Fired heaters allowance

Under a more general frame, the ehmination of fuel combustion in fired heaters allows the use of multiple sources of low temperature heat as solar-heated molten salts or helium from a nuclear power [8] and not only the gas turbine exhausts. The cogenerative scheme presented for steam reforming could easily be extended to the other chemical processes that have been analyzed in this book. 

Direct-fired heater A heat generator that allows the combustion products to be mixed with the air to be heated. 

Volume 1, Chapter 9 explains the criteria for choosing a diameter and wall thickness of pipe. This procedure can be applied to choosing a coil diameter in an indirect fired heater. Erosional flow criteria will almost always govern in choosing the diameter. Sometimes it is necessary to check for pressure drop in the coil. Typically, pressure drop will not be important since the whole purpose of the line heater is to allow a large pressure drop that must be taken. The allowable erosional velocity is ffiven bv ... 

The crude phthalic anhydride is heated and held at 260°C to allow some byproduct reactions to go to completion. Purification is by continuous distillation in two columns. In the first column, maleic anhydride and benzoic and toluic acids are removed overhead. In the second column, pure phthalic anhydride is removed overhead. High-boiling residues are removed from the bottom of the second column. The reboilers of both distillation columns are serviced by a fired heater via a hot oil circuit. 

Provisions should be made to direct flammable or combustible liquid spills away from fired heaters. Spills from other equipment that flow into or under fired heaters can and have been ignited by the hot surfaces or flames of fired heaters. Conversely, in the past, spills from ruptured tubes in process heaters have allowed burning liquid to flow around and damage other equipment. Both process heaters and nearby equipment should be protected from each other due to possible spills of flammable or combustible liquids using one of the following methods ... 

The selected system comprises three direct-fired hot oil heaters, surge vessels ond circulation pumps. The hot oil is circulated in a closed loop through the tube bundles in the glycol reboilers. The system allows accurate control of the glycol tempercture ond by designing the fired heaters os a redundant utility system cvailoble to the three process troins the reliability of the system is thereby improved. 

Allowing a fired heater, boiler, or furnace to operate with insufficient air is hazardous because... 

Suppose a fired heater in an oil refinery, in service to heat crude oil, is fitted with a new air preheater. The firebox was running hot before the air preheater was installed, but the management was convinced that the installation of the air preheater would allow them to heat more crude oil. When the heater with its new air preheater came back on line, the following changes were noted ... 

The overhead from the second stage is heated by an exchange with hot solvent. The fired heater further raises the temperature of the solvent/demetallized oil mixture to a point above the critical temperature of the solvent. This causes the demetallized oil to separate. It is then flashed and steam-stripped to remove all traces of solvent. The vapor streams from the demetallized oil and asphalt strippers are condensed, dewatered, and pumped up to process pressure for recycle. The bulk of the solvent goes overhead in the supercritical separator. This hot solvent stream is then effectively used for process heat exchange. The subcritical solvent recovery techniques, including multiple effect systems, allow much less heat recovery. Most of the low grade heat in the solvent vapors from the subcritical flash vaporization must be released to the atmosphere requiring additional heat input to the process. 

Most hydrotreating processes are equipped with reboiled product strippers to remove hydrogen and light ends produced in the reactions. These strippers are of normal design however a few details may be noted. Because of the high bottom column temperatures required, fired-heater reboilers may be useful because they may allow vaporization with minimum exposure of bottom product to thermal degradation. 

Based on the tuned models, equipment performance could be evaluated and compared with design. The models could then be readily adapted to simulate an additional process fired heater (to unload the methanol superheater) and to evaluate it s inpact on the overall MTG unit performance. Multiple case studies were performed to determine optimum heater size with regard to maximising heat recovery and providing unit heat balance flexibility. The additional heat balance flexibility allows the ZSM-5 catalyst outlet temperature to be reduced hence maximising gasoline yield. 

In-line banks of high-finned tubes are not usually used in air-cooled heat-transfer equipment because the preferential flow path between the fin tips of adjacent tubes allows flow bypassing and reduces the apparent heat-transfer coefficient. This effect tends to disappear in the deeper tube banks used in convection sections of fired heaters. 

Decomposition. As column pressure rises, so does column temperature. When the materials distilled are heat-sensitive, decomposition or thermal cracking of the liquid may set in. Decomposition is likely to yield gaseous products which behave as noncondensables. Decomposition will therefore increase the relief requirement and may reduce the credit that can be allowed for cooling. The rate of decomposition may be particularly high upon a heat input controller failure. This problem is most severe when the reboiler is a fired heater or one that has a high temperature difference. 

The potential of MRs is enormous. In partieular, they play a key role in reversible endothermic reactions where their use can provide high reactant conversion at relatively lower temperatures. This will make possible to avoid fuel combustion in fired heaters to supply the required reaction heat and could allow multiple heat sources at low temperature to be used, ranging from gas turbine exhausts to solar heated molten salts or even helium from a nuclear power [24, 25, 29]. 

