Washing, counter-current

The cycle of a Tilting Pan Filter that includes three counter-current washing stages consists of the following zones ... 

This unit is good to pre-thicken feed to centrifugal filters and similar applications. One cyclone may satisfy a requirement, or the units can be arranged in parallel for large capacities or in series for removal of extreme fines. See Figure 4-53 for a counter-current wash system. Solids as small as 10 microns can be separated. 

In wet scrubbing the dust is removed by counter-current washing with a liquid, usually water, and the solids are removed as a slurry. The principal mechanism involved is the impact (impingement) of the dust particles and the water droplets. Particle sizes down to 0.5 /i.m can be removed in suitably designed scrubbers. In addition to removing solids, wet scrubbers can be used to simultaneously cool the gas and neutralise any corrosive constituents. 

The biocatalyst may be supported on a Lewis acid. Elemental sulfur is removed from the liquid hydrocarbons and the recovered solvent is counter-currently washed with water in a separate unit. Prior to reuse, the solvent is distilled to decontaminate it from remaining water or sulfur slurry. The treated product not only has a reduced concentration of organic sulfur compounds, but also its viscosity is reduced. 

A system is now considered consisting of n washing thickeners arranged for counter-current washing of a solid from a leaching plant, in which the whole of the soluble material is dissolved. The suspension is separated in a thickener and the underflow from this thickener is fed to the washing system as shown in Figure 10.12. 

A liquid/liquid separation of product and catalyst is done in separate vessels after the reaction has taken place. The reaction mixture is sent to a gas separator and from there to a counter current washing tower (a simple phase separator is shown in the figure) in which the effluent is treated with aqueous Na2C03. The acidic HCo(CO)4 is transformed into the water soluble conjugate base NaCo(CO)4. The product is scrubbed with water to remove the traces of base. The oxo-crude goes to the distillation unit. 

String discharge filter. (7) Two or three stages of counter-current washing. ... 

Uses of Dorr and Other Thickeners.—Outside of the separation use to which these devices can be put is their employment in what is termed counter current washing/ To show the extractive and recovery possibilities a hypothetical computation based on three tanks will be made. It is assumed for convenience that the tanks hold 110 tons of liquid and solid in the proportion by weight of 10 parts of liquid to one of solids. It is desired to save the liquid which is valuable and reject the solids with the minimum of valuable liquid. Let it be further assumed that the solids will settle to a ratio of liquid to solids of 1 1 in each tank. 

The bottom of this tower, a mixture of C4 and ether produced in the second stage, is recycled to the first column while the overhead stream (C4 and ethanol) is sent to the alcohol extraction tower. In this column, ethanol is removed by counter-current washing with alcohol-free water while C4 raffinate, isobutene-free, is sent to battery limits. The stream water/ethanol is finally sentto an additional fractionation tower where the ethanol-free water, recovered as bottom stream, is recycled to the washing tower, and the overhead ethanol/water azeotropic stream is recycled to the reaction stages. 

The reaction effluent, mainly consisting of isobutylene, methanol and unconverted MTBE, is sent to a counter-current washing tower (2) to separate out methanol, and then to two fractionation towers to separate isobutylene from unconverted MTBE, which is recycled to the reactor (3) and from light compounds (4). The produced isobutylene has a product purity of 99.9-" wt%. 

The raw material which is pre-heated to around 115 °C, is fed at atmospheric pressure to the extraction column (e.g. rotating disc contactor, sieve tray extractor), where the aromatics are preferentially dissolved by sulfolane. The solvent is separated from the aromatics mixture at a temperature of around 190 ""C and a pressure of just 1 bar. The sulfolane which boils at 287 °C, remains at the base of the column and is recycled to the top of the extraction column. The aliphatics are recovered by counter-current washing with sulfolane and subsequent distillation. 

It is necessary to remove all the solvent from the fiber, usually carried out by counter current washing with hot water in conjunction with fiber stretching. 

The recycled solvent is usually pumped to a feed stock tank for re-use, after appropriate counter-current washing, if required. Stable solvents can usually be selected which do not require replacement, except to make up for process losses. 

Hermia J., 1981. Calculation method for counter current washing on continuous vacuum filters, Hydrometallurgy 81, Soc. Chertu Ind., C5, 1-10. 

Hermia J. and Letesson Ph., 1982. The universal washing curve and its application to multi-staged counter current washing processes, Proc. 3rd World Filtration Congress, 426-435, Downingtown. 

Finally, multiple-series arrangements are also extensively used in counter-current washing of solids. The reader is referred to chapter 15 for the description and design of the washing trains used for this purpose. 

Horizontal belt filters are well suited to either fast or slowly draining solids, especially where washing requirements are critical. Multi-stage counter-current washing can be effectively carried out due to the sharp separation of filtrates available here. Horizontal belt vacuum filters are classified according to the method employed to support the filter medium. 

Just like washing generally can be done by washing filter cakes in situ (whilst they are still on the filter medium), by reslurrying of filter cakes or sediments, or by successive dilutions of slurries, the same is done in counter-current washing. Applications and examples exist in industry for all of the above three types. 

The rest of this chapter, however, is concerned with continuous counter-current washing. The separators used may be sedimenting centrifuges, gravity thickeners or filters but the preferred choice is often hydrocyclones. The inherent advantages of hydrocyclones are compactness, small hold-up of liquid, high shear forces and turbulence in the flow, the latter two of which improve mass transfer and dispersion. 

Removal of superfine solids from coarser particles. This brings counter-current washing into the realms of particle classification as considered in chapter 16. An example of this application is separation of the two sizes of... 

White, D. A. and Bashir, S., Counter current washing of ion exchange floes— theory and optimisatiou , Solid-Liquid Separation Practice III, I.ChemL. Symposium Series No. 113, The Institution of Chemical Engineers, Rughy, 41-50 (1989)... 

Svarovsky, L., Counter-Current Washing of Solids , Chapter 15, in Solid-Liquid Separation, 3rd edn, Ed. L. Svarovsky, Butterworths, London, 533-540 (1990)... 

Svarovsky, L. and Potter J. K., Counter-current washing with hydrocyclones . Paper J2, 3rd International Conference on Hydrocyclones, Oxford, 30 September to 2 October, 1987, BHRA, Cranfield, 235-247 (1987)... 

Trawinski, H., Counter-current washing of thickened suspensions by repeated dilution and separation by sedimentation , Verfahrenstechnik, Iio. 1,28 31 (1974)... 

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