Advanced Materials for Membrane Separations - American Chemical

Chapter 11

Pure- and Mixed-Gas Permeation Properties of Poly(p-tert-butyl diphenylacetylene) 1

1

2

Ingo Pinnau , Zhenjie He , Toshio Masuda , and Toshikazu Sakaguchi Downloaded by COLUMBIA UNIV on September 12, 2012 | http://pubs.acs.org Publication Date: April 20, 2004 | doi: 10.1021/bk-2004-0876.ch011

2

1

Membrane Technology and Research, Inc., 1360 Willow Road, Suite 103, Menlo Park, C A 94025 Department of Polymer Chemistry, Kyoto University, Kyoto 606-8501, Japan 2

Poly(p-tert-butyl diphenylacetylene) [PptBDPA] is an amorphous, glassy, substituted acetylene-based polymer. PptBDPA ranks amongst the most permeable polymers known. The oxygen permeability o f PptBDPA at 35°C is 1,930 x 10 cm (STP)cm/cm s cmHg. A s expected for a high permeability glassy polymer, the selectivity of PptBDPA is low for the separation of supercritical gases; for example, its oxygen/nitrogen selectivity is only 1.9. O n the other hand, PptBDPA shows very high permeabilities for organic vapors and high organic-vapor/supercritical-gas selectivity. The permeability o f supercritical gases, such as nitrogen or methane, was significantly reduced by co-permeation o f a condensable gas. This behavior is very similar to that of other high-free-volume, glassy acetylene-based polymers and results from blocking o f the excess free volume o f the polymer by preferential sorption o f the condensable gas mixture component. -10

2.

.

© 2004 American Chemical Society

In Advanced Materials for Membrane Separations; Pinnau, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

167

168

Downloaded by COLUMBIA UNIV on September 12, 2012 | http://pubs.acs.org Publication Date: April 20, 2004 | doi: 10.1021/bk-2004-0876.ch011

Introduction The separation of organic vapors from supercritical gases with membranes has recently gained significant commercial importance in the chemical process industry. Examples of current large-scale industrial applications include (i) separation o f olefins, such as ethylene and propylene, from nitrogen in polyolefin polymerization purge-gas streams (1,2), (ii) recovery o f vinyl chloride monomer from polyvinyl chloride) off-gas streams (3,4), and (iii) recovery of gasoline vapor from storage tank operations (5). Future applications that could significantly expand the commercial use of membranes for vapor separations include the recovery o f hydrocarbons from hydrogen in petrochemical processes and removal of C + hydrocarbons from natural gas. Recently, the pure- and mixed-gas permeation properties of high-freevolume, glassy acetylene-based polymers, such as poly( 1-trimethylsilyl-1propyne) [PTMSP] (6-10) and poly(4-methyl-2-pentyne) [PMP] (11,12), were reported for hydrocarbon/methane and hydrocarbon/hydrogen separations. These polymers are characterized by high glass transition temperatures, typically > 200°C, very high fractional free volume (> 0.25), and extremely high supercritical gas and organic vapor permeabilities. Specifically, p o l y ( l trimethylsilyl-l-propyne) [ P T M S P ] , has the highest gas and vapor permeabilities o f all known polymers (13,14). The extraordinarily high permeability of P T M S P results from its very large amount of excess free volume and interconnectivity of free-volume-elements. P T M S P exhibits very unusual organic vapor permeation properties. In contrast to conventional low-freevolume glassy polymers, such as bispenol-A polycarbonate, P T M S P is significantly more permeable to large, organic vapors than to small, supercritical gases. In particular, for C + hydrocarbon/methane as well as C + hydrocarbon/hydrogen mixtures, P T M S P exhibits both the highest C + hydrocarbon permeability and the highest C +/methane and C +/hydrogen selectivity of any known polymer (6,7,9). 2

2

2

2

2

2

In this study, the pure-gas permeation properties o f poly(p-rer*-butyl diphenylacetylene) [PpfflDPA], a glassy, diphenyl-substituted acetylene polymer, were determined for supercritical gases as well as for a series of hydrocarbon vapors. In addition, gas permeation experiments with PprBDPA were carried out with ethylene/nitrogen, propylene/nitrogen, and nbutane/methane mixtures.

Experimental Polymer Synthesis, Characterization, and Film Formation. Poly(/?-tert-butyl diphenylacetylene) was synthesized as described previously by Masuda et al. (15,16). The polymerization was carried out in toluene at 80°C in a Schlenk tube under dry nitrogen using a TaCls-w-BiuSn co-catalyst system. The polymer yield was 84%. The chemical structure o f P/tfBDPA is shown in Figure 1. The


169 molecular weight of the polymer, as determined by gel chromatography, was 3.6 x 10 g/mole. 6

