Flammability and volatility attributes of binary mixtures

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The vapor pressure of fuels and fuel blends has been used to determine the safe operating ... pressure fuels result in starting difficulties, sluggish warm-up and ... country, region, and season, e.g., within a range of 48.2–103 kPa ... bility limits of fuel–air mixtures in the gas phase from fundamental ..... ence (absolute basis).
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Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel 5 6

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Flammability and volatility attributes of binary mixtures of some practical multi-component fuels

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Ibrahim M. Algunaibet a,⇑, Alexander K. Voice b, Gautam T. Kalghatgi a, Hassan Babiker a

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a b

R&D Center, PO Box 62, Saudi Aramco, Dhahran 31311, Saudi Arabia Aramco Research Center – Detroit, United States

h i g h l i g h t s  Measured the flash point and volatility of binary multi-component fuel mixtures.

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 Light naphtha blended at >20% vol. with diesel has a flash point below 40 °C.

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 n-Butane can be used with diesel/gasoline mixtures to meet flammability needs.

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 Changing ambient temperature does not significantly alter the flammability limits.  An equation has been developed to predict flash point from other fuel properties.

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a r t i c l e 2 3 2 6 23 24 25 26 27 28 29 30 31 32 33 34 35

i n f o

Article history: Received 9 August 2015 Received in revised form 21 December 2015 Accepted 5 January 2016 Available online xxxx Keywords: Flash point Vapor pressure Flammability limits Naphtha Diesel Gasoline compression ignition (GCI)

a b s t r a c t Gasoline compression ignition (GCI) engines could be more efficient than most advanced SI engines while running on lower octane fuel. GCI engines may utilize a mixture of different fuels and fuel components such as gasoline and diesel or diesel and naphtha. The risks and hazards associated with such mixtures must be studied to ensure safe fuel storage, shipping and dispensing. In this work, flash point and vapor pressure measurements of different binary multi-component hydrocarbon mixtures are presented along with calculated lower and upper flammability limits. An equation has been developed to correlate flash point with other fuel properties. The flash point of a mixture approaches the flash point of the more volatile component, falling rapidly in some cases, as the more volatile component concentration increases. Vapor pressure is inversely related to flash point for a given mixture. Diesel/light straight run naphtha mixtures and diesel/gasoline mixtures exhibit similar flash point versus vapor pressure trends. Flammability limits were calculated using Le Chatelier’s Mixing Rule and modified Burgess–Wheeler Law. Hydrocarbon mixtures have similar lower and upper flammability limits over a range of temperatures. The vapor pressure of fuels and fuel blends has been used to determine the safe operating region as a function of blending formula and temperature. This work demonstrates that normal butane can be used to formulate blends of gasoline and naphtha with diesel, which are safe to handle and meet seasonal vapor pressure requirements. Ó 2016 Published by Elsevier Ltd.

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1. Introducterion

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Nearly all transport energy is derived from liquid fuels made from crude oil, and transport fuels account for 60% of the total oil consumption worldwide [1]. The demand for transport fuels is growing but this growth is heavily skewed toward commercial transport and hence toward diesel and jet fuel (as opposed to gasoline) [1–3]. This demand shift, combined with a push to higher

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⇑ Corresponding author at: Saudi Aramco, PO Box 10017, Dhahran 31311, Saudi Arabia. Tel.: +966 138724718; fax: +966 138744444x197388. E-mail address: [email protected] (I.M. Algunaibet).

octane requirements for spark ignition engines, is likely to result in a surplus of low octane components in the gasoline boiling range. One possible solution is to develop engine/fuel systems that can utilize such light and low octane components of gasoline in the most beneficial manner. Examples of these new engine/fuel systems are gasoline compression ignition (GCI) [2] and octane on demand (OOD) [4]. GCI engines can be as efficient as current diesel engines, while operating on low cost, low octane gasolines (such as naphtha). In the short term, such new combustion systems may have to run on existing market fuels and available fuel components. This can be accomplished by blending diesel and gasoline

http://dx.doi.org/10.1016/j.fuel.2016.01.023 0016-2361/Ó 2016 Published by Elsevier Ltd.

Please cite this article in press as: Algunaibet IM et al. Flammability and volatility attributes of binary mixtures of some practical multi-component fuels. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.01.023

