Thermal Degradation Kinetic Study of Rubber Seed Oil and Its Methyl

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Thermal degradation kinetic study of rubber seed oil and its methyl esters under inert atmosphere Ali Shemsedin Reshad, Pankaj Tiwari, and Vaibhav V. Goud Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02249 • Publication Date (Web): 24 Aug 2017 Downloaded from http://pubs.acs.org on August 29, 2017

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Thermal degradation kinetic study of rubber seed oil and its methyl esters under

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inert atmosphere

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Ali Shemsedin Reshad, Pankaj Tiwari*, Vaibhav V. Goud*

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Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati,

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Assam, 781039, India

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∗Corresponding

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E-mail address: [email protected] (Pankaj Tiwari), [email protected] (V. V. Goud)

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ABSTRACT: Non-edible vegetable oil feedstocks are promising for sustainable production of

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biodiesel. Thermal decomposition characteristics of the feedstocks and its biodiesel are crucial

11

for handling and quality control. Thermal degradation of rubber seed oil (RSO) and rubber seed

12

oil methyl esters (ROME) were investigated with the help of thermogravimetry. The samples

13

were pyrolysed from 30 °C to 800 °C for heating rates of 10 °C/min to 50 °C/min with 10

14

°C/min increment under nitrogen atmosphere. The temperature window for thermal degradation

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of RSO and ROME were shifted towards higher range as the heating rate increased from 10

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°C/min to 50 °C/min. Transesterification reaction leads to decrease the molecular weight of

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triglycerides present in sample (RSO) and this causes to lower the thermal stability of the

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produced product (ROME). Fourier transform infrared (FT-IR) analysis of evolved gaseous

19

products during pyrolysis revealed the formation of water, carbon dioxide, carbon monoxide,

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saturated (alkanes), and unsaturated (alkenes) aliphatic hydrocarbons. Friedman (FRD), Flynn-

21

Wall-Ozawa (FWO), modified Coat-Redfern (MCR), Kissinger (KM) methods and Avrami

22

theory were applied to calculate the values of activation energy (E), order of reaction (n) and

author: Tel.: +91 361 2582263/2272; fax: +91 361 2582291

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enthalpy (∆H). Furthermore, pre-exponential factor (A), entropy (∆S) and Gibbs free energy

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(∆G) were also calculated.

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INTRODUCTION

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Biodiesel has gained worldwide attention to partially substitute fossil based diesel fuel.

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Chemically, biodiesel is a mixture of long chain fatty acid alkyl esters derived from triglyceride

28

present in vegetable oil, animal fat and waste cooking oil through transesterification process.

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Transesterification process improves the physico-chemical, thermal and flow properties of the

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feedstock. Soybean oil, sunflower oil, palm oil, coconut oil, corn oil, rapeseed oil and olive oil

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are the most widely used first generation feedstocks for the production of biodiesel. First

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generation biodiesel feedstocks are usually categorized as a part of food chain and account

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around 60%–80% of the total biodiesel production cost

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biodiesel production can be overcome by using of non-edible feedstocks. Jatropha curcas L.

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(Jatropha)

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pinnata L. (Karanja), Calophyllum inophyllum L. (Polanga) 7, Croton megalocarpus (Musine) 7,

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Cocos nucifera (coconut)

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materials for biodiesel synthesis.

1, 2

, Ricinus communis (Castor)

7

and

3, 4

1, 2

. The challenge for higher cost of

, Hevea brasiliensis (Rubber tree)

Mesua ferrea (Nahor)

1, 4

5, 6

, Pongamia

are sustainable non-edible raw

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Beside edibility of the feedstock, thermal degradation characteristics of both the feedstock

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and product are of great concern for scientific applications 8-11. Thermal degradation analysis can

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be used to estimate the thermal properties such as activation energy, enthalpy, Gibbs free energy,

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entropy, heat capacity as well as the quality of the produced biodiesel

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(TG) technique monitors the physical and chemical changes of sample happens with

44

temperature. Thermal degradation of vegetable oils and their methyl esters mainly involves 2 ACS Paragon Plus Environment

12, 13

. Thermogravimetry

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physico-chemical processes of volatilization and decomposition. The gaseous products evolved

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during thermal decomposition of the sample can be identified in real time by coupling Fourier

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transform infrared spectroscopy (FT-IR) with TG

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such coupled technique has been used in various research fields to estimate sample structure and

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composition. Li et al.

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behavior of biodiesel samples derived from peanut oil, palm oil and waste cooking oil. TGA

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results showed that the onset decomposition and peak temperatures for palm oil methyl esters are

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higher than both peanut oil and waste cooking oil methyl esters due to lower content of

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unsaturated fatty acids components in palm oil methyl esters. The real time analysis of evolved

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products using attached FT-IR revealed the formation of alkanes, cyclic and aromatic

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compounds along with CO, CO2 and H2O. Santos et al.13 reported thermal decomposition of

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sunflower oil and its biodiesel using non-isothermal thermogravimetric analysis under inert

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atmosphere. The values of kinetic parameters have been estimated as 155.62–200.12kJ/mol (E)

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and 0.95–1.82 (n) for sunflower oil and 61.32kJ/mol–115.35kJ/mol (E) and 0.69–1.89 (n) for its

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biodiesel. Souza et al.

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under air and nitrogen atmospheres. The values of activation energy (E) for cotton oil have been

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found higher than that of cotton oil based biodiesel for both air and nitrogen atmosphere.

10

15

10, 14

. The analysis of evolved gas by using

employed thermogravimetry analysis (TGA) to estimate thermal

evaluated thermal and kinetic behavior of cotton oil and its biodiesel

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The investigations on thermal degradation of rubber seed oil and its biodiesel have been

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rarely reported. In the present work, thermal degradation behavior of rubber seed oil and its

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biodiesel under nitrogen atmosphere was studied. There are varieties of kinetic methods

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available in open literature to deduce the kinetic parameters. However, Friedman model (FRD),

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Flynn-Wall-Ozawa (FWO), and Coats-Redfern (CR) kinetic methods are consider more reliable

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and widely used. Hence, in the present study, Friedman (FRD), Flynn-Wall-Ozawa (FWO), 3 ACS Paragon Plus Environment

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Modified Coats-Redfern (MCR) and Kissinger (KM) kinetics methods were applied to estimate

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the value of activation energy (E) and enthalpy (∆H) for thermal degradation reactions.

