The significance of bubble point reflux in distillation calculations during exams
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This post was last edited by jacques0920 on 2016-3-9 15:00. Hello everyone! In distillation calculations, the requirement for bubble-point reflux is sometimes given in the problem statement – what impact does this have on solving the problem? Thank you.
It seems that bubble point reflux is the basis for the equal-molar-flow assumption in chemical engineering principles; without bubble point reflux, it cannot be assumed that the liquid phase flow rate inside the tower is equal to the reflux flow rate. That’s what it seems to be
The definition of the bubble point is the point at which a bubble reaches equilibrium with the liquid phase, that is, full liquid-phase reflux! Dew point is the point at which a liquid droplet and the gas phase reach equilibrium.
For factories, this means that certain equipment comes from different suppliers. For example, if the design condition for your distillation system is feed at the bubble point, then for materials at normal temperature and pressure (1 atm, 25°C), the additional equipment needed to heat (cool) or increase (decrease) the pressure to reach the desired bubble point temperature and pressure will not be taken into account by the distillation system manufacturers. Conversely, the designers of the distillation system need to take this into account; they have to carry out calculations for the pressure/temperature adjustment devices and charge a design fee for that. If it is merely a student project, it may mean that there is no need to consider the selection of equipment for heating/cooling or pressure increase/decrease, nor to carry out parameter calculations——in other words, the scope of the design is limited, and accordingly the cost associated with the design is reduced. Therefore, many manufacturers, in order to save costs, are willing to make arbitrary decisions regarding details that they consider simple and not requiring design calculations, in order to reduce design expenses
If it’s not a batch reflux, then wouldn’t the assumption of equilibrium molar flow no longer hold?
Conditions under which the assumption of constant molar flow holds: 1. The molar vaporization enthalpies of all components are equal； 2. The sensible heat exchanged due to temperature differences during gas-liquid contact can be ignored ； 3. It has good heat retention, and the heat loss of the tower can be disregarded. Condition 2 applies to bubble-point reflux, requiring the liquid to return in a saturated state; under this condition, the condensation of 1 mol of vapor can approximately cause 1 mol of liquid to vaporize. Only at this point does the gas-liquid flow satisfy the constant molar flow assumption.
This post was last edited by Higee on 2017-6-5 at 16:16. In fact, whether to use reflux or not has no inherent relationship with the assumption of constant molar flow. A prerequisite for the assumption of a constant molar flow is that the molar vaporization enthalpies of the light and heavy components are approximately equal, and this has nothing to do with whether bubble backflow occurs or not; this can be understood by referring to textbooks on chemical engineering principles. So, what is the difference between bubble point reflux and non-bubble point reflux, that is, subcooled reflux? It mainly depends on whether the amount of liquid returning outside the tower is equal to the amount of liquid flowing downward in the distillation section of the tower; in other words, it’s the difference between the amounts of liquid inside and outside the tower that corresponds to the reflux ratios inside and outside the tower. It is the internal reflux ratio that actually plays a decisive role in the separation process within the tower. When reflux is below the bubble point, the reflux ratio used in the calculations is not equal to the amount of liquid reflux outside the tower divided by the amount of vapor rising from the top of the tower.
@ifgotmoney, your understanding seems a bit incorrect. In the case of bubble flow, the amount of liquid flowing down on the first plate in the distillation section, L1, is equal to the amount of reflux flowing in from outside the tower, L. In the case of non-bubble flow, L1 is not equal to L. The concept of constant molar flow means that the amount of liquid flowing down on each plate in the distillation section is the same, i.e., L1=L2=..., and it has nothing to do with whether L1 is equal to L or not
The assumptions for the full version of the constant molar flow concept are as follows: 1. Constant molar vaporization: In the distillation section or stripping section, the molar flow rate of vapor rising per tray is equal； V1=V2=V3=...=V=constant value. 2. Constant molar overflow: In the distillation section or stripping section, the molar flow rate of the liquid overflowing from each tray is identical ； L1=L2=L3=...=L is a constant value; constant-molar overflow and constant-molar vaporization are collectively referred to as the constant-molar flow assumption. (Note: L = L1.) Here’s the key point: the ultimate essence of the constant molar flow assumption is that \"it is assumed that when the gas and liquid phases come into contact, the heat released by the condensation of 1 mole of vapor is just enough to vaporize 1 mole of liquid.\" Only such a vapor-liquid flow satisfies the constant molar flow assumption. Since the gas-liquid phase leaving each theoretical plate is in a saturated state, if the reflux from the top condenser is not saturated, it necessarily fails to meet the fundamental requirement that \"the heat released by the condensation of 1 mole of vapor upon contact with liquid is just enough to vaporize 1 mole of liquid.\" Thus, it does not satisfy the assumption of a constant molar flow.
@ifgotmoney, you’re correct about the red-colored part, but it should be noted that whether it’s a constant-molar flow can only be determined after the tower operation reaches a steady state. After the distillation column is operating stably, the heat difference between the cold reflux and the bubble point reflux is ultimately provided by the reboiler at the bottom of the column; this heat is transferred through the vapor rising from each tray, without affecting the assumption of constant molarity. Furthermore, how is the non-saturated reflux, that is, the cold reflux distillation process, calculated? Simply replace the external reflux ratio with the internal reflux ratio, write the operating line equation, and then solve it using either the plate-by-plate method or graphical methods. The premise for such a solution is, of course, a constant molar flow; if it is a non-constant molar flow, the operating line is not linear.