Cake release

Another advantage of the membrane plate is its flexibiUty to cake thickness, ie, thinner cakes can be easily handled without loss of dryness. Cake release characteristics are also improved by deflation of the membrane prior to cake discharge. Alternating arrangements, in which the membrane plates and the normal recessed plates alternate, have been used to reduce cost. 

A recently developed medium known as a double weave incorporates different yarns in warp and fill in order to combine the specific advantages of each type. An example of this is Style 99FS, made by SCAPA Filtratiou (formerly P S Filtratiou), in which multifilameut warp yarns provide good cake release properties and spun staple fill yarns contribute to greater reteutivity. 

When good cake release assisted by squeezing is required. 

The filtrate that is back-pulsed from the dome first expands the filter socks thereby cracking the cake. The movement of the socks and the subsequent liquid flow through them serves to dislodge the filter cake solids from the GORE-TEX membrane surface. Because of the expanded PTFE material of the membrane, the cake releases from... 

The liquid left after distillation is subjected to centrifugation, where most of the suspended solids are separated. The clear liquid can be recycled by adding it back to the starch conversion stage. The moist cake released by the centrifuges is mixed with the syrup produced by the evaporator to form a homogenous mixture and is dewatered in dryers. The dryers produce a Distillers Dried Grains with Solubles (DDGS) meal, which can be formed into pellets. These are used in many applications, most prominently in animal feed. 

Retention of particles SufQcient flow rate Easy cake release Gasket action... 

Other types of woven fabrics used as filter media are the twill and satin weaves. In the twiU weave, yam is woven over two and under one for a 2/1 twill and over three and under one for a 3/1 twill weave. The twill weave fabrics allow good flow rates and do not retain particles as well as the plain weave as a result, the filters do not blind as fast as the plain weave fabric filters. In the satin weave fabrics, the yams are woven over one and under four in both directions. These types of fabrics are not very effective in retaining particles but have the best cake release characteristics therefore, the filters can be cleaned with comparative ease. Woven fabric area densities are typically in the range of 200-500 gm" ... 

Filter fabrics, especially nonwoven fabrics, which are produced from short staple fibres, invariably possess surfaces with protruding fibre ends. Since such protrusions may inhibit the cake release by clinging to the dust, it is a common practice to remove them. This is achieved by singeing, a process in which the fabric is passed, at relatively high speed, over a naked gas flame, or, in another technique, over a heated copper plate. The heat of the flame causes the fibres to contract to the surface of the fabric where, in the case of thermoplastic fibres, they form small hard polymer beads (Fig. 3.8). Singeing conditions (i.e. speed and gas pressure) will normally be adjusted according to polymer type and the intensity required by either the end-use application or the individual manufacturer s preference. 

Staple-fibre yams - at least in the weft direction. In operation, the fabric is pulled over a series of rotating rollers termed pile and connter pile , each of which is clothed with card wire and monnted concentrically on a large cylinder of approximately 1.5 m diameter. As the cylinder rotates, the pile rollers raise the fibres proud of the surface, whereas the counter pile rollers stroke them into a more orderly fashion. Raised fabrics may comprise 100% staple-fibre yams or a combination of multifilament and staple-fibre yams, the latter being woven in satin style in which the face side is predominantly multifilament yams and the reverse side predominantly of staple-fibre yams. The smooth surface provided by the multifilaments will aid cake release, whilst the raised staple-fibre yams on the reverse side will enhance the particle collection efficiency. A significant measure of the width contraction invariably takes place during this operation and proper attention will have to be given to this when designing the fabric. 

This category of treatments is devoted to improving still further the fabric s filtration efficiency and cake release characteristics. In this respect, there are basically two types of treatment (1) attachment of a more efficient membrane, for example biaxi-ally stretched PTFE (Fig. 3.9) in a lamination operation and (2) the application of a low-density microporous foam (Fig. 3.10). 

At the end of the filtration cycle, the dewatered filter cake must be removed from the fabric in preparation for the next cycle. It is important that the cake is effectively discharged at this point since any delays will lead to extended filtration cycle times and therefore reduced process efficiency. This is particularly apt in filter press operations, where manual intervention may be necessary to remove sticky cakes. As a consequence, in addition to longer cycle times, the cost of the operator must also be considered. To some extent this topic may be linked to the cake moisture content because, broadly speaking, wetter cakes will adhere more tenaciously to the cloth. This problem has been partly addressed by the equipment manufacturers with the incorporation of high pressure wash jets and brush cleaning devices, and the filter media producers also continue to pursue the development of fabrics that will facilitate the ultimate goal of perfect, unassisted cake release and hence the achievement of a fully automated operation. 

