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Dokumentenidentifikation EP1017759 17.08.2000
EP-Veröffentlichungsnummer 1017759
Titel VERMINDERUNG VON METALLBEANSPRUCHUNGEN IN ÖFEN ZUR VERZÖGERTEN VERKOKUNG
Anmelder Conoco Inc., Ponca City, Okla., US
Erfinder NELSEN, K., David, Ponca City, US
Vertreter derzeit kein Vertreter bestellt
Vertragsstaaten AT, DE, DK, ES, FI, FR, GB, IT, NL, SE
Sprache des Dokument EN
EP-Anmeldetag 11.06.1998
EP-Aktenzeichen 989312822
WO-Anmeldetag 11.06.1998
PCT-Aktenzeichen US9812445
WO-Veröffentlichungsnummer 9913023
WO-Veröffentlichungsdatum 18.03.1999
EP-Offenlegungsdatum 12.07.2000
Veröffentlichungstag im Patentblatt 17.08.2000
IPC-Hauptklasse C10B 25/00
IPC-Nebenklasse C10B 25/18   C10B 39/00   C10B 47/00   C10B 49/00   C10B 51/00   

Beschreibung[en]
Background of the Invention 1. Field of the Invention

This invention relates to delayed coking, and more particularly to a method of reducing the metal stresses in delayed coking drums during the cooling and quenching part of the coking cycle.

In a typical delayed coker unit, a pair of coke drums are alternately filled and emptied, with coker feed being pumped into one of the drums while the other drum is being emptied of coke and prepared for the next filling cycle.

2. Background Art

A conventional coking operation includes, in the process of emptying the filled drum, the steps of steaming out the filled drum to remove residual volatile material from the drum, quenching the steamed out coke bed with water, draining quench water from the drum, opening the top and bottom of the coke drum (unheading the drum), drilling a pilot hole in the coke bed from the top, drilling out the remaining coke with a radially directed water jet drill, removing the drilled out coke from the bottom of the drum, closing the top and bottom openings of the coke drum, and preheating the empty coke drum by passing hot vapors from the other drum being filled with hot coker feed. The preheating step is necessary to bring the empty coke drum temperature up prior to switching the hot coker feed to the recently emptied drum, as otherwise the thermal stresses from feeding hot feed into a relatively cool drum would cause serious damage. In my U.S. patent No. 5,891,310, filed on June 20, 1997, a method of reducing the time required for the preheating step is described. That method includes the application of external heat to a critical area of the coke drum during the preheat step of the coking cycle.

A typical coke drum is supported by a skirt which is welded to the drum near the junction of the drum shell and the lower cone of the drum. As described in my aforementioned U.S. Patent, the maximum thermal stresses occur at the time the hot oil feed, at about 482°C (900°F), is switched to the preheated drum. These thermal stresses are partly due to the fact that the interior surface of the preheated drum is hotter than the exterior of the drum, including the area where the supporting skirt is welded to the drum shell. The expansion rate of the interior of the shell, upon being contacted with hot oil feed, is initially greater than the expansion rate of the cooler exterior portion. If sufficient time is available, the preheat step can be carried out over a time period sufficient to heat the drum exterior to a temperature near that of the drum interior. However, this is a problem if preheat time is to be minimized in order to reduce the overall cycle time.

There is another point in the coking cycle during which high metal stresses develop in the area of the junction between the coke drum and its supporting skirt. This occurs when quench water is introduced into the drum to quench the steamed out coke. At the time the quench water is introduced, the drum exterior is much hotter than the quench water, and the temperature differential between the drum interior and the drum exterior sets up large thermal gradients which result in high metal stresses. This is particularly critical in the area of the drum where the supporting skirt is attached. The top portion of the support skirt remains at a higher temperature than the cooling cone and shell. The resulting temperature differences in the components results in the cone and shell contracting at a faster rate than the skirt. The differential of expansion rates creates high metal stresses when the contracting cone and shell pull away from the hotter skirt.

US patent 4,634,500 discloses a method of quenching heated coke in a coke drum, whereby the rate of feeding the quench water into the coke drum is regulated to prevent the stress in the coke drum wall, which is monitored by measuring either the longitudinal thermal gradient or the rates of change in the drum wall temperature over time, from exceeding a predetermined limit.

US patent 3,167,486 discloses a method of retarding skirt weld cracking in coking vessels by reducing temperature gradients in and about the skirt weld by creating a coke depositing and retention zone in a limited region within the vessel near the skirt weld and utilizing the deposited coke itself as an internal layer of insulation.