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MATERIALS AND METHODS

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Materials. Rubber seeds, collected from Assam India were de-shelled manually and the

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obtained kernels were subjected for oil extraction. Rubber seed oil (RSO) and rubber seed oil

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methyl esters (ROME) were obtained through Soxhlet extractor and ultrasonic- assisted

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transesterification, respectively. The detail can be found in our previous studies

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extracted oil and produced biodiesel under optimum conditions were used for thermal analysis.

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The physico-chemical properties of obtained RSO and ROME are presented in Table 1

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13

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(biodiesel) are depicted in Fig. S1 and Fig. S2, respectively. The signal at 69 ppm and 62 ppm in

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13

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O–) and 62 ppm (CH2–C–O–)) (Fi. S1) while the signal is absent in the biodiesel (ROME) (Fig.

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S2). It can be clearly seen that the glyceride backbone of triglyceride is totally absent in the

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ROME sample. The methoxy carbon of methyl esters of ROME illustrates the signal at 51.49

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ppm. The unsaturation signal (–C=C–) obtained between 133–120 ppm in

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the

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(monounsaturated) fatty acids and ester in RSO (Fig. S1) and ROME (Fig. S2), respectively.

16, 17

. The

16

. The

C Nuclear magnetic resonance (NMR) spectra of rubber seed oil and its methyl esters

C NMR spectrum of rubber seed oil are due to the carbonyl methylene groups (69 ppm (H–C–

presence

of

linoleic

(polyunsaturated),

linolenic

13

C NMR are due to

(polyunsaturated)

and

oleic

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Methods. Thermogravimetric analysis. Thermal degradation of rubber seed oil (RSO) and

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rubber seed oil methyl esters (ROME) were evaluated using TG analyzer (Netzch STA449F300)

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at various heating rates (10, 20, 30 40 and 50 °C/min) under nitrogen atmosphere. The samples

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were weighed ~10 mg in alumina crucible. TGA experiments were conducted from 30 °C to 800 4 ACS Paragon Plus Environment

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°C for respective heating rates, where the nitrogen gas (99.999% purity) flow rate was set at 60

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ml/min. The evolved gas products during thermal decomposition at heating rate of 40 °C/min

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were continuously monitored and measured using Perkin-Elmer TGA and FT-IR coupled system.

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Gas transfer tube and gas cell were heated up to 250 °C to prevent condensation of evolved

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products.

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Thermal degradation study. The values of activation energy (E) for thermal decomposition of 18

RSO and ROME were calculated using Friedman (FRD)

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Kissinger (KM)

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rate, considering Arrhenius temperature dependency for constant heating rate can be expressed

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by the following Eq.1.

20

, Flynn-Wall-Ozawa (FWO)

19

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and Modified Coats-Redfern (MCR) methods

(

11, 21

,

. Thermal decomposition

)

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β ⋅ dα

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Where β, f(α), A, α, E, T and R refer to heating rate (°C/min), reaction mechanism model, pre-

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exponential factor (1/min), degree of decomposition of the samples (conversion), activation

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energy (J/mol), temperature (K) and gas constant (8.314 J/mol K), respectively. The value of α at

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appropriate time/temperature can be calculated using TG data (Eq.2)

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α=

106

Where: wo, wt and wf are initial weight, weight at time t, and final weight, respectively.

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dT

= f (α ) ⋅ A exp − E

( w0 − wt )

RT

(Eq.1)

(Eq.2)

( w0 − w f )

Friedman model (FRD) (Eq.3) is the first and general isoconversional method on the basis of

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model free differential technique, and obtained by taking natural logarithm both side of Eq. 1

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and becomes (Eq. 3).


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(

)

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ln dα

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The aspects of Friedman 18 and Coats-Redfern 21 methods can be combined for estimation of

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kinetic parameters for multiple heating rates. The general expression modified Coats-Redfern for

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nth order is as follows (Eq. 4) 22-24;

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  −E ln  β 2  = ln A ⋅ R E ⋅ g ( ) R ⋅ Ti, j α  T i , j i, j

dt i, j

= ln ( f (α ) ⋅ A ) − E

(

(Eq.3)

RTi, j

)

(Eq.4)

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Where: g(α) is the integral form of the reaction model.

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Beside the differential approaches, the fundamental rate expression (Eq.1) can also be used

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by integral method of Flynn-Wall-Ozawa model (Eq. 5) using Doyle approximation to estimate

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the kinetic parameters 19.

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ln βi, j = C − 1.052 E

(Eq.5)

RTi, j

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Kissinger proposed that the maximum rate occurs when d(dα/dt)/dt is zero. Therefore, the

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differentiation of fundamental Arrhenius expression (Eq. 1) for constant heating rate at which

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maximum rate occurs (at peak temperature Tmax) is equal to zero. The simplified Kissinger model

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for first order thermal decomposition at peak temperature (Tmax) expression is as follow (Eq. 6);

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 β   AR   E   1  ln  = ln     −   T2  E    R   Tmax  j  max  j

(Eq.6)

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Based on the same degree of thermal degradation (i) at different heating rates (j), linear plots

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of ln(dα/dt)i,j versus 1/Ti,j (FRD, Eq. 3), ln(βj/T2i,j) versus 1/Ti,j (MCR, Eq. 4), ln(βj) versus 1/Ti,j


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(FWO, Eq. 5) and ln(β/Tmax2) versus 1/Tmaxj (KM, Eq. 6) were constructed. The slope of the

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straight line was used to calculate the value of activation energy.

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The thermodynamic parameters such as enthalpy (∆H) (Eq.7), Gibbs free energy (∆G) and

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entropy (∆S) of RSO and ROME samples were calculated at the maximum peak temperatures 10,

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13, 25

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calculated using Eq. 8 10, 25. The obtained pre-exponential factors were used to calculate ∆G (Eq.

