Heat exchanger

Abstract

The heat exchanger comprises a plurality of cells formed by an alternating stack of planar flow guide plates (1) and heat transfer plates (2), each heat transfer plate having at least three openings (3, 4, 6) therethrough, each opening defining a portion of a respective one of at least three fluid flow paths in the heat exchanger, each flow guide plate having openings therethrough corresponding to at least two of the flow paths and larger openings therethrough (5, 7, 8) configured to guide fluid in the remaining flow paths across the faces of the heat transfer plates between which it is located, with each successive flow guide plate in the stack forming a portion of a different flow path than the previous one in the stack.

Description

Technical Field

[0001] The present invention relates to a heat exchanger for use, for example, in a cryogenic system for separating electrolytically generated oxygen and hydrogen.

Background Art

[0002] Recently, the efficient and cost-effective production of hydrogen as a fuel to replace fossil fuels has become remarkably active, and as a result, the demand for stable, cost-effective, and efficient electrolysis technology has increased exponentially. For example, the acquisition of solar energy by using a solar cell array has become an important power source in many countries. However, since the maximum power generation by such an array does not necessarily match the demand, energy storage is required. The use of storage batteries is costly, relatively inefficient, and due to their weight, is not an ideal solution for road and railway vehicles, and the intervals for charging are a significant limitation. Suitable locations for installing arrays with continuous sunlight and land availability are not necessarily close to where electricity is needed, so energy storage becomes an even more important issue.

[0003] By using electricity from a solar array or the like to electrolyze water into hydrogen and oxygen, for example, hydrogen can be produced as an easily transportable fuel together with oxygen used in industrial processes. This makes it possible to place the solar cell array at a location far from residential areas, where strong sunlight is received or on land unsuitable for production.

[0004] Electrolysis systems for hydrogen gas generation are described in International Publication No. 2014 / 170337A1 and GB2515292A. These need to generate a mixed gas stream and then separate the mixed gas stream, for example, by cryogenic distillation of oxygen. International Publication No. 2015 / 118073A1 discloses an alternative configuration having a stack (laminated) cell and using a flow of electrolyte to carry gases to respective degassing units by separate fluid circuits and remove the generated hydrogen or oxygen.

[0005] When generating a mixed gas stream, the cryogenic distillation system requires a heat exchanger to lower the temperature of the incoming mixed gas stream in order to use the cooled hydrogen and oxygen streams. The heat exchanger requires the following characteristics. 1. Provision of three independent separate streams (most are two-stream units). 2. Operability at cryogenic temperatures (about 70 K). 3. Pressure resistance during operation, with an internal pressure of 50 bar(g) or more when housed in a vacuum vessel. 4. Ability to provide turbulent flow to all streams to ensure maximum heat transfer between streams. 5. Ability to withstand internal explosions of gases within the system. 6. A completely leak-free state throughout the pressure-thermal transient phenomenon. 7. Relatively easy to manufacture. 8. Low manufacturing cost.

Summary of the Invention

Means for Solving the Problems

[0006] The present invention provides a heat exchanger including a plurality of cells formed by alternately stacking flat flow guide plates and heat transfer plates. Each heat transfer plate has at least three openings therethrough, each opening defining a part of each of at least three fluid flow paths within the heat exchanger. Each flow guide plate has openings therethrough corresponding to at least two fluid flow paths and a larger opening therethrough configured to guide the fluid in the remaining other flow path across the surface of the heat transfer plate between which the flow guide plate is located. Successive flow guide plates in the stack constitute a part of a fluid flow path different from that of the preceding flow guide plate in the stack.

[0007] At least some of the cells can contain a turbulence-induced matrix therein. The matrix can include a welded or woven mesh insert within the cell, or formations on at least one surface of a heat transfer plate within which the cell is defined therebetween.

[0008] The plates are suitably formed from stainless steel and are preferably joined to each other by copper brazing. As the stainless steel, 316L stainless steel having a linear thermal expansion coefficient similar to that of copper is suitable. It should be understood that the plates may generally be rectangular in plan view, but other outer shapes can also be adopted.

[0009] A pair of end plates are suitably provided, between which a stack of plates is clamped, and the end plates have flow connections therethrough.

[0010] The cells within the stack need not be evenly divided among the three flow paths. For example, since the thermal conductivity of oxygen is lower than that of hydrogen, it may be desirable to make the reflux path for oxygen longer than that for hydrogen. Alternatively, or additionally, the internal arrangement of the cells may be changed taking into account different thermal conductivities.

[0011] It may be desirable to reverse the orientation of the cells in the stack in order to change the direction of flow across the surfaces of the heat transfer plates within each flow path along the length of the stack.

[0012] The heat exchanger of the present invention can be configured to handle three or more flow paths so as to be usable when separating a plurality of gases from a mixed gas flow, and can be used, for example, in a separator for hydrogen and oxygen generated from the hydrolysis of water.

