High Pressure Phase Equilibria of CO 2 with Limonene and Other Components Present in the Light Naphtha Cut of Tyre Derived Oil

Transcription

1 High Pressure Phase Equilibria of CO 2 with Limonene and Other Components Present in the Light Naphtha Cut of Tyre Derived Oil C.E. Schwarz a, * and C. Latsky a a Department of Process Engineering, Stellenbosch University, Private Bag X1, Matieland, 7602, South Africa * cschwarz@sun.ac.za ABSTRACT Waste tyres are causing a serious disposal problem and therefore methods to recycle and re-use waste tyres are gaining more attention. One such method is pyrolysis. To improve the economic viability of the pyrolysis process limonene is to be derived from the tyre derived oil produced by pyrolysis. While distillation can easily produce a limonene rich light naphtha cut, obtaining pure limonene is not trivial. Supercritical fluid fractionation was therefore investigated as an alternative separation technique. The aim of this paper is to present and evaluate high pressure phase equilibria of CO2 plus limonene and other components in the light naphtha cut of tyre derived oil. In particular phase equilibria data for CO2 + m-cymene, CO2 + p-cymene, CO2 + indane and CO2 + 1,2,3- trimethylbenzene is measured on a static synthetic high pressure view cell and compared with previously published CO2 + limonene data. Analysis of the phase behaviour data revealed that supercritical fluid fractionation may be a suitable separation technique to achieve separation between limonene and some components of similar boiling point in the light naphtha cut of tyre derived oil. 1. INTRODUCTION The international increase in the number of vehicles along with a lack of effective and economical methods to recycle waste tyres, is causing serious waste disposal problems [1]. Recent attempts to re-use and recycle waste tyres includes the use of ground tyre rubber as an additive in, for example, artificial sport fields and floor mats. Furthermore, waste tyres are also being used as a solid fuel to power cement kilns and power plants. A more environmentally friendly recycling approach is pyrolysis [2]. Pyrolysis is a process in which the volatile components in tyres are thermally degraded in the absence of oxygen [3]. The process produces three products namely pyrolytic char, flammable gas and tyre derived oil, all of which are potentially useable [3, 2]. Although pyrolysis produces useable products, a drawback of this recycling method is that pyrolysis products are in low demand and therefore the process is not economically attractive to pursue [4]. Literature indicates that a possible method to improve the economic viability of the pyrolysis process is to recover valuable chemicals, in particular limonene, from the tyre derived oil [3, 4, 1]. 1

2 Various studies have investigated the extraction of limonene from the naphtha distillation cut of tyre derived oil, but it has been found that obtaining high purity limonene is not a trivial tasks [3]. A study conducted by Pakdel, et al. [5] determined that a limonene enriched fraction, also known as the light naphtha cut, could easily be obtained through distillation [5]. Further purification was however determined to be more complex due to the composition of the light naphtha cut. The study revealed that the major impurities in the light naphtha cut are m-cymene, indane and 1,2,3-trimethylbenzene. These components are formed due to thermal decomposition of limonene during the pyrolysis process [5, 3]. A study conducted by Danon, et al. [3] reported that p-cymene would also be present in the light naphtha cut, as limonene would most likely be aromatized to form p-cymene [3]. The above mentioned impurities all have similar boiling points to limonene and therefore traditional separation techniques such as distillation are ineffective to separate the components [3, 5]. An alternative separation technique receiving much attention is supercritical fluid extraction [6]. The popularity of this separation technique mainly stems from the good solvent properties of supercritical solvents and the fact that the solvent can easily be regenerated with minimal solvent residue in the product [7]. Furthermore, the use of CO2 as solvent, which is non-toxic, non-flammable, chemically stable and relatively inexpensive [8], increases the advantages associated with the process. The aim of this work was to determine whether supercritical CO2 fractionation is a technically viable method to separate limonene from the major impurities in the light naphtha distillation cut of tyre derived oil. In order to achieve this aim the phase behaviour of limonene and the major impurities in the light naphtha cut, namely m-cymene, p-cymene, indane and 1,2,3- trimethylbenzene, with supercritical CO2 was investigated. Due to a lack of phase behaviour data for the major impurities with supercritical CO2, high pressure phase equilibria experiments were conducted to generate data for these systems. 2. MATERIALS AND METHODS The high pressure phase equilibria experiments were conducted on two separate, previously constructed variable volume equilibrium cells. The design of these two cells are similar, with the only difference being the internal volume. The internal volume of the large and small cells are 80cm 3 and 45cm 3, respectively [9]. The maximum operating temperature and pressure of both cells are 200⁰C and 25 MPa, respectively. For the detailed design of the smaller equilibrium cell the reader can refer to the work done by Schwarz [7]. During the experiments a known amount of solute, along with the magnetic stirrer bar was added to the equilibrium cell. Once the solute was loaded in the cell, the piping and the cell were evacuated and then the system was flushed with CO2. Thereafter, a known amount of CO2 was transferred to the cell. Once the solute and the CO2 were loaded, the magnetic stirrer was turned on, the set point temperature of the thermostat bath was set to the first temperature and the cell content was pressurised to the one phase region. Once thermal equilibrium was reached the pressure of the system was slowly reduced to determine the transition point. The transition point, where the system moves from the one phase region to the two phase region, was visually observed on the monitor and the pressure, temperature, piston position and the number of phases were recorded. The process was repeated until the transition point was 2

