DISTILLATION TOWERS Eng. Ahmed Deyab Fares Senior Process Engineer Special thanks to: Eng. M. Abd El-Raof
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DISTILLATIONTOWERS
Eng. Ahmed DeyabFares
Senior Process EngineerSpecial thanks to: Eng. M. Abd El-Raof
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Ahmed Deyab
Senior Process Engineer
Deliver Several Training Courses & Consultancy for process enginee
rs from several Oil, Gas & Petrochemicals companies in Egypt & Gulf countries.
Contact details:
Mobile (Egypt): +20 –1227549943
Mobile (a!di "#abia): +9$$500297503
E%ail: adeyab&adeyab'co%
ebsite: 'adeyab'co%
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Courses Conducted to engineers from:
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*!nda%entals o epa#ation
,oe# -nte#nals
C#!de .istillation
,#o!ble /ooting
Contents
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FUNDAMENTALS OFSEPARATONN TO!ERS
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Disti""ation
Separation by distillation implies a diference in
boiling points o two or more materials
We separate many things by detecting a
diference in a physical properties
color, size, weight, shape
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The components or com#o$nds ma%ing $# cr$de oi" are
n$mbered in tho$sands
Many o& these com#onents ha'e simi"ar #hysica"
#ro#erties inc"$ding boi"ing #oints that may di(er by
on"y a &e) degrees. Thereore, it is dicult to separate
some pure compounds rom the complex mixture o
components in crude oil by distillation alone
There are other methods o& se#aration $sed in are*nery &or e+am#"e, e+traction )ith a so"'ent,
crysta""i-ation, and absor#tion
Fortunately, rare"y need #$re com#o$nds and it is oten
enough to separate groups o compounds rom each other by
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we separate many compounds in crudeoil into groups we !nd that these groupsha"e characteristics that ma#e themconsiderably more "aluable than the whole
crude oilSome o these groups are products Some may be eedstoc# to other processing units where they are chemically changedinto more "aluable products
These products, in turn, are usually
separated or puri!ed by distillation
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The basic principle o distillation issimple
When a solution o two or morecomponents is boiled
The lighter component $the one most
"olatile or the one with the greatesttendency to "aporize %"aporizespreerentially
rinci#"es O& Disti""ation
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10
Tow component mixture is contained in a "essel
When heat is add until the more "olatile material
$ red dotes % start to "aporize. &ow the "apor
contains a higher proportion o red dots than dose
the original li'uid
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t is important to note that an e'uilibrium in composition will
be established
at a gi'en tem#erat$re and #ress$re
(y e'uilibrium we mean there is a gi"en concentration as )red
dots* in the "apor and in the li'uid depending upon the
original concentration o each component in the li'uid and
their respecti"e properties in relation to each other
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No), "ets de'e"o# this sim#"e disti""ation
conce#t into a #ractica" o#eration as it is $sedin the re*nery
First , let+s separate and remo"e the product
This results in the "apor abo"e the li'uid being
relati"ely rich in the lighter $ more "olatile material %
nd the li'uid is let with proportionately more o theless "olatile $ hea"ier li'uid %
Thus a separation, to some degree, has ta#en place
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(y cooling the o"er head"apor, we condense andremo"e it rom the originalmixture
Thus to ha"e made a partialseparation, partial becauseyou will note that there are aew )blue dotes* in the
distillate product
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This has occurred because at the temperature and
pressure we are conducting the distillation, the
hea"ier component still "aporizes to some extent
This is because the components o interest in a gi"en
distillation usually ha"e airly close boiling points
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Thereore, to #$ri&y the
disti""ate #rod$ct, )e
may ha'e to cond$ct
a second disti""ation
as shown
-b"iously, we can
continue to cascade
these simple distillations
until we achie"e the
desired purity o product
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The distillations depicted so ar are those we call
patch, and normally practical in the re!nery,
although it is done re'uently in the laboratory
Let $s ma%e o$r disti""ation e/$i#ment "oo%
more "i%e re*nery #ieces o& e/$i#ment and
"et $s ma%e contin$o$s instead o& #atch
o#eration
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741$
This is called F"ash
0a#ori-ation s shown.
