Nanofiltration (NF)Polymeric Membrane Solution
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What is Nanofiltration?
Nanofiltration refers to a classification of membranes where separation range is categorized based on rejection characteristics of known solutes, such as magnesium sulfate (MgSO4) and sodium chloride (NaCl). Typical ranges of rejection are typically 90-99.5% and 30-70%, respectively. Unlike MF and UF technologies, separation of NF is based on the diffusion of dissolved species through the membrane and can be highly dependent on pH and chemical charge at or near the membrane surface along with other operating conditions.
The manufacturing process for nanofiltration is quite complex and typically involves the application of thin film (e.g. polypiperazine) on a polyethersulfone (PES) or polysulfone (PS) UF substrate.
Examples of materials that will more freely pass through the membrane into permeate include monovalent salts (such as sodium, chloride, potassium, etc.), monosaccharides (glucose, fructose, galactose, etc.) and small dissolved organics. Nanofiltration separation is better characterized based on component rejection, but in general it is typically classified as having a molecular weight cut-off between 150-1,000 Daltons) NF is utilized to purify either the retained concentrate components or the permeate that passes through the membrane, making it attractive membrane format for enrichment of numerous types of these molecules.
Although there are a couple form factors (e.g. tubular, spiral wound) and materials of construction (e.g. polymeric, ceramic) for Nanofiltration, the most commonly used is polymeric spiral-wound technology. Solecta is proud to offer polymeric spiral-wound membranes in a variety of pore sizes and feed spacers to accommodate the needs of numerous process applications.
How do you ensure performance from nanofiltration membranes?
Variables to monitor performance
There are several variables which are used to monitor NF system performance. These include the following operational parameters:
- Feed flow, pressure, and conductivity
- Permeate flow, pressure, and conductivity
- Retentate flow, pressure, and conductivity
- Other –parameters such as protein concentration and COD are utilized in process applications to measure performance. However, since these tests typically require offline measurement and/or more advanced analytical procedures, conductivity and Brix are oftentimes used as a proxy.
Calculations to validate performance
Calculations can also be performed to understand rejection and passage. Simplified formulas are provided below:
% Rejection = (Feed Conductivity – Permeate Conductivity) / (Feed Conductivity )
Note that conductivity is an effective way to get a quick read on NF performance, but ultimately, measuring the specific rejection of the process component (e.g. protein or sugar) is the best way to measure true process performance.
Passage % = 1- Rejection %
Passage is simply the inverse of rejection. Typically, nanofiltration is used to purify either the concentrate or permeate, so monitoring both the rejection and passage of the components is important to understand how if the process is operating properly. When changes in the operation arise, it typically means there is some process issue that requires addressing via CIP, mechanical inspections, or potentially replacement.
Recovery % = (Permeate Flow Rate / Feed Flow Rate) * 100
While recovery is a typical calculation used to measure water treatment performance (e.g, polishing) , it is also valuable in process applications to understand how much process stream is being recovered on a % basis. It is also helpful in understanding how well the system is concentrating the process stream.
Concentration Factor % = 1 / (1 – Recovery %)
Since concentration is typically the main goal of NF in process applications, this is a good way to validate the effectiveness of your application. As is the case with other variables, changes in performance over time should be monitored to ensure optimal system performance.
Benefits of Nanofiltration
What are some key benefits of Nanofiltration?
When properly designed and operated, NF and specifically spiral-wound membranes can offer several benefits over traditional separation process:
With advances in element construction and system design, substantial surface area can be designed into a membrane solution vs other traditional filtration technologies
Lower energy consumption
These systems generally consume less energy than other complex separation processes, particularly if they are thermally-driven
Minimized waste generation
With proper operational protocols, including cleaning procedures, NF membranes can generally run with a higher proportion of runtime vs cleaning/downtime
Ease of operation
NF membrane operations are well understood, and control systems can ensure smooth, safe separation operations
Lower cost of operation
when considering capital and operating costs, including those mentioned above, NF membranes offer an attractive solution for filtration based on size exclusion of 0.001 – 0.01 µm or 10-100 Da.
