Explosion Characteristics of Propanol Isomer–Air Mixtures

energies

Article Explosion Characteristics of Propanol Isomer–Air Mixtures

Jan Skˇrínský * and Tadeáš Ochodek

Energy Research Center, VŠB-Technical University of Ostrava, 17. listopadu 2172/15, 708 00 Ostrava, Czech Republic; [email protected] * Correspondence: [email protected]; Tel.: +420-597-324-931

Received: 15 March 2019; Accepted: 21 April 2019; Published: 25 April 2019

Abstract: This paper describes a series of experiments performed to study the explosion characteristics of propanol isomer (1-propanol and 2-propanol)–air binary mixtures. The experiments were conducted in two different experimental arrangements—a 0.02 m3 oil-heated spherical vessel and a 1.00 m3 electro-heated spherical vessel—for different equivalence ratios between 0.3 and 1.7, and initial temperatures of 50, 100, and 150 ◦C. More than 150 pressure–time curves were recorded. The effects of temperature and test vessel volume on various explosion characteristics, such as the maximum explosion pressure, maximum rate of pressure rise, deflagration index, and the lower and upper explosion limits were investigated and the results were further compared with the results available in literature for other alcohols, namely methanol, ethanol, 1-butanol, and 1-pentanol. The most important results from evaluated experiments are the values of deflagration index 89–98 bar m/s for · 2-propanol and 105–108 bar m/s for 1-propanol/2-propanol–air mixtures. These values are used to · describe the effect of isomer blends on a deflagration process and to rate the effects of an explosion.

Keywords: explosion characteristics; vessel; mixtures; propanol; isomers

1. Introduction Increasing attention has been paid to the use of non-petroleum-based fuels, preferably from renewable sources, including alcohols with up to five carbon atoms. These alcohols are successful in the practical use as gasoline and diesel fuel additives and this has motivated the full investigation of their combustion chemistry [1–4]. Compared to methanol and ethanol, propanol has many superior properties as an alternative fuel (lower hygroscopicity, higher boiling point, and higher energy density). Compared to butanol and pentanol, propanol has a higher percent oxygen content in the molecule. Due to propanol’s isomeric structure its combustion properties could be even further tuned by its blends with no change in the fuel’s physical-chemical properties [5–8]. Apart from the bio-fuel interest, alcohol fuels use involves critically evaluating their combustion and explosion characteristics under various conditions. Devastating chemical accidents can occur in chemical storage and process industry and transport applications as the flammable liquid expands and increases the risk of vapor cloud explosion. Such accidents have essential social and economic impact [9,10]. Explosion of vapors in closed vessel is a complex process involving fuel atomisation, vaporisation, mixing, ignition and combustion. To characterize this effect and to develop the effective explosion protection systems against overpressure the key closed-vessel deflagration characteristics are used: maximum explosion pressure, pmax, maximum rate of pressure rise, (dp/dt)max and the deflagration index, KG [11–16].

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Small-scale characterisation in the gas phase towards understanding explosion deflagration over a range of fuel concentrations, temperatures and pressures has been published starting from 2009. Zhang et al. presented a study on the explosion characteristics of methanol–air premixed mixtures in a 5.5 L cylindrical vessel for Φ = 0.7–1.8, with initial pressures of 0.1 MPa, 0.25 MPa, 0.5 MPa and three initial temperatures (373, 423, 473 K) [11]. Mitu and Brandes investigated the explosion parameters of methanol-air mixtures measured for initial temperatures of 50 ◦C and 150 ◦C and initial pressures of 75 kPa and 100 kPa and supplemented the measurements with a calculation for temperatures of 25 and 110 ◦C in a 5 L spherical stainless steel vessel [12]. Li et al. conducted a series of experimental studies on the explosion characteristics of ethanol, 1-butanol, 1-pentanol, 2-pentanol and 3-pentanol with air mixtures at an initial temperature up to 200 ◦C and an initial pressure up to 0.75 MPa [13]. Parameters were measured in a cylindrical explosion chamber with a diameter D = 180 mm and a length L = 210 mm (V = 5.3 dm3). The same authors deepened their research through a comparative study of the explosion characteristics of four isomers of pentanol [14]. More recently, Mitu et al. [15] studied the effect of 0.005 m3 and 0.02 m3 vessel volumes on ethanol–air mixtures for different initial conditions. Later, Mitu et al. investigated ethanol–air mixtures in the presence of different diluents in a 0.02 m3 vessel [16]. Most recently, Skrinsky et al. investigated the influence of temperature and vessel volume on explosion characteristics of 1-propanol-air mixtures in 0.02 m3 and 1.00 m3 closed spherical vessels [8]. The results of these experimental studies are summarized in Table1.

Table 1. Published pmax and KG values.

K Volume Temperature Equivalence Chemical p (bar) G Reference max (bar m/s) (m3) ( C) Ratio · ◦ CH3OH 7.1–8.2 105–107 0.005–0.020 50–100 1.00–1.26 [11,12] C2H5OH 7.9–8.7 95–135 0.005–0.020 100–120 1.00–1.10 [15,16] 1-C3H7OH 7.7–8.7 86–97 0.020–1.000 50–150 1.06 [8] 1 1-C4H9OH 5.5–5.7 95–100 0.0053 120 1.00 [13] 1 1-C5H11OH 5.6–5.7 90–95 0.0053 120–160 1.00 [13,14] 1 lengths/diameter = 1.17.

In comparison with Table1 for the 2-propanol isomer and 1-propanol /2-propanol blends, no studies are available. Objective of this paper is to describe the explosion characteristics of 2-propanol isomer mixtures with air with different equivalence ratios, depending on three initial temperatures of 50 ◦C, 3 100 ◦C, and 150 ◦C and two volumes (0.02 and 1.00 m ). Emphasis is given on the deflagration index of propanol isomer–air mixtures directly obtained by explosion experiments in volume of 1.00 m3. Based on the knowledge of these characteristics, it is possible to evaluate the influence of the carbon chain structure and blending ratio on the explosion characteristics of these alcohol mixtures and compare them with the values from small-scale experiments. This work allowed a systematic comparison and extending knowledges about the explosion properties among C1-C5 alcohols.

