Apr-2012

High-performance trays alone do not guarantee performance improvements

A high-performance tray does not determine the effectiveness of the whole column. Other distillation equipment plays a vital role in a column revamp

SOUNHO LEE, GTC Technology US
LIBOR HOJSIK, GTC Technology Europe
MYUNGHYUN KIM and DAEYOUNG JUNG, Hyundai Oil Bank

Article Summary

High-performance trays are widely used for projects to improve the performance of distillation columns. These types of trays can enhance column performance without complex modifications to the existing tray supports and column. However, utilising high-performance trays is not a guarantee of successful column performance. Other distillation equipment plays a vital role in a successful column revamp and overlooking these critical items often brings inferior results. These overlooked items are discussed and evaluated, and remedies are implemented in an actual hydrocracker revamp case study, which examines how targeted distillation column performance is achieved through careful analysis and design procedures.

Critical tray design issues
Since the early 1990s, high-performance trays have been applied to commercial distillation columns, especially where structured packing is not able to provide a noticeable performance gain. Most high-pressurised distillation services where high liquid and low vapour traffic are formed inside the column are good applications for these types of trays. A simple change-out from conventional trays to high-performance trays can provide a noticeable capacity gain without the need for increasing the existing column diameter. Tray efficiency improvement can also be achieved in certain applications utilising some high-performance tray devices available on the market. In addition, column shell modifications, known as field construction hot work, are typically eliminated. This shortens the unit downtime offline, as field construction is reduced, thereby resulting in overall cost savings.

Due to the simplicity of the field fit-up and easily attainable performance gains, high-performance trays have become a common option for distillation column revamps. However, there are cases where the post-modification column performances do not meet targeted objectives in spite of the installation of high- performance trays. Since a large number of design/operation parameters influence the resulting column performance, it is not easy to generalise the typical root causes of inferior performance. Two major areas have often been observed in revamp failures: improper evaluation and selection of optional high-performance tray features; and inadequate or incorrect design of peripheral distillation components within the column.

Optional high-performance tray features
Most high-performance trays in the industry have been designed with various performance-enhancing features to achieve the targeted column performance. These include specialised shaped downcomers, liquid inlet momentum breakers, tray inlet vapour/liquid contact initiation devices, and directional valves positioned in the tray periphery areas. A tray designer’s knowledge and experience is critical for the correct selection of the appropriate performance-enhancing features. Keep in mind that while some features help to improve column performance in certain applications, these features can also cause performance losses or unit troubles in other applications. For example, fouling service applications require precise, careful selection of performance-enhancing features. Poor application know-how often causes improper optional features selection in the high-performance tray design.

The shape of valve units utilised on high-performance trays has been developed as a mechanism to improve vapour and liquid contact, minimise pressure drop and maintain desired fouling resistance. The low pressure drop valve units can reduce the overall distillation column pressure drop compared to conventional tray valve units. Lower column pressure drop can improve distillation column energy consumption and reduce the chance of thermal degradation. Therefore, many distillation column revamps aim for minimum column pressure drop through high-performance tray implementation. While this is an important tray design principle, what is often ignored is that low tray vapour velocity can downgrade tray efficiency due to insufficient vapour/liquid contact volume. The tray designer should take this into consideration when optimising the tray pressure drop to prevent undesired efficiency loss.

Design of peripheral distillation components
The other common root cause of performance failure is poor design of the other distillation equipment in the column. Improperly designed peripheral distillation equipment can result in inferior performance even though the fractionation trays are designed correctly. It is essential to optimise the complete distillation equipment design, including fractionation trays and peripheral equipment, to ensure optimum column performance. The following discussion outlines the components often overlooked in distillation column design.

Chimney tray
The chimney tray is a common device for an intermediate stream draw-out of a distillation column, such as a refinery multi-product fractionator. A chimney tray can also be utilised as a transition device where the number of tray passes is changed in a particular section of the column. Since no vapour and liquid contact is created, liquid is collected and redistributed through a chimney tray. The chimney tray does not cause side draw product loss, which is a common problem when attempting to utilise a fractionation tray in direct side draw applications. In addition, a chimney tray provides better buffering against tray upset conditions and increases residence time for vapour component disengagement.

Chimney overall open area and liquid hold-up volume parameters are usually considered in most industry-standard chimney tray designs. It is well known that a chimney tray also functions as a liquid collection device. On the other hand, it is not well known that chimney trays provide vapour distribution through the column. Vapour distribution is an important factor for column efficiency. Ignoring this factor can cause vapour maldistribution and result in decreased column performance. The vertical distance between the chimney hat and the tray above is critical to ensure proper vapour distribution. An insufficient distance can lead to poor vapour distribution. Constructing an accurate hypothetical angle of vapour distribution can help to anticipate vapour distribution performance through the chimney tray.¹ A wide hypothetical angle between the chimney hat and the fractionating tray positioned above risks a higher chance of vapour maldistribution.