Membrane separation process and membrane plant for energy-efficient production of oxygen

Abstract

The invention relates to a membrane separation process for energy-efficient generation of oxygen from fresh air. In the process, mixed conducting membranes in vacuum operation are used, the fresh air is discharged as waste air after separation of the oxygen, at least 85% of the thermal energy required for heating the fresh air is acquired by utilizing the waste heat of the waste air and/or of the obtained oxygen, the rest of the heating of the fresh air being realized through external energy supply, and a ratio of fresh air to generated oxygen in normal operation is adjusted to a range of from 6:1 to 25:1.

Claims

1. A membrane separation process for energy-efficient generation of oxygen from fresh air, wherein the process is a stand-alone process, mixed conducting membranes in vacuum operation are used, the fresh air is discharged as waste air after separation of the oxygen, at least 85% of the thermal energy required for heating the fresh air is acquired by utilizing the waste heat of the waste air and/or of the obtained oxygen, the rest of the heating of the fresh air is realized through external energy supply, and a volume ratio of fresh air to generated oxygen in normal operation is adjusted to a range of from 6:1 to 25:1.

2. The membrane separation process of claim 1, wherein the rest of the heating is carried out by electric heating or a combustion process.

3. The membrane separation process of claim 1, wherein the thermal energy required for heating the fresh air is obtained through the use of regenerative heat exchangers.

4. The membrane separation process of claim 1, wherein the oxygen is removed by vacuum on the permeate side, the feed gas is introduced at ambient pressure, and vacuum generation is carried out through at least one of an electromechanical vacuum pump, a mechanical vacuum pump or a steam jet pump.

5. The membrane separation process of claim 1, wherein the air throughput is controlled such that an oxygen partial pressure in the waste air is not higher than 100 mbar above a vacuum pressure on a permeate side.

6. The membrane separation process of claim 5, wherein the oxygen partial pressure in the waste air is less than 20 mbar above the vacuum pressure on the permeate side.

7. The membrane separation process of claim 2, wherein the air throughput is controlled such that an oxygen partial pressure in the waste air is not higher than 100 mbar above a vacuum pressure on a permeate side.

8. The membrane separation process of claim 7, wherein the oxygen partial pressure in the waste air is less than 20 mbar above the vacuum pressure on the permeate side.

9. The membrane separation process of claim 1, wherein more than 95% of the thermal energy required for heating the fresh air is acquired by utilizing the waste heat of the waste air and/or of the obtained oxygen.

10. A membrane plant for energy-efficient production of oxygen from fresh air, wherein the plant comprises a housing with an input and an output, MIEC (Mixed Ionic Electronic Conductor) membranes and a vacuum pump, a metal connection plate is arranged in the housing, which metal connection plate comprises a vacuum-tight hollow space structure in which the MIEC membranes which are closed on one side are arranged in a gastight manner, at least one dividing wall for dividing into chambers is present, each chamber comprising a stationary regenerator, a supplemental heater and a portion of the MIEC membranes, and an orifice is present in every dividing wall to ensure passage for the fresh air from a chamber downstream of the input to a chamber upstream of the output, an upstream fan is arranged upstream of the input, a downstream fan is arranged downstream of the output, the upstream fan and the downstream fan having opposite suction directions, a regenerative heat exchanger is present, partial regions being associated with the input and other partial regions being associated with the output, and the vacuum pump communicates with the hollow space structure for sucking out the obtained oxygen.

11. The membrane plant of claim 10, wherein the housing is not pressure-tight.

12. The membrane plant of claim 10, wherein the opposite suction directions are reversibly adjustable.

13. The membrane plant of claim 10, wherein the upstream fan and the downstream fan are arranged on a rotary slide such that during rotation of the rotary slide the input moves from the chamber downstream of the input to the respective adjacent chamber and, consequently, the output moves from the opposite chamber to the adjacent chamber.

14. The membrane plant of claim 13, wherein the input and, consequently, the output extend over a plurality of adjacent chambers.

