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Understanding the liquefaction mechanism of Beypazarı lignite in tetralin with ultraviolet irradiation using discrete time models
Fuel Processing Technology 198 (2020) 106227
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Research article
Understanding the liquefaction mechanism of Beypazarı lignite in tetralin with ultraviolet irradiation using discrete time models
T
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Emir Hüseyin Şimşeka, Fatih Güleçb, , Fatma Söğüt Akçadağc a
Ankara University, Faculty of Engineering, Department of Chemical Engineering, 06100 Ankara, Turkey University of Nottingham, Faculty of Engineering, Department of Chemical Engineering, Energy Technologies Building, Nottingham NG7 2TU, UK c TUBITAK National Meteorology Institute (UME), 5441470 Kocaeli, Turkey b
A R T I C LE I N FO
A B S T R A C T
Keywords: Coal liquefaction models Kinetics Ultraviolet irradiation Discrete-time models Kalman filter
This study has proposed four different liquefaction models consisting of both reversible and irreversible reaction steps with three lumped parameters, asphaltenes, preasphaltenes, and oils, for the liquefaction of Turkish lignite (Beypazarı-Çayırhan). The coal had been liquefied in tetralin using a solvent/coal ratio of 5/1 at four different ultraviolet irradiation light sources of 90, 120, 150, and 180 W. The validity of the proposed models is specified by first order linear discrete-time models with the experimental data. Furthermore, the reaction rates of the proposed liquefaction models are determined using a Matlab program with the use of Kalman filter. The validation of proposed models for the liquefaction of Beypazarı lignite is defined using the sum of the squared differences of the models and experimental data. The results demonstrate that models which consist of reversible steps provide a better fit with experimental data compared to models consisting of irreversible steps. While the liquefaction step from reactive coal to oils demonstrates a maximum reaction rate constant of 2*10−2 h−1, the other steps from reactive coal to either preasphaltenes or asphaltenes show lower reaction rate constants, about 1*10−2 h−1. In addition to the fact that the formation of preasphaltenes and asphaltenes from reactive coal takes place slowly, a major proportion of the oils are formed directly from reactive coal.
1. Introduction As one of the most abundant fossil fuels, coal has been identified one of the most important fuel sources in industrial applications such as power generation, steel, cement etc. [1–4]. Due to the depletion of the natural gas and liquid petroleum resources, clean coal technologies are getting more attention with the aim of environmentally friendly and economical use of coal. Liquefaction of coal is an advantageous process in which the coal can be converted into more valuable and clean liquid hydrocarbons; liquid fuels and petrochemicals [5–8]. Coal liquefaction process usually categorised as either (i) “direct liquefaction” which is based on the carbonization, pyrolysis and hydrogenation [3,6,9,10] or (ii) “indirect liquefaction” where the coal is firstly gasified to hydrogen and carbon monoxide known as syngas which is then converted to liquid hydrocarbons [3]. In the direct coal liquefaction process, the extractable molecules are firstly extracted from reactive coal and then the larger molecules are cracked into the smaller molecules. Finally, the resultant free radicals are stabilised by hydrogen [11–15]. Due to the complexity of coal liquefaction, studies have focussed extensively on understanding the kinetics and mechanisms of the process which is the
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utmost importance for reactor design and process optimization [16–22]. Because of the wide product range, kinetic studies of coal liquefaction have successfully investigated using lump parameter kinetic models, where the products are classified in similar groups such as preasphaltenes, asphaltenes, and oils [11,13,14,20,23–31]. In the 1970s, Liebenberg and Potgieter [25] suggested a coal liquefaction model which consists of one series; coal → asphaltenes → heavy oil, and two parallel; coal → asphaltenes and coal → heavy oil. The simplistic liquefaction model was enhanced by Squires [27] as coal is firstly liquefied to oils, asphaltenes, and asphaltols in parallel reactions and then products are converted to either semi coke + residue or oil in a complex liquefaction pathway. Another complex irreversible coal liquefaction model having parallel and series reactions with three lumped parameters; oils, asphaltenes, and preasphaltenes was investigated by Shalabi et al. [32]. In this model, while the preasphaltenes is produced by only the liquefaction of reactive coal, asphaltenes are produced by the liquefaction of both reactive coal and preasphaltenes. Additionally, oil is produced from reactive coal and liquefied products, asphaltenes and preasphaltenes [32]. Cronauer et al. [17] enhanced the model suggested by Shalabi et al. [32] with an
Corresponding author. E-mail address: [email protected] (F. Güleç).
https://doi.org/10.1016/j.fuproc.2019.106227 Received 12 April 2019; Received in revised form 14 September 2019; Accepted 25 September 2019 0378-3820/ © 2019 Elsevier B.V. All rights reserved.
Fuel Processing Technology 198 (2020) 106227
E.H. Şimşek, et al.
ultraviolet (UV) irradiation light sources proposing four different liquefaction models having three lumped parameters; preasphaltenes, asphaltenes and oil. The validity of the proposed models is determined by forming first order linear discrete-time models with the experimental data. Formation rates of preasphaltenes, asphaltenes, and oil are also determined with the program written in Matlab with the use of the Kalman filter.
additional step from reactive coal to gases. The liquefaction model of reactive coal was assumed as irreversible and first order in the studies of both Cronauer et al. [17] and Shalabi et al. [32]. Additionally, the formation rate of preasphaltenes, asphaltenes and oil during the liquefaction of six different reactive coals in tetralin as hydrogen donor solvent using microwave heating were investigated in our previous study [30]. Five different liquefaction model defined as irreversible and pseudo first order has been proposed and tested. While the models which consist of parallel liquefaction from coal to preasphaltenes, asphaltenes and oils demonstrate the best fit for the coal presenting higher carbon, the models which contain series and parallel liquefaction reactions illustrate better fit for the coals which have lower carbon. Another liquefaction model having two lumped parameters; preasphaltenes+asphaltenes and oils+gas were suggested for the liquefaction of Shenhua coal by Li et al. [11]. According to the rate constants of liquefaction model, the suggested model is valid for heating up and isotherm stages. Additionally, the rate limiting step for the liquefaction of coal was defined as the reaction from preasphaltenes+asphaltenes to oils+gas. The same model was also suggested as valid for biochar liquefaction by Feiner et al. [33]. It is also known that although heat energy is commonly used in the coal liquefaction process, ultraviolet irradiation light is another applicable energy source for the liquefaction process. Photochemical energy has been used in the oxidation [12], depolymerisation [13], and the solubilisation [34] of coals but no studies on the model and kinetics of liquefaction with Ultraviolet (UV) irradiation energy can be found in the literature. It is known that the use of UV radiation and microwaves as an energy source to energise depolymerisation and solubilisation of lignites has been growing technology [30,35]. The coal liquefaction is running on free radicals in a hydrogen donor solvent. These free radicals can be generated by not only thermal energy but also photochemical energy. The light absorption of the matter is an extremely progressive and highly selective process which enable to raise new regulations within the molecules, which cannot be obtained by any other way. The absorption of electromagnetic radiation is the basis of photochemistry. This phenomenon can be explained as a molecule which is stimulated by absorbing a photon (light beams). In this new state, the distribution of electrons and the geometry of the molecule may be changed and so it can take on a new form defined the basic state different from the initial state. The excited state can, therefore, be regarded as an isomer of the basic state; indeed, this molecule now has different spectroscopic and chemical properties [36,37]. The amount of energy absorbed on the molecular scale by the photochemical process is much larger than that by thermal stimulation. This energy is then stored selectively for a bond or part of a molecule. In the next step, that bond will be broken or the molecule part will be stimulated. In this case, it can be said that photochemical transformation is completed in two stages; energy absorption and subsequent molecular movements [37]. The main reason for the using of UV radiation in the application of coal liquefaction process is, therefore, to optimize the oil products and minimize others such as asphaltene and preasphaltenes. Further, due to the UV radiation process, the temperature can keep low which may hinder the formation of gases products [35]. Although there are many studies investigating the kinetics and mechanisms of coal liquefaction, defining a general mechanism for the liquefaction is very difficult because of the differences in the petrographic compositions, ranks, mineral components of coals and the differences of experiment conditions [6,38,39]. In our previous studies [15,40], the importance of the reversible reaction pathways for the liquefaction of Zonguldak, Soma, and Tunçbilek lignites under microwave power was firstly presented and the model consisting of reversible reaction steps with the irreversible parallel and series liquefaction steps demonstrated the best fit with the experimental results. In this study, the importance of reversible reaction pathways has been firstly investigated for the liquefaction of a Beypazarı lignite liquefied using
