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When is flash distillation used?

flash distillation a single equilibrium stage

1. when very crude separation is needede.g., oil/water separation in crude oil refining

2. when volatilities of components in the

mixture are very different

e.g., water desalination (4000 plants worldwide,

producing 3.4 billion gallons potable H2O daily)

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http://www.edwardsmoving.com/Flash%20Drum.html

Flash drum 126' x 12' @ 179,000 lbs.,

being moved into position on 4 self propelled steerable dollies at a major oil refinery.

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10 x 5.9 MIGD MSF units in Saudi ArabiaDESALINATION OF SEA WATER BY MULTISTAGE FLASH DISTILLATION

There are several consecutive stages (flash drums) maintained at decreasing pressures from the first

stage (hot) to the last stage (cold). Sea-water flows through the tubes of the heat exchangers,

where it is warmed by condensation of the vapor produced in each stage. Its temperature increases

from sea temperature to inlet temperature of the brine heater. The sea water then flows through

the brine heater where it receives the heat necessary for the process (generally by condensing

steam). At the outlet of the brine heater, when entering the first flash drum, sea water is

overheated compared to the temperature and pressure of stage 1. Thus it will immediately "flash,i.e., release heat, and thus vapor, to reach equilibrium with stage conditions. The vapor is

condensed into fresh water on the tubular exchanger at the top of the stage. The process takes

place again when the water is introduced into the following stage, and so on until the last and

coldest stage. The fresh water accumulates as the distillate, which is extracted from the last

(coldest) stage. Sea water becomes slightly concentrated from stage to stage and becomes the

brine flow, which is also extracted from the last stage.

http://www.sidem-desalination.com/en/process/MSF/

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BIODIESEL MANUFACTURING

Biodiesel is made by transesterification of triglycerides

with methanol, using NaOH as the catalyst:

The products are glycerol and fatty acid methyl esters

(FAMEs). The latter are biodiesel.

The glycerol is much denser than the biodiesel, and

separates by gravity in a settler.

The biodiesel is purified by separating it from

methanol/water using simple flash distillation.

http://www.enerclean.biz/Equipment/Biodiesel/BiodieselFlash/BiodieselFlash.html

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GLYCOL DEHYDRATION UNIT

The DPS Delta glycol dehydration unit removes water vapor from a gas stream to allow further

treatment and transportation without risk of hydrate formation or corrosion in the presence of

H2S or CO2, using ethylene glycol as a liquid desiccant.

http://www.dps-delta.com/portfolio/glycol.htm

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GLYCOL DEHYDRATION PROCESS

Wet gas containing HC droplets enters the integral scrubber section of the contactor tower where free liquid is removed.

Saturated gas flows up through the mass transfer section of the tower mixing with the downward flowing lean glycol. Dry

gas will exit the tower via a demister pad and the rich glycol goes to a coil within the still column mounted on the reboiler.

The condensing vapors provide reflux for the still column.

The warmed rich glycol flows via the lean/ rich glycol exchanger to the Flash Drum to remove entrained gas and separateHC liquid from the rich glycol. The rich glycol then passes through a solids filter to remove particulates and a carbon filter to

remove traces of aromatic compounds. After filtering the rich glycol is heated by the lean glycol from the Surge Vessel .Lean

glycol flows from an integral gas stripping column via the Surge Drum to the Lean/ Rich Glycol Exchanger before flowing to

the Lean Glycol Pump which sends the glycol under high pressure to the Glycol Contactor via the Lean Glycol Cooler.

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Flash drum schematic

feed easiest to pump if liquid

if necessary, increase pressure to keep it liquid (TF< Tbp)

to make feed partially vaporize, need Tdrum> Tbp, and Pdrum< PF flash is usually adiabatic (no heat transfer across the system boundary)

partial condensor operates in the same way, with hot vapor feed which partiallycondenses when cooled.

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Case 1: specified Tdrum, PdrumTMB: F = L + V

CMB: F zi= L xi+ V yi

VLE: Ki= yi/ xi

solve for xi: xi=Fz

i

L+VKi

=

Fzi

(F V)+VKi

=

zi

1+ (Ki 1)V

F

can also express in terms of yi:

knowns: zi, Ki(Tdrum) unknowns: xi(or yi), V/F

xii =1 yi

i =1oralso need to use:

where V/F is the fraction of feed vaporized, 0 V/F 1

yi=

Kizi

1+ (Ki 1)V

F

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Rachford-Rice Solutiontrial-and-error method (except binary); easy to program

xi

i

=1

iterate until convergence

then calculate xi, yivalues then use TMB to calculate V, L

possible solution:

faster convergence:

Rachford-Rice equation

andxi= zi

1+ (Ki 1)V

F

zi

1+ (Ki 1)V

Fi =1

yii xi=

i

Kizi

1+ (Ki 1)V

Fi

zi

1+ (Ki 1)V

Fi = 0

(Ki 1)z

i

1+ (Ki 1)VFi

= 0

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Finding V/F

f VF( )=

(Ki 1)z

i

1+ (Ki 1)V

Fi = 0

algorithm for Newtonian convergence (Wankat, p. 37-8):

pick a value for V/F

evaluate f(V/F)

if f(V/F) 0, choose a new value for V/F

VF( )next =

VF( )prev

fprev

dfprev

dVF( )

where

df

dVF( )

