Energy and Energy Balances

1
Energy and Energy Balances

• Every chemical process involves the transfer of
  energy
• combustion ? power generation
• distillation ? energy required for volatilization
  (reboiler) and energy removed at
  condenser
• reactors ? breaking and forming chemical bonds
• phase changes ? evaporator and condenser
• Energy balances are used to
• determine the amount of energy that flows into or
  out of each process unit
• calculate the net energy requirement for the
  process
• assess ways of reducing energy requirements in
  order to improve process profitability

2
Units of Energy

• Energy has units of force times distance
  (mass?length/time2)
• Energy also defined in terms of the amount of
  heat required to raise the temperature of a
  specified mass of water by one degree at 1 atm

• Unit conversions found inside the front cover of
  FR

3
Forms of Energy

• The total energy of a process system has three
  components
• Kinetic Energy (Ek) energy due to the
  translational motion of the system as a whole
• Potential Energy (Ep) energy due to the
  position of the system in a potential field
  (e.g., earths gravitational field (g9.8 m/s2))

4
Example

• Water flows into a process unit through a 2 cm ID
  pipe at a rate of 2.00 m3/h. Calculate for
  this stream in joules/second.

5
Example

• Crude oil is pumped at a rate of 15.0 kg/s from a
  point 220 meters below the earths surface to a
  point 20 meters above ground level. Calculate
  the attendant rate of increase of potential
  energy.

6
Forms of Energy

• 3. Internal Energy (U) all energy possessed by
  system other than kinetic and potential energy,
  including the energy due to the
• rotational and vibrational motion of molecules
  within the system
• interactions between molecules within the system
• motion and interactions of electrons and nuclei
  within molecules
• Internal energy (U) is related to enthalpy (H) ?
• U and H are a function of temperature, chemical
  composition, physical state (solid, liquid or
  gas) and only weakly a function of pressure
• U and H are relative quantities
• absolute values are unknown
• values must be defined with respect to their
  reference state

7
Intensive Versus Extensive Variables

• Extensive Variables depend on the size of the
  system
• e.g., mass, number of moles, volume (mass or
  molar flow rate and volumetric flow rate),
  kinetic energy, potential energy and internal
  energy
• Intensive Variables independent of the size of
  the system
• e.g., temperature, pressure, density, specific
  volume, composition (mass or mole fraction)

Specific Property an intensive quantity
obtained by dividing an extensive property (or
its flow rate) by the total amount (or flow rate)
of the process material denoted by ?
specific volume ( ) units of m3/kg enthalpy
and internal energy commonly reported as
intensive quantities ? (kJ/kg),
(kJ/kg) ?
8
Example

• The specific internal energy of helium at 300 K
  and 1 atm is 3800 J/mol, and the specific molar
  volume at the same temperature and pressure is
  24.63 L/mol. Calculate the specific enthalpy of
  helium at this temperature and pressure, and the
  rate at which enthalpy is transported by a stream
  of helium at 300 K and 1 atm with a molar flow
  rate of 250 kmol/h.

9
Transfer of Energy

• Heat (Q) energy that flows due to a temperature
  difference between the system and its
  surroundings
• always flows from high to low temperature
• defined to be positive if it flows to a system
  (i.e. input)
• Work (W) energy that flows in response to any
  driving force (e.g, applied force, torque) other
  than temperature
• defined as positive if it flows from the system
  (i.e. output)
• in chemical processes, work may come from a
  moving piston or moving turbine

In a closed system (no mass transferred across
the system boundaries (i.e., batch system)),
energy may be transferred between the system and
the surroundings in two ways
A system does not possess heat or work. Heat or
work only refer to energy that is being
transferred to the system.
10
Example

• A certain gasoline engine has an efficiency of
  30 that is, it converts into useful work 30 of
  the heat generated by burning a fuel. If the
  engine consumes 0.80 L/h of gasoline with a
  heating value of 3.5 x 104 kJ/L, how much power
  does it provide? Express the answer both in kW
  and horsepower.

11
First Law of Thermodynamics

• The First Law of Thermodynamics states that
  energy can neither be created or destroyed (just
  like total mass!)

Accumulation In Out Generation
Consumption
But generation0 and consumption0 since energy
cannot be created or destroyed so the general
balance becomes
Accumulation In Out