrolysis.20,21 A few literature references reported the moisture transfer characteristics of biomass under nonisothermal conditions at a constant heating rate.22,23 However, a comparative study of isothermal and nonisothermal drying of biomass has not been reported. The objectives of this study are to evaluate the isothermal and nonisothermal conditions by thermogravimetric analysis, to compare the drying characteristics of biomass under different drying conditions, and to determine the drying kinetics of powdered biomass particles. Received: February 29, 2012 Revised: May 29, 2012 Published: May 30, 2012 4189

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2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Rice straw obtained from a local farm was used as the biomass material in this study. The material was ground and sieved prior to the experiments, and particles ranging from 0.125 to 0.3 mm in size were chosen for the drying experiments. TGA requires small and uniform particles that are generally less than 1 mm in size to diminish thermal lag and sudden weight loss problems in the system. The size of the rice straw used in this study is similar to that of the materials in many previous studies and meets the requirements of TGA.3,6,9 The initial moisture content of rice straw was determined as 0.095 kg water/kg dry matter by using the vacuum oven method at 120 °C for 6 h. 2.2. Drying Experiments under Different Conditions. A thermogravimetric analyzer (TGA Q5000IR, TA Instruments) was used to perform the drying experiments. The analyzer has a precise temperature control capability that could perfectly achieve isothermal or nonisothermal drying conditions. The main technical parameters of the TGA are as following: weighing accuracy, ±0.1%; weighing range, 0−100 mg; specification of sample pan, 100 μL; size of sample pan, 1 mm high and 1 cm cylindrical diameter; heating rate, 0.1 °C/ min to 500 °C/min; and isothermal temperature accuracy, ±1 °C. For each experiment in this study, about 5 mg of rice straw was used and an air flow rate of 100 mL/min was adopted. These selected conditions are in agreement with many drying studies by TGA.6,10,20,22 According to many previous studies, the temperature range of isothermal drying is generally from 40 to 90 °C.9,11,17,24 Such moderate temperatures can produce a very good drying effect, and the results of kinetics analysis can also be used to predict values beyond the experimental temperature range. Thus, hightemperature drying was not selected in this study, and the isothermal drying experiments were performed at 50, 60, 70, and 80 °C. Nonisothermal drying of biomass has attracted increased attention due to its importance not only as a single process but also as the first step to biomass pyrolysis and gasification. In the literature, nonisothermal drying is generally performed between room temperature to a temperature of 150 °C at a low heating rate.6,20,22 Thus, the nonisothermal drying experiments in this study were performed at a heating rate of 10 °C/min from 30 to 140 °C.

Table 1. Drying Models of Rice Straw under Isothermal Drying Conditions model

M − Me M 0 − Me

MR MR MR MR

= = = =

ref

exp(−ktn) exp(−kt) a + b exp(−kt) a exp(−kt)

6 25 11 17

during the falling rate period.26 It has been widely used to interpret the experimental drying data as the drying process is dominated by internal mass transfer. The mathematical solution of eq 2 is shown in eq 3. ∂MR = ∇[Deff (∇MR)] ∂t MR =

8 π2

∞

∑ n=0

(2)

⎛ (2n + 1)2 π 2D t ⎞ 1 eff ⎜− ⎟ exp 2 2 (2n + 1) 4L ⎝ ⎠

(3)

Equation 3 could be further simplified into a straight-line equation shown in eq 4. ⎛ 8 ⎞ ⎛ π 2Deff ⎞ ln MR = ln⎜ 2 ⎟ − ⎜ t⎟ ⎝ π ⎠ ⎝ 4L2 ⎠

(4) 2

where Deff is the effective moisture diffusivity (m /s), and L is half of the sample thickness (m). A straight line is obtained from eq 4 by plotting ln MR versus drying time, and the Deff for each temperature can be calculated from the slope k0. k0 =

π 2Deff 4L2

(5)

The temperature dependence of the Deff is generally described by an Arrhenius relationship eq 6.27 The drying activation energy can be calculated by plotting ln(Deff) versus 1/(T + 273.15). ⎛ ⎞ Ea Deff = D0 exp⎜ − ⎟ ⎝ R(T + 273.15) ⎠

(6)

where Ea is the drying activation energy (kJ/mol), D0 is the preexponential factor (m2/s), T is the drying temperature (°C), and R is the ideal gas constant (J/mol K). 3.3. Nonisothermal Drying Kinetics Analysis. The moisture ratio (MR) of rice straw under nonisothermal condition is also calculated using eq 1. Under both isothermal and nonisothermal conditions, Me was equal to the final moisture content. However, most of the moisture is removed under nonisothermal conditions as the drying temperature is high. Thus, the value of Me was 0 in this case, and eq 1 can be simplified into eq 7. It is should be noted that MR in eq 7 is a function of dying temperature, whereas MR in eq 1 under isothermal conditions is a function of drying time.

