TW201718066A - Air fractionation plant, operating method and control facility - Google Patents

Abstract

The invention relates to an air fractionation plant (100) in which a cooling water circuit (10) having a recooling apparatus (11) is provided for cooling compressed air, where the recooling apparatus (11) is configured for cooling water using cooling air. The recooling apparatus (11) is configured so as to cool the cooling water, at least at a wet bulb temperature of the cooling air of more than 289 K, to a temperature which is not more than 3 K above the wet bulb temperature. A corresponding operating method and a control facility are likewise provided by the invention.

Description

Air fractionation device, operation method and control device

In accordance with the respective preambles of the scope of the independent patent application, the present invention relates to an air fractionation apparatus, a method of operating an air fractionation apparatus, and a control apparatus for such an air fractionation apparatus.

The production of air products in liquid or gaseous form by low temperature fractionation of air in an air fractionation plant is known and described, for example, in H.-W. Häring (ed.), Industrial Gases Processing, Wiley-VCH, 2006, Specifically, Section 2.2.5, “Cryogenic Rectification”. The invention is applicable to various embodiments of corresponding air fractionation equipment.

The air fractionation apparatus has a distillation column system that can be, for example, configured as a two-column system, specifically a classic Linde two-column system, and a three-column or multi-column system. In addition to a distillation column for separating nitrogen and/or oxygen in a liquid and/or gaseous state (for example, liquid oxygen LOX, gaseous oxygen GOX, liquid nitrogen LIN, and/or gaseous nitrogen GAN) (ie, a distillation column for nitrogen-oxygen separation) In addition to the distillation column for separating other components of the air, in particular the rare gases helium, neon and/or argon.

The distillation column system of the air fractionation apparatus is operated in its distillation column at different operating pressures. Known two-column systems have, for example, (high) pressure columns and low pressure columns. The operating pressure of the high pressure column is, for example, from 4.3 bar to 6.9 bar, in particular about 5.5 bar. The low pressure column is operated at an operating pressure of, for example, 1.2 bar to 1.7 bar, specifically about 1.4 bar. The pressure indicated herein is the absolute pressure at the bottom of the corresponding distillation column. The pressure indicated will be below Also known as "distillation pressure", this occurs because the fractionation of the individual feed air in the distillation column occurs at these pressures. This does not exclude other pressures that can dominate at different locations in the distillation column system.

Cooling compressed air (feed air) that is pressured by a combination of various air compressors or various air compressors (eg, primary air compressor and afterburner) is fed into the distillation column system. All air compressors can be multi-stage. Since about 95% of the energy consumption in the air fractionation plant comes from the air compressor mentioned above, there is a maximum potential savings here. There is a fundamental need in the industry for more efficient processes and equipment for the energy of cryogenically fractionated air.

In view of this background, the present invention provides an air fractionation apparatus, a method of operating an air fractionation apparatus, and a control apparatus for such an air fractionation apparatus having the respective features of the independent patent application. Embodiments of the invention are set forth in the scope of the dependent claims and the following description.

Typically, the feed air that has reached the pressure in the air compressor of the air fractionation plant is recooled in a different configuration of the cooling device to remove the heat of compression generated during compression. The cooling devices comprise, for example, intercoolers and aftercoolers between one or more compression stages and downstream, as is known per se. Re-cooling of the air compressed in the main air compressor of the air fractionation plant can be accomplished, inter alia, in a direct contact cooler operating with cooling water from a cooling water circuit. In addition, it is possible to provide an indirect heat exchanger that can also operate using cooling water. In known processes, the feed air is then brought to an extremely low temperature in the main heat exchanger, i.e., significantly below 0 °C.

In particular, recooling is performed for the purpose of reducing the power consumption of the air compressor. Here, the lower the temperature of the cooling water, the further the process air can be cooled, which makes the power consumption of the air compressor lower. In addition, process air thus enters the actual process of air fractionation at lower temperatures, including entering the main heat exchanger. Therefore, the heat to be transferred in the main heat exchanger is low, whereby the heat exchanger can be designed to have a small volume and, in addition, Less cooling must be produced by decompression. Producing a cold to a suitable low temperature of, for example, 120K to 200K results in a substantial energy loss that is significantly greater than would be the case with cooling by cooling water that is implemented at near ambient temperature. In addition, the need to make larger cryogenic components (heat exchangers, turbines, valves) is more costly.

