Thermal treatment of heavy petroleum stocks

Abstract: This invention relates to a process in which the addition of a substantially inert solvent to a heavy petroleum stock permits high conversion hydrocracking and hydrogen addition to be accomplished in the absence of catalyst.

Description: This invention relates to thermal treatment of heavy petroleum stocks. Conversion of heavy petroleum stocks into distillates requires substantial boiling range reduction via thermal, catalytic, or hydrocatalytic cracking. One of the major difficulties in such conversion processes is that these heavy petroleum fractions are hydrogen deficient compared to the distillates into which they are to be converted. Their upgrading, particularly catalytic upgrading, is further complicated by substantial levels of heteroatoms (S+N), metals (Ni+V) and asphaltenes. Contaminant levels must be reduced and hydrogen content enriched by processes in which carbon is rejected or hydrogen is added. Carbon rejection processes necessarily limit liquid yield due to the constraints of stoichiometry. Furthermore, at high temperatures typical of thermal processing, thermodynamic equilibrium favors the formation of coke and high H-content gas. However, thermal carbon rejection processes are well known in the prior art. For example, coking easily converts 100% of a wide range of feedstocks, but is limited to relatively low coker gas oil yield. The products are unstable and require subsequent hydrotreating. Although coking is a flexible process, the quality and marketability of the coke and the degree of treatment required to upgrade the coker gas oil are dependent on feedstock quality. Another thermal process, visbreaking is a low conversion process mostly used to reduce feed viscosity and minimize heavy fuel oil production. Typical conversions are resticted to less than 40% to avoid significant coke formation. Fluid catalytic cracking is a carbon rejection process which uses a catalyst to maximize gasoline and gas oil quality and yield. However, the most advanced resid crackers are limited to feedstocks with less than about 10% Conradson carbon (CCR) and 60 ppm Ni+V. These specifications include almost all heavy stocks, and include only a limited number of higher quality or treated atmospheric resids. Due to stoichiometric limitations, gasoline and distillate yield is less than obtained in catalytic hydrocracking, but is greater than that in coking due to the action of the catalyst. A number of commercial and exploratory processes for hydrotreating or hydrocracking heavy stocks are known. These are catalytic processes in which hydrogen is added, but feed metals can poison the catalyst and shorten its life. Pressure drop may increase unacceptably in fixed bed processing from accumulation of particulate matter plus cake formed during reaction of the heavy stock, especially at high temperatures. These considerations result in poor cycle length and catalyst life. Moving bed configurations can avoid bed plugging, but fresh catalyst make-up rates, needed to maintain a constant activity level, depend on feedstock quality and are often unacceptably high. Furthermore, these catalytic processes can lead to unselectively high hydrogen consumption, resulting in, for example, hydrogenation of the high-octane aromatic components of gasoline. Hydrocracking processes such as CANMET and Veba Combicracking can obtain high resid conversions using a relatively wide range of feedstock qualities. Although these processes do not employ a fixed or moving bed of "conventional" catalyst, they do use additives which impart catalytic functionality. Conversion may occur substantially by a thermal mechanism, assisted by the "additive" which may catalytically stabilize the products, preventing their further degradation to coke and gas. US Patent No. 3,549,519 disclosed a process for thermally cracking a vacuum resid fraction which has been initially subjected to catalytic desulfurization and hydrogenation, in which the thermal cracking is affected in the presence of a lower-boiling, normally liquid diluent obtained from the thermally cracked product effluent. The process is effected in the absence of a gaseous hydrogen cofeed although, by virtue of the initial catalytic hydrogenation, the liquid charge stock contains a small amount of dissolved hydrogen. The present invention is a thermal process which uses a solvent to facilitate the addition of hydrogen to the heavy stock while suppressing coke and gas make. The ability of the present invention to add hydrogen in the absence of a catalyst of catalytic "additives" eliminates catalyst fouling and make-up rate problems which are inherent in typical catalytic processes. The current invention permits conversions in excess of 50%, typically 60-80%, with hydrogen addition of up to about 700 SCFB (heavy stock basis), and coke selectivities below about 1 wt%. This allows extended continuous operation with a wide range of feedstocks. A process for treating a heavy petroleum stock containing a fraction boiling above 540°C, comprising the steps of combining the stock