Process for the selective oxidation of methane

[0001] Despite its natural abundance, only a small portion of the extracted natural gas is used in chemical production. Due to the fact that most natural gas sources are located in remote areas or offshore areas far away from its use, the lack of infrastructure is the biggest barrier to increasing the use of natural gas in the world. At present, natural gas occurring with oil is being re-introduced to increase oil production or is burned. Although the distribution of natural gas takes place through pipelines, this process still requires that mining areas be easily reachable and pipelines installed on easily accessible substrates. This natural gas is stored at 8-30 MPa. Another transport option uses natural gas liquefaction (LNG) at low temperatures (-160 ° C), which requires the use of properly equipped tanker ships. Transporting gas from remote areas is associated with high costs. Due to the large investment and transport costs, there is a great interest in the conversion of natural gas to obtain more interesting products such as liquid oxygen compounds or higher hydrocarbons. The process of the present invention relates to a method for the selective oxidation of methane, preferably to obtain methanol and / or valuable oxygen compounds.

[Background of the invention] [0002] US 4618732 describes a method of direct conversion of natural gas to obtain methanol. This is achieved by reacting natural gas with oxygen or air in the absence of a catalyst in an inert reactor at elevated temperature and pressure. In order to obtain high methanol yield, gaseous reagents, i.e. natural gas and oxygen or air, are first mixed thoroughly. In another aspect, this prior art document describes an apparatus for conducting a method of converting natural gas to methanol. The device includes an inert reactor and means for thoroughly mixing oxygen or air and natural gas before entering the reactor. Means are also provided to ensure that the reactor gases reach the appropriate elevated temperature and pressure. By the term "inert reactor" is meant a reactor whose internal surfaces are produced or coated with a substance that does not have any substantial adverse effect on methanol yield or selectivity. Preferably the reactor is made of stainless steel lined with glass or polytetrafluoroethylene. The reactor pressure used is generally in the range of 1.013-10.133 MPa (10 to 100 atmospheres), more preferably 1013-6.080 MPa (10 to 60 atmospheres), and even more preferably 1.013-5.066 MPa (10 to 50 atmospheres). The temperature used in the reactor is generally in the range 300 ° C to 500 ° C, more preferably 350 ° C to 450 ° C. The contact time of the gases depends to some extent on the temperature, pressure and relative oxygen concentration, but usually the contact time is in the range of 2 to 1000 seconds, preferably 5 to seconds, and more preferably about 10 seconds. According to fig. 5 at 350 ° C the CO and CO2 yields are similar. According to Fig. 6 at 65 atmospheres and 410-430 ° C, the CO to CO2 ratio is about 2.

[0003] US 4982023 describes the synthesis of methanol by single-phase direct partial oxidation of a natural gas or other methane source when the reactor space is filled with inert, refractory inorganic particles. The reactor is a pipe lined with Pyrex material with an internal diameter of 16.5 mm. Both the efficiency and the selectivity of the direct single-phase oxidation of the gas cartridge containing methane and oxygen gas are improved when the unfilled reactor is filled with a low surface solid substance such as sand. The cycles were carried out using a charge in the form of a natural gas containing 95.66 wt. methane. [0004] According to Example 1, a blank tube is used at 68 bar, 360 ° C, 6.4% O2 in the charge and 4 minutes residence time. The conversion is 5.5%, CO selectivity 49.4%, CO2 selectivity 21.8%, methanol selectivity 25.8% and other oxygen compounds 3%.

[0005] According to Example 2, a sand pipe is used, at 68 bar, 400 ° C, 7% O2 in the charge and 4 minutes residence time. The conversion is 5.9%, CO selectivity 40%, CO2 selectivity 21.7%, selectivity methanol 27.2% and other oxygen compounds [0006] WO 00-007718 describes a catalytic composition, optionally placed on an inert substance, characterized by this that includes (i) first metal oxides and / or hydroxides (M1) and (ii) second metal halides (M2) in which M1 and M2 are the same or different, selected from metals belonging to groups IIa, IIb, IVb , VIII, Ib, Va, lanthanides and mixtures thereof. It also relates to the selective conversion of methane using this catalytic composition, in example 7 the reactor is made of quartz.

[0007] Publications US 4918249, GB 1244001, US 5414157 and GB 1398385 also relate to the oxidation of methane using catalysts.

[0008] It has now been found that oxidation of methane to obtain methanol can be carried out in a silica tube, preferably a quartz tube, preferably unfilled. Preferably, the quartz tube is treated with HF.

