Chapter 9Tubular Flow Reactors

Fabiola Monge, Veronica Aranda, Angela Millera, Rafael Bilbaoand María U. Alzueta

Abstract Tubular flow reactors for studying combustion chemistry areextensively used due to their operational flexibility. In these reactors, conditions oftemperature, pressure, and gas fluid residence time can be carefully controlled. Thedifferent real reactors (turbulent and laminar regimes, with and without tempera-ture profiles) can attain almost ideal behavior (plug flow), diminishing the math-ematical difficulties in the simulation of such environments. Nevertheless, theadvantages and disadvantages, as well as the deviations of real reactors fromideality, must be considered in order to choose the most suitable reaction system ineach case. Tubular flow reactors are proposed as one of the possible facilities toinvestigate the oxidation of oxygenated compounds, which have been suggested asadditives to diesel fuel in order to reduce the formation of soot. Among oxygen-ates, this chapter focuses on the study of some alcohols (methanol, ethanol, pro-panol, and butanol) and ethers (dimethylether and dimethoxymethane), aiming toshow the results that can be obtained by using a tubular flow reactor.

List of Important Symbols and AbbreviationsA ReactantC Concentrationdt Tube diameterD Dispersion coefficientD Diffusion coefficientE(t) Residence time distributionF Molar flow ratek Reaction rate constantL Length of the reactorp PressureQ Volumetric flow rate

F. Monge � V. Aranda � A. Millera � R. Bilbao � M. U. Alzueta (&)Aragón Institute of Engineering Research (I3A), University of Zaragoza, Río Ebro Campus,C/Mariano Esquillor s/n 50018 Zaragoza, Spaine-mail: uxue@unizar.es

F. Battin-Leclerc et al. (eds.), Cleaner Combustion, Green Energy and Technology,DOI: 10.1007/978-1-4471-5307-8_9, � Springer-Verlag London 2013

211

r Radius(-rA) Rate of reactionR Radiust TimeT Temperatureu Velocity of fluidV VolumeXA Fraction of A converted

Greek Symbolsl Viscosity of fluidq Density of fluid

Dimensionless GroupsBo Bodenstein numberRe Reynolds numberSc Schmidt number

AbbreviationsDME DimethyletherDMM DimethoxymethanePFR Plug flow reactorRTD Residence time distribution

9.1 Introduction

A bench scale tubular flow reactor consists of a cylindrical tube with a constantdiameter, where the reactant mixture flows continuously along the tube. This typeof reactor is easy to construct and operational problems as leaks, repairs, etc. arereduced. These reactors can be used to study photochemistry, high temperaturechemistry (Cutler et al. 1988), and to obtain experimental results that allow us tocarry out kinetic studies and mainly to validate detailed kinetic models.

Some of the chemical processes that can be studied in tubular flow reactors (Leeet al. 2000) are coupled reaction systems, gas phase elementary reaction rates, andisolated elementary reactions by means of the consumption of specific reactiveradicals.

Tubular flow reactors are suitable, in particular, for the study of the hydro-carbon oxidation processes. This kind of process is characterized by radical

212 F. Monge et al.

reactions that require high temperature and short reaction times because of theirhigh reaction rate. The tubular flow reactors enable operational flexibility and aresuitable to work with low residence times and high temperature and pressureconditions (Santamaría et al. 2002). They can easily be used with gas chroma-tography and Fourier transform infrared spectroscopy (FTIR) analyses.

Various questions have been raised concerning the qualities of data derivedfrom tubular flow reactors. The questions deal mainly with the fluid flow pattern inthe reactor and the existence of temperature gradients. These important questionsare further analyzed in the following sections.

One of the processes, that is being extensively studied, is the gas phase oxi-dation of oxygenated compounds, mainly because of their relevance as dieseladditives. Some studies of oxidation of oxygenated compounds in tubular flowreactors will be described later within this Chapter.

9.2 Fluid Flow Pattern in Tubular Flow Reactors

Three factors configure the contacting or fluid flow pattern.

