Production Of Coker Feedstocks

Abstract

Coker feedstocks having minimum amounts of sulfur or metals are produced from residuum feeds using an ebullated bed reaction zone. The qualities of the coker feedstocks are controlled through the use of selected operating conditions within the limits of converting 30 to 60 percent of the material in the feed boiling above 975.degree. F to lighter boiling products.

Description

BACKGROUND

With a large increase in light hydrocarbon product demand in the United States, the petroleum refiner has had to increase his throughput rate. As a result, he is faced with the problem of discarding the increased residual fractions which arise from refining more crude. Elimination of residuals has been the subject of considerable study by the refining industry. This has led to a substantial investment in processing facilities which provide a wide variety of approaches to the solution of this problem. Among the several processes either currently in operation or being installed are:

1. coking (both delayed and fluid)

2. propane deasphalting

3. partial oxidation

4. residue hydrocracking; and

5. residue desulfurization.

U.S. Pat. No. 2,871,182 discloses one method of making coke from long and short residua. This patent like other prior art teachings fails to solve the problem of how to produce a coke with minimum levels of metals or sulfur or how to handle the increased volume of residua without requiring increased coker units. The metals and sulfur content of the coke produced is dependent upon the characteristics of the coker feedstocks. In today's ecology oriented society, the restriction of pollutants such as sulfur and metals in fuel sources poses a situation that requires new methods by which fuels, such as coke, can be produced so that they will be low in sulfur and metals. At the same time different consumers require coke having different characteristics. This then leaves one with the task of producing a coker feedstock by a method which will allow one to control the sulfur and metals levels in the product. The prior art teaches that there is increased desulfurization and demetalization of residua feeds with increased severity of hydrogenation conditions and conversions. This teaching does not prove true when producing coker feedstocks from residua feeds.

SUMMARY

In this invention it was found that residua feeds could be converted into coker feedstock having minimum levels of sulfur or metals which would enable one to produce coke having specific desired characteristics. It was discovered in the operation of this invention that the level of sulfur and metals in a coke and, therefore, in a coker feedstock is dependent upon the percent conversion of the fraction in the feed boiling above 975.degree. F to material boiling below 975.degree. F. It was further discovered that the levels of sulfur and metals in the coker feedstock reach a minimum, dependent upon feed and space velocity, level between 30 and 60 percent conversion of the 975.degree. F.sup.+ fraction.

The problems of handling the increasing volume of residue solved by hydrogenating the residua through an ebullated bed reaction zone prior to using it as a feedstock to a coker. The ebullated bed is a three phase reaction zone in which the gas and liquid are passed upwardly through the particulate solids in random motion in the liquid. The operation of the ebullated bed reaction zone is disclosed in U.S. Pat. Re. 25,770.

It was discovered that in the ebullated bed reaction zone the resid is hydrocracked under conditions that can control the resid yield to match existing coker capacity. No additional cokers are necessary to handle the increased residua and operating conditions can be further varied to control the level of sulfur and metals in the coker feed.

A process has been discovered in this invention, wherein coke having less than 200 ppm vanadium and less than 2.5 percent sulfur can be produced from the residual fractions discarded in the refining of crude petroleum. The levels of sulfur and metals, hereinafter referred to as vanadium, vary with the source of the crude which in turn affects the levels of the sulfur and metals in the resid and, therefore, in the coker feedstock and the resulting coke.

In this process the minimum sulfur and vanadium that can be present in the product, coker feedstock, obtained from the hydrotreating of the resid can be determined experimentally for each type of crude. It has been determined that coke with the desired minimized sulfur and vanadium content can be obtained by operating the ebullated bed for the hydrotreating of the resid under pressures between 1,500 and 3,000 psig hydrogen partial pressure, temperatures between 700.degree. and 900.degree. F and preferably 750.degree. to 840.degree. F, a liquid space velocity of 0.3 to 1.5 volumes of feed per hour per volume of reactor, V.sub.f /hr/V.sub.r, with a suitable hydrotreating catalyst as hereinafter described, and a conversion of between about 30 and about 60 percent of the 975.degree. F plus fraction, in the resid, to lower boiling hydrocarbons. Within these conversion percentages the minimum of sulfur or vanadium in the product is a function of the temperature, space velocity, pressure and catalyst activity, i.e., replacement rate.

When the resid is hydrotreated under the above conditions, a liquid effluent is obtained from the reaction zone which has a 975.degree. F plus fraction with less than 2 percent sulfur and less than 70 ppm vanadium.

The liquid effluent undergoes a separation to provide the coker feedstock which will give a coke having the desired range of sulfur and vanadium content.

DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of the process for producing a coker feedstock from a hydrocarbon residuum.

FIG. 2 shows the change in the level of sulfur in the 975.degree. F+ fraction with a change in liquid space velocity and H.sub.2 pressure at a given percent conversion of the 975.degree. F+ material in the feed.

