EP3594596A1 - Method for operating a heat exchanger, assembly with a heat exchanger and air processing installation with such an assembly - Google Patents

Abstract

The invention proposes a method for operating a heat exchanger (100) which has a heat exchange zone (10) which extends between a first end (11) and a second end (12), wherein fluids at different temperature levels are used in a first operating mode in a first quantity per unit of time through the heat exchange zone (10), whereby the first end (11) of the heat exchange zone (10) is brought to a first temperature level and the second end (12) of the heat exchange zone (10) to a second temperature level, which is below the first temperature level is brought, wherein in a second operating mode, the passage of the fluids, which are conducted in the first operating mode in the first amount per unit time through the heat exchange zone (2), is at least partially prevented, whereby a temperature transition from the first end (11) to the second end (12) of the heat exchange zone (10), and wherein in the method is switched several times from the first operating mode to the second operating mode and from the second operating mode to the first operating mode. It is envisaged that the switchover from the second operating mode to the first operating mode comprises the fluids, which are conducted in the first operating mode in the first amount per unit of time through the heat exchange zone (10), up to an increase time initially in a second amount per Unit of time which is less than the first amount per unit of time to pass through the first heat exchange zone (10) and only to pass through the first heat exchange zone (10) in the first amount per unit of time from the time of increase. A corresponding arrangement, which is designed in particular as an air processing system, is also the subject of the invention.

Description

The invention relates to a method for operating a heat exchanger and an arrangement with a correspondingly operable heat exchanger according to the preambles of the respective independent claims.

State of the art

Heat exchangers with cryogenic fluids, i.e. Operate fluids at temperatures significantly below 0 ° C, in particular significantly below -100 ° C. In the following, the present invention is mainly described with reference to the main heat exchangers (also referred to as "main heat exchanger" or "main heat exchanger") of air separation plants, but it is also fundamentally suitable for use in other fields of application, for example for plants for storing and recovering energy using of liquid air or natural gas liquefaction.

For the reasons explained below, the present invention is also particularly suitable in systems for liquefying gaseous air products, for example gaseous nitrogen. Corresponding plants can be supplied with gaseous nitrogen and liquefy it, in particular by air separation plants. The liquefaction is not followed by a rectification like in an air separation plant. Therefore, when the problems described below are overcome, for example when there is no need for corresponding liquefaction products, these systems can be switched off completely and kept in standby until the next use.

For the construction and operation of main heat exchangers in air separation plants and other heat exchangers, refer to the relevant specialist literature, for example H.-W. Häring (ed.), Industrial Gases Processing, Wiley-VCH, 2006 , in particular Section 2.2.5.6, "Apparatus". Details of heat exchangers in general can be found, for example, in the publication " The Standards of the Brazed Aluminum Plate-Fin Heat Exchanger Manufacturers' Association ", 2nd edition, 2000 , in particular Section 1.2.1, "Components of an Exchanger".

Without additional measures, heat exchangers from air separation plants and other heat exchangers through which warm and cryogenic media flow flow heat up when the associated plant is at a standstill and thus the heat exchanger is decommissioned, or the temperature profile formed in a corresponding heat exchanger cannot be maintained in such a case. If, for example, fluid is subsequently fed into a heated or temperature-balanced heat exchanger as explained below, which has a large temperature difference from the local metal temperature, this causes high thermal stresses due to the thermal expansion or contraction of the metal, which can damage the heat exchanger or require a disproportionately high amount of material or production. This applies both when warm fluid meets colder metal and when cold fluid meets warmer metal.

In particular, when a heat exchanger is decommissioned before it heats up overall, due to the good heat conduction (longitudinal heat conduction) in its metallic material, the temperatures at the previously warm end and at the previously cold end are equalized. In other words, the previously warm end of the heat exchanger becomes colder over time and the previously cold end of the heat exchanger warmer until the temperatures mentioned are at or close to an average temperature. This is also attached in the Figure 1 illustrated again. The temperatures, which were around -175 ° C or +20 ° C at the time of decommissioning, converge over several hours and reach almost a medium temperature.

This behavior is observed in particular when the main heat exchanger, which is housed in a cold-insulated manner, is blocked together with the rectification unit when an air separation plant is switched off, ie when no more gas is supplied from the outside. In such a case, typically only gas that is produced due to thermal insulation losses is blown off cold. The same also applies if a system for liquefying a gaseous air product, for example liquid nitrogen, is switched off.

If hot fluid is subsequently fed into the cooled, warm end of the heat exchanger when it is restarted, the temperature there suddenly increases. Accordingly, the temperature at the heated cold end drops suddenly when restarting, if corresponding cold fluid is fed in there, suddenly. This leads to the material stresses already mentioned and thus possibly to damage.

