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of Applied Sciences Faculty of Mechanical and Process Engineering Brno University of Technology Faculty of Mechanical Engineering Institute ID: 269425

design distillation columns column distillation design column columns liquid tray vapor pressure stripping systems absorption conceptual reppich prof drop

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Slide1

Augsburg University

of

Applied

Sciences | Faculty of Mechanical and Process EngineeringBrno University of Technology | Faculty of Mechanical Engineering | Institute of Process and Environmental Engineering Projektování a řízení procesů (KPJ) Conceptual Design of Distillation, Absorption and Stripping Systems Prof. Dr.-Ing. Marcus Reppich Room D5/249  marcus.reppich@hs-augsburg.de Important notice: These documents are to be used exclusively for study purposes, they are made available to participants of the lecture Conceptual Design of Distillation, Absorption and Stripping Systems (Projektování a řízení procesů, KPJ) at the Institute of Process and Environmental Engineering at the Brno University of Tech-nology only. Cover image: Copyright by BASF SESlide2

Conceptual

Design of

Distillation, Absorption and

Stripping SystemsTimetable and Contents of Lectures and ExercisesLectures19.11.2013P 09

Fundamentals

of

Binary

Distillation

26.11.2013

P 10

Types

of

Distillation

Columns

03.12.2013

P 11

Design

of

Distillation

Columns

Exercises

19.11.2013

Assignment

date

26.11.2013

C 10

Design

of

a

Multicomponent

Distillation

System

Using

the

Process

Simulation Software CHEMCAD

(

group

work

of

two

students

,

elaboration

of

a final

project

report

)

03.12.2013

C 11

10.12.2013

C 12

13.12.2013

Due

dateSlide3

3 Design

of Distillation Columns

Determination

of the Number of Actual Stages In the previous analysis, we have assumed that the vapor leaving each stage was in equilibrium with the liquid leaving the same stage. However, in practice, the trays are not perfect, i.e. there are devia-tions from ideal conditions. The assumption of thermal equilibrium is reasonable, but the assumption of equilibrium with respect to the mass transfer is seldom justified due to:insufficient time of contact between the liquid and vapor phasesinsufficient degree of mixing of the both phases (the presence of stagnant zones on large-diameter distillation trays)the effects of entrainment and weeping.To determine the actual number of trays required for a given separation, the number of theoretical stages must be adjusted with a overall column efficiency and a safety coefficient:Neff

number

of

actual

stages

N

th…number of theoretical stagess…safety coefficient (s = 1,3 … 2)EOV…overall column efficiency

|

Prof. Dr. M. Reppich

|

Conceptual Design of Distillation, Absorption and Stripping Systems

|

3

|Slide4

3 Design

of Distillation

Columns

Determination of the Number of Actual Stages Overall Column Efficiency, Murphree Tray Efficiency The overall column efficiency EOV depends on the geometry and design of the trays, flow rates and flow paths of vapor and liquid streams, compositions and properties of both phases. Values of EOV can be predicted by comparsion with performance data from industrial columns for similar systems, by use of empirical correlations or semitheoretical tray models or by scale-up

from laboratory data. Guideline

values

are

:

The

Murphree

tray efficiency

Ej that describes the separation achieved on individual tray j is usually based on the vapor phase:Tray TypeTunnel TrayBubble Cup TraySieve TrayValve TrayEOV0,5 – 0,7 0,6 – 0,80,7 – 0,8 0,7 – 0,9

j

+ 1

j

j –

1

z

z

Equilibrium

Curve

Operating Line

a

b

|

Prof. Dr. M. Reppich

|

Conceptual Design of Distillation, Absorption and Stripping Systems

|

4

|Slide5

3 Design

of Distillation Columns

Determination

of the Tray Column Height Once the overall column efficiency EOV is known, it can be used on the McCabe-Thiele graphical me-thod in the form of a pseudo-equilibrium curve. In stepping off stages, the overall vapor-side effi-ciency EOV = a/b, can be used to dictate the percentage of the vertical distance taken from the operating line to the equilibrium curve. Following the staircase construction between the pseudo-equilibrium curve and the operating lines the number of actual stages Neff required for the given separation is determined:The height of trayed portion of column H and the column height Htot, required to meet the product specifications, are where z is the tray spacing, typically z = 0,2 … 0,6 m.