• the quartz tube treated with the HF water solution is characterized by greater activity and selectivity in the selective oxidation of methane mainly to obtain methanol and carbon monoxide. Only small amounts of formaldehyde and carbon dioxide are produced.

• preferably the quartz tube reactor is unfilled. When filled with quartz particles, the methane conversion is significantly reduced.

• if quartz tubes with smaller diameters are used, the conversion and selectivity are increased so that the surface-to-volume ratio becomes significant.

[Summary of the invention] [0009] The present invention relates to a method for converting methane to obtain methanol according to claim 1.

[0010] The term silica means a composition consisting essentially of silica and not including any component having an adverse effect on the conversion of methane to obtain methanol. Preferably it refers to pure silica according to the customary term used by a person skilled in the art.

[0011] Silica may be amorphous, crystalline or of any structure or it may be quartz. The inner surface can be made partly from some type of silica and partly from some other type of silica. The inner surface may be partially coated with some type of silica and partly with another type of silica. The inner surface may be a combination in part made of some type of silica and partly coated with another or the same type of silica.

[0012] Preferably the inner surface is made of quartz or coated with quartz.

[0013] Preferably the reactor is operated at a pressure of 0.1 to 7.5 MPa. Preferably, the reactor is operated at a temperature from 300 ° C to 600 ° C. Preferably, the reactor is operated at a residence time of 0.1 to 100 seconds. Preferably the reactor operates at a molar ratio of methane to oxygen of 1 to 50.

[0014] Preferably, the reactor operates at a pressure of 0.1 to 7.5 MPa, a temperature of 300 ° C to 600 ° C, a residence time of 0.1 to 100 seconds, and a molar ratio of methane to oxygen of 1 to 50.

[0015] The present invention also relates to the use of a reactor having an internal surface of silica (preferably quartz) or coated silica (preferably quartz) and treated with HF. The inner surface of the reactor may be made partly of some type of silica and partly of another type of silica. The inner surface of the reactor may be made partly of some type of silica and partly of another type of silica. The inner surface of the reactor may be coated partly with some type of silica and partly with another type of silica.

[0016] The inner surface of the reactor may be a combination of a part made of some type of silica and a part coated with another or the same type of silica.

[Description of the figures] [0017]

Figure 1 is a drawing of a reactor configuration consisting of multiple quartz tubes placed in a larger reactor tank.

Figure 2 is a drawing of a reactor configuration consisting of multiple quartz tubes placed in a larger reactor tank with countercurrent flow directions.

Figure 3 is a drawing of a reactor configuration consisting of multiple plates made of quartz or any other suitable quartz-coated material placed in a larger reactor tank.

Figure 4 is a drawing of a reactor configuration consisting of multiple plates made of quartz or any other suitable quartz-coated material placed in a larger reactor vessel. The plates are arranged in such a way that heat exchange between the cold inlet gas and the hot exhaust gas can be used.

[Detailed description of the invention] [0018] Although the reactor can be made of any type of silica or coated with silica, the following reactor description focuses on quartz as an example. This is only to illustrate the present invention without limiting its scope.

[0019] The reactor may consist of a tubular quartz reactor, which is in principle simple. The pipe may consist of ordinary quartz or any other suitable material coated with quartz and treated with an acidic HF solution.

The industrial reactor may consist of a plurality of pipes of a certain diameter arranged parallel to each other in a large reactor tank, so that a multi-tube reactor with a defined surface to volume ratio is obtained. The internal diameter of the individual quartz tubes may be from 0.1 to 1000 mm, preferably from 1 to 100 mm, and most preferably from 2 to 10 mm. The length of the reactor pipes is such that the desired diameter and the desired residence time can be used. The diameter is the most important of the operating parameters and you can control the residence time by adjusting the length of the pipe. The wall thickness is such that the mechanical strength of the pipe is sufficient to allow manipulation and placing it in a commercially available reactor tank.

