4 FLOWSHEETING
4.4. SPECIFICATION OF COMPONENTS AND PHYSICAL PROPERTY MODELS
and the cost routines allow quick economic comparisons to be made. Some programs include optimization routines. To make use of a costing routine, the program must be capable of producing at least approximate equipment designs.
In a sequential-modular program, the executive program sets up the flowsheet sequence, identifies the recycle loops, and controls the unit operation calculations, while interacting with the unit operations library, physical property data bank, and the other subroutines. The executive program also contains procedures for the optimum ordering of the calculations and routines to promote convergence.
In an equation-oriented simulator, the executive program sets up the flowsheet and the set of equations that describe the unit operations and then solves the equations using data from the unit operations library and the physical property data bank and calling on the file of thermodynamics subroutines.
All process simulators use graphical user interfaces to display the flowsheet and facilitate the input of information to the package. The entry of data is usually intuitive to anyone familiar with the MS WindowsTMoperating systems.
4.4. SPECIFICATION OF COMPONENTS AND PHYSICAL
Some guidelines to keep in mind when building a component list include
1. Always include any component that has a specified limit in any of the products if that component is present in any of the feeds or could be formed in the process.
This is critical to determining whether the separations are meeting product specifications.
2. Always include any component that has a specified limit in any of the feeds.
These components can be a source of byproducts or can act as catalyst or enzyme inhibitors. They must be tracked to ensure that they do not accumulate in the process or make it difficult to meet product specifications. In some cases, an additional separation may be needed to remove a feed contaminant.
3. Always include components that are expected to be formed in side reactions or consecutive reactions. It is important to understand where these components will accumulate or leave the process, even if their yield is not yet known.
4. Always include any compounds that are expected to be present and are known to have significant health, safety, or environmental concerns, such as com- pounds with high toxicity or explosivity, known carcinogens, or listed hazard- ous air pollutants (see Chapter 14). These compounds must be tracked to make sure that they do not reach unsafe levels in any stream and to understand where they might be released to the environment.
5. Usually include any compound that might be present at a mass or mole fraction greater than 2% in any stream in the process.
6. Do not include isomers unless the process specifically requires distinction be- tween isomers (for example, if the process is selective for one isomer, gives different products for different isomers, or is designed to separate isomers).
Considering all of the possible isomers of organic compounds becomes combi- natorially explosive at high carbon numbers. For fuels and bulk petrochemical processes that are carried out at relatively high temperatures, it is often reasonable to assume an equilibrium distribution of isomers. For fine chemical and pharmaceutical processes, it is usually important to track isomers separately, particularly enantiomers, as the desired product is often only one of the isomers.
In general, pure component models solve more efficiently with fewer than about 40 components. If the number of components becomes too large and there are many recycles, then it may be necessary to build two models. The first is a high-level model that contains only the main bulk components. This model is then used to initialize a second, more detailed model that has the full component list.
4.4.2. Pseudocomponents
Pseudocomponents (hypocomponents) are components created by the simulator to match the boiling curves of petroleum mixtures.
Crude oil; fuels such as gasoline, kerosene and diesel; and most intermediate streams in oil refinery consist of many different hydrocarbon compounds. The number of possible hydrocarbon isomers present depends on the carbon number, and both increase with boiling range. For diesel, crude oil, and heavy fuel oils, the
number of possible compounds can be from 104to>106. At the time of writing, there is no characterization method that can identify all of these compounds, so it would be impossible to include them all in a model even if the resulting model could be solved.
Instead, a large number of possible compounds with boiling points in a given range are
‘‘lumped’’ together and represented by a single pseudocomponent with a boiling point in the middle of that range. A set of 10 to 30 pseudocomponents can then be fitted to any petroleum assay and used to model that oil.
Pseudocomponent models are very useful for oil fractionation and blending prob- lems. They can also be used to characterize heavy products in some chemical pro- cesses such as ethane cracking. Pseudocomponents are treated as inert in most of the reactor models, but they can be converted or produced in yield-shift reactors (see Section 4.5.1).
Some of the commercial simulation programs use a standard default set of pseu- docomponents and fit the composition of each to match a boiling curve of the oil that is entered by the user. This can sometimes lead to errors when predicting ASTM D86 or D2887 curves for products from a feed that has been defined based on a true boiling point (TBP) curve, or when making many subcuts or cuts with tight distillation specifications. It is often better to work back from the product distillation curves and add extra pseudocomponents around the cut points to make sure that the recoveries and 5% and 95% points on the product distillation curves are predicted properly.
