Kinetics and Mechanism of Plasma Oxidative Desulfurization in Liquid

Yahya S. Al-Degs , Amjad H. El-Sheikh , Ramia Z. Al Bakain , Alan P. Newman , Mohammad A. Al-Ghouti. Energy Technology 2016 4 (6), 679-699 ...
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Energy & Fuels 2001, 15, 38-43

Kinetics and Mechanism of Plasma Oxidative Desulfurization in Liquid Phase Wan-Ying Liu,* Zheng-Lan Lei, and Jin-Kun Wang Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu, 610041 Received February 28, 2000. Revised Manuscript Received September 18, 2000

Factors affecting the plasma oxidative desulfurization of organic sulfides in the liquid phase were investigated. The organic sulfides, including mercaptan, thioether, and thiophene, are mixed with n-heptane severally or jointly. The plasma power load, i.e., the ratio of oxygen flow to plasma power, and the reaction time are the primary factors affecting the desulfurization reaction when the reaction temperature was controlled at about -85 °C. Under suitable conditions, the degree of desulfurization for mercaptan, thioether, thiophene, and/or their mixtures can exceed 99%, 80%, 79%, and 88%, respectively. On the basis of the conception that the electron energy function f() and the molar ratio of the reactants R control the reaction rate and the product composition, a kinetic model was developed to describe the plasma oxidative desulfurization reaction in the liquid phase using macroscopic test parameters. The good agreement of the calculated results with the experimental data indicates that the plasma reaction in the liquid phase is essentially the gaseous reaction occurring under special conditions. The normal viewpoint that the reaction takes place between energetic electrons and the surface molecule of the liquid may be incorrect. The reaction mechanism for plasma oxidative desulfurization of mercaptan, thioether, and thiophene in the liquid phase is discussed.

Introduction Crude petroleum contains sulfur in the form of mercaptan, thioether, and thiophene. The desulfurization technique normally used is catalytic hydrodesulfurization, which fails or becomes uneconomical if the material contains metals, such as V, Ni, etc. A nonequilibrium plasma has a high activation ability and avoids catalyst poisoning. Plasma desulfurization has been proposed as an alternative method for petroleum desulfurization. The plasma desulfurization of mercaptan, thioether, and thiophene in the gas phase has given some interesting results, and it has been found that the degree of desulfurization can be improved when oxygen was added to the plasma.1,2 Plasma reaction in the liquid phase is obviously superior to plasma reaction in the gas phase, because cool liquids can provide rapid quenching of the primary products and may avoid and/ or suppress consecutive reactions.3 In this study, the advantages of plasma reaction in the liquid phase and the role of oxygen in the desulfurization reaction are combined. The reaction of oxygen plasma with liquid reactants was carried out to investigate the fundamental kinetic and mechanism of the desulfurization reaction. Better results than in the gas phase were obtained. It is generally agreed that the initiation of a plasma reaction in the liquid phase is due to an effective impact between the energetic electron and the surface mol* To whom correspondence should be addressed. (1) Suhr, H.; Henne, P.; Iacocca, D.; Ropero, M. J. Liebigs Ann. Chem. 1980, 411. (2) Suhr, H.; Schmid, H.; Walter, H. G. Plasma Chem. Plasma Pro. 1981, 1, 179. (3) Liu, W. Y. Chinese J. Org. Chem. 1992, 12, 377.

ecules of the liquid.3,4 This view may be unsound, because the energy of the free electron in a nonequilibrium plasma (a few eV) is not enough to activate liquid molecules at a temperature close to the melting point. Certainly, it is impossible for a chemical reaction to take place. We suppose that the plasma reaction in the liquid phase is essentially the gaseous reaction occurring under special conditions; that is, the molecule is activated by the impact between an energetic electron and vapor molecules from the liquid reactant, and then a chemical reaction takes place. Under this presupposition, a kinetic model was derived by using macroscopic test parameters to describe the plasma reaction in the liquid-phase based on the concept that the electron energy function f() and molar ratio of reactants control the reaction rate and product composition.5-7 Some reasonable assumptions were also adopted in the derivation of the model. The kinetic model was tested against the experimental results of the desulfurization of mercaptan, thioether, and thiophene. Good agreement between the results of calculation and experiment provide a reliable basis for us to study the mechanism of the desulfurization reaction. At present, there are two views of the reaction mechanism of the plasma chemistry, i.e., ionic mechanism and free radical mechanism. We hold that the interaction of various activated radicals should be a major process in organic plasma (4) Suhr, H.; Schmid, H.; Pfeundschuh, H.; Iacocca, D. Plasma Chem. Plasma Process. 1984, 4, 285. (5) Hollahan, J. R.; Bell, A. T. Techniques and Application of Plasma Chemistry; J. Wiley and Sons: New York, 1974; Chapter 1. (6) Liu, W. Y.; Lei, Z. L.; Wang, J. K. J. Natural Gas Chem. 1992, 1, 77. (7) Tezuka, M.; Miller, L. L. J. Am. Chem. Soc. 1978, 100, 4201.