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to achieve the lower octane number fuel needed by GCI engines [2,5]. Eventually, GCI engines will be well positioned if they are able to consume surplus low octane gasoline blendstocks like naphtha. It is also possible that mixtures of diesel fuel and naphtha would provide the optimum fuel quality needed by GCI, while providing lower cost and carbon emissions relative to diesel fuel. The hazards and risks associated with such blends need to be studied. The headspace of a fuel tank contains a mixture of fuel vapors and air. Vapor pressure is an important property of automotive and aviation gasoline [6]. It is used to set a standard at which the fuel would be safe for handling. Vapor pressure is also important for cold-start and warm-up in spark ignition engines [7]. Low vapor pressure fuels result in starting difficulties, sluggish warm-up and acceleration reduction [8]. While high vapor pressure fuel could cause vapor lock, resulting in decreased fuel flow to the engine. Vapor pressure and ambient pressure will determine the gas composition at the vapor liquid interface according to Henry’s law. Gasoline standards limit the vapor pressure depending on the country, region, and season, e.g., within a range of 48.2–103 kPa in the U.S. [7]. There are different standards to measure vapor pressure. These include dry vapor pressure equivalent (DVPE), ASTM test method D5191 [9], and Reid vapor pressure (RVP), ASTM test method D323 [10]. Both methods have been designed to determine the vapor pressure of gasoline and other volatile crude products. DVPE uses an automated vapor pressure instrument to detect the total vapor pressure applied in vacuum [9]. Moreover, DVPE has the ability to detect vapor pressure of a fuel whether it contains oxygenates or not. In contrast, RVP is limited to fuels that do not contain oxygenates. DVPE depends on a statistical equation that applies a correction factor to the measured total vapor pressure [9]. The vapor pressure of mixtures can also be estimated using Raoult’s Law, which states that the vapor pressure of an ideal mixture is equal to the vapor pressure of each component within that mixture times its mole fraction [11]. Petroleum-based fuels contain hundreds of different hydrocarbons, each with a different boiling point [7]. Therefore, each fuel will have a unique boiling range (rather than a boiling point as with a single component). The relationship between temperature and the amount of fuel evaporated is known as the distillation curve [1]. There are several ASTM test methods that can be used to produce such distillation curves – D86 [12], D7345 [13] and D2887 [14]. All these test methods can be used to determine the range at which the petroleum products will boil, although they cover different boiling ranges. In this study, ASTM test method D2887 was used to determine the distillation curve for the fuels of interest. Most industrial processes consider flammability limits to be a major safety concern [15]. Many hydrocarbons are volatile at normal conditions, thus knowledge of flammability limits is needed to prevent explosive hazards. At a given temperature, the lower flammability limit (LFL) and the upper flammability limit (UFL) are, respectively, the lowest and highest concentration of a hydrocarbon (volume %) in air at which a flame can be initiated by an ignition source [16]. In other words, concentrations below the LFL are too lean, whereas the ones above the UFL are too rich, to support a propagating flame. Fire protection engineers therefore utilize flammability limits when assessing the potential risk posed by a flammable substance [15]. There are many methods to obtain reliable flammability limits, to handle flammable gasses safely [15]. These flammability limits can be measured. For example, an ASTM test method E681 can identify flammability limits through the visual detection of a propagating flame initiated by an electric ignition source [17]. This test is costly. Therefore, correlations have been proposed to predict them in the absence of experimental measurements. Shimy

demonstrated that flammability limits of all hydrocarbons with multiple molecular structures depend mainly upon the required energy for two carbon atoms to be disengaged [18]. In 1891, Le Chatelier demonstrated an empirical equation to predict flammability limits of fuel–air mixtures in the gas phase from fundamental properties [19]. Zhao established a comparison between experimental flammability limits and the predicted ones using Le Chatelier’s Mixing Rule [15]. The probability of ignition varies from 0% to 100% depending on the vapor concentration at a given temperature [16]. At a certain point slightly below the calculated upper flammability limit, the mixture would have a 100% probability in terms of producing a sustainable, propagating flame [16]. Various other correlations have established relationships between flammability limit and other parameters, such as molecular weight and molecular structure [20,21]. If the mixture is between the upper and lower flammability thresholds, an accidental spark could create an explosion. One measure used to quantify the risk of explosion is the flash point, which is the lowest temperature at which the fuel can vaporize to form an ignitable mixture in air [22]. In other words, it is approximately the minimum temperature at which the vapor pressure of the fuel exceeds the lower flammability threshold. Lower flammable limit, compared to flash point, requires lower concentration of flammable vapors [20]. Commercial gasoline in the market has a flash point less than 40 °C. At ambient temperatures the air/fuel vapor mixture is too rich to ignite because the vapor pressure at these conditions exceeds the upper flammability threshold. Diesel fuel has a flash point of around 60 °C and the air/fuel mixture in the head space is too lean to ignite at normal ambient temperatures. Blending light fuels like naphtha or gasoline with diesel changes the vapor pressure characteristics and therefore the flashpoint – potentially creating a hazardous situation. Flash point can be measured in a standardized test such as IP 170 [22]. Several studies have also developed correlations to predict flash points of binary hydrocarbon mixtures. Such methods are unsuitable for mixtures that have a wide boiling range such as gasoline/naphtha and diesel. Establishing a sound understanding of the flash point would assist in achieving safer fuel formulations. There are two procedures used to measure flash point temperature. These are the closed cup and open cup methods. Since vapors are vulnerable to escape in the open cup method compared with the closed cup, the closed cup flash point temperature is lower than the open cup [23]. Flash point temperatures can be determined from the literature for pure liquids, but not for mixtures [23]. Prediction models have been established to approximate the flash point temperatures for both pure components and mixtures. Wickey and Chittenden introduced an index based model to calculate the closed cup flash point [24]. Catoire and Paulmier also developed an empirical model to predict the flash point temperature assuming a non-ideal solution [25]. Rowley proposed a structural group contribution model based on the well-known Clausius– Clapeyron equation (which relates the vapor pressure dependence on temperature of a pure component to its heat of vaporization) [26]. Shimy correlated the flash point of any compound of paraffinic hydrocarbons, paraffinic isomers, olefins, benzene series, acetylene and alcohols with the ignition temperature of that compound [18]. These predictive equations for flammability limits are limited to 25 °C and atmospheric pressure. An understanding of flammability limit changes with respect to temperature must also be established to address hazards associated with fuel storage and distribution. The modified Burgess–Wheeler Law with Spakowski’s assumption can relate the flammability limit with temperature change [27,28]. Flash point measurements for blends of a Saudi gasoline and a Saudi diesel fuel were reported in [5].

