Ch i l E i i P i i l Ch i l E i i P i i l 22Chemical Engineering Principles Chemical Engineering Principles 22((09052120905212))((09050905 ))

Energy BalanceEnergy BalanceEnergy BalanceEnergy Balance

Dr.Dr.--IngIng. Zayed Al. Zayed Al--HamamreHamamregg yy

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ContentContent

IntroductionIntroductionForms of Energy: Forms of Energy: 11stst law of thermodynamicslaw of thermodynamicsEnergy Balance on Closed SystemEnergy Balance on Closed SystemEnergy Balance on Closed SystemEnergy Balance on Closed SystemEnergy Balance on open System at st.st.Energy Balance on open System at st.st.Tables of Thermodynamic DataTables of Thermodynamic DataEnergy Balance ProceduresEnergy Balance ProceduresgygyMechanical Energy BalancesMechanical Energy Balances

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IntroductionIntroduction As an engineer designing a process, one of your principal jobs would be

o To account carefully for the energy that flows into and out of each process unit and

o To determine the overall energy requirement for the process

This by be done by writing energy balances on the process, in much the same way that you

write material balances to account for the mass flows to and from the process and its units

In process industries, wasting energy leads to reduced profits

Coal, natural gas and petroleum are the major energy sources being utilized.

Sun, winds, and tides; are sources for renewable energy.

Nuclear power generation is feasible, but the need for safe disposal of radioactive wastes

from nuclear reactors is a serious unresolved problem.

The tremendous increase in fuel prices and the environmental problems associated with the The tremendous increase in fuel prices and the environmental problems associated with the

combustion of fossil fuel are the deriving force for energy research.

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Energy FormsEnergy Forms1. Kinetic energy: Energy due to the translational motion of the system as a whole relative to

some frame of reference (usually the earth's surface) or to rotation of the system about some

axisaxis.

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ExampleExample

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Energy FormsEnergy Forms2. Potential energy: Energy due to the position of the system in a potential field (such as a

gravitational or electromagnetic field).

The gravitational potential energy of an object of mass m is

The change in potential energy when a body or fluid moves from one elevation to another

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Energy FormsEnergy Forms3. Internal energy:

Energy due to the motion of molecules relative to the center of mass of the system, to the

i l d ib i l i d h l i i i f h l l drotational and vibrational motion and the electromagnetic interactions of the molecules, and

to the motion and interactions of the atomic and subatomic constituents of the molecules.

The internal energy of a system depends almost entirely on: The internal energy of a system depends almost entirely on:

o The chemical composition,

o State of aggregation (solid liquid or gas) ando State of aggregation (solid, liquid, or gas), and

o Temperature of the system materials.

o It is independent of pressure for ideal gases and nearly independent of pressure foro It is independent of pressure for ideal gases and nearly independent of pressure for

liquids and solids.

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ExampleExample

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Energy balanceEnergy balance Open system: is a system in which mass crosses the system boundary during the period of

time as the process occurs.

o Semibatch and continuous systems are open

Closed system: is a system in which no mass crosses the system boundary while the process

i t ki lis taking place.

o A batch process system is closed.

Energy may be transferred between such a system and its surroundings in two ways:

1. As heat, or energy that flows as a result of temperature difference between a system and

its surroundings.

o The direction of flow is always from a higher temperature to a lower one.

i d fi d i i h i i f d h f h dio Heat is defined as positive when it is transferred to the system from the surroundings.

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Energy balanceEnergy balance2. As work, or energy that flows in response to any driving force other than a temperature

difference, such as a force, a torque, or a voltage.

o work is defined as positive when it is done by the system on the surroundings

o For example, if a gas in a cylinder expands and moves a piston against a restraining force,

the gas does work on the piston (energy is transferred as work from the gas to itsthe gas does work on the piston (energy is transferred as work from the gas to its

surroundings, which include the piston).

o In chemical processes, work may come from pumps, compressors, moving piston and p , y p p , p , g p

moving turbines

The terms "work" and "heat" refer only to energy that is being transferred

It is the law of conservation of energy

11stst law of thermodynamicslaw of thermodynamics It is the law of conservation of energy.

