Liquid-Liquid Extraction: Process and Equipment

Liquid-liquid extraction moves a target compound out of one liquid and into a second, immiscible liquid that dissolves it better. You mix the two, let them settle, and drain the layers apart. The physics is simple. Scaling it past bench glassware to a 50 L vessel is where the engineering starts.

What is liquid-liquid extraction, and what makes it work?

Put a solute in contact with two liquids that will not mix and it does not sit still. It distributes between the two phases according to how well each one dissolves it, and at equilibrium that split settles at a fixed ratio: the partition coefficient. Every liquid-liquid extraction is an attempt to make that ratio work in your favour, then physically separate the layers.

Three conditions have to hold. The liquids must stay immiscible, or you have one phase and nothing to drain. They need a density difference, or the layers never settle cleanly. And the solute has to prefer your solvent over the feed, or you are just diluting it.

The partition coefficient decides how much work the process is. A strong preference for the solvent means one contact stage gets you most of the way. A mild one means more solvent, or several stages washing the same feed with fresh solvent each time. Nobody can pick your vessel until you know how many stages you are running and how much solvent each one eats.

The other half of the job, the half most explanations skip: the solvent leaves with your product. Extraction does not finish at the drain valve. It finishes when the solvent is stripped back off and you are holding the compound you wanted.

How do you pick the solvent?

Pick the solvent by how you plan to get it back out, not only by how well it dissolves your target. That single reframe kills most bad solvent choices at the whiteboard stage.

Run the candidates through five filters:

• Selectivity. It has to pull your target and leave impurities behind. A solvent that takes everything gives you a different mixture, not a product.
• Immiscibility with the feed. Partial miscibility means a fuzzy interface, a slow settle, and solvent lost into the raffinate.
• Density gap. The wider the gap, the faster the phases split. A narrow gap is a settling-time problem you pay for on every batch.
• Recovery route. A volatile solvent strips off under vacuum. A high boiler needs short-path distillation, or it only comes off by cooking your product.
• Material contact. The solvent and the feed both touch glass, the seal and the valve for the whole run.

If your target is ionizable, you get a lever the textbooks under-sell: a pH swing. Move the pH so the compound turns neutral, and it walks into the organic phase. Move it back, and it walks into the aqueous phase. Two extractions with opposite pH give you a clean-up that no single solvent choice can match.

What do the steps look like, feed to recovered solvent?

A batch extraction is five steps, and the equipment exists to hold each of them steady.

Charge. Feed and solvent go into the same vessel at a known ratio, set by the partition coefficient and the number of stages. It is fixed before you start, not adjusted by eye.

Contact. You disperse the solvent through the feed so the solute has interface to cross. On a bench you shake. On a 20L vessel you stir, and the FY dispensers give you 60-600rpm to do it with.

Settle. Stop the stirrer and stand back; the density difference does the work. Stir harder than the phases can recover from and you buy an emulsion — the most common reason a scale-up batch runs long.

Split. Drain the heavy phase out of the bottom and stop at the interface. The valve, not the operator’s wrist, controls the cut.

Recover. Strip the solvent off the extract. On the FY vessels that means pulling the pressure down to -0.095Mpa so the solvent leaves at a lower temperature. For heat-sensitive or high-boiling material it means short-path distillation instead.

Where does the separatory funnel stop?

Every account of this process starts at the separatory funnel, and the funnel is genuinely the right tool for a bench check. Shake, vent, settle, drain, done. What almost nobody tells you is what breaks the moment you want more than a bench check.

The funnel has no motor. Mixing energy comes from your arms, so it is neither measurable nor repeatable — and it is what separates a quick settle from an emulsion. The funnel is open to atmosphere every time you vent it: fine with a drop of solvent, not fine with litres. There is no vacuum, so recovery has to happen somewhere else in different glassware. And the interface cut depends on an operator watching a stopcock, which stops being reliable when the vessel is heavy and the boundary is dark.

So the honest answer is that the funnel does not scale. It gets replaced. The scale-up is not a bigger funnel — it is a sealed, stirred, vacuum-rated vessel with a bottom discharge valve doing the same five steps under control.

What equipment runs a 10 L to 50 L extraction?

This is the vessel we build for that band. The parameters below come straight off the FY spherical vacuum glass dispenser sheet, and each row maps onto a step of the process above.

Read it as an extraction rig rather than a parts list. The 60-600rpm stirrer replaces your arms and makes the contact step a number you can write into a batch record. The -0.095Mpa vacuum puts the recovery step in the same vessel you extracted in, and lets you degas the feed before you start. Thickened borosilicate 3.3 plus PTFE sealing is what keeps a solvent-and-acid contact honest for the whole run; the source sheet lists both for the FY models. The stainless steel frame carries a 50L glass body full of liquid, which is not a trivial load. And 220V/50Hz means normal single-phase lab supply — worth checking against your site before anyone assumes it.

One thing in that table deserves a blunt reading. Motor power is 90W on the FY-10L and 120W on the FY-20L — and it stays 120W at 30L and at 50L. The stirrer does not scale with the vessel above 20 L. For a low-viscosity aqueous-organic pair that is fine, because you are dispersing, not kneading. For a viscous feed or a low solvent-to-feed ratio at 50 L, tell us the viscosity before you spec it, because that is the case where the same 120W has to work a lot harder. Skip the FY-50L if your batch never exceeds 20 L; you gain nothing and you settle a taller column of liquid every cycle.

