CHE111P - 3 - C HE111P Chemical Engineering Calculations [...

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CHE111P Chemical Engineering Calculations [ CHAPTER 3: GAS BALANCES ] Chapter 3: Gas Balances I. Ideal Systems: Gases Ideal pure gases For gases, the  ideality  assumption is that there are no interactions at all  between the molecules.  This sounds pretty poor in light of the interactions  that we talked about on the previous page when dealing with liquids and  solids.   However,   remember   that   the   density   of   gases   is   very,   very   low  compared to that of liquids and solids. Thus, the molecules are much further  separated in gases. Interactions between molecules are fairly short-ranged,  and so the density of the gas (hence the average separation between the  molecules)   makes   a   big   difference   in   whether   or   not   the   molecular  interactions affect the properties of the fluid. Experiment indicates that at higher densities, the product of the pressure,  P and   the   molar   volume,  v ,   is   different   for   different   gases.   However,   at  sufficiently   low   densities,  Pv  becomes   identical   for  all  gases   at   the   same  temperature  (see   sketch) .   Because   the   product   is   proportional   to   the  temperature, so in the limit of low pressures we can write Pv = RT      or      PV = nRT We  use  the  lower case  v  to represent the  molar  volume   and  the  upper  case  V  to represent the total volume.   Thus  v  =  V/n .   This is the ideal gas  equation of state.   As mentioned, it is an idealization of the case where  molecules do not interact.  While it is strictly true only at very low densities, it  is often used as an approximation at moderate gas densities.  When the ideal  gas   equation   is   applicable   depends   upon   the   fluid.   It   is   not   a   good  approximation   anywhere   near   the   temperature   at   which   the   gas   would  condense   or   near   its   critical   point.   It   can   often   be   used   as   a   good  approximation for compounds that boil well below room temperature if the  pressure is below 3 atm or 4 atm.  We will see on a later page how to use  equations of state that are more accurate for other fluids. The ideal gas constant has the following values in various units: R  = ideal gas constant = 8.314 J/mol*K = 1.987 cal/mol*K = 10.l73  psia*ft 3 /lbmol*R = 8.314 m 3 Pa/mol*K = 82.06 cm 3 atm/mol*K It is generally wise to have all of these values memorized. The student should be familiar with the ideal gas equation from chemistry 
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This note was uploaded on 04/27/2011 for the course CHM 111P taught by Professor Marquez during the Spring '11 term at Mapúa Institute of Technology.

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CHE111P - 3 - C HE111P Chemical Engineering Calculations [...

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