高分子概論
Gas separation by
membrane
班級:化材三乙
姓名: 余成航
學號:49940056
一、 原理說明
Gas separation by polymeric membranes is oftenperformed in single-stage
cells. The
relation ofsuch a configuration is motivated by its simple operation
and control [ l-31. Large separation cells with membrane areas of 1 m2 have
been used in order to obtain higher production rates. Depending upon the
separation to be achieved, such systems may be operated in either the
concurrent or countercurrent mode [4,5]. Further demand for lower production
costs and higher product purity has prompted the use of two-stage systems for
gas separation [68]. The second stage is applied to process the higher
pressure stream (reject from the first stage). The final permeate product is
constituted by mixing the permeate stream from the first and second stages.
Modifications of the serial configuration of the two-stage system can be
achieved through proper selection of the feed location, recycling of
the permeate stream, and withdrawal of side streams from the reject or
permeate streams. Such modifications are analogous to those applied in
distillation columns [ 9, lo]. Two original configurations of the two-stage system
are the continuous membrane column (CMC) and the two-strippersin- series
permeator (TSSP). In a previous study, Bhide and Stem [ 111 analyzed
the separation characteristics for various module configurations in membrane
separation. These configurations include the single-stage permeator, the
conventional two-stage systems, CMC and TSSP, as well as others.
Comparisons have been made by McCandless [ 51 for four single-stage
separation configurations including cocurrent flow, countercurrent flow,
complete mixing and cross flow. These comparisons were based on the overall,
heads, tails, and Sandell separation factors as well as the Rony extent of
separation. His results indicate that comparisons can be made based on any
of the above criteria as long as all the systems are operated at the same stage
cut. McCandless’ results [5] show that a selection of the best operating
conditions for a specific configuration at different stage cuts is only possible
through the use of Rony’s extent of separation. Additional analysis was
presented by McCandless [7] on separation configurations with recycle
streams. The configurations include CMC, TSSP and single-stage units with
permeate recycle operating at high or low pressure feeds. The results
demonstrate that the four systems are similar at low recycle ratios. However,
the performance of the CMC and the TSSP are superior to those of a
single-stage system at higher recycle ratios. Under such conditions, the
required membrane area of the two-stage systems was lower and the
separation characteristics were better. Extensive mathematical analysis was
later performed by Kao et al. [9] for the CMC and TSSP systems. Their results
reveal substantial differences between these two systems. The CMC case was
found to operate better than the TSSP system at lower stage cuts and higher
permeate enrichment. Further economic analysis performed by Qiu et al. [lo]
shows that the CMC system was better forseparations where the permeate
was the desired product. The opposite was found to hold for the TSSP system.
Pettersen and Lien [ 121 used a robust design model developed by
themselves [ 131 to examine seven separation configurations. These systems
included single- and two-stage separation modules with recycling of either the
retentate or the permeate streams as well as using the bypass stream to
control the purity of the final product. Their results show that retentate
recycling was essential to the improvement of recovery and product purity. On
the other hand, permeate recycling was found to reduce considerably the
compressor duty, which in turn should improve the economics of the
process. Other studies reported in the literature include a comparison of CO2
removal from natural gas by membranes and by conventional diethanol amine
absorption. The cost of applying a singlestage arrangement was found to vary
considerably with the CO2 content in the feed stream [2]. Accordingly, it was
concluded that proper selection and combination of a single-stage membrane
system and an amine unit may provide the optimum operating configuration
(i.e. the lowest production cost). However, the use of more sophisticated
membrane systems, including a threestage arrangement with permeate
recycling, was found to be superior to the stand-alone amine
system [ 141. A study performed by Tsuru and Hwang [ 151 considered the
use of a combination of a membrane system with a pressure swing adsorption
unit for oxygen production. The membrane configurations used in the analysis
were a conventional CMC system as well as a modified configuration in the
TSSP mode. The feed stream was obtained from the PSA unit. Their results
show that higher oxygen purity was obtained at lower feed flow, rates, higher
pressures, lower stage cuts and a longer enricher section. Also, using the
modified CMC configuration gave better separation efficiency and higher
oxygen purity. The modified CMC configuration proposed by Tsuru and Hwang
[ 151 is similar to that recently proposed by Agrawal [ 161. In such a
configuration, the permeate from the first enriching stage is not fed to the next
stage. Instead, it is mixed with the permeate from the second enriching stage.
Consequently, a higher separation efficiency is obtained. In contrast to the
relatively large number of numerical analyses and calculations on the optimum
conditions of membrane separation operation, there is a scarcity of relevant
experimental data, especially data involving gas mixtures with more than two
species. In an effort to rectify this deficiency, the present study was undertaken.
