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Solecta - Model Nanofiltration (NF) -Polymeric Membrane Solution
Nanofiltration (NF) products are gaining traction in a number of industrial separation processes due to their specific separation capabilities as opposed to to reverse osmosis (RO) which tends to concentrate all components except water. Separation processes, such as purification and demineralization, are ideally suited for NF given the nature of the molecular weight cutoff of these materials. NF typically follows an ultrafiltration (UF) or ion exchange (IX) treatment step. In some instances, NF can also be followed by reverse osmosis (RO), to further remove dissolved salts prior to wastewater treatment or discharge, or water recovery.
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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.
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
- Temperature
- 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 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.
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:
Compact footprint
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.
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)