5 Things You Need to Consider When NAPL Governs the Mass Present at Your Thermal Site

August 15, 2024

Thermal remediation technologies have been used for more than two decades to address highly contaminated source zones. In the beginning, thermal was typically utilized to address volatile organic compound (VOC) contaminations, such as contaminant mixtures dominated by chlorinated solvents and the lighter end of the hydrocarbons such as BTEX mixtures. In these applications, the objective of heating the targeted source zone was enhancing volatilization and co-boiling of the VOCs to efficiently remove mass and achieve remedial goals. Now days, thermal treatment is also frequently used to address sites contaminated with tar, creosote and viscous petroleum hydrocarbons.  These contaminants are far less volatile, and the objective of heating is mass removal by enhancing free-product or Non-Aqueous Phase Liquid (NAPL) recovery. In this blog post, we’ll cover five things you need to consider during the project planning and design stages, when NAPL governs the mass present at your site.

If you’d like a deeper dive head over to our webinar library to learn about the, Thermal Remediation of High Mass Hydrocarbon Sites: When NAPL Capture Governs the Mass Recovery.

 

When Volatilization is the Main Removal Mechanism

For VOCs, the governing removal mechanism is volatilization and mass removal in the vapor phase. Increasing the subsurface temperature from ambient to the boiling point of water effectively drives the majority of the VOCs to the vapor phase (>95%), where the mass can be easily captured by the vapor extraction and treatment system. A way of understanding if a chemical is susceptible for vaporization around the boiling point of water is to focus on the vapor pressure increase in the temperature range considered. For example, the vapor pressures of TCE and PCE increase by factors of 25 and 35 times, respectively between ambient and boiling.  Benzene’s vapor pressure also increases ~25 times over the same temperature range.

When Physical Removal is the Main Removal Mechanism

In recent years, the focus for thermal projects has broadened to include higher boiling point contaminants like tar, creosote, and petroleum hydrocarbons. When  these chemicals are heated to a treatment temperature around the boiling point of water, volatilization will not be the governing removal mechanism. For creosote, coal tar, PCBs and general oil-based source zones, the reduction in liquid viscosity with temperature can be several orders of magnitudes, and therefore NAPL mobility and direct removal play a major role in design and implementation of thermal remedies for sites where these contaminants are the primary targets. For example, a treatability study we recently conducted on creosote showed a decrease in viscosity of  more than 1,200 times between 24 °C and 90 °C (75 °F and 194 °F). That’s an amazing reduction of more than three orders of magnitude!   

For these sites, physical mass removal in the NAPL phase is the governing mass removal mechanism, and volatilization is secondary. Thus, it is crucial for a successful thermal design for  source zones with large amounts of non-volatile NAPL to include provisions for proper NAPL removal and management – to both capture the chemical mass in the subsurface, and to properly handle the mobilized mass in the treatment system when brought to the surface. Recently we have seen sites where more than 80-98% of the mass came out in the NAPL phase.

What to Consider During Planning and Design

Understanding the phase behavior of the NAPL and contaminants heavily impacts the thermal approach, and there are five major things you need to consider when planning your project strategy.

1. Vapor treatment concept
   If only a small fraction of the mass comes out in the vapor phase, a granular activated charcoal (GAC) based or steam regen GAC based system may be a better choice than a more robust thermal oxidation system.
2. Extraction strategy
   When NAPL removal governs the mass removal, a well thought out multi-phase extraction strategy is required. Large viscosity changes with temperatures affect the design, types and selection of pumps as well as the liquid and piping manifold system.
3. Phase separation
   Proper phase separation in the treatment system is required, to separate both LNAPL and DNAPL phases from the overall aqueous liquid stream.
4. Bio and emulsion management
   Depending on the source of the NAPL, generation of substantial bio-mats and emulsions at various points in the treatment system may heavily affect the ability to properly handle the extracted NAPL.
5. Storage and disposal
   Substantial amounts of NAPL may be accumulated in the treatment system, and a proper storage system needs to be put in place. For example, at previous sites, we had to manage more than a million pounds of generated NAPL. Additionally, a disposal strategy needs to be put in place, since some NAPL may contain chemicals like PCBs, PAHs and PFAS, that may require disposal of the NAPL as a hazardous waste.

There are several key points to consider at high mass sites with non-volatile NAPL to ensure proper design of the heating, extraction, and treatment components of an effective thermal remedy. If you’d like to learn more about how to identify solutions for proper removal, please watch our webinar, Thermal Remediation of High Mass Hydrocarbon Sites: When NAPL Capture Governs the Mass Recovery, or contact us.