PROPYLENE GLYCOL FUEL PROCESSOR COMPONENT AND PROPYLENE GLYCOL FUEL PROCESSOR

Abstract

The invention relates to a fuel processor component for a propylene glycol fuel processor, comprising at least one housing having at least two inlets and two outlets, a plurality of first plates having a first side (S1) and a second side (S2) and second plates having a third side (S3) and a fourth side (S4) being arranged in the form of a stack in the housing, the first and second plates stacked forming at least first cavities and second cavities, the first inlet being fluidly connected to the first outlet by the first cavities and the second inlet being fluidly connected to the second outlet by the second cavities.

Description

The invention relates to a fuel processor component for a propylene glycol fuel processor as well as a propylene glycol fuel processor. The prior art includes fuel processors for other fuels. Such a fuel processor for LPG, octane, kerosene, and diesel is described, for example, in the publication by M. O'Connell et al., 4th World Hydrogen Technologies Convention, 2011, Glasgow, UK. The fuel processor comprises reactors. Such a reactor may consist of a plurality of identical plates, each equipped, for example, partially with a catalyst. There is a need for fuel processor components adapted for use with propylene glycol. Furthermore, for applications in vehicles, such as aircraft, there is a need for compact, lightweight, and small-sized fuel processors that optimally utilize available waste heat. The aim of the invention is to provide a fuel processor component for a propylene glycol fuel processor that has small dimensions, low weight and high power density. This objective is achieved by the invention through a fuel processor component for a propylene glycol fuel processor comprising at least one housing having at least two inlets and two outlets, wherein a plurality of first plates having a first side and a second side and second plates having a third side and a fourth side are arranged in a stack within the housing. Furthermore, the stacked first and second plates form at least first cavities and second cavities, the first inlet being fluidly connected to the first outlet via the first cavities and the second inlet being fluidly connected to the second outlet via the second cavities. The fuel processor component according to the invention is lightweight and compact. Heat input or heat dissipation can to be optimally adapted to the power range of the fuel processor component by adapting the plate suite. A fuel processor typically comprises a plurality of fuel processor components, such as a reformer, a gas-to-water conversion reactor, a PrOx reactor, an evaporator, a heat exchanger, and a starting device. A liquid fuel is evaporated in an evaporator. In a propylene glycol fuel processor, the fuel to be evaporated consists of a propylene glycol-water mixture. The fuel is transformed in the reformer into a gas containing hydrogen and various residual gases, including carbon monoxide. The gases obtained in the reformer, particularly the carbon monoxide produced, are transformed in the gas-to-water conversion reactor and in the PrOx reactor into carbon dioxide, so that the exhaust gas from the PrOx reactor no longer contains, or only a small proportion of, carbon monoxide. The heat exchanger carries one or possibly two different gases in the same or opposite directions. The heat exchanger serves to regulate the temperature of the gas before it is introduced into the next reactor or to dissipate the heat of reaction from a reactor. The fuel processor component has at least two separate gas lines comprising, on the one hand, the first cavities with a first associated inlet and outlet, and on the other hand, the second cavities with a second inlet and outlet. The fuel processor component may be provided on the inlet side with a gas / liquid splitter and on the outlet side with a gas / liquid collector. In an advantageous embodiment, the first and / or second plates have a surface structure, the surface structure comprising a pattern and / or channels, a distribution structure, and a collection structure. The pattern is a regular arrangement of reliefs and hollows that conduct a flow from the distribution structure to the collection structure. Such surface structuring can be achieved by a plurality of known processes, for example stamping, injection molding, laser structuring, additive manufacturing, for example 3D printing, and others. In another preferred embodiment, a first number of the first and second plates are arranged in a predefined order, the first number of first and second plates forming a repeating unit. A second number of repeating units is thus preferably housed in the casing. Preferably, in the casing is arranged a plurality of third plates which each have a fifth side and a sixth side, the third plates forming an element of the repeating unit. A fuel processor component may in particular comprise from 2 to 500, preferably from 5 to 250, preferably from 50 to 200, preferably from 100 to 150 repeating units. Preferably, the first side can have a structure configured for a gaseous reaction and the second side a cooling or evaporation side. In another advantageous embodiment, the third side has the same surface structure as the second side and the fourth side has the same surface structure as the first side. In another embodiment, the fuel processor component is an evaporator in which at least one side of a first plate is designed as an evaporator and a second side of the plate is designed to carry a heating gas. The heat from the heating gas is thus transferred directly to the opposite side of the same plate to evaporate the fuel. For the purposes of this invention, "evaporator" can also mean "superheater" and "evaporate" can mean "superheat." In one embodiment, the fuel processor component is an evaporator in which the first and second sides of the first plate are designed as evaporator surfaces. A third side of the