Catalyst deactivation by carbon deposition has been investigated for the catalytic propane dehydrogenation reaction using computational fluid dynamics to couple the transport and reaction processes occurring inside the cylindrical pellet to the gas flow of reactants in the bed. The pellet scale reaction and carbon laydown are shown to be strongly affected by the bed scale tube wall heat flux supplied for the endothermic reactions, and the species distributions on the pellet surface are affected by the ease or otherwise of reactant access to the particle.

To produce hydrogen as a clean fuel and a new source of energy, catalytic reactions are widely used in industry [1]. The deactivation of catalysts is an important problem in industry, such as steam reforming and the catalytic dehydrogenation of alkanes, which are both strongly endothermic reactions. Different publications reported on the deactivation of steam reforming of methane (SRM) as a significant source of hydrogen production [2] and propane dehydrogenation (PDH) as a commercial and interesting reaction [3] to understand the effect of deactivation on the reactor situations and performance generally. The local carbon deposition on catalysts can cause particle breakage and strongly decrease reaction rates. Catalyst deactivation in heated tubes removes the heat sink and can result in local hot spots that weaken the reactor tube. This is particularly a problem for a low tube-to-particle diameter ratio (N) fixed bed reactor, where a large fraction of the catalyst particles are located next to the heated tube wall. Computational fluid dynamics (CFD) has been reviewed as a suitable tool to simulate packed bed tubes [4]. Previous works [5] have introduced the coupling of gas flow and resolved species and temperature gradients inside pellets by CFD for SRM and PDH without considering deactivation. In this work, the focus is on using CFD to obtain better comprehension of the role of catalyst deactivation on reactions, mass and heat transfer of the gas-phase fluid flow around and inside of cylindrical catalyst particles. SRM and PDH reactions were simulated by CFD to study the interaction between local flow and reaction/deactivation. CFD simulations of flow, heat transfer, diffusion and reaction were carried out using the commercial CFD code Fluent 6.3 in a 3D 120-degree periodic wall segment of an N=4 tube. The mesh used boundary layer prism cells at both the inside and outside particle surfaces and at the tube wall, with tetrahedral cells in the main fluid volume. These reactions were represented in the solid particles using user-defined scalars to mimic species transport and reaction, with user-defined functions supplying reaction rates. Diffusion in the particles was modeled by Fick's law using an effective diffusivity, given by Hite and Jackson's approximation of the Dusty Gas Model [6]. Catalyst activity is related to the coke accumulation with a suitable initial coking expression and causes decrease of reaction rates with deposition of carbon. Carbon deposition can be calculated by first running a CFD simulation at base case (undeactivated) conditions, then accumulating carbon production rates over a 60-second interval to obtain the local carbon build-up. This is then inserted into the activity factor of the catalyst, which is used to modify the fresh catalyst reaction rates to give the reaction rates after 60 s. The CFD simulation is then run for a further 60-second time period to obtain increased values of carbon accumulation and the process repeated. The CFD simulations show local details of carbon laydown both on the surface of, and inside near-wall catalyst particles. The transient development of particle internal gradients and carbon accumulation were studied for the early stages of deactivation. Carbon concentration is initially strongest close to the surface and in the high temperature regions of the catalysts and affected by the wall heat flux. Deactivation of the endothermic reactions causes a slow increase in the average catalyst temperature. The results present a decrease of the product mole percentages at the outlet with time. References:

Catalyst deactivation by carbon deposition has been investigated for the dehydrogenation of propane to propene on a Cr2O3/Al2O3 catalyst. Computational fluid dynamics was used to couple the 3D transport and reaction processes occurring inside the cylindrical pellet to the gas flow around the pellet. The pellet scale reaction and carbon laydown are shown to be strongly affected by the bed scale tube wall heat flux supplied for the endothermic reactions, and the species distributions on the pellet surface are also affected by the ease of reactant access to the particle. The development of particle internal gradients and carbon accumulation are illustrated for the early stages of deactivation. Carbon deposition is initially strongest in the high temperature regions close to the tube wall. As time progresses, the increased deactivation caused by the carbon acts to reduce all rates of reaction, and propene production and coke formation shift to other regions of the pellet.

