Numerical Simulation On Products Distribution Of Fischer-Tropsch Synthesis Using Aspen Plus

Based on the principle of minimal Gibbs free energy,the Aspen Plus software were used to analyze the global thermodynamic equilibrium of Fischer-Tropsch synthesis products.Under thermodynamic equilibrium,the phenomenon of Fischer-Tropsch synthesis products distribution and the trend of olefin/alkane ratios（O/P）,olefin/alcohol ratio andα-olefin weight were predicted and compared with the actual products distribution.At the same time,olefin（ethylene）and alcohol（propanol）were introduced to simulate the effect on the product distribution,The model on the Fischer-Tropsch synthesis product chain growth factorαabout temperature（T）,space velocity（SV）,and hydrogen/carbon monoxide ratio(P_H2?P_CO)was constructed.With Marquardt method and global optimizing method,according literature experimental data,the parameters of model were valued.Theαvalue predicted by model was compared with experimental data to analyze the prediction accuracy of model,meanwhile,the product distribution were predicted by the associated power function kinetic model.Using the component data analysis（ADA）of the Aspen software,the virtual component cutting method（PCS）and the API-METH virtual component algorithm,thermodynamic properties such as lump average molecular weight（MW）,API gravity,specific gravity,and average boiling point（NBP）were obtained to directly define the virtual components.At the same time,the Fortran language was used to compile the five lump kinetic model and the nine lump kinetic model subroutine,the Aspen software externally called the subroutine and shared the database with Fortran.The product distribution was calculated through the built-in loop iteration.Conclusions have been shown as follows:1.In the thermodynamic equilibrium and the actual situation,the products of C₃-C₁₅ showed a linear ASF distribution and there was a phenomenon that ethylene was low and the methane was high;Increasing ethylene residence time favored the formation of long-chain hydrocarbons.When the retention time（ethylene/CO）was 0.6,theαvalue was closer to the experimental value;the introduction of alcohol would lead to a significant increase of C₃-C₄ hydrocarbons and to some extent inhibited the Fischer-Tropsch synthesis.For a complex Fischer-Tropsch synthesis reaction with heterogeneous multicomponent mixing,a single chain growth mechanism and reaction theory cannot fully explain the deviation of the product distribution from the ASF distribution.The Fischer-Tropsch synthesis process was likely to be a multi-mechanism,and controlled by both kinetics and thermodynamics.2.Based on the existingα-model and the distribution characteristics of Fischer-Tropsch synthesis products,the ternary doubleαmodel for predicting the alpha values of two-stage products was established:4)=1+0)[4),1（1）4),2+4),3（1（1）4）,4+4),5（2）4),6+4),7]（4）=1,2)The new model has higher prediction accuracy and wider application range,and the deviation from the experimental value was basically less than±10%.Combining the doubleαmodel and power function dynamic model,the kinetic equations of Fischer-Tropsch synthesis products were obtained by fitting the two-stages reaction rate with literature experimental data.For the first stage（2<n<10）,in the±10%range,the product distribution was predicted more accurately;the second-stage product（n≥10）,in the range of±15%of the relative residual,the model can be accurately predicted and the experimental value can fit well.3.By using a five lump and nine lump kinetic models,Fischer-Tropsch wax oil hydrocracking reaction was simulated under T=600K,P=3.5 MPa,WHSV=1g_H2/(g_Cat.·h),R_H2/wax=0.105g_H2/g_wax.The result of five lump and nine lump kinetic models are different in prediction ability.For the fuel gas(C_1-4),gasoline(C_5-9),and heavy diesel(C_15-22),the predicted values of the five lump kinetic model were more consistent with the experimental values,especially for the fuel gas and heavy diesel components.But for light diesel(C_10-14),the nine lump kinetic model was relatively more adaptable.Under T=580-640K,P=3.5-6MPa,WHSV=0.67-2g_H2/(g_Cat.·h),R_H2/wax=0.06-0.3g_H2/g_wax,the sensitivity analysis shows that the increase of temperature and hydrogen/wax ratio was beneficial to the hydrocracking process of the wax.When the pressure and space velocity increased,it was harmful to the cracking process and the conversion rate of the wax was reduced.The influence of temperature and space velocity on the wax conversion rate was relatively greater.When the temperature was approximately600K and the space velocity was 1h^-1,the product was mai NLy dominated by gasoline with a mass fraction of up to 30%.