Stirred tank impellers location

As discussed in Chapter 6, high-energy dissipation zones have been identified for certain stirred tank/ impeller configurations. These zones are often small, and they can move enough so that the exact location of and linear velocity from an addition dip pipe ate very difficult to optimize. When very intense micro- and/or mesomixing are required, stirred tanks are not the ideal type of equipment to catty out a robust, reproducible process. 

Determine the reactor volume required for one reactor and that for two equal-sized reactors in series for 80 percent conversion of A. And if the capital cost of a continuous-flow stirred-tank reactor unit is given by 200,000(17/100)° 6 (where V is reactor volume in m3), the life is 20 years with no salvage value, and power costs 3 cents per kilowatt-hour, determine which system has the economic advantage. Assume that overhead, personnel, and other operating costs, except agitation, are constant. The operating year is 340 days. Each reactor is baffled (with a baffle width to tank diameter of 1/12) and equipped with an impeller whose diameter is one-third the tank diameter. The impeller is a six-bladed turbine having a width-to-diameter ratio of 1 /5. The impeller is located at one-third the liquid depth from the bottom. The tank liquid-depth-to-diameter ratio is unity. 

A stirred tank fermenter consists of a centrally mounted agitation system inside a cylindrical vessel. Typically, the agitation system is composed of either multiple radial flow impellers (see Fig. 4A) or a combination of radial and axial flow impellers as shown in Fig. 4B. A gas sparger is located below the radial gas dispersing impeller. The role of the bottom radial impeller... 

Stirred tank paddles power input suspend solids, 0.2 to 1.6 kW/m UD = 0.7 to 1.05/1. Baffle, four 90° baffle width = 0.08 x tank diameter off-the-wall distance = 0.015 x tank diameter. Minimum level of liquid = 0.15 x tank diameter for impeller tank diameter 0.28 1 and minimum level = 0.25 x tank diameter for impeller tank diameter = 0.4 1. Use a foot bearing plus a single, main axial hydrofoil impeller diameter = 0.28 x tank diameter located 0.2 x tank diameter from the bottom plus a pitched blade impeller diameter = 0.19 x tank diameter located 0.5 x tank diameter from the bottom. Liquid fluidized bed in general, particle diameter 0.5 to 5 mm with density and diameter of the particle dependent on the application. The superficial liquid velocity to fluidize the bed depends on both the diameter and the density difference between the liquid and the particle. Usually, the operation is particulate fluidization. Particle diameter 0.2 to 1 mm reactors superficial liquid velocity 2 to 200 mm/s. Fluidized adsorption bed expands 20 to 30% superficial liquid velocity for usual carbon adsorbent = 8 to 14 mm/s. Fluidized ion exchange bed expands 50 to 200% superficial liquid velocity for usual ion exchange resin = 40 mm/s. Backwash operations fixed-bed adsorption superficial liquid velocity = 8 to 14 mm/s fixed-bed ion exchange superficial backwash velocity = 3 mm/s. 

The reactor, or Contactor, is basically a special type of a continuous-flow stirred tank reactor, as shown in Figure 1 (6). It is a cylindrical vessel positioned horizontally in which the acid/hydrocarbon dispersion is repeatedly circulated over and around heat transfer coils (tube bundle). The impeller employed to promote the dispersion of the feed mixture of isobutane and olefins in the acid phase is located at one end of the reactor. The impeller causes the dispersion to enter the annular region between the shell of the reactor and the tube bundle the dispersion flows rapidly in this region, which extends over most of the length of the reactor. As the dispersion reaches the exit end of the annulus, a small portion of the dispersion is withdrawn and fed to the decanter, which is discussed later. The remainder of the dispersion leaving the annular region makes a 180° turn at the end of the reactor and flows back toward the impeller. As it returns, the dispersion passes over and around U-shaped heat transfer coils that remove the exothermic heats of reaction and the energy added to the reactor by the impeller. 

The Ballestra system consists of a number of stirred-tank reactors, arranged in a cascade sequence or train. Each reactor is equipped with a high-speed turbine impeller to disperse the gas and to mix and circulate the organic phase within the reactor. Cooling is facilitated by cooling coils located within the body of the reactor and a cooling jacket around the reactor. 

Figure 3-29 The evolution of a fast reaction in a mixing tank stirred with three Rushton impellers at Re = 20. The reactive zones in a stirred tank are identieal to the location of the intermaterial contact area in a mixing after 10, 20, 40, and 60 impeller revolutions. Each figure shows half of the vertical cross-section, where the shaft and three impeller blades are seen on the left. The upper half is a photograph of the reactive flow and the bohom is the computed chaotic mixing structure.

Fignre 5-lfl shows the outline of a simple baffled stirred tank containing a Rushton turbine on a centrally mounted shaft. The tank has diameter T. The impeller has diameter D and is located a distance C off the bottom of the tank. These symbols are used throughout the chapter. 

What is the minimum agitator speed to suspend the solids In stirred tanks, there is always an impeller speed below which settling solids will tend to accumulate on the bottom of the vessel. This speed is different for different types of impellers and for identical impellers located at different clearances from the bottom of the vessel. It also depends on the properties of the solid and liquid phases. The minimum speed may be estimated for certain impeller and tank geometries using the Zwietering correlation. It is advisable, however, to determine this value experimentally for processes where solid-liquid mixing is deemed critical. See Section 10-2.2 for details. 

Figure 20-6 Effect of yield stress on suspension motion in a stirred tank. Cm = 0.02 FBK suspension. The vessel is 30 cm in diameter with the suspension height set at 30 cm. A D = 10 cm diameter Rushton turbine was located 10 cm from the vessel floor. Impeller speeds are N = (a) 4, (b) 7, and (c) 14 rps. The red dye shows regions of suspension motion. In image (a) the cavan has not reached the vessel wall. See insert for a color representation of this figure.

The cylindrical mixing tank simulated in this study has an ellipsoidal bottom with four equally spaced, wall-mounted baffles extending from the vessel bottom to the free surface, stirred by two centrally located six-blade Rushton turbine impellers. The tank diameter measured 0.138 m, and the baffle width was 0.008 m. The impeller diameter was 0.046 m (PIT=2) for both impellers. The distance between the impellers was 0.061 m. The bottom impeller center was positioned at a distance C=T/3 off the tank bottom. The liquid level was equal to the tank diameter, Z/7 =1.3. 

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