12Experimental Study of Liquid Hydrocarbons Pyrolysis

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O R I G I N A L P A P E R

Experimental Study of Liquid Hydrocarbons Pyrolysis

to Acetylene in H2/Ar Plasma

Binhang Yan Pengcheng Xu Xuan Li Cliff Yi Guo

Yong Jin Yi Cheng

Received: 15 May 2012 / Accepted: 24 June 2012 / Published online: 12 July 2012 Springer Science+Business Media, LLC 2012

Abstract Liquid hydrocarbons including n-hexane, cyclohexane and toluene are pyro-

lyzed in H2/Ar plasma to investigate the effects of feedstock properties and key operating

conditions (e.g., the feedstock specific input power and residence time) on the reaction

performance. The experiments verify that the non-aromatic hydrocarbons show better

chemical reactivity than partially aromatic substances. Meanwhile, the straight-chain

alkanes and cycloalkanes have better yields of ethylene during the pyrolysis. The results

also demonstrate that the pyrolysis reactions are almost completed within the first 0.8 ms inAr/H2 plasma independent of the feed substances (i.e., liquid hydrocarbons), where the

increased feedstock specific input power enhances the reactant conversions and corre-

spondingly raises the yields of acetylene. At a feedstock specific input power of

4.7 9 104 kJ/kg, the n-hexane conversion is over 90 % and the yield of acetylene reaches

70 %. In addition, when using n-hexane as the feedstock, very little coke is formed during

the course of reaction. Comprehensive comparisons of the current experiments with the

data reported in the literature are made to point out the key influencing factors, i.e., the

effective mass ratio of C/H (RC/H) in the gaseous phase and the quench temperature. Both

two factors would need to be enhanced in order to get a better performance. Finally, the

improvements on the specific energy requirement of this process are discussed.

Keywords Thermal plasma Liquid hydrocarbons Pyrolysis Acetylene Specific energy requirement (SER)

B. Yan P. Xu Y. Jin Y. Cheng (&)Department of Chemical Engineering, Beijing Key Laboratory of Green Chemical Reaction

Engineering and Technology, Tsinghua University, Beijing 100084, Peoples Republic of China

e-mail: [email protected]

X. Li C. Y. GuoNational Institute of Clean-and-Low-Carbon Energy, P.O. Box 001, Shenhua NICE, Future Science &

Technology City, Chaoyang District, Beijing 102209, Peoples Republic of China

123

Plasma Chem Plasma Process (2012) 32:12031214

DOI 10.1007/s11090-012-9400-1


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Introduction

Pyrolysis of hydrocarbons in thermal plasma provides a direct route to make acetylene

(C2H2) [1]. Once the gaseous or liquid hydrocarbons are rapidly heated to the temperatures

above 1,800 K, a gaseous mixture with C2H2 as the principal hydrocarbon constituent aswell as H2, C2H4, and CH4 as by-products will be produced. This is attributed to the free

energy data that acetylene is more stable than other light hydrocarbons at above 1,500 K

[2]. The first ever reported pilot production was the Huels process using an 8 MW plasma

reactor with a feed rate of 2,344 Nm3/h natural gas in the early 1940s [3, 4]. The reported

methane conversion was 70.5 %, the yield of acetylene was 51.4 % and the specific energy

requirement (SER) was 12.1 kWh/kg C2H2. A similar process with a 9 MW plasma reactor

was built by DuPont in the United States in 1960s for producing acetylene from a wide

range of hydrocarbons [4]. When operated on methane, the yield of acetylene and SER of

the DuPont process were 70 % and 8.8 kWh/kg C2H2, respectively. Although further

attempt on scale-up was terminated, the pyrolysis process of hydrocarbons was proved

technologically viable and economically favorable.

