Advanced fired heater unit for use in refinery and petro-chemical applications (Exxonmobil)

CATEGORY: FIRED HEATERS
PATENT
Advanced fired heater unit for use in refinery and petro-chemical applications (Exxonmobil)
Publication number US8490581 B2
Application number US 11/808,973
Publication date Jul 23, 2013
Also published as CN101490214A
Inventors
San Chhotray, Thomas M. Rudy, Dennis L. Juedes, James P. Norton, Charles E. Benson
Original Assignee
Exxonmobil Research And Engineering Company, Tiax, Llc.
Abstract
A fired heater unit is disclosed having at least one radiant heating section with each of the at least one radiant heating section having a heating element located therein. The unit includes a convection section operatively connected to the at least one radiant heating section, wherein the convection section having at least one vertically oriented convection tube. Each of the at least one convection tube being operatively connected to the at least one radiant heating section. Each of the at least one convection tube includes an inner tube having process fluid flowing therethrough, wherein the process fluid being heated by the at least one radiant heating section, and an outer tube extending along a predetermined portion of the inner tube, wherein the outer tube is spaced from the inner tube to form a flow path therebetween.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high efficiency, low emissions fired heater unit utilizing positive pressure with a vertically arranged convection section. The use of positive pressure permits forced convective heat transfer which reduces the necessary size for heating, which results in a compact furnace design. It is contemplated that the fired heater unit in accordance with the present invention will be used in refinery and petro-chemical applications.
2. Discussion of Related Art
In petroleum refineries and petro-chemical plants, fired heaters are typically employed because these heaters can provide a higher level of heat, which typically can not be obtained from other utility sources. The conventional fired heaters presently in commercial practice contain a radiant section, a convection section, an air preheater and ultra low NOx burners. The convection section is located on top of the radiant section. The convection tubes of the convection section have a horizontal orientation.
The conventional fired heaters utilize preheated air to maximize heater efficiency. The conventional fired heaters require a balanced draft design with a forced draft blower and an induced draft blower. The forced draft blower feeds the ambient air to the preheater and then to the heater. The induced draft blower withdraws the flue gas. This arrangement, however, is prone to the ingress of additional ambient air. The ingress of ambient air has a negative impact on the operation of the heaters. As a result, the conventional fired heater must be operated at 3% or higher excess O2 levels to prevent combustion related instabilities. This reduces heater efficiency such that only a typical maximum achievable heater efficiency of is 91-92% is attained. Reductions in efficiency result in increases in total NOx and greenhouse gas emissions.
With an ever increasing demand on fuel, its rising cost, and at the same time tighter regulatory control on emissions reduction, there is a need for a fired heater that maximizes heater efficiency while reducing emissions. This need especially exists in larger heaters (greater than 100 MBtu/hr).
BRIEF SUMMARY OF THE INVENTION
Aspects of embodiments of the invention relate to a fired heater unit with high efficiency, low emissions and a compact design. Increases in heater efficiency will result in reduced fuel consumption leading to energy savings and a corresponding reduction in NOx and green house gas emissions.
It is an aspect of the present invention to utilize positive pressure (i.e. above atmospheric) within the fired heater unit to prevent the ingress of ambient air. It is contemplated that the entire fired heater unit may be operated under positive pressure. The utilization of positive pressure operation permits the operation of the heater unit with lower excess O2. The reduction in excess O2 results in higher efficiency and lower emissions.
The utilization of positive pressure operation also permits forced convective heat transfer from the flue gas to the process fluid in the convection section of the heater. The forced convective heat transfer decreases the effective convective heat transfer surface needed to heat the process fluid, which reduces the unit size.
It is another aspect of the present invention to provide a convection section having longitudinally finned vertical tubes. The tubes have radially extending spaced fins. This arrangement enhances heat transfer to the process fluids.
It is another aspect of the present invention to provide a fired heater having a compact construction. The unique arrangement is accomplished by locating the convection section between the radiant sections.
Since the heater in accordance with the present invention operates with positive pressure throughout the heater, only forced draft blowers are necessary. No induced draft blowers are needed, which are more prone to maintenance problems than the forced draft blowers.
Utilizing forced draft (only) throughout the heater, the heater can be operated at lower excess O2 levels of 1-2%. As such a potential thermal efficiency of 95% can be achieved. Additionally, as a result of higher efficiency, an estimated 3% lower NOx and green house gas emissions can be realized due to lower fuel firing.
