WO2014161713A1

WO2014161713A1 - Process for the enrichment of air

Info

Publication number: WO2014161713A1
Application number: PCT/EP2014/055107
Authority: WO
Inventors: Flavio MANENTI; Carlo PIROLA
Original assignee: Politecnico Di Milano; Universita' Degli Studi Di Milano
Priority date: 2013-04-04
Filing date: 2014-03-14
Publication date: 2014-10-09
Also published as: ITLO20130002A1

Abstract

The present invention is aimed to a process for producing enriched air from water containing dissolved oxygen and nitrogen. In particular, water is preferably salt water or fresh water; more preferably water is seawater. In particular, the present invention relates to a process for producing oxygen enriched air from water, the process comprising heating (HE1) said water to a temperature below the boiling point and separating (FDS1) a stream (S4) comprising incondensable gases from a stream (S5) of degassed water. Furthermore the present invention relates to an apparatus for producing oxygen enriched air, according to the process cited above, and to plants which employ water and comprising said apparatus. In particular, the present invention is directed to a process for obtaining an oxygen enriched air from seawater within a seawater desalination plant.

Description

PROCESS FOR THE ENRICHMENT OF AIR

Summary of the invention

[0001] The present invention is aimed to a process for producing enriched air from water containing dissolved oxygen and nitrogen . In particular, water is preferably salt water or fresh water; more preferably water is seawater.

[0002] A stream enriched in oxygen is obtained by heating water at a temperature below its boiling point.

[0003] The present invention relates thus to a process for producing oxygen enriched air from water, the process comprising heating said water to a temperature below the boiling point and separating a stream comprising incondensable gases from a stream of degassed water.

[0004] Furthermore the present invention relates to an apparatus for producing oxygen enriched air, according to the process cited above, and to plants which employ water and comprising said apparatus.

[0005] In particular, the present invention is directed to a process for obtaining an oxygen enriched air from seawater within a seawater desalination plant.

Background of the invention

[0006] At constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

[0007] This principle is ruled by Henry's law and is expressed as:

wherein c is the solubility of dissolved gas, k_H is the proportionality constant, depending on the nature of the gas, the solvent and temperature, and p_g is the partial pressure of the gas.

[0008] Air is dissolved in water and, at the equilibrium with atmosphere, each gas composing air is present in water in a percentage according to Henry's Law. The solubility of oxygen in water is higher than the solubility of nitrogen. Air dissolved in water contains approximately 35.6% oxygen at ambient conditions compared to about 21% oxygen contained in air.

[0009] As the temperature increases, the solubility of gases decreases. Therefore, when providing heat, nitrogen and oxygen (together with the other atmosphere's components) are released from water together with water vapor. In particular, when heating at a temperature below boiling point, the stream of vapor obtained from water is rich of incondensable gases. The composition of released gases is related to the operating temperature and it differs from the atmospheric composition .

[0010] The present invention takes advantage of the different dissolution of atmospheric oxygen and nitrogen at equilibrium with water, wherein Henry's constant is favorable to a higher dissolution of oxygen. The present invention is in fact based on the separation of oxygen enriched air by heating water at a temperature below the boiling point.

[0011] Conventional processes for production of enriched air comprise, for example, compression, refrigeration and separation, when the final specifications are particularly tight and the requests are large. Otherwise, when the specifications are relaxed, molecular sieves and zeolites are sufficient to concentrate the air in oxygen by few percent.

[0012] Compared to conventional processes, the present invention is very cost effective, particularly when used in combination with processes which generate vapor or wherein heating of water is provided.

[0013] For example, it is possible to recover enriched air in vapor generation systems before water reaches the boiling point. Recovery of enriched air from these processes does not have any additional operational cost, but simply exploits differently the existing energy supply.

