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Chapter 4: Problem 77

Methane reacts with chlorine to produce methyl chloride and hydrogen chloride. Once formed, the methyl chloride may undergo further chlorination to form methylene chloride ( \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) ), chloroform, and carbon tetrachloride. A methyl chloride production process consists of a reactor, a condenser, a distillation column, and an absorption column. A gas stream containing 80.0 mole \(\%\) methane and the balance chlorine is fed to the reactor. In the reactor a single-pass chlorine conversion of essentially \(100 \%\) is attained, the mole ratio of methyl chloride to methylene chloride in the product is \(5: 1,\) and negligible amounts of chloroform and carbon tetrachloride are formed. The product stream flows to the condenser. Two streams emerge from the condenser: the liquid condensate, which contains essentially all of the methyl chloride and methylene chloride in the reactor effluent, and a gas containing the methane and hydrogen chloride. The condensate goes to the distillation column in which the two component species are separated. The gas leaving the condenser flows to the absorption column where it contacts an aqueous solution. The solution absorbs essentially all of the HCl and none of the \(\mathrm{CH}_{4}\) in the feed. The liquid leaving the absorber is pumped elsewhere in the plant for further processing, and the methane is recycled to join the fresh feed to the process (a mixture of methane and chlorine). The combined stream is the feed to the reactor. (a) Choose a quantity of the reactor feed as a basis of calculation, draw and label a flowchart, and determine the degrees of freedom for the overall process and each single unit and stream mixing point. Then write in order the equations you would use to calculate the molar flow rate and molar composition of the fresh feed, the rate at which HCI must be removed in the absorber, the methyl chloride production rate, and the molar flow rate of the recycle stream. Do no calculations. (b) Calculate the quantities specified in Part (a), either manually or with an equation-solving program. (c) What molar flow rates and compositions of the fresh feed and the recycle stream are required to achieve a methyl chloride production rate of \(1000 \mathrm{kg} / \mathrm{h} ?\)

Short Answer

Expert verified
Calculation of the degrees of freedom, writing of the relevant equations, calculation of specified quantities to understand process flow, and finally calculation of molar flow rates and compositions required to achieve a specific production rate are needed to fully solve this application problem in chemical engineering.

Step by step solution

01

Determination of the degrees of freedom

To begin, determine the degrees of freedom for the entire process, each unit, and mix points by considering the number of unknowns and the number of independent equations available.
02

Write equations without calculation

Write the equations of molar flow rate, molar composition of the fresh feed, the rate at which HCl must be removed in the absorber, the methyl chloride production rate, and the molar flow rate of the recycle stream. All equations are based on the conservation of mass law across each unit operation, according to the problem statement and the flow chart drawn.
03

Perform the Calculations

With the equations from step two handy, perform the necessary calculations, either manually or using an equation-solving program, to determine the quantities specified in the problem's part (b). The calculated results reveal the molar flow rate and molar composition of the fresh feed, the rate at which HCl must be removed in the absorber, the methyl chloride production rate, and the molar flow rate of the recycle stream.
04

Calculate Production Rate

From the analysis of the process and the earlier calculations, determine the molar flow rates and compositions of the fresh feed and the recycle stream that are required to achieve a methyl chloride production rate of 1000 kg/hr.

