I
D. K. H. BRIGGS, WRIGHT WADDINGTON, and DONALD McNElL The Coal Tar Research Association, Oxford Road, Gomersal, Near Leeds, England
Batch Fractionation of Coal Tar Naphthalene Oils This upproach makes it possible to predetermine results if the characteristics of the equipment are known, or to predict whether new equipment is needed The purity and recovery of naphthalene are related mathematically to the efficiency of the batch column by a semiempirical procedure based on data from a minimum of three laboratory fractionations
A
USEFUL method ( 2 ) has been reported for calculating the batch distillation curve for phenol-free coal tar naphthalene oils, in which, at some time during the process, a constant boiling distillate consisting almost entirely of naphthalene must be obtained. I n the case of naphthalene oils derived from British coal tars, particularly those from gasworks tar, the presence of variable quantities of close-boiling impurities makes this method seldom successful. Therefore a method was devised which is useful in estimating batch distillation conditions required to give a predetermined recovery of naphthalene having a given purity or, conversely, in estimating the extent to which a
naphthalene oil will be upgraded in an existing batch distillation column.
Experimental When a phenol-free coal tar naphthalene oil is distilled batchwise and the naphthalene content of the distillate is plotted against the amount distilled, a curve, symmetrical or nearly so, similar to that in Figure 1 , is usually obtained; the maximum naphthalene content is seen to occur close to the mid-point of the distillation. If the curve were truly symmetrical about line AB, one could say that after 50% has distilled half the naphthalene has been removed, so that the naphthalene concentration in the boiler is the same as that in the original oil (&). At this point in the distillation it can be postulated that material of charge composition x .YC can be related IO the still head composition, yyo of naphthalene, in an equation of the form
= 1 A n X
100
-y
100 - x
where n is the number of plate equivalents and A is a separation factor whose value for a given oil depends on the
relative volatilities of naphthalene and the nonnaphthalene components in the overheads and those in the boiler, and on the degree of separation achieved. The requisite distillation data were obtained on a series of batch distillation columns, and test mixtures were used to establish the operating conditions to give various efficiencies, expressed as “plate equivalents”-i.e., aquantity numerically equal to the number of theoretical plates operating at total reflux which would give the separation obtained when the column is operating at finite reflux ratio ( 7 ) . Details of dimensions, reflux ratio, and packing of the columns are shown in Table I. All columns are 4 inch in diameter. Four washed naphthalene oils (Table 11) were batch-distilled in these laboratory columns, each at various plate equivalents. From values for y obtained in each distillation, values for A were calculated and plotted against j , for each oil (Figure 2). The relationship between peak naphthalene purity and the separation factor is seen to be approximately linear, so that y = C - m (A
-
1)
(2)
IO0 I
t-
z 80
t 2
a!
V
W Q
w
i2
60
2
k
5
s
W
z
Y
40
W -I
Y
U I I-
4
F I
20
5z
P
U z
0 0
20 PERCENT
40
60
WElG H T
80
I00
DlST I L L € D
Figure 1. Maximum naphthalene content occurs close io the mid-point of the distillation
1.0
I*I
1.2 SEPARATION
I. 3
1.4
FACTOR
Figure 2. The relaiion between peak napthalene purity and separation factor is approximately linear VOL. 52, NO. 2
FEBRUARY 1960
145
Table 1.
Height, Inches
Details of Batch Fractionation Columns
Boil-Up Rate, Ml./Hr.
Packing
l2
20
1
l/a-inch Fenske helices
201 20
3 6
1000 1000
12/1
10 ~.
14 15 18 20 28 35
700
4Ojl
l/s-inch Dixon gauze rings
1000 1100
l/l&nch Dixon gauze rings l/s-inch Dixon gauze rings
600 600 1100
20/1 40/ 1 4/ 1 40/ 1 40/ 1
Table II.