(a) State of aggregation of the flowing stream

It is its tendency to clump and for a group of molecules move about together.The state of aggregation of the flowing material depends on its nature. In theextremes, these states are microfluid and macrofluid. In the microfluid state,individual molecules are not attached to their neighbors and each one movesindependently. Some examples are gases and not very viscous liquids. In themacrofluid state, molecules move about in clumps and each clump is uniform incomposition. Some examples are noncoalescing disperse set droplets, very viscousliquids, and solid particles.

(b) Earliness of mixing

The fluid elements of a single flowing stream can mix with each other early orlate in their flow through the vessel. Usually, this factor has little effect on theoverall behavior for a single flowing fluid. However, it can be important for asystem with several entering reactant streams if the mixing occurs with no suffi-cient time for reaction.

(c) The residence time distribution (RTD) of the material that is flowing throughthe vessel. Individual molecules can stay different times in the reactor. TheRTD or exit age distribution function E(t) fulfills that:

Z 10

EðtÞdt ¼ 1 ð9:1Þ

9 Tubular Flow Reactors 213

9.3 Ideal Plug Flow Reactor Model

Models are useful for representing material flow in real vessels. The resultsobtained with two ideal flow patterns, plug flow, and mixed flow, are simple totreat. A tubular flow reactor can be a source of high quality rate data when the flowpattern can be idealized as a plug flow.

Steady state plug flow reactors consider that the fluid is perfectly mixed in theradial direction and, consequently, each ‘‘plug’’ or cross-section of the reactor hasuniform fluid properties (velocity, composition, temperature and pressure) (Lev-enspiel 1999). If the radial flow velocity is uniform, all the elements will have thesame residence time in the reactor.

It is supposed that fluid flows along the axial direction with negligible diffusivetransport in that direction and there is no mixing between the thin ‘‘plugs’’ (dif-ferential volume of a fluid element, dV), Fig. 9.1. It is assumed that fluid com-position changes progressively through the reactor.

Since the fluid composition changes progressively along the length in a plugflow reactor, the material balance, Eq. (9.2), is made for a fluid element (dV) andapplied for the reactant A:

In� Out ¼ Accumulationþ Consumption ð9:2Þ

Considering the steady state (the accumulation term is zero) and the nomen-clature shown in Fig. 9.1, it results:

FA � ðFA þ dFAÞ ¼ ð�rAÞdV ð9:3Þ

Taking into account that:

dFA ¼ d FA0ð1� XAÞ½ � ¼ �FA0dXA ð9:4Þ

It results:

FA0dXA ¼ ð�rAÞdV ð9:5Þ

Therefore:

ZV

0

dV=FA0 ¼ZXAf

0

dXA=ð�rAÞ ð9:6Þ

Fig. 9.1 Sketch of a plug flow reactor

214 F. Monge et al.

Where the reaction rate per unit volume (–rA) depends on the temperature andthe reactant concentration. FA is the molar flow rate of the reactant, FA0 the inletmolar flow rate of the reactant and XA the fraction of A converted.

9.4 Flow Regimes in the Vessel

The radial velocity profile depends on the flow regime on the vessel. Figure 9.2shows the radial velocity profiles corresponding to turbulent and laminar flowscompared with the plug flow.

Turbulent flow appears at high velocity gas flows. It is the flow regime that bestapproaches plug flow, because the high gas velocity results in an almost flatvelocity profile on the cross section. Dryer (1972) found wall effects to beunimportant and that radial concentration and temperature variations were small.Turbulent flow also offers advantages relatives to the isothermal assumption, sincemixing favors the heat transfer and the radial temperature uniformity. However,turbulent flow also has some disadvantages. For example, one disadvantage is thatrelated to the high flow velocities needed to obtain turbulent flow. Sometimes,these high flow rates may not be achievable in laboratory reactors, or involve highcost of reactants. In addition, in some cases and in order to have a significant fluidresidence time, the installation results in very long reactors. Additionally, theformation of eddies in the turbulent regime can produce mixing in longitudinaldirection, moving away from an assumption of a plug flow reactor, and the the-oretical differential volume of a fluid element, dV, would become thicker.

