PTQ Q3 2024 Issue

Ultra-low entrainment spray nozzles for use in packing wash applications

Development and performance of new spray nozzle technology can greatly reduce entrainment from vacuum distillation wash bed sprays

Alejandro Lago and Ashwin Patni Lechler Inc.

E ntrainment of liquid gasoil droplets in the wash section of a vacuum distillation column can lead to critical process reliability issues and distillate purity issues. Entrainment can be reduced by using a new spray nozzle technology designed to generate coarser diameter droplets, ensuring a larger percentage of wash oil makes it into the wash bed packing. Extensive testing shows that in comparison to traditional maximum free passage-style spray nozzles, the new spray nozzle technology can pro- duce a much more favourable spectrum of droplets, which can be correlated to a significant reduction in entrainment from a wash oil spray header. Theoretical Stokes’ Law- based entrainment calculations based on this droplet data being sprayed in a counter-current tower of ~0.4°C factor result in an entrainment reduction of around 300% in com- parison to traditional spray nozzles commonly used in this service. VDU wash section Vacuum distillation plays a critical role in various industries, such as petroleum refining and chemical processing, where the extraction of volatile components under reduced pres- sure is necessary. Refinery vacuum distillation units (VDUs) allow for the processing of heavier oil feeds slumped from upstream atmospheric distillation units. By drawing a vac- uum in this column and thus reducing the boiling point of the feed, these heavier oil feeds separate more effectively. A typical VDU is comprised of multiple levels of pack - ing, trays, and/or spray nozzles. Spray nozzles are gener- ally found in two regions: wash sections and pumparound sections. The focus of this article will be the VDU wash section, typically comprised of a short section of packing and a wash oil spray header located above this packing. The wash zone in a distillation column is responsible for removing non-volatile, entrained heavy metals and heavy end contaminates from the rising vapour. In theory, the wash zone spray header feeds liquid gas- oil to the packing to facilitate contact between the vapour and liquid on the surface area of the wash bed packing. The intent of this spray header is to provide evenly distrib- uted liquid to the wash packing, which provides the wetted interfacial surface area for the liquid contact of incoming vapours. It is imperative that this packing remains wetted,

as liquid dry-out can lead to operational upsets due to pre- mature coking or fouling. As refiners push VDU towers with the intent of optimising yields, they often face challenges in the wash section since high vapour velocities coupled with high wash rates create a perfect environment for the carryover of small droplets created by conventional, maximum free passage- style spray nozzles. This article aims to address the issue of entrainment of liquid gasoil in the wash section by using a new spray noz- zle technology that produces a favourable droplet spectrum. By increasing the percentage of large droplets and decreasing the percentage of small droplets created by the spray nozzle, more of the total volume of gasoil will overcome the drag forces of incoming vapour and allow for more effective washing of the packing. This can con- tribute to more efficient distillation in a column, extend the As refiners push VDU towers with the intent of optimising yields, they often face challenges in the wash section runtime of a distillation column, and help avoid unplanned shutdowns due to premature coking of the bed, which is generally caused by unwetted packing. Entrainment in vacuum distillation Entrainment is an undesirable phenomenon in which liquid droplets generated by a spray nozzle are carried away, or entrained, by a vapour phase in a distillation column in a counter-current arrangement. In vacuum distillation, drop- let entrainment occurs when the velocity and momentum of descending liquid droplets are overcome by the drag forces of an ascending vapour stream. Spray nozzles generate a population of droplet sizes with corresponding velocities and mass. A fraction of droplets are entrained if their mass, veloc- ity, and droplet size are not sufficient to overcome the drag forces generated on the droplets by incoming vapour. For this reason, it is important to minimise the volume fraction

PTQ Q3 2024