Editorial Technical Reference

Separation unit for continuous flow processes: in-line membrane and gravity liquid–liquid and gas–liquid separators

In-line separation units for continuous flow chemistry · Membrane liquid–liquid and gas–liquid separators, gravity settlers, flow range, pressure and wetted materials

A separation unit in a continuous flow process is the in-line device that takes a two-phase stream, such as an organic product phase with an aqueous wash, or a liquid carrying gas, and sends each phase to its own outlet while the line keeps running. Membrane separators do this by wetting a porous membrane with one phase; gravity separators let the phases settle and draw them off at a detected interface. Buyers should check three things first: whether the separation principle suits the phases and any solids, whether the flow range and pressure rating match the line, and which wetted materials touch the process fluids.

In-line liquid–liquid and gas–liquid separation units used downstream of continuous flow reactors, quench and extraction steps: membrane separators, tube-in-tube gas–liquid separators and continuous gravity separators, with their membranes and pressure control. Continuous flow reactors for pharmaceutical synthesis, with their residence time, heat transfer and ICH Q13 control strategy, are covered on the Continuous flow reactor for pharmaceutical synthesis page. Air separation units, centrifuges and distillation columns are outside this page.

CNFX editorial research · Sources checked October 12, 2026 · 4 sources

Choose the separation principle Size flow and pressure

Worker looking through the sight glass of a vertical separator vessel on a process skid, with a stainless filter housing, pump, plate heat exchanger and piping alongside — Skid-mounted separator vessel with sight glass, filter housing and plate heat exchanger

Start with your task

Choose the separation principleMembrane or gravity. Size flow and pressureRange and hold-up.

Choose the separation principle: membrane wetting or gravity settling

Zaiput describes its separators as separating an immiscible liquid–liquid or gas–liquid stream by differences in how the phases wet a porous membrane: the wetting phase fills the pores and is pushed through by a pressure differential set by an internal pressure controller, while the non-wetting phase is retained. Because separation is interfacial rather than gravity-based, Zaiput states the devices work regardless of the density difference between the phases. Membranes are PTFE, hydrophobic or hydrophilic, in different pore sizes; the lower the interfacial tension, the smaller the pore size needs to be, and a smaller pore reduces the maximum viscosity the membrane can pass. [1]

For gas–liquid separation between two telescoped reactors, a team at the University of Leeds with AstraZeneca used a tube-in-tube separator, a porous ePTFE tube encased in a stainless-steel tube, to separate the phases at 7 bar; only the liquid permeated the tubing, so only liquid was sampled to the HPLC and the gas flow did not alter residence time in the second reactor. [3]

Membranes have limits. In a University of Liège continuous synthesis of nitrofuran pharmaceuticals, membrane separation could not be sustained because solid particles formed sporadically, and gravity separation with optical interface detection was hampered because both phases were dark. The team built an automated filtration and gravity separation unit that tracks the phase boundary magnetically and drives automated valves. [4]

Editorial note: tell the supplier whether the stream carries solids, how dark or opaque the phases are, the interfacial tension if known, and the viscosity of each phase, because these decide between a membrane unit and a gravity unit with filtration.

Give the supplier the two phases and their compositions, flow rate of each phase, temperature, line pressure and outlet pressures, solids content, interfacial tension and viscosity if known, which phase is the product, wetted-material restrictions, and whether one unit or a multistage countercurrent arrangement is needed.

Size the unit for flow range, pressure and hold-up volume

Zaiput publishes three sizes. SEP-10 (laboratory) handles a total flow of 0.05–10 mL/min, has about 0.5 mL internal volume, a 2 MPa (290 psi) maximum pressure, 90 °C maximum temperature and perfluorinated polymer wetted parts. SEP-200 (pilot) handles 20–200 mL/min with about 35 mL internal volume, 130 °C for the stainless steel 316 and Hastelloy C276 models and 90 °C for the fluoropolymer model. SEP-3000 (production) handles 200–3000 mL/min with about 610 mL internal volume, 2 MPa at 20 °C falling to 1.2 MPa at 100 °C, and ½-inch Swagelok ports. Listed maximum gas flows are about 500, 20,000 and 300,000 sccm. [1]

Zaiput adds that the outlet pressures of the two phases should not differ considerably, and that membranes may need replacing sooner when the fluid carries a significant amount of solids. [1]

Editorial calculation: dividing internal volume by maximum total flow gives a hold-up of about 3 s for SEP-10 (0.5 mL at 10 mL/min), about 10.5 s for SEP-200 (35 mL at 200 mL/min) and about 12.2 s for SEP-3000 (610 mL at 3000 mL/min); 3000 mL/min equals 0.18 m³/h. [1]

Separation performance also depends on contact before the separator. Ghent University recovered dihydrolevoglucosenone from water into ethyl acetate or 2-MeTHF using a SEP-10 at 1.5 mL/min with in-line static mixers ahead of it. After five consecutive extractions, recovery reached 56% at 25 °C and 92% at 65 °C with ethyl acetate, and 99% at 75 °C with 2-MeTHF. Five separators combined in a countercurrent platform reached around 55–60% with ethyl acetate at room temperature while using one fifth of the extraction solvent of five consecutive extractions. [2]

Compare separation unit types

The reviewed sources describe these arrangements.