Since coal is the most abundant fossil fuel in the world and its price is much lower than

those of gaseous or liquid hydrocarbons, deep research on the coal pyrolysis process would

make a significant sense and generate a remarkable economic benefit. Therefore, since

1960s, coal has been tested in lab- and pilot-scale plasma reactors as an alternative

feedstock [516]. However, compared to the gaseous or liquid hydrocarbons, the mixing

efficiency of coal with the hot gas is more inefficient due to that the heat transfer from

plasma to coal particle is the rate-limiting step of acetylene formation. It is considered that

acetylene is formed principally from the aliphatic and alicyclic components of coalstructure. Thus, the reactions of the volatiles released from coal play an essential role in

acetylene production in thermal plasma. For this reason, various aliphatic and aromatic

hydrocarbons that are similar to the primary volatiles of coal could be employed as model

substances to help understanding the pyrolysis behavior of volatiles. Therefore, besides the

process of Huels and DuPont, more and more attention has been focused on the lab-scale

thermal plasma conversion and utilization of gaseous and liquid hydrocarbons. Methane is

one of the most frequently studied feedstock in thermal plasma reactor [1723]. The

conversion is generally in excess of 90 % while the yield of acetylene ranges from 76 to

86 %; the measured SER ranges from a high value of 88 kWh/kg C2H2 to a low value of

9 kWh/kg C2H2. Fincke et al. [23] made a systematic re-examination of the direct plasmathermal conversion of methane to acetylene. The results showed that methane conversion

could approach 100 % and yield of acetylene was 9095 %. The improvements in terms of

conversion and yield were due primarily to the improved injector design, reactant mixing

and minimization of temperature gradients. Chen and Xie [24] carried out the hydropyr-

olysis experiment of liquefied petroleum gas (LPG) in H2/Ar plasma jet at atmosphere

pressure and found that the conversion of LPG was 76 %, the selectivity of C2H2 was

above 80 %, and the yield of C2H2 was 74 %. Beiers et al. [25] used some gaseous and

liquid hydrocarbons, such as CH4, C2H4, C2H6, cyclohexane, toluene, benzene and so on,

as model substances to study their reactions in a hydrogen plasma jet. The experimentalresults showed that the hydrocarbons underwent very fast reactions with highly reactive

plasma species and then stopped to react. The product distribution depended on the

hydrocarbons employed.

Therefore, the research of hydrocarbon pyrolysis in thermal plasma is not only useful

for heavy hydrocarbon cracking, aromatics conversion and fuel upgrading, but also fun-

damental to better understand the coal pyrolysis process. However, the pyrolysis

1204 Plasma Chem Plasma Process (2012) 32:12031214

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mechanism of various aliphatic and aromatic liquid hydrocarbons is still not known in

detail. In this work, the lab-scale experiments of liquid hydrocarbons pyrolysis to acetylene

were carried out in a H2/Ar plasma reactor at atmosphere pressure. Since the volatiles are

composed primarily of aliphatic, alicyclic and aromatic structure, n-hexane, cyclohexane

and toluene, as the typical representatives of the aliphatic, alicyclic and aromatic fragmentsreleased from coal respectively, are used as the feedstock. Our goal is to obtain the

chemistry of liquid hydrocarbons pyrolysis to acetylene. The influences of various

parameters, i.e., the input power, mass flow rate of reactants and reactor length (residence

time) on the conversions of feedstock, the yields of main gaseous products and the SER of

this process were studied. Furthermore, we wish to compare the current experimental

results with coal/hydrocarbons pyrolysis data reported in the literature, in order to establish

the solid fundamental knowledge to understand the basic rules in coal/hydrocarbon

pyrolysis to acetylene and obtain the appropriate operating conditions to improve the

reactor performance.

Experimental

The plasma torch and its associated equipments of the experimental set-up for liquid

hydrocarbon pyrolysis are shown in Fig. 1.

The central device of the experimental set-up is the plasma reactor with a maximum

electric power input of 10 kW. It consists of four parts: the plasma torch, the mixing

section, the reaction chamber and the quench device. The plasma torch shown in Fig. 1

Cooling Water

Quench Media

Product gas

ArH2

Mass Flowmeter

Gas cylinders

Mass Flow controllerOscillograph

Power supply

Plasma torch

Filter

Gas

chromatograph

Soot

Liquid hydrocarbons

Reactor

Quench

Gas-solid Separator

Mixing zoneSyring pump

1

3

57

9

2

4

6

8

10

Fig. 1 Schematic drawing of the lab-scale thermal plasma reactor for liquid hydrocarbons pyrolysis