The "i/$id is #$m#edcontin$o$s"y thro$gh aheater and into a dr$m
)here the #ress$re is"o)er
The lighter material ashes
instantaneously $"apor andli'uid ow rom the drumcontinuously%
The same system is showndiagrammatically in the in
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S$##ose )e ha'e 123 o& the charge ta%en o'erhead
That is, we set the temperature and the pressure o the
system in such a way that hal the charge is boiled of
nd urther, s$##ose the res$"ting o'erhead #rod$ct
does not contain the desired concentration o& the
"ighter #rod$ct
s we ha"e seen beore, )e can increase the #$rity by
adding a stage o& disti""ation
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19
Suppose we add twomore stages odistillation
lthough this isaccomplishing ourgoal o increasingthe #$rity o& the"ight &riction, we arealso ma#ing large
amounts o theintermediate product,each o whichcontains the same
light riction
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n ob"ious simpli!cation in e'uipment can be made
i we allow the hot "apor rom the stage abo"e the
next higher $the intermediate product%
This eliminates the need or the intermediate
condensers and heaters
No) )e ha'e the contin$o$s, m$"ti4stage
disti""ation
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To)er SectionsWe ha"e described stagingor the purpose oconcentrating the lightercomponent in the o"erhead
The same principles apply to
concentrating the hea"iercomponent in the bottomproduct
The upper stages are calledrecti&ying stages
These below the eed arecalled stri##ing stages
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The upper rectiying section increases the
purity o the o"erhead product.
The lower stripping section increases the
reco"ery o the o"erhead product.n many cases, the bottom product is the
one o primary interest
For the bottom, or hea"y, product the
rectiying section impro"es reco"ery
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/$i"ibri$m Stage
stage, or more speci!cally, an e'uilibrium stage,is de!ned as /
Any #ortion o& the disti""ation co"$mn s$chthat the "i/$id and 'a#or "ea'ing it ha'e
com#osition in e/$i"ibri$m )ith each other.
(y de!nition, then, a stage should be designed insuch a way as to pro"ide intimate contact, or
mixing, o the rising "apor and the descendingli'uid. The concept o an e'uilibrium stage iscon"erted to an actual mechanical separation trayby using an eciency actor which is less than oneand depends on the tray design.
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Column Internals:
The plates or trays are contacting devices and used to hold up the liuid to
provide !etter contact !et"een vapour and liuid, hence !etter separation.
1.Trays !lates "# !ac$ings
V A L V E T R A Y S
S I E V E T R A Y S
B U B B L E C A P T R A Y S
T R A Y S W I T H D O W N C O M E R
D U A L F L O W T R A Y S
B A F F L E T R A Y S
D I S C & D O N U T T R A Y SR I P P L E T R A Y S
T R A Y S W I T H O U T D O W N C O M E R
P R O P R I E T A R Y D E S I G N S ( M V G , S U P E R F R A C )
T R I T O N , P R O V A L V E
N Y E T R A Y S
M U L T I D O W N C O M E R T R A Y S
N O N - F R A C T I O N A T I O N T R A Y S
C O L L E C T O R / C H I M N E Y T R A Y S
T R A Y S
R A S C H I GL E S S I N G
C R S S ! A R " I " I N R I N G
# E R L S A $ $ L E S
I G E N E R A " I N
! A L L R I N GH % ! A &
I M " !
C M R
N ' " " E R R I N G
I I G E N E R A " I N
R A N $ M
G E M ! A &M E L L A ! A &
I N " A L (
! A R L ! A &
I I I G E N E R A " I N
S " R ' C " ' R E $
! A C & I N G
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Si
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Sie%es
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Sieve Tray This tray is a sheet of light metal &ith a large number of holes dril
led through it#
'a(or rising through the holes $ee(s the li)uid on the tray and bubbles u( through it#
The o%erflo& &eir $ee(s a constant de(th of li)uid on the tray#
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A sieve tray is:
Inexpensive,Easy to clean, and
Maintains good liquid and vapor contact
as long as it is operated at its design load.
Because the sieve tray has fixed openings
and does not have covers over the holes, it
does not perform well if tower loads are
constantly changing.
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#u!!le Cap $ %alve trays
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#u!!le Cap $ %alve trays
In %al%e trays *ubble Ca( trays+ (erforations are co%ered
by liftable ca(s# 'a(or flo&s lifts the ca(s+ thus self creati
ng a flo& area for the (assage of %a(our# The lifting ca(
directs the %a(or to flo& hori,ontally into the li)uid+ thus
(ro%iding better mi-ing
ale ,#ays
iee ,#ays
!bble Cap
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Valve Tray
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Valve Tray
A valve tray has a variale opening for vapors to flowthrough.
!he hole has a cover
that consists of a capheld in place y guides
which go down through
the plate, or tray and
hoo" underneath it.
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#hen there is no vapor flow, the caps sits over the
hole and close it.
