membrane concentration i : reverse osmosis technology

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Membrane CONCENTRATION (art of technology) :

Membrane CONCENTRATION (art of technology) Dr. Shilpi Verma Mr.Tinsae Belay Mr. Habtamu Geberi Institute of Nutrition, Food Science & Technology Department of Food Science and Post Harvest Technology Hawassa , Ethiopia

Definition:

Definition CA Cellulose acetate, most often di - or tri acetate. CIP Cleaning In Place. The ability to clean a system without dismantling. Concentrate The volume of liquid exiting a membrane system after flowing over the membrane, not through it. It is expressed as feed less permeate; it is also called brine, reject and retentive.

Slide 3:

Flux Volume of permeate per unit area and time. lmh Flux as liter per m 2 per hour: lmh = gfd * 1.7 based on US gallons. gfd Gallons per ft 2 per day: gfd = lmh / 1.7 based on US gallons . HF Hyper filtration HMWC High Molecular Weight Component, such as a protein molecule . LMWC Low Molecular Weight Component, such as NaCl .

Slide 4:

PAN Polyacrylonitrile Permeate The volume of liquid passing through the membranes (also called filtrate). PPM Parts per million. Strictly speaking, mg solute per 1000 gram solution. Used as the equivalent for mg per liter. Pressure psi (pounds per square inch): ( unit ) 14.5 psi = 1 bar. Bar: 1 bar = 0.1 mPa = 100 kPa

Slide 5:

TS Total solids; the total of dissolved and suspended solids. TDS Total dissolved solids. Both values are mostly expressed as mg/l, ppm or %.

Introduction:

Introduction Reverse osmosis (RO) (or ‘ hyperfiltration ’) and ultrafiltration (UF) are both unit operations in which water and some solutes in a solution are selectively removed through a semi-permeable membrane. They are similar in that the driving force for transport across the membrane is the pressure applied to the feed liquid. However, reverse osmosis is used to separate water from low-molecular-weight solutes ( e.g salts , monosaccharide and aroma compounds), which have a high osmotic pressure . A high pressure, five to ten times that used in UF (4000–8000 _103 Pa), is therefore necessary to overcome this (hence the term reverse osmosis).

Application of membrane :

Application of membrane Membrane processing is a technique that permits concentration and separation without the use of heat. Particles are separated on the basis of their molecular size and shape with the use of pressure and specially designed semi-permeable membranes. There are some fairly new developments in terms of commercial reality and is gaining readily in its applications: proteins can be separated in whey for the production of whey protein concentrate (WPC) milk can be concentrated prior to cheese making at the farm level

Slide 8:

Apple juice and wine can be clarified . Waste treatment and product recovery is possible in edible oil, fat, potato, and fish processing Fermentation broths can be clarified and separated whole egg and egg white ultrafiltration as preconcentration prior to spray drying Concentrating fluids by removal of water at low temperatures in the dairy, fruit juice and sugar processing industries etc.

The main limitations of membrane concentration are: :

The main limitations of membrane concentration are: variation in the product flow rate when changes occur in the concentration of feed liquor. higher capital costs than evaporation. a maximum concentration to 30% total solids. fouling of the membranes (deposition of polymers), which reduces the operating time between membrane cleaning.

THEORY:

THEORY In both reverse osmosis and ultra filtration the flow rates through the membrane depend on : 1.The resistance of the membrane material, 2.The resistance of boundary layers of liquid on each side of the membrane , and 3. The extent of fouling. Movement of molecules through reverse osmosis membranes is by diffusion and not by liquid flow. The molecules dissolve at one face of the membrane, are transported through it and then removed from the other face.

Slide 11:

The flow rate of liquid (the ‘transport rate’ or ‘flux’) is determined by the solubility and diffusivity of the molecules in the membrane material, and by the difference between the osmotic pressure of the liquid and the applied pressure. The pressure difference across the membrane (the trans-membrane pressure) is found using: P = _ Pf+Pr _ P p 2 Where P (Pa) = trans-membrane pressure, P f (Pa) = pressure of the feed (inlet), P r (Pa) =pressure of the retentate (outlet) ( high molecular weight fraction) and P p (Pa) = pressure of the permeate ( low molecular weight fraction).