Fired heaters have either forced draft fans or induced draft fans to control air to the burners. This allows control of oxygen amount by direct measurement of air and fuel... 

Eliminating or minimizing hazards (for example, avoid proximity of fired heaters to potential sources of LPG leakage, close off areas where rocks may fall, and so on) and allowing adequate fire-fighting access. 

Tube materials in CRU fired heaters are usually of 2V4Cr 1 Mo specification, although ferritic tubes with higher chromium content, SCrlMo and 9CrlMo are encountered, the last mainly in CCR service. Tube dimension design and corrosion allowance generally follow API RP-530 (various dates). 

Boiler and fired heater tubes develop scale on the fired side of the tube. Past procedures required furnace shutdown for cleaning on a regular basis. On-line cleaning with combustible abrasives allows treatment without shutdown. With on-line cleaning, one refinery experienced a CO2 emissions reduction of 1,800 toimes per year, 60,000 fuel per year savings, 300,000 per year yield improvement, 800,000 per year throughput increase, and an overall 1.5% improvement in efficiency for the unit. 

Figure 1.6 shows a typical direct fired heater. Oil flows through an inlet distributor and is heated directly by a fire box. The heat may be supplied by a heating fluid medium, steam, or an electric immersed heater. Direct heaters are quick to reach the desired temperature, are efficient (75-90%), and offer a reasonable initial cost. Direct fired heaters are typically used where fuel gas is available and high volume oil treating is required. On the other hand, they are hazardous and require special safety equipment. Scale may form on the oil side of the fire tube, which prevents the transfer of heat from the fire box to the oil emulsion. Heat collects in the steel walls under the scale, which causes the metal to soften and buckle. The metal eventually ruptures and allows oil to flow into the fire box, which results in a fire. The resultant blaze, if not extinguished, will be fed by the incoming oil stream. 

The simplest unit employing vacuum fractionation is that designed by Canadian Badger for Dominion Tar and Chemical Company (now Rttgers VFT Inc.) at Hamilton, Ontario (13). In this plant, the tar is dehydrated in the usual manner by heat exchange and injection into a dehydrator. The dry tar is then heated under pressure in an oil-fired hehcal-tube heater and injected directly into the vacuum fractionating column from which a benzole fraction, overhead fraction, various oil fractions as side streams, and a pitch base product are taken. Some alterations were made to the plant in 1991, which allows some pitch properties to be controlled because pitch is the only product the distillate oils are used as fuel. 

Fire tubes, especially in heater treaters, where they can be immersed in crude oil, can become a source of ignition if the tube develops a leak, allowing crude oil to come in direct contact with the flame. Fire tubes can also be a source of ignition if the burner controls fail and the tube overheats or if the pilot is out and the burner turns on when there is a combustible mixture in the tubes. 

In subsequent years, Savery made important improvements that benefited future steam inventions. In June 1699 he demonstrated to the Royal Society a pump with two receivers, each with a separate, hand-controlled steam supply. This ensured improved continuity of operation, allowing one receiver tn operate in its vacuum stage and the other under steam pressure. In 1701, he added two more critical steps a second boiler, avoiding the need to shut down the fire and pump, between stages and he replaced the two interconnected steam cocks with a single valve, run with a manually operated long lever. This may have been the inspiration for the modern slide valve and his inventiveness created, in effect, the world s first feed-water heater. 

With hot water units, time clock control can operate satisfactorily as automatic bypass valves built into the distribution system will help the heater to achieve its working temperature quickly. With steam boilers, it is important that the boiler reaches a reasonable working pressure before steam is allowed into the distribution system. For example, if boilers are left open to a system for an extended length of time while not firing they will quickly lose their pressure. This is not only wasteful of energy but eventually creates a problem on start-up. To... 

Many of the conservation measures require detailed process analysis plus optimization. For example, the efficient firing of fuel (category 1) is extremely important in all applications. For any rate of fuel combustion, a theoretical quantity of air (for complete combustion to carbon dioxide and water vapor) exists under which the most efficient combustion occurs. Reduction of the amount of air available leads to incomplete combustion and a rapid decrease in efficiency. In addition, carbon particles may be formed that can lead to accelerated fouling of heater tube surfaces. To allow for small variations in fuel composition and flow rate and in the air flow rates that inevitably occur in industrial practice, it is usually desirable to aim for operation with a small amount of excess air, say 5 to 10 percent, above the theoretical amount for complete combustion. Too much excess air, however, leads to increased sensible heat losses through the stack gas. 

Oil and emulsion rise user heater fire tubes to a coalescing section where sullicicnt retention tune is provided to allow small water particles in the oil continuous phase to coalesce and settle to the bottom Treated oil Hows out the oil outlet Gas. flashed from the heated oil. flows through an equalizing line to the gas spare above Oil level is maintained by pneumatic or lever-operated dump valves Oil-water inter-fare height is regulated hy an interface controller or by an adjustable external water leg... 

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