2. Experimental 2.1. Experimental data The experimental data which were used in the Discrete-time models of coal liquefaction models in this study had been produced by Söğüt [41] who investigated the effects of UV irradiation on the solubility of Beypazarı lignite which was obtained from the Turkish Mineral Research and Exploration Institute in Ankara. The lignite sample was ground in a porcelain ball mill and sieved to −0.2 mm. The samples were, then, stored in plastic containers under nitrogen atmosphere. The lignite samples were liquefied under nitrogen in a 500 ml two-necked quartz flask equipped with a magnetic stirrer, a reflux condenser and a thermometer. The flask was first charged with a mixture of 125 g tetralin (Alfa Aesar, 98+%) and 25 g of ground, dried lignite and then exposed to four different light power of 90, 120, 150, and 180 watts for various periods of time i.e., 24, 48, 72, 96 and 120 h at atmospheric pressure with a solvent/lignite ratio of 5/1. The temperature was measured as 35 °C and the formation of gaseous products was not observed at the end of the liquefaction. Furthermore, the baseline experiments demonstrate that there is no liquefaction the absence of UV power. Liquid products are divided into fractions as in our previous studies [15,40]. The solid residue was removed by filtration and exhaustively extracted with Tetrahydrofuran (THF, Merck, 99%) in a Soxhlet apparatus. THF was then removed by rotary evaporation and the residue was recombined with the tetralin solution. Tetralin and naphthalene were distilled off at 60 °C under vacuum. The residual extract was defined three lumped parameter, preasphaltenes (toluene insoluble), asphaltenes (toluene soluble) and oils (hexane soluble) [41]. After adding 200 ml of n-hexane (Merck, 96+%), it was left overnight, and the hexane soluble oils were separated. Hexane was removed from the oil fraction by a rotary evaporator. The residue was treated with 200 ml toluene (Merck, 99.9%), left overnight to dissolve asphaltenes and then filtered. Evaporation of toluene under vacuum from the fraction gave asphaltenes and the residue insoluble in toluene gave preasphaltenes. The yields of liquid products, preasphaltenes, asphaltenes, and oils versus time for different UV irradiations are calculated using the weight percentage of liquified products (preasphaltenes, asphaltenes, and oils) per weight of used lignite (daf) and the results are shown in Table 1. Additionally, both ultimate and proximate analysis of the lignite are presented in Table 2. 2.2. Proposed coal liquefaction models A lot of reaction models consisting of the formation of the pseudo components have been published to identify the liquefaction of coals showing different properties [16–22,28,30,42–46]. Four different coal liquefaction models based on three lumped parameters; preasphaltenes (C), asphaltenes (B) and oils (D) have been investigated for the liquefaction of reactive coal (A), Beypazarı lignite. In these models, it was assumed that the liquified products were formed from reactive coal in a group of parallel and series irreversible-reversible reaction steps as shown in Fig. 1. However, due to the low liquefaction temperature, the formation of gas products has not been observed. The possible reaction pathways of gas products are, therefore, eliminated in the proposed models. On the other side, it is known that there are a few studies where the liquefaction models involved a reaction steps from coal to gas [13] 2
Fuel Processing Technology 198 (2020) 106227
E.H. Şimşek, et al.
Table 1 Yields of liquid products of Beypazarı lignite liquefaction versus time. UV irradiation
Time (h)
Oil %
Asphaltene %
Preasphaltene %
Liquid product %
Residue %
90 W
0 24 48 72 96 120 0 24 48 72 96 120 0 24 48 72 96 120 0 24 48 72 96 120
0 11.95 14.53 20.90 23.79 23.79 0 9.96 11.05 17.52 20.51 25.51 0 6.77 14.43 15.63 25.18 22.70 0 5.77 8.07 11.05 20.30 20.81
0 1.09 1.99 2.69 5.27 11.25 0 3.88 6.67 5.85 6.57 3.48 0 1.79 2.99 3.28 2.69 5.77 0 2.59 3.88 4.68 7.37 9.26
0 2.99 2.89 4.28 4.98 5.57 0 4.18 5.67 5.38 5.47 5.97 0 2.89 3.88 4.88 6.87 8.26 0 3.39 4.18 5.17 6.37 7.76
0 16.03 19.41 27.87 34.04 40.61 0 18.02 23.39 28.75 32.55 35.04 0 11.45 21.30 23.79 34.74 36.73 0 11.75 16.13 20.90 34.04 37.83
100 83.97 80.59 72.13 65.96 59.39 100 81.98 76.61 71.25 67.45 65.04 100 88.55 78.70 76.21 65.26 63.27 100 88.25 83.87 79.10 65.96 62.17
120 W
150 W
180 W
reaction steps between coal ↔ asphaltenes, asphaltenes ↔ preasphaltenes and asphaltenes ↔ oils in addition to Model-3.
Table 2 Ultimate and proximate analysis of Beypazarı lignite. Proximate analysis (wt%)b
Ultimate analysis (wt% daf)
2.3. Comparison of the models with the experimental data C
H
N
S
Oa
VMc
FCd
Ash
Me
62.50
5.29
2.07
9.14
21.00
25.10
23.23
37.77
13.90
a b c d e
The compliance of the proposed liquefaction reaction models with the experimental data is figured out by forming first order linear discrete-time models. Additionally, the reaction rate constants for each of the reactions in the models are determined using a Matlab program (written in ver 7.11, The MathWorks Inc. Natick, MA, USA) with the use of the Kalman filter. For instance, the discrete-time model used in this study for Model-1 is given the Eq. (17)–(20).
O content was determined by difference. Air dried. Volatile matter (VM). Fixed carbon (FC). Moisture (M).
or from coal to oil+gas [20], have been suggested at high temperatures. Therefore, the models suggested in this need to be updated with an additional reaction step from coal to gas if they are used for the definition of the liquefaction process occurring at high temperatures. The reaction rate equations belonging to these models are also presented in Eqs. (1)–(16) in Fig. 1. Additionally, the validity of the models has been assessed by comparing against experimental data. Model-1 is defined in a group of series and parallel irreversible reactions where the reactive coal is firstly liquefied to preasphaltenes and then asphaltenes and oils are simultaneously produced from the preasphaltenes as suggested by Shalabi et al. [20]. Additionally, oils are also produced from preasphaltenes. Contrary to Model-1, Model-2, which has been put forward by Radomyski and Szczygiel [47], consists of a group parallel irreversible reactions where the oils, asphaltenes, and preasphaltenes, are directly liquefied from reactive coal in simultaneously. An additional reaction where the oils are also produced from preasphaltenes may help to explain the highest oil yield in the experimental data (Table 1). In addition to these models, Model-3, which seems a combination of Model-1 and Model-2, was also found out by Shalabi et al. [20]. In the Model-3, the irreversible parallel reactions from reactive coal to preasphaltenes, asphaltenes and oils are defined as they were on Model-2. Furthermore, oils are also produced from preasphaltenes and asphaltenes, as assumed on Model-1. In our previous study [15], it was mentioned that reversible reactions have importance to explain the liquefaction of the coals. A new model, Model-4, having reversible and irreversible liquefaction reactions is now proposed suggested by our group. Model-4 consists of reversible
CA (i + 1) = CAi (1 − k1 ∆t )
(17)
CB (i + 1) = CBi (1 − k 4 ∆t ) + k2 CCi ∆t
(18)
CC (i + 1) = CCi (1 − (k2 + k3) ∆t ) + k1 CAi ∆t
(19)
CD (i + 1) = CDi + k3 CCi ∆t + k 4 CBi ∆t
(20)
where: CA is % unreacted coal (daf) and CB, CC, CD, are % asphaltenes, % preasphaltenes, and % oils yields, respectively. Kalman filter is an iterative mathematical algorithm which uses a set of equations and consecutive data inputs in order to estimate the unknown variables that tend to be more precise than those based on a single measurement [48]. Although the understanding of the liquefaction of coals is of the utmost importance, the measurement of variables is nearly impossible in dynamic systems such as the liquefaction studies. However, the Kalman filter provides the prediction of the unknown variables from the known parameters of the period with a minimal variance of state variables. For example, the reaction rate constants of the complex liquefaction models can be predicted by using the Kalman filter method. More information about the Kalman filtering theory and application on the liquefaction model practice using Matlab have been presented in previous studies [15,40,49,50]. 3. Results and discussion The consistency of the experimental data with the proposed coal liquefaction models is figured out using first order linear discrete-time models. Furthermore, the rate constants of the proposed liquefaction 3
Fuel Processing Technology 198 (2020) 106227
E.H. Şimşek, et al.
