EP3124902A1 - Air separation facility, operating method and control device - Google Patents

Abstract

The invention relates to an air separation plant (100) in which compressed air for cooling a cooling water circuit (10) with a recooling plant (11) is provided, wherein the recooling plant (11) is adapted to cool cooling water using cooling air. The recooling plant (11) is designed such that, at least at a wet bulb temperature of the cooling air of more than 289 K, the cooling water cools to a temperature which is at most 3 K above the wet bulb temperature. A corresponding operating method and a control device are likewise provided by the invention.

Description

The invention relates to an air separation plant, a method for operating an air separation plant and a control device for such an air separation plant according to the respective preambles of the independent claims.

State of the art

The production of air products in the liquid or gaseous state by cryogenic separation of air in air separation plants is known and, for example at H.-W. Haring (ed.), Industrial Gases Processing, Wiley-VCH, 2006, especially Section 2.2.5, "Cryogenic Rectification The present invention is suitable for different embodiments of corresponding air separation plants.

Air separation plants have distillation column systems which can be designed, for example, as two-column systems, in particular as classic Linde double-column systems, but also as three-column or multi-column systems. In addition to the distillation columns for the recovery of nitrogen and / or oxygen in the liquid and / or gaseous state (for example, liquid oxygen, LOX, gaseous oxygen, GOX, liquid nitrogen, LIN and / or gaseous nitrogen, GAN), so the distillation columns for nitrogen-oxygen Separation, distillation columns can be provided for obtaining further air components, in particular the noble gases krypton, xenon and / or argon.

The distillation column systems of air separation plants are operated at different operating pressures in their distillation columns. Known double column systems have, for example, a so-called (high) pressure column and a so-called low-pressure column. The operating pressure of the high-pressure column is, for example, 4.3 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 to 1.7 bar, in particular about 1.4 bar. The pressures stated here are absolute pressures in Bottom of corresponding distillation columns. The pressures mentioned below are also referred to as "distillation pressures" because they are subjected to the fractional distillation of the respectively fed air within the distillation columns. This does not exclude that other pressures may be present elsewhere in a distillation column system.

In the distillation column systems cooled compressed air (feed air) is fed, which is brought by means of different air compressor or combinations of different air compressors (for example, main air compressors and booster) to pressure. All air compressors can be multi-stage. Since about 95% of the energy demand in an air separation plant is created by the air compressors mentioned, the greatest potential for saving is present here. There is a general need for more energy efficient processes and systems for the cryogenic separation of air.

Disclosure of the invention

Against this background, the present invention provides an air separation plant, a method for operating an air separation plant and a control device for such an air separation plant with the respective features of the independent claims. Embodiments of the invention are the subject of the dependent claims and the following description.

The feed air pressurized in the air compressors of an air separation plant is typically recooled in various cooling devices to remove the heat of compression generated during the compression. These cooling devices include, for example, intermediate and aftercoolers between and downstream of one or more compression stages, as known in the art. The recooling of the air compressed in a main air compressor of an air separation plant can take place, for example, inter alia in a direct contact cooler operated with cooling water from a cooling water circuit. In addition, indirect heat exchangers can be provided, which can also be operated with cooling water. Subsequently, the feed air is brought in known processes in a main heat exchanger to cryogenic temperatures, ie temperatures of well below 0 ° C.

The recooling is in particular for the purpose of reducing the power consumption of the air compressor. The lower the cooling water temperature is, the further the process air can be cooled, resulting in lower power consumption of the air compressors. In addition, the process air already enters the process of air separation colder, including in the main heat exchanger. Therefore, the heat to be transferred to the main heat exchanger is lower, which can therefore be carried out with a smaller volume and in addition less cold must be generated by relaxation. Refrigeration at correspondingly low temperature levels of, for example, 120 to 200 K results in considerable exergetic losses, which are significantly greater than in the cooling of cooling water, which occurs at near ambient temperature. In addition, larger-sized cryogenic components (heat exchangers, turbines, valves) are extremely costly.

Cooling water circuits of air separation plants typically include recooling plants in which the heated cooling water of the cooling water circuit is cooled by cooling air by evaporative cooling. In particular, cooling towers of known type can be used as recooling units, as also explained below. Corresponding air separation plants are for example in the EP 0 644 390 A1 and the JP 5 885093 A1 shown. The cooling air used in this case is typically derived from the environment of the air separation plant and therefore has a dependent on the environment temperature, a dependent on the environment pressure and a dependent on the environment moisture content. From these three parameters, the wet bulb temperature can be determined.

