WO2008003520A1

WO2008003520A1 - Evaporation device, component comprising an evaporation device and method for evaporating an oxidizable liquid

Info

Publication number: WO2008003520A1
Application number: PCT/EP2007/006068
Authority: WO
Inventors: Wolfgang Hahnl; Christoph Hossfeld; Jürgen Klement; Marco Ranalli
Original assignee: Emcon Technologies Germany (Augsburg) Gmbh
Priority date: 2006-07-07
Filing date: 2007-07-09
Publication date: 2008-01-10
Also published as: DE102006031544A1

Abstract

The invention relates to a component (10) that is arranged upstream or downstream from a user, comprising a main pipe (12) for a gas flow, in addition to an evaporation device (14) for evaporating an oxidizable liquid and for introducing the evaporation vapour produced from the oxidizable liquid into the gas flow. Said evaporation device (14) has a heating element (18) that is arranged in a housing (16) that comprises an inlet (20) for the liquid, at least one evaporation chamber (21) and an outlet for the evaporation. Said heating element (18) is surrounded at least partially by a covering (28) that is arranged between the heating element (18) and a wall (30) of the housing (16). The oxidizable liquid is provided on the lateral side of the covering (28) when seen from the heating element. The component is, in particular, part of a waste gas system of an internal combustion engine or a reforming device for producing a gas containing hydrogen. The invention also relates to a method for evaporating an oxidizable liquid.

Description

Vaporizing device, assembly with an evaporation device and method for vaporizing an oxidizable liquid

The invention relates to an evaporation device for evaporating an oxidizable liquid and introducing a vapor generated from the oxidizable liquid in a gas stream, in particular upstream or downstream of a consumer, with a heating element disposed in a housing, wherein the housing has an inlet for the liquid , at least one evaporation space and an outlet for the steam. In particular, the evaporation device for an exhaust system of an internal combustion engine (the latter in this case represents the consumer) or provided for a reforming device for generating a hydrogen-containing gas, which is also coupled to a consumer, for example, a (H ₂ consuming) fuel cell. Furthermore, the invention relates to an assembly with an evaporation device and a method for evaporating an oxidizable liquid.

A generic evaporation device is known from EP 1 369 557 A1. This has as a heating element to a glow plug, evaporated by means of the diesel fuel and upstream of a diesel particulate filter or _NOx - storage catalyst is introduced into the exhaust stream to regenerate the filter or catalyst at periodic intervals.

As it turns out, a comparatively large amount of electrical energy is needed to vaporize fuel with the aid of a glow plug, with about 60% of the energy already being needed to heat the fuel from ambient to boiling. If the fuel is not completely evaporated, then enters a mixture of liquid and vapor fuel in the exhaust stream, which is not completely evaporated in the exhaust gas under certain circumstances and therefore can not mix satisfactorily with this. In this case, the regeneration of the exhaust gas purification system may be prolonged or incomplete. Furthermore, so-called hot spots on the catalyst can arise. For safety reasons and In order not to excessively burden the vehicle electrical system such as a vehicle by the power supply of the glow plug, it is not possible to increase the performance of the glow plug indefinitely.

The invention provides an evaporation device that is distinguished from the prior art by an improved evaporation of an oxidizable liquid while maintaining low energy consumption.

According to the invention this is achieved in an evaporation device of the type mentioned above in that the heating element is at least partially surrounded by a sheath which is disposed between the heating element and a wall of the housing, wherein seen from the heating element laterally outside the envelope, the oxidizable liquid is provided , The embodiment of the invention is based on the recognition that the largest heat exchange between the glow plug and the liquid during the bubble boiling (ie the formation of bubbles in the boiling liquid) takes place. By wrapping the bladder boiling is favored. Thus, the evaporation can be improved by simple means. If the coating is provided on the heating element, the surface available for the heat exchange with the liquid can be increased, whereby more heat can be transferred to the liquid. If the sheath is slightly spaced from the heating element, it acts as a flow guide surface, by means of which liquid is selectively guided to the heat source.

In particular, the heating element is a glow plug, which is available at low cost as a mass-produced item.

Preferably, the envelope is at least partially liquid-permeable, so that it is possible to flow through the oxidizable liquid.

