User:Tfausloos/Automatic Pressure Filter

Automatic Pressure Filter
Key Words: Automatic Pressure Filter, Flat Bed Pressure Filter, Indexing Filter, Belt Filter Press, Cake Filtration, Backwash Filter

The Automatic Pressure Filter (APF) has been recognized as an effective means of separating solids from liquid in the machine tool/automotive industry and chemical processing industry for more than 40 years. The APF’s mechanical simplicity results in low purchase costs. The APF’s payback is usually found when evaluating it’s automatic features, dry solids discharge, reduced need for operator attention, reduced exposure to hazardous materials, and solids waste disposal.

Operation

The Automatic Pressure Filter (APF) consists of two horizontal compartments – an upper chamber (slurry compartment) and lower chamber (clean filtrate compartment). These chambers are separated by a filter belt or filter media that lays flat across the interface between the two chambers. At the start of the filter cycle the upper chamber descends down over the lower chamber and forms a seal around the perimeter of the filter media. The slurry is then pumped into the upper chamber and forces the liquid through the filter media that is now holding back the solids. The filtered liquid is collected in the lower chamber and quickly drained out. During the feed step solids accumulate on top of the filtering media to form what is known as a “cake”. By continually passing the slurry through the depositing solids the now growing “cake” now functions as the primary filter medium. This process is known as “cake filtration”, which a very efficient and economical means of separation. As the cake increases in depth the filter pressure rises until the pressure in the upper chamber reaches 30-50 psi. At that point feed flow is stopped and pressurized air or inert gas is fed into the upper chamber to dry the filter cake. After a timed period the upper chamber is then pneumatically lifted and the filter cake is discharged using a conveyor drive or by a re-roller drive pulling media through. New or cleaned filter media is automatically replaced and a new filtration cycle is automatically restarted. The dry filter cake and filtrate is collected and handled by the user. The entire process is usually PLC controlled. Filter aids, stabilizing agents, or flocculants can be added in-line, if required. A schematic of the pressure filter is provided in Fig. 1.

The APF can be set up to utilize Reusable or Disposable Media. A Reusable Media or often referred to as the filter belt, is selected for applications where solids easily separate from the media and belt cleaning is effective. Belt replacement is fast and inexpensive when compared with other types of belt filters. A Disposable Media is used where the solids have poor media release or where filtrate quality is critical. Filter media is available in rolled form. The filter can be set up for easy conversion between reusable and disposable media if process conditions change. With proper filter media selection the Automatic Pressure Filter can remove solids ranging in size from over 1000 microns (strainer) down to 0.5 microns (microfiltration). If the slurry can be pumped the filter can be designed to handle it.

Process Conditions

The filter can be designed to handle a variety of process conditions. For corrosive solutions wetted surfaces can be provided with coated carbon steel, stainless steel, alloy 20, C-276 and titanium. With proper material selection the APF can work in operating temperatures up to 300 degrees F. with special applications up to 400 degrees F. For volatile environments this type of filter can be fitted with explosion proof controls and housing ventilation. Cake washing and inert gas drying features can be provided.

Automatic for Operator Safety

The filter operates on a batch basis because of the need to stop feed flow to dry and discharge solids. The filter can handle continuous flows by either duplexing or by providing the sufficient surge capacity in the feed hold tank. The automatic sequencing reduces or eliminates the need for operator attention. Sequencing can be interrupted by the operator or automatically through remote process signals such as tank level. This automatic feature frees up the operator as well as eliminates unnecessary exposure to the solid and liquid. The dry solid discharge feature minimizes liquid loss and makes for easier solids handling for disposal or continued processing. Cake dryness depends on the solid particle being removed and can range between 25- 45% for metal hydroxides to over 95% for crystalline and granular particles.

Applications

Suitable applications include straining, crystallizer blowdown, evaporator purges, reactor precipitates, clarifier underflow sludge purges, wet scrubber blowdown, backwash sluices from pressure leaf and tubular filters, membrane pre-filtration, cooling tower purges and low level radioactive processes. This filter type has been successfully demonstrated in the Superfund Innovative Technology Evaluation program and is an EPA approved technology.

Installation

The filter usually comes as a packaged system with all necessary valving and controls for automatic operation. No special foundations are required for support mounting.

Testing

The same solid-liquid separation principles and limitations that apply to filtration also apply to the APF. With any filter application the solution should be tested by the equipment manufacturer to determine fluxrate, filtrate/solid quality, media selection, and any pre- or post treatment.

References Dupont-Oberlin Microfiltration Technology Applications Analysis Report, EPA doc 540-A5-90-007 October 1991