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Frequently Asked Questions

Here is a list of questions about the BHS Thin-Cake Filtration Technologies that we commonly receive. The answers below provide general information and links to other parts of our website. For answers to your specific application, please contact us via the website or by telephone. We look forward to beginning our process discussions.

1. General Filtration Questions

2. Continuous Filtration Questions

3. Batch Filtration Questions

1. General Filtration Questions

1.1: What is the definition of thin-cake filtration?

Thin-cake filtration is defined as cakes that are normally between 3 mm to 50 mm. The benefit of thin-cake filtration is that the filtration can be done at high pressures or high vacuum without the potential of cloth blinding. If a cake is compressible such that a nutsche filter-dryer, filterpress or centrifuge has a long cycle time, then the process solution would be thin-cake filtration, as described in the “Thin-Cake Filtration” article.

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1.2: How can we test for thin-cake filtration?

The BHS pocket leaf filter (PLF) is ideally suited for laboratory tests. The PLF can be used to test thin-cake filtration for pressure or vacuum operation. It is jacketed so hot (150 º C) filtration or cold filtration can be tested. Different cake thickness and different filter media are also included as the test parameters. Finally, cake washing and drying can also be tested on the PLF. BHS conducts these tests in our process laboratories in Charlotte, NC or at your facility.

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1.3: Can cake washing be conducted on BHS filters?

Yes, all BHS filters provide a cake washing process step. With thin-cake processing, cake washing, especially displacement washing, is very efficient, as described in the “Cake Washing” article.

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1.4: How is the cake discharged from the BHS filters?

The five patented BHS filters all use different discharge techniques depending upon the filter technology, the characteristics of the cake and the specific process and downstream equipment at your plant. As a general overview, the BHS technologies offer automatic discharge, complete discharge with no residual heel and full containment to eliminate operator exposure. Cake discharge can be a wet cake, dry cake or a slurry.

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1.5: With five technologies, how do we select the optimum choice for our plant?

BHS always recommends beginning the process discussion with PLF testing (Please see question 2.). Our process engineers then ask specific questions from the data application sheet.

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1.6: How do our engineers categorize the BHS filters?

The BHS filters are categorized as thin-cake technologies. The units can handle applications from high solids (up to 60%) slurries to clarification for slurries with trace amounts of solids. They can be pressure or vacuum filtration for continuous or batch operations. The BHS process engineers are experts in determining the optimum technology for your application.

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1.7: What applications are the best for the BHS thin-cake technologies?

The BHS technologies have been installed in many industries since the 1950’s. We serve three general marketplaces: chemical, food & pharmaceutical and energy & environmental.

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1.8: We use centrifuges, nutsche filters, filter presses as well as manual plate filters, bag filters and other conventional separation equipment. What would BHS recommend to improve our operation?

All of the BHS pressure and vacuum filtration technologies are alternatives for your current equipment. Once we know the percent solids in the slurry feed as well as several other process parameters, we can assist your team. Please submit the application data sheet and one of our application sales engineers will contact you to begin the process discussions.

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1.9: We have a process that has a cycle time of over 24 – 48 hours for filtration, washing and drying. What would BHS recommend?

As discussed in question 1, thin-cake processing in the 3 mm – 50 mm range is a solution to this process problem. Please submit the application data sheet and one of our application sales engineers will contact you to begin the process discussions.

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1.10: We have a batch process that would benefit from being upgraded to a continuous process. What would BHS recommend?

The BHS rotary pressure filter or continuous-indexing vacuum belt filter are solutions to this process problem. Please submit the application data sheet and one of our application sales engineers will contact you to begin the process discussions.

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2.1. Continuous Filtration Questions: Rotary Pressure Filter

2.1.1: Is the BHS rotary pressure filter the same technology as the FEST filter?

Yes. The FEST rotary pressure filter was invented in the 1950’s by Dr. Fest who was employed by BHS in Sonthofen, Germany, which is our worldwide headquarters. There are over 1000 rotary pressure units in operation producing 1-2 Kg/hour to over 100 tons / hour of dry cake. The “FEST” name was dropped from the product name in 2005. BHS retains all of the patents and rights from the original design through the subsequent improvements over the past 50 years. Many of our earlier clients still refer to the units as the FEST filter.

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2.1.2: What operations can be conducted on the RPF?

Continuous pressure filtration operations up to 6 bar are conducted. After pressure filtration, cake washing including counter-current washing, and drying are accomplished.

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2.1.3: What cake thickness is possible on the RPF?

The cake thickness can be set at 3 mm and up to 6 inches of cake for fast filtering products.

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2.1.4: What production rates are possible?

This, of course, depends upon the specific product parameters. The BHS units are from 0.12 m2 to 7.6 m2 of filter area, which can result in 1-2 kilograms of dry solids/hour to over 100 tons/hour.

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2.2. Continuous Filtration Questions: Vacuum belt filter

2.2.1: What is the principle of operation of the continuous – indexing vacuum belt filter?

In the BHS vacuum belt filter, the slurry feed is continuous while the filter cloth is moved intermittently and the trays fixed in place. For cloth movement, the vacuum is broken by butterfly valves and the cloth moves (indexes), by pneumatic cylinders, in the space above the vacuum trays. The belt moves along and relaxes but never touches the trays, which allows for long filter cloth life.

