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Apr-2006

Residue upgrading

An examination into the feedstock and process conditions where solvent deasphalting is the most appropriate technology for upgrading residue

Michael J McGrath, Foster Wheeler USA
Edward J Houde, UOP LLC
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Article Summary
Solvent deasphalting (SDA) has a key role in today’s refinery, as the technology can be used in a variety of ways for residue upgrading. It is less expensive to build and operate than other residue conversion processes and is especially useful in recovering large quantities of high-quality oils, which can be further upgraded via traditional FCC and hydrocracking units. The fit for a particular location is dependent on finding a use for the SDA pitch, and various options are discussed.

While SDA has been used for more than 50 years to upgrade non-volatile residues, the technology continues to evolve over time. It is a robust, economical process that uses an aliphatic solvent to separate the typically more valuable oils and resins from the more aromatic and asphaltenic components of its vacuum residue feedstock. The earliest commercial applications of SDA used propane as the solvent to extract high-quality lubricating oil bright stock from vacuum residue. These applications were called propane deasphalting (or propane deresining when used to separate high molecular weight resins from Pennsylvania-grade vacuum residue). SDA processes have gradually extended to include the preparation of feedstocks for 
catalytic cracking, hydrocracking and hydrodesulphurisation units, as well as the production of specialty asphalts.

In 1996, UOP and Foster Wheeler USA Corporation (FW) entered into a collaboration agreement to merge their SDA technologies. The merging of both companies’ experience in designing solvent deasphalters represents a total capacity of more than 500 000 bpsd and more than 50 commercial units, with capacities ranging from 500–45 000 bpsd These include all applications of solvent deasphalting, such as:
•    Production of lube oil feedstocks
•    Recovery of incremental feedstock for downstream FCC and hydrocracking units
•    Production of road bitumen, including two-product (deasphalted oil [DAO] and pitch) and three-product (DAO, resin and pitch) process configurations.

Technology overview
SDA, whether for the production of lubricating oil or cracking stocks, uses a light hydrocarbon solvent specifically tailored to ensure the most economical deasphalting design. For example, propane solvent may be specified for a low deasphalted oil yield operation such as lube production, while a solvent containing hydrocarbons as heavy as hexane may be used to obtain a high deasphalted oil yield for the production of additional conversion unit feedstock. Plant designs have been developed using heavy solvents at elevated temperatures in order to maximise the yield of usable deasphalted oil and minimise the yield of pitch having a softening point of 350°F or higher.

UOP’s SDA experience has principally focused on the use of butane and heavier-type solvents that can obtain higher DAO recoveries. There is a distinct advantage to the use of supercritical separation for the recovery of the solvent and DAO when using these types of solvents.

Consequently, UOP developed supercritical solvent-DAO separation technology. The other area UOP focused its development efforts on involved minimising the solvent-to-oil ratio while still producing a reasonably high-quality DAO.

FW SDA technology development emphasis was initially more focused on lower lift, very high-quality applications, such as the production of lube oil feedstocks for hydrocracking and further solvent refining. Consequently, its technology originally focused on propane/butane deasphalting using optimised extraction techniques for those specific applications. In addition, FW has made available its detailed design and construction experiences from a multitude of SDA projects.

SDA advantages
SDA technology has several advantages, including:
•    Extraction devices tailored to the specific application:
    ν    UOP/FW/Sulzer’s structured packing and proprietary internals in both the multi-stage counter current extractor as well as the DAO and resin separators. This technology provides state-of-the-art contacting and separation devices to maximise extraction efficiency as well as the optimal recovery of clean products
     ν    FW’s multi-stage rotating disk contactor (RDC). The RDC is specifically designed to achieve high product yields and quality by incorporating both stripping and rectification of the oil feed. Superior-quality DAO is obtained from the RDC even at DAO recovery rates exceeding 85%, with an even greater difference in quality being seen at lower DAO yields.
•    Supercritical solvent recovery (of the solvent) allows more efficient utilisation of the system’s thermodynamic characteristics, while also reducing the unit’s operating costs
•    Multiple product recovery designs that take advantage of the changes in liquid-liquid equilibrium, which result from changes in operating conditions between those utilised during extraction and those used for solvent recovery
•    Lower solvent requirements used to achieve processing objectives. Although increasing the amount of solvent used in the extraction improves the extraction efficiency, it also has a major impact on the unit’s operating costs. Consequently, the lowest solvent-to-oil ratio necessary to achieve the desired product separation is typically specified
•    Optimal design of heat-exchange systems. Combined design experience in optimising SDA heat-exchange systems allows the SDA user to select a multitude of heat-exchange options, depending on the project-specific objectives and opportunities.

Extraction devices
The efficiency of the extraction process is the key equipment design variable impacting both the capital and operating costs of SDA. The extractor’s role in SDA is to separate the precipitate (pitch phase) from the continuous fluid stream (DAO/solvent).

Both single-stage co-current extraction, where the bulk of the solvent is mixed with the feed prior to the extractor, and multi-stage counter-current extraction, where the bulk of the solvent enters the bottom of the extractor separate from the feed, have been used in the industry.
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