Chemical Engineering June 2012 - 53

Liquid reinjection
Condensate stripper
CGC = Charge gas compressor
PFO = Pyrolysis fuel oil
RPG = Raw pyrolysis gasoline
Acid
gas
Quench
Charge area
gas
CGC
PFO RPG
Water and
condensed
hydrocarbons
(1st three stages)
Condensed
hydrocarbons
(4th and 5th stages)
Condensate
Stripper
C3+ to
depropanizer
Liquid
dryer
Acetylene
converters
Demethanizer
Charge
gas
dryer
H2 CH4
Chill
train
Fractionator
C2
NOMENCLATURE
AE = Pipe-equivalent slot area, ft2
AD = Pipe-equivalent of downcomer
area, ft2
Ah = Hole area, ft2
aD = Downcomer area, ft2
C = Constant given by Equation (9)
Cv
= Discharge coefficient, unitless
Deq = Equivalent diameter, ft
F1 = Factor, equivalent to unity
(F1 = 1) for most tower larger
than 4 ft in diameter [6]
F2 = Factor given by Equation (11)
g
= Gravitational acceleration,
32.2 ft/s2
Deethanizer
K1 = Constant (0 for sieve deck)
K2 = Dry pressure drop coefficient,
in./(ft/s)2
C3
+ to
depropanizer
FIGURE 2. This low diagram shows the liquid re-injection mode and condensate
stripper operating mode
Rectifying section
(2-pass sieve trays)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Relative density
Startup condition
Normal rates
Relative density
FIGURE 3. This gamma scan of the depropanizer compares normal operating load
to the startup conditions
cooling to the overhead condenser.
As shown in Figure 1, the tower has
two main feed points at Tray 34 and
Tray 39. The feed is distributed by
trough distributors at those locations.
A third minor feed point, which accommodates
a small recycle stream from
the C3 rerun tower is located on Tray
22. The tower normally receives feed
from the deethanizer bottoms at Tray
34 and the condensate stripper bottoms
at Tray 39. External C4s can be processed
from time to time and are mixed
with the condensate-stripper feed. The
olefins unit has the ability to run with
or without the condensate stripper.
As shown in Figure 2, condensed hydrocarbons
from the charge-gas compressor's
4th and 5th stage are dried
in liquid dryers and can either be processed
by the condensate stripper or
mixed with dry charge gas, upstream
of the chill train (liquid re-injection
path [1]). Re-injection of this liquid
stream to the charge gas enhances
condensation in the first propylene
chiller. Process loads are then reduced
in the chill train but increased in the
deethanizer and depropanizer.
In the liquid re-injection configuration,
the depropanizer is entirely
fed from the deethanizer bottom. Because
of nozzle size and distributor
limitations the feed is split between
Tray 34 and Tray 39. Tray 34 feed is
cooled through cross exchange with
Stripping section
(high-capacity trays)
55
56
57
58
59
60
61
62
KC = Number of velocity head lost
LD = Downcomer liquid rate, lb/h
lD = Downcomer length, ft
Nslot = Number of downcomer slots
P1 = Lower tray pressure,
in. of liquid
P2 = Upper tray pressure,
in. of liquid
∆PD = Downcomer pressure drop, in.
of liquid
∆PT = Tray deck pressure drop, in. of
liquid
V = Total vapor rate, lb/h
VD = Downcomer vapor rate, lb/h
(VD)cr = Downcomer critical vapor rate,
lb/h
VT = Tray vapor rate, lb/h
ƒ = Pipe friction factor, unitless
ƒn
ρV = Vapor density, lb/ft3
ρL = Liquid density, lb/ft3
15A
the depropanizer overhead product.
Feed to Tray 39 is cooled against cooling
water. Without cooling, the feed
would enter the tower with a significant
vapor fraction, which would shift
the tray loads from the stripping to
the rectifying section.
From a startup standpoint, the liquid
re-injection configuration (Figure
2), offers two advantages. First, given
the low feedrate during startup, this
arrangement provides more load on
the deethanizer and depropanizer,
which helps in meeting turndown. Second,
the movement of material from
the front to the back end of the plant is
sequential. As such, the depropanizer
does not see any feed until the deethanizer
has been inventoried. The plant
is therefore normally started with the
liquid re-injection configuration.
Depropanizer startup problems
Sequence of events. Feed from the
deethanizer to the depropanizer
started ramping up roughly 15 h
after the charge gas compressor was
CHEMICAL ENGINEERING WWW.CHE.COM JUNE 2012 49
= Fraction of VD flowing to the
downcomer n
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Chemical Engineering June 2012

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