WO2010108677A2

WO2010108677A2 - Ozone assisted mist control unit

Info

Publication number: WO2010108677A2
Application number: PCT/EP2010/001868
Authority: WO
Inventors: Finn Petersen
Original assignee: Haldor Topsøe A/S
Priority date: 2009-03-26
Filing date: 2010-03-25
Publication date: 2010-09-30
Also published as: WO2010108677A3

Abstract

A process for the production of particles in form of an oxide compound of a size of 0.1 to 1000 nm. A chemical compound including for instance silicon is chemically oxidized into oxide particles by means of ozone.

Description

Title: Ozone Assisted Mist Control Unit

The invention • relates to a process for the production of particles of a size of 0.1 to 1000 nm (nanometre).

The particles are produced from chemical compounds being oxidized into oxides by an oxidizing chemical compound, optionally during heating. As a result, stable oxides are produced within said particle sizes.

Hitherto known techniques for generating particles primarily employ thermal oxidation of chemical compounds by way of electrical heating, an open flame or laser, cf. for instance US Application No. 2008011876 (Al) . The US applica- tion involves the use of an electrical oven.

The expression thermal oxidation is here to be interpreted as a heating to such high temperatures that the chemical compound reacts with the surrounding oxygen while forming oxide.

Although possible to induce the thermal decomposition of for instance silicone oil vapour in a electrically heated air stream, the temperatures required are as high as 700°C, thus requiring a high electrical input.

Hence, a common feature of these processes is that they require rather high temperatures to cause an oxidation. Therefore, these processes are encumbered with various draw-backs, viz. they involve heavy costs involved in building the plant, a high consumption of energy per pro- duced amount of particles, difficult choices of material, as well as a possible fire risk.

A majority of these thermal oxidation processes present such a complex structure that they are unsuited for a stationary surveillance of the operation for a long period of time .

WO-A-9904441 describes an apparatus and a method for pro- ducing vanadium oxide nano-particles having size in the range of 5 to 1000 nm for use in batteries. The method comprises pyrolysis of a molecular stream including the vanadium precursor in an oxidizing or inert atmosphere, the oxidizing atmosphere comprising oxygen, ozone, carbon mon- oxide, carbon dioxide or combinations thereof. The method includes the use of a radiation absorbing gas in a reaction chamber in which the pyrolysis is driven with heat absorbed from a laser beam.

WO-A-2009032654 describes an apparatus and a method for producing photoactive films through the production of nano- particles in a flame aerosol reactor. Vaporized fuel and vaporized oxidizer are combusted in a burner to form a flame. Oxidizers include air, ozone, oxygen, fluorine, sul- phur, chlorine, bromine and iodine. Oxygen is preferred when films comprising oxidized metal nano-particles are required. A metal precursor is fed to the burner. Thus, metal species-based nano-particles are formed in the combustion zone and are deposited as film on a substrate surface. The metal encompasses a wide range of alternatives. It is disclosed that a predominant metal compound such as tin or silicon is used together with less than about 10 mol % of another metal to provide doped particles with unusual optical, magnetic or electrical properties.

In our patent US 5,198,206 we disclose a process for pro- ducing sulphuric acid from gases containing sulphur dioxide such as off-gases from power stations. The sulphur dioxide is first converted to sulphur trioxide and thereafter condensed in the presence of excess steam into sulphuric acid in a Wet-Sulphuric Acid-condenser (WSA) . A substantial de- crease of the amount of minute droplets of sulphuric acid, so-called acid mist, escaping to the surroundings is obtained when minute nucleation cores are incorporated into the gas. The nucleation cores are generated by combusting in a burner together with a hydrocarbon fuel a silicone- containing component, in such a manner that the smoke from the burner contains particles of SiO₂. The SiO₂ particles are formed as a result of the high flame temperatures. Normally 50 to 100 nm silica particles are produced in one or more mist control units (MCU) by decomposition of silicone oil vapour in a gas burner and added to the main process gas flow.

