Urea Extractive Crystallization of
Straight-Chain Hydrocarbons
development
WM. A. BAILEY, Jkl, R. A. B A N N E R O T , L. C. FETTERLY: AND A. G. SMITH SHELL O I L CO., WILMINGTON, CALIF,
Extractive crystallization, based on the selective reaction of urea with straight-chain hydrocarbons, results in the separation of n-paraffins or olefins from petroleum fractions ranging from gasoline to heavy lubricating oil. This new separation process has been developed in a pilot plant of 2 barrels per day capacity, and the results of the study indicate that continuous operation on a commercial basis will be practical. In the pilot plant nearly complete removal of n-paraffin was achieved except for low boiling feed stocks, and extracts of better than 95% n-paraffin content were produced in a single stage. Potential applications of the process include production of pure n-paraffins, recovery of straight-chain olefins from cracked stocks, pour point reduction of oils and fuels, and octane number improvement.
T
HE formation of crystalline complexes between urea and normal paraffins was first reported by Bengen (9)whose work has been confirmed and greatly extended by this and other investigating groups in these laboratories. Recently Smith and Redlich et al. have published information relative to t,he crystal structure (9) and the composition and physical chemistry (6) of the complexes of urea with various straight-chain compounds. Other workers have also been active in this field: Zimmerschied and coworkers of the St,andard Oil Co. (Indiana) reported Iaboratory studies of these complexes (Y),and reports of further German work a t the.Badische iinilin und Soda Fabriken were published by Bengen and Schlenk ( 3 )and by Schlenk (8). Analogous complexes of thiourea with certain branched chain and cyclic hydrocarbons were discovered by one of the present authors ( 4 ) ,and are described in a recent paper ( 5 ) ; an independent study of thiourea complexes by Angla in France has also appeared ( I ) . The purpose of this paper is to describe the continuous process which was developed to utilize the urea reaction in practical large scale separation of straight-chain hydrocarbons from petroleum fractions. The name “extractive crystallization” has been adopted for this new type unit process, which comprises selective recovery of solid compounds formed by specific molecular types with urea, thiourea, and similar complexing agents. BASIC CONSIDERATIONS
The stability of the urea complexes of straight-chain compounds increases as the length of the carbon chain is increased (6). 1 Present address, Shell Oil Co., Martinez Research Laboratory, Martinez, Calif. 9 Present address, Shell Development Co., Emeryville, Calif.
Their stability increases also as thc activity-i.c., the conccntration-of urea in the reactant phzses is increased and decrcmcs as the temperature is raised. Hencc t,he longer the chain, thc lower will be the equilibrium concentration of the straight-chain compound in the unreacted residue; also a low equilibrium concentration is favored by high urea concentrat.ion and by low reaction temperature. Figures 1 and 2 , constructed from published data (e),show the concentration of n-paraffin in a hydrocarbon mixture, in equilibrium with saturated aqueous urea solution, and as a function of the number of carbon atoms and temperature. When several straight-chain hydrocarbons are present, the equilibrium concentration of each is reduced approximately in proportion t o its partial molal concentration in the total extract hydrocarbons. The region of stability of the complex is limited. It is usually necessary t,hat the urea solution be nearly saturated before the complex can form. Conversely, if the equilibrium reaction mixture is diluted or heated the complex will readily dissociate into urea and hydrocarbons. These factors must be carefully evaluated in selecting operating conditions for a given feed.
2 f0
10
t a z e W 0
z
1.0
0
E
lL
a E
0.1
?