10.1021/ef000039p CCC: $20.00 © 2001 American Chemical Society Published on Web 11/18/2000

Plasma Oxidative Desulfurization

Energy & Fuels, Vol. 15, No. 1, 2001 39

Table 1. Desulfurization of Isobutyl-mercaptan vs Reaction Time and Power Loada

Table 2. Desulfurization of Ethyl-thioether vs Reaction Time and Power Loada

reaction time conversion DdeSb power load conversion DdeS (min) (%) (%) (cm3 min-1 w-1) (%) (%)

reaction time conversion DdeS power load conversion DdeS (min) (%) (%) (cm3 min-1 w-1) (%) (%)

10.0 20.0 40.0 60.0 120.0

51.70 95.00 100.00 99.50 100.00

51.70 89.70 97.40 98.40 98.00

0.00 0.13 0.20 0.25 0.50 1.00

52.00 98.80 100.00 97.50 95.70 94.80

52.00 95.90 98.80 98.40 91.40 84.40

a The power load is 0.25 cm3 min-1 w-1 on the left. The reaction time is 60 min on the right. b DdeS is equal to the degree of desulfurization.

reactions. On the basis of the kinetic study, the desulfurization reaction mechanisms of ethyl mercaptan, ethyl thioether, and thiophene are proposed. Experimental Section Oxygen(99.9%), after passing a massflowmeter, was fed into a discharge tube surrounded by a comby capacitance which was connected through a matching network to a 13.56 MHz rf generator. The discharge tube ended about 4 mm above the liquid surface. The distance between the lowest circle of coupling capacitance and the liquid surface could be changed from 55 to 275 mm. To suppress gas-phase reactions, the liquid reactants were cooled close to their melting points in a thermostated bath with magnetic stirring. The products, having melting points higher than the liquids, were condensed in the reactor. Products with melting points lower than the liquid were captured in a liquid nitrogen trap, and the tail gas was absorbed into water. Attention is focused on the conversion and the degree of desulfurization of the organic sulfides. Analysis of the total sulfur in the reactant and/or the product mixture was carried out by gas chromatography (GC) with a flame photometric detector (FPD). All sulfides were transformed passing a combustion chamber into one sulfide before the product mixture was fed into chromatographic column. The degree of desulfurization was calculated using the difference in the total amount of sulfur between the reactants and the product mixture. The conversion of each sulfide was calculated from the difference in the amount of this sulfide between the reactant and the product mixture. All reactants, ethyl/isobutyl mercaptan, ethyl thioether, thiophene, and n-heptane are commercial reagents used without further purification. To simulate crude petroleum, the starting material consisted of isobutyl (or ethyl) mercaptan (1.20%), ethyl thioether (1.44%), and thiophene (1.32%) mixed with n-heptane together or separately.

Results and Discussion 1. The Desulfurization of Isobutyl Mercaptan. As a simple simulation of crude petroleum, the starting material consists of isobutyl mercaptan (1.20%) and n-heptane (98.80%). The starting material was cooled to about -85 °C in an ethanol-liquid nitrogen bath with magnetic stirring. The pressure of the reaction system varied from (1.1-1.2) × 102 Pa. Factors affecting conversion and the degree of desulfurization were investigated, such as reaction time, plasma power load, i.e., the ratio of oxygen flow rate to plasma power. Under reasonable conditions, the conversion and the degree of desulfurization of isobutyl mercaptan can reach 99% and 98%, respectively (Table 1). These results show that the conversion and the degree of desulfurization of isobutyl mercaptan increase with

15.0 30.0 45.0 60.0

22.92 59.72 54.17 88.89

13.19 23.61 38.89 78.86

0.05 0.06 0.10 0.13 0.20 0.25

25.00 58.33 72.22 79.44 45.14 40.94

18.06 32.33 34.72 43.06 33.53 20.09

a The plasma power load is 0.08 cm3 min-1 w-1 on the left. The reaction time is 60 min on the right.