These and other aspects can be realized by the present invention, which is directed to a fired heater unit that includes a twin cell vertical tube box type radiant section with a convection section of vertical tube orientation located between the two radiant cells. The convection section design concept in this invention is unique to fired heaters as follows. Each tube in the convection section consists of a bare and a segmented finned section with longitudinally finned tubes. The finned tube section of each tube is encased within an externally refractory lined metal sleeve forming a flue gas shell and thus providing a path for flue gas flow. The flue gas from the radiant section enters into the top of the convection section, exchanges heat with the process fluid inside the tube, first with a bare section and then with a finned section within the flue gas shell of each tube, similar to the flow path in a conventional double pipe heat exchanger. Thus the flue gas flow with respect to the process fluid flow in each tube is a combination of alternating co-current and counter-current flow in each pass. The flue gas shell around each tube is welded into a tube sheet which in turn is welded into a flue gas plenum. Flue gas from each shell exits into the plenum and flows through a high efficiency air pre-heater unit and then ducted into the exhaust stack. Ambient air from the forced draft blower is preheated in the air pre-heater unit and then ducted into the burners located on the heater floor. The forced draft blower(s) provides the preheated combustion air flow and the required head (typically 25 inches of water) for the air and flue gas flow circuit throughout the heater. There is no induced draft blower in this invention.
In accordance with aspects of the present invention, the fired heater unit includes at least one radiant heating section. Each radiant heating section includes a heating element. The fired heater unit further includes a convection section operatively connected to the at least one radiant heating section. The convection section is vertically oriented and includes at least one vertically oriented convection coil. Vertical tube orientation with top supported tubes results in lower maintenance. Elimination of induced draft blower and associated ducting, add to a lower cost design. A portion of each convection tube is operatively connected to the at least one radiant heating section.
In accordance with the present invention, each of the at least one convection tube includes an inner tube having process fluid flowing therethrough. The process fluid is heated by the at least one radiant heating section. Each convection tube also includes an outer tube extending along a predetermined portion of the inner tube. The outer tube is spaced from the inner tube to form a flow path therebetween. The flow path may include at least one radially extending fin located therein which extends outwardly from the inner tube toward the outer tube. Each fin extends the length of the outer tube. The combination of the forced convection and wider spacing between the fins practically eliminates fouling of the finned tubes. The reduction and/or elimination of fouling helps maintain design efficiency and lower maintenance costs. Each of the vertical tubes preferably includes an internal tube surrounded by longitudinally extending fins. The tube and the fins are surrounded by a refractory lined metal sleeve. The outer tube preferably includes a refractory layer surrounding the outer tube.
In accordance with an aspect of the present invention, the flow path has a first open end and a second open end. The first open end of the flow path is operatively connected to the at least one radiant heating section. With this arrangement, flue gas from the at least one radiant heating section enters the flow path through the first open end. The flue gas acts to further heat the process fluid contained within the inner tube. The second open end is operatively connected to a plenum such that flue gas traveling through the flow path exits the flow path though the second open end into the plenum. The plenum is operatively coupled to a preheater unit such that the flue gas exiting the flow path travels through the plenum to the preheater unit where the flue gas preheats the incoming ambient air.
In accordance with another aspect of the present invention, the fired heater unit includes a forced draft blower. The forced draft blower supplies ambient air to the preheater unit, which is then fed to the at least one radiant heating section. In accordance with the present invention, only a forced draft blower is needed to supply ambient air and move the same through the heater unit. As such, the heater unit operates under positive pressure.
In accordance with another aspect of the present invention, the fired heater unit includes a first radiant heating section and a second radiant heating section. The first and second heating sections are located on opposing sides of the convection unit such that the convection unit is located therebetween. The first and second radiant heating sections heat the process fluids as the process fluid flows through the convection unit.
It is contemplated that each convection tube may have a generally U-shaped inner tube. Each inner tube may include a first vertically extending section, a second vertically extending section, and a connecting section connecting the first vertically extending section to the second vertically extending section. The process fluid flows through the first vertically extending section into the connecting section and then into the second vertically extending section. Each of the vertically extending section may include an outer tube extending along a predetermined portion of the vertically extending section. Preferably, a first outer tube extends along a predetermined portion of the first vertically extending section; and a second outer tube extends along a predetermined portion of the second vertically extending section. A flow path is located between the inner tube and the first outer tube and the inner tube and the second outer tube. Each flow path has a first open end and a second open end. Each first open end of the flow path is operatively connected to the first and second radiant heating sections such that the flue gas in each of the first and second radiant heating sections enters the flow path through the first open ends. Each of the second open ends is operatively connected to a plenum such that flue gas traveling through the flow path exits the flow path though the second open end.
These and other aspects of the invention will become apparent when taken in conjunction with the detailed description and appended drawings.
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