I
G. T. GWIN, R. L. HEINRICH, E. J. HOFFMANN, R. S. MANNE, H. W. J. R. MILLER, and C. Le THORPE
H. MEYER,
Humble Oil & Refining Co., Baytown, Tex.
Hydrogenation of Asphalt for Fuel Products A catalytic process with favorable outlook for commercial application
DURING
the past five years the petroleum industry has become increasingly active in developing processes for converting vacuum crude residua into high quality gasolines and middle distillates, and thereby increasing the yield of prime fuel products from crude oil. From an economic standpoint, residuum conversion processes that result in a high selectivity to catalytic cracking feed stock, nominally a 650' to 1000° F. fraction, are of the greatest interest. Thermal cracking, or coking, of residua is in commercial use, and several coking processes are available for licensing (7, 9 ) .
Deasphalting the residuum, using light hydrocarbon solvents, is another means of increasing the yield of catalytic cracking feed stocks from crude petroleum. The deasphalted oil is a high quality catalytic cracking feed, and the asphalt can be blended into merchantable bunker fuel oil. A process for converting the asphalt fraction to catalytic cracking feed stock and lighter fuel products is highly desirable. The chemical structure of asphalt is complex and it is generally believed to consist of a complex colloidal system of hydrocarbons (4, 5 ) , which can be considered as dispersions of micelles in heavy ails, peptized to different degrees. The dispersed particles in asphalt are commonly referred to as asphaltenes; the concentration of asphaltenes is considered t o consist of that portion insoluble in
668
petroleum naphtha under certain specified conditions. Asphaltenes are probably aromatic and are known to be materials of high molecular weight (from 1O3 to 105). I t has been postulated that the micelle structure is stabilized by compounds containing polar groups, such as those involving oxygen, nitrogen, and sulfur atoms. I t follows then, that if these polar groups could be removed, the stability of the micelle could be destroyed with a resultant disassociation to smaller, Iess complex molecules. Hydrogenation, with removal of oxygen, nitrogen, and sulfur, offers a means for accomplishing this objective. It also improves the general quality of the oil by increasing the mole ratio of hydrogen to carbon in the average hydrocarbon molecule. The conversion of high boiling hydrocarbons by destructive hydrogenation has generally required pressures in the region of 3000 pounds per square inch. Economic considerations dictate use of lower pressures. The advent of hydrogenation catalysts which are insensitive to sulfur deactivation and allow operations a t relatively low pressures (200 to 1000 pounds per square inch gage) has resulted in considerable research in the field of hydrodesulfurization-hydroconversion of both high and low boiling petroleum fractions. Mixtures of cobalt and molybdenum oxides on alumina are particularly useful catalysts (7-3, 6, 8). The development of a low-pressure hydrodesulfurization-
INDUSTRIAL AND ENGINEERING CHEMISTRY
hydroconversion process would have advantages over more conventional thermal processes for residuum conversion-production of stable products of superior quality, low in sulfur content, with much higher yields of the more useful and valuable distillate products. The residual fraction, after hydrogenation, in addition to being produced in low yield, in most instances is suitable for blending into merchantable bunker fuel oil. For commercial operation the hydrogenation process must be operable for long periods of time at a constant and high level of catalyst activity, and the catalyst must be amenable to periodic regeneration. The results of pilot and bench-scale experimental studies to develop such an asphalt hydrogenation process, and the place of the hydrogenation process in petroleum refining, are the subject of this discussion. A simplified flowsheet for a commercial asphalt hydrogenation unit is shown on page 669.