second plate is designed as the heating gas side, and a fourth side of the second plate is designed as a flat surface. Optionally, a fifth side of the third plate is designed as the heating gas side for passing a heating gas or air, and a sixth side of the third plate is designed as a flat surface. Each evaporator surface faces a flat surface, and two heating gas sides face each other. In this embodiment, two, or optionally three, different gases or fluids are conducted through the evaporator. A first cavity -4- is in this case an evaporation cavity. The respective adjacent cavity is heated by a heating gas. Such an evaporator can be designed as a superheater. The structuring of the third side, the heating gas side, and the fifth side can be carried out identically. Preferably, the evaporator according to the invention has first to sixth sides and comprises a repeating unit having the following sequence: first side, second side, fourth side, third side, third side, fourth side, first side, second side, fourth side, third side, third side, fourth side, sixth side, fifth side, fifth side, sixth side. This set comprises eight plates, the eight plates each comprising two or three different plates. In one embodiment, the fuel processor component is a heat exchanger in which at least one side is designed to carry a first gas and a second side to carry a second gas. The first and second gases may be identical. However, they are conducted in separate fluid paths. Advantageously, the first and second gases may be conducted in the same direction or in opposite directions through the fuel processor component. In one embodiment, the fuel processor component according to the invention is a gas-to-water conversion reactor in which the first plate has a first side designed as a cooling plate and a second side designed as a reaction side. The gas-to-water conversion reactor according to the invention thus has a heat exchanger integrated within the reactor to dissipate the heat of reaction. Advantageously, in a repeating unit, every sixth plate or side has, or is designed as, a cooling side. The term "reaction side" refers to a side with a plurality of channels and / or a pattern designed to facilitate a reaction. Such a reaction side may, in particular, be coated with a catalyst. The number of cooling sides depends on the amount of reaction heat that is produced and must be dissipated during the reaction in the gas-to-water conversion reactor. In another embodiment, the fuel processor component according to the invention is a PrOx reactor in which a first plate has a first side designed as a cooling side and a second side designed as a cooling side, in which a second plate has a third side designed as a reaction side and a fourth side designed as a gas inlet side, in which a third plate has a fifth side designed as a reaction side and a sixth side designed as an unstructured surface. The "gas inlet side" is understood to be a side in which a conduit for an incoming gas is integrated. The conduit is fluidically connected to the opposite side of the plate. The aforementioned fourth side has an air inlet conduit that is fluidly connected to a plurality of distribution channels on the aforementioned third side.The distribution channels contained in the third side are designed so that each has at least one opening towards the reaction side. The surface designed as a cooling side may, in one embodiment, have a structure for the passage of a cooling gas. In another embodiment, a cooling side may be designed as an evaporation side, so that the heat of reaction produced in the PrOx reactor can be used to evaporate the fuel. In one alternative embodiment, the second plate on the fourth side can be designed as a flat surface. Advantageously, a repeating unit in the PrOx reactor has the following sequence: first side, second side, sixth side, fifth side, third side, fourth side, fourth side, third side, fifth side, sixth side. The invention further relates to a propylene glycol fuel processor, comprising a heat exchanger and a series arrangement of an evaporator, a reformer, a gas-to-water conversion reactor, and a PrOx reactor. At least one of the fuel processor components is a fuel processor component designed according to the invention as an evaporator, a fuel processor component designed according to the invention as a heat exchanger, a fuel processor component designed according to the invention as a gas-to-water conversion reactor, and / or a fuel processor component designed according to the invention as a PrOx reactor. -6- Advantageous, all the aforementioned components are designed according to the invention and as described previously. In this embodiment, the propylene glycol fuel processor comprises relatively few different plates to form the various fuel processor components. The integration of a cooling or evaporation unit into the gas-to-water conversion reactor or the PrOx reactor already achieves a high level of integration. In an advantageous embodiment of the propylene glycol fuel processor, at least two of the fuel processor components are arranged one above the other such that the plate stacks of said at least two fuel processor components form a superstack. A "superstack" is understood to be a stack interrupted only by bottom plates or top casing plates arranged between the individual component stacks. In a particularly integrated form, the top casing plate of a fuel processor component arranged as the lower fuel processor component and a bottom casing plate of a fuel processor component arranged above it can be replaced by a single separating plate. In another advantageous embodiment, a heat exchanger, a gas-to-water conversion reactor, and a PrOx reactor are arranged one above the other such that the plates of the heat exchanger, the gas-to-water conversion reactor, and the PrOx reactor form a superpile. The gas-to-water conversion reactor is located in a central zone of the superpile, and a first gas can successively pass through the heat exchanger, the