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Membranes: Introduction

Principles, materials and module designs

Separation Processes Laboratory - Prof. Mazzotti - Rate Controlled Separations : Basics

FEED RETENTATE

Driving force

SWEEP PERMEATE (optional)

• A membrane is an interfacial structure that restricts the movement of some species while allowing other species to permeate through (if there is a driving force, e.g., gradient of , concentration, or chemical potential).

• Membranes are used in a wide range of applications, because: 1. Allow controlling carefully the permeation rate of a chemical species 2. Are inherently low-energy consumers (if pressure-energy is available) 3. Have simple design and no moving parts

Separation Processes Laboratory - Prof. Mazzotti - Rate Controlled Separations Membranes: Definitions

FEED RETENTATE

Driving MEMBRANE force

SWEEP PERMEATE (optional)

• Feed: stream entering the membrane separation module. • Permeate: stream that is separated from the feed and therefore crosses the membrane barrier. • Permeate flux: term describing the rate at which a product passes through a membrane. Flux is specific to the membrane, the application, the operating conditions, and usually varies in time. • Retentate: amount of the feed stream that is not separated. • Sweep: (optional) stream entering the membrane on the permeate side and conveying the permeate. • Permeability: coefficient linking the flux to the driving force (and to the membrane thickness). • Selectivity: ratio of permeability coefficients of two species (grater or equal to 1).

Separation Processes Laboratory - Prof. Mazzotti - Rate Controlled Separations Membranes: Basic Requirements

• High permeability for the component to be separated (smaller area for given permeate flow)

• High selectivity toward the component to be separated in relation to other components (higher purity)

• Low effective thickness of the active portion of the membrane (to ensure a high permeation and low cost)

• Good mechanical strength to support the physical structure

• High membrane stability in real working conditions

• Uniformity-freedom from pinholes or other defects

Separation Processes Laboratory - Prof. Mazzotti - Rate Controlled Separations Membrane Separation Processes

1. (MF): of micron particulates from liquid and gases (e.g., separation, juice clarification, heavy metal removal)

2. (UF): Removal of macromolecules and colloids from liquids (e.g., treatment of product streams in the food and beverage industry, recovery of useful material from coating or dyeing baths in the automobile and textile industries)

3. (RO): Removal of all material from water or other (e.g., seawater desalination  pressure gradient)

4. Electric (ED): Selective transport of only ionic species (e.g., seawater desalination  electrical potential gradient)

5. Gas Separation (GS): Selective separation of mixtures of gases and vapors (e.g., bio-gas upgrading)

6. (PV): Separation of mixtures of miscible liquids (e.g., extraction of volatile organic compounds from water or glycols)

Separation Processes Laboratory - Prof. Mazzotti - Rate Controlled Separations Membrane classification

• Synthetic membranes can be classified based on their (i) selective barrier, (ii) structure and morphology, (iii) membrane material • Selective barrier:  Porous (mean pore size diameter 5000-1 nm)  Driving force: pressure gradient  Nonporous (mean pore size diameter less than 1nm)  Driving force: concentration gradient  Charged or with special chemical affinity  Carried-mediated transport

Different transport mechanism

Separation Processes Laboratory - Prof. Mazzotti - Rate Controlled Separations Transport mechanisms

• Porous membranes: transport by viscous flow or diffusion. The selectivity is based on size exclusion. • Nonporous membranes: transport based on the solution-diffusion mechanism. • Charged membranes: separation, either with nonporous (swollen gel) or porous (fixed charged groups on the pore wall) membranes, based on charge exclusion (Donnan effect  or molecules having the same charge as the fixed ions in the membrane will be rejected, whereas species with opposite charge will be taken up by and transported through the membrane). • Carrier-mediated membrane: transport based on molecules or moieties with special affinity for substances in the feed.

Selectivity

Technical Development… …Evolution (already there!)

Pore Diffusion + Convection: Active transport: • Dominant in porous membranes • Against chemical potential gradient • Selectivity based on molecular size • In biological membranes • …-Filtration • Highly specific transport („channels“)

Solution diffusion: Facilitated diffusion: • Dominant in dense mebranes • Carrier in the membrane (red) vs. Permeating molecule (blue) • high solubility and diffusivity  high flux • Concentration of blue species in membrane is enhanced by carrier • In gas separation, pervaporation and reverse osmosis • Oxygen and Hemoglobin (blood)

• CO2 in a solution of carbonate  „Liquid membranes“ Separation Processes Laboratory - Prof. Mazzotti - Rate Controlled Separations Structure and morphology

• Isotropic (“symmetric”) membrane wrt the cross-section: Schematic representation of membrane cross-sections1. self-supported nonporous membranes (mainly - exchange) and macroporous microfiltration (MF) membranes o Dense nonporous isotropic membranes: not commonly used because the flux is too low for practical separation processes o Microporous membranes: widely used as microfiltration membranes • Anisotropic membrane ( “asymmetric”): thin porous or nonporous selective barrier supported mechanically by a much thicker porous substructure o Reduces the effective thickness of the selective barrier o The permeate flux can be enhanced without changes in selectivity • In composite membranes, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties.

1Mulder, M., 2012. Basic principles of . Springer Science & Business Media.

