John
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the c els, USA spen
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oppo allenges discusse
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insul tunities nd
in pr
ating
o
dela
yed perly
coke
rs.
O
Walking the
tightrope
perators of delayed coker units (DCU) walk a
tightrope when setting their furnace outlet
temperature. Running the feedstock too hot
accelerates the formation of coke inside the
furnace tubes, causing more frequent shutdowns and lost
production time. Running it too cold shifts the yield curve
away from refinable liquid and gas components, and toward
less valuable solid coke. While discussions around process
temperature control have typically focused on the operation
and design of furnaces, this article examines a more prosaic
approach that virtually any DCU operator can implement with
payback times of one to four years: better coke drum
insulation.
DCU operations and the role of
insulation
Delayed coker units heat heavy, residual oil to its thermal
cracking temperature (typically 460 - 500°C, or 860 - 930°F),
extracting useful liquid and gas components, and leaving
behind solid coke as a byproduct. While most of the
heating takes place inside a tube furnace, chemical kinetics
delay the thermal cracking until further downstream, when
the feedstock has entered one or more large, vertical
drums (Figure 1). These drums operate in batch mode,
alternately filling with solid coke, then being quenched,
emptied and reheated on a 12 – 24 hour cycle. These drums
are vertical vessels enclosed within a lattice like structure, and
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Reprinted
Reprinted from
from March
March 2015
2015
Figure 1. Delayed coker unit (©iStock.com/
Phil Augustavo).
thermomechanical movement, with some drums growing by
more than 20 cm (7.9 in.) in both length and circumference,
often in unusual, banana shaped modes. Add to this trifecta of
misery exposure to high winds, infrequent or nonexistent
insulation maintenance, and flat, vertical geometry that makes
mechanical support difficult, and you have a perfect recipe for
failure. And that is exactly what one sees on many coke drums:
insulation that is wet, sagging and, in many cases, falling off
entirely (Figure 2), often within just the first few years in
service.
And yet, the payoff for getting the insulation right is
enormous. While heat lost through the drum insulation
typically only accounts for approximately 1% of overall DCU
energy spend, these losses occur at the most temperature
sensitive part of the entire the coking process. Industry rule of
thumb indicates that a 5°C (9°F) increase in drum temperature
can provide an incremental liquid yield improvement of
0.5 - 1%. To put this into perspective, a 25 000 bpd DCU could,
with just a 1°C increase in drum temperature, earn an extra
US$185 000/yr based on the current price spread between
refined products and solid coke. In other words, as an
economic driver of insulation design, the potential for yield
improvement is 15 - 30 times more powerful than the avoided
cost of energy. This key insight opens the door to some very
different thinking about coke drums insulation design. In
particular, it opens the door to the use of flexible aerogel
blanket materials.
Next generation coke drum
insulation
Figure 2. Failing coke drum insulation
(©Insultherm, Inc.).
can be as large as 8 m (26 ft) in diameter and over 40 m (130 ft)
tall.
From an insulator’s perspective, coke drums are among the
hardest pieces of equipment to dress properly because they
harbor the three main enemies of thermal insulation: heat,
water, and mechanical abuse. The typical drum operates above
the binder burn out temperatures for traditional fibrous
insulations, then cycles down to temperatures at which liquid
water can pool at the drum surface. Sources of water are
abundant, from rainfall to steam leaks to the hydraulic cutting
operations used to decoke the drum. The frequent
temperature swings also result in extraordinary
Reprinted from March 2015
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Derived from a wet gel precursor, aerogels are a lightweight
silica solid in which the liquid component has been replaced
with air. When the liquid is removed, what remains is
essentially ‘puffed up sand’, with up to 99% porosity. The result
is a lightweight material with the lowest thermal conductivity,
or ‘k value’, of any solid known to man. While aerogels have
been a laboratory curiosity since the 1930s, it is only within the
last decade that they have become available in an economical
and convenient flexible blanket form suitable for service up to
650°C. This particular form is manufactured by Aspen Aerogels
in East Providence, RI (USA), and is called Pyrogel XT-E.
