FCC Riser Reactor Research Articles

Fluidized catalytic cracking with novel emulsified feeding for increasing light oil yield and reducing additional CO2 emission

The fluid catalytic cracking unit (FCCU) is widely used in the processing of heavy oil. Fast and sufficient vaporization of heavy oil can maximize the light oil yield and reduce the coke yield. However, the inferior quality generally makes the heavy oil difficult to be vaporized. Unvaporized oil remaining on the catalyst surface will lead to high coke yield and subsequently high carbon dioxide emissions in the regenerator. The novel emulsified feeding technique can achieve secondary atomization of oil droplet through micro-explosion, which can reduce the droplet diameter and enhance the droplet vaporization. Computational Fluid Dynamics (CFD) simulation was performed to investigate the effect of emulsified feeding on the droplet vaporization and cracking reaction in the FCC riser reactor. It was found that when the reaction temperature is constant, the emulsified feeding can promote the droplet vaporization and cracking reaction. The average vaporization ratio can be improved by 1.2%-1.7%, and the light oil yield can be improved by 0.6%-0.8% compared to the conventional atomized feeding. The volume fraction of droplet near the side wall is largely reduced. The additional carbon emissions produced by unvaporized oil can therefore be reduced in the regenerator. In addition, the emulsified feeding technique will not have great impact on solids fluidization and pressure balance in the riser. The probability of micro-explosion of emulsion droplets is further discussed with respect to its effect on the vaporization and cracking reaction. The maximum light oil yield can be obtained with micro-explosion probability of 85%.

Simulation analysis of micro-explosion during emulsification feeding of residue fluidized catalytic cracking

Residue Fluidized Catalytic Cracking (RFCC) is one of the key processes to convert heavy oil into light products in refineries. The fast and sufficient vaporization of the feedstock in the FCC riser reactor can promote the catalytic cracking reaction and is beneficial for increasing the yield of light products and decreasing the yield of coke. However, fast vaporization of residue oil is extremely challenging due to its high density and viscosity. Recently, emulsified feeding technology is seen as a potential method that can effectively enhance the vaporization of RFCC feedstock. Computational Fluid Dynamics (CFD) simulations were performed in order to reveal the micro-explosion mechanism of emulsion droplet in the riser reactor. The CFD model was validated against experimental results from literature. The model was used to study micro-explosion of emulsified droplets in the gas phase and at the catalyst particle surface separately. It was found that due to the micro-explosion of emulsion oil droplets in producing large number of tiny droplets, the secondary atomization of droplets can be realized. In the gas phase, the interfacial area of emulsion droplet can be improved by 5 to 15 times. At the catalyst surface, the percentage of liquid film coverage was reduced by 32.6 %, and the droplet diameter was reduced by 92 % for the emulsified feeding technology. These results indicate that the vaporization efficiency of the residual oil can be greatly improved by this technology.

A Four-Lump Kinetic Model for Atmospheric Residue Conversion in the Fluid Catalytic Cracking Unit: Effect of the Inlet Gas Oil Temperature and Catalyst-Oil Weight Ratio on the Catalytic Reaction Behavior

The effects of the inlet gas oil temperature and catalyst-to-oil weight ratio on the catalytic cracking behaviors and the distribution of the product and the yield of the riser outlet main products as unconverted gas oil, gasoline, and coke were investigated on a real industrial FCC Riser reactor with a model LRC-99 catalyst and atmospheric residue derived from Niger’s crude oil of Agadem bloc was used as the feedstock. A four-lumped kinetic mathematical model is developed to predict the feedstock conversion and the product distribution by using the deactivation function coke-on- catalyst approach. The model was validated with a good agreement against real industrial fluidized bed riser reactor plant data from Niger and industrial riser data from literature sources. The results show that the inlet gas oil temperature and catalyst-to-oil weight ratio obviously affect the cracking reactions behavior and govern the reaction rates.

CFD modeling of hydrodynamics and kinetic reactions in a heavy oil riser reactor: Influence of downward feed injection scheme

Downward feed injection scheme is more promising than traditionally upper feed injection scheme for FCC riser reactors, however, its effects on the whole riser performance have not been elaborated. This study aims at CFD modeling of hydrodynamics and chemical reactions in an industrial-scale riser reactor, with focus on the influence of downward feed injection scheme. For this purpose, a CFD model, verified earlier in a real industrial riser reactor, is extended to the present work. The hydrodynamics, temperature profile and species concentration distribution in the riser reactor with the downward feed injection scheme are numerically studied and compared to those in the upper feed injection scheme. The results indicate that different from the smooth evolution in the upward feed injection scheme, the gas velocity, particle content and riser temperature in the downward injection scheme exhibit local maximum value in the feed injection zone. In the middle and upper zones of the riser reactor, the downward 45° and 60° injections show lower gas velocity and riser temperature than the upward 60° injection while the downward 30° injection shows an opposite trend. The downward feed injection scheme with an angle of 45° and a velocity of 60 m/s is optimal for the present industrial-scale riser reactor. Compared to the traditionally upper feed injection scheme, the new downward feed injection scheme could enhance the yields of the diesel and gasoline species by 0.93 and 0.29% point and reduce the yields of the dry gas and coke species by 0.61 and 0.96 unit.

