Technical field

[0001] The present invention is related to a solid by-product reducing additive for a NO _X reducing solution. The invention is also related to a NO _X reducing solution comprising such a solid by-product reducing additive. The invention is further related to the use of such a solid by-product reducing additive and NO _X reducing solution in the catalytic treatment of exhaust gases.

Background art

[0002] In the light of environmental pressure, exhaust gases are in many cases treated to remove or at least reduce the content of harmful and potentially harmful components. For example, the exhaust gases of engines, in particular of diesel engines, comprise significant amounts of nitrogen oxides (abbreviated as NO _X ). NO _X include nitric oxide (NO); nitrogen dioxide (NO2) and nitrous oxide (N2O). Nitric oxide oxidises readily in the atmosphere to nitrogen dioxide. NO2 is a major pollutant and component of smog. [0003] Different methods exist to remove NO _X from exhaust gases.

Selective catalytic reduction (SCR) is the most common method used in diesel engine exhausts. SCR makes use of a NO _X reducing agent and a catalyst to convert NO _X gases into nitrogen and water. Typical NO _X reducing agents are ammonia or an ammonia precursor, such as urea, which is converted to ammonia at temperatures above 150 °C before reacting with the NO _X . The catalyst typically comprises platina (Pt) and palladium (Pd), or copper and a zeolite. Ammonia and/or urea is/are injected, often as an aqueous composition, into the exhaust gases over the catalyst. An elevated temperature between 150 °C and 400 °C causes the ammonia and/or urea to evaporate. The evaporated ammonia and/or urea react(s) with the NO _X , thereby reducing the NO _X to nitrogen and water.

[0004] A disadvantage of the SCR process is that solid by-products are formed during the conversion of urea and water into ammonia, leading to a reduced amount of ammonia that can be used for reduction of NO _X in the exhaust gases. The formation of by-products has been noticed in particular at the place where urea is injected into the exhaust pipes of the (diesel) engine. These solid by-products, for example having the form of crystals, can be large enough to cause a partial and even a total blocking of the exhaust duct, which can lead to loss of engine power. [0005] At temperatures below 250 °C to 300 °C, the solid by-products are made up predominantly of crystallised urea, while at temperatures above 300 °C they are made up predominantly of cyanuric acid. Cyanuric acid may be converted to ammonia by sublimation, but this requires temperatures above 450 °C, which is rarely reached at the location of the by-products in the exhaust pipes. It is also known that the formation of solid deposits is higher when the temperature of the exhaust gases is lower, such as below 200 °C.

[0006] W02008/125745 discloses an aqueous solution comprising a component that releases gaseous ammonia at temperatures below 200 °C and a polyfunctional additive of which the HLB (hydrophilic/lipophilic balance) varies between 7 and 17 to reduce the formation of cyanuric acid solid by-products. The polyfunctional additives are in particular polyalcoxylated fatty alcohols and esters thereof.

[0007] EP2488283 discloses polyalcoxylated fatty alcohols as additive for urea solutions to reduce the formation of solid by-products.

[0008] WO2011/046491 discloses the use of compounds according to the formula C _x +i H2x+3(C2H4O) _y OH as additive to urea solutions for catalytic converters in exhaust after-treatment. The additive is used to reduce the formation of solid byproducts.

[0009] WO2018/178592 discloses an aqueous composition comprising at least a NO _X reducing agent or a precursor thereof, at least one paraffin and at least one additive chosen from mono- or polyalkylene glycol hydrocarbyl ethers, polyol hydrocarbyl ethers, mono- or polyalkylene glycol fatty acid esters, mono- or polyglycerol fatty acid esters, and the mixtures of these compounds. The additives have for purpose to reduce the formation of foam when the aqueous solution is introduced in the exhaust piping. A limited number of examples are given. One such example is the combination of a fatty alcohol polyethylene glycol ether and a C22-32 paraffin, wherein the presence of the paraffin seems essential for the reduction of foam formation.

[0010] It has however been noticed that the abovementioned NO _X reducing compositions still form solid by-products. Further, certain known additives are considered toxic, have a limited degree of dissolution in, for example, electrolytes, and/or have a limited wettability. A limited wettability of the additive leads to a higher surface tension of the NO _X reducing composition. It has been noticed that such a higher surface tension increases the size of the droplets that are formed when the NO _X reducing composition is sprayed, e.g. via a nozzle, into the flow of exhaust gases. Such larger droplets are known to decrease the efficiency of the catalytic NO _X reduction of the exhaust gases. Summary of the invention

[0011] The present invention aims to overcome one or more of the above drawbacks. It is an aim of the invention to provide a harmless additive for a NO _X reducing solution, said additive contributing to a reduction of solid by-products formed by the NO _X reducing solution, in particular in the treatment of exhaust gases.