$nder low pressure
the cap start to rise.
As the flow of vapors
increases, the cap rise
until it is stopped ythe guide tas.
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!he valve tray is similar to the ale cap tray.
Both are more adaptale to variations in tower
loads than a sieve tray.
!he valve trays and ule cap trays are designed
to perform well with variale tower loads.
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T
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Trays Bubble Cap Tray
!he vapor is ro"en into small ules whichincreases the surface area for vapor%liquid contact.
!he ule cap sits on top of a riser.
!he riser channels vapors into the ule cap.
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$5
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&election of !ray !ype
!he principal factors to consider when comparingthe performance of ule%cap, sieve and valve
trays are:
'ost,
'apacity,
(perating range,
Maintenance and
)ressure drop.
'ost:
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'ost:
Bule%cap trays are apprecialy more expensive
than sieve or valve trays.
!he relative cost will depend on the material of
construction used*
+or mild steel the ratios,
ule%cap: valve: sieve, are approximately
.- : ./ : .-
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'apacity:
!here is little difference in the capacity rating for the
three types 0the diameter of the column required for
a given flow%rate1.
!he ran"ing is:
sieve, valve, and ule%cap
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(perating range:
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(perating range:
!his is the most significant factor.
By operating range is meant the range of vapour and
liquid rates over which the plate will operate
satisfactorily 0the stale operating range1.
&ome flexiility will always e required in an
operating plant to:
Allow for changes in production rate, and
'over start%up and shut%down conditions.
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!bblecap trays have a positive liquid seal and cantherefore operate efficiently at very low vapour
rates.
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iee t#ays rely on the flow of vapour through the
holes to hold the liquid on the tray and cannotoperate at very low vapour rates, ut, with good
design, sieve trays can e designed to give a
satisfactory operating range*
!ypically, from /- to 2- 3 of design capacity.
4alve trays are intended to give greater flexiility
than sieve trays at a lower cost than ule%caps.
Maintenance5
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Maintenance5
For dirty ser"ices, bubble0caps are not
suitable as they are most susceptible to
plugging. Sie"e trays are the easiest to
clean.
Press$re Dro#5
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Press$re Dro#5
!he pressure drop over the trays can e an importantdesign consideration, particularly for vacuum
columns.
!he trays pressure drop will depend on the detaileddesign of the tray ut.
In general,
sieve plates give the lowest pressure drop, followed y
valves, with ule%caps giving the highest.
&ummary
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&ummary
&ieve trays are the cheapest and are satisfactory formost applications.
4alve trays should e considered if the specified
turn%down cannot e met with sieve trays.
Bule%caps should only e used where:
4ery low vapor 0gas1 rates have to e handled
and
A positive liquid seal is essential at all flow%
rates.
Typical Tray SectionTypical Tray Section
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Typical Tray SectionTypical Tray Section
!tlet ei#
-nlet ei#
eal 6ot
.onco%e#
,#ays
Inlet Weirs
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3
Inlet Weirs
!hese contriute to the uniform distriution of
liquid as it enters the tray from the down comer.
It is not recommended for
fluids that are dirty or tendto foul surfaces.
If a more positive seal is
required at the downcomer atthe outlet, an inlet weir can
e fitted or a recessed seal pan
used.
Do"ncomer sealing can !e achieved primarily !y means'
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()* inlet "eirs
(* recessed seal pan.
These devices provide a positive
seal on the tray. The disadvantage is that they create a poc+et of s
tagnant liuid "here dirt, sediments, etc can !uild up. large a
mount of such !uild up can restrict the do"ncomer outlet area a
nd lead to premature flooding. Thus+ the use of these de%ices is n
ot recommended in fouling or corrosi%e ser%ices#
Outlet Weirs
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Outlet Weirs
The function of a "eir is to maintain a desired liuid leve
l on the tray + thus insuring bubbling of %a(ors through li
)uid# Typical "eir height is !et"een - inches. /o" "
eirs are freuently used in lo" pressure or vacuum colu
mns. 0otched (rectangular or %-shaped* "eirs are com
monly used for lo" liuid loads
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Do)ncomers
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Do)ncomers5eflux flows down from one
tray to the next throughdowncomers.
6owncomers must e large
enough to allow for drainagefrom one tray to the next or
flooding might occur on some
of the trays.
6owncomers can e designed in
several different ways to provide
smooth flow from tray
to tray.
