Exercise 07 ENG

Pagina 1

Chemical Processes and Technologies (Prof. Laura Pellegrini) 1. Absorption of H2S A gas stream containing 0.3 m ol% of hydrogen sulfide (H2S) and 99.7 mol% of nitrogen (N2), at the pressure of 1 atm, has to be treated to remove hydrogen sulfide. Because of its low cost, water is used as solvent in a plate absorption column. The gas stream has to be purified to reduce the H2S content to a molar fraction of 0.0001. Considering to have a uniform temperature of 21°C into the column and that water remains in the liquid phase:  determine the minimum water flow rate required, expressed in kmol/kmol of inlet gas;  compute the number of equilibrium stages using a water flow rate equal to 1.2 times the minimum one;  evaluate the composition of the output streams. Data: Henry constant: HH2S = 500 atm. Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta” POLITECNICO di MILANO 2. Non-linear absorption A gas stream consisting of 5 mol% benzene and 95 mol% air is fed to the bottom of a plate absorption column, where it is contacted with a downward -flowing stream of a non-volatile oil containing 0.2 wt% of benzene , that enters the top of the column . The unit is operated at 300 K and at atmospheric pressure. A flow rate of 1500 Nm 3/h of polluted air has to be treated with the aim to absorb 90% of the contained benzene. Assuming to adopt an oil flow rate equal to 1.5·Lmin, compute:  the oil flow rate flowing within the column;  the number of ideal stages required to perform the desired separation. Data: Component A B C Molecular weight [kg/kmol] benzene 15.9008 2788.51 -52.36 78.114 oil 16.1510 4294.55 -124.0 240.475 Antoine equation: 0ln( ) BPA TC  , with T in [K] and P° in [mmHg].