Multi-Component Distillation

Lecture 2

1

Objectives

�To understand thetheories

theories

theories

theories associated with

multicomponent distillation.

�To be familiar with various methods

various methods

various methods

various methods

multicomponent column design e.g. shortcut

methods, rigorous method.

2

methods, rigorous method.

�Shortcut Method i.e. Smith Brinkley, FUG

�Rigorous Method –Lewis M

atheson (Design Approach)

�Rigorous Method –Thiele Geddes (Rating Approach)

�Some Examples

General Considerations -MCD

�Feed -more than two components

�Not possibleto specify the complete composition of

the top and bottom products independently.

�Separation between the top and bottom products

is specified by setting limits

limits

limits

limitson two ‘key’

two ‘key’

two ‘key’

two ‘key’

components

components

components

components(desired separation)

3

components

components

components

components(desired separation)

�Methods:

1.1.1.1.

Stage

Stage

Stage

Stage- ---bybybyby- ---stage

stage

stage

stagecalculation is too complex

complex

complex

complexand tedious.

2.2.2.2.

Short

Short

Short

Short- ---cut methods

cut methods

cut methods

cut methodsto simplify the task of designing

Multicomponent columns

�Short-cut methods are useful for the preliminary

preliminary

preliminary

preliminary

design work

Short-cut Methods

�Two classes of short-cut methods:

1.1.1.1.

Simplifications

Simplifications

Simplifications

Simplificationsof the rigorous stage-by-stage procedures

–E.g. Hengstebeck(1961) and Smith-Brinkley method

(1960)

2.2.2.2.

Empirical methods

Empirical methods

Empirical methods

Empirical methods, which are based on the perform

ance of

operating columns, or results of rigorous designs

4

operating columns, or results of rigorous designs

–E.g. Gilliland’s correlation, Erbar-M

addoxcorrelation

�Before commencingdesign of multicomponent

distillation, the designer has to choose the two

two

two

two‘keykeykeykey’

components

Shortcut method

1.1.1.1.

Scoping

Scoping

Scoping

Scopingstudies suitable for prelim

inarycosts

2.2.2.2.

Evaluation

Evaluation

Evaluation

Evaluationof operating variables

Separations with coarse purity

coarse purity

coarse purity

coarse purityrequirem

ents (I.e.

5

3.

Separations with coarse purity

coarse purity

coarse purity

coarse purityrequirem

ents (I.e.

contaminants >0.5 wt%)

4.

Detailed designs for approximately idealsystem

s

5.

Design when equilibrium data

equilibrium data

equilibrium data

equilibrium dataare unavailable (minimum

data requirem

ent)

Rigorous Design Procedures

�High

High

High

Highproduct purity is required.

�Non

Non

Non

Non- ---ideal

ideal

ideal

idealsystem

s and good equilibrium data

available.

6

�Relative volatility

Relative volatility

Relative volatility

Relative volatilitybetween key components < 1.3

< 1.3

< 1.3

< 1.3.

�One or more components is near the critical

critical

critical

critical

pressure

pressure

pressure

pressure.

Key Components

�Key componentsare the components between which it is

desired to make the separation

separation

separation

separation

�Light key

Light key

Light key

Light key

at the top

�Heavy key

Heavy key

Heavy key

Heavy key

at the bottom

�Adjacent

Adjacent

Adjacent

Adjacentkeys and Split

Split

Split

Splitkeys

7

�Adjacent

Adjacent

Adjacent

Adjacentkeys and Split

Split

Split

Splitkeys

�Non-keys are non-distributed components

�Non-keys are distributed components

�For complete separation of N components, (N-1) columns

are required

�Order of columnssequence will determine the capital and

operating costs

Heuristic rules –optimum sequencing

�Rem

ove the components one at a time

�Rem

ove any components that are presents in large excess

large excess

large excess

large excessearly in the

sequence

1.

With difficult separations, involving close boiling components

close boiling components

close boiling components

close boiling components,

postpone the most difficult separation to late in sequence

�Difficult separation require many stages, to reduce cost, the column

diameter should be made as small as possible. Since column diameter

8

diameter should be made as small as possible. Since column diameter

depend on flow-rate, the further down the sequence the sm

aller the

amount of material that the column has to handle.

