TECHNICAL FIELD:

The invention generally relates to a particulate filter for treatment of a gas flow. In particular, the invention relates to a particulate filter that is applied to purify exhaust gas from an internal combustion engine, preferably a diesel engine.

BACKGROUND ART:

The need for reducing the amount of particulates (ash, soot, organic fractions etc.) in exhaust gas emanating from diesel engines has become increasingly important.

Generally, prior art particulate filters makes use of a ceramic monolithic structure with porous walls in which every second channel is plugged so that adjacent channels are plugged either at the inlet side or at the outlet side. Thereby the gas is forced to flow through the walls in which the particulates get stuck. A problem with this type of filter is clogging of the walls leading to increasing pressure drop which makes it necessary to frequently regenerate the filter. In addition, clogging of the walls increases the risk of uncontrolled regeneration that may destroy the filter. Further, service stops are normally required for removing ash from the filter.

DISCLOSURE OF THE INVENTION:

An object of the present invention is to provide a particulate filter that separates particulates from a gas flow in an efficient way and that is resistant towards clogging. This objective is achieved by the technical features contained in claim 1. The dependent claims contain advantageous embodiments, further developments and variants of the invention.

The invention concerns a particulate filter for treatment of a gas flow, comprising a body provided with a plurality of substantially parallel gas channels separated by porous walls. The invention is characterized in that the filter comprises a second body that also is provided with a plurality of substantially parallel gas channels separated by porous walls, in that the second

body is positioned at an outlet side of the first body as to farther lead a gas flow leaving the first body, and in that said first and second bodies are arranged to create turbulence in a gas flowing from the first body to the second body. Thus, the two bodies are arranged relative to each other in such a way that the flow of gas through the filter is disturbed so that turbulence is created at the entrance of the second body, i.e. inside the filter. An advantageous effect of this arrangement is that it leads to an increased mass transport to the walls resulting in a larger fraction of the particulates contained in the gas flow getting trapped in or onto the porous walls, i.e. the filtration becomes more efficient. The invention thus makes filters with open gas channels very effective and thus makes it possible to overcome the problems related to conventional plugged filters.

In a first advantageous embodiment of the invention, the first and the second body are arranged at a distance from each other. A gas flow leaving the first body can thus for some distance flow more freely which will create turbulence at the inlet side of the second body. Preferably, the bodies are kept in their proper positions by a joining element, such as a ring. Alternatively, the bodies may be kept in place by a casing that at least partly encloses the bodies.

In a second advantageous embodiment of the invention, the second body is arranged such that at least a portion of the porous walls in the second body have at least a part of their end parts located in a position corresponding to an imaginary elongation of at least a portion of the channels in the first body. Thus, the walls of the second body splits and disturbes the gas flow coming from the gas channels of the first body. In addition to the turbulence created, an advantageous effect of this design is that the distance to the wall is shortened for particulates flowing in the centre of the channels of the first body resulting in that particulates that are affected by molecular diffusion increase their probability to reach the wall by random walk. In contrast, a particulate in laminar flow in a monolith channel is very unlikely to migrate to the wall. The inventive feature thus decreases the average particulate-wall distance which increases the filtering efficiency further. Another advantageous effect of this design is that acoustic pulses present in an exhaust gas flow are reflected in the filter. The wave propagation tends to orient in the flow direction, unless actions are taken. By disturbing the acoustic waves in a direction not parallel to the flow direction in the gas channel, i.e. by displacing the walls of the second body in relation to the first body and letting the acoustic waves interact with the end part of the walls, the particulates are brought in to a motion enabling them to

enter the walls. A further advantageous effect of this design is that particulates that are sufficiently heavy, i.e. particulates that are less affected by the induced turbulence, will flow straight into the wall end parts that are positioned in their way where they will get stuck.

BRIEF DESCRIPTION OF THE DRAWINGS:

The invention will now be described in more detail with reference to the following drawings where:

Figure 1 schematically shows, in an exploded view, a first advantagous embodiment of the invention, Figure 2 schematically shows a detail of the first advantagous embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION:

Figure 1 shows schematically a first advantagous embodiment of the invention, wherein the particulate filter 1 is made up of a plurality of (in this example four) bodies 2, 2', 2", 2'" in the form of cylindrical monolithic structures. Each body 2, 2\ 2", 2'" is provided with parallel gas channels 3 separated by porous walls 4. Further, each body 2, 2', 2", 2'" is rotationally displaced 45° relative to the adjacent body or bodies such that the porous walls 4 in one body have their end parts 9 located in a position corresponding to an imaginary elongation of the channels 3 in an adjacent body. The filter 1 is preferably enclosed in a casing (not shown) provided with an inlet and an outlet.

The arrow 10 represents an incoming gas flow to the filter 1 and the arrow 11 represents an outgoing gas flow from the filter 1. During operation of the filter 1 gas flows through the gas channels 3 from an inlet side 5, 5' to an outlet side 6, 6' of all bodies 2, 2', 2", 2'". As the gas flow leaves the gas channels 3 of the first body 2 and enters the second body 2' it will encounter the end parts 9 of the walls in the second body 2' on its way into the gas channels 3 of the second body 2'. This will bring about an entrance disturbance of the gas flow leading to a turbulent flow in at least a first part of the gas channels 3 in the second body 2'. A similar entrance effect will arise also when the gas enters the third body 2" and the fourth body 2'". Of course, an entrance effect can also be achieved when the gas flow enters the first body 2. The inventive particular filter 1 will thus give rise to multiple entrance effects.

It may be noted that the filter according to the invention may make use of similar porous walls as state-of-the-art filters, which also use pores for trapping particulates. Further, the use of porous walls is something that differentiates the inventive filter from catalyst designs in general and state-of-the-art metallic open particulate filters in particular.