increasing reaction time, but that the plasma power load has an optimum range. The latter is because the electron energy and the electron density (i.e., electron energy function f()) are key factors which control the reaction rate and the product composition in the plasma reaction. When a discharge plasma is generated steadily, f() depends on the pressure (p) of the discharge system, the input power (P), the frequency, the discharge time, etc. With a fluidized tubular system and fixed frequency generator, f() ∝ P/p.7 However, the pressure(p)is usually difficult to measure exactly, but p and the flow rate r show a good linear dependence over a wide range, so f() ∝ P/r. It is known that plasma power load is a reciprocal of P/r, so an optimum value must exist for the plasma power load in its influence on the plasma reaction. The power load of zero in Table 1 indicates that oxygen was not introduced into the reaction system. The corresponding low conversion and degree of desulfurization show that oxygen is very important to the desulfurization reaction. 2. The Desulfurization of Ethyl Thioether. In the case of ethyl thioether(1.44%) and n-heptane, the influence of the reaction time and the power load on the conversion and the degree of desulfurization were investigated at a reaction temperature of about -85 °C (Table 2). The results listed in Table 2 are the average of two or three experiments. It can be seen from Table 2 that the influence of reaction time and plasma power load on the conversion and the degree of desulfurization of ethyl thioether are quite similar to that of isobutyl mercaptan. In mercaptans and thioethers, the C-S bonds are weaker than the C-C and C-H bonds and thus broken preferably, so the reaction of desulfurization is easier than following thiophene desulfurization. 3. The Desulfurization of Thiophene. In thiophene, the C-S bond is no longer the weakest bond, thus the oxidative desulfurization of thiophene is more difficult than the desulfurization of mercaptans and thioethers. The desulfurization of thiophene was carried out using the nonequilibrium oxygen plasma and some easily separated solid products were obtained. A few factors affecting the plasma oxidative desulfurization of thiophene in the liquid phase were investigated (Table 3 and Table 4). Table 3 shows that the desulfurization of thiophene by plasma in the liquid phase is best when the reaction temperature approaches the melting point. The results show that the influence of the power load on the degree of desulfurization is quite similar to that observed with mercaptan and thioether. An optimum value of power load exists. But the influence of reaction time on the desulfurization of thiophene is different

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Liu et al.

Table 3. Desulfurization of Thiophene vs Reaction Temperaturea reaction temperature (°C)

degree of desulfurization (%)

selectivity of solid (%)

-85 -75 -65 -55

79.26 56.03 46.67 37.93

47.83 30.61 21.59

Table 6. Desulfurization of Mixed Sulfides vs Power Loada conversion (%) power load degree of (cm3 min-1 w-1) C2H5SH (C2H5)2S C4H4S desulfurization (%) 0.05 0.10 0.17 0.20 0.25 0.50

a The reaction time and plasma power load are 60 min and 0.25 cm3 min-1 w-1, respectively. a

Table 4. Desulfurization of Thiophenevs Reaction Time and Power Loada reaction time DdeS selection power load DdeS selection (min) (%) of solid (%) (cm3 min-1 w-1) (%) of solid (%) 20.0 40.0 60.0 80.0 100.0

44.84 54.76 79.26 57.35 61.15

20.84 35.58 47.83 46.06 42.42

0.10 0.17 0.25 0.50 0.63

56.48 69.11 79.26 61.10 54.22

37.12 43.57 47.83 43.51 41.72

a Reaction temperature are all -85 °C. The power load is 0.25 cm3 min-1 w-1 on the left; the reaction time is 60 min on the right.