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9) and ∆S (Eq.10) 26-28.

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∆ H = E − RT

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A=

136

∆G = E + RT ln 

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∆S =

. For this purpose , the value of pre-exponential factor (A) for RSO and ROME were

(Eq.7)

β ⋅E

 E  ⋅ exp    RT  R ⋅T 2

(Eq.8)

 KB ⋅T    h⋅ A 

(Eq.9)

∆ H − ∆G

(Eq.10)

T

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Where KB is Boltzmann constant (1.3806×10-23 m2 kg s-2 K-1) and h is Planck constant

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(6.6261×10-34 m2 kg s-1).

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Beside the values of activation energy and pre-exponential factor of thermal degradation,

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reaction order is also an important index. Avrami theory was applied to calculate the order of

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thermal degradation of both RSO and ROME at various temperatures using (Eq.11) 10, 13, 29:

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α = 1 − exp 

144

Eq.11 can be simplified to the following expression (Eq.12);

 A ⋅ exp ( − E / RT )    βn  

(Eq.11)


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E − n ln β RT

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ln(− ln(1 − α )) = ln A −

146

At a particular degradation temperature, T, the points of ln(-ln(1-α)) versus lnβ at various heating

147

rates could be fitted to a linear line. The reaction order (n) can be calculated from the slope of the

148

linear line.

(Eq.12)

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RESULTS AND DISCUSSION

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TGA analysis. Thermal decomposition behavior of both, RSO and ROME samples was

151

investigated. TGA and DTG profiles for RSO and ROME at various heating rates are shown Fig.

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1. Volatilization occurs during early stage, when the lighter components evolved. During

153

decomposition the heavier components break to low molecular weight components. Further, the

154

evolved products, low molecular weight components during decomposition go through the

155

volatilization process. Thus, in active thermal degradation stage both the phenomena take place.

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A change in the slope of TG profile was considered as the beginning of new stage. However,

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only active thermal degradation stage was subjected for kinetic analysis. Therefore, overall 3

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stages were found for both the samples, RSO and ROME (Fig. 1). Active pyrolysis stages for

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RSO (stage I, IIa and IIb) and for ROME (stage I and II) were selected for kinetic analysis. The

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two split peaks of RSO (stage IIa and IIb) and ROME (stage Ia and Ib) were considered as single

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stage (II for RSO and stage I for ROME) for kinetic analysis. The main (active) thermal

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decomposition of ROME sample at all the heating rates considered (10, 20, 30, 40 and 50

163

°C/min) occurred in single stage, that describes the decomposition and volatilization

164

phenomenas. However, the two-stage thermal decomposition was observed for RSO sample

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(Table 2). The first stage (RSO-I) showed the presence of higher free fatty acids in the sample. In

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addition to free fatty acids decomposition, mass loss due to moisture removal and degradation of 8 ACS Paragon Plus Environment

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light volatile compounds also takes place within this stage. The active decomposition of RSO in

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second stage (RSO-II) is due to the degradation and volatilization of triglyceride. At a heating

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rate of 20 °C/min, the thermal decomposition of RSO and its methyl ester (ROME) occurred in

170

the temperature range of 242–480 °C (95.7 wt%) and 165–456 °C (97.2 wt%), respectively. The

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active thermal decomposition and volatilization of RSO sample were started relatively at higher

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temperature values, started around 242 °C and completed around 480 °C compared to its methyl

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ester (ROME) (i.e. started around 165 °C and completed around 456 °C). This is due to the fact

174

that RSO has higher molecular weight compounds and stronger intermolecular force (higher

175

viscosity) as compared to ROME. The value of onset temperature for ROME (165 °C) obtained

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in the present study is proximity similar with palm oil methyl ester (164.5 °C), peanut oil methyl

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ester (155.8 °C), waste cooking oil methyl ester (142.2 °C) as reported by Li et al.10. The onset

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temperature values for palm oil methyl ester (164.5 °C) 10, peanut oil methyl ester (155.8 °C) 10,

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waste cooking oil methyl ester (142.2 °C) 10 were approximately similar with ROME (165 °C).

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It can be seen from Fig. 1(A–D) that the onset temperature and the temperature at which the

181

rate of mass loss is maximum (Tmax) were shifted towards higher temperatures with increasing

182

heating rate. This is due to low heat distribution (heat transfer limitation) 10, 22, 30. With respect to

183

heat transfer phenomenon, the initial thermal degradation temperature values for rubber seed oil

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in first stage (RSO-I) and second stage (RSO-II), and for rubber seed oil methyl esters (ROME)

185

were shifted from 223 °C to 264 °C, 253 °C to 295 °C and 120 °C to 193 °C as the heating rate

186

increased from 10 °C/min to 50 °C/min, respectively (Table 2). Furthermore, the values of peak

187

temperature, Tmax were also changed from 271 °C to 328 °C, 416 °C to 446 °C and 250 °C to

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287.9 °C for thermal degradation of RSO-I, RSO-II and ROME, respectively. Similarly, some

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other parameters such as Tf and wmax values were also increased (Table 3). From Fig. 1(B and D), 9 ACS Paragon Plus Environment

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it can also be clearly observed that the heating rate has a significant effect on the rate of thermal

191

degradation of the samples.

