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Carbon molecular sieve gas separation membranes-II. Regeneration following organic exposure
Carbon, Vol. 32, No. 8, pp. 1427.-1432, 1994 Copyright0 1994ElsevierScienceLtd Printed in Great Britain.All rightsreserved 0008.6223/94 $6.00+ .OO
Pergamon
CARBON MOLECULAR SIEVE GAS SEPARATION MEMBRANES-II. REGENERATION FOLLOWING ORGANIC EXPOSURE CHERYL W. JONES and WILLIAM J. KOROS Separations Research Program, Center for Energy Studies, The University of Texas at Austin, Austin, TX 78712, U.S.A. (Received 28 ~bruary 1994; accepfed in revisedform 8 June 1994)
Abstract-Carbon molecular sieving (CMS) membranes have been found to have exceptional gas separation properties with high-purity feeds. The basic nature of the carbon itself, however, makes these membranes vulnerable to compounds typically found in industrial process streams. Because of their organophilic nature, CMS materiaIs are excellent adsorbents for organics, and this results in significant problems in membrane applications. The studies detailed in this paper show that CMS membranes are vulnerable to adverse effects from exposure to organic contaminants. Membrane performance losses were severe, and occurred with feed stream concentrations of organics as low as 0.1 ppm. The pattern was consistent and observed for a number of different organic compounds. For various reasons, regeneration techniques used for carbon adsorbents were not suitable for the CMS membranes. However, a very promising regeneration process has been identified that uses pure propylene at unit or near-unit activity as a cleaning agent. This property appears to be unique to propylene, and may have significant implications for a number of application areas. Key Words-Carbon
molecular sieve membranes, organic contamination,
1. INTRODUCTION For many years molecular sieving materials have been successfully used as adsorbents in the field of gas separations, and carbon molecular sieves have been shown to be highly effective for separating gas pairs with very similar molecular dimensions[l,2]. In recent years, carbon molecuIar sieving (CMS) membranes with exceptional gas separation properties have been produced by the pyrolysis of polymeric materials[3,4]. Although these membranes have demonstrated exceptional separation properties with high purity feed gases, the basic nature of the carbon itself makes these membranes vulnerable to compounds typically found in industrial process streams. Carbons generally have nonpolar surfaces, and as a result are organophilic. This characteristic makes them excellent adsorbents for removing organics from process streams, but rcsults in significant problems in membrane applications. A large body of research exists regarding the nature of sorption in carbon materials[%131, and it is well established that sorption in carbons is determined both by the chemical nature of the carbon and by its porous structure. Numerous studies have shown that although the sorption mechanisms may vary, organic compounds generally have a very high affinity for carbon adsorbents. In addition, sorption in micropores is enhanced due to higher interaction potentials resulting from the close proximity of pore walls. Based on the adsorption characteristics of organics in microporous carbons, it would be reasonable to expect that ultramicroporous carbon membranes would be very vulnerable to adverse effects from exposure to organic contaminants. As will be shown, this is the case.
regeneration.
Even if the use of carbon membranes is restricted to applications in which organics are not normally present, inadvertent exposure from unsuspected contaminants can drastically reduce membrane performance. Thus, another factor in the successful use of these membranes is the ease with which the membranes can be regenerated through the removal of sorbed contaminants. For various reasons, a number of regeneration techniques commonly used to regenerate carbon beds11 l] are not suitable for our ultramicroporous carbon membranes. As will be demonstrated, however, a unique regeneration technique has been developed that looks very promising for removing a number of organic contaminants.
2. EXPERIMENTAL
Carbon membranes are produced by pyrolyzing hollow-fiber polymeric materials in a tube furnace. The membranes described in this study were produced from the vacuum pyrolysis of an aromatic polyimide, at temperatures of 500 and 55O’C. The resulting membranes are 295.0 atomic % carbon in a hollow-fiber configuration, with outer diameters of 170-180 pm and wall thicknesses of 30-35 km. The membranes are highly effective in separating gas pairs with very similar dimensions through a molecular sieving mechanism. More detailed descriptions of the polyimide precursor, the pyrolysis process, and the gas separation properties of the resulting carbon membranes are given in Part 1.
1427
1428
C.
W. JONESand W. J. KOROS
2.2 Membrane characterization Characterization work was done with single-fiber test modules constructed in the same manner as described in Part I. The membrane test system used for the organic work was basically the same test system used for the dry gas studies (Part I), but with the addition of a gas saturation step. The organic compound of interest was placed in the stainless steel “saturation” canister, and dry air was passed through at a very slow flow rate to obtain an organic saturated or nearsaturated air feed. Flux and composition measurements were made in the same manner as described previously. The molecular sieving constrictions of the membrane allow only very small molecules to penetrate the membrane completely, so permeate flux calculations based on the ideal gas law were still valid for these runs. A flame ionization detector in conjunction with the thermal conductivity detector was used for hydrocarbon detection, and confirmed that the larger hydrocarbons (2C6) were not present in the permeate. Membrane performance changes resulting from exposure to organits were characterized in terms of changes to 0,/N* selectivity and O2 flux.
