Experimental study on explosion limit of M15 methanol-gasoline mixture

TAN Yingxin, SUN Yanlong， ZHANG Huarong, LIU Haihong， RAO Yonghong

(1. School of Environment and Safety Engineering, North University of China, Taiyuan 030051, China；2. PLA Unit 32382, Beijing 100072, China)

Abstract： The explosion characteristics of M15 methanol-gasoline mixture was experimental test by using FRTA explosion limit instrument. The effect of temperature on the explosion area of the sample was studied. The results show that the lower explosion limit of M15 methanol-gasoline mixture is 1.716% and the upper explosion limit is 11.451% at the initial temperature of 80 ℃. The lower explosion limit of M15 methanol-gasoline is in the range of 1.711%-1.760% with the initial temperature from 25 ℃ to 100 ℃, and the upper explosion limit of the sample changes between 11.253% and 11.451%. Considering experimental error and precision, it can be approximated that the temperature has little influence on the explosion area of M15 methanol-gasoline mixture.

Key words： M15 methanol-gasoline mixture； explosion limits； vapor explosion； gas explosion

0 Introduction

Nowadays our country is facing a serious situation of energy and environment. M15 methanol-gasoline is one of the most important new energy. It is consisted by 85% gasoline and 15% methanol. As flammable and explosive chemical, M15 methanol-gasoline has an explosion hazard in the process of allocation, storage, use and so on. There may be a serious blasting accident with a slight mistake. Therefore, it is important to assess its safety in order to prevent the burning explosion accidents of M15 methanol-gasoline mixture.

Explosion limit, as one of the important parameters of flammable and explosive chemicals, is the main indicator of assessing the explosion risk of M15 methanol-gasoline and also an important basis for developing relevant standards and norms. Domestic researches on explosion characteristics of combustible liquid vapors are abundant. Besides, test tools and methods are also more complete. Explosion limits and critical oxygen content of petroleum vapor were researched at different temperatures by Zhenyi Liu, and the results showed that critical oxygen content decreased as well as explosion limits were variable wide with temperature increasing[1]. Ganbing Yao researched on explosive limits and inerting mechanism of RP-5 and RP-3 oil, used flammable liquid explosion limit testing device designed by themselves, and had got the explosion areas and inerting rules of inerting inhibition of inert gas[2]. Weibing Chen[3]introduced determination method and affecting factors of explosion limits of oil, and pointed out that it was necessary to consider certain safety factors when the explosion limits were applied in practice. Wenhui Liu[4]designed a testing system for explosion limits of petroleum gas, which could be used to simulate different environmental conditions. Research on methanol-gasoline mixtures at home and abroad focuses on physical and chemical properties, fuel properties and emission characteristics[5-10], however, less research focuses on its combustion hazard characteristics.

In view of this, the aim of this study is to determine the explosion limits and affecting factors of M15 methanol-gasoline by using FRTA explosion limits instrument and referring to relevant test standards[11-12]. In addition, this study makes an analysis, comparison and summarizes on the test results in both qualitative and quantitative ways according to the combustion and explosion mechanism of combustible liquid vapor. The results have an important significance in evaluating safety, preventing burning explosion accidents and supplementing technical indicators of M15 methanol-gasoline.

1 Experimental device

The FRTA instrument was used to test the explosion limits of M15 methanol-gasoline. The schematic diagram of the structure is shown in Fig.1. The apparatus consists of a glass pressure test vessel (the volume is 5 L) placed inside a stainless steel oven (main unit) equipped by temperature sensor and pressure transducer for monitoring of gas temperature and pressure within test vessel. Two pcs of needle valves are used for fine flow regulation of vacuum or air inlet into glass vessel. M15 methanol-gasoline is inserted into the vessel through serum-bottle septum by micro-syringe. Pair of tungsten electrodes of spark gap are inserted inside testing vessel and connected to ignition device (high voltage power supply 15 kV/30 mA). Time duration of spark is precisely controlled by timer in range 0.001 s-0.4 s.

A—Needle valve; B—Vacuum pump interface; C—Threaded rod holder; D—High voltage cables feedthrough; E—Serum bottle septum; F—Pressure transducer; G—External vent bezel (including bursting membrane); H—Thermocouple; I—Stirring bar; J—Main unit; K—Magnetic stirrer; L—Ignition electrodes; M—Glass vessel; N—PTFE Head; O—Thermocouple socket; P—Pressure transducer socket; Q—Intake

The oven is equipped by barometric pressure transducers, magnetic stirrer and safe viewing glass door made from safety glass and ventilation hole on rear panel. There are two touch panels on front side of the main unit. One serves for control and display values of the temperature inside the oven, second for displaying temperature and pressure inside vessel, barometric pressure and for control and configuration of ignition device. The maximum operating temperature of the instrument is about 150 ℃[13].

