Pyrolysis kinetics of waste automobile lubricating oil

Fuel 79 (2000) 1943–1949 www.elsevier.com/locate/fuel

Pyrolysis kinetics of waste automobile lubricating oil S.S. Kim a,*, S.H. Kim b a

Department of Environmental Systems Engineering, Korea University, 1, 5-Ka, Anam-dong, Seongbuk-Ku, Seoul 136-701, South Korea b Department of Chemical Engineering, Korea University, 1, 5-Ka, Anam-dong, Seongbuk-Ku, Seoul 136-701, South Korea Received 26 July 1999; received in revised form 10 January 2000; accepted 20 January 2000

Abstract The kinetics of waste automobile lubricating oil have been studied experimentally and modeled mathematically. Experiments were carried out in the tubing bomb microreactor at a temperature of 420–440⬚C and reaction times of 5–50 min. Volatile pyrolysis products were identified and quantitatively determined by gas chromotography. A lump model of combined series and parallel reactions for oil formation is proposed. Conversion data fitted first order kinetics for C5 –C11 and C12 –C25. The kinetic parameters were determined by nonlinear leastsquares regression of the experimental data. These calculated values of the product distribution were found to be in agreement with the experimental data. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: Pyrolysis; Waste automobile lubricating oil; Lump kinetic model

1. Introduction Billions of gallons of waste automobile lubricating oil are generated every year in the world. A number of studies to remove various pollutants existing in the waste lubricating oil and simultaneously to reuse these resources as valuable products have been attempted. Some of the technologies successfully applied the operation of the newly developed regenerating plants and obtained the re-refined lubricating oil, which has equivalent quality to the new lubricating oil [1–3]. The Phillips re-refining process combines chemical demetallization with hydrotreatment to produce high yields of base oils from waste lubes [1]. Kim et al. [4,5] investigated a novel technology for obtaining more valuable product oils from the waste lubricating oils. The waste lubricating oils were mixed with atmospheric distillation residuum from crude oil and then were distilled under a vacuum. These methods of disposal are no longer practicable, because these treatment processes have several problems such as low yield, environmental pollution and sludge disposal. Therefore, it is necessary to seek a suitable method to solve the disposal problems. In recent years, many pyrolysis studies have been applied * Corresponding author. Present address: Department of Environmental Engineering, Tong Hae University, 119 Jiheung-dong, Tong Hae, Kangwondo 240-713, South Korea. Tel.: ⫹ 82-33-521-9400; fax: ⫹ 82-33-5219907. E-mail addresses: [email protected] (S.S. Kim), kimsh @kuccnx.korea.ac.kr (S.H. Kim).

to organic materials such as coal, waste tires, waste plastics, waste woods, and oil shales [6–14]. Pyrolysis process has certain advantages over other treatment methods of waste disposal [6,14,18]. Pyrolysis products such as gases, liquid oils and char can be used as fuels. Some studies have elucidated the mechanism of the degradation of organic materials. Carniti et al. [15] proposed that thermal degradation of polystyrene to volatile products can be interpreted by the following consecutive reactions: PS (and heavy products of partial degradation) ! C13 –C24 ! C6 –C11. Martinez et al. [16] and Yoshida et al. [17] studied the thermal cracking of coal residues. They proposed the lump model, which considers parallel reactions for oil ⫹ gas and coke formation. Ballice et al. [18] and Prakash [19] proposed a combined parallel and series reaction scheme for the pyrolysis mechanism of oil shale. However, although extensive knowledge has been accumulated about lumped kinetics, no attempt has been made so far to apply the kinetic model to waste automobile lubricating oil. The main purpose of this work is to propose a reaction pathway for the pyrolysis of waste automobile lubricating oil and to develop a kinetic model for the pyrolysis. In view of the need to develop commercially practicable processes for deriving liquid fuels from waste automobile lubricating oil, a description of pyrolysis kinetics is necessary for reactor design and scale-up. This paper reports a lumped kinetic of oil conversion (i.e. those with a number of carbon atoms

0016-2361/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(00)00028-4

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S.S. Kim, S.H. Kim / Fuel 79 (2000) 1943–1949

Nomenclature waste automobile lubricating oil, component of ⬎C25 CB component of C12 –C25 CC, CC1, CC2 component of C5 –C11 CA0 initial component of waste automobile lubricating oil initial component of C12 –C25 CB0 k1, k2, k3 pyrolysis rate constant, s ⫺1 Yexperimental,i experimental value Ycalculated,i calculated value CA

of ⬍C26) by thermal decomposition of waste automobile lubricating oil. Oil products were lumped in two groups of C5 –C11 and C12 –C25. A lump model of combined series and parallel reactions for oil formation is proposed. Conversion data fitted first order kinetics for C5 –C11 and C12 –C25. The kinetic parameters were determined by nonlinear leastsquares regression of the experimental data. This study is important because it will give more insight into the pyrolysis process of waste automobile lubricating oil.

