(187c) A Design Solution to Feed Flow Maldistribution to Multiple Reboilers in Large Diameter Distillation Towers

In refineries, petrochemical, and gas plants it is common to have large diameter distillation towers, in which multi-pass trays are employed and the heat is provided by multiple parallel reboilers. Providing adequate distribution to multiple parallel reboilers has been particularly challenging. Conventional designs have often been plagued with heat transfer limitations, accelerated reboiler fouling, operating instability, transients and swings, and plant bottlenecks.

Recognizing these issues, while designing two large-diameter towers with multiple once-through thermosiphon reboilers, a task force of Tengizchevroil and KPJV engineers was assembled to develop a design for a collector tray, gathering the liquid from the bottom 4-pass tray, that will provide equal liquid distribution to the reboilers over the full operating range of 40 to 110% of design flows, permit full production rate even when one of the reboilers is removed for cleaning with equal distribution to remaining reboilers, completely decouple liquid distribution from hydraulic disturbances in the reboiler circuits to positively prevent swings and transients, handle reasonable tilt, handle reasonable maldistribution on the 4-pass tray above, and provide good gas distribution to the tray above.

The KPJV-TCO MOTR (Multiple Once-Through Reboilers) Collector-distributor design developed consists of an internal liquid reservoir maintained by an annular shroud with notches cut in the shroud. Liquid from the inner reservoir overflows the rectangular notches to an annular sump ring. The annular sump ring is divided into reboiler feed compartments by segmenting radial baffles. The number of annular sump ring compartments is a function of the number of reboiler draw nozzles required. Each compartment feeds one reboiler draw nozzle.

The liquid level in the inner reservoir is set by the liquid flow rate and heights of the notches. The annular sump ring floor elevation is well-below the floor of the inner reservoir, so the liquid overflowing the notches falls through the notches onto the annular sump ring. In operation there is no liquid continuity via the notches between the inner reservoir and annular sump ring. The annular sump ring bottom deck is level with the bottom of the reboiler draw nozzles to prevent stagnant liquid hold up on the tray.

Just like any design, the details will make or break it. With the challenging design at hand, a multitude of considerations exist, and a failure to adequately address any of these details can make the difference between success and failure.

To the best of our knowledge, our innovative improved design has not previously been applied. Therefore our team decided to audit the design using CFD and water-test it. The water test was performed at the Sulzer shop in Tulsa, OK, on a full scale prototype of the collector tray design for each of the two columns. Both the CFD audit and the water tests demonstrated that our design met all the objectives described above. For once, a design is available that permits even distribution of liquid to any combination of multiple parallel once-through thermosiphon reboiler circuits even when one is out of service for maintenance.

Our paper provides detailed description of the new MOTR splitter as well as the lengthy list of considerations that need to be addressed to achieve a successful design. Our paper also describes the CFD audit and the water test and its results. It is believed that this development will help engineers struggling with similar issues to achieve good liquid distribution and trouble-free operation of multiple reboiler circuits.