Genera I Economics
The hydrogenation process for residuum upgrading can be applied alternatively to one or more of several residual fractions, such as vacuum crude residuum, reduced crude (atmospheric tower bottoms), asphalt, vis-breaker tar, and certain heavy deasphalted oils obtained from the deasphalting of crude residuum. The selection of the optimum
H Y D R O G E N I N THE PETROLEUM I N D U S T R Y
BY-PROWCT HYDROGEN
Q MAKE-UP COMPRESSORS
HZS TO SULFUR RECOMRY
.E RECYCLE COMPRESSORS
TAIL GAS TO
AMINE SYSTEM FRACTIONATOR
SEPARATOR
ASPHALT TO FUEL OIL
Process flow in a commercial plant such as proposed in this article would take a shape similar to this
Y
feed for processing becomes a rather complex economic problem when consideration is given to maximum utilization of available equipment in an existing refinery, the desire to minimize the investment for new facilities, and the need in some instances to maintain a specified eve1 of fuel oil production in order to meet sales commitments. A new refinery processing high-sulfur crudes may find it advantageous to hydrogenate the reduced crude. Although the investment for hydrogenation will be high, owing to the capacity required, the investment for vacuum crude distillation and for certain finishing operations will be eliminated. Furthermore, this operation partially desulfurizes the virgin gas oil ahead of catalytic cracking. However, when hydrogenation conditions are severe enough for sizable conversion of residuum, a substantial amount of virgin gas oil in the reduced crude feed is converted to lighter products. A mild operation with recycle of the unconverted bottoms minimizes the gas oil conversion. For an existing refinery that has vacuum distillation equipment, the vacuum residuum may be preferable to the reduced crude for hydrogenation, because the new investment will be much lower. Deasphalting of vacuum residuum with propane or butane can be combined with hydrogenation of the asphalt produced, to obtain the maximum yield of heavy gas oil for catalytic cracking. The hydrogenation of asphalt is particularly applicable to refiners that already have deasphalting
facilities for recovering gas oil from residuum. The asphalt yield from an average vacuum residuum usually is in the range of 40 to 50%. Hence, when asphalt is processed, the investment for hydrogenation is reduced substantially because of the lower capacity involved. In practice, it probably will be advantageous to flux the asphalt charge to hydrogenation with a refinery heating oil fraction to facilitate handling. I n the general application of hydrogenation to the conversion of residual stocks, it is desirable to maximize the yield of gas oil for catalytic cracking, as opposed to naphtha and light distillate heating oil products. The naphtha produced is low in octane quality; as a consequence, hydroforming or other catalytic reforming capacity must be provided for improving its octane rating, if a sizable amount of this naphtha is produced, in order to blend the naphtha into the refinery gasoline pool. Directionally, the selectivity to gas oil product is increased by hydrogenating a t relatively low temperatures, and obtaining the desired conversion level by either recycling the 1000" F.-and-heavier product or using low charge rates. The return for asphalt hydrogenation becomes most attractive when the hydrogenation is carried to a high conversion of 50 to 70010, by using rather severe process conditions or by product recycle. The 1000° F.-and-heavier bottoms product from the high conversion operation cannot be blended to merchantable fuel oil on account of quality limitations,
but can be burned as refinery fuel. Alternatively, moderate conversion of 25 to 45y0 will yield a bottoms product that can be blended to Bunker C fuel. This mild hydrogenation operation shows a lesser but reasonably attractive monetary return. The mild operation that produces a salable fuel oil will be of most interest to many refiners a t present. The monetary return for asphalt hydrogenation will vary over an appreciable range, depending on crude oil type, yield of asphalt taken from a given crude, conversion level at which the hydrogenation process is conducted, and the pre- . vailing market price of the basic fuel products. For certain specific cases, over-all product value credits in the order of 50 to 80 cents per barrel of asphalt feed have been calculated. In a number of typical refining situations, a reasonable return on investment can be expected from asphalt hydrogenation. T h e availability of by-product hydrogen a t low cost from hydroforming or catalytic reforming of naphthas has a markedly advantageous effect on the economics of the asphalt process, The increase in product values obtained from the process is due to a combination of credits based on improvement in the yield, distribution, and quality of the ultimate fuel products. Most credits are associated with catalytic cracking of the gas oil product. An additional product credit may be obtained by recovering elemental sulfur from the tail gas produced in hydrogenation. The hydrogen sulfide produced during hydrogenation is recovered from the residue gas, using an amine scrubber. Sulfur is then produced from the hydrogen sulfide by conventional methods such as the Klaus process. Sulfur recovery is of added interest in obviating atmospheric pollution in the refinery area. Process Conditions and Yields
I n general, the asphalt hydrogenation process is conducted a t 725' to 825" F. temperature, 400 to 800 pounds per square inch (gage) total pressure, feed rates from 0.5 to 2.0 volumes of liquid feed per hour per volume of catalyst, and from 1500 to 4500 standard cubic feet of hydrogen per barrel of feed. Milder conditions result in impractically low conversions to distillate products; under more severe conditions catalyst activity maybe rapidly lost. Cobalt molybdate on alumina catalyst, under properly selected conditions, may be employed continuously without air regeneration for 500 hours or more at conversion levels up to 60 volume Yo of the feed. Hydrogen consumption depends principally on the conversion level attained, and usually ranges between 300 and 1000 cubic feet per barrel of feed. Typical product yield data from a West Texas and a VOL. 49,
NO. 4
APRIL 1957
669
mixed asphalt (obtained by propane deasphalting a mixture of vacuum residua from West Texas, Salt Flat, Coastal, and mixed sweet crudes) are shown in Table I. The mixed asphalt feed, owing to the distillation and deasphalting conditions used in producing this stock, contains oil boiling below 1000 F. The sulfur and naphtha-insolubles inspections on the feed and total product indicate the high degree of desulfurization and the conversion of asphaltenes to lower molecular weight materials, respectively. The increase in API gravity also is an indication of conversion. The processing of these two significantly different asphalt feeds results in yields of up to 60 barrels of distillate prodducts per 100 barrels of asphalt feed. A significant expansion in liquid volume takes place, with a total recovery of up to 105 barrels of liquid product per 100 barrels of asphalt feed. The high yield of catalytic cracking feed stock (gas oil) is especially attractive. The dry gas yield in each case is only about 2 weight yoof the feed. About 80% of the sulfur in the feed is removed, and about 40y0 of the asphaltenes, as reflected by the naphtha-insolubles test, are converted to materials of lower molecular weight, that are more amenable to subsequent refinery processing. In general, the over-all quality of the hydrogenated products declines as the conversion is increased. The conversion may be varied over a wide range, by adjusting the process conditions such as temperature and space velocity. In experimental studies, once-through conversions of 1000° F.-and-heavier in the range of 20 to 7Oy0have been obtained and evaluated.
