Drum filters

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Rotary vacuum drum filter (RVDF) is also called as drum filters used in the industrial liquid-solids separation is which are one of the oldest filters.

It offers provides a flexible application of dewatering, washing and/or clarification and a wide range of industrial processing flow sheets.

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

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/ solids suspension as a cake. When the cake is rotating out, it is dewatered in the dying 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.

Rotary vacuum filter

DESCRIPITION

Rotary vacuum filter drum consists of a drum rotating in a tub of liquid to be filtered.

The technique is well suited to slurries, and liquids with a high solid content, which could clog other forms of filter.

The drum is pre-coated with a filter aid. After pre-coat has been applied, the liquid to be filtered is sent to the tub below the drum.

The drum rotates through the liquid and the vacuum sucks liquid and solids onto the drum pre-coat surface, the liquid portion is "sucked" by the vacuum through the filter media to the internal portion of the drum, and the filtrate pumped away.

The solids adhere to the outside of the drum, which then passes a knife, cutting off the solids and a small portion of the filter media to reveal a fresh media surface that will enter the liquid as the drum rotates. The knife advances automatically as the surface is removed.

PRINCIPLE

Rotary drum filters works on the principle or function of filtering the slurry through sieve leke mechanism on a rotating drum surface under the condition of the vacuum. In addition compression drying (using hoy air) and removing the filter cake (using a knife) are possible.

CONSTRUCTION

  • The construction of a rotary drum filter consist of a metal cylinder mounted horizontally.
  • The drum may be up to 3 meters in diameter and 3.5 meters in length and give a surface area of 20 meter square.
  • The curved surface is a perforated plate, which supports a filter cloth.
  • The drum is radially partitioned dividing the annular space into separate compartments.
  • Each of it connected by an internal pipe to the center of the drum through a rotating valve.

DESIGN AVAILABLE

Basically there are five types of discharge that are used for the rotary vacuum drum filter such as belt, scraper, roll, string and pre coat discharge.

Belt discharge

The washing should be done on both sides of the filter cloth while discharging filter cakes for each rotation of the drum. The products for this mechanism are usually sticky, wet and thin thus, requiring the aid of a discharge roll. Belt discharge is used for the easy filtration of the slurry.

Belt discharge

Scraper Discharge

This is the standard drum filter discharge. A scraper blade, which serves to redirect the filter cake into the discharge chute. 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.

Scraper discharge

Roll Discharge

This are thin and have the tendency to stick with one another and it is a suitable discharge option for cakes. 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.

Roll Discharge

Roller discharge

String Discharge

The filtrate cakes that are thin and fragile are usually the end products of this discharge lie. The materials are capable to change 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. In the pharmaceutical and starch industries the string discharge can be seen.

String discharge

Pre coat Discharge

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 formation.

Precoating discharge

WORKING

The drum is rotated at a speed less than one revolution per minute. The drum just enters the slurry in the trough.

As it dips vaccum is applied in this segment so that the solid is build up an the surface. The liquid passes through the filter cloth into the internal pipe and valves. Finally the filtrate reaches the collecting tank.

As the drum leaves the slurry section it enters the drainage zone. Here excess of the liquid is drawn inside.

Special cake compression rollers may be included at this stage, so that the cake is consolidated by the compression of the cake. This improves the efficiency of washing and drying process.

uVacuum is applied to carry the slurry along with the drum

uDrainage zone

uWater washing arrangement

uDrying zone-dry air supply

uCake rmoval zone

As the drum leaves the drainage zone it enters the water wash section . water is sprayed on the cake.

A separate system of vacuum is applied on the panel in order to suck the wash the liquid and air through the cake solid.

Wash the liquid is drawn through the filters in to a sepatare collecting tank. Then the cake enters the drying zone where hot air is blown on the cake .

The cake may have the moisture content less than one percent. Finally the cake is removed using a doctor knife and discharged.

All the steps are completed in one rotation of the drum. Now the drum is ready to receive a fresh lot of slurry.

Operation of the rotary filter

DISCHARGE MECHANISM ADJUSTMENT

Endless belt

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.

Scraper

Select filter cloth to obtain good wear and solid binding characteristics. Use moderate blow back pressure to avoid high wear. Adjust duration of blow back pressure short enough to remove the cake form 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.

Roll

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.

String

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

Pre coat

Select filter cloth based on the type filter aid used, adjust the advancing knife to optimize the knife advance rate per drum revolution.

Working of rotary vacuum filter

PHARMACEUTICAL APPLICATIONS

As a basic separation operation, rotary vacuum drum filter is used in a wide range of applications:

dewatering slurries of food, pulp, pharmaceutical and chemical, applications of metallurgical and the treatment of waste water.