All of the simulators have the option to add pseudocomponents to the default set or use a user-generated curve.
4.4.3. Solids and Salts
Most chemical and pharmaceutical processes involve some degree of solids handling.
Examples of solids that must be modeled include
& Components that are crystallized for separation, recovery, or purification;
& Pharmaceutical products that are manufactured as powders or tablets;
& Insoluble salts formed by the reaction of acids and bases or other electrolytes;
& Hydrates, ice, and solid carbon dioxide that can form in cryogenic processes;
& Cells, bacteria, and immobilized enzymes in biological processes;
& Pellets or crystals of polymer formed in polymerization processes;
& Coal and ash particles in power generation;
& Catalyst pellets in processes in which the catalyst is fluidized or transported as a
slurry;
& Mineral salts and ores that are used as process feeds;
& Fertilizer products;
& Fibers in paper processing.
Some solid phase components can be characterized as pure components and can interact with other components in the model through phase and reaction equilibrium.
Others, such as cells and catalysts, are unlikely to equilibrate with other components, although they can play a vital role in the process.
In Aspen Plus, solid components are identified as different types. Pure materials with measurable properties such as molecular weight, vapor pressure, and critical temperature and pressure are known as conventional solids and are present in the MIXED substream with other pure components. They can participate in any of the phase or reaction equilibria specified in any unit operation. If the solid phase partici- pates only in reaction equilibrium but not in phase equilibrium (for example, when the solubility in the fluid phase is known to be very low), then it is called aconven- tional inert solidand is listed in a substream CISOLID. If a solid is not involved in either phase or reaction equilibrium, then it is anonconventional solidand is assigned to substream NC. Nonconventional solids are defined by attributes rather than molecular properties and can be used for coal, cells, catalysts, bacteria, wood pulp, and other multicomponent solid materials.
In UniSim Design, nonconventional solids can be defined as hypothetical compo- nents (see Section 4.4.4). The solid phases of pure components are predicted in the phase and reaction equilibrium calculations and do not need to be identified separately.
Many solids-handling operations have an effect on the particle size distribution (PSD) of the solid phase. The particle size distribution can also be an important product property. Aspen Plus allows the user to enter a particle size distribution as an attribute of a solid substream. In UniSim Design, the particle size distribution is entered on the ‘‘PSD Property’’ tab, which appears under ‘‘worksheet’’ on the stream editor window for any stream that contains a pure or hypothetical solid component.
Unit operations such as yield-shift reactor, crusher, screen, cyclone, electrostatic precipitator, and crystallizer can then be set up to modify the particle size distribution, typically by using a conversion function or a particle capture efficiency in each size range.
When inorganic solids and water are present, an electrolyte phase equilibrium model must be selected for the aqueous phase, to properly account for the dissolution of the solid and formation of ions in solution.
4.4.4. User Components
The process simulators were originally developed for petrochemical and fuels appli- cations; consequently, many molecules that are made in specialty chemical and pharmaceutical processes are not listed in the component data banks. All of the simulators allow the designer to overcome this drawback by adding new molecules to customize the data bank.
In UniSim Design, new molecules are added as hypothetical components. The minimum information needed to create a new hypothetical pure component is the normal boiling point, although the user is encouraged to provide as much information as is available. If the boiling point is unknown, then the molecular weight and density are used instead. The input information is used to tune the UNIFAC correlation to predict the physical and phase equilibrium properties of the molecule, as described in Chapter 8.
User-defined components are created in Aspen Plus using a ‘‘user-defined compo- nent wizard.’’ The minimum required information is the molecular weight and normal boiling point. The program also allows the designer to enter molecular structure, specific gravity, enthalpy, and Gibbs energy of formation, ideal gas heat capacity, and Antoine vapor pressure coefficients, but for complex molecules usually only the molecular structure is known.
It is often necessary to add user components to complete a simulation model. The design engineer should always be cautious when interpreting simulation results for models that include user components. Phase equilibrium predictions for flashes, decanters, extraction, distillation, and crystallization operations should be carefully checked against laboratory data to ensure that the model is correctly predicting the component distribution between the phases. If the fit is poor, the binary inter- action parameters in the phase equilibrium model can be tuned to improve the prediction.