In today’s lesson, we are going to discuss the following topics:
1. Basic Chemical Engineering Units as they relate to the mass and energy balances course, and common equations
2. Unit Conversions and Conversion Factors
Common Units and Equations
There are many different units we will run across in the chemical engineering, however in this introductory course there are some specific units we should know more than others. These are the following:
(a) Mass Flow Rate and Volumetric Flow Rate: Mass Flow rate is the mass flowing per unit time, and volumetric flow rate is the volume flowing per unit time. These units are respectively mass/time and volume/time.
(b) Density and Specific Volume: Density is the ratio of a substance’s mass to volume or mass flow rate to volumetric flow rate, and specific volume is the inverse. Units of density: mass/volume. Units of specific volume: volume/mass. Special units of density also include degrees API, degrees Baume and degrees Twaddell, but these are rarely ever used for this intro course.
(c) Specific Gravity (SG): It is the ratio of density of a substance to density of a reference substance, where the reference density is usually that of water, with values shown below. Units of SG: Unitless
(d) Gram-mole (g-mole or just mole): It is the amount of a substance that contains 6.02×1023 molecules. Units of gram-mole: moles
(e) Pound-mole (lb-mole or lbm): Is the amount of substance that contains 6.02×1023 * 453.59237 molecules. Units of lb-mole: moles
(f) Mass Fraction and Mole Fraction: Ratio of the mass (or moles) of a component in a mixture to the mass (or moles) of the total mixture. Units of mass or mole fraction: Unitless
(g) Percent by Mass (or weight percent): It is the mass fraction times 100, to convert to units of percentage.
(h) Average Molecular Weight (or Molar Mass): It is the ratio of the total mass of a mixture to the total number of moles in that mixture. Units of average molecular weight: mass/moles
(i) Concentration and Molarity: Concentration is the ratio of the mass or moles of a substance to the total volume of the mixture. Units of concentration: Moles/Volume or Mass/Volume. Molarity is just in terms of Moles/Volume, but concentration can be both.
(j) Pressure: It is the force per unit area that a fluid would exert on a surface. Units of pressure: N/m^2, mmHg, Pa, kPa, atm, psi, etc.
(k) Atmospheric or Barometric Pressure: It is the pressure within the atmosphere of the Earth. If it is not given, we usually assume 1 atm. Units of pressure: N/m^2, mmHg, Pa, kPa, atm, psi, etc.
(l) Absolute Pressure: Absolute Pressure of a fluid is the pressure relative to a perfect vacuum, when P = 0. Units of pressure: N/m^2, mmHg, Pa, kPa, atm, psi, etc.
(m) Gauge Pressure: It is the difference between absolute and atmospheric pressure. Units of pressure: N/m^2, mmHg, Pa, kPa, atm, psi, etc.
(n) Temperature: A measure of the heat in a substance. Units: Degrees Farenheit, Celsius, Kelvin or Rankine
Unit Conversions
When we are converting units, there is often a trick I use where I make a chart, like the following example where I convert 10 g/cm^3 to kg/m^3. If we look at this example below, I first write down all necessary conversion factors. These are conversions between different units that I know I will need to use to obtain my final answer, as shown below.
Use the above conversion factors to set up a conversion table. I make sure the correct units are on lined up in the chart so they cancel out, then I multiply the remaining values across the top and bottom, and we get the answer. I cover this example more in-depth in the video above.