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CONVERSION OF REAL ASSOCIATED GASES BY CARBON DIOXIDE OVER THE KMR-8 СATALYST

УДК 665.612.2

Новый цеолит-содержащий нанесенный катализатор исследован в реакции взаимодействия между диоксидом углерода и реальным попутным нефтяным газом (ПНГ) при варьировании температуры 350-800oC. Показано, что катализатор проявляет высокую активность в риформинге ПНГ диоксидом углерода. При 800oC фракция углеводородов C2+ полностью конвертируется, степень конверсии метана - 91.7, а диоксида углерода - 90.2%. Основным продуктом реакции при 800oC является синтез-газ (смесь оксида углерода и водорода) с соотношением H2/CO=1.3. Также образуется вода и небольшие количества (2.8%) кислородсодержащих продуктов (в основном уксусная кислота) при T ≤ 600oC. Кроме активности и селективности еще одним из достоинств синтезированного катализатора является его устойчивость к коксообразованию.

Бұл жұмыста цеолитке қондырылған жаңа катализатор синтезделінді. Катализаторда әр түрлі процесс температурасы (350-800oC) алмастыру кезінде ілеспе газ және көміртек диоксидінің конверсиясы зерттелді. Катализатордің активтілігі ілеспе газ және көміртек диоксиді әрекеттескенде синтез-газ, соның ішінде сутегі көбірек мөлшерде түзілген жағдайда көрінетіңдігі анықталынды. C2+ фракция 800оС температурада толық конверсияға түскенде Н2/СО қатынасы 1.3 құрайды.

Abstract

The new supported zeolite-containing catalyst has been investigated in the reaction of interaction between carbon dioxide and real associated petroleum gases (APG) at varying experiment temperature within a range of 350-800oC. It has been shown that the catalyst performs the high activity in carbon dioxide reforming of APG. At 800oC, the C2+ hydrocarbon fraction is completely converted, degree of methane conversion is 91.7 and carbon dioxide conversion is 90.2%. The main product of reaction is synthesis-gas (mix of carbon oxide and hydrogen) with a ratio of H2/CO=1.3 at 800oC. Also water and small amount (2.8%) of oxygenates (basically acetic acid) are produced at T ≤ 600oC. One of the advantages of the synthesized catalyst in addition to its activity and selectivity is its resistance to coke formation.

Key words: zeolite-containing catalyst, associated gases, carbon dioxide, synthesis-gas

Introduction

Associated petroleum gas (APG) is still burned in Kazakhstan. Kazakhstan ranks the fifth place on amount of burned APG. Information on total amount of burned gases in Kazakhstan is not complete and has an inconsistent character. In 2008 according to the official data the gross output was 33.5 bln. m3 at that volume of burned associated gases was decreased to 1.8 bln. m3 [1].

From 25 to 1000 m3 of associated gases is extracted at producing 1 ton of oil at the Kazakhstans’ oil and oil-gas condensate fields. In a best case they are squeezed into oil reservoir. This approach does not solve the problem of associated gases utilization. Moreover as a rule it leads to raising the gas amount at the following oil extraction and increasing a cost of re-squeezing.

At APG combustion, the high value hydrocarbon raw material is aimlessly burned that is accompanied with carbon dioxide and hazardous sulfur and nitrogen oxides formation. Carbon dioxide among these anthropogenic gases is a main greenhouse gas. The amount of carbon dioxide formed exceeds twice the amount of burned fuel.

Obviously that burning of APG is unacceptable from a point of view of environment protection and resource-saving. One of decisions of this problem is creation of infrastructure for APG collection, transportation, purification, separation, and squeezing into a main pipe line that requires the huge investments [2].

Other way is processing of APG into motor fuel, methanol, and other high value products directly at the oil fields [2-4]. The light hydrocarbons in composition of APG can be converted into liquid products by so called GTL process (Gas-To-Liquid) [4].Also they can be a source for hydrogen production [5]. The GTL process has been considered as a clean and alternative process in an environmental respect. By GTL the light hydrocarbons are converted into syngas. Syngas, a mixture of H2 and CO is a major feedstock for methanol, ammonia and Fischer–Tropsch (F–T) synthesis.

There are three main catalytic ways for syngas production from hydrocarbon feed with involving: 1) water - steam reforming (equation 1), 2) half oxygen - partial oxidation (equation 2); and 3) carbon dioxide - dry reforming (equation 3). The last one attracts an interest because of allows utilizing greenhouse gases – methane and carbon dioxide with producing syngas with a ratio of Н2/СО = 1 [6-10].

CH4 + H2O → CO + 3H2 (eq.1)

CH4+1/2O2 → CO + 2H2 (eq.2)

CH4 + CO2 → 2CO + 2H2 (eq.3)

The aim of this study was the development and test of the new catalyst – KMR-8 for converting the real associated gases by involving into process carbon dioxide (dry reforming) with producing syngas.

