Method and system for supplying gas

Abstract

Method and system for supplying gas The invention relates to a method for supplying gas, wherein a fluid in liquid phase (b) is received in a heat exchanger (120), wherein the fluid is heated in the heat exchanger (120) using a heating medium (1d20), which is provided in the heat exchanger, wherein the fluid is provided in gaseous phase (c) for further use, wherein the heating medium (d) is, at least temporarily, heated by means of a heating means (116), and wherein one or multiple elements (130) comprising a phase-change material are provided in the heat exchanger (120), so as to be in contact with the heating medium (d), wherein the heating medium (b) is heated by means of heat released by the one or at least one of the multiple elements (130) upon activation.

Description

[0001] Description

[0002] Method and system for supplying gas

[0003] The present invention relates to a method for supplying gas like nitrogen or oxygen, preferably as a back-up, to a gas supply system, and to an air separation system comprising an air separation unit and such gas supply system.

[0004] Background

[0005] Air separation units (ASUs), typically, provide different gaseous products like nitrogen and oxygen. These products can be used for different applications. While such applications may rely on a continued operation of these gaseous products, potential failures of the air separation units can happen. In order to provide the applications with the reguired gaseous product even if any failure in the air separation unit occurs, a back-up gas supply system can be provided.

[0006] Such back-up gas supply system can be based on heating a fluid that is received in liguid phase in a heat exchanger so as to provide the fluid in gaseous phase. Such back-up gas supply system can be used to provide the gas instead of the air separation unit, preferably, during a period where the failure is present and until the air separation unit is operable again.

[0007] In such back-up gas supply system, a heating medium used in the heat exchanger has to be heated in order to provide stable operation of the back-up gas supply system. Heating means used for heating the heating medium, however, can also fail. Typical reguirements for applications, for such cases, are that the back-up gas supply system shall be able to provide the fluid in gaseous phase for a certain or pre-defined period of time. A way to cope with such failure of heating means is to provide a sufficient amount of heating medium in a bath vaporizer used as the heat exchanger. The longer the reguired period is, however, the larger the amount of heating medium has to be, which in turn results in large bath vaporizers and high costs.

[0008] Disclosure of the Invention This object is achieved by providing a method for supplying gas, a gas supply system and an air separation system with the features of the independent claims.

[0009] Embodiments of the invention are the subject of the dependent claims and of the description that follows.

[0010] The invention relates to supplying gas, preferably by way of a back-up gas source for an air separation unit. In the following, the invention will basically be described with respect to such back-up gas source for an air separation unit, however, the gas can be supplied for any intended use, irrespective of whether or not this gas is supplied as a back-up or not, and irrespective of whether this is for an air separation unit or not.

[0011] A fluid in liquid phase is received in a heat exchanger. The corresponding gas supply system comprises a tank configured to store such fluid in liquid phase, and to supply the fluid to the heat exchanger. In an embodiment, the fluid is a cryogenic fluid, preferably one of the following fluids: nitrogen, oxygen, helium, hydrogen, carbon dioxide, argon, ethylene, propylene, natural gas. Typically, the gas supply system comprises a pump, e.g., a cryogenic pump, by means of which the fluid is pumped from the tank to the heat exchanger.

[0012] The fluid is heated in the heat exchanger using a heating medium, which is provided in the heat exchanger. The fluid is then provided in gaseous phase for further use. For example, the gas supply system can comprise an outlet or other line for providing the fluid gaseous phase, i.e. , as a gas.

[0013] Further, the heating medium is, at least temporarily, heated by means of a heating means; this is required to keep a temperature of the heating medium at a certain or desired temperature or in a certain range of temperature. According to the present invention, a bath vaporizer is used as the heat exchanger. This can be, e.g., a water bath vaporizer where water is used as the heating medium. However, also other mediums like water-glycol or an exhaust gas-water mixture can be used.

[0014] Further, one or multiple elements comprising a phase-change material (PCM) are provided in the heat exchanger, so as to be in contact with the heating medium. When a bath vaporizer is used, the elements can be placed therein. The heating medium is heated by means of heat released by the one or at least one of the multiple elements upon activation. Note that the elements can, for example, be directly made of the PCM, or the elements can have a housing or shell including the PCM, where the housing or shell is made of a different material.

