F=D+W, FXF=DXD+WXW – is it necessary to convert these values to moles for calculation?
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For some problems, if mass conservation is used directly, the answer will differ slightly from that obtained by converting to moles, and I’ve never been able to figure out why. Seeking help from experts
In chemical reactions, mass is conserved, but this is not necessarily true for moles; the change in molecules before and after a chemical reaction equation represents the change in moles. Processes without chemical reactions are physical changes; in such cases, mass is conserved, and moles remain unchanged as well.
There are no chemical reactions; for example: continuous atmospheric distillation column: toluene (A), benzene (B). Slurry: 20000 kg.h-1, wA=0.62, wB=0.38. Requirements for the distillate: It should be able to recover 0.97 of the benzene present in the raw material. Requirements for the residue remaining in the reactor: The amount of benzene there should not exceed 0.02. Determine the flow rates and compositions of the distillate and the bottom stream.
The premise for deriving and establishing this formula is a constant molar flow; therefore, all formulas in this series must be expressed in terms of molar quantities. Additionally, it is recommended to go through the textbook from scratch first, based on the principles of chemical engineering.
F = D + W, FXF = DXD + WXW; these two formulas are essentially material balance equations. Distillation is a physical process without any chemical reactions. When mass is used, mass units should be applied (with X representing the mass fraction), and when moles are used, mole units should be used (with X representing the molar fraction). As for the difference you mentioned, it is likely due to variations in the selection of significant figures or in the values of molecular weights during conversion; neither approach is correct or incorrect. Additionally, the author believes that the explanations regarding other constant-mole assumptions are unnecessary.
There is no need to necessarily switch to mole units. Normal distillation is considered to involve no chemical reactions, thus mass conservation holds. In = Out. In my opinion, switching to mole units is done for subsequent calculations; the common unit for heat is kJ/mol.
Moles or mass are both fine; as long as they are consistent. Use the same unit for flow rate as it is for concentration
This conservation principle applies to both mass-based and molar-based calculation methods, with no objections; it should be noted that the values need to be converted to a unified format during calculation
This conservation principle applies to both mass-based and molar-based calculation methods, with no objections; it should be noted that the values need to be converted to a unified format during calculation