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Nonisothermal Decomposition Kinetics of Trona Ahmet Ekmekyapar Department of Chemical Engineering, Engineering Faculty, Ino¨ nu¨ University, 44100 Malatya, Turkey
Hu 1 rriyet Ers¸ ahan* and Sinan Yapıcı Department of Chemical Engineering, Engineering Faculty, Atatu¨ rk University, 25240 Erzurum, Turkey
In the present work, experiments on the nonisothermal decomposition of trona in thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) devices were carried out, and kinetic analysis of the process was performed by employing four different approaches. It was determined that the process fits a first-order reaction kinetic model and that the value of the activation energy changes, to some degree, depending upon the method used in the analysis, with a maximum deviation of 8.9% from the average value. The activation energy was found to be approximately 112 kJ mol-1, which is in good agreement with the data from similar studies in the literature. Introduction Natural trona found in the region of Ankara, Turkey, mainly consists of sodium sesquicarbonate having the chemical formula Na2CO3‚NaHCO3‚2H2O. The trona resources of Turkey are predicted to be approximately 108 tons (Okutan et al., 1986). In the production of Na2CO3, a much-needed substance in the chemical industry, the Solvay method has been employed for years. However, the exploration of natural sodium carbonate resources in the United States revealed that Na2CO3 may be produced in a much cheaper way. When many of the substances containing carbonate and/or hydrated water are heated to a certain temperature, they give off CO2 and/or H2O. These processes are called calcination and dehydration, respectively. Calcination and dehydration processes are used for different purposes: to eliminate carbonate for reducing acid consumption if an acidic treatment is to be applied, to decrease the weight of a material for reducing transportation costs in the case that it includes hydrate water in large quantities, or as a necessary stage of a chemical process as in the production of Na2CO3 from trona. In the production of Na2CO3 from trona, the monohydrate process is the most developed method. The first stage of this method is the thermal decomposition of the mineral at elevated temperatures in the range of 200 to 600 °C. The product obtained from the decomposition is crude Na2CO3 via the following reaction:
2(Na2CO3‚NaHCO3‚2H2O)(s) f 3Na2CO3(s) + CO2(g) + 5H2O(g) (1) As seen, the hydrate water and CO2 are produced by the decomposition process. To make an accurate and reliable design of a calcination-dehydration reactor, the designer should possess kinetic knowledge of the process. Thus, the present work was aimed to investigate the kinetics of the thermal decomposition of trona in TGA and DSC devices. Many types of thermal analysis methods have been developed in almost all fields of materials science. The thermogravimetric methods are effective and commonly used in kinetic investigations. In these methods, the property change of a material heated at a constant heating rate is observed continuously. The thermal analysis techniques have commonly been used in the kinetic studies of decomposition processes. Suzuki et 0888-5885/96/2635-0258$12.00/0
al. (1978) worked on the thermal decomposition of organic substances precipitated on active carbon and developed two methods for the investigation of decomposition kinetics. Salvador and Calvo (1992) investigated the dehydration kinetics of zinc acetate dihydrate by using the Coats-Redfern method and found that the activation energy of the process was about 85 kJ mol-1. Khadikar et al. (1993) studied the thermal decomposition of tallium(III) citrate by using both the CoatsRedfern and Horowitz-Metzger methods. ErsoyMericboyu et al. (1993) studied the calcination kinetics of some calcite minerals in TG using the Coats-Redfern method and found that the activation energies for the thermal decomposition of calcites were between 125 and 238 kJ mol-1. The kinetic investigation of solid copper sulfate reduction with hydrogen and carbon monoxide gases in TG was carried out by Van and Habashi (1974) using Coats-Redfern method, and they found that the activation energies of the reduction processes with hydrogen and carbon monoxide were 63 and 100 kJ mol-1, respectively. In a study on the use of data obtained from thermal decomposition in DSC for kinetic analysis, Duswalt (1974) obtained the kinetic parameters of thermal decomposition of 10 different materials by using the approaches of McCarty and