Chapter 3. Process Variables 1. What is a chemical process? Adapted from Wikipedia:  A chemical process is a method or means of somehow changing one or more chemicals.  In an "engineering" sense, a chemical process is a method intended to be used in manufacturing to change the composition of chemical(s) or material(s), using technology similar or related to that used in chemical plants.  Chemical process may sometimes involve only one step, but often multiple steps, referred to as unit 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: Primary Dimensions: Mass, Temperature, Mole Secondary Dimensions: Secondary Dimensions: Volume, Density, Pressure, Flow rate, Mass/Mol fraction, Molecular Weight, Concentration, etc. Mass and Energy Balances - Chapter II1

2 2. Mass, Volume, Density, etc.

Mass and Energy Balances - Chapter II3 3. Mass and Volumetric Flow Rate The rate at which a material is transported through a process line. See Ex. 2 For 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: Liquid water flowing through the pipe on the L.H.S. Mass flow rate is 5.0 kg/s Cross sectional area is m 2 at the enterance, and m 2 at the exit. Find the volumetric flow rate and velocity at the enterance and the exit

Mass and Energy Balances - Chapter II5 4. Chemical Composition i. 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 12 C 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 u (6 s.f.), while molecular oxygen (O 2 ) has a molecular weight of 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/lb m is numerically equal to its molecular weight. Mass of 1 g-mole O 2 is equal to 32 g. Mass of 1 lb-mole O 2 is equal to 32 lb m. One g-mole of any species contains approximately 6.02  (Avagadro’s number) molecules of that species. See Ex. 3

Mass and Energy Balances - Chapter II6 ii. Variables related to solutions: Mass/mol fraction, Average MW, ppm, etc. Ex. 4 Pg. 68 Solution on board

Mass and Energy Balances - Chapter II7 Ex. 5 Pg. 68 Solution on board

Mass and Energy Balances - Chapter II8 Concentration of a component in a solution is the amount of that component per unit volume of the solution. Mass concentration  m/V (  kg/m 3, g/cm 3, lb m /ft 3 ) Note that mass concentration is similar to density, but applies to solutions. Molar concentration  mol/V (  kmol/m 3, g-mol/cm 3, lb-mol/ft 3 ) 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. See Ex. 8 ppm (= y i  10 6 ) and pbm (= y i  10 9 ) 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 pg. 52