Methods for operating a steam reforming plant and steam reforming plant

Abstract

A method for operating a steam reforming plant (100) in which a hydrocarbon-rich feedstock (b) is reacted with steam (c) to form a synthesis gas (k) using at least one steam reforming reactor (2), with at least one processing unit (4-7) with which a hydrogen-rich fraction (u) is obtained from the synthesis gas (k) by separating carbon dioxide (m, n) and carbon monoxide (t), and with at least one pressure swing adsorption plant (9) with which a residual gas (h) is separated from the hydrogen-rich fraction, wherein at least one recycling compressor (10) is used with which, in a first operating mode, carbon dioxide (n, o) separated from the synthesis gas (k) is at least partially mixed with the hydrocarbon-rich feedstock (b), characterized in that, in a second operating mode, the recycling compressor (10) is not used for compressing carbon dioxide (m) separated from the synthesis gas (k), but rather forto mix at least part of the residual gas (w, x) separated in the pressure swing adsorption plant (9) with the hydrogen-rich fraction (u) upstream of the pressure swing adsorption plant (9), wherein the steam reforming plant (100) is operated in the first or second operating mode depending on a desired hydrogen / carbon monoxide product ratio.

Description

The invention relates to a method for operating a steam reforming plant and a steam reforming plant that is equipped for such operation. State of the art Steam reforming is the most widely used process for producing synthesis gas from light hydrocarbons. In steam reforming, a hydrocarbon feedstock such as natural gas, liquefied petroleum gas (LPG), or naphtha is reacted endothermically with steam in a catalytic tubular reactor, hereinafter referred to as a "steam reforming reactor," to produce synthesis gas. Process heat and flue gas heat are used for steam generation. Further details can be found, for example, in Chapter 2 of the article "Gas Production" in Ullmann's Encyclopedia of Industrial Chemistry (doi:10.1002 / 14356007.a12_169.pub2, electronic edition 2007). The purified, especially desulfurized, hydrocarbon feedstock is first mixed with superheated process steam according to the steam / carbon ratio required for the reforming reactions. The resulting hydrocarbon / steam mixture is heated and distributed into the catalyst-filled, externally fired tubes of the steam reforming reactor. As it flows through the tubes, it reacts according to the following reactions: CnHm + n H₂O → n CO + ((n + m) / 2) H₂(1) CH₄ + H₂O ↔ CO + 3 H₂(2) CO + H₂O ↔ CO₂ + H₂(3) To minimize the methane content in the synthesis gas, maximize the hydrogen yield and avoid the formation of elemental carbon and its deposition on the catalyst, the steam reforming reactor is operated in practical operation with a higher steam / carbon ratio than theoretically required. The actual steam reforming process is typically followed by several processing steps to obtain more or less pure end products from the synthesis gas. The synthesis gas is first cooled. This is followed by an initial step to remove carbon dioxide in a carbon dioxide removal unit. In this unit, some of the carbon dioxide is scrubbed out, for example, with methanol and / or diethanolamine. Further carbon dioxide can be removed, for example, by subsequent temperature swing adsorption. This is usually followed by a cryogenic separation of the synthesis gas into a hydrogen-rich and a carbon monoxide-rich fraction. The latter can be compressed and discharged at the plant boundary. After cryogenic separation, the hydrogen-rich fraction still contains impurities such as carbon monoxide, carbon dioxide, and light hydrocarbons like methane, ethane, propane, ethylene, and propylene. These can be separated in a pressure swing adsorption (PSA) system, yielding so-called tail gas, so that a hydrogen product, preferably pure hydrogen, can be obtained from the hydrogen-rich fraction. Since the heat balance for the main reactions (1) - (3) listed above is endothermic, the required heat must be supplied by external combustion. Among other things, residual gas from the pressure swing adsorption system and heating gas from the system boundary can be used as fuel gas. The influence of different parameters on the composition of the synthesis gas is summarized in tabular form, for example, in WO 2005 / 040704 A2. Corresponding processes are also disclosed in WO 03 / 086965 A1, EP 1 544 166 A2 and EP 0 790 212 A1. Depending on the desired hydrogen / carbon monoxide product ratio in the synthesis gas, a carbon dioxide fraction from the carbon dioxide separation unit can, for example, be recycled via a so-called recycling compressor and added to the hydrocarbon feed. In a plant using natural gas, a hydrogen / carbon monoxide product ratio of approximately 2.5 can be achieved with complete recycling of the separated carbon dioxide fraction and depending on other boundary conditions. If even less hydrogen is required, additional carbon dioxide