Fig. 1. Suggested liquefaction models for Beypazarı lignite. A; Reactive coal, B; asphaltene, C; preasphaltene, D; oil.
models are estimated using Kalman filter written a program in Matlab [51]. The best model explaining the liquefaction of Beypazarı lignite is defined by the sum of the squared differences of the values calculated with the models and experimental data, using Eq. (21) [15].
linkages such as methylene, disulphide, sulphide, or ether. Thanks to the photochemical decomposition of these weak bonds, a large number of free radicals can be formed, and they can either react with the hydrogen supplied by hydrogen donor solvent or polymerize with other molecules. The hydrogen-rich products which have low molecular weight can be produced with the stabilization of the free radicals by hydrogen i.e. oils. Otherwise, the free radicals may polymerize and produce high molecule weight products in the absence of hydrogen i.e. asphaltenes, preasphaltenes, and char, reversible and irreversible reactions. The sum of the squared differences calculated with the experimental data and Model-1, and the reaction rate constants (h−1) for Model-1, at various UV powers, are presented in Table 3. Additionally, the comparison of the liquefaction experimental data with Model-1 is also presented in Fig. 2 for four different UV powers including 90, 120, 150, and 180 W. Table 3 demonstrates that Model-1 fails to numerically approximate the experimental data. Furthermore, the incompatibility between the Model-1 and experimental data is also clearly seen in
∑ (ymi − yei )2 = (PASm − PASe )2 + (ASm − ASe )2 + (YAm − YA e)2 i
(21) ymi and yei; the values calculated from the model and the experimental data, PASm and PASe; the preasphaltenes yields ASm and ASe; the asphaltenes yields YAmand YAe; the oils yields calculated using the model and the experimental data. The currently accepted coal liquefaction models assume the organic fraction as an amorphous, cross-linked polymeric structure. Condensed aromatic ring systems of the high molecular weight containing small amounts of heteroatomic species such as nitrogen, oxygen, and sulphur, comprise a significant portion of this macromolecule. The first step of coal liquefaction may attribute to the cross-linking which is provided by 4
Fuel Processing Technology 198 (2020) 106227
E.H. Şimşek, et al.
toluene, (iii) the next weakest bonds become ‘crackable’ only when preasphaltenes are formed. The other assumption of the model is that the toluene insoluble fragments can change to either toluene or hexane soluble products. However, it is known that the liquefaction of the coals usually run through free radicals in hydrogen donor solvent environments. Thus, the free radicals formed from liquefaction of coal with UV irradiation energy are stabilised by hydrogen transfer from the hydrogen donor solvent and smaller molecular weight products can be directly produced [15]. The incompatibility of the Model-1 with the experimental data proves that the main radicals may arise from coal rather than the products which are defined as toluene insoluble. Therefore, the parallel coal liquefaction reactions from coal (A) to preasphaltenes (C), asphaltenes (B) and oils (C) may demonstrate a better explanation for the liquefaction of Beypazarı lignite, as proposed Model-2. In addition to these three parallel reactions, the formation of oils is also supported by the reaction of preasphaltenes to oils. As clearly seen from Fig. 3, although Model-2 illustrates better results for the explanation of the liquefaction compared with Model-1, it is still far from a well-defined model for the liquefaction of Beypazarı lignite. Additionally, the sum of the squared differences also proves this inconsistency between Model-2 and the experimental data in Table 4. However, the results of Model-2 show the importance of the other parallel reactions; coal to asphaltenes and coal to oils, apart from the reaction from coal to preasphaltenes for the definition of liquefaction model. The assumption in this model is that the liquefaction process runs through with a group of parallel reactions along with series reactions. First of all, the fragments such as toluene insoluble, toluene soluble and hexane soluble, are formed in a group of parallel reactions. Additionally, the toluene insoluble molecules may change to hexane soluble products. Although the model tries to explain the higher oils
Table 3 The sum of the squared differences of the values calculated with the Model-1 and experimental data and reaction rate constants (h−1) for various UV power. UV power (W)
∑i (ymi − yei )2
90 120 150 180
1122 960 907 765
Reaction rate constants (h−1)*102 k1
k2
k3
k4
2.49 2.49 2.46 2.36
1.64 1.57 1.48 1.56
1.97 2.09 1.93 1.85
1.05 1.14 1.08 1.07
Fig. 2. As in this model, the coal is first converted to preasphaltenes (C) and then asphaltenes (B) and oils (C) are produced from the preasphaltenes in parallel reactions. Additionally, oils are also produced from the asphaltenes in addition to being formed from preasphaltenes. The assumption in this model that the process of coal liquefaction would cause the scission of the bond in a specific way [15]. First, a group of fragments consisting of polar groups would be produced by cracking of coal with the UV energy. The preliminary cracking products with UV irradiation energy in the coal liquefaction reaction can be, therefore, defined as preasphaltenes (insoluble in toluene), that would impose strict requirements on the molecular structure of the coal. Although neither the Arrhenius-type description of cracking probability nor the general description of the chemical constitution of coal support such requirements, the implications for the liquefaction are that (i) the weakest bonds on the structure are the only bonds undergoing bond dissociation under the liquefaction conditions, (ii) the weakest bonds are in a position to lead to two product molecules, both of which are insoluble in
(a) Conversion (%)
Conversion (%)
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
Time (h)
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
(b)
Time (h)
30
% Asfalten Asphaltene (Model) % (Model) % Preasfalten Preasphaltene (Model) % (Model) % (Model) %Yağ Oil (Model) % %Asfalten Asphaltene % %Preasfalten Preasphaltene % Yağ
(c)
% Oil
20
Conversion (%)
Conversion (%)
25
15
10
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
(d)
5
0 0
20
40
60
80
100
120
Time (h)
Time (h)
Fig. 2. Comparison of experimental data with Model-1 at a) 90 W, b) 120 W, c) 150, and d) 180 W. 5
Fuel Processing Technology 198 (2020) 106227
E.H. Şimşek, et al.
