Problems in multicomponent distillation often stem from components that have volatilities that lie between the light key and heavy key. The relative volatility of component a to the heavy key, h, is:
α = [ya/xa]/[yh/xh]
where y is the vapor-phase mole fraction and x as the liquid-phase mole fraction.
A component that has a relatively volatility greater than one but less than that of the light key may accumulate inside a process. Depending upon the specifics, the material may build up until the process doesn't work or becomes unstable (for one example, see: "Watch Out for Trapped Components in Towers"). Adding a vapor or liquid side-draw to purge the trapped component often can solve this entrapment problem.
Side-draws also may provide benefits in other situations. One potentially valuable use is for "pasteurization" sections. Such sections have the side-draw as the major component. This differs completely from the side-draw as a purge. Pasteurization sections take a liquid draw above the feed point or a vapor draw below the feed point. They may require control changes because the large side-draw can make achieving stable rates inside the column difficult. A previous column illustrated some of the control issues for a solvent recovery unit ("Sidestep Side-Draw Control Surprises").
Let's look at the use and possible benefit of a pasteurization section in an ethylene/ethane (C2) splitter. Commonly, the main ethylene product is a liquid side-draw taken 5–10 trays below the top of the column (Figure 1).
The feed to the splitter also contains some hydrogen and methane. These light molecules have very high relative volatility compared to ethylene. However, despite their high α values, too much of them remain in the overhead product. The hydrogen and methane level of the overhead liquid can be cut — but only by using a large vent purge, which then must get recycled for ethylene recovery. However, excessive recycling reduces unit capacity. Another solution is to impose extremely tight specifications on the upstream demethanizer — but this has capacity and recovery consequences.
A pasteurization section provides a reasonable alternative. With their high α values, little hydrogen and methane from the reflux will get back down the tower to the liquid draw. At the draw point, the equilibrium content of hydrogen and methane in the side-draw is similar to that found in a simple flash drum. However, the reflux provides a diluent, reducing the effective concentration of light material. The effective concentration, feed concentration × (reflux rate/ethylene rate), sets the draw composition.
In columns with a high reflux rate, pasteurization sections can be very effective.
Keep in mind, though, the light material will go to the top of the tower in the vapor. The amount of trace material varies more linearly with changes in reflux rate than in typical distillation. Of course, the same applies for an inverted system below the feed used to reject trace amounts of heavy material.
In specific cases, pasteurization sections can cut capital and energy costs. However, they are sensitive to upsets in the feed composition. Because they work on a dilution effect, such sections require large reflux-rate or boilup changes to compensate for feed composition variations. Before using pasteurization, check that the column still can meet product specifications in upset and startup conditions.
ANDREW SLOLEY is a Chemical Processing Contributing Editor. You can e-mail him at Asloley@putman.net