[0015] In this way, the elements comprising a phase-change material can provide heat to keep the heat exchanger running for a certain period of time, even if the main heating means for the heat exchanger fails, for example.

[0016] A phase-change material (PCM) is a substance which releases or absorbs sufficient energy at phase transition to provide useful heat or cooling. Typically, the transition is from one of the first two states of matter - solid and liquid - to the other. The phase transition may also be between non-classical states of matter, such as a conformity of crystals, where the material changes from conforming to one crystalline structure to conforming to another.

[0017] The energy released or absorbed by phase transition from solid to liquid, or vice versa, the heat of fusion is generally much higher than the sensible heat. By melting and solidifying at the phase-change temperature (PCT), a phase-change material is capable of storing and releasing large amounts of energy compared to sensible heat storage. Heat is absorbed or released when the material changes from solid to liquid and vice versa or when the internal structure of the material changes; phase-change materials are, thus, also referred to as latent heat storage (LHS) materials.

[0018] There are two principal classes of phase-change material: organic (carbon-containing) materials derived, e.g., either from petroleum, from plants or from animals; and inorganic materials like salt hydrates, which generally either use natural salts from the sea or from mineral deposits or are by-products of other processes. A third class is solid to solid phase change.

[0019] Initially, solid-liquid PCMs behave like sensible heat storage (SHS) materials; their temperature rises as they absorb heat. Unlike conventional SHS materials, however, when PCMs reach their phase change temperature (their melting point) they absorb large amounts of heat at an almost constant temperature until all the material is melted. When the ambient temperature around a liquid material falls, the PCM solidifies, releasing its stored latent heat; the phase-change material is thus activated to release its heat.

[0020] Phase change materials are available in different required temperature ranges; thus, it is possible to allow activation (releasing heat) of the phase-change material when a temperature of the heating medium falls below a predefined threshold.

[0021] A typical operating temperature range for the heating medium of a water bath vaporizer is from 40°C to 80°C. Thus, a phase-change material can be chosen, for example, that activates at 35°C or at 40°C. Note that elements having different PCMs can also be used. In this way, not all elements provide their heat at the same time but subsequently or only if the temperature falls further.

[0022] Thus, instead of providing the required amount of standby energy solely via the water holdup in a water bath vaporizer, or another heating medium, one or more elements with PCM can be used on the water or heating medium side. The latent heat of those PCMs is significantly higher than the sensible heat of water (or other typical heating mediums) which means that less water holdup and therefore less volume is required; a typical PCM has, e.g. 184MJ / m3whereas water has 4.2MJ / m3. This minimizes the cost of the whole heat exchanger while still keeping the desired standby time requirements.

[0023] In addition, the temperature of the water bath can be kept high during the standby time which counteracts icing on the tubes. A further advantage is that using PCM together with electric heaters, for example, can provide the whole electric system with sufficient time to come into operation. This could indeed take some time because the amount of electric energy required can be rather large (e.g., more than 10MW). Further, using PCM makes the control of the heat source (steam, electricity etc.) less sensitive and more robust since a too low supply of heat over time would be compensated by the PCM.

[0024] Preferred PCMs are non-toxic or biodegradable ones. However, other types of PCMs can also be used in order to achieve the effects described herein.

[0025] In an embodiment, multiple elements (comprising the PCM) are provided in the heat exchanger, which are regularly distributed, e.g., stacked above each other. Basically, however, such elements can also be irregularly distributed. PCMs or elements comprising PCM can be provided in special forms (e.g., for optimized surface-to-mass ratio) including stackable plates or cells, cigar shaped ellipsoids, or sphere-like micro or macro capsules.

[0026] The PCM elements can be used with different types of heat exchangers or respective heating means. In an embodiment, the heating means comprises at least one of the following ones: a steam injector (e.g. a steam lance), one or more electrical heaters, one or more sources of warm fluid, one or more fired heaters. The heat exchanger is a bath vaporizer. Such bath vaporizer can comprise a coiled tube bundle, for example. However, the heat exchanger or bath vaporizer geometry can vary, e.g. having straight tubes, coiled tubes, plates etc. The orientation of the heat exchanger or its vessel can be vertical or horizontal, for example.

