(261g) Modeling, Simulation and Optimization of Single-Cell Protein Production in a U-Loop Reactor

With the increasing world population, the demand for proteins increases. Proteins and amino acids are essential for growth of humans and animals. Proteins cannot be substituted by other food components. Soya beans and fish in the form of fish meal have been important sources of protein for animal feed. In turn, the animals have served as protein sources in human nutrition. However, with the diminishing fish reserves in the ocean, it will be a challenge to sustain the protein demand of an increasing population. Technological advancements must provide new ways to synthesize proteins and produce them in a cost efficient manner.

The microorganism Methylococcus capsulatus can grow on cheap carbon sources such as methane and methanol. The protein content of M. capsulatus is approximately 70% on a dry mass basis. This protein is called Single Cell Protein (SCP) as it is produced in single cells. In addition to methanol, the process requires oxygen and sources for nitrogen and minerals. We use nitric acid as the nitrogen source. Consequently, proteins for animal feed may be produced by fermentation of M. capsulatus using methanol, oxygen, nitric acids, and minerals as the feed stocks. Finally, the cells are hydrolyzed to improve digestibility of the proteins.

Unlike many other bio-chemicals, Single Cell Protein is a commodity. Therefore, it is essential that the process equipment for manufacturing of Single Cell Protein is energy efficient and able to utilize the raw materials with a high yield. The principle problems facing the manufacture of Single Cell Protein is transfer of oxygen to liquid phase and removal of the high amount of heat produced by the exothermic process. Conventional stirred tank fermenters cannot provide sufficient mass transfer of oxygen nor sufficient area for removal of the heat produced. The U-loop reactor is a reactor designed to have high degrees of gas-liquid mixing and heat removal. A U-loop reactor pilot plant has been build at the Technical University of Denmark. The legs (the u-loop) of the reactor are equipped with static mixers to have high gas-liquid mass transfer rates. The u-loop is also equipped with a heat exchanger for removal of the extensive heat produced by the fermentation. The tank on the top of the legs is essentially a degassing unit. It is used to separate the produced CO2 from the liquid. Furthermore, as long as the substrates methanol, oxygen, nitric acid, and minerals) are present, the micro organism will also grow in the tank at the top. The U-Loop reactor at DTU is used for production of SCP based on methanol, oxygen, nitric acid (potassium nitrate), and minerals.

Based on experimental data for the U-Loop reactor at DTU, we develop a distributed dynamic mathematical model for SCP production using M. capsulatus. The model describes evolution of biomass, substrate, dissolved oxygen, and gaseous oxygen in the system. This model is used for static and dynamic simulations. Static simulations of SCP production using M. capsulatus in an U-loop reactor have been used to determine the optimal operation. The optimal operating point is located close to both washout and oxygen limitation. With oxygen being the most expensive reactant, the U-Loop reactor is operated in the oxygen limited mode and substrate feed is controlled according to the oxygen feed. The maximum oxygen feed is determined by the maximum biomass concentration that can be tolerated in the reactor. Higher biomass concentration is believed to give a more viscous fluid that requires more pumping energy for circulation in the U-loop and has a lower gas-liquid oxygen transfer rate. The optimal dilution rate is relative constant around 0.2 1/hr. This operating strategy gives the highest SCP productivity, approximately 6 kg biomass/m3/hr.