Percent (%) Evaluation Attendance and Class Participation 5% ENERGY BALANCE - SKF2123 SESSION 2006/2007 Evaluation Percent (%) Attendance and Class Participation 5% Assignments Quizzes 10% HYSYS Computer Assignments Two Major Exams 40% Final Exam 35% TOTAL 100% Chapter Topic Lectures Chapter 7 Chapter 8 Chapter 9 Laboratory Computer Class Energy and Energy Balances Balances on Nonreactive Processes Balances on Reactive Processes Aspen HYSYS – 10 5 LECTURER : DR AZEMAN MUSTAFA

Major Exam Major Exam I: 28 Feb 2013, 10:00 AM Major Exam II: 4 April 2013, 10:00 AM Final Exam: homepage

Energy & Energy Balances Dr. Shamsuzzoha, PhD ENERGY BALANCE - SKF2123 SESSION 2006/2007 ENERGY BALANCE (ChE 202) Chapter 7 Energy & Energy Balances Dr. Shamsuzzoha, PhD LECTURER : DR AZEMAN MUSTAFA

7.1-7.2 Energy & 1st Law of Thermodynamics - What is energy? - Forms of Energy - Kinetic energy (KE) - Potential energy (PE) - Internal energy (U) Total Energy , E = KE + PE + U Energy due to the translational motion of the system as a whole relative to some frame of reference Energy due to the position of the system in a potential field energy due to the motion of molecules relative to the center of mass of the system, to the rotational and vibrational motion and the electromagnetic interactions of the molecules

Change in kinetic energy: Change in potential energy Change in Internal energy Note: Δ means “change” and is always calculated as “final value minus initial value”

TEST YOURSELF What forms of energy may a system possess? In what forms may energy be transferred to and from a closed system? Kinetic, potential, internal; heat, work 2. Why is it meaningless to speak of the heat possessed by a system? Heat is only defined in terms of energy being transferred. 3. Suppose the initial energy of a system (internal + kinetic + potential) is Ej, the final energy is Ef , an amount of energy Q is transferred from the environment to the system as heat, and an amount W is transferred from the system to the environment as work. According to the first law of thermodynamics, how must Ei, Ef , Q, and W be related? E; + Q - W = Ef

How energy can be transferred between a system and its surroundings How energy can be transferred between a system and its surroundings? Heat – energy that flows as a result of temperature difference between a system and its surrounding ; heat is defined positive when it is transferred to the system from the surroundings. Work – energy that flows in response to any driving force other than a temperature difference. ; work is defined positive when it is done by the system on the surroundings.

Types of Work Flow work (Wfl) - energy carried across the boundaries of a system with the mass flowing across the boundaries (i.e. internal, kinetic & potential energy) Shaft work (Ws) - energy in transition across the boundaries of a system due to a driving force other than temperature, and not associated with mass flow (an example would be mechanical work due to a piston, pump or compressor)

ENERGY – CONVERSION UNITS 1 newton (N) = 1 kg. m/s2 1 dyne = 1 g.cm/s2 1 lbf = 32.174 lbm.ft/s2

Kinetic Energy Transported by a Flowing Stream Example 7.2-11: Water flows into a process unit through a 2-cm ID pipe at a rate of 2.00 m3/h. Calculate Ėk for this stream in joules/second. SOLUTION

Potential Energy Increase of a Flowing Fluid Example: 7.2-2: Crude oil is pumped at a rate of 15.0 kg/s from a point 220 meters below the earth's surface to a point 20 meters above ground level. Calculate the attendant rate of increase of potential energy. SOLUTION

Problems 7.1. 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 a 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. 7.2. Consider an automobile with a mass of 5500 Ibm braking to a stop from a speed of 55 miles/h. How much energy (Btu) is dissipated as heat by the friction of the braking process? (b) Suppose that throughout the United States, 300,000,000 such braking processes occur in the course of a given day. Calculate the average rate (megawatts) at which energy is being dissipated by the resulting friction.

General Balance Equation A balance on conserved quantity (i.e. mass, energy, momentum) in a process system may be written as: Input + generation - output - consumption = accumulation A system is termed open or closed according to whether or not mass crosses the system boundary during the period of time covered by the energy balance. A batch process system is, by definition, closed, and semibatch and continuous systems are open.

7.3 Energy Balance on Closed Systems How do you describe a closed system control volume? What effect does this have on the mass and energy balances?

There is no mass transfer into a closed system The only way energy can get into or out of a closed system is by heat transfer or work Heat transfer (Q): Work (Ws): Note: * Work is any boundary interaction that is not heat (mechanical, electrical, magnetic, etc.) Ws Q

First Law of Thermodynamics Energy can neither be created nor destroyed ; It can only change forms Input + generation - output - consumption = accumulation  Input - output = accumulation

In a closed system, no mass crosses the boundary, hence the input & output terms are eliminated energy can be transferred across the boundary as heat & work, hence the accumulation term may be defined as the change in total energy in the system, i.e.

