RO water Desalination Units Design

Ro Water Desalination Units Design Yamen Al-Jajan – Alfurat University – Petrochemical Engineering

Abstract Water Desalination was always the basis of the environmental researches , and lots of studies were done to achieve acceptable water quality , and increase the productivity with the least cost , in addition to find solutions for fouling and depression efficiency of the membrane units . In this research , membrane units fundamentals ,the most important membrane processes (MF,UF,NF,RO) , membrane modules were discussed , at the end the RO design procedure was explained clarified with a simple example was analysied and designed with Reveres Osmosis Systems Analysis Program ROSA .

Keywords : water desalination – membrane technology – membrane processes – membrane modules – Reverse Osmosis – ROSA program .

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1. Introduction to membrane Technology : Recently , there was a necessity for operation units that could consume less energy , not environmental harmful and separate components easily . membrane technology is one of the most important methods to separate liquid & gaseous mixtures that has many of industrial applications from water treatment , reactors and advanced bio-separation processes …… First membrane processes back to the WWI 1940 in Germany with artificial kidneys depending on studies of Zsigmonoly and Bachmann in 1918 . Membrane technology has many features : - It could be operated continuously . - Low energy consuming comparative with other operation units such as distillation an crystallization . - It could be integrated with other operation unit to form Hybrid Processes . - It is operated with moderate conditions . - Easily scaling-up . - Easily modified according to process demand . 1-1-

There is no need for chemical addition .

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Membrane technology : Definition and applications

In Unit operation field , when we say "membrane" it means those units with good selectivity permeable that allow to transport for specific component particles from one side to another . In another words , they help us to separate a stream parts depending on the membrane selectivity .

Figure (1) : membrane separation principle

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Generally , membrane could be classified - according to its nature – into : Natural (Bio-membrane) and synthetic membrane , according to the chemical structure into : organic and inorganic , or according to its morphological statue into : symmetrical and asymmetrical membrane . Usually , polymers (natural/ synthetic) are used as membrane , examples for natural ones : wool , natural rubber and Cellulous, examples for synthetic polymers : 6,6 Nylon , polyamides and polycarbonates . Membranes are used in cold temperature conditions as in the food technology , biotechnology , pharmaceutical industries and other fields that difficult to use conventional thermal separation units . for instance , it is very complicated to separate isotropic mixtures or amorphous solutions by conventional separation units such as distillation and recrystallization so we resort to membrane units . This technology is being used actually to produce drinking water rely on reverse osmosis , while 7 million cubic meters of drinking water are produce globally . it also used as filtration processes in the food industries and organic vapor recover in petrochemical industries . This technology is also used in wastewater treatment with help of ultra/microfiltration to remove particles and colloids , so that wastewater can be disinfected in this way before it is being casted into surrounding environment . About half of the market is in medical applications such as use in artificial kidneys to remove toxic substance by hemodialysis and artificial lung for bubble-free supply of oxygen in the blood . The importance of membrane technology is growing in the field of environmental protection . Even in modern energy recovery techniques membranes are increasingly used , for example in fuel cells and in osmotic plants .

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2- Membrane Mechanisms : As previously discussed , substance particles transporting throughout membrane is demand a driving force that could be gradient in potentiality , concentration or pressure , and it could be a mixture of these forces . Table (1) : membrane processes according to driving force [8] Process Pore size Driving force Microfiltration 1-2 bar , pressure 0.05 – 10 𝝁m Ultrafiltration 2-5 bar , pressure 0.01 –0.05 𝝁m Nanofiltration 5-15 bar , pressure < 2.0 nm Reverse osmosis 15-100 bar , pressure < 1.0 nm Flux is in proportional to driving force according to the following relation :

𝑱𝐢 = 𝐀. 𝐗 … (𝟏) Whereas :

𝑱𝐢 – is the producte flux . 𝐀 – is proportional factor . 𝐗 – is the driving force . The importance of the proportion coefficient (A) is about define particles velocity transporting through the membrane , so it indicates resistence of the membrane against particles transporting when a driving force is applied . Many studies were done to understand the relation between the proportion coefficient and the driving force , and we will discuss two of them that were be famous :  Solution-Diffusion Model : This theory explain membrane mechanisms with : when permeate substance particles dissolve into membrane , they transport through it under the driving force effect . The feed stream components are separate by effect of two factor : variation of dissolution and diffusion .

