Retreat-curve impellers

In glass-lined vessels, the 3 blade Pfaudler Retreat Curve Impeller (RCI) is often used for liquid-liquid dispersions although the myriad of impeller styles currently available with glass... 

If the application requires high interfacial area (i.e., small drop diameters), a high-shear impeller, such as the Rushton or radial disk turbine (RDT), is a good choice (Fig. 1). Acceptable substitutes include the Scaba and Chemineer s BT-6 and CD-6 impellers, commonly used for gas-liquid dispersion. If moderate, yet gentle shear is required, such as for emulsion polymerization, the retreat-curve impeller is commonly chosen. When larger drops of a narrow size distribution are required, the loop impeller is a reasonable choice. Broad-blade paddles are also used. 

Power Number, Np The power number, Np, sometimes referred to as Po, is a measure of the relative drag of the impeller. Streamline curved blades, like hydrofoils and retreat-curve impellers, have less drag than flat blades consequently, their power numbers are lower than those for flat-blade impellers. Power numbers of some of the more popular impellers are given in Table 9.1. The calculation of power from impeller diameter, speed, and liquid density is given by Equation (9.1). 

Impellers available in the pre-1960 era would have been limited to four- and six-blade disc turbines (also known as radial-flow turbines or RFT or Rushton turbines), the four- and six-blade 45° pitch blade turbines (PBT), the four- and six-blade flat-blade turbines (FBT), and the three-blade retreat-curve impellers (RCI). 

FIGURE 9.3 Commonly used impellers (L-R) (row 1) Lightnin A-310, Chemineer HE-3 (row 2) Rushton (R-lOO, D-6), Chemineer CD-6 (row 3) Pfaudler TurbofoU T, Lightnin C-102 Mark It (row 4) Lightnin A-315, Lightnin A-320 (row 5) 45° pitched hlade turbine, Lightnin R-500 (row 6) Prochem Max Flow T, Pfaudler 4-blade, curved blade turbine, retreat-curve impeller RCI. 

Impeller clearance affects total discharge flow and its direction. For example, when axial-flow impellers are placed close to the bottom, they produce a radial discharge. Upward-angled, retreat-curve impellers are always located at the bottom of the vessel, where they produce strong radial flow and produce good circulation while handling level changes and solids effectively. 

Temperature control by this strategy is not balanced for adiabatic cooling to allow recovery if the target temperature is undershot however, it does permit higher heat transfer rates without wall fouling encountered from jacket cooling. An additional caution is required for vessels with retreat curve impellers. These have adequate pumping capacity for solids suspension but seldom... 

Specialty Retreat curve impeller, sweptback impeller, spring impeller,... 

For glass-lined vessels, one is no longer limited to the Pfaudler crowfoot, also known as the retreat blade or retreat curve impeller (RCl). Most impeller... 

Any impeller in a vessel capable of pumping fluid and providing shear can produce liquid-liquid dispersions. The impellers commonly used for immiscible liquid-liquid systems include disk turbines, pitched blade turbines, propellers, hydrofoils, paddles, retreat curve impellers, and other proprietary designs. We showed in Section 12-2 that drop size depends on maximum energy dissipation rate. More specifically, eq. (12-23) shows that the power number of an impeller affects drop size. In this section we deal with equipment used for two common industrial applications creating the maximum interfacial area and creating uniformly sized drops. 

Recommended impeller diameter-to-tank diameter D/T) ratios, for liquid-liquid operations vary from 0.25 to 0.40 for radial disk turbines, from 0.4 to 0.6 for hydrofoils and propellers, and from 0.5 to 0.8 for retreat-curve, glassed-steel impellers. Vertical placement of the impeller depends on the vessel shape and the application. For dispersion by continuous addition of a dense phase fluid into a less dense fluid, the impeller should have a relatively small impeller clearance off the reactor bottom, C, with respect to the final height of the dispersion, H, i.e., the impeller should be placed low in the vessel (C H/A to H/S). For dispersion of light liquids, it is good practice to place a single impeller between 0.20 < C/H < 0.50. 

Given that the impeller is a glass-coated retreat-curve type, the approximate calculation for the minimum suspension speed is 115 rpm. A recommended design speed would be 130-150 rpm. This would guarantee more than minimum suspension conditions. The power at 150 rpm based on a power number of 0.6 would be 0.323 kW ( 0.5 hp). 

Ackley [79] suggests use of Equation (9.87) for unbaffied retreat-curve (blade) impellers typically used in glass-lined vessels ... 

For glass-lined vessels and retreat curve blade impellers and glass-lined turbines, five different types of baffles (shown in Figure 6-7) are commonly used Anger, flattened pipe, h style, concave baffle, and fin. These baffles can be conveniently supported in the vessel heads of glass-lined reactors. Of these, the fin baffle has become a more standard choice. 

It is important to note that some impellers and mixer configurations do not pump weU. For example, the retreat curve blade impeller in an unbaffled vessel creates solid body rotation and poor pumping. Pumping with close-clearance impellers such as anchors and helical ribbon can be very high or, sometimes, very poor, depending on conditions and the materials being pumped. Turbulent impellers in laminar applications only pump locally. Often, the rest of the tank goes unmixed. 

Retreating blade turbine Yes No limitation 0.33 (Nre)0-67 (Np,-)0 33 (Pb/P.)0 14 Glassed-steel impeller. Three retreating blades. The lower constant (0.33) for the glassed-steel impeller is attributed to greater slippage around its curved surfaces than around the sharp comers of the alloy-steel impeller. 

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