Distillation sieve tray efficiencies

• Main Author: Kalbassi, Mohammed Ali
• Published: University of Nottingham 1987
• Subjects: 660; TP Chemical technology
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.376628

The distillation point efficiencies for the alcohol-water binary, ternary and quaternary systems were measured using a modified Oldershaw column. This column is expanded above the tray to separate the newly formed bubbles from the column wall, thus eliminating the surface tension induced wall effects for positive systems and discouraging wetted wall effects. The excessive and recirculating foam and froth found in the conventional Oldershaw column is due to these wall effects and does not represent conditions in large scale distillation. The point efficiencies measured using this column for the system methanol/water were lower than the point efficiencies deduced from the composition profiles across a large and narrow rectangular distillation column using an eddy diffusion model. The narrow rectangular column had a liquid flow path length of about one meter, thus avoiding stagnant zones and flow non-uniformities. The lower efficiencies were due to the shorter contact time between the gas and the liquid. This contact time was increased markedly by fitting an outlet weir to the modified Oldershaw column, thus increasing the tray liquid hold-up and the point efficiencies. These point efficiencies were about 10 per cent lower than those on the large tray at a similar value of the F. Factor. The eddy diffusion model predicted rectangular tray efficiencies about 10 to 20 per cent lower than those measured, when using the improved modified column point efficiencies. Using a suitable model, the improved point efficiencies were scaled-up to the conditions existing on the rectangular tray. This resulted in the large tray values of 2 to 4 per cent lower tray efficiencies than those measured. The surface tension effect on the point efficiencies of the binary systems MeOH/n.PrOH using the original modified Oldershaw column in the absence of wall effects using the concept of the Marangoni stabilising index. The surface tension of these systems were measured using a glass thermometer. The system MeOH/H20 had the highest Marangoni index and showed the highest point efficiencies throughout the composition range, with the EtOH/H2O/n.PrOH, with low values of the Marangoni index, showed comparable point efficiencies throughout the composition range. These systems demonstrate all the possible types of surface tension behaviour. The effects of the outlet weir height and hole size on the point efficiencies in the rectangular column operating under similar hydrodynamic conditions were also investigated using the system MeOH/H2O. There was an increase in point and tray efficiencies on increasing the outlet weir height from 2 mm to 12.7 mm. There was also small increase in point and tray efficiencies on decreasing the hole size from 6.4 mm to 1 mm at the expense of higher pressure drops. The point efficiencies of these trays under different hydraulic conditions were in the range 85 to 95 per cent, with subsequent high tray efficiencies. This provides further evidence of the high tray efficiencies available to the design engineer if the detrimental effects of stagnant zones and flow non-uniformities were eliminated. Two highly non-ideal ternary systems and quaternary system were also studied using the original modified Oldershaw and the rectangular columns. Considerable differences between the individual component point efficiencies were observed. These differences are probably caused by the interactive nature of the mass transfer in these systems. These systems also exhibited equal component point efficiencies in parts of the composition range, which illustrates the composition dependency of these systems. The individual component tray efficiencies for these systems were noticeably different, even with equal component point efficiencies operating across the tray. These differences were simulated using the eddy diffusion model, highlighting the effects of limited liquid back mixing on the tray. The composition profile for the system MeOH/EtOH/H20 were predicted and compared with the measurements across the rectangular column using three methods derived from the original Maxwell and Stephan mass transfer equations. These predictions were in good agreement with the measurements. However, as the comparison is only based on a one meter flow path length, the actual design of distillation column using these methods would be conservative. The prediction of the composition profiles using the point efficiencies from the original version of the modified 0ldershaw column yielded a similar observation for both the ternaries and the quaternary system. An expanded aluminium tray (Expamet 607A) was also subject to preliminary efficiency tests in the rectangular column. This material has corrugated angled holes, thus encouraging the liquid flow across the tray by using the vapour momentum. This material showed much lower pressure drops, due to its high open area compared with conventional sieve trays, and discourages weeping and entrainment.