Abstract:
A process for stabilization of heavy hydrocarbons to reduce sludge formation in storage tanks and/or transportation lines and to enhance the hydrocarbon yield includes mixing a paraffinic or heavy naphtha solvent having carbon numbers in the range 10 to 20 with the feedstock to solvent-flocculate a relatively small, predetermined portion of asphaltenes present in the feedstock, separating and flashing the sediment to recover a light hydrocarbon fraction, flashing the heavy hydrocarbon/solvent phase and recycling the solvent to stabilize the heavy hydrocarbons without significantly affecting the yield of valuable products.

Description:
RELATED APPLICATIONS 
     This application claims priority to provisional patent application U.S. Ser. No. 61/513,457 filed Jul. 29, 2011, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process for stabilization of heavy hydrocarbons by efficiently preventing sludge formation in storage tanks and/or transportation lines. 
     2. Description of Related Art 
     The composition of crude oils and their heavy hydrocarbon fractions varies greatly depending upon their geographic origins and types. Properties of several sample vacuum residues derived from various crude oils are shown in Table 1. As can be seen from Table 1, vacuum residues can have a sulfur content that ranges from 0.2 to 7.7 W % and a nitrogen content that ranges from 3800 to 7800 parts per million by weight (ppmw). Vacuum residues can also contain metals such as nickel and vanadium which make them difficult to process, since they deactivate or poison the catalysts used. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Properties of Sample Vacuum Residues 
               
             
          
           
               
                 Source 
                 Taching 
                 Brent 
                 Kirkuk 
                 Safaniya 
                 Athabasca 
                 Boscan 
                 Rospomare 
               
               
                   
               
             
          
           
               
                 Specific Gravity 
                 0.932 
                 0.984 
                 1.021 
                 1.04 
                 1.038 
                 1.035 
                 1.065 
               
               
                 API Gravity 
                 20.3 
                 12.3 
                 7.1 
                 4.6 
                 4.8 
                 5.2 
                 1.4 
               
               
                 Viscosity @ 100° F. 
                 175 
                 380 
                 870 
                 4000 
                 1300 
                 4000 
                 3500 
               
               
                 Sulfur W % 
                 0.2 
                 1.57 
                 5.23 
                 5.42 
                 4.94 
                 5.56 
                 7.66 
               
               
                 Nitrogen ppmw 
                 3800 
                 4700 
                 4000 
                 4300 
                 5700 
                 7800 
                 4200 
               
               
                 Conradson Carbon 
                 9.4 
                 16.5 
                 18 
                 24.6 
                 16.7 
                 19.3 
                 26.3 
               
               
                 Residue (CCR) 
                   
                   
                   
                   
                   
                   
                   
               
               
                 C 5 -Insolubles 
                 0.8 
                 3.5 
                 15.7 
                 23.6 
                 17.9 
                 23.2 
                 35.2 
               
               
                 C 7 -Insolubles 
                 0.3 
                 1 
                 7.7 
                 13.6 
                 10.2 
                 14.1 
                 23.9 
               
               
                 Nickel (Ni) 
                 10 
                 11 
                 52 
                 44 
                 101 
                 121 
                 71 
               
               
                 Vanadium (V) 
                 7 
                 38 
                 125 
                 162 
                 280 
                 1330 
                 278 
               
               
                   
               
             
          
         
       
     
