Jun-2014

Operating challenges associated with fines in the delayed coker

The coking process has been around since the early days of the refining industry. Over the years it has undergone continuous improvements to make it a primary process to provide the upgrading needs for heavy oils.

Article Summary

Other processing options are equally viable such as ebullated bed residua hydrocrackers, residua solvent deasphalting or residua FCC, and each fits a specific need in the upgrading chain. The use of coking gives the operator a reliable unit that provides a yield slate fitting the desired product market. The synthetic crudes are partially upgraded or completely upgraded products from Athabasca Bitumen or other heavy crudes.  Upgrading of these heavy crudes occurs around the world and is a critical part of the worlds energy needs. Current installed capacity for coking is 7 to 8 MMBPD with about 46-48% in North America. Poor reliability can have a negative impact on the refinery or upgrader, and understanding operability issues and how to avoid them increases refinery profitability.

The effect of fines in the Delayed Coker feedstocks presents a processing challenge which is not well understood in the industry. The fines reduce the ability of the coker to reach full capacity utilisation. Fines are present in the delayed coker feed from several different sources. Both inorganic and organic fines can significantly impact the delayed coker reliability and overall refinery economics. In some cases, sudden changes in the refinery or a major upset in the refinery’s desalter can bring the delayed coker down within hours. In most cases, issues with solids fed to the coker are not as dramatic and the solids problem is a slightly slower but still significant ongoing issue.

Types of Fines and Their Sources
Fines can be divided into two groups, organic and inorganic; however there is significant interaction between these two categories.

• Organic solids are coke particles or coke fines and asphaltenes. Organic solids are at times a true solid (coke fines, corrosion products) and are also precipitated asphaltenes both of which form a third solid phase.
• Inorganic fines can be from oil sands bitumen, FCC catalyst or corrosion from up-stream equipment.

Inorganic or organic salts that can form from amine and chloride components can also be thought of as solids and can definitely affect the operations of the delayed coker. Salt formation in the fractionator and downstream equipment are not discussed here as this topic warrants a separate discussion.

Organic Fines
Organic fines are primarily hydrocarbons but typically have some inorganic constituents, such as metals, sulphur, oxygen and nitrogen. Coke fines are the single largest type of fines in the delayed coker. Coke fines can be a problem in both the primary process operations and in the ancillary operations such as coke cutting and coke handling.

When the coke drums are being filled, coke fines can be entrained in the drum overhead into the fractionator due to high vapor velocities and high foam fronts. The fines are relatively consistent in shape and size. The very small fines, less than 50 microns, are solid rectangular shapes with a specific gravity of about 1.7. A significant quantity of larger particles can also exit with the vapor due to high velocities, full drums and large foam fronts. These larger particles can be as much as a quarter of an inch or 50 millimetres. At some locations these very large fines can actually be heard exiting the top of the drum. These large particles are less dense and can be spherically shaped or like small shot coke particles. These larger particles can be recovered in the fractionation bottoms circulation system.

Sometimes due to phase incompatibility between cracked products sediment forms. The sediment can be asphaltene precipitation, wax formation or other molecules that form solids as composition changes. Sedimentation can be reversible (waxes) or non-reversible (asphaltene precipitation). Sedimentation is a subject about which a great deal is written but is not well understood. Sedimentation can manifest itself as a solid or in a sticky pitch or gum.

Historically, the delayed coker feed asphaltene content, in conjunction with the Concarbon Carbon content, is used to predict the propensity for sedimentation. Canadian feeds have relatively high asphaltenes content compared to typical crudes. Other feed types such as a deasphalted feed might have significantly less asphaltenes while ebullated bed vacuum residua would have more. Asphaltenes are particles in the submicron range and can agglomerate to form micron sized particles. These are often neglected in the discussion of fines and the impact on operations.
The vapours leaving the coke drum also entrain very small pitch particles in the form of an aerosol spray or mist. Again, the amount of unconverted heavy oils or pitch carried out of the drum is a strong function of the drum vapor velocities, the level in the drum and the amount of foam, or unconverted coke, at the top of the drum. Figure 1 illustrates a full drum with solids and pitch carry over.

KBC’s newly developed coke quality simulation also incorporates an estimate of the height of the foam front in a coke drum. The foaming calculations give a rough estimate of the foaming potential of different crude blends and how these blends interact under different coke drum operations such as temperature, recycle, pressure, cycle time and drum utilisation or coke level in the drum. Figure 2 shows an example of how two dissimilar crudes will interact as the coke drum temperature is increased.
 
Inorganic Fines
Inorganic fines are FCC catalyst, ebullated bed catalyst fines, corrosion deposits, bitumen sand, and other <100 micron solids entering the coker via the feed.

FCC slurry contains catalyst fines typically in the 20 to 60 micron range. Catalyst fines have a very high surface area and relatively low density and are not removed by the bottoms filtration circuit. The concentration can vary over a wide range. Most are less than 0.1 wt%.

FCC catalyst slurry fines are measurable using many techniques. The particle size and distribution are not routinely tested but this can be done by the licensor upon request. The quantity of fines should be measured daily and is a monitoring point for the coker operation.

Ebullated bed fines come from the vacuum tower bottoms of an ebullated bed heavy oil hydrocracker and are in the 100 micron range. This catalyst has a higher density and lower surface area than FCC catalyst. Due to their larger size and higher density, some of these fines would be removed by the bottoms filtration circuit. Concentration is low, <0.02 w%.