[0014] The process of seawater desalination to produce fresh water is of particular interest. Depending on the temperature, seawater can dissolve from about 6 to 20 liters of air every cubic meter of water. A seawater desalination plant to produce fresh water for domestic use generally processes thousands of tons of water per day. Briefly, by means of multiple-effect evaporators, water is progressively vaporized and desalinated. It is possible to obtain enriched air by separating incondensable gases and water vapor before seawater is vaporized. Otherwise enriched air can be recovered between the desalinated steam and its condenser (two-steps condensing process to (1) recover enriched air at high temperature (below the boiling point) and (2) cool the desalinated water to ambient conditions).

[0015] In general, any process that makes use of water for thermal recovery or steam generation can be integrated with the invention (intensified process) with a small investment, without any increment of operational costs and with benefits coming from the use of the enriched air produced. Examples of these processes are methanol synthesis processes, wherein tubular reactors are cooled with water, thermal recovery processes, or steam generation processes. The process of the present invention can be used upstream the boiling of water. For example, in steam generation systems recovery of enriched air may occur before reaching the boiling point. The present invention can be also used downstream the boiling of water, and recovery of enriched air may be obtained during cooling step. Whatever kind of chemical and industrial process that uses water as coolant is a potential application for the invention. For instance, upstream systems of refrigerating towers can be intensified with the invention. From a general viewpoint, all the exothermic processes, such as methanol synthesis, require a coolant to preserve the temperature for safety and industrial reasons. The cooling is performed either directly jacketing the reaction environment, as in the methanol synthesis case, or immediately downstream, such as in the case of combustion processes. Usually, the coolant is water for several reasons.

[0016] Specifically, pyrolysis and combustion systems that adopt water to quench the outlet gas streams are potential applications. For instance, Claus processes are characterized by a thermal reactor that operates at 1100-1300°C to partially oxidize acid gases. The outlet gas is cooled to 350°C in a waste heat boiler that uses boiling water as coolant, generating medium pressure steam.

[0017] Appealing applications are represented by chemical processes that have an open loop of water. In practice, the water that enters the process is active in the reaction to generate special commodities and chemical products. Typical processes with open loop of water are the steam reforming, steam cracking, and gasification (for instance of coal, biomass, waste) to quote a few. These processes already have scavenger/degassing processes that can be used to introduce the invention lowering the installation costs.

The invention will be described hereafter in more details.

Brief description of the invention

[0018] The present invention is aimed to the production of enriched air from water. Preferably, the present invention is aimed to the production of enriched air from salt water or fresh water, more preferably from seawater. The solubility of oxygen in water is higher than the solubility of nitrogen. Increasing water temperature, the solubility of both gases decreases and it is possible to remove oxygen enriched air from water.

[0019] Enriched air comprises an oxygen concentration of from 22 to 35 % volume (vol), preferably of from 23 to 30 % volume (vol), more preferably of from 24 to 27 % volume (vol) .

[0020] By heating water at a temperature below the boiling point, a stream is obtained which is enriched in incondensable gases dissolved in it. For example, a stream can be obtained with an oxygen to nitrogen ratio similar to the one dissolved in the origin fluid. Oxygen is present in water in a ratio to nitrogen equal or greater than 1 : 2.

Brief description of figures

[0021] Figure 1 : Qualitative layout of the invention applied to a desalination plant.

In the present simulation, the invention consists in two separators (FDS), two heat exchangers (HE) and the respective linking lines, together with the instrumentation connected. Briefly, the seawater S2 enters the first heat exchanger (HE1) and is heated to a temperature lower than the boiling point. The heated water stream then enters the first separator (FDS1) wherein the incondensable gases dissolved in the water are separated and leave the unit through gas line S4, while seawater partially devoid of incondensable gases exits through line S5. S4 stream, which is rich in incondensable gases originally dissolved in seawater and water vapor, is cooled in the second heat exchanger (HE2) . The majority of water vapor is thus condensed, re-absorbing only a minor amount of incondensable gases, since the water amount in line S4 is much smaller compared to the original seawater stream S2. The cooled stream is then fed to the second separator (FDS2) and the final separation occurs. The condensed stream S7 is mixed with S5, while enriched air S6 is partially compressed by a blower (BL), in order to be ready-to-use on-site. For example, the enriched air stream can be compressed to a pressure of 1.6 bar. Said pressure is suitable for use in combustion chambers operating at atmospheric pressure.

