The Engineering Fundamental Research Of LB Energy-saving High Temperature Shift Catalyst

With the scientific and technological progress and sustained development of the economy,the R & D of energy-saving high temperature shift catalyst becomes very active,it is imperative to accelerate industrialization process of energy-saving high-temperature shift catalysts which possess independent intellectual rights.As one part of "2005 State Project of Major Industrial Technology Development and National High-tech Industry Specific Investment Projects",the LB energy-saving high temperature shift catalyst is prepared in this paper,the basic studies of optimization of the composition and preparation conditions, fundamental properties,the thermodynamic behavior in adsorption process,TPD,TPR, tortuosity factor and effective diffusion,the intrinsic kinetics,industrial reactor simulation are studied,which have very important theoretical significance and application value in industry.Learning from the R & D experience of the modern catalysis,the preparation of Fe-Cr-Ce-La-Cu multi-component LB energy-saving high temperature shift catalysts through nitric acid-method and coprecipitation and impregnation technology is studied in this paper. The composition and preparation conditions of catalysts were coupled in the view of engineering,and the orthogonal tests were done.The key influential factors obtained from statistical test of orthogonal experiments were regarded as characteristic input vectors,output vectors was activities of catalysts.The Bayesian Automated Regularization Algorithm was utilized to build Back-propagation artificial neural network model.The optimal components, preparation conditions to manufacture LB catalyst and the catalytic activity 0.777 were obtained by simulations.The catalytic activities are stability at 0.776 when the steam-gas ratio is greater than 0.40 and in a considerable scope,the catalytic activity is also up to 0.685 even if the steam-gas ratio is only 0.30.The specific surface area,pore structure,pore size distribution and adsorption-desorption isotherms of LB catalyst are measured by dynamic adsorption method.The results show that pore size distribution of LB catalyst is relatively concentrated,the main pores is mesoporous, and the pore diameter increases after reduction;The type of N₂ adsorption-desorption isotherms falls into IUPAC classificationâ…¢,and the hysteresis loop belongs to H3-type which represents a typical multi-layer adsorption;The pore structure has obvious fractal features,which indicates that internal surface structure is between the smooth one and very rough one.The results of XRD show that catalyst phase is ofγ-Fe₂O₃-based,the other compositions(Cerium,lanthanum,copper) enter the Fe-Cr solid solution,the characteristic peaks become more sharper after reduction,the crystal phase become more integral. Scanning electron microscope showed that the catalyst surface is composed with the loose particles.The chromatographic retention parameters of CO,CO₂,and H₂O on LB catalyst in H₂ surroundings were determined by means of inverse phase gas chromatography(IGC) in accordance with chromatography process thermodynamic.Adsorption enthalpiesΔH,Gibbs adsorption free energiesΔG and adsorption entropiesΔS of different gases were estimated by their retention volumes.The adsorption enthalpyΔH of CO,CO₂ and H₂O on LB catalyst are negative.It indicates the adsorption process is exothermic.The adsorption enthalpyΔH of CO,CO₂ and H₂O increase in turn,theΔH of H₂O is much bigger than that of CO and CO₂.ΔG＜0,which means the adsorption of CO,CO₂ and H₂O on the LB catalyst is spontaneous. Also it reveals that CO,CO₂ and H₂O are easy to be adsorbed by the catalyst.The numerical value of Gibbs absorption flee energy decreases with the increase of temperature,which suggests that the spontaneity of absorption process increases with temperature.The reason whyΔS is positive is that the relative molecule quality and the relative molecule volume of H₂ are less than those of CO,CO₂ and H₂O in the inverse phase gas chromatography.After being adsorbed on the catalyst,the movement of CO,CO₂ and H₂O molecules are more regular than that in H₂.So the absorption entropy should decrease when CO,CO₂,and H₂O molecules were absorbed on catalyst.But at the same time,the rearrangement of surface molecules might cause a lot of H₂ molecules leaving the surface of the catalyst.It results in an increase in the degree of freedom and a considerable increase in the disorder of the system.Therefore the entropy in this process increases greatly and the entropy of absorption is great.As a result, the total absorption entropy of the process in which the catalyst adsorbs CO,CO₂ and H₂O molecules is positive.The temperature-programmed desorption curves of different gases(CO,H₂O,CO₂) on LB catalyst,the temperature-programmed reduction curves and H₂-TG-DSC curves were measured by temperature-programmed techniques.The adsorption activation energy, pre-exponential factor and reaction orders were calculated.There are not multi-absorption centers for CO,there are multi-adsorption centers for H₂O,and there are lower-temperature adsorption centers and high-temperature absorption centers for CO₂ in LB catalyst.The adsorption of CO,H₂O and CO₂ on LB catalyst belongs to one-order dynamics.The temperature of CO desorption peak is higher,and the desorption activation energy of CO is larger.The activation energy of main reduction-peaks of LB catalyst was lower than that of the commercial catalyst.The activation energy of overreduction-peaks of LB catalyst was higher than that of the commercial catalyst.Those facts show that LB catalyst is easier to be reduced to active state,and is more difficult to be overreduced than the commercial catalyst. The exothermic peak in H₂,which represents the reduction procession from Fe₂O₃ to Fe₃O₄ and the interaction among all composition,hadn't been widened.This shows that the catalyst particles don't grow up,which is possible because that the synergistic effect of Ce and La improves the thermal stability of catalyst and the dispersibility of active composition.The binary and ternary counter-current diffusion fluxes of different gas-pairs(consisting of N₂,CO and CO₂) through LB catalyst were measured at 313 K,333K and ambient pressure with steady-state method.The tests were done in a self-made modified Wicke-Kallenbach(MWK) diffusion cell.The molar ratio of diffusion flux accorded with Graham's law,which shows that the diffusion in LB catalyst is in transition region.Based on the cross-linked pore model,the tortuosity factors of reduced LB catalyst were calculated based on mean transport-pore diameters((?)) and pore size distribution(δ) respectively.The effective diffusivities of gases in different gas-pairs were calculated.The(?) andδof the same gas pairs differ from each other,it indicates that LB catalyst has wider pore size distribution,which is in accordance with experimental pore size distribution curve.The intrinsic kinetics experiments were designed by orthogonal experimental design,and were conducted at different temperature,different gas flow,different steam/gas ratio and different gas component over LB energy-saving high temperature shift catalyst.The kinetics data were fitted with formate mechanism model,power-law model and simplify power-law model,and the parameters in the models were estimated.The statistical examination shows that the three models are highly acceptable and creditable,the simplify power-law model is the best one.The industrial measurements of LB catalyst,including the conversion rate,bed temperature distribution,by-products amount,the resistance of reactor,were done in a large-scale ammonia factory which manufactures 1000 tons of ammonia every day. One-dimension heterogenous model to simulate the HTS reactor was built combining with simplify power-law intrinsic kinetics model.The model parameters were obtained by use of the configuration method and Merson method which is used to solve nonlinear equations models.The simulative distributions of bed temperature,intra-particle diffusion efficiency factor,CO content and shift ratio with the height of the catalyst bed were obtained,and the results shows that they are accordant with that of industrial measurements.The results will speed up both the industrialization process of LB energy-saving high temperature shift catalyst and promote the development of related fields at a certain extent.