On the crystallization of solid formers during liquefaction of gases

Abstract

Besides well-known technologies for the liquefaction of air components (N2, Ar, and O2) that of CH4, H2, and CO2 has recently caught attention for the energy transition and climate change targets. Gases undergoing cryogenic processes (cooling, liquefaction, cryo-compression) can contain a certain number of impurities, thus one challenge encountered in liquefaction is the solidification of heavy compounds present in the feed. In dealing with the crystallization risk of solid formers during liquefaction, attention is nowadays addressed to their solubility limits in the liquefied gas at the lowest temperature of the process, because solubility is known to decrease for decreasing temperatures. These limits are then used for tailoring the purification steps upstream the liquefaction train or modifying the operative conditions of the plant to avoid solid formation. In this work, the modeling and the analysis of the available data of several binary mixtures (like H2+N2, N2+CO2, CH4+CO2, and CO2+nC10H22) allow drawing insights about the thermodynamic behavior of such mixtures at low temperatures, showing that the lowest solubility limits are not always encountered at the lowest temperature, and this may have an impact on the design of industrial liquefaction processes. According to the liquefaction pressure, the first risk of solidification of the heaviest component could indeed be related to the deposition from the vapor phase rather than the solidification from the liquid phase. As a matter of fact, solubility limits of a solid former could be lower in a warmer gaseous/supercritical solvent than in a colder liquid solvent, thus solid formation could occur at temperatures that are up to tens of degrees higher than the final liquefaction temperature. Consequently, front-end purification units reducing the content of solid formers in the feed below their solubility limits in the liquefied gas could not be sufficient to completely avoid the risk of crystallization as temperature decreases.