L
0.01 IO
12
14
16
18
NUMBER OF CARBON ATOMS
Figure 1. Equilibrium Concentration of n-Paraffins in Hydrocarbon Mixtures Contacted with Saturated Urea Solution
Any given straight-chain hydrocarbon of sufficient chain length will combine with urea in a definite but nonintegral molc ratio (6-8),which increases with molecular weight of the hydrocarbon. The ratio of urea weight t o n-paraffin volume is nearly constanf; the average value calculated for n-paraffins from C I through CIS is 2.48 grams of urea per ml. of n-paraffin. This ratio is convenient for process calculations. 2125
INDUSTRIAL AND ENGINEERING CHEMISTRY
2126
T h e use of aqueous and alcoholic solvents has been previously W r i b e d (6). Study of reaction rates and crystal properties in various solvents led to the discovery that the lower molecular weight ketones with water have special advantages in the applicattim of the urea reaction to continuous operation. The choice among the several members of the ketone series depends on the boiling range of the feed stock as well as other considerations. Methyl isobutyl ketone is convenient to use, being commercially available and suitable for hydrocarbon mixtures ranging from !heavy naphtha to lubricating oil. This is the solvent used in the pilot plant operation described here.
lool
.
40
25
9c
50
60
/ OJ'
60°F
Figure 2.
60
80
too
TEMPERATURE
120
140
Effect of Temperature on Equilibrium Concentration
Figure 3 shows that methyl isobutyl ketone and water are only slightly soluble in each other and that uma is soluble in water but only slightly so in methyl isobutyl ketone. When hydrocarbons are added, methyl isobutyl ketone is partitioned l-,rgely into the hydrocarbon phase; it serves as a diluent for the hydrocarbon and operates to reduce the reaction rate barrier which exists between the aqueous urea solution and the oil phase; the urea remaim largely in the aqueous phase. Thus a rapid reaction can be effected by bringing together a saturated aqueous solution of urea and a solution of the hydrocarbon feed in methyl isobutyl ketone. The equilibrium distribution of stwight-chain hydrocarbom between the solid complex and the liquid phase ie, of course, influenced by the pieeence of the ketone, but the equilibrium data presented in Figures 1 and 2 are still approximately valid.
Vol. 43, No. 9
cooling was provided to remove sensible heat as well 8 s the heat of reaction. Consider, for example, a feed stock from which 15% n-paraffins must be removed by reaction with urea using 3 volumes of solution per volume of feed. In order to maintain the solution saturated under terminal conditions, i t was then sufficient to have the original solution saturated a t a teniperature 15' above the terminal temperature. The reactor section consisted of a single vessel with four discrete compartments which by test was equivalent to about two discrete equilibrium stages. However, a single compartment sufficed when maximum reaction of n-paraffins was not required, Each compartment was equipped for intensive mixing and contained heat-transfer surface. The temperature was reduced progressively in the several compartments in the direction of
flow.
The reaction product was a fluid slurry of the crystalline urea complex in the two-phase liquid system consisting of: (1) aqueous urea solution and (2) unreacted hydrocarbons (the residue) plus the ketone solvent. The crystalline complex was usually recovered by a rotary filter as shown in the diagram; however a centrifugal filter was also employed. (In some instances it was possible by simple settling to separate the residue phase clearly from the slurry of urea complex in urea solution.) The complex was washed on the filter or centrifuge with fresh ketone solvent to remove entrained residual hydrocarbons. The spent wash solvent was then used as the diluent in the reaction step. The pilot plant rotary filter was housed, and an atmosphere of inert gas was maintained within the housing. The gas drawn out through the filter and vacuum pump was recycled to the housing. The filtrate waa passed through a phase separator, from which were withdrawn (1) the ketone solution of the residual hydrocarbons and (2) the aqueous urea solution. After separation the former was washed with water to remove a small amount of dissolved and entrained urea and then was passed to the solvent recovery ssction. The urea solution from the separator was washed with fresh solvent to remove traces of dissolved or entrained hydrocarbons. It was then recombined with the washed filter cake of urea complex, sufficient heat being supplied to bring the temperature of the mixture to 40" to 70" F. above the reaction temperature. The combined effect of the increased temperature and the decreased urea activity (decreased percentage saturation) caused the complex to dissociate under these conditions, liberating the straight-chain hydrocarbons (the extract) as an oil phase and returning the urea to the aqueous solution.