[0027] In an embodiment, the fluid is provided in gaseous phase for further use as a back-up for an air separation unit; the gas supply system can, thus, be provided near the air separation unit and also be part of an air separation system, which comprises the air separation unit and the gas supply system. In this case, the gas supply system can also be referred to as a back-up gas supply system. In this way, a double-back-up system is provided; the gas supply system serves as back-up for the air separation unit, and the PCM elements serve as back-up for the heating means of the gas supply system or its heat exchanger.

[0028] It is noted that the fluid, which is provided in the gaseous phase for further use, can also be provided as a back-up (i.e. back-up fluid supply) for another application or system that provides gas (i.e. another application than the air separation unit) and, for example, shall be provided with a back-up system. Such gas supply systems can, thus, not only be used in or for ASUs, but also in or for other types of industrial plants to compensate for short-term production downtimes. For example, such gas supply systems can also be used in ethylene crackers to keep the ethylene header pressure constant for consumers (such as PE or PP systems) in the event of a problem in the cracker. Alternatively, such gas supply systems can also be used for peak shaving, i.e. intercepting peak loads, e.g., in the LNG sector. Furthermore, these gas supply systems can not only be used in local but also mobile operation (e.g., virtual pipeline). In an embodiment, the fluid is a cryogenic fluid, preferably nitrogen, oxygen, helium, hydrogen, carbon dioxide, argon, ethylene, propylene, or natural gas. In general, the gas supply system can be used as a standalone system for a specific kind of gas, if so required, or in combination with an air separation unit as back-up source for gas. IN this case, the gas supply system has to provide the required fluid. Note that also two or more gas supply systems, each with a different fluid, can be provided as back-up for an air separation unit, e.g., if back-up for two or more different gases is required.

[0029] Further advantages and embodiments of the invention will be apparent from the description and the accompanying drawing. The invention is illustrated schematically by means of embodiments in the drawing and is described below with reference to the drawing.

[0030] Short description of the figures

[0031] Fig. 1 illustrates an air separation system according to an embodiment.

[0032] Detailed description of the figures

[0033] Fig. 1 schematically illustrates an air separation system 100 in an embodiment, which can be used for method in an embodiment as described herein. The air separation system 100 comprises an air separation unit 102, which provides a gas as stream a. Note that this is only for explanation, typical air separation units can also provide different gases.

[0034] The air separation system 100 further comprises a gas supply system 110 which is used to provide gas as a back-up for the air separation unit 102, i.e. in case of failure of the air separation unit 102, for example. As mentioned above, such gas supply system 110 can also be used standalone, if so required.

[0035] The gas supply system 110 comprises a tank 112, a cryogenic pump 114, a heat exchanger 120, and a heating means 116. The tank 112 is configured to store a fluid or cryogenic fluid b in liquid phase, and to supply the fluid to the heat exchanger 120. For example, such fluid can be oxygen or nitrogen, stored in liquid phase at the respective low temperature in the tank 112. The cryogenic pump 114 is used to pump the fluid b from the tank 112 to the heat exchanger 120. In particular, pipes for the liquid b can be used to allow the fluid to be pumped to the heat exchanger 120.

[0036] The heat exchange 120 is a bath vaporizer. This bath vaporizer 120 comprises a vessel 122 and a lid 124. Inside the vessel 122, a heating medium d is stored. This heating medium can be, for example, water or water-glycol as mentioned earlier. The fluid b which is to be heated by or in the bath vaporizer 120 is guided through the heating medium d by means of pipes or the like which are provided inside the vessel 122.

[0037] The heating means 116 comprise, in an embodiment a water steam source and pipes or steam lances by means of which steam e can be introduced into the heating medium d. IN this way, the heating medium d can be kept at a specified temperature or within a temperature range of, e.g., 40°C to 80°C. Note that temperature sensors and a controller can be used to regulate the temperature of the heating medium d. Valves may be provided in the pipes or steam lances, for example, in order to allow regulation of the steam flow and, thus, temperature control.

[0038] In that the fluid b in liquid phase is introduced into the bath vaporizer and the heating medium d, the fluid is heated and transformed into gaseous phase; the fluid in gaseous phase is denoted by c. The fluid c in gaseous phase is then provided for further use, e.g., as back-up source for gas in case of a failure of the air separation unit 102. This allows continued gas supply for a specific application, for example.