(a) Conversion (%)
Conversion (%)
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
Time (h)
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
(b)
Time (h)
30
%Asfalten Asphaltene (Model) % (Model) % (Model) %Preasfalten Preasphaltene (Model) % %Yağ Oil (Model) (Model) % %Asfalten Asphaltene % Preasfalten % Preasphaltene % Yağ
20
(c)
% Oil
Conversion (%)
Conversion (%)
25
15
10
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
(d)
5
0 0
20
40
60
80
100
120
Time (h)
Time (h)
Fig. 3. Comparison of experimental data with Model-2 at a) 90 W, b) 120 W, c) 150 W, and d) 180 W.
provide a good definition of liquefaction of the Beypazarı lignite. The experimental results presented in Table 1 illustrate that although the formation of oils consistently and saliently increases with the liquefaction time increasing until 96 h, the formation of both preasphaltenes and asphaltenes demonstrates fluctuations and may assume going to a steady line at the further liquefaction times. The consistent increase in the oils yield may be attributed to the irreversible liquefaction reaction step from reactive coal to oils. Moreover, the high oils yield may also be supported by the further reactions from asphaltenes and preasphaltenes to oils. On the other hand, the stability of the preasphaltenes and asphaltenes yields may be explained by the reversible reaction steps between coal ↔ asphaltenes and preasphaltenes ↔ asphaltenes. The crucial role of the reversible reactions in the coal liquefaction models was also demonstrated in our previous papers [15,40]. In Model-4, the liquefaction of coal has, therefore, been improved by three reversible reactions; between coal ↔ asphaltenes, asphaltenes ↔ oils, and asphaltenes ↔ preasphaltenes, in addition to Model-3. As demonstrated in Fig. 5, the Model-4 fits best with experimental data for the UV power of 90, 120, 150 and 180 W compared with the other models. Furthermore, the sum of the squared differences illustrated in Table 6 also proves that Model-4 has a better explanation for the liquefaction of the lignite compare with the other three models. Moreover, it was also demonstrated the models show better fit with the experimental results at higher UV powers, 180 W, compared with the low UV powers, 90 W. Thanks to these three reversible reactions, which is the only difference from Model-3, Model-4 demonstrates much better fitting with the experimental data. Thus, it can be said that the main
Table 4 The sum of the squared differences of the values calculated with the Model-2 and experimental data and reaction rate constants (h−1) for various UV power. UV power (W)
90 120 150 180
∑i (ymi − yei )2
145 162 118 95
Reaction rate constants (h−1)*102 k1
k2
k3
k4
0.86 0.96 1.00 0.96
0.99 0.91 0.73 1.01
2.65 2.47 2.48 2.05
1.05 1.10 1.05 1.06
yields in the liquefaction of Beypazarı lignite via the reaction from both coal and preasphaltenes to oils, the model has still failed to the formation of hexane soluble products, oils, seen from Fig. 3. Model-3 is defined as a combination of Model-1 and Model-2. In addition to Model-2, Model-3 consists of the formation of asphaltenes are build up from preasphaltenes. Additionally, oils are also formed from preasphaltenes and asphaltenes along with reactive coal. However, Fig. 4 demonstrates that the model, Model-3, also fails to provide a good definition of liquefaction of Beypazarı lignite for four different UV powers. The discrepancy between Model-3 and experimental data is more prevalent in the formation of hexane soluble fragments, oils, as observed on Model-2. Additionally, the sum of squared differences presented in Table 5 demonstrates similar values with Model-2. In terms of the UV power, although the models, Model-1, Model-2, and Model-3, demonstrate lower sum of the squared differences at higher UV powers, the values are still not low enough to 6
Fuel Processing Technology 198 (2020) 106227
E.H. Şimşek, et al.
(a) Conversion (%)
Conversion (%)
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
(b)
Time (h)
Time (h) 30
Conversion (%)
25
20
(c) Conversion (%)
%Asfalten Asphaltene (Model) % (Model) % (Model) %Preasfalten Preasphaltene (Model) % Yağ (Model) % Oil (Model) % Asfalten % Asphaltene % Preasfalten % Preasphaltene % Yağ % Oil
15
10
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
(d)
5
0 0
20
40
60
80
100
120
Time (h)
Time (h)
Fig. 4. Comparison of experimental data with Model-3 at a) 90 W, b) 120 W, c) 150 W, and d) 180 W.
with the hydrogen transfer, and then either (iii) the formation of the smaller molecular weight liquefied molecules or (iv) the re-polymerization of the free radicals due to the hydrogen deficiency. The formation of high molecular weight hydrogen deficient polyaromatic species similar to char may result in re-polymerization. The increase in the oil yield during the liquefaction process can be attributed to the stabilization of free radicals by hydrogen shuttling from hydrogen-rich hydrocarbons instead of hydrogen donor solvent [15]. The presence of reversible reactions in the liquefaction model can prove that the hydrogen donor solvent may not be able to transfer enough hydrogen to the free radicals [40]. This study demonstrates that the reversible reactions steps are the determinative steps in the liquefaction of the Beypazarı lignite under UV power. Furthermore, the models having reversible reaction steps demonstrate the best fit with the experimental data regardless of coal types and power sources such as microwave [15,40].
Table 5 The sum of the squared differences of the values calculated with the Model-3 and experimental data and reaction rate constants (h−1) for various UV power. UV power (W)
90 120 150 180
∑i (ymi − yei )2
145 160 118 94
Reaction rate constants (h−1)*102 k1
k2
k3
k4
k5
k6
0.99 0.92 0.73 1.01
0.89 0.98 1.03 0.99
2.63 2.43 2.46 2.02
1.01 1.12 1.06 1.05
1.08 1.07 1.03 1.07
1.00 1.00 1.00 1.00
free radicals have been formed from the reactive coal as stated above. It is known that during the liquefaction of reactive coals, many competitive and concurrent chemical reactions may occur. The reaction rate constants of Model-4 at four different UV powers including 90, 120, 150, and 180 W, have been determined using multiple regression analysis and the results are also presented in Table 6. It is clearly seen from the table that the reaction rate from reactive coal to oils are almost double compared with the other reaction rate constants demonstrated on the Model-4. The reaction rate constant is approximately 2.0*10−2 h−1 for the liquefaction reaction from reactive coal to oils while the reaction rate constants for the other reactions on the model are about 1.0*10−2 h−1. Additionally, the reaction rate constants are insignificantly affected by the UV powers used in the liquefaction of Beypazarı lignite. The general liquefaction steps of a reactive coal may consist of a group of steps; (i) bonds cleavages between structural species, and then (ii) stabilization of the free radicals formed by the breaking of bonds
4. Conclusion The models, Model-2, Model-3, and Model-4, where the products are formed from reactive coal in parallel reactions illustrate better fit with the experimental data compared with the model, Model-1, where the products are liquefied in a series reaction from coal to preasphaltenes. A much better explanation for the liquefaction of Beypazarı lignite has been brought over with Model-4 consisting of reversible reaction steps in addition to parallel liquefaction steps. Furthermore, it was found that the liquefaction step from coal to oils has the highest reaction rate constant compared with other reactions suggested in the Model-4. Moreover, the reaction rate constants are 7
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E.H. Şimşek, et al.
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
(a) Conversion (%)
Conversion (%)
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
(b)
Time (h)
Time (h) 30
25
Conversion (%)
20
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
(c) Conversion (%)
% Asfalten Asphaltene (Model) % (Model) % Preasfalten Preasphaltene (Model) % (Model) % %Yağ Oil (Model) (Model) % %Asfalten Asphaltene % Preasfalten % Preasphaltene % Yağ % Oil
15
10
(d)
5
0 0
20
40
60
80
100
120
Time (h)
Time (h)
Fig. 5. Comparison of experimental data with Model-4 at a) 90 W, b) 120 W, c) 150 W, and d) 180 W. Table 6 The sum of the squared differences of the values calculated with the Model-4 and experimental data and reaction rate constants (h−1) for various UV power. UV power (W)
90 120 150 180
∑i (ymi − yei )2
58 60 49 38
Reaction rate constants (h−1)*102 k1
k2
k3
k4
k5
k6
k7
k8
k9
2.10 1.97 2.02 1.89
1.03 1.10 1.05 1.02
1.11 0.73 0.79 1.08
1.01 0.78 0.89 0.93
1.00 1.10 1.04 1.04
0.92 0.82 1.02 1.01
1.03 1.07 1.03 1.05
1.02 0.99 0.99 1.01
0.98 1.02 1.01 0.99
important to investigate the determination of reaction orders with other parameter estimation methods for the best model.
