Solids modelling conveys solid performance

Ignoring or underestimating the need for rigorous design and optimal operation of the solids section of processes in industries such as mines, food processing, chemical, agrichemical, and pharmaceutical industries, may reduce design efficiency, encourage over-design of equipment, add unnecessary operating energy costs, and result in reduced throughput and lower product quality.

In addition to fluids, many industrial processes involve solids processing steps that often have a significant influence on the overall process performance, product quality, or energy demand.

In general, most industrial processing plants have liquids and solids sections. The solids section has to fulfil one or more of the following tasks: adjust the particle size distribution (crushing/grinding, classifying, compacting), formulate particles (granulation, crystallization), adjust the moisture content (drying), change the composition by chemical reactions (fluidised bed reactor), and separating solids from liquid or gas streams (cyclone, centrifuge, filters).

The solids section is typically only a small part of an overall production process, but may have a significant influence on the overall process performance and the quality of the final product. 

The process engineers model the fluid part of a process in a simulator software tool, while particle scientists within the same company model the solids part of the process in a spreadsheet tool. 

There is limited sharing of process data with these disparate software tools resulting in data redundancy, inconsistency, and inaccuracy. Optimisation of the overall process design is more or less impossible using this historical workflow.

The second challenge is that to reduce the risk of bottlenecks, equipment is often over-designed or installed in excess leading to inflated capital costs.

Rigorous solids’ modelling encourages the selection of appropriately sized solids processing equipment and reduced recycle streams to minimise the load of equipment. Examples of solids processing equipment that is often overdesigned include crushers, compactors, and dryers.

Thirdly, for many applications, intermediate or final products need to fulfil very tight moisture content specifications to be suitable for use in subsequent process steps, or to be sold as final products. Managing the moisture content requires energy, and without detailed modelling, energy costs can be unnecessarily high.

Most importantly the design of a profitable process hinges on the throughput and the quality of the final product. The formulation of particles is an example of where both quality and quantity can be improved. Starting from a solution or slurry, particles are formulated with the aim to produce a dust-free, free-flowing powder with well-defined properties. 

This is done in most cases by crystallization, granulation/agglomeration, or spray drying. The design and operating conditions of these units determine the particle size distribution (PSD) and moisture content of the product, and incorrect design and operation results in reduced throughput and inferior quality.

The solids section of a process plant, in many instances, is considered a minor part of an overall production process, but may have a significant influence on the overall process performance and the quality of the final product.

In response to the lack of a commercially available industrial simulator to rigorously model the solids section of an industrial process, the standalone solids simulator SolidSim was developed by solids experts and industry participants.

SolidSim introduced a generally applicable flowsheet simulation system to rigorously describe granular solids and the machines and equipment of particle technology.

Subsequently Aspen Tech acquired SolidSim, and the Aspen Plus V8 model library was enhanced with the SolidSim technology, incorporating unit operations models, including models for crushing and grinding, classification, drying, crystallization, granulation and agglomeration, 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 visualising particle size distributions (cumulative, density, or RRSB), and apparatus-specific results with the click of a button. Also, characteristic diameters such as d25, d50, or the Sauter Mean Diameter (SMD) are shown as stream results.

The new solids modelling library is a combination of the legacy solids modelling unit operations and the new models introduced from SolidSim.  Currently 20 solids unit operations are available, representing over 70 different types of equipment.

Solids unit operations available in Aspen Plus V8.4

Figure 1: Solids unit operations available in Aspen Plus V8.4

Also included are the conveying and fluidised bed models and the spray dryer model, and the ability to model reactions in the fluidised bed model. 

Since Aspen Plus has historically focussed on fluids modelling, this enhancement enables the user, without any additional software costs, to model processes that contain both fluids and solids in one simulation environment using consistent physical properties and industry leading optimisation techniques.

One example of this more holistic workflow is the process model of the entire urea production process, shown in figure 2 below. 

Urea is a fundamental ingredient of lawn fertiliser, is used to produce some plastics, and is a component of detergents, and some healthcare products. Since this model describes the upstream urea synthesis (fluid part) and the downstream urea granulation section (solids part), the influence of each part is considered in a rigorous way. 

If, for example, the air flow rate to the fluidised bed coolers in the granulation section of the urea process needs to be increased, this will lead to a higher entrainment of fines from the coolers.

The entrained particles will be removed from the gas stream by the venturi scrubber, dissolved in the wash liquid, and then recycled back to the synthesis section.  Therefore, the change of the air flow rate to the cooler will have an influence on the upstream urea synthesis which will then have an impact on the downstream solids part.

New workflow: Holistic process model of the urea synthesis and granulation in Aspen Plus V8

Figure 2: New workflow: Holistic process model of the urea synthesis and granulation in Aspen Plus V8

To further streamline the workflow, new conceptual models have been introduced with the latest release of Aspen Plus.

The conceptual models are an enhancement of the existing solids blocks and allow users to model solids processing steps at different levels of accuracy from conceptual to rigorous without changing the structure of the flowsheet. With conceptual models, process engineers that are not familiar with solids modelling can be eased into learning how to use the capabilities.

The conceptual model also offers the possibility that process engineers and particle scientists can collaborate more closely. 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.

Having analysed the first simulation results, the process engineer can decide what parts of the solids section need to be modelled more rigorously, and if necessary, ask the particle scientist to help select and parameterise the rigorous model.

The Aspen Plus user environment is surrounded by a suite of integrated products called aspenONE engineering. 

Aspen Plus supports integrated workflows, such as Activated Economics (for estimating capital and operating cost) and Activated Energy Analysis (for pinch analysis), as well as Activated Exchanger Design and Rating (for sizing and rating heat exchangers).

By modelling solids using Aspen Plus, users have access to all of these simultaneous conceptual design features. This provides an environment for fast, collaborative plant and process design.

Incorporating granular solids and the corresponding solids processing steps, with the fluids section, when modelling any chemical process plant has, until now, been a vital missing link.

Modelling the solids section of a process is important for many common processes including metals and mining, food processing, specialty chemicals, agrochemicals, pharmaceuticals, biofuels, and more. 

Aspen Plus V8 incorporating SolidSim addresses all of these challenges, and although it has only recently been available, usage of solids modelling in Aspen Plus has grown at a surprisingly fast rate. Over 140 organisations have started using solids modelling in Aspen Plus.

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