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Fourier transform infrared (FT-IR) analysis of evolved products. FT-IR spectra obtained

193

for RSO and ROME were found with similar characteristics due to similar nature of chemical

194

structure of functional group present in the samples (Fig. S3, supplementary data). However, the

195

signals specific to hydroxyl group of free fatty acid can be observed only in the spectrum of RSO

196

at 3480 cm-1. In addition, single peak at 1456 cm-1 was observed for bending vibration of CH2

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and CH3 group only in RSO sample. Furthermore, a signal peak specific to ester functional group

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for triglycerides for RSO and methyl esters of ROME was clearly observed at wave number

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1735–1740 cm-1. Fig. 2 shows FT-IR signatures of evolved products at various mass loss

200

temperatures during thermal volatilization and decomposition of RSO and its methyl esters for

201

heating rate of 40 °C/min. Absorbance peaks corresponding to gaseous and liquid water

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molecules can be seen only for RSO thermal degradation at 3500–3950 cm−1 and 3400–

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3500 cm−1, respectively

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oxygen-containing group in ROME (i.e. R1COOR2) mainly decomposed into C=O– and C–O 10,

205

12

206

clearly shown in Fig. 2. Symmetric and asymmetric stretching vibrations of –CH– and, –CH3

207

asymmetric deformation vibration in the range of wave number, 3000–2700 cm-1 and 1475–1000

208

cm-1, respectively revealed the presence of alkanes in the evolved products during thermal

209

degradation of RSO and ROME. Carbonyl groups of aldehydes and ketones were also observed

210

as H–C=O– and –C=O– in plane bending vibrations at 1720–1740 cm-1 and 1735–1750 cm-1

211

(Fig. 2), respectively. Furthermore, C–O–C (stretching vibration at 1000–1300 cm-1), C=O

212

(bending vibration at 2250–2400 cm-1 and 580–730 cm-1), C=O (stretching vibration at 2000–

31

. This shows the moisture content for RSO higher than ROME and

. The characteristics infrared absorption peaks for volatile components functional group were


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2250 cm-1) and C–O (stretching vibration at 2200–2100 cm-1) were observed in the evolved

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gaseous product (Fig. 2) 14.

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Taking the absorbance of identified volatile compounds such as alkanes, alkenes,

216

aldehydes, ketones, ethers and CO2, the intensity of the evolved compounds with increasing

217

thermal decomposition temperature is presented in Fig. 3. As the thermal decomposition

218

temperature increased, ether and aldehydes were formed due to de-oxygenation of ester12.

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Presence of CO2 in the evolved product revealed that the de-carboxylation of ester and

220

triglyceride occurred during the thermal decomposition of ROME and RSO.10. It can be seen

221

from Fig. 3 that the maximum absorbance characteristic peaks for alkenes, alkanes, aldehydes

222

and ether occurred at same temperature (448 °C) and, for ketones and CO2 at temperature of 536

223

°C during thermal degradation of RSO. Similarly, during the thermal degradation of ROME, the

224

maximum rate of alkenes, alkanes, aldehydes and ether production occurred at temperature of

225

307 °C while for ketones and CO2, the maximum rate values were found at 571 °C. As can be

226

observed in TGA profiles (Fig. 1) and FT-IR spectra (Fig. 2), beyond 500°C, the mass losses

227

during thermal decomposition of ROME (1.44 wt%) and RSO (1.19 wt%) are mainly due to the

228

formation of CO2 and ketone (Fig. 3). Taking the Lambert-Beer law into consideration, the

229

concentrations of alkanes in evolved products were found maximum during the thermal

230

decomposition of RSO and ROME

231

ROME with temperature (Fig. 3) were found similar with that of DTG curves (Fig. 1).

12, 14

. The appearance of absorbance profiles of RSO and

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Kinetic parameter calculation. It can be seen from Fig. 4 that the degree of conversion of

233

both the samples were greatly varied with temperature and heating rate. To estimate the

234

dependency of activation energy on temperature and degree of conversion during the active

235

decomposition (pyrolysis) process, nine conversion fractions from 0.1 to 0.9 with increment of 11 ACS Paragon Plus Environment

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0.1 were selected at all heating rates. Based on FRD, FWO and MCR isoconversional methods,

237

the activation energies (E) for selected conversions were calculated from the slopes of linear

238

regression (Fig. 5–7).

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The deduced regression lines for RSO-I, RSO-II and ROME are presented in Fig. 5–7 and

240

the values of activation energies (E) are summarizes in Table 3. Parallel lines shown in Fig. 5–7

241

indicate that the values of activation energy (E) for thermal degradation of the samples (RSO and

242

ROME) for free fatty acids (RSO-I), triglycerides (RSO-II) and fatty acid methyl esters (ROME)

243

in the respective sample follow the same reaction rate or intensity. In other words, the values of

244

activation energy (E) obtained at different degree of conversions and temperatures were

245

proximity similar for RSO-I and ROME. This suggests that a single mechanism or unification of

246

multiple reactions mechanism was followed in the active thermal degradation of RSO-I and

247

ROME

248

to each other’s and this shows the change in activation energy at different degree of conversions

249

due to multiple and parallel reactions during thermal decomposition of triglycerides of RSO.

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Change in the slope of the lines at different conversion show that the rates of thermal

251

decomposition differ due to multiple reactions occurring. The higher values of activation

252

energies were found at later stage of conversion (α=0.9). The values of R2 were found to be more

253

than 0.99 for the selected conversions (0.1 to 0.9) which show the fitness of the methods

254

considered, FRD, FWO and MCR.

255

The values of activation energy (E) for RSO-I and RSO-II vary from 69.8–92.7 kJ/mol and 144–

256

433.3 kJ/mol, respectively. The difference in the values of activation energy at lower (α=0.1) and

257

higher (α=0.9) degree of conversion for RSO-II reveals that incomplete decomposition of free

258

fatty acids occurred in first stage. Further, it also suggests that the thermal decomposition of

10, 11

. However, for RSO-II, fitted lines at different conversion were slightly not parallel


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Energy & Fuels

259

RSO samples is a complex reaction which involves several parallel, competitive and consecutive

260

types of reactions. Activation energy (E) is the critical energy barrier to be overcome to generate

261

a chemical reaction and also it represent the minimum energy required to break the chemical

262

bond between atoms 29. Hence, the higher values of activation energy for RSO-II as compared to

263

RSO-I indicate that more difficult reactions have taken place during secondary stage (RSO-II).

