Olefin/paraffin separation represents a class of most important and also most
costly processes in the petrochemical industry. Cryogenic distillation has been
used for over 60 years for this separation (Keller et al., 1992). It
continues to be the most energy-intensive method because of the close relative
volatilities of the compounds to separate (Humphrey et al., 1991). The
most important olefin/paraffin separations are for the binary mixtures of ethane/ethylene
and propane/propylene. A number of alternatives have been investigated (Eldridge,
1993). The most promising is separation based on π-complexation. The metal-olefin
bonding described by the Dewar-Chatt model is commonly known as π-bond
complexation. The Dewar-Chatt description, as applied to Cu(I)- or Ag(I)-ethylene
complexes, is shown in Fig. 1. The complex is formed by double
bonding of a Cu(I) or Ag(I) atom with the olefin (Quinn, 1971; Long, 1972; Herberhold,
1974; Bochmann, 1977; Yamamoto, 1986). Both the metal and alkene act as an electron
donor and acceptor in the complexation interaction. A σ component of the
bond results from overlap of the vacant outermost s atomic orbital of the metal
with the full π (bonding) molecular orbital of the olefin. This new molecular
orbital, formed by donation of electrons from olefin to metal, has electron
density concentrated between the bond members. In Cu(I) and Ag(I) ions, the
outermost s orbital is empty because the single electron present in the metal
is lost upon ionization to a+1 valence. In nonionizing facilitators, the metal
is often bound to an electronegative atom (as oxygen atom in ether bond).
||Dewar-Chatt model of π-bond complexation
These electronegative atoms withdraw electron density from the metal, resulting
in a partial positive charge and a substantially vacant outermost s orbital.
A π component of the metal-olefin bond is formed by backdonation of electrons
from the full outer d atomic orbital of the metal to the vacant π* (antibonding)
molecular orbital of the olefin. This new molecular orbital has a nodal plane
of electron density between the members of the bond.
The effects of metal ions and weight percentage of metal salts on the ethylene/ethane selectivity of PA12-co-PTMO based membrane were studied. The gas permeation properties and gas sorption properties of PA12-PTMO/AgBF4 and PA12-PTMO/CuBF4 membranes were compared.
MATERIALS AND METHODS
Chemicals: The PA12-PTMO granulates (PebaxTM 2533) were kindly provided by Arkema Corp. Its density as measured with a Micrometrics Accupyc 1330 instrument was 1.01 g cm¯3 and its Shore D hardness was 25. This PebaxTM grade contains 25 wt. % of PA12 blocks and 75 wt. % of PTMO blocks. All other chemicals were purchased from Aldrich and used without further purification.
Ethylene and ethane N. 35 quality gases were purchased from Air liquide/Alphagaz Company.
Membranes: The membranes were prepared by dissolving first the copolymer in ethanol at 75°C to obtain a 9 wt. % solution. Then appropriate mass of AgBF4 or CuBF4 was dissolved in the PebaxTM solution and the obtained dopes were cast onto a glass plate with a Gardner knife and the membranes were slowly dried in ambient air first, then under vacuum. The flat membrane samples were used shortly after their preparation, as the membrane darkens in long storage.
The resulting membranes contain 35% wt. of silver salt and 32, 43, 50, 57 and 60% wt. of copper salt in the polymer matrix. The thickness of the membranes obtained was about 100 μm.
Gas permeation measurements: Ethane and ethylene permeation properties of films were determined using the time-lag permeation apparatus previously described (Joly et al., 1999). Before measurement, the air present in the permeation cell was completely evacuated by applying a vacuum on both sides of the film for one night. Thus, the pressure in the permeation cell had a constant value < 5x10¯3 mbar. Then the upstream side was exposed to the gas under test.
The upstream pressure applied to the samples p1 = 1 bar was chosen in order to make measurements in a reasonable time allowing comparisons between the different films. The increase of pressure p2, in the calibrated downstream volume, was measured using a sensitive pressure gauge (0-10 mbar, Effa AW-10-T4) linked to a data acquisition system.
The permeability coefficient, P, was calculated using the variable pressure
method (Glatz et al., 1994) assuming p1 >> p2:
with Jst the steady-state gas flux obtained from the slope of the
steady-state part of the curve p2 versus time t:
where, dQ is the quantity of gas permeated at STP in a time interval dt in the steady state of gas flow, A is the effective film area for gas permeation.
