Agitated vessels circulation rate

Heating or cooling of process fluids in a batch-operated vessel is common in the chemical process industries. The process is unsteady state in nature because the heat flow and/or the temperature vary with time at a fixed point. The time required for the heat transfer can be modified, by increasing the agitation of the batch fluid, the rate of circulation of the heat transfer medium in a jacket and/or coil, or the heat transfer area. Bondy and Lippa [45] and Dream [46] have compiled a collection of correlations of heat transfer coefficients in agitated vessels. Batch processes are sometimes disadvantageous because ... 

The gas-liquid flow characteristics of stirred vessels depend both on the level of agitation and the rate of gas flow and can vary from the case of bubble column type operation to that of a full circulating tank, as shown in Fig. 5.123. The mixing characteristics and gas distribution obtained, obviously exert a considerable influence on the rate of mass transfer obtained (Harnby et al., 1985). 

Pitched-blade turbines (Fig. 18-3) are used on top-entering agitator shafts instead of propellers when a high axial circulation rate is desired and the power consumption is more than 2.2 kW (3 hp). A pitched-blade turbine near the upper surface of liquid in a vessel is effective for rapid submergence of floating particulate solids. 

A single, isothermal reactor design more reliably yields gypsum crystals than multiple-reactor designs with poorer temperature control [39]. This benefit has also been claimed for a two-vessel loop system. The effect of surfactants on crystallization of phosphogypsum has been examined [40]. Higher reactor circulation rates and increased agitation coupled with other refinements also improve acid recovery [41]. 

Skelland (Ref. 1, p. 106) gives the results of a theoiy of penetration with surface renewal in which it is assumed that the fractional rate of surface renewal s is related to the age of the surfoce throu the constants and n as r = where (, n + 1) > 0. A theoiy of penetration with periodically vaiying rates of surface renewal has been developed by Skelland and Lee for two-phase dispersions in turbine-agitated vessels. This attempts to allow for the varying degrees of turbulence encountered by bubbles or drops as they circulate through all regions of the vessel. 

An agitator acts like a centrifugal ptimp impeller without a casing and gives a flow at a certain pressure head. This circulation rate Q in m /s from the edge of the impeller is the flow rate perpendicular to the impeller discharge area. Fluid velocities have been measured in mixers and have been used to calculate the circulation rates. Data for baffled vessels have been correlated using the dimensionless flow number Nq (Ul). 

The performance of an agitator is frequently rated by its pumping capacity. In this way it is treated as if it were a pump circulating fluid from the bottom back to the top of the vessel at a uniform rate. This rate of circulation is labeled Fa in Fig- 3.8. It will then be seen that the time required for a particle to travel from top to bottom of the vessel, i.e., the dead time, is... 

In forced convection, circulating currents are produced by an external agency such as an agitator in a reaction vessel or as a result of turbulent flow in a pipe. In general, the magnitude of the circulation in forced convection is greater, and higher rates of heat transfer are obtained than in natural convection. 

II. Chemostat In a chemostat, the medium is delivered at a constant rate, to keep constant growth rate. The nutrient medium is supplied to the culture vessel at a constant rate by a peristaltic pump used to control the washout rate. The rate of media flow is often set at approximately 20% of culture volume per day. Air is pumped into the algal culture vessel through an air compressor controlled by a flow meter and carried in two flasks of sterile water. This bubbling air has three effects (i) it supplies C02 and 02 to the culture, (ii) allows circulation and agitation of the cultures and (iii)... 

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