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Distillation Column Design Syed Zaheer Abbas

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Page 1: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:

Distillation Column Design

Syed Zaheer Abbas

Page 2: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:

Distillation:

• Process in which a liquid or vapour

mixture of two or more substances

is separated into its component

fractions of desired purity, by the

application and removal of heat”

Page 3: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:

The design of a distillation column can be divided into

the following steps:

� Specify the degree of separation required: set product

specifications.

� Select the operating conditions: batch or continuous;

operating pressure.

� Select the type of contacting device: plates or

packing.

� Determine the stage and reflux requirements: the

number of equilibrium stages.

� Size the column: diameter, number of real stages.

� Design the column internals: plates, distributors,

packing supports.

� Mechanical design: vessel and internal fittings.

Page 4: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:

The separation of liquid

mixtures by distillation

depends on differences in

volatility between the

components. The greater the

relative volatilities, the easier

the separation.

In the section below the feed,

the more volatile

components are stripped

from the liquid and this is

known as the stripping

section. Above the feed, the

concentration of the more

volatile components is

increased and this is called

the enrichment, or more

commonly, the rectifying

section.

Page 5: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:

Reflux considerations

The reflux ratio, R, is normally defined

as:

R = (flow returned as reflux)/(flow of top

product taken off)

The number of stages required for a

given separation will be dependent on

the reflux ratio used.

Page 6: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:

Total reflux

Total reflux is the condition when all the

condensate is returned to the column as

reflux: no product is taken off and there

is no feed.

At total reflux the number of stages

required for a given separation is the

minimum at which it is theoretically

possible to achieve the separation.

Page 7: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:

Minimum reflux

As the reflux ratio is reduced a pinch

point will occur at which the separation

can only be achieved with an infinite

number of stages. This sets the

minimum possible reflux ratio for the

specified separation.

Page 8: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:

Optimum reflux ratio

Practical reflux ratios will lie somewhere

between the minimum for the specified

separation and total reflux.

For many systems the optimum will lie

between 1.2 to 1.5 times the minimum

reflux ratio.

Page 9: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:
Page 10: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:

Flooding and Weeping

Flow weeps through

the holes at low vapor

velocity

Weeping

Liquid cannot get

down the column

at high vapor velocity

Flooding

Page 11: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:

Feed-point location

• The precise location of the feed point

will affect the number of stages

required for a specified separation

and the subsequent operation of the

column. As a general rule, the feed

should enter the column at the point

that gives the best match between the

feed composition (vapor and liquid if

two phases) and the vapor and liquid

streams in the column.

Page 12: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:

Dew points and bubble points

• Dew points and bubble points can be

calculated from a knowledge of the

vapor-liquid equilibrium for the

system. In terms of equilibrium

constants, the bubble point is defined

by the equation:

Page 13: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:

Choice b/w Tray & Packed Column

�Plate column are designed to handle wide range

of liquid flow rates without flooding.

�For large column heights, weight of the packed

column is more than plate column.

�Man holes will be provided for cleaning in tray

Columns. In packed columns packing must be

removed before cleaning.

�When large temperature changes are involved as

in the distillation operations tray column are often

preferred.

�Random-Packed Column generally not designed

with the diameter larger than 1.5 m and diameters

of commercial tray column is seldom less than

0.67m

Page 14: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:

Selection of Trays:

Page 15: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:

DESIGNING STEPS OF

DISTILLATION COLUMN

�Calculation of Minimum number of stages, Nmin

�Calculation of Minimum Reflux Ratio Rm.

�Calculation of Actual Reflux Ratio.

�Calculation of theoretical number of stages.

�Calculation of actual number of stages.

�Calculation of diameter of the column.

�Calculation of weeping point, entrainment.

�Calculation of pressure drop.

�Calculation of the height of the column.

Page 16: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:

Calculation of Minimum no. of Plates:

The minimum no. of stages Nmin is obtained from Fenske equation which is,

Nmin = LN[(xLK/xHK)D(xHK /xLK)B]

LN (αLK/HK) average

Page 17: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:

Theoretical no. of Plates:

+−

−=+

−566.0

minmin

1175.0

1 R

RR

N

NN

Page 18: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:

Problem

• A butane-pentane splitter is to operate at 8.3 bar with

the following feed composition:

Page 19: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:

• For a specification of not more

than 1 mol of the light key in the

bottom product and not more

than 1mol of the heavy key in

the top product, and a reflux

ratio of 2.5, make a stage-by-

stage calculation to determine

the product composition and

number of stages required.

Page 20: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:
Page 21: Distillation Column Design - libvolume2.xyzlibvolume2.xyz/chemicalengineering/.../multicomponentdistillation/...Distillation Column Design Syed Zaheer Abbas . ... Mechanical design:

• To estimate the dew and bubble

points, assume that nothing

heavier than the heavy key

appears in the tops, and nothing

lighter than the light key in the

bottoms.