The issue of temperature changes when feeding propane into the distillation tower
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Dear experts: Hello, by consulting the chart of saturated vapor pressures of propane at different pressures, it was found that the temperature corresponding to 0.6 Mpa is 8℃； Therefore, before introducing propane (room-temperature propane) into the system, fill it with nitrogen to a pressure of over 0.6 Mpa ； Can it be assumed that, in theory, the temperature will not drop below 8 due to the vaporization of propane?℃ ； However, in actual operation, for example when the speed is high, temperatures lower than 5°C can be observed at the temperature sensing points of the refined version of the distillation tower, which is somewhat difficult to understand; if the feed rate is not reduced, the temperatures might drop even further. Could everyone please help analyze this? Thank you.
Although the system was filled with nitrogen to 0.6 MPa or slightly higher before introducing ambient-temperature propane feed into the tower, the nitrogen in the system was not completely displaced at the beginning of propane introduction. With the introduction of propane, a portion of it vaporizes, causing the partial pressure of propane in the system to rise gradually from 0 to a certain value (lower than the total system pressure). Therefore, the equilibrium temperature on the tray actually corresponds to the propane partial pressure in the gas phase, rather than the total system pressure; this equilibrium temperature is thus lower than the propane saturation temperature at the system’s total pressure, and the initial temperature for propane vaporization will be even lower than 5°C. For example, if the gas phase in the system that has not yet been completely replaced still contains 10% nitrogen, then the partial pressure of propane is approximately 0.6 X (1–10%) = 0.54 MPa; the corresponding saturated temperature for propane is therefore around 5°C, rather than 8°C. Once the nitrogen in the system is gradually displaced and removed, the equilibrium temperature on the tray also rises to the normal equilibrium temperature.
Thank you very much for sharing this. It seems that it’s not possible to calculate things in exactly this way; during the material introduction process, the system is filled with nitrogen at a pressure of 0.6 MPa. Then propane in liquid form is introduced. If we calculate based on the partial pressure of propane, its partial pressure will be 0 at the beginning, which would mean that the temperature would drop to minus 100°C or even lower. Even at room temperature for propane and the ambient temperature of the distillation tower itself, it can’t withstand the heat exchange due to propane vaporization, right?
The cooling capacity (or heat absorption) of that amount of propane that vaporizes initially upon its introduction is not sufficient to reduce the temperature of the nitrogen in the system, as well as the heavy tower equipment itself, its internal components, and the insulation materials, to very low levels. As the amount of propane vaporized increases and the mixture of propane and nitrogen in the system is displaced and removed, the partial pressure of propane in the gas phase of the system gradually rises, and accordingly, the vaporization equilibrium temperature of propane also increases. In short, during the initial stage of propane introduction, the system is in a unsteady state; parameters such as temperature and composition change over time, so it is not possible to analyze the situation using the concepts of steady state and equilibrium.