According to a first embodiment of the invention, the (liquid-permeable) envelope abuts directly on the heating element and thus increases the effective surface area for the heat exchange between the heating element and the liquid. Blistering occurs in this embodiment on the surface of the glow plug and inside the liquid pervious enclosure. According to another embodiment of the invention, the enclosure is spaced from both the heating element and the wall of the housing. In this embodiment, blistering occurs between the heating element and the enclosure, with the vapor bubbles rising along the heating element due to their lower density relative to the liquid. Above the heating element, the vapor bubbles unite due to surface tension, and the liquid which has been torn up by the vapor bubbles flows down again outside the envelope, the envelope serving to separate the regions of the rising vapor bubbles and the effluent to separate. In this way, a closed liquid circuit is formed in the housing of the evaporation device.

Of course, a (liquid-permeable) casing can be arranged directly on the heating element and additionally a sheath with a distance from the heating element.

As already mentioned, the oxidizable liquid is fuel, in particular diesel fuel.

The (liquid-permeable) sheath is preferably a porous material, in particular a metal foam, which has a comparatively large surface which promotes nucleate boiling and heat transfer.

Alternatively, the (liquid-permeable) sheath may be a wire mesh. This is also relatively inexpensive available.

The housing is according to a preferred embodiment, an elongated tube in which the likewise elongated heating element is arranged vertically. In this way one obtains a particularly favorable for the nucleate boiling geometry.

In order to separate the liquid entrained by the rising bubbles from the bubbles, the evaporation space preferably has a vapor collecting section located above the heating element and having a comparatively large volume. The outlet of the housing is advantageously arranged in the region of the vapor collecting section into which only the vaporous phase passes. In this way it is largely prevented that liquid droplets get into the gas stream.

To increase the heat transfer in the liquid phase, the

Heating element may be divided over its length into a plurality of heating zones, wherein the zone with the highest energy density is arranged at the bottom, where the liquid collects.

To ensure that no unvaporized liquid leaves the evaporation device, according to a second aspect of the invention in an evaporation device of the type mentioned above, a drip element is provided downstream of the heating element, in particular in the form of a metal foam or wire mesh. Such a drip element is preferably combined with the enclosure described so far, but can be used advantageously in any conventional evaporation device.

The evaporation device according to the invention is preferably used in an assembly with a main line in which a gas stream is guided.

Advantageously, a portion of the housing of the evaporation device, on which the outlet is arranged, projects into the main line. In this way, the heat of the gas stream can be used in addition to heating the liquid.

In order to achieve a particularly high heat transfer from the gas stream to the evaporation device, the section projecting into the main line preferably forms a majority of the housing.

According to a further development, the section projecting into the main line has a plurality of integrally formed ribs which increase the surface of the housing which is effective for the heat transfer.

In a first variant of the invention, the main line is an exhaust pipe in an exhaust system for an internal combustion engine.

In this case, the evaporation device is preferably upstream of an exhaust gas purification device, in particular a particulate filter with switched oxidation catalyst or a NO _x storage catalyst arranged. The oxidizable liquid serves as a regeneration agent for the exhaust gas purification system.

According to an alternative application, the main line is a supply line of a reforming device for generating a hydrogen-containing gas. Again, the oxidizable liquid is preferably fuel that is mixed with air in the vapor state and reacts in the reforming device to form a hydrogen-containing gas.

According to a further aspect of the invention, a method for evaporating an oxidizable liquid by means of an evaporation device is provided, with a heating element arranged in an evaporation space, which is at least partially surrounded by an envelope. The method comprises the following steps. First, the liquid is introduced into the evaporation space and heated by means of the heating element. In the region of the heating element, bubbles are formed which rise with the entrainment of liquid to the top of the heating element. It forms a foam that covers the tip of the heating element at least almost completely. The foam is separated into a vapor portion and a liquid portion, the liquid portion being returned to a portion of the heating element surrounded by liquid, and the vapor portion being discharged from the evaporation space. In the method according to the invention, therefore, the heating element is (almost) completely covered by liquid or foam, to which the heat of the heating element can be transferred better than to steam. In addition, a separation of liquid and vapor is provided.

In particular, the evaporation space has a downstream of the

Heater arranged outlet, wherein the liquid is introduced into the evaporation chamber, without this reaches the outlet. Thus, there is only steam in the area of the outlet, which can leave the evaporation space.

