Page 569 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Applied Process Design 535

that could aggravate the flammability problem of flam- P O = beginning pressure in vessel, 14. 7 psia
mability limits. Also see [75]. P1_1 = high pressure of the purge nitrogen
Yo= (0.21) [14.7/(70 + 14.7)] = 0.03644
Pressure Purgi.ng
The final oxygen concentration,
The inert gas is added under pressure to the system lo be
purged. This is then vented or purged to the atmosphere, Yr is to be 1 ppm (1 o- lb mol/total mols)
6
usually more than one cycle of pressurization followed by
venting is necessary to drop the concentration of a specific (7-98)
flammable or toxic component to a pre-established level.
To determine the number of purge cycles and achieve 10-· = 0.21 [14..7/(70 + 14.7)]j
6
a specified component concentration after y purge
cycles of pressure ( or vacuum) and relief [29]: Solving by taking logarithms:

(7-96)

Repeat the process as required to decrease the oxidant In [10- /0.21] = j In [14.7/84.7]
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concentration to the desired level.
j = 6.99 cycles
where PH = initial high pressures, mmHg
P1. = initial low pressure or vacuum, mmHG Use seven minimum, perhaps use eight, for assurance
y0 = initial concentration of component (oxidant) that purging is complete. Note that the above relation-
under low pressure, mol fraction ships hold for vacuum purging. Keep in mind the rela-
nH = number of mols at pressure condition tionships between high and low pressure of the system
n1. = number of mols at atmospheric pressure or low and use mmHg for pressure if more convenient. For
pressure conditions
j = number of purge cycles (pressuring and relief) sweep-through purging, see Ref. [29].
Total mols nitrogen required [29]:
Note: The above equation assumes pressure limits PH
and PL are identical for each cycle and the total mols of (7-99)
nitrogen added for each cycle is constant [29].
= 7.0 (84.7 - 14.7) [(800/7.48)/10.3 (80 + 460)]
Example 7-19. Purge Vessel by Pressurization following = 8.98 mols nitrogen
the method of Ref. [29].
lb nitrogen = 8.98 (28) = 125. 72 lbs
A process vessel of 800 gallons capacity is to have the
V = 800 gal volume
oxygen content reduced from 21 % oxygen (air). The sys- Rg. = 10.73 psi cu ft/lb mo! 0
R
tem before process startup is at ambient conditions of T = nitrogen temperature, 8Q°F + 460 = 54Q°R
14.7 psia and 80°F. Determine the number of purges to
reduce the oxygen content to 1 ppm (10- lb mol) using Static Electricity
6
purchased nitrogen and used at 70 psig and 80°F LO pro-
tect the strength of the vessel. How much nitrogen would Static electrical charges cause major damage in chemi-
be required? cal and refining plants, yet they are not often recognized
Using Equation 7-96:
in the planning and design details of many plant areas.
One of the least commonly recognized situations is that
y0 = initial mo! fraction of oxygen. This is now the concentra-
tion of oxygen al end of the first pressuring cycle (not venting dusts generated in plant operations can be ignited to
or purging). explosive violence by static electrical charges built up on
the small particles. Of course, there are other dangers of
At high pressure pressurization: explosions/fires being ignited by static discharges involv-
ing flammable vapor and mists and liquid particles (larg-
y 0 = 21 lb mols oxygen/100 total mols in vessel (initial) er than mists). (Also see Pratt [86].)
Although static discharges are small electrical phe-
y 0 = (0.21) (P 0 /Pu), composition for the high pressure nomena, they are significantly different from a high volt-
condition (7-97) age electrical discharge to ground from a power system or