The present invention therefore has as its object to provide measures which enable a corresponding heat exchanger to be put into operation again after a prolonged decommissioning without the disadvantageous effects mentioned.

Disclosure of the invention

Against this background, the present invention proposes a method for operating a heat exchanger and an arrangement with a correspondingly operable heat exchanger, which can be designed in particular as an air processing system, system for storing and recovering electrical energy or system for liquefying an air product, with the features of respective independent claims. Refinements are the subject of the dependent claims and the following description.

First, some terms used to describe the present invention are explained and defined below.

In the parlance used here, a "heat exchanger" is an apparatus which is designed for the indirect transfer of heat between at least two fluid streams, for example in countercurrent to one another. A heat exchanger for use in the context of the present invention can be formed from a single or a plurality of heat exchanger sections connected in parallel and / or in series, for example from one or more plate heat exchanger blocks. A heat exchanger has “passages” which are set up for fluid guidance and are fluidically separated from other passages or are only connected on the input and output sides via the respective headers. These are referred to below as "heat exchanger passages". Frequently The terms "heat exchanger" and "heat (exchanger)" are used synonymously in the professional world. This also applies here.

The present invention relates in particular to the apparatuses designated as fin-plate heat exchangers in accordance with the German version of ISO 15547-2: 2005. If the term "heat exchanger" is used below, this should be understood to mean in particular a fin-plate heat exchanger. A fin-plate heat exchanger has a large number of superimposed flat chambers or elongated channels, which are each separated from one another by corrugated or otherwise structured and interconnected, for example soldered plates, usually made of aluminum. The plates are stabilized by means of side bars and connected to each other. The structuring of the heat exchanger plates serves in particular to increase the heat exchange area, but also to increase the stability of the heat exchanger. The invention relates in particular to brazed aluminum fin-plate heat exchangers.
As mentioned, the present invention can be used in air separation plants of known type, but also, for example, in plants for storing and recovering energy using liquid air and in plants for liquefying gaseous air products. The storage and recovery of energy using liquid air is also referred to as Liquid Air Energy Storage (LAES). A corresponding system is for example in the EP 3 032 203 A1 disclosed.

At times of high electricity supply, air is compressed, cooled, liquefied and stored in an insulated tank system in a first operating mode in a first operating mode. At times of low electricity supply, the liquefied air stored in the tank system is heated in a second operating mode, in particular after a pressure increase by means of a pump, and thus converted into the gaseous or supercritical state. A pressure stream obtained in this way is expanded in an expansion turbine which is coupled to a generator. The electrical energy obtained in the generator is fed back into an electrical network, for example.

Appropriate storage and recovery of energy is fundamentally not only possible using liquid air. Rather, other cryogenic liquids formed using air can also be stored in the first operating mode and used to generate electrical energy in the second operating mode. Examples of corresponding cryogenic liquids are liquid nitrogen or liquid oxygen or component mixtures which predominantly consist of liquid nitrogen or liquid oxygen. In corresponding systems, external heat and fuel can also be coupled in in order to increase the efficiency and the output power, in particular using a gas turbine, the exhaust gas of which is expanded together with the pressure stream formed from the air product in the second operating mode. The invention is also suitable for such systems.

Classic air separation plants can be used to provide appropriate cryogenic liquids. If liquid air is used, it is also possible to use pure air liquefaction systems. The term “air treatment plants” is therefore also used below as a generic term for air separation plants and air liquefaction plants.

Advantages of the invention

In principle, cold gas from a tank or exhaust gas from the stationary system can flow through a heat exchanger while the associated system is at a standstill, in order to avoid heating or to maintain the temperature profile formed. However, such an operation can only be implemented with great effort in conventional methods.

In particular in the case of small amounts of corresponding cold gases or low flow velocities in the heat exchanger, incorrect distribution within a heat exchanger block and in particular over several heat exchanger blocks cannot be ruled out. In principle, however, it is desirable to keep the gas quantities used low, for example in order to avoid product losses or the consumption of appropriate cryogenic media. Furthermore, implementation is more appropriate Measures always require certain amounts of fluids that are additionally used to temper an appropriate heat exchanger.

The present invention proposes a method of operating a heat exchanger having a heat exchange zone extending between a first end and a second end, wherein in a first mode of operation fluids at different temperature levels are passed through the heat exchange zone in a first amount per unit time whereby the first end of the heat exchange zone is brought to a first temperature level and the second end of the heat exchange zone is brought to a second temperature level which is below the first temperature level. Such a first operating mode corresponds to a normal operation of the heat exchanger, which is used for the temperature control of corresponding fluids, which can be provided in the form of one or more identical or different fluid flows. As is customary in this respect, at least one first fluid to be cooled is passed through the heat exchange zone from the first to the second end and at least one second fluid to be heated is passed from the second to the first end. Corresponding temperature levels can in particular be at least partially in a cryogenic range. For example, the first temperature level can be in particular from 0 to 100 ° C, for example at approximately 20 ° C, and the second temperature level can be in particular from -100 to 200 ° C, for example at approximately -175 ° C.

