Flexible upgrading of heavy feedstocks
Adding tailor-made options for upgrading heavy crudes to existing assets can adapt refineries to changes in market conditions and regulatory demands.
JACINTHE FRECON, DELPHINE LE-BARS and JACQUES RAULT
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With the commercial introduction of delayed coking in 1929 by Standard Oil of Indiana, the refining industry achieved 100% conversion of vacuum residue to distillates and coke. For the next 40 years crude oil prices remained low (less than $20/bbl) and the US became the world’s largest market for delayed coking. As crude oil prices started to climb after the first oil shock in 1972, refiners started to shift their focus away from conversion and towards technologies that could maximise the liquid yield of transportation fuels. With crude oil prices today around $60/bbl, a 20% increase in liquid yield could net a refiner more than $175 million per year for a 40000 b/d residue hydrocracker relative to a delayed coker of the same capacity. This has been the driving force behind the hydroprocessing of atmospheric and vacuum residues around the world.
The introduction of stricter regulations concerning marine fuel oil by the International Maritime Organization means that demand for high sulphur residual fuel oil will continue to decline in the near future because of difficulty in meeting the new low sulphur standard for residual fuel oil. Consequently, refiners will now have an incentive for complete residual oil destruction for heavy high sulphur crudes. The PIRA energy consulting firm forecast the net supply of high sulphur fuel oil could decline by 1.4 million b/d from 2020 and low sulphur (0.5 wt% or less) fuel oil will grow by 900000 b/d. Upgrading margins have not recovered in recent years but could do so in the future. However, refiners must look at a wide range of alternatives to meet their target return on investment in light of this uncertain market.
One option available in today’s market includes integration with downstream petrochemicals to increase the margin over the entire upgrading chain from crude oil to finished products. Another option includes the development of tailor-made solutions that take advantage of existing assets at specific refinery locations along with local market opportunities for fuel outlets and environmental regulations.
Axens has developed a standard technology screening methodology that helps refiners to define the basis of a feasibility study. Key criteria to consider are existing assets at the refinery (delayed coker, residual fuel oil boiler, solvent deasphalting (SDA) unit, and so on), types of crudes to be processed in the future, source of hydrogen and its cost. During the feasibility study, in close collaboration with the petroleum refiner and technology provider, alternative schemes are defined and carefully evaluated. The most appropriate option is selected taking into account the price of crude oil, finished and intermediate products, local market outlets for the products, local and national environmental regulations, capex, operating cost and plant operability.
Tailor-made solutions for achieving high conversion
The H-Oil suite of technologies is based on the proven H-Oil platform and provides a refiner with many options to increase overall liquid yield while meeting a good rate of return on its investment. Figure 1 illustrates the relative performance of the H-Oil process, the H-Oil+ and the H-Oil2-stage configuration that make up the suite of hydroprocessing technologies available to a refiner. Each technology option offers advantages in terms of conversion and ability to process various types of feed difficulty. Conversion is defined as the cracking of vacuum residue into valuable transportation fuels while feed difficulty typically refers to heavy crude oil with high concentrations of asphaltenes and other heteroatoms which require special selection of operating conditions and catalyst.
The H-Oil process is based on the ebullated bed reactor system that was invented in the 1950s. The first patent was issued in 1961. A demonstration plant started up in 1963 at the Cities Service refinery in Louisiana and its successful operation led to the first large scale commercial plant which started up in 1968 at the KNPC Shuaiba refinery in Kuwait. The process has been described in many publications. Over 90% of the world’s vacuum residues that are hydrocracked use the ebullated bed reactor. The ebullated bed reactor operates at a constant temperature and catalyst activity. The exotherm generated inside the reactor is quenched by the cold feed and the catalyst activity is controlled by varying the amount and type of catalyst that is added on a daily basis. The conversion of vacuum residue is normally set between 75 wt% and 90 wt% when production of a stable residual fuel oil is desired from the unconverted residue. A typical configuration is shown in Figure 2.
The unconverted vacuum residue has other commercial applications, including fuel for an on-site boiler for steam and power production, which is applied commercially at PKN Orlen refinery in Plock, Poland; for gasification for hydrogen production at Shell Convent, LA refinery; or as feed to a delayed coker for production of either fuel grade or anode grade coke, an option followed at the Husky Energy Upgrader in Canada.
In many cases, conversion can be increased to higher levels depending on the application and the types of crudes being processed. In the application of feeding unconverted vacuum residue to a gasification unit, the optimal conversion level is sometimes set at 86% in order to produce enough unconverted oil to the gasification unit to put the complex in hydrogen balance. This operation was economically attractive when margins were high. When margins shrank, the refinery took advantage of spreads between low sulphur and high sulphur opportunity crudes. During this period, throughput was maximised and the unconverted oil was routed to the residual fuel oil pool.
Research by Nova Husky Research Center has also demonstrated that unconverted bottoms from high conversion operation could be successfully blended to make road grade asphalt. The level of blending is determined by two main factors: the conversion level, and the grade of road asphalt desired by the end user.
It has been 50 years since the start-up of the first large scale commercial plant. During this time, Axens has been working to enhance the operation and reliability of this technology through innovative designs, the use of improved equipment and the development of catalysts. Some of the technology improvements include:
• Improved recycle cup design inside the reactor to reduce gas hold-up and to improve the efficiency of the ebullated-bed reactor
• Inter-stage separation (ISS) for a design which includes two reactors in series. By off-loading the gas from the first reactor, the hydrogen partial pressure can be increased in downstream reactors or the hydrogen partial pressure can stay the same by lowering the overall system pressure. This allows for a decrease in overall capex
• Catalyst cascading whereby the spent catalyst from the lag reactor is cascaded to the front reactor, thus reducing the overall catalyst addition rate, which results directly in a reduction in operating costs. This is an effective tool for feedstocks with low to medium levels of metals in the crude oil
• High performance catalyst has enabled refiners to raise conversion with minimal increase in sedimentation or decrease in stability of the unconverted oil. In addition, catalyst can now be tailor-made for specific crude oils in situations that are tied to those crude oils
• Automated weighted average bed temperature (wabt) controlled by adjusting the feed oil temperature to the reactor in the control panel
• Automated ebullated bed level control by detecting the bed level through nuclear detectors on the reactor and then adjusting the variable frequency drive that controls the speed of the ebullating pump.
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