Q = heat transferred to the system Ws = work done by the system

DE = DU + DPE + DKE = Q – W Note: (Summation of all heat transfer across system boundary) (Summation of all work across system boundary)

For a closed system what is DE equal to? Is it steady state ? (if yes, DE = 0) Is it adiabatic? (if yes, Q = 0) Are there moving parts, e.g. do the walls move? (if no, Ws = 0) Is the system moving? (if no, DKE = 0) Is there a change in elevation of the system? (if no, DPE = 0 ) Does temperature, phase, chemical composition change, or pressure change less than a few atmospheres ? (if no to all, DU = 0)

Example 2 A closed system of mass 5 kg undergoes a process in which there is work of magnitude 9 kJ to the system from the surroundings. The elevation of the system increases by 700 m during the process. The specific internal energy of the system decreases by 6 kJ/kg and there is no change in kinetic energy of the system. The acceleration of gravity is constant at g=9.6 m/s2. Determine the heat transfer, in kJ. heat is defined positive when it is transferred to the system from the surroundings. work is defined positive when it is done by the system on the surroundings.

Solution

7.4 Energy Balances on Open Systems How are open systems control volumes different from closed systems What effect does this have on the energy balance?

7.4 ENERGY BALANCES ON OPEN SYSTEMS AT STEADY STATE 7.4a Flow Work and Shaft Work 7.4b Specific Properties and Enthalpy 7.4c The Steady-State Open-System Energy Balance

Flow Work and Shaft Work The net rate of work done by an open system on its surroundings may be written as Ẇ= Ẇs + Ẇfl Ẇs = shaft work, or rate of work done by the process fluid on a moving part within the system (e.g., a pump rotor) Ẇfl = flow work, or rate of work done by the fluid at the system outlet minus the rate of work done on the fluid at the system inlet

heat is defined positive when it is transferred to the system from the surroundings. work is defined positive when it is done by the system on the surroundings.

To derive an expression for Ẇfl ,we initially consider the single-inlet-single-outlet system The fluid that enters the system has work done on it by the fluid just behind it at a rate If several input and output streams enter and leave the system. the products for each stream must be summed to determine Ẇfl

7.4b Specific Properties and Enthalpy The properties of a process material are either extensive (proportional to the quantity of the material) or intensive (independent of the quantity) Mass, number of moles, and volume (or mass flow rate, molar flow rate and volumetric flow rate for a continuous stream), and kinetic energy, potential energy, and internal energy (or the rates of transport of these quantities by a continuous stream) are extensive properties Intensive: temperature, pressure, and density

We will use the symbol ᶺ to denote a specific property A property that occurs in the energy balance equation for open systems is the specific enthalpy, defined as

Calculation of Enthalpy 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. Solution

TEST YOURSELF The specific internal energy of a fluid is 200 cal/g. 1. What is the internal energy of 30 g of this fluid? 2. If the fluid leaves a system at a flow rate of 5 g/min, at what rate does it transport internal energy out of the system? 3. What would you need to know to calculate the specific enthalpy of this fluid?

Example 3 1. Air at 300oC and 130 kPa flows through a horizontal 7 cm ID pipe at a velocity of 42 cm/sec Write and simplify the energy balance Calculate the rate of kinetic energy (W), if the air is heated to 400oC at constant pressure, assuming ideal gas behaviour Why would be correct to say that the rate of heat transfer to the gas equals the rate of change of kinetic energy? Why? 1 2 Air T1 =300oC P1=130 kPa V1 = 42 m/s T2 =400oC P2=130 kPa V2 = ? m/s Q 7 cm ID

Write and simplify the energy balance DH + DEk + DEp = Q - Ws Calculate Ek (W), assuming ideal gas behaviour

If the air is heated to 400oC at constant pressure what is DEk (300oC  400oC)? Why would be incorrect to say that the rate of heat transfer to the gas in part (c) must equal the rate of change of kinetic energy? Q = DH + DEk ….hence Q ≠ DEk

7.4b Specific properties and Enthalpy Total Energy of a flowing fluid (open system) The fluid possesses an additional form of energy –the flow energy (flow work) Shaft work

7.4c Energy balance on an open system at steady state Input - Output = Accumulation This work represents everything but the flow work The flow work is included in the enthalpy term

Energy Balance on Open Systems at Steady State

For an open system what is DE equal to? Is it adiabatic? (if yes, Q = 0) Are there moving parts, e.g. pump, compressor, turbine ? (if no, Ws = 0) Does the average velocity of the fluid change between the input and the output? ? (if no, DKE = 0) Is there a change in elevation of the system between the input and the output? ? (if no, DPE = 0 ) Does temperature, phase, chemical composition or pressure change? (if no to all, DH = 0)

Single Stream Steady Flow System Nozzles Diffusers Often the change in kinetic energy of the fluid is small, and the change in potential energy of the fluid is small

Nozzles and Diffusers A nozzle is a device that increases the velocity of a fluid at the expense of pressure nozzle diffuser A diffuser is a device that slows a fluid down

Usually it can be ignored Is there work in this system? Is there heat transfer? Usually it can be ignored Does the fluid change elevation? NO enthalpy is converted into kinetic energy

Recall… E is always measured relative to reference point! Reference plane for PE Reference frame for KE Reference state for Û or Ĥ (i.e. usually, but not necessarily Û or Ĥ = 0) And… Changes in E are important, not total values of E DE depends only on beginning and end states Q and W depend on process path (could get to the same end state with different combinations of Q and W)