Petroleum Refining Processes: Separation and Characterization

Ketone Dewaxing Process Details

Ketone dewaxing is a solvent dewaxing process utilized in petroleum refineries to eliminate paraffin wax from lubricating oil fractions. It employs a solvent mixture primarily composed of Methyl Ethyl Ketone (MEK) and Toluene.

• MEK effectively dissolves the oil but not the wax, facilitating easy wax crystallization.
• Hot lube oil is combined with the MEK–Toluene solvent and then chilled to very low temperatures (between −20°C and −30°C).
• At these low temperatures, wax forms solid crystals and separates from the oil.
• The mixture is filtered using a rotary drum filter, removing solid wax as a "wax cake."
• The filtrate (dewaxed oil plus solvent) proceeds to solvent recovery, where the solvent is evaporated, condensed, and recycled.
• The final output is dewaxed lubricating oil, possessing a low pour point suitable for cold-weather applications.
• The separated wax undergoes further processing to yield high-purity paraffin wax.
• Ketone dewaxing is widely adopted due to its efficiency, economy, and capacity to produce high-quality oils and wax.

HF Alkylation for High-Octane Gasoline

HF alkylation is a refinery process that reacts isobutane with light olefins using a hydrofluoric acid catalyst to generate high-octane alkylate gasoline.

1. Feed olefins from the FCC gas plant are dried and mixed with isobutane to maintain a high C₄/olefin ratio, ensuring good selectivity.
2. This mixture enters the HF reactor, where hydrofluoric acid functions as a liquid homogeneous catalyst under low temperature and moderate pressure.
3. Olefins react with isobutane to form highly branched iso paraffins (C₇–C₉ range), resulting in premium-quality alkylate.
4. Reactor effluent moves to an acid settler where HF acid separates from hydrocarbons based on density difference and is recycled as the catalyst.
5. The hydrocarbon phase is sent to a fractionation section to separate and recycle unreacted isobutane back to the reactor.
6. The remaining heavier fraction is stabilized to remove light ends, leaving the high-octane alkylate product.
7. The resulting alkylate is clean, sulfur-free, and aromatic-free, boasting a high octane number, making it an excellent gasoline blending component.
8. HF alkylation is favored for its high yield, high octane (95+ RON), excellent stability, and contribution to low-emissions gasoline.
9. Major concerns involve HF toxicity and corrosion, necessitating specialized materials, safety systems, and acid-handling protocols.

Furfural Extraction in Lube Oil Refining

Furfural extraction is a solvent extraction process used to remove undesirable aromatic compounds from lubricating oil fractions, thereby enhancing viscosity index and oxidation stability.

Process Steps

1. Hot lubricating oil contacts furfural solvent in an extraction tower; aromatics preferentially dissolve into the furfural, while paraffinic and naphthenic components remain in the raffinate.
2. The mixture separates into two liquid layers due to density difference: the extract phase (furfural + aromatics) and the raffinate phase (purified lube oil).
3. The raffinate (desired lube oil) is withdrawn and sent to a stripper to remove residual furfural for recycling.
4. The extract phase is heated in a solvent recovery column to separate aromatics from furfural, enabling solvent reuse.
5. The final raffinate product exhibits an improved viscosity index, better oxidation stability, lower carbon residue, and higher-quality lubricating characteristics.
6. Furfural extraction is widely employed because it achieves deep extraction of aromatics, yielding premium-grade lube oil base stocks.

Furfural Advantages

• Furfural is less toxic and safer to handle compared to phenol.
• It demonstrates higher selectivity for aromatics, resulting in better removal of undesirable components.
• Furfural provides a higher-quality raffinate with improved viscosity index and oxidation stability.
• Furfural is less corrosive, requiring simpler material handling than phenol.

Fluid Catalytic Cracking (FCC) Fundamentals

FCC is a primary refinery conversion process that cracks heavy gas oil into lighter products, such as gasoline, LPG, and light olefins, utilizing a fine powdered zeolite catalyst.

1. Preheated feed enters the riser, instantly vaporizing and undergoing catalytic cracking upon contact with very hot, regenerated catalyst.
2. The mixture flows to reactor cyclones where the catalyst is separated from the cracked hydrocarbon vapors.
3. Vapors enter the main fractionator, which separates LPG, gasoline, light cycle oil, and slurry oil.
4. Spent catalyst, containing coke deposits, moves to the regenerator where air burns off the coke and reheats the catalyst.
5. The hot, regenerated catalyst circulates back to the riser, ensuring continuous operation and heat balance.
6. FCC is widely used because it delivers high-octane gasoline, valuable LPG, and high conversion efficiency.