3. RESULTS AND DISCUSSION 3.1 C6 and higher hydrocarbons Initial runs were made with an air feed saturated with n-hexane. The vapor pressure of hexane is 152 mmHg (20.3 KPa) at 24”C, resulting in a feed stream of about 18% hexane at a total feed pressure of 817 mmHg (108.9 KPa). Upon exposure to hexane saturated feed, membrane performance deteriorated very rapidly. In just 9 minutes the oxygen flux was reduced by 80%, dropping from 39.0 CPU to 7.6 CPU. The 02/N2 selectivity also dropped, but at a slower rate. As shown in Fig. 1, the membrane had essentially shut down after 17 hours of hexane exposure. Membrane performance was also evaluated under
4011
.
,
I
I
I
FoadP-1.1
run conditions that more closely resembled a case of inadvertent contamination. Dry air feed was saturated with Fisherbrand* 19 Mechanical Pump Oil@. Paraffinic hydrocarbons are listed as a major constituent of this oil, and its vapor pressure at room temperature is given as 8.5 x lop5 mmHg. At a total feed pressure at 817 mmHg, the concentration of pump oil vapor was 0.1 ppm. As shown in Fig. 2, even this low concentration of hydrocarbon in the feed stream drastically reduced membrane function. Additional runs with air feeds saturated with decane and hexadecane showed the same patterns as observed above. The Cl0 and Cl6 runs were made at higher feed pressures, and results are given in Tables 1 and 2. Although the hydrocarbon concentrations are low, the activity levels are unity or near unity, and the effects on membrane performance are extremely detrimental. Not surprisingly, the hydrocarbon concentration in the feed stream appears to influence the rate of membrane performance loss, but not the end result. If the membrane is viewed in the same manner as a carbon bed for adsorption, the membrane has a certain loading capacity for hydrocarbon sorption. As the sorption of organics proceeds in the micropores, less capacity is available for other compounds. Once a hydrocarbon monolayer is complete, a significant resistance layer to other permeating species has been established, and the membranes are effectively shut down. 3.2 Propane and propylene Although the above runs demonstrated that exposure to paraffinic hydrocarbons of C6 and higher would result in near total loss of membrane function, no adverse effects had been observed in earlier runs with methane mixtures (Part I). Runs were next made to evaluate membrane performance with lower carbon number hydrocarbons, such as propane and propylene. In previous runs, air feeds were saturated or near saturated with the various hydrocarbons. Thus, the hydrocarbons were at unit or near-unit activity levels.
12 1
I
prb
b 0
I
0
with hexane-saturated
I
10
I
15
Run Time (hours)
Run Tlmo (houn) Fig. 1. (a) O2 flux stability
5
air feed; (b) 02/N2 air feed.
selectivity
with hexane-saturated
20
Carbon molecular sieve gas separation membranes-II
1429
Feed P = 1.1 pslg
FeadP=f.ipslg
-
5 ii : ii 8
0
a I
0
I
I
.
10
20
30
0
‘)U
10 20 30 Run lima (hourrl
Run Time (hour@
Fig. 2. (a) 0, flux stability with vacuum pump oil saturated air feed; (b) 0,/N, pump oil-saturated air feed.