In this study, the potential of biobutanol was evaluated as an alternative to bioethanol which is currently the predominant liquid biofuel in the US. Life-cycle assessments (LCAs) suggest that the net energy generated during corn-to-biobutanol conversion is 6.53 MJ/L, which is greater than that of the corn-derived bioethanol (0.40 MJ/L). Additionally, replacing corn with lignocellulosic materials in bioethanol production can further increase the net energy to 15.90 MJ/L. Therefore, it was interesting to study the possibility of using domestically produced switchgrass, hybrid poplar, corn stover, and wheat straw as feedstocks to produce liquid biofuels in the US. By sustainable harvest based on current yields, these materials can be converted to 8.27 billion gallons of biobutanol replacing 7.55 billion gallons of gasoline annually. To further expand the scale, significant crop yield increases and appropriate land use changes are considered two major requirements.

Hydrogen production from steam reforming of methanol for fuel cell application was modeled in a wall coated micro channel reactor by CFD approach. Heat of steam reforming (SR) was supplied from catalytic total oxidation (TOX) of methanol on Cu/ZnO/Al2O3 catalyst and Heat conducts from TOX to SR zone through Steel divider wall between two channels. Heat integration was compared in zigzag and straight geometry of microreactor by CFD modeling. The model is two dimensional, steady state and containing five zones: TOX fluid, TOX catalyst layer, steel wall of the channel, SR catalyst layer and SR fluid. Set of partial differential equations (PDEs) including x and y momentum balance, continuity, partial mass balances and energy balance was solved by finite volume method. Stiff reaction rates were considered for methanol total oxidation (TOX), methanol steam reforming (SR), water gas shift (WGS) and methanol decomposition (MD) reactions. The results show that zigzag geometry is better than straight one because heat and mass transfer in zigzag reactor are more than straight. Conversion of methanol in zigzag geometry is greater than straight one. In the outlet of zigzag micro channels, carbon monoxide selectivity is less and hydrogen mole fraction is more than straight one.

Hydrogen production from steam reforming of methanol for fuel cell application was modeled in a wall coated micro channel reactor by CFD approach. Heat of steam reforming was supplied from catalytic total oxidation (TOX) of methanol in neighboring channel of steamreforming (SR) channel and conduct from TOX to SR channel through metal divider wall of the channels. Heat integration was compared in zigzag and straight geometry of microreactor by CFD modeling. The model is 2 dimensional, steady state and containing five zones: TOX fluid, TOX catalyst layer, steel wall of the channel, SR catalyst layer, SR fluid. Set of partial differential equations (PDEs) including x and y momentum balance, continuity, partial mass balances and energy balance was solved by finite volume method. And stiff reaction rates were considered for methanol total oxidation (TOX), methanol steam reforming (SR), water gas shift (WGS) and methanol decomposition (MD) reactions, in the model. The results showthat zigzag geometry is beeter than straight geometry because of increasing of heat and mass transfer, mixing.

Hydrogen production from steam reforming of methanol for fuel cell application was modeled in a wall coated micro channel reactor by CFD approach. Heat of steam reforming was supplied from catalytic total oxidation (TOX) of methanol in neighboring channel of steamreforming (SR) channel and conduct from TOX to SR channel through metal divider wall of the channels. Heat integration was compared in co-current and counter-current microreactor by CFD modeling. The model is 2 dimensional, steady state and containing five zones: TOX fluid, TOX catalyst layer, steel wall of the channel, SR catalyst layer, SR fluid. Set of partial differential equations (PDEs) including x and y momentum balance, continuity, partial mass balances and energy balance was solved by finite volume method. And stiff reaction rates were considered for methanol total oxidation (TOX), methanol steam reforming (SR), water gas shift (WGS) and methanol decomposition (MD) reactions, in the model. The results showthat mole fraction of hydrogen and carbon monoxide in the outlet of counter-current microreactor is greater than co-current and co-current is better than counter-current for fuel cell application.

Producing hydrogen from natural gas for a mini scale fuel cell is a new challenge for researchers. Therefore, modeling of hydrogen production microreactors should be helpful for designing and developing new microreactors. Experimental sensing of velocity, concentration, temperature and reaction rates in numerous points of the microreactor is impracticable. A microreactor in special geometry was considered for hydrogen production and a CFD model was developed in order to incorporate the mechanism of autothermal reforming. This mechanism includes three main reactions and Langmuir-Hinshelwood type kinetic rates. A three dimensional reformer model was developed to simulate the reactive laminar flow model of this microreactor. Effects of styles of feed entrance, air to fuel ratio and adding water to methane were studied. This model shows that there are hot spots near the entrance of the microreactor where the total oxidation of methane occurs and air distribution along the microreactor is a good solution for hot spot problems. The model shows that air distribution is good for fuel cell application because of high hydrogen production and low CO content in the outlet.