【Daily Question】Chemical Engineering Principles 183: Constant Volumetric Flow Assumption (September 25)
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Constant molar flow: In the rectifying section of a distillation column, the molar flow rate of the liquid descending from each tray is equal; the same holds true in the stripping section, but these values need not be identical. Constant-molar vapor flow: In the rectifying section of a distillation column, the molar flow rate of vapor rising from each tray is equal; the same holds true in the stripping section, but these values need not be identical in the two sections.
In a constant-mole-flow gas stream, during distillation operations, within the distillation section of the distillation column, the molar flow rate of the rising vapor is equal at each tray. The same holds true in the stripping section as well.
Constant molar flow rate: In the rectifying section of a distillation column, the molar flow rate of the liquid descending from each tray is equal; the same holds true in the stripping section, but these values need not be identical in the two sections. Constant molar flow rate of vapor: In the rectifying section of a distillation column, the molar flow rate of vapor rising from each tray is equal; the same holds true in the stripping section, but these values need not be equal in the two sections.
Constant molar flow rate: In the rectifying section of a distillation column, the molar flow rate of the liquid descending from each tray is equal; the same holds true in the stripping section, but these values need not be identical in the two sections. Constant molar flow rate: In the distillation section of a distillation column, the molar flow rate per liter of vapor is equal; the same holds true in the stripping section, although these values need not be identical.
Assumptions of the constant molar flow hypothesis: ① Constant molar vapor flow: During distillation, in the distillation section of the distillation column, the molar flow rate of the vapor rising through each tray is equal. The same holds true for the stripping section. However, the molar flow rates of the vapor in these two sections are not necessarily equal. That is: V1 = V2 = ... = Vn = V, while V1’ = V2’ = ... = Vm’ = V’. Here, V represents the molar flow rate of the vapor rising in the distillation section (kmol/h), and V’ represents the molar flow rate of the vapor rising in the stripping section (kmol/h). ② Constant molar liquid flow: During distillation, in the distillation section of the column, the molar flow rate of the liquid falling through each tray is equal. The same applies to the stripping section as well. Yet, the molar flow rates of the liquid in these two sections are not necessarily equal. That is: L1 = L2 = ... = Ln = L, while L1’ = L2’ = ... = Ln’ = L’. Here, L represents the molar flow rate of the liquid falling in the distillation section (kmol/h), and L’ represents the molar flow rate of the liquid falling in the stripping section (kmol/h). For most organic homologues and many similar ideal systems, the molar heat of vaporization of various components differs little from one another. Therefore, it can be assumed that the molar heat of vaporization of the mixture is independent of its composition, and that this value is approximately constant across all trays. The change in the enthalpy of the liquid due to variations in composition, resulting from changes in the bubble point, is relatively large compared to its molar heat of vaporization and can thus be ignored. Hence, it can be assumed that the molar enthalpy of the liquid phase is approximately constant.
The molar vaporization enthalpies of the volatile and non-volatile components are equal, while other heat effects can be ignored or cancel each other out; as a result, the heat required for liquid vaporization and gas condensation exactly compensates for one another, ensuring that the molar flow rates of the liquid and gas phases passing through each tray remain constant.
Constant molar flow rate: In the distillation section of a distillation column, the molar flow rate of the liquid descending from each tray is equal; the same holds true in the stripping section, but these values need not be identical in the two sections. Constant molar flow rate of vapor: In the rectifying section of a distillation column, the molar flow rate of vapor rising from each tray is equal; the same holds true in the stripping section, but these values need not be equal in the two sections.
The so-called constant vapor and liquid flow assumption means that the amount of vapor rising on each tray is equal, and the amount of liquid falling is also equal.
Constant-molar vaporization: The molar flow rate of the vapor rising per tray in the rectifying section of the distillation column is equal, and the same holds true in the stripping section. Constant-molar overflow: The liquid flow rate per tray decreases equally in the rectifying section of the distillation column, and the same holds true in the stripping section. Condition: The molar vaporization of the components is equal or similar ； The sensible heat transferred due to temperature differences when the vapor and liquid phases come into contact can be ignored ； The heat loss of the tower is negligible.
In the rectifying section of a distillation column, the molar flow rate of the liquid descending from each tray is equal; the same holds true in the stripping section, although these values need not be identical. Constant-molar vapor flow: in the rectifying section of a distillation column, the molar flow rate of the vapor rising from each tray is equal, and the same applies to the stripping section as well, though again these values may not be equal.