In general, the current MCU technology in sulphuric acid plants, such as plants using WSA technology, works satisfactorily. However, the presence of gas-fired burners com- plicates the implementation since a significant amount of documentation is required to fulfil the local and often strict legislation for burners, particularly in refineries requiring WSA technology, thus reducing significantly the commercial applicability of this technique. Since a gas burner is also quite complicated, it is also desired to provide a less expensive and more robust particle generator (MCU) process which is not dependent on the fuel gas available.

Therefore we provide a process for the production of particles which allows oxidation of chemical compounds by way of a heating in combination with a chemical oxidation, alternatively without involving heating, i.e. exclusively by way of chemical oxidation.

The expression chemical oxidation is here to be interpreted as oxidation by means of an added chemical other than air.

The features of the invention are:

1. Process for the production of particles of a size of 0.1 to 1000 nm (nanometre) within a conduit containing a carrying gas, comprising forming an oxide by chemically oxidizing a chemical compound with an oxidizing chemical without the use of burners or lasers, and wherein said oxidizing chemical is ozone.

The carrying gas can be air or any other carrying gas. The conduit is preferably a pipe or conduct within which the chemical compound can be chemically oxidized by ozone, and within which particles can be generated before introduction into a downstream application, such as an aerosol in a gas containing a sulphuric acid vapour.

2. Process according to feature 1, comprising oxidizing the chemical compound by the combination of heating and oxidation by said oxidizing chemical. The heating enables an increase of the reaction speed.

3. Process according to any of features 1 and 2, comprising generating the chemical compound through evaporation in a closed container, preferably a heated container, optionally by way of bubbling through a flow of air or gas.

4. Process according to any of features 1, 2 and 3, comprising adding a flow of air or gas into the conduit. Op- tionally, said flow of air or gas is heated for instance by means of electrical heating members.

As a result the flow of air or gas can assist in producing an increased total flow and an improved adjustment of the particle size as well as of the amount of particles in the resulting flow of air or gas.

5. Process according to any of features 1 to 4, wherein the chemical compound is a silicon-containing component se- lected from the group consisting of silicone oils, methyl- triethoxysilane (MTES) and tetraethyl orthosilicates (TEOS) .

This enables the oxide-forming portion to be silicon with the effect that the particles produced present the chemical structure SiO₂ which is a completely harmless substance, yet without any combustion in air or laser treatment.

6. Process according to any of features 1 to 5, compris- ing introducing the chemical compound in the form of a liquid into the flow of carrying gas. 7. Process according to any of features 1 to 6, comprising introducing ozone by way of evaporation from a closed container, preferably a heated container, optionally by way of a bubbling through of a flow of air or gas.

8. Process according to any of features 1 to 7, comprising adding ozone in the form of a liquid into the conduit.

9. Process according to any of features 1 to 8 wherein the particles are prepared by: (a) forming a silicon/air mixture by mixing the silicon-containing component with air; forming an ozone/air mixture by mixing ozone and air; and subsequently forming a single stream by mixing the silicon/air mixture with the ozone/air mixture and then in- jecting the single stream into the carrying gas in the conduit, or (b) forming a silicon/air mixture by mixing the silicon-containing component with air; forming an ozone/air mixture by mixing ozone and air, and injecting each mixture separately, but at the same point into the carrying gas, or (c) forming a silicon/air mixture by mixing the silicon- containing component with air, forming an ozone/air mixture by mixing ozone and air, and injecting each mixture separately and at different points into the carrying gas.

Accordingly, it would be understood that ozone is the active oxidizing chemical, but still it can be used together with another gas, such as air. Likewise, the chemical compound to be oxidized can also be used together with a gas, such as air. The separate points are preferably the intake of a blower, which also provides for the carrying gas in the conduit, and a point downstream said blower.