can be imported from the plant boundary to further reduce the hydrogen / carbon monoxide product ratio. If a higher hydrogen / carbon monoxide product ratio is desired, the recirculation of the carbon dioxide fraction can be reduced to zero. This allows the hydrogen / carbon monoxide product ratio to be increased to approximately 4.1. If even more hydrogen is to be produced, the plant can be operated with modified parameters (e.g., a higher steam fraction), which, however, directly affects the plant size and thus the investment costs. Alternatively, a side-flow shift or a separate shift line can be installed, but this also leads to an increase in the investment. The object of the present invention is therefore to create a way to produce a correspondingly hydrogen-rich synthesis gas using a steam reforming plant without significant additional effort. Disclosure of the invention Against this background, the invention proposes a method for operating a steam reforming plant and a steam reforming plant configured for such operation, comprising the features of the independent claims. Preferred embodiments are the subject of the dependent claims and the following description. Advantages of the invention The invention relates to a method for operating a steam reforming plant in which a hydrocarbon-rich feed is reacted with steam to form a synthesis gas using at least one steam reforming reactor, with at least one processing unit in which a hydrogen-rich fraction is obtained from the synthesis gas by separating carbon dioxide and carbon monoxide, and with at least one pressure swing adsorption plant in which a residual gas is separated from the hydrogen-rich fraction. As explained above, such processes can employ a compressor, referred to here as a recycling compressor, which, in a first operating mode, at least partially compresses carbon dioxide separated from the synthesis gas and mixes it with the hydrocarbon-rich feedstock. The carbon dioxide separated from the synthesis gas can be extracted from a carbon dioxide separation unit, in particular a so-called carbon dioxide scrubber. Such a process is known from WO 2005 / 040704 A2. Publications WO 2012 / 140994 A1, DE 102006006281 A1, and US3943236 A propose the simultaneous recirculation of several gas streams obtained during synthesis gas separation, for which WO 2012 / 140994 A1 uses one recycling compressor and DE 102006006281 A1 uses two. According to the invention, it is now provided that in a second operating mode the recycling compressor is not used for the compression of carbon dioxide separated from the synthesis gas, but rather to mix the residual gas separated in the pressure swing adsorption plant at least partially with the hydrogen-rich fraction upstream of the pressure swing adsorption plant. According to the invention, the recycling compressor, which is typically used to add carbon dioxide separated from the synthesis gas to the hydrocarbon-rich feed to achieve a low hydrogen / carbon monoxide product ratio, but which is not needed if a high hydrogen / carbon monoxide product ratio is desired and the carbon dioxide fraction is reduced to zero for this purpose, is used to return a portion of the residual gas from the pressure swing adsorption (PSA) plant to the hydrogen-rich fraction upstream of the PSA plant, thereby increasing the hydrogen yield. The process according to the invention is based on the fact that the typically used PSA plants only achieve a hydrogen yield of approximately 90%. The remaining 10% of hydrogen ends up in the residual gas.By recirculating at least part of the residual gas and feeding it upstream of the pressure swing adsorption system, approximately 90% of the hydrogen contained therein is transferred to the hydrogen product, thus increasing the hydrogen yield accordingly. In the present application, substances and mixtures of substances in a plant or process, for example, a hydrocarbon feedstock and the synthesis gas obtained therefrom, are referred to as "streams" and "fractions." A stream is typically conveyed as a fluid in a line specifically designed for this purpose. A fraction refers to a portion of a feedstock mixture that has been separated from it. A fraction can form a stream if it is conveyed accordingly. Conversely, a stream can, for example, serve to provide a feedstock mixture from which a fraction can be separated. A stream or fraction can be “rich” or “poor” in one or more of its components, e.g., hydrogen, where “rich” can refer to a proportion of more than 75%, 80%, 85%, 90%, 95%, 99%, 99.5% or 99.9% and “poor” to a proportion of less than 25%, 20%, 15%, 10%, 5%, 1%, 0.5% or 0.1%, in each case on a weight or volume basis. The process according to the invention enables an increase in the hydrogen / carbon monoxide product ratio of the synthesis gas produced in the steam reforming plant through simple and cost-effective measures. The use of a so-called shift, i.e., an additional unit by which a carbon monoxide or water-gas shift reaction of at least a portion of the carbon