US 3562146 A
Description (OCR text may contain errors)
Feb. 9, 1971 R. M. DEANESLY 3,562,146
STEAM CRACKING PROCESS Filed Dec. 12, 1968 x Q Q i g 0 h 1 g; 1Q t .7
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I II Q s m b b Yr 0; m a fi INVENTOR' Richard M. Deanes/y yaw? 2) ATTUR/VEYS United States Patent Q i 3,562,146 STEAM CRACKING PROCESS Richard M. Deanesly, Hinsdale, lll., assignor to Universal Oil Products Company, Des Plaines, 111., a corporation of Delaware Filed Dec. 12, 1968, Ser. No. 783,254 Int. Cl. (310g 9/36 US. Cl. 208-130 Claims ABSTRACT OF THE DISCLOSURE APPLICABILITY OF INVENTION The invention described herein is adaptable to a process for the thermal conversion of petroleum crude oil residua into lower-boiling hydrocarbon products. More specifically, the present invention is directed toward steam cracking hydrocarbonaceous residuals including atmospheric tower bottoms, vacuum tower bottoms, crude oil residuum, topped crude oils, coal oil extracts, crude oils extracted from tar sands, etc., all of which are commonly referred to in the art as black oils.
Petroleum crude oils, particularly the heavy oils extracted from tar sands, topped or reduced crudes, vacuum residuum, coal oil, etc., contain high molecular weight sulfurous compounds in large quantities. In addition, such black oils contain excessive quantities of nitrogenous compounds, high molecular weight organo-metallic complexes, comprising nickel and vanadium, and asphaltic material. Currently, an abundant supply of such hydrocarbonaceous material exists, much of which has a gravity less than API, and a larger proportion of which has a gravity less than 100 API. This material is generally further characterized by a boiling range indicating that 10.0% or more, by volume, boils above a temperature of about 1050 F., considered to be non-distillable. From an economic standpoint, the conversion of black oil into distillable hydrocarbons has not generally been considered feasible. However, the abundant supply of black oil virtually demands conversion, especially for the purpose of satisfying the ever-increasing need for greater volumes of distillable hydrocarbons.
The present invention is adaptable to the thermal conversion of black oils into distillable hydrocarbons in the presence of superheated steam. 'Exemplary of black oils, for the conversion of which the present scheme is applicable, include a vacuum tower bottoms product having a gravity of 7.1" API, containing about 4.0% by weight of sulfur and about 23.7% by weight asphaltics, a topped Middle-East Kuwait crude oil, having a gravity of 110 API, containing about 10.0% by weight of asphaltenes and about 5.2% by weight of sulfur; and, a vacuum residuum having a gravity of about 8.8 API, containing 3.0% by weight of sulfur and 4,300 ppm. of nitrogen, and having a 20.0% volumetric distillation point of 1055 F. The principal difficulties, respecting the conversion of black oils, stem from the presence of asphaltic material, and excessive quantities of metallic contami nants. This asphaltic material consists primarily of high molecular weight, non-distillable coke precursors which 3,562,146 Patented Feb. 9, 1971 are insoluble in light hydrocarbons such as pentane or heptane, and which are often found to be complexed with nitrogen, sulfur and metals.
The invention described herein is also adaptable to a process for the steam-thermal cracking of mixed-phase product effluent from a prior conversion process, wherein the charge to the thermal zone can comprise as much as 60.0 mol percent propane and lighter hydrocarbons, and hydrogen, in addition to a non-distillable asphaltic residuum in an amount as high as 25.0 mol percent. Such mixed-phase charge stocks, to which the present invention is applicable, generally stem, or are recovered from a prior conversion process such as hydrorefining of heavy hydrocarbonaceous material, conversion of contaminated black oils of low metals content, reduced crude desulfurization, etc. These conversion processes are conventionally conducted catalytically in the presence of hydrogen. The processes are designed to convert contaminated hydrocarbonaceous material into lower-boiling hydrocarbons substantially decreased in the concentration of contaminants, especially sulfur.