Industrial Applications of Nanofiltration
NF is used broadly across process industries, most namely dairy, food ingredients, biotechnology/life sciences, beverages, and automotive manufacturing operations. Some of the key applications across these industries include the following:
- UF permeate processing (lactose concentration)
- Milk production (concentration)
- WPC and WPI production (solids concentration)
- Polishing (purification of COW water and other process streams)
- Sugar/sweetener processing (concentration)
- Other fermentation processes (concentration, water recovery)
- Polishing (condensate purification)
- Cell mass removal (downstream processing of bulk fermentation)
- Juice production (concentration)
- Utility water (purification)
FAQs on Nanofiltration
The typical Nanofiltration separation process is based on an ionic exclusion process, where a feed solution is pumped through a semi-permeable membrane, where the pores sizes range between 0.001 – 0.01 µm, or 10-100 Da.
As fluids pass through, usually in a cross-flow configuration with low transmembrane pressures (TMPs) of 3.5-8 bar or 50-120 PSI, the membranes retain those particles in a process stream called retentate. The fluid that passes through the membrane is referred to as permeate.
As the membranes separate solids, particles can agglomerate at the membrane surface, causing what is referred to as fouling. This phenomenon can slow down the flow rate and/or increase pressure across the membrane, if the process is not optimized.
Additionally, harsh cleaning chemicals can degrade the integrity of a membrane, as well. Maintaining proper operational best practices, such as backwashing in some membrane formats and/or cleaning procedures in others, can prevent fouling. Automation can also help detect this by monitoring flow and/or pressure drop across the membrane.
Nanofiltration systems are very robust and typically require little routine maintenance. When they do, it is recommended that qualified technicians work on these systems to maintain optimal performance. Some of the typical maintenance tasks around Nanofiltration system maintenance include:
- Pre-treatment system – some membrane systems might include a pre-treatment step, such as cartridge filters or MF/UF membranes. These will need to be exchanged periodically to maintain Nanofiltration system performance.
- Gauges and other instrumentation – as these systems typically include both manual and automated instrumentation, it is important to check and ensure these are operating correctly. In the case of automation, it is important to calibrate these instruments per the manufacturers’ recommended protocol.
- Valves, solenoids, and other wear parts – it is not uncommon for valves to become stuck and/or freeze, particularly if they are not exercised during normal operations. It is important to turn valves off and on periodically, as well as check to ensure solenoids are operating correctly, to maintain system reliability.
- Element replacement – when a membrane has gone past its useful life, it will need to be replaced. Loss of performance will typically manifest itself in reduced rejection or clarification results and/or through changes in flow rate. Ensure that technicians are qualified to replace elements, so they don’t damage elements upon installation, and also ensure proper seals to prevent system leakage.
Otherwise, Nanofiltration systems will typically have an on-going cleaning protocol, which maintains the health and reliability of the membrane system. These cleaning chemicals will ensure proper flow and rejection, as well as prevent unwanted microbiological contamination – ensuring optimal performance and sanitary conditions.
Nanofiltration removes particles that are greater than 0.01 µm or 100 Da. These particles include, but are not limited to:
- Residual materials left from previous membrane steps (cells, microorganisms, proteins)
- Dissolved sugars
- Dissolved organics
- Some dissolved salts
Nanofiltration membranes can be constructed of either ceramic or polymeric materials. In the case of polymeric Nanofiltration membranes, the specific polymers are thin film (e.g. polypiperazine) laid on top of a polyethersulfone (PES) or polysulfone (PS) substrate.
Form factors for Nanofiltration include tubular (shell and tube design) and spiral-wound (alternating layers of flat sheet, feed spacer, and permeate carrier).
The most common Nanofiltration membranes are polymeric, spiral-wound configurations.
The process stream that enters the membrane for clarification and/or fractionation
The rate of extraction of permeate, which is typically measured in LMH (liters per square meter of membrane surface per hour – l/m2/h) or GFD (gallons per square foot of membrane surface per day – gal/ft2/day)
The deposition of solids on the surface of a membrane
The liquid stream that passes through the membrane (aka filtrate)
The liquid stream that is rejected by the membrane
The ratio of initial feed volume to retentate or concentrate, which is an indication of target volume reduction achieved by membrane filtration