Comparative analysis of full-scale membrane distillation contactors - methods and modules
A new multi-channel spiral-wound membrane distillation (MD) module with a membrane surface area of 27.5m2 was developed, constructed and characterized. For comparison, a set of different conventional single-channel direct contact membrane distillation (DCMD) module prototypes is introduced and experimentally evaluated, as well. Furthermore, a comprehensive assessment on DCMD module design and operation as well as methods for comparative thermodynamic MD module evaluation has been conducted. The methodical core of this work specifically addresses an integrative method for the derivation of the thermal energy demand of DCMD systems, comprising specifications of the external heat recovery system. A new strategy for the optimization of energy demand in DCMD is identified, which aims on the formation of parallel temperature profiles by adjusting the flow streams on the evaporator and the condenser channel. Suitable methods for a comparative analysis of different MD performan ce results are suggested and fundamental MD specifications are derived and applied to evaluate a set of different DCMD prototypes. The methods involve the analysis of the permeate output rate, flux and thermal energy consumption with respect to feed flow rate conditions. The optimization of flux and thermal energy consumption is identified to rely on conflicting requirements, which cannot be satisfied at the same time. The optimal combination of flux and thermal energy consumption depends on the projected scenario and may not be identified by a technical assessment alone. Economic considerations must be included. As final outcome, a method of specifying MD results by a representation of flux versus thermal energy consumption is suggested in order to allow a meaningful evaluation and to achieve global comparability among different MD configurations and other thermal separation technologies.