Pure gas permeance

Fig. 4.11 Change of pure gas permeance with kinetic diameter (a) membrane carbonized at 700°C, (b) carbon membranes oxidized at different temperatures. (From [47])...

Pure gas permeation data of membranes prepared using these additives were obtained from a constant pressure permeation system for CO2, CH4, O2 and N2. In order to know the effect of the structure of the nonsolvents, nonlinear regression analysis was attempted. Each additive was split into structural components groups. Several linear polynomial first and second order equations as well as nonlinear polynomial equations were attempted to derive an empirical correlation between the number of structural components and gas permeation data. A second order polynomial equation was derived to predict the pure CO2/CH4 permeance ratio from the structural components of the nonsolvents. From the structural studies the authors concluded that nonsolvent additives that possess a long straight hydrocarbon chain such as 2-ethyl-l-hexanol, 1-octanol and 2-decanol showed the highest pure gas permeance ratio. 

Among a series of either monovalent or divalent cations, the permeance ratio decreases as the ionic radius increases. Therefore, it may be inferred that there is a general correlation between the ionic charge density of the cation and the pure gas permeance ratio, i.e., the permeance ratio increases with an increase in the charge density of the cations. This is probably due to the enhancement of the electrostatic crosslinking between charged polymers and metal cations with an increase in the cationic charge density. 

The surface of the membrane was observed by AFM and the polymer morphology parameters such as the size of the polymer nodules and the surface roughness were collected and correlated to the membrane performance data. While the pure gas permeances could not be correlated with the AFM data, the correlation between pure gas permeance ratios and the AFM data provided some important insights into the performance of the membrane. 

Surface tension data was collected for a number of solvent-nonsolvent additive systems related to PPO gas separation asymmetric membranes. These membranes were made from different types of nonsolvent additives and were characterised from gas permeation experiments. The results of pure gas permeance ratios for O2/N2 and CO2/CH4 are shown in Figures 44 and 45. 

The effect of the surface tension of chloroform-nonsolvent additive systems on the formation of nodules are reflected in the correlation between surface tension and pure O2/N2 (Figure 44) and CO2/CH4 (Figure 45) permeance rate ratios. Both figures show that as the surface tension increased, the pure gas permeance rate ratios increased. Also, most of the membranes with merged nodules are found in the range of surface tensions greater than... 

Nevertheless, if mixtures of gas and vapour of higher molecular mass species, or liquid mixtures of two species with different volatility and surface tension, are considered, the separation factors and permeation fluxes can be very interesting, but such separations cannot be predicted from the pure gas permeance. Silicalite membranes are hydrophobic and preferentially adsorb organic molecules that are small enough to enter the pore openings. Therefore, they can be used to separate hydrocarbon mixtures with relatively high separation factors. The selectivity for n-heptane isooctane has a maximum of 138 at 373 K for the ternary mixture of isooctane, n-heptane and n-hexane (Funke et al, 1996). 

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. 

Table 4.3 Comparison of permeances from the pure gas and mixed gas experiments...

Morphology studies done by tapping mode atomic force microscope revealed that membranes prepared from a PPO solution with 2-ethyl-1-hexanol as the additive had the lowest mean diameter of nodules. Interestingly these were the membranes that exhibited the highest permeance ratio of 5.5 for O2/N2 24.5 for CO2/CH4 (second highest after 1-octanol). Pure gas... 

The selectivity, ttHj/othergas defined as the ratio between the permeance of pure Hj versus the permeance of the other (pure) gas. 

Transport properties (permeance and separation factor) of the modified inorganic membranes were measured through steady state pure and mixed gas experiments. In pnre gas experiments, the feed gas was pressurized and permeate flow rate was measured using a bubble flow meter, which in turn was used to calculate the permeance using Eq. 7.2. 

A very large body of data on the gas permeability of many rubbery and glassy polymers has been published in the literature. These data were obtained with homopolymers as well as with copolymers and polymer blends in the form of nonporous dense (homogeneous) membranes and, to a much lesser extent, with asymmetric or composite membranes. The results of gas permeability measurements are commonly reported for dense membranes as permeability coefficients, and for asymmetric or composite membranes as permeances (permeability coefficients not normalized for the effective membrane thickness). Most permeability data have been obtained with pure gases, but information on the permeability of polymer membranes to a variety of gas mixtures has also become available in recent years. Many of the earlier gas permeability measurements were made at ambient temperature and at atmospheric pressure. In recent years, however, permeability coefficients as well as solubility and diffusion coefficients for many gas/polymer systems have been determined also at different temperatures and at elevated pressures. Values of permeability coefficients for selected gases and polymers, usually at a single temperature and pressure, have been published in a number of compilations and review articles [27—35]. 

Gas testing of these membranes for carbon capture applications showed an enhanced CO2 permeance up to 1830 GPU, without a significant drop in CO2/N2 selectivity at 35°C and 350 kPa, relative to a pure PEBAX upper layer. The impacts of temperature and pressiue on membrane performance were investigated for temperatures from 25°C to 55°C and pressures from 100 kPa to 500 kPa. 

---