System and method for compressing a medium, in particular hydrogen

Abstract

The invention relates to a method for operating a system (1), wherein the system (1) has a first compressor (2) and a second compressor (9), wherein the system (1) is designed to compress a medium, wherein the first compressor (2) is designed for a mass flow rate mV1 which leads to a first compression output pressure pendV1, wherein the second compressor (9) is designed for a mass flow rate mV2 which leads to a second compression output pressure pendV2, wherein pendV2 > pendV1, wherein the first compressor (2) is designed to compress a medium from a first compressor input pressure pein to the first compressor output pressure pendV1, wherein the first compressor (2) is operated in such a way that, when the mass flow rate mV1 flowing through the first compressor (2) is reduced, a rotational speed of the first compressor (2) is reduced in such a way that the quotient of the compressor output pressure pendV1 / compressor input pressure pein runs along a surge limit line (13) until a minimum pressure paus,min is reached, wherein the second compressor (9) is fluidically connected to the first compressor (2), wherein the second compressor (9) is operated in such a way that, at the minimum pressure paus,min, the second compression output pressure pendV2 is reached.

Description

Description TITLE Plant and process for compressing a medium, in particular hydrogen TECHNICAL AREA

[0001] The invention relates to a method for operating a system, wherein the system comprises a first compressor and a second compressor, wherein the system is designed for compressing a medium, wherein the first compressor is designed for a mass flow rate mvi which leads to a first compression outlet pressure p e ndvi leads to, where the second compressor is designed for a mass flow rate mv2, which leads to a second compression outlet pressure PendV2, where p e ndV2 > Pendvi , where the first compressor is used to compress a medium from a first compressor inlet pressure p e in which the pressure pendvi is formed relative to the first compressor outlet pressure.

[0002] The invention also relates to a plant. BACKGROUND

[0003] The importance of hydrogen has increased significantly and it is of great relevance as a future technology. The hydrogen density in the atmosphere is approximately 90 g / m³. 3 very low. Therefore, hydrogen must be compressed to achieve a usable energy density. Compressors designed for hydrogen compression are essential for realizing a future-oriented hydrogen economy.

[0004] Unfortunately, the operating principle of compressors, such as turbo compressors, has disadvantages when compressing hydrogen. Due to the comparatively low molecular weight of hydrogen, a very low possible pressure ratio per compressor stage is achieved. For example, in single-shaft compressors, the pressure ratio is... Essentially 1.05 per stage. Therefore, a very large number of stages is required to achieve a high final pressure. An example calculation shows that to achieve a final pressure of 5 bar with a suction pressure of 1 bar and a pressure ratio of 1.05, essentially 33 stages would be needed. This means that a very large number of compressors or casings are required for the desired compression task, which would be very costly.

[0005] Efforts to reduce the number of stages required for hydrogen compression could involve increasing the enthalpy input per stage. However, this requires increasing the impeller peripheral speeds and thus the rotational speeds. However, increasing impeller peripheral speeds and rotational speeds could lead to insufficient material strength in the impeller material when used with hydrogen. Furthermore, shrink-fit connections between a shaft and an impeller in single-shaft compressors, especially with toe-fit connections (which offer reduced impeller shrinkage compared to heel-fit connections) and high rotational speeds, might prove inadequate. Similarly, rotordynamic problems could arise due to a high number of stages and high rotational speeds. The increase in bearing sliding speeds resulting from higher rotational speeds is also problematic. Additionally, high rotational speeds lead to higher gas seal sliding speeds. Finally, increased mechanical power losses could be expected due to high rotational speeds.

[0007] One way to efficiently compress hydrogen is through a combination solution featuring a first and a second compressor. The first compressor is a turbo compressor, and the second is a piston compressor. This allows, for example, the hydrogen to be pre-compressed in the first compressor and then compressed to the desired final pressure in the second.

[0008] Another challenge arises in the production of hydrogen from renewable energies. The hydrogen produced in water electrolysis Hydrogen production can be subject to significant fluctuations in mass flow due to changing wind conditions and / or variations in solar intensity. While hydrogen production is subject to significant fluctuations, the requirements on the demand side remain constant with respect to the required pressure. A reduction in the delivery rate of turbo compressors is generally possible to as low as 70% of the design rate without a process bypass, which involves recirculating a certain amount of the process gas from the pressure side to the suction side. Without this bypass, damaging pump surges or pressure fluctuations can occur within the compressor. For this reason, individual compressor stages must be equipped with bypass lines to ensure that a minimum permissible process gas volume at the compressor stage inlet is not undercut. This volume is also the minimum process gas volume that can be operated without bypass.

