The Design and Operation of Rotary Drum Vacuum Filters - Assignment Example

Introduction The rotary drum vacuum filters are used in various applications and industries including food and chemical industries, and treatment of effluent. Wakeman and Tarleton (2005, p.244) has noted that there has been slight changes in the basic design of the filters since 1872, when the basic rotary drum filter became a patent. This paper examines a standard rotary drum vacuum filter and makes a discussion of the design and operation of the filter, while pointing out future development of the technology. The design A standard rotary drum vacuum filters consist of seven main parts, but primarily of a drum that rotates in a tub the fluid to be filtered. These components include a drum, a rotary valve, the piping, the drives, the agitator, a tank or trough, and the discharge mechanism. The drum and piping The drum, which is a hollow cylinder, is mounted on the support frame of the trough such that it dips horizontally into the trough and the slurry. It is positioned such that one third of it dips into the slurry or the liquid/solid suspension (Wakeman & Tarleton 2005, p.244). The face of the drum is divided into several lateral sections that hold the filter medium. In standard drum set up, internal drain-pipes link the lateral sections to the rotary valve assemblage, while in end- flow set up, the drain-pipes are placed at the ends of the drum such that vacuum is not applied to the inside part of the drum. Drainage grids of either nylon or polypropylene are fitted on to the to the drum face to allow the filtrates across the lateral sections to flow freely to the drain-pipes. A shaft passes horizontally at both ends of the drum so that its one end links to the drive gearbox, while the other has the drain-pipes bundle linked to the rotary valve assemblage. Figure 1. a diagrammatic representation of the rotary drum vacuum filter with a knife discharge As stated earlier, the surface of the drum or the drum deck is compartmentalized, and these compartments or sections further divided by grooved strips that run along the surface of the drum and around its circumference. The strips holds synthetic grids that cover the whole drum face while supporting the filter cloth. This filter cloth is fixed to the drum with special ropes that are inserted into the grooves. The rotary valve A valve together with a bridge setting is included in the drum vacuum filters to control the cycle sequence such that each of the drum sector goes through the blow, vacuum, and dead-time phases. In some filters, the valve has adjustable blocks, while in others it has a fixed ring. The adjustable block allow the optimization of the drum submergence as well as the form to dry ratio in the filtration cycle. Many drum filters include a valve that has three bridge brocks and a single-row pipe plate as illustrated below. Figure 2. A diagram showing a drum with a valve and the three bridge blocks One bridge separates the blow and vacuum zones. Another bridge, the dead zone bridge, opens to the vacuum during submergence of a compartment, while the start-up-assist bridge that controls the opening of the opening during start up. The agitator An agitator is used to gently excite the slurry suspension in the tank to allow a homogenous feed, while preventing sedimentation. The agitator oscillates between the bottom of the tank and the drum surface at approximately 12 to 16 cycles per minute. This frequency is suitable to so that cake formation is not obstructed (Wakeman & Tarleton 2005, p.244). The agitator is characterised by an assembly of two arms joined by a plough like shaft, with the arms being pivoted on the drum shaft. Push rods connected to the agitator drive are used to drive this assembly. The trough The trough or tank is used to hold the slurry or the solid/liquid suspension, and it also houses the agitator and the drum. In most drum vacuum filters, the trough is designed to allow about one third the drum to submerge in the suspension, although in some application it can submerge by about 50% (Svarovsky 2000, p.425). The trough also has a slurry feed connection, a drain connection and an adjustable spill-over box that allows regulation of the drum submergence into the slurry. The ends of the trough are higher so as to accommodate the drive shaft stuffing box on one end and the valve end box. Discharge mechanism. There are several types cake discharge mechanism that are used in the rotary drum vacuum filters. These include the scraper or knife mechanism, belt mechanism, string mechanism, roller mechanism, and pre-coat mechanism. The choice of these discharge mechanism is dependent on the type of cake formed. The scraper or knife discharge mechanism is suitable for cake that are thick or more than 3mm think and which release easily (Svarovsky 2000, p.425). Belt filter are usually used if the caked formed is thin and tends to adhere to the filtering medium (Wakeman & Tarleton 2005, p.246). The string