hybrid inorganic membrane technology shows promise in gas processing
TRANSCRIPT
Hybrid inorganic membrane technology shows promise in gas processing By Susanne Olsen, Bhavana Tripathi, Helen Mill* and Edward Gobina, Centre for Process Integration and Membrane Technology, School of Engineering, The Robert Gordon University, Aberdeen, Scotland, UK
This paper examines how the use of novel hybrid inorganic membrane technology is enabling natural gas processing and the production of high density hydrogen without any impurities.
Operators in the hydrocarbon industry (HPI)
will increasingly require flexibility in the control
over key parameters such as reaction
temperature, pressure and flow rate as the
chemical reaction proceeds. Downstream
requirements also mean that separation costs
should be kept as low as possible.
By having the capability to manage key
operational variables such as temperature, and
simultaneously minimize downstream separation
costs, petrochemical manufacturers and gas
processing operators can optimize other
processing variables, thus increasing productivity
and minimizing wastes and by-products.
Exponential growth In common with most modern technology
fields, the development of inorganic membranes
is currently witnessing exponential growth.
Inorganic membrane technology offers great
opportunities to those companies and
individuals who can find innovative ways of
using new materials, and new preparation
technologies to create the designer membranes
of the future.
Because of their size, the mobility of the
gaseous specie is strongly affected by molecular
collisions. Each molecule of gas then has a
random component to its motion as well as
macroscopic motion of the gas stream. This
*Research Commercialization Unit.
makes it much more likely for the gas molecule
to collide with the membrane surface even when
the macroscopic flow is parallel to that surface.
Materials matter An example would be the use of a membrane in
which the density changes with depth. The low
density pores of the more coarse support are
quite effective in maintaining strength. Since
more selective membranes invariably cost more,
ir is important to compare the economics,
maintenance needs and efficiencies of using a
single, non-complex material as opposed to
separate multiple components that have
individual maintenance and efficiency cycles.
The materials that can be used to make
inorganic membranes also include a new
dimension to separation design. Manufacturing
capabilities have improved to allow membrane
preparation from these new materials.
Alternatively, they can be combined with
conventional materials to provide multilayered
membranes with enhanced properties.
Limited application The wider application of hybrid inorganic
membranes has been limited by high cost, and,
in dense metallic systems, the relatively low
permeability of monolithic systems. These
barriers have therefore restricted applications to
very specialized markets where a particular
product is required at exceptionally high purity.
In order to expand the technology base of
inorganic hybrid membranes, the cost and
permeability must be drastically altered to the
advantage of the end-user.
Step-change To produce this step-change in gas processing,
the Cencre for Process Integration and
Membrane Technology (CPIMT) at Robert
@ Membrane Technology June 2002
Gordon University, UK, established a research
and development initiative with funding from
the Scottish Enterprise Proof of Concept. The
main goal of the projects being studied is to
advance the technology base of inorganic hybrid
membrane technology by increasing
productivity and reducing costs, and thus
facilitate commercialization.
Two approaches have been devised to achieve
this. One aspect involves the preparation and
testing of hybrid inorganic membranes for
natural gas processing, while the other involves
the preparation, characterization and testing of
all-metallic hybrid membranes for high density
hydrogen production at 100% purity.
Market opportunities for hybrid inorganic membranes Natural gas processing
Low quality natural gas upgrading research can
have a significant impact on the liquid fuel
market by increasing the use of natural gas where
liquid fuels are currently used such as in
transportation.
Natural gas for industrial and power plant
energy needs will reduce gas emissions, enabling
the UK to meet international agreements and
targets on climate change.
In the offshore industry, hybrid inorganic
membranes will be useful in enabling early
development of marginal fields, reducing
inventory on the platform by separating natural
gas from inert cushion gas in gas storage
operations, and upgrading gas to meet specific
quality requirements for end-use applications.
