How Does a Büchner Funnel Work?
The Büchner funnel is a common piece of chemical lab equipment designed for filtration. The mechanism is relatively straightforward: the funnel contains either a perforated plate (over which a paper filter is placed) or a fritted glass plate which can be used without a paper filter. The narrow part of the funnel is typically sealed to a side-arm flask (also known as a Büchner flask), with the arm of the flask connected via a tube to a vacuum pump. The liquid to be filtered is poured into the top of the funnel and is drawn through the paper filter/fritted glass by a combination of gravity and suction provided by the vacuum pump. Suspended solids are filtered out, and the filtrate collects in the bottom of the side-arm flask.
Typically made out of porcelain, glass, or plastic, Büchner funnels are inexpensive pieces of equipment that provide much faster filtration than simply allowing gravity to draw liquid through a filter. Büchner funnels support a relatively wide range of applications. For example, funnels of an inch or so in diameter are a common sight on chemistry lab benchtops. In contrast, large models of up to around a meter in diameter can support pilot plant scale applications (often with the aid of “trowels” to manually smooth over fissures in the compacted solid material). Troweling the cake in this manner introduces the problem of operator variability and human error.
However, for industrial applications – especially pharmaceuticals or fine chemicals – Büchner funnels are typically avoided in favour of agitated Nutsche filter dryers.
How Agitated Nutsche Filter Dryers Work
An agitated Nutsche filter dryer is a specialist piece of equipment designed to provide a more sophisticated filtration method than Büchner funnels.
Agitated Nutsche filter dryers use a bladed drive assembly to provide low-speed, high-torque agitation/mixing of the slurry, ensuring the height of the “cake” (i.e. filtered solids) doesn’t impact the filtration rate. One key difference between an agitated Nutsche filter dryer and a Büchner funnel is that while Büchner funnels only provide suction from below, agitated Nutsche filter dryers provide both suction from below and pressure from above. The result is faster filtration and much lower moisture content in the filtered solid.
As filtration advances, the blades inside the agitated Nutsche filter dryer rise to the top of the cake to automatically smooth out cracks and/or preferential channels. Therefore, operator variability is not a problem as with the smoothing process in the Büchner funnel.
Once solids have been fully separated, an agitated Nutsche filter dryer will wash out any remaining filtrate and impurities. Büchner funnels do support washing, but agitated Nutsche filter dryers provide superior washing either via traditional methods (in which a solvent seeps through the product cake) or by re-slurry washing (similar to conventional washing but using the agitator to plough the solvent into the cake before pressing – this reduces the amount of solvent required and optimises the removal of impurities).
The similarities between the two different types of filters diverge here, with agitated Nutsche filter dryers gaining added benefits. Products discharged from a Büchner funnel need drying in a secondary piece of equipment, such as a tray dryer or desiccator. Both methods of drying are small-scale, which naturally makes them unsuitable for many industrial applications.
By comparison, filtered and washed solids are dried within an agitated Nutsche filter dryer. This can be carried out by either pressurised gas heating or vacuum-assisted heating. The agitator plays a vital role by providing drying homogeneity and breaking down difficult products.
Finally, the product is discharged from the agitated Nutsche filter dryer. The agitator once again plays a vital role by gradually pushing the cake to the discharge valve.
For all applications that demand high-performance filtration, agitated Nutsche filter dryers enable fast and effective processing in batches of up to 1000L.
It is also worth noting that agitated Nutsche filters provide a fully closed environment which enables certain processes that can be difficult – if not impossible – using a Büchner filter. Many toxic, highly potent products, and a range of volatile solvents must be handled with care using a Büchner filter, possibly requiring external ventilation. This is not the case with an agitated Nutsche filter.
Powder Systems Ltd. provides a full range of high-performance agitated Nutsche filter dryers to suit all applications, from R&D scale right through to commercial manufacturing. Get in touch with PSL today to learn more.