Slide 12:

Water flux increases with an increase in applied pressure, increased permeability of the membrane and lower solute concentration in the feed stream. It is calculated using: J = kA (▲P -▲∏) where J (kg h _1 ) = flux, K (kg m _2 h _1 Pa _1 ) = mass transfer coefficient, A (m 2 ) = area of the membrane, P▲ (Pa) = applied pressure and ▲ (Pa) = change in osmotic pressure.

Slide 13:

Osmotic pressure is found for dilute solutions using: ∏= MRT where T ( 0 K) (where 0 K 0 C + 273) = absolute temperature, R (Pa m -3 mol -1 K -1 )= universal gas constant, M (mol m -3 ) = molar concentration and ∏ (Pa) = osmotic pressure .

Slide 14:

Many foods have high osmotic pressures (for example (6–10) 10 5 Pa for fresh fruit juice), and a high applied pressure is therefore needed. Solutes that are ‘rejected’(retained) by the membrane either have a lower solubility than water in the membrane material or diffuse more slowly through the membrane.

The important factors in determining the performance of a membrane:

The important factors in determining the performance of a membrane its thickness chemical composition and molecular structure

Other factors :

Other factors liquid velocity, viscosity Temperature and pH

Types of membrane :

Types of membrane RO NF UF

Slide 19:

Comparing Four Membrane Processes Reverse Osmosis Nanofiltration Ultrafiltration Micro filtration membrane Asymmetrical Asymmetrical Asymmetrical Symmetrical Asymmetrical Thickness Thin film 150 µm 1 µm 150 µm 1 µm 150-250 µm 1 µm 10-150 µm Pore size <0.002 µm <0.002 µm 0.2-0.02 µm 4-0.02 µm Rejection of HMWC, LMC Sodium chloride glucose amino acids HMWC mono- di -and oligosaccharides Poval neg.ions Macromolecules , proteins, polysaccharides Particles clay bacteria Membrane material(s) CA Thin film CA Thin film Ceramic PSO,CA Thin film Ceramic PP,PSO,PVDF Membrane module Tubular, spiral wound, plate-and-frame Tubular, spiral wound, plate-and-frame Tubular,hollow fiber,spira wound, plate-and frame Tubular, hollow fiber Operating pressure 15-150bar 5-35bar 1-10bar <2 bar

Product and Process:

Product and Process A vast array of products are being treated using membranes, but water desalination is using over 80% of all membranes having ever been sold. The better portion of the remaining 20% are used for dairy processing, while the remaining are sold for use with many different liquids.

Slide 21:

Type of Membrane Process for Several Products Permeate Concentrate RO Dyeing effluent Clean water BOD, salt, chemicals, waste products Water low salinity water Salty water whey low BOD permeate Whey concentrate NF Antibiotics Salty waste product Desalted, concentrated antibiotics Dyeing effluent Clean, salty water BOD/COD, color water Softened water Waste product whey Salty waste water Desalted whey concentrate UF antibiotics Clarified fermentation broth Waste product Bio-gas waste Clarified liquid for discharge Microbes to be recycled carrageenan Waste product Concentrated carrageenan enzymes Waste product High value product milk Lactose solution Protein concentrate for cheese production Oil emulsion Oil free water (<10ppm) Highly concentrated oil emulsion Washting effluent Clarified water Dirty water(waste product) Water Clarified water Waste product whey Lactose solution Whey protein concentrate xanta Water product Concentration xantan

MEMBRANES - MATERIALS :

MEMBRANES - MATERIALS The molecular structure of reverse osmosis membranes is the main factor that controls the rate of diffusion of solutes. The materials should have a high water permeability and a high solute rejection and durability. The main requirement of an ultrafiltration membrane is the ability to form and retain a ‘ microporous ’ structure during manufacture and operation.

Slide 23:

The pore size in the inner skin determines the size of molecules which can pass through the membrane; larger molecules are retained on the inside of the membrane.

