Overview of Concepts, Practices & Applications in Liquid Filtration Dr Abdoulaye DOUCOURE, Ablo Senior Scientist, Hollingsworth & Vose Abdoulaye.Doucoure@hovo.com FILTRATION 2011 TUTORIALS November 15 th - Chicago
Outline Fluid filtration basics Liquid filtration processes Liquid filter materials ( media ) Filter characterization Applications
Fluid Filtration Basics Gas Separation Liquid Separation Merits of Liquid Filtration
Solid/gas Separation Filtration (filter) Indirect interception Direct sieving Separation (no filter) Inertia Electrostatic Centrifugation Solid /gas separation is predominantly conducted by air filtration. Collection mechanisms for indirect interception Inertial impaction Diffusion Interception Electrostatic In direct sieving, particles larger than the medium openings are filtered out
Solid/Liquid Separation Filtration (filter used) Straining (= sieving capture) Cake (surface) & Depth Filters Separation (no filter) Gravity settling Centrifugal settling Flotation Others (magnetic, electrostatic) Flotation: Lighter particles float to liquid top and are separated. Gravitation, centrifugation: particles are drawn to liquid bottom (higher density than liquid phase) and are separated.
Driving Forces in Liquid Filtration Gravity - small scale, laboratory (e.g.: particle capture on filter paper) Vacuum small & large scales Driving forces Pressure small & large scales Note: Liquid filtration involves size-based particle separation. The particle of interest can either pass through or be retained by the filter. The liquid properties (ph, viscosity, polarity, surface tension) play a key role in liquid filtration performance.
Merits of liquid filtration (municipal water) Conventional Process (mainly based on liquid separation) Raw Water Coagulation and Mixing Clarification Filtration (sand) Disinfection Distribution System Low Pressure Filtration Alternative Backwash Sludge (Membrane) Raw Water Disinfection Distribution System Liquid filtration (synthetic media) only demands 1 step prior disinfection!
2 Modes of Filtration Feed DEAD-END FILTATION Feed CROSSFLOW FILTRATION Retentate Permeate Permeate Crossflow mode: Less fouling but the feed stream is not fully recovered in the permeate Dead-end flow: More fouling & flux decline but the feed stream is 100% filtered
Spiral wound Filter configurations & geometries Geometries and Configurations Bundle of hollow fibers Disc tube Hollow Fiber Spiral Wound Plate and Frame Capillary Tubular Disc tube Single hollow fiber Capillary filter Plate and frame Tubular filter
Liquid Filtration Processes Liquid filtration System Microfiltration & Ultrafiltration Nanofiltration & Reverse Osmosis
Liquid Filtration System & Key Process Parameters FEED Prefiltration Feed stream Qf Pf Pp Permeate stream Qp Qf: feed flow rate Qc: concentrate flow rate Qp: permeate flow rate Pf: upstream pressure (feed) Pp :downstream pressure (permeate) P = Pf Pp = Differential pressure Concentrate stream Qc Recovery = Qp / Qf (Percent) Process Parameters Membranes Pressure ( P), Temperature (feed) Recovery (%) Feed stream quality Flow rates (feed, concentrate) System performance Permeate quality Throughput, filter fouling Specific energy (KWH/volume permeate) Filter service life Downtime (maintenance)
Filtration types >40 Bar 10 Bar < 1 Bar High Pressure Membranes Reverse Osmosis Nanofiltration Ultrafiltration Low Pressure Membranes Microfiltration Filters 0,0001 0,001 Organic 0,01 0,1 1 10 Compound Colloids s Salts Organic Macromolecules Yeasts Algae Pollens 100 µm Virus Bacteria Protozoa Polio Virus Smallest Bacteria Red Cell Hair
Microfiltration (MF) & Ultrafiltration (UF) MF, UF are the most widely used membrane filtration processes. MF, UF operate at a recovery rate ~ 100% (no waste) MF, UF require moderate operating pressure, ranging 1 to 6 bars MF process is primarily used in dead-end configuration for clarifying suspensions. MF separates particles with a size of 0.1-10 um. UF operates in cross flow mode and is used to concentrate or separate dissolved molecules. From 1kDa to 1,000 kda - e.g. treatment of proteins in biotech or virus removal in water.
