Page 274 - Pipeline Rules of Thumb Handbook

Gas—General 261

t°F K, Btu/hrft°F If the calculation requiring diffusivities is a crucial one, we
recommend that the engineer consult the principal literature
80 0.01985 11
reference.
100 0.0208
120 0.0216
200 0.0258
Thermodynamic properties

Diffusivity The most extensive, reliable compilation of the thermody-
13
namic properties is the one by Matthews and Hurd. In addi-
The transport property, mass diffusivity (D), is of impor- tion to a pressure-enthalpy diagram, tabulations are provided
tance in calculations usually entering computations in combi- for saturated methane from -280°F to -115.8°F, the critical
nation with viscosity and density as the Schmidt No., m/r D. temperature, and for superheated methane to 500°F and
Reliable estimates of D 12 , the diffusivity of species 1 through 1,500psia. This tabulated information is reproduced in the
species 2 are best made by the method of Hirschfelder, Bird, Chemical Engineers’ Handbook 3rd Ed. 14 In the 4th Ed.
and Spotz. 11 (1963) superheat tables are omitted.
Corresponding to the pressure-temperature pair, values are
32
0 001858 T [ ( M 1 + M 2 ) M M 2] 12 given for:
.
1
D 12 = 2 (4)
Ps 12 W D 13
a. Ratio fugacity (where fugacity refers to “escaping ten-
dency”) to pressure
2
where D = diffusivity, cm /sec b. Specific volume of liquid and vapor
T = temperature, °K c. Enthalpy of liquid, vaporization, and vapor
M = molecular weight, M 1 of species 1, M 2 of d. Entropy of liquid and vapor
species 2
P = pressure, atm
A few numbers of immediate interest to the process engi-
s 12 = (s 1 + s 2 )/2 = collision diameter, A°
neer are:
W D = collision integral = function of KT/Π12
K = Boltzmann’s constant
a. Latent heat of vaporization at normal boiling point =
Π= a force constant
Œ 12 = ŒŒ 2 219.22Btu/lb
1
b. Liquid density at normal boiling point = 26.6lb/ft 3
c. Vapor density at normal boiling point = 0.113lb/ft 3
Force constants and collision diameters for air and methane
are:
The liquid density is required to compute the volume of a
diked area to contain a spill. Latent heat is useful in estimat-
Œ/K, °K s, A°
ing the time required for a spill to be vaporized. The vapor
Air 97 3.617 density points up the fact that, although methane has a spe-
Methane 136.5 3.822 cific gravity referred to air of about 0.55, vapors generated at
a spill are about 1.5 times as heavy as ambient air; a matter
These are known as the Lennard-Jones force constants and of importance in various studies.
an extensive table is given. 11 W D , which depends upon the
12
temperature through KT/e, is tabulated in the reference. A
few values are: Heat capacities

KT/ΠW D If heat capacities are required for process calculations, they
can be extracted from the thermodynamic tables mentioned
0.30 2.662
above by the following procedure:
0.50 2.066
1.00 1.439
1.50 1.198 H -
2.00 1.075 C p = 1 H 2 , Btu lb ∞F (5)
T -
4.00 0.8836 1 T 2
10 0.7424
50 0.5756
100 0.5130 Example. Find the heat capacity for superheated methane
400 0.4170 at 15psia and 100°F.