The flammable mixture of M15 methanol-gasoline-air is prepared by the measuring method of partial pressure. Temperature and pressure parameters are recorded respectively before and after adding M15 methanol-gasoline to the test vessel. According to the explosion criterion, high voltage discharge can be determined after ighition. If there is an explosion after ignition, the methanol-gasoline vapor concentration is reduced to repeat test. If the explosion does not occur after ignition, the methanol-gasoline vapor concentration is increased to repeat test. The methanol-gasoline vapor concentration is changed until there is an acceptable sample concentration difference between the ignition and non-ignition. The sample difference is determined by the sample, and is usually 0.1 mL or 0.05 mL. The difference of sample amount between the ignition and non-ignition in this test is 0.02 mL, and the smaller the difference of sample amount is, the more accurate the test result is. The test of non-ignition is repeated at least 5 times. On the basis of GB/T 12474(2008), explosion limits are calculated by

(1)

whereφis explosive limits;φ1is flame propagation concentration;φ2is flame non-propagation concentration.

2 Explosion criterions

In the test process, the basis of judging whether the methanol-gasoline explosion occurs is shown in Fig.2. Firstly, observe whether there is flame propagation in the glass test vessel. If the angleβof the flame propagation is bigger than or equal to 90°, it is judged as burning, otherwise, judged to be unburned.

Fig.2 Burning or explosion criterion

Secondly, refer to theP(T) curve displayed on the touch screen in the upper right corner of the test device, and then to determine whether an explosion will occur by changing the pressure. Finally, an explosion can also be determined according to whether there is a “sound” of pressure relief in the test vessel after ignition[14].

3 Experimental results

3.1 Determination of explosion limits

3.1.1 Determination of lower explosion limit

In order to ensure the testing process safe and reliable, according to the test instrument requirements, the lower explosion limit of M15 methanol-gasoline is tested by FRTA explosion limits instrument from the lower concentration of the sample at the beginning, and then the sample concentration is gradually increased until the flame spreads. In this test, the initial volume of M15 methanol-gasoline is 0.20 mL, and then the sample volume is increased with 0.02 mL as the gradient. Detailed test data is shown in Table 1.

Table 1 Testing data of lower explosion limit of M15 methanol-gasoline

The initial test temperature is 80 ℃, and the main reasons why selecting this test temperature lie in the following 3 factors. Firstly, methanol-gasoline is not volatile at room temperature. Therefore, if the initial temperature is low, the effective flammable components of methanol-gasoline are less, and the lower explosion limit is difficult to test. Secondly, in the actual working environment, ambient temperature can reach 40 ℃-50 ℃ in the extreme conditions, and reservoir temperature is generally higher than 70 ℃[1]. Therefore, 80 ℃ is selected as the initial test temperature. Among them, safety factors should be considered and actual working conditions should be satisfied. Finally, the limitations of the explosion limits instrument and the test temperature of other documents are taken into account[15].

According to the Table 1, the volume concentrations of flame propagation and flame non-propagation of M15 methanol-gasoline are respectively 1.757% and 1.674%. Therefore, according to Eq.(1), the lower explosion limit of M15 methanol-gasoline can be calculated by

(2)

3.1.2 M15 Determination of upper explosion limit

In the actual production and life, the concentration of the combustible liquid vapor does not reach its upper explosion limit under normal circumstances. Compared with the lower explosion limit, the scope of actual application of the upper explosion limit is narrow and the application value is small. Moreover the upper explosion limit of flammable liquid vapor is difficult to test and the test process has a certain degree of risk. M15 methanol-gasoline most commonly used is chosen as research subject in this paper to test its upper explosion limit at 80 ℃. It should be noted that the test of lower explosion limit is from a lower concentration, gradually increasing the concentration of M15 methanol gasoline until it just explodes, but the test of upper explosion limit is to start from a higher concentration, then decreasing the concentration of M15 methanol-gasoline until the explosion occurs.

The Eq.(1) is used to calculate the upper explosion limit of M15 methanol-gasoline. Test results are shown in Table 2.

According to the Table 2, the volume concentrations of flame propagation and flame non-propagation of M15 methanol-gasoline are respectively 11.329% and 11.574%. Therefore, according to Eq.(1), the upper explosion limit of M15 methanol-gasoline can be calculated by

(3)

That is to say, the upper explosion limit of M15 methanol-gasoline is 11.451% at the initial temperature of 80 ℃. The application value of the upper explosion limit is small, generally used for prevention of blasting accidents as well as investigation and analysis of fire and explosion accidents later.

3.2 Effect of temperature on explosion limits

The initial temperature is an important factor influencing the methanol-gasoline explosion limits. Generally speaking, the higher the temperature is, the wider the range of methanol gasoline explosion is. Firstly, the higher the initial temperature is, the stronger the volatility of methanol-gasoline is. That is to say, the greater the contents of the vapor from the liquid is, namely the greater the concentration of the vapor is. Secondly, the internal energy of the molecules becomes large and the effective collision frequency between molecules increases when the temperature rises. So that the reaction occurs more easily as well as the lower explosion limit decreases and the upper explosion limit increases. However, the rising range of the upper explosion limit is not obvious due to the oxygen content is relatively insufficient at the concentration of the upper explosion limit.