Energy can neither be created nor destroyed, but converted from one Energy can neither be created nor destroyed, but converted from one form to anotherform to another

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10form to anotherform to another

Energy balanceEnergy balance In more general form, the first law states that the rate at which energy (kinetic + potential +

internal) is carried into a system by the input streams, plus the rate at which it enters as heat,

minus the rate at which it is transported out of the system by the output streams, minus the

rate at which it leaves as work, equals the rate of accumulation of energy in the system

Energy balance on closed systemsEnergy balance on closed systems

No mass crosses the boundaries of a closed system No mass crosses the boundaries of a closed system.

Is possible, however, for energy to be transferred across the boundaries as heat or work

The accumulation term equals the final value of the system energy minus the initial value of The accumulation term equals the final value of the system energy minus the initial value of

this quantity. (As with mass balances)

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Energy balance on closed systemsEnergy balance on closed systems

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Energy balance on closed systemsEnergy balance on closed systems

Or the system is perfectly insulated (adiabatic system)

Work done on or by a closed system is accomplished by movement of the system boundary Work done on or by a closed system is accomplished by movement of the system boundary

against a resisting force or the passage of an electrical current or radiation across the system

boundary

If there are no moving parts or electrical currents or radiation at the system boundary,

If no temperature changes, phase changes, or chemical reactions occur in a closed system and if pressure changes are less than a few atmospheres,

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ExampleExample

A gas is contained in a cylinder fitted with a movable piston. The initial gas temperature is 25°C. g y p g p

1. The cylinder is placed in boiling water with the piston held in a fixed position. Heat in the

amount of 2.00 kcal is transferred to the gas, which equilibrates at 100°C (and a higher

pressure).

2. The piston is then released, and the gas does 100 J of work in moving the piston to its new

equilibrium position The final gas temperature is 100°Cequilibrium position. The final gas temperature is 100°C.

neglect the change in potential energy of the gas as the piston moves vertically, and assume the

b h id ll i h b l i f h f h f higas behaves ideally. Write the energy balance equation for each of the two stages of this process.

and in each case solve for the unknown energy term in the equation.

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Example Cont.Example Cont.

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Example Cont.Example Cont.

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Energy balance on open systemsEnergy balance on open systems An open process system by definition has mass crossing its boundaries as the process occurs.

Work must be done on such a system to push mass in, and work is done on the surroundings

b hby mass that emerges.

Both work terms must be included in the energy balance

Flow Work and Shaft WorkFlow Work and Shaft Work The work or the rate of energy transferred as work is required to move fluid through a

The net rate of work done by an open system on its surroundings

The work or the rate of energy transferred as work is required to move fluid through a

continuous process system,

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Flow WorkFlow Work

The fluid that enters the system has work done on it by the fluid just behind it at a ratey y j

The fluid leaving the system performs work on the surroundings at a rate

The net rate at which work is done by the system at the inlet and outlet is therefore

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Specific Properties and EnthalpySpecific Properties and Enthalpy The properties of a process material are either

o Extensive (proportional to the quantity of the material) or

o Intensive (independent of the quantity), T, P and ρ

A specific property is an intensive quantity obtained by dividing an extensive property (or its

fl ) b h l ( fl ) f h i lflow rate) by the total amount (or flow rate) of the process material.

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Specific Properties and EnthalpySpecific Properties and Enthalpy

Define

: The specific enthalpy

ExampleExample

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Example Cont.Example Cont.

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SteadySteady--State OpenState Open--System Energy BalanceSystem Energy Balance The first law of thermodynamics for an open system at steady state

Input here signifies the

o Total rate of transport of kinetic energy, potential energy, and internal energy by all

process input streams plus

o The rate at which energy is transferred in as heat,

Output is

o The total rate of energy transport by the output streams plus

o The rate at which energy is transferred out as work

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SteadySteady--State OpenState Open--System Energy BalanceSystem Energy Balance

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SteadySteady--State OpenState Open--System Energy BalanceSystem Energy Balance

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SteadySteady--State OpenState Open--System Energy BalanceSystem Energy Balance

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SteadySteady--State OpenState Open--System Energy BalanceSystem Energy Balance The net rate at which energy is transferred to a system as heat and/or shaft work

equals the difference between the rates at which the quantity (enthalpy + kinetic energy +

potential energy) is transported into and out of the systempotential energy) is transported into and out of the system

For single input - single output stream steady state process

E lE lExampleExample

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Example Cont.Example Cont.

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Example Cont.Example Cont.