[0020] The reactants are introduced from one side of the reactor tubes and the reaction products leave it from the opposite side. The flow direction can be directed in any direction, from top to bottom, from below up or even horizontally (see figure 1). The reactor pipes can also be placed in parallel and connected to the bottom or top with the collector, so that only reagents can flow in one direction inside the pipe and in the other direction outside the pipe. The reagent inlet is therefore located transversely in the reactor vessel. This configuration allows heating of the reagent mixture flowing outside the pipe by exchanging heat through the pipe wall with the hot reaction product flowing inside the reactor pipe. The reagent flow direction can also be reversed: cold reagent flowing inside and hot reaction product outside the pipes (see figure 2). In yet another reactor configuration, the reactor consists of a plurality of plates, made of quartz or some other suitable material coated with a layer of quartz and treated with an HF acid solution. The distance between the plates is such that an optimal ratio of surface to volume and residence time is obtained (see figure 3). Also in this case, the plates can be configured in such a way that heat exchange can be used between cold inlet gas and hot outlet gas (see figure 4).

[0021] According to a specific embodiment, the reactor is a microreactor as described in the prior art but coated with silica, preferably quartz.

[0022] Regarding quartz, a person skilled in the art can easily make a choice from materials available on the market by routine experimentation (see example 1).

Similar experiments can be carried out using quartz-coated surfaces. Oxidation of methane is highly improved when the inner surface, made of quartz or coated with quartz, is treated with HF.

[0023] As for the treatment with HF, it is carried out using an aqueous HF solution. Preferably, this aqueous HF solution contains from 0.1 to 25 wt. HF, more preferably from 3 to 20%, and most preferably from 5 to 10%. The aqueous solution may also contain other acids that have complexing properties that can remove metals from the surface. They are sulfuric acid, nitric acid and phosphoric acid or phosphonic acid. They can be present in a concentration of 0.1 to 25% by weight, preferably 3 to 20%, each. The duration of the treatment may be from 0.01 to 5 hours, most preferably from 0.05 to 2 hours. The processing temperature may be from 0 to 100 ° C, most preferably from 10 to 50 ° C.

A typical treatment in terms of the concentration of individual acids is as follows:

<td>Acid solution</td><td>quantity</td><td>active acid concentration</td>

<td>HF at 25%</td><td> 20%</td><td> 5%</td>

<td>H₂PHO₃ phosphonic acid at 75%</td><td> 10%</td><td> 7.5%</td>

<td>H₃AFTER₄ phosphoric acid at 60%</td><td> 10%</td><td> 6%</td>

This treatment is carried out in a polypropylene tank for 10 minutes at room temperature.

HF treatment improves methane conversion and improves the selectivity of oxygenates. It also increases the methanol / formaldehyde ratio.

[0024] As for methane, it may be a natural gas or any gas containing mostly methane. It does not depart from the scope of the invention that there is more hydrocarbons besides methane.

[0025] The molar ratio of methane to oxygen is preferably from 1 to 50, more preferably from 2 to 20, and most preferably from 3 to 10. The reactor can be filled with gaseous air or oxygen or gaseous air enriched with oxygen.

[0026] The residence time in the reactor at the desired reaction temperature and pressure is preferably from 0.1 to 100 seconds, more preferably from 1 to 75 seconds, and most preferably from 2 to 20 seconds. [0027] The reaction pressure is preferably from 1 to 75 bar, more preferably from 2 to 50 bar, and most preferably from 4 to 25 bar.

[0028] The temperature is preferably between 300 ° C to 600 ° C and more preferably between 400 and 450 ° C.

[0029] The operating conditions can be any combination of different ranges of the above parameters.

[0030] As the temperature increases, the methane conversion increases, but the selectivity in methanol decreases.

[0031] The reaction is carried out in the absence of any added substance in this reaction zone that would measurably affect the reaction rate or product yield. For example, when the reaction is carried out in the inner part of the pipe, the pipe is preferably unfilled.

[0032] The method of the present invention produces more CO than CO2. CO still has some value and can be used to produce hydrogen in the reaction of converting carbon monoxide with steam:

CO + H2O CO2 + H2

CO can also be included in the conventional methanol synthesis process. When catalytic steam conversion of methane is used, the synthesis gas has an SN = (H2-CO2 / (CO + CO2) ratio close to 3 or an H2 / CO ratio of 3 or more. Methanol synthesis only requires an SN ratio slightly above 2. By adding CO, produced as a result of the selective oxidation of methane to methanol, to the synthesis gas derived from the catalytic conversion of methane with steam, more methanol can be obtained. In addition, strongly exothermic selective methane oxidation using oxygen can be integrated with endothermic catalytic steam conversion of methane.

[Examples] [0033] A partial methane oxidation reaction (PMO) was carried out in a continuous flow reactor by sending a mixture of methane and oxygen through a tubular reactor. Conversion and selectivity values ​​are reported for carbon.

Example 1 not according to the invention: comparison of commercially available quartz tubes.

