Rotary vacuum-drum filter

Rotary vacuum drum filter (RVDF), patented in 1872,[1] is one of the oldest filters used in the industrial liquid-solids separation. It offers a wide range of industrial processing flow sheets and provides a flexible application of dewatering, washing and/or clarification.

A rotary vacuum filter consists of a large rotating drum covered by a cloth. The drum is suspended on an axial over a trough containing liquid or solids slurry with approximately 50-80% of the screen area immersed in the slurry.

As the drum rotates into and out of the trough, the slurry is sucked on the surface of the cloth and rotated out of the liquid or solids suspension as a cake. When the cake is rotating out, it is dewatered in the drying zone. The cake is dry because the vacuum drum is continuously sucking the cake and taking the water out of it. At the final step of the separation, the cake is discharged as solids products and the drum rotates continuously to another separation cycle.

The advantages and limitations of rotary vacuum drum filter compared to other separation methods are:

• The rotary vacuum drum filter is a continuous and automatic operation, so the operating cost is low.
• The variation of the drum speed rotating can be used to control the cake thickness.
• The process can be easily modified (pre-coating filter process).
• Can produce relatively clean product by adding a showering device.

• Due to the structure, the pressure difference is theoretically limited to atmospheric pressure (1 bar), and in practice somewhat lower.
• Besides the drum, other accessories, for example, agitators and vacuum pump, are required.
• The discharge cake contains residual moisture.
• High energy consumption by the vacuum pump.

The filter cloth is washed on both sides with each drum rotation while discharging filter cakes. The products for this mechanism are usually sticky, wet and thin thus, requiring the aid of a discharge roll. Belt discharge is used if slurry with moderate solid concentration is used or if the slurry is easy to filter to produce cake formation or if a longer wear resistance is desired for the separation of the mentioned slurry.[2][3]....

This is the standard drum filter discharge. A scraper blade, which serves to redirect the filter cake into the discharge chute, removes the cake from the filter cloth just before re-entering the vat. Scraper discharge is used if the desired separation requires high filtration rate or if heavy solid slurry is used or if the slurry is easy to filter to produce cake formation or if a longer wear resistance is desired for the separation of the mentioned slurry.[2][3]

It is a suitable discharge option for cakes that are thin and have the tendency to stick with one another. Filter cakes on the drum and discharged roll are pressed against one another to ensure that the thin filter cake is peeled or pulled from the drum. Removal of solids from the discharge roll is done via a knife blade. Roll discharged is used if the desired separation requires high filtration rate, if high solid content slurry is used or if the slurry is easy to filter to produce cake formation or if the discharged solid is sticky or mud-like cake.[2][3]

The filtrate cakes that are thin and fragile are usually the end products of this discharge lie. The materials are capable of changing phases, from solid to liquid, due to instability and disturbance. Two rollers guide the strings back to drum surface and at the same time separation of the filtrate cake occurs as they pass the rollers. Application of the string discharge can be seen at the pharmaceutical and starch industries. String discharge is used if the high solid concentration slurry is used or if the slurry is easy to filter to produce cake formation or if the discharged solid is fibrous, stringy or pulpy or if a longer wear resistance is desired for the separation of the mentioned slurry.

Application of this discharge are usually seen where production of filter cakes that blind the filter media thoroughly and processes that have low solid concentration slurry. Pre coat discharge is used if slurry with very low solid concentration slurry is used that resulted in difficult cake formation or if the slurry is difficult to filter to produce cake .[2][3]

Vat level and drum speed are the two basic operating parameters for any rotary vacuum drum filter. These parameters are adjusted dependently to each other to optimize the filtration performance. Valve level determines the proportion filter cycle in the filter. The filter cycle consist of the filter drum rotation, release of cake formation from slurry and the drying period for the cake formation shown in figure 1. By default, operate the vat at its maximum level to maximise the rate of filtration. Reduce vat level if discharged solid is in the form of thin and slimy cake or if the discharged solid is very thick.[3][6][7]

Decrease in the vat level eventually leads to a decrease in the portion of the drum being submerge under the slurry, more surface exposure for the cake dying surface hence, larger cake formation to dry time ratio. This result in less moisture content of formed solid and lessen the thickness of the form solid. In addition to operating at lower vat level, the flow rate per drum revolution decreases and ultimately thinner cake formation occurs. In the case of pre coat discharge the filter aid efficiency increases. Drum speed is the driving factor for the filter output and its units is in the form of minutes per drum revolution. At steady operating conditions, adjusting the drum speed gives a proportional relationship with the filter throughput as shown as in figure 2.

