Patent Application
20220170695
The invention relates to a method for condensing a gas, in particular natural gas, and to a corresponding system in accordance with the respective preambles of the independent claims.
Methods and systems for condensing natural gas are known and are, for example, described in the article “Natural Gas” in Ullmann's Encyclopedia of Industrial Chemistry, online publication, Jul. 15, 2006, DOI: 10.1002/14356007.a17_073.pub2, in particular section 3, “Liquefaction,” or in Wang and Economides, “Advanced Natural Gas Engineering,” Gült Publishing, 2010, DOI: 10.1016/C2013-0-15532-8, in particular chapter 6, “Liquefied Natural Gas (LNG).”
In particular, mixed refrigerants consisting of various hydrocarbon components and nitrogen can be used in natural gas condensing processes. For example, one, two or even three mixed refrigerant circuits can be used (single mixed refrigerant, SMR; dual mixed refrigerant, DMR; mixed fluid cascade, MFC). Mixed refrigerant circuits with propane precooling (C3MR) or more generally using a pure refrigerant (see below) are also known.
Although the present invention is described below predominantly with reference to the condensing process of natural gas, the proposed measures are in principle also suitable for condensing other gas mixtures. Natural gas and corresponding other gas mixtures may, in particular, have more than 70, preferably more than 90, mole percent methane and in the remainder (inter alia) non-hydrocarbon gases, such as nitrogen and acid gases. They may also contain higher hydrocarbons, in particular ethane. Higher hydrocarbons, such as ethane, propane, butane, etc. are preferably contained at less than 10 mole percent. Such higher hydrocarbons may, for example, be removed upstream of the actual condensing process. A natural gas used for condensing or another gas mixture is preferably essentially free of water and/or carbon dioxide.
Methods for condensing natural gas are energy-intensive. Depending on the technology selected, between 5 and 15% of the energy contained in the feed gas are consumed internally, in order to produce the required cold. Increased process efficiency frequently leads to additional investments, since technically more sophisticated systems must be used.
Large refrigeration circuit compressors are usually driven by gas turbines, which convert only 30 to 45% of the energy of the fuel gas, i.e., its calorific value, into mechanical shaft power. The remainder, i.e., 55 to 70% of the energy, is lost when the waste heat of the turbine waste gas is not utilized.
Various concepts for utilizing the waste heat of turbine waste gases exist. Simple systems comprise the recovery of the waste heat in the form of process heat, e.g., in a hot oil system, which transfers the heat from the turbine waste gas at an appropriate temperature level, for example to reboilers of regeneration columns in amine washes, regeneration gas heaters for dryers or any other heat users.
More complex waste heat utilization systems comprise a closed steam circuit. The steam produced by the waste heat can be expanded in a steam turbine in a work-performing manner. Any refrigeration circuit compressors can be driven with a corresponding steam turbine, including, for example, those of precooling circuits with, for example, propane, carbon dioxide or ammonia as refrigerant. Support of a gas turbine for the main compressor is also possible.
Overall, there is the desire to increase the efficiency in natural gas condensing and other gas condensing methods without the complex installation of a circuit on the basis of an additional working fluid, such as steam.
Against this background, the present invention proposes a method and a system with the features of the independent claims. Embodiments of the present invention are respectively the subject matter of the dependent claims and the following description.
Within the scope of the present invention, a method for condensing a gas is proposed, wherein the gas is subjected to a heat exchange with a refrigerant, and at least a part of the refrigerant is subjected, after the heat exchange with the gas, with which the refrigerant can in particular be at least partially evaporated, to compression using a drive that produces waste heat and to a partial or complete condensing process. Within the scope of the present invention, a refrigerant circuit is thus used, which comprises the steps known per se of heating and evaporation (against the fluid to be cooled, here the gas to be condensed), recompression (using the drive that produces waste heat) and (partial) condensation in the circuit.
Below, the term “evaporation” generally always refers to partial or complete evaporation. Accordingly, the term “condensation” is also to be understood as partial or complete condensation, even if this is not explicitly indicated in each case. The heat exchange of the refrigerant “with the gas” can take place in the form of indirect heat exchange between the gas and the refrigerant without an intermediate further refrigerant, i.e., via a common heat exchange surface of a heat exchanger, but also via an additional refrigerant. Heat exchange “with the gas” thus also takes place when heat is withdrawn from the gas via a further refrigerant, and the further refrigerant is precooled with the refrigerant considered here. The term “heat exchange” is always used herein synonymously with the scientifically more correct term “heat transfer” and the term “heat exchanger” is used synonymously with the term “heat transfer device.”
As also known in this respect, the heating and evaporation, the recompression and the (partial) condensing process can take place in the form of any (pressure or temperature) stages or in the form of a plurality of partial flows in parallel to one another, wherein corresponding partial flows may be combined with one another at any locations or formed from an output flow. The present invention relates in particular to closed refrigerant circuits as known for condensing natural gas from the aforementioned prior art.
According to the invention, after the partial or complete condensing process of the refrigerant, a first portion of the refrigerant is subjected to heat exchange with the gas in the sense just explained, whereas a second portion of the refrigerant is subjected, in succession, to pressurization (in the liquid state), heating (in particular superheating) using the waste heat of the drive, and work-performing expansion and is fed back to the partial or complete condensing process. In other words, after its work-performing expansion, with which evaporation in particular takes place, the second portion of the refrigerant is thus returned to the refrigerant circuit and is thereby in particular combined with the first portion of the refrigerant, which was previously subjected to the heat exchange with the gas and was likewise evaporated thereby. A partial circuit is thus created. In principle, the second portion can again be returned to the refrigerant circuit and combined with the first portion at any location; specific positions are explained below.
