Dynamically optimized design of a solar thermal driven distillation process

Abstract:
This contribution addresses a dynamic design optimization of a distillation process that is exclusively driven by solar thermal energy. The optimization is set up to find a minimum investment cost for the solar distillation process that is able to deliver a fixed amount of product while satisfying a minimum integrated product purity at the end of a defined time period. This also involves process optimization considerations, as the size of the equipment depends on the magnitude of the process dynamics. A literature review is carried out to understand the necessary solar thermal energy components and to identify different industrial solar driven approaches. The process model consists of a solar Parabolic Trough field as its energy source, a Thermocline thermal energy storage system to account for fluctuations and to ensure nighttime operability, as well as a distillation column whose reboiler heat duty is supplied by solar energy. PI controllers are used to control the dumping of excess heat in a Trimm heat exchanger and to ensure the desired energy supply to the column. A separation of benzene and toluene with an average reboiler load of 378 kW was chosen as an example process. An optimal design was determined to fulfill the specified requirements that corresponds to investment costs of roughly 1,897,000 € and to a dynamic load range of 49-100%. Given the cost figures used, this small scale example is not cost-competitive compared to a fossil fueled separation as the lifetime cost exceeds those of a conventional column by roughly 570,000 €. Due to economies of scale, however, bigger solar driven columns become favorable, especially if CO2 emissions are being priced. The thesis closes with considerations for model and approach expansions in future work.

Keywords:
solar thermal, dynamic optimization, renewable energy, distillation