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Liquid Row Rata. GPM

F'fev'RE 12.7.2 Start-up Stability Diagram Olefins Low-Temperature FractJonator (Baaed on Raf 1)

(Contributed by T.C. Hower Jr., Santa Fe Braun Inc., Alhambra. Ca.)

INSTALLATION A lean oil still in an absorption-refrigeration gas plant (Figure 12.8.1). This still is the last step of purification of the absorption oil before the oil is returned to the plant absorber to absorb heavy components from natural gas. Feed to the still is the absorption oil, containing the absorbed gasolines and some LPG. Lighter components are removed from the oil upstream of the still. Lean oil leaves as the still bottom product, while gasolines and LPG are the top product. The still is operated at 210 psig. Note the large temperature difference between the bottom and top of the still.

The main quality control objective of the column is to keep the gasoline out of the column bottom. This product quality is controlled by the furnace outlet temperature controller. The reflux rate is trimmed by manually adjusting the flow controller set point, so as to give a reasonably constant column overhead temperature. The reflux drum is flooded, and liquid level in the condenser is used to control column pressure.

PROBLEM The plant absorber appeared to be malfunctioning. It did not absorb all the heavy ends out of the gas.

INVESTIGATION The Initial Boiling Point (IBP) of the lean oil leaving the still was low, which indicated the presence of a substantial quantity of gasolines in the still bottom. This suggested that the still was malfunctioning. The still showed no signs of flooding. The control temperature, the overhead temperature, and the reflux rate appeared to be at their design values. The composition of the top-product was not analyzed.

SOLUTION The problem was caused by insufficient reflux rate. The low reflux rate was unnoticed because of an incorrectly sized orifice plate in the still reflux line. When the orifice plate was replaced and the correct reflux flow set, the plant observed a large permanent increase in fuel usage and a large drop in the apparent quantity of absorption oil, indicating that the gasoline was being stripped off the bottom. Following this, the plant absorber started functioning normally and absorbing heavy ends out of the gas.

ANALYSIS The problem was particularly difficult to detect because of the unusual behavior of the overhead and bottom temperatures. Normally, when a substantial amount of light impurity is present in the bottom, one would expect the bottom temperature to drop; when a substantial amount of heavy impurity is present in the column overhead, one would expect this temperature to rise. Over-reboiling can bring the bottom temperature back up, but in such a case, one would expect the top temperature to rise further above design.

The above considerations are generally valid for binary distillation, and often, but not always, for multicomponent separations. This case is an example of a multicomponent distillation where the above considerations do not apply.

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