ERT 313
BIOSEPARATION ENGINEERING
DISTILLATION
Prepared by:
Miss Hairul Nazirah Abdul Halim
Topic Outline
• 1.0 Vapor-Liquid Equilibrium Relations
• 2.0 Distillation: Introduction & Applications
• 3.0 Types of Distillation
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.
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 11.1-1 shown the benzene (A) – toluene (B) system at total
pressure of 101.32 kPa.
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 11.1-1 at 101.32
kPa.
2.0 Distillation:
Introduction & Applications
•
•
a)
b)
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
3.0 Types of Distillation
•
a)
b)
2 types of operations:
Batch distillation
Continuous distillation
•
a)
b)
2 types of continuous distillation process:
Flash distillation (without reflux)
Rectification distillation (with reflux)
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
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
3.2 Continuous Operation:
Flash Distillation
• Figure 21 .1 shows the elements of a flash distillation plant.
3.3 Continuous operation:
Rectification Distillation
3.3.1
3.3.2
3.3.3
3.3.4
3.3.5
Types of Plate Columns
Action on an ideal plate
Combination rectification and stripping
Material balances in Plate Column
Number of Ideal Plates (McCabe-Thiele Method) &
Feed Plate Location
3.3.6 Total & Minimum Reflux Ratio
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
3.3.1.1
Sieve Plate Column
• Sieve plate is designed to bring a rising stream of vapor into
intimate contact with a descending stream of liquid
• 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)
Bubble Cap Plate
3.3.1.2
Valve-Tray
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
3.3.3
Combination rectification and
stripping
3.3.4
Material Balances in Plate Columns
Overall Material Balances for two-component systems
• Total material balance:
• Material balance on component A
• Eliminating B from these equations gives
• Eliminating D gives
3.3.4.1
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:
Net Flow Rates in the Lower Section of the Column
3.3.4.2
Operating Lines
• There are 2 operating lines: for rectification and stripping section.
Rectification Section
General operating line equation:
Substitute:
DxD = Vaya – Laxa
• Eliminate Vn+1 from Eq. 21.16 by Eq. 21.11, gives:
(21.12)
Stripping Section
• Material balance over control surface II
• Operating line for stripping section:
• Eliminate Vm+1 from Eq. 21.19 by Eq. 21.13, gives:
3.3.4.3
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.
• For constant molal overflow (divide Eq. 21.17 by D):
• Eq. 21.22 is the operating line of the rectifying section.
3.3.4.4
•
•
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:
1.
Cold feed : It is assumed that the entire feed stream adds to the
liquid flowing down the column.
2.
Feed at bubble point: no condensation is required to heat the
feed.
3.
Feed partial vapor: the liquid portion of the feed becomes part of
the L and the vapor portion becomes part of V
4.
Feed saturated vapor the entire feed becomes part of the V
5.
Feed superheated: part of the liquid from the rectifying column
is vaporized to cool the feed to a state of saturated vapor.
• Feed types can be characterized by the use of a single factor, q
• q is the moles of liquid flow in the stripping section
•
1.
2.
3.
4.
5.
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
3.3.4.5 Feed Line
Feed line Equation
• If xq = xF, and yq =xF then; The point of intersection of the two
operating lines lies on the straight line of slope (q/q -1) and
intercept (xF, yF)
• Where the value of q for cold-liquid feed is:
• Where the value of q for superheated vapor feed is:
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.
3.3.4.6
Construction of Operating Lines
Construction the operating lines:
1.
2.
3.
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.
3.3.5
Number of Ideal Plates &
Feed Plate Location
Number of Ideal Plates
•
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.
Heating & Cooling Requirement
• If saturated steam is used as the heating medium, the steam
required at the reboiler is:
• If water is used as the cooling medium in the condenser and the
condensate is not subcooled, the cooling water requirement is:
3.3.6
Total & Minimum Reflux Ratio
Total Reflux
• 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
• The rate of feed, overhead and bottom products are zero.
• Since there is no feed in a column operating under total reflux,
there is no discontinuity between the upper and lower sections.
Minimum Reflux
•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.
•This corresponds to the minimum vapor flow in the tower, and
hence minimum reboiler and condenser sizes.
•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:
x D  y'

R m  1 x D  x'
Rm
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 11.7-1.