Distillation Flashcards
What happens to the concentration (in terms of the more volatile component) of the vapor phase and the liquid phase per stage in distillation?
Concentration of the vapor phase increases
Concentration of the liquid phase decreases
Cooled liquid stream returned to the column to a cool plate or stage
Reflux
Location of the highest pressure in a distilling column
At the bottom
- Due to high temperature provided by the reboiler
- Provides pressure gradient that pushes the vapor to flow upwards
Two heat transfer equipment used in distillation column
Condenser — to convert the vapor at the top of the column to liquid
Reboiler — to convert the liquid into vapor; provides heat at the bottom of the column
What happens to the temperature in a distilling column from bottom to top?
Temperature decreases
Assumption of the McCabe-Thiele Method
Constant molal latent heat of vaporization (there is small difference, about <10%, between the latent heats of the two components)
- Streams entering a tray have negligible heat of solution (small sensible heat differences) — latent heat becomes important
- Vapor entering = vapor exiting
- Liquid entering = liquid exiting
Definition of q of the feed line
Energy to convert 1 mol of feed to saturated vapor per molar heat of vaporization
q values and behavior of the slope of the feed line for various thermal conditions of the feed
Subcooled liquid — q > 1 (m is positive)
Saturated liquid — q = 1 (m is undefined; line is vertical)
Partially flashed (mixture) — 0 < q < 1 (m is negative)
Saturated vapor — q = 0 (m is zero; line is horizontal)
Superheated vapor — q < 0 (m is positive)
Number of stages for distillation column operating at minimum reflux ratio and total reflux ratio
Minimum reflux ratio (D»_space;> L) : Infinite number of stages
Total reflux ratio (L»_space;> D): Minimum number of stages
Number of plate increases as the reflux ratio decreases
What happens to the enriching, feed, and stripping lines at minimum reflux ratio and total reflux ratio?
Minimum reflux ratio:
- The three lines intersect at a common point (pinch point, zone of constant composition) in the equilibrium curve
Total reflux ratio:
- No feed, enriching, and stripping lines; stripping and enriching lines become the 45 degree line
Assumption of the Fenske equation (used to determine the minimum number of theoretical stages)
Constant relative volatility
Effect of increasing the reflux ratio (increasing reflux to the column) on the column diameter, condenser and reboiler duties
Column diameter, condenser duty, and reboiler duty increases
(since too much liquid is returned and too much heat is required to heat/cool the liquid)
The ease of difficulty of separating components by distillation is called the relative volatility, which is the ratio between the vapor pressure of the more volatile component to the least volatile component. What happens to the relative volatility and the curvature of the equilibrium curve as the pressure or the temperature increases?
Relative volatility decreases and the curvature of the curve flattens
- Strictly speaking, relative volatility is greatly affected by temperature (as it affects vapor pressure)
- The greater the distance between equilibrium line and 45 line, greater difference between y and x, greater separation (greater relative volatility)
A plot that shows the vapor-liquid relations, as well as the thermodynamic condition, for a binary mixture of A and B.
Boiling-point diagram
Applicability of Raoult’s Law
Ideal solutions
It relates the partial pressure of a component of a solution and its vapor pressure and mole fraction. If it is pure, the partial pressure equals vapor pressure (since x = 1). In case of mixture, there is a vapor pressure lowering and the partial pressure of the vapor of the liquid is less than the vapor pressure.
Example of nonideal system that shows maximum-boiling azeotrope and minimum-boiling azeotrope.
Maximum-boiling azeotrope — acetone-chloroform
Minimum-boiling azeotrope — ethanol-water
Distillation used for a system with two miscible components that have a relative volatility close to 1.00 (both vapor pressures are similar) rendering ordinary distillation methods impractical. It is done by altering the components of the feed by adding another liquid.
Extractive Distillation
In batch distillation, the liquid charged is boiled slowly and the vapors are withdrawn as rapidly as possible as they form to condenser. What happens to the concentration the more volatile component in the vapor condensed as vaporization proceeds?
Concentration decreases (since the least volatile component eventually vaporizes after some time)
- First portion of the vapor condensed will be the richest
In batch distillation, the boiling point of the distillate product compared to the residue is
Lower
(Distillate contains more volatile component than the residue that lowers its boiling point)
What happens to the separation when the operating pressure is increased in a distilling column.
Separation becomes difficult (since boiling point increases and it will take a long time to vaporize the components)
Relative volatility also decreases.
What is the effect of changing the feed composition of the binary mixture in a flash distillation at constant P and T?
Fraction vaporized (V/F) changes but not the product composition (since P and T are fixed)
Applicability of steam distillation.
High-boiling liquids as the liquid may decompose at the high temperatures required by ordinary distillation.
Way to reduce the amount of steam (energy cost) per unit quantity of distillate in case of steam distillation
Raising the temperature and lowering the pressure
Describe the sizes of the reboiler and condenser in minimum and total reflux ratio.
Minimum reflux ratio
* Minimum reboiler and condenser sizes (since vapor flow in the tower is minimum for infinite number of plates)
Total reflux ratio
* Infinite reboiler and condenser sizes (all of the vapor will be condensed and all of the liquid at the bottom stream will be vaporize)
In McCabe-Thiele method, the triangle whose one corner coincides with the enriching line and the outer corner is on the stripping line is called
Feed plate
A single stage distillation wherein a liquid mixture is partially vaporized. The vapor is allowed to come to equilibrium with the liquid, and the vapor and liquid phases are then separated, either batchwise or continuously.
Flash or equilibrium distillation
A distillation in which the liquid is first charged to a heated kettle and then boiled slowly. The vapors are withdrawn as rapidly as they form to a condenser, where the condensed vapor (distillate) is collected.
Batch or differential distillation
Application of flash distillation
Separation of components having very wide boiling points
Example of a large scale usage of flash distillation.
Petroleum refinning
In case of steam distillation, why is the steam leaving the liquid is not completely saturated with distillate vapor?
Mixing of steam with the material being vaporized is not so intimate as to result in equilibrium condition
Role of steam in steam distillation.
Steam lowers the partial pressures of the components, allowing them to vaporize out of the mixture at temperatures lower than their normal boiling point.
Cause of back-trapping in a distillation column
Excessive liquid gradient
Back-trapping in a distillation column results due to ________.
Excessive liquid gradient
