Agitated tank example

As a first approximation, suppose that the concentration of oxygen in the gas phase changes instantly from 20.9% oxygen to 100% oxygen. Then a j will change instantly from 0.219 to 1.05mol/m, and the gas-phase balance is not required. The parameter k , = 0.1 s was specified in Example 11.1 so the only unknown parameter is the liquid holdup, VijV. A typical value for a mechanically agitated tank is 0.9. The liquid-phase balance becomes... 

Repeat Example 24-2 for the xylene (B) oxidation reaction carried out in an agitated tank reactor (instead of a bubble-column reactor). Use the data given in Example 24-2 as required, but assume the diameter D is unknown. Additional data are the power input without any gas flow is 8.5 kW the impeller rotates at 2.5 Hz the height and diameter of the tank are the same (h = D) the impeller diameter is DI3, and the impeller contains 6 blades assume ubr = 1.25usg. In addition to the vessel dimensions for the conversion specified (/B = 0.16), determine the power input to the agitator (P,). 

While the Dove composition described in Table 9.4-2 was processable at reasonable line speeds on a conventional soap processing line (roll mills, extruders, stampers), some equipment modifications were necessary. For example, whereas soap is normally mixed in large agitated tanks, the Dove mixture had a much greater viscosity and therefore required use of a steam-jacketed kneader mixer such as those used to make bread dough, pastes or mastics. 

The fu st step in the solution of the example is the design of the agitated tank, which will determine its hydraulic parameters and thus the mass transfer coefficients, which are an input in the model of the reactor. 

Thus, both interfacial area and hold-up should be high. For example, an agitated tank will give a high value of JA. 

In some cases, as in Example 4.4 that follows, a bubble column which is relatively short in relation to its diameter may be used (see Fig. 4.6). The bubbling gas will then generate sufficient circulation and turbulence in the liquid phase for the liquid to be assumed to be well mixed and uniform in composition (except, in principle, in the thin Alms immediately surrounding the bubbles). The circulating liquid will also drag down smaller bubbles of gas which can then mix with fresh gas. Under these circumstances, the gas phase can also be assumed to be well mixed, just as in the case of the agitated tank in Example 4.3. The question of taller bubble columns will be considered in the following section. 

As in Example 4.3 with an agitated tank, let the fraction of chlorine passing through the reactor unreacted be / and, because CI2 is replaced by HCI, / is also the mole fraction of chlorine in the off-gas. Since the total pressure is 1 bar, the partial pressure of the chlorine will be fu bar. Because the gas phase in the reactor is assumed to be well mixed, the equivalent interfacial chlorine concentration Ca, is fJStS, i.e. /u/0.45 = 2.22/ kmol/m3. Considering unit volume, i.e. 1 m3 of dispersion, and following equation 4.17, the rate of mass transfer across the interface is now equated to the rate of the reaction in the bulk of the liquid where the concentration of the chlorine is Qnt ... 

The overall rate of reaction calculated for the three-phase fluidised-bed reactor above is approximately one tenth of the rate calculated for the agitated tank slurry reactor in Example 4.6. The main reasons are the very poor effectiveness factor and the relatively smaller external surface area for mass transfer caused by using the larger particles. Even the gas-liquid transfer resistance is greater for the three-phase fluidised-bed, in spite of the larger particles being able to produce relatively small bubbles these bubbles are not however as small as can be produced... 

An agitated tank is often used as an example of a first-order lag process. However, mixing in real tanks falls far short of the ideal well-mixed tank. Real tanks have composition responses that are a combination of a first-order lag and deadtime. If the pumping rate of the agitator Fa) is known, the deadtime (T ) of the real tank may be estimated by the following equation Tj = V/ F - - Fa), where V is the volume of the tank and F is the flow through it. 

Not all crystallizers are agitated with impellers in the vessel itself in agitated tank configuration. A common arrangement is to use a pump (or an axial flow impeller in an external pipe) to generate flow in a loop external to, but drawing from, and discharging to the main vessel. Such crystallizers are known as forced circulation crystallizers. These are widely used, for example, in sodium chloride crystallization (see Chapter 5, section... 

Here we will pay attention to the gas-liquid reactors. The reaction takes place usually in the liquid phase. Three main types of contact may be distinguished following the phase ratio (1) gas bubbles dispersed in liquid, (2) liquid drops dispersed in gas, and (3) gas and liquid in film contact. In the first category we may cite gas-liquid bubble columns, plate or packed absorption columns, agitated tanks, agitated columns, static mixer columns, pump-type reactors. As examples in the second class we may name spray columns or liquid injection systems. The third category can be used with very exothermic reactions or viscous liquids. 

Re is the Reynolds number and Fr is the Froude number. Numerous flow problems can be solved by applying similarity relations. For the individual problems, the various scaling parameters given in Eqs. (11.8) to (11.9) must be chosen in a proper way. For example. Bird [31, pp. 101-102] exemplifies how the depth of a vortex in an agitated tank can be found out by similarity calculations. 

For the above reasons it can be seen for example why the assumption of the same value of the interfacial area in physical and chemical absorption can lead to some incertainty especially if the mass transfer coefficient is obtained from the ratio k a/a where kLa is measured by physical absorption or desorption and a from chemical absorption in two different series of experiments. The effective interfacial area in the case of the fast reaction system where the absorbing capacity is increased by a chemical reactant is probably much larger than the effective interfacial area for physical absorption or desorption and in fact the semi-stagnant liquid zones in a packing or in the bulk of an agitated tank can be more and less effective to mass transfer depending on the ratio between the absorbing capacity and the rate of absorption, as pointed out by Joosten and Danckwerts (33). These authors introduced a parameter y... 

The simplest type of crystallizer is the non-agitated tank into which hot liquor is poured and the contents allowed to cool naturally in a batch operation. Significant local and transient variations in supersaturation occur and in consequence the product mass can contain large interlocking crystals, agglomerates and fines which are difficult to separate, wash and handle. Solar saltmaking ponds provide the earliest example of this type of crystallizer (see Jones etal., 1981) but, with a few exceptions, most modern crystallizers incorporate some form of agitation. 

The condition of the curd on precipitation is important. As the milk starts to gel, agitators in the coagulation tanks are started as the temperature is raised to about 65°C. Under these conditions the protein is thrown out in fine particles. Too slow an agitation will produce large clots difficult to wash whilst too fine a curd also presents washing problems. In order to obtain the requisite consistency of the precipitate it may be necessary to add inorganic material to the skimmed milk. For example, the addition of phosphate ions will prevent undesirable flaky polymer. Similarly, calcium-deficient casein will not coagulate satisfactorily and the addition of calcium ions may be necessary. 

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