Multi Component Distillation 3l1w1h

MULTICOMPONENT DISTILLATION

DISTILLATION 

Mass transfer process of separation based on distribution between the vapor and liquid phases. Distillate (D)

Feed (F) ABC

Vapor rate (V)

bottoms

3

ABSORPTION AND DISTILLATION BOTH involve mass transfer & equilibrium Differences1) All components in the mixture transfer during distillation (this complicates equilibrium calculations). Vap/Liq Equilibrium = All components exist in both phases Gas/Liq Equilibrium = Only 1 component exists in both phases 2) Addition of heat is required for distillation 3) Degrees of freedom for distillation of a binary mixture (2 components) F=C-P+2 = 2-2+2= 2 (Absorption has 3 degrees of freedom) 4) Only 1 feed stream in distillation (2 in absorption/stripping)

INTRODUCTION TO MULTICOMPONENT DISTILLATION •

•

•

•

Most of the distillation processes deal with multicomponent mixtures Multicomponent phase behaviour is much more complex than that for the binary mixtures Rigorous design requires computers

Short cut methods exist to outline the scope and limitations of a particular process

MULTICOMPONENT DISTILLATION As in binary mixtures calculations of equilibrium stages usage multi component mixtures also requires 

material balances for each component overall and for each stage



one enthalpy balance overall and one for each stage



complex phase Equilibria

DEGREES OF FREEDOM IN MCD variables

No. to be specified

Feed rate (F)

1

Feed composition

3

Quality of feed

1

Distillate

1

Bottom product

1

Reflux ratio

1

Reflux condition

1

Optimum feed plate

1 Total 10 (= C+6)

PHASE EQUILIBRIA IN MULTI-COMPONENT MIXTURES Distribution Coefficient or K factors K's = fn(T, P, comp) Ki = yie/xie

If Dalton’s law and Roult’s law hold then Ki = Pi’/P

We can Use Relative volatility for each component in system instead of K's

Relative volatility for each component based on one base component key

 IJ

mI  mJ

VAPOR LIQUID EQUILIBRIUM 

Bubble point



yi*  1.0

 i , j xi yi    i , j xi * x  i  1.0



Dew point xi 

yi /  i , j

y

i

/ i , j

MULTI COMPONENT FLASH DISTILLATION Vaporizing a definite fraction of liquid  Evolved vapor in equilibrium with residual liquid  Separate vapor from liquid and then condense  No reflux 

FLASH DISTILLATION

GOVERNING EQUATIONS

yDi

xFi 1 f   xBi f f

 yDi 1  xFi  Ki   f  1  xBi f  xBi  Nc

x i 1

Nc

Bi

1  i 1

xFi f ( K i  1)  1

DISTILLATION COLUMN DESIGN

FRACTIONATION OR COLUMN DISTILLATION

DISTILLATION COLUMN DESIGN

DISTILLATION COLUMN DESIGN Tools: Material balance Energy balance Thermodynamic equilibrium Bubble point / dew point summation

Specifications:

Purity Recovery

In multi component separation involving n species, n-1 columns are needed to totally separate all n species Computers usually used because of the large number of variables (T,P, composition, flow rates) and because iterative solution is necessary Two different Methods are commonly used to specify computer input I.

specify feed condition, desired separation between 2 components, and reflux ratio II. specify feed condition, no. of stages, and reflux ratio

KEY COMPONENTS Light key : designated by L  Heavy key : designated by H 

Only KEY components are present in significant amounts in both Distillate and Bottom Usually, L & K are adjacent in rank order of volatility. This is called a "sharp" separation

DISTRIBUTED AND UNDISTRIBUTED COMPONENTS • Components that are present in both the distillate and the bottoms product are called distributed components - The key components are always distributed components • Components with negligible concentration (<10-6) in one of the products are called undistributed

A

B

light non-distributed components (will end up in the overhead product)

C

D

key components

E

G

heavy non-distributed components (will end up in bottoms product)

FENSKE EQUATION FOR MULTICOMPONENT DISTILLATIONS Assumption: relative volatilities of components remain constant throughout the column yN

N m in

 xD , LK xB , HK  ln   xB , LK xD , HK    1 ln  LK , HK

LK – light component HK – heavy component

N

1 yo Total Reboiler

K LK (T )  LK , HK (T )  K HK (T )

xN+1

x1

FENSKE EQUATION FOR MULTICOMPONENT DISTILLATIONS Choices for relative volatility:

T

K LK (T )  LK , HK (T )  K HK (T ) B 1) Relative volatility at saturated feed condition

 LK , HK   F

LK ,HK

(TF )