  • It is a continuous operation and is utilised to filter slurries containing high proporation of solids up to 15 to 30 percent.
  • It is used extract the pencilline from the mycelium or cell mass by the drum filters.
  • These are used for collecting calcium carbonate, starch and magnesium carbonate.
  • Drum filter is a large and typically used in industrial applications to filter liquids carrying high concentrations of suspended solids. The rotating drum filter functions by drawing water through its external filter surface into its interior space by means of an internal vacuum. Suspended solids in the water are trapped on the surface of the drum and the filtered water is then pumped away. Drum filters may utilize removable screen type filter panels or have a sieve pattern of holes in their surfaces for use with a filter aid coating. Drum filters feature a minimum of moving parts and are cheap to operate and maintain.
  • The drum filter rotates against a flexible plate or knife which scrapes the trapped sediment off for later use or disposal. This knife removes a thin layer of filter aid at the same time exposing fresh, uncontaminated material to continue the filtering cycle. This causes gradual depletion of the filter aid layer necessitating regular re-coating of the drum. Both types of drum filters are not only very effective at separating heavy, dense suspensions but can be cost effective due to the simplicity of the systems.
  • Perforated drum filters are often used in water treatment plants that remove large amounts of fine sediment from water. They can also be used to remove waste water from suspended slurry products. These filters operate on the same basic principle as removable panel filters but employ a regularly applied layer of filter aid to trap the sediment. The drum is perforated with numerous holes which create a sieve-like external surface through which the water is drawn.
  • Drum filters which make use of fine mesh panels are usually fitted with a spray system which washes the collected solids off the filter elements as the drum rotates. This variant is commonly known as the micro-screen drum filter and is typically used in applications where the solids are to be discarded. The micro-screen drum filter is particularly useful for cleaning water contaminated with coarser solids, such as animal hair or feathers, which would clog conventional filters. Filter panels are also simple to replace once worn.

ADVANTAGES

The advantages of rotary vacuum drum filter are:

  • The rotary vacuum drum filter is a continuous and automatic operation, so the operating cost is low.
  • It controls the cake thickness by showing the varitation in drum speed rotating.
  • The process can be easily modified (pre-coating filter process).
  • Can produce relatively clean product by adding a showering device.
  • Cake is removed simultaneously during operation. Therfore suitable for use with concentarted slurries.
  • The labour cost is very low on account of automatic and continuous operation of the rotary filter.
  • The filter has large surface area.
  • The speed of the rotation can be varied and the cake thickness can be controlled.

For example if the solids form an impenetrable cake, the thicknessmay be limited to less than 5 mm. On the other hand if the solids are coarse and form a porous cake , the thickness of 100 mm or more can be obtained.

DISADVANTAGES

  • Due to the structure, the pressure difference is limited up to 1 bar.
  • Besides the drum, other accessories, for example, agitators and vacuum pump, are required.
  • The discharge cake contains residual moisture.
  • High energy consumption by vacuum pump.
  • Rotary drum filter is expensive equipment with the complex functioning. It contains moving parts and also requires a number of the accessories uch as vacuum pumps, vacuum receivers, traps etc.
  • The cake tends to crack due to the air drawn through by the vacuum system. This makes washing and drying procesess inefficient.
  • The rotary filter is suitable only for straight forward slurries. It is less satisfactory, if the solids form an impermeable cake or it is difficult to remove the cake adequately.

Maintenance

  • The slow rotation of the drum and reciprocation of the agitator reduce maintenance requirements to a minimum but the following should be inspected periodically:
  • The strip liner of the trunnion bearing at the valve end will normally wear at the lower half. However, in cases when the slurry has a high specific gravity, the drum may become buoyant causing a wear to the upper half. At this point it should be mentioned that one way to remove the lower half of the liner, when hoisting facilities are not available or operational, is to float the drum by filling the tank with a sufficiently concentrated solution.
  • The stuffing boxes on high submergence filters should be inspected for leakage and, if necessary, the stud nuts should be tightened. It should be noted that excess tightening can increase substantially the load on the drum drive so the use of a torque wrench is recommended.
  • The face of the wear plate should be checked periodically and remachined if necessary. A whistling noise during operation is an indication the wear plate is worn out or the valve spring requires tensioning.
  • The drum has a bailer tube that protrudes from the drive end shaft and must be kept open to atmosphere at all times since its blockage may cause the collapse of the drum. The bailer tube is a tell-tale indication to the following:
    • If a lighter flame is drawn through the bailer tube to the inside of the drum it indicates that a vacuum leak exists in the drum shell or the internal piping and the explosives may occurs. In such cases the use of aerosol type smokes or a light tissue paper should be used instead of an open flame to identify a vacuum leak.
    • If liquid leakage is observed from the bailer tube it indicates that a hole exists in the drum head causing penetration of slurry from the tank into the drum.
  • The filter checked periodically for pressure build-up due to progressive blockage. REFERENCE
  1. C.v.s subrahmanyam et al., pharmaceutical engineering principles and practices-filtration, 2001, page no:272 - 274.
  2. http://en.wikipedia.org/wiki/Rotary_vacuum-drum_filte

About the Author

Saraswathi.B's picture
Author: Saraswathi.B

Assistant Professor in St. johns college of pharmacy

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