Hydrocarbon Processing - January 2022 - 79

Heat Transfer
(clogging), the TLE should
be shut down due to pressure
drop limit rather than outlet
gas temperature limit; yet,
if the overall pressure drop
across the coil and TLE
is still acceptable, plant
operators will likely continue
running the furnace and,
therefore, the TLE working
tubes will be overloaded.
Additionally, gas velocity in tubes,
chiefly at the inlet, increases proportionally
with the number of clogged tubes, up
to inadmissible values.
Suggestions and takeaways. The primary
TLE installed in steam cracking furnaces
suffers from heavy gas-side fouling:
the formation and growth are governed
by several parameters and indeterministic
phenomena. Accordingly, operating runs
and cleaning of the TLE should rely on
plant operators' experience and observations.
Regular inspections on the gas side
can help clarify how a TLE is running
and if on-line cleaning is effective. The
author has noted that on-line cleaning is
not always successful in unclogging the
tubes and removing the fouling along the
tube length. The following suggestions
are for plant operators:
* Conduct gas-side inspections
at least twice per year.
* Report the type and extension of
fouling during gas-side inspections,
specifying if a prior on-line
cleaning has been done or not.
* Correlate different feedstocks
and cracking severity with
fouling type and extension.
* Consider installing a pressure
measurement both at the
coil and TLE outlets, if
not yet implemented.
* A high pressure drop across the
TLE due to tubes clogging is
a risky and tricky condition-
worse than heavy fouling
spread along the tube length.
* When pressure drop across the
TLE is rising fast, and is rising
faster than the increase of the outlet
gas temperature, it likely indicates
that tube clogging is occurring.
* If gas-side inspections reveal that
clogged tubes are present also
after on-line cleaning of the TLE,
consider starting on-line cleaning
FIG. 6. Trends of pressure drop (in ordinates) at different fouling thicknesses in tubes
(in abscissas) and at a different percentage of clogged tubes.
at relatively low pressure drop
across the TLE (1.5-2 times the
allowable one in clean conditions)
provided that the gas outlet
temperature is still acceptable.
* If gas-side inspections indicate
that clogging of a tube inlet is not
promoted or not important and the
rise of pressure drop across the TLE
is moderately slow, the cleaning of
the TLE can be started at a higher
pressure drop (3-4 times the
allowable one in clean conditions)
provided that the gas outlet
temperature is still acceptable.
* If gas-side inspections indicate
that clogging of the tube inlet
is important and some tubes
are even clogged after on-line
cleaning, operators should
decide for frequent mechanical
cleaning, at least at each
inspection. A TLE should not be
re-started with clogged tubes.
* On-line cleaning of the TLE due
to end-of-run (EOR) conditions
(high pressure drop or high outlet
temperature) should restore startof-run
(SOR) conditions-except
for water-side fouling, which is,
in any case, slow. Plant operators
must assess if SOR conditions are
efficiently restored (if feedstock
has not been changed); if no, this
means that the on-line cleaning
has not been effective, and that
the TLE is becoming more and
more fouled, so a mechanical
cleaning should be performed.
* SOR conditions after on-line
cleaning should be always
compared (if feedstock has not been
changed) with SOR conditions
after mechanical cleaning or SOR
conditions of the 1st startup.
* When on-line cleaning becomes
more frequent (with the same
feed and cracking conditions), it
is time for mechanical cleaning.
* The author has noted that
some fouling along tubes is so
hardened that strong and insistent
mechanical cleaning can locally
damage the tube surface.
* TLE design with no anti-erosion
devices at the inlet are preferred to
avoid impingement and turbulence.
* TLE design with flushed tubeto-tubesheet
joints (no welding
step) are preferred to avoid
impingement and turbulence.
* TLE design with tubes of larger
diameter should be used when
tube clogging is expected.
NOTES
These suggestions are given for reference only and
are irrespective to any economic consideration and
furnace conduction practices.
LITERATURE CITED
Complete literature cited available online at
www.HydrocarbonProcessing.com
GIOVANNI MANENTI has more than
20 yr of experience in process and
heat transfer design and R&D
activities, and in engineering and
pressure vessels manufacturing
companies. He is focused on special
shell-and-tube heat exchangers,
such as process gas boilers for the syngas industry
and transfer-line exchangers for steam cracking
furnaces, and on heat transfer prototype equipment.
Dr. Manenti earned a PhD in chemical engineering
from Politecnico di Milano, Italy.
Hydrocarbon Processing | JANUARY 2022 79
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