It has been found that an acceptable degree of desulturization, with respect to the quantity of distillable hydrocarbons produced, can be achieved catalytically at relatively mild operating severities which favor extended catalyst life. This has been improved further, with respect to the overall quantity of distillables produced, through the integration of a non-catalytic thermal reaction zone, or coil, with the fixed-bed catalytic system. That is, the fixed-bed product efiluent, or more often a portion thereof after separation, is subjected to thermal cracking in order to produce greater yields of lowerboiling distillables, while also converting at least a portion of the unconverted asphaltic residuum.
My invention further involves a process by which the thermal cracking of the previously-described mixed-phase product eflluent is effected. Through the use of my invention, the asphaltic residuum is concentrated, substantially free from distillable hydrocarbons, and greater yields of the latter are produced.
OBJECTS AND EMBODIMENTS An object of my invention is to provide a process for effecting the steam-thermal cracking of a hydrocarbonaceous charge stock containing an asphaltic residuum. A corollary objective is to provide a process for converting a greater proportion of the asphaltic residuum, contained in the mixed-phase effluent from prior hydrohydrogenative conversion processes, into distillable hydrocarbons.
Therefore, in a broad embodiment, the present invention affords a process for thermally cracking a heavy carbonaceous charge stock containing an asphaltic residuum, which proces comprises the steps of: (a) heating said charge stock and a lower-boiling, normally liquid diluent to a temperature below that at which thermal cracking of said charge stock is effected; (b) separately superheating steam to a temperature above that at which thermal cracking of hydrocarbons is effected; (c) admixing the heated charge stock and superheated steam, and reacting the resulting mixture in a reaction chamber maintained under an imposed pressure within the range of from about 15 to about 300 p.s.i.g.; (d) separating the resulting thermally-cracked product efiluent, to provide a first liquid phase principally comprising heptane and higher boiling hydrocarbons; (e) further separating said first liquid phase to provide at least a second, normally liquid phase substantially free from asphaltic residuum; and, (f) recycling at least a portion of said second liquid phase to combine with said charge stock as said lowerboiling diluent.
In a more limited embodiment, my invention affords a process for the steam-thermal cracking of a hydrocarbonaceous charge stock containing an asphaltic residuum fraction, which process comprises the steps of: (a) heating said charge stock and a lower-boiling, normally liquid diluent, in a combined liquid feed ratio of from about 1.111 to about 4.0:1, to a temperature in the range of from about 700 F. to about 900 F.; (b) separately superheating steam to a temperature of from about 950 F. to about 1500 F.; (c) admixing the heated charge and superheated steam, and reacting the resulting mix ture in a reaction chamber at a temperature of from 925 F. to about 1000 F. and a pressure of from to about 300 p.s.i.g.; (d) separating the resulting thermally-cracked product effluent, at a lower temperature, to provide a first liquid phase and a first vapor phase; (e) further separating said first liquid phase to provide at least a second liquid phase and to concentrate an asphaltic residuum substantially free from distillable hydrocarbons; (f) recycling a portion of said second liquid phase to combine with said charge stock as said lowerboiling diluent; (g) condensing said first vapor phase, at a temperature of from 60 F. to about 140 F., to provide a third liquid phase; and, (h) combining at least a portion of said third liquid phase with said thermallycracked product effluent.
Other embodiments, as hereinafter set forth in greater detail, are primarily concerned with particular process variables and processing techniques. The precise levels to which the individual streams are heated is, of course, dependent upon the characteristics of the charge stock as well as the relative concentrations of charge stock and superheated steam; however, the temperatures and concentrations will be such that the temperature of the charge stock-steam mixture, which is introduced into a reaction chamber, is within the range of from about 925 F. to about 1000 F. It will be recognized that these temperature levels are considerably higher than those permitted in present-day processes wherein charge stock mixture is heated to cracking temperatures in a reaction coil.
Other objects and embodiments relating to the present inventive concept will become evident from the following additional description of the process.