By adding the silicone/air mixture and ozone/air mixture as described in (a) and (b) and a point downstream the intake of a blower, a higher number of particles is generated. Hence, preferably the injection of the single stream or injection of each mixture separately is conducted with a 1 to 10 mm, more preferably 2 to 5 mm, most preferably 2 mm co- current nozzle. The resulting concentration of particles in the conduit is on average about 25% higher when adding the silicone/air and ozone/air mixtures downstream the intake of the blower.

The chemical oxidation according to the invention renders it possible to carry out the above process at significantly lower temperatures than by way of a thermal oxidation with the effect that the thermal part of the particle generation process can be completely omitted, if desired, or be manufactured at comparatively reduced costs with respect to the material, insulation as well as heating source.

The particle generation process of the invention has sig- nificantly lower energy consumption per generated amount of particles; it is basically maintenance-free and needs in specific embodiments only electricity for stationary operation during prolonged periods without surveillance or service.

In addition, the particle generation process according to the invention constitutes a much lower fire hazard than the known thermal oxidation techniques due to the reduced temperatures in the oxidation zone, i.e. within the conduit carrying a gas into which the chemical component and ozone are added.

More specifically, we have found i.a. that not only is the reaction temperature without ozone injection higher, i.e. 630 to 65O⁰C compared to when adding ozone, about 340°C, but also that twice as many particles are now generated when using ozone. In particular, when using silicone oils, we find that particle production begins at 275°C and reaches a maximum at 390 to 410⁰C. The invention encompasses also these features, feature 10 and 11, as recited below:

10. Process according to any of features 5 to 9 wherein the silicon-containing component is one or more silicone oils and the reaction temperature is 390 to 410⁰C.

Particular suitable silicone oils include one or more of octamethylcyclotetrasiloxane (Abil K4 , CAS nr . 556-67-2), hexamethyldisiloxane (Dow Corning 200, CAS nr. 107-46-0), decamethylcyclopentasiloxane (component 1, Dow Corning 245, CAS nr. 541-02-6) and dimethylcyclosiloxanes (component 2, Dow Corning 245), decamethylcyclopentasiloxane (Dow Corning 246, CAS nr. 541-02-6), decamethyltetrasiloxane (component 1, Dow Corning 2-1184, CAS nr .141-62-8 ), octamethyltrisi- loxane (component 2, Dow Corning 2-1184, CAS nr. 107-51-7), dodecamethylpentasiloxane (component 3, Dow Corning 2-1184, CAS nr. 141-63-9), polydimethylsiloxane (component 4, Dow Corning 2-1184, CAS nr. 63148-62-9), decamethylcyclopenta- siloxane (GF silicone, CAS nr. 541-02-6), and combinations thereof.

11. Process according to feature 10, wherein the silicone oil is octamethylcyclotetrasiloxane (Abil K4 , CAS nr. 556-

67-2), hexamethyldisiloxane (Dow Corning 200, CAS nr. 107- 46-0), or combinations thereof.

We have found that with these particular silicone oils, the amount of particles generated at about 400⁰C is about twice as high as when for instance using decamethylcyclopentasi- loxane (GF silicone, CAS nr. 541-02-6) or decamethylcy- clopentasiloxane (component 1, Dow Corning 245, CAS nr. 541-02-6) and dimethylcyclosiloxanes (component 2, Dow Corning 245) .

Another feature of the invention encompasses the use of Tetraethylorthosilicate (TEOS) and Methyltriethoxysilane (MTES) , as recited below:

12. Process according to any of features 5 to 9, wherein the silicon-containing compound is TEOS, MTES or combinations thereof, and the reaction temperature is 200 to 400⁰C, preferably 280 to 390°C, more preferably 380°C.

We have found that Tetraethylorthosilicate (TEOS) cannot be oxidized thermally at temperatures below 530⁰C, yet by adding ozone the oxidation can be conducted at 38O⁰C with particle production already taking place at 200⁰C and particle production reaching its maximum at 38O⁰C. Methyltriethoxysilane (MTES) cannot be oxidized thermally at temperatures below 600⁰C, yet by adding ozone the oxidation can be con- ducted at 230⁰C with particle production already taking place at 230⁰C and particle production reaching its maximum at 270°C. For TEOS the particle yield per added amount of Si is about the same as for the silicone oil Abil K4 , but about six times lower for MTES.