Compared to traditional coke-drum insulation materials
such as mineral wool, Pyrogel offers a number of advantages.
When applied at the same thickness, Pyrogel’s lower k value
cuts heat loss during peak coking operations by a factor of 2.3
(Figure 3). The greater thermal efficiency not only improves
process yields, but can also speed pre heating of a typical drum
by 10 - 30 min, shortening the overall cycle time and improving
throughput. Conversely, the material can be installed at par
thermal efficiency, but at less than half the original thickness.
This helps resolve many of the mechanical interference issues
that arise, particularly when refurbishing new coke drums
within existing structures (Figure 4).
Furthermore, because Pyrogel does not use organic binders,
it is not susceptible to the binder burnout and subsequent
mechanical breakdown that plagues fibrous insulation systems.
The outermost, weather facing layers of Pyrogel also retain
significant hydrophobicity, protecting the inner layers of
insulation and the drum itself from water ingress. This
combined resistance to both mechanical breakdown and liquid
water makes Pyrogel one of the toughest, most durable
insulation products on the market today.
Case study 1
Nowhere was this toughness demonstrated more convincingly
than in a DCU located along the US Gulf Coast. In 2009, a
hurricane stripped the coke drums of their existing, fibrous
insulation, posing significant operational and safety issues, and
jeopardising the restart of the refinery. The owner and
insulation contractor had recently had success using Pyrogel for
routine insulation maintenance elsewhere in the refinery. The
decision was made to apply Pyrogel to the drums as a
temporary stopgap measure until the unit could be returned to
service and a more permanent solution devised. So urgent was
their need that the material did not even receive the customary
sheet metal jacketing that is typically applied over insulation in
the field. It was simply applied, as quickly as possible, and left
exposed to the elements.
Five years later, the temporary stopgap insulation was still in
place. Periodic thermal surveys showed the material to be
performing well, despite having zero weather protection, and only
the barest level of mechanical support. As of this writing, the four
particular drums in question are finally being refurbished with a
permanent insulation system that, not surprisingly, has been
designed from the ground up around Pyrogel.
Exploring potential impact
Consider a 25 000 bpd, two drum DCU operating on a 24 hr fill
cycle. At a nominal peak temperature of 468°C (875°F), the
baseline coke yield is 30 wt%. These drums were originally
designed with 110 mm (4.5 in.) of mineral wool, and mechanical
interference with the surrounding structure prevents going any
thicker. The existing system has degraded to the point that the
insulation is wet and sagging, and the cladding has opened up
beneath several of the insulation support rings. As a result, the
drum is peaking 4.3°C (7.7°F) below the design temperature. For
reasons of unit reliability, the furnace cannot compensate by
providing a hotter feedstock. Can a better insulation system
provide an economical solution?
When considering which materials, and how much, to use
for the reinsulation of the drums, it is useful to think about it in
terms of setting a dial. The dial controls the thermal resistance
of the insulation; turning it to the right provides greater
efficiency and more favorable yields, but at a higher
construction cost. Turning it all the way to the right provides a
perfectly insulated, or adiabatic, drum. While economics and
space, not to mention the second law of thermodynamics,
prevent the actual attainment of the adiabatic ideal, it is
nonetheless a useful benchmark. It turns out that for the drums
in question, a perfectly insulated design would raise the peak
temperature by 3.2°C (5.8°F) above the target value. Add that to
the current underperformance of 4.3°C, and the total possible
scope for temperature improvement is 7.5°C (13.5°F). If achieved,
that would favorably shift the liquid yield by 1.1 wt%. But the
real question is how much of that theoretical potential can be
afforded?
Given the geometric constraints on these drums, the total
insulation thickness is capped at 110 mm. The only way to
increase thermal resistance is by using more efficient materials,
or combinations thereof. For example, several DCU operators
Figure 3.
