Energy Balance on Reaction System Using Heat of Reaction

LearnChemE

27 Jan 202104:36

EducationalLearning

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TLDRThe video script discusses a common method for performing an energy balance in a chemical reactor. It involves using the heat of reaction and setting reference states at 25°C and one atmosphere. The process is visualized through three steps: lowering reactants to reference temperature, carrying out reactions, and raising products to outlet temperature. The equation provided allows for calculating heat added or determining the extent of reaction based on known variables.

Takeaways

🔍 The script discusses a common method for performing an energy balance on a chemical reactor, focusing on the use of heat of reaction and reference states.
🌡️ The reference states for this method are reactants and products at 25 degrees Celsius and one atmosphere.
📉 The process involves adjusting the temperature of reactants to the reference temperature before the reaction occurs.
⚗️ The heat of reaction is calculated using standard conditions, and the products are assumed to be at the reference temperature post-reaction.
🔥 The energy balance includes steps of lowering reactants to the reference temperature, the heat of reaction, and raising the temperature of products to the outlet temperature.
⚙️ The calculation involves summing the enthalpy changes over three steps, equating them to other terms in the energy balance equation.
📈 The heat of reaction term includes the sum of molar flow rates and heat capacities, integrated over the temperature range from the reference to the outlet temperature.
📚 The script mentions that the heat capacity can have various forms, such as constants multiplied by temperature, squared, or cubed.
🔄 The enthalpy of reactants and products is calculated by integrating the heat capacities from the reference temperature to the inlet and outlet temperatures, respectively.
🔢 The extent of reaction is defined in a specific way in the script, relating to the flow rates and stoichiometric coefficients of reactants.
🔄 The script provides an equation that can be used to calculate either the heat added or the outlet temperature, given the necessary parameters.
🔑 Knowing the outlet temperature and the heat added allows for the determination of the extent of reaction in a chemical reactor.

Q & A

What are the two most common ways of doing an energy balance on a chemical reactor?
-The script does not specify the two most common ways but focuses on one method that involves using heat of reaction and energy balance with reactants and products at 25 degrees Celsius and one atmosphere as reference states.
Why is 25 degrees Celsius and one atmosphere chosen as the reference state for the energy balance calculation?
-The reference state of 25 degrees Celsius and one atmosphere is chosen for standardization, allowing for the use of standard heat of formation values and simplifying the calculation of enthalpy changes.
What is the significance of the term 'T reference' in the energy balance process?
-T reference refers to the reference temperature, which is set at 25 degrees Celsius in this context, to which the reactants' and products' temperatures are adjusted for the calculation of enthalpy changes.
How does the energy balance calculation account for multiple reactant streams at different inlet temperatures?
-The concept remains the same regardless of the number of reactant streams or their different inlet temperatures. Each stream is considered individually, and the overall energy balance is calculated by summing the enthalpy changes for all streams.
What is the role of the heat of reaction term in the energy balance equation?
-The heat of reaction term represents the energy change that occurs during the chemical reaction at standard conditions. It is used to calculate the energy absorbed or released during the reaction process.
How is the extent of reaction defined in the script?
-The extent of reaction is defined as the difference between the flow rate out and flow rate in of a reactant, divided by its stoichiometric coefficient, resulting in a dimensionless value.
What is the significance of considering kinetic and potential energy changes in the energy balance of a chemical reactor?
-In most chemical reactors, changes in kinetic and potential energy are not significant and are often considered negligible, simplifying the energy balance equation.
What are the typical forms of heat capacity functions used in the energy balance calculations?
-Typical forms of heat capacity functions include a constant, a linear term (first constant times the temperature), a quadratic term (second constant times the temperature squared), a cubic term (third constant times the temperature cubed), and higher order terms if necessary.
How does the energy balance equation relate to the calculation of heat added or outlet temperature?
-The energy balance equation can be used to calculate the heat added to the system if the outlet temperature is known, or to determine the outlet temperature if the heat added is known, given the extent of reaction.
What additional information is required to determine the extent of reaction if the outlet temperature and heat added are known?
-If the outlet temperature and heat added are known, the extent of reaction can be determined using the energy balance equation, which requires knowledge of the molar flow rates, heat capacities, and stoichiometric coefficients of the reactants and products.
How does the script differentiate between the enthalpy calculations for reactants and products?
-The script differentiates by integrating the heat capacity of each component from the reference temperature to the outlet temperature for products, and from the inlet temperature to the reference temperature for reactants, with a minus sign indicating the direction of the calculation.

Outlines

00:00

🔍 Energy Balance in Chemical Reactors

This paragraph discusses a common method for performing an energy balance on a chemical reactor, focusing on the use of heat of reaction and energy balance equations. The process begins with reactants at various inlet temperatures, which are then hypothetically cooled to a reference temperature of 25 degrees Celsius. The reactions occur at this standard temperature, and the products are also maintained at 25 degrees Celsius. The energy balance involves calculating the enthalpy changes over three steps: cooling reactants, reaction heat, and heating products. The equation provided integrates molar flow rates, heat capacities, and temperature changes to determine the heat added or the extent of reaction. This method allows for calculating either the heat required or the outlet temperature, given sufficient information.

📚 Understanding the Equations for Energy Balance

This section delves deeper into the equations used in the energy balance calculation. It explains the significance of the extent of reaction, which is defined as the ratio of flow rates to stoichiometric coefficients, resulting in a dimensionless value. The heat of reaction is calculated using standard heats of formation at 25 degrees Celsius. The enthalpy terms in the equation account for the heat capacities of each component, integrated over the temperature range from the reference temperature to the outlet temperature, and vice versa for the inlet temperature. The paragraph clarifies that the equation can be used to find the heat added or the outlet temperature, provided the extent of reaction is known. Additionally, it mentions that if both the outlet temperature and the heat added are known, the extent of reaction can be determined.