monoxide contained in the synthesis gas can be carried out, is no longer necessary. Nor is a conventionally required significant increase in the steam fraction in the steam reforming reactor, which demands additional energy. This allows for an increase in the maximum hydrogen yield of a corresponding plant without a significant increase in investment costs. The measures according to the invention allow the hydrogen / carbon monoxide product ratio to be significantly increased without requiring a larger design for the rest of the system. When using natural gas as a hydrocarbon-rich feedstock, this value can be increased to approximately 4.8. This corresponds to a yield increase of approximately 5–10% through recirculation, meaning that either a larger quantity of hydrogen product can be extracted from the system overall, or the system load can be reduced accordingly: With a constant quantity of the hydrogen-rich fraction fed into the pressure swing adsorption system (and thus a constant quantity of the carbon monoxide product), the quantity of extractable hydrogen product increases accordingly. If this is not desired, the throughput can be reduced due to the increased hydrogen / carbon monoxide product ratio and the corresponding reduction in the carbon monoxide product. The process according to the invention advantageously comprises operating the steam reforming plant in the first or second operating mode, depending on a desired hydrogen / carbon monoxide product ratio – either by feeding separated carbon dioxide upstream of the steam reforming reactor on the one hand, or by feeding separated residual gas into the hydrogen-rich fraction upstream of the pressure swing adsorption plant on the other. A corresponding process can therefore be very easily adapted to the respective product requirements. Advantageously, the second operating mode – feeding separated residual gas into the hydrogen-rich fraction upstream of the pressure swing adsorption unit – is always selected when the desired hydrogen / carbon monoxide product ratio is at least 4.0. Such values ​​and higher can only be achieved in conventional systems with considerable additional effort. The hydrogen-rich fraction fed into the pressure swing adsorption (PSA) system typically has a hydrogen content of at least 85 mol%, and in particular at least 95 mol%. A tail gas is separated from this fraction in the PSA system, typically containing at least 2 to 50 mol% carbon monoxide, 0 to 5 mol% carbon dioxide, 30 to 90 mol% hydrogen, and 0 to 20 mol% light hydrocarbons. These values ​​depend significantly on the specific configuration of the carbon monoxide removal unit used. In particular, natural gas is used as a hydrocarbon-rich feedstock in a corresponding process because this allows for a particularly effective increase in the hydrogen / carbon monoxide product ratio. Natural gases are known to consist mainly of methane. Due to its favorable hydrogen / carbon ratio of 4:1, this enables a particularly high hydrogen yield. Other feedstocks with corresponding contents of light, gaseous hydrocarbons can also be used. The steam reforming plant according to the invention is equipped to carry out a corresponding process. It comprises at least one steam reforming reactor, which is equipped to convert a hydrocarbon-rich feedstock with steam to produce synthesis gas, at least one processing unit, which is equipped to obtain a hydrogen-rich fraction from the synthesis gas by separating carbon dioxide and carbon monoxide, and at least one pressure swing adsorption plant, which is equipped to separate a residual gas from the hydrogen-rich fraction.It further comprises at least one recycling compressor, which in a first operating mode is connected to the processing unit and the inlet side of the steam reformer, from which it can be disconnected in a second operating mode to be connected to the pressure swing adsorption (PSA) plant, so that in the first operating mode, carbon dioxide separated from the synthesis gas can be at least partially added to the hydrocarbon-rich feed, and in the second operating mode, the residual gas separated in the PSA plant can be at least partially added to the hydrogen-rich fraction upstream of the PSA plant. The plant benefits from the advantages described above, which are therefore expressly referenced. Advantageously, such a steam reforming plant also includes at least one control unit designed to switch the steam reforming plant from the first operating mode to the second operating mode. This is advantageously done fully automatically by means of a corresponding user input, making the plant particularly easy to operate. The steam reforming plant advantageously comprises, as at least one processing unit, at least one carbon dioxide separation unit, at least one temperature swing adsorption unit and / or at least one cryogenic separation unit. The invention and further aspects of the invention are explained below with reference to the attached figures. Brief description of the drawings Fig. 1 schematically