gas-to-water conversion reactor, and the PrOx reactor. Other gases intended to pass through the respective reactors can be conveyed through only one or more of the reactors in a fluidically separate system. The invention is described in more detail below with the aid of drawings. The drawings are schematic representations. Figure 1 shows a schematic representation of a fuel processor component according to the invention. Fig. 2 a cross-section of the fuel processor component according to Fig. 1 along the section line 11-11', Fig. 3 a series of plates in the conversion reactor of the water gas, Fig. 4A, 4B a series of plates in an evaporator, Fig. 5a, 5b a series of plates in a PrOx reactor, Fig. 6 a schematic representation of an evaporator, Fig. 7 a schematic representation of a fuel cell composed of a PrOx reactor, a gas-to-water conversion reactor and an evaporator, and Fig. 8 is a schematic representation of a fuel processor according to the invention. Fig. 1 shows the housing G of a fuel processor component comprising a first inlet El, a second inlet E2, a first outlet Al, and a second outlet A2. The first inlet El and the second inlet E2, and the first outlet Al and the second outlet A2, respectively, are arranged at opposite ends of the housing G. In this embodiment, the housing G has a basic rectangular shape with chamfered corners, with the first inlet El and the second inlet E2, as well as the first outlet Al and the second outlet A2, arranged on a lateral surface of the chamfered corners. In a three-gas embodiment, a third inlet E3 and, opposite it, a third outlet A3 can be further arranged on the head sides of the housing G. This allows for either same-direction or opposite-direction flow in the area of ​​plates P1, P2, and P3. Fig. 2 shows a cross-section along the section line 11-11' of Fig. 1. Only the first cavities H1 are fluidically connected via the first inlet E1. The first cavities H1 are fluidly connected to the first outlet A1. Similarly, only the second cavities H2 are accessible via the second inlet E2 and the second outlet A2. Fig. 3 shows a series of plates of a fuel processor component implemented as a gas-to-water (GWW) reactor. The plate series comprises plates P1, P2, and P3, each having a first side SI, a second side S2, a third side S3, and a fourth side S4. On the first side SI, a distribution structure 12 is accessible from the front left side as shown in the figure. The first side SI includes channels 11 and a collection structure 13 which leads, on the rear right side, to a first outlet Al. The third side S3, on the other hand, has a distribution structure 12 open to the side -8-rear left, channels 11 and a collection structure 13 open towards the front right side. The channels 11 on the first side SI, the second side S2, the third side S3, and the fourth side S4 are identical in this embodiment. A cooling cavity formed between a fourth side S4 and a first side SI is traversed, in the view shown in Fig. 3, in the opposite direction to the observer's direction; a reaction cavity formed by the second side S2 and the third side S3, or by the third side S3 and the fourth side S4, is traversed in the observer's direction. Figures 4A and 4B show a series of plates of a fuel processor component implemented as an evaporator V in a slanted view from below (Fig. 4A) and in a slanted view from above (Fig. 4B). The first side S1 and the second side S2 are designed as the evaporation side. They have a feed channel in the middle leading to a distribution structure 12. The evaporation side is provided with a pattern 10 or channels (not shown). The distribution structure 12 is arranged centrally on one side, and the collection structure 13 is arranged perpendicularly to it. The third side, S3, is designed as the heating gas side, while the fourth side, S4, located behind the third side, S3, is designed as a flat surface. In this embodiment, the fourth side, S4, faces the second side, S2, and thus forms the evaporation cavity with the second side, S2. Two third sides S3 are arranged opposite each other so that the heating gas is conducted in channels 11 formed by the two third sides S3. Optionally—as shown—a third plate P3 with a fifth side S5 and a sixth side S6 is provided, the fifth side S5 being designed as a heating gas conduit for air passage and the sixth side S6 as a flat surface. The stack in this example has four second plates P2 and two third plates P3, the second plates P2 and third plates P3 being able to be identical. The stack shown, comprising the first plates P1, the second plates P2, and the third plates P3, forms a repeating unit. Figures 5a and 5b show a series of plates in a PrOx R2 reactor. Figure 5a shows a slanted view from above, Figure 5b shows a slanted view from below. The first plate, PI, with a The first side, S1, and a second side, S2, are designed as either the cooling side or the evaporation side. When designed as the evaporation side, they include a pattern (not shown) adapted for evaporation. A second plate, P2, includes a third side, S3, and a fourth side, S4. The third side, S3, is designed as the reaction side, and the fourth side, S4, is either a flat surface or—as shown in the example embodiment—includes an air supply channel 14 to supply air to the opposite side of the second plate, P2. The third plate, P3, includes the fifth side, S5, and the sixth side, S6. The sixth side, S6, is designed as a flat surface, and the fifth side, S5, is designed as the reaction side.The repeating unit of the plate stack in the PrOx R2 reactor therefore consists of a first plate PI, two second plates P2 and two third plates P3, evaporation of a propylene glycol-water mixture between the first side SI and the second side S2 of the first plate PI being possible to cool the PrOx R2 reactor. Fig. 6 shows a fuel processor component implemented in the form of an evaporator V. The evaporator V comprises a housing G having a first inlet E1, a second inlet E2, and a third inlet (not shown), as well as