Pyrolysis of Acetylene for Vacuum Carburizing of Steel: Modeling with Detailed Kinetics

INTERNATIONAL JOURNAL OF CHEMICAL

REACTOR ENGINEERING

Volume 7 2009 Article A10

Pyrolysis of Acetylene for VacuumCarburizing of Steel: Modeling with

Detailed Kinetics

Rafi U. Khan∗ Dominic Buchholz†

Frank Graf‡ Rainer Reimert∗∗

∗Universitat Karlsruhe (TH), rafi [email protected]†Universitat Karlsruhe (TH), [email protected]‡Universitat Karlsruhe (TH), [email protected]∗∗Universitat Karlsruhe (TH), [email protected]

ISSN 1542-6580

UnauthenticatedDownload Date | 7/12/16 4:18 AM

Pyrolysis of Acetylene for Vacuum Carburizing ofSteel: Modeling with Detailed Kinetics∗

Rafi U. Khan, Dominic Buchholz, Frank Graf, and Rainer Reimert

Abstract

Acetylene was pyrolyzed in a thermogravimetric reactor system under the vac-uum carburizing conditions of steel. The products of pyrolysis were collected andanalyzed by gas chromatography. The reactor was numerically simulated by usinga detailed reaction mechanism. Sensitivity analysis and reaction flow analysis wasperformed under the investigated conditions to identify important reaction steps.The comparison of experimental and modeling results is discussed in this paper.

KEYWORDS: acetylene, pyrolysis, modeling, simulation, carburizing, detailedkinetics

∗The authors are thankful to Professor O. Deutschmann and K. Norinaga for providing necessarydata for the model and to Professor J. Warnatz and U. Mass (University of Heidelberg) for provid-ing the HOMREA software and support to run the software. The authors are also thankful to thedeveloper team of Cantera.


1 IntroductionVacuum carburizing or low pressure carburizing process, as its name implies, iscarried out in a vacuum furnace at pressures below atmospheric pressure. Vac-uum carburizing using methane (CH4) as the carburizing gas was introduced in the1960s. This process requires comparatively higher pressures up to 500 mbar. Prob-lems experienced with this process were achieving the uniformity and repeatabilityrequired to meet the quality specifications for precision parts. Other drawbacks in-clude the formation of soot and higher hydrocarbons which can settle on furnacewalls requiring considerable maintenance time and cost. To overcome these prob-lems, propane (C3H8), ethylene (C2H4) or acetylene (C2H2) are being used for car-burizing. The steel parts are exposed to the carburizing gas at temperatures between900-1050 ◦C and total pressures between 2-20 mbar. Under the high temperaturethe carburizing gases are pyrolyzed and form atomic carbon on the steel surface.The carbon diffuses into the steel and locally increases the carbon concentration.At the surface the concentration of carbon is about 1 mass- % and then decreases tothe core concentration of typically 0.2 mass-% depending on the steel type [1–15].The vacuum carburizing process has some advantages as compared to gas carburiz-ing at atmospheric pressure, e.g.

• High temperature carburizing resulting in shorter carburizing time or increasedproductivity

• Creation of a surface free of oxides

• Carburization of complex shapes such as blind holes

• Reproducible and uniform results

• Environment friendliness

In the conventional gas carburizing at atmospheric pressure, the carbon po-tential is controlled by adjusting the flow rate of the carburizing gas. Carbon po-tential of the furnace atmosphere can be related to partial pressure of CO2 or O2

or vapour pressure of water by equilibrium relationships and a sensor can be usedto measure it. This method of carbon-potential control cannot be used for vacuumgas carburizing due to the absence of thermodynamic equilibrium which is one ofthe main difficulties to control the vacuum carburizing process. The formation ofsoot during carburization is also undesirable and the process parameters should beselected such that the formation of soot is minimized. The amount of carbon avail-able for carburizing the steel depends on the partial pressure of the carburizing gas,

1Khan et al.: Pyrolysis of Acetylene for Vacuum Carburizing of Steel


carbon content in the carburizing gas and the pyrolysis reactions of the carburizinggas.