2. Experimental Setup and Chemicals Explosion characteristic values of the pure liquids and their binary mixtures were determined experimentally according to the EN 15967:2011 [17] and EN 1839:2017 [18] standards. Two methods were used in the present study; both of them were conducted at the VSB-Technical University of Ostrava [8]. One was the 0.02 m3 (SN: 497-OZM-15, OZM Research, s.r.o., Hrochu˚v Týnec, Czech Republic) oil-heated spherical vessel setup to record pressure–time curves between 50 and 150 ◦C. The other was the 1.00 m3 (SN: 284-OZM-12-01, OZM Research, s.r.o.) electro-heated spherical vessel setup. Both test apparatuses consisted of a test vessel, an external heating system, a setup for special atmosphere development (vacuum pump, dispersion nozzle, and high-pressure air), pressure and ﬂash measuring systems, a discharge type ignition system, a temperature measuring device, a user interface, and a data storage system. The test vessels were designed to withstand a working pressure of 30 bar. For testing, the explosion vessels were heated to the desired temperatures of 50–150 ◦C. Energies 2019, 12, 1574 3 of 15

3 EnergiesThe 2019 0.02, 12, m x FORapparatus PEER REVIEW incorporated a digitally adjustable external control device Presto A303 (SN:of 15 10291377, JULABO GmbH, Seelbach, Germany) that heated the oil in the instrument to the specified temperaturetemperature range. The The 1.00 1.00 m m3 apparatus3 apparatus used used an anelectro-heated electro-heated system system with with a power a power of 25 of kW 25 (HT kW (HTIndustry, Industry, HTH8, HTH8, Polič Poliˇcka,Czechka, Czech Republic). Republic). The initial The initial temper temperatureature of mixtures of mixtures was measured was measured with withan accuracy an accuracy of within of within 1 K. 1 The K. The test test mixture mixture was was prepared prepared by by a apartial partial pressure pressure method withwith maximum samplingsampling uncertaintyuncertainty ±0.2%.0.2%. TheThe evaporation evaporation of of samples samples in in both both vessels vessels was was supported supported by ± twoby two systems. systems. First, First, the the procedure procedure started started with with evacuation evacuation to to the the initial initial conditions conditions underunder aa pressurepressure of 0.400.40 barbar inin thethe 0.020.02 mm33 setupsetup oror 0.890.89 barbar inin thethe 1.001.00 mm33 apparatus.apparatus. The sample was blown into thethe evacuatedevacuated chamber by a dispersiondispersion pressurepressure ofof 55 barbar inin bothboth vessels.vessels. Second,Second, thethe dispersiondispersion nozzlenozzle forfor aerosolaerosol generationgeneration waswas used.used. This combination was applied to effectively effectively evaporate the samples, 3 especiallyespecially at temperaturestemperatures belowbelow 8484 ◦°CC (the(the boilingboiling pointpoint ofof 2-propanol).2-propanol). InIn thethe 0.020.02 mm3 systemsystem thethe dynamic pressure inin thethe vesselvessel waswas measuredmeasured byby aa pairpair ofof quartzquartz pressurepressure sensorssensors (SN:(SN: 45128214512821 andand SN: 4512822,4512822, model 701A, Kistler, Winterthur, Switzerland)Switzerland) and recorded withwith aa chargecharge amplifieramplifier (Kistler,(Kistler, modelmodel 5041E0).5041E0). In In the the 1.00 1.00 m m33 systemsystem the the dynamic dynamic pressure pressure in in the the vessel vessel was was measured measured by by a apair pair of of quartz quartz pressure pressure sensors sensors (SN: (SN: 4353089 4353089 and and SN: SN: 4353090, 4353090, Kistler, Kistler, model 701A) and recordedrecorded withwith a charge amplifier amplifier (SN: (SN: 50734370760, 50734370760, Kistler, Kistler, model model 5073). 5073). The The pressure pressure measuring measuring range range for the for thesensors sensors was was within within 25 25bar. bar. The The data data acquisition acquisition sampling sampling period period was was 0.02 0.02 ms ms with with a samplingsampling frequencyfrequency 5050 kSkS/s./s. ProgrammableProgrammable logic controllers (model(model 12151215 inin 1.001.00 mm33 and 5073A211 in 0.020.02 mm33, Siemens, Munich, Germany) connected to a PC werewere used with the interfaceinterface (PROMOTIC system, MICROSYS, spol.spol. s.r.o., s.r.o., Ostrava, Ostrava, Czech Czech Republic) Republic to automatically) to automatically control thecontrol whole the testing whole procedure. testing Bothprocedure. experimental Both experimental setups are schematically setups are schematically introduced inintroduced Figures1 andin Figures2. 1 and 2.

FigureFigure 1.1. Experimental setupsetup ofof thethe 0.020.02 mm33 spherical vessel.

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FigureFigure 2. 2.Experimental Experimental setup of the 1.00 1.00 m m3 3sphericalspherical vessel. vessel.

TheThe properties properties of of the the experimental experimental materialsmaterials used are summarized summarized in in Table Table 2.2 .

TableTable 2. 2.Physical Physical propertiesproperties of experimental materials. materials.

ChemicalChemical Formula Formula Pur Purityity (Mass (Mass %) %) Company Company 1-propanol C3H7OH >99.7 Merck KGaA (Darmstadt, Germany) 1-propanol C3H7OH >99.7 Merck KGaA (Darmstadt, Germany) 2-propanol C3H7OH >99.8 Penta (Katowice, Poland) 2-propanol C3H7OH >99.8 Penta (Katowice, Poland) 3. Data Evaluation 3. Data Evaluation The pex was determined as the highest value of the pressure–time curve divided by the initial pressureThe p exforwas the actual determined equivalence as the ratio. highest The valuepmax is ofthe the highest pressure–time explosion pressure curve divided for all equivalence by the initial pressureratios divided for the actualby the equivalence initial pressure ratio. [17]. The The pmax valueis the of highest(dp/dt)max explosion decreases pressure with increasing for all equivalence vessel 3 3 ratiosvolume. divided Therefore, by the (dp/dt) initialmax is pressure for 0.02 m [ 17recalculated]. The value to a vessel of (dp volume/dt)max (V) decreasesof 1 m , using with Equation increasing (1) 3 3 vesselfor the volume. deflagration Therefore, index: (dp/dt)max is for 0.02 m recalculated to a vessel volume (V) of 1 m , using Equation (1) for the deﬂagration index: KG = V⅓·(dp/dt)max. (1)

