Direct Partial Oxidation of Methane to Methanol in a Specially

Chapter 18

Direct Partial Oxidation of Methane to Methanol in a Specially Designed Reactor 1,2

1,2,*

1,2

1,2

Qijian Zhang , Dehua He , Xin Zhang , Qiming Zhu , and Shuiliang Yao Downloaded by COLUMBIA UNIV on September 13, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0852.ch018

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1

State Ley Laboratory of C Chemistry and Technology, Department of Chemistry, Tsinghua University, Beijing 100084, China Tsinghua University and ABB Joint Chemistry Laboratory on Greenhouse Gas Control Technology, Beijing 100084, China Research Institute of Innovative Technology for the Earth, Kyoto 619-0292, Japan 1

2

3

Studies on direct gas-phase partial oxidation of methane to methanol were carried out in a specially designed reactor. A comparatively high yield of methanol was obtained over a wide temperature range. When the O in the feed gas was depleted, the reaction would quickly terminate and the secondary reaction of the produced methanol would not take place significantly. The total feed gas flow rate had little effect on methanol yield while the pressure was an important influencing factor. 2

Introduction The direct partial oxidation of methane to methanol was first found possible in die early 20 century (7-J). Since then, it has been sporadically investigated for about 80 years because of the great industrial potential for the use of natural gas. During the 1980's, the investigation was stimulated by the two "energy crises" that occurred in the 1970s and early 1980s. In 1984, Liu et al. (2) and th

280

© 2003 American Chemical Society

In Utilization of Greenhouse Gases; Liu, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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281

Gesser et al. (3) reported high selectivity of methanol (more than 80%) at methane conversion of higher than 8% from the catalytic and non-catalytic oxidation of methane. These promising results inspired more researchers to carry out further studies into this reaction in the following decades. Some quite good and important results have been achieved and several reviews printed (4-9). Over temperature ranges of 400-500°C and pressure ranges of 3.0-5.0 MPa, homogeneous partial oxidation can result in quite good methane conversion and methanol selectivity. Till now, the homogeneous reaction has produced the most promising yields of methanol plus formaldehyde. The yields are at least as good as those reported for catalytic processes (J). Therefore studies on homogeneous reactions have attracted much attention. The effect of many factors, such as temperature, pressure, feed gas ratio, residence time, reactor type and reactor wall material, have been studied, but different researchers reported different results. There is no general agreement on how to control the oxidation of methane to favor the production of methanol, and how the reaction conditions affect the product distribution. Some results and explanations have even been contradictory (6-9). In our previous work, a comparatively high yield of methanol from the partial oxidation of methane was obtained (7). The high yield was attributed to the special structure of the reactor, in which the feed gas was isolated from contact with the metal wall of the stainless steel. In our recent works, the reaction has been further investigated. In this paper, the results are reported and a "one pass" concept is proposed for producing methanol from pipeline natural gas.

Experimental The gas phase partial oxidation of methane was carried out in a specially designed quartz lined stainless steel tubular reactor at 300-500 °C and 1.05.0MPa. The inner quartz line (460 mm long, 6.7 mm i.d.) was tightly fixed in the stainless steel line (Figure 1). A Viton O-ring pressed by a locking nut was used in the top of the quartz line for preventing the gas from leaking into the ringed gap between the quartz line and the stainless steel line. The reactor temperature (outer-wall temperature of the stainless steel) was controlled by a temperature controller with a thermocouple. The reaction temperature (center temperature of the inner quartz line) was measured by a thermocouple inside a Φ 3 stainless steel tube which was covered by a quartz tube (5.5 mm o.d.) with an upper dead end (called shelth tube). The assembly was designed to prevent the contact of the reactants with the metal walls of the stainless steel so as to eliminate the influence of the metal wall on the reaction and thus avoid the deep oxidation caused by the metal surface.