Another disadvantage of turbulent flow reactors is the unknown position of thereaction start within the mixing region. Thus, the reaction zone is not totallydefined. Usually, the mixing time is determined by fitting experimental data.

Laminar regime assumes that the flow of every element fluid is orderly andparallel to the walls of the reactor, without any perpendicular flow to that direction.This leads to a parabolic velocity (u) profile, Fig. 9.2, given by the equations:

u ¼ umax 1� r=Rð Þ2h i

ð9:7Þ

umax ¼ uðr ¼ 0Þ ¼ P1 � P2=4lLð ÞR2 ¼ 2um ð9:8Þ

Fig. 9.2 Radial velocity profiles in tubular reactor

9 Tubular Flow Reactors 215

where um is the average velocity, R the total reactor radius, r the radius where themeasure is taken, L the total length, l the viscosity of the fluid, and P1-P2 thefluid pressure drop.

For laminar flow in tubes, different regimes can be obtained.

(a) If the tube is long enough, the molecular diffusion in the lateral direction willhave enough time to disturb the parabolic velocity profile and plug flow ordispersion model can be applied. In the dispersion model, a diffusion-likeprocess superposed on plug flow is assumed. To distinguish this process frommolecular diffusion, it is called dispersion or longitudinal dispersion. Tocharacterize the dispersion model the dimensionless group (D/uL) is used,where D is the dispersion coefficient (m2/s). When D/uL = 0, ideal plug flowis obtained. When D/uL value increases, larger deviations from plug flow areobtained.

(b) If the tube is short enough or the material is very viscous, the moleculardiffusion has not enough time to act and pure convection regime is obtained.The pure convection model assumes that each element of fluid slides part itsneighbor with no interaction by molecular diffusion. Thus, a spread in the fluidresidence according to the radial parabolic velocity profile must be considered.

In this case, the fluid residence time distribution, E(t), is used to calculate thereactant concentrations, CA, according to the equation:

�CA=CA0 ¼Z1

0

ðCA=CA0ÞEðtÞdt ð9:9Þ

For the radial parabolic velocity profile, the residence time distribution is:

EðtÞ ¼ �t2�

2t3 for t [�t=2 ð9:10Þ

Some examples for different kinetic order of a Newtonian fluid in a pipe are:

• Zero order

�CA=CA0 ¼ 1� k�t=2CA0ð Þ2 ð9:11Þ

• First order

�CA=CA0 ¼ �t2=2� � Z1

�t=2

e �kt=�t3ð Þdt ð9:12Þ

• Second order

�CA=CA0 ¼ 1� kCA0�t 1� kCA0�t=2ð Þ ln 1þ 2=kCA0�tð Þð Þð Þ½ � ð9:13Þ

216 F. Monge et al.

where k is the reaction rate constant, t the mean residence time of fluid in the pipe,CA0 the inlet reactant concentration and �CA the mean concentration of the reactant.

(c) If the flow velocity is so slow, then the main movement of the fluid is bymolecular diffusion, not by bulk flow, pure diffusion regime is obtained.

Usually, gases are likely to be in the dispersion regime approaching to plugflow, liquids in dispersion regime or pure convection regime, and the very viscousliquids (such as polymers) in the pure convection regime. The pure diffusionregime is rarely met outside of reservoir engineering.

To characterize the different regimes in laminar flow, the Bodenstein number(udt/D) can be used. D is the diffusion coefficient of the fluid and it is not the axialdispersion coefficient except in the pure diffusion regime. It measures the flowcontribution made by molecular diffusion.

Bo ¼ Re � Sc ¼ dtuq=l � l=qD ð9:14Þ

Figure 9.3 shows the different regimes as a function of Bo and L/dt, where L isthe vessel length and dt the vessel diameter. The point on the chart corresponds tothe fluid being used (Schmidt number, Sc), the flow conditions (Reynolds number,Re) and vessel geometry (L/dt). q is the density of the fluid. This chart only hasmeaning if laminar flow is obtained. It does not apply for turbulent flow.