Pure propane and propylene feed gases were used for
the following runs, and feed pressures were chosen to reflect activity levels of about 95% of unit activity. A run was made with a pure propane feed at 110.5 psig (862.8 KPa), and the propane flux was measured over time. The carbon membrane used for this run was first checked with dry air feed and found to have an Oz/N, selectivity of 10.4 and an O2 flux of 23 CPU. Following the dry air feed, the membrane was evacuated and then exposed to the pure propane feed. As shown in Fig. 3, propane flux decreased significantly over time. A run with pure propylene feed at 140 psig (1066.1 KPa) was carried out in the same manner as above, and the carbon membrane used had an 0,/N, selectivity of 11.4 and an 0, flux of 29 CPU prior to exposure to propylene. The results of this run are shown in Fig. 4 and again show a flux decrease over time. The decrease in flux with propylene feed, however, was less than that seen with propane. 3.3 Regeneration A surprising observation was made after the propylene run was complete and the module was again exposed to a dry air feed. After a few hours of air feed, no propylene was detected in the permeate, and the Oz/N, selectivity was 10.4 with an O2 flux of 35.4 CPU, a 22% increase over the pre-propylene exposure value of 29 CPU. Not only was propyIene effectively removed from the membrane by the air purge, but the
Table 1. Membrane performance with decane-saturated feed: Feed P = 95.5 psig/Decane concentration
Run time (h)
Q/N2
selectivity
0,
flux
(CPU)
= 298 ppm
Total flux (cc/set)
Total flux reduction (7s)
1 4 21
10.9 3.5
20.9
1.779 8.787 9.634 4.735
x x x x
10-s IO-’ 10-6 10-s
0 95.1 99.5 99.7
selectivity with vacuum
propylene exposure apparently caused a small “opening up” of the pore structure. Further studies described below indicate that propylene most likely acted as a cleaning agent, removing other sorbed compounds from the carbon surface. Attempts to regenerate membranes after hydrocarbon exposure had been unsuccessful to this point. The thermal limits of the epoxy used in module construction prevented the use of high-temperature regeneration processes, and attempts to use lower temperatures (90°C) in conjunction with vacuum proved ineffective. Although in some cases function was slowly restored with a dry air purge, the rate was much too slow to be practical. Most options for solvent extraction[l4] were discounted because of the solvent’s high affinity for the carbon. Thus, the prospect of membrane regeneration with propylene was highly attractive. Runs to evaluate the regeneration potential of propylene were made with membranes shut down by exposure to hexane and hexadecane. The shut-down membranes were first exposed to a dry air feed for a period of time to establish a dry air purge regeneration rate as a basis of comparison. Membranes were next exposed to pure propylene at feed pressures between 140 and 150 psig (9.5100% unit activity), for time periods ranging from 2 to 3 hours. After each exposure, dry air feed was resumed and changes in membrane properties were checked. Results of these runs are shown in Figs. 5 and 6, and show that membrane recovery was significantly boosted by exposure to pro-
Table 2. Membrane performance with hexadecane-saturated feed: Feed P = 98.0 psig/hexadecane concentration = 0.1 ppm Total flux Run time
(h) 0
40
0
0.25 5.5
WN2
0,
flux
selectivity (CPU) Il.6
2.9
20.3
Total flux (cc/set)
reduction Vo)
1.641 x 10-j 2.367 x 1O-4 2.533 x lo-*
0 85.6 98.5
C.
1430
0.00l~ 0
1
2
3
4
W. JONES and W. J. KOROS
5
6
Run Tlmo (hour@)
Fig. 3. Propane flux at 110.5 psig.
F
0.00
I
I
I 0
S 2
1 Run Tlma
7
1
I 3
4
(hours)
Fig. 4. Propylene flux at 140 psig.
pylene. In fact, two exposures to propylene completely restored the membrane shut down by hexane to its original condition in terms of selectivity and O2 flux. To explore propylene regeneration further, the scope of the study in terms of the number and type of contaminants was expanded. Compounds other than
paraffins and olefins studied to date include phenol, isopropyl alcohol, toluene, ally1 alcohol, and diethylamine. Membrane performance with air feeds saturated with the various compounds was first evaluated, and in all cases membrane function was lost over a period of time. Once the membranes were shut down, they were exposed to dry air feeds for a period of time to establish baseline recovery curves. In the same manner as described above, membranes were exposed to pure propylene and then dry air feed was resumed. Results of these runs are shown in Figs. 7-l 1, and demonstrate that in most cases recovery was enhanced by propylene exposure. The ease or difficulty with which various compounds are removed from the carbon membrane reflects differences in the initial nature of sorption. A number of factors determine the interaction forces between a given adsorbate and the carbon adsorbent. These factors include the nature and number of surface groups on the carbon, surface area, and pore size distribution, as well as the chemical nature and physical characteristics of the adsorbate. These factors combine to determine whether physical adsorption (van de Waals) or chemisorption occurs. The compounds more easily removed from the carbon during regeneration will be those held by weaker interaction forces. It can be seen from Figs. 5 and 8 that hexane and isopropyl alcohol were both completely removed from the carbon membrane with dry air purge and propylene regeneration. In contrast, essentially no function was restored in the membrane shut down by diethylamine (DEA), as shown in Fig. 11. In the other cases, membrane function was restored to varying degrees. It is clear from these runs that, with the exception of DEA sorbed membranes, exposure to propylene significantly enhanced membrane recovery after organic exposure. It was consistently observed during these runs that most of the recovery enhancement realized with propylene was achieved with the initial exposure. Although small additional increases were sometimes
OrIgina 02 Flux I 26 GPU ’ 0
40
60
Rogenerallon
20
tlmo
80
100
120
140
(bra) with air food
Fig. 5. Membrane regeneration after hexane exposure.