1 The maximum explosion pressures, the deflagratio/3 n indexes, and the explosion delay times were KG = V (dp/dt)max. (1) obtained from pressure time records. Upper expl·osion limit, UEL, and lower explosion limit, LEL, evaluationThe maximum is based explosion on the highest pressures, and lowest the deﬂagration concentrations, indexes, respectively, and the explosion at which delaythe material times wereis obtainedexplosive from by using pressure the 5.0 time vol.% records. criterion Upper [18]. explosion The explosion limit, delay UEL, time, and t lowerex, is the explosion time at which limit, the LEL, evaluationmaximum is explosion based on pressu the highestre is reached and lowest [15]. concentrations, respectively, at which the material is The experimental uncertainty in a statistical sense is given by the used experimental method and explosive by using the 5.0 vol.% criterion [18]. The explosion delay time, tex, is the time at which the is 10% for explosion characteristics according to EN 15967:2011 [17]. The flammable substance content maximum explosion pressure is reached [15]. in the test mixture lies within an uncertainty of measurement of ±10% relative for a flammable The experimental uncertainty in a statistical sense is given by the used experimental method and is substance content up to 2 mol.%, and +0.2% absolute for a flammable substance content above 2 10% for explosion characteristics according to EN 15967:2011 [17]. The ﬂammable substance content in

Energies 2019, 12, 1574 5 of 15 the test mixture lies within an uncertainty of measurement of 10% relative for a ﬂammable substance ± content up to 2 mol.%, and +0.2% absolute for a ﬂammable substance content above 2 mol.% according to ENEnergies 1839:2017 2019, 12, x [FOR18]. PEER With REVIEW 2-propanol and 1-propanol/2-propanol in air mixtures 3 measurements5 of 15 were performed for each sample concentration at T0 = 50–150 ◦C and p0 = 1 bar. For example, with the 3 compositionmol.% according of 7.0 vol.%to EN 2-propanol1839:2017 [18]. in air,With 3 tests2-propanol were performed and 1-propanol/2-propanol in 0.02 m at T0 = in50 air◦ Cmixtures and p0 3= 1 bar.measurements Pex was determined were performed to be 8.69 for bar each0.14 sample bar, (dpconcentration/dt)ex to be at 349.2 T0 = bar50–150/s 24.1°C and bar p/s,0 = K 1G bar.to be For 94.6 ± ± 3 6.5example, bar m/ swith and the tex compositionto be 89.0 of3.7 7.0 ms. vol.% As a2-propanol result the meanin air, values3 tests were 8.7 bar, performed 349 bar/ s,in 950.02 bar mm at/s T and0 ± · ± · 89.0= 50 ms °C were and takenp0 = 1 forbar. further Pex was evaluations. determined to The be maximum8.69 bar ± 0.14 measurement bar, (dp/dt) uncertaintiesex to be 349.2 bar/s obtained ± 24.1 for G ex pexbar/s,, (dp/ dt)K exto andbe 94.6 tex are± 6.5 given bar·m/s in Tableand t3. to be 89.0 ± 3.7 ms. As a result the mean values 8.7 bar, 349 bar/s, 95 bar·m/s and 89.0 ms were taken for further evaluations. The maximum measurement uncertainties obtained for pexTable, (dp/dt) 3. Maximumex and tex are measurement given in Table uncertainty. 3.

0.02Table m3 3. Maximum measurement uncertainty. 1 m3 2-Propanol 1-Propanol/2-Propanol 2-Propanol 1-Propanol/2-Propanol 0.02 m3 1 m3 T0 (◦C) pmax 2-PropanolKG tex 1-Propanol/2-Propanolpmax KG tex pmax2-PropanolKG tex 1-Propanol/2-Propanolpmax KG tex 50T0 (°C) 1.6%pmax 6.9%KG 4.2%tex p 1.7%max 7.1%KG 4.5%tex p 1.9%max K 7.9%G tex 4.9% pmax 1.8% KG 7.2% tex4.6% 10050 2.0% 1.6% 7.0%6.9% 4.2% 5.0% 1.7% 2.1% 7.1% 8.0% 4.5% 5.1% 1.9% 2.2% 7.9% 7.2% 4.9% 5.2% 1.8% 2.3% 7.2% 8.2% 4.6% 5.3% 150100 2.2% 2.0% 8.4%7.0% 5.0% 5.1% 2.1% 2.1% 8.0% 8.5% 5.1% 5.2% 2.2% 2.2% 7.2% 8.5% 5.2% 5.3% 2.3% 2.4% 8.2% 8.5% 5.3% 5.6% 150 2.2% 8.4% 5.1% 2.1% 8.5% 5.2% 2.2% 8.5% 5.3% 2.4% 8.5% 5.6% The measurement uncertainties in Table3 are well within the respective ranges given in EN The measurement uncertainties in Table 3 are well within the respective ranges given in EN 15967:2011 [17]. The maximum experimental uncertainty for LEL and UEL was taken 0.2% absolute. 15967:2011 [17]. The maximum experimental uncertainty for LEL and UEL was taken 0.2% absolute. TheThe pressure–time pressure–time curves curves were were smoothed smoothed to to removeremove noises from from the the signal signal using using the the Savitzky–Golay Savitzky–Golay methodmethod [19 [19]] by by signal signal processing processing polynomial polynomial functionsfunctions implemented implemented in in the the OriginPro OriginPro 8 program 8 program [20]. [20 ]. ToTo perform perform smoothing smoothing we we used used 20 points20 points of windowsof windows with with a periodic a periodic boundary boundary condition condition for for the the ﬁfth polynomialfifth polynomial order optimized order optimized by R-squared by R-squared maximization. maximization. The pressure–timeThe pressure–time curve curve changed changed little little after theafter smoothing the smoothing process butprocess the maximumbut the maximum rate of pressure rate of rose, pressure change rose, greatly. change Figure greatly.3 illustrates Figure the3 comparisonillustrates ofthe the comparison raw and ﬁlteredof the raw data and for filtered the pressure–time data for the pressure–time curve determination curve determination of the explosion of pressurethe explosion rise. pressure rise.

FigureFigure 3. 3.An An illustration illustration of of thethe smoothingsmoothing ﬁlterfilter on the pressure–time curve curve determination determination of ofthe the explosionexplosion pressure pressure rise. rise.

Energies 2019,, 1212,, 1574x FOR PEER REVIEW 66 of of 15 Energies 2019, 12, x FOR PEER REVIEW 6 of 15 4. Results and Discussion 4.4. ResultsResults andand DiscussionDiscussion 4.1. Influence of Temperature 4.1.4.1. InﬂuenceInfluence ofof TemperatureTemperature The influence of temperature on explosion pressures at 11 concentrations for pure 1-propanol– TheThe inﬂuenceinfluence ofof temperature temperature on on explosion explosion pressures pressures at at 11 11 concentrations concentrations for for pure pure 1-propanol–air 1-propanol– air mixtures have been described in [8]. The results of 1-propanol–air mixtures were used to predict mixturesair mixtures have have been been described described in [8 in]. [8]. The The results result ofs 1-propanol–air of 1-propanol–air mixtures mixtures were were used used to predict to predict the the initial values for pure 2-propanol–air measurements, as presented in Figures 4 and 5. initialthe initial values values for pure for pure 2-propanol–air 2-propanol–air measurements, measurements, as presented as presented in Figures in Figures4 and 45 .and 5.