Thermal
performance
comparison
of aerogel
vs mineral
wood.
have used
composites of
mineral wool
and Pyrogel.
Varying the
thickness ratio
between 0
(100% mineral
wool) and 1
(100% Pyrogel)
allows us to
explore the
performance
and cost
implications in a
more or less
continuous
fashion. This
process is
Figure 4. Mechanical clearance issues restrict the
thickness of typical coke drum insulation designs
to between 75 and 150 mm (©Insultherm, Inc.).
illustrated in Figure 5, which shows the sensitivity of
operational and economic parameters to the thermal resistance
of various insulation designs. In its deteriorated state, the
current insulation is costing the unit an extra
US$70 000/y in lost energy. If this were the only performance
metric at play, replacing the insulation would be an iffy financial
investment, with paybacks in excess of five years. But the
energy costs pale in significance to the US$800 000 loss due to
the depressed peak temperature and resulting 0.8 wt% yield
penalty.
So the insulation is clearly worth replacing, even just to get
back to baseline performance. But is it worth going further still
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Reprinted from March 2015
Figure 5. Performance benefits and payback forupgrading coke drum insulation.
by using a premium, aerogel based insulation system? The green
line in Figure 5 indicates the incremental payback for various
combinations of Pyrogel and mineral wool. For example, a
design using 30 mm (three layers) of Pyrogel with 80 mm of
mineral wool, while more costly up front, would recoup the
incremental investment in 13 months.
Case study 2
That is exactly what a major US refinery did recently, when they
upgraded their six new drums to a combination of mineral wool
and Pyrogel. The reduced thickness was not only more thermally
efficient than the baseline design, but it also allowed them to
put larger drums into the existing structure, increasing
throughput. By using a panel system from Insultherm in La Porte,
TX (USA), the impact of the insulation change on mechanical
design and construction was negligible. One additional benefit
of thermal composites is that they actually render the mineral
wool more durable. Placing Pyrogel at the cold face helps
protect the system from water ingress. Placing it at the hot face
shields the mineral wool from temperatures that would
otherwise cause binder burnout, loss of mechanical
consolidation, and the subsequent drop in performance.
Shifting all the way to a 100% Pyrogel design would
provide the most efficient system possible for the available
space, and one that would pay for itself in under four years.
That is the subject of the next case study.
Case study 3
One of the largest refineries in the US recently added two
new drums to their six drum DCU. While the existing drums
were insulated with a traditional mineral wool design, the
Reprinted from March 2015
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new drums were insulated with 70 mm (2.8 in) of Pyrogel
using another Insultherm panel system. Since beginning
operation in 2012, the owner reports coke drum outlet
temperatures running 4.5°C (10°F) hotter than those of the
existing units. As of this writing, four more drums at the
same complex are being upgraded from mineral wool to the
70 mm Pyrogel design. In addition, four drums at one of the
owner’s other facilities have adopted the Pyrogel panel
system as well. In this particular case, the economics are
favorable enough that the design has been pushed all the
way to 100 mm (3.9 in) of Pyrogel, more than twice the
thickness required for simple energy cost avoidance.
Conclusion
Less than six years have passed since the hurricane that first
inaugurated Pyrogel’s usage in delayed coker units. In that
time, more than 10% of the world’s delayed coker units have
begun using the material. Applications include not just
drums, but also feed and overhead lines, inspection
windows, level sensors, and passive fire protection for skirts.
One of the world’s largest DCU engineering firms has
standardised on the use of Pyrogel for all top heads and
bottom cones, and several US and international DCU
operators now use Pyrogel as their default insulation
material. By the end of 2015, Pyrogel will be in service on
more than 50 drums, up from four in 2011. For DCU operators
around the world, the question is no longer whether to use
Pyrogel, but simply where, and how much. Operating a DCU
is still a high wire act but, with new insulation materials, the
tightrope can at least be made a little easier, and more
profitable, to walk.