Temperature and Composition Controllers

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. 

Composition measurements often involve the use of a chromatographic column, and these devices exhibit significant deadtimes because of both sample-line and column-cycle 

Relay-Feedback Testing. A relay-feedback test is an easy and accurate method for experimentally determining important dynamic parameters of a process. The test can be used on a real process or on a simulation. It is widely used in both industrial and academic applications. Relay-feedback tests can be easily performed in Aspen Dynamics simulations. 

This dynamic test gives very accurate estimates of the ultimate gain and ultimate period of the loop, which can then be used for controller tuning. The method inserts an on-off relay in the feedback loop that positions the controller output signal at a specified percent higher or lower than the initial steady-state value. For example, if the OP signal to the valve is 50% at steady state and the specified displacement is 5%, the controller output will fluemate back and forth between 45 and 55%. 

The height of this displacement is called h. The loop will settle into a periodic oscillation because the relay fires up or down every time the PV signal crosses the SP signal (see Fig. 4.39). The period of the oscillation is the ultimate period Pu- The amplitude of the PV signal is called a and is read off the strip chart plot. Knowing h and a, the ultimate gain can be calculated from Eq. (4.17). 

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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. 

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