Contactor design and operation

This section discusses the design and operation of a single stage (Figure 10.2), followed by a multistage contactor (Figure 10.10). Other design and operational considerations are also reviewed. 

After a single contactor stage has been designed and the unit has been built and successfully tested with the desired process fluids at the required flow rates, a multistage unit can be designed and built. This section looks at the overall design and operation of the many stages usually required for a process flowsheet. 

This section concerns other design and operational matters for both single- and multistage contactors. 

North and Wells (N7) described the design and operation of a rotary-film contactor for the solvent extraction of metals directly from leached ore slurries. It consists of an enclosed rotating assembly of closely spaced disks centrally mounted on a common horizontal shaft. The transfer of the metal to the solvent layer is effected by the rotating disks where a film of slurry is formed on contact with the leached ore at the bottom of the vessel. This contactor gave excellent uranium recovery from sulfuric... 

Gas-liquid contacting operations, which transfer one or more components between a gas phase and a liquid phase, are important to numerous industrial chemical processes. Their significance is reflected in the abundance of different contactor designs and review articles. The importance of these operations to the chemical industry is affirmed by their global prevalence and involvement in annually producing hundreds of millions of tons of basic chemicals. The various gas-liquid contactor designs attempt to optimize controlling parameters or such specific domains as the gas-liquid interface or continuous-phase residence time. 

Table 2.5 Correlations estimated for contactor designs and mode operations...

A systematic, rational analysis of both isothermal and nonisothermal tubular systems in which two fluids are flowing must be carried out, if optimal design and economic operation of these pipeline devices is to be achieved. The design of all two-phase contactors must be based on a firm knowledge of two-phase hydrodynamics. In addition, a mathematical description is needed of the heat and mass transfer and of the chemical reaction occurring within a particular system. 

The types of equipment used, which range from stirred tanks and mixer-settlers to centrifugal contactors and various types of columns, affect both capital and operating costs [9]. In the decision to build a plant, the choice of the most suitable contactor for the specific situation is most important. In some systems, because of the chemistry and mass transfer rates involved, several alternative designs of contacting equipment are available. In the selection of a contactor, one must consider the capacity and stage requirements solvent type and residence time phase flow ratio physical properties direction of mass transfer phase dispersion and coalescence holdup kinetics equilibrium presence of solids overall performance and maintenance as a function of contactor complexity. This may appear very complicated, but with some experience, the choice is relatively simple. 

This subsection describes the design of a contactor stage and how that design affects contactor operation. It will be shown when to add or avoid a particular feature. The discussion follows the flow of the two immiscible liquids through a contactor stage liquid entry, mixing, separation, and liquid exit. Finally, criteria for motor selection are reviewed. 

The 3.3-cm Russian contactors were used to test a cobalt-dicarbollide-based solvent-extraction process for separating Cs, Sr, and the actinides from dissolved HLW (Law et al., 2001, Herbst et al., 2002). These contactors were designed and fabricated in Moscow, Russia, by the Research and Development Institute of Construction Technology (NIKIMT). They are operated at 2700 rpm and have a nominal throughput of 417 mL/min (25 L/h). Figure 10.25 shows the 26-stage 3.3-cm contactor bank used in these tests. A recent summary of this work is given by Romanovskiy et al. (2005). 

For standard types of finite-stage contactor columns operated in the range of allowable velocities where the overall column efficiencies are essentially constant, O Connell has correlated efficiency data on the basis of liquid viscosity and relative volatility (or gas solubility). The results for fractionators and absorbers are presented in Fig. 16-9. This correlation is based, primarily, on experimental data obtained with bubble-cap columns having a liquid path of less than 5 ft and operated at a reflux ratio near the minimum value. Figure 16-9 is adequate for design estimates with most types of commercial equipment and... 

---