of Applied Sciences Faculty of Mechanical and Process Engineering Brno University of Technology Faculty of Mechanical Engineering Institute ID: 269425
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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 = 1Operating 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 110-3 to 1510-3 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/(kmolK)| 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 [Pas]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 n1 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
|