A model is a mathematical representation of a chemical process. A process consists of chemical components being heated, cooled, mixed, or separated by unit operations. These components are transferred from unit to unit through process streams. The model is a heat and material balance, while mass and energy are not created or lost.

There are several types of models as described below.

Types of models

Classification after content

A chemical process is defined as a Unit operation, Process Equipment or a Process Area.

• A Unit Operation is the basic step in a process. A process may have many unit operations to obtain the desired product.
• A Process Equipment is a single unit operation or a collection of unit operations and is, physically, a single, self-contained entity.
• A Process Area is a collection of process equipments and can be a subset of an entire process or the entire process. Process areas are areas in a manufacturing process defined either by responsibility or function. Normally, process areas have a distinct function in the production of a product.

A model is then a single unit operation, a collection of unit operations or a collection of process equipments. A model can then be further defined as a collection of process areas.

Classification after the way they are specified

• Parameterized Models
  • Fundamental Models (specified directly through their function Y=F(X))
  • Composite Models (specified as a combination of other composite and/or fundamental models)
• Case Models

The chemical engineer designs a new process model in two situations:

• A new chemical plant is needed, with given constraints and functionality. Before constructing it, a model must be created and tested.
• A chemical plant already exists, but its model is needed, for simulations and other purposes.

The designing of a complex and large model is usually approached by using the divide and conquer method: the process is divided into smaller parts (e.g. process areas or unit operations), those parts are modeled and tested separately, and afterwards linked together into the final model. Since many times the same process areas or unit operations appear in many cases, there are situations where the same smaller model is used in building a larger one. Therefore, it makes sense to define Parameterized Models, which are usable in defining other models.

A Parameterized Model is a model that has some of its inputs undefined. Those inputs are defined on a case by case basis, later, when the model is used. Its solely purpose is to be used as a building block for other models.

A Composite Model is a Parameterized Model that is specified as a network of Fundamental Models and/or other Composite Models. The chemical engineer will use the fundamental models that she defined (or were previously defined by others) by importing them into his composed, larger model.  

Since some of its inputs are undefined, the model cannot be simulated standalone. The final purpose of chemical engineering design is to obtain a fully defined model that can be simulated. This model is constructed by using parameterized models in its structure, that have their attributes defined depending on the respective case. Such a model is called a Case Model. Since Case Models inputs are fully specified, it can be simulated standalone.

The design goal is complete when the chemical engineer has obtained a high level model with the requested constraints and functionality for its corresponding chemical process.

 

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