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The application of loop-seals for the catalyst transfer from the Regenerator to the Riser on pilot-plant scale FCC applying gas-particle simulation


Computational fluid dynamics (CFD) is robust in predicting and analyzing complex multiphase flow hydrodynamics, especially on fluid catalytic cracking (FCC) phenomena in fluid-solid reaction. This study presented the replacement valve as a regulator of the catalyst's distribution from the Regenerator to the Riser in a pilot-scale fluid catalytic cracking (FCC). On a large scale, FCC applies Slide valves/plug valves as a regulator of the catalyst rate from the Regenerator to the Riser also stops nitrogen flow Riser. However, Slide valves are carefully designed with abrasion-resistant protection for improving the reliability of the valve. Internal insulation allows using of carbon steel for the body of the valves. It is impossible on a pilot plant scale because the dimeter pipe is small, making the catalyst often stuck in the valve and nitrogen escapes to the Riser. The solution is loop-seal pipe by flowing the air below to regulate the catalyst and solve the Riser nitrogen leakage. This study uses the CFD approach, specifically the MMPIC applying CFDSOF, including the preliminary validation with catalyst dimension based on the Geldart group A and A ' Miyauchi.

1. Introduction

During 2019 Indonesia reached 51.8 million tons of CPO. From this source, it is processed into green fuel. Before becoming Green Fuel, CPO is processed first to become Refined Bleached Deodorized Palm Oil (RBDPO) next processed utilising the Fluid Catalytic Cracking method ( FCC). The FCC unit process consists of the feed injection system, Riser, riser outlet separator system, disengager/stripper, regenerator, catalyst cooler (optional), catalyst withdrawal well, catalyst transfer lines, and control systems. The main processing products for RBDPO using the FCC are Gasoline, LCO (light cycle oil) and LPG.

The FCC process is a hydrocarbon reaction between crude oil (RBDPO) and a catalyst based on Particles' Geldart Classification. The proper selection of catalyst is essential to successful residue cracking operations. The importance of magnifies as the percentage of residual oil increases in the feedstock. Several properties of the catalyst should be examined for a particular feed. The properties are, Zeolite content, Micro-activity, Rare earth content, Unit cell size, Coke selectivity, Particle size distribution, Bulk density, Thermal stability, Surface area, Pore volume and pore distribution (strippability), Attrition resistance, Metals resistance, Gasoline octane properties. Regenerator and Riser are equipment that determines the FCC's product yield. This paper describes the transport of catalysts from the regenerator to the Riser.

Hydrodynamics through the Computational Fluid Dynamics (CFD) depicts the phenomenon of catalysts in the FCC. There are several types of gas and particle simulation approaches Eulerian, MPPIC, and DEM. Each type of method has its characteristics in solving gas-particle problems. MPPIC is the best an approach with the parcel in cell where the solids model tracks the position and trajectory of computational parcels, statistical groups of particles that share the same physical characteristics (e.g. diameter and density). Different diameter particles of the same material must be defined as separate solid phases in the present formulation, each with its statistical classification. This paper applies CFDSOF as a tool to calculate the catalyst phenomenon in the FCC.

The multiphase particle-in-cell (MP-PIC) is one of the numerical methods for predicting dense gas-solids flow. The gas-phase is treated as a continuum in the Eulerian reference frame. The solids are modelled in the Lagrangian reference frame tracking computational particles. The MPPIC is a derivative of the Particle-in-Cell (PIC) method for multiphase flows. The method employs a fixed Eulerian grid, and Lagrangian parcels are used to transport mass, momentum, and energy through this grid in a way that preserves the identities of the different materials associated with the particles. The main distinction with traditional Eulerian-Lagrangian methods is that the particles' interactions are calculated on the Eulerian grid. The Eulerian-Lagrangian method and the multiphase particle in cell (MPPIC) method have been used in this study. The model of the MPPIC is a Model Collision in CFDSOF, first of all, PIC models are derived from a Liouville equation describing the time evolution of a particle distribution function.

Mathematical model MPPIC

The word particle, a single piece of material, spherical in nature, having physical characteristics that can be uniquely defined (like density, chemical composition, etc.); the word parcel indicates a statistical collection of particles of similar physical characteristics.

Conservation of Mass

The conservation of mass (or continuity equation) for the p th MPPIC parcel is given by managing the statistical particle weight, Wp, and considering its mass change, dm/dt, under a chemical effects reaction.

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Rpn is the production/consumption rate of the n th chemical species, and Np is the number of chemical species. Specifically, the right-hand side of (eq above) accounts for interphase mass transfer because of heterogeneous chemical reactions or physical processes, like evaporation. In non-chemically reactive simulations (or those without phase change), the right side of Equation (eq above) equals zero.

Conservation of Species Mass

The n th species mass conservation equation for the MMPIC parcel is given by:

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Xpn is the n th chemical species mass fraction, and Rpn is the rate of formation of species mass attributed to chemical reactions or physical processes. In non-chemically reactive simulations (or those without phase change), the right side of Equation (above) equals zero.

Conservation of Translational Momentum

The general conservation of translation momentum for the p th MMPIC parcel in i th coordinate direction is given by:

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where ܷU i is the parcel velocity, and ݃g i is the gravity body force. The first term on the right-hand side is the gravitational body force. The second term is a PIC-specific term derived from interparticle stress, described in detail in the section Interparticle Stress below. As expected, the position of a parcel is related to its velocity through:

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where Xi is the parcel position in the ݅i th coordinate direction.

Interparticle Stress

The interparticle stress variable follows the form suggested by Snider (2001). Specifically,

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where E cp indicates a pre-determined, problem-specific, close-pack volume fraction for the solids phase. P s is an empirical pressure constant relatable to the scale and unit of the problem under evaluation. B is a practical unitless exponent, usually between 2 and 5. alpha is a tiny constant (e.g. 1e-7) to assure a non-zero denominator in calculations.

Conservation of Internal Energy

The general conservation of internal energy for the p th MMPIC parcel follows the same theoretical underpinnings as DEM. The internal energy is present in terms of temperature. For an isothermal parcel (a PIC assumption),

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where Cp and ܶT are parcel specific heat and temperature (same as particle values). The first term on the right-hand side represents changes in internal energy accompanying species formation or destruction from a chemical reaction or phase change (h pn is the n the species-specific enthalpy). The last term, S p is a general source term. By using the MMPIC approaches can explain the process phenomena on the catalyst. A sliding valve controls transferring the catalyst from the regenerator to the riser in FCC plant. However, on FCC pilot-plant with a transfer pipe diameter, only 2 inches is impossible to use a sliding valve. The valve gets stuck during the opening and closing process to adjust the volume of the catalyst to the riser and the abrasion properties of the gas and catalyst that make the valve easily damaged. For this reason, this study describes the application of seal loops to regulate catalysts on a pilot plant scale FCC.

2. Simulation conditions

Transferring the catalyst from the regenerator to the riser is one of the crucial steps in the fluid catalytic cracking(FCC) process. Because the catalyst and crude oil (RPDPO) ratio determine the product's quality, the catalyst supply process must be considered, especially on the pilot-plant scale FCC plan. Figure 1 shows the FCC process diagram, in which in this paper, the sliding valve will be replaced with a loop-seal for pilot-plant scale FCC.