J. K. LI, Xiamen University, Xiamen City, China; and H. LI, Hualu Engineering & Technology Co. Ltd., Xian, China
Flashpoint refers to the minimum temperature at which a liquid emits enough vapor to form a combustible mixture with air, either near the surface of the liquid or within the container itself. The vapor pressure of a flammable liquid is influenced by its temperature. As the temperature of the liquid increases, its vapor pressure increases correspondingly, leading to an increased concentration of flammable vapor in the surrounding air.
The liquid’s temperature plays a key role in determining its explosion potential—liquids that have a flashpoint lower than the ambient temperature pose a greater hazard compared to those with a higher flashpoint. For this reason, the flashpoint serves as a crucial factor in classifying flammable liquids according to their fire and explosion risks. According to NFPA 30 standards,1 liquids are categorized based on their flashpoint to determine whether they are flammable or combustible. Combustible liquids are defined as those with a flashpoint of ≥ 100°F (≥ 37.8°C), whereas flammable liquids have a flashpoint of < 100°F (< 37.8°C). Hazard levels are further classified in TABLE 1, with flammable liquids falling into Class I and combustible liquids into Classes II and III.
As illustrated in TABLE 1, the flashpoint is a crucial metric for assessing the fire and explosion hazards of liquids. It holds significant practical importance when dealing with the handling and transportation of large quantities of liquid mixtures. Therefore, flammability is a key consideration in developing safe practices for the storage and handling of these mixtures.
In numerous manufacturing processes involving flammable chemicals, flashpoint and flammability limits are essential to ensure the safety of process design, equipment layout and operational procedures. Regulatory authorities use the flashpoint test, conducted with small-scale testing equipment, to classify flammable and combustible liquids. This classification predominantly relies on the closed-cup flashpoint temperature to determine the flammability class.3 Experimentally determined flammability limits, obtained under conditions similar to those in practice, are the most reliable for designing safe equipment and evaluating potential gas explosion risks. Based on these classifications, regulatory bodies provide guidelines or specify methods for the safe transportation, handling, packaging, storage, distribution and protection of these materials.
While the flashpoint is applicable to most pure liquids and can be referenced in the material safety data sheet (MSDS) for some, it is particularly critical for mixtures of flammable liquids. This is because the composition of the gas phase often differs from that of the liquid phase.
Calculation procedure of flashpoints. The flashpoint of a liquid is a key physical property closely linked to other characteristics, such as vapor pressure, explosion limits, ignition temperature and boiling point. These interrelated properties are critical for calculating the flashpoint. The flashpoint is defined as the temperature at which the vapor concentration in the air reaches the minimum required for an explosion, commonly referred to as the lower explosive limit (LFL), below which ignition cannot occur. In cases where experimental data is unavailable, theoretical methods are employed to predict the flashpoint of liquid mixtures.
Typically, the flashpoint of mixtures is calculated using the Le Chatelier equation, in conjunction with a vapor-liquid equilibrium model to assess vapor composition in the presence of liquids. Various methods that are only based on ideal solutions or water mixtures to calculate mixture flashpoints are discussed, as well as the assumptions and conditions required for their application.4 For binary mixtures of non-ideal solutions, the necessary data includes the flashpoint of Component A [(TFA), see nomenclature below] and its mole fraction in the liquid phase (XA), along with the flashpoint of Component B (TFB) and its mole fraction in the liquid phase (XB). The total pressure is generally assumed to be atmospheric.
The Antoine equation applies to pure substances. When handling liquid mixtures, it is important to consider the gas phase composition. Several studies on vapor-liquid equilibrium and activity coefficients provide experimental data that should be referenced as a starting point. The Wilson equation is commonly used to estimate activity coefficients in the liquid phase. The process for estimating the flashpoint of a liquid mixture is detailed in the steps below.
Step 1: Enter the mole fraction of flammable liquid species i in the liquid phase (Xi), along with the flashpoint temperature (TF) for the pure liquid and the overall pressure. The vapor pressure of the pure liquid can then be calculated using the Antoine equation (Eq. 1):
ln(Pisat ) = A – [B / (C + T)] (1)
Calculate the value of the constants A12 and A21 of the Wilson equation, according to vapor-liquid equilibrium data from the literature. The activity coefficient of binary mixed liquid can be calculated using Eqs. 2 and 3：
lnγA = –ln(XA + A12 XB) + XB [A12 / (XA + A12 XB) – A21 / (XB + A21 XA)] (2)
lnγB = –ln(XB + A21 XA ) + XA [A21 / (XB + A21 XA) – A12 / (XA + A12 XB)] (3)
Step 2: Guess a flashpoint temperature (TG) of the liquid mixture, assume a flashpoint of the liquid mixture (usually the initial value is the mean of the pure component flashpoint). Calculate the saturated vapor pressures PA0 and PB0 of the pure components at TG using Eq. 1.
Step 3: The activity coefficients γA and γB were calculated using the Wilson Eqs. 2 and 3.