Separation Processes Laboratory - Prof. Mazzotti - Rate Controlled Separations Structure and morphology: Anisotropic membranes

 Functionality  • Anisotropic membranes combine the high selectivity of a dense membrane with the high permeation rate of a very thin membrane  Support  • The discovery and development of anisotropic membranes (for reverse osmosis seawater mechanical strength desalination, by Loeb and Sourirajan, 1963) was a critical breakthrough in membrane technology

 The cross sectional microstructures of an asymmetric membrane: (a) the general view, (b) dense layer, (c) dense layer and porous support, (d) the porous substrate.

Chen, L. et al., (2018). Asymmetric membrane structure: An efficient approach to enhance hydrogen separation performance. Separation and Purification Technology, 207, 363-369.

Separation Processes Laboratory - Prof. Mazzotti - Rate Controlled Separations Membrane classification: Summary

Porous

Microporous membrane

TiO2¨membrane Polymeric Isotropic Anisotropic

Thin, selective top layer + thick highly permeable micro-porous support Dense membranes substrate Non-porous Separation Processes Laboratory - Prof. Mazzotti - Rate Controlled Separations Membrane Separation Processes

Particle Size 1 A 10 A 100 A 0.1 µm 1 µm 10 µm

• N2 • Cl • Sucrose • Viruses • Colloidal silica • Bacteria Low molecular • H2 • OH • Pesticide • Carbon black • Coal dust material • O2 • Na • Herbicide • H2O

Gas and vapor Micofiltration separation Ultrafiltration Separation Liquid method separation Reverse Osmosis Electro dialysis

Reverse Osmosis Ultrafiltration Microfiltration Dyalisis membranes membrane membrane type Ion exchange

Membrane Non-porous Nano-porous Micro-porous structure membrane membrane membrane Chemical structure is important Physical structure and chemical properties are important

• Blood osmosis • N2 separation • Sterilization, clarification Main • Blood filtration • H2 separation • Waste water treatment application • Organic/water • Water desalination and purification

Separation Processes Laboratory - Prof. Mazzotti - Rate Controlled Separations Synthetic membrane materials

Organic Inorganic

Polymer Metallic Carbon membranes membranes membranes membranes (, PTFE, (metal oxide, metal (palladium and (graphene, CNTs, PVDF, PP) carbide, zeolite) palladium alloys) coal)

• Rigid in glassy form or flexible • Chemically and thermally stable, mechanically robust, in rubbery state operational under harsh feed conditions • Cost-effective, good selectivity, • Withstand harsh chemical cleaning, ability to be sterilized and easy autoclaved, high temperature (up to 500°C) and water processability resistance, well-defined and stable pore structure, high • , chemical stability, long life time chemically non resistant, limited • Fragile, rigid operating T & P, short life time

Kayvani Fard, A. et al., (2018). Inorganic membranes: Preparation and application for water treatment and desalination. Materials, 11(1), 74. Separation Processes Laboratory - Prof. Mazzotti - Rate Controlled Separations Membrane configuration

• Flat configuration i. Plate-and-frame modules ii. Spiral-wound modules

• Tubular configuration i. Tubular modules (D>10.0 mm) ii. Capillary modules (D: 0.5-10.0 mm) iii. Hollow-fiber modules (D<0.5 mm)

Separation Processes Laboratory - Prof. Mazzotti - Rate Controlled Separations Plate-and-frame modules

• Sets of two membranes are placed in a sandwich- like fashion with their feed sides facing each other Permeate • One of the earlier approaches to module design • Some small scale applications (also filtration units) • Still used in electrodyalisis

Permeate

Separation Processes Laboratory - Prof. Mazzotti - Rate Controlled Separations Spiral-wound modules

• Plate-and-frame system wrapped around a central collection pipe • Fairly low manufacturing costs • High surface to volume ratio • Modules can be easily connected in series inside a tubular pressure vessel

Separation Processes Laboratory - Prof. Mazzotti - Rate Controlled Separations Tubular modules

Typical tubular ultrafiltration module design (commercial modules).

• Not self-supporting • High cost (low surface / volume ratio) • Turbulent flow  good resistance to fouling • Application limited to ultrafiltration (in cases where concentration polarization and fouling are problematic)

Separation Processes Laboratory - Prof. Mazzotti - Rate Controlled Separations Hollow-fiber modules

• High surface to volume ratio • Used e.g. in hemodialyis • Shell side feed for high pressure applications (e.g. gas separation), bore side feed to avoid concentration polarization (no stagnant volumes).

Porous, dense and double layer hollow fibers.

Polymeric hollow fiber membranes can be readily prepared by solution spinning with a wide range of fiber diameters (50µm – 3000µm).

Depending on bore fluid and coagulation bath, selective skin layers can be formed on the inside, the outside or on both sides of the hollow fiber.

Separation Processes Laboratory - Prof. Mazzotti - Rate Controlled Separations Simple well-mixed module

• No chemical reactions

• ni: molar flow rate of species i • xi: molar fraction of species i

Overall mass balance:

Single species overall mass balance:

with:

Total mass balance across the membrane:

• J is the total molar flux of permeating species: • A is the membrane section area where mass transfer occurs

Separation Processes Laboratory - Prof. Mazzotti - Rate Controlled Separations