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2.2.2: Do the trays move?

No, the trays are fixed in place and every tray has a filtrate outlet pipe. Compared with moving tray designs, the BHS unit requires no additional hardware such as rails, rollers or flexible pressure-vacuum rated hoses.

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2.2.3: Does the BHS Vacuum Filter require a rubber carrier belt?

No, the BHS filter eliminates the need for rubber carrier belts and motor/VFD to move the filter belt. The benefit is higher vacuum levels for increased filtration rates and much lower operating costs and spare part costs.

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2.2.4: Does the BHS Vacuum Filter require a motor and VFD?

No, the filter belt is moved by pneumatic cylinders.

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2.2.5: What is the benefit of indexing?

For the process operation, due to the stepwise operation of the belt, washing and drying efficiencies are maximized, as the belt is stopped and the mechanism of “plug-flow” for gases and liquids is in effect. Finally, because the trays are fixed, the mother liquid and the wash filtrates can be recovered individually and recirculated and/or recovered and reused which allows for a more efficient overall operation.

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2.2.6: Can the Indexing Vacuum belt filter replace a rubber carrier vacuum belt filter?

Yes, the BHS vacuum belt filter is a direct replacement for a rubber carrier vacuum belt filter.

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2.2.7: What materials of construction can be used?

The units can be enclosed, dust-tight, or pressure tight for inerting or gas-blanket and are manufactured in stainless steel, Hastelloy, synthetic or reinforced - synthetic components depending upon the solvents, solids, temperatures, etc. BHS completes the turnkey systems with feed pumps, liquid transfer and recirculation pumps, separator and receiver tanks, liquid ring vacuum pumps, instrumentation, pre-piped and pre-wired skids and PLC control systems. Heating and cooling packages for liquids and/or gases and solids handling can also be included.

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3.1. Batch Filtration Questions: Candle Filter

3.1.1: What operations are conducted on the BHS Candle Filter?

The BHS candle filter provides for pressure filtration, cake washing, drying, reslurry and automatic discharge as well as heel filtration in an enclosed and pressure vessel.

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3.1.2: Can you describe the candles?

The filter candles consist of three components: single-piece filtrate pipe, perforated core with outer support rods and filter media. The filtrate pipe is the full length of the candle and ensures high liquid flow as well as maximum distribution of the gas during cake discharge. The perforated core can be a synthetic material, stainless steel or Hastelloy and is designed for the full pressure of the vessel. The outer support rods provide for an annual space between the media and the core for a low pressure-drop operation and efficient gas blowback for cake discharge.

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3.1.3: What type of filter media can be used and what is the finest removal efficiency:

The filter media is a synthetic media such as Teflon, polyester, polypropylene, PEEK, duplex media, Nylon and others. The clean removal efficiency to 1-3 microns but as the cake builds up on the candles; removal efficiency can be below 0.5 microns.

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3.1.4: How are the Filter Vessel & Candle Registers constructed?

Vessel is constructed of various types of stainless steel, Hastelloy or can be lined with EPDM or other similar coating. Within the vessel are candle registers. Each candle is connected to a register with a positive seal to prevent bypass. Each register may contain from 1 – 10 candles depending upon the filter size. The registers convey the liquid filtrate as well as the pressure gas for blowback. Each register is controlled with automated valves to ensure optimum flow in both directions.

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3.1.5: What are the Automatic Process Cycles?

• Filling: The slurry feed enters the bottom of the filter vessel.
• Filtration: The slurry is pumped under pressure into the vessel. Cake will deposit on the outside of the candle; the separated filtrate will flow through the filtrate pipe and the registers. This process continues to a maximum pressure drop, the maximum cake thickness, or the minimum flow.
• Washing: Displacement washing or recirculation washing.
• Drying: Blowing gas, steam or “shock” drying.
• Heel (Falling-Film) Filtration: The liquid remaining in the vessel cone after filtration or washing is completely filtered.
• Cake Discharge: Gas flows through the register pipes, and down the filtrate pipe. The filter media gently expands allowing for cake discharge. Alternatively, the cake can be discharged as a slurry.

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3.1.6: What are the Process Benefits?

• Full containment of solids, liquids and vapors
• Automatic operation
• Replaces filter presses, bag filters & manual plate filters
• Eliminate pre-coat and/or body feed
• Clean-in-place
• Recovery of catalysts and activated carbon
• Clarification of pharmaceutical and chemical solutions

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3.1.7: Does BHS manufacture completely skid-mounted systems including piping, valves instruments and PLC controls?

Yes, this is a standard scope of supply based upon an approved P & ID.

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3.2. Batch Filtration Questions: Pressure Plate Filter

3.2.1: What operations are conducted on the BHS Pressure Plate Filter?

The BHS pressure plate filter provides for pressure filtration, cake washing, drying, reslurry and automatic discharge as well as heel filtration in an enclosed and pressure vessel.

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3.2.2: What is the Description and Operation of the Pressure Plate Filter?

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3.2.3: How is the Cake Discharged?