Pseudo-

equilibrium

curve

for

E

OV

=

a/b = const.Equilibrium curve for EOV = 1Operating linese.g., for

0

1

1

a

b

a

b

|

Prof. Dr. M. Reppich

|

Conceptual Design of Distillation, Absorption and Stripping Systems

|

5

|Slide6

3 Design

of Distillation Columns

Operating Region

of Tray ColumnsTray columns can only be operated within certain limits of gas and liquid flow. The operating region of a tray column can be represented in a diagram with x-coordinate and y-coordinate . Often these two loads are referred to the active area Aac. The upper borders for the gas and liquid flow (bold lines) are absolute borders that can never be crossed without causing mechanical damages. The lower bor-ders (dashed lines) may be exceeded to a certain extent without encountering any flow problems. However the mass transfer efficiency may gradually decrease. The shape and size of the operating re-gion depends on the design parameters. The operation point should be chosen so that a sufficient sa-fety margin to the operation limits remains.Note: See https://www.youtube.com/watch?v=D0H9FWsk_Ck, https://www.youtube.com/watch?v=Ch68_F-G9z8, https://www.youtube.com/watch?v=I6G8yGBpX5I for videos showing different operation modes of tray columns.

D

A

ac

Entrainment

Froth

height

Downcomer

capacity

WeepingMinimum crest over weirOperating region| Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 6 |Slide7

3 Design

of Distillation Columns

Operating Region

of Tray Columns As described below the gas and liquid loads has to be kept between a maximum and a minimum value. Therefore, four limitations can be defined: at low gas velocities either the gas no longer flows uniformly though all the tray openings (bypassing part of the tray) or the liquid leaks though the tray (weeping) both modes of operation should be avoided due to tray efficiency losses because of insufficient degree of mixing of the both phases the main factor that affects weeping is the hole diameter (the minimum gas load increases with increasing hole diameter)at high gas velocities the gas blows the liquid off the tray in form of fine droplets (entrainment, jet flood)the liquid flows no longer countercurrently to the gas, and proper column operation endsthe maximum feasible gas load depends on system properties (den-sity

of gas and liquid, surface tension) as well as on tray design

entrainment flooding of trays is decisive at very large tray

spacings

z

, for smaller tray

spacings

z the froth height on the tray sets a lower limitation Minimum Gas Load:Maximum Gas Load:| Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 7 |Slide8

3 Design

of Distillation Columns

Operating Region

of Tray Columnsat extremely low liquid loads, liquid flows unevenly across the tray (maldistribution), which decreases the mass transfer efficiencyminimum height of the weir overflow hwo  5 mmthis corresponds to a minimum liquid weir load of:the liquid flow downward through the downcomers is enforced by gravity forces which results in a limitation of the maximum liquid loadthe following four empirical rules are often used to determine the maximum liquid flow rate the weir load should be the liquid velocity in the downcomer should not exceed a value of 0,1 m/s the volume of the downcomer should permit a liquid residence time of more than 5 s the height of the clear liquid in the downcomer should not exceed half of the tray spacing (hl  z/2) Ideally, the column

is

operated

in

the

range

of

60

to 90 % of the flooding vapor velocity.Minimum LiquidLoad:Maximum LiquidLoad:lw

h

wo

z

h

l

|

Prof. Dr. M. Reppich

|

Conceptual Design of Distillation, Absorption and Stripping Systems

|

8

|Slide9

3 Design

of Distillation Columns

Comparsion

of Common Tray Types Source: RASCHIG GmbH, Ludwigshafen * within optimum operating range123

1

2

3

|

Prof. Dr. M. Reppich

|

Conceptual Design of Distillation, Absorption and Stripping Systems

|

9

|Slide10

3 Design

of Distillation Columns

Summary

of the Geometry and Layout of Common Tray Types at Different Operating Pressures * DS column diameter in m, hw outlet weir height in m23

