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# Thermal Desorption - Types of Thermal Desorption and Processes Involved

The basic idea of thermal desorption requires heating the soil and thereby vaporizing the contaminants with a low boiling point, collecting these contaminants, and outputting the remediated soil. ”Three major principles control this phenomenon: volitilization; adsorption-desorption; and diffusion,” (Sharma et al., 2004). The heat applied by the desorber directly affects the low boiling temperatures of organic contaminants, and readily turns them into vapor, or volatilizes them. Adsorption is a property of the soil; it is the ability of the grains of soil to accumulate the contaminants on the surface of the particle. “As volatilization and temperature increase, the contaminants start to lose their ‘hold’ from the soil particle surface,” (Sharma et al., 2004). Less energy is needed to adsorb organic contaminants than it is to remove them (which is known as desorption) so adding energy to the system is required to remove contaminants from soil particle surfaces. The rate of desorption is also a function of diffusion, based off the characteristics of the contaminant. The overall thermal desorption process is usually carried out in three general steps. These include materials handling , post treatment, and desorption (Sharma et al., 2004).

The success of the remediation process largely depends on four factors: temperature, soil matrix, contaminant, and moisture content. Higher temperatures have been shown to greatly reduce the final concentrations of contaminants, but require more energy and result in higher cost. The soil matrix controls the complexity of the desorption process: sands and coarse grained materials desorb more easily than fine grained soils because there is less surface area. Some contaminants have the capacity to be adsorbed by soils more easily than others. The moisture content dictates the ability of all contaminants’ adsorption; moister materials adsorb contaminants better (Sharma et al., 2004). Many of these processes vary from site to site depending on the unique characteristics of the project. The general schematic for an ex-situ thermal desorption unit is shown in Figure 2.

Sorption occurs as a result of enthalpy related and entropy related forces. Entropy driven forces (such as hydrophobic bonding) are often much weaker than enthalpy forces (including polar chemicals). Usually sorption decreases with increasing temperature before desorption can occur. Temperatures needed for desorption are usually about 300 ⁰F or greater (Delle Site, 1999).

Figure 2: Generalized Schematic Diagram for ex-situ thermal desorption (NFESC, 1998a)

##### Materials Handling

It is the responsibility of this process to prepare the contaminated materials for treatment. This includes excavating (for an ex-situ thermal desorption method), drying, and screening the material before its input to the system.

The material is not always contaminated in a homogeneous manner so there are often different requirements for different contaminant concentrations. To run a more efficient clean-up program, the soils with the same level of contamination are usually stockpiled together, which requires a knowledge of where the contamination areas are located and is usually carried out by mapping the soil stratum. An example of this process is examined in the Freeman’s Bridge site case study (Floess et al, 2011).

Excess water content in the material can often increase costs, damage equipment, or decrease efficiency, and as a result there is sometimes a drying treatment that takes place prior to desorption treatment. This can be done with the aid of lime or sand addition, which is often mechanically mixed into the soil. While sand is most commonly used, it is not always the best option since additional energy must be used to account for increased mass being processed through the system. Lime, however does not add to the bulk density of the material and is often a favored addition. Yet, reactions can occur with sulfur and chlorine in contaminants to form salts at incineration temperatures, but these are minimized at desorption temperatures (Smith, 2001).

Screening is often important if there are large particles present in the material. To adequately desorb the contaminants, it is important to break down the material and expose the finer grains. When clay is present, a crusher can be used to break down large chunks into pieces of a more manageable size. It can also be used to remove debris, which is often present in dump cleanups (Floess et al, 2011).

##### Post Treatment Processes

This process is necessary to prepare the vapors produced from the desorption unit for disposal, including small particulate matter, as well as treating any remaining contaminants. Several solutions are common in many thermal desorption units, including bag houses, cyclones, afterburners, venturi scrubbers, wet scrubbers, and carbon adsorption units. An example of a thermal desorption system with several post treatment processes can be viewed in Figure 3.

Figure 3: Example of several post treatment systems (RASCO, 1993).

###### Bag Houses

Bag houses utilize a system of permeable bags, which collect small particulate matter. Gases entering a bag house must be cooled with a separate gas cooling system. Usually filtered particles are on the order of magnitude of 10 micrometers or less (Sharma et al., 2004). If the particulate matter is contaminated, the permeable bags require further decontamination or special disposal (RASCO, 1993).