Catalyst The catalyst employed for all the asphalt hydrogenation studies discussed was cobalt molybdate supported on an
Table I.
Regeneration of Catalyst after Asphalt Hydrogenation Hours on asphalt since regeneration Carbon on catalyst, wt. Yo
37 18.0
alumina base, commercially available in quantity from a number of catalyst manufacturers. The cost of cobalt molybdate on alumina is such that the debit to the process for catalyst is considerable, Catalyst life and susceptibility to regeneration are, therefore, of prime importance, and major emphasis is placed on maximizing catalyst life so as to reduce the catalyst cost per barrel of asphalt processed. The catalyst is subject to activity loss due to build-up of carbon on the surface during normal operation. This type of deactivation may be overcome by periodic air burning to remove the contaminants from the surface. I n addition to this temporary deactivation, the deposition of noncombustible ash materials and certain nonreversible changes in the catalyst itself, such as loss of surface area, result in eventual permanent deactivation to a degree which necessitates discarding the catalyst. In the first 100 to 150 hours of operation with fresh catalyst or after regeneration, there is a fairly rapid and substantial decline in catalyst activity for both conversion of asphalt and desulfurization. Beyond this point, however, by suitable selection of conditions, activity can be held at a fairly constant level for several hundred hours. A 600-hour run was completed with little or no loss in activity. The pattern of this run indicates that run lengths between regenerations might be 1000 hours or greater, particularly if no departure from optimum conditions was permitted. A catalyst life study employing as feed stock the residuum from which the asphalt feed was produced has demon-
Typical Product Yields from Asphalt Hydrogenation West Texas Asphalt Product
Feed Product yields, vol. % of feed Dry gas (Cs-). wt. % Gasoline (C4-430' F.) Heating oil (430450' F.) Gas oil (650-1000' F.) Residuum (1000°+ F.) Total, wt. % vol. % Desulfurization, yo Conversion of 1000°+ F., vol. %
+
2.3 12.1 16.4 31.2 45.2 107.2 80 54.8
Mixed Asphalta Feed Product 2.3 9.0 13.7 42.9 38.5 106.4
83 61.5
Inspections on total liquid 0.6 12.1 6.5 16.5 Gravity, OAPI 5.10 1.01 2.80 0.48 Sulfur, wt. Yo 7.1 12.2 7.2 Naphtha insolubles, wt. Yo 12.3 a .Feed and total product inspections include flux oil. Yields and conversion on flux-free basis.
670
INDUSTRIAL AND ENGINEERING CHEMISTRY
146 25.4
439 29.7
506 21.4
strated that total catalyst life in excess of 6 months can be achieved with little permanent activity loss. During the 4000-hour demonstration run, 25 regenerations were made. An over-all catalyst life of one year or more probably could be obtained. Regenerations are carried out by admitting controlled concentratioiiv of air in admixture with nitrogen to the catalyst bed under carefully regulated temperature conditions. Oxygen concentration is limited to 1%, in order that catalyst surface temperatures will not approach those at which a phase change from gamma- to alpha-alumina occurs. Any such change severely diminishes activity. The carbon and occluded oil are burned off the catalyst and, after the flame front passes through the bed, the bed is purged and hydrogenation operations are resumed. That an equilibrium carbon on catalyst value is reached early in a hydrogenation run is shown by data based on determination of carbon dioxide in regeneration gas. The carbon content of the catalyst is no greater after 500 hours than after much shorter periods of operation. Reducing the particle size of the catalyst improves conversion and desulfurization, but may lead to pressure drop limitations in a reactor of commercial size.
