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New Workflow: A Holistic Fluid and Solids Model in OneSimulation EnvironmentIn response to a lack of an industrial simulator to rigorously model the solids section of an industrial process, the stand-
alone solids simulator SolidSim was developed by solids experts and industry participants in Germany in the early 2000s
to 2010 timeframe. SolidSim introduced a generally applicable flowsheet simulation system to rigorously describe granular
solids and the machines and apparatuses of particle technology3. The development of SolidSim was focused mainly on
solids so users of that tool couldn’t rigorously model the fluid section of a process.
In February 2012, Aspen Technology acquired SolidSim Engineering GmbH, the company that was developing and
marketing SolidSim in order to unify these two modeling environments. With the release of Aspen Plus V8 in December
2012, the Aspen Plus model library was enhanced with the SolidSim technology incorporating 25 unit operations models,
including models for drying, crystallization, granulation and agglomeration, crushing and grinding, classification, gas/solid,
and solid liquid separation. In addition, an easy-to-use workflow for the definition of particle size distributions was
introduced with an enhanced results representation that allows visualizing particle size distributions (e.g. cumulative,
density, or RRSB) and apparatus-specific results (e.g. separation efficiency curve of a screen or a gas cyclone) with the
click of a button. Also, characteristic diameters such as d25, d50, or the Sauter Mean Diameter (SMD) are now shown in
Aspen Plus in the stream results.
This enhancement enables the user of Aspen Plus, without any additional software use costs, to model processes that
contain both fluids and solids in one simulation environment using consistent physical properties and avoiding errors and
inefficiencies that may result from the transfer of data from one simulation system to the other. Considering the entire
process, rather than only smaller subsections, allows the user to avoid sub-optimal design due to localized optimization.
By overcoming this challenge, as well as introducing detailed solids modeling, users can address capital costs due to
overdesign, energy and other operating costs, and reduce product quality or throughput. Furthermore, Aspen Plus V8
enables the user to optimize the entire process and use integrated features such as Activated Economics and Activated
Energy.
One example of this more holistic workflow is shown in Figure 4 and displays the process model of the entire urea
production process containing both the synthesis and the granulation part in Aspen Plus V8.
Figure 4: New workflow: Holistic process model of the urea synthesis and granulation in Aspen Plus V8
rigorous the model is may differ in appropriateness. For instance, when a rough estimate is quickly needed, the
consideration of exact solids processing equipment may not be necessary. With conceptual modeling, particle scientists
and process engineers can model solids processes in various degrees of detail, from rough sketches to the Mona Lisa of
models. Once a user is ready to switch a unit from conceptual to rigorous, it can be done without changing the flowsheet.
With conceptual models, process engineers that are not savvy with solids modeling can be eased into learning how to use
the capabilities.
Another great opportunity that the conceptual models offer is the possibility that process engineers and particle scientists
can collaborate more closely by using the same simulation environment. When setting up the model of a combined fluids
and solids process, the process engineer can use the conceptual models to describe the solids section of the process.
After having the first simulation results, the process engineer can decide what parts of the solids section need to be
modeled more rigorously, and if necessary, ask the particle scientist to help select and parameterize the rigorous model.
Particle Characterization: The Key to Understanding SolidsProcessingIn Aspen Plus, dispersed solids can be characterized in detail as schematically shown in Figure 6. Users can define
multiple particle types with different distributed properties, such as composition and particle size. In addition to this, it is
also possible to define the moisture content of the particles as a loading of one or more fluid components. The defined
moisture content has an influence on the particles heat capacity and on its density and settling velocity. As a result, wet
particles, for example, will be separated differently in a classifier as dry particles with the composition and same particle
size distribution.
Figure 6: Schematic of the solids characterization in Aspen Plus V8
In addition to this, an intuitive and easy-to-use workflow for the definition of particle size distributions has been integrated
into Aspen Plus that consists of the following steps:
Definition of the particle size classes (particle size mesh)
Aspen Plus offers an automated and a manual mode to generate the particle size classes that should be used to describe
the PSD. The user can select a pre-defined mesh type (e.g. equidistant, geometric, or logarithmic) in automated mode or
the user can import measured data from a particle size analysis in manual mode.
Definition of the mass fraction within the different particle size classes
For the definition of the mass fractions, Aspen Plus offers an automated or manual mode. In the automated mode, the
user can define mass fractions by selecting a distribution function (e.g. GGS, RRSB (Rosin Rammler Sperling Bennet), log-
normal, or normal distribution) he wants to use to define the PSD and enters a value for the shape (e.g. dispersion
parameter) and the position parameter (e.g. characteristic diameter d63 or d50). In the manual mode, the user defines
the mass fractions as tabular data such as from measured data found during a particle size analysis.