Select filter cloth to obtain a good surface for cake formation. Use twill weave variation in the construction pattern of the fabric for better wear resistance. The belt tension, de-mooning bar height, wash water quantity and discharge roll speed are carefully tuned to maintain a good path for the cake formation to prevent excessive wear of the filter cloth.

Select filter cloth to obtain good wear and solid binding characteristics. Use moderate blowback pressure to avoid high wear. Adjust duration of blow back pressure short enough to remove the cake from the filter cloth. The tuning of valve body is important for the blow back to prevent the excess filtrated being force back out of the pipe to with the release cake solid as this minimises wear and filter media maintenance.

Select filter cloth to obtain solid binding resistance and good cake release. Use coated fabric for more effective cake release and have a longer-lasting cloth media due to solid binding filter cloth. Both the discharge roll speed and drum speed must be the same. Adjust the scraper knife to leave a significant heal on discharge roll to produce a continuous cake transfer.

Minimise the lateral pressure of the strings by adjusting the alignment tine bar to avoid the string being cut off. Have ceramic tube place over each aligning tine bar to act as bearing surface for the strings.[3]

Select filter cloth based on the type filter aid used (refer Filter aid selection), adjust the advancing knife to optimize the knife advance rate per drum revolution. (Detail explained in Advance blade section)

Filter aid selection: filter aid are recoat cake that act as the actual filter media and there two different types which are diatomaceous earth or perlite. Important parameter to consider is the solid penetration into the pre coat cake and its limits 0.002 to 0.005 inch penetration thickness. Large amount of filter aid is used i.e. “open”, more filter aid is aid removed which lead to higher disposal cost. If little amount of filter aid is used i.e. “tight” will lead to no flow rate into the drum. This comparison can be illustrated in figure 5 as below.[3]

The approximate knife advance rate can be determined for a set of operating conditions using table 6 below. The table indicates the number of hours that the filter can operate in a one-inch pre coat cake; the required condition is that the advance blade must be at a constant position. This method can be used to check for optimum operation range.

If the operating parameter is higher than the optimum range, the user can reduce the knife advance rate and use a tighter grade of filter aid. This will result in less filter aid used (lower capital cost) and less filter aid being removed (lower disposal cost). However, if the operating parameter is lower than the optimum range, the user can increase the knife advanced rate (more production) and decrease the drum speed for less filter air usage (reduced operating cost).[3]

Most commonly used post treatment, where chlorine is dissolved in water to form and hydrochloric acid hypochlorous acid. The latter act as a disinfectant that is able to eliminate pathogens such as bacteria, viruses and protozoa by penetrating the cell walls.[8]

The waste stream is irradiated with Ultraviolet radiation. The UV radiation disinfect by disrupting the pathogen cell to be mutated and prevent the cell from replicating. Eventually the mutated cell becomes extinct and this process eliminates odour.[8]

The stream is exposed to ozone and ozone is unstable at atmospheric condition. The ozone (O3) decomposes into oxygen (O2) and more oxygen is dissolved into the stream. The pathogen is oxidised to form carbon dioxide. This process eliminates the odour of the stream but result in slightly acidic product due to the effect of carbon dioxide present.[8]

The waste discharge can be used as land stabilizer as dry bio-solids that can be distributed to the market. The land stabilizer is used in reclaiming marginal land such as mining waste land. This process will help to restore the land to its initial appearance.[8]

The waste discharge can be sent into incineration plant, where the organic solid undergoes combustion process. The combustion process produces heat that can be used to generate electricity.[8]