[0022] Figure 2 : Flow-sheet using PRO/II process simulator.

[0023] Figure 3 : Flow-sheet using AspenHysys process simulator.

[0024] Figure 4: Traditional (upper scheme) and intensified with the invention (lower scheme) process layout for steam generation. The steam generated can be used for, but not limited to, steam reforming, steam cracking, gasification, power generation. The invention units are shown in dark. Traditional water generation systems consist in an economizer for water pre-heating, a boiler wherein water vapor is generated, one or more superheaters wherein vapor is heated before use. Integration of the invention with traditional water generation systems allows concomitant generation of enriched air. The invention requires only small modifications of the plant, such as revamping of economizer and installation of heat exchanger HE2 and of separators (FDS 1 and 2). [0025] Figure 5: Half-slice of conventional thermal cracker (the transfer line exchanger, TLE, is the steam generator) .

[0026] Figure 6: Intensified thermal cracking system with on-site reuse of enriched air. The direct use of enriched air allows to achieve the same temperature of steam cracking with less fuel consumption.

[0027] Figure 7a and 7b: Traditional (7a) and intensified (7b) multi-effect distillation for seawater desalination (as per example 1) . In this case, the invention is applied upstream. The invention units are shown in dark.

Detailed description of the invention

[0028] The present invention relates to a process for producing oxygen enriched air, comprising : providing water with dissolved oxygen and nitrogen; heating water to a temperature of from 1°C to 70°C lower than the boiling point, preferably of from 1°C to 50°C lower than the boiling point, more preferably of from 1°C to 25°C lower than the boiling point; feeding the heated water to a separator; separating in said separator from said heated water a stream comprising incondensable gases and a stream of degassed water; recovering enriched air from the stream comprising incondensable gases. Preferably, the water is salt water or fresh water; more preferably the water is seawater.

[0029] Water vapor is comprised in the stream enriched in incondensable gases and it is possible to recover oxygen, nitrogen and other atmospheric components dissolved in water, together with water vapor.

[0030] Optionally, the process further comprises the steps of cooling the stream comprising incondensable gases to obtain a cooled stream comprising a condensed phase. Preferably, the cooled stream is fed to a separator to separate the condensed phase and a stream of enriched air. Preferably, said condensed phase comprises a minor amount of the incondensable gases and said stream of enriched air comprises the major amount of incondensable gases.

[0031] Preferably streams are separated in flash drum separators.

[0032] In one embodiment of the present invention, heating and/or cooling of the streams of the inventive process occur through heat exchangers.

[0033] In preferred embodiments water is heated to the boiling temperature in one or more evaporators; then the evaporated water is separated with separation apparatuses, preferably membranes, condensers or cold traps. In these embodiments the stream comprising incondensable gases may be separated before evaporation of the heated water and/or during condensing of the evaporated water. When speaking of evaporated water, it is intended that the majority of water is evaporated. Thus, in a preferred embodiment, at least 50% of water is evaporated in the process of the present invention, most preferably at least 80%.

[0034] In preferred embodiments, the heating energy required to heat water at a temperature below the boiling point is obtained from processes wherein high amounts of water are used, for example vapor generation systems, seawater desalination plants, refrigerating systems.

[0035] The present invention relates also to an apparatus for producing enriched air, said apparatus comprising : a heat exchanger for heating a stream of water to a temperature below boiling point; a separator for separating heated water into a stream comprising incondensable gases and a stream of degassed water; optionally the apparatus further comprises a heat exchanger for cooling the stream comprising incondensable gases to provide a cooled stream; a separator to separate the cooled stream into a condensed phase and a stream of enriched air.

[0036] The present invention is particularly suitable for being used together with existing processes wherein water is heated to the boiling point, since enriched air can be obtained intensifying said existing processes with low or no extra charges. By intensifying existing processes with the process of the present invention, costs can be lowered and energy efficiency of the processes and plants intensified can be increased.

