Feed-Splitting Technique in Cryogenic Distillation

In distillation columns with condenser temperatures significantly below room temperature, such as in demethanizer towers, it is essential to minimize the expensive energy requirements of the refrigeration cycle that produces the tower reflux. A partial solution can be found by expanding the gaseous distillate to decrease its temperature; another solution is to use the cooler distillate to reduce the temperature of the tower feed. In this way, it has been shown that precooling only part of the feed brings about a significant decrease of its temperature, as well as decreases of the related minimum reflux ratio and finally the condenser duty. The fraction of the precooled feed has an optimum value that must be determined by means of a sensitivity analysis. By repeating the simulation of the tower and its auxiliaries, it is possible to find the optimal configuration, i.e., the configuration that minimizes the energy requirements at the condenser, by varying the feed-splitting ratio and by adjusting the tower geometry (i.e., identity of the feed trays) for each trial. A typical example to be applied in a demethanizer tower is provided here to illustrate the effect of feed splitting on the condenser and reboiler duties.