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Chapter 6: Problem 9

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.

Short Answer

Expert verified
The solvent with a flash point of 15掳C is riskier than the 75掳C solvent because it takes a lower temperature to emit sufficient vapors that could ignite. It thus requires more careful handling. The lower flammability limit (LFL) of methanol at specific temperature implies that at this temperature methanol emits sufficient vapors that can be ignited. The flash point is lower than the LFL temperature which makes it unsafe to store methanol in open containers at a temperature below LFL if there are ignition sources nearby. Typical ignition sources in a laboratory include electrical appliances, open flames, static electricity and hot surfaces.

Step by step solution

01

Compare solvent safety based on flash point

The solvent with a flash point of 15掳C is more risky than the one with a 75掳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掳C takes a lower temperature to ignite than the one at 75掳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鈥檚 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掳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 鈥� one with a flash point of \(15^\circ \mathrm{C}\) and another at \(75^\circ \mathrm{C}\) 鈥� 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.

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Most popular questions from this chapter

A fuel cell is an electrochemical device in which hydrogen reacts with oxygen to produce water and DC electricity. A 1-watt proton-exchange membrane fuel cell (PEMFC) could be used for portable applications such as cellular telephones, and a \(100-\mathrm{kW}\) PEMFC could be used to power an automobile. The following reactions occur inside the PEMFC:Anode: \(\quad \mathrm{H}_{2} \rightarrow 2 \mathrm{H}^{+}+2 \mathrm{e}^{-}\) Cathode: \(\quad \frac{1}{2} \mathrm{O}_{2}+2 \mathrm{H}^{+}+2 \mathrm{e}^{-} \rightarrow \mathrm{H}_{2} \mathrm{O}\) Overall: \(\quad \overline{\mathrm{H}}_{2}+\frac{1}{2} \mathrm{O}_{2} \rightarrow \mathrm{H}_{2} \mathrm{O}\) A flowchart of a single cell of a PEMFC is shown below. The complete cell would consist of a stack of such cells in series, such as the one shown in Problem 9.19.The cell consists of two gas channels separated by a membrane sandwiched between two flat carbonpaper electrodes- -the anode and the cathode- -that contain imbedded platinum particles. Hydrogen flows into the anode chamber and contacts the anode, where \(\mathrm{H}_{2}\) molecules are catalyzed by the platinum to dissociate and ionize to form hydrogen ions (protons) and electrons. The electrons are conducted throughthe carbon fibers of the anode to an extemal circuit, where they pass to the cathode of the next cell in the stack. The hydrogen ions permeate from the anode through the membrane to the cathode.Humid air is fed into the cathode chamber, and at the cathode \(\mathrm{O}_{2}\) molecules are catalytically split to form oxygen atoms, which combine with the hydrogen ions coming through the membrane and electrons coming from the external circuit to form water. The water desorbs into the cathode gas and is carried out of the cell. The membrane material is a hydrophilic polymer that absorbs water molecules and facilitates the transport of the hydrogen ions from the anode to the cathode. Electrons come from the anode of the cell at one end of the stack and flow through an extemal circuit to drive the device that the fuel cell is powering, while the electrons coming from the device flow back to the cathode at the opposite end of the stack to complete the circuit. is important to keep the water content of the cathode gas between upper and lower limits. If the content reaches a value for which the relative humidity would exceed \(100 \%,\) condensation occurs at the cathode (flooding), and the entering oxygen must diffuse through a liquid water film before it can react. The rate of this diffusion is much lower than the rate of diffusion through the gas film normally adjacent to the cathode, and so the performance of the fuel cell deteriorates. On the other hand, if there is not enough water in the cathode gas (less than \(85 \%\) relative humidity), the membrane dries out and cannot transport hydrogen efficiently, which also leads to reduced performance. 400-sell 300-yolt PEMFS anerates at stady state witha nonwer outnul of 36 k W, The air fod to It is important to keep the water content of the cathode gas between upper and lower limits. If the content reaches a value for which the relative humidity would exceed \(100 \%,\) condensation occurs at the cathode (flooding), and the entering oxygen must diffuse through a liquid water film before it can react. The rate of this diffusion is much lower than the rate of diffusion through the gas film normally adjacent to the cathode, and so the performance of the fuel cell deteriorates. On the other hand, if there is not enough water in the cathode gas (less than \(85 \%\) relative humidity), the membrane dries out and cannot transport hydrogen efficiently, which also leads to reduced performance.A 400-cell 300-volt PEMFC operates at steady state with a power output of 36 kW. The air fed to the cathode side is at \(20.0^{\circ} \mathrm{C}\) and roughly 1.0 atm (absolute) with a relative humidity of \(70.0 \%\) and a volumetric flow rate of \(4.00 \times 10^{3}\) SLPM (standard liters per minute). The gas exits at \(60^{\circ} \mathrm{C}\). (a) Explain in your own words what happens in a single cell of a PEMFC. (b) The stoichiometric hydrogen requirement for a PEMFC is given by \(\left(n_{\mathrm{Hz}}\right)_{\text {conanmad }}=I N / 2 F,\) where \(I\) is the current in amperes (coulomb/s), \(N\) is the number of single cells in the fuel cell stack, and \(F\) is the Faraday constant, 96,485 coulombs of charge per mol of electrons. Derive this expression. (Hint: Recall that since the cells are stacked in series the same current flows through each one, and the same quantity of hydrogen must be consumed in each single cell to produce that current at each anode.) (c) Use the expression of Part (b) to determine the molar rates of oxygen consumed and water generated in the unit with the given specifications, both in units of mol/min. (Remember that power = voltage \(\times\) current.) Then determine the relative humidity of the cathode exit stream, \(h_{\mathrm{r} \text { rout. }}\) (d) Determine the minimum cathode inlet flow rate in SLPM to prevent the fuel cell from flooding ( \(h_{\mathrm{r}, \text { out }}=100 \%\) ) and the maximum flow rate to prevent it from drying \(\left(h_{\mathrm{r}, \text { out }}=85 \%\right)\) .