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Tables of thermodynamic dataTables of thermodynamic dataReference States and State PropertiesReference States and State Properties

determineddetermined

(temperature pressure and phase)(temperature, pressure. and phase)

For example, assume that the enthalpy changes for carbon monoxide going from a reference

state of 0oC and 1 atm to two other states are measured

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Tables of thermodynamic dataTables of thermodynamic data

A property whose change of value in any process depends only on its initial and final states and

not on the path between them, i.e. depends only on the state of the system and not on how the

t h d th t t t

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30system reached that state

Tables of thermodynamic dataTables of thermodynamic data

Their values depend only on the state of the species primarily on its temperature and state of Their values depend only on the state of the species-primarily on its temperature and state of

aggregation (solid, liquid, or gas) and,

To a lesser extent, on its pressure and , p

For mixtures of some species, on its mole fraction in the mixture).

A state property does not depend on how the species reached its state. Consequently, p p y p p q y,

When a species passes from one state to another, both for the process are

independent of the path taken from the first state to the second one.

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ExampleExample

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Example Cont.Example Cont.

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Example Cont.Example Cont.QuizQuiz

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Steam tablesSteam tables Tables that contains the compilations of physical properties of liquid water, saturated steam,

and superheated steam.

The reference point for the tabulated values is again liquid water at the triple point

the phase diagram for water

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35p g

Steam tablesSteam tables

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ExampleExample

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Example Cont.Example Cont.

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ExampleExample

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Example Cont.Example Cont.

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Energy balance procedureEnergy balance procedure1. Draw a labeled flowchart

2 Include all of the information needed to determine the specific enthalpy of each stream2. Include all of the information needed to determine the specific enthalpy of each stream

component including known temperatures and pressures.

3. Show states of aggregation of process materials according to whether water is present as a

solid, a liquid, or a vapor.

When process streams contain several components, the specific enthalpies of each component

must be determined separately and substituted in the energy balance equation when

For mixtures of near-ideal gases or of liquids with similar molecular structures (e.g., mixtures

of paraffins) it might be assumed that for a mixture component is the same as for theof paraffins), it might be assumed that for a mixture component is the same as for the

pure substance at the same temperature and pressure.

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ExampleExample

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Example Cont.Example Cont.

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Example Cont.Example Cont.

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Example Cont.Example Cont.

It is not uncommon to neglect kinetic and potential energy changes relative to enthalpy

changes for processes that involve phase changes, chemical reactions, or large temperature

h

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ExampleExample

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Example Cont.Example Cont.are required

No material balances are necessary since there is only one input stream and one output stream

and no chemical reactions

The enthalpies of each stream equal to the sums of the individual component enthalpies

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Example Cont.Example Cont.

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ExampleExample

Specific enthalpies of the two feed streams and the product stream are obtained from the

t t blsteam tables

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Example Cont.Example Cont.1 material balance.

1 energy balance1 energy balance

The material and energy balances must therefore be solved simultaneously to

d t i th t fl tdetermine the two flow rates

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Example Cont.Example Cont.

The volumetric flow rate

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Type of chemical process Type of chemical process In chemical process units such as reactors. distillation columns, evaporators. and heat

exchangers, shaft work and kinetic and potential energy changes tend to be negligible

d ith h t fl d i t l d th l hcompared with heat flows and internal energy and enthalpy changes

Energy balances on such units therefore usually take the simple form

Another important class of operations is one for which the opposite is true-heat flows and

internal energy changes are secondary in importance to kinetic and potential energy changes

d h ft kand shaft work.

Most of these operations involve the flow of fluids to, from, and between tanks. reservoirs,

wells, and process units.wells, and process units.

Accounting for energy flows in such processes is most conveniently done with mechanical

energy balances.

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Mechanical energy balanceMechanical energy balance For a single incompressible liquid flowing into and out of a process system at steady state

is called the friction loss .It may appear due to:

f h f d f h dio Any amounts of heat are transferred to or from the surroundings.

o Any little change in temperature from inlet to outlet.

Th i f ki i i l h l l fo The conversion of some kinetic or potential energy to thermal energy as a result of

friction due to the movement of the fluid through the system.

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Mechanical energy balanceMechanical energy balance

For frictionless processes

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ExampleExample

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Example Cont.Example Cont.

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ExampleExample

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Example Cont.Example Cont.

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ExampleExample

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Example Cont.Example Cont.

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