[0010] The volume flow rate supplied to the compressor can still be reduced. However, the compressor is then operated in recirculation mode, and the cooled differential volume is transferred from the pressure side to the suction side. Cooling the recirculated gas volume flow is essential, as otherwise unacceptably high temperatures would occur during the compression process. During the recooling of the hydrogen gas, a large portion of the water vapor contained in the gas condenses. However, this reduces the molecular weight. Due to the recirculation of the cooled hydrogen volume flow, throttled to suction pressure and with a significantly reduced molecular weight (resulting from the lower water content after recooling), hydrogen with a high water content (molecular weight around 3 g / mol) and hydrogen with a low water content (molecular weight min. 2 g / mol) mixes.Due to the reduction in molecular weight after the mixing of the two hydrogen streams before the compressor stage, the original final pressure can no longer be achieved. This drop in the achievable final pressure cannot be compensated for either in the compressor design or during operation. This can also occur in dynamic plant operating situations. In addition, a sudden increase in pressure loss (e.g., caused by closing control valves) may necessitate operating the compressor housings in bypass mode. This also leads to a decrease in molecular weight and compressor discharge pressure.

[0011] By combining a turbo compressor solution on the low-pressure side and a piston compressor solution on the high-pressure side, the operating limitations of the turbo compressor resulting from fluid mechanics can be balanced with the advantages of the flexibility of the piston compressor. SUMMARY OF THE INVENTION

[0012] Against this background, the invention aims to provide a system and a method for compressing hydrogen that takes into account a reduced volume flow rate.

[0013] The problem is solved by a method for operating a plant, wherein the plant has a first compressor and a second compressor, wherein the plant is designed for compressing a medium, and wherein the first compressor is designed for a mass flow rate mvi, which leads to a first compression outlet pressure p e ndvi leads to, where the second compressor is designed for a mass flow rate mv2, which leads to a second compression outlet pressure PendV2, where p e ndV2 > Pendvi , where the first compressor is used to compress a medium from a first compressor inlet pressure p ein relation to the first compressor outlet pressure pendvi, wherein the first compressor is operated in such a way that, when the mass flow rate through the first compressor decreases, the rotational speed of the first compressor is reduced such that the quotient of the compressor outlet pressure pendvi / compressor inlet pressure p e in runs along a pumping limit line until a minimum pressure p is reached. a us,min is reached, wherein the second compressor is fluidically connected to the first compressor, wherein the second compressor is operated in such a way that the minimum pressure p aus , min the second compression output pressure p e ndV2 is reached.

[0014] Advantageous embodiments are the subject of the dependent claims. DESCRIPTION OF THE INVENTION

[0015] When designing the turbo compressor solution on the low-pressure side of the hydrogen compression system, a typical configuration is either 2 x 50% branch arrangement or 1 x 100%. Subsequent compression is then carried out with one or more piston compressors, usually arranged in parallel.

[0016] The design of the piston compressors is then carried out taking into account the turn-down scenarios that arise from the operation of the turbo compressors, utilizing the entire speed map. The turn-down of the turbo compressors is performed as follows: Reduction of the mass flow rate to the surge line at a constant target pressure. Further reduction of the delivery rate by further reducing the speed along the surge line with a decreasing final pressure until the minimum possible speed and achievable final pressure are reached.

[0017] In a 2x 50% configuration, one of the two 50% compressors can be shut down, reducing the achievable turn-down. The resulting volume flow to the piston compressor is reduced on the one hand by the turn-down of the turbo compressors, and on the other hand increased again by the reduced intermediate pressure. This can result in the turbo compressor's turn-down state, at reduced pressure, representing the largest volume flow for the piston compressor. Simultaneously, the piston compressor must be designed to deliver the greatest relative pressure rise due to the reduced inlet pressure and constant target pressure. This can be achieved in the piston compressor without switching cylinder stages on or off.

