REVERSE OSMOSIS ÏU AZOGUE JAVIER PÉREZ December 1st 2003

WHAT IS REVERSE OSMOSIS? (I) But, first, what is osmosis? SOLUTE C1 < C2 SOLVENT

WHAT IS REVERSE OSMOSIS? (II) This is osmosis: Osmotic pressure difference Dp SOLVENT

WHAT IS REVERSE OSMOSIS? (III) P > Dp SOLVENT

1748. J.A. Nollet discovers osmosis phenomena. 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.

There are some process which both solvent and solute are recovered. 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.

Production of drinking water Treatment of urban waste water 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

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 fresh water turbine 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 Qp Cp Pa pa Pp pp Qr Cr Pr pr Q - Flux C - Concentration P - Pressure p - Osmotic pressure

A, permeability coeficient (for T and salinity fixed) [m3/d·m2·bar] PARAMETERS A, permeability coeficient (for T and salinity fixed) [m3/d·m2·bar] Y, recovery R, rejection

Fc, concentration factor PARAMETERS Ps, pass of salts Fc, concentration factor

There are two forces which rule solvent and solute fluxes: 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

Ja = A(DP - Dp) Ja, solvent flux SOLVENT TRANSPORT Ja = A(DP - Dp) Ja, solvent flux A, permeability coeficient [m3/d·m2·bar] DP, applied pressure difference Dp, osmotic pressure difference

Js = B·DC + M·JaCm Js, solute mass flux B, permeability [m3/d·m2·bar] 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

Rejection and solvent flux

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.

PRESSURE

FEED SOLUTE CONCENTRATION

FEED TEMPERATURE

RECOVERY

FEED PH

Is the most important application of reverse osmosis 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

SCHEMATIC DIAGRAM DESALINATION

REVERSE OSMOSIS PLANTS DESALINATION

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

TECHNOLOGY COMPARISON

THECNOLOGY COMPARISON

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 SELECTION CRITERIA: Chemical tolerance Mechanical suitability Price Cleanibility Separation performance GOOD DESIGN: Consistent performance Needs less frequent membrane cleaning Reasonable consum of power Little operational attention

Pore size in reverse osmosis Salts and low molecular weight compounds

Pressure and flux range Membrane process Pressure range (bar) Flux range (l/m2·h·bar) Microfiltration 0,1 - 2,0 >50 Ultrafiltration 1,0 - 5,0 10 – 50 Nanofiltration 5,0 – 20 1,4 - 12 Reverse Osmosis 20 - 100 0,05 - 1,4 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.

CTA membranes (Cellulose TriAcetate) Main Membrane Types CTA membranes (Cellulose TriAcetate) Low stability against chemicals, temperature and bacteria. Typically needs a sediment prefilter Low duration (18-24 months) High sensibility to hydrolysis High work pressures High permeability Allow contact with chlorine in water. Low cost High removal percentage of sales. 86-94%

TFC membranes (Thin Film Composite) 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

Membranes comparative

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 Model l/m2 · h Size Mem-S-25 3,94 1.8" x 12" 69$ Mem-S-40 6,30 2.0" x 12" 79$ Mem-S-60 9,50 89$ Mem-S-90 14,2 99$ TFC Membranes Model l/m2 · h Size Price XTFCH-150 23,7 2.5" x 14" 112$ XTFCH-350 55,1 2.5" x 21" 174$ XTFCH-750 118,3 2.5" x 40" 247$ XTFCH-1000 157,7 4.0" x 21" 361$ XTFCH-2000 315,4 4.0" x 40" 489$ XTFCH-2600 410 540$

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

MFI fouling test: Integrating the before expression: The use of MFI values has the next advantages: By comparing various solution, different fouling behavior can be observed A maximum allowable MFI value can be given for a specific plant Flux decline can be predicted to some extent Other tests used: silting index (SI), plugging index (PI), silt density index (SDI)

Reducing fouling Pretreatment of the feed solution: ph-adjustement, pre-microfiltration, pre-ultrafiltration, adsorption onto active carbon … Membrane properties: use of hydrophilic membranes and dense membranes reduces the fouling. Module and process conditions: fouling decreases as concentration polarization decreases. In this order is convenient to work with high flow velocities. Cleaning: Hydraulic Mechanical Chemical Electric

Cleaning needs As a general rule is necessary to realize a cleaning of the membranes when one of the next situations occurs: Salts filtration increases in more than 15% over the last value. Production changes (increase or decrease) in more than 10% Reject flow changes in more than 10% Drop pressure in the modules increases in more than 20% Feed pressure increases in more than 10% Long plant stops (> 1week) Before applying any reactant to regenerate membranes.

Chemical cleaning Problem identification: the chemical product to be used will depend on the nature of the deposit, so the first thing to do is identify the type of fouling. Types of fouling Precipitations: mineral salts and metallic oxids Deposits: high size particles, colloids, biological developments Others: colloidal sulfur, oils, organic compounds .. Fouling effects We can also identify the effect produced by the effects produced in the pressure drop of the equipment, the flow permeate and the filtrate of salts.

Fouling effects

Chemical cleaning The cleaning is realized recirculating through the modules the prepared solutions:

TFC membrane formulations

CTA membrane formulations

Membrane configurations in RO (I) Spiral-wound configuration Next logical step from a flat membrane but with higher packing density 300 – 1000 m2/m3 Permeate is collected in the central tube

Membrane configurations in RO (III) Tubular configuration Not self supporting in contrast to hollow fiber –modules Permeate crosses the membrane layer to the outside Low surface-volume ratio Usually the active layer is inside

Membrane configurations in RO (II) Capillary/hollow fiber configuration Fibers diameter: <0.5 mm • Flux velocity: low (up to 2.5 m/s) • Feed: inside-out or outside-in • Surface area/volume: high • Pressure drop: low (up to 1 bar) • Maintenance: hard • Cleaning: poor

Comparison of module configurations Tubular Plate and frame Spiral-wound Capillary Hollow-fiber Packing density LOW VERY HIGH Investment HIGH Fouling tendency Cleaning GOOD POOR Membrane replacement YES NO