2) Geometric mean relative volatility

 LK , HK   D

B ( T )  (TB ) D LK , HK LK , HK

 LK , HK  3  F

D B ( T )  ( T )  (TB ) F D LK , HK LK , HK LK , HK

NON KEY COMPONENT DISTRIBUTION FROM THE FENSKE EQUATION

x D ,i x B ,i



N m in 1 i , HK

 xD , HK     xB , HK 

 DxD , HK  FxF ,i     BxB , HK     DxD , HK  N m in 1 1   i , HK     BxB , HK  N m in 1 i , HK

DxD ,i

 i , HK

Ki  K HK

MINIMUM REFLUX RATIO ANALYSIS

•Maximum ratio which require infinite no. of trays for desired separation • •At the minimum reflux ratio condition invariant zones occurs above and below the feed plate, where the number of plates is infinite and the liquid and vapour compositions do notchange from plate to plate • Unlike in binary distillations, in multicomponent mixtures these zones are not necessarily adjacent to the feed plate location

y y1

zf yB xB xN

zf

xD x

MINIMUM REFLUX RATIO ANALYSIS Underwood method * Relative volatility of each component has to be the same for each invariant zone * Constant molar overflow * αi=Ki/Kref (Usually Kref=KHK) The operating line equations for each section of the column become:

L Vyi ,n 1  yi ,n  Dxi , D  i K REF L V yi ,m 1  yi ,m  Bxi , B  i K REF

rectifying section

stripping section

Minimum reflux ratio analysis Underwood method

L yi ,n  Dxi , D  i K REF L V yi ,m 1  yi ,m  Bxi , B  i K REF Vyi ,n 1 

In the invariant zones:

rectifying section

stripping section

yi ,n 1  yi ,n  yi ,

 L  V   yi ,  Dxi , D  i K REF    L  V   y i ,  Bxi , B    K i REF  

 i xi , D  i xi , D V yi ,   D  i  L VKREF   i  A  i xi , B  i xi , B V  y i ,   D  i  L VKREF  i  A





Minimum reflux ratio analysis Underwood method

 i xi , D V V  D yi ,   D     A i  i xi , B V V   y i ,     D B i  A

We are looking for a condition where this is correct. In general there are multiple solutions

But consider the following

 i xi , D  i xi , B V  V  D  B  F (1  q) i  A i  A

Minimum reflux ratio analysis Underwood method

 i xi , D  i xi , B V  V  D  B  F (1  q) i  A i  A In other words:

1 x1, D  1 x1, B  2 x2, D  2 x2, B F (1  q)  D B D B  1  A 2  A 1  A 2  A Under Underwood conditions: A=Ā,

i   i

1 x1, F  2 x2, F  i xi , F (1  q )     1  A  2  A i  A

MINIMUM REFLUX RATIO ANALYSIS Underwood method

 i , HK xF ,i (1  q)   i  i , HK  A

 i , HK xD,i V Rm  1    D i  i , HK  A

For a given q, and the feed composition we are looking for A satisfies this equation (usually A is between αLK and αHK. Once A is found, we can calculate the minimum reflux ratio

GILLILAND CORRELATION: NUMBER OF IDEAL PLATES AT THE OPERATING REFLUX

 RD  RDm  N  N min   f  N 1  RD  1 

Kirkbride equation: Feed stage location

  xF , HK NR   N S  xF , LK 

 xB , LK   x  D , HK

NR  NS  N

 B    D   2

0.206

Complete short cut design: Fenske-Underwood-Gilliland method Given a multicomponent distillation problem: a) Identify light and heavy key components

b) Guess splits of the non-key components and compositions of the distillate and bottoms products c) Calculate

 LK , HK

d) Use Fenske equation to find Nmin e) Calculate distribution of non key components f) Use Underwood method to find RDm g) Use Gilliland correlation to find actual number of ideal stages given operating reflux h) Use Kirkbride equation to locate the feed stage

RIGROUS METHODS Two broad categories 1. Equilibrium methods 2.

Rate based models



Equilibrium methods solve MESH equation simultaneously



Rate based method solve mass and heat transfer equations in of available driving force

MESH equations M- Material balance equations Total material balance:

Ln 1  Vn 1  Fn  ( Lsn  Ln )  (Vsn  Vn )  0 Component i balance:

Ln 1 xn 1  Vn 1 yn 1  Fn . zi ,n  ( Lsn  Ln ). xi ,n  (Vsn  Vn ). yi ,n  0

E- Equilibrium relations

EMG ,i n .Ki ,n . xi ,n  yi ,n  (1  EMG ,i n ). yi ,n 1  0 S- summation of mole fractions

