Prediction of Flash Points of Middle Distillates R. 31. BUTLER, G. 31. COOKE, G. G. LUKK, ANP) B. G. JAMESON Imperial Oil Research D e p a r t m e n t , Sarnia, Ontario, Canada
B
ECAUSE flash point is frequently given as a specification for various products, methods for predicting the flash point of petroleum fractions and blends of petroleum fractions are of considerable interest. The present paper deals with the prediction of this property for the so-called middle distillates-Le., petroleum fractions boiling in the range 200" to 700" F. Very little work has been done on this subject in the past. The flash point of lubricating oil blends has been discussed by Thiele (9). Although Thiele's method is, in some respects, similar to the work described in the first part of this paper, it contains several assumptions which are invalid when applied to middle distillates. Selson ( 7 ) gives a simple relation for the flash point of a petroleum fraction, which is similar. t o that proposed by the associated Factory Mutual Fire Insurance Companies ( g ) . ?;either of these equations gives the required accuracy. A method of predicting the flash points of pure organic materials has been described by Leslie and Geniesse ( 5 ) .
lated to the vapor pressure of the components in the liquid mixtule. This view is substantiated in Figure 1, where flash point has been plotted against boiling point for a nuniber of pure hydrocarbons. The data for the closely fractionated narrou cuts, designated RJI7 246/250 etc., fall on the same line. This graph indicates that flash point increases continuously IT ith boiling point (for pure hydrocarbons) according t o the equation tp
11.
=
flash point (closed cup),
t~ = noimal boiling point,
119
F.
O
F.
The available data do not indicate any great variation between hydrocarbon types. Normal paraffins, cycloparaffins, aromatics, and the sharply separated fractions all fit the above equation The equation is used later for the prediction of the flash pointof a wide cut from an assay curve, such as the 15/5 distillation line (15 theoietical plates operated a t 5 to 1 reflux ratio). In Table I the vapor pressures ( 1 ) a t the flash points of a nuniber of pure hydrocarbons are listed. There is a considerable vaiiation in this vapor pressure. The higher boiling compound. of higher molecular weight exert lower vapor pressures a t the flash point than the lighter compounds. Fewer large molecule> are required t o form a combustible mixture with air than small ones. The values in the last column in Table I are arrived a t by niultiplying the molecular weights and vapor pressures of the respective compounds a t their flash points. These values are reasonably constant (average value = 19.19). The flash point of a pure hydrocai bon occurs approximately at the temperature mhere
I t is assumed that the flash point (values obtained by means of the Pensky-hlartens closed cup flash point tester) occurs a t that temperature where the hydrocarbon concentration in the air above the liquid surface becomes great enough for coinbustion t o occur. It is apparent that the flash point must be re-
250
200
MP"
=
15.19 pounds per square inch
JT here
LL.
150
21
R 0
=
molecular weight
and
I7n 2
Po
a
2
-
and
THEORETICAL B A S I S
5-
= 0.68318
where
100
=
vapor pressure
For n mixture of hydrocarbons it is assuyed that each hydrocarbon exerts a vapor pressure equal to s,P,, mhere xLis its mole fraction in the liquid and P,' is its vapor pressure. The flash point is assumed to occur when
R 0 3
n W
50 0
zx&f,PP
The Clausius-Clapeyron equation may be written for each compound as follows:
0
Pp = Aie
-CA
4"
= 15.19 pounds per square inch
100
200
300
Figure 1.
500
400
BOILING POINT
600
70
Li --RT
where
OF.
A , is a constant Li = molar latent h5at of evaporation of component i T = temperature, R.
Flash points of pure hydrocarbons Data from (3, 4, 8 )
808
809
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1956
Then for the whole mixture Table I. Materia n-CsHl4 Cyclohexane n-C7His Methylcyolohexane Toluene n-CsHla n-CsHm n-CioHzz n-CiiHzn n-C1zHze n-Ci4Hso
Physical Properties of Hydrocarbons Boiling Point, F.
Flash Vapor Pressure, Point, at Flash Point, Mol. F. Lb./Sq. Inch Wt.
-
hIol. W t . X V.P., Lb./Sq. I n c h
15 6
15 1 25 25
16.0
17.9 18.1 15.8 15.4 15.5 14.2 13 9 12.6 12.2
40
56 88 115 142
165 212
sa3 =
15.19e
Therefore
n1
200 180 I60 150 140
nl
n2
eTF,
12%
+
n2
8500 _-
scale of this chart is proportioned to e T F and the horizontal scale is mole fraction. On this chart the flash point of a blend lies on a straight line joining the flash points of the two mixtures which are blended. Numerical Example. What is the flash point of a mixture containing 42.8 volume yoheavy naphtha and 57.2 volume 70heavy gas oil, given the following data?
I30
120 I O
y
105
z
00
2
+
8500
+-
The constant, 15.19, drops out in this equation. To simplify the use of the relation, the flash point blending chart shown in Figure 2 has been prepared. The vertical
240
5
8800 _ -.. eTFi
8500
Av. 15.19
-
TF
98 96 94
92
$ 90
4
88 86 84
82
80 78 76 74 72 70
0
10
30
20
40
50
60
70
80
90
IW
MOLE %
Figure 2.