264

This is maybe due to higher free fatty acids content in RSO sample. Considering all three

265

methods (FRD, FWO and MCR), the overall average values of activation energy (E) for thermal

266

decomposition of RSO were found to be 167.8 kJ/mol (FRD), 144.85 kJ/mol (FWO) and 142.05

267

kJ/mol (MCR). The obtained overall average values of activation energy for RSO thermal

268

decomposition are in good agreement with sunflower oil (170–210 kJ/mol)

269

160.2 kJ/mol)

270

kJ/mol)

271

RSO-I and RSO-II. The values of activation energy obtained by Kissinger method (E) (67.89

272

kJ/mol, RSO-I; 183.85 kJ/mol, RSO-II) are lower than that of obtained by FRD (76.7 kJ/mol,

273

RSO-I; 258.9 kJ/mol, RSO-II), FWO (80.9 kJ/mol, RSO-I; 208.8 kJ/mol, RSO-II) and MCR

274

(75.8 kJ/mol, RSO-I; 208.3 kJ/mol, RSO-II) methods.

11

11

, karanja seed oil (156.5–160.7 kJ/mol)

11

13

, soybean (146.6–

and mustard seed oil (142.4–148.1

. Fig. 6C shows the linear plots for Kissinger method for thermal decomposition of

275

Table 3 demonstrates that the average values of activation energy (E) of ROME were

276

estimated as 87.7kJ/mol, 85.5 kJ/mol and 80.9 kJ/mol calculated using FRD, FWO and MCR

277

approaches, respectively. The values are lower than that of RSO-II. HIgher molecular weight of

278

triglycerides of RSO require high energy for thermal decomposition and volatilization

279

differences in the values of activation energy (E) of RSO and ROME revealed that

280

decomposition or/and volatilization mechanism of the samples occur in different manner, and

281

RSO was chemically modified through transesterification process (Table 3). However, the


13

. The

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Page 14 of 35

282

activation energy range for first stage decomposition of RSO was found to be similar with

283

ROME thermal degradation with 5–12% error. Fig.7D shows the fitness of Kissinger method for

284

ROME thermal decomposition and the activation energy was found to be 92.5 kJ/mol.

285

Considering all the above methods, the average activation energy for ROME was obtained as

286

86.65 kJ/mol. The calculated average activation energy is in good agreement with for sunflower

287

oil methyl ester

288

cooking oil methyl ester (50.07 kJ/mol) and palm oil methyl ester (54.09 kJ/mol) 10. This may be

289

due to the fact that the physico-chemical-thermal properties of the parent feedstocks and

290

produced biodiesels differ.

291

13

. However, it is higher than peanut oil methyl ester (49.71 kJ/mol), waste

Order of reactions for RSO and its methyl ester. Most of the investigations assumed zero11

292

order or first-order reaction for thermal decomposition of oil and biodiesel samples

293

present study, the dependency of order of reaction (n) on temperature for thermal degradation of

294

RSO and ROME were evaluated through the Avrami theory. The regression plots of RSO (stage

295

I and II) and ROME are shown in Fig. 8(A–C) and the calculated values of order of reaction for

296

thermal degradation of samples are presented in Table 4. Based to the R2 values (Table 4), the

297

Avrami theory is suitable and well fitted to estimate the values of n for thermal degradation of

298

RSO and ROME. As the decomposition temperature increased from 267 °C to 307 °C (within

299

stage I), the reaction order of RSO first decreased from 1.59 to 1.26 and then increased to 1.42.

300

Further increasing the decomposition temperature for RSO, 402 °C to 457 °C (within stage II),

301

the value of n decreased from 1.41 to 0.38. The average order of reaction for RSO-I (1.37) was

302

found higher than that of RSO-II (0.92). The signatures of different compounds in FTIR spectra

303

of RSO-I, RSO-II and ROME also suggest that the values of overall order of the reactions may

304

differ significantly. The values of n for sunflower oil reported Santos et al. 13 vary in the range of 14 ACS Paragon Plus Environment

. In the

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Energy & Fuels

305

0.95 to 1.82. Similarly, for overall decomposition of RSO, the n value is within the range 0.39 to

306

1.59. Varying decomposition temperature for ROME, the reaction order (n) initially increased

307

from 0.92 (227 °C) to 1.10 (257 °C) and then decreased to 0.19 (317 °C). Li et al. 10 and Santos

308

et al.

309

1.77 to 1.95 (257 °C–277 °C), 0.71 to 1.13 (257 °C–277 °C) and 0.69 to 1.89 for peanut oil,

310

palm oil, waste cooking oil and sunflower methyl esters, respectively 10, 13. Similarly, the order of

311

reaction (n) was found to be varied from 0.96 to 1.1 (237 °C–267 °C) for ROME thermal

312

degradation. Furthermore, the average value of order of reaction (n) for ROME (0.69) is in good

313

agreement with soybean (0.5)

314

esters thermal decomposition.

13

reported that the values of order reactions (n) varied from 1.6 to 1.68 (269 °C–277 °),

32

, higuereta (0.7)

32

, babassu (1.4)

33

and palm (0.4)

33

oil ethyl

315

Thermodynamic parameter calculation for RSO and its methyl esters. In addition to the

316

values of activation energy and order of reaction, important thermodynamic parameters (∆H, ∆G

317

and ∆S) for thermal decomposition of RSO and it biodiesel were calculated using Eq. 7–10. It

318

can be seen from Fig. 8D that all the calculated ∆H values are positives; thermal decomposition

319

of RSO and ROME within active degradation stages is endothermic processes. Due to high

320

molecular weight of triglycerides of vegetable oil as compared to that of their fatty acid esters,

321

∆H for vegetable oil is higher than that of biodiesel

322

calculated by FWO and MCR methods for RSO within conversion interval of 0.1 to 0.9 fractions

323

are higher than that of ROME. The values of ∆H by FWO and FRD were found within the range

324

of 69.27–79.34 kJ/mol (RSO-I), 138.39–318.01 kJ/mol (RSO-II) and 61.76–84.52 kJ/mol

325

(ROME). The average values of ∆H, considering the active thermal degradation for above two

326

methods, were determined as 146.26 kJ/mol and 78.69 kJ/mol for RSO and ROME, respectively.

