Continuous thickening

The controlled thermal decomposition of dry aromatic diazonium fluoborates to yield an aromatic fluoride, boron trifluoride and nitrogen is known as the Schiemann reaction. Most diazonium fluoborates have definite decomposition temperatures and the rates of decomposition, with few exceptions, are easily controlled. Another procedure for preparing the diazonium fluoborate is to diazotise in the presence of the fluoborate ion. Fluoboric acid may be the only acid present, thus acting as acid and source of fluoborate ion. The insoluble fluoborate separates as it is formed side reactions, such as phenol formation and coupling, are held at a minimum temperature control is not usually critical and the temperature may rise to about 20° without ill effect efficient stirring is, however, necessary since a continuously thickening precipitate is formed as the reaction proceeds. The modified procedure is illustrated by the preparation of -fluoroanisole ... 

Sedimentation is also used for other purposes. For example, relative motion of particles and Hquid iacreases the mass-transfer coefficient. This motion is particularly useful ia solvent extraction ia immiscible Hquid—Hquid systems (see Extraction, liquid-liquid). An important commercial use of sedimentation is ia continuous countercurrent washing, where a series of continuous thickeners is used ia a countercurrent mode ia conjunction with reslurrying to remove mother liquor or to wash soluble substances from the soHds. Most appHcations of sedimentation are, however, ia straight sohd—Hquid separation. 

The primary function of a continuous thickener is to concentrate sus-penaed solids by gravity settling so that a steady-state material balance is achieved, solids being withdrawn continuously in the underflow at the rate they are supphed in the feed. Normally, an inventory of pulp is maintained in order to achieve the desired concentration. This volume will vary somewhat as operating conditions change on occasion, this inventoiy can be used for storage of sohds when reed and underflow rates are reduced or temporarily suspended. 

Operating Costs Power cost for a continuous thickener is an almost insignificant item. For example, a unit thickener 60 m (200 ft) in diameter with a torque rating of 1.0 MN-m (8.8 Mlbf in) will normally require 12 kW (16 hp). The low power consumption is due to the very slow rotative speeds. Normally, a mechanism vi l be designed for a peripheral speed of about 9 m/min (0.5 ft/s), which corresponds to only 3 r/h for a 60-m (200-ft) unit. This low speed also means veiy low maintenance costs. Operating labor is low because little attention is normally required after initial operation has balanced the feed and underflow. If chemicals are required for flocculation, the chemical cost frequently dwarfs all other operating costs. 

Font, R. and Laveda, M.L., 1996. Design method of continuous thickeners from semibatch tests of sedimentation. Chemical Engineering Science, 51, 5007-5015. 

Scott, K.J., 1968b. Experimental study of continuous thickening of a flocculated silica slurry. Industrial and Engineering Chemistry, 7, 582. 

The lamella thickening depends sensitively on the initial lamella thickness as well as on the annealing temperature. We first considered the thickening of very thin lamella of about 18-bonds thick this thickness nearly corresponds to that of the lamella crystallized at 0 K (Fig. 7). The temperature of annealing Ta was taken between 20 K and 150 K a quick jump to a higher temperature resulted in partial melting and re-crystallization and a continuous thickening process could not be observed. 

In a continuous thickener, the area required for thickening must be such that the total solids flux (volumetric flowrate per unit area) at any level does not exceed the rate at which the solids can be transmitted downwards. If this condition is not met, solids will build up and steady-state operation will not be possible. If no solids escape in the overflow, this flux must be constant at all depths below the feed point. In the design of a thickener, it is therefore necessary to establish the concentration at which the total flux is a minimum in order to calculate the required area. 

At any horizontal plane in a continuous thickener operating under steady-state conditions, the total flux of solids xfrT is made up of two components ... 

In a batch sedimentation experiment, the sediment builds up gradually and the solids which are deposited in the early stages are those which are subjected to the compressive forces for the longest period of time. In the continuous thickener, on the other hand, all of the particles are retained for the same length of time with fresh particles continuously being deposited at the top of the sediment and others being removed at the same rate in the underflow, with the inventory thus remaining constant. Residence time distributions are therefore not the same in batch and continuous systems. Therefore, the value of tR calculated from equation 5.59 will be subject to some inaccuracy because of the mismatch between the models for batch and continuous operation. 

The limiting operating conditions for continuous thickeners has been studied by a number of workers including Tiller and Chen(51), and the height of the compression zone has been the subject of a paper by Font(52). 

Yoshioka, N., Hotta, Y., Tanaka, S., Naito, S. and Tsugami, S. Kagaku Kogaku (J. Soc. Chem. Eng., Japan 2) 21 (1957) 66. Continuous thickening of homogeneous flocculated slurries. 

Uassett, N. J. Industrial Chemist 37 (1961) 25. Theories of the operation of continuous thickeners. 

Go Volumetric feed rate of suspension to continuous thickener m3/s L3T- ... 

U Mass ratio of liquid to solid in underflow from continuous thickener ... 

Y Mass ratio of liquid to solids in feed to continuous thickener — —... 

The bottom of Figure 4 indicates one system for the handling of the crude with silt from the bottom of the column before, after, or without a heat exchanger to recover the sensible heat of the bottoms from this still. A continuous thickener, of the usual rotating type with hoes, thickens the suspended silt into an oil-mud which is removed, washed, and steamed before discard. The solvent recovered from this washing of mud goes back to the feed dissolution. The clear crude kerosene solution, now free of water and silt, has kerosene removed by distillation, or it is pumped to the refinery. Often where the crude does not contain too much water in a permanent emulsion or is not too difficult to pump, it would not require the initial dilution. 

The primary function of a continuous thickener is to concentrate suspended solids by gravity settling so that a steady-state material balance... 

Thickeners are used to concentrate dilute suspensions in preparation for further separation in filters and centrifuges. As previously shown in Table 3, thickeners frequently lead to removal of a large fraction of the liquid in a slurry. A schematic continuous thickener is shown in Fig. 24. In the figure, a clear liquid overflows the top while a thickened sediment flows out of the bottom as underflow. Thickeners are widely used in fields sueh as wastewater, aluminum, coal, pulp and paper, cement, and sugar. 

Tiller, F.M. Chen, W. Limiting operating conditions for continuous thickeners. Chem. Eng. Sci. 

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