Hydrocarbon Processing - January 2022 - 77

Heat Transfer
temperature cannot truly give
indications about fouling in the
TLE and, more specifically, if
the fouling is mostly deposited
along the exchanging tubes or
at the tubes entry. In other words,
it is not possible to infer if the
TLE tubes are all working with
fouling spread along the tube
length or if the TLE works
with clogged tubes.
2. The pressure measured at the
TLE outlet is acceptable, the
temperature measured at the
TLE outlet is acceptable, and
no pressure measurement is
available at the coil outlet. In this
case, the overall pressure drops
across the coil and the TLE can
be within the allowable limit, but
there is no clear indication how
the pressure drop is distributed
between the coil and the TLE. In
other words, most of the pressure
drop may occur across the TLE
and, consequently, several tubes
and/or inlet grids/shields may be
clogged, leading to non-uniform
distribution of gas in the TLE
tubes and overloading some tubes.
3. The pressure measured at the
coil outlet is acceptable, the
temperature measured at the
TLE outlet is acceptable, and the
pressure measured at the TLE
outlet is high or the pressure
drop in the TLE increases faster
than in the coil. In this case,
the operating conditions of the
coil and the TLE are known
and acceptable since the sum of
pressure drops across the coil
and TLE is within the overall
allowable limit. Therefore, rather
than shutting down and cleaning
the TLE, it may be decided to
proceed with production and to
use available pressure drop in the
coil for the TLE. As a result, the
TLE works with a high pressure
drop, which could mean that
several tubes are clogged.
For the above three operating conditions,
the risk is represented by the overloading
of the TLE tubes. When some
tubes are clogged due to fouling growth
at their inlet, the cracked gas (probably
non-uniformly) distributes into the remaining
tubes. Yet, the TLE outlet temperature
may stay below the allowable
limit, since the tube bundle is provided
with a heat transfer surface overdesign
or because the remaining working tubes
are clean or partially fouled. Because
of thermal-hydraulics and thermal-mechanical
design of the TLE are carried
out considering all exchanging tubes
in operation, a TLE with clogged tubes
works outside its design conditions;
consequently, the design life of the TLE
can be jeopardized. It should be emphasized
that, even if general observations
indicate that fouling growth at tubes
inlet is more pronounced for gaseous
cracking furnaces, all TLEs are potentially
exposed to tubes clogging since
fouling at the TLE inlet is unpredictable.
TLE performance. Results of heat transfer
performance calculations of a typical
TLE are shown in FIGS. 3-6. The TLE is
receiving gas from an ethane cracking
furnace at 830°C and 205 kPa(a), and
with a molecular weight of 18.7 kg/kmol.
Nominal design of the TLE is made for
an outlet gas temperature of 373°C and
a pressure drop of 6 kPa, with waterside
fouling resistance set to 0.000086
m2
°C/W and clean conditions on the gas
side. Boiling water temperature is 325°C.
Exchanging tubes are made of low-alloy
steel and have outside and inside diameters
of 50.8 mm and 40 mm, respectively.
Tube length is 7.5 m.
The performance calculations are made
for two boundary operating conditions:
1. Clean, which means fouling
on the water side and no
fouling on the gas side
2. Fouled, which means fouling
on both sides, with gasside
fouling assumed to be
uniformly distributed in tubes
and along the tube length.
The maximum allowable outlet temperature
for the cracked gas is 430°C:
accordingly, all the calculations in fouled
conditions are made in design mode by
imposing a gas-side fouling coefficient
that provides for an outlet gas temperature
of 430°C.
In FIGS. 3-5, percentages in abscissas
represent the number of TLE working
tubes: 100% means that all TLE tubes are
working, whereas 70% means that 30% of
tubes are clogged by fouling and therefore
do not contribute to heat exchange.
For FIGS. 4 and 5, it is assumed that, for
fouled conditions, the gas-side fouling has
no effect on gas pressure drop; in other
words, FIGS. 4 and 5 show trends as if fouling
has no thickness. Conversely, for FIG. 6,
an imposed thickness for the gas-side fouling
(in abscissas) has been considered for
calculations, always according to the above
assumption that fouling distribution in
tubes and along tube length is uniform.
The following considerations can be
drawn from the results of performance
calculations:
a. FIG. 3 illustrates the gas pressure
drop and outlet temperature,
in clean conditions, against
the number of working tubes.
The two quantities increase
by increasing the number of
clogged tubes exponentially.
The pressure drop trend in
case of fouled conditions-
FIG. 3. The gas pressure drop and outlet temperature, in clean conditions, against the number
of working tubes.
Hydrocarbon Processing | JANUARY 2022 77

Hydrocarbon Processing - January 2022

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