PROCESS FOR THE PREPARATION OF LIGHT HYDROCARBON DISTILLATES BY HYDROCRACKING AND CATALYTIC CRACKING

Abstract: Process for the preparation of a light hydrocarbon oil distillate by (1) hydrocracking a heavy vacuum distillate, (2) separating the product of (1) into distillates and a residue, (3) catalytic cracking the residue obtained in (2) and (4) isolating a light hydrocarbon oil distillate from the product of (3), in which process the residue obtained in (2) is catalytic­ ally cracked in (3) together with a further quantity of said heavy vacuum distllate.

Description: The invention relates to a process for the preparation of one or more light hydrocarbon oil distillates by applying the following steps:- step 1: hydrocracking a heavy vacuum hydrocarbon oil distillate, step 2: separating the product obtained in step 1 by means of distillation into one or more distillates and a residue, step 3: catalytically cracking the residue obtained in step 2, and step 4: isolating one or more light hydrocarbon oil distillates from the product obtained in step 3. In the atmospheric distillation of crude mineral oil, as applied on a large scale in refineries in the preparation of light hydrocarbon oil distillates, for example gasoline fractions, a residual oil is obtained as a by-product. Gasolines, as referred to herein, are those fractions having a boiling range at atmospheric pressure between that of n-pentane and 220 °C. To increase the yield of light hydrocarbon oil distillates from the crude oil concerned, a heavy hydrocarbon oil distillate can be separated from said residual oil by vacuum distillation, which heavy vacuum hydrocarbon oil distillate can be converted in a relatively simple way by hydrocracking or by catalytic cracking into one or more light hydrocarbon oil distillates. A process to which the invention relates is described in "Oil & Gas Journal", Feb. 16, 1987, pages 55-66 and is directed at meeting the increasing demands for middle distillates, i.e. those having an atmos­ pheric boiling range between 180 °C and 370 °C. It has now been found that, among the light hydro­ carbon oil distillates, gasoline fractions are obtained in a surprisingly high yield when making a proper use of the catalytic cracking in step 3. Accordingly, the invention provides a process for the preparation of one or more light hydrocarbon oil distillates by applying the following steps:- step 1: hydrocracking a heavy vacuum hydrocarbon oil distillate, step 2: separating the product obtained in step 1 by means of distillation into one or more distillates and a residue, step 3: catalytically cracking the residue obtained in step 2, and step 4: isolating one or more light hydrocarbon oil distillates from the product obtained in step 3, characterized in that the residue obtained in step 2 is catalytically cracked in step 3 together with a further quantity of said heavy vacuum hydrocarbon oil distillate. The process according to the present invention is first elucidated by means of the accompanying drawing in which Figures 1 and 2 schematically represent the process according to the present invention and the prior art process described hereinbefore, respectively. Referring to Figure 1, a heavy vacuum hydrocarbon oil distillate (hereinafter also referred to as "vacuum distillate") is introduced via a line 1a and a line 1 into a hydrocracker 2 in which the oil is hydrocracked (step 1). The product obtained in hydrocracker 2 is conducted through a line 3 and introduced into a distillation column 4 in which it is distilled with formation of a residue (step 2) which is withdrawn from column 4 via a line 5. This residue is introduced via the lines 5 and 5a into a catalytic cracker 6 in which the residue is catalytically cracked (step 3). The product obtained in catalytic cracker 6 is withdrawn therefrom via a line 7 and introduced via this line into a distillation column 8 from which a gasoline fraction is withdrawn via a line 9 (step 4) and a middle distillate fraction via a line 10. According to the present invention, vacuum distillate is introduced into the catalytic cracker 6, in the case as shown by branching off from the line 1a, conducting it via a line 11 and introducing it into line 5a where it is mixed with the residue conducted through the line 5. From the distillation column 4 a gas fraction is withdrawn