Descriptions
CRUDE CHARGE HEATER:
The typical refinery contains one or more Crude Heaters which serve as "fractionator feed heaters" or Charge Heaters for the Crude (or Atmospheric) Distillation Column. The desalted crude oil is first preheated in a series of heat exchangers, utilizing waste heat from other parts of the refinery. It then flows to the direct-fired Crude Heater. The oil is heated in the furnace up to temperatures of about 700 °F and fairly high percentage vaporization (from lows of 30% to 35% to as high as 80% in some cases). Despite these challenging operating conditions, Crude Heaters have been successfully designed with both vertical and horizontal tubes. Another important factor is that, due to the presence of trace amounts of sulfur and various metallic compounds in the feedstock, internal corrosion of the furnace tubes is a major concern. The metallurgy of furnace tubes has continually been upgraded over the years, with traditional Chrome-Moly alloys such as A335 P5 or P9 being supplanted in some modern-day units, especially those designed to process heavy crudes, by stainless steels such as TP317L.
VACUUM CHARGE HEATER:
Downstream of many Crude or Atmospheric Distillation Units is a Vacuum Distillation Unit, which gets more light hydrocarbons out of every barrel of oil by operating at a pressure slightly below atmospheric. A Vacuum Heater serves as "fractionator feed heater" or Charge Heater for the Vacuum Distillation Column. Bottoms from the Crude Tower are processed in the Vacuum Heater Although it typically operates at lower pressures and, most often, lower percent vaporization than the Crude Heater, the Vacuum Heater must heat the fluid to higher temperatures (up to about 800 °F). Also, because of the low pressure, even the lower percent vaporization creates a very high fluid volume and thus it is common for large-bore tubes to be used near the furnace outlet. For these reasons, Vacuum Heater designs usually require horizontal tubes. As in a Crude Heater, the presence of trace amounts of sulfur and various metallic compounds in the feedstock makes internal corrosion of the furnace tubes a major concern. The metallurgy of tubes in a Vacuum Heater has continually been upgraded over the years, with traditional Chrome-Moly alloys such as A335 P5 or A335 P9 being supplanted in some modern-day units, especially those designed to process heavy crudes, by stainless steels such as TP317L.
HCU AND HTU REACTOR CHARGE HEATERS:
These heaters process a mixture of hydrocarbons and hydrogen at high pressures, usually in excess of 1000 psig and often as high as 2000 to 3000 psig. The high pressures require heavy-wall tubes, the expense of which favors designs utilizing double-fired tubes (i.e. tubes which are heated from both sides). The tubes may be either vertically or horizontally disposed. Furnace thermal efficiency is often maximized by recovering heat in the form of low-pressure or medium-pressure steam.
COLUMN REBOILERS:
This is typically a very mild process service, and the most cost-effective approach is usually a Vertical Cylindrical design. Tube metallurgy can vary depending on the process. The percent vaporization in the outlet can be 50% or more. Reboilers are also sometimes extremely large in terms of absorbed duty (heat absorbed by the process fluid, typically expressed in MM BTU / hr.
HOT OIL HEATERS:
Hot oil is used, like steam, as a circulating heat transfer medium that can then be utilized in a variety of small, locally-placed heat exchangers. Like column reboilers, the typical Hot Oil Heater (sometimes called a Heat Medium Heater) represents a very mild service. Most modern heat transfer fluids such as Dowtherm, Therminol, etc. have been specifically designed to allow use of carbon steel tubes in the Hot Oil Heater. Further, these fluids can tolerate fairly high heat fluxes without breaking down chemically. For these reasons, the typical Hot Oil Heater is a Vertical Cylindrical design.
COKERS & VISBREAKERS:
Coking and Visbreaking are both processes in which a moderate amount of thermal cracking (breakdown of long chain molecules into shorter-chain molecules) takes place inside the furnace tubes themselves. Typically they are installed downstream of the Vacuum Distillation Unit, and utilize the bottoms of the Vacuum Tower as feedstock. Controlled input of heat is of critical importance, to avoid overheating of tubes as well as the fluids contained within the tubes. They are characterized by fairly low (and uniform) heat fluxes, horizontally-disposed tubes, and often require multiple-cell configurations that allow accurate control of the heat input to each parallel pass of fluid.
