Patent Application
20080173261
Direct fired heaters find wide application, particularly in oil refineries, where they are used for the purpose of preheating petroleum or petroleum derived feed-stocks for further processing to produce such products as fuel gas, gasoline, diesel fuel, heavy fuel oil and coke. The feed-stocks are of variable composition and boiling range and require that they be preheated to varying temperatures for further processing. Some of the applications considered would be as follows:
Vacuum Heater Service, wherein residuum from atmospheric distillation is preheated prior to being processed in a distillation tower operated under vacuum, to separate such products as lower boiling point liquids and very high boiling point bottoms liquids from one another.
Visbreaking Heater Service, wherein high boiling point feed-stocks are subject to heat treatment in a fired heater at temperatures lower than those used in a delayed coking heater, resulting in a product slate consisting of fuel gas, gasoline and heavy fuel oil.
Reboiling Heater Service, wherein relatively low boiling point feed-stocks are preheated to temperatures at which permit separation of the feedstock constituents in a distillation column is made possible.
Fully Integrated Steam Generating-Steam Superheating-Boiler Feed-Water Service, the direct fired heater for which is the second focus of the subject invention. The direct fired heaters used for the above services are usually provided with two sections, a radiant section and a convection section. The radiant section consists of a refractory lined enclosure wherein is disposed one or more tubular heating coils thru which the process fluid flows. The heating coils are arranged so as to surround a grouping of one or more burners fueled by gas. The heating coils are arranged so as to form a combustion chamber into which high temperature combustion products generated by the burners are discharged. Heat is transferred from the combustion products to the heating coils, and the process fluid which they contain, principally by radiation.
Process fluid is usually preheated in a convection section prior to entering the radiant section, the convection section consisting of a refractory lined enclosure containing multiple rows of tubes, the rows and the tubes comprising the rows are closely spaced, forming channels thru which combustion products, leaving the radiant section, pass at relatively high velocity. In so doing, heat is transferred from the combustion products to the heating coils and contained process fluid, principally by convection. Ideally, the spent combustion products leave the convection section at low temperature corresponding to a high overall heater thermal efficiency.
Because of the high temperature to which hydrocarbon process fluids in the radiant section are subjected, fluid at the inside wall of the tubular heating elements at this location experience a degree of thermal decomposition, leaving behind adherent coke deposits which reach maximum thickness at the outlet of the coil. These deposits restrict the flow of heat from the tube wall to the contained process fluid so that the tube wall eventually reaches design temperature. At this point, referred to as an end of run condition, the heater must be shut down and de-coked to avoid tube damage. The time interval between shutdowns for decoking is referred to as run length.
This invention relates to the design of direct fired heaters, in general, and more specifically, to delayed coking heaters and fully integrated heaters for steam generating, steam superheating and boiler feed-water preheating.
The delayed coking heaters consist of a lower radiant section and upper convection section. The convection section consists of a refractory lined enclosure containing a plurality of closely spaced horizontal tubes arranged to form closely spaced planes. Because combustion products passing thru the convection section are relatively low, heat is transferred from the combustion products to the heating coils, and the process fluid flowing through said coils, primarily by convection. Several coils may be contained in the convection section, one of which consists of a process coil, the outlet of which is connected to the radiant section, so that process fluid can be preheated in the convection section, raised to the design temperature required at the inlet of the radiant section, and heated further in said radiant section to design temperatures required at the radiant section outlet, as required for further processing. Additional convection section coils can be added to generate steam, superheat steam, preheat boiler feed-water or for like purposes.
The radiant section is comprised of a refractory lined enclosure having horizontal tubes, arranged in parallel, to form serpentine process heating coils located at each of two parallel and opposed sidewalls of the heater. Rows of closely spaced burners, located at the bottom of the heater, midway between the parallel tube planes and firing vertically upward and with gaseous fuels, provide the heat necessary to raise the process fluid, contained in the heating coils, from design inlet to design outlet temperature. Heat from the high temperature combustion products, generated by the burners, is transferred to the heating coils and process fluid contained therein primarily by radiation. The size and placement of the burners is such as to very substantially limit firebox re-circulation of combustion products. As a result, combustion product temperature at the bottom of the radiant section are very high, much higher than that in heaters of conventional design, and temperatures at the top of the heater are much lower, yet high enough to transfer significant amounts of heat to the process heating coils by radiation.
The arrangement described results overall radiant heat transfer rates that are some 75% higher than heat transfer rates in heaters of conventional design, and accordingly reduce the size and cost of the heater.
Despite the higher radiant heat absorption rates characteristic of heaters designed in accordance with the subject invention, it is nevertheless possible to provide for heater run lengths, as are limited by the deposition of coke on inside surfaces of tubular heating elements, that are essentially equal to those obtained in heaters of conventional design. This is accomplished thru use of heating coil tube sizes consistent with sufficiently high inside heat transfer coefficients to maintain fluid film temperatures in contact with internal high temperature tube surfaces or coke deposits at acceptable levels and, in addition, by locating low temperature radiant coil inlets at the bottom of the radiant section, where combustion product temperature is high, and by locating high temperature radiant coil outlets at the top of the radiant section where combustion product temperature is low.