1  yi ,n and i

x

i ,n

1

i

H- Heat (Enthalpy) balance

Ln1H L,n1  Vn1HV ,n1  Fn .H F ,n  ( Lsn  Ln ).H L,n  (Vsn  Vn ).HV ,n  Qn  0

RATE BASED MODEL 

Consider that murphee efficiency of plates varies from plate to plate



A simulation program RATEFRAC available in ASPEN PLUS

EQUIPMENT AND COLUMN SIZING 



In order to have stable operation in a distillation column, the vapor and liquid flow must be managed. Requirements are:

vapor should flow only through the open regions of the tray between the downcomers  liquid should flow only through the downcomers  liquid should not weep through tray perforations  liquid should not be carried up the column entrained in the vapor  vapor should not be carried down the column in the liquid  vapor should not bubble up through the downcomers 

SINGLE SIEVE TRAY

TRAY DECK

MULTIPLE TRAYS

Single

Two

Four

TRAY HYDRAULICS

Types of trays 1. Sieve plates 2. Bubble-cap plates 3. Valve plates

Types of trays

In order to get a preliminary sizing for distillation column, we need to obtain values for  the tray efficiency  the column diameter  the pressure drop  the column height

Stage efficiency analysis Step 1: Thermodynamics data and methods to predict equilibrium phase compositions Step 2: Design of equilibrium stage separation

Step 3: Develop an actual design by applying the stage efficiency analysis to equilibrium stage design

Stage efficiency analysis In general the overall efficiency will depend: 1) Geometry and design of stages 2) Flow rates and patterns on the tray 3) Composition and properties of vapour and liquid streams

Stage efficiency analysis Lin,xin

Vout,yout

What are the sources of inefficiencies? Lout,xout

Vin,yin

For this we need to look at what actually happens on the tray

Local efficiency

Emv

yn  yn1  * yn  yn1

Actual separation Separation that would have been achieved on an ideal tray

Stage efficiency analysis Point efficiency stagnation points

Depending on the location on the tray the point efficiency will vary

The overall plate efficiency can be characterized by the Murphree plate efficiency:

high concentration gradients

low concentration gradients

When both the vapour and liquid phases are perfectly mixed the plate efficiency is equal to the point efficiency

EmV

yn  yn1  * yn  yn1

EmV  Emv

Stage efficiency analysis In general a number of empirical correlations exist that relate point and plate efficiencies Peclet number

N Pe

Z L2  Detc

eddy diffusivity

length of liquid flow path

residence time of liquid on the tray

Stage efficiency analysis In addition we need to take in effects of entrainment

Entrained liquid droplets

Dry Murphree efficiency can be corrected for the entrainment effects by Colburn equation:

EmV Ea     1  EmV   1 

entrainment fraction = entrained liquid/gross liquid flow

Stage efficiency analysis Finally the overall efficiency of the process defined as

N actual EO  N theoretical

TRAY DIAMETER

U flood

 r L  rV  C   rV 

s  FST    20

FHA  1.0

1/ 2

C  FST FF FHA CF

0 .2

FF  foam factor

for

  4V   DT   f U flood p 

Ah

 0.10

Aa   M vapor    A  1  d  rV   A 

FHA

0.50

CF 

 Ah   5   0.5  Aa 

where:

Ad A

 0.1

for 0.06 

Ah

 0.10

Aa

(typical value)

PRESSURE DROP

TRAY COLUMN HEIGHT  Column

Height = # actual stages x tray spacing + space allowance for feed/draws + sump + top volume

 Tray

spacing for most applications is 18-24 inches

REBOILERS

Reboilers Top Tray

Top Tray

Heating Medium

Heating Medium

Bottoms Product Bottoms Product

Circulating Pump

Forced Circulation

Vertical Thermosiphon

Reboilers Top Tray

Top Tray

Heating Medium

Heating Medium

Bottoms Product

Bottoms Product

Kettle

Horizontal Thermosiphon

FEED DISTRIBUTORS

THE “REAL” WORLD

Fouled Structured Packing

Damaged Valve Tray

THE “REAL” WORLD

Plugged Distributor

Tray “Blanking” Strips

VALVE TRAY DECK

MAJOR TRAY DAMAGE

FOULING RESISTANT DESIGN

FOULED BUBBLE CAP TRAY

Tray Number

Distance from tower top

GAMMA SCAN TECHNOLOGY Tower Scan

CLOSE BOILING OR AZEOTROPIC MIXTURE 

Binary mixtures having nearly equal to 1



Separation difficult even of ideal mixtures



Third component is used



Two types: Extractive distillation Azeotropic distillation

I. II.

EXTRACTIVE DISTILLATION 

non volatile solvent is used



Associate with one of the component and increase



Solvent selectivity: ability to enhance the separation of key component

AZEOTROPIC DISILLATION 



Entrainer is added Entrainer forms an low boiling azeotrope with one component

CRUDE OIL DISTILLATION