If you want that vessel in front of you, the FY spherical vacuum glass dispenser runs 10L through 50L on thickened borosilicate 3.3 with the -0.095Mpa vacuum rating, and the rest of the family — including the discharge and port variants — sits in our glass liquid separator category.

Spherical body or drum body?

We list the same 10L-50L extraction duty in two body shapes, and the sheets differ less than people expect. Both are thickened high borosilicate 3.3 with PTFE sealing and vacuum operation — that comes from the same source sheet for the FY models. The choice is geometry and ports, not glass grade.

The spherical body is the one specified above, sold across FY-10L, FY-20L, FY-30L and FY-50L. The drum type vacuum glass dispenser parameter sheet lists FY-10L, FY-20L and FY-30L at the same 3.3 material, with the ports called out individually — port A as the stirring port, plus B and C.

Decide it on two questions. How does your interface behave? A body that narrows toward the drain gives a sharper cut on a small heavy phase, which matters when the phase you want is the minor one. And how many things go through the top? Every thermometer, feed line, condenser take-off and sampling point needs a port, and running out of ports mid-project is a real way to be wrong. If you want the two shapes set against the funnel and the evaporative routes in one view, we laid that out in our comparison of liquid-liquid separation equipment by principle, feedstock and vacuum rating.

How do you get the solvent back out?

Your extract is product dissolved in solvent. Something has to take the solvent away, and there are two answers.

The first is a vacuum strip in the extraction vessel itself. Pull to -0.095Mpa and the solvent boils off at a lower temperature than it would at atmosphere. If your product tolerates that, stop reading this section — you already own the equipment.

The second is short-path distillation, for material that does not tolerate it. Here is what our molecular distillation sheet commits to:

The line that matters is the application row: heat-sensitive or high-boiling-point materials requiring gentle separation. That is the whole reason the equipment exists. A high-boiling solvent needs so much heat at ordinary vacuum that you degrade the thing you spent the extraction isolating. Short-path drops the evaporator and condenser close together under high vacuum, so the molecule makes a short jump at a lower temperature and shorter residence time.

Do not buy it out of caution. If a vacuum strip works, the strip is the right answer and the short-path molecular distillation set with its evaporator, condenser, receiving system and vacuum support is a step you do not need. If it does not work, our molecular distillation selection guide walks the feed rate, vacuum stability and condenser questions that decide the configuration.

How does this differ from solid-liquid and botanical extraction?

The names get used interchangeably and they are not the same operation.

Solid liquid extraction pulls a solute out of a solid matrix — leaching. There is no partition between two liquids because there is only one liquid. The rate limit is the solvent getting into the solid and the solute getting back out, so particle size, contact time and temperature drive it, not a density gap. No settling step either; you filter or press.

Botanical extraction is usually both, in sequence. You start solid-liquid, washing solvent through plant material. The downstream clean-up then turns into liquid-liquid work — washing the crude extract against a second immiscible phase — and ends in distillation to take the solvent off. A botanical line typically owns all three pieces of equipment, and the extraction vessel is the middle one.

The practical consequence: if someone quotes you an extraction vessel for a botanical project, ask what handles the first step and what handles the last. A vessel alone finishes nothing. Our FAQ covers the configuration questions that come up when the three steps have to line up.

FAQ

What is the partition coefficient in liquid-liquid extraction?

It is the ratio of a solute’s concentration in the two phases once they reach equilibrium. It tells you how much of your target moves per contact stage. A strong preference for the solvent means one stage does the job; a weak one means more solvent or more stages.

Can you run liquid-liquid extraction under vacuum?

Yes. Our FY spherical vacuum glass dispensers are rated for normal or negative pressure down to -0.095Mpa across the 10L, 20L, 30L and 50L models. In practice the vacuum earns its keep on the solvent recovery step and on degassing the feed, rather than during the mix itself.

What vessel size should I choose for a pilot extraction?

Size it on batch volume including both phases, not on the feed alone — the solvent takes up room too. The FY range covers 10L, 20L, 30L and 50L. If your largest realistic batch is 20 L of combined liquid, the FY-20L is the right vessel and the 50L only adds settling height.

Why does my extraction keep forming an emulsion?

Usually too much mixing energy for the density difference between your phases. Stirring past what the pair can recover from disperses droplets too fine to coalesce. The FY stirrer runs 60-600rpm precisely so you can dial the contact down and repeat it, rather than shaking to a different result every batch.

Do I need molecular distillation after extraction?

Only if a vacuum strip will not get the solvent off without damaging the product. Short-path distillation is specified for heat-sensitive or high-boiling-point materials needing gentle separation. If your solvent is volatile and your product is robust, the extraction vessel’s own -0.095Mpa vacuum finishes the job.

What to confirm before you spec the setup

Get the process settled first: which solvent, how many stages, what solvent-to-feed ratio, and whether the product survives a vacuum strip. Those four answers determine the equipment. Reversing the order — picking a vessel and then fitting a process to it — is how a project ends up with a 50L body running bench-scale batches.

When you come to us for the vessel, we will ask for vessel volume, vacuum requirement, stirring speed, discharge valve, frame material and pump matching. That is the selection basis on the glass liquid separator sheet. If a short-path step is in scope, add feed material, vacuum stability, heating temperature, feed rate, condenser support and receiving setup to the same conversation.

Bring the viscosity and the density difference of your actual phase pair, not a similar one — they decide the stirring and the settling, and they are what customers most often leave out. Our selection guide sets out the same questions in order if you would rather work through them first.