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2.12: Untitled Page 24

  • Page ID
    18157
  • Chapter 2

    2‐11. (Adopted from Safety Health and Loss Prevention in Chemical Processes by AIChE). The level of exposure to hazardous materials for personnel of chemical plants is a very important safety concern. The Occupational Safety and Health Act (OSHA) defines as a hazardous material any substance or mixture of substances capable of producing adverse effects on the health and safety of a human being. OSHA also requires the Permissible Exposure Limit, or PEL, to be listed on the Material Safety Data Sheet (MSDS) for the particular component.

    The PEL is defined by the OSHA authority and is usually expressed in volume parts per million and abbreviated as ppm. Vinyl chloride is believed to be a human carcinogen, that is an agent which causes or promotes the initiation of cancer. The PEL for vinyl chloride in air is 1 ppm, i.e., one liter of vinyl chloride per one million liters of mixture. For a dilute mixture of a gas in air at ambient pressure and temperature, one can show that that volume fractions are equivalent to molar fractions. Compute the PEL of VC in the following units: (a) moles of VC/m3

    (b) grams of VC/m3

    (c) moles VC/mole of air

    2‐12. This problem is adopted from Safety Health and Loss Prevention in Chemical Processes by the AIChE. Trichloroethylene (TCE) has a molecular mass of 131.5 g/mol so the vapors are much more dense than air. The density of air at 25 C and 1 atm is

    3

      1.178 kg/m , while the density of TCE is

    air

    3

    . Being much denser than air, one would expect TCE to TCE

    5.37 kg/m

    descend to the floor where it would be relatively harmless. However, gases easily mix under most circumstances, and at toxic concentrations the difference in density of a toxic mixture with respect to air is negligible. Assume that the gas mixture is ideal (see Sec. 5‐1) and compute the density of a mixture of TCE and air at the following conditions:

    (a) The time‐weighted average of PEL (see Problem 2‐11) for 8 hours exposure, 100 ppm .

    (b) The 15 minute ceiling exposure, 200 ppm .

    (c) The 5 minute peak exposure, 300 ppm .

    Here ppm represents “parts per million” and in this particular case it means moles per million moles. Answer (a):

    3

     1 17842 kg m

    mix

    .

    (

    )

    Units

    35

    2‐13. A liquid has a specific gravity of 0.865. What is the density of the liquid at 20 C in the following units:

    (a) kg/m3

    (b) lbm/ft3

    (c) g/cm3

    (d) kg/L

    Section 2.3

    2‐14. In order to develop a dimensionally correct form of Eq. 2‐13, the appropriate units must be included with the numerical coefficients, 1920 and 597.

    The units associated with the first coefficient are given by Eq. 2‐15 and in this problem you are asked to find the units associated with 597.

    2‐15. In the literature you have found an empirical equation for the pressure drop in a column packed with a particular type of particle. The pressure drop is given by the dimensionally incorrect equation

     0.15

    0.85

    1.85

    H

    v

    p  4.7 

    1.2

    d

    p

    which requires the following units:

    p = pressure drop,

    2

    lb / ft

    f

     = fluid viscosity, lbm/ft s

    H = height of the column, ft

     = density, lbm/ft3

    v = superficial velocity, ft/s

    dp = effective particle diameter, ft

    Imagine that you are given data for  , H,  , v and d in SI units and you wish to p

    use it directly to calculate the pressure drop in

    2

    lb / ft . How would you change

    f

    the empirical equation for  p to obtain another empirical equation suitable for use with SI units? Note that your objective here is to replace the coefficient 4.7

    with a new coefficient. Begin by putting the equation in dimensionally correct form, i.e., find the units associated with the coefficient 4.7, and then set up the empiricism so that it can be used with SI units.

    2‐16. The ideal gas heat capacity can be expressed as a power series in terms of temperature according to

    2

    3

    C

    A A T A T A T A 4

    T

    p

    1

    2

    3

    4

    5

    36