FLASH DRUM DESIGN
Vertical vessel
ChE 4253 - Design I
Horizontal Vessel
FLASH DRUM DESIGN
Installed Units
ChE 4253 - Design I
FLASH DRUM DESIGN
Deflectors, and Diffusers
ChE 4253 - Design I
FLASH DRUM DESIGN
Demisters
ChE 4253 - Design I
FLASH DRUM DESIGN
Control Schemes
ChE 4253 - Design I
FLASH DRUM DESIGN
Dimensions to decide
D
hV
hL
hf
ChE 4253 - Design I
FLASH DRUM DESIGN
Calculations to make
F = L +V
Fzi = Lxi + Vyi
yi = K i xi
∑
xi = 1
∑
yi = 1
i
i
zi
⎧
=
x
⎪ i ⎡ V
1+
( K − 1) ⎤
⎪
F i
⎣
⎦
⇒⎨
⇒
K
z
i i
⎪y =
i
⎪
⎡1 + V ( K i − 1) ⎤
F
⎣
⎦
⎩
ChE 4253 - Design I
Ratchford Rice Equation (look for it!!)
FLASH DRUM DESIGN
Dimensions to decide
D
hV
hL
hf
related to vapor velocity.
related to vapor velocity.
related to level control
related to flooding
ChE 4253 - Design I
FLASH DRUM DESIGN
D related to vapor velocity.
Permitted velocity
K =e
v perm = K
A+ B ln Flv + C (ln Flv ) 2 + d (ln Flv )3 + E (ln Flv )4
Where do these come from ?
ChE 4253 - Design I
( ρ L − ρV )
ρV
Flv =
WL
WV
ρL
ρV
A = −1.877478
B = −0.814580
C = −0.187074
D = −0.014523
E = −0.001015
FLASH DRUM DESIGN
D related to vapor velocity.
Fvis Fdrag
Fg
Stokes
Fvis = 6πμ Rd vd
1
Fdrag = CD AρV vd2 drag
2
4
Fg = ( ρ L − ρV ) g π Rd3 gravity-Buoyancy
3
Fdrag + Fvis ≈ Fdrag
Fdrag >> Fvis
1
4
2
2
= Fg ⇒ CD (π RD ) ρV vd = ( ρ L − ρV ) g π Rd3
2
3
⇒ v perm = K
ChE 4253 - Design I
( ρ L − ρV )
ρV
8 gRd
K=
3CD
FLASH DRUM DESIGN
Dimensional analysis for drag: Force is dependent on velocity, cross sectional area,
density and viscosity.
f a ( Fdrag , vd , A, ρV , μ ) = 0
Two nondimensional numbers:
Re =
vd A / π
μ
CD =
Fdrag
1
ρV Avd2
2
Therefore
f b (Re, CD ) = 0
Thus
CD
Î
CD =
FD
1
ρV Avd2
2
= f c (Re)
is a function of the particle Reynolds number. Î
ChE 4253 - Design I
8 gRd
K=
3 f c (Re)
FLASH DRUM DESIGN
But, what Rd should be used? The criteria is that 5% of the liquid
is entrained.
Thus
K =e
A+ B ln Flv + C (ln Flv )2 + d (ln Flv )3 + E (ln Flv ) 4
W
ρ
; Flv = WL ρ L
V
V
were obtained
fitting experimental data.
Therefore
D=
4
π
AD =
Demisters should take care of 4% (or less) of the 5%.
ChE 4253 - Design I
4
V
π v perm ρV
FLASH DRUM DESIGN
1
hV = 36"+ diameter of feedline
2
Who came up with this rule and why?
What is so magic about 3 ft? (Room for demister?)
1
h f = 12"+ diameter of feedline
2
Again, why 2ft? (Residence Time, Failure Analysis,
Other Control issues?)
hL =
Finally:
V pool
π D2
or ~2 minutes residence time.
4
L = hV + h f + hL
ChE 4253 - Design I
Nozzle size
FLASH DRUM DESIGN
Final considerations
IF
L
<
D
3 increase Vpool (Why???)
IF
L
>
D
5 Use horizontal drum (Why???)
Different design protocol: Why?
ChE 4253 - Design I