3 Figure 4. pex/p0/time for 1-propanol and 2-propanol deflagration in a 0.02 m3 vessel, p0 = 100 kPa. FigureFigure 4. 4.p pexex//pp00//timetime forfor 1-propanol1-propanol andand 2-propanol2-propanol deﬂagrationdeflagration inin aa 0.020.02 mm3 vessel,vessel, pp00 = 100100 kPa.

3 Figure 5. (dp/dt)(dp/dt)max/time/time for for 1-propanol 1-propanol and and 2-propanol deﬂagrationdeflagration inin aa 0.020.02 m m3chamber, chamber,p p00= = 100 kPakPa.. Figure 5. (dp/dt)max/time for 1-propanol and 2-propanol deflagration in a 0.02 m3 chamber, p0 = 100 kPa. Figure 4 illustrates the effects of initial temperature on the explosion pressure, explosion rate of Figure 4 illustrates the effects of initial temperature on the explosion pressure, explosion rate of pressure rise, and explosion delay time of 1-propanol and 2-propanol at the initial temperatures of pressure rise, and explosion delay time of 1-propanol and 2-propanol at the initial temperatures of

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EnergiesFigure 20194, 12 illustrates, x FOR PEER the REVIEW e ﬀects of initial temperature on the explosion pressure, explosion7 rateof 15 of pressure rise, and explosion delay time of 1-propanol and 2-propanol at the initial temperatures of50 50 and and 150 150 °C,◦C, atmospheric atmospheric pressure, pressure, a p a0 pof0 100of 100 kPa, kPa, and and the thesame same equivalence equivalence ratio, ratio, ϕ = 1.06.φ = 1.06.With Withincreasing increasing temperature, temperature, the theexplosion explosion pressure pressure decreased decreased and and the the explosion explosion delay timetime slightlyslightly decreaseddecreased for for both both isomers. isomers. TheThe explosionexplosion pressurepressurechanges changesonly only slightly slightly in in the the maximum maximum values values whenwhen comparing comparing 1-propanol 1-propanol and and 2-propanol 2-propanol for for both both temperatures. temperatures. FigureFigure5 gives5 gives the the measured measured explosion explosion pressure pressure rise–time rise–time curves curves of 1-propanol of 1-propanol and 2-propanoland 2-propanol for near-stoichiometricfor near-stoichiometric equivalence equivalence ratios ratios and initial and temperaturesinitial temperatures of 50 and of150 50 ◦andC. With 150 the°C. increased With the initialincreased temperature, initial temperature, the values the of thevalues maximum of the maximum rates of pressure rates of rise pressure decreased rise decreased for both isomers. for both Theisomers. results The of the results rate ofof pressure the rate rise–time of pressure curve rise ﬁts–time well withcurve the fits data well for with ethanol the publisheddata for ethanol in [15] andpublished data for in C [15]nH2n and+1OH data (n for= 1,2,4,5) CnH2n+1 [13OH]. (n = 1,2,4,5) [13].

4.2.4.2. Inﬂuence Influence of of Volume Volume and and Isomer Isomer Structure Structure

FiguresFigures6 and6 and7 plot 7 examplesplot examples of the normalized of the normalized explosion pressure,explosion p maxpressure,/p0, versus pmax the/p0 equivalence, versus the ratioequivalence for 2-propanol–air ratio for and2-propanol–air 1-propanol/ 2-propanol–airand 1-propanol/2-propanol–air mixtures. The normalized mixtures. explosion The normalized pressures wereexplosion obtained pressures at two vesselwere obtained volumes at (0.020 two mvessel3 and volumes 1.00 m3) (0.020 and three m3 and initial 1.00 temperatures m3) and three (50, initial 100, andtemperatures 150 ◦C). (50, 100, and 150 °C).

FigureFigure 6. 6.pex p/pex0/pversus0 versusΦ = 0.3–1.7Φ = 0.3–1.7 for 2-propanol–air for 2-propanol–air mixtures atmixtures various initialat various volumes, initial temperatures, volumes, andtemperatures, mixture compositions, and mixture p compositions,0 = 100 kPa. p0 = 100 kPa.

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Figure 7. pex/p/p00 versusversus ΦΦ == 0.3–1.70.3–1.7 for for 1-propanol/2 1-propanol/2-propanol–air-propanol–air mixtures (f00 = 0.5)0.5) at various initial volumes, temperatures, andand mixturemixture compositions,compositions, pp00 = 100 kPa.