The cooling water circuit of the air fractionation apparatus usually comprises a recooling device, wherein the warming water of the cooling water circuit is cooled by evaporative cooling by means of cooling air. In particular, a known type of cooling tower (also as explained below) can be used as a recooling device. Corresponding air fractionation equipment is disclosed in, for example, EP 0 644 390 A1 and JP 5 885 093 A1. The cooling air used herein is typically derived from the surrounding environment of the air fractionation apparatus and therefore has ambient temperature dependent humidity, ambient dependent pressure, and ambient dependent humidity. The wet bulb temperature can be determined from these three parameters.

The wet bulb temperature is a measure of the cooling limit temperature, i.e., the cooling water can be theoretically obtained by direct evaporation in the corresponding recooling device. It is known that the release of water on a wet surface is in equilibrium with the ability to absorb water from the surrounding atmosphere. Due to the amount of cold produced by evaporation, the cooling limit temperature is lower than the air temperature as a function of relative atmospheric humidity. The greater the temperature drop during evaporative cooling, the drier the surrounding air. The temperature difference between the wet bulb temperature and the cooled cooling water actually obtained in the corresponding recooling device is referred to in the art as the cooling limit difference. The performance of a recooling device, such as a cooling tower, is determined by the specific surface area of the packing, the ratio of liquid to gas, and the pressure drop. In order to obtain a small cooling limit difference, as is expected in principle due to the advantages of the lower cooling water temperature mentioned above, the capital cost for installing the recooling device is significantly greater.

Therefore, the cooling limit difference used is determined by economic considerations including the mentioned aspects. In previous publications on forced air recooling devices in industrial equipment, the cooling limit difference of 3K to 5K is usually indicated as economically viable, see, for example, ZKMorvay and DDGvozdenac, "Applied Industrial Energy and Environmental Management, Part III: Toolbox-Fundamentals for Analysis and Calculation of Energy and Environmental Performance, Toolbox 12: Cooling Towers", Chichester, Wiley, 2008. However, this number is usually given without indicating the corresponding environmental conditions and the resulting wet bulb temperature. Recooling devices with cooling limit differences significantly below 3K can also be technically achieved, but are generally considered uneconomical. Accordingly, lower cooling limit differences are typically only used on a laboratory scale, as in, for example, according to VDPapaefthimiou et al. publication "Thermodynamic Study of Wet Cooling Tower Performance", Int. J. Energ. Res. 30 (6) ), 2006, 411-426.

Further details on the recooling unit and its design can be found in the relevant specialist literature (for example) H.-D.Held, H.-G.Schnell, Kühlwasser: Verfahren der Systeme der Aufbereitung und Kühlung von Süßwasser, Brackwasser-und Meerwasser zur Industriellen Kühlung, 5th edition, Vulkan, 2000; H. Rietschel, K. Fitzner, Raumklimatechnik, Vol. 2: Raumluft und Raumkühltechnik, 16th edition, Springer, 2008; JJMcKetta, Encyclopedia of Chemical Processing and Design, Volume 58 , Marcel Dekker, 1997; PNAnanthanarayanan, Basic Refrigeration and Air Conditioning, 3rd edition, Tata McGraw-Hill, 2006; and B. Buecker, Power Plant Water Chemistry: A Practical Guide, PennWell, 1997. In particular, it can be emphasized that the cooling limit difference achievable by means of the recooling device can be reliably predicted by those skilled in the art based on known calculation methods. Accordingly, it is stated below that the recooling device is configured such that it cools the cooling water to a temperature above the maximum temperature of the wet bulb temperature, and it is recommended for those skilled in the art to tailor the size of the recooling device such that it has the above The properties mentioned are that there is a corresponding cooling limit difference. In particular, those skilled in the art will herein account for or provide the specific surface area of the filler, the ratio of liquid to gas, and pressure drop.

Advantages of the invention

Surprisingly and with a forced air recooling device (see, for example, cited above) Contrary to the dominant view of the subject matter of the publications ZKMorvay and DDGvozdenac), it has been recognized in accordance with the present invention that based on the total operating cost (Total Cost of Ownership (TCO)), a cooling limit difference of less than 3K Provides economic advantages for many air fractionation equipment. Here, under design conditions, at a given wet bulb temperature, the cooling limit difference should be chosen as a function of capital value (expressed in currency units per kW, Net Present Value (NPV)). Thus, equipment having the same capital value can obtain recooling devices that remove the same specific heat regardless of individual environmental conditions. This makes it possible to systematically select a cooling tower that varies with the value of capital. As mentioned, according to the prevailing view, in particular for practical industrial applications, such as air fractionation equipment, a cooling limit difference significantly above 3K is considered appropriate. The publications of Z.K. Morvay and D.D. Gvozdenac, which have been cited several times, propose a series of efficiency improvements but have not proposed a reduction in the cooling limit.

