Thursday 10 March 2011

INSTRUMENTATION AND CONTROL OF CRACKING FURNACE



CONTROL OBJECTIVES

The operation and control of the .cracking heaters is central to the overall economics of plant operation since feedstock costs are the largest component of production costs. The general objectives for heater control are to meet the desired severity and feedstock targets without violation of any operating constraints. Severity fluctuations are reduced by reducing the variations in steam to hydrocarbon ratio, (S/HC), balancing the coil outlet temperatures (COT), equalizing the coil feeds, and compensating for fuel composition changes. Furnace constraints are monitored and control targets selected which do not violate the constraints.

BASIC BLAIER CONTROL

The hydrocarbon Feed rate to each coil is regulated by a flow controller; Steam is mixed with the feed through four steam flow controllers. After cracking, the hotter effluent is cooled rapidly in a transfer line exchanger (TLE). An analog temperature controller adjusts the overall fuel flow through pressure ratio controllers, to hold the outlet temperature of the target coil constant. Four pressure ratio controllers on the fuel gas are provided to allow balancing of the filing in the different coils of the heaters. Outing the hunter cycle, coke deposit on the coils and TLE's eventually constraint the operation of the heater and force & shut down for decoking which is indicated by the rise in the tube metal temperature of the coils being monitored by the installed TI's in the transfer line.



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A chromatograph system is also installed analyzing heater effluent streams and gas feed stream. The GC's are micro-computer controlled and linked via the gas chromatograph computer with the process computer system; Extensive validity checking is performed on the analyzer and analyses.

COMPUTER IN OPERATIONAL CONTROL

The computer in operational control steers the actual operation of the plant, it does this by continually logging the important operational parameters such as flow, pressure, and temperature and making adjustments to keep these within the range of predetermined set points by manipulating controller set points that directly affect these parameters. In the case that the computer is unable to control the process within given limits, or when the computer detects unsafe conditions, it seeds out an alarm to alert the plant operators. The following description refers to method of Computer control frequently used.
The computer monitors and controls the temperature at each heater coil outlet by the flow of hydrocarbons to each coil. This manipulation balances the coil outlet temperatures when each coil operates on the same set point, otherwise, for biased set points; it just maintains coil outlet temperatures. The coil outlet set point is determined based upon the severity calculated from heater effluent gas chromatographic analysis. The hydrocarbon flow is checked for coil-to-coil deviations to ensure that the computer logic does not severely unbalance the mass flows while balancing coil outlet temperatures. When the constraints are violated, the computer so notifies the operator to take corrective-action. It does not manipulate the flow beyond the set range.
The computer calculates total heater throughput by summarizing the individual mass flows to each coil. The heater throughput is controlled by adjusting the fuel gas flow to the heater. If, for example, total heater throughput decreases while the coil outlet temperatures are being held within range, the computer will increase fuel gas pressure (i.e., fuel gas flow) which will cause coil outlet temperatures to rise. The COT logic will then increase the flow to compensate. The reverse logic is used when the heater throughput increases above the set point. In addition, the computer also regulates the dilution steam flow in accordance with hydrocarbon flow rate,
In addition to the control functions, a number of performance calculations are also performed by the computer such as around furnace with excess air monitoring and transfer line exchanger temperature monitoring etc.
Other control functions performed in the hot section by a process computer include compressor and distillation column control. Anti surge control can be implemented on a compressor by maintaining the minimum bypass flow rates, This will both conserve energy by avoiding over-recycling and preventing surge conditions.

To summarize, while the computer used for design is mainly a high-speed calculating tool, the computer in operation (the process computer) is an executive tool that automatically, through a built-in logic, monitors and performs sequential operations in the proper order so as to control at or near optimum conditions and to maintain operational stability.

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