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Key Concepts

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

Mass Balance Calculations
To start with, mass balance calculations are a fundamental part of any chemical engineering process analysis. These calculations are essential because they help us understand the flow of mass in and out of a process. In a methyl chloride production process, we can apply the conservation of mass principle to every reactor, condenser, distillation and absorption column.
To perform these calculations, consider the input and output streams for each unit.
  • Identify all components entering and leaving each unit.
  • Write balance equations based on the conservation law: input = output + accumulation.
  • Account for chemical reactions by incorporating stoichiometry into the mass balance equations.
This analysis is crucial in arriving at the molar flow rates and compositions of various streams within the process, thus ensuring the safe and efficient operation of the system.
Methyl Chloride Production
Methyl chloride production is a significant process in organic chemistry and industrial applications due to its use in creating various chemicals and polymers. In the given process, methane and chlorine react to produce methyl chloride and hydrogen chloride as primary products. However, further chlorination can lead to the formation of side products like methylene chloride.
It's essential to control the conditions within the reactor to optimize methyl chloride production and minimize unwanted reactions. The process typically involves:
  • A high single-pass conversion of chlorine, up to 100%, which is crucial for maximizing yield.
  • Maintaining a specific mole ratio (5:1) of methyl chloride to methylene chloride to ensure process efficiency.
Through careful monitoring and systematic analysis, chemical engineers can enhance the production rate and quality of methyl chloride while controlling the formation of less desirable by-products.
Reactor Operations
Reactor operations are at the heart of any chemical synthesis process, determining the overall success of product formation. In the methyl chloride process, the reactor is designed to handle a stream of methane and chlorine and facilitate their reaction.
Critical factors in reactor operations include:
  • Temperature and pressure control to favor the desired reaction path.
  • Ensuring complete chlorine conversion by optimizing the residence time of reactants within the reactor.
  • Handling products and by-products through well-designed removal systems like condensers and absorption columns.
In essence, efficient reactor operations require balancing the chemical kinetics and thermodynamics to maximize the desired output while minimizing energy consumption and waste generation.
Flowchart Analysis
Flowchart analysis is a powerful tool in process engineering, providing a visual representation of the entire production process flow. In the context of methyl chloride production, a well-developed flowchart shows each major step, including the reactor, condenser, distillation, and absorption columns.
By examining this flowchart, we can:
  • Identify key unit operations and understand their functions within the broader process.
  • Pinpoint critical control points for mass balance and efficiency adjustments.
  • Visualize interactions between different streams, aiding in the identification of potential bottlenecks or recycle points.
Flowchart analysis not only serves as a roadmap for process design and optimization but also aids in troubleshooting by clearly outlining the paths and transformations of materials throughout the system.

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Most popular questions from this chapter

In the production of soybean oil, dried and flaked soybeans are brought into contact with a solvent (often hexane) that extracts the oil and leaves behind the residual solids and a small amount of oil. (a) Draw a flowchart of the process, labeling the two feed streams (beans and solvent) and the leaving streams (solids and extract). (b) The soybeans contain 18.5 wt\% oil and the remainder insoluble solids, and the hexane is fed at a rate corresponding to \(2.0 \mathrm{kg}\) hexane per \(\mathrm{kg}\) beans. The residual solids leaving the extraction unit contain 35.0 wt\% hexane, all of the non-oil solids that entered with the beans, and \(1.0 \%\) of the oil that entered with the beans. For a feed rate of \(1000 \mathrm{kg} / \mathrm{h}\) of dried flaked soybeans, calculate the mass flow rates of the extract and residual solids, and the composition of the extract. (c) The product soybean oil must now be separated from the extract. Sketch a flowchart with two units, the extraction unit from Parts (a) and (b) and the unit separating soybean oil from hexane. Propose a use for the recovered hexane.

Chlorobenzene \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}\right),\) an important solvent and intermediate in the production of many other chemicals, is produced by bubbling chlorine gas through liquid benzene in the presence of ferric chloride catalyst. In an undesired side reaction, the product is further chlorinated to dichlorobenzene, and in a third reaction the dichlorobenzene is chlorinated to trichlorobenzene. The feed to a chlorination reactor consists of essentially pure benzene and a technical grade of chlorine gas (98 wt\% \(\mathrm{Cl}_{2}\), the balance gaseous impurities with an average molecular weight of 25.0 ). The liquid output from the reactor contains \(65.0 \mathrm{wt} \% \mathrm{C}_{6} \mathrm{H}_{6}, 32.0 \% \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}, 2.5 \% \mathrm{C}_{6} \mathrm{H}_{4} \mathrm{Cl}_{2},\) and \(0.5 \%\) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{Cl}_{3} .\) The gaseous output contains only \(\mathrm{HCl}\) and the impurities that entered with the chlorine. (a) You wish to determine (i) the percentage by which benzene is fed in excess, (ii) the fractional conversion of benzene, (iii) the fractional yield of monochlorobenzene, and (iv) the mass ratio of the gas feed to the liquid feed. Without doing any calculations, prove that you have enough information about the process to determine these quantities. (b) Perform the calculations. (c) Why would benzene be fed in excess and the fractional conversion kept low? (d) What might be done with the gaseous effluent? (e) It is possible to use 99.9\% pure ("reagent-grade") chlorine instead of the technical grade actually used in the process. Why is this probably not done? Under what conditions might extremely pure reactants be called for in a commercial process? (Hint: Think about possible problems associated with the impurities in technical grade chemicals.)

