(8.3)Distillation Column

8/10/2019 (8.3)Distillation Column

1/21

HYDROCRACKING 85

Design of Distillation Column

Distillation:

In process industries , it is often desired to separate the components of liquid mixture . theeasiest way is to carry out this separation is by distillation .this unit operation makes use of thedifference in the boiling or the relative volatilities of the components. Distillation is consideredto be the preferred separation technique if the relative volatility difference between the two keycomponents (that are required to be separate from each other ) is greater than .!

Selection of the Distillation Column:

Batch and Continuous Columns.

Distillation columns may be batch or continuous, based on the feed is introduced.In batch columns, a batch of feed is charged and operating carried out till the desired degree ofseparation is achieved. "he material removed and the next batch is charged. "hese columns aresuitable for very low throuputs and for system where very high purity is required. #ontinuouscolumns process continuous feed streams. "he are widely used in industries for high throuputs.

$ere we are concerned with the later type of operation.

Vacuum Distillation Column

I have selected the continuous vacuum distillation column because.

%or heat sensitive material (gasoline) &elative volatility of components is increased "o avoid thermal decomposition of '. . ("*+ - #) /nergy economical process.

Selection of Vacuum System:

Steam jet ejector Vacuum pumps0imple design , with no moving parts and

practically no wear.$igh power requirement.

1owest capital cost among vacuum producing devices.

1ow operating cost but high capitalinvestment .


2/21

HYDROCRACKING 86

0imple repair and maintenance $ard to repair and maintain.ffers the largest throuput capacity of any

vacuum producing devices,#apacity range varies depending upon typeof pump.

#an handle more than ---,--- ft + 2min of process fluid.

#an handle overload in capacity at the costof power

3referred when steam is available.

0everal devices are available for producing vacuum at a chemical process plant. /ach has its ownadvantages and disadvantages , among these e4ectors are workhorses. "he simples and probablymost widely used vacuum producer is the steam 4et e4ector.

Ejectors Offer a Range of Attraction: 0imply design with no moving parts and particularly no wear. #an be mounted in any orientation #an be fabricated of virtually any metal 5o special start up or shut down procedure required #an handle condensable loads and corrosive vapors 0imple repair and maintenance .

Steam Jet Ejector:

0team 4et e4ectors are designed to convert the pressure energy of a motivating fluid to avelocity energy to entrain suction fluid ant then to recompress the mixed fluid by convertingvelocity energy back in to the pressure energy. "his is based on the theory that a properlydesigned no66le followed by a properly designed throat or venture will make economically makeuse of high pressure fluid to compress from a low pressure region to a high pressure. "his changefrom pressure head to velocity head is the basis of the 4et vacuum principle.

Ejectors Range:

/4ectors range from single stage up to six stage units, and can be either condensing or non7condensing types. "he numbers of e4ector stages required are usually determined by the economyof the e4ectors and the level of vacuum required. 8acuum ranges for each stage are as follows.9

st stage : -mm$g'7+-mm$g'! nd stage +-mm$g'7+mm$g'+rd stage ! mm$g'7-.:mm$g'; th stage ; mm$g'7< microns $g '

th stage -.; mm$g'7 - microns $g'=th stage -. mm$g'7+ microns $g'


3/21

HYDROCRACKING 87

Ejector Performance Curve:

"his curve is used to find out the required water or steam flow rate to create the desired suction pressure in the upstream of the motive fluid.

Tray and Packed Column:"he performance of a distillation column depends upon the intimate contact of liquid

and vapor steams. "wo configurations are widely used in this respect. "hese are the plate and the packed columns. ' general comparison of the two configuration is made below.

. 3acked columns are continuous contacting units. n the other hand , vapor liquid contactin a tray column occurs only at discrete location i.e., on the trays.

Fig .

Fig .


4/21

HYDROCRACKING 88

!. "he liquid and vapor streams never reach equilibrium in a packed column. In a traycolumn, the steams leaving any stage are assumed to be in the equilibrium.

