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.

Niket S. Kaisare

Department Of Chemical Engineering

AMMONIA DECOMPOSITION

COUNTER-CURRENT REACTOR

COUPLING MODE

Cross flows

Flow direction

HEAT UTILIZATION

Higher efficiency

Hydrogen generations

MICRO REACTOR

Operating condition

OPERATING DIAGRAM

Operating regions

PROCESS INTENSIFICATION

PROPANE COMBUSTION

Reactor walls

Simulation

SPATIAL COUPLINGS

Transverse directions

Ammonia

combustion

Energy efficiency

flow rate

Hydrogen

Hydrogen production

propane

Thermal conductivity

Chemical reactors

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Chemical Engineering Journal

The Sabatier reaction, which is the catalytic reduction of CO2 to methane, is a reversible and exothermic reaction. The reaction provides a classical trade-off, with low temperature thermodynamically favored, and high temperature required for the occurrence of the reaction. It is also accompanied by side reactions that produce CO. In this work, the autothermal operability of the Sabatier reaction is analyzed in a thermally integrated U-bend microreactor using computational fluid dynamics (CFD) simulations. Internal recirculation of heat from the hot product stream to the cold inlet stream results in a favorable temperature profile. Results show that there is sufficient preheating near the inlet to overcome the kinetic barrier so that the reaction lights-off; and simultaneous cooling of the product stream near the outlet, which thermodynamically favors higher conversion to methane. This preheating through internal heat recirculation enables the U-bend reactor to be used even with feed at ambient temperature, a unique feature not present in a non-heat-recirculating straight-channel reactor with otherwise similar properties. Comparisons with other modes of operation, and the effects of the operating conditions, namely inlet velocity, temperature and composition, are also presented. © 2019 The Royal Society of Chemistry.

Reaction Chemistry and Engineering

Analysis of the autothermal operability of the Sabatier reaction in a heat-recirculating microreactor using CFD

Numerical CFD simulations are used to demonstrate the performance characteristics of a novel spiral geometry, which is used for catalytic micro-combustion in this work. This reactor has catalytic channel, with inlet at the center and spiraling outwards. The spiral microreactor geometry allows preheating of the cold inlet owing to heat exchange with the hot products flowing in co-current direction in the adjacent channel. Results indicate that the spiral is more stable than the straight channel reactor, due to the transverse heat transfer between adjacent channels in the spiral reactor. 2D simulations are performed to comprehensively analyze the role of this heat recirculation on the thermal management and stability of catalytic combustion of propane/air mixtures; and the spiral microreactor is compared with the well-studied straight channel microreactor. Axial and transverse temperature profiles at various design and operating conditions are analyzed for a better understanding of heat recirculation in this geometry. Effect of various parameters viz. equivalence ratio (ϕ), inlet velocity (u0), wall thermal conductivity (ks) and number of turns (N) on the spiral reactor's stability and performance is presented. © 2018 Elsevier Ltd

Chemical Engineering Science

A spiral microreactor for improved stability and performance for catalytic combustion of propane

The aim of this paper is to analyze cross-flow operation in a microreactor with posted catalytic inserts for Pt-catalyzed propane combustion under fuel-lean conditions. The insert consists of 150 posts arranged in-line in six rows. The posts are static pillar-like structures in the flow channel, which act as static mixers and provide higher catalytic surface area. Computational Fluid Dynamics (CFD) simulations are used to compare the post microreactors for axial- and cross-flow configurations. In cross-flow case, where the bulk flow is in transverse direction across the six rows of posts, significant flow mal-distribution is observed. Tapering of the inner walls of the microreactor solid structure, at an angle of 5.766°, is proposed for obtaining more uniform flow distribution. For the cross-flow microreactors with tapered inner walls (referred to as "tapered-post"), the flow distribution becomes more uniform as the flow rate is decreased. The axial- and tapered-post microreactors show similar propane conversion for a wide range of operating conditions. Tapered-post microreactors have lower pressure drop and more uniform wall temperatures because the reaction zone is spread over the entire length of the catalytic insert. We also show that tapered-post microreactors have lower heat losses, and therefore are more stable towards device extinction. Consequently, the primary advantages of tapered-post microreactors are better thermal management and ability to operate at more fuel-lean conditions. © 2011 Elsevier Ltd.

Propane combustion in non-adiabatic microreactors: 2. Flow configuration in posted microreactors

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.

Propane combustion in non-adiabatic microreactors: 1. Comparison of channel and posted catalytic inserts

A pseudo-2-dimensional model is used for modeling a multifunctional microreactor for hydrogen generation by coupling catalytic ammonia decomposition on ruthenium with catalytic propane combustion on platinum. The two reactions are carried out in adjacent parallel plate channels in a cocurrent flow mode. Operating lines defining the attainable region are computed. The high temperatures and fast heat transfer ensure that both reactions go to completion in as low as submillisecond contact times and enable compact hydrogen production for portable and distributed power generation. The ammonia decomposition reaction tends to be limited by the intrinsic rate of reaction, whereas catalytic combustion is also affected by mass and heat diffusion. We have found that moderate and high conductivity materials are preferable because they support a rather wide attainable region. We show that one such device could enable variable hydrogen supply for variable power needs. A simple operating strategy to dial in the desirable power is proposed, which ensures high thermal efficiency (∼75% once-through efficiency of the integrated device and ∼85% reformer efficiency, both without any heat recuperation), wall isothermicity, and high conversions. © 2009 American Chemical Society.

Industrial and Engineering Chemistry Research

Millisecond production of hydrogen from alternative, high hydrogen density fuels in a cocurrent multifunctional microreactor

AIChE Journal

Optimizing the catalyst distribution for countercurrent methane steam reforming in plate reactors

Industrial & Engineering Chemistry Research

Scale-out of Microreactor Stacks for Portable and Distributed Processing: Coupling of Exothermic and Endothermic Processes for Syngas Production

Hydrogen generation in spatially coupled cross-flow microreactors

Journal	Chemical Engineering Journal
ISSN	13858947
Open Access	No