Robeson plots

Figure 8.11 Robeson plot of CO2/CH4 selectivity versus membrane permeability and permeance [12]. The points shown are based on low-pressure, pure-gas measurements. The performance of commercial membranes when used to separate carbon dioxide from high-pressure natural gas is shown on the same figure for comparison.

The problem with use of polymeric membranes in this application is plasticization, leading to much lower selectivities with gas mixtures than the simple ratio of pure-gas permeabilities would suggest. For this type of separation, a Robeson plot based on the ratio of pure-gas permeabilities has no predictive value. Although membranes with pure-gas propylene/propane selectivities of 20 or more have been reported [43, 44], only a handful of membranes have been able to achieve selectivities of 5 to 10 under realistic operating conditions, and these membranes have low permeances of 10 gpu or less for the fast component (propylene). This may be one of the few gas-separation applications where ceramic or carbon membranes have an industrial future. 

FIGURE 4.1 Selectivity for the gas pair O2-N2 as a function of O2 permeability. Properties of materials like molecular sieves and mixed matrix are expected to be found in the upper right comer (hatched area modified Robeson plot). (From Singh A., Koros W.J., Ind. Eng. Chem. Res., 35, 1231, 1996. With permission.)... 

Fig. 14 Robeson plot showing high separation selectivities for PIM-1 and PIM-7, (a) [77], Relation between CO permeability and COj/CH selectivity of theimally rearranged polyimides, (b) [78]...

FIGURE 7.1 Selectivity for the gas pair CO2/CH4 as a function of CO2 permeability and the Robeson plots (squares for CMS membranes, solid circles for TR polymers, triangles for FSC membranes, and solid diamonds for PIMs). (Robeson plots from Ref. [5] data from Refs. [8-13].)... 

Although these rules can lead to increased permeability and selectivity, an extensive compilation of the data in the literature by Robeson suggests an upper bound exists on transport properties - increases in permeability eventually lead to a decrease in selectivity and vice versa [38]. Figure 10 illustrates the upper bounds that exist for a number of gas pairs such plots of selectivity versus permeability are referred as Robeson plots . As one might expect, a similar upper bound exists for pervaporation membranes used to separate benzene-cyclohexane mixtures [39]. 

Mehta and Zydney [41] show a similar relationship exists for ultrafiltration membranes where transport through the membrane occurs by convective pore flow. A Robeson plot was created by taking the selectivity of an ultrafiltration membrane as the reciprocal of the protein sieving coefficient (the ratio of protein concentration in the permeate to that in the fluid adjacent to the membrane surface) and the permeability as the solvent hydraulic permeability. A plot of literature data for bovine serum albumin separation shows the existence of an upper bound. The location of the upper bound was predicted assuming the... 

Figure 15.1 Comparing the CO2/CH4 Robeson upper bound for dense and thermally rearranged (TR) polymer membranes to the carbon membranes/ and the region for industrial applicability was suggested by Hillock et alf (Data for CMS membranes and industrial applicability region added to the original Robeson plot.)...

In the early 1990s Robeson (1993) found an upper limit to the performance of polymer membranes in the commercially important separation of oxygen and nitrogen from air. On a log-log plot of selectivity versus oxygen permeability (a Robeson plot), the upper bound plots as a straight line (see Problem 17.D17 for more details). Although theoretical reasons for this limit have not been found, very few new membranes have been developed that are able to perform better than Robeson s limit. Membrane research has focused on ways to do better than Robeson s upper limit. 

D17. On a Robeson plot (a log-log plot of selectivity versus oxygen permeability in Barrers) the upper bound for separation of oxygen from nitrogen plots as a straight line. Approximate values of the end points are for Pq2 = 0.0001 Barrers, 002. 2 = 42 and for Pq2 = 10,000 Barrers, ao2-N2 ... 

Figure 2.7 Robeson plot illustrating the tradeoff between selectivity (a, ALPHA) and permeability (P) for the separation of carbon dioxide from nitrogen with polymer membranes [47]. The circles indicate all literature data considered relevant. The upper bound line is an empirical judgment of the outermost range of reliable data. Reprinted from Robeson IM. The upper bound revisited. J Membr Sci 2008 320(1—2) 390—400. Copyright (2008), with permission from Elsevier.

Figure 3.8 Robeson plot for comparison of O2-N2 separation properties of investigated polymers with Matrimid and Ultem .

Robeson s well-known tradeoff curve shows the strong inverse relationship between gas permeation flux (permeability) and selectivity [66], Robeson s plot also shows a line Unking the most permeable polymers at a particular selectivity. This Une is called the upper bound. Comparison of gas permeability (permeability of O2 and permeability of CO2) and the separation performances for different gas pairs (O2/N2 and CO2/CH4) of PAs have been shown in terms of Robeson plots. Permselectivity values of O2/N2 gas pair versus O2 permeabiUty values and permselectivity values of CO2/CH4 gas pair versus... 

Figure 5.12 Robeson plots for PIM-PIs and PIM-PIs and the gas pairs (a) H2/N2, (b) O2/N2 and (c) CO2/CH4. The solid lines represent Robeson s 1991 upper bounds. ...

The performances (selectivity/permeability) of the membrane were considered to be part of the optimisation parameters of the study and were left free to vary following the Robeson plot (permeability-selectivity upper boundary) formula of the best available materials at the time, i.e. carbon membranes ... 

Another study focused in that case on ethylene/ethane separation came to the conclusion. It was shown that the coupling of a carbon-based membrane in parallel with the distillation column could lead to a little bit more than 100000 US dollars annual savings in operating costs. As in the previous study, the membrane performances were considered as a floating optimisation parameter following the Robeson plot upper limit equation. The authors chose to correlate the selectivity with the permeability after the following equation ... 

The nature of the polar substituent was significant impacts on the permeability of CO2, N2 and CH4. OEG functionalities, when included in polylLs, produced membranes that were several times more permeable than those with C CN functional groups. OEG-functionalized polylLs exhibited CO2 permeabilities on par with polylLs with n-all l group , but with improved CO2/N2 selectivities that exceeded the "upp>er bound" of the "Robeson Plot". CO2/CH4 separation was also enhanced in each of these second-generation polylLs. 

Yun et al reported a comparison in terms of selectivity/permeance ratio of Pd-based and polymeric membranes for hydrogen permeators, as shown in Fig. 4.1. The polymeric membranes exhibit low hydrogen permeance with moderate Hj/Nj selectivity, and the selectivity/permeance upper limit of the polymeric membranes is given by the Robeson plot. From this analysis, it is... 

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