Patent Application
20150267122
This invention relates to a method of extending the fatigue life of delayed coking coke drums used for the thermal processing of heavy petroleum oils and more particularly, to the use of internal linings in delayed coking coke drums for extending their fatigue life.
Delayed coking is a process used in the petroleum refining industry for increasing the yield of liquid product from heavy residual oils such as vacuum resid.
In delayed coking, the heavy oil feed is heated in a furnace to a temperature at which thermal cracking is initiated but is low enough to reduce the extent of cracking in the furnace itself. The heated feed is then led into a large drum in which the cracking proceeds over an extended period of residence in the drum. The cracking produces hydrocarbons of lower molecular weight than the feed which, at the temperatures prevailing in the drum, are in vapor form and which rise to the top of the drum where they are led off to the downstream product recovery unit with its fractionation facilities. The thermal cracking of the feed that takes place in the drum also produces coke, which gradually accumulates in the drum during the delayed coking cycle. When the coke reaches a certain level in the drum, the introduction of the feed is terminated and the cracked products remaining in the drum are removed by purging with steam. After this, the coke is quenched with water, the drum is depressurized, the top and bottom heads are opened, and then the coke is discharged through the bottom head of the drum through use of a high pressure cutting water system. The cracking cycle is then ready to be repeated. Typically the process itself is achieved by heating the heavy oil feed to a temperature in the range that permits a pumpable condition in which it is fed into the furnace and heated to a temperature in the range of 380 to 525° C.; the outlet temperature of a coker furnace is typically around 500° C. with a pressure of 4 bar. The hot oil is then fed into the coke drum where the pressure is held at a low value in order to favor release of the vaporous cracking products, typically ranging from 1 to 6 bar, more usually around 2 to 3 bar. Large volumes of water are used in the quench portion of the coking cycle: one industry estimate is that for a typically large coke drum about 8 m in diameter and 25 m high, about 750 tonnes of water are required for quenching alone with even more required for the cutting operation after the drum is opened and the coke discharged. A useful and widely cited summary of the delayed coking process is available online in “Tutorial: Delayed Coking Fundamentals”, Ellis et al, Great Lakes Carbon Corporation, Port Arthur, Tex., AlChE 1998 Spring National Meeting, New Orleans, La., 8-12 Mar. 1998, Paper 29a, Copyright ©1998 Great Lakes Carbon Corporation.
Delayed coking coke drums are conventionally large vessels, typically at least 4 and possibly as much as 10 m in diameter with heights of 10 to 30 m or even more. The drums are usually operated in twos or threes with each drum sequentially going through a charge-quench-discharge cycle, with the heated feed being switched to the drum in the feed phase of the cycle. The drums are typically made of unlined or clad steel, with base thicknesses that can range from about 10 to 30 mm thick. The internal cladding thickness is nominally 1-3 mm and is used for protection against sulfur corrosion. The present common commercial practice is to use 401S clad or unclad CS, C-1/2 Mo, or low chromium drums for delayed coking service. In form, the drums comprise vertical cylinders with either an ellipsoidal or hemispherical top head and a conical bottom head. The bottom head has either a flange or, alternatively, a mechanical valve arrangement as described, for example, in U.S. Pat. No. 6,843,889 (Lah). The feed inlet and steam/water connections are located in this lower conical section of the vessel. Operating envelopes and inspection/repair strategies are the mechanisms used to manage fatigue cracking in this equipment.
Delayed Coker coke drums are inherently exposed to pressure boundary fatigue cracking due to the thermal stresses imposed on the steel primarily during the quench/fill process. The drums are prone to thermal fatigue due to the through-wall thermal stresses that are developed prior to the drum reaching steady state. Additionally, at the skirt-to-shell junction, the transient temperature differentials between the pressure boundary and the skirt also set up high stresses that can lead to weld and base metal cracking. This is a transient effect, and data analysis has shown that the other delayed coking steps (e.g., drum warm-up, feed introduction, coking, steam out, etc.) have less impact on pressure boundary stresses. As noted by Ellis, op. cit., the rate of cooling water injection is critical. Increasing the flow of water too rapidly can “case harden” the main channels up through the coker without cooling all of the coke radially across the coke bed. The coke has low porosity which then allows the water to flow away from the main channels in the coke drum, leading to the problem of drum bulging during cool down. If the rate of water is too high, the high pressure causes the water to flow up the outside of the coke bed cooling the wall of the coke drum. Coke has a higher coefficient of thermal expansion than does steel (154 for needle coke versus 120 for steel, cm/cm/° C.×10−7). While drum support systems such as that described in U.S. Pat. No. 8,221,591 (de Para) may be capable of reducing the mechanical stresses generated by the differential cooling, it would nevertheless by desirable to minimize the transient thermal stress in both the coke drum Shell/cone as well as at the skirt-to-shell junction.
