How to understand the difference between single-stage heat pumps and two-stage heat pumps

I haven’t seen the process, but judging from the description of the UOP propane-propylene tower and the picture, it is estimated that there are some parameter requirements for the propane-propylene tower when choosing different heat pump processes (direct overhead gas compression heat pump or tower split heat pump process). Back to the question, why is the tower pressure of a single-stage heat pump higher than that of a two-stage heat pump? This needs to be discussed in conjunction with the specific process. ； From a mechanical perspective, there are two aspects: one is to meet the heat source requirements of the reboiler, and the other is that the compressor outlet gas must be condensed by (water) ； If it is a first-level heat pump, the tower pressure is low, the first-level heat pump compressor pressure ratio is also low, and the compressed gas cannot be condensed by water, while the second-level heat pump has a higher compressor pressure ratio (note: The power of the compressor will also increase), which can meet the reboiling and condensation conditions, so the tower pressure can be reduced. This is also confirmed by the pressure value in the picture.

I agree with the explanation above. The main thing is to consider that the compressor outlet gas can be cooled down by water, because the heat pump is mainly adjusted by the temperature-adjusting condenser.