An introduction to fouling in fired heaters: part 1
Fouling is the accumulation and formation of unwanted materials on the surfaces of processing equipment.
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It is an extremely complex phenomenon and considered the major unresolved problem in heat transfer (Bott, 1995). Fouling in refineries and petrochemical plants has an impact on safety, reliability, operation, the environment and profitability. The cost of fouling is believed to be more than $2 billion per year in U.S. refineries alone (source: www.energy.gov) due to increased production costs, production losses, unit shutdowns and high maintenance costs. Fouling is further exacerbated by continued increase in use of heavy, unconventional oil sources, deeper residue conversion to light ends, tightening environmental demands and fuel standards and increased production complexity.
The goal of this article is to provide an overview of the most common types of fouling in fired heaters and preventive strategies. It addresses areas inside the tubes, on the outside of radiant tubes, convection section, burners, air preheaters and selective catalytic reduction (SCR).
Fouling is generally classified by six categories. The first three are commonly found in fired heaters and related equipment.
Fouling inside fired heater tubes
Particulate, reaction, and corrosion fouling occur inside fired heater tubes. The most common one is reaction fouling in the form of coking, which is driven by temperature, residence time, velocity, and feed composition. The heaters that are the most prone to this type of fouling are the ones that process crude feeds, due to the wide range of components they require. This is especially true for refineries that turned to alternative heavy feedstocks that are more cost-effective to process. For example, bitumen (asphalt) is a highly viscous semi-solid form of petroleum. Canada has the largest reserve of natural asphalt (“tar”) in the tar sands, which is a combination of clay, sand, water, and bitumen. Tar sands can be mined and processed to extract the bitumen, which is then refined into oil.
The first problem that occurs when heating Tar Sands Bitumen is the potential for particulate fouling by clay particles. Alumina Silicate clay particles that are normally dispersed in a colloidal system lose solubility and deposit upon heating. The silicate deposits have low thermal conductivities and form a significant resistance against heat transfer. This type of fouling usually occurs in the convection section or the top of the radiant section. Clay deposits can only be removed by pigging, not by spalling or steam air decoking.
This type of fouling is particulate fouling and therefore driven by concentration and velocity. To minimise particle deposits, it is recommended to keep cold oil velocity above 6 ft/s at a minimum and ideally above 10 ft/s. Consider adding velocity steam to the convection inlet to increase the tube side velocity.
A second problem is that bitumen typically contains 16 — 25% asphaltenes. Asphaltenes are heavy, polyaromatic molecules that contain sulphur, nitrogen, and heavy metals. Asphaltene molecular weight is in the range of 500 — 3,000 but the apparent molecular weight can be up to 300,000 due to association by polar constituents. Their weight and molecular structure make asphaltenes strong fouling precursors and must be kept in solution as much as possible to prevent excessive deposition.
On the other end of the spectrum, we have shale oil. The production of shale oil, also known as Light Tight Oil (LTO), has grown exponentially in the last 10 years. Shale oil has many features that are attractive to refiners; it is a light oil with a low viscosity, a low asphaltene concentration (typically less than 0.1 wt%), a low sulphur content, and due to its recent abundance, has become a very economic feedstock. Unfortunately, there are downsides to using LTO as well. It is highly paraffinic, with long chain alkanes of 20 to 50 carbon atoms). This has significantly increased the risk of wax deposition on cold walls of tanks and processing units.
Another downside for vaporisation of LTO inside a fired heater is that its physical properties are very inconsistent. Day to day variations in density and solids content can be very wide, even for shale oil coming from the same basin. A high variability in vaporisation potential can lead to excessive vaporisation inside the tubes, which can lead to dry points. Dry points should always be avoided since they leave behind residue and cause excessive fouling.
Refineries in the US are not designed to process either heavy bitumen or shale oil. They are historically designed for medium crudes and cannot readily process very light or very heavy crudes without significant (and expensive) changes. For economic reasons, many refineries blend LTO and bitumen to achieve the characteristics of a medium type crude. This has introduced yet another fouling problem inside fired heaters. Blends of LTO with heavy asphaltenic crude can result in asphaltene instability and precipitation, resulting in a strong increase in coking rate when onset of precipitation occurs early. Crude, vacuum and delayed coker heaters that used to run for years on Arabian crudes without decoking now show runlengths of several months or less before the tubes are too fouled to continue. Studies show that the paraffinic character of LTO causes the ashpaltenes to lose solubility, an instability that is hard to manage with the daily changes in LTO composition, and even harder when sourcing many different heavy crudes to blend with the LTO. Blending will have to be done extremely carefully to maximise asphaltene stability. Toluene solubility tests according to ASTM D 7157 (“Standard Test Method for Determination of Intrinsic Stability of Asphaltene-Containing Residues, Heavy Fuel Oils, and Crude Oils”) help the refiner determine the optimum ratio of crude to shale. Since coking is a type of reaction fouling, it is strongly dependent on the process film temperature, which in turn is dependent on the incident flux profile from the flames.
See Figure 1 for two different kinds of flame interactions, leading to two very different incident flux profiles. The flames that merge (on the left side of the figure) create a long and uniform flux profile with a peak near the top of the firebox. The flame interactions on the right create a much more intense peak near the bottom of the heater due to flames impinging on the tubes. Note that the flux profiles are normalised and that the absolute flux values of the flames on the right are much higher.
Neither flame behavior is desirable; the long merging flames will lead to poor fuel efficiency, while the short impinging flames create hotspots and very high coking rates. The difference in peak film temperature between the two cases is well over 100°F even though they are the same heater and the same burner design. Flame behavior and flux profiles from burners can be studied using CFD and manipulated by changing burner type, quantity, and/or location.
The mechanism of coking
There are two main types of coking mechanisms. The first one is catalytic coking which takes place at the tube wall itself. The shape (‘morphology’) of catalytic coke is filamentous, which means that a network of fine carbon threads is formed on the tube inner wall. Small metal particles can be found on the ends of these filaments. The process of catalytic coking is demonstrated in Figure 2.
Catalytic coke is the major form of coke formed in high temperature processes like gas cracking (ethane, propane) to produce ethylene. The radiant tubes in these heaters typically contain 35 — 45 % nickel. During operation, the catalytic coke layer continuously dehydrogenates and changes into a very hard graphite-like material that is difficult to spall and gasify. Due to its hardness and rigidness, it poses a risk of tube rupture during a thermal shock.
Pyrolytic coke, also called condensation coke, is softer and less structured than catalytic coke. It is formed in the bulk of the gas by several mechanisms, including dehydrogenation, polymerisation, and condensation of aromatic and olefinic compounds.
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