Hydrocarbon Processing - January 2022 - 73

Heat Transfer
closer together, which increases the risk
of flame-flame interactions. Several CFD
iterations were used to determine the
optimal burner circle diameter (FIG. 3).
In this scenario, some interactions are
still apparent, causing the flames to
lean slightly to the center. Nonetheless,
the new layout produced an acceptable
flame pattern and flux profile. The improved
flux profile resulted in improved
heat flux and temperature uniformity,
allowing the heater to operate at greater
capacity. Since neither the burner liberation
rate nor the heater surface area
increased, this capacity increase is limited.
Absence of flame interactions will
reduce NOx
emissions. This solution requires
a new heater floor.
Option 2: Convection replacement.
Replacing the damaged/fouled
convection section with a new and more
efficient version would increase the fuel
efficiency by several percentage points.
However, with a feed inlet temperature
range of 170°C-200°C (338°F-392°F),
the minimum achievable stack temperature
is ~250°C (~482°F), which limits
the fuel efficiency to a maximum of 86%.
The 8% gain could lower fuel consumption
and CO2
emissions or increase the
heater capacity. This option does not address
the flame interference problems, so
it also requires Option 1 or 3 to de-constrain
the firebox operation.
Option 3: Forced-draft air reheat
(FDAPH). The stack temperature of
a heater with 78% fuel efficiency is
~400°C (~750°F). This heat can be used
to heat the combustion air to 300°C-
350°C (572°F-662°F). More heat can be
extracted from the flue gas than in Option
2, pushing fuel efficiency to more
than 90%. The maximum efficiency is
only limited by the dewpoint of sulfuric
acid, which, in this case, restricts
the stack temperature to a minimum of
165°C (329°F).
In this case, the burners must be replaced
with forced-draft type burners,
which have a few advantages:
* The burner capacity can be
drastically increased, which
allows the number of burners
to be reduced from 15 to five.
This allows the use of the inner
burner circle without making
major modifications to the
heater floor other than plugging
the outer ring of burners.
* The high air side pressure drop
of forced-draft (FD) burners
results in a more compact and
robust flame, which provides
more freedom in locating
them without causing flame
interaction problems.
* FD burners are not individually
controlled by air dampers,
but by the FD fan damper or
variable speed motor, which
simplifies the operator's job.
* FD burners are not sensitive to
ambient effects like wind or rain,
which reduces the variability
in firebox draft and oxygen.
This allows the heater to be
operated more consistently and
on less excess air, which further
improves the system efficiency.
The scope of this option is more substantial
than the others, resulting in the
highest overall CAPEX:
* New FD burners and pilots
* Fuel piping
* Flue gas/air exchangers
* FD and ID fans with
variable-speed drives
* Flue gas and air ducting
with structural support
* New transition duct and
bypass dampers.
While Option 2 (convection replacement)
showed the quickest payback
FIG. 6. Model from laser scan (shown as
mesh) integrated with new equipment
(shown in grey).
time, it only made sense in combination
with Option 1 or Option 3, which reduced
the economic feasibility. Also, the
combination of Options 1 and 2 would
not allow for the capacity increase that
the customer desired.
Option 3 showed the biggest benefits
and the most attractive economics.
Purely based on fuel savings and CO2
tax
FIG. 5. Temperature contours with five FD
burners.
FIG. 7. Model of revamped heater.
Hydrocarbon Processing | JANUARY 2022 73
Hydrocarbon Processing - January 2022

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