Filtration candle filters

Cylindrica.1 Element Filters. These filters, often referred to as candle filters, have cylindrical elements or sleeves mounted vertically and suspended from a header sheet, which divides the filter vessel into two separate compartments (Fig. 16). The filtration takes place on the outside of the sleeves. The inlet is usually in the bottom section of the vessel and the filtrate oudet in the top section above the header sheet. A less usual design is to locate the filtrate outlet at the bottom of the elements and thus allow the top chamber to be opened for each inspection of the elements during operation. 

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. 

Pall Process Filtration Ceramic Candle Filter 1000°C (max.) 99.99% Supported both sides clay bonded silicon carbide filter... 

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 candle filter element can be textile cloth over a supporting tube, sintered metal, ceramic or spaced rings. It can also be used for air or gas filtration. Metallic candles are particully suited for high temperature applications. 

Tiller, F.M., and Wenping Li. 2001. Optimizing candle filters for super-compactible materials. Advances in Filtration and Separation Technology 15. 

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. 

This analysis is relevant to the cake filtration mode. Filtration of non-Newtonian liquids is common in clarification such as oil or polymer filtration by cartridge and candle filters. In these instances an alternative mode of filtration is likely to occur. 

In the brewing process diatomite is used as precoat and body aid on candle filters and horizontal leaf filters. Hermia Brocheton [1993] give a conparison of these methods and cite typical precoat dosage as 1.5 kg/rn and body feed dosage as 1 kg/rn with a cake average specific resistance of lO m/kg. Other aspects of diatomite filtration are discussed comprehensively in a historical review by Cummins [1973]. 

Rushton, A, 1985, Clarification of water with a self cleaning candle filter, Filtrat. and Separat, 22, pp 388-391. 

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). 

Candle filters have in general proved highly efficient, even for very fine particles of sizes down to the order of 1 pm. However, a number of problems remain to be solved, such as the build-up of dust cake on the filters leading to occasional cake bridging between candles, filter failure and breakage. A number of hot gas filtration systems have been developed and tested under industrially relevant conditions an advanced particle filter (APF) containing 284 candles was installed at the 70 MW (electric) Tidd PFBC demonstration plant at Brilliant, Ohio gas flow and particle deposition have been modelled by Ahmadi and Smith [53] for the Tidd filter vessel and by Mazaheri and Ahmadi [54] for the Siemens-Westinghouse particulate control device (PCD). 

Zuber, J. P. (2009). Industrial results of precoat filtration on a candle filter with legenerable filter aid. In Proceedings of the European Brewing Convention, Hamburg (abstract published online). 

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. 

If filtration takes place at constant pressure then the equations presented for the tube press in Section 6.2.1.3 are also valid for the multi-element candle filter noting that = n,ndh where n, is the total number of candles in the filter. When filtration takes place at variable pressure then it is necessary to impose the pump curve characteristics to relate Ap and q and a numerical solution to equation (6.17) is generally required. For the special case of variable pressure/constant rate filtration with a positive displacement pump... 

Prior to spinning it is necessary to filter various impurities out of the solution. Most of the impurities are introduced with the pulp feedstock, the principle ones being undissolved pulp fibres or inorganic compounds such as sand and ash. The solution is passed through two stages of filtration. The primary filtration is centrally located and consists of sets of sintered stainless steel media candle filter elements. The secondary stage filtration is achieved by candle filter elements associated with each spinning machine position. 

Other types of filters, such as the rotating table filter, the candle filter, filter presses, etc., are described in detail in the recent filtration literature (Dickenson, 1994 Schweitzer, 1997 Svarovsky, 1990). 

Centrifugation or filtration in candle filters or in disk filters precoated with charcoal gives sodium hydroxide solutions containing mercury concentrations of < 0.05 ppm (mgAg of 50 % caustic soda). 

There is a large variety of conventional filtration equipment. One of the most common types of batch filter is the plate-and-frame filter (Figure 12.4). This filter consists of a number of filter chambers with medium-covered filter plates alternated with frames that provide space for the cake. The chambers are closed and tightened by a hydraulic ram or screw which pushes the plate and frame together. Other types of batch filters are horizontal or vertical leaf filter, tray filter, tube or candle filter, and Nutsche filter. These are all enclosed pressure filters. 

Several bags can be housed in one vessel, supported from the same tube plate, and operated in parallel. This arrangement (very common in the bag houses used for gas filtration) is akin to a candle filter, and is described later in this section. 

There is much confusion over the precise meaning of the term candle filter, so let it be said that, in the present context, a candle filter is a process filter, mainly used for the recovery of process residues from liquid streams. It almost always has a vertical cylindrical housing, inside which a multitude of cylindrical filter elements, closed at the bottom end, are suspended from a tube plate. How of liquid is from the outside of the element to its inside, and thence through the upper, open end of the element into the filtrate space above the support plate. Some typical arrangements for a candle filter are shown in Figure 3.47. 

---