New way for processing of light hydrocarbons, accumulated in the form of hydrates.
Recently the surveys of the capabilities of the unconventional sources of gas exploration are becoming more and more topical, including the investigation of the natural gas-hydrate bearing fields. The resources of natural gas in the form of hydrates are compared to the explored reserves of usual natural gas which allows to examine hydrates of natural gas as the most promising alternative source of energy and hydrocarbons [9,10,6,1].
The relative stability of the lower paraffin carbons is the primary factor that limits a wide use of natural gas as a raw material for chemical processing. Mechanical activation is one of the unusual methods to initiate chemical conversions of the hydrocarbons. The surveys of the sequences of natural gas mechanical processing showed that there is degradation to the more low-molecular homologs and hydrogen [2,13]. One of the factors which impact the mechanical conversions of the matters is the medium in which activation takes place. Mechanical reactions of natural gas hydrates (NGHs) take place in the air medium with the participation of water in comparison with the processes of the mechanical activation (MA) of natural gas. It is known that the water molecules during the intense processing on the metal surface dissociates into radicals of hydrogen and hydroxyl. These radicals are capable to initiate chemical processes in the reactive medium.
The aim of this work is the investigation of the mechanical conversions of natural gas aquatic clathrates, as well as the scale and ways of the chemical conversions of the hydrocarbons in the process of the mechanical activation of gas hydrates.
Mechanical activation was conducted in the centrifugal and planetary mill AGO-2C (ATO-2C). 40 grams of the synthesized hydrate was loaded in the reactor drum with the proportion of the masses loading / balls being 1/4. Mechanical processing was conducted during 60 and 300 seconds.
3 phases of the products were received during the mechanical processing of natural gas hydrate: gaseous (G), liquid (L) and solid (S). The solid phase was separated from the liquid phase by filtering. The liquid phase was divided into organic and aquatic phases through the method of extraction.
The component analysis of natural gas and the gaseous products received during the process of the mechanical activation of the hydrates were analyzed by the method of the gas-solid chromatography according to GOST 23781-87 on the analytical software complex "GS2010Plus" (Shimadzu).
The composition of the liquid organic products was examined by the method of gas chromatography-mass spectrometry on the system which includes gas chromatograph <<Agilent 6890>> that is equipped by the interface with a highly efficient mass selective detector (MSD) <<Agilent 5973N>>.
To carry out mechanical and chemical examinations 2 samples of hydrates were synthesized from distilled water and natural gas of Irelyakhsky GOF (see table 1). The samples of hydrates were taken under T=268, 15 K and P=2.0 MPa in the high pressure chambers. Sample 1 was synthesized during 5 days, whereas sample 2 was synthesized within 20 days. As natural gas has 0.6 of ethane and 0.2% mole of propane (see table 1), synthesized hydrates have crystalline structure CS-2 .
To examine the composition of gas in the clathrate phase of the samples, synthesized hydrates were decomposed under the temperature of 278 K and pressure of 0.1 MPa. The dissociation of the hydrates samples resulted in different volumes of hydrating agents: thus gas volume received under the degrading of the hydrate sample 1 is equaled to 326 mL, the value for sample 2 is totaled to 856 mL. The component composition of the gas phase is shown in the table 1.
Two samples of the synthesized natural gas hydrates which differ in the quantity of hydrating agents were subjected to the mechanical activation (see table 1). After the mechanical activation 3 phases of the products were released: gaseous, liquid and solid.
There is shown in table 2 a component composition of the gaseous products received as a result of the mechanical activation of natural gas hydrates within 300 seconds which have different composition.
The survey of the liquid organic phase showed that the mechanical processing resulted in previously unknown chemical conversions of hydrocarbons--natural gas components, which are involved in the formation of hydrates. Thus, there were found in the liquid phase hydrocarbons with a longer chain than the hydrating agents have (see table 3).
Mechanical conversions of the hydrates samples lead to the deterioration of the reactor drums and milling agents with the formation of the so-called "rubbings". The drums are made of the corrosion proof (stainless) heat-proof steel produced under 40X13 mark. The steel contains from 0,35 up to 0,44% of carbon. The milling agents--balls--made of constructional bearing--grade steel produced under fflX15 mark.
The qualitative and quantitative composition of the solid phase ("rubbings") examination is in compliance with the chemical composition of the steels from which the milling agents and the steels are made of. It is established that the solid phase received during mechanical activation contains FeO oxide or FeOOH, [Fe.sub.3][O.sub.4] oxide, a-Fe Fe-Cr alloy. Thus, the solid phase is the combination of the lightly oxidized steels of these 2 marks.
