Chemical Engineering June 2012 - 52

Feature Report
Engineering Practice
Distillation: Avoid Problems
During Tower Startup
Practical procedures for
both effective startup
and problem analysis
are discussed here for
a depropanizer that
experienced downcomer
seal loss
André Bernard
NOVA Chemicals (Canada) Ltd.
A
variety of performance problems
can arise in distillation towers
during startup. Many are caused
by equipment, hardware or process
conditions. Hardware issues may
be related to instrumentation, tower
internals or ancillaries. Instrumentation
malfunctions are also common
during startup and are predominantly
related to liquid level indicators, online
analysis of key components and in
some cases, flowmeters. If not properly
compensated, flowmeters can mislead
the operator on streams operating
outside the normal design envelope.
Vessel-isolation blinds, valve alignments
or unexpected obstructions in
valves can also restrict or misdirect
flows. Any of these issues could lead to
hazardous conditions and ultimately
process incidents.
Anomalies in tower internals, such
as obstructions, mechanical damage
or poor installation can also cause
towers to perform poorly. Meanwhile,
issues related to process conditions
could arise in all operations conducted
from the point of shutdown to getting
the tower ready for startup, during
the startup itself, and ultimately during
steady-state operation. The presence
of unexpected impurities, such as
Depropanizer
1
22
34
39
C3's to methyl
acetylene,
propadiene
converters
Condenser
Reflux
drum
43
38
External
C4's
62
Reboiler
Condensate
stripper
bottoms
To debutanizer
Deethanizer
bottoms
From C3
rerun tower
FIGURE 1. Shown here is a schematic process low diagram of the depropanizer
and its ancillaries
water, are also known to cause problems
during distillation, or in some
cases may result in pressure surges.
Meanwhile, oxygen-freeing practices,
and methodologies to introduce
the feed stream can lead to cold
temperatures that are below the minimum
allowable temperature of the
piping or tower material. Unstable
thermosyphon or reboiler stalling,
caused by low loads or premature
start of the reboiler, can also
cause tower instability. Similarly,
tray or packing hydraulic loads
that are outside the equipment capability
can significantly impact
fractionation efficiency by allowing
either flooding or operation
below turndown.
Although some of these problems ultimately
require the process to be shut
down so that the tower can be inspected
internally, that should be the last resort.
Good startup planning and procedures
can go a long way toward preventing
operating problems in the first
place. If problems do arise, appropriate
knowledge of the system is essential for
identifying the corrective actions that
48 CHEMICAL ENGINEERING WWW.CHE.COM JUNE 2012
are effective and the least invasive to
process operations. Specifically, the use
of diagrams that define the tray-stability
limits and expected operating lines,
and support effective startup planning,
can help to demystify the analysis of
poor tower performance.
This article illustrates the use of
these practical tools through a troubleshooting
exercise with a depropanizer
tower at NOVA Chemicals' Corunna
site. The Corunna site is an olefins
plant with a front-end crude unit
and a back-end aromatics unit. The
plant has the flexibility to crack naphtha
and heavy atmospheric gas oil
(HAGO), produced by the crude unit.
It also has the capability of cracking
natural gas liquids (NGLs).
Depropanizer description
The depropanizer is a 147-ft tower
with a diameter of 7.5 ft and a total of
62 trays. The first 33 trays (from the
top) are conventional two-pass sieve
trays. Trays 34 to 62 are high-capacity
trays. Heat is supplied to the bottom
reboiler by low-pressure steam. Propylene
refrigeration (3°C) provides
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