via a line 12, a gasoline fraction via a line 13, a kerosine fraction via a line 14 and a gas oil fraction via a line 15. Coke is withdrawn from the catalytic cracker 6 via a line 16. From the distil­ lation column 8 a residue is withdrawn via a line 17 and a gas fraction via a line 18. Hydrogen is intro­ duced into the hydrocracker 2 via a line 19. The reference numbers in Figure 2 have the same meaning as the corresponding reference number in Figure 1; the differences with Figure 1 are that line 11 is not present in Figure 2 and that line 5 runs from distillation column 4 to catalytic cracker 6. The proper use of the catalytic cracking in step 3, mentioned hereinbefore, means that the residue of treated vacuum distillate obtained in step 2 (conducted through the line 5, see Figure 1) is catalytically cracked in step 3 together with a further quantity of untreated vacuum distillate (conducted via the line 11, see Figure 1). This use of the catalytic cracker results in a surprisingly high yield of gasoline, taking into account the yields of gasoline obtained by (1) the prior art process represented by Figure 2, and (2) a prior art process in which all of the vacuum distillate conducted through line 1 (see Figure 2) is not sent to the hydrocracker 2 but introduced directly in the catalytic cracker 6. The yield of gasoline in the process according to the present invention is surprisingly high, because it is significantly higher than could be expected on the basis of linear interpolation between the gasoline yields obtained in processes (1) and (2) mentioned hereinbefore. The vacuum distillate to be hydrocracked in step 1 may be any vacuum distillate obtained from crude mineral oil. Preferably, the vacuum distillate is a vacuum gas oil having a boiling range at atmospheric pressure in the range of from 200 °C to 600 °C. Such gas oils may be a mixture of gas oils obtained by vacuum distillation (that is to say at sub-atmospheric pressure) and gas oils obtained by distillation at atmospheric pressure. In the hydrocracking in step 1 lighter products are formed. This hydrocracking is mild, that is to say only a part of the vacuum heavy hydrocarbon oil distillate is cracked. The products formed are mainly in the kerosine and gas oil range, but gasoline and gas are also formed. Furthermore, sulphur compounds and nitrogen compounds, which are usually present in the vacuum distillate, are simultaneously converted in step 1, in hydrogen sulphide and ammonia, respectively. Hydrocracking is preferably carried out at a tempera­ ture in the range of from 375 °C to 450 °C, a pressure in the range of from 10 to 200 bar, a space velocity in the range of from 0.1 to 1.5 kg of vacuum distillate per litre of catalyst per hour and a hydrogen to vacuum distillate ratio in the range of from 100 to 2500 Nl per kg. In step 1 a catalyst is suitably applied which contains nickel and/or cobalt and, in addition, molyb­ denum and/or tungsten on a carrier, which contains more than 40% by weight of alumina. Very suitable catalysts for application in step 1 are catalysts comprising the combination cobalt/molybdenum on alumina as carrier or nickel/molybdenum on alumina as carrier. Step 2 is preferably carried out so as to obtain a residue having a boiling point at atmospheric pressure of at least 300 °C. In the process according to the present invention a considerable portion of the feed to step 3 is converted into distillate fractions. In the catalytic cracking process, which is preferably carried out in the presence of a zeolitic catalyst, coke is deposited on the catalyst. This coke is removed from the catalyst by burning off during a catalyst regeneration step that is combined with the catalytic cracking, whereby a waste gas is obtained substantially consisting of a mixture of carbon monoxide and carbon dioxide. Catalytic cracking is preferably carried out at a temperature in the range from 400 °C to 550 °C and a pressure in the range of from 1 to 10 bar. Furthermore, catalytic cracking is preferably carried out at a severity, indicated with "V s ", in the range of from 2.0 to 5.0, "V s " being defined as "t" being the contact time in seconds, between the catalyst and the feed, and α being equal to 0.30. The process according to the present invention may be carried out using a weight ratio of vacuum distillate (originating from the line 11) which is catalytically cracked in step 3 to vacuum distillate which is hydrocracked in step 1 (originating from the line 5) which is not critical and may vary within wide ranges. This weight ratio is suitably in the range of from 0.05 to 0.8 and is preferably in the range of from 0.1 to 0.6. The following Examples further illustrate the invention. In the Examples "%wt" and "ppm" mean "per cent by weight" and "parts per million by weight", respectively. The boiling points given are at atmos­ pheric pressure. A number of experiments are carried out in the manner as described hereinbefore with respect to Figures 1 and 2. The vacuum distillate conducted through line 1 has the following properties: The total content of carbon in aromatic structure and hydrogen bound to carbon in aromatic structure is 14.79 %wt. The conditions in the hydrocracker 2 are: Hydrocracking is carried out in the presence of a commercially available catalyst containing 3.0 %wt of nickel and 12.9 %wt of molybdenum (both calculated as metals on total catalyst) on alumina as the carrier. The catalyst has a surface area of 160 m²/g, a pore volume of 0.45 ml/g and a compacted bulk density of 0.82-0.83 kg/l. The catalyst is used as three-lobed extrudates having a largest dimension of 1.2 mm. The residue withdrawn from the distillation column 4 via the line 5 has the following properties: The total content of carbon in aromatic structure and hydrogen bound to carbon in aromatic structure is 11.15 %wt. Nickel and vanadium could not be detected in the residue. The residue in line 5 is obtained in a yield of 59.5 %wt, calculated on vacuum distillate in line 1. In all experiments described hereinafter the catalytic cracker 6 is operated so as to obtain the maximum gasoline yield and to produce in total 6.0 %wt of coke. Six experiments are carried out, according to the present invention, and are referred to hereinafter as Examples 1 to 6. In the Examples 1-6 140.5 parts by weight of the vacuum distillate is conducted via the line 1a (see Figure 1) and split into 100 parts by weight through line 1 and 40.5 parts by weight through line 11. The residue withdrawn from the distillation column 4 (see Figure 1, 59.5 parts by weight) is mixed with 40.5 parts by weight of vacuum distillate, orginitating from the line 11 and the mixture thus obtained (100 parts by weight) is conducted via the line 5a into the catalytic cracker 6. Catalytic cracking is carried out in the presence of a zeolitic catalyst and at a pressure of 2 bar. In each of the Examples 1-6 a different temperature is used in the catalytic cracker 6. Table 1 hereinafter states these temperatures in column 1 and presents in column 5 the yield of gasoline (withdrawn via the line 9), expressed in per cent by weight on the mixture conducted through the line 5a. Six further experiments are carried out, not according to the present invention, and are referred to herein as Comparative Experiments A1 to F1. The experiments A1-F1 were a repetition of the Examples 1-6, respectively, with the difference that the residue of the treated vacuum distillate withdrawn from the distillation column 4 (see Figure 2) is not mixed with untreated vacuum distillate, 100 parts by weight of vacuum distillate being conducted into the hydrocracker 2. The yield of gasoline found in each of these experiments A1-F1 is stated in Table 1 in column 3. Six other experiments are carried out, not according to the present invention, and are referred to herein as Comparative Experiments A2 to F2. In these experiments the vacuum distillate (100 parts by weight) is introduced directly into the catalytic cracker 6, no hydrocracking applied at all. The yield of gasoline found in each of these experiments A2-F2 is stated in Table 1 hereinbefore in column 9. Subsequently, the yields obtained in Comparative Experiments A1 and A2 are used to predict the yield of gasoline which could be expected for Example 1 on the basis of this yield being directly proportional to the fraction of untreated vacuum distillate in the feed to the catalytic cracker 6. For example, on this basis, the yield of gasoline which can be expected in Example 1 is 0.595 × 53.9 + 0.405 × 45.6 = 50.5%. This percentage is mentioned in Table 1 herein­ before in the top of column 7 and is referred to as "I1". Similar