Comparison of reverse flow and counter-current reactors in the case of selective catalytic reduction of NOx
Abstract The paper is focused on the comparison between a reverse flow reactor (RFR) and a counter-current reactor (CCR) in terms of the performances that may be achieved in these kinds of reactors when selective catalytic reduction (SCR) of NO x with ammonia is carried out. In order to investigate the performances of both reactor configurations, a mathematical model based on heat and mass balances for gas and solid phase was used. The possibility of achieving auto-thermal operation, even when a gas with a low temperature is going to be fed to the reactor was investigated. The temperature profiles, the reactants conversion, as well as the response to disturbances in the feeding flow have been used as criteria for comparison. Numerical simulations have shown that the CCR model is able to provide the same results as the RFR model in terms of temperature profiles when certain values of heat transfer coefficient, switching time, flow conditions and reactor geometry are taken into consideration. As a consequence of such similar thermal behaviour, the CCR model could be applied as a limiting case of RFR operation when the fast switching of the flow direction conditions is taken into account. Nevertheless, this analogy provides a simple basis for short-cut calculations since the steady-state profile of a CCR can be computed much easier than the periodic steady-state of a RFR.
Advanced control strategy for NOx reduction in forced unsteady-state reactors
Abstract The environment safety and climate changes reduction are two important aspects which partially can be achieved by lowering gaseous emissions emerged from chemical industry. These emissions are characterized by low pollutant concentration, high volumes of gases and low temperatures. Their treatment could emerge in economical sustainable technologies only if the reactors used are capable of high yields, high selectivity and autothermal behavior. The forced unsteady-state reactors are an excellent choice for treating this type of emissions because allow trapping of the moving heat wave inside the catalytic bed when exothermic reactions take place; the catalytic bed acts as a regenerative heat exchanger creating the premises of a sustain autothermal behavior even if the temperature of the feed is low. Nevertheless, the forced unsteady state reactors are characterized by a remarkable complex dynamic behavior, as a consequence of the complex interaction between heat and mass transport phenomena inside the reactor. This dynamic behavior gives rise to a set of spatial-temporal patterns, chaotic changes in concentration and traveling waves of heat and chemical reactivity, difficult to deal with. Therefore, an advance control strategy is needed to be applied in order to tackle with all issues that characterize reactors with improved contact between reactants, in which the thermal wave is stored and which allow improved kinetic activity of the catalyst used. The catalytic activity is the main operation concern for processes with low feed temperatures and constraints related to low emissions of unconverted reactants. Therefore, predictions of reactor pseudo or steady-state performances regarding conversion, selectivity and thermal behavior and the dynamic reactor response during exploitation are important aspects in finding the optimal control strategy. Moreover, the switching time and the reagents concentrations in the feeding zone are the main operating parameters that may be adapted or changed to fulfill the condition of zero emissions when using forced unsteady-state reactors. Nevertheless, due to unsteady state reactors characteristics, the adaptation of these parameters to process requirements reveals important control design problems. Therefore, in this paper the possibility of using an advanced control strategy, combining model predictive control (MPC) with an on-off control was studied in order to handle with problems that arise when using a forced unsteady-state catalytic reactor network (RN) in case of selective catalytic reduction of NOx with ammonia. The RN system consists of a network of three plug-flow reactors in series, operated in a closed loop configuration. The RN dynamic model was developed using MatLab/Simulink software. The results revealed that the proposed control strategy (i.e. the on-off control and MPC) can successfully handle the NOx emissions of this kind of reactors.
Household economy and gendered labor in the 17th centurya.d. on Guam
Abstract The gendered division of labor in household economies is well known in documentary accounts of many societies, although archaeological evidence for it is often elusive. Our study compares ethnohistorical accounts of household organization with archaeological patterns at a 17th-century village on the island of Guam in the Marianas archipelago to determine if these different sources of evidence provide similar insights. We investigated archaeological assemblages from two latte (megalithic) buildings to document their economic activities. Unexpected differences in their assemblages revealed that economic activities varied between the two latte buildings. They were domiciles of a single economically integrated household, but their disparate functions likely signaled a gendered division of labor. This study reveals aspects of gendered labor that documentary accounts do not fully describe. Our findings suggest that the assumption that domestic buildings were functionally redundant in traditional societies must be tested on a case-by-case basis.