Membrane Separation

Types of materials which are difficult and expensive to

separate:

(a) Finely dispersed solids, especially which have

a density close to that of the liquid phase, have high viscosity, or are gelatinous.

(b) Low molecular weight, non-volatile organics or pharmaceuticals and dissolved salts.

(c) Biological materials which are very sensitive to their physical and chemical environment.

Membrane separations

•Membrane separations are techniques used industrially for removal of solutes and emulsified substances from solutions by application of pressure onto a very thin layer of a substance with microscopic pores, known as a membrane.

What is the membrane?an interphase separating two phases and selectively

controlling the transport of materials between those phases.

interphase not interface

Types of membrane? • Permeable membrane .• Semipermeable membrane.

• What is the difference ?

Two wayOne way

CLASSIFICATION OF MEMBRANE PROCESSES:

according to the size range of materials which they are to separate and the driving force used in eparation.

(their limiting size of retention)

• according to separating capability is by means of a molecular weight rating, expressed in terms of a rejection coefficient against a species of specific molecular weight.

•(molecular weight cutoff) (MWCO)

[Daltons]

• The use of reverse osmosis and ultrafiltration in the dairy industry.

• Reverse Osmosis is used world-wide for the desalination of brackish water, with more than 1,000 units in operation. Plants capable of producing up to 10^5 m3/day of drinking water are in operation.

The osmotic pressure π fordilute solutions

•π = MRT:M is the molar concentration of

the solution.:R the universal gas constant.:T the absolute temperature.

The flow rate Q• The flow rate Q through a membrane separation

process may be represented by

Q = kA(ΔP − Δπ): k is a membrane permeability coefficient. A is the membrane superficial area. ΔP is the pressure drop.

Δπ the difference in osmotic pressure between the feed and the permeate.

• Transmembrane Pressure (TMP):is the pressure gradient that exits through the membrane, from feed size to permeate size, at each point along the membrane surface.

Recovery Percent:

• Recovery percentage : the relation of the permeate flow rate to the feed flow rate.

• %Recovery =

• Recovery percentages is inversely proportional with the feed concentration.

Fators affect the separation:- The separation ability of a synthetic material depends

on its physical, chemical properties.

Pore size and structure.

Design.

Chemical characteristics.

Electrical charge.

Natural membranes:

THE NATURE OF SYNTHETIC MEMBRANES

cellulosic materials polymeric

materials. Inorganic materials carbon fibers

• Membranes have most commonly been produced by a form of phase inversion known as

“immersion precipitation” This process has four main steps:

(a) the polymer is dissolved in a solvent to 10–30 per cent by mass.

(b) the resulting solution is cast on a suitable support as a film of thickness, approximately 100 μm.

(c) the film is quenched by immersion in a non-solvent bath, typically water or an aqueous solution.

(d) the resulting membrane is annealed by heating.

solute rejection coefficient

“ R”

• R =

• R = 1 − ( Cp/Cf )

:Cf is the concentration of solute in the feed stream.

:Cp is the concentration of solute in the permeate.

• There is a relationship between R and the solute molecular weight.

• The molecular weight cut-off is defined as :

the molecular weight of a solute for which R = 0.95.

• High resolution electron microscopy does not allow the resolution of an extensive pore structure.

Gas separation• The most important application of

membrane gas separation is the generation of N2 from air. The production of oxygen from air is also significant.

• Other substantial applications are the recovery of hydrogen from refinery off-gases (CO, N2, C1, C2) and vapours (C3+, CO2)

• the removal of carbon dioxide from natural gas.

Advantages Energy savings. The energy consumption is very low as there is no phase change.

Low temperature operation. Almost all processes proceed at room temperature, thus they can deal with compounds that are not resistant at high temperatures.

Recovery. Both the concentrate and the permeate could be recovered to use.

Water reuse. When applied to recover water, they avoid the transport of large water volumes and permit the reduction of the Chemical Oxygen Demand (COD) loading in sewage plants.

Disadvantages High cost. Membranes (and associated systems) are costly, but for low selective separations.

Low fluxes. The permeat flowrate available are still too low for some applications.

Sensitive to chemical attack. Many materials can be damaged by acids, oxidants or organic solvents.

Lack of mechanical resistance. Many materials do not withstand abrasion, vibrations, high temperatures or pressures.

Thanks for your

attention

• Prepared by /

Ahmed M. NagyAbdullah Sayed

References:• Coulson and Richardson’s CHEMICAL

ENGINEERING VOLUME 2 FIFTH EDITION Particle

Technology and Separation Processes.

• Unit Operations of Particulate Solids Enrique

Ortega-Rivas Theory and Practice