282 The flow rate of each reactant was controlled by a mass controller and the pressure was controlled by a back-pressure regulator. The reaction effluent was kept at about 150 °C to prevent condensation of liquid products. The products were analyzed by an on-line gas chromatography (SfflMADZU GC-8A, TCD, H as carrier gas). 2


L o c k i n g nut O-ring Stainless steel line Quartz line

Thermal couple Inner stainless steel tube Quartz sheath tube

Outlet

Figure L Illustration of the structure of tubular reactor

Results and Discussion In the specially designed reactor, comparatively high yields of methanol (ca 7-8%) were obtained at 430-470 °C (Table 1). At the temperature of 422°C, only 1% CH4 and 20% 0 was converted. When the temperature was increased to 429 2


283 Table 1 Effect of rezction temperatures on the conversion of methane and oxygen and the selectivities of products. Selectivity/%

Conversion /%

CH3OH

Τ/Ό* • o

CH

CO

co

20 100 100 100 100 100 100 100 100

1.0 13.0 13.0 12.7 13.1 12.8 11.7 9.6 7.3

80.1 34.4 33.8 33.7 33.7 34.4 38.3 47.8 65.7

0 5.0 4.4 4.5 4.8 6.4 6.3 4.6 7.1


2

422 429 435 445 455 466 475 485 495

4

2

CH OH

C Hg

Yield/%

19.9 60.7 61.8 61.7 62.3 59.0 57.0 47.0 26.3

0 0.0 0.1 0.2 0.2 0.3 0.4 0.6 0.9

0.2 7.9 8.0 7.8 8.2 7.6 6.6 4.5 1.9

3

2

P=5.0 MPa, CH4/O /N =100/10/10 (mL/min), id-6.5 mm *The thermocouple was placed in the center of the inner quartz line at the point 14 cm longfromthe furnace bottom. 2

e

2

e

C (just 7 C higher), the conversion of oxygen increased quickly to 100% and conversion of methane increased to 13%. Such results showed typical character of a free radical reaction. The very rapid rise in conversions has also been observed by many other researchers (5-77). It has also been observed by other researchers that a further increase in reactor temperature, generally results in a slight drop in the conversion and methanol selectivity (8-11). In our experiments, oxygen was depleted at temperatures above 430°C, but methane conversion was kept constant at ca. 13% and methanol selectivity was kept at ca. 60% before the decreasing at temperatures above 470 °C. It is supposed that the reaction mainly took place in a very short zone, and the oxygen in the feed was used up in this zone. In the downstream of the zone, there was no oxygen that could be used. There might be no oxidizing free radical to react with produced methanol which leads to the deep oxidation. We measured the axial temperature profiles (inside temperatures of the quartz line) during the reactions by moving the thermocouple inside the long quartz sheath tube along the axis of the reactor (Figure 2A). The temperature profiles obtained at different reactor temperatures (outer-wall temperatures of the stainless steel) are shown in Figure 2B. Considerable reaction began to occur in


284

A


Quartz sheath tube

0

2

4

Reaction zone

6

8

10

12 14

16 18

20 22 24 26

28

30

Distancefrombottom of reactor / cm

Figure 2. Temperature profiles of reactor at different reaction temperatures reaction condition: CH/O /N =100:10:10 (mL/min), P=5.0MPa W stands for outer-wall temperature of the stainless steel line 2

2



285 the inner quartz line when the reactor temperature (outer-wall of the stainless steel) increased to 420 °C (while the measured temperature in the center of the quartz line at the point 14 cm long from the furnace bottom was 432 °C). At this temperature, oxygen was only partially consumed, and the temperature profile had little difference with those measured at lower reactor temperatures. When the reactor temperature (outer-wall temperature of the stainless steel line) increased to 430 °C, the oxygen was depleted, and there appeared an abrupt temperature increase in the inner quartz line (as shown in Figure 2B Curve #). Because the oxidation of methane is a exothermic reaction, such an increase in temperature may indicate that the reaction proceeded primarily in a short zone along the reactor (reaction zone, Figure 2 A) and a great deal of heat was released in this zone causing the temperature to increase. Because the CH4/O2 ratio of the feed gas was high, the oxygen was the limiting species. It was used up in the fierce reaction zone. In the downstream of the zone, there was no oxygen existing so little secondary reaction could occur.