Summary of the Invention

According to the present invention, the metal stresses in a coke drum during the quenching step of the coking cycle are reduced by applying a cooling fluid to the external part of the coke drum adjacent the area where the drum and its supporting skirt are connected. This external cooling fluid reduces the temperature differential between the drum interior and the supporting skirt connection, thereby reducing the metal stresses during the quenching step.

Description of the Drawings

  • Figure 1 is a schematic view of a delayed coker unit showing a pair of coke drums and associated equipment.
  • Figure 2 is a chart showing the coke drum schedule for a coking cycle.
  • Figure 3 is a side elevation, partly in cross section, showing details of a coke drum and its supporting structure.
  • Figure 4 is a side elevation, partially cut away, showing details of the junction of a coke drum and its supporting skirt.
  • Figure 5 is a cross section showing a coke drum supported by a skirt welded to the knuckle section on the cone of the drum.
  • Figure 6 is a cross section showing a coke drum supported by a skirt welded to the shell of the drum.

Description of the Preferred Embodiments

The primary object of the present invention is to decrease the metal stresses in a coke drum during the quenching step of the coke cycle.

Figure 1 shows a typical coker unit comprised of a pair of coke drums 10 and 12. Coker feed from feed line 14 enters coker fractionator 16 and is pumped to furnace 54 and then fed to one of the coke drums. Overhead vapors from the drum being filled return to fractionator 16 where they are separated into product streams.

Referring to Figure 2, a typical cycle schedule is shown. The example illustrated is for an eighteen hour cycle, but longer and shorter cycles are common.

The means for applying external cooling fluid to the drum are best shown in Figure 3. A cooling fluid jacket 48 encircles drum 10 around the area of the skirt-to-drum junction. A cooling fluid inlet 50 and outlet 52 are provided for passing cooling fluid, preferably water or low pressure steam, through the cooling jacket 48.

As seen in Figure 3, a coke drum 10 includes a bottom cone section 34 and a removable lower plate 36. Between the drum shell and the bottom cone section 34 there is a transition or knuckle section 44. As shown in Figures 3 and 6, near the junction of the drum shell and knuckle section 44, a supporting skirt 38 is welded to the drum, in what is sometimes referred to as a tangent line connection.

As shown in Figure 5, a knuckle section 44 is welded between the drum shell and lower cone section 34. A supporting skirt 38 is welded to the knuckle section 44 at weld 22, in what is sometimes referred to as a knuckle connection.

In one popular variation as shown in Figure 4, the skirt includes a series of fingers 40 formed by slots extending from the top of the skirt, and each finger has a curved top 46 to present a scalloped shape, and the curved finger tops are welded to the drum shell. It is common to include rounded lower ends in slots in the skirt to prevent stress risers from forming at the slot ends. In cases where the cooling jacket 48 extends over part of the slots extending from the top of the skirt as shown in Figure 4, it may be desirable to apply a packing material in the slots to prevent leakage of cooling fluid.

Whichever type of skirt-to-drum system is used, the junction between the drum shell and skirt is very hot when the quench step is initiated. The exterior drum surface, and especially the welded junction of the drum shell and the supporting skirt, does not cool down at the same rate as the interior of the drum. High metal stresses then develop because of the thermal shock that occurs when quench water is introduced into the bottom of the drum. This thermal shock can potentially damage the skirt-to-drum connection.

To illustrate the process of the invention, the coking cycle including the use of external drum cooling will now be described with reference to Figures 1 and 3.

Hot coker feed from furnace 54 is fed to the bottom of coke drum 10. At the time feed to drum 10 is initiated, coke drum 12, which is full of coke, is steamed with low pressure steam to strip residual volatile hydrocarbons from the coke bed in the drum. The steam also removes some heat from the coke. After the steamout step, the coke is quenched by filling the drum with quench water. Before the thermal gradient caused by the quench water reaches the level of the drum-to-skirt connection, a cooling fluid such as water, air or other gas, or low pressure steam, is injected into cooling jacket 48 from inlet 50. The cooling fluid exits outlet 50, providing external cooling to the drum at the area of the drum-to-skirt junction, and reducing the metal stresses in the drum. Once the coke bed is covered with water, the drum drain is opened and water is drained out. The top and bottom drum head covers are then removed. A pilot hole is drilled through the coke bed from the top, and then a rotating high pressure water jet drill passing down through the pilot hole directs a cutting stream horizontally against the coke bed. The drilled out coke falls downwardly out of the drum. After the coke cutting is completed and the coke has been removed from the drum, the head covers are reinstalled and the drum is purged with steam and tested for leaks. Part of the hot vapor from the top of the on-line drum is diverted into che cleaned drum to warm the drum to a predetermined temperature. Hot feed from furnace 54 is then switched into the cleaned drum.

