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Chapter 3 – Process Variables Mass and Volume

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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 3 Density: mass per unit volume of a material (e.g. lb m/ft ); symbol ρ 3 Specific Volume: volume per unit mass (e.g. ft /lb m), equals 1/ ρ ; symbol Vˆ 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 (ρref ) of a reference substance. The reference substance is often water at 4 oC, whose density is 1.000 g/cm 3 = 62.43 3 lb m/ft . Symbol: SG. Note that SG is dimensionless. SG = ρ/ρref 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: 25 o 100 o The following data are available for a liquid: SG1 = 0.95 , SG 2 = 0.94 4o 4o The reference substance is water. What is the density of the liquid at 25 oC in AES units? *Example 3.1-2. One kg of Hg occupies 7.36 × 10 -5 m3 at 0 oC. Given that the volume of a mass of mercury changes according to -3 o -6 2 o V(T) = V0(1 + 0.18182 × 10 T( C) + 0.0078 × 10 T ( C)) o o what is the density of mercury at 100 C? V0 is the volume of mercury at 0 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 6H6? (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 6H6? 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 6H6? 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, O2 has 16 protons, 16 neutrons, 16 electrons, for a total mass of about 5.32 × 10 -26 kg. *How many lb-moles are in 150 g of O 2? (1 lb m = 453.59 g) Mass fraction : the fraction of total mass occupied by a component i of a mixture or solution. Symbol: usually xi or ωi. Given: 100 lb m of solution of NaCl in water. If the mass of NaCl in the solution is 5 lb m, what is its mass fraction? What is the mass percent of NaCl present? Mole (or molar) fraction : the fraction of total moles attributable to a component i of a mixture or solution. Symbol: usually yi or xi. Given : 200 g-moles of a solution that contains 20 g-moles of substance A and 180 g- moles of substance B. What are the mole fractions of A and B? What are the mole percents of A and B? n NOTE : For both mass and mole fractions, we must have ∑ xi =1 i=1 where xi is the mass or mole fraction of species i and there are n species present in the mixture. 3 Mass concentration : mass of a species per unit volume of solution (e.g. 0.3 lb m water/ft of solution). Molar concentration : number of moles of a species per unit volume of solution (e.g. 0.2 kg-mole water/m 3 of solution). Molarity is molar concentration expressed in units of g- mole solute/L of solution. The symbol M is used to indicate units of molarity (e.g. 1 M solution of NaCl in water means 1 g-mole NaCl/1 L of solution). Parts per million (ppm) and parts per billion (ppb): these units are sometimes used when the concentration of a species is low. One needs to specify whether a molar or mass concentration is intended. Ppm of a species equals its mass or mole fraction times one million (1 × 10 6); ppb of a species equals its mass or mole fraction times one billion (1 × 9 10 ). Thus, if xi is mass or mole fraction of i, 6 9 ppm i = xi × 10 ppb i = xi × 10 Example : A “solution” consists of pure benzene. What are the molar and mass ppm and ppb of benzene in the “solution”? 1 ng of KOH is present in 1 g of solution. What are the mass ppm and ppb of KOH? A gas mixture contains 1000 moles total, including 1 mole of HCl. What is the molar ppm of HCl? *Example 3.3-3. A gas mixture possesses following mass fractions of species: Mass fraction molecular weight (g/g-mol) O2 0.16 32 CO 0.04 28 CO 2 0.17 44 N2 0.63 28 What is the molar fraction of O 2? Note: the easiest way to start is by assuming a basis of calculation . Average molecular weight : The average molecular weight M of a solution is the mass of solution per mole of particles it contains. If we have a solution of n species that contains moles i of species i, the molecular weight of which is Mi, then: M = mass of solution / (moles of particles in solution) = (M1×moles 1 + M2×moles 2 + …Mn×moles n) / (moles 1 + moles 2 + …moles n) Thus: n n moles i M = ∑ M i = ∑ M i yi (1) i=1 moles total i=1 Flow Rates When materials are transported from one location to another, for example between two process units, the rate at which this transport takes place is quantified by their flow rates. A flow rate can be expressed in mass , molar , or volumetric units. As with all “rates,” time must be in the denominator. Mass flow rate : symbol m& . Example: 0.5 lb m air/s Molar flow rate : symbol n& . Example: 10 kg-moles toluene/h Volumetric flow rate : symbol V& . Example: 50 ft 3 water/min In future courses, you will also encounter fluxes of materials, which can also be in mass, molar, or volumetric units. Fluxes are flow rates per area. For example, a mass flux of 1 kg/m 2 ⋅ s means that 1 kg of material passes through an area of 1 m 2 each second. *Given : Fluid flows through a pipe of radius 1 ft. The average volumetric flux is 10 ft 3/ft 2⋅s (note that volumetric flux has units of speed). What is the volumetric flow rate of the fluid? Approximate measurement of liquid flow rates can be accomplished with a bucket and a timer - just measure how much liquid (expressed in units of mass, moles, or volume) flows into the bucket within a specified time period. Devices such as rotameters, orifice meters, turbine flow meters, ultrasonic flow meters, and others are available for more sophisticated measurement and control of liquid and gas flow rates. *Example 3.3-5. A 0.50 molar solution of sulfuric acid (H 2SO 4) in water flows into a reactor at a rate of 1.25 m 3/min. The specific gravity of the solution is 1.03 (relative to water at 4 oC). What is the total mass flow rate? 3 What is the mass concentration of H2SO 4 in the stream (in kg/m )? (MH2SO4 = 98 g/mol) What is the mass flow rate of H 2SO 4 (in kg/s)? What is the mass fraction of H 2SO 4? What is the molar flow rate of H 2SO 4 (in g-mole/s)? PRESSURE Pressure : pressure is, by definition, force per area. Common units of pressure are: N/m 2, 2 2 2 2 dynes/cm , lb f/in . N/m is otherwise known as a Pascal (Pa), and lb f/in as psi (“pounds per square inch”). In a static fluid , no part of the fluid is in motion relative to any other part of the fluid. If the only body force acting on a static fluid is that of gravity, then the pressure P at a depth h below the free surface of the fluid is equal to P = P0 + ρgh (2) where P0 is the pressure at the free surface of the fluid (i.e. at a depth of “0”), ρ is the density of the fluid, and g is gravitational acceleration. The pressure inside a static fluid is sometimes referred to as hydrostatic pressure .
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