Runaway Criterion in Fixed Bed Catalytic Reactors with Radial Temperature Profile

The discrepancy between pseudo-homogeneous one-dimensional model and pseudo-homogeneous two-dimensional model is studied. It is found that there are great differences between two models. This paper compares the maximum and minimum values of the radial temperature in the hot spot in case that a single exothermic reaction is carried out, a correlation is obtained with pseudo-homogeneous one-dimensional model to describe the entire reactor behavior. A new runaway criterion, based on the occurrence of inflection in the hot spot locus, is developed for the case of pseudo-homogeneous two-dimensional model. This criterion predicts the maximum allowable temperature for safe operation and the regions of runaway, respectively. The calculated results show that, compared with the results based on pseudo-homogeneous one-dimensional model, runaway will easily occur when the radial temperature gradient has to be considered.

Oxidative coupling of methane in a fixed bed reactor over perovskite catalyst:A simulation study using experimental kinetic model

The oxidative coupling of methane （OCM） to ethylene over a perovskite titanate catalyst in a fixed bed reactor was studied experimentally and numerically. The two-dimensional steady state model accounted for separate energy equations for the gas and solid phases coupled with an experimental kinetic model. A lumped kinetic model containing four main species CH4, O2, COx （CO2, CO）, and C2 （C2H4 and C2H6） was used with a plug flow reactor model as well. The results from the model agreed with the experimental data. The model was used to analyze the influence of temperature and feed gas composition on the conversion and selectivity of the reactor performance. The analytical results indicate that the conversion decreases, whereas, C2 selectivity increases by increasing gas hourly space velocity （GHSV） and the methane conversion also decreases by increasing the methane to oxygen ratio.

Modeling the oxidative coupling of methane: Heterogeneous chemistry coupled with 3D flow field simulation

The oxidative coupling of methane （OCM） over titanate perovskite catalyst has been developed by three-dimensional numerical simulations of flow field coupled with heat transfer as well as heterogeneous kinetic model. The reaction was assumed to take place both in the gas phase and on the catalytic surface. Kinetic rate constants were experimentally obtained using a ten step kinetic model. The simulation results agree quite well with the data of OCM experiments, which were used to investigate the effect of temperature on the selectivity and conversion obtained in the methane oxidative coupling process. The conversion of methane linearly increased with temperature and the selectivity of C2 was practically constant in the temperature range of 973-1073 K. The study shows that CFD tools make it possible to implement the heterogeneous kinetic model even for high exothermic reaction such as OCM.

Reaction performance of fluidized bed catalytic reactor of Group C^(+)particles

Group C particles are often regarded as non-fluidizable but have proven to effectively fluidize with nanoparticle addition,which results in small bubbles and a high gas holdup in the dense phase during the experiments.Group C^(+)particles provide an increased surface area for gas-solid contact and improve the reaction performance,especially for gas-phase catalytic reactions.On the basis of a previous study of the ozone decomposition reaction using Group C^(+)particles,a two-phase model was used to evaluate the reactor contact efficiency,and was used to compare the partial oxidation performance of the n-butane to maleic anhydride reaction in fluidized-bed catalytic reactors of Group C^(+)and Group A particles.The reactor with Group C^(+)particles achieved a higher n-butane conversion and MAN yield compared with that using Group A particles,based on the identical catalyst quantity or on the same gas residence time.Therefore,the reactor with Group C^(+)particles can achieve the same reaction conversion and yield with fewer catalysts or a smaller reactor size,or both.Therefore,the fluidized bed catalytic reactor of Group C^(+)particles is expected to be of major significance in industrial processes,especially for gas-phase catalytic reactions.

Catalytic effect of platinum and silver in a hydrogen peroxide monopropellant ceramic microthruster

Ceramic microthrusters with an embedded Pt resistive heater,two temperature sensors,and a Pt or Ag catalytic bed were made of high-temperature co-fired alumina ceramics.To increase the surface area by a factor of 1.21,and so the catalytic effect,the Pt catalytic bed was made porous by mixing the Pt paste with 15—20vol.%graphite sacrificial paste before screen printing it.Ag was in-situ electroplated on the porous Pt surface after sintering.Decomposition of 50wt.%hydrogen peroxide as a monopropellant was studied both qualitatively and quantitatively by changing the catalyst(between Ag and Pt),flow rate(15—55 μl/min),and operating temperature(115—300℃).A reference device without catalyst exhibited an unstable behavior as a result of no,or vety little,decomposition,whereas the Ag catalyst was more stable,and the Pt one even more stable.Also,Pt was found to be slightly more effective.Quantitatively,there were small differences between Pt and Ag in the power needed to maintain the temperature.The inventive methods to make the Pt bed porous as well as in-situ electroplating Ag were success-fully demonstrated.

