1. ERT 313 BIOSEPARATION ENGINEERING DISTILLATION
Prepared by:
Miss Hairul Nazirah Abdul Halim

2. Topic Outline 1.0 Vapor-Liquid Equilibrium Relations
2.0 Distillation: Introduction & Applications
3.0 Types of Distillation

3. 1.0 Vapor – Liquid Equilibrium (VLE) Relations
Raoult’s Law
Raoult’s Law for V-L phase in equilibrium is defined as:
pA = PA xA
Where:
pA is the partial pressure of component A in the vapor in Pa (atm)
PA is the vapor pressure of pure A in Pa (atm)
xA is the mol fraction of A in the liquid
This law holds only for ideal solutions such as benzene-toluene, hexane-heptane and methanol-ethanol, which are usually substances very similar to each other.

4. Boiling-Point Diagrams and x-y Plots
Often the VLE relations for a binary mixture of A and B are given as a boiling-point diagram
Figure shown the benzene (A) – toluene (B) system at total pressure of kPa.

5. Example: Use of Raoult’s law for Boiling Point Diagram
Calculate the vapor and liquid composition in equilibrium at 95oC for benzene-toluene using the vapor pressure in Table at kPa.

8. Relative Volatility of Vapor-Liquid Systems
Relative volatility, αAB is relative volatility of A with respect to B in the binary system.
High αAB – the separation easier to occur.
yA = concentration of A in vapor
xA = concentration of A in liquid

9. 2.0 Distillation: Introduction & Applications
The function of Distillation is to separate, by vaporization, a liquid mixture of miscible and volatile substances into individual/group components.
Examples:
The separation of a mixture of alcohol and water into its component
The separation of crude petroleum into gasoline, kerosene and fuel oil

10. 3.0 Types of Distillation 2 types of operations: Batch distillation
Continuous distillation
2 types of continuous distillation process:
Flash distillation (without reflux)
Rectification distillation (with reflux)

11. 3.1 Batch Distillation
It is a simple distillation where the liquid is charged to a reboiler (still), is boiled slowly and the vapors are taken directly from the condenser.
Batch Distillation is used in some small plant to recover volatile products from liquid solution

12. Batch Distillation with Reflux
Batch distillation with a simple still does not give a good separation unless the relative volatility is high.
Reflux is used to improve the performance of the batch still

13. 3.2 Continuous Operation: Flash Distillation
Figure shows the elements of a flash distillation plant.

14. 3.3 Continuous operation: Rectification Distillation
3.3.1 Types of Plate Columns
3.3.2 Action on an ideal plate
3.3.3 Combination rectification and stripping
3.3.4 Material balances in Plate Column
3.3.5 Number of Ideal Plates (McCabe-Thiele Method) & Feed Plate Location
3.3.6 Total & Minimum Reflux Ratio

15. 3.3.1 Types of Plate Columns 3 main types: Sieve Plate Column
Bubble-cap Plate Column
Valve Tray Column
Usually used in petroleum industry and chemical plant
Tower diameter: 0.3 – 9 meter
Number of plates from a few to about a hundred
Plate spacing may vary from 150 mm to 2 m

16. Sieve Plate Column
Sieve plate is designed to bring a rising stream of vapor into intimate contact with a descending stream of liquid

17. The liquid flows across the plate and passes over a weir to a downcomer
The flow pattern on each plate is crossflow.
The flow pattern in the column is countercurrent flow of liquid and vapor.
The downcomers are the segment-shaped region between the curved wall of the column and the straight chord weir.
Each downcomer usually occupies 10 – 15% of the column cross section, leaving 70 – 80% of the column area for bubbling and contacting.
The vapor pass through the perforated region of the plate (occupies most of the space between downcomers)

18. Bubble Cap Plate

19. Valve-Tray

20. 3.3.2 Action on an Ideal Plate
On an ideal plate, the vapor and liquid leaving the plate are brought into equilibrium.
Consider plate n in the figure.
yn = concentration of vapor leaving plate
xn = concentration of liquid leaving plate
yn+1 = concentration of vapor entering plate
xn+1 = concentration of liquid entering plate

21. 3.3.3 Combination rectification and stripping

22. Combination of Rectification & Stripping
Auxilary equipments in distillation process:
1. Distillation Column
2. Reboiler
3. Condenser
Feed plate – the plate on which the feed enters to the column
If the feed is liquid, it flows down the column by gravity to the reboiler
The volatile component is stripped by vapor rising from the reboiler.