Mindmap

Keywords

💡Energy Balance

Energy balance is a fundamental concept in thermodynamics and chemical engineering, referring to the process of accounting for all energy inputs and outputs in a system to ensure the conservation of energy. In the video, energy balance is used to analyze a chemical reactor, where the sum of enthalpy changes for different steps in the process is equated to other terms in the energy balance equation, such as heat added and shaft work.

💡Heat of Reaction

Heat of reaction, also known as enthalpy change, is the amount of heat absorbed or released during a chemical reaction. It is a key parameter in the energy balance calculation for a chemical reactor. The script mentions using the heat of reaction at standard conditions (25 degrees Celsius and one atmosphere) to calculate the enthalpy change when reactants are transformed into products.

💡Reference States

Reference states are defined conditions used to measure changes in thermodynamic properties. In the context of the video, reactants and products at 25 degrees Celsius and one atmosphere are chosen as the reference states to simplify the calculation of enthalpy changes during the energy balance of a chemical reactor.

💡Inlet Temperature

Inlet temperature refers to the temperature of the reactants as they enter the chemical reactor. The script discusses the concept of lowering the temperature of the reactants to the reference temperature before the reaction takes place, emphasizing the importance of inlet temperature in the energy balance calculation.

💡Outlet Temperature

Outlet temperature is the temperature of the products and any inerts as they leave the reactor. The script explains that after the reaction, the products' temperature is raised to the outlet temperature, which is a critical variable in determining the heat added to the system or calculating the extent of the reaction.

💡Enthalpy Change

Enthalpy change is the difference in enthalpy between the final and initial states of a system. In the video, enthalpy change is calculated for three steps: lowering the reactants to the reference temperature, the heat of reaction, and raising the products to the outlet temperature. These steps are crucial for the overall energy balance equation.

💡Molar Flow Rates

Molar flow rates are the amounts of substances passing per unit time, measured in moles per second. The script uses molar flow rates in the calculation of the heat of reaction term, emphasizing their importance in determining the heat added to the system based on the stoichiometry of the reaction.

💡Heat Capacities

Heat capacities are properties of substances that define the amount of heat needed to change their temperature by one degree. The script mentions integrating the heat capacities over a temperature range from 25 degrees Celsius to the outlet temperature for both reactants and products, which is essential for calculating the enthalpy change.

💡Extent of Reaction

Extent of reaction is a measure of how far a chemical reaction has proceeded. It is defined in various ways in the literature, but in the video, it is shown as a dimensionless quantity derived from the difference between the flow rates in and out of the reactor, divided by the stoichiometric coefficient.

💡Shaft Work

Shaft work refers to the mechanical work done by or on a system through a rotating shaft. In the script, it is mentioned as one of the terms in the energy balance equation, representing the work done per time in kilojoules per second.

💡Kinetic and Potential Energy

Kinetic and potential energy are forms of mechanical energy. Kinetic energy is the energy of motion, while potential energy is stored energy due to an object's position or condition. The script notes that for most chemical reactors, changes in kinetic and potential energy are not significant and can be considered negligible in the energy balance calculations.

Highlights

Discussing one of the two most common ways of performing an energy balance on a chemical reactor.

Using heat of reaction and energy balance with reactants and products at 25 degrees Celsius and one atmosphere as reference states.

Visualizing calculations by starting with reactants at an inlet temperature and lowering it to the reference temperature.

Heat of reaction occurs during the reaction process using standard conditions.

Products are considered at the same temperature as the reference state after the reaction.

Raising the temperature of products and reactants to the outlet temperature.

Calculating the enthalpy change in three steps for energy balance.

Heat of reaction term is calculated as the sum of molar flow rates and heat capacities integrated from 25 degrees to the outlet temperature.

Integration from the inlet temperature to 25 degrees for reactants and inerts in the feed.

Equation presented for calculating energy balance, including work per time and heat added per time.

Neglecting changes in kinetic and potential energy for most chemical reactors.

Extent of reaction defined in a specific way, with a focus on dimensionless units.

Heat of reaction calculated using standard heats of formation at 25 degrees Celsius.

Molar flow rates and enthalpy values are essential for calculating the energy balance.

Enthalpy integrals are calculated from the reference temperature to the outlet and inlet temperatures.

Equation allows for calculating heat added or outlet temperature, given certain parameters.

Extent of reaction can be determined with known outlet temperature and heat added.

Transcripts

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Takeaways

Q & A

What are the two most common ways of doing an energy balance on a chemical reactor?

Why is 25 degrees Celsius and one atmosphere chosen as the reference state for the energy balance calculation?

What is the significance of the term 'T reference' in the energy balance process?

How does the energy balance calculation account for multiple reactant streams at different inlet temperatures?

What is the role of the heat of reaction term in the energy balance equation?

How is the extent of reaction defined in the script?

What is the significance of considering kinetic and potential energy changes in the energy balance of a chemical reactor?

What are the typical forms of heat capacity functions used in the energy balance calculations?

How does the energy balance equation relate to the calculation of heat added or outlet temperature?

What additional information is required to determine the extent of reaction if the outlet temperature and heat added are known?

How does the script differentiate between the enthalpy calculations for reactants and products?