Revamps 2025 Issue

relies heavily on a high surface area packing to promote the stripping of the methanol from the methanol-water solu- tion coming from the cold section and stabilisation section. Given the different temperature levels and nature of the available methanol solutions, two strippers are required to regenerate the methanol; these are referred to as the Ifpexol stripper and water wash stripper (see Figure 1 ). The technology licensor specified the use of wire gauze structured packing with a bed height of about 6,000 mm, one inlet feed distributor for the saturated wet gas, and an overhead distributor for the methanol solution liquid feed. Although the fluid properties and operational conditions were not optimal for the use of wire gauze structured pack - ing – due to the high surface tension and relatively high vis- cosity of the liquid phase, as the liquid would not be able to harness the advantage of the capillary effect of the woven material – the high surface area of wire gauze structured packing required (in the range of 500 m2/m³) makes it an appealing option. It allows for enough stripping stages from a rather small amount of liquid compared with a sizeable flow of gas. After the commissioning of the unit, the operator started struggling with stripping efficiency in both towers, leading to a higher fresh methanol consumption and an unsteady operation, especially in the Ifpexol stripper. Similar situa- tions presented after the operator commissioned analo- gous units in different locations. According to the licensor models, the methanol recovery performance of the strip- pers was about 50% lower than expected, and the metha- nol losses were at least twice as high as the design target. While studying the preliminary information received from the field, it was highlighted that the fouling of the distribu - tors (and consequently their cleaning) was more prevalent in the stripper, which operates at cryogenic conditions in the overhead of the tower. This, along with evidence from elsewhere in the plant, supported the idea that the fouling was generated by an accumulation of paraffinic compounds with a tendency to solidify, especially at low temperatures. These compounds were inherent to the gas composition, even if they were not expected to be present at the design stage of the plant. As a result of these findings, it was determined that the cause of the unexpected poor performance was due to the inefficiency of the original internals in the strippers, rather than being inherent to the technology or process design. Consequently, The operator decided to collaborate with Koch-Glitsch to leverage its expertise in addressing the plant’s operational issues. Process evaluation The operational challenges were varied, affecting the process in multiple ways. Operators faced a higher-than- expected pressure drop in the packing, poor methanol stripping, freezing in the cold section due to insufficient methanol circulation, blockages in the liquid feed distrib- utor, and increased consumption of make-up methanol. To cope with this situation, operators had to constantly adjust the process to try to optimise mass transfer in the strippers. Some of the usual actions included increasing the use of

fresh methanol upstream of the cold process to mitigate freezing, manipulating the gas split ratio of the gas feed between the strippers to optimise the methanol recovery, and performing ‘blowbacks’ in the tubular liquid distributor to try to improve the quality of the distribution. Preliminary hydraulic studies were conducted at design and operational conditions, and no indications of evident constraints were found in the packing or the internals. However, the saturated gas was not able to produce enough stripping of the water-methanol feed. Even after a series of operational adjustments, the methanol recovery targets were not met. A grid gamma scan was conducted on both strippers but given the very low liquid flux in both towers, the results of the scan were deemed inconclusive. Knowing the packing was not hydraulically constrained, and the issues could be summarised as a lack of efficiency and operational relia - bility, additional efforts were aimed at studying the liquid distribution. In theory, the original design for the tubular liquid dis- tributors made sense, but they generated significant short - comings during the operation of the strippers. To achieve high drip point density, essential for effective liquid distri- bution in tubular designs, the holes in the laterals were set at 1 mm in diameter, which led to persistent clogging and poor distribution, making the operation unreliable, imprac- tical, and inefficient. Even when the original distributor showed a high drip point density of 200 points/m2, a distribution quality assessment using a widely accepted method in the indus- try5 produced a rating of 64%, whereas the expectation for wire gauze packing is to use high-performance distributors with distribution qualities surpassing the 90% threshold. The distributors frequently became plugged, and meth- anol losses grew unsustainable due to decreased stripping efficiency. Operators would stop the methanol-water flow and use the tower’s high pressure to clear obstructions back into the feed piping system, temporarily restoring flow. This process subjected the system to process and reliabil- ity issues, potentially compromising its mechanical stability and losing stripping capabilities for a few hours at a time, at least once a week, with all the attached consequences this brings to the operation downstream. However, even after cleaning, the efficiency was not on target, and it quickly declined from there, starting the cycle again. Although the Ifpexol and wash water (WW) strippers aim for similar results, their operations differ. The Ifpexol strip- per is designed to contact the cold water-methanol mixture (>70 wt% methanol) from the cold process with saturated gas. In contrast, the WW stripper is focused on recovering any remaining methanol that may have migrated from the cold process to the C 3 + stabilisation section. The liquid feed to the WW stripper is largely dependent on the feed gas composition and is not directly linked to the Ifpexol stripper’s overhead liquid feed. Despite these differ- ences, having the same diameter enabled the original equip- ment manufacturer (OEM) to make a design shortcut, where a single-design distributor was used in both towers instead of using two distinct distributors calibrated for each.

Revamps 2025