Process Technology: An Introduction - Haan A.B. 2015
10 Solid-liquid separation
10.5 Filtration equipment
An important factor in the optimization of particular processes is the thickness of the filter cake. Too thick filter cakes lead to an uneconomic lengthening of the filtercycle due to low filtering, dewatering, and washing rates. On the other hand, thin filter cakes may be difficult to remove from the equipment, again increasing the filter cycle time. The productivity of all filters is related to the time required to complete a full filtration cycle and may be described as
(10.28)
Besides filtration, additional time is required for dewatering, cake washing, and finally the discharge of the filter cake and the cleaning/reassembly/filling of the filter. It is usual to lump the latter two and other not-mentioned operations into a total downtime period tDW,. The overall cycle time is then given by
(10.29)
Fig. 10.27: Schematic of a rotating vacuum filter.
10.5.1 Continuous large-scale vacuum filters
Vacuum filters are the only truly continuous filters that can provide for washing, drying, and other process requirements on a large scale. Examples include the rotary drum, rotary disc, horizontal tilting pan, and horizontal belt filters. Many of them use a horizontal filtering surface with the cake forming on top. The suspension is delivered to the filter at atmospheric pressure, and vacuum is applied on the filtrate side of the medium to create the driving force for filtration. The rotary vacuum drum filter is the most popular vacuum filter. As depicted in Fig. 10.27, the filtering surface of rotating-drum filters is usually situated on the outer face of a cylindrical drum that rotates slowly about its horizontal axis and is partially submerged in a slurry reservoir. The drum surface is covered with a cloth filter medium and divided into independent longitudinal sections, which are connected to the vacuum source by a circular connector or rotary valve. Filtration takes place when a section is submerged in the feed slurry. After cake formation, dewatering and washing by vacuum displacement take place, followed by cake discharge at the end of the rotational cycle. In some applications, compression rolls or belts are used to close possible cracks or to further dewater the cake by mechanical compression. Cake discharge can be affected by knife, belt, or string and roller discharge. Compared to drum filters, significant savings in required floor space and costs is possible with vertical-disc filters. Rotary-disk filters use a number of disks mounted vertically on a horizontal shaft and suspended in a slurry reservoir. The feed suspension is supplied continuously into troughs, in which the liquor flow is arranged in the same direction as the rotating discs. A particular disadvantage of disk filters is inefficient cake washing and difficult cloth washing.
Horizontal filters largely circumvent the geometric constraints inherent in the design of rotary-drum and vertical-disc units. They have the advantage that gravity settling can take place before the vacuum is applied, and they are ideal for cake washing, dewatering, and other process operations such as leaching. Of the available industrial units in this category, the horizontal-belt filter is the most popular. A schematic diagram of a typical unit is shown in Fig. 10.28. The feed slurry is supplied to the upper surface of the horizontal filter cloth that is supported by an endless belt situated above the vacuum box filtrate receivers. The top strand of the endless belt is used for filtration, cake washing, and drying. There is appreciable flexibility in the relative areas allocated to each cycle step. Efficient cake discharge can be affected by separation of the belt from the cloth and directing the latter over a set of discharge rollers. Here the produced cake is cracked and discharged by a sharp turn in the cloth over a small diameter roller. In module-type horizontal-belt filters, stainless steel or plastic trays replace the endless rubber belt. Horizontal-belt filters are well suited to either fast or slowly draining solids, especially where washing requirements are critical. The primary advantages of this filter are its simple design and low maintenance costs. The main disadvantage is the difficulty of handling very fast filtering materials on a large scale.
Fig. 10.28: Schematic of a horizontal belt filter. Adapted from [1].
