Temperature and Composition Control

The function of composition control extends beyond assuring adequate product purity. The composition controller manipulates a stream such as reboil, reflux, or product. Unstable composition control will distvirb this stream, and the disturbances vdll unsettle the column. Two primary methods are used for composition control temperature control or analyzer control. Temperature control is cheaper, faster, and far more popular. Analyzers provide more direct and more accurate control of product composition and are favored where these qualities translate into profits. Both temperature and analyzer controls have serious limitations. Failure to recognize these limitations and plan for them will render composition control poor or unstable. 

This chapter reviews the application of temperature and analyzer control in distillation columns, highlights the problem areas for each technique, examines the pitfalls of composition controls and the consequences of overlooking them, and provides guidelines for good composition control practices. 

Column temperature control is perhaps the most popular means of controlling product composition. The control temperature is used as a substitute to product composition analysis. A change in control temperature represents a corresponding variation in the concentration of key components in the product. For instance, a rise in top section control temperature represents a rise in the concentration of the heavy key component in the top product. 

Temperature control is an easy and inexpensive means of controlling product composition. It uses a high-reliability, low-maintenance 

Effect of concentration of nonkey components on the control temperature 

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At the microscale level, detailed local temperature and composition control through the staged feed and supply of reactants or the removal of products would result in a higher selectivity and productivity than would the conventional approach. Indeed, this conventional approach imposes boundary conditions and lets a system operate under spontaneous reaction and transfer. To produce a local energy supply, microwave and ultrasound can be used instead of heat. To operate the relevant models on these energies, local sensors and actuators as well as close computer control will absolutely be needed. 

Since the syngas passes over multiple stacks arranged in series, it is only partially converted over each stack and temperature and composition control can be more easily achieved. This is similar to the design intent of the Linde reactor... 

Installing temperature and composition controllers is somewhat more involved than installing level and flow controllers because of three issues. First, we need to include additional dynamic elements in the loop. Temperature and composition measurements... 

Once the file has been exported into Aspen Dynamics, controllers are installed to achieve the desired control structure and dynamic simulations are mn to check the stability and performance of the control system. Various types of disturbances should be imposed on the system, such as throughput changes, feed composition changes, and changes in the set-points of the product-quality controllers (temperature and composition controllers). In this section we demonstrate, in a detailed step-by-step fashion, how these operations are performed in Aspen Dynamics. 

Deadtime and Lags. Most temperature and composition controllers need to be tuned because the dynamics lags in these loops carmot be neglected. Deadtimes and lags degrade dynamic performance, so not including realistic dynamic lags in the simulation of these loops can lead to a prediction of dynamic performance that is unrealistically better than what will actually be seen in the plant. 

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