Mineral oils: processes and products – Chemical conversion of hydrocarbons – Cracking
Reexamination Certificate
2001-09-07
2004-07-20
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Chemical conversion of hydrocarbons
Cracking
C202S096000
Reexamination Certificate
active
06764592
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods of and apparatuses for warming coking drums in petroleum coking systems.
BACKGROUND OF THE INVENTION
Coking systems are commonly used in petroleum refineries for converting vacuum tower bottoms and/or other heavy (i.e., high boiling point) residual petroleum materials to petroleum coke and other products. The greater part of each barrel of resid material processed in the coker will typically be recovered as fuel gas, coker gasoline
aphtha, light cycle oil (also commonly referred to by various other names such as light coker gas oil), and heavy cycle oil (also commonly referred to by various other names such as heavy coker gas oil).
A typical delayed coking system comprises: a combination tower or other fractionator; a fired heater; and at least one vertical coking drum. Most coking systems include at least a pair of vertical coking drums. The heavy coker feed is typically delivered to the bottom of the fractionator wherein it is combined with a heavy, liquid, residual bottom product (commonly referred to as a “recycle”) produced in the fractionator. The resulting mixture is drawn from the bottom of the fractionator and then pumped through the heater and into at least one coking drum. Typically, multiple coking drums are operated in alternating cycles such that, while one drum (referred to herein as the “live” drum) is -operating in a fill cycle, another drum is operating in a second cycle typically comprising a steaming stage, a cooling/quenching stage, a hydraulic decoking stage, a pressure testing stage, and a warm-up stage. During the warm-up stage, a portion of the product vapor produced in the live drum is used to warm the empty drum.
In the fill cycle, the hot feed material from the coker heater typically flows into the bottom of the live coking drum. Some of the heavy feed material vaporizes in the heater such that the material entering the bottom of the coking drum is a vapor/liquid mixture. The vapor portion of the mixture undergoes mild cracking in the coking heater and experiences further cracking as it passes upwardly through the coking drum. The hot liquid material undergoes intensive thermal cracking and polymerization in the coking drum such that the liquid material is converted to cracked vapor and petroleum coke. The resulting combined overhead vapor product produced in the coking drum is typically delivered to the fractionator wherein it is separated into gas, naphtha, light cycle oil, and heavy cycle oil, which are withdrawn from the fractionator as products, and the heavy recycle/residual material which flows to the bottom of the fractionator. The light and heavy cycle oil products are typically taken from the fractionator as side-draw products which are further processed (e.g., in a fluid catalytic cracker) to produce gasoline and other desirable end products. The heavy recycle material combines with the heavy feed material in the bottom of the fractionator and, as mentioned above, is pumped with the heavy feed material through the coker heater.
By way of example, but not by way of limitation, typical coker operating conditions and product specifications include: a heater outlet temperature in the range of from about 905 to about 935° F.; coke drum pressures in the range of from about 20 to about 40 psig; live drum overhead temperatures in the range of from about 800° to about 820° F.; a fractionator overhead pressure in the range of from about 10 to about 30 psig; a fractionator bottom temperature in the range of from about 750° to about 780° F.; a light cycle oil draw temperature in the range of from about 450° to about 550° F.; a light cycle oil initial boiling point (ASTM D-1186) in the range of from about 300° to about 325° F.; a light cycle oil end point (D-1186) in the range of from about 600° to about 650° F.; a heavy cycle oil draw temperature in the range of from about 600° to about 690° F.; a heavy cycle oil initial boiling point (D-1186) in the range of from about 470° to about 500° F.; and a heavy cycle oil end point (D-1186) in the range of from about 960° to about 990° F.
One of the most serious and commonly encountered problems in delayed coking operations is foamover. Foamover typically results from the formation of an excessive volume of foam in the live coking drum during the fill cycle. When foamover occurs, partially coked resid is carried into the coke drum overhead line and, depending on the amount of such overflow, can result in: coke lay-down in the coke drum overhead lines; partial plugging of the combination tower bottoms screen; complete plugging of the combination tower bottom screen and a resultant sudden loss of feed to the coker heater; plugged (i.e., coked) heater tubes resulting from the sudden loss of flow therethrough; and plugging of the coker blowdown system. A massive foamover can even carry coke into the upper portions of the combination tower.
Foam is primarily formed from waxy, paraffinic condensed hydrocarbons present in the live coking drum during the fill cycle. A primary source of such material comprises condensate which forms in the live drum when hot resid is first switched into the drum at the beginning of the fill cycle. Although the empty drum is warmed prior to beginning the fill cycle, the warmed drum will typically still be very cool compared to the hot resid material flowing from the coker heater. Thus, some of the vapor condenses, particularly on the interior surface of the coking drum.
Each barrel of the condensed hydrocarbon material can form up to 1,200 barrels of foam in the live drum. The foam material travels up the coking drum on top of the coke layer.
Several factors promote the formation and expansion of foam material within the filling drum. These include: the amount of condensed and/or entrained liquid hydrocarbon material present in the drum; pressure swings in the live coking drum; a significant drop in overhead vapor product temperature; failure of the anti-foam chemical addition system; and over-filling the live drum. Particularly significant pressure losses typically occur in the live drum when a portion of the vapor product therefrom is diverted to warm up an empty drum.
Besides causing foam problems, condensate remaining or formed in the drum during the fill cycle detrimentally affects the quality and value of the coke product. Coke containing a significant amount of condensate material is commonly referred to as “sticky” or “green” coke.
The procedures heretofore used in the art for preventing foamovers have commonly included: attempting to ensure that the unit operators drain completely the warm, empty coking drum before beginning the fill cycle; injecting silicone anti-foam chemicals when a high foam level is detected in the live drum; restricting the fill rate so that the final level of the coke product is significantly below the top of the coking drum; and significantly limiting the amount of warm-up vapor taken from the live drum.
These approaches for reducing foam formation and expansion have serious shortcomings and are typically highly susceptible to operator error. Restricting fill rates and product levels significantly reduces unit capacity and, by necessitating the use of larger drums and/or a greater number of drums to achieve a given capacity, significantly increases construction costs. Silicone anti-foam chemicals are costly, unreliable, and can significantly poison catalysts used in fluid catalytic crackers and other downstream processing systems.
Most significantly, attempting to maintain live drum pressure by reducing the amount of warm-up vapor taken from the live drum can result in the empty drum being not sufficiently warmed before beginning the fill cycle. Foam formation rates increase rapidly with decreasing switchover temperatures, particularly below 600° F. Unfortunately, however, due to the inadequacy of the warm-up procedures heretofore used in the art, switchover temperatures of as little as 450° F. or less are common. Low switchover temperatures can also produce large pressure and temperatu
Fellers Snider Blankenship Bailey & Tippens, P.C.
Griffin Walter D.
LandOfFree
Drum warming in petroleum cokers does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Drum warming in petroleum cokers, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Drum warming in petroleum cokers will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3245581