Hydrocarbon Processing - January 2021 - 49

Heat Transfer
A. V. KARRE, Worley Group Inc, Baton Rouge,
Louisiana; and K. T. VALSARAJ, Cain Department
of Chemical Engineering, Louisiana State University,
Baton Rouge, Louisiana

Selection and comparison of thermic fluids
in a fired heater
Hot oil, also called thermic fluid, is a
heat transfer fluid (HTF) used in oil refineries and chemical plants. HTF transfers heat to the process fluid using sensible heat. Many limitations exist to using
steam as a heat source. The steam pressure
must be high enough to get the high temperatures required for the process; a steam
system also requires water treatment and
condensate collection systems; and steam
systems are also a common leak point
within plants. Conversely, steam is nontoxic and non-flammable, so there is zero
negative impact to the environment.
Hot oil can also leak, but it does not require a high-pressure system. By selecting
an appropriate HTF, high temperatures
can easily be reached. The hot oil process
is a closed-loop process. HTFs undergo
thermal degradation over time due to the
high-temperature process and will often
need to be refilled by an external make-up.
As shown in FIG. 1, a hot oil system consists
of a fired heater, circulation pumps, expansion tank, filters, a hot oil cooler, process
users and a slop oil system. The respective
process equipment is located close to its
process user to minimize both the piping
and the required hot oil volume. Efficient
design of hot oil equipment and the proper selection of the HTF is crucial to the
performance of the hot oil system.1
Circulating hot oil systems can limit
CAPEX, depending on the chosen HTF.
These hot oil systems are most advantageous to areas of the plant in which multiple users require process temperatures
above what can be achieved with highpressure steam. In some select cases, a
circulating hot oil system can be utilized
in which only one fired heater is required

vs. installing independent fired heaters for
each of the process users.
Many HTFs are available from manufacturers based on various properties, such
as temperature range, density, thermal
conductivity, heat capacity, vapor pressure, viscosity, corrosivity, flammability,
flash point, fire point, thermal expansion,
thermal stability, freezing point, flash
point and autoignition temperature. The
main parameters, such as heat transfer coefficient and line pressure drop, are used to
determine the suitability of the hot oil.
A refinery client of the author's company uses a proprietary base oila as the hot oil
medium. The base oil degrades rapidly at
an operating temperature of 575°F-600°F
(302°C-316°C), causing fouling issues
in the pumps, pipelines and exchangers.
As an example, fouling caused by rapid
degradation of hot oil in a heat exchanger

is shown in FIG. 2. A client refinery condensate fractionation project required
a reboiler duty of 48.1 MMBtu/hr. The
additional duty for the project required a
comparative study of different HTF systems for the service in the naphtha splitter
reboiler, as the plant experienced degradation problems with the hot oil. The project team evaluated different hot oils that
remain stable and are expected to have a
20-yr life span for operating temperatures
≥ 680°F (≥ 360°C). This study evaluates
the hot oils by comparing their heat transfer characteristics, hydraulic benefits, cost
benefits and other (vapor pressure, thermal stability, etc.) vital process parameters. The aims of the study are:
1. Investigate thermal and hydraulic
performance and suitability
of the proprietary oil and ultra-low
sulfur diesel (ULSD) hot oils at
Temperature bypass control

External makeup
Hot oil
fired heater
Hot oil cooler
Hot oil expansion tank

Hot oil pump

Filter

Hot oil user
exchanger 01

Temperature/
flow control

Fuel gas

Hot oil user
exchanger 02

Slop oil sump tank

FIG. 1. A hot oil system consists of a fired heater, circulation pumps, expansion tank, filters,
a hot oil cooler, process users, and a slop oil system.
Hydrocarbon Processing | JANUARY 2021 49

Hydrocarbon Processing - January 2021

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