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CHE 205 – Chemical Process Principles Section 3: EPCP, Chapter 5

EQUATIONS OF STATE FOR GASES

Questions   A gas enters a reactor at a rate of 255 SCMH. What does that mean? An orifice meter mounted in a process gas line indicates a flow rate of 24 ft3/min. The gas temperature is 195oF and the pressure is 62 psig. The gas is a mixture containing 70 mole% Co and the balance H2. What is the mass flow rate of the hydrogen in the gas? A reactor feed stream consists of O2 flowing at 32 kg/s. The gas is to be compressed from 37oC and 2.8 atm absolute to 54oC and 284 atm. What are the volumetric flow rates at the inlet and outlet (needed to rate the compressor)? A pitot tube indicates that the velocity of a stack gas is 5.0 m/s at 175oC. The stack diameter is 4.0 m. A continuous stack analyzer indicates an SO2 level of 2500 ppm (2500 moles SO2/106 moles gas). At what rate in kg/s is SO2 being discharged into the atmosphere? A 70.0 m3 tank is rated at 2000 kPa. If 150 kg of helium is charged into the tank, what will the pressure be? How much more helium can be added before the rated pressure is attained?

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Answers: Need an equation of state: relationship between temperature (T), pressure (P), volume (V), and number of moles (n) of a gas.  In Chapter 4, streams on flow charts labeled like this:

100 mol/s 0.600 mol A/mol 0.400 mol B/mol

In this chapter, stream data just as likely to look like this:

250 L/s @ 37oC, 800 mm Hg pA = 420 mm Hg (partial pressure of A)

For material balances, however, we still need moles and mole fractions. The job now becomes one of converting volumetric flow rates (or volumes) to molar flow rates (or moles), and (for gases) partial pressures to mole fractions. (Latter is easy: yA = pA/P )  Convert volumes to moles – Solids & liquids: use tabulated densities (volume to mass) & molecular weights (mass to moles). Mixtures—either look up mixture density data or assume volume additivity & calculate density from Eq. (5.1-1). Gases, can’t...