We now propose the use of a thermal buffering system to reduce or minimize the transient thermal stress that occurs in the steel during the portions of the coking cycle when the thermal stresses arise. The application of a lining system applied to the internal surface of the coke drum pressure boundary will be effective to reduce stresses on the drum during the operation of the process, particularly during the cooling/quench portion of the cycle. Coverage of the pressure boundary with internal lining can vary from a few meters of vessel height to all of the pressure boundary depending on (1) the level of protection needed in historically problematic areas (i.e., at the skirt-to-shell junction, in the bottom cone, near the outage level, etc.), and/or (2) to address efforts to minimize cycle time via shorter quench phases, feed introduction at lower drum warm-up temperatures, etc.
According to one embodiment of the present invention, the delayed coker coke drum has a monolithic, thermal shock-resistant, erosion-resistant refractory lining on the inner surface of the drum, especially in the areas subject to pressure boundary stress. The monolithic lining, applied by ramming in a similar manner to air-setting erosion-resistant refractory, is held in place by a suitable anchoring system, preferably a single point anchoring system as discussed further below. Anchoring systems of this type are customarily used for anchoring erosion-resistant refractory linings in petroleum processing vessels and may be used for the present purposes.
In another embodiment of the present invention, the delayed coker coke drum includes the same aforementioned anchoring system, but does not include the air setting erosion-resistant refractory. In this embodiment, the coke being fed into the coke drum fills the anchoring system and the two form an internal lining on the inner surface of the drum. This allows the transient thermal stress to be dissipated across a layer of coke rather than across the coke drum pressure boundary.
In another embodiment of the present invention, the delayed coker coke drum includes a in and plate assembly. In this assembly, pins are provided extending inward from the outer wall of the coke drum. Attached to the pins are protective plates. The plates are arranged such that they create an air gap that will fill with a protective layer of coke between the coke being fed into the coke drum and the inner surface of the drum. This allows the transient thermal stress to be dissipated across the coke and the protective plates rather than across the coke drum pressure boundary. The protective plates prevent the removal of the protective coke layer during the cutting cycle.
Zones in the drum subject to pressure boundary stress are indicated in
According to the present invention, the delayed coker coke drum has a thermal shock-resistant lining applied to the inner surfaces of the drum. The lining has the function of reducing the thermally-induced mechanical stresses from the transient temperature cycles occurring during the delayed coking process, particularly common during the cooling/quench phase of the cycle, but present to a lesser extent during other phases. The lining is effective to minimize the transient thermal stress that occurs in the shell and bottom head and to reduce the high thermal stress resulting from temperature differentials at the skirt-to-shell junction.
In one embodiment of the invention, thermal barrier 23 is a refractory material. The cyclic service of the drum is such that a brick lining is unlikely to be satisfactory due to its inability to handle the thermal loads in the through-thickness direction. Additionally, a heat-resistant, monolithic refractory lining is also unlikely to handle such thermal cycling loads due to an inadequate anchorage system common for such refractory types. According to one embodiment of the invention the use of a thin-layer (¾-2 inch (1.9-5 cm) nominally), thermally-shock resistant and erosion-resistant refractory lining is contained in appropriate anchorage that resists transient thermal loading.
