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Introduction

Fluidized-Bed Catalytic Cracking (FCC) is needed to transform low-value heavy oil into more useful lighter chained products (LPG, fuel gas, gasoline etc.).  The FCC unit will give an approximate conversion of 75% of the feedstock into gasoline and lighter products.  The catalytic process has been shown to be more efficient than the thermal cracking process.  The FCC unit is the heart of the modern refinery (Paul et al., 2015). 

Although the FCC unit is the most effective unit in the refinery, its feed comes from other units.  The distillation unit separates the raw crude into several intermediates, namely, gasoline, kerosene, naphtha, diesel and gas oil (Sadeghbeigi, 2012).  The heaviest part of the crude, which cannot be refined in the barometrical tower, is warmed and sent to the vacuum tower, where separation into gas oil and residue occurs.  The residue is then sent units such as the de-asphalting unit, the residue cracker, the delayed coker for further processing.  The residue can also be used as asphalt for tarring roads.  The gas oil coming primarily from the atmospheric column, the vacuum tower, and the delayed coker is known to be the feed of the FCC unit (Lababidi and Al Humaidan, 2011).

A broad range of products is obtained from the FCC unit, namely, Liquefied Petroleum Gas (LPG), high-octane gasoline, fuel gases, etc. (Lababidi and Al Humaidan, 2011).

Due to its high sensitivity to various boundary conditions and several continuous disturbances from different sources, the complexity of the operation process of the FCC unit is very high.  The FCC unit is responsible for the production of almost 50% of all used gasoline in the world (Negrao and Baldessar, 2006).

The FCC unit consists of three main sub-units, namely;

• The riser reactor
• The stripping section/ dis-engager
• The regenerator reactor

The riser-reactor is a key sub-unit in the Fluidized-bed Catalytic Cracking Unit (FCCU), as this is where the catalytic cracking of the gasoline occurs. The liquid feed enters the riser reactor through a nozzle at the bottom of the reactor to ensure optimum atomization. The feed droplets subsequently contact the hot catalyst particles and, as a result of heat transfer, are vaporized. The cracking reaction begins instantaneously. The resulting vapor-particles mixture moves upwards due to a pressure differential along the reactor length. As the motion occurs, a cracking reaction also occurs simultaneously. As a result of cracking, the velocity of the vapors increases along with the riser height. The reactions along the reactor length are known to be endothermic (Heydari et al., 2010).  The rate at which the feed vaporizes at the entry zone is a key performance indicator of the catalytic cracking reaction. At a point in the riser-reactor, the cracking reaction halts due to the deposition of coke on the catalyst surface, causing catalyst deactivation and a short contact time between the catalyst and the vaporized feed. The spent catalyst moves from the riser reactor to the top of the regenerator.  It is termed ‘deactivated’ as a result of the accumulation of coke and hydrocarbons on its surface. The accumulated hydrocarbons on the surface of the spent catalyst are separated from the catalyst before leaving the reactor by stripping with steam (Du et al., 2014). The hydrocarbon vapours generated are then sent to a fractionating column. In the regenerator, coke is burnt off from the catalyst with the passage of air for catalyst reactivation.  The flue gas leaving the regenerator contains a large quantity of carbon (II) oxide, which is burned to carbon (IV) oxide in a CO furnace or alternatively with a waste heat boiler so that available heat can be recovered. The catalyst leaves the regenerator through the bottom to be mixed with the feed stream. To reactivate the catalyst, the regenerator oxidizes the coke on the spent catalyst to form CO, CO₂, and H₂O. Compressed air enters the regenerator from the bottom of the regenerator through a grid distribution pattern.  The regenerated catalyst leaves the regenerator and is mixed with the feedstock at the riser reactor's base, ensuring a continuous cycle.  Fresh makeup catalyst could be added to maintain continuity and for provisions of withdrawal of used up catalyst over time (Ahmed et al., 2013). 

Modern FCC catalysts usually comprise four components: matrix, binder, filler, and the active component (Zeolite). Zeolite ranges from about 15-50 wt% of the catalyst. Industrially, zeolite-based FCC catalysts are usually termed faujasite or type Y (Kumar and Reddy, 2011). The catalytic sites in the zeolite are known to be strong acids providing most of the catalytic activity in the catalyst.  The nitrogen present usually restrains the catalytic activity of the catalyst in trace quantities in the feedstock; the Nitrogen content of a typical Vacuum Gas Oil (VGO) is in the range of 1000-2000 ppm (Dasila et al., 2012). Higher Nitrogen content would result in a significant reduction in conversion rate.  Amorphous alumina constitutes the matrix component of the FCC catalyst, enabling cracking of higher-boiling, larger feedstock molecules.  Mostly, silica sol is used as the binder and kaolin, the filler, providing the physical strength of the catalyst.  Contaminant metals, e.g., nickel, vanadium, iron, copper, etc., present in the feedstock also contribute to the catalyst deactivation (Nace et al., 1971; Paraskos et al., 1976; Shah et al., 1977; Weekman, 1968).

