0263–8762/06/$30.00+0.00
# 2006 Institution of Chemical Engineers
Trans IChemE, Part A, May 2006
Chemical Engineering Research and Design, 84(A5): 350– 354
www.icheme.org/cherd
doi: 10.1205/cherd05021
ADSORPTIVE SEPARATION OF LIGHT
OLEFIN/PARAFFIN MIXTURES
A. VAN MILTENBURG , W. ZHU, F. KAPTEIJN and J. A. MOULIJN
Reactor and Catalysis Engineering, DelftChemTech, Delft University of Technology, Delft, The Netherlands
A
n effective adsorbent for the ethylene/ethane separation has been prepared by the
dispersion of CuCl in large NaX crystals. The results from the thermo gravimetric
analysis (TGA) indicate a maximal dispersion capacity of 36 wt% of CuCl for the
NaX crystals. Single component isotherms of ethane and ethylene on CuCl/NaX show that
the CuCl containing adsorbents are highly favourable to the olefin, which is ascribed to the
strong interaction due to the formation of p-complexes between ethylene and CuCl.
Breakthrough experiments demonstrate that CuCl/NaX can be used as an effective adsorbent
for the separation of ethylene and ethane mixtures and would allow the use of more
sustainable processes.
Keywords: propane; propylene; Faujasite zeolite; separation; CuCl.
INTRODUCTION
obtain the desired purity levels. Therefore considerable
amounts of compression power will be needed to achieve
this separation, which could end up being 85% of the
total energy cost needed for the entire process (Moulijn
et al., 2001).
Although the traditional cryogenic distillation is reliable,
the rising demand of light olefins and the large energy cost
involved, makes it necessary to search for alternative and
more sustainable separation schemes. Compared to cryogenic distillation adsorptive separation is a more sustainable alternative in terms of low energy costs and process
economics. One component can selectively be adsorbed
in a bed of adsorbent particles, while the other passes
through. The adsorbed component can later on be recovered by temperature swing adsorption (TSA) or pressure
swing adsorption (PSA) (Ruthven et al., 1994; Thomas
and Crittenden, 1998). In these processes the separation is
achieved at ambient temperature and no additional compression power will be needed to cool the gas mixtures,
as was the case for the cryogenic distillation.
Continuous separation of light olefin/paraffin mixtures
could be achieved with a membrane consisting of a selective adsorbent (Kotelnikov et al., 2004; Bryan, 2004). In
this continuous process temperature and pressure can
remain constant, which could further reduce the required
energy demands for the temperature or pressure change.
The use of a continuous membrane operation allows new
integrated process schemes in which the separation and
for instance the direct dehydrogenation process are combined. The selective removal of one component continuously preserves a driving force for conversion, reducing
recycle streams and energy cost.
For the adsorptive separation of light olefin/paraffin
mixtures it is of utmost importance to find a cheap and
Light olefins, like ethylene and propylene, are important
feedstocks in the chemical industry for the production of
rubbers, plastics, fuel components and other valuable
chemical products. In the last decade the demand of these
olefins has increased by more than 50% to a production
of almost 25 trillion tons of ethylene per annum (Moulijn
et al., 2001). With a continuing growth of the world population and the increased demand from developing countries,
the demand of these olefins is expected to increase further.
The light olefins are nowadays mainly produced by steam
cracking of hydrocarbons and by fluid catalytic cracking
(FCC) of gas oils (Moulijn et al., 2001; Aitani, 2004).
The individual olefins are obtained after separation in
downstream cryogenic distillation towers.
A more recent technology is the dehydrogenation of the
corresponding paraffin (Weyten et al., 2000; Kotelnikov
et al., 2001, 2004; Buyanov and Pakhomov, 2001). For
this reaction several reaction configurations are being
developed (Moulijn et al., 2001). However, single pass
conversions are low and limited by the thermodynamic
equilibrium to a conversion of 20– 40%. In order to
obtain the olefin and for the complete dehydrogenation of
the paraffin feed, separation of the olefin/paraffin mixtures
is required.