In other words, the present invention thus relates to a gas condensing method with which at least one compressor is used in a refrigerant circuit used to provide cold. A drive of the compressor produces waste heat. In particular, a gas turbine is used as the drive, so that the waste heat is provided in particular with the turbine waste gas that is extracted from an expansion stage of the gas turbine. In the present invention, work-performing expansion of a partial flow of the refrigerant, of the mentioned “second portion,” is carried out. The latter is both further pressurized and heated before the work-performing expansion, so that the refrigerant is capable of absorbing the waste heat contained in the turbine waste gas of the gas turbine or in another waste heat carrier. The heated, in particular superheated, refrigerant, which is obtained by utilizing the waste heat, is used as an energy source by the work-performing expansion, so that in this way the waste heat can be converted into a different energy form. The work performed during the work-performing expansion can be used as explained below. The work-performing expansion can also take place in two or more stages with or without intermediate superheating using the waste heat.
Within the scope of the present invention, as explained below in embodiments, it is in particular provided that the work performed during the work-performing expansion is used for compressing the same or a different refrigerant. Although certain compressors in the embodiments below are driven by means of the work performed during the work-performing expansion, it is not excluded that other compressors can also be driven in this way. In the specific embodiments of the invention, in some cases, the compressors that compress to the highest pressure in each case in the refrigerant circuit (designated C2 in the figures) are, for example, coupled to corresponding expansion machines. Alternatively, however, any other compressors or compressor stages that are configured to compress to a lower pressure (designated C1, C1A or C1B in the figures) can also be driven via the work-performing expansion. Nevertheless, it is possible to operate compressors connected in parallel, of which one is driven by means of the work performed during the work-performing expansion and another is driven in another way, and which compress parallel partial flows of the refrigerant.
In various embodiments of the present invention, the work performed during the expansion can also be used at least in part to drive an electric generator.
In medium-size systems for natural gas condensing with a capacity of, for example, approximately 0.3 to 2 megatons per year, the mentioned SMR circuits are frequently used, since a limited number of components is required in these systems and an adequate thermodynamic efficiency is present. However, the investment costs for a steam system to utilize the turbine waste heat are not necessarily economical for such a system size if the possible energy savings do not compensate for the additional costs. The present invention can particularly be used in such cases and creates an alternative and advantageous possibility for waste heat utilization here. By the use of the present invention, the efficiency of an SMR process can be improved by at least 10 to 15 percent points by correspondingly relieving the gas turbine used to drive the refrigerant compressor.
On the other hand, the present invention can also be advantageously used to condense natural gas on a larger scale, for example in systems with a capacity of approximately 2 to 10 megatons per year. In such systems, more than one refrigerant compressor is typically required in order to achieve the specified capacity. The optimal rotational speed of the different refrigerant compressors is not necessarily similar or identical, so that transmissions between the individual compressors may have to be used if the latter are to be driven by means of a common gas turbine. However, even when a plurality of independent gas turbines is used, an imbalance in the required shaft power for each compressor can occur. In certain situations, the present invention can be used advantageously in that the work performed during the work-performing expansion is used in support of the drive and imbalances in rotational speed or power are thus compensated.
In the method according to the invention, a mixed refrigerant in one or more mixed refrigerant circuits can in particular be used as the refrigerant. The refrigerant mixture typically consists of light hydrocarbons having one to five carbon atoms and at most 20 mole percent nitrogen. The invention can be used in the mentioned SMR circuits, but also in DMR, MFC or C3MR refrigeration circuits, along with other refrigeration circuits in which, in addition to a mixed refrigerant, a pure refrigerant is used, as in principle known from the prior art cited at the outset. The term “pure refrigerant” is understood here to mean a refrigerant that has or essentially consists of at least 95 mole percent, in particular at least 99 mole percent, of a single hydrocarbon, in particular ethane, ethylene, propane or propylene, or another compound with a suitable vapor pressure curve, such as ammonia or carbon dioxide. If, for example, “propane” or “propane refrigeration circuit” is mentioned below, the related explanations are always to be understood as meaning that they also more generally relate to a pure refrigerant. Reference to a specific pure substance is merely illustrative. A corresponding pure refrigerant can in particular be one that is treated in the manner explained, i.e., from which the first and second portions are formed in the form of corresponding partial flows.
As mentioned several times, within the scope of the present invention, natural gas or a gas mixture formed using natural gas (for example deacidified hydrocarbons, dried hydrocarbons and/or hydrocarbons difficult to boil, in particular hydrocarbons having three or more carbon atoms, freed natural gas) can be used as the gas to be condensed, and/or a gas turbine can be used as the drive that produces waste heat.
Particular advantages result in embodiments of the invention if work performed during the work-performing expansion is used in addition to the drive in the compression of the same refrigerant, which is also expanded in a work-performing manner and is used to form the first and second portions. In this way, a drive that is otherwise used for compression can be relieved by the work performed during the work-performing expansion, and corresponding energy savings result, which can be attributed directly to the utilization of the waste heat. The liquid pressurization of the second portion of the refrigerant expanded later in a work-performing manner requires comparatively significantly less energy as a result. Such embodiments are initially explained below.
Within the scope of the present invention, i.e., in a first group of embodiments, exclusively mixed refrigerants, but no pure refrigerant in the aforementioned sense, are used. However, these embodiments can absolutely also be embodiments with which precooling takes place using a mixed refrigerant. In this first group of embodiments, the compression of the refrigerant comprises in particular a first compression step to a first pressure level and a second compression step to a second pressure level, which is in particular above the first pressure level, wherein the drive is used in the first compression step and the work performed during the work-performing expansion is used in the second compression step. Thus, the first compression step can in particular be carried out using one or more first compressors or one or more first compressor stages that is or are at least partially driven using the drive, and the second compression step can in particular be carried out using one or more second compressors or compressor stages that is or are driven at least partially using the work performed during the work-performing expansion. The second compression step is in this case in particular driven without using the drive that produces waste heat, but advantageously only using the work performed during the work-performing expansion. In this way, both compression steps can be realized by machines that can be operated independently of one another, and no mechanical couplings are required. As also explained below, however, the work performed during the work-performing expansion can also be used correspondingly at any other location.