[0013] Accordingly, another aspect of the present invention provides an apparatus for separating hydrogen and oxygen from a mixed gas stream, the apparatus comprising a mixed gas inlet connected to a condensation coil at least partially immersed in liquid nitrogen within an insulated container, a liquid oxygen container connected to the condensation coil, a hydrogen gas outlet from the liquid oxygen container, an oxygen flow control valve connected to the liquid oxygen container, and a heat exchanger according to any of the preceding claims, wherein the mixed gas inlet is connected to a first flow path of the heat exchanger, the hydrogen gas outlet is connected to a second flow path of the heat exchanger, and the oxygen flow control valve is connected to a third flow path of the heat exchanger.

[0014] The heat exchanger is preferably mounted within a vacuum chamber above the liquid nitrogen container.

[0015] The mixed gas stream can be generated by the electrolytic decomposition of water. Embodiments of the present invention are illustrated by the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

[0017] First, referring to FIG. 1, the heat exchanger is formed by alternately stacking (laminating) a series of thin plates of two general types, namely, flow guide plates 1 (typically about 1.5 mm to 2.5 mm thick) and heat transfer plates 2 (typically about 0.5 mm to 1.5 mm thick). In the illustrated embodiment, there are three different flow guide plates 1a, 1b, 1c and three different heat transfer plates 2a, 2b, 2c, each having an opening therethrough, and each opening forming a part of one of three fluid flow paths through the heat exchanger. In the figure, the openings related to the three flow paths are respectively shown by black, gray, and white circles. FIG. 1 shows a series of 12 plates in the stack, but the number is not limited to this. A typical heat exchanger can include 48 cells when the flow rate is small and can include more than 200 cells in a larger system. Looking at the plates in the illustrated order (this is for explanation and other configurations are possible), the first flow guide plate 1a is in the form of a generally square frame with rounded corners, and each plate has the same outer shape. One edge of the frame is widened to accommodate an opening 3 forming part of a second path at one corner of the frame and an opening 4 forming part of a third flow path at the other corner. The region 5 within the frame is shaped to align with spaced-apart inlets and outlets for the first flow path shown as black circles with the direction of flow between them indicated by arrows, but the inlets and outlets do not actually form part of plate 1a.

[0018] The next plate 2a in the stack is a heat transfer plate provided with three through openings 3, 4, and 6. Openings 3 and 4 are adjacent to one edge of the plate and align with openings 3 and 4 of the flow guide plate 1a when the plates are stacked. The other opening 6 opens into the region 5 of the first flow guide plate 1a on one side and forms part of the first flow path through the heat exchanger.

[0019] The flow guide plate 1b is the third plate in the stack and is in the form of a frame with a wide edge to accommodate the openings 3, 6 that align with the openings 3, 6 of the heat transfer plate 2a in the previous stage. The space region 7 surrounded by the frame communicates with the opening 4 of the heat transfer plate 2a, allowing gas to flow across the surface of the plate from the opening 4 of the preceding heat transfer plate to the opening 4 of the next heat transfer plate 2b located adjacent to the opposite corner of the square.

[0020] The third flow guide plate 1c is in the shape of a frame having an opening 6 located at the center on one side of the frame and penetrating into the frame, and an opening 4 at the corner on the opposite side of the frame, which is correspondingly widened. The space region 8 within the frame communicates with the opening 3 of the preceding heat transfer plate 2b and the opening 3 at the opposite corner of the third heat transfer plate 2c. This heat transfer plate 2c has an opening 6 corresponding to the opening 6 of the third flow guide plate 1c, and openings 3, 4 at the two opposite corners.

[0021] The same series of flow guide plates 1 and heat transfer plates 2 are repeated in the next six plates, but these plates are reversed so that the flow direction across the surface of the heat transfer plates is opposite to that of the first six plates. The next six plates return to the pattern of the first six plates, and this pattern is repeated throughout the stack. Thus, it can be seen that in this particular configuration, only three different designs of flow guide plates and three different designs of heat transfer plates need to be manufactured.

[0022] Depending on the gas flow rate, the number of cells in the heat exchanger stack can be substantially varied.

[0023] Figure 2 shows a stack of plates assembled between two end plates 20 and 21, with alignment pins 22 extending between the end plates to hold the plates in alignment, and threaded tie rods 23 and nuts 24 clamping the assembly. Also, each end plate 20, 21 is provided with connection plugs 25, 26, 27 for connecting three flow paths in the exchanger to external gas flow path pipes.