Plasma Chem Plasma Process (2012) 32:12031214 1205

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comprises three main elements: the cathode, the plasma-forming gas injection stage, and

the anode. The electrodes are protected by cooling systems to reduce electrode wear. The

button-type cathode, which is made of thoriated tungsten, is embedded in a water-cooled

copper holder located at the center of the torch. The tapered tungsten cathode is to center

the arc root in the middle of the button. This plasma torch is restricted to nonoxidizinggases (e.g., H2, Ar or a mixture of H2/Ar) since tungsten oxide is volatile over 1,800 K,

which is lower than the cathode working temperature of 3,4003,800 K [ 26]. The insu-

lating gas injection is made of Teflon and the plasma-forming gas is injected along the

cathode axis. The water-cooled copper anode in the form of nozzle-shape, which can work

with both nonoxidizing and oxidizing gases, is used. The inter diameter of the nozzle is

6 mm. The arc voltage depends on the arc length linked to arc current, nozzle diameter, gas

composition and flow rate. The direct current for the arc is supplied from a power supply

with an open-circuit voltage of 120 V. For 10 %H2/90 %Ar plasma, however, the oper-

ation voltage is at about 4060 V. The power supply is capable of delivering current up to

100 A, but most of the experiments are carried out in the current range 2080 A corre-

sponding to a power input of 1.53.0 kW to the arc.

Each of the feed gases is controlled by a set of Mass Flow Controllers (MFCs). To

prevent any back flow, a check valve is located at downstream of each MFC. The total flow

rate of the arc gas is 10 L/min (10 %H2 and 90 %Ar). The feed substances are injected into

the mixing section perpendicular to the side wall below the plasma torch so as not to

influence the stability of the arc. The inside diameters of the mixing section and the

reaction chamber are 14 and 20 mm, respectively. At the bottom of the reactor, a quench

operation, using argon as the quench material, efficiently terminates the decomposition of

acetylene to hydrogen and soot.The measurement/analysis system consists of two parts: one is the temperature mea-

surements, and the other is gas analysis system. The temperature variation of cooling water

is measured by thermocouples. According to the temperature data and energy balance of

the system, the average temperature of plasma can be estimated, which is in the range of

4,60010,000 K at the nozzle exit with a power input of 1.63.0 kW. The gas measure-

ments are performed when the reactor has reached a stationary state. Gas samples are taken

for analysis at ten different points of the reaction chamber, which are monitored qualita-

tively by an on-line mass spectrometer (AMETEK Corp., DCU 200) and analyzed quan-

titatively by two gas chromatographs (Shanghai Tianmei Corp., GC7890-II and Shimadzu

Corp., GC2014). The concentrations of gaseous hydrocarbons such as CH4, C2H2, C2H4,C2H6, and so on, are detected by GC-Hydrogen flame ionization detector (FID) with a

capillary column (HP-plot Al2O3, Agilent Corp.). H2, Ar, CO and CO2 are determined by

GC-thermal conductivity detector (TCD) with a packed column (TDX01, Lanzhou

Chemical Engineering Research Institute). In order to prevent the GC from being polluted,

a filter is installed before it to collect a small amount of carbon black and unconverted

liquid hydrocarbons.

According to the measurement results, the conversion of reactant is defined as

Conversion 1 Foutfeedstock

Finfeedstock

100% 1

where Ffeedstock is the mass flow rate of liquid hydrocarbons (including n-hexane, cyclo-

hexane and toluene) at standard temperature and pressure, the superscripts out and in

denote the product stream and feedstock inflow respectively.


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The yield (y) of a particular product is generally defined as

Yield Mass flow rate of a particular product

Mass flow rate of feedstock inflow 100% 2

Two yield numbers are particularly important, i.e., the yields of ethylene and acetylene,which for n-hexane feedstock are

YC2 H4 FoutC2 H4

Finnhexane 100% 3

YC2 H2 FoutC2 H2

Finnhexane 100% 4

The feedstock specific input power and specific energy requirement (SER) that express

the energy density in the reactor and the economic value of the process are defined as

Specific input power kJ=kg Input power kW

Mass flow rate of feedstockkg=s5

SER kwh=kg C2H2 Input power kW

Mass flow rate of C2H2 kg=hr6

Results and Discussion

It has been acknowledged that thermal energy is the driving force for acetylene formation.

Therefore, the operating conditions which determine the heat transfer between the liquid

hydrocarbons and surrounding gases, such as the feedstock specific input power and reactor

length (residence time), appear to be of primary importance. For this reason, we focus on

how these operating conditions affect the reactant conversions, yields of products and the

SER of the process in this work.