The essence of the invention is in externally applying cooling fluid to the junction of the coke drum and its supporting skirt during and/or prior to introducing quench water into the drum. The application of external cooling fluid allows the area of the drum-to-skirt junction to more nearly approach the temperature of the drum interior during the quench step, and allows the introduction of quench water without the damaging metal stresses that would result if the exterior of the drum, particularly around the drum-to-skirt welds, is at a much higher temperature than the quench water.

The foregoing description of the preferred embodiments of the invention is intended to be illustrative rather than limiting of the scope of the invention, which is to be defined by the appended claims.


Anspruch[de]
  1. Verzögerter Verkokungsprozess, bei dem ein Paar Kokstrommeln jeweils durch einen Umrandungsabschnitt gestützt wird, der mit der Trommel verschweißt ist, die abwechselnd befüllt und entleert werden, und wobei der Entleerungsabschnitt des Zyklus folgende Schritte umfasst:
    • (a) Ausdampfen der befüllten Kokstrommel, um restliche flüchtige Bestandteile aus der Trommel abzuführen;
    • (b) Ablöschen des heißen Koksbetts mit Wasser;
    • (c) Ablassen des Löschwassers aus der Kokstrommel;
    • (d) Öffnen der Oberseite der Kokstrommel und Hineinbohren eines Vorbohrlochs durch das Koksbett;
    • (e) Ausbohren des Kokses aus dem Koksbett zwischen dem Vorbohrloch und der Kokstrommelwand durch radial gerichtetes Bohrwasser und Austragen des Kokses durch eine Öffnung in der Unterseite der Kokstrommel;
    • (f) Schließen der oberen und unteren Öffnung der Kokstrommel; und
    • (g) Vorerhitzen der leeren Trommel durch Hindurchleiten von heißen Kokstrommeldämpfen durch die Trommel vor dem Einführen von Beschickungsgut in die leere Trommel;
    dadurch gekennzeichnet, dass die Metallspannungen an der Verbindungsstelle der Kokstrommel und der Umrandung durch Aufbringen von Kühlfluid auf den Außenbereich der Kokstrommel benachbart zu der Verbindungsstelle der Trommelhülle und der Umrandung der Trommel während des Einführens von Löschwasser in die Trommel verringert werden, um dadurch übermäßige thermische Spannungen zu verhindern.
  2. Verfahren nach Anspruch 1, wobei das Kühlfluid auf die Außenseite der Trommel während des Einführens des Löschwassers durch Verwenden eines Kühlmantels aufgebracht wird, der die Trommel nahe der Verbindungsstelle der Hülle und der Stützumrandung umgibt.
  3. Verfahren nach Anspruch 2, wobei das Kühlfluid ein Gas ist.
  4. Verfahren nach Anspruch 3, wobei das Kühlfluid Luft ist.
  5. Verfahren nach Anspruch 3, wobei das Kühlfluid ein Niederdruckdampf ist.
  6. Verfahren nach Anspruch 2, wobei das Kühlfluid eine Flüssigkeit ist.
  7. Verfahren nach Anspruch 6, wobei das Kühlfluid Wasser ist.
  8. Verfahren zum Verringern von Metallspannungen in einer Kokstrommel, welche während des Löschschrittes auftreten, umfassend das Aufbringen eines Kühlfluids auf den Außenteil der Kokstrommel benachbart zu dem Bereich, wo die Kokstrommel und die Kokstrommel-Stützumrandung verbunden sind und Verringern der Temperaturdifferenz zwischen der Trommelinnenseite und der Stützumrandungsverbindung, um dadurch Spannungen während des Löschschrittes zu verringern.
Anspruch[en]
  1. A delayed coking process in which a pair of coke drums each supported by a skirt section welded to said drum are alternately filled and emptied, and in which the emptying portion of the cycle comprises the steps of:
    • (a) steaming out the filled coke drum to remove residual volatile matter from the drum;
    • (b) quenching the hot coke bed with water;
    • (c) draining quench water from the coke drum;
    • (d) opening the top of the coke drum and drilling a pilot hole through the coke bed therein;
    • (e) drilling out the coke from the coke bed between the pilot hole and the coke drum wall by radially directed drill water and removing the coke through an opening in the bottom of the coke drum;
    • (f) closing the top and bottom openings of the coke drum; and
    • (g) prior to introducing feed into the emptied drum, preheating the empty drum by passing hot coke drum vapors through the drum;