The small difference in relative volatility between olefins
and paraffins and their low boiling points requires large
cryogenic distillation towers of over 100 trays in order to
Correspondence to: Mr A. van Miltenburg, Reactor and Catalysis Engineering, DelftChemTech, Delft University of Technology, Julianalaan 136,
2628 BL, Delft, The Netherlands.
E-mail: [email protected]
350
ADSORPTIVE SEPARATION OF LIGHT OLEFIN/PARAFFIN MIXTURES
effective adsorbent. The selective adsorption can be
achieved using differences in adsorption affinity of the
adsorbing components. The double bond of the olefin can
form p-complexes with some transition metals and a difference in adsorption affinity between olefin and paraffin can
be achieved (Yang and Kikkinides, 1995). The p-complex
is formed by the donation of p-electrons of the olefin to the
empty s-orbital of a transition metal and the backdonation
of d-electrons of the transition metal to the p -orbitals of
the olefin. Potential transition metals are salts containing
cations such as Cuþ and Agþ. To obtain a large number
of adsorption sites, the transition salt will have to be dispersed over a large surface area of a support. Supports
investigated by others are ion-exchange resins (Wu et al.,
1997), g-Al2O3 (Yang and Kikkinides, 1995; Blas et al.,
1998), clays (Choudary et al., 2002) and activated carbon
(Mei et al., 2002).
In the present study Faujasite zeolites were chosen as
supports. The dispersion of CuCl is characterized by
TGA. The adsorption properties of ethane and ethylene
on the CuCl dispersed in NaX (CuCl/NaX) are determined
with the tapered element oscillating microbalance (TEOM)
(Zhu et al., 1998). The separation performance of an ethylene/ethane mixture is investigated by means of breakthrough column experiments. To avoid the large pressure
drop over the breakthrough column large zeolite particles
will be required. The use of pressed pellets of small zeolite
crystals (,2 mm) would introduce additional diffusion
terms, which would complicate modelling. Therefore, the
synthesis of large NaX crystals is a primary activity.
METHODS AND MATERIALS
Large zeolite NaX crystals were synthesized using a
modified version of the recipe reported in the literature
(Qiu et al., 1998). An Al-solution was made by dissolving
NaAlO2 (Riedel-de Haën) in a sodium hydroxide solution
in water. Trietholamine (TEA) (J.T. Baker) was added as
a stabilizing and buffering or complexing agent. The solution was filtered twice through a 0.2 mm filter to remove
remaining particulates and other impurities. A Si-solution
was made by dissolving Aerosil 90 (Degussa) in water. A
few milligrams of Na2SiO3.9H2O (Acros) were added to
the solution as earlier experiments showed it would suppress the formation of large zeolite NaP crystals. Finally
triethanolamine was added to the Si-solution. Both solutions were aged for about 1 h after which they were
mixed together. The molar ratio of the ingredients was:
SiO2 : NaAlO2 : NaOH: TEA : H2O ¼ 1 : 1.4 : 4.3 : 2 : 222.
The gel was kept at 353 K so the crystallization of the zeolites could occur. After 2 weeks the crystals were filtrated
and dried at 353 K for 1 day. To remove small silica particles attached to the crystals, they were ultrasonically
cleaned in ethanol and dried afterwards. The obtained crystals were divided into several sieve fractions. In this study
only the 56– 63 mm sieve fraction was used.
The synthesized NaX crystals were confirmed with XRD
(Bruker-AXS D5005, CuKa) and SEM (Phillips XL20,
15 kV). The Si/Al-molar ratio of the zeolite was determined by the inductively coupled plasma-optical emission
spectroscopy (ICP-OES) (PerkinElmer Optima 3000DV).
The Na content in the zeolite was determined by the
atomic adsorption spectroscopy (AAS).
351
Physical mixtures were prepared by mixing different
amounts of CuCl (Fluka) with the NaX crystals. These
physical mixtures were slowly heated (1 K min21) in the
quartz reactor to 623 K in flowing argon with a rate of
100 ml (STP) min21 and at this temperature the samples
were heated for 4 h. Thereafter heating was stopped and
the temperature slowly returned to room temperature.
These heat-treated mixtures, referred as to CuCl/NaX,
will be used for adsorption measurements.
The TGA experiments of NaX and of physical mixtures
of CuCl and NaX were performed in a Mettler Toledo
TGA/SDTA851e. Depending on the composition for the
experiments, the sample amount ranging from 15 to
40 mg was inserted in an alumina TGA cup of 70 ml. For
all the TGA experiments the volume of the sample in the
cup was approximately equal; therefore the amount of
NaX was similar for all the experiments. Once the
sample was inserted in the TGA, it was purged for 3 h at
298 K with helium with a rate of 100 ml (STP) min21.
The temperature was then slowly raised (1 K min21) to
423 K and this temperature was kept for 1.5 h. Then the
temperature was further increased to 523 K at 1 K min21
and this temperature was kept for 1.5 h. Then the temperature was further increased to 623 K at 1 K min21. After that
a temperature of 623 K was kept for 5 h.