23. Combination of Rectification & Stripping
Rectifying section – all plates above the feed plates
Stripping section – all plates below the feed plates (including the feed plate itself)
Vapor rising through the rectifying section are completely condensed in condenser
Condensate is collected in accumulator
Reflux pump deliver the liquid from accumulator to the top plate of tower
Reflux – downflowing liquid in the rectifying section
Overhead product – condensate which is not picked up by the reflux pump

24. 3.3.4 Material Balances in Plate Columns

25. Overall Material Balances for two-component systems
Total material balance:
Material balance on component A
Eliminating B from these equations gives
Eliminating D gives

26. Net Flow Rates
Net Flow Rates in the Upper Section of the Column
Material balance around the condenser and accumulator:
Material balance for component A:
A total material balance around the control surface I:

27. Net Flow Rates in the Lower Section of the Column

28. Operating Lines
There are 2 operating lines: for rectification and stripping section.
Rectification Section
General operating line equation:
Substitute: DxD = Vaya – Laxa (21.12)
Eliminate Vn+1 from Eq by Eq , gives:

29. Stripping Section
Material balance over control surface II
Operating line for stripping section:
Eliminate Vm+1 from Eq by Eq , gives:

30. Reflux Ratio
Reflux is the returning of a portion of the condensate to the reboiler.
2 quantities of reflux ratio:
a) ratio of the reflux to the overhead product
b) ratio of the reflux to the vapor
*In this text RD will be used.
RD, the reflux ratio, is an operating variable that can be controlled by adjusting the split between reflux and overhead product.

31. For constant molal overflow (divide Eq. 21.17 by D):
Eq is the operating line of the rectifying section.

32. Condition of Feed
The conditions of the vapor rate or the liquid rate may change depending of the thermal condition of the feed.
Various types of feed conditions:
Cold feed : It is assumed that the entire feed stream adds to the liquid flowing down the column.
Feed at bubble point: no condensation is required to heat the feed.
Feed partial vapor: the liquid portion of the feed becomes part of the L and the vapor portion becomes part of V
Feed saturated vapor the entire feed becomes part of the V
Feed superheated: part of the liquid from the rectifying column is vaporized to cool the feed to a state of saturated vapor.

33. Feed types can be characterized by the use of a single factor, q
q is the moles of liquid flow in the stripping section
Various types of feed conditions:
Cold feed, q > 1
Feed at bubble point (saturated liquid), q = 1
Feed partially vapor, 0 < q < 1
Feed at dew point (saturated vapor), q = 0
Feed superheated vapor q < 0

34. 3.3.4.5 Feed Line Feed line Equation
The point of intersection of the two operating lines lies on the straight line of slope (- q/1-q) and intercept (xF, yF)
Where the value of q for cold-liquid feed is:

35. Where the value of q for superheated vapor feed is:

36. Effect the feed condition on feed line
If the feed is a cold liquid, the feed line slopes will be upward and to the right;
if the feed is a saturated liquid, the line is vertical;
if the feed is a mixture of liquid and vapor, the lines slopes upward and to the left and the slope is the negative of the ratio of the liquid to the vapor;
if the feed is saturated vapor the line is horizontal and
if the feed is superheated vapor. The lines slope downward and to the left.

38. 3.3.4.6 Construction of Operating Lines
Construction the operating lines:
Locate the feed line
Calculate the y-axis intercept xD/(RD + 1) of the rectifying line and plot that line through the intercept and the point (xD, xD)
Draw the stripping line through point (xB,xB) and the intersection of the rectifying line with the feed line.

39. 3.3.5 Number of Ideal Plates & Feed Plate Location
After the operating lines have been plotted, the construction of the number of ideal trays is found by the usual step-by-step construction.
The process can begin at the top of the rectifying line or at the bottom of stripping line
Feed Plate location
The feed plate is the intersection of operating lines.

41. Heating & Cooling Requirement
If saturated steam is used as the heating medium, the steam required at the reboiler is:

42. If water is used as the cooling medium in the condenser and the condensate is not subcooled, the cooling water requirement is:

43. 3.3.6 Total & Minimum Reflux Ratio
Total Reflux
Returning all overhead condensed vapor from top of the tower back to the tower.
The slope of rectifying line increase with RD
When RD is infinite, V = L, the slope is 1 (this condition is called total reflux)
The operating line for rectification = operating line for stripping
= 45o line
At total reflux the no. of plates required is minimum
but the tower diameter is infinite

45. Minimum Reflux
Minimum vapor flow in the tower and hence minimum reboiler and condenser sizes.
The number of plates becomes very large as the RD becomes smaller.
At definite minimum (minimum reflux ratio, Rm) the number of plates becomes infinite.
The slope of rectifying line decrease with RD
The slope closer to the equilibrium line.
When both of operating lines touch the equilibrium line, a ‘pinch point’ occurs where the no. of steps required becomes infinite.
The slope of rectification operating line is as follows:

47. Multicomponent Distillation
Separation of more than two components
The general principles of design for multicomponent distillation towers are the same as described for binary system.
The calculation of equilibrium stages uses mass and enthalpy balances and VLE.
In binary distillation, one tower is used to separate the two components A and B.
In multicomponent mixture of n components, n – 1 fractionators will be required for separation.
For example, for 3 components mixture of A, B and C where A is the most volatile and C is least volatile, two column will be needed as shown in Figure