Table 5. Desulfurization of Mixed Sulfides vs Reaction Timea conversion (%) reaction time degree of (min) C2H5SH (C2H5)2S C4H4S desulfurization (%) 20.0 40.0 60.0 80.0 100.0 a

96.10 97.16 99.45 99.67 100.00

53.40 62.51 85.87 84.25 86.30

44.85 54.25 78.70 76.15 72.24

60.20 75.06 88.56 87.85 89.00

The power load is 0.17 cm3 min-1 w-1.

from that of mercaptan and thioether. In the case of mercaptan and thioether, the degree of desulfurization increases with increasing reaction time, but there exists an optimum reaction time in thiophene desulfurization. This may result from the high activating ability of the nonequilibrium plasma, which can cause thiophene ring to fragment and the fragments may rearrange into some insoluble sulfides. The fragmentation rate of thiophene ring is higher than forming rate of insoluble sulfides at the initial stage of desulfurization reaction and may produce some gaseous products to escape from the liquid mixture, so the sulfur content of liquid mixture will decrease with increasing reaction time. After a particular reaction time, the concentration of thiophene in the product mixture was very low. After that, its fragmentation is not a major process. Now, the high activating ability of the plasma can cause the solid polymeric sulfide to depolymetize into soluble sulfides. Thus, the amount of sulfur in the product mixture again increases slowly with increasing reaction time. 4. The Desulfurization of Mixed Sulfides. For a more complex simulation of crude petroleum, the starting material consists of ethyl mercaptan (1.20%), ethyl thioether (1.44%), thiophene (1.32%), and n-heptane (96.04%). The influence of reaction time and plasma power load on the oxidative desulfurization were investigated at about -85 °C (Table 5 and Table 6). When the desulfurization reactions of mercaptan, thioether, and thiophene take place simultaneously, the relation between the conversion and the reaction time is the same as that when they were desulfurized separately

65.10 91.25 95.34 100.00 98.47 96.71

24. 12 44.10 69.69 72.31 65.28 40.06

20.15 50.29 57.03 61.24 57.65 35.20

25.04 50.13 68.24 85.30 75.52 51.45

The reaction time is 40.0 min.

(Table 5). The total degree of desulfurization increases slowly with increasing reaction time although for the desulfurization of thiophene there exists an optimum reaction time (Table 5). This is because the contribution of mercaptan and thioether to the total desulfurization reaction surpass as thiophene. Table 6 shows that the conversion and the total amount of desulfurization vs plasma power load are the same as single desulfurization, an optimum value for the power load also exists, when three sulfides desulfurize simultaneously. It means that the probability of the collision between each sulfide and an energetic electron is equal in the nonequilibrium plasma. For plasma reactions in the liquid phase, the product can be quenched rapidly; therefore, the possibility of reaction between sulfides can be neglected. The reason for the existence of an optimum power load may be interpreted as follows: when the power load is quite small, although the electrons have high kinetic energy and longer lifetime, their density is very low, so the probability of an effective collision between an energetic electron and the reactant molecule is very small. But when the electron density is higher, the kinetic energy is small and the lifetime must be short. Obviously, the above two cases are unfavorable for initiating chemical reactions. In other words, only when the power load is suitable do the electrons in the system possess higher kinetic energy, longer lifetime and suitable density at the same time. This is the reason only optimum power load is effective. 5. Kinetics of Oxidative Desulfurization. Reaction kinetics are of great importance for explaining the reaction mechanism. The determination of experimental parameters, such as electron energy, electron density, and the density of various activated species, is generally complex and requires expensive instrumentation for plasma diagnosis. In a reactive plasma, not only is such work troublesome, but also it may be unreliable. The description of reaction rates and product composition using the easily accessible plasma parameters and test data is more practical. For this reason L. L. Miller7 and Liu6 developed respectively kinetic models to describe plasmolysis and methane oxidation in the gas phase. These models can describe the conversion and/or selectivity using plasma power P, flow rate r, and the molar ratio of the reactants R, and with good agreement with experimental results. The scientific basis of the methods are as follows: Organic molecules which pass through the plasma region are primarily activated by electron impact; the reaction rate and the type of reaction products will depend on the number and energy of the electrons, i.e., the electron energy function5 f(), and molar ratio of the reactants6 R.