Two streams flow into a 500 -gallon tank. The first stream is 10.0 wt\% ethanol and \(90.0 \%\) hexane (the mixture density, \(\rho_{1},\) is \(0.68 \mathrm{g} / \mathrm{cm}^{3}\) ) and the second is \(90.0 \mathrm{wt} \%\) ethanol, \(10.0 \%\) hexane \(\left(\rho_{2}=0.78 \mathrm{g} / \mathrm{cm}^{3}\right) .\) After the tank has been filled, which takes 22 \(\mathrm{min}\), an analysis of its contents determines that the mixture is 60.0 wt\% ethanol, \(40.0 \%\) hexane. You wish to estimate the density of the final mixture and the mass and volumetric flow rates of the two feed streams. (a) Draw and label a flowchart of the mixing process and do the degree-of- freedom analysis. (b) Perform the calculations and state what you assumed.

A fuel distributor supplies four liquid fuels, each of which has a different ratio of ethanol to gasoline. Five percent of the demand is for E100 (pure ethanol), 15\% for E85 (85.0 volume\% ethanol), 40\% for E10 (10.0\% ethanol), and the remainder for pure gasoline. The distributor blends gasoline and ethanol to produce E85 and E10, and the four products are produced continuously. (a) Draw and label a flowchart for the blending operation, letting \(\dot{V}\) represent the combined volumetric flow rate of all four fuels and \(\dot{V}_{\mathrm{G}}\) and \(\dot{V}_{\mathrm{E}}\) represent the volumetric flow rates of gasoline and ethanol sold as fuels and sent to the blending operation. (b) Assuming volume additivity when blending ethanol and gasoline, determine the volumetric flow rates of all streams when delivery of 100,000 L/d of \(\mathrm{E} 10\) is specified. (c) Tank trucks are to transport the fuel from the blending operation to service stations in the area. The gross weight of a loaded truck, which has a tare (empty) weight of \(12,700 \mathrm{kg}\), cannot exceed \(36,000 \mathrm{kg} .\) Assuming the specific gravity of pure gasoline is \(0.73,\) estimate the maximum volume (L) of each fuel that can be loaded onto a truck.

A liquid mixture contains \(60.0 \mathrm{wt} \%\) ethanol \((\mathrm{E}), 5.0 \mathrm{wt} \%\) of a dissolved solute \((\mathrm{S}),\) and the balance water. A stream of this mixture is fed to a continuous distillation column operating at steady state. Product streams emerge at the top and bottom of the column. The column design calls for the product streams to have equal mass flow rates and for the top stream to contain 90.0 wt\% ethanol and no S. (a) Assume a basis of calculation, draw and fully label a process flowchart, do the degree-of-freedom analysis, and verify that all unknown stream flows and compositions can be calculated. (Don't do any calculations yet.) (b) Calculate (i) the mass fraction of \(S\) in the bottom stream and (ii) the fraction of the ethanol in the feed that leaves in the bottom product stream (i.e., \(\mathrm{kg} \mathrm{E}\) in bottom stream/kg \(\mathrm{E}\) in feed) if the process operates as designed. (c) An analyzer is available to determine the composition of ethanol-water mixtures. The calibration curve for the analyzer is a straight line on a plot on logarithmic axes of mass fraction of ethanol, \(x\) (kg E/kg mixture), versus analyzer reading, \(R\). The line passes through the points \((R=15, x=\) 0.100) and \((R=38, x=0.400)\). Derive an expression for \(x\) as a function of \(R(x=\cdots\) ) based on the calibration, and use it to determine the value of \(R\) that should be obtained if the top product stream from the distillation column is analyzed. (d) Suppose a sample of the top stream is taken and analyzed and the reading obtained is not the one calculated in Part (c). Assume that the calculation in Part (c) is correct and that the plant operator followed the correct procedure in doing the analysis. Give five significantly different possible causes for the deviation between \(R_{\text {measured and }} R_{\text {prediced }}\), including several assumptions made when writing the balances of Part (c). For each one, suggest something that the operator could do to check whether it is in fact the problem.
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