Doyle. Subramanian et al. (1972) studied the thermal decomposition of sodium bicarbonate in DTA, taking heating rate and particle size as parameters. They found that the activation energy for the process varied from 83 to 100 kJ mol-1. Hu et al. (1986) investigated the thermal decomposition kinetics of sodium bicarbonate both isothermally and nonisothermally and found that the process fitted to a first-order kinetic model and that the activation energy of the process was 102 kJ mol-1. Van Dooren and Mu¨ller (1983) investigated the thermal decomposition of sodium bicarbonate in DSC and found that the activation energy of the process was between 105 and 113 kJ mol-1. In spite of this large number of studies on the thermal decomposition of different materials, there are only a couple of investigations on the kinetics of thermal decomposition of trona. Logvitsenko and Bobkova (1979) studied the thermal decomposition kinetics of trona from 0.5 to 38% conversion and found that the activation energy of the degradation was approximately 146 kJ mol-1. Glagoza et al. (1981) studied the thermal decomposition kinetics of trona coated on sodium bicar© 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996 259 Table 1. Chemical Analysis of Trona
1 - (1 - x)1-n
component
wt %
Na2CO3 NaHCO3 Na2SO4 insolubles hydrate water others
46.53 34.82 0.568 2.98 14.92 0.182
)
2
T (1 - n)
(5)
-ln(1 - x) )
T2
k0R exp(-E/RT) qE
Trona mineral obtained from the Ankara region in Turkey was first ground and then sieved to obtain a nominal particle size of 0.165 mm. Its chemical analysis is given in Table 1. TGA and DSC analyses of the mineral were performed by using a Shimadzu Model 50 TGA and a Shimadzu Model 50 DSC. For the work in TGA, a sample of 7.23 mg was put into a platinum capsule, and its decomposition, i.e., the weight loss, was recorded at a constant heating rate of 0.17 K s-1 and a N2 gas flow rate of 0.83 mL s-1. For the analysis in DSC, samples of 20 mg were used for the observation of thermal decomposition at heating rates of 0.083, 0.167, 0.333, and 0.500 K s-1. In this analysis, the sample was put into an aluminum cell and alumina was used as the reference material. The kinetic analysis were performed using four different methods, namely, Coats-Redfern, Suzuki, and Doyle’s and McCarty’s approximations. Theoretical Background For kinetic analyses using the thermal decomposition data, some methods have been developed. Four of these methods were used in the present study. Coats-Redfern Method (Ersoy-Meric¸ boyu et al., 1993). In this method, the decomposition rate of a solid in the form of aAs f bBs + cCg can be expressed as
(2)
where x is the conversion fraction, t is time, k is the rate constant, and n is the reaction order. If the heating rate is expressed as q ) dT/dt, and the rate constant as k ) k0 exp(-E/RT), eq 2 can be written as
where k0 is the frequency factor, T is absolute temperature, E is the activation energy, and R is the universal gas constant. Integration of this equation for the boundary conditions of 0 f x for the conversion fraction and T0 f T for the temperature and rearrangement gives
1 - (1 - x)1-n 2
T (1 - n)
)
k0R 2RT 1exp(-E/RT) qE E
(
1 - (1 - x)n
f(x) )
T2
f(x) )
-ln(1 - x) T2
)
for (n * 1) (4) where k0R/qE is constant for any particular value of n and heating rate. By assuming 2RT/E , 1, eq 4 reduces to
(n * 1)
(7)
(n ) 1)
(8)
the following general expression can be written
ln(f(x)/T2) ) ln(k0R/qE) - (E/RT)
(9)
From the slope of the plot of ln(f(x)/T2) vs 1/T, the activation energy, E, can be calculated, and the intercept of the straight line predicts the frequency factor. Suzuki Method (Suzuki et al., 1978). In this method, a conversion vs temperature plot is constructed using TGA data. From this plot, after the temperature corresponding to 50% conversion, T1/2, is determined and ∆T, which is the slope of the TGA plot at the point of 50% conversion, is calculated, the following function is defined:
( )
(10)
E ) 1 - [zezE1(z)] RT1/2
(11)
ξ)
∆T 2 E φ ) T1/2 ln 2 RT1/2
where
φ
( )
where z ) (E/RT1/2). The value of z is calculated from the plot prepared by Suzuki et al. (1978), giving us the tool to calculate the value of the activation energy. In this method, the frequency factor is obtained by the following equation:
k0 ) (3)
for (n ) 1) (6)
If the following functions are defined
Experimental Section
dx k0 ) (1 - x)n exp(-E/RT) dT q
for (n * 1)
For n ) 1, the following expression can be obtained from eq 3 with the same assumptions:
bonate crystals and determined that the activation energy was between 88 and 96 kJ mol-1.