9.5 Temperature Gradients in the Reactor

The existence of temperature profiles can also complicate the analysis of theexperimental data obtained. Usually, the reactor is heated by an electrical oven and

Fig. 9.3 Map showingwhich flow models should beused in any situation

9 Tubular Flow Reactors 217

longitudinal and radial temperature gradients can exist in the reactor. Longitudinaltemperature gradients depend on the way that the fluid is heated or cooled and onthe influence of the endothermicity or exothermicity of the reaction.

It is important to achieve fast heating of the fluid at the inlet of the reactor andconversely, fast cooling that at the outlet to prevent further reaction. These factscan allow us to obtain an important zone with a constant temperature. An exampleof a longitudinal temperature profile is shown in Fig. 9.4.

The influence of the endothermicity or exothermicity on the longitudinaltemperature gradient can be minimized if the reactant is highly diluted andtherefore low heat is released during the reaction.

The radial temperature profile is not significant if the fluid flow is in turbulentregime, since mixing favors heat transfer and the radial temperature uniformity. Ifthe fluid flow is in a laminar regime, a radial temperature gradient can be created.These gradients can be minimized by decreasing the tube diameter.

9.6 An Example of a Laboratory Scale Tubular FlowReactor

Figure 9.5 shows an example of a laboratory scale tubular flow reactor developedby Kristensen (1997), and that is used by the Combustion and Harmful EmissionControl group of the Technical University of Denmark and by the Thermo-Chemical Processes Group in the University of Zaragoza, Spain.

It is reasonable to approximate the laminar flow field to plug flow if the gas ispremixed and the radial velocity gradients are sufficiently small to allow fluid

Fig. 9.4 Temperature profilefor 400 K along the reactorlength measured on a tubularflow reactor with temperaturegradient

218 F. Monge et al.

elements to exhibit similar residence times. Based on correlations shown inFig. 9.3, in the present reactor, low axial dispersion conditions are obtained atvolumetric flow rates of 1–3 standard liter per minute (Rasmussen et al. 2008a).Thermal gradients along the reactor can be minimized if the reaction zone islocated far enough from the ends of the oven. Moreover, reactants must be pre-heated before being mixed.

The reactor shown in Fig. 9.5 has one main reactor inlet (1) and three injectorinlets (2). The main gas and the three inlet gases are heated separately and mixedin point number 3. The gases react along the reaction zone (4). The reaction isfrozen at the reactor outlet using an external addition of air, which is fed throughthe point number 6. The product gases go out of the reactor through the exitnumbered as 5.

As it can be observed in Fig. 9.5, the reaction zone does not show any sig-nificant temperature profile, as it was intended.

9.7 Oxygenated Compounds

Typical additives for diesel fuel are compounds that contain oxygen in theirmolecular constitution which alters the electronic structure and the C/H ratio. The

Fig. 9.5 Temperature profile for 400 K along the reactor length on a laminar flow reactor with aflat temperature gradient within ±5 K

9 Tubular Flow Reactors 219

influence of oxygen as a component of the fuel in the formation of soot has beenstudied in numerous works and, the results show that generally an increase of theoxygen content in the fuel composition results in a diminution of particulate matter(Choi and Reitz 1998; Pepiot-Desjardins et al. 2008; Demirbas 2009). Althoughoxygenated compounds can lead to the formation of soot due to their carboncontent, the oxygen that is present in the molecule structure can participate inreactions forming new intermediate compounds or in the oxidation of soot pre-cursors and soot which have been already formed (Sison et al. 2007).

Oxygenates that are usually blended with diesel fuel are alcohols, ethers andesters because of their physical and chemical properties. In the present study aliterature review of the studies of the oxidation of methanol (CH3OH), ethanol(C2H5OH), propanol (C3H7OH), butanol (C4H9OH), dimethylether (C2H6O,DME), and dimethoxymethane (C3H8O2, DMM) under flow reactor conditions ispresented. The use of oxygenated additives affects directly some importantproperties of the fuel such as density, viscosity, volatility, behavior at low tem-peratures, and cetane number (Ribeiro et al. 2007). For that reason and, since theengine performance and emissions produced can be altered, it is important tooptimize the amount of additive to be blended.