The adsorption isotherms of ethane and ethylene on
36 wt% CuCl/NaX were measured with the TEOM 1500
mass analyser (100 mg sample volume). A sample of
24.1 mg of the adsorbent powder was used. A detailed
description of the TEOM operating principles and procedures is given elsewhere (Zhu et al., 1998).
Breakthrough experiments were performed using a 1400 SS
tube with a length of 60 mm and an internal diameter of
3.9 mm filled with the adsorbent. In order to be able to
increase the temperature, the tube was installed inside a
ceramic oven. The physical mixture of CuCl and NaX
was retained in the tube using quartz wool. To remove
the adsorbed water and other impurities from the zeolite
and to allow the dispersion of CuCl on NaX, the sample
was preheated to 623 K at 1 K min21 in flowing helium
at a rate of 100 ml (STP) min21 and remained at this temperature for 4 h. Thereafter the temperature was lowered to
358 K. For the breakthrough experiments a total flow rate
of 8 ml (STP) min21 of an ethane, ethylene, and helium
(25 : 25 : 50) mixture was fed to the column at 358 K and
a total pressure of 200 kPa. The desorption was initiated
by rapidly switching the sample gas stream to a pure
helium stream with a rate of 8 ml (STP) min21.
The concentrations in the effluent of the breakthrough
column were analysed using a CompactGC of Interscience.
Although the breakthrough of single components could
easily be measured with a mass spectrometer, the measurement of binary mixtures is more complex (Senkan et al.,
2003; Hagemeyer et al., 2001). The measurement of
those binary mixtures could easily be achieved with gas
chromatography. The CompactGC was equipped with
three separate Rt Qplot columns with a length of 8 m
with their own FID detectors. With this configuration and
the continuous injection of gas samples in the GC
column, it was possible to monitor the concentrations in
the effluent every 8 s.
The gases used in the experiments were supplied by
HoekLoos and had the following purities: helium 4.6
Trans IChemE, Part A, Chemical Engineering Research and Design, 2006, 84(A5): 350 –354
352
VAN MILTENBURG et al.
(.99.996%), argon 4.6 (.99.996%), ethane 3.0 (.99.9%),
and ethylene 3.0 (99.9%).
RESULTS AND DISCUSSION
The XRD patterns of the synthesized crystals agree with
those in the literature for NaX (Baerlocher and McCusker,
2005). Additional peaks in the pattern indicate the presence
of small quantities of NaP and NaA particles. The analysis
of the ICP-OES indicates a Si/Al ratio of 1.3, which is
within the range of zeolite X. A Na/Al ratio of 1.0 was
found with AAS. The SEM revealed that still a small
amount of agglomerated 30 mm NaP crystals was present
in the sieve fraction of 56– 63 mm. A SEM picture of
the obtained NaX crystals in this sieve fraction is shown
in Figure 1.
The TGA patterns of NaX and of the physical mixtures
of CuCl and NaX are shown in Figure 2. The hydrated
NaX powder lost 20 wt% of the initial mass of the zeolite
sample upon heating from 298 to 423 K, corresponding to
regions I– III in Figure 2. A temperature increase from
423 K to 623 K resulted in an extra mass loss of 3 wt%
(regions IV –VI). So the total mass loss of the NaX is
about 23 wt%, mainly attributed to the desorption of the
adsorbed water on the zeolite, as heated up to 623 K, at
which the zeolite is assumed to be dry. Most of the mass
loss occurs in temperatures up to 423 K, while the last
3 wt% mass loss takes place at higher temperatures. This
behaviour agrees with that expected; first the desorption
of weakly adsorbed water and then the desorption of
strongly adsorbed water occurs.
For all the physical mixtures of CuCl and NaX, the mass
loss below 623 K is about 23 wt% on the basis of the initial
mass of the NaX in the mixtures, which is in good agreement with the observation with the pure zeolite sample.
For an amount of CuCl in the mixture below 36 wt%, the
mass of the mixture sample remains constant at 623 K for
5 h, while for a higher amount of CuCl in the mixture, a
decrease in the mass still occurs and the excess of CuCl
slowly sublimes into the flowing helium stream. Therefore
the amount of CuCl dispersed onto NaX is limited to the socalled dispersion capacity of 36 wt%.
The single component isotherms of ethane and ethylene
on 36 wt% CuCl/NaX at 373 K are shown in Figure 3.
Figure 1. SEM image of a synthesized NaX crystal.
Figure 2. TGA patterns of NaX and of physical mixtures of CuCl and
NaX. The values correspond to the weight percentages of CuCl.
