Step 2. Calculate the “equivalent dry air load” to the first ejector stage. Using the
equation
W a ¼
W i
Rm i  Rt i
þ
W s
Rm s  Rt s
(13)
where
W a = Equivalent air flow in lb/h.
W i = Actual lb/h of overhead (includes air, and light ends).
Rm i and Rt i = Ratio factors for component i from ejector Figs. 28 and 29.
W s = Weight flow of steam in lb/h.
Rm s and Rt s = Ratio factors for steam from Figs. 30 and 31.
In this case Ws will be zero, as no steam will be used in the distillation.
Step 3. Calculate the steam consumption to the first ejector using tower top pressure
as the suction pressure. The consumption is calculated by
W ms ¼ R a M p W a
(14)
where
W ms = Weight flow of motive steam in lb/h.
R a = Ratio of lb motive steam/lb air equivalent using Fig. 30.
M r = Steam usage multiplier from Fig. 31.
W a = Air equivalent flow in lb/h.
2.0
1.8
1.6
1.4
1.2
1.0
0.8
MOLECULAR WEIGHT ENTRAINMENT RATIO R
m
0.6
0.4
0.2
0
0
10
20
30
40
50
ENTRAINMENT RATIO =
60
MOLECULAR WEIGHT
70
80
90 100 110 120 130 140 150
WEIGHT OF GAS
WEIGHT OF AIR
Fig. 28 Ratio factors for steam
184
D.S.J. Jones