Membrane Separation
Membrane separation is a process in which a semi-permeable membrane selectively allows certain molecules or ions to pass through while retaining others, driven by a concentration, pressure, or electrical potential gradient. Common forms include reverse osmosis, nanofiltration, ultrafiltration, microfiltration, and pervaporation, each distinguished by the size range of species separated. Membrane processes are highly energy-efficient alternatives to thermal separation methods and are critical in water purification, food processing, and pharmaceutical applications.
Key Formula
J = Lp × (ΔP − Δπ)
LaTeX: J = L_p (\Delta P - \Delta \pi)
| Symbol | Meaning | Unit |
|---|---|---|
| J | Permeate flux (water flow per unit area) | m³/(m²·s) |
| Lp | Hydraulic permeability of membrane | m³/(m²·s·Pa) |
| ΔP | Applied transmembrane pressure difference | Pa |
| Δπ | Osmotic pressure difference across membrane | Pa |
Worked Example
Problem
A reverse osmosis membrane has a hydraulic permeability Lp = 5×10⁻¹² m³/(m²·s·Pa). The applied pressure is 60 bar and the osmotic pressure difference for seawater is 27 bar. Calculate the water flux through the membrane.
Solution
Step 1: Convert pressures to Pa — ΔP = 60 × 10⁵ = 6×10⁶ Pa; Δπ = 27 × 10⁵ = 2.7×10⁶ Pa. Step 2: Net driving pressure = 6×10⁶ − 2.7×10⁶ = 3.3×10⁶ Pa. Step 3: J = 5×10⁻¹² × 3.3×10⁶ = 1.65×10⁻⁵ m³/(m²·s).
Answer
Water flux J = 1.65×10⁻⁵ m³/(m²·s) ≈ 59.4 L/(m²·h), a typical value for seawater RO membranes.
Membrane Process Classification by Pore Size and Separation Target
| Process | Pore Size | Retained Species | Driving Force | Typical Pressure |
|---|---|---|---|---|
| Microfiltration (MF) | 0.1–10 µm | Bacteria, suspended solids | Pressure | 0.1–2 bar |
| Ultrafiltration (UF) | 1–100 nm | Proteins, viruses, colloids | Pressure | 1–5 bar |
| Nanofiltration (NF) | 0.5–2 nm | Divalent ions, small organics | Pressure | 3–20 bar |
| Reverse Osmosis (RO) | <0.5 nm | Monovalent ions, dissolved salts | Pressure | 20–80 bar |
| Pervaporation | Dense polymer | Organic mixtures (e.g., ethanol/water) | Concentration | Vacuum permeate |
Interactive Tools
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Related Terms
Industrial Filtration
Industrial filtration is a mechanical separation process that removes solid particles from a liquid or gas stream by passing the mixture through a porous medium that retains the solids (filter cake) while allowing the fluid (filtrate) to pass through. It is fundamental to chemical processing, wastewater treatment, food and beverage production, and pharmaceutical manufacturing. The efficiency of filtration depends on particle size, filter medium properties, applied pressure, and the characteristics of the slurry.
Engineering Adsorption
Engineering adsorption is a surface-based separation process in which molecules (adsorbate) from a fluid phase adhere to the surface of a solid material (adsorbent) via physical or chemical interactions, enabling removal or recovery of target species from gases or liquids. It is used industrially for air purification, solvent recovery, water treatment, and chromatographic separation. The process is characterized by equilibrium isotherms and mass transfer kinetics, with the adsorbent regenerated by temperature or pressure changes to allow repeated cycles.
Ion Exchange (engineering)
Ion exchange is a reversible chemical process in which ions of the same charge are exchanged between a solution and an insoluble solid ion-exchange resin, allowing selective removal, concentration, or substitution of specific ions. Cation exchange resins remove positively charged ions (e.g., Ca²⁺, Mg²⁺, heavy metals) while anion exchange resins target negatively charged species (e.g., NO₃⁻, SO₄²⁻). Ion exchange is fundamental to water softening, demineralization, pharmaceutical purification, and nuclear waste treatment.
From Latin "membrana" (skin, parchment), related to "membrum" (member, body part). The scientific study of osmosis dates to the 18th-century work of Abbé Nollet, while engineered membrane processes were commercialized in the 1960s following the development of the Loeb–Sourirajan asymmetric membrane.