The bottoms liquid from any gas plant may be sold as a mixed product. This is common for small, isolated plants where there is insufficient local demand. The mixed product is transported by truck, rail, barge or pipeline to a central location for further processing. Often it is more economical to separate the liquid into its various components and sell it as ethane, propane, butane, and natural gasoline. The process of separating the liquids into these components is called fractionation.
Figure 9-4 shows a typical fractionation system for a refrigeration or lean oil plant. The liquid is cascaded through a series of distillation towers where successively heavier and heavier components (fractions) are separated as overhead gas. In this figure the liquid from the still of an absorption plant or the de-methanizer (or de-ethanizer) tower of an expansion or refrigeration plant is routed to a de-propanizer. If there is too high a fraction of butanes-plus in the propane, this can be reduced by adjusting the de-propanizer pressure upward or reflux condensing temperature downward. If the vapor pressure of the propane exceeds the required specification this means that the fraction of methane and ethane in the inlet stream is too high. This fraction can be adjusted downward by increasing the temperature or decreasing the operating pressure of the still or tower that feeds liquid to the de-propanizer.
The de-butanizer works in a similar manner. The upstream tower (depropanizer) determines the maximum vapor pressure of the butane product. If the concentration of propane-minus is too large in the inlet stream, the vapor pressure of the butane overheads will be too high. Similarly, the concentration of pentanes-plus in the butane will depend upon the reflux condensing temperature and tower operating pressure. If the pentanes-plus exceed specifications, further reflux cooling or a higher operating pressure will be needed to condense pentanes-plus from the butane overheads.
The temperature at the base of the de-butanizer determines the vapor pressure of the gasoline product. If its vapor pressure is too high, the temperature must be increased or the tower pressure decreased to drive more butanes-minus out of the bottoms liquids.
If the feed to the fractionator contains recoverable ethane, such as is likely to be the case with a cryogenic plant, then a de-ethanizer tower would be installed upstream of the de-propanizer.