Accordingly, the present invention proposes an air fractionation apparatus in which a cooling water circuit having a recooling means for cooling compressed air is provided, wherein the recooling means is configured to use cooling air to cool the cooling water. The air fractionation apparatus of the present invention is characterized in that the recooling means is configured to cool the cooling water to a temperature above the wet bulb temperature of no more than 3K at least at a wet bulb temperature of the cooling air of greater than 289K. In other words, under the specified conditions, a cooling limit difference of 3K or less, specifically 2K or less or 1K or less is achieved by means of the recooling device of the air fractionation apparatus of the present invention.

As has been found in the context of the present invention, the statement that the refrigerating device with a cooling limit difference of less than 2K, which is often found in the literature, is not technically feasible is erroneous. As such, it has been found that a typical profile of a cooling limit difference of only 3K to 5K is economically viable. As has been recognized in the context of the present invention, the indication of the feasibility and economic utility of a recooling device having a fixed cooling limit difference and ignoring the wet bulb temperature has no predictive value.

As has been recognized in accordance with the present invention and as described below, the reduction in the cooling limit difference to a value below 3K provides a significant improvement in the economics of the air fractionation apparatus at temperatures greater than 289 K and at high wet bulb temperatures. Therefore, the present invention is based on the general air fractionation process and low An important new assessment of the knowledge of the design of the cooling tower design.

In the context of the present invention, it has been shown that an efficient operation of recooling the cooling water to a temperature that is extremely close to the thermodynamically possible minimum (i.e., wet bulb temperature) allows for significant energy savings in the air fractionation apparatus. This is clearly shown in Figures 2 and 3 and other details are explained. The additional capital cost (CAPEX) of an effectively operated and therefore generally larger recooling device is amortized over an average of one year by saving operating costs (OPEX). The short amortization time of larger recooling units is due to their small proportion (usually about 2%) of the total cost of the air fractionation equipment. Table 1 gives an overview of the capital cost and operating costs of a conventional recooling device and a recooling device (herein, a corresponding cooling tower) according to the present invention.

In the economic evaluation, for example, a recooling device of a conventional design compared to the recooling device designed according to the present invention according to Figs. 2 and 3 can be viewed. It is assumed that the (forced ventilation) cooling tower having a water tank for accommodating the reflux is a re-cooling device. Lower cooling limit temperatures result in larger recooling units with equally increased tanks and thus higher capital costs. The mass flow of water is the same in both cases. The decisive fact is that in the case of large cooling towers, for the same amount of cooling water, a larger amount of air can flow through the recooling device; this air absorbs the evaporating water and at the same time makes greater convection cooling possible. This reduces the temperature of the cooling water in the cooling tower according to the present invention and, due to the lower power consumption of the air compressor and the energy optimized cooling tower, the operating cost is lower. In the case considered, it is assumed that the power cost is 0.07 €/kWh in each case. The results of a recooling unit using a 500,000 standard cubic meter process air/hour air fractionation plant are reported in Table 1 and the operating costs relate to one year.

For the purposes of the present invention, it is advantageous to use a recooling device such that it cools the cooling water to a temperature above the wet bulb temperature of at least 0.5 K, for example also at least 1 K, at least 1.5 K or at least 2 K. Configuration. From the above considerations, the optimal range of values for the minimum and maximum cooling limit differences can be derived.

The air fractionation plant according to the invention can in principle have a recooling device of any configuration, but this preferably comprises a cooling tower. Recooling devices or cooling towers, in particular with forced ventilation, have proven to be frequently used in air fractionation plants and have low maintenance requirements. As explained above, the cooling tower specifically allows a relatively simple reduction in the cooling limit temperature by increasing.

As mentioned above, the cooling water which has been cooled using the recooling device is particularly suitable for cooling downstream of the compressor in the respective air fractionation plant, so that the cooling water circuit of the air fractionation plant according to the invention advantageously comprises the air compressor Or a heat exchanger downstream of the stage of the corresponding air compressor. For the purposes of the present invention, an "air compressor" is a single or multi-stage configuration that is configured to increase pressure, in particular a radial compressor or a turbo compressor. One or more heat exchangers may be present downstream of one or more compressor stages.