The analysis of the gas from the samples showed that with the formation of natural gas hydrates there is redistribution of the components of the initial gas mixture as in case of other multi--component gas mixture. With the formation of the natural gas hydrates the combined hydrate enrich with the components simple hydrates which are characterized by the minimal values of the dissociation pressure [P.sub.[TEXT NOT REPRODUCIBLE IN ASCII]]. It is ascertained that redistribution of hydrocarbons [C.sub.1]-[C.sub.4] in the samples with the different duration of the synthesis is carried out in compliance with the increase of dissociation pressure [P.sub.dissociation] of simple hydrates formed by the individual components of natural gas (see table 4). The less the dissociation pressure [P.sub.dissociation] of the simple hydrate of the mixture component, the higher its concentration in the clathrate phase.
Thus, with the formation of natural gas hydrates in the solid phase there is an accumulation of those components of the mixture with the equilibrium pressure of the formation of hydrates being less than the pressure of the formation of hydrates of natural gas, i.e. less than 2.0 MPa.
It is ascertained that with 5 and 20 days of the synthesis in the solid phase there is an accumulation of i-[C.sub.4][H.sub.10] and [C.sub.3][H.sub.8], as it is the heavy hydrocarbons which run into the natural gas hydrate. The process of n-[C.sub.4][H.sub.10] concentration can be explained by the steric factor, i.e. a larger Wan der Waals size of molecule n-[C.sub.4][H.sub.10] comparing to the sizes of the other hydrating molecules [7,8].
The sufficient enrichment of the first phase with ethane is the specific feature of the rearrangement of natural gas components accompanied by the formation of its hydrates. If the ethane concentration in solid phase is 5.49% mole with the 5 days' hydrate synthesis, the ethane concentration is 10.5% mole with the 20 days' hydrate synthesis; i.e. the ethane concentration in the second sample is virtually twice as much than in the first sample. Thus a longer process of synthesis of natural gas hydrates leads to the consistent increase of the ethane concentration, n-butane, propane and i-butane in the clathrate phase.
The hydrates do not contain n- and iso-pentanes, as the Wan der Waals size of their molecules is larger than the size of the empty cavities of the hydrate with the structure CS-2 [7,8]. It explains why pentanes do not form individual hydrates and are not contained in the mixed hydrates. He and [H.sub.2] cannot be contained in the hydrates with the specified experimental criterion for the formation of hydrates. The linear sizes of these molecules are less than 3.5 [Angstrom] which leads to their inability to stabilize molecular cavities of the hydrates.
The research showed that there are changes in the qualitative and quantitative compositions of the gas phase in the process of the mechanical activation. The components which are not involved in the formation of the hydrates were found out in the gas phase. Thus during the mechanical activation of the samples there was found out the formation of hydrogen. With the increase in the hydrate synthesis duration the concentration of hydrogen in the products of decomposition virtually increases twice. With the mechanical activation of natural gas hydrates hydrogen isolates through mechanical conversions of the water molecules which form clathrate framework of the hydrate and hydrating agents--hydrocarbons.
I- and n-pentane form in the gas phase after the mechanical activation of the hydrate sample II. It is proved that n-butane concentration increases after the mechanical activation of sample II hydrate. The concentration of hydrocarbons C1-C3 decreases whereas iso-butane entirely decomposes in the mechanical activation of the hydrates samples.
1,2 -dimethylcyclopentane and n-heptane were received under both 60 and 300 seconds (see table 4) in the liquid organic phase of the mechanical processing of sample I. By the correlation of their concentration we can assume that n-heptane is the product of the 1,2-dimethylcyclopentane hydrogenation. The concentrations of 1,2dimethylcyclopentane and n-heptane do not change if the conversions are under more than 60 seconds. It leads to the fixed composition and concentration of the organic products received during the mechanical activation.
The mechanical activation of sample II during 60 seconds results in the formation of cyclohexane, 1methyl-3-propylbenzene and 1-butylbenzene (table 3). The increase in the duration of the mechanical impact on sample II leads to the lower concentration of cyclohexane down to its contents in the extraction agent and higher concentrations of the alkylbenzenes. Therefore chemical conversions of sample II hydrates with different time periods of the mechanical activation leads to the formation of the same aromatic hydrocarbons. The change in concentration of the cyclohexane starting from the time of gas hydrates activation allows us to assume that it is the intermediary product of the alkylbenzenes formation reaction.