[0034] Different types of quartz tubes with identical diameters (internal diameter 3 mm), obtained from different additions or with different quartz composition (Table 1) were tested for a single phase gas phase reaction.

Table 1: Concentrations of impurities and hydroxyl (in ppm) in various types of quartz tubes obtained from the manufacturer.

<td>Type quartz</td><td>Manufacturer</td><td>IN</td><td>Al</td><td>Ca</td><td>fe</td><td>K</td><td>Li</td><td>mg</td><td>Mn</td><td>On</td><td>ti</td><td>Zr</td><td>OH</td>

<td>GEI</td><td>General Electrics</td><td>Yes</td><td> 15</td><td> 0,5</td><td> 0,3</td><td> 1,5</td><td> 1</td><td> 0,2</td><td> 0,1</td><td> 1,3</td><td> 0,9</td><td> 1,5</td><td> &lt;5</td>

<td>GE2</td><td>General Electrics</td><td>no</td><td> 15</td><td> 0,5</td><td> 0,3</td><td> 1,5</td><td> 1</td><td> 0,2</td><td> 0,1</td><td> 1,3</td><td> 0,9</td><td> 1,5</td><td> 1545</td>

<td>HSQ *</td><td>Heraeus</td><td>no</td><td> -</td><td> 0,21</td><td> 0,10,3</td><td> 0,10,5</td><td> 0,51</td><td>0 0 1</td><td> 0</td><td>0 0 Bovine 1</td><td></td><td> 0</td><td> 30</td>

<td>PH</td><td>Philips</td><td>Yes</td><td> 16</td><td> 0,8</td><td> 0,8</td><td> 0,9</td><td> 0,7</td><td> 0</td><td> 0</td><td> 0,9</td><td> 1,5</td><td> -</td><td> &lt;5</td>

<td>PN</td><td>IImenau</td><td> -</td><td> 15</td><td> 0,8</td><td> 0,3</td><td> 0,9</td><td> 0,7</td><td> -</td><td> -</td><td> 0,9</td><td> 1,4</td><td> 0</td><td> 1545</td>

<td>PS</td><td>Ilnenau</td><td> -</td><td> 8</td><td> 0,2</td><td> 0,4</td><td> 0,3</td><td> 0,4</td><td> -</td><td> -</td><td> 5</td><td> &lt;0,2</td><td> -</td><td> 5-15</td>

<td colspan="14">- unknown * Cr, Cu, As concentrations were below 0.06, 0.02 and 0.002 ppm, respectively.</td>

[0035] The results are shown in Figure 5. The temperature at which some methane conversion or 100% oxygen conversion is achieved clearly depends on the type of quartz tube used. There is no PMO reaction (partial methane oxidation) for General Electrics 1 (GE1) and Philips (PH) quartz tubes at 0.5 MPa and temperatures below 500 ° C, while there is obvious activity in terms of single-phase gas reaction in other quartz tubes . There is already methane conversion of 0.9% at 450 ° C for the Heraeus 300 (HSQ) quartz tube. The order in terms of activity within the various reactor quartz tubes is as follows: HSQ&gt; GE₂ = PS&gt; PN &gt;&gt; PH = GE₁.

As can be seen in Figure 6, the type of quartz used in the reaction tubes has some effect on selectivity. For example, in the HSQ pipe, the C1 oxygen selectivity is 28% at 9.41% CH4 conversion while this value is only 24% at a 8.6% conversion at PS pipes.

The study of the composition of quartz tubes showed some correlation between the activity and selectivity obtained for the PMO reaction in quartz tubes. Table 1 shows various compositions with specific levels of impurities and hydroxyl concentrations in quartz tubes.

There seems to be no obvious trends between the activity / selectivity in the PMO reaction in the quartz tube and the content of quartz impurities in terms of alkali, redox or other substances. However, the hydroxyl concentration in the quartz tubes appears to differ in parallel with the order of activity of the different quartz tubes. Low activity PH and GE1 contain hydroxyls in concentrations below 5 ppm, while more active quartz tubes (e.g. HSQ and GE2) contain substantially higher OH concentrations. Generally, quartz tubes are obtained from molten quartz solidifying around a tungsten rod. This can lead to slight contamination of the quartz tubes with tungsten. The skilled person is aware that these surface contaminants can be removed using an aqueous HF solution. However, not for General Electrics 1 (GE1) quartz tubes and Philips (PH) tubes. These pipes still contain W impurities from the rod on which they were made. Only with these two types of quartz tubes no activity was found at 0.5 MPa at temperatures below 500 ° C.

