Phase Equilibrium Data of Tetrabutylurea, Tetramethylurea, and Tetramethylthiourea/Carbon Dioxide at Pressures up to 200 bar at 313.15 and 333.15 K

High-pressure phase equilibrium (vapor-liquid and solid-supercritical fluid phase equilibria) data for three binary systems were determined at two temperatures (313.15 and 333.15 K) and at pressures of carbon dioxide of up to 250 bar for tetrabutylurea/carbon dioxide, up to 200 bar for tetramethylurea/carbon dioxide, and up to 110 bar for tetramethylthiourea/carbon dioxide using a variable-volume high-pressure optical view cell. The measurements have been performed for all of the three systems at a constant temperature with increasing pressure. Furthermore, a melting point curve (solid-supercritical fluid phase equilibrium) of the solid sample tetramethylthiourea was determined using the modified capillary method in a high-pressure optical view cell at a pressure of carbon dioxide of up to 300 bar. The highest mutual solubilities for these binary systems under the same process conditions were in the following order: tetrabutylurea/carbon dioxide > tetramethylurea/carbon dioxide > tetramethylthiourea/ carbon dioxide. The results determined with the modified capillary method showed that at a pressure higher than 10 bar, the liquefaction temperature decreased significantly.