The objective of the present work is to delineate and establish the relationships
between the various operating variables (pressure, flow rate, composition and
flow mode) and the separation characteristics (stage cut, species permeance
and separation factors). The experimental work performed, the results
obtained and their interpretation are described in what follows.
二、 應用/用途
Water separation
The removal of water from raw natural gas is extremely common and is usually
performed close to the wellhead to avoid hydrate formation and corrosion in
the downstream piping. The most widely used technology for water separation
is through absorption by diethylene or triethylene glycol [2], which is
anoperationally simple and very low cost approach. Membranes can be used
to remove water, with many glassy polymers showing very high selectivities for
water, generally orders of magnitude greater than both CO2 and CH4 (Fig. 7).
It has been estimated that a simple economical membrane system could lower
the dew point of the raw gas by 30–50 _C [130]. This approach has been
commercialized by Air Products, using their Prism_ polysulfone membrane
materials.
Nitrogen separation
Significant volumes of natural gas are considered low quality because of the
large nitrogen content, generally well above the <4% pipeline specification.
Removal of this nitrogen could allow access to an estimated 10 trillion scf
additional natural gas in the USA alone [9]. To date N2 separation has been
achieved through cryogenic distillation, which also has the added benefit of
removing many of the heavier hydrocarbons. However, cryogenic natural
gas-nitrogen separation plants are capital intensive, and must be very large to
take advantage of economy of scale. Hence, this approach is more suited to
large fields that can deliver 50–500 million scf per day [138]. Membrane-based
nitrogen separation has a potential market in smaller natural gas operations,
where cryogenic distillation is uneconomical. Lokhandwala et al. [138] indicate
that twelve membrane-based nitrogen removal systems had been installed by
2010 using this approach. For this separation, membranes can either be N2
permeable or CH4 permeable. Typically glassy polymers are nitrogen
permeable, while rubbery polymers are methane permeable (see Table 13).
For a 10% N2 in CH4 mixture, Baker [2] has shown that a membrane requires
a CH4/N2 selectivity of 6, or a N2/CH4 selectivity of 17 to achieve the same
level of separation in a single stage. However, there are no membranes that
can achieve this level of selectivity. To overcome this Dow Chemical [139] has
utilized low temperature operation of their glassy membranes to increase the
selectivity. However, temperatures low enough to liquefy methane are
necessary to achieve the necessary selectivity which defeats the purpose of
the process. There is considerable interest in perfluoropolymers in this
application, given their unique solubility properties, and resilience to
hydrocarbons. However, a CH4-permeable rubbery membrane process
remains the most feasible, though it requires considerable recompression of
the permeate gas for delivery to the pipeline [138]. Baker [2] has stated that
the cost of recompression is not high enough to significantly impact on the
process economics. Indeed, Lokhandwala et al. [138,140] show that the
process economics can be highly favorable, relative to both process swing
adsorption and cryogenic distillation (Table 14). To achieve the level of
separation required, considerable recycling between multiple membrane units
can be required which is heavily influenced by membrane selectivity, with a
number of configurations patented [141,142].
Helium separation
Natural gas is a considerable source of helium, especially in locations in the
USA. A large installation has also recently been installed in northern Australia,
which will produce 900 tonnes a year of helium from nitrogen reject gases of
an LNG facility [145]. Currently, helium is removed with nitrogen by cryogenic
distillation [2] and isolated in a separate cryogenic process. However, given
the marginal economics of cryogenic distillation, membrane separation for
helium has been considered since the 1960s [146], with cellulose acetate the
first material trialed [147]. The permeability and selectivity of a range of
polymeric membranes to helium is provided in Table 15 and the Robeson’s
plot in Fig. 9. A significant problem is that the majority of reservoirs have very
low He concentrations, and therefore to achieve good separation of He with
minimal CH4 loss, very high He selectivies are required (1000 against CH4) [7].
Alternatively multi-membrane cascade processes [148,149] or hybrid systems
with other separation technologies [150–153] have been considered. These
processes have limitations, such as the considerable energy penalty because
of permeate recompression.
三、 參考文獻
1. 作者: Colin A. Scholes, Geoff W. Stevens, Sandra E. Kentish
2. 文獻名稱: Membrane gas separation applications in natural gas processing
3. 年代: June 2012
1. 作者 Hisham M. Ettouney, Usman Majeed
2. 文獻名稱;Effect of cell configuration on separation characteristics of gas
mixtures by polymeric membranes
3. 年份 2 June 1997
1. 作者 M. Le Digabel, D. Ducret, C. Laquerbe, P. Perriat, J.-C. Niepce
2. 參考文獻Application of gas separation membranes to detritiation
systems
3. 年份 10 September 2002