FAQ's about Nutsche Filtration and Drying

External_spray_ring (1)Filtration and drying are critical operations in a variety of industrial processes. These unit operations are handled separately in many applications, but can also be performed within the same unit when advantageous. The Nutsche Filter/Dryer allows for solid-liquid separation and drying operations to take place in a single process vessel.

Filtration can be broken down into two types of processes – continuous and discontinuous (or batch) operations. Nutsche filtration is a batch filtration technique that uses vacuum and/or pressure in a closed vessel. (We've outlined the steps of the filtration and drying process in this post about nutsche filter/dryers.)

Once you have an understanding of nutsche filtration, you might still have some questions about it. We've compiled a list of frequently asked questions to help further your understanding of the equipment and how it can be beneficial to your process.


Q: What are some benefits of using a nutsche filter/dryer?

A: Some benefits of using this technology include product isolation, minimal operator exposure, reduced product handling, and environmental protection against solvent vaporization. If you have a batch process where containment is an issue and you require a closed system for the protection of your workers or the integrity of your product, a nutsche filter is something to further consider.


Q: Who uses nutsche filter/dryers?

A: Due to the benefits listed above, the level of containment achieved by a nutsche filter/dryer can comply with GMP and other health and safety requirements that companies often face. For these reasons, nutsche filter/dryers are commonly used in pharmaceutical, fine chemical, dye and paint production, and waste water treatment applications.


Q: How do you select the right size filter for your process?

A: The filter should be sized appropriately to handle the solids volume that is being processed. A general rule of thumb to follow is that the maximum solids height should be equal or less than the agitator stroke, typically 12 to 20 inches. However, the final cake thickness achievable will depend on the characteristics of the filter cake. To accurately size a unit, you would need to provide some basic process information, including slurry volume per batch, wet cake volume per batch, and preferred material of construction. Using these three main factors, we can determine the model and size that is most suitable for your process. It is also important to know the filtration rate as that can impact the size selected.


Q: How is filtration rate and cake depth determined?

A: Filtration rate is controlled mainly by the product’s physical characteristics so the particle shape, cake porosity, and compressibility need to be taken into account. These characteristics are not easily measured, so tests must be run to measure their effect on pressure drop, filtration rate, and cake depth. As a general rule, the minimum allowance for cake depth is three inches and the minimum cake buildup rate is 1 inch per hour, giving you a minimum filtration rate of 4 liter/min-M² (0.1 gpm/ft²).


Q: What are recommended testing options for determining the proper equipment selection?

A: There are multiple options for testing your product to ensure efficient filtration. What method you choose usually depends on your product characteristics and details of your process. There are scale-up filter/dryers that you can rent, which allow you to run small batch tests; Our pocket filter is available for purchase or rental, which allows benchtop filtration testing; Or, you can have your product tested at our pilot plant facility in Semur, France. A DDPS representative can help you determine which is the right testing method for your application.


Q: When are filter/dryers advantageous to use over centrifuges?

A: To assess what separation technology will best fit the needs of your specialized application you really need to look at your specifications and requirements. However, when doing a general comparison between filter/dryers and centrifuges, filter/dryers are superior in the areas of containment, maintenance and operating costs, efficient washing, capacity flexibility, and drying capacity. For more detailed information, read our Centrifuge and Filter/Dryer Product Comparison post.


Q: When is hastelloy used over stainless steel?

A: Hastelloy, a nickel alloy, is a more exotic and expensive material of construction than stainless steel, which is an iron alloy. When you are dealing with an extremely corrosive product and stainless steel is not deemed suitable for the process, hastelloy is usually the best alternative.


Q: How do you select which type of filter media is best for your process?

A: There are various types of filter media available in nutsche filters – cloth, single layer metal screen, or multi-layer sintered metal. The media should be determined based on the characteristics of the slurry including particle size and shape, cake porosity, and compressibility (which are all factors that, when taken into account with the filter media selected, will determine filtration rate).


Q: What is the purpose of washing in the filtration process?

A: There are two types of washing in nutsche filtration. First, displacement washing, serves several purposes – it removes the liquid and its impurities while keeping the cake intact and it replaces the previous liquid with fresh liquid. There is also the opportunity prior to this step to try and close cracks if the cake has any via smoothing. The re-slurry process is an optional wash that can be used when a long contact time is needed between the wash fluid and the solids or the displacement wash does not provide the required wash quality.


Q: Besides closing cracks in the cake, what is the value of the smoothing step?

A: This function will help to achieve uniform flow of liquid or gas through the filter cake, while helping to eliminate liquid and gas channeling that reduces the efficiency of displacement washing and gas blow through. For this reason, smoothing may be used after any filtration or wash, especially after the final wash.


Q: What is flexible discharge?

A: There are certain applications where the desired product to be discharged is not a dry solid. Nutsche filter/dryers are designed to facilitate flexible discharge, which allows for wet solids, slurries, or even liquid to be discharged.


Q: What are the differences between vacuum and convection drying methods?

A: There are two types of drying that can be conducted; the method used depends on the product behavior. Vacuum drying, the most common method, involves a vacuum source, agitation, and dust filter. This type of drying utilizes vacuum to reduce the temperature at which the solvent evaporates, reducing the average drying temperature. During convection drying, hot, pressurized gas (usually nitrogen) is blown down through solids and out of filtrate lines (and even recirculated as necessary); this will eventually dry the solids. Convection drying is unique to filter/dryers due to their porous filter plate. Unlike vacuum drying, it does not require agitation. Whether vacuum or blow-through drying is selected, a filter/dryer can allow you to get completely dry material out from your process, with less than 0.1% final product moisture. At this point, the vessel and product may need to be cooled to an ambient temperature prior to discharge.


Whether in the chemical, pharmaceutical, food, or other industry, filters and filter/dryers provide a means to efficiently accomplish washing and isolating solids. There are many different options available to suit various process requirements based on scope and product characteristics. If you think nutsche filtration is a viable solution for your process, fill out our Filtration Questionnaire and a DDPS filtration expert can contact you to discuss your process needs in more detail.


Filtration Questionnaire Link