Process Variables The pilot unit asphalt hydrogenation operations studied included the principal process variables, such as pressure, temperature, space velocity, hydrogen rate, hydrogen purity, the use of diluents to flux the asphalt, and the use of product recycle. Both conversion and desulfurization are increased as the total operating pressure is increased from 200 to 800 pounds per square inch gage. The advantages of operating at higher temperatures, such as increased conversion and desulfurization, have to be weighed against the disadvantages of a higher rate of temporary catalyst deactivation and the production of a residual stock of lower quality after recovery of the converted material. An interchange between temperature and space velocity can be made-Le., for a certain reactor temperature and space velocity a higher temperature and higher space velocity will result in the same degree of conversion and desulfurization. In general, increasing the hydrogen rate from 1000 to 3000 cubic feet per barrel of feed increased conversion and desulfurization, and dilution of the inlet hydrogen
H Y D R O G E N IN THE PETROLEUM I N D U S T R Y with light hydrocarbons such as those found in by-product hydrogen made it necessary to operate a t higher pressures to obtain a n equivalent hydrogen partial pressure and realize comparable results. When recycle hydrogen is used, a small advantage has been shown for removal of hydrogen sulfide from the gas prior to recycling. By recycle of the 1000" F.and-heavier product, over-all conversions as high as 8OYo can be obtained, but the quality of the 1000'F.-and-heavier material is degraded by each successive recycle. During these asphalt hydrogenation operations corrosion rates in pilot units constructed of 18-8 stainless steel were not excessive, as long as the temperatures during catalyst regeneration were kept below about 1050' F. High corrosion rates were encountered when carbon steel was used. For commercial operations it probably will be necessary to use ceramic or alloy steel liners in the reactor vessels and in certain piping in the product and regeneration systems.
Product Quality Table I1 shows properties of the products from catalytic hydrogeneration of asphalts (1) from a plant nonlubricating oil residuum mixture which does not contain Hawkins crude residuum, and (2) from a plant residuum containing about 2570 of Hawkins residuum (a very heavy crude of high asphalt content). These inspections are typical. Both asphalt feeds contained heating oil, which was used as a flux to aid in handling the asphalt. Yields are based on the fluxed asphalt. Naphtha. The yields of the total (7-430 ' F. naphtha fraction varied with conversion and usually were from 2 to 12 volume 7 0 based on the asphalt feed. The sulfur contents varied from 0.03 to 0.2 weight % and were lower than those of virgin naphtha from the same crude. I n general, the products from the more active 10- to 20-mesh catalysts were much lower in sulfur content than those from the l/s-inch pilled catalyst. The aromatic contents, which were relatively independent of catalyst activity, varied from 20 to 40 volume yo. The olefin contents, which were lower with the more active catalyst and increased with increase in the operating temperature, varied from 2 to 34 volume Yo. The octane qualities were approximately the same as for virgin naphthas and in many applications would require octane improvement by some other process. The oxidation stability and carbon-hydrogen ratio of these naphthas are more favorable than those of naphthas produced by thermal processes. The naphthas from hydrogenation should require less sweetening, acid treating, or caustic washing than
Table II.
Properties of Products from Hydrogenation of Asphalts Feed Humble Hawkins containing asphalt fluxed with thermal heating oil
Humble asphalt fluxed with catalytic heating oil Conversion of 1000°+ F., yo IBP-430' F. naphtha yield, vol. yo Naohtha insoections bravity, O-API Sulfur, wt. yo Res, O.N. 1.5 cc. TEL Res. O.N. 3 cc. TEL Motor O.N. 1.5 cc. TEL Motor O.N. 3 cc. TEL ~
+ + + +
430-650' F. heating oil yield, vol. yo Heating oil inspections Gravity, ' API Sulfur, wt. Yo Amline point, ' F. Neutralization value Copper strip corr., 1 hr. at 212' F. 650-1000' F. gas oil yield, vol. yo Gas oil inspections Gravity, ' API Sulfur, wt. yo Aniline point, ' F. Diesel index Viscosity, SUS at 210' F. Conradson carbon, wt. % Nitrogen, wt. % Nickel, p.p.m. Vanadium, p.p.m.
+
1000° F. bottoms yield, vol. To 1000° F.+ bottoms inspections Gravity, API Sulfur, wt. % Viscosity at 275' F., SSF Conradson carbon, wt. % Ash, wt. 70 Naphtha insolubles
16
44
40
3.89
10.44
11.24
43.3 0.116
44.5 0,026 68.8 77.4 66.0 70.0
45.2 0.12
14.83
19.68
18.42
28.7 0.645 119 0.048 2
29.0 0.055 126 0.021 3
28.5 0.61 129 0.108 3
17.27
30.54
29.40
15.1 1.40
17.3 0.35 161 29 63.0 1.28 0.336 0.22