Table 2 Partial oxidation of methane in stainless steel line ^

Conversion /%

11

Selectivity /% CH

CH OH Yield/%

7.9

0.0

0.040

29.8

15.2

0.3

1.047

55.5

29.5

14.4

0.6

1.044

7.7

54.2

30.2

14.7

0.9

1.126

8.1

52.3

28.4

18.2

1.2

1.477

0

CH

CO

C0

386

5.3

0.5

31.0

61.1

417

100.0

6.9

54.8

438

100.0

7.2

459

99.9

480

100.0

4

2

2

CH3OH

3

2

6

P=5.0MPa, CH4/O/N=100/10/10 (ml/min), id=6.5mm *The thermocouple was placed at the point 14 cm long from furnace bottom. 2

2

It also can be seen that C O was the initial product and C 0 was produced only when oxygen was completed converted (Table 1). In the experiment with just the stainless steel line reactor (no inner quartz line), C 0 was found to be produced at the temperature when no significant reaction occured, and its selectivity was higher than C O selectivity (Table 2). In our earlier research (9), C 0 was the initial product at low temperatures, and this was explained by the catalytic deep oxidation by the metal surface in the ringed gap between the inner lining quartz tube and the stainless steel tube. This indicated that the feed gas could leak into the ringed gap and result in the contact of the feed gas with the metal wall. The selectivity of methanol was about 39% when oxygen was 2

2

2


286 completely consumed (Ρ), which was lower than that obtained in the specially deigned reactor. Compared to the reaction conditions, the ultimate controlling factor was the reactor structure. The leak of feed gas into the ringed gap might be an important factor that influences the partial oxidation of methane. In one of the our papers, it has been stated that the isulation of the ringed gap between the inner quartz line and the SS tube is of great importance (7). The effect of feed gas total flow rates on the partial oxidation of methane was also investigated. The results are shown in Table 3. When the total flow rate was increased from 120 mL/min to 180 mL/min, the conversion of CH4 and yield of CH3OH decreased slightly. Further increase in the total flow rate did not affect the conversion of CH4 and yield of C H O H markedly. Therefore, the total gas flow rate had very little effect on the oxidation of methane when the conversion of oxygen reached 100%. It is worth noting that the loss of methanol resulted in the increase of CO while the selectivity of C 0 was not affected. Such results indicate that CO and C H O H were produced via the same intermediate(s).


3

2

3

Table 3. Effect of total flow rate on the gas phase partial oxidation of methane to methanol. Total flow rate Conversion/% /(mL/min) CH O2 4

120 180 240 360

13.1 11.4 11.5 11.9

100 100 100 100

selectivity/%

CH3OH

CH3OH

CO

C0

2

CH

yield/%

62.3 57.9 57.4 55.7

33.7 37.1 37.4 35.1

4.8 4.4 4.5 8.2

0.2 0.5 0.7 1.0

8.2 6.6 6.6 6.7

2

6

CH4/O/N=100:10:10 (mL/min), P=5.0 MPa, T=450 °C 2

2

We also measured the axial temperature profiles of the reactor during the reaction under different flow rates (Figure 3). At the reactor temperature of 430 C , the oxygen was completely consumed for the flow rates of 120 and 240 ml/min, and the temperature increases in the reaction zone were observed during the reaction. It can be seen from the temperature profiles that the length of the reaction zones, in which the temperature changes were observed, were almost the same (about 4 cm long) for both 120 and 240 mL/min (Figure 3). But the temperature increase in the zone was more fiercely in flow rate of 240 mL/min than in that of 120 mL/min and the hotspot temperature in the reaction zone reached 475 °C for the 240 mL/min flow rate. These results indicate that increasing total flow rate only resulted in the increase of the amount of feed gas e


287 4cm

500 450 Ρ

400

S 350 Έ S 300 Β β 250 200 Downloaded by COLUMBIA UNIV on September 13, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0852.ch018

150 0

5

10

15

20

25

30

Distance from bottom of reactor /cm Figure 3. Temperature profiles of reactor under differentflow rates. Reaction conditions: P=5.0MPa, CHJOJNi=l0/1/1, Wall Temperature=430 V reacted in the reaction zone which caused the temperature to increase to a higher value. The phenomenon of the temperature changes in the reaction zone under different flow rates also suggests that the oxidation of methane occurred primarily and proceeded fiercely and very quickly in the zone. The reaction pressure is an important factor for partial oxidation of methane to methanol (Figure 4). At pressure of l.OMPa, the oxygen was not consumed significantly (that is the reaction did not take place significantly) until 470°C, while at pressure of 2.0MPa, the oxygen was consumed completely at less than 450°C And increasing the pressure resulted in the complete oxygen conversion at even low temperature (420-430 C). These results were in agreement with the general conclusion drawn by Foulds et al. (8) and Burch et al. (10). E

