PTQ Q2 2022 Issue

Revamp of a methanol wash column

Installation of high performance trays and internals enabled a successful revamp for a syngas purification process


L inde Engineering’s Rectisol is a process for purifica - tion of syngas with selective removal of carbon dioxide (CO 2 ) and hydrogen sulphide (H 2 S)/car- bonyl sulphide (COS). It involves the physical absorption of acid gases using methanol at high pres- sure and low temperature in the main column – methanol wash col- umn. In 2018, Linde Engineering was planning for an expansion of a Rectisol plant in Singapore (see Figure 1 ) after 15 years of operation to achieve 30% higher plant capac- ity with more stringent efficiency requirements. The modification plan included replacing existing trays in the methanol wash column with state-of-the-art tray technology – Shell HiFi(*) and Sulzer VGPlus(**) trays – at reduced tray spacing to provide for more absorption stages. The great teamwork between Sulzer and Linde Engineering was pivotal to the success of the revamp – all the revamp targets were achieved. This article discusses various challenges of this success story, including the careful selection and hydrau- lic design of the high performance trays, the complexity of the mechan- ical design to retrofit the trays with - out hot works in the column, as well as the installation of the column internals within a record time of 17 days by Sulzer tower field services. Methanol wash column and revamp targets The revamp targets of the Rectisol plant in Singapore were for 30% higher plant capacity while using the original solvent quantity at the same syngas specifications. This column, of inner diameter 1750mm,

to be brought through manholes of diameter 500mm and the modifi - cation work was to be executed in the tight column space of diameter 1750mm. The target turnaround time was only 21 days, a week short of the ‘standard’ of 30 days. This revamp not only posed process and mechanical challenges in the design of the column internals but also challenged Sulzer’s field service team carrying out the installation. Sections A and B: four-for-three replacement with Shell HiFi trays In normal operation, lean methanol is fed into the top tray of Section A to absorb CO 2 from the gas; the loaded methanol leaving Section A is refrigerated before being fed into Section B to absorb CO 2 from the rising vapour. Before the revamp, Section A had 39 two-pass trays with tray spacing of 400mm, and Section B had nine two-pass trays with tray spacing of 450mm. The modification plan was to have a four-for-three replacement with high performance trays for more theoretical stages. Every four exist- ing trays, with three-tray spacing between them, will be replaced by five new trays, each with reduced tray spacing. After the revamp, Section A had 49 trays at reduced tray spacing of 300mm, while Section B had 11 trays with reduced tray spacing of 337.5mm. The feed pipes for Sections A and B were also replaced to optimise liquid distribu- tion onto the new HiFi trays. Sulzer and Linde Engineering did a detailed hydraulic evaluation and selected the Shell HiFi trays (see Figure 3 ) for the following reasons: 1. They are multiple downcomer

Figure 1 Rectisol plant in Singapore

had four trayed sections, segregated by three chimney trays with total draw off. CO 2 and H 2 S are removed in separate sections, allowing for a pure CO 2 stream and a H 2 S/COS enriched Claus gas fraction. In Figure 2 , the feed gas enters the bot- tom of the column – the pre-wash section (Section D) – while lean methanol is fed onto the top of the column. CO 2 is removed in the top two sections (Sections A and B) and H 2 S is removed in the third section (Section C). As per Linde Engineering’s detailed revamp study, the existing column internals had to be modi- fied to handle the increased plant load. To handle the higher vapour loads, Sections A, B and C required a replacement by high performance trays. The number of trays had to be increased in Sections A and B, by reducing tray spacing, to pro- vide additional absorption stages. Hot works were not allowed on the column shell. All internals and additional support structures were

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