Further development of an existing absorption driven multiple effect evaporator.
This project is a continuation of a past contract EN3E-0138-GR where three problems were identified which need to be resolved in order to improve the performance of the unit:
The observed overall temperature difference between the absorber (first effect of the evaporator) and the last effect (fourth effect) is only 70-80% of the theoretical Boiling Point Elevation (BPE) of the absorptive solution. This is probably due to concentration polarization and the temperature gradient across the NaOH film in the absorber. The use of turbulence promoters in the absorber is expected to eliminate this problem.
Another problem is crystallization of strong NaOH solutions at low temperatures. Mixtures of NaOH/KOH solidify at lower temperatures without seriously affecting the BPE of the solution. The use of NaOH/KOH mixtures is expected to increase the capacity of the unit since higher concentrations can be used and higher overall temperature differences can be realized without the danger of crystallization.
In continuous operation, it is necessary that the amount of water evaporated from the absorbing solution in the regenerator equals the amount of water that is absorbed in the absorber. In past experiments, this condition was not fulfilled and an automatic controller, which controls the rate of solution regeneration, was necessary to keep the solution concentration constant for long runs.
Tasks in this project have been distributed in the following way: redesign the absorber of the first effect using turbulence promoters (CRIFIC); rebuild the first effect of the absorption- driven evaporator according to the new design; incorporate a system to control the water evaporation rate from the NaOH solution in the regenerator (Hellas Energy): operate the system using NaOH/KOH mixtures instead of NaOH alone (Hellas Energy). Develop a theoretical model for the absorber (CRIFIC); make an economic assessment of the system and evaluate experimental results (Hellas Energy).
The contract have been given an extension which consists in replacing NaOH or KOH by propylene glycol which is not corrosive and has better heat transfer characteristics. Due to the volatility of glycol, this required the construction of a rectification column.
Experiments to date show that energy savings of 30-40 % can be obtained in comparison with a conventional evaporator.
Funding SchemeCSC - Cost-sharing contracts