Hydrocarbon Processing - January 2022 - 75

Heat Transfer
G. MANENTI, Alfa Laval Olmi SpA, Suisio, Italy
Gas-side fouling effects on transfer-line exchangers
Transfer-line exchangers (TLEs) are
specific shell-and-tube heat exchangers
installed out of hydrocarbons steam
cracking furnaces for olefins production.
Their purpose is to cool process gas coming
from the furnace with a minimum
residence time-TLEs are also known as
quenchers. The quenching is achieved by
means of high-pressure vaporizing water
as cooling fluid, flowing on the shell side.
Up to three TLEs in series can be installed
downstream of a furnace coil, depending
on the process and feedstock. The first
one of the series, called the primary TLE
(FIG. 1), is the most critical from an operating
and thermal-mechanical standpoint
due to harsh operating conditions.
Gas-side fouling in a TLE is a major
source of operating and maintenance issues;
chiefly, massive deposits of tar and
coke on the internal surface of tubes lead
to a rapid degradation of quenching. It
then becomes vital to perform frequent
cleaning operations with a consequent
loss of production yield. This article focuses
on gas-side fouling effects and consequential
potential issues that are not
well known or that are sometimes intentionally
or unintentionally overlooked.
A proper and exhaustive understanding
of fouling consequences on quenching
can help to extend the design life of a
TLE and reduce annoying and costly operations,
such as repairs. The information
presented here is based on the author's
company's knowledge and experience as
a global supplier of a proprietary design
TLE, but these concepts can be extended
to any other TLE design.
Fouling in TLE. Heat transfer resistance in
the primary TLE due to gas-side fouling is
one of the largest incidences in heat transfer
technology for the petrochemical industry,
since it can reach (before cleaning)
values of 0.001 m2
°C/W-0.005 m2
°C/W.
It is even more remarkable that such fouling
levels can be reached, from clean conditions,
in 40 d-60 d of operations.
Of course, fouling is experienced as well
in the upstream furnace coil, even at higher
levels. Practically, operations and plant
production yield are strongly linked to gasside
fouling effects. For this reason, several
attempts to catalogue feedstocks in terms
of fouling source, to understand mechanisms
of fouling growth, and to introduce
techniques or solutions to mitigate fouling
deposition have been proposed.
According to literature, the type and
amount of gas-side fouling mainly depend
on furnace feedstock and cracking operating
parameters. Furnace feedstock can be
either gaseous (ethane to butane) or liquid
(naphtha to gasoil) hydrocarbons-it
is generally understood that fouling in
TLEs is a consequence of the formation
of coke and tars and, more specifically,
that gaseous cracking mainly promotes
fouling growth at the TLE inlet, whereas
liquid cracking mainly promotes fouling
growth along the tube length. The author,
according to feedback received from operating
units based on different processes,
agrees with such a general understanding,
although (1) observations are generally
done after on-line cleaning so there is
not a direct picture of fouling, and (2) no
clear observations can be obtained when
mixed cracking is performed.
Among operating parameters, the
cracked gas temperature and velocity,
the tube metal temperature and surface
roughness play a role in fouling growth.
From a thermodynamics and kinetics
standpoint, two major root mechanisms
are deemed responsible for gas-side fouling
formation:
1. Catalytic reactions on the
metallic surface of the TLE tube
forming coke and tar compounds
directly on the tube surface1
2. Polymerization reactions in
the bulk of gas forming coke
and tar compounds, which
impinge or migrate towards
the TLE tube surface and
then condense and stick.2
The first mechanism is likely promoted
on the hottest metallic surfaces (at the
inlet of the TLE) and at startup (clean surfaces),
whereas the second mechanism is
likely promoted by the presence of local
turbulence and/or colder metallic surfaces
(along the tube length). Models of
fouling formation and deposition in TLE
tubes are available in literature.3
However, possible fluctuations in
chemical composition and temperature of
cracked gas, turbulence in the TLE inlet
channel, and flowrate unbalance between
tubes can all introduce a stochastic contribution
to the fouling deposition and
growth at the TLE inlet. In practice, this
FIG. 1. TLEs are specific shell-and-tube heat
exchangers designed to cool process gas
coming from the furnace with a minimum
residence time.
Hydrocarbon Processing | JANUARY 2022 75
Hydrocarbon Processing - January 2022

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