Abstract: The industrial fluidized bed reactor (FBR) described here is designed to convert an aqueous solid laden stream into a consistent granular product. The FBR is heated to 650 °C and chemically reducing conditions are achieved by using steam as the fluidizing gas and coal as a source of carbon. The objective of this study is to use the MFiX (Multiphase Flow with Interphase eXchanges) two-fluid model as a screening tool to investigate the effects of off-nominal conditions to establish performance guidelines. This information will be helpful in assessing whether the identified boundaries of the system are steep or gradual. Initially, 13 independent operating parameters were sampled in a one-at-a-time manner using high, low, and base values and ranked against a set of performance criteria indicative of defluidization. Five operating parameters were found to have the largest effect (bed particle size, bed particle density, coal particle size, spray feed flow rate, and fluidizing gas flow rate) on three quantities of interest—bed differential temperature, low solids velocity, and bed voidage. Latin-hypercube sampling was used to generate a minimal number of random values for various combinations of input parameters. The spray feed flow rate was the most significant parameter affecting the temperature differences, followed by the coal particle and bed particle size. The bed particle size had the largest effect on the low velocity of the bed particles, with the coal particle size as a contributing second effect. The high solid packing shows the coal particle size with the largest effect, and significant secondary contributions from the feed flow rate and fluidizing gas flow rate. The fits to the five-dimensional Gaussian Process models were 0.7797, 0.8664, and 0.9440 for the temperature, velocity, and solids packing, respectively.