I
HARRY E. ClER and M.
T. WADDELL
Humble Oil & Refining Co., Baytown, Tex.
Producing High Purity Cyclohexane Cyclohexane of 99% purity has been produced in continuous pilot scale operations. Higher purities can be produced, but at a considerable sacrifice in yield
T m continuing increase in product purity demands for cyclohexane indicated the need for a suitable process for manufacturing 98 to 99Yc cyclohexane from native cyclohexane available in certain refinery streams. Recovery by distillation alone gives a product containing only 85YG cyclohexane. Such a fractionation preceded by a n extractive distillation step employing anhydrous phenol gives the desired product. This combination process has been demonstrated in continuous operation on a pilot scale basis. Experimental
T h e preferred feed stock for this study, a 76% concentrate, was produced in existing plant fractionating equipment. T h e composition of the product \vas established by an infrared-mass spectrometer procedure especially developed for this purpose (Table I ) . T h e preferred solvent, anhydrous phenol, \vas selected on the basis of its separating efficiency coupled with other factors such as cost and stability. Calculations were made to establish the composition of the several streams (Table 11). T h e paraffin classification emploved is a grouping of compounds boiling above or below 180' F. T h e maximum tolerable amounts of anv
component in the product could not exceed a few tenths of 1%. This imposed a right requirement of accuracy for both the calculational procedure and the relative volatility data. These data were experimentally determined on key components for the fractionation and extractive distillation sections of the process. Several laborator). procedures were used in this study, including a technique developed especially for systems of 1 0 ~relative . volatility. Included was a n evaluation of the activity coefficients of certain contaminants a t values approaching infinite dilution. This established relative volatility values in the high purity cyclohexane region. Process requirements to produce 93 Lveight % cyclohexane were calculated (Table 111). Operating conditions specified that for each 100 barrels of 76% feed, charged to the 45th plate of the extractive column a t its dew point, 1320 barrels of phenol would be charged at the 75th plate. .4 vapor rate a t the top of the column of 640 barrels and raffinate at 18.2 barrels was specified.
Table II.
Methylcyclopentane Benzene Cyclohexane Light paraffins Heavy paraffins Dimethylcyclopentanes Stream yields, vol. based on 76% concentrate
Table I. Composition of Feed Cyclohexane Concentrate Was Established by Infrared-Mass Spectrometer Boiling YolPoint. unie Component O F . c/o 161.3 174.6 176.2 176.9 177.3 177.6 186.9 190.1 193.6 194.1 197.1/ 195.4 211.11 197.4 197.3
0.2 3.7 0.1 6.3 75.8 2.3 1.3 1.5
Table 111.
Extractive Distillation, Yol. 7 0 Kaffinate Extract 1.09 0.00 19.82 66.72 12.33 0.03 18.2
0.01 0.06 88.24 0.25 7.10 4.34 81.8
F r n c t io~iat~ioii Vol. vc Overhead Rottoiii\ ~
0.02 0.08 98.94 0.30 0.48 0.18 66.0
0.00 0.00 43.43
0.00 34.78 21.79 15.8
Calculated and Actual Operating Requirements W e r e Generally in Good Agreement Frart ioiiatioii .Extractire Distillation __ ~~
Calcd.
1.9 3.9 1.5
Number of actual plates Hydrocarbon feed plate Solvent feed plate Reflux ratio Solvent charge-hydrocarbon charge
0.5
1.0 100.0
Calculated Stream Compositions
Extract yield should approximate 82%
Compound
Methylcyclopentane 2,2-Dimethylpentane Benzene 2,4-Dimethylpentane Cyclohexane 2,2,3-Trimethylbutane 3,3-Dimethylpentane 1,l-Dimethylcyclopentane 2,3-Dimethylpentane 2-Methylhexane cis/trans-1,3-Dimethylcyclopentane cis/trans-1,2-Dimethylcyclopentane 3-Methylhexane
T h e extract solution. consisting of 1320 barrels of phenol and 81.8 barrels of hydrocarbon, \vould be transferred to the 20th plate of the stripper. The stripped extract would be charged at its bubble point to the 35th plate of the fractionator and the vapor rate a t the top of the column adjusted to 659 barrels. Overhead and bottoms product kvould average 66 and 15.8 barrrls. respectively. Pilot distillation facilities tvcre fabricated from conventional glass Oldershaw columns. The specified sequencr of operations \vas first to distill the 76%. concentrate extractively, strip the cyclohexane fraction from the phenol, and distill the phenol-free extraci in a fractionating column. Typical operating data are shown in Table I\.. T h e extractive distillation column and the stripper were run simultaneously. Because the production rate of extract did not match the charge rate of the fractionator. the fractionator was operated separately. Feed and vapor rates had to be very accurately controlled
a
C'alcd.
requirements
Pilot iunit
require-
Pilot
nieiits
unit
85 40 75 34.211 13.2
85 45 75 35,'l 13.0- 1 3 . 7
96" 30
110" 35
9,'l
9'1
...
