The many faces of horizontal vacuum-belt filters

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.

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  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.