Process heaters

Ratio and Multiplicative Feedforward Control. In many physical and chemical processes and portions thereof, it is important to maintain a desired ratio between certain input (independent) variables in order to control certain output (dependent) variables (1,3,6). For example, it is important to maintain the ratio of reactants in certain chemical reactors to control conversion and selectivity the ratio of energy input to material input in a distillation column to control separation the ratio of energy input to material flow in a process heater to control the outlet temperature the fuel—air ratio to ensure proper combustion in a furnace and the ratio of blending components in a blending process. Indeed, the value of maintaining the ratio of independent variables in order more easily to control an output variable occurs in virtually every class of unit operation. 

Once an undesirable material is created, the most widely used approach to exhaust emission control is the appHcation of add-on control devices (6). Eor organic vapors, these devices can be one of two types, combustion or capture. AppHcable combustion devices include thermal iaciaerators (qv), ie, rotary kilns, Hquid injection combusters, fixed hearths, and uidi2ed-bed combustors catalytic oxidi2ation devices flares or boilers/process heaters. Primary appHcable capture devices include condensers, adsorbers, and absorbers, although such techniques as precipitation and membrane filtration ate finding increased appHcation. A comparison of the primary control alternatives is shown in Table 1 (see also Absorption Adsorption Membrane technology). 

While process design and equipment specification are usually performed prior to the implementation of the process, optimization of operating conditions is carried out monthly, weekly, daily, hourly, or even eveiy minute. Optimization of plant operations determines the set points for each unit at the temperatures, pressures, and flow rates that are the best in some sense. For example, the selection of the percentage of excess air in a process heater is quite critical and involves a balance on the fuel-air ratio to assure complete combustion and at the same time make the maximum use of the Heating potential of the fuel. Typical day-to-day optimization in a plant minimizes steam consumption or cooling water consumption, optimizes the reflux ratio in a distillation column, or allocates raw materials on an economic basis [Latour, Hydro Proc., 58(6), 73, 1979, and Hydro. Proc., 58(7), 219, 1979]. 

Vlanv oil biirnor.s arc do.SLgnod a.s cornbination ga.s/oil biirnor.s, An example of a modern low-N(), oil/ga.s forced-draft burner Ls. shown in Fig, 27-30, This is an air-staged design, with the air divided into pri-rnaiv, secondarv, and tertiarv streams. An air-staged natural draft process heater oil/gas burner is illustrated in Fig, 27-3L... 

Fuel-Staged Burners Use of fuel-staged burners is the preferred combustion approach for NO control because gaseous fuels typically contain little or no fixed nitrogen. Figure 27-36 illustrates a fuel-staged natural draft refineiy process heater burner. The fuel is spht into primaiy (30 to 40 percent) and secondary (60 to 70 percent) streams. Furnace gas may be internally recirciJated by the primaiy... 

Due to the great variation in pressures, flux rates, materials of construction, heat recovery, burner configuration, etc., correlation of process heaters is difficult even with large amounts of data. For similar furnaces, heat absorption vs. cost gives the best correlation. It is again recommended that vendor help be obtained for estimating process furnaces, unless data on similar furnaces is available. Data can be found in References 24 and 25. 

Woodward, A. M. Reduce Process Heater Fuel, Hydrocarbon Processing, July 1974. 

Compounds considered carcinogenic that may be present in air emissions include benzene, butadiene, 1,2-dichloroethane, and vinyl chloride. A typical naphtha cracker at a petrochemical complex may release annually about 2,500 metric tons of alkenes, such as propylenes and ethylene, in producing 500,000 metric tons of ethylene. Boilers, process heaters, flares, and other process equipment (which in some cases may include catalyst regenerators) are responsible for the emission of PM (particulate matter), carbon monoxide, nitrogen oxides (200 tpy), based on 500,000 tpy of ethylene capacity, and sulfur oxides (600 tpy). 

Air emissions from coking operations include the process heater flue gas emissions, fugitive emissions, and emissions that may arise from the removal of the coke from the coke drum. The injected steam is condensed and the remaining vapors are typically flared. Wastewater is generated from the coke removal and cooling operations and from the steam injection. In addition, the removal of coke from the drum can release particulate emissions and any remaining hydrocarbons to the atmosphere. 

