Most material streams in process units are mixtures of compounds. We describe the composition of the stream in various ways.
Composition fractions can be based on mass or on moles.
The units of mass measurement used don't make a difference, as long as the top and bottom of the ratio use the same units:
First, you need to find the mass of each component (given), the total mass (add them up). You'll also need to calculate the moles of each component (divide mass by molecular weight) and the total moles.
A concentration unit often seen in environmental usage is parts per million or ppm. It is the grams of solute in 1 million grams of solution. PPM (or ppb) is a special kind of mass fraction.
Often, you will be given a composition in percent or fraction form, but to solve the problem you will need to know the masses of the individual components (if only to convert to molar composition). Take care of this by assuming a basis of 1 kg, 100 mol, etc. and work from there. After all, if a mixture is 21 mole percent oxygen, it doesn't make a difference if you've got 5 g or 30 lb or 200 mol -- the percentage or fractional composition is the same.
You don't know how much (total moles or total mass) air you have -- but it doesn't make a difference. So choose a basis amount that will make the calculation easy. When compositions are given in percentages, a basis of 100 is always nice since it requires no multiplication or division. I think I'll work this in lbmoles.
BASIS: 100 moles air WRITE IT DOWN!
The Average Molecular Weight of a mixture is computed from the molar composition and the molecular weight. It is a weighted average -- the molecular weights are averaged using the mole fractions as weights.
Assume air is 79 mole % nitrogen, 21 mole % oxygen.
BASIS: 1 gmol air
You should NOT try to calculate average densities or average specific gravities using a weighted arithmetic mean. If you look at what this does, the units don't work out. You have to use a weighted harmonic mean.
A lot of times, the terms "composition" and "concentration" are used interchangeably. At this point, we want to make clear the difference. Concentration is based on volume and is one way of expressing composition. The mass concentration is the mass of a component per unit volume, similarly molar concentration is the moles per unit volume.
Assume additive volumes.
Concentration is grams solute divided by the volume of the mixture (water and solute).
In the example, I assumed additive volumes. Generally speaking, if we add 1 m3 of component A to 1 m3 of component B, we cannot be sure to get 2 m3 of the mixture. When it is true, we say that the "volumes add" or that "volume is additive".
Volumes are additive only if the mixture is "ideal". The details of ideality will be discussed in ChE thermodynamics, but for the time being we will usually assume ideal solutions. This is probably ok if all components are similar and if the temperatures and pressures are not extreme, but the assumption of ideality needs to be stated.
When concentration is calculated in terms of gmol/liter, it is called Molarity, abbreviated M.
Concentration is grams solute divided by the volume of the mixture (water and solute).
You MUST be able to switch between volumetric, mass, and molar compositions and flows quickly and without struggle. Otherwise all the problems in this class will take a lot longer than they should.
The smart engineer will usually work problems in mass or mole units, converting in and out of volume units if necessary. Trying to work problems primarily in volume units is often a source of problems.
References:
R.M. Price
Original: 6/2/94
Modified: 8/24/95, 8/14/96, 8/26/98; 5/24/2004
Copyright 1998, 2004 by R.M. Price -- All Rights Reserved