Gasification technologies aimed at producing
synthetic natural gas will benefit significantly
from these hybrid membranes. In the UK about
80% of CO, emissions come from stationary
sources, such as thermal power plants, therefore
the availability of suitable technologies to
separate, recover and possibly re-use CO, have
recently been identified as necessary steps to
reduce global warming - the causes and effects of
which are now well documented.
In landfill sites, hybrid ceramic membranes
will be useful in upgrading landfill gas to
pipeline quality and eliminating the current
practice of flaring and venting that is damaging
to the environment.
Hydrogen purification
The demand for hydrogen at 100% purity
is growing at a rare of 7.6% per year. This is
likely to increase with the development of fuel
cells, which could make hydrogen the fuel of
the future, and with the upgrading of fuels
required to meet increasingly more demanding
environmental standards.
However, the lack of a cost-effective technique
to ‘clean’ bulk quantities of readily available
hydrogen at 99% purity is a limiting factor. In
addition, contaminated gas may reduce the
service life of the fuel cell. As a result, the search
for a cost-effective source of ultrapure hydrogen
could play a key part in the future development
of fuel cell technology.
Ultrapure hydrogen is required for
manufacturing processes across a range of
industries. In particular, the microelectronics
industry requires hydrogen with purity in excess
of 99.99999%, which is costly to achieve using
existing technologies.
Hybrid inorganic membranes for high-density
hydrogen production are a key technology
component to enable more widespread hydrogen
use, either in clean power generation or as a
primary chemical feedstock.
Membrane reactors Membrane reactors combine the functions of
chemical reaction and separation within a single
unit.
Because of this unique attribute, these reactors
have been considered for a wide range of
chemical reactions such as dehydrogenations,
which are equilibrium limited,“‘*’ catalytic
selective oxidations, where the control of the
oxidant concentration is very important,[gx41 and
in reaction coupling processes where the
utilization of the permeable species to produce
another important product can be thermo-
dynamically beneficial.[5z”l
Independent economic analyses of hydrogen
permselective membrane reactors as an alternative
to traditional, non-membrane processes include
methane steam reforming”] and the water-gas
shift reaction.“’ In both studies, results indicate
good returns if membrane productivity increases
Membrane Technology June 2002
can be realized. Uniquely, it is the reduction of the
number of chemical reaction steps that leads to
the significant capital cost savings.
Fuel cell markets
Hybrid inorganic membranes developed for high
density, pure hydrogen may be applied to a wide
range of applications across the technology
spectrum where ultrahigh purity hydrogen or
high efficiency hydrogen separation/recovery is
required.
Among these applications are semiconductors
processing, commercial gas purification, metals
processing, chemical and polymer synthesis, and
environmental remediation including denitrifi-
cation of drinking water supplies.
However, the technology of increasing impor-
tance is the proton exchange membrane fuel cell
(PEMFC). In the PEMFC, hydrogen and oxygen
are reacted to generate electricity with pure water
as the by product. While the PEMFC is capable of
producing high power densities, high chemical
energy to electrical energy conversion, and fast
and easy start-up in cars for example, a major
drawback is that the anode of the fuel cell is easily
poisoned by the presence of carbon monoxide
even as low as in the part per million (ppm) range
within the hydrogen stream.
As a result, either an ultrahigh purity hydrogen
supply is needed or a lower purity gas supply must
pass through shift and partial oxidation reactors
(POXD) to effectively oxidize the carbon
monoxide to carbon dioxide. The use of a hybrid
inorganic membrane separation system capable of
producing high density hydrogen at 100% can
replace the need for the shift and POXD because
the hybrid would exclude any carbon monoxide
and only allow hydrogen to pass.
Hybrid inorganic membrane technology Tailored pore size hybrid membrane In this study, the repeated dip-coating technique
has been used to apply a thin continuous gas
separating layer on the outside surface of a coarse
ceramic support tube (pore size - 0.4 pm; o.d. =
1 mm; and i.d. = 7 mm).