As shown in Figure 5, the motors on the top of the filter operate at different frequencies and the plates gently vibrate for cake discharge. The plates vibrate in the vertical and horizontal planes and the solids are conveyed in an elliptical pattern to the outside of the vessel. Gas assist helps in the discharge process. There are no rotating plates; gears or bushings and mechanical seals are not required.

Cake Discharge

Figure 5: Cake Discharge in the Pressure Plate Filter

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3.2.4: How does BHS select the candle or the pressure plate filter technologies?

BHS examines the cake structure and process parameters to determine the optimum technology. The major difference between the two technologies depends on the cake structure that is formed. Some cakes are better handled in the horizontal and some in the vertical.

Cake Thickness and Filtration: The candle filter is limited to cake structures that can be formed to about 5 – 20 mm. The pressure plate filter can handle cakes up to 75 mm. Both units can conduct filtration up to 150 psig.

Filter Media: The candle filter uses only synthetic media with a clean removal efficiency from 1 –3 micron range and finer down to 0.5 microns. The pressure plate filter can also use metal media. For the pressure plate filter, the clean micron range removal efficiency is also 1 – 3 microns and finer.

Cake Washing: If the process requires washing to remove the API from the solids, then generally the pressure plate filter is a better alternative. If washing is not as critical, then the candle filter may be the optimum technology for clarification and recovery.

Heel Filtration: The remaining liquid in the vessel (liquid heel) after filtration or washing can be removed from the candle filter or pressure plate filter by circulation, heel filter in the cone of the vessel, or additional heel filter plates in the pressure plate filter.

Cake Drying: The candle filter can produce cakes with approximately 10% moisture. This moisture level depends upon the specific cake but the moisture lower limit is that moisture just above the cake cracking point. The pressure plate filter can produce bone dry cakes.

Cake Discharge: Both designs can easily discharge most cakes equally with no residual heel.

Clean-In-Place (CIP)/ Steam-In-Place (SIP): Both units conduct CIP / SIP operations in identical manners by filling and circulating cleaning fluids while blowing gas in the reverse direction to the filtration direction, which crates a turbulent mixture or a quasi-ultrasonic cleaning effect. The pressure plate filter further enhances this operation with plate vibration.

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3.2.5: What is the difference between the BHS pressure plate filter and the “spinning disk” filters?

Spinning disk filters are also commonly used for clarification. While these units overcome some of the special handling requirements of manual filters, they add mechanical complexity to the process as well as special cleaning requirements. Spinning disk filters require drive motors, gear box assemblies (including gear box housing, gear reducers, bearings, shaft bearing arrangement and bushings, torque loadings, etc.), mechanical seals (either single or double with special maintenance and cleaning) and unique installation concerns such as center of gravity due to the spinning plates, static and dynamic balancing and bearing lubrication and design to ensure no exposed threads and cleanability issues, and overall maintenance of these components.

The BHS units use vibrating plates which eliminate motors, gears, rotating seals, etc. and many of the operational issues of the spinning units.

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3.3. Batch Filtration Questions: Autopress Filter

3.3.1: What is the description and operation of the BHS AUTOPRESS Pressure Filter?

The AUTOPRESS technology provides for thin-cake, as small as 5 mm, and multipurpose production in a single vessel. Pressure filtration, forward flow or counter-current washing and vacuum or hot-gas drying steps are tailored to meet specific batch process requirements.

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3.3.2: What is the design of the AUTOPRESS?

The AUTOPRESS consists of specially designed circular filter plates with synthetic or sintered metal filter media. The filter plates are sealed to each other to eliminate solids, gas or liquid bypass. The filter plates are contained in a pressurized filter housing where a gas-inflated membrane seals the annular space between the housing and the filter plates. All operations are contained and can be from full vacuum to 150 psig. The entire filter housing is enclosed in a pressure-tight outer housing for complete product containment.

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3.3.3: What are the operating steps of the AUTOPRESS?

The operation of the AUTOPRESS begins with slurry filling and pressure filtration. The cake can then be mechanically compressed on spacer elements to eliminate cracking to ensure maximum washing efficiency in the forward or counter-current direction. Finally, the cake can be dried either by vacuum or blowing gas through the cake in either direction. Final moisture contents of less than 0.1 percent have been achieved. This gentile drying without agitation or tumbling is important for fragile crystals and thixotropic cakes.

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3.3.4: How is the AUTOPRESS cleaned?

The AUTOPRESS is easily cleaned with CIP/SIP systems and is used for specialty products and chemicals and bulk and final pharmaceutical products. The AUTOPRESS has been FDA validated with automatic cleaning to less than 1 ppm residual product cross-contamination. Riboflavin tests document this performance.

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3.3.5: What are the process benefits compared with nutsche filter-dryers and centrifuges?

The fully automatic solids discharge system of the AUTOPRESS eliminates residual product heels, which is an advantage over “nutsche” type filters. The AUTOPRESS also provides for increased flexibility, higher filtration areas and is more easily installed and controlled than peeler or inverting-basket centrifuges. It is an alternative to manual operations where solids are transferred to tray, shelf or vacuum dryers or into drums.

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