Crude Oil Distillation Tower Arrangement

The crude distillation tower is a vertical fractionating column engineered to separate crude oil into various boiling-range products using trays and reflux.

1. Hot crude enters the tower's flash zone; heavy components descend while lighter vapors ascend.
2. The upper section features rectifying trays that produce light fractions like LPG, naphtha, and kerosene, aided by reflux for enhanced separation.
3. The middle section draws side-cut products, such as kerosene, diesel, and AGO, through side strippers to boost purity.
4. The lower section contains stripping trays that use steam stripping to remove light ends from heavy gas oil and residue.
5. The tower bottom yields the heaviest product—reduced crude or atmospheric residue—which is directed to the vacuum distillation unit.
6. Proper tower arrangement ensures efficient vapor-liquid contact, sharp product separations, and maximum recovery of valuable refinery fractions.

Atmospheric Distillation Unit (ADU)

ADU is the initial major refinery unit, heating crude oil and separating it into fractions based on boiling points under atmospheric pressure.

1. Hot crude enters the flash zone; lighter components (LPG, naphtha, kerosene, diesel) rise and are collected at different tray levels.
2. Side strippers utilize steam to improve product purity by removing lighter ends.
3. Heavy residue (atmospheric residue) remains at the bottom and is transferred to the vacuum distillation unit.
4. ADU provides the essential primary separation required for subsequent downstream processing and maximizes the recovery of valuable light products.

Vacuum Distillation Unit (VDU) Operation

VDU processes atmospheric residue under vacuum to prevent thermal cracking and separate heavy fractions at lower vaporization temperatures.

1. Reduced pressure enables high-boiling components to vaporize without thermal decomposition.
2. The tower produces light vacuum gas oil (LVGO), heavy vacuum gas oil (HVGO), and vacuum residue.
3. VGO streams are typically sent to FCC or hydrocrackers for conversion into lighter fuels.
4. Vacuum residue is utilized for bitumen production, coking, or other residue upgrading processes.
5. VDU significantly improves refinery yield by recovering valuable heavy gas oils unobtainable in the atmospheric unit.

Petroleum Characterization Factors

UOP Characterization Factor (K)

The UOP Characterization Factor (K) is a property used to identify the type and nature of crude oil or petroleum fraction based on its molecular structure.

• It is calculated using the formula: K = (TB)¹.⁵⁴³ / SG, where TB is the average boiling point (°R) and SG is the specific gravity.
• A high K value (12.5–13) signifies paraffinic crude.
• A medium K value (10.5–12.5) indicates naphthenic crude.
• A low K value (<10) suggests aromatic crude.
• This factor aids refineries in predicting product quality, refining behavior, catalyst performance, and suitability for lube oil production.
• Paraffinic oils have a high K factor, naphthenic oils have a moderate K, and aromatic oils have low values, allowing for quick identification of crude characteristics.

The Correlation Index (CI)

The Correlation Index (CI) is an empirical number indicating the aromaticity or paraffinic nature of a petroleum fraction.

1. It is calculated from refractive index (RI) and specific gravity (SG) using the formula: CI = (RI − 1.475) × 1000 − 450 × SG.
2. Low CI values (0–20) indicate paraffinic hydrocarbons.
3. Medium CI values (20–50) indicate naphthenic hydrocarbons.
4. High CI values (>50) indicate aromatic hydrocarbons.
5. CI assists in classifying fuels and predicting their behavior in refining, combustion, and lubricant formulation.

Duo-Sol Solvent Extraction Process

The Duo-Sol process is a solvent extraction method for lubricating oil refining, designed to remove aromatic compounds and enhance viscosity index and oxidation stability.

1. It employs a dual-solvent system, typically an aromatic solvent like phenol combined with a paraffinic solvent like propane, for superior selectivity and phase separation.
2. The lube oil feed mixes with the solvent blend in the extraction tower; aromatics preferentially dissolve into the phenol-rich extract phase, while paraffinic components remain in the raffinate.
3. The two liquid layers naturally separate by density, allowing recovery of purified raffinate lube oil.
4. The extract containing aromatics is sent to solvent recovery where phenol and propane are distilled and recycled.
5. The Duo-Sol raffinate exhibits an improved viscosity index, stability, and lube quality due to the deeper removal of undesirable aromatics.
6. The Duo-Sol process is often preferred for heavy lube stocks as it offers deeper extraction and cleaner separation than single-solvent methods.