[0039] Again, note that this requires the fluid c in gaseous phase being the same gas as gas a provided by the air separation unit 102.

[0040] Further, in the heat exchanger 120, specifically in the vessel 122, elements 120 are provided, which elements comprise a phase-change material. By means of example, three elements 130 are shown which a stacked above each other.

[0041] The phase-change material can be chosen such that when the temperature of the heating medium d falls below a certain threshold, e.g., 40°C, heat is released by the elements 130 due to phase transition as has been explained in more detail above. Thus, if for whatever reason the heating means 116 fails, the heating medium d still is heated for a certain period of time, i.e., as long as the heat is released by the elements 120 comprising a PCM.

Claims

Patent Claims1. A method for supplying gas, wherein a fluid in liquid phase (b) is received in a heat exchanger (120), wherein the fluid is heated in the heat exchanger (120) using a heating medium (1d20), which is provided in the heat exchanger, wherein the fluid is provided in gaseous phase (c) for further use, wherein the heating medium (d) is, at least temporarily, heated by means of a heating means (116), and wherein one or multiple elements (130) comprising a phase-change material are provided in the heat exchanger (120), so as to be in contact with the heating medium (d), wherein the heating medium (b) is heated by means of heat released by the one or at least one of the multiple elements (130) upon activation, characterized in that a bath vaporizer is used as the heat exchanger (120).

2. The method of claim 1 , wherein the heating medium (d) is heated by means of heat released by the one or at least one of the multiple elements when at least one of the following activation criteria is met: a failure in operation of the heating means (116) occurs, a temperature of the heating medium (d) falls below a predefined threshold.

3. The method of claim 1 or 2, wherein multiple elements (130) are provided in the heat exchanger (120), which multiple elements are regularly distributed, in particular stacked above each other.

4. The method of any one of the preceding claims, wherein one of the following mediums is used as the heating medium (d): water, water-glycol or an exhaust gas-water mixture.

5. The method of any one of the preceding claims, wherein the heating means (116) comprises at least one of the following ones: a steam injector, one or more electrical heaters, one or more sources of warm fluid, one or more fired heaters.

6. The method of any one of the preceding claims, wherein the fluid is a cryogenic fluid, preferably one of the following fluids: nitrogen, oxygen, helium, hydrogen, carbon dioxide, argon, ethylene, propylene, natural gas.

7. The method of any one of the preceding claims, wherein the fluid is provided in gaseous phase (c) for further use as a back-up for an air separation unit (102).

8. A gas supply system (110) comprising: a tank (112), a heat exchanger (120), and a heating means (116), wherein the tank (112) is configured to store a fluid in liquid phase, and to supply the fluid to the heat exchanger (120), wherein the heat exchanger (120) is configured to receive the fluid from the tank, to heat the fluid via heat exchanger with a heating medium (d) provided in the heat exchanger, and to provide the fluid in gaseous phase for further use, wherein the heating means (1116) is configured to heat the heating medium, wherein the gas supply system (110) further comprises one or multiple elements (130) comprising a phase-change material, provided in the heat exchanger, being in contact with the heating medium and configured to release heat to the heating medium upon activation, wherein the heat exchanger (120) is configured as a bath vaporizer.

9. The gas supply system (110) of claim 9, comprising multiple elements (130) which are regularly distributed, in particular stacked above each other.

10. The gas supply system (110) of any one of claims 8 to 9, wherein heating means comprises at least one of the following ones: a steam injector, one or more electrical heaters, one or more sources of warm fluid, one or more fired heaters.

11. The gas supply system (110) of any one of claims 8 to 10, wherein the fluid is a cryogenic fluid, preferably nitrogen, oxygen, helium, hydrogen, carbon dioxide, argon, ethylene, propylene, or natural gas.

12. The gas supply system (110) of any one of claims 8 to 11 , configured to perform the method of any one of claims 1 to 7.

13. An air separation system (100) comprising an air separation unit (102) and the gas supply system (110) of any one of claims 9 to 12, wherein the heat exchanger of the gas supply system is configured to provide the cryogenic fluid in gaseous phase instead of a corresponding gas of the air separation unit.

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