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The straight, segmental, vertical do"ncomer is &idely us
ed as it provides good utili1ation of column area for do
"nflo" and has cost and sim(licity ad%antage. Sloped d
o"ncomer can !e used if vapour-liuid disengagement i
s difficult (e.g. due to foaming*. Sloped do"ncomer also
provide a slightly larger active area for vapour-liuid con
tact, !ut it is also more e2pensive.
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do"ncomer must !e sufficiently large to allo" liuid to
flo" smoothly "ithout cho+ing. Sufficient time must also
!e provided in the do"ncomer to allo" proper vapour dis
engagement from the do"n-flo"ing liuid, so that the li
uid is relatively free of vapour !y the time it enters the tra
y !elo". Inade)uate do&ncomer area &ill lead to do&nc
omer cho$ing+ &hereby li)uid bac$s u( the do&ncomer i
nto the tray abo%e and e%entually flood the column#
Weep Holes
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Weep Holes7oles for drainage must e adequate to drain the tower
in a reasonale time, yet not too large to interfere withtray action.
6raining of the tower
through the trays isnecessary efore any
internal maintenance
can e started.
!he ma8ority of the holes are placed ad8acent to the
outlet or down comer weir.
Bottom &trainer
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&
6uring the operation of a tower:
!he ule caps,
Bolts, and
(ther foreign o8ects
may e dislodged and carried along with ottom
stream.
!o prevent these o8ects leaving the tower and
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!o prevent these o8ects leaving the tower and
damaging pumps, a strainer is installed in the
ottom outlet line.
&trainers must have openings
small enough to catch small
o8ects, ut large enoughnot to hinder the flow of
liquid, or product, or oil
out of the tower.
!he holes in the strainers must e "ept open so that
the flow of liquid out of the tower will not e
stopped, or hindered.
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5eflux distriutor
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5eflux entering the top of the tower should e
spread evenly across the top tray to avoid dead spots.
(ne way to disperse
reflux is to place a
reflux distriutor in
front of the inlet line.
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A reflux distriutor is simply a plate or affle that
prevents liquid from spraying across the tray.
5eflux entering the
tower is forced to flowunder the affle so that
the liquid is distriuted
evenly across the tray.
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!op !ower 6emister
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p
&ometimes small drops of liquid suspended in vapor
are carried up from one tray to the next or into theoverhead vapor line.
!his is called entrainment.
#hen the overhead product must e a dry vapor or
gas, entrainment is a more serious prolem.
Entrainment etween trays can usually e
prevented y controlling vapor velocity.
E t i t t th t f t t d
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103
Entrainment at the top of a tower can e cut down
y placing a demister on the vapor outlet line.
6emisters are constructed
of fine%gauge wire "nitted
into mesh.
6emisters must e "ept
clean of dirt and foreign
matter, or the flow of
vapor will e restricted, or stopped.
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Classification of Trays
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#ased on the 0um!er of liuid paths
2. Two Pass
3. Three Pass
4. Four Pass
1. Single Pass
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!ac$ing
Pac+ing:
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6"88 -;
"C<-; -;
-,"8= "..8E
1' ando% 6ac>ing
2' t#!ct!#ed 6ac>ing
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Str$ct$red
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Pac%ingStructured packing are considerablymore expensiveper
unit volume than random packing. They come with
different sizes and are neatly stacked in the column.
Structure packing usually offer:
- less pressure drop and
- have higher efficiency and capacity than random
packing.
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Reflu- . Reboiling
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Reflu- . Reboiling
Re6$+ The word reux is de!ned as *6o)ing
bac% )
pplying it to distillation tower, reux is
the li'uid owing bac# down the tower
rom each successi"e stage
inds o& Re6$+
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Co"d Re6$+
1old reux is de!ned as li'uid that is supplied at temperature a
little below that at the top o the tower
2ach pound o this reux remo"es a 'uantity o heat e'ual to the
sum o its latent and sensible heat re'uired to raise its temperature
rom reux drum temperature to the temperature at the top o the
tower
constant 'uantity o reux is recirculated rom the reux drum
into the top o the tower
t is "aporized and condensed and then returns in li#e 'uantity to
the reux
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8ot
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8otRe6$+
t is the reux that isadmitted to the tower atthe same temperature asthat maintained at the
top o the tower
t is capable o remo"ingthe latent heat because
no diference intemperature is in"ol"ed.
