The trays in the V-Grid family are used successfully in applications such as separation or absorption columns, which are common in the petrochemical industry. Following extensive research and testing, Sulzer Chemtech has now dramatically improved the high-performance VGPlus trays with the help of new technologies like computational fluid dynamics (CFD).
Markus Fischer, Loris Tonon
The internals in distillation columns, which are widely used for separation in the petroleum and petrochemical industries, strongly influence the maximum capacity and the quality of the products. In high-pressure applications, where structured packings are unsuitable, sieve or valve trays are used. In each tray stage, liquid containing a certain fraction of the prod-ucts to be separated flows over the tray, while vapour rises upwards through the valves. In the active or bubbling area of the tray, the vapour and liquid come into contact and the concentrations of the required product in the vapour and liquid phases converge. This increases the concentration of the highly volatile product in the vapour on its way up through the tray stages (150 or more stages can be necessary for the fractionation of ethylene and ethane). It is essential for reliable operation of the trays that all liquid flows down through the downcomers, in other words no liquid must be allowed to drip through the valves.
Design confirmed by CFD study
The proven MVGT tray combines V-Grids with a high-performance downcomer and is Sulzer Chemtech’s proprietary solution for maximum efficiency and capacity over a wide operating range. As there are no moving parts, these trays exhibit no mechanical wear and a very good fouling resis-tance. Sulzer Chemtech has recently enhanced the existing V-Grid series of trays, making the VGPlus type one of the best performing trays currently available in the marketplace. In collaboration with Sulzer Innotec, the two-phase simulation of the flow on the tray deck has been dramatically improved, allowing complex phenomena like froth flow and liquid entrainment in the active area to be studied. By strategically designing and placing baffle bars on the tray deck, stagnation of the liquid flow has been eliminated.
A comparison of pilot plant results and CFD studies reveals a very good match. The redirecting system distributes the flow from the downcomer such that the full valve area is evenly covered with liquid, thus increasing the capacity and efficiency of the tray.
Flow capacity maximised
Another important feature of the VGPlus trays is the downcomer technology. The high-performance downcomers maximise the bubbling area and increase the capacity. In combination with the V-Grid valves and their entrainment-reducing properties, VGPlus has proved to have exceptional vapour and liquid handling capabilities.
Due to their reported high performance and commercial success, the VGPlus trays were selected for testing by FRI. FRI is a non-profit-making research consortium based in Oklahoma (USA) and financed by its members, which include the world’s largest petroleum and petrochemical companies. In its Category 1 program, leading distillation internals are selected for performance validation at FRI’s industrial scale test facility. In these tests, VGPlus achieved the best performance of any tray ever tested at FRI in terms of both efficiency and capacity. The VGPlus trays integrate the design features necessary to achieve maximum capacity and efficiency. These features are not simply welcome when designing new columns: they are almost unbeatable when it comes to planning the re-vamp of an existing tower. Compared to conventional sieve trays, the capacity can be increased by as much as 30 % – without changing the diameter of the column.
Example of a successful revamp
In a major American refinery, the existing depropanizer was partially revamped with VGPlus trays. The customer’s stipulation that the feed capacity be augmented by 20 % necessitated a 20 % increase in the reflux ratio from the condenser to the top of the column and thus also in the internal loads.
It was possible to reuse the existing column with its 3.2 m diameter by replacing 44 conventional 2-pass trays in the bottom section with VGPlus trays. Since the revamp, the column has been operating at more than 25 % higher capacity with product pu-rities remaining within the specification. Furthermore, the ultimate capacity of the trays has not yet been reached, because no additional feed is available.
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