Please cite this article in press as: Algunaibet IM et al. Flammability and volatility attributes of binary mixtures of some practical multi-component fuels. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.01.023

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This paper reports flash point measurements for blends of some refinery naphthas with diesel fuel. It also considers the flammability limits of these blends to get a broader understanding of the implications for safety.

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2. Fuels used and methods

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2.1. Fuels used

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Diesel, 91 RON and 95 RON gasoline, light straight run naphtha (LSRN) and heavy straight run naphtha (HSRN), obtained from Ras Tanura refinery in Saudi Arabia have been considered in this study (further details of the fuels in Appendix A). Mixtures containing ethanol were also tested but have not been considered in this paper because ethanol was not fully miscible in diesel at concentrations greater than 5% by volume of ethanol.

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2.2. Method

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All flash point experimental data presented in this paper was acquired following the IP 170 standard [22]. All vapor pressure measurements presented in this paper were obtained in accordance with ASTM test method D5191 [9]. The distillation curves presented in this paper were obtained in accordance with ASTM test method D2887 [14]. A test method developed in-house was used to obtain the PIONA analysis of vapors at the headspace. In this method, a quarter of a 2 mL tube is filled with the liquid of interest. Prior to starting the test, the sample is placed in a water bath at 25 °C. Then, the sample is transferred into the GC where the syringe takes the vapors from the headspace located in the top quarter of the tube. The obtained results have been used to calculate the flammability limits of different binary mixtures using Le Chatelier’s Mixing Rule.

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3. Theory

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3.1. Dry vapor pressure equivalent [9]

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ASTM test method D5191 correlates DVPE with the total pressure measured using Eq. (1):

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must not exceed a critical value of 1.9 °C for this test method to produce reliable results. The equations in ASTM test method D7215 are:

T^ IBP ¼ 2:75 þ 0:944T IBP þ 0:163T 5%  0:124T 10% ½ C;

ð2Þ

T^ 5% ¼ 2:21 þ 0:163T IBP þ 0:363T 5% þ 0:445T 10% ½ C;

ð3Þ

T^ 10% ¼ 3:71  0:124T IBP þ 0:455T 5% þ 0:694T 10% ½ C;

ð4Þ



1 MSPEx ¼ 3

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ð1Þ

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where Pt is the measured total vapor pressure in kPa at 37.8 °C [9]. The total pressure measured is the sum of the absolute vapor pressure of the liquid and the partial pressure of the dissolved gases (air).

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3.2. Calculated flash point from simulated distillation [29]

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ASTM test method D7215 correlates the simulated distillation analysis with flash point [29]. This method can be used only for middle distillates (diesel and jet fuel) that have an initial boiling point (IBP) from 90 °C to 162 °C (Table 1). Reconstruction values of TIBP, 5% and 10% recovered temperatures have to be calculated and substituted into Eq. (5) to obtain the mean sum of prediction error (MSPEx) of the recovery temperatures. Furthermore, MSPEx Table 1 IBP, 5% and 10% recovered temperatures requirements to use Eq. (6) [29]. Test methods

D93 Diesel

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 ½T IBP  T^ IBP  þ ½T 5%  T^ 5%  þ ½T 10%  T^ 10%  ;

IBP

5%

10%

Min temp (°C)

Max temp (°C)

Min temp (°C)

Max temp (°C)

Min temp (°C)

Max temp (°C)

103

163

144

210

159

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257

258 260 263 264 266 267

ð5Þ

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CFPD93 ¼ 51:7 þ 0:403T IBP þ 0:163T 5% þ 0:214T 10% ½ C

ð6Þ

where T^ IBP , T^ 5% and T^ 10% are the calculated reconstruction values of T IBP , T 5% and T 10% , which are the recovered temperatures of the sample using simulated distillation according to ASTM test method D2887 [14] and CFPD93 is the estimated flash point temperature in accordance with ASTM test methods D93 [29]. Certain requirements have to be met for this equation to produce reliable results. These requirements are summarized in Table 1.

