3/16 Temperature Effects Example

Overview of Temporary Effects in Industrial Reactions

This section focuses on the concept of temporary effects as they pertain to industrial reactions, particularly in the context of a group project due on the 27th. A review of prior materials is necessary, especially for those who missed the previous class.

Importance of Understanding Temporary Effects

• Definition of Temporary Effects: These are changes in reaction conditions that temporarily affect the outcome of chemical processes.
• Typical Scenarios in Industrial Reactions:
  • Reactions may not reach completion.
  • The process may not be isothermal, meaning temperature varies.
  • The presence of both innate species and multiple concurrent reactions complicates the analysis.

Exercise Introduction

• The upcoming exercise will illustrate two complications in reaction analysis:
  • The presence of nitrogen from air used in combustion (reacting with hydrocarbon fuels).
  • The reactants are not present in their main proportions, impacting the balance calculations.

Molar Balance

• Understanding Molar Balance: A foundational concept for approaching temperature effect problems, particularly vital because various reactions may influence reactants and products.
• Typical Strategy in Steady-State Industrial Reactions:
  1. Set accumulation term to zero because there is no change in the amount of reactants/products over time at steady state.
  2. Focus on inert outlet flows and the production/consumption terms from reactions.

Molar Balance Equation

• Molar balance is typically expressed as:
  
  ext{No accumulation} = ext{Molar Out} - ext{Molar In} + ext{Production} - ext{Consumption}
  
• During an industrial operation, this means that:
  • Accumulation = 0
  • Molar Out = Molar In + Production - Consumption

Example Reaction: Reverse Water Gas Shift Reaction

• Chemical Equation:
  $ext{CO}<em>2 + ext{H}</em>2 \rightarrow ext{CO} + ext{H}_2 ext{O}$
• Key Concepts for Calculating Stoichiometric Amounts:
  • Stoichiometric coefficients help quantify amounts of products and reactants in equations.
• Consumption/Production Relations:
  • For CO2:
  • $ext{Molar Balance for CO2} = ext{Molar Out (CO2)} - ext{Molar In (CO2)} = -\zeta$
  • For CO and Water:
  • $ext{Molar Balance for CO} = \zeta$
  • $ext{Molar Balance for H2O} = 0.54 + 2\zeta$

Enthalpy Calculation Methods

• Direct Way:
  • $\Delta H = ext{Enthalpy In} - ext{Enthalpy Out}$
• Indirect Way: More favored due to ease.
  • Combining sensible heat contributions and heat of reaction:
  • $ext{Sensible Heat Gain} = C<em>p (T</em>f - T_i)$
  • Two distinct pathways:
  • Temperature increase/decrease of products vs. reactants.

Class Exercise Follow-Up

• The today’s in-class exercise focuses on a problem based on earlier demonstrations, aimed at reinforcing concepts discussed prior.
• Key points raised during setup include:
  • Reactants burned completely.
  • Use of 30% excess air in the reaction, indicating more oxidant than necessary for complete combustion.
  • The presence of water vapor in combustion air.
  • The inert nitrogen present alongside oxygen in the air.

Setting up the Equation for Molar Balance in the Exercise

1. Write the balanced equation for combustion:
   $ext{CH}<em>4 + 2 ext{O}</em>2 \rightarrow ext{2CO} + 2 ext{H}_2 ext{O}$
2. Establish moles based on one mole of methane:
   • Moles of O2 required: 2 moles are typical; with 30% excess:
   • $m=1 \times (2 + 0.3 \cdot 2) = 2.6$ moles of O2
3. For nitrogen in air (79% N2, 21% O2):
   • Compute nitrogen based on total O2 present:
   • $N<em>{2} = \frac{2.6 \text{moles O}</em>2}{0.21} \approx 9.78$ moles of nitrogen.
4. Consider water vapor present at 4.2 mole percent, leading to:
   • $H<em>2O</em>{in} = 4.2 \% \cdot (moles \ O<em>2 + moles \ N</em>2)$ leading to needed adjustments for molar balance formulation.

Spreadsheet Illustrations and Techniques

• Use of Name Boxes in Excel for clarity in formula applications.
  • Define a name for specific data cells to reference clearly across calculations.
• Formula implementation illustrated with reversible reactions and direct output relationships in spreadsheet setups.
• Usage of functions like SUM to total enthalpy across components for both input and output phases.

Final Remarks and Homework

• Students encouraged to replicate processes learned in class exercises, with additional practice problems provided in course materials.
• Emphasis on iterative learning, revising concepts through practical spreadsheet scenarios and preparation for upcoming evaluations.