Oct-2004

Increasing vacuum tower performance

Advantages of using gravity flow liquid distributors in services where, historically, spray nozzles have been used, specifically in refinery vacuum tower wash sections where liquid entrainment must be avoided if capacity is to be increased

Mark Pilling, Nina Prohorenko & Alan Szymanski
Sulzer Chemtech USA Inc

Article Summary

Spray nozzle distributors are commonly used in heat transfer services and wash sections, but have the inherent disadvantage of generating small liquid droplets that tend to be entrained upward with the tower vapour. In vacuum tower wash sections, where vapour velocities are high and entrainment is critical, this can create serious process problems. Compared to spray nozzles, gravity flow distributors have better distribution characteristics and generate little, if any, liquid entrainment. Because of this, replacing vacuum tower spray nozzle distributors with gravity flow distributors can achieve capacity increases of over 25% while maintaining existing product yield and quality. Testing data highlight the distinctly different operating characteristics of spray nozzle and gravity flow distributors.

Since the introduction of structured packings and grid into refinery towers, the most commonly used liquid distributor for refinery vacuum tower wash sections has been the spray nozzle distributor. However, for several years gravity flow distributors have been successfully used to achieve significant performance increases in vacuum tower wash sections with no loss in column performance or run length. 

When evaluating equipment for replacement, it is a good idea to compare the important characteristics and capabilities of each type of equipment. Spray nozzle distributors are widely used with packed beds. Obviously, there are advantages in their use. The major advantages include:
— High open area producing very little vapour-side pressure drop
— Some degree of heat transfer from the interfacial area generated from the droplet formation
— Relatively insensitive to out-of-levelness
— Low residence time.

However, spray nozzle distributors also have several inherent disadvantages, including relatively small orifices that are often susceptible to fouling, relatively poor distribution quality and lower turndown range.

Accordingly, there are several benefits in using gravity distributors in most services, particularly with regard to less entrainment, high quality distribution and fouling resistance.

The vapour-side pressure drop across a distributor is often an important consideration in vacuum towers. A spray nozzle distributor has a typical open area of 85–90%. A narrow trough gravity flow distributor (Figure 1) used in vacuum service will have an open area of about 70–75%. Both distributors have high open areas.

Using a simple orifice equation, the vapour-side pressure drop for both devices is calculated to be less than 0.1in of water (0.15mm Hg). This is similar to the pressure drop taken by 1ft (305mm) of grid packing. Vapour-side pressure drops of this magnitude are quite low and should not adversely affect tower operations or economics.

Distribution quality
Spray nozzle distributors have a poorer distribution quality than gravity flow distributors due to their geometric layout and also from flow variances within the spray pattern itself. The nozzle pattern for a spray nozzle distributor header is arranged to create concentric rings of nozzles. The number of rings varies with column diameter, nozzle spray angle, and nozzle elevation above the packed bed. The conical spray patterns from the individual nozzles then overlap on the top of the packed bed.

Typical designs use anywhere from 150 to 200% coverage for the spray patterns in order to ensure the complete bed coverage. The problem that arises with this design is that the overlap patterns are never uniform across the bed. In order to get a more uniform overlap pattern, arrangements with more nozzles must be used. As more nozzles are used, the nozzle orifices must get smaller, creating a potential for fouling. Also, as more nozzles are used, the complexity and cost of the distributor increase.

Levelling of distributors should also be an important consideration. As stated earlier, spray nozzle distributors are not especially sensitive to out-of-levelness. This is due to the fact that the pressure drop taken across the nozzle itself is much larger than any typical level differences that may be seen. Therefore, liquid head deviations from out-of-levelness are generally negligible.

Most gravity flow distributors require a levelling tolerance of ±1/8in (3mm). This tolerance is generally not difficult to achieve. In some very large towers (>30ft or 9.1m), levelling can be a challenge, but is achievable with proper design and installation techniques.

Regarding turndown, distribution quality for all distributors suffers when the effective turndown range is exceeded. In wash section service, spray nozzles generally operate in the range of 10 to 15psi (0.7 to 1.0 bar) pressure drop with a maximum pressure drop of 20psi (1.4 bar). Above this range, the nozzles create larger amounts of small droplets that are susceptible to entrainment. Below this range, nozzle spray angles tend to decrease. There is also a possibility that the spray header pressure drop may become a significant portion of the overall distributor pressure drop and maldistribution may occur within the header itself. The pressure drop range of 10 to 20psi (0.7 to 1.4 bar) limits the allowable turndown, which will be the square root of 10/20, or 71%. A gravity flow distributor can easily be designed to allow for a 35% turndown or twice that of a spray nozzle.

The gravity flow distributor is clearly superior with respect to distribution quality and turndown. This is very important in the case of the vacuum tower wash section where improved distribution increases the packing efficiency and lowers the asphaltene and heavy metals concentration in the (HVGO) product drawn from above the wash section.

Fouling resistance
A spray nozzle generally takes a significant pressure drop on the liquid side due to its relatively small orifice. Because many refinery services are fouling or coking, spray nozzle distributors are typically installed with a filtering system to remove fouling material. A gravity flow distributor in a fouling service should be designed with an overflowing pre-distributor box and with flow orifices that are located in the vertical plane and elevated above the floor of the distributor.