1

2

3

Source: RASCHIG GmbH, Ludwigshafen

1

2

3

1

|

Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 10 |Slide11

3 Design

of Distillation Columns

Determination

of the Tray Column DiameterThe tower diameter and, consequently, the cross-sectional area of the column must be sufficiently large to handle the gas and liquid rates within the operating region. The diameter of a distillation column is generally controlled by the vapor velocity.For designing a column the vapor velocity of the inside cross-sectional area of the empty tower is used. The vapor flows vertically upward usually at velocity from 0,5 to 2,5 m/s, and from 3 to 6 m/s in bubble-cup or tunnel tray columns. In contrast, the downflow velocity range of the liquid is from 110-3 to 1510-3 m/s.The required free cross-sectional area of the column is determined using the maximum vapor volume-tric flow rate during the operation and the allowable vapor velocity referred to the total column cross-sectional area:

A

Q

free

(total)

cross-sectional

area

of the column [m²]Di…column internal diameter [m]…maximum vapor volumetric flow rate [m³/s]wG zul

allowable

vapor

velocity

referred

to

the

area

A

Q

[m/s] (0,5…6 m/s)

maximum

vapor

mass

flow

rate

[kg/s]

maximum

vapor

molar

flow

rate [

kmol

/s]

G

avarage

density

of

the

vapor

phase

[kg/m³]

M

G

average

molecular

weigth

of

the

vapor

phase

[kg/

kmol

]

|

Prof. Dr. M. Reppich

|

Conceptual Design of Distillation, Absorption and Stripping Systems

|

11

|Slide12

3 Design

of Distillation Columns

Determination

of the Tray Column DiameterColumn internal diameter Di can be expressed as:Assuming ideal gas behavior for the vapor phase, the average vapor density can be substituted:Thus, the column internal diameter at a given operating pressure and operating temperature is: where Di [m], [kmol/s], T [K], p [Pa], wG

zul [m/s]

p

operating

pressure

[

Pa

]T…operating temperature [K]R…universal gas constant(R = 8314,5 J/(kmolK)| Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems |

12

|Slide13

3 Design

of Distillation Columns

Determination

of the Tray Column DiameterThe allowable vapor velocity referred to the total column cross-sectional area wG zul depends mainly on the tray type and its geometry, on the liquid load, and on the physical properties of the both phases.The usual design limit is entrainment flooding, which is caused by excessive carry-up of suspended liquid droplets by rising vapor to the tray above. At low vapor velocity, a droplet settles out; at high vapor velocity, it is entrained. At flooding velocity wG max

, the droplet is suspended

such

that

the

vec-tor

sum

of the buoyant force FA, drag force FW, and gravitational force FG acting on the droplet will be zero.From the balance of forces at a liquid doplet and a safety margin (fraction of flooding) results the allowable vapor velocity wG zul referred to the total column cross-sectional area:f…fraction of flooding, e.g. f=0,7kV…capacity

parameter

of

Souders

/Brown

[m/s]

A

ac

active

area

of

a

tray

[m²]

A

Q

total

column

cross-sectional

area

[m²]

L

liquid

density

[kg/m³]

G

vapor

density

[kg/m³]

z

F

A

F

W

F

G

Two-phase

Layer

z

tray

spacing

liquid

surface

tension

|

Prof. Dr. M. Reppich

|

Conceptual Design of Distillation, Absorption and Stripping Systems

|

13

|Slide14

3 Design

of Distillation Columns

Pressure

Drop in Tray ColumnsGenerally, the column pressure drop should be as low as possible because obtaining the number of theoretical trays using the McCabe-Thiele graphical method assumes that the pressure is constant over the whole columnlow pressure drop leads to a reduced energy requirement and to a heat

supplied by

the

reboiler

at

the

bottom at a lower boiling temperature level.Pressure loss of the vapor significantly depends on both gas and liquid load. The total column pressure drop is the sum of the hydrostatic pressure loss caused by the clear liquid holdup on the trays, the pressure drop due to the friction for vapor flow through the openings in the trays, and a loss due to the formation of bubbles by the gas:The first term in the equation above accounts for the liquid head on a tray, the second term refers to the dry pressure loss of the tray, the third term is small compared with pcol and is usually negligible. Due to the numerous variables such as tray geometry, physical

properties

of

vapor

and liquid, gas

and liquid loads,

operating pressure, etc. a

general equation

to calculate

the columns

pressure drop has

not yet been

developed. In most

cases, the

pressure drop

must be

found depending on

the tray type

experimentally.