     In addition, the vacuum residues shown in Table 1 contain asphaltenes that can range from 0.3 to 35 W %, depending upon the source of the crude oil. Asphaltenes are defined as the particles precipitated by addition of a low-boiling paraffin solvent such as normal-pentane. They are solid in nature and comprise polynuclear aromatic hydrocarbons. 
     The chemistry of asphaltenes is complex. It is known that the asphaltene molecular composition differs from one asphaltene to another depending on the solvent type used, operating conditions and the oil source. It is also known that the amount of asphaltenes decreases with an increase in the carbon number of the solvent used to separate the asphaltenes, but with a loss in the quality of the treated oil. The asphaltenes recovered using high carbon number solvents are highly condensed structures and are likely to form sediment when there is a change of conditions, i.e., in processing or during storage. 
     The structure of the oil phase is well explained by Pfeiffer and Saal, who proposed a colloidal model of petroleum as schematically illustrated in  FIG. 1 . According to this model, asphaltenes are dispersed by resin molecules and small molecules such as aromatics that act as a solvent for the asphaltenes-resin dispersion; hydrocarbons are present as a non-solvent. If the oil composition is altered, i.e., by adding more hydrocarbon saturates or removing resins by means of reaction or physical separation, the equilibrium between the oil components changes, in which case asphaltenes start to flocculate out of the solution and can coalesce and precipitate. 
     Asphaltenes start to precipitate in oil storage tanks and/or transportation lines once they flocculate out of the solution. The accumulated precipitate of asphaltenes form a hard sediment, also referred to as “sludge.” The technical problems created by sludge formation include blockage of pipelines and burner nozzles, reduction in storage capacity, pump malfunctions, corrosion, false measurements and plugging. The factors controlling the sludge formation are oxidation, electrostatic charging, coagulation, volatility and the precipitation of wax and solid components, which usually result from changed conditions. Routine industrial maintenance of storage tanks unavoidably means the temporary inoperability of equipment. Furthermore, when conventional treatments are used to remove sludge, there is a potential for a significant negative environmental impact. 
     Solvent deasphalting is a process employed in oil refineries to extract valuable components from residual oil. The extracted components can be further processed in the refinery where they are cracked and converted into lighter fractions, such as gasoline and diesel. Suitable residual oil feedstocks which can be used in solvent deasphalting processes include, for example, atmospheric distillation bottoms, vacuum distillation bottoms, crude oil, topped crude oils, coal oil extract, shale oils, and oils recovered from tar sands. Solvent deasphalting processes are well known and described, for instance, in U.S. Pat. No. 3,968,023, U.S. Pat. No. 4,017,383 and U.S. Pat. No. 4,125,458, all of which disclosures are incorporated herein by reference. 
     In a typical solvent deasphalting process, a light hydrocarbon solvent, which can be a combination of one or more paraffinic compounds, is admixed with a residual oil feed to flocculate and separate the solids formed from the oil. Common solvents and their mixtures used in the deasphalting process include normal and/or iso-paraffins with carbon numbers ranging from 1 to 7, preferably from 3 to 7, including most preferably, propanes, normal and/or iso butanes, pentanes, hexanes, and heptanes. Under elevated temperatures and pressures, generally below the critical temperature of the solvent, the mixture is separated into two liquid streams, including (1) a substantially asphaltenes-free stream of deasphalted oil, and (2) a mixture of asphaltenes and solvent that includes some dissolved deasphalted oil. 
     While the solvent deasphalting process can be effective in removing almost all of the asphaltenes from the feedstock and thereby reduce sludge formation, a large portion of feedstock is rejected as asphalt due to the nature of the low carbon number paraffinic solvent used, resulting in a large loss in yield. 
     The problem addressed by the present invention is how to efficiently process heavy hydrocarbon feeds to prevent sludge formation in storage tanks and/or transportation lines while minimizing any adverse effects on the quality and yield losses of the hydrocarbon stream that is treated. 
     SUMMARY OF THE INVENTION 
     The present invention broadly comprehends a process for the stabilization of heavy hydrocarbons that prevents sludge formation in storage tanks and/or transportation lines by removing a portion of asphaltenes that are sediment precursors and preventing further sediment formation, the process including the steps of: 
     a. mixing a solvent with a heavy hydrocarbon feedstock containing asphaltenes to solvent-flocculate a portion of the asphaltenes that are sediment precursors present in the feedstock; 
     b. heating the combined stream of feedstock and solvent to produce feedstock containing solvent-flocculated asphaltenes; 
