Analysis of thermally coupled distillation sequences without vapor transfer (Liquid-Only-Transfer)

Distillation is one of the most important separation methods in the chemical industry, however it has a high energy demand. The energy demand of distillation sequences can be reduced by introducing thermal coupling links through the replacement of heat exchangers at the top or bottom of the columns with bidirectional vapor-liquid transfer streams. Due to the significant energy saving potential of thermally coupled sequences, thermal coupling has established itself as an established intensification option with hundredfold industrial implementation. However, both design and operation of thermally coupled sequences are more challenging than those of uncoupled sequences. This is primarily due to the vapor transfer stream, which is typically uncontrolled but has a significant impact on the saving potential of the sequence. Additionally, the individual adjustment of operating pressure is limited due to the vapor connection. To address these challenges, the vapor transfer stream between the columns can be eliminated by duplicating and strategically rearranging column segments. This results in so-called liquid-only-transfer sequences (LOT), which are thermodynamically equivalent to the original structures and thus maintain the energy saving potential. The following graphic illustrates the development of a LOT sequence from a conventional thermally coupled sequence.

Within the scope of this project, LOT sequences are being investigated both by means of model-based methods as well as experiments. For this, methodologies for optimal design (Grassroot and retrofit) are being developed. Furthermore, the practical operability of LOT sequences is investigated. To achieve this, an LOT sequence at a pilot plant scale is operated and comprehensively analysed.