Three-way catalytic converters typically consist of an encased cordierite honeycomb structure, coated with an alumina washcoat impregnated with platinum group metals (PGMS). While effective under normal operating conditions, there is typically a significant warm-up time during which the TWC is ineffective. Aerogels are nanoporous materials that have a large surface area, low density, and low thermal conductivity. The use of aerogel in place of the more dense and thermally conductive alumina washcoat might reduce the time needed to heat up the TWC and decrease overall pollutant emissions. This idea was investigated using a one-dimensional model to simulate heat transfer in a cordierite wall with three different coatings: silica aerogel washcoat, a catalytically active copper-alumina (CuAl) aerogel washcoat and a traditional alumina washcoat. Simulations were performed using a transient finite difference model in MATLAB and confirmed in Abaqus. The exhaust gas was assumed to flow over the surface at a temperature of 600 K with a heat transfer coefficient of 35 W/m2K, which is typical for a catalytic converter. Two different honeycomb structures were analyzed, 75-µm-thick and 150-µm-thick cordierite walls, to simulate 400 and 300 cells per square inch (CPSI) honeycombs. A nominal washcoat thickness of 20 µm was modelled, and the surface temperature of the washcoat in direct contact with the exhaust was analyzed over time. A number of scenarios were examined including: (a) the effect of washcoat properties; (b) the effect of the percent (0-100%) of the washcoat thickness in the total wall composition; (c) the effect of washcoat thickness (10-100 µm) for fixed cordierite thickness; and (d) the effect of a transient exhaust temperature. The results show that there is a large initial increase in surface temperature for the aerogel-based washcoats (compared to that of the alumina) which then levels off as the heat penetrates into the cordierite layer. The aerogel-based washcoat reaches light-off temperature (the temperature at which the TWC converts 50% of the pollutants) 2-3 sec faster than the alumina washcoat for the 20-µm layer. For a 100-µm layer, the aerogel washcoats reach light off 13-14 sec faster. Increasing the percentage of aerogel in the total wall composition significantly reduces the time to light-off by 29 sec (300 CPSI honeycomb) and by 16 sec (400 CPSI); however, light-off time increases with washcoat layer thickness. Therefore, using a thinner coating is better under all conditions modelled. When using a more realistic model with an exhaust temperature that increases with time, similar trends are observed. Although there are a number of challenges associated with using an aerogel-based washcoat, these results indicate that their use could reduce TWC light-off time. Finding a way to maintain the initial surface temperature rise would allow even shorter light-off times to be achieved.
“ … [8]. Allison M. Stanec, Ann M. Anderson Department of Mechanical … ”
Abstract
The unique characteristics of silica aerogels allow them to be used in a wide variety of applications. They are extremely porous and have a low density, low thermal conductivity, large surface area and are relatively translucent. They can be used in a wide variety of applications such as in window systems, acoustic devices and in insulation. Making a large monolithic aerogel requires the use of a supercritical extraction process. At Union, we use the patented Rapid Supercritical Extraction (RSCE) method which uses a hydraulic hot press and confined metal mold, and have produced a 10 cm x 11 cm x 1.5 cm aerogel in a little over 10 hours. Various studies have been completed to understand what factors lead to the formation of cracks in the silica aerogel monoliths under some processing conditions. In this work, we statistically analyzed the factors that have the largest effect on the quality of the aerogel, i.e., the amount of cracking in each sample. The goal was to minimize cracking, thus maximize the quality. An initial screening design studied the effects of the heating rate, cooling rate, maximum temperature, sealing force, amount of grease, catalyst gel time, and force release rate, and found that the heating and cooling rates were the most important factors. A low heating rate and high cooling rate were found to best prevent crack formation. From this, a second screening was set up to look more closely at the cooling rate (how fast) and the length of the dwell time used at high temperature and pressure before the actual supercritical extraction step. In the end, we developed a set of parameters to produce a crack free aerogel that included a heating rate of 2 ¬?F/min, cooling rate of 30 ¬?F/min, and a dwell time of 60 min. Using these settings, it only takes a little over 6 hours in the hot press to produce an aerogel which is a 40% reduction in production time.
“ … instruments needed to complete this thesis. Thank you, Allison Stanec and Sri Teja Mangu, for training me how to … three … Experiments, 9th Ed., John Wiley & Sons 2017. [15] Stanec, A. M.; Hajjaj, Z.; Carroll, M. K.; Anderson, A. M. … ”
Abstract
The loss of heat through windows accounts for 25% of in-home heating and cooling losses. Due to climate change, conserving energy is more important than ever. If the interspace of windows was filled with silica-based aerogel, which has low thermal conductivity (0.012- 0.020 W/mK), it would result in significant energy savings. Monolithic silica-based aerogels have been previously synthesized using tetraethyl orthosilicate (TEOS) with transparency from 700-800 nm of 45%-73%. This presentation focuses on using a design-of-experiments approach to testing the conditions needed to produce monolithic TEOS-based aerogels with sufficient transparency for window applications by a patented rapid supercritical extraction (RSCE) method. Two designs of experiments (DOE) were carried out. The variables (1) quantity of time mixed in hydrolysis step, (2) quantity of time waiting in the hydrolysis step, (3) quantity of ethanol, (4) concentration of oxalic acid, (5) concentration of ammonia catalyst, and (6) quantity of water were tested to determine their effects on translucency and cracking of the resulting aerogel. The DOE results indicated that the concentration of ammonia catalyst and the quantity of time waiting in the hydrolysis step had the largest effect on the resulting aerogel's translucency.