Generally, a laminar flow will be established at a certain distance downstream the entrance of a smooth gas channel. With a laminar flow the movement of particulates in a direction towards the wall is too low to achieve efficient trapping in through-flow filters. However, if the gas flow is made turbulent the masstransport to the walls increases resulting in a larger fraction of the particulates contained in the gas flow getting trapped in and/or onto the porous walls 4. The advantage of creating a turbulent gas flow in the gas channels 3, i.e. the advantage of creating multiple entrance effects, is thus that the filtering efficiency increases. With a sufficient number of bodies 2, 2\ 2", 2"', i.e. at least two, arranged in the inventive manner, there is no need for conventional plugging of every second channel, i.e. the filter 1 may be an open structure, i.e. a through-flow filter.

Figure 2 gives a schematic enlarged view of the interface between the first and the second body 2, 2' during operation of the filter 1. As can be clearly seen in figure 2 the channels 3 in the second body 2' are displaced relative to the channels 3 in the first body 2. A laminar gas flow in the gas channels 3 of the first body 2 encounter the end parts 9 of the walls in the second body 2'. This forces the flow to distribute between the channels 3 in the second body 2' creating at the same time a turbulent flow. A turbulent zone 12 will be created in the second body 2' extending from the channel entrances to a certain point downstream the channels 3. The length of the turbulent zone 12 depends on e.g. mass flow, flow velocity, temperature, gas flow content and body/channel geometry. If the gas channels 3 of the second body 2' are sufficiently short no laminar flow will be established in the second body 2'. Of course, figure 2 also applies to the interfaces between the second and the third body T, 2" and the third and the fourth body 2", 2'". A typical length of the turbulent zone 12 for a conventional type of filter applied to purify the exhaust gas of a diesel engine may be around 1-2 cm.

Figure 2 also shows other advantageous effects of the embodiment according to the invention. More heavy particulates that are less affected by the turbulence will flow straight into the wall end parts 9 and get stuck. Acoustic pulses, e.g. diesel engine exhaust pulses, in the gas flow, will be reflected by the wall end parts 9 and give rise to gas and particulate movements

towards the walls 4 which will further increase the amount of particulates trapped in the filter 1. These acoustic reflections are indicated by the dotted lines 13.

In principal, the higher the number of bodies, the higher the filtering efficiency induced by entrance effect, acoustic reflection and other mechanisms. A continously turbulent flow may be achieved by using sufficiently short bodies, i.e. equal to or shorter than the turbulent zone 12. However, in order to optimize a particulate filter according to the invention one must also consider e.g. the type of application, the pressure drop, the available space, the mechanical stability of individual bodies and the filter, and the cost.

The inventive way of rotationally displace bodies 2, 2', 2", 2'" of similar structure so that the gas channels 3 of adjacent bodies are displaced is an advantageous way of creating multiple entrance effects in extruded monolithic ceramic structures. In principal, a conventional monolith may be cut into a suitable number of sections and be sintered together again after a suitable rotational displacement.

The particulate filter 1 according to the invention is particularly advantageous for using NO ₂ to regenerate particulates in a so-called CRT (Contineous Regeneration Trap) process according to the reaction: NO ₂ + PM => NO + CO ₂ , where PM denotes Particulate Matter (particulates).

In a conventional filter the gas flows into a first type of channel that is plugged at the outlet side of the filter, through the porous wall at some point on its way through the filter, and out from the filter via a second type of channel that is plugged at the inlet side of the filter. With such filters it is essential that NO _x -molecules in the gas are in the form of NO ₂ when they enter the filter since they, in principle, have only one chance to take part in the above reaction, i.e. when they pass through the wall where the particulates are located. Normally, such a filter must be positioned downstream an oxidation unit that, inter alia, oxidizes gas flow contents of NO into NO ₂ . hi order to speed up the above reaction it is common to coat the filter walls with an oxidation catalyst. A fraction of the NO may be oxidized to NO ₂ while passing through the walls but such NO _x -molecules will most unlikely be available for taking part in the above reaction a second time.

In contrast to conventional filters, the filter according to the invention has a significantly higher potential of "re-using" NO _x -molecules. By applying oxidation catalysts to at least parts of the walls 4 NO-molecules are highly likely to be oxidized into NO ₂ on their way through the filter and since these oxidized molecules will encounter one or several particulate containing regions at the inventive body-body interface(s) downstream it is a considerable chance of "re-using" these molecules in the PM-consuming reaction mentioned above. Thus, the invention will make the regeneration more efficient compared to conventional filters. Further, it is not necessary that an oxidation unit is arranged upstream the filter 1 since NO ₂ that is available for taking part in the above reaction may be produced in the filter 1 itself.

In a preferred embodiment of the invention the cell density, i.e. the number of gas channels per area unit, and/or the wall thickness increases downstream the filter 1. Thereby one obtains a better filtration of the small particles present in a large fraction further downstream and at the same time one avoids clogging of the upstream part of the filter 1. A filter 1 with such properties can be achieved by using bodies 2, 2', 2", 2'" with different internal structures.

The invention is not limited to the above described embodiments, but a number of modifications are possible within the frame of the following claims. For instance, the rotational displacement does not need to be 45° as exemplified in figure 1. Sufficient turbulence may be created at any displacement having the effect that at least a portion of the porous walls in one body have at least a part of their end parts located in a position corresponding to an imaginary elongation of at least a portion of the channels in another body.

Further, two or more of the bodies 2, 2', 2", 2'" in the embodiment shown in figure 1 may be arranged at a distance from each other.

To further increase the efficiency of the filter 1 it maybe combined with other methods or devices, such as directing of acoustic waves for inducing particulate movements towards the porous walls 4.