Reprinted from March 2015HYDROCARBON ENGINEERING

Walking the

Operators 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 insulationDelayed 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

John Williams, Aspen

Aerogels, USA, discusses

the challenges and opportunities in properly

insulating delayed cokers. tightrope

Reprinted from March 2015HYDROCARBON ENGINEERING

Reprinted from March 2015 HYDROCARBON ENGINEERING

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

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 insulationDerived 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

Figure 1. Delayed coker unit (©iStock.com/Phil Augustavo).

Figure 2. Failing coke drum insulation (©Insultherm, Inc.).

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water makes Pyrogel one of the toughest, most durable insulation products on the market today.

Case study 1Nowhere 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 impactConsider 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

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

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

Figure 3. Thermal performance comparison of aerogel vs mineral wood.

Figure 4. Mechanical clearance issues restrict the thickness of typical coke drum insulation designs to between 75 and 150 mm (©Insultherm, Inc.).

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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 2That 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 3One 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

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.

ConclusionLess 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.

Figure 5. Performance benefits and payback forupgrading coke drum insulation.