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Worldwide Refinery Processing Review (Quarterly Issues)

Publication date:1Q 2018
Item#: B21801

Sulfur Plant, and Alkylation and Olefin Oligomerization

Sulfur Plant

The need for on-purpose sulfur production has become non-existent, as byproduct sulfur production from refineries and upstream oil and gas production sites more than meets the current demand for sulfur in the market. Further, major new supplies of byproduct sulfur are expected to be introduced into market over the coming years due to tightening product specifications for transportation fuels, developing sour gas fields in the Middle East, and increasing oilsands production in Canada.

Typically, refinery sulfur plants consist of an acid gas removal unit, a Claus sulfur recovery unit, a tailgas treatment unit (to achieve sulfur recovery levels >99.99%), and, in some instances, sulfur degasification and finishing processes. The amount of sulfur produced by each refinery will differ based on a number of factors including sulfur content in the feed coupled with the final product slate and final product specifications. This section of the Review will focus on the recovery of sulfur in a refinery setting; specifically omitted from the discussion is sulfur recovery technologies that are focused on upstream applications, such as oil and gas production.

Continued sulfur plant technology developments have focused on improving the energy efficiency of the acid gas removal unit, Claus unit, and tailgas treatment unit in order to lower operating costs as sulfur removal is done at a cost to the refiner and offers little back in terms of value, so minimizing costs is necessary to improve margins. Additionally, the utilization of Claus plants that can recover sulfur while mitigating the effects of high levels of ammonia and BTX was also discussed as deeper levels of HDS needed to meet more stringent gasoline sulfur requirements tend to increase ammonia production. Also, the use of separate processing units to process sour water stripper gas to free existing Claus capacity has been commercialized. Finally, processes that can produce sulfuric acid from recovered sulfur may become more popular due to the expected sulfur glut that will occur over the coming years as an uptick in high-quality gasoline demand growth has fueled growth for alkylate. The sulfur plant section also features the latest trends and technology offerings, including:

Alkylation and Olefin Oligomerization

Globally, octane requirement is a dominant issue in major gasoline-consuming countries (such as the US, Japan, and China). Europe is diesel-centric and higher octane specification can be met by blending of MTBE, reformate, and to lesser extent ethanol.

In the US, the need for higher-octane blending components in the gasoline pool is growing as more manufactures are designing their new cars with higher compression engines in order to meet the 2025 Corporate Average Fuel Economy (CAFE) regulations. This is in spite of recent announcements by the Trump administration to place the higher fuel efficiency requirement on hold. Another contributing factor to rising octane demand in the US is the potential for significant octane loss as refiners attempt to comply with tighter standards such as the Tier 3 vehicle and fuel requirements that went into effect in the US in Jan. of 2017. Also exports of gasoline in the US more than doubled from 2010 to 2016 and the RON of most of these export grades ranged from 91 to 95.

One option to recover octane loss is isoparaffin alkylation, which comprises the reaction of isobutane with C3-C5 olefins in the presence of an acid catalyst to produce a high-quality gasoline blendstock product. The product known as alkylate is made up of a mixture of isoheptanes and isooctanes with high octane, low sulfur, and low vapor pressure.

The ongoing shale boom in the US has also created opportunities for operators of alkylation units as the shale boom is providing abundant stocks of cheap butane. Furthermore, the paraffinic nature of tight oils is resulting in a FCC gasoline product that is seeing on average an 8-10 point reduction in octane value meaning refiners will need to account for that octane loss with a high-octane gasoline blending pool component like alkylate.

China has already experienced a significant boost in alkylate demand as evidenced by the construction of several new alkylation plants in the last five years. This country has traditionally relied on the FCC unit for gasoline and diesel production. However, the octane of the gasoline that is obtained is below regulatory standards. Another impetus for increased use of alkylation technology in China is the increased availability of isobutane feedstock obtained from the isomerization of butane in imported liquefied natural gas. The country is also looking into ethanol blending to meet the octane requirement via imports particularly from the US. However, a recent trade spat between China and the US in early 2018 could limit the import of US ethanol due to a potentially higher tariff. MTBE is not a viable option to satisfy the octane requirement in the future as China is banning MTBE sale though conversion of MTBE units to ETBE units is acceptable.

Olefin oligomerization provides another option for the production of gasoline blending components particularly when isobutane is unavailable. The feedstock for this process is primarily propylene and/or butene and the reaction occurs between one olefin and one or more other olefins of the same or different type to produce dimers, trimers and other oligomers. Oligomerization is not utilized as often as in the past and most refineries instead choose alkylation as a more cost-effective process. However, selective dimerization of isobutene can yield high-octane isooctane and can be used to revamp existing facilities used to produce MTBE thereby replacing this gasoline additive with isooctane. The alkylation and olefin oligomerization section also features the latest trends and technology offerings, including:

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Keywords: SRU, acid gas removal, AGR, Claus unit, tailgas treating, TGT, SOX, desulfurization, CO2, COS, clean fuels specification, H2S, ultra-low sulfur, ULSD, ULSG, clean fuels, elemental sulfur, direct oxidation, amine scrubbing, amine solvent, advanced process control, acid gas corrosion, foaming, amine loss, fuel gas sweetening, ammonia destruction, BTX destruction, sour crude, sub dew-point Claus, oxygen-enriched Claus, sulfur degasification, sulfur finishing, sulfur granulation, analyzers, instrumentation, sour water stripper gas, Alkylation and Olefin Oligomerization, alkylate, ULSG, high octane, gasoline, acid catalyst, hydrofluoric acid, sulfuric acid, HF, H2SO4, solid acid, ionic liquid, renewable alkylation, supported liquid-phase, alkylation safety, corrosion, fouling, acid regeneration, acid storage, acid runaway, acid release, mitigation, tight oil, paraffinic, propylene, butylenes, olefin oligomerization, dimerization, dimers, trimmers, oligomers