[0018] This optimized design allows for a turn-down of up to 75% (to 25% of the original hydrogen volume) instead of approximately 35%. Furthermore, it compensates for pressure drops that can occur due to fluctuations in molecular weight (e.g., during recirculation operation). Start-up processes). Such a combination of turbo and piston compressors not only extends the range of turn-down operation without blow-off but also saves up to 15% of power compared to operation with blow-off.

[0019] The solution according to the invention is flexible and takes start-up processes into account. Furthermore, the solution according to the invention extends the operating range and avoids recirculation and the resulting fluctuations in the process gas composition. A further advantage is achieved by the solution being more energy-efficient due to the absence of power loss from recirculation and the improved efficiency of the piston compressor. In addition, the solution according to the invention is more economical because the intercooler on the pressure side can be smaller and only one intercooler is required. This also leads to lower operating costs, since only one intercooler is needed and not a second one that has to be operated continuously, even when recirculation is not taking place. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] An embodiment of the invention will be explained in more detail below with reference to the following figures.

[0021] The properties, features and advantages of this invention described above, as well as the manner in which they are achieved, will become clearer and more easily understood in connection with the following description of the exemplary embodiments, which are explained in more detail in conjunction with the drawings.

[0022] Identical components or components with the same function are marked with the same reference numerals.

[0023] Exemplary embodiments of the invention are described below with reference to the drawings. These drawings are not intended to represent the exemplary embodiments to scale; rather, where helpful for explanation, the drawing is presented in a schematic form. and / or slightly distorted form. Regarding additions to the teachings immediately apparent in the drawing, reference is made to the relevant prior art.

[0024] They show:

[0025] FIG 1 a schematic representation of a system according to the invention

[0026] FIG 2 shows a schematic representation of a characteristic curve field. DESCRIPTION OF THE EMBODY FORM EN Figure 1 shows a schematic representation of a system 1 according to the invention. The system comprises a first compressor 2, which is supplied with a medium via an inlet opening 3. In one embodiment, the medium is hydrogen, which is produced in an electrolyzer 4. The electrolyzer 4 is supplied with water 5, which is separated into hydrogen 6 and oxygen 7 in an energy-intensive process. The energy required for this is indicated by arrow 8.

[0028] The energy used in this process is primarily derived from renewable energy sources. The hydrogen produced in this way is then also referred to as green hydrogen. The renewable energy sources can be derived from wind or solar power. If the renewable energy sources are sufficient, the hydrogen production in the electrolyzer is such that the quantity remains consistently high.

[0029] The system 1 comprises a first compressor 2 and a second compressor 9. The system 1 is designed to compress the medium, in particular hydrogen.

[0030] After the electrolyzer 4, the hydrogen reaches the first compressor 2, which is designed for a mass flow rate mvi. At the outlet 10 of the first compressor 2, the hydrogen is converted into a first Compression outlet pressure p eThe hydrogen is compressed. After the first compressor 2, the hydrogen reaches the second compressor 9, which is designed for a mass flow rate mv2. The second compressor 9 is fluidically connected to the first compressor 2. At the outlet 11 of the second compressor 9, the hydrogen is compressed to a second compression outlet pressure p. e ndV2 is compressed. The first compressor 2 and the second compressor 9 are designed such that p e ndV2 > Pendvi applies.

[0031] The first compressor 2 is used to compress the medium from a first compressor inlet pressure p. e in the first compressor outlet pressure pendvi formed.

[0032] Provided sufficient renewable energy 8 is available, hydrogen is produced in an adequate quantity, which is then compressed in plant 1. However, the situation changes if renewable energy 8 is not available in sufficient quantity. The amount of hydrogen produced is thereby reduced, which affects the first compressor 2 and the second compressor 9. The first compressor 2 is capable of compressing the hydrogen up to a minimum mass flow rate. If the minimum mass flow rate at the inlet is not reached, damage can occur in the first compressor 2. In this state, a so-called surge line is reached, which will be explained later in connection with Figure 2.

[0033] A remedy could be achieved by arranging a bypass line 12 around the first compressor. A cooling unit (not shown) in the bypass line 12 must also be taken into account. However, this solution is not necessarily efficient from an energy perspective. The invention aims to remedy this, which will now be explained in more detail.

