Relationship between temperature and vapor pressure of ...
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Professional Degree Theses Student Theses and Dissertations
1927
Relationship between temperature and vapor pressure of light oils Relationship between temperature and vapor pressure of light oils
William Orval Keeling
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Recommended Citation Recommended Citation Keeling, William Orval, "Relationship between temperature and vapor pressure of light oils" (1927). Professional Degree Theses. 210. https://scholarsmine.mst.edu/professional_theses/210
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RELATIONSHIP BETWEEN TEMPERA.TURE
AND VAPOR PRESSURES
OF LIGHT OILS
BY- W. O. Keeling.
A THESIS
IN PARTIAL FULFILLMENT OF THE
REQ,UlREMENTS FOR THE DEGREE OF
CHEMICAL ENGINEER
June 24, ~927
Thesis: Relations~iP betw4en tempe~ature an~vapor preFsures ot light O~lS. 1\'Keellng. 1 1927 \ I I
- 1 -
mTRODUCTION
There are a few data scattered through the lit
erature on the vapor pressures of pure hydrooarbons, but
the author has never seen a systematic study of the rela
tionship between temperature and vapor pressures of oompli
cated mixtures such as the various light oila encountered
in refinery praotioe.
suoh data would be useful in designing equipment
for distillation, steam stilling, vaouum distillation and
in oaloulating heat balanoe, etc.
An attempt has been made to link the vapor pres
sure o£ a light oil, at various temperatures, to the aver
age boiling point of the oil.
Apparatus Used
The apparatus used was an ordinary mercury baro
meter, jaoketed by a two inch Pyrex tube. An electric heat
ing coil was placed in the space between the barometer an4
pyrex tubing. A meter stick was placed behind the barometer.
One end of & small metal rod was inserted into one ena of
this stiok" the other end 'of the rod passing through the cork
whiah held in the bath oil. !He distanoe from the end of the
meter stiok to the end of the rod was measured. The free end
of the rod was made to barely touch the surfaoe of the mer
cury in the barometer well by raising or lowering the latter.
Agitation of the bath oil was acoomplished by in-
- 2 -
troducing air into the lower part of the annular space.
By regulating the electrio current in the heater and the
amount of air used for stirring, the temperature was main8
tained at the desired point.
Kind o~ Oils Used
The oils seleoted for the experimental work were:-
1. Light, Straight-Run gasoline
2. Heavy, Straight-Run gasoline
3. 40-60 Blend of Cracked andStraight-Run gasoline
4. Pressure Distillate
5. Beluslne
6. Crude Oil
These oila represent most of the light oils encountered in
the average refinery praotioe. The vapor pressure of each
oil was first measured at different temperatures. Each 011
was then fraotionated into 10% cute and the vapor pressures
of each cut were determined at different temperatures.
Procedure
The prooedure used was to introduoe the oil to be
tested into the barometer by means of a ourved medicine
dropper. A suffioient amount of oil was introduoed into the
barometer as to insure some of the lighter ends remaining
in a liqUid form at the higher temperatures. At least 10 00
were used for eaoh test. After the 1ntroduotion of the 011,
- 3 -
the eleotrio current was turned on and the air "jet start
ed. As the bath warmed up, the height of the barometer
was read at 20~F intervals. The resulta are tabulated in
the following tables.
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74
34
..... l\')
CORR
ECTE
DVA
POR
PRES
SURE
SOF
FRA
CTIO
NS
FROM
BEN
ZIN
EI
Fa
hr.
%%
%%
%%
%Te~p.
Ben
zin
eO-~O
10
-20
20
-30
30
-40
40
-50
50
-60
50
-70
--
10
04
217
3la
B60
37
37
35
12
04
52
13
15
07
347
403
714
:05
62
70
15
77
65
642
.3
91
60
71
37
51
66
796
350
42-
18
08
05
20
18
08
36
66
544
200
98
Atm
.2
06
89
76
84
502
20
12
0-
227
11
595
11
07
624
01
45
-2
4e
14
01
28
14
51
10
26
017
4-
39
11
76
17
51
76
15
328
02
24
-4
95
23
62
30
20
81
70
ZOO
26
1-
59
03
15
273
24
51
97
32
0~21
-A
tm.
41
2~13
~O4
23
334
0'~76
--
48
04
15
37
42
79
Bel
owar
eo
urv
esp
lott
ed
trom
the
fore
go
ing
data
.
l3enz/I"Je
~~u
t.....
VI~,~~~
~
~
~ Sf6\~
~.t>
;.40
36
.2°
Gra
vit
yCR
UDE
OIL
Tem~;.OF
10
01
£0
14
016
01
80
200
22
02
40
260
280
vapo
rP
ress
ure
sM
.M.
