Washing filter cakes efficiently

A 100-mL volume of benzene is added to the 20 g of air-dried soil and the mixture is shaken vigorously for 2h. After extraction twice with 100 mL of benzene, the combined extract is filtered through filter paper and the filter cake is washed with an additional 20 mL of benzene. The benzene extracts are dried over anhydrous Na2S04 and concentrated to dryness using a vacuum rotary evaporator. The residue is dissolved in an appropriate volume followed by GC/ECD analysis. For the monitoring of pesticide residues in soil, methanol for bifenox and oxyfluorfen and acetonitrile for nitrofen were recommended as the solvents for efficient extraction. ... 

The very nature of process development necessitates the contributions of all members of a typical development team. Thus, reaction engineers determine reaction kinetics and select the best reactor type, while filtration experts measure the filter cake resistance and washing efficiency. To reduce development time, it is crucial that all of these activities be performed in a coordinated manner. Proper workflow automates such a development process, in whole or part, during which documents, information, or tasks are passed from one participant to another for action, according to a set of procedural rules. 

These filters (Fig. 9) have the advantages of low cost, near indestructibility, and ease of internal inspection. They have the lowest liquid volume-to-area ratio, which makes them most efficient for the washing of filter cakes. Because of this low ratio, they will also have the smallest unfiltered heel remaining at the end of the cycle. 

Any washing operation will reduce the content of downstream colloidal material. Screening is an efficient method of removing organic contaminants from the fibres. The removal can be aided by dispersants that will shift the equilibrium of depositable material adsorbed onto the fibre to depositable material in the aqueous phase. Deaerators, additives that will remove entrained air from the furnish, will increase dry content of the fibre cake and thereby reduce the carry over of water over the wash filter. This will lead to an overall downstream reduction in colloidal material. 

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. 

Similar considerations apply to filtration where only reslurry washing is involved. Usually however, additional washing is given on the filter and the efficiency is not then amenable to calculation without detailed knowledge of the manner in which the wash water displaces the filtrate from the filter cake in each individual case. 

A number of these leaves are hung in parallel in a closed tank. The slurry enters the tank and is forced under pressure through the filter cloth, where the cake deposits on the outside of the leaf. The filtrate flows inside the hollow framework and out a header. The wash liquid follows the same path as the slurry. Hence, the washing is more efficient than the through washing in plate-and-frame filter presses. To remove the cake, the shell is opened. Sometimes air is blown in the reverse direction into the leaves to help in dislodging the cake. If the solids are not wanted, water jets can be used to simply wash away the cakes without opening the filter. 

Continuous rotary disk filter. This filter consists of concentric vertical disks mounted on a horizontal rotating shaft. The filter operates on the same principle as the vacuum rotary drum filter. Each disk is hollow and covered with a filter cloth and is partly submerged in the slurry. The cake is washed, dried, and scraped off when the disk is in the upper half of its rotation. Washing is less efficient than with a rotating drum type. 

To ensure the same washing efficiency on large scale as on lab scale, it is important to learn about the washing behavior of the filter cake. In case the filter cake tends to crack, this may be not critical on lab scale if the crack is closed, for example, with a spatula, but on large scale this happens to a much higher extent and the cracks may cause the wash to preferentially flow this way and the rest of the cake will not be washed properly. 

Values regarding specific filtration capacity, residual moisture in the filter cake and washing efficiency (if washing of the filter cake is necessary) as well as then-dependency on cycle time and immersion depth established in the laboratory can, within the limitations set by the operational aggregate and taking into account of safety factors, reliably be applied to the production unit. The laboratory experiment also provides important information regarding the most appropriate method of discharging the filter cake, and the filter cloth to be selected. 

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