10.5.2 Batch vacuum filters
The best-known batch vacuum filter units include Nutsche, horizontal-table“ and vertical-leaf filters. The Nutsche filter (Fig. 10.29) is a scaled-up version of the simple Buchner funnel consisting of a tank divided into two compartments by a horizontal filter medium supported by a filter plate. A vacuum is applied to the lower compartment where the filtrate is collected. These filters are particularly advantageous for separations where it is necessary to keep batches separated and when rigorous washing is required. They are simple in design but laborious and prone to high wear in cake discharge. This problem is circumvented with the mechanized Nutsche shown in Fig. 10.30, which is provided with a shaft carrying a stirrer passing through the cover. The stirrer can sweep the whole filter area and can be lowered or raised vertically as required. The agitators are fitted to facilitate slurry agitation, cake smoothening prior to washing, and cake removal. A discharge door is provided at the edge, and the rotor moves the cake towards the door. Enclosed agitated filters are useful when volatile solvents are in use, or when the solvent gives off toxic vapor or fume. Another advantage is that their operation does not require any manual labor. The horizontal table filter has overall features similar to the other horizontal vacuum filters, except that during filtration, washing, and dewatering the filter element is stationary. This facilitates optimization of filter cake thickness, wash times, etc. The totally closed system is opened to allow band movement for cake discharge.
Fig. 10.29: Nutsche filter.
Fig.10.30: Mechanized Nutsche filter.
10.5.3 Pressure filters
In circumstances where large separating areas may be necessary because of slow settling characteristics, poor filterability, high solids content, or other factors, the use of pressure filtration is beneficial. Pressure filters can be operated at constant pressure differential or at constant flow rate. At constant pressure the liquid flow rate will decrease with time, while at constant flow rate the pressure differential need to increase to compensate for increasing cake resistance. Within the extremely large variety of pressure filters three main groups may be distinguished:
· (1) plate and frame filter presses;
· (2) pressure vessels containing tubular or flat filter elements;
· (3) variable-chamber presses.
Various types of plate-and-frame filter presses, designed for cake formation and squeezing, are available for use in the chemical industry. The conventional plate-and-frame filter press shown in Fig. 10.31 contains a sequence of perforated square or rectangular plates mounted on suitable supports, alternating with hollow frames, and pressed together with hydraulic screw-driven rams. The plates are covered with a filter cloth, which also forms the sealing gasket. Most units are operated batchwise. After filling frames with slurry, the filtrate is drained through the plates, and the machine is disassembled or opened for cake discharge. Washing is performed by introducing the wash liquid either through the main feed port or through a separate port behind the filter cloth. Plate-and-frame filter presses are most versatile, since their effective area is easily varied by blanking off some of the plates and cake holding capacity altered by changing the frame thickness. In deciding the overall economics of the process the time taken in discharging the cake and refitting the filter is of great importance.
Fig.10.31: Typical arrangement of a plate-and-frame filter press.
All pressure vessel filter units consist of a multitude of leaf, candle, or cartridge filter elements mounted horizontally or vertically in a pressure vessels housing (Fig.10.32). Vertical vessels with vertical leaf or candle filters are the cheapest of the pressure vessel filters and have the lowest volume-to-area ratio. In order to avoid filter cake bridging between the elements, serious attention has to be given to candle or leaf spacing. Deposition on the outer surface is advantageous in view of the increase in the area with cake growth. Horizontal leaf filters consist of a stack of rectangular horizontal trays mounted inside the vessel that can be withdrawn for cake discharge. They have the disadvantage that half the filtration area is lost because the underside of the leaf cannot be used for filtration. Cartridge filtration is limited to liquid polishing or clarification, in order to keep the frequency of cartridge replacements down.
Fig. 10.32: A vertical candle (a) and vertical leaf (b) pressure vessel filter.
Most continuous pressure filters have their roots in vacuum filtration technology. They have been adapted to pressure by being enclosed in a pressure cover. As such one finds continuous disk, drum, and belt pressure filters. Special designs of continuous pressure filters are belt presses and screw presses. Belt presses combine gravity drainage with mechanical squeezing of the cake between two running belts. In screw presses a screw mounted inside a perforated cage conveys the material along the barrel (Fig. 10.33). The available volume for transport diminishes continuously along the length of the screw in order to compress the filter cake. Washing liquid can be injected at points along the length of the cage. Screw presses are only suitable for the dewatering of high solids containing pastes or sludges, because no filtration stage is included.
Fig.10.33: Schematic of a screw press.