Hg
.
29
83
26
36
73
97
42
34
51
47
84
98
52
55
45
CO
RR
EC,T
EDV
.KPO
RPR
ESS
UR
ES
OF
10
%C
UTS
FRO
MC
RU
DE
Tem
p.%
%%
%F
aM
.0
-10
10
-20
20
-30
30
-40
10
012
01
40
16
01
80
20
022
.0
240
260
28
03
00
32
03
40
~60
11
81
70
25
03
28
41
24
85
55
562
56
68
Atm
.
2.0
38
50 60 71
11
81
60
218
'30
04
15
51
56
35
Atm
.
25
26
28
30
40
50
78
11
31
55
216
283
37
64
70
25
2& 28 30
32
34
36
40
43
45 54 95
12
61
65
f-'
(>l I
The
foll
ow
ing
curv
esar
ep
lott
ed
from
the
abov
ed
ata
.
I~
la.O
/bt>
Crud t2
/0/0 Cl//s Fro"" Crude (!)/"/
_h.c~~~f'1d .X '-O-l¢ 7l>~t.Jf-4h. - l~~).~j;~'!-t
1lf:- .~¢ -30%C,tlt'_.•'..1Z:_':d3C>-:'iP~?tlt .. ..
- 14 -
DISCUSSION
]Tom the very nature of, the substances worked
with, the regularity of results oan not equal the results
to be obtained from pure substances. Gasolinea of exact
ly the same gravity. or even the same initial and end points
may di~far from eaoh other in vapor pressures at any given
temperature. beoause of differenoes in oomposition. !he
author recognizes this and simply offers the data obtained
in the hope that it will be useful for average engineering
oomputations.
It will be observed that the first two or three
cuts from eaoh oil. partioular1y pressure gasoline. gives
vapor pressures that seem to be 8 little out of line with
the remainder of the outs. This is probably due to one or
two things ,- dissolved permanent gases. or wide variation
in composition. This is shORn also in the A.S.T.M. distil
lations of the l~~ outs from eaoh 011. The tendenoy 1s for
the differenoe in temperature between the initial point and
end point to be greatest for the lightest outs and least for
the heaviest outs even though the same fractionating oolumn
was used and the same rate of distillation maintained. Then.
too. thQ're is a tendenoy for the overlap between the end
point of the first fraotion and the in1tial point of the
seoond to be larger than the overlap between two sucoessive
fractions of higher boiling cute, although the differenoe
- 15 -
shows lass regularity than the difference in temperature
between the initial and end points.
Rejecting the figures obtained for the first
three cuts of eaoh oil, excepting benzine and crude oil,
the remaining data seems to anow a general relationship
between temperature and vapor pressure. By use of this
relationship, this data can be extended from the partic
ular produots tested (with the inoorrect figures thrown
out in any case) to general use on all usual light oils
and possibly the heavier. This relationship can be .%~
pressed mathematioally &s:
P • 600 f~ ~ tg8J 9Equation No. 1
p : Vapor pressure in m.m. of mercury at
temperature t in degrees Fahrenheit
t =-Temperature Fahrenheit
T =Average boiling point of fraction in
degreeS 19hrenheit
The following comments are in order about this empirical
equation:
1. It gives average 'figures for this data. On any
one fraction from a particular oil it may show some error.
Much of our engineering work covers average con4itions,
wi th the probable error within the aoouraoy of other fac
tors, so this equation is available for use with judgement.
Further work would probably permit the uee of the equation
for any kind of oonditions with aooeptable aocuraoy.
- 16 -
2. The equation applies with greates~ accuracy to
olose cut fractions. Plant practioe rarely has to do
with olose outs. henoe a modification is necessary for
fractions of wide boiling point ranges.
Let t
T'a Value of T to be used in Equation No.1
T =Actual mean boiling point '
D : Distillation range, or differenoe be
tween initial and end points in degrees
Fahrenheit
then,T' • T - ~
-!;;....(T-'"''''''''''4-60-)Equation No. n
3. EVen this modifioation .ill not give correctly
the vapor pressure if there is an appreciable quantity ot
dissolved gas in the oil. This dissolved gas adds to the
vapor pressure of the out itself. In this case the vapor
pressure of the gas mnst be oons'idered separately and mast
then be aided to that of the oil itself.
Use of the Kqaatlons Illustrated
Assume a 010S8 out naphtha fraction wi tb. an '&'.13 .P.
of 3750F. The vapor pressure is required at 2QOoF.
t II 2000'.
