Mass balancing is a fundamental part of bioprocessing that is used to determine the amount of a component coming in or out of a system. A process stream is usually made up of multiple components. For example, feed entering the reactor will contain nutrients, water, growth hormones, etc. whilst the stream leaving the reactor will contain product, cells, waste (ammonia, carbon dioxide) and many other components!
We want to know how much of each component is entering, leaving or inside the reactor or system so that we can characterise the process and understand it better. However, with so many reactions occurring in biological processes, we won’t always know how much of each component there is off the top of our head! This is where mass balancing comes in.
Mass balancing is the calculation of what is going in or out of the system based on what information we have. For example, if we know x amount of cells and x amount of feed are going into the reactor, what is the amount of product and waste produced?
Remember: we need to know one side of the equation (in or out) to figure out the other!
In broader terms, mass balances refer to the balancing of mass going in and out of a system. A system can be anything that has a clearly defined boundary that separates it from the external environment. For example, a cell has a cell membrane, a water bottle has plastic walls, a bioreactor has metal walls. There are 3 main types of systems:
- Open: Allows energy and matter exchange freely with external environment
- Closed: Only allows energy exchange freely with external environment
- Isolated: Does not allow energy or matter exchange with external environment
In most bioprocesses, the system (bioreactor) is closed – matter cannot be exchanged freely with the surroundings. When matter is exchanged in the reactor, it is because we intentionally add or remove components.
Remember: matter cannot disappear! It can only be transferred. Therefore when we add a component to a system we expect to either stay in the system, come out or be transformed into another component.
Now that we know what mass balances are, lets move on to how to do them! If this post wasn’t clear, please leave a comment and I will try to explain it in simpler terms.
Bioprocessing Explained 