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Reducing Energy Cost through Improved Disturbance Rejection
Two of the most popular architectures for improving regulatory performance and increasing profitability are 1) cascade control and 2) feed forward with feedback trim. Both architectures trade off additional complexity in the form of instrumentation and engineering time for a controller better able to reject the impact of disturbances on the measured process variable. These architectures neither benefit nor detract from set point tracking performance. This paper compares and contrasts the two architectures and links the benefits of improved disturbance rejection with reducing energy costs in addition to improved product quality and reduced equipment wear. A comparative example is presented using data from a jacketed reactor process.
The cost per barrel of crude oil has risen dramatically, increasing the burden on process facilities for both quality and profitable production. Adjusted for inflation, the cost of oil averaged $19.61 from 1945 thru 2003. October 2004 saw the per barrel cost of oil rise to $55.67, rising 70% over a 10-month timeframe and negatively impacting the profitability of companies across the process industries. According to the U.S. Department of Energy, 43% of all energy consumed by the average pulp and paper mill is production related. This percentage is small when compared to other industry segments such as chemicals (74%), glass (89%), and aluminum (93%). In all cases, the higher cost of energy suggests that all process companies need to examine ways of curbing energy consumption and unnecessary increases to their cost of goods sold. Improving disturbance rejection through cascade control or feed forward with feedback trim provides one way of achieving those objectives.
Improved disturbance rejection is linked to increased product quality and decreased equipment wear. These are important benefits, indeed. Consider the market value of high quality white paper produced by an average mill. On-spec production is sold at a premium of approximately $2,000 per ton whereas "seconds" are sold on the aftermarket at a discounted rate. Of the 6%-8% that fails to meet spec, only 2% is classified as "broke" and able to be re-pulped Next consider the investment in production facilities. With initial costs of $400-$500 million and annual maintenance budgets approaching 10%, mills must operate 24 x 7 in order to recoup the investment. Effective disturbance rejection provides a valuable means of achieving a return on those investments through increased quality and decreased equipment wear. Additionally, it offers significant value in terms of reduced energy consumption and lower cost of goods sold.
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