independent of the liquefaction power in the liquefaction process where the UV is used as the liquefaction power source. It is the first study to demonstrate that the models having reversible steps are crucial in the liquefaction of Beypazari lignite using UV power source. Furthermore, the study is also demonstrated that the Kalman filter is one of the useful methods to estimate the model parameter for the liquefaction of coals using minimum experimental results. After this study and previous studies [15,40], it can be concluded that the models having reversible reaction steps demonstrate best fit with the experimental data regardless of coal types and power sources such as microwave or UV. It is believed that although the applicability of UV irradiation for the coal liquefaction in mass production seems too difficult due to the long reaction time, this study may guide for the UV Lamb Photochemical Reactor design of small-scale liquefaction processes. Furthermore, this study may enable the formation of better quality (low molecular weight) liquid products in the thermal coal liquefaction process assisted by UV irradiation energy. Additionally, as future work, it is highly
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9
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Research article
Understanding the liquefaction mechanism of Beypazarı lignite in tetralin with ultraviolet irradiation using discrete time models
T
⁎
Emir Hüseyin Şimşeka, Fatih Güleçb, , Fatma Söğüt Akçadağc a
Ankara University, Faculty of Engineering, Department of Chemical Engineering, 06100 Ankara, Turkey University of Nottingham, Faculty of Engineering, Department of Chemical Engineering, Energy Technologies Building, Nottingham NG7 2TU, UK c TUBITAK National Meteorology Institute (UME), 5441470 Kocaeli, Turkey b
A R T I C LE I N FO
A B S T R A C T
Keywords: Coal liquefaction models Kinetics Ultraviolet irradiation Discrete-time models Kalman filter
This study has proposed four different liquefaction models consisting of both reversible and irreversible reaction steps with three lumped parameters, asphaltenes, preasphaltenes, and oils, for the liquefaction of Turkish lignite (Beypazarı-Çayırhan). The coal had been liquefied in tetralin using a solvent/coal ratio of 5/1 at four different ultraviolet irradiation light sources of 90, 120, 150, and 180 W. The validity of the proposed models is specified by first order linear discrete-time models with the experimental data. Furthermore, the reaction rates of the proposed liquefaction models are determined using a Matlab program with the use of Kalman filter. The validation of proposed models for the liquefaction of Beypazarı lignite is defined using the sum of the squared differences of the models and experimental data. The results demonstrate that models which consist of reversible steps provide a better fit with experimental data compared to models consisting of irreversible steps. While the liquefaction step from reactive coal to oils demonstrates a maximum reaction rate constant of 2*10−2 h−1, the other steps from reactive coal to either preasphaltenes or asphaltenes show lower reaction rate constants, about 1*10−2 h−1. In addition to the fact that the formation of preasphaltenes and asphaltenes from reactive coal takes place slowly, a major proportion of the oils are formed directly from reactive coal.
1. Introduction As one of the most abundant fossil fuels, coal has been identified one of the most important fuel sources in industrial applications such as power generation, steel, cement etc. [1–4]. Due to the depletion of the natural gas and liquid petroleum resources, clean coal technologies are getting more attention with the aim of environmentally friendly and economical use of coal. Liquefaction of coal is an advantageous process in which the coal can be converted into more valuable and clean liquid hydrocarbons; liquid fuels and petrochemicals [5–8]. Coal liquefaction process usually categorised as either (i) “direct liquefaction” which is based on the carbonization, pyrolysis and hydrogenation [3,6,9,10] or (ii) “indirect liquefaction” where the coal is firstly gasified to hydrogen and carbon monoxide known as syngas which is then converted to liquid hydrocarbons [3]. In the direct coal liquefaction process, the extractable molecules are firstly extracted from reactive coal and then the larger molecules are cracked into the smaller molecules. Finally, the resultant free radicals are stabilised by hydrogen [11–15]. Due to the complexity of coal liquefaction, studies have focussed extensively on understanding the kinetics and mechanisms of the process which is the
⁎
utmost importance for reactor design and process optimization [16–22]. Because of the wide product range, kinetic studies of coal liquefaction have successfully investigated using lump parameter kinetic models, where the products are classified in similar groups such as preasphaltenes, asphaltenes, and oils [11,13,14,20,23–31]. In the 1970s, Liebenberg and Potgieter [25] suggested a coal liquefaction model which consists of one series; coal → asphaltenes → heavy oil, and two parallel; coal → asphaltenes and coal → heavy oil. The simplistic liquefaction model was enhanced by Squires [27] as coal is firstly liquefied to oils, asphaltenes, and asphaltols in parallel reactions and then products are converted to either semi coke + residue or oil in a complex liquefaction pathway. Another complex irreversible coal liquefaction model having parallel and series reactions with three lumped parameters; oils, asphaltenes, and preasphaltenes was investigated by Shalabi et al. [32]. In this model, while the preasphaltenes is produced by only the liquefaction of reactive coal, asphaltenes are produced by the liquefaction of both reactive coal and preasphaltenes. Additionally, oil is produced from reactive coal and liquefied products, asphaltenes and preasphaltenes [32]. Cronauer et al. [17] enhanced the model suggested by Shalabi et al. [32] with an
Corresponding author. E-mail address: [email protected] (F. Güleç).
https://doi.org/10.1016/j.fuproc.2019.106227 Received 12 April 2019; Received in revised form 14 September 2019; Accepted 25 September 2019 0378-3820/ © 2019 Elsevier B.V. All rights reserved.
Fuel Processing Technology 198 (2020) 106227
E.H. Şimşek, et al.
ultraviolet (UV) irradiation light sources proposing four different liquefaction models having three lumped parameters; preasphaltenes, asphaltenes and oil. The validity of the proposed models is determined by forming first order linear discrete-time models with the experimental data. Formation rates of preasphaltenes, asphaltenes, and oil are also determined with the program written in Matlab with the use of the Kalman filter.
additional step from reactive coal to gases. The liquefaction model of reactive coal was assumed as irreversible and first order in the studies of both Cronauer et al. [17] and Shalabi et al. [32]. Additionally, the formation rate of preasphaltenes, asphaltenes and oil during the liquefaction of six different reactive coals in tetralin as hydrogen donor solvent using microwave heating were investigated in our previous study [30]. Five different liquefaction model defined as irreversible and pseudo first order has been proposed and tested. While the models which consist of parallel liquefaction from coal to preasphaltenes, asphaltenes and oils demonstrate the best fit for the coal presenting higher carbon, the models which contain series and parallel liquefaction reactions illustrate better fit for the coals which have lower carbon. Another liquefaction model having two lumped parameters; preasphaltenes+asphaltenes and oils+gas were suggested for the liquefaction of Shenhua coal by Li et al. [11]. According to the rate constants of liquefaction model, the suggested model is valid for heating up and isotherm stages. Additionally, the rate limiting step for the liquefaction of coal was defined as the reaction from preasphaltenes+asphaltenes to oils+gas. The same model was also suggested as valid for biochar liquefaction by Feiner et al. [33]. It is also known that although heat energy is commonly used in the coal liquefaction process, ultraviolet irradiation light is another applicable energy source for the liquefaction process. Photochemical energy has been used in the oxidation [12], depolymerisation [13], and the solubilisation [34] of coals but no studies on the model and kinetics of liquefaction with Ultraviolet (UV) irradiation energy can be found in the literature. It is known that the use of UV radiation and microwaves as an energy source to energise depolymerisation and solubilisation of lignites has been growing technology [30,35]. The coal liquefaction is running on free radicals in a hydrogen donor solvent. These free radicals can be generated by not only thermal energy but also photochemical energy. The light absorption of the matter is an extremely progressive and highly selective process which enable to raise new regulations within the molecules, which cannot be obtained by any other way. The absorption of electromagnetic radiation is the basis of photochemistry. This phenomenon can be explained as a molecule which is stimulated by absorbing a photon (light beams). In this new state, the distribution of electrons and the geometry of the molecule may be changed and so it can take on a new form defined the basic state different from the initial state. The excited state can, therefore, be regarded as an isomer of the basic state; indeed, this molecule now has different spectroscopic and chemical properties [36,37]. The amount of energy absorbed on the molecular scale by the photochemical process is much larger than that by thermal stimulation. This energy is then stored selectively for a bond or part of a molecule. In the next step, that bond will be broken or the molecule part will be stimulated. In this case, it can be said that photochemical transformation is completed in two stages; energy absorption and subsequent molecular movements [37]. The main reason for the using of UV radiation in the application of coal liquefaction process is, therefore, to optimize the oil products and minimize others such as asphaltene and preasphaltenes. Further, due to the UV radiation process, the temperature can keep low which may hinder the formation of gases products [35]. Although there are many studies investigating the kinetics and mechanisms of coal liquefaction, defining a general mechanism for the liquefaction is very difficult because of the differences in the petrographic compositions, ranks, mineral components of coals and the differences of experiment conditions [6,38,39]. In our previous studies [15,40], the importance of the reversible reaction pathways for the liquefaction of Zonguldak, Soma, and Tunçbilek lignites under microwave power was firstly presented and the model consisting of reversible reaction steps with the irreversible parallel and series liquefaction steps demonstrated the best fit with the experimental results. In this study, the importance of reversible reaction pathways has been firstly investigated for the liquefaction of a Beypazarı lignite liquefied using
