US3491018A - Hydrocracking start-up procedure - Google Patents

Description

United States Patent 3,491,018 HYDROCRACKING START-UP PROCEDURE Seymour C. Schuman, Rocky Hill, N.J., assignor to Cities Service Research and Development Company, New York, N.Y., a corporation of Delaware No Drawing. Filed Nov. 30, 1967, Ser. No. 686,813 Int. Cl. C10g 13/02 U.S. Cl. 208108 2 Claims ABSTRACT OF THE DISCLOSURE A start-up procedure for a hydrocarbon oil hydrocracking process in which a heavy, high boiling feed oil is initially fed to the reactor at a space velocity and temperature below normal operating conditions but under conditions such that a high level of conversion is obtained. Temperature and space velocity of the high boiling material are then increased until normal operating conditions are achieved while maintaining at all times a high conversion of high boiling materials to low boiling material. Recycle oil or cutter stock may be added along with the desired feed material so that the total space velocity of all oil fed to the reactor may be maintained relatively constant throughout the start-up process. This procedure is useful in reducing fouling of equipment and catalyst where it is desired to convert at least 75 volume percent of hydrocarbon oil boiling above 975 F. to material boiling below 975 F.

The techniques for conducting liquid phase hydrocracking of hydrocarbon oils under a wide variety of operating conditions are well known and established in the art. These techniques have been used in the treatment of both light and heavy hydrocarbon oils. In the hydrocracking of heavy hydrocarbon oils boiling above 975 F. to produce material boiling below 975 F. difficulty has been encountered due to fouling of equipment and catalyst under certain operating conditions. This fouling is particularly noticeable during start-up of the hydrocracking processes and in many instances has prevented operation at desired conversion levels. (As used in this application, the term conversion refers to the volume percent of hydrocarbon oil boiling above 975 F. which is converted to material boiling below 975 F.)

In the hydrocracking of heavy hydrocarbon feeds, such as residual fractions which substantially boil above about 975 F., the feed is reacted with hydrogen over a hydrogenation catalyst at elevated temperatures and pressures to hydrocrack high molecular weight hydrocarbon oils to lower molecular weight hydrocarbons. At the same time the sulfur present in the oil combines with hydrogen and is removed as hydrogen sulfide.

High conversion reactions for the hydrocracking of heavy hydrocarbon oils at conversion levels exceeding about 75% are normally conducted at liquid hourly space velocities (LHSV) between about 0.5 and about 5 based upon feed oil boiling above 975 F. Temperature between about 825 and about 900 F., pressures between about 1,000 and about 3,000 p.s.i.g. and hydrogen rates between about 1,000 and about 10,000 standard cubic feet of hydrogen per barrel (s.c.f./b.) of feed boiling above 975 F. are normal for such operations.

In starting up such a process for the hydrocracking of heavy hydrocarbon oils, it is generally necessary to heat the reactants to the temperature at'which the hydrocracking of heavy oils takes place, and then, taking advantage of both external heat and exothermic heat of reaction, gradually raise the temperature to the desired operating temperature needed for hydrocracking the heavy oil to the desired conversion level.

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In conventional start-up procedures for such a process, a low boiling oil such as cycle oil from catalytic cracking is fed into the system together with hydrogen at desired operating pressure, and the reactor heated to a temperature lower than desired operating temperatures, usually at least about 50 F. and frequently at least about 200 F. below desired operating temperature. Preheating of the system to normal operating temperature before cutting in the regular feed material is impracticable because of the excessive requirements for preheat furnaces. Thus, using the lighter oil, the reactor may be heated to about 600 to 800 F. At this point in the start-up procedure the low boiling feed is discontinued and the desired heavy feed material containing oil boiling above 975 F. is cut in at normal feed rates. Because of the low temperature and the fact that normal feed rates are employed, the initial conversion is quite low, frequently below about 50%. The temperature of the reactor is then gradually raised, utilizing both exothermic heat of reaction and external preheat. As temperature is brought up, conversion gradually rises until the desired conversion level of greater than has been attained at the normal operating temperature. Fouling of equipment and catalyst frequently occurs, sometimes to the extent that the desired conversion level is never reached.