Within the scope of the present invention, in a preferred embodiment, which is also referred to below as “first embodiment,” the refrigerant can be subjected at least partially to the first compression step and subsequently at least partially to a first partial condensing process to obtain a first liquid fraction and a first gas fraction, wherein in this first embodiment, the first gas fraction is at least partially subjected to the second compression step and subsequently at least partially to a second partial condensing process to obtain a second liquid fraction and a second gas fraction. In this first embodiment, the entire refrigerant can in particular be subjected to the first compression step after it has been evaporated in heat exchange with the gas to be condensed. The method can thus be used simply and without great additional effort in connection with known methods for condensing gas, in which corresponding steps are provided. Reference is made to the cited prior art.
In the first embodiment, the first compression step is carried out in particular using a single, though possibly multistage, compressor, which however does not compress the refrigerant to different pressures and which is provided with reference sign C1 throughout the relevant figures. In this and the following embodiments, the second compression step is carried out in particular using a compressor, which is operated independently of the first compression step and which is provided with reference sign C2 throughout the figures.
After its work-performing expansion, the second portion of the refrigerant in the first embodiment can be at least partially combined with the refrigerant compressed in the first compression step, before the latter is cooled for the first partial condensing process. In this way, the second portion of the refrigerant can be returned to the refrigerant circuit and can there again be subjected to the required compression and condensation steps.
In particular, in the first embodiment, the second portion of the refrigerant used according to the invention can be brought in the liquid state from a pressure level of 10 to 40 bar to a pressure level of 60 to 120 bar for the subsequent expansion. The heating by means of the waste heat in particular carries out heating from a temperature level of 10 to 50° C. to a temperature level of 200 to 400° C. For example, a turbine waste gas of a gas turbine used as a drive or another substance flow can be present at 400 to 600° C. In the first embodiment, the work-performing expansion takes place, in particular, starting from the mentioned pressure level or a higher pressure level to a pressure level of 10 to 40 bar, whereby the temperature is in particular reduced by about 30 to 100° C. In the first embodiment, the first compression step can in particular take place to a pressure level of 10 to 40 bar and the second compression step to a pressure level of 30 to 70 bar. The respective subsequent partial condensation steps in particular take place to a temperature level of 10 to 50° C. in each case. The second portion of the refrigerant, which is ultimately supplied to the work-performing expansion, comprises in particular 40 to 80% of the first liquid fraction.
In the first embodiment, before its work-performing expansion, the second portion of the refrigerant can be partially or completely subjected to indirect heat exchange with the second portion of the refrigerant or a part thereof (i.e., at least partially “with itself”) that has already been subjected to the work-performing expansion, before the latter is combined with the first gas fraction.
If the second portion of the refrigerant is only partially subjected to the mentioned heat exchange with itself, this takes place in the form of a first partial flow of the second portion, whereas a second partial flow of the second portion is not subjected to this heat exchange with itself. The first and second partial flows can be subjected separately from one another, and in particular at different temperature levels, to the heating using the waste heat and can thereafter, and before the work-performing expansion, be combined with one another again. For example, the first partial flow of the second portion can be heated at a higher temperature level with a turbine waste gas in a first waste heat exchanger, wherein the already partially cooled waste gas of the gas turbine is supplied to a second waste heat exchanger, in which the second partial flow can be heated at a lower temperature level. In this way, advantageous preheating for the subsequent further heating or cooling for the subsequent feeding to the first gas fraction after its compression can take place.
In the method according to the invention, in the first embodiment, the second liquid fraction can be at least partially expanded and, downstream of the first compression step, combined with the refrigerant or a part thereof after a corresponding cooling, before the latter is phase-separated.
In the first embodiment, a heat exchanger having a plurality of sections or a plurality of heat exchangers can be used for cooling the gas in indirect heat exchange with the refrigerant, wherein the first portion of the refrigerant and the second gas fraction or parts thereof can be further cooled to different temperature levels and reheated after expansion. The heat exchanger or the plurality of heat exchangers can in particular be designed as coiled shell-and-tube heat exchangers or as soldered plate heat exchangers or can comprise a plurality of such heat exchangers, even heat exchangers of different types.
For example, in the first embodiment, the first portion of the refrigerant and the second gas fraction or respectively parts thereof (the same also applies, without explicit mention, to the other fluids mentioned below) can be supplied at an inlet temperature level of, for example, 10 to 50° C. to the heat exchanger designed as a coiled heat exchanger and can be cooled by separate heat exchanger tubes. The first portion of the refrigerant can be extracted from the heat exchanger at a first intermediate temperature level, below the inlet temperature level, of, for example, −20 to −60° C., expanded, and fed back to the heat exchanger on the shell side. In this case, the second gas fraction can likewise be extracted from the heat exchanger at the first intermediate temperature level, at which it is present in partially condensed form. After phase separation outside the heat exchanger, the liquid phase and the gas phase are fed back separately from one another at the first intermediate temperature level to the heat exchanger and further cooled by separate heat exchanger tubes. The liquid phase is extracted at a second intermediate temperature level, below the first intermediate temperature level, of, for example, −70 to −100° C., expanded, and fed back to the heat exchanger on the shell side. The gas phase is extracted at a third intermediate temperature level, below the second intermediate temperature level, of, for example, −120 to −160° C., expanded, and likewise fed back to the heat exchanger on the shell side. The fluids combined in this way on the shell side are fed back to the compression.
