Lesson 11 - Carbon Dioxide Recovery Technology SAQ

*/**An employee is conducting an audit of the CO₂ recovery plant to identify running cost reduction opportunities. The employee has identified that the costs of compressing the CO₂ to 14-20 bar a are significant and is proposing to reduce the compressor discharge pressure to 5 bar a. Extensive cost calculations support the case. How would you go about critiquing their proposal?

Response:

There is more to this question than it might seem. In fact, it is not about cost savings, but rather, is to test your understanding of the significance of the triple-point for CO₂. The triple-point pressure is 5.2 bar a. If we cool CO₂ gas to any temperature below this pressure, we will not condense it. Any attempt to reduce the compressor discharge pressure to 5 bar a will make it impossible to condense the gas later.

But what would happen if we dropped the compressor discharge pressure to just above the triple-point, perhaps 6 bar a, could we still condense the gas? The answer is that we could, and in all likelihood, there would be a compressor running cost saving as well. To do this, we would have to cool it to around 220 K, compared to the more typical 240-250 K. This would incur a running cost increase for the condenser (i.e. a running cost increase of the ammonia refrigeration plant). However, such a compressor saving is likely to be wiped out by the increased ammonia plant costs. The CO₂ plant and ammonia refrigeration plant are very much intertwined. Any proposed changes to one must be carefully considered so that they do not adversely affect the other.