Motors&Drives.book Page 231 Monday, January 10, 2005 1:54 PM
Chapter 6: Drive System Control Methods
231
Drive System Control Methods
Introduction
Up to this point, basic drive theory, component hardware, and interface devices have been discussed. It is n
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 desali
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Packed columns
structured p acking:
random p acking:
larger surface area, for b etter contact b etween liquid and vapor
preferred for column d iameters < 2 .5
packing is considerably more expensive than trays
change
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Downcomers
Dualflow tray ( no d owncomer)
Both liquid and vapor p ass through holes
Narrow operating range
In large diameter columns, u se multipass trays
to reduce liquid loading in d owncomers
Crossflow tray ( si
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Column internals (Wankat, ch. 10)
Sieve tray
Also called a p erforated tray
Simple, cheap, easy to clean
Good for feeds that contain s uspended s olids
Poor turndown performance ( low efficiency when operated b elow d esig
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Open steam distillation
If the b ottoms s tream is p rimarily water,
then the b oilup is p rimarily s team.
Can replace reboiler with d irect s team
heating ( S).
L, xR D, xD
mostly M eOH
MeOH/H 2O
feed
F, z
Top operati
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Intermediate condensers and reboilers
Intermediate condensers/reboilers can improve
the energy efficiency of column distillation:
1. by decreasing t he heat t hat must be supplied
at t he bottom of t he column, providing
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Slope of middle operating line
2feed mass balances:
TMB: F 2 + V = L + D
CMB: F 2 z2 + V y j+1 = Lxj + DxD
middle operating line equation:
y = (L/V)x + (DxD  F 2 z2 )/V
obtain slope from:
L = F 2 q2 + L = F 2 q2 +
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 bio
Flash drum schematic
feed must be heated, but is easiest to pump if it remains liquid
if necessary, increase pressure P1 to keep it liquid (T1 < Tbp)
to make feed partially vaporize, need Tdrum > Tbp, and Pdrum < P1
flash is usually adiabatic (no heat
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 wellinsulated, flash is adiabatic
for ideal mixtures and arbitrary Tref:
hL (T,x
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 simultaneously satisfies CMB
(operating line) and VLE
Vertical drum size
1. Calculate maximum permissible vapor velocity, uperm (m/s)
rL  rV
rV
uperm = K drum
where Kdrum is an empirical constant (see Wankat p. 49)
typically 0.1 Kdrum 0.35
2. Calculate crosssectional area, Ac (m)
V(MWv )
Ac =
uperm rV
wher
Case 2: specify Pdrum and V/F
Guess Tdrum
Calculate Ki values
Evaluate V/F using RR 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 =
RachfordRice Solution
trialanderror method (except binary); but easy to program
xi =
zi
1+ (K i 1)V
possible solution:
i
x
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
faste
Finding V/F
( )
f V
(K i 1)zi
=
F
i 1+ (K 1)V
i
F
algorithm for Newtonian convergence (Wankat, p. 3738):
( )
V
pick an initial 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)
= 
i
( F)
= V
prev

fprev
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 entrai
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
zi
Fzi
=
=
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:
knowns: zi,
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Additional column inputs/outputs
Column with
two feeds:
Column with
three products:
L
V
L
V
distillate
D, xD
feed 2
F2, z 2, q 2
feed 1
F1, z 1, q 1
L
V
L
z 2 > z 1
and/or
q 2 > q 1
L
V
distillate
D, xD
sidestream
S, xS or
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Partial condensers
1
A p artial condenser can b e u sed when a
vapor d istillate is d esired:
0.9
0.8
2
0.7
D, yD
L, x0
L
PC
yD
0.6
y(MeOH)
V, y1
1
0.5
0.4
0.3
V
0.2
A p artial condenser is an equilibrium s tage.
0.1
0
CMB:
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Ex.: MeOHH2O enriching column
Enriching column with total condenser
Specifications: xD = 0 .8, R = 2
Find N required to achieve xB = 0 .1
NEVER step over the VLE line.
1
0.9
stage 1
(x1,y1)
0.8
2. D raw y=x line
stage 2
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Splits
Sometimes used instead of s pecifying
compositions in p roduct streams.
What is the fractional recovery (FR) of
benzene in the d istillate?
What is the fractional recovery (FR) of
toluene in the b ottoms?
Mo
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Enriching column
Feed enters at the bottom, as a vapor.
No reboiler required.
L, xR
D, xD
Can give very pure distillate; but bottoms
stream w ill not be very pure.
Mass balance around top of column,
down to and in
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Column diagram
feed
F, xF
to keep the liquid flow rate
constant, p art of the d istillate
must b e returned to the top of
the column as reflux
reflux d rum
(accumulator)
reflux
L, xR
feed s tage
L
V
temperature
for
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External energy balance
assume column is w ell
insulated, adiabatic
EB:
distillate
D, xD
F hF + QC + QR
= D hD + B hB
F, hF are known
feed
F, xF
D and B are saturated liquids
so hD, hB are also known
unknowns: QC, QR
bot
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Optimum reflux ratio
total cost
cost/lb
s tages
capital cost
operating ( energy) cost
min. heat
required
Rmin
Ropt
external reflux ratio, R
Ruleofthumb:
1.05 R opt/R min 1.25
R actual can be specified as a m ultiple of R