Selection between Separation Alternatives: Membrane Flash Index (MFLI)

Abstract

Chemical process design is a creative step of engineers that should be supported by different computer-aided design tools. Such tools should be simple and easy to use because process synthesis often means the investigation of a huge number of alternatives. During the design of the separation of liquid mixtures, among many others, distillation and pervaporation are usually simultaneously considered because pervaporation is frequently considered as an alternative to distillation. To easily compare the efficiencies of continuous pervaporation and flash distillation, we propose a new and simple method, the so-called membrane flash index (MFLI). The comparison is based upon vapor–liquid equilibrium data and the permeation data of a pervaporation membrane and therefore can be plotted on a common chart in the case of binary mixtures. The permeation values of organophilic and hydrophilic pervaporations can be calculated with the help of known separation factors and feed concentrations. The MFLI is dimensionless and can be determined by dividing the permeate concentration by the corresponding vapor equilibrium data obtained from a simple flash distillation. Only the feed concentration, separation factor, and refereed equilibrium data are necessary for the calculations. If the MFLI is <1, the application of pervaporation can be worse than that of flash distillation. In our study, the efficiencies of pervaporation and flash distillation are compared using three mixtures, methanol/water, ethanol/water, and isobutanol/water. After determining the MFLI for different types of membranes, we can conclude that the separation capability of hydrophilic membranes is significantly better than that of the organophilic ones and also better than that of flash distillation by far. Apart from azeotrope braking, the MFLI, as a single number, gives clear information about the separation capabilities of pervaporation and flash distillation in the course of process design.

PVA-Based MMMs for Ethanol Dehydration via Pervaporation: A Comparison Study between Graphene and Graphene Oxide

“…MFLI was also used to evaluate the PV membranes' capabilities in liquid separation compared to traditional distillation. It can be seen that the graphene/PVA MMMs prepared in this work exhibited comparable or even higher MFLI values compared with other PVA-based MMMs reported in the literature, which indicated that pervaporation with these MMMs possessed better separation efficiency over flash distillation [44,45]. According to the results, it was presumed that graphene intercalated with an appropriate number of oxygen-containing groups might be a good choice to prepare PVA-based MMMs for ethanol dehydration, which would combine the advantages of GR's high interlayer diffusion selectivity and GO's high permeation properties and open a door to achieve both high permeation flux and the separation factor of MMMs by tuning the microstructure of graphene.…”

“…GR showed a strong diffraction peak at 2θ = 26.55 • , and the corresponding interplanar spacing of GR was only 0.33 nm due to the strong π-π interaction between GR lamellae [41]. The GR interplanar spacing (0.33 nm) was exactly in between the kinetic diameters of ethanol and water molecules, which was 0.43 nm and 0.26 nm, respectively [45]. This might facilitate the selective permeation of water molecules.…”

“…Membrane flash index (MFLI) [44,45] is another simple and valuable method to evaluate the PV membranes' capabilities in liquid separation compared to traditional distillation, which is mainly based on the comparison of available theoretical maximum distillate compositions.…”

“…The total flux of PVA-based MMMs increased with raising the operation temperature, and the separation factor followed the reversed order. This may be attributed to the following reasons:(1) Increasing temperature enhanced the membrane swelling and the mobility of PVA chains, creating more and larger free volume available for small molecules [42], which might contribute to the improvement of permeability; (2) The driving force, which was strongly related to the saturated vapor pressure, was significantly improved due to the increased saturation vapor pressure with feed temperature rising, as shown in Table S2 [45]. (3) The diffusion coefficient of ethanol/water molecules increased with the temperature rising as reported in many studies in the literature [43,52], which also resulted in the increase in total flux.…”

Membrane Flash Index: Powerful and Perspicuous Help for Efficient Separation System Design

“…In steady-state operations, the combinations of two phases are permanent and in equilibrium. The liquid and vapor phases are fed into a decanter (phase separator) where they are treated separately. , …”