CFD Simulation IDs Cause Of Thermal Oxidizer Failure
Idaho National Engineering and Environmental Laboratory (INEEL), operated jointly by Bechtel BWXT Idaho LLC and the Inland Northwest Research Alliance, is a multiprogram laboratory supporting the U.S. DOE's missions of environmental quality, energy resources, science and national security. INEEL operates recuperative flameless thermal oxidizers (RFTOs) to remediate volatile organic compounds (VOCs) contamination in the vadose zone (zone of aeration in the earth's crust above the ground-water level).
The system extracts organic contaminants from soil under vacuum using a positive displacement blower. Contaminant vapor is fed into the inlet of the oxidizer vessel under positive pressure (Fig. 1). The gaseous material enters a plenum through a side-mounted inlet pipe near the base of the vessel (Fig. 2). From the plenum, feed flow is diverted upward through 12 vertically mounted tubes packed with low void fraction ceramic balls in the bottom section and high void fraction saddles in the upper section. The flow exits at the top of the tubes, reverses direction and passes through the ceramic saddle-packed oxidation chamber surrounding the tubes before being exhausted through a side-mounted pipe opposite the inlet.
When the unit works to specification, the combustion front is maintained in the upper section of the oxidation chamber outside of the feed tubes. Heat flux from the oxidation reaction through the tube walls is imparted to the inlet gas, effecting feed preheat and heat recovery. The process oxidizes more than 99.5% of hazardous VOCs before releasing exhaust to the atmosphere. However, the unit fails catastrophically when operation goes out-of-specification. One unit failed three times, upon which the heat exchanger tubes melted (Fig. 3) and had to be rebuilt in each instance, according to INEEL advisory engineer Thomas Foust.
Because it was unclear whether the tube failures were caused by operating problems, design problems or a combination of both, INEEL performed a detailed failure analysis to determine the root cause of the failure. The oxidizer had a very narrow window of acceptable operating conditions. Propane feed was required between a lower limit necessary to sustain the oxidization reaction and an upper limit for safety reasons. This translated to operating propane feeds between 30 and 50% of the lower flammability limit (LFL) of propane in air. It was theorized that if propane levels exceeded this limit, the oxidation front, which was supposed to be localized downstream of the tubes in the outer packed bed region, could enter the tubes, rapidly causing an over-temperature condition and tube damage. If the oxidation front entered the top of the tube, melting would be observed in that area.
Operator logs did not indicate excessively high propane feed levels. In addition, tube damage occurred in the lower rather than the upper portion of the tubes, inconsistent with the expected result of excessive propane feed.
Studying the failure on a physical model was not feasible due to cost and instrumentation limitations. INEEL used computational fluid dynamics (CFD) analysis instead to simulate unit operation. A CFD simulation provides fluid velocity, temperature and species concentration values throughout the solution domain for problems having complex geometries and boundary conditions. Such analysis allows making changes in system geometry and boundary conditions (e.g., inlet velocity), to see the effect on fluid flow patterns, temperatures and concentration distributions. A CFD program from Fluent Inc. (Lebanon, N.H.) with a combustion model that handles oxidation well was used for the analysis.
Failure scenario reconstruction
A simulation of the inlet plenum in the lower part of the oxidizer vessel focused on the airflow from that location into the feed tubes to determine whether or not there was an even distribution of feed into the 12 tubes. Flow in the system was specified to exceed 16 cfm for each tube, and a flow rate below that would allow propane to remain in the tube longer. This would be a problem because it would raise the heat flux, ultimately allowing the combustion front to proceed back into the feed tubes. The manufacturer of the unit assumed even flow into each tube, but development of turbulence and secondary eddies was likely as the incoming gas flowed from the inlet pipe into the large open plenum. The analysts determined that it was not safe to assume uniform flow.
Fluent's GAMBIT preprocessor was used to model the geometry of the plenum and the openings to the heat exchanger tubes and to generate an unstructured analysis mesh. Nonuniform mesh was constructed to account for differences in the size of the components being analyzed. For example, a very fine mesh was created around small, critical areas (such as the tubes), and was progressively coarsened to regions where the components were larger (such as the plenum).
The next steps were to specify boundary conditions and solve the problem. A mass flow rate was specified at the inlet boundary where the propane/air mixture entered the oxidizer inlet pipe. Propane and air temperatures were determined from physical tests. A Reynolds stress model was used because it is better at picking up secondary flows such as eddies.
The results of the simulation showed that the airflow into the tubes was not evenly distributed. A plot of velocity vectors (Fig. 4) traced the cause to a 90-degree bend in the inlet pipe. When the gas/air mixture impinged the bend, its velocity was too high to make the turn uniformly. Some feed hit the wall at an angle, causing recirculation eddies, which resulted in uneven flow into the heat exchanger tubes. Flow into one tube was clearly below the manufacturer's recommended 16 cfm minimum flow rate.
A simulation of the upper part of the unit was designed to observe heat transfer in the tubes. A near-wall meshing technique was used for the heat exchanger tubes, which allowed more accurate resolution of heat transfer through the tube walls than a wall function approach.
Fluent's porous media model was used for simulation because the oxidation vessel is a packed bed chamber filled with noncatalytic, porous, inert ceramic material (Fig. 5). The tubes also contained this ceramic material, but in two different shapes: spheres in the bottom one-third of the tubes and saddles in the top two-thirds (Fig. 6). Based on the void fractions for the regions filled with spheres and saddles, the Ergun equation was used to compute loss coefficients for use in the porous media model. Airflow results from the previous inlet plenum simulation provided the inlet boundary conditions. Fluent's premixed combustion model and the ___ model for turbulence were used. The _-_ model was used because not much secondary flow was expected inside the tubes.
Multiple CFD analyses were run using various startup scenarios from operator logs. For example, there was a normal temperature profile in the oxidizer for a month after the first rebuild. However, the normal profile could not be achieved after a shut down for training purposes followed by an immediate system restart. These conditions were modeled to determine whether they could lead to the failure. The startup procedure required higher than normal propane feed (about 60% of LFL) for a few minutes, and due to the shutdown followed by immediate restart, the bed was in a hot soak condition. Operation logs suggested that normal operating parameters were difficult to maintain after this startup with normal propane feed rates, indicating that tube damage had occurred.
CFD analysis showed that an oxidation front was established in the low flow tubes at the transition between the spheres and the saddles indicated by the distinct change in temperature at that location (Fig. 7). This correlates with a drop off in velocity at that spot as air moved into the region containing the saddle-shaped material, which had a higher void fraction than the spherical material. Because the tubes were thin-walled, melting and tube failure occurred very quickly at the combustion front, then spread to the other tubes. With melting and failure of the wall in the first tube, feed flow short-circuited to the exhaust creating a preferential flow path developed, which decreased flow in the remaining tubes, resulting in damage to the remaining tubes.
CFD analysis was consistent with the physical evidence; that is; uneven airflow through the plenum caused the flow rate into one tube to be too low. When propane levels were above the LFL range on startup and the packing was in a hot soak condition, the oxidation front established in the tube, the tube overheated and the wall failed.
CFD analysis identified the cause of the failure of the 12-tube oxidizer and allowed INEEL engineers to propose simple design changes to prevent the problem from reoccurring. IH