�For tall columns, it may be necessary to split a column into two

separate columns to reduce the height of the column. Alternatively,

reduce the column pressure

Details: Douglas (1988), Conceptual Design of Chem

ical Processes.

Details: Douglas (1988), Conceptual Design of Chem

ical Processes.

Details: Douglas (1988), Conceptual Design of Chem

ical Processes.

Details: Douglas (1988), Conceptual Design of Chem

ical Processes.

Smith-Brinkley method

()

()

()

()

()

11

11

11

+−

−

−

−+

−+

−

−+

−=

ss

rs

r

sr

N s

NN r

r

NN r

r

NN r

SG

SS

RS

SR

S

fb

feed

t

he

abo

ve

st

ages

m

equ

ilib

riu

o

fn

um

ber

bo

tto

m

and

fe

ed e

bet

wee

n t

hco

mp

on

ent

t

he

of

spli

t

frac

tio

nal

Nfb

r

===

9

con

dit

ion

fe

ed

on

th

e

dep

end

s

se

ctio

n

st

rip

pin

g

fact

or,

st

rip

pin

g

se

ctio

n

re

ctif

yin

g

fact

or,

st

rip

pin

g

feed

t

he

bel

ow

st

ages

m

equ

ilib

riu

o

fn

um

ber

G

'L

/'

V'

KS

L/

VK

SN

is

ir

s

==

===

isr

ii

iSS

'L

K

L'

KG

−−=

11

isr

iSS

'LL

G

−−=

11

Main

ly l

iqu

id:

Main

ly v

ap

or:

Smith-Brinkley Procedures

1.

Estimate the flow rates L, V and L’, V’

L, V and L’, V’

L, V and L’, V’

L, V and L’, V’from the specified

component separations and reflux ratio.

2.

Estimate the top

top

top

topand bottom

bottom

bottom

bottomtemperatures by calculating

the dewdewdewdew

and bubble

bubble

bubble

bubblepoints for assumed

assumed

assumed

assumed

top and bottom

compositions

compositions

compositions

compositions.

10

compositions

compositions

compositions

compositions.

3.

Estimate the feed

feed

feed

feed

temperature

4.

Estimate the average component K values

K values

K values

K valuesin the stripping

and rectifying sections

5.

Calculate the values of

values of

values of

values of S SSSr,i

r,i

r,i

r,ifor the rectifying section and S SSS

s,i

s,i

s,i

s,i

for the stripping section

Smith-Brinkley Procedures…

6.

Calculate the fractional split

fractional split

fractional split

fractional splitof each component, and

hence the top and bottom compositions

top and bottom compositions

top and bottom compositions

top and bottom compositions.

7.7.7.7.

Compare the calculated

Compare the calculated

Compare the calculated

Compare the calculatedwith the

with the

with the

with the assumed values

assumed values

assumed values

assumed valuesand

check the overall column

overall column

overall column

overall column material balance.

material balance.

material balance.

material balance.

8.

Repeat the calculation until a

calculation until a

calculation until a

calculation until a satisfactory material balance

satisfactory material balance

satisfactory material balance

satisfactory material balance

is obtained

is obtained

is obtained

is obtained.

11

is obtained

is obtained

is obtained

is obtained.

•The usual procedure is to adjust the feed tem

perature up

adjust the feed tem

perature up

adjust the feed tem

perature up

adjust the feed tem

perature up

and down

and down

and down

and downtill asatisfactory

satisfactory

satisfactory

satisfactorybalance is obtained.

�Note:

Note:

Note:

Note:This is a rating approach

rating approach

rating approach

rating approach, suitable for determining

the perform

ance

perform

ance

perform

ance

perform

ance

of an existing

existing

existing

existingcolumn, rather than a

design method

design method

design method

design method.

