11.1 Energy Balances for Non-Reactive Processes

Energy is the lifeblood of chemical processes. The first law of thermodynamics governs how energy flows and changes forms in steady-state systems, where conditions remain constant over time.

In non-reactive processes, energy transfers occur through heat, work, and changes in kinetic and potential energy. Understanding these transfers is crucial for analyzing and optimizing various industrial equipment and operations.

Thermodynamic Principles and Energy Transfers

First law in steady-state systems

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• First law of thermodynamics asserts energy conservation principle stating energy cannot be created or destroyed, only converted between forms (heat, work, internal energy)
• Steady-state operation maintains constant system conditions over time with no accumulation of mass or energy within system boundaries (input rates equal output rates)
• Energy balance equation $\Delta E = Q - W$ quantifies energy changes where $\Delta E$ represents internal energy change, $Q$ denotes heat added to system, and $W$ signifies work done by system
• System boundaries define scope of analysis:
  • Closed systems allow only energy transfer, no mass exchange (steam turbine)
  • Open systems permit both mass and energy transfer across boundaries (heat exchanger)

Energy transfers in non-reactive processes

• Heat transfer occurs through:
  1. Conduction in solid materials (heat flow through metal pot)
  2. Convection in fluids (hot air rising in a room)
  3. Radiation via electromagnetic waves (solar energy reaching Earth)
• Work manifests as:
  • Shaft work in rotating machinery (turbines, pumps)
  • Flow work from fluid movement (compressed air systems)
  • Electrical work due to current flow (electric motors)
• Kinetic energy relates to velocity of moving objects or fluids, calculated as $KE = \frac{1}{2}mv^2$ (flowing water in pipes)
• Potential energy exists as:
  • Gravitational potential $PE = mgh$ (water in elevated tanks)
  • Elastic potential stored in compressed springs or gases (pneumatic systems)
• Internal energy encompasses all microscopic energy forms within a system (molecular vibrations, rotations)
• Enthalpy $H = U + PV$ proves useful for constant pressure processes (steam power plants)

Energy balances for non-reactive systems

• General energy balance equation states $E_{in} + E_{generated} = E_{out} + E_{accumulated}$
• Steady-state simplification reduces to $E_{in} = E_{out}$
• Process equipment energy balances:
  1. Heat exchangers: $Q = m c_p \Delta T$ (shell and tube heat exchangers)
  2. Pumps and compressors: $W = m(h_{out} - h_{in})$ (centrifugal pumps)
  3. Turbines: $W = m(h_{in} - h_{out})$ (gas turbines)
• Reference states establish baselines for energy calculations (standard temperature and pressure)
• Psychrometric charts aid in analyzing air-water vapor mixtures (HVAC systems)
• Steam tables provide water and steam properties at various conditions (boiler operations)

Efficiency of non-reactive processes

• Energy efficiency defined as ratio of useful output energy to input energy $\eta = \frac{E_{out}}{E_{in}} \times 100\%$
• Carnot efficiency represents maximum theoretical efficiency for heat engines $\eta_{Carnot} = 1 - \frac{T_C}{T_H}$
• Identifying energy losses helps improve efficiency:
  • Friction in moving parts (bearings, seals)
  • Heat leakage through insulation (furnaces, reactors)
  • Irreversibilities in real processes (throttling valves)
• Process integration techniques enhance overall efficiency:
  • Heat integration through pinch analysis optimizes heat exchanger networks
  • Cogeneration combines heat and power production (combined cycle power plants)
• Equipment modifications boost performance:
  • Improving insulation reduces heat losses (steam pipes, storage tanks)
  • Upgrading to high-efficiency motors decreases electrical consumption (conveyor systems)
• Waste heat recovery utilizes exhaust gases to preheat incoming streams (economizers in boilers)
• Alternative energy sources promote sustainability:
  • Integrating renewable energy (solar panels, wind turbines)
  • Switching to cleaner fuel options (natural gas instead of coal)