Within the scope of the method proposed according to the invention, in a second operating mode, the passage of the fluids, which in the first operating mode are conducted through the heat exchange zone in the first quantity per unit of time, is at least partially prevented. A corresponding heat exchanger can in particular also be “blocked in” completely, ie there is no longer any flow of fluid and any evaporating fluid that may be present is removed. A corresponding heat exchanger can be arranged in a so-called cold box, in particular together with other apparatus. A corresponding decommissioning can be particularly advantageous in the case of a plant for the liquefaction of a gaseous air product, for example gaseous nitrogen, since this is not connected to a rectification column system like an air separation plant.

By at least partially preventing the passage of the fluids, a temperature transition from the first end to the second end of the heat exchange zone or an increasing temperature compensation, as has already been explained several times before, is brought about. For more details on Figure 1 and the related explanations referenced below.

In the method, the system switches repeatedly from the first operating mode to the second operating mode and from the second operating mode to the first operating mode. Corresponding switching processes can in principle take place in different systems, but they are particularly important in systems in which an alternating mode of operation takes place routinely, for example systems for storing and recovering electrical energy using liquid air or other liquid air products. The present invention is particularly advantageous in plants of this type. In principle, the present invention can be used in any system in which a heat exchanger can be operated accordingly. For example, these can be plants for liquefying natural gas and separating natural gas, the aforementioned LAES plants, plants for air separation, all types of liquefaction circuits (in particular for air and nitrogen) with and without air separation, ethylene plants (in particular separation plants that process gas mixtures) Steam crackers are set up), systems in which cooling circuits are used, for example with ethane or ethylene at different pressure levels, and systems in which carbon monoxide and / or carbon dioxide circuits are provided.

Within the scope of the present invention, it is provided that the switchover from the second operating mode to the first operating mode comprises the fluids which are conducted through the heat exchange zone in the first amount per unit time in the first operating mode, initially in a second amount until an increase in time per unit time, which is less than the first amount per unit time, to pass through the heat exchange zone and only to pass through the first heat exchange zone in the first quantity per unit time from the time of increase. It goes without saying that the switchover from the second operating mode can also take place in the form of a gradual or ramp-shaped transition. Even in such a case, the second quantity per unit of time (with the ramp-shaped increase) less than the first quantity per unit of time (after the increase). Ramps with different gradients, gradients with ramps and plateaus and the like can also be used. The same applies to a ramp-like increase after the time of the increase.

The present invention is based on the knowledge that the thermally induced voltages when restarting a heat exchanger, in particular a fin-plate heat exchanger from a temperature-balanced state, as can be present in particular after longer phases of an explained second operating mode, strongly depend on the speed of the restart can depend. While high mass flows can lead to high thermal voltages, the thermal stress can be significantly reduced at low start-up speeds with sufficiently small mass flows. More is particularly with reference to Figure 3 illustrated below.

Since cold and warm currents are usually directed in countercurrent to one another in a corresponding heat exchanger, the temperature profile when the apparatus starts up is set from the two ends as time progresses to the interior of the heat exchanger or its heat exchange zone. If the heat exchanger has already experienced the greatest temperature changes occurring during the transition to normal operation in sensitive areas, which are typically located in the terminal areas of a corresponding heat exchange zone, for example in a sensitive area of module connections, then only reduced gradients occur in the further course and thus reduced thermal stresses. The present invention therefore proposes, with the measures mentioned above, first to pass fluids through the heat exchange zone with a smaller amount per unit of time and only afterwards, namely when the temperature change has already taken place sufficiently in appropriate sensitive areas, to increase or increase the amount to set a maximum amount.

The present invention enables, in particular, a significant service life optimization or improvement of fin-plate heat exchangers during restart processes from temperature-balanced conditions or at high temperature differences between incoming currents and metal temperatures of the Heat exchanger. Commissioning processes proposed according to the invention can in particular also be carried out for existing topologies (possibly by retrofitting surface temperature measurements or sensors and / or corresponding sensors at entry and / or exit points of the heat exchanger, in particular in connection with turbines), since the present invention essentially involves the optimization of the dynamic approach can be implemented. In particular, the main heat exchangers of air separation plants can better withstand a load-flexible operating mode (for use, for example, electricity market prices) over the life of the plant through the use of the present invention. In this way, the present invention makes it possible to avoid unplanned downtimes, repair costs and procurement of spare parts.