discharge is almost similar to the belt discharge, but uses strings and is useful in sticky and string filter cakes. The roll discharge mechanisms uses driven rollers that are mounted to the surface of the drum. The speed of the roller is often higher than that of the drum to allow shearing of the cakes from the drum. Roll discharge is appropriate for sticky cakes (Wakeman & Tarleton 2005, p.246). Pre-coat filter mechanism are suitable for filtration of fine and highly dilute slurry, but one that has low solids (Svarovsky 2000, p.425). Operation The operation of the rotary drum vacuum filter is characterised by filtration cycles that involves continuous rotation of the drum and functionality of other components. The drum rotates during the cycles to allow initiation of various phases at their scheduled time, while the valve controls the subjection of the drum section into blow, dead-time, and vacuum zone. The revolving speed of the drum varies with the suspension filtration characteristics, although in most applications it lies between zero and two revolutions per minute (Wakeman & Tarleton 2005, p.244). The filtration cycle can be categorized into various phases namely the filtration or form phase, the initial and final dewatering or drying phase, the discharge phase, and the dead-time phase (Shammas & Wang 2007, p.16; Wakeman & Tarleton 2005, p.244). In the form or cake formation phase, a drum sector or compartment enters the submergence and vacuum is applied. The cake begins to forms until the compartment emerges out of the suspension. The part of the cycle that is available for cake formation is referred to as the effective submergence. Note that the duration of cake formation depends on the bridge settings, number of drum compartments, and the level of the slurry in the trough. It is also worth to note that as the filtrate is sucked, it is channelled to its receiver from where it goes to the a holding tank. The dewatering or the drying stage of the cycle begins when the drum sector emergences from the suspension. The final and initial dewatering phases can be put together, although this depends on the configuration of the bridge in the rotary valve (Wakeman & Tarleton 2005, p.246). Thus for applications that do not require washing the dewatering phase is continuous until vacuum is cut off, while in those that require washing, the washing duration separates the initial and the final drying when vacuum is terminated. The cake discharge phase follows the drying phase, and after vacuum in the drum compartment is cut off. In this stage, air blow begins to facilitate discharge of the cake. However, note that blow is applied on filters that have roll and scraper discharge mechanism and not to those using belt or string discharge. After the blow stops, the drum compartment passes via a zone that is blocked by bridges such that air is not drawn via the bare filter media that can result in vacuum loss on the whole surface of the drum. The dead-time refers to the time interval between the discharge of the cake and the start of the subsequent cycle. Future developments Future development of the rotary drum vacuum filter will aim to improve it performance in terms of the number of application it can handle and the rate of processing. In future filters, the design of its components such as the valves will require improvement to allow retention of higher vacuums. This will increase the rates of processing as well as the range of applications that the filter would handle. The development of the filter might also need to preserve the advantages of continuous pressure filters, while minimizing its disadvantages. A continuous pressure filters does not have interruptions in its operation. It has the advantages of low moisture in the cake and increased throughputs. The trend of vigorously developing continuous pressure drum filters will certainly continue in the future (Svarovsky 2000, p. 394), to minimize their limitations such as increased weight, high costs, problem in discharging the cake, and not be able to observe filtration progress. Conclusion The discussion has examined the standard design of a rotary drum vacuum filter, its operation, and future development of the technology. It consist mainly of a drum, valve, piping, drive, discharge mechanism, the agitator and the trough. These components are necessary to allow its basic operation of forming the cake, drying the cake, discharging the cake, allowing a dead zone, and channelling the filtrate to the appropriate holder. Future development will focus on improving the performance of the filter in term of holding higher vacuum and thus higher rate processing rate. Reference list Shammas, NK & Wang, LK 2007, Biosolids treatment processes, Handbook of Environmental Engineering. Vol. 6. Humana Press. Svarovsky, L 2000, Solid-liquid separation, Butterworth-Heinemann, United States. Wakeman, Richard J. & Tarleton E. S. 2005. Solid/liquid separation: scale-up of industrial equipment, Elsevier, Australia. Read More