Table 1 Properties of waste lubricaing oil Dynamic viscosity (m 2/s) Density (at 20⬚C) Gravity (15.4/4) Paraffin (wt%) Naphthane (wt%) C12 –C25 (wt%) ⬎ C25 (wt%)

0.50 0.8797 0.8833 58 42 16.58 83.42

Other pyrolyzed products were separated into oil (hexane soluble) and coke (hexane insoluble) by solvent extraction technique; the details of the procedure are available in the literature [6]. The solid yield is defined as (weight of hexane insoluble) × 100/(weight of feed), while oil yield is defined as (100-gas yield-solid yield). The solid was separated by micro filter paper of 0.45 mm. Liquid products were analyzed by a Gas Chromotography (GC) (Young Lin-M600D) which was equipped with a flame ionization detector. They were analyzed for simulating boiling point distribution (ASTM D-2887) [20]. Capillary column a HP-1 (0.53 mm i.d., 5 m length) was employed with a programmed temperature from 50 to 350⬚C. Peaks were identified by comparing retention times with those of known reference compounds.

2. Experiment The experimental raw material was waste automobile lubricating oil from an automobile maintenance shop. The characteristics of waste automobile lubricating oil are represented in Table 1. Their properties were characterized by ASTM D2140. A tubing bomb microreactor was employed to elucidate the pyrolysis of waste automobile lubricating oil. A schematic diagram of the experimental equipment is given in Fig. 1. The volume of the tubing reactor was 39 ml. It was used to test the effect of residence time on the pyrolysis of waste automobile lubricating oil under a constant temperature. A sample mass of 5 g was charged into the tubing reactors for all experimental runs. Molten salt bath, which has excellent heat transfer properties, was used for the pyrolysis of waste automobile lubricating oil. Eutectic salt of KNO3 (59 wt%) ⫹ Ca(NO3)2 (41 wt%) has been used. The temperature of the molten salt bath (420, 430, and 440⬚C) was constant within ^1⬚C. The pyrolysis temperatures of 420, 430 and 440⬚C were selected from temperature of maximum peak for variation of instantaneous reaction rate with temperature at three different heating rates of 0.5, 1.0 and 2.0⬚C/min. Samples were taken at different times (5, 10, 15, 20, 30, 40, and 50 min) and cooled to room temperature. The reaction products were analyzed for the amount of gases formed, amount of oil formed, and the amount of coke formed. Tubing reactors were opened after cooling to room temperature to allow gases to evolve gently. Gas yield was obtained by weighing the tubing reactor before and after venting gases. Gas yield is defined as (gas weight) × 100/(feed weight).

3. Result and discussion The distribution of oil, gas, and coke formation according to solubility of n-hexane is shown in Table 2. The results of Table 2 provide evidence for the effects of pyrolysis reaction temperature and residence time on the yields of gas, oil and coke. Variation of residence time does not make a significant effect on the formation of gas, oil and coke for three different reaction temperature (420, 430 and 440⬚C) and time (5, 10, 15, 20, 30, 40, and 50 min). The yield of gas is in the range of 1– 3 wt% and that of oil is 95–90 wt% at a residence time of 5– 50 min and reaction temperature of 420, 430 and 440⬚C. The fraction of coke formation is 1–2 wt%. Gas compounds were not analyzed by gas chromatogram, because gas yield was very low values of 1–3 wt% compared with liquid oil products. Thus GC analysis limited to liquid oil product. A characteristic GC of the waste automobile lubricating oil and pyrolysis product for the reaction temperature 440⬚C at 20 and 50 min is given in Fig. 2. The waste automobile lubricating oil consisted of mostly C20 –C42 with small amount of C13 –C19 as shown in Fig. 2. However, major organic cracking products of waste automobile lubricating oil was C6 –C20 hydrocarbons at the reaction temperature of 440⬚C at 20 min and 50 min. Ballice et al. [18] analyzed carbon number distribution of the organic pyrolysis product at 440⬚C for oil shale. The product of C1 –C26 was broadly distributed and hexane and heptene were produced in much higher quantities as compared with the other products. The carbon number distribution of waste automobile

S.S. Kim, S.H. Kim / Fuel 79 (2000) 1943–1949

1. Microscale tubing bomb reactor

4. Stirrer

2. Salt bath

5. Thermocouple

3. Heater

6. PID temperature controller

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Fig. 1. Schematic diagram of experimental apparatus.

