Dr Saad Al-ShahraniChE 334: Separation Processes Multistage Distillation Multistage flash technique Reasons for devising multistage process are: a)Increase.

Dr Saad Al-ShahraniChE 334: Separation Processes

Multistage Distillation

Multistage flash technique

Reasons for devising multistage process are:

a) Increase product purity

b) To reduce consumption of separating agents

A single stage flash distillation generally produces a vapor that is only somewhat richer in the lower-boiling constituent than the feed.

Further enrichment can be achieved by a series of flash distillation where the vapor from each stage is condensed then re-flashed.

In principle, any desired product purity could be obtained by such a multi-stage flash technique provided a suitable number of stages are employed.


Multistage Distillation.

Consider a process in which a benzene-toluene mixture is to be separated into benzene and toluene

Suppose, however, that the feed consists of 50 mol % benzene and 50 mol % toluene The first small amount of vapor generated will have a composition given by:

B-T = 2.25 at 121oC and P= 1 atm

BTB

BTBB x

xy

)1(1


Feed

zB=0.5

zT=0.5

yB

xB

steam


One way to obtain a richer benzene product is to condense a portion of vapor generated in the first stage and re-flashed again

Feed

zB=0.5

zT=0.5

yB=0.691

xB

y`B=0.838

x`B

steam

water


Feed

zB=0.5

zT=0.5

yB=0.691

xB

steam


By extending this concept, we can obtain the benzene product of the required purity

Feed

VO

LO

V1

L1

steam

water

Vn-2

Vn-1

Ln-1

Vn

Ln

water

water

Process for production of pure benzene



Example: Benzene- Toluene mixture

Concentration of more volatile component in vapor phase

Feed

zB=0.55

zT=0.45

B-T=7.0

V1=50=F/2

L2= 25

y1

y2

L1= 50

V2=25=V1/2

V3=8.3=L3/2

L3= 16.7

y3




Plot y1 Vs x1

1

11 61

7

)1(1 x

x

x

xy

BTB

BTB

1

11 61

7

x

xy

11

11

11 z

V

Fx

V

Ly

55.0*50

100

50

5011 xy

1.111 xy

1.111 xy

Stage#1

x

y

X1=0.33

y1 =0.78

z=0.55

Slope =-1

y1= 0.78, x1=0.330

0

1.0

1.0

111 VyLxFz



Stage # 2

78.0*225

2522 xy

5.122 xy

78.0*25

502

2

22 x

V

Ly

y1

V1

V2

y2= 0.92, x2=0.64

222 VyLxFz

1

11 61

7

x

xy

1.111 xy

x

y

X1=0.33

y1 =0.78

z=0.55

1

2

5.122 xy

X2=0.64

y2=0.92

Slope =-1

3

stage V L xB yB xT yT

1 50 50 0.33 0.78 0.67 0.22

2 25 25 0.64 0.92 0.36 0.08

3 8.3 16.7 0.9 0.98 0.1 0.02




Distillation of Binary Mixture

In distillation (fractionation), a feed mixture of two or more components is separated into two or more products, including, and often limited to, an overhead distillate and a bottoms whose compositions differ from that of the feed.

The feed is a liquid or a vapor-liquid mixture. The bottoms product is almost always a liquid, but the distillate may be a liquid or a vapor or both.

The separation requires that:

(1) a second phase be formed so that both liquid and vapor phases are present and can contact each other on each stage within a separation column.


(2) The components have different volatilities so that they will partition

between the two phases to different extents.

(3) The two phases can be separated by gravity or other mechanical

means

Example: Distillation of a binary mixture of benzene and toluene.

The purpose of the 25 sieve-tray (equivalent to 20 theoretical stages plus

a partial re-boiler that acts as an additional theoretical stage) distillation

column is to separate the feed into a liquid distillate of 99 mol% benzene

and a liquid bottoms product of 98 mol% toluene.



Δ P = 0.1 psia / tray

The pressure in the reboiler =

18 + 0.1 (25) =20.5 psia

In this range of pressure,

benzene and toluene form

nearly ideal mixture with a

relative volatility:

B-T = 2.26 at the bottom.

T-B = 2.52 at the top tray.



Table: shows a list of the more commonly binary distillation operations in decreasing order of difficulty of separation.




Equipment AND DESIGN CONSIDERATIONS

Industrial distillation operations are most commonly conducted in trayed tower, but packed columns are finding increasing use. Occasionally, distillation columns contain both trays and packing. Types of trays and packing

1. Feed flow rate, composition, temperature, pressure, and phase condition.

2. Desired degree of separation between two components.

3. Operating pressure (which must be below the critical pressure of the

mixture.

4. Vapor pressure drop, particularly for vacuum operation.

5. Minimum reflux ratio and actual reflux ratio.

Factors that influence the design or analysis of a binary distillation operation include:



6. Minimum number of equilibrium stages and actual number of

equilibrium (stage efficiency)

7. Type of condenser (total, partial, or mixed)

8. Degrees of su-bcooling, if any, of the liquid reflux

9. Type of re-boiler (partial or total)

10. Type of contacting (trays or packing or both)

11. Height of the column

12. Feed entry stage

13. Diameter of the column

14. Column internals.



Trayed tower Packed tower



Plate Towers

The most common

single-vessel device for

carrying out distillation

is plate tower.F



5. If the mixing of vapor and liquid on the plates were sufficient to provide equilibrium between the vapor and liquid streams leaving the plate, each plate acts as separator vessels.

6. The portion of the tower above the feed is called the rectifying or enrichment section. and The portion of the tower below the feed is called (stripping section)

In the Plate Tower:

1. plates, or trays, on each of which vapor and liquid are contacted.

2. The liquid flows down the tower under the force of gravity.

3. the vapor flows upward under the force of as light pressure drop from plate to plate. The highest pressure is produced by the boiling in the bottom steam heater, called the re-boiler.

4. The vapor passes up ward through openings in each plate and contacts the liquid flowing downward across the plate.



Types of Plates (Trays)

1. Sieve tray

As shown in figure (4.8, King).

It is the simplest type of plate,

which consists of a metal

plate with 3-15 mm holes,

spaced in a regular pattern.



Details of a contacting tray in a trayed tower.

A 2.1-m-diameter perforated (sieve) tray. The down-comer to the next tray is located on the left.

(Tray diameter)



2. Bubble-cap tray

A bubble cap tray has

bubble caps that consists

of a fixed cap, 3 – 6

inches in diameter,

mounted over and above

a riser of 2 – 3 inches in

diameter. The cap has

rectangular or triangular

slots cut around its side

cap. The vapor flows up

through up through the

tray opening into the riser,

turns around and pass.

Bubble-cap



3. valve-cap tray

The riser of the valve

cap is supported by the

momentum of the up

following vapor. At the

velocities the riser is fully

open, while at lower

vapor velocities the riser

is partially or completely

lowered.



Perforation (sieve)

Valve cap Bubble cap

Common packing shapesGrid type packing 1.8 m in diameter and 2.4 m deep





Down-comer

A separate compartment provide sufficient volume and a long enough residence time for the liquid to be freed of entrained vapor before reaching the bottom and entering the next plate.

weir

Connected with each tray in order to give good mixing between phases and provide the necessary disengagement of vapor and liquid between stage.

Dr Saad Al-ShahraniChE 334: Separation Processes Multistage Distillation Multistage flash technique Reasons for devising multistage process are: a)Increase.

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