The world is quickly evolving towards the production of low/ultra low sulfur, high quality fuels while at the same time continually demanding reduced refinery

emissions. The key driver for these transformations is the regulatory environment becoming increasingly stringent.

One direct consequence of required reductions in emissions of criteria pollutants (SOx, NOx, VOCs, CO, particulate matter and ozone) is that refiners are faced with extensive capital investment to reduce emissions and improve reliability of existing abatement systems, including redundancy for sulfur gas recovery capacity. It is also expected that the global refining community will experience a general trend towards heavier, high sulfur crude supply which will require further upgrades to existing desulfurisation infrastructure and consequentially additional sulfur gas recovery capacity as well.

A second direct consequence of required reductions are the increased restrictions on mobile source emissions of criteria pollutants, with the sulfur content of transportation fuels being reduced to virtually zero. Use of other gasoline blending compounds, such as aromatics,

cleaner futureCarlos Cavalca, Monty Davis, Nick Confuorto, Glenn Liolios, Tom Hightower and Stuart Thomas, DuPont Clean Technologies, USA, discuss integrated environmental solutions for refinery sulfur, nitrogen and clean fuel challenges.

Solutions for a

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olefins, butane and, in some regions, oxygenates, are also being restricted due to their impact on increased higher criteria pollutant emissions. In the USA another driver is the elimination of MTBE as a gasoline blending compound, resulting in changes in fuel specification and the increased use of ethanol, all of these resulting in increased RFG demand.

All the above complex environmental and fuels market dynamics, coupled with the intensification in overall fuel demand coincident with the rapid economic growth in several regions of the world (Asia Pacific, Latin America, etc.) will require robust, integrated sulfur management, emission control and clean fuel environmental solutions to effectively address the challenges associated with today’s low sulfur and high fuel quality requirements.

integrating environmental solutionsDuPont Clean Technologies provides science based, customised environmental solutions to refiners’ sulfur management, nitrogen management, emission control and clean fuel needs. Today DuPont’s building blocks for integrated environmental solutions include DuPont Sulfur Gas Recovery (SGR) and Spent Acid Regeneration (SAR) Technologies, DuPont™ STRATCO® Clean Fuel Technologies, DuPont™ BELCO® Clean Air Technologies and DuPont Clean Water technologies.

SAR-SGR acid plant - alkylation integrated solutionsDuPont’s sulfur management takes place via an onsite SAR-SGR sulfuric acid facility where DuPont builds, owns, operates and maintains (BOOM) a sulfuric acid plant located on or adjacent to a refinery, as shown in Figure 1. The objective of this plant is to provide total or partial sulfur gas recovery (SGR) to the refinery, including SRU backup, in addition to alkylation spent acid regeneration (SAR) service. Process design improvements make investment in new onsite acid plants a viable alternative to Claus with tail gas treatment. The acid plant is designed to optimise the conversion balance of refinery sulfur gases and alkylation spent acid, as the acid gas is used as the fuel in the spent regeneration process. Sulfur gases include amine gas, tail gas, SWS gas, gasifier acid gas, and stack scrubbed gases. The DuPont plant can also process a variety of chemical spent sulfuric acid. In addition, the operating philosophy and reliability of the SAR-SGR plant is consistent with refinery turnaround schedules. Furthermore, benefits of on-site SAR arrangements at the refinery include no logistics issues for spent acid (e.g. elimination of transport of spent acid offsite), management of acid gas for the refinery and high pressure steam production. Figure 1 shows DuPont's Delaware City, DE SAR-SGR plant. Two more similar facilities are currently under construction in El Paso, Texas, and

Linden, New Jersey.DuPont™ STRATCO® Clean Fuels Technologies

are based on STRATCO® sulfuric acid alkylation technology. One of DuPont’s strategies for clean fuels involve the production of alkylate. Alkylate enjoys inherent advantages, such as high RON and MON with low sensitivity, negligible amount of sulfur, olefins, aromatics/benzene and can be produced to low RVP. When compared to other olefin upgrading processes (such as ETBE, dimerisation, etc.) alkylation offers overall improved yield (as volume product/volume olefin used) and improved RON & MON numbers. STRATCO® sulfuric acid alkylation is the safest, proven alkylation technology and is being constantly improved through DuPont R&D programmes and operating experience.