SUMMARY OF THE INVENTION The present process is effected in a manner such that the mixture of liquid charge and recycled diluent is never at a thermal cracking temperature in the heating coil. The liquid feed, consisting of fresh charge stock and a heavy recycled fraction, is separately heated to a temperature below that at which thermal cracking of hydrocarbons is eifected, or below about 900 -F. The first, or combined feed heater, raises the liquid charge to a temperature level below about 900 F., and preferably to a temperature within the range of about 700 F, to about 900 F. In a second, separate heater, the steam in an amount of about 5.0% to about 20.0% by weight, (based upon fresh hydrocarbon charge exclusive of recycle) is superheated to a temperature above that at which thermal cracking of hydrocarbons is normally effected. Therefore, the steam will be separately heated to a temperature within the range of about 950 F. to about 1500 -F. or higher. The thus-heated streams are then admixed just prior to introduction into a reaction chamber. In a preferred mode of operation, the individually heated streams are introduced into the upper portion of a reaction chamber through different feed points such that the mixture initially is formed within the reaction chamber. The reaction chamber will be maintained under an imposed pressure within the range of about 15 to about 300 p.s.i.g. Intermediate pressure levels are generally preferred, and include pressures within the range of about 50 to about 150 p.s.i.g.
Residence time, within the reaction chamber, may be obtained by providing the same with any of the well-known mechanical devices such as side-to-side pans, sieve decks, disc and donut trays, etc. The residence time will generally range from about thirty seconds to about two minutes. The precise residence time for a given charge stock is a function of temperature and the UOP K- factor. The latter is defined in detail in Chemical Process Principles, Part I, Hougen and Watson, p.p.m. 330-331, John Wiley & Sons, (1947). As an example, a charge stock having a K-factor of 12.6, characteristic of a highly parafiinic, easily cracked material, will require a lower residence time, at a given temperature, than a charge stock having a K-factor of about 11.2, characterstic of a highly refractory material.
In most applications, the thermal cracking process encompassed by the present invention utilizes, as principal charge stocks, either a virgin asphaltic residual fraction, or an asphaltene-containing heavy portion of a product effluent resulting from a prior conversion process. With respect to the latter, as a consequence of preferred processing techniques integrated into these processes, the total product eifiuent is generally separated to provide a hydrogen-rich gaseous phase, utilized as a recycle stream to the catalytic conversion zone, and a principally liqiud phase containing dissolved hydrogen and an asphaltic residuum. Thus, some of the applications of my invention will utilize that portion of the total product efiiuent not being recycled to the original catalytic conversion zone. In this context, the use of the term virgin asphaltic residual fraction, is intended to allude to that portion of a naturally-occurring petroleum crude oil, or coal oil extract, oil extracted from tar sands, etc., which remain following conventional separation in an atmospheric crude column, vacuum column, etc.
Coke deposition, particularly within the reaction coil, is avoided by separately heating the normally liquid charge stock to a temperature which does not exceed that temperature at which thermal cracking reactions are effected. Within the reaction chamber, the coke deposition is avoided or at least minimized by the presence of steam and the low partial pressure of the hydrocarbons within the zone. As a result of the relatively low partial pressure, maximum quantities of hydrocarbon components are recovered in the vaporous phase. Through the adjustment of the quantity of steam, within the range of from about 5.0% to about 20.0% by weight of the fresh charge stock, its temperature and that of the hot oil, any combination of temperature, pressure and residence time, appropriate to the charge stock being processed, can be readily obtained.
Other operating conditions and preferred operating techniques will be given in conjunction with the following description of the present process. In further describing this process, reference will be made to the accompanying figure which illustrates one embodiment of my invention.
DESCRIPTION OF DRAWING In the drawing, the embodiment is presented as a simplified flow diagram in which such details as pumps, instrumentation and controls, heat-exchange and heat-recovery circuits, start-up lines, compressors, valving and similar hardware have been omitted as nonessential to an understanding of the techniques involved. The utilization of such miscellaneous appurtenances, to modify the process, are well within the purview of those possessing expertise in the art of petroleum refining technology.