Top

Flo

wra

te:

L &

V

Bott

om

Flo

wra

te:

L’

& V

’

Sp

ecif

y C

om

pon

ents

&

Ref

lux R

ati

o

Dew

Poin

t -

Top

Bu

bb

le P

oin

t -

Bott

om

Ass

um

e: T

op

&

Bott

om

Com

posi

tion

Est

imate

: K

i&

Fee

d T

(1)

Com

pare

: A

ssu

med

&

12

Calc

ula

te:

Sr,

i&

Ss,

i

Calc

ula

te:

Fra

ctio

nal

Sp

lit

(b/f

)

Calc

ula

te:

Top

& B

ott

om

Com

posi

tion

s

(1)

Com

pare

: A

ssu

med

&

Calc

ula

ted

Valu

es –

Top

&

Bott

om

Com

posi

tion

s

(2)

Ch

eck

–O

ver

all

Mate

rial

Bala

nce

Sto

p:

Ass

um

ed =

Calc

ula

ted

&

Over

all

Mate

rial

Bala

nce

is

met

Shortcut Method -Fenske Equation

�Calculation of multicomponent separation at total reflux

total reflux

total reflux

total reflux

�Stage equilibrium assumption

�Partial condenser and reboiler

Dx

Dx

)/

(

x

x

AB

RB

A

dis

tB

A

Bx

Bx

Dx

Dx

Nα

ln

)/

(

)/

(ln

min

=

()

()

()

()

()

()

AB

bo

tB

dis

tA

bo

tB

dis

tA

FR

FR

FR

FR

Nα

ln

11

ln

min

−−

=

13

AB

RBA

dis

tBA

xx

xx

Nα

ln

/ln

min

=

----

----

----

-(A

)

Fenske Equation…

()

Ad

ist

AA

Fz

FR

Dx

=

()

()

()

min

min

1

N CB

bo

tBb

ot

B

N CB

dis

tC

FR

FR

FR

α

α

+−

=

----

----

--(B

)

[]

[]

2/1

1

/1

12

1m

in..

.R

N

NN

AB

αα

αα

αα

α≈

=−

0

,,

,=

=b

ot

LN

KL

NK

dis

tL

NK

xF

zD

x

14

Ass

um

e non-d

istr

ibuti

ng n

on-k

eys: 0

,,

,=

=d

ist

HN

KH

NK

bo

tH

NK

xF

zB

x

Stage calculation at Total Reflux

1.

Calculate average relative volatility

average relative volatility

average relative volatility

average relative volatility

2.

Calculate N NNN

min

min

min

minfrom eqn(A)

3.

Calculate fractional recovery of non-key component C

component C

component C

component C

with eqn

eqn

eqn

eqn(B)

(B)

(B)

(B)based on key component A.

4.

Calculate fractional recovery

fractional recovery

fractional recovery

fractional recoveryof non-key component C

component C

component C

component C

15

4.

Calculate fractional recovery

fractional recovery

fractional recovery

fractional recoveryof non-key component C

component C

component C

component C

with eqn

eqn

eqn

eqn(B)

(B)

(B)

(B)based on key component B

component B

component B

component B.

5.

If fractional recovery of C in (3) and (4) are similar then

calculation is consistent (valid assumption)

Note:

Note:

Note:

Note:A

ccuracy of Fenske calculation depends on accurate

estimation of relative volatility

relative volatility

relative volatility

relative volatility.

Underwood Equation-Minimum Reflux

∑ =

−=

C ii

dis

ti

iDx

V1

,

min

φα

α

HK

KV

L

min

min

=φ

Rec

tify

ing s

ecti

on

----

----

-(9

.29)

∑ =

−=

−C i

i

bot

iiB

xV

1

,m

in

φαα

HK

KV

L

min

min

=φ

16

Str

ippin

g s

ecti

on

----

----

-(9

.30)

Underwood equation…

Assume CMO and constant relative volatilities, there are

common values Ø ØØØof that satisfy both equations (9.29 and

9.30):

∑ =

−+

−=

−=

∆C i

i

bot

ii

i

dis

ti

i

feed

Bx

Dx

VV

V1

,,

min

min

φαα

φα

α--

-(9.3

1)

17

feed

feed

V

calc

ula

te

to

feed

at

the

n

calc

ula

tio

flas

h

U

sed

stag

e.

feed

at

the

ra

te

flow

in v

apor

ch

ange

- V

∆

∆

=

−

−i

ii

1φ

αφ

α

()

qF

V

Fz

V

feed

C ii

ii

feed

−=

∆

−=

∆∑ =

1

1φ

αα--

----

---(

9.3

3)

----

----

-(9.3

4)

Underwood equation -Case A

Case A

Case A

Case A

�Assume all non

all non

all non

all non- ---keys do not distribute.

keys do not distribute.

keys do not distribute.

keys do not distribute.

