Oil Bleaching Vessel

Overview

Oil bleaching removes color-causing pigments and trace impurities from degummed oil through adsorption onto activated clay. The pigments are carotenoids (yellow), chlorophyll (green), and oxidation products (brown). They have no nutritional value and worsen product appearance and shelf life. Food-quality oils require color reduction to < 5 Red units (LOVIBOND scale).

The Oil Bleaching Vessel is a batch vessel where degummed oil is heated under partial vacuum, mixed with activated clay, held briefly, then filtered. The clay absorbs the pigments; filtration removes the clay and its loaded contaminants.

Chemistry of bleaching

Activated clay is a porous adsorbent created by acid-leaching natural clay (montmorillonite or similar) to increase surface area. Surface area typically exceeds 200 m²/g. When added to hot oil, the pigment molecules, which are polar, preferentially adsorb on the clay surface through van der Waals forces.

Temperature is critical: color removal improves 5–10% per 10°C rise up to 100–110°C, due to faster diffusion and enhanced adsorption kinetics. However, above 120°C oil oxidation accelerates, darkening the product and wasting effort.

Vacuum (30–100 mbar) is applied for two reasons: it lowers water boiling point, allowing indirect removal of residual moisture from the Oil Degumming System; and it reduces oxygen partial pressure, slowing oxidation during the hold time.

Bleaching vessel design

The Bleaching Tank is jacketed stainless steel, typically 500–2000 L capacity. Degummed oil is charged first; the Temperature Control System raises temperature via steam jacket to setpoint (typically 95–105°C). Once stabilized, the Clay Dosing System system meters bleaching clay (1–3% by mass) into the tank.

A Agitator Paddle rotating at 20–50 rpm gently mixes the clay suspension, ensuring uniform contact. The oil is held at temperature for 15–40 minutes, longer residence time gives better color reduction but increases oxidation risk.

The Vacuum and Vapor Recovery draws down the tank headspace, eliminating oxygen and assisting water removal.

Filtration

Once the hold time expires, the oil-clay suspension is transferred to the Filter Press, a plate-and-frame or cartridge filter. A Filter Feed Pump pushes the oil through Filter Plates (typically 10–20 μm) at 50–200 bar pressure.

The clay cake builds on the filter media; pressure rises as the cake thickens. When pressure exceeds a set limit (typically 5–7 bar), the filter is stopped, the spent clay cake is discharged, and fresh media are installed or cleaned.

A single set of filter cartridges typically handles 5–10 batches before replacement or cleaning.

Vapor handling

The Vapor Condenser removes water vapor from the vacuum line. Recovered condensate is drawn off at the condenser bottom; dissolved solvents or aldehydes (if present) are recycled to the Oil Deodorizer stage for recovery.

Integration in refining

Bleaching follows Oil Degumming System and precedes Oil Deodorizer:

Crude → Degumming → Bleaching → Deodorizing → Finished Oil

In integrated facilities, degumming and bleaching may occur in the same vessel via sequential clay treatments, reducing equipment count and footprint.

Product quality outcomes

Properly bleached oil exhibits:

• Color < 5 Red (LOVIBOND)
• Moisture < 200 ppm (residual from process)
• Oxidation (peroxide value) < 5 meq/kg
• Iodine value (C=C content) unchanged

These parameters are measured by quality control labs to verify bleaching effectiveness before proceeding to deodorization.

Waste disposal

The clay cake discharged from the filter is typically 30–50% oil-saturated by weight. Industrial facilities often dry and combust this waste as fuel, or sell it to oil-extraction services that recover residual oil via solvent re-extraction. Some clay batches are designed for animal feed inclusion, allowing direct reuse.