p

col

total

column

pressure

drop

[

Pa

]

p

st

hydrostatic

pressure

drop

of

clear

liquid

[

Pa

]

p

dyn

pressure

drop

due

to

vapor

flow

resistance

[

Pa

]

p

pressure

drop

due

to

surface

tension

[

Pa

]

|

Prof. Dr. M. Reppich

|

Conceptual Design of Distillation, Absorption and Stripping Systems

|

14

|Slide15

3 Design

of Distillation Columns

Pressure

Drop in Tray ColumnsThe hydrostatic pressure drop of liquid pst depends on the mass of the clear liquid inside the column, as given by: For tray columns the hydrostatic pressure drop is given by the sum of the pressure drops across the trays:

m

L

total

mass

of

the

liquid in the column [kg]g…gravitational constant g = 9,81 m/s²AQ…total cross-sectional area

of

the

column

[m²]

N

eff

number

of

actual

trays

[

]

S

average

density

of

the

two-phase

layer

[kg/m³]

L

density

of

clear

liquid

[kg/m³]

relative gas/

vapor

fraction

in

the

two-phase

layer

[

]

h

S

average

heigth

of

the

two-phase

layer

[m]

|

Prof. Dr. M. Reppich

|

Conceptual Design of Distillation, Absorption and Stripping Systems

|

15

|Slide16

3 Design

of Distillation Columns

Pressure

Drop in Tray ColumnsThe pressure drop due to the friction for vapor flow up the column pdyn can be expressed approxima-tely by: The orifice coefficient  depends on the type and geometry of the column internals, and on the surface tension of the liquid. In tray columns, the orifice coefficient can be taken from:

If

the

column

is

operated

at 85 % of the flooding vapor velocity, the pressure drop per tray is, depen-ding on tray type, approximately from 2 to 8,5 mbar.…orifice coefficient of column internals []G…vapor density [kg/m³]wG

vapor

velocity

referred to the column cross-sectional area

[m/s]

B

orifice

coefficient

of

a dry

tray

[

]

|

Prof. Dr. M. Reppich

|

Conceptual Design of Distillation, Absorption and Stripping Systems

|

16

|Slide17

3 Design

of Distillation

Columns Rate-

Based Method for Packed Columns With the availibilty of economical and efficient packings, packed towers are finding increasing use in new distillation processes and for retrofitting existing trayed towers. They are particularly useful in applications when the separation is relatively easy and the required column diameter is not very large, where pressure drop must be low, as in low-pressure distillation, and where liquid holdup must be small, such as when separating heat-sensitive materials whose

exposure to high temperatures must

be

minimized

.

Packed

columns

are continuous, differential-contacting devices that do not have the physically distin-guishable, discrete stages found in trayed towers. Thus, packed columns are better analyzed by mass-transfer models than by equilibrium-stage concepts. However, in practice, packed-tower performance is often presented on the basis of equilibrium stages using a packed height equivalent to a theoretical plate, called the HETP and defined by the equationValues of the HETP depend mainly on packing type and size, liquid viscosity, surface tension, and operating conditions. In the absence of detailed information on the HETP, following rough approxima-tions are sufficient: HETP  0,6 m for random packings, HETP  0,3 m for structured packings.The required height of the packing within the column H and the total height of the column Htot are: with Hmin  (0,5...1 m) + (1...2 m)| Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 17 |Slide18

3 Design

of Distillation

Columns HETP

Estimation for Random and Structured Packings For rough estimates of the HETP the following relations can be used (all values are in ft, 1 ft = 0,3048 m)1. Random packings of second and third generation with low-viscosity liquids dP … nominal packing diameter [in] (1 in = 25,4 mm)2. Structured packings at low-to-modarate pressures with low-viscosity liquids

a

packing

surface

area

per

packed

volume [ft²/ft³]3. Distillation with viscous liquid4. Vacuum service5. Structured packings at high pressures6. Small-diameter columns with internal diameter Di < 2 ft| Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 18 |Slide19

3 Design

of Distillation

Columns

Characteristics of Random Packings Sources: W. D. Seider et al.: Product and Process Design Principles. 3. Aufl., John Wiley, Hoboken 2010; Vereinigte Füllkörper-Fabriken GmbH & Co. KG., Ransbach-BaumbachType Packing

