Equivalent Reactor Network Model for the Modeling of Fluid Catalytic Cracking Riser Reactor

Modeling description of riser reactors is a highly interesting issue in design and development of fluid catalytic cracking (FCC) processes. However, one of the challenging problems in the modeling of FCC riser reactors is that sophisticated flow-reaction models with high accuracy require time-consuming computation, while simple flow-reaction models with fast computation result in low-accuracy predictions. This dilemma requires new types of coupled flow-reaction models, which should own time-efficient computation and acceptable model accuracy. In this investigation, an Equivalent Reactor Network (ERN) model was developed for a pilot FCC riser reactor. The construction procedure of the ERN model contains two main steps: hydrodynamic simulations under reactive condition and determination of the equivalent reactor network structure. Numerical results demonstrate that with the ERN model the predicted averaged error of the product yields at the riser outlet is 4.69% and the computation time is ∼5 s. Contrast to the ERN model, the predicted error with the plug-flow model is almost three times larger (12.79%), and the computational time of the CFD model is 0.1 million times longer (6.7 days). The superiority of the novel ERN model can be ascribed to its reasonably simplifying transport process and avoiding calculation divergences in most CFD models, as well as taking the back-mixing behavior in the riser into consideration where the plug-flow model does not do so. In summary, the findings indicate the capabilities of the ERN model in modeling description of FCC riser reactors and the possibilities of the model being applied to studies on the dynamic simulation, optimization, and control of FCC units in the future

Numerical Simulation of the Flue Gas and Process Side of Coking Furnaces

A numerical simulation for the flue gas and process sides of a coking furnace with floor gas burners was conducted. The computational fluid dynamics (CFD) approach was employed to simulate flow, combustion, and heat transfer in the furnace. The process-side conditions were calculated with a special program. The simulation provides detailed information about the flue gas velocity, temperature fields, and process conditions for this type of coking furnace. Good agreement is obtained between industrial measurement and simulated excess air coefficient, outlet temperature of flue gas, and outlet pressure on the process side. Moreover, the simulated results indicate that there are hot spots on the tubes, located at the height of 1.5–2.5 m. That is consistent with the actual phenomenon of industrial coking furnaces. To investigate the effect of furnace structure on physical field distribution and process-side conditions, a comparative simulation case with more wide spacing of burners to walls was conducted. Results indicate that the comparative case improves the uniformity of heat flux distribution, obviously, which is beneficial for the run length of coking furnaces

Practical Model for the Induction Period of Heavy Oil during Thermal Reaction

The induction period is a significant concept for design and operation of the delayed coking process. To simplify the prediction of the induction period and make it applicable to operational analysis and real-time control of the thermal cracking process, a practical model for the induction period of heavy oil during thermal reaction, which is suitable for the non-isothermal reaction in the industrial conditions, was presented. The development of this model was based on the experimental data of five heavy oils. The model parameters were correlated with microcarbon residue (MCR) of heavy oil to improve the model universality. Furthermore, the validity of correlation between MCR and cracking kinetic parameters was proven. The model-predicted cracking conversion agrees well with the experimental values. The induction period model was employed to predict the induction period of two feedstocks reported in the literature. Results indicated that the induction period model is available for the thermal reaction of these heavy oils

Multifunctional Two-Stage Riser Catalytic Cracking of Heavy Oil

The continuous deterioration of feedstocks, the increasing demand of diesel, and the increasingly strict environmental regulations on gasoline call for the development of fluid catalytic cracking (FCC) technology. To increase the feed conversion and the diesel yield as well as produce low-olefin gasoline, the multifunctional two-stage riser (MFT) FCC process was proposed. Experiments were carried out in a pilot-scale riser FCC apparatus. Results show that a higher reaction temperature is appropriate for heavy cycle oil (HCO) conversion, and the semispent catalyst can also be used to upgrade light FCC gasoline (LCG). The synergistic process of cracking HCO and upgrading LCG in the second-stage riser can significantly enhance the conversion of HCO while reducing the olefin content of gasoline at less expense of gasoline yield. Furthermore, the novel structure riser reactor can increase the conversion of olefins in gasoline. Because of the significant increase of HCO conversion, the fresh feedstock can be cracked under mild conditions for producing more diesel without negative effects on the feed conversion. Compared with the TSR FCC process, in the MFT FCC process, the increased feed conversion, diesel and light oil yields can be achieved, at the same time, the olefin content of gasoline decreased by approximately 17 wt %

Adsorption and Separation Mechanism of Thiophene/Benzene in MFI Zeolite: A GCMC Study

Selective removal of thiophene from aromatic components is one of the key challenges facing the petrochemical industry. The adsorption and separation mechanism from the molecular viewpoint can guide and upgrade the relative adsorption based technology. Therefore, we performed the grand canonical ensemble Monte Carlo (GCMC) simulation to investigate the adsorption performance and mechanism of competitive adsorption. Density distribution and radial distribution functions (RDF) analysis give a more detailed description of the adsorption sites. For pure component adsorption, donut-shaped adsorption sites were obtained for both benzene and thiophene from the straight channel point. From the viewpoint of the zigzag channel, the sorbates follow the straight line shape distribution at low loading and the S shape distribution at high loading. As for the binary component adsorption, more benzene adsorbs in the zeolite than thiophene at low pressure; however, thiophene competes successfully at high pressure. This can be explained by the key factor: at low pressure, the size effect plays an important role. While the pressure increases, the interaction energy dominates the process. Analyzing RDFs of the binary adsorption, we observed that when benzene competes with thiophene, the preferential adsorption sites do not change; however, the emergence possibility of benzene gets smaller