The hybrid composite membrane has been
characterized by scanning electron microscopy
(SEM) (Figure l), nitrogen adsorption and X-
ray diffraction (XRD).
Pure and mixed gas transport charac-teristics
of the membrane have also been studied. The
method of resistance combination has been used
to estimate the separation layer thickness and the
estimated value of 1.6 km agrees with the actual
value of 1.62 Frn observed by SEM.
Chemically modified hybrid membrane
For these hybrids, a chemical MgO modification
was conducted after a pore size modification
with gamma alumina.
The MgO modification also involved a dip
coating technique, and the amount of MgO was
adjusted to achieve a surface concentration of 4
mmols of magnesium per square metre of
membrane surface area. A scanning electrode
micrograph of this type of hybrid membrane is
shown in Figure 2.
Metallic composite hybrid membrane The technology is based on an enhanced
multi-metallic composite membrane, which
appears to overcome the life cycle limitations
of existing membranes for high throughput
hydrogen recovery.
The project examines the preparation and
characterization testing of the membrane,
including the design of a hydrogen reactor that
will permit on-site processing of ultrapure
hydrogen at moderate temperatures, using only a
tiny amount of precious metal.
l Knudsen Flow
l Surface Flow
mol - m’s 1
Viscous flow gas flux Knudsen flow gas flux Membrane pore size Absolute pressure Atmospheric pressure Molar gas constant Avogadro number (6.023~10’“) Porosity Viscosity Tortuosity factor Molecular weight Membrane wall thickness Surface area adsorbed molecule Atmospheric temperature Permeability of surface diffusion Differential surface coverage
Membrane Technology June 2002
n-Paraffins
mparaffins recycle
Net hydrogen
t Benzene
I (
Detergent _ t Dehydrogenation -b Hydrogenation + a,ky,ation
mOlefins mParaff ins Diolefins
mOlefins n-Paraffins
J LAB
The project also investigates the
requirements for a range of target markets with
a view to developing appropriate configurations
to suit the feed gas, flow rate and purity
requirements of each. Ultimately, the aim of
the project is to produce a reactor design that is
cost-effective, is easy to maintain, and flexible
to operate.
The factors that are dominant in the
diffusivity and permeability of hydrogen in
these membranes are examined. The
minimum thickness of platinum and
palladium required to provide improved
hydrogen fluxes will be experimentally
determined, together with the maximum
thickness of the support metal that is required
for optimum hydrogen flow.
The effect of prolonged usage of the
composite membrane will be evaluated by
running the membrane continuously for over
100 hours at a temperature of 300°C to yield
data of hydrogen flow against time. Any
improvement in the crystallinity will be observed
by X-ray diffraction results.
Development modelling Confirming the benefits of hybrid membrane
technology for a given process requires exploring
technical and economic benefits with a range of
tools and exercises - from mathematical
modelling to commercial demonstration.
Before the demonstration membrane reactor
unit used in the study was designed, extensive
development work was required to set design
parameters.
Characterization of fundamental transport mechanisms The primary mechanisms for gas separations in
microporous membranes include:
separation based on the differences in
molecular weight of the components as
described by Equation 2 - Knudsen
diffusion through pores (see box);
separation based on molecular size (molecular
sieving through molecule sized pores);
separation based on surface flow, where one
of the components to be separated adsorbs
preferentially onto the pore walls described
by Equation 3; and
capillary condensation where a species
selectively condenses in the pores and is
preferentially transported.[9l
Separations based on the surface flow concept can
offer a number of advantages for gas separation.“‘]
These advantages include the following:
. the adsorption of a more strongly inter-
acting component can be high even if its
partial pressure in the gas mixture is low
l larger molecules can be selectively adsorbed
and preferentially transported through the
membrane while largely excluding smaller
molecules; and . molecules of similar sizes can be separated if
one molecule specifically adsorbs on the
pore wall.
