Chapter 2. Introduction to Engineering Calculations

Chapter 3. Process Variables1. What is a chemical process?Adapted from Wikipedia:Achemical processis a method or means of somehow changing one or morechemicals.In an "engineering" sense, achemical processis a method intended to be used inmanufacturing to change the composition of chemical(s) or material(s), using technology similar or related to that used inchemical plants.Chemical process may sometimes involve only one step, but often multiple steps, referred to asunit operations. Chemical processes can be illustrated generally via process flow diagrams (to be covered in the next Chapter).Which variables characterize the operations of chemical processes?Primary Dimensions: Mass, Temperature, MoleSecondary Dimensions: Volume, Density, Pressure, Flow rate, Mass/Mol fraction, Molecular Weight, Concentration, etc.Mass and Energy Balances - Chapter II11Mass and Energy Balances - Chapter II22. Mass, Volume, Density, etc.Mass and Energy Balances - Chapter II33. Mass and Volumetric Flow RateThe rate at which a material is transported through a process line.

See Ex. 2For an incompressible fluid, e.g. liquid:Mass flow rate is constant.Volumetric flow rate is constant.Velocity will change due to changes in Cross Sectional Area.Mass and Energy Balances - Chapter II4

Ex. 2.(iv)

Animation taken from:https://www.youtube.com/watch?v=k8IRxT1sM6sLiquid water flowing through the pipe on the L.H.S.Mass flow rate is 5.0 kg/sCross sectional area is 0.250 m2 at the enterance, and 0.500 m2 at the exit.Find the volumetric flow rate and velocity at the enterance and the exitMass and Energy Balances - Chapter II54. Chemical Compositioni. Variables related to pure substances: Atomic & Molecular Weight, Mole, etc.Atomic weight (atomic mass unit amu u Da) of an element is the mass of an atom on a scale that assigns 12C a mass of exactly 12.Molecular weight of a compound is the sum of the atomic weights of the atoms that constitute a molecule of the compound.Atomic oxygen (O) has an atomic weight of 15.9994 u (6 s.f.), while molecular oxygen (O2) has a molecular weight of 31.9988 u (6 s.f.).A gram-mole (g-mole, or mol)/lb-moles of a species (atomic or molecular) is the amount of that species whose mass in grams/lbm is numerically equal to its molecular weight.Mass of 1 g-mole O2 is equal to 32 g. Mass of 1 lb-mole O2 is equal to 32 lbm.One g-mole of any species contains approximately 6.021023 (Avagadros number) molecules of that species.See Ex. 3Mass and Energy Balances - Chapter II6ii. Variables related to solutions: Mass/mol fraction, Average MW, ppm, etc.

Ex. 4Pg. 68Solution on boardMass and Energy Balances - Chapter II7

Ex. 5Pg. 68Solution on boardMass and Energy Balances - Chapter II8Concentration of a component in a solution is the amount of that component per unit volume of the solution.Mass concentration m/V ( kg/m3, g/cm3, lbm/ft3) Note that mass concentration is similar to density, but applies to solutions.Molar concentration mol/V ( kmol/m3, g-mol/cm3, lb-mol/ft3)If molar concentration is given in g-mol/L, then it is called Molarity.

Parts per Million (ppm) and Parts per Billion (pbm) are used to express the concentration of trace species, i.e. species present in very small (minute) amounts. Especially used for environmentally hazardous species.ppm (= yi106) and pbm (= yi109) may refer to mass or mol fractions, depending on how concentration is reported. Recall that it is customary to use mass fractions for solids&liquids, and mol fractions for gases. See Ex. 8

Ex. 73-3.5 pg. 52