Carbon
OT 0
molecular
’
-
20
sieve gas separation
membranes-11
*
-
’
’
40
Regonrrrtion
60 tlmo
Fig. 6. Membrane
(hrr)
regeneration
after a second exposure, no further improvements were seen with additional exposures. It was also observed that membrane selectivity recovery paralleled that of the O2 flux, and that the original selectivity was achieved only after the flux was fully restored. At this point, we are unable to explain why propylobserved
.
’
’
80
J
*
120
100
with
air
1431
food
after hexadecane
exposure
ene is so effective in removing sorbed contaminants. Limited studies with other related compounds have not yielded similar results. Regeneration attempts with propane, ethylene, and 1,3-butadiene have all been unsuccessful. At pressures required to reflect unit activity, no regenerative properties were observed for either propane or 1,3-butadiene. Due to the pressure limits of the system, runs with ethylene were not made at a
26 Orlglnrrl 02 Flux . 23.1 QPU 02IN2
20
Soloctlvlty I
11.8
OQhalO2
Flux.
23.1 GPU 12.8
02/N2 SoIocW~y I 15
Allr c3tle lxPoauro
J(10
6 0
0
IO
20
40
30
60
60
Rogonomtlon ltmo (hn) with rk hod Fig. 7. Membrane
regeneration
after phenol
0
IO
20
30
40
Rogonoratlon tlmo (hrr)
exposure.
Fig. 9. Membrane
regeneration
SO
60
with air
after toluene
food
exposure.
25
20
I
.
I
.
I
.
I
.
OrlglnnlO2 Flux I 20.9 GPU 02IN2 Smloctivlty = 11.7
After C3H6 __+ regenoratlon
15
10 After 2nd /J-
5
/ -I
.
Origlnel 02 Flux - 21.1 GPU 02IN2 Selectlvlty - 12.1
f
I
0 0
40
20
Rogonoratlon
tlmo
.
60
(hrr)
1
SO
wlth
. 100
air food
O&&-+-d0
10
20
30
Rogenorallon Hmo (hr.) Fig. 8. Membrane
regeneration exposure.
after
isopropyl
alcohol Fig. 10. Membrane
regeneration
40
wllh air lead
1
I 50
after ally1 alcohol exposure.
C. W. JONES and W. J. KOROS
1432 l.OY *
.
,
.
,
.
,
.
,
.
,
.
Oti@7d02 Flux - 15.3 OPU
08 _ 02m2 solocnvlty . 11A
S
h
0.6
:
After 2nd c2l-B rxp.
0.4 Z 0.2
10 20 30 40 50 60 Rogonoratbn tlmo (hrm) with air food
mance losses occur rapidly. Once a monolayer has been established, a prohibitive resistance to other permeating species exists and the membrane is effectively shut down. This pattern has been observed for a number of different organic compounds. For various reasons, regeneration techniques commonly used for carbon adsorbents are not suitable for the carbon molecular sieving membranes. However, a very promising regeneration process has been identified that uses pure propylene at unit activity. In addition to being a useful cleaning process for carbon membranes, the propylene regeneration process may also find wide application in a number of other areas.
Fig. 11. Membrane regeneration after diethylamine exposure. Acknowledgement-The
authors would like to gratefully acknowledge the financial support from Medal.
equivalent to unit activity (850 psig or 5.96 MPa). At the same pressure as used for propylene, however, no regenerative properties were observed with ethylene. Although it is still possible that regenerative effects could be achieved with ethylene at a 95 100% activity level, the feed pressure requirement would still make ethylene a less desirable regeneration agent. Thus, based on results to date, propylene appears to be unique as a cleaning agent. Although the regenerative properties of propylene need to be explored further, initial results are very promising and may have significant implications for regenerating carbon adsorbents and other materials. pressure
4. CONCLUSIONS
Because of their organophilic nature, carbon molecular sieving membranes are highly vulnerable to adverse effects from exposure to organic contaminants. Membrane performance losses in terms of selectivity and flux are severe, and occur with feed stream concentrations of organics as low as 0.1 ppm. Much like a carbon adsorbent bed, the carbon membranes appear to have a finite loading capacity for organic adsorption. As organic sorption proceeds, capacity for other compounds is diminished and membrane perfor-
REFERENCES
1. J. Koresh and A. Soffer, J.C.S. Faraday I 76, 2457 (1980). 2. A. Kapoor and R. T. Yang, Chemical Engineering Science 44, 1723 (1989). 3. J. Koresh and A. Soffer, Sep. Sci. and Tech. 18, 123
(1983). 4. A. Soffer, J. Koresh, and S. Saggy, U.S. Patent 4,685,940
(1987). 5. C. Pierce, J. W. Wiley, and R. N. Smith, J. Phys. Chem.
53, 669 (1949). 6. M. M. Dubinin, In Chemistry and Physics of Carbon
(Edited by Philip L. Walker, Jr.), Vol. 2, p. 51. Marcel Dekker, New York (1966). I. J. S. Mattson and H. B. Mark, Jr., Activated Carbon: Surface Chemistry and Adsorption from Solution.
