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Fluidized Bed Reactor (Gas-Solid) CFD Simulation

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The problem is going to simulate a two-phase flow (gas-solid) inside a fluidized bed reactor with simple geometry.

 

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Description

Fluidized Bed Reactors

A reactor is a device for performing chemical reactions (conversion, composition, decomposition, etc.) using catalysts and, as a result, the conversion of raw materials into desired products. We can build reactors on both large-scale industrial and small-scale laboratory scales. Due to the economic aspects in the production of reactors, it is necessary to design reactors with higher efficiency but lower cost and lower energy consumption; therefore, in the design of reactors, parameters such as volume, temperature, pressure, particle concentration, particle residence time, heat transfer coefficient, and reaction rate are important.

Chemical reactors have different classifications in various respects, including continuous and discontinuous reactors (in terms of continuity in the entry of reactive materials and output of products), homogeneous and heterogeneous reactors (in terms of phases involved in the reaction), pipe and tank reactors (in terms of reactor housing structure), and fixed bed and fluidized bed reactors (in terms of the behavior of the solid particles in the reactor).

Fluidized Bed Reactors

In fixed bed reactors, the solids are stationary inside the reactor as a catalyst, and the reactants pass through these materials, reacting and leaving the reactor, while in fluidized bed reactors, the solids are as reactants or catalysts, suspended in the flow under pressure.

The fluidized bed type reactor has advantages such as high heat transfer rate and high mass transfer rate, need for lower heat transfer surface, uniform temperature distribution, proper temperature control, as well as complete and rapid mixing of reactors and catalysts. In fact, the main advantage of the fluidized bed type reactor in comparison with the fixed bed type is the ability to control the temperature and prevent the formation of hot spots, which is necessary for any reaction. Fluidized bed reactors have many industrial applications, including the petrochemical, chemical, electricity generation, incinerator, etc.

The following figure shows a schematic of a fluidized bed reactor.

fluidized bed

Project Description

The problem is going to simulate a two-phase flow inside a fluidized bed reactor with simple geometry. Since the performance of reactors is inherently based on the mixing of flows and particles as reactants and catalysts in the chemical reaction process, a multi-phase model has been used to define the fluid flow within the model. Also, considering that the present model is related to simulating the suspension of solid particles within a fluid flow like a fluidized bed model, choosing the Eulerian multi-phase flow model is the most appropriate option.

Therefore, the model includes a gas flow defined in the software as the primary phase with a density of 21.56 kg.m-3 and a viscosity of 0.00001081 kg.m-1.s-1 and suspended solids defined as a secondary phase with a density of 910 kg.m-3 and the viscosity is 0.000017894 kg.m-1.s-1. The simulation process is such that in the initial state inside the chamber, the solid suspended particles are only up to a height of 20 cm from the chamber bottom with a volume fraction of 0.63 inside the gas flow. The operating pressure of the system is defined as 1400000 pa.

The gas flow enters vertically and upwards at a speed of 0.3 m.s-1, while no solid particles enter the reactor. The simulation process was performed over a period of 4 s with a time step of 0.001 s. The aim of the present study is to investigate the behavior of suspended solid particles in the gas flow over time as well as the pressure drop generated from the reactor inlet to the outlet.

Geometry & Mesh

The 2-D geometry of the present model is generated using Design Modeler software. The geometry of the model consists of a simple rectangle with dimensions of 33 and 90 cm, which has fixed walls on both sides and the direction of flow from the entrance to the exit of the model is upwards. The figure below shows an overview of the model’s geometry.

fluidized bed

The meshing of the present model has been done using ANSYS Meshing software. The mesh type is structured and the element number is 24,750. The figure below shows a view of the mesh.

fluidized bed

CFD Simulation

To simulate the present model, several assumptions are considered, which are:

  • The solver is Pressure-Based.
  • Simulation has only examined fluid behavior; in other words, heat transfer simulation has not been performed.
  • The present model is unsteady in terms of time because the nature of the model is such that the behavior of the particles in the model changes over time.
  • The effect of gravity is considered to be 9.81 m.s-2 along the y-axis in the present model.

The following is a summary of the steps for defining a problem and its solution:

Models
k-epsilon Viscous model
standard k-epsilon model
standard wall function near-wall treatment
mixture turbulence multiphase model
eulerian Multiphase model
implicit formulation
gas and particles phases
implicit formulation
Boundary conditions (Fluidized Bed)
velocity-inlet Inlet
0 pascal supersonic/initial gauge pressure mixture
0.3 m.s-1 velocity magnitude gas
1 volume fraction
0 m.s-1 velocity magnitude particles
0 volume fraction
Pressure outlet Outlet
101325 pascal gauge pressure mixture
1 backflow volume fraction gas
0 backflow volume fraction particles
wall right and left wall
stationary wall wall motion
Solution Methods (Fluidized Bed)
Phase Coupled SIMPLE   Pressure-velocity coupling
PRESTO pressure Spatial discretization
first order upwind momentum
first order upwind turbulent dissipation rate
first order upwind turbulent kinetic energy
modified HRIC volume fraction
Initialization (Fluidized Bed)
Standard, patch Initialization method
0 m.s-1 gas velocity (x)
0.3 m.s-1 gas velocity (y)
0 pascal gauge pressure
0 m.s-1 particle velocity (x,y)
0 particle volume fraction
0.63 particle volume fraction patch

Fluidized Bed Reactor Results

After the solution process is complete, two-dimensional contours related to the mixture pressure, gas flow velocity and suspended solids, and volume fraction of the gas flow and solid suspended particles from zero to four seconds are obtained. Also, the amount of pressure in the inlet and outlet sections and as a result the pressure drop in the model is obtained at different times and is presented in the form of a pressure drop as a function of time.

Fluidized Bed Reactor Validation

In the present model, the amount of pressure drop is obtained from the input to the output section, so that the amount of pressure drop is equal to the static pressure difference between the inlet and outlet. The amount of pressure drop at different times (every 0.2 seconds) was obtained using facet average and the graph of points obtained in excel was plotted and compared with the results in the article.

fluidized bed

 

All files, including Geometry, Mesh, Case & Data, are available in Simulation File. By the way, Training File presents how to solve the problem and extract all desired results.

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