shows a steam reforming plant according to a preferred embodiment of the invention in a first operating mode. Fig. 2 schematically shows a steam reforming plant according to a preferred embodiment of the invention in a second operating mode. In the figures, corresponding elements bear identical reference symbols. Further explanation is therefore unnecessary. Embodiments of the invention Fig. 1 schematically shows a steam reforming plant according to a preferred embodiment of the invention in a first operating mode. The steam reforming plant is designated as 100. A hydrocarbon-rich feed can be supplied to plant 100 via line a. A purification unit 1 is provided for the purification, in particular desulfurization, of the hydrocarbon-rich feed. A steam reforming reactor 2 is configured to convert the purified, hydrocarbon-rich feedstock, supplied via line b, into synthesis gas using steam supplied from a steam system 3 via line c. Condensate generated in the steam reforming reactor 2 and the subsequent cooling unit 4 (see below) can be returned to the steam system 3 via line d. The steam system 3 can also be configured to provide export steam via line e. It can be supplied with preferably demineralized water via line f. The steam reforming reactor 2 can be fired with a portion of the hydrocarbon-rich feed that can be diverted upstream of the purification unit 1 via line g. Additional combustible gases, described below, can also be supplied as fuel via lines h and i. Synthesis gas obtained in steam reforming reactor 2 can be fed into cooling unit 4 via line k. It can then be fed to a carbon dioxide separation unit 5 via line I. In this unit, a first portion of the carbon dioxide contained in the synthesis gas can be scrubbed out, for example with methanol and / or diethanolamine. The scrubbed carbon dioxide can then be vented to the atmosphere via line m or transferred for further processing. In particular, the washed-out carbon dioxide can be fed, at least in part, to a recycling compressor 10 via a line n, thereby enabling the operation of the plant referred to as the "first operating mode" within the scope of this invention. The carbon dioxide compressed in the recycling compressor 10 is mixed, at least in part, with the hydrocarbon-rich feed in line b via a line o. This reduces the hydrogen / carbon monoxide ratio in the synthesis gas and, consequently, the hydrogen / carbon monoxide product ratio. The synthesis gas, freed from some of the carbon dioxide, can be fed via a line p into a temperature swing adsorption plant 6, which serves to separate the remaining carbon dioxide from the synthesis gas. The further processed synthesis gas is then fed via a line q to a cryogenic separation unit 7, where it can be separated into a hydrogen-rich fraction and a carbon monoxide-rich fraction. The operation of corresponding cryogenic separation units 7 is generally known. The cooling unit 4, the carbon dioxide separation unit 5, the temperature swing adsorption unit 6, and the cryogenic separation unit 7 together serve to obtain a hydrogen-rich fraction from the synthesis gas by separating carbon dioxide and carbon monoxide. Within the scope of this application, they are collectively referred to as "processing units" 4-7. The carbon monoxide-rich fraction can be fed into a carbon monoxide compressor 8 via a line r. Part of it is fed back into the cryogenic separation unit 7 via a line s, and the remainder is discharged as a product via a line t at the plant boundary. The hydrogen-rich fraction can be fed into a pressure swing adsorption (PSA) unit 9 via line u and further purified there. A hydrogen product obtained in PSA unit 9 can be discharged at the plant boundary via line v. So-called tail gas obtained in PSA unit 9 can, for example, be used to fuel the steam reforming reactor 2 via line h. The flow in line i is a mixture of light hydrocarbons (so-called fuel gas), which is separated in the cryogenic separation unit 7. Fig. 2 schematically shows the steam reforming plant 100 according to a preferred embodiment of the invention in a second operating mode. The second operating mode, shown in Fig. 2, differs from the first operating mode, shown in Fig. 1, in that the residual gas separated in the pressure swing adsorption unit 9 is at least partially mixed with the hydrogen-rich fraction upstream of the pressure swing adsorption unit 9 using the recycling compressor 10. Lines w and x are used for this purpose. The carbon dioxide separated from the synthesis gas continues to be drawn off via line m, but is no longer mixed with the feedstock. Line h remains and is used at least intermittently, since only a portion of the residual gas (e.g., 50%) is ever returned to the pressure swing adsorption unit 9; otherwise, impurities would accumulate. The remainder continues to be combusted in the steam reforming reactor 2. In both figures a schematic representation of a control unit 20, which is set up to switch the steam reforming plant 100 from the first operating mode to the second operating mode.