2. Experimental 2.1. Experimental data The experimental data which were used in the Discrete-time models of coal liquefaction models in this study had been produced by Söğüt [41] who investigated the effects of UV irradiation on the solubility of Beypazarı lignite which was obtained from the Turkish Mineral Research and Exploration Institute in Ankara. The lignite sample was ground in a porcelain ball mill and sieved to −0.2 mm. The samples were, then, stored in plastic containers under nitrogen atmosphere. The lignite samples were liquefied under nitrogen in a 500 ml two-necked quartz flask equipped with a magnetic stirrer, a reflux condenser and a thermometer. The flask was first charged with a mixture of 125 g tetralin (Alfa Aesar, 98+%) and 25 g of ground, dried lignite and then exposed to four different light power of 90, 120, 150, and 180 watts for various periods of time i.e., 24, 48, 72, 96 and 120 h at atmospheric pressure with a solvent/lignite ratio of 5/1. The temperature was measured as 35 °C and the formation of gaseous products was not observed at the end of the liquefaction. Furthermore, the baseline experiments demonstrate that there is no liquefaction the absence of UV power. Liquid products are divided into fractions as in our previous studies [15,40]. The solid residue was removed by filtration and exhaustively extracted with Tetrahydrofuran (THF, Merck, 99%) in a Soxhlet apparatus. THF was then removed by rotary evaporation and the residue was recombined with the tetralin solution. Tetralin and naphthalene were distilled off at 60 °C under vacuum. The residual extract was defined three lumped parameter, preasphaltenes (toluene insoluble), asphaltenes (toluene soluble) and oils (hexane soluble) [41]. After adding 200 ml of n-hexane (Merck, 96+%), it was left overnight, and the hexane soluble oils were separated. Hexane was removed from the oil fraction by a rotary evaporator. The residue was treated with 200 ml toluene (Merck, 99.9%), left overnight to dissolve asphaltenes and then filtered. Evaporation of toluene under vacuum from the fraction gave asphaltenes and the residue insoluble in toluene gave preasphaltenes. The yields of liquid products, preasphaltenes, asphaltenes, and oils versus time for different UV irradiations are calculated using the weight percentage of liquified products (preasphaltenes, asphaltenes, and oils) per weight of used lignite (daf) and the results are shown in Table 1. Additionally, both ultimate and proximate analysis of the lignite are presented in Table 2. 2.2. Proposed coal liquefaction models A lot of reaction models consisting of the formation of the pseudo components have been published to identify the liquefaction of coals showing different properties [16–22,28,30,42–46]. Four different coal liquefaction models based on three lumped parameters; preasphaltenes (C), asphaltenes (B) and oils (D) have been investigated for the liquefaction of reactive coal (A), Beypazarı lignite. In these models, it was assumed that the liquified products were formed from reactive coal in a group of parallel and series irreversible-reversible reaction steps as shown in Fig. 1. However, due to the low liquefaction temperature, the formation of gas products has not been observed. The possible reaction pathways of gas products are, therefore, eliminated in the proposed models. On the other side, it is known that there are a few studies where the liquefaction models involved a reaction steps from coal to gas [13] 2
Fuel Processing Technology 198 (2020) 106227
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Table 1 Yields of liquid products of Beypazarı lignite liquefaction versus time. UV irradiation
Time (h)
Oil %
Asphaltene %
Preasphaltene %
Liquid product %
Residue %
90 W
0 24 48 72 96 120 0 24 48 72 96 120 0 24 48 72 96 120 0 24 48 72 96 120
0 11.95 14.53 20.90 23.79 23.79 0 9.96 11.05 17.52 20.51 25.51 0 6.77 14.43 15.63 25.18 22.70 0 5.77 8.07 11.05 20.30 20.81
0 1.09 1.99 2.69 5.27 11.25 0 3.88 6.67 5.85 6.57 3.48 0 1.79 2.99 3.28 2.69 5.77 0 2.59 3.88 4.68 7.37 9.26
0 2.99 2.89 4.28 4.98 5.57 0 4.18 5.67 5.38 5.47 5.97 0 2.89 3.88 4.88 6.87 8.26 0 3.39 4.18 5.17 6.37 7.76
0 16.03 19.41 27.87 34.04 40.61 0 18.02 23.39 28.75 32.55 35.04 0 11.45 21.30 23.79 34.74 36.73 0 11.75 16.13 20.90 34.04 37.83
100 83.97 80.59 72.13 65.96 59.39 100 81.98 76.61 71.25 67.45 65.04 100 88.55 78.70 76.21 65.26 63.27 100 88.25 83.87 79.10 65.96 62.17
120 W
150 W
180 W
reaction steps between coal ↔ asphaltenes, asphaltenes ↔ preasphaltenes and asphaltenes ↔ oils in addition to Model-3.
Table 2 Ultimate and proximate analysis of Beypazarı lignite. Proximate analysis (wt%)b
Ultimate analysis (wt% daf)
2.3. Comparison of the models with the experimental data C
H
N
S
Oa
VMc
FCd
Ash
Me
62.50
5.29
2.07
9.14
21.00
25.10
23.23
37.77
13.90
a b c d e
The compliance of the proposed liquefaction reaction models with the experimental data is figured out by forming first order linear discrete-time models. Additionally, the reaction rate constants for each of the reactions in the models are determined using a Matlab program (written in ver 7.11, The MathWorks Inc. Natick, MA, USA) with the use of the Kalman filter. For instance, the discrete-time model used in this study for Model-1 is given the Eq. (17)–(20).
O content was determined by difference. Air dried. Volatile matter (VM). Fixed carbon (FC). Moisture (M).
or from coal to oil+gas [20], have been suggested at high temperatures. Therefore, the models suggested in this need to be updated with an additional reaction step from coal to gas if they are used for the definition of the liquefaction process occurring at high temperatures. The reaction rate equations belonging to these models are also presented in Eqs. (1)–(16) in Fig. 1. Additionally, the validity of the models has been assessed by comparing against experimental data. Model-1 is defined in a group of series and parallel irreversible reactions where the reactive coal is firstly liquefied to preasphaltenes and then asphaltenes and oils are simultaneously produced from the preasphaltenes as suggested by Shalabi et al. [20]. Additionally, oils are also produced from preasphaltenes. Contrary to Model-1, Model-2, which has been put forward by Radomyski and Szczygiel [47], consists of a group parallel irreversible reactions where the oils, asphaltenes, and preasphaltenes, are directly liquefied from reactive coal in simultaneously. An additional reaction where the oils are also produced from preasphaltenes may help to explain the highest oil yield in the experimental data (Table 1). In addition to these models, Model-3, which seems a combination of Model-1 and Model-2, was also found out by Shalabi et al. [20]. In the Model-3, the irreversible parallel reactions from reactive coal to preasphaltenes, asphaltenes and oils are defined as they were on Model-2. Furthermore, oils are also produced from preasphaltenes and asphaltenes, as assumed on Model-1. In our previous study [15], it was mentioned that reversible reactions have importance to explain the liquefaction of the coals. A new model, Model-4, having reversible and irreversible liquefaction reactions is now proposed suggested by our group. Model-4 consists of reversible
CA (i + 1) = CAi (1 − k1 ∆t )
(17)
CB (i + 1) = CBi (1 − k 4 ∆t ) + k2 CCi ∆t
(18)
CC (i + 1) = CCi (1 − (k2 + k3) ∆t ) + k1 CAi ∆t
(19)
CD (i + 1) = CDi + k3 CCi ∆t + k 4 CBi ∆t
(20)
where: CA is % unreacted coal (daf) and CB, CC, CD, are % asphaltenes, % preasphaltenes, and % oils yields, respectively. Kalman filter is an iterative mathematical algorithm which uses a set of equations and consecutive data inputs in order to estimate the unknown variables that tend to be more precise than those based on a single measurement [48]. Although the understanding of the liquefaction of coals is of the utmost importance, the measurement of variables is nearly impossible in dynamic systems such as the liquefaction studies. However, the Kalman filter provides the prediction of the unknown variables from the known parameters of the period with a minimal variance of state variables. For example, the reaction rate constants of the complex liquefaction models can be predicted by using the Kalman filter method. More information about the Kalman filtering theory and application on the liquefaction model practice using Matlab have been presented in previous studies [15,40,49,50]. 3. Results and discussion The consistency of the experimental data with the proposed coal liquefaction models is figured out using first order linear discrete-time models. Furthermore, the rate constants of the proposed liquefaction 3
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Fig. 1. Suggested liquefaction models for Beypazarı lignite. A; Reactive coal, B; asphaltene, C; preasphaltene, D; oil.
