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The current global reliance on fossil fuels for heating, transportation, and electric power generation raises concems regarding the release of \(\mathrm{CO}_{2}\) and \(\mathrm{CH}_{4},\) which are greenhouse gases thought to lead to climate change, and NO, which contributes to smog. One potential solution to these problems is to produce transportation fuels from renewable biomass. You have been asked to evaluate a proposed process for converting forest residues to alcohols that may be used as transportation fuels. In the first stage of the process, steam and dry wood from hybrid poplar trees (which grow between five and eight feet a year and can be harvested roughly every five years) are fed to a gasifier in which the biomass is converted to light gases in the following reactions: $$\begin{aligned} \mathrm{C}+\mathrm{H}_{2} \mathrm{O} & \rightarrow \mathrm{CO}+\mathrm{H}_{2} \\\ \mathrm{CO}+\mathrm{H}_{2} \mathrm{O} & \rightarrow \mathrm{CO}_{2}+\mathrm{H}_{2} \\ \mathrm{C}+\mathrm{CO}_{2} & \rightarrow 2 \mathrm{CO} \\ \mathrm{C}+2 \mathrm{H}_{2} & \rightarrow \mathrm{CH}_{4} \\ \mathrm{CH}_{4}+\mathrm{H}_{2} \mathrm{O} & \rightarrow \mathrm{CO}+3 \mathrm{H}_{2} \end{aligned}$$ The effluents from the reactor are a gas stream containing \(\mathrm{H}_{2}, \mathrm{CO}, \mathrm{CO}_{2}, \mathrm{CH}_{4},\) and \(\mathrm{H}_{2} \mathrm{O},\) and a solid char stream that contains only carbon and hydrogen. The char is discarded and the gases go through additional steps in which the hydrogen and carbon monoxide are converted to mixed alcohols. This problem only concerns the gasifier. \(\cdot\) Elemental composition of biomass: 51.9 mass \(\%\) C \(, 6.3 \%\) H, and \(41.8 \%\) O \(\cdot\) Pressure and temperature of entering steam: \(155^{\circ} \mathrm{C}, 4.4 \mathrm{atm}\) \(\cdot\) Feed ratio of steam to biomass: 1.1 kg steam/kg biomass \(\cdot\) Yield and dry-basis composition of product gas: 1.35 kg dry gas/kg biomass at \(700^{\circ} \mathrm{C}, 1.2\) atm; 50.7 mol\% \(\mathrm{H}_{2}, 23.8 \%\) CO, \(18.0 \% \mathrm{CO}_{2}, 7.5 \% \mathrm{CH}_{4}\) (a) Taking a basis of \(100 \mathrm{kg}\) of biomass fed, draw and completely label a flowchart for the gasifier incorporating the given data, labeling the volumes of the steam fed and the gases produced. Perform a degree-of-freedom analysis. (b) Calculate the mass and mass composition of the char and the volumes of the steam feed and product gas streams. (c) List advantages and possible drawbacks of using biomass rather than petroleum as a fuel source.

Step by step solution

01

- Creating a flowchart and Performing a degree of freedom analysis

One can create a flowchart quite simply by starting at the beginning of the process (the introduction of dry wood and steam) and following the process through to the end (the output of a gas stream and solid char). Remember to incorporate the given data.Perform a degree-of-freedom analysis using the formula:\[DOF = c - s - p + 1\]where \(c\) is the number of components, \(s\) is the number of species and \(p\) is the number of phases present.In this case: \(c = 4\) (H2, CO, CO2 and CH4), \(s = 5\) (H2, CO, CO2, CH4 and H2O) and \(p = 1\)So, \[DOF = 4 - 5 - 1 + 1 = -1\]This suggests that there is one unknown too many and thus additional information would be required to solve this problem. In this case, the 'additional' information is given as the product gas composition.