If a soldered plate heat exchanger is used, the first portion of the refrigerant and the second gas fraction or respectively parts thereof can also be supplied together at an inlet temperature level in the aforementioned range to the heat exchanger and cooled in common passages. After extraction at the cold end of the heat exchanger at an extraction temperature level of, for example, −120 to −160° C., expansion can be carried out, and the refrigerant further cooled in this way to a temperature level of, for example, −130 to −170° C. is returned through separate passages and fed back to the compression after heating to a temperature level in the range of the inlet temperature level.
In a further preferred embodiment of the present invention, hereinafter also referred to as the “second embodiment,” the first compression step can in particular be designed differently and can be carried out using two compressor stages, namely a first compressor stage and a second compressor stage, which are, however, advantageously driven together by the drive that supplies waste heat. The first compressor stage, which may also be structurally designed in the form of a plurality of compressor stages of a compressor, is designated throughout the figures with reference sign C1A, the correspondingly designed second compressor stage with reference sign C1B. The second embodiment relates in particular to a DMR process. This process advantageously uses two or three heat exchangers or heat exchanger sections, which can each be designed as coiled heat exchangers or corresponding sections of a coiled heat exchanger. Hereinafter, only for the sake of simplicity, two or three “heat exchangers” are mentioned, which term, however, also includes corresponding sections of a common heat exchanger. In the language used herein, they are a first, a second, and a third heat exchanger in the direction of decreasing temperature of the gas to be condensed. In the embodiments with three heat exchangers, the first and second heat exchangers use the same refrigerant at different evaporation pressures and may therefore also be combined, in particular in the case of low-cost systems, or the first heat exchanger may be dispensed with in such systems. The invention also relates to such methods and systems, even if reference is not made separately below and the invention is described with reference to methods and systems with three heat exchangers.
In the second embodiment, refrigerant flows evaporated correspondingly from the first and the second heat exchanger are supplied to the first compressor stage of the first compression step at pressure levels of, for example, 5 to 20 bar or 2 to 10 bar. Compression to, for example, 15 to 50 bar takes place in the first compressor stage of the first compression step; compression to, for example, 40 to 80 bar takes place in the second compressor stage of the first compression step. Recooling takes place in each case downstream of the compression stages. The first or second portions of the refrigerant previously mentioned several times is formed from the fluid that is compressed in the first compressor stage and that can, in addition to the specified refrigerant, also comprise further refrigerant. The second portion thereof also in particular comprises 40 to 80%.
The first portion is initially passed through the first heat exchanger on the tube side and cooled there to a temperature level of, for example, 0 to −20° C. A partial flow can be expanded downstream of the first heat exchanger and fed on the shell side into the first heat exchanger. This partial flow in particular represents the entire refrigerant evaporated in the first heat exchanger. In the mentioned embodiments with only two heat exchangers, the measures described for the first heat exchanger are omitted. The non-expanded remainder of the first portion of the refrigerant can be used to form a further partial flow, which can be used in a separate further heat exchanger to cool the fluid compressed in the second compressor stage of the first compression step and can thereafter be fed to the first compressor stage of the first compression step. A remainder of the first portion still remaining thereafter is initially passed through the second heat exchanger on the tube side and cooled in it to a temperature level of, for example, −30 to −70° C. This remainder can now be expanded downstream of the second heat exchanger and fed on the shell side into the second heat exchanger. This remainder in particular represents the entire refrigerant evaporated in the second heat exchanger.
In the second embodiment, the second portion of the refrigerant can be essentially treated in the manner explained for the first embodiment and can in particular be fed to the refrigerant compressed in the first compressor stage of the first compression step, before the latter is cooled and condensed. The second portion is circulated in this way. The refrigerant compressed in the second compressor stage of the first compression step can in particular be supplied to the second compression step and compressed there in principle as explained for the first embodiment. In particular, compression to a pressure level of 70 to 110 bar takes place. The correspondingly compressed refrigerant is cooled and initially guided on the tube side through the first to third heat exchangers for further cooling. Downstream thereof, this refrigerant portion is expanded and fed on the shell side into the third heat exchanger. This refrigerant portion in particular represents the entire refrigerant evaporated in the third heat exchanger.
A yet further preferred embodiment of the present invention, hereinafter also referred to as the “third embodiment,” comprises the first compression step being carried out using two compressors, which are now advantageously driven by two separate drives that supply waste heat. These are operated largely similarly to the corresponding compressor stages in the second embodiment and therefore bear the corresponding designations. The third embodiment likewise relates to a DMR process. As in the second embodiment, two or three heat exchangers or heat exchanger sections are advantageously used so that the explanations above continue to apply. The above features and explanations regarding the second embodiment also relate to the third embodiment, wherein, however, the remainder of the first portion of the refrigerant not expanded downstream of the first heat exchanger is optionally not used to form a further partial flow, which serves to cool the fluid compressed in the second compressor of the first compression step. The second portion of the refrigerant, which is ultimately expanded in a work-performing manner, is heated with the waste heat of both drives.
As mentioned, in the embodiments just explained, work performed during the work-performing expansion is used in addition to the drive in the compression of the same refrigerant, which is also expanded in a work-performing manner and which is used to form the first and second portions, although it is used in different circuits in the DMR circuits. In contrast, in other embodiments of the invention, advantages arise if work performed during the work-performing expansion is used in the compression of a further refrigerant, i.e., not the same refrigerant, which is expanded in a work-performing manner and which is used to form the first and second portions. For better differentiation, the refrigerant expanded in a work-performing manner and used to form the first and second portions is referred to as the “first” refrigerant, and the further refrigerant is referred to as the “second” refrigerant.
The first to third embodiments are part of the aforementioned first group of embodiments, in which exclusively mixed refrigerants are used. These are SMR and DMR circuits, i.e., even circuits with which a mixed refrigerant is used for precooling. A second group of embodiments, which is now explained, comprises embodiments in which a pure refrigerant is additionally used in a precooling circuit. These therefore include C3MR circuits, inter alia.