     c. separating the feedstock containing solvent-flocculated asphaltenes in a contact vessel into a solvent/hydrocarbon phase and a sediment phase; 
     d. flashing the solvent/hydrocarbon phase to produce a sediment-free hydrocarbon fraction and a solvent fraction; 
     e. flashing the sediment phase to produce a sediment bottom fraction and a light hydrocarbon fraction; 
     f. flashing the light hydrocarbon fraction to produce a sediment-free hydrocarbon fraction and a solvent fraction; 
     g. recycling the solvent fractions produced in steps (d) and (f) to step (a); and 
     h. recovering the sediment-free hydrocarbon fractions produced in steps (d) and (f). 
     As used herein, the term “sediment-free” fraction is used for convenience and means a fraction that has been treated in accordance with the process of the invention, which fraction is substantially free of sediment, but can contain a small proportion of sediment. 
     Solvents suitable for use in the process include paraffinic solvents having the formula C n H 2n+2 , where n=10 to 20, and heavy naphtha solvents having a carbon number in the range of from 10 to 20, and mixtures thereof. 
     The heavy hydrocarbon feed can be stabilized by removing from as little as 0.1 W % and up to 10 W % by the solvent-flocculation and treatment process of the invention. 
     The process and system described herein provide the following benefits: 
     1. Heavy hydrocarbons are stabilized during production, storage, transportation and refining processes. 
     2. High carbon number paraffinic or heavy naphtha solvents, e.g., C 10  to C 20  are used only to remove asphaltenes that are sediment precursors and to prevent further sediment formation. The sludge formation is reduced while yield loss is minimized. 
     3. The relatively low temperature and pressure operating conditions in the contact vessel allows addition of the equipment required for the practice of the process at a relatively low cost. The choice of the types of contact vessels that are suitable for use in the process to be used is very broad. 
     4. The process has broad application to heavy hydrocarbons, particular whole crude oil and its heavy fractions. 
     Other aspects, embodiments, and advantages of the process of the present invention are discussed in detail below. Moreover, it is to be understood that both the foregoing information and the following detailed description are merely illustrative examples of various aspects and embodiments, and are intended to provide an overview or framework for understanding the nature and character of the claimed features and embodiments. The accompanying drawings are included to provide illustration and a further understanding of the various aspects and embodiments. The drawings, together with the remainder of the specification, serve to explain principles and operations of the described and claimed aspects and embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing summary, as well as the following detailed description will be best understood when read in conjunction with the attached drawings, in which: 
         FIG. 1  is a schematic illustration that is representative of the nature of the colloidal dispersion of a petroleum mixture; and 
         FIG. 2  is a schematic flow diagram of a heavy hydrocarbon feedstock stabilization system and process in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 2 , a heavy hydrocarbon stabilization process and apparatus  10  is schematically illustrated. Apparatus  10  includes a heating vessel  20 , a contact vessel  30 , a first flash vessel  40 , a second flash vessel  50 , a third flash vessel  60 , and a solvent tank  70 . In another embodiment, apparatus  10  optionally includes a sediment-free hydrocarbon storage tank  80  and a sediment bottoms storage tank  90 . 
     Heating vessel  20  includes an inlet  21  for receiving the heavy hydrocarbon feedstock. Inlet  21  is in fluid communication with a conduit  73  which is in fluid communication with an outlet  72  of the solvent tank  70  for transferring the solvent. Heating vessel  20  also includes an outlet  22  for discharging heated feedstock containing solvent-flocculated asphaltenes. 
     Contact vessel  30  includes an inlet  31  in fluid communication with outlet  22  of heating vessel  20 , an outlet  32  for discharging a solvent/hydrocarbon phase and an outlet  34  for discharging the sediment phase. 
     First flash vessel  40  includes an inlet  41  in fluid communication with outlet  32  of contact vessel  30 , an outlet  42  for discharging sediment-free hydrocarbon for further downstream processing or for storage in optional tank  80 , and an outlet  44  for discharging solvent stream to storage tank  70 . 
     Second flash vessel  50  includes an inlet  51  in fluid communication with the outlet  34  of contact vessel  30 , an outlet  52  for discharging the light hydrocarbon fraction and an outlet  54  for discharging a sediment bottom to optional storage tank  90 . 
     Third flash vessel  60  includes an inlet  61  in fluid communication with the outlet  52  of second flash vessel  50 , an outlet  62  for discharging sediment-free hydrocarbon to optional storage tank  80  and an outlet  64  for discharging solvent stream to tank  70 . 
