Page 394 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

362 Applied Process Design for Chemical and Petrochemical Plants

Example 6-4: Steam Air Mixture Temperature in Ejector 3. The 70°F air equivalent correcting for temperature
is found as previously described, using the air curve
Steam used to draw air out of a vessel is: of Figure 6-17.
240 psig (255 psia) @ 440°F total temp
This is: 440 - 402 sat temp = 38 superheat Example 6-5: Actual Capacity For Process Vapor Plus
Temp of air from vessel: 75°F N on-Condensable
Steam flow: 475 lbs/hr
Air flow: 175 lbs/hr A distillation system is to operate with a horizontal over-
Ejector suction pressure: 1.5 in Hg abs head condenser, Figure 6-19, and pressures are as marked.
Then: Enthalpy of steam = 1226.5 Btu/lb (from super- The estimated air leakage into the system is 7 lbs/hr. The
heat vapor tables @ 255 psia) molecular weight of the product vapor going out the con-
Corresponding steam temperature at 1.5 in. Hg abs denser into the ejector (at S0°F) is 53. The vapor pressure
and enthalpy of 1226.5 = 366°F (interpolation on super- of the condensing vapors is 3 mm Hg abs at 80°F.
heated steam tables at 1.5 in. Hg abs) Partial pressure air= 5 - 3 = 2 mm Hg (See Figure 6-19)
Vapor required to saturate at 80°F and 5 mm abs total

t = (475) (0.45) (366) + (175) (0.24) (.75) pressure.
rn ( 475) (0.45) + (175) (0.24)
= 318° F
3
w 7 53 ) ( ) = 19. 2 lbs I hr (6- 2)
(
Reference [11] provides a complete procedure for test- \' (29) (2)
ing ejector units in vacuum service, and the charts and
calculation procedures for the tests. Average molecular weight of mixture:
Another design approach for calculating saturated gas
loads for vacuum systems is given in Reference [28].
Air 7 lbs/hr = 0.241 mols/hr
Process vapor 19.2 = 0.362
--mos
Total Weight of a Saturated Mixture of Two Vapors: One Total vapor 26.2 lbs/hr = 0.603 1 /h r
Being Condensable
Often when the non-condensable quantity is known or
estimated, it is important to state whether these gases are
in the presence of water or other process liquid. In this
case, the amount of condensable vapor above the liquid
must be considered as it also will enter the ejector suction.
Steam

(6-2)

Second Stag�
where �.!.£!2!
n refers to the non-condensable component
v refers to the condensable vapor.
Distillation
Non-Condensables Plus Process Vapor Mixture Column

Many process systems fall in this group. They are han- 34
Feet
dled in a similar manner to the other systems, correcting �Column Bottoms Seal
for temperature and molecular weight. Leo
To Sume.
l. Calculate the average molecular of the mixture. or After-
2. The air equivalent is determined from Figure 6-18 6= Absolute Pressure, mm. Hg. Abs. Condenser
O= Temperature , °F
using the average molecular weight. Note :Some Manufacturers might Recommend
this as a Three Stage Sy&tem.
lbs/hr of mixture
Air Equivalenl =
Ratio, from Fig. 6- 18
(uncorrected for temperature) Figure 6-19. Vacuum system for distillation.