The wet bulb temperature is a measure of the cooling limit temperature, ie the lowest temperature that the cooling water can theoretically achieve by direct evaporative cooling in a corresponding recooling plant. As is known, the release of water from a moist surface is in equilibrium with the water absorption capacity of the surrounding atmosphere. Due to the evaporative cooling, the cooling limit temperature is dependent on the relative humidity below the air temperature. The lower the temperature in the evaporative cooling, the drier the surrounding air is. The temperature difference actually reached in a corresponding recooling plant between the Wet bulb temperature and the cooled cooling water is referred to in the art as the cooling limit distance. The quality of a recooling plant, for example a cooling tower, is determined by the specific packing surface, the ratio of liquid to gas and the pressure loss. In order to achieve a low cooling limit distance, as it is basically desirable due to the above-mentioned advantages of a lower cooling water temperature, therefore, in principle, higher investment costs for the preparation of recooling come on.

The used cooling distance is therefore based on economic considerations that include the above aspects. In previous publications on forced-air recooling plants in industrial plants usually a cooling limit distance of 3 to 5 K is given as economical, see for example Z. K. Morvay & DD Gvozdenac, "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 information is conventionally given without mentioning the corresponding environmental conditions and the resulting wet bulb temperature. Recooling plants with a cooling limit distance of well below 3 K are technically feasible, but this is conventionally referred to as uneconomical. Correspondingly small cooling boundary distances are conventionally used only on a laboratory scale, as for example according to the publication of VD Papaefthimiou et al., "Thermodynamic Study of Wet Cooling Tower Performance", Int. J. Energ. Res. 30 (6), 2006, 411-426 , the case.

For further details regarding recooling and their interpretation is in relevant literature, such as H.-D. Held, H.-G. Fast, cooling water: process of systems of treatment and cooling of Fresh water, brackish water and sea water for industrial cooling, 5th edition, Vulkan, 2000, H. Rietschel, K. Fitzner, Raumklimatechnik, Volume 2 : Room air and room cooling technology, 16th edition, Springer, 2008 . JJ McKetta, Encyclopedia of Chemical Processing and Design, Vol. 58, Marcel Dekker, 1997 . PN Ananthanarayanan, Basic Refrigeration and Air Conditioning, 3rd Edition, Tata McGraw-Hill, 2006 and B. Buecker, Power Plant Water Chemistry: A Practical Guide, PennWell, 1997 , referenced. In particular, it should be emphasized that the cooling limit distance achievable by means of a recooling plant can be reliably predicted by the person skilled in the art on the basis of known calculation methods. is Therefore, the following is the speech that a recooling is designed such that it cools cooling water to a temperature which is a maximum temperature above the Feuchtkugeltemperatur, this is an instruction to those skilled in dimensioning a recooling such that it Has property, so has a corresponding cooling limit distance. In particular, the person skilled in the art will take appropriate account of the specific packing surface, the ratio of liquid to gas and the pressure loss.

Advantages of the invention

According to the invention was surprisingly and in contrast to the prevailing opinion on forced-ventilated recooling (see, for example, already cited publication ZK Morvay & DD Gvozdenac) recognized that, based on the total cost of ownership (TCO), a cooling limit distance below 3 K for a variety offers economic benefits from air separation plants. In this case, the cooling limit distance should be selected depending on the capital value (in monetary units per kW, net present value, NPV) for a given wet bulb temperature under design conditions. Systems with the same net present value can thus receive a recooling unit with the same amount of specific heat dissipated regardless of the respective ambient conditions. This allows a systematic selection of the cooling towers depending on the net present value. As mentioned, according to prevailing opinion, especially for real industrial applications such as air separation plants, a cooling boundary distance well above 3 K is considered useful. In the cited publication by Z.K. Morvay & D.D. Gvozdenac proposes a number of efficiency improvements, but not the lowering of the cooling margin.

The present invention therefore proposes an air separation plant in which a cooling water circuit with a recooling plant is provided for cooling compressed air, wherein the recooling plant is adapted to cool cooling water using cooling air. The air separation plant according to the invention is characterized in that the recooling plant is set up to cool the cooling water to a temperature at least 3 K above that at least at a wet bulb temperature of the cooling air of more than 289 K Wet bulb temperature is. In other words, by means of the recooling plant of the air separation plant according to the invention, a cooling boundary distance of 3 K or less, in particular also 2 K or less or 1 K or less, is achieved under the stated conditions.

As has been found in the context of the present invention, the frequently found in the literature statement, recooling units with a cooling limit distance of less than 2 K are not technically feasible, wrong. Likewise, the commonly used generalization, only a cooling limit distance of 3 to 5 K would be economically useful, as not recognized correctly. An indication of the feasibility as well as the economic benefit of recooling plants with a fixed cooling boundary distance in disregard of the wet bulb temperature, as recognized in the context of the present invention, not meaningful.

