Waste tyre pyrolysis in a conical spouted bed reactor under vacuum conditions

Transcription

1 Waste tyre pyrolysis in a conical spouted bed reactor under vacuum conditions Gartzen Lopez, Maider Amutio, Gorka Elordi, Maite Artetxe, Aitziber Erkiaga, Astrid Barona, and Martin Olazar Abstract Pyrolysis of scrap tires has been studied at 425 and 5 ºC and three pressure levels, atmospheric one.5 and.25 atm. The experimental runs have been carried out in a pilot plant conical spouted bed reactor provided with a system for continuous feeding of scrap tires. The feeding rate was 3 g/min and the mass fed in each run was 1 g. The main effect of the vacuum over the product distribution is the increase in diesel fraction, which is an interesting result given the increasing trend of fuel demand. Moreover a char of better quality is obtained when the pyrolysis is carried out under vacuum. Keywords carbon black, conical spouted bed reactor, tire pyrolysis, vacuum pyrolysis. W I. INTRODUCTION aste tires are a serious environmental problem due to their potential landfill hazard. At present, 3.1 million tonnes per year of waste tires are generated in EU, 4.4 in USA and 1.2 in Japan, and according to estimations, these figures will increase in future decades [1]. Amongst the routes for the large-scale valorisation of waste tires, encouraging results have been obtained in the studies concerning their use as crumb in cement-based materials and road asphalts and as powder in the preparation of thermoplastic elastomers [2]. Waste tire has high volatile and fixed carbon contents with heating values greater than that of coal and biomass and, consequently, it is an ideal raw material for thermochemical processes (combustion, pyrolysis and gasification) [3]. The interest of the tire pyrolysis process lies in the fact that the products obtained by this process may be easily handled, stored and transported and then valorised separately according to different objectives. The non-condensable gas is made up of light olefins) together with H 2, CO, CO 2 and H 2 S, and can be used to provide the energy requirements of the process, contributing to the design of a cost-effective and thermally integrated process. Tire pyrolysis oil (TPO) can be used as a source of refined chemicals (such as benzene, toluene, xylenes, isoprene and limonene) [4], and as a substitute for diesel fuel it helps to reduce the demand for natural resources. Tire-derived residual carbon blacks have two different applications: on the one hand, for reuse as carbon black, although a reduction in the sulphur content (approximately 5 wt% of the S in the tire) is needed for that purpose and, on the other hand, active carbons can be obtained from residual carbon black by carrying out an activation process using steam or carbon dioxide as activation agents. The active carbons derived from waste tire are of high quality, mainly mesoporous and have been successfully applied to the adsorption of different pollutants [5,6]. Different technologies have been applied in the tire pyrolysis process under atmospheric pressure, such as fixed bed reactors [7], rotatory kilns [8] moving beds [9] and fluidised beds [1]. The conical spouted bed reactor (CSBR) is an alternative technology to conventional (bubbling) fluid beds due to its excellent performance in handling sticky and irregular materials, such as scrap tires. The CSBR is characterized by its versatility for operation with high gas velocities and, consequently, a vigorous gas-solid contact is generated, which enhances heat and mass transfer between phases, increases the heating rate of the solid, allows attaining an isothermal bed, and avoids bed defluidization by agglomeration of particles, even under severe conditions involving very sticky particles, as happens in the pyrolysis of waste plastics [11,12]. A conical spouted bed reactor (CSBR) has been used in this study for tire pyrolysis under vacuum and with continuous feed, in order to compare product yields and compositions with those obtained in a previous paper under ambient pressure [13,14]. The main objective, which is a key factor for process viability, is to reduce the mass flow rate of inert gas. Consequently, the condensation section is simpler and less energy is required to cool the outlet stream. II. EXPERIMENTAL Based on the hydrodynamics studies carried in a cold unit, on the experience acquired in the pyrolyis of other types of wastes, such agroforest residues [15], and plastic wastes [11,12] and on the literature information, a continuous pyrolysis unit for tire processing has been set up and finetuned, Figure 1. The unit consists of the following components: 1) Solid feeding device. 2) Gas feeding device. 3) Gaseous stream ISSN: ISBN:

2 preheater. 4) Pyrolysis reactor. 5) A cyclone and a filter for retaining the fine particles from the stream of volatile products. 6) A condenser and a coalescence filter for liquid collection. 7) A vacuum pump. 8) A system for gaseous product analysis. Fig. 1 scheme of the pilot plant. The feeding system is pneumatically actuated and is able to feed up to 3 g/h of tire. The nitrogen flow is controlled by a mass flow controller that allows for feeding up to 3 L min-1. Prior to entering the reactor, it is heated to the reaction temperature by means of a preheater. The reactor is the main element of the unit and is a spouted bed of conical geometry with a cylindrical upper section. The total height of the reactor is 34 cm, the height of the conical section 2.5 cm, and the angle of the conical section 28º. The diameter of the cylindrical section is 12.3 cm, the bottom diameter is 2 cm and the gas inlet diameter 1 cm. The reactor may operate from the regime of spouted bed to a vigorous regime of jet spouted bed (or dilute spouted bed) The volatile products leave the reactor together with the inert gas and the finest carbon black particles. These particles are retained in a high efficiency cyclone followed by a 25 µm sintered steel filter. Both cyclone and filter are placed inside a forced convection oven at 28 ºC to avoid the condensation of heavy hydrocarbons in those elements. The gases leaving this filter circulate through a volatile condensation system consisting of a condenser and a coalescence filter. The condenser is a double shell glass tube cooled by tap water. The coalescence filter ensures total collection of volatile hydrocarbons. Finally, vacuum is attained in the pyrolysis plant by means of a Vacuubrand mz2d vacuum pump placed downstream the coalescence filter. The power of the vacuum pump cannot be controlled so a valve has been placed between the plant and the pump to control the pressure drop of the plant and maintain the desired vacuum inside the reactor. The tire studied is that commonly used for cars and light lorries. It is mainly constituted by natural rubber (NR, %), stirene-butadiene rubber (SBR, %), carbon black (29.59 %) and additives like zinc oxide, phenolic resin and, to a lesser degree, aromatic oil. This material has been shredded by a cryogenic method to a size lower than 1mm. The thermal behavior of this material was studied in a previous paper by thermogravimetric analysis [16]. In order to ensure stable spouting, the nitrogen flowrate has been set at 1.2 times the minimum spouting velocity, both for the vacuum and atmospheric operation. Continuous pyrolysis has been carried out by feeding 3 g/min of scrap tires. The reactor outlet stream has been analysed on-line by means of a GC Agilent 689. The line from the reactor outlet to the chromatograph is heated to a temperature of 25ºC in order to avoid condensation of heavy hydrocarbon compounds. The liquid collected has also been analysed by GC/MS Shimadzu UP-21S, in order to identify individual compounds in the reactor outlet stream. Non-condensed gases have also been identified by means of the same equipment (GC/MS Shimadzu UP-21S), given that it is provided with a port for gas injection. Once a continuous run has been finished (1 g of tire have been fed), the char collected by the lateral outlet and retained in the cyclone and filter has been weighted and the liquid has also been collected for weighing and for its subsequent analysis. III. RESULTS The pyrolysis of tire particles has been studied at 425 and 5 ºC, and the effect of pressure has also been studied by operating under both vacuum (.25 and.5 atm) and atmospheric pressure. The products have been grouped into five lumps: char (or carbon black), gas (C 1 -C 4 hydrocarbons), non-aromatic liquid fraction (non-aromatic C 5 -C 1 hydrocarbons), aromatic liquid fraction (single ring C 1 - aromatic hydrocarbons) and tar (including C 11 +, independently of their aromatic or non-aromatic nature). Figure 2 shows the evolution of the lumps obtained under the four different conditions studied, Figure 2a shows the results obtained at 425 ºC and Figure 2b the results obtained at 5 ºC. The more important effect of reducing the operating pressure is the increase in the yield of tar or C Moreover, as tar yield increases, that of the C 5 -C 1 fraction (both aromatics and non-aromatics) decreases. Vacuum operation causes a slight increase in the gas yield and this effect is more pronounced at 425 ºC. The decrease in the yield of singlering C 1 - aromatic hydrocarbons is attributed to the negative ISSN: ISBN:

3 effect of vacuum on the cyclization and aromatization reactions of pyrolysis primary products [3]. This interpretation is in line with the increase in the gas yield, which is explained by the attenuation of olefin condensation by Diels-Alder reactions to give aromatic compounds. Yield (wt %) Yield (wt %) a gases b gases non-aromatics C 5 -C 1 non-aromatics C 5 -C 1 P = 1 atm P =.5 atm P =.25 atm aromatics C 1 - C 11 + P = 1 atm P =.5 atm P =.25 atm aromatics C 1 - C 11 + char char Fig. 2 effect of pressure on the yields of the different product lumps. Graph a, 425 ºC. Graph b, 5 ºC. The effect of vacuum is explained by the fact that vacuum enhances diffusion towards the outside of the volatiles formed within the porous structure of the tire particle, which is due to the positive pressure gradient generated by vacuum for that flow. The faster diffusion of the volatiles inside the particle reduces their residence time and, consequently, limits the secondary and cracking