Further features and advantages of the invention will become apparent from the following description of several preferred embodiments with reference to the accompanying drawings. In these show: - Figure 1 is a schematic view of an assembly with an evaporation device according to a first embodiment of the invention;

FIG. 2 shows a schematic view of an assembly with an evaporation device according to a second embodiment of the invention;

FIG. 3 shows a schematic view of an assembly with an evaporation device according to a third embodiment of the invention;

FIG. 4 shows a schematic view of an assembly with an evaporation device according to a fourth embodiment of the invention;

- Figure 5 is a schematic view of an assembly with a vaporizing device according to a fifth embodiment of the invention;

Figure 6 is a diagram illustrating heat transfer during boiling of the oxidizable liquid;

- Figure 7 is a schematic representation of an exhaust system with an assembly according to the invention; and

- Figure 8 is a schematic representation of a reforming device with an assembly according to the invention.

FIG. 1 shows an assembly 10 according to the invention which has a main line 12 in which a gas flow (indicated by the arrow P ₁ ) is guided. The assembly 10 further comprises an evaporation device 14, in which an oxidizable liquid, in particular fuel, evaporated and via which the vapor generated from the oxidizable liquid is introduced into the gas stream, as indicated by the arrow P ₂ . For this purpose, the evaporation device 14 has a housing 16 and a heating element 18 arranged therein in the form of a glow plug. The housing 16 has an inlet 20 for the liquid, which is outside the main line 12, an evaporation chamber 21 and an outlet 22 for the steam, which is arranged on a projecting into the main line 12 portion 24 of the housing 16. As can be seen, makes the projecting into the main line 12 section 24 in the axial direction of the housing 16 a major part of the housing 16. In order to improve the heat exchange between the gas flow and the evaporation device 14, the portion 24 has a plurality of molded ribs 26 which are parallel to the Main flow direction P _{1 of} the gas flow to the housing 16 are arranged. If the ribs 26 are oriented obliquely to the direction P ₁ , after the evaporation device 14, the vortex formation could increase, resulting in a better distribution of the vapor in the gas stream.

The heating element 18 is partially surrounded by a sheath 28 which is disposed between the heating element 18 and a wall 30 of the housing 16. In the embodiment shown in Figure 1, it lies in particular directly on the heating element 18 and is liquid-permeable. The sheath 28 is a porous material in the form of a metal foam, alternatively it may be a wire mesh or a perforated plate.

The housing 16 is an elongated tube in which the likewise elongate heating element 18 is arranged upright or inclined to 45 °. During operation of the evaporation device 14, liquid flows through the inlet 20 into the housing 16 and boils in the region of the heating element 18 and the cover 28. In this case, bubbles are formed on the porous and therefore very large surface of the heating element 18 or the cover 28 Figure 1 are indicated by the reference numeral 32. The temperature of the vapor bubbles is about 15 ° C higher than the temperature of the surrounding liquid. Due to their lower density, the vapor bubbles in the housing 16 rise rapidly along the heating element 18. From small bubbles arise when rising (reference numeral 34) larger bubbles and form a two-phase mixture of liquid and vapor, which can lead to foaming. It can even create a plug flow, so a plug of bubbles, which displaces the liquid. The two-phase mixture is now separated into two separate phases. With 36 the dividing line between the gaseous and the liquid phase is designated. Above the heating element 18, the evaporation space 21 has a vapor collection section 38, which has a comparatively large volume. In this area, the phase separation takes place in that the fluid entrained by the gas bubbles forms drops which flow down in the evaporation space 21 along the peripheral wall 30 (reference numeral 40) and mix with the newly introduced liquid. This preheated liquid then comes (again) in contact with the heating element 18, causing bubbles to form again and the process described starts again. In the evaporation chamber 21 thus forms a circulating flow.

FIG. 2 shows an assembly 10 with an evaporation device 14 according to a second embodiment of the invention. The same or functionally identical components bear the same reference numerals, and in the following, only the differences from the previously described solution will be discussed.

In contrast to the previously described embodiment, in the evaporator device 14 according to FIG. 2, the sheath 28 is arranged at a distance both from the heating element 18 and from the wall 30 of the housing 16. In this way, a narrow gap is formed between the sheath 28 and the heating element 18 in which the liquid boils and forms bubbles. The bubbles rise up again and, due to the narrow gap, carry a large amount of liquid, which is separated from the vapor in the vapor collection section 38. The enclosure 28 here serves to separate between the outer area where the liquid flows down the wall 30 and the inner area where the bubbles rise upwards. For this purpose, the wrapper 28 may be at least partially liquid-permeable, but need not. It is sufficient if the sheath 28 is arranged at a distance from the bottom of the evaporation space 21, so that there is a passage for the liquid. Alternatively, a tubular element with a few individual openings in the lower area is conceivable. The bubbles can not come down again. On the easy for the skilled person to be realized storage of the enclosure 28, in particular on the heating element 18 or the housing 16, will not be discussed here.