FIG. 3 shows the change in the level of vanadium in the 975.degree. F+ fraction with a change in the liquid space velocity and H.sub.2 pressure at a given conversion of the 975.degree. F+ material.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Generally, the ebullated bed hydrotreating of a hydrocarbon residuum material is carried out as diagrammed in FIG. 1 and may be described as follows:

The feed material which normally has at least 25 volume percent of components boiling above about 975.degree. F at 1 together with hydrogen in 5 passes in line 3 to reactor 2. Such a reactor will be charged with a suitable catalyst having the required hydrotreating, desulfurization or demetalization, characteristics. The catalyst particles will have a narrow size distribution with a diameter in the range from about 20 to about 325 USS mesh. Alternatively, catalysts in the form of extrudates of about 1/2 inch to about 1/32 inch diameter may be used.

The catalyst may be of any type of material which can affect hydrogenation of the sulfur and metal compounds in the feed. Particular examples would be cobalt molybdate on alumina (preferred), nickel molybdate on alumina, nickel tungstate on alumina, alumina and the like as defined in the prior art. Normally, the catalyst consists essentially of alumina promoted with metals and compounds of metals selected from groups VIb and VIII of the periodic table.

The residuum and hydrogen pass by upflow through the bed of catalyst such that the bed will tend to expand to at least 10 percent of the rest volume of the bed and such that the catalyst particles are all in random motion in the liquid. In such condition, they are described as ebullated. As stated in the Johanson patent cited hereinabove, one can operate the particular process so as to cause the mass of contact material in the fluid to become ebullated and to calculate the percent expansion of the ebullated mass at any given set of reaction conditions.

Under the preferred conditions of temperature, and pressure set forth, a total effluent is removed in 11 and passes with hydrogen in 7 through 9 to reactor 4 for further hydrogenation in an ebullated bed under the preferred operating conditions of reactor 2. Reactor 4 will also contain a suitable catalyst as hereinbefore described. The conversion of the 975.degree. F plus fraction in the resid feed by the hydrotreating process is between 30 and 60 percent.

A vapor overhead effluent is removed in 17 and a liquid effluent is removed in 13 to separator 6. The liquid effluent coker feedstock in 13 has less than 2 percent sulfur and less than 70 ppm vanadium in the 975.degree. F plus fraction. An overhead is removed in 21 and the desired coker feed is passed in 25 to coker 10. Coker 10 may be any of several known coking operations existing in the art. The 650.degree. F and lighter products from the coker are removed in 29 while the heavy gas oils leave coker 10 in 33. The coke leaves in 37. It has been found that a two or more stage hydrogenation is preferred although a single stage may be used as well.

FIG. 2 is a graph showing the effect of liquid space velocity and hydrogen pressure on the sulfur content of the 975.degree. F+ fraction in the product. Point A represents the sulfur content of the resid feed. The feed is subjected to operating conditions under a liquid space velocity in volume of feed per hour per volume of reactor having a value "Q" and at a hydrogen partial pressure having a value "H." Curve B represents the sulfur level in the 975.degree. F+ product fraction when operating with a liquid space velocity of Q and a hydrogen partial pressure of 0.67 H. Curve D was operated at 0.5 Q and 0.67 H, curve C had operating conditions of Q and H and curve E had operating conditions of 0.5 Q and H. Line F is the locus of minimum points at a hydrogen partial pressure of H while line G is the locus of minimums of sulfur in the 975.degree. F+ fraction in the product at 0.67 H.

FIG. 3 is a graph showing the effect of liquid space velocity and hydrogen pressure on the vanadium content of the 975.degree. F+ fraction in the product. Point Z represents the vanadium content of the feed. The feed is subjected to operating conditions under a liquid space velocity in volume of feed per hour per volume of reactor, having a value "Q" and at a hydrogen partial pressure having a value "H." Curve X represents the vanadium level in the 975.degree. F+ product fraction when operating with a liquid space velocity of Q and a hydrogen partial pressure of H. Curve Y was operated at Q and 0.67 H, curve W had operating conditions of 0.5 Q and 0.67 H and curve V had operating conditions of 0.5 Q and H. Line T is the locus of minimum vanadium content points at a hydrogen partial pressure of 0.67 H while line U is the locus of minimum points at a hydrogen partial pressure of H.

All minimums in FIGS. 2 and 3 fall between 30 and 60 percent conversion of the 975.degree. F+ material in the feed.

In general, it is envisioned that the reaction conditions utilized in hydrogenation process of this type would be within a temperature range of between 700.degree. and 900.degree. F and preferably between 750.degree. F and 840.degree. F, a hydrogen partial pressure range of between 1,500 and 3,000 psig, a liquid space velocity range of between 0.3 and 1.5 V.sub.f /hr/V.sub.r and a catalyst replacement rate between 0.02 and 0.4 lbs/bbl.

