PRESENTATION NAME

Chemical Engineering & Basic Principles and Calculations CHAPTER 3

OutcomeAt the end of this chapter, you are be able to:Apply the basic principles of chemical engineering in mass balance

2Chapter ContentsUnit and dimensionSystem and unitsConversion of unitsChoosing a basisThe concept of material balanceOpen and closed systemSteady state and unsteady stateIntroduction to reactive and non reactive system9. Introduction to recycle, by-pass and purge34Units and DimensionEvery measured or quantity has a numerical value and a unit. What is numerical value?? It is just a number .any number (e.g.1, 500, 0.8, .. )What is Unit?? The means of expressing the dimensions (meter, kilogram, minutes)What is Dimension? Basic concept of measurement or property that can be measured (e.g. length, temperature, mass, time) or calculated by multiplying or dividing other dimensions such as velocity (length/ time), volume (lentgh3) or density (mass/ lentgh3).Measurable unit : Specific values of dimensions that have been defined by convention, custom or law e.g. gram (mass), seconds (time) & cm (length). Units can be treated like algebraic variables when qty are added, subtracted, multiplied or divided. 5 i. Numerical values of 2 qty may be added or subtracted (if the units are same).3 cm 1 cm = 2 cm(3x x = 2x) ii. Numerical value and their corresponding units also may always be combined or division. 3 N x 4 m = 12 Nm 5 km/ 2 hr = 2.5 km/h 7 m x 8 m = 56 m2 6 g/ 3 g = 2 (dimensionless)

1.2 UNITS (S.I & AMERICAN UNITS)1.2.1 System of UnitA system of units has 3 components: i. Base units : mass, length, time, temperature, electrical current and light intensity. ii. Multiple unit : multiples or fractions of based units

6 iii. Derived units : a. By multiplying and dividing base or multiple units. Referred as compound units. (cm2, ft/min, kg.m/s2) b. As defined equivalents of compound unit (e.g., 1 W = 1.341 x 10-3 hp, 1 Ibf = 32.174 Ibm.ft/s2)SI units (Systme Internatinale dUnit) widespread acceptance in scientific and engineering community (e.g., meter (m) for length, kilogram (kg) for mass, kelvin (K) for temperature.American Engineering System (e.g., feet (ft) for length, pound-mass (Ibm) for mass.

Units and Dimensions7

Prefixes and suffixesIt is often convenient to use units which are a multiple or fraction of the basic unit.The metric system, formally known as the International System of Units, defines a number of prefixes to denote powers of ten.8

Conversion of UnitsConversion of UnitsThe same unit can be converted to another unit. Example :ft/smile/hr cm/yrIt can be done using conversion factor.Conversion factor (c.f) : a ratio of equivalent values of a qty expressed in different units.1 m/ 100 cm(1 meter per 100 centimeters)100 cm/ 1 m(100 centimeters per 1 meter)

977 mg1 g1000 mg10Example: To convert 77 mg to its equivalent in grams = (77 mg) x 1 g = 0.077 g 1000 mg

Alternative way to write this equation instead of the multiplication symbol use a vertical line : = = 0.077 g

If qty having a compound unit [e.g., miles/h, cal/g.(C)] convert to its equivalent in terms of another set of units set up a dimensional equation : 1. Write the given qty & its units (old unit) on the left side 2. Write the unit of c.f (new unit/old unit) to cancel out the old unit 3. Replace the c.f until desired unit 4. Fill the value of the c.f. 5. Do the arithmetic.1 cm36002 s2242 h23652 day21 m1 kms212 h212 day212 yr2102 cm103 m11Example : Convert an acceleration of 1 cm/s2 to its equivalent in km/yr2

=

= (3600 x 24 x 365)2 km 102 x 103 yr2 = 9.95 x 109 km/yr2

12

Numerical Calculation and EstimationScientific notationBoth very large and very small numbers are commonly encountered in process calculations.A convenient way to represent such numbers is to use scientific notation. Examples: 123,000,000 = 1.23 x 108 (or 0.123 x 109)

13Scientific figuresA number are the digits from the first nonzero digit on the left to either: (a) the last digit on the right if there is a decimal point. (b) the last nonzero digit of the number if there is no decimal pointExample: 2300 or 2.3 x 103 (2 significant figures)2300. or 2.300 x 103 (4 significant figures)2300.0 or 2.3000 x 103 (5 significant figures)0.035 or 3.5 x 10-2 (2 significant figures)0.003500 or 3.500 x 10-2 (4 significant figures)

