More than one recirculation loop may be used for thermal drying, or in cases in which the process requires a different solvent for cake washing and the two solvents need to be separated. In the latter case, each loop would have its own vacuum source.

Gas-tight filters are available with the same design options as the fume-tight machines for ultrapure applications and corrosion resistance.

Ultrapure gas-tight operation

In a filter designed for pharmaceutical or ultraclean applications (Fig.4), the housing design must angle all surfaces, including nozzles, windows, window openings, and even the housing floor, so they can self-drain. Additionally, all internal surfaces must be rounded or profiled to prevent dead spots, and finished to industry standards. In addition, a built-in CIP system must cover the entire internal volume of the filter.

Figure 4. Built-in Cleanroom

This modular system features a housing that functions as its own cleanroom, enclosing its own controlled inert atmosphere. Whenever the filter is opened up, the unit starts up again with an automated validation procedure to bring the filter's internal volume back to cleanroom specs.

The HVBF offers another advantage for ultrapure processing: its filter cloth is continuously cleaned once the cake has discharged, eliminating carryover of product from batch to batch. The filter can be run in batch or semi-continuous mode to ensure batch identity, and to facilitate automation of cleaning cycles and other process steps. In addition, cloth wash liquid can be recycled back onto the filter or returned with the fed slurry, making it possible to recover virtually all of the solids. In solvent operations, the cloth wash liquid is typically the solvent being processed, so there is no risk of product contamination.

When gas-tight horizontal belt units are installed in the pharmaceutical industry on final products, a full validation protocol for the equipment is required.

HVBFs Run Rings Around Centrifuges

 

Gas-tight HVBFs have displaced centrifuges in carboxymethyl cellulose (CMC) production. The components and steps required for both are outlined here. CMC is processed in alcohol/water solvent ," water washes out the salt, while alcohol prevents the cellulose from gelling ," and involves 3-7 countercurrent washing steps. With HVBF, all washing can be accomplished in one filter.

Ease of cleaning is critical
As with all sanitary equipment, the primary concern in building a belt filter is ease of cleaning. Designs must address the most obvious areas such as polished surfaces, and rounded corners, and must eliminate all dead spots. Angled nozzles and window openings, filter trays and filter housings must all be made to be self-draining.

When it is time to clean the filter, the cloth wash system can be combined with the slurry feed system and the cake wash boxes for filter tray and filter cloth cleaning. If a 360 nozzle CIP system also is added, cleaning can be extremely thorough and achieved in a relatively short time.

Pressure-tight operation

HVBFs also have been designed to withstand high pressures, for example, in processes with solvents that would otherwise be in the vapor phase. In addition, filtration systems have been developed for processes that run at high temperatures, with too high a solvent vapor pressure for conventional filtration. In these situations, selective separation can be achieved by keeping certain products in solution with the right combination of solvent, temperature and pressure.

Still, the HVBF separation process only uses the equivalent of vacuum (maximum 1 atmosphere) as the differential pressure across the filter. The filter's operating principle is identical to that of a conventional atmospheric filter, except that it takes place at an elevated pressure.

To date, filters have been built and installed with design pressures for the housing up to 225 psig and temperatures to 250°C (482°F).

Pressure filters also can be operated with air or nitrogen atmospheres, and aqueous or non-aqueous processes can be handled as well. As with the gas-tight design, the pressure-tight filter can be built to sanitary conditions for final product pharmaceuticals.

From supersize to modular

The largest pressure filters built to date, each roughly 28 m² in area, have been installed at a monomer plant in the Far East. Each filter has a housing design pressure of 225 psig and design temperature of 250°C, with operating conditions around 150 psig and 200C. The filters carry out filtration and cake washing at elevated pressure, enabling removal of impurities that would remain in the filtrate if washing occurred at atmospheric pressure.

On the other extreme, the HVBF filter lends itself to compact modular designs and packaging, especially since gas-tight and pressure-tight systems are installed as closed loops. Filtrate receivers, pumps and vacuum pumps can be packaged on one skid, combined with the filter and support structures and access platforms.

Brian Mawson is the vice president of Pannevis Inc., the U.S. subsidiary of Royal Pannevis of the Netherlands.