HYSIS system

The process simulation code Aspen-HYSYS 3.2 has been used for residential PEM fuel cell system calculations. Natural gas has been simulated as three different sources for hydrogen production. The chemical compositions of the natural gas fuel are summarized in Table 1. The average molecular weight of natural gas is around 16.6 kg/kmol. All simulation studies are performed based on this composition. 

The major units of the Aspen-HYSYS simulation for natural gas steam reforming based fuel cell system are presented in Figure 3. 

Table 1 gives the components present in the crude DDSO and their properties critical pressure (Pc), critical temperature (Tc), critical volume (Vc) and acentric factor (co). These properties were obtained from hypothetical components (a tool of the commercial simulator HYSYS) that are created through the UNIFAC group contribution. The developed DISMOL simulator requires these properties (mean free path enthalpy of vaporization mass diffusivity vapor pressure liquid density heat capacity thermal conductivity viscosity and equipment, process, and system characteristics that are simulation inputs) in calculating other properties of the system, such as evaporation rate, temperature and concentration profiles, residence time, stream compositions, and flow rates (output from the simulation). Furthermore, film thickness and liquid velocity profile on the evaporator are also calculated. 

System characteristics—crude DDSO (13) properties of the components and of the mixture calculated from the properties obtained from HYSYS (see Table 1) through the equations presented. Note that all errors mentioned, one for correlation, were suggested by the authors of the equations. 

The simulator packages such as Aspen Plus and Hysys may be useful in analyzing distillation column systems to improve recovery and separation capacity, and to decrease the rate of entropy production. For example, for the optimization of feed conditions and reflux, exergy analysis can be helpful. A complete exergy analysis, however, should include both an examination of the exergy losses related to economic and environmental costs and suggestions for modifications to reduce these costs. Otherwise, the analysis is only theoretical and less effective. 

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]. 

Throughout this book, we have seen that when more than one species is involved in a process or when energy balances are required, several balance equations must be derived and solved simultaneously. For steady-state systems the equations are algebraic, but when the systems are transient, simultaneous differential equations must be solved. For the simplest systems, analytical solutions may be obtained by hand, but more commonly numerical solutions are required. Software packages that solve general systems of ordinary differential equations— such as Mathematica , Maple , Matlab , TK-Solver , Polymath , and EZ-Solve —are readily obtained for most computers. Other software packages have been designed specifically to simulate transient chemical processes. Some of these dynamic process simulators run in conjunction with the steady-state flowsheet simulators mentioned in Chapter 10 (e.g.. SPEEDUP, which runs with Aspen Plus, and a dynamic component of HYSYS ) and so have access to physical property databases and thermodynamic correlations. 

System is shown in Fig. 15. A flowsheeting program, HYSIS or other comparable one, is used to develop the process flow diagram. The flowsheeting program determines the operating conditions and the utilities required—steam and cooling water. Then, a value-added economic analysis is performed to estimate the profitability of the plant. If the profitability is acceptable. 

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. 

The HYSYS program, like most other software, is continually being developed and new versions are released frequently. This book covers HYSYS, Version 2004.1. It should be emphasized, however, that this book covers the basics of HYSYS which do not change that much from version to version. The book covers the use of HYSYS on computers that use the Windows operating system. It is assumed that the software is installed on the computer, and die user has basic knowledge of operating the computer. 

The property packages available in HYSYS allow you to predict properties of mixtures ranging from well defined light hydrocarbon systems to complex oil mixtures and highly nonideal (non-electrolyte) chemical systems. HYSYS provides enhanced equations of state (PR and PRSV) for rigorous treatment of hydrocarbon systems semiempirical and vapor pressure models for the heavier hydrocarbon systems steam correlations for accmate steam property predictions and activity coefficient models for chemical systems. All of these equations have their own inherent limitations and you are encouraged to become more familiar with the application of each equation. 