Further, compared to current flame producing techniques such as those involving the use of burners for generating particles, it is now possible to produce a higher amount of particles at the same reaction temperature. At a reaction temperature of for instance 360°C it is now possible with ozone assisted particle production to produce twice as many particles/injected-silicone-amount as when using a burner. This is particularly advantageous when operating with sul- phuric acid plants, since as described above conventional use of gas-fired burners complicates their implementation i.a. because a significant amount of documentation is required to fulfil the local and often strict legislation for burners .

We provide also therefore yet another feature of the invention, feature 13:

13. A process for producing sulphuric acid comprising con- densing sulphuric acid vapour in a gas by direct or indirect cooling including the step of adding to the gas before said cooling an aerosol containing particles produced according to any of features 1 to 12.

Further features of the invention are recited below: 14. Process according to feature 13 wherein the cooling of the sulphuric ' acid vapour in the gas is conducted in vertical, externally cooled tubes in which the gas flows from the bottom of the tubes in an upward direction counter- currently with an external coolant, or the gas flow from the top of the tubes in downward direction counter- currently with an external coolant, and wherein the external coolant is preferably air.

15. Process according to feature 13 wherein the cooling of the sulphuric acid vapour in the gas is conducted in a packed sulphuric acid tower counter-currently or co- currently with circulating sulphuric acid as coolant.

The invention is described in greater detail below with reference to the accompanying drawings.

Fig. 1 shows a particularly advantageous embodiment of the invention, where the chemical compound is evaporated in a closed and heated container.

Fig. 2 shows a further particularly advantageous embodiment of the invention where the conduit is subjected to heating.

Fig. 3 shows yet another particularly advantageous embodi^¬ ment of the invention, where the process of the above embodiments shown in Figs. 1 and 2 involves admixing of air or gas to the conduit .

Fig. 4 shows yet another particularly advantageous embodiment of the particle generator according to the invention, where the chemical compound 1 is introduced in the form of a liquid.

Fig. 5 shows yet another particularly advantageous embodi- ment of the invention, where the chemical causing the chemical oxidation by way of evaporation is introduced from a closed, heated container.

Fig. 6 shows yet another particularly advantageous embodi- ment of the invention, where the chemical causing the chemical oxidation is introduced in form of a liquid.

Fig. 7 shows temperature profiles when using TEOS without ozone (Fig. 7a) and with ozone (Fig. 7b). X-axis: time in min; Y-axis: heater temperature.

Fig. 1 shows a particularly advantageous embodiment of the process according to the invention, where the chemical compound is evaporated in a closed, optionally heated 13 con- tainer 2 by being subjected to a bubbling through of a flow 1 of air or gas. The evaporated chemical compound/air/gas 3 exits the container and is admixed a flow 6 of air or gas, said flow including ozone generated by the ozone generator 5 under the addition of an air or gas stream 4. The chemi- cal oxidation is carried out inside the conduit or pipe 7 thus resulting in an air or gas stream 12 containing oxide- particles .

Fig. 2 shows a further particularly advantageous embodiment of the invention where the conduit or pipe 7 is subjected to heating by means of electrical heating members 9 in or- der to increase the reaction speed of the chemical oxidation.

Fig. 3 shows yet another particularly advantageous embodi- ment, where the process of the above embodiments shown in Figs. 1 and 2 involves admixing of a flow 10 of air or gas to the pipe 7. The flow 10 of air or gas is optionally heated by means of electrical heating members 14. The flow 10 of air or gas assists in producing a comparatively in- creased total flow and an improved adjustment of the particle size and amount of particles in the resulting flow 12 of air or gas containing the oxide particles.

