1 INSTALLATION A Benfield Process hot pot absorber. The absorber 1 uses hot pc.assium carbonate solution to absorb C0a and K2S from ¡concentrated sour gas (Figure 12.9.1). Because of the corrosive ¡environment, the column was packed with 2 inch polypropylene Raschig rings.
HISTORY The absorber plugged on start-up. It was losing capacity during the first couple of days, until it was virtually plugged. When it was opened up, the plant observed that the plastic rings had melted, thus plugging the column. The heat of reaction caused the packings to melt. It is possible that the melting started in a few !hot spots; once started, the hot spots moved to other points, thus ¡melting the whole bed.
!The plastic rings were replaced by stainless steel Raschig rings. When the column was re-started, it operated well, but flooded below |design rates. Flooding was recognized by liquid carryover into the !overhead stream.
|The carbonate solution was checked for foaming tendency. This |appeared reasonably small. Shortly later, it was realized that although the solution itself did not have a large foaming tendency, the gas contained a surface active agent, which was injected downhole into the gas well as a corrosion inhibitor. When this material came j in contact with the solution, it foamed, causing premature flooding.
|An antifoam injection at a concentration of 10 ppm was started. |Field tests at solution temperature and atmospheric pressure showed |that this concentration was sufficient to eliminate foaming. Once j the injection began, column capacity was increased by 50 percent, but j it was still short of design capacity. It was then decided to repeat | the foaming tests under actual operating conditions. These were j carried out in a level glass set up in the field so it could be I operated at system conditions. When gas was bubbled through the glass at 1000 psi and 180°F (process conditions), the solution broke into foam.
Field tests at system conditions showed that an antifoam injection of 1000 ppm was needed to suppress the foaming. When antifoam injection into the column was increased to this concentration, design capacity was reached.
F»€ur£ u.q.i Berrfield Process Hot Pot Absorber _
(Contributed by T.C. Hower Jr., Santa Fe Braun Inc., Alhair.bra, Ca.)
INSTALLATION A gas plant refluxed deethanizer (Figure 12.10.1). Feed to the column was rich absorption oil saturated with absorbed gas components (C^ to gasoline). Reflux was condensed using C. refrigeration and entered the column at -30°F.
PROBLEM At unpredictable time intervals, a slug of water would empty out either from the top or from the bottom of the column.
Emptying out from the top appeared to occur by massive vaporization of water, which carried over the fluids above the point of vaporization and out of the top rapidly. Some absorption oil, water, j and gasoline were found in equipment downstream of the reflux drum I following emptying out from the top. Emptying out of the bottom appeared to take place by a massive slug of fluids. This slug caused a large increase in feed to a downstream depropanizer, resulting in a major upset in the column train downsteam.
CAUSE Trace quantities of water, absorbed in the absorption oil, | would enter the column. Top temperature was too cold, while bottom I temperature was too hot, to permit the water to leave, so it accumulated in the column. The accumulation continued to a point where a water slug would empty out.
CURE Refluxing the column was discontinued. Instead, presaturated absorption oil (i.e., absorption oil that was previously contacted with C. and C_s to eliminate heat of absorption effects) was injected onto trie top tray, so that tray temperature could be raised to -20°F (Figure 12.10.2). This was sufficient to ensure that the water vapor left with the top product. This eliminated the water slug problem. Absorption oil losses in the column overhead stream were negligible. The only unfavorable effect of this modification was to increase the absorption oil circulation throughout the plant by 1-2 percent.
An alternative solution would have been to install water draw-off trays in the column; however, this solution would have been less economical, and would have also suffered from the difficulty in predicting the location of the main points of water accumulation.
figure il. 10.i Deethanizer - Original Arrangement
Cftie S~oy Nie ie
Cl, C2 To Fuel
** Z 12. io. Z Deethanizer - Modified Arrangement
CASE STuoy NO. 40
INSTALLATION An isopropyl acetate solvent recovery column (Figure 12.11.1). The solvent was concentrated in the overhead stream. The product flowed into a decanter, where the insoluble isopropyl acetate was finally separated from water.
PROBLEM After a certain period, poor phase separation was observed in the decanter.
CAUSE Slow hydrolysis of isopropyl acetate into isopropanol, which was soluble in both phases.
CURE At regular intervals the system needed to be purged to a separate still for removing the isopropanol.
RECycLG TO FEED
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