1. Field of the Invention
This invention relates to the coking of heavy petroleum fractions or residues. More particularly, this invention relates to heat generation and transfer in delayed coking processes and to subsequent treatment of the delayed petroleum coke product.
2. Description of Prior Art
Delayed cokers are furnace-type coking units wherein the feed is rapidly heated to temperatures at which it will thermally decompose and the effluent from the furnace discharges (before decomposition) into a large "coke drum", where it remains until it either cracks and passes off as vapor or condenses into coke.
In the usual application of the delayed coking process, residual oil is heated by exchanging heat with liquid products from the coking process and is then fed into a fractionating tower where any light products which might remain in the residual oil are distilled out. The oil is then pumped through a furnace where it is heated to the required temperature and discharged into the bottom of the coke drum. The first stages of thermal decomposition reduce this oil to a very heavy tar or pitch which further decomposes into solid coke. The vapors formed during this decomposition produce pores and channels in the coking mass through which the incoming oil from the furnace may pass. This process continues until the drum is filled with a mass of coke. The vapors formed in the process leave from the top of the drum and are returned to the fractionating tower where they are fractionated into desired cuts.
The delayed coking heater outlet temperature is controlled in the temperature range of 900.degree. to 950.degree. F. Higher temperatures may cause rapid coking in the coking heater and shortened on-stream time. Lower temperatures produce soft coke with a high VCM content. Sufficient pressure to avoid vaporization of the feed is maintained in the heater. The residence time must be long enough to bring the oil up to the desired temperature but excess time in the heater may cause coking and result in clogging the heater coil. A method frequently used for controlling the velocity and residence time in the heating coil is to inject water (or steam) into the high-boiling petroleum oil entering the heating coil. Water or steam injection is controlled at a rate sufficient to maintain the oil velocity in the heating coil to prevent coke from forming and depositing in the coil. Nevertheless, it has not been possible to raise the temperature of residual oil charged to the coking drum above about 950.degree. F. without encountering unacceptable rates of coke deposition in the coker heating means.
Coke formation reactions are essentially endothermic with the temperature dropping to 780.degree. to 900.degree. F., more usually to 780.degree. to 840.degree. F., in the coke drum. Coke drum pressures are maintained in the range from 10 to 70 psig.
To avoid the temperature limitations of delayed coking units, both moving bed and fluidized bed units have been proposed for reduced crude coking operations. Because they generally operate at lower pressures and higher temperatures than delayed cokers, more of the feed charge to fluid and contact or moving bed cokers is vaporized. The higher temperatures of fluid and contact or moving bed units also result in higher octane gasoline than that from delayed coking and in more olefinic gases. However, despite the development of these higher temperature coking processes, most commercial coking operations currently employ the delayed coking process.
The principal charging stocks for coking operations are high boiling virgin or cracked petroleum residues which may or may not be suitable as heavy fuel oils. An important use of coke is as domestic or industrial fuel although a substantial tonnage is processed and used in making carbon or graphite electrodes for use in the metals industries. However, the dynamic manner in which fluid coke is formed yields a solid product having physical properties which make it undesirable for this latter application. Delayed coking, on the other hand, when processing a sufficiently aromatic feedstock, can provide a premium quality coke product.
A primary objective of all of the various known coking processes has been to convert as large a proportion as possible of the feedstock to lighter hydrocarbon fractions while keeping coke formation to a minimum. The coker feedstock is completely converted to lighter and heavier materials. The lighter products (resulting from cracking) are gas, some gasoline, and gas oil. The heavier product (resulting from condensation reactions) is coke. The various product yields are affected by the coking tendency of the charge stock (e.g., as indicated by the Conradson Carbon Number), by the process employed (delayed or fluid) and by the process conditions. The yield of distillates is maximized by coking at low pressures. At higher pressures more gas and coke are produced, and the liquid product contains more gasoline. The yields of gas and gasoline also increase with increasing temperatures; the yield of gas oil decreases. Moreover, the research octane number of the gasoline increases linearly with temperature: for example, from 72.degree. at 930.degree. F. to 87.degree. at 1057.degree. F. Gasolines produced at higher temperatures are unstable and require finishing operations such as clay treating or mild hydrogenation. The gases produced at higher temperatures are more olefinic: at an average temperature of 955.degree. F. they are 50% olefinic, as compared with 15% at temperatures of about 850.degree. F.
Present delayed coker reactors must be operated within a relatively narrow range of conditions which limits the degree of control over product yield distribution and over product qualities. As noted above, a principle limitation of delayed cokers is the furnace outlet temperature which in turn limits the temperatures in the delayed coking drums. This limitation is of relatively minor importance in plants where the more valuable gaseous and liquid products produced by delayed coking are a relatively small percentage of the total volume of similar products produced in the complete refinery. However, improved product flexibility would be a considerable asset to the process and is particularly important in refineries processing heavy crudes such that the coker products have a major influence on overall refinery yields. Inasmuch as high quality crudes are becoming increasingly scarce and expensive, the processing of heavy crudes is becoming increasingly important today.
Accordingly, a principal object of the present invention is an improved delayed coking process having enchanced temperature flexibility. A related object is to provide a delayed coking process wherein the temperature of the resid charge may be raised rapidly to at least 50.degree. F. above the maximum temperature achievable by conventional delayed coking processes. An attendant increase in coke drum temperature would also ensue. A further object is to increase gas and liquid yields from delayed coking processes without sacrificing the highly desirable properties of delayed petroleum coke product. These objects and others apparent from this specification are accomplished by adding hot coke to feed charged to the coking drum of delayed coking units.
Although addition of heat carrying solids to raise the coke drum temperature has not been suggested by the prior art, addition of solids such as coke to coking feedstocks is generally known. U.S. Pat. No. 3,673,080 suggests dispersing particulate carbon in a delayed coker feed, heating the resulting mixture in a heater to about 900.degree.-930.degree. F. and charging the heated mixture to a coke drum. Clusters of spheroidal-shaped, solid petroleum coke pellets having improved compressive strength are said to be produced. U.S. Pat. No. 3,704,224 suggests adding relatively small quantities (0.2-20 ppm weight) of graphite particles to heated coker feedstock as it passes from the furnace to the coking drum (column 3, lines 4-9) to improve the qualities of the delayed coke product.
U.S. Pat. No. 3,116,231 described a delayed coking process wherein coke fines, formed by attrition when decoking the drums, are slurried with a gas oil stream and added to the heated feedstock stream as it enters the delayed coke drums. U.S. Pat. No. 4,082,650 teaches that coke yield and quality may be improved if the fines are at a temperature approximately equal to the temperature of the liquid feedstocks and are added to the coke drums either prior to the addition of liquid feedstock or continuously during the coking operation. The examples of the '650 patent show that coking when fines are added according to the process disclosed therein yields results which are substantially equivalent to those obtained when no fines were added. Furthermore no claim is made that either the temperature of the added coke or the amount of heated coke results in any appreciable enhancement of coker feed and drum operating temperature.
U.S. Pat. No. 2,717,865 teaches adding a naphtha-boiling range diluent and seed solids to a heavy oil feed prior to heating the feed to coking temperatures to prevent coke deposition on apparatus surfaces in systems for the liquid phase coking of heavy hydrocarbonaceous residues. A two-stage coking process wherein the first stage is similar to that of the '865 patent is disclosed in U.S. Pat. No. 2,899,376.