HYSYS.Plant 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. 

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. 

---