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.
Limiting cases and approximate solutions for fixed-bed reactors with periodic flow reversal
Fixed-bed reactors with periodic flow reversal are used to carry out reactions with low exothermicity in an autothermal way. Contrary to conventional autothermal reactors, the preheating of the feed gas is accomplished by regenerative heat exchange between the gas and fixed bed. The feedback of heat and direct coupling of heat exchange and chemical reaction give rise to an unusual reactor behavior. This behavior can be understood through consideration of the limiting cases of very long and very short cycle periods. Both cases can be calculated easily with the methods presented. Especially for the case of rapid flow reversal, simple equations can be derived that allow for a good physical understanding of the process and for a preliminary reactor design. In this case, it is also possible to compute the unstable solution, which is a lower bound of the temperature profile necessary for reactor startup.
Damage initiation and propagation in composite shells subjected to impact
Abstract The paper investigates the impact-induced damage initiation and propagation for laminated composite shells under low-velocity impact. A three-dimensional eight-node non-conforming element is described to analyze interlaminar stress distribution, and progressive failure. The damage analysis is performed by using Tsai–Wu quadratic failure criterion, Tsai’s damage modes and an additional delamination formula at all Gaussian points. The damage modes considered are matrix cracking, fiber breakage and delamination. The progressive failure is expressed by reducing stiffness of the material at all failed Gaussian points. Special attention is centered on curved laminates. Impact energy threshold for the onset of damage and transient damage size are predicted for composite shells. The analyses of the flat and curved laminates are compared for discussing their different damage mechanism. In addition, the influence of the stacking sequence, the thickness and the radius of curvature on damage behavior of composite shells are studied.