Modeling and Simulation of a Bubble Column Reactor for Single-Cell Protein Production from Methane
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The methane fermentation of the bacterium Methylococcus capsulatus (Bath) to produce single-cell protein has been studied by the use of mathematical modeling and simulations. Protein production by fermentation can substitute corn or soya in conventional animal feed and is a sought-after product due to the high protein content. An axial dispersion model describing a bubble column has been implemented in MATLAB and solved numerically by orthogonal collocation. The bubble column reactor model showed increased biomass flow rate and biomass reaction rate for decreased values of the superficial liquid velocity and increased values of the superficial gas velocity. The optimum inlet gas ratio was found to be a gas mixture of 26.5% methane and 73.5% oxygen on mass basis. The reaction is mass transfer limited and increased mass transfer rate is desired to increase the biomass production and utilize as much of the gaseous substrates as possible. Vertical internal rods sparging the gaseous substrates, or several gas inlets are modifications that can increase the production throughout the column. The overall single-cell protein plant design has been studied and modifications based on the model simulations have been made. The simulations done in Aspen HYSYS are the basis for calculating energy usage and thereby performing a simple energy integration to obtain maximum energy recovery. The process equipment is cost estimated and the total investment, operating costs and revenue are used to find the process profitability. This has been done on the basis of re-sizing the reactor to increase the size of the plant. With a market price of 1.1 $/kg and a biomass productivity of 10.9 kg/(m^3h), the process will not be economical sustainable based on the current estimations presented.