Revamp of a methanol wash column

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

ANG CHEW PENG and TAN HIAN MIN, Sulzer Singapore
CARLOS ARGUELLES, Sulzer Chemtech Switzerland

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Article Summary

Linde Engineering's Rectisol is a process for purification of syngas with selective removal of carbon dioxide (CO2) and hydrogen sulphide (H2S)/carbonyl sulphide (COS). It involves the physical absorption of acid gases using methanol at high pressure and low temperature in the main column“ methanol wash column. 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 capacity 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 hydraulic design of the high performance trays, the complexity of the mechanical design to retrofit the trays without 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, had four trayed sections, segregated by three chimney trays with total draw off. CO2 and H2S are removed in separate sections, allowing for a pure CO2 stream and a H2S/COS enriched Claus gas fraction. In Figure 2, the feed gas enters the bottom of the column the pre-wash section (Section D)“ while lean methanol is fed onto the top of the column. CO2 is removed in the top two sections (Sections A and B) and H2S is removed in the third section (Section C).

As per Linde Engineerings detailed revamp study, the existing column internals had to be modified 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 provide additional absorption stages. Hot works were not allowed on the column shell. All internals and additional support structures were to be brought through manholes of diameter 500mm and the modification 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 Sulzers 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 CO2 from the gas; the loaded methanol leaving Section A is refrigerated before being fed into Section B to absorb CO2 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 existing 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 distribution 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 trays, ideal for high liquid loads. They allow for multiple downcomers even at small column diameters. Their large downcomer top area can handle high liquid loads, while the longer weir length results in a lower weir loading, which may additionally improve the vapour handling capacity in some cases.
2. These trays are self-balanced. The liquid to each downcomer is distributed proportionally to the weir length of each downcomer. They also allow for liquid communication on the trays. Unlike multi-pass trays such as two-pass and four-pass trays, where the liquid from different panels does not mix, Shell HiFi trays allow the liquid to equalise and balance over the entire tray, enhancing the mass transfer.
3. Shell HiFi trays, which use full support rings, are easier to install for a four-for-three retrofit, especially when hot works are not permitted on the column shell.

Hydraulic design
The hydraulic design of the HiFi trays was challenging due to the high liquid loads and the low tray spacing. The high loads required a large downcomer top area; the number and size of the HiFi boxes was optimised to avoid small flow path lengths. The HiFi trays in Sections A and B were designed with three HiFi boxes of top width 300mm. The downcomer inlet velocity at the maximum case was 0.105 m/s. Typical for trays with low tray spacing (below 400mm), the limiting hydraulic parameter was the downcomer froth backup. In order to have a taller downcomer height to handle the froth, the downcomers of these trays were untypically positively sealed with a clearance height less than the outlet weir height. For positively sealed trays, the amount of backup in the downcomers is constituted by three main factors: the head loss (or resistance) of the liquid as it exits the downcomers, the tray pressure drop, and the hydrostatic head on the tray decks.

A tall weir height was designed to maintain a high tray clear liquid height, which is essential for absorption. Even though the trays were handling 30% higher gas loads, it was important to keep the tray pressure drop low to reduce the froth backup. The dry pressure drop of the trays is dependent on the perforation type and size. After various discussion with Linde Engineering and Shell Global Solutions, it was eventually decided to apply sieve holes, which can achieve a larger open area (compared to mini valves) for a lower pressure drop hence downcomer froth backup.

Other than the above-mentioned challenges in hydraulic design, the restriction of hot works on the column shell for this four-for-three retrofit posed further challenges in the mechanical design of the column internals and attachments.

Mechanical design of attachments
Shell HiFi trays need full support rings, and expansion rings were used for the trays which do not sit on existing support rings. The enveloped HiFi downcomers were pre-assembled and brought into the column through a manhole for easy installation.

In Section A, every three tray spacings were divided into four tray spacings. Figure 4 shows a section sketch of the overlay of new trays #112-116 (in blue) against the existing trays #101-104 (in grey). New tray #116 was on the support ring of existing tray #104, while new tray #112 was on the support ring of existing tray #101. The existing support rings, which were originally designed for two-pass trays, had to be completed for a full ring. The new trays #113-115 were supported by full expansion rings. Figure 5 shows the new expansion rings (in blue) and new support struts (in light blue) on existing column attachments. These support struts were to be welded on-site to  an existing support ring and downcomer bolting bar for mechanical strength.

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