120712ChE128-3-FlashDist

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When is flash distillation used?
flash distillation  a single equilibrium stage
1. when very crude separation is needed
e.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)
Flash drum 126' x 12' @ 179,000 lbs.,
being moved into position on 4 self propelled steerable dollies at a major oil refinery.
http://www.edwardsmoving.com/Flash%20Drum.html
DESALINATION 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/
10 x 5.9 MIGD MSF units in Saudi Arabia
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
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
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 separate
HC 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.
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 partially
condenses when cooled.
Case 1: specified Tdrum, Pdrum
TMB:
CMB:
VLE:
F=L+V
F zi = L x i + V y i
Ki = yi / xi
solve for xi:
Fzi
Fzi
zi
xi =
=
=
L +VK i (F -V ) +VK i 1+ (K -1)V
i
F
where V/F is the fraction of feed vaporized, 0 ≤ V/F ≤ 1
can also express in terms of yi:
yi =
also need to use:
K i zi
1+ (K i -1)V
åx
i
i
=1
F
or
• knowns: zi, Ki (Tdrum)
• unknowns: xi (or yi), V/F
åy
i
i
=1
Rachford-Rice Solution
trial-and-error method (except binary); easy to program
xi =
zi
1+ (K i -1)V
possible solution:
å
i
åx
and
i
=1
i
F
zi
1+ (K i -1)V
F
• iterate until convergence
= 1 • then calculate xi, yi values
• then use TMB to calculate V, L
faster convergence:
å y - å x =å
i
i
i
i
i
K i zi
1+ (K i -1)V
(K i -1)zi
å
i 1+ (K -1)V
i
=0
F
-å
F
i
zi
1+ (K i -1)V
=0
F
Rachford-Rice equation
Finding V/F
( )
f V
(K i -1)zi
=å
F
i 1+ (K -1)V
i
=0
F
algorithm for Newtonian convergence (Wankat, p. 37-8):
( )
V
( F)
= V
• pick a value for V/F
F next
• evaluate f(V/F)
• if f(V/F) ≠ 0, choose a new value for V/F
where
df
d V
( F)
= -å
i
prev
-
fprev
dfprev
( F)
d V
(K i -1)2 zi
é1+ (K -1)V ù
êë
i
F úû
2
Case 2: specify Pdrum and V/F
•
•
•
•
Guess Tdrum
Calculate Ki values
Evaluate V/F using R-R equation
If f(V/F) ≠ 0, guess new Tdrum (recall Tbp calculation)
Case 3: specify Pdrum and one xi (or yi) value
• Guess Tdrum
• Calculate a Ki value
• Calculate V/F using
xi =
zi
1+ (K i -1)V
• Evaluate f(V/F) for this Tdrum
• If f(V/F) ≠ 0, guess new Tdrum
or
F
yi =
K i zi
1+ (K i -1)V
F
Ready! To design the flash drum
meaning, how large?
• Consider the flash drum as a vertical cylinder:
V
F
h
important size parameters:
diameter D
height h
placement of feed nozzle
D
L
• If the flash drum is too small:
– bubble entrainment in L
– liquid entrainment in V (demister helps)
• Separation is compromised
materials issues:
high temperature?
high pressure?
corrosive feed?
• If the flash drum is too large:
- wasted expense
• If the liquid volume is large:
– Use a horizontal drum instead
Vertical drum size
1. Calculate maximum permissible vapor velocity, uperm (m/s)
rL - rV
rV
uperm = K drum
where Kdrum is an empirical constant, typically 0.1 ≤ Kdrum ≤ 0.35
2. Calculate cross-sectional area, Ac (m)
V(MWv )
Ac =
uperm rV
where V is vapor flow rate and MWV is the vapor average molecular weight
3. Calculate drum diameter, D, and height, h
D=
4Ac
p
rule-of-thumb: 3 ≤ h/D ≤ 5
Graphical solution
for binary mixtures
CMB:
F zi = L xi + V yi
solve for yi:
yi = -(L/V) xi + (F/V) zi
equation of a straight line
the “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
McCabe-Thiele diagram
yE = -(L/V) xE + (F/V) zE
• simultaneous
solution of
CMB and VLE
yE
•
Find Tdrum?
yE/xE = KE (Tdrum)
xE
Figure 2-2 McCabe-Thiele diagram for ethanol-water
From Separation Process Engineering, Third Edition by Phillip C. Wankat
(ISBN: 0131382276) Copyright © 2012 Pearson Education, Inc. All rights reserved.
Drawing the operating line
all operating lines must
pass through this point
yi = -(L/V) xi + (F/V) zi
yint = (F/V)zE
for specified feed (zi), Pdrum and 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
Figure 2-2 McCabe-Thiele diagram for ethanol-water
From Separation Process Engineering, Third Edition by Phillip C. Wankat
(ISBN: 0131382276) Copyright © 2012 Pearson Education, Inc. All rights reserved.
Limits of separation
• as x  xmin, or y  ymax, product yield  0
• to achieve finite yield, must have less separation
yi = -(L/V) xi + (F/V) zi
for a given feed (zi), 0 ≤ (V/F) ≤ 1
(V/F) = 1
V/F = 0
(vaporize all the feed)
(L/V) = (F-V)/V = (F/V) – 1 = 0
 operating line is horizontal
 corresponds to xmin
(V/F) = 0
yE,max
•
(vaporize no feed)
(L/V) = (F/V) – 1 = ∞
 operating line is vertical
 corresponds to ymax
•
•z
E
V/F = 1
xE,min
Figure 2-2 McCabe-Thiele diagram for ethanol-water
From Separation Process Engineering, Third Edition by Phillip C. Wankat
(ISBN: 0131382276) Copyright © 2012 Pearson Education, Inc. All rights reserved.
Energy balance (EB)
EB:
0
FhF + Qflash = VHv + LhL
where
hF, hL are liquid enthalpies
HV is vapor enthalpy
all are functions of temperature, composition
assume flash drum is well-insulated, flash is adiabatic
for ideal mixtures and arbitrary Tref:
hi (T,x i ) = å xiCP (T -Tref )
L,i
i
Hi (T,y i ) = å y i éëli +CP (T -Tref )ùû
i
V ,i
for a particular component i,
CP,L is molar heat capacity of pure liquid
CP,V is molar heat capacity of pure vapor
l is latent heat of vaporization at T = Tref
Feed temperature
What temperature must the feed be to cause the flash?
EB:
Case 1:
FhF = VHv + LhL
for specified TF, value of hF is known
this determines values of HV, hL necessary to satisfy
EB
i.e., Tdrum is determined
Case 2:
for specified Tdrum, values of HV, hL are known
this determines value of hF necessary to satisfy
EB
i.e., TF is determined
Heating the feed
if the feed is not already at temperature TF, how much heat must
be supplied?
F, zi, T1
F, zi, TF
energy balance on heater:
F h1(T1) + Qh = F hF(TF)
Qh = F (hF – h1)
(dictates size of heater required)

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