15. The membrane plant of claim 11, wherein the opposite suction directions are reversibly adjustable.

16. The membrane plant of claim 11, wherein the upstream fan and the downstream fan are arranged on a rotary slide such that during rotation of the rotary slide the input moves from the chamber downstream of the input to the respective adjacent chamber and, consequently, the output moves from the opposite chamber to the adjacent chamber.

17. The membrane plant of claim 16, wherein the input and, consequently, the output extend over a plurality of adjacent chambers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be described more fully in the following with reference to embodiment examples. The accompanying drawings show:

(2) FIG. 1 the oxygen throughput in vacuum operation and overpressure operation (BSCF, 850? C.) for different O.sub.2 recoveries;

(3) FIG. 2 the equilibrium oxygen partial pressure and energy demand for air heating (at 85% heat recovery), oxygen compression, oxygen cooling;

(4) FIG. 3 the equilibrium oxygen pressure and vacuum pressure and the total energy demand W.sub.total for oxygen compression, oxygen cooling and air heating at different heat recoveries (HR);

(5) FIG. 4 a schematic diagram of a membrane module for oxygen production in vacuum operation with stationary regenerative heat exchangers; and

(6) FIG. 5 a schematic diagram of a MI EC membrane module for oxygen production in vacuum operation with rotatingly traversed regenerative heat exchangers and rotary slide.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(7) FIG. 1 shows the oxygen permeation for vacuum operation and overpressure operation for tubular BSCF membranes and different O.sub.2 recoveries, which oxygen permeation is calculated according to a simplified Wagner equation. It is clear that increasing O.sub.2 recoveries lead to a sharp reduction in oxygen permeation. It is also clear that an oxygen permeation first occurs below a determined vacuum pressure on the permeate side or above a determined overpressure on the feed side, and this limiting equilibrium oxygen partial pressure p.sub.OEq is determined by the O.sub.2 recovery.

(8) It is clear from FIG. 1 that the vacuum process leads to an ever greater increase in oxygen permeation as the vacuum pressure decreases. In contrast, in the overpressure process the further increase in oxygen permeation flattens out increasingly. Further, the overpressure process requires recovery of compression energy which would mean a disproportionately high expenditure for small plants. Corresponding compressors and expansion turbines with a sufficiently high efficiency are not available to date. Therefore, contrary to the majority of current publications, the construction of an energy-efficient MIEC membrane plant for oxygen generation is oriented to a vacuum process.

(9) Modelings of the overall process show that the entire energy demand of the process depends decisively on the O.sub.2 recovery and all of the process parameters relevant for energy can be calculated directly from the O.sub.2 recovery. Accordingly, FIG. 2 shows the curve of the equilibrium oxygen partial pressure p.sub.OEq and the energy demand for air heating, incl. 85% heat recovery, for cooling the oxygen and for compressing the oxygen to ambient pressure. Calculation of the compression energy in the vacuum process was based on the compression energy of conventional vacuum pumps resulting from suction power and nominal throughput. The most energy-efficient commercial vacuum pumps achieve minimum values of 0.015 kWh.sub.el./Sm.sup.3 (Sm.sup.3=suction m.sup.3). A value of 0.018 kWh.sub.el./Sm.sup.3 was used for the calculation.

(10) All energy values indicated in FIG. 2 are normalized to the amount of oxygen generated. Therefore, they can easily be used to calculate the total energy demand W.sub.total. FIG. 3 shows the total energy demand W.sub.total of the vacuum process for different heat recoveries of the utilized heat exchangers which use the waste heat of the oxygen-depleted waste air to heat the fresh air. At 85% HR (heat recovery) of the heat exchanger and 0.018 kWh.sub.el./Sm.sup.3, the total energy consumption already lies just below that of a decentralized PSA plant even without utilizing the waste heat from the oxygen cooling.