+. 3acked columns can operate over a relatively wide range of vapor flow rates. n theother hand , plated columns offer wide operating range with respect to the liquid flowrates.

;. 3acked columns cannot work efficiently under stressed condition of temperature and pressure.

. 1iquid distribution can be a problem in packed columns and cause channeling.=. %or diameters less than about -.= m , a plate column cannot be constructed. "herefore a

packed column has to be used.


5/21

HYDROCRACKING 89

"hree basic types of trays are used in plate columns. "hese are sieve trays, bubble captrays, and valve trays. "he trays selected for the column are sieve trays. "he supportingfactors are.

. "hey are light weight and the cheapest available, and easiest to fabricated and install.

!. "hey have higher capacity and lower pressure drop than other tray types+. 0ufficient design data is available;. "he maintenance cost is lower because of the ease of cleaning.

Com onent Of Distillation Column:

Aa4or components of distillation system are.

Fig


6/21

HYDROCRACKING 90

. "he distillation column itself , as a tray or packed column where the vapor and liquidstreams are brought in contact with each other, and the separation occurs.

!. ' reboiler to provide the heat required for vapori6ation.+. ' condenser to remove the heat from the system and condenses the vapors leaving from

top of the column.

;. ' reflux drum to hold the condensed liquid after the separator and provide continuousliquid reflux to the column.

Design Of Distillation Column:

"he design steps for a column design are9

#alculation of Ainimum number of stages. 5 min

#alculation of Ainimum &eflux &atio & m.

#alculation of 'ctual &eflux &atio.

#alculation of theoretical number of stages.

#alculation of actual number of stages.

#alculation of diameter of the column.

#alculation of weeping point, entrainment, etc

#alculation of pressure drop.

#alculation of the height of the column.


7/21

HYDROCRACKING 91

!rom "aterial #alance $e %ave: &ta'le( )*+,

Com onent !eed

!raction

- f

#ottom

!raction

- '

To

!raction

- d

C+.C/ *012 0 *000+

C1 *03/ 0 *00)2

4(C3 *03) 0 *01/

5(C3 *036 0 *0/1)

7asoline *1+1 0 *312

5a t8ta &9 , *1/; *0


8/21

HYDROCRACKING 92

$ater 0 0 *03

Selection Of ey On T8e #asis Of Relative =olatility*

# ,#! B lighter than lighter key

#+ B lighter than lighter key

i7#; B lighter than light key

n7#; B lighter than light key

gasoline B lighter than light key

naphtha B light key

unconverted. '. . B heavy key

5ature Of !eed:

%eed is entering in column as a saturated liquid at "B - - # and 3B+- kpa

#omponents Cf @i @i Cf # ,#! -.-+> =- 0.115648

#+ -.-;! + 0.321437i7#; -.-;: = 0.340852n7#; -.-;< 0.208026

asoline -.+ + .-< 0.33628 5aphtha -.+!= -. 0.042272E.'. . -. := .+ /7 2.42E-06

?y bubble point Cf @iB . -F it is verified that feed is entering at saturated liquid. ref9 (col 8= page ;>:.)

Estimation Of To Tem erature:

?y dew point calculation , (@i2Cd)B

"B>- P=13 kpa

Table


9/21

HYDROCRACKING 93

#omponents Cd @i @i2Cd# ,#! .--- 147.5 1.62E-05#+ .--:> 32 9.59E-04i7#; .-+! 18 3.92E-03n7#; .-!+: 17 3.04E-03

asoline .;+> 22.1923 1.75E-02 5aphtha .; 2.2077 2.06E-01E.'. . .-- + 0.0014 8.25E-01Gater .-; 0.25E-02

$ence by dew point calculation (@i2Cd)B .! app . it is verified the top temperature

Estimation Of #ottom Tem erature:

?y bubble point calculation Cw @iB "B!-- # and 3B+- kpa

#omponents Cw @i Cw @i 5aphtha -.- : 13.025 0.8597041En.'. . -.>;! 0.094866667 0.0893334

$ence by bubble point calculations the bottom temperature is verified. Cw @iB-.:> !

