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Modeling Urea Processes: A New Thermodynamic Model and Software Integration Paradigm
(Special Shared Content with Virtual Materials Group)

Software Implementation

Process simulation is a tool that shows its power when widely available to process engineers, allowing them to perform better understand the process, propose changes to the process to fine tune performance based on particular characteristics of their plants, markets and economic situation. Therefore, although a rigorous thermodynamic model of the process is a necessary condition for success, it is not sufficient. Some proprietary urea simulation programs exist (7, 26), but they seem to be available only to a handful of users. Our objective was to combine excellence in science with excellence in software to create a solution, which in turn can be used by a wide audience, made up mostly of process engineers, not thermodynamic specialists. Therefore, a robust implementation of the mathematical model would have to be combined with a robust software implementation to allow physical property calculations to be available over a wide range of software platforms (27). 

Our choice was to create a central core of classes programmed using the C++ programming language (26). This central core implements all the necessary support functions for the creation of physical property systems, and it is very flexible in terms of physical properties it can handle. Complete support for pure component property configuration, interaction parameter configuration, physical property definition and property package definition (which is nothing more than a collection of physical properties, pure components and interaction parameter matrices) is provided. This allows us to create complete property packages with maximum reuse of parts in an extremely efficient and fast manner.

On top of this core set of classes, software interfaces are implemented allowing our property package system to be used by many diverse applications, ranging from a steady state and dynamic simulator such as HYSYS to batch process simulators, operator training simulators and productivity tools such as Excel, Java and Visual Basic. This functionality is shown schematically in Figure 11.

Figure 11: Property Package System Interfaces

This modern approach to software engineering provides immediate benefit to the user. Usually a process engineer will be familiar with a process simulator. By installing our property package system in his or her computer, the user has immediate access to the physical property calculations provided by our physical property system without having to learn how to use a new process simulator. For example, the HYSYS process simulator was built with an open software architecture in mind, and our property package provides an interface, which HYSYS can recognize. From that point on, the user can select our property package system directly from inside the simulator:

Figure 12: Selecting Virtual Materials Group Property Package System from inside the HYSYS Process Simulator

Figure 12 shows what the user of the HYSYS process simulator would see in the basis environment. The basis is the section of the program where thermodynamic models and components are selected for the simulation. Note the Virtual Materials Group Property Package System appears in the Base Property Package Selection scrollable list. With VMG Property Package selected, the form displayed on the right appears, where the user can select the appropriate Virtual Material to represent his or her system of interest as shown in Figure 13.

Figure 13: Selecting a Virtual Material to represent a specific process.

Hardware Like Software

We believe that software to be useful for engineers should behave like a piece of hardware. This has two significant conceptual implications:

1. Software should be able to be plugged into other applications without any intervention from the software vendor. This was explored in the previous item.
2. Users should know how good and how bad the software is when used to simulate his process. Virtual Materials Group performs extensive validations on its property packages and is able to create a "boiler plate" for each property package it produces, thus allowing engineers to know a-priori the performance of a property package. This in turn allows the user to critically evaluate the performance of the property package and interact with Virtual Materials in the event errors are not reported for a specific condition of interest.

Validation of property package systems is a fundamental part of an industrial grade property package system and considerable effort is spent in this feature before a property package is actually released. For example, UREA++ 2.0's boilerplate is reproduced in Table 3.

Table 3:  UREA++ Version 2.0 Boiler Plate – Temperatures (K) and Pressures (kPa)

Putting It All Together

Several different urea production flowsheets were successfully modeled using the physical behavior modeled by Virtual Materials Property Package System and the calculation sequencing provided by AEA's Software Engineering process simulator HYSYS. Engineers can be productive from the start by having access to rigorous thermodynamic calculations while not having to learn a new process simulation tool. Table 4 summarizes the typical errors found when modeling a urea plant using UREA++.

Table 4: Model versus actual plant comparisons

Conclusions and Future Work

A rigorous, general-purpose model for urea production processes was successfully implemented and experimentally verified against actual plant data. The model was implemented using modern software technology, which allows the model to be used in process simulators or other applications such as spreadsheets or operator training software. Currently work is being done in further refining the low and medium pressure thermodynamic models and in the creation of a mass transfer based high-pressure steady state decomposer model.

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By: Marco A. Satyro, Yau-Kun Li, Rajeev K. Agarwal, and Oscar J. Santollani
Virtual Materials Group, Inc., Calgary, Alberta, Canada
Email: Mr. Marco A. Satyro