Pharma applications of depth filtration

While depth filter sheets act like a screen that hold larger particles at the surface, its channel and void-like structure, coupled with the electro-kinetic nature of the sheet's constituents, allow very fine and electromagnetically charged particles to be removed.

A typical depth filter sheet has about 50;80% void volume within its structure. This allows for slow flow rates through the structure and long residence times so that very high absorbency and adsorbency levels can be achieved. This type of structure also offers the advantage that when the sheet's capacity has been filled, it does not block or 'blind' like a surface filter.

The sheet's end point is usually marked by breakthrough of impurities, an end point that should be monitored. Fig. 1 shows a section through a typical depth filter sheet.

For Duffy and his counterparts, however, the main challenge comes from the fact that different customers have their own particular requirements. 'Much of our effort goes into experimenting with hundreds of possible wood and other cellulose sources, powder additive and resin systems as well as varying manufacturing conditions to find the best solutions,' he says. 'We also spend a lot of time developing specific products for particular customers. These products are based on the recipes for our standard products but with individual tailored constituents incorporated in the sheet. These allow particular types of impurities to be removed.'

Activated carbon is a typical example of such a constituent to be incorporated into a depth filter sheet, which is used to remove colour bodies, colour precursors and other impurities from filtrates. Many manufacturers used to use activated carbon in powder form, which was added to a stirred vessel to remove the colour and then filtered out using a standard type of filter such as a filter press or horizontal leaf filter. Whilst this method produces the required degree of colour removal, it is very uneconomical, environmentally unfriendly and presents major clean up and dust problems.

Duffy adds, 'We have put an enormous amount of effort into incorporating a range of adsorbents including various activated carbon types into our sheets. The challenge has been to incorporate the adsorbents in such a way that they are available for adsorption but remain within the sheet when in use.'

optimising sheet life

Optimising flow rates and depth filter sheet life present interesting challenges to the depth filter sheet manufacturer. In fact they are linked, because flow rate is linked to sheet capacity, which in turn tends to govern sheet life.

Duffy approaches this issue from a particular standpoint. 'Our approach has been to look at getting the right filtrate purity first and then to maintain this while we optimise flow rates and filter sheet life. So once we are happy with the sheet 'recipe', we work on flow and life mainly by varying manufacturing conditions.'

If the depth filter sheet manufacturer is to make individual products for individual customers while maintaining manufacturing efficiency, rapid process batch changeover times must be achieved. This has required a considerable amount of process development effort, and Carlson has invested heavily in this area.

Another important development over the last few years has been the successful introduction of depth filter sheets in a lenticular cartridge format. These cartridges are used in a simple housing and present an excellent alternative to depth filter sheets in a filter press.

Compared with using filter sheets in a conventional filter press, the development of the lenticular filter cartridge format has brought a number of advantages.

An interlocking disc design improves lateral strength and rigidity and ensures that only polypropylene-to-polypropylene contact occurs throughout the internal core of the filter. This alleviates any compressibility problems and maximises internal filter volume, which in turn improves filter capacity.

improved resistance

A heavy duty nonwoven polyester scrim gives improved resistance to back pressure shock and a provides much improved back washing capability, as well as providing an appreciable degree of pre-filtration prior to depth filtration

The design of the polypropylene separator discs and sealing rings promotes even liquid flow within each cell. This in turn tends to reduce the pressure drop across the filter, leading to longer filter life, shorter filtration cycles and more consistent filtrate quality.

A good example of how a particular customer requirement has been met can be found at BASF, the multinational chemicals and pharmaceuticals manufacturer. As well as removing particulate impurities, BASF has a requirement to remove colour bodies and pre-cursors from a range of pharmaceutical intermediates and finished products.

A spokesperson for BASF said, 'Traditionally, we used to use carbon powder to remove colour, and a horizontal leaf filter to remove the carbon after use. Not only was this a very dirty procedure but the time taken to empty, clean and redress the leaf filter added considerably to the batch times. By using a lenticular filter with the carbon incorporated into it, we have significantly reduced both batch and product changeover times. This has effectively increased our manufacturing capacity by at least 25%.'

BASF added that since they need all the manufacturing capacity they can get, this development saved them hundreds of thousands of pounds in capital costs.

Their spokesperson added, 'Carlson worked closely with us over an intensive three month period to incorporate a specific type of carbon into their lenticular format. In addition they have developed a particular type of elastomer for the filter seals, which is compatible with our filtrate. Even though the cost of using this filtration method is higher than our traditional method, we are very happy with both product quality and overall savings.'

An important feature of depth filter sheet manufacture is that a Drug Master File should cover products designed for use in the pharmaceuticals industry.

Following its successful submission to the United States Food and Drug Administration, Carlson was assigned a Type I Drug Master File Number - DMF 14255 in mid-1999.

lenticular cartridges

Duffy points out, 'Pharmaceuticals and fine chemicals manufacturers can use Carlson's depth filter sheets and lenticular cartridges with confidence. The facilities, materials and manufacturing methods, as well as the skills of Carlson people in Barnoldswick, have been catalogued in the DMF. It is important that all pharmaceutical manufacturers should check that their filter system supplier has filed a DMF with the US FDA.'