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irc$"ating Re6$+
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t is also able toremo"e only thesensible heat which isrepresented by its
change in temperatureas it circulates
The reux is withdrawn
and is returned to thetower ater ha"ingbeen cooled
Re6$+ Ratio
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t is de!ned as the amount o actual
reux di"ided by the amount o top
product
t is denoted by 3 which e'uals 456
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)o #oints to consider
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7. minimum number o plates $stages% re'uired at total reux
8. There is a minimum reux ratio below which it is impossible toobtain the desired enrichment howe"er many plates are used
Tota" Re6$+
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Total reux is the conclusion when all thecondensate is returned to the tower asreux, no product is ta#en of and there isno eed
t total reux, the number o stagesre'uired or a gi"en separation is theminimum at which it is theoreticallypossible to achie"e the separation
Total reux is carried out at /
7. Towers start0up
8 Testin o the tower
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O#tim$m Re6$+ Ration
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9ractical reux ratio will lie between
The minim$m or the speci!ed separation and Tota" reux
The optimum "alue will be the one at which thespeci!ed separation is achie"ed at the lowest annualcost
For many systems, the optimum "alue o reux ratio
will lie between
7.8 to 7.: times the minimum reux ratio
Reboi"ing
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n all distillations processes
;eat been added by <eans the eed
<eans o a reboiler
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Types of re!oilers
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yp
The most critical element of re!oiler design is the selection of th
e proper type of re!oiler for a specific service. 4ost re!oilers ar
e of the shell and tu!e heat e2changer type and normally steam
is used as the heat source in such re!oilers. 7o"ever, other hea
t transfer fluids li+e hot oil or Do"therm (T4* may !e used. 8ue
l-fired furnaces may also !e used as re!oilers in some cases.
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:ettle re!oilers
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Thermosyphon re!oilers
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8orced circulation re!oilers
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8orced circulation re!oilers are used for re!oiler duties "here viscous
and$or heavily contaminated media are to !e e2pected in the !otto
m product.
7igh liuid velocities in the tu!es and the resulting shearing forces e
nsure that this type of heat e2changer is operated "ithin its optimu
m performance range, "hile +eeping fouling to a minimum. Pump s
election influences performance and efficiency. 8orced circulation re
!oilers can !e designed for either hori1ontal or vertical installation.
1 ? 'olumn
2 ? ,#ays
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3 ? .onco%e#
4 ? eboiling
ci#c!lation line
5 ? Man/ole
$ ? +orced
circulation
reoiler
7 ? tea% inlet
? ales
9 ? <eating t!bes
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h b il i h h h h
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The 3eboiler is a heat exchanger through
which the bottom li'uids circulate
;eat is transerred to the bottom materialswhich cause "aporization o the lighter
components
This "apor tra"els up the column to pro"ide
The stripping action and The additional heat necessary to
"aporize the down coming reux
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9. CRUDE DSTLLATON
The #$r#ose o& cr$de oi"disti""ation
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disti""ation
is primarily to split the crude into se"eral
distillate ractions o a certain boiling range
Sharpness o ractionation is o secondaryimportance
crude distillation tower, producing =ractions has >? to :? trays
into
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gasoline,
naphtha,
#erosene,
diesel oil,
gas oil, and other products, by
distillation at atmospheric
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147
1rude is generally pumped to the unit
rocess Descri#tion
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1rude is generally pumped to the unitdirectly rom a storage tan#, and it isimportant that charge tan#s bedrained completely ree rom waterbeore charging to the unit
water is entrained in the charget will "aporize in the exchangers and
in the heater, and cause a highpressure drop through that e'uipment
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The mixture o oil and water ist d i d lt hi h i
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150
separated in a desalter, which is a
large "essel in which may beaccelerated by the addition ochemicals or by electrical de"ices
the oil entering the desalter is notenough heated, it may be too "iscousto permit proper mixing and completeseparation o the water and the oil,and some o the water may be carriedinto the ractionators
th th h d th il i t h t
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, on the other hand, the oil is too hot,
some "aporization may occur, and theresulting turbulence can result inimproper separation o oil and water
The desalter temperature is thereore'uite critical, and normally a bypass ispro"ided around at least one o theexchangers so that the temperaturecan be controlled
The optimum temperature depends upon
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The desalter pressure and'uantity o light material in the crude
but is normally about :;2<C :2<C
being lower or low pressure and lightcrudes
The a"erage water inAection rate is
B0:C o the charge
±
8eat E+change
d t d th t ti
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n order to reduce the cost o operating a
crude unit
s much heat as possible is reco"ered rom
the hot streams by heat exchanging them
with the cold crude charge
The number o heat exchangers within the
crude unit and cross heat exchange with
other units will "ary with unit design
Cr$de F"ashing =F$rnace>
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6esalted crude is heat exchanged against what e"er
