Withdrawing solids

Feeding and withdrawing solids from high-pressure units is not a trivial problem. For instance, if one wishes to withdraw hot reduced iron from an iron ore reduction unit operating at high pressure and temperature without withdrawing too much process gas and without plugging the withdrawal line, the operation is a difficult one. 

Assemble the apparatus shown in Fig. 1 V, 67, 1 this is self-explanatory. The distilling flask has a capacity of 250 ml. and the beaker contains 150 ml. of 10 per cent, sodium hydroxide solution. All corks must fit well and should be coated with paraflSn wax (by dipping into molten wax, and allowing to drain). Place half of the yield of the dry phenyldiazonium fluoborate in the distilling flask. Heat the solid gently with a small luminous flame at one point near its surface until decomposition begins withdraw the flame and allow the reaction to continue... 

Sample Withdrawal from Process A number of considerations are involved in the design of sample-withdrawal devices that wih provide representative samples. For example, in a horizontal pipe that conveys process fluid, a sample point on the bottom of the pipe wih collect a maximum amount of rust, scale, or other solid materials being carried along by the process fluid. In a gas stream, such a location will also collect a maximum amount of liquid contaminants. A sample point on the top side of a pipe will, for liquid streams, collec t a... 

In some crystalhzation apphcations it is desirable to increase the solids content of the shiny within the body above the natural consis-tencw, which is that developed by equilibrium cooling of the incoming feed solution to the final temperature. This can be done by withdrawing a stream of mother liquor from the baffle zone, thereby thickening the shiny within the growing zone of the crystallizer. This mother liquor is also available for removal of fine ciystals for size control of the product. 

A thickener has several basic components a tank to contain the slurry, feed piping and a feedwell to allow the feed stream to enter the tank, a rake mechanism to assist in moving the concentrated sohds to the withdrawal points, an underflow solids-withdrawal system, and an overflow launder. The basic design of a bridge-supported thickener mechanism is illustrated in Fig. 18-86. 

Automated control schemes employ one or more sets of controls, which will fit into three categories (1) control loops which are used to regulate the addition of tlocciilant, (2) control loops to regulate the withdrawal of Iindertlow, and (3) rake dri e controls. Usually, the feed to a thickener is not controlled and most control systems ha e been designed with some tlexibility to deal with changes in feed characteristics, such as an increase in oliirne or alteration in the nature of the solids thernseh es. In se ere cases, some equalization of the feed is required in order to allow the control system to perform effecth ely. 

On carbon dioxide service, rapid withdrawal of gas may result in plugging by solid CO2. Close the valve, if possible, to allow the metal to warm up this will prevent a sudden gas discharge. 

Absorption - Processes water can be removed from a material by the capillary action of porous bodies. An example is the cream of clay and water used for casting pottery, which is deprived of the greater part of its water by placing it in molds of plaster of Paris. The capillary character of this mold withdraws the water from the liquid clay mixture and deposits upon itself a layer of solid clay, the thickness of which is controlled by the time of standing. Certain types of candies, such as gumdrops, are dried mainly by contact with the starch molds in which they are cast. The drying effect of sponges, towels and materials of this kind is due to this same action. 

The arm rake automatically raises when periods of heavy sludge are encountered. Continuous raking action moves the solids down to the withdrawal point and then the arm is raised back into an optimal position automatically. 

Imidazole solution, and solid state Equivalent the IH tautomer Electron-withdrawing group prefers 4-... 

Treatment of nitrofurazans bearing an electron-withdrawing group with an anhydrous solid inorganic base (Na2C03, NaOAc, KCN, and other) in dry acetonitrile... 

Electrodeposition This method of paint application is basically a dipping process. The paint is water-based and is either an emulsion or a stabilised dispersion. The solids of the paint are usually very low and the viscosity lower than that used in conventional dipping. The workpiece is made one electrode, usually the cathode, in a d.c. circuit and the anode can be either the tank itself or suitably sized electrodes sited to give optimum coating conditions. The current is applied for a few minutes and after withdrawal and draining the article is rinsed with de-ionised water to remove the thin layer of dipped paint. The deposited film is firmly adherent and contains a minimum of water and can be stoved without any flash-off period. This process is used for metal fabrications, notably car bodies. Complete coverage of inaccessible areas can be achieved and the corrosion resistance of the coating is excellent (Fig. 14.1). 

When possible it is best to withdraw the fluid from a propeller-mixed system directly below the propeller. This allows removal of all solids and mixed liquids. 

For many reactions in the subsequent sections, the product of dehydration constitutes the reactant in a decomposition. Account must be taken of the changes in structure and texture which accompany or follow withdrawal of water from the lattice (perhaps in several steps) in considering the kinetics of subsequent decompositions or interactions of the anhydrous solid. 

Metal carbonate decompositions proceed to completion in one or more stages which are generally both endothermic and reversible. Kinetic behaviour is sensitive to the pressure and composition of the prevailing atmosphere and, in particular, to the availability and ease of removal of C02. The structure and porosity of the solid product and its relationship with the reactant phase controls the rate of escape of volatile product by inter-and/or intragranular diffusion, so that rapid and effectively complete withdrawal of C02 from the interface may be difficult to achieve experimentally. Similar features have been described for the removal of water from crystalline hydrates and attention has been drawn to comparable aspects of reactions of both types in Garner s review [ 64 ]. 

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