According to Sugiyama et al. [3], acetylene rapidly dissociates into carbonand hydrogen when it comes into contact with hot steel. Carbon immediately formsiron carbide on the surface of the steel which then diffuses into the steel. Thefollowing reaction rapidly occurs when acetylene gas is introduced into a vacuumcarburizing furnace:

2Fe + C2H2 → 2Fe(C) + H2 (1)

Of course, much more reactions occur during vacuum carburizing of steel. Thethermal decomposition of acetylene has been studied by many researchers in staticsystems[16, 17], in flow systems [18–22], in shock tubes [23–32] and in flames[33, 34]. The temperature range covered in these studies is about 625 K to 4650 K.A radical chain mechanism was proposed in the 1970s [35, 24, 36] assuming

C2H2 + C2H2 → C4H3 + H (2)

as the initiation reaction.According to Kiefer et al. [37] acetylene pyrolysis proceeds differently de-

pending on the temperature regime as follows:(i) T < 1100 K where the homogeneous reaction is a molecular polymerization.(ii) 1100 < T < 1800 K where the process is still dominated by a molecular poly-merization.(iii) T > 1800, where a fragment chain carried by C2H and H drives a polymeriza-tion to polyacetylene.The core mechanism of acetylene pyrolysis has been reported as follows:

C2H2 + C2H2 → C4H4 (3)C2H2 + C2H2 → C4H2 + H2 (4)C2H2 + C2H2 → C4H3 + H (2)

C4H4 → C4H2 + H2 (5)C4H4 → C4H3 + H (6)

Frenklach and coworkers [28] identified the isomers n-C4H3 and i-C4H3 andproposed that the reaction (7) shown below is the initiation reaction.

C2H2 + C2H2 → n− C4H3 + H (7)C2H2 + C2H2 → i− C4H3 + H (8)

Wu et al. [29] proposed that i-C4H3 is the product after noting the discrepancybetween endothermicity of the reaction (7) and the observed activation energy. Du-ran et al. [38] suggested that acetylene polymerizes by isomerization to vinylidene

2 International Journal of Chemical Reactor Engineering Vol. 7 [2009], Article A10


which is further converted to vinylacetylene. Colket et al. [39] proposed a de-tailed radical chain mechanism for acetylene pyrolysis with acetone as a source ofinitiation reaction. The initiation by acetone is described as follows:

CH3COCH3 = CH3 + CH3CO (9)CH3CO = CH3 + CO (10)

C2H2 + CH3 = CH3CHCH (11)

Kruse and Roth [32] studied the pyrolysis of acetylene at high temperaturein shock tube and proposed a detailed mechanism for high temperature pyrolysis ofacetylene. The initiation reaction consists of successive abstraction of H atoms :

C2H2 + M = C2H + H + M (12)C2H + M = C2 + H + M (13)

Krestinin [40] studied the kinetics of heterogeneous pyrolysis of acetyleneto explain the carbon film formation on the hot cylinder surface. The work of Cal-lear and Smith [41], who investigated the addition of hydrogen to acetylene, pro-vides the evidence of radical chain mechanism.The effect of acetone on the pyrolysis of acetylene has been also studied by Dim-itrijevic et al. [20] at 914-1039 K and 6-47 kPa. The presence of acetone was foundto accelerate the formation of vinyl acetylene and benzene.Graf [42] studied the pyrolysis of acetylene in a tubular flow reactor and proposeda reaction mechanism consisting of 7 species and 9 non-elementary reactions.

Recently Norinaga and Deutschmann [43] have studied the pyrolysis ofacetylene at 900 ◦C for chemical vapour deposition of carbon and developed adetailed mechanism comprising of 227 species and 827 reactions. Acetylene isconsumed by dimerization to C4H4 (68 % ), C4H2 (17 %) and formation of ben-zene by the combination of C4H4 and C2H2(7%).

In the current work, investigations are carried out to achieve a better under-standing of the pyrolysis of acetylene under the vacuum carburizing conditions ofsteel. The objective of this research is to development a model for the predictionof gas phase composition resulting from the homogeneous pyrolysis of acetyleneunder the vacuum carburizing conditions of steel. The developed model will beused for further investigations of heterogeneous pyrolysis of acetylene to predictthe carbon flux on steel parts being carburized and optimize the behavior of thewhole reactor.