reactions, increasing the yield of the heavier fraction. This point is reinforced in a previous paper on tire pyrolysis kinetics under vacuum [16] in which a positive effect of vacuum on the thermal degradation process of the tire material was observed and, consequently, the pyrolysis reaction begins at lower temperatures and is faster when performed under vacuum (at the same temperature). This effect of vacuum is attributed to the enhancement of the volatilization of primary products and their diffusion within the particle, reducing their residence time. Consequently, the secondary reactions of repolymerization and carbonization and char pore blockage are minimized. The concentration of volatile pyrolysis products in the reaction medium is similar in both the vacuum and atmospheric processes, and no effect is expected on the secondary reactions. The effect of vacuum reported by Zhang et al [17] involved a reduction in the residence time in the reactor (fixed bed), which prevented the secondary cracking of volatile vapours. Thus, they obtained a condensed pyrolysis oil mainly made up of compounds with a high boiling point and low naphtha content. The residence time of the volatiles in the CSBR decreases slightly when the operation is carried out under vacuum. A slight reduction in char yield is obtained operating under vacuum due to the reduction in the polymerization reactions on the surface of the char particle. This effect is related to the reduction of volatile residence time in the reactor, given that this variable has a significant effect on the degradation of tar components. The main effect of temperature is an increase in the gaseous and aromatic fractions and the resulting decrease in the yield of non-aromatic liquid fraction. The increase in the gaseous fraction with temperature is attributed to the more severe cracking at high temperatures, which gives way to the formation of gases, mainly olefinic compounds. A clear increase in gas yield with temperature has been observed by several authors using different technologies [8,18,19]. The yield of aromatics in the pyrolysis of tires is enhanced by high temperatures (due to secondary reactions) and by the presence of aromatics in the original formulation of the tire, as is our case (styrene-butadiene rubber). The yields of tar, or C 11 + fraction, and char do not change significantly in the temperature range studied. Regarding the composition of the volatile fraction, the gases are mainly made up of methane and C 2 -C 4 olefins when pyrolysis is performed under atmospheric pressure. However, a significant increase in the yield of alkanes is obtained operating under vacuum. The content of CO 2 is very low and negligible in the case of CO, H 2 and H 2 S. The major component in the gaseous phase is 1,3-butadiene and its yield increases with temperature and vacuum, with the maximum yield being 2.84 wt% at 5 ºC under vacuum (.25 atm). Although it is higher in pyrolysis under vacuum than under atmospheric pressure, the yield of the gas fraction is low and best burnt to produce energy for the pyrolysis process. The liquid fraction is a complex mixture of hydrocarbons whose individual composition is generally low, but it also contains interesting hydrocarbons in a high proportion, such ISSN: ISBN:

4 as isoprene, limonene and styrene. Vacuum operation has a positive effect on the yield of isoprene at both temperatures studied, with a maximum yield of 7.52 wt% at 5 ºC and.25 atm. The opposite occurs in the case of the other main compound, limonene, whose yield is lower under vacuum conditions. The reduction in limonene yield is approximately 6 % at both temperatures studied operating at.25 atm. The yields of isoprene and limonene are closely related, i.e. isoprene is formed by β-scission of polyisoprene in the degradation of the polymer and it undergoes dimerization in the reaction medium to yield limonene. It seems that vacuum inhibits the dimerization reaction of isoprene to yield limonene, resulting in an increase in the yield of isoprene. A similar trend as that of limonene is observed in the case of styrene, whose yield is considerably reduced operating under vacuum (around 35 % reduction at both temperatures studied when pyrolysis is carried out under.25 atm). The yield of certain aromatic compounds, such as toluene, ethylbenzene and xylenes, increases with temperature, which has also been reported by other authors. That increase in aromatic products is related to Diels-Alder reactions that promote the formation of aromatic compounds from olefins [2]. However, the yield of these compounds is not greatly affected by the operating pressure in the range studied in this paper. The tar is made up of hydrocarbons heavier than C 1, whose characterization is complex due to the low concentration of its components and to the limitations of GC/MS techniques. Vacuum operation favors the formation of most of the compounds in the C 11 + range. Most of the components in the C 11 -C 13 range are of aromatic nature, but those heavier than C 14 are mainly long-chain paraffins. Tire pyrolysis oil (TPO) has a rather high content of olefinic and aromatic compounds, which