Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Molecular oxygen or ozone component
Reexamination Certificate
2001-06-04
2003-09-23
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Molecular oxygen or ozone component
C060S039120, C060S772000, C060S774000, C060S801000, C095S045000, C095S054000, C095S288000
Reexamination Certificate
active
06623714
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of separating oxygen from an oxygen containing gas with the use of an ceramic membrane unit. More particularly, the present invention relates to such a method in which the oxygen containing gas is compressed by a compressor powered by the expansion of a cooled process stream made up at least in part by a retentate formed in the ceramic membrane unit. Even more particularly, the present invention relates to such a method in which the expansion of the process stream is carried out in stages with interstage heating.
BACKGROUND OF THE INVENTION
Oxygen transport membranes have demonstrated an ability to separate high-purity oxygen from an oxygen containing stream with a purity of at least about 99% and with an oxygen recovery of about 60%. Such oxygen transport membranes are formed from a ceramic that is capable of transporting oxygen ions when both heated to a suitable operational temperature and the opposite sides of the membrane are subjected to an oxygen partial pressure differential. The oxygen ions are formed by oxygen atoms in an oxygen containing feed gaining two electrons at one surface of the membrane. The oxygen is reconstituted at the opposite surface of the membrane by the loss of the electrons from the oxygen ions thus to complete the separation of the oxygen from the feed. Typically, multiple oxygen transport membranes are housed in a ceramic membrane unit that functions to separate oxygen from the oxygen containing feed to produce both an oxygen permeate from the separated oxygen and a retentate from the feed after the separation of oxygen therefrom.
Suitable oxygen transport membrane materials are known as either mixed conducting or ionic. Mixed conducting materials conduct both the oxygen ions and the electrons that are formed upon reconstitution of elemental oxygen from the oxygen ions. Ceramic mixed conducting materials include but are not limited to perovskites. Ionic materials conduct only oxygen ions and thus require an external electric circuit for the return of the electrons. Common materials used in an ionic membrane include, but are not limited to, Yttrium Stabilized Zirconia.
In order to compress the oxygen containing feed, for instance, air, the feed is compressed in a compressor that is powered at least in part by the work extracted from a turboexpander. Typically, a retentate stream, composed of a retentate formed upon separation of the oxygen within the ceramic membrane unit, is expanded in the turboexpander. An example of this is shown in U.S. Pat. No. 5,516,359 in which feed air is compressed and then heated in a direct-fired burner. The resultant heated feed gas is introduced into the ceramic membrane unit to separate oxygen from the feed. The retentate is heated by another direct fired burner prior to its introduction into the turboexpander. The turboexpander is used to power the compressor.
U.S. Pat. No. 5,643,354 discloses an integrated process in which oxygen is recovered from an oxygen-containing feed gas and subsequently is consumed in a coal gasifier. The hot oxygen product exiting the ceramic membrane unit is cooled through indirect heat exchange with water and an expander is used to recover the work needed to drive the feed gas compressor.
“Ion Transport Membrane Technology for Oxygen Separation and Syngas Production”, 134 Solid State Ionics, Dyer et al. pp 21-33 (2000), discloses a process in which a hot, low-pressure oxygen product gas, produced by a ceramic membrane unit, is cooled by indirect heat exchange with a cooling medium. After being cooled, the oxygen is compressed to a final delivery pressure. Fuel is used both to heat the feed gas to the desired inlet temperature of the oxygen transport membranes contained in the ceramic membrane unit and to heat the non permeate (i.e., retentate) to the desired inlet temperature to the expander. The work recovered from the expander is used to drive both the feed gas compressor and an oxygen product blower or compressor.
“Advanced Oxygen Separation Membranes”, Report No. TDA-GRI-90/0303, Wright et al., The Gas Research Institute, pp 33-61 (1990), illustrates various schemes for integrating ceramic membrane units with electrical generation systems. In one such integration, feed air is compressed and heated by combustion supported by oxygen contained in a retentate stream that is produced in a ceramic membrane unit. The retentate stream is then fed into a turboexpander that is used to drive a feed air compressor.
U.S. Pat. No. 5,753,007 discloses a process for oxygen recovery from an oxygen-containing feed gas by the use of a ceramic membrane unit in which the retentate stream is cooled and then expanded to recover useful work. In this patent, the degree of cooling is sufficiently high that the work can be extracted for the use of processes that are less energetic than those in which electrical power also is generated. The feed gas can be heated through indirect heat exchange with both the retentate and oxygen product streams. Additionally, the feed stream may be heated further by a combustor interposed prior to the ceramic membrane unit.
An important consideration in the fabrication of any equipment that is used to separate oxygen is its cost. The cost of acquiring a turboexpander increases with its operating temperature due to the use of more exotic and/or more expensive materials. It therefore would be desirable from the standpoint of cost to be able to utilize a turboexpander at a lower temperature, for instance, preferably in a range of between about 300° C. and about 650° C. However, as the inlet temperature to the turboexpander decreases, there is less energy that can be extracted from a stream to be expanded and, therefore, less energy that is available to drive the feed air compressor. The energy able to be extracted from a stream sufficiently cooled to allow the use of turboexpanders designed to operate at low temperatures can be less than that required to operate the feed air compressor.
As will be discussed, the present invention provides a method of separating oxygen from an oxygen containing feed that is particularly applicable to the use of temperature limited turboexpander components and that can generate sufficient energy from the turboexpansion to drive the feed air compressor as well as other components. Other advantages will become apparent from the following discussion.
SUMMARY OF THE INVENTION
A method of separating oxygen from an oxygen containing gas is provided in which a feed stream containing the oxygen containing gas is compressed to produce a compressed feed stream. The compressed feed stream is heated. A ceramic membrane unit also is heated to an operational temperature. Oxygen is separated from the compressed feed stream within the ceramic membrane unit to produce both a retentate that contains residual components of the feed stream and an oxygen permeate formed by the separated oxygen. A process stream composed of at least a portion of the residual components of the retentate is cooled to a temperature below the operational temperature of the ceramic membrane unit. The process stream is expanded with the performance of work in an initial stage of expansion. An expansion stage, as described herein, is comprised of all system components that may be utilized to recover work from an inlet stream. Initial and subsequent stages of expansion are separated by a separate reheating step in which the expanded stream is reheated prior to entering the next stage of expansion. The process stream, after the initial stage of expansion, is reheated, then expanded with the performance of work in a subsequent stage of expansion.
The work of expansion produced by the initial stage of expansion is insufficient to meet the power requirements for the compression of the feed stream, and a sum of the work of expansion of the initial and subsequent expansion stages is at least sufficient to meet the power requirements for the compression of the feed stream. At least a part of a sum of the work of expansion of the initial a
Eardley Edward Paul
Shah Minish Mahendra
Shreiber Eric Hunter
Srinivasan Vijayaraghavan
van Hassel Bart Antonie
Praxair Technology Inc.
Rosenblum David M.
Silverman Stanley S.
Vanoy Timothy C.
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