An essential advantage of the fluid catalytic cracking process is the fluidity of the catalyst between the reactor and regenerator, especially when fluidized with appropriate vapor phases. The fluidization of the catalyst ensures intimate interaction between the catalyst and hydrocarbons for better and more efficient cracking reactions (Amhammed, 2013). The feedstock quality, operating conditions of the riser reactor-regenerator sections, and the type of catalyst are factors that strongly determine the conversion and yield pattern of the FCC unit.  The interactions between the riser and regenerator reactors, the uncertainty in the cracking reactions, and the coke deposition and burning kinetics influences the complexity of the FCC unit (Dagde and Puyate, 2012).

Problem Statement

Several works have shown proposed models for the dynamic situation in the FCCU but have failed to imitate the industrial situation in reality for several reasons successfully. These reasons include the complexity of the reactions in the reactor and the inconsistency between the proposed models and the industrial situation (Ahmed et al., 2014).  This inconsistency can be attributed to the assumption of isothermal conditions in the reactor, the lumped model characteristics, the mode in which heat and mass transfer resistances are accounted for and in some cases not accounted for, the catalyst deactivation model, and the model accounting for the adsorption effects of aromatics (Bollas et al., 2007).

This work seeks to account for some of these ignored discrepancies. It imitates the steady-state situation in the FCCU successfully to analyze the optimal length of the riser reactor using the operating conditions of the Fawley refinery’s FCCU. (Berry et al., 2004; Das et al., 2003; Gupta et al., 2007; Gupta and Kumar, 2008).

Aim and Objectives

This study aims to model and simulate the riser-reactor of Fawley Refinery’s Fluid Catalytic Cracking Unit (FCCU) using a modified twelve-lump kinetics model. It has the following objectives;

• Objective 1: Development of a feasible and comprehensive model for the riser reactor of Fawley’s FCCU, accounting for the mass and heat transfer resistances, catalyst deactivation function, the poisoning effect of basic Nitrogen, and the adsorption effect of aromatics, asphaltenes, and resins.
• Objective 2:  Development of a detailed hydrodynamic model for the riser reactor of Fawley’s FCCU.
• Objective 3: Obtaining numerical solutions of the developed models using MATLAB built-in ode23 function, needed to predict the respective yields of vacuum gas oil, gasoline, liquefied petroleum gas, fuel gas, and coke.
• Objective 4:  Comparison of the simulated results with plant data gotten from Fawley refinery to reveal the accuracy and efficiency of the developed models to imitate the real-time situation in the FCCU.  This comparison would be done with respect to the yield of vacuum gas oil, the cracked products, the fluids pressure drop, solid-phase temperature, and other critical process parameters.
• Objective 5:  Carrying out sensitivity analysis on the developed models.

Research Questions

The identified research questions for this project are provided below:

• What energy models are incorporated into the developed model to take cognizance of the heat transfer resistance, instant vaporization of feed, and one-dimensional vaporization of feed?
• What hydrodynamic model is incorporated into the developed model to account for the pressure drop along the reactor length?
• Are models incorporated to account for the catalyst deactivation, poisoning effect of basic Nitrogen, and adsorption effects of aromatics?
• How would the operating conditions and parameters of Fawley Refinery’s FCCU be gotten?
• Are the representation of the viscosities and the specific heat capacities of vacuum gas oil and other lumped components a function of temperature in the developed model, since in reality, there is a temperature variation along the reactor length as a result of heat exchange? (Leon-Becerril et al. 2004; Sharma, 2011).

Deliverables

The deliverables of this project are a project report, mathematical models, and simulated results. The models should consistently mimic the real-time situation in the riser-reactor of Fawley refinery’s FCCU. Also, the report should contain complete documentation of how the mathematical models were arrived at and tables of the simulated results along the reactor length.

Relevance

This project mainly focuses on mimicking the real-time situation in the riser-reactor of Fawley Refinery’s FCCU.

Methodology

This project focuses on secondary research, development of mathematical models, and model simulation, and they are discussed below:

Secondary research

The secondary research in this project will utilize a systematic approach (Johnson et al., 2016) to review the works of literature. The steps involved in the systematic review of the literature are provided below:

• Step 1: Identify the research questions that can be used for the project.
• Step 2: Identify the keywords that should be used to research the works of literature.
• Step 3: Extract the journals and books that are appropriate for this project.
• Step 4: Write the literature review chapter.

Model development

The development of mathematical models for the riser-reactor situation in Fawley refinery’s FCCU and simulating the developed models. The development of the mathematical models are in stages:

• Stage 1:  Model development for the vaporizer subsystem
• Stage 2: Using a preferred and appropriate kinetics of catalytic cracking in the FCCU’s riser reactor.
• Stage 3:  Developing an appropriate catalyst deactivation model (Dagde and Puyate, 2012).
• Stage 4: Developing an appropriate hydrodynamic model along the reactor length (American Petroleum Institute, 1992).
• Stage 5: Taking an appropriate mass balance around the riser reactor of Fawley refinery’s FCCU (Geankoplis, 2011).
• Stage 6: Taking an appropriate energy balance around the riser-reactor of Fawley refinery’s FCCU.
• Stage 7: Carrying out an appropriate force balance around the riser-reactor of Fawley refinery’s FCCU.

Model simulation

The developed models would be transformed into appropriate codes on the MATLAB platform, and the codes would also be run and simulated on the MATLAB platform.

The codes and simulated results would be documented both as MATLAB and Excel spreadsheets.

Evaluation

The risk assessment conducted for this project is provided in the table below:

Table 1:  Risk assessment