dx ) k(1 - x)n dt
k0R exp(-E/RT) qE
2q exp(E/RT1/2) ∆T
(12)
Doyle’s Approximation (Duswalt, 1974). This method requires the determination of the maximum peak decomposition temperature, Tmax, for each heating rate and the construction of a log q vs 1/Tmax plot. The slope of the straight line obtained with this procedure allows the calculation of the activation energy with the equation ∆log q/∆(1/T) ≈ 0.457E/R. The frequency factor is obtained by the following equation:
k0 )
Eq exp(E/RTmax) ∆RTmax2
(13)
McCarty’s Approximation (Duswalt, 1974). This is an improved version of Doyle’s approximation to calculate activation energy more precisely. In this approximation, activation energy is calculated by iteration using the following equation:
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Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996
[(
E)R
∆ln q/∆(1/T) 1 1 1 1 - -1 z+3 z z+1 z+4
)
]
(14)
where z ) E/RT. Since z includes both E and T as variables, the expression is iterated until the change in E reaches an acceptable minimum value. The frequency factor is calculated by using eq 13. Results and Discussion The reaction taking place in the decomposition process is given in reaction 1. The conversion fractions were calculated as the ratio of weight loss at a temperature to the total weight loss at the end of process. The TGA diagram of the sample is given in Figure 1. The diagram in this form was converted into conversion fraction vs temperature to be able to employ the CoatsRedfern and Suzuki methods for the calculation of kinetic parameters. As mentioned earlier in the Coats-Redfern method, the reaction order of the process must be determined first in order to calculate the kinetic parameters. The reaction order was determined as the value giving the best fit for the plot ln(f(x)/T2) vs (1/T) by iteration. This was done by increasing the value of the reaction order by a certain interval and then testing the fit of the ln(f(x)) vs (1/T) plot. The results indicated that the reaction fit a first-order kinetic model. The plot is given in Figure 2, with a standard deviation of 0.02347 and a regression coefficient of 0.9990. By using the slope of the straight line in Figure 2 and eq 9, the activation energy was found to be 123 kJ mol-1. The frequency factor was calculated to be 1.192 × 1015 s-1. In the case of the Suzuki method, the values of ∆T and T1/2 were found to be 30.3 and 378.4 K, respectively, as shown in Figure 3. The activation energy and the frequency factor were calculated by using eqs 10-12, and their values were found to be 110 kJ mol-1 and 1.684 × 1013 s-1, respectively. DSC diagrams obtained at four different heating rates are given in Figures 4-7. Doyle’s approximation requires the construction of a log q vs 1/Tmax plot using DSC diagrams. In this method, the activation energy is calculated from the slope of the constructed plot, given in Figure 8. With this method, the activation energy and frequency factor were calculated to be 112.3 kJ mol-1 and 3.192 × 1014 s-1, respectively, while McCarty’s approximation gave an activation energy of 113.2 kJ mol-1. The predicted values of the activation energies from different methods are given in Table 2. As seen from the table, the values are in very good agreement with one another, with the exception of the one calculated by the Coats-Redfern method. If the value obtained using the first method is discarded because of its large deviation compared with the values calculated using other methods, the activation energy of the thermal decomposition of trona is found to be approximately 112 kJ mol-1. If the thermal decompositions of trona and sodium bicarbonate are considered to be similar processes, it can be seen that the activation energy values found in the present study are in close agreement with the values for sodium bicarbonate and the values of Galogaza (1981) for trona, as shown in Table 3. The deviation of these values from that of the present study is a maximum of 21%. If the methods used here are compared with one another according to practicality of their calculations,
Figure 1. TGA diagram of trona.
Figure 2. Graph for the Coats-Redfern method.
Figure 3. Graph for the Suzuki method.
it can be said that the Coats-Redfern method requires more calculation but only one TGA study. The Suzuki method has the advantage of both simplicity and only one TGA study. Doyle’s and McCarty’s approximations require at least three DSC experiments and more calculations than first two methods. Conclusion In the present study, the kinetic parameters of the thermal decomposition of Turkish trona in TGA and DSC were investigated by employing four different
Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996 261
Figure 4. DSC diagram of trona (q ) 5 K min-1).
Figure 7. DSC diagram of trona (q ) 30 K min-1).
Figure 8. Graph for Doyle’s approximation. Figure 5. DSC diagram of trona (q ) 10 K min-1).