In the second group of embodiments, the compression of the pure refrigerant, which here represents a “first” refrigerant in the sense just explained, is carried out in the precooling circuit in a first compressor or a first compressor stage, and the compression of the mixed refrigerant in the mixed refrigerant circuit, which in this sense represents the “second” refrigerant, is carried out using a second compressor or a second compressor stage and a third compressor or a third compressor stage in the manner explained below. The work performed during the work-performing expansion is used to drive the third compressor or the third compressor stage. Merely for the sake of clarity, compressors are mentioned below, which are also to be understood as compressor stages.
In a corresponding embodiment of the invention, hereinafter also referred to as the “fourth embodiment,” the first and second compressors (C1A and C1B in the figures) are driven by two separate drives, wherein only the drive of the second compressor is a drive, such as a gas turbine, that supplies waste heat (at least to a considerable and usable extent). The first compressor can be driven electrically, for example, producing significantly lower (and not usable) quantities of waste heat.
In deviation from the second and third embodiments, a soldered plate heat exchanger and a coiled shell-and-tube heat exchanger are used in the fourth embodiment to cool the gas to be condensed. As mentioned, two separate refrigerant circuits are realized, namely a pure substance circuit with pure refrigerant for precooling and a refrigerant circuit with mixed refrigerant. As already mentioned, the pure substance circuit comprises the first compressor, whereas the mixed refrigerant circuit comprises the second and the third compressors.
The pure refrigerant of the pure substance circuit is supplied to the first compressor in a plurality of partial flows, which are in particular heated against the mixed refrigerant from the second compression step and thus precool the mixed refrigerant, and compressed there. After subsequent cooling and condensing, the first and second portions of the refrigerant are also formed here. In distinction from the embodiments explained above, the first and second portions are thus formed from the pure refrigerant, the “first” refrigerant, and not the mixed refrigerant, the “second” refrigerant. The first portion is initially cooled, subsequently expanded, heated against the mixed refrigerant, and fed back to the first compressor. The second portion is treated as already mentioned above and thereby heated with the waste heat of the drive of the second compressor.
After its precooling with the pure refrigerant of the pure substance circuit, in particular to a temperature level of −20 to −40° C., the mixed refrigerant is cooled further on the tube side in the coiled heat exchanger, in particular to a temperature level of −120 to −160° C. Downstream thereof, it is expanded and supplied on the shell side to the coiled heat exchanger. After extraction from the coiled heat exchanger and corresponding heating, further heating is carried out in the soldered plate heat exchanger, and compression subsequently takes place in the second and third compressors.
A variant of the fourth embodiment just explained, referred to as the “fifth embodiment,” comprises the first and second compressors being driven via a common drive that produces waste heat.
In all cases, work performed during the work-performing expansion can be used in the compression of a further refrigerant, with which the gas is subjected to cooling in indirect heat exchange. This may be the case, for example, when using a pure substance or C3MR refrigerant circuit, or in variants of the first group of embodiments.
In a further embodiment of the invention, which is referred to herein as the “sixth embodiment,” a mixed refrigerant is used as the first refrigerant and nitrogen is used as the second refrigerant. In this embodiment as well, the first and second portions are portions of a first refrigerant, namely the mixed refrigerant, and work performed during the work-performing expansion is used in the compression of a second refrigerant, namely the nitrogen.
In principle, in the sixth embodiment, as previously explained, for example, for the first embodiment, the mixed refrigerant can be at least partially subjected to a first compression step and subsequently at least partially subjected to a first partial condensing process to obtain a first liquid fraction and a first gas fraction. The first gas fraction can be at least partially subjected to the second compression step and subsequently at least partially subjected to a second partial condensing process to obtain a second liquid fraction and a second gas fraction. The further treatment can also be identical.
Generally speaking, in the fifth embodiment, the nitrogen is subjected to expansion and compression, wherein the compression of the nitrogen takes place using the work performed during the work-performing expansion of the second portion of the mixed refrigerant. In the fifth embodiment, the expansion of the nitrogen can take place in a work-performing manner, wherein work performed during the work-performing expansion of the nitrogen can likewise be used in the compression of the nitrogen.
The compressed nitrogen is, in succession, cooled, subjected to a first indirect heat exchange and thereby cooled, subjected to expansion, subjected to a second indirect heat exchange and thereby heated, thereafter subjected to the first indirect heat exchange and thereby heated, and fed back to the compression. In the second indirect heat exchange, the gas subjected to the partial or complete condensing process is supercooled.
A further embodiment of the present invention, referred to herein as the “seventh embodiment,” differs from the sixth embodiment in that the compression of the nitrogen is carried out in two stages in a first and thereafter a second compression step, wherein the first compression step takes place using the work performed during the work-performing expansion of the nitrogen and the second compression step takes place using the work performed during the work-performing expansion of the second portion of the mixed refrigerant.
The invention also extends to a system for condensing a gas, wherein the system comprises means configured to subject the gas to cooling in indirect heat exchange with a refrigerant and, after the heat exchange with the gas, to subject at least a part of the refrigerant to compression using a drive that produces waste heat, and to subsequently subject it to a partial or complete condensing process. According to the invention, the system has means configured, after a partial or complete condensing process, to subject a first portion of the refrigerant to the heat exchange with the gas and a second portion of the refrigerant, in succession, to pressurization, to heating using the waste heat of the drive and to work-performing expansion, and thereafter to feed it back to the partial or complete condensing process.
Reference is expressly made to the above statements with regard to features and advantages of a corresponding system, which is advantageously configured to carry out a method in accordance with the present invention and any previously explained embodiments.
