Across all forms of chemical processing, fluid filtration serves several very important roles including protecting process equipment from foreign particulates, optimizing process system performance, and ensuring high finished product quality. Chemical filtration solutions are available in many forms, but one particular design has served the chemical processing industry for most of its history by being both incredibly reliable as well as cost-effective, known as bag filtration. Bag filtration uses a simple fabric bag installed inside of an inline filter housing, which chemicals flow through. As these chemicals flow, the filter bag’s porous fabric extracts unwanted particulates out of the fluid stream and allows only filtered fluids to pass forward. Chemical filtration applications can be highly aggressive, hazardous, and dynamic, requiring engineers to be very intentional and informed when selecting bag filter materials for these systems. Polypropylene and Nylon are two of the most common filter bag materials chosen for chemical processing applications, which we’ll examine in detail below.
Chemical Process Filtration Applications
Before we compare filter bag materials, it will be helpful to understand where filtration technologies are utilized in chemical processing applications. Filtration solutions will vary in their design, specifications, and ratings depending on where in the process they’re installed and what process steps they’re to serve, which we can define as follows:
- Raw Materials – raw material filtration removes large contaminants and particulates from incoming ingredients, helping to standardize materials before they enter downstream processes as well as to protect equipment from unsuitable foreign materials.
- Pre-Process – initial pre-process steps may include material heating, cooling, stabilizing, sizing, grading, and clarification, all of which can benefit from additional filtration to continue pulling out oversized particulates and impurities.
- In-Process – once processing begins, chemicals can undergo any number of changes and reactions, acquiring new physical properties while also producing unwanted byproducts. Inline filtration here is crucial, separating out particulates to maintain the integrity of the overall process. This is especially vital in molecular size-dependent reactions such as fractionation, distillation, reduction, catalysis, and refining.
- Finished Products – once all chemical reactions are complete, most of the resulting finished products still require a polishing process to bring their quality up to final specifications such as chemical final filtration, purification, color and haze standardization, and sterilization.
- Utility Services – virtually all chemical process systems rely on key utility services such as compressed air, plant water, industrial gasses (IE carbon dioxide, oxygen, nitrogen, argon, helium), steam, and fuel gas. All these utility services require filtration, keeping each stream free of harmful foreign materials that can threaten the reliability of the process systems they serve.
Understanding Bag Filtration Solutions
Bag material is only one part of a larger set of variables that come into play when selecting a bag filter solution. To better understand how filter bag materials fit into the larger equation of filtration efficacy, let’s have a look at the main components that make up a bag filtration package:
- Bag Filter Housing – a bag filter housing is the overall vessel that holds all other components inside, and through which all chemical or utility streams flow. Housings are selected for the flowrate, pressure, chemical compatibility, and bag volume required to suit the application. Most housings are selected with flanged, threaded, or weld stub piping connections, and are made from stainless steel, carbon steel, aluminum, or plastic materials.
- Bag Filter – filter bags are consumable fabric components that are installed within the above filter housings, and act as the physical barrier that extracts contaminants out of the liquid stream flowing through the housing. Filter bags are specified by their overall size (diameter and length), filtration rating (measured in microns or µm), and material. Most bag filters are nominally rated with efficiencies not provided. Common filter bag ratings range from 1 µm to 1,500 µm.
- Filter Screen – filter bags are lightweight fabrics that easily deform, fold, and tear under the force of fast-moving liquids, and so they need a more resilient surface to rest against. A filter screen provides this surface in the form of a thin, perforated or mesh sheet metal tube that the filter bag installs within. Once installed, the filter bag will expand up against the inside wall of the cylindrical filter screen tube, protecting the bag from damage.
- Seals – most bag filters rely on integral bag flanges or separate elastomeric seals to properly contain hydraulic forces within the housing, as well as to keep the upstream and downstream liquid zones separated (assuring that all process liquid flows through the filter bag and not around it). Elastomers may come in the form of O-rings, square rings, gaskets, or unique seal strips, and must be selected for compatibility with the liquids and pressures to be encountered. Common elastomer options include EPDM, silicon, Viton, nitrile, Teflon, and Buna. Common integral flange options include stainless steel, steel, and various plastics.
