Niket S Kaisare

The aim of this work is to analyze output feedback distributed control framework based on model predictive control (MPC) for multi-unit systems with multirate and delayed measurements. Primary controlled variable(s) is sampled infrequently, and the measurement is available after a delay. The distributed, unit-specific estimators and controllers communicate through selective information transfer based on process knowledge. The distributed state estimator is designed based on sampled state augmentation method to efficiently compute improved state estimates on arrival of the delayed infrequent measurements. The multirate estimators are appropriately modified to incorporate the disturbance model to handle model-plant mismatch (MPM). The proposed implementation is analyzed using two case studies. A linearized, reactor-separator system is used to demonstrate that multirate estimation is required for offset-free tracking. A more complex variant of the reactor-separator system with recycle demonstrates the ability of nonlinear distributed estimator-MPC framework to incorporate infrequent and delayed primary measurements.

The reduction in size of catalytic microreactors results in high heat and mass transfer rates and a significant increase in the surface area to volume ratio. A further increase in the catalytic surface area can be achieved in a scaled-down version of fixed bed reactors. Since micro-fixed bed reactors are often deemed impractical due to their large pressure drops, one could use precisely structured inserts to increase the surface area, enhance mixing and manipulate the flow distribution. Catalytic propane combustion in microreactors with multi-channel and posted inserts, which consist of multiple static structures (walls separating various channels and pillar-like structures, respectively) in the flow channel of a microreactor, is considered in this series of two papers. In this first paper, we present numerical comparison of multi-channel and posted catalytic inserts for non-adiabatic self-sustained propane combustion. The inserts are oriented axially along the flow direction. We show that channel and post microreactors have similar performance for low thermal conductivity of the inserts. The in-line arrangement of the posted structures is preferred over a staggered arrangement because the former provides higher propane conversion and more stable combustion. The role of thermal conductivity of the microreactor wall structure and the catalytic inserts is investigated. The thermal conductivity of the microreactor structure affects the performance of the posts but not the channels; this is contrary to the effect of catalyst insert thermal conductivity where it is vice-versa. The channel microreactor is more stable towards high flow-rate blowout limit, whereas the post microreactor is significantly more stable at the lower flow-rate extinction limit. This results in stable operation of the post microreactor under more fuel-lean mixtures than the channel microreactor. © 2010 Elsevier Ltd.

This work deals with state estimation in the presence of delayed and infrequent measurements. While most measurements (referred to as secondary measurements) are available frequently and instantaneously, there might be a delay associated with acquiring other measurements (primary measurements) due to long analysis times involved. The primary measurements are usually sampled at irregular intervals and the exact delay is also unknown. The traditional fixed-lag smoothing algorithm, which has been applied for a variety of chemical processes systems, can be computationally inefficient for such situations and alternate methods to handle delays are necessary. In this paper, we analyze several existing methods to incorporate measurement delays and reinterpret their results under a common unified framework (for Extended Kalman Filter). Extensions to handle time-varying and uncertain delays, as well as out of sequence measurement arrival are also presented. Simulation studies on a linear distillation column and a nonlinear polymerization reactor are used to compare the performance of these methods based on RMSE values and computation times. A large scale nonlinear reactive distillation column example is also used to illustrate the practicality of the suggested method. © 2010 Elsevier Ltd. All rights reserved.

Ignition of methane-air and propane-air mixtures over platinum catalyst in a parallelplate microburner is studied numerically and a comparison of their ignition characteristics is presented. The ignition behaviour of the two fuels is compared for the case of heated feed and the strategy of using propane-methane mixed fuel is analysed. We show that adding small quantities of propane reduces the ignition temperature of lean methane-air mixture. Transient response of the mixed methane-propane fuel reveals sequential ignition of propane followed by methane. Sensitivity analysis on physical properties of methane and propane shows that the higher apparent activation energy of methane combustion accounts for most of the observed differences in their ignition behaviour. Ignition by resistive preheating, specifically the effect of locally preheating initial section of the burner is investigated. The amount of electric power required for ignition decreases with decrease in the electrical preheating length. This reduction in ignition power is especially significant for low conductivity walls, compared to highly conducting walls. Finally, the gap size of the channel has a relatively small effect on ignition in catalytic microburners. © 2010 Taylor & Francis.

The potential of methane steam, reforming at microscale is theoretically explored. To this end, a multifunctional catalytic plate microreactor, comprising of a propane combustion channel and a methane steam reforming channel, separated by a solid wall, is simulated with a pseudo 2-D (two-dimensional) reactor model. Newly developed lumped kinetic rate expressions for both processes, obtained front a posteriori reduction of detailed microkinetic models, are used. It is shown that the steam, reforming at millisecond contact times is feasible at microscale, and in agreement with a recent experimental report. Furthermore, the attainable operating regions delimited from the materials stability limit, the breakthrough limit, and the maximum, power output limit are mapped out. A simple operation strategy is presented for obtaining variable power output along the breakthrough line (a nearly iso-flow rate ratio linej, while ensuring good overlap of reaction zones, and provide guidelines for reactor sizing. Finally, it is shown that the choice of the wall material depeneis on the targeted operating regime. Low-conductivity materials increase the methane conversion and power output at the expense of higher wall temperatures and steeper temperature gradients along the wall. For operation close to the breakthrough limit, intermediate conductivity materials, such as stainless steel, offer a good compromise between methane conversion and wall temperature. Even without recuperative heat exchange, the thermal efficiency of the multifunctional device and the reformer approaches -65% and -85%, respectively. © 2008 American Institute of Chemical Engineers.

