(327a) Hybrid PSA-Distillation Process for Propane/Propylene Separation

Polymer grade propylene is important and widely used in the manufacture of many chemicals and plastics; it is used especially as a monomer feedstock for polypropylene elastomer production. Its purity cannot be less than 99.5 mol%. The mixture of propane and propylene that results from the thermal or catalytic cracking of hydrocarbons is separated by distillation in a C₃ splitter.

In this traditional distillation, the relative volatility for this system is between 1.05 and 1.22 in the temperature range of 100-160 °F and in the pressure range of 189-454 psia. Thus, the separation is commonly performed in columns with more than 200 trays, with reflux ratios of about 13, and with high operating pressures of around 14 atm. The separation of propane and propylene mixtures represents one of the most energy consuming processes in the chemical process industries. This makes it highly desirable to develop new, more energy efficient processes for this very difficult separation.

Several pressure swing adsorption (PSA) processes have been developed by others for propane/propylene separation with several different adsorbents, such as 13X zeolite, 4A zeolite, carbon molecular sieve, and silica gel. However, the performance of a single PSA unit is not comparable with traditional distillation. This stems from it being difficult to produce two pure products from a single PSA process. The objective of this on-going project at USC has been to develop a hybrid PSA-distillation process for propane/propylene separation. Unfortunately, the energy savings could not be achieved, since none of the aforementioned commercial adsorbents work well enough, even in a hybrid process configuration.

With this in mind, a hypothetical adsorbent was conceived that has all the desirable and none of the undesirable properties of the commercial adsorbents already tested for this separation. Several PSA cycle configurations that utilize this hypothetical adsorbent under different operating conditions have been investigated via simulation. The results were very encouraging. They show that a hybrid PSA-distillation process is able to achieve significant energy saving compared to the traditional distillation process for propane/propylene separation. The goal now is to go to the lab and fabricate this novel adsorbent. The latest results of this work will be discussed during this presentation.