Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2001-09-27
2003-05-13
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C095S039000
Reexamination Certificate
active
06562105
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a combined method of separating oxygen from an oxygen containing gas and generating power. More particularly, the present invention relates to such a combined method in which the oxygen is separated by an oxygen transport membrane purged with superheated steam and the power is generated through a Rankine cycle. Even more particularly, the present invention relates to such a combined method in which heat is transferred from an oxygen product stream produced by the oxygen transport membrane to a process fluid used within the Rankine cycle.
BACKGROUND OF THE INVENTION
Integration of power and oxygen generation cycles involving the use of oxygen transport membranes are particularly attractive from the standpoint of thermal efficiency. This is because oxygen transport membranes are effective to separate oxygen in a temperature range that encompasses the high temperatures involved in many power generation cycles.
Oxygen transport membranes are formed from a variety of well known ceramics, for example, perovskites and perovskite-like materials. At a high temperature, between about 400° C. and about 1000° C., such ceramics are capable of conducting oxygen ions while remaining impervious to oxygen molecules and substances containing oxygen in a combined form. In an oxygen transport membrane, the oxygen in an oxygen containing gas ionizes on a cathode side of the membrane. The oxygen ions can be transported across the membrane under the impetus of a positive ratio of partial oxygen pressures applied across the membrane. The oxygen ions emerging at the opposite, anode side of the membrane, recombine to liberate electrons that are used to ionize the oxygen at the cathode of the membrane. In some materials, known as mixed conductors, the electrons are transported back to the cathode directly within the ceramic. In dual phase conductors, electrons are conducted by a metallic phase or electron conducting ceramic phase located within the ceramic.
Application of a countercurrent (to the direction of retentate side flow) steam purge to the anode of an oxygen transport membrane lowers the oxygen partial pressure along the length of the membrane to increase the driving force for oxygen transport. This permits higher oxygen recovery and/or a more efficient cycle in that the degree of compression of cathode side gases or anode side gases that otherwise would be necessary to produce the driving force is reduced and can permit withdrawing an oxygen product at elevated pressure. The molar ratio of steam to oxygen at the anode side exit of the oxygen transport membrane unit determines the maximum pressure at which the oxygen product can be recovered; the higher the ratio the higher the possible oxygen product pressure. Unfortunately significant reductions in the partial pressure of oxygen at the anode and or high oxygen product pressures require high ratios of steam to oxygen. For instance, at a retentate or cathode side air pressure of 12 Bar the partial oxygen pressure at the cathode or retentate side inlet will be about 2.4 Bar. At a minimum partial oxygen pressure ratio (driving force for ion transport across the membrane) of 1.5 at the pinch point, the corresponding required partial oxygen pressure at the permeate side will be about 1.6 Bar. If, for instance it is desired to recover oxygen at a pressure of 6 bar, the steam to oxygen molar ratio has to be about (6−1.6)/1.6=2.75.
To recover oxygen at pressure, the permeate product stream, that consists of steam and oxygen, is cooled to condense the steam against a heat sink such as cooling water. Unfortunately, the amount of heat required to generate the large quantities of steam makes the process economically unattractive because the latent heat of condensation cannot be recovered effectively. If the steam-oxygen mixture is expanded in a turbine, the oxygen is recovered at low pressure. This is a problem when the oxygen product is subsequently required at high pressure and requires recompression. Also if one wants to avoid compressing oxygen from a high vacuum level, a significant fraction of the power producing potential, that can be recovered in the turbine, is lost.
For instance, in U.S. Pat. No. 5,562,754, air is compressed and heated in an in-line combustor. The oxygen is separated from the air in an oxygen transport membrane to produce a retentate. A stream of the retentate is expanded in a gas expander that is used to drive the air compressor and optionally, an electric generator. A steam purge is used at the anode side to produce an oxygen product containing steam that is used to preheat the feed water. Aside from such preheating, the latent heat of condensation is not recovered in the illustrated cycle and is thereby lost to the cycle. In U.S. Pat. No. 5,964,922, water is pressurized by pumping and then used as a steam purge for an oxygen transport membrane. The pressurized oxygen product that contains both permeated oxygen and steam is cooled in a water cooled or air cooled condenser to allow water to be condensed from the steam and recycled. As a result, the latent heat of condensation is thereby lost to the cooling mediums. U.S. Pat. No. 5,954,859 discloses purging the permeate side of an oxygen transport membrane with a high pressure purge gas stream containing steam to produce a high pressure gas stream containing oxygen and steam. The resultant stream is introduced into a turbine to recover shaft work. Hence if thereafter, the stream or portions of it were required at high pressure, it would require recompression with a concomitant energy outlay.
As will be discussed, the present invention encompasses an energy efficient method of producing an oxygen product stream at pressure that allows for the recovery of work from the latent heat of condensation of steam contained in such product stream. Other advantageous aspects of the present invention will become apparent from the following discussion.
SUMMARY OF THE INVENTION
The present invention provides a combined method of separating oxygen from an oxygen containing gas and generating power. In accordance with a method of the present invention, oxygen is separated from the oxygen containing gas into permeated oxygen and an oxygen depleted retentate by an oxygen transport membrane unit. The oxygen transport membrane unit includes at least one oxygen transport membrane operating at an elevated operational temperature and having a cathode side and an anode side. The anode side of the at least one oxygen transport membrane is purged with a pressurized purge stream comprising pressurized, superheated steam. A pressurized oxygen product stream is discharged from the anode side of the at least one oxygen transport membrane. The pressurized oxygen product stream comprises the permeated oxygen and the steam. At least part of the steam in the pressurized oxygen product stream is condensed by transferring heat to a process fluid that boils at a boiling temperature lower than the condensing temperature of the steam contained in the oxygen product stream. As a result, the process fluid boils and the at least part of the steam within the pressurized oxygen product stream condenses. The condensed water is separated from the pressurized oxygen product stream and energy is extracted from the process fluid as shaft work.
Preferably, the oxygen containing gas is heated prior to its being subjected to oxygen separation within the oxygen transport membrane unit. A retentate stream composed of the oxygen depleted retentate can be heated in an inline combustor by combustion of fuel supported by at least a portion of residual oxygen contained in the retentate stream to produce a heated retentate stream. The oxygen containing stream is at least partially heated by indirect heat transfer from the heated retentate stream.
The oxygen containing gas can be air that can be compressed to form a compressed air stream. A retentate stream composed of the oxygen depleted retentate can be expanded in a gas expander. Also the retentate stream can be coole
Praxair Technology Inc.
Rosenblum David M.
Spitzer Robert H.
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