The tested vessel volumes did not significantlysignificantly in influencefluence the values of the explosion pressure. The maximum explosion pressure measured in the 0.020.02 mm33 vessel showed the same trend and was 3 slightly higherhigher atat 150 150◦ C°C than than the the maximum maximum explosion explosion pressure pressure measured measured in thein the 1.00 1.00 m vessel.m3 vessel. For For the equivalencethe equivalence ratio ratio of Φ of= Φ1.00 = 1.00 and and the the initial initial temperature temperature 150 150◦C, °C, the the values values of of p pmaxmax werewere 7.67.6 bar and 7.1 bar. Comparable results, as shown in Figure 66,, havehave beenbeen reportedreported inin [[8]8] forfor 1-propanol–air1-propanol–air deflagrationsdeflagrations andand in [in15 ][15] for ethanol–airfor ethanol–air mixtures mixtures at various at initial various temperatures, initial temperatures, mixture compositions, mixture andcompositions, p0 = 100 kPa. and The p0 experiments= 100 kPa. ofThe both experiments propanol isomer–airof both propanol mixtures isomer–air had equivalence mixtures ratios had of Φequivalence= 0.3–1.7. Theratios maximum of Φ = 0.3–1.7. value The of the maximum explosion value pressure of the for explosion 2-propanol pressure was found for 2-propanol at Φ = 1.06 wasfor allfound temperatures. at Φ = 1.06 This for all means temperatures. that isomer This equivalence means that ratios isomer corresponding equivalence to theratios maximum corresponding explosion to pressuresthe maximum arenot explosion influenced pressures by the are initial not influenced temperature. by the The initial measured temperature. results forThe p measuredex values showresults a reasonablefor pex values agreement show a at reasonable near-stoichiometric agreement concentrations at near-stoichiometric with the four concentrations pentanol isomer–air with mixturesthe four 3 frompentanol previous isomer–air studies mixtures [14] measured from previous in 0.005 studies m at 160 [14]◦C. measured in 0.005 m3 at 160 °C. The shapeshape of of the the explosion explosion pressure pressure versus versus 1-propanol 1-propanol/2-propanol–air/2-propanol–air equivalence equivalence ratio at diratiofferent at initialdifferent temperatures initial temperatures with varying with concentrations varying concen wastrations similar was at all similar investigated at all initialinvestigated temperatures, initial astemperatures, for pure 2-propanol as for pure and 2-propanol pure 1-propanol and pure [8]. 1-propanol The maximum [8]. The value maximum of the explosion value of the pressure explosion was foundpressure at Φwas= 1.06 found for bothat Φ volumes= 1.06 for and both all temperatures.volumes and Theall temperatures. increase in the The initial increase temperature in the lowersinitial thetemperature explosion lowers pressure, the as explosion discussed pressure, in [12]. At as equivalence discussed ratiosin [12]. close At equivalence to the LEL and ratios UEL, close the valuesto the ofLEL pex and/p0 decreaseUEL, the tovalues the value of pex close/p0 decrease to initial to pressure. the value After close reaching to initial this pressure. concentration After thereaching explosion this canconcentration no longer propagate.the explosion The can explosion no longer pressures propagat closere. The to theexplosion upper explosionpressures limitcloser decrease to the upper more thanexplosion in pure limit 2-propanol–air decrease more but than similar in pure to the 2-propan trend observedol–air but for similar pure 1-propanol–air. to the trend observed Figures for6 and pure7 indicate1-propanol–air. the tendency Figures of 6 2-propanol and 7 indicate to lower the tendency the explosion of 2-propanol pressure of to the lower 1-propanol the explosion/2-propanol–air pressure mixture.of the 1-propanol/2-propanol–air The presence of the OH mi groupxture. in The the secondpresence carbon of the of OH the group 2-propanol in the second molecule carbon makes of thisthe result2-propanol interesting molecule to compare makes this with result 1-propanol, interesting which to compare has the OH with group 1-propanol, at the end which of the has molecule. the OH Thisgroup eff atect the of end propanol of the ismolecule. attacked This by unstable effect of and prop reactiveanol is speciesattacked through by unstable different and mechanisms.reactive species throughThe different measured mechanisms. results show good agreement at near-stoichiometric concentrations with the previous 1-propanolThe measured studies [8 ]results with a 0.5show bar good difference agreement at temperatures at near-stoichiometric 50 ◦C and 100 ◦ C.concentrations The explosion with pressure the previous 1-propanol studies [8] with a 0.5 bar difference at temperatures 50 °C and 100 °C. The explosion pressure is only slightly dependent on the vessel volume, which plays a role in the

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Energies 2019, 12, x FOR PEER REVIEW 9 of 15 is only slightly dependent on the vessel volume, which plays a role in the assessment of explosion hazard.assessment From of theseexplosion results hazard. it was From concluded these thatresults the it isomer was concluded structure doesthat the not isomer play a signiﬁcantstructure does role innot maximum play a significant explosion role pressure in maximum development explosio andn pressure for LEL development and UEL. The and slight for diLELﬀerence and UEL. between The 0.02slight m difference3 and 1.00 mbetween3 is attributed 0.02 m to3 and heat 1.00 lost m in3 is the attributed vessel walls to heat [13]. lost Figure in the8 plots vessel a comparison walls [13]. Figure of the pure8 plots 1-propanol–air a comparison and of 2-propanol–air the pure 1-propanol–air mixtures at variousand 2-propanol–air initial temperatures, mixtures 11 equivalenceat various ratios,initial andtemperatures, p0 = 100 kPa. 11 equivalence ratios, and p0 = 100 kPa.

Figure 8. (dp/dt)(dp/dt)ex versusversus ΦΦ == 0.3–1.70.3–1.7 for for propanol propanol isomer–air isomer–air mixtur mixtureses at at various various initial temperatures and mixture compositions,compositions, pp00 = 100100 kPa. kPa.

The rate of pressure rise reached the maximum at Φ = 1.06 (7.0 0.2 vol.%) for both volumes The rate of pressure rise reached the maximum at Φ = 1.06 (7.0 ± 0.2± vol.%) for both volumes and andall temperatures. all temperatures. The Themaximum maximum rate rateof pressure of pressure rise risefor 2-propanol for 2-propanol is in is good in good agreement agreement with with the thevalues values published published for for hydroxyl-substituted hydroxyl-substituted Cn CHn2n+1H2nOH+1OH analogues analogues (where (where n n= =1–5)1–5) in in [13]. [13]. Table Table 44 shows that there is only a slight didifferenceﬀerence inin thethe trend presented duedue toto the explosion vessel volume according toto thethe deﬂagrationdeflagration indexindex values.values.

Table 4. 2-propanol explosion characteristics at Φ = 1.06, p = 100 kPa. Table 4. 2-propanol explosion characteristics at Φ = 1.06, p00 = 100 kPa. 3 3 0.020.02 m m3 1.001.00 m3 m CharacteristicCharacteristic UnitUnit 5050◦C °C 100 100◦ C°C 150 150 °C◦C 50 50 °C◦ C 100 100°C ◦C 150 °C 150 ◦C max 0 pmax/p /p 8.7 8.70.1 ± 0.1 8.38.3 ±0.2 0.2 7.4 7.4 ± 0.20.2 8.6 8.6 ± 0.20.2 8.1 8.1± 0.2 0.27.3 ± 7.30.2 0.2 0 ± ± ± ± ± ± (dp/dt)(dp/dt)max max bar/bar/ss 349 34924 ± 24 338338 ±24 24 329 329 ± 2828 361 361 ± 2929 354 354± 26 26336 ± 336 29 29 ± ± ± ± ± ± K KG bar bar·m/sm/s 95 957 ± 7 9292 ±6 6 89 89 ± 77 98 98 ± 7 796 ± 96 7 791 ± 8 91 8 G · ± ± ± ± ± ± LEL 1LEL 1 %% 3.5 3.50.2 ± 0.2 3.53.5 ±0.2 0.2 3.0 3.0 ± 0.20.2 2.5 2.5 ± 0.20.2 2.0 2.0± 0.2 0.22.0 ± 2.00.2 0.2 2 ± ± ± ± ± ± UEL 2 % 14.0 0.2 14.0 0.2 14.5 0.2 13.0 0.2 13.0 0.2 13.5 0.2 UEL % 14.0± ± 0.2 14.0± ± 0.2 14.5 ± 0.2 13.0 ± ±0.2 13.0 ± 0.2± 13.5 ± 0.2 ± tex ms 89 4 87 4 85 4 215 11 181 9 176 9 tex ms 89± ± 4 87± ± 4 85 ±± 4 215 ± ±11 181 ± 9 ± 176 ± 9 ± 1 2 1 LowerLower explosion explosion limit, 2Upper Upper explosion explosion limit. limit.

