Predicting the influence of operating conditions on DCMD flux and thermal efficiency for incompressible and compressible membrane systems

Abstract

Membrane distillation (MD) is a hybrid of thermal distillation and membrane separation. As a thermal distillation process, MD is an energy intensive technique. Therefore, it is important to select optimum operation parameters to maximise both the flux and energy efficiency, even though low grade or waste heat maybe employed in the process. In this paper, a model originally developed for Direct Contact Membrane Distillation (DCMD) using a compressible membrane is employed to predict the thermal efficiency and flux for both compressible and incompressible membrane systems. The predicted thermal efficiency was also compared with experimental results, and the errors were mostly within the range of experimental variation (±5%). As a result of membrane compressibility, the thermal conductivity and vapour permeability of the membrane were altered, causing a change in flux and energy efficiency. If a compressible membrane is used, it was found from the predicted results, that increasing stream velocity will reduce the thermal efficiency and even reduce flux once the increased pressure associated with the higher flows is in the range to compress the membrane. Increasing temperature either on feed side or on permeate side will increase thermal efficiency. In scaling up the process using a compressible membrane, it is necessary to reduce the pressure drop across the module in the DCMD process to maximise energy efficiency and flux. Since the operating temperature is normally not adjustable when low grade waste heat is employed, optimising the flowrates and pressure drops are the key factors for optimising flux and thermal efficiency.

Scale reduction and cleaning techniques during direct contact membrane distillation of seawater reverse osmosis brine

“…A pressure gauge (PI) of range 0-100 kPa was installed to measure hot feed pressure into the DCMD module. This was done to ensure operating pressure was under critical values that lead to compression of the membrane or pore wetting and subsequent loss of performance [25]. All pressures were measured less than 5 kPa in this study and significant membrane compression is not anticipated at these pressures [26].…”

Evaluating energy consumption of air gap membrane distillation for seawater desalination at pilot scale level

“…Increasing the water circulation rate from 150 to 350 L/h resulted in an increase in T from 3.0 to 4.5 ºC; thus, the distillate production rate increased from 9.5 to 19 L/h. The positive influence of water circulation rate on permeate flux, and thus distillate production rate in MD, has been widely reported [23,[25][26][27].…”

Performance and Fouling Study of Asymmetric PVDF Membrane Applied in the Concentration of Organic Fertilizer by Direct Contact Membrane Distillation (DCMD)

“…The membranes used in this study are asymmetric PVDF and commercial PTFE membranes. The characteristics of the PVDF membrane are given in Table 1 ; the characteristics of the commercial PTFE membrane were provided by the manufacturer and the relevant literature [ 33 , 34 ]. The PVDF membrane has a much smaller mean pore size (r m ) of 34 nm than that of PTFE (450 nm) [ 33 , 34 ], leading to a much higher liquid entry pressure of water (LEP w ) of 3.5 bar, indicating excellent anti-wetting properties.…”

“…Firstly, the effects of operating parameters in direct contact membrane distillation (DCMD) were investigated with the as-prepared PVDF membrane, such as flow rate, membrane orientation, and solution salinity. Secondly, the application of the membrane in the dewatering of real organic fertilizer stream to the desired concentration was examined in terms of the process stability and membrane fouling behavior, which was then compared with the commercial polytetrafluoroethylene (PTFE) membrane after previous systematic research into industrial applications [ 33 , 34 , 35 ].…”