Plasma Oxidative Desulfurization

Energy & Fuels, Vol. 15, No. 1, 2001 41

It is generally agreed that plasma reactions in the liquid phase take place between the energetic electrons and reactant surface molecules in the liquid.3,4 This view may be incorrect because the intermolecular forces at a temperature close to the melting point is no negligible. As it is known, intermolecular force decreases rapidly with increasing intermolecular distance (F. London theory: intermolecular force varies inversely as the seventh power of intermolecular distance). In the liquid, this force can elongate to a distance that a few times of molecular diameter. In this case electronic energy of nonequilibrium plasma must first overcome intermolecular force, and then activate the molecule. We hold that the electronic energy in the nonequilibrium plasma is not enough to activate liquid molecules. We assume that the plasma reaction in the liquid phase is caused by impact between an energetic electron and vapor molecules from the liquid reactant, though the vapor pressure of the liquid reactant may be very small at the moment. As mentioned above, f() and R are key factors affecting the reaction rate and the type of reaction products. It has been known that f() varies directly as P/r, i.e., f() ∝ P/r. We will describe plasma reactions in the liquid phase using macroscopic test parameters, such as P, r, and R, provided that we know the value of the molar ratio R. In fluidized reaction systems, R can be described using the ratio of the partial pressure of the reactants. In oxidative desulfurization, R is just the ratio of the sulfide partial pressure p1 to oxygen partial pressure p2. Under general conditions, p2 may be described using the system pressure. It is generally 1.1 × 102 to 1.2 × 102 pa in this study, the sulfide partial pressure p1, which is difficult to measure in the presence of a large quantity of oxygen, can be calculated approximately from a plot of log P vs 1/T. To take the plasma oxidative desulfurization of thiophene in the liquid phase as an example, we try to obtain the R value by some reasonable assumptions and simplification. First of all, we take the mixture which contains 1.32% thiophene as a pure solution of nheptane so the partial pressures are about 1.6 Pa and 2.0 × 10-2 Pa respectively for n-heptane and thiophene at -85 °C. The pressure of the system, i.e., oxygen partial pressure p2, is about 102 Pa. The partial pressure of thiophene, p1 amounts to about 1.7 × 10-4 of the system pressure, so that p1 can be taken as constant within the experiment pressure limit. So we know the value of p1/p2, i.e., the R value. Because the flow rate is linearly dependent on the pressure over a wide range for the fluidized system, we may use p1/r instead of p1/ p2. Because p1 is constant, R must show a linear dependence on r. Assuming that R is 0.1 when r is equal to 5, a series of R values with changing r is established. On the basis of the idea that f() and R control the reaction rate and the product composition, we developed a kinetic model to describe plasma oxidative desulfurization in the liquid phase from a large amount of the experiment data. 2

2

2 2

-log A/A0 ) kP /r (R + R/n )

(1)

where A/A0 is the conversion or the degree of desulfurization, P is plasma power, r is the mass flow of oxygen, R is the molar ratio of sulfide to oxygen, n is the number

Table 7. Relation of Thiophene DdeS and Selectivity to P, r, and Ra P (w)

r (cm3 min-1)

R

P2/r2 (R + R/n2)2

DdeS (%)

selectivity (%)

40.0 60.0 80.0 90.0 120.0

30.0 30.0 8.0 15.0 30.0

0.017 0.017 0.063 0.033 0.017

3937 8858 16125 21157 35438

57.4 61.1 65.5 69.1 79.3

29.0 43.5 37.1 43.6 47.8

a The reaction time is 60.0 min; the temperature is about -85 °C. The selectivity is the selectivity of solid sulfide formation.

Figure 1. Kinetics on plasma oxidative desulfurization of single sulfide.

Figure 2. Kinetic of plasma oxidative desulfurization of mixed sulfides.

of the reactants, and k is a proportional constant. The relations among P, r, R, and the test results are given in Table 7∼10, respectively. These results are from the plasma oxidative desulfurization of thiophene, thioether,

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Liu et al.

Table 8. Relation of Ethyl Thioether Conversion and DdeS to P, r, and Ra P (w)

r (cm3 min-1)

R

P2/r2 (R + R/n2)2

conversion (%)

DdeS (%)

40.0 50.0 70.0 80.0 100.0

20.0 10.0 12.0 5.0 10.0

0.025 0.050 0.042 0.100 0.050

4096 6400 12346 16384 25600

43.1 45.6 52.1 58.3 72.2

26.4 38.3 29.2 33.3 34.7

a

Reaction time is 30.0 min; the temperature is about -85 °C.

