Simulator unit description operation

The structure of the recipes and the parameters of the unit operations (heating power, cooling power, mass flows, etc.) were taken from the process description and had to be slightly adapted during the first simulation runs. 

Steady-state process simulation or process flowsheeting has become a routine activity for process analysis and design. Such systems allow the development of comprehensive, detailed, and complex process models with relatively little effort. Embedded within these simulators are rigorous unit operations models often derived from first principles, extensive physical property models for the accurate description of a wide variety of chemical systems, and powerful algorithms for the solution of large, nonlinear systems of equations. 

As I stated in Chapter 2, computation of the Fourier transform is the lens-simulating operation that a computer performs to produce an image of molecules in the crystal. The Fourier transform describes precisely the mathematical relationship between an object and its diffraction pattern. The transform allows us to convert a Fourier-series description of the reflections to a Fourier-series description of the electron density. A reflection can be described by a structure-factor equation, containing one term for each atom (or each volume element) in the unit cell. In turn, the electron density is described by a Fourier series in which each term is a structure factor. The crystallographer uses the Fourier transform to convert the structure factors to p(.x,y,z), the desired electron density equation. 

A library of generalized models is supplied in ASPEN to allow the user to simulate coal conversion processes as well as chemical and petroleum processes. A listing of ASPEN s unit operations models is given in Table I. Space does not permit descriptions of the models here, however, the ASPEN project reports (2) discuss their capabilities. 

The FCC simulator also requires a description of the unit and its operation in terms of equipment dimensions and constraints, operating variables and unit parameters. Standard or default values for all of these are stored in the preprocessor database. The user, however, can change the values of any of the following in specifying the base and alternate FCC operations ... 

Simulation techniques suitable for the description of phenomena at each length-scale are now relatively well established Monte Carlo (MC) and Molecular Dynamics (MD) methods at the molecular length-scale, various mesoscopic simulation methods such as Dissipative Particle Dynamics (Groot and Warren, 1997), Brownian Dynamics, or Lattice Boltzmann in the colloidal domain, Computational Fluid Dynamics at the continuum length-scale, and sequential-modular or equation-based methods at the unit operation/process-systems level. 

Computer simulation models have been formulated for cascade and Stratco sulfuric acid alkylation units. These complete models incorporate mathematical descriptions of all the interacting parts of the units, including reactors, distillation columns, compressors, condensers, and heat exchangers. Examples illus-strate diverse model applications. These Include identifying profitable unit modifications, comparing cascade to Stratco performance, evaluating optimal unit capacity and determining optimal deisobutanizer operation. 

Simulation in Process Engineering requires specific scientific knowledge among we may cite accurate description of physical properties of pure components and complex mixtures, models for a large variety of reactors and unit operations, as well as numerical techniques for solving large systems of algebraic and differential equations. 

From the above description, we may conclude that flowsheeting software is a very sophisticated computer-based system, and not a collection of algorithms for solving different unit operations. A process simulator must be designed with computer science development and management tools. It is interesting to note that in the total cost the software maintenance (typically more than 70 %) is by far more important than the cost of programming (typically under 10 %). 

The mercury removal performance of pilot-scale ICDAC and of Norit s FGD carbon were determined in a 0.236 m% (0.25 MWe) pilot plant operated by CONSOL, Inc., Library, PA. The pilot plant can simulate flue gas conditions downstream of the air preheater in a coal fired utility power plant. The flue gas mercury concentration studied (10-15 pg/m ) is typical of utility flue gas concentration. Mercury removals were evaluated in the flue gas duct, which provided a gas residence time of approximately 2 seconds, and in the baghouse, where the solids retention times can be as long as 30 min. Common test conditions were flue gas flow, 0.165 m /s flue gas wet bulb temperature, 50-53°C flue gas composition, 1000 ppmv dry SO2, 10 vol% dry O2, and 10 vol% dry CO2. All tests were conducted with a fly ash obtained from a coal-fired utility boiler firing an eastern bituminous coal. The fly ash feed rate was 4.5 kg/hr (solids loading of 90.6-104.7 gm/dcm ). Mercury removal was determined from the mercury feed rate, the solids (carbon and fly ash) feed rate, and mercury analysis of the feed and recovered solids (by combustion followed by cold vapor atomic absorption spectroscopy). Except where noted, all mercury removal results discussed in this paper include mercury removal by the carbon sorbent and the fly ash. A more detailed description of the pilot test unit is given elsewhere (27]. 

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