[0012] It is a further aim of the present invention to provide a solid byproduct reducing additive for a NO _X reducing solution which is easy to handle - pouring, adding, transporting, and storing for example - in particular upon preparing a NO _X reducing solution comprising the additive, compared to additives of the art.

[0013] The invention further aims to provide a NO _X reducing solution comprising such an additive, wherein the NO _X reducing solution reduces more efficiently the NO _X in exhaust gases. It is a further aim to provide a NO _X reducing solution that forms less solid by-products during the treatment of exhaust gases. It is a further aim to provide a NO _X reducing solution that allows easier handling and use in the treatment of exhaust gases, compared to NO _X reducing solutions of the art.

[0014] According to a first aspect of the invention, there is therefore provided a solid-by product reducing additive for a NO _X reducing solution as set out in the appended claims. The additive is tripropyleneglycol-n-butylether. Preferably, the tripropyleneglycol-n-butylether is selected from the group consisting of tri(1 ,2- propyleneglycol)-n-butylether, tri(1 ,3-propyleneglycol)-n-butylether, and mixtures thereof.

[0015] Advantageously, the solid by-product reducing additive is also an anti-foaming additive for a NO _X reducing solution. Advantageously, the solid by-product reducing additive is capable of reducing the formation of foam in a NO _X reducing solution, for example upon preparation thereof.

[0016] According to a second aspect of the invention, there is provided a

NO _X reducing solution as set out in the appended claims. The NO _X reducing solution comprises a solid by-product reducing additive according to the first aspect of the invention. Advantageously, the NO _X reducing solution comprises a NO _X reducing agent. An example of such a NO _X reducing agent is urea.

[0017] Advantageously, the NO _X reducing solution comprises between 0.01

% by volume and 5 % by volume of the additive, based on the total volume of the NO _X reducing solution, preferably between 0.1 % by volume and 1.0 % by volume.

[0018] According to a third aspect of the invention, there is provided the use of a NO _X reducing solution according to the second aspect of the invention as set out in the appended claims. [0019] A use of the additive and of a NO _X reducing solution as described herein comprises the catalytic conversion of exhaust gases, i.e. the NO _X reducing solution is used in the catalytic conversion of exhaust gases. Advantageously, the catalytic conversion of exhaust gases comprises or substantially consists of the catalytic reduction of NO _X of exhaust gases. Advantageously, the exhaust gases are exhaust gases of an engine, in particular a diesel engine.

[0020] Advantages of the solid by-product reducing additive of the present invention, compared to prior art additives, are harmlessness, an easier solubility in electrolytes and an improved wettability.. An improved wettability of the additive leads to a lower surface tension of the NO _X reducing solution. The inventors have surprisingly discovered that the additive of the present invention contributes to the formation of smaller droplets when NO _X reducing solutions, comprising the additive according to the invention, are sprayed in exhaust gases compared to prior art NO _X reducing solutions, sprayed in exhaust gases under the same conditions and using the same nozzle. It was also noticed that smaller droplets of NO _X reducing solution improve the miscibility of the NO _X reducing solution in the exhaust gases. An improved miscibility has shown to improve the efficiency of the reduction treatment of the exhaust gases.

[0021] A further advantage of the solid by-product reducing additive of the present invention is that it reduces the formation of solid by-products by the NO _X reducing solution during treatment of NO _X comprising exhaust gases. In particular cases no solid by-products were formed. Consequently, a reduction of solid by-products reduces significantly the risk of clogging or blocking of the piping and tubes of the exhaust system of an engine, leading to an improved performance of the engine. This further leads to a decreased need for maintenance of the exhaust system. Hence, for the end consumer, this also reduces the maintenance cost, and provides thus an economic advantage as well.

[0022] Yet a further advantage of the solid by-product reducing additive of the present invention is that, upon preparing the NO _X reducing solution comprising the solid by-product reducing additive, any formation of bubbles or foam is largely reduced, when compared to existing additives being added to a same solution. In certain cases substantially no bubbles or foam is formed. In particular, no further additives to keep the formation of foam or bubbles under control, e.g. to a minimal level, are required to obtain a NO _X reducing solution that is easy to handle. Description of embodiments

[0023] According to an aspect of the present invention, a solid by-product reducing additive for a NO _X reducing solution is provided, wherein the additive is tripropyleneglycol-n-butylether (abbreviated as TPnB), which is known as a mixture of isomers with CAS number 55934-93-5.

[0024] Preferably, the tripropyleneglycol-n-butylether (TPnB) is selected from the group consisting of tri(1 ,2-propyleneglycol)-n-butylether, tri(1 ,3- propyleneglycol)-n-butylether, and mixtures thereof.

[0025] By tri(1 ,2-propyleneglycol)-n-butylether, the present invention means one isomer or a mixture of at least two isomers of tri(1 ,2-propyleneglycol)-n- butylether.