lubricating oil was represented in Table 1. The composition of ⬎C25 of the unused lubricating oil is 100% [21]. However, the composition of ⬎C25 of the waste automobile lubricating oil was 83.42 wt%. It is thought that 16.58 wt% (⬍C25) of the waste lubricating oil was produced during the lubrication in automobile engines. The most significant results in terms of product distribution according to boiling point of liquid products are represented in Table 3. To analyze the reaction products, the samples were dissolved in CS2. The volatile products were quantitatively determined by GC. The selectivity of the C5 –C25 compounds increased with reaction time at each temperature. To simplify the kinetic model of the experimental data, the fraction of gas and coke was neglected, due to very low value compared with the oil fraction. Oil products were Table 2 Distribution of oil, gas and coke according to solubility of the pyrolysis products(wt%) Residence time (min) 5

10

15

20

30

40

50

Experiments at 420⬚C Gas (C1 –C4) 0.81 Oil 98.15 Coke 1.04

0.98 97.50 1.52

1.09 97.88 1.03

2.62 96.25 1.13

2.20 96.48 1.32

1.97 96.70 1.33

1.98 95.97 2.05

Experiments at 430⬚C Gas (C1 –C4) 1.55 Oil 95.96 Coke 2.99

1.95 96.99 1.06

2.65 96.30 1.05

2.40 96.34 1.26

1.68 96.69 1.63

1.29 97.26 1.45

1.59 96.91 1.50

Experiments at 440⬚C Gas(C1 –C4) 1.07 Oil 97.29 Coke 1.64

2.06 96.84 1.10

3.23 95.74 1.03

3.18 95.25 1.57

2.99 95.10 1.91

2.42 95.61 1.97

2.40 95.65 1.95

lumped in two groups of C5 –C11 and C12 –C25 (the carbon number of volatile gasoline is C5 –C11, and the carbon number of fuel oil such as gas oil and heavy gas oil is C12 –C25). Figs. 3–5 represent the reactant and the product selectivity of two groups as functions of time at the temperature of 420, 430, and 440⬚C The fraction of 83.42 wt% (⬎C25) of waste lubricating oil was considered as the reactant components, because waste lubricating oil contained 16.58 wt% of C12 –C25 of the product components. The fraction of waste automobile lubricating oil (⬎C25) decreased exponentially with increasing residence time for temperature of three different reactions. The yield of C5 –C11 and C12 –C25 shows an increase depending on time. The fraction of C5 – C11 and C12 –C25 increased up to reaction times of 20 min at each temperature. However, the fraction of C12 –C25 did not increase much despite an increase of retention time above 20 min. This suggests that the degradation of waste automobile lubricating oil can be interpreted by the consecutive reactions. The pyrolysis mechanisms assumed for the kinetic model development in this study are shown below.

…1†

1. A combination of series and parallel reactions were

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S.S. Kim, S.H. Kim / Fuel 79 (2000) 1943–1949

Fig. 2. Sample chromatogram of pyrolysis product.

2. 3. 4.

5.

assumed for the pyrolysis mechanism of waste automobile lubricating oil. All the reactions are irreversible and first order. The formation of gas and coke is negligible. The primary reaction proceeds directly in an initial step from A (waste automobile lubricating oil) to C (C5 – C11). The secondary reaction from waste automobile lubricating oil proceeds consecutively from B (C12 –C25) to C (C5 –C11).

Based on these assumptions the rate of formation/ depletion of various components can be written as follows: dCA ˆ ⫺…k1 ⫹ k2 †CA dt

…2†

where CA is the concentration of ⬎C25 of raw sample material, and k1 and k2 are the kinetic constant for C5 – C11 formation and C11 –C25 formation, respectively.

Table 3 Carbon number distribution of oil according to boiling point distribution of the pyrolysis products (wt%) Residence time (min) 5

10

15

20

30

40

50

420⬚C

Oil

C5 –C11 C12 –C25 ⬎ C25

4.83 31.56 63.61

12.69 29.53 57.78

21.84 36.48 41.68

23.61 41.96 34.43

32.90 47.38 19.72

43.27 43.28 13.45

39.48 47.97 12.55

430⬚C

Oil

C5 –C11 C12 –C25 ⬎ C25

12.02 35.80 52.18

16.37 44.70 38.93

19.88 47.87 32.25

29.05 49.56 21.39

33.11 52.72 14.17

29.83 56.66 13.51

34.01 54.58 11.41

440⬚C

Oil

C5 –C11 C12 –C25 ⬎ C25

13.86 31.02 55.12

23.13 33.16 43.71

20.73 49.44 29.83

30.90 50.16 18.94

34.37 55.67 9.66

37.56 56.03 6.41

39.38 55.24 5.38

S.S. Kim, S.H. Kim / Fuel 79 (2000) 1943–1949

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Fig. 3. Effect of residence time on product distribution at 420⬚C.