�Solve eqn(9.33) for one value of Ø ØØØbetween the relative

volatilities of the two keys, α

HK< Ø ØØØ

< α

LK.

�Use the value of Ø ØØØto calculate V

minusing eqn(9.29)

�Calculate L

min= V

min-D

�Note: Non-distributing non-keys assumption is probably invalid

�Note: Non-distributing non-keys assumption is probably invalid

for sloppy separations or when a sandwich component is present.

With sandwich component, two value of Ø ØØØ

between α

HKand

αLK

()

[]

HK

bo

tH

Kd

ist

HK

LK

dis

tL

Kd

ist

LK

LN

Kd

ist

LN

Kd

ist

HN

K

Fz

FR

Dx

Fz

FR

Dx

Fz

Dx

Dx

)(

1

an

d 0

,,

,,

−==

==

18

----

----

---(

9.3

6)

----

----

---(

9.3

7)

Underwood Equation –Case B

Case B

Case B

Case B

�Assume non

non

non

non- ---distributing non

distributing non

distributing non

distributing non- ---keys

keys

keys

keysdetermined from

Fenske equation at total reflux is valid at minimum

minimum

minimum

minimum

reflux.

reflux.

reflux.

reflux.

�Dx N

K,distvalues obtained from Fenske Eqn.

�Solve eqn(9.33) for Ø ØØØvalue between relative volatilities

19

�Solve eqn(9.33) for Ø ØØØvalue between relative volatilities

of two keys.

�Use eqn(9.36) and (9.37) to obtain LK and HK in the

distillate

�Solve eqn(9.29) to find V

min

�Calculate L

min= V

min+ D

Underwood Equation –Case C

Case C

Case C

Case C

�Exact solution without further assumption

Exact solution without further assumption

Exact solution without further assumption

Exact solution without further assumption–solve the polynomial

eqn(9.33)

�Solve eqn(9.33) for all values of Ø ØØØlying between the relative

volatilities of all component

�αLNK,1< Ø ØØØ

1< α

LNK,2< Ø ØØØ

2< α

LK< Ø ØØØ

3< α

HK < Ø ØØØ

4< α

HNK,1

�C-1 valid roots

20�C-1 valid roots

�Write eqn(9.29) C-1 times, for each value of Ø ØØØ

�C-1 equations and C-1 unknowns

�Solve the simultaneous equations

�Note:

Note:

Note:

Note: A

ccuracy

Accuracy

Accuracy

Accuracydepends on

depends on

depends on

depends on relative volatility

relative volatility

relative volatility

relative volatilityand CMO

and CMO

and CMO

and CMO

Gilliland Correlation

1.

Calculate N

minfrom Fenske eqn

2.

Calculate (L/D) m

infrom Underwood eqn

3.

Choose actual (L/D); usually multiplication of (L/D) m

inby

a factor between 1.05 to 3.0

4.

Calculate the abscissa [(L/D)-(L/D)

]/[(L/D)+1]

21

4.

Calculate the abscissa [(L/D)-(L/D) m

in]/[(L/D)+1]

5.

Determine the ordinate value (N-N

min)/(N+1)

6.

Calculate the actual number of stages, N

Note:

Note:

Note:

Note:G

illiland correlation is only for rough estimates

rough estimates

rough estimates

rough estimates. C

alculated number

of stages can be off by ±30%,

30%,

30%,

30%,but norm

ally within ±7%.

7%.

7%.

7%.

Gilliland Correlation

00

27

43

.0

N-

N

0.0

1x

0fo

r

57

1.

18

0.1

1N

N-

Nm

in

<<

+−

=

≤≤

−=

+x

22

1.0

x0

.90

for

16

59

5.

01

65

95

.0

1N

N-

N

0.9

0x

0.0

1fo

r

00

27

43

.0

59

14

22

.0

54

58

27

.0

1N

N-

N

min

min

≤≤

−=

+

<<

+−

=+

x

xx

Feed Stage –Gilliland criteria

1+

≤

≤

fH

K

LK

FE

ED

HK

LK

fH

K

LK

xx

xx

xx

23

HK

LK

HK

LK

dis

tH

K

LK

zz

xx

−

=α

/ln

N

stag

e

feed

t

he

wh

ere

esti

mat

e

eqn

to

F

ensk

e

Use

min

F,

FUG

Approach

Fenske

Equation

To

tal R

efl

ux -

Nm

in

24

Underwood

Equation

Case A

Case B

Case C

To

ob

tain

r min

Gilliland

Correlation

To

ob

tain

actu

al

N

Kirkbride empirical equation

N NNNand N

and N

and N

and N

are the number of theoretical stages above

20

60

2.

