Because distillation is the most used separation technique in the chemical process industries, improvements can have significant impact. Distillation systems with vapor feeds containing a modest amount of the more volatile component can usually be improved by the use of two enthalpy feed (2F) that condenses a fraction fL of the feed and inputs this liquid above the vapor feed. The two feeds are at the same composition but are not in equilibrium. For the same energy use more stages are needed, but the diameter of the column is reduced. Rigorous Aspen Plus simulations were done for argon-oxygen, alcohols-water, and light hydrocarbons for rectifiers and complete columns. The total annual cost (TAC) was calculated for standard and 2F systems for alcohols-water and light hydrocarbons. The 2Fdistillation system has the following advantages for vapor feeds with a modest amount of the more volatile component: 1. TAC values for 2F rectifiers and complete columns are often lower than for standard designs for new construction although the difference can be very modest. 2. Part of the condensation is of feed which condenses at a higher temperature than distillate - for refrigerated systems condensation at higher temperature reduces 2F cooling costs and TAC considerably. 3. The 2F rectifier has superior turndown properties. With fL = 0 and flow rate reduced to (1 - fL)(original feed rate) the design will operate as a vapor feed rectifier designed for the lower feed rate. Complete columns have improved turndown operation in the rectifying section but % flooding, although higher than in turndown of standard columns, can be quite low in stripping section. 4. Retrofitting normal rectifiers and complete vapor feed distillations to 2F can significantly increase capacity at much lower TAC values than building an additional standard column.

Improving distillation: Not an oxymoron

249th National Meeting (2015)

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