Optimization of Fast Separator Structure of FCC Riser Reactor

Optimization of Fast Separator Structure of FCC Riser Reactor

Numerical simulation of the turbulent gas–solid flow and reaction in a polydisperse FCC riser reactor

National Ministry of Science and Technology of China [2012CB21500402]; National Natural Science Foundation of China [21076171]; State Key Laboratory of Heavy Oil Processing (China University of Petroleum) [2012-1-08]

Advanced Reaction-Terminating Technique for FCC Riser Reactor

Numerical simulation of the performance of a commercial FCC riser reactor was carried out by using a three-dimensional gas-particle, two-phase, turbulent flow-reaction model. Results showed that overcracking of desired products, such as diesel or light fuel oil, occurs and the optimum yields of desirable FCC products could be achieved at a critical riser height of less than 20 m above the feed inlet. This suggests that injection of quenching media above the critical riser height can be an effective option for optimizing the yields of desirable products. The reaction-terminating technique conducted in a commercial FCC riser reactor, using light waste oil, straight-run gasoline, and desalted water as quenching media produced favorable results. Light fuel oil yields increased by 1.3 wt% and 0.95 wt% for using the first two quenching media. Liquefied cracking gas yields increased by 0.43 wt% and 1.03 wt%, whereas the yields of dry gas and coke decreased slightly. Desalted water produced better results than either light waste oil or straight-run gasoline. The improvements of gasoline and liquefied gas yields using desalted water are 0.95% and 0.49% more than those of using straight-run gasoline, respectively.

Single-Event MicroKinetics for coke formation in catalytic cracking

A Single-Event MicroKinetic model taking into account coke formation based on elementary steps has been developed for the catalytic cracking of hydrocarbons. Coke is proposed to be formed out of coke precursors. Both coke precursors and coke formation are accounted for in terms of the same families of elementary steps as those involved in the cracking process. The coke precursors are defined as three-ring aromatic molecules. The actual and critical step for coke formation is considered to be the alkylation of coke precursors that are present in the feedstock or that are formed during the catalytic cracking. The inclusion of coke precursors formation in the reaction network leads to an important expansion of the network. Appropriate rules are defined to keep the size of the network within tractable limits. The rate equations for the initial coke formation do not contain supplementary kinetic parameters. The only extra parameter required to be determined from experimental coking data is the average molecular mass of the coke. The developed model is implemented in a simulator of an industrial FCC riser-reactor. The calculated product and coke yield profiles are typical for industrial practice.

Modeling of vaporization and cracking of liquid oil injected in a gas–solid riser

Vaporization and cracking of liquid oil injected in a gas–solid riser (fluid catalytic cracking riser reactor) was computationally studied in this work. Evaporation of a single drop injected in a stream of gas–solid mixture was analyzed first. A model for simulating evaporation of a drop considering heat transfer from the gas phase as well as from the solid particles was developed. The model relates the evaporation rate of droplet with rate of collisions of solid particles, specific heat capacities of solid and liquid, latent heat of vaporization, relative velocity of gas and liquid and temperatures of three phases. The understanding gained from such a model was then extended to simulate evaporation of liquid drops injected in FCC risers. The Eulerian–Lagrangian approach was used to simulate simultaneous evaporation and cracking reactions occurring in FCC riser reactors. A commercial CFD code, FLUENT (of Fluent Inc., USA) was used. Four and ten lump models were used for simulating cracking reactions. Appropriate user defined functions were developed to implement heterogeneous kinetics and heat transfer models in FLUENT. A special algorithm was developed to calculate accumulated coke on catalyst particles. A boiling point range was considered for simulating realistic oil feedstock. The model was first evaluated by comparing predicted results with published industrial data. The simulations were then carried out to understand influence of key design and operating parameters on performance of FCC riser reactors. The parameters studied included; initial oil droplet distribution, catalyst inlet temperature, catalyst to oil ratio and thermal cracking. The approach, model and results presented here would be useful for optimization of FCC operation, cost to benefit analysis of new FCC nozzles and related decision-making.