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3.3. Le Chatelier’s Mixing Rule [19]

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Le Chatelier proposed an empirical equation to predict the lower and upper flammability limits using the following equations:

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100 LFLmix ¼ PN C ; i

Xn

C ¼ 100; i¼1 i

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ð7Þ 286

i¼1 LFLi

and

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100 ¼ PN C i

i¼1 UFLi

DVPE ðkPaÞ ¼ 0:965P t  3:78

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and

UFLmix

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n X ; C i ¼ 100

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ð8Þ 290

i¼1

where C i and LFLi are the percentage composition and lower flammable limit of the ith component, respectively, considering only combustible species [27]. Algebraically, the minimum and maximum flammability limits of pure components within a mixture represent the flammability limits of that mixture [30]. Le Chatelier’s Mixing Rule is limited to 25 °C and atmospheric pressure.

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3.4. Modified Burgess–Wheeler Law [27,28]

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Flammability limits can be extrapolated into a wider range of temperatures using Modified Burgess–Wheeler Law. These equations can be defined as:

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LFLT ¼ 1  0:000721ðT  25  CÞ LFL25  C

ð9Þ

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UFLT ¼ 1 þ 0:000721ðT  25  CÞ UFL25  C

ð10Þ

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where LFLT is the lower flammable limit at the temperature of interest, LFL25  C is the lower flammable limit at 25  C, UFLT is the upper flammable limit at the temperature of interest, UFL25  C is the upper flammable limit at 25  C and T is the temperature of interest in °C [27,28].

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3.5. Vapor pressure calculations

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Vapor pressure calculations were carried out in Aspen HYSYS using the Peng–Robinson equation of state for hydrocarbon-only systems, and the PRSV (modified Peng–Robinson) equation of state for hydrocarbon plus oxygenate systems. A vapor pressure surro-

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gate was constructed for each of the fuels of interest, using the exact composition for C1–C6, normal paraffin amounts matching the overall carbon distribution for C7–C30 (Appendix B).

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4. Results and discussion

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4.1. Flash point of different binary mixtures

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Fig. 1 shows the measured flash point temperatures of different binary fuel mixtures. The flash point of a mixture approaches the flash point of the more volatile component (falling rapidly in some cases) as the more volatile component concentration increases. For instance, for diesel and LSRN blends as the concentration of LSRN, which is very volatile compared with diesel, increases the flash point temperature drops dramatically. 20% of LSRN by volume is sufficient to reduce the flash point of the mixture below 40 °C. In HSRN/LSRN mixtures, 70% of LSRN by volume is required to reduce the flash point below 40 °C. Diesel has a freezing point of 1 °C, whereas LSRN and HSRN have freezing points below 80 °C. Thus when the sample is cooled below the expected flash point temperature, the diesel may not contribute to the total flash point of the mixture at all. Both HSRN and LSRN have continuing contributions to the total flash point of the mixture as a result of having low freezing points. Flash point alone cannot be used as a measure of the combustibility of fuels. The flash point does not provide any information about the upper flammable temperature, which is the highest temperature at which a flammable mixture exists over the liquid surface. A fuel with a flash point similar to gasoline may not necessarily have the same upper flammable temperature.

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4.2. Vapor pressure of different binary mixtures

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Fig. 2 demonstrates the obtained DVPE measurements for each binary mixture. The vapor pressure increases as the concentration of the volatile components increases. Fig. 3 shows that the measured absolute vapor pressure of LSRN increases with the temperature.

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Fig. 1. Flash point (IP170) of different binary mixtures. The x-axis represents the percentage of the second component into the blend. For example in diesel/LSRN blend, the second component would be LSRN. Gasoline data have been obtained from [5].

Fig. 2. Dry vapor pressure equivalent (D5191) of different binary mixtures. The x-axis represents the percentage of the second component into the blend. For example in diesel/LSRN blend, the second component would be LSRN. Gasoline data have been obtained from [5].

4.3. Correlation between flash point and vapor pressure

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Fig. 4 shows that flash point decreases with increasing vapor pressure. Some mixtures tend to have similar flash point versus vapor pressure trends, although they have different components. For instance diesel/LSRN and diesel/gasoline blends almost share a similar vapor pressure trend. HSRN/LSRN blends extend the exact trend of diesel/HSRN blends. Diesel/HSRN blends are in the low vapor pressure and high flash point region, while HSRN/LSRN blends are in the high vapor pressure and low flash point region.

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4.4. Distillation curves of different binary mixtures

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Fig. 5a presents the boiling range of diesel/LSRN mixtures using ASTM test method D2887 [14]. Similar curves are shown for diesel/ HSRN (Fig. 5b) and HSRN/LSRN mixtures (Fig. 5c). LSRN and HSRN are within the boiling range of gasoline. Furthermore, LSRN/HSRN

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Fig. 3. Absolute vapor pressure of light straight run naphtha at different temperatures.

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Fig. 4. Flash point versus dry vapor pressure equivalent of different binary mixtures. Gasoline data have been obtained from [5]. Fig. 5b. Boiling range distribution of diesel/heavy straight run naphtha mixtures (ASTM D2887).

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mixtures that have 70% or less of LSRN by volume exhibit a boiling range that is similar to that of gasoline. Since GCI engines are likely to prefer a fuel with at least 50% by volume gasoline to provide sufficient chemical ignition delay for air–fuel mixing to occur, LSRN/ HSRN blends that exhibit volatility characteristics similar to that of gasoline can be useful [1].