3. KINETICS ANALYSIS OF BIOMASS DRYING 3.1. Isothermal Drying Kinetics Analysis. The moisture ratio (MR) of rice straw can be determined by the following equation.25 MR =

isothermal model equation

Page Newton Logarithmic Henderson

(1)

where M, M0, and Me are the moisture content (kg water/kg dry matter) at time t, the initial moisture content (kg water/kg dry matter), and the equilibrium moisture content (kg water/kg dry matter) of the sample, respectively. The value of Me was equal to the moisture content at the end of drying when the weight of sample became constant with time. The drying curves obtained by TGA under isothermal conditions were fitted with four widely used drying models listed in Table 1. 3.2. Determination of the Effective Diffusivity and Activation Energy. Fick’s second law shown in eq 2 is a wellknown mass-diffusion equation to describe the drying process

MR =

M M0

(7)

The temperature dependence of drying rate constant could be presented by an Arrhenius-type equation:6 ⎛ ⎞ Ea k = k 0 exp⎜ − ⎟ ⎝ R(T + 273.15) ⎠ 4190

(8)

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where k0 is the pre-exponential factor (1/min), and k is the drying constant (1/min). The relationship between temperature and time under nonisothermal drying conditions can be described by eq 9. T = T0 + βt

(9)

where T0 is the initial drying temperature (°C), β is the heating rate (°C/min), and t is the drying time (min). The corresponding nonisothermal models were then obtained by substituting eqs 8 and 9 into the isothermal models and are listed in Table 2. Table 2. Drying Models of Rice Straw under Nonisothermal Drying Conditions model

nonisothermal model equation

⎡ ⎛ ⎞⎛ T − T0 ⎞n⎤ Ea MR = exp⎢− k 0 exp⎜− ⎟⎜ ⎟⎥ ⎢⎣ ⎝ R(T + 273.15) ⎠⎝ β ⎠ ⎥⎦

Page

Figure 1. Temperature profiles of rice straw under isothermal and nonisothermal drying conditions.

⎡ ⎛ ⎞ T − T0 ⎤ Ea ⎥ MR = exp⎢− k 0 exp⎜− ⎟ ⎝ R(T + 273.15) ⎠ β ⎦ ⎣

Newton

Logarithmic

⎡ ⎛ ⎞ T − T0 ⎤ Ea ⎥ MR = a + b exp⎢− k 0 exp⎜− ⎟ ⎝ R(T + 273.15) ⎠ β ⎦ ⎣

Henderson

⎡ ⎛ ⎞ T − T0 ⎤ Ea ⎥ MR = a exp⎢− k 0 exp⎜− ⎟ ⎝ R(T + 273.15) ⎠ β ⎦ ⎣

4.2. Comparison of Drying Characteristics under Different Conditions. The results of drying under isothermal conditions are shown in Figures 2 and 3. The drying

3.4. Statistical Evaluation of Drying Models. Drying models were fit to the experimental data under different conditions. The chi-square (χ2) and the coefficient of determination (R2) were used to evaluate the fitting goodness of predicted values to experimental values. χ2 can be calculated by eq 10. N

∑ χ2 =

(MR pre, i − MR exp , i)2

i=1

N−n

(10)

where MRexp,i and MRpre,i are experimental and predicted moisture ratios, respectively; N is number of observations, and n is the number of drying constants.

Figure 2. Drying curves of rice straw under isothermal conditions.

4. RESULTS AND DISCUSSION 4.1. Evaluate the Isothermal and Nonisothermal Conditions. A computer connected to TGA online recorded temperature data of the material. The temperature profiles of rice straw under different drying conditions are illustrated in Figure 1. The isothermal drying conditions were found to be quickly achieved in the drying experiments. The time needed to heat the biomass material to the desired drying temperatures (50, 60, 70, and 80 °C) was only about 40 s, and the temperature of the material did not change during the later stages of the drying process. Owing to precise temperature control of TGA and minimal material used in experiment as well as low heat capacity of material, the temperature gradient in material was negligible. Figure 1 also shows that for the nonisothermal drying experiments, the material was successfully heated at a constant heating rate of 10 °C/min. Therefore, determination of the drying kinetics and Deff of powdered rice straw was possible since the required isothermal or nonisothermal drying conditions were well-established.