Never before has this further treatment of quartz with HF been found to affect the chemical reactions occurring in such quartz tubes.

Example 2 not according to the invention: reactor configuration and significance of the unfilled reactor pipe.

[0036] To determine the actual residence time, i.e. the time during which the charge reacts in the hot zone, the quartz tube was filled at different places with quartz granulate. The quartz reactor tube is usually placed in a vertical furnace, divided into 5 different sections, each 6 cm long (Figure 7). Each one of the HSQ quartz tube sections was filled with 6 cm quartz granules (250-500 μm) and tested for PMO reaction at 0.5 MPa (Figure 8). Occasionally, the pipe was also completely filled with quartz granules. [0037] When the quartz granulate was placed in section 2, 3 or 4, this affected methane conversion. The temperature-conversion curve shifted to higher temperatures when the quartz substrate was placed in one of these sections. The placement of quartz granules in sections 1 and 5 did not affect PMO activity. The length of the hot zone in which the single-phase reaction in the gas phase takes place is therefore 18 cm (sum of sections 2, 3 and 4), which corresponds to a reaction volume of 1.27 ml for a quartz tube with an internal diameter of 3 mm.

Under standard conditions, a total flow of 25 ml / min (STP) was used, and therefore the residence time in the hot zone in which the reaction took place was 5.8 seconds at 0.5 MPa and at 500 ° C.

Example 3: effect of treatment using HF and estimation at 0.5 MPa.

[0038] This example shows that treatment using HF improves methane conversion and improves the selectivity of oxygenates. It also increases the methanol / formaldehyde ratio from below 10 without HF treatment to above 15 after treatment with

HF.

HSQ and Philips pipes were treated with a mixture of 20% hydrofluoric acid (HF) together with 10% phosphonic acid (H2PHO3) and 10% phosphoric acid (H3PO4).

Without being bound by theory in this regard, when HF reacts with SiO2, volatile SiF4 (1) is formed, or in excess of aqueous HF, which is usually used in quartz treatment processes, reaction 2. This HF treatment may remove some silicon from quartz surface and create special places (surface defects) that affect the activation of methane or oxygen.

4HF + SiO_;&lt;&gt; SiF,: + H_;(1) O SiO2 + 6HF θ H2SiF6 + 2H2 (2) O [0039] The pipes were subjected to static treatment in a polypropylene tank for 10 minutes. They were then rinsed thoroughly with deionized water and dried at room temperature. They were not roasted at high temperatures, but were immediately used for the reaction.

The conversion of methane-oxygen in quartz tubes with an internal diameter of 3 mm was studied, which corresponded to a reactor volume of 1.27 ml and a residence time of 5.8 seconds at 0.5 MPa and 500 ° C. The results are shown in Figures 9 and 10. Higher methane conversion was found in the PMO reaction for HF treated quartz tubes as compared to untreated quartz tubes. The effect was very significant in the case of Philips pipes that did not show activity at 0.5 MPa and temperatures below 500 ° C, in the absence of treatment with HF. The PMO reaction has already started at 440 ° C in Philips quartz tubes treated with HF (PH + HF) as opposed to untreated Philips tubes (PH).

Figure 10 shows a conversion-selectivity graph for various quartz tubes. Table 2 provides a closer look at the results, and more specifically on product distribution from both HSQ and HSQ + HF pipes. The methanol / formaldehyde ratio increases with increasing methane conversion, e.g. in HSQ pipes from 0.7 at 0.6% XCH4 to 7.7 at 9.4% XCH4 in HSQ pipes.

Table 2: Selectivity of C1 oxygen compounds and methanol / formaldehyde molar ratio for reactions in HSQ and HSQ + HF quartz tubes with comparable methane conversion (0.5 MPa, CH4 / O2 = 9, undiluted, residence time = 5.8 seconds)

<td></td><td></td><td>HSQ</td><td></td><td></td><td></td><td colspan="3">HSQ + HF</td>

<td>^XCH4 (%)</td><td>^SHCHO (%)</td><td>^SCH 3 OH (%)</td><td>CH 3 OH / HCHO</td><td></td><td>^XCH4 (%)</td><td>^SHCHO (%)</td><td>^SCH 3 OH (%)</td><td>CH 3 OH / HCHO</td>