Advantages and limitations of different types of membrane :

Advantages and limitations of different types of membrane Cellulose acetate Advantages High permeate flux ,Good salt rejection ,Easy to manufacture. Limitations Break down at high temperatures ,pH sensitive (can only operate between pH 3–6) and Broken down by chlorine , causing problems with cleaning and sanitation

Polymers (e.g. polysulphones, polyamides, poly-vinyl chloride, polystyrene, polycarbonates, polyethers) :

Polymers (e.g. polysulphones , polyamides, poly-vinyl chloride, polystyrene, polycarbonates, polyethers ) Advantages Polyamides have better P H resistance than cellulose acetate . Polysulphones have greater temperature resistance (up to 75ºC), wider pH range (1–13) and better chlorine resistance . Easy to fabricate Wide range of pore sizes. Limitations Do not withstand high pressures and restricted to ultrafiltration . Polyamides more sensitive to chlorine than cellulose acetate.

Composite or ceramic membranes ( e.g. porous carbon, zirconium oxide, alumina):

Composite or ceramic membranes ( e.g. porous carbon, zirconium oxide, alumina) Advantages Very wide range of operating temperatures and pH Resistant to chlorine and easily Cleaned Limitations Expensive

The design criteria for modules ( to resist the high pressures and the membrane plus support material) should include::

The design criteria for modules ( to resist the high pressures and the membrane plus support material) should include : •Provision of a large surface area in a compact volume. •Configuration of the membrane to permit suitable turbulence, pressure losses, flow rates and energy requirements. •No dead spaces and capability for cleaning-in-place (CIP) on both the concentrate and permeate sides. •Easy accessibility for cleaning and membrane replacement .

Membrane Configurations:

Membrane Configurations • Spiral Wound • Hollow Fiber • Tubular

Tabular and plate:

Tabular and plate Tubular membranes are held in cylindrical tubes mounted on a frame with associated pipe work and controls. The two main types are the hollow fibre and wide tube designs. The fibres are attached at each end to a tube sheet to ensure that the feed is uniformly distributed to all tubes.

Slide 30:

These systems have a large surface area to volume ratio and a small holdup volume. They are used for RO applications such as desalination, but in UF applications the low applied pressure and laminar flow limits this system to low viscosity liquids that do not contain particles. Flat plate systems can be either plate-and-frame types or spiral-wound cartridges

spiral-wound system:

spiral-wound system In the spiral-wound system , alternating layers of polysulphone membranes and polyethylene supports are wrapped around a hollow central tube and are separated by a channel spacer mesh and drains.

RO: Spiral Wound Membrane Element:

RO: Spiral Wound Membrane Element Page 32

Spiral Membrane RO Element:

Spiral Membrane RO Element Page 33 Feed Water Feed channel = 26 - 31 mils (0.00026 - 0.00031 inches) Permeate Tube

Flow Pattern for a Spiral Wound Element :

Flow Pattern for a Spiral Wound Element

Slide 35:

Page 35 Permeate Concentrate Reject 20 m3/h @ 4 Bar Inlet Water 100 m3/h @ 16 Bar Housing Spiral Modules= Membrane element + Membrane Housing 80 m3/h @ 0 Bar RO: Spiral Wound Membrane Housing

Slide 36:

Tubular Membrane

Slide 37:

Hollow Fine Fiber Element Construction

Safety and Hygiene Considerations:

Safety and Hygiene Considerations These revolve round cleaning and disinfecting procedures for the membranes and auxiliary equipment, as well as the monitoring and controlling the microbial quality of the feed material.

Effect on foods:

Effect on foods The effects on nutritional value are more difficult to assess in most operations and are usually incidental to the main purpose of altering eating qualities. The main losses occur as a result of the physical removal of food components(not even heat sensitive). Both types of membrane retain proteins, fats and larger carbohydrates, but the larger pore size of ultrafiltration membranes allows sugars, vitamins and amino acids to be lost.