[0034] The first compressor 2 is operated in such a way that if the mass flow rate mvi flowing through the first compressor 2 decreases, the rotational speed of the first compressor 2 is reduced so that the quotient 14 of the Compressor outlet pressure p e ndvi / compressor inlet pressure p e in runs along a pumping limit line 13 until a minimum pressure p a us,min is reached.

[0035] The second compressor 9 is operated in such a way that the second compression outlet pressure p is achieved with the minimum pressure Paus,min. endV2 is reached.

[0036] The method according to the invention will now be explained with the aid of a so-called characteristic curve field 16. Provided that the renewable energies 8 generate sufficient energy, a sufficiently high quantity of hydrogen is produced in the electrolyzer 4, which is compressed in the first compressor 2. In the characteristic curve field 16, this situation would be found, for example, at operating point 17. The x-axis represents the flow rate 18 or the mass flow rate mvi of the medium, here hydrogen, through the first compressor 2. The y-axis represents the quotient 14 of the compressor outlet pressure p. eThe compressor inlet pressure pein of the first compressor 2 is shown. To the left of the surge line 13 are operating points that should not be reached during operation, as otherwise damage to the compressor could occur due to surges. The lines labeled ni, n2, etc., show operating points at constant speeds. If less renewable energy 8 is available, the flow rate 18 through the first compressor also decreases, since less hydrogen is produced in the electrolyzer 4. As a result, the operating point 17 in the characteristic curve field 16 shifts to the left, which is indicated by line 19. To avoid the area to the left of the surge line 13, the operating point is guided along the surge line 13 or, better yet, parallel to it at a distance to the right of the surge line 13. This is shown by line 21. This is achieved by operating the first compressor 2 at decreasing speeds.

[0038] The operating point is guided along or parallel to the pumping limit line 13 until a minimum quotient 22 or minimum pressure p is reached. a us,min is reached at the outlet of the first compressor 2. The second compressor 9 is fluidically connected to the first compressor 2, and the second compressor 9 is operated in such a way as to that with the minimum pressure p a us,min the second compression outlet pressure PendV2 is reached. The minimum pressure p a The initial compression pressure (us,min) is between 1 and 4 bar, preferably between 2.2 and 2.7 bar. The second compression outlet pressure (p) e ndV2 is between 25 and 35 bar, preferably between 28 and 32 bar.

[0039] The first compressor 2 is preferably designed as a turbo compressor, e.g., as an axial or radial compressor. The second compressor 9 is preferably designed as a piston compressor.

[0040] Although the invention has been illustrated and described in detail by the preferred embodiment, the invention is not limited by the disclosed examples and other variants can be derived by a person skilled in the art without leaving the scope of protection of the invention.

Claims

Claims 1. Method for operating a plant (1) wherein the plant (1) comprises a first compressor (2) and a second compressor (9), wherein the plant (1) is designed for compressing a medium, wherein the first compressor (2) is designed for a mass flow rate mvi which leads to a first compression outlet pressure p e ndvi leads to the second compressor (9) being designed for a mass flow rate mv2 which results in a second compression outlet pressure p en dV2 leads, where p e ndV2 > Pendvi , where the first compressor (2) is used to compress a medium from a first compressor inlet pressure p ein relation to the first compressor outlet pressure pendvi, characterized in that the first compressor (2) is operated such that when the mass flow rate mvi flowing through the first compressor (2) decreases, the rotational speed of the first compressor (2) is reduced such that the quotient of the compressor outlet pressure pendvi / compressor inlet pressure p e in runs along a pumping limit line (13) until a minimum pressure p a us,min is reached, wherein the second compressor (9) is fluidically connected to the first compressor (2), wherein the second compressor (9) is operated such that the minimum pressure p a us,min the second compression output pressure p e ndV2 is reached.

2. The method of claim 1, wherein the medium comprises hydrogen.

3. Method according to claim 1 or 2, wherein the first compressor (2) is designed as a turbo compressor, in particular as a radial compressor.

4. Method according to claim 1, 2 or 3, wherein the second compressor (9) is designed as a piston compressor.

5. Method according to one of the preceding claims, wherein the second compression output pressure p e ndV2 is between 25 and 35 bar, especially at 30 bar.

6. System (1) which is designed to carry out a process according to any of the preceding claims.

Images