<td> 0,6</td><td> 32</td><td> 22</td><td> 0,7</td><td></td><td> 0,76</td><td> 30</td><td> 28</td><td> 0,9</td>

<td> 2,0</td><td> 13</td><td> 29</td><td> 2,2</td><td></td><td> 2,4</td><td> 8</td><td> 38</td><td> 4,8</td>

<td> 9,4</td><td> 3</td><td> 23</td><td> 7,7</td><td></td><td> 9,6</td><td> 1</td><td> 28</td><td> 28,0</td>

[0040] The methanol / formaldehyde ratio with some XCH4 conversion clearly differs for the reaction in HSQ compared to HSQ + HF pipes. With a methane conversion of about 9.5%, i.e. 100% oxygen conversion, the methanol / formaldehyde ratio is 7.7 and 28, respectively for HSQ and HSQ + HF pipes. In other words, while the total selectivity of the oxygen compound C1 is only slightly different, the methanol selectivity is increased from 23 to 28% when the PMO reaction is carried out in the HSQ + HF reactor quartz tubes.

Example 4: influence of pipe diameter [0041] This example shows that the smaller the inner diameter, the higher the methane conversion at a given reaction temperature and the higher the selectivity of oxygen compounds.

To further investigate the effect of the reactor wall on activity and selectivity and determine whether this effect is beneficial or not, various quartz tubes of the same type (PN) but with different internal diameters were tested. To determine the effect of the reactor wall on gas phase oxidation, the linear velocity or residence time in the hot reaction zone must be identical for all pipes. Thus, the gas flow was adjusted in the 2, 3 and 4 mm pipes until the residence time was 5.8 seconds.

[0042] Figures 11 and 12 show PMO results for untreated and treated PN quartz tubes with HF with different internal diameters (2, 3 and 4 mm). The methanol selectivity and yield at 100% O2 conversion obtained in the three PN pipes was very similar, although lower compared to the non-HF treated PN pipes. However, the increase in methanol yield and selectivity at 100% oxygen conversion due to the treatment of the quartz tube with HF also depends on the diameter of the quartz tube (Table 3). For a 2mm PN tube, the methanol yield increases from 2.1 to 2.8% after HF treatment, while it only increases from 2.1 to 2.5% in a 4mm PN tube.

The temperature at which PMO activity begins and at which 100% O2 conversion is achieved is lower for smaller pipe diameter and HF treatment. The impact is smaller for a pipe with a larger internal diameter. Activity decreases as the diameter increases, which may indicate that there is some positive (catalytic) effect of the wall on methane activation. The selectivity values ​​are very similar. The increase in activity after HF treatment is greater for smaller PN diameter quartz tubes, which again indicates the presence of a favorable (catalytic) wall effect. Table 3: Impact of internal diameter and quartz tube processing using HF on selectivity, efficiency and temperature for 100% oxygen conversion in PMO reaction (0.5 MPa, CH4 / O2 = 9, residence time = 5.8 seconds).

<td rowspan="2">inner diameter x outer diameter (mm)</td><td></td><td></td><td>PN</td><td></td><td></td><td></td><td colspan="3">PN + HF</td>

<td>T₁₀₀^*(° C)</td><td> $ ^XCH4 (%)</td><td>Sci (%)</td><td>YCI (%)</td><td></td><td>T₁₀₀^*(° C)</td><td> $ ^XCH4 (%)</td><td>Sc1 (%)</td><td>YC1 (%)</td>

<td>2x6</td><td> 490</td><td> 9,4</td><td> 22,8</td><td> 2,1</td><td></td><td> 470</td><td> 9,6</td><td> 28,9</td><td> 2,8</td>

<td>3x6</td><td> 490</td><td> 9,2</td><td> 22,7</td><td> 2,1</td><td></td><td> -</td><td></td><td> -</td><td> -</td>

<td>4x6</td><td> 490</td><td> 9,5</td><td> 22,0</td><td> 2,1</td><td></td><td> 480</td><td> 9,9</td><td> 25</td><td> 2,5</td>

<td colspan="10">^* T100 = temperature (° C) for 100% oxygen conversion _$^$ Methane conversion at 100% oxygen conversion SC1 and YC1 stand for the selectivity and yield of one carbon atom oxygen compounds</td>

Example 5: effect of filling the reactor with particles [0043] This example shows that the reactor pipes must be unfilled. Filling the reactor with quartz particles both without and with HF treatment significantly reduces methane conversion. Methane-oxygen conversion was tested in quartz tubes with a 3 mm internal diameter corresponding to a reactor volume of 1.27 ml and a residence time of 5.8 seconds at 0.5 MPa and 500 ° C. These tests were carried out at the same residence time in the reactor of 5.8 seconds by adjusting the flow rate when the reactor was filled with particles. The quartz particles had a degree of filling of 46%.

