Engineering the future

As the refining and chemical industries evolve, so does the focus on maximising energy efficiency and operational capacity. Distillation accounts for approximately 40% of energy consumption in refining and chemical processing¹ and 6% of total energy use in the US.² The design of distillation internals, particularly the selection of tray valve types, is an important decision that can strongly influence the energy efficiency and capacity goals of a plant. A poorly operating tower can significantly increase its energy consumption. Understanding how different valve types perform under varying conditions can simplify valve selection decisions and better align column design with long-term energy and capacity goals.

History of valves

Distillation trays have been used for centuries, with the earliest consisting of simple holes in the deck (sieve trays). Another early design was the ‘bubble cap’, a large, formed cap patented by Cellier-Blumenthal in 1815.³ Both devices served the industry for over a century, with the bubble cap tray used in services with a wide operating range. In the 1950s, movable valve trays were developed. These were smaller devices than bubble caps and improved on sieve trays with additional capacity and turndown by providing a cover over the hole. In the early 1990s, valves that are punched directly from the deck material were developed, which enhanced both capacity and reliability. In recent years, valve performance has been further optimised, and several new valve devices have been developed to increase capacity and efficiency compared to earlier generations.

Operating conditions

Valve performance is directly related to the active area performance of the tray. While the valve type has some effect on the downcomer performance, the main impact of valve performance is on the deck area. Tray efficiency depends on effective vapour and liquid interaction, which is achieved through uniform contact across the tray and thorough mixing at the deck level. Any inefficiencies in vapour and liquid contact across the tray will result in a greater energy requirement to make the separation, leading to energy inefficiency which is counter to plant sustainability goals. The upper limit of efficient operation is defined by jet flooding, which is where a large percentage of the liquid hits the tray above (entrainment). Eventually, this leads to the column filling with liquid which makes the column inoperable. Weeping, the lower limit of efficient operation, occurs when there is insufficient vapour pressure and liquid falls down onto the tray below through the openings in the deck, bypassing the contacting area on the tray. Weeping at the inlet of the tray is the worst type since the liquid misses contact on two tray levels, dropping near the downcomer below. This can cause inefficiency at minimum rates where more energy is required to maintain the vapour pressure and keep the efficiency at an operable level.

Valve features

The feature set of an active device will have a direct influence on its performance. By examining the specific features a device has, operators can directly correlate this to the performance in the tower.

The most basic deck device is the sieve hole. This is simply a hole in the deck without any added features to direct vapour flow or prevent liquid from weeping through the opening.

Movable valves, such as type T valves, have a cage and moving valve cap over the deck hole. This gives the hole some protection from liquid weep at low vapour rates and blocks the hole as the valve closes.

The latest generation of valves have a variety of features to enhance efficiency, capacity, and turndown performance. FLEXIPRO^® floating valve trays have a shaped cap that directs the vapour flow to leave the valve in a downward fashion. The valve shape, with a narrower downstream leg, helps create a forward pushing action which helps minimise gradients in the froth. The hole is extruded upward, creating a barrier to help prevent liquid from weeping through the hole. The floating valve also includes a moving cap that is able to close at reduced vapour rates to improve vapour distribution and further prevent the liquid from weeping through the cap.

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References

1. WHITE, D., ‘Optimize Energy Use in Distillation’, Chemical Engineering Progress, (March 2012).

2. CAHILL, J., ‘Reducing Distillation Column Energy Usage’, https://www.emersonautomationexperts.com/2010/industry/downstream-hydrocarbons/reducing_distil/

3. NIEUWOUDT, I. and PENCIAK, J., ‘Best of both’, Hydrocarbon Engineering, (July 2007), pp. 85 - 91.

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/08092025/engineering-the-future/