Flash point blending chart
and
R
= gas constant = 1.986 B.t.u. per lb -mole, The flash point occurs when
' F. I
I
I
LL -_
ZzliM,A,e
RT =
15.19
Figure 3. Predicted and observed flash points of blends of refinery stocks
If the simplifying assumption is made that L, is the same for each component, we may write L -
Zz,M,Ai = 15.19eRTF
If L is taken as 16,900 B.t.u. per 1b.-mole, the value for n-octane at 125' F., then
It is recommended that this relation be used in the forms discussed in the next few sections. DEVELOPMENT OF FLASH POINT BLENDING CHART
Suppose nt moles of a mixture having a flash point Tp1 are to be mixed with nz moles of a mixture having a flash point of TQ. Then if xil is the mole fraction of i in mixture 1, etc. 8500 -
ZxilMilAil = 15.19e TF1 and similarly 8500 -
ZxigMizAiz = 15.198 TF1
Heavy gas oil.
Gravity, 30.3' A.P.I., mid-boiling point,
648' F., flash point, 200' F.
Heavy naphtha. Gravity, 48.4" A.P.I., mid-boiling point, 320" F., flash point, 90" F. SOLUTION. From the "Data Book on Hydrocarbons" ( 6 ) , the average molecular weights are found to be 2 i 2 for heavy gas oil and 130 for heavy naphtha, and their specific gravities are, respectively, 0.874 and 0.786. Consider 100 cc. of the blend co. H.N. H.G.O. Total
42.8
57.2 100.0
Grama 33.7 49.9
GramMoles 0.259 0.184 __ 0.443
Mole Fraction 0,584 0.416
A straight line drawn from O%, 200' F., t o loo%, 90" F., on Figure 2 passes through 58.4Y0, 107' F. Therefore the predicted flash point of the blend is 107' F. The above blend was prepared and an experimental value of 106' F. was obtained. Comparison of Method with Experimental Data. I n Figure 3 experimental flash points for various blends of heavy gas oil and heavy naphtha, and light gas oil and heavy naphtha, have been
810
INDUSTRIAL AND ENGINEERING CHEMISTRY Table 11. 200
Vol. 48, No. 4
Calculation of Flash Points of Crude Fractions
r
(Basis 10,000 grams of crude) Boiling Range Wt. in Vapor Temp., Crude, O F. G. 'A.P.1.
Mol. Wt. 108 125 131 136 141 149 156 164 170 177 185 193 20 1 213
Moles
200
1
$
POINT
,
1
400
300
BOILING
Figure 4.
1
IN
"E
I
l
For 280' F. I.V.T., F,P,F. = 309.24 lli,03
- 117 - 1,,6
l
500
Relation of flash point factor to boiling point
plotted against the liquid composition. Curves, predicted bJmeans of the flash point blending chart, Figure 2, have also been plotted. The agreement is satisfactory. A number of such comparisons has been made for middle distillate stocks from various sources and similar agreement has been obtained in every case. A statistical analysis of the difference between the predicted and measured flash point in 39 different experiments involving middle dist,illates from three different sources gave the following results:
=
12.58
For 300° F. I.V.T., 129.24 61.1 = F.P.F. = 15.28 - 1.36
9,42
-
66.5 45 29 20.8 14.5 10.2 7.2 5.4 4.2 3.1 2.3 2.1 1.2
117 61.1 42.0 20.4 17.5 13.7 8.3 5.95 4.45 3.16 2.37 1.95 1.08 1.18 309.14
0.9
3107i:
= 18.15
~
338' F Figure 4) 112O F:!Figure 1)
Correspondina boiling point Corresponding flash point
1'"'A
Factor X Moles
1.76 1.36 1.45 1.41 1.21 1.34 1.15 1.10 1.06 1.02 1.03 0.93 0.90 1.31 __ 17.03
Flash point factor for mixture =
Ill1
Flash Point Factor (F.P.F.) at Mid. B. P t .
Hence Bash point = 127' F.
Flash point = 13S0 F .
For 320° F. I.V.T., F,P,B, = 131.14 - 4 2 . 0 13.92 - i . 4 5 =
7.15
Flash point = 150° F.
For 340" F. I.V.T., 8!4.i4 F.P.€. = 1 2 . 4 7
- 21 9. 4. 41
=
5'40
Flash point = 162' F.
For 360" F. I.V.T., F,P,F. = 5 9 . 7 4 11.06
-
17.5 1.21 =
4 29
Flash point = 174O F.
180
r---
/ /
TESTS WITH HEAVYGASOIL A N D HEAVY KAPHTIIA.Average difference between measured value and predicted value = + l o F.
Standard deviation of differences from mean = 3' F. TESTS WITH LIGHTGAS OIL ASD E i s a v ~NAPHTHA.Average difference between measured value and predicted value = - 1" F. Standard deviation of differences from mean = 1.5' F.
z
8500 ~
TF
for each narrow cut.
I n order
$
120
-J L
I i
le,/
63
/"
80
1
I
1
250 I'iITIAL
EXPERIMENTAL
POINTS
Q 751100 D I S T I L L A T I O N S 0 36/30 DISTILLATIOVS
L
loo
I
300 \/ADOR
350
TEYPEF?ATLt