When the initial temperature changes from 50 to 100 ◦°C,C, as shown in FigureFigure8 8,, the the maximum maximum rate ofof pressurepressure riserise increasesincreases andand thethe didifferenﬀerencece betweenbetween thethe 1-propanol1-propanol andand 2-propanol2-propanol isomersisomers appears, especially in near-stoichiometricnear-stoichiometric concentrations. There is also some small deviation in thethe leanlean explosionexplosion mixturemixture concentrationsconcentrations closeclose toto thethe valuevalue ΦΦ == 0.7. The The observed observed trend is different diﬀerent from that described in [21]. Such difference from our results is attributed to the heat losses to the wall in both 0.02 m3 and 1.00 m3 vessels in comparison with the heat lost in a 0.005 m3 vessel, being

Energies 2019, 12, 1574 10 of 15 Energies 2019, 12, x FOR PEER REVIEW 10 of 15 thatrespectively described four in [21 and]. Such 20 ditimesfference lower from in our volume results iscompared attributed to thethe heat vessels losses employed to the wall in bothour 0.02experiments. m3 and 1.00 m3 vessels in comparison with the heat lost in a 0.005 m3 vessel, being respectively four andValues 20 timesclose to lower the lower in volume explosion compared limit todiffer the vesselsin trend employed in comparison in our to experiments. the values close to the higherValues explosion close limit. to the This lower behavior explosion is influenced limit differ by in the trend initial in comparison temperature. to the values close to the higherFigure explosion 9 shows limit. an This example behavior of the is influencedrate of pressu by there rise initial versus temperature. the equivalence ratio under the sameFigure initial9 conditions, shows an examplei.e., p0 = 1 of bar the and rate a blending of pressure ratio rise of versus 0.5 for thetwo equivalence initial temperatures ratio under of 50 the °C sameand 100 initial °C. conditions,The present i.e., data p0 of= 1-propanol/2-propa1 bar and a blendingnol–air ratio of mixtures 0.5 for two are initialin good temperatures agreement with of 50 the◦C andexplosion 100 ◦C. characteristics The present dataof pentanol of 1-propanol isomer–air/2-propanol–air mixtures [14]. mixtures The maximu are in goodm values agreement of the withmixture the explosionlie in between characteristics the values of pentanol reported isomer–air for pure mixtures 1-propanol [14]. and The maximumpure 2-propanol values offor the both mixture initial lie intemperatures. between the values reported for pure 1-propanol and pure 2-propanol for both initial temperatures.

Figure 9. (dp/dt)(dp/dt)exex versusversus Φ Φ= 0.3–1.7= 0.3–1.7 for for1-propanol/2-propanol–air 1-propanol/2-propanol–air mixtures mixtures (f0 = (f0.5)0 = at0.5) various at various initial initialtemperatures temperatures and mixture and mixture compositions, compositions, p0 = 100 p0 kPa.= 100 kPa.

Figures 10 10 and and 11 give11 give an example an example of the combustionof the comb delayustion time delay characteristic time characteristic versus 11 equivalence versus 11 3 ratiosequivalence at two initialratios temperatures,at two initial 50temperatures,◦C and 100 ◦C, 50 for °C pure and isomers 100 °C, at for p0 =pure100 kPaisomers measured at p0 = in 100 0.02 kPa m . Themeasured shortest in combustion0.02 m3. The delayshortest times combustion were found delay at thetimes equivalence were found ratio at theΦ = equivalence1.06. The presented ratio Φ = trends1.06. The are presented in agreement trends with are the in CagreementnH2n+1OH with analogues the CnH (where2n+1OH n analogues= 1,2,4,5) in(where [13] and n = are1,2,4,5) similar in [13] for theand 1 are m3 similarexplosion for vesselthe 1 m volume.3 explosion vessel volume. Figure 1111 illustrates illustrates the the effect effect of blending of blending on time toonreach time the toexplosion reach the maximum explosion characteristics. maximum Physicochemicalcharacteristics. Physicochemical properties of pure properties alcohols influencedof pure alcohols the explosion influenced delay the time explosion mainly indelay lean time and richmainly regions. in lean The and explosion rich regions. delay The time explosion of the 1-propanol delay time/2-propanol–air of the 1-propanol/2-propanol–air mixture is influenced mixture by the mixture’sis influenced individual by the mixture’s components. individual Especially componen in the leants. Especially region, the inexplosion the lean region, delay time the explosion is mainly influenceddelay time byis mainly the behavior influenced of 2-propanol. by the behavior of 2-propanol. Figures 10 and 11 show that the time to reach the maximum explosion pressure at lean and rich explosion mixtures decreased with increasing initial temperature and fixedfixed initial pressure for bothboth purepure alcoholsalcohols andand their their mixtures. mixtures. The The results results reveal reve thatal that the the minimum minimum explosion explosion delays delays times times are only are slightlyonly slightly influenced influenced by the by di thefferent different chemical chemical structures structures of the of studiedthe studied isomers. isomers. The The lowest lowest value value of explosionof explosion delay delay time time is near is near the the stoichiometric stoichiometric ratio ratio for both for both the 0.02 the and0.02 1.00and m1.003 volumes. m3 volumes. Figures Figures8–11 show8–11 show that with that the with increase the increase of the initial of the temperature, initial temperature, the rate of explosionthe rate pressureof explosion rise decreasedpressure andrise thedecreased explosion and durationthe explosion decreases. duration The decreases. rate of explosion The rate of pressure explosion rise pressure and the rise explosion and the delay explosion time delay time have reverse effects. The maximum rate of explosion pressure rise does not have the same trend with increasing initial temperature for lean 1-propanol/2-propanol–air mixtures.

Energies 2019, 12, 1574 11 of 15 have reverse eﬀects. The maximum rate of explosion pressure rise does not have the same trend with Energies 2019, 12, x FOR PEER REVIEW 11 of 15 Energiesincreasing 2019,initial 12, x FOR temperature PEER REVIEW for lean 1-propanol/2-propanol–air mixtures. 11 of 15

Figure 10. tex versus Φ = 0.7–1.3 for propanol isomer–air mixtures at various initial temperatures and Figure 10. ttex versusversus ΦΦ == 0.7–1.30.7–1.3 for for propanol propanol isomer–air isomer–air mixtur mixtureses at at various various initial initial temperatures and mixture compositions, p0 = 100 kPa. mixture compositions, p0 = 100100 kPa. kPa.

