1734
Ind. Eng. Chem. Res. 2006, 45, 1734-1738
Process for the Separation of Gas Products from Waste Tire Pyrolysis Ramo´ n Murillo, Asuncio´ n Aranda, Elvira Aylo´ n, Marı´a S. Calle´ n, and Ana M. Mastral* Instituto de Carboquı´mica, CSIC, M Luesma Castan 4, 50018-Zaragoza, Spain
In recent years, pyrolysis has proven to be an effective thermochemical process for the recycling of waste tires. After pyrolysis, three fractions are obtained: carbonaceous solids, liquid hydrocarbons, and the socalled noncondensable gases. In this paper, the starting point is the pyrolysis reactor outlet that provides a gas stream with a temperature of 600 °C and a pressure of 2 bar, comprising both the condensable and the noncondensable fractions. Although the pyrolysis process is technically and environmentally feasible, its economic viability will clearly depend on the possible salability or use of the three main derived products. Focusing on liquid hydrocarbons, usually the most abundant and valuable fraction, they are a mixture of aliphatic, aromatic, and polar compounds with a boiling point ranging between approximately 70 and 400 °C. Therefore, it is difficult to find possible applications different from their direct combustion without a purification or separation step for their commercialization. In this paper, a complete process for the separation of valuable gas and liquid fractions is described by using commercial process simulation software. After a condensation step, the liquid stream is derived to a conventional crude distillation column where it is fractionated and the gas stream is conducted to an absorption column where H2S and CO2 are removed to obtain a clean gas. In addition to the complete flow sheet description, an energy balance is provided and the liquid fractions are compared to those obtained in crude distillation. Introduction The disposal of used automotive tires is an increasing economic and environmental problem for most of the developed countries. It is estimated that 2.5 million metric tons per year are generated in the European Union, 2.5 million metric tons in North America, and around 1 million metric tons in Japan.1 Unfortunately, most of the scrap tires generated are dumped in the open or in landfill sites.2 It is well-known that tires are made of rubber materials (polybutadiene, styrene-butadiene rubber, and polyisoprene or natural rubber), carbon black, and some fibrous materials.3,4 Tires have high volatile and fixed carbon contents with a heating value greater than that of coal. This makes rubber from old tires a good raw material for thermochemical processes.5 Combustion has been considered a feasible alternative for energy recovery from old tires because of the high calorific value of this residue, around 28-37 MJ/kg.6 However, environmental problems are associated with this process mainly because high concentrations of toxic organic pollutants (polycyclic aromatic hydrocarbons) have been detected in the flue gas.6 In addition, there are other thermochemical processes that could take advantage of the rubber high chemical potential. Waste tire pyrolysis has been widely studied for years. This process seems to be an alternative to combustion processes because no hazardous emissions are produced and the recovery of solid and liquid material is achieved.7-8 Mastral et al.9 studied the thermochemical recycling of rubber from old tires by pyrolysis and hydropyrolysis using a swept fixed bed reactor. They analyzed the effect of the main process variables (temperature, heating rate, gas flow, reaction time, and hydrogen pressure) on oils, gases, and solid residue produced. They found that, while the main variable affecting tire conversion is temperature, oil composition is influenced mainly by hydrogen pressure, with the oils becoming lighter as the pressure is raised. They performed simulated distillations in a capillary column, * To whom correspondence should be addressed. Phone: 34 976 733977. Fax: 34 976 733318. E-mail:
[email protected].
finding that tire liquids are a mixture of gasoline, kerosene, gas oil, heavy gas oil, vacuum gas oil, and vacuum residue. Although the composition depended on process variables, they always found a complex mixture of products with a wide boiling point range. De Marco et al.1 did not find significant influence of temperature on the amount and characteristics of pyrolysis products above 500 °C and proved that tire pyrolysis liquids are a complex mixture of C5-C20 organic compounds, with a great proportion of aromatics. Benallal et al.10 performed the thermal decomposition of used tires under vacuum in a process development unit. They found that, at 510 °C and a total pressure of 2-20 kPa, the process yielded 50 wt % oil, 25 wt % carbon black, 9 wt % steel, 5 wt % fibers, and 11 wt % gas. Distillation of the pyrolytic oil yielded ∼20 wt % light naphtha (initial boiling point 160 °C), 6.8 wt % heavy naphtha (160-204 °C), 30.7 wt % middle distillate (204-350 °C), and 42.5 wt % bottom residue (>350 °C). The analysis of the naphtha fraction showed that this fraction has a higher octane number than petroleum naphtha but must be hydrofined and reformed. Laresgoiti et al.11 performed waste tire decomposition in a batch reactor at different temperatures. Distillation data of the 500 °C oils showed that ≈20 wt % have the boiling range of light naphtha (