(11) If the waste heat from oxygen cooling is also utilized for preheating air, an appreciably lower specific energy consumption of 0.55 kWh/Nm.sup.3 O.sub.2 can already be achieved in the range of optimal O.sub.2 recovery. Obviously, a higher heat recovery efficiency of the air/waste air heat exchanger leads to an appreciable reduction in the specific energy consumption of the process. Further, with higher HR values the range of the minimum specific energy consumption expands, the minimum is less pronounced and is shifted to lower O.sub.2 recoveries. Accordingly, compared to lower HR values of the heat exchangers, it is still possible to separate oxygen in an energy-efficient manner at appreciably higher vacuum pressure. For example, while the maximum permissible vacuum pressure is about 90 mbar at the minimum of the energy demand curve for 85% FIR, it climbs to 133 mbar at 97% HR. Accordingly, it is possible to use smaller vacuum pumps with higher heat recovery and to further decrease the share of energy for compression in the total specific energy demand.

(12) Already at 97% HR, the specific energy demand of the MIEC membrane process can fall below a value of 0.3 kWh/Nm.sup.3 O.sub.2 if O.sub.2 recoveries of 20% to approximately 70% and resulting ratios of fresh air amount to produced oxygen of 24:1 to 6.8:1 are maintained. Therefore, a considerable energy advantage is achieved according to the invention compared to the prior art (cryogenic air separation plants, decentralized PSA plants).

(13) As an alternative to controlling the air amount in proportion to the oxygen produced, the oxygen partial pressure at the feed output p.sub.OFout can be used for realizing an energy-efficient operating mode. To this end, the oxygen partial pressure at the feed output p.sub.OFout and the vacuum pressure or the oxygen partial pressure on the permeate side p.sub.OSout, which is identical to the latter, are continuously measured. The air throughput is adapted by control technology in such a way that the oxygen partial pressure at the feed output p.sub.OFout lies above the vacuum pressure p.sub.OSout by no more than 100 mbar, but, in a preferred constructional variant, by no more than 20 mbar. Therefore, a sufficiently low air throughput and a correspondingly high O.sub.2 recovery are realized within a wide operating range in order to ensure an energy-efficient operation.

(14) A highly energy-efficient MIEC membrane plant is characterized according to the invention by heating the fresh air via regenerative heat exchangers which utilize more than 85%, in a preferred constructional variant more than 95%, of the thermal energy contained in the waste air and which utilize the waste heat released during the cooling of the oxygen to heat air. The driving force for the oxygen transport is generated through application of a vacuum because, in this way, the compression energy to be expended is minimized and need not be recovered. According to the invention, the membrane plant is operated such that in normal operation a ratio of the entering fresh air amount to the produced oxygen amount of 25:1 is not exceeded and does not fall below a ratio of 6:1. The required residual heat for maintaining the operating temperature of the membrane plant is acquired through supplemental electrical heating or by metered injection of small amounts of fuel. Accordingly, in the latter case, a further reduction in the consumption of electrical energy can be achieved.

EMBODIMENT EXAMPLE 1

(15) The membrane plant for energy-efficient oxygen production shown schematically in FIG. 4 comprises, according to the invention, a housing 1 which is not pressure-tight and which has a metal connection plate 2 in which tubular BSCF membranes 3 which are closed on one side were inserted by means of silicon seals. A dividing wall 15 with orifice 16 divides the interior of the housing into two chambers which are constructed in a mirror-symmetrical manner with respect to the path of fresh air. In cycle phase A, the fresh air is sucked in via a speed-controlled fan 4 arranged upstream of the input 11, preheated by a regenerative heat exchanger 5 and guided through the partitioned connection plate 2 in order to absorb the heat of the extracted oxygen. Subsequently, the fresh air is guided through a stationary regenerator 6 for further heating and is post-heated to operating temperature by the supplemental heater 7. The air stream passes the tubular BSCF membranes 3 and the second supplemental heater 8 and delivers its heat to the further stationary regenerator 9. The oxygen-depleted air stream which is already highly cooled is subsequently guided through the metal connection plate 2 to the regenerative heat exchanger 5 to those regions upstream of the output 12, where further heat is extracted from it. The speed-controlled fan 10 downstream of the output 12 works in suction mode in cycle phase A. The metal connection plate 2 contains a vacuum-tight hollow space structure 13 which communicates with the tubular BSCF membranes 3. The hollow space of this hollow space structure 13 is suctioned through an external vacuum pump 14. The pure oxygen is subsequently available at ambient pressure.