D,H

K

B,L

K

FL

K

HK

sr

DB

xx

xx

NN

=

25

�N NNN

r rrrand N

and N

and N

and N

s sssare the number of theoretical stages above

(rectifying section) and below (stripping section) the feed

plate respectively.

�Match as closely as possible the composition of the feed

composition of the feed

composition of the feed

composition of the feed

and

that of the appropriate stream

(vapor or liquid) leaving the

leaving the

leaving the

leaving the

feed stage

feed stage

feed stage

feed stage.

Rigorous MCD Methods

�Lewis

Lewis

Lewis

Lewis- ---M

atheson

Matheson

Matheson

Mathesonmethod (LM) is rigorous analog of FUG

rigorous analog of FUG

rigorous analog of FUG

rigorous analog of FUG

shortcut method.

�Number of stages is determined from specified split between

two key components.

Basis of LM method:

26�Basis of LM method:

()

()

() D

in

in

ix

VDx

VLy

+=

+1

()

()

() B

im

im

x'

V

'B

x'

V

'L

y−

=+

11

Rec

tify

ing s

ecti

on

Str

ipp

ing

sec

tio

n

Thiele-Geddes method (TG)

�Rating method which is analog to SB shortcut

analog to SB shortcut

analog to SB shortcut

analog to SB shortcut.

�Distribution of components

Distribution of components

Distribution of components

Distribution of componentsbetween distillate and bottom is

calculated for a specified number of stages.

�Approach starts with an assumed number of stages

assumed number of stages

assumed number of stages

assumed number of stagesand

reflux ratio, and provides as output the separation

output the separationthat can

27

reflux ratio, and provides as output the separation

output the separation

output the separation

output the separationthat can

be made.

�It rates a given column instead of designing it.

�Has convergence advantages

convergence advantages

convergence advantages

convergence advantages, hence norm

ally used for

computer solutions to multicomponent, multi-tray

distillation systems.

LM Method –Design Approach

�Starts with a given separation and reflux ratio

given separation and reflux ratio

given separation and reflux ratio

given separation and reflux ratioand

determines the number of theoretical stages

number of theoretical stages

number of theoretical stages

number of theoretical stagesrequired to

make the separation.

�Stage

Stage

Stage

Stage- ---bybybyby- ---stage calculations

stage calculations

stage calculations

stage calculationsstarting at thebottom

bottom

bottom

bottomup

up

up

up the

28

�Stage

Stage

Stage

Stage- ---bybybyby- ---stage calculations

stage calculations

stage calculations

stage calculationsstarting at thebottom

bottom

bottom

bottomup

up

up

up the

column, or from the top down

top down

top down

top downthe column.

�Difficulty is the distribution of components

distribution of components

distribution of components

distribution of componentsbetween the

distillate and the bottoms will not be known until the

calculation is completed.

LM method –Design Procedure

1.

Estimate reflux ratio

reflux ratio

reflux ratio

reflux ratio, product compositions

compositions

compositions

compositions, tem

perature

temperature

temperature

temperature

profile,

2.

And number of theoretical trays

theoretical trays

theoretical trays

theoretical traysand feed location

feed location

feed location

feed location(by

various shortcut methods).

3.

Check the number of theoretical trays, feed trays, feed tray

location and tem

perature profile by LM method

4.

Calculate the product composition by TG method

294.

Calculate the product composition by TG method

Note:

Note:

Note:

Note:

LM -design method. TG -rating method. LM does not give accurate

product compositions

product compositions

product compositions

product compositions, but provides a better check for fixing the

better check for fixing the

better check for fixing the

better check for fixing the

specification

specification

specification

specificationof tower design (number of theoretical stages required

at a given reflux ratio).

TG allows the designer to rate the perform

ance

rate the perform

ance

rate the perform

ance

rate the perform

ance

of a given design by

checking the theoretical product compositions.

