OCR Text |
Show by passing through a series of concentric tubes. The preheated gas exits through the top of the exchanger, entering the bottom of the soil-bed assembly, which is 3 inches in diameter. A "sandwich" of solid, a 0.5 inch thick solid bed supponed between two wiremesh screens, is placed in the bottom of this assembly. The gas passes up through the solid bed and exhausts out the top to a Hewlett Packard 5840A gas chromatograph, equipped with a flame ionization detector (GC/FID). Gas samples are taken semicontinuously, every 2-3 minutes, using a 6-pon sampling valve. A thermocouple is placed in the center of the solid bed for temperature measurement. Prior to an experiment, the heat exchanger assembly is preheated approximately 20°C above the desired operating temperature. A solid "sandwich" is then loaded, the soil-bed assembly is placed in the furnace, and desorbent gas flow is initiated. The preheating of the furnace and heat exchanger assembly minimizes the time required for the reactor to come to equilibrium temperature; the solid bed comes to within 10% of its operating temperature in approximately 10 minutes. Bed Characterization Reactor The bed characterization reactor is used to characterize interparticle mass and heat transfer in a dense phase environment with temperatures up to 1200°C. These studies focus on defming contaminant and solid composition effects with the specific objective of characterizing the behavior of a bed of particles. The effective thermal conductivity of the solid phase and diffusivity of the contaminant within a solid phase are detennined in a one dimensional experiment by measuring the overall contaminant evolution rate and the temperature profile for a symmetric bed of known composition, particle size, and porosity. A bed of solid is placed in the reactor and a unifonn sweep gas passes over the top of the bed; the bottom is insulated. In this system, the boundary conditions are welldefmed with the temperature and concentration gradients zero at the bonom of the bed, and the temperature and concentration known at the top of the bed (see Figure 2). Two separate 2 kg. solid beds are positioned symmetrically inside a high temperature electric furnace, shown in Figure 3. One of the beds is connected to an electronic balance which provides a record of contaminant and moisture evolution by measuring the continuous weight loss. The second bed contains a thennocouple matrix which is used to measure effective thennal conductivity. The exhaust gas is semi-continuously analyzed using the GC/FID and batch gas samples are analyzed using a gas chromatograph/ion trap detector (GCIITD) which quantifies the chemical speciation of the evolved components. Rotary Kiln Simulator The third experimental facility, a 73 kW natural gas frred rotary kiln simulator, is used to evaluate the overall incineration process where heat and mass transfer are coupled as in a full scale unit with the element of solids mixing considered. In a typical commercial kiln, a control volume of solid charge moves progressively from the entrance to the exit in a predetennined residence time, based on the rotation speed, angle, and length of the kiln, as shown in Figure 4. The rotary kiln simulator is designed to follow the fate of this control volume of solid by replacing the variable of distance with time; thus, it is possible to directly characterize the evolution of hazardous components as they "pass through" a kiln. In the simulator, a batch of solid is loaded into the kiln, and combusted for a predetennined residence time, corresponding to the time required for the solid to flow from the entrance to the exit of the commercial unit being simulated. The position of the solid with respect to the flame can be adjusted by moving the burner back away from the solid with time for co-current flow (closer to the solid with time for counter-current flow). The simulator is constructed in three refractory-lined steel sections, as shown in Figure 5, with the core of the main section being 60.96 cm. in diameter and |