Influence of Feedstock Specific Input Power

In the first series of experiments, n-hexane, cyclohexane and toluene, which represent the

typical aliphatic, alicyclic and aromatic fragments of volatiles respectively, were fed into the

H2/Ar plasma reactor by a syringe pump with the volume flow rate of 26 mL/min. Figure 2

shows the effect of feedstock specific input power on the conversions of these three liquid

hydrocarbons. As illustrated, the reactant conversions increase gradually with the increase of

feedstock specific input power. The conversions ofn-hexane and cyclohexane increase sharply

with increasing feedstock specific input power from 2.5 9 104 to 6.0 9 104 kJ/kg, and then

change slowly when the value higher than 6.0 9 104 kJ/kg. The variation of toluene conversion

shows the same trend but the value is much lower under the same operating conditions, whichmay chiefly be because of its highly condensed aromatic structure. Therefore, the feedstock

specific input power of 1.0 9 105 kJ/kg seems to be the optimal value under the specified

operating conditions. Then-hexane conversions can reach up to 93 % while the conversions of

cyclohexane and toluene can only achieve 77 and 65 % respectively when the feedstock

specific input power in excess of 1.0 9 105 kJ/kg. This might be attributed to the CC bond

energy data that the carbon bonds in aromatic structures (BDEs: *426.8 kJ/mol) and


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cycloalkanes (BDEs: *377.0 kJ/mol) is larger than that in straight-chain alkanes (BDEs:

*363.6 kJ/mol). Both the theoretical analysis and experimental results reveal the sequence of

chemical reactivity: n-hexane (n-C6H14)[ cyclohexane (C6H12)[ toluene (C7H8).

The influence of feedstock specific input power on the yield of C2H2 and C2H4 are

shown in Figs. 3 and 4, respectively. As showed in Fig. 3, the yield of C2H2 increases

gradually with the increase of feedstock specific input power. The yield of C2H2 in

n-hexane and cyclohexane pyrolysis process reach 50 and 60 % respectively when the

feedstock specific input power is higher than 1.0 9 105

kJ/kg, while the corresponding

value in toluene pyrolysis process stays at a low level (about 14 %). Figure 4 shows that anoptimal yield of C2H4 exists but the value varies greatly in these three liquid hydrocarbon

pyrolysis systems. The maximum yield of C2H4 can reach up to 32 % in n-hexane pyro-

lysis process and 16 % in cyclohexane pyrolysis system, while is lower than 1 % when

using toluene as feedstock. This information indicates that the sequence of ethylene pro-

duction ability: n-hexane (n-C6H14)[ cyclohexane (C6H12)[ toluene (C7H8). The

experimental results of AVCO arc-coal process [11] also showed that compared to water

quench, the propane quench seems to be particularly effective, increasing the yield of

ethylene from 0.3 to 21.4 % and the combined yield of acetylene and ethylene to 51.4 %

(from 37.9 %). This suggests that straight-chain alkanes might be more suitable to be used

as quench materials due to its better ethylene production ability to realize integrated energyutilization. It is worth mentioning that very little coke was formed during the course of

reaction in n-hexane pyrolysis experiments. However, serious coking occurred on the

pyrolysis process with toluene as feedstock. This also indicates that acetylene is formed

principally from the aliphatic and alicyclic components of coal in coal pyrolysis process.

On the other hand, online coke cleaning seems particularly important when using the

hydrocarbons that contain a comparatively higher level of aromatic structures as feedstock

to produce acetylene.

Influence of Reactor Length (Residence Time)

Figure 5 shows the yields of main gaseous products as a function of the reactor length,

which also means as a function of the residence time. The average temperatures of plasma

at the outlet of the plasma torch were estimated to be*6,200 K. Gas samples are taken for

analysis at ten different points of the reaction chamber corresponding to ten different

2.50x104

5.00x104

7.50x104

1.00x105

1.25x105

20

40

60

80

100

n-Hexane

Cyclohexane

Toluene

Conversio

n(%)

Specific input power (kJ/kg)

Fig. 2 Effect of the feedstock

specific input power on the

conversions of reactants (reaction

conditions: flow rate of Ar for

torch = 9 L/min, flow rate of H2

for torch = 1 L/min, flow rate ofAr for quench = 5 L/min)


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residence times calculated from the gas velocity, not taking into account the radial velocity

distribution. The gap between two sampling points is 10 mm, which corresponds to a gas

residence time of about 0.8 ms at a power input of 2.0 kW.