CO2 methanation is the catalytic reduction of CO2 to methane, which is accompanied by side reactions that produce CO. Being an equilibrium-limited exothermic reaction, thermal management is essential for good reactor performance. We analyze efficient thermal coupling by transferring the heat of reaction from outlet to the inlet stream in a single thermally-integrated microreactor with counter-current flow. We use CFD simulations to compare two modes of heat removal, viz. external cooling and feed dilution, over a wide range of operating conditions. In one mode of operation, undiluted feed is processed with external cooling using air in counter flow (air-cooled reactor) and in the other, diluted reactant mixtures in alternate channels exchange heat counter-currently as the reaction proceeds (counter-current reactor). Depending on the inlet temperature, a favorable temperature profile that improves conversion to methane forms in each reactor. For the base case, 67 % CO2 conversion is obtained, with CH4 selectivity of 88 % and 91 % in air-cooled and counter-current reactors, respectively. Counter-current reactor shows autothermal operation with feed at ambient temperature while air-cooled reactor gives good performance at higher inlet temperatures. It is possible to attain high selectivity to methane through appropriate choice of operating conditions.

Ignition of propane-air combustion in a Pt-coated microburner is numerically investigated. Three startup modes are compared: heating the inlet feed above its ignition temperature, resistive heating using electric power, and spatially distributed (stratified) resistive heating. Depending on wall conductivity, velocity, and inlet temperature (for preheated feed) or power supplied (for resistive heating), the fuel lights off either at the entrance (front-end ignition), towards the exit (back-end ignition) or in the middle of the reactor. The cumulative propane emissions are the highest for resistive heating of microburners. Promoting front-end ignition, via locally heating the initial section of the reactor or by feed preheating, significantly reduces the ignition time and the emissions. Lower conductivity materials show shorter ignition times and lower emissions for all start-up modes. The time to steady state depends on start-up mode and materials' conductivity. A good start-up strategy would be to ignite the microburner with a low flow rate and then increase it. © 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

The objective of this work is to elucidate controlling mechanisms in NO x reduction, develop reduced-order reaction models, and analyze the reactor performance using the reduced-order reaction model for the NO-CO reaction. We start with the microkinetic model on platinum, which describes the mechanism of catalytic reduction of NO by CO. The formation of the main product N 2O and the competitive formation of the side product N 2 are accounted for in the microkinetic model. Sensitivity and reaction path analysis have been carried out to determine the rate-limiting steps as well as the most abundant reactive intermediates in the system. Owing to the differences between system performance at high and low temperatures, the model has been analyzed in detail in these temperature regimes. Two closed-form expressions, corresponding to the two global reactions involved, have been derived. The characteristic features of the microkinetic model such as the sharp increase in NO conversion and the selectivity to N 2O are captured well by the reduced model. The reduced-order model has been extended to the rhodium catalyst as well. © 2012 Wiley Periodicals, Inc.

Thermal management of microreactors is very important for process intensification to achieve higher efficiency and for maximum heat utilization. In this work, a cross-flow spatial coupling between endothermic ammonia decomposition reaction and exothermic propane combustion in a microreactor is explored. Unlike the conventional co-current or counter-current reactors, the combustion channels in the cross-flow microreactor are aligned in transverse direction, perpendicular to the ammonia flow direction. The cross-flow coupling mode is parametrically analyzed to delineate the operating region and efficiency of the microreactor. The effect of inlet flow rates of propane and ammonia, reactor wall thermal conductivity on the performance of cross-flow microreactor are studied in detailed, and design guidelines are depicted through operating diagrams, which are quantitatively valid for the reactor dimensions and flow rates simulated in this work. The performance of the cross-flow coupling is compared with co-current coupling in terms of energy efficiency and the range of feasible operating conditions. The results show that energy efficiency of co-current coupled microreactor is higher than that of cross-flow coupled microreactor, whereas cross-flow coupled microreactors show advantages when operating at lower ammonia flow rates (i.e., lower hydrogen throughput). © 2012 Elsevier B.V.

Sabatier reaction, the catalytic reduction of CO2 to methane, is a reversible and exothermic reaction that produces CO as a by-product. We use two-dimensional computational fluid dynamics (CFD) simulations to analyze the performance of a three-turn spiral microreactor, useful for portable or modular applications. Spiral reactor has inlet at the center followed by three turns that spirals outwards. It allows thermal integration via co-current self-coupling of Sabatier reaction, which enables utilizing the exothermic heat of methanation for preheating of the cold reactants by hot products. Due to compact nature of the geometry, heat released spreads out in the reactor causing an overall moderation of temperature. A favorable cooling effect is achieved in spiral geometry without using external cooling or feed dilution, merely by the property of reactor configuration. For the base case, 71% conversion with 92.6% selectivity to CH4 is attained. Comparison with counter-current operating strategy in U-bend reactor is presented.