POWNFLOW RKEA CASE STUDy NO. it
(Contributed by T.C. Hower Jr., Santa Fe Braun Inc., Ainambra, Ca.)
INSTALLATION A lean oil still in an absorption-refrigeration gas jplant (Figure 12.17.1). Feed to the still is rich absorption oil, containing gasolines and some LPG absorbed from the natural gas. Lean oil leaves as the still bottom product, while gasolines and LPG are the top product. The still was reboiled by a fired heater, and reflux was supplied by an air condenser. Top temperature was about 175°F, bottom temperature 520°F, and column pressure was about 140-160 psig. To reduce the duty of the fired reboiler, the still feed was preheated by exchanging heat with the still bottom stream. The preheater had a valved bypass on the feed side (valve VI in Figure 12.17.1). The preheater performed better than expected and preheated the feed to 400-450°F.
PROBLEM Most of the time, the column operated normally. However, every four to 12 hours or so, the column would suddenly unload or "hiccup" and empty itself out either through the top or into the bottom surge section. It was difficult to predict through which end the column would unload. Unloading occurred with no prior warning. Each occurrence of unloading lasted less than one minute, and was accompanied by a rapid rise in column differential pressure. The column then stabilized itself automatically over a 15- to 20-minute period. When the column unloaded out of the top, the bottom temperature rose and cut back the heater fuel supply. When the column unloaded out of the bottom, the heater fuel consumption greatly increased, and even this could not supply sufficient heat to maintain the bottom temperature.
ANALYSIS There appeared to be some vague connection, but no direct cause-and-effeet relationship, between the column feed composition and the occurrence of the problem. An increase in the heavies content of the feed appeared to aggravate the problem. The still feed composition varied widely, depending on the gas wells feeding the plant. In addition, the raw gas lines were pigged about every ! six hours to clear condensate which settled in the lines, and this temporarily increased the heavies content of the feed to the still.
The plant experimented with several variables in an an attempt to overcome the problem. Different bottom temperatures, varying rates of circulation through the reboiler, changing reflux rate, opening and closing the feed bypass valve were all tried, but no improvement was observed. Plant rates also appeared to have little effect; the problem occurred at plant rates as low as 40 percent of design.
THEORY The feed temperature of about 400-450°F was sufficiently high to boil a significant fraction of the heavier components of the gas contained in the rich oil. These heavy components would normally end up in the column bottom and add up to the lean oil. Column temperature at the top was too cold to let these components escape with the LPG and gasoline, while feed tray temperature was too hot to allow them to flow toward the bottom of the column. These components therefore accumulated in the top section until the column would flood and empty itself - either by massive entrainment or by post-flood dumping.
SOLUTION To eliminate the accumulation, it was desired to lower the still feed temperature. Opening the feed-bottom interchanger bypass did not drop the temperature significantly because of the good performance of the exchanger. A valve V2 was installed in the feed 1 line to the exchanger, and was throttled to force more feed via the j bypass. Adjustment of both valves enabled feed temperature to be i reduced to 350-375°F. Once the feed temperature was lowered, the column no longer emptied itself.
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