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[4]

Solution-diffusion model is described by Fick's law that can be mathematically expressed as following :

𝐉𝐢 = −𝐃𝐢

𝒅𝑪𝒊 … (𝟐) 𝐝𝐱

Whereas :

𝐃𝐢 – is the diffusivity of (i) and expresses the mobility throughout membrane . 𝒅𝑪𝒊

– concentration gradient along (x) axis that is perpendicular on the 𝐝𝐱 membrane surface .

High production ratio could be achieved by decrease the membrane thickness and increase the concentration difference between the two sides of the membrane .

 Pore-flow model : This theory explain membrane mechanisms with : The particles diffuse through tiny pores under convective effect of the feed stream , the separation happen according the variation of mobility through pores . Pore-flow model was been famous in the 1940s , while solution-diffusion model was published later explaining gas separation using polymers membranes .

[4,13]

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Figure (2) : Molecular transport membranes can be described by a flow through permanent pores or by the solution diffusion mechanism Pore-flow model is explicated by Darcy's law that is mathematically expressed :

𝐉𝐢 = 𝐤 ′ 𝐂𝐢

𝒅𝑷 … (𝟑) 𝐝𝐱

Whereas : 𝒅𝑷 𝐝𝐱

–

is the pressure gradient through the porous media .

𝐤 ′ - is a constant rely on the nature of the membrane . Flux through membrane according to pore-flow model is greater than the one according solution-diffusion model , the two model vary pursuant to pores sizes and their stability . Membrane described by Fick's law , the pores between the polymeric chains are very tiny , in addition they open and close through time pursuant to the thermal motion of the polymeric chains . On the other side , the membrane described by Darcy's law , its pores are relatively wide and stable through time . So , whenever the pores were wider , the membrane get closer to described by Darcy's law and pore-flow mechanism . [4,7]

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Figure (3) : Solvent flux vs. solute size and solute type for various membrane processes

It could be understood – from the previous figure- that RO membrane with pores size less than 0.1 nm so water molecules – with 0.1 radius – can get through it easily contrariwise dissolved ions and organic matter like sucrose . [7,8]

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Figure (4) : The relative size of different solutes removed by each class of membrane

When pores size come tinier , membrane will get close to dense membrane behavior , so that the mechanical behavior changes from hydrodynamic to solution-diffusion one . [4] The Ratio between product flow to feed flow is called Recovery , and it is one of the most important factors that should be considered while Ro , NF units designing , because it expresses the productivity of the unit , mathematically it could be expressesd as following : % Recovery =

product flow × 100 feed flow

… (4)

There is also what it called Rejection ratio that is expressed as following : % 𝑅𝑒𝑗𝑒𝑐𝑡𝑖𝑜𝑛 =

solute concentration in feed − solute concentraion in permeat × 100 … (5) solute concentration in feed

And there is the Retention ration that is expressed as following : % 𝑅𝑒𝑡𝑒𝑛𝑡𝑖𝑜𝑛 =

solute concentration on memb. surface − solute concentraion in permeat × 100 … (6) solute concentration in feed

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So , membrane units are based on two concept : the selectivity and the productivity , and whenever the selectivity increase , the productivity will decrease and vice versa . [7]

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Figure (5) : RO membrane performance data for various polymers