In the context of the present invention, the cooling zone of the recooling device can range from 5K to 25K, in particular from 8K to 12K, typically about 10K.

The invention further extends to a method of operating an air fractionation apparatus wherein a cooling water circuit having a recooling device is provided for cooling compressed air, wherein the recooling device is configured to use cooling air to cool the cooling water. The method of the present invention is characterized in that the recooling device cools the cooling water to at least a wet bulb temperature of the cooling air of more than 289K. Operates in a manner that the temperature of the wet bulb does not exceed 3K. As such, the invention extends to a control device for an air fractionation apparatus that is configured for carrying out a method of this type. In both cases, reference may be made to what has been described above with respect to features and advantages.

1‧‧‧Compressor level

2‧‧‧Compressor level

3‧‧‧Intercooler

10‧‧‧Cooling water circuit

11‧‧‧Recooling device

100‧‧‧Air fractionation equipment

101‧‧‧Filter

102‧‧‧Main air compressor, compressor

103‧‧‧Direct contact with coolers, heat exchangers

104‧‧‧Evaporation cooler

105‧‧‧Adsorber group

106‧‧‧Regeneration gas heating devices and regenerative gas heaters for electrical and/or steam operation

107‧‧‧After compressor, compressor

108‧‧‧Main heat exchanger

110‧‧‧Distillation column system

111‧‧‧High pressure column

112‧‧‧Low pressure column

113‧‧‧ argon enrichment column

114‧‧‧Pure argon column

201‧‧‧ data points

202‧‧‧data points

203‧‧‧data points

A‧‧‧feed air flow, compressed and cooled feed air flow

b‧‧‧Cooling water flow, water flow

C‧‧‧flow, current

d‧‧‧Water flow

e‧‧‧Water flow

F‧‧‧air flow, flow

g‧‧‧Water flow

h‧‧‧Compressed and cooled feed air flow

i‧‧‧Regeneration gas flow

K‧‧‧流

l‧‧‧Compressed air flow

m‧‧‧Substream, flow

n‧‧‧Substream, flow

o‧‧‧Gaseous nitrogen flow

P‧‧‧ flow

Q‧‧‧ flow

r‧‧‧Liquid Oxygen-Enriched Flow

S‧‧‧ flow

Figure 1 shows an air fractionation apparatus in accordance with an embodiment of the present invention in the form of a schematic process flow diagram.

Figure 2 depicts cooling water temperatures and corresponding wet bulb temperatures to illustrate embodiments of the present invention.

Figures 3A and 3B show additional cooling of the cooling water and associated energy savings that can be achieved in accordance with the present invention.

The invention will now be illustrated with reference to the accompanying drawings, in which: FIG.

In Figure 1, an air fractionation apparatus in accordance with a preferred embodiment of the present invention is shown in the form of a schematic process flow diagram and is generally designated 100.

Feed air stream a is fed to air fractionation apparatus 100 via filter 101, compressed by main air compressor 102 and cooled in direct contact cooler 103, which is supplied, in particular, from evaporative cooler 104. Cooling water stream b. The water stream b is introduced into the direct contact cooler 103 by means of a pump which is not separately indicated. To provide a cooling water stream b, water to stream e is supplied to evaporative cooler 104, which may also be partially fed into direct contact cooler 103 without pre-cooling in evaporative cooler 104. The water flow d is taken out from the direct contact cooler 103.

The illustrated water streams b, c, and d are also integrated into the cooling water circuit (shown herein as 10) in the direct contact cooler 103 and the evaporative cooler 104. The cooling water circuit may also include any other water flow not shown, Pu, direct and indirect heat exchangers, etc. For example, as shown herein in a greatly simplified form, the primary air compressor 102 can have at least two compressor stages 1 And 2, intermediate cooling is performed between the two compressors by means of the intercooler 3. A typical main air compressor 102 of an air fractionation plant contains 5 to 9 compressor stages and a corresponding number of intercoolers. Cooling water in the form of a stream s can be fed into the illustrated intercooler 3, which is configured for indirect heat exchange. In particular, the flow s can be a substream of flow c, ie cooling water which is likewise circulated in the cooling water circuit 10. A similar situation applies to other (post) coolers as explained below. For example, to compensate for evaporation losses, other water streams may be fed into the cooling water circuit 10 anywhere, as indicated herein as water flow e. Additionally, cross-connections between water flow, conditioning devices, metrology sensors, and the like can be placed at advantageous locations in the cooling water circuit 10.