The mode of operation of a corresponding system proposed according to the invention can be observed operationally, for example by means of surface temperature measurements. The start-up process can thus be monitored and the time at which the start-up process can be accelerated can be well predicted.

A corresponding time of increase can therefore be determined in the context of the present invention at least in part on the basis of one or more temperature measurements at one or more points in the heat exchange zone. It is particularly advantageous here if such temperature measurements relate to a sensitive zone such as the aforementioned area of the module connection. However, other sensitive areas resulting from the design of the heat exchanger can also be taken into account within the scope of the present invention. A corresponding area can be defined in particular via a longitudinal coordinate, which corresponds to the end of a module connection. A measurement at another point is also possible in principle, provided that a temperature at a corresponding sensitive point can be inferred in this way, for example on the basis of known material properties and possibly model calculations with regard to heat spreading and fluid dynamics. In particular, one or more surface temperature sensors can be used in the context of the present invention, which can be easily and inexpensively retrofitted in existing heat exchangers.

If a corresponding heat exchanger is characterized with sufficient accuracy, for example with regard to material and thermal properties, and if the temperatures and fluid flows used are known, temperature measurement may also be dispensed with, because it can be assumed that after a certain time in the sensitive zones a corresponding temperature value has been reached. Therefore, as an alternative or in addition, it is also possible to base the increase in time at least in part on the basis of a period of time that has elapsed since the switchover was initiated and / or on the basis of the start of the supply of the fluids that are used in the second operating mode in the second quantity per unit time to set the elapsed period. For example, it is also possible to determine the switchover time on the basis of a total amount of the fluids which are used in the second operating mode in the second amount per unit of time.

As mentioned several times, the present invention unfolds its particular advantages when the heat exchanger is designed as a fin-plate heat exchanger. Such a fin-plate heat exchanger can be made in particular of aluminum and / or stainless steel. A corresponding heat exchanger can also be produced, for example, by means of 3D printing. In the case of such heat exchangers in particular, the potentially high thermal stresses mentioned may occur during repeated start-up processes.

The method according to the invention can be used particularly advantageously when the heat exchange zone has a first terminal partial zone extending from the first end, a second terminal partial zone extending from the second end and a central partial zone arranged between the first terminal partial zone and the second terminal partial zone , the point in time of the increase being determined as having been reached if it is established or predicted that one or more temperature values in at least one of the terminal sub-zones will exceed or fall below a predetermined temperature.

In particular, the heat exchanger can have a number of modules which are connected to one another by means of module connections, one or more of the module connections being or being arranged in the terminal sub-zones, and the central sub-zone being free of the module connections. The present Due to the slow approach, the invention permits targeted protection of the areas with the module connections or other sensitive zones, that is to say the first and second terminal sub-zones, which are particularly critical with regard to rapid temperature changes. Module connections are particularly critical due to their notch effect, but the terminal end zones are fundamentally sensitive to thermally induced stresses, even if there is no module connection here. For details, please refer to the explanations above.

In the method according to the invention, as mentioned, the point in time of the increase can in particular be determined to be reached when it is determined or predicted that one or more temperature values in at least one of the terminal sub-zones will exceed or fall below a predetermined temperature. More precisely, in such a case the time of increase can be determined as reached if it is determined or predicted that one or more temperature values in the first terminal sub-zones exceed a predetermined temperature and / or if it is determined or predicted that one or more temperature values in the fall below a predetermined temperature in the second terminal sub-zone.

The changeover between the first and the second quantity per unit of time can take place suddenly or gradually. In other words, in a particularly preferred embodiment of the present invention, switching from the second operating mode to the first operating mode may include an amount per unit time of the fluids that are conducted through the heat exchange zone in the first operating mode in the first amount per unit time to increase the time of increase continuously or gradually. In this way, temperature jumps can be further reduced.

For clarification, it is stated that, in a method of the present invention, in particular in the first operating mode, one or more first fluids are fed to the heat exchange zone at its first end at the first temperature level, passed through the heat exchange zone and removed from the heat exchange zone at its second end at the second temperature level will or will, and that in the first operating mode one or more second fluids of the heat exchange zone at its second end at the second temperature level fed, passed through the heat exchange zone and the heat exchange zone at the first end or is removed at the first temperature level.

