(184d) Optimal Design of Reactors for Pyrolytic Remediation of Petroleum-Contaminated Soils

While marine oil spills from offshore platforms or tankers attract most of the public attention, the vast majority of oil spills (about 98%) occur on land. Every year about 10-25 million gallons of petroleum products are spilled mostly from pipelines and fixed facilities. Petroleum hydrocarbons pose long term threats to groundwater quality, inhibit the germination and root elongation rates of plants and are extremely toxic to important soil microbes. Moreover, polycyclic aromatic hydrocarbons (PAH) present in petroleum can damage the immune systems of aquatic organisms and wildlife.

Current remediation methods for petroleum-contaminated soils are either relatively slow or have unintended consequences in the form of soil damage and high-energy usage. Even worse, some processes such as aerobic bioremediation can activate toxic hydrocarbons and transform them to more noxious byproducts such as PAH derivatives. For all these reasons, there is still a pressing need for more efficient and sustainable remediation of petroleum-contaminated soils.

Over the past several years and in collaboration with Chevron ETC, we have developed a novel pyrolytic method for ex-situ remediation of soils contaminated with heavy petroleum hydrocarbons. This approach can rapidly and reliably remove total petroleum hydrocarbons (TPH) with lower energy requirements and better post-treatment soil fertility than other ex-situ thermal remediation approaches [1-3]. Pyrolytic treatment of contaminated soils at 420^oC with only 15-min residence time in a continuous reactor reduced TPH by 99.9%, lowered the total PAH concentration by almost 95%, and restored soil fertility to 98% of the clean soil level [3]. These results clearly suggest that pyrolysis has the potential for improved ecosystem restoration following remediation compared to traditional thermal technologies. However, the same study also revealed potential tradeoffs between pyrolytic treatment intensity, soil detoxification efficacy and soil fertility restoration. Specifically, treatment at 470^oC for 15- or 30-min reduced soil fertility to 51% and 39% of the clean soil level, which was only marginally higher that the fertility of the contaminated soil.

The existence of an optimal treatment intensity for some contaminated soils underscores the need for a deeper understanding of the pyrolysis kinetics and the development of models that will allow us to design reactors that can:

• Detoxify contaminated soils by removing the petroleum hydrocarbons;
• Restore the fertility of the treated soils; and
• Minimize the energy requirements.

Reactor Design and the Effect of Operating Conditions

Using the kinetic model for the pyrolysis of petroleum-contaminated soils we have recently developed [4], we simulated the operation of non-isothermal rotary kiln reactors that will be used for ex situ treatment of petroleum-contaminated soils. To preserve anoxic conditions, the reactors were heated externally while a stream of nitrogen gas flowed through the rotary kiln to sweep the desorbing hydrocarbons and pyrolysis products. A finite stage or mixing cell model with separate solid and gas phases was used to describe the operation of the pyrolysis reactor.

Model predictions were compared to experimental data we obtained with a continuous rotary kiln reactor [3]. The TPH reductions predicted by the model agreed very well with the experimental data for all the conditions considered in our earlier study: pyrolysis temperatures, 370, 420 and 470^oC; residence times 15, 30 and 60 min; solid flow rates 7, 14, and 25 lb/h.

More importantly, however, our model was able to explain the trade-offs observed in our earlier study between pyrolysis treatment intensity, soil detoxification and soil fertility restoration [3]. The clay components of the soil used in [3] undergo significant dehydration between 350 and 450^oC, a range that overlaps with that of pyrolysis reactions. As contaminated soil moves through the reactor zone kept at 420^oC, the reaction rate for the heavy hydrocarbon fraction is fast enough to achieve almost complete conversion within 15 or 30 min. But, the same conditions are not severe enough to completely remove the water from the clay components of the soil. If the soil is treated at 470^oC, on the other hand, the clay components are almost completely dehydrated, causing irreversible soil damage and loss of fertility.

Using the results of our pilot-scale studies, we will then extend our theoretical models to full-scale kiln reactors that can be used for ex-situ pyrolytic treatment of oil spills. Our focus here will be on providing guidelines for estimating the optimal operating conditions (pyrolysis temperature and residence time) for some of the petroleum-contaminated soils we have studied.

Conclusions

By emphasizing the importance of soil composition and by raising the possibility of trade-offs between detoxification and fertility of treated soils, this study has significantly advanced our fundamental understanding of the multiple and concurrent physicochemical processes taking place during the pyrolytic treatment of petroleum-contaminated soils.

Our kinetic and reactor models can become an invaluable engineering tool for designing pyrolysis reactors optimized for specific petroleum-soil systems, enabling robust operation and facilitating acceptance of this novel technology by both regulatory agencies and stakeholders.

References

1. Vidonish, J. E.; Zygourakis, K.; Masiello, C. A.; Gao, X.; Mathieu, J.; Alvarez, P. J.,Environmental Science & Technology 2016, 50, (5), 2498-2506.
2. Vidonish, J. E.; Alvarez, P. J. J.; Zygourakis, K.,Industrial & Engineering Chemistry Research 2018, 57, (10), 3489-3500.
3. Song, W.; Vidonish, J. E.; Kamath, R.; Yu, P. F.; Chu, C.; Moorthy, B.; Gao, B. Y.; Zygourakis, K.; Alvarez, P. J. J.,Environmental Science & Technology 2019, 53, (4), 2045-2053.
4. Gao, Y., Zygourakis, K., Industrial & Engineering Chemistry Research, 2019, 58, 10829-10843.