Membrane Modules

Modeling Membrane Transport

Reverse Osmosis

Solvent flux A = permeability constant =

Solute flux

B =

Gas Separation

Di = Diffusivity of gas in membrane

Ki(g) = Sorption Coefficient

PG = permeability coefficient

Membrane Modules

Industrial membrane processes require large area

Module is the smallest unit with max membrane area

Module types:

Flat : plate and frame, spiral

Tubular: tubular, capillary, hollow fiber

Configuration Packing Density (m2/m3)

Plate and frame 100-400

Spiral Wound 300-1000

Capillary 600-1200

Hollow fiber Upto 30,000

Area/unit volume for various membrane modules

Plate and Frame Modules

Feed In

Feed Out

Tortuous path can be incorporated in the gasket

Increase residence time

Improves mass transfer

Reduces concentration polarization

Simplest to implement

Packing density (100-400 m2/m3)

Flat membrane sheets are placed in sandwich pattern

Alternate compartments for feed and permeate sealed with gaskets

Need stop disks to prevent channeling

Easier to clean, dairy applications

Spiral Wound Modules

Schematic of Spiral Wound module

Essentially plate and frame module wound around a central pipe

Feed side spacer acts a turbulence promoter

Packing Density: 300 1000 m2/m3

Feed passes axially down and across the membrane

Permeate flows radially towards the central pipe

Multi-envelope spiral modules are used to decrease permeate side pressure drop

Are connected in series of 4-6 modules in a single pressure vessel

Used for seawater desalination, Gas separation pervaporation

Tubular Modules

contain generally 4-

18 tubes per module

Feed is usually through the tubes

Permeate flows across the tubes in to the module housing

Packing Density: 10.0

Capillary 0.5 10.0

Hollow Fiber

Hollow Fiber Modules Construction is same as capillary

modules

Difference in the tube diameter

Fiber diameters: typically 50 m ID,

100-200 m OD

Very high packing density possible

Upto 30000 m2/m3

Hollow fiber modules are used when feed is relatively clean, pre-treated

Feed can enter in the shell side or tube side

Outside-in configuration has higher surface area compared to inside-out

Channeling may occur in outside-in configuration

Membrane skin-layer is better protected in inside-out configuration

Higher ID of fibres needed for inside-out configuration to reduce pressure drop

Outside-in configuration is generally used for Gas Separations

To avoid high pressure losses inside the fiber

Attain high membrane area

Inside-out is generally used for RO, Pervaporation

System Design

Can be dead-end (sparkler filter) or crossflow

Dead-end is simplest, but highest fouling, concentration polarization (CP)

Crossflow applications preferred to reduce CP and fouling

Can operate crossflow filtration in various configuration Co-current

Counter-current (gives best performance)

Cross-flow with permeate mixing

Mixing on permeate and feed side

feed retentate

permeatepermeate

Co-current

feed retentate

permeatepermeate

Counter-current

feed retentate

permeate

Cross-flow with Permeate mixing

Cascade Operations

Used when single stage is does not give enough separation e.g. in 235U separation through porous membranes

Feed

permeate retentate

Feed

permeate

retentate

Two stage membrane process

Several examples are given in Mulder: Chapter VIII

Flow Scheme

Two basic flow schemes in single/multi-stage process Single pass system

Recirculation system

Feed pump permeate

Single-pass system

feed pump permeate

Recirculation system

recirculation pump

Feed solution passes only once Volume of feed decreases in multi-

stage design

Modules are arranged in tapereddesign

Volume reduction factor: ratio of initial feed volume and retentate

determined by configuration and not by applied pressure

feed

retentate

Schematic of single-pass system (tapered cascade)

Flow Scheme

Feed is pressurized using pump and circulated multiple times

Each stage is fitted with a recirculation pump

Pressure drop over single stage is low

Pump is used to maximize hydrodynamic conditions

Flow velocity can be adjusted at each stage

Feed recycle system is more flexible

Preferred for high fouling applications

permeate

feed

permeate

retentate

Stage 2Stage 1

Process Parameters

Feed

permeate

retentate

cf qf cr qr

cp qp