Mathematical modelling Several models have been developed and
incorporated into the overall design tool. Using
the model, for example, it is possible to predict
the product distribution in the permeate and
retentate streams for a given membrane area.
Based on physical modelling, a geometry for
the internals, such as the membrane area, is
selected and simulation results are generated for
that area. Such plots are presented in Figures 3
and 4, respectively. In these plots X0, X, YP
represent the molar fractions of carbon dioxide
in the retentate, feed and permeate streams,
respectively.
S, represents the normalized surface area of
the hybrid membrane. These are plotted against
the stage cut which is the ratio of the flow rate of
the permeate divided by that of the feed. The
hydrocarbon recovery and carbon dioxide
selectivity of the membrane compare very well
with the experimental values.
Commercial unit Once the conceptual design was completed, the
demonstration unit was designed as a portion of
the commercial unit to ensure scalability.
A schematic of the demonstration unit is
shown in Figure 5. All critical parameters
between demonstration and commercial units
are maintained. Commercial demonstration will
be done to:
l verify feasibility of the inorganic hybrid
membrane concept in gas processing;
l demonstrate the ability to handle a wide
range of concentrations; . confirm mechanical integrity of membrane
structure; . confirm heat and mass transfer
characteristics; and . explore process variables effect on
membrane performance.
Case studies Endothermic reaction The dehydrogenation of paraffins to
monoolefins is a good fit when exploring
membrane reactor applications. The process
requires a low pressure drop and ease of catalyst
loading and unloading, and is equilibrium
limited.
The typical installation includes a dehydro-
genation unit, a hydrogenation step for any
diolefins formed, and an alkylation unit that
combines the paraffins unit with benzene
to form linear alkylbenzene (LAB) for
sulphonation to produce detergents (Figure 6).
A conventional dehydrogenation process
contains an adiabatic radial flow reactor. Since
the reaction is endothermic and equilibrium
limited, the all-metal hybrid inorganic
membrane (with its capability of removing
hydrogen at the reaction temperature)
simplifies the process because it eliminates a
recycle stream and a costly separation process.
Energy consumption is reduced along with
equipment size.
Exothermic reaction The selective oxidation of vapour phase
hydrocarbons over catalysts, to produce
epoxides, anhydrides, carboxylic acids and other
MembraneTechnology June 2002
products, is another good fit for membrane Also, any propensity for a temperature runaway
reactor applications (Figure 7). reaction is minimized at the lower reaction
These processes benefit from a low pressure
drop and enhanced heat removal capability.
Although conventional vapour phase oxidation
reactor technology is predominantly tubular,
some operators exploit fluidized bed technology
for oxidation applications. For these exothermic
reactions, the maximum allowable temperature
is usually limiting.
temperatures.
The complete oxidation of hydrocarbons
to carbon dioxide and water is an example
in which an undesirable reaction forms by-
products. Better catalyst usage can be achieved
because higher average bed temperatures
can be maintained with lower maximum
temperatures.
By using a hybrid inorganic membrane
with a tailored pore geometry design, the
amount of oxygen supplied into the
reaction zone can be stoichiometrically
controlled, enabling a reduction in the reac-
tor’s maximum temperature, thus decreasing
the rate of sequential reactions that convert
desired product into undesired by-products.
Natural gas processing and advanced power generation One of the most promising features of
hybrid inorganic membranes is to selectively
r
Ceramic support (alumina/Stainless steel/
Separation layer (gamma alumina-molecular sieve/ alumina-silica)
C)
permeate carbon dioxide from natural gas while
retaining the high pressure methane to facilitate
transportation by pipeline and also reduce the
cost of processing (as shown in Figure 8).
Using a mixed gas stream of methane and
carbon dioxide it is possible to achieve more
than 95% recovery of the methane from the raw
gas. Such high recovery rates are economical for
incorporating hybrid inorganic membranes to
enhance productivity in monodiameter wells
and reduce the costs of facility automation
techniques, multilateral wells and under-
balanced drilling.