with a substantially inert solvent, selected from at least one of a paraffin, olefin, cycloparaffin, aromatic compound and alkylaromatic compound, then feeding the combination to a hydrocracking zone together with gaseous hydrogen in an amount of at least 500 SCFB (90 Nm³/m³), and then thermally hydrocracking the stock in the presence of gaseous hydrogen and a substantially inert solvent. This invention relates to a process for treating heavy petroleum stocks comprising thermally hydrocracking a heavy stock in the presence of gaseous hydrogen and a substantially inert solvent. The item "thermal hydrocracking" is used herein to refer to a process in which the cracking occurs in the absence of a catalyst. This invention embodies a thermal process in which the addition of solvent permits heavy stock hydrocracking at greater than about 40%, preferably greater than about 50% conversion of 1000° F +(540° C+) material with hydrogen addition accomplished in the absence of a catalyst or additive. It is believed that the solvent helps reduce physical barriers to direct addition of gas phase hydrogen to free radicals formed in thermal cracking. In a preferred embodiment, the solvent is in its super-critical or dense super-critical phase regime. Dense super-critical means that at the operating conditions of the invention the pure solvent would be above its critical pressure but below its critical temperature, while super-critical would mean that the solvent is above both its critical pressure and its critical temperature. Heavy stocks, as defined in this invention, include whole crudes and distillation residues thereof having one or more of the following characteristics: (1) API gravity less than about 20 °; (2) hydrogen content below about 11 wt%; (3) metals concentration greater than about 100 ppm; and (4) heteroatom content greater than about 3 wt%. Increased upgrading of such heavy stock is expected in the future. The operating conditions for this new process are presented in the following table: Operating Ranges General Preferred Temperature (°F) 700 - 950 770 - 870 (°C) 370 - 510 410 - 470 Pressure (psig) 500 - 3,000 1000 - 2,000 (kPa) 3550 - 20800 7000 - 13900 LHSV (hr⁻¹) 0.1 - 5 0.5 - 3 Solvent (wt% of feed mixture) 10 - 90 15 - 40 H₂ Circulation (SCFB) 500 - 10,000 2,500 - 5,000 (Nm³/m³) 90 - 1780 450 - 900 Examples of solvents useful in the present invention include paraffins and olefins such as dodecane, hexadecane and hexadecene, cycloparaffins such as cyclohexane and decalin, aromatics such as benzene and naphthalene and alkylaromatics such as toluene, xylenes, propylbenzene and methylnaphthalene. Preferably, the solvent is in its super-critical or dense super-critical state at the operating conditions specified in Table 1, (for example, hexadecane, methylnaphthalene and toluene). The solvents are those which are not hydrogen donors, i.e., they are substantially inert under the reaction conditions specified in Table 1. The prior art indicates that high vacuum resid conversions (> about 50%) cannot be achieved in the presence of an inert solvent alone or the use of H₂ alone without substantial coke make. Applicants have discovered that through the combined use of an inert solvent and H₂, that elevated conversions (up to about 75% conversion at 1000°F + (540°C+) material) can be sustained with less than 1% coke selectivity. The invention will now be explained with reference to the examples and the accompanying drawings, in which: Fig. 1 is a graph comparing total volume of converted product with time on stream for a vacuum resid feed containing varying quantities of 1-methylnaphthalene solvent; Fig. 2 is a graph showing hydrogen consumption versus conversion with and without solvent present; Fig. 3 is a graph showing hydrogen consumption from the gas phase versus the H-content of liquid product formed from heavy stock conversion, the latter being calculated after subtracting the H-content of the solvent; and Fig. 4 is a graph comparing the level of hydrodesulfurization with conversion at various solvent levels. EXAMPLE 1 1-methylnaphthalene was combined with Arab Light vacuum resid in a 1:2 weight ratio and fed at 15 cc/hr over 10cc of an inert vycor packing with 150 SCCM H₂, at 1900 psig (13200 KPa) total pressure and 870°F (466°C). Vacuum resid conversion of 65% to 75% was sustained for greater than 10 days of operation at which time the run was arbitrarily terminated. At these conditions, 1-methylnaphthalene was above its critical pressure and below its critical temperature. This demonstrated sustained operability for vacuum resid conversions > 50% in the presence of solvent and H₂. Moreover, 1-methylnaphthalene conversion was less than 1 wt%, indicating that the solvent was substantially inert. EXAMPLE 2 When Arab Light vacuum resid was processed as in Example 1, but without solvent, vacuum resid conversion was limited to about 35% during the first 2 days. Increasing severity to achieve higher conversion led to unit plugging due to coke deposition and forced shutdown with 2 hours. The results of a series of runs similar to those of