Jul-2024

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.

Article Summary

Entrainment 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 produce 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 comparison 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 pressure is necessary. Refinery vacuum distillation units (VDUs) allow for the processing of heavier oil feeds slumped from upstream atmospheric distillation units. By drawing a vacuum 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 packing, trays, and/or spray nozzles. Spray nozzles are generally 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 gasoil 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 distributed 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 premature 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 nozzle 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 contribute to more efficient distillation in a column, extend the 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, droplet 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, velocity, 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 of small droplets, which have a higher tendency of becoming entrained. Moreover, as vapour side velocity increases, it exerts a greater drag force on spray droplets, essentially increasing the threshold of entrained droplets to a larger droplet diameter.

Spray nozzle hydraulics also impact entrainment. As differential pressure across the spray nozzle increases, more energy for atomisation is introduced, creating finer droplets that are more easily entrainable. Entrainment thus becomes more pronounced when both the liquid feed rates and vapour feed rates are relatively high. High levels of entrainment in a vacuum distillation column can pose undesirable consequences, such as a higher propensity for coking and reduced yield, which are detrimental to the overall distillation efficiency of a tower.

Guiding principles: Full cone spray nozzle design
In the field of spray nozzle development, several interrelated design levers can be adjusted to reach desired performance characteristics. The shape of internal and external spray nozzle geometries plays a major role in droplet sizes, droplet trajectories, spray angle, and clog resistance. However, certain guiding principles in full cone spray nozzle development cannot be circumvented. For instance, under identical operating conditions (same flow rate and differential pressure), two nozzles with different exit orifices will generate different droplet sizes.

A nozzle with a larger exit orifice will generate larger droplet sizes and vice versa. Moreover, in the case of two full-cone nozzles delivering 10 gallons per minute (GPM) at 10 pounds per square inch gauge (psig) differential, the free passage of a nozzle with a 90-degree spray angle will be larger than that of a nozzle with a 120-degree spray angle. Also, it is generally observed that the spray angle of a nozzle is inversely related to its free passage, assuming all other factors remain constant.

Regarding droplet size, when two spray nozzles operating at the same differential pressure are observed, the nozzle with a greater flow capacity produces larger droplets than a smaller capacity nozzle. For example, a nozzle designed to spray 10 GPM at 10 psig will produce larger droplets than a mechanically identical nozzle designed for 5 GPM at 10 psig. It is important to consider all these guiding principles in chorus during the conceptual design phase of a new spray nozzle in an effort to satisfy both process and operational expectations.

Maximising free passage
Conventional spray nozzles that are widely used today in the wash sections of vacuum distillation columns were primarily designed to be fouling resistant. The design of these nozzles centres on maximising the ‘free passage’ of the spray nozzle in the hope of reducing nozzle clogging with little consideration for their atomisation characteristics. Traditional maximum free passage-style nozzles tend to create a significant percentage of fine droplets, especially when operating at higher differential pressures. In an effort to overcome limitations tied to conventional wash bed spray nozzles, Lechler embarked on a product development journey to design a spray nozzle specifically for use in counter-current wetting applications. During the conceptual design phase, a wish list of design parameters was gathered from key distillation stakeholders and ordered in terms of importance to both process and operational considerations: entrainment reduction, clog resistance, good distribution, and self-draining axial design.