The effective integration of this state of the art alkylation technology with DuPont’s SAR-SGR offering allows the optimisation of synergies between processes, thus allowing lower energy costs, reduction of capital investment, and improved operational flexibility for alkylate production and sulfur gas recovery (including processing of amine gas and sour water stripper gas). Examples of synergies of integration for these systems include, for example, elimination of redundant equipment, such as neutralisation systems between units, minimising capital investment. Another example is the supply of steam for the alkylation unit de-isobutaniser column by the SAR-SGR acid plant optimising operating cost and energy integration as

Figure 2. Onsite DuPont SAR-SGR sulfuric acid plant integrated to a DuPont STRATCO® sulfuric acid alkylation plant.

Figure 1. Onsite DuPont SAR-SGR sulfuric acid plant. The picture on the right shows DuPont's Delaware City, DE plant.

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well as varying the rate of fresh acid supply to the alkylation unit by the SAR plant to optimise alkylate product octane properties.

In this integrated sulfur management-alkylation conversion configuration DuPont can BOOM both the SAR-SGR and sulfuric acid alkylation plants located on or adjacent to a refinery. In addition to sulfur recovery, DuPont can conduct toll (fee-based) conversion of refinery-owned alkylation feedstocks, returning to the customer alkylate product and byproducts. As DuPont toll-alkylates feeds for the refinery, the spent acid regeneration is now ‘invisible’ to the customer. Figure 2 depicts this integrated solution.

When this integrated model is deployed regionally, by consolidation of alkylation feeds from multiple refineries into a regional alkylation-SAR-SGR installation, improved economies of scale driven by the increased size of the alkylation plant can enable alkylation of feedstocks for a number of refineries. Harnessing of this alkylation economy of scale can result in lower per unit alkylation costs. This regional SAR-SGR-alkylation model is particularly applicable in situations where local refineries do not have enough alkylation feedstocks to justify an alkylation unit and/or where economies of scale of a smaller unit makes owning an alkylation plant not economically feasible.

In this business model, depicted in Figure 3, a regional DuPont BOOM SAR-SGR-alkylation unit is deployed in a host refinery where the larger alkylation and SGR-SAR acid plant is built, providing not only toll alkylation and SGR service to the host refinery but also toll alkylation service for feedstocks from regional refineries. Alkylation feedstocks and alkylate product for these regional refineries can be transported by truck, rail or marine.

Ammonia SWS gas recovery - SGR acid plant integrated solutionsDuPont recently developed a technology to separate and process ammonia from sour water stripper (SWS) gas. This technology is very suitable for refineries that process crudes with a high level of nitrogen-bound species and/or larger amounts of sour water. The former is particularly true in Central and South America, where the indigenous crudes are typically high in nitrogen, for example, Marlim (Brazil) containing 4900 ppmw N and Maya (Mexico) averaging 3600 ppmw N.

In this technology, ammonia in the SWS gas is converted into ammonium sulfate solution by scrubbing the sour gas with sulfuric acid and by steam stripping the liquid sulfate product of residual H2S and mercaptans. The ammonia free, dry SWS scrubbed gas containing 90%+ H2S can be fed into a Claus unit or a SGR sulfuric acid plant while the ammonium sulfate liquor product can be used as a green (non-hydrocarbon derived) nitrogen fertiliser feedstock (Figure 4 depicts the process).

This technology of refinery nitrogen management allows improved Claus capacity and minimisation of

downstream problems in the SRU due to nitrogen salt formation. By elimination of ammonia from the Claus feed, ca 3.75 t additional sulfur processing capacity per t of ammonia removed can be gained. This also provides for elimination of SWS NH3 separation costs and elimination of ammonia incineration and associated SOx emissions linked with the incineration of H2S-containing NH3 streams.

As the process of ammonia recovery from the sour gas stream requires sulfuric acid, this SWS gas scrubbing plant can be integrated to an on-site SGR sulfuric acid plant which can provide sulfur management to the host refinery. This SGR plant can convert sulfur gases into sulfuric acid product, providing incremental or total sulfur recovery capacity and/or SRU backup to the refinery and non-fuming sulfuric acid to the NH3 scrubbing/ammonium sulfate plant. It should be noted that this ammonia scrubbing plant can be fed with a variety of acid strengths, including diluted (waste) acid streams from the SGR plant, thus maximising the usage of sulfur streams. Figure 5 presents the integration concept for this installation.