As shown in Fig. 5a, independent of the feed substances, a very fast conversion in thefirst 10 mm of the reaction chamber is observed (e.g., leading to a high yield of C 2H2 over

68 %) corresponding to a residence time of less than 0.8 ms. After this time the yield of

C2H2 changes slightly but has a maximum value with the increase of reactor length (or

residence time). The yield of C2H4 in n-hexane pyrolysis process is roughly twice as high

as that of using cyclohexane as feedstock. This is in good concordance with the mentioned

sequence of ethylene production ability. The yield of C2H4 decreases quickly (nearly

50 %) when the reactor length increases from 10 to 50 mm and then keeps on a steady

value, whether using n-hexane or cyclohexane as feed substance. The yield of CH4 has a

minimum value with the variation of reactor length (residence time) but this phenomenon

is not noticeable, as shown in Fig. 5c. The yield of H2 in n-hexane pyrolysis process

increases quickly when the reactor length increases from 10 to 40 mm and then keeps on a

steady value; while when using cyclohexane as feedstock, this value increases gradually

with the increase of the residence time. It might be attributed to the different coking ability

ofn-hexane and cyclohexane. During the course of cyclohexane pyrolysis, small amount of

coke is formed, leading to a sustained growth in the yield of H2.

2.50x104

5.00x104

7.50x104

1.00x105

1.25x105

10

20

30

40

50

60

n-Hexane

Cyclohexane

Toluene

YieldsofC2H

2(wt%)



specific input power on the yield

of acetylene (reaction conditions:

flow rate of Ar for torch =

9 L/min, flow rate of H2 for

torch = 1 L/min, flow rate of Arfor quench = 5 L/min)

2.50x104

5.00x104

7.50x104

1.00x105

1.25x105

1

10

40

n-Hexane

Cyclohexane

Toluene

Y

ieldsofC2H4(wt%)


0.2


specific input power on the yield

of ethylene (reaction conditions:

flow rate of Ar for torch =

9 L/min, flow rate of H2 for

torch = 1 L/min, flow rate of Ar

for quench = 5 L/min)


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It can be concluded from Fig. 5 that 50 mm shows an optimal reactor length for liquid

hydrocarbons pyrolysis to acetylene under the specific operating conditions in this work.

The reason can be directly attributed to the combined influences of thermodynamics and

reaction kinetics of liquid hydrocarbon pyrolysis. According to the thermodynamics

analysis [27], the concentration of C2H2 is very sensitive to the temperature. Once the

temperature is below 1,800 K, the concentration of C2H2 drops rapidly. A fast reaction of

feed substances occurs when it is well mixed with the hot plasma under ultrahigh tem-

peratures. Together with the heat loss of the reactor, the temperature drops with the

increase of reactor length. Therefore, the temperature might be 1,800 K, the lowest tem-perature at which C2H2 can keep as the principal hydrocarbon constituent, at the reactor

length of 50 mm. After that, further reaction of liquid hydrocarbons does not take place

and the acetylene and ethylene would decompose to soot, methane, hydrogen, and so on.

Therefore, both the yields of CH4 and H2 increase while the yields of C2H2 and C2H4decrease when the reactor length increases from 50 to 100 mm.

A Comprehensive Understanding of Available Experimental Data

Table 1 lists the typical operating conditions and experimental results of liquid hydro-

carbons pyrolysis to acetylene under lab-scale power input together with the performance

of coal pyrolysis process. Based on our previous work [28], known two of the three

variables, i.e., the effective mass ratio of C/H (RC/H), the quench temperature and the

volume ratio of C2H2/CH4 for an arbitrary CHO system, the third unknown variable

could be reasonably estimated using Fig. 6. Therefore, according to Fig. 6, the gas

60

64

68

72

5

10

15

20

25

10 20 30 40 50 60 70 80 90 1003.0

4.5

6.0

7.5

9.0

10 20 30 40 50 60 70 80 90 100

4.5

5.0

5.5

6.0

(a)

(d)(c)

YieldsofC2

H2(wt%)

(b)

n-Hexane

Cyclohexane

YieldsofC2H

4(wt%)

YieldsofCH4(wt%)

Reactor length (mm)

Yields

ofH2(wt%)