3.2.1 Effect of temperature on lower explosion limit

The lower explosion limit of M15 methanol-gasoline is tested at different temperatures (25 ℃, 40 ℃, 60 ℃, 80 ℃ and 100 ℃) to research the effect of temperature on the lower explosion limit. Table 3 gives the test data of the lower explosion limit of M15 methanol-gasoline at different temperatures.

Table 3 shows the lower explosion limit of M15 methanol-gasoline at the temperature from 25 ℃ to 100 ℃, and indicates that the effect of temperature on M15 is very small. For gasoline, the distillation range is from 30 ℃ to 220 ℃. Therefore, in the range of 25 ℃ to 100 ℃, only a small part of the light fraction has evaporated, namely in the range of initial temperature, the temperature has little influence on gasoline evaporation, and the main component of M15 is gasoline, so the lower explosion limit is less affected by the initial temperature.

Table 3 Lower explosion limit of M15 at different temperatures

According to the effect of initial temperature on flammable gas explosion limits[16-21], it can be known that compared with the effect of initial temperature on the lower explosion limit of flammable gas, the effect of initial temperature on the lower explosion limit of flammable liquid vapor is much smaller. Influence of initial temperature on the lower explosion limit of flammable gas is mainly manifested in following: when the temperature increases, the molecule energy of flammable gas increases, the number of activated molecules in the system increases, and the chemical reaction is accelerated and easy to be carried out. In macroscopic view, as the temperature rises, the lower explosion limit of combustible gas decreases.

In terms of the lower explosion limit of flammable liquid vapor, the temperature not only affects the internal molecule energy of the liquid vapor, but also mainly affects its volatility. The lower the boiling point is, the greater the impact of the temperature on the lower explosion limit of volatile flammable liquid is. On the contrary, the temperature has little effect on the lower explosion limit of flammable liquid which is difficult to be volatile. For methanol-gasoline, the effect of temperature on the lower explosion limit of methanol vapor is greater than that of gasoline vapor. That is to say, with the increase of methanol concentration, the effect of temperature on the lower explosion limit of methanol-gasoline becomes obvious.

3.2.2 Effect of temperature on upper explosion limit

In order to study the influence of temperature on the upper explosion limit of methanol-gasoline, M15 methanol-gasoline is selected as the object, and the upper explosion limit of M15 is tested at different initial temperatures (40 ℃, 60 ℃, 80 ℃, 100 ℃ and 120 ℃). Test results data are shown in Table 4.

Table 4 Upper explosion limit of M15 at different temperatures

Table 4 shows that the upper explosion limit of M15 methanol-gasoline changes between the minimum (11.253%) and maximum (11.451%) in the temperature range from 40 ℃ to 120 ℃. Considering experimental error and precision, it can be approximated that the upper explosion limit of M15 methanol-gasoline is almost unchanged. That is, the upper explosion limit of M15 is substantially unaffected by temperature within the range from 40 ℃ to 120 ℃.

For M15 methanol-gasoline, in the vicinity of concentration of the upper explosion limit, content of M15 methanol-gasoline vapor is relatively adequate, but the oxygen content is relatively insufficient. With the increase of temperature, the evaporation of M15 methanol-gasoline is accelerated as well as the molecular energy and the number of effective collisions between molecules increases. The chemical reaction is accelerated and easy to be carried out. However, the oxygen content does not increase with the temperature increases. Therefore, the change range of the upper explosion limit is not obvious; On the other hand, the main component of M15 methanol-gasoline is gasoline, and 50% evaporation temperature of gasoline is not higher than 120 ℃, that is to say, in the temperature range of the test, only about 50% of the light fraction of M15 methanol-gasoline is volatilized. So the temperature has little effect on the content of evaporation. In addition, gasoline is a mixture of C5-C12hydrocarbons, and relatively difficult to volatilize. The above factors together lead to the upper explosion limit of M15 methanol-gasoline basically remains unchanged in the temperature range from 40 ℃ to 120 ℃.

4 Conclusions

In this paper, the explosion limits of M15 methanol-gasoline mixture and the influence of temperature on that are studied, and the experimental phenomena and results are analyzed in both qualitative and quantitative ways. The main conclusions are as follows:

1) The lower explosion limit of M15 methanol-gasoline is 1.716% and the upper explosion limit of that is 11.451%. That is to say, the explosion area of M15 sample is 1.716%-11.451% at a temperature of 80 ℃.

2) The lower explosion limit of M15 methanol-gasoline changes between 1.711% and 1.760% at the initial temperature from 25 ℃ up to 100 ℃. That is, the temperature has little influence on the lower explosion limit of M15 methanol-gasoline.

3) The upper explosion limit of M15 methanol-gasoline changes between the minimum (11.253%) and maximum (11.451%) in the temperature range from 40 ℃ to 120 ℃. Considering experimental error and precision, it can be approximated that the upper explosion limit of M15 methanol-gasoline is almost unchanged.