The present invention also extends to an arrangement having a heat exchanger which has a heat exchange zone which extends between a first end and a second end, and technical means are provided which are set up in a first operating mode to handle fluids at different temperature levels a first amount per unit time through the heat exchange zone, thereby bringing the first end of the heat exchange zone to a first temperature level and the second end of the heat exchange zone to a second temperature level which is below the first temperature level, with technical means being provided for this are configured, in a second operating mode, to at least partially prevent the passage of the fluids, which are passed through the heat exchange zone in the first amount per unit time in the first operating mode, as a result of which a temperature transition from the first end to the end is effected in the second end of the heat exchange zone, and technical means are provided which are set up to switch repeatedly in the method from the first operating mode to the second operating mode and from the second operating mode to the first operating mode.

The arrangement is characterized by technical means which are set up to carry out the switching from the second operating mode to the first operating mode in such a way that the fluids which are passed through the heat exchange zone in the first amount in the first quantity per unit time up to at a time of increase initially in a second amount per unit time, which is less than the first amount per unit time, are passed through the first heat exchange zone and are only passed through the first heat exchange zone in the first amount per time unit from the time of increase.

For features and advantages of a corresponding arrangement, which is set up in particular to carry out a method as explained above, reference is expressly made to the above statements. In particular, such a system has a control device which is designed to switch between the first and the second operating mode when required, for example according to a fixed switching pattern, on the basis of a sensor signal or on request.

A corresponding arrangement can in particular have suitable sensors, in particular temperature and / or strain sensors.

As mentioned, the present invention also extends to an arrangement which has means for the liquefaction and / or low-temperature separation of air and / or at least one gaseous air product. According to the invention, this is characterized in that it represents an arrangement with a heat exchanger, as has just been explained. In particular, the arrangement can be designed as an air separation plant. In this case, it comprises a distillation column system of basically known type. A corresponding arrangement can in particular also be designed as a system for storing and recovering energy. A corresponding arrangement can also be designed as a plant for the liquefaction of nitrogen or as another plant of the type explained above. Regarding features and advantages, reference is made to the above explanations regarding the method according to the invention.

The invention is explained in more detail below with reference to the accompanying drawings, which show an embodiment of the invention and corresponding heat exchange diagrams.

Brief description of the drawings

• Figure 1 illustrates temperature profiles at the warm and cold end of a heat exchanger that can be operated according to the invention after decommissioning.
• Figure 2 illustrates a fin-plate heat exchanger that can be operated using a method according to an embodiment of the invention.
• Figure 3 illustrates a relationship between fluid flows and thermal stresses in a fin-plate heat exchanger.
• Figure 4 illustrates temperature gradients in a fin-plate heat exchanger at different times of flow.
• Figure 5 illustrates a nitrogen liquefaction plant that can be operated using a method according to an embodiment of the invention

In the figures, elements that are identical or correspond to one another functionally or in terms of meaning are given identical reference numerals and are not explained repeatedly for the sake of clarity.

Detailed description of the drawings

Figure 1 illustrates temperatures in a heat exchanger, in particular a finned plate heat exchanger, after decommissioning, ie in an operating mode referred to above and below as "second operating mode", in which the passage of fluids through the heat exchanger is prevented, in the form of a temperature time diagram.

In the in Figure 1 The temperature-time diagram shown are a temperature denoted by H at the warm end of the heat exchanger or its heat exchange zone (previously and hereinafter also referred to as "first end") and a temperature denoted by C at the cold end ("second end") in each case in ° C on the ordinate versus a time in hours on the abscissa.

How out Figure 1 can be seen, the temperature H at the first (warm) end of the heat exchange zone at the beginning of the decommissioning, and thus the temperature in a regular operation of the heat exchanger or at the end of the operating mode referred to above and below as the "first operating mode", in which corresponding fluids pass through the heat exchanger, about 20 ° C and the temperature C at the second (cold) end about -175 ° C. These temperatures become increasingly similar over time. The high thermal conductivity of the materials used in the heat exchanger is responsible for this. In other words, heat flows from the first (warm) end towards the second (cold) end. Together with the heat input from the environment, this results in an average temperature of approx. -90 ° C. The significant temperature increase at the second (cold) end of the heat exchange zone is largely due to the internal temperature compensation in the heat exchanger and only to a lesser extent due to external heat input.

As mentioned several times, strong thermal stresses can occur in the case shown if the first (warm) end of the heat exchanger is again exposed to a warm fluid of approximately 20 ° C. in the example shown after some time in the second operating mode , The same applies to a second (cold) end.

Figure 2 illustrates a fin-plate heat exchanger that can be operated using a method according to an embodiment of the invention. This is designated by a total of 100 and is fundamentally designed in a known manner, as documented, for example, in the specialist literature mentioned at the beginning. The heat exchanger is designed for heat exchange between two fluids. However, the present invention can in particular also be designed to operate corresponding heat exchangers in which more than two fluids are subjected to heat exchange.