Material

Nominal Diameter

d

P

[in]

Packing Factor

F

P

[ft²/ft³]Raschig ringsCeramic1,02,03,01575833Raschig ringsMetal1,02,03,01657140Intalox saddlesCeramic1,02,03,0923015Intalox saddlesPlastic1,02,03625Pall ringsMetal1,01,52,03,556292716Pall ringsPlastic1,02,03,5532515

|

Prof. Dr. M. Reppich

|

Conceptual Design of Distillation, Absorption and Stripping Systems

|

19

|Slide20

3 Design

of Distillation

Columns

Characteristics of Random and Structured Packings Source: E. J. Henley et al.: Separation Process Principles. 3. Aufl., John Wiley, Hoboken 2011 PackingMaterialSize

[mm]

Packing Factor

F

P

[ft²/ft³]

Mass-transfer Surface Area per Unit Volume

a [m²/m³]

Void

Fraction [-]Random PackingsHiflow ringsCeramicMetalPlastic505050291620 89,7 92,3117,10,8090,9770,924Nor-Pac ringsPlasticPlasticPlastic5035151421 86,8141,8311,40,9470,9440,918TellerettesPlastic2540190,0

0,930

Top-Pak

rings

Aluminium

50

105,5

0,956

VSP rings

Metal

Metal

50

25

104,6

199,6

0,980

0,975

Structured

Packings

Euroform

Gempak

Koch-Sulzer

Koch-Sulzer

Mellapak

Montz

Montz

Montz

Montz

Montz

Plastic

Metal

Metal

Metal

Plastic

Metal

Metal

Metal

Plastic

Plastic

PN-110

A2 T-304

CY

BX

250 Y

B1-100

B1-200

B1-300

C1-200

C2-200

70

21

22

33

110,0

202,0

250,0

100,0

200,0

300,0

200,0

200,0

0,936

0,977

0,970

0,987

0,979

0,930

0,954

0,900

|

Prof. Dr. M. Reppich

|

Conceptual Design of Distillation, Absorption and Stripping Systems

|

20

|Slide21

3 Design

of Distillation

Columns

Determination of the Packed Column Diameter The column diameter is determined so as to safely avoid flooding and to ensure that pressure drop is below 1,2 kPa/m of packed height. At the flooding point, the pressure drop increases infinitely with increasing vapor velocity. The internal column diameter Di

is based

again

on a

fraction

f

of

flooding velocity wG max by: f … fraction of flooding 0,65 < f < 0,9 (f  0,7)The generalized correlation of Leva gives reasonable estimates of the flooding gas velocity wG max [ft/s]: and with packing factor [ft²/ft³] (usually a … packing surface area per packed determined experimentally) volume [m²/m³] g … gravitational constant  … void fraction [m³/m³, %, -] (g = 32,174 ft/s²) … avarage density of the vapor phase [kg/m³] … average density of water [kg/m³] ( = 999,5 kg/m³ at 20 °C, 1 bar) The above regression model for the dimensionless flooding velocity factor Y = f (FLG) is valid for 0,01  Y  10.

|

Prof. Dr. M. Reppich

|

Conceptual Design of Distillation, Absorption and Stripping Systems

|

21

|Slide22

3 Design

of Distillation

Columns

Determination of the Packed Column Diameter The functions F and F are corrections for liquid properties given by: … average density of water [kg/m³] ( = 999,5 kg/m³ at 20 °C, 1 bar) L … average density of liquid [kg/m³] L … average dynamic viscosity of liquid [cP]

For a certain value

of

the

flow

parameter

FLG can firstly be calculated the dimensionless factor Y, and the superficial gas velocity at flooding wG max = f (Y) is then estimated for a given packing type and size (FP = f (a, )) and the correction functions F and F.Finally, using the allowable vapor velocity wG zul, the internal diameter of the distillation column Di can be determined. The tower inside diameter should be at least 10 times the nominal packing diameter and preferably closer to 30 times, dP … nominal packing diameter [mm] .Under these conditions, the negative effect of maldistribution on mass-transfer efficiency is minimised. Therefore, the column diameter may need to be adjusted accordingly.| Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems

|

22

|Slide23

3 Design

of Distillation

Columns

Pressure drop in Packed Columns An estimation of pressure drop in Pa/m can be made by using the generalized pressure drop correla-tion for packed beds according to Sherwood et al. and Leva: … liquid flow rate [kg/s] … gas flow rate [kg/s] L … liquid density [kg/m³] 

