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When a flammable liquid (e.g.. gasoline) ignites, the substance actually buming is vapor generated from the liquid. If the concentration of the vapor in the air above the liquid exceeds a certain level (the lower flammability limit), the vapor will ignite if it is exposed to a spark or another ignition source. Once ignited, the heat released is likely to cause additional vaporization of the liquid, and the resulting fire may continue until all combustible material has been consumed.(a) The flash point is defined as the minimum temperature at which a flammable liquid or volatile solid gives off sufficient vapor to form an ignitable mixture with air near the surface of the liquid or within a vessel (page \(2-515,\) Perry's Chemical Engineers' Handbook, see Footnote 1 ). For example, the flash point of \(n\) -octane at 1.0 atm is \(13^{\circ} \mathrm{C}\left(55^{\circ} \mathrm{F}\right)\), which means that dropping a match into an open container of octane is likely to start a fire in a laboratory, but not outside on a cold winter day. (Do not try it! One reference- -L. Bretherick, Bretherick's Handbook of Reactive Chemical Hazards, 4th Edition, Butterworths, London, 1990, p. 1596 - points out there is "usually a fair [our emphasis] correlation between flash point and probability of involvement in fire.")Suppose you are keeping two solvents in your laboratory, one with a flash point of \(15^{\circ} \mathrm{C}\) and the other with a flash point of \(75^{\circ} \mathrm{C}\). How do these solvents differ from the standpoint of safety? What differences, if any, should there be in how you treat them?(b) The lower flammability limit (LFL) of methanol in air is 6.0 mole \(\%\). Calculate the temperature at which a saturated methanol-air mixture at 1 atm would have a composition corresponding to the LFL. What is the relationship of this value to the flash point, and what value would you assign the flash point of methanol?(c) Give reasons why it would be unsafe to maintain an open container of methanol in an environment below the LFL (i.e., the value calculated in Part (b)) if there are ignition sources nearby. List common ignition sources that may be found in a laboratory.

Step by step solution

01

Compare solvent safety based on flash point

The solvent with a flash point of 15°C is more risky than the one with a 75°C flash point. The risk comes from the fact that a lower flash point means that it takes a lower temperature for the solvent to emit sufficient vapors that can be easily ignited by a spark or another ignition source. As such, the solvent with a flash point of 15°C takes a lower temperature to ignite than the one at 75°C, thus making it more prone to start a fire. The treatment for each of these solvents should therefore be different with the one with the lower flash point requiring more careful handling like avoiding sparks or flames.

02

Calculate the LFL of a methanol-air mixture

Methanol’s LFL in air is 6.0 mole%. This denotes the minimum concentration at which methanol vapors in the air can ignite. From the Antoine equation, we can determine the vapor pressure of methanol at different temperatures then convert that vapor pressure into mole fraction in air. The Antoine constants for methanol are: A=7.89750, B=1473.11, C=-15. For an LFL of 6.0 mole%, this corresponds to a vapor pressure of 0.06 atm (as the total pressure is 1 atm). Solving the Antoine equation for temperature will provide the temperature that corresponds to the LFL.

03

Discuss the relationship between LFL and flash point

The flash point of a substance is generally lower than the temperature at which the LFL is reached. The flash point of methanol is about 11°C but the temperature that corresponds to the LFL from the calculations is less. It means methanol will start producing flammable vapors at a lower temperature than it will ignite.

04

Reasons to avoid keeping open container of methanol in environment below LFL

Keeping an open container in an environment below the LFL means the methanol is under a temperature at which it is producing enough vapors that can ignite upon the presence of a spark. Therefore, if there are ignition sources nearby, it can easily result in a fire or explosion. Ignition sources in a laboratory can be numerous: electrical appliances, open flames, static electricity, hot surfaces, among others.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Flash Point

Understanding the flash point of a substance plays a crucial role in evaluating potential fire hazards. It is defined as the minimum temperature at which a flammable liquid or volatile solid can generate enough vapor to form an ignitable mixture with air near the surface of the substance. This concept directly correlates with fire safety, particularly in environments like laboratories where flammable materials are common. For instance, considering the different flash points of two solvents – one with a flash point of \(15^\circ \mathrm{C}\) and another at \(75^\circ \mathrm{C}\) – significantly impacts their safety handling protocols. The lower the flash point, the greater the risk of ignition at a lower temperature. Therefore, the solvent with the \(15^\circ \mathrm{C}\) flash point is more susceptible to catching fire and requires more stringent safety measures to prevent accidents, such as thorough ventilation and avoiding any heat sources or open flames.

Particularly in a laboratory setting, distinguishing between chemicals based on flash point can inform the way they are stored and handled. Chemicals with lower flash points should not only be kept in well-ventilated areas but may also require refrigeration to maintain temperatures below the flash point, thus reducing the risk of a vapor-related fire.

Methanol Safety

Methanol is a commonly used solvent and fuel, which, due to its properties, necessitates specific safety precautions. These safety measures stem from understanding methanol's lower flammability limit (LFL), which is the minimum concentration of vapor in the air that can propagate flame when an ignition source is present. Methanol's LFL is 6.0 mole%, a critical value to consider when working with it to prevent fire or explosion hazards.

With this in mind, safety protocols for methanol include proper labeling and storage, as well as the use of flame arrester-equipped containers to prevent vapor ignition. Workers should use personal protective equipment, such as safety goggles and gloves, when handling methanol to prevent skin and eye contact. Additionally, in the case of a spill, immediate action should be taken to clean it up, and proper disposal procedures should be followed to prevent environmental contamination or further safety risks. Understanding and adhering to these safety measures are key to preventing accidents and ensuring a safe working environment.

Laboratory Fire Prevention

Fire prevention in a laboratory involves a combination of practices, equipment, and knowledge designed to reduce the risk of accidental fires. Common ignition sources in a laboratory setting include electrical equipment that might malfunction, open flames from Bunsen burners, static discharge which can spark unexpectedly, and hot surfaces like heating plates. To avoid fires, it's essential to maintain proper separation between flammable materials and ignition sources.

Regular safety audits and training are vital to ensure all laboratory personnel are aware of how to handle flammable substances safely, including understanding the importance of flash points and LFLs. Additionally, installing safety equipment such as fire extinguishers, fire blankets, and having an accessible emergency shower and eyewash station are critical components of a comprehensive fire safety plan. Most importantly, however, is cultivating a culture of safety where all lab workers know to prioritize safe handling practices, including the proper storage of chemicals in flameproof cabinets and routine checks for leaks or any other potential hazards.