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Achieve high, aim low

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Hydrocarbon Engineering,

Over the last two decades, the refining industry has weathered volatile market conditions and increasingly stringent fuels regulations. In North America, refiners are tasked with meeting challenging ultra-low sulfur gasoline regulations (Tier 3). Also, as more turbocharged vehicles enter the market, the demand for premium, higher octane gasoline will also increase as automakers begin recommending or requiring the use of higher octane gasoline.

Tier 3 regulations will create challenges as refiners turn to higher gasoline post-treatment severity for increased desulfurisation, which will negatively impact octane. Under new regulations, refiners are required to produce gasoline with an annual average sulfur content of ≤10 ppm, with a maximum gasoline sulfur cap at the refinery gate of ≤80 ppm. Fluid catalytic cracking (FCC) naphtha is the main ‘problem stream’, as it comprises 40 – 50 vol% of finished gasoline volume and contributes 80 – 90% of the total sulfur in the refinery gasoline pool.

Meeting sulfur limits will require operating adjustments, catalyst formulation changes, and/or capital investment. Operating strategies that reduce sulfur may limit gasoline octane, reducing the flexibility of the refinery’s gasoline blending system. Incremental production from the reformer or alkylation complex can alleviate some of this pressure, depending on the refinery configuration and capabilities.

Capital solutions around the FCC include feed and/or product hydrotreating, or other sulfur removal units. Most refiners have some form of feed and/or product hydrotreating in place, and these systems are effective at reducing the sulfur content of gasoline streams. However, there are some tradeoffs; increased hydrotreating severity can reduce gasoline octane, and octane preservation is a key feature in the design of these systems. Also, increasing hydrotreating severity increases the refinery H2 consumption and reduces the hydrotreater run length. For some, pre-treating FCC feed sufficiently for sulfur minimisation results in a heat balance challenge for the FCC unit. Some of these challenges can be exacerbated at refineries using shale-derived crudes, which are naturally light and produce low sulfur, but also low octane gasoline.

FCC operating strategies include undercutting gasoline for sulfur control, and control of FCC unit heat balance parameters to ensure that the FCC produces enough feedstock to keep the alkylation unit full. Often, refiners can face FCC unit limitations (e.g. wet gas compressor capacity) when pursuing these changes, but they are quick and often easy to implement.

Written by Andy Huang, Gary Cheng, Ann Benoit, and Bob Riley, W. R. Grace & Co., USA.

This article was originally published in the September 2017 issue of Hydrocarbon Engineering. To read the complete article, please sign in or register for a free trial subscription.

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