For the purpose of demonstrating the illustrated embodiment, the drawing will be described in connection with the conversion of a virgin vacuum residuum. It is understood that the charge stock, stream compositions, operating conditions, design of fractionators, separators and the like, are exemplary only, and may be varied widely without departure from the spirit of my invention, the scope of which is defined by the appended claims. With reference now to the drawing, the vacuum residuum is introduced into the process via line 1. Pertinent properties of the vacuum residuum include a gravity of about 8.8 API, sulfur in an amount of about 3.0% by Weight, a Conradson carbon residue factor of 16.0 wt. percent, an initial boiling point of about 690 F. and a 20.0% volumetric distillation temperature of about 1055 F. The fresh charge stock in line 1 is admixed with a recycle stream in line 2, the source of which is hereinafter described, and the amount of which provides a combined liquid feed ratio to charge heater 3 of from about 1.1:1 to about 4.021. Following suitable heat-exchange with various hot efiluent streams, which technique is not illustrated, the combined liquid charge continues through line 1 into heater 3, wherein the temperature thereof is raised to a level of from 700 F. to about 900 F.
Steam is introduced into the process via line 6, being admixed therein with a recycle stream of condensed steam in line 7, the mixture continuing through line 6 into steam superheater 8. The temperature of the steam is raised to a level above that at which thermal cracking of hydrocarbons is effected, and generally within the range of from about 950 F. to about 1500 F. The thus-heated superheated steam passes through line 9 into an unheated reaction zone 5. As indicated in the drawing, the heated liquid charge in line 4 is separately introduced into unheated reaction zone 5. Where desired, unheated reaction zone 5 can be designed as a vertical, elongated and substantially cylindrical vessel into which the two streams (lines 4 and 9) can be individually and separately introduced into the upper portion, such that the initial mixing takes place therein. This scheme is preferred from the standpoint of coke formation; obviously, there is a lesser risk than with in-line mixing. In any event, the temperature of the liquid charge super-heated steam mixture is in the range of from about 925 F. to about 1000 F. In addition, unheated reaction zone 5 is maintained under a pressure of from about 15 to about 300 p.s.i.g. Intermediate pressures are generally preferred, and are within the range of from about 50 to about 150 p.s.i.g.
The thermally-cracked product effluent, in mixedphase, is removed by way of line 10, and is quenched to a lower-temperature in the manner and to the degree hereinafter set forth. The thus-cooled, thermally-cracked product effluent is separated, preferably in a flash fractionator, or in a flash zone, having one or more distillation trays in the upper portion thereof, to provide a normally liquid phase relatively low in normally gaseous components, and a heavier normally liquid hydrocarbon stream, including the unconverted asphaltic residuum. A vaporous phase from flash zone 11 is withdrawn by way of line 12, and is further separated, in cold receiver 13, at substantially the same pressure imposed upon flash zone 11, but at a lower temperature of from about 60 F. to about 140 F. The temperature of the material leaving the upper portion of flash zone 11 via line 12, is maintained below about 850 F. in order to insure that none of the unconverted asphaltic residuum is carried over" into the cold receiver. The functions of the cold receiver are (1) to concentrate the normally gaseous components in a principally vaporous phase which can be sent to a light ends recovery system, along with other similarly constituted refinery streams, (2) to provide a principally liquid phase comprising butanes and heavier normally liquid hydrocarbons and (3) to condense and separate Water for internal recycle within the system. As indicated in the drawing, the principally vaporous phase is withdrawn through line 14, the normally liquid hydro carbon stream is withdrawn by way of line 15 and the water is removed through dip-leg 16 by way of line 7 for recycle therethrough to steam superheater 8. At least a portion of the liquid phase is diverted through lines 17 and 24 to quench the thermally-cracked product effluent in line 10 prior to the effluent being separated in flash zone 11. Portions of this quench stream can also be introduced at intermediate loci of the flash zone, especially located above one or more of the distillation trays in the upper rectifying portion thereof, as illustrated in the drawing by that portion which continues through line 17.