Mar-
cel Dekker, New York (1971). 8. M. M. Dubinin, Carbon 18, 355 (1980). 9. S. J. Gregg and K. S. W. Sing, In Adsorption, Surface Area and Porosity. Academic Press, London (1982). 10. M. Rozwadowski, K. E. Wisniewski, and R. Wojsz, Carbon 22, 273 (1984). 11. D. M. Ruthven, Principles of Adsorption and Adsorption Processes. John Wiley & Sons, New York (1984). 12. R. C. Bansal, J.-B. Donnet, and F. Stoeckli, Active Carbon. Marcel Dekker, New York (1988). 13. S. Sircar, Carbon 25, 39 (1987).
J. Rivera14. M. A. Ferro-Garcia, E. Utrera-Hidalgo, Utrilla, C. Moreno-Castilla, and J. P. Joly, Carbon 31, 857 (1993).
Pergamon
CARBON MOLECULAR SIEVE GAS SEPARATION MEMBRANES-II. REGENERATION FOLLOWING ORGANIC EXPOSURE CHERYL W. JONES and WILLIAM J. KOROS Separations Research Program, Center for Energy Studies, The University of Texas at Austin, Austin, TX 78712, U.S.A. (Received 28 ~bruary 1994; accepfed in revisedform 8 June 1994)
Abstract-Carbon molecular sieving (CMS) membranes have been found to have exceptional gas separation properties with high-purity feeds. The basic nature of the carbon itself, however, makes these membranes vulnerable to compounds typically found in industrial process streams. Because of their organophilic nature, CMS materiaIs are excellent adsorbents for organics, and this results in significant problems in membrane applications. The studies detailed in this paper show that CMS membranes are vulnerable to adverse effects from exposure to organic contaminants. Membrane performance losses were severe, and occurred with feed stream concentrations of organics as low as 0.1 ppm. The pattern was consistent and observed for a number of different organic compounds. For various reasons, regeneration techniques used for carbon adsorbents were not suitable for the CMS membranes. However, a very promising regeneration process has been identified that uses pure propylene at unit or near-unit activity as a cleaning agent. This property appears to be unique to propylene, and may have significant implications for a number of application areas. Key Words-Carbon
molecular sieve membranes, organic contamination,
1. INTRODUCTION For many years molecular sieving materials have been successfully used as adsorbents in the field of gas separations, and carbon molecular sieves have been shown to be highly effective for separating gas pairs with very similar molecular dimensions[l,2]. In recent years, carbon molecuIar sieving (CMS) membranes with exceptional gas separation properties have been produced by the pyrolysis of polymeric materials[3,4]. Although these membranes have demonstrated exceptional separation properties with high purity feed gases, the basic nature of the carbon itself makes these membranes vulnerable to compounds typically found in industrial process streams. Carbons generally have nonpolar surfaces, and as a result are organophilic. This characteristic makes them excellent adsorbents for removing organics from process streams, but rcsults in significant problems in membrane applications. A large body of research exists regarding the nature of sorption in carbon materials[%131, and it is well established that sorption in carbons is determined both by the chemical nature of the carbon and by its porous structure. Numerous studies have shown that although the sorption mechanisms may vary, organic compounds generally have a very high affinity for carbon adsorbents. In addition, sorption in micropores is enhanced due to higher interaction potentials resulting from the close proximity of pore walls. Based on the adsorption characteristics of organics in microporous carbons, it would be reasonable to expect that ultramicroporous carbon membranes would be very vulnerable to adverse effects from exposure to organic contaminants. As will be shown, this is the case.
regeneration.
Even if the use of carbon membranes is restricted to applications in which organics are not normally present, inadvertent exposure from unsuspected contaminants can drastically reduce membrane performance. Thus, another factor in the successful use of these membranes is the ease with which the membranes can be regenerated through the removal of sorbed contaminants. For various reasons, a number of regeneration techniques commonly used to regenerate carbon beds11 l] are not suitable for our ultramicroporous carbon membranes. As will be demonstrated, however, a unique regeneration technique has been developed that looks very promising for removing a number of organic contaminants.
2. EXPERIMENTAL
Carbon membranes are produced by pyrolyzing hollow-fiber polymeric materials in a tube furnace. The membranes described in this study were produced from the vacuum pyrolysis of an aromatic polyimide, at temperatures of 500 and 55O’C. The resulting membranes are 295.0 atomic % carbon in a hollow-fiber configuration, with outer diameters of 170-180 pm and wall thicknesses of 30-35 km. The membranes are highly effective in separating gas pairs with very similar dimensions through a molecular sieving mechanism. More detailed descriptions of the polyimide precursor, the pyrolysis process, and the gas separation properties of the resulting carbon membranes are given in Part 1.