The accuracy on p-values (cm3 (STP) cm¯2 sec¯1 cmHg¯1) is about 6%.
A time-lag diffusion coefficient, D, was calculated from the time-lag, tL, given by the extrapolation of the steady-state asymptote to the time axis: D = L2/6tL. A solubility coefficient S, which links at equilibrium gas concentration in the material to its pressure, is given by the ratio S = P/D.
The ethylene/ethane permeation selectivity coefficient α was determined
the permeability coefficients of ethylene and ethane respectively:
This coefficient can be expressed as the product of the diffusion selectivity
coefficient by the solubility selectivity coefficient:
IR spectrometry: FTIR spectra were recorded in Attenuated Total Reflectance mode on a Nicolet Avatar 360 instrument equipped with a germanium crystal. 64 scans were accumulated with a resolution of 4 cm¯1 for each spectrum and IR spectra present absorbance from 3800 to 900 cm¯1.
RESULTS AND DISCUSSION
From appearance point of view, PA12-co-PTMO membranes were colourless whereas
the metal incorporated polymer membranes obtained were coloured, the colour
differing for different metals and were also transparent. The colour indicates
that the metals are certainly incorporated in the ionic form making charge transfer
type complexes with the polymer structure. Scanning electron microscopy (SEM)
was then performed to detect particles in the materials. The SEM micrographs
show crystal-like particles (20 lm needles, Fig. 2).
Membrane structure IR characterization: FTIR spectra of pure PA12-co-PTMO
and metal salt-PA12-co-PTMO composite membranes with low salt contents are shown
in Fig. 3. It can be observed that all the metal-copolymer
samples show only a slight change in the overall FTIR picture compared to the
pure PA12-co-PTMO. This could be probably due to the large number of polymer
repeat units which suppress the vibrational effects shown by the low concentration
of the metal incorporated in the copolymer.
||SEM photograph of the Pebax films with 32 wt. % of CuBF4
Permeability and selectivity of PA12-co-PTMO-Salt membranes: The permeation
properties of ethylene/ethane gases in CuBF4-PA12-co-PTMO complex
membranes are shown in Table 1 as a function of the copper
salt content (32, 43, 50, 57 and 60 wt. % salt in the polymer matrix).
Pure PA12-co-PTMO membrane exhibits a poor performance for ethylene/ethane separation; its ethylene/ethane selectivity is only 1.25. Composite membranes containing up to 32 wt. % CuBF4 also has poor olefin/paraffin separation properties. However when the CuBF4 concentration exceeds 43 wt. %, the complexation effect of the copper ions in the polymer membrane is clearly evident. The ethylene flux now increases dramatically with increasing copper salt concentration.
The ethylene permeability of membranes containing 60 wt. % copper salt are
18 times higher than those of pure PA12-co-PTMO membrane and 13 times higher
than PA12-co-PTMO membranes containing 32 wt. % CuBF4. These results
give direct indications on the transport mechanism occurring in the PA12-co-PTMO
based composite membranes under test. Based on the pure-gas permeation properties,
facilitated ethylene transport occurred only in membranes containing at least
50 wt. % dissolved CuBF4.
||Effect of CuBF4 salt concentration on the permeation of pure
ethylene and ethane in PA12-co-PTMO/ CuBF4 composite membranes
(p1 = 1 Atm. and T = 25°C)
||FTIR spectra of PA12-co-PTMO (Pebax) membranes loaded with AgBF4
and CuBF4 at low salts contents
That suggests that the copper ions are not freely mobile in the polymer matrix
and that therefore the facilitated transport in this kind of solid polymer electrolyte
membrane is likely to occur by a hopping mechanism. In this hypothesis olefin
molecules move from copper-ether site to site across the membrane, similar to
that suggested for fixed-site carrier membrane (Cussler et al., 1989;
Noble, 1990). Based on this mechanism, the copper ions must be close enough
to allow olefin molecules to diffuse from site to site. Accordingly, a threshold
concentration of copper ions exists, below which no facilitation of olefin molecules
occurs. This hypothesis is supported by our pure-gas permeation data in Table
1 which show that the ethylene fluxes increased dramatically for PA12-co-PTMO
membranes containing more than 43 wt. % CuBF4.
