Maximizing refinery energy optimisation

Flexicoking™ technology is a highly differentiated unique resid upgrading process resulting in significantly lower environmental emissions than other conventional coking technologies, and production of a clean burning fuel gas available for refinery energy optimisation.

Sebastian K. Seider, ExxonMobil Research and Engineering
Martin De Wit, ExxonMobil
Stylianos Kyriakou, Hellenic Petroleum

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Article Summary

These environmental and energy utilisation advantages result in an improved economic resid conversion solution for processing difficult heavy feedstocks.

This paper focuses on two main themes:
1. An overview of the technology highlighting over 40+ years of process performance with a focus on flexigas utilisation.
2. The real world benefits as well as project implementation and operational considerations associated with the Hellenic Petroleum (HELPE) Elefsina Refinery Upgrade project including Flexicoking technology and services.

Flexicoking technology is a commercially proven fluid bed resid upgrading process, designed for thermal coking all types of residue feeds with minimal coke production; as low as 1% of feed is converted to coke product. The unique design eliminates the open coke pits and associated particulate emissions with enclosed coke transfers. Also, there is no high sulphur petcoke production to market and no coke drum deheadings associated with delayed coking. Flexicoking technology gasifies the majority of the produced coke with steam and air to produce a fuel gas referred to as flexigas. During gasification, sulphur in the coke is converted into H2S which is removed via amine absorption included within the battery limit of the unit. The clean flexigas supplements the refinery fuel gas and can be used in process heaters, utility boilers and power generation. Thus, Flexicoking provides an environmentally friendly technology solution for upgrading all types of refinery residual streams to liquid products and converting coke to clean burning fuel for optimising the refinery energy integration.

This article provides a comprehensive overview of ExxonMobil’s technology and utilisation of flexigas for refinery energy requirements. This article will also discuss the application of the technology in the HELPE Elefsina Refinery in its 2012 Refinery Upgrade Project and subsequent energy balance optimisations facilitated by the project.

Exxonmobil Flexicoking technology overview
Resid conversion technology alternatives

Resid or residuum materials are high boiling hydrocarbons that are not suitable for transportation fuels or lubes unless they can be converted to lighter, more hydrogen rich hydrocarbon types. There are two basic approaches to processing these feedstocks to more valuable lighter products. These are referred to as hydrogen addition or carbon rejection.

Hydrogen addition processes usually operate at high pressure and depend upon catalysis and hydrogen gas to achieve the desired reactions, which simultaneously crack and hydrogenate the large residuum molecules. These heavy feedstocks generally cause catalyst deactivation at a significant rate due to the presence of hetero atoms and metals, requiring high fresh catalyst make-up rates. Hydrogenation is indiscriminate and the resultant high hydrogen demand can be expensive at locations where hydrogen is costly. These processes also produce a bottoms stream of very low quality that is difficult to dispose of in a cost-effective manner.

Carbon rejection processes generally refer to thermal coking processes which operate at low pressure (less than 0.4 MPa-g/60psig) and utilise thermal cracking reactions to achieve the desired conversion of the high boiling molecules. Thermal cracking reactions refer to several types of reactions including cracking, condensation, polymerisation and isomerisation. This chemistry results in a redistribution of hydrogen in the feed to yield lighter liquid products with higher hydrogen-carbon ratios and a byproduct of solid coke with low hydrogen-carbon ratio. Typical commercial coking processes include delayed coking, fluid coking, and ExxonMobil’s unique Flexicoking technology. Solvent deasphalting is an alternative carbon rejection process which separates carbon rich asphaltenes by solvent extraction from a more hydrogen rich deasphalted oil that can be processed in conventional FCCs or hydrocrackers. The asphaltene pitch or “rock” is typically disposed of in fuel oil blending or a coking process.

Process description
Flexicoking technology utilises a low pressure process that integrates fluid bed thermal coking with a fluid bed steam and air coke gasification. The feed is converted to high value full-range liquid products, conventional fuel gas and a CO/H2 based fuel gas referred to as flexigas. Process heat for the thermal conversion and gasification steps is provided by partial oxidation of carbonaceous coke formed in the coking reactor. Most of the coke is gasified and the resulting gas is desulphurised using the proprietary Flexsorb™ technology. The significant volume of clean flexigas can be used in refinery fired equipment, for power generation or for other energy needs. The unit at HELPE Elefsina Refinery utilising Flexicoking technology is shown in Figure 1.

Process description
the process consists of three main vessels: coking reactor, coke gasifier, and reaction heater. The typical process flow and energy balance for the Flexicoking technology is shown in Figure 2.

The vacuum residue feed enters a scrubber section located at the top of the coking reactor for direct contact heat exchange with the reactor overhead product vapours. The higher-boiling point hydrocarbons (~975°F+/525°C+) present in the reactor product vapours condense in the scrubber and return to the reactor in a mixture with the fresh feed. The feed is thermally cracked in the reactor fluidised coke bed to a full range of gas and liquid products and coke.

Lighter overhead product vapours from the reactor/scrubber go to conventional fractionation and light ends recovery where the liquid products and LPG are produced.

Coke inventory in the reactor and heater is maintained by circulating coke from the reactor to the heater via the cold coke transfer line. In the heater, the coke is heated by contact with the gasifier products and circulated back to the reactor via the hot coke transfer line to supply the process heat that sustains the thermal cracking reaction. The excess coke in the heater is transferred to the gasifier where it reacts with air and steam to produce CO/H2 rich syngas called flexigas. The gasifier products, consisting of a mixture of flexigas and unconsumed metals rich coke, return to the heater to heat up the circulating coke.

Flexigas exits the heater vessel and is fed to steam generators before going to dry and wet particulate removal and then to H2S removal in the integrated Flexsorb technology amine absorption process.

Flexsorb technology is designed for selective removal of H2S in the presence of CO and CO2 and utilises a proprietary severely sterically hindered amine. This allows the Flexsorb technology solvent to achieve high H2S clean-up selectively at low solvent circulation rates. The flexigas product contains less than 10 vppm H2S and is ready for use as fuel in fired process heaters and/or boilers for steam and power generation.

Flexigas benefits and utilisation
Integration of coke gasification into the fluid bed coking process has multiple benefits:
Reduced coke handling and by-product disposition: Minimal coke production reduces the requirement for containing and managing sales or disposal of large quantities of high sulphur coke produced in the traditional delayed coking processes or residual bottoms streams produced in hydrogenation processes.

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