1427
1428
C.
W. JONESand W. J. KOROS
2.2 Membrane characterization Characterization work was done with single-fiber test modules constructed in the same manner as described in Part I. The membrane test system used for the organic work was basically the same test system used for the dry gas studies (Part I), but with the addition of a gas saturation step. The organic compound of interest was placed in the stainless steel “saturation” canister, and dry air was passed through at a very slow flow rate to obtain an organic saturated or nearsaturated air feed. Flux and composition measurements were made in the same manner as described previously. The molecular sieving constrictions of the membrane allow only very small molecules to penetrate the membrane completely, so permeate flux calculations based on the ideal gas law were still valid for these runs. A flame ionization detector in conjunction with the thermal conductivity detector was used for hydrocarbon detection, and confirmed that the larger hydrocarbons (2C6) were not present in the permeate. Membrane performance changes resulting from exposure to organits were characterized in terms of changes to 0,/N* selectivity and O2 flux.
3. RESULTS AND DISCUSSION 3.1 C6 and higher hydrocarbons Initial runs were made with an air feed saturated with n-hexane. The vapor pressure of hexane is 152 mmHg (20.3 KPa) at 24”C, resulting in a feed stream of about 18% hexane at a total feed pressure of 817 mmHg (108.9 KPa). Upon exposure to hexane saturated feed, membrane performance deteriorated very rapidly. In just 9 minutes the oxygen flux was reduced by 80%, dropping from 39.0 CPU to 7.6 CPU. The 02/N2 selectivity also dropped, but at a slower rate. As shown in Fig. 1, the membrane had essentially shut down after 17 hours of hexane exposure. Membrane performance was also evaluated under
4011
.
,
I
I
I
FoadP-1.1
run conditions that more closely resembled a case of inadvertent contamination. Dry air feed was saturated with Fisherbrand* 19 Mechanical Pump Oil@. Paraffinic hydrocarbons are listed as a major constituent of this oil, and its vapor pressure at room temperature is given as 8.5 x lop5 mmHg. At a total feed pressure at 817 mmHg, the concentration of pump oil vapor was 0.1 ppm. As shown in Fig. 2, even this low concentration of hydrocarbon in the feed stream drastically reduced membrane function. Additional runs with air feeds saturated with decane and hexadecane showed the same patterns as observed above. The Cl0 and Cl6 runs were made at higher feed pressures, and results are given in Tables 1 and 2. Although the hydrocarbon concentrations are low, the activity levels are unity or near unity, and the effects on membrane performance are extremely detrimental. Not surprisingly, the hydrocarbon concentration in the feed stream appears to influence the rate of membrane performance loss, but not the end result. If the membrane is viewed in the same manner as a carbon bed for adsorption, the membrane has a certain loading capacity for hydrocarbon sorption. As the sorption of organics proceeds in the micropores, less capacity is available for other compounds. Once a hydrocarbon monolayer is complete, a significant resistance layer to other permeating species has been established, and the membranes are effectively shut down. 3.2 Propane and propylene Although the above runs demonstrated that exposure to paraffinic hydrocarbons of C6 and higher would result in near total loss of membrane function, no adverse effects had been observed in earlier runs with methane mixtures (Part I). Runs were next made to evaluate membrane performance with lower carbon number hydrocarbons, such as propane and propylene. In previous runs, air feeds were saturated or near saturated with the various hydrocarbons. Thus, the hydrocarbons were at unit or near-unit activity levels.
12 1
I
prb
b 0
I
0
with hexane-saturated
I
10
I
15
Run Time (hours)
Run Tlmo (houn) Fig. 1. (a) O2 flux stability
5
air feed; (b) 02/N2 air feed.
selectivity
with hexane-saturated
20
Carbon molecular sieve gas separation membranes-II
1429
Feed P = 1.1 pslg
FeadP=f.ipslg
-
5 ii : ii 8
0
a I
0
I
I
.
10
20
30
0
‘)U
10 20 30 Run lima (hourrl
Run Time (hour@
Fig. 2. (a) 0, flux stability with vacuum pump oil saturated air feed; (b) 0,/N, pump oil-saturated air feed.
