Reverse Osmosis. Background to Market and Technology

Transcription

1 Reverse Osmosis Background to Market and Technology 1

2 Technology and Applications Reverse osmosis has been commercial for over 25 years. 60MLD plants built in Saudi Arabia 20 years ago. Current sales of RO membranes world-wide wide are around $250 million per annum excluding Japan. RO System sales could be as high as $1.0b per annum 2

3 Membrane Market Growth sector of the water treatment industry driven by reduced energy consumption and increased awareness of environmental impact/cost of ion exchange operation Growth at 15-18% 18% per annum Main manufacturers are from USA - Dow/Hydranautics Hydranautics/Fluid Systems USA - Koch/Dupont/Osmonics/Trisep Japan - Nitto Denko/Toray/Toyobo 3

4 Fundamentals of Membranes and Reverse Osmosis 4

5 Membrane Separation Angstrom Aqueous Salts Sugars Viruses Colloids Bacteria Pollens Beach Sand Nano- Filtration Reverse Osmosis Ultrafiltration Microfiltration Particle Filtration 5

6 Pressure Driven Membrane Processes - Pressures Membrane Process Typical Operating Pressure Range (PSI) Reverse Osmosis seawater brackish water Nanofiltration Ultrafiltration Microfiltration

7 Impurities in Water Ionic Non Ionic Particulate Microbiological Gases 7

8 RO Removes Ionic Non ionic Particulate Microbiological 8

9 When to Consider RO Water with TDS greater than 150 mg/l Regenerant cost reduction Waste cost reduction Water conservation or recovery 9

10 Pressure Filtration Not feasible for reverse osmosis A rises to equal B sparingly soluble solutes precipitate and foul the membrane 10

11 Cross Flow Filtration Required for reverse osmosis and nanofiltration A sweeps away membrane foulants B minimizes concentration polarization (maintains difference) C generates a concentrate stream and a permeate stream 11

12 Cross-Section of Thin-film Composite Membranes Polyamide Polysulfone 0.2 µm Ultrathin Barrier Layer Microporous Polysulfone Reinforcing Fabric 40 µm 120 µm 7 //

13 Thin-film Composite Membrane Chemistries Typical composition Fully aromatic polyamide (Dow FT30) Polypiperazineamide (Dow NF45) Polyvinyl alcohol (Hydranautics( Hydranautics) Sulfonated polysulphone (Ionpure) 13

14 Schematic Cross Section Permeate Channel HOLLOW FINE FIBRE PERMEATORS Barrier Layer 14

15 Spiral Wound Reverse Osmosis Brine Channel Spacer Product Water Water Flow Permeate Channel Spacer Membranes Brine Seal Product Water Brine Brine Feed 15

16 Reverse Osmosis Water Flow Product Feed Ions Concentrate 16

17 Nanofiltration Definition Minimum size rejected on order of one nanometer Between RO and UF Operates at ultra-low low pressure Selective permeation of ionic salts and small solutes 17

18 Nanofiltration Water Flow Product Feed Concentrate Macromolecules Polyvalent Ions Monovalent Ions 18

19 Osmotic Processes Concentrated Solution Pure Water 19

20 Osmotic Processes Concentrated Solution Pure Water Osmosis 20

21 Osmotic Processes Pressure Concentrated Solution Pure Water Reverse Osmosis 21

22 Reverse Osmosis Involves Application of pressure greater than osmotic pressure of solution Diffusion of water but not salt through a semipermeable membrane in direction opposite of natural flow Crossflow filtration to sweep away concentrated salts 22

23 Reverse Osmosis: What It Can Do Remove purified water from a feed stream (permeate) Concentrate chemicals in a feed stream (reject) Selectively separates small ions and molecules 23

24 Reverse Osmosis: What It Cannot Do Cannot concentrate to 100% Cannot separate to 100% Cannot reject gases and is Not always the most cost effective method 24

25 Mass Balance Equations Recovery (%) = Permeate flow Feed flow x 100 Salt Passage (%) = Permeate Salt Concentration Feed Salt Concentration x 100 Salt Rejection (%) = Salt Passage 25

26 Normalization of Field Operating Data Feedwater pressure Temperature Ionic concentration System recovery 26

27 ph vs Flux and Salt Rejection Water Flux (GFD) Salt Rejection (%) Data from DOW Filmtec ph

28 Pressure vs Flux+Rejection Water Flux (GFD) Salt Rejection (%) Data from DOW Filmtec Pressure (PSI) 28

29 Temperature v Flux+Rejection Water Flux (GFD) Salt Rejection (%) Data from DOW Filmtec Temperature ( C) 29

30 Salinity vs Flux and Rejection Water Flux (GFD) Salt Rejection (%) Data from DOW Filmtec Percent 30

31 Factors Which Affect Performance of Membranes Feedwater Pressure Feedwater Temperature Feedwater Concentration Increased Recovery 31

32 Simplified RO System Feed Water Pump Concentrate Permeate 100 to 400 psi (brackish water) 800 to 1,200 psi (seawater) 32