MODELING AND SIMULATION OF FLUID CATALYTIC CRACKING UNIT

In the present work, a new approach of kinetic scheme for the FCC riser is introduced which considers cracking of one lump (pseudo component) giving two other lumps in one single reaction step. The proposed model falls under the category of models in which lumps are formed on the basis of boiling point. In this approach, each individual lump is considered as a pure component with known physico-chemical properties. This scheme facilitates the consideration of cracking reactions’ stoichiometry. Since a new cracking reaction mechanism is introduced, a new semi empirical approach based on normal probability distribution is developed to estimate the cracking reactions’ rate constants. This approach makes the kinetic model more versatile. Six tunable parameters have been introduced to adjust more than ten thousand reaction rate constants needed to explain complete reaction mechanism in a typical FCC riser reactor. The model also incorporates two phase flow and catalyst deactivation. The various products’ yields, catalyst activity, and riser temperature are predicted all along the riser height. Plant data reported in the literature is used for the validation of the developed model. A sensitivity analysis of the proposed tuning parameters is done which suggested that the products’ yields were insensitive to one tuning parameter out of the proposed six tuning parameters. Also, the value obtained for one of the tuning parameter out of the proposed six tuning parameters was zero. These two tuning parameters were than dropped for the subsequent case studies for the simulation of the riser reactor and FCC unit. A regenerator model adopted from the literature is integrated with the proposed riser model to simulate the entire FCC unit. The steady state simulation of the FCC unit is then done to study the effect of operating parameters on the performance of this unit.

NUMERICAL SIMULATION OF FCC RISERS

The catalytic cracking of hydrocarbons in a FCC riser is a very complex
physical and chemical phenomenon, which combines a three-dimensional,
three-phase fluid flow with a heterogeneous catalytic cracking kinetics.
Several researchers have carried out the modeling of the problem in different
ways. Depending on the main objective of the modeling it is possible to find
in the literature very simple models while in other cases, when more
accurate results are necessary, each equipment is normally treated separately
and a set of differential and algebraic equations is written for the problem.
The riser reactor is probably the most important equipment in a FCC plant.
All cracking reactions and fuel formation occur during the short time (about
4-5s) that the gas oil stays in contact with the catalyst inside the riser. This
work presents a simplified model to predict the, temperature and
concentrations in a FCC riser reactor. A bi-dimensional fluid flow field
combined with a 6 lumps kinetic model and two energy equations (catalyst
and gas oil) are used to simulate the gas oil cracking process. Based on the
velocity, temperature and concentration fields, it is intended, on a next step,
to use the second law of thermodynamic to perform a thermodynamic
optimization of the system.

NUMERICAL SIMULATION OF FCC RISERS

The catalytic cracking of hydrocarbons in a FCC riser is a very complex physical and chemical phenomenon, which combines a three-dimensional, three-phase fluid flow with a heterogeneous catalytic cracking kinetics. Several researchers have carried out the modeling of the problem in different ways. Depending on the main objective of the modeling it is possible to find in the literature very simple models while in other cases, when more accurate results are necessary, each equipment is normally treated separately and a set of differential and algebraic equations is written for the problem. The riser reactor is probably the most important equipment in a FCC plant. All cracking reactions and fuel formation occur during the short time (about 4-5s) that the gas oil stays in contact with the catalyst inside the riser. This work presents a simplified model to predict the, temperature and concentrations in a FCC riser reactor. A bi-dimensional fluid flow field combined with a 6 lumps kinetic model and two energy equations (catalyst and gas oil) are used to simulate the gas oil cracking process. Based on the velocity, temperature and concentration fields, it is intended, on a next step, to use the second law of thermodynamic to perform a thermodynamic optimization of the system.

Predicting Vaporization of Residua by UNIFAC Model and Its Implications to RFCC Operations

Supercritical fluid extraction and fractionation (SFEF) was used to prepare various narrow-cuts from a variety of petroleum vacuum residua. Molecular weights of SFEF narrow-cuts were used to derive the modified normal paraffin, UNIFAC group contribution model for predicting vaporization of residua. The predicted amount of residue vaporization at the feed injection point of a commercial FCC riser reactor agrees with that estimated by the residue boiling point correlation and the amount of carbonaceous matter on catalysts obtained in a field test. The predicted results show that a significant amount of residua (about 50 wt %) is not vaporized at the feed injection point of the FCC riser reactor. Parametric sensitivity tests using the UNIFAC model suggest that increasing temperature or quantity of atomization steam at the feed injection point of the FCC riser reactor has a higher impact on increasing residue vaporization than decreasing pressure. This newly developed UNIFAC model can be used as a tool for op...

