Distillation, stripping, absorption and extraction columns
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Design methods and information
Simulation using Aspen Plus
Many tutorials are available on-line.
Simulation using HYSYS and UniSim
· Defaults: There are several defaults that are not true for real columns:
· Linear pressure change through column based on specified top and bottom pressures.
· 10 trays
· For distillation, feed on tray 5
· 1.5 m diameter
· 100% efficiency
1. For the desired separation, select the heavy and light key components, i.e. the split where you want lower boiling components above the light key to go almost entirely overhead and the higher boiling components below the heavy key to go almost entirely out in the bottoms. You may want to use a “Short Cut Distillation” unit first to do approximate calculations that will give you an idea of what the number of trays, reflux ratio, and feed tray should be. Alternately, you can use Heuristics to estimate the required number of trays and reflux ratio.
2. If you have non-condensables such as nitrogen and oxygen in a distillation column, it may be necessary to use a partial condenser with the vent rate specified in order to avoid an extremely low temperature in the condenser. Sometimes it is helpful to use a simple flash vessel before the column in order to eliminate most of the non-condensables.
3. Pressures: Unless the feed is 100% vapor, set the bottom pressure to the nominal feed pressure. (This is because the feed pipe normally starts near the reboiler height, and then goes up to the feed tray causing a static head loss, so the pressure of the feed is actually smaller.) Use heuristics to estimate the pressure drop from tray to tray. To get the overhead pressure, multiply the number of trays by the pressure drop / tray and then add this to the bottom pressure.
4. Start with a reflux ratio and a number of trays greater than those from short cut, heuristics, or McCabe Thiele diagram. Remember that the separation increases with increasing reflux ratio and number of trays. The optimal feed tray can be estimated by looking at how the temperature and composition vary from tray to tray. For example, if the feed is saturated liquid, the best feed tray is probably that with nearly the same liquid composition.
5. Track the calculations using the Design/Monitor page. Add additional specifications to monitor the results, but you must have only enough of these “active” that the degrees of freedom is 0. Convergence may be easier to obtain by specifying product flows rather than compositions. However, if your design requires a recycle stream flow specifications will cause problems. To avoid these problems, change the column flow rate specifications to something else, such as compositions.
6. Once you have a converged column that gives your desired results, use the tray-sizing utility to get the column diameter and pressure drop. Since this pressure drop will likely differ from that estimated in step 3 above, you will have to change the pressure specifications for the column.
7. Size the reboilers and condensers For heat exchangers that condense steam or boil water fed at saturation, the temperature of the water-steam is fixed by the pressure and is a constant. The flow rate of the water-steam is calculated by dividing the Q by the latent heat of evaporation at that pressure. If the boiler feed water (bfw) is below saturation, calculation of the heat exchanger should be broken into two parts -- one to heat the bfw up to the saturation temperature and a second to evaporate all of it. Similarly, if superheated steam is fed, for calculation purposes break the heat exchanger into one that cools the steam to saturation and a second that condenses it to saturated water.
· Procedure for liquid-liquid extractors, strippers and absorbers: If you encounter a zero-diagonal matrix error message, cut the number of trays to 2. Then, without resetting the calculations, increase the number of trays until you reach the number desired. This should avoid the matrix inversion problem. To efficiently obtain the desired separation, you will have to experiment with changing the column pressure and the flow rate of the extraction, stripping or absorbing agent. Do not use the tray-sizing utility to find the diameter or pressure drop of a LLE column. Instead use Perry’s Handbook or search sites given at General. For supercritical fluid extraction, see Supercritical Fluid Extraction.
Last revised January 13, 2012. Please email questions, comments and suggestions to W.R. Wilcox
Disclaimer: The material on these pages is intended for instructional purposes by Clarkson University students only. Neither Clarkson University nor Professor Wilcox is responsible for problems caused by using this information.