PILOT PLANT DESIGN AND OPERATION
Process Flow. Using the ketone-water solvent system, a continuous process was achieved, in which the only solid handled W&B the urea-hydrocarbon complex itself. Figure 4 is a simplified flow diagram of the 2-barrel pilot plant, showing the major steps in the process. The streams charged to the reactor were the hydrocarbon feed, the ketone solvent, and aqueous urea solution. The aqueous phase of the reaction mixture was kept substantially saturated with urea, during and after reaction, by taking advantage of the high temperature coefficient of urea solubility in water (Figure 5 ) . A suitable amount of urea solution, saturated a t a temperature higher than the intended final reaction temperature, was admixed in the reactor with the feed and the organic solvent, and sufficient
Figure 3.
Phase Diagram for Urea-WatercMethyl Isobutyl Ketone at 85' F.
These two phases were separated. The extract hase, including some ketone solvent carried through in the fi&r cake, waa washed with water and passed to the solvent recovery section. The urea solution was returned to the reaction step. Solvent was removed from the extract and residue by distillation. It is important, of course, that there be a gap between the boiling point of the solvent and the initial boiling point of the
September 1951
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
hydrocarbon. In some runs on high boiling feed st'oclis, s k a m was injected a t the bottom of the columns to improve t,he recovery of solvent from the products. However, this steam stripping was found to be optional, and its use in a commercial plant would depend on the feed boiling range and on the maximum reboiler temperature available. A solvent-drying column was provided to remove water from the recovered solvent. I n runs in which steam stripping was not employed this drying column was operated intermittently or on a side stream, as i t served then only to e1iminat.e the small amount of water introduced via the feed and the product water washes.
2127
Convtruction can be of mild steel tliroughout, except that s h i n less steel is preferred for some of the filter parts. The tendency
of urea solution to corrode steel in the presence of air was overcome in the pilot plant by the addition of an inhibitor, preferably ammonium chromate, to the solution. Copper-bearing mrtals must be avoided as they are attacked by urea and by thr ammonia formed as a hydrolysis product. Rubber and many types of pIaatic materials are weakened or dissolved by the ketone solvent; therefore gasket and how materials must be spc~~ially selected. Polythene and Teflon were used in many locations in the pilot plant. Process Conditions. Optimum process conditions were found to vary a(.cording to the type of feed stock pro(*essed and the result required. Tnt)lrb I illustrates t-ypiral conditions e m p l o \ d uhen processing a straight run s t o w oil containing 12% n-paraffins. Temperatures somewhat hidlei t Iim those indicatcd could be employed 'ticcrssfully for high-boiling stocks, w l i i l ~a~ reactor temperature as low as 50" F. was necessary for 300" to 400" F. naphtha when high recovery of n-ptraffins was required. Solution-to-feed ratio was varied according to the content of n-hydrocarbons in the feed in order trJ eupply sufficient urea for the reaction and also to maintain a flowsble slurry. The solventto-feed ratio was found t o be important to reaction rate and (TVPtal size. Also, it was necessary t o maintain a sufficient solvent rate to thr filter to effect adequate washing of the filter cake. So Ivent-to-feed ratios from u 0.5 to above 1.0 were employed H U C C ~ S P Figure 4. Simplified Flow Diagram of Extractive Crystallization Process fullv for iced stocks containing 10 to 20% n-hydrocarbons. Several process details which could be applied in a commercial Reaction rate and crystal sin, wrre greatly influenced d s o h.v the mixing intensity in the reactor. Power input of thv order plant have been omitted from the pilot plant flow diagram for the 0.05 to 0.1 hp. per daily barrel of fced was required to obtniii :4 sake of simplicity. Thus the extract and residue water washes satisfactory slurry. might consist of'two countercurrent stages each, as a means of reducing water usage. Solvent would be recovered from the spent wash water by a single-stage contact of the latter with the entering feed. The wash water would be finally discarded, since 7 60 40 SO II0.C 20 30 the amount of urea which it contains would not justify recovery. Heat exchange between the filtrate and the initial reactor stages would probably be employed to reduce the cooling load. Equipment Requirements. The pilot plant studies led to the following conclusions regardlug equipment design:
n
1J';
s"
1. The rzactor must provide, in addition t o adequate residence time and power requirement for phase contact, an adequate amount of heat transfer surface. 2. Under the conditions cited, about 2000 B.t.u. per hour must he removed for each barrel of feed processed per day. A heat tranosfer coefficient of 200 to 300 B.t.u. per hour per s uare foot per F. was realized in the pilot plant. Process consaerations dictate a mean temperature difference of not over 10' F. The required heat tzansfer surface, assuming 200 B.t.u. per hour square foot per F., is accordingly between 1.0and 1.5 square eet per barrel of feed per day. The other parts of the proposed commercial plant can be largely of conventional design. The rotary filter, as indicated, can be in most respects standard. Lines containing urea sohtion should be heat-traced to avoid crystallization during shutdowns; this was found necessary in the pilot plant. If high molecular weight feeds are t o be processed the extract system must be heat-traced, Instrumentation should include temperature, flow, and level controllers.
