Heat And Mass Transport Characteristics Of Methanol Steam Reforming In Reactors With Fractal Tree-shaped Channels

Due to the methanol steam reforming(MSR)possesses low operating temperature,and provides higher hydrogen yield and lower CO formation,it has become the ideal hydrogen source of the Proton Exchange Membrane Fuel Cell(PEMFC)in small mobile electronic device.Developing a novel and high performance methanol steam reformer has been one of the key technique problems that need to be solved for miniature mobile power supply.Compared with conventional channel structures(such as parallel,serpentine),the tree-shaped flow architecture possesses highly branched structure.Due to the scale effect and bifurcation effect,the heat and mass transfer process in reactors is intensified,which finally improves the reaction performance of steam reforming for hydrogen production.Therefore,the studies on the transports processes and reaction in methanol steam reforming reactor with fractal tree-like microchannel network can reveal the heat and mass transport characteristics as well as the kinetic behavior of the methanol steam reforming,which have important academic value and application prospect.Recent researches on the methanol steam reactors mainly focus on the catalyst choice and catalytic mechanism,and the performance enhancement inside the reactors.The results indicate that the geometric has significant influence on the hydrogen production performance of the micro-methanol steam reformer.The heat and mass transfer process can be enhanced when the channel structure is designed properly,which results in an improvement in the efficiency of reforming process.However,the most commonly used microchannel structures in a microreactor are based on single channel(straight and serpentine)or parallel channels.Inspired by the successful applications of fractal tree-shaped networks in cooling of electronics,the tree-shaped structure is introduced into the design of a methanol steam microreactor in the context of optimization of flow reaction configuration.Furthermore,the kinetics of the methanol steam reforming used in previous reaseraches are based on empirically correction factors,which means the reaction mechanism is not considered in the simulation.In this case,it is still unclear how the operating parameters affects the reforming and decomposition reactions.Therefore,in this paper,a three-dimensional model is developed and analyzed numerically to study the transport characteristics of the MSR in micro reactors with tree-shaped channels.Then based on a complete Langmuir-Hinshelwood(LH)kinetic approach,an improved three-dimensional model of methanol steam reforming with considering of chemical reaction mechanisms is developed and numerically analyzed to investigate the reaction kinetic behavior.Meanwhile,the experiment on a reactor with tree-shaped microchannels is preformed to study the methanol steam reforming in the reactor.Additionally,the adsorption of methanol molecules as well as the mechanism of the first step in the dehydrogenation reaction on the surface of Cu(111),Pd(111)and PdZn(111)is studied.The major research work and conclusions are summarized as follows:(1)The theoretical study on the transport characteristics of the MSR in a disc microreactor with tree-shaped flow architecture is preformed.The effect of branching level,steam to methanol ratio(SMR),and inlet velocity on the reaction performance of the microreactor with a constructal tree-shaped network are investigated and discussed.The results indicate that the methanol conversion in the disc tree-shaped microreactor is more than 10%better than that of a parallel microreactor.Furthermore,the yield of hydrogen at the outlet of the disc tree-shaped microreator is greater than the parallel flow configuration.The CO concentration in the products of the disc tree-shaped microreactor is higher than that of parallel microreactor.Increases in the branching level lead to a large surface-to volume ratio and enough time for the methanol to contact with the catalyst,which introduce a superior performance in the microreactor.(2)Based on a complete Langmuir-Hinshelwood(LH)kinetic model,the reaction kinetic behavior of the methanol steam reforming in tree-shaped microchannels have been investigated,The effects of temperature,pressure and bifurcation angle on the reaction performance are examined and discussed.The results indicate that,due to the redevelopment of velocity boundary layer and the species boundary layer,the higher concentration of the left unreacted mixture at the center region in former branch toward to solid surface and maintain contact with the catalyst in latter branch channel,which is beneficial to the improvement of methanol steam reforming reaction.The "figure of merit"(FoM)in a fractal tree-like channel network is more superior when compared with serpentine channel,so the fractal tree-like flow configuration is a preferred design for hydrogen production by methanol steam reforming.In a fractal tree-like microchannel network,the reaction rates of methanol steam reforming(MSR)and reversed water gas shift(rWGS)are improved by increasing temperature.For a higher working pressure,the chance of effective collision among reactant molecules is increased,as a result,the reaction rate of methanol steam reforming is enhanced.Increases in catalyst thickness provide more active sites for methanol steam reforming reaction,which contributes to enhancing the reaction rates and hence increases the methanol conversion rate.Unlike the significant role in liquid flow in fractal tree-like microchannel networks,the effect of bifurcation angle on the chemical reaction performance in the fractal tree-like microchannel networks is herein found to be less significant.(3)A methanol steam reformer has been designed and fabricated.The experiment on the reactor with tree-shaped microchannels is preformed to study the methanol steam reforming performance of the reactor.The effects of working conditions on the reaction performance are obtained.The results indicate that increases in temperature can enhance the methanol conversion rate,however,the concentration of H2 in the dry product gas is decreased while the concentration of CO is increased.Increases in steam-to-methanol molar ratio(SMR)not only improve methanol conversion rate but also reduce CO concentrations in the product.A larger velocity of the mixture liquid leads to the reduction of time for the methanol to contact with the catalyst.Therefore,the conversion rate in the reactor decreases as the inlet velocity increases.When the inlet velocity is between 1 mL/h and 3 mL/h,the effect of inlet velocity on the methanol steam reforming performance is quite slight,while the inlet velocity is more than 3 mL/h,the influence of inlet velocity on methanol steam reforming performance becomes more obviously.The concentration of CO slightly increases first and then begins to decline when the inlet velocity increases.When the reaction temperature is 270 ?,the inlet velocity is 2 mL/h,the optimal SMR is correspondingly 1?1.2.Compared with a parallel channel reactor and a serpentine channel reactor which have the same inlet geometric and react area,the methanol conversion rate of the reactor with tree-shaped channels is the highest,therefore the reactor with tree-shaped channels is a preferred design for methanol steam reformer.(4)Base on the Density Function Theory(DFT)method,the adsorption of methanol molecules as well as the mechanism of the first step in the dehydrogenation reaction on the surface of Cu(111),Pd(111)and PdZn(111)is studied.The cause of different atomic bond break position is discussed.The results indicate that,on the Cu(111)and PdZn(111)surfaces,the activation energy barriers and reaction energies of the O-H cleavage pathway is lower than those of the C-H cleavage pathway.Therefore the methanol molecule removes the first H atom through the O-H cleavage with the formation of co-adsorbed methoxyl and hydrogen atom on the Cu(111)and PdZn(111)surfaces.On the Pd(111)surface,the lower energy values are required for C-H cleavage pathway,so it performs through the C-H cleavage with the formation of co-adsorbed hydroxymethyl and hydrogen atom.The surface energies of the M(111)slabs can characterize the ability of M(111)surfaces to bind with methanol molecule and its fragments,the PdZn(111)possesses the highest surface energy when compared with the Cu(111)and Pd(111)slab.The DOS of d-electron distribution for the Cu(111)and PdZn(111)slabs is below the fermi level,while for the Pd(111)slab the DOS of d-electron distribution across the fermi level.The O-H cleavage pathway is preferred both on the Cu(111)and PdZn(111)slabs as the DOS of d-electron is similar on the Cu(111)and PdZn(111).In summary,the current work reveals the transport characteristics,reaction kinetic behavior in reactor with fractal tree-shaped channels systematically.The correspongding reasarch results can provide a theoretical guidance and technical support for the optimal design and engineering applications of the microreactors.