In the example shown, the heat exchanger 100 is constructed from two modules 1, 2, which can in principle be configured identically. Instead of two modules 1, 2, heat exchangers with more than two modules can also be used within the scope of the present invention. In the example shown, the modules 1, 2 are connected to one another by means of module connections, which, however, are only provided at the two ends of the two modules 1, 2. The module connections 1, 2 can be designed, for example, as elements which are each soldered to the modules 1, 2.

The modules 1, 2, alternatively also a corresponding heat exchanger 100 as a whole, are each constructed from heat exchanger plates 4, of which only one is specifically designated in the example shown. The heat exchanger plates 4 can in particular be soldered to one another. In particular, they are combined alternately in groups, which can be flowed through separately.

The two modules 1, 2 can be supplied with a warm or a cold fluid via headers 5 and 7, respectively. Corresponding fluids are fed into the respective headers by means of connecting pieces 51 and 71. A warm fluid is distributed by means of the header 5 to a group of heat exchanger plates 4 of the modules 1, 2.

After the fluid fed in by means of the header 5 has flowed through the modules 1, 2, it is collected by means of the header 6 and, in the cooled state, is discharged via a connection piece (not visible here). Correspondingly, a cold fluid is distributed to another group of heat exchanger plates 4 of the modules 1, 2 by means of the header 7. After the fluid fed in by means of the header 7 has flowed through the modules 1, 2, it is collected by means of the header 8 and, in the heated state, is discharged via the nozzle 81. As mentioned, a corresponding heat exchanger 100 can also be set up to process further fluid flows. Corresponding groups of heat exchanger plates 4 and headers are provided for this.

For heat exchange, corresponding fluids flow through a heat exchange zone 10 of the heat exchanger 100, designated here as 10, which extends between a first end, here designated 11, and a second end, designated here 12. In a regular (“first”) operating mode, fluids at different temperature levels are passed through the heat exchange zone 10 in a specific (“first”) amount per unit of time in the manner previously explained. In this way, the first end 11 of the heat exchange zone 10 is brought to a certain ("first") temperature level and the second end 12 of the heat exchange zone 10 is also brought to a certain ("second") temperature level which is below the first temperature level.

If the heat exchanger 100 is taken out of operation (“second” operating mode), the passage of the fluids, which in the first operating mode are conducted through the heat exchange zone 2 in the first quantity per unit of time, is at least partially prevented. In this way, a temperature transition is effected from the first end 11 to the second end 12 of the heat exchange zone 10. In a corresponding method, there is a multiple switchover from the first operating mode to the second operating mode and from the second operating mode to the first operating mode. This can result in very strong thermal stresses without the use of the measures proposed within the scope of the present invention, as explained several times before. This applies in particular in the case of a heat exchanger 100 which is constructed from a plurality of modules 1, 2 and is connected to one another by means of corresponding module connections 3.

The heat exchanger 100 shown here is characterized in particular by the fact that the heat exchange zone 10 has a first terminal sub-zone 13 extending from the first end 11 and a second terminal sub-zone 14 extending from the second end 12 and in the terminal sub-zones 13, 14 each of the module connections 3 are arranged. A central subzone of the heat exchange zone 10, however, is free of the module connections 3.

In the course of investigations, as also mentioned, it has been shown that the thermally induced voltages when restarting a corresponding heat exchanger 100, in particular a fin-plate heat exchanger, from a temperature-balanced state, i.e. if the second operating mode has been carried out for a long time, it may depend heavily on the speed of the restart. While high mass flows can lead to high thermal voltages, thermal stress can be almost completely avoided at low start-up speeds with sufficiently small mass flows.

Figure 3 illustrates a relationship between fluid flows and thermal stresses in a fin-plate heat exchanger. In Figure 3 are a normalized cold mass flow in dimensionless units, i.e. a quantity of a cold fluid supplied to the heat exchanger per unit time, on the abscissa and a normalized maximum thermal stress in dimensionless units on the ordinate.

As can be seen, the thermal stresses induced at low mass flows are significantly lower than at higher mass flows. The present invention makes use of this knowledge and, in particular, proposes to start up a corresponding heat exchanger again using smaller amounts of fluid per unit of time. In particular, an amount of fluid is only increased when the areas in which module connections of a finned plate heat exchanger constructed from several modules, for example a heat exchanger as shown in Figure 2 shown, are arranged, are already sufficiently temperature-controlled, since there are particularly negative effects of the thermal stresses in such areas. This is with reference to Figure 4 further explained.