A bottoms, normally liquid stream is withdrawn from flash zone 11, by way of line 18 and introduced into a separation zone. Although the separation can be effected at pressures up to about p.s.i.g., the component analysis of this stream indicates that vacuum separation, at pressures of about 20 to 60 mm. of Hg, absolute, constitutes a preferred technique. Thus, as illustrated in the drawing, the bottoms liquid stream is introduced into vacuum column 19, and preferably at a temperature of about 700 F. to about 800 F. A light vacuum gas oil (LVGO) boiling from about 320 F. to about 750 F., a heavy vacuum gas oil (HVGO), containing 750 F.-plus distillables, are removed as product streams through lines 21 and 22 respectively. The unconverted asphaltic residuum, in an amount of about 15.0% by weight of the total fresh charge stock, is removed from vacuum column 19 through line 23. Any hydrocarbonaceous material boiling below about 320 F. is removed by way of the vacuum jets through line 20. A typical stream, emanating as the bottoms liquid from flash zone 11, contains about 5.1 mol percent normally liquid hydrocarbons boiling below a temperature of about 320 F. As hereinbefore set forth, a preferred technique is to withdraw a third principal product stream from vacuum column 19, a slop-wax cut containing distillable hydrocarbons boiling above a temperature of 980 F., for use as the recycled diluent, by way of line 2, to combine with the fresh charge stock in line 1. Since particularly desired product distributions sometimes demand, the illustrated embodi ment indicates that a portion of the HVGO is utilized as part of the diluent with the slop-wax out. In any event, the diluent will be in an amount such that the combined liquid feed ratio to charge heater 3 is about 1.1:1 to about 4.0:1.
With respect to a commercially-scaled unit designed for a charge capacity of about 20,000 barrels per day of fresh charge stock, the various component yields, exclusive of non-hydrocarbonaceous material, are presented in the following Table I:
TABLE I.OOMPONENT YIELDS By way of further illustrating my invention, this eX- ample is presented to indicate its application in converting a portion of a product effluent from a prior conversion process. The charge stock is a vacuum column bottoms product, having a gravity of about 60 API, and containing about 5.5% by weight of sulfur, and is intended for conversion into maximum distillable hydrocarbons having a sulfur concentration less than about 1.0% by Weight.
The vacuum bottoms is initially subjected to desulfurization and hydrogenation in a fixed-bed catalytic reaction zone. The product effluent is separated, at a pressure of'about 3,000 p.s.i.g. and a temperature of about 775 F., to provide a hydrogen-rich gaseous phase, at least a portion of which is employed as recycle to the catalytic reaction zone. The remaining portion of the principally liquid phase from this initial separation serves as the charge stock to the present process.
With reference to the drawing, based upon a commercially-scaled unit having a capacity of about 23,000 bbl./ day, the liquid phase charge stock enters the process through line 1 in the amount of 20,700 bbL/day (about 1246 mols/hr.), at a temperature of about 775 F. and a pressure of about 3,000 p.s.i.g. A recycled, lower-boil ing diluent, in an amount of 10,350 bbl./ day, the source of which is hereafter described, is admixed therewith, (combined liquid feed ratio of 1.5 :1) by way of line 2. The mixture continues through line 1 into heater 3 at a temperature of about 770 F. and a pressure of about 120 p.s.i.g., and is heated therein to 900 F. A component analysis of the 20,700 bbl./ day charge in line 1 is presented in the following Table II, in terms of mols/hr., for convenience.
Table II.-Fresh Charge Stock to Thermal Coil Based upon a fresh feed charge rate (line 1) of about 275,904 lbs./hr., 27,600 lbs./hr. of steam (10.0% by weight) are superheated, in a separate heater, to a temperature of about 1250 F., and admixed with the heated charge stock as both streams are introduced into the unheated reaction chamber; the temperature of the initial mixture is about 940 F.
