Agitated vessels mixing

Agitated vessel Complete mixing Integral Liquid Absorption... 

Mixing in Agitated Vessels Agitated vessels may frequently be used for either batch or continuous service and for the latter may be sized to provide any holding time desired. They are useful for liquids of any viscosity up to 750 Pa s (750,000 cP), although in contacting two liquids for reaction or extraction purposes viscosities in excess of 0.1 Pa s (100 cP) are only rarely encountered. 

Computation fluid mixing and computational fluid dynamic techniques have increasingly been used to elucidate solids distribution in agitated vessels [44],... 

Blend time and ehemieal produet distribution in turbulent agitated vessels ean be predieted with the aid of Computational Fluid Mixing... 

Alternative microcrystallizer configurations are being developed that seek to avoid the mixing problems asociated with conventional agitated vessels and offer the potential of consistent precipitation of high quality crystal products. 

Mersmann, A. and Laufliiitte, H.D., 1985. Scale-up of agitated vessels for different mixing processes. 5th European Conference on Mixing 1985, Wurzburg, pp. 273-284. 

In a mixed agitated vessel with high agitation rate, at the centre of the vessel a vortex often forms. To prevent a central vortex in tanks less than 3 m in diameter, four baffles each with a baffle width of 15-20 cm are necessary. A basic assumption is to select a ratio of liquid height to tank diameter from 2 1 to 6 1. 

It is well understood that mixing and mass transfer are affected by agitation and aeration rates. Typically, in an agitated vessel, the working volume is about 75% of nominal CSTR volume. 

A reaction is to be carried out in an agitated vessel. Pilot scale tests have been carried out under fully turbulent conditions in a tank 0.6 m in diameter, fitted with baffles and provided with a flat-bladed turbine, and it has been found that satisfactory mixing is obtained at a rotor speed of 4 Hz when the power consumption is 0.15 kW and the Reynolds number 160,000. What should be the rotor speed in order to achieve the same degree of mixing if the linear scale of the equipment if increased by a factor of 6 and what will be the Reynolds number and the power consumption ... 

Mixing times in mechanically agitated vessels typically range from a few seconds in laboratory glassware to a few minutes in large industrial reactors. The classic correlation by Norwood and Metzner for turbine impellers in baffled vessels can be used for order of magnitude estimates of... 

A real continuous-flow stirred tank will approximate a perfectly mixed CSTR provided that tmix h/i and tmix i. Mixing time correlations are developed using batch vessels, but they can be applied to flow vessels provided the ratio of throughput to circulatory flow is small. This idea is explored in Section 4.5.3 where a recycle loop reactor is used as a model of an internally agitated vessel. 

The archetypal, stagewise extraction device is the mixer-settler. This consists essentially of a well-mixed agitated vessel, in which the two liquid phases are mixed and brought into intimate contact to form a two phase dispersion, which then flows into the settler for the mechanical separation of the two liquid phases by continuous decantation. The settler, in its most basic form, consists of a large empty tank, provided with weirs to allow the separated phases to discharge. The dispersion entering the settler from the mixer forms an emulsion band, from which the dispersed phase droplets coalesce into the two separate liquid phases. The mixer must adequately disperse the two phases, and the hydrodynamic conditions within the mixer are usually such that a close approach to equilibrium is obtained within the mixer. The settler therefore contributes little mass transfer function to the overall extraction device. 

Mixed-flow Mixed-flow Mechanically agitated vessel 10 0.02-0.2... 

Figure 14.4 Some examples of mixing compartment networks to represent agitated vessels. (Reproduced from Zhang J and Smith R, 2004, Chem Eng Sci, 59 459, by permission of Elsevier Ltd).

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