A fresh feed stream consisting of ethylene gas (63 mol %) and pure O2 gas (37 mol %) at 20 °C and 303 kPa enters an oxidation reactor system with a molar flowrate of 120 kmol/hr plus recycled gasses/vapors (estimated by HYSYS). The reaction is promoted by a solid catalyst and occurs isothermally at 230 °C. The feed stream must therefore be pre-heated to 230 °C before it is fed into the oxidation reactor. 

The hot effluent is cooled to -1°C (in practice this very large temperature difference can only achieved by direct contact heat exchange, ie a quench system). The pressure drop across the large condenser is 50 kPa. Under these conditions, the product stream has a vapour fraction of about 0.8 and the task of recovering condensable liquid ethylene oxide begins. The cool product stream is fed into a 3-phase separator and the light liquid phase is separated from the heavy liquid phase and vapour residual. HYSYS normally puts water in the heavy phase when there is a non-zero water stream. 

The MESH equations constitute a nonlinear and strongly coupled system of algebraic equations since the equilibrium ratios Ki j and the enthalpies and are complex functions of temperature and concentrations. The system (5.2-71) is numerically solved by the iterative Newton-Raphson algorithm. Commercial software packages (e.g., ASPEN, HYSYS, CHEMCAD) contain both the mathematical solver and the required system properties, such as vapor liquid equilibria and enthalpies. 

In recent years, property information systems have become widely available in computer packages. Some are available on a stand-alone basis, such as PPDS2 (1997), while others are available within the chemical process simulators, such as ASPEN PLUS, HYSYS.Plant, PRO/n, CHEMCAD, BATCH PLUS, and SUPERPRO DESIGNER. Commonly, constants and parameters are stored for a few thousand chemical species, with programs provided to estimate the property values of mixtures, and determine the constants and parameters for species that are not in the data bank using estimation methods or the regression of experimental data. Virtually all of the property systems estimate the properties of mixtures of organic chemicals in the vapor and liquid phases. Methods are also provided for electrolytes and some solids, but these are less predictive and less accurate. 

Each of the property information systems has an extensive set of subroutines to determine the parameters for vapor pressure equations (e.g., the extended Antoine equation), heat capacity equations, etc., by regression and to estimate the thehnophysical and transport properties. The latter subroutines are called to determine the state of a chemical mixture (phases at equilibrium) and its properties (density, enthalpy, entropy, etc.) When calculating phase equilibria, the fugacities of the species are needed for each of the phases. A review of the phase equilibrium equations, as well as the facilities provided by the process simulators for the calculation of phase equilibria, is provided on the CD-ROM that accompanies this book (see ASPEN- Physical Property Estimation and HYSYS Physical Property Estimation). 

To improve the control system in Figure 21.35, controllability and resiliency analysis has two roles. These involve the use of (1) the RGA to aid in selecting the appropriate pairing between the controlled outputs and manipulated variables when interactions are anticipated, and (2) the DC to assist in checking that the operating ranges of the manipulated variables are sufficient to ensure adequate disturbance rejection. To provide data for these two analytical methods, a dynamic simulation of the MCB separation process is developed using HYSYS.Plant. 

Aspen IPE usually begins with the results of a simulation from one of the major process simulators (e g., ASPEN PLUS, HYSYS, CHEMCAD, and PRO/II), it being noted that users can, alternatively, provide equipment specifications and request investment analysis without using the process simulators. In these notes, only results from ASPEN PLUS are used to initiate Aspen IPE evaluations and only capital cost estimation is emphasized. Readers should refer to the Aspen IPE User s Guide (press the Help button in Aspen IPE) for detailed instructions, explanations, and for improvements in new versions of the software system. 

In the thermodynamic study of the reaction, one can also use the HYSYS software version 3.1, particularly the Gibbs reactor module, with the thermodynamic package Peng-Robinson and the method of minimization of the Gibbs free energy (G) of the system, given by the following equation ... 

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