Refinery process furnace

Details Refinery process furnace Refinery process furnace

2000-04-07

2001-05-29

Wilson, Gregory (Department: 3749)

Liquid heaters and vaporizers

Water tube

Having specific tube patterns

C122S247000, C122S511000

Reexamination Certificate

active

06237545

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to refinery process furnaces, and more particularly to furnace improvements relating to the radiant coil and to the tube supports for horizontal sections of the coil.
BACKGROUND OF THE INVENTION
Refinery process furnaces are widely used to heat hydrocarbons in a variety of services, for example, crude oil feed to an atmospheric tower, crude residuum from the atmospheric tower for feed to a vacuum tower, and the like. Perhaps the most severe service is the heating of feedstock to a delayed coker. While coke deposition can be a problem in any refinery process furnace, because of the high temperatures employed and the residual nature of the coker feedstock, there is a pronounced tendency for the formation of coke deposits on the inside wall of the radiant tubing through the coker preheat furnace.
Regardless of service, the formation of coke deposits is not desirable. Coke deposits can lead to increased pressure in the tubes due to the restriction of flow, and to higher tube wall temperatures due to the insulative effects of the coke deposits. Both higher pressure and higher temperature lead to premature failure of the tubes. Furthermore, it is often necessary to periodically remove the tube from service and remove the coke deposits by burning off the deposited coke by oxidation with air or another oxidant that is passed through the tube at a high temperature. This periodic burn-off can result in severe thermal cycling which also reduces the life of the tube.
One factor that has been identified as contributing to high coke formation rates and high tube metal temperatures is the presence of heat flux imbalances. See Martin, G. R., “Heat-Flux Imbalances in Fired Heaters Cause Operating Problems,”
Hydrocarbon Processing,
pp. 103-109 (May 1998). Heat-flux imbalances can be caused by many factors, such as, for example, furnace design and furnace operating conditions, including such things as the ratio of radiant section height to width, burner-to-tube distances, number and type of burners, flame shape, air preheat and air temperature, radiant section tube layout, one or more burners out of service, insufficient air to burners, fuel gas composition, burner fuel pressure, higher-than-normal firing rates, fouled burner tips, eroded burner tip orifices, insufficient draft, and the like.
In multi-pass heating arrangements, the tube layouts in existing heaters are generally different for each pass, i.e. each pass is positioned in a different place in the furnace. A pass located at the bottom of the furnace will see a flame temperature of 3000-3500° F. (1650-1930° C.) near its outlet, but a pass located near the arch at the top of the radiant section will see much lower temperatures. One way to compensate for heat-flux imbalances is to control the relative rates of feedstock supplied to each pass so that the outlet temperatures are about the same. However, this still does not avoid the existence of hot spots in individual tube passes that can lead to localized coke deposition.
An improvement over wall-mounted tube runs in the radiant section is the double-fired heater in U.S. Pat. No. 5,078,857 to Melton. This uses a bank of tubes running centrally through the firebox with a row of burners on either side thereof. The tubes pass through slots in the end wall of the radiant section, and an insulated header cover encloses the conventional returns. Unfortunately, this tube design still does not allow the placement of multiple passes within a single firebox that have the same heating pattern and fluxes in each pass.
It would be desirable to have available a multi-pass furnace design which allows the various passes to have about the same heating pattern and heat fluxes. It would also be desirable to increase the effective tube-heating surface of each pass, while at the same time reducing the pressure drop through the tubing. Further, it would also be desirable to eliminate the need for insulated end-tube header boxes and provide an improved manner of supporting the horizontal tubes. The present invention addresses these and other needs in the refinery process furnace art.
SUMMARY OF THE INVENTION
The present invention provides a radiant coil for a refinery process furnace. The coil has an inlet pipe section and an outlet pipe section. A plurality of essentially straight horizontal pipe sections are arranged in at least two vertical banks. The vertical banks are parallel and horizontally spaced apart. A plurality of bent pipe sweep return bends are arranged in vertical banks at either end of the straight pipe banks. Each bend connects a pair of straight pipe sections in adjacent vertical banks thereof. The return bends are sloped between horizontal and vertical, and one of the straight pipe sections in the pair connected by a return bend is elevated with respect to the other. A tubeside fluid flow path is provided from the inlet pipe section through an alternating series of the straight pipe sections and the return bends to the outlet pipe section. The coil preferably includes first and second vertical straight pipe banks and opposing return bend banks, wherein the straight pipe sections and the return bends are evenly spaced from adjacent sections and bends above and below except for uppermost and lowermost pipe sections and return bends. The return bends at either end of the adjacent tube banks can be oppositely sloped so as to provide a generally horizontal-helical flow pattern. The coil preferably has first and second nested passes wherein the fluid flow paths of the first and second passes each comprise a series of alternating straight pipe sections in each of said vertical banks thereof, wherein the straight pipe sections of the first pass in the first and second banks are horizontally spaced opposite the straight pipe sections of the second pass in the respective second and first banks. The first and second pass straight pipe sections in each vertical tube bank can be alternated every other one from top to bottom.
In an alternate embodiment, the coil also has nested third and fourth passes wherein the fluid flow paths of the third and fourth passes each comprise a series of alternating straight pipe sections in each of said vertical banks thereof wherein the straight pipe sections of the third pass in the first and second banks are horizontally spaced opposite the straight pipe sections of the fourth pass in the respective second and first banks. The first, second, third and fourth pass straight pipe sections in each vertical tube bank can be alternated every fourth one from top to bottom.
In a further alternate embodiment, the coil can also have nested fifth and sixth passes wherein the fluid flow paths of the fifth and sixth passes each comprise a series of alternating straight pipe sections in each of said vertical banks thereof wherein the straight pipe sections of the fifth pass in the first and second banks are horizontally spaced opposite the straight pipe sections of the sixth pass in the respective second and first banks. The first, second, third, fourth, fifth and sixth pass straight pipe sections in each vertical tube bank can be alternated every sixth one from top to bottom.
In another aspect, the present invention provides a refinery process furnace having a firebox, a coil and floor-mounted burners. The firebox has a floor, opposing vertical sidewalls, and opposing vertical end walls. The coil described above is disposed with the straight pipe banks and the return bend banks within the firebox. Endmost vertical straight pipe banks are evenly spaced from and generally parallel to the vertical side walls, each of the return bend banks are evenly spaced from the end walls, and the inlet and outlet sections are disposed for introducing a relatively cold fluid into the flow paths and discharging a heated fluid therefrom, respectively. Multiple rows of the floor-mounted burners are arranged alternately, and evenly spaced in plan with respect to the vertical straight pipe banks. The refinery process furnace preferably comprises a

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