Residue Catalytic Cracking Process for Maximum Ethylene and Propylene Production

Effects of operating conditions on residue fluid catalytic cracking (RFCC) were studied in a pilot-scale FCC unit. Experimental results indicated that both high reaction severity and long residence time promoted the production of ethylene and propylene. A novel RFCC process for maximum ethylene and propylene (MEP) production was further proposed, which was characterized by high operating severity, application of olefin-selective catalyst, and stratified reprocessing of light gasoline and butenes. Simulation experiments of the MEP process demonstrated that both light cycle gasoline and recycled butenes were effectively converted; meanwhile, the semispent catalyst still retained sufficient activity to further crack residue feedstock. When treating Daqing AR, the MEP process yielded up to 8.85 wt % ethylene and 25.97 wt % propylene. In contrast, due to elevated catalyst activity in a second-stage riser, the two-stage riser MEP process produced more propylene and LPG at the expense of light oil. Also, ethylene yield was still up to a comparative level

Mechanistic Insights into the Pore Confinement Effect on Bimolecular and Monomolecular Cracking Mechanisms of <i>N</i>‑Octane over HY and HZSM‑5 Zeolites: A DFT Study

Bimolecular and monomolecular cracking mechanisms of alkanes simultaneously occur and have a competitive relationship, which strongly influences the product distribution. In this work, the density functional theory (DFT) calculation is first carried out to elucidate two cracking mechanisms in HZSM-5 and HY zeolites. It is found that the overall apparent reaction barrier for the monomolecular cracking reaction at 750 K in the HZSM-5 zeolite is 5.30 kcal/mol, much lower than that (23.12 kcal/mol) for bimolecular cracking reaction, indicating that the monomolecular mechanism is predominant in the HZSM-5 zeolite. In contrast, the bimolecular mechanism is predominant in the HY zeolite because of a lower apparent reaction barrier energy barrier (6.95 kcal/mol) for bimolecular cracking reaction than that (24.34 kcal/mol) for the monomolecular cracking reaction. Moreover, the intrinsic reason for the different mechanisms is further elucidated. The confinement effect can effectively decrease the energy barrier when the size of transition states is comparable to the pore size of zeolite. The insights in this work will be of great significance to the understanding of confinement on catalytic cracking mechanism and to the design of highly efficient cracking catalysts

In Situ Upgrading of Light Fluid Catalytic Cracking Naphtha for Minimum Loss

The key to reducing the olefin content in fluid catalytic cracking (FCC) gasoline is to upgrade the olefin-rich light FCC naphtha (LCN). To minimize the naphtha loss, several parameters were investigated in a pilot-scale riser FCC apparatus. The results indicate that, besides the reaction temperature, the catalyst-to-oil ratio, and the catalyst type, the boiling range and the olefin content of LCNs also have significant influence on the upgrading effect. Moreover, a relatively short residence time is beneficial for efficiently upgrading LCNs. In addition, the influence of the reactor structure should be brought to our attention. When a novel structurally changed reactor with a multinozzle feed system was used, significantly increased olefin conversion and decreased naphtha loss can be achieved. The calculation of hydrogen balance indicates that, because of the decrease of dry gas and coke yields, more hydrogen in the feed can be distributed into the desired products

Fluid Catalytic Cracking Study of Coker Gas Oil: Effects of Processing Parameters on Sulfur and Nitrogen Distributions

To investigate the effects of operating conditions and the catalyst activity on the transfer regularity of sulfur and nitrogen during the cracking process of coker gas oil (CGO), the CGO was catalytically cracked in a pilot-scale riser fluid catalytic cracking (FCC) apparatus at different test environments. Then the cracked liquid products were analyzed for sulfur and nitrogen distributions with boiling point, from which the sulfur and nitrogen concentrations of gasoline, light cycle oil (LCO), and heavy cycle oil (HCO) fractions were determined. The sulfur and nitrogen compounds in each product cut, and their possible reaction pathways were reviewed and discussed. The results show that sulfur-containing species are easier to crack but more difficult to be removed from the liquid product, while nitrogen compounds are easier to form coke, then be removed from the liquid product. The sulfur distribution of CGO is different from that of conventional feedstocks. Different processing parameters can significantly affect the sulfur and nitrogen distribution yields and concentrations in liquid products. Increasing the reaction temperature and the catalyst-to-oil ratio as well as shortening the residence time cannot only increase the light oil yield but also improve the product quality and reduce the SO_<i>x</i> and NO_<i>x</i> emissions in the regenerator

Isomerization of <i>n</i>‑Butane over SO₄^2–/Al₂O₃–ZrO₂ in a Circulated Fluidized Bed Reactor: Prospects for Commercial Application

The stability of alumina-promoted sulfated zirconia (SZA) was investigated to achieve the isomerization of <i>n</i>-butane in a circulating fluidized bed (CFB) unit. The pilot-scale evaluation in a CFB unit showed high stability of the SZA catalyst and that the catalytic activity was dominated by the residence time of <i>n</i>-butane rather than its linear velocity. Increases in the reaction and regeneration temperature both led to an increase in the conversion of <i>n</i>-butane and a decrease in the selectivity to isobutane, caused by increasing side reactions. Although the regeneration was conducted in air, a trace of SO₂ evolved during the regeneration, which could be minimized at the appropriate gas stripping temperature, low regeneration temperature, and high space time of the feed. Compared with conventional fixed-bed technologies, the CFB process shows lower selectivity to isobutane due to the inevitable axial back-mixing and severe “dimerization-cracking” reaction