There is much to be said regarding pas-through distillation as a separation tool. This post is going to deal with only one aspect of the topic.The internals of the SAM may constitute a brand new type of fractional distillation apparatus, eliminating the reboiler in favour of heated “packing”.
Let’s first of all review what we know about two simple forms of distillation which will serve as points of reference: the flash tank and the stripping column.
FIGURE 1 Flash Distillation
As shown in Figure 1 above, a flash distillation unit heats the feed outside the vessel to a temperature above its boiling point at the pressure in the tank. When the liquid passes through the pressure reducing valve, some of the liquid vapourizes, and the liquid quickly drops to its boiling point. A stream of liquid L is removed from the bottom and a stream of vapour V from the top. These two streams are in equilibrium with each other both thermally and chemically.
FIGURE 2 Stripper Column
Figure 2 shows a stripping column with its reboiler. The column is contains trays or packing upon which descending liquid and ascending gases can exchange mass and energy. Like the flash operation, it too generates a liquid stream at the bottom and a vapour stream at the top, but this time the vapour stream will be in thermal and chemical equilibrium not with the bottoms product but rather with incoming feed at the top.
Now consider Figure 3, a SAM. The acronym stands for Stripper/Absorber Module. We are considering the left-hand compartment which I often call the evaporator section but which is, arguably, a stripper column. The heat pipes serve as a coarse packing, causing mass exchange between ascending vapours and descending liquids. The feed liquid is the last thing the vapours (shown in blue) see before leaving the compartment. We should expect then that the vapour stream should be in equilibrium with the feed, or at least nearly so. Most importantly, the vapour stream will be richer in the most volatile components than the bottoms. So is this a stripper? I say yes, but one you will never encounter in a chemical engineering textbook.
FIGURE 3 Stripper/Absorber Module
What makes it distinctive is that at every “tray” heat is added. This leads to strange flow behaviour. At the top tray both the descending liquid stream L and the rising vapour stream V are at their maximum. A portion of the feed is evaporated on this top tier of heat pipes, but the vapour flow rate is the cumulative total evaporation on this “tray” and all the trays beneath it. The flow of liquid descending from the top row of heat pipes to the second is reduced from the feed flow rate by the amount that evaporated. Similarly the amount falling from all subsequent tiers will be reduced until a minimum flow rate is reached at the bottom. At that point no further evaporation takes place and the flow of vapour is zero.
McCabe-Thiele analysis will not work on this “stripper”. Its underlying assumption of constant molal overflow does not apply. Other tools will have to be used to describe its behaviour.