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4.5. Correlation between flash point and other fuel properties

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The measured and predicted flashpoint (using ASTM D7215) of binary fuel blends are shown in Fig. 6. Although the method is not intended for fuels with an initial boiling point (IBP) less than 103 °C, blends of HSRN with LSRN and diesel mixtures show a good agreement with the model. The model is not able to reliably predict flash points for diesel/LSRN and diesel/gasoline mixtures. For this reason, regression analysis was used to develop a new equation that can predict the flash point of fuel mixtures with IBP in the gasoline range – using the distillation curve and the vapor pressure measurements (Appendix C). It can be defined as:

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CFPmodified ¼ 63:2 þ 0:671T IBP þ 0:361T 5%  0:003T 10% 383

 0:156DVPE

ð11Þ

Fig. 5a. Boiling range distribution of diesel/light straight run naphtha mixtures (ASTM D2887).

Fig. 5c. Boiling range distribution of heavy straight run naphtha/light straight run naphtha mixtures (ASTM D2887).

T IBP , T 5% and T 10% are the recovered temperatures in °C of the sample using simulated distillation analysis following ASTM test method D2887 [14], DVPE is dry vapor pressure equivalent in kPa obtained by ASTM test method D5191 [9] and CFPmodified is the calculated flash point temperature in °C. Fig. 7 shows a better correlation between the measured and calculated flash points than the one obtained by ASTM test method D7215 (Fig. 6). The predicted and measured flash points of all mixtures approximately agree with each other, including diesel/gasoline (from [5]) and diesel/LSRN mixtures. However, fuel composition varies across the globe and more work is necessary to resolve the issues associated with the impact of differing fuel composition on its properties.

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4.6. Calculating flammability limits using Le Chatelier’s Mixing Rule

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Figs. 8 and 9 show the calculated lower and upper flammable limits, LFL and UFL, using Le Chatelier’s Mixing Rule. The green and red points in both figures represent the results for 91 RON gasoline and 95 RON gasoline, respectively. The LFL for gasoline ranges from 1.2% to 1.3% by volume in air – similar to the LFLs

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Fig. 8. Lower flammable limits of different binary mixtures at 25 °C and atmospheric pressure obtained using Le Chatelier’s Mixing Rule. The x-axis represents the percentage of the second component into the blend. For example in diesel/LSRN blend, the second component would be LSRN.

Fig. 6. Measured flash point versus predicted flash point of different binary hydrocarbon mixtures based on ASTM correlation in Eq. (6) [29]. Gasoline data have been obtained from [5].

Fig. 9. Upper flammable limits of different binary mixtures at 25 °C and atmospheric pressure obtained using Le Chatelier’s Mixing Rule. The x-axis represents the percentage of the second component into the blend. For example in diesel/LSRN blend, the second component would be LSRN.

Fig. 7. Measured flash point versus predicted flash point of different binary hydrocarbon mixtures, based on Eq. (11). Gasoline data have been obtained from [5].

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for the binary hydrocarbon mixtures at 25 °C and atmospheric pressure. The UFL of these blends ranges between 7.5% and 7.8% by volume in air and are comparable to gasoline. Figs. 10 and 11 show the LFL and UFL of gasoline and binary hydrocarbon mixtures that were determined using the modified Burgess–Wheeler law from 45 °C to 95 °C. The LFL of gasoline and the hydrocarbon mixtures was very similar over the entire temperature range, with a difference of no more than 0.3% difference (absolute basis). The UFL of the mixtures was also very similar

to the gasolines considered across the full temperature range, with all of the fuels falling in a band of 1.1% by volume in air. Ambient temperature was predicted to have a minimal impact on the LFL and UFL of all of the fuels modeled in this study.

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4.7. Vapor pressure and flammability of fuels and fuel mixtures

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Flash point and flammability limits alone are not sufficient to describe the hazardous behavior of a fuel, since its hazardous behavior is determined by whether it produces a flammable mixture at typical ambient conditions. For the purposes of this study, a fuel is presumed safe if its true vapor pressure at a given temperature falls above the upper flammability limit or below the lower flammability limit. This vapor pressure corresponds to the fuel mole fraction in the vapor at the vapor–liquid interface, according to Raoult’s law. As a simplification, one and eight percent are chosen as a conservative estimate for the mole fraction at the lower flammable and upper flammable limits, respectively, for all temperatures and fuels. Accordingly with Raoult’s law, a ‘‘danger zone” is defined where,

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0:01 < Pv ap ðatmÞ < 0:08

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Fig. 10. Lower flammable limit versus temperature of different binary mixtures obtained using modified Burgess–Wheeler Law.

Fig. 11. Upper flammable limit versus temperature of different binary mixtures obtained using modified Burgess–Wheeler Law.

