REVERSE OSMOSIS ÏU AZOGUE JAVIER PÉREZ December 1st 2003
SOLUTE SOLVENT WHAT IS REVERSE OSMOSIS? (I) • But, first, what is osmosis? C1 < C2
Osmotic pressure difference Dp SOLVENT WHAT IS REVERSE OSMOSIS? (II) • This is osmosis:
SOLVENT WHAT IS REVERSE OSMOSIS? (III) P > Dp
HISTORY • 1748. J.A. Nollet discovers osmosis phenomena. • 1855. A. Fick enunciates a law to describe membrane diffusion. 1887, J.A. Van’t Hoff expose first theorical explanations. • Gibbs adduce a scientific base. 1920 these theories are completed. However, osmosis is banished from one’s mind.
HISTORY: Desalination • 1953-59. First attempt, J.E. Breton and C.E. Reid who is recognized as inventor of reverse osmosis. Nevertheless, they obtain low volume of drinking water. • 1960. S. Loeb and S. Sourirajan prepare cellulose acetate membranes. • 1962. Pilot plant in California. 1965, 4th june: Incorporation to water supply.
1963. U. Merten propound mathematical equations which describe solute and solvent flow across membrane. • 1968. J. Westmoreland and D.T. Bray design and patent spiral configuration. • 1917. Patent of aromatic polyamide. • First industrial plants to produce drinking water appear in the second half of 70’s.
WHY IS USEFUL? • There are many applications for reverse osmosis. These can be included in two general groups: • Recovery of solvent • Recovery of solute • There are some process which both solvent and solute are recovered.
SOME USES • Production of drinking water • Treatment of urban waste water • Production of water for industrial uses • Treatment of different wastes • Concentration of fruit juices, white of an egg, whey... • Fermentation
turbine fresh water PRESSURE RETARDED OSMOSIS • This process enables to generate energy from a concentration difference. • The water flow at a pressure water P < • Power per unit membrane area: • Practical problems: • Salt flux • Concentration polarisation saline water
PRESENT AND FUTURE • Reverse osmosis has have a high increase since 60’s. This increase has been promoted by its application to desalation and waste treatment • Nowadays water market hold majority economic resources for investigation in osmosis field.
PRESENT AND FUTURE • Removal and selective concentration of certain substances are less developed than production of clean water • However, their low development may involve espectacular innovations and new applications during next years.
PARAMETERS Qa Ca Papa Qp Cp Pppp Qr Cr Prpr Q - Flux C - Concentration P - Pressure p - Osmotic pressure
PARAMETERS • A, permeability coeficient (for T and salinity fixed) [m3/d·m2·bar] • Y, recovery • R, rejection
PARAMETERS • Ps, pass of salts • Fc, concentration factor
EQUATIONS OF PROCESS • There are two forces which rule solvent and solute fluxes: • Solvent: Pressure gradient • Ja = A(DP - Dp) • Solute: Concentration gradient • Js = B·DC + M·JaCm
SOLVENT TRANSPORT Ja = A(DP - Dp) • Ja, solvent flux • A, permeability coeficient [m3/d·m2·bar] • DP, applied pressure difference • Dp, osmotic pressure difference
SOLUTE TRANSPORT Js = B·DC + M·JaCm • Js, solute mass flux • B, permeability [m3/d·m2·bar] • DC (= Cm-Cp), concentration difference in solute • M, distribution constant
Js = Ja·Cp • From this equation, Cp: • Is proportional to concentration gradient • Is inversely proportional to pressure gradient
SOLVENT FLUX AND REJECTION • Solvent flux and rejection are two important parameters • Study of these and their relation with other parameters are useful to optimize the process. Some of these parameters are pressure, Ca, feed temperature, recovery, feed pH.