Experimental

The catalyst – KMR-8 with total metal content - 5 weight % supported on alumina promoted with zeolite has been synthesized and tested in conversion of real associated gases (APG). The process was carried out in a flow quartz reactor at atmospheric pressure and varying experiment temperature from 300 to 800oC. Space velocity (S.V.) was 1500 hr-1. The catalyst loading was 10-30 mL. Ratio of CO2:APG in dry reforming was constant - 1:1.

The composition of associated gases extracted from one of oil wells of the West Kazakhstan is presented in Table 1.

Table 1. Hydrocarbon composition of real associated gas extracted from one of oil wells of the West Kazakhstan

Hydrocarbons

С1

С2

С3

С4

С5

С6

С7

Content, vol. %

52.1

20.1

14.0

8.1

4.4

1.1

0.2

The set for APG conversion was combined with gas chromatographs (GC) equipped with thermal conductivity detector for on-line analysis of Н2, Ar, СО, СН4, О2, СО2 (columns: molecular sieves and activated coal) and flame-ionization detector (FID) for on-line analysis of hydrocarbons (column: modified alumina). Liquid phase was collected in a special cooled trap (separator) and then analyzed by GC equipped with FID on columns: Carbowax/Carbopak and Poropak N, as well as by IR-spectroscopy. The carbon formation was controlled by thermogravimetric analysis (TGA), thermo-programmed reduction (TPR) and electron microscopy.

The catalyst was studied with using BET, IR-spectroscopy and electron microscopy.

Results and discussion

The conversion of hydrocarbons at dry reforming of APG (interaction with carbon dioxide) over the KMR-8 catalyst already runs at 350оС. With increase in temperature from 350 to 800оС the conversion degree raises for all hydrocarbons (C1-C7) and carbon dioxide as it is shown in Figure 1. It needs to note that then higher the molecular weight of hydrocarbon then less temperature of its complete conversion. Thus, 100% conversion of C7 is occurred at 350оС, С6 - 550оС, С5 - 600оС, С4 - 700оС, С3 - 750оС, and С2 - 800оС (Figure 1).

Figure 1. Effect of temperature on conversion degree of hydrocarbons and CO2 at dry reforming of associated gases over the KMR-8 catalyst

At raising temperature from 350 to 800оС, methane conversion increased from 20.9 to 91.7%, and СО2 conversion – from 51.6 to 90.2% (Table 2).

Table2. Effect of temperature on dry reforming of associated gases over the KMR-8 catalyst (APG /СО2=1/1, Р= 1 atm, space velocity – 1500 hr-1)

Т, оС

Degree of conversion (K), %

Н2/СО

КС1

КС2+

KСО2

350

20.9

51.2

51.6

1.9

400

28.7

56.6

67.2

1.2

450

32.5

59.0

68.0

1.1

500

43.6

74.2

67.2

0.8

550

47.8

86.6

68.5

0.9

600

49.8

97.6

69.7

1.0

650

56.5

98.6

69.6

1.1

700

79.4

99.5

69.8

1.2

750

84.0

99.6

77.5

1.2

800

91.7

100

90.2

1.3

The main product of APG conversion by carbon dioxide is syngas. At lower temperatures – 350-450oC syngas is reached with hydrogen, because production of carbon oxide is started later. With following increasing temperature to 550oC the yield of carbon oxide is surpassed hydrogen and Н2/СО <1. At 600oC the yield of hydrogen and carbon oxide becomes equal and ratio of Н2/СО=1. At higher process temperatures 650-800оС, the hydrogen production is prevailed and maximum ratio of Н2/СО=1.3 is observed at 800оС.

Except synthesis-gas oxygenates are formed over the KMR-8 catalyst. Acetic acid is prevailed among them (30-40%). The total yield of oxygenates is 2.8% at T=600оС. There is no oxygenates at higher temperature – 800oC (Table 3).

Table 3. Composition of products formed at conversion of associated gases (Р=1 atm, S.V=1500 hr-1)

Т,о С

Degree of conversion, %

H2/CO

Yield of oxygenates, %

СН4

С2+

СО2

600

49.8

> 97

69.7

1.0

2.8

800

91.7

100

90.2

1.3

not observed

The catalyst demonstrates the high resistance to coke formation. By electron microscope it was shown that the catalyst keeps its high dispersed state after reaction.

Conclusions

The KMR-8 catalyst performs the high activity in dry reforming of associated gases. The conversion degree XCH4 = 91.7% and XCO2 ~ 90.2%. Syngas produced has an appropriate ratio for Fisher-Tropsch synthesis - Н2/СО=1.3. The advantages of the synthesized catalyst also are the stability and resistance to coke formation. The catalyst worked with the stable activity during all period of its exploitation (more than 50 hours).

The catalyst can be recommended for the industrial application to produce syngas from APG. In whole, the introduction of the technology of APG utilization into practice will promote the mitigation of carbon dioxide emissions.

Acknowledgement

Author is grateful to academician of NAS of RK, Prof. G.D. Zakumbaeva for her assistance and consultation at implementing this study. The special thanks to the Laboratory of physico-chemical study of catalysts of IOCE for carrying out the catalysts investigation.

 

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