[0026] Tripropyleneglycol-n-butylether (TPnB, CAS number 55934-93-5) is known to be of low toxicity. For example, TPnB is designated as a low-priority substance in view of its toxicity potential by the EPA (US Environmental Protection Agency), considered as not PBT/vPBT (persistent, bioaccumulative and toxic/ very persistent, bioaccumulative and toxic) by the ECHA (European Chemicals Agency), and is not considered as presenting a hazard in view of the Classification, Labelling and Packaging (CLP) Regulation ((EC No 1272/2008)).

[0027] According to a further aspect of the invention, a NO _X reducing solution comprising a solid by-product reducing additive according to the first aspect is provided. Advantageously, the NO _X reducing solution further comprises a NO _X reducing agent. Preferably, the NO _X reducing agent is urea, but other compounds can also be used.

[0028] The NO _X reducing solution can further comprise water, preferably demineralised water, i.e. the NO _X reducing solution can be an aqueous solution or an aqueous composition.

[0029] Advantageously, the NO _X reducing solution comprises between 0.01

% by volume and 5 % by volume of the additive, based on the total volume of the NO _X reducing solution, preferably between 0.05 % by volume and 2.5 % by volume, more preferably between 0.1 % by volume and 1.0 % by volume, for example between 0.2 % by volume and 0.5 % by volume.

[0030] Advantageously, the NO _X reducing solution comprises between 20

% by volume and 50 % by volume of the NO _X reducing agent, based on the total volume of the NO _X reducing solution, such as between 25 % by volume and 40 % by volume, preferably between 30 % by volume and 35 % by volume, more preferably between 31 % by volume and 34 % by volume, such as between 31.5 % by volume and 33.5 % by volume, for example 31 .8 % by volume, 32.5 % by volume or 33.3 % by volume.

[0031] The inventors have surprisingly discovered that when preparing the

NO _X reducing solution, the additive of the present disclosure does not lead to the formation of bubbles or foam within the NO _X reducing solution. This was in particular noticed upon adding the additive to the NO _X reducing solution. In other words, the solid by-product reducing additive of the present invention comprise anti-foaming properties, i.e. they are anti-foaming. They allow easy handling and addition to the NO _X reducing solution. Further, they contribute to an easy handling of the NO _X reducing solution.

[0032] Advantageously, the NO _X reducing solution comprising the solid byproduct reducing additive does not require the addition of further foam-formation reducing additives or anti-foaming additives. Advantageously, and in particular, the NO _X reducing solution does not comprise paraffin. In the light of the present invention, “not comprising paraffin” means that the amount of paraffin present in the NO _X reducing solution is below the detection limit of analysis techniques that can be used to determine the amount of paraffin, such as liquid chromatography (LC), optionally combined with mass spectroscopy (LC-MS) or Fourier transform infrared spectroscopy (FTIR).

[0033] According to a third aspect of the invention, there is provided the use of a NO _X reducing solution, respectively, for the (post-)treatment of exhaust gases. Advantageously, the exhaust gases are NO _X comprising exhaust gases. The exhaust gases can be engine exhaust gases, such as from a diesel engine.

[0034] Advantageously, the (post-)treatment of exhaust gases comprises the catalytic conversion of exhaust gases, in particular the catalytic reduction of NO _X of exhaust gases. Advantageously, the catalytic conversion of exhaust gases comprises selective catalytic reduction (SCR).

Examples

Example 1

[0035] An aqueous NO _X reducing composition comprising 32.5 % by volume of urea and 67.5 vol% of demineralized water was prepared. To this aqueous NO _X reducing composition different amounts (volumes) of additives were added according to Table 1 , and the surface tension of the obtained NO _X reducing compositions was measured, as well as the surface tension of the aqueous NO _X reducing composition without any additive. [0036] As reference additive dipropyleneglycol-methylether (DPM; CAS

34590-94-8) was added in different % by volume. 0.2 % by volume of tripropyleneglycol- n-butylether was tested as inventive additive.

Table 1 : surface tension for various additives

[0037] From Table 1 it is clear that for similar % by volume of the reference additive and the inventive additive, the surface tension with the inventive additive is clearly lower than for the reference additive. The surface tension for 0.2 vol.% tripropyleneglycol-n-butylether is even lower than for a solution comprising 1.0 vol.% of the reference additive.

Example 2

[0038] The capability of reducing the formation of solid by-products by an

NOx reducing solution was tested by simulating the conditions in which the NO _X reducing solution of example 1 is used during treatment of exhaust gases.

[0039] As reference additive isotridecanol, ethoxylated (CAS 69011-36-5) was used. Tripropyleneglycol-n-butylether (CAS 55934-93-5), also a mixture or isomers, was tested as additive according to the invention.