other heat sources are a"ailable to reco"ermaximum heat beore crude is charged to theheater, which ultimately supplies all the heatre'uired or operation o the crude unit
The heater transer temperature is merely acon"enient control, and the actual temperature,which has no great signi!cance, will "ary rom B8:D1to as high as >B?D1, depending on
The ty#e o& cr$de and
The #ress$re at the bottom o& the&ractionating to)er
Fractionation
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1rude entering the ash zone o the ractionating
column ashes into/
The "apor which rises up the column and The li'uid residue which drops downwards
This ash is a "ery rough separation
The "apors contain appreciable 'uantities o hea"y
ends, which must be reAected downwards intoreduced crude, while the li'uid contains lighterproducts, which must be stripped out
Prod$ct Stri##ing =SideStri##ers>
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Stri##ers>
The ashed residue in the bottom o theractionators and the side cut products ha"ebeen in contact with lighter boiling "apors
These "apors must be remo"ed to meet ashpoint speci!cations and to dri"e the lightends into lighter and more "aluable products
Steam, usually superheated steam, is used tostrip these light ends
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?enera""y on"y eno$gh steam is$sed to meet a 6ash #oints#eci*cation
!hi"e &$rther increases in the/$antity o& steam may raise the@P o& the #rod$ct s"ight"y
ll the stripping steam is condensed inthe o"erhead recei"er and must be
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the o"erhead recei"er and must be
drained of
3euxing water will upset the
ractionators
& the end#oint o& the o'erhead#rod$ct is 'ery "o), )ater may not#ass o'erhead, and )i""acc$m$"ate on the $##er traysand ca$se the to)er to 6ood
Prod$ct Dis#osa"
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ll products are cooled beore beingsent to storage
4ight products should be below =?D1
to reduce "apor losses in storage, but
;ea"ier products need not be as cold
a product is being charged to anotherunit, there may be an ad"antage in
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sending it out hot
product m$st ne'er "ea'e a $nit ato'er :22<C
there is any possibility o it entering atan# with water bottoms
The hot oi" co$"d readi"y boi" the)ater and b"o) the roo& o(
)roduct &pecifications
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The composition o a distillation product is
determined by perorming laboratory
tests on samples o that product.
These test results are then compared with
product speci!cations or standards that
ha"e been set or the product
the product is meeting speci!cations, column
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operations do not ha"e to be adAusted
(ut, i the products are of0speci!cation, a change
in column operations must be made
-ne can see that the control o the tower is a
rather complicated simultaneous solution o
material and heat balances
<aterial (alance
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t each draw we must draw the 'uantity omaterial in the crude that boils within thespeci!ed boiling range
we draw too much, or too little, theproduct abo"e or below will ha"e to shit bythat amount, thereby possibly putting it ofspeci!cation
To stay on speci!cations the materialbalance must be maintained
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Suppose speci!cations on the bottomproduct call or an (9 between 7??
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product call or an (9 between 7??0
77?EF
4ab tests show an (9 o :EF
Gou #now that light material boils atlower temperatures than hea"ymaterial
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o@ t/e botto% p#od!ct in t/is eAa%ple
contains %ate#ial t/at is too lig/t
n order to raise the (9 o a product
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n order to raise the (9 o a product,
we must ma#e the product hea"ier
-ne way is to strip some lightcomponents of with steam
nother way is to increase thetemperature o the eed or the
reboiler temperature so more lightcomponents are "aporized
End @oi"ing Point =EP>
h hi h h l
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s the temperature at which the last
drop o li'uid "aporizes during the test
n a mixture o hydrocarbons, the
last molecules to "aporize are thehea"y ones
So, the 2(9 $29% test is used to chec#or hea"y hydrocarbons that arepresent in a product
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Speci!cations call or an o"erheadproduct with an 29 between 7:?07=?EF
4ab results indicate an 29 o 7H?E F
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Bo! >no t/at /eay %ate#ial boils at
/ig/e# te%pe#at!#es t/an lig/t %ate#ial
o@ t/e top p#od!ct does not %eet
speciications beca!se it contains %ate#ial
t/at is too /eay
n order to lower the 29 o a product,
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we must ma#e the product lighter
-ne way is to decrease the eed orreboiler temperature so that ewer
hea"y components "aporize
nother way is to lower the top
temperature by increasing the reuxrate
F"ash Point
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s the temperature at which a petroleumproduct generates ignitable "apors
4ight hydrocarbons tend to ash moreeasily than hea"y hydrocarbons
sample that contains traces o light
hydrocarbons ashes at a lowertemperature than a sample without thesetraces
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side draw product carries ash pointspeci!cations o 78:07B?E F
The lab test shows a ash point o77?E F
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,/e sa%ple contains %ate#ial t/at is too lig/t
e can b#ing t/e p#od!ct bac> to speciication by
dec#easing t/e #el!A #ate@ o#
!sing %o#e st#ipping stea%@ o#
inc#easing t/e #eboile# te%pe#at!#e
AP ?ra'ity
s used to designate the *hea"iness* or
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s used to designate the hea"iness or
*lightness* o products
Ierosene is measured at about >8E 9Jasoline is measured at about =?E 9
The lighter the oil, the higher 9
gra"ity
o 1rude oil 9 gra"ities typically range from 7 to 52 corresponding
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to about H? #g5mBto H:? #g5mB,
o (ut most fall in the 20 to 45 API gra"ity range.