Some important advantages of the use of alcohols as fuel additives or substi-tutes come from their low viscosity that favors the injection and atomization; thereduction of pollutant emissions due to high oxygen content, low C/H ratio, andlow sulfur content; high evaporative cooling that reduces the temperature in thesystem; and a high laminar propagation speed that improves the engine thermalefficiency (Sayin 2010).

DME and DMM also present good effects on the ignition and combustion whenmixed with diesel fuel because of the high oxygen content, low C/H ratio, and theabsence of C–C bonds. Regarding DME, it has a higher cetane number than dieselfuel, what is seen as an advantage from the point of view of the ignition timingdelay. However, DME presents some disadvantages which are related to its lowlubricity, low calorific value, and high vapor pressure (Park 2009). To solve thelatter problem, DME should be pressurized to liquefy for being blended with dieselfuel, what makes it difficult to be readily used in current engines. For this reason,DMM is an interesting fuel additive because it is a liquid miscible with diesel fuel(Maricq et al. 1998). Nevertheless, DMM presents a low cetane number if it iscompared to DME, leading to an increase of the ignition timing delay and the burnrate (Lu et al. 2007).

On the other hand, the selected oxygenated compounds in this study have anadditional interest because all of them can be obtained from biorefinery processes.This fact provides an attractive use of these compounds, since governments arepromoting the use of biofuels in order to decrease the dependency of the transportsector on fossil fuels (Wiesenthal et al. 2009).

220 F. Monge et al.

9.8 Alcohols: Methanol, Ethanol, Propanol, Butanol

9.8.1 Methanol

Several studies have been focused on the study of the oxidation of methanol underflow reactor conditions aiming to describe the reaction pathways of such processand identify and obtain important kinetic data. In this chapter, some of the mostrelevant research works related to this issue are presented.

The first study including a systematic study of the oxidation of methanol underflow reactor conditions was that carried out by Westbrook and Dryer (1979). Theauthors developed a detailed chemical kinetic model that can describe the oxi-dation of methanol under flow reactor and shock tube conditions (see Chap. 6),between 1000 and 2180 K. The data obtained in the reactors were used to estimatethe values of several reaction kinetic constants, such as the thermal decompositionof methanol or the CH2OH radical, which are key reactions in the oxidation ofmethanol. On the other hand, the contribution of the interactions of radical CH3Owas neglected since they were considered as insignificant.

In Fig. 9.6, some of the experimental methanol pyrolysis results (Aronowitzet al. 1977) obtained in a turbulent flow reactor, used to develop the mechanismabove, are shown. Figure 9.6 shows the reaction product distribution, as well asthe temperature profile, along the turbulent flow reactor.

Later works (e.g., Cathonnet et al. 1982; Norton and Dryer 1989, 1990) con-tributed to progress in the understanding of the reactions involved in the methanoloxidation process. Among them, the work of Held and Dryer (1998) includes anupdated and optimized model previously released (Westbrook and Dryer 1979)and a detailed chemical kinetic mechanism which was validated through experi-mental data at different pressure and temperature conditions. This mechanism hasbeen successfully used in a later research work focused on the study of thepyrolysis and oxidation of methanol in a laminar flow reactor (Jazbec and Haynes2005).

The work of Alzueta et al. (2001) on the oxidation of methanol was carried outin a flow reactor and between 700 and 1500 K for different oxidation regimes,finding the carbon oxides as the main reaction products. The gas phase chemicalkinetic mechanism developed in this study reproduces satisfactorily the experi-mental results, matching the main reaction pathways found with some of thepublished previously studies (Westbrook and Dryer 1979; Norton and Dryer 1989;Held and Dryer 1998).

