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Chapter 5: Problem 35

The lower flammability limit (LFL) and the upper flammability limit (UFL) of propane in air at 1 atm are, respectively, 2.3 mole \(\% \mathrm{C}_{3} \mathrm{H}_{8}\) and 9.5 mole \(\% \mathrm{C}_{3} \mathrm{H}_{8} .^{17}\) If the mole percent of propane in a propane-air mixture is between \(2.3 \%\) and \(9.5 \%,\) the gas mixture will burn explosively if exposed to a flame or spark; if the percentage is outside these limits, the mixture is safe-a match may burn in it but the flame will not spread. If the percentage of propane is below the LFL, the mixture is said to be too lean to ignite; if it is above the UFL, the mixture is too rich to ignite. (a) Which would be safer to release into the atmosphere- -a fuel-air mixture that is too lean or too rich to ignite? Explain. (b) A mixture of propane in air containing 4.03 mole \(\% \mathrm{C}_{3} \mathrm{H}_{8}\) is fed to a combustion furnace. If there is a problem in the furnace, the mixture is diluted with a stream of pure air to make sure that it cannot accidentally ignite. If propane enters the furnace at a rate of \(150 \mathrm{mol} \mathrm{C}_{3} \mathrm{H}_{8} / \mathrm{s}\) in the original fuel- air mixture, what is the minimum molar flow rate of the diluting air? (c) The actual diluting air molar flow rate is specified to be \(130 \%\) of the minimum value. Assuming the fuel mixture (4.03 mole\% \(\mathrm{C}_{3} \mathrm{H}_{8}\) ) enters the furnace at the same rate as in Part (b) at \(125^{\circ} \mathrm{C}\) and 131 kPa and the diluting air enters at \(25^{\circ} \mathrm{C}\) and \(110 \mathrm{kPa}\), calculate the ratio \(\left(\mathrm{m}^{3} \text { diluting air) } /\right.\) (m \(^{3}\) fuel gas) and the mole percent of propane in the diluted mixture. (d) Give several possible reasons for feeding air at a value greater than the calculated minimum rate.

Short Answer

Expert verified
a) A too lean mixture is safer to release. b) Minimum molar flow rate of the diluting air can be calculated by subtracting the propane's molar flow rate from total molar flow rate. c) Volume ratio of diluting air to fuel gas is equal to the molar flow rate ratio. The mole percent of propane in the diluted mixture can be calculated from the propane molar flow rate and the total molar flow rate. d) It provides safety margin, accounts for fluctuations and errors in the molar flow rate of propane, and assists in efficient dispersion and dilution of any unburned propane.

Step by step solution

01

Explain the safeness of too lean or too rich fuel-air mixture

A fuel-air mixture that is too lean to ignite is safer to release into the atmosphere. This is because it contains too little fuel (propane in this case) to sustain a flame. A mixture that is too rich, on the other hand, although it doesn't ignite, it still contains more fuel which may pose a greater environmental hazard.
02

Minimum molar flow rate calculation

To find the minimum molar flow rate, divide the molar flow rate of propane by the mole percent of the lower flammable limit. This gives the total molar flow rate of the mixture. Since air makes up the remainder of the mixture, subtract the propane's molar flow rate from the total to get the minimum molar flow rate of air: \( Minimum \, molar \, flow \, rate = \frac{150 \, mol}{2.3 \, \%} - 150 \, mol \)
03

Actual diluting air molar flow rate calculation

The problem states that the actual flow rate is 130% of the minimum value. Thus take 130% of the value obtained in Step 2 to get the actual diluting air molar flow rate.
04

Calculate the ratio of diluting air to fuel gas and the mole percent of propane in the diluted mixture

Assuming the gases behave ideally, the volume ratio of diluting air to fuel gas is equal to the molar flow rate ratio calculated in Step 3. The mole percent of propane in the diluted mixture can be calculated from the propane molar flow rate and the total molar flow rate, which is the sum of the propane and diluting air molar flow rates. Simply divide the propane molar flow rate by the total molar flow rate and multiply by 100.
05

Explain possible reasons for feeding air at a value greater than the calculated minimum rate

Feeding air at a rate higher than the minimum value provides an extra safety margin to account for fluctuations in the molar flow rate of propane. It also accounts for potential errors in the measurement or calculation of the minimum flow rate. Additionally, having a higher air flow rate can help in more efficient dispersion and dilution of any unburned propane in the exhaust stream.

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Key Concepts

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

Fuel-Air Mixture Safety
Ensuring the safety of fuel-air mixtures in chemical processes is paramount to prevent accidental explosions and fires. The flammability limits of a gas like propane are critical to this endeavor. These limits define the range of concentration in which the gas mixture can ignite -- for propane, between 2.3% and 9.5% by mole in air. A mixture below the lower flammability limit (LFL) is too lean and is considered safer to release into the atmosphere, as it lacks sufficient fuel to support combustion. Conversely, mixtures exceeding the upper flammability limit (UFL) contain too much fuel and, while not ignitable, could pose environmental risks. It's important to ensure any released mixtures are outside the flammable range, hence understanding and managing these limits contributes significantly to the safety of chemical processes.
Molar Flow Rate Calculation
Calculating the molar flow rate is essential for optimizing industrial processes like combustion in furnaces. To illustrate, consider a scenario where propane enters a furnace at a specific rate, and it鈥檚 crucial to add enough diluting air to avoid the mixture falling within the flammable range. Using the LFL of propane, you can determine the minimum molar flow rate of the diluting air required for safety. The calculation comprises dividing the propane's molar flow rate by the mole percent at the LFL, then subtracting the propane flow rate itself, as demonstrated in the exercise. Accurate molar flow rate calculations are necessary not only for safety but also for economic and environmental reasons, reducing waste and emissions.
Composition of Gas Mixtures
Understanding the composition of gas mixtures is fundamental in various chemical process applications, from combustion to materials synthesis. In the context of fuel-air mixtures for combustion, the molar percentages of each component must be managed to avoid explosive scenarios. As seen in our propane example, maintaining a certain molar percentage of propane ensures that the mixture falls outside of its flammable range when fed to a combustion furnace. The ratio of diluting air to fuel gas, along with the overall molar percentage, dictates the mixture's behavior when exposed to an ignition source. Accurately determining this composition is not only key to safety but it also influences the efficiency and emissions of the combustion process.
Combustion Furnace Safety Measures
When operating a combustion furnace, a host of safety measures must be in place to mitigate risks. These include ensuring the molar flow rate of the fuel-air mixture is controlled and maintained outside flammable limits, as discussed earlier. Providing excess diluting air can serve as a buffer against process variability and potential measurement inaccuracy. Additionally, monitoring systems are often used to continuously measure the composition of the gas mixture. Alarms and automatic shutdown features are integrated to act in case unsafe conditions are detected. Ensuring efficient dispersion of unburned fuel in the exhaust and maintaining system integrity through regular inspections are also part of comprehensive safety strategies. These measures together enhance the safety and reliability of operating combustion furnaces in chemical plants.

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Most popular questions from this chapter

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