Flowsheeting definition

The preceding definitions of economic potential and total annual cost can be simplified if it is accepted that they will be used to compare the relative merits of difierent structural options in the flowsheet and difierent settings of the operating parameters. Thus items which will be common to the options being compared can be neglected. 

Project Control (factored and take off) 5% Mechanical Flowsheets, Preliminary Plot Plans, Location, Off Site Definition... 

Johns and Romero use a very different strategy to create a flowsheet. As stated earlier their approach is based on finding the best transformations among the small number of streams which can exist, regardless of the process structure. Obviously they need a crude definition for a stream to keep the number of streams small. For separation sequences, where each separator sharply splits the feed into two products, each of which have no species in common, the crude stream approximation is in fact accurate and was the first problem solved using this approach (Johns (1977)). 

As the process flowsheet is the definitive document on the process, the presentation must he clear, comprehensive, accurate, and complete. The various types of flowsheets are discussed in the following sections. 

On the model level, we distinguish between a type (or class) level and an instance level, like standard UML does. On the type level, document models for specific types of documents are defined. They are expressed as class hierarchies describing the documents underlying type systems. In our example, documents containing simulation models for Aspen Plus and flowsheets for Comos PT are defined. To be able to perform an integration of these documents, link types that relate classes contained in the documents class hierarchies are defined. All occurrences of links in further definitions on lower levels are instances of these link tjqjes and are thereby constrained by these types. 

However, a management configuration should not evolve in arbitrary ways. There are domain-specific constraints which have to be met. In particular, activities can be classified into types such as requirements definition, design, simulation, etc. (likewise for products and resources). Furthermore, the way how activities are connected is constrained as well. For example, a flowsheet can be designed only after the requirements have been defined. Such domain-specific constraints should be taken into account such that they restrict the freedom of evolution. 

Since the Estimation task is not defined in the class diagram, the manager makes use of the unconstrained type Task to insert it into the task net as an untyped task. This modification degrades the consistency level of the task net to weakly consistent. Unfortunately, inconsistencies are introduced, as well The flowsheet design task has to be supplied with the estimation as input parameter. This was excluded in the process definition by the value false of the tag AllowStandardParameters. Therefore, the manager has to switch off consistency enforcement explicitly to make the modification feasible. 

The task net after migration is shown in Fig. 3.80. The control flow from the estimation task to the flowsheet design task is marked as behaviorally inconsistent (emphasized by red color). Both tasks are currently active, while the revised process definition prescribes a sequential control flow. This illustrates that migration does not necessarily result in a task net which does not contain inconsistencies. Migration can always be performed - even if inconsistencies persist. 

Additionally, a new construct for modeling alternative subgraphs [180] is realized. This is a powerful construct to specify multiple alternative correspondence structures in one rule. For example, there are different options to map a process stream to a pipe which contains different types of valves in different order. All these alternatives can be now specified within one rule. Concepts for the parametrization of integration rules will be defined. Parameters can be set at runtime as well as at rule definition time. Figure 7.19 shows an example of a rule with a runtime parameter in combination with a set-valued pattern A column in the simulation model is mapped to a column in the flowsheet, which has a number of column trays each having a port. The parameter n controls the number of trays, that are added to the column it is provided by the user at runtime. Up to now, the parametrization works with attributes occurring in a rule whose values can be retrieved at runtime without user interaction. 

Feedback controller may be placed inside or outside of a flowsheet convergence loop. There are cases when the manual definition of the computation sequence is useful. As typical applications we cite ... 

Optimisation of the flowsheet is the ultimate goal. This task is difficult, and should be preceded by careful analysis regarding the definition of the objective function and its sensitivity to different variables. 

Summing up, a conceptual design problem is always open-ended. The result is never unique. The optimality of a flowsheet depends not only on the definition of the objective function and constraints, but also on numerous design decisions that the designer must consider. Moreover, only static evaluation of the design performances (steady-state behaviour) is not sufficient. Dynamic characteristics and flexibility in manufacturing should be also considered in assessing the optimality. 

Problem definition. Firstly the key components are identified. These are products, sub-products and intermediates, as well as impurities with significant effect on product quality and operation. Then the impurities are traced by means of tables containing sources, sinks, exit streams, transit units and process streams. Formation and depletion of impurities must be supported by a consistent stoichiometry. Then an operating window is defined in terms of production rate, operation parameters and technological constraints. In principle, this step may identify a number of flowsheet alternatives, but supplementary alternatives may arise during the application of the procedure. 

An important forthcoming step ofvaHdating the input data, i.e., the problem definition in PNS Studio, is to check whether the major overall flowsheet alternatives assumed by the designer are included in the optimization. Algorithm SSG serves this target by enumerating the structurally feasible process flowsheets. First, we have to ensure that the resulting number of structures is not Hmited to a small number is Options/Default values in software PNS Studio (Fig. 9.9). 

Selected stream captures from the Aspen Plus example for CO2 removal from flue gas using the AMINES package, (a) The FORTRAN code and variable definitions for specifying the makeup water flow rate (b) The FORTRAN code and variable definitions for specifying the makeup MEA flow rate (c) The final process flowsheet (d) Selected control panel output, indicating where Aspen Plus has selected the tear stream. 

Problem definition. The key impurities are traced by means of tables containing sources, sinks, exit streams, and transit units. Their formation and depletion must be supported by consistent stoichiometry. This step may lead to flowsheet alternatives by changing the... 

The word process in the second definition refers to that part of the flowsheet that... 

---