Fig. 4 shows yet another particularly advantageous embodi- ment of the particle generator according to the invention, where the chemical compound 1, such as silicon oil, is introduced in form of a liquid into the flow 1 of air or gas of the embodiments of Figs. 1, 2 or 3.

Fig. 5 shows yet another particularly advantageous embodiment of the particle generator according to the invention, where the chemical causing the chemical oxidation by way of evaporation is introduced into the embodiments of Figs. 1, 2, 3 or 4 from a closed, optionally heated container 17, through a bubbling through of a flow 15 of air or gas.

Fig. 6 shows yet another particularly advantageous embodiment of the invention, where the chemical causing the chemical oxidation is introduced in form of a liquid 1 into the flow of air or gas in pipe 7 in the embodiments of Figs. 1 to 5. EXAMPLES

Example 1: Burner vs ozone

A silicone-air mixture is led into a burner, and the products are introduced to conduit via the intake of a blower. A silicone/ozone mixture is introduced into the conduit via the intake of a blower and subsequently heated to 36O⁰C. The samples were diluted with dry, filtered air in three stages and the particle concentration was determined by a condensation particle counter (CPC3010 from TSI).

Results: with the gas burner 1.1- 10¹⁶ particles/g Siθ2 are produced whereas with ozone the number is 2.4-10¹⁶ parti- cles/g Siθ2, i.e. at least twice as many particles are produced with ozone and without the use of burner.

Example 2: Silicone oils

Different silicone oils, see table below, are tested. The tests are carried out in such a way that the oil is applied to a cloth in a region of about 5 cm² and is kept about 1 cm from the intake of a blower.

Initially, tests are made without ozone. Subsequently the ozone generator is switched on, 0.5 g/h 03, and the amount of particles is measured again. All tests are conducted at a reaction temperature (temperature within conduit - pipe) of 405⁰C.

The results are shown in the second table below. At 405⁰C it is not possible to produce particles without the presence of ozone. With all oils it is now possible to produce particles at 405⁰C or even at lower temperatures.

The best silicone oils are Abil K4 (Octamethylcyclotetrasi- loxane) and particularly Dow Corning 200 (Hexamethyldisiloxane) ; the latter shows more than twice as many particles as for instance GE Silicones (Decamethylcyclopentasiloxane) and about 14% more particles than the second best oil, Abil K4:

Example 3: Use of TEOS and ozone

TetraEthylOrthoSilicate (TEOS) is used as silicon- containing compound together with ozone. TEOS is introduced to the conduit/pipe via the intake of a blower.

The temperature profile results are shown in Fig. 7. Fig. 7a shows the temperature profile without ozone; the ozone generator is switched on in the period 21:03 to 21:04. Fig. 7b shows the temperature profile with ozone; the ozone generator was switched off in the period 21:32 to 21:33.

It is shown that whereas no particles are generated without ozone, it is now possible to generate particles in the presence of ozone at 28O⁰C, reaching a maximum at 380°C. It is not possible to thermally oxidize TEOS at temperatures below 530⁰C, but with ozone it is possible to oxidize TEOS at 380⁰C. Example 4: Injection points

Experiments are conducted to study the effect of the injection points.

Injection/position point A: via the intake of a blower. Injection/position point B: a downstream blower with a 2 mm co-current nozzle.

All tests are carried out at a reaction temperature of 400⁰C with 0.26 g/h Abil K4 and 0.5 g/h ozone.

The addition of silicon and ozone is conducted by:

(a) forming a silicon/air mixture by mixing the silicon- containing component with air, forming an ozone/air mixture by mixing ozone and air, and subsequently forming a single stream by mixing the silicon/air mixture with the ozone/air mixture and then injecting the single stream into the carrying gas in the conduit, or

(b) forming a silicon/air mixture by mixing the silicon- containing component with air, forming an ozone/air mixture by mixing ozone and air, and injecting each mixture sepa- rately but at the same point into the carrying gas, or

(c) forming a silicon/air mixture by mixing the silicon- containing component with air, forming an ozone/air mixture by mixing ozone and air, and injecting each mixture separately and at different points into the carrying gas.

