Chemical Processing - March 2008 - (Page 38)

Steam valve position, % >> The process operating line 300 Linear valve >> Effect of throughput 300 Equal-percentage valve Liquid ﬂow 1,000 lb/min 2,000 lb/min 220 180 140 100 4,000 lb/min Liquid outlet temperature, °F 220 200 180 140 100 0 6 14 20 29 51 80 100 Liquid outlet temperature, °F 276 260 Equal-percentage valve 260 40 60 Steam valve position, % 0 20 29 40 60 Steam valve position, % 37 42 51 59 64 80 100 Figure 6. The limits on operation provide crucial insights. Figure 7. Usable valve position range remains at about 20% regardless of flow. above 50%. Even if the controller gain is increased dramatically, the liquid outlet temperature won’t be controlled very effectively in this region. Regions where the operating line is vertical. The oper300 ating lines in Figure 6 valve exhibit this characteristic. Liquid ﬂow Equal-percentage don’t 1,000 lb/min Effect of process operating variables. Throughput has 260 the most significant influence on the operating lines. Other variables such as steam supply pressure and liquid inlet 2,000 lb/min 220 temperature for the exchanger also influence the operating lines. Usually the effect of such variables is less than the ef4,000 lb/min 180 fect of throughput — but this is a generalization for which exceptions definitely exist. So, for any process in which 140 you expect a significant change in some operating variable, evaluate its impact on the process operating lines. 100 20 60 80 100 Effect 0 throughput 40 of Steam valve position, % Figure 7 presents the process operating lines (equal-percentage valve only) for liquid flows of 1,000, 2,000 and 4,000 lb/ min for our exchanger. The heat transfer generally increases as the flow increases but not in a linear fashion. The shape of the operating lines is basically the same for all, the major difference being that the increase in temperature from liquid inlet to liquid outlet decreases as the liquid flow increases. Figure 7 also illustrates the effect of throughput on the valve positions corresponding to the minimum and maximum heat transfer rates. The effect of increasing throughput is to increase these valve positions; however, the difference between the valve positions for maximum and minimum heat transfer remains at about 20%. Over this range, all of the operating lines exhibit only a modest departure from linearity. Most utility processes are expected to handle significant changes in throughput. For example, most industrial boilers are designed for a turndown ratio of at least 4:1. Batch processes also can exhibit extreme variations in throughput. Consider a batch reactor with a jacket for re38 • March 2008 29 37 42 51 59 64 moving heat. The contents of the reactor, that is, the reacting media, determine the dynamics of a production-scale reactor. The dynamics associated with the jacket are far shorter, which means that the jacket is essentially at an equilibrium state that reflects the conditions within the reactor. As the conditions within the reactor change, the jacket basically tracks those conditions. For batch reactors, the turndown ratio pertains to the heat transfer rate between the reactor and the jacket. For many batch applications, this heat transfer rate varies substantially during the batch, typically being the highest in the early stages and dropping off considerably in the later stages. Turndown ratios of 50:1 are experienced in practice. For operating lines such as those in Figure 7, the temperature controller usually must be tuned to give acceptable performance for the throughput for which the slope of the operating line is the steepest. This is where the process will have its highest sensitivity, which in turn requires the smallest value for the controller gain. The steepest slope decreases in magnitude as the throughput increases. Consequently, the higher the throughput, the lower the process sensitivity and the higher the controller gain required to achieve consistent loop performance. This is another potential application for scheduled tuning. A measurement is required for the liquid flow through the exchanger. For low liquid flows, the process gain is high, so a low controller gain is appropriate. As the liquid flow increases, the controller gain should be raised (approximately proportional to the increase in flow). These applications of scheduled tuning usually are successful. But when other operating variables are added to the mix, the logic becomes more complex and success less assured. CP Cecil L. Smith is president of Cecil L. Smith, Inc., Baton Rouge, La. E-mail him at cecilsmith@cox.net. www.chemicalprocessing.com Liquid outlet temperature, °F http://www.chemicalprocessing.com

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Chemical Processing - March 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Distillation is Bubbling Feel Secure About Vulnerability Assessments The Door Opens For Membranes Achieve Effective Heat Exchanger Control Epoxy Maker Finds the Right Glue for Its Business Process Puzzler Plant InSites Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - March 2008

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