The experiments showed that natural gas in hydrated condition under the intense mechanical actions subjected to the previously unknown chemical conversions--formation of hydrocarbons with the longer carbon chain than the chain of the hydrating agents. The absence of carbon and ferric carbide in the solid phase show that lower paraffin hydrocarbons and, chiefly, methane without subjecting to the degradation with the formation of hydrogen and carbon can take part in the chemical addition reactions and cyclization of hydrocarbons. This paper presents an investigation of the natural gas mechanical activation, which causes difficulties in establishing the mechanism of chemical transformations of the hydrating agents. Therefore it is necessary to conduct experimental surveys of the mechanical conversions of the individual gases hydrates.
Received 15 April 2014
Received in revised form 22 May
Accepted 25 May 2014
Available online 15 June 2014
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Liudmila Petrovna Kalacheva and Aitalina Fedorovna Fedorova
Institute of Oil and Gas Problems, SB RAS, Yakutsk, 1 Oktyabrskaya street, 677980, Russia
Corresponding Author: Liudmila Petrovna Kalacheva, Institute of Oil and Gas Problems, SB RAS, Yakutsk, 1 Oktyabrskaya street, 677980, Russia
Table 1: Component composition of natural gas and gas stage of the hydrates samples. Component Composition, % mole Natural gas Sample I Sample II C[H.sub.4] 87.1 86.8 77.4 [C.sub.2][H.sub.6] 3.49 5.54 10.6 [C.sub.3][H.sub.8] 1.11 4.09 4.84 i-[C.sub.4][H.sub.10] 0.070 0.41 0.59 n-[C.sub.4][H.sub.10] 0.18 0.36 0.64 i-[C.sub.5][H.sub.12] 0.075 0 0 n-[C.sub.5][H.sub.12] 0.083 0 0 C[O.sub.2] 0.01 0.075 0.028 [N.sub.2] 7.53 2.64 6.05 [H.sub.2] 0.16 0 0 He 0.056 0 0 Table 2: Component composition of the gaseous phase received after the mechanical activation of the hydrates within 300 seconds. Component Composition, % vol. Sample I Sample II C[H.sub.4] 53.4 5.08 [C.sub.2][H.sub.6] 2.67 8.68 [C.sub.3][H.sub.8] 2.34 3.55 i-[C.sub.4][H.sub.10] 0 0 n-[C.sub.4][H.sub.10] 0 1.01 i-[C.sub.5][H.sub.12] 0 0.02 n-[C.sub.5][H.sub.12] 0 0.019 C[O.sub.2] 0.06 0.042 [N.sub.2] 3.4 6.4 [H.sub.2] 38.1 75.2 Table 3: Composition of the liquid organic products received after the mechanical activation of the hydrates. Activation time, s 60 component Composition, % mole Sample I 1,2-dimethylcyclopentane 0.108 n-heptane 7.10 Sample II cyclohexane 10.683 1-methyl-3-propylbenzene 12.093 1-butylbenzene 6.35 Activation time, s 300 component composition, % mole Sample I 1,2-dimethylcyclopentane 0.108 n-heptane 7.10 Sample II - - 1-methyl-3-propylbenzene 14.812 1-butylbenzene 6.45 Table 4: The dependence of the redistribution of natural gas components in hydrated phase starting from [P.sub.dissociation] of individual hydrates. Contents in sample Molecules - [P.sub.dissociation] I/contents in "guests" (M) MPa natural gas i-[C.sub.4][H.sub.10] 0.113 5.94 [C.sub.3][H.sub.8] 0.134 3.68 n-[C.sub.4][H.sub.10] * 0.152 2.00 [C.sub.2][H.sub.6] 0.350 1.59 C[H.sub.4] 2.510 0.99 Molecules - Contents in sample II/ "guests" (M) contents in natural gas i-[C.sub.4][H.sub.10] 8.55 [C.sub.3][H.sub.8] 4.37 n-[C.sub.4][H.sub.10] * 3.56 [C.sub.2][H.sub.6] 3.04 C[H.sub.4] 0.89 * As n-[C.sub.4][H.sub.10] does not have an individual hydrate, [P.sub.dissociation] is calculated for the mixture with 99.9 % mole of n-[C.sub.4][H.sub.10] and 0.1 % mole of C[H.sub.4] according to the methods of Sloan.
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|Author:||Kalacheva, Liudmila Petrovna; Fedorova, Aitalina Fedorovna|
|Publication:||Advances in Environmental Biology|
|Date:||Jun 1, 2014|
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