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The vapor pressure at different temperatures can be calculated, and then the temperature range where the vapor will be in the ‘‘danger zone” can be estimated. In Fig. 12 and 91 RON gasoline is observed to cross below the eight percent threshold and into the danger zone at 13 °C, a reasonable result given that typical ambient temperatures in Saudi Arabia are unlikely to dip below this temperature. LSRN showed similar behavior to 91 RON gasoline, with a safe operating temperature above 12 °C. HSRN crossed through the danger zone between 3 °C and 36 °C, sug-

gesting it would be unsafe under most conditions without further formulation. Diesel remained below the LFT at all temperatures, suggesting a flash point above 60 °C (the minimum required for diesel fuel in Saudi Arabia) (Fig. 12). Vapor compositions at the vapor–liquid interface were also determined for diesel/91 RON gasoline. The data are represented in Fig. 13 showing vapor composition as a function of temperature and amount of diesel blended, where the region between the onepercent and eight-percent contours (unshaded area) indicating the

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Fig. 12. Vapor-pressure curves for Ras Tanura light and heavy naphtha (RTRLN and RTRHN, respectively), Saudi Arabia market diesel (SA Diesel), and Saudi Arabia market 91 RON gasoline (SA 91RON).

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Fig. 14. Vapor pressure for Saudi Arabia market diesel and gasoline blends with normal butane at 13 °C. The contour shows vapor pressure in atm.

50 40

0.08

20 10

8

0.0

0 -10

0.08

0.08

-20

Danger Zone

-30

0.01 < Vapor Pressure < 0.08 atm

-40

0

0.01

Temperature (°C)

30

20

40

60

1 0.0 80

Diesel (LV%) Fig. 13. Vapor pressure as a function of temperature and blending for Saudi Arabia market gasoline and diesel blends. The contours show vapor pressure in atm. 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471

danger zone. The results show that diesel blending has little impact on the vapor pressure of 91 RON gasoline up to 50% by volume. Thus a 50/50 blend of diesel and 91 RON gasoline would be safe at temperatures above 10 °C because the vapor would be too rich to ignite. After this point, the vapor pressure at a given temperature decreases and the danger zone expands significantly (Fig. 13). One way to avoid traversing into the danger zone under certain conditions is to blend additional normal butane to compensate for the reduction in vapor pressure from diesel blending. Normal butane is a low-value refining product blended into gasoline at the terminal to meet seasonal Reid vapor pressure (RVP) specifications (minimum 45, maximum 60 or 70 kPa for winter and summer gasoline in Saudi Arabia, respectively). Blending between zero and four percent by volume of n-butane allows diesel/gasoline blends (and diesel/naphtha though the results are not explicitly shown here), to exceed the eight percent threshold at 13 °C at all diesel blend levels (Fig. 14). Thus around 0.5% vol. of n-butane would need to be added to a gasoline/diesel mixture containing 20% vol. diesel to get the vapor to be too rich to ignite at 13 °C. The exact amount must be adjusted seasonally and according to the amount of diesel being blended to avoid exceeding the maximum RVP specification (Fig. 15). Thus, fuel producers should have

Fig. 15. Vapor pressure for Saudi Arabia market diesel and gasoline blends with normal butane at 37.8 °C. The contours show vapor pressure in atm, where 0.691 and 0.592 atm are the maximum RVPs for winter and summer gasoline, respectively, and 0.444 is the minimum RVP for summer and winter gasoline.

both the means and (due to the low value of normal butane), the desire to produce diesel/gasoline or diesel/naphtha blends that are both safe and meet seasonal RVP requirements.

472

5. Conclusion

475

Flash point and vapor pressure measurements of different binary hydrocarbon mixtures are presented along with calculated lower and upper flammability limits. The flash point of a mixture approaches the flash point of the more volatile component, in some cases falling rapidly, as the concentration of the more volatile component increases. 20% of LSRN or gasoline blended with diesel by volume is sufficient to pull down the flash point below 40 °C. Flash point was found to be inversely related to vapor pressure at 37.8 C. Vapor pressure at a single temperature alone was not sufficient to accurately predict the flash point of fuel mixtures. It can

476

Please cite this article in press as: Algunaibet IM et al. Flammability and volatility attributes of binary mixtures of some practical multi-component fuels. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.01.023

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be accurately predicted if the distillation properties of the mixture were also considered as suggested in [29]. All of the binary hydrocarbon mixtures considered have similar lower and upper flammability limits to gasoline, and neither the UFL or LFL of any of the fuels or fuel mixtures considered were found to significantly depend on temperature. The results show that diesel blending has little impact on the vapor pressure characteristics of gasoline up to 50% by volume. This suggests that gasoline can be safely blended with diesel at lower concentrations, although hazards must still be evaluated for each system where the fuel will be transferred or stored. For blends with greater amounts of diesel fuel, a third component (normal butane) can be used to increase the volatility of the mixture and ensure it meets the required vapor-pressure characteristics.

500

Acknowledgements

501

504

We thank our colleagues in Crude & Product Characterization Unit and Ahmad Al-Radhwan in Fuel Technology R&D Division at the Saudi Aramco Research & Development Center; without their help this study would not have been possible.

