Revamps 2025 Issue

Improper initial liquid distribution can severely impair the efficiency of packed towers. Severe forms of initial maldistribution, such as those induced by inadequate liquid distrib- utors, lead to inconsistent liquid flow across the packing. This maldistribu- tion manifests as chordal, peripheral, and central blanking profiles, each severely affecting column efficiency by creating over- and under-irrigated zones within the packed bed. When a scenario of initial gross maldistri- bution is presented, a packed bed cannot self-correct this situation⁴, and the performance is destined to bear the consequences. Once the issues with the distributor designs were iden- tified, the task was to produce a new set of distributors that aligned with the internal rates and operational ranges of the towers. After receiving feedback on a set of new operational conditions from the technology licensor, a new simulation model of the strippers was generated using operational data, updated gas compositions, and current saturated gas flows to each of the strippers. This new set of information revealed the nuances between the operation of each tower, informing the design choices in each case. Selecting the right liquid distributor Packed towers operate as a system, with the performance of the packing being directly linked with the type and design of the liquid and vapour distributor. Improper feed distributor design accounts for a considerable number of troubleshooting issues in mass transfer columns, even for relatively simple distillation and absorption systems. The choice of a liquid distributor for a packed tower is informed by several factors, such as the packing type, pro- cess application, liquid load, operational range, and fouling tendency. However, sometimes variables like the available overhead space above the packing, existing tower attach- ments, manway size, and installation time constraints can play a role in defining the final design of a liquid distribu - tor, potentially compounding the complexity of the retrofit. Hence, a one-size-fits-all approach is rare, especially in revamp scenarios. Considering that the original liquid distributors in the strip- pers were pressurised, a compact design was possible, as no static head of liquid was required as a driving force. As a result, the vertical spacing for a new installation was limited. Designs for liquid distribution systems require thorough hydraulic knowledge and a deep understanding of the operational windows and process conditions. Therefore, close collaboration between the parties involved is crucial to the success of the design. There are generic rules for the selection of liquid distrib- utor types, but in general, these are just guidelines and should be treated as such. Therefore, looking at the dis- tributor types just in terms of flow range is simplistic, and this approach should not be used as a replacement for a comprehensive evaluation of the system.

Figure 2 Comparison of liquid distributors Models 166 (left) and 156 (right)

The final selection of a liquid distributor must be the result of a judicious assessment of operational conditions, packing type, fouling potential, mass transfer requirements, available installation spacing, and operational flexibility, among others. To adapt to all these parameters, there are several levers that can be moved. Although having two identical distributors in towers with different operational conditions can save money during the project phase, it can prove costly in the long term due to poor performance dur- ing operation. Once the operational conditions and expectations were revised between the operator and Koch-Glitsch, the deci- sion was made to use trough distributors for both tow- ers, as they offered the most comprehensive solution, addressing most of the issues captured during the evalu- ation. However, if a single design was required (following the original design philosophy), high flow variations were expected for the WW stripper at intermediate and low rates, which would produce severe maldistribution and poor stripping efficiency. Redesign approach Several issues needed to be fixed within the confines of the mechanical constraints of the strippers. For instance, the disruptive cleaning process of the liquid distributor using blowbacks (backflushing the feed distributors into the feed line utilising the high internal pressure of the gas in the tower) was addressed by removing the pressurisation from the distributors and using an open pipe to transport the liq- uid feed into the tower. However, without pressure as a driving force to spread the fluid through the packing, the distribution would rely on any static head the new design could generate in the limited available space. Even if one would aim for a very compact design, a precise distribution was not possible at face value, given the overhead space available. The available vertical distance to install the distributor was about 600 mm from the centreline of the feed nozzle to the top of the packing. The preliminary assessment showed that about twice this distance was required to allow spac- ing for pre-distribution feed piping, the main parting box, and the secondary distribution trough, but the overhead space did not allow for this increase. An additional step to complement the solution was pro- posed to the operator, consisting of replacing the packing

Revamps 2025