Table 9. Relation of Isobutyl Mercaptan Conversion and DdeS to P, r, and Ra P (w)

r (cm3 min-1)

R

P2/r2 (R + R/n2)2

conversion (%)

DdeS (%)

10.0 20.0 40.0 50.0

10.0 10.0 10.0 10.0

0.05 0.05 0.05 0.05

256 1024 4096 6400

94.8 95.7 97.5 100.0

84.0 91.7 94.0 99.0

a The reaction time is 60.0 min; the temperature is about -85 °C.

mercaptan, or their mixtures in the liquid phase in n-heptane. Comparisons of the calculated results with the experimental data is shown in Figures 1 and 2. The good linear relationship demonstrated the applicability of eq 1 to the plasma oxidative desulfurization of thiophene, thioether and mercaptan or their mixtures in the liquid phase. The significance of kinetic equation is as follows: First, what is called a plasma reaction in the liquid phase is essentially a gas-phase reaction occurring under special conditions. The initiation of the reaction is the energetic electron impact on gaseous molecules in the reactant vapor, resulting in molecule activation, although the vapor pressure may be very small. In other words, the common knowledge that the energetic electron impact on surface molecules of the liquid may be unsound. Second, the reaction occurs mainly in a thin layer on top of liquid reactant. In this thin layer, the products may be quenched rapidly, and may be quickly absorbed by cool liquid so that consecutive reactions can be avoided or suppressed. That is the reason the selectivity of plasma reaction in the liquid phase is far superior to that in the gas phase. At the same time, new gaseous molecule continuously move into the thin layer from liquid reactant. Owing to the universal applicability of the theory employed in the development of kinetic model, it may be applicable to other plasma reaction in the liquid phase, but more tests are needed to verify it. It must be pointed out that the mechanism of initiation of the plasma reaction in the liquid phase suggested above do not rule out the possibility of a few molecules of surface liquid taking part in the reaction, but it is undoubtedly not the major process during the reaction.

6. The Reaction Mechanism of Desulfurization. The foregoing kinetic discussion shows that the plasma reaction in the liquid phase is essentially a gas phase reaction occurring under special conditions, therefore, its reaction mechanism is bound to follow the general law of plasma reactions in the gas phase. It is generally agreed that the plasma reactions of organic substances involves three steps: molecule excitation, bond breaking, and stabilization of the intermediate. The rate of reaction and the type of products depend on the electron energy function f() and molar ratio of the reactants R. The product selectivity depended predominantly on bond breaking and the activity of intermediates. Bond breaking will control the product selectivity if one bond in the reactant molecule is considerably weaker than all others. Stabilization will control the selectivity if the intermediate is fairly unreactive. In addition, creating favorable conditions for quenching products is an effective method for increasing selectivity, and this can be attained using liquid-phase reaction. There are two possible mechanism classes for plasma reactions: radical reactions and ionic pathways. We hold that radical reactions should be the major process in organic plasma reactions. That is to say, the organic molecule first forms a variety of active radicals by way of energetic electron impact and then radicals react with each other to produce more stable products. In mercaptans and thioethers, the C-S bonds are weaker than the C-C and C-H bonds, the former is about 60-70 kcal/mol, the latter about 83 and 99 kcal/ mol respectively. In a nonequilibrium plasma, the C-S bond will break preferably and form radicals such as R•, R:, •SH or RS•, R• and S: and so on. The interaction of these radicals with O (3P), O2 (a1∆g) and H can produce some small hydrocarbons, SO2, H2S, H2, elementary sulfur, and some sulfur-bearing or oxygenbearing compounds. The mechanism of heptane oxidation was simplified in the following, because it is not a main part of this discussion. On the basis of previous work,8-10 the mechanisms of plasma oxidative desulfurization in the liquid phase of ethyl mercaptan, ethyl thioether, and thiophene are proposed below. (1) The Desulfurization Mechanism of Ethyl Mercaptan

O2 + e f O(3P), O (a1∆g) C2H5SH + e f C2H5•, HS• C7H16 + e f C7H15•, C7H14:, C4H9•, C3H7•, C2H5•, H HS• + O (3P), O2 (a1∆g) f SO2, SO3, H2O, H2 HS• + H f H2S, S, H2

Table 10. Relation of Conversion and DdeS of Mixed Sulfides to P, r, and Ra conversion (%) P (w) 25.0 40.0 60.0 100.0 a

r

(cm3

min-1)

5.0 10.0 10.0 10.0

R 0.10 0.05 0.05 0.05

P2/r2

(R +

R/n2)2

2214 5669 12755 35432

The reaction time is 40.0 min; the temperature is about -85 °C.

