UNIT OPERASI BIOPROSES (UOB)

TPE4211

YUSRON SUGIARTO KULIAH 4

MATERI KULIAH No Pokok Bahasan Sub Pokok Bahasan Waktu

(Jam) 1. Pengantar 2. Satuan dimensi Sistem satuan, dimensi

dan konversi 2 x 50

3 Kesetimbangan Massa 2 x 50

4. Kesetimbangan Energi 2 x 50 5. Kesetimbangan Unsteady-State

Massa dan Energi 2 x 50

6. Dasar-dasar perpindahan momentum

Viskositas dan macam-macam fluida, fluida statis, aliran fluida, tipe aliran dan faktor gesekan

2 x 50

7. Lanjutan Pengukuran aliran fluida, kebutuhan tenaga untuk aliran, persamaan Bernoilli dan penerapannya

2 x 50

ENERGY BALANCE

• Energy is expensive…. – Effective use of energy is important task for

bioprocess engineers.

• Topics of this chapter – Energy balance

– Energy and energy transfer • Forms of energy : Kinetic / Potential / Internal Energy

• Energy transfer : Heat and Work

– Using tables of thermodynamic data

– Mechanical energy balances

Unlike many chemical processes, bioprocesses are not particularly energy intensive. Fermenters and enzyme reactors are operated at temperatures and pressures close to ambient; energy input for downstream processing is minimized to avoid damaging heat-labile products. Nevertheless, energy effects are important because biological catalysts are very sensitive to heat and changes in temperature. In large-scale processes, heat released during reaction can cause cell death or denaturation of enzymes if it is not quickly removed. For rational design of temperature-control facilities, energy flows in the system must be determined using energy balances. Energy effects are also important in other areas of bioprocessing such as steam sterilisation.

INTRODUCTION

The law of conservation of energy means that an energy accounting system can be set up to determine the amount of steam or cooling water required to maintain optimum process temperatures. In this chapter, after the necessary thermodynamic concepts are explained, an energy-conservation equation applicable to biological processes is derived.

INTRODUCTION

SYSTEM

SYSTEM

Forms of Energy – The First Law of Thermodynamics

A balance on conserved quantity (i.e. mass, energy, momentum) in a process system may be written as:

Input + generation - output - consumption =accumulation

Forms of Energy – The First Law of Thermodynamics

• Forms of energy – Kinetic energy : due to the motion of the system

– Potential energy : due to the position of the system

– Internal energy : due to the motion of internal molecules

• U : from thermodynamic calculation

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2

2

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Forms of Energy – The First Law of Thermodynamics

Change in kinetic energy:

Change in potential energy

Change in potential energy

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.

Forms of Energy – The First Law of Thermodynamics

Forms of Energy – The First Law of Thermodynamics

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

Energy balance on closed systems 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

Q

Ws

a. Heat transfer (Q) b. Work (Ws) Note: * Work is any boundary interaction that is not heat (mechanical, electrical, magnetic, etc.)

1

The law of conservation of energy can be written as:

energy in through system boundaries

energy out through system boundaries

energy accumulated within the system - =

• Balance equation

(Final System Energy) – (Initial System Energy)

= (Net Energy Transfer)

• Initial System Energy

• Final System Energy

• Net Energy Transfer

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WQ

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The first law of thermodynamics for closed systems

Energy balance on closed systems 1

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For a closed system what is ∆E equal to?

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, ∆KE = 0) Is there a change in elevation of the system? (if no, ∆PE = 0 ) Does temperature, phase, chemical composition change, or

pressure change less than a few atmospheres ? (if no to all, ∆U = 0)

Energy balance on closed systems 1

Energy Balance Procedures • Solve material balance Get all the

flow rate of streams

• Determine the specific enthalpies of each stream components – Using tabulated data

– Calculation

• Construct energy balance equation and solve it.

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Exercise

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 700m 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=9.6m/s2. Determine the heat transfer, in kJ!

Energy balance on open systems 2

How are open systems control volumes different from closed systems

What effect does this have on the energy balance?

Some common open system steady flow devices

Some common open system steady flow devices

Energy balance on open systems

*Steady State • Flow work and shaft work

– Flow work : work done on system by the fluid itself at the inlet and the outlet

– Shaft work : work done on the system by a moving part within the system

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Process Unit )/(

)/(

2

3

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in

in

)/(

)/(

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out

out

outoutininf VPVPW

2

Application of First Law - Conservation of energy for a control volume

The Steady-State Open System Energy Balance

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outoutoutinininf VPmVPmW ˆˆ

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K

UmU ˆVPUH ˆˆˆ

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Air at 300oC and 130 kPa flows through a horizontal 7 cm ID pipe at a velocity of 42 m/s

a) Write and simplify the energy balance b) Calculate the rate of kinetic energy, if the air is heated to 400oC at constant pressure, assuming ideal gas behaviour c) Why would be correct to say that the rate of heat transfer to the gas equals the rate of change of kinetic energy? Why?

Exercise

Enthalpy • It is convenient to define the following property

for the calculation of energy balance for flowing systems. – Enthalpy

– Specific Enthalpy PVUH

VPUH ˆˆˆ

What is the enthalpy of 150 g formic acid at 70oC and 1 atm relative to 25oC and 1 atm?

Exercise

Tables of Thermodynamic Data • U, H, S, V,… Thermodynamic function

• Tables of Thermodynamic Data – Tabulation of values of thermodynamic functions (U, H,

V,..) at various condition (T and P)

– It is impossible to know the absolute values of U , H for process materials Only changes are important ( U, H,…)

– Reference state • Choose a T and P as a reference state and measure changes of U

and H from this reference state tabulation

Steam Tables • Compilation of physical properties of water

(i) Steady-stateflow process. At steady state, all properties of the system are invariant. Therefore, there can be no accumulation or change in the energy of the system: AE = 0

(ii) Adiabatic process. A process in which no heat is transferred to or from the system is termed adiabatic; if the system has an adiabatic wall it cannot receive or release heat to the surroundings. Under these conditions Q- 0 and becomes:

Water at 25oC enters an open heating tank at a rate of 10 kg h-1. Liquid water leaves the tank at 88 oC at a rate of 9 kg h-1; 1 kg h-1 water vapor is lost from the system through evaporation. At steady state, what is the rate of heat input to the system?

Exercise

Exercise 1. Assemble (i) Select units for the problem. kg, h, kJ, oC (ii) Draw the flow sheet showing all data and units. (iii) Define the system boundary by drawing on the flowsheet.

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