HSQ tubes treated with HF were filled with quartz granules (250-500μm) over the entire length of the hot reaction zone (see Example 2)

The results are shown in Figure 13. The conversion of CH4 is much lower compared to unfilled HSQ quartz tubes. It seems that a larger surface area obtained by filling promotes the formation of radical-radical endings, which necessitates the use of higher temperatures.

The selectivity for both tubes filled with quartz granules treated with HF and untreated is much lower compared to unfilled quartz tubes. For the unfilled HSQ tube and the HSQ tube filled with quartz granules treated with HF, the C1 selectivity at 1% CH4 conversion was 53 and 29%, respectively. In addition, the CO / CO2 ratio is much lower for HSQ tubes filled with quartz granules compared to unfilled quartz tubes.

Example 6: methanol stability [0044] This example shows what conditions are required to maintain methanol stability under reaction conditions. The methanol-oxygen mixture is subjected to typical methane conversion conditions. The methanol-oxygen conversion was tested using quartz tubes with a 3 mm internal diameter corresponding to a reactor volume of 1.27 ml and a residence time of 5.8 seconds at 0.5 MPa and 500 ° C. The results show that below 475 ° C methanol is hardly oxidized in the presence of excess oxygen.

Methanol stability experiments were carried out by placing in the reactor a mixture of 10% O2 and 5% CH3OH diluted in nitrogen. The results are shown in Table 4 for unfilled PH pipes and HF etched HSQ pipes. Between 425 and 450 ° C, methanol is mainly converted to give formaldehyde and some CO. Above 450 ° C, much more methanol is converted and mainly COx is produced, which is caused by additional HCHO oxidation. However, this does not mean that during the PMO reaction some methanol is converted to HCHO and / or CO / CO2. At high oxygen conversion, less oxygen is available for methanol oxidation and there is competition between the formation of primary products due to methane oxidation and methanol oxidation.

Table 4 also shows that the reactor wall or its treatment with HF has no effect on methanol stability. The difference in methanol selectivity cannot therefore be explained by the difference in methanol oxidation / degradation.

Table 4: Methanol conversion in unfilled PH pipe (above) and HSQ + HF quartz pipes (below) (0.5 MPa, N2 / O2 / CH3OH = 85/10/5, total flow = 25 ml.min^-1).

<td>Temperature (° C)</td><td> 350</td><td> 375</td><td> 400</td><td> 425</td><td> 450</td><td> 475</td><td> 500</td>

<td>XcH3OH,%</td><td> 0,0</td><td> 0,2</td><td> 0,3</td><td> 3</td><td> 4</td><td> 49</td><td> 96</td>

<td>Sco,%</td><td> 0</td><td> 10</td><td> 23</td><td> 25</td><td> 29</td><td> 50</td><td> 67</td>

<td>SCO2,%</td><td> 0</td><td> 0</td><td> 2</td><td> 3</td><td> 2</td><td> 27</td><td> 33</td>

<td>Shcho,%</td><td> 0</td><td> 90</td><td> 75</td><td> 72</td><td> 69</td><td> 23</td><td> 0</td>

<td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Temperature (° C)</td><td> 350</td><td> 375</td><td> 400</td><td> 425</td><td> 450</td><td> 475</td><td> 500</td>

<td>XcH3OH,%</td><td> 0,0</td><td> 0,3</td><td> 1</td><td> 3</td><td> 5</td><td> 52</td><td> 93</td>

<td>Sco,%</td><td> 0</td><td> 12</td><td> 23</td><td> 26</td><td> 22</td><td> 48</td><td> 70</td>

<td>SCO2,%</td><td> 0</td><td> 0</td><td> 3</td><td> 5</td><td> 5</td><td> 30</td><td> 30</td>

<td>Shcho,%</td><td> 0</td><td> 88</td><td> 74</td><td> 69</td><td> 73</td><td> 22</td><td> 0</td>

Example 7: effect of working pressure [0045] This example shows that at higher pressure, methane conversion is higher at lower temperature and the selectivity of oxygen compounds is also higher.

