Thermal desorption is an environmental remediation technology that utilizes heat to increase the volatility of contaminants such that they can be removed (separated) from the solid matrix (typically soil, sludge or filter cake). The volatilized contaminants are then either collected or thermally destroyed. A thermal desorption system therefore has two major components; the desorber itself and the offgas treatment system. Thermal desorption is not incineration.
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Temperature-Programmed Desorption
Transcription
I am going to describe a technique that us used very extensively in heterogeneous for characterizing catalyst as well as studying reactions. It is called temperature programmed desorption. What I want to do is describe it and I will solve the equation that describe the behavior we absorb experimentally. So the idea is pretty simple. We start of with some gas that is adsorbed on any surface. As we raise the temperature that gas is going to desorb into the gas phase. So what we want to do is study this desorption and what we are going to do with the temperature- programmed desportion is measure the rate of desorption as a function of time or as a function of temperature. So the rate of desorption is going to be proportional to some pre-exponential form. Some exponential term containing an activation energy, and then we will consider the case were it is proportional to the concentration. So this is first order adsorption. It does not have to be first order, but we are going to look at the simple case. This is the rate, and this is the rate that in the actual experiment we would plot the rate vs temperature as we raise the temperature.So this is a surface concentration, not a gas phase concentration, but a surface concentration, and the E is the activation energy for the desorption is in many cases just equal to, that is often referred to as the heat of adsorption, were this is a positive value. So this would be the same minus delta H of adsorption. Since its adsorption is typically exothermic. So what happens is we raise the temperature. The most common way is a linear ramp, which means the derivative of temperature respect to time is a constant. We are going to use beta to indicate this. So as we raise the temperature. We typically start at low enough temperature that there is no desorption. So down here we can wait without being concerned that the molecule is being desorbing that rate. As we raise the temperature the rate starts to increase exponentially, because of this term, and then what is going to happen is that you will go through a maximum and it is going to go through a maximum because we desorbed enough. So this term is increasing as we raise the temperature. This term is decreasing, and eventually this term dominates the rate decreases. Wee get to the point were now everything has desporbed. So this is what we would adsorb in a temperature programmed desorption experiment. The temperature increases. We often measure this heat temperature as the indication of how strongly the molecules are adsorbed on the surface, and of course that temperature depends on how fast we carry out the heat. So lets look at some just some sample numbers as a way to determine how this curve shape actually looks like. So I have picked the rate of heating of 10 degrees per second, and in an ultra-high vacuum system this might be a reasonable rate. We are going to start at 300 K. At this point 300 K. I am going to use a dimensionless surface concentration. I can certainly do it in terms of molecules per squared cm. It tends to be a large number. So lets just do it dimensionless to make it easier to see the behavior, and essentially what I am doing is solving two equations. The mass balance, change in concentration on the surface with respect to time. Is equal to the rate of desorption, which of course has this exponential dependence on temperature, and using a first order case, and the change in temperature with respect to time is equal to beta. Then we are going to solve these equations simultaneously. We will do this numerically, because of this exponential term. We want to plot the change in concentration of A with respect to time. vs the temperature. This is what we referred to temperature-programmed desportion. This would be what we typically plot. So what I am going to do now is solve these two different differential equations numerically. We used Polymath, and I will show you the program and the output. On the left is the Polymath program with a number of comments of what we are doing, but basically we are solving 2 order differential equations simultaneously, and plotting the rate of desorption. That is what we measure experimentally as the function of temperature, and what I am showing in these three plots are for 3 starting concentrations. So in this case I used values of 150, and 25. The important thing to notice is that the curves look identical expect scaled by their initial concentration. In other words if I take this curve and multiply by 2 I get this curve. If I take this curve and multiply by 4 again. I get this curve. The peak temperature does not change for this first order process. By this point of course everything has desporbed, and this again is what we would measure experimentally as a way to characterize how strongly something is adsorbed on a surface. The area under these curves is proportional to how much is adsorbed on this surface initially. So that is a brief description of temperature program desorption. Remember this is for first order. The second order behavior or 0 order will have different temperature dependence, but the idea is the same. We can certainly with the same program change to 2nd order or 0 order and adsorb what the behavior is.
History
Thermal desorption first appeared as an environmental treatment technology in 1985 when it was specified in the Record of Decision for the McKin Company Superfund site within the Royal River watershed in Maine.[1]
It is frequently referred to as "low temp" thermal desorption to differentiate it from high temperature incineration. An early direct fired thermal desorption project was the treatment of 8000 tons of toxaphene (a chlorinated pesticide) contaminated sandy soil at the S&S Flying Services site in Marianna Florida in 1990, with later projects exceeding 170,000 tons at the Cape Fear coal tar site in 1999. A status report from the United States Environmental Protection Agency shows that thermal desorption has been used at 69 Superfund sites through FY2000. In addition, hundreds of remediation projects have been completed using thermal desorption at non-Superfund sites.
For in-situ on-site treatment options, only incineration and stabilization have been used at more Superfund sites. Incineration suffers from poor public acceptance. Stabilization does not provide a permanent remedy, since the contaminants are still on site. Thermal desorption is a widely accepted technology that provides a permanent solution at an economically competitive cost.
The world’s first large-scale thermal desorption for treatment of mercury-containing wastes was erected in Wölsau, for the remediation of the Chemical Factory Marktredwitz (founded in 1788) was considered to be the oldest in Germany. Operation commenced in October 1993 including the first optimising phase. 50,000 tons of mercury-contaminated solid wastes were treated successfully between August 1993 and June 1996. 25 metric tons of mercury had been recovered from soil and rubble. Unfortunately the Marktredwitz plant is often misunderstood in the literature as a pilot-scale plant only.
