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8.3 Control Systems That Did Not Work

CASE STUDY No. 1 Analyzer Located in the Next Column Product Line

(Reference 6)

INSTALLATION An absorption-refrigeration gas plant. Rich absorption oil contained absorbed hydrocarbons from C_ to gasoline. In order to regenerate the absorption oil, the deethanizer stripped out the C. and C2 hydrocarbons. Gasolines, LPG (C.-C.), and C, impurities from the bottom of the deethanizer were recovered ifl the still overhead product (Figure 8.3.1A). There was an economic incentive to recover as much C_ as possible in the deethanizer bottoms (or still overheads) without exceeding the LPG purity specs for C_. As the deethanizer bottom stream was difficult to analyze, a Canalyzer was installed in the still overhead product line. The analyzer measurement for C-/C- ratio was used to adjust the set point of the deethanizer temperature controller.

PROBLEM The system did not work. Instead of getting better control of the C_/C_ ratio in the deethanizer bottom, the control was worse than without the analyzer.

INVESTIGATION System response was tested by taking the analyzer off-control, then increasing the deethanizer temperature set point by a 5°F step. The analyzer reading changed steadily over a period of five hours following the change, and only then stablized.

SOLUTION The analyzer was installed on the deethanizer bottom line (Figure 8.3.IB) to eliminate the lag problem. A small liquid stream was sampled from the deethanizer bottom line at a fixed flow rate. The liquid flowed into a little flash pot, to which a constant amount of heat was supplied. The analyzer was installed on the vent line from this pot. The correlation between the C./C. ratio in this stream and the deethanizer control temperature was determined experimentally. This system worked well with no further problems.

Deethanizer Processes

Figure a Analyzer Located in Next Column Product Una

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