In this text, we will generally concern ourselves with the analysis of systems of the type illustrated in Figures 1‐1 through 1‐4. For example, if we know how much $$\ce{SO2}$$ is entering the scrubber shown in Figure 1‐2 and we can measure the amount of $$\ce{CaSO3}$$ leaving with the slurry, then we can use material balance techniques to determine the amount of $$\ce{SO2}$$ leaving the scrubber in the “clean” gas. The design of a limestone slurry scrubber is a much more difficult problem. In that problem we would be given the amount and composition of the stack gas to be treated, and the allowable amount of $$\ce{SO2}$$ in the clean gas would be specified. The task of the chemical engineer would be to determine the size of the equipment and the flow rate of the limestone slurry required to produce the desired clean gas. The design of a stack gas scrubber is not a trivial problem because the rate of transfer of $$\ce{SO2}$$ from the gas to the liquid is influenced by both the homogeneous and heterogeneous chemical reactions. In addition, this rate of transfer depends on the bubble size and velocity, the viscosity of the slurry, and a number of other parameters. Because of this, there are many possible designs that will provide the necessary concentration of $$\ce{SO2}$$ in the stack gas; however, it is the responsibility of the chemical engineer to develop the least expensive design that minimizes environmental impact, protects the health and safety of plant personnel, and assures a continuous and reliable operation of the chemical plant.