By reducing the cooling boundary distance to values below 3 K, the efficiency of air separation plants can be markedly increased at medium and high wet bulb temperatures of more than 289 K as recognized and subsequently documented in accordance with the invention. The present invention is thus based on a substantial revaluation of the state of knowledge of cooling tower design with respect to air separation processes and cryogenic processes in general.

In the context of the present invention it has been shown that a more efficient recooling plant, which cools the cooling water very close to the thermodynamically possible minimum (ie the wet bulb temperature), enables a significant energy saving of the air separation plant. This will be in the Figures 2 and 3 clarified and further details explained. The additional investment costs (CAPEX) for a more efficient and thus generally larger recooling plant are amortized on average in about one year by savings on the running costs (OPEX). The short amortization period of the larger recooling plants is due to their small share (typically about 2%) of the total costs for an air separation plant. Table 1 gives an overview of investment costs and operating costs with a conventional and a recooling plant according to the invention, here a corresponding cooling tower.

Within the scope of a profitability analysis, for example, a conventionally designed recooling plant can be designed in comparison with a recooling plant designed according to the invention Figures 2 and 3 to be viewed as. The recooling plant was assumed to be a (forced-ventilated) cooling tower with a water basin for receiving the return. A lower cooling limit temperature leads to a larger recooling plant with a likewise enlarged basin and thus to a higher investment price. The water mass flow is the same in both cases. It is crucial that in a large cooling tower with the same amount of cooling water, a larger amount of air can flow through the recooling, which absorbs the evaporating water and simultaneously allows a stronger convective cooling. This lowers the cooling water temperature in the cooling tower according to the invention and, due to the lower power consumption at the air compressors and an energy-optimized cooling tower, leads to lower operating costs. In the case considered, electricity costs of € 0.07 / kWh were assumed. The results for the recooling plant of an air separation plant with 500,000 standard cubic meters per hour of process air are given in Table 1, with the operating costs relating to one year. <b> Table 1: Investment and operating costs of a conventional and a cooling tower designed according to the invention </ b> investment costs conventional inventively additional costs pool 50.444 € 81,806 € 31,362 € cooling tower 211,500 € 475,500 € 264,000 € total 295,362 € operating cost conventional inventively saving cooling tower 89,250 € 91,630 € 2,380 € water pumps 228,985 € 228,985 € 0 € air compressor 13,315,505 € 13,110,446 € -205,059 € total -202,679 €

In the context of the present invention, a recooling unit is advantageously used which is designed such that it cools the cooling water to a temperature which is at least 0.5 K, for example but also at least 1 K, at least 1.5 K or at least 2 K above the wet bulb temperature is. An optimal one The range of values for minimum and maximum cooling limit distances results from the above considerations.

In principle, an air separation plant according to the invention can have an optionally designed recooling plant, but in particular this comprises a cooling tower. In particular, recooling or cooling towers with forced ventilation are often used in air separation plants and are proven and low maintenance. In particular, a cooling tower allows a comparatively simple reduction of the cooling limit temperature by enlargement, as explained above.

As already mentioned above, the cooling water cooled using the recooling plant is suitable in particular for aftercooling downstream of compressors in a corresponding air separation plant, so that the cooling water circuit of an air separation plant according to the invention advantageously comprises a heat exchanger downstream of an air compressor or a stage of a corresponding air compressor. An "air compressor" is here understood to mean a single-stage or multi-stage arrangement which is set up to increase the pressure, in particular a radial or turbo compressor. One or more heat exchangers may be downstream of one or more compressor stages.

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

The invention further extends to a method for operating an air separation plant, in which a cooling water circuit is provided with a recooling unit for cooling compressed air, wherein the recooling unit is adapted to cool cooling water using cooling air. The inventive method is characterized in that the recooling is operated such that it cools the cooling water to a temperature which is at most 3 K above the wet bulb temperature at least at a wet bulb temperature of the cooling air of more than 289 K. Likewise, the present invention extends to a control device of an air separation plant, which is adapted to perform a corresponding method. In both cases reference should be made to the above explanations regarding features and advantages.

The invention will be explained in more detail below with reference to the accompanying drawings which show preferred embodiments of the invention.

Brief description of the drawings

In FIG. 1 an air separation plant according to an embodiment of the invention is illustrated in the form of a schematic process flow diagram.

In FIG. 2 For example, the cooling water temperatures and the corresponding wet bulb temperatures are shown to illustrate one embodiment of the invention.