505

Appendix A. Detailed hydrocarbon analysis of the fuels used

486 487 488 489 490 491 492 493 494 495 496 497 498

502 503

506

See Tables A.1–A.4. Table A.1 The mole fraction distribution by group and carbon number of the 91 RON gasoline. Group

% Mol

Carbon number

% Mol

Paraffin I-Paraffins Aromatics Naphthenes Olefins Oxygenates Unidentified

22.5 27.1 32.3 3.9 0.1 14.1 0

C3 C4 C5 C6 C7 C8 C9 C10 C11

0.1 2.7 38.4 18.4 18.1 13.7 6.5 1.7 0.4

Table A.2 The mole fraction distribution by group and carbon number of the 95 RON gasoline. Group

% Mol

Carbon number

% Mol

Paraffin I-Paraffins Aromatics Naphthenes Olefins Oxygenates Unidentified

18.2 30.8 26.9 6.2 1.3 16.3 0.4

C3 C4 C5 C6 C7 C8 C9 C10 C11 C12

0.8 4.4 35.0 15.2 20.2 11.7 4.7 6.7 0.9 0.1

Table A.3 The mole fraction distribution by group and carbon number of the RTR light straight run naphtha. Group

% Mol

Carbon number

% Mol

Paraffin I-Paraffins Aromatics Naphthenes Olefins Oxygenates Unidentified

50.5 37.5 5.0 6.8 0.2 0 0

C5 C6 C7 C11 C12 C13 C14

37.9 56.5 0.6 1.1 2.7 0.9 0.3

Table A.4 The mole fraction distribution by group and carbon number of the RTR heavy straight run naphtha. Group

% Mol

Carbon number

% Mol

Paraffin I-Paraffins Aromatics Naphthenes Olefins Oxygenates Unidentified

36.4 35.4 13.3 13.1 1.9 0 0

C6 C7 C8 C9 C10 C11 C12

10.6 31.5 31.5 19.2 6.0 1.2 0.1

Table B.1 Surrogates used for modeling vapor pressure of fuels and fuel mixtures, where the number indicates the amount of each component in mole fraction. Component

RTRLN

RTRHN

SA Diesel

SA 91 RON

Ethanol Propane i-Butane n-Butane i-Pentane n-Pentane n-Hexane n-Heptane n-Octane n-Nonane n-Decane n-C11 n-C12 n-C13 n-C14 n-C15 n-C16 n-C17 n-C18 n-C19 n-C20 n-C21 n-C22 n-C23 n-C24 n-C25 n-C26 n-C27 n-C28 n-C29 n-C30 2,3-Dimethylbutane 2-Methylpentane 3-Methylpentane Methylcyclopentane Cyclopentane Benzene Ethane 2,2-Dimethylpropane Methanol Cyclohexane 2,2-Dimethylbutane MTBE

0.00000 0.00000 0.00000 0.00000 0.09752 0.24633 0.23934 0.00698 0.00000 0.00000 0.00000 0.01522 0.03856 0.01535 0.00581 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.02111 0.15019 0.09694 0.03005 0.01437 0.00763 0.00000 0.00000 0.00000 0.01144 0.00316 0.00000

0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.04761 0.29000 0.31842 0.21285 0.07582 0.01600 0.00105 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00580 0.00879 0.01024 0.00000 0.00292 0.00000 0.00000 0.00000 0.01051 0.00000 0.00000

0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00070 0.00940 0.01980 0.04241 0.06351 0.06431 0.08392 0.09182 0.09362 0.08242 0.08342 0.07481 0.06731 0.05501 0.02841 0.03881 0.03271 0.02370 0.01900 0.01050 0.00710 0.00390 0.00200 0.00140 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000

0.00000 0.00099 0.00437 0.01786 0.10813 0.11680 0.05792 0.18245 0.14484 0.07284 0.02227 0.00467 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.04929 0.03190 0.01175 0.00407 0.02274 0.00000 0.00000 0.00000 0.00457 0.00505 0.137489

Appendix B. The mole fraction of the considered fuel mixtures See Table B.1.

Appendix C. The measured and predicted properties of fuel mixtures See Table C.1.

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Table C.1 Measured fuel properties of different fuels and fuel mixtures. Calculated flash points have been obtained using Eq. (11).

a b

512 513 514 515 516 517

Sample

MFPa (°C)

IBP (°C)

T5% (°C)

T10% (°C)

DVPE (kPa)

CFPb (°C)

