(535g) Upgrading Tire-Derived Pyrolysis Oil Via Distillation for Blending with Fuel Oils

Worldwide waste tires are stockpiled resulting in environmental problems. In addition, significant calorific value is captured in these spent tires. A possible solution to the problem of spent tires is to pyrolyze the rubber fraction of spent tires to produce a crude oil commonly known as tire derived oil (TDO), which has a significant calorific value. The production of TDO therefore not only addresses the problem of spent tires but also harnesses the calorific value captured therein and can be used as a fuel source.

During the vulcanization process of the production of tires significant quantities of sulphur are introduced into the rubber to improve the durability of the tires. The sulphur propagates through to the TDO during the pyrolysis process, therefore resulting in a high sulphur content of the TDO (typically 0.82 mass %, as in the sample used here). Further, the TDO has a broad molecular mass range, typically broader than that of crude oil, therefore requiring fractionation before it can be used as a fuel source. The aim of this paper is to present results in the upgrading of TDO via distillation to investigate whether distillation is a viable process to obtain fractions that can be blended with current fuel fractions.

In this work commercial tire derived oil was upgraded via fractional distillation in a batch distillation column. Two sets of experiments were conducted, namely bench scale batch distillation tests at atmospheric pressure and pilot plant scale tests varying the pressure, reflux ratio and heating input, and the results evaluated for application as fuel fractions.

Methodology

The bench scale setup consisted of a single stage 500 mL pot setup containing 100 mL of TDO. The setup operated at atmospheric pressure and five fractions were collected namely distillates at temperatures below 140^oC (naphtha), between 140^oC and 200^oC (Diesel 1), between 200^oC and 245^oC (Diesel 2) and between 245^oC and 300^oC (Marine Bunker Oil, MBO), and the bottoms product. The products were analyzed for sulphur content and fuel properties. Five repeats of the experiment were conducted.

The column consisted of a 50 L pot loaded with 15 kg of TDO, a 1.7 m high packed bed (68 mm diameter, 1/4” Rashig rings random packing) and reflux was controlled via a solenoid valve between 5 and 20. The column was operated up to 300 ^oC and at pressures from atmospheric down to 30 kPa absolute. Pressure was controlled using a 1.5 kW vacuum pump. Experiments were conducted varying pressure, reflux ratio and heating rate (1.24 to 2.48 kW). A central composite design was used to obtain the optimum conditions resulting in 16 experimental runs. Distillate fractions were obtained at temperatures below 140°C (Naphtha), between 140°C and 245°C (Diesel), and between 245°C and 300°C (MBO) resulting in four final products (3 distillate fractions and remaining bottoms). These fractions were analyzed for sulphur content and fuel properties to evaluate their suitability to be blended as fuel fractions.

Results and Discussion

Mass balance results showing the various fractions obtained for the repeats of the bench scale study are shown in Table 1. The results show that significant quantities of each of the fractions can be obtained with the MBO fraction being the larges.

Analysis of the various fractions show that the sulphur content increased with increased boiling point ranges with that of the naphtha fraction being 0.56 wt.% increasing to 0.77 wt.% and 0.72 wt.% in the two diesel fractions and 0.75 wt.% in the MBO. Except for the MBO, these values are well above specification. Further, legislation for MBO specifications is changing and the sulphur content will be in excess of specification.

The other fuel properties (viscosity, density, water content and calorific value) of all fractions are either within specification or close to. Where these properties are not within specification it is believed that finetuning of the process will result in on-spec products.

The results for the masses obtained for each of the fractions for the pilot plant experiments are shown in Table 2. The mass balances in Table 2 typically account for between 92.6 and 96.7 % of the mass. Differences in the mas balance are typical for pilot plant scale studies and can be accounted for through accuracy of measurements, losses through the vacuum systems and column hold-up, amongst others.

Analysis of the naphtha fractions showed that the sulphur content was lower than for the batch distillation runs (0.24 – 0.41 wt.%) but still not within specification. Also, the fuel properties were again either close to or within specification with fine-tuning expected to yield on spec properties. Similar results were obtained for the Diesel fraction with sulphur content between 0.41 and 0.61 wt.% and physical properties close to or within specification.

For the MBO fraction the sulphur content was higher than in batch processes (0.84 to 1.07 wt.%) which is also within current specifications. Further the other properties were also close to or within specification.

Considering the study conducted here the optimum conditions depend on the target product. The most naphtha was obtained at 70 kPa, a reflux ratio of 10 and a 2kW heating rate for 15 L of oil (equivalent to 1.3 approximately kg.s^-1.m^-2). The most diesel was obtained at 101 kPa, a reflux ratio of 10 and a 2kW heating rate (approximately 1.6 kg.s^-1.m^-2) and the most MBO at 101 kPa, a reflux ratio of 20 and a heating rate of 2.48 kW (approximately 1.9 kg.s^-1.m^-2). Further alternative optimums may result if the sulphur content and/or the fuel properties are targeted.

It should also be noted that the experimental results presented here were conducted at a range of pressures but the temperatures at which the cuts were obtained remained the same. Alternative optimums may be obtained if the temperatures ranges are adjusted at the various pressures to align with the vapor pressures.

Conclusions

From this study it can be concluded that TDO can be considered a viable source for fuel fractions. The crude oil can be fractionated into fractions that have similar specifications than various fuel fractions. However, due to the sulphur present due to the vulcanization process, the sulphur content is currently too high. Options for the reduction of sulphur include the pre-treatment of the tire rubber before pyrolysis and the post-pyrolysis desulphurization of the oil.

A significant quantity of further work still needs to be done before viable fractions are produced. In addition to the aforementioned desulphurization, further optimization of the distillation process is also required. Additionally, fractional condensation of the pyrolysis outlet gasses to produce a pre-fractionated TDO should be considered. All these factors need to be considered and optimized for the process as a whole.