Bidirectional information flow

Understand degrees of freedom in modeling process imits and flowsheets, and be able to make design specifications and follow the iterations implemented to satisfy them. When using HYSYS-Plant, the reader will learn that its implementation of bidirectional information flows is very efficient in satisfying many specifications. 

In HYSYS-Plant, the models are programmed to reverse the information flow when appropriate, that is, to accept values for the variables of the product streams and to compute the variables of the feed streams. HYSYS.Plant implements the so-called bidirectional information flow, as described next. 

In nearly all of the flowsheet simulators, the material and energy balances for the process units are solved given specifications for the inlet streams and the equipment parameters, along with selected variables of the outlet streams (e.g., temperatures and pressures). The unknown variables to be computed are usually those of the outlet streams (typically, the flow rates and compositions). The HYSYS.Plant simulator is a notable exception in that most combinations of specifications are permitted for each simulation model. With this flexibility, HYSYS.Plant can implement a reverse information flow, in which specifications are provided for the product streams and the unknown variables of the inlet streams are computed. More commonly, HYSYS.Plant implements a bidirectional information flow, involving the calculation of the unknown variables associated with the inlet and outlet streams. Whenever a stream variable is altered, the adjacent process units are recomputed. This causes the information to flow in parallel to the material streams, when a unit downstream is recomputed, or opposite to the material streams when a unit upstream is recomputed. 

For the cooler in Figure 4.5, it is desired to specify the vapor fraction of the effluent stream, <()2, the heat duty, Q, and the pressure drop. Can this be accomplished with bidirectional information flow ... 

In the HYSYS.Plant simulator, bidirectional information flow is utilized to compute the vapor fraction of the feed stream, other process simulators, where, instead, an iteration loop is created in which a guess is provided for d>i and iterations are carried out until the specified value of 4>2 is obtained, as discussed in the next subsection. Note that for the heater or cooler model in most simulators, the vapor fraction can be specified for both streams and the heat duty computed. ... 

For assistance in the use of the Adjust and Set objects, the reader is referred to the module HYSYS -> Principles of Flowsheet Simulation Getting Started in HYSYS -> Convergence of Simulation on the multimedia CD-ROM that accompanies this text. As was discussed in the subsection on bidirectional information flow, for all of its subroutines, HYSYS.Plant provides a bidirectional information flow, that is, when product stream variables are specified, the subroutines calculate most of the unknown inlet-stream variables. In CHEMCAD, a control unit, with one inlet stream and one outlet stream (which may be identical to the inlet stream), is placed into the simulation flowsheet using the CONT subroutine. The parameters of the control unit are specified so as to achieve the desired value of a stream variable (or an expression involving stream variables) or an equipment parameter (or an expression involving equipment parameters) by manipulating an equipment parameter or a stream variable. This is the feed-backward mode, which requires that the control unit be placed downstream of the units being simulated. The CONT subroutine also has afeed-forward mode. 

As mentioned above, the Wodd Wide Web is a hypertext-based database system of multimedia content. It can be used for building either local or global information systems. It is interactive, which means that the information flow can be bidirectional (from the database to the user and from the user to the database). It integrates all the other traditional information system technologies, such as ftp, gopher (a predecessor of the WWW in organizing and displaying files on dedicated servers), and news. 

The next process step considers the environment in which the product will subsequently be used in the field. All relevant influences on the product to be developed are modeled in the environment model. The influences can be modeled with three different flow relationships material flow (e.g., spray), energy flow (e.g., heat), and information flow (e.g., control signal). Flows can be unidirectional or bidirectional. A distinction can be drawn between wanted and unwanted flows. The latter are termed parasitic flows. Some of these parasitic flows are defined in VDI4005. They include vibration, temperature, and relative humidity. Along with the influences on the product as such, the geometrical boundary conditions and interfaces to other module parts are important constituents of the environment model, particularly so in the case of product development in MID technology. Initial requirements can be identified on the basis of the environment model. 

It is now well ascertained that dendrites are capable of propagating action potentials not only in distal to proximal direction, but also in the reverse direction by back-propagation after initiation at the cell body (Ludwig and Pittman, 2003). The so-called law of dynamic polarization enunciated by Cajal (see Berlucchi, 1999) was aimed at stating the unidirectional propagation of excitations within the nervous system, and assumed that nerve impulses are conducted from the dendrite or soma to axon terminals. This dogma is now being reconsidered, not only in view of the evidence of dendrodendritic synapses, but also in view of the existence of electrical synapses in which the flow of information can be bidirectional. 

Furthermore, REACH includes new or increased demands on the bidirectional flow of data and information in the supply chain (Tide IV) and places demands on downstream users (Tide V). For instance, REACH includes provisions on safety data sheets for substances on the candidate list (Article 31 and Annex II) and forces suppliers of articles to actively provide information for the safe use of the articles (Article 33). In addition, REACH entities consumers to, without charge, request information within 45 days on the safe use of articles containing SVHCs in concentrations above 0.1 wt% (Article 33). It remains to be seen to what extent the increased flows of information will impact on the management of chemicals. However, companies that work with environmental management systems and companies located closer to consumers in commodity chains will probably seek more actively to decrease chemical-related risks. 

SMC, though, as mentioned above, these two characteristics are not necessarily linked. Since endothelial cells and SMC are quiescent in the adult, it might be that a potential continuous, bidirectional flow of information, at a low level between these cells, is partecipating in the reciprocal regulation of growth and differentiation (see Fig. 14). 

Sequential modular approach has some clear advantages for process flowsheeting that explain why it still dominates the technology of steady-state simulation over the simultaneous or equation-oriented approach. Table 8.2 shows a list of pros and cons about sequential modular process simulators. In order to cope with the disadvantages, a few process simulators have improved the flow of information and avoid redundant computations. As an example, Aspen HYSYS has implemented the bidirectional transmission of information technology. 

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