       characterised in that the metal stresses at the junction of the coke drum and skirt are reduced by applying cooling fluid to the exterior portion of said coke drum adjacent the junction of the drum shell and the skirt of said drum during the introduction of quench water into said drum, thereby preventing excessive thermal stresses.
  2. The process of Claim 1 wherein cooling fluid is applied to the exterior of said drum during the introduction of quench water by utilizing a cooling jacket surrounding said drum near the junction of the shell and the supporting skirt thereof.
  3. The process of Claim 2 wherein said cooling fluid is a gas.
  4. The process of Claim 3 wherein said cooling fluid is air.
  5. The process of Claim 3 wherein said cooling fluid is low pressure steam.
  6. The process of Claim 2 wherein said cooling fluid is a liquid.
  7. The process of Claim 6 wherein said cooling fluid is water.
  8. A method for reducing metal stresses in a coke drum which occur during the quenching step comprising applying a cooling fluid to the external part of the coke drum adjacent to the area where the coke drum and the coke drum supporting skirt are connected, reducing the temperature differential between the drum interior and the supporting skirt connection, thereby reducing metal stresses during the quenching step.
Anspruch[fr]
  1. Procédé de cokéfaction retardée dans lequel une paire de tambours à coke, chacun supporté par une section de jupe soudée audit tambour, sont remplis et vidés en alternance, et dans lequel la partie de vidage du cycle comprend les étapes consistant à :
    • (a) faire passer de la vapeur dans le tambour à coke rempli pour éliminer la matière volatile résiduelle du tambour ;
    • (b) éteindre le lit de coke chaud avec de l'eau ;
    • (c) évacuer l'eau d'extinction du tambour à coke ;
    • (d) ouvrir le haut du tambour à coke et percer un trou de positionnement à travers le lit de coke dans celui-ci ;
    • (e) expulser le coke du lit de coke entre le trou de positionnement et la paroi de tambour à coke grâce à de l'eau de forage dirigée radialement et enlever le coke par une ouverture dans le fond du tambour à coke ;
    • (f) fermer les ouvertures supérieure et inférieure du tambour à coke ; et
    • (g) avant l'introduction de la charge dans le tambour vidé, préchauffer le tambour vide en faisant passer des vapeurs de tambour à coke chaudes à travers le tambour,
       caractérisé en ce que les contraintes métalliques au niveau de la jonction du tambour à coke et de la jupe sont réduites en appliquant un fluide de refroidissement à la partie extérieure dudit tambour à coke de façon adjacente à la jonction de l'enveloppe de tambour et de la jupe dudit tambour durant l'introduction de l'eau d'extinction dans ledit tambour, en empêchant ainsi des contraintes thermiques excessives.
  2. Procédé selon la revendication 1, dans lequel le fluide de refroidissement est appliqué à l'extérieur dudit tambour au cours de l'introduction de l'eau d'extinction en utilisant une chemise de refroidissement entourant ledit tambour à proximité de la jonction de l'enveloppe et de sa jupe de support.
  3. Procédé selon la revendication 2, dans lequel ledit fluide de refroidissement est un gaz.
  4. Procédé selon la revendication 3, dans lequel ledit fluide de refroidissement est de l'air.
  5. Procédé selon la revendication 3, dans lequel ledit fluide de refroidissement est de la vapeur à basse pression.
  6. Procédé selon la revendication 2, dans lequel ledit fluide de refroidissement est un liquide.
  7. Procédé selon la revendication 6, dans lequel ledit fluide de refroidissement est de l'eau.
  8. Procédé destiné à réduire les contraintes métalliques dans un tambour à coke qui se produisent au cours de l'étape d'extinction comprenant l'application d'un fluide de refroidissement à la partie externe du tambour à coke de façon adjacente à la zone où le tambour à coke et la jupe de support de tambour à coke sont reliés, la réduction de la différence de température entre l'intérieur du tambour et le raccordement de jupe de support, en réduisant ainsi les contraintes métalliques au cours de l'étape d'extinction.






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G Physik
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