100% diesel 99% diesel + 1% LSRN 98% diesel + 2% LSRN 97% diesel + 3% LSRN 96% diesel + 4% LSRN 95% diesel + 5% LSRN 94% diesel + 6% LSRN 93% diesel + 7% LSRN 92% diesel + 8% LSRN 91% diesel + 9% LSRN 90% diesel + 10% LSRN 88% diesel + 12% LSRN 86% diesel + 14% LSRN 84% diesel + 16% LSRN 82% diesel + 18% LSRN 99% diesel + 1% HSRN 98% diesel + 2% HSRN 97% diesel + 3% HSRN 96% diesel + 4% HSRN 95% diesel + 5% HSRN 94% diesel + 6% HSRN 93% diesel + 7% HSRN 92% diesel + 8% HSRN 91% diesel + 9% HSRN 90% diesel + 10% HSRN 88% diesel + 12% HSRN 86% diesel + 14% HSRN 84% diesel + 16% HSRN 82% diesel + 18% HSRN 80% diesel + 20% HSRN 77% diesel + 23% HSRN 74% diesel + 26% HSRN 71% diesel + 29% HSRN 68% diesel + 32% HSRN 65% diesel + 35% HSRN 62% diesel + 38% HSRN 59% diesel + 41% HSRN 56% diesel + 44% HSRN 53% diesel + 47% HSRN 50% diesel + 50% HSRN 46% diesel + 54% HSRN 42% diesel + 58% HSRN 38% diesel + 62% HSRN 34% diesel + 66% HSRN 30% diesel + 70% HSRN 26% diesel + 74% HSRN 22% diesel + 78% HSRN 18% diesel + 82% HSRN 14% diesel + 86% HSRN 10% diesel + 90% HSRN 6% diesel + 94% HSRN 2% diesel + 98% HSRN 1000% HSRN 95% HSRN + 5% LSRN 90% HSRN + 10% LSRN 80% HSRN + 20% LSRN 68% HSRN + 32% LSRN 59% HSRN + 41% LSRN 50% HSRN + 50% LSRN 38% HSRN + 62% LSRN 95% diesel + 5% gasoline 90% diesel + 10% gasoline

60.00 34.00 21.50 12.50 5.00 0.00 3.50 8.00 15.00 20.50 29.50 30.50 31.50 33.00 34.50 54.50 49.50 47.00 43.00 42.00 38.50 35.50 34.00 32.00 31.00 28.00 26.00 23.00 22.00 20.00 18.00 16.50 14.50 12.50 10.00 10.00 8.50 8.00 8.00 6.00 5.00 3.50 2.50 2.50 2.00 0.50 0.00 0.00 0.00 0.00 2.00 2.50 2.50 11.0 15.0 25.0 27.5 33.5 36.0 40.0 7.0 19.0

116.7 37.3 36.3 35.8 35.1 33.9 33.3 32.4 32.1 32.0 31.7 31.4 31.4 31.2 31.3 80.9 81.1 77.6 85.2 83.5 77.5 75.1 73.1 70.2 69.2 68.9 68.6 68.1 68.0 67.5 67.2 66.7 66.6 66.2 64.8 64.1 62.5 62.3 61.8 61.7 61.1 60.8 60.5 60.1 60.0 59.6 58.9 61.0 55.6 56.1 57.2 57.8 57.8 35.6 32.2 31.7 31.1 30.6 30.6 30.6 31.0 31.0

159.4 150.9 147.2 142.8 133.9 120.3 111.6 69.1 67.4 62.9 60.4 55.8 54.0 46.3 37.6 151.6 150.3 147.4 142.9 139.9 137.0 132.5 126.1 125.2 120.4 115.8 111.8 105.4 102.0 98.6 97.8 96.8 96.0 92.2 89.8 89.1 88.2 87.7 87.2 86.8 86.1 85.6 84.9 83.9 83.1 80.7 76.1 76.4 73.9 70.6 70.0 70.0 69.4 68.3 57.8 37.2 35.6 35.6 34.4 32.8 176.0 132.0

173.3 169.8 168.5 167.1 164.8 162.9 161.6 156.6 150.6 144.8 139.2 91.6 68.9 67.9 64.4 170.5 168.7 168.3 165.4 164.1 161.7 158.9 156.3 151.2 150.2 143.3 138.9 133.3 126.1 124.9 117.8 114.7 111.9 106.3 102.6 98.9 98.4 98.1 97.8 97.6 96.7 94.1 92.0 90.4 89.8 89.2 88.9 89.9 87.8 87.2 87.2 87.2 86.7 75.6 69.4 58.3 37.2 36.7 36.1 35.6 200.0 177.0

1.40 0.30 0.90 1.90 3.10 3.90 5.20 6.10 6.70 7.40 8.40 10.10 12.30 14.10 16.50 1.00 0.80 0.70 0.50 0.40 0.40 0.30 0.10 0.10 0.20 0.40 0.60 1.10 1.20 1.60 2.00 2.50 2.80 3.10 3.40 3.80 4.10 4.40 4.70 5.30 5.70 5.90 6.30 6.70 7.00 7.50 7.90 8.20 8.60 8.80 9.20 9.50 9.60 13.1 16.8 23.2 32.0 38.0 43.5 50.9 2.5 8.2

72.22 15.71 13.54 11.45 7.58 1.77 1.95 18.01 18.92 20.65 21.90 23.83 24.79 27.94 31.43 45.35 45.00 41.58 45.04 42.81 37.73 34.46 30.78 28.50 26.15 24.28 22.58 19.90 18.61 16.98 16.43 15.69 15.28 13.63 11.82 10.99 9.60 9.24 8.68 8.35 7.62 7.23 6.73 6.07 5.63 4.42 2.25 3.73 0.89 1.75 1.27 0.91 1.16 17.0 23.6 32.3 34.6 35.9 37.1 38.9 20.0 3.3

MFP stands for measured flash point. CFP stands for calculated flash point.

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