Thermal maintenance in delayed cokers
An engineered approach to line heating avoids plugging in a delayed coker’s residuum lines. Refiners recognise the need for delayed coking to keep up with the growing demand for transportation fuels, more stringent legislation and the opportunity for significant improvement in refinery profit margins.
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Delayed coking is a mature process, but improvements continue to be made in key areas such as operability, safety and reliability. One recent improvement involves applying an engineered approach to the heating of residuum lines to prevent plugging.
Traditional approaches to line heating do not address the thermal strain of no-flow and restart conditions. It is common to see ceramic heat blankets and steam hoses deployed on an emergency basis for unplugging lines “heated” with non-engineered thermal maintenance products. As a result, refineries lose hundreds of thousands of dollars every year due to plugging-induced lost production and recovery efforts. This article considers the performance and economic implications of plugged coker lines, and discusses the real thermal requirements of the heating system. It also evaluates the economic benefits of a system that is engineered to prevent plugging and, in the event of a utilities loss, has the capability to re-melt the line without employing external heat supplements.
Refineries use atmospheric and vacuum distillation towers to separate various useable gas and oil products from crude oil. The product remaining after these lighter products are separated is commonly known as residuum. Residuum is a high molecular weight material with long carbon chains and a high viscosity. Due to the relationship between viscosity and temperature, it is widely held that residuum must be kept at a high temperature to preserve its flowability. While actual viscosities vary with feedstock and processing technology, the target temperature range to maintain its flowability is typically 300–325°F (150–160°C). The historical approach to keeping residuum within this temperature range is through some form of pipe heating.
Survey of delayed coker units
To extract useful, lighter products from this heavy residuum, today’s refineries are making increasing use of the delayed coking process. Over the past few years, Controls Southeast has worked with a wide variety of delayed coker units throughout North America. These units represent a spectrum of geographic locations, crude feedstock and coker licensing technology. However, despite their individual differences, all the units share a common problem: coker lines plug, resulting in downtime, lost production and maintenance costs.
Based on a survey of these units, plugging can be grouped into two broad categories: bypass and main line plugging. On average, bypass line plugging occurs two to three times a year when operations attempt to redirect flow from the main line to one of the many bypass lines in a coker unit. It is reported that the typical downtime experienced with each bypass line plugging event is 4–12 hours. Main line plugging occurs less frequently at a reported rate of once every 2–10 years. In this case, plugging is discovered when operations attempt to restart flow following an extended downtime, usually associated with a maintenance event. Due to the considerable length of main lines, identification of the actual plug location is onerous and time consuming. The typical downtime due to main line plugging is reported to be one week. For both bypass and main lines, plugging is reported to occur most commonly at the location of pumps, valves and instruments.
It is widely believed that plugging represents a heating problem. In fact, all units represented in the survey had implemented some form of pipe heating to prevent plugging. The most common forms of heating are steam and electric tracing. In steam tracing, steam flows through tubing that is banded to the piping. Heat is transferred from the tubing to the piping via convection. Some installations use heat transfer compound to improve heat transfer via conduction. In electric tracing, current flows through a cable banded to the piping and generates heat based on the resistance of the cable. The amount of current flow through the tracing is regulated based on a temperature measurement at a particular location on the piping.
Root cause of plugging
As previously reported, most units only experience plugging several times each year. So, what is different about the times when conventional tracing systems fail to work? The answer lies in the fact that most of the time the heating system is unnecessary. This is because residuum exits the vacuum distillation column at a temperature 300–400°F (150–205°C) above the target temperature range. As long as the residuum is flowing, it is virtually impossible for it to freeze before reaching the fractionator or coker furnace. Simply put, no heating system is necessary when the residuum is flowing. This widely misunderstood truth leads to a false sense of security in a conventional tracing system. Only in time, when residuum stops flowing, is the inadequacy of the tracing exposed.
The real design requirement for a residuum line heating system is the no-flow condition. Bypass lines are normally no-flow since they are only used when the main line is down. No-flow is also caused by both planned (maintenance) and unplanned downtime. The no-flow condition can be particularly problematic at the location of pumps, valves and instruments. Due to their considerable surface area and complex geometries, these components can rapidly dissipate heat from the line during no-flow conditions, and it is very difficult to heat them via conventional tracing. A system for maintaining high temperatures during no-flow is crucial to avoiding plugging. Additionally, the system must be capable of reheating the residuum following loss of utilities.
A closer review of the heat transfer capability of steam tracing provides insight into its no-flow capability. According to a major steam tracing provider, one conductive steam tracer (with heat transfer compound) is needed to maintain residuum at 325°F (160°C) in a 4in pipeline heated by 150 psig steam. To evaluate this, a finite-element thermal model was constructed for this scenario. This model was then used to evaluate the decay in residuum temperature over time in a no-flow line, assuming an initial temperature of 700°F (370°C). Steam tracing cannot prevent the residuum temperature from falling below the 325°F (160°C) target (see Figure 1). It should also be noted that if the tracing cannot maintain the temperature at target, it certainly cannot heat the residuum back to target following a loss of utilities.
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