The composite membrane containing Ag+ ions shows a higher ethylene/ethane ideal selectivity than the membranes containing Cu+ ions (Hsiue and Yang, 1996). The ethylene/ethane selectivity is obviously enhanced by the incorporation of the metal ion, such as Ag+ and Cu+. Though the PA12-PTMO/AgBF4 membranes were accompanied with a lower permeability coefficient. This is due to the fact that the high crystallinity of the PA12 matrix reduces the effective permeating area as well as the permeability of the film (Yang and Hsiue, 1991; Hsiue and Yang, 1993, 1994).
Effect of solubility and diffusivity: Membrane selectivity is related
to the ideal separation factor which is partitioned between the preferential
sorption, expressed by Solefin/Sparaffin and the ratio
of diffusivities, Dolefin/Dparaffin. Therefore, the olefin/paraffin
selectivity (α) can be analyzed by the former solubility selectivity (αS)
and latter diffusivity selectivity or mobility selectivity (αD)as
follows (Nakagawa, 1992):
The effects of S and D for various complex membranes and the corresponding
non-Ag membranes are shown in Table 2. The gas diffusivity
in complex membranes are decreased as compared with the non-Ag membranes. The
αS and αD are increased as compared with the
corresponding non-Ag membranes since membrane modification not only increases
the affinity of olefin, but also increases the sieving effect. As compared with
PA12-PTMO membrane, higher olefin/paraffin selectivity in PA12-PTMO/Ag+
membrane is mainly due to the contribution of αD that was caused
by the molecular sieving effect.
||The solubility, diffusivity and their corresponding selectivity in complex
membranes at 1 atm and 25°C
||Gas permeability coefficients, ideal selectivities and % metal-content
of ordinary and metal incorporated Pebax membranes
||Pure-gas permeation properties a PA12-co-PTMO (Pebax)/CuBF4
composite membrane (60 wt. % CuBF4) as a function of permeation
time: ethylene and ethane permeability coefficient
Both the S and D values are important for olefin/paraffin transport through PA12-PTMO/AgBF4 membrane since αS and αD are higher than those in PA12-PTMO/CuBF4 membrane. The lower D in silver complex membrane as compared with copper complex membrane also indicates that the relative amounts of gas transport by diffusion in PA12-PTMO/AgBF4 membrane are lesser than that in PA12-PTMO/CuBF4 membrane. It means that the diffusivity determined PA12-PTMO membrane (αD>αS) can be altered to solubility determined PA12-PTMO/AgBF4 membrane (αS>αD).
Gas permeability coefficients, ideal selectivities and % metal-content of ordinary
and metal incorporated Pebax membranes are shown in Table 3.
Stability of pure-gas permeation properties: Mechanical and chemical
instability are major concerns with conventional facilitated transport membranes.
Therefore, the pure-gas permeation properties of a PA12-co-PTMO/60 wt. % CuBF4
composite membrane were evaluated continuously for 16 h with ethylene and ethane
gases at a feed pressure of 3 atm as a function of operating time. The results
are shown in Fig. 4. Although the ethylene and ethane permeabilities
decrease over time, the ethylene/ethane selectivity of near 2 is essentially
constant over the permeation period.
During 16 h of continuous permeation, both ethylene and ethane permeabilities
decreased to 50% of their original values. This loss may be a result of slow
crystallization of the copper salt in the polymer electrolyte. In future work,
we will investigate this hypothesis further using X-ray diffraction and FTIR.
The dense complex membranes of PA12-co-PTMO/AgBF4 and PA12-co-PTMO/CuBF4 with homogeneous Ag+ and Cu+ distribution were developed for olefin/paraffin separation. The two ions are coordinated onto the carbonyl of the PA12 and behave as fixed carriers for facilitating olefin transport.
The High-flux, solid polymer electrolyte composite membranes were prepared by coating a solution containing PA12-PTMO, silver and copper tetrafluoroborate onto a microporous poly(ether imide) support. Formation of a polyether/silver, copper salts complex resulted in the formation of a solid polymer electrolyte membrane. Permeation tests with pure gases showed a significant increase in ethylene and propylene permeance of PA12-PTMO-AgBF4 membranes with a silver salt concentration of more than 43 wt. % over those of the pure PA12-PTMO membrane, confirming that olefin transport results from interaction of the olefin with metal ions. In addition, the high carrier concentration also hinders paraffin transport. An increase in metal salt concentration in the polymer matrix increases the olefin permeance and olefin/paraffin selectivity. A Pebax-based electrolyte membrane containing 60 wt. % CuBF4 had a pure-gas ethylene permeance of about 810 barrer and an ethylene/ethane selectivity of 22.5.