(16) After a corresponding cycling time, the streaming direction of the air stream is reversed either through suitable air flaps, not shown, or by reversing the running direction of the fans 4 and 10. Consequently, all of the gas streams are reversed in this cycle phase B. In this way, the heat contained in the hot waste air and the heat transferred through the oxygen to the metal connection plate 2 is extensively recovered. The air throughput is controlled through the speed-controlled fans 4 and 10 such that the oxygen partial pressure after membrane contact is a maximum of only 100 mbar, preferably, according to the invention, only about 20 mbar, above the oxygen partial pressure on the permeate side. Therefore, when these specifications are complied with, a low air surplus and mean O.sub.2 recovery of 30 to 70% and, accordingly, an energy-efficient operation are ensured.

EMBODIMENT EXAMPLE 2

(17) The membrane plant for oxygen production shown schematically in FIG. 5 comprises, according to the invention, a housing 1 which is not pressure-tight and which has a metal connection plate 2 in which tubular BSCF membranes 3 which are closed on one side were inserted by means of cable fittings. The housing 1 is octagonal and is divided into eight chambers by eight dividing walls 15, each dividing wall being provided with an orifice 16 in the region of the connection plate 2 so that fresh air can stream through all of the chambers. The fresh air is sucked in via the speed-controlled fan 4 which is arranged upstream of the input 11. The input 11 is located on a rotary slide 17 and has dimensions sufficient to allow fresh air to stream into three chambers simultaneously. The output 12 is likewise located on the rotary slide 17 and is preferably dimensioned identically to input 11 and lies opposite input 11. The regenerative beat exchanger 5 directly downstream of the input 11 is dimensioned at least such that areas of the input 11 and output 12, respectively, are constantly covered as the rotary slide revolves by 360?. The fresh air sucked in through the input 11 by the upstream fan 4 is initially heated through regions of the regenerative heat exchanger 5. Further heating takes place by sweeping past the segmented connection plate 2 and the downstream stationary regenerators 6 of the three chambers downstream of the input. The supplemental heaters 7 arranged in the cover region are used to post-heat the air stream. As an alternative to electrical heating, heating can also be carried out by small amounts of fuel gas. The heated fresh air subsequently streams downward between the dividing walls 15 and through the orifices 16 into the opposed chambers. The fresh air which is already depleted is now directed upward past the tubular BSCF membranes 3 and supplemental heaters 8 via the stationary regenerators 9 of the respective chambers. Finally, the extensively cooled fresh air is directed through the partitioned connection plate 2 and streams through those areas of the regenerative heat exchanger 5 which are opened by the output 12. In so doing, additional heat is extracted from the waste air stream. The speed-controlled downstream fan 10 operates continuously in suction mode. The metal connection plate 2 contains a vacuum-tight hollow space structure 13 in which the obtained oxygen collects and is accordingly extracted through an external vacuum pump 14. A permanent reversal of the fresh air feed direction with respect to the opposed chambers is effected through the constant rotation of the rotary slide 17, i.e., the input 11 lies at the output 12, and vice versa, after 180-degree rotation of the rotary slide 17.

(18) As a result of the construction shown here, the heat contained in the hot waste air and the heat transferred through the oxygen to the metal connection plate 2 is extensively recovered. The air throughput is controlled through the fan 4 upstream of the rotary slide 17 and the downstream fan 10 with variable speed such that the air throughput is 12 to 18 times the oxygen production rate.

LIST OF REFERENCE NUMERALS

(19) 1 housing 2 connection plate 3 tubular BSCF membranes 4 upstream fan 5 regenerative heat exchanger 6 regenerator 7 supplemental heater 8 supplemental heater 9 regenerator 10 downstream fan 11 input 12 output 13 hollow space structure 14 vacuum pump 15 dividing wall 16 orifice 17 rotary slide