Osmosis:

Osmosis Page 40 OSMOSIS. . . the spontaneous flow of water from less concentrated to a more concentrated solution through a semi-permeable membrane until energy equilibrium is achieved Less Strong Salt Concentration 150 gallons Less Weak Salt Concentration 50 gallons Time = Equilibrium atmospheric pressure semi-permeable membrane Strong Salt Concentration 100 gallons Weak Salt Concentration 100 gallons Time = 0 Osmotic Pressure

Reverse Osmosis:

Reverse Osmosis Page 41 semi-permeable membrane Strong Salt Concentration 100 gallons Weak Salt Concentration 100 gallons Stronger Salt Concentration 50 gallons Weaker Salt Concentration 150 gallons applied pressure

Reverse Osmosis: Membranes:

Reverse Osmosis: Membranes Membrane : natural or highly engineered synthetic selectively-permeable material, containing controlled distribution of pores First commercial membranes were “asymmetric” (non-uniform density) cellulose acetate (CA) modified CA membranes are still in use (cellulose di - and tri-acetate) usually cheaper than polyamide/thin-film composites, but lower rejection, and are biodegradable (require chlorine protection) asymmetric membranes are subject to compaction, which leads to loss of flux/performance Next came aromatic polyamides (PA), also asymmetric higher rejection wider operating temperatures and pH intolerant to oxidants (require dechlorination )

Reverse Osmosis:

Reverse Osmosis Reverse osmosis is often used as the model of comparison for other membrane techniques. R.O. can be designed to remove: Typically 95-99+% of most inorganics Mid to large sized organics Turbidity, color, some sensory-active compounds Suspended, colloidal particulates Microorganisms It can be a fully automated system that can reach demineralized water quality, yet requires no chemicals for regeneration like ion exchange

Slide 44:

Page 44 Feed Water 100% Brine/Concentrate 25% Permeate - pure water 75% Reverse Osmosis Pass water by a semi-permeable membrane Permeate - High quality treated water Brine/Concentrate with dissolved solids, suspended solids, microbes goes to waste RO Membrane RO: Process Overview

Reverse Osmosis: Membranes:

Reverse Osmosis: Membranes Thin-film composite (TFC) membranes very similar to PA, but differ in the way they are cast formed by interfacial polymerization on a separate microporous layer (usually polysulfone ) slightly cationic character, so adversely sensitive to anionic coagulants and surfactants/detergents wide operating temperature and pH range not susceptible to hydrolysis or biodegradation limited tolerance to oxidants 0.2 u 40 u 120 u Reinforcing fabric (polyester) Microporous polysulfone Ultrathin barrier Polyamide Polysulfone

RO Construction:

RO Construction Page 46 segmented ring feed water segmented ring permeate permeate plug endcap end cap/membrane adapter brine seals o-rings interconnector thrust ring Membrane Housing brine membrane

Flow Through a Membrane:

Flow Through a Membrane Page 47 feed semi-permeable membrane Mineral Ions Colloids Bacteria Particles “Glue line” R.O. Permeate “Glue line” feed channel spacer Permeate spacer brine backside of membrane

Reverse Osmosis: Membrane Comparison:

Reverse Osmosis: Membrane Comparison Page 48

RO: Fluid Flow:

RO: Fluid Flow Page 49 Concentrate 10 m3/h Permeate 50 m3/h Bypass Stream 50 m3/h Raw Water Concentrate 20 m3/h Permeate 100 m3/h Raw Water Full Flow Partial Bypass Pump 120 m3/h Pump 60 m3/h Treated Water 100 m3/h For partial by-pass operations, the quality of the by-pass stream is critical, and must be considered. This is not recommended for any PCI beverage applications.