In other words, the present invention proposes to switch from the second operating mode to the first operating mode in such a way that the fluids which are conducted in the first operating mode in the first quantity per unit time through the heat exchange zone of a corresponding heat exchanger up to an increase time first in a second quantity per unit time, which is less than the first quantity per unit time, to pass through the first heat exchange zone and only from the time of increase in the first quantity per unit time through the first heat exchange zone.

Since cold and warm currents in a corresponding heat exchanger, as in that in Figure 2 Example shown, typically directed in countercurrent to each other, the temperature profile when starting, that is, from the transition from the second to the first operating mode, from the two ends starting with progressive time to the interior of the heat exchanger or the heat exchange zone, is set. If the heat exchanger, for example in a sensitive area of module connections, has already experienced the greatest temperature changes occurring during the transition to the first operating mode, then only reduced gradients and thus greatly reduced thermal voltages then occur in the further course. In the context of the present invention, the temperature changes of corresponding sensitive areas are brought about in particular with reduced amounts of fluid. Only then is a corresponding heat exchanger operated with the full amount of fluid.

In Figure 4 Diagrams 410 to 460 are each shown, in each of which temperature profiles 401 to 406 in a heat exchange zone of a heat exchanger, for example of the heat exchanger 100 according to FIG Figure 2 shown at different times. The times are each after a point in time at which a balanced temperature profile has been established due to a heat transfer from the warm to the cold end because the fluid supply has been cut off, that is to say after some time in the second operating mode. For better clarity and comparability with Figure 2 the heat exchange zone and its partial zones are also designated here in the diagram 410 with 10, 13 and 14.

At the point in time illustrated with diagram 410, there was only a slight change in temperature at the outermost ends of the heat exchange zone 10, which initially only affects the partial zones 13 and 14. Temperature profiles 402 to 406 arise with increasing time. The highest voltages occur in particular when the temperature gradient which is established is present at the inner end of the module connections, which is roughly at the times indicated here with diagrams 430 and 440 , the case is.

In the context of the present invention, corresponding thermal voltages are reduced, in particular, by using lower mass flows in a targeted manner in the periods in which the areas of the module connections experience large temperature changes (corresponding to diagrams 410 to 440). Has the local temperature gradient already formed over the module connections (corresponding to diagram 550), the approach speed can be accelerated again if necessary and thus based on common procedures without generating any further significant voltage peaks.

In Figure 5 is a nitrogen liquefaction plant that can be operated using a method according to an embodiment of the invention, is schematically illustrated and is designated overall by 500.

In the Figure 1 The illustrated system 500 has, in particular, a heat exchanger 100 of the type explained above or a comparable heat exchanger. Plants for nitrogen liquefaction are known in principle and are not restricted to the exemplary embodiment shown.

In the example shown, plant 500 is supplied with gaseous nitrogen (stream a), which can be provided, for example, by means of an air separation plant. The gaseous nitrogen is fed to a multi-stage compressor 510 and compressed. A portion of the compressed gaseous nitrogen (stream b) is further compressed in turbine boosters 520, 530, which are each provided with aftercoolers, and fed to the heat exchanger 100 on the warm side. The rest (stream c) remains undensified and is also fed to the heat exchanger 100 on the warm side.

A partial flow d of the flow b is taken from the heat exchanger 100 at an intermediate temperature level, expanded in a relaxation turbine of the turbine booster 520 and fed into a container 540. Another partial stream e of stream c is taken from the heat exchanger 100 on the cold side and expanded into the tank 540 via a throttle (not specifically designated).

The stream c is taken from the heat exchanger 100 at an intermediate temperature level, expanded in a expansion turbine of the turbine booster 530, fed to the heat exchanger 100 at an intermediate temperature level and returned together with gaseous nitrogen from the container 540 as stream f to the compressor 510 at an intermediate pressure level.

Liquid nitrogen from the container 540 is subcooled in a subcooler 550, which is cooled with a part of this nitrogen (stream g), and expanded into a storage tank 560 as stream h. Due to evaporation, gaseous nitrogen is now formed in the storage tank 560 and can be discharged unused as current i via a line and a valve if required. In addition, when the liquid nitrogen is transported from the subcooler 550 via the corresponding line (stream h) into the storage tank 200, flash gas is formed, which is also undesirable.

A further line can now be provided, via which gaseous and cold nitrogen can be fed back from the storage tank 560 as stream k into the liquefaction process. In the case shown here, this gaseous nitrogen is combined with the stream g upstream of the subcooler 550.

In this way, the cooling energy of the electricity k in the heat exchanger 100 can be used, as a result of which the entire liquefaction process becomes more efficient. A current I formed by combining the currents g and k can also be returned to the current a after heating, i.e. the amount of gaseous nitrogen carried in stream k is fed via stream I to stream a and thus again to the liquefaction process.