By opening the stream results, users can visualize the defined and calculated particle size distributions with a click of a
button in the form of a cumulative mass, density, or RRSB distribution. It is also possible to show the PSD for different
particle types and streams in the same plot and compare inlet and outlet streams of a unit operation to fully understand
how a certain unit operation changes the particle size distribution of its feed material. An example of how a double-deck
screen changes the PSD is shown in Figure 7.
Figure 7: Cumulative particle size distributions for the stream entering (purple) and the three streams leaving (fines: green, midsize: pink, coarse:blue) the screen
In addition to the plot of the particle size distribution of the streams, the user can easily generate plots that are specific to
a unit operation. This could be the solids volume concentration profile of a fluidized bed, or the solids temperature and
moisture profile in a convective dryer. As an example, Figure 8 shows the separation efficiency curve of the double-deck
screen used in the example above.
Figure 8: Separation Curves for the two decks used in the screen in the granulation example
the separation efficiency curve based on the more rigorous model, therefore at a higher level of fidelity. A comparison of
the calculated separation efficiency curve using the conceptual and the rigorous model is shown in Figure 11. The plot
shows that the conceptual model predicts the classification in the centrifuge with accuracy that already may be sufficient
for different use cases, for example a feasibility study.
Figure 10: The top image shows a decanter centrifuge described with a conceptual model and the bottom image shows the same decanterdescribed with a rigorous equipment model
Figure 11: Comparison of the calculated separation efficiency curve forthe conceptual centrifuge model (red curve) and the rigorous decantermodel (blue curve)
From Sequential to Simultaneous Conceptual DesignAspen Plus is surrounded by a suite of integrated products called aspenONE engineering. Aspen Plus supports integrated
workflows, such as Activated Economics and Energy Analysis, as well as Exchanger Design and Rating (EDR). By modeling
solids using Aspen Plus with aspenONE engineering, users have access to these features and can optimize the energy
demand of the entire process with Activated Energy and can assess the capital and utility costs of solids modeling
alternatives with Activated Economics. Using Activated EDR, users can appropriately size heat exchangers required in the
fluid part of the process.
Figure 12: Schematic showing tasks available with the integrated features and layered products surrounding and including Aspen Plus and AspenHYSYS®
Economics for solids allows users to consider the capital and utility costs for all solids processing blocks. The impact of
design alternatives and their associated configurations and equipment specifications can be instantly seen. An example of
the activated economic interface and a portion of equipment cost list for the urea synthesis and granulation example is
shown in Figure 13.
Figure 13: Economics for solids helps users determine capital and utility costs
Use in the IndustryAlthough Aspen Plus V8 with solids has only been available to customers since December 2012, usage of solids modeling
in Aspen Plus has grown at a surprisingly fast rate. Over 100 organizations have started using solids modeling in Aspen
Plus, Evonik Industries AG, Dow Chemical5, and Yara Technology Centre6, just to name a few. In May of 2013, Dow
engineers reported at AspenTech’s OPTIMIZETM 2013 conference that they significantly improved their understanding of
the solids behavior in a process involving solid reactants. They were also able to better understand the growth mechanism
they were looking at and relate them with batch cycle time constants. This improved understanding helped them to
optimize the entire process.
Examples Designed for Getting Started FasterModeling the solids section of a process using Aspen Plus V8 has many unique benefits, as previously described. Several
self-service examples have been developed by experts at Aspen Technology for the purpose of providing users a start in
building the models using solids processing operations in Aspen Plus V8 and to allow new and experienced users to get
started faster. These examples were generated using real challenges experienced with solids processing. In addition to the
earlier referenced urea synthesis and granulation example, two examples will be briefly discussed, including a belt dryer
example and a potassium chloride production example.
The belt dryer example illustrates how the energy demand of an industrial multi-stage belt dryer, as shown in Figure 13,
can be optimized using Aspen Plus V8. The convective dryer model in Aspen Plus is used to model the different chambers
of the belt dryer. As a simulation result, the temperature and moisture profile along the dryer, as shown in Figure 14, as
well as the overall energy demand are obtained. The example shows how adding a cooling stage to the dryer and using the
optimization capability of Aspen Plus to determine the optimal drying agent flow rate can help to reduce the energy
demand in the present case by over 23%.
Figure 14: View of the belt dryer layout with the proposed design alternative