The dispersion of CuCl into the zeolite results in a much
higher adsorbed amount for ethylene than for ethane in
the whole pressure range investigated. A two-step adsorption behaviour for ethylene is observed; in the very low
pressure range the adsorbed amount sharply increases
with pressure while its increase becomes slow in the high
pressure range. The same behaviour was observed in an
earlier study with propylene (Van Miltenburg et al.,
2005). The high loadings at low pressures are attributed
to specific interactions between ethylene and the monolayer
of CuCl on the zeolite. After these specific adsorption sites
are saturated, ethylene molecules can only fill into the
remaining pore space, a kind of physical adsorption.
Since ethane can also adsorb in the remaining pore space
the ideal selectivity for ethylene over ethane will decrease
with pressure. This ideal selectivity indeed dramatically
decreases with pressure in the low-pressure range, while
it remains almost constant in the high pressure range, see
Figure 3. Therefore, the strong affinity to the adsorption
of the olefin is completely ascribed to the p-complex
formation of ethylene with Cuþ in CuCl.
In order to verify the separation performance of the
CuCl/NaX adsorbent, binary adsorption was investigated
in terms of breakthrough column experiments. Figure 4
Figure 3. Adsorption isotherms of ethane (O) and ethylene (†) on 36 wt%
CuCl/NaX and the ideal selectivity (V) for ethylene over ethane at 373 K.
Trans IChemE, Part A, Chemical Engineering Research and Design, 2006, 84(A5): 350– 354
ADSORPTIVE SEPARATION OF LIGHT OLEFIN/PARAFFIN MIXTURES
353
weak. Since ethylene has a much stronger affinity to the
adsorbent, its loading in the column is much higher. In
addition, the release of ethylene is slower, compared to
that of ethane, due to the stronger interaction in terms of
the p-complex formation.
The breakthrough and desorption curves clearly show the
preferential adsorption of the olefin on the CuCl/NaX
adsorbent. Only a limited amount of ethane is adsorbed.
These results demonstrate the capability of the CuCl/
NaX as an adsorbent for the separation of ethane and ethylene mixtures.
Figure 4. Breakthrough curves of an ethane (O) and ethylene (†) mixture
in helium (25 : 25 : 50) at 358 K and 200 kPa through 1.32 g of 36 wt%
CuCl/NaX.
shows the breakthrough curves of a mixture of ethane and
ethylene in helium (25 : 25 : 50) through a 60 mm column
filled with 1.32 g of 36 wt% CuCl/NaX at 358 K. Practically from the beginning of the experiment, ethane free
of ethylene is obtained until about 10 min. It is interesting
to notice that the partial pressure of ethane exceeds its feed
pressure of 50 kPa. This overshoot can be explained by two
phenomena occurring along the breakthrough column. First
of all the complete adsorption of ethylene will change the
gas composition to a mixture containing only ethane in
helium. This would yield 67 kPa ethane. The second
phenomenon occurring in the breakthrough column is the
displacement of the weaker adsorbed ethane by ethylene.
When ethylene progresses through the column, the
adsorbed ethane is displaced by the stronger adsorptive
ethylene. The desorption of ethane results in a further
increase of its partial pressure. This provides an explanation on the overshoot of ethane in its breakthrough
curve. Once ethylene approaches the end of the breakthrough column, binary equilibrium will be obtained and
the partial pressures of both components will return to the
feed composition.
After binary equilibrium has been obtained desorption is
initiated by rapidly switching to a helium flow. These desorption curves are shown in Figure 5. As seen in the figure
the partial pressure of ethane quickly drops to zero, which
indicates that the adsorbed amount of ethane is rather small
and the interaction between ethane and the adsorbent is
Figure 5. Desorption curves at 358 K and 200 kPa of the adsorbed ethane
(O) and ethylene (†) after the breakthrough experiment presented in
Figure 4.
CONCLUSIONS
Large zeolite NaX crystals with a Si/Al ratio of 1.3 were
successfully synthesized. A simple and effective method
was developed to prepare the dispersion of CuCl into
the synthesized NaX crystals. The TGA results indicate a
dispersion capacity of 36 wt% of CuCl onto NaX. The
single component isotherms of ethane and ethylene show
that the CuCl dispersed zeolite has the preferential adsorption of ethylene, which is ascribed to the strong interaction
with CuCl via a p-complex. The breakthrough experiments
show a selective removal of the olefin from ethane and
ethylene mixture streams, demonstrating the applicability
of the CuCl/NaX for the separation of ethylene/ethane
mixtures, which would allow more sustainable processes.
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The manuscript was received 30 September 2005 and accepted for
publication after revision 25 January 2006.
Trans IChemE, Part A, Chemical Engineering Research and Design, 2006, 84(A5): 350– 354