[0037] The process of the present invention can intensify processes wherein high amounts of water are used, like methanol synthesis processes, wherein tubular reactors are cooled with water, or thermal recovery processes, or steam generation processes. This requires a small investment, without any increment of operational costs and with benefits coming from the use of the enriched air produced. For example the process of the present invention can be used upstream the boiling of water, in vapor generation systems; recovery of enriched air may occur before reaching the boiling point. In one embodiment the present invention can be used downstream the boiling of water and recovering of enriched air may be obtained during cooling step.

[0038] In preferred embodiments the process of the present invention can intensify the processes of seawater desalination, whereas the target is still the separation of desalinated water from the brine. The process to desalinate water is energy intensive; significant power consumption or significant greenhouse emissions, when fossil fuels are burnt for it, are the unavoidable result of the traditional technologies. Whenever separation is performed by evaporation, for example in multi-effect distillation (MED) technology, an additional target to intensify the plant can be introduced without additional operational costs and energy consumption.

[0039] The typical layout of desalination plants consists of several evaporators where the steam generated in the n-th evaporator is used as duty for the (n+l)-th evaporator. For example, seawater is fed to the first evaporator at ambient temperature (of from about 5°C to about 30°C for Mediterranean Sea conditions). A certain amount of heat is provided to increase the temperature and evaporate a quantity of water in the first effect of MED desalination plants. The water evaporated in the first effect is the driving force for the subsequent evaporators. Focusing on the first effect, the seawater partially evaporates and the desalinated water and brine are obtained (see figure 7a) .

[0040] It is possible to intensify the process of water desalination by inserting the process of the present invention between the seawater and the feeding of the first vaporizer without any additional operational cost (see figure 7b) . Alternatively, it is possible to insert the invention between the desalinated steam and its condenser (two-steps condensing process to ( 1) recover enriched air at high temperature (below the boiling point) and (2) cool the desalinated water to ambient conditions). If the duty is supplied in two steps, it is thus possible to collect the first vapor obtained at lower temperature, which is rich in incondensable gases, and produce the desalinated water at higher temperature in a second step. Enriched air can be recovered before the second step. In one embodiment, the enriched air can be recovered during the condensation of desalinated water. In such cases, the first condensation step is carried out at a temperature slightly lower than the boiling point, whereas the complete cooling of desalinated water is carried out in a second step.

[0041] Whenever the desalination process is thermally supplied by fossil fuel combustion, variables costs and fuel consumption can be reduced making the process more sustainable.

[0042] Thus, the present invention further relates to an industrial plant comprising the apparatus for producing enriched air.

[0043] In particular, the present invention further relates to a plant for desalinating seawater comprising an apparatus for producing enriched air; said apparatus comprising a heat exchanger for heating a stream of water to a temperature below boiling point; a separator for separating heated water into a stream comprising incondensable gases and a stream of degassed water; optionally the apparatus further comprises a heat exchanger for cooling the stream comprising incondensable gases to provide a cooled stream; a separator to separate the cooled stream into a condensed phase and a stream of enriched air.

[0044] In other embodiments the present invention relates to a plant for steam cracking comprising an apparatus for producing enriched air, according to the present invention.

[0045] Benefits of the intensification are tangible when the enriched air can be used directly within the plant that produces it, for instance in combustion chambers, partially diluted by air (if the oxygen content is particularly high). It allows to reduce the operational costs, to reduce the thermal inertia of air in combustion processes for steam generation, with the unavoidably reduction of fossil fuel consumption, and to reduce the volumes of unit operations. The enriched air can be appropriately stored or used locally.

Examples

[0046] Example 1

[0047] The simplified seawater desalination plant designed for domestic use of fresh water in Pantelleria Island has been adopted as industrial case study. The intensified plant of the example consists of three evaporators and the apparatus for producing enriched air according to the invention (see on figure 7b). The simulation has been solved using PRO/II commercial package. The operating conditions are: seawater at 22°C with Mediterranean Sea salinity content; inlet flow of seawater 25 kg/s; pressure drops at the second and third evaporators of 0.35 bar; final temperature of desalinated water of 51°C. The simulation provides the following results: 13.09 kg/s of desalinated water, 11.91 kg/s of brine with about double salt concentration according to local restrictions, and 0.034 kg/s of highly enriched air. The power requirement to supply the first evaporator is estimated in the order of 23.176 MW.

