Rotary Vacuum Drum Filter

From the book, Fermentation and Biochemical Engineering Handbook  ROTARY VACUUM DRUM F!TER 

Raw fermen fermentat tation ion broth broth is an example example of a large large volume volume produc productio tion. n. Rotary Rotary drum drum vacuum filters (RVF’s) have traditionally been found in this service. Slowsettling materials or  more difficult filtrations with large scale production re!uirements are typical applications for this type of e!uipment. "he basic principle on an RVF is a hollow rotating cylindrical drum driven by a variable speed drive at #.$$# revolutions per minute. %nethird of the drum is submerged in a slurry trough. &s it rotates' the mycelia suspension is drawn to the surface of the drum by an internal vacuum. "he surface is the filter medium mounted on top of a grid support structure. other li!uor and wash are pulled through the vacuum line to a large chamber and evacuated by a pump. pump. &ppli &pplicabl cablee to a broad broad range range of proces processes ses'' e.g.' e.g.' pharma pharmaceu ceutic tical' al' starch starch'' cerami ceramics' cs' metall metallurg urgica ical' l' salt' salt' etc.' etc.' many many variat variation ionss of the RVF have been been develop developed' ed' however however'' the fundamental cylinder design remains the same. "he cylinder is divided into compartments lie pieces of a pie (see Figure *.+)' and drainage pipes carry fluid from the cylinder surface to an internal manifold. Filter diameters range from three to twelve feet' with face lengths of one to twentyfour feet' and up to $### ft+ of  filtration area. Filtration rates range from , -/ per s!uare foot to $,# -/ per s!uare foot. oisture levels are' of course' dependent upon particle si0e distribution and tend to range from +,1 to 2,1 by weight and cae thicness tends to be in the $3*$3+ inch range' as most applications are for slow filtering materials. 4ith the exception of the precoat applications' RVF’s do not usually yield absolutely clear filtrate. <hough still widely used' rotary vacuum filters are' in some cases' being replaced by membrane separation technology as the method of 

choice for clarification of fermentation broths and concentrating cell mass. embranes can yield more complete filtration clarification' but often a wetter cell paste. "he drum is positioned in a trough containing the agitated slurry' whose submergence level can be controlled. &s the drum rotates' a panel is submerged in the slurry. "he applied vacuum draws the suspension to the cloth' retaining solids as the filtrate passes through the cloth to the inner piping and' subse!uently' exiting the system to a vaporli!uid separator with high3low level control by a pump. 5ae formation occurs during submergence. %nce formed' the cae dewaters above the submergence level and is then washed' dewatered and discharged. 6ischarge mechanisms will vary depending upon cae characteristics. Friable' dry materials can use a 7doctor8 blade as in Figure *.9. 6ifficult filtrations re!uiring thinner caes incorporate a string discharge mechanism. "his is the primary method for starch and mycelia applications. & series of $3+ inch spaced strings rest on the filter medium at the two o’cloc   position. "he cae is lifted from the drum as shown in Figure *.:. Fermentation broths containing grains' soybean hulls' etc.' are applications for this type of discharge mechanism. "he solids may be used for animal feed stoc' or incinerated. String or belt discharge mechanisms facilitate cae removal and' therefore' can eliminate the need for filter aid. 5ontinuous belt discharge (Figure *.,) is employed for products that have a propensity for blinding the filter  medium. & series of rollers facilitate cae removal in this case. recoated rotary vacuum drum filters (Figure *.;) are used by filtering slurry of filter aid and water first' then subse!uent  product filtration. 6ifficult filtering materials' which have a tendency to blind' are removed with a doctor blade. recoat is removed along with the slurry to expose a new filtration surface each cycle. "he progressively advancing blade moves #.#, to #.+ mm per revolution. Vacuum is maintained throughout the cycle' instead of
retained. %nce the precoat is expended' the RVF must be thoroughly cleaned' and a fresh coat reapplied.