In FIGS. 3A and 3B are the present invention possible additional cooling of cooling water and the associated energy savings illustrated.

Detailed description of the drawings

In FIG. 1 an air separation plant according to a particularly preferred embodiment of the invention is illustrated in the form of a schematic process flow diagram and denoted overall by 100.

The air separation plant 100 is fed via a filter 101, a feed air stream a, compressed by a main air compressor 102 and cooled in a direct contact cooler 103, which among other things with a cooled water flow b from an evaporative cooler 104 is charged. The water flow b is thereby applied via a not separately designated pump to the direct contact cooler 103. In order to provide the cooled water flow b, water is supplied to the evaporative cooler 104 of a stream c, which can also be fed into the direct contact cooler 103 partially without prior cooling in the evaporative cooler 104. The direct contact cooler 103, a stream of water d is removed.

The illustrated water flows b, c and d as well as the direct contact cooler 103 and the evaporative cooler 104 are incorporated into a cooling water circuit designated here by 10, which also contains any other water flows, not illustrated, Pumps, direct and indirect heat exchangers, etc. may include. For example, the main air compressor 102, as illustrated here in a highly simplified manner, have at least two compressor stages 1 and 2, between which an intermediate cooling takes place by means of an intercooler 3. Typical main air compressors 102 of air separation plants include five to nine compressor stages and a corresponding number of intercoolers. The illustrated intercooler 3, which is set up for an indirect heat exchange, can be supplied with cooling water in the form of the flow s. The stream s may in particular be a partial stream of the stream c, that is, cooling water which also circulates in the cooling water circuit 10. The same applies to further (after-) cooler as explained below. In the cooling water circuit 10 more water streams can be fed at any point, for example, to compensate for evaporation losses, as illustrated here with the water flow e. At an appropriate point in the cooling water circuit 10 also cross connections between water streams, control devices, sensors and the like may be arranged.

Central component of the cooling water circuit 10 is a recooling 11, which is illustrated here as a wet cooler and may be formed, for example, as a cooling tower with forced ventilation. As mentioned, however, any other configurations are possible. The recooling plant 11 is set up for operation in accordance with a previously explained embodiment of the invention. The recooling plant 11 is supplied with a current f atmospheric air with a prevailing at the location of the air separation plant 100 Feuchtkugeltemperatur. The recooling plant 11 is designed, for example, such that it cools water of a water stream g to be cooled formed in the example shown from the water streams d and e to a temperature level which is at most 3 K above the wet bulb temperature of the air stream f. This is especially true when the wet bulb temperature of the airflow f is above 289K.

• Der verdichtete und gekühlte Einsatzluftstrom h, wird einem Adsorbersatz 105 zugeführt, der im Wechsel betrieb betriebene Adsorberbehälter umfasst und mittels eines Regeneriergasstroms i regeneriert werden kann. Der Regeneriergasstrom i kann mittels einer elektrisch und/oder mit Dampf betriebenen Regeneriergasheizung 106, erwärmt werden. Zur Bereitstellung des Regeneriergasstroms i kann ein Strom k verwendet werden, dessen Bereitstellung unten näher erläutert wird.

The further processing of the compressed and cooled feed air flow a, which is now designated h, largely corresponds to that in conventional air separation plants, for example in an air separation plant, as described in H.- W. Haring (ed.), Industrial Gases Processing, Wiley-VCH, 2006, especially Section 2.2.5, "Cryogenic Rectification "is described:

• The compressed and cooled feed air stream h is fed to an adsorber set 105, which comprises 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 and / or steam-operated regeneration gas heater 106. To provide the Regeneriergasstroms i, a current k can be used, the provision of which is explained in more detail below.

A dried in the Adsorbersatz 50 compressed air flow is denoted by I. Depending on the embodiment of the air separation plant 100, the compressed air flow I can be provided at a pressure which makes recompression necessary or dispensable (the latter in so-called high-air pressure processes). In the example shown, a partial flow m of the compressed air flow I is fed to a secondary compressor 107. A not separately designated aftercooler of the after-compressor 107 can also be cooled with water from the cooling water circuit 10.

The partial flow m and a non-compressed partial flow n of the compressed air flow I are, according to the illustrated embodiment, supplied to a main heat exchanger 108 and taken from it at different temperature levels. The stream m can be expanded by means of a generator turbine 109 and, after being combined with the stream n, fed into a high-pressure column 111 of a distillation column system 110. Further partial streams of the compressed air stream I can be advantageously formed, cooled, re-compressed, expanded and likewise fed into columns of the distillation column system 110, for example a known, but not illustrated here, inductor current.

