Performance comparison between overlapped and woven spacers for membrane distillation

Abstract

a b s t r a c tThe sustainable production of freshwater from seawater desalination is receiving increasing attention. Recently, some desalination technologies are taking advantage from the coupling with renewable resources; among them, membrane distillation (MD) is one of the most promising since it can be easily powered by low-grade thermal energy. MD being an emerging technology, efforts are required to optimize geometry and operating conditions of real units in order to reduce the unitary freshwater production cost. In particular, temperature polarization is a well-known detrimental effect for the process driving force; spacers are traditionally used to enhance mixing and make temperature boundary layers thinner, at the cost of higher pressure losses. The present work is devoted to testing and comparing the performance of two different two-layer net spacers: overlapped and woven. Investigations were carried out both by experiments and by computational fluid dynamics (CFD) at different Reynolds numbers, ranging from creeping flow to turbulent flow regimes. Experiments (for intermediate to high Re) made use of thermochromic liquid crystals along with digital image processing. Computational results (for low to intermediate Re) were obtained via steady state (low Re) or direct numerical simulations (intermediate Re) along with the unit cell approach. A good agreement between experiments and CFD results was obtained in the range of superposition. Results showed that woven spacers guarantee a better mixing than overlapped ones, especially in the low to intermediate Re range, thus resulting in Nusselt numbers 2.5-3 times higher. On the other hand, the less disturbed flow field induced by overlapped spacers was found to yield friction coefficients up to 4 times lower, thus allowing lower pumping costs. The choice between the two configurations depends crucially on the relative importance attributed to savings in membrane surface area and in pumping energy for any specific application.

CFD prediction of flow, heat and mass transfer in woven spacer-filled channels for membrane processes

“…Unsteadiness and transition to turbulence were also studied for the same geometry by time-dependent direct numerical simulation [17]. In comparison with the corresponding nonwoven spacers, the woven ones exhibited better mixing at the investigated flow attack angles of 0 and 45° [18].…”

Modelling and cost analysis of hybrid systems for seawater desalination: Electromembrane pre-treatments for Reverse Osmosis

“…The ED/RED model here adopted can be used for the three electromembrane processes considered in this paper. It uses a one-dimensional representation of the stack, but it includes also local results from 3-D Computational Fluid Dynamics (CFD) simulations (friction coefficients [40][41][42], mass transfer coefficients [43,44], and Ohmic resistances). This coupling between CFD and a one-dimensional model was already presented in previous works [45,46].…”

“…In regard to the spacer geometry, in the feed channel of an RO module, usually non-woven spacers are adopted [47][48][49]. Due to the lack of detailed data on the spacer geometry adopted in the commercial RO modules considered in this work, we used a friction factor value obtained from previous CFD simulations of overlapped spacers reported in [42]. In the range of the Reynolds numbers investigated here (100-300), the friction factor for the overlapped spacer with an orientation of 45° with respect to the flow, can be estimated as 𝑓 ≈ 10 • (96 𝑅𝑒 ⁄ ).…”

Friction and Heat Transfer in Membrane Distillation Channels: An Experimental Study on Conventional and Novel Spacers

“…Therefore, most of the ongoing research on better spacer designs is focused on the double purpose of increasing the fluid mixing, and thus, the heat transfer coefficients, while keeping the frictional pressure losses within the acceptable values [7][8][9][10][11]. A comparison of two of the most popular spacer designs, i.e., the overlapped and woven ones, is given by Gurreri et al [12] for a pitch-to-height ratio P/H = 2. The results showed that the woven spacers provided higher Nusselt numbers, but there were also higher friction coefficients with respect to the overlapped spacers.…”