The core component of the cooling water circuit 10 is a recooling device 11, which is shown herein as a wet cooler and can be configured, for example, as a forced cooling cooling tower. However, as mentioned above, any other embodiment is also possible. The re-cooling device 11 is configured for operation in accordance with the embodiments of the invention mentioned above. The flow of atmospheric air f having a wet bulb temperature which is dominant at the position of the air fractionation apparatus 100 is fed into the recooling unit 11. The re-cooling device 11 is, for example, configured to cool the water of the water stream g to be cooled (formed by the water streams d and e in the illustrated example) to a temperature above the wet bulb temperature of the air stream f not exceeding 3K. Level. This is especially true when the wet bulb temperature of the air stream f is above 289K.

Further processing of the compressed and cooled feed air stream a (now designated h) corresponds to a large extent to conventional air fractionation equipment, for example as in H.-W. Häring (ed.), Industrial Gases Processing, Wiley-VCH, 2006, specifically the air fractionation equipment described in Section 2.2.5 "Cryogenic Rectification".

The compressed and cooled feed air stream h is fed to an adsorber stack 105 comprising alternately operated adsorber vessels and can be regenerated by means of a regeneration gas stream i. The regeneration gas stream i can be heated by means of an electrically operated and/or steam operated regeneration gas heating device 106. To provide a regeneration gas stream i, a stream k can be used, the provision of which will be explained in more detail below.

The compressed air flow that has been dried in the adsorber set 50 is indicated as one. End-view air fractionation In the configuration of apparatus 100, the compressed air stream 1 can be provided at a pressure that makes post-compression necessary or unnecessary (the latter being in the case of a high air pressure process). In the illustrated example, substream m of compressed air stream 1 is fed to post compressor 107. After the post compressor 107 is not individually designated, the cooler can similarly be cooled using water from the cooling water circuit 10.

According to the illustrated embodiment, the substream m of the compressed air stream 1 and the substream n that is not post-compressed are fed to the main heat exchanger 108 and are withdrawn therefrom at different temperature levels. The stream m can be depressurized by means of the generator turbine 109 and fed to the high pressure column 111 of the distillation column system 110 after being combined with the stream n. The other substreams of the compressed air stream 1 can be formed in an advantageous manner, cooled, post-compressed, depressurized and likewise fed to the column of the distillation column system 110, such as the known throttling stream not shown herein.

The high pressure column 111 and the low pressure column 112 together form a twin column system of a known type. In the example shown, the distillation column system additionally includes an argon enrichment column 113 and a pure argon column 114, but these need not be provided. Other distillation columns are available.

The operation of distillation column system 110 is known and will therefore not be explained. In the illustrated example, the distillation column system 110 is especially supplied with a gaseous nitrogen stream o ("impure nitrogen" in the form of stream p) from which stream k and/or stream q can be heated in the main heat exchanger 108 It is formed and can be fed to the regeneration gas heater 106 or the evaporative cooler 104, and the liquid oxygen-enriched stream r can be taken out. Instead of stream q, it is also possible to use, for example, a cold nitrogen-rich stream. Other streams will not be explained in detail. Any stream may be heated in the main heat exchanger 108, compressed or pressurized upstream or downstream of the main heat exchanger 108, combined with other streams, and divided into substreams.

Figure 2 shows the average cooling water temperature and corresponding wet bulb temperatures for each month of the year to illustrate an embodiment of the invention. The cooling water temperature (expressed in K) is plotted on the ordinate relative to the wet bulb temperature (indicated by K) plotted on the abscissa. In the figure, the wet bulb temperature is shown in the form of data points 201, and the cooling water temperature in the conventional design of the recooling device configured as a cooling tower is shown in the form of data points 202 and is implemented in accordance with the present invention. The cooling water temperature in the design of the example is shown in the form of data points 203.