Desorbers
Numerous desorber types are available today. Some of the more common types are listed below.
- Indirect fired rotary
- Direct fired rotary
- Heated screw (hot oil, molten salt, electric)
- Infrared
- Microwave
Most indirect fired rotary systems use an inclined rotating metallic cylinder to heat the feed material. The heat transfer mechanism is usually conduction through the cylinder wall. In this type of system neither the flame nor the products of combustion can contact the feed solids or the offgas. Think of it as a rotating pipe inside a furnace with both ends sticking outside of the furnace. The cylinder for full-scale transportable systems is typically five to eight feet in diameter with heated lengths ranging from twenty to fifty feet. With a carbon steel shell, the maximum solids temperature is around 1,000 °F, while temperatures of 1,800 °F with special alloy cylinders are attainable. Total residence time in this type of desorber normally ranges from 30 to 120 minutes. Treatment capacities can range from 2 to 30 tons per hour for transportable units.
Direct-fired rotary desorbers have been used extensively over the years for petroleum contaminated soils and soils contaminated with Resource Conservation and Recovery Act hazardous wastes as defined by the United States Environmental Protection Agency. A 1992 paper on treating petroleum contaminated soils estimated that between 20 and 30 contractors have 40 to 60 rotary dryer systems available. Today, it is probably closer to 6 to 10 contractors with 15 to 20 portable systems commercially available. The majority of these systems utilize a secondary combustion chamber (afterburner) or catalytic oxidizer to thermally destroy the volatilized organics. A few of these systems also have a quench and scrubber after the oxidizer which allows them to treat soils containing chlorinated organics such as solvents and pesticides. The desorbing cylinder for full-scale transportable systems is typically four to ten feet in diameter with heated lengths ranging from twenty to fifty feet. The maximum practical solids temperature for these systems is around 750 to 900 °F depending on the material of construction of the cylinder. Total residence time in this type of desorber normally ranges from 3 to 15 minutes. Treatment capacities can range from 6 to over 100 tons per hour for transportable units.
Heated screw systems are also an indirect heated system. Typically they use a jacketed trough with a double auger that intermeshes. The augers themselves frequently contain passages for the heating medium to increase the heat transfer surface area. Some systems use electric resistance heaters instead of a heat transfer media and may employ a single auger in each housing. The augers can range from 12 to 36 inches in diameter for full-scale systems, with lengths up to 20 feet. The auger/trough assemblies can be connected in parallel and/or series to increase throughput. Full scale capabilities up to 4 tons per hour have been demonstrated. This type of system has been most successful treating refinery wastes.
In the early days, there was a continuous infrared system that is no longer in common use. In theory, microwaves would be an excellent technical choice since uniform and accurately controlled heating can be achieved with no heat transfer surface fouling problems. One can only guess that capital and/or energy costs have prevented the development of a microwave thermal desorber at the commercial scale.
Offgas treatment
There are only three basic options for offgas treatment available. The volatilized contaminants in the offgas can either be discharged to atmosphere, collected or destroyed. In some cases, both a collection and destruction system are employed. In addition to managing the volatilized components, the particulate solids (dust) that exit the desorber must also be removed from the offgas.
When a collection system is used, the offgas must be cooled to condense the bulk of the volatilized components into a liquid. The offgas will exit most desorbers in the 350–900 °F range. The offgas is then typically cooled to somewhere between 120 and 40 °F to condense the bulk of the volatilized water and organic contaminants. Even at 40 °F, there may be measurable amounts of non-condensed organics. For this reason, after the condensation step, further treatment of the offgas is usually required. The cooled offgas may be treated by carbon adsorption, or thermal oxidation. Thermal oxidation can be accomplished using a catalytic oxidizer, an afterburner or by routing the offgas to the combustion heat source for the desorber. The volume of gas requiring treatment for indirect fired desorbers is a fraction of that required for a direct fired desorber. This requires smaller air pollution control trains for the gaseous process vent emissions. Some thermal desorption systems recycle the carrier gas, thereby further reducing the volume of gaseous emissions.
The condensed liquid from cooling the offgas is separated into organic and aqueous fractions. The water is either disposed of or used to cool the treated solids and prevent dusting. The condensed liquid organic is removed from the site. Depending on its composition, the liquid is either recycled as a supplemental fuel or destroyed in a fixed base incinerator. A thermal desorber removing 500 mg/kg of organic contaminants from 20,000 tons of soil will produce less than 3,000 US gallons (11,000 L) of liquid organic. In essence 20,000 tons of contaminated soil could be reduced to less than one tank truck of extracted liquid residue for off-site disposal.
Desorbers using offgas destruction systems use combustion to thermally destroy the volatilized organics components forming CO, CO2, NOx, SOx and HCl. The destruction unit may be called an afterburner, secondary combustion chamber, or thermal oxidizer. Catalytic oxidizers may also be used if the organic halide content of the contaminated media is low enough. Regardless of the name, the destruction unit is used to thermally destroy the hazardous organic constituents that were removed (volatilized) from the soil or waste.
See also
References
- ^ "Site Information McKin Company Superfund Site Gray Maine". United States Environmental Protection Agency. 1985-07-22. Retrieved 2009-07-21.
T. McGowan, T., R. Carnes and P. Hulon. Incineration of Pesticide-Contaminated Soil on a Superfund Site, paper on the S&S Flying Services Superfund Site remediation project, Marianna, FL, presented at HazMat '91 Conference, Atlanta, GA, October, 1991
This page was last edited on 20 November 2023, at 13:48