Jul-2004

Further advances in light hydrocarbon fractionation

Recent developments in high capacity distillation trays focus on increasing capacity in fixed tower sizes in refineries and chemical plants. C2 and C3 splitter revamp case studies illustrate the potential of improved fractionation technology

Waldo de Villiers and Jose Bravo, Shell Global Solutions
Peter Wilkinson, Shell Global Solutions
Daniel Summers, Sulzer Chemtech

Article Summary

Revamps of high-pressure distillation columns, such as those found in light hydrocarbon service, can effectively make use of high capacity tray technology. Features of high capacity trays are:
Truncated downcomers that maximise the bubbling area;
Low entrainment bubblers such as the proprietary Sulzer MVG valves;
Long weir lengths providing high liquid handling capacity;
Good tray efficiency ensured by providing adequate flow path length and avoiding stagnant areas through good liquid distribution;
Ability to function at low tray spa-cing; and
Devices at the inlet of downcomers that produce a lower and more uniform froth height.

High capacity devices such as Shell’s proprietary Calming Section Plus (CS Plus) and HiFi Plus trays make use of these features. Figure 1 illustrates the expected capacity and range of application of these devices relative to conventional trays and packing. The design principle is that for every flow parameter (F) there is an optimal tray layout – one that maximises vapour capacity while maintaining an acceptable efficiency.

At high system pressures downcomer choking may occur, resulting in increased froth height and an earlier limitation on tray capacity. Hence, the maximum useful capacity is really that determined by the minimum of jet flood, downcomer choking and downcomer inlet velocity restrictions. The concept of “maximum useful capacity” (MUC) rather than a hydraulic flood is used to indicate the maximum tray capacity. The column may still be oper-able beyond 100% of MUC but the efficiency will be severely degraded.

The flow parameter is defined as: 
       F =  L     rV
                   V ÷ rL
where L/V represents the liquid-to-vapour mass flow ratio. The density correction factor is the ratio of vapour-to-liquid density.

MVG valves (low entrainment bubblers) are utilised in the CS Plus and HiFi Plus trays to offer increased vapour capacity and reduced entrainment. The lateral discharge of the vapour jets helps to collapse the froth and reduce entrainment. The directional nature of the MVG valves helps to sweep liquid across the tray deck.

Turndown capability is between that of sieve and moving valves. MVG tray panels are mechanically more robust than sieve or moving valve tray panels.

In the case of the HiFi Plus tray, additional downcomer liquid handling capacity can be obtained by using a proprietary Crown Inlet Device (CID). Typically, high-pressure columns have to handle high liquid loads and are often capacity-limited through the mechanism of downcomer inlet choking. The CID is a special downcomer inlet device that reduces the hydraulic restriction at the downcomer inlet. Note the typical tray layouts illustrated in Figure 2.

Calming Section (CS) trays are typ-ically used for flow parameters below 0.2 such as main fractionators, de-isobutanisers, C6/C7/C8 fractionations, atmospheric and mild vacuum chem-ical columns as well as any place where entrainment is generally a concern. A single tray design can be used for both odd and even trays.

The HiFi trays similar to those shown in Figure 2 are typically used for flow parameters above 0.1, such as deethanisers, depropanisers, debutanisers, rectified absorbers, C2 and C3 splitters, pump-around sections in fractionators as well as any place where liquid capacity is generally a concern. Again, a single tray design suffices for odd and even trays. Since their development, there have been more than 2000 applications of CS and HiFi trays worldwide.

Installation time and effort
When revamping C2 and other splitters, the downtime required to install new internals can often become the critical path. The length of the shutdown can have a significant impact on the revamp economics. Techniques that reduce installation effort are therefore of particular interest. Shell high capacity trays use envelope downcomers that are fully supported by the tray deck.

The downcomers are dropped into place with no need for modifications to downcomer bolting bars. Missing support ring segments can be filled in using expansion rings, thus avoiding welding to the vessel wall. Various time and cost saving techniques can be utilised with the high capacity trays [Bravo J L et al, Design and revamp of modern C2 splitters using high capacity internals and fast installation techniques; ERTC Petrochemical Conference, Amsterdam, Netherlands, February 2002].

 For example, the proprietary Lip-slot panel connection (Figure 3, top left) eliminates the need for bolting at panel overlaps. This dramatically reduces the time required to fasten panels to each other. Only the last panel is required to be bolted or connected with wedge-type connections. Proprietary Split-wedge tray support ring and downcomer connections further reduce the need for bolting. After installation, the wedges are secured by bending them outwards.

Existing support rings can be used to support new expansion rings with vertical struts and lattice beams to avoid having to weld to the column wall. This practice (Figure 3, bottom left) elim-inates the need for post welded heat treatment or pressure testing after the work has been completed. In addition, up to two HiFi trays can be supported by a single lattice beam at low tray spacings. The use of expansion rings, vertical bars and lattice beams combined with Lip-Slot panel connections may save up to 35% of the installation time, resulting in significant money savings over conventional methods.