Flowsheet simulation equation-based

Clearly define, in your own words, the terms design variables and state variables, sequential modular flowsheet simulation, equation-based flowsheet simulation, tear stream, convergence block, and design specification. 

Basically, DESIGNER can use different physical property packages that are easy to interchange with commercial flowsheet simulators. For the case considered, the vapor-liquid equilibrium description is based on the UNIQUAC model. The liquid-phase binary diffusivities are determined using the method of Tyn and Calus (see Ref. 72) for the diluted mixtures, corrected by the Vignes equation (57), to account for finite concentrations. The vapor-phase diffusion coefficients are assumed constant. The reaction kinetics parameters taken from Ref. 202 are implemented directly in the DESIGNER code. 

Presently, several equation-based flowsheeting systems exist perhaps the best known of these is SPEED-UP, which continues to be developed at Imperial College by Sargent and Perkins and their students. Now commercially available, its strongest attraction in industry seems to be as a dynamic simulator. TISFLO at Dutch State Mines is also often mentioned in the literature. As a complete flowsheeting system, no equation-based approach is yet very popular. One might wonder why. 

Kuru, S., Dynamic Simulation with an Equation Based Flowsheeting System. Ph.D. thesis, Carnegie Mellon University, Pittsburgh, 1981. 

Ihese difficulties vanish if the system equations are simply collected and solved for all unknown variables. Several powerful equation-solving algorithms are available in commercial programs like Maple , Mathematica , Matlab , Mathcad , and E-Z Solve that make the equation-based approach competitive with the sequential modular approach. Many researchers in the field believe that as this trend continues, the former approach will replace the latter one as the standard method for flowsheet simulation. (Engineers are also working on simultaneous modular methods, which combine features of both sequential modular and equation-based approaches. We will not deal with these refinements here, however.)... 

Equation-based flowsheet simulators have not reached the level of commercialization of their sequential modular counterparts as of the time of this writing, and the development of efficient equation-solving algorithms is the subject of much continuing research. 

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. 

Achieving dynamic simulations that rigorously capmre the neat heat integration require the use of Flowsheet Equations in Aspen Dynamics. Two conditions must exist at each point in time during the dynamic simulation. First, the heat transfer in the condenser/reboiler must be equal to the product of the area, the overall heat-transfer coefficient, and the current temperature difference between the reflux drum of the high-pressure column and the base of the low-pressure column. These two temperatures both change dynamically as compositions and pressures vary. The pressure in the high-pressure column is not controlled but floats. 

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