(142e) Olefin-Paraffin Separation by Reversible Reactive Distillation

Olefins, especially ethylene and propylene, are large-volume industrial chemicals that are produced commercially by the separation from petroleum refining, natural gas liquids, and on-purpose chemical syntheses.  A key step is the separation of the corresponding (same carbon-number) paraffins from the olefins, conventionally done by fractional distillation in “splitter” columns, e.g. ethylene from ethane (“C2 Splitter”), propylene from propane (“C3 Splitter”), etc.  Due to the very close relative volatilities of the olefins and their corresponding paraffins, separation by distillation, while widely practiced industrially, is energy- and capital- intensive:  They are carried out in high-pressure distillation columns, often requiring more than 150 theoretical stages and high reflux ratios.  The latter results in large energy requirements, and large-diameter, heavy-walled columns, because of the large internal hydraulic traffic.  It is therefore highly desirable to find an alternative olefin-paraffin separation scheme that requires substantially less energy and preferably smaller, less expensive equipment as well.

It is proposed to more efficiently separate olefins from paraffins by means of reversible reactive distillation process.  An olefin-paraffin mixture and suitable reactants are fed to a first reactive distillation column, operating at suitable conditions to convert the olefin to a high degree to a high-boiling adduct, taking the paraffin as the overhead distillate product, and taking the adduct as the bottoms product.  The olefin-adduct is fed to a second reactive distillation column, operating at suitable conditions to back-convert the adduct to the original olefin, which is taken overhead as the distillate product, and the original reactants, which are taken as the bottom and/or side-draw product and recycled to the first distillation column.   If the olefin distillate from the second column contains residual amounts of the adduct and/or reactants, it may be sent to another separation unit to purify the olefin and recover and recycle the adduct and/or reactant back to the reactive distillation system. 

A conceptual design study examined the feasibility of olefin-paraffin separation by reversible reactive distillation, for the specific case of C3 (propylene-propane) splitting.  The calculated total reboiler duty per pound of propylene product was 35 – 45 % lower for the reversible reactive distillation process compared to a conventional C3 Splitter.  Even though 3 columns are required in place of a single conventional C3 Splitter, in aggregate, they require ~40 % fewer total theoretical separation stages, ~ 85% less total tray area, and ~80% less total column volume,  In addition to the energy savings, the projected capital investment is ~20% lower for the reversible reactive distillation system compared to a conventional steam-reboiled C3 splitter.