o Although light crude (ie 40!45 degrees API" is considered
the #est , lighter crude $i.e., >= degree 9 and abo"e% is generally
no better or a typical re!nery.
o As the crude gets lighter than 40!45 degrees API, it contains
shorter molecules, which means a lower carbon number. This also
means it contains less of the molecules useful as high octane
gasoline and diesel fuel$ the %roduction of &hich most
re'ners try to maimi)e.
o If a crude is heavier than *5 degree API, it contains longer and
#igger molecules that are not useul as high octane gasoline and
diesel uel without urther processing
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!ppose speciications call o# a p#od!ct it/ an "6- g#aity o 3035
,/e p#od!ct sa%ple tests 2
,/e p#od!ct is too /eay
Co"or
4ight hydrocarbons are light colored
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g y g
while ;ea"y hydrocarbons are dar# incolor
light hydrocarbon product that is dar#
colored probably contains too many hea"ymolecules
2xcessi"e "apor rates can cause small
drops o li'uid to become entrained in the"apor and be carried up the tower
2ntrainment o hea"y materials maycontaminate the o"erhead product
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p
and ma#e it too dar# in color
;ydrocarbons will decompose andchange color at "ery hightemperatures
So, an of0color product may
indicate that a tower is operating attoo high a temperature
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RUDE DSTLLATON OPERAT
Re6$+ Rate Changing
S h i i d
9$E&!I( 01
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193
Suppose the reux rate is increasedrom 7,??? to 7,8?? barrels per hour,and the other tower operatingconditions are held constant which of the
following will occur
"' 8ig/te# oe#/ead@ botto%@ and side d#a
p#od!cts'' <eaie# oe#/ead and side d#a p#od!cts'
C' <eaie# botto% p#od!ct'
. M / d d t
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,/is eAt#a #el!A loing don t/e toe#
ca!ses t/e te%pe#at!#e on eac/ t#ay to
dec#ease
Some o the hea"ier hydrocarbons in the upward
owing "apors will now condense and all bac# down
the tower
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741$
195
the tower
The extra reux owing down the tower reduces the
temperature o the li'uid at the bottom o the
column. When the bottom temperature decreases,
the amount o light material "aporized out o the
li'uid at the bottom o the tower is dec#eased
(ecause ewer "apors are now going o"erhead, the
amount o top product ormed is decreased, or less.
4ighter o"erhead, bottom, and side draw products
are produced by increasing the reux rate
9$E&!I( 021
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we decrease the reux raterom 7,??? barrels to K??
barrels,
,/ t i t / d ,/
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,/e c!t point c/anges a#e #ee#sed' ,/e
te%pe#at!#e on eac/ o t/e t#ays inc#eases@
and a /ig/e# toe# te%pe#at!#e %ean
/eaie# p#od!cts' o oe#/ead@ botto%@and side d#a p#od!cts beco%e /eaie# '
,/e a%o!nt o oe#/ead p#od!ct p#od!ced
inc#eases and t/e a%o!nt o botto% p#od!ct o#%ed dec#eases
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pe#ation 6#oble%s
+looding
(ccurs when the pressure drop across a tray is so high
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(ccurs when the pressure drop across a tray is so high
that the liquid cannot flow down the tower as fast asrequired.
!he pressure drop across the tray increases to very
high values, and the tray efficiency drops mar"edly.
#hen the froth and foam in the down comer ac" up
to the tray aove and egin accumulating on this tray.