[0048] Example 2

[0049] A process simulation has been performed. The most conservative conditions are selected not to overestimate oxygen concentration in the enriched air stream. On figure 3, a scheme of the apparatus for producing enriched air is shown. The invention consists of two separators, V- 101 and V-102, and two heat exchangers, E-100 and E- 101. The first unit, V-100, is used to simulate the seawater in contact with the atmosphere. According to the use of enriched air obtained in the stream 8, a blower or compressor can be needed. The separations can be performed with membranes, flash drums, atomizers or other units. In this general simulation, ideal flash drum separators are adopted. The seawater (stream 2) enters the unit E-100 where it is heated to a temperature lower than the boiling point. Then, it enters the unit V- 102 for the degassing operation (a stripper could be used in this case) to separate the wet flow of incondensable gases (stream 11) from the degassed seawater (stream 5), which is sent to the MED desalination process (not shown on figure

3) . The stream 11 is cooled in the unit E-101 to condensate the most of the water within the stream. By doing so, a negligible portion of the incondensable gases are re-absorbed in water (stream 4) since the water amount is significantly smaller than the original one. The condensed phase (stream 6) is separated from the enriched air (stream 7). The total amount of enriched air estimated with the most conservative simulation is estimated in the order of 1.4 kg of enriched air with 24.5% of oxygen per 1 m³ of seawater. Considering the large volumes processed in a typical desalination plant, a relevant flow of enriched air can be recovered with the proposed intensification. It is worth remarking that the energy supply required to operate the intensified process is exactly the same of the traditional desalination process.

[0050] Example 3

[0051] An experimental pilot plant has been predisposed. The plant is continuous and it is designed to process up to 30 l/h of fresh water. In this plant water is drawn from a tank of about 25 liters, kept open and in contact with the air in order to ensure the dissolution equilibrium at ambient temperature (20°C) of oxygen and nitrogen. The flow of water (10 l/h) is sent by a pump to a heat exchanger to raise its temperature at a fixed value of 80°C. This flow then enters in a stripping column for the degassing operation in order to separate the wet flow of oxygen and nitrogen from the degassed water. This column, with a height of 48 cm and with an internal diameter of 4.5 cm, is filled with Sulzer Mellapak CX type structured packing and maintained at 80°C by an external heating jacket. The flow of water is fed at the top of the column and the stripping gas, helium, at the bottom. The stripping gas is useful to favor the separation of enriched air and helium has been selected not to spoil the gas chromatograph measurements. The flow outgoing from the column then enters in a cold trap for the separation of the evaporated water and it is finally sent to a micro-gas chromatograph for the on-line quantification of oxygen and nitrogen. These analyses are performed using an Agilent mod. 3000A micro-GC with molecular sieves column, operating with helium as carrier and an isothermal analysis at 80°C. Different flows of helium are used and specifically 4 Nl/h, 10 Nl/h and 20 Nl/h which give the different concentrations of nitrogen and oxygen : the experiments confirm that the intensification leads to the production of a highly enriched air with the 32%, 28% and 26%vol of oxygen, respectively. The flow of enriched air obtained in these runs is in the range 0.05-0.08 Nl/h . In industrial applications, the helium can be replaced by other gases. Preferably air or the same enriched air could be adopted to favor the separation.

