Distillation is a common, energy-intensive separation method, but its control technology hasnt changed much since the 19th century. The thermal energy needs of distillation are enormous, while the thermodynamic efficiency of its processes is less than 10%, so serious savings can be gained by better controlling and optimizing it. Here and in the January and March 2007 issues, Ill describe the distillation process and PID-based controls, review optimization strategies, and show that advanced controls can cut operating costs of distillation by more than one third.
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Pressure Control Optimization, Part 2
In this series on distillation controls, Im discussing control and optimization strategies for pressure control. Distillation columns must be operated under constant pressure if temperature is to indicate composition. The primary advantage of floating pressure control is that, at minimum pressure, the required heat needed at the reboiler is thereby minimized. Other advantages include increased reboiler capacity and reduced reboiler fouling. The disadvantage is that the operating temperature is no longer an indication of composition. It needs compensation.
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Distillation Control and Optimization, Part 3
Reboilers can be inserted into the column or can be external. The process fluid circulation in them can be natural or forced. The kettle reboiler is the most common for external forced circulation applications. Thermosyphon reboilers (vertical or horizontal) are operated by natural circulation, induced by the hydrostatic pressure imbalance between the heavier liquid inside the tower and the lighter (two-phase) mixture in the reboiler tubes.
Read more...In the first three parts of this series, I discussed the individual loops used to control the distillation process. Now it is time to discuss the overall control strategies for the complete column, treated as a single unit operation. Here, the individual variables will only be treated as constraints and so long as their values are within acceptable limits, the column will be optimized to maximize production, minimize operating cost or meet other optimization criteria.
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Distillation is a common and energy-intensive method of separation in the petroleum, chemical, food, paper, and pharmaceutical industries. Globally, more than 80 million barrels of crude oil are refined daily. In the United States, 146 refineries operate, employing over 65,000 people and producing a total value that exceeds $150 billion. Yet, in the U.S., no new refinery has been built since 1976, and the control technology in use today is not much different from that used on the first distillation columns.
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Globally, more than 80 million barrels of crude oil are refined daily. The amount of energy used for distillation is approximately 8% of the total energy used in the industrial sector of the United States. American refineries spend 50 to 60 percent of their operating costs (i.e., excluding capital costs and depreciation) on energy, in Venezuela, Russia and the Middle East that number is even higher, about 70%. Compared to refineries, the much more advanced chemical industry spends only 30 to 40%. This difference shows the savings that can be obtained just by implementing the state-of-the-art controls and optimization described in this series.
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Globally, more than 80 million barrels of crude oil are refined daily. American refineries spend 50 percent to 60 percent of their operating costs on energy, while the chemical industry spends only 30 percent to 40 percent. If the optimization strategies described in this series are implemented, refineries can cut their energy costs by a third, while improving product quality. Ethanol and other bio-fuels are also products of distillation, and the efficiency of these processes can similarly benefit from the described strategies. In the United States six billion gallons of ethanol were produced in 2006 and 35 billion is projected by 2017.
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