More recently, Rasmussen et al. (2008b) studied the oxidation of methanolunder different oxidation regimes in the 650–1350 K temperature interval, aimingto identify the branching ratio of the reaction of methanol with OH and H todecompose into CH3O and CH2OH given the existing discrepancy. This workreasserts the findings that establish that methanol mainly decomposes throughCH2OH radical.

9 Tubular Flow Reactors 221

The oxidation of methanol mixed with other hydrocarbons has not beenexhaustively studied, and most of the research works found were carried out inflames and shock tubes (e.g., Nakamura et al. 1982; Böhm and Braun-Unkhoff2008). A recent research work of Esarte et al. (2012) presented an experimentaland modeling study of the interaction of methanol mixed with acetylene, one of themain soot precursors, focusing on the gas phase interactions and soot formationfrom the reacting mixtures.

9.8.2 Ethanol

Given that ethanol has been considered as a fuel substitute and additive for a longtime, there are a number of studies focused on the investigation of ethanol oxi-dation processes, including studies under flow reactor conditions.

The first detailed work on the oxidation of ethanol (Dagaut et al. 1992) presenteda chemical kinetic mechanism able to describe the pyrolysis of ethanol. In the sameyear, and based on the previously described study of the oxidation of methanol(Norton and Dryer 1990, 1991), a kinetic model was proposed for the oxidation of

Fig. 9.6 CH3OH, CO, CH4, C2H4, HCHO and H2 concentration profiles and temperature profilealong the turbulent flow reactor for the methanol pyrolysis at 1158 K. Reprinted from Aronowitzet al. (1977), Copyright 1997, with permission from American Chemical Society

222 F. Monge et al.

ethanol under flow reactor conditions at 1100 K (Norton and Dryer 1992). Authorsidentified that most of ethanol reacts with OH radicals and demonstrated theimportance of considering the three isomeric forms of C2H5O radical and thebranching ratio in the mechanism, which was identified as is shown in Fig. 9.7:

Given the limitations of the mechanism developed by Norton and Dryer (1992),some years later, Marinov (1999) presented a detailed chemical kinetic mecha-nism, which is still nowadays one of the most accepted for describing ethanoloxidation in different reaction systems, including flow reactors.

The conversion reactions for ethanol oxidation from the mechanism of Marinov(1999), together with the mechanism of Glarborg et al. (1998), updated by Alzuetaet al. (2001), were used to develop a new chemical kinetic mechanism able todescribe the oxidation of ethanol and its interaction with NO in a flow reactor(Alzueta and Hernández 2002). This mechanism reproduces satisfactorily theexperimental data for the conversion of ethanol and the formation of CO and CO2

as the major products of the reaction. As an example, Fig. 9.8 shows the goodagreement between experimental and simulation results for the concentration ofethanol, CO and CO2 as function of temperature, for almost stoichiometricconditions.

The mechanism by Alzueta and Hernández (2002), and updated by Abián et al.(2008) was validated in a recent work on the pyrolysis of ethanol under flowreactor conditions (Esarte et al. 2011a). In this work, the capability of ethanol todecompose into soot precursors and its tendency to form soot was demonstratedand evaluated.

The role of ethanol as fuel substitute and additive has led to the development ofsome research works on the oxidation of ethanol mixed with hydrocarbons inreaction systems such as flames, jet-stirred reactors, or shock wave reactors (e.g.,Alexiou and Williams 1996; Song et al. 2003). However, not many worksregarding the oxidation of hydrocarbon/ethanol mixtures under flow reactor con-ditions are available in the literature (Abián et al. 2008; Esarte et al. 2009, 2011b).In those works, the effect of ethanol as fuel substitute and additive was evaluated.As an example of the results obtained in the work of Esarte et al. (2009), Fig. 9.9,shows the amount of soot collected for different percentages of ethanol in the fuelblend. The results indicate that low concentrations of ethanol in the initial blendhave a strong influence in the decrease of the amount of soot collected, while thediminution is less sharp when the ethanol concentration increases above5000 ppm.