[0052] Example 4

[0053] Economic calculations have been done on the plant of example 1. It is assumed that the steam generation to supply the first evaporator of seawater desalination plant is obtained using methane with stoichiometric air in a combustion chamber. The calorific value of methane is assumed to be 55,255 kJ/kg. To generate the power of about 23 MW required to the first evaporator to initialize the overall desalination plant, 0.4194 kg/s of methane are necessary. If we assume that the methane is completely burnt with stoichiometric air within the combustion chamber according to the general oxidation path CH₄ + 20₂ - C0₂ + 2H₂0, the total amount of oxygen required is 0.8388 kg/s. It means that 3.995 kg/s of air must be fed to the combustion air to generate the required power. With the intensification according to the present invention 0.034 kg/s of highly enriched air (about 1/3 of oxygen) is recovered. It means that the required flow becomes 3.964 kg/s for the combustion air. Consequently, the total inlet gas flow is reduced only in the amount of inert gas (nitrogen). Therefore, with a simple energy balance a corresponding reduction of fossil fuel required to achieve the same temperature in the combustion chamber is obtained . The performance improvement in terms of operational costs obtained with the most conservative process simulation is 0.69% for the overall traditional combustion section. If we suppose that the energy supply with fossil fuel is supported by some renewable energy source, which is the modern trend to make sustainable the desalination processes (i.e. with geothermal or solar source), the contribution of enriched air is much more relevant and it can lead to a significant decrease of fossil fuel, preserving the combustion chamber performances. For instance, suppose that renewable energy covers the 90% of the energy needs, whereas the 10% is still supplied by fossil fuel. Consider that fossil fuel is always in spare to bridge possible gaps of discontinuous nature of renewable energy, but it is kept operating for prompt actions. In such a case, the combustion air is less than 10% (total air + enriched air = 0.38 kg/s) with respect of the previous case (total air + enriched air = 3.995 kg/s) and to obtain the same temperature in the combustion chamber, the fuel flow required is reduced by 9.51%. It leads to an even decrease of variable costs for fossil fuel, less and better use of fuel, and further significant reduction of greenhouse emissions.

Claims

1. Process for producing oxygen enriched air, comprising :

providing water which contains dissolved oxygen and nitrogen,

heating said water to a temperature of from 1°C to 70°C lower than water boiling point, preferably to a temperature of from 1°C to 50°C lower than water boiling point, more preferably to a temperature of from 1°C to 25°C lower than water boiling point;

feeding the heated water to a separator;

separating in said separator from said heated water a stream comprising incondensable gases and a stream of degassed water;

recovering enriched air from the stream comprising incondensable gases.

2. The process of claim 1, wherein the water is seawater.

3. The process of claim 1, wherein the water is salt water.

4. The process of claim 1, wherein the water is fresh water.

5. The process of claims 1-4 further comprising the step of:

cooling the stream comprising incondensable gases to obtain a cooled stream comprising a condensed phase.

6. The process of claim 5 further comprising the steps of:

feeding the cooled stream to a separator;

separating in said separator from the cooled stream a condensed phase comprising a minor amount of the incondensable gases and a stream of enriched air comprising the major amount of incondensable gases.

7. The process of claims 1-6 wherein the water is heated through a heat exchanger.

8. The process of claims 1-7 wherein the stream comprising incondensable gases is cooled through a heat exchanger.

9. The process of claims 1-8, further comprising the steps of:

evaporating the heated water in one or more evaporators; and

condensing the evaporated water.

10. The process of claim 9 wherein the stream comprising incondensable gases is separated before evaporation of the heated water.

11. The process of claim 9 wherein the enriched air is recovered during condensing of the evaporated water.

12. The process of claims 1-11 wherein the separator is a flash drum.

13. An apparatus for producing enriched air, said apparatus comprising :

one or more heat exchangers for heating a stream of water to a temperature of from 1°C to 70°C lower than water boiling point, preferably to a temperature of from 1°C to 50°C lower than water boiling point, more preferably to a temperature of from 1°C to 25°C lower than water boiling point;

one or more separators for separating the heated water into a stream comprising incondensable gases and a stream of degassed water;

optionally said apparatus comprising one or more heat exchangers for cooling the stream comprising incondensable gases to a cooled stream; and one or more separators to separate the cooled stream into a condensed phase and a stream of enriched air.

14. A plant for seawater desalination comprising the apparatus of claim 13.

15. A plant for steam cracking comprising the apparatus of claim 13.