BHS-Sonthofen Inc.

A Review of Filter Press Basics Versus Alternative Technologies

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Source: BHS-Sonthofen Inc.

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INTRODUCTION
Filtration experts, over the years, have discussed and debated filter presses and have indicated that soon they would be a thing of the past; the last filler press would be replaced by more modern equipment. Given that filler presses were amongst the oldest mechanical dewatering devices, this was a fairly plausible suggestion. However, now. almost half a century later, any chemical engineering exhibition shows that filter presses are alive and well and are going to be around for years to come. True, they look different from the ones in the 1950's, but essentially they are still the same device. Their continued presence is a tribute to the filler press manufacturers who embraced new materials and upgraded the mechanics and configurations. However, the basics are still the same and much of the issues surrounding filler presses remain valid.

This article discusses the perceived shortcomings of the filter press as well as batch or continuous filtration alternatives to them based upon the process conditions and requirements of the chemical operation.

HISTORICAL PROBLEMS WITH FILTER PRESSES

  • The cake has lo be scraped manually oul of Ihe frames
  • The frames cannot be too thin otherwise the feed ports block. This point applies to the plate and frame presses. For the rest, almost all filler presses now have recessed or chamber plates.
  • If the plates are too large or there are too many of them, the weight is excessive. Almost every plate nowadays is made out of a plastic material, usually polypropylene which also solves many chemical resistance problems, although this does pose limitations on temperatures of the feed, the wash liquids and the cleaning liquids.
  • If the cake sticks to the cloth, there is no discharge. The much better filler cloths have reduced this problem considerably; in addition most manufacturers offer a range of cloth scrapers, plate 'bumpers'', plate shakers or any other mechanical devices to induce the cake to drop away.
  • If some of the cake does, in fact, stick to the cloth, especially the edges, the next cycle may result in leaks and the plates may distort when closing the plate pack.
  • Many manufacturers offer cloth-washing systems with trays to channel the water away, whereas others offer compensators to allow for misalignment.
  • If the filter cycle is a bit loo short, sloppy cakes with wel centers may result.
  • However, advanced electronics allow for a fairly accurate interplay between time, pressure, backpressure, filtrate clarity etc. so that this risk is reduced.
  • Opening and closing the filter press is time consuming.
  • However, almost all modern presses are equipped with automatic plate moving systems, which select one or several plates, allowing them to discharge the cake.
  • If there is only a partial batch remaining in the reactor, the chambers cannot be filled and the result is a partially filtered mess, which cannot be washed or dried. If the press is designed with a membrane compression device, this can be overcome.
  • Admittedly this may come at the cost of reduced filtration area and definitely at the cost of reduced chemical resistance, higher capital cost and more maintenance, but it does resolve the problem.

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