Table 2. Activation Energy of Trona Calculated by Different Methods method
E (kJ mol-1)
frequency factor (s-1)
Coats-Redfern Suzuki Doyle McCarty
99.0 110.0 112.3 113.2
4.337 × 1011 1.684 × 1013 3.192 × 1014 3.192 × 1014
Table 3. Activation Energies of Some Similar Studies from Literature material
E (kJ mol-1)
trona trona NaHCO3 NaHCO3
146.3 88-96 100-103 97-125
NaHCO3
84-100.3
method
Suzuki Ozawa BorchardtDaniels
reference Logivtsenko et al. (1979) Galogaza et al. (1981) Hu et al. (1986) Van Dooren and Mu¨ller (1983) Subramanian et al. (1972)
Figure 6. DSC diagram of trona (q ) 20 K min-1).
Nomenclature
procedures in the calculations. It was determined that the process fits a first-order kinetic model, and the values of the kinetic parameters might change with the method of calculation used to some degree. The activation energy was found to be approximately 112 kJ mol-1, which is in close agreement with the data of sodium bicarbonate from the literature.
E ) activation energy, kJ mol-1 k ) rate constant, s-1 k0 ) frequency factor, s-1 n ) reaction order q ) heating rate, K s-1 R ) universal gas constant, 8314 kJ mol-1 K-1
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T ) temperature, K x ) conversion fraction
Literature Cited Duswalt, A. A. The Practice of Obtaining Kinetic Data by Differential Scanning Calorimetry. Thermochim. Acta 1974, 8, 57-68. Ersoy-Meric¸ boyu, A.; Ku¨c¸ u¨kbayrak, S.; Du¨ru¨s, B. Evaluation of the Kinetic Parameters for the Thermal Decomposition of Natural Turkish Limestones from Their Thermogravimetric Curves Using a Computer Programme. J. Thermal Anal. 1993, 39, 707-714. Galogaza, V. M.; Mitrovic, M. V.; Prodan, E. A. Kinetics of Thermal Decomposition of Trona Coating the Surface of NaHCO3 Crystals. Vestri. Akad. Navk 1981, 3, 69-72. Hu, W.; Smith, J. M.; Dogˇu, T.; Dogˇu, G. Kinetics of Sodium Bicarbonate Decomposition. AIChE J. 1986, 32 (9), 1483-1490. Khadikar, P.; Joshi, A.; Parnerkar, S.; Karmarkar, S.; Karmarkar, S. Thermogravimetric (TG, DTG, DTA) Studies on ThalliumIII Citrate. Chim. Acta Turcica 1993, 21, 117-130. Logvitsenko, V. A.; Bobkova, A. A. Thermolysis of Trona under Nonisothermal Conditions. Therm. Anal. Tarby. Dokl. Vses. Sov. 7th 1979, 1, 156-157. Okutan, H.; O ¨ ner, G.; Nasu¨n, G.; Savguc¸ , A. B.; C¸ ataltas¸ , I.; Okutan, F. Yerinde Degˇerlendirme Yo¨ntemleri (In-Situ Processes) ve Tu¨rkiye Maden Rezervlerine Uygulanabilirligˇinin
Aras¸ tiril. Eng. Soc. Turkey, Chem. Eng. Sect. 1986, 15 (118), 7-14. Salvador, A. R.; Calvo, E. G. Kinetic Analysis of Nonisothermal Decomposition of Solids: Dehydration of Zinc-Acetate Dihydrate. Int. Chem. Eng. 1992, 32 (4), 726-731. Subramanian, K. S.; Radhakrishnan, T. P.; Sundaram, A. K. Thermal Decomposition Kinetics of Sodium Bicarbonate by Differential Thermal Analysis. J. Thermal Anal. 1972, 4, 8993. Suzuki, M.; Misic, M. D.; Koyama, O.; Kawazoe, K. Study of Thermal Regeneration of Spent Activated Carbons: Thermogravimetric Measurements of Various Single Component Organics Loaded on Activated Carbons. Chem. Eng. Sci. 1978, 33 (3B), 271-279. Van, K. V.; Habashi, F. Kinetics of Reduction of Solid Copper Sulphate by Hydrogen and Carbonmonoxide. Can. J. Chem. Eng. 1974, 52, 369-373. Van Dooren, A. A.; Mu¨ller, B. W. Effects of Experimental Variables on the Determination of Kinetic Parameters with Differential Scanning Calorimetry: 1. Calculation Procedures of Ozawa and Kissinger. Thermochim. Acta 1983, 8, 257-267.
Received for review March 10, 1995 Revised manuscript received July 14, 1995 Accepted August 7, 1995X IE950171Q X Abstract published in Advance ACS Abstracts, November 15, 1995.
