/*! This file is auto-generated */ .wp-block-button__link{color:#fff;background-color:#32373c;border-radius:9999px;box-shadow:none;text-decoration:none;padding:calc(.667em + 2px) calc(1.333em + 2px);font-size:1.125em}.wp-block-file__button{background:#32373c;color:#fff;text-decoration:none} Problem 72 A Claus plant converts gaseous s... [FREE SOLUTION] | 91影视

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Chapter 4: Problem 72

A Claus plant converts gaseous sulfur compounds to elemental sulfur, thereby eliminating emission of sulfur into the atmosphere. The process can be especially important in the gasification of coal, which contains significant amounts of sulfur that is converted to \(\mathrm{H}_{2}\) S during gasification. In the Claus process, the \(\mathrm{H}_{2}\) S-rich product gas recovered from an acid-gas removal system following the gasifier is split, with one-third going to a furnace where the hydrogen sulfide is burned at 1 atm with a stoichiometric amount of air to form SO \(_{2}\). $$\mathrm{H}_{2} \mathrm{S}+\frac{3}{2} \mathrm{O}_{2} \rightarrow \mathrm{SO}_{2}+\mathrm{H}_{2} \mathrm{O}$$ The hot gases leave the furnace and are cooled prior to being mixed with the remainder of the \(\mathrm{H}_{2}\) S-rich gases. The mixed gas is then fed to a catalytic reactor where hydrogen sulfide and \(\mathrm{SO}_{2}\) react to form elemental sulfur. $$2 \mathrm{H}_{2} \mathrm{S}+\mathrm{SO}_{2} \rightarrow 2 \mathrm{H}_{2} \mathrm{O}+3 \mathrm{S}$$ The coal available to the gasification process is 0.6 wt\% sulfur, and you may assume that all of the sulfur is converted to \(\mathrm{H}_{2} \mathrm{S}\), which is then fed to the Claus plant. (a) Estimate the feed rate of air to the Claus plant in \(\mathrm{kg} / \mathrm{kg}\) coal. (b) While the removal of sulfur emissions to the atmosphere is environmentally beneficial, identify an environmental concern that still must be addressed with the products from the Claus plant.

Short Answer

Expert verified
a) The feed rate of air to the Claus plant is 0.0402 kg air/kg coal. b) The management and disposal of elemental sulfur and other chemical wastes produced by the Claus process is an environmental concern.

Step by step solution

01

Determine Quantity of Sulfur

First, one must calculate the amount of sulfur in a one kg of coal. From the given data, it is known that coal consists of 0.6 wt\% sulfur. Hence, the quantity of sulfur in 1 kg of coal will be 0.006 kg.
02

Convert Sulfur to \(H_{2}S\)

The problem states that all sulfur is converted to \(H_{2}S\). So, calculate the number of moles. The molar mass of \(H_{2}S\) is approximately 34 g/mol or 0.034 kg/mol. Therefore, 0.006 kg of sulfur is equivalent to 0.006 kg/0.034 kg/mol = 0.176 moles of \(H_{2}S\).
03

Calculate Quantity of Oxygen Needed

The reaction is \(H_{2}S + \frac{3}{2}O_{2} \rightarrow SO_{2} + H_2O\). From the stoichiometry of the reaction, 1 mol of \(H_{2}S\) reacts with 1.5 mol of \(O_2\). Therefore, 0.176 moles of \(H_{2}S\) will require 0.176 * 1.5 = 0.264 moles of \(O_2\).
04

Convert Quantity of Oxygen to Mass

The molar mass of oxygen is 32 g/mol or 0.032 kg/mol. Therefore, 0.264 moles of \(O_2\) is equivalent to 0.264 * 0.032 = 0.00845 kg.
05

Calculate Air Feed Rate

Air consists of approximately 21 wt\% oxygen. Therefore, the mass of air needed to provide 0.00845 kg of oxygen is 0.00845/0.21 = 0.0402 kg. Hence, the feed rate of air to the Claus plant is 0.0402 kg air per kg of coal.
06

Identify an Environmental Concern

The Claus process produces waste that includes water and elemental sulfur as byproducts. Though sulfur is less harmful than its gaseous forms, the disposal and management of such chemical waste is an environmental concern.

