MFIX

MFIX (Multiphase Flow with Interphase eXchanges) is a general-purpose computer code developed at the National Energy Technology Laboratory (NETL) for describing the hydrodynamics, heat transfer and chemical reactions in fluid-solids systems. It has been used for describing bubbling and circulating fluidized beds and spouted beds. MFIX calculations give transient data on the three-dimensional distribution of pressure, velocity, temperature, and species mass fractions. MFIX code is based on a generally accepted set of multiphase flow equations. The code is used as a "test-stand" for testing and developing multiphase flow constitutive equations.

Consider a fluidized bed coal gasification reactor, in which pulverized coal is fed into the bottom of a vertical, cylindrical reactor tube along with air and steam. As the coal particles rise in the tube and are heated, they bump into each other, into the sidewalls of the reactor, and into hot gas molecules in the air.  Chaos prevails. Reactions between the coal particles and gases produce the desired syngas combination of CO and hydrogen, but also carbon dioxide, sulfur and nitrogen oxides, and various hydrocarbons. Reaction products change if you modify the ratio of air to coal volumes, the particle size of the coal, the velocity of the air, the temperature or pressure of the reactor, the diameter of the reactor tube, or other variables.

How to optimize such a chaotic process? Experimentally, an engineer could carefully change one variable at a time, run the reactor for a day or two, and monitor the reaction products that emerge from the process. But this can be difficult, especially if you’re at the stage where you’re trying to determine the optimum diameter of the reactor—not an easy thing to change. Computationally, you can easily change any variable—including the tube diameter—and let the supercomputer and the MFIX software simulate the experiment.  Furthermore, you’ll end up with more detailed data. MFIX can tell you the position, velocity, temperature, pressure, and chemical composition of each tiny volume (called a computational cell) inside the gasifier every few seconds.  In one simulation, the NETL researchers divided a small region of the gasifier into 12 million of these computational cells to gain a high-resolution picture of the region. By collecting each of these “snapshots” of the state of the reactor and looking at them sequentially using visualization software, you can watch a movie of the simulated experiment, and see how the process changes with time. The main goal here is simulate high-efficiency, near-zero emissions processes to evaluate proposed system design and performance.

MFIX’s versatility has found applications in such diverse areas as volcanology, nuclear fuel particle coating, polyethylene production, and fluid catalytic cracking—anywhere solids and fluids (liquids and gases) come into contact.