1415PRESSURE Atmospheric Pressure, Absolute Pressure & Gauge PressureAir is a fluid the air above the earth exerts a Phydrostatic on the surface Patmospheric The P exerted will depend on the height of the column and the air. Typical value of the Patmospheric at sea level are : 1 atm = 14.696 psi = 760 mmHg = 101.325 kPa

It will decrease as the altitude increases. For many calculations, it is useful to have a fixed reference value for Patmospheric sea-level value (standard atmosphere). This value is also used as a unit of pressure measurement (atm). Absolute vs Gauge The Pfluid referred to absolute pressures P of zero corresponds to a perfect vacuum. Many P measuring devices give the Pgauge of a fluid, or the P relative to Patmospheric. A Pgauge = 0 indicates Pabsolute of the fluid = Patmospheric.

16The relationship for converting btw Pabsolute & Pgauge Pabsolute = Pgauge + Patmospheric

The abbreviation for Pabsolute & Pgauge psia & psig are commonly used in lbf/in2 unit.

17Process: any operation or series of operations.The material enters a process Input or FeedThe material which leaves the process Output or Product

Law of conservation of mass: mass can neither can created nor destroyedIn other words: total mass of input = total mass of outputThis is called Mass/material balances18PROCESSFEEDINPUTS PRODUCTOUTPUTThe concept of Material BalanceChoosing a basisPrior to do material balance calculations,basis of calculation has to be chosen. The basis is a reference you choose for the calculations you plan to make. Your basis is a fixed number all other values are determined relative to it. Basis: an amount of (mass/moles) or flowrate (mass/molar) of one stream or stream component in a process. Your basis can be a given mass of material or a flow rate. When a weight analysis of solids or liquids are given, it is often most convenient to choose 1 or 100 (e.g. kg, g, Ibm) as your basis and for gases, 1 or 100 (e.g. kmol, gmol, Ibm-mol) is useful.19Choosing a basisThe most question that you may ask:What is the most convenient basis to use?If a stream amount or flowrate is given in a problem statement, it is usually most convenient to use this quantity as a basis of calculations.There are 2 types of basis : a. A known basis : based on a process which the amount is given. b. Unknown basis : when all the process stream are unknown ASSUME the basis of calculation. Choose the amount or flowrate base on the composition given:

20Choosing a basis i. If x (mass fraction) are know total mass or mass flow rate ii. If y (mole fraction) are know total mol or molar flow rate

Before solve the problems, start the solution with state your basis.21Mass & Mole Fractions

Mass Process flow usually is more than 1 substance. Mass fraction :

x A = mass A kg A or g A or Ibm A tot mass tot kg tot g tot Ibm2223 Mole fraction :

y A = mole A kmol A or mol A or Ib-mole A tot mole tot kmol tot mol tot Ib-mole Mass & mole percent A or chemical composition of A : Mass % (A) = x A x 100, Mole% (A) = y A x 100

Note : numerical value of x and y not depend on the mass unit in the numerator or denominator as long as the unit same. For example, if the x of Benzene (C6H6), in a mixture is 0.25, then xC6H6 = 0.25 C6H6 kg/ kg tot, 0.25 Ibm C6H6 / Ibm tot etc.

Mass & Mole Fractions A set of x can be converted to an equivalent set of y by assuming as a basis of calculation a mass of mixture (e.g., 100 kg or Ibm) and vice versa with a tot number of moles also can be taken as basis of calculation (e.g., 100 mole or Ib-moles).24MATERIAL BALANCE CALCULATIONSolving the equation is usually a matter of simple algebra. However, deriving them from a description of a process and a collection of process data may be difficulties.This section outlines the procedure for reducing description of a process to a set of equation that can be solved for unknown variables.

FlowchartsOne ways for organizing information about a process to sketch a flowchart of the process.Use boxes to symbolize unit processes or process units (reactors, mixers, separation units, etc) & directed lines (arrows) to represent material or information flows (inputs and outputs).

17Steps of constructing and labeling the flowchart: 1. Label the flow diagram with proper symbols. 2. Rates or amount of process streams are normally written on the arrows. 3. Composition of process streams are written beneath the arrow. 4. Unknowns rates, amount and compositions must be labeled by a certain symbols. 5. All streams must be use the same unit. 6. Stream flows correspond to composition.

Example :

450 kg/h waterP kg/h product sugarx kg dry sugar/ kg product sugar(1- x) kg water/ kg product sugar1000 kg/h wet sugar0.5 kg dry sugar/ kg wet sugar0.5 kg water/ kg wet sugarDRYER1826Open & Closed SystemThe system : any process or portion of a process chosen by the engineer for analysis. Open system: material flows across the system boundary during the time interval.

