PTQ Q2 2022 Issue

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 veloc- ity at the maximum case was 0.105 m/s. Typical for trays with low tray spacing (below 400mm), the lim- iting 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 posi- tively 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 absorp- tion. Even though the trays were handling 30% higher gas loads, it was important to keep the tray pres- sure drop low to reduce the froth backup. The dry pressure drop of the trays is dependent on the perforation type and size. After various discus- sion with Linde Engineering and Shell Global Solutions, it was even- tually decided to apply sieve holes, which can achieve a larger open area (compared to mini valves) for a lower pressure drop hence down- comer froth backup. Other than the above-mentioned challenges in hydraulic design, the restriction of hot works on the col- umn shell for this four-for-three ret- rofit 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

Treated gas

T.L.

Lean methanol

Section A: CO removal

Section B: CO removal

57000

Section C: HS removal

Section D: Pre-wash

Gas feed

ID1750

T.L. ± 0

Loaded methanol

Figure 2 Methanol wash column

trays, ideal for high liquid loads. They allow for multiple downcom- ers 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 addition- ally improve the vapour handling capacity in some cases. 2. These trays are self-balanced. The liquid to each downcomer is dis- tributed proportionally to the weir length of each downcomer. They also allow for liquid communica- tion 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 equal- ise 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, espe - cially when hot works are not per- mitted on the column shell. Hydraulic design The hydraulic design of the HiFi trays was challenging due to the

Figure 3 Sample sketch of Shell HiFi tray

34 PTQQ 2 2022