...
... *..
Theoretiral plat,es for two cases are same.
VOL. 5 1 , NO. 3
8
MARCH 1 9 5 9
259
Figure 1. Extractive distillation of 76y0 cyclohexane concentrate removes no heavy paraffins when extract yield i s 84y0 or more 3-
-/
b
IL - _ _ - ~ I
Figure 2. Increasing cyclohexane purity increases cyclohexane loss
HEAVY PARAFFINS REJECTED TO THE EXTRACT
g o r.-~-I
'0 08 2 --
06
_ _ 96
951
-
/-
A97
sa
33
100
P b r i t y SICycloberane Overheac Product
vapor a t condenser 'ivater temperature. but material balance data were not corrected to account for this. Discussion
week. The unit operated Jvith a good degree of reliability and for long periods of operation a t essentially constant conditions. Material balance data for these operations Lvere obtained on a weight basis. 1-olunie per cent yields were calculated from charge and product weights. I n the extractive distillation step, material balances ranged from about 97 to 99.5 weight %: in the fractionation step, from about 94 to 96%,. A significant portion of the total loss experienced in these runs is believed to be due to a small stream of nitrogen purge gas introduced into the system between the still and the manometers. T h e exit purge gas \vas saturated with hydrocarbon
to ensure rhe proper ratio of solvent to hydrocarbon. continuous and uniform transfer of extract solution to the stripper and stripped solvent to the extrac. tive distillation column, and external feed rates as required for the internal vapor rates generated in che column. Precise control of numerous operating variables was mandatory to ensure maximum yield of a given high purity product. Several such critical variables werc solvent charge temperature (a factor affecting column reflux ratio), column 1,b !a measure of vapor rates), and reboiler temperature (an index of still composition). Operation of this pilot unit \vas conducted on a 24-hour basis for a 5-day Table IV.
Typical Operating Data
Extractive C'olumn Hours on stream Feed rate, cc.'hr. at 60' F. Solvent Hydrocarbon Overhead Bottoms
260
€rac,tionator
68-90
68-90
53-60
5230 385.2 59.9
317.4
230.8 161.8 60.0
...
Reflux ratio
35 '1
Temperature, O F. Hydrocarbon feed plate Solvent feed plate Reboiler
199 184 291
Recovery, wt. % Cyclohexane concentrate, normalized, vol. %
Stripper
-~
INDUSTRIAL AND ENGINEERING CHEMISTRY
...
Variable ; instrument controlled
9 '1
182
.*.
254 365 97.6---84 1
194 ~.
96.1 73.8
From each operation of satisfactor!. reliability, a complete component analysis of the extract, raffinate, fractionator overhead. and fractionator bottoms was obtained by the infrared-mass spectrometer technique. After the component analyses of the various streams Lvere obtained, a normalized component balance \\;as calculated. A s a first step, the normalized yields of extract. raffinate, fractionator overhead, and bottoms obtained from the operating units were revicwed. and compared with stream yields calculated on the basis of a cyclohexane balance from the various analyses. Generally there was good agreement between these material balances. From these values? adjusted yields of products were obtained. Component balances were then calculated on the basis of these adjusted yields. I n determining component balances all possible sources of data were used. In most instances duplicate samples of the run composites were analyzed. Numerous spot samples were subjected to control testing, including naphthene analysis by refractive index and cyclohexane purity by density and crystal point measurements. Due cognizance \vas given to both the frequency of testing and accuracy of the method \vhen the final component balances were determined. -4s a result of this extensive analytical program, normalized component balances pvere obtained which present a detailed balance of all streams throughout this entire operation. These normalized values represent the best possible approximation of composition changes occurring within the system. In the subsequent discussions of the data. these values are used as a basis for thr correlations developed. hfethylcyclopentane is essentially completely rejected in the raffinate layer of
CHEMICALS the extractive distillation column. Beiizene is carried through the process and appears in the cyclohexane product. For data analysis it appears advisable to divide the paraffinic components arbitrarily into a light a n d heavy classification. I n these discussions light paraffins include 2,2-dimethvlpentane, 2,4-dimethylpentane, and 2.2,3-trimethylbutane, boiling at 174.6". 176.9", and 177.6" F., respectively. T h e heac? paraffinic materials include .3.?-dimethylpentane, 2,?-dirnethylpentane? ?-methy-lhexane, and ?-methylhexane: lvhose boiling points are 186.9", 133.6": 194.1", and 197.3" F.. respectively. T h e normal boiling point of cyclohexanc is 177.3" F. T h e items uf pimar!