In many small to medium sized production processes, which aretypical in industries like pharmaceuticals, food and beverageand waste minimization, solid-liquid separation in the form oftraditional cake filtration is often a critical operation. Because ofthe amount of material being used in processes of this sizetraditionally only batch equipment was feasible. Unfortunately,this does not allow these users to take advantage of the processimprovements associated with continuous cake filtrationequipment which are available to those in larger operations. Tofill this gap, the disposable rotary drum filter (Figure 1) has beendeveloped. It is small enough to use for traditionally batchquantities, yet operating in a continuous manner it delivers allthe advantages of continuous-mode operation.

How it worksThe disposable rotary drum filter operates on the same basicprinciples as all vacuum drum filters. The filter is made up of aninner rotating drum encased in an outer cylinder, of which thebottom half serves as a trough. The slurry enters the outercylinder and settles in the bottom. The level is controlled so thatthe inner drum's surface is covered to 30-35%. The surface of theinner rotating drum is covered with a filter fabric. A vacuum ispulled on the inside of the rotating drum, pulling filtrate into theinside and depositing the solids on the fabric surface. The filtrateis pulled into channels in the interior of the drum and routed outof the device. The system is made continuous by rotating thedrum. As it rotates out of the liquid, the solids start to dry as airpasses though them via the vacuum on the rotating drum. Whenthe drum completes its revolution the surface comes in contactwith a scraper or knife blade, which removes the solids that haveaccumulated. The drum continues rotating back into the slurryand the process starts all over again.

The key difference in the disposable rotary drum filter is thatit has a drum that is only 8 cm in diameter and 11 cm in length.The whole device measures only about 15 cm in diameter by 18cm long. The small size of this device allows it to be fabricatedout of low-cost injection moulded plastics, and therefore can beoffered as a single-use, disposable device targeted at operatingcontinuously to process a single batch of material. Of course themain advantage in moving from a batch process to a continuous

one is that theactual solid-liquidprocessingequipment can besignificantlysmaller than itstraditional batchequipmentequivalent.Essentially, inconverting frombatch equipmentto continuousones, you aretrading equipmentsize for operatingtime. As a result,when looking atthe processingcapacity of thedisposable rotarydrum filter, bothflow rate andprocessing time must be examined, in contrast to batchequipment where gross size is the primary consideration.

The disposable rotary drum filter has a typical processingrate, depending on the slurry being processed, of 1 l/min to 3 l/min. This can a give processing batch capacity of over 700 l ina typical eight hour shift (Figure 2).

Enhancing processing capabilitiesAs noted above, the main advantage of this new drum filter is itsability to process small to medium sized batches of slurry in arelatively small footprint using continuous processingmethodology. This small footprint translates into reducedcapital, operating, and facility costs initially. In addition to this,certain inherent properties of continuous processes produceadded advantages.

One of these advantages is closed equipment. The size andcontinuous operation of this filter, make it suitable for totally or

26 November 2003 ISSN 0015-1882/03 2003 Elsevier Ltd. All rights reserved

Small Scale Continuous CakeFiltration using the Disposable

Rotary Drum Filter

In many small to medium sized production processes batch equipmenthas traditionally been the only feasible option because of the amount ofmaterial being handled. This has prevented users from taking advantageof the process improvements associated with continuous cake filtrationequipment. However, according to John Kossik this is about to change

with the introduction of the disposable rotary drum filter from Steadfast Equipment, USA.

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Figure 1: Althougth small, thefilter has the capacity of batch

equipment of a much larger size and cost.

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partially enclosed applications. This ability for enclosing thematerial throughout the operating process can have somesignificant advantages over other batch equipment that must beopened before and/or after the processing is completed. Significantadvantages in avoiding contact between the process materials andthe operations personnel can result. Limiting this exposure hasdistinct advantages in many industries. In pharmaceuticals it keepsbatches from being contaminated and in industries handlinghazardous or toxic materials it keeps personnel exposure to aminimum. As an added benefit, this process enclosure can limitthe heating, ventilation and air-conditioning (HVAC) requirementsrequired for the processing space, whether it was originally neededfor protection of the product or personnel, or both.

Another advantage of continuous operation is processingstability. Batch processes are in an unsteady state, with theconditions in the equipment changing as the processing progresses.In some cases, this unsteady state behavior results in a process thatis more difficult to control, making upsets more likely. Continuousprocesses operate in a constant or steady-state manner. This lendsitself to easier operation, minimizing upsets and maximizing theability to control the properties of the final product.

What specific uses?The introduction of this design opens up a number of processingpossibilities in a wide range of industries. Initial applications inpharmaceuticals and intermediates are first evident, as theseindustries have many processing needs entailing smaller batches.Applications from clarification of fermentation broths torecovery of precipitated or crystallized intermediates are onesthat first comes to mind. Outside these industries, the disposablefilter can be used to provide better modelling of large scalevacuum drum filters than simple static filtration tests. This couldbe of particular advantage in industries like starch processing.

The small size of this device can open up applications that werepreviously not feasible. Some that are possible include removal ofsolids from metal plating baths and caustic scrubber towers tolengthen the life of these solutions and minimize the amount ofhazardous waste that is generated. In an analogous fashion,applications in non-processing industries like inks and dyes, drycleaners and laundry facilities, paint reclamation and photoprocessing become economically possible.

Typical solid-liquid separation applications are just the initialpossibilities for this type of device. Its small size and continuousoperation also lend it to be the centerpiece of more extensive, non-traditional applications like continuous precipitation andcrystallization systems, continuous chromatography systems andcontinuous absorption systems. Moving continuous operations ofprocesses like these into a realm of smaller operations opens upgreat advantages specifically in industries like biotechnology.

Where not to use it?Although the small size and continuous operation of this filtermake it an ideal replacement for some processes that now rely onlarger more labour and capital intensive batch equipment, itcannot replace them all. There will always be processes where,either because of a large variability in input materials, specialhanding requirements or just the worth of the product itself, itwill always be more economical to stay with traditional batchsolid-liquid processing equipment. Also, for slurries with solidsthat form cakes that are highly compressible and/or cannot bedislodged easily by a simple scraper blade method, traditionalbatch filtration equipment or even centrifuges will still beneeded.

Concluding remarksFor many other applications though, the ability to use a piece ofequipment like the disposable rotary drum filter, opens up alarge number of previously unexplored processing possibilities.These exist both in traditional industries and those thatnormally do not think of themselves as in the market of findingnew ways of separating large amounts on solids form thesolutions that they handle day to day. Figure 3 shows a typicalprocess flowsheet for the disposable rotary drum filter.

Contact: John Kossik, Technical Manager,

Steadfast Equipment, Mill Creek, WA 98012, USA.Tel: +1 206 409 7594; Fax: +1 425 743 4420;

E-mail: jmk@steadfastequipment.com; Website: www.steadfastequipment.com.

Filtration+Separation November 2003 27

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Figure 2: Operating in a continuous fashion, itcan filter over 700 l of slurry, with an effective

filtration area of 275 ft2 over 8 hours.

800

700

600

500

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300

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100

00 2

Bat

ch c

apac

ity (li

ters)

Hours of Operation

Batch Capacity Filtration Area

Filtr

atio

n Ar

ea (s

q.ft.

)

4 6 8050100

150200250

300350400450500

10

Figure 3: The filter is the centrepiece of acontinuous solid-liquid separation process

that produces clarified filtrate & dried solids.

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