Oil Clarification

The objective is narrow and mechanical: get the insoluble solids out. Crude oil from a screw press carries seed and meal fines; oil recovered from an extractor carries its own fines. These particles are a problem for three reasons. They settle wherever the oil is held, forming deposits that have to be removed. They foul heat-transfer surfaces and pumps. And they carry oil with them wherever they end up, so a stream of solids leaving with oil attached is a stream of product leaving in the wrong place.

The principle is a difference in physical properties, exploited one of three ways. Solids are denser than oil, so given quiescent time they settle out under gravity; given a centrifugal field instead, they separate far faster on the same density difference. Alternatively, solids are larger than the oil molecules around them, so a filter medium can retain them while oil passes. Which route a plant uses is a matter of scale, solids loading, and what happens to be installed — the underlying separation is the same in each case.

It is easy to overstate what this step achieves, so it is worth stating the boundary plainly. Clarification removes what is suspended in the oil. It does nothing whatever to what is dissolved in it. Phospholipids, free fatty acids, colour bodies, and the oil’s minor components are in solution, and they pass through a settling tank, a centrifuge, and a filter alike, entirely unaffected.

This is the reason clarification is not the whole of refining but the first step of it. Degumming, and the steps that follow it, exist precisely to address the dissolved fraction, and each works on a different principle because a different kind of impurity is in question. An oil that has only been clarified is a clean crude oil — clean in the sense of free of grit, not in the sense of ready for the table.

The recovered solids are oil-bearing and are not discarded as a matter of course. Where the plant is configured for it, they are returned into the process to recover the oil they carry, or directed into the meal stream. A stream that goes back into the process is not a waste stream.

The quality effect of clarification is largely protective. Solids left in oil are not inert: fine seed material carries enzymes and moisture, and oil held in contact with them can degrade over time in ways that a later refining step then has to correct. Removing solids early removes that ongoing risk. What clarification cannot do is reverse degradation that has already happened — an oil that arrives with elevated free fatty acids leaves with them.

• Oil retained with the separated solids — recovered where the plant is configured to return them, lost where it is not
• Oil held up in filter media and in equipment at changeover
• Incomplete solids removal, carrying fines forward to foul downstream equipment
• Quality degradation where crude oil is held in prolonged contact with fines before clarification

Clarification is a modest consumer relative to the steps around it: its demands are electrical energy for pumping and centrifuging, and filter media as a consumable. Its solids stream is oil-bearing and is generally routed back into the process or into the meal rather than discarded, so the step tends to add little to a plant’s residue burden.