     Solvent tank  70  includes an inlet  74  for receiving fresh solvent and an inlet  71  in fluid communication with outlet  44  of first flash vessel  40  and outlet  64  of third flash vessel  60  for receiving recovered solvent. Solvent tank  70  also includes an outlet  75  for discharging excess solvent and an outlet  72  which is in fluid communication with conduit  73  for conveying solvent to heating vessel  20 . 
     In the practice of the method of the invention, a heavy hydrocarbon feedstock containing asphaltenes is mixed with the solvent in a ratio of solvent-to-feedstock of from 1:1 to 10:1 by volume. The ratio is based on an analysis of the feedstock and targeted stability of the treated stabilized feedstock in accordance IP-390 test method. The heavy hydrocarbon feed can be stabilized by removing from as little as 0.1 W % and up to 10 W % by the solvent-flocculation and treatment process of the invention. The combined stream is introduced into inlet  21  of heating vessel  20  and heated to from 100° C. to 300° C. to form solvent-flocculated asphaltenes in the feedstock. The heated feedstock containing solvent-flocculated asphaltenes is passed to contact vessel  30  where it forms a solvent/hydrocarbon phase and a sediment phase. 
     The solvent/hydrocarbon phase is passed to the first flash vessel  40  for the recovery of a solvent stream which is recovered via outlet  44  and stored in tank  70 ; a sediment-free hydrocarbon stream is discharged via outlet  42  and is either stored in tank  80 , or subjected to further downstream processing. The sediment phase is passed to the second flash vessel  50  for recovery of a light hydrocarbon fraction that is discharged via outlet  52 , and a sediment bottom that is discharged via outlet  54  and either stored in tank  90  or removed for appropriate disposition. The light hydrocarbon fraction is passed to the third flash vessel  60  for recovery of a sediment-free hydrocarbon stream that is discharged via outlet  62  and optionally stored in tank  80 ; the solvent stream is discharged in tank  70 . 
     In certain embodiments, a feedstock such as whole crude oil is flashed prior to the addition of the solvent to remove light naphtha and other light components. The remaining portion that is substantially free of light naphtha is passed to the crude oil stabilization apparatus  10  and processed in accordance with the process described above. 
     In certain embodiments, before the sediment bottom is recovered and stored in tank  90 , it is washed with hexadecane at a hexadecane-to-feedstock ratio of 5:1 by volume and/or a C 5  to C 7  light solvent such as pentane at a solvent-to-feedstock ratio in the range of about 1:1 by volume to remove remaining hydrocarbon feedstock and any other contaminants. The solvent can be recovered in a flash vessel for reuse. 
     The feedstocks for the heavy hydrocarbon stabilization process described herein are hydrocarbons derived from natural sources including whole crude oil, shale oils, coal liquids, bitumen, and tar sands, or those from refinery processes including vacuum gas oil, atmospheric or vacuum residue, products from coking, visbreaker and fluid catalytic cracking operations. The hydrocarbon feedstock has a boiling point above 36° C. 
     Suitable solvents include paraffinic solvents and heavy naphtha solvents. The paraffinic solvents have the general formula C n H 2n+2 , where n=10 to 20. Suitable paraffinic solvents include n-decane, n-undecane, n-dodecane, n-tridecane, n-tetradecane, n-pentadecane, n-hexadecane, n-heptadecane, n-octadecane, n-nonadecane, and n-eicosane. The heavy naphtha solvents can have a carbon number ranging from 10 to 20 and can be derived from crude oil or other intermediate refining processes, i.e., hydrocracking. 
     The contact vessel can be a batch vessel with an impeller, an extraction vessel, i.e., a centrifugal contactor, or contacting columns such as tray columns, spray columns, packed towers, rotating disc contactors and pulse columns. In general, the operating conditions for the contact vessel include a temperature of from 80° C. to 300° C., and in certain embodiments from 100° C. to 200° C.; a pressure of from 1 bar to 40 bars; a residence time of from 15 to 180 minutes, in certain embodiments from 35 to 90 minutes, and in further embodiments about 60 minutes. 
     The process of the invention represents an improvement over the prior art sludge treatment processes that is achieved by reducing sludge formation associated with heavy hydrocarbons by mixing one or more paraffinic or heavy naphtha solvents having carbon numbers in the range of from 10 to 20 with the feedstock to flocculate a predetermined and relatively small proportion of asphaltenes in the feedstock. In accordance with the present process, the heavy hydrocarbons are stabilized and the yield and quality of the treated hydrocarbon feed is not significantly affected by the solvent added. 
     EXAMPLES 
     Example 1 
     A hydrocarbon sample having an initial boiling point of 560° C., the properties of which are given in Table 2, was mixed with hexadecane at a 1:1 ratio by volume and maintained at 100° C. and atmospheric pressure for one hour. The combined product was filtered through a sintered glass filter having a 145 to 175 micron pore size, and 0.1 W % of asphaltenes were recovered. 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
             