Calculation Of "inimum 5um'er Of Stages:

"he minimum no. of stages 5min is obtained from %enske relation which is

5 min H B ln (x1@ 2x$@ )D2(x1@ 2x$@ )? J

ln (K1@ 2$@ ) average

"o find average geometric relative volatility of light key to heavy key9

so Average geometri re!ative vo!ati!it" = 3.01

5 min H B:.


10/21

HYDROCRACKING 94

Calculation Of "inimum Reflu- Ratio r m

Esing Enderwood equation

's feed is entering as saturated liquid so,

qB

?y trial, B .!! (root of equation)

Esing equation of minimum reflux ratio

3utting all values we get,

R m > 0*1

Actual Reflu- Ratio:

"he rule of thumb is

& B ( .!777777777 .


11/21

HYDROCRACKING 95

%rom which the theoretical no of stages to be

5> +-)2! B ; o#

%eed viscosity at average temperature B avg B-.-- < m5s2m !

0o E o >

Calculation Of !eed Plate:

"he irk 'ride method is used to determine the ratio of trays above and below the feed point.

ref #oulson 8= page !=

%&ere # r = $'m(er o) *tage* a(ove t&e )ee+, i$ !'+i$g a$" partia! o$+e$*er,# * = $'m(er o) *tage* (e!o% t&e )ee+, i$ !'+i$g t&e re(oi!er,

= mo!ar o% (ottom pro+' t,/ = mo!ar o% top pro+' t,

), = o$ e$tratio$ o) t&e &eav" ke" i$ t&e )ee+,), = o$ e$tratio$ o) t&e !ig&t ke" i$ t&e )ee+,+, = o$ e$tratio$ o) t&e &eav" ke" i$ t&e top pro+' t,


12/21

HYDROCRACKING 96

(, = o$ e$tratio$ o) t&e !ig&t ke" i) i$ t&e (ottom pro+' t

C 1k(f)B. +: ?B+; . kmol2hr

C 1k(?)B. - : DB !->kmol2hr

C $k(?)B. -- +

C $k(f)B. :;

%rom which,

5umber of 3lates above the feed trayB5r B 5D B -

5umber of 3lates below the feed trayB5s B 5? B >

0o feed is entering at plate from top

To And #ottom Condition

To conditions #ottom conditions1nB=-;kmol2hr 1wB1nH%B!+:!kmole2hr1nNB1mNB :.>kg2hr 8wB1w7GB!-+=.+ kmole2hr8nB1nHDB : +kmole2hr 1mB1wNB!:; =!kg2hr8nNB8mNB =:- kg2hr"B>-#B+=;@ ,3B +kpa "B!--#B;


13/21

HYDROCRACKING 97

1 mB1 nB!:; =!kg2hr

8 mB8 nB >=:- kg2hr

vB .=:kg2m +

1B< =. kg2m+

%18 B 1iquid 8apor %actor B .!

'ssumed tray spacing B + inch (-.> m)

5et vapor velocity at flooding.

%rom %ig ( .!


14/21

HYDROCRACKING 98

RvB >=:- 2( .=: +=--)B >.=m +2s

5ow, net area B R v2vn B +;ft

(based upon bottom conditions)

in the similar way we calculate the top diameter which come out to be 3*31m .so we willdecrease the perforated area for uniform diameter of m

Provisional Plate Design :#olumn Diameter D cB m

#olumn #ross7sectional 'rea(' c) B !- m !

Down comer area Ad B -. ' c B+ m!

5et 'rea (' n) B ' c 7 ' d B < m!

'c;


15/21

HYDROCRACKING 99

'ctive area ' aB' c7!' d B ; m !

$ole area A h take -Q A a B -. T ;

B .; m !

To find $eir lengt8

'd 2 'c B + 2 !- B -.