G … gas density [kg/m³]

F

P

packing

factor

[ft²/ft³]

wG … gas velocity [m/s], wG = f  wG max L … liquid viscosity [Pas]The flooding curve in the above figure corresponds to a pressure drop of 1200 Pa/m of packed height and can be accurately described by the polynomial regression: J. Benítez: Principles and Modern Applications of Mass Transfer Operations. 2. Aufl., John Wiley, Hoboken 2009FLGY‘| Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 23 |Slide24

3 Design

of Distillation

Columns

Charts for the Design of Random-Packed Columns: HETP Estimation INTALOX® Metal Tower Packing (IMTP®) Source: Koch-Glitsch, LP, Wichita| Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 24 |Slide25

3 Design

of Distillation

Columns

Charts for the Design of Random-Packed Columns: Estimation of Pressure Drop INTALOX® Metal Tower Packing (IMTP®) Source: Koch-Glitsch, LP, Wichita| Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems |

25 |Slide26

3 Design

of Distillation

Columns Summary

of Distillation Column Design Concept of equilibrium stages: determination of the number of theoretical stages Nth McCabe-Thiele graphical equilibrium-stage methodKremser equation (analytically, assuming straight equilibrium and operation lines 

absorption/ stripping

)

Estimation

of

the

actual

number of contacting trays Neff applying an overall column efficiency EOV: 0,1  EOV  0,9Estimation of HETP depending on packing type and size, liquid viscosity, and surface tension:Height of the actual equipment:Internal Column Diameter:

Vapor-side

pressure

drop

:

GPDC

charts

,

correlations

Distillation

/Absorption/Stripping

Tray Columns

(

sieve

,

valve

,

bubble-cup

trays

)

Random-/Structured-

Packed

Columns

stagewise

contact

between

the

phases

continuous

contact

between

the

phases

|

Prof. Dr. M. Reppich

|

Conceptual Design of Distillation, Absorption and Stripping Systems

|

26

|Slide27

3 Design

of Distillation

Columns

Summary of Distillation Column DesignSketch the column, set the given feed and product specificationsDetermine the operating temperature and pressure by the available utilitiy temperatures, the boiling tem-perature of the mixture

, the desired

purity

of

the

separation

, and any contraints on the stability of the mixture Make a material balance over the column to determine the unknown top and bottom compostions and/or flow ratesPlot the vapor-liquid equilibrium curve from data available at the column operating pressure and the diagonal line on the equilibrium diagram, mark given compositions of feed, distillate and bottom product on the diagramDraw the q-lineDetermine the minimum reflux ratio rmin from intersection of

rectifying

section

operating

line

and the

equilibrium curve

or by

calculationDetermine

the optimum

reflux ratio

, e.g. r = 1,2 

rmin

Draw operating lines

for

rectifying and

stripping section

Determine

the number

of theoretical

stages N

th using

the equilibrium

diagram

or by

analytical

methods

Select the type

of contacting

device:

trays or

packings

Determine the

actual

number of

trays

Neff or HETP

and specify

the

column

height H

tot

Determine the

allowable

vapor velocity

wG zul Design the column

: inner

diameter, column

internals (

trays, packing, liquid

and vapor

distribution

systems, packing

supports, etc.)

Estimate

the total column

pressure drop 

pcol

Evaluate

pressure drop

according

to

p

col

 

p

alow

,

if

necessary

select

another

column

internals

and

return

to

step

(11)

Determine

the

energy

requirements

,

calculate

the

size

of

the

heat

exchangers

(

basic

design

of

condenser

and

reboiler

)

and

the

utiliy

flows

Estimate

the

annual

total

costs

C

tot

(

r

) =

C

op

(

r

) +

C

cap

(

r

)