               
                   
                 Sulfur 
                 1.3 
                 W % 
               
               
                   
                 Hydrogen 
                 10.0 
                 W % 
               
               
                   
                 Nitrogen 
                 4,000 
                 ppmw 
               
               
                   
                 Conradson Carbon Residue 
                 29 
                 W % 
               
               
                   
                 Pentane Asphaltenes 
                 6 
                 W % 
               
               
                   
                 Aromatics 
                 60 
                 W % 
               
               
                   
                   
               
             
          
         
       
     
     Example 2 
     A hydrocarbon sample having an initial boiling point of 290° C., the properties of which are given in Table 3, was mixed with hexadecane at a 1:1 ratio by volume and maintained at 100° C. and atmospheric pressure for one hour. The combined product was filtered through a sintered glass filter having 145 to 175 micron pore size, and 0.4 W % of asphaltenes were recovered. 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
             
               
                   
                 Sulfur 
                 1.5 
                 W % 
               
               
                   
                 Hydrogen 
                 11.2 
                 W % 
               
               
                   
                 Nitrogen 
                 2,200 
                 ppmw 
               
               
                   
                 Conradson Carbon Residue 
                 15 
                 W % 
               
               
                   
                 Pentane Asphaltenes 
                 3 
                 W % 
               
               
                   
                 Aromatics 
                 48 
                 W % 
               
               
                   
                   
               
             
          
         
       
     
     Example 3 
     A hydrocarbon sample having an initial boiling point of 210° C., the properties of which are given in Table 4, was mixed with hexadecane at a 1:1 ratio by volume and maintained at 100° C. and atmospheric pressure for 1 hour. The combined product was filtered through a sintered glass filter having 145 to 175 micron pore size, and 0.5 W % of asphaltenes were recovered. 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
             
             
               
                   
                 Sulfur 
                 1.0 
                 W % 
               
               
                   
                 Hydrogen 
                 10.7 
                 W % 
               
               
                   
                 Nitrogen 
                 2,000 
                 ppmw 
               
               
                   
                 Conradson Carbon Residue 
                 15 
                 W % 
               
               
                   
                 Pentane Asphaltenes 
                 3 
                 W % 
               
               
                   
                 Aromatics 
                 44 
                 W % 
               
               
                   
                   
               
             
          
         
       
     
     Example 4 
     A crude oil sample having an initial boiling point of 36° C. and an API gravity of 27.2°, the properties of which are given in Table 5, was mixed with hexadecane at a hexadecane-to-crude oil ratio of 1:1 by volume and maintained at 100° C. and atmospheric pressure for one hour. The combined product was filtered through a sintered glass filter having 145 to 175 micron pore size. The residue was washed with hexadecane at a hexadecane-to-crude oil ratio of 5:1 by volume and then with pentane at a pentane-to-crude oil ratio of 1:1 by volume and 1.4 W % of asphaltenes were obtained. 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 5 
               
               
                   
                   
               
             
             
               
                   
                 Sulfur 
                 3.0 
                 W % 
               
               
                   
                 Nitrogen 
                 1,430 
                 ppmw 
               
               
                   
                 Conradson Carbon Residue 
                 15 
                 W % 
               
               
                   
                   
               
             
          
         
       
     
     Example 5 
     A sample of the same crude oil used in Example 4 was mixed with hexadecane at a hexadecane-to-crude oil ratio of 1:5 by volume and maintained at 100° C. and atmospheric pressure for one hour. The combined stream was filtered through a sintered glass filter having 145 to 175 micron pore size. The residue was washed with pentane at a pentane-to-crude oil ratio of 5:1 by volume. 2.9 W % of asphaltenes were obtained. 
     The method and system of the invention have been described above and in the attached drawings; however, modifications will be apparent to those of ordinary skill in the art from this description and the scope of protection for the invention is to be determined by the claims that follow.