Reactor length (mm)

Fig. 5 Effect of the reactor length (residence time) on the yields of main gaseous products, the products

are: a C2H2; b C2H4; c CH4; d H2 (flow rate of Ar for torch = 9 L/min, flow rate of H2 for torch = 1 L/min,

flow rate of Ar for quench = 5 L/min, input power = 2.0 kW, n-hexane specific input power =

4.7 9 104 kJ/kg, cyclohexane specific input power = 6.1 9 104 kJ/kg)


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Table1

Typicaloperatingconditionsandperformanc

eoflab-scalepyrolysisprocess

Unit

TsinghuaUniversity

TaiyuanUniversity[29]

Case

1

2

3

4

5

6

7

8

9

10

11

Feedstock

Coal

Coal

n-Hexane

n-Hexane

Cyclohexane

Cyclohexane

Toluene

To

luene

n-Hexane

Cyclohexane

Toluene

Inputpower

W

1,9

07

2,2

60

2,52

0

3,3

00

2,2

55

2,5

00

2,0

40

2,2

80

40,0

00

40,0

00

40,0

00

Feedrate

g/min

2.8

9

1.6

5

3.96

3.3

0

3.1

0

2.3

2

3.4

7

2.6

0

23.0

8

27.0

8

30.3

4

Specificinput

power

kJ/kg

3.9

69

104

8.2

29

104

3.82

9

104

6.0

19

104

4.3

79

104

6.4

69

104

3.5

39

104

5.2

69

104

1.0

49

105

8.8

69

104

7.9

19

104

H2

v/v%

18.1

8

12.8

2

9.45

12.2

3

11.5

1

12.1

3

7.4

6

9.0

4

38.9

0

34.3

9

38.2

3

C2H2

v/v%

0.9

3

1.0

4

3.96

5.8

9

6.1

4

5.9

3

1.7

7

2.2

7

3.6

0

2.2

3

3.8

3

CO

v/v%

1.9

56

1.8

02

0.08

50

0.0

811

0.0

77

0.0

75

0.0

75

0.0

77

0.0

90

5.8

60

0.1

80

CH4

v/v%

0.1

9

0.0

5

2.25

2.8

3

1.0

8

0.6

2

0.6

2

0.7

7

0.6

5

0.4

0

0.2

3

C2H4

v/v%

0.0

7

0.0

3

3.80

4.4

0

2.1

4

0.8

7

0.0

7

0.0

9

0.5

5

0.3

2

0.4

1

YieldofC2H2

wt%

6.2

16.7

20.5

39.1

41.6

52.6

9.4

16.5

20.0

8.5

8.5

RC/H

0.6

3

0.9

0

4.15

4.2

8

4.3

6

4.0

3

2.4

1

2.5

1

1.1

1

0.8

6

1.1

6

C2H2/CH4

4.9

19.3

1.8

2.1

5.7

9.5

2.8

3.0

5.5

5.6

16.7


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temperature before quench of each case can be estimated and the system performance of

the current experiments can be compared with other data reported in the literature. It can beseen from Fig. 6 that the quench temperatures of our plasma reactor are in the range of

1,5801,750 K, below the suggested appropriate temperature (i.e., 1,800 K). This might

demonstrate that the heat loss of the reactor is too much to maintain the temperature over

1,800 K. Therefore, to shorten the reactor length to 50 mm, as mentioned above, or to

properly increase the energy input would prevent the decomposition of C2H2, leading to a

high yield of C2H2.

The performance of the reactor described here is superior to that reported in the liter-

ature [29] according to the data listed in Table 1. The big difference between these two

processes is probably due to the RC/H in the gaseous phase. The RC/H in Case 9Case 11 isbelow 1.2, so the corresponding yield of C2H2 remains at a fairly low level, even though

the quench temperature is higher than 1,800 K. The improvements in conversions of liquid

hydrocarbons and yield of C2H2 are primarily due to the improved mixing section design

and higher energy efficiency. As a rule of thumb, improving the initial mixing of reactants

with hot gas and extending the residence time in the high temperature zone would always

benefit this millisecond process and probably result in a better yield of acetylene product.