models are estimated using Kalman filter written a program in Matlab [51]. The best model explaining the liquefaction of Beypazarı lignite is defined by the sum of the squared differences of the values calculated with the models and experimental data, using Eq. (21) [15].
linkages such as methylene, disulphide, sulphide, or ether. Thanks to the photochemical decomposition of these weak bonds, a large number of free radicals can be formed, and they can either react with the hydrogen supplied by hydrogen donor solvent or polymerize with other molecules. The hydrogen-rich products which have low molecular weight can be produced with the stabilization of the free radicals by hydrogen i.e. oils. Otherwise, the free radicals may polymerize and produce high molecule weight products in the absence of hydrogen i.e. asphaltenes, preasphaltenes, and char, reversible and irreversible reactions. The sum of the squared differences calculated with the experimental data and Model-1, and the reaction rate constants (h−1) for Model-1, at various UV powers, are presented in Table 3. Additionally, the comparison of the liquefaction experimental data with Model-1 is also presented in Fig. 2 for four different UV powers including 90, 120, 150, and 180 W. Table 3 demonstrates that Model-1 fails to numerically approximate the experimental data. Furthermore, the incompatibility between the Model-1 and experimental data is also clearly seen in
∑ (ymi − yei )2 = (PASm − PASe )2 + (ASm − ASe )2 + (YAm − YA e)2 i
(21) ymi and yei; the values calculated from the model and the experimental data, PASm and PASe; the preasphaltenes yields ASm and ASe; the asphaltenes yields YAmand YAe; the oils yields calculated using the model and the experimental data. The currently accepted coal liquefaction models assume the organic fraction as an amorphous, cross-linked polymeric structure. Condensed aromatic ring systems of the high molecular weight containing small amounts of heteroatomic species such as nitrogen, oxygen, and sulphur, comprise a significant portion of this macromolecule. The first step of coal liquefaction may attribute to the cross-linking which is provided by 4
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toluene, (iii) the next weakest bonds become ‘crackable’ only when preasphaltenes are formed. The other assumption of the model is that the toluene insoluble fragments can change to either toluene or hexane soluble products. However, it is known that the liquefaction of the coals usually run through free radicals in hydrogen donor solvent environments. Thus, the free radicals formed from liquefaction of coal with UV irradiation energy are stabilised by hydrogen transfer from the hydrogen donor solvent and smaller molecular weight products can be directly produced [15]. The incompatibility of the Model-1 with the experimental data proves that the main radicals may arise from coal rather than the products which are defined as toluene insoluble. Therefore, the parallel coal liquefaction reactions from coal (A) to preasphaltenes (C), asphaltenes (B) and oils (C) may demonstrate a better explanation for the liquefaction of Beypazarı lignite, as proposed Model-2. In addition to these three parallel reactions, the formation of oils is also supported by the reaction of preasphaltenes to oils. As clearly seen from Fig. 3, although Model-2 illustrates better results for the explanation of the liquefaction compared with Model-1, it is still far from a well-defined model for the liquefaction of Beypazarı lignite. Additionally, the sum of the squared differences also proves this inconsistency between Model-2 and the experimental data in Table 4. However, the results of Model-2 show the importance of the other parallel reactions; coal to asphaltenes and coal to oils, apart from the reaction from coal to preasphaltenes for the definition of liquefaction model. The assumption in this model is that the liquefaction process runs through with a group of parallel reactions along with series reactions. First of all, the fragments such as toluene insoluble, toluene soluble and hexane soluble, are formed in a group of parallel reactions. Additionally, the toluene insoluble molecules may change to hexane soluble products. Although the model tries to explain the higher oils
Table 3 The sum of the squared differences of the values calculated with the Model-1 and experimental data and reaction rate constants (h−1) for various UV power. UV power (W)
∑i (ymi − yei )2
90 120 150 180
1122 960 907 765
Reaction rate constants (h−1)*102 k1
k2
k3
k4
2.49 2.49 2.46 2.36
1.64 1.57 1.48 1.56
1.97 2.09 1.93 1.85
1.05 1.14 1.08 1.07
Fig. 2. As in this model, the coal is first converted to preasphaltenes (C) and then asphaltenes (B) and oils (C) are produced from the preasphaltenes in parallel reactions. Additionally, oils are also produced from the asphaltenes in addition to being formed from preasphaltenes. The assumption in this model that the process of coal liquefaction would cause the scission of the bond in a specific way [15]. First, a group of fragments consisting of polar groups would be produced by cracking of coal with the UV energy. The preliminary cracking products with UV irradiation energy in the coal liquefaction reaction can be, therefore, defined as preasphaltenes (insoluble in toluene), that would impose strict requirements on the molecular structure of the coal. Although neither the Arrhenius-type description of cracking probability nor the general description of the chemical constitution of coal support such requirements, the implications for the liquefaction are that (i) the weakest bonds on the structure are the only bonds undergoing bond dissociation under the liquefaction conditions, (ii) the weakest bonds are in a position to lead to two product molecules, both of which are insoluble in
(a) Conversion (%)
Conversion (%)
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
Time (h)
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
(b)
Time (h)
30
% Asfalten Asphaltene (Model) % (Model) % Preasfalten Preasphaltene (Model) % (Model) % (Model) %Yağ Oil (Model) % %Asfalten Asphaltene % %Preasfalten Preasphaltene % Yağ
(c)
% Oil
20
Conversion (%)
Conversion (%)
25
15
10
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
(d)
5
0 0
20
40
60
80
100
120
Time (h)
Time (h)
Fig. 2. Comparison of experimental data with Model-1 at a) 90 W, b) 120 W, c) 150, and d) 180 W. 5
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(a) Conversion (%)
Conversion (%)
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
Time (h)
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
(b)
Time (h)
30
%Asfalten Asphaltene (Model) % (Model) % (Model) %Preasfalten Preasphaltene (Model) % %Yağ Oil (Model) (Model) % %Asfalten Asphaltene % Preasfalten % Preasphaltene % Yağ
20
(c)
% Oil
Conversion (%)
Conversion (%)
25
15
10
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
(d)
5
0 0
20
40
60
80
100
120
Time (h)
Time (h)
Fig. 3. Comparison of experimental data with Model-2 at a) 90 W, b) 120 W, c) 150 W, and d) 180 W.