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Key Concepts

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

Chemical Engineering and the Claus Process
Chemical engineering plays a crucial role in transforming raw materials into valuable products while addressing environmental concerns. One notable application is the Claus process, which is effectively harnessed to mitigate pollution by converting hazardous sulfur compounds into elemental sulfur. This catalytic chemical process, typically used in refineries and chemical plants, highlights the field's commitment to sustainability and public health.

The Claus process, in its operation, involves two main chemical reactions. Initially, a portion of hydrogen sulfide (\(H_2S\)) from the acid-gas is combusted with oxygen to form water (\(H_2O\)) and sulfur dioxide (\(SO_2\)). Subsequently, in a catalytic reactor, the remaining \(H_2S\) reacts with the produced \(SO_2\) to yield elemental sulfur. This dual-stage approach ensures that sulfur, a detrimental pollutant, is not released into the atmosphere but is instead captured as a solid. By tackling the problem of sulfur emissions, chemical engineers utilize the Claus process as a testament to their expertise in designing processes which are both efficient and environmentally considerate.
Environmental Impact of Gasification and Sulfur Recovery
The environmental impact of gasification processes, which convert carbonaceous materials into synthetic gas, cannot be overlooked. One critical aspect of gasification, particularly of coal, is the release of sulfur compounds such as \(H_{2}S\), which can lead to acid rain and respiratory problems. The integration of the Claus process in gasification plants showcases the industry's efforts in reducing the environmental footprint of such operations.

While the Claus process significantly diminishes the emission of sulfur into the atmosphere, an issue still persists with the handling of byproducts. The process generates a considerable amount of elemental sulfur, which must be managed responsibly to prevent environmental degradation. Thus, environmental engineers and chemical engineers must collaborate to develop robust strategies for sulfur waste disposal or repurposing, ensuring that the solutions implemented do not trade one environmental hazard for another.
Understanding Stoichiometry in the Claus Process
Stoichiometry, the cornerstone of chemical reactions, defines the proportional relationship between reactants and products. In the context of the Claus process, stoichiometry is pivotal to determining the precise quantities of reactants required for the efficient conversion of \(H_{2}S\) to sulfur.

In the exercise provided, we explore the stoichiometric calculations necessary for the Claus process to function. To find the air feed rate, one must determine the mass of sulfur in the coal, convert it to the equivalent mass of \(H_{2}S\), and then calculate the required oxygen. This meticulous approach, guided by stoichiometric principles, ensures the complete reaction of \(H_{2}S\) while minimizing excess reactants, which is fundamental for both economic and environmental optimality. Such exercises not only train students in computational skills but also instill an understanding of the precision required in chemical engineering practice.

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Most popular questions from this chapter

\- An equimolar liquid mixture of benzene and toluene is separated into two product streams by distillation. A process flowchart and a somewhat oversimplified description of what happens in the process follow: Inside the column a liquid stream flows downward and a vapor stream rises. At each point in the column some of the liquid vaporizes and some of the vapor condenses. The vapor leaving the top of the column, which contains 97 mole\% benzene, is completely condensed and split into two equal fractions: one is taken off as the overhead product stream, and the other (the reflux) is recycled to the top of the column. The overhead product stream contains \(89.2 \%\) of the benzene fed to the column. The liquid leaving the bottom of the column is fed to a partial reboiler in which \(45 \%\) of it is vaporized. The vapor generated in the reboiler (the boilup) is recycled to become the rising vapor stream in the column, and the residual reboiler liquid is taken off as the bottom product stream. The compositions of the streams leaving the reboiler are governed by the relation $$\frac{y_{\mathrm{B}} /\left(1-y_{\mathrm{B}}\right)}{x_{\mathrm{B}} /\left(1-x_{\mathrm{B}}\right)}=2.25$$ where \(y_{\mathrm{B}}\) and \(x_{\mathrm{B}}\) are the mole fractions of benzene in the vapor and liquid streams, respectively. (a) Take a basis of 100 mol fed to the column. Draw and completely label a flowchart, and for each of four systems (overall process, column, condenser, and reboiler), do the degree-of-freedom analysis and identify a system with which the process analysis might appropriately begin (one with zero degrees of freedom). (b) Write in order the equations you would solve to determine all unknown variables on the flowchart, circling the variable for which you would solve in each equation. Do not do the calculations in this part. (c) Calculate the molar amounts of the overhead and bottoms products, the mole fraction of benzene in the bottoms product, and the percentage recovery of toluene in the bottoms product \((100 \times\) moles toluene in bottoms/mole toluene in feed).