Figure 3. Drying rate curves of rice straw under isothermal conditions. 4191

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role on moisture evaporation at temperatures below 58 °C. As the drying process continued, the internal resistance became much larger than the drying effect of temperature. Thus, the drying rate was observed to decrease with the increase in temperature. 4.3. Isothermal Drying Kinetics. 4.3.1. Modeling of Drying Curves. Four drying models, as listed in Table 1, were used to fit the isothermal drying data obtained by TGA. The best model describing the drying kinetics of rice straw was chosen as the one with the highest R2 and the least χ2. The statistical analysis results are summarized in Table 3. The

temperature significantly affected the drying rate and drying time. More moisture can be removed with high-temperature drying because temperature is the main driving force of moisture evaporation.24 As seen from Figure 3, there was no constant rate period. Therefore, the drying process can be divided into two periods, namely, the rising rate period and the falling rate period. The rising rate period of isothermal drying was very short and occurred just at the beginning of the drying process. This short period of rising rate probably resulted from the rising temperature of the biomass material (see Figure 1), which directly improved the evaporation of free moisture on the biomass surface. The falling rate period was the main drying process, during which internal diffusion dominated the moisture transfer in the material. Similar results have been reported by different authors.8,10,11 The results of drying under nonisothermal conditions are shown in Figure 4. The trend of water loss in rice straw under

Table 3. Statistical Results Obtained from the Isothermal Models model Page

Newton

Logarithmic

Henderson

Figure 4. Drying curve and drying rate curve of rice straw under nonisothermal conditions.

temperature (°C)

drying constant (1/ min)

R2

50 60 70 80 50 60 70 80 50 60 70 80 50 60 70 80

0.463 0.391 0.370 0.560 0.483 0.435 0.407 0.562 0.530 0.478 0.439 0.615 0.510 0.469 0.434 0.581

0.986 66 0.981 77 0.987 60 0.985 97 0.986 08 0.980 25 0.985 64 0.985 97 0.998 67 0.998 02 0.998 89 0.998 36 0.989 10 0.992 40 0.987 83 0.993 68

χ2 2.0199 2.7607 1.6296 2.1530 2.3708 3.9817 2.9521 2.1515 8.0228 9.9351 7.5339 8.7549 1.1875 1.0735 7.9274 1.7721

× × × × × × × × × × × × × × × ×

10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−5 10−5 10−5 10−5 10−4 10−4 10−4 10−4

logarithmic model gave higher R2 and lower χ2 than the three other models during fitting of the experimental data. Figure 5

nonisothermal conditions was observed to be different from that under isothermal conditions even though the drying curves were similar in both conditions. In nonisothermal drying, the maximum drying rate was lower but the evaporation of water was more thorough. By the end of the drying process, the moisture content was very close to 0. As seen from Figure 4, the drying process also mainly took place during the falling rate period. However, the rising rate period of nonisothermal drying was much longer than that of isothermal drying, and more than one-third of the water in the biomass was removed during this period. This phenomenon could be attributed to the different product alterations that occur under the different conditions. Owing to the relatively low temperatures during isothermal drying (below 60 °C), the removed moisture could be generally considered to be the free moisture that was weakly bound to the biomass. As the temperature increased, the bound moisture, which is distributed inside the biomass with relatively strong bonding, began to evaporate in the drying process. Thus, the removed moisture increased with increasing temperature. For example, 55% of the total moisture was removed at 50 °C in isothermal drying, whereas 84% was removed at 80 °C. During nonisothermal drying, the drying rate should theoretically increase as the temperature increases. In reality, the drying rate only continued to rise until the temperature reached 58 °C, which was then followed by a long falling rate period. This finding indicated that the drying temperature played a decisive

Figure 5. Comparison of experimental and predicted moisture ratios by the Logarithmic model under isothermal conditions.

shows the comparisons of experimental and predicted moisture ratio values. According to this figure, it is clear that the Logarithmic model fits the drying curves of rice straw very well. 4.3.2. Calculation of Effective Diffusivity and Activation Energy. The shape of straw particles does not change significantly until the temperature rising to 200 °C when 4192

