Hydrocarbon Processing - January 2022 - 76

Heat Transfer
is impossible to be exhaustively predicted;
moreover, the cracked gas at high velocity
can also strip off fouling from the furnace
coil and bring it to the TLE inlet. This is
corroborated by the fact that fouling deposition
at the tubes inlet is not uniform
and can even change between subsequent
inspections-the position and number of
clogged tubes at the inlet appear to be stochastic
rather than deterministic. Accordingly,
the author thinks that turbulence
and back-mixing at the tubes inlet first led
to localized tiny layers or spots of fouling
that act as a substrate for further fouling
sticking and the possible clogging of tubes.
Contrary to the fouling deposited at
tubes inlet, of which the extension and
aspect appear stochastic, the fouling along
the tube length appears more uniform,
both from tube-to-tube and from run-torun.
Therefore, the fouling is more linked
to deterministic phenomena or linked
to a lesser number of concurrent phenomena;
actually, along the tube, the gas
path is more regular and the turbulence is
smoother. Yet, quantitative measurements
(thickness, morphology, etc.) would require
samples collection along the tube.
FIG. 2 shows two examples of gas-side
fouling after on-line cleaning of a TLE:
* FIGS. 2A, 2B and 2C refer to a TLE
working with gaseous feedstock
(propane/butane).
* FIG. 2A shows clogging in a tube,
60 mm-70 mm from the inlet.
* FIG. 2B shows residual fouling along
the length of a tube.
* FIG. 2C shows an inlet tubesheet in
clean condition except for some
outer tubes that remain clogged.
* FIG. 2D shows a TLE working with
liquid feedstock (heavy naphtha)
during mechanical cleaning.
Thick, compact and uniform
fouling along the tube length is
apparent; this TLE was difficult to
mechanically clean due to hardened
fouling along thetube length.
* FIGS. 2A, 2B, 2C and 2D indicate
that on-line cleaning can be
unsuccessful. FIGS. 2A, 2B and 2C
indicate that some tubes are still
clogged or contain residual fouling
after on-line cleaning, whereas
FIG. 2D indicates that fouling along
the tube length leads to hardening
and aging along runs.
TLE operations. The hot cracked gas
is usually quenched in the primary TLE
from 780°C-850°C to 350°C-600°C,
depending on the process and feedstock.
Typically, the gas temperature at the TLE
outlet increases by 50°C-200°C due to
fouling, then production operations are
shut down and on-line/mechanical cleaning
of equipment is undertaken. Due to
complex thermodynamics and kinetics
of steam cracking and to high fouling levels,
a tight process control is inherently
difficult. Gas temperature and feedstock
conversion at the furnace coil outlet are
major control parameters; the cracking
furnace is also controlled by measuring
the pressure of cracked gas at the inlet
and/or outlet of the TLE and by measuring
the temperature of cracked gas at the
TLE outlet. Usually, production is halted
and cleaning is performed as one of the
following constraints due to fouling:
* The cracked gas pressure drop
approaches the maximum allowable
* The cracked gas temperature at
the TLE outlet approaches the
maximum allowable
* The maximum coil metal
temperature approaches the
maximum allowable.
Pressure instruments can be installed
either at the coil or TLE outlet, or both.
In the first case (coil), the pressure drop
across the cracking coil is accurately measured,
whereas the pressure drop across the
TLE can be overlooked if the downstream
pressure measurement is far from the primary
TLE. In the second case (TLE outlet),
the overall pressure drops across the
coil and TLE are accurately measured, but
no clear indication exists about specific
drops. Finally, an accurate and exhaustive
valuation of pressure drops can be done if
a two-points measurement-at both the
coil and TLE outlet-is carried out.
However, latent and tricky problems
may arise from typical furnace process
control when run lengths are determined
by TLE conditions. The pressure drop
across the coil/TLE and the temperature
of the cracked gas at the TLE outlet are
often weakly coupled; usually, cleaning
operation is decided once one of the two
parameters (pressure drop, outlet temperature)
approaches the relevant allowable
limit, whichever occurs earlier without
inferring on the other parameter. In other
words, the following risky operating conditions
may occur:
1. The pressure measured at the
coil outlet is acceptable, the
temperature measured at the
TLE outlet is acceptable, and
no accurate measurement for
FIG. 2. Examples of gas-side fouling after on-line cleaning of a TLE.
76 JANUARY 2022 | HydrocarbonProcessing.com
pressure at the TLE outlet
is available. In this case,
measurement of the TLE outlet
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