Scale-out and stability of microreactor stacks for syngas production
Date
2010
Authors
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Publisher
University of Delaware
Abstract
Computational fluid dynamics (CFD) simulations are used to simulate stacks of different sizes in order to understand nonlinear effects which arise in scale-out of microchemical systems. As an example process, syngas production from methane is studied using a multifunctional, parallel plate reactor with alternating combustion and steam reforming channels. A scale-out strategy is proposed which creates larger stacks from a base-unit. Stacks are evaluated in terms of efficiency, maximum wall temperature and stability under external heat loss for both high and moderate wall thermal conductivities. We find that smaller stacks are unstable under laboratory heat loss conditions. Stacks with high wall thermal conductivities are more stable than those with moderate wall conductivities under our conditions. At high heat loss coefficients, significant transverse thermal gradients exist between interior and edge channels of the stacks that result in significant loss of efficiency. A transition from all ignited to some ignited and extinguished and finally to all extinguished channels is discovered as criticality is approached in moderate size stacks. Microsystems provide one to three orders of magnitude larger volumetric and gravimetric throughputs than conventional technology irrespective of model uncertainty, and such intensification is central to portable and distributed processing. They exhibit energy efficiency that is a strong function of size and heat loss but can outperform conventional processing under many conditions. However, they result in higher cost per unit syngas volume unless system optimization is carried out. Stability improvement techniques such as different reactor sizes, reducing wall thermal conductivity, increasing the Pt catalyst loading and burning hydrogen rather than methane in the outermost channels are investigated. To simulate fuel-lean hydrogen combustion in a computationally efficient manner, a single step rate expression for hydrogen combustion on Pt is derived from a previously published microkinetic model. The most effective means for improving stability is found to be combusting hydrogen or increasing the Pt catalyst loading in the outermost combustion channel. White-tailed deer (Odocoileus virginianus) are overabundant in many areas of the country and increased deer numbers have caused damage to agronomic crops to increase. Winter wheat (Triticum aestivum) is a commonly grown crop and food source for white-tailed deer on the Delmarva Peninsula. The effect of deer browsing on corn and soybean yields has been documented, but research is lacking on the effect of deer browsing on wheat yield. In 2007-2009, I investigated the effect of browse timing and wheat type (bearded and unbearded) on wheat yield in Delaware. I placed 1680, 4.4 m2 plots in the middle of 10 m distance classes starting from the forested edge out to 60 m. In 2007, I systematically assigned 1 of 2 treatments (i.e., no protection or protected at planting) to the plots. In 2008-2009, I added a third treatment: protected prior to heading. After head emergence, I conducted weekly browse surveys on all unprotected plots to determine browse rates. I collected weekly biomass samples to estimate the amount of wheat biomass removed by deer. I harvested a 1 m2 area in the middle of each plot to determine the deer impact on yield. I also tested if browsing affects the test weight of wheat during 2008 and 2009. Browsing increased in intensity as head development progressed with most browsing occurring on the unbearded wheat. I found no interaction between wheat and treatment (P > 0.10) during the study. Unprotected plots yielded 195 kg/ha more wheat then protected plots in 2007 (P = 0.08). I found no difference between treatments for 2008 and 2009 (P = 0.38). Bearded wheat yielded 379 kg/ha higher in 2007 and 399 kg/ha higher in 2008 and 2009 than unbearded wheat (P < 0.001). I found no difference in test weight among treatments (P = 0.42) or between wheat types (P = 0.997) in 2008 and 2009. My browse surveys demonstrated avoidance of bearding but the overall browsing was not intense enough to reduce yield. My results differed from past research documenting a reduction in yield but the lower deer density (~15 deer/km2) on my study area was the likely cause of the difference.