Reverse Osmosis (RO) & Nanofiltration (NF) RO, NF target the removal of small organics, ions in solvents, often water. RO, NF separation mechanism is based on size and charge exclusion. RO operational pressure is > 40 bars, while NF operates at 10-15 bars. RO recovery is ~30-50% (seawater desalination). NF recovery > 90%. Disc tube module Spiral wound filter NF/RO membrane (side view) RO and NF operate in cross flow mode configuration. RO filters reject >95% monovalent ions (Na+, Cl-) and only the solvent can pass through NF pore size varies in the 0.4-1.5 kda range. It rejects particles, polyvalent ions over 90% but let most mono-valent ions and the solvent pass through.
Application Areas Water (municipal, industrial, wastewater) Industrial Processes (fuels, chemicals, inks) Microelectronics (ultrapure water, cleaning chemicals) Food & Beverages (dairy products, wine, beer) Biopharmaceuticals (antibodies, proteins, vaccines) Biomedical (blood, plasma filtration, hospital water)
Boiler Water Treatment Process
Major Liquid filter Applications (MF,UF) Microfiltration Ultrafiltration Hemodialysis Potable Water Drinking Water production, removal of turbidity, parasites, bacteria, cysts Pretreatment prior to RO and NF Industrial Process Water Ultrapure water for semiconductors, electronics, electric power and pharmaceuticals Process water treatment/recycling Separation, recovery of catalysts caustics, degreasers, dyes, sizing agents Solvent purification Separation of oil/water emulsions Biotech, Biopharma Bioprocessing: cell harvesting protein concentration and clarification Production of pharmaceutical makeup water Wastewater Wastewater treatment/recycling Pretreatment prior to RO and NF Separation, recovery of catalysts caustics, degreasers, dyes, sizing agents Separation of oil/water emulsions Solvent purification Food & Beverage Clarification of juice wine beer, vinegar, sugar Cold sterilization of wine & beer Whey filtration Milk fractionization, extended shelf life Chemical purification Copper slurry filtration Semiconductor Potable Water Drinking Water production, removal of turbidity, parasites, bacteria, cysts, Virus Pretreatment prior to RO and NF Industrial Process Water Ultrapure water for semiconductors, electronics, electric power and pharmaceuticals Process water treatment/recycling Separation, recovery of catalysts caustics, degreasers, dyes, sizing agents Solvent purification Separation of oil/water emulsions Biotech, Biopharma Bioprocessing: cell harvesting protein concentration and clarification Production of pharmaceutical makeup water Wastewater Wastewater treatment/recycling Pretreatment prior to RO and NF Waste water in food processing plants especially meat and poultry. to reduce BOD and COD levels Separation, recovery of catalysts caustics, degreasers, dyes, sizing agents Separation of oil/water emulsions Solvent purification Food & Beverage Clarification of juice wine beer, vinegar, sugar Cold sterilization of wine & beer Whey filtration Milk fractionization, extended shelf life Grain milling application, corn sweetener and ethanol production from biofuels Protein fractionation in soy products Chemical purification Copper slurry filtration Semiconductor
Liquid Filter Media Fibrous Materials Membranes
Filter products & Filtering processes Filter Products Processes Micron Rating Spiral Wound Hollow Fiber Tubular Plate & Frame Pleated Depth Reverse Osmosis (RO) Nano- Filtration (NF) Ultra- Filtration (UF) Micro- Filtration (MF) <0.001 < 0.001 < 0.1 <10.0 Coarse Prefiltration <100.0
Fibrous Media *Abbreviations: PP: polypropylene - PE: polyester Wet laid ufiber glass, cellulose Meltblown, *PP, *PE, Nylon, Aramid Dry laid, Spunbond (PE, PP, nylon etc) Nanofibers fiber size~ 100 s nm DEPTH, SURFACE DEPTH, SURFACE Substrates (for membranes) SURFACE, DEPTH Coarse prefiltration, MF pores ~ 1-10 um Pore 1-200 um Fine prefiltration & MF Pores ~ 0.3-1.0 um Porosity 40-95 % Basis weight 0.5-300 gsm
Nanofiber Media Nanofiber size varies in the 50 nm to 1,000 nm range typical size < 500 nm. Nanofibers have found more applications in gas filtration so far (enhanced efficiency). In liquid filtration, nanofiber mats can be leveraged as membrane-like surface filters (pore < 1um), while exhibiting a low hydraulic resistance and interconnected porous structure similar to depth non-woven filters. Depth filter (Meltblown polypropylene) Nanofiber media is a bridge between these 2 platforms Surface filter (Polypropylene membrane) Finer pore size than standard depth media ENHANCED EFFICIENCY Higher specific area than standard depth media GREATER BINDING CAPACITY More open pores than standard membranes INCREASED PERMEABILITY
Nanofibers for liquid filtration
Membrane types (MF, UF, NF & RO) SEM Top view Polymer, Ceramic, Metal Porous Dense Composites Symmetric Asymmetric Standard polymeric membranes Polyethersulfone (PES), polysulfone, Polyvinydene fluoride, PVDF Cellulose Nylon Polypropylene, PP Polytetrafluoroethylene, PTFE Polyacrylonitrile (PAN) SEM Cross sections of PES membranes
Filter Characterization Pore size determination Solvent transport (basic definition) Wettability and Extractables
Filter pore Size Pore size : largest media opening that lets a contaminant pass through. No universal method of size determination. It dependents on the function, such as non-bio clarification (suspended solids), biological treatment (microorganisms), or high purity fluid processing (total particle removal). Common techniques used to measure pore size: Visual method : SEM imaging (surface pores) Porometry & Porosimetry (Mean Pore, Max Pore sizes) Particle challenge (monodisperse spheres of latex, silica) When Pore size is less ~ 50 nm (UF, NF and RO membranes) Membrane rejection is expressed as Molecular Weight Cut off (MWCO) in Dalton Da If MWCO of a membrane is 70 kda (proteins, small organics) 90% of 70 kda molecules are retained, while 10% pass through.
