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Two-fluid models for fluidized bed reactors: Latest trends and challenges Yassir Makkawi Chemical Engineering. Contents . Two-fluid model for multiphase flow simulation Limitations and challenges Example of results Conclusion and recommendations. Two-fluid model.

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slide1

Two-fluid models for fluidized bed reactors: Latest trends and challenges

Yassir Makkawi

Chemical Engineering

contents
Contents
  • Two-fluid model for multiphase flow simulation
  • Limitations and challenges
  • Example of results
  • Conclusion and recommendations
two fluid model
Two-fluid model
  • Mathematical formulation to describe the interaction of two fluids by treating the phases as interpenetrating continua
  • e.g. solid momentum
  • Kinetic energy
    • PDE :
    • Algebraic :

Gas-solid drag

fluid

Solid-solid drag

Solid stresses

Solid-solid energy exchange

l imitations
Limitations
  • Slightly wet or cohesive particles
  • Intermediate flow
  • Poly-dispersed particles
  • Various constitutive relations
  • Adjustable parameters
  • Size change during processing
schematic of flow regimes and modelling
Schematic of flow regimes and modelling

soil mechanics principles

Kinetic theory of granular flow

slightly wet or cohesive particles

?

R

h

Slightly wet or cohesive particles

Cohesive particles

dry wet

Slightly wet particles

Kinetic+ collision contacts

Enduring contact

polydispersed mixture1
polydispersed mixture

Solution of the Energy equation

Comparison of predicted and measured cross-sectional average solid velocity for the case of a polydispersed binary mixture of glass beads (755 µm,2500 kg/m3) and wood (500 µm, 585 kg/m3) with the mixing ratio of 83 wt% to 17 wt%.

Granular temperature predicted by two different solution methods of the energy equation. Data produced with particle size of 755 µm fluidized by air at 4.7 m/s at the solid circulation rate of 36.g/s.

Positron Emission Particle Tracking (PEPT)

building a biomass gasifier model
Building a biomass gasifier model
  • 3D model is considered to simulate the gasification of Biomass using Fluent.
  • two solid phases are modelled as mixture:
    • Gas phases: O2, N2, CO, H2, CH4, CO2, tar, and H2O
    • Solid phases: Biomass mixture of C(s), volatiles and ash.
  • Sand is introduced as an inert solid phase
  • The gasification model is based on three main steps: (i) Drying (ii) Devolatilization and tar cracking (iii) Partial combustion and gasification reactions
latest trends modelling of reactive system
Latest trends- modelling of reactive system
  • Drying
    • Is modelled as mass transfer mechanism:
  • Devolatilization and tar cracking
  • Partial combustion and gasification reactions
    • Combustion reactions
    • Heterogeneous reactions
    • Homogenous reactions
building the reaction model continue
Building the reaction model- continue
  • Combustion reactions

C+0.5O2→CO

2CO+O2→2CO2

  • Heterogeneous gasification reactions

C + 2H2→ CH4

C + CO2→ 2CO

C + H2O → CO + H2

  • Homogenous reactions

CO + H2O → H2 + CO2

CH4 + H2O → 3H2 + CO

results hot flow hydrodynamics
Results: hot flow hydrodynamics
  • Gasifier operating at: Inlet sand temperature of 900 oC; ER=0.1; biomass-to-steam ratio of 0.6; biomass feed rate of 20 g/s (7.2 kg/h)
results product gas composition
Results: product gas composition
  • Steady exit gas composition at 900 oC solid inlet temperature; ER=0.1; steam-to-biomass ratio = 0.6
  • Tar content in the exit gas is 3.7 g/Nm3.
  • Contours of gas concentration in the reactor. Solid inlet temp 1200 oC, ER=0.1, steam-to-biomass ratio =0.6, biomass feed=18 kg/h.
results of parametric analysis effect of temperature
Results of parametric analysis- Effect of temperature
  • H2 content independent of operating temperature
  • CO2 decreases and CO increases with increasing temperature
  • Consistent increase in the product gas heating value (HHV) with increasing the temperature
  • The improved product gas quality (high H2 and HHV) here is due to the increase in the gasiferthroughput, which in this case: 50 g/s (18 kg/h) for biomass and 30 g/s (108 kg/h) for sand.
  • The operating temperature of ~900 oC appear to be reasonable for high quality fuel.
conclusion and recommendations
Conclusion and recommendations
  • Two-fluid modelling is so far the most reliable for the simulation of solid-gas fluidized bed reactors.
  • The development and improvement of predictive capabilities of the two-fluid model is moving at a faster pace than the alternative Discrete Element Modelling.
  • Great success in simulating complex reactive system.
  • More effort is required:
    • To reduce computational time
    • Inter-particle forces
    • Particle size distribution and physical change