Zanfir, Monica; (2002) Catalytic Plate Reactors for exothermic-endothermic reaction coupling. Doctoral thesis (Ph.D), UCL (University College London).

Preview

Text Catalytic_Plate_Reactors_for_e.pdf Download (23MB) | Preview

Abstract

Current technologies in chemical engineering focus on process intensification in order to remain cost efficient in an environmentally responsible manner. Catalytic Plate Reactors (CPRs) emerge as a novel design for intensification of conventional chemical reactors. Such a design consists of catalytically coated metal plates so that exothermic and endothermic reactions can take place in alternate channels, which have the channel height of order of millimetres and the catalyst thickness of order of micrometers. CPRs combine reaction with heat transfer in an intensified manner, representing a feasible alternative to conventional reactors. The reduced dimensions of the reacting channels minimise heat and mass transfer resistances leading to significant volume and catalyst amount reduction in comparison to conventional reactors. Suitable applications for CPRs include autothermal coupling by means of indirect heat transfer of strong endothermic processes such as hydrocarbon steam reforming, dehydrogenation, or catalytic cracking with an exothermic process, usually catalytic combustion. Despite their attractiveness CPRs have not been yet implemented in practice, and only a few experimental and theoretical studies exist. Although CPRs show important advantages compared to conventional reactors, in order to become a viable alternative it is necessary to demonstrate their feasibility and reliability. This work is an initiation in the study of CPRs. Its main objective is oriented towards formulation of accurate mathematical models utilised further to investigate such reactors for illustrative case studies. Different reaction systems are considered and explored using parametric studies. The study of the system ethane-dehydrogenation - methane combustion revealed that the ratio of catalyst loading for the two reactions is a key variable and must be carefully adjusted in order to avoid severe hot or cold spots that can lead to either reactor run-away or extinction A metallic wall with a thickness of 2 mm, due to its high thermal conductivity, makes possible an efficient heat transfer between the endothermic and exothermic channel for small temperature differences. Among all potential applications for CPR, small-scale hydrogen production is favoured due to an increasing demand of hydrogen for fuel cells. Thus, hydrogen production in a CPR from steam-reforming of methane coupled with methane catalytic combustion was investigated. It was shown that a reduction in the reformer volume by a factor of 150 and reduction for the necessary amount of catalyst by a factor of 85 could be achieved. The effectiveness factors for the chemical reactions of the reforming process are about one order of magnitude higher than in the conventional process, proving a significant reduction of intraphase resistances. The short distance between the heat source and heat sink increases the efficiency of heat transfer. The influence of channel height and catalyst thickness on reactor behaviour was also addressed. It was shown that the intraphase resistances are important and cannot be neglected. In addition, the size of channel height has to be correlated with care with the flowrates and the amount of catalyst necessary to achieve desired CPR performance. A comparison between operation in co-current and counter-current showed that overall, the heat generation and consumption is balanced better for co-current operation than for counter-current one. Although, the counter-current flow arrangement may provide better thermal efficiency, due to opposite reactant concentration depletion along the reactor local heat balance proves difficult to achieve. Successful counter-current operation needs to use a non- uniform catalyst distribution as a degree of flexibility in adjusting the rate of heat generated and consumed. Another objective of this thesis is to find criteria to identify suitable ranges for CPR design parameters and to evaluate if the reactor has a reliable and stable operation. Parametric sensitivity analysis was utilised to define such criteria and to identify the most important parameters that can affect CPR behaviour. Among the parameters studied the strongest influence comes from the activation energies followed by the inlet temperature. Sensitivity to inlet composition and velocities have moderate effect, while the lowest sensitivity was found with respect to geometrical parameters and wall thermal conductivity. A generic procedure for CPRs design is summarised based on the understanding gained during the present work. The procedure includes preliminary, detailed and optimal design steps which are shortly described and discussed. It was also aimed to use mathematical modelling in order to aid experimental work concerned with the study of catalytic combustion in small channels. Simulation of a 2 mm-diameter reacting channel having the catalyst coated on the walls, demonstrated that immersing the channel in a fluidised bed can keep the channel wall in almost isothermal conditions. It was also shown that the axial velocity profile does not affect the model accuracy. In addition, CFD simulations showed that an expansion-contraction geometry for the channel inlet eliminates recirculation patterns and minimises the experimental errors due to entrance effects.

Download activity - last month

Export as XMLCSVJSON

Download activity - last 12 months

Export as CSVJSONXML

Downloads by country - last 12 months

Export as CSVJSONXML

Archive Staff Only

View Item