O2/N2 permeance ratio

Fig. 8.6. Pure O2/N2 permeance ratio of asymmetric poly(phenylene oxide) membranes as a function of surface tension of chloroform/nonsolvent additives mixtures. Nonsolvent additives include 2-ethyl-l-hexanol (1m), 1-octanol (2m), 2-propanol (3d), 2-decanol (4m), 3,5,5-trimethyl-1-hexanol (5m), 2,4-dimethyl-3-pentanol (6d), 2,4,4-trimethyl-1-pentanol (7d), 2-methyl-3-hexanol (lOd), 3-ethyl-3-pentanol (12m), and 2-methyl-2-hexanol (13d). Merged is indicated by m discrete is indicated by d. Reprinted from [22], with kind permission from J.Tan...

Fig. 8.9. Pure O2/N2 permeance ratio versus etching time. Reprinted from [22], with kind permission from J. Tan...

Table 8.7 shows the 02/N2and CO2/CH4 permeance ratio for the as-cast P3AcET membrane and the base- or acid-treated membrane. As the table shows, the base or acid treatment resulted in a dramatic increase in the O2/N2 permeance ratio, while... 

Development of integrally skinned asymmetric membranes from polyphenylene oxide for gas separation to the author s knowledge dates back to 1973 when Kimura of General Electric Company in his patent work disclosed the method of such membrane preparation and use for air separation. He used a solution of PPO in chloroform (volatile and good solvent) and dichlorobenzene (non-volatile and poor solvent). The cast membrane was allowed to desolvate for 30 seconds after which it was precipitated in methanol. The dried membrane was tested for oxygen and nitrogen permeances. Kimura observed O2 permeance of 5.6 GPU with O2/N2 permeance ratio of 4.3. The patent did not mention about the molecular weight of PPO. 

Figure 39. Pure O2/N2 permeance ratio as a function of mean nodule diameter of numbered nonsolvent additives. Legend (1) 2-ethyl-1-hexanol (2) 1-octanol (3) 2- propanol (4) 2-decanol (5) 3,5,5-trimethyl-1-hexanol (6) 2,4-dimethyl-3-pentanol (7)2,4,4-trimethyl-l-pentanol (8) 2,2-dimethyl-3-pentanol (9) 3-ethyl-2,2-dimethyl-3-pentanol (12) 3-ethyl-3-pentanol m merged d discrete.

Figure 49. Pure O2 /N2 permeance ratio as function of etching time. Results for four coupons etched at different times. Etching times are listed beside coupon code.

In Figure 44, the use of 2-ethyl-l-hexanol, 1-octanol and 2-decanol as nonsolvent additives produces higher surface tensions resulting in higher pure O2/N2 permeance rate ratios. On the other hand, lower surface tensions resulting in lower pure O2/N2 permeance rate ratios are reflected in the use of 2-propanol, 2,4-dimethyl-3-pentanol, 2-methyl-3-hexanol and 2-methyl-2-hexanol as nonsolvent additives. 

In Figure 49, the pure O2/N2 permeance rate ratios of coupons CIO and Cl 1 increased as the etching time was increased from 0 to 30 seconds. oo2/n2 then decreased when the etching time of all the coupons was increased from 40 to 190 seconds. 

It should be however kept in mind that the membrane preparation procedure could influence its structure and gas transport properties. Thus, casting of the integrally skinned asymmetric membranes from p-DMePO solution using different nonsolvent additives produced the nodule structures in the surface skin layer of the membranes, which affected the permeance ratios for O2/N2 and CO2/CH4 [72]. In the homogeneous films of polyphenylene oxides considered in this chapter such structures apparently do not exist. 

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. 

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... 

Memb. Type Solvent Solution concentration wt/wt % Permeance, GPU CO2 O2 Permeance ratio CO2/CH4 O2/N2 Ref. 

The membranes tested here showed very high permeance ratio for O2/N2 gas pair. The permeance for O2 was on an average of 10.5 GPU. The permeance ratio for CO2/CH4 gas pair in these cases was also high and close to the intrinsic permeability ratio of 59 in most of the membranes tested. 

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... 

From both these figures, it can be seen that long chain alcohols of nonsolvent additives tend to result in solvent-nonsolvent additive solutions with higher surface tension and membranes with merged nodules. This results in higher pure O2/N2 and CO2/CH4 permeance rate ratios. Lower pure O2/N2 and CO2/CH4 permeance rate ratios are expected when shorter and highly branched chain alcohols were used. They produced membranes with discrete nodules due to lower surface tensions in their solvent-nonsolvent additive solutions. 

The permeance ratios for both O2/N2 and CO2/CH4 gas pairs show maxima as the etching time increases. 

---