Jun-2021

Delivering drop-in renewable fuels

A series of pretreatment, hydrotreating, and distillation steps converts fresh or used lipid based feedstocks to full range, drop-in renewable fuels.

CHUCK RED and ED COPPOLA, Applied Research Associates
ROBERT VALENTE, CHRISTINE CONWAY and LIN ZHOU, Chevron Lummus Global

Article Summary

Hydrothermal Cleanup
Hydrothermal Cleanup (HCU) is a pretreatment step that uses traditional refinery components to remove inorganic materials from waste fat, oil, and grease feedstocks. During HCU, water is combined with feed oil and then fed to the HCU reactor system at the temperature and pressure necessary to maintain a hydrothermal/liquid-phase environment. Metals that are present in waste fats, oils, and greases (FOG) are mostly in the form of free salts and soaps. Phosphorus is mostly in the form of phospholipids. HCU relies on three primary mechanisms to achieve metals reduction:
1) Removal of soluble, free salts — similar to conventional desalting 2) Rapid acidulation of metal soaps (Na, K, Mg, Ca, Fe, others) using a weak acid
3) Hydrolysis of phospholipids into phosphate salts and phosphate-free lipids. Since oil and water are only partially soluble in each other at operating conditions, flow in the HCU reactor is maintained at a high Reynolds Number to achieve rapid mass transfer between each phase. Rapid mass transfer facilitates metals reduction by the mechanisms identified above.

Biofuels Isoconversion
The Biofuels Isoconversion (BIC) process converts freshly produced or used lipid based feedstocks, such as FOG into renewable fuels including diesel, jet, and naphtha. This process is unique because it produces fuels that are molecularly nearly identical to fuels produced from petroleum. The nearly identical chemistry results in jet and diesel fuels that are true ‘drop-in’ fuels which do not require blending with petroleum. Therefore, the BIC process can be applied as an insertable unit to existing hydrotreating units which enables co-processing, or built as a stand-alone unit for renewable fuel production.

There are four main steps in the BIC process (see Figure 1): a cleanup step (HCU) where contaminants are chemically removed to make suitable feed oil for the next step; the conversion step, called catalytic hydrothermolysis (CH), wherein the feed oil molecules are converted to molecules that are nearly identical to those found in petroleum; a hydrotreating step that is identical to the petroleum hydrotreating processes that removes any remaining heteroatoms (oxygen, sulphur, nitrogen, and metals) down to acceptable levels; and a final distillation step that separates the hydrotreated product into the naphtha, jet, and diesel fuels.

Catalytic hydrothermolysis
In the CH process, water is combined with clean FOG products, pressurised, and heated in excess of supercritical water conditions. At these conditions, water and the feed oil become one phase. Water mediates the conversion of free fatty acids into CH crude, containing compounds such as normal and branched paraffins, high-density cycloparaffins, and aromatic molecules ideal for diesel and aviation turbine fuels.

Key reactions that occur during CH conversion include, but are not limited to:
- Hydrolysis of glycerides to produce free fatty acids
- Cracking of fatty acids into lower molecular weight acids and hydrocarbons
- Cyclisation of fatty acids into alkyl cyclohexane compounds
- Cyclisation of fatty acids into alkyl benzene compounds
- Dehydrogenation of naphthenic compounds into aromatic compound,
- Decarboxylation of fatty acids
- Skeletal isomerisation of intermediate and product compounds

Typical CH reactions of unsaturated fatty acids, such as oleic acid and linolenic acid, are shown in Figure 2. Those reactions that occur in the CH unit eliminate the need for further catalytic reforming, hydroisomerisation, or hydrocracking, and also consume less hydrogen than a route that uses all hydroprocessing.

The process occurs in a single step for less than two minutes of residence time. Product yields and product composition are controlled by adjusting the process variables, which include reaction temperature, reaction pressure, residence time, and water to oil ratio.

Isoconversion
The renewable CH crude contains oxygenated compounds, mainly carboxylic acids. In order to meet the fuel specifications, element oxygen must be removed from those oxygenates to produce pure hydrocarbons, and nearly all of the olefins must be saturated to produce paraffin compounds. For example, the total acid number (TAN, expressed as mg KOH/g) of jet fuel or jet fuel blending component under the CHJ Annex to D7566 must be less than 0.015. Hydrotreating achieves deoxygenation and reduces the TAN to meet the jet fuel specification. This results in jet fuel that has very good thermal and oxidative stability.

The BIC process uses commercial hydrotreating catalyst that results in near-complete oxygen removal (TAN <0.01) without saturating aromatic rings to cycloparaffins, opening cycloparaffin rings to form paraffins, or cracking of jet and diesel range hydrocarbons into naphtha and gaseous hydrocarbons. This results in fuels that contain aromatic and naphthenic isomers similar in concentration and type to petroleum derived fuels. Because cycloparaffin and aromatic compounds are produced and retained in the products, the BIC process uses less hydrogen than hydrotreated esters and fatty acids (HEFA) type processes.

Product distillation
The BIC process produces the entire range of hydrocarbons from naphtha through diesel boiling ranges. To produce jet fuel, the whole hydrotreated product is distilled to meet several specification requirements that include distillation, distillation slope (T90-T50 and T90-T10), flash point, and freezing point.

BIC technology has features that overcome deficiencies with technologies that practise direct catalytic hydrogenation of triacylglycerides-rich feedstocks using fixed bed hydroprocessing catalysts. They include the following.

Unlike direct catalytic hydrogenation, which essentially converts triacylglycerides to their corresponding straight-chain n-alkanes (primarily high concentrations of n-hexadecane and n-octadecane), the BIC process incorporates cyclisation and aromatisation reaction chemistry to produce renewable jet and diesel fuels containing n-alkanes, iso- alkanes, naphthenes, and aromatics, much more analogous to those in petroleum derived distillate fuels.

The catalytic hydrothermolysis reactor acts as a guard device to remove/transfer any inorganics and potential hydroprocessing catalyst foulants into the aqueous phase product such that the downstream Isoconversion catalysts will sustain relatively longer catalyst lives.
Unlike direct catalytic hydrogenation, which converts the glyceryl backbone of the triacylglycerides to propane, and thereby results in consuming additional hydrogen, the BIC process converts the glyceryl backbone non-hydrogenatively into fuel gas for captive use in the CH reaction system.

Chemical hydrogen consumption in the BIC process is significantly lower than that in direct hydrogenation processes due to the supercritical water reforming of olefinic bonds in the unsaturated triglycerides via cyclisation and aromatisation reactions that produce lower hydrogen content naphthenes and aromatic hydrocarbons. By way of example, for a feedstock having a 13/1 unsaturates/saturates ratio, such as that in conventional rapeseed oil, the chemical hydrogen consumption for the direct hydrogenation route can be as high as 148% that of the BIC process.

In the production of jet fuels, aromatic content (ASTM D6379) is required to be 8-20 vol%. BIC technology produces aromatic rings, which enable drop-in jet fuel production independent of feedstock compositions.