Closed system: no material flows in or out.

27PROCESSFEEDINPUTS PRODUCTOUTPUTPROCESSSteady & Unsteady StateProcess can operate at;steady state (static models) the values of all the variables in a process (T, P, V, flow rates) do not change with time (except possibly for minor fluctuations about constant mean values). In other words: process conditions remain constant with time. Continuous process is one of in which material enters and/or leaves the system with interruption.

28Steady & Unsteady StateTransient/ unsteady state (dynamic models) the values of process variables change with time. Since models are functions of time, so the system can track fluctuations and changes in behavior.Batch processes require transient operations (it never reach a steady state). Continuous processes may be either steady-state or transient

29A balance on a conserved qty (tot mass, mass of a particular species, energy, momentum) in a system (a single process unit, a collection of units or & entire process) written as :

Input + Generation Output Consumption = Accumulation

Accumulation (acc) + (material is increasing), (material decreasing) or 0 (steady state). For steady state; acc = 0, thus :

Input Output = Generation (gen) Consumption (con)

13The gen and con terms consequence of chemical rxn. For non- reactive species (reactant or product) or no chemical rxn gen & con = 0. Thus, the equation :

Input = Output

Note : The total mass of a system & elements (or atoms) is conserved but the individual species are not conserved.

14Balances on Continuous Steady State Processes

For continuous processes at steady state, the acc within system = 0, so the equation :

Input + Generation = Output + Consumption

If the balance is on a non-reactive species or on total mass, the gen & con terms = 0, so the equation reduces to:

Input Flow Rates = Output Flow Rates

15Balances on Batch ProcessesIn a batch process:

Initial Input + Generation = Final Output + Consumption

The steady state process has no acc, thus batch process can be assumed operate at steady state if all inputs and outputs come in and come out all at once. Thus there are no acc of mass at the end of process.

Amount of Inputs = Amount of Outputs

16Introduction to reactive & non-reactive systemProcess : any operation, treatment, or series of operations, treatments that cause the raw materials to undergo physical or chemical changes into finished products. Physical Change Change in physical attribute of material especially physical state of the materials (phases). This changes result to either weakening or strengthening of intermolecular bonds that are responsible to hold discrete molecules together. The strength of these intermolecular bonds determines the physical state of materials. The molecules retain their original chemical structure and the chemical properties.

34Example process: drying, crystallization, screening, filtration

Chemical Change material undergoes chemical change interatomic chemical bonds that hold atoms in molecules of the materials are broken and then structured to form new interatomic bonds. These new interatomic bonds hold atoms new molecules are totally different, physically and chemically.Example process: combustion of methane in air, oxidation of methylbenzene

35RECYCLE, BYPASS & PURGING36RECYCLE & BYPASSOne of the more common processing configuration is the material recycle structure.A recycle stream is a part of an output process stream that has been spitted and returned to mix with an input process stream.

37Recycle stream Fresh feed Feed Output Final productPROCESS UNITRECYCLERecycle allow better control of reactor selectivity when multiple reactions occur. In order to study recycle systems recycle ratio have to be calculated dividing mass flow of the recycle stream by the mass flow of the "fresh feed" entering the system.In the industrial, recycle ratios have important consequences for system performance & operating costs.

38RECYCLEA common recycle structure is the reactor/separator

In figure above, separator is used to recover unreacted material & return it to the reactor. The "separator" may be a single piece of equipment or it may be an entire process on its own.

39REACTORSEPARATORFresh feed ProductRECYCLEThe usage of recycle: a. To recover and reuse unconsumed reactants that come and out n the output stream. b. To recovery and reuse catalysts. c. To dilute the process streams. d. To control the process variables. e. To circulation the working fluids.

40BYPASSA related process structure is the bypass system. A bypass stream is a part of a feed stream to a process unit that has been diverted around the unit and then combined with an output stream.One use of bypass is to obtain precise control of the output Stream, as when a small wet air stream bypasses a drier so that the output humidity can be regulated.

41Bypass stream Fresh feed Feed Output Final productPROCESS UNITPURGINGRecycle can results in build up of unwanted species especially inert.Purge stream are used to release a small portion of the recycle stream. Therefore, unwanted species have a path for release.

42ReactorSeparatorm1m3m2PURGINGPurge stream: a stream bled off from the process to remove an accumulation of inert/unwanted material that might otherwise build up in the recycle stream. 43