Simulations of gas‐liquid‐solid 3‐phase flow and reaction in FCC riser reactors

AbstractA 3‐D, three‐phase flow‐reaction model was developed to predict the performance of FCC riser reactors. This model incorporates the effect of feed vaporization into an earlier 3‐D, two‐phase flow‐reaction model (Gao et al., 1999). The synergistic effects of hydrodynamics, heat transfer, and feed vaporization on FCC reactions are elucidated. The results show that a complex phenomenon occurs in the zone up to 4m above the point at the bottom of the riser reactor where feed is injected. The changes in the spray droplet diameter, volumetric concentration and temperature of the feed in the feed injection zone are illustrated. The model was used as a design tool for an operation modification study. Results indicate that FCC reaction selectivity for the desired products (gasoline and light fuel oil) can be improved by reducing initial droplet diameter in the feed spray from 80 to 40 μm.

Feedstock atomization effects on FCC riser reactors selectivity

Feedstock atomization affects catalytic cracking reactions that take place in an FCC riser reactor, and is considered a critical parameter especially when processing heavy fuels. A finely atomized feed allows instantaneous flash vaporization and quick quenching of the catalyst, conditions that allow the feed components to be subjected to an ideal mixing temperature. Mixing conditions at the inlet of the riser reactor also affect cracking reactions selectivity. An atomization modeling scheme has been incorporated into a CFD model developed by Theologos & Markatos (1993, A.I.Ch.E. Journal, 39( 6), 1001.) and Theologos, Nikou, Lygeros & Markatos (1997, A.I.Ch.E. Journal, 43( 2), 486) and has been used to evaluate atomization effects on feedstock vaporization rates, cracking reactions initiation, reactor selectivity and overall reactor performance. Cracking reactions are simulated using a simplified 3-lump model. The atomization modeling scheme takes into account the initial feed droplet size at nozzle's exit, as well as droplet size reduction along with vaporization. Droplet vaporization parameters from the literature have been considered. To evaluate the degree of feedstock atomization effects on reactor performance, a parametric study is carried out considering three different initial feed droplet sizes, namely 30, 100, and 500 μm. The area where feedstock vaporization takes place inside the reactor is predicted and the evolution of atomized droplet size is illustrated. The effect of the initial degree of atomization on cracking reactions selectivity is discussed.

Advanced model for turbulent gas–solid flow and reaction in FCC riser reactors

AbstractA 3‐D two‐phase flow‐reaction model was developed for predicting the performance of FCC riser reactors. This model combines two‐phase turbulent flow with 13‐lump reaction kinetics. The simulated results show that the gas–particulate turbulent reacting flow regime in the riser reactor is extremely complex, especially at the feed inlet zone of the riser reactor. The distributions of velocities, temperature, and yields are not homogeneous in the riser reactor. The model predictions were used as a design tool for operational case studies. The optimum yields of desirable FCC products, gasoline and light fuel oil were achieved at a critical riser height that is less than 10 m above the feed inlet. Beyond the critical riser height, excessive cracking reactions resulted in the increased yields of by‐products, cracking gas and coke, at the expense of desirable products. Model simulations carried out to determine the parametric effects on product yields suggest that injection of water as a reaction‐terminating medium above the critical riser height can be an effective option for optimizing the yields of desirable products.

97/00990 Thermal and catalytic cracking reactions of heavy oils. (1)

A model for predicting conversions and yield patterns in FCC riser reactor is developed considering heat transfer, gas oil vaporization, catalyst entrainment hydrodynamics, mass transfer, catalytic cracking kinetics and deactivation. The effect of the initial average droplet size generated by feed atomization nozzle on conversion and yield pattern is discussed.

Two phase cluster model in riser reactors: impact of radial density distribution on yields

Recent cold flow data (Miller et al., 1994) have shown that the manner in which the oil and catalyst are initially contacted in fluid catalytic cracker risers has a major impact on the radial density profile at the riser bottom. These results show that circumferential oil injection has a much more uniform radial density distribution than the traditional axial oil injection. In order to assess the impact of the improvements in radial density distribution on conversion and product selectivity, a two-phase heterogeneous model for riser reactors operating in pneumatic transport has been developed. The model consists of a dispersed cluster phase which contains all the catalyst and is where the bulk of the catalytic reactions occur, and a continuous oil phase with mass transfer between the phases. A simple three- component kinetic model is used to describe the rates of reaction which occur in the cluster phase. The average concentration of any component at any axial or radial position in the riser is then determined by the density-weighted average of the cluster phase and the oil phase at that position using experimental values for the radial density distribution. The radial density distributions were obtained in a cold flow riser unit simulating FCC riser reactor hydrodynamic performance. The resulting model matches commercial radial and axial conversion and gasoline yield profiles. This model provides a basis to determine the incentives for improving the mass transfer between the two phases in the riser by making hardware modifications. For example, the model predicts that the circumferential feed nozzle configuration would increase conversion by about 3 wt% over the traditional axial configuration.