Nov-2021

High capacity valves for distillation towers

Advanced valves deliver higher column capacity while meeting process requirements such as wider operating range and higher fouling resistance

ANG CHEW PENG
Sulzer

Article Summary

Distillation is an essential unit operation in the refining, petrochemical and chemical industries. Mass transfer equipment applied in distillation towers can be trays or packings. Trays are usually the preferred choice of column internals for towers operating above atmospheric conditions and are handling moderate to high liquid loads or potentially fouling process fluids. As plants ramp up their capacity to meet increasing product demands in a cost effective manner, column equipment specialists are driven to employ high performance separation equipment as revamps. Sulzer has always been focused in the research and development of high performance trays to meet the new requirements of these fractionating towers. Sulzer’s latest valves, part of the UFM family, have proven to handle higher column capacity while meeting the process requirements such as wider operating range or higher fouling resistance. This article will discuss these advanced valves — the high capacity UFM movable valve, high performance UFMPlus trays with UFM PV, the anti-fouling UFM AF fixed valve — from their conceptualisation to practical application.

Development
Movable round valves, developed in the 1940s, have been popular and are seen in many distillation towers today. Movable valves allow the tray open area to vary in response to the changing vapour loads, thus widening the operating range of the trays. There have been several variations on these conventional movable valves in the industry. As refineries and chemical processing plants expand their production capacity, the hydraulic demands of these distillation towers increase. Towers which were originally equipped with conventional movable valves require higher capacity valves without compromising on operating range and separation efficiency. This led Sulzer to develop a high performance movable valve, UFM.

In the early development stages, Sulzer looked extensively into the shape of the valve and how it impacts fluid dynamics on the tray deck. The new valve had to meet the following objectives:
-    The valve should direct the vapour flow closer to the tray floor, to maximise the interaction of vapour and liquid for mass transfer.
-    The valve should reduce the froth height on the tray, to increase the vapour handling capacity of the tray.

Following research and laboratory work, Sulzer designed the UFM valve with the signature dome shape. Comparing the CFD simulation results of vapour flow from conventional valves and UFM valves (see Figure 1), this shape directs vapour towards the tray decks resulting in high velocity zones close to the decks while the region above the UFM valves has approximately 40% lower velocity than that of conventional valves.

The capacity of UFM valves was first tested in the Sulzer laboratory in a chlorobenzene/ethylbenzene test system operating at atmospheric pressure under total reflux conditions. Results are compared with trays with conventional movable valves and, as Figure 2 shows, UFM valves have a 15-20% higher capacity than conventional movable valves, while maintaining higher efficiency through the entire operating range. This indicates good mixing on the tray deck and minimal entrainment until hydraulic limitation is reached. 

The advantages of UFM valves are further validated in the tests conducted at a leading independent commercial scale testing facility, Fractional Research, Inc. (FRI). The UFM trays were tested in cyclohexane/n-heptane at 23 psia (1.62 bar) and o-xylene/p-xylene at 14.7 psia (1.0 bar). For both systems, the UFM trays achieved high useful capacities while maintaining a consistent high efficiency over a wide operating range and are regarded as one of the best performing valves tested by FRI to date.

Since the launch of UFM valves in 2012, they have been installed in over 1200 towers. They can be applied on tray decks in all clean services including super fractionators, light hydrocarbon fractionators and splitters in chemicals and petrochemical applications where movable valves are desired. Their performance has been demonstrated in more than 90 different applications, meeting hydraulic demand while maintaining high efficiency throughout the required operating range.

One successful case study is the revamp of a reformate splitter with UFM trays to meet target revamp loads. A new environmental regulation required the blended gasoline to have less than 0.62% benzene by volume, leading to a 15% increase in feed rate to the reformate splitter. The splitter, originally equipped with high capacity fixed valve trays, was at its hydraulic limits and could not handle the new loads. Sulzer revamped the tower with UFM tray decks and optimised downcomers, and the tower was able to meet the required revamp capacity and product specifications.

Another application is the revamp of a diesel stripper with UFM trays. The revamp targets were to increase the feed rate by 14% while maximising the diesel yield and reducing H2S slippage. The bottom section of the diesel stripper, equipped with four-pass round valve trays, was evaluated to be the bottleneck of the tower. Sulzer supplied new four-pass UFM trays which were able to meet the target revamp capacity as well as the product specifications.

Boosting column capacity
While high capacity valves can boost the hydraulic capacity of the trays, the application of high performance downcomers can further optimise the tray bubbling area, increasing the vapour handling capacity and efficiency of the trays. UFMPlus high performance trays (see Figure 3) combine high capacity UFM valves, and enhanced downcomers to maximise tower performance and profitability.

To further boost the capacity of UFMPlus trays, Sulzer developed UFM PV directional movable valves which are strategically positioned at the tray periphery and elsewhere depending on the flow rates. These push valves promote uniform flows and aeration throughout the tray deck, especially in the dead zone near the column wall. The net effect is stable high efficiency at high rates as well as turndown conditions. 

The capacity enhancement effect of UFM PV was also validated in Sulzer’s laboratory, in the same chlorobenzene/ethylbenzene test system operating at atmospheric pressure under total reflux conditions. In the 1m test column, the use of UFM PV on the tray deck increased the capacity of the UFMPlus trays by nearly 8% while maintaining high efficiency throughout the whole operating range (see Figure 4).

UFMPlus high performance trays have been successfully applied in numerous commercial applications. One application is in the ethylene oxide purification column of an ethylene oxide/ethylene glycol (EOEG) plant. The plant operator wanted to increase ethylene oxide throughput by 50%. Sulzer replaced the existing conventional movable valve trays with UFMPlus trays with high performance downcomers. The column was able to achieve the target capacity increase of 50% with less impurity in the ethylene oxide product.

Another case story is the revamp of benzene tower in a phenol plant. The plant was planning for a 50% increase in phenol production rates, but the existing sieve trays could not handle the target loads. UFMPlus high performance trays with UFM PV were supplied and the tower was able to meet the revamp capacity and separation requirements.
High capacity valve for fouling applications

Some distillation towers in certain applications are more prone to fouling; the main fouling mechanisms may include deposition of solids such as corrosion products or precipitated salts, and chemical reactions such as polymerisation. If methods such as filtration, process control, and the use of anti-foulants cannot effectively mitigate fouling in the towers, advanced column internals can be used to handle the fouling conditions and prolong the run length between turnarounds.¹