It has now been found that fouling of equipment and catalyst during start-up of processes for the hydrocracking of heavy hydrocarbon oil can be avoided if the heavy oil feed is initially fed to the reactor at a space velocity substantially below that required for normal operation at the desired conversion level. According to the present invention, the high boiling feed oil is first introduced to the reactor at relatively low space velocities so that high conversion is maintained at all times. Then, as the temperature is raised, the amount of heavy feed oil used is also increased, until both temperature and space velocity with respect to heavy feed oil are at normal operating conditions. During this procedure, conversion of feed oil above 975 F. to oil boiling below 975 F. is maintained above 75% at all times. During the initial stages of introduction of high boiling oil, its LHSV may be between about 0.05 and about 0.25 and is thus between about one twentieth and about one-half of the LHSV utilized at normal operating temperatures to obtain the desired conversion.

Fouling of catalyst and equipment during conventional start-up procedures is believed to be due to solid carbonaceous material formed during the reaction. A good measure of such material is obtained by conventional BS & W measurements of the product in accordance with ASTM Test No. D179662. It has been found that for some unknown reason this sludge is formed at much greater rates in the conversion range between about 55 and about 70%. By starting up and maintaining conversion at all times above this range in accordance with the present invention, the unit is never operated within this conversion range and the excessive fouling caused by sludge build-up. when operating in this conversion range is thereby avoided. This is in contrast to conventional start-up procedures in which the unit must be operated within this undesirable conversion range for a significant amount of time, possibly from 8 to 48 hours, frequently sufficient to foul catalyst and equipment to the point where the process becomes inoperable. More rapid increase of conversion levels during conventional start-up procedures, although possibly helpful, is not feasible due to practical limitations of available heat needed to raise reactor temperatures to desired operating levels at full design feed rate of high boiling material. In any case. with some heavy hydrocarbon feedstocks, using a conventional start-up procedure, even if the reactor is operated within the conversion range fouling is irremediable. In contrast in accordance with this invention, the unit is never operated within this conversion range.

In order to introduce a high boiling feed oil at lower than normal space Velocities for start-up in accordance with the present invention, it is usually desirable to continue feeding lower boiling oil along with the high boiling feed materal so that the total rate of addition of oil to the reactor is maintained approximately constant at all times. This lower boiling oil can be the same oil used prio to introduction of the heavy feed oil into the reactor and may be obtained from any suitable source. For instance, cycle oils obtained from catalytic cracking operations or recycle oils obtained from the hydrocracking reaction itself are entirely suitable for this purpose. Such oils frequently boil between about 450- and about 1,000 F. and when used for this purpose may be referred to as cutter stock. Sufficient cutter stock is preferably used so that the total rate of addition of oil to the reactor varies by no more than about 50% during the start-up procedure.

Heavy hydrocarbon oils having significant proportions, usually at least ten volume percent, boiling above 975 F., are suitable feed oils for practicing the present invention and include for instance, residual fuel oil, uncracked gas oil, shale oil, bitumen (including that which occurs naturally, such as that found in the Athabasca tar sands), coal tar and other so-called bottom of the barrel materials.

Catalyst suitable for use in hydrocracking processes of the present invention may be any suitable hydrocracking catalyst, either nature or synthetic, the composition size and quantity of such catalyst particles forming no part of the present invention. Suitable catalysts include for example, cobalt, iron, molybdenum, nickel, tungsten, cobaltmolybdate, as well as their sulfides and oxides, used alone or together with other suitable catalysts such as naturally occurring silicates, etc. on suitable bases such as alumina or silica-alumina. The catalyst may be in the form of finely divided particles such as those used in fixed bed operations. Catalysts may be employed in any suitable form such as fixed bed, slurry, or the ebullated bed described in U.S. patent to Johanson, Re. 25,770.

