Sample Points for Top Product Analyzer Control

The sample point location affects the correlation between the composition measurement and the product composition. It also affects the measurement accuracy, speed of response, the nature of the sampling (sometimes also the analyzer), and the reliability of the system. The pros and cons of different sampling locations for column composition control are discussed below. 

Recommendations for best sample location assume that the analyzer controls composition without cascading onto a temperature controller. In this case, the dynamics of the analyzer system is a prime composition control consideration. If the analyzer cascades onto the column temperat lre controller (Sec. 18.13), the dynamics of the ana- 

The overhead vapor line or top vapor space. Dynamically, this is the most desirable location. Measurement lags, both those between the column and the sampling point and those in the sample line (vapor sample) are minimized. However, this location also has several drawbacks. 

Samples drawn at this location often suffer from nonreproducibility. Liquid droplets due to entrainment or atmospheric condensation may affect sample composition. The amount of liquid caught by the sample may vary, and this too affects the measurement. Large composition gradients may exist in the vapor phase above the top tray and persist in the overhead line. In one case (309), the heavy key concentration measured near the wall of the overhead line was half the concentration measured near the centerline, and the concentration gradient was unsteady. To mitigate the above problems it is best to sample from the top of a horizontal leg in the overhead line, and to provide a sample knockout pot for removing droplets, but this may not be sufficient to entirely eliminate the nonreproducibility problem. 

Samples from the overhead line give an inferior correlation with 

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The general comments at the opening of the discussion on locating sample points for top product analyzer control also apply here and will not be repeated. The following sampling locations are used for controlling bottom product composition. 

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