Rating method

�Input

�Number of stages

�Feed stage number

�Feed rate

�Feed composition

�Output

�Distillate composition

�Bottom composition

30

�Feed composition

�Feed enthalpy

�Reflux ratio

�Distillate to feed ratio

�Pressure

Design method

�Input

�Distillate composition

�Bottom composition

�Feed rate

�Feed enthalpy

�Output

�Number of stages

�Feed stage

�Reflux ratio

�Distillate rate

31

�Feed enthalpy

�Design/m

inimum reflux

ratio

�Optimum feed stage

�Pressure

�Distillate rate

TG method-Rating Approach

�Assume a temperature profile throughout

temperature profile throughout

temperature profile throughout

temperature profile throughoutthe column

(number of stages given).

�Starting at either end and working along the profile, arrive at

a composition at the other end.

32

a composition at the other end.

�If the calculated split

calculated split

calculated split

calculated splitdoes meet material balance

material balance

material balance

material balance

requirem

ent, the temperature profile is varied

temperature profile is varied

temperature profile is varied

temperature profile is varied

through

successive iterations.

TG method

�Assume number of theoretical plates, reflux ratio, temperature

on each plate.

�Procedure:

�Obtain V VVV- ---L distribution coefficient

L distribution coefficient

L distribution coefficient

L distribution coefficientas a function of T and P for

each component

�Perform

plate

plate

plate

plate- ---to tototo- ---plate calculations

plate calculations

plate calculations

plate calculationsfrom reflux condenser AND

from reboiler to fed plate

33

Perform

plate

plate

plate

plate- ---to tototo- ---plate calculations

plate calculations

plate calculations

plate calculationsfrom reflux condenser AND

from reboiler to fed plate

�Match results at feed plate

Match results at feed plate

Match results at feed plate

Match results at feed plate, calculate distillate and bottom

compositions

�With the obtained compositions, check the assumed T

�Adjust T and repeat until convergence.

Note:

Note:

Note:

Note:See Dr. M

artynRay note, pp 1.17-1.18 for mathem

atical operation of TG method.

Problem Specification

�C+6 variables must be specified, and C+2 are always

specified:

�Feed composition (C-1)

�Feed rate (1)

�Feed enthalpy (1)

34

�Feed enthalpy (1)

�Pressure (1)

�What are other four variables?

TG method –rating approach

1.

Number of stages

2.

Feed stage number

3.

Reflux ratio

4.

Distillate to feed ratio

Alternatively

Alternatively,

35

�Alternatively

Alternatively

Alternatively

Alternatively,

1.

Rectifying stages and Stripping stages or

2.

Number of stages and Feed stages

3.

Fractional recovery of LK in distillate

4.

Concentration of LK in distillate

Design Approach –LM method

1.

Distillate composition

2.

Bottom composition

3.

Design/m

inimum reflux ratio

4.

Feed location (usually at optimum)

�Alternatively,

Alternatively,

Alternatively,

Alternatively,

36

�Alternatively,

Alternatively,

Alternatively,

Alternatively,

1.

Composition of LK in distillate

2.

Composition of LK in bottom

3.

Reflux ratio

4.

Feed location (usually at optimum)

Other Methods

1.

Underwood’s Group M

ethod, A.J.V., Trans. Inst. Chem

. Eng. (London), Vol.10, p.112 (1932); and J. Inst. Petrol.,

Vol.31, p.111 (1945); Vol.32, p.598 and p.614 (1946)

2.

Tridiagonal-M

atrix Algorithm, (see Perry 6

thed, pp.13.44

to 13.47 for further detail)

37

to 13.47 for further detail)

3.

Matrix Techniques for Multicomponent Distillation

•Mathem

atical models for multicomponent equilibrium

separation systems in matrix notation (see Dr. M

artynRay,

pp1.22 –1.23 for further detail).

Summary

�FUG

FUG

FUG

FUG- ---method

method

method

methodto estimate the number of theoretical stages at

a given reflux ratio.

�Operating reflux ratio is usually chosen to be between 1.05

1.05

1.05

1.05

to 3 times that of minimum value

to 3 times that of minimum value

to 3 times that of minimum value

to 3 times that of minimum value.

38

�LM method is a design approach

LM method is a design approach

LM method is a design approach

LM method is a design approach(output are number of

stages, feed stage, reflux ratio, distillate).

�TG is a rating approach

TG is a rating approach

TG is a rating approach

TG is a rating approach(output are distillate composition

and bottom composition).