Flowsheet architectures

The main objective of design is the flowsheet architecture. We mean by this type of units, performance and connections by material and energy streams. Systemic techniques are capable of calculating optimal targets for subsystems and components without the need of the detailed sizing of equipment. 

Integrated flowsheet architecture for a cost-effective process is the main objective. Appropriate systemic techniques are capable of determining close-to-optimum targets for components without the need for detailed design and sizing. 

It is clear that the reaction and separation systems are interrelated and, in principle, their design should be examined simultaneously. In practice, this approach turns to be very difficult It is possible to apply, however, a simplified approach. Indeed, from the systemic viewpoint the functions and the connections of units have priority on their detailed design. Therefore, a functional analysis based on the RSR structure only is valuable. This analysis should primarily show that the flowsheet architecture is feasible and appropriate for stable operation. On this basis design targets for the subsystems may be assigned. 

Modular-based flowsheeting exhibits several advantages in design. The flowsheet architecture is easily understood because it closely follows the process flowsheet. Individual modules can easily be added and removed from the computer package. Furthermore, new modules may be added to or removed from the flowsheet without any difficulty or affecting other modules. Modules at two different levels of accuracy can be substituted for one another as mentioned above. Modular-based flowsheeting also has certain drawbacks. 

The two basic flowsheet software architectures are sequential modular and equation-based. In sequential modular, we write each unit model so that it calculates output(s), given feed(s), and unit parameters. This is the most commonly used flowsheeting architecture at present, and examples include Aspen+ plus Hysys (AspenTech), ChemCAD, and PROll (SimSci). In equation-based (or open-system) architectures, all equations are written describing material and energy balances as algebraic equations in the form/(x) = 0. This is the preferred architecture for new simulators and optimization, and examples include Speedup (AspenTech) and gPROMS (PSE pic). Each is discussed in turn. 

The flowsheet architecture is most easily understood as it closely follows the process flow diagraiiL... 

Sparse Matrix Methods. In order to get around the limitations of the sequential modular architecture for use in design and optimization, alternate approaches to solving flowsheeting problems have been investigated. Attempts to solve all or many of the nonlinear equations simultaneously has led to considerable interest in sparse matrix methods generally as a result of using the Newton-Raphson method or Broyden s method (22, 23, 24 ). ... 

Many of the recently suggested architectures for flowsheeting systems have been reviewed by Motard ( 58). Table 1 is a list of selected system architectures and some representative systems. 

Sequential Modular. By far the most experience with flowsheeting systems has been with the sequential modular architecture (59- 3). It is this architecture that is most easily understood by the process engineer. Each module calculates all output streams from input streams subject to module parameters. Generally, the stream variables consist of component flows, temperature (or enthalpy) and pressure as the independent variables. Other dependent variables such as total flow, fraction vapor and total enthalpy (or temperature) are often carried in the stream. 

The computational architecture is a sequential modular approach with advanced features. To model a process, each equipment module is simulated by a program module. The overall process is simulated by connecting the models together in the same way as the equipment in the flow sheet. When the input streams are known then the outputs can be calculated. The entire flowsheet can be calculated "sequentially" in this manner. Advanced features are discussed below in connection with an example. 

Figure 3.6 depicts the coarse-grained architecture of the flowsheet editor... 

In order to support the central role of flowsheets during engineering design, we have implemented a prototypical flowsheet-centered architecture (Fig. 3.7). The central element of this architecture is the VISIO based flowsheet editor that, based on the PRIME process integration mechanism, is operationally linked to other domain-specific tools and acts as the prominent communication medium among developers. 

Bayer, B., Marquardt, W., Weidenhaupt, K., Jarke, M. A flowsheet centered architecture for conceptual design. In Gani, R., Jprgensen, S.B. (eds.) Proceedings of the European Symposium on Computer Aided Process Engineering - ESCAPE 11, pp. 345-350. Elsevier, Amsterdam (2001)... 

The most important part of process optimization is linking the process flowsheeting tool to the optimization algorithm. With an equation-based architecture, the unit equations (material and energy balances, operating constraints, and specifications) are constraints in a general nonlinear programming formulation. The main problems are... 

The main objective is the architecture of the process (flowsheet structure). Systemic design techniques are today available that are capable to determine optimal targets for sub-systems and components without detailed modelling of equipment. In this way, the detailed equipment sizing becomes a downstream activity. 

In the period 1970-80 s the major engineering bureaus, as well as some large manufacturing companies in refining and petrochemical industries developed in-house flowsheeting programs. Mostly adopted the modular sequential architecture. However, some were based on the equations oriented approach, as SPEEDUP at Imperial College in London (UK) and TISFLO at DSM in The Netherlands. 

The architecture of a flowsheeting software is determined by the strategy of computation. Three basic approaches have been developed over the years ... 

In Simultaneous-Modular approach the solution strategy is a combination of Sequential-Modular and Equation-Oriented approaches. Rigorous models are used at units level, which are solved sequentially, while linear models are used at flowsheet level, solved globally. The linear models are updated based on results obtained with rigorous models. This architecture has been experimented in some academic products. 

Sequential-Modular approach is mostly used in steady state flowsheeting, among we may cite as major products Aspen Plus, ChemCad, Hysys, ProII, Prosim, and Winsim (see Table 2.2 for information). However, there are some dynamic simulators built on this architecture, the most popular being Hysys. 

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. 

The algorithmic treatment depends on the architecture of the flowsheeting system. In Equation-Oriented mode, the approach consists of solving all the equations describing the problem simultaneously. In Sequential-Modular approach the mathematical solution must take into account the convergence of units and tear streams, as well as of all design specifications. Supplementary equations must be added, so that the general formulation of the optimisation problem (3.10) becomes ... 

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-... 

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