The MFIX (Multiphase Flow with Interphase eXchanges) computer model is a general-purpose hydrodynamic model used to simulate chemical reactions and heat transfer in dense or dilute fluid-solids flows, commonly found in energy conversion and chemical processing reactors. MFIX provides detailed information on pressure, temperature, composition, and velocity distributions within reactors, enabling engineers to visualize reactor conditions, conduct parametric studies, and assist in design processes. The model includes mass and momentum balance equations for gas and multiple solids phases, energy equations for gas and solids phases, species balance equations, granular stress equations, and user-defined chemistry subroutines. It supports three-dimensional Cartesian or cylindrical coordinate systems, nonuniform mesh sizes, and various boundary conditions. The model also includes extensive error reporting and post-processor codes to animate results and manipulate data. MFIX is based on hydrodynamic theory, which includes conservation equations, constitutive relations, and initial and boundary conditions. The model accounts for fluid-solids and solids-solids momentum transfer, heat transfer, and granular energy dissipation. It uses a mixture theory approach to derive multiphase flow equations, assuming phases are interpenetrating continua. The model incorporates constitutive relations such as fluid-phase equations of state, stress tensors, and drag correlations. It also includes a granular stress formulation based on kinetic theory and frictional flow theory. The model has been used to simulate various fluidization conditions, including circulating fluidized-bed reactors, fluidized beds with immersed heat transfer tubes, and fluidized beds with filters. It has been validated against experimental data and has been used to study increasingly complex fluidization conditions. The model's capabilities include handling multiple solids phases, accounting for particle segregation, and simulating granular energy transfer. It also includes a user-friendly input data file and multiple output files that minimize disk storage and accelerate data retrieval. The model's hydrodynamic theory is based on fundamental laws of mass, momentum, energy, and species conservation. It has been developed and refined over several years, incorporating various numerical techniques and constitutive relations. The model's accuracy is limited by factors such as incomplete formulation of governing equations, insufficient knowledge of constitutive relations, and numerical treatment of partial differential equations. However, the model's ability to synthesize data from various experiments and predict reactor conditions makes it a valuable tool for industrial reactor design. The model requires significant computational resources but is more affordable with the availability of faster and cheaper computers. It is designed to be used by expert users who can conduct simulations and analyze results, with detailed manuals and post-processor codes to assist in its use.The MFIX (Multiphase Flow with Interphase eXchanges) computer model is a general-purpose hydrodynamic model used to simulate chemical reactions and heat transfer in dense or dilute fluid-solids flows, commonly found in energy conversion and chemical processing reactors. MFIX provides detailed information on pressure, temperature, composition, and velocity distributions within reactors, enabling engineers to visualize reactor conditions, conduct parametric studies, and assist in design processes. The model includes mass and momentum balance equations for gas and multiple solids phases, energy equations for gas and solids phases, species balance equations, granular stress equations, and user-defined chemistry subroutines. It supports three-dimensional Cartesian or cylindrical coordinate systems, nonuniform mesh sizes, and various boundary conditions. The model also includes extensive error reporting and post-processor codes to animate results and manipulate data. MFIX is based on hydrodynamic theory, which includes conservation equations, constitutive relations, and initial and boundary conditions. The model accounts for fluid-solids and solids-solids momentum transfer, heat transfer, and granular energy dissipation. It uses a mixture theory approach to derive multiphase flow equations, assuming phases are interpenetrating continua. The model incorporates constitutive relations such as fluid-phase equations of state, stress tensors, and drag correlations. It also includes a granular stress formulation based on kinetic theory and frictional flow theory. The model has been used to simulate various fluidization conditions, including circulating fluidized-bed reactors, fluidized beds with immersed heat transfer tubes, and fluidized beds with filters. It has been validated against experimental data and has been used to study increasingly complex fluidization conditions. The model's capabilities include handling multiple solids phases, accounting for particle segregation, and simulating granular energy transfer. It also includes a user-friendly input data file and multiple output files that minimize disk storage and accelerate data retrieval. The model's hydrodynamic theory is based on fundamental laws of mass, momentum, energy, and species conservation. It has been developed and refined over several years, incorporating various numerical techniques and constitutive relations. The model's accuracy is limited by factors such as incomplete formulation of governing equations, insufficient knowledge of constitutive relations, and numerical treatment of partial differential equations. However, the model's ability to synthesize data from various experiments and predict reactor conditions makes it a valuable tool for industrial reactor design. The model requires significant computational resources but is more affordable with the availability of faster and cheaper computers. It is designed to be used by expert users who can conduct simulations and analyze results, with detailed manuals and post-processor codes to assist in its use.