Early Detection of Distillation Column Flooding Conditions

The distillation process is one of the most common means of separating two or more components from a fluid. Ever handy Wikipedia defines it:

Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction.

One of the things that can go awry is column flooding. I found this definition from a U.K.-based introductory distillation site:

Flooding is brought about by excessive vapour flow, causing liquid to be entrained in the vapour up the column. The increased pressure from excessive vapour also backs up the liquid in the downcomer, causing an increase in liquid holdup on the plate above. Depending on the degree of flooding, the maximum capacity of the column may be severely reduced. Flooding is detected by sharp increases in column differential pressure and significant decrease in separation efficiency.

I share this as background to highlight a whitepaper by Emerson’s Roger Pihlaja, Distillation Column Flooding Diagnostics with Intelligent Differential Pressure Transmitter. Roger documents his work in how statistically based diagnostics can provide early warning to help operators take steps to avoid distillation column flooding conditions.

Not only helping avoid the flood condition, but also having this diagnostic allows the plant to operate the columns closer to their operational limits. In the whitepaper, Roger noted:

The process of distillation is very energy intensive and can contribute to more than 50% of plant operating costs [p. 63]. Thus, there is great interest in operating distillation columns as efficiently and reliably as possible.

Avoiding abnormal situations and improving energy efficiency are two key areas where these diagnostics can help. In developing these advanced diagnostics, Roger and the Rosemount measurement technology team worked with the University of Texas at Austin’s Separation Research Program. He highlighted the equipment used in the testing:

During this field experiment, the column was operating in 100% reflux mode. Under this mode, the reboiler heat duty could be increased incrementally until flooding was observed. The distillation column is instrumented with Foundation Fieldbus devices, and DeltaV is used as the distributed control system…

In this experiment, the structured packing only filled the bottom half of the column. A Rosemount 3051S Foundation Fieldbus differential pressure transmitter (Range 1: -25 to 25 inches of water) was installed on the distillation column across the packing section. A specialized data acquisition system was used to log the pressure signal at the same rate as sampled within the transmitter (approximately 22 Hz). By logging the raw-sampled process data, it would be possible to determine optimal SPM [statistical process monitoring] configuration settings (e.g. filter and sample window size) for detecting flooding.

Click to enlarge

The testing included adding heat into the reboiler in steps:

…the reboiler duty was increased further, each time moving the column closer to the Runaway Flooding condition. At 11:45 AM the reboiler duty was raised to 1.025 MMBtu/hr, and at 11:55 the reboiler duty was raised again to 1.05 MMBtu/hr. Both of these were operating points at which it was expected, based on operator knowledge, that the column was likely getting close to the Runaway Flooding condition. However, at both of these points, the DP leveled off to a steady-state value… Because the column was getting close to flooding, but flooding was not actually observed, we will call this time segment the “Incipient Flooding” condition.

Roger described how the SPM diagnostics operate:

The Advanced Diagnostics feature of both the HART and Fieldbus Rosemount 3051S pressure transmitters contains a user selectable, first-order digital high-pass filter (the differencing filter), which cuts off the lower frequency signals (including setpoint or transient changes), while passing the higher frequency noise.

The bottom plot of Figure 9 shows the standard deviation (using a 5-minute rolling window) of the highpass filtered pressure signal. Each time the reboiler duty is increased, the standard deviation also increased. In the Incipient Flooding condition, the standard deviation is significantly greater than in the Normal condition. Note; that, because a 5-minute rolling window was used for the standard deviation calculation, it takes at least that long for the standard deviation to reach its final value, after the reboiler duty was increased. Note also; that, the standard deviation is even greater when the distillation column transitions to the Runaway Flooding condition.

Therefore, one can clearly see that the filtered standard deviation gives an indication that the distillation column is approaching the flooded condition. By trending this filtered standard deviation, the plant operator is given an early warning that the distillation column is approaching the flooding point. In this way, an abnormal situation may be prevented.

Roger concluded:

Distillation Column Flooding is a phenomenon that can cause loss of separation and negatively impact the performance and energy efficiency of the distillation process. The onset of distillation column flooding is associated with a change in the flow regimes of the gas and liquids flowing inside the column. The flow regime associated with flooding generates more high frequency white noise, which can be detected in the DP signal across the column… When an operator is made aware that the column is approaching flooding, adjustments can be made to prevent the column from becoming completed flooded.

Give the whitepaper a read to better understand the column flooding conditions and how these diagnostics can help prevent this abnormal condition and allow the column to be operated closer to constraints.

3 comments

Leave a Reply