Acetaldehyde is synthesized by the catalytic dehydrogenation of ethanol:$$ \mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH}\rightarrow\mathrm{CH}_{3}\mathrm{CHO}+\mathrm{H}_{2}.$$ Fresh feed (pure ethanol) is blended with a recycle stream (95 mole\% ethanol and 5\% acetaldehyde), and the combined stream is heated and vaporized, entering the reactor at \(280^{\circ} \mathrm{C}\). Gases leaving the reactor are cooled to \(-40^{\circ} \mathrm{C}\) to condense the acetaldehyde and unreacted ethanol. Off-gas from the condenser is sent to a scrubber, where the uncondensed organic compounds are removed and hydrogen is recovered as a by- product. The condensate from the condenser, which is 45 mole\% ethanol, is sent to a distillation column that produces a distillate containing 99 mole\% acetaldehyde and a bottoms product that constitutes the recycle blended with fresh feed to the process. The production rate of the distillate is \(1000 \mathrm{kg} / \mathrm{h}\). The pressure throughout the process may be taken as 1 atm absolute. (a) Calculate the molar flow rates ( \(\mathrm{kmol} / \mathrm{h}\) ) of the fresh feed, the recycle stream, and the hydrogen in the off-gas. Also determine the volumetric flow rate \(\left(\mathrm{m}^{3} / \mathrm{h}\right)\) of the feed to the reactor. (Suggestion:Use Raoult's law in the analysis of the condenser.)(b) Estimate (i) the overall and single-pass conversions of ethanol and (ii) the rates ( \(\mathrm{kmol} / \mathrm{h}\) ) at which ethanol and acetaldehyde are sent to the scrubber.

The vapor pressure of an organic solvent is \(50 \mathrm{mm}\) Hg at \(25^{\circ} \mathrm{C}\) and \(200 \mathrm{mm} \mathrm{Hg}\) at \(45^{\circ} \mathrm{C}\). The solvent is the only species in a closed flask at \(35^{\circ} \mathrm{C}\) and is present in both liquid and vapor states. The volume of gas above the liquid is \(150 \mathrm{mL}\). (a) Estimate the amount of the solvent \((\mathrm{mol})\)contained in the gas phase. (b) What assumptions did you make? How would your answer change if the species dimerized (one molecule results from two molecules of the species combining)?

The solubility of sodium bicarbonate in water is \(11.1 \mathrm{g} \mathrm{NaHCO}_{3} / 100 \mathrm{g} \mathrm{H}_{2} \mathrm{O}\) at \(30^{\circ} \mathrm{C}\) and \(16.4 \mathrm{g}\) \(\mathrm{NaHCO}_{3} / 100 \mathrm{g} \mathrm{H}_{2} \mathrm{O}\) at \(60^{\circ} \mathrm{C} .\) If a saturated solution of \(\mathrm{NaHCO}_{3}\) at \(60^{\circ} \mathrm{C}\) is cooled and comes to equilibrium at \(30^{\circ} \mathrm{C},\) what percentage of the dissolved salt crystallizes?

A stage of a separation process is defined as an operation in which components of one or more feed streams divide themselves between two phases, and the phases are taken off separately. In an ideal stage or equilibrium stage, the effluent (exit) streams are in equilibrium with each other.Distillation columns often consist of a series of vertically distributed stages. Vapor flows upward and liquid flows downward between adjacent stages; some of the liquid fed to each stage vaporizes,and some of the vapor fed to each stage condenses. A representation of a section of a distillation column is shown below. (See Problem 4.42 for a more realistic representation.) Consider a distillation column operating at 0.4 atm absolute in which benzene and styrene are being separated. A vapor stream containing 65 mole\% benzene and 35 mole\% styrene enters stage 1 at a rate of \(200 \mathrm{mol} / \mathrm{h}\), and liquid containing 55 mole\% benzene and 45 mole\% styrene leaves this stage at a rate of 150 mol/h. You may assume (1) the stages are ideal, (2) Raoult's law can be used to relate the compositions of the streams leaving each stage, and (3) the total vapor and liquid molar flow rates do not change by a significant amount from one stage to the next.(a) How would you expect the mole fraction of benzene in the liquid to vary from one stage to another, beginning with stage 1 and moving up the column? In light of your answer and considering that the pressure remains essentially constant from one stage to another, how would you then expect the temperature to vary at progressively higher stages? Briefly explain. (b) Estimate the temperature at stage 1 and the compositions of the vapor stream leaving this stage and the liquid stream entering it. Then repeat these calculations for stage 2 . (c) Describe how you would calculate the number of ideal stages required to reduce the styrene content of the vapor to less than 5 mole\%.
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