The invention is explained in more detail below with reference to the accompanying drawings, which illustrate arrangements in accordance with embodiments of the present invention.
In the figures, elements corresponding to one another are indicated by identical reference signs and are not explained repeatedly for the sake of clarity. Identical elements are not designated separately in all figures.
The method serves to condense a gas that is supplied to the method in the gaseous state as substance flow 1 and is provided in condensed form as substance flow 2. An overall highly simplified heat exchanger or cryogenic part 10 is used here for the condensing process. In order to illustrate general applicability, the heat exchanger part 10 is shown in highly simplified form.
Refrigerant is discharged from the heat exchanger part 10 in the form of a heated (“warm”) refrigerant flow W. Remaining condensate is separated in a separator D1. The refrigerant of the substance flow W is compressed in a first compression step using a compressor C1, which is driven by a gas turbine GT1. In the gas turbine GT1, air of an air flow A is compressed in a compressor stage, which is not designated separately, and is combusted with fuel F in a combustion chamber (not shown). Hot gas is expanded in an expansion stage, which is likewise not designated separately, and is discharged via a heat exchanger E4 for heat recovery. Auxiliary firing using further fuel AF can also take place.
The refrigerant compressed in the compressor C1 is cooled in a heat exchanger E1, thereby partially condensed, and subjected to phase separation in a separator D2. The gas and liquid phases are supplied to the heat exchanger part 10 in the form of separate substance flows, wherein a part of the liquid phase is supplied to the heat exchanger part 10 as the “first portion” of the refrigerant, which was previously referred to correspondingly several times, and a further part as the “second portion” in the form of a substance flow R is increased in pressure by means of a pump P1, heated in a heat exchanger E3 and thereafter in the heat exchanger E4, then expanded in a work-performing manner in an expansion machine X1, passed through the heat exchanger E3, and subsequently combined with the refrigerant compressed in the compressor C1 before its cooling.
A compressor C2 is coupled to the expansion machine X1 via a transmission G. A mixed refrigerant in the form of a heated refrigerant flow W1 can be supplied to the compressor C2 from the heat exchanger part 10, so that utilization of the waste heat of the gas turbine GT1 is possible in this way.
The refrigerant flow W1 in accordance with
The substance flow W is compressed in the compressor C1 downstream of the separator D1 and subsequently, after cooling in a heat exchanger E1, is subjected to a partial condensing process in a separator D2 to obtain a first liquid fraction and a first gas fraction. The first gas fraction, which is not separately designated, from the separator D2 is compressed in the second compressor C2 and subsequently, after cooling in a heat exchanger E2, is subjected to a partial condensing process in a separator D3 to obtain a second liquid fraction and a second gas fraction.
The first liquid fraction from the separator D2 is partially treated in the form of the substance flow R as already explained above. The remainder, like the second gas fraction from the separator D2, is supplied to the coiled heat exchanger 11 in the form of a substance flow which is not separately designated. The specified refrigerant flows are passed through separate heat exchanger tubes and cooled.
The first liquid fraction from the separator D2 not used in the form of the substance flow R is extracted from the heat exchanger 11 at a first intermediate temperature level below the corresponding inlet temperature level, expanded and fed back to the heat exchanger 11 on the shell side. The second gas fraction can likewise be extracted from the heat exchanger at the first intermediate temperature level, expanded and thereby partially condensed, wherein, however, a phase separation into a liquid phase and a gas phase is carried out outside the heat exchanger 11 in the separator 12.
The liquid phase formed in the separator 12 and the gas phase are fed back separately from one another to the heat exchanger 11 at the first intermediate temperature level and further cooled by separate heat exchanger tubes. The liquid phase is extracted at a second intermediate temperature level below the first intermediate temperature level, expanded and fed back to the heat exchanger 11 on the shell side. The gas phase is extracted at a third intermediate temperature level below the second intermediate temperature level, expanded and likewise fed back to the heat exchanger 11 on the shell side. The fluids combined on the shell side in this way are fed back to the compression in the form of the substance flow W.
After its work-performing expansion, the substance flow R is combined with the refrigerant that was compressed in the compressor C1, before the latter is cooled for the first partial condensation. The second liquid fraction from the separator D3 is expanded via a valve V1 and returned to the separator D2.
As illustrated here, the portion of the first liquid fraction from the separator D2 not used in the form of the substance flow R and the second gas fraction from the separator D3 can be supplied together to the heat exchanger 13 and cooled in common passages. A pump 14 boosts the portion of the first liquid fraction thus used to the pressure of the second gas fraction, so that both fractions can be fed together to the heat exchanger 13. After extraction at the cold end, expansion can be carried out via a valve 15 and the refrigerant further cooled in this way can be returned through separate passages and, after corresponding heating, can be fed into the separator D1 again.
Furthermore, three heat exchangers 16, 17, 18, each designed as a coiled heat exchanger, are used. In the language used herein, they are a first heat exchanger 16, a second heat exchanger 17 and a third heat exchanger 18 in the direction of decreasing temperature of the gas 1 to be condensed. The first heat exchanger 16 may be omitted, as explained in detail above.
Correspondingly evaporated refrigerant flows are supplied from the first and second heat exchangers 16, 17 to the first compressor stage C1A and compressed there. An evaporated refrigerant flow is supplied from the third heat exchanger 18 to the second compressor stage C1B and compressed there. Recooling takes place in each case downstream of the compressor stages. The first and second portions of the refrigerant previously mentioned several times are formed from the fluid that is compressed in the first compressor stage C1A and that, in addition to the specified refrigerant, can also comprise further refrigerant, which is extracted from the separator also designated here with D2.