a first outlet A1, a second outlet A2, and a third outlet A3. The series of plates according to Fig. 4A and Fig. 4B can be arranged in the housing G, with one repeating unit shown in Fig. 4A and Fig. 4B. A plurality of repeating units according to Fig. 4A and Fig. 4B is arranged in the housing G as shown in Fig. 6. Specifically, from 2 to 500, from 5 to 250, and from 50 to 100 repeating units can be arranged. Reference signs 20, 22 and 23 designate means for distributing fluid and reference signs 21, 24 and 25 designate means for collecting fluid, in particular gas.A propylene glycol-water mixture is introduced through the first inlet E1 and is conducted by the fluid distribution means 22 and also by the side designed as an evaporator surface, thereby evaporating the propylene glycol-water mixture. The vapor thus produced is collected in the gas collection means 24 and conducted to the outside through the first outlet AL. Air is introduced through the second inlet E2 and can be directed through the evaporator V either to preheat the evaporator V or to heat the air. The air is... - 10 - Conduit through the sides of the heating gas supply provided for this purpose. The air is conveyed by means of gas collection 25 to the second outlet A2. The third inlet E3 is used to supply heating gas which can be heated in another fuel processor component, for example with heat from the reformer or another burner. The heating gas is conveyed via the gas collection line 20 to the third outlet A3. Fig. 7 shows a fuel cell consisting of three fuel processor components, including a heat exchanger W, a gas-to-water conversion reactor RI, and a PrOx reactor R2. The heat exchanger W can be designed as a counterflow heat exchanger in which a hydrogen-containing fuel gas, here a reformate, is cooled by another gas. The inlet for the other gas is not shown. The other gas is conveyed to the cooling gas outlet 31 by a collection means 43. The fuel gas, in particular the reformate, can be conveyed by the distribution means 42 into the heat exchanger W and delivered to a combined gas collection / distribution means 4L. The fuel gas, in particular the reformate, is conveyed by the combined gas collection / distribution means 41 into the gas-to-water conversion reactor RI and then conveyed into the PrOx reactor R2 by a combined gas collection / distribution means 44. Air can be conveyed into the cavities provided for cooling the gas-to-water conversion reactor RI by the inlet 32 ​​and the distribution means 45. The PrOx R2 reactor includes an inlet 33 with a distribution means 46 for introducing a gas, in particular fuel for evaporative cooling, and another inlet 35 to which a distribution means 47 is connected for introducing air. The gas produced in the PrOx R2 reactor is conducted through a gas collection means 40 to an exhaust gas outlet 34. A fuel cell can be connected to this exhaust gas outlet 34. The steam produced by evaporative cooling is discharged through another outlet (not shown). The reactor combination shown in Fig. 7, consisting of three fuel processor components, including the so-called "CO-clean up" reactors, features a particularly compact design and high power density because all components of - 11 -fuel processor, namely in the heat exchanger W, the gas-to-water conversion reactor RI and the PrOx reactor R2, are arranged plates PI, P2, P3 of the same size which are separated only by intermediate bottoms 50, 51 or spacers. Figure 8 shows a propylene glycol fuel processor comprising a fuel processor component designed as an evaporator V, a subsequent reformer R, and a CO-clean-up reactor unit R3. The CO-clean-up reactor unit R3 consists of a heat exchanger W, a gas-to-water conversion reactor RI, and a PrOx reactor R2. The CO-clean-up reactor unit R3 is specifically implemented as shown in Figure 7. The evaporator V is shown in detail in Figure 6. The reformer R transforms the evaporated propylene glycol-water mixture, specifically by means of a catalyst applied in the reformer R, into a gas containing hydrogen and various residual gases. The carbon monoxide content of the gas is reduced in the CO-clean-up reactor unit R3, in the gas-to-water conversion reactor RI, and in the PrOx reactor R2 contained within it.In particular, evaporator V and CO-clean up reactor unit R3 include other inlets and outlets which are not shown for clarity. The connecting line between evaporator V, reformer R and CO-clean up reactor unit R3 is the line for the fuel or the fuel transformed by reaction. The parts, components, objects, elements and means constituting the invention are referenced as follows in the attached figures: 10 pattern 11 channel 12 distribution structure 13 collection structure 14 air supply channel 20 distribution means 21 collection means 22 distribution means 23 distribution means 24 collection means 25 collection means 30 additional inlet 31 cooling outlet - 12- 32: Inlet 33: Evaporation gas inlet 34: Exhaust gas outlet 35: Air inlet 5 40: Gas collection means 41: Collection / distribution means 42: Distribution means 43: Collection means 44: Collection / distribution means 10 45: Distribution means 46: Distribution means 47: Distribution means 50: First intermediate bottom 51: Second intermediate bottom 15 A1: First outlet A2: Second outlet A3: Third outlet El: First inlet E2: Second inlet 20 E3: Third inlet G: Housing H1: First cavity H2: Second cavity PI: First plate 25 P2: Second plate P3: Third plate R: Reformer RI: Gas-to-water conversion reactor R2: PrOx reactor 30 R3: CO-clean-up reactor unit SI: First side S2: Second side S3: Third side S4: Fourth side 35 S5 : fifth side S6 : sixth side V: evaporator - 13 - W: heat exchanger Of course, the invention is not limited to the embodiment described and shown in the accompanying drawings. Modifications remain possible, particularly with regard to the composition of the various five elements or by substitution of technical equivalents, without departing from the scope of protection of the invention.

