Lesson 10 - Carbon Dioxide Recovery Technology

Not all plants have a stripper, but if one is present the gas is stripped (essentially distilled), to increase the purity of the CO₂. This is a form of purification.

An option for plants where the purity of the liquefied CO₂ (principally the dissolved oxygen) concentration may not be low enough for brewery use, is to use a stripper unit. This, essentially a distillation unit, will increase the concentration of the CO₂ (or reduce the concentration of the impurities).

The analogy of an ethanol distillation system is possibly something we have more experience of, so let us use that in explaining the function of a CO₂ distillation unit. In an ethanol distillation unit used to produce a concentrated ethanol-water solution (we are grossly simplifying the world of distilled products here), pure water has a higher boiling point (100 °C at sea level) than pure ethanol (78 °C at sea level). When we boil a mixture of ethanol and water, the first vapour that is boiled off is richer in ethanol. If we condense that vapour, we will have produced a liquid richer in ethanol than we originally had and that’s the concept of how we create a concentrated ethanol-water solution. So, how about a solution of CO₂ and O₂?

With a solution of CO₂ and O₂, pure CO₂ has a higher boiling point (250 K at 18 bar) than pure O₂ (130 K at 18 bar). When we boil a mixture of the two, the first vapour formed will be richer in O₂. Conversely, the remaining boiling liquid mixture will be leaner in O₂, i.e. we will have reduced the dissolved oxygen in the boiling liquid by boiling off some of the oxygen – purification!

A simplified schematic for this process is shown below.

Figure: The purification of impure liquid CO₂ by boiling

We can see two streams are coming from our boiling process: a vapour stream which is higher in oxygen and a liquid stream which is lower in oxygen.

Such a simple process is not normally efficient enough in removing sufficient quantities of oxygen from the boiling liquid. However, if we consider adding another boiling process (as shown below), then we would get an additional level of oxygen removal, i.e. better purification. Note that in this diagram, we have just one place where we only feed the liquid CO₂ from the condenser (or storage tanks) at the top of process. However, we still have the purified liquid CO₂ stream exiting at the bottom, and a single CO₂ vapour stream, which is rich in oxygen, exiting at the top.

Figure: The further purification of impure liquid CO₂ using two idealised boiling stages

If we take the idea of layering boiling processes on top of one another even further, until we get the required level of oxygen purification, then the most practical means of creating such a process is to use a packed column. This would typically have an equivalent stack upwards of ten single processes in a single column. Furthermore, we can include a single input of heat, at the bottom of the column, and have a single exit of oxygen-rich gaseous CO₂ at the top of the column, as shown in the figure below. The figure also shows – in red – the CO₂ gas feed, the incondensable vent, and the purified liquid CO₂ lines to aid our understanding.

Figure: A simplified CO₂ stripper unit