Distillation Tower with Reflux
Figure 6-5 shows a stabilizer with reflux. The well fluid is heated with the bottoms product and injected into the tower, below the top, where the temperature in the tower is equal to the temperature of the feed. This minimizes the amount of flashing. In the tower, the action is the same as in a cold-feed stabilizer or any other distillation tower. As the liquid falls through the tower, it goes from tray to tray, and gets increasingly richer in the heavy components and increasingly leaner in the light components.
The stabilized liquid is cooled in the heat exchanger by the feed stream before flowing to the stock tank.
At the top of the tower any intermediate components going out with the gas are condensed, separated, pumped back to the tower, and sprayed down on the top tray. This liquid is called “reflux,” and the two-phase separator that separates it from the gas is called a “reflux tank” or “reflux dram,” The reflux performs the same function as the cold feed in a coldfeed stabilizer. Cold liquids strip out the intermediate components from the gas as the gas rises.
The heat required at the reboiler depends upon the amount of cooling done in the condenser. The colder the condenser, the purer the product and the larger the percentage of the intermediate components that will be recovered in the separator and kept from going out with the gas. The hotter the bottoms, the greater the percentage of light components will be boiled out of the bottoms liquid and the lower the vapor pressure of the bottoms liquid.
A condensate stabilizer with reflux will recover more intermediate components from the gas than a cold-feed stabilizer. However, it requires more equipment to purchase, install, and operate. This additional cost must be justified by the net benefit of the incremental liquid recovery; less the cost of natural gas shrinkage and loss of heating value, over that obtained from a cold-feed stabilizer.
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