!rom figure ++*1+ Coulson B Ric8ardson ;t8 volume 3t8 edition age


16/21

HYDROCRACKING 100

B =+.!@g2sec

IwB;m

OlB< =kg2m+

't Aaximum rate ( h ow)B =+ mm 1iquid

't Ainimum rate (h ow) B ;.= mm 1iquid

hw H how B ! H ;.= B mm 1iquid

from fig ++*10. Coulson and Ric8ardson =ol*;

@ ! B !:

0o, putting the value for Emin we will have.

E (min) B +. m2sec

5ow maximum volumetric flow rate (vapors) ?ase B >.= m +2sec

' hB .;m !

't :-Q turn down ratio

'ctual minimum vapor velocity

B minimum vapor rate 2 ' h

B . m2sec

0o minimum vapor rate will be well above the weep point.

5ow we well calculate the pressure drop (3.D)

Plate Pressure Dro &P*D,:

#onsist of dry plate 3.D (orifice loss), 3.D due to static head of liquid and residual 3.D (bubblesformation result in energy loss H froth formed in operating plates)

8t>8d &8 8o , 8r

Dry Plate Dro &8d,:

Fig


17/21

HYDROCRACKING 101

Aax. 8apor velocity through holes (Eh) B

Aaximum 8olumetric %low &ate 2 $ole 'reaB>.=2 .;B =.: m2sec

3erforated area 'p (active area) B ; m !

'h2'p B -. --

!rom fig* ++*13 &Coulson B Ric8ardson ;t8 volume3t8 edition, for

plate thickness2hole diameter B .-- Ge get, # o B -.:;

hd B ! mm 1iquid

Residual %ead &8 r ,:

&r = 12.5 10 3

O1 B< =kg2m+ so hr B =. mm 1iquid

Total Pressure DrohtB! H(! H ;)H =.

ht B + .+ mm liquid

"otal column pressure drop 3a, (52m ! )

Y3 B (>.: - 7+) h tO1 B :>-3a B -.:>- k3aB-. psi

'nd allowable p.d is -. psi per tray.

Do n Comer 9i@uid #acku ::a'*e+ (" P./ over t&e p!ate a$+ re*i*ta$ e to o% i$ t&e +o%$ omer it*e!).

8 ' > &8 8 o , 8 t 8 dc

"he main resistance to flow in down comer will be caused by constriction in the down comeroutlet, and head loss in the down comer can be estimated using the equation given as,

Fig


18/21

HYDROCRACKING 102

where 1 wd is the liquid flow rate in down comer, kg2sec(kg2sec)

OlB


19/21

HYDROCRACKING 103

h bcB-.;-;m

OlB< =kg2m+

1 maxBkg2sec

tr B ! sec

It should be \ + sec. so, result is satisfactory

C8eck Entrainment:

(un) actual velocity (based on net area) B (max volumetric flow rate at base 8 m 2 net area ' n)

unB>.=2


20/21

HYDROCRACKING 104

%eig8t of distillation column:

$eight of column $ cB (5 act7 ) $ sH ^$H plates thickness

5o. of plates B !>

"ray spacing $ s B -.>- m

^$Btop clearance H bottom clearance.

^$B . H . m

"otal thickness of trays B -.-- !> B -.+; m

0o,

$eight of column B (!>7 ) -.>-H +H-.+;

> /2 meters

;ig < 8.7


21/21

HYDROCRACKING 105

S ecification s8eet

4dentification:

4tem vacuum Distillation column

5o* re@uired:

Tray ty e: 0ieve tray

!unction:

separation of gasoline and naphtha from unconverted '.. .

O eration: #ontinuous

"able Z :.=

5o. of trayB !> 'ctive holes B m Geir length B ; m

Diameter of columnB m &eflux ratio B -.

3ressure drop pertrayB-.:>>@paB-. psi

"ray spacing B-.> m

$ole si6e B mm 'ctive area B ; m !

"ray thickness B mm %looding 3ercent B:- Q

/ntrainment B -.-! Q &esidence timeB ! sec

Documents

(8.3)Distillation Column