Jul-2014

Tray design for high load applications

Distillation is an essential separation technology in refineries, gas processing, and the chemical industries. Internals for distillation columns can be trays, structured packing, or random packing, with trays being often applied in column services with moderate to high liquid loads.

Ang Chew Peng, Sulzer Chemtech Pte Ltd
Mark Pilling Sulzer Chemtech USA, Inc.

Article Summary

In the last decades, larger and larger columns have been built as more world scale plants are constructed around the world to meet the downstream demands for petrochemical intermediates or chemical end products.¹ With these larger columns, the frequency for use of high liquid rate trays increases. This article discusses the unique requirements for high liquid tray applications as well as the different tray designs and selection criteria that can be used to operate successfully in those systems.

Tray design and operation fundamentals
Design concerns for conventional cross flow trays can be broadly categorised into two sections: tray deck design and downcomer design. Tray decks can have a variety of orifice types such as bubble caps, sieve holes, float or fixed valves to allow vapour passage for contacting with liquid on the tray deck to effect mass transfer. Downcomers serve to disengage vapour from the frothy mixture exiting the active tray deck and then carry the ideally clear liquid to the tray below as feed. Since tray decks and downcomers both fit within a given column cross section, they both must be proportioned to allow for adequate vapour and liquid handling capabilities. As a result, tray design for maximum capacity is a trade off to accommodate the vapour and liquid flows. If the downcomers are too big, then the deck’s active bubbling area will limit vapour capacity. If the downcomers are too small, the column will flood prematurely from a liquid side restriction.

Since the vapour must travel through the liquid on each tray, the more liquid on the tray, the less vapour that can be processed. The general liquid loading on a given tray panel is typically evaluated by the weir load. Weir load is the measure of volume of liquid that must travel across a given weir length on a tray. In the simplest sense, weir load directly affects the liquid level on the tray deck due to frictional losses. Not surprisingly, vapour capacity diminishes gradually with liquid weir loading up to a certain maximum. Beyond that weir loading, liquid builds up quickly on the tray deck and the vapour side capacity degrades even more quickly. Once the maximum weir load is approached, tray performance will almost always improve if the number of tray passes is increased. Hence, weir load is an important parameter used in multi pass tray design.

Cross flow trays have liquid flow passes typically ranging in number from one to four (but can be as high as eight), with the selection of passes depending on the column diameter and weir loading. Based simply on geometry, as the column diameter increases, the liquid weir loading increases also. As a result, larger diameter columns typically require the use of two or more liquid flow passes. Figures 1a-d show schematic diagrams of liquid cross flow trays with different numbers of passes.

Tray design for optimal performance
Choosing the proper number of passes for a tray usually involves some additional trade-offs. Increasing the number of passes for highly liquid loaded trays will give higher capacity but also increases mechanical complexity and equipment cost. More tray passes also reduces the liquid flow path length which can reduce the tray efficiency. It is not uncommon for a four-pass tray to have 5-10% lower efficiency than a comparable two pass tray. Regardless of the number of passes, it is critical that the resulting tray design perform satisfactorily, away from hydraulic limits such as jet flooding and downcomer flood while ensuring the base efficiency of the service is maintained.

Downcomer top area is usually sized based on the liquid volumetric flow rate, density difference between liquid and vapour, as well as any foaming tendency of the process. The downcomer downward flow velocity is generally the key design parameter. Also, the physical characteristics of the froth leaving the tray deck and entering the downcomer play a critical role in the downcomer sizing. If the vapour and liquid tend to separate easily (i.e., a large difference between the vapour and liquid densities and/or a low foaming tendency), then the allowable downcomer velocity can be higher. Conversely, if the vapour and liquid are more difficult to separate, then the downcomer design velocity must be lower, requiring trays with larger downcomers.

As mentioned earlier, weir loading is another very important hydraulic parameter in tray design. Weir loading affects the hydrostatic head on the tray and is used as a key parameter to determine the best number of tray passes.

Hydrostatic head adversely affects tray vapour capacity. The modified Colwell equation is commonly used to calculate the resulting hydrostatic clear liquid head on the tray deck.² The equation is shown below.

hcl = αhw + k1(α0.5/Cd)2/3(QL/WL)2/3

Where: hydrostatic head (hcl) is a function of weir height (hw), relative froth density (α), weir coefficient (Cd), liquid volumetric flow rate (QL) and weir length (WL).

Too high a weir loading (>110 m3/h.m), due to high liquid loads and/or inadequate weir length, results in high hydrostatic head, increased tray pressure drop and potential entrainment issues. If a tray design has a weir load of more than 110 m3/h.m, it is recommended to increase the number of passes. In some cases, an optimal design cannot be obtained with conventional cross flow trays for high liquid rate application. This may be due to liquid loads or the inability to mechanically fit the necessary number of flow passes in a given column diameter. In those cases, the designer can often use Shell HiFi™ trays with multiple downcomer boxes to achieve an improved design.5 HiFi trays have a very long effective weir length and can be used to successfully reduce weir loads to a more optimal value to maximise tray hydraulic performance.³