The high-pressure column 111 together with a low-pressure column 112 forms a double-column system of known type. In the example shown, the distillation column system additionally comprises an argon enrichment column 113 and a pure argon column 114, which, however, need not be provided. Additional distillation columns may be provided.

The operation of the distillation column system 110 is known and therefore not discussed. In the example shown, the distillation column system 110 may include, inter alia, a gaseous stream of nitrogen o, so-called impure nitrogen in the form of the stream p, from which, after heating in the main heat exchanger 108, the flow k and / or a stream q can be formed and supplied to the regeneration gas heater 106 or the evaporative cooler 104, and a liquid, oxygen-rich stream r can be removed. Instead of the stream q, for example, a cold, nitrogen-enriched stream can be used. Further currents are not explained in detail. Any streams may be heated in the main heat exchanger 108, pressurized upstream and / or downstream of the main heat exchanger 108, combined with other streams, and divided into sub-streams.

In FIG. 2 Fig. 2 shows the average cooling water temperatures of the months of a year and the corresponding wet bulb temperatures for illustrating an embodiment of the invention. The cooling water temperature is plotted in K on the ordinate against the wet bulb temperature in K on the abscissa. The diagram shows the wet bulb temperature in the form of the data points 201, the cooling water temperature in the conventional design of a cooling tower formed as a cooling tower in the form of the data points 202 and the cooling water temperature in a design according to an embodiment of the invention in the form of the data points 203.

The conventional design produces a cooling limit of 8K at a wet bulb temperature of 289K. According to the illustrated embodiment of the invention, the cooling margin has been reduced by five Kelvin to 3K. The use of a more efficient cooling tower and thus a lowering of the cooling limit temperature leads to two effects, on the one hand to colder cooling water and on the other hand to a lower relative distance of the cooling water temperature to the wet bulb temperature. This means that the efficiency loss of the cooling towers for a design with a smaller cooling boundary distance is generally lower in colder months. The reason for the lower efficiency loss of a large cooling tower in the cold months is the water-air ratio, which can be shifted in favor of the air. The water mass flow is the same for both types of cooling tower, it is crucial that with a large cooling tower with the same amount of cooling water, a larger amount of air can flow through the cooling tower, which absorbs the evaporating water and at the same time allows a stronger convective cooling. Especially at low air temperatures, where the air can absorb little water, this effect has a positive effect.

In FIGS. 3A and 3B are the possible additional cooling of cooling water according to the invention ( FIG. 3A ) and the associated energy savings ( FIG. 3B ). In FIG. 3A is a temperature difference in K, in FIG. 3B an energy difference in kW on the ordinate against the months of January (J) to December (D) plotted on the abscissa.

How out FIG. 3A can be seen, results in an average of almost 5 K colder cooling water. The accordingly off FIG. 3B The apparent energy saving is between 270 and 450 kW per month, resulting in an average annual saving of 340 kW. The minimization of the compressor power consumption by 340 kW corresponds to 1.5% of the total compressor power consumption.

Claims (8)

Air separation plant (100), in the cooling of compressed air, a cooling water circuit (10) is provided with a recooling plant (11), wherein the recooling plant (11) is adapted to cool cooling water using cooling air, characterized in that the recooling (11 ) is designed such that, at least at a wet bulb temperature of the cooling air of more than 289 K, the cooling water cools to a temperature which is at most 3 K above the wet bulb temperature. Air separation plant (100) according to claim 1, wherein the recooling unit (11) is designed such that it cools the cooling water to a temperature which is at least 0.5 K above the wet bulb temperature. Air separation plant (100) according to claim 1 or 2, wherein the recooling plant (11) comprises a cooling tower. Air separation plant (100) according to claim 3, wherein the recooling plant (11) has forced ventilation. An air separation plant (100) according to any one of the preceding claims, wherein the cooling water circuit (10) comprises a heat exchanger (103) disposed downstream of a compressor (102, 107). Air separation plant (100) according to one of the preceding claims, in which a cooling zone width of 5 to 25 K is provided. A method for operating an air separation plant (100), in which for the cooling of compressed air, a cooling water circuit (10) with a recooling plant (11) is provided, wherein the recooling plant (11) is adapted to cool cooling water using cooling air, characterized in that the recooling plant (11) is designed and operated such that, at least at a wet bulb temperature of the cooling air of more than 289 K, the cooling water cools to a temperature which is at most 3 K above the wet bulb temperature. The method of claim 7, wherein the specific packing surface and / or the ratio of liquid to gas and / or the pressure loss in the recooling unit (11) is selected and / or adjusted such that at least at a wet bulb temperature of the cooling air of more than 289 K the cooling water is cooled to the temperature which is at most 3 K above the wet bulb temperature.

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