The fully integrated steam generating, steam superheating, boiler feed water preheating heater follows much the same principals as those used in the design of the delayed coking heater. The radiant section, however, consists of a plurality of vertically oriented parallel tubes dedicated only to the generation of steam, the horizontal tube convection section being dedicated only to the superheating of steam, from saturation temperature at design pressure, to design superheat temperature, to the preheating of boiler feed-water, from design inlet temperature to design outlet temperature, and with a flow-rate in each case being consistent with the design quantity of steam produced.
One embodiment of the invention, a delayed coking heater, as shown in
A side elevation as viewed from a vertical plane passing thru the centerline between the two horizontal parallel rows of tubes located at the opposed parallel side walls of the radiant section, the same vertical plane passing thru the horizontal parallel rows of tubes between the two parallel opposed sidewalls of the convection section;
An end elevation as viewed from a vertical plane passing thru the centerline between the two parallel end walls of radiant and convection sections, and perpendicular thereto;
Section A-A as viewed from a plane perpendicular to the centerline between the two rows of burners and passing thru the burners;
Another embodiment of the invention, a fully integrated steam generator, steam super-heater and boiler feed water pre-heater as shown in
A side elevation as viewed from a vertical plane passing thru the centerline between the two parallel rows of vertical tubes located at the opposed parallel side walls of the radiant section, the same vertical plane passing thru the horizontal parallel rows of tubes between the two opposed parallel sidewalls of the convection section;
And
One embodiment of the invention, a delayed coking heater, as shown in
Heaters configured using design parameters proposed by this invention differ in a number of ways from heaters configured using design parameters in current usage, this being evident by comparing configuration data for a delayed coking heater designed in accordance with current practice with a heater designed in accordance with the subject invention, the heaters in each case being vertically oriented with horizontal serpentine coils located at either sidewall and with burners located midway between heating coils and firing upwards from the bottom of the heater. The two heaters being compared although having different configurational differences, do have operating conditions in common, these being as follows:
The following are configurational and performance data for a delayed coking heater designed in accordance with conventional practice which differ from that for the same heater designed in accordance with the subject invention.
The above data demonstrate several important advantages obtainable thru use of heaters designed in accordance with the subject invention, these being;
These data also indicate the fundamental differences between the two heaters resulting thru use of the design concepts made available by the subject invention, a key concept being the reduction in radiant section cross-sectional flow area perpendicular to the flow of combustion products generated by the burners. This strategy decreases re-circulation of combustion products, increases combustion product temperature at the bottom of the heater, close to the burners, increases heat transfer rates by radiation and convection, from combustion products to radiant section process heating coils, reduces process heating coil surface requirements and the overall size of the heater. That these effects are in fact obtainable is validated by examination of the performance of conventionally designed heaters. Thus, the commonly used assumption and experimental observation is the near equality of top and bottom combustion product temperature in a bottom fired vertical heater. It can be reasoned that this is due to the use of relatively small burners firing into a very large combustion chamber, so as to provide space sufficient to allow for a large amounts of combustion product re-circulation and top to bottom mixing. Use of a simplified re-recirculation model using as a basis the effect of the combustion chamber size relative to burner size has demonstrated that the aforementioned reasoning is very nearly correct and can be used to calculate re-circulation rates for heater configurations that are markedly different from those in a conventionally designed heater as in the case of the subject invention.
Despite the higher heat transfer rates characteristics of heaters designed in accordance with the subject invention, it is nevertheless possible to maintain run lengths for such heaters at low levels, comparable to those obtainable in heaters of conventional design. This is accomplished by providing; co-current flow for combustion products and process fluid; by maintaining process fluid film temperatures at low levels, thru use of low process fluid inlet temperatures of 600 F or less, and by providing for inside tube heat transfer coefficients of not less than 400 BTU per hour per square foot per degree F.
A second embodiment of the invention, a fully integrated steam generator, steam super-heater, and boiler feed water pre-heater, as shown in
Configurational and performance data for a fully integrated direct fired steam generator, steam super-heater, and boiler feed water pre-heater, of a design in accordance with the subject invention, are summarized as follows:
Performance data which differ for heaters of conventional design and heaters of a design in accordance with the subject invention, are as follows:
If heaters of conventional design were to operate at the same overall average heat transfer rates as heaters designed in accordance with the subject invention, the latter heaters would have a distinct advantage, as can be noted from the above performance data. Thus, for equal transfer rates, the heater of conventional design requires that a sizeable fraction of the total steam generation absorption be shifted to the horizontal tube convection section. This is most undesirable because natural circulation steam generation would then be impossible, because of the resulting high overall pressure drop for both the radiant and convection section coil combination. Additionally, a satisfactory homogeneous flow regime in the horizontal tube convection section would be difficult to acquire, unless use of a high velocity, high pressure drop arrangement, requiring forced circulating pump usage were resorted to. In contrast, the design in accordance with the subject invention confines steam generation to the vertical tube radiant section so that all the advantages of a natural circulation system are achieved.
