(263c) Thermodynamic Models As CAPE OPEN Equilibrium Calculators

Thermodynamic
models as CAPE OPEN equilibrium calculators

C. G. Pernalete ¹, J. M. Contreras ¹, R. E.
López ^1,2, A. Nagel ^1,3

¹Gerencia Técnica de
Refinación. PDVSA Intevep. Venezuela.

² Departamento de
Ingeniería Química. Vice-Rectorado de Barquisimeto. Universidad Nacional
Experimental Politécnica ?Antonio José de Sucre?. Venezuela.

³ Departamento de
Ingeniería Química. Universidad Metropolitana. Venezuela.

Abstract

Nowadays, software technology for process simulation has been developed
in such an important way that they have become a fundamental tool for the correct
operation of several processes, specially in the oil field. The use of this
technology had reached a level that enhances human work or indeed substitutes it,
improving its efficiency and decreasing the time and effort to carry on
activities like process monitoring, control and optimization.  In a daily basis, the amount of commercial
proposals available in the market for tools to process simulation is
increasing.

One of the main strengths of most commercial process simulators is their
flexibility to make different kinds of thermodynamic calculations. In the oil
industry the equation-of-state approach (EOS) has had an important role in
thermodynamic modeling of  vapor - liquid
equilibrium (VLE) for hydrocarbons. However, these models do not offer good results
under certain conditions, like under the presence of hydrogen or at high
temperature or pressure conditions, making the system far from an ideal
situation. In these cases it is necessary to use special models like the
correlation proposed by Chao and Seader in 1961, and the modification proposed
by Grayson and Streed in 1963 or the modification made by Chorng H. Twu in 1995
to the Peng-Robinson equation of state.

In spite of all the benefits and advantages that offer the use of
commercial software technology for chemical process simulation, from a
practical point of view there are some drawbacks. Commercial proposals for
process simulation require a license that in most cases is expensive and must
be renewed every year. If there is no renewal or purchase of this license it is
not possible to develop new simulation projects or to have access to models
previously built. This situation generates a problem of technological
dependence.

It is important to point out that, chemical process simulators are
software tools that provide a set of functions such as physical and
thermodynamic property databases, thermodynamic calculations, numerical
methods, unit operation and kinetic models. At the beginning, process
simulators combined these functions with a programming paradigm where there
were not well defined boundaries among these functions.  The current trend is to have a process
simulator formed as a group of software components that interact or
interoperate simultaneously, where each component have well defined duties.

Since 2001, the CO-LaN organization has promoted the creation of the
CAPE-OPEN standards, where a set of free-to-use non-proprietary rules and
interfaces that allow CAPE (Computer-Aided
Process Engineering) applications or components to interoperate, are defined.
Currently, most process simulators are somehow compliant with this standard;
however, COCO (CAPE OPEN to CAPE
OPEN) simulator is the CAPE OPEN
compliant reference tool. COCO simulator is a
non-commercial software for process simulation that was fully designed inspired
by this standard.

According to the philosophy proposed in the CAPE OPEN
standards, in the architecture for process modeling tools in steady state, there
must be a ?Process Modeling Executive? (PME), which is the component
responsible for the building of the entire model and execute the overall
calculations to solve the system of equations. In order to meet this goal, the
PME must communicate with the other components that describe each unit
operation or other calculations like thermodynamic equilibrium, specialized
algorithms for solving the entire system or physical properties determination.
These components are called ?Process Modeling Components? (PMC) since they
?wrap? specific tasks in a well defined software unit. PMC's can be implemented
as independent software components but able to communicate among themselves.

In this work, we have implemented some thermodynamic methods, as CAPE OPEN
compliant components: process modeling components following the aforementioned
strategy. The idea is to have these components available for using in
non-commercial process simulation tools in order to solve specific problems for
the oil industry. We have developed three components that implement the
following methods: Chao-Seader (1961), Grayson Streed (1963) and Peng Robinson
? Twu (1995). Specifically, we have developed them as equilibrium calculators that
could be used in a property package as described in the 1.1 version of the
standard for physical and thermodynamic properties.

In this study the component TEA (Thermodynamic for engineering
application) was used as the property package. TEA is a component conceived as
a property package manager that comes with the distribution of COCO simulator. TEA is the only property package
available as non-commercial software on the network that is able to deal with
external CAPE OPEN equilibrium calculator
components.  Otherwise it would have been
necessary to create a property package and this study would have considered the
calculation of chemical-physical properties in addition to equilibrium
calculations.

In that sense, as a first step we described the original thermodynamic
models considered in this work, including the temperature, pressure and
compositions constraints associated to each model. After that we designed the
algorithms of each model using UML (Unified Modeling Language). The
implementation of the algorithms was made using C++ as programming language
taking into account the interoperability standards. As a result, we achieved
three interoperable thermodynamic models as CAPE OPEN
equilibrium calculators that could be used in commercial and non-commercial
process simulation tools, even though the simulator implements the standard.

For testing and validation of the developed components we used COCO simulator process modeling environment. In first
place, we tested the validity of the models through the comparison of its
performance in predicting Vapor-Liquid Equilibriums (VLE) in systems composed
only by pure components. Secondly, it was selected an experimental system
composed by petroleum fractions and whose modeling is made through the use of
pseudo compounds. It is important to clarify that temperature and pressure conditions
of those systems were within the constraints established for the original
models.  Finally, a comparison between
the results obtained with the developed models and the results showed by a
commercial simulation tool that consider the models in study, was made.

We found that the developed models are able to predict with good
agreement experimental data about VLE used in the validation step, both in case
of pure compounds and in case of petroleum fractions. Moreover, we found that
the error prediction of the commercial software were always similar or larger
than the reported by the in-house models.

Finally, we noticed a robust performance of each one of the software
components of COCO simulator, specially TEA which
was responsible for establishing the communication with the equilibrium
calculators developed.