327

The values of ∆H are positive which indicate endothermic reactions. High values of ∆H for RSO

12

, similar to E values. The values of ∆H


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34, 35

Page 16 of 35

. Li et al.12 evaluated the average values of ∆H for

328

infer the high degree of endothermicity

329

thermal decompositions of peanut oil (118.54 kJ/mol) and its biodiesel (48.08 kJ/mol) using

330

applying different kinetics methods. Oliveira et al.

331

kJ/mol) and babassu (80.38 kJ/mol) oil biodiesel. Additionally, ∆S and ∆G at To and Tmax for

332

RSO and ROME thermal degradation were evaluated and, the obtained data are presented in

333

Table 5. The negative values of ∆S and positive values of ∆G confirm that the thermal

334

decomposition of both the samples are non-spontaneous process10,

335

since the samples were subjected to forced thermal decomposition by non-isothermal conditions.

336

The values of ∆S obtained at To and Tmax for RSO and ROME thermal degradation are negative,

337

which indicates that the activated complex has a more ordered structure than the reactants, and

338

that the reactions are slower

339

required to reduce the degree of disorder at To as compared to disorder degree at Tmax for RSO-I,

340

RSO-II and ROME (Table 5). The higher value of ∆G reveals lower favorability of a reaction10,

341

12, 25, 37, 38

342

to be 145.01–201.37 kJ/mol and 115.73–152.19 kJ/mol, respectively. Higher value of ∆G for

343

RSO indicates that larger amount of heat is required for thermal decomposition as compared its

344

biodiesel, which is similar with the average activation energy values (E).

34-36

33

reported ∆H values of for palm (90.53

12, 25, 33

. This was expected

. The absolute values of ∆S indicate that higher energy is

. The favorability order for thermal degradation process of RSO and ROME were found

345

CONCLUSIONS

346

Thermal decomposition of RSO and its methyl esters (ROME) under inert atmosphere was

347

taken place in two and one stage, respectively. The thermal stability of RSO was found greater

348

than that of its biodiesel. Rate of maximum weight loss (wmax) was increased from 12.1%/min to

349

76.7%/min (RSO) and 15.9 %/min to 76.2%/min (ROME) as the heating rate increased from 10

350

°C/min to 50 °C/min. Similarly, the active thermal degradation temperature range were also 16 ACS Paragon Plus Environment

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Energy & Fuels

351

shifted from 223 °C–471 °C to 264 °C–488 °C for RSO and 120 °C–407 °C to 193 °C–480 °C

352

for ROME. The values of activation energy (E) and enthalpy (∆H) of RSO, calculated by several

353

methods (FRD, FWO, MCR and KM) were found greater than that of ROME. The values of

354

average order of reaction obtained using Avrami theory were found to be 1.14 and 0.69 for RSO

355

and ROME, respectively. The values of order of favorability (∆G) for thermal decomposition of

356

RSO and ROME were 145.01–201.37 kJ/mol and 115.73–152.19 kJ/mol, respectively.

357

Furthermore, the positive value of ∆G and negative value of ∆S at initial (To) and maximum

358

(Tmax) thermal degradation temperatures indicate that thermal decomposition for RSO and

359

ROME are non-spontaneous process. The absorbance peaks for alkanes, alkenes, aldehydes,

360

ketones, ethers, water, carbon dioxide and carbon monoxide were detected in the evolved

361

products. The absence of absorbance peak for water in ROME degradation shows the quality of

362

produced ester from RSO through transesterification. Thermal decomposition of RSO and

363

ROME after 500 °C (~1.19 wt% loss for RSO and 1.44 wt% loss for ROME) was mainly due to

364

the formation of ketone and CO2. From TGA-FTIR, it can be concluded that, absorbance

365

characteristics for the formation of evolved product such as alkenes, alkane ethers and aldehydes

366

were maximum within temperature range of 400 °C–500 °C and 200 °C–350 °C for RSO and

367

ROME, respectively.

368

ACKNOWLEDGMENTS

369

Authors would like to acknowledge the Central Instruments Facility (CIF) and Center for

370

Energy at Indian Institute of Technology Guwahati (IITG) for providing the characterization

371

facility to conduct the sample analyses. Authors also would like to acknowledge the Center of

372

Excellence on Sustainable Polymers at IITG for providing access to TGA-FTIR instrument.


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373 374 375 376 377 378 379

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(4) No, S. Y. Renewable and Sustainable Energy Reviews 2011, 15, 131–149.

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(5) Ahmad, J.; Yusup, S.; Bokhari, A.; Kamil, R. N. M. Energy Conversion and Management

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2014, 78, 266–275. (6) Reshad, A. S.; Barman, P.; Chaudhari, A. J.; Tiwari, P.; Kulkarni, V.; Goud, V. V.; Sahoo, N. Energy and Fuels 2015, 29, 5136–5144. (7) Atabani, A. E.; Badruddin, I. A.; Mahlia, T. M. I.; Masjuki, H. H.; Mofijur, M.; Lee, K. T.; Chong, W. T. Energy Technology 2013, 1, 685–694.

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(8) Math, M. C. Energy for Sustainable Development 2007, 1, 100–104.

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Calorimetry 2012, 109, 1145–1150.

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(10) Li, H.; Niu, S.; Lu, C.; Cheng, S. Energy Conversion and Management 2015, 98, 81–88.

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(11) Volli, V.; Purkait, M. K. Fuel 2014, 117, 1010–1019.

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(12) Li, H.; Niu, S.; Lu, C.; Wang, Y. Energy and Fuels 2015, 29, 5145–5153.

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(13) Santos, A. G. D.; Caldeira, V. P. S.; Farias, M. F.; Araujo, A. S.; Souza, L. D.; Barros, A.

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K. Journal of Thermal Analysis and Calorimetry 2011, 106, 747–751.


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S. M.; Teixeira, L. S. G.; Novak, C. Journal of Thermal Analysis and Calorimetry 2007, 90,

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945–949.

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(16) Reshad, A. S.; Panjiara, D.; Tiwari, P.; Goud, V. V. Journal of Cleaner Production 2017, 142, 3490–3499.

