Unit operation models

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. 

Modular simulators are frequently constructed on three levels. The lowest level consists of thermodynamics and other physical property relations that are accessed frequently for a large number of flowsheeting utilities (flash calculations, enthalpy balances, etc.). The next level consists of unit operations models as described above. The highest level then deals with the sequencing and convergence of the flowsheet models. Here, simultaneous... 

The Input Translator is completely table driven. This means that all of the information needed to process input statements (such as names of keywords, default values of data items, etc.) is stored in tables in a file called the System Definition File. Therefore, it is easy to add keywords or change defaults by changing entries in the System Definition File. In addition to the Input Language tables, almost any "changeable" information related to Input Translation is stored in the System Definition File. This includes unit conversion tables, attribute descriptions, physical property option models, data structure, unit operation model data, and stream requirements, etc. Thus it is easy to add new system parameters without changing any code in the Input Translator. 

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. 

Flowsheet simulators consist of unit operation models, physical and thermodynamic calculation models and databanks. Consequently, the simulation results are only as good as the underlying physical properties and engineering models. Many steady-state commercial simulators [2.1, 2.2] have some dynamic (batch) models included, which can be used in steady-state simulations with intermediate storage buffer tanks. 

Unsteady-state or dynamic simulation accounts for process transients, from an initial state to a final state. Dynamic models for complex chemical processes typically consist of large systems of ordinary differential equations and algebraic equations. Therefore, dynamic process simulation is computationally intensive. Dynamic simulators typically contain three units (i) thermodynamic and physical properties packages, (ii) unit operation models, (hi) numerical solvers. Dynamic simulation is used for batch process design and development, control strategy development, control system check-out, the optimization of plant operations, process reliability/availability/safety studies, process improvement, process start-up and shutdown. There are countless dynamic process simulators available on the market. One of them has the commercial name Hysis [2.3]. 

If a process hydraulic model is built, then care must be taken to specify pressure drop properly in the unit operation models. Rules of thumb are adequate for initial estimates, but in a hydraulic model these should be replaced with rigorous pressure drop calculations. 

These concepts are implemented in the integration platform CHEOPS Component-Based Hierarchical Explorative Open Process Simulator) [252, 409, 462]. The platform provides generic component prototypes and interfaces for the integration of models, solvers, and tools. The generic components are instantiated at run-time by concrete software components and classes representing actual unit operation models, solvers, etc. That way, arbitrary components from the list of available components can be used in the simulation. The list of model and solver components can easily be extended with the components that comply with the abstract structure and interface definitions. 

The basic element in a modular simulator is the unit operation model. A simulation model is obtained by means of conservation equations for mass, energy and momentum. These lead finally to a system of non-linear algebraic equations as ... 

Figure 2.7 General layout of unit operation model...

Unit operation model (black box models such as mixers, separators, component splitters, etc. models of phase separation and relaxation, heat-transfer model, multistage models, pumps and compressors, reactor models such as equilibrium reactor, stoichiometric reactor, tubular reactor, etc. see Chapter 2). 

Take case 2 in Table 60.2, for example. One can evaluate the heat loss of a burner by measuring the actual flue gas temperature and excess air% and then input these measured values into the unit operation model to get the heat loss% of the burner calculated as displayed in Figure 60.7. 

The heat exchanger models in Simprosys are based on Knudsen et al. (1997), Richard et al. (1997), Walas (1990), Kuppan (2000), Kakac and Liu (2002), McCabe et al. (2(XX)), and Minton and Morrison (2003). The cyclone models are based on Pell and Dunson (1997), Zenz (1999), and Reynolds et al. (2002). The electrostatic precipitator model is based on Pell and Dunson (1997) and Reynolds et al. (2002). The wet scrubber model is based on Pell and Dunson (1997) and Schifftner and Hesketh (1983). All the other unit operation models of Simprosys are based on Perry (1997). For more general information about Simprosys, please refer to Gong and Mujumdar (2008). For detailed information about the burner unit operation, please refer to Gong and Mujumdar (2014). 

Davis, R.A. (2002) Simple gas permeation and pervaporation membrane unit operation models for process simulators. Chemical Engineering Technology, 25 (7), 717-722. 

The development and the design of new processes are today extensively based on mathematical modelling and simulation. The entire process is modelled through combining individual unit operation models. Several flowsheeting programs (Aspen+ , HySim , ChemCAD Pro II etc.) have already found their place in engineering... 

Reaction models are necessary in the chemical process industries for a number of purposes which are most often related to the modeling, simulation and control of production processes process synthesis, process simulation, plant optimization and production control are typically some of the domains concerned with the use of reaction models within unit operation models. To provide interoperability of reaction models within a number of software applications, a specific part of the CAPE-OPEN standard has been devoted to these simulation components called Reactions Packages. CAPE-OPEN Reactions Packages are described in terms of the interfaces that they must support, their interaction with a process modelling environment and the functionality they are expected to support. The interfaces defined support both kinetic and electrolyte reactions. 

A reaction model is a typical component of simulation systems, along with unit operations, thermodynamic servers, physical properties databanks, etc. The reaction model may provide information on how it is built, or can choose not to provide such information but Just to provide computation mainly of reaction rates so that these terms may be readily used in mass balances within unit operation models. The same applies for energy terms. By implementing a common interface standard, a reaction model component may be deployed on its own, independently of the process simulator it is used in. That develops the reusability of reaction models throughout unit operations and process simulators. A reaction model is contained by a Reactions Package software component exhibiting the specific CAPE-OPEN interfaces discussed here. 

Unit Operations Models for Process Analysis using ASPEN... 

While most of the batch unit operation models involve ordinary differential equations some unit operations like batch adsorption column encounters partial differential equations, orthogonal collocation method can be used to reduce set of partial differential equations to ordinary differential equations. 

No elaborate prerequisites are required however, an understanding of unit operation modelling is assumed, it is inevitable that modelling involving differential equations will, by necessity, be involved in parts of the theory and workshops. Quite often, mathematics is a barrier that prevents a clear understanding of control concepts and implementation of process control theory. It is anticipated that the real-time approach will remove, or at least minimize, these barriers. 

Gabbar, H.A. (2000h), Suzuki, K., Shimada, Y. Unit Operation Modeling Approach For Plant Safety Practices, Conference Proceedings of Society Of Plant Engineers in Japan (SOPEJ), Nov-2000, No. 3, pp 27-32. 

Another example is the systematic analysis undertaken by Palsson et al. on combined SOFC and gas turbine cycles [36]. In combination with a robust and accurate 2-D SOFC model, the system-level model attempts to provide an unbiased evaluation of performance prospects and operational behaviours of such systems. The 2-D SOFC model was integrated into a process simulation tool. Aspen Plus , as a user-defined model, whereas other components constituting the system are modelled as standard unit operation models. Parametric studies can be carried out to gain knowledge of stack and system behaviour such as the influence of fuel and air flow rate on the stack performance and the mean temperature and the effects of cell voltage and compressor pressure on the system efficiency. The pressure ratio is shown to have a large impact on performance and electrical efficiencies of higher than 65% are possible at low-pressure ratios. 

To generate ternary plots and to use them for design. Aspen Split is used. This software is imbedded in Aspen Plus and can be accessed by going to the toolbar and clicking Library and References. The window shown in Figure 8.11 opens in which the Aspen Split box should be checked. A new page tab will appear at the bottom of the process flow diagram next to those of the standard unit operation models, which is shown in... 

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