following:

The rate of C12 –C25 formation is expressed by: dCB ˆ k2 CA ⫺ k3 CB dt

…3†



where CB is the concentration of C12 –C25 formation. The rate of C5 –C11 formation is expressed by: dCC ˆ k1 CA ⫹ k3 CB dt

CA ˆ CA0 exp‰⫺…k1 ⫹ k2 †tŠ CB ˆ CB0 ⫹ k2 CA0

exp‰⫺…k1 ⫹ k2 †tŠ exp…⫺k3 t† ⫺ k3 ⫺ …k1 ⫹ k2 † k3 ⫺ …k1 ⫹ k2 †

 …6†

…4†

where CC is the concentration of C5 –C11, and k3 is the kinetic constant for C5 –C11 formation. Thus, the kinetic equations for the reaction of Eqs. (2)–(4) in terms of yields are the

…5†

CC ˆ CA0 ⫺ CA ⫺ CB

…7†

The reaction rate constants of k1, k2, and k3 were obtained by applying an optimization procedure (Levenberg–Marquardt

Fig. 4. Effect of residence time on product distribution at 430⬚C.

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S.S. Kim, S.H. Kim / Fuel 79 (2000) 1943–1949

Fig. 5. Effect of residence time on product distribution at 440⬚C.

method) to the values of each component of produced oil at the three reaction temperatures investigated. The parameter estimation problem can be mathematically formulated into the following constrained nonlinear optimization program as follows: minimize S ˆ

N X

…Ycalculated;i ⫺ Yexperimental;i †2

…8†

iˆ1

where Yexperimental,i and Ycalculated,i refers to the experimental value at a given temperature and time of reaction, and the corresponding value calculated by Eqs. (5)–(7). The reaction rate constants are estimated from the application of Eq. (8) in Figs. 3–5 The estimated reaction rate constants are shown in Table 4. The reaction rate constants are higher for k2 (⬎C25 ! C12 –C25 formation) than k1 (C12 – C25 ! C5 –C11 formation) and k3 (C12 –C25 ! C5 –C11formation) at each temperature. The reaction rate constant k1 increased with the pyrolysis temperature increase from 420 to 440⬚C. The reaction rate constant k2 increased with the pyrolysis temperature increase from 420 to 430⬚C, but it decreased with the pyroysis temperature increase from 430 to 440⬚C, k3 decreased with increasing temperature, and decreased highly while the temperature increased from Table 4 Reaction rate constants (min ⫺1) for waste automobile lubricating oil Temperature (⬚C)

420 430 440

Rate constant (min ⫺1) k1

k2

k3

0.0190 0.0202 0.0313

0.0253 0.0432 0.0387

0.0110 0.0111 0.0034

430 to 440⬚C. The kinetic constant k1 increased with increasing temperature from 430 to 440⬚C. However, k2 decreased slightly. These results suggest that waste automobile lubricating oil decompose into lighter hydrocarbons with an increase of reaction temperature. Pyrolysis of waste automobile lubricating oil becomes a predominant reaction pathway from A (waste automobile lubricating oil) to C12 –C25 formation rather than from A (waste automobile lubricating oil) to C5 –C11 formation. First-order lumped kinetics provide an excellent fit for the products obtained at 420, 430, and 440⬚C. The lumped reaction scheme proposed can be rationalized by the pyrolysis mechanism of waste automobile lubricating oil in Eqs. (2)– (7). This mechanism is in agreement with the experimental results and accounts for the formation of the final products of C5 –C11 fraction and C12 –C25 fraction.

4. Conclusions 1. The yields of gas (C1 –C4) increased until 20 min of reaction time and thereafter decreased with a further increase in the reaction time at the experimental temperatures. The coke content of the products was 1–2% and the yield of oil was 95–98% at different residence times and temperatures. 2. A lumped kinetic model was proposed to represent the pyrolysis reaction of waste automobile lubricating oil in a tubing reactor. Model products were separated into two groups of C5 –C11 and C12 –C25. The kinetic parameters of the proposed model were evaluated by a nonlinear leastsquares regression method. The proposed kinetic model was found to well represent the experimental results.

S.S. Kim, S.H. Kim / Fuel 79 (2000) 1943–1949

3. It was found from the reaction kinetic constants that the predominant reaction pathway was A (waste automobile lubricating oil) to C12 –C25 formation rather than A to C5 – C11 formation at a temperature of 420–440⬚C.

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