Step 4: Substitute the calculated activity coefficients and vapor pressures in the third step into the gas-liquid equilibrium formula of the non-ideal solution for further calculation. The vapor phase mole fraction of mixtures liquids (YA and YB）is defined using Eqs. 4 and 5:
YA = [(αAB XA) / (1 + (αAB –1)XA)] (4)
αAB = [(γA PA0 ) / (γB PB0 )] (5)
Step 5: The saturated vapor pressure Pisat at the flashpoint of pure liquid (TF) was calculated using the Antoine equation of pure components (Eq. 1).
Step 6: The Le Chatelier formula calculates the vapor pressure PHS at the flashpoint of the liquid mixture. This method is known as Le Chatelier's rule. Eqs. 6 and 7 are the empirical equations for determining the flammability limits:
This rule offers flammability limit estimates that closely match experimental values for many simple hydrocarbons. This method is applicable only to mixtures with air.
Eq. 6 is equivalent to Eq.8 at atmospheric pressure:
Step 7: Calculate the partial pressures PA and PB of Components A and B in the equilibrium vapor phase at the assumed flashpoint of the binary liquid mixture. Use a vapor-liquid equilibrium model to estimate the partial pressure of the flammable vapor above the liquid mixture (Pi ) using Eq. 9:
Pi = Yi PFS (9)
Step 8: Calculate the vapor pressure Pisat (PAFS and PBFS). Utilize a vapor-liquid equilibrium model to estimate the partial pressure of the flammable vapor above the liquid mixture (Pisat ). Non-ideal liquid is determined using Eq. 10:
Pi = γixi Pisat (10)
Step 9: The vapor pressures PAFS and PBFS were substituted into Eq. 1, respectively, to calculate whether the temperatures (TmA ) of Component A were equal to the temperatures (TmB ) of Component B. If they are equal, this is the flashpoint of the liquid mixture; if not, the average value was taken and the iteration was repeated.
An example of a MATLAB procedure for flashpoint calculation. Calculations were made for the flashpoint of a mixed liquid with different methanol concentrations, and the liquid mixture consisting of Component A (methanol) and Component B (1-butanol).These compounds do not constitute ideal solutions. Activity coefficients have been computed using the Wilson equation. Refer to data from NFPA497 as follows:5
The flashpoint of methanol is 12°C with an LFL = 6%, and the flashpoint of 1-butanol is 36°C with an LFL = 1.4%
According to literature, the input of XA, γA and γB (methanol-butanol 760 mmHg) is detailed in TABLE 2.
The methanol antoine equation (Eq. 11):
lgPA = 8.0724 – [1,574.99 / (T + 238.87)] (11)
The n-butanol antoine equation (Eq. 12)：
lgPB = 7.92484 – [1,617.52 / (T + 203.296)] (12)
Calculating results. Based on the vapor-liquid equilibrium data in TABLE 2, a program was written using MATLAB to plot FIG. 1. The calculation of the Wilson value constant A12 and A21 were 1.757 and 0.592, respectively. The agreement between experiments and calculation for the flashpoints of the methanol-butanol liquid mixtures with different molecular volumes is shown in TABLE 3.
The data shown in FIG. 2 compares the experimental flashpoints and the calculated ones with the equation. The mean absolute deviation is 0.46°C, and the maximum absolute deviation is 1.1°C.
Takeaways. A calculation method has been introduced for estimating the flashpoint of liquid mixtures. This method has been demonstrated to accurately predict the flashpoints of binary mixtures, even for highly non-ideal ones. A key feature of this approach is that, in some cases, the flashpoint of the mixture may fall between the flashpoints of its two pure components. The method requires knowledge of the LFLs of the pure components, the temperature dependence of these LFLs, and the application of Le Chatelier's law.
Therefore, when using flammable or combustible liquid, it is necessary to know its flashpoint so that appropriate safety measures can be adopted. The flashpoint can also be calculated from other properties in the liquid because the liquid flashpoint is a physical property of that liquid. Particularly when the data is incomplete or the data discrepancy is large, the flashpoint can be determined by this calculation method. HP
LITERATURE CITED
National Fire Protection Association (NFPA) 30, “Flammable and combustible liquids code,” 2015.
Sutton, I., Plant design and operations, Gulf Professional Publishing, 2015.
Wray, H. A., Manual on flashpoint standards and their use: Methods and regulations, ASTM Publication, 1992.
Crowl, D. A., Understanding Explosions, CCPS Publication, 2003.
National Fire Protection Association (NFPA) 497, “Recommended practice for the classification of flammable liquids, gases, or vapors and of hazardous (classified) locations for electrical installations in chemical process areas,” 2012.
LI JINGKAI is a postgraduate student at Xiamen University in Xiamen City, Fujian Province, China. He is proficient in numerical calculation functions of MATLAB.
LI HAN is a Professor Senior Engineer and the Deputy Chief Engineer at Hualu Engineering & Technology Co. Ltd. He holds an MSc degree in chemical engineering from Northwest University in Xian, China. He has more than 34 yr of experience in process engineering and project management. He is senior member of AIChE. He lives in Xian, Shaanxi Province, China. The author can be reached at lh1720@chinahualueng.com.