Control valve selection for SAF production

Save to read list Published by Poppy Clements, Editorial Assistant Hydrocarbon Engineering, Wednesday, 30 August 2023 12:00

Green or sustainable aviation fuel (SAF) is a renewable replacement for the fossil-based aviation fuels that have historically powered most aircraft. SAF is a drop-in replacement for standard aviation fuel, but it is created from sustainable feedstocks, so it boasts significant carbon emission reductions over its lifetime. Although it is currently more expensive than fossil-based aviation fuel, SAF does offer a variety of green fuel credits for both producers and aviation companies, so production rates are rising rapidly.

Similar to green diesel, SAF has process differences that can pose significant challenges for control valves in the conversion units. This article discusses the SAF production process, and offers suggestions for choosing the best control valves for the more difficult applications.

While the first flight using bio-based aviation fuels occurred in 2008, SAF did not become an approved fuel replacement until 2011 when ASTM International officially allowed commercial airlines to blend up to 50% of certain biofuels with their conventional jet fuel. Since then, a large number of bio-derived aviation fuels have entered the market and are being utilised by an increasing number of airlines. As of 2022, 450 000 flights have used some combination of sustainable fuels, although the price for these products can be as much as three times the cost of kerosene-based aviation fuel.

SAF is different to other bio-based aviation fuels in that it must be certified by a third party to verify that the fuel satisfies certain environmental, social and economic considerations. This does elevate the cost of production, but has driven strong product demand as producers and consumers are offered an array of governmental credits for the fuel’s production and use.

While it is technically possible to create SAF from a wide variety of feedstocks, including sugar-bearing plants, algae, and even woody biomass, most of the SAF that is currently produced comes from the lipid conversion process. Extensive research is underway for the other pathways, but the similarity of lipids conversion to refinery hydrotreating/cracking makes it relatively easy to repurpose existing refining facilities for the production of SAF. Lipid feedstocks for SAF production are primarily used cooking oil, oil-bearing plants (such as rapeseed, carinata, soybeans, etc.), and tallow.

Lipid conversion utilises feedstocks that already contain significant amounts of oil, so the chemical processing methods and equipment are familiar and well understood in the refining industry.

Regardless of the pathway used, nearly all SAF feedstocks require some kind of pretreatment to refine and break down the feedstocks and convert them into the common intermediates that feed the hydrotreater processing units downstream. These pretreatment processes are quite different depending on the particular feedstock involved, but generally all pretreatment processes strive to eliminate contaminants that can poison the reactor catalysts and chemically break down the feedstock into common and consistent intermediates to feed the hydrotreater section.

Once pretreated, the SAF intermediates are fed into a hydrotreating process that is very similar to that used in most refineries. Several different licensors are offering conversion methodologies. Honeywell’s Ecofining process includes a deoxygenation reactor followed by deisomerisation and hydrocracking, and then a product separation section to create light fuels, green diesel and SAF.

Another lipid conversion process uses the Axens Vegan HVO process to deoxygenate the feedstocks and hydrotreat the resulting intermediate to create SAF, green diesel, and naphtha.

In either of these processes, there are certain critical valves that are extremely important, must endure particularly gruelling service conditions, or both. Although similar to the standard hydrocracking process in many ways, there are differences that pose challenges for SAF production. The intermediates tend to be more corrosive and stickier than with conventional feedstocks, and they are prone to paraffin and/or waxy build-up. The process also requires significantly more hydrogen, elevating process pressures and temperatures. Finally, the intermediate feed quality tends to be variable, so reaction chemistry is impacted and less consistent. All of these issues can complicate process control and increase the need for constant maintenance and intervention to keep the plant in operation.

Choosing the right control valves for these services is critical, as proper selection can extend time between outages and increase uptime. The more challenging applications will be discussed in the next section of this article.

This article was originally published in the August 2023 issue of Hydrocarbon Engineering magazine. To read the full article, sign in or register for a free subscription.

Written by Scot Bauder and Janelle Prusha, Emerson.

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/30082023/control-valve-selection-for-saf-production/

Aadam F. Aryan, Distillation Equipment Company Ltd, UK, discusses the selection of appropriate feed arrangements for two-phase feeds, and lists several industrial examples.

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