Process engineers devote their time to finding the appropriate application of separation techniques. There’s need for effective solid-liquid separation, cake washing, and drying steps across industries. In many chemical and pharmaceutical processes, the production operations are further complicated by the nature of the process, especially if the process is air-sensitive or toxic.
The solid-liquid separation step may be accomplished by pressure, vacuum, or centrifugation in a batch or continuous mode. In this step, further choices need to be made regarding the type of filter media and the thickness of the cake or the cake depth during which the separation occurs. To optimize the production process, I’ve found value in thin-cake (2-25 mm) pressure separation technology for full containment, no residual heel.
Importance of Thin-Cake Filtration
Thin-cake solid-liquid separation can be defined as the formation of a cake in the 2-20 mm thickness range. In this range, cake compressibility becomes less important in the cake building stage of a separation process. Compressible cakes can be better handled at thinner cake depths and higher pressures.
For example, an amorphous crystal that does not centrifuge well or requires long filtration times on Nutsche Filter-Dryers can be filtered at 45 psig with a cake thickness of 2 – 3 mm. Thin-cakes also lend themselves to more effective washing and drying as there is less of a chance of channeling and the mechanism of “plug-flow” of liquids or gases is enhanced.
Impacting Filtration Performance
There are several parameters that can impact filtration performance:
- Filtration pressure
- Temperature
- Particle size/Particle size distribution
- Particle shape
- Cake washing
- Drying of the filter cake.
BHS’s Autopress technology can conduct filtration, cake washing, pressure and vacuum drying all in a contained environment. Cake discharge is complete. There is no residual liquid or solid heel, which is an important benefit for air-sensitive and toxic products.
Understanding Autopress Technology
This fully enclosed filter press, with circular filter plates, allows flow in forward and reverse directions. The filter plates (which can use synthetic or metal media) are contained in pressurized filter housing with a gas-inflated membrane sealing the annular space. Thus, all operations are contained from full vacuum to 150 psig.
The operation of the AP Filter begins with slurry filling to form thin filter cakes of typically 5 – 25 mm thickness. Pressure filtration continues operating up to 8 barg. The cake can then be mechanically compressed to eliminate cracking to ensure maximum washing efficiency in the forward or reverse direction. Finally, the cake can be pre-dried or fully dried either by vacuum or blowing gas through the cake. Gentle drying without agitation or tumbling is especially important for fragile crystals and thixotropic cakes. Elastomeric knives sequentially and automatically discharge the circular cakes after which the filter begins a new cycle.
Read more about this topic in an article I wrote for PharmaChem. My take-away is that with close collaboration between the client and the vendor, we can do the kind of creative problem-solving that applies the separation technique needed to achieve production objectives.