Dynamic flowsheeting

Figure 6.37 Aspen dynamics flowsheet with default controllers.

Material and energy balances remain the most important results in flowsheeting. The stream report displays the way in which the raw materials are transformed in products for given performances of units. In addition, dynamic flowsheeting can mirror the time-variation of the component inventory in different locations, as well as the dynamics of energy streams. 

In Computer Aided Operation we can mention the real time monitoring of material and energy balance, managed nowadays by means of data reconciliation programs. The plant operation can be adapted and optimised in real time by means of computerised tools based on dynamic flowsheeting. Other advanced applications are simulators for safety studies and operator training. 

Aspen Dynamics dynamic flowsheeting interfaced with Aspen Plus. 

Flowsheeting is still dominated by the Sequential-Modular architecture, but incorporates increasingly features of the Equation-Oriented solution mode. A limited number of systems can offer both steady state and dynamic flowsheeting simulators. 

Dynamic flowsheeting is based on the unsteady state balance of mass and energy, and may be formulated in general terms by the following equation ... 

Dynamic flowsheeting consists of performing the dynamic simulation of the whole process plant rather than of a single unit. Commercial software for dynamic flowsheeting may be classified in two categories ... 

Advantages and disadvantages of each approach have been discussed in Chapter 2. For steady state flowsheeting most of the simulators have adopted the SM architecture. The situation is more complex in dynamic flowsheeting, mainly because of the way in which the time-derivatives are handled. Thus, the time may be seen locally, at unit level, or globally, at flowsheet level. In the case of flowsheets without recycles, the time is seen at the same horizon from every unit. The information is transmitted sequentially in a synchronised manner. In the case of flowsheet with recycles the count of time is less obvious. The integration should take place simultaneously in all units, with a time-... 

Chemical conversion is an effective way to counteract the accumulation of impurities due to positive feedback. Also, changing the connectivity of units may be used to modify the effect of interactions, for example by preventing an excessive increase in recycles due to snowball effects. Effective plantwide control structures may imply controlled and manipulated variables belonging to different but dynamically neighbouring units. The methodology to evaluate the dynamic inventory of impurities consists of a combination of steady state and dynamic flowsheeting with controllability analysis. This is used to assess the best flowsheet alternative and propose subsequent design modifications of units. Case Study 3 in Chapter 17 will present this problem in more detail. 

Dynamic flowsheeting. The detail of modelling depends on the dynamics of units involved in the plantwide control problem. However, the availability of suitable dynamic models for the wide variety of unit operations Involved in practice is questionable. The simplification of the steady-state Plant Simulation Model to a tractable dynamic model, but still able to represent the relevant dynamics of the actual problem, is a practical alternative. In this case detailed models are necessary for the key units, where impurities are generated and eliminated, as kinetic models for reactors and dynamic models for some distillation columns. For other units, steady-state models are sufficient. 

Figure 16.4 shows the Aspen Dynamics flowsheet with controllers installed. A steam-to-feed ratio is used with the ratio changed by the temperature controller. The need for this ratio to improve load performance is illustrated in Figure 16.5. A 50% increase in feed flow rate is the disturbance. The solid lines show responses without the QR/F ratio. There are very large drops in Stage 55 temperature that result in large transient increases in the C4 impurity in the bottoms (xB). The units of the multiplier must be metric in the Aspen Dynamics simulation (GJ/kmol).

When this script is compiled in Aspen Dynamics a message appears stating that the flowsheet is overspecified. The input signal to the dead time block must be changed from fixed to free" for the dynamic simulation to run. Figure 16.8 shows the modified Aspen Dynamics flowsheet. 

Figure 18.6 shows the Aspen Dynamics flowsheet with a standard PI flow controller (no external reset feedback) and a high-level override controller. Liquid level is normally controlled by the liquid valve. The normal level is at 2 ft, so the setpoint of the normal level controller is 2 ft. The level transmitter span is 4 ft. A simple P-only high-level override controller is set up to come into action when the level gets too high. The setpoint of the... 

The external reset feedback control structure discussed in Figure 18.3 is inserted in the Aspen Dynamics flowsheet for the flash tank process as shown in Figure 18.8. The feed flow controller has its output signal OPpc sent to a low selector. The other input to the low selector is the output signal OPqrc of the high-level override controller. The override controller is proportional-only, so it does not need anti-reset windup protection. 

However, for the practical solution of realistic, large-scale dynamic optimization problems from the field of chemical engineering, the solution methods must be tailored to the domain-specific, inherent problem structures. Also, they must be interfaced with the models generated by dynamic flowsheeting packages. In this paper, we discuss some of what is currently available, and we identify major areas for future research. 

For the example of the circulation loop reactor it was demonstrated, how continuation methods can be used for efficient and comprehensive parameter studies. An algorithm for the continuation of steady state and periodic solutions was presented. While the steady state continuation is implemented for large sparse DAEs, the algorithm for periodic continuation exists only as a test program for the continuation of ODEs. Further studies will be concerned with the extension to the periodic continuation of DAEs and the implementation into the dynamic flowsheet simulator Diva. 

Aylott, M.R., Ponton, J.W., Lott, D.H. (1985) Development of a dynamic flowsheeting program. Proceedings of Process Systems Engineering, IChemE Symposium Series 92,... 

Figure 11.12 Aspen Dynamics flowsheet equations for heat-integrated extractive process and pressure-compensated temperature.

Vogel, E. F. An industrial perspective on dynamic flowsheet simulation. In Proceedings, CPC IV, Padre Island, Texas, 17-22 February. CAChE, AIChE, New York, 1991, pp. 181-208. 

Dosta M, Heinrich S, Werther J Fluidized bed spray granulation analysis of the system behaviour by means of dynamic flowsheet simulation, Powder Technol 204 71—82, 2010. 

Dosta M Dynamic flowsheet simulation of solids processes and its application to fluidized bed spray granulation, Gottingen, 2013, CuviUier Verlag, p. 142. 

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