Process Heater Control

The problem with this type of control loop is the lag time (three to twelve minutes) between the changing of a setpoint and a recorded temperature change. 

Using manual control with the single-loop process heater control system, a 10 percent change in 

Effect of Fuel Pressure. Another major problem with a conventioned single-loop control is that if the fuel pressure drops, less fuel will flow through the control valve. Eventually, the liquid flowing through the process heater will have a lower temperature which is finally affected by the thermocouple in the heater outlet. When this happens, the recorder-controller signals the fuel valve to open wider letting more fuel into the process heater. Unfortunately, more time passes before the controller knows precisely how much to open the fuel valve for correct temperature control. 

Effects of Process Flow Change. If the amount of process fluid to be heated suddenly decreases, some time will elapse before the thermocouple detects a temperature change and notifies the recorder-controller to reset the fuel valve. 

Effect of Fuel Heat Content. Because process heaters often bum two or more fuels—natural gas, process plant off-gas, heavy fuel oil, or light fuel oil (each having a different Btu content)—a change in fuel to the fired heater can dramatically change the heat developed. Some time will elapse before the change in process fluid temperature is registered on the heater outlet thermocouple. 

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Which cascade system is best Although the PRC and the PRC control loops are faster than the TRC loop, they do not take into account all the disturbances described under the description of conventional single-loop control. Therefore, the TRC/TRC cascade control loop is usually best for process heater control. 

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). 

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... 

Fired heaters are extensively used in the oil and gas industry to process the raw materials into usable products in a variety of processes. Fuel gas is normally used to fire the units which heat process fluids. Control of the burner system is critical in order to avoid firebox explosions and uncontrolled heater fires due to malfunctions and deterioration of the heat transfer tubes. Microprocessor computers are used to manage and control the burner system. 

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. 

Increasingly, newer fired process heater installations are adding more fuel-air combustion controls and safety instrumentation systems. However, the decision on the extent of fired heater combustion controls, instrumentation, and safety systems to employ is fundamentally a loss prevention and risk tolerance issue, rather than a fire protection one. The following recommended practices, codes and standards apply to fired heater and dryer controls and instrumentation ... 

API RP 556, Fired Heaters and Steam Generators, provides current guidance on the recommended practice for controls and instrumentation for fired process heaters for liquid and gas stream heating in the petroleum and petrochemical industries. 

Cracking (FCC) units have been established through consent decree between the EPA and the rehnery or by the application of new source review (NSR) provisions of the Clean Air Act when making rehnery modihcations that result in a signihcant emission increase. NSR standards require facilities to apply best available control technology (BACT) in ozone attainment areas and the lowest achievable emissions rate (LAER) in ozone nonattainment areas. In addition, in mid-2009, the U.S. EPA revised the Standards of Performance for Petroleum Rehneries (40 CFR 60 Subpart Ja) to include NO, standards for process heaters and FCCUs. 

Fio. 7.66. Cascade control or the temperature of the outlet stream from a process heater... 

R 18] [A 1] Each module is equipped with a heater (H3-H8) and a fluidic cooling (C03-C06). Temperature sensors integrated in the modules deliver the sensor signals for the heater control. Fluidic data such as flow and pressure are measured integrally outside the micro structured devices by laboratory-made flow sensors manufactured by silicon machining. The micro structured pressure sensor can tolerate up to 10 bar at 200 °C with a small dead volume of only 0.5 pi. The micro structured mass flow sensor relies on the Coriolis principle and is positioned behind the pumps in Figure 4.59 (FIC). For more detailed information about the product quality it was recommended to use optical flow cells inline with the chemical process combined with an NIR analytic or a Raman spectrometer. 

This cycling can be eliminated by mounting the control valve in the condensate pipe, but this creates new problems, because when the load decreases, the process is slow steam has to condense before the condensate level is affected, and when the load increases, the process is fast, because blowing out liquid condensate is fast. With such "nonsymmetrical" process dynamics, control is bound to be poor. A better option is to use lifting traps to prevent condensate accumulation. These pumping traps will make temperature control possible even when the heater is under vacuum, but will not improve the problem of low rangeability, and the possible use of two control valves in parallel can still be necessary. 

Production of heat energy at Rohm Haas has been improved by better control of combustion in boilers, cleaning and maintenance of boilers and process heaters to maintain their efficiency, elimination of steam leakages, steam trap maintenance, and improved condensate recovery (4). 

TEMPERATURE HIGH Ambient Conditions Fouled or Failed Exchanger Tubes Fire Situation Cooling Water Failure Defective Control Valve Heater Control Failure Internal Fires Reaction Control Failures Heating Medium Leak into Process Faulty Instrumentation and Control... 

Step 8. Several control valves now remain unassigned. Steam flow to the trim heater controls reactor inlet temperature. Cooling water flow to the trim cooler is used to control the exit process temperature and provide the required condensation in the reactor effluent stream. Liquid recirculation in the absorber is flow-controlled to achieve product recovery, while the cooling water flow to the absorber cooler controls the recirculating liquid temperature. Acetic acid flow to the top of the absorber is flow-controlled to meet recovery specifications on the overhead gas stream. Cooling water flow to the cooler on this acetic acid feed to the absorber is regulated to control the stream temperature. Cooling water flow in the column condenser controls decanter temperature. 

CO entering cleaning vessel was consistently below process temperature Heater control for inlet was poorly designed Installed controller and thermocouples... 

Alternative Control Techniques Document - NO Emissions from Process Heaters (Revised) U. S. Environmental Protection Agency Bulletin EPA-453/R-93-034, September 1993. 

Most of the system interlocks are handled by the embedded NI controller. This is similar to that used in other laboratory-scale reactor systems (Mills and Nicole, 2005) except the Siemens 545 PLCs have been replaced with the National Instruments system. The National Instruments controller monitors the status of the AIMS process variables and takes appropriate action if any interlock conditions are found. In addition to these interlocks, the external heater controllers that are used for the system heating tapes and sampling valve box contain hardwired overtemperature interlocks built-in to these controllers. However, these interlocks only interrupt power to the affected heating device, so the NI controller must still take action to shutdown the rest of the process. 

Control motor on each burner. (From Bussman, W., and Baukal, C., "Ambient Condition Effects on Process Heater Emissions," Proceedings of the Inti Mechanical Engineering Congress Exhibition, Paper 1MECE2008-68284, Boston, MA, November 2008. With permission.)... 

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