Dr Saad Al-ShahraniChE 334: Separation Processes Multistage Distillation Multistage flash technique...
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Transcript of Dr Saad Al-ShahraniChE 334: Separation Processes Multistage Distillation Multistage flash technique...
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.
Dr Saad Al-ShahraniChE 334: Separation Processes
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
Multistage Distillation
Feed
zB=0.5
zT=0.5
yB
xB
steam
Dr Saad Al-ShahraniChE 334: Separation Processes
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
Multistage Distillation
Feed
zB=0.5
zT=0.5
yB=0.691
xB
steam
Dr Saad Al-ShahraniChE 334: Separation Processes
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
Multistage Distillation
Dr Saad Al-ShahraniChE 334: Separation Processes
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
Multistage Distillation
Dr Saad Al-ShahraniChE 334: Separation Processes
Multistage Distillation
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
Dr Saad Al-ShahraniChE 334: Separation Processes
Multistage Distillation
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
Dr Saad Al-ShahraniChE 334: Separation Processes
Multistage Distillation
Dr Saad Al-ShahraniChE 334: Separation Processes
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.
Dr Saad Al-ShahraniChE 334: Separation Processes
(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.
Distillation of Binary Mixture
Dr Saad Al-ShahraniChE 334: Separation Processes
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.
Distillation of Binary Mixture
Dr Saad Al-ShahraniChE 334: Separation Processes
Table: shows a list of the more commonly binary distillation operations in decreasing order of difficulty of separation.
Distillation of Binary Mixture
Dr Saad Al-ShahraniChE 334: Separation Processes
Distillation of Binary Mixture
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:
Dr Saad Al-ShahraniChE 334: Separation Processes
Distillation of Binary Mixture
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.
Dr Saad Al-ShahraniChE 334: Separation Processes
Distillation of Binary Mixture
Trayed tower Packed tower
Dr Saad Al-ShahraniChE 334: Separation Processes
Distillation of Binary Mixture
Plate Towers
The most common
single-vessel device for
carrying out distillation
is plate tower.F
Dr Saad Al-ShahraniChE 334: Separation Processes
Distillation of Binary Mixture
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.
Dr Saad Al-ShahraniChE 334: Separation Processes
Distillation of Binary Mixture
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.
Dr Saad Al-ShahraniChE 334: Separation Processes
Distillation of Binary Mixture
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)
Dr Saad Al-ShahraniChE 334: Separation Processes
Distillation of Binary Mixture
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
Dr Saad Al-ShahraniChE 334: Separation Processes
Distillation of Binary Mixture
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.
Dr Saad Al-ShahraniChE 334: Separation Processes
Distillation of Binary Mixture
Perforation (sieve)
Valve cap Bubble cap
Common packing shapesGrid type packing 1.8 m in diameter and 2.4 m deep
Dr Saad Al-ShahraniChE 334: Separation Processes
Distillation of Binary Mixture
Dr Saad Al-ShahraniChE 334: Separation Processes
Distillation of Binary Mixture
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.