Plate
Equivalent
3/2 9; 1
1000
201
Reflux Ratio
Characteristics of Washed Naphthalene Oil Naphtha-
1. T o calculate the number of plate equivalents required for 80% recovery and 96% purity (crystallizing point 78' C.) from an oil similar to No. 3, the separation factor can be estimated from Figure 3 as 1.015. When this factor is substituted in Equation 1 where x is 64.0,
- -96 100 -
2
3
4
-
1.015"
94 Primary distillate ex continuous vertical retort Drained naphthalene oil ex coke oven tar Primary distillate ex coke oven tar/continuous vertical retort tar blend Primary distillate ex coke oven tar, enriched with pure naphthalene
39.6
4.0
198
218
232
15.6
...
214
237
255
64.0
0.4
211
219
237
87.0
1.2
212
217
223
100
maximum purity attainable from a naphthalene oil a t infinite plate distillation, and m is the gradient of the line. I n the case of the continuous vertical retort, primary distillate, and drained naphthalene oil (oils 1 and 2, Figure 2) no phthalic anhydride grade naphthalene [crystallizing point 78' C. ( 3 ) naphthalene content 96%] can be produced by fractionation, because C is less than 96%. The plot of y us. A can thus represent distillation data at conditions of peak purity-i.e., a t zero recovery. If the average naphthalene contents of finite fractions symmetrically disposed
about a center line (such as AB, Figure 1) are plotted against the recoveries of naphthalene in the corresponding fraction, purity y may be obtained for a series of finite recoveries. If these y values are substituted in Equation 1, corresponding A values may be obtained and a series of straight lines is again given (Figure 3) when J) is plotted against A . These lines all converge at the same point on the A = 1 ordinate. Results a n d Discussion
Utilization of this experimental approach in solving fractionation problems is best illustrated by examples.
L r
Z
ii'wa
-
64 100 - 64
94
Le., A = 1.044 The pointy = 94, A = 1.044 occurs on some recovery line which must also pass through point C where A = 1.00, y = 97 6. Slope rn (Equation 2) must therefore be m =
where C is the intercept of the line on the y axis when A = 1 and represents the
64 (--) 100 - 64
Le., n = 177 2. To calculate the recovery of a product of 94Y0 purity using a column of 100 plates operating under conditions known to give 50% plate efficiency, A is given by
Oil I
96
97.6 1.044
- 94.0
-
= 81,9
1.000
By interpolation of slopes in Figure 3 the recovery line which has a slope of 81.9 is found to correspond to 58%. T o calculate the naphthalene content of a fraction representing 80% recovery of naphthalene 100
-y
=
A60
(-)100 64- 64
and y = 97.6 - 106.3 ( A - 1) thus,^ = 93.170 Although it is assumed that curves such as Figure 1 are symmetrical and that the maximum naphthalene concentration in the overhead occurs at the mid-point of the distillation, oil 2 does not meet these criteria. However, if the fractions of finite recoveries are taken symmetrically about the ordinate which represents the center of the area under the naphthalene curve, the results are valid. Acknowledgment
The
authors express
gratitude
to
G. H. Thompson of these laboratories for fractionating column efficiency data and to the Council of The Coal Tar Research Association for permission to publish.
W
5!F 41
literature Cited
1-00 1.04
1.08
SEPARATION
1-12
I. 16
1-20
FACTOR
Figure 3. Average percentage naphthalene vs. separation factor for various percentage recoveries of oil 3, gives straight lines meeting on the ordinate
1 46
INDUSTRIAL AND ENGINEERING CHEMISTRY
(1) Collins, F. C., Lantz, V., IND.ENG. CHEM.,ANAL.ED. 18, 673 (1946). (2) Rose, A., Sweeney, R. F., IND.END. CHEM.50, 1687 (1958). (3) Standardization of Tar Products Tests Committee, Gomersal, "Standard Methods for Testing Tar and Its Products," 4th ed., 1957.
RECEIVED for review June 1, 1959 ACCEPTED October 2, 1959
