J. Chem. Eng.

Data 1996, 41, 947-950 947

The Process Simulation Revolution: Thermophysical Property

Needs and Concerns

C. L. Rhodes
Union Carbide Corporation, P.O. Box 8361, South Charleston, West Virginia 25303-0361

Chemical process simulators have undergone tremendous development in recent years. The new
capabilities of these tools have changed the way chemical companies approach many aspects of process
design and development, including thermophysical properties. What are the important thermophysical
property needs in process simulation remaining to be addressed? What weaknesses in the application of
thermophysical properties should users of process simulators be concerned about? Will chemical process
simulators become the primary stewards of thermophysical property technology for the chemical industry?
How can expertise in the thermophysical property area best be applied to optimize the impact on process
design, development, and operation?

Introduction with dedicated skill groups and a reduction in the staffing

Over the last decade or so, the chemical process simula- of those remaining. A number of companies have recently
tor has evolved from many primarily proprietary programs determined that some of these reductions were overopti-
with dedicated development and maintenance staffs at mistic and are now reconstituting or “beefing up” such skill
major chemical and petroleum companies to a few com- groups, but the “steady-state” level is not yet apparent and
mercial products licensed by software companies and may well be a characteristic of each corporation’s personal-
accepted nearly universally by industry. During this time, ity based on their product lines and other competitive
advances in computation technology have greatly expanded advantages.
the range of applications as well as the ease of use. Where As already implied, developments in process simulation
once a few specially trained individuals labored over fixed have had a great impact on the thermophysical property
format input records to simulate a single column or reactor, skill area. Early process simulators had no built-in data-
nearly all process engineers now have the capability to bases of pure component data or multicomponent interac-
simulate whole chemical processes or plants through a tion parameters, so that inclusion of such data in the input
graphical interface on their desktop. to each process simulation was a requirement. In this era,
Clearly, the majority of the chemical and petroleum it was clear that a staff devoted to collection and develop-
industry has determined that development of process ment of thermophysical property data was an integral part
simulation tools is not one of the critical skills which can of any process simulation effort in the chemical industry.
be cost-effectively maintained within each corporation. In many cases, in fact, it was impossible to get any result
Indeed, the continuing rapid development of the com- from a process simulator without considered thought and
mercial simulators seems to confirm the validity of this some degree of expertise in the area of thermophysical
choice. A number of related skill areas, including the field