[0024] This designed heat exchanger uses materials with very similar linear thermal expansion coefficients to avoid stresses that can accumulate during the thermal cycle and create a gradient across the length of the heat exchanger. For this purpose, the heat exchanger is entirely composed of 316L stainless steel parts, and high-temperature copper brazing joints are applied between its components. Both 316L stainless steel and copper have a linear thermal expansion rate of approximately 16×10 -6 mm / °C, which is suitable for collaborative use in these applications.

[0025] For the unit to operate at the pressure required by the hydrogen production system, the heat exchanger must be constructed with an appropriate wall thickness. This requirement is further increased by the need to withstand an explosion at any point within the gas transport system and resist the forces in the radial direction outside the radius associated with the explosion.

[0026] Similarly, this unit must be designed to withstand the linear cell separation forces under such conditions. This requires a high-reliability brazed interface of a considerable area, assisted by end plates and tie rods of a considerable thickness, at each joint of the unit. Also, between each flow guide plate and each heat transfer plate, a copper foil is pre-formed to form a high-strength joint between the stainless steel plates in the high-temperature vacuum brazing process that finally joins the unit.

[0027] The connectors at each end of each gas passage need to withstand the above-mentioned pressures and pressure spikes without failure. That is to say, it is also necessary to ensure a high level of integrity at the joints between the end plates of the heat exchanger and each connector. Such connectors may depend on fitting techniques and may have a threaded or tubular structure.

[0028] To improve heat transfer, the cells may have a metal matrix that allows gas to flow freely while creating more turbulence. This may be in the form of a welded or woven mesh, or may be directly assembled onto the outer surface of the cell plate. As shown in Figure 3, the formed insert 30 is made from a stainless steel mesh that can be welded or woven. Alternatively, the heat transfer plate may have a form that is assembled and formed by pressing or stamping to achieve the same result.

[0029] These inserts have three functions. a. The inserts generate turbulence in the gas passing across their outer shape, thereby increasing the potential for heat energy transfer. b. The inserts provide additional surface area for the gas to improve heat transfer. c. The inserts provide resistance to deformation of the heat transfer plate in the event of a significant differential pressure between cells.

[0030] The orientation of the flow manifolds (connecting pipes) within the stack can be changed to adapt to assembly in a particular environment if required. A further advantage of this configuration is that the number of cells in a given gas flow can be varied to accommodate differences in the heat transfer rate from fluid to fluid, whether the fluid is a gas or a liquid.

[0031] Referring now to FIG. 4, one configuration of a cryogenic system using the heat exchanger of the present invention includes a Dewar flask 40 for containing liquid nitrogen. The flask 40 is provided with a cold head 41 for maintaining the interior of the flask at a low temperature in a conventional manner. The condensation coil 42 has at least most of it immersed in liquid nitrogen and communicates with a container 43 for liquid oxygen. The gaseous hydrogen is directly guided to a first inlet of a heat exchanger 44 mounted in a vacuum chamber 45 located above the Dewar flask 40. The liquid oxygen container 43 communicates with a flow control valve 47 in the vacuum chamber 45 via a pipe 46, and the gaseous oxygen exiting therefrom is guided to a second inlet of the heat exchanger. The mixed gas flow from the electrolytic cell is input into a third flow path of the exchanger at the opposite end.

[0032] By housing a unit with such a configuration in a vacuum container, efficient heat energy exchange can be performed with one side of the unit approaching room temperature and the other side at an extremely low temperature of about 70K to 90K (-203.15°C to -183.15°C). An example of a system equipped with the heat exchanger according to the present invention is shown in FIG. 4, where a Dewar flask of liquid nitrogen is maintained in a cooled state by a cold head. A container for recovering oxygen is immersed in the liquid nitrogen (LN). The oxygen obtains cooling power from the LN, and the cold head maintains the temperature.

[0033] The above-described heat exchanger is located in a vacuum chamber above the Dewar flask, taking in hydrogen and oxygen flows at the bottom and taking in the mixed gas flow from the top. As the flow passes from cell to cell, heat energy is transferred, the incoming flow is cooled to about 70K to 90K, and the outgoing flow is warmed to around ambient temperature, thereby reducing the power required to maintain the temperature of the LN in the Dewar flask to an absolute minimum. Therefore, the efficiency of the entire process of hydrogen and oxygen production by electrolytic water decomposition can be optimized.

[0034] Other designs and configurations of the three-stream heat exchanger can be derived from the above units. Crucially, the present application encompasses the use of a heat exchanger operating according to the aforementioned criteria in combination with a cryogenic system.

[0035] In the following figure, the mixed gas stream enters the heat exchanger from the top, passes through it and moves downward, being cooled as the mixed gas stream passes from cell to cell. Finally, the condensation coil located within the LN cools the gas stream, ensuring the separation of more than 99% of the oxygen collected in the oxygen container near the bottom of the dewar flask.

[0036] Liquid oxygen is pushed back from the O2 container by the differential pressure between the inflowing and outflowing gas streams, passes through the liquid O2 flow control valve, and then enters the bottom of the heat exchanger.