The first portion is initially passed through the first heat exchanger 16 on the tube side and cooled there. A partial flow can be expanded downstream of the first heat exchanger 16 and fed on the shell side into the first heat exchanger 16. The non-expanded remainder of the first portion of the refrigerant can be used to form a further partial flow, which can be used in a separate further heat exchanger E5 to cool the fluid compressed in the second compressor stage C1B of the first compression step and can thereafter be fed to the first compressor stage C1A of the first compression step. A remainder of the first portion still remaining thereafter is initially passed through the second heat exchanger 17 on the tube side and cooled therein. This remainder can now be expanded downstream of the second heat exchanger 17 and fed on the shell side into the second heat exchanger 17.
The second portion of the refrigerant can be treated essentially as described above in the form of the substance flow R and can in particular be fed to the refrigerant compressed in the first compressor stage C1A of the first compression step, before the latter is further cooled and condensed. The second portion is circulated in this way.
The refrigerant compressed in the second compressor stage C1B of the first compression step can in particular be supplied to the second compression step with the compressor C2 and compressed there in principle as explained for the first embodiment. The correspondingly compressed refrigerant is cooled in a further heat exchanger E6 and initially passed through the first to third heat exchangers 16, 17, 18 on the tube side for further cooling. Downstream of the last one, this refrigerant portion is expanded and fed on the shell side into the third heat exchanger 18.
Yet another preferred embodiment of the present invention is illustrated in
A further embodiment of the present invention is illustrated in
The compression of a pure refrigerant (illustrated here by way of example as propane C3H8) in a precooling circuit is carried out here in a first compressor C1A, and the compression of a mixed refrigerant in a mixed refrigerant circuit takes place using a second compressor C1B and a third compressor C2. The work performed during the work-performing expansion is used to drive the third compressor C2. The first and second compressors C1A, C1B are driven by two separate drives, wherein only the drive of the second compressor C1B is a drive, such as a gas turbine GT1, that supplies waste heat (at least to a considerable and usable extent). The first compressor C1A can, for example, be driven by means of a motor M, producing significantly lower (and not usable) quantities of waste heat.
In deviation from the previously explained embodiments, a soldered plate heat exchanger 19 in addition to a coiled heat exchanger 11 is used to cool the gas 1 to be condensed. The refrigerant of the pure substance circuit is supplied to the first compressor C1A in a plurality of partial flows, which are in particular heated and evaporated against the mixed refrigerant from the second compression step and thus precool the mixed refrigerant, and compressed there. After subsequent cooling and condensing, the first and second portions of the refrigerant are also formed here. The first portion is initially supercooled, subsequently heated and evaporated against the mixed refrigerant from the second compressor, and fed back to the first compressor C1A. The second portion R is treated as already mentioned above and thereby heated with the waste heat of the drive of the second compressor.
After its precooling, the mixed refrigerant is cooled further with the refrigerant of the pure refrigerant circuit on the tube side in the coiled heat exchanger 11. Downstream thereof, it is expanded and supplied on the shell side to the coiled heat exchanger 11. After extraction from the coiled heat exchanger 11 and corresponding heating, further heating is carried out in the soldered plate heat exchanger 19, and compression subsequently takes place in the second and third compressors C1B and C2.
A variant of the embodiment just explained is illustrated in
In all cases, work performed during the work-performing expansion can be used in the compression of a further refrigerant, with which the gas is subjected to cooling in indirect heat exchange. This may be the case, for example, when using a mixed refrigerant circuit precooled with a pure refrigerant, or in further variants of the invention illustrated in
The treatment of the mixed refrigerant results directly from
In the embodiment in accordance with
The compressed nitrogen is, in succession, cooled, subjected to a first indirect heat exchange in the heat exchanger 19B and thereby cooled, subjected to the expansion, subjected to a second indirect heat exchange in the heat exchanger 19A and thereby heated, thereafter again subjected to the first indirect heat exchange in the heat exchanger 19B and thereby heated, and fed back to the compression. In the second indirect heat exchange in the heat exchanger 19A, the gas previously subjected to the partial or complete condensing process is supercooled. A heat exchanger E7 is provided for recooling the nitrogen in the nitrogen circuit downstream of the compressor C3.
In the embodiment in accordance with
The invention described above and its explained embodiments in particular shown in the figures are described again below in other words. The terms used below may be synonymous with the terms used above for the method steps, devices and media referred to in each case. The following explanations describe the same inventive concept with corresponding advantageous developments as the above explanations in at least partially deviating formulation.
The present invention relates to a method for collecting or recovering waste heat produced in a gas condensing process, comprising condensing a gas by a heat exchange process using a refrigerant fluid, compressing the spent refrigerant fluid from the condensing process by a method that produces excess heat, condensing at least a part of the compressed refrigerant fluid, pumping a part of the condensed compressed refrigerant fluid to a higher pressure, heating the part of the condensed compressed compressed refrigerant fluid at a higher pressure by absorbing the excess heat produced by the compression of the spent refrigerant fluid, whereby the part of the compressed refrigerant fluid is superheated at a higher pressure, and using the superheated compressed refrigerant fluid to supply a mechanical process.
One embodiment of the present invention applies to a natural gas condensing method with at least one compressor used in the refrigerant circuit for the cryogenic process of the natural gas condensing process. The present invention uses a compressor in the refrigerant circuit, wherein the compressor is driven by a gas turbine or a similar energy source that produces waste heat during the generation of power for operating the compressor. The present invention uses a work expander, wherein the fluid circuit for the work expander is used to absorb the waste heat of the gas turbine or a similar power source that drives the compressor in the refrigerant circuit. In one embodiment of the invention, the fluid circuit for the work expander is both pressurized and heated so that the fluid circuit can absorb the waste heat present in the waste gas flow of the gas turbine or other waste heat of the power source that drives the compressor in the refrigeration circuit. The resulting superheated fluid, which arises from the recovery process for the waste heat energy, is then used as an energy source for the drive of the work expander.