Particularly advantageously, the embodiments of Figures 1 and 2 can be combined with each other, i. an enclosure 28 may be disposed both directly on the heating element 18 and at a distance from the heating element 18.

The embodiments of Figures 3 and 4 are in principle as the embodiment of Figure 2 formed, but without the integrally formed on the housing 16 ribs 26. Also, these evaporation devices 14 do not protrude so far into the main line 12. Both evaporation devices 14 additionally have a drip element 42 above (ie downstream) of the heating element 18, which is a metal foam or a Wire mesh acts. The drip element 42 ensures that no liquid drops from the evaporation device 14 get into the main line 12. The drip element 42 may be a continuous plate extending over the entire cross-sectional area of the evaporator 14 (FIG. 3), or it may consist of a plurality of individual and tilted parts, as shown in FIG.

Finally, FIG. 5 shows a final embodiment of the invention, which in turn is very similar to the embodiment according to FIG. 2, the heating element 18 being divided over its length into three different heating zones 44, 46, 48. The highest energy density zone 48 is located at the bottom, the lowest energy density zone 44 at the top. In this way it is possible to provide more heat in the area of the liquid than in the area of the vapor, since it can absorb less heat anyway. The electrical connections of the heating element 18 (or of the housing 16) are designated by 50.

Figure 6 illustrates the heat transfer during boiling of the liquid. For this purpose, the heat transfer coefficient α is plotted against the temperature difference .DELTA.t between the wall and the medium (ie liquid, foam or steam), wherein the following applies to the transferred heat quantity Q:

Q = α * a * Δt,

with a: effective area for heat transfer.

In the area from A to B heat is transferred to the (still unvaporised) liquid, whereby first bubbles can arise; the area B to C marks the heat transfer of the bubbling, which is ten to twenty times larger than in the area AB. In area C there is steam overheating. Depending on the amount of liquid supplied, these zones shift along the height of the heating element 18. Bubbles and liquid can be entrained up to the top. The aim is to have nucleate boiling over a large area of the heating element. The section C to D represents a phase in which the bubbles combine to form a film and form a vapor film on the surface of the heating element. If this is the case, there may be little liquid be evaporated. The diagram illustrates once again how important it is for the evaporation of the liquid to remain in the area of bubble boiling.

In the embodiment according to FIG. 1, the surface 28 which is effective for heat transfer and the heat transfer coefficient α is increased by the covering 28 made of porous material, which increases the amount of heat Q transferred.

The embodiments according to FIGS. 2 to 4 also have a heat transfer coefficient α which is higher than that of the prior art due to the circulating flow that forms and the phase separation, as a result of which more heat is also transferred.

The assembly 10 according to the invention, as shown in Figure 7, be part of an exhaust system 52 for an internal combustion engine 54, wherein the main line 12 then represents an exhaust pipe. The exhaust system 52 has an exhaust gas purification device 56 in the form of a particulate filter with upstream oxidation catalyst or a NO _x storage catalytic converter. The vaporizing device 14 is disposed upstream of the exhaust gas purifying device 56 with respect to the exhaust gas flow in the main pipe 12 to regenerate the same at periodic intervals.

Alternatively, the main line 12 may be a supply line of a reformer 58 for producing a hydrogen-containing gas, wherein the evaporator 14 is preceded by a catalyst 60, which is a hydrogen-containing from a fuel-air mixture

Gas generated (see Figure 8).

Claims

claims

An evaporation device for vaporizing an oxidizable liquid and for introducing a vapor generated from the oxidizable liquid into a gas stream which lies in particular upstream or downstream of a consumer, in particular for an exhaust system (52) of an internal combustion engine (54) or a reforming device (58). for producing a hydrogen-containing gas, comprising a heating element (18) arranged in a housing (16), the housing (16) having a liquid inlet (20), at least one evaporation space (21) and an outlet (22) for the steam characterized in that the heating element (18) is at least partially surrounded by a sheath (28) between the

Heating element (18) and a wall (30) of the housing (16) is arranged, wherein seen from the heating element (18) laterally outside the enclosure (28) the oxidizable liquid is provided.

