Abstract
An integrated membrane distillation (MD) flowsheet, consisting of direct contact membrane distillation (DCMD) and vacuum membrane distillation (VMD) units, was proposed and analysed in terms of thermal performance and water recovery factor, for the first time. The same lab-scale membrane module (40 cm2) was used for carrying out experiments of DCMD and VMD at fixed feed operating conditions (deionised water at 230 L/h and ~40 °C) while working at the permeate side with deionised water at 18 °C and with a vacuum of 20 mbar for the DCMD and the VMD configuration, respectively. Based on experimental data obtained on the single modules, calculations of the permeate production, the specific thermal energy consumption (STEC) and the gained output ratio (GOR) were carried out for both single and integrated units. Moreover, the calculations were also made for a flow sheet consisting of two DCMD units in series, representing the “traditional” way in which more units of the same MD configuration are combined to enhance the water recovery factor. A significant improvement of the thermal performance (lower STEC and higher GOR) was obtained with the integrated DCMD–VMD flowsheet with respect to the DCMD units operating in series. The integration of DCMD with VMD also led to a higher permeate production and productivity/size (PS) ratio, a metric defined to compare plants in terms of the process intensification strategy.
Highlights
The potential of membrane distillation has been successfully investigated in different fields of industrial interest, such as wastewater treatment, desalination, agro-food and beverage, and biomedical applications [1–7]
To promote the liquid evaporation, the feed is usually heated while at the other side of the membrane: (i) A colder aqueous stream is sent at atmospheric pressure, which is blocked at the membrane hydrophobic surface (DCMD configuration); (ii) An air gap is created between the membrane and a condensing surface; (iii) A vacuum is applied (VMD configuration); (iv) A cold sweep gas is sent
Considering the relevant number of studies on direct contact membrane distillation (DCMD), this paper focuses on a possible alternative strategy to improve the thermal performance of DCMD when operating at low feed temperatures (~40 ◦C)
Summary
The potential of membrane distillation has been successfully investigated in different fields of industrial interest, such as wastewater treatment, desalination, agro-food and beverage, and biomedical applications [1–7]. In DCMD, during the evaporation the feed becomes colder while the permeate increases its temperature because of the vapor condensation With this configuration, the heat recovery is not possible inside the module, and it is performed in an external heat exchanger where the feed is pre-heated by the permeate stream, which, in turns, is pre-cooled. It is known that in MD the water recovery factor per pass is quite low (maximum 8% [25]) and, membrane distillation plants must work with feed recirculation and with different modules in series, in order to ensure an acceptable productivity. The higher efficiency in permeate production of VMD was confirmed in a theoretical work of Guan et al [34] who simulated the performance of DCMD and VMD to Energies 2021, 14, 7405 treat a 7 wt% NaCl feed in a hollow fibre membrane distillation module. In the work of Méricq et al [35], the heat energy demand for VMD applied to seawater desalination is more than 98% of the total energy requirements
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