In a further embodiment of the present invention, the fluid used in the fluid circuit for the work expander is also used for the refrigerant circuit. In this embodiment of the invention, a second compressor is additionally used in the refrigerant circuit, wherein the second compressor is driven by the work expander. Accordingly, in this embodiment of the invention, the refrigerant fluid used in the cryogenic process for condensing natural gas is also used for absorbing waste heat, which is produced for driving the first compressor in order to provide power for driving the work expander, which in turn drives the second compressor in order to further compress the refrigerant fluid. Accordingly, this embodiment of the present invention offers advantages over other systems for collecting waste heat energy. This embodiment of the present invention thus requires neither the introduction of additional working liquids, such as water, nor the addition of other liquids (e.g., steam, ammonia, propane, etc.) in closed circuits.
In a natural gas condensing process (not illustrated) in accordance with the prior art, with which a single mixed refrigerant (SMR) with a two-stage SMR compression process is used, it can be provided that two compressors C1 and C2 are driven by a single gas turbine GT1. In this case, a cryogenic part of the process carries out the condensing process of the natural gas by a heat exchange process with a mixed refrigerant. In the natural gas condensing process, the mixed refrigerant is compressed, cooled and partially condensed before it is recycled in the cryogenic process. Mixed refrigerant discharged by the cryogenic part can be collected in a container D1 and is then conducted into the first compressor C1 and the heat exchanger E1. In a corresponding two-stage compression process, the liquid portion of the first compressor C1 and of a heat exchanger E1 is collected in a storage container D2, wherein the vapor portion of the first compressor C1 is fed into the second stage of the process via the second compressor C2 and a heat exchanger E2. The resulting portion is combined from the second compressor C2 and the heat exchanger E2 and collected in a container D3. The two fractions collected in the containers D2 and D3 may be fed into the cryogenic part, in order to carry out the condensing process of natural gas by a heat exchange process.
In one embodiment of the present invention, the cryogenic part can be designed with coil-wound heat exchangers (CWHEs), soldered plate heat exchangers (PFHEs) or a combination thereof.
In one embodiment of the invention, which is shown in
As explained with respect to the embodiment of the invention shown in
In other embodiments of the invention, the shaft power generated by the work expander X1 could be used to drive other processes, such as a power generator, a feed gas compression, a terminal flash gas compression, any type of refrigerant compression or any other service that requires power.
The entire cooling system will have at least one refrigerant consisting of either a pure component or a mixture of components, wherein the refrigerant in one embodiment of the invention can be at least partially condensed at ambient temperature. In one embodiment of the invention, the permissible refrigerant components could include nitrogen and light paraffinic or olefinic hydrocarbons of C1 to C5 (such as CH4, C2H4, C2H6, C2H6, C3H6, C3H8, iC4H10, nC4H10, nC4H10, iC5H12, nC5H12, nC5H12, etc.). The cooling system can also include more than one circuit, wherein the additional circuits are pure refrigerant circuits and/or mixed refrigerant circuits and/or gas expansion circuits.
In other embodiments of the invention in which the cooling system includes more than one circuit, the additional circuits can be pure refrigerant circuits, mixed refrigerant circuits and/or gas expansion circuits. In addition, in other configurations, one or more gas turbines can be operated in parallel or in series.
In accordance with a first aspect, the present invention comprises a method for separating waste heat produced in a gas condensing process, comprising condensing a gas by a heat exchange process using a refrigerant fluid, compressing the spent refrigerant fluid from the condensing process by a method that produces excess heat, condensing at least a part of the compressed refrigerant fluid, pumping a part of the condensed compressed refrigerant fluid to a higher pressure, heating the part of the condensed compressed refrigerant fluid at a higher pressure by collecting the excess heat produced by the compression of the spent refrigerant fluid, whereby the part of the compressed refrigerant fluid is superheated at a higher pressure, and using the superheated compressed refrigerant fluid to carry out a mechanical process.
In accordance with a second aspect, a method for recovering waste heat produced in a gas condensing process, according to the first aspect is provided, furthermore comprising the mechanical process representing a further compression of the compressed refrigerant fluid.
In accordance with a third aspect, a method for recovering waste heat produced in a gas condensing process, according to the first aspect is provided, wherein the mechanical process is furthermore the operation of a work expander.
In accordance with a fourth aspect, a method for recovering waste heat produced in a gas condensing process, according to the third aspect is provided, furthermore comprising heating the part of the condensed compressed refrigerant fluid at a higher pressure by heat exchange with the fluid discharged by the work expander.
In accordance with a fifth aspect, a method for recovering waste heat produced in a gas condensing process, according to the fourth aspect is provided, wherein the fluid from the work expander used in heat exchange is furthermore combined with the condensed compressed refrigerant fluid.
In accordance with a sixth aspect, a method for recovering waste heat produced in a gas condensing process, according to the third aspect is provided, furthermore comprising the mechanical process representing a further compression of the compressed refrigerant fluid.
In accordance with a seventh aspect, a method for recovering waste heat produced in a gas condensing process, according to the sixth aspect is provided, furthermore comprising the further compression refrigerant fluid being the refrigerant fluid in the condensing step.
In accordance with an eighth aspect, a method for recovering waste heat produced in a gas condensing process, according to the first aspect is provided, furthermore comprising the mechanical method generating electrical energy.
In accordance with a ninth aspect, a method for recovering waste heat produced in a gas condensing process, according to the first aspect is provided, furthermore comprising heating the part of the condensed compressed refrigerant fluid at a higher pressure, auxiliary firing of an additional heat source into the collected excess heat produced by the compression of the spent refrigerant fluid.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1904525.1 | Apr 2019 | GB | national |
| 19020458.6 | Aug 2019 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/025127 | 3/12/2020 | WO | 00 |
