Indirect fired heaters

Fresh reducing gas is generated by reforming natural gas with steam. The natural gas is heated in a recuperator, desulfurized to less than 1 ppm sulfur, mixed with superheated steam, further preheated to 620°C in another recuperator, then reformed in alloy tubes filled with nickel-based catalyst at a temperature of 830°C. The reformed gas is quenched to remove water vapor, mixed with clean recycled top gas from the shaft furnace, reheated to 925°C in an indirect fired heater, and injected into the shaft furnace. For high (above 92%) metallization a CO2 removal unit is added in the top gas recycle line in order to upgrade the quaUty of the recycled top gas and reducing gas. 

Reducing gas is generated from natural gas in a conventional steam reformer. The natural gas is preheated, desulfurized, mixed with steam, further heated, and reformed in catalyst-filled reformer tubes at 760°C. The reformed gas is cooled to 350°C in a waste heat boiler, passed through a shift converter to increase the content, mixed with clean recycled top gas, heated to 830°C in an indirect-fired heater, then injected into reactor 4. 

The next chapter discusses the use of LTX units to melt the hydrates as they form, and the use of indirect fired heaters to keep the gas temperature above the hydrate formation temperature. Chapter 8 describes processes and equipment to dehydrate the gas and keep free water from forming. 

Indirect fired heaters (sometimes called line heaters) heat the gas stream before and/or after the choke so that the gas is maintained above the hydrate temperature. Indirect fired heaters can also be used to heat crude oil for treating, heat a hot fluid circulating medium (heat medium) that is used to provide process heat, etc. 

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 previous discussion focused on the use of indirect fired heaters as line heaters to provide the necessary heat to avoid hydrate formation at wellstream chokes. Indirect fired heaters have many other potential uses in production facilities. For example, indirect fired heaters can be used to provide heat to emulsions prior to treating, as reboilers on distillation towers, and to heat liquids that are circulated to several heat users. The sizing of indirect fired heaters for these uses relies on the same principles and techniques discussed for wellstream line heaters. 

The reboiler provides the heat input to an amine stripper, which reverses the chemical reactions and drives off the acid gases. Amine reboilers may be either a kettle reboiler (see Chapter 3) or an indirect fired heater (see Chapter 5). 

Loss of fuel gas to catalytic heater., ow ambient temp in xil Idfng Possible equipment failure TSL Initiates ESO Indirect fire heater should provide heat source ... 

Indirect-fired heaters of the box type, also called fired heaters, process heaters, and furnaces, are commonly used to heat and/or vaporize non-reacting process streams at elevated temperatures beyond where steam is normally employed. The fuel for combustion is either gas or... 

Figure 16.12 Base f.o.b. purchase costs for indirect-fired heaters of the box type.

Both direct-fired and indirect-fired natural gas heaters are used by the manufacturers. In a direct fired heater, natural gas is burned directly into the reactivation air stream and the exhaust gas is used for regeneration. Although 90% to 95% heating efficiency can be achieved, there is always a danger of local hot spots that can scorch the desiccant and other components as the temperature can be as high as 977 K. These types of problems can be avoided in indirect-fired heaters, but the heating efficiency for indirect-fired heaters is in the range of 50% to 70%. 

Heaters are vessels used to raise the temperature of the liquid before it enters a gunbarrel, wash tank, or horizontal flow treater. They are used to treat crude oil emulsions. The two types of heaters commonly used in upstream operations are indirect fired heaters and direct fired heaters. Both types have a shell and a fire tube. Indirect heaters have a third element, which is the process flow coil. Heaters have standard accessories such as burners, regulators, relief valves, thermometers, temperature controllers, etc. 

Figure 1.5 shows a typical indirect fired heater. Oil flows through tubes that are immersed in water, which in turn is heated by a fire tube. The heat may be supplied by a heating fluid medium, steam, or electric immersed heaters. Indirect heaters maintain a constant temperature over a long period of time and are safer than the direct heater. Hot spots are not as likely to occm if the calcium content of the heating water is controlled. The primary disadvantage is that these heaters require several hours to reach the desired temperature after they have been out of service. 

Steam is the usual heating medium, and a standard heater arrangement consists of a main heater before the circulating fan. When steam is not available or the diying load is small, electricalheat can be used. For temperatures above 450 K, products of combustion can be used, or indirect-fired air heaters. 

Indirect-Fired Equipment (Fired Heaters) Indirect-fired combustion equipment (fired heaters) transfers heat across either a metallic or refractory wall separating the flame and products of combustion from the process stream. Examples are heat exchangers (dis-... 

Fired heaters differ from other indirect-fired processing equipment in that the process stream is heated by passage through a coil or tubebank enclosed in a furnace. Fired heaters are classified by function and by coil design. 

In an indirect bath heat exchanger, the heating medium provides Iil u to an intermediary fluid, which then transfers the heat to the fluid h)cuig heated. An example of this is the common line heater used on many gas well streams to keep the temperature above the hydrate formal ion lem perature. A fire tube heats a water bath, which provides heat to tlie v.all siieam flowing through a coil immersed in the bath. Details pertaining to dcsi jit of indirect bath heaters are presented in Chapter 5. 

There are many other types of heat exchanger devices that can be used to heat the gas above the hydrate temperature. These could include shell and tube heat exchangers, electrical immersion heaters, furnaces, etc. However, the most common equipment type used to heat a well stream is the indirect fired water bath heater. 

Indirect or direct fired heaters are widely used in the process industries. Heat loss is kept to a minimum by refractory coatings on the furnace wall. Any material in the fuel that is corrosive or forms excess soot has to be avoided. Usually 20-25% excess air is required for fuel oil vs. 5-10% for gaseous fuel, hence the latter is more economic. Energy in the exit flue gas not used to heat water or a product can be recovered by heat exchangers that generate additional steam or preheat the entering air. 

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