Week 3 Flashcards
3 types of distillation
- Differential distillation
- Flash/equilibrium distillation
- Recification
Type of distillation the Rayleigh equation relates to
Differential distillation
Simple one step distillation process
flash distillation
Overall flash distillation balance
F = V + L
F = feed, V = top, L = bottom
In a continuous rectification process, temperature ________ as you go up the column
increases
In a continuous rectification process, temperature increases as you go ____ the column
up
The fractionating column is mounted on a large reboiler or still which holds the charge. The charge is boiled and the vapour evolved passes up the column, is condensed and partially returned as reflux
Differential distillation
Differential distillation: as the vapour passes ____ the column, it becomes increasingly rich in the more volatile component.
up
Differential distillation: as the vapour passes up the column, it becomes ________ rich in the more volatile component.
increasingly
Differential distillation: as the vapour passes up the column, it becomes increasingly rich in the ____ volatile component.
more
Differential distillation: charge remaining the still becomes ________rich in the less volatile component
increasingly
Differential distillation: charge remaining the still becomes increasingly rich in the ____ volatile component
less
In this distillation method, the composition of the distillate falls below the specification and the distillation is stopped
Differential distillation/ batch distillation
flash distillation
Assuming a binary system
y = F/V z - x(F/V - 1)
Rich in more volatile components
Distillate
Rich is less volatile components
Bottoms
Vapour flows up, liquid flows down.
Vapour and liquid are brough into contact on plates or packing.
Some of the condensate is returned to the column to provide liquid flow above the feed point.
Some of the liquid at the base of the column is vapourised to provide vapour flow.
In the section below the feed, the more volatile components are stripped out.
Above the feed the concentration of the more volatile components is increased.
Continuous rectification
Section below the feed where the more volatile component is stripped out
stripping section
Above the feed where the concentration of the more volatile component is increased
enrichment/ rectifiying section
theoretical plate
a distillation column plate such that the vapour leaving the plate is in equilibrium with the liquid on and leaving the plate
heat entering a plate =
heat leaving a plate + losses + heat of mixing
If component A is more volatile
Yn > Yn-1
Xn+1>Xn
Mass balance for plate n
Continuous rectification
Vn-1 + Ln+1 = Vn + Ln
Component balance for the more volatile component
Vn-1 Yn-1 + Ln+1 Xn+1 = VnYn + LnYn
Requires you to perform two mass balances on the column
Lewis Sorel Method
1) Balance around the top of the column
2) Balance around the bottom of the column
Lewis Sorel top of column overall balance
Vn = Ln+1 + D
Lewis Sorel bottom of column overall balance
Lm+1 = Vm + W
Lewis Sorel top of column component balance
yn = (Ln+1/Vn) (Xn+1) + (D/V) Xd
where feed is introduced into the column
feed plate
Lewis Sorel top of column slope
Lm/Vm
Lewis Sorel bottom of column slope
Ln/Vn
McCabe Thiele method simplfying assumptions
- Constant molar heat of vapourisation
- No heat losses
- No heat of mixing
q value equation
heat to vapourise 1 mol of feed/ mol latent heat of the feed
Feed at boiling point q value
q = 1
Cold feed as liquor q value
q > 1
Party vapour q value
0 < q < 1
Saturated vapour q value
q = 0
Superheated q value
q < 0
Diagram plotted for McCabe Thiele method
xy plot
The points the top operating line in McCabe-Thiele plot goes through
(xD, xD) (0, DxD/Vn)
q line slope
q/q-1
The points the q-line line in McCabe-Thiele plot goes through
(xw, xw)
The points the bottom operating line in McCabe-Thiele plot goes through
(xw, xw) and intersection of top operatng line and the q-line
the total number of steps on the McCabe Thiele graph is equal to
number of theoretical pates required for the separation
Reflux ratio equation
Ln/D
McCabe Thiele equation for top operating line in terms of reflux ratio
R/R+1 xn+1 + xd/R+1
Occurs when all the distillate is returned to the column and no product is withdrawn
total reflux
occurs when the top operating line coincides with the y=x line
McCable Thiele
total reflux
occurs when both the q line and the top operating line intersect the equilibrium line at the same point (pinch point)
McCable Thiele
condition of minimum reflux
The two things that intersect at pinch point
Minimum reflux condition
q-line
top operating line
Any ____ in R above this Rmin value will produce a workable column
increase
If the q-line is vertical then it can be shown that the slope of the operating line is:
Rmin = xd - yf/ yf - xf
If the q-line is horizontal then it can be shown that the slope of the operating line is:
Rmin = xd - xf/xf - xc
Differential mass balance equation
Rayleigh equation
- Relative volatility is constant
- lets you find R min for the system
- you do not need to use graph to find R min with this method
Underwood equation
- Relative volatility is constant
- Way to calculate the minimum number of stages needed operating under the condition of total reflux
- actual number of stages > number of stages determined in this method
Fenske Equation
