Vacuum rotating filters

Vacuum Filters, Continuous Type.— Vacuum leaf filters which dipped into a tank or basin of the material to be filtered were the predecessors in point of time of the continuous rotating vacuum filters of today, practically the only type of continuous vacuum filter which exists today. The old leaf type of vacuum filter was intermittent in operation and one of two sets of operations were necessary with them (omitting for brevity the washing operations). In one set the leaves were stationary and after the tank or basin was filled suction was applied and a cake of the proper thickness accumulated. It was then necessary to run off the balance of the material in the tank or basin, reverse the air current and blow off and wash out the cake which had accumulated. After this was done fresh material was allowed to enter and the cycle of operations was repeated- In the other set of operations and better from a mechanical point of view the filter leaves with their accumulated cake were removed by machinery and after the cake was dropped the leaves could be returned to the tank for further solid accunaulation. 

After the enzymatic treatment the juice can be clarified. Flocculation aids and fining aids (gelatine, silica sol, etc.) help to coagulate the cloudy substances and facilitate their separation by settling and filtration. Bentonites can be used to eliminate proteins and other cloudy substances. Filtration (Kieselgur precoat filtration with plate and frame filters, rotating vacuum filters, and sheet filters as police filters ) is used to produce a crystal clear juice. 

Ultrafiltration and microfiltrafion are employed successfully on an industrial scale for the clarification of juices (e.g. apple juice, pear juice, etc.). Ultrafiltration saves filter aids and fining aids which have to be used when operating conventionally with rotating vacuum filters or plate filters and precoat filters. On the other hand, the power consumption of an ultrafiltration system is high and so is the replacement cost of membranes. 

For separation of large volumes of suspension, vacuum rotating filters are recommended (Figure 2.5). The operation of the rotating vacuum filter is characterized by producing dried cakes of small thickness (less than 1 cm). Furthermore, the cake is easily removed and the operating parameters are easy to control. 

Among the disadvantages of the rotating vacuum filter are the high energy consumption due to the vacuum pump and the constant washing required to the filter media. 

Figure 2.5 Schematic diagram of a rotating vacuum filter, modified from Geankoplis (2003). ...

Horizontal Rotating Pan Filters. These filters (Fig. 10) represent a further development of the tipping pan filter for continuous operation. They consist of a circular pan rotating around the central filter valve. The pan is divided into wedge-shaped sections covered with the filter medium. Vacuum is appHed from below. Each section is provided with a drainage pipe which connects to a rotary filter valve of the same type as in dmm filters. 

The disk filter is similar to the dmm in operation, but filtration is conducted using a series of large diameter filter disks that carry the filter medium on both sides of the disk. They are connected to the main horizontal shaft and partly immersed in the feed slurry. The central shaft is connected by a set of valves which serve to provide vacuum and air as in dmm filters. As the disk sections submerge during rotation, vacuum is appHed to form a cake on both sides of the disk. The cycle of operation is similar to that in a dmm filter. One unit can have as many as 12 disks of up to 5-m diameter. Disk filters, both compact and cost effective, are used extensively in the iron ore industry to dewater magnetite concentrates. 

There are two commercial solvent crystaUi2ation processes. The Emersol Process, patented in 1942 by Emery Industries, uses methanol as solvent and the Armour-Texaco Process, patented in 1948, uses acetone as solvent. The fatty acids to be separated are dissolved in the solvent and cooled, usually in a double-pipe chiller. Internal scrapers rotating at low rpm remove the crystals from the chilled surface. The slurry is then separated by means of a rotary vacuum filter. The filter cake is sprayed with cold solvent to remove free Hquid acids, and the solvents are removed by flash evaporation and steam stripping and recovered for reuse (10). 

Disk Filters A disk filter is a vacuum filter consisting of a number of vertical disks attached at intervals on a continuously rotating horizontal hollow central shaft (Fig. 18-127). Rotation is by a gear drive. Each disk consists of 10 to 30 sectors of metal, plastic, or wood, ribbed on both sides to support a filter cloth and provide drainage via an outlet nipple into the central shaft. Each sector may be replaced individually. The filter medium is usually a cloth bag slipped over the sectors and sealed to the discharge nipple. For some heavy-duty applications on ores, stainless-steel screens may be used. 

Procedure 2 The culture medium was frozen at -18°C for 24 hr s. The precipitated polysaccharide was filtered through a cloth, washed as in Procedure 1 and dried (Sample 2). The filtrate was concentrated 10 fold on a rotation vacuum - evaporator at 45°C Then 96 % ethanol was added to the concentrated filtrate and the sample was kept overnight at 4°C. The precipitated polysaccharide was filtered, washed as in procedure 1, and dried (Sample 3)... 

Filtration separates components according to their size. Efficiency depends on the shape and compressibility of the particles, the viscosity of the liquid phase and the driving force, which is the pressure created by overpressure or by vacuum. Filtration can be performed either as dead-end filtration, where the feed stream flows perpendicular to the filter surface (Lee, 1989) or as tangential flow filtration, where the feed stream flows parallel to the filter and the filtrate diffuses across it. Examples of the former are the continuous rotaiy vacuum dram filter, where a rotaiy vacuum filter has a filter medium covering the surface of a rotating drum and the filtrate is drawn through the dram by an... 

Figure 11.11. Continuous horizontal vacuum filters especially suited to free settling and draining solids, (a) Principle of the conveyor belt filter units may operate up to 0.S m/sec with a cycle time up to 10 min and produce cake thicknesses up to 15 cm. (b) Showing the construction of a grooved rubber belt support for the filter cloth of the belt filter (Purchas, 1981). (c) Rotating horizontal vacuum filter the unit has readily accessible piping and is amenable to thorough washing of free draining solids (Dorr-Oliver Inc.).

Vacuum filters include the nulschc filter, enclosed agitated vacuum fillers, the vacuum leaf filter, the tipping pan filter, horizontal rotating pan filters, horizontal belt vacuum fillers, rotary vacuum drum lilters. and rotary vacuum disk filters. 

The liquid, together with the crystals, is run into a crystallizer equipped with a lead cooling coil and a stirrer rotating at a low speed. Here the rest of the picric acid is precipitated and then separated from the spent acid on a vacuum filter. 

Figures 7.5 to Figure 7.7 pertain to the use of rotary vacuum filters in vacuum filtration. In vacuum filtration, a drum wrapped in filter cloth rotates slowly while the lower portion is submerged in a sludge tank (Figure 7.7a). A vacuum applied in the underside of the drum sucks the sludge onto the filter cloth, separating the filtrate and, thus, dewatering the sludge.

A popular variant of the rotating drum vacuum filter has been widely applied especially for the palm oil industry. The Florentine vacuum filter uses a horizontal stainless steel perforated belt with vacuum applied under the belt. This design includes a recycle from the first filter section, which is said to allow the first layer of stearine to act as a continuous precoat (25). 

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