Oct-2006

Distributor design and testing

A new main fractionator designed to separate a range of feed streams incorporates multiple layers of structured packing and a slurry section. Flow testing to verify the distribution quality of the column distributors is essential

Deepak Dhabalia and Mark Pilling
Sulzer Chemtech Inc

Article Summary

When a large US Gulf Coast FCC unit was revamped, it required a new main fractionator. The new column was 22ft (6.7m) in diameter and comprised of ten “beds” with fractionation and pumparound sections for HFCC naphtha, jet fuel, LCO and HCO, plus a slurry pumparound section. All sections other than the slurry section were packed with structured packing and fed by Sulzer VEP gravity flow distributors. The slurry section used the proprietary Mellagrid structured packing and was fed by a Sulzer VES distributor. This combination of grid packing and v-notch distributor is commonly used for slurry sections because of its resistance to fouling.

The design of distributors in any fouling service is always a balance between resistance to fouling and distribution quality. For resistance to fouling, flow orifices typically need to be large and preferably elevated off the floor of the distributor, while good distribution quality requires uniform wetting of the packing (ie, a high enough drip point density) as well as controlled momentum near the orifices to ensure equal flows. Adequate liquid head in the distributor plays a significant role in dissipating momentum and minimising the effects of flow anomalies throughout the distributor.

Ideally, a distributor with large orifices, a moderate drip point density and a moderate head level would provide an efficient, fouling-resistant design. However, a large number of big holes in a distributor generally results in a low liquid head, which typically means low distribution quality. As a result, the proper design for fouling-resistant distributors often requires a compromise between hole size, distribution density and acceptable distribution quality. Since a fouled distributor is generally inoperable, resistance to fouling usually takes precedence over anything more than adequate distribution quality.

A good practice for most distributor designs is to perform a flow test prior to installation in order to verify distribution quality. A simple hydraulic check at the manufacturing facility can help identify potential problems that can be easily corrected prior to actual tower installation. Although these types of problem are relatively rare, when a problem is found during a flow test the benefits are tremendous compared with finding a problem during startup or subsequent operation.

Distributor design and testing
Slurry services typically use v-notch gravity distributors to handle the slurry and extremely high liquid rates. Sulzer’s general recommendation for this service is a VES distributor. This distributor has a main channel-parting box to calm the initial liquid feed and then proportionally distribute the liquid to a series of individual troughs (or arm channels) for final distribution. Liquid leaves these troughs through a number of v-notch orifices located on the lower portion of the vertical side panels. Liquid flowing out of these v-notches impinges onto a vertical baffle plate located outside the troughs. The baffle serves to disperse the liquid into an even flow and then directs it onto the packing immediately below. Since the baffle provides additional redistribution of the liquid from the troughs, the troughs can be designed with fewer and, more importantly, larger, more fouling-resistant flow orifices.

Distributors of this type are normally designed with a distribution density of approximately 4 pts/ft2 (40 pts/m2), depending on the minimum slot size at the bottom and the minimum head requirements.

V-notch distributors generally have the maximum possible hole size for resistance to fouling, so they usually do not have high liquid heads in the distributor channels. A typical liquid head level in a v-notch distributor is 1–3in (25–75mm) over the entire operating range, while a more standard orifice distributor will maintain a head of 3–12in (75–300mm). Since liquid head above the orifice is a major component of flow uniformity, the lower head distributors typically have a lower-quality distribution than higher head designs. As a result, careful design of the feeds, main channel and troughs is mandatory to ensure the liquid head, and resulting liquid distribution, is uniform across the entire distributor.

Based on the diameter and flow rate to be handled in this column, a dual main channel design was selected. This is similar to the single main channel design shown in Figure 1, except that it uses two parallel main channels rather than a single channel on the centreline. Another common feature with large-diameter, high-liquid distributors is the use of smaller channels within the trough that transport liquid delivered to the centre of a channel or trough laterally out towards the ends. This alleviates much of the horizontal momentum in the lower portion of the trough and leads to better flow quality from the trough orifices. This particular design initially used a perforated v-shaped channel along the main channel and the arm troughs to improve flow characteristics.

Distributor flow test requirements
Sulzer’s standard test criteria for all distributors are based on the liquid head measured at each end of the trough. For larger-diameter distributors, other random points in the arm trough at the centre (close to the main channel) are tested as well. For this particular design, it was agreed to take measurements at 60 locations on each side of the distributor, meaning a total of 120 measurements. These tests are performed for all design rates, including maximum and minimum. The design rates for this application are shown as follows:

Operating range of VES distributor
Minimum flow: 1985 gpm (451 m3/hr)
Design flow: 3971 gpm (902 m3/hr)
Maximum flow: 4523 gpm (1027 m3/hr)

During testing, these measurements are checked for standard deviation, where a coefficient of variation, or Cv, is calculated. Most well-designed distributors for non-fouling services can achieve Cvs well below 10% for design conditions. Turndown conditions typically have lower head levels and achieve Cv values near 10%. Acceptance criteria for these distributors are generally 5–10%, depending on the flow rates and service.