Processing heavy ends: part II
Improvements in the SDA technology incorporated into ROSE units, together with expanding options for asphaltene product utilisation, have led to an increased rate of capacity for producing high-quality FCC feedstocks from asphaltenes
Phillip K Niccum and Aldrich H Northup
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Recent advances in supercritical separator design have led to improvements in solvent deasphalting (SDA) technology that have been incorporated into KBR’s residuum oil supercritical extraction (ROSE) units. These improvements in separation technology, developed with Koch-Glitsch, are also incorporated into KBR’s proprietary separators.1 Purpose-designed internals are provided to address the special needs of a unit producing lube base stocks or deasphalted oil (DAO) for conversion feedstock.2
The proprietary Lubemax internals are utilised in ROSE units that produce lube base stock. These units typically produce high-quality DAO at a relatively low yield. This tray-type solution offers superior yield and quality benefits compared to other low superficial velocity solutions such as rotating disk contactors (RDC), packing or other tray types. The following process gains are expected for a typical application:
- 20–30% increase in capacity compared to RDC columns
- 2x increase in mass transfer efficiency
- 0.5–1.0% DAO yield increase.
This tray-type solution is designed to carefully manage the flow of light- and heavy-phase liquids in order to minimise undesirable backmixing. The design technique is the key to providing improved capacity and efficiency compared to other design alternatives.
The proprietary Rosemax extraction internals primarily address the specific needs of ROSE units intended to produce DAO for use as a conversion feedstock. These units must extract deeply to obtain a high yield of DAO while maintaining a reasonable level of contaminants, typically carbon residue and metals. Maximising throughput for a given vessel diameter is often a significant objective. Rosemax internals represent a structured packing-based solution to achieve a high capacity while maintaining low impurity levels. Benefits provided by these internals compared to random packing are:
- 20–30% increase in capacity
- 1%+ increase in DAO yield at maximum extraction
- 7–10% improvement in quality at constant yield.
Asphaltene utilisation options
Economic utilisation of the asphaltene product from a ROSE unit is the key to its process economics. Some of the options refineries are using to maximise the value of the asphaltene product include fuel oil blend components, speciality commercial asphaltenes, conversion (coker) feedstock, partial oxidation feedstock and solid fuel.
Asphaltene product to fuel oil: It is sometimes possible to burn the asphaltene product directly as fuel oil, but in most cases it is first blended with other low-value streams to produce a lower viscosity product that meets fuel oil specifications. The fuel oil production can often be cut to less than half by installing a ROSE unit and blending asphaltenes instead of vacuum residuum into fuel oil.
Some refiners blend the asphaltenes with distillate materials to produce No 6 fuel oil. Light cycle oil (LCO) and slurry oil from the FCCU make excellent blending stocks because of their high aromatic content. A visbreaker can be used to reduce the viscosity of the asphaltene and thus reduce the required amount of blending stock. However, the high sulphur content of the asphaltene may limit its use in No 6 fuel oil production.
Asphaltene quality depends on crude slate, and as the crude slate becomes heavier and more sour the asphaltene produced from these crude oils will also contain a higher quantity of sulphur. Environmental regulations will therefore dictate how much flue gas clean-up is required and, hence, the viability of direct firing of asphaltenes.
Commercial asphaltenes: Speciality products, such as paving or roofing asphalt, can be made by blending the ROSE asphaltenes with aromatic oils.
Asphaltene coking: Refiners are now successfully cracking asphaltenes in their cokers. Asphaltene is normally blended with vacuum residuum to achieve good flow properties. The blend is then cracked in cokers. Asphaltene cracking is being carried out by both delayed and fluid coker operators. Thermal cracking of ROSE asphaltenes instead of vacuum residuum reduces the total coke make by 10–20%. The liquid yield also improves. At the 2003 NPRA Technology Q&A, some refiners reported use of more than 50% asphaltenes in their coker feedstocks.3
Asphaltenes have been processed in KBR’s coker pilot plant, as discussed in NPRA paper AM-06-18: “Economic extraction of FCC feedstock from residual oil”. Typical pilot plant yields from coking the asphaltenes are provided in Table 1. The coke yield is much less than would be expected from a traditional feed with high Conradson carbon residue (CCR). The ratio of feed CCR to coke yield is about 1.2 for this feedstock. Lower CCR feedstocks can be expected to have a feed CCR-to-coke yield ratio of 1.5–1.6 under similar conditions.
Asphaltene to partial oxidation unit: The asphaltene can be fed to a partial oxidation unit to produce synthesis gas. Hydrogen in the synthesis gas can be used for hydroprocessing units. The remaining synthesis gas is fired to produce steam and power. There are presently two ROSE units in operation feeding partial oxidation units and three more planned as the prime outlet for their asphaltene product.
Solid fuel: There are existing commercial technologies to produce solid fuel from the solvent asphaltene. However, these processes are generally high in maintenance, low in reliability and manpower intensive. The KBR Aquaform process produces solid pellets. These are stable, can be stored and are easily transportable. The pellets have a high heating value and low attrition rates. The solid fuel produced from heavy and sour oils will be high in sulphur. Therefore, it can be used only in boilers with stack-gas clean-up or in fluidised-bed boilers that use limestone to capture sulphur oxides from the combustion products. The potential users of these pellets are cement, steel, partial oxidation units (gasifiers), cokers and utility companies.
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