Characterization of Petroleum Residuurns by Vacuum Flashing PAUL H. JOHNSON AND K. L. MILLS, JR. Phillips Petroleum Co., Research Division, Bartlesville, Okla.
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OR the past few years one of the main objectives of the refiner has been the greater utilization of the heavier fractions of crude oil. Factors contributing to this trend include the instability of the fuel oil market, the unprecedented demand for motor fuel, and the continuing effort directed toward crude oil conservation. Because it can efficiently handle high boiling feed stocks with a minimum of fuel oil production, catalytic cracking has given further impetus to the investigation of the high boiling portions of petroleum. Along with the increased use of heavy oils, the problems of separating and evaluating mixtures of exceedingly high boiling hydrocarbons, coping with metal contaminants and high sulfur content of the high boiling oils, and disposing of crude residuums which cannot be processed economically face the refining industry. ' The laboratory investigator finds that his working tools for examination of the high boiling portions of crude are inadequate. The complexity of the hydrocarbons and the molecular structures which may be present in the heavy oil tax the methods of identification, so that conventional analytical tests may be misleading. Similarly, conventional methods for precise separation can be used only to a limited degree. For lack of a better tool, vacuum flash vaporization has been used in this laboratory to separate high boiling gas oil and asphalt fractions from various crudes for the purpose of making a closer examination of these materials and to ascertain the best and most economical disposition of them. I t is realized that flash vaporization cannot produce precise separation either as to boiling point or hydrocarbon type. Be/. P R E S S U R E G A G E S cause of the type of separation i n v o 1v e d , care must be exercised in comparing data obtained from dissimilar crudes. Although flash vaporization leaves much to be dea i r e d as a r e s e a r c h BRONZE t o o l i t d o e s effect 0LOCU separations similar to those expected from TEYPERATURE refinery e q u i p m e n t , WELL/ and to that extent the results justify the use of vacuum flashing in characterizing h e a v y oils and tars. ~
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used include vacuum fractionation, short path or molecular distillation, and equilibrium vacuum flash vaporization. M o s t fractionation equipment, although successfully used over a rather wide range of subatmospheric pressures, is impractical a t very low pressure levels because of pressure drop limitations. In this type of equipment there is always the possibility of cracking because of the relatively long residence time, and a t extremely low pressures it is doubtful if any great degree of fractionation can be effected at all. The short path or molecular distillation has proved valuable in handling petroleum fractions and other materials which decompose easily; however, in this type of apparatus it is difficult to handle high-pour or viscous materials and the units are highly complicated. The equilibrium vacuum flash vaporizer has proved to be a simple efficient tool for eeparating high boiling petroleum fractions into cuts for subsequent examination of physical and chemical properties (1).
FLASH STILL. Vhile various units have been used in this laboratory for vacuum flash studies, only the one used most recently will be described here. This unit consists essentially of a coiled tubular preheater and a flash chamber, both cast in solid metal blocks which act as heat reservoirs. The preheater consists of 13 feet of l/Z-inch tubing wound into a 6-inch diameter coil; the flash chamber is a 6-inch cylinder with a cone bottom and rounded top, the total volume being approximately 1 gallon. A tangential feed entry is used and a single baffle near the top reduces carry-over to a low value without introducing seriom pre+ sure droo. Flash conditions are determined w i t h t h e u s e of shielded t hermocouples and pressure taps in the flash zone proper. A diagram of the flash still and associated equipment is shown in Figure 1. Since successful operation a t low pressures PREHEAT requires that pressure drop be negligible, a MEASURING detail on the type of EPUIPNENT c o n d e n s e r u s e d is shown in Figure 2. AUXILIARY E Q U I -P MENT. A 1abqrator.y vacuum pump is normally used to maintain pressures in the region of 0.5 to 10 mm., while for lower pressure (micron range) a Distillation Products M C 275 diffusion pump has been used. The pressure of the system is controlled by means of an aneroidtype regulator which admits air downstream of the condenser. A closed system is used so that the exhaust gas can be recycled and a n y exces6 g a s Schematic Diagram of Vacuum Flash Unit
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EQUIPMENT
I n making use of the d i f f e r e n c e in vapor pressure of high boiling petroleum components to effect a separation, several methods c o m m o n l y
Figure 1.
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produced in the flash zone is 3 - I N G I LINES the actual crude oil containvented through a wet-test ing these topped crudes were meter. Usual sources of exnot readily available; howcess gas are dissolved gases in the charge, leakage, and ever, inspection data for re.hydrocarbon cracking. Since finery d e l i v e r i e s of W e s t the leakage can be determined Kansas, West Texas, Panprior t o a test, it is possible to determine the dissolved handle, and Venezuela crudes and cracked gas produced are given in Table 11. under each test condition. The topped crudes were Flash conditions n o r m a 11y VICUUM are selected t o preclude flashed primarily to provide PUUP cracking. information on the yield and For moderately low presquality of relatively clean gas sures an aneroid-type vacuum oils that could be used for gage (0 to 50 mm.) and a 05-mm. McLeod-type gage are cracking stocks, to obtain used. F o r t h e v e r y l o w data for commercial vacuum pressures an expanded-scale unit design, and to determine McLeod gage and a thermoproperties of the vacuum cou le gage are normally use$. The higher pressure inresiduums. Of i m m e d i a t e struments may be connected interest were the carbon resiFigure 2. Detail of Condenser and Diffusion Pump at the condenser outlet and a due and heavy metal conbutyl phthalate differential tent of the vacuum gas oils manometer used to make pressure drop corrections. The low pressure instruments are and the properties of the vacuum residuums which affected final connected directly to the flash zone and must be suitably protected disposition and fuel oil yields. from vapor contamination. A summary of pertinent data obtained in flashing each of the METHODOF OPERATION.The topped crude or residuum topped crudes will be found in Tables I11 through VI. being studied is flashed over a range of temperatures and pressures to effect various depths of flash up to the gas-producing region. If desired a series of cuts can be made by using the flash TABLE11. PROPERTIES OF PIPELINE SAMPLES OF CRUDES chamber bottoms as feed to a succeeding flash operation. The West West Venezuela Panhandle Texas Kansas residuum is pumped a t the rate of 0.6 to 0.7 gallon per hour 42.7 37.8 41.0 until sufficient sample is obtained for analytical or test purposes. 46.4 To ensure good recovery and to prevent coke formation in the 0.1 0 0.1 1.9 0.27 0.45 0.06 0.39 preheater coil, the system is purged with nitrogen a t the end of - 25 - 20 - 40 +75 each test. Occasionally the unit is washed with hot kerosene 46.8 57.4 39.4 ... and any coke accumulations are removed by burning out with 40.7 39.4 35.7