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(17) Reshad, A. S.; Tiwari, P.; Goud, V. V. Fuel 2015, 150, 636–644.

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(18) Friedman, H. L. Journal of Polymer Science: Part C 1964, 6, 183–195.

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(19) Flynn, J. H.; Wall, L. A. Journal of Research of the National Bureau of Standards —A.

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Physics and Chemistry 1966, 6, 487–523.

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(20) Kissinger, H. E. Analytical Chemistry 1957, 29, 1702–1706.

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(21) Ebrahim-Kahrizsangi, R.; Abbasi, M. H. Transactions of Nonferrous Metals Society of

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China 2008, 18, 217–221. (22) Niu, S.; Liu, M.; Lu, C.; Li, H.; Huo, M. Journal of Thermal Analysis and Calorimetry

2014, 115, 73–79.

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(23) Niu, S.; Han, K.; Lu, C.; Sun, R. Applied Energy 2010, 87, 2237–2242.

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(24) Vayazovkin, S.; Sbirrazzuoli, N. Analytica Chimica Acta 1997, 355, 175–180.

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(25) Kim, Y. S.; Kim, Y. S.; Kim, S. H. Environmental Science and Technology 2010, 44,

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5313–5317.

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2007, 61, 4670–4673.

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(29) Gai, C.; Dong, Y.; Zhang, T. Bioresource Technology 2013, 127, 298–305.

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2009, 96, 897–901. (33) Oliveira, L. E.; Giordani, D. S.; Paiva, E. M.; De Castro, H. F.; Da silva, M. L. C. P. Journal of Thermal Analysis and Calorimetry 2013, 111, 155–160.

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(35) Aravindakshan, K. K.; Muraleedharan, K. Thermochim Acta 1990, 159, 101–107.

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Lyubchev, L. A. Journal of the Chinese Chemical Society 2010, 57, 411–416. (37) Mo, Y.; Zhao, L.; Chen, C.; Tan, G. Y. A. T.; Wang, J. W. Journal of Thermal Analysis and Calorimetry 2013, 111, 781–788. (38) Vecchio, S.; Cerretani, L.; Bendini, A.; Chiavaro, E. Journal of Agricultural and Food Chemistry 2009, 57, 4793–4800.

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Energy & Fuels

441

LIST OF TABLE CAPTIONS

442

Table 1: Physico-chemical properties of rubber oil (RSO) and its methyl esters (ROME) samples

443

Table 2: TG characteristic properties for active pyrolysis stages of RSO and ROME samples

444

Table 3: The value of activation energy deduced from FRD, FWO and MCR methods

445

Table 4: The value of reaction order for thermal decomposition of RSO and ROME calculated

446

using Avrami theory

447

Table 5: Thermodynamic parameters of RSO and ROME samples at TO and Tmax for heating rate

448

of 10 °C/min

449 450 451 452 453 454 455 456 457 458 459 460 461 462


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463

Table 1: Physico-chemical properties of rubber oil (RSO) and its methyl esters (ROME) samples Properties

Unit –

Specific Gravity @ 24 °C

464 465

Page 22 of 35

RSO

ROMEα Biodiesel#

Diesel§

0.91

0.883

0.86–0.9*

0.846*

Kinematic Viscosity @24°C

mm2/s

30

5.82

1.9–6*

1.9–4.1*

Kinematic Viscosity @40°C

mm2/s

13.13

3.81

1.9–6

1.9–4.1

Calorific value

MJ/kg

39.34

39.53

35min

45.62–46.48

Iodine Value

g I2/100g

113

114

120max

N/A

Acid Value

mg KOH/g oil

24

0.4

0.5max

0.35

Saponification value

mg KOH/g oil

235.28

190

N/S

N/A

Refractive index @ 24°C

–

1.47

1.45

N/S

N/S

Cloud point

°C

3

2.5

(-3) –12

(-15) –5

Pour point

°C

2**

-3 **

(-15) –10

-20

Cetane No.

–

44

49.9

47min

40min

Moisture content

%

0.27

0.33

0.05max

0.05max

Flash point

o

273

131

130min

52 – 96

Fire point

o

282

146

N/S

N/S

C C

Rubber seed oil methyl ester, *measured at 40 oC, #Standards ASTM test methods, §Fossil fuel, ** DSC method, N/S: not specified, N/A: not applicable α

466 467 468 469 470 471 472 22 ACS Paragon Plus Environment

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473 474

Table 2: TG characteristic properties for active pyrolysis stages of RSO and ROME samples Sample

Properties

Heating rate (°C/min) 10

20

30

40

50

Stage

I

II

I

II

I

II

I

II

I

II

To, °C

223

353

242

373

249

382

253

384

264

395

Tf, °C

309

471

334

480.0

345.6

483

350

487

365

488

∆wt, %

20

69.5

20

69.5

20

69.5

20

69.5

20

69.5

Tmax, °C

271

416

296

424

310

438

318

443

328

446

wmax,%/min 3.26

12.1

5.9

35.2

8.8

44.6

11.7

60

13.6

76.7

RSO

To, °C

120

–

165

–

172

–

181

–

193

–

Tf, °C

407

–

456

–

470

–

474

–

480

–

∆wt., %

97.2

–

97.2

–

97.2

–

97.2

–

97.2

–

Tmax, °C

250

–

268.5

–

277.5

–

283.5

–

287.9

–

wmax,%/min 15.9

–

29.1

–

41.4

–

55.2

–

76.2

–

ROME

475

To: Initial temperature for the main mass loss (°C), Tf: Final temperature for the main mass loss

476

(°C), Tmax: Temperature for maximum rate of mass loss (°C), ∆wt: change of mass loss and wmax:

477

maximum mass loss rate (%/min)


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478

Page 24 of 35

Table 3: The value of activation energy deduced from FRD, FWO and MCR methods Activation Energy (kJ/mol) at Conversion (%)* Average Sample

