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Section 8: Process ControlBenefits of Advanced Control
...The economics of most processes are determined by the steady-state operating conditions. Excursions from these steady-state conditions generally average out and have an insignificant effect on the economics of the process, except when the excursions lead to off-specification products. In order to enhance the economic performance of a process, the steady-state operating conditions must be altered in a manner that leads to more efficient process operation.
The following hierarchy is used for process control:
- Level 0: Measurement devices and actuators
- Level 1: Regulatory control
- Level 2: Supervisory control
- Level 3: Production control
- Level 4: Information technology
- Level 1: Regulatory control
For most processes, the optimum operating point is determined by a constraint. The constraint might be a product specification (a product stream can contain no more than 2 percent ethane); violation of this constraint causesoff-specification product. The constraint might be an equipment limit (vessel pressure rating is 300 psig); violation of this constraint causes the equipment protection mechanism (pressure relief device) to activate. As: the penalties are serious, violation of such constraints must be very infrequent.
If the regulatory control system were perfect, the target could be set exactly equal to the constraint (that is, the target for the pressure controller could be set at the vessel relief pressure). However, no regulatory control system is perfect. Therefore, the value specified for the target must be on the safe side of the constraint, thus giving the control system some "elbow room." How much depends on the following:
- The performance of the control system (i.e., how effectively it responds to disturbances). The faster the control system reacts to a disturbance, the closer the process can be operated to the constraint.
- The magnitude of the disturbances to which the control system must respond. If the magnitude of the major disturbances can be reduced, the process can be operated closer to the constraint.
In a flew applications, improving the control system leads to a reduction in off-specification product and thus improved process economics. However, in most situations, the process is operated sufficiently far from the constraint that very little, if any, off-specification product results from control system deficiencies. Management often places considerable emphasis on avoiding off-specification production, so consequently the target is actually set far more conservatively than it should be.
In most applications, simply improving the control system does not directly lea to improved process economics. Instead, the control system improvement must be accompanied by shifting the target closer to the constraint. There is always a cost of operating a process in a conservative manner. The cost may be a lower production rate, a lower process efficiency, a product giveaway, or otherwise. When management places extreme emphasis on avoiding off-spec production, the natural reaction is to operate very conservatively, thus incurring other costs.
The immediate objective of an advanced control effort is to reduce the variance in an important controlled variable. However, this effort must be coupled with a commitment to adjust the target for this controlled variable so that the process is operated closer to the constraint. In large throughput (commodity) processes, very small shifts in operating targets can lead to large economic returns. ...
Table of ContentsConversion Factors.Physical and Chemical Data.Mathematics.Thermodynamics.Heat and Mass Transfer.Fluid and Particle Mechanics.Reaction Kinetics.Process Control and Instrumentation.Process Economics.Transport and Storage of Fluids.Heat Transfer Operations and Equipment.Drying, Humidification and Evaporative Cooling.Distillation.Gas Absorption and Other Gas-Liquid Operations and Equipment.Solid-State Operations and Equipment.Size Reduction and Size Enlargement.Handling of Bulk Solids and Packaging of Solids and Liquids.Alternative Separation Processes.Chemical Reactors.Biochemcial Engineering.Waste Management.Safety and Handling of Hazardous Materials.Energy Sources, Conversion and Utilization.Materials of Construction.Process Machinery Drives.Analysis of Plant Performance.
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