33 Membrane Performance Feed Permeate 33 Reject (Brine)

34 Membrane Performance 75% Recovery Feed 100 m 3 /hr Permeate 75 m 3 /hr 25 m 3 /hr 34 Reject (Brine)

35 RO Pretreatment Options Suspended Solids Removal Clarification Filtration Primary Membrane UF/MF/EDR Control of biological activity Chlorination/dechlorination Chloramines Non Oxidising Biocides Ultraviolet Light 35

36 RO Pretreatment Options Scale control & ph adjustment Antiscalant Addition Acid Addition Ion Exchange Pretreatment 36

37 RO Systems Design Module (Element): Contains Membrane Tube: Modules in Series (1-7) Stage: Set of Tubes in Parallel Array: Train: No. of Stages, Tubes/Stage Set of all of the Above 37

38 RO System Flow Diagram 2 STAGES 1 TRAIN 50% MAXIMUM RECOVERY per STAGE 4:2 ARRAY 20 m 3 /hr PERMEATE 75% RECOVERY 10 m 3 /hr FEED 20 m 3 /hr 40 m 3 /hr 1st stage 2nd stage 10 m 3 /hr CONCENTRATE 38

39 Other Factors to be Aware of.. One to seven elements per pressure vessel Maximum feed flow physical limitations Minimum brine flow or maximum ratio of permeate flow to feed flow concentration polarization Recovery 39

40 Other Factors to be Aware of.. System Design Guidelines Fouling and/or scaling tendency of feed most influences system design Tendency for fouling increases with increasing permeate flux and increasing element recovery Only experience can set limits on permeate flux and element recovery for specific feed Use system design guidelines when previous experience is not available 40

41 Other Factors to be Aware of.. Feed Composition on System Recovery Seawater recovery limitations High osmotic pressure Osmotic pressure limits recovery to 35-45% Brackish water recovery limitations Brackish water chemistry tends to contain many sparingly soluble salts which cause scaling Usually limits recovery to 70-85% 41

42 Factors Which Affect Performance of Membranes Feedwater Pressure Feedwater Temperature Feedwater Concentration Increased Recovery 42

43 Troubleshooting 43

44 Overview The Importance of Record Keeping The General Rule of Troubleshooting Signs of Trouble Causes and Corrective Measures of Trouble Signs Taking the Total System Approach 44

45 Why Keep Records? Necessary for observing trends Valuable tool for troubleshooting Required in the event of a warranty claim 45

46 General Rule of Troubleshooting First Stage Problem - Fouling Last Stage Problem - Scaling 46

47 Troubleshooting Signs of trouble Loss of permeate flow Increase in salt passage Increase in differential P 47

48 Probing Procedure to determine problem area in pressure vessel without unloading elements from vessel Probe if one pressure vessel shows a significantly higher permeate TDS than other vessels of the same array Plot TDS measurements on a conductivity profile to determine problem area in vessel 48

49 Troubleshooting - Probing Feed Conc Permeate conductivity 49

50 Troubleshooting - System Best recorded by preparing a series of circles arranged similar to the vessel rack assembly and writing each vessel s s reading in its respective circle. Example: 24:12 array at 75% recovery

51 High Differential Pressure ²P P is a measure of the resistance to the hydraulic flow of water through the system. This is very dependent on flow rates through the element brine flow channels and on water temperature Lead element brine flow channels will show debris, foulants,, and scalants 51

52 High Differential Pressure Causes Cartridge filter by-pass Media filter breakthrough Pump impeller deterioration Scaling Brine seal damage / improper placement Biological fouling Precipitated antiscalants 52

53 High Differential Pressure Cause: Cartridge Filter By-pass Filter improperly installed Avoid cellulose-based filters Corrective Measure: Properly install cartridge filter Clean filter housings when replacing filters 53

54 Taking the Total System Approach Troubleshooting Steps Investigate Evaluate Solve Prevent 54

55 Troubleshooting Investigate entire system Review normalized operating data Check feedwater quality Confirm chemical dose rates Calculate material balance Calibrate instruments, i.e. flow meters Try to localize problems for further in-depth evaluation 55

56 If Source of Problem is Not Identified Check conductivities and probe if necessary Remove and inspect first element, first stage and last element, last stage Look for mechanical damage (torn O-ring, O cracked fiberglass) Visually inspect elements; send to Anjou Recherche for autopsy if necessary Determine effect of first high ph then low ph cleaning Analyze cleaning solutions for metals and TOC 56

57 If Source of Problem is Still Not Identified Conduct a destructive autopsy of the elements: Check for metals and organics on membrane surface Conduct dye test for oxidative damage to the membrane Visually examine the element for physical damage (wrinkles, glue line separation, etc.) 57

58 Clean In Place System 9 m3/hr per pressure vessel >4 bar Pressure In Line Filter Heater System Litres per element to be cleaned Return Line below liquid in Tank 58

59 Design Faults with CIP Insufficient Flow Excessive Pressure Tank Heating Capacity too Small or Omitted Lack of Appropriate Monitoring - Flow/Pressure Plant Cannot be Cleaned in Stages Contents of Tank Cannot be Diverted Procedure Recommends incorrect Products for Fouling 59

60 Monitoring Requirement Feed System (by stage) Salinity Concentration ph Temperature Permeate Concentration/Flow/PressurePressure Concentrate Flow/Pressure and Concentration (optional) 60