Because the formation of acetylene from liquid hydrocarbons is strongly endothermic,

relatively large amounts of energy are required per unit mass of acetylene formed. Figure 7

shows the influences of reactor length on the SER of this lab-scale process. It is not obvious

that the SER changes with reactor length. As shown in Fig. 7, the SER of n-hexanepyrolysis process is much lower than that of using cyclohexane as feedstock. The minimum

0 1 2 3 4 50.1

1

10

100

2000 K

1900 K

1800 K

1700 K

1600 K

1500 KMoleratioof

C2

H2

/CH

4

Effective mass ratio of C/H

Case 1_ Coal

Case 2_ Coal

Case 3_n-C6H

14

Case 4_n-C6H

14

Case 5_C6H

12

Case 6_C6H12

Case 7_C7H

8

Case 8_C7H

8

Case 9_n-C6H

14

Case 10_C6H

12

Case 11_C7H

8

Fig. 6 Variations of quench

temperature with different

effective mass ratio of C/H and

mole ratio of C2H2/CH4 and

evaluations of quench

temperature in different coal/hydrocarbons pyrolysis system

10 20 30 40 50 60 70 80 90 100

14

16

18

20

22

24Cyclohexane, C

2H

2

Cyclohexane, C2H

2+C

2H

4

n-Hexane, C2H

2

n-Hexane, C2H

2+C

2H

4

Specificenergyrequirement

(k

Wh/kg)

Reactor length (mm)

Fig. 7 Effect of reactor length

on the specific energy

requirement (SER) of the lab-

scale process (flow rate of Ar for

torch = 9 L/min, flow rate of H2for torch = 1 L/min, flow rate of

Ar for quench = 5 L/min,

input power = 2.0 kW,

n-hexane specific inputpower = 4.7 9 104 kJ/kg,

cyclohexane specific input

power = 6.1 9 104 kJ/kg)


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SER of this laboratory processes is approximately 18 kWh/kg C2H2 and 13.6 kWh/

kg (C2H2 ? C2H4). As the plasma reactors are scaled to larger sizes, with more favorable

volume to surface area ratios, the system thermal efficiency can be improved. Therefore,

process heat recovery might further reduce the SER by about 30 % or so to around

12 kWh/kg C2H2, where the energy efficiency is high enough to meet the industrialrequest, such as the Huels process (12.1 kWh/kg C2H2) and DuPont process (8.8 kWh/

kg C2H2).

Conclusions

This work presented a comprehensive study on liquid hydrocarbons including n-hexane,

cyclohexane and toluene pyrolysis to acetylene in H2/Ar plasma. The experiments verify

that the non-aromatic hydrocarbons show better chemical reactivity than partially aromatic

substance. All the pyrolysis reactions are almost completed within the first 10 mm of the

reaction chamber corresponding to a residence time of 0.8 ms. The reactant conversions

and yield of acetylene increase gradually with the increase of feedstock specific input

power. At a feedstock specific input power of 4.7 9 104 kJ/kg, the n-hexane conversion

exceeds 90 % and the yield of acetylene reaches 70 %. With the features of high con-

version, better ethylene production ability and very little coke formation during the whole

reaction process, the straight-chain alkanes are proposed to be used as quench materials to

realize integrated energy utilization of such ultrahigh temperature process. In order to

better understand the basic rules of the hydrocarbon pyrolysis process, comprehensive

comparisons of the current experiments with the coal/hydrocarbons pyrolysis data reportedin the literature were made. The comparisons indicate that the quench temperatures of our

plasma reactor should be raised to the suggested appropriate temperature (i.e., 1,800 K).

Meanwhile, compared to solid feedstock, liquid hydrocarbons are easier to obtain a high

RC/H in the gaseous phase. As a rule of thumb, improving the initial mixing of feedstock

with hot gas and extending the residence time in the high temperature zone would benefit

this millisecond process. The estimation of SER in the current small apparatus is just a hint

to understand the economics of the process. More reasonable and reliable improvements on

the system heat loss would be made after the optimization of reactor configuration at a

larger scale, where the energy efficiency is high enough to meet the industrial request.

Acknowledgments Financial supports from National Basic Research Program of China (973 Program no.

2012CB720301), National Natural Science Foundation of China (NSFC) under the grant of no. 20976091

and no. 21176137, National Institute of Clean-and-Low-Carbon Energy (NICE) and Xinjiang Tianye

(Group) Co. Ltd. are acknowledged.

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12Experimental Study of Liquid Hydrocarbons Pyrolysis