Fig. 9.7 Reaction pathways involved in the formation of the three isomeric forms of C2H5Oradical

9 Tubular Flow Reactors 223

9.8.3 Propanol

There exist some old studies on the thermal decomposition of the isomers ofpropanol mainly in discontinuous reaction systems from the 1950 s (e.g., Smithand Gordon 1956; Trenwith 1975) and some recent works on the oxidation ofpropanol isomers in flames and shock tubes (e.g., Sinha and Thomson 2004;Frassoldati et al. 2010) given that the oxidation of the isomers of propanol asoxygenated fuels has recently attracted attention. However, very little researchwork on the oxidation of propanol isomers under flow reactor conditions has beenfound in the literature.

Fig. 9.8 Concentration of C2H5OH, CO and CO2 as a function of temperature under almoststoichiometric conditions. Symbols represent experimental data and lines correspond to simulatedresults. Reprinted with permission from Alzueta and Hernández (2002), Copyright 2002, withpermission from American Chemical Society

Fig. 9.9 Amount of sootcollected as a function of thepercentage of ethanolvolume. Reprinted fromEsarte et al. (2009),Copyright (2009), withpermission from Elsevier

224 F. Monge et al.

A study found on the oxidation of n-propanol and iso-propanol was carried outunder flow reactor conditions in the 1020–1200 K temperature interval (Nortonand Dryer 1991). In this work the authors established that n-propanol presents ahigher tendency to dehydrogenation instead of dehydration due to the presence ofaC-H bonds forming alkenes. Meanwhile, iso-propanol goes through dehydrationprocesses leading to the formation of alkenes and dehydrogenation leading toketones. The proposed mechanism is used successfully in a later work on theoxidation of n-propanol and iso-propanol flames (Sinha and Thomson 2004).

A recent research work (Esarte et al. 2012) included experimental and modelingdata on the oxidation of acetylene mixed with iso-propanol under flow reactorconditions under sooting and nonsooting conditions. From this work, it was con-cluded that the presence of iso-propanol affects the combustion of acetylene,enhancing oxidation reactions that lead to the diminution of the formation of sootprecursors, but they also promote its formation by the decomposition into smallintermediate hydrocarbons.

Much work remains to be done regarding the pyrolysis and oxidation of pro-panol isomers and mixtures of these compounds with different hydrocarbons sincethe combustion mechanism and their contribution to pollutants formation is stillnot clear.

9.8.4 Butanol

Although the interest in the oxidation of butanol is very recent, mainly because ofits potential use as an oxygenated fuel, there was some work done in the 1950 s(Barnard 1957). More recently, some research works on the oxidation of thedifferent isomers of butanol have been carried in different combustion systems,mainly in shock tubes and flames (e.g., Moss et al. 2008; Sarathy et al. 2009,Frassoldati et al. 2012). The outcome of these studies highlighted the importanceof studying the differences in the oxidation processes of the different isomers ofbutanol.

Only a few research works on the oxidation of butanol under flow reactorconditions have been found in the literature. The work of Norton and Dryer (1991)was focused on the oxidation of tert-butanol between 1020 and 1120 K and it wasconcluded that the oxidation process comes mainly through dehydration and themajor products are alkenes and small amounts of ketones that are formed sec-ondarily by means of dehydrogenation. Recently, Lefkowitz et al. (2012) studiedthe oxidation of tert-butanol in a variable pressure flow reactor at 12.5 atm andtemperatures of 675–950 K. In these conditions, the tert-butanol oxidation comesthrough hydrogen abstraction and methane and acetone are observed as reactionproducts.

The aforementioned work of Esarte et al. (2012) also includes an analysis of theeffect of n-butanol on the oxidation of acetylene. The observations of this studyindicated that the presence of n-butanol in the reacting mixture alters the

9 Tubular Flow Reactors 225

combustion of acetylene, enhancing oxidation reactions that disfavor the formationof soot precursors. However, the presence of this alcohol also promotes the for-mation of small hydrocarbons that favor the growth of larger hydrocarbons andPAH that could finally lead to the formation of soot.