###### Cyclones

These filters are also designed to remove entrained particles; however these are specialized to remove larger particles than bag houses. Through inertial separation, cyclones can usually extract particles in the range of 15 micrometers or larger (Sharma et al., 2004).

###### Afterburners

Afterburners are used as another decontamination treatment. Although they are not always necessary depending on site and design regulations, without them, gas is usually required to be treated offsite, which can add additional costs. Afterburners use high heat to destroy the residual organic compounds, and as a result, exit gases have very high temperatures (Sharma et al., 2004).

###### Venturi Scrubbers

These filters are used to extract sulfur dioxide and hydrogen chloride, but are also used to remove particles greater than or equal to 5 micrometers. One disadvantage of venture scrubbers is that water is required to remove the particles (Sharma et al., 2004).

###### Wet Scrubbers

Wet scrubbers utilize an alkali reagent to neutralize acids left over in the exit gases. This is done to prevent corrosion on steel throughout parts of the desorption system (Sharma et al., 2004).

These can replace afterburners by reducing the amount of gas that requires offsite treatment. Usually, these use activated carbon, which has an extremely high surface area to accumulate contaminants.

###### Testing

Post treatment testing is also carried out to ensure quality control and consistent remediation. This is also done to track the efficiency of the method used as well as the effectiveness (Sharma et al., 2004).

##### Desorption Processes

There are a variety of methods for desorption processes. For ex-situ processes, these include batch or continuous feed reactors, co-current or countercurrent systems, direct or indirect fired heaters, and low or high temperature reactors. Thermal desorption usually refers to ex-situ processes but several in-situ processes exist. For each of these types, there are a several design parameters that go into designing the process and equipment.

###### Batch and Continuous

A batch-feed reactor works by accepting and heating a discrete amount of material. This allows the residence time to be exactly the amount of time that the material is placed in the reactor. It also allows for the residence time to be varied easily, as each batch load can be different. The batch reactor can also utilize a vacuum system since it is sealed in the reactor. This is useful because the gas stream exiting the reactor is made up of only the contaminants, including some particulate matter, the exiting transfer gas (usually an inert gas to prevent the contaminants from oxidizing), and water. This significantly reduces the volume of gases, which needs to be condensed. In addition, a vacuum environment reduces the temperature for desorption, which reduces the energy to heat the reactor as well to cool the gases (Smith et al., 2003). Batch feed systems are further divided into mixing or static system. Mixing within a drier or during the drying process can increase the amount of heat the soil is exposed to, thereby increasing the efficiency of the desorption (USEPA, 1994). Mixing systems often include a rotating drum that mixes the soil as heating occurs. Static systems are often rectangular structures that can be sealed as heating occurs.

Continuous-feed reactors can take advantage of large processes of material, and operate continuously until maintenance is needed. These systems usually utilize either a rotating drum or conveyor to transport soil through the desorber. An advantage of a continuous-feed reactor is that direct or indirect heating can be applied. This offers more variability so the system can be tuned to remediate a specific case of contamination or soil (NFESC, 1998a). Much like batch-feed systems, continuous-feed systems can be either mixing or static. Mixing is achieved using the rotating drum while static systems use a conveyor.

Continuous-feed systems can adjust the residence time for each case. This is done by adjusting the slope, S, and the rotation rate, rpm:

$$t=\frac{0.19\cdot L_{T}}{\left (rpm\right )\cdot D\cdot S}$$ (Sharma et al., 2004)

Where t is the residence time, LT is the length of the reactor, and D is the diameter. The residence time can also be determined based on the relationship of the initial and final concentrations of the contaminant and the desorption rate:

$$\frac{C_{sf}}{C_{si}}=e^{k\cdot t}$$ (Sharma et al., 2004)

Where the Csf and Csi are the desired (final) and initial concentrations of the contaminant, respectively, k is the desorption rate and t is the residence time. In many cases, however, residence time is most accurately determined through tests of the equipment using contaminated soil samples (Sharma et al., 2004).