limits its direct applications as automotive fuel. Murugan et al. [21] studied the performance of the tire pyrolysis oil in a diesel engine. The results obtained were promising in terms of efficiency, emissions and lack of operational problems, but they concluded that the main challenge is to reduce the aromatic content and viscosity for its use as a fuel in diesel engines. Consequently, a mild hydrocracking treatment would be sufficient for upgrading the liquid, or it may also be fed with other feedstocks from wastes into refinery units like those for FCC, coking or thermal cracking. Furthermore, given that it has a high heating value (4 MJ kg-1) and a relatively low sulphur content, another option is the use of this liquid in industrial furnaces. Figure 3 shows the distillation curves for the pyrolysis oils obtained operating at 425 ºC (Figure 3a) and 5 ºC (Figure 3b). The distillation curves have been obtained with a HYSYS commercial simulator, using over 5 compounds heavier than C 5 (prevailing ones in the GC analysis) to define the different liquid fractions. The distillation curve of the liquids obtained operating under atmospheric pressure is characterized by a long plateau at a temperature around 175 ºC. This plateau is due to the high concentration of limonene and to other C 1 compounds in the liquid. This plateau is shorter for the liquid obtained under vacuum, which is due to the increase in the light and heavy fractions in the liquid. Thus, the fraction of the liquid that distils below 1 ºC is 9 % higher in the liquids obtained operating under vacuum at both temperatures studied, which is explained by the higher isoprene yield obtained operating under vacuum. The increase in the heavier fraction is more significant when operating under vacuum. Thus, the liquid fraction with a boiling point higher than 2 ºC (diesel fraction) increases, especially at 5 ºC and.25 atm, which accounts for 36 wt% of all the liquid, whereas this fraction accounts for only 13 wt% under atmospheric conditions. Temperature (ºC) Temperature (ºC) a.25 atm.5 atm 1 atm % Distilled b.25 atm.5 atm 1 atm % Distilled Fig. 3 comparison of the simulated distillation curves of the pyrolysis oils obtained under different pressures. Graph a, 425 ºC. Graph b, 5 ºC. ISSN: ISBN:

5 Consequently, these results show that the good performance of the atmospheric conical spouted bed for fuel production by waste tire pyrolysis is improved by operating under vacuum, which means that pressure is a significant variable for fitting the range of boiling points in the fuels to market requirements. The amount of char obtained is slightly higher than the sum of the original carbon black plus the inorganic components in the tire. This implies that there is a certain degree of coking or degraded tire deposition on the carbon black. The residual carbon black obtained has an overly high sulphur content that may limit its direct reuse. This sulphur content is around 3 wt%. Nevertheless, the quality of the char obtained operating under vacuum conditions is better than that under atmospheric pressure. At 425 ºC, BET surface area increases from 46 m 2 g -1 under atmospheric pressure to 96 m 2 g - 1 under.25 atm vacuum. The results at 5 ºC follow the same trend, an increase from 65 to 91 m 2 g -1 by reducing the pressure from 1 to.25 atm. The results in terms of BET surface area obtained by other authors operating under atmospheric conditions are below the values obtained in this paper with pyrolysis under vacuum [3,7,19,22]. The improvement in the porous structure properties of the residual carbon black obtained operating under vacuum reinforces the possibilities for carbon black recycling. The positive effect of vacuum on the porous structure of the char is related to two factors: i) vacuum facilitates the devolatilization and diffusion of the volatiles within the particle; ii) vacuum minimises the deposition of carbon material on the porous structure and so reduces the blockage of pores. IV. CONCLUSIONS The continuous vacuum pyrolysis of waste tires in a conical spouted bed reactor is interesting for increasing process viability. Thus, nitrogen mass flow rate is lower and, consequently, the condensation of the liquid product is easier and less energy is required. Vacuum has a significant effect on the distribution of products and their composition, but does not have any negative consequences of importance, which means that vacuum operation maintains the good performance of the conical spouted bed reactor for waste tire pyrolysis. The main advantages of vacuum operation over atmospheric operation are the increase in the yield of the liquid fraction corresponding to diesel fuel and the improvement in the surface area of the residual carbon black, although the high content of aromatics and sulphur in the former requires a hydroreforming treatment for commercial use. Moreover, an increase in the yield of isoprene has been obtained operating under vacuum, but limonene yields are lower. The increase in the yield of tar fraction (or C11+ fraction) may be tempered by increasing temperature. 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