Type	What the source describes	Ask about
Membrane liquid–liquid separator	Porous PTFE membrane wetted by one phase, internal pressure controller, 0.05 to 3000 mL/min across three sizes. [1]	Membrane type and pore size; flow range.
Multistage countercurrent platform	Five separators linked, aqueous outlet of one feeding the next, organic flowing the opposite way. [2]	Number of stages; solvent ratio.
Tube-in-tube gas–liquid separator	Porous ePTFE tube inside a stainless-steel tube, operated at 7 bar. [3]	Pressure; gas and liquid flows.
Gravity separator with interface tracking	Continuous gravity settling with magnetic phase-boundary detection, automated valves and upstream filtration. [4]	Interface detection method; solids handling.

Where phases split: chamber, membrane, pressure controller and interface detector

Part	Role	Fix in the order
Separation chamber or shell	Holds the two phases; metal shell allows in-line pressure up to 2 MPa. [1]	Material and internal volume.
Membrane or porous tube	Passes the wetting phase or the liquid only. [1] [3]	Hydrophobic or hydrophilic; pore size.
Pressure controller or diaphragm	Holds the differential across the membrane. [1]	Operating window; spare diaphragm.
Interface detector	Locates the phase boundary in a gravity unit; magnetic in the Liège unit. [4]	Detection principle for your phases.
Inlet mixer	Static mixers ahead of the separator to raise extraction. [2]	Mixer type and contact time.
Ports and fittings	¼-28 flat-bottom on SEP-10; Swagelok on larger sizes. [1]	Connection standard and size.

Flow, pressure drop and separation efficiency through the in-line unit

Name both phases, the flow range and the pressure, and say whether a membrane or gravity unit is wanted.

Field	What it means and published values	Ask the supplier for
Nominal diameter (mm)	Port and line size; ½-inch OD Swagelok on SEP-3000. [1]	Port size and standard.
Flow rate (m³/h)	0.05–10, 20–200 and 200–3000 mL/min across three sizes; 3000 mL/min is 0.18 m³/h by editorial calculation. [1]	Range for each phase.
Separation efficiency (%)	Recovery depends on extraction conditions: 56% to 99% after five extractions in one study. [2]	Test data on your phases.
Operating temperature (°C)	90 °C maximum for fluoropolymer units; 130 °C for stainless steel and Hastelloy. [1]	Maximum for membrane and seals.
Operating pressure (MPa)	Up to 2 MPa; SEP-3000 1.2 MPa at 100 °C. Tube-in-tube example at 7 bar. [1] [3]	Rating at your temperature.
Pressure drop (MPa)	Outlet pressures of the two phases should not differ considerably. [1]	Allowed outlet back-pressures.
Material grade	SS 316, Hastelloy C276, ETFE, PTFE, PFA. [1]	Wetted-material list and certificates.
Connection type	¼-28 flat-bottom or Swagelok. [1]	Fittings.
Weight (kg)	Not stated in the reviewed sources.	Weight and mounting.
Seal material	FFKM listed for metal and fluoropolymer models. [1]	Seal compatibility with your solvents.

Send the phase, flow and material data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Phases: [organic; aqueous; gas; compositions]
Flow: [each phase, minimum and maximum]
Conditions: [temperature; line pressure; outlet pressures]
Properties: [interfacial tension; viscosity; density; colour]
Solids: [present or not; particle size]
Materials: [wetted parts; seals]
Arrangement: [single unit; multistage countercurrent; gas–liquid]

See manufacturer records Send an enquiry through CNFX

Buyer questions

Does a membrane separator need a density difference between phases?

Zaiput states its separation is interfacial, not gravity-based, so it can be used regardless of density difference. [1]

What flow rates do in-line separators cover?

Zaiput lists 0.05–10 mL/min (SEP-10), 20–200 mL/min (SEP-200) and 200–3000 mL/min (SEP-3000). [1]

When is a gravity separator chosen over a membrane?

In the Liège nitrofuran process, sporadic solids ruled out a membrane, so a filtration and gravity separation unit with magnetic interface tracking was built. [4]

Sources and what still needs a direct check

Sources checked 2026-10-12. Zaiput Flow Technologies publishes specifications for its own separators. Ghent University, the University of Leeds with AstraZeneca, and the University of Liège report separation units used in published continuous flow studies.

Still to check yourself

Check solids and phase colour before choosing membrane or gravity.
Match flow range for each phase.
Confirm pressure rating at operating temperature.
List wetted materials and seals.
Verify the legal supplier and factory.