To

minimize

the

annual

total

costs

modify

the

reflux

ratio

,

return

to

step

(8)

and

cycle

through

steps

(8)

to

(17)

until

C

tot

(

r

)

min

Carry

out

the

mechanical

design

of

the

column

considering

the

operating

temperature

and

pressure

and

the

corrosion

properties

of

the

mixture

|

Prof. Dr. M. Reppich

|

Conceptual Design of Distillation, Absorption and Stripping Systems

|

27

|Slide28

3

Design of

Distillation Columns

Distillation of Multicomponent Mixtures without Azeotropes Sequences of Simple Columns Multicomponent mixtures are often separated in more than two products. In this case, there is a choice of order in which the products are separated. Consider the design of distillation systems comprising only simple columns, which have a single feed and two products. If there is a mixture of three components A, B, C (in order of increasing boiling

point) to be separated

into

three

relatively

pure

products

, then the decision is between two distillation sequences: direct sequence (the lightest component b) indirect sequence (takes the heaviest com- is taken overhead in each column) ponent as bottom product in each column)In general, to separate an n-component mixture into nearly pure products n1 simple columns are sufficient, and the number of different distillation sequences is . The following table shows some sequences of splits that can be used to separate multicomponent nonazeotropic mixtures. These can all be accomplished in simple columns, which have a single feed and two products. The com-ponents are A, B, C, D, E in order of increasing boiling point. Each of the splits listed corresponds to one simple column. Actually, more columns can be used, and this is sometimes more economical. | Prof. Dr. M. Reppich | Conceptual Design of Distillation, Absorption and Stripping Systems | 28

|Slide29

3 Design

of Distillation

Columns

Distillation of Multicomponent Mixtures without Azeotropes Sequences of Simple Columns Sequences for three components A, B, CColumn 1Column 21A / BCB / C

2

AB / C

A / B

Sequences

for

four

components A, B, C, DColumn 1Column 2Column 31A / BCDB / CDC / D2A / BCDBC / DB / C3AB / CDA / BC / D4ABC / DA / BCB / C5ABC / DAB / CA / B

Sequences

four

five

components

A, B, C, D, E

Column

1

Column

2

Column

3

Column

4

1

A / BCDE

B / CDE

C / DE

D / E

2

A / BCDE

B / CDE

CD / E

C / D

3

A / BCDE

BC / DE

B/ C

D / E

4

A / BCDE

BCD / E

B / CD

C / D5

A / BCDE

BCD / E

BC / D

B / C

6

AB / CDE

A / B

C / DE

D / E

7

AB / CDE

A / B

CD / EC / D8ABC / DE

A / BC

D / E

B / C

9

ABC / DE

AB / C

D / E

A / B10

ABCD / E

A / BCD

B / CD

C / D11

ABCD / E

A / BCD

BC / D

B / C

12

ABCD / E

AB

/ CD

A / B

C / D

13

ABCD / E

ABC / D

A / BC

B / C

14

ABCD / E

ABC / D

AB / C

A / B

|

Prof. Dr. M. Reppich

|

Conceptual Design of Distillation, Absorption and Stripping Systems

|

29

|Slide30

3 Design

of Distillation

Columns

Distillation of Multicomponent Mixtures without Azeotropes Sequences of Simple Columns General Heuristics:1)Remove corrosive, hazardous, chemically reactive, or thermally unstable

components

as

early

as

possible

.

2)Remove final products one-by-one as distillates (prefer the direct sequence) or as vapor streams from total reboilers.3)Prefer to reduce the number of columns in a recycle loop. 4)Lump pairs of components with relative volatilities less than 1,1 and remove these as a single product to be separated

using

another

separating

technology

the

relative

volatility

between

the

two

selected

key

components

for

the

separation in

each

column

is

> 1,05.

Heuristics

for

Simple Columns:

1)

Remove the components of greatest molar percentage in the feed first.

2)

Remove

the

lightest

component

first

.

3)

Make splits with the highest recoveries last.

4)

Sequence

separation

points

in

the

order

of

decreasing

relative

volatility

make

the

most

difficult

separation

in

the

absence

of

the

other

components

last.

5)

Favor

splits

which

give

molar

flows

of

distillate

and

bottom

products

as

near

equal

as

possible

.

6)

Make

the

cheapest

split

next

in

selecting

a

sequence

of

columns

.

|

Prof. Dr. M. Reppich

|

Conceptual Design of Distillation, Absorption and Stripping Systems

|

30

|