Study On Catalysts And Reactor Modeling For Acetic Acid Hydrogenation To Ethanol

With limited supply from petroleum resources and increasing demand of energy from gobal industrialization,a new transformation technology for alternative fuel need to be developed.Ehtanol,as a renewable fuel,has few environmental issue and has been promoted in USA and Brazil.Nowadays,ethanol systhesis from biomass has been well developed in research and industry,which has large propotion in the market of ethanol production.In present,main trendency for producing fuel ethanol should be a transition from fermentation with high cost to fuel processing technology with low cost.In ethanol's chemical systhesis,ethylene hydration has low conversion per pass and direct systhesis from syngas had poor selectivity to ethanol,which have limited prospect.From 20 century 50s,methanol carbonylation has been applied to manufacture acetic acid massively.Fatty acid hydrogenation to corresponding alchol has been realized in theory.Hence,acetic acid hydrogenation to ethanol with high selectivity and conversion can be rekoned as a reasonal route for producing fuel ethanol massively.The researches on catalysts for acetic acid hydrogenation were focused on noble metal catalysts.This paper developed a new Rh-based catalyst system with modification of Sn for acetic acid hydrogenation.Experiments has been carried out over Rh/Al2O3 and RhSn/Al2O3 with various Sn/Rh ratios and different loadings.The effect of Rh and Sn on acetic acid hydrogenation was explored by the characterization of catalyst structure and adsorption of acetic acid on catalyst.After reduction at 623 K,catalyst surface in RhSn/Al2O3 had metallic Rh,Rh oxides and Sn oxides.During H2-TPD process,spillover of hydrogen tan be observed and high Sn/Rh promoted hydrogen spillover.After adsorption of acetic acid at 548K,the existence of Sn species was benefit for cleavage of C-OH bond and made top adsorption of acetyl species more stable.The RhSn catalyst with high Sn/Rh inhibited methane generation and produced more ethoxy species.The performance of acetic acid hydrogenation to ethanol can also be improved by high loadings of Sn and Rh.The best performance over 3Rh5Sn/Al2O3 achieved 73%selectivity to ethanol and 98%conversion of acetic acid.In past,there were many researches on PtSn catalysts.For modification of acetic acid hydrogenation catalysts,PtSn/Al2O3 catalysts modified by K with different loading amounts and various impregnation sequences were prepared by co-impregnation and continuous impregantion.The modification of K deacreased vibaration frequency of CO adsorbed peak,and Pt 4d5/2 binding energy and adding K can also improved adsorption of carbonyl group leading to high selectivity to ethanol.K on PtSn surface can increased electron density of PtSn and K modified support also had similar effect.Electronic modification by K can weaken Pt-C bond inhibiting methane generation.Potassium blocked acid sites on catalyst surface and restrained side reactions,such as esterification and ethanol dehydration.Further co-impregnation of Pt and Sn to K-doped support decreased dispersion of Pt active sites leading to low catalytic performance.Adding 0.5 wt.%K into PtSn catalyst improved selectivity to ethanol from 37%to 64%and conversion of acetic acid from 88%to 93%.Further addition of potassium had little positive effect on catalytic performance.Based on K modified PtSn catalysts,two sites Langmuir-Hinshelwood kinetic models with hydrogenation and esterification were established and adsorption behaviors for acetic acid hydrogenation over PtSn/Al2O3,K/PtSn/Al2O3 and PtSn/K/Al2O3 were characterized.Various adsorption species over different catalysts has been discussed from the perspective of kinetics.On the condition of 230-280?,1-3MPa,0.45-1.35h-1 and 7-11,the experiments over different catalysts have been carried out and kinetic models can be fitted with experimental data by differential evolution and quasi-Newton to obtain kinetics parameters.In comparison of the experimental and calculated data,kinetic models can well predict performance over three catalysts.0.5 wt%K adding to PtSn/Al2O3 decreased activation energy of hydrogenation from 20.8 kJ/mol to 11.5 kJ/mol and increased activation energy of esterification from 22.3 kJ/mol to 31.7 kJ/mol.The results of dissociation energy and adsorption heat from H2-TPD and DRIFTS reflected that K/PtSn/Al2O3 could promoted hydrogen spillover and desorption of ethanol.In contrast to K/PtSn/Al2O3,PtSn/K/Al2O3 produced more ethyl acetate.Potassium also enhanced dissociation of C-OH bond to form more acetyl species.The experiments for acetic acid hydrogenation over PtSn/Al2O3,K/PtSn/Al2O3 and PtSn/K/Al2O3 catalysts were performed with respect to different reaction conditions.High temperature,pressure and low WHSV can increase conversion of acetic acid.The reaction pressure had further benefit for the enhancement of K on selectivity to ethanol.On background of annual 100,000 tons production for acetic acid hydrogenation to ethanol,one and two dimensional pseudo-homogenous simulations for tubular fixed-bed reator were performed and simulation results were obtained by Runge-Kutta method and orthogonal collocation under typical and various operating conditions.The profiles of concentrations along axial show increasing conversion of acetic acid and decreasing selectivity to ethanol in reactor tubes,while temperature of catalyst bed firstly increased then decreased along axial,which existed hotspot.In radia,concentrations had little difference and temperature had obvious change.In the results of one dimensional modeling,high reaction pressure,inlet temperature of feed,inlet temperature of thermal oil and low WHSV can increase conversion of acetic acid and have little effect on selectivity to ethanol.Flow rate of thermal oil had little influence on concentrations.Increasing pressure,inlet temperature of feed and decreasing WHSV led to higher temperature and forward position of hotspot because of high radial temperayure difference.Higher inlet temperature and lower flow rate of thermal oil had increasing temperature and backward position of hotspot due to low radial temperature difference.