Methane-oxygen conversion was tested in quartz tubes with a 3 mm internal diameter corresponding to a reactor volume of 1.27 ml and a residence time at 500 ° C of 5.8 seconds at 0.5 MPa, 9.3 at 0.8 MPa and 13 , 8 at 1.2 MPa. Figures 14 and 15 show the effect of pressure on the activity and selectivity of PMO in HSQ quartz tubes treated with HF. The methane conversion increases with increasing pressure. Already at 390 ° C some activity is detected.

The selectivity of C1 oxygenates also increases with pressure. The increase in selectivity is greater at a pressure in the range of 0.5-0.8 MPa than in the range of 0.8-1.2 MPa. The maximum yield of C1 oxygenates, which is almost exclusively methanol, obtained at 0.5, 0.8 and 1.2 MPa is 2.8, 3.4 and 3.6%, respectively. Figure 16 shows the effect of working pressure on product yield at 100% oxygen conversion. The methanol yield increases with pressure while the HCHO yield decreases. Thus, the CH3OH / HCHO ratio increases with increasing pressure. The sum of ethylene and ethane yields decreases as the temperature at which 100% oxygen conversion is obtained decreases as the pressure increases. These temperatures are 480, 460, 440 and 430 ° C at 0.3, 0.5, 0.8 and 1.2 MPa, respectively.

[0046] The following table 5 gives the selectivity values ​​obtained at 1.2 MPa. Table 5: Results obtained under the test conditions of Figure 14.

<td>Quartz: HSQ + HF</td><td colspan="6">12 bar CH4 / O2 = 9/1</td>

<td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Reaction temperature ° C</td><td> 390</td><td> 400</td><td> 410</td><td> 420</td><td> 430</td><td> 440</td>

<td>CH conversion₄ %</td><td> 0,30</td><td> 1,40</td><td> 4,60</td><td> 7,64</td><td> 8,95</td><td> 9,94</td>

<td>C% selectivity values</td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>WHAT</td><td> 7,7</td><td> 24,3</td><td> 43,3</td><td> 50,3</td><td> 55</td><td> 58,6</td>

<td>CO2</td><td> 0,3</td><td> 6,7</td><td> 3,1</td><td> 6,4</td><td> 5,4</td><td> 3,8</td>

<td>C2</td><td> 0,0</td><td> 0,0</td><td> 0,0</td><td> 0,0</td><td> 0,7</td><td> 2,1</td>

<td>HCHO</td><td> 57,7</td><td> 26,0</td><td> 14,2</td><td> 7,7</td><td> 4,6</td><td> 3,5</td>

<td>CH 3 OH</td><td> 34,3</td><td> 43</td><td> 39,4</td><td> 35,6</td><td> 34,3</td><td> 32</td>

[0047] These data show that a small amount of CO2 is produced. CO still has some value and can be used to produce hydrogen in a carbon monoxide vapor conversion reaction:

CO + H2O θ CO2 + H2

Example 8: effect of reactor pipe diameter [0048] Table 6 shows the results of the PMO reaction at 0.8 MPa in PN pipes treated with HF with different internal diameters (2 and 4 mm), but at the same residence time of 9.3 seconds in a hot zone. As with 0.5 MPa, a PN pipe treated with a smaller diameter HF showed greater activity and selectivity compared to a PN pipe treated with a 4 mm diameter HF. The maximum methanol yield is 2.8% at 29% selectivity for a 4 mm diameter pipe while it is 3.3% at 33% selectivity for a 2 mm diameter pipe.

Table 6: Influence of internal diameter and HF quartz tube treatment on the selectivity, efficiency and temperature of 100% oxygen conversion in PMO reaction at 0.8 MPa (CH4 / O2 = 9, undiluted, residence time = 9.3 seconds)

<td rowspan="2">Inner diameter x outer diameter (mm)</td><td colspan="4">PN + HF</td>

<td>T100 * (° C)</td><td>XcH4 $ (%)</td><td>Sci (%)</td><td>Yci (%)</td>

<td>2x6</td><td> 450</td><td> 9,7</td><td> 34</td><td> 3,3</td>

<td>4x6</td><td> 460</td><td> 9,6</td><td> 29</td><td> 2,8</td>

<td colspan="5">* T100 = temperature (° C) for 100% oxygen conversion^$ Methane conversion at 100% oxygen conversion SC1 and YC1 means the selectivity and yield of one carbon atom oxygen compounds</td>

Total Research &amp; Technology Feluy Representative:

PL-PAT-2012-89

EP 2 170 792 B1