DESALINATION • Is the most important application of reverse osmosis • There are many osmosis desalination plants constructed or in process of construction • In 1987, reverse osmosis represented 25% of total worldwide desalination capacity by all methods
FEED CONSIDERATIONS (DESALINATION) • Depending water source: • Superficial • Well • Salinity of water used. Feed of process is usually seawater or brine: Seawater: 30-50 g/l Brine: 50-200 g/l
PRINCIPAL SECTIONS: DESALINATION • Water feed • Pretreatment • Reverse osmosis • Complementary treatment
TECHNOLOGY COMPARISON • There are another technologies as destilation and electrodialysis to desalinate water • Comercialy, destilation is used for seawater, electrodialysis for saltwater and reverse osmosis for both kind of water
REVERSE OSMOSIS MEMBRANES ÏU AZOGUE JAVIER PÉREZ December 1st 2003
Membrane Selection • Membrane accounts for 15 to 40 percent of the price in reverse osmosis. • Membranes must be replaced periodically • CAREFUL MEMBRANE • SELECTION IS ESSENTIAL • GOOD DESIGN: • Consistent performance • Needs less frequent membrane cleaning • Reasonable consum of power • Little operational attention • SELECTION CRITERIA: • Chemical tolerance • Mechanical suitability • Price • Cleanibility • Separation performance
Salts and low molecular weight compounds Pore size in reverse osmosis
Pressure and flux range The pressures used in reverse osmosis range from 20 to 100 bar and the flux from 0,05 to 1,4 l / m2·h
Features of membranes in RO • In contrast to MF and UF the choice of material directly influences the separation efficiency through the constants A and B. A hydrodynamic permeability coefficient B solute permeability coefficient • The types of membranes used are porous, and it can be asymmetric or composite. Usually they are formed by a toplayer (about 1 mm) and a sublayer (150 mm) • The flux through the membrane is as important as its selectivity towards various kinds of solute. The flux is approximately inversely proportional to the membrane thickness, and for this reason membranes have an asymmetric structure. • The resistance towards transport is determined mainly by the dense toplayer. • The function of the sublayer is mainly support.
Types of membranes (I) • INTEGRAL MEMBRANES • Both toplayer and sublayer consist of the same material. • Materials used: cellulose esters (especially diacetate & triacetate), polybenzimidazoles. • Prepared by phase inversion techniques. • COMPOSITE MEMBRANES • Toplayer and sublayer composed of different polymeric materials such as aromatic polyamides • The support material is commonly polysulfones while the thin film is made from various types of polyamides, polyureas, etc • Prepared by dip coating, in-situ and interfacial polymerization.
Main Membrane Types CTA membranes (Cellulose TriAcetate) • High permeability • Allow contact with chlorine in water. • Low cost • High removal percentage of sales. 86-94% • Low stability against chemicals, temperature and bacteria. • Typically needs a sediment prefilter • Low duration (18-24 months) • High sensibility to hydrolysis • High work pressures
Main Membrane Types TFC membranes (Thin Film Composite) • Synthetic membranes • Higher rejection to many chemicals than CTA membranes • High duration (3-5 years) • High removal percentage 94-99% • Low work pressures. • High chemical stability • High sensibility to the oxidants and chlorine. • Typically needs a carbon prefilter. • Easy fouling • High cost
Types of membranes (II) • Very low pressure membranes: • Work pressures between 5-10 bars used to desale water with a salt content in a range of 500-1500 mg/l • Designed to compete with the ion exchange resins • Low pressure membranes • Range pressure: 10-20 bars Salt content (1500-4000 mg/l) • Desalation of water or removal of compounds such as nitrates, organic substances .. • Medium pressure membranes • Range pressure: 20-40 bars Salt content (4000-10000 mg/l) • At the beginning used in desalation of waters with high salt content, but now used in multiple processes of separation and concentration. • High pressure membranes • Conceived to obtain potable water from the sea water in a single pass. • Osmotic pressure of sea water (till 35 bar in the Red Sea) Range pressure (50-80 bar) • Recommendations of O.M.S (salt < 500 mg/l) salt reject 99%
Prices of membranes CTA Membranes TFC Membranes
How to prepare RO membranes Phase inversion techniques Chemical PI is a process whereby a polymer is transformed in a controlled manner from a liquid to solid state. The process is very often initiated from the transition from one liquid state in two liquids. One of the liquid phases (the high polymer one) will solidify forming a matrix. Immersion Precipitation is the most used PI technique currently specially for flat or tubular membranes. The only requirement is that the polymer must be soluble in a solvent or a solvent mixture.
How to prepare RO membranes Dip coating • Very simple and useful technique for preparing composite membranes with a very thin but dense toplayer. • An asymmetric membrane (often of the type used in UF) is immersed in the coating solution where a thin layer adheres to it • The film is put in a oven where the solvent evaporates and crosslinking occurs.
How to prepare RO membranes Interfacial polimerisation • A polymerisation reaction occurs between two very reactive monomers at the interface of two immiscible solvents. • Support layer (usually a UF membrane) immersed in an aqueous solution. Film immersed in a water immiscible solvent. • Reaction of the two monomers forms a dense polymeric toplayer. • The process takes advantage of the self-inhibiting character of the reaction.
Polarisation & Membrane fouling • Performance in RO is diminished by polarization and fouling phenomena. • Polarisation: reversible processes related with the increase of concentration over the bulk when we are closed to the membrane. • Fouling:deposition of retained particles, colloids, macromolecules, salts, etc … on or in the membrane. Typically more important in micro and ultrafiltration, but it becomes important also when we use hollow fiber or spiral wound configurations.
Fouling description and tests • The cake layer resistance can be written from a mass balance as: • Now the flux may be written as: or