2 ExperimentalThe flow sheet of the thermogravimetric apparatus is shown in Fig. 1. The ap-paratus consists of the thermobalance (type NETZSCH STA-409 CD) connectedwith the gas feed system and with the gas analysis system. In the gas feed sys-tem, flows of six different gases can be regulated and mixed by means of mass flowcontrollers (MFC) (type Bronkhorst EL-FLOW). Over three-way Valves they canbe directed either to a calibration system or via a common line into the reactor ofthe thermo balance. The weighing mechanism is not separated from the reactionspace of the thermobalance, therefore the balancing system must be protected fromdamage caused by the entrance of particles, hot, corrosive or reactive gases. Theweighing mechanism is also thoroughly flushed with an inert gas (argon). The flowrate of the flush gas is measured by a mass flow controller in a pulse box. Withthis pulse box two sample gas streams with a specified volume can be fed into thethermobalance . Over a valve the reactor can be flooded also directly with inertgas. When the flow rate exceed 9 l/h(NTP), the sample carrier begins to swing andthe weighing accuracy is affected from the incident flow. From the reactor exit, theproduct or exhaust gases flow through a bypass to the outlet or through a 200 ◦Cheated line to a Fourier transform infrared spectrometer (type Bruker Tensor 27).In order to protect the following analytic devices against tar-like hydrocarbons andsoot particles, a heated fine filter made of sinter metal with a pore diameter of 15µm is used upstream. After going through the IR gas measuring cell, the hydrogencontent of the exhaust gas is measured in a heat conductivity detector (type ABBCaldos 17). In addition, part of the exhaust gas passes through a Micro Gas Chro-matograph (type Varian CP 4900). After leaving the analyzers, the exhaust gasesare led to the outlet. An oil-free vacuum pump (BOC Edward XDS5-S) is attachedto the thermo balance, with which the equipment including the gas measuring cellof the FTIR can be evacuated. The pressure in the apparatus should not exceed 0.1bar above ambient pressure.

In the present investigations, the flow rate of acetylene was 3 l/h (NTP) and9 l/h (NTP), respectively, while the concentration of acetylene was varied from 0.25to 1.6 vol.%. The experiments were performed at controlled temperatures of 950and 1000 ◦C. Fig. 2 shows the sketch of the thermo balance used for the experi-ments. The thermo balance has a measuring range from 0 to 18 gram and a measur-ing accuracy of ± 5 µg. A sample carrier rod holds a ceramic crucible containingthe test sample. Two axial temperature profiles for temperature controller valuesof 950 and 1000 ◦C are shown in Fig. 2. The reactor, the sample carrier and theprotection shields in the heating zone are all made of Al2O3 for operating temper-atures up to 1600 ◦C. At the upper end of the sample carrier a thermocouple (typeS) measures the temperature in the sample. The reactor is heated from the outside



Figure 1: Simple process flow diagram of the thermogravimetric apparatus

with an electrical resistance heating. The furnace temperature is regulated by thetemperature measurement at the sample carrier. The reaction gas passes through anannular ring from downside of the reactor and after passing through the protectionshields flows toward the sample. The highest temperature is reached at the end ofthe sample carrier. By the interior pipe at the reactor entrance, the cleaning gasflows through the balancing system into the reactor. In order to exclude the pos-sibility that carburizing is disturbed by nitriding of the steel sample with nitrogen(N2) , argon (Ar) is used as a carrier gas.

3 Reaction Mechanism and ModelingThe reactor was numerically simulated using the detailed mechanism of Norinagaand Deutschmann [43]. The mechanism has been developed to describe the chem-ical vapour deposition (CVD) of carbon from light hydrocarbons. The mechanismwas used with the 0D model of HOMREA [44, 45] software package that performs



Figure 2: Sketch of the thermobalance (NETZSCH STA-409 CD) with typical tem-perature profiles



Figure 3: Sensitivity analysis at 950 ◦C with respect to acetylene

computational analysis of time-dependent homogeneous reaction systems. The re-action mechanism consists of 227 species and 827 reactions most of which arereversible. The HOMREA requires the forward reaction rate parameters and thethermodynamic data for all of the participating species and calculates the backwardrate constants for each reversible reaction in the mechanism. The temperature pro-file in the reactor was not considered. This can also be justified to some extent bythe fact that temperature in the heated section of the reactor has small variation.Further due to the presence of radiation protection shields at the inlet of the reactor,the temperature is much lower at the inlet section than in the middle section and theconversion of acetylene is negligible at these temperatures. So the volume of thereactor used for isothermal simulations corresponds to the hot section of the reactorwhere the variation of the temperature is small enough to assume it to be isothermal.Since acetylene used for experimental investigations was of high purity, presenceof acetone was neglected in simulations.

The detailed mechanism was also used with the 0D model of an open sourcereactive flow modeling tool Cantera [46] and the results of both the modeling toolsCantera and HOMREA are discussed.