In identifying the filtration requirements, it is recognised that, in some cases, the filter fabric may require additional assistance, for example, by way of filter aids, body feeds, or even filter papers. The use of filter aids, of which there are many types, is designed to precoat the fabric with a layer of powder, such as diatomaceous earth. This is carried out in order (1) to protect it from blinding (2) to assist in the collection of particularly fine particles or (3) to enable more efficient cake release. In special circumstances, filter papers may also be used for similar reasons, especially where absolute clarity is essential. Body feeds, on the other hand, are added to the slurry to be filtered in order to enable the formation of a more porons cake than would otherwise be the case, thereby enhancing the rate of filtration flow. 

By producing fabrics with different components in warp and weft, it may be possible to create a stmcture that utiUses the best features of each. The most popular combinations in this respect are multifilament warp and staple-fibre weft yams (Fig. 3.24) and monofilament warp and multifilament weft yams. In both cases the ratio of warp to weft threads is at least 2 1 and usually considerably higher. This facilitates the production of fabrics with a smooth warp-faced surface for efficient cake release. 

Finishing treatments for fabrics employed in liquid filtration applications are designed for three basic reasons (1) to ensure dimensional stability during use (2) to modify the surface for more efficient cake release and (3) to regulate the permeability of the fabric for more efficient particle collection. 

With a small sample of slurry and a laboratory pressure vacuum leaf or piston press, stiU further refinements can be made. (This of course presumes that the nature of the slurry will be a representative sample whose character will not change irreversibly on leaving the manufacturing plant.) From such tests, speedy comparisons can be made on the parameters, such as throughput, filtrate clarity, cake moisture content, and a subjective assessment of cake release. Note that the medium/long-term blinding will not normally be apparent from such procedures. 

In order to produce apertures finer than mentioned above, changes in the weave are available to alter the size (and shape) of the cloth pores. Thus in the production of various twill cloths, weaves wch as one over-two under, three over-one under, two over-two under, can be arranged to produce different cloths, as shown in Figure 4.4. Cloth of this type, e.g. sateen weaves, possess very smooth surfaces which are optimal for cake release. 

New fiibrics are contimia% being developed by manu cturers aiming to inqtrove particle retention, cake release and filtrate drainage. 

The fiber cloth is the deciding ctor in the success or failure of all press operations. In view of the wide range of process variables involved in the filtration process, it is virtually in ossible to select a filter medium that will satisfy all process requirements and the usual limited time scale available for cloth selection is used to find an acceptable medium, i.e. one that will satisfy most, if not all of the requirements. In this reject, one particular requirement (e.g. filtrate clarity) may have to be relaxed, if other specifications (e.g. filtrate rate, absence of blinding) are to be maximised. Thus the more open weave Mcs will be superior in nonblinding characteristics, but may have poor particle retention. The latter will in rove in the order monofilament < muldfilament < staple fibre. Tabulated information is presented in Tables 4.2,4.3 and 4.4 below on the effect of yam properties, weave patterns, etc. on the processes of cake release, productivity, resistance to blinding, etc.. 

The critical in 5)ortance of filter cloth selection in filter press systems has been stressed in the literature [Regan, 1977], As discussed above, methods of filter cloth selection have been based on selection factors [Purchas, 1967, 1981]. These factors include textile fibre type, yam type, fabric geometry and weave pattern. Other factors include cloth shrinkage and stretching, filter cake release, cloth sur ce characteristics and cleaning. These added factors are considered below. 

Adequate cake release is a fundamental pre-requisite in efficient pressing operations, in maintaining a low down-time, t/m. the overall batch time. The overall productivity is given by the quotient (Vf/U) where is the filtrate produced per cycle and the cycle time tc = tf+tw+ U tf and U are the fillin filtration time and wash time, respectively. 

Applications into the traditional wov i cloth area are increasmg in view of the in roved performance (better ga et action around edges/bosses of plate) following surface treatments for pore size and cake release control Skinned felts, for presses, are sometimes supplied with a surface finish on both sides of the cloth. 

Both the permeability and filtration characteristics of nonwovens are dependent on the feh porosaty and fibre diameter. A medium which as been heavily calendered on both sides will possess the lowest poro. Sur ce treatments and/or use of laminations of different porosities, are aimed at inqtroving cake filtration performance and cake release. Generally speaking, the filtration efficiency at a particular particle size is inversely proportional to the fibre diameter, other Actors bdng the same. 

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