In offshore natural gas liquefaction, inorganic
hybrid membranes can also be integrated with
molecular sieve adsorption systems that are used
to remove carbon dioxide from the natural gas
prior to liquefaction, enabling favourable
marginal field economics.
Sweep gas It is inreresting to note that in the experimental
tests, a sweep gas of nitrogen was used in the
bore of the membrane tube to strip the
permeated carbon dioxide.
While the nitrogen served as a diluent to
increase the driving force for the carbon dioxide,
analysis of the retentate did not reveal any
measurable quantities of nitrogen in the retentate,
indicating that nitrogen does not permeate the
membrane to any significant degree. This has
significant implications in advanced power
generation involving the use of fossil fuels.
Hybrid inorganic membranes can be used to
capture carbon dioxide in power plants as a
result of their high permeability for carbon
Membrane Technology June 2002
(a)
Air inject Air bleed
Methane --) Reformer + feed
Shift 1 Prox 1 Fuel z reactor(s) reactor cell
Burner
p
Anode off-gas
Combustion air
@I
Combustion air
dioxide and low selectivity for nitrogen,
coupled with the fact that they can operate
effectively at high temperatures. The extent to
which the present cost of these hybrids could
be reduced is not clear.
One attraction is that they utilize less energy for
operanon than other methods. Another attraction
is that they can be used as gas absorption
membranes in conjunction with a solvent, which
helps enhance the transfer of carbon dioxide
through the membrane and strips it faster.
A recent market study[“l has shown that
carbon dioxide concentrations in the 4-15%
range are most important either for natural gas
processing or in capture projects involving
advanced power generation projects. Our main
focus currently is to carry out extensive
experimental tests.
New energy production systems Development of reformer technology and
experimental work on reforming natural gas is
one of the research areas at the CPIMT.
The aim is to produce a hydrogen-rich gas
mixture as the feed for the PEMFC. The
development of an all-metallic hybrid
inorganic membrane will greatly simplify the
process for integration into propulsion systems
for zero and ultra-low emission vehicles as
shown in Figure 9.
Conclusions Hybrid inorganic membranes have been prepared
and tested for gas processing applications.
The two-tiered approach will enable the
eflicient processing of natural gas on one hand,
and allow the production of high density
hydrogen, at 100% purity on the other.
As the chemical and process industries
improve standards to reduce waste and increase
productivity, hybrid inorganic membrane
designs will cut costs and the time needed for
chemical transformations, improving and
facilitating yield and selectivity.
Computer modelling and interdisciplinary
teamwork involving chemists, chemical
engineers and metallurgists is helping to show
the possibilities.
Acknowledgements The CPIMT would like to express gratitude to
the Scottish Enterprise Proof of Concept Fund
(Rounds 2 and 3) for their continued support.
We wish to thank all the scientists at the
Centre for Materials Surfaces and Interfaces
Group for the characterization of the
membranes.
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J Membrane Sri. 105 163-176.
2. Yildrim, Y., Gobina, E. and Hughes, R.
(1997) J Membrane Sci. 135 107-l 15.
3. Aasberg-Peterson, K., US Patent US 6 338
833 Bl (15 January 2002).
4. Gobina, E. (2000) Membrane Technology 125
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5. Gobina, E. and Hughes, R. (1996) Chem.
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6. Gryaznov, V.M. (1992) P/at. Met. Rev. 36
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7. Aasberg-Peterson, K. (1998) et al., CataLysis
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For more information, contact: Edward Gobina,
Centre for Process Integration and Membrane
Technology, School of Engineering, The Robert Gordon
University, Aberdeen, Scotland ABlO 1 FR, UK. Tel: +44
1224 262348, Fax: +44 1224 262444, Email:
Membrane Technology June 2002 0