# Optimized Water Distillation Layout for Detritiation Purpose
## 2 The Heat Pump Assisted Distillation  
Several strategies were investigated and adopted to reduce the power consumption of distillation systems. The simplest and least expensive is the use of waste heat from the same or other processes [18]. This solution was foreseen in the preliminary design of the EU-DEMO water CPS, where waste heat from the Chemical and Volume Control System (CVCS) is used to feed the reboiler of the distillation column [10]. Furthermore, focusing on various heat integrated distillation techniques, the heat pump system is recognized as one of the most promising ways to reduce power consumption [18, 19]. In a conventional distillation column, heat is provided at the bottom in the reboiler, and approximately the same quantity is removed at the top by the condenser at a lower temperature. A more efficient use of the energy is conceived in the Heat Pump-Assisted Distillation (HPAD) that upgrades the discharged energy to reduce the overall consumption of valuable utilities [20]. Several HPAD concepts have been studied, each one giving the maximum power saving only at specific conditions [21; 22; 23]. Distillation technology is used for a wide range of separation tasks, involving different species, and the assessment of the most appropriate HPAD strategy for the given task can be a topic of great concern for the engineers. [PERSON] et al. [11] tried to solve this issue, proposing a novel practical approach. Based on an extensive literature review, they provided a selection scheme that suggested the most appropriate HPAD solution based on the conditions of the separation task and performance required [11]. Although it applies well to industrial application, the detritiation task of the CPS, due to the presence of tritium, has peculiarities that exceed the capabilities of this selection scheme. Nevertheless, it can be considered as basis for the selection procedure for the EU-DEMO water CPS. The HPAD concepts can be divided in two macro groups, i.e., the Absorption Heat Pump (AHP) and the Mechanical Heat Pump (MHP) [11]. The AHP uses thermo-chemical conversion to enhance the efficiency of the distillation column, adopting pairs as heat transfer fluids (e.g., ammonia and waters, or lithium bromide and water), whereas MHP adopts mechanical machines to upgrade discharged energy and reuse it. One of the main drivers in the design of the EU-DEMO water CPS is the simplicity, and thus the safety, of the system. This is the main advantage of water distillation among other separation technologies such as the Combined Electrolysis and Catalytic Exchange (CECE) [24]. From this perspective, the AHP could affect the simplicity of the system, introducing an additional refrigerant loop with its peculiar complexities and where tritium can migrate and build up. Therefore, the MHP is currently considered the most viable solution for the EU-DEMO water CPS. Different layouts of the MHP can be considered [11]. In the External Vapor Compression (EVC), a working fluid runs between the heat source, i.e., the reboiler, and the heat sink, i.e., the condenser, in an isolated loop, completed by the compressor, providing the required work input, and the flash valve, closing the loop. On the other hand, in the Mechanical Vapor Recompression (MVR) and in the Bottom Flashing (BF) concepts, the working medium is the same fluid evolving in the distillation column. In the MVR, the overhead vapor (i.e., the distillate) is compressed through the compressor, and its latent heat is used for boiling the bottom product in the reboiler. A throttling valve closes the loop. The BF exploits the reverse cycle of the MVR, flashing the bottom product and using it as cooling medium in the condenser. More details about the different concepts can be found in [11].