Previous figure illustrate the relation between productivity (solvent flux) and selectivity (Rejection%) for several commercial membrane to separate sodium chloride solution with different concentrations at 25°C by RO process . Membranes from various manufacturers: A, Hollosep–cellulose triacetate hollow fibre membrane (Toyobo); B,sulphonated polysulphone composite hollow fibre membrane (Albane International); C, B10–aromatic polyamide hollow fibre membrane (Du Pont); D, PEC-1000–composite flatsheet membrane (Toray); E, NS-200–composite polyfurfuryl alcohol membrane; F, FT-30–composite polyamide flat-sheet membrane (Film Tec/Dow); G, NTR-7199–composite polyamide/polyurea flat-sheet membrane (Nitto Denko); H, TFC-803 (PA-300)–composite polyetheramide flat-sheet membrane (Fluid Systems/Signal; I, NS-100–composite polyurea flat-sheet membrane; J, TFC-801 (RC-100)–composite polyetherurea flat-sheet membrane. (fluid systems/Signal); K, NTR-7197–composite polyamide/polyurea flat-sheet membrane (Nitto Denko); L, B-15— asymmetric polyamide flat-sheet membrane (Du Pont); M, asymmetric cellulose acetate flat-sheet membrane (FilmTec/Dow); P, Romembra SU-composite polyamide flat-sheet membrane (Toray); Q, NF-50–composite flat-sheet membrane (FilmTec/Dow); and R, NTR-7250–composite polyvinyl alcohol flat-sheet membrane (Nitto Denko). [7,8] 3- Membranes structure & materials : Membranes usually are made of ceramic materials or polymers ( natural / synthetically ) such as wool , rubber or Cellulous . Choosing the basic structural of membrane is one the most distinctive thing in determining the performance of the membrane unit , since the chosen membrane must have specific characters to do their rule acceptable , for examples : -

-

Forming-able like films : because membrane must have so slim thickness as possible , specially with dense polymeric membranes because flux is inversely proportional with thickness of membrane . High flux insurance . High selectivity . Chemical and biological resistance .

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Detergents and disinfection resistance . Mechanical stability : to afford high feed pressure (7-140 KPa) . Thermal resistance . Ease properties control and manufacturing. Being cheap .

According to the structure , membrane can be classified into two main categories : symmetrical and asymmetrical . in symmetrical membrane , properties are uniform with membrane section contrariwise the other category while membrane is consist of a fine layer and support one to give the first layer the strength . [11]

Figure (6) : different types of membrane

Asymmetric membranes are in general superior compared to symmetric membrane because the flux determining top layer can be very thin . Asymmetric membranes offer greater possibilities in optimizing the membrane separation properties by varying the preparation parameters of especially the thin top layer . Also the ongoing development in the field of polymers throughout the decades resulted in the use of new polymers , like polysulfone (1965) , polyether-ketone (1980) and polyetherimide (1982) Porous membranes are used in MF & UF processes while dense membrane are used in NF & RO . However , some UF facilities have Anisotropic structure consist of a thin porous layer then a relatively thick layer with high porosity to mechanical stability insurance . [12]  Cellulous Membranes : These membranes were used mainly in UF and RO applictions . The structure of cellulous component is Honey Comb structure . They have

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acceptable flux , high salt and stains rejection ability and ease to forming . On the other hand , their faults reject them from industries filed , like :

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Low thermal resistance (maximum 30°C ) Low acidity resistance (PH=3-6) so there will be deterging problems . Low chemical resistance (especially against chlorine ) . Low efficiency at high pressure .

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Sensitive against bacterial attacking .

-

[4,8]

Figure (6) : Cellulous membrane microscopic structure  Ceramic Membranes : Most ceramic membranes have an asymmetrical membrane structure with either a dense or a porous skin layer. The rough porous support is made of sintered ceramic particles (alumina (A12O3), titania (TiO2), and zirconia (ZrO2)), in which the pores are subsequently reduced in size (in three to five deposition steps) before the top layer is formed. Typically the final layer of the support has a pore size between 1 and 5 gm. The preferred shape of ceramic membranes is a rod, because flat discs have shown to be too brittle. Building the ceramic rod starts with making the support by injection molding the inner core with the largest particle size. As long as the rod is not yet sintered (i.e. green phase), the paste of ceramic materials can be shaped freely. The rod is being prebaked (fired) and subsequently a finer particle coating is being applied by dip coating. This process of dip coating