Stage

I RSO

II

ROME

479 480

I

Method #

10

20

30

40

50

60

70

80

90

FRD

72.8

70.7

69.8

73.5

74.1

75.2

77.72

83.7

92.7

76.7

FWO

83.8

81.6

80.2

79.6

79.4

79.5

79.9

81.1

83.7

80.9

MCR

79.3

76.8

75.2

74.4

74.1

74.1

74.3

75.5

78.1

75.8

FRD

164.2

196.9

220.7

212.1

231.2

261.1

283.1

328.6

433.3

258.9

FWO

147.8

162.8

177.9

188.9

197.2

209.4

225.4

250.9

319.2

208.8

MCR

144

160

176.3

187.3

196

209.5

225.3

252.1

324.1

208.3

FRD

71.8

89.6

93.9

93.3

90.9

87.1

83.9

78.2

100.9

87.7

FWO

70.5

78.4

84.6

88.1

89.9

90.4

90.1

88.1

89.7

85.5

MCR

65.9

74.0

80.3

83.8

85.6

86.1

85.5

83.3

84.2

80.9

*

Stage by stage conversion (α) of active pyrolysis of the samples, # average R2 was found to be ~0.994 and overall average activation energy (E) for RSO decomposition was found to be 167.8 kJ/mol (FRD), 144.9 kJ/mol (FWO) and 152.5 kJ/mol (MCR)

481 482 483 484 24 ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

485 486

Energy & Fuels

Table 4: The value of reaction order for thermal decomposition of RSO and ROME calculated using Avrami theory RSO-I

RSO-II

ROME

Temp (°C)

n

R2

Temp (°C)

n

R2

Temp (°C)

n

R2

267

1.59

0.99

402

1.41

0.99

227

0.92

0.99

272

1.41

0.99

407

1.24

0.99

237

0.96

0.99

282

1.31

0.99

417

1.16

0.99

247

1.04

0.99

292

1.26

0.99

427

1.06

0.99

257

1.10

0.99

297

1.26

0.98

432

0.88

0.99

267

1.05

0.99

307

1.42

0.98

442

0.69

0.98

277

0.86

0.99

452

0.50

0.98

287

0.61

0.98

457

0.38

0.99

297

0.39

0.98

307

0.24

0.98

317

0.19

0.99

327

0.19

0.99

Average (n)

1.37

0.92

487 488 489 490 491 492 493 494


0.69

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

495 496

Page 26 of 35

Table 5: Thermodynamic parameters of RSO and ROME samples at To and Tmax for heating rate of 10 °C/min Sample

Temp (°C)

Log A (s-1)

∆H (kJ/mol)

∆G (kJ/mol)

∆S (kJ/mol K)

=223

4.89

+63.75

+145.01

-163.82

Tmax =271

5.67

+78.02

+159.41

-149.62

To

=353

7.46

+122.33

+195.35

-116.64

Tmax =416

11.83

+178.11

+201.37

-33.76

To

=120

1.01

+22.89

+115.73

-236.22

Tmax =250

7.2

+89.44

+152.19

-119.97

To RSO-I

RSO-II

ROME 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523


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Energy & Fuels

524

LIST OF FIGURE CAPTIONS

525

Fig. 1: TGA and DTG profiles (A, B) for RSO and (C, D) for ROME samples

526

Fig. 2: FT-IR spectra for evolved products during thermal decomposition of (A) RSO and (B)

527

ROME at heating rate of 40 °C/min

528

Fig. 3: Formation of evolved products during thermal decomposition of (A) RSO and (B) ROME

529

at heating rate of 40 °C/min

530

Fig. 4: TGA mass conversion for (A) RSO-I, (B) RSO-II and (C) ROME samples

531

Fig. 5: Fig.5: Regression plots based on (A) FRD for RSO-I, (B) FRD for RSO-II, (C) FWO for

532

RSO-I and (D) FWO for RSO-II

533

Fig. 6: Regression plots based on (A) MCR for RSO-I, (B) MCR for RSO-II and (C) KM for

534

RSO-I and RSO-II

535

Fig. 7: Regression plots based on (A) FRD, (B) FWO, (C) MCR and (D) KM for ROME thermal

536

degradation

537

Fig. 8: Regression plots to calculate order of reaction proposed by Avrami theory for (A) RSO-I,

538

(B) RSO-II and (C) ROME; (D) Enthalpy change for RSO and ROME based on FRD and MCR

539

methods

540 541 542 543 544 27 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

545

546 547

Fig.1: TGA and DTG profiles (A, B) for RSO and (C, D) for ROME samples

548 549 550 551 552 553 554 555


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Energy & Fuels

556 557 558

559 560 561

Fig. 2: FT-IR spectra for evolved products during thermal decomposition of (A) RSO and (B) ROME at heating rate of 40 °C/min

562 563 564 565 566 29 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

567 568 569 570 571 572

573 574

Fig.3: Formation of evolved products during thermal decomposition of (A) RSO and (B) ROME

575

at heating rate of 40 °C/min

576 577 578 579 580 581


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Energy & Fuels

582 583 584 585 586 587

588 589

Fig.4: TGA mass conversion for (A) RSO-I, (B) RSO-II and (C) ROME samples


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

590 591

Fig.5: Regression plots based on (A) FRD for RSO-I, (B) FRD for RSO-II, (C) FWO for RSO-I and (D) FWO for RSO-II


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Energy & Fuels

592

593 594

Fig.6: Regression plots based on (A) MCR for RSO-I, (B) MCR for RSO-II and (C) KM for RSO-I and RSO-II 33 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

595

596 597

Fig.7: Regression plots based on (A) FRD, (B) FWO, (C) MCR and (D) KM for ROME thermal degradation 34 ACS Paragon Plus Environment

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Energy & Fuels

598 599 600 601

602 603

Fig.8: Regression plots to calculate order of reaction proposed by Avrami theory for (A) RSO-I,

604

(B) RSO-II and (C) ROME; (D) Enthalpy change for RSO and ROME based on FWO and MCR

605

methods

606


Thermal Degradation Kinetic Study of Rubber Seed Oil and Its Methyl

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