Pure propane and propylene feed gases were used for
the following runs, and feed pressures were chosen to reflect activity levels of about 95% of unit activity. A run was made with a pure propane feed at 110.5 psig (862.8 KPa), and the propane flux was measured over time. The carbon membrane used for this run was first checked with dry air feed and found to have an Oz/N, selectivity of 10.4 and an O2 flux of 23 CPU. Following the dry air feed, the membrane was evacuated and then exposed to the pure propane feed. As shown in Fig. 3, propane flux decreased significantly over time. A run with pure propylene feed at 140 psig (1066.1 KPa) was carried out in the same manner as above, and the carbon membrane used had an 0,/N, selectivity of 11.4 and an 0, flux of 29 CPU prior to exposure to propylene. The results of this run are shown in Fig. 4 and again show a flux decrease over time. The decrease in flux with propylene feed, however, was less than that seen with propane. 3.3 Regeneration A surprising observation was made after the propylene run was complete and the module was again exposed to a dry air feed. After a few hours of air feed, no propylene was detected in the permeate, and the Oz/N, selectivity was 10.4 with an O2 flux of 35.4 CPU, a 22% increase over the pre-propylene exposure value of 29 CPU. Not only was propyIene effectively removed from the membrane by the air purge, but the
Table 1. Membrane performance with decane-saturated feed: Feed P = 95.5 psig/Decane concentration
Run time (h)
Q/N2
selectivity
0,
flux
(CPU)
= 298 ppm
Total flux (cc/set)
Total flux reduction (7s)
1 4 21
10.9 3.5
20.9
1.779 8.787 9.634 4.735
x x x x
10-s IO-’ 10-6 10-s
0 95.1 99.5 99.7
selectivity with vacuum
propylene exposure apparently caused a small “opening up” of the pore structure. Further studies described below indicate that propylene most likely acted as a cleaning agent, removing other sorbed compounds from the carbon surface. Attempts to regenerate membranes after hydrocarbon exposure had been unsuccessful to this point. The thermal limits of the epoxy used in module construction prevented the use of high-temperature regeneration processes, and attempts to use lower temperatures (90°C) in conjunction with vacuum proved ineffective. Although in some cases function was slowly restored with a dry air purge, the rate was much too slow to be practical. Most options for solvent extraction[l4] were discounted because of the solvent’s high affinity for the carbon. Thus, the prospect of membrane regeneration with propylene was highly attractive. Runs to evaluate the regeneration potential of propylene were made with membranes shut down by exposure to hexane and hexadecane. The shut-down membranes were first exposed to a dry air feed for a period of time to establish a dry air purge regeneration rate as a basis of comparison. Membranes were next exposed to pure propylene at feed pressures between 140 and 150 psig (9.5100% unit activity), for time periods ranging from 2 to 3 hours. After each exposure, dry air feed was resumed and changes in membrane properties were checked. Results of these runs are shown in Figs. 5 and 6, and show that membrane recovery was significantly boosted by exposure to pro-
Table 2. Membrane performance with hexadecane-saturated feed: Feed P = 98.0 psig/hexadecane concentration = 0.1 ppm Total flux Run time
(h) 0
40
0
0.25 5.5
WN2
0,
flux
selectivity (CPU) Il.6
2.9
20.3
Total flux (cc/set)
reduction Vo)
1.641 x 10-j 2.367 x 1O-4 2.533 x lo-*
0 85.6 98.5
C.
1430
0.00l~ 0
1
2
3
4
W. JONES and W. J. KOROS
5
6
Run Tlmo (hour@)
Fig. 3. Propane flux at 110.5 psig.
F
0.00
I
I
I 0
S 2
1 Run Tlma
7
1
I 3
4
(hours)
Fig. 4. Propylene flux at 140 psig.
pylene. In fact, two exposures to propylene completely restored the membrane shut down by hexane to its original condition in terms of selectivity and O2 flux. To explore propylene regeneration further, the scope of the study in terms of the number and type of contaminants was expanded. Compounds other than
paraffins and olefins studied to date include phenol, isopropyl alcohol, toluene, ally1 alcohol, and diethylamine. Membrane performance with air feeds saturated with the various compounds was first evaluated, and in all cases membrane function was lost over a period of time. Once the membranes were shut down, they were exposed to dry air feeds for a period of time to establish baseline recovery curves. In the same manner as described above, membranes were exposed to pure propylene and then dry air feed was resumed. Results of these runs are shown in Figs. 7-l 1, and demonstrate that in most cases recovery was enhanced by propylene exposure. The ease or difficulty with which various compounds are removed from the carbon membrane reflects differences in the initial nature of sorption. A number of factors determine the interaction forces between a given adsorbate and the carbon adsorbent. These factors include the nature and number of surface groups on the carbon, surface area, and pore size distribution, as well as the chemical nature and physical characteristics of the adsorbate. These factors combine to determine whether physical adsorption (van de Waals) or chemisorption occurs. The compounds more easily removed from the carbon during regeneration will be those held by weaker interaction forces. It can be seen from Figs. 5 and 8 that hexane and isopropyl alcohol were both completely removed from the carbon membrane with dry air purge and propylene regeneration. In contrast, essentially no function was restored in the membrane shut down by diethylamine (DEA), as shown in Fig. 11. In the other cases, membrane function was restored to varying degrees. It is clear from these runs that, with the exception of DEA sorbed membranes, exposure to propylene significantly enhanced membrane recovery after organic exposure. It was consistently observed during these runs that most of the recovery enhancement realized with propylene was achieved with the initial exposure. Although small additional increases were sometimes
OrIgina 02 Flux I 26 GPU ’ 0
40
60
Rogenerallon
20
tlmo
80
100
120
140
(bra) with air food
Fig. 5. Membrane regeneration after hexane exposure.