Taking into consideration factors such as capital required to BOOM an SGR acid plant, availability of spent acid and availability of acid gas and sour water, the optimal, most cost effective configuration for integration of this type of technology may be a cluster of NH3 scrubbing/ammonium sulfate plants located strategically at nearby satellite refineries with a single

Figure 3. Regional onsite DuPont SAR-SGR - alkylation plant.

Figure 4. DuPont modular plant for on-site NH3 SWS gas scrubbing and ammonium

sulfate production.

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DuPont BOOM SGR sulfuric acid plant located at a host refinery converting acid gas into sulfuric acid to supply the network of ammonium sulfate plants. The logistics of acid transport and uptake of ammonium sulfate product to/from the nearby satellite refineries will also be an important factor when considering this option. Figure 5 shows this distributed NH3 SWS gas scrubbing/ammonium sulfate model.

SO2 scrubbing - SGR-SAR acid plant - alkylation integrated solutionsDuPont™ BELCO® Clean Air Technologies are based on world class scrubbing technologies for abatement of SOx, NOx and particulate matter emissions from refinery processes. BELCO® provides an extensive array of proven scrubbing solutions (standard, such as the EDV® Wet Scrubbing System, or regenerative, such as LABSORB™) for refinery FCCU, coking units, power

boilers, fired heaters, etc. The EDV® scrubber can also be integrated with the LoTOx™ process for additional control of NOx emissions. For applications with very high levels of SO2, such as fuel switching in boilers or power plants where lower cost high sulfur fuels can be used instead of natural gas or sulfur-lean fuels, or for those applications where disposal of liquid or solid scrubber byproducts are an issue, BELCO® provides Dual Alkaly and LABSORB™ scrubbing technologies.

Strong synergies exist between the regenerative LABSORB™ system and the DuPont SGR sulfuric acid plant offering. In cases where a LABSORB™ regenerative scrubbing system is deployed to clean FCCU flue gases or boiler/power plant exhaust (typically when the refinery is faced with requirements of minimal solids or liquid byproducts discharge), the regenerated, concentrated, SO2 stream from the LABSORB™ stripper column can be sent to the DuPont SGR plant for conversion into sulfuric acid product. In this integrated configuration, operating costs can be reduced if the steam generated in the SGR plant can be sent back to the LABSORB™ system to be used in the regeneration process. For this application the design of the SGR acid plant could be simplified (with a resultant reduction in capital investment) if the only source of sulfur is SO2. Finally, this integrated configuration allows the straightforward conversion of waste SO2 into a valuable commodity, sulfuric acid, instead of conversion into sulfur in a Claus unit requiring H2S feedstock. Figure 6 shows the LABSORB™ regenerative scrubber-SGR acid plant integrated concept.

Finally, the optimal combination of synergies for the integration of DuPont environmental solutions can be found on the DuPont onsite scrubbing, SAR-SGR sulfuric acid, toll alkylation configuration. In this full integration case the design of the acid plant is optimised by the presence of spent acid, acid gas and scrubber sulfur dioxide streams, with the design of this unit being simultaneously optimised with the design of the alkylation unit and scrubber system. The synergies derived from the full integration of the above solutions translate into lower capital investment, overall lower operating costs, improved environmental footprint (lower emissions) and improved sulfur recovery/toll alkylation benefits to the refiner. Figure 7 presents this full integration concept.

ConclusionThe effective integration of sulfur management, nitrogen management, emission control and clean fuels technologies allows the creation and implementation of innovative environmental solutions that:

Optimise synergies between technologies.

Offers the lowest environmental footprint.

Maximises benefit to the refiner.

notesDuPont™ BELCO® and BELCO® are trademarks of BELCO Technologies Corp.LABSORB™ is a trademark of BELCO Technologies Corp.EDV® is a trademark of BELCO Technologies Corp.LoTOx™ is a trademark of the BOC Group, Inc.The LoTOx™ process is available exclusively for FCCU and Fluid Coker applications from BELCO® under license from BOC.DuPont™ STRATCO® and STRATCO® are trademarks of DuPont.

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Figure 7. DuPont onsite integrated SO2 scrubbing - SGR-SAR sulfuric acid – alkylation installation.

Figure 6. DuPont onsite integrated SO2 scrubbing - SGR sulfuric acid plant facility.

Figure 5. NH3 SWS gas scrubbing/ammonium sulfate - SGR sulfuric acid plant integrated installation concept.