Figure 5 .
Sdubilitg of Urea in Water
The slurry produced under properly chosen conditiona was frcefiltering, the crystals being uniform needles about 0.1 mm. in length. The cake was voluminous and somewhat cornprcssit)lr; a final cake volume of about 50 cubic feet per barrel of n-hydrocarbons extracted was typical. Careful trmprrature con t ro'i within the filter housing was necessary to offsrt t h r trndrncy of saturated urea solution to rryst:tlliw :ind vlog thcb filtcr.
Vol. 43, No. 9
INDUSTRIAL AND ENGINEERING CHEMISTRY
2128
Chemical Consumption. Hydrolysis of urea proceeds a t an appreciable rate a t the temperatures used in the process. The rate of hydrolysis is approximately doubled for each 10' F. increase in temperature. Hydrolysis rates in the range of 0.1 to 1.0 pound of urea per barrel of feed were experienced in the pilot plant, for operation a t reaction temperatures of 50' to 90' F. with correspondingly higher temperatures in other parts of the system and a normal solution inventory. It was found that the hydrolysis product, ammonium carbonate, could be tolerated in the solution without ill effect, and in fact tended to supprc'ss the hydrolysis. With most petroleum feed stocks the rate of accumulation of objectionable impurities in the solution was low, so that except for replacement of urea lost mechanically and by hydrolysis, solution replenishment should not be a major factor. Products. Normal hydrocarbons of 95% purity were readily obtained under the operating conditions indicated. Purity depenaed largely on the esciency of washing the filter cake. By increasing solvent usage above the usual range, a purity approaching 99% was usually obtainable in the process as described. I t has been shown (6) that in the higher molecular weight range slightly branched chains are appreciably reactive with urea. Extracts above about were found to contain such slightly branched hydrocarbons in appreciable quantity. Table I1 shows the properties of three typical feed stocks (a naphtha, light stove oil, and heavy stove oil derived from straight-run Los Angeles basin crude), with properties of the extracts and residues obtained from pilot plant operation.
TABLEI. TYPICAL OPERATING CONDITIONS FOR STOVE OIL
-
(%-Paraffincontent 12% volume) Final reaction temp., F. Regeneration temp F. Initial saturation t&p. of urea solution, O F. Urea solution circulation rate, gal./gal. feed Solvent circulation rate (methyl isobutyl ketone), gal./gal.. feed . Reactor residence time, minutes
80
140
95 3 0.7 12
The n-paraffiri content of residues was also estimated from the pour point or from the cloud point of the material after dilution with a dewasing solvent. An empirical correlation curve [vas first established for each stock using synthetic blends of ultimate residue with extract. Low temperature treatment with urea and methanol served t o produce a residue of nearly negligible nparaffin content for this purpose, and further, the n-paraffin content of such residues could be calculated from equilibrium data for the individual pure n-paraffins.. APPLICATIONS
Separations by single-stage urea extractive crystallization are sufficiently selective and complete to provide both a valuable normal hydrocarbon extract and an improved raffinate. Several practical applications of extractive crystallization are evident. The first is recovery of n-paraffin mixtures in nearly pure form. These have possibilities as odorless solvents, special lubricants, and chemical raw materials. Mixed n-paraffins can be separated by distillation into substantially pure individual compounds. Those above about CU are, of course, solid a t room temperature. An extract of 99% n-paraffin content obtained from light stove oil had the following composition :
ANALYTICAL M ETHOIIS
The extractable n-paraffin content of feed was estimated ( a )by actual laboratory scale extractive crystallization under controlled conditions or ( b ) by preparing urea extract and residue of ultimate quality, measuring their densities or refractive indices, and interpolating the corresponding property of the feed. It was assumed in ( b )that there was no volume change on mixing. In determining its purity the extract was reextracted with urea, or else was exhaustively sulfonated with fuming sulfuric acid, to obtain an ultimate extract; the density of the extract was then compared with that of the extract under test. A density value was estimated or measured for the non-normal paraffins removed by assuming the same composition as the non-normal paraffins present in the original feed.
