Pressured lab-scale filtration and drying

Camille Flores-Kilfoyle discusses how advanced filter dryer technology is enabling new product development.

Lab-scale filter dryer technology has not always been available for accurate scaling up. During laboratory studies and R&D product development, chemists need to work on similar technology to evaluate and determine the appropriate process parameters.

Traditional filtration and drying methods usually found in laboratories are Buchner filtration, centrifuge and oven dryer. These technologies cannot be transferred successfully onto commercial production scale.

Therefore, smaller agitated nutsche filter dryers have been developed to provide the same technology for 10 to 500 grams batch of final product. They can be found in glass for complete process visibility and now also in stainless steel for higher pressure requirements (Fig. 1).

These lab filter dryers perform the same filtration under vacuum and pressure, washing and re-slurry capabilities and thermal drying through a heating jacket. The GFD design allows a total recovery of the final dried powder thanks to a filtration basket that can be removed for efficient product harvesting.

The new lab filter dryer is pressure rated at 4 bar(g) for a typical pressured filtration at 2 bar(g). The glass version still allows up to 0.5 bar(g), ideal when applying low pressure during the filtration process.

The GFD products range from the smallest size Mini Lab GFD with 0.002m² filtration basket for a total volume of 0.3L and can filter a cake volume up to 0.1L; this is a suitable piece of equipment for lab-scale trials. Then the Lab GFD size includes a removable filtration basket of 0.01m² for a cake volume around 0.5L for small scale synthesis. This is the most popular size of the range thanks to its polyvalent use. 

The Maxi Lab GFD is used within Kilo-Laboratory for a cake volume of 5L with 0.05m² filtration area.

The full range uses the same accessories, including control box and drive system for the agitator, allowing true flexibility between the different vessels in glass borosilicate 3.3 or stainless steel (see Fig. 2).

How does it work?

The slurry enters the vessel through one of the nozzles located on the vessel lid. During the filtration phase, vacuum is pulled at the bottom of the vessel whilst the filter basket collects the product. It is then possible to wash the cake, again using a nozzle of choice on the lid to introduce the solvent and the heated vessel jacket if required.

It is common practice to apply pressure to the top of the filter cake during filtration whilst the bi-directional motor is running in a clockwise direction to smooth the cake. Optimal filtration is achieved in this way as you can pressure filter with no cracks in the cake.

The heated jacket is then used to heat the vessel for the drying phase. The vessel is rated to temperatures between 25°C and 150°C, and it is not uncommon for the GFD to also be used for the filtration of unstable products at low temperatures. The bi-directional motor is lowered into the cake during this phase and is now used in the anti-clockwise direction to dig into the cake thus resulting in the homogeneous drying of the product.

Technology transfer

When scaling up the processes, the following key parameters must be taken into consideration for a positive technological transfer on a filter dryer: the heat transfer area; the agitator type; and the product cake depth.

The heat transfer area efficiency must be determined for both size agitated nutsche filter dryers. This can be performed through various tests. When scaling up, it is important to also determine the heat transfer area to the maximum cake volume for each piece of equipment.

The agitator design is the next important factor. Filter dryer agitators are cantilevered and subjected to high torque of the solids. Their maximum speeds are therefore very low, usually in the range of 5 to 25 rpm. 

When scaling up, it is important to match the tip speed of the agitator to simulate the same agitator to product contact times. 

When simulating a filtration scale-up the final factor is to maintain the same cake depth. As the product builds up on the filter media, the product cake starts to act as the filter media. As long as the cracks and preferential channelling are eliminated with the agitator, the filtration time should be comparable with the same parameters of cake depth and applied pressure. This has been proven to work from lab scale to large-scale production.

The applications of the filter dryer are as vast as with a commercial agitated nutsche filter dryer that is widely used in the production processes of pharmaceutical, chemical and laboratory industries. They are designed for the filtration and drying in the same unit of active pharmaceutical ingredients (API) and chemical compounds. PSL’s experience shows that these filter dryers are installed in API production plants but also used for formulation of microsphere drugs, isolated peptides, isolated crystals and even cosmetic applications.

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Camille Flores-Kilfoyle is with Powder Systems Limited (PSL) in Liverpool, UK. 

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