As an example of the results obtained in Esarte et al. (2012), Fig. 9.10 showssoot and gas yields obtained for different temperatures in the pyrolysis experimentsof acetylene, acetylene-methanol, acetylene-ethanol, acetylene-isopropanol, andacetylene-n-butanol mixtures from 1275 K. The soot yield increases (Fig. 9.10a)and gas yield decreases (Fig. 9.10b) as temperature does for a given fuel mixture.Moreover, substituting in the reacting mixture part of acetylene by alcohols leadsto a diminution of soot yield, being methanol the most effective alcohol.

9.9 Ethers: Dimethylether and Dimethoxymethane

9.9.1 Dimethylether

As has been already mentioned, apart from alcohols, ethers are also proposed assubstitutes or additives to conventional fuels, being dimethylether (DME) thesimplest ether.

First results of DME in flow reactors are those related to DME pyrolysis underdifferent experimental conditions (e.g., Benson and Jain 1959; Pacey 1975; Heldet al. 1977). These experimental data were used to obtain kinetic information aboutDME oxidation. Nevertheless, one of the most important detailed chemical kineticmodels for the oxidation of DME is the mechanism developed by Curran et al.(1998). This mechanism was able to predict the correct distribution and concen-trations of intermediate and final products formed in the oxidation of DME, both ina jet-stirred reactor and in a shock tube. Although in this study the mechanism was

Fig. 9.10 a Soot yield and b Gas yield obtained in the pyrolysis of pure acetylene and thepyrolysis of acetylene-alcohol mixtures at 1275, 1375 and 1425 K. Reprinted from Esarte et al.(2012) Copyright (2012), with permission from Elsevier

226 F. Monge et al.

not tested for flow reactor, the developed model has been used in later worksincluding flow reactors.

For instance, Alzueta et al. (1999) developed a new mechanism using thereaction subset of DME oxidation presented by Curran et al. (1998) and combinedit with the mechanism of Glarborg et al. (1998), which describes the interactionsbetween C1/C2 hydrocarbons and NO. In this study, experiments of the oxidationof DME in a flow reactor at atmospheric pressure in the 600–1500 K temperatureinterval, for different air/fuel ratios and in the presence of nitrogen oxides werecarried out. As an example of some other information provided by a chemicalkinetic mechanism, Fig. 9.11 shows a complete reaction diagram for the DMEoxidation under the conditions of this study.

Since DME has been proposed as a substitute or an additive of fuel, differentinvestigations about mixtures DME/hydrocarbons have been found in the litera-ture. Usually, the hydrocarbons studied are those that show a significant tendencyto form soot and its precursors. Most of these studies have been performed inflames (e.g., Song et al. 2003; Park 2006), and only some investigations have beencarried out under flow conditions (Amano and Dryer 1998; Tamm et al. 2009;Esarte et al. 2010; Brumfield et al. 2013).

9.9.2 Dimethoxymethane

Despite having been concluded that dimethoxymethane (DMM) can be used as anadditive in reformulated fuels (Sirman et al. 2000), it has not been studied asextensively as DME. DMM oxidation studies have been performed in staticreactors (Molera et al. 1974a), jet-stirred reactor (Daly et al. 2001) and flames(Molera et al. 1974b). However, to our knowledge, there is not any study in theliterature about DMM oxidation under flow reactor conditions, except for the studycarried out by Monge et al. (2012). This research has been performed at atmo-spheric pressure, in the temperature interval of 373–1473 K, different air excessratios, and in the absence and the presence of nitric oxide. The experimental results

CH3OCH3

CH3OCH3 CH3OCH2 CH2O

CH4

C2H6 CH3O2

+H/OH/CH3

+M+M

+M +M+CH3OCH3

+CH3

+O2

+O2

+M

Fig. 9.11 Reaction pathways for the DME oxidation

9 Tubular Flow Reactors 227

show a low influence of the stoichiometry on the DMM oxidation, being shifted tolower temperatures under oxidizing conditions and NO presence. The mainproducts found are methane, methanol, methyl formate, ethane, ethylene, acety-lene, carbon monoxide, and carbon dioxide.

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