Suitable refractories are those normally used for erosion-resistant linings in thermal processing units, such as those used in Fluid Catalytic Cracking Units (FCCUs), but with the essential qualifications that the erosion-resistant nature of the refractory also be thermal-shock resistant and capable of withstanding the cutting water pressure required to remove the coke from the drum as part of the normal decoking cycle. In all cases, the refractory should be selected to be as durable as possible. In view of the service requirements, three conceptual approaches are possible:
The specific refractory material used to implement these approaches may be selected on an empirical basis from the many castable refractories of this type that are commercially available. Selection of the specific refractories may be made according to experience in other petroleum refining applications, relations with suppliers, etc., as is normally the practice. Qualification of the lining should be established by transient thermal cycle tests (simulating actual delayed coker quench/fill steps) to ensure optimized refractory/anchor system reliability.
An important feature of the drum linings is the anchoring system. Hexagonal mesh has been the preeminent thin layer lining system, typically available in standard thicknesses of ¾ inch (19 mm), 1 inch (25 mm) and 2 inch (50 mm), although other thicknesses can be custom made. Hexagonal mesh is composed of long ribbons and the resultant lining system is comprised of discrete refractory cells bound by a metallic cell formed by the ribbons. Attachment of these long ribbons to the base material results in accumulation of thermal strain across the attachment welds (typically at 25 mm distances) resulting in failure. For this reason, hexagonal mesh is unlikely to be optimal as an anchoring system for service in the coke drum and will not be preferred. Experience in FCC units with hexmesh in coking service has shown that when the welds start to break, coke accumulates with each thermal cycle until all the welds break and the section falls off as a sheet. If used, hexagonal mesh should be installed in discrete sections that could pass through the outlet nozzle and not impede unloading if they became detached.
Alternatives to hexagonal mesh are single point anchoring systems in which thermal strain is accumulated only across the individual weld (3-10 mm diameter): stud weldable anchoring systems that minimize the potential for accumulated thermal strain across multiple attachment welds are preferred. The resultant systems provide a continuous refractory system with discrete anchoring points where the failure of a single anchor is less detrimental to the lining system than failure of a sheet secured by hexmesh. Individual I Anchors such as the Silicon CVC anchors, Hex-Alt anchors (e.g., K-bars™, Half Hex™, etc.), such as those shown, for instance, in U.S. Pat. No. 6,393,789 (Lanclos), U.S. Pat. No. D393,588 (Tuthill), may be considered for potential use. An extensive range of refractory anchors is supplied commercially by the Hanlock-Causeway Company of Tulsa, Okla. and Houston, Tex. Wear-resistant anchors such as Hanlock, Flexmesh™, Tabs, hex cells, S-Anchor™ and stud gun weldable half hex cell anchors may also be useful. Typical anchoring systems are welded, usually by spot or stud welds to the underlying metal surface prior to application of the lining. Anchors should be welded directly to the surface (can be clad or unclad) of the coke drum, or alternatively, stud-welding technology may be employed for improved installation efficiency. These refractory anchors will typically extend directly out to the surface of the refractory lining. A description of refractory lining techniques including refractory materials and anchoring systems may be found in Refractories Handbook, Charles Schacht (Ed), CRC Press Content, August 2004, ISBN 9780824756543, to which reference is made for a description of refractory material, systems and application techniques such as may be used for forming the refractory linings in coke drums.
The refractory material will typically be installed by hand packing, ramming or hammering an air-setting refractory mix into place within the anchoring system attached to the shell wall of the drum. Refractory ramming mixes usually contain a plastic clay which is tempered with water (typically 2-5 percent). They are commonly supplied in a damp granular form ready for installation by hand packing or by using pneumatic rammers. The mix, containing refractory minerals and clay, can also include organic plasticizers to facilitate installation. Suitable mixes can be determined upon consultation with refractory suppliers as noted above when the specific site and service duties are fixed. Typical commercial ramming mixes include Rescobond AA-22S™, Actchem™ 75, Actchem™ 85, and the ONEX™ ramming products. As noted above, selection of the specific refractory material may be made on an empirical basis in light of the applicable service specifications.
Still referring to
The present invention offers potential benefits in the following problem areas:
Embodiments with refractory lining have the potential to reduce or eliminate localized erosion incurred by the high pressure cutting water.
This application claims priority to U.S. Provisional Application Ser. No. 61/951,614 filed Mar. 12, 2014 and U.S. Provisional Application Ser. No. 61/992,316 filed May 13, 2014, both herein incorporated by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61951614 | Mar 2014 | US | |
| 61992316 | May 2014 | US |