Solvent transport Definitions Flow rate Q = Volume / time Flux J = Volume / (time x area) = Q/area Permeability Pe = Volume / (time x area x pressure) = J/pressure Flow P profiles (3 different liquid cartridges) P (Pressure drop) Filter 3 - smallest pores Filter 2 medium pores Filter 1 largest pores Flow rate (liter per minute, Q)
Wettability and Extractables Wettability affects the amount of pressure needed to force the solvent through the pores. Thus, liquid penetration is made easier with a wettable filter. Hydrophobic polymers are widely used to filter aqueous solutions due to their good stability (PTFE, PVDF, PP etc). But they need to be chemically modified to be rendered water wettable, i.e. hydrophilic. We can check the wetting properties of a Hydrophilic PVDF membrane by pouring a water droplet on its surface. Water wettability (through-pore) is verified when the membrane becomes transluscent!
Extactables Extractables are found in the filtrate (downstream of filter) and originate in the filter. It s critical to be aware of their presence as they can be harmful to an application. Extractables are measured by soaking a filter in a solvent under known conditions and analyzing the (liquid) filtrate composition. Sources of extractables can be associated with : Shedding of filter materials. Chemicals produced in the manufacturing process. Additives, wetting agents washed off the filter (liquid-filter interaction).
Opportunities/limitations in liquid filtration Opportunities Limitations Efficient filtration of very polluted streams Need to understand filter plugging cycles High consistency in filter performance Filter compatibility, stability issues Highly selective processes Highly versatile process (many applications) Large filter product offerings Abundant filter testing protocols among manufacturers Need expert to design the right liquid filtration system (large scale application) Very modular in scale expansion; Reduced foot print & energy consumption Too many products with a broad range of filter quality/performance Ease of operation
ACKNOWLEDGEMENT INDA FILTRATION 2011 ORGANIZERS TUTORIOALS S HOSTS & SPONSORS HOLLINGSWOTH & VOSE
References R.H. Perry, D.W. Green, Perry s Chemical Engineer s Handbook, 8 th ed., Mcgraw-Hill, N.Y., 2007 L. Svarovsky, Solid-Liquid Separation, 3 rd ed., Butterworths, 1990 D. Purchas, Solid-Liquid Separation Technology, Uplands Press, Croydon, 1981 W.S. Winston Ho & K.K. Sirkar, Membrane Handbook, Chapman and Hall, N.Y., 1992 S. P. Nunes & K.V. Peinemann, Membrane Technology in the Chemical Industry, 2 nd ed., 2006 M. Mulder, Basic Principles of Membrane Technology, Springer, 1996
Pore size Determined by the manufacturer. It is the largest opening through which a particle can pass. Characterized using: Scanning electron microscope, SEM : visual analysis. Small sample size = a few cm^2. Porometry: Gas is forced through a fully wetted media. Measurement of pore gas flow distribution. Porosimetry: Liquid is forced through a dry media. Measurement of pore volume distribution. Calculation of Mean pore size and Max pore size Particle challenge : Particles (liquid suspension) of fixed size are used to determine the smallest particles retained by a filter only for MF and coarser media Beta ratio : # particles in feed # particles in filtrate Log reduction value Log (#particles in feed / #particles in filtrate) Fractional efficiency 1 - #particles in feed # particles in filtrate
Membrane Process Capabilities