C2H5SH

(C2H5)2S

C4H4S

total DdeS (%)

100.0 98.5 95.3 91.3

72.3 65.3 60.7 44.1

61.2 57.7 57.0 50.3

85.3 75.5 68.2 50.1

Plasma Oxidative Desulfurization

C7H16 + O (3P), O2 (a1∆g) f C4H9COC2H5, C4H9OHC2H5 C3H7• or C2H5• + nS f (C2H2S)n, (C3H3S)n, H2S, H2

Energy & Fuels, Vol. 15, No. 1, 2001 43

reaction mechanism of n-heptane oxidation is omitted. The fragmentation of thiophene ring and the interaction among the fragments and O (3P), O2 (a1∆g) are described separately. a. Fragmentation of Thiophene Ring and Rearrangements.

nS f Sn (2) The Desulfurization Mechanism of Ethyl Thioether

O2 + e f O (3P), O2 (a1∆g) C2H5SC2H5 + e f C2H5•, C2H5S•, S: C7H16 + e f C7H15•, C7H14:, C4H9•, C3H7•, C2H5•, H C2H5S• + e f C2H5•, C2H4:, HS•, S:

C2H5S C2H5 + O (3P), O2 (a1∆g) f C2H5SO C2H5, C2H5SO2 C2H5

b. Interaction among Fragments and O (3P), O2 (a1∆g). Complex reactions can take place among the fragments of thiophene as well as the small hydrocarbons molecules and O (3P), O2 (a1∆g), and H. The following are only the processes involving the sulfur-bearing fragments.

S: + O (3P), O2 (a1∆g), f SO2, SO3

S + H, O (3P), O2 (a1∆g) f H2S, SO2, SO3‚‚‚H2O

S: +H f HS• + H f H2S, S

CS2 + H, O (3P), O2 (a1∆g) f H2S, CO, CO2, SO2, SO3‚‚‚H2O

C2H5S• + C2H5S• f C2H5SS C2H5

C7H16 + O (3P), O2 (a1∆g) f C4H9COC2H5, C4H9COHC2H5

H2CdCdS + H, O (3P), O2 (a1∆g) f C2H5S•, C2H5•, S, SO2, SO3, CO2‚‚‚H2O

HS• + O (3P), O2 (a1∆g), H f SO2, SO3, S, H2S, H2 •

C2H5S• f C2H5SS C2H5

•

C3H7 , C2H5 + nS: f (C2H2S)n, (C3H3S)n, H2S, H2 nS f Sn The oxidation of n-heptane into heptanone and heptanol has been demonstrated in a previous paper,11 and heptanone-3 and heptanol-3 were the main products. Other products and intermediates may be interpreted using some qualitative experiment as follows. The reaction tail gas from the desulfurization shows a strong smell of rotten duck eggs, and its absorption into water given an acid, which are evidence for the existence of H2S, SO2, and SO3 in the tail gas. In addition, the elementary analysis of the brown solid substance in the product mixture and on the reactor wall shows 38-50% sulfur. The known sulfides in this brown solid substance are sulfone, sulfoxide and disulfide, respectively. The unknown may be sulfur-bearing polymer. (3) The Desulfurization Mechanism of Thiophene. In thiophene, the C-S bond is no longer the weakest and the oxidative desulfurization is difficult. A nonequilibrium oxygen plasma can initiate the oxidative desulfurization of thiophene because its high activity can cause the thiophene ring to fragment through the impact of energetic electrons. For convenience of discussion, the (8) Gousset, G.; Touzeau, M.; Vialle, M.; Ferreira, C. M. Plasma Chem. Plasma Process. 1989, 9, 189. (9) Patin˜o, P.; Herna´ndez, F. E.; Rondo´n, S. Plasma Chem. Plasma Process. 1995, 15, 159. (10) Foote, C. S.; Peters, W. J. Am. Chem. Soc. 1971, 93, 3795.

C2H5S• + H, O (3P), O2 (a1∆g) f C2H5SO3H, H2S, SO2 ••• H2O H2Cd C -CH2-CS + H, O(3P), O2(a1∆g) f | S• C2H5S C2H5, C2H5SO3H, H2S, SO2 ••• H2O nS f Sn H2CdCdS f polymer The qualitative interpretation on the reaction mechanism of thiophene desulfurization is quite similar to that of desulfurization of mercaptan and thioether [see section (2) in the Results and Discussion]. It must be pointed out that although we have given a better qualitative interpretation for the above desulfurization mechanism, a large amount of experimental data are needed to verify it. This requires further investigation. Acknowledgment. The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant 29573129.) EF000039P (11) Liu W. Y.; Lu, W.; Wang, J. K.; Lei, Z. L. J. Nat. Gas Chem. (China) 1992, 3, 27.