Claims

CLAIMS 1. Use of a propylene glycol fuel processor for operation with propylene glycol in the form of a propylene glycol / water mixture as a liquid fuel to be evaporated, the fuel processor comprising a heat exchanger (W) and a series connection of fuel processor components in the form of an evaporator (V), a reformer (R), a water gas shift reactor (RI) and a PrOx reactor (R2), wherein in the propylene glycol fuel processor at least one fuel processor component is used for operation with propylene glycol, which comprises at least one housing (G) having at least two inlets (E1, E2) and two outlets (A1, A2), a plurality of first plates (PI) having a first side (SI) and a second side (S2) and second plates (P2) having a third side (S3) and a fourth side (S4) being arranged in the form of a stack in the housing (G), and the stacked first and second plates (PI, P2) forming at least first cavities (H1) and second cavities (H2), the first inlet (El) being fluidically connected to the first outlet (Al) by the first cavities (H1) and the second inlet (E2) being fluidically connected to the second outlet (A2) by the second cavities (H2).

2. Use according to claim 1, characterized in that the first plates (PI) and / or the second plates (P2) have a surface structuring, the surface structuring comprising a pattern (10) and / or channels (11) with a distribution structure (12) and a collection structure (13).

3. Use according to claim 1 or 2, characterized in that a first number of first and second plates (PI, P2) is arranged in a predefined order, the first number of first and second plates (PI, P2) forming a repeating unit.