4 Results and DiscussionsThe analysis and the comparison are based on the concentrations of acetylene,methane, ethylene, vinyl acetylene, diacetylene and benzene measured at the exitof the reactor. The amount of soot formed was not measured but the hydrocarbonsother than the measured were regarded as soot and carbon content for these com-pounds was calculated by material balance. Sensitivity analysis and reaction flowanalysis was performed for 26.3 sec at 950 ◦C with 0.25 volume% of acetylene atthe inlet to identify the important reaction steps of the detailed mechanism. Theresults of reaction flow analysis are shown in Table 1. Most of the acetylene isconsumed by the molecular mechanism forming vinyl acetylene and diacetylene.Methane is formed mainly by the reaction of methyl radical with hydrogen. Mostof the ethylene is formed by the reactions of hyrogen with vinyl radical and acety-lene respectively. Vinyl acetylene is formed mainly from acetylene by its reactionswith another molecule of acetylene and vinyl radical, respectively. Reaction flowanalyses reveal that most of the diacetylene (about 96%) formed results from thereaction no. 4. This is also the important reaction causing most of the consump-tion of acetylene (about 76%) under these conditions. In Table 1, symbols C4H51,C4H512 represent isomers which contain 4 carbon and 5 hydrogen atoms, the sym-bols C4H61, C4H612 represent isomers which contain 4 carbon and 6 hydrogen atomsand the symbol A1C2H3 reprents styrene (C8H8) [43].

The sensitivity analysis result for acetylene is shown in the Fig.3. This anal-ysis is performed to identify the reactions whose rates require accurate determina-tion. The reactions having higher sensitivity coefficients have great influence on theresults of mathematical modeling [44]. The analysis reveals that the consumption ofacetylene has higher sensitivity towards the bimolecular combinations of acetyleneleading to the formation of diacetylene and vinyl acetylene. In the previous studyby Norinaga and Deutschmann [43], it has been reported that at a temperature of900 ◦C most of the acetylene consumption is by dimerization to vinylacetylene byreaction no.3 (see section no. 1). In the present study, higher temperature favoursreaction no. 4 compared to reaction no. 3 resulting in the consumption of acetyleneto diacetylene. This has been also illustrated in Fig. 4 produced by Cantera. Mainreaction paths are shown at 950 ◦C for a residence time of 26.3 sec with 0.25 vol-ume % acetylene at the inlet. The edge values when multiplied by scale provide theflow of carbon along that path. The comparison of experimentally measured yieldsof carbon and model predictions under vacuum carburizing conditions of steel withacetylene is shown in Figures 5 to 9.



Table 1: Reaction flow analysis at 950 ◦C, 26.3 sec and0.25 volume % of acetylene at the reactor inlet

Consumption of C2H2 Formation of C2H2

C2H2 + C2H2 = C4H4 13% C2H3 + H = C2H2 + H2 1%C2H2 + C2H2 = C4H2 + H2 71% C4H2 + C2H = C2H2 + C4H 26%C2H2 + C4H = C6H2 + H 3% I-C4H3 + H = C2H2 + C2H2 61%C2H2 + H +M = C2H3 +M 1% C2H2 + C2H = C4H2 + H 9%C2H + H2 = C2H2 + H 1%C2H2 + C4H4 = C6H6 1%C6H3 + H = C2H2 + C4H2 2%Formation of CH4 Consumption of CH4

H + CH3 + M = CH4+ M 2% C4H6 + CH3 = CH4 + N-C4H5 61%H2 + CH3 = CH4 +H 95% C4H6 + CH3 = CH4 + I-C4H5 31%

C4H6 + CH3 = CH4 + I-C4H5 1%C4H612 + CH3 = CH4 + C4H512 2%C4H61 + CH3 = CH4 + N-C4H51 1%

Formation of C2H4 Consumption of C2H4

H2 + C2H2 = C2H4 12% C2H4 +H + M = C2H5 + M 4%C2H4 + M = H2 + C2H2 + M 4% C2H4 + C2H3 = C4H6 + H 52%C2H4 + H = H2 + C2H3 70% C4H6 = C2H2 + C2H4 42%C2H4 + C2H = C4H4 + H 2%N-C4H51 = C2H4 + C2H 5%Formation of C4H4 Consumption of C4H4

C2H2 + C2H2 = C4H4 85% CH3 + C3H2 = C4H4 + H 4C2H2 + C2H3 =C4H4 + H 13% C4H4 = H2 + C4H2 8

C4H4 + H = H2 + N-C4H3 19%C4H4 + H = H2 + I-C4H3 32%C2H2 + C4H4 = C6H6 26%C4H4 + C4H4 = A1C2H3 1%C4H4 + C4H4 = C8H8 2%N-C4H51 = C4H4 + H 1%