Air emissions from hydrotreating may arise from process heater flue gas, vents, and fugitive emissions. Figure 6 provides a simplified flow diagram. 

Air emissions from refineries include fugitive emissions of the volatile constituents in crude oil and its fractions, emissions from the burning of fuels in process heaters, and emissions from the various refinery processes themselves. Fugitive emissions occur throughout refmeries and arise from the thousands of potential fugitive emission sources such as valves, pumps, tanks, pressure relief valves, flanges, etc. 

Fignre 9-1 is a schematic drawing of the major eqnipment and protective measnres that comprise the vent manifold system. In the system shown, the vent vapors (offgas) from the condensers of two vacnnm col-nmns are collected in a manifold which goes to the vacnnm pnmp system. From the vacnnm pnmp system, the vapors go to a seal dmm (hydranlic flame arrester), and then to the firebox of a process heater, where they are incinerated. 

A detonation flame arrester with an integral thermocouple at the inlet to the process heater firebox to prevent backflash into the vacuum system. 

An interlock system (sensors and valves) which isolates offgas flow to die process heater firebox and routes the offgas to atmosphere on detection of low nitrogen flow or high temperature at the detonation flame arrester outlet. 

PH — Process Heater or Furnace R — Reactor S — Separator St — Strainer ST — Steam Tutbine Str — Steam trap SV — Safety Valve Tr — Trap V — Valve... 

The combustors affected by this rule detoxify or recover energy from hazardous waste and include incinerators, cement kilns, lightweight aggregate kilns, boilers and process heaters, and hydrochloric acid production furnaces. U.S. EPA estimates that 145 facilities operate 265 devices that burn hazardous waste. These technology-based standards reduce emissions of hazardous pollutants, including lead, mercury, arsenic, dioxin and furans, and HC1 and chlorine gas. In addition, emissions of PM are also reduced. 

Basis of chart Process heater type Cylindrical construction Carbon steel tubes Design pressure, 500 psi. 

Air emissions from a petroleum distillation unit include emissions from the combustion of fuels in process heaters and boilers, fugitive emissions of volatile constituents in the crude oil and fractions, and emissions from process vents. The primary source of emissions is combustion of fuels in the crude preheat furnace and in boilers that produce steam for process heat and stripping. When operating in an optimum condition and burning cleaner fuels (e.g., natural gas, refinery gas), these heating units create relatively low emissions of sulfur oxides, (SO c), nitrogen oxides (NO c), carbon monoxide (CO), hydrogen sulfide (H2S), particulate... 

Emissions from catalytic reforming (Figure 4.14) include fugitive emissions of volatile constituents in the feed and emissions from process heaters and boilers. As with all process heaters in the refinery, combustion of fossil fuels produces emissions of sulfur oxides, nitrogen oxides, carbon monoxide, particulate matter, and volatile hydrocarbons. 

The numerous process heaters used in refineries to heat process streams or to generate steam (boilers) for heating or steam stripping can be potential sources of sulfur oxides (SO2, and SO3), nitrogen oxides (NO and NO2), carbon monoxide (CO), particulates, and hydrocarbons emissions. When operating properly and when burning cleaner fuels such as refinery fuel gas, fuel oil, or natural gas, these emissions are relatively low. If, however, combustion is not complete, or heaters are fired with refinery fuel pitch or residuals, emissions can be significant. 

FIG. 24-33 Low-NO fuel-staged burner for a natural draft refinery process heater. Callidus Technolo es, Inc.)... 

Fired process heaters and boilers, incinerators, flares, and other equipment with flame burners are located at an appropriate distance from high value operating or processing areas, large volume storage of flammable or combustible materials, control rooms, operating offices, and their occupants. 

At least one potassium bicarbonate dry chemical extinguisher (120-B C) should be provided at each fired process heater handling liquid fuel or a liquid process stream. They should be installed on opposite sides, or ends, and adjacent to fire aisles. 

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