Numerical comparison of two modes of gas-solid riser operation： Fluid catalytic cracking vs CFB combustor

Two modes of gas-solid riser operation, i.e., fluid catalytic cracking （FCC） and circulating fluidized bed combustor （CFBC）, have been recognized in literature; particularly in the understanding of choking phenomena. This work compares these two modes of operation through computational fluid dynamics （CFD） simulation. In CFD simulations, the different operations are represented by fixing appropriate boundary conditions： solids flux or solids inventory. It is found that the FCC and CFBC modes generally have the same dependence of solids flux on the mean solids volume fraction or solids inventory. However, during the choking transition, the FCC mode of operation needs more time to reach a steady state; thus the FCC system may have insufficient time to respond to valve adjustments or flow state change, leading to the choking. The difference between FCC and CFBC systems is more pronounced for the systems with longer risers. A more detailed investigation of these two modes of riser operation may require a three-dimensional full loop simulation with dynamic valve adjustment.

Experimental analysis of volatile liquid injection into a fluidized bed

Experiments were conducted on a lab-scale fluidized bed to study the distribution of liquid ethanol injected into fluidized catalyst particles. Electrical capacitance measurements were used to study the liquid distribution inside the bed, and a new method was developed to determine the liquid content inside fluidized beds of fluid catalytic cracking particles. The results shed light on the complex liquid injection region and reveal the strong effect of superficial gas velocity on liquid distribution inside the fluidized bed, which is also affected by the imbibition of liquid inside particle pores. Particle internal porosity was found to play a major role when the changing mass of liquid in the bed was monitored. The results also showed that the duration of liquid injection affected liquid-solid contact inside the bed and that liouid-solid mixin~ was not homogeneous durin~ the limited liouid injection time.

An investigation of reaction furnace temperatures and sulfur recovery

In a modern day sulfur recovery unit(SRU),hydrogen sulfide(H_(2)S)is converted to elemental sulfur using a modified Claus unit.A process simulator called TSWEET has been used to consider the Claus process.The effect of the H_(2)S concentration,the H_(2)S/CO_(2) ratio,the input airflow rate,the acid gasflow of the acid gas(AG)splitter and the temperature of the acid gas feed at three different oxygen concentrations(in the air input)on the main burner temperature have been studied.Also the effects of the tail gas ratio and the catalytic bed type on the sulfur recovery were studied.The bed temperatures were optimized in order to enhance the sulfur recovery for a given acid gas feed and air input.Initially when the fraction of AG splitterflow to the main burner was increased,the temperature of the main burner increased to a maximum but then decreased sharply when theflow fraction was further increased;this was true for all three concentrations of oxygen.However,if three other parameters(the concentration of H_(2)S,the ratio H_(2)S/CO_(2) and theflow rate of air)were increased,the temperature of the main burner increased monotonically.This increase had differ-ent slopes depending on the oxygen concentration in the input air.But,by increasing the temperature of the acid gas feed,the temperature of the main burner decreased.In general,the concentration of oxygen in the input air into the Claus unit had little effect on the temperature of the main burner(This is true for all parameters).The optimal catalytic bed temperature,tail gas ratio and type of catalytic bed were also determined and these conditions are a minimum temperature of 300°C,a ratio of 2.0 and a hydrolysing Claus bed.

Fluidization science,its development and future

By revisiting the three stage theory for the progress of science proposed by Taketani in 1942, the footmarks of fluidization research are examined. The bubbling and fast fluidization issues were emphasized so that the future offluidization research can be discussed among scientists and engineers in a wider perspective. The first cycle of fluidization research was started in the early 1940s by an initial stage of phenomenology. The second stage of structural studies was kicked off in the early 1950s with the introduction of the two phase theory. The third stage of essential studies occurred in the early 1960s in the form of bubble hydrodynamics. The second cycle, which confirmed the aforementioned three stages closed at the turn of the century, established a general understanding of suspension structures including agglomerating fluidization, bubbling, turbulent and fast fluidizations and pneumatic transport; also established powerful measurement and numerical simulation tools.After a general remark on science, technology and society issues the interactions between fluidization technology and science are revisited. Our future directions are discussed including the tasks in the third cycle, particularly in its phenomenology stage where strong motivation and intention are always necessary, in relation also to the green reforming of the present technology. A generalized definition of ＇fluidization＇ is proposed to extend fluidization principle into much wider scientific fields, which would be effective also for wider collaborations.

CFD simulation of gas-solid flow patterns in a downscaled combustor-style FCC regenerator

To investigate the gas-solid flow pattern of a combustor-style fluid catalytic cracking regenerator, a laboratory-scale regenerator was designed. In scaling down from an actual regenerator, large-diameter hydrodynamic effects were taken into consideration. These considerations are the novelties of the present study. Applying the Eulerian-Eulerian approach, a three-dimensional computational fluid dynamics （CFD） model of the regenerator was developed. Using this model, various aspects of the hydrodynamic behavior that are potentially effective in catalyst regeneration were investigated. The CFD simulation results show that at various sections the gas-solid flow patterns exhibit different behavior because of the asymmetric location of the catalyst inlets and the lift outlets. The ratio of the recirculated catalyst to spent catalyst determines the quality of the spent and recirculated catalyst mixing and distribution because the location and quality of vortices change in the lower part of the combustor. The simulation results show that recirculated catalyst considerably reduces the air bypass that disperses the catalyst particles widely over the cross section. Decreasing the velocity of superficial air produces a complex flow pattern whereas the variation in catalyst mass flux does not alter the flow pattern significantly as the flow is dilute.