As mentioned, the system 500 can, if the measures proposed according to the invention are implemented, be switched on and off as required, depending on the need for liquid nitrogen.

Claims (12)

Method for operating a heat exchanger (100) which has a heat exchange zone (10) which extends between a first end (11) and a second end (12), wherein in a first operating mode fluids at different temperature levels in a first quantity per unit of time are passed through the heat exchange zone (10), whereby the first end (11) of the heat exchange zone (10) is brought to a first temperature level and the second end (12) of the heat exchange zone (10) to a second temperature level which is below the first temperature level , in a second operating mode, the passage of the fluids, which are conducted in the first operating mode in the first quantity per unit time through the heat exchange zone (2), is at least partially prevented, whereby a temperature transition from the first end (11) to the second end (12) of the heat exchange zone (10) is effected, and wherein in the process several times of first operating mode is switched to the second operating mode and from the second operating mode to the first operating mode, characterized in that the switching from the second operating mode to the first operating mode comprises the fluids in the first operating mode in the first amount per unit of time by the Heat exchange zone (10) are passed through the first heat exchange zone (10) until an increase in time, first in a second amount per unit of time, which is less than the first amount per unit of time, and only in the first amount per unit of time from the time of increase to direct the first heat exchange zone (10). The method of claim 1, wherein the time of increase is determined based at least in part on one or more temperature measurements at one or more points of the heat exchange zone (10). The method of claim 1 or claim 2, wherein the time of increase is determined based at least in part on a period of time that has elapsed since the switching was initiated. Method according to one of the preceding claims, in which the heat exchanger (100) is designed as a fin-plate heat exchanger. Method according to one of the preceding claims, in which the heat exchange zone (10) comprises a first terminal partial zone (13) extending from the first end (11), a second terminal partial zone (14) extending from the second end (12) and has a central sub-zone arranged between the first terminal sub-zone (13) and the second terminal sub-zone (14), the point in time of the increase being determined as having been reached if it is determined or predicted that one or more temperature values in at least one of the terminal sub-zones (13 , 14) exceed or fall below a predetermined temperature. Method according to Claim 5, in which the heat exchanger (100) has a number of modules (1, 2) which are connected to one another by means of module connections (3), one or more of the module connections (13, 14) in each of the terminal sub-zones (13, 14) 3) is or are arranged, and wherein the central subzone is free of the module connections (3). A method according to any preceding claim, wherein switching from the second mode of operation to the first mode of operation comprises an amount per unit time of the fluids passed through the heat exchange zone (10) in the first mode of operation in the first amount per unit time to increase the time of increase continuously or gradually. Method according to one of the preceding claims, in which, in the first operating mode, one or more first fluids are fed to the heat exchange zone (10) at their first end (11) at the first temperature level, passed through the heat exchange zone (10) and to the heat exchange zone (10) the second end (12) of which is or are removed at the second temperature level, and in which, in the first operating mode, one or more second fluids are fed to the heat exchange zone (10) at the second end (12) of the second temperature level through the heat exchange zone (10 ) passed and the heat exchange zone (10) at its first end (11) at the first temperature level is or will be removed. Arrangement with a heat exchanger (100) which has a heat exchange zone (10) which extends between a first end (11) and a second end (12), wherein technical means are provided which are set up to handle fluids in a first operating mode at different temperature levels in a first quantity per unit time through the heat exchange zone (10), whereby the first end (11) of the heat exchange zone (10) to a first temperature level and the second end (12) of the heat exchange zone (10) to a second temperature level , which is below the first temperature level, are provided, technical means being provided which, in a second operating mode, are provided for the passage of the fluids which are conducted in the first operating mode in the first quantity per unit of time through the heat exchange zone (2) to at least partially prevent, whereby a temperature transition from the first end (11) to de m second end (12) of the heat exchange zone (10) is effected, and wherein technical means are provided which are adapted to switch in the procedure several times from the first operating mode to the second operation mode and the second operation mode to the first mode of operation, characterized by technical means which are set up to switch from the second operating mode to the first operating mode in such a way that the fluids which are conducted in the first operating mode in the first quantity per unit time through the heat exchange zone (10) up to an increase time are first passed through the first heat exchange zone (10) in a second quantity per unit of time, which is less than the first quantity per unit of time, and are only passed through the first heat exchange zone (10) from the time of the increase in the first quantity per unit of time. Arrangement according to Claim 9, which has technical means set up for carrying out a method according to one of Claims 1 to 8. Arrangement (100) according to claim 9 or claim 10, which has means for liquefying and / or low-temperature separation of air and / or one or more gaseous air products. Arrangement (100) according to claim 11, wherein the means for liquefying and / or low-temperature separation of air comprise a distillation column system (20).

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