High performance trays for a fractionator in an aromatics plant
Distillation is a very important separation technology in refineries and downstream chemical processing. Mass transfer internals in distillation columns can be equipped with trays, structured packing or random packing.
Ang Chew Peng, Senthil Krishnamoorthy and Mario Roza
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Trays are generally the preferred option in columns with high liquid rates or in services with high fouling tendencies. After the acquisition of Nutter Engineering in 1998, Sulzer Chemtech expanded its tray product portfolio with inclusion of the acclaimed BDHTM rectangular float valves and the V-GridTM fixed valves. The technological alliance with Shell Global Solutions since 2000 has brought Shell high performance trays into Sulzer Chemtech’s range of tray products. This case study highlights an integrated refinery complex in the East Asia, where various tray options were evaluated and Shell tray technology was eventually applied for a new xylene rerun column in an aromatic unit.
Increased demand for synthetic fibre, partly fuelled by the rise in price of natural cotton, has led to a proliferation of world scale aromatics plants in recent years. Para-xylene is the key raw material in production of purified terepthalic acid (PTA) which is the main ingredient in polyester manufacturing. A typical aromatic unit has more than 30 columns of various sizes, among them small strippers of diameters less than 1 meter and large super-fractionators of diameters spanning more than 10 meters. The biggest columns in an aromatic unit include xylene rerun column, xylene splitters in the xylene fractionation unit as well as extract and raffinate columns in the para-xylene adsorption unit. A simplified process flow diagram of a typical aromatic unit is shown in Figure 1.
An integrated refinery complex in the East Asia planned for an expansion of its aromatic unit to increase aromatics production, to meet market demand. A process licensor was engaged for the project study. Based on the detailed evaluation and analysis, it was concluded that a new xylene rerun column was required to handle the new, increased, production loads. The xylene rerun column, in the xylene fractionation unit, is one of the heaviest loaded columns in the aromatic plant. It usually takes the bottom stream of deheptaniser, together with the effluent from the transalkylation unit. This column separates the mixed xylenes (ortho-xylene, meta-xylene and para-xylene) from the heavies, and the mixed xylenes are then sent to the xylene splitters for further separation.
The new xylene rerun column T-4001 was simulated with 88 theoretical stages including condenser and reboiler. It was designed to handle 2 feeds, with the top feed entering onto Stage 22 and the bottom feed entering onto Stage 61. The column had a specification of the C9 aromatics and heavies content in the distillate not to be more than 300ppm by weight and the recovery of ortho-xylene to be more than 99.90% by weight. Figure 2 shows the simplified column sketch of T-4001.
Tray Design Options
The selection of the most appropriate tray type requires detailed evaluation and the two main factors are tray performance and economics. Over the last decade, there has been a trend to equip new large fractionators with high performance trays. Use of advanced tray designs with high capacity valves and enhanced downcomers to handle the high vapour and liquid internal loadings inside the columns, can result in a substantial reduction in column sizes which translates into immediate savings in capital investment for plant owners.
For the new xylene rerun column T-4001, Sulzer Chemtech evaluated three tray designs.
• Design A: Conventional Trays
• Design B: High Performance Trays with Chordal Downcomers
• Design C: High Performance Trays with Multi-Downcomers
For the base tray Design A, BDH conventional valves with straight downcomers were considered. BDH valves are Sulzer Chemtech’s proprietary rectangular float valves, the lateral vapour release of these valves minimize the tendency of liquid weeping from upstream edge of the valves. In the process of tray design optimization, it was apparent that the high weir loading was the limiting factor for these chordal downcomer trays. Weir loading is defined as the liquid volumetric flowrate [in m3/hr] per unit weir length [in m]. It is a common design practice not to design trays with weir loading more than 120m3/(hr.m) and it would be ideal to keep the value less than 90m3/(hr.m) for increased safety margins in the tray design. The most effective method to reduce the weir loading is to increase the tray weir length by increasing the number of liquid passes on the tray. Hydraulic evaluation suggested that the diameters of these BDH conventional trays would be approximately 12 meters if they were designed with 4 liquid passes per tray. By increasing the number of liquid passes per tray to 6, the column diameter could be reduced to slightly over 10 meters.
Design B is for high performance trays with chordal downcomers. It uses proprietary MVG fixed valves with sloped downcomers, for an increased active area. MVG is the most established member of the V-Grid family which comprises of MMVGTM, MVGTM, SVGTM and XVGTM in the order of increasing sizes. V-Grid fixed valves are formed integrally on the tray decks and they release vapour from the side open area. MVG, being smaller than the conventional valves, offers higher vapour handling capacity due to improved distribution of vapour from the tray deck which reduces the froth height. The combination of MVG fixed valves with sloped downcomers resulted in a reduction in column diameter of more than 1 meter from the conventional tray design with 6 liquid passes per tray.
Design C is for Shell HiFiTM high performance trays. These trays are equipped with multiple envelope downcomers which are oriented offset to the tray’s centre line. This layout allows for the placement of more number of downcomers; the long weir length of these downcomers reduces the clear liquid height on the tray which results in a higher vapour handling capacity. Shell HiFi Plus trays are enhanced version featuring high capacity valves (such as MVG, MMVG) to further boost the tray deck performance.
The direction of vapour and liquid flows on a Shell HiFi tray is illustrated in Figure 4. The unique orientation of the downcomers allows uniform liquid distribution on each active tray deck (blue arrows) which then enables the vapour to get distributed uniformly underneath the tray deck and released laterally through the MVG valves perpendicular to the liquid flow (red arrows). The liquid flow path lengths are the same for all liquid passes, which bring about equal mass transfer on each active panel. With no obstruction between the various compartments of the trays, liquid and vapour are naturally self-balanced all over the tray for the most uniform mixing, contacting and highest separation efficiency achievable for multi-downcomer trays.
With the use of Shell HiFi Plus trays, the column diameter of xylene rerun column T-4001 was reduced to 8300mm for the top and middle sections and 8600mm for the bottom section. Compared to the conventional tray design, there was a reduction of more than 2 meters in column diameters and 32% in estimated column weight including column internals.
The xylene rerun column T-4001 was eventually designed with 117 Shell HiFi Plus trays with 80 trays in the top and middle sections (of diameter 8300mm), and 37 trays in the bottom section (of diameter 8600mm). As the trays in the bottom section had to handle much larger liquid loads, the HiFi Plus trays in the bottom section were designed with 12 downcomers per tray whereas the each HiFi Plus tray in the top and middle section was designed with 10 downcomers. Hence, a redistribution tray was required for the transition, to ensure that the liquid from the last tray of the middle section was properly distributed to the first tray of the bottom section. Sulzer Chemtech also added a collector tray below the last HiFi Plus tray. The collector tray was built to collect and direct the liquid from multiple downcomers to the column sump. Use of the collector tray prevented the potential re-entrainment of the liquid from the last tray by the vapour from the reboiler return.
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