CH3-C-CH3


CH

3

Figure 1. Repeat unit ofpoly(p-tert-butyl diphenylacetylene). A dense, isotropic film of P/tfBDPA was ring-cast from a polymer solution (0.5 wt% in toluene) onto a Teflon-coated glass plate. The film was dried gradually at ambient conditions for 72 hours and then under vacuum at 80°C for three days to completely remove the solvent. To ensure that the film was completely solvent-free, the film was removed periodically from the vacuum oven and weighed on an analytical balance. The PpfBDPA film used for the permeation measurements had a thickness of 63 pm (± 0.5 pm). The density of P/tfBDPA was determined by a gravimetric method. Three film samples were weighed on an analytical balance and the density was determined from the known area and the thickness of the films, as determined with a precision micrometer. The density of P/7/BDPA was 0.91 (±0.01) g/cm . The fractional free volume [FFV] (cm free volume/cm polymer), commonly used as a measure for the free volume available for chain packing can be determined from 3

3

3

FFV=-%-

-

(1)

3

where v is the specific volume (cm /g) of the polymer, as determined from density measurements, and v is the van der Waals volume, estimated from van Krevelen's group contribution method. The fractional free volume of P/tfBDPA is 0.27, which ranks amongst the highest F F V values of glassy polymers reported to date. sp

w

Permeation Experiments. The pure-gas permeabilities of P/tfBDPA to helium, hydrogen, nitrogen, oxygen, methane, ethane, ethylene, propane, propylene, and H-butane were determined using the constant pressure/variable volume method. The gas permeation experiments were carried out at 35°C. The feed pressure was 50 psig (except for w-butane: p = 10 psig); the permeate-side pressure was atmospheric (0 psig). Volumetric permeate flow rates were determined with soap-bubble flowmeters. The steady-state flux, J (cm (STP)/cm s), was calculated from: feed

3


2

170 J ^ ^ P a dtAT-U

)

(2

where (dV/dt) is the volumetric displacement rate o f the soap film in the flowmeter (CWLI$\A is the membrane area (cm ), T is the gas temperature (K), and p is the atmospheric pressure. The permeability, P, (cm (STP)cm/cm scmHg), of the film was determined by: 2

a

3

2

J *£ J

P=

(3)


(Pi-Pi) where £ is the film thickness and p and pi are the feed and permeate pressure, 2

respectively. The selectivity, c^*, was calculated by:

(4) The mixed-gas permeation properties o f PptBDPA were also determined using die constant pressure/variable volume method. The following gas mixtures were used: i) 20 v o l % ethylene/80 v o l % nitrogen, (ii) 10 v o l % propylene/90 vol% nitrogen, and (Mi) a series of ^-butane/methane mixtures containing 1,2,4, and 6 v o l % ^-butane in methane, respectively. The feed pressure was 150 psig; the permeate pressure was atmospheric (0 psig). The mixed-gas permeability was calculated by: P ~ "mix

u

c

^pmrn perm ^perm I " I (Pfeed* feed)~~ (Pperm' perm) — •

x

x

sc\ D

\ *

where p/ d and p are the feed and permeate pressure (cmHg absolute), and Xfeed and Xperm are the feed and permeate volume fractions, respectively. The gas mixture selectivity was then calculated from Eq. 4. ee

perm

Results and Discussion Pure-Gas Permeation Properties of P/tfBDPA The gas permeabilities of PpfBDPA as a function of the critical gas volume, a relative measure of penetrant size, are shown in Figure 2. In PpfBDPA the permeability of larger gas molecules, specifically hydrocarbons, increases as the molecular size of the gases increases. This result is in qualitative agreement with the pure-gas permeation properties o f other high-free-volume glassy


171 disubstituted acetylene-based polymers, namely, poly( 1-trimethylsilyl-1propyne) [PTMSP], poly(4-methyl-2-pentyne) [PMP], poly [1-pheny 1-2-[p(trimethylsilyl) phenyljacetylene], and poly[l-phenyl-2[triisopropyl)phenyl]acetylene] (6,8,17-18). In these high-free-volume polymers, diffusion coefficients show a relatively weak dependence on gas size and, hence, permeability is affected significantly less by differences in diffusion coefficients but is more dependent on differences in gas solubility (8).


100.000

x E

fS

P ||

10,000

r

J V

to 5Z 9& 8 E 3 O tL e

1,000

100 100

150

20q

250

300

Critical Volume (cm3/mol)

Figure 2. Pure-gas permeability of PptBDPA for a series of gases and hydrocarbon vapors as a function of critical gas volume.

The pure-gas permeation properties of Pp/BDPA for various gases in terms of permeability and selectivity over nitrogen are summarized in Table I. A s observed previously for other high-free-volume, disubstituted acetylene-based polymers, PptBDPA exhibits very high gas permeabilities. For example, the oxygen permeability o f PprBDPA is 1,930 x 10" cm (STP)cm/cm scmHg. The high gas permeabilities of PptBDPA result from its very high fractional free volume ( F F V = 0.27). Typical for a high-free-volume disubstituted acetylene polymer, the selectivity o f P/tfBDPA for supercritical gases is low; its oxygen/nitrogen selectivity is only 1.9. It is important to note, however, that P/tfBDPA is more permeable to large, condensable C + hydrocarbons than to supercritical gases, such as nitrogen or methane. For example, the pure-gas nbutane/nitrogen selectivity of P/tfBDPA is 19. Hence, the hydrocarbon/nitrogen solubility selectivity of Ppf&DPA is higher ( » 1 ) than the hydrocarbon/nitrogen diffusivity selectivity (

Advanced Materials for Membrane Separations - American Chemical

Recommend Documents