F"ooding is brought about by excessi"e "apour
ow, causing li'uid to be entrained in the "apour up
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the column. The increased pressure rom excessi"e
"apour also bac#s up the li'uid in the downcomer,
causing an increase in li'uid holdup on the plate
abo"e. 6epending on the degree o ooding, the
maximum capacity o the column may be se"erely
reduced. Flooding is detected by sharp increases in
column diferential pressure and signi!cant decreasein separation eciency.
Entrainment +looding *o# a constant liD!id #ate@ inc#easing
t/e gas #ate #es!lts eent!ally in eAcessie ent#ain%ent and looding'
"t t/e lood point it is diic!lt to obtain net dona#d lo o liD!id
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"t t/e lood point it is diic!lt to obtain net dona#d lo o liD!id@
and any liD!id ed to t/e col!%n is ca##ied o!t it/ t/e oe#/ead gas'
*!#t/e#%o#e@ t/e col!%n inento#y o liD!id inc#eases@ p#ess!#e d#op
ac#oss t/e col!%n beco%es D!ite la#ge@ and cont#ol beco%es diic!lt'
6ownflow +looding *looding %ay also be b#o!g/t on by
inc#easing t/e liD!id #ate /ile /olding t/e gas #ate constant' EAcessie
liD!id lo can oe#taA t/e capacity o donco%e#s o# ot/e# passages@
it/ t/e !lti%ate #es!lt o inc#eased liD!id inento#y@ inc#eased
p#ess!#e d#op@ and t/e ot/e# c/a#acte#istics o a looded col!%n'
#eeping;6umping
,/is p/eno%enon is ca!sed by lo apo!# lo' ,/e
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p#ess!#e eAe#ted by t/e apo!# is ins!icient to /old !p t/eliD!id on t/e t#ay' ,/e#eo#e@ liD!id sta#ts to lea> t/#o!g/
pe#o#ations' EAcessie eeping ill lead to d!%ping' ,/at
is t/e liD!id on all t#ays ill c#as/ (d!%p) t/#o!g/ to t/e
base o t/e col!%n (ia a do%ino eect) and t/e col!%n
ill /ae to be #esta#ted' eeping is indicated by a s/a#p p#ess!#e d#op in t/e col!%n and #ed!ced sepa#ation
eiciency'
#eeping;6umping
(ccurs at:
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7igh liquid rates, and<ow vapor loads.
&ome of slots or holes will dump liquid instead of
passing vapor, resulting in poor tray efficiency.
+or towers with conventional down comers,
dumping usually occurs at the upstream raw of caps
or holes, where the liquid has the largest head and"inetic energy.
+oaming
*oa%ing #ee#s to t/e eApansion o liD!id d!e to passage o
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204
apo!# o# gas' "lt/o!g/ it p#oides /ig/ inte#acial liD!id
apo!# contact@ eAcessie oa%ing oten leads to liD!id
b!ild!p on t#ays' -n so%e cases@ oa%ing %ay be so bad
t/at t/e oa% %iAes it/ liD!id on t/e t#ay aboe' /et/e#
oa%ing ill occ!# depends p#i%a#ily on p/ysical
p#ope#ties o t/e liD!id %iAt!#es@ b!t is so%eti%es d!e to
t#ay designs and condition' /atee# t/e ca!se@ sepa#ation
eiciency is alays #ed!ced'
'oning
(ccurs at low liquid rate or seals.
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(ccurs at low liquid rate or seals.
!he vapor pushes the liquid ac" from the slots or
holes and passes upward with poor liquid contact.
!his causes poor tray efficiency.
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5educing the liquid flow rate will usually eliminate
pu"ing.
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)u"ing should not e confused with carryover.
)u"ing occurs almost instantaneously.
+urthermore, if the liquid rate is not reduced, the
tower will pu"e again when the liquid stac"s up.
'arryover is usually caused y a high vapor flow rate0It happens continuously, whereas pu"ing is an
intermittent thing1.
(perating 5ange
&atisfactory operation will only e achieved over a
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20
&atisfactory operation will only e achieved over a
limited range of vapour and liquid flow rates.
!he upper limit to vapour flow is set y the condition
of flooding.
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At flooding there is:
A sharp drop in plate
efficiency and
Increase in pressure drop.
+looding is caused y either:
Excessive carry over of liquid to the next plate
y entrainment, or
<iquid ac"ing%up in the down comers.
!he lower limit of the vapour flow is set y condition
of weeping.
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p g
#eeping occurs when the vapour flow is insufficient
to maintain a level of liquid on the plate.
='oning= occurs at low liquidrates, and is the term given
to the condition where the
vapour pushes the liquid
ac" from the holes and 8ets
upward, with poor liquid contact.
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Than$ 0ou