3 measured to within 0.02 MPa [10, 7]. The measuring procedure was repeated at each set temperature. Upon completion of the experiments the cell was unloaded and cleaned. For further detail regarding the experimental procedure the reader is referred to the work conducted by Schwarz & Nieuwoudt [11]. The materials used, along with their suppliers and purity is presented in Table 1. Table 1: Supplier and purity of required materials Material Supplier Purity p-cymene Sigma-Aldrich 99% m-cymene Tokyo Chemical Industry Co.,Ltd >99% indane Sigma-Aldrich 95% 1,2,3-trimethylbenzene Finetech Industry Limited 98% carbon dioxide Air Products % In order to quantify the effect of the inaccuracies on measurements reported in this work, the uncertainty in the measurement of the temperature, pressure and mass fraction was evaluated [11, 12]. The standard uncertainty in the temperature and pressure measurements were determined to be better than 0.2 K and 0.06 MPa, respectively. Furthermore, the relative uncertainty in the mass fraction was determined to be better than 0.01 of the mass fraction value. 3. RESULTS 3.1 TEMPERATURE CORRELATIONS AND VALIDATION Due to changes in ambient conditions, temperature fluctuations between the different data sets were unavoidable [10, 9]. In order to generate isothermal pressure-composition diagrams, as presented in Figure 1 (a), polynomial curves were fitted to the experimental pressure and temperature data at constant composition. This approach to generate isothermal data, has been used in various studies and it is mostly reported that the pressure-temperature relation at constant composition can be adequately described by linear correlations [9, 13, 14]. Based on this approach, all experimental data was regressed using a linear relationship between temperature and pressure. The linear correlations were then analysed according to the acceptance criteria presented in equations 1 to 3 [9]. If the linear correlation did not meet all the acceptance criteria limits, a second order polynomial was implemented. R 2 > 0.98 (1) P predicted - P measured = 0.2 MPa (2) P predicted - P measured P measured x 100 2% (3) In order to validate the results obtained in this work, the repeatability and reproducibility of the results were analysed and the experimental data was compared to available literature. The repeatability of the data was evaluated by repeating each phase transition measurements at least twice, before confirming it to be the phase transition point [7]. The reproducibility of the data obtained was evaluated by repeating an experiment under different conditions, that is with a slight deviation in solute mass fraction and different ambient conditions [15]. In Figure 1 (b) the results obtained from the experiments conducted with p-cymene mass fractions of and are compared. From Figure 1 (b) it is noted that the data obtained from the different 3