The following examples will illustrate the advantage of the present invention as applied to start-up of a hydrocracking operation designed to convert 80% of feed material boiling above 975 F. to material boiling below 975 F. In both of the following examples, the feed material is an 11 degree API West Texas Vacuum Residuum about 90 volume percent of which boils above 975 F. This material has a sulfur content of about 3.2 wt. percent. The portion of this material boiling above 975 F. has an API gravity of 8 and contains 3.5 wt. percent sulfur. The operating unit in which this material is being hydrocracked to form lighter lower boiling products is a 2500 barrel per day plant designed to process this feed material at a space velocity of 1.0 v./hr./v. so that 80% of the residuum material boiling above 975 F. will be converted to material boiling below 975 F. This particular operating unit is designed to operate upflow with cobalt-molybdate catalyst in the so-called ebullated state as described in the above-mentioned Johanson patent.

EXAMPLE 1CONVENTIONAL START-UP In the conventional start-up procedure the unit was first purged with inert gas and then with hydrogen. The feed preheater was fired to gradually heat the reactor to about 450 F. At this time, 2500 barrels per day of a 22 API light catalytic cycle oil were fed to the system and the feed preheater was increasingly fired to gradually bring the reactor to about 600 F. At this time the light cycle oil was replaced with the same quantity of 15 API heavy catalytic cycle oil and the preheater fired further to raise the reactor temperature to 750 F. When this point was reached, the heavy cycle oil was replaced by the design feed, i.e. the 11 API West Texas Vacuum Residuum, which was now fed to the reactor at the design rate of 2500 barrels per day, corresponding to a space velocity of 1.0 v./hr./v. based upon material boiling above 975 F.

At a reactor temperature of 750 F. with the residuum feed, the conversion of material boiling above 975 F. to material boiling below 975 F. was less than 10% and the product BS & W less than 0.1 wt. percent. By firing the preheater further the reactor temperature was raised to 800 F. at which point inspection of the product indicated a conversion of about 40% with a product BS & W of 0.1 wt. percent. The reactor temperature was then increased more slowly to 825 F. (by still further firing of the preheater) where the first signs of fouling of the feed-product exchanger were apparent. This required still further firing of the preheater to maintain temperature at 825 F. Inspection of the product at this point indicated the residuum conversion to be about 56% with the product BS & W now increased to 0.5 wt. percent. In this particular unit, this represented the highest conversion at which the unit could be successfully operated for long periods of time with the conventional start-up procedure used in this example. Attempts to further increase conversion by increasing reaction temperature resulted in a continually worsening fouling of the product exchanger as BS & W value of the product increased. At a temperature of 840 F. with conversion at about 65 to 70% the exchanger was fouled to the point where insufficient heat transfer was available in the exchanger and the reactor thermocouples indicated partial slumping of the catalyst bed due to agglomeration of catalyst. At the same time the product BS & W was up to 1.0 wt. percent and the unit was shut down since it is impracticable and dangerous to operate under these conditions. Inspection of the unit after shutdown indicated catalyst agglomeration up to one foot in diameter, severe fouling of the product exchanger (which had to be burned out) and additional fouling of pipes and vessels downstream, apparently from the high BS & W value of the product. As indicated by the above example, it proved impossible to attain the design conversion with conventional start-up procedure.

EXAMPLE 2 This example illustrates the use of the novel start-up procedure of the present invention in avoiding the difficulties experienced in Example 1. The operating unit, feed stock and design conditions for normal operation are the same as described in Example 1.

