Oxygen separation through hydroxide-conductive membrane

Details Oxygen separation through hydroxide-conductive membrane Oxygen separation through hydroxide-conductive membrane

2001-04-17

2004-07-27

Ryan, Patrick (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S010000, C429S303000, C521S027000

Reexamination Certificate

active

06767663

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of gas separation, and more particularly to gas separation using a hydroxide ion conductive membrane.
2. Description of the Prior Art
There is an ever-increasing need for improved systems and methods for efficient and rapid separation of selective components from mixtures. In particular, techniques for the separation of selective components from gaseous mixtures have many significant technical applications. Many important chemical, environmental, medical and electronics processing technologies require pure gas, particularly pure oxygen gas. For example, oxygen is used in semiconductor fabrication for chemical vapor deposition, reactive sputtering and reactive ion etching. It finds wide application in health services, for resuscitation, or, in combination with other chemicals, for anesthesia. Oxygen can also be used to achieve environmental benefits by reducing the sulfur emissions of oil refineries and helping pulp and paper manufacturers meet regulations relating to bleaching, delignification and lime kiln enrichment. However, the high cost of pure oxygen typically limits the wide adoption of such beneficial processes in the chemical, electronics and medical industries. Further, nitrogen is used, for example, to protect perishable goods, to protect oxygen-sensitive materials and to facilitate oxygen sensitive processes.
High-purity oxygen (e.g., 99.5%+) is largely produced in cryogenic air separation plants, where the air is cooled down to the melting point of nitrogen (−210° C.) and its components separated in large condensation columns. This process requires expensive, bulky equipment and high-energy consumption, which tends to militate against the use of oxygen to generate energy. Cryogenic air separation is typically commercially feasible only on large scale generation systems.
Adsorption, both pressure-swing absorption (PSA) and vacuum-swing absorption (VSA), is widely used for producing moderate purity oxygen (e.g., 85-90%). The gas separation is generally based on selective adsorption of nitrogen by synthetic zeolites. Nitrogen, which is more readily polarizable than oxygen, interacts more strongly with the electrostatic fields in the zeolites structure. Nitrogen is thus retained by the zeolites, while oxygen passes through. When the zeolite is saturated with nitrogen, it must be stripped, generally by reducing the pressure.
Polymer membrane air separation has also been used to produce air slightly enriched in oxygen (e.g., 28-35%) or for nitrogen enriched blanketing (e.g., 95-99%). Polymeric air separation membranes are typically selectively permeable, whereby the polymeric materials are more permeable to oxygen than nitrogen. Transport through the membrane is induced by maintaining the vapor pressure on the permeate side of the membrane lower than the vapor pressure of the feed mixture. The driving force is typically the difference in partial vapor pressure of each species across the membrane.
One typical configuration includes a system of hollow fibers, wherein air is passed over the exterior of the fibers, and enriched oxygen permeates to the interior of the fibers. Another typical configuration used a spiral wound system, wherein air is passed through an edge of a configuration of feed channels, membranes and a permeate channel wound about a porous tube. The permeate exits the system from the porous tube. Production of relatively high purity oxygen with air separation based on selective permeability is not commercially feasible, and is only suitable for applications where only oxygen-enriched air is required.
The permeability of a membrane for a particular gas species is related to the volumetric flow (cubic centimeters (at standard pressure and temperature) per second (cc(STP)/s)) of the desired species through a unit length of membrane thickness (in centimeters) per area of membrane (in centimeters squared) per pressure differential (expressed in centimeters of mercury). The permeability is generally expressed in Barrer units, where:
1
⁢
 
⁢
Barrer
=
1
×
10
-
10
⁡
[
cc
⁡
(
STP
)
·
cm
cm
2
·
s
·
cm
⁢
 
⁢
Hg
]
.
(
1
)
Additionally, selectivity is critical to effective permeability based gas separation, such that a desired species permeates at a greater rate than another species. Selectivity (alpha) is generally expressed as:
α
A
,
B
=
Permability
A
Permability
B
.
(
2
)
A relatively new technology that has emerged for relatively high purity gas separation involves selective membranes which pass only the desired components, such as described by H. J. M. Bouwmeester, A. J. Burggraaf, in “The CRC Handbook of Solid State Electrochemistry,” Ed. P. J. Gellings, H. J. M. Bouwmeester, chapter 11, CRC Press, Boca Raton, 1997, which is incorporated by reference herein. Various known membranes include ionic conducting membranes and mixed ionic-electronic conducting membranes, which rely on the transportation of oxide (O
2−
) ions to separate the oxygen from air. Although these approaches may offer some advantages relative to cryogenic oxygen separation, practical application of the MIEC membrane is hindered by a number of drawbacks intrinsic to oxide (O
2−
) conductive membranes. These problems include: low oxygen throughput (typically caused by both low ionic conductivity and low surface oxygen exchange rate); relatively high operating temperature (>800° C.); costly materials and costly fabrication; tendency to degrade over time; and system equipment that is relatively complex and expensive to build and maintain.
Turning now to
FIG. 1
a
, a general process that occurs in a conventional ionic-electronic membrane
10
is shown. Air arrives at a cathode
12
, where oxygen is reduced, but other species do not react. The oxygen is shuttled across a membrane
10
in the form of an ion such as O
2−
(the process of ionic conduction). At an anode
14
, a complementary chemical reaction evolves pure oxygen, which is released. Functionally, this type of membrane
10
has three primary components: a backbone
16
, which provides the membrane's structure; an ionic conductor (not shown), which conducts the ions across the membrane; and a catalyst (not shown), which aids the reduction of oxygen at the cathode
12
and the evolution of oxygen at the anode
14
.
The overall oxygen throughput is determined primarily by two parameters (the conductivity of the electrons is generally too fast to be a limiting factor). The first is the ionic conductivity (how fast the ions can travel across the membrane), which is dependent on the electrolyte properties. The second is the surface oxygen exchange rate (how quickly the oxygen is reduced and evolved on each side), which is dependent on both the ionic conductor and the catalyst properties of the electrodes.
Referring now to
FIG. 1
b
, prior art mixed ionic-electronic conducting membranes
110
typically utilize only a single component, i.e., a ceramic-noble metal composite, to play all three functional roles, namely, backbone, ion conduction and catalyst. This material provides the physical membrane structure
116
, reduces and evolves the oxygen, and relays the O
2−
ions and charge-compensating electrons in opposite directions as shown. To drive the reaction, a pressure differential is required, with the air pressure at the cathode
112
exceeding the oxygen pressure at the anode
114
, i.e., by a factor of about 2 to 10.
As shown in
FIG. 1
c
, a similar approach known as “active oxygen pumping” utilizes a membrane
110
in combination with an external circuit
118
, instead of a pressure differential, to drive the reaction.
As mentioned hereinabove, there are several disadvantages associated with the current ceramic-based membranes (ionic conducting and mixed ionic-electronic conducting) approach, which are intrinsic to oxide-conducting membranes.
One such disadvantage is their relatively high operating temperatures. The chemistry of the ce

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