Dodecane Tridecane Tetradecane Pentadecane Hexadecane (cetane)
Per Cent 5 10 15 16 14
Heptadecane Octadecane Nonadecane Oicosane Higher boiling
Per Cent 12 10 6 5
7
The n-paraffins recoverable by extractive crystallization with urea by the specific process described above vary from about CI or
PROPERTIES OF FEEDS AND PRODUCTS FROM EXTRACTIVE CRYSTALLIZATION TABLE 11. TYPICAL Feed Stock Reaction temp., O F. Yield, % vol. of feed n-Hydrocarbons, % vol. Properties Specific gravity, d 20/4O C. Refractive index, n 20/D Color..A.S.T.M. Viecositv a t 100' D155-45T F., CS. Viscosity Ir t -65' F., 08. t " F Pour pain Freezing 6dint,'FSB 141.1, F.
Feed
...
9.9
0.8121 1.4501 21 . 3 0 14.3
...
- 97
(Oilsstraieht - run fractions from Loa Angeles basin crude) Naphtha Light Stove Oil Extract Residue Feed Extract Residue 80 7 50 89.1 10.9 8.1 96.8 ... 11 97.0 1
...
0.7461 1.4199 30 b 1.30
... ...
-22
0.8182 1.4528 2 -1.27
a
b C
356 366 383 424 470
0.8441 1.4691 4 3.00
14.1
...
Below 100
-
46.8 Distillation, A.S.T.M. D158-41, * F. 1 B .P.. 349 356 369 396 452
...
346 356 371 410 454
-6
...
... ...
428 460 49 6 598 668:
0.7740 1.4269 30b 3.07
... ...
+53
... 453 486 530 614 668
Solid a t 20" C data obtained at 25' C. and extrapolated by use of API Research Project 44 data. Saybolt color {ASTM D 156-49) after finishi.ng treatment with 10 lb. 98% sulfuric acid per bbl. Blending octane number in a 5 0 % ~blend . with iso-octane.
0.8542 1.4755 4 3.26 nnn JZJ
-85 ...
... 438 464 498 590 666
Feed 7
ii
'
0.8621 1.4806 5 5.82
+is'.
... ... 420 506 580 668 726
Heavy Stove Oil Extract Residue 65 13.7 86.3 95 0 1
--.
0.78654 1,44230 176 4... .47
...
+77
... 530 558 595 667 709
0.8769 1.4857 7 6 .30 aonn
DO""
- 80 ...
... 444 51 6 580 674 734
September 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
CSto at,least C ~ Othe ; n-paraffins of highest molecular weight are found in heavy lubricating stocks or in the crude wax obtained therefrom by conventional methods. Such waxes contain large percentages of high melting hydrocarbons other than normal and slightly branched paraffins, so that extractive crystallization pi ovides a means of selectively recovering the latter types. Production of low pour point distillate fuels is readily accomplished by extractive crystallization. This can be done in conjunction with the n-paraffin production described. Thus extraction of n-paraffins from light and heavy stove oils, which are also typical Diesel fuel components, reduced pour point from original values of - 5 " and +25" F. to -85Oand -80" F., respectively. A reaction temperature of 70" F. or below, in a light stove oil,(was found satisfactory for the production of a residue of pour point below -100' F. A substantial amount of n-paraffins may remain in the residue, especially a t higher reaction temperatures, but these consist predominantly of the lowest boiling n-paraffins present in the feed, which have the least effect on pour point. With heavy stove oil a reaction temperature of 65' F. was used in the listed run in order to obtain nearly complete removal of n-paraffins, since it was desired to achieve the ultimate pour point possible with this heavy stock. Octane number improvement of gasoline stocks can be obtained by extractive crystallization. Thus in the naphtha shown in Table I1 removal of 8% n-paraffins raised the octane number by 11 to 12 units. Extractive crystallization may be used as an alternative to solvent extraction for cetane number improvement of distillate fuels. Straight-chain hydrocarbons can be extracted from low quality straight run or cracked intermediates and used as an ingredient of the finished fuel. For this application an extract of high purity is not required. Recovery of straight-chain olefins from cracked stocks is a particularly interesting application, as there are many chemical uses for such olefins. For example, a 328' t o 672' F. A.S.T.M.