Carbon
OT 0
molecular
’
-
20
sieve gas separation
membranes-11
*
-
’
’
40
Regonrrrtion
60 tlmo
Fig. 6. Membrane
(hrr)
regeneration
after a second exposure, no further improvements were seen with additional exposures. It was also observed that membrane selectivity recovery paralleled that of the O2 flux, and that the original selectivity was achieved only after the flux was fully restored. At this point, we are unable to explain why propylobserved
.
’
’
80
J
*
120
100
with
air
1431
food
after hexadecane
exposure
ene is so effective in removing sorbed contaminants. Limited studies with other related compounds have not yielded similar results. Regeneration attempts with propane, ethylene, and 1,3-butadiene have all been unsuccessful. At pressures required to reflect unit activity, no regenerative properties were observed for either propane or 1,3-butadiene. Due to the pressure limits of the system, runs with ethylene were not made at a
26 Orlglnrrl 02 Flux . 23.1 QPU 02IN2
20
Soloctlvlty I
11.8
OQhalO2
Flux.
23.1 GPU 12.8
02/N2 SoIocW~y I 15
Allr c3tle lxPoauro
J(10
6 0
0
IO
20
40
30
60
60
Rogonomtlon ltmo (hn) with rk hod Fig. 7. Membrane
regeneration
after phenol
0
IO
20
30
40
Rogonoratlon tlmo (hrr)
exposure.
Fig. 9. Membrane
regeneration
SO
60
with air
after toluene
food
exposure.
25
20
I
.
I
.
I
.
I
.
OrlglnnlO2 Flux I 20.9 GPU 02IN2 Smloctivlty = 11.7
After C3H6 __+ regenoratlon
15
10 After 2nd /J-
5
/ -I
.
Origlnel 02 Flux - 21.1 GPU 02IN2 Selectlvlty - 12.1
f
I
0 0
40
20
Rogonoratlon
tlmo
.
60
(hrr)
1
SO
wlth
. 100
air food
O&&-+-d0
10
20
30
Rogenorallon Hmo (hr.) Fig. 8. Membrane
regeneration exposure.
after
isopropyl
alcohol Fig. 10. Membrane
regeneration
40
wllh air lead
1
I 50
after ally1 alcohol exposure.
C. W. JONES and W. J. KOROS
1432 l.OY *
.
,
.
,
.
,
.
,
.
,
.
Oti@7d02 Flux - 15.3 OPU
08 _ 02m2 solocnvlty . 11A
S
h
0.6
:
After 2nd c2l-B rxp.
0.4 Z 0.2
10 20 30 40 50 60 Rogonoratbn tlmo (hrm) with air food
mance losses occur rapidly. Once a monolayer has been established, a prohibitive resistance to other permeating species exists and the membrane is effectively shut down. This pattern has been observed for a number of different organic compounds. For various reasons, regeneration techniques commonly used for carbon adsorbents are not suitable for the carbon molecular sieving membranes. However, a very promising regeneration process has been identified that uses pure propylene at unit activity. In addition to being a useful cleaning process for carbon membranes, the propylene regeneration process may also find wide application in a number of other areas.
Fig. 11. Membrane regeneration after diethylamine exposure. Acknowledgement-The
authors would like to gratefully acknowledge the financial support from Medal.
equivalent to unit activity (850 psig or 5.96 MPa). At the same pressure as used for propylene, however, no regenerative properties were observed with ethylene. Although it is still possible that regenerative effects could be achieved with ethylene at a 95 100% activity level, the feed pressure requirement would still make ethylene a less desirable regeneration agent. Thus, based on results to date, propylene appears to be unique as a cleaning agent. Although the regenerative properties of propylene need to be explored further, initial results are very promising and may have significant implications for regenerating carbon adsorbents and other materials. pressure
4. CONCLUSIONS
Because of their organophilic nature, carbon molecular sieving membranes are highly vulnerable to adverse effects from exposure to organic contaminants. Membrane performance losses in terms of selectivity and flux are severe, and occur with feed stream concentrations of organics as low as 0.1 ppm. Much like a carbon adsorbent bed, the carbon membranes appear to have a finite loading capacity for organic adsorption. As organic sorption proceeds, capacity for other compounds is diminished and membrane perfor-
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