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The many faces of horizontal vacuum-belt filters

Sept. 11, 2003
More ultrapure, corrosive and hazardous chemical processes are specifying HVBFs
Enclosed and capable of continuous operation, horizontal vacuum-belt filters (HVBFs) are being used in a wide variety of liquid-solid separations, in applications ranging from basic chemical processes to the manufacture of ultrapure pharmaceuticals. Attracting users are the facts that the filters are easy to clean, and that they allow multiple washing stages to be handled by a single filter, reducing equipment needs.

In addition, the filters can wash in either co-current or countercurrent mode, and can incorporate compression and/or thermal drying features, to recover drier solids. In some chemical processing applications, the filters are replacing such traditional separation equipment as centrifuges, Nutsche and pressure filters.

HVBFs can be operated in one of three modes, depending on the degree of vacuum required:

  • Fume-tight, running at atmospheric pressure with a normal air atmosphere;
  • Gas-tight, running at atmospheric pressure with an inert atmosphere; or
  • Pressure-Tight, running at elevated pressures with either an air or an inert atmosphere.

    Fume-tight operation

    In fume-tight mode, as shown in Fig. 1, an HVBF operates within its housing at near-atmospheric levels, using air as the housing atmosphere. When preventing contamination of the process is the key goal, the filter can run at a slight overpressure, as shown in Fig. 2. Any leakage will flow outside the filter, protecting the integrity of the process ingredients. When plant personnel need an extra measure of protection, the filter can run at a slight underpressure, ensuring that any leakage will flow into the filter (Fig. 3).

    Figure 1. Fume-tight Operation for Pharmaceuticals

    The HVBF shown here operates in fume-tight mode. Hot air is drawn through the filter cake to dry it. This installation produces a citrus peel extract that has pharmaceutical applications. Since high levels of cleanliness are required, air is used in a closed-loop recirculation system.

    Figure 2. Overpressure Prevents Contamination

    The HVBF shown here is operating in fume-tight mode with overpressure. The unit produces an artificial sweetener, so strict standards of cleanliness are required. The overpressure ensures that product is not contaminated by outside sources.

    Figure 3. Underpressure Protects Workers and Equipment

    Designed to operate in corrosive environments, this HVBF is used in fume-tight mode with underpressure. Note the inlet air filter on unit's roof. The filter also features polycarbonate or polymethyl methacrylate openings that serve as windows and access points for inspection and maintenance.

    The fume-tight operating mode allows only a very small amount of in- or out-leakage at the housing seals, because the differential pressure across the housing is so small. It is still necessary, however, to draw air into the enclosure, since air is drawn through the filter cake by the vacuum source. This air can either be recycled via the vacuum pump discharge or drawn into the housing via a filter element, or both approaches can be combined.

    For filters operating with slight overpressure in sanitary or pharmaceutical processes, any air coming into the housing from a source other than the vacuum pump loop is clean air that enters via a supply duct.

    Clean scene

    Sanitary operation is achieved by building the filter to the requisite level of internal surface finish, and providing comprehensive clean-in-place (CIP) capability within the housing. The CIP system cleans both the vacuum filter and all surfaces within the enclosure. All internal surfaces, including nozzles, window openings and mounting brackets, must be designed to be self cleaning if the filter is to pass muster for ultrapure pharmaceutical applications.

    Since the fume-tight filter mode works with an air atmosphere, doors or entry points can be built into the housing to allow for easy access.

    Standing up to corrosion

    For corrosive processes, the trend has been to fabricate the filter and housing from nonmetallic materials whenever possible. Typically, the HVBF filter housing is made from fiberglass-reinforced plastic (FRP), while internals can be a combination of FRP and such polymers as polypropylene and high-density polyethylene. The few internal metallic components necessary for successful operation of the filter are usually made of Hastelloy, Titanium, coated stainless steel or other materials appropriate for the given application.

    As with all enclosed filters, those components that require the most maintenance, including filter drives, bearings, valves and actuators, should be placed outside the filter housing. Not only does this placement eliminate potential corrosion, but it minimizes the need for operators to access the inside of the housing.

    Gas-tight operation

    Gas-tight mode primarily is used for solvent applications, but also can be used with an inert atmosphere in potentially explosive or hazardous situations where absolutely no vapor can be emitted from the filter. To achieve the inert atmosphere, the filter is set up as a closed loop, together with the ancillary equipment and the vacuum source, with the inert atmosphere fully contained within the loop.

    All seals on the filter, windows, shafts and other components are designed to withhold the internal pressure -- still close to atmospheric -- inside the housing. A small amount of inert gas exits the filter with the cake, but a pressure-control device ensures that the correct level of makeup gas is introduced into the housing to compensate.

    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.

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