The conventional design resulted in a cooling limit of 8K for a wet ball temperature of 289K. According to the illustrated embodiment of the invention, the cooling limit difference is reduced by 5 grams kelvin to 3K. The use of more efficient cooling towers and thus lowering the cooling limit temperature results in two effects, namely a smaller relative difference between the cooler cooling water and its secondary cooling water temperature and the wet bulb temperature. This means that for designs with relatively small cooling limit differences, the efficiency loss of the cooling tower is substantially lower during the relatively cold months. The reason for the lower efficiency loss of large cooling towers in the colder months is the water/air ratio that can be biased toward air movement. For the two cooling tower variants, the mass flow of water is the same, and the key factor is in the case of a large cooling tower. For the same amount of cooling water, a larger amount of air can flow through the recooling device, and this air absorption Evaporate water while allowing for large convective cooling. This effect produces a positive contribution especially at low air temperatures where air can absorb very little water.

Figures 3A and 3B show additional cooling of the cooling water (Figure 3A) and associated energy savings (Figure 3B) that can be achieved in accordance with the present invention. The temperature difference (indicated by K) in Figure 3A and the energy difference (in kW) in Figure 3B are plotted on the ordinate, relative to the month plotted on the abscissa (January (J) to December ( D)).

As can be seen from Figure 3A, substantially 5K cooler cooling water is obtained. The corresponding energy savings seen in Figure 3B are 270 kW to 450 kW/month and result in an average annual savings of 340 kW. The 340 kW compressor power consumption reduction corresponds to 1.5% of the total compressor power consumption.

1‧‧‧Compressor level

2‧‧‧Compressor level

3‧‧‧Intercooler

10‧‧‧Cooling water circuit

11‧‧‧Recooling device

100‧‧‧Air fractionation equipment

101‧‧‧Filter

102‧‧‧Main air compressor, compressor

103‧‧‧Direct contact with coolers, heat exchangers

104‧‧‧Evaporation cooler

105‧‧‧Adsorber group

106‧‧‧Regeneration gas heating devices and regenerative gas heaters for electrical and/or steam operation

107‧‧‧After compressor, compressor

108‧‧‧Main heat exchanger

110‧‧‧Distillation column system

111‧‧‧High pressure column

112‧‧‧Low pressure column

113‧‧‧ argon enrichment column

114‧‧‧Pure argon column

A‧‧‧feed air flow, compressed and cooled feed air flow

b‧‧‧Cooling water flow, water flow

C‧‧‧flow, current

d‧‧‧Water flow

e‧‧‧Water flow

F‧‧‧air flow, flow

g‧‧‧Water flow

h‧‧‧Compressed and cooled feed air flow

i‧‧‧Regeneration gas flow

K‧‧‧流

l‧‧‧Compressed air flow

m‧‧‧Substream, flow

n‧‧‧Substream, flow

o‧‧‧Gaseous nitrogen flow

P‧‧‧ flow

Q‧‧‧ flow

r‧‧‧Liquid Oxygen-Enriched Flow

S‧‧‧ flow

Claims (8)

An air fractionation apparatus (100), wherein a cooling water circuit (10) having a recooling device (11) for cooling compressed air is provided, wherein the recooling device (11) is configured to use cooling air to cool the cooling water characterized in that the re-cooling device (11) via at least configured so that the cooling air is greater than the wet-bulb temperature of 289K of the cooling water to a temperature above the temperature does not exceed the wet bulb temperature of the 3K. The air fractionation apparatus (100) of claim 1, wherein the recooling means (11) is configured to cool the cooling water to a temperature above the wet bulb temperature of at least 0.5K. The air fractionation apparatus (100) of claim 1 or 2, wherein the recooling means (11) comprises a cooling tower. The air fractionation apparatus (100) of claim 3, wherein the recooling means (11) has forced ventilation. An air fractionation apparatus (100) according to any of the preceding claims, wherein the cooling water circuit (10) comprises a heat exchanger (103) arranged in the compressor (102, 107) Downstream. An air fractionation apparatus (100) according to any of the preceding claims, wherein a cooling zone range of 5K to 25K is provided. A method of operating an air fractionation apparatus (100), wherein a cooling water circuit (10) having a recooling device (11) is provided for cooling compressed air, wherein the recooling device (11) is configured for use with cooling air The cooling water is cooled, characterized in that the recooling device (11) is configured and operated to cool the cooling water to a temperature above the wet bulb temperature of no more than 3K at least at a wet bulb temperature of the cooling air greater than 289K. The method of claim 7, wherein the specific surface area of the filler in the recooling device (11) and/or the ratio of liquid to gas and/or pressure drop is such that at least greater than 289K The cooling water at the wet bulb temperature is selected and/or set by cooling the cooling water to a temperature above the wet bulb temperature of no more than 3K.