FigureFigure 11. 11. ttexex versus Φ == 0.7–1.30.7–1.3 for for 1-propanol/2-propanol–air 1-propanol/2-propanol–air mixturesmixtures atat variousvarious initialinitial temperaturestemperatures Figure 11. tex versus Φ = 0.7–1.3 for 1-propanol/2-propanol–air mixtures at various initial temperatures andand mixturemixture compositions, compositions, p p00= = 100100 kPa.kPa. and mixture compositions, p0 = 100 kPa.

TablesTables4 4 andand5 5 presentpresent the the experimentally experimentally derived derived data dataof 2-propanol of 2-propanol and 1-propanol/2- and Tables 4 and 5 present the experimentally derived data of 2-propanol and 1-propanol/2- 1-propanolpropanol–air/2-propanol–air mixtures in 0.02 mixtures and 1.00 in 0.02 m3 vessel and 1.00 volumes m3 vessel for three volumes initial for temperatures three initial temperatures of 50 °C, 100 propanol–air mixtures in 0.02 and 1.00 m3 vessel volumes for three initial temperatures of 50 °C, 100 of°C, 50 and◦C, 150 100 °C.◦C, and 150 ◦C. °C, and 150 °C.

Energies 2019, 12, 1574 12 of 15

Table 5. 1-propanol/2-propanol–air mixture explosion characteristics at Φ = 1.06, p0 = 100 kPa.

0.02 m3 1.00 m3 Characteristic Unit 50 ◦C 100 ◦C 150 ◦C 50 ◦C 100 ◦C 150 ◦C

pmax/p 8.2 0.1 7.9 0.2 7.6 0.2 8.2 0.2 7.9 0.2 7.1 0.2 0 ± ± ± ± ± ± (dp/dt)max bar/s 399 28 393 31 387 33 402 29 396 33 382 33 ± ± ± ± ± ± K bar m/s 108 8 107 9 105 9 109 9 107 9 104 9 G · ± ± ± ± ± ± LEL 1 % 3.5 0.2 3.5 0.2 3.0 0.2 3.5 0.2 3.0 0.2 2.5 0.2 ± ± ± ± ± ± UEL 2 % 13.0 0.2 13.0 0.2 13.5 0.2 13.5 0.2 13.5 0.2 13.0 0.2 ± ± ± ± ± ± tex ms 77 4 75 4 72 4 166 8 156 8 142 8 ± ± ± ± ± ± 1 Lower explosion limit, 2 Upper explosion limit.

The presented explosion pressures averages for pure 2-propanol and 1-propanol/2-propanol–air mixtures reported for different temperatures differs considering the reproducibility given in [17] and are consistent with the data published in [8] for 1-propanol. The reported (dp/dt)max of 2-propanol is close to that reported for 1-propanol in [8]. The explosion characteristics dissimilarity could be attributed to the chain structure. As the chain length of the alcohols increases, thermal decomposition through C–C bonds becomes more prevalent. Because C–C bonds are weaker than the C–OH bond, 1-propanol (unlike 2-propanol) does not lose the OH group in an initiation step [22]. However, because of the relative weakness of the C–OH bond at a central part, 2-propanol loses its –OH group easily. This trend is similar in all initial conditions. The values of tex presented are influenced by the volume and are comparable between 1-propanol and 2-propanol and only slightly differ for the 1-propanol/2-propanol–air mixture. Table6 compares the data of maximum explosion pressure, maximum rate of pressure rise, and combustion time experimentally derived in this study with the values adopted from the literature.

Table 6. Presented and published pmax and KG values.

K Volume Temperature Equivalence Chemical p (bar) G Reference max (bar m/s) (m3) ( C) Ratio · ◦ CH3OH 7.1–8.2 105–107 0.005–0.020 50–100 1.00–1.26 [11,12] C2H5OH 7.9–8.7 95–135 0.005–0.020 100–120 1.00–1.10 [15,16] 1-C3H7OH 7.7–8.7 86–97 0.020–1.000 50–150 1.06 [8] 2-C3H7OH 7.3–8.7 89–98 0.020–1.000 50–150 1.06 Present 1P + 2P 1 7.6–8.2 105–108 0.020–1.000 50–150 1.06 Present 2 1-C4H9OH 5.5–5.7 95–100 0.0053 120 1.00 [13] 2 1-C5H11OH 5.6–5.7 90–95 0.0053 120–160 1.00 [13,14] 1 2 1P + 2P = 1-C3H7OH + 2-C3H7OH mixture for f = 0.5; lengths/diameter = 1.17.

The values in Table6 are close to the literature-derived values. The values of p max 1-C4H9OH and 1-C5H11OH differ from other published data. In Table6, for example, the values of p max for methanol adopted from [12] (7.1 bar and 8.2 bar) clearly appear to be quite different based on initial conditions (temperature, pressure, equivalence ratios). At the initial temperature, the value published by [14] was 5.6 bar, which is similar to value of 5.5 bar obtained by [13]. The (dp/dt)max of 1-propanol and 2-propanol show agreement with the values for ethanol, 1-butanol, and 1-pentanol measured 3 in a 0.005 m cylinder. From Table6, it is obvious that the (dp /dt)max values are highly sensitive to the experimental conditions, mainly the vessel volume and shape used for their determination. The closer the volume of V = 1.00 m3 to the standard explosion chamber, the smaller the deviation and the more accurate the application of the cubic law [23]. The measured values of deflagration index were 89–98 bar m/s for 2-propanol and 105–108 bar m/s for 1-propanol/2-propanol–air mixtures. · · Thus, in terms of reactivity, 2-propanol is characterized as less reactive than 1-propanol/2-propanol–air mixtures. As shown in Table6, the values reported on deflagration index, K G, of 2-propanol are in good agreement with the presented values for ethanol, 1-propanol, 1-butanol, and 1-pentanol but are lower than the published values for methanol. Meanwhile, the values of KG for 1-propanol/2-propanol–air Energies 2019, 12, 1574 13 of 15 mixtures are similar to the values of methanol. Considering these results, the oxidation and overall stoichiometry of the mixture seems to follows the dominant pathways as methanol in the given initial temperature ranges more than 1-propanol and 2-propanol. The described behavior needs more rigorous experimental investigation from the chemical point of view to elucidate the possible mechanisms of action behind the presented experiments. Table7 summarizes the experimentally derived data for the explosion limits of alcohols reported in the literature. The maximum experimental uncertainty for LEL and UEL in Table7 is 0.2% absolute.