=(K

i 1)2z

i

1+ (Ki 1)V

F

2i

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Case 2: specify Pdrumand V/F

Guess Tdrum

Calculate Kivalues

Evaluate V/F using R-R equation

If f(V/F) 0, guess new Tdrum(recall Tbpcalculation)

Case 3: specify Pdrumand one xi(or yi) value

Guess Tdrum

Calculate a Kivalue

Calculate V/F using

Evaluate f(V/F) for this Tdrum

If f(V/F) 0, guess new Tdrum

orxi=

zi

1+ (Ki 1)V

F

yi=

Kizi

1+ (Ki 1)V

F

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Ready! To design the flash drummeaning, how large?

Consider the flash drum as a vertical cylinder:

important size parameters:

diameter D

height hplacement of feed nozzle

materials issues:

high temperature?

high pressure?corrosive feed?

D

hF

V

L

If the flash drum is too small: bubble entrainment in L

liquid entrainment in V (demister helps)

Separation is compromised

If the flash drum is too large:- wasted expense

If the liquid volume is large: Use a horizontal drum instead

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Vertical drum size

1. Calculate maximum permissible vapor velocity, uperm

(m/s)

uperm

= Kdrum

r

L r

V

r

V

where Kdrumis an empirical constant, typically 0.1 Kdrum 0.35

2. Calculate cross-sectional area, Ac(m)

Ac=

V(MWv)

uperm

r

V

where V is vapor flow rate and MWVis the vapor average molecular weight

3. Calculate drum diameter, D, and height, h

D=4A

c

rule-of-thumb: 3 h/D 5

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Graphical solutionfor binary mixtures

CMB: F zi= L xi+ V yi

solve for yi: yi= -(L/V) xi+ (F/V) zi

equation of a straight linethe operating line

slope = -(L/V)

solution to the flash drum problem must simultaneously

satisfy CMB (operating line) and VLE (equilibrium line)

for a binary mixture, we can represent both on a 2D graph

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McCabe-Thiele diagram

From Separation Process Engineering, Third Editionby Phillip C. Wankat

(ISBN: 0131382276) Copyright 2012 Pearson Education, Inc. All rights reserved.

Figure 2-2 McCabe-Thiele diagram for ethanol-water

yE= -(L/V) xE+ (F/V) zE

yE

xE

Find Tdrum?

yE/xE= KE(Tdrum)

simultaneoussolution of

CMB and VLE

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Drawing the operating line




yi= -(L/V) xi+ (F/V) ziyint= (F/V)zE

for specified feed (zi), Pdrumand V/F:

yint= (F/V) zi

-(L/V) = -(F-V)/V = 1 - (F/V)

when yi= xi,

yi= -(L/V) yi+ (F/V) zi

yi(1 + L/V) = (F/V) zi

yi(V + L)/V = (F/V) zi

i.e., yi= xi= zi

zE

all operating lines must

pass through this point

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Limits of separation



yi= -(L/V) xi+ (F/V) zi

for a given feed (zi), 0 (V/F) 1

(V/F) = 1 (vaporize all the feed)

(L/V) = (F-V)/V = (F/V)1 = 0

operating line is horizontal

corresponds to xmin

V/F = 0

(V/F) = 0 (vaporize no feed)

(L/V) = (F/V)1 =

operating line is vertical

corresponds to ymax

V/F = 1

xE,min

yE,max

zE


as x xmin, or yymax, product yield 0

to achieve finite yield, must have lessseparation

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Energy balance (EB)

EB: FhF + Qflash= VHv + LhLwhere hF, hLare liquid enthalpies

HVis vapor enthalpy

all are functions of temperature, composition

0

assume flash drum is well-insulated, flash is adiabatic

for ideal mixtures and arbitrary Tref:

hi(T,x

i) = x

iC

PL,i

(T Tref)

i

Hi(T,y

i) = y

i

i+C

PV,i

(T Tref)

i

for a particular component i,

CP,Lis molar heat capacity of pure liquid

CP,Vis molar heat capacity of pure vapor

lis latent heat of vaporization at T = Tref

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Feed temperature

EB: FhF = VHv + LhL

What temperature must the feed be to cause the flash?

Case 1: for specified TF, value of hFis knownthis determines values of HV, hLnecessary to satisfy

EB

i.e., Tdrumis determined

Case 2: for specified Tdrum, values of HV, hLare knownthis determines value of hFnecessary to satisfy

EB

i.e., TF is determined

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Heating the feed

if the feed is not already at temperature TF, how much heat mustbe supplied?

energy balance on heater:

F h1(T1) + Qh = F hF(TF)

Qh = F (hFh1)

(dictates size of heater required)

F, zi, T1 F, zi, TF

120712che128-3-flashdist

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