Propylene refrigeration
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Why is the temperature at the cryogenic station for low-temperature methanol washing -20 degrees, while the gaseous propylene coming out of the low-temperature methanol washing process is at -40 degrees? According to the law of conservation of energy, liquid propylene absorbs heat and turns into a gas; therefore, the temperature of the gaseous propylene coming out of the low-temperature unit should be above -20 degrees, right? So why is it -40 degrees?
One can’t just look at temperature; pressure also needs to be considered
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You only considered the sensible heat and not the latent heat; a large amount of heat is absorbed during the vaporization of a liquid, and both the liquid and the gas experience a decrease in temperature. The refrigeration plants used in low-temperature methanol washing systems that I’ve come across before use an ammonia-ammonium water cycle; I’ve never seen an propylene cycle used in such systems, and their operating costs are likely lower than those of ammonia-based systems
Latent heat is the source of the coolness. For ammonia cooling or propylene cooling, it depends on one’s own products: propylene is more cost-effective for olefin plants, while ammonia is obviously the better choice for ammonia synthesis.
Liquefying propylene requires mechanical pressure and cooling for condensation, right? Ammonia can be absorbed by water, then reheated to produce high-pressure steam, which is subsequently condensed to yield a liquid. The cost of heating is much lower than that of mechanical compression
1. Throttling refrigeration. 2. Phase transition.