[0040] 5.0 g of each additive was placed in a Petri dish, which were then placed in an oven with forced circulation at 200 °C. The oven was placed in a fume hood. [0041] The mass reduction of each additive was measured by weighting the residual additive after a certain duration in the oven, and calculating the weight reduction.

[0042] The inventive additive tripropyleneglycol-n-butylether had disappeared after 20 minutes in the oven, i.e. a weight reduction of 100 %. However, the reference additive isotridecanol, ethoxylated showed a weight reduction of only 41 % after 120 minutes.

[0043] From this, it is clear that the additives according to the invention are transferred completely and faster in the NO _X environment than existing additives. Consequently, the inventive additives can intervene faster as solid by-product reducing reagent compared to existing additives, leading to a significant reduction of any solid byproducts formed.

Example 3

[0044] The capability of reducing the formation of solid by-products by an

NOx reducing solution was further tested by heating NO _X reducing solutions at different temperatures and evaluating the formation, if any, of solid by-products.

[0045] The NO _X reducing solution of example 1 was used, with and without additives. Reference solution 1 did not comprise any additives. Reference solution 2 comprised 1 vol.% propyleneglycol methyl ether. The inventive solution comprised 1 vol.% tripropyleneglycol-n-butylether.

[0046] Each solution was heated at 250 °C and at 350 °C for up to 48 hours, and the formation of any solid deposits was visually evaluated. Table 2 summarizes the results.

Table 2: visual inspection of solid by-product formation

[0047] From Table 2 it is clear that without any additive, large white deposits were formed already after 24 hours at both 250 °C and 350 °C. The deposits had a size in the order of a few cm. With the reference additive, deposit was still formed already after 24 hours at both 250 °C and 350 °C, but it was less and smaller in size. However with the additive of the present invention, no significant or no deposit was visually noticed, not even after 48 hours at 250 °C and 350 °C.

Example 4

[0048] The formation of foam upon adding the additive to water was tested.

8 gram of additive was added to 192 g demineralised water, thereby obtaining a water- additive mixture having a total weight of 200 g. The water-additive mixture was then stirred in a 500 ml measuring cup at 1000 rpm for 1 minute using a I KA™ EUROSTAR 60 Control Overhead Stirrer with a blade diameter of 45 mm. Immediately afterwards, the stirred mixture was transferred to a calibrated measuring cylinder (Dispolab) and then left unstirred.

[0049] The amount of foam formed was visually inspected by noting the height of foam (expressed in ml) in the measuring cylinder directly after transfer of the mixed mixture (time = 0), and after 10 minutes leaving unstirred (time = 10 min).

[0050] As a first reference additive, a commercial product comprising between 2.5 and 10 % by weight n-pentane and less than 2.5 % by weight ethanol, based on the total weight of the additive, was used. As a second reference additive, propyleneglycol methyl ether was used. Tripropyleneglycol-n-butylether was used as the inventive additive. The results are presented in Table 3.

Table 3: foam formation results

[0051] It was noticed that at time = 0, no foam was formed by using tripropyleneglycol-n-butylether, while with the reference additives significant amounts of foam had formed. Even after 10 minutes leaving unstirred, some foam was still remaining in the measuring cylinder for the mixtures with the reference additives. This clearly demonstrates the reduction of foam formation and easier handling.

[0052] The same test was repeated by adding the additive to the

NO _x reducing solution of example 1 . Similar results were obtained.

Example 5

[0053] The formation of foam upon handling the NO _X reducing solution was also tested. 200 ml of a NO _X reducing solution of example 1 was stirred in a 500 ml measuring cup for 1 minute at 1000 rpm using a I KA™ EUROSTAR 60 Control Overhead Stirrer with a blade diameter of 45 mm. Immediately afterwards, the stirred mixture was transferred to a calibrated measuring cylinder (Dispolab) and then left unstirred. The amount of foam formed was visually inspected by inspecting the height or volume of foam in the measuring cylinder over time upon leaving the mixture unstirred. [0054] Three solutions were tested. Reference solution 1 did not comprise any additive. Reference solution 2 comprised 1 vol. % propyleneglycol methyl ether. The inventive solution comprised 1 vol. % tripropyleneglycol-n-butylether.

[0055] Directly after transfer to the measuring cylinder, reference solution 1 had at least 5 times more foam than the inventive solution. Reference solution 2 had at least 3 times more foam than the inventive solution.

[0056] After leaving unstirred for 5 minutes, the reference solutions still clearly showed the presence of foam, whereas the foam of the inventive solution had almost completely disappeared. [0057] After leaving unstirred for 10 minutes, the reference solutions still showed the presence of some foam, whereas no foam was visible in the inventive solution.

[0058] This clearly demonstrates the anti-foaming effect of the additive of the present disclosure.