[0037] Similarly, the hydrogen stream exits the top of the O2 container and passes through the heat exchanger. Other designed cryogenic systems can be used in the liquefaction process, but a three-stream heat exchanger is required to optimize efficiency.

[0038] In the cell configuration shown in FIGS. 5 and 6, four flow guide plates 50a, 50b, 50c, 50d with different internal formations and three heat transfer plates 51a, 51b, 51c (one of which is used twice in each 4-cell group) are adopted. The mixed gas flow is represented by black ports and arrows, the oxygen flow is represented by gray ports and arrows, and hydrogen is represented by white ports. FIG. 6 shows a series of plates forming a group of cells. The heat transfer plate 51 is disposed on top of the flow guide plate 50 arranged on the left side of the figure, and the first one in the next column is disposed immediately above the last one in the previous column. Each flow guide plate is in the form of a frame having a fabric mesh insert 52 within the frame and openings 53, 54, 55 that define separate flow paths through the frame. Thus, the flow guide plate 50a has an opening 53 that constitutes part of the oxygen flow path and an opening 54 that constitutes part of the hydrogen flow path, and the region within the frame is adapted to direct the flow of the mixed gas flow across the surface of the adjacent heat transfer plate 51a. The heat transfer plate 50a has three through openings 53, 54, 55. The central opening 55 receives the mixed gas flow, and this mixed gas flow passes through the aligned openings 55 of the next two plates 50b, 51b and then enters the space defined by the next series of flow guide plates 50a, where the mixed gas flow is rotated 90° with respect to the first flow guide plate so that it flows across that space in the reverse direction. Similarly, the oxygen flow passes through the cell defined by the flow guide plate 50b and then flows across the flow guide plate 50d in the opposite direction. As can be seen from FIG. 6, in this series of eight cells thus defined, the oxygen flow exchanges heat with the mixed gas flow, while hydrogen simply flows through the opening 54 without transferring its thermal energy. Therefore, as an entire heat exchanger, a group of cells can be configured such that there are fewer cells through which hydrogen passes than cells through which oxygen passes, and heat exchange from the oxygen flow with a lower thermal conductivity than hydrogen balances the heat exchange of hydrogen.

Claims

1. A cryogenic system for separating oxygen and hydrogen from an electrolytically generated mixed gas stream, comprising a heat exchanger configured to be used in the cryogenic system and configured to effect heat exchange between the mixed gas stream and oxygen and hydrogen, wherein the heat exchanger comprises a plurality of cells formed by a stack in which flat flow guide plates and heat transfer plates are alternately arranged, each heat transfer plate has at least three openings passing through the heat transfer plate, and each opening defines a part of each of at least three fluid flow paths in the heat exchanger, each flow guide plate has an opening passing through the flow guide plate and corresponding to at least two fluid flow paths, and a larger opening passing through the flow guide plate and configured to guide the fluid in the remaining other flow paths across the surface of the heat transfer plate where the flow guide plate is located, successive flow guide plates in the stack constitute a part of a fluid flow path different from that of the preceding flow guide plate in the stack, a cryogenic system.

2. The cryogenic system according to claim 1, wherein the heat exchanger further comprises a pair of end plates that sandwich the stack in which the flat flow guide plates and the heat transfer plates are alternately arranged by tie rods.

3. The cryogenic system according to claim 2, wherein the tie rod is a threaded tie rod, and the stack in which the flat flow guide plates and the heat transfer plates are alternately arranged is clamped by the threaded tie rod and nuts.

4. The cryogenic system according to claim 2 or claim 3, wherein the heat exchanger further comprises alignment pins that extend between the end plates and hold each plate in an aligned state.

5. The cryogenic system according to any one of claims 2 to 4, wherein the end plate has a flow connection portion passing through the end plate.

6. The cryogenic system according to claim 5, wherein the heat exchanger further comprises connection plugs provided on each of the end plates for connecting three fluid flow paths in the heat exchanger to an external gas flow path pipe.

7. The cryogenic system according to any one of claims 1 to 6, wherein the cells in the stack are not evenly divided among the three fluid flow paths.

8. The three fluid flow paths include one mixed gas flow path, one hydrogen flow path, and one oxygen flow path, and the mixed gas flow path and the oxygen flow path are longer than the hydrogen flow path. The cryogenic system according to any one of claims 1 to 7.

9. The direction of the flow across the surface of the heat transfer plate in each of the flow paths changes along the length of the stack. The cryogenic system according to any one of claims 1 to 8.

10. The cryogenic system according to any one of claims 1 to 9, comprising a liquid oxygen container connected to the oxygen flow path.

11. The cryogenic system according to any one of claims 1 to 10, comprising an electrolytic cell connected to the mixed gas flow path.

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