CATALYTIC REFORMER CHARGE HEATERS:
These are a form of Reactor Charge Heater, often referred to as Reformers, which can create some confusion with Steam-Hydrocarbon reformer furnaces (see below). They are distinguished by their large size, and are unique in terms of operating at fairly low pressures, reasonably high temperatures, and (most critically) extremely low pressure drop through the furnace coil. Because of the extremely low allowable pressure drop, the total flow of process fluid is typically first divided into many small streams (called parallel passes) which then each flow through fairly small furnace tubes. The furnace configurations favored by these process requirements are traditionally the arbor-coil design (with large manifolds on the bottom, supplying a multitude of small-bore tubes arranged in an arch-like formation) or the inverted arbor design (same as above, but with the large manifolds located at the top of the radiant section.
STEAM-HYDROCARBON REFORMERS:
These are typically large furnaces, designed either to produce large volumes of hydrogen for use in other refinery processes such as hydrotreating or hydrocracking, or to supply syngas (a mixture of hydrogen, carbon monoxide, and residual methane and steam) for downstream conversion to products such as ammonia or methanol. The feedstock, typically natural gas but sometimes light liquids such as naphtha, is preheated and then combined with steam. It then flows through furnace tubes packed with catalyst, over which the following desired chemical reaction takes place:
CH4 + H2O -> 3H2 + CO
Trace amounts of carbon dioxide and unreacted steam & methane are also present.
Steam-Hydrocarbon or Steam-Methane Reformers are extremely complex from both process and mechanical points of view. Process outlet temperatures can be as high as 1650 °F. Regardless of the manufacturer, the furnace tubes are invariably vertically disposed and almost without exception double-fired (i.e. heated from both sides). Many different burner configurations are available, each with a claim to some unique advantages. The most common are top-fired (burners mounted on the roof or "arch" of the radiant section), sidewall-fired (with special burners yielding very compact flames), and terrace-wall fired (with two or more levels of burners firing vertically upwards on either side of a row of tubes).
PYROLYSIS HEATERS (CRACKING FURNACES):
Cracking furnaces are utilized to produce olefins such as ethylene and propylene from light hydrocarbons such as ethane, propane, and naphtha. The olefins are then processed downstream to create other products, including many plastics and synthetic fibers. Even more complex than Steam-Hydrocarbon Reformers, typical Cracking Furnaces operate with extremely high outlet temperatures. The tubes are not filled with catalyst; the desired reactions are initiated merely by the high temperatures involved. At the same time, undesirable side reactions can occur, so the "residence time" of the fluid within the heater must be minimized. This results in a staggering variety of unique, proprietary designs utilizing the most advance metallurgy and complex coil configurations.
SPECIALTY FURNACES:
Onquest also provides a variety of process furnaces and fired heaters for very specific applications, such as high-temperature Styrene Steam Superheaters, EDC Crackers for Vinyl Chloride Monomer (VCM) plants, CO Boilers for Fluid Catalytic Cracker (FCC) units, Acetic Acid Cracking furnaces, etc.
WASTE HEAT RECOVERY UNITS:
Onquest can provide Waste Heat Recovery Units (WHRU's) and Heat Recovery Steam Generators (HRSG's) for fired heaters, process furnaces, and gas turbines (cogen plants).
SCR SYSTEMS AND OTHER NOx TECHNOLOGIES:
Onquest has experience providing the most advanced NOx technology including implementation of Selective Catalytic Reduction (SCR) units and/or replacement of existing burners with modern, ultra-low Nox burners. Often, a furnace modification can combine an environmentally-driven retrofit with other process improvements such as increased throughput / capacity, higher thermal efficiency, lower pressure drop, or others.
FURNACE REVAMPS AND UPGRADES:
Operating companies looking to improve the operation of their existing combustion equipment know that Onquest is an expert at revamping furnaces for increased throughput, improved efficiency, or both. Downtime required to complete the field modifications can be planned to take into account scheduled shutdowns and planned maintenance turnarounds. Often an existing furnace can be modified to perform at a like-new level, at a fraction of the cost of a completely new heater.
EMERGENCY REBUILDS, REPAIRS, AND SPARE PARTS
Onquest's experience and our ongoing contact with furnace pipe & tube suppliers, steel and coil fabricators, foundries, etc. puts us in a unique position to be of assistance in emergency situations. Our experienced staff can respond immediately to a customer's needs, plan the best approach considering both cost and schedule requirements, and execute the engineering, supply of materials, and/or construction on a fast-track basis.
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