Examples 1 and 2 are shown in the accompanying drawings. Thus, as shown in Figure 1, total product converted at 1 LHSV increased from about 130 ml in the absence of solvent to over 1300 ml in the presence of 1-methylnaphthalene. As shown in Figure 2, insignificant H₂ consumption, less than 0.3 wt%, was observed in the absence of solvent, whereas in the presence of 1-methylnaphthalene, gas phase H₂ consumption was observed, increasing from about 0.6 wt% to 1.2 wt% with increasing resid conversion. Moreover, as shown in Figure 3, the gas phase H₂ consumption was largely incorporated in the liquid product, with the liquid H-content increasing by 0.83 wt% for every 1 wt% H₂ consumed. As shown in Figure 4, the presence of the 1-methylnaphtalene also increased the amount of hydrodesulfurization from about 20% (no solvent) to 37 - 53% (depending on the level of resid conversion). EXAMPLE 3 When Arab Light vacuum resid was processed, as in Example 1, but H₂ was replaced with N₂, vacuum resid conversion was limited to about 45%. Increasing severity did not increase conversion; forced shutdown due to excessive pressure drop occurred within 3 days of attempting the more severe condition. EXAMPLE 4 Since high conversion of Arab Light vacuum resid could not be achieved in fixed bed operation without solvent as described in Example 2, thermal conversion was carried out in a stirred autoclave with H₂ at 2000 psig (13890 KPa) and no solvent. In this mode, 67% vacuum resid conversion was achieved in the absence of solvent with corresponding coke selectivity of about 11%. Thus, coke selectivity in the absence of solvent, but in the presence of H₂ is about 11% while coke selectivity according to the present invention is less than 0.7% at comparable conversion. EXAMPLE 5 As in Example 1, but following a 10 day time-temperature protocol at 1 LHSV, maximum conversion was 57%. Initial conversion at 840°F was about 49%, and dropped to 39% after the 10-day test. Reduced conversion with time on stream was due to coke deposition in the bed of inert packing, increasing effective LHSV. Toluene, being above both its critical pressure and temperature under reaction conditions, was substituted for 1-methylnaphthalene in the same 10-day test. Performance was generally similar with toluene reaching the same maximum conversion with about the same percentage loss of conversion over the course of the 10-day test. EXAMPLE 6 The process of Example 1 was repeated, except that toluene was substituted for 1-methylnaphthalene, and solvent recovery from the product was greater than 97%. Even at relatively severe reaction conditions of 1 LHSV and 870°F (466°C), solvent conversion was less than 0.1 wt%. Thus, the solvent was substantially inert and did not participate directly in reactions with resid components. EXAMPLE 7 In Example 1, gas make was modest, changing little with conversion. With increasing conversion and hydrogen consumption, selectivity to liquid products in the gasoline and gas oil range increased as shown in the following Table: Temperature 870°F (466°C) 870°F (466°C) LHSV 1.0 0.4 1000°F⁺(540°C) Conversion, % 51 74 Selectivity, % C₄- gases 6.0 5.1 C₅/650°F (C₅/343°C) 51.1 63.4 650/1000°F (343/540°C) 41.2 29.1

Claims: A process for treating a heavy petroleum stock containing a fraction boiling above 540°C, comprising the steps of combining the stock with a substantially inert solvent, selected from at least one of a paraffin, olefin, cycloparaffin, aromatic compound and alkylaromatic compound, then feeding the combination to a hydrocracking zone together with gaseous hydrogen in an amount of at least 500 SCFB (90 Nm³/m³), and then thermally hydrocracking the stock in the presence of gaseous hydrogen and a substantially inert solvent. The process according to claim 1, wherein thermal hydrocracking occurs at greater than the critical temperature and critical pressure for the solvent. The process according to claim 1, wherein thermal hydrocracking occurs at greater than the critical pressure but less than the critical temperature of the solvent. The process according to any preceding claim wherein the solvent is 1-methylnaphthalene or toluene. The process according to claim 1, wherein the thermal hydrocracking occurs at a temperature between 700°F and 950°F (370°C and 510°C). The process according to any preceding claim wherein the thermal hydrocracking occurs at a pressure between 500 and 3,000 psig (3550 and 20800 Kpa). The process according to any preceding claim wherein the thermal hydrocracking occurs at LHSV between 0.1 and 5 hr⁻¹. The process according to any preceding claim wherein the thermal hydrocracking occurs wherein feed mixture contains between 10 and 90 wt% of solvent. The process according to any preceding claim wherein the thermal hydrocracking occurs with gaseous hydrogen circulating between 500 and 10,000 SCFB (90 and 1780 Nm³/n³).

Inventors: SMITH CHARLES MORRIS, WALSH DENNIS EDWARD, SMITH, CHARLES MORRIS, WALSH, DENNIS EDWARD
Applicants: MOBIL OIL CORP, MOBIL OIL CORPORATION
Application year: 1990
Main classification: C10G47
IPC codes: C10G47