Pumps air lift

Loop reactor modified air lift, pump transport the air and fluid through the vessel. 

A gas-liquid mixture will have a lower density than the liquid alone. Therefore, if in a U-tube one limb contains liquid and the other a liquid-gas mixture, the equilibrium height in the second limb will be higher than in the first. If two-phase mixture is discharged at a height less than the equilibrium height, a continuous flow of liquid will take place from the first to the second limb, provided that a continuous feed of liquid and gas is maintained. This principle is used in the design of the air lift pump described in Chapter 8. 

The principal flow patterns are shown in Figure 5.1. In general, the flow pattern map (Figure 5.2) is also applicable to vertical flow. Further reference to flow of gas- liquid mixtures in vertical pipes is made in Section 8.4.1 with reference to the operation of the air-lift pump. 

There are a number of important applications of the air-lift pump in the process industries due to its simplicity. It is particularly useful for handling radioactive materials as there are no mechanical parts in contact with the fluid, and the pump will operate virtually indefinitely without the need for maintenance which can prove very difficult when handling radioactive liquids. 

An air-lift pump raises 0.01 m3/s of water from a well 100 m deep through a 100 mm diameter pipe. The level of water is 40 m below the surface. The air flow is 0.1 m3/s of free air compressed to 800 kN/m2. Calculate the efficiency of the pump and the mean velocity of the mixture in the pipe. 

In an experimental study of a small air-lift pump(6), (25 mm. diameter and 13.8 m overall height) the results were expressed by plotting the efficiency of the pump, defined as the useful work done on the water divided by the energy required for isothermal compression of the air, to a basis of energy input in the air. In each case, the curve was found to rise sharply to a maximum and then to fall off more gradually. Typical results are shown in Figure 8.37. 

Figure 8,44. Efficiency of air-lift pump as function of energy input from air, showing effect of throttling water...

RlCHARDSQN, J. F. and Higson, D- J. Trans. Inst. Chem. Eng. 40 (1962) 169. A study of the energy losses associated with the operation of an air-lift pump. 

In-well aeration is the process of injecting air into the lower portion of a dual-screened well with perforations at the bottom and above the water table. As the bubbles rise, they expand, which causes the mixed mass of air and water to have less density. The result is an air-lift pump effect. When the water rises and exits the upper perforations, replacement water enters the bottom of the well. The result is a circulation cycle. Free air does not enter the aquifer, but dissolved air (and oxygen) travels with the circulating water. Figure 9.4 is a schematic diagram of in-well aeration. 

An improved perfusion unit. Perfusion units of earlier design used air-lift pumps operated by reduced pressure to circulate the aqueous solution. Such units are often used in banks of about ten and were difficult to adjust so that all of them circulated solution at the same rate. With this design, which uses compressed air, the air-lift pumps are easily adjusted and more positive in action. The solution can be readily sampled without dismantling the components. Additives to the solution can be introduced without disturbing the flow setting (Fig. A. 10). 

An in-well aeration unit with an air-lift pump achieved 97% removal of vinyl chloride. 

The investigation of gas bubbles rising in vertical, cylindrical tubes filled with liquid is of technical interest, for example for the design of air-lift pumps and circulation evaporators among others. However the calculation of this process is difficult, even when it is highly idealised. 

Harris, E.G. (1895). Theory of the air-lift pump. Journal of the Franklin Institute 140(1) 32-52. Harris, E.G. (1903). Theory of centrifugal pumps and fans Analysis of their action, with suggestions for designers. Trans. ASCE 51 166-252. 

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