ERT 313BIOSEPARATION 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 LawRaoult’s Law for V-L phase in equilibrium is defined as:

pA = PA xAWhere:

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.

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.

)1/()1(

/

/

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AA

AA

BB

AAAB xy

xy

xy

xy

P

xPy

P

xPy

BBB

AAA

B

AAB P

P

AB

A

AA x

xy

)1(1

yA = concentration of A in vaporxA = concentration of A in liquid

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:

a) The separation of a mixture of alcohol and water into its component

b) The separation of crude petroleum into gasoline, kerosene and fuel oil

3.0 Types of Distillation

• 2 types of operations:

a) Batch distillation

b) Continuous distillation

• 2 types of continuous distillation process:

a) Flash distillation (without reflux)

b) 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 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

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

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

• On an ideal plate, the vapor and liquid leaving the plate are brought into equilibrium.• Consider plate n in the figure.

3.3.3 Combination rectification and stripping

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.

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

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 RatesNet 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 (21.12)

• Eliminate Vn+1 from Eq. 21.16 by Eq. 21.11, gives:

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

• Various types of feed conditions:

1. Cold feed, q > 1

2. Feed at bubble point (saturated liquid), q = 1

3. Feed partially vapor, 0 < q < 1

4. Feed at dew point (saturated vapor), q = 0

5. Feed superheated vapor q < 0

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:

3.3.4.5Feed Line

• 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 LinesConstruction the operating lines:

1. Locate the feed line

2. Calculate the y-axis intercept xD/(RD + 1) of the rectifying line and plot that line through the intercept and the point (xD, xD)

3. 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• 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

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:

'

'

1 xx

yx

R

R

D

D

m

m

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.