4 experiments correlate well. The deviation in the data is deemed to be within an acceptable range when considering the reported accuracy of the measurements and the acceptance criteria limits for the temperature-pressure correlations. Figure 1: Phase equilibria data for p-cymene + CO2 system: (a) isothermal pressure-composition curves (b) repeatability data Due to a lack of reliable literature data, no conclusion as to the validity of the experimental data, when compared to literature, could be made. However, as the working principle and the measuring technique used to generate the experimental data presented in this work has been validated by previous studies [10, 6] the results presented in this work are deemed to be credible. 3.2 PRESSURE-COMPOSITION DIAGRAMS In order to evaluate the phase behaviour of the different compounds isothermal binary pressurecompositions diagrams, as presented in Figures 2 (a) to (d), were generated. The limonene data presented in Figures 2 (a) to (d) was obtained from a study conducted by Madzimbamuto, et al. [13] and the reason for using this source mainly stems from the fact that the data presented by the source was obtained using the same experimental method, equipment and measurement range as the data presented in this work. When analysing the experimental pressure-composition curves it will be noted that for some systems the data points at lower solute mass fraction has been neglected. This is due to the fact that the phase transition could not be visually detected and the polynomial curves fitted to the data are not suited for extrapolation. A study conducted by Schwarz & Knoetze [14] found that the reason for the difficulty in measuring the phase transition at the low solute mass fractions is due to the low solubility in the low solute mass fraction region. The low solubility results in a steep pressure-composition gradient and therefore a slight change in the pressure near the transition point results in the formation of a small quantity of the second phase, which is difficult to detect [14]. 4

5 Figure 2:Pressure-composition diagrams for limonene+co2, p-cymene+co2, m-cymene+co2, indane+co2 and 1,2,3 trimethylbenzene+co2 systems at (a) K, (b) K, (c) K and (d) K 3.3 SEPARABILITY ANALYSIS Based on a study conducted by Schwarz [7] and theory presented by Brunner [16], the technical viability of a supercritical fractionation process can be evaluated by investigating the solvent loading capabilities and by analysing the separability of the components [7, 16]. Due to the fact that the operating conditions of supercritical extraction processes is represented by the low solute mass fraction section, that is the vapour-like region, of the pressure-composition diagrams, the solute loading and separability of the components in this region was evaluated to determine whether supercritical fractionation is technically viable [13]. 5

6 The solvent loading capability was evaluated by analysing the binary phase behaviour of the individual components with supercritical CO2. From the analysis it was determined that the phase behaviour of all the components allow for significant solute loading (up to 15 mass % at 358.2K) in the vapour-like region. Based on this, the solvent loading capability criteria for the proposed extraction process was deemed to be met. The separability of the components were evaluated by comparing the vapour-like phase behaviour (up to 0.15 mass fraction solute) of the different components. When analysing the pressure-composition diagrams, presented in Figure 2 (a) to (d), it is seen that below 328.2K, there is very little difference between the solubility of the components. As the temperature increases, the pressure-composition curves of the different components start to separate. At K a noticeable distinction can be made between the solubility of the different components. As the temperature increases from K, the difference between the pressure-composition curves of the different components further increases. When evaluating the pressurecomposition diagram constructed at K, it is evident that p-cymene is the most soluble component in supercritical CO2. Furthermore, it is seen that the pressure-composition curve for limonene closely corresponds to that of m-cymene, suggesting that there is little difference in solubility between these components. Moreover, it is also noted that although there is very little difference in solubility between indane and 1,2,3-trimethylbenzene, both of these components are less soluble in supercritical CO2 than limonene. These findings suggest that the ease and selectivity with which the components can be separated increases as temperature increases. Furthermore, the results suggest that all of the components, except for m-cymene, can possibly be separated from limonene at temperatures from K, using supercritical CO2 fractionation. 4. CONCLUSION AND RECOMMENDATIONS The aim of this work was to investigate whether supercritical CO2 fractionation is a technically viable method to separate limonene from the major impurities, namely m-cymene, p-cymene, indane and 1,2,3-trimethylbenzene, in the light naphtha distillation cut of tyre derived oil. In order to achieve this aim, the phase behaviour of limonene and the major impurities with supercritical CO2 was evaluated. From the phase diagrams it was determined that the phase behaviour of all the components allow for significant solute loading in the vapour-like region. Furthermore, the results suggest that all of the components, except for m-cymene, can possibly be separated from limonene at temperatures from 358.2K, using supercritical CO2 fractionation. Based on these findings supercritical CO2 fractionation is concluded to be a technically viable method to separate limonene from some of the major impurities in the light naphtha cut of tyre derived oil. In order to further evaluate the possibility of using supercritical CO2 fractionation to separate limonene from the major impurities in the light naphtha distillation cut of tyre derived oil, it is recommended that pilot plant experiments be conducted. The experiments will determine the optimal operating conditions for the supercritical fractionation process and evaluate the degree of separation attainable. 6

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