Study On Heat Transfer Of Fixed-Bed And Mathematical Simulation Of Fischer-Tropsch Synthesis Reactor

Energy is important to human survival and development. Developments of coal to liquid and gas to liquid can achieve clean utilization of coal and optimal utilization of gas, which are good for relieving the imbalance between supply and demand. Pilot experimental device for 3000 t/y synthesis oil has been set up in Zhenhai Refining & Chemical Company by China Petrochemical Corporation. Project researches were undertaken by Zhenhai Refining & Chemical Company, Ningbo Engineering Company and East China University of Science and Technology.The paper was focused on the study on heat transfer of fixed-bed for Fischer-Tropsch synthesis and two dimensional mathematical simulation of above poject.Thermal conductivity of cobalt-based catalyst in the temperature range 170-265℃is measured via an DRL-Ⅱthermal conductivity measuring apparatus based on steady-state heat flow method. The effects of powder size, specimen density, porosity, parallelism, ruggedness, and moisture content on the thermal-conductivity measurements are investigated, respectively. A random distribution heat transfer model is used to calculate the effective thermal conductivity of cobalt-based catalyst based on the experimental data in reaction conditions. The results indicate that the thermal conductivity of the catalyst specimens increases linearly with temperature and density and is practically independent of the particle size of the powder in an atmosphere of air, while the porosity dependence of the thermal conductivity is inverse to that of density. The reliability of thermal conductivity of cobalt-based catalyst specimens is influenced easily by the parallelism, specimen ruggedness and moisture content. The effective thermal conductivity of cobalt-based catalyst increases with temperature and reaction pressure increasing, while the composition of mixed gas has weak effect on the effective thermal conductivity.The effective thermal conductivity experiment apparatus of fixed bed is set up when gas is stagnant. The effective thermal conductivity of stagnant fixed bed packed with cobalt-based catalyst is measured using the steady state method in the temperature range of 165-265℃at normal pressure. The correlation of the stagnant effective thermal conductivity with the thermal conductivity of fluid and catalyst is obtained based on the steady state theory. And the stagnant effective thermal conductivity from experiment is also compared with the predicted values. Meanwhile, the effective thermal conductivity of fixed bed is measured in H2, N2 and He, respectively. The radial effective thermal conductivity and wall heat transfer coefficient are calculated according to the stagnant effective thermal conductivity, Rep and Pr. The results show that the maximum deviation and standard deviation of the experimental results from correlation are 1.74% and 0.43%, respectively. The experimental results and model values proposed by Kunii et al are in good agreement. The relations of the effective thermal conductivity of fixed-bed in different gases are as follows:λe0,air<λe0,N2<λ0,He<λe0,H2。An experimental study is conducted on the heat transfer characteristic in a fixed bed packed with cobalt-based catalyst. A two dimensional pseudo-homogeneous phase model is adapted to describe the heat transfer progress in fixed bed. The radial effective conductivity and the wall individual heat transfer coefficient are obtained by using orthogonal collocation method and Levenberg-Marquardt method to solve the mathematical model. The heat transfer parameters are correlated as a function of Reynolds number Rep. The obtained correlations are applied to the comparison of heat transfer parameters correlation of the correlations which are gained based on the stagnant effective thermal conductivity, Rep and Pr with the same packing catalyst particle. The results show that they are in good agreement.A two-dimensional pseudo-homogeneous reactor model is proposed to simulate the performance of fixed-bed Fischer-Tropsch synthesis (FTS) reactors by lumped thought based on the heat transfer parameter correlations of fixed-bed. CO consumption kinetics equation and carbon chain growth probability model are incorporated into the reactor model. The model equations discretized by two-dimensional orthogonal collocation method are solved by Broyden method. The concentration and temperature profiles are obtained. The validity of the reactor model is investigated against the pilot-plant test data. Satisfactory agreements between model predictions and experiment results are obtained.Further simulations are carried out to investigate the effect of operating conditions on the reaction behavior of fixed-bed FTS reactor. The results show that the increases of reaction pressure from 3.0MPa to 5.0MPa have a significant effect on the reactor temperature and the increase of reactant conversions, and what's more it favors the enhancement of the oil yield. The effect of inlet temperature from 209.50℃to 224.50℃on reactor operation is not remarkable. The bed temperature keeps stable. The conversions of reactants and oil yield all have a slight increase. The increase of boiling water temperature from 199.4℃to 205.4℃can increase the reactor bulk temperature and the reactant conversions, however it suppresses the oil yield. Although increasing the feed gas flow from 3400Nm3/h to 5950Nm3/h decreases the reactant conversions and oil yield, it contributes favorably to maintaining the heat stability of the reactor system. Increase of the ratio of H2/CO from 1.9 to 2.5 makes the conversion of CO increase and the conversion of H2 decrease, but the oil yield change little.