Pressure filter candles

The advantage of candle filters is that as the cake grows on the tubular elements the filtration area increases and the thickness of a given volume of cake is therefore less than it would be on a flat element. This is of importance where a thick cake is being formed the rate of increase in the pressure drop is less with tubular elements. 

Some PFBC boiler designs incorporate high-temperature, high-pressure (HTHP) filter devices in the flue-gas stream. These are installed primarily to protec t the gas turbine from erosion damage by the fine particles that escape the cyclones, but as the filters remove virtually all the suspended particulates, they also eliminate the need for back-end removal. The commonest HTHP filter elements used are rigid ceramic candles. 

Tuhtdar Presses As the name implies, this press is composed of a candle filter inside a cyhndrical hydrauhc casing (Fig. 18-192). The filter cloth is wrapped around the filter candle, and a diaphragm is attached to the inner side of the outer casing. During the filtration step, the space in between two cylinders is filled with slurry, and pressure filtration is conducted. At the end of the filtration step, the diaphragm is inflated to squeeze the cake around the filter candle. At the end of expression, the bottom of the hydrauhc casing tube is opened and the filter assembly is lowered. Air is then introduced to pulse the cake off the candle. Alter the cake is discharged, the inner filter candle moves back, and the bottom is closed for the next filtration cycle. 

Candle FilterS Tubular filter elements contained in a matching vessel are known as candle filters. The actual filter vessel may contain one or more filter candles, and may be used as pressure or suction filters for the filtration of liquids and gases. A particular advantage offered is that candles may readily be changed to different types, to suit particular requirements or applications. A typical candle filter is shown in Fig. 11. Materials of candles are selected to fit a particular process. 

The pressure chamber consists of a perforated candle filter. On top of the screen is a filter cloth and a membrane constructed of EPDM, BUNA, or Viton. At present these are the only available materials of construction. The... 

Pressure filters are usually operated batch-wise. The batch pressure filters can be classified as tank (pressure vessel) filters or presses. Tank filters have filter elements of different types mounted in pressure vessels. Tank filters are divided into pressure nutsches, leaf filters, candle or tubular filters, bag filters, and cartridge filters. Presses (see Figure 22.38) consist of a series of filter surfaces (plates). The elements are mounted on a frame and are pressed together mechanically. 

Plate and frame filters are therefore more readily precoated and less susceptible to malfunction due to pressure surges. Again, the effective filtration area in a cellulose sheet represents 50 % of the total while corresponding values for leaf septa and candles are 35% and 10% respectively. This means that localized flow rates in leaf and candle filters are undesirably far greater than in plate and frame filters. 

These units are sometimes described as pressure vessel filters the vessels used to contain the filter dements are cylindrical designs, positioned vertically or horizontally. These vessels operate in batch mode, with filter cycles similar to plate and fimne filters. The variations available in the type of elements (leaf or candle) and its position in the pressure vessel, lead to a large number of available designs [Purchas, 1981], General operating features of pressure vessel filters have been described [Bosley, 1986] these units are considered to be cleaner and more reliable than other pressure filters, provided adequate attention is given to filter cake properties. 

Control of the back-pressure level required for efllbctive back-flushing has bear claimed with the use of flat perforated plate elements in the Cricket filter [Filtration and Separation, Sep/Oct., 1991]. Automatic, semicontinuous cleaning of tubular candle filters can also be realised by momentary reversal of the filtrate flow a resulting bump is defivered to the cake, which is dModged and Ds to the cone-shaped bottom of the unit, Bgure 11.53. The effectiveness of these filters is inproved by the use of filter media whkh can handle veiy dilute suspensions of fine (< 0.5 pm) particles, e.g. PIPE membranes). 

Additional extensive reviews of high-temperature gas cleanup by means of, among other devices, candle filters have been summarized by Di Carlo and Foscolo [51]. Smith and Ahmadi [52] review the field of hot gas filtration in relation to pressurized fluidized bed combustion (PFBC) and integrated combined cycle gasification (IGCC). 

Many types of filter are supplied for this sort of operation. In chlor-alkali brine treatment, the two most frequently encountered are the pressure leaf filter and the candle filter. Both types are capable of removing submicron particles and producing a filtrate with less than 1 ppm of suspended solids. A leaf filter, as the name implies, contains a number of thin, flat elements that are active on both sides. In chlor-alkali brine plants, the leaves normally are suspended vertically in a tank. The tank may be horizontal, in which case the leaves are circles or rounded squares, or vertical, in which case the leaves are approximately rectangular and of different widths. 

A more specialized application is the filtration of mercury-cell liquor to remove mercury from the product. We consider two types of filter here, the candle filter using carbon tubes and the pressure-leaf filter precoated with activated carbon. The operation of candle filters also has been described in Section 7.S.4.2. 

Compared with the previously mentioned systems, pressure filters consume by far the greatest volume of fabric, being employed in a host of industries including mining applications, food processing, chemical process industries and effluent treatment. The most extensively used type of pressure filter is the filter press (of which there are several variations), though other pressure filters are also widely found with titles such as candle filter tower belt press , pressure leaf filter tube press , multi-roll press and pressure... 

Single leaf (pressure Nutsche) filter Multi-element (tubular candle) 6 6 6 6 8 -... 

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