Claims

Method for operating a steam reforming plant (100) in which a hydrocarbon-rich feedstock (b) is reacted with steam (c) to form a synthesis gas (k) using at least one steam reforming reactor (2), with at least one processing unit (4-7) with which a hydrogen-rich fraction (u) is obtained from the synthesis gas (k) by separating carbon dioxide (m, n) and carbon monoxide (t), and with at least one pressure swing adsorption plant (9) with which a residual gas (h) is separated from the hydrogen-rich fraction, wherein at least one recycling compressor (10) is used with which, in a first operating mode, carbon dioxide (n, o) separated from the synthesis gas (k) is at least partially mixed with the hydrocarbon-rich feedstock (b), characterized in that, in a second operating mode, the recycling compressor (10) is not used for compressing carbon dioxide (m) separated from the synthesis gas (k), but rather forto mix at least part of the residual gas (w, x) separated in the pressure swing adsorption plant (9) with the hydrogen-rich fraction (u) upstream of the pressure swing adsorption plant (9), wherein the steam reforming plant (100) is operated in the first or second operating mode depending on a desired hydrogen / carbon monoxide product ratio. Method according to claim 1, characterized in that the steam reforming plant (100) is operated in the second operating mode when the desired hydrogen / carbon monoxide product ratio is at least 4.

0. Method according to one of claims 1 or 2, characterized in that the hydrogen-rich fraction (u) has a hydrogen content of at least 85 mol%, in particular at least 95 mol%. Method according to one of claims 1 to 3, characterized in that the residual gas (h) comprises at least 2 to 50 mol% carbon monoxide, 0 to 5 mol% carbon dioxide, 30 to 90 mol% hydrogen and 0 to 20 mol% light hydrocarbons. Method according to one of claims 1 to 4, characterized in that the hydrocarbon-rich feedstock (b) is obtained from natural gas (a). Steam reforming plant (100) configured for carrying out a process according to one of the preceding claims, comprising at least one steam reforming reactor (2) configured to react a hydrocarbon-rich feed (b) with steam (c) to form a synthesis gas (k), at least one processing unit (4-7) configured to obtain a hydrogen-rich fraction (u) from the synthesis gas by separating carbon dioxide (m, n) and carbon monoxide (t), and at least one pressure swing adsorption plant (9) configured to separate a residual gas (h) from the hydrogen-rich fraction (u), wherein at least one recycling compressor (10) is provided which can be connected in an operating mode to the processing unit (4-7) and the inlet side of the steam reformer (2) so that carbon dioxide (n, o) separated from the synthesis gas (k) can be at least partially mixed with the hydrocarbon-rich feed (b).characterized in that, in a second operating mode, the connection of the recycling compressor (10) with the processing unit (4-7) and the inlet side of the steam reformer (2) is interrupted and the recycling compressor (10) can be connected to the pressure swing adsorption unit (9), so that no carbon dioxide (m) separated from the synthesis gas (k) is compressed, but the residual gas (w, x) separated in the pressure swing adsorption unit (9) can be at least partially mixed with the hydrogen-rich fraction (u) upstream of the pressure swing adsorption unit (9). Steam reforming plant (100) according to claim 6, characterized in that it has at least one control unit (20) which is configured to switch the steam reforming plant (100) from the first operating mode to the second operating mode. Steam reforming plant (100) according to one of claims 6 or 7, characterized in that the at least one processing unit (4 - 7) comprises at least one carbon dioxide separation unit (5), one temperature swing adsorption plant (6) and / or one cryogenic separation unit (7).

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