Simulation Approach Prof. Dr. Marco Mazzotti - Institut für Verfahrenstechnik

In this case we have a polluted gas current that we would like to clean. On the other hand we have an absorption equipment, and we would like to evaluate whether the operation can be performed with this equipment and in which conditions. The temperature and the pressure at which the process takes place must be chosen. The thermodynamics of the process at different conditions must be studied in order to decide the most convenient. The solvent can be used pure or can come from a recycle, and thus contain some pollutant. The initial composition of the solvent (x o ) must be obtained by analytical methods, or can be set by mixing pure and recycled solvent. G, y n+1 Process 1. Variables L, x 0 The geometry of the equipment is well known. G, y 1 ? T, p n L, x n ?

Once temperature, pressure and initial composition of the solvent are set we can consider them as data. This is a list of known variables: Data and set valuesSpecificationsUnknowns xnxn y n+1 T y1y1 x0x0 Temperature, T Pressure, p Gas molar flow-rate, G Gas initial composition, y n+1 Solvent initial composition, x 0 Equilibrium data, y=m x Solvent final composition, x n p G L GL y = m x n Solvent molar flow-rate, L Number of ideal stages, n Gas final composition, y 1

As we have seen before, the problem can be represented in the x-y plan. The equilibrium line can be drawn to start. Considering the initial compositions of the gas and solvent flows and the specification made for the gas outlet composition, we can draw in the diagram: 2. Diagram y = m x y x y n+1 x0x0

Because we know both of the molar flow rates, the absorption factor is fixed, and so the slope of the operating line is known: There are infinite number of lines that having the slope L/G pass by a point of y-coordinate y n+1 and by a second point of x-coordinate x o... x y = f(x) x0x0 y n+1 L /G

But actually there is only one line of those infinite that fulfills the last condition: the number of ideal stages is fixed to the number set by the equipment. The procedure is the following: the operating line is moved up and down, until the number of stages obtained coincides with the number of stages that the equipment provides. On the diagram, we see how by moving the operating line, we go from one to six stages. If the available equipment provides six stages, we have found the right operating line in the second case. When the line is fixed, the final concentration of the gas is obtained. This concentration can be then evaluated in order to clarify whether the process has for instance, the desired performance. The outlet composition of the solvent is also obtained. x y = f(x) xoxo y n+1 L /G y 1 xnxn xnxn

Have a look to the design Flowsheetdesign Flowsheet