02

- Calculating mass and composition of the char and volumes of steam feed and product gas streams

The mass of biomass feed is given as 100 kg, with mass composition of 51.9 % C, 6.3 % H, and 41.8 % O by mass. The overall feed also contains steam and the mass ratio of steam to biomass is given as 1.1. Thus, the mass of steam equals 1.1 x 100 kg = 110 kg.Based on the given yield, 1.35 kg dry gas is produced per kg biomass, so for 100 kg biomass, it would be 1.35 x 100 = 135 kg dry gas. The remaining mass would be the char as the gases produced are known.The volumes of the steam and gas streams can be approximated using the ideal gas law together with the conditions of pressure and temperature given, with the molar fraction of each component, but taking into account that the volume is not necessarily additive.

03

- Listing advantages and drawbacks of using biomass.

Advantages of using biomass instead of petroleum as a fuel source can include: its renewability, reduced greenhouse gas emissions (as the CO2 released is offset by the CO2 absorbed during the growth of the biomass), and its potential to cause less damage to the environment in the event of a spill. Potential drawbacks could include: the requirement of land and resources for growing biomass, possible interference with food production, possible increase in water usage, and the need for complex and potentially costly technologies for processing and conversion.

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Key Concepts

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

Chemical Process Design

Understanding Chemical Process Design is pivotal when considering the transformation of renewable resources, like biomass, into fuel. The process involves the strategic conversion of raw materials—such as dry wood from trees—into more valuable products using chemical reactions. This transformation is managed within a chemical reactor, here, a biomass gasifier.
When designing such a chemical process, considerations include reaction stoichiometry, energy balances, and material flows. Efficiency, safety, and environmental impact are also significant concerns due to the greenhouse gases involved. In the context of the given exercise, the design must efficiently convert the biomass to gas while minimizing the output of harmful substances.
To improve understanding, visual aids like flowcharts and step-by-step exercises enable students to methodically follow the complex process.

Flowchart Creation

Creating a Flowchart is a fundamental step in visualizing the sequence and interaction of events in a chemical process. Flowcharts provide a bird's-eye view of the entire process, showcasing each step from the initial inputs to the final outputs.
For the biomass gasifier process, it's key to construct a clear diagram detailing the inputs (steam and dry wood), the chemical reactions, and the outputs (gas stream and solid char). Labeling each element, such as flow rates, stream composition, and operating conditions, can make the system easier to analyze and understand. This visual representation helps in identifying process relationships and systems, essential in enabling students to recognize the significance of each step in the conversion of biomass to fuel.

Degree-of-Freedom Analysis

Degree-of-Freedom Analysis is a quantitative tool employed to ascertain if the available information about a process is sufficient to calculate unknowns. It's essentially a count of variables versus equations available. The term 'degree-of-freedom' refers to the number of independent variables that can be changed without affecting others.
In chemical process calculation, it's determined by the formula \[DOF = c - s - p + 1\]. This concept alerts you to either an excess or deficit of information required to solve a problem. In the gasifier scenario, the analysis revealed an additional piece of data needed, pointing to the necessity of product gas composition for fully determining outputs. This analytical approach is crucial in troubleshooting and ensuring the completeness of information in process design.

Gas Stream Composition Analysis

Gas Stream Composition Analysis is vital in assessing the efficiency and quality of the chemical processes. In the case of the gasifier, it entails examining the mixture of gases produced—hydrogen (\(H_2\)), carbon monoxide (\(CO\)), carbon dioxide (\(CO_2\)), and methane (\(CH_4\)). Knowing the molar or mass percentage composition of these gases is critical for optimizing the performance of the subsequent alcohol synthesis process and for environmental compliance.
Analysis of gas stream composition aids in determining if the reaction has proceeded as intended or if adjustments in process conditions or feedstocks are necessary. For students, understanding how to perform such an analysis can shape the way they approach problems requiring accuracy of composition for successful outcomes.

Renewable Energy Sources

Considering Renewable Energy Sources is imperative due to the burdensome environmental and economic impacts of fossil fuels. Biomass, as showcased in the exercise, represents a renewable source that can potentially lead to the production of cleaner fuels. Unlike fossil fuels, biomass can be replenished quickly and continuously, aligning with sustainable practices.
While biomass offers benefits like reducing greenhouse gas emissions, the process of converting biomass to fuel requires careful evaluation of its own environmental impacts, such as land use and water consumption. For students learning about renewable energy, understanding the characteristics, benefits, and drawbacks of various sources, including biomass, provides a comprehensive view of existing and future energy solutions.