The typical residence times are highly variable depending on the type of thermal desorption system, and are most accurately determined on site.  Typical values for the solids residence times can vary from 3 to 70 minutes.  Rotary dryer systems tend to have residence times closer to 3 to 7 minutes while thermal screws have much higher residence times (3- to 70 min) (Environment Canada, 2002). Dimensions of these systems can range from 2 to 4 feet in diameter, and up to 20 feet in length. Desorption rates depend on the dimensions of the thermal desorption unit, residence time, contaminants, and soil characteristics.

###### Co-current and Countercurrent

This refers to the flow direction of the combustion gases. If the flow of gases follows the direction of the material going through the desorber, then it is considered to be a co-current desorption system. If the gases and soil flow in opposite directions, then it is considered a countercurrent desorption system. Co-current desorption systems produce combustion gases that are at much higher temperatures than countercurrent desorption systems, usually about 10⁰ C to 38⁰ C higher. Soils also exit the desorber at much higher temperatures. Because of this high exit gas temperature, exit gases typically enter an afterburner before being cooled for a filtering through a bag house. Countercurrent desorption systems usually pass the combustion gases through a bag house before the afterburner. The main advantage to using a countercurrent flow system is the small amount of contaminated particulate matter present in the gas stream. This results in less downstream cleaning equipment (RASCO, 1993). An example of a countercurrent system can be seen in Figure 4, below.

Figure 4: An example of a countercurrent design (Mechati et al., 2004)

###### Direct and Indirect

Thermal desorption units can be further classified as either direct-contact or indirect-contact. Direct-contact desorbers are designed to allow direct contact between an open flame and the soil. These direct-contact systems are usually used for continuous-feed systems that use an inclined rotating drum. Soil is fed into one end of the drum, comes into contact with an open flame, and exits the drum on the downstream end.

Unlike direct-contact systems, indirect-contact systems do not allow contact between the flame and the soil. Indirect-contact systems can be implemented for either batch-feed or continuous-feed systems, but often use heated steam to enhance heat transfer (Midwest Soil Remediation, 2013a).

###### Low and High Temperature

The temperature plays one of the most crucial roles in the thermal desorption process. While the pressure of the reactor is also important, only a temperature increase can volatize some of the more potent pollutants (Mechati et al., 2004). Low temperature thermal desorption uses temperatures ranging from 93⁰ C to 316⁰ C, while high temperature thermal desorption ranges from 316⁰ C to 538⁰ C. Low temperature desorption preserves organic components of the soil and its physical characteristics. Thus, the soil can be reused for biological purposes. High temperature desorption can reduce potent contaminants to less than 5 ppm, even though many of the natural soil properties may be altered (Sharma et al., 2004).

###### In-Situ

Thermal desorption can also be classified as in-situ or ex-situ depending on the location of the treatment. All of the above classifications are used for ex-situ desorption and require excavation of the soil. In-situ solutions do not require excavation as treatment occurs in the ground.

In-situ thermal desorption solutions are much like soil vapor extraction solutions in that they heat the contaminated earth in-situ, and trap the volatilized contaminants for treatment. In-situ heating can be achieved by using a thermal blanket, or thermal wells. Thermal blankets are usually about 8 feet by 20 feet and are overlapped to cover the entire area (Sharma et al., 2004). Thermal blankets offer a short set up time, but are limited to reaching roughly 3 feet below ground surface. Thermal wells involve lowering a heater and a vacuum into a well to a depth below the surface to effectively treat a large contaminated volume. The vacuum is used to collect the vapor produced during the desorption process (Sharma et al., 2004).

Two methods are used to heat the soil in an in-situ case. These include powerline frequency heating (PLH), and radio-frequency heating (RFH). The PLH method involves using resistive heat generated by an alternating current, while the RFH method uses high energy radio waves to transfer heat through the soil strata. Because PLH works by using the soil moisture as the conductive path for the energy, it only works to about 100⁰ C, when the water vaporizes. At this point, the soil resistance becomes too great for the method to be effective. RFH works based on the dielectric properties of the soil and the frequency of the radio waves. If the frequency of the heater matches the impedance of the soil, the soil can reach temperatures of 250⁰ C or greater. However, lower frequencies can reach greater depths (Sharma et al., 2004).

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