calculations have been made for the combinations B1-B2, C1-C2, D1-D2, E1-E2 and F1-F2. The results of these calculations are mentioned in Table 1, column 7 and are referred to as "I2", "I3", "I4", "I5" and "I6". A comparison between the yield obtained in Example 1 (52.2%) and that calculated as "I1" (50.5%) shows that the former is significantly higher. This higher percentage illustrates the synergistic effect of the process according to the present invention. Table 1 shows a similar synergistic effect by comparing the yield of Example 2 with "I2", of Example 3 with "I3", of Example 4 with "I4", of Example 5 with "I5" and of Example 6 with "I6". In Figure 3 of the attached drawing, the gasoline yield withdrawn from the catalytic cracker 6 via line 9, expressed in %wt, and the temperature applied in the catalytic cracker 6 are plotted along the vertical and horizontal axis, respectively. In Figure 3, the Examples 1-6 are indicated with a square, the Comparative Experiments A1-F1 with a + (plus), the Comparative Experiments A2-F2 with a # and the calculated yields I1-I6 with a * (asterisk). The numerals next to a square refer to the Examples having the same numeral. The indications A1-F1 next to a + refer to the Comparative Experiments having the same indication. The indications A2-F2 next to a # refer to the Comparative Experiments having the same indication. The indications I1-I6 next to a * refer to the same indications in the Table hereinbefore. The synergistic effect of the process according to the present invention is demonstrated by the hatched area in Figure 3.

Claims: 1. A process for the preparation of one or more light hydrocarbon oil distillates by applying the following steps:- step 1: hydrocracking a heavy vacuum hydrocarbon oil distillate, step 2: separating the product obtained in step 1 by means of distillation into one or more distillates and a residue, step 3: catalytically cracking the residue obtained in step 2, and step 4: isolating one or more light hydrocarbon oil distillates from the product obtained in step 3, characterized in that the residue obtained in step 2 is catalytically cracked in step 3 together with a further quantity of said heavy vacuum hydrocarbon oil distillate. 2. A process as claimed in claim 1 in which step 1 is carried out at a temperature in the range of from 375 °C to 450 °C, a pressure in the range of from 10 to 200 bar, a space velocity in the range of from 0.1 to 1.5 kg of heavy vacuum hydrocarbon oil distillate per litre of catalyst per hour and a hydrogen to heavy vacuum hydro­ carbon oil distillate ratio in the range of from 100 to 2500 Nl per kg. 3. A process as claimed in claim 1 or 2 in which in step 1 a catalyst is used containing the combination nickel-molybdenum on alumina as carrier or cobalt-­ molybdenum on alumina as carrier. 4. A process as claimed in any one of the preceding claims in which the residue obtained in step 2 has an initial boiling point at atmospheric pressure of at least 300 °C. 5. A process as claimed in any one of the preceding claims in which the catalytic cracking is carried out at a temperature in the range of 400 °C to 550 °C and a pressure in the range of from 1 to 10 bar. 6. A process as claimed in any one of the preceding claims in which the catalytic cracking is carried out at a severity Vsin the range of from 2.0 to 5.0, Vsbeing defined as "t" being the contact time in seconds, between the catalyst and the feed, and α being equal to 0.30. 7. A process as claimed in any one of the preceding claims in which in step 3 a zeolitic catalyst is used. 8. A process as claimed in any one of the preceding claims in which a weight ratio of heavy vacuum hydro­ carbon oil distillate which is catalytically cracked to in step 3 heavy vacuum hydrocarbon oil distillate which is hydrocracked in step 1 in the range of from 0.1 to 0.6 is applied. 9. A process for the preparation of one or more light hydrocarbon oil distillates as claimed in claim 1 substantially as hereinbefore described with reference to the Examples. 10. Light hydrocarbon oil distillates whenever prepared by a process as claimed in any one of the preceding claims.

Inventors: DE VRIES, AUKE FIMME, STORK, WILLEM HARTMAN JURRIAAN
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1988
Main classification: C10G69
IPC codes: C10G11, B01J29, B01J23, C10G69