The thermally-cracked product effluent, at a pressure of about 100 p.s.i.g., passes through line 10, is quenched with 374.22 mols/hr. of a condensed liquid hydrocarbon stream from line 24. In this particular operation, the quench liquid in line 24 is employed in an amount such that the temperature of the overhead fraction, removed by way of line 12, is at a temperature of about 800 F.; the pressure of this stream, as it enters receiver 13 is slightly less than 100 p.s.i.g. as a result of the normally experienced pressure drop through the system, The component analyses of the thermally-cracked effluent stream, and that of the quench liquid in line 24, exclusive of the 306.50 mols/hr. of water, are presented in the following Table III:
TABLE III.CRACKED EFFLUENT AND QUENCH LIQUID E [fluent Quench Component, mols/hr.:
Ammonia 9. 96 Hydrogen 402. 24 1. 24
750 F.-980 F 980 F.plns (distillable) Residuum (non-distillable) As hereinbefore stated, although not illustrated in the drawing, in addition to light and heavy gas oil streams,
TABLE IV.-FLASH ZONE STREAM ANALYSES Line number Component, mols/hr.:
Ammonia 9. 96 2. 70 490. 78 0. 72 97. 20 0. 88 153. 64 1. 00 71. 12 1. -14 92. 68 1. 30 68. 42 0. 94 36. 38 1. 98 62. 56 8. 06 219. 34 25. 98 266. 00 29. 26 92. 46 41. 68 59. 750" F.-980 F 147. 42 52.14 980 F.plus (distillable) 31. 32 Residuum (non distillableLflu 118. 16
With respect to the illustrative embodiment, the LVGO in line 21, 320 F. to 750 F., is in an amount of about 96.92 mols/hr., and the HVGO in line 22, 750 F.-plus distillables, exclusive of the amount of slop-wax recycled as diluent, is about 147.42 mols/hr. The asphaltic residuum is, obviously, the 118.16 mols/hr. shown in Table IV.
The separation being effected in receiver 13, exclusive of the quench liquid which continues through lines 17 and 24, is indicated in the following Table V:
TABLE V.COLD RECEIVER STREAM ANALYSES Line number Component, mols/l1r.:
Ammonia Hydrogen Hydrogen Sulfid 980 rI-pius (distillable) Residuum (non distillable) An overall product distribution, exclusive of the foregoing quench and recycle streams, but inclusive of the material subject to recovery from the vacuum jets and the light ends recovery system, is presented in the following Table VI:
Table VI0verall Product Distribution Component: Mols/hr. Methane 152.08 Ethane 68.10 Propane 81.78 Butanes 53.30 Pentanes-320 F. 203.58 320 F.-750 F. 352.30 750 F.-plus (distillable) 209.10 Residuum 118.16
The foregoing specification, and especially the example integrated into the drawing, indicates the method by which my invention is effected, and clearly illustrates the benefits afforded through the utilization thereof. It should be particularly noted that the asphaltic residuum has been reduced from 212.82 mols/hr. to 118.16 mols/hr.
I claim as my invention:
1. A process for thermally cracking a heavy carbonaceous charge stock containing an asphaltic residuum, which process comprises the steps of:
(a) heating said charge stock and a lower-boiling, normallly liquid diluent to a temperature below that at which thermal cracking of said charge stock is efiected;
(b) separately superheating steam to a temperature above that at which thermal cracking of hydrocarbons is effected;
(c) admixing the heated charge of sock with a sufficient amount of the superheated steam to supply the heat required for the cracking of said charge stock, reacting the resulting mixture in an unheated reaction chamber maintained under an imposed pressure within the range of from about 15 to about 300 p.s.i.g. and retaining the mixture in said chamber for a sufficient time to crack the charge stock by the heat of the superheated steam;
(d) separating the resulting thermal1y-cracked product efiluent, to provide a first liquid phase principally comprising heptane and higher boiling hydrocarbons;
(e) further separating said first liquid phase to pro vide at least a second, normally liquid phase substantially free from asphaltic residuum; and
(f) recycling at least a portion of said second liquid 10 phase to combine with said charge stock as said lower-boiling diluent.
2. The process of claim 1 further characterized in that said second liquid phase is recycled in an amount to provide a combined feed ratio to said thermal reaction zone above about 1.1:1.
3. The process of claim 1 further characterized in that said charge stock is heated to a temperature of from about 700 F. to about 900 F.
4. The process of claim 1 further characterized in that the temperature of the charge stock-superheated steam mixture, introduced into said reaction chamber, is within the range of from about 925 F. to about 1000 F.
5. The process of claim 1 further characterized in that said steam is superheated to a temperature above about 950 F.
References Cited UNITED STATES PATENTS 2,135,332 11/1938 Gary 208130 2,752,290 6/1956 Beattie 208l02 2,847,359 8/1958 Beuther et a1. 208l02 2,904,502 9/1959 Shapleigh 208130 3,371,030 2/1968 Penisten et a1. 208l02 HERBERT LEVINE, Primary Examiner US. Cl. X.R. 208102, 106