(184b) Negative Inlet Downcomer Seal Impact On Tray Efficiency: A Study of Past Commercial Data

Downcomer (DC) hydraulics form an integral part to the successful efficient operation of distillation cross flow sieve and valve trays.

Conventional wisdom for tray designers is to ensure downcomer (DC) seal liquid to facilitate stable liquid entry on to the tray deck. DC seal depends on tray designs including the DC clearance and outlet weir height and operating conditions. Parameters including DC backup, tray pressure drop, and tray liquid holdup all influence the DC seal. Downcomer backup (clear liquid height at bottom of DC) comprises of clear liquid height at DC exit, DC head loss, tray clear liquid height, outlet weir height.

Under certain conditions, when downcomer seal is lost, vapor short circuits through the downcomer, depriving the liquid passage through the downcomer on to the tray deck. A negative inlet downcomer seal is detrimental to overall column efficiency, particularly at turndown.

The paper comprises of two parts. Part 1 is a forensic analysis of four tray clear liquid height predictions from the literature compared against measured old FRI data in reports held in a special section of Oklahoma State University. When plugged into the clear liquid height at bottom of DC correlation, one can determine which literature correlations predict a positive or negative seal, particularly at turndown. Finally, it will be shown whether a negative DC seal deleteriously affects measured tray efficiencies documented in those same FRI reports.