Single cell protein production in U-loop bioreactors: Fundamentals, Modeling & Control

In the years to come, the number of persons that inhabit planet earth is going to continue to increase, and with the increase in population, an equal rise in consumption of animal protein is expected to follow. As conventional livestock farming already consumes a large degree of the earths arable land, which is unlikely to change -let alone expand- in the future, it is desirable to tap into the pool of unconventional raw materials in the context of livestock feed.
One option with a lot of promise is to convert methane gas, an otherwise potent greenhouse gas, into a sustainable protein supplement for livestock nutrition. This is possible through a microbial fermentation process with methanotrophic bacteria known as single cell protein pro-duction (SCP). Through this approach it is not only possible to turn a potent greenhouse gas into a sustainable product, but also to shift a large part of livestock feed production away from conventional farming onto the industry, freeing up large areas of precious land.
In this dissertation some of the obstacles in producing SCP at a commercial scale are out-lined with the overall goal of control and automation of the process in mind. Mass transfer of poorly water soluble gasses (oxygen and methane), and a limited knowledge of the microbial metabolism are identified as the primary bottlenecks preventing commercial production.
An unconventional bioreactor named ”the U-loop fermentor” after its ”U” like shape is proposed as a solution to the mass transfer problem. It is discovered, after rigorous testing of power transmission, mixing time and mass transfer rate in two U-loop reactors of 0.15m³ and 2.2m³ working volumes respectively, that the rate of mass transfer from a gas phase to a liquid is both fast and energy efficient if the right geometry is applied in the reactor construction phase.
Next the primary and co-metabolic processes in the microorganism of choice Methylococcus capsulatus was studied through ammonia pulse experiments. It is discovered that Methylococcus capsulatus is capable of executing a full nitrification cycle and oxidize ammonia all the way to nitrate, not just nitrite as previously believed. More importantly however, is that the observed metabolism could be modeled with a first (bio)principles model, with a high degree of accuracy.
Finally the knowledge gained on the reactor performance and on the microorganism was comable size finite volume approach is applied to the ”U” part of the reactor model. The model was combined with knowledge of feed and effluent streams in an analysis of system observability and controllability with promising results for future control algorithm design.