4. Use according to claim 3, characterized in that in the housing (G) there is arranged a plurality of third plates (P3) which each have a fifth side (S5) and a sixth side (S6), the third plates (P3) forming an element of the repeating unit.

5. Use according to one of the preceding claims, characterized in that the first side (S1) has a structure configured for a gaseous reaction and the second side (S2) is a cooling or evaporation side.

6. Use according to one of claims 2 to 5, characterized in that the third side (S3) has the same surface structure as the second side (S2) and the fourth side (S4) has the same surface structure as the first side (S1).

7. Use according to one of the preceding claims, characterized in that the fuel processor component is an evaporator (V) in which at least a first side (SI) of a first plate (PI) is designed as an evaporator (V) and a second side (S2) for passing a heating gas.

8. Use according to one of the preceding claims, characterized in that the fuel processor component is an evaporator (V) and a first side (S1) and a second side (S2) of a first plate (P1) are designed as evaporator surfaces, wherein a third side (S3) is designed as a heating gas side and a fourth side (S4) of a second plate (P2) as a flat surface, wherein, optionally, a fifth side (S5) is designed as a heating gas side for passing air, and wherein each evaporator surface faces a flat surface and two heating gas sides face each other.

9. Use according to claim 8 having first to sixth sides (S1, S2, S3, S4, S5, S6), characterized in that a repeating unit has the following sequence: first side (SI), second side (S2), fourth side (S4), third side (S3), third side (S3), fourth side (S4), first side (SI), second side (S2), fourth side (S4), third side (S3), third side (S3), fourth side (S4), sixth side (S6), fifth side (S5), fifth side (S5), sixth side (S6).

10. Use according to one of claims 1 to 6, characterized in that the fuel processor component is a heat exchanger (W) in which at least a first side (SI) is designed to pass a first gas and a second side (S2) to pass a second gas.

11. Use according to claim 10, characterized in that the fuel processor component is designed such that the first gas and the second gas can be conducted in the same direction or in opposite directions through the fuel processor component.

12. Use according to one of claims 1 to 6, characterized in that the fuel processor component is a water gas conversion reactor (RI) in which the first plate (PI) has a first side (SI) designed as a cooling side and a second side (S2) designed as a reaction side.

13. Use according to claim 12, characterized in that, in a repeating unit, every sixth side (S6) is designed as a cooling side.

14. Use according to one of claims 1 to 6, characterized in that the fuel processor component is a PrOx reactor (R2), that a first plate (PI) has a first side (SI) designed as a cooling side and a second side (S2) designed as a cooling side, that a second plate (P2) has a third side (S3) designed as a reaction side and a fourth side (S4) designed as a gas supply side, and that a third plate (P3) has a fifth side (S5) designed as a reaction side and a sixth side (S6) designed as an unstructured surface.

15. Use according to claim 14, characterized in that a repeating unit has the following sequence: first side (SI), second side (S2), sixth side (S6), fifth side (S5), third side (S3), fourth side (S4), fourth side (S4), third side (S3), fifth side (S5), sixth side (S5).

16. Use according to one of the preceding claims, wherein the propylene glycol fuel processor comprises a heat exchanger (W) and a series connection of an evaporator (V), a reformer (R), a water gas conversion reactor (RI) and a PrOx reactor (R2) for operation with propylene glycol, characterized in that at least one of the fuel processor components is a fuel processor component designed as an evaporator (V) according to one of claims 7 to 9, a fuel processor component designed as a heat exchanger (W) according to one of claims 10 or 11, a fuel processor component designed as a water gas conversion reactor (RI) according to one of claims 12 or 13 and / or a fuel processor component designed as a PrOx reactor (R2) according to one of claims 14 or 15.

17. Use according to claim 16, characterized in that at least two of the fuel processor components are arranged one above the other such that the plate stacks of said at least two fuel processor components form a superstack.

18. Use according to claim 17, characterized in that a heat exchanger (W), a water gas conversion reactor (RI) and a PrOx reactor (RI) are arranged one above the other in such a way that their plates (PI, P2, P3) form a superpile, that the water gas conversion reactor (RI) is arranged in a middle zone of the superpile, that the propylene glycol fuel processor is designed such that the heat exchanger (W), the water gas conversion reactor (RI) and the PrOx reactor (R2) can be successively crossed by a first gas.

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