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and firing is repeated several times until the final support coating is applied. Ceramic Membranes characteristic : -

-

-

They do not absorb water nor swell : while swelling is a common problem for many membrane materials , when membrane absorp water so pores sizes increase which serve to decreasing in particles rejection ability of the membrane and this leads to decreasing in selectivity . Thermal stability : sometimes it should apply high temperature in membrane units espically while petroleum oils treatment with high density to decrease it . Erosion resistance : so cross-flow mode - that help to remove filtration cake - could be applied without membrane hurting . High chemical resistance : and that is help to treat lots of active chemicals without membrane hurting

[4,7,8]

Figure (7) : alumina ceramic membrane with a pore size of 800 nm  Polymeric Membranes : A significant research effort has been made to develop polymeric membrane in the past decades in a larg varity of polymeric materials . Also a wide range of fabrication processes is available for polymeric membrane . For example , Nylone 6,6 , polyacrylonitrel , polyamides , polypropylenes , polystyrene , polyisoperenes , polycarbonates , polytetrafluroethelenes (PTFE) and polysulfones . [4,8]

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Figure (8) : comparison between polymeric membrane (on the left) and ceramic membrane (on the right) 

Template Leaching Membranes :

Template leaching membrane is a type of isotropic microporous membrane that manufactured starting from insoluble polymers such as polyethylene , polypropylene and poly(tetrafluoroethlene) . In this process a homogeneous melt is prepared from a mixture of the polymeric membrane matrix material and a leachable component. To finely disperse the leachable component in the polymer matrix, the mixture is often homogenized, extruded, and pelletized several times before final extrusion as a thin film. After formation of the film, the leachable component is removed with a suitable solvent, and a microporous membrane is formed . The leachable component can be a soluble, lowmolecular-weight solid, a liquid such as liquid paraffin, or even a polymeric material such as polystyrene. [4,7,8,12] 

Stretched polymeric membranes :

This kind of membrane is manufactured from crystalline or semicrystalline polymers , while these polymers are being applied to orientation and annealing processes . In the first step , the oriented polymeric film is formed by extruding it at a temperature close to melting point with high velocity , so that polymer crystals are grouped according the applied orientation . After cooling and annealing , the film is being stretched to 300% , and during the process the amorphous regions between crystals is being deformed . pores sizes are controlled by the extent and velocity of the stretching at the last step . [4,7,12]

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Table (2) : Characteristics of common polymeric membrane materials [7,8]

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4- Membranes Processes : 4-1- Microfiltration : This process is used to remove micro-particles out the feed stream . The pores sizes are between 0.1 – 10.0 𝛍m so they can reject microorganisms , but there are some organic/inorganic components materials still can cross through . The MF process is based on Sieving Mechanism , so the particles with size lager than the pores size can not pass through . Usually , MF membranes are polymeric (natural/synthetically) such as cellulose acetate , polycarbonate , and polysulfones , however ceramic membranes could be used . There are two configurations form MF : dead-end and cross-flow . In the first configuration , the whole solution is being pushed through the membrane , during the process progress filtration cake will start forming , this cake is consist of the rejected particles mainly , this will lead to a hydraulic resistance against the process , so we will be forced to renew the membrane because the productivity level is less than the minimum required one with maximum pressure applying . In the second configuration , the feed stream is being parallel to membrane with 0.5-5 m/s so filtration cake can not form . [4,7,8]

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Figure (10) : representation of (a) in-line (dead-end) and (b) cross-flow filtration with microfiltration membranes

MF processes are used in drinking water , wastewater and swage treatment . Recent researches that MF can disinfect water from viruses with adsorption them on clay particles surface so that they will be rejected although their size less than the pores sizes . Chemical treatment also can help gathering the particles to the perfect size to be rejected . Recently , Blocher et al . were able to design a hybrid separation unit consist of MF unit and flotation reactor . furthermore, Futanura et al. have developed the MF in hollow fiber module and integrated it with water treatment with activated sludge units , the results were satisfactory . [11] Generally , smallness of the pores size forces the flow configuration to be laminar Re