Hydrocarbon Processing - January 2022 - 72

Heat Transfer
observe the flames, measure stack emissions
and collect operating data. This is
also an excellent opportunity to interview
operators about difficulties they
experience controlling the heater. This
information is used to generate thermohydraulic
models to identify bottlenecks
in present or future operations. Comparing
the model results with actual operating
data can reveal issues that are not
readily observed, like fouling inside the
tubes or loss of heat transfer in the convection
section.
Key findings of the survey of the VC
heater included:
* Temperature/heat flux
maldistribution. The peak skin
temperature difference between
the hottest and coldest pass
was ~50°C (~90°F) at startof-run
(SOR). This imbalance
increased to about 150°C
(270°F) at end-of-run (EOR).
* Short run length. The
radiant tube skin temperatures
increased by ~120°C
(~216°F) in only 3 mos.
* Low fuel efficiency. The field
data indicated an efficiency
between 78% and 79%, despite
good mechanical condition
of the heater and efficient
operation of the burners.
* Loss of heat transfer. Comparison
with the thermal models showed
that the actual fuel efficiency at
SOR (i.e., in clean condition)
should have been 2% higher.
Visual observation of the firebox
showed flame interactions, which almost
always result in a performance penalty.
Flames rolling into tubes (FIG. 1) cause
hotspots, excessive coke formation,
oxidation and tube damage from oxidation,
creep and carburization. Unruly
flames are a safety concern, produce
high emissions and shorten the life of
many components.
The root cause of the flame interacFIG.
2. Temperature contours on the center
plane of the VC heater.
tions was investigated by a computational
fluid dynamics (CFD) analysis of the
heater. CFD is a common tool to evaluate
the performance before and after a
fired heater revamp (FIG. 2).
In the original design of the firebox,
the natural draft burners were arranged
in two circles. The outer ring consisted
of 10 burners while the inner ring consisted
of five burners. This layout caused
poor flue gas recirculation patterns and
flames coalescing in an unsteady flame
cloud. Additionally, the outer burners
were too close to the radiant tubes, causing
flames to locally impinge, creating
hotspots. The CFD model replicated
the poor flame patterns and explained
the difference in coil temperatures, high
coking rates and tube degradation issues.
Decision analysis. Armed with information
from an engineering study and
CFD analysis, the contractor and the
operator jointly evaluate the potential
revamp opportunities to reach an achievable
and optimal retrofit target. For a
heater revamp, these include (but are not
limited to):
* Improve safety, reliability
and operability
o Eliminate flame
interaction problems
o Simplify burner control
o Eliminate ambient effects
(e.g., wind, rain)
* Reduce operating costs
o Increase fuel efficiency
o Reduce fouling in tubes
* Increase profitability
o Increase capacity
and throughput
o Switch to more economic
fuels or feedstocks
* Reduce emissions
o NOx
o Greenhouse gases (CO2
)
o Unburned hydrocarbons
[carbon monoxide (CO), CxHy]
Ideally, several scenarios are developed,
including estimates for CAPEX
and OPEX, while considering credits or
penalties for CO2
emissions. For this example,
the following solutions were considered,
also shown in TABLE 1.
Option 1: Change existing burner
FIG. 3. Streamlines of the optimized burner
circle with 15 burners.
72 JANUARY 2022 | HydrocarbonProcessing.com
FIG. 4. Overview of revamp scope.
layout. Since flame impingement is a
crucial factor in operational and reliability
issues of the heater, the author's
company investigated whether moving
the natural draft burners to a single
burner circle would improve the overall
flame performance. This optimization
can be difficult since moving the burners
farther from the radiant coils puts them
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