Handout2 - Chapter 3 Process Variables Process to a...

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Chapter 3 – Process Variables Process: to a chemical engineer, the set of tasks or operations that accomplish a chemical or material transformation to produce a product Feed or inputs: raw materials and energy that go into a process Product or output: the desired outcome (e.g. a material) a process is used to make Process units: hardware used by a process to accomplish specific tasks – for example, a mixing tank, a heat exchanger, a reactor, an absorption column, etc. Process streams: the liquid, solid, or gas flows that move material from one process unit to another Process variables: the physical and chemical properties of process streams, such as temperature, pressure, and composition ________________________________________________________________________ Mass and Volume Density: mass per unit volume of a material (e.g. lb m /ft 3 ); symbol ρ Specific Volume: volume per unit mass (e.g. ft 3 /lb m ), equals 1/ ρ ; symbol V ˆ For solids and liquids, changes in temperature ( T ) and pressure ( p ) have a relatively small effect on density; for gases, changes in T and p cause large density changes. Solids and liquids, in this course, will be usually assumed to be incompressible, i.e. ρ = constant. Specific Gravity : ratio of density ( ρ ) of a substance to that ( ρ ref ) of a reference substance. The reference substance is often water at 4 o C, whose density is 1.000 g/cm 3 = 62.43 lb m /ft 3 . Symbol: SG. Note that SG is dimensionless. SG = ρ / ρ ref For clarity, the temperatures at which the density and reference density are evaluated need to be specified. Also, the reference substance must be given. *Example: The following data are available for a liquid: SG 1 = 0.95 o o 4 25 , SG 2 = 0.94 o o 4 100 The reference substance is water. What is the density of the liquid at 25 o C in AES units?
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*Example 3.1-2 . One kg of Hg occupies 7.36 × 10 -5 m 3 at 0 o C. Given that the volume of a mass of mercury changes according to V ( T ) = V 0 (1 + 0.18182 × 10 -3 T ( o C) + 0.0078 × 10 -6 T 2 ( o C)) what is the density of mercury at 100 o C? V 0 is the volume of mercury at 0 o C. ________________________________________________________________________ Composition Atomic weight : mass of an atom, measured on a scale on which carbon 12 ( 12 C) has a mass of exactly 12. If an atom has twice as much mass as 12 C, what is its atomic weight? Molecular weight : the sum of the atomic weights of all the atoms making up a molecule. Symbol M . *What is the molecular weight of C 6 H 6 ? (atomic weight of C = 12.01, atomic weight of H = 1.01). gram - mole (g-mole) of a substance: an amount of the substance whose mass, measured in grams , equals its molecular weight. What is the mass of 1 g-mole of C 6 H 6 ? kg-mole of a substance : an amount of the substance whose mass, measured in kg, equals its molecular weight. What is the mass of 1 kg-mole of C 6 H 6 ? lb - moles are similarly defined. Example : *How many molecules are in 1 g-mole of O 2 ? Take the molecular weight of O 2 to be 32.0. Also, O 2 has 16 protons, 16 neutrons, 16 electrons, for a total mass of about 5.32 × 10 -26 kg.
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