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pyrolysis reactions occur. Thus sample shrinkage is negligible below 150 °C. The results in Figures 1−3 have shown that isothermal drying condition is accomplished quickly and the internal transfer dominates the drying process. Therefore, it was suitable to determine the Deff by eq 4. The calculated values of Deff for rice straw were 4.35 × 10−8, 4.55 × 10−8, 4.94 × 10−8, and 5.13 × 10−8 m2/s at 50, 60, 70, and 80 °C, respectively. The Deff values for rice straw increased as the temperature increased and the values were similar to those reported in the literature for other biomass: 8.42 × 10−9 to 1.69 × 10−8 for rice husk drying at 30−60 °C;28 2 × 10−11 to l × 10−9 m2/s for sawdust drying at 65−105 °C;12 3.4 × 10−11 to 1.8 × 10−10 for husk of parboiled paddy drying at 50−110 °C;13 1.52 to 3.67 × 10−10 m2/s for rubber wood sawdust drying at 40−60 °C;14 1 to 10 × 10−10 m2/s and 1 to 6 × 10−8 m2/s for agriculture residues (rice husk and coconut husk, respectively) drying at 30−70 °C.15 The activation energy was determined by eq 6. A linear relationship between ln(Deff) and 1/(T + 273.15) is presented in Figure 6 due to the Arrhenius type dependence. The value of

Figure 7. Comparison of experimental and predicted moisture ratios by the nonisothermal Henderson model under nonisothermal conditions.

Henderson model. Both Figure 5 and Table 4 demonstrate that the drying kinetics of rice straw can be successfully described by the nonisothermal Henderson model. The activation energy was calculated to be 6.5 kJ/mol for rice straw under nonisothermal drying conditions, which was slightly higher than that under isothermal drying conditions. The activation energy of rice straw in this study is similar to those proposed for corn stalk but lower than those for rice husks and sawdust in previous studies: 6.1 and 14.1 kJ/mol for corn stalk and wheat straw, respectively,6 12.3 kJ/mol in poplar sawdust,9 12.34 kJ/mol in olive-waste cake,17 14.1 kJ/mol in fir sawdust,21 15.1 and 9.2 kJ/mol in cotton stalk and rice husk, respectively.22

5. CONCLUSIONS The drying kinetics of rice straw under isothermal and nonisothermal drying conditions were studied using a thermogravimetric analyzer. With online weight measurements and precise temperature control as well as minimal material requirement, the thermogravimetric approach was suitable for determining the drying kinetics and effective moisture diffusivity of powdered biomass particles. The drying process under both conditions mainly occurred during the falling rate period, in which moisture transfer was dominated by internal diffusion. The trend of water loss under nonisothermal conditions has some differences from that under isothermal conditions, especially in terms of the drying rate and the final moisture content. On the basis of four widely used isothermal models, the nonisothermal drying models were developed in order to describe the nonisothermal drying characteristics of rice straw. The Logarithmic model was found to be the best for describing isothermal drying, whereas the nonisothermal Henderson model gave the best fit to experimental results under nonisothermal conditions. The effective moisture diffusivity ranged from 4.35 × 10−8 to 5.13 × 10−8 m2/s within the given temperature range. The values of drying activation energy were 5.3 and 6.5 kJ/mol for isothermal and nonisothermal drying conditions, respectively.

Figure 6. Arrhenius type relationship between Deff and drying temperature.

the activation energy was obtained from the slope of the line and was found to be 6.5 kJ/mol for rice straw under isothermal drying conditions. 4.4. Nonisothermal Drying Kinetics. The four nonisothermal drying models in Table 2 were used to fit the experimental data of rice straw under nonisothermal conditions. The statistical results are summarized in Table 4, which shows that the nonisothermal Henderson model yields higher R2 and lower χ2 than the three other models. The nonisothermal Henderson model can thus be concluded to provide the best predicted values for describing the drying characteristics of rice straw. Figure 7 shows a comparison of the experimental and predicted moisture ratios based on the nonisothermal Table 4. Statistical Results Obtained from the Nonisothermal Models model

R2

Page Newton Logarithmic Henderson

0.989 78 0.988 63 0.990 29 0.999 83

■

χ2 2.2773 1.4519 7.5889 1.7529

× × × ×

10−4 10−4 10−5 10−5

AUTHOR INFORMATION

Corresponding Author

*Phone: +86 551 3600040. Fax: +86 551 3606689. E-mail: [email protected]. 4193

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Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research is supported by National Natural Science Foundation of China (Grant 50930006), National High Technology Research and Development Program (Grant 2012AA051803), National Key Technology R&D Program (Grant 2011BAD22B07), and The CAS Special Grant for Postgraduate Research, Innovation and Practice.

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