Considering Alternative API Filtration Technologies

Different types of filtration equipment can be used for removing catalyst residues from API slurries. While conventional filtration equipment is operated manually, both candle filters and pressure plate filters are operated as automated systems, notes Barry A. Perlmutter, president and managing director of BHS-Filtration in Charlotte, NC, a subsidiary of BHS-Sonthofen. He notes that pharmaceutical manufacturers are increasingly looking for automated equipment with in-line process control, and that automated filtration meets the demand for eliminating manual operation for improved safety and reliability. Pharmaceutical Technology spoke withPerlmutter to understand the difference between candle filters and pressure-plate filters and learn best practices for choosing and operating these systems.

Choosing a filter type

PharmTech: How do candle filters and pressure-plate filters differ from traditional filters?

Perlmutter (BHS): Conventional or traditional filters can be defined as bag filters, cartridge filters, manual plate filters, and plate and frame filter presses. These are all manually operated filters. They are not really sealed-especially not when solids get discharged. Candle filters and pressure plate filters are improvements over these types in terms of reproducible quality, multiple process steps, cleanable and reusable filter media, and full containment for solids recovery. A majordifference is that the operation of plate filters and candle filters is 100% automated. Solids discharge is provided in a sealed and safe way.

PharmTech:What are the differences between how candle filters and pressure-plate filters work and the types of applications they can be used in?

Perlmutter (BHS):The differences between candle and pressure plate filters are dependent upon the cake structure that is developed by the process solids. Cake structures that can maintain their integrity in a vertical form are suited for candle filters. If the cakes themselves are too dense or too light or tend to crack, they can easily fall off the vertical candles; in this case, a horizontal plate filter is the better choice of technology. Finally, the thickness of the cake structure is another decision parameter. Candle filters typically have maximum cake thickness of 20 mm, while plate filters can handle up to 75 mm.

Generally, the candle filters and pressure plate filters can be used interchangeably based upon the cake structure itself. Some cakes can be handled in either form (vertical or horizontal), and then the process dictates the choice. Processes that have stringent cake washing and drying requirements will most often use horizontal pressure plate filters, while processes that require only clarification for a clean liquid will tend more towards candle filters. 

PharmTech:What types of filtration are best with continuous or semi-continuous processing?

Perlmutter (BHS):The upstream and downstream equipment will determine the filtration process. Both candle filters and pressure plate filters are batch operations. For continuous or semi-continuous operations, either multiple units are required or buffer/holding tanks can be installed.

PharmTech: What types of filter media are available, and what are some of the parameters for choosing different types of filter media?

Perlmutter (BHS): Choices of media depend upon filtrate quality, liquids and solids material compatibilities, temperature, and cake release properties. Media includes synthetic as well as metallic.

Best practices for disposal and recycling

PharmTech: What are some best practices for disposal or recycling?

Perlmutter (BHS): For non-hazardous disposal, the cakes can be first washed to remove all of the toxic or hazardous compounds and then dried to a standard of no free liquids. The cakes can be fully discharged in a contained and dust-free manner to totes or drums. For recycling, the process solids can be reslurried within the candle filter or pressure plate filter to be pumped back as a slurry to the process. The process liquids or filtrates can also be pumped back to the upstream reactors for reuse.