It has been found, in accordance with this invention, that by controlling the quality, that is to say sulfur, and vanadium content of the coker feedstock, that the quality of the coke produced therefrom is controlled. The quality of the coker feedstock is, in turn, controlled by the operating conditions used in the ebullated bed, hydrogenation reactor and the feedstock characteristics.

In accordance with this invention the necessary quality of the coke is obtained by controlling the quality of the coker feedstock. Variations of the operating conditions in the ebullated bed hydrotreating reactors results in conditions that will produce a coker feedstock with the desired properties. That is to say, from a given amount of feedstock, a known coker capacity and coke yield requirements, the desired level of conversion of the 975.degree. F+ material in the ebullated bed unit is estimated to thereby produce a coker feedstock that will produce coke having the desired levels of sulfur and metals.

EXAMPLE I

To illustrate the hereinabove disclosure, the following cases on West Texas resids are presented.

In Table I the yields and coke product properties obtained from direct coker processing of virgin vacuum bottoms are given.

The yields and product properties obtained from the ebullated bed hydroconversion processing of the virgin vacuum bottoms followed by the coker processing of the ebullated bed pretreated vacuum bottoms coker feedstock are summarized in Tables 2 and 3 respectively.

Tables 2 and 3 show that if one wanted a coke with about 1.5 percent sulfur and about 140 ppm of vanadium then one would need a coker feedstock with about 1.2 percent sulfur and about 41 ppm of vanadium. Starting with a residuum with 2.65 percent sulfur and 70 ppm of vanadium one would then find that ebullated bed reactor should be operated at 780.degree. F, 2250 psig of hydrogen with a space velocity of 1.0 V.sub.f /hr/V.sub.r to give the desired coker feedstock. This results in a minimum sulfur of 1.2 percent in the 975.degree. F+ coker feedstock at the given operating conditions when about 33 percent of the 975.degree. F+ fraction is converted to lower boiling fractions.

Comparing the results from Tables 1 and 3 shows that; (1) the sulfur and metal contents of the coke obtained wherein the ebullated bed processing of the coker feedstock is performed are lower than those of the coke obtained by direct coking of the virgin resid; (2) the yield of coke by direct coking of the virgin resid is about 30 percent of the total resid, as compared to 18.5 percent when processing the virgin resid through the ebullated bed reactor prior to coking, thereby reducing the more valuable lower boiling hydrocarbons obtained from the resid.

The sulfur and metals contents and the yield of coke depends respectively on the sulfur content and metals content of the coker feedstock. These are fixed properties of the virgin vacuum resid and hence the coke yield and its sulfur and metals contents are defined when the virgin vacuum resid is directly coked. On the other hand, the properties of the coker feedstock are easily changed in the ebullated bed by manipulating the operating variables. Thus the coke yield and the sulfur and metals contents of the coke can be adjusted to meet with different coke needs. ##SPC1## ##SPC2## ##SPC3##

Although the above example and discussions disclose a preferred mode of embodiment of this invention, it is recognized that from such disclosures, many modifications will now be made obvious to those skilled in the art and it is understood, therefore, that this invention is not limited to only those specific methods, steps or combinations or sequence of method steps described, but covers all equivalent steps or methods that may fall within the scope of the appended claims.

Claims

We claim:

1. In a process for the production of coker feedstocks having minimum amounts of sulfur or metals wherein a petroleum resid, containing at least 25 percent of hydrocarbons boiling above 975.degree. F, and a hydrogen containing gas are fed in upflow through an ebullated bed reaction zone while maintaining a particulate hydrotreating catalyst in said reaction zone at a hydrogen partial pressure between 1500 and 3000 psi, a temperature between 700.degree. and 900.degree. F and at a space velocity between 0.3 and 1.5 V.sub.f /hr/V.sub.r wherein the improvement comprises:

a. converting, in said reaction zone, 30 to 60 percent of the fraction boiling above 975.degree. F in the feed to lower boiling fractions;

b. removing a liquid effluent from said reaction zone;

c. and separating, from said liquid effluent, the bottom fraction boiling above 975.degree. F, having less than two weight percent sulfur and less than 70 ppm vanadium.

2. The process of claim 1 wherein said liquid effluent removed in step b) is passed to a second reaction zone, wherein said second reaction zone is operated under conditions substantially the same as the first reaction zone, before completing step c).

3. The process of claim 2 wherein the reaction zones are operated at a temperature between 750.degree. and 840.degree. F and the catalyst is replaced at a rate between 0.02 and 0.4 pounds of catalyst per barrel of resid fed wherein said bottom fraction is coked to yield a coke having less than 200 ppm vanadium and less than 2.5 percent sulfur.