T • 3750 )1,
P • 600 (200 i 46019(316 /1601
• 600 %(.79)9 • 600 % .121 =73 m••• Kg.
- 17 -
~-
OheCking Over the data, temperatures at whlan this vapor
pressure will be exerted by outs of about this mean boil
ing point seem to vary within 100F of 2000F.
If, instead of a close cut fraction, we have a
naphtha of about 200o~ I.P_, 4500 F E.P., 3750 FA.B.Pe, as
would be met with in actual praotice, the correction for
temperature will be required. Here t and T are as in above
problem.
D =450 - 200 • 250oF. Then
T r =T - D2 =375 - 25022(Tr460) 2(375r460)
• 375 - 62600 =375 - 37 • 33SoF, and2x835
P =600 (t ~ 460)9(T r 460)
= 600 (200 ~ 460)9 : 600 i;;g}933B 460)
: 600 %(.82,)9 =600 x.182 : 115 m.m. Kg.
instead of 73 m.m. for olose cut fraction
Qalcn1ationa for cl at ermining the degree of vacuum
required in vaouum distillation of light oils would be de
termined by the same oaleula't,ions as above t but it should
be borne in mind tha't these data and equatioM should be
used only for "flash" distillstion and not for batoh.
Use of Data ~or steam Distillation caloulations
For Light Gasoline
The £ollowing formula w111 giva the theeret10al
- 18 -
amount of steam to use in steam distillation.
p -Px 18IS t al
or a - (P-p) 18- PMwhere p Partial ~reasure of oil.
P Absolute ~ressur8 on surfaoe of 011.
l{ lloleoular weight of oil.
a Quanti ty of steam to one part of 011.
18 Molecular weight of water.
Take t~e light gasoline. ~h1s has an average boiling point
of 2590~. ~his correspOnds to Ootane Whioh has a boiling
point of 259 and a molecular weight of 114.
Assume the all 1s to be distilled with steam at
atmospheric pressure at 240°1'. Required the amount of steam
to 11se.
P 760 K.K•• atmospheric pressure.
M 114, moleoular weight of oil.
18 Molecular weight of water.
a Unknown.
a =(760-675) 18 = .15 Ibs. steam, to one part676x114
of 011. Actual praotioe using open ani olosed 0011s shows
about double the oalculated amount of steam. Then .04 Ibs.
of steam per pound of oil, or 12 pounds of steam per barrel
of gasoline will 'be necessary when steam 8tll1in~ at 840°)'.
- 19-
In practioe,distillation is usually oarried on
under pressures of from 5 to 10 pounds per square inch.
Suppose wa steam still under 10 pounda per square inoh.
gage pressure the other factor's remaining the same. What
amount of steam will be necessary?
a =(1340 ... 675) x 18 • .16, 1bs. steam per lb.675x114 of 011 theoretioally.
Aotually .3 pounds of at eam per pound o-L oil oX' 90 pounds
of steam per barrel of oil.
Now let us take a heavier gas and sea What the
steam consumption w111 be. Let us take the 90-100 per aent
cut of heav7 straight-run gasoline. The average boiling
point is 4150:r. whioh corresponds to the boiling point of
Dodeoane whiah has a molecular weight of 1'70. SUppose the
distillation 1a to be carried out at 280o~. and atmospherio
pressure.
p =110 X.M.
P =760 Y.U.
K =170
a : {'760 - 110~ 18 :.6 pounds of steam per pound of Oil110 % 1 0
theoretlcaD3 or 1.20 pounds of steam or 360 pounds of steam
per barrel of oil actually.
Now let us distill this oil at 10 pounds gage
pressure at t.he same temperature.
a : (1340 - 1106 18 =1.88 pounds o~ steam per pound of, 110 x 1'1
011 theoretioally or 3.~6 pounds of steam per pound of oil
- 20 -
or 1128 pounds of steam per barrel of oil under actual
conditions.
Suppose that the distillation is carrie4 on under
a vacuum of 300 M.M., than.
a = (300 - 110) 18 = .18 pound of steam per pound of 011110 x 170
theoretically or .36 pounds of steam per barrel of oil or
108 pounds of steam per barrel of Oil, actually.
CONCLUSIONS
1. There is a definite relationship between the
vapor pressure of a light oil, at any given temperature,
and its average boiling point.
8. This relationship is expressed in the Equation No.
I for close aut oila, and in Equation No. II for oils o~
wide boiling point range.
3. An aOQuraoy of about 5% oan be had by use of theee
equations, Whioh is aoourat e enough tor most engineering
oaloulations.
4. Examples have been given showing the useo! the
equations.