Case Study Detail

How do I calculate the saturation concentration of a pollutant in air?

A common thermodynamic problem is the calculation of the concentration of a particular chemical in air that is in contact with a liquid stream containing a known concentration of the substance (usually a pollutant in a waste stream). The difficulty with this calculation is that one does not know how much air is present in the thermodynamic system and so an iterative approach must be used. Microsoft’s EXCEL spreadsheet software together with the PPDS EXCEL add-in can be set up to easily solve this type of problem.

The problem is to determine the amount of Chlorobenzene in the equilibrium vapour phase of a saturated organic mixture in air and to check this against a recommended emissions limit ( e.g. 100 ppm of Chlorobenzene). The initial liquid concentration of the organic is known but the amount of water in the system is not known beforehand except that it is in excess in the liquid phase (water is the solvent for the liquid waste stream). In setting the problem up with the PPDSEXCEL add-in library calls, the amount of water is initially set to zero.

The calls to the PPDS library (these are fully described in the on-line help and manuals) are:

• Cells B6:E6use CompName to get the component name given the bank number and the PPDS component identification number.
• Cell B15 uses VLETemp to get the dew temperature at the specified pressure (1 atm, cell B11).
• Cells B17:E17 use VLEComp to get the compositions of the equilibrium vapour

Speadsheet using the PPDS Excel Interface to determine pollution values

As the water liquid feed composition is unknown, the problem is to adjust it until the calculated bubble temperature equals the specified process temperature (in this case 320K, set in cell B10). This can be achieved by manually adjusting the value in cell E8, but a better option is to use the Solver add-in supplied with Excel.

Screenshot showing the Excel solver parameters for solving the pollution calculation

The parameters for this Solver case are shown above, using a simple constraint to check that the composition does not go negative during the iteration process. In this example the solution converges quickly. The water feed at equilibrium is given in cell E8, the mixture compositions in cells B17:E17 and the Chlorobenzene composition is converted to ppm in cell B18. Comparison of the contents of cell B18 with the target level of 100 ppm indicates that this process is exceeding emission limits.