provide a good definition of liquefaction of the Beypazarı lignite. The experimental results presented in Table 1 illustrate that although the formation of oils consistently and saliently increases with the liquefaction time increasing until 96 h, the formation of both preasphaltenes and asphaltenes demonstrates fluctuations and may assume going to a steady line at the further liquefaction times. The consistent increase in the oils yield may be attributed to the irreversible liquefaction reaction step from reactive coal to oils. Moreover, the high oils yield may also be supported by the further reactions from asphaltenes and preasphaltenes to oils. On the other hand, the stability of the preasphaltenes and asphaltenes yields may be explained by the reversible reaction steps between coal ↔ asphaltenes and preasphaltenes ↔ asphaltenes. The crucial role of the reversible reactions in the coal liquefaction models was also demonstrated in our previous papers [15,40]. In Model-4, the liquefaction of coal has, therefore, been improved by three reversible reactions; between coal ↔ asphaltenes, asphaltenes ↔ oils, and asphaltenes ↔ preasphaltenes, in addition to Model-3. As demonstrated in Fig. 5, the Model-4 fits best with experimental data for the UV power of 90, 120, 150 and 180 W compared with the other models. Furthermore, the sum of the squared differences illustrated in Table 6 also proves that Model-4 has a better explanation for the liquefaction of the lignite compare with the other three models. Moreover, it was also demonstrated the models show better fit with the experimental results at higher UV powers, 180 W, compared with the low UV powers, 90 W. Thanks to these three reversible reactions, which is the only difference from Model-3, Model-4 demonstrates much better fitting with the experimental data. Thus, it can be said that the main
Table 4 The sum of the squared differences of the values calculated with the Model-2 and experimental data and reaction rate constants (h−1) for various UV power. UV power (W)
90 120 150 180
∑i (ymi − yei )2
145 162 118 95
Reaction rate constants (h−1)*102 k1
k2
k3
k4
0.86 0.96 1.00 0.96
0.99 0.91 0.73 1.01
2.65 2.47 2.48 2.05
1.05 1.10 1.05 1.06
yields in the liquefaction of Beypazarı lignite via the reaction from both coal and preasphaltenes to oils, the model has still failed to the formation of hexane soluble products, oils, seen from Fig. 3. Model-3 is defined as a combination of Model-1 and Model-2. In addition to Model-2, Model-3 consists of the formation of asphaltenes are build up from preasphaltenes. Additionally, oils are also formed from preasphaltenes and asphaltenes along with reactive coal. However, Fig. 4 demonstrates that the model, Model-3, also fails to provide a good definition of liquefaction of Beypazarı lignite for four different UV powers. The discrepancy between Model-3 and experimental data is more prevalent in the formation of hexane soluble fragments, oils, as observed on Model-2. Additionally, the sum of squared differences presented in Table 5 demonstrates similar values with Model-2. In terms of the UV power, although the models, Model-1, Model-2, and Model-3, demonstrate lower sum of the squared differences at higher UV powers, the values are still not low enough to 6
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(a) Conversion (%)
Conversion (%)
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
(b)
Time (h)
Time (h) 30
Conversion (%)
25
20
(c) Conversion (%)
%Asfalten Asphaltene (Model) % (Model) % (Model) %Preasfalten Preasphaltene (Model) % Yağ (Model) % Oil (Model) % Asfalten % Asphaltene % Preasfalten % Preasphaltene % Yağ % Oil
15
10
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
(d)
5
0 0
20
40
60
80
100
120
Time (h)
Time (h)
Fig. 4. Comparison of experimental data with Model-3 at a) 90 W, b) 120 W, c) 150 W, and d) 180 W.
with the hydrogen transfer, and then either (iii) the formation of the smaller molecular weight liquefied molecules or (iv) the re-polymerization of the free radicals due to the hydrogen deficiency. The formation of high molecular weight hydrogen deficient polyaromatic species similar to char may result in re-polymerization. The increase in the oil yield during the liquefaction process can be attributed to the stabilization of free radicals by hydrogen shuttling from hydrogen-rich hydrocarbons instead of hydrogen donor solvent [15]. The presence of reversible reactions in the liquefaction model can prove that the hydrogen donor solvent may not be able to transfer enough hydrogen to the free radicals [40]. This study demonstrates that the reversible reactions steps are the determinative steps in the liquefaction of the Beypazarı lignite under UV power. Furthermore, the models having reversible reaction steps demonstrate the best fit with the experimental data regardless of coal types and power sources such as microwave [15,40].
Table 5 The sum of the squared differences of the values calculated with the Model-3 and experimental data and reaction rate constants (h−1) for various UV power. UV power (W)
90 120 150 180
∑i (ymi − yei )2
145 160 118 94
Reaction rate constants (h−1)*102 k1
k2
k3
k4
k5
k6
0.99 0.92 0.73 1.01
0.89 0.98 1.03 0.99
2.63 2.43 2.46 2.02
1.01 1.12 1.06 1.05
1.08 1.07 1.03 1.07
1.00 1.00 1.00 1.00
free radicals have been formed from the reactive coal as stated above. It is known that during the liquefaction of reactive coals, many competitive and concurrent chemical reactions may occur. The reaction rate constants of Model-4 at four different UV powers including 90, 120, 150, and 180 W, have been determined using multiple regression analysis and the results are also presented in Table 6. It is clearly seen from the table that the reaction rate from reactive coal to oils are almost double compared with the other reaction rate constants demonstrated on the Model-4. The reaction rate constant is approximately 2.0*10−2 h−1 for the liquefaction reaction from reactive coal to oils while the reaction rate constants for the other reactions on the model are about 1.0*10−2 h−1. Additionally, the reaction rate constants are insignificantly affected by the UV powers used in the liquefaction of Beypazarı lignite. The general liquefaction steps of a reactive coal may consist of a group of steps; (i) bonds cleavages between structural species, and then (ii) stabilization of the free radicals formed by the breaking of bonds
4. Conclusion The models, Model-2, Model-3, and Model-4, where the products are formed from reactive coal in parallel reactions illustrate better fit with the experimental data compared with the model, Model-1, where the products are liquefied in a series reaction from coal to preasphaltenes. A much better explanation for the liquefaction of Beypazarı lignite has been brought over with Model-4 consisting of reversible reaction steps in addition to parallel liquefaction steps. Furthermore, it was found that the liquefaction step from coal to oils has the highest reaction rate constant compared with other reactions suggested in the Model-4. Moreover, the reaction rate constants are 7
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% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
(a) Conversion (%)
Conversion (%)
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
(b)
Time (h)
Time (h) 30
25
Conversion (%)
20
% Asphaltene (Model) % Preasphaltene (Model) % Oil (Model) % Asphaltene % Preasphaltene % Oil
(c) Conversion (%)
% Asfalten Asphaltene (Model) % (Model) % Preasfalten Preasphaltene (Model) % (Model) % %Yağ Oil (Model) (Model) % %Asfalten Asphaltene % Preasfalten % Preasphaltene % Yağ % Oil
15
10
(d)
5
0 0
20
40
60
80
100
120
Time (h)
Time (h)
Fig. 5. Comparison of experimental data with Model-4 at a) 90 W, b) 120 W, c) 150 W, and d) 180 W. Table 6 The sum of the squared differences of the values calculated with the Model-4 and experimental data and reaction rate constants (h−1) for various UV power. UV power (W)
90 120 150 180
∑i (ymi − yei )2
58 60 49 38
Reaction rate constants (h−1)*102 k1
k2
k3
k4
k5
k6
k7
k8
k9
2.10 1.97 2.02 1.89
1.03 1.10 1.05 1.02
1.11 0.73 0.79 1.08
1.01 0.78 0.89 0.93
1.00 1.10 1.04 1.04
0.92 0.82 1.02 1.01
1.03 1.07 1.03 1.05
1.02 0.99 0.99 1.01
0.98 1.02 1.01 0.99
important to investigate the determination of reaction orders with other parameter estimation methods for the best model.
independent of the liquefaction power in the liquefaction process where the UV is used as the liquefaction power source. It is the first study to demonstrate that the models having reversible steps are crucial in the liquefaction of Beypazari lignite using UV power source. Furthermore, the study is also demonstrated that the Kalman filter is one of the useful methods to estimate the model parameter for the liquefaction of coals using minimum experimental results. After this study and previous studies [15,40], it can be concluded that the models having reversible reaction steps demonstrate best fit with the experimental data regardless of coal types and power sources such as microwave or UV. It is believed that although the applicability of UV irradiation for the coal liquefaction in mass production seems too difficult due to the long reaction time, this study may guide for the UV Lamb Photochemical Reactor design of small-scale liquefaction processes. Furthermore, this study may enable the formation of better quality (low molecular weight) liquid products in the thermal coal liquefaction process assisted by UV irradiation energy. Additionally, as future work, it is highly
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