The results of the table below show that by adding the silicone/air mixture and ozone/air mixture as described in (a) and (b) and at a point downstream the intake of a blower, a higher number of particles is generated. The average concentration of particles in the injection/position points B-B (silicon injection point: B; ozone injection point: B) is 3.5-10⁸, while for the other injection/position points B-A (silicon injection point: B; ozone injection point: A), A-B (silicon injection point: A; ozone injection point: B), A-A (silicon injection point: A; ozone injection point: A) the average particle concentration is 2.8-10⁸. Thus, the resulting concentration of particles in the conduit is on average about 25% higher when adding the silicone/air and ozone/air mixtures downstream the intake of the blower (injection/position point B-B) .

Claims

1. Process for the production of particles of a size of 0.1 to 1000 nm within a conduit containing a carrying gas, comprising forming an oxide by chemically oxidizing a chemical compound with an oxidizing chemical without the use of burners or lasers, and wherein said oxidizing chemical is ozone.

2. Process according to claim 1, comprising oxidizing the chemical compound by the combination of heating and oxidation by said oxidizing chemical.

3. Process according to any of claims 1 and 2, comprising generating the chemical compound through evaporation in a closed container, preferably a heated container.

4. Process according to any of claims 1, 2 and 3, comprising adding a flow of air or gas into the conduit.

5. Process according to any of claims 1 to 4, wherein the chemical compound is a silicon-containing component selected from the group consisting of silicone oils, methyl- triethoxysilane (MTES) and tetraethyl orthosilicates (TEOS) .

6. Process according to any of claims 1 to 5, comprising introducing the chemical compound in the form of a liquid into the flow of carrying gas.

7. Process according to any of claims 1 to 6, comprising introducing ozone by way of evaporation from a closed container, preferably a heated container.

8. Process according to any of claims 1 to 7, comprising adding ozone in the form of a liquid into the conduit.

9. Process according to any of claims 1 to 8 wherein the particles are prepared by: (a) forming a silicon/air mix- ture by mixing the silicon-containing component with air, forming a ozone/air mixture by mixing ozone and air, and subsequently forming a single stream by mixing the silicon/air mixture with the ozone/air mixture and then injecting the single stream into the carrying gas in the conduit, or (b) forming a silicon/air mixture by mixing the silicon- containing component with air, forming an ozone/air mixture by mixing ozone and air, and injecting each mixture separately, but at the same point into the carrying gas, or (c) forming a silicon/air mixture by mixing the silicon- containing component with air, forming an ozone/air mixture by mixing ozone and air, and injecting each mixture separately and at different points into the carrying gas.

10. Process according to any of claims 5 to 9 wherein the silicon-containing component is one or more silicone oils, and the reaction temperature is 390 to 410⁰C.

11. Process according to claim 10, wherein the silicone oil is octamethylcyclotetrasiloxane, hexamethyldisiloxane, or combinations thereof.

12. Process according to any of claims 5 to 9, wherein the silicon-containing compound is TEOS, MTES or combinations thereof, and the reaction temperature is 200 to 400⁰C, preferably 280 to 39O⁰C, more preferably 380⁰C.

13. A process for producing sulphuric acid comprising condensing sulphuric acid vapour in a gas by direct or indirect cooling including the step of adding to the gas before said cooling an aerosol containing particles produced ac- cording to any of claims 1 to 12.

14. Process according to claim 13 wherein the cooling of the sulphuric acid vapour in the gas is conducted in vertical, externally cooled tubes in which the gas flows from the bottom of the tubes in an upward direction counter- currently with an external coolant, or the gas flow from the top of the tubes in downward direction counter- currently with an external coolant, and wherein the external coolant is preferably air.

15. Process according to claim 13 wherein the cooling of the sulphuric acid vapour in the gas is conducted in a packed sulphuric acid tower counter-currently or co- currently with circulating sulphuric acid as coolant.