Liquid methanol is fed to a space heater at a rate of \(12.0 \mathrm{L} / \mathrm{h}\) and burned with excess air. The product gas is analyzed and the following dry-basis mole percentages are determined: \(\mathrm{CH}_{3} \mathrm{OH}=0.45 \%\) \(\mathrm{CO}_{2}=9.03 \%,\) and \(\mathrm{CO}=1.81 \%\) (a) Draw and label a flowchart and verify that the system has zero degrees of freedom. (b) Calculate the fractional conversion of methanol, the percentage excess air fed, and the mole fraction of water in the product gas. (c) Suppose the combustion products are released directly into a room. What potential problems do you see and what remedies can you suggest?

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 \(1000 \mathrm{kg} / \mathrm{h} ?\)

A liquid mixture of acetone and water contains 35 mole\% acetone. The mixture is to be partially evaporated to produce a vapor that is 75 mole \(\%\) acetone and leave a residual liquid that is 18.7 mole \(\%\) (a) Suppose the process is to be carried out continuously and at steady state with a feed rate of 10.0 kmol/h. Let \(\dot{n}_{\mathrm{v}}\) and \(\dot{n}_{1}\) be the flow rates of the vapor and liquid product streams, respectively. Draw and label a process flowchart, then write and solve balances on total moles and on acetone to determine the values of \(\dot{n}_{\mathrm{v}}\) and \(\dot{n}_{\mathrm{l}}\). For each balance, state which terms in the general balance equation (accumulation \(=\)input \(+\)generation \(-\)output\(-\)consumption ) can be discarded and why. (See Example 4.2-2.) (b) Now suppose the process is to be carried out in a closed container that initially contains 10.0 kmol of the liquid mixture. Let \(n_{\mathrm{v}}\) and \(n_{1}\) be the moles of final vapor and liquid phases, respectively. Draw and label a process flowchart, then write and solve integral balances on total moles and on acetone. For each balance, state which terms of the general balance equation can be discarded and why. (c) Returning to the continuous process, suppose the vaporization unit is built and started and the product stream flow rates and compositions are measured. The measured acetone content of the vapor stream is 75 mole \(\%\) acetone, and the product stream flow rates have the values calculated in Part (a). However, the liquid product stream is found to contain 22.3 mole \(\%\) acetone. It is possible that there is an error in the measured composition of the liquid stream, but give at least five other reasons for the discrepancy. [Think about assumptions made in obtaining the solution of Part (a).]

Methane and oxygen react in the presence of a catalyst to form formaldehyde. In a parallel reaction, methane is oxidized to carbon dioxide and water: $$\begin{aligned} \mathrm{CH}_{4}+\mathrm{O}_{2} & \rightarrow \mathrm{HCHO}+\mathrm{H}_{2} \mathrm{O} \\ \mathrm{CH}_{4}+2 \mathrm{O}_{2} & \rightarrow \mathrm{CO}_{2}+2 \mathrm{H}_{2} \mathrm{O} \end{aligned}$$ The feed to the reactor contains equimolar amounts of methane and oxygen. Assume a basis of \(100 \mathrm{mol}\) feed/s. (a) Draw and label a flowchart. Use a degree-of-freedom analysis based on extents of reaction to determine how many process variable values must be specified for the remaining variable values to be calculated. (b) Use Equation 4.6-7 to derive expressions for the product stream component flow rates in terms of the two extents of reaction, \(\xi_{1}\) and \(\xi_{2}\) (c) The fractional conversion of methane is 0.900 and the fractional yield of formaldehyde is 0.855 . Calculate the molar composition of the reactor output stream and the selectivity of formaldehyde production relative to carbon dioxide production. (d) A classmate of yours makes the following observation: "If you add the stoichiometric equations for the two reactions, you get the balanced equation $$2 \mathrm{CH}_{4}+3 \mathrm{O}_{2} \rightarrow \mathrm{HCHO}+\mathrm{CO}_{2}+3 \mathrm{H}_{2} \mathrm{O}$$ The reactor output must therefore contain one mole of \(\mathrm{CO}_{2}\) for every mole of HCHO, so the selectivity of formaldehyde to carbon dioxide must be \(1.0 .\) Doing it the way the book said to do it, \(I\) got a different selectivity. Which way is right, and why is the other way wrong?" What is your response?
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