2. evaporation device according to claim 1, characterized in that the heating element (18) is a glow plug.

3. evaporation device according to claim 1 or 2, characterized in that the envelope (28) is at least partially liquid-permeable.

4. evaporation device according to claim 3, characterized in that the sheath (28) directly on the heating element (18).

5. evaporation device according to one of the preceding claims, characterized in that the sheath (28) at a distance from both

Heating element (18) and from the wall (30) of the housing (16) is arranged.

6. Vaporizing device according to one of the preceding claims, characterized in that it is the fuel in the oxidizable liquid, in particular diesel fuel is.

7. evaporation device according to one of the preceding claims, characterized in that the sheath (28) is a porous material, in particular a metal foam.

8. evaporation device according to one of the preceding claims, characterized in that the sheath (28) is a wire mesh.

9. evaporation device according to one of the preceding claims, characterized in that the housing (16) is an elongated tube in which the likewise elongate heating element (18) is arranged vertically.

10. Vaporizing device according to one of the preceding claims, characterized in that the evaporation chamber (21) one above the

Heating element (18) arranged Dampfsammeiabschnitt (38).

11. Vaporizing device according to claim 10, characterized in that the outlet (22) in the region of the Dampfsammeiabschnitts (38) is arranged.

12. Vaporizing device according to one of the preceding claims, characterized in that the heating element (18) over its length in several

Heating zones (44, 46, 48) is divided, wherein the zone (48) is arranged with the greatest energy density at the bottom.

13. vaporizing device, preferably according to one of the preceding claims, for vaporizing an oxidizable liquid and for introducing a vapor generated from the oxidizable liquid into a gas stream, in particular upstream or downstream of a consumer, in particular for an exhaust system (52) of an internal combustion engine ( 54) or a reforming device (58) for generating a hydrogen-containing gas, with a in a housing (16) arranged heating element (18), wherein the housing (16) has an inlet (20) for the liquid, at least one evaporation chamber (21) and an outlet (22) for the steam, characterized in that downstream of the heating element (18) a drip element (42), in particular in the form of a metal foam or wire mesh, is provided.

14 assembly with a main line (12) in which a gas stream is guided, and at least one evaporation device (14) according to one of the preceding claims.

15. An assembly according to claim 14, characterized in that a portion (24) of the housing (16) of the evaporation device (14) on which the

Outlet (22) is arranged, projects into the main line (12).

16. An assembly according to claim 15, characterized in that in the main line (12) projecting portion (24) forms a major part of the housing (16).

17. An assembly according to claim 15 or 16, characterized in that in the main line (12) projecting portion (24) has a plurality of molded-in ribs (26).

18. An assembly according to any one of claims 14 to 17, characterized in that the main line (12) is an exhaust pipe in an exhaust system (52) for an internal combustion engine (54).

19. An assembly according to claim 18, characterized in that the evaporation device (14) upstream of an exhaust gas purification device (56), in particular a particulate filter with upstream oxidation catalyst or a NGySpeicher catalytic converter is arranged.

20. An assembly according to any one of claims 14 to 17, characterized in that the main line (12) is a supply line of a reforming device (58) for generating a hydrogen-containing gas.

21. A method for vaporizing an oxidizable liquid by means of a

Evaporating device (14) having a heating element (18) arranged in an evaporation space (21), which is at least partially separated from a heating element (18)

Enclosure (28) is surrounded, in particular according to one of claims 1 to 13, characterized by the following steps:

Introducing the liquid into the evaporation space (21), heating the liquid by means of the heating element (18), - forming bubbles in the region of the heating element (18), Rising of the bubbles while carrying liquid to the top of the heating element (18),

Forming a foam which at least substantially completely covers the tip of the heating element (18), separating the foam into a vapor portion and a liquid portion,

Returning the liquid portion in a region of the heating element (18), which is surrounded by liquid, and

Omitting the vapor portion from the evaporation space (21).

A method according to claim 21, wherein the evaporation space (21) comprises an outlet (22) located downstream of the heating element (18), characterized by the step:

Introducing liquid into the evaporation chamber (21) without this until the outlet (22) passes.