- Control Devices – filtration components present restriction to flowing liquids to strip out undesired particulates, and as these particulates build up within the filter bag’s pores, the level of restriction increases. This restriction is measured in terms of pressure drop, and once the pressure drop gets too high, the filter bag must be cleaned or replaced. Control devices such as pressure gauges, differential pressure sensors, or electronic pressure transmitters are used to monitor a filter’s restriction.
Digging Further into Bag Filter Materials – Polypropylene vs Nylon
At this point, readers can see that bag filtration packages have many critical parts that must work in tandem to provide reliable fluid filtration. The filter bag itself is the workhorse in the application, responsible for reliably retaining particulates under the physical strains presented by the process fluid’s flow rate, pressure, and physical material properties. The two most common choices selected for chemical applications are Polypropylene and Nylon bag materials, which offer the following general features:
Polypropylene Bag Filter Features
- Ideal Use: High Performance Water, Chemical, and Aggressive Media Filtration
- Compatible With: Water, Organic Acids, Alkalies, Inorganic Acids
- Incompatible With: Oxidizing Agents, Oils
- Temperature Rating: 0-194°F
- Micron Rating: 1 µm to 800 µm
- Construction Types: Polypropylene Needle Felt, Extended-Life Needle Felt, Monofilament Mesh, Melt-Blown
- Other Key Features: FDA Food-Grade, High Wear Resistance, Optional Oil Absorption Fabrics, Bacteria and Mildew Resistant
Nylon Bag Filter Features
- Ideal Use: Economical Chemical and Mild Media Filtration
- Compatible With: Organic Solvents, Oils, Microorganisms, Alkalies
- Incompatible With: Oxidizing Agents, Mineral Acids, Inorganic Acids
- Temperature Rating: 0-325°F
- Micron Rating: 5 µm to 1,000 µm
- Construction Types: Nylon Monofilament Mesh, Multifilament Mesh
- Other Key Features: FDA Food-Grade, Cleanable / Reusable, High Tensile Strength
Selecting between Polypropylene and Nylon Bag Filter Materials
Choosing between Polypropylene and Nylon materials for chemical bag filtration applications can be simplified into four core decisions:
Chemical Compatibility
Polypropylene and Nylon filter bag materials generally overlap in their chemical compatibility, though polypropylene is far better suited for use with water, acids, and oxidizers. The choice here will come down to compatibility with a user’s specific chemical properties.
Filtration Grade
Polypropylene filter bags are available in smaller micron ratings and with higher filtration efficiency ratings than nylon filter bags. For the smallest particulate size and highest filtration consistency, polypropylene is the ideal choice. On the other hand for large particulate or pre-process filtration, nylon bags are preferred.
Application
Referring to our list of applications earlier in this article, nylon bags are often selected for raw material, pre-process, and utility filtration applications based on their larger particulate range and reusability (where cross-contamination between product runs is not a factor). Polypropylene bags are often selected for in-process and finished product applications based on their smaller particulate range, higher efficiency, and higher purity.
Cost
In general, nylon filter bags are more expensive than polypropylene filter bags. With that said, buyers should consider the total lifecycle cost of their filter bags, as nylon bags tend to last longer and can be reused, offering a lower net lifecycle cost over polypropylene bags (depending on the application).
A Final Note on Filter Bag Materials
Filter bags are a competitive market, with many manufacturers supplying their own unique styles, models, and materials to attract new business. Manufacturers tend to differentiate themselves by balancing their filter bags’ technical features and cost points to a targeted market segment – which is to say, no two filter bags are created equal. While the above comparison between polypropylene and nylon filter bag materials outlines the general differences between these two options, buyers are encouraged to always check the specifications of any filter bags being considered for purchase against their application’s requirements, as these specifications will vary.