Instarting up the unit, the hydrogen flow is initiated and the unit heated to 750 F. with light and heavy catalytic cycle oil as described in Example 1. However, when the reactor has attained a temperature of 750 F. and it is desired to feed the West Texas Vacuum Residuum, the latter is initially fed to the reactor at the rate of only 200 barrels per day instead of the full design rate of 2500 barrels per day as in Example 1. This represents a space velocity of approximately 0.08 v./hr./v. based upon material boiling above 975 F. as opposed to a space velocity of about 1.0 v./hr./v. based upon the design capacity of 2500 barrels per day of the residuum feed. Since the catalyst in this case must be maintained in random motion by the velocity of a liquid stream passing through the reactor, 2300 barrels per day of the catalytic cycle oil is continued in the feed so as to provide a total feed of 2500 barrels per day. Under these conditions, conversion of residuum is 83%, and the BS & W of the product is only 0.15 wt. percent, and no operating diificulties are noted. By further firing of the preheater, the reactor temperature is increased to 760 F., after which residuum feed rates are increased to 250 barrels per day (corresponding to a space velocity of 0.1 v./hr./v.) and feed rate of cycle oil is decreased to 2250 barrels per day. At this point the conversion is about, 82% and the product BS & W still less than .2 wt. percent. A further increase of reactor temperature to 770 F.

followed by an increase in residuum feed rate to 300 barrels per day (0.12 v./hr./v.) and corresponding decrease in cycle oil feed to 2200 barrels per day again maintains conversion and product BS & W at previously established levels with no operating difiiculties. Similarly, the temperature of the reactor is continually raised by corresponding increases in residuum feed rate and decreases in cycle oil feed rate, so as to maintain conversion at about 82% and total feed rate at 2500 barrels per day. When residuum feed of 2500 barrels per day has been attained, with a corresponding reactor temperature of 865 F., the unit is on stream at the design feed rate and design conversion of 82%, with the BS & W of the product only 0.2 wt. percent and no operating difiiculties, fouling of equipment or agglomeration of catalyst experienced. At this point also, the design space velocity of 1.0 v./hr./v. with respect to material boiling above 975 F. has been attained.

It can be seen from the above examples, that start-up of the reactor in accordance with the present invention enables the unit to be operated at all times at high conversion levels and avoids the undesirably high BS & W values of product which tends to foul equipment and catalyst in the conversion range between about 55 and about 70% While the invention has been described above with respect to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention and it is intended to cover all such changes and modifications in the appended claims.

I claim:

1. In the starting up of a process for hydrocracking heavy hydrocarbon feed oil at least volume percent of which boils above 975 F. in the presence of hydrogencontaining gas to convert at least 75 volume percent of the feed material boiling above 975 F. to material boiling below 975 F. at liquid hourly space velocities between about 0.5 and about 5.0, the improvement which comprises:

(a) initially feeding such heavy hydrocarbon oil to a hydrocracking reaction zone at a liquid hourly space velocity less than about one-half desired normal operating space velocity while maintaining the reaction zone at a temperature at least about F. below normal desired operating temperatures of 825 F. to 900 F.; and then (b) gradually increasing the temperature of the reaction zone to the normal desired operating temperature of 825 F. to 900 F. while simultaneously gradually increasing the space velocity of material boiling above 975 F. to normal desired operating space velocity of 0.5 to 5.0 v./hr./v. and at all times maintaining operating conditions such that conversion of material boiling above 975 F. to material boiling below 975 F. is at least about volume percent.

2. The process of claim 1 in which lower boiling hydrocarbon oil boiling below 975 F. is initially fed to the reaction zone together with feed oil boiling above 975 F. with the rate of addition of such lower boiling oil to the reaction zone being decreased as the rate of addition of feed oil boiling above 975 F. is increased so that the total rate of addition of oil to the reaction zone varies by no more than about 25% during the startup procedure.

References Cited UNITED STATES PATENTS 3,244,617 4/1966 Galbreath 2082l6 DELBERT E. GANTZ, Primary Examiner A. RIMENS, Assistant Examiner US. Cl. X.R. 208-2 1 6