The effect of pressure on the products distribution are shown in Table 4. The selectivity and yield of methanol increased as the pressure increased and began to level off when the pressure was above 4.0 MPa. A similar trend has been observed (8) though Burch et al. (10) reported that no smooth trend about the effect of pressures on methanol selectivity was found. At pressure of 1.0 MPa, the complete oxygen conversion occurred at temperature as high as 500 ° C . The dominant product was C O , and there was significant C HÔ observed in the products. Bearing in mind that the oxidative coupling of methane to ethane and ethene was carried out at low pressure and high temperature, high pressure should be favorable to the production of methanol from partial oxidation of methane. 2



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Table 4. Effect of pressure on the products distribution for the partial oxidation of methane to methanol Pressure Conversion/% /MPa CH o 4

5.0 4.0 3.0 2.0 1.0"

13.1 12.3 9.3 8.0 8.6

2

100 100 100 100 100

Selectivity/% CH OH

CO

co

62.3 53.9 32.4 17.1 6.3

33.7 35.1 52.2 66.6 77.3

4.8 10.6 14.6 15.1 9.6

3

2

CH

CH OH yield/%

0.2 0.4 0.9 1.2 5.8

8.2 6.6 3.0 1.4 0.5

3

2

6

a

CH4/O/N=100:10:10 (mL/min), T=450 *C, T=500 °C 2

2

At present, natural gas is mainly used as a fuel for the public or for manufacturers of electricity. Based on the above-mentioned results, we proposed a new concept called "one pass" for producing methanol from the pipeline natural gas (as shown in Figure 5). The natural gas taken from the pipeline is mixed with oxygen, heated and passed through the reactor to produce methanol. The effluent mixture is then passed through a condenser to separate the liquid products. Then the outlet gas could be pumped back to the pipeline.



289

y MeOH Figure 5. 'One pass " concept for producing methanol from pipe-line natural gas.

Conclusion Comparatively high yields of methanol were obtained in a specially designed reactor. The reaction was completed in a very short zone, and when oxygen in the feed was depleted, the production would be stable. Therefore, high yields can be obtained over wide temperature ranges. The total feed gas flow rate had little effect on the partial oxidation while pressure was an important influencing factor. A "one pass" concept was provided for producing methanol from pipeline natural gas.

Acknowledgement We acknowledge the financial supportfromthe National Key Fundamental Research Project Foundation of China (G199902240-06), the Research Foundation of Tsinghua University and the Analytical Foundation of Tsinghua University.

References 1.

Bone, W.A. and Vheeler, R.V. J. Chem. Soc. London 1902, 81, 535.


290


2.

Liu, H.F.; Liu, R.S.; Lunsford, J.H. J. Am. Chem. Soc. 1984, 106, 41174121. 3. Gesser, H.D. International Chemical Congress of Pacific Basin Societies, 1984, Honololu, Paper 03002. 4. Foster, R. Appl. Catal. 1985,19, 1. 5. Feng, W.Y.; Knopf, F.C.; Dooley,K.M.Energy Fuels 1994,8, 815-822. 6. Omata, K.; Fukuoka,N.;Fujimoto, K . Ind. Eng. Chem. Res. 1994, 33,784789. 7. Zhang, Q.; He, D.; Li, J., et al. Appl. Catal. A: General 2002,224, 201-207. 8. Foulds, G.A.; Gray, B.F.; Miller, S.A.; Walker, G.S. Ind. Eng. Chem. Res. 1993, 32, 780-787. 9. Han, Z. PhD. thesis, Department of Chemistry, Tsinghua University, China, 1999. 10. Burch, R.; Squier, G.D.; Tsang, S.C. J. Chem. Soc. Faraday Trans. 1989,1, 3561-3568.


Direct Partial Oxidation of Methane to Methanol in a Specially

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