Table 7. Presented and published explosion ranges (in vol.%).

IFA 1 ILO 2 Merck Penta SFPE 3 DIPPR 4 Yaws, C.L. Chemical Present [24] [25] [26] [27] [28] [29] [30]

CH3OH 6.0–50.0 5.5–44.0 5.5–44.0 6.0–36.0 6.7–36.5 7.2–36.5 6.0–36.0 C2H5OH 9.0–44.0 3.3–19.0 3.1–27.7 3.5–15.0 3.3–19.0 3.3–19.0 4.3–19.0 1-C3H7OH 2.1–19.9 2.1–13.5 2.1–19.2 2.1–13.7 2.2–13.5 2.1–14.0 2.0–12.0 2-C3H7OH 2.0–13.4 2.0–12.7 2.0–13.4 2.0–12.7 2.0–11.8 2.0–12.7 2.0–12.7 2.0–14.0 1P + 2P 5 2.5–13.5 1-C4H9OH 1.4–11.3 1.4–11.3 1.4–11.3 1.4–11.3 1.4–11.3 1.7–11.3 1.4–11.2 1-C5H11OH 1.3–10.5 1.2–10.5 1.6–8.0 1.2–10.0 1.2–10.0 1.4–10.0 1.2–10.0 1 Institut für Arbeitsschutz der Deutschen Gesetzlichen Unfallversicherung, Sankt Augustin, Germany, 2 International Labour Organization, Geneva, Switzerland, 3 Society of Fire Protection Engineers, Gaithersburg, USA, 4 Design 5 Institute for Physical Property Research, Fort Washington, USA, 1P + 2P = 1-C3H7OH + 2-C3H7OH mixture for f = 0.5.

The present experimentally derived explosion range of 2.0–14.0 vol.% for 2-propanol corresponds well with the trend found in different data compilations provided by chemical databases, chemical company material safety data sheets, and the National Fire Protection Association. There are small differences between the presented higher explosion limit of 2-propanol and the published values due to an elevated initial temperature. Our experimental explosion range of 2.5–13.5 vol.% for the tested 1-propanol/2-propanol–air mixtures is close to the ILO and Penta-derived range of 2.1–13.5 and 2.1–13.7 vol.% for 1-propanol. As compared with the experimental results, the lower explosion limit of the mixture is slightly higher than the explosion limit of 2-propanol and at the same time the higher explosion limit of the mixture is slightly lower than the explosion limit of both 2-propanol and 1-propanol. Such diversity is mainly due to the physicochemical properties of the mixed alcohols. The main limitation of this work is the more rigorous insightful chemical interpretation. It is lacking because the presented closed spherical methods do not allow straight and deeper experimental view into a chemical combustion problem. Such view could be given by the studies that involves experiments using for example a shock tube set-up, direct laser absorption using a mid-infrared QCL system, flow reactor pyrolysis, rapid compression machines, and premixed flat flame set-up. Even in these studies the mechanism of propanol isomers oxidation is not established to compare with methanol or ethanol. Nevertheless, there are still many safety aspects for the premixed flames of the alcohol–air mixtures needed to be clarified, such as analysis of heat release (or mass burning rate) and burning velocities (flame development duration and combustion duration) in which the presented data plays important role.

5. Conclusions In this work, the experimental study of the explosion characteristics of propanol isomer–air mixtures at diﬀerent equivalence ratios, volumes, and initial temperatures was performed. 2-propanol and 1-propanol/2-propanol with air mixtures have been characterized in the gas phase by 0.02 m3 and 1.00 m3 for the ﬁrst time. The presented data allowed to compare the explosion characteristics of the monoalcohol-substituted compounds CnH2n+1OH (n = 1–5) at elevated temperatures. An extensive set of explosion characteristics was obtained, allowing a systematic comparison of the explosion behavior properties to be made among the entire series. The main conclusions are summarized as follows: Energies 2019, 12, 1574 14 of 15

The propanol–air mixtures gave the maximum explosion pressures and the highest deflagration • index values at the equivalence ratio of 1.06. Among propanol isomer–air mixtures, the maximum explosion pressure decreased in the order • of 1-propanol–air, 2-propanol–air, and 1-propanol/2-propanol–air mixtures. The maximum explosion pressure decreased with increasing initial temperature and slightly decreased with increasing volume. The change of initial temperature only slightly influences the maximum rates of pressure rise and • deflagration indexes (in both explosion vessel volumes). The maximum rate of pressure rise slightly decreased with an increasing initial temperature and slightly decreased with increasing volume. For 2-propanol–air mixture lower explosion limit values decreased with an increasing temperature • from 3.5 0.2% to 3.0 0.2% for 0.02 m3 and from 2.5 0.2% to 2.0 0.2% for 1.00 m3. ± ± ± ± Upper explosion limit values increased with increasing temperature from 14.0 0.2% to 14.5 0.2% ± ± for 0.02 m3 and from 13.0 0.2% to 13.5 0.2% for 1.00 m3. ± ± For 1-propanol/2-propanol–air mixtures lower explosion limit values decreased with an increasing • temperature from 3.5 0.2% to 3.0 0.2% for 0.02 m3 and from 3.5 0.2% to 2.5 0.2% for ± ± ± ± 1.00 m3. Upper explosion limit values increased with increasing temperature from 13.0 0.2% to ± 13.5 0.2% for 0.02 m3 and decreased with increasing temperature from 13.5 0.2% to 13.0 0.2% ± ± ± for 1.00 m3. The time to reach the maximum explosion pressure decreased with increasing initial temperature in • the order of 1-propanol, 1-propanol + 2-propanol mixture, and 2-propanol. Moreover, it increased greatly with an increasing explosion vessel volume.

The most important results from evaluated experiments are in normalised maximum rates of pressure rise, K , 89–98 bar m/s for 2-propanol and 105–108 bar m/s for 1-propanol/2-propanol–air G · · mixtures given by the combination of 0.20 and 1.00 m3 vessels at different temperatures regimes. These values are used to describe the effect of isomer blends on a deflagration process that is the paper’s added value to the existing knowledge. The experiments carried out for this paper enable further studies for insightful chemical interpretation.

Author Contributions: Conceptualization, J.S. and T.O.; Data curation, J.S.; Methodology, J.S.; Supervision, T.O.; Visualization, J.S.; Writing—original draft, J.S. Funding: This research was funded by the European regional development fund project “Increase of research capacity and quality of INEF center”, grant number CZ.1.05/2.1.00/19.0407, and the APC was funded by “Research of conversion processes and utilization of waste heat in the framework of fuel-energy technological complexes”, grant number SP 2019/89. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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