Slide 50:

Page 50 An “Array” is a series of parallel vessels with common feed, product and reject lines Pump Recycle Permeate (Product) Concentrate (Reject) RO: An Array Feed

Slide 51:

Page 51 Refers to a configuration where permeate from the first array passes as feed to another. Pump Feed Water Concentrate (drain) Concentrate (sidestream) Permeate Pass 1 Pass 2 RO: Double Pass For very high quality treated water (very low TDS) or very poor quality raw water

Slide 52:

Page 52 ~85% recovery Reject from first array feeds into second array Pump Permeate Concentrate 3 Array in a ratio of 4:2:1 RO: Multi-Array

Slide 54:

RO: Operating Criteria

RO: Operating Criteria:

RO: Operating Criteria Feed Flow = Permeate + Concentrate % Recovery = Permeate Flow x 100 Feed Flow Typical recovery rates are 75% to 85% Page 55

RO: Percent Rejection:

RO: Percent Rejection Rejection may also be compared for individual solutes : %R = (C F - C P ) 100 C F Where... %R = percent solute rejection C F = feedwater concentration of solute C P = permeate concentration of solute Overall salt rejection may be calculated from TDS measurements: Salt Rejection = (Feed TDS - Permeate TDS) X 100 Feed TDS Typical salt rejection is > 99% for modern spiral wound polyamide membranes

Slide 57:

Performance Problems

RO: Performance Problems:

RO: Performance Problems Page 58 Time Performance 100% Particulates Mineral scale Colloids Microorganisms All of these characteristics, and others, could result in a loss of membrane performance over time pH Temperature Degradation

RO: Performance Problems:

RO: Performance Problems Membrane Fouling Caused by biologic and organic material that binds to the membrane surface and causes blockage. Membrane Scaling Caused by the increase in concentration of salts beyond their saturation level - this factor directly affects recovery levels. For example, i f a water source has a high calcium sulfate content, water recovery levels can be as low as 60% to prevent m embrane scaling. Membrane Degradation Caused by hydrolysis, microbial attack, or chemical solubilization of the membrane material Page 59 Each of these will affect pressures, flow rates, and % salt rejection of your individual system in different ways. The cleaning procedure must address the appropriate foulant

RO: Particulate Fouling:

RO: Particulate Fouling Where in system? Typically in the front end of the first stage Symptoms? Reduced permeate flow and increased differential pressure Cleaning procedures? Physical/mechanical removal of sand and particles off ends How to avoid? Proper care and maintenance of pretreatment system, direct filtration, pre-RO polishing filters Proper well construction, development, equilibration, and monitoring What to monitor? Feed water turbidity, suspended solids Page 60

RO: Colloidal Fouling:

RO: Colloidal Fouling Where in system? Typically most severe in the first stage Symptoms? Reduced permeate flow and increased differential pressure Cleaning procedures? Removed by using an alkaline cleaning solution. May require more than one cleaning. See Appendix section for more detail How to avoid? Direct filtration, ferric sulfate feed, antiscalant What to monitor? Weekly SDI testing Page 61

RO: Biologic Fouling:

RO: Biologic Fouling Where in system? Can occur anywhere Symptoms? Rapid pressure drop, membranes covered with slime, potential permeate flow declines and differential pressure increases Cleaning procedures? Removed chemically via a clean-in-place (CIP) system How to avoid? Keep system running, maintain proper upstream chlorine levels, UV disinfection of RO feed, proper use of metabisulfite Appropriate sanitization frequency What to monitor? RO influent and effluent microbial levels Page 62

RO: Mineral Scaling:

RO: Mineral Scaling Where in system? Typically in the last stages where concentrations of scale forming ions and pH are higher. Symptoms? Significant losses in permeate flow and salt rejection. Increases in differential pressure Cleaning procedures? Mineral scaling is removed by cleaning with an acid-based cleaner How to avoid? pH control (acid feed), antiscalant , softening of feed water, limiting the recovery What to monitor? Langelier Saturation Index (LSI) measurements Page 63

RO: Membrane Degradation:

RO: Membrane Degradation Where in system? Typically in the first stages, but may propagate throughout, depending on the nature of the chemical attack. May be due to oxidant attack, pinpoint corrosion from metallic oxides, solubilization of the membrane by organic contaminants, hydrolysis of the membrane due to pH extremes (mostly CA) Symptoms? Significant increases in permeate flow and loss of salt rejection. Decreases in differential pressure Cleaning procedures? Usually not applicable, due to the gravity of the situation How to avoid? Proper membrane selection, dosing of chemicals, pH control (acid feed), and chlorine/oxidant control What to monitor? pH, chemical feed, infeed quality Page 64

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