Chemical simulation, steady-state

The persistent organochlorine compounds, once released, will partition between environmental media according to their physical and chemical properties. Steady state equilibrium partitioning between these media has been considered as the simplest model simulating their behaviour. Over recent years, modelling the environmental partitioning and fate of these compounds has led to a broad... 

FIG. 9 Simulated steady-state tip current vs. distance curves showing the effect of y on SG/TC responses, y = 1 (o), 0.5 (A), and 2 ( ). The solid and dotted lines are calculated from Eq. (49). For y = I the SG/TS approach curve is indistinguishable from the corresponding feedback mode approach curve. (Reprinted with permission from Ref. 35. Copyright 1998 American Chemical Society.)... 

The method of Dantzig and Johnson et al. marked a breakthrough, both in the time required to solve chemical composition problems and in the complexity of problems that could be handled. Their method constituted the first version of what has become RAND s chemical composition program. Subsequently, refinements have been made in numerical techniques and human engineering, resulting in the present program. RAND mathematicians, notably H. E. Kanter, R. J. Clasen, N. Z. Shapiro, M. Shapley, and L. Cutler, have developed programs to simulate steady—state systems and certain types of transients. (See References for a full list of contributors... 

Fig. 16 Coupled chemo-electro-mechanical simulation Steady-state results (a) of the mobile concentrations and c and (b) of the electric potential after chemical stimulation...

Additionally, there is process simulation steady-state flow sheet simulators and dynamic flow sheet simulators. Steady-state flow sheet simulators have been widely used in chemical process engineering since the 1960s. Steady-state simulators describe the process as a set of modules connected by flows of material and energy between them. The modules correspond to mass and energy balances together with physical and thermodynamic data necessary for calculations. The calculations may be performed using one of two basic techniques. The sequential approach computes modules one by one, in a direction that generally follows that of the physical flows in the system (Leiviska, 1996). 

PLOW 1 RAN was made available in 1974 by Monsanto Co. for steady-state simulation of chemical processes based on sequential modular technology. It requires specification of feed streams and topology of the system. In 1987, an optimization enhancement was added. 

Classification Process simulation refers to the activity in which mathematical models of chemical processes and refineries are modeled with equations, usually on the computer. The usual distinction must be made between steady-state models and transient models, following the ideas presented in the introduction to this sec tion. In a chemical process, of course, the process is nearly always in a transient mode, at some level of precision, but when the time-dependent fluctuations are below some value, a steady-state model can be formulated. This subsection presents briefly the ideas behind steady-state process simulation (also called flowsheeting), which are embodied in commercial codes. The transient simulations are important for designing startup of plants and are especially useful for the operating of chemical plants. 

The overall system that we will analyze comprises the unbleached Kraft pulp line, chemicals and energy recovery zones of a specific paper mill (Melville and Williams, 1977). We will employ a somewhat simplified but still realistic representation of the plant, originally developed in a series of research projects at Purdue University (Adler and Goodson, 1972 Foster et al., 1973 Melville and Williams, 1977). The records of simulated operation data, used to support the application of our learning architecture, were generated by a reimplementation, with only minor changes, of steady-state models (for each individual module and the system as a... 

Distillation is a well-known process and scale-up methods have been well established. Many computer programs for the simulation of continuous distillation columns that are operated at steady state are available. In fine chemicals manufacture, this concerns separations of products in the production of bulk fine chemicals and for solvent recovery/purification. In the past decade, software for modelling of distillation columns operated at non-steady state, including batch distillation, has been developed. In the fine chemicals business, usually batch distillation is applied. 

The simultaneous integration of the two continuity equations, combined with the chemical kinetic relationships, thus gives the steady-state values of both, Ca and T, as functions of reactor length. The simulation examples BENZHYD, ANHYD and NITRO illustrate the above method of solution. 

A plant simulation is the set of equations necessary to approximate the response of a chemical plant to various changes. A steady- state plant simulation is one that predicts the eventual outputs when the inputs and all the internal variables are held constant. It does not say how the outputs are reached. A dynamic plant simulation is one that predicts how the outputs of a plant will change when a known change in the input occurs. It gives the path the process follows in going from one steady state to another. 

Crowe et al. have written a book entitled Chemical Plant Simulation 2 that gives the details of the steady-state simulation of a contact sulfuric acid plant. It uses an executive program named PACER. This and many other such programs as COPS, Flowsim, GPFS, and PDA are for sale.3... 

The Level II calculation simulates a situation in which a chemical is continuously discharged into the multimedia environment and achieves a steady-state and equilibrium condition, at which input and output rates are equal. The task is to deduce the rates of loss by reaction and advection and the prevailing concentrations and masses. 

Mahalec, V. H. Kluzik and L. B. Evans. Simultaneous Modular Algorithm for Steady State Flowsheet Simulation and Design. Paper presented at the 12th European Symposium on Computers in Chemical Engineering. Montreaux, Switzerland (1979). 

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