boiling range, thermally cracked distillate containing 15% straight-chain hydrocarbons gave a 15% yield of extract containing 96% straight-chain hydrocarbons and rich in straightchain olefins. CONCLUSION
Extractive crystallization Las unique advantages in several practical applications. The process has undergone extensive development on pilot plant scale so that its practical operability is ensured, and a commercial plant could be designed. Extractive crystallization constitutes a new and unique separation tool added to those available to the petroleum and chemical industries. In view of the widely varying potential applications, this process should find a place in future petroleum and chemical technology. ACKNOWLEDGMENT
The help and cooperation of A. L. Johnson, S. York, R. E. Melrose, J. L. Maloney, and R. W. Barnes are particularly acknowledged. LITERATURE CITED
(1) Angla, B., Ann. Chim., 4, 639-98 (1949). (2) Bengen, F., U. S. Tech. Oil Mission, Reel 6, p. 263 [Ger. Patent Application 0.2.12438 (March 18, 1940)l. (3) Bengen, F., and Schlenk, W., Jr., Experientia, V/5,200 (1949). (4) Fetterly, L., U. S. Patent 2,499,820 (March 7, 1950). (5) Redlich, O., Gable, C. M., Beason, L. R. and Millar, R. W., J . Am. Chem. SOC..7 2 . 4 1 6 1 (1950). (6) Redlich, O., Gable, 'C. 'M., Dunlop, A. K., and Millar, R. W., Ibid., 72, 4153 (1950). (7) Zimmerschiedl W. J., Dinerstein, R. A., Weitkamp, A. W., and Marschner, R. F., Ibid., 71, 2947 (1950): IND. ENC).CHEM..42, 1300 (1950). (8) Schlenk, W., Jr., Ann., 565, 2 0 4 4 0 (1949). (9) Smith, A. E., J. Chem. Phys., 18, 150 (1950). RECEIVED October 16, 19.50.
Blowoff of Flames from Short Burner Ports CHANNING W. WILSON
2129
AND
Enginnyring pOCeSS
development
NORVAL J. HAWKINS
CONSOLIDATED GAS, ELECTRIC LIGHT AND POWER CO. OF BALTIMORE, BALTIMORE 3, MD.
Among the factors having important influence on the performance of gas appliance burners, the form of the burner ports, and the character of fluid flow through them, have received least attention. Means for quantitative evaluation of their influence has not been available. An analytical expression ,is here developed, through application of the concept of a ('critical boundary velocity gradient" at the limits of the stable flame region, which may be used to correlate the occurrence of blowoff on burners having short cylindrical ports. The ports studied experimentally approximate closely in size and shape those found in many domestic gas appliance burners. The validity of the expression has been confirmed with two fuel gases, for several nort sizes, at flow rates in the viscous region.
Through this method of approach, especially by extension to include other port forms, a clearer understanding of this element of burner design and performance may be reached.
G
AS appliances provided for consumers in different parts of the country must be designed and adjusted so that they will give safe and satisfactory performance, when supplied with the type of fuel gas locally available. The fuel gases available in different localities may have widely different properties. The chemical composition, calorific value, density, and the distribution pressure of gas in a given community will depend on the relative availability of the necessary raw materials and other economic factors, and in one community the properties of the fuel may