C2H2 + C2H2 = C4H2 + H2 96% H2 + C4H = C4H2 + H 45%C4H2 + C2H = C2H2 + C4H 2%C4H2 + C2H = H + C6H2 13%C4H2 + C4H2 = H2 + C8H2 9%H + C6H3 = C2H2 + C4H2 28%




C2H2 + C4H4 = C6H6 96% C6H6 + H = C6H5 + H2 97%C6H6 + C2H3 = A1C2H3 + H 1%

The model predictions are in agreement with the experimental measure-ments except for diacetylene. As shown in Fig. 6, the model significantly over pre-dicts the formation of diacetylene under the investigated experimental conditions.This is probably due to the inaccurrate estimation of the Arrhenius parameters forthe reaction no. 4 or some missing reaction pathways in the reaction mechanism.

Scale = 1.5e-007 reaction path diagram following C

C4H2

C2H

0.0831

C6H2

0.187C6H3

0.177

C4H

0.517

0.0933

C2H2

1

0.276

C4H4

0.152

0.0884

C6H6

0.0665

0.349 0.552

Figure 4: Main reaction paths showing flow of Carbon at 950 ◦C with 0.25 vol-ume % acetylene at the inlet

However the amount of diacetylene plus rest of the hydrocarbons predicted by themodel when compared to the experimental yields of soot are in agreement as shownin Fig.7 to Fig.9. Mole fraction profiles as function of time are shown in Fig.10 toFig. 16 at 950 ◦C produced by the modeling tools HOMREA and Cantera. Thedetailed kinetics modeling approach which was used for modeling the pyrolysisof propane under vacuum carburizing conditions in our previous [15] work lookspromising also for the pyrolysis of acetylene.



inlet C2H2 in vol.%

%Y

ield

ofC

arbo

n(C

1)

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

10-2

10-1

100

101

102 sim_CH4sim_C2H2sim_C2H4sim_C4H4sim_C6H6exp_CH4exp_C2H2exp_C2H4exp_C4H4exp_C6H6

C2H2

temperature=950 °C

C2H2 flow rate = 3 l/h (NTP)

C6H6

C4H4

C2H4

CH4

temperature=950 °C


Figure 5: Comparison of experimental and simulations results

inlet C2H2 in vol.%

%Y

ield

ofC

arbo

n(C

1)

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

10-2

10-1

100

101

102

temperature=950 °C


temperature=950 °C


sim_C4H2

exp_C4H2

Figure 6: Comparison of experimental and simulations results



inlet C2H2 in vol.%

%Y

ield

ofC

arbo

n(C

1)

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

10-2

10-1

100

101

102 sim_CH4sim_C2H2sim_C2H4sim_C4H4sim_C6H6sim_sootexp_CH4exp_C2H2exp_C2H4exp_C4H4exp_C6H6exp_soot

CH4

C2H2

soot

temperature=950 °C


C6H6

C4H4

C2H4

Figure 7: Comparison of experimental and simulation results

inlet C2H2 in vol.%

%Y

ield

ofC

arbo

n(C

1)

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

10-2

10-1

100

101


CH4

C2H2

soot

temperature=1000 °C


C6H6

C4H4

C2H4




inlet C2H2 in vol.%

%Y

ield

ofC

arbo

n(C

1)

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

10-2

10-1

100

101


CH4

C2H2

soot

temperature=1000 °C


C6H6

C4H4

C2H4


Figure 10: Predicted mole fraction profile for acetylene at 950 ◦C



Figure 11: Predicted mole fraction profile for methane at 950 ◦C

Figure 12: Predicted mole fraction profile for ethylene at 950 ◦C



Figure 13: Predicted mole fraction profile for vinyl acetylene at 950 ◦C

Figure 14: Predicted mole fraction profile for diacetylene at 950 ◦C



Figure 15: Predicted mole fraction profile for benzene at 950 ◦C

Figure 16: Predicted mole fraction profile for hydrogen at 950 ◦C



5 ConclusionThe homogeneous pyrolysis of acetylene was investigated under vacuum carburiz-ing conditions of steel in a thermogravimetric reactor. Major products of pyrolysiswere measured at the reactor outlet and results were compared to the simulationresults of a detailed kinetic model. The important reactions which contribute to theformation or consumption of major species have been identified by performing thereaction flow analysis. Sensitivity analysis with respect to acetylene has been per-formed. Experimental results were found in agreement with the simulations exceptfor diacetylene. The model overpredicts the formation of diacetylene.

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Pyrolysis of Acetylene for Vacuum Carburizing of Steel: Modeling with Detailed Kinetics

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