- concern in this process are the distribution of the light and heavy paraffinic components in the several streams. c>.clohexane recovery? and purity of thc recovered product. Figure 1 presents the results of the extractive distillation of the 76% cyclohexane concentrate. \\-hen the yield of extract is 84% or greater, no heavy paraffins are removed in the extraction distillation. T h e light paraffinic material remaining in the extract ranges from 0 to 2.5Yc of the total amount of light paraffins charged. There is some scatter of points in this case, but this is largel) a reflection of the small quantity of light paraffinic material involved. Inclusion of light paraffins to the above extent in the extract represents a maxim u m concentration of only 0.37, of these several light components. I t is interesting to compare the experimental data in Figure 1 \vith the calculated values in Table 11. Calculated data indicate that the extract yield should approsimate 82%. At this level 72% or the heavy paraffins should be found in the extract. Experimental data show that the average amount of heav) paraffins contained in the extract layer approximates 967,. Recovery of cyclohexane is approsimately 2 to 3 7 , below
Table V. Calculated and Experimental Product Composition
Figure 3. The yield of product of given purity i s generally within 0.5% of the calculated value
A
80
Y ~ e l d c:
H , g h P u r i t y C y c l c h e ~ c n e , B a r r e l s p e r 100 B a r r e l s c i 76%
the predicted value. This sug-gests that the ;dative volatilities used cn- the calculations pertaining to heavy paraffins \\-ere in error to some degree. T h e data obtained in the fractionation step are summarized in Figure 2 . Of particular interest is the rapid loss in product recovery with increasing purity-i.e.: increasing the cyclohexane purity from 99.0 to 99.5% increases cyclohexane loss in this step from about 10% to about 20%. T h e product whose composition is given in Table 1. was obtained from the run summarized in Table 1L7. Product purity is somewhat higher than predicted and results primarily from more efficient removal of light paraffins than anticipated. O n the basis of the experimental data obtained, the relationships bet\veen yield of cyclohexane and product purity are presented graphically in Figure 3. T o indicate the usefulness and accuracy of this relationship, a summary of experimental data obtained in this study is presented in Table 1'1; lvith data obtained from these correlation curves. T h e yield of a given purity product predicted from this curve is generally ivithin 0.5Yc of the experimental value. A uniform distribution of positive and negative values results.
M ethylcyclopentane 2,2-Dimethylpentane
Benzene 2,4-Dimethylpentane Cyclohexane 2,2,3-Trimethylbutane 3,3-Dimethylpentane 1,l-Dimethylcyclopentane 2,3-Dimethylpentane 2-Methylhexane 1,3-Dimethylcyclopentane 1.2-Dimethylcyclopentane 3-Methylhexane
% 1-oluine 7 I-xptl. C'alcd. 0.00 0.00 0.09
0.02 0.05 0.08 0.00 0.16 99.22 98.94 0.00 0.09 0.33 0.25 0.09 0.17 0.00 0.15 0.18 0.07 0.00 0.01 0.09 0.00 0.00 0.01
Co-,centrate
Conclusions
-4combination estractive distillationfractionation separation process is suitable for producing high purity cyclohexane. Extended periods of continuous operation \\-ere realized bvith product purities of 99.2yc; spot samples of as high as 99.6YC \\-ere obtained. T h e over-all process should be conducted to produce a raffinate containing a maximum amount of light paraffins and a minimum amounr of heavy paraffins. Yields of 84% extract give sarisfacturily high product purities a t the best over-all yield, T h e absolute maximum value of extract that might be produced free of light paraffins is approximatel>-87.5 since the feed contains about 1 2 . 5 light paraffins. Kemoval or heavy paraffins in the extiactive step is costly, in so far as cyclohexane recoveries are concerned. These can be more advantageously removed in the subsequent fractionation step. T h e separation efficiency achieved in the extraction step was somewhat less than calculated, \\hereas in the fractionation step efficiency was somewhat higher. The net effect of these factors \vas to give a slightly lower y i d d of product of a given degree of purity than had heen anticipated. Acknowledgment
Product purity is somewhat higher than predicted ('oingonent
FROM PETROLEUM
Table VI.
Yield vs. Purity
Higher purity is obtained at a sacriflce of yield YlCld of Produt t Pxoduct E x t r a c t Product PrePurity, Tieid, Yield, di( ted, T o 1 % Vol % T 01 % Vol R
96.70 98.63 98.67 98.99 99.00 99.22 99.25
78.1 78.1 78.1
75.2 77.7 84.0 75.2
68.6 64.4 64.0 58.2 60.7 62.0 54.4
69.0 63.8 63.4 58.0 60.3 62.5 56.0
A
+0.4 -0.6 -0.6 -0.2 -0.4 +0.5
+1.6
T h e authors thank the management of the Humble Oil & Refining. Co. for permission to publish this work. Special mention is due to T. hi. Ne\vsom for assistance in the dis rillation calculations. T h e efforts and invaluable assistance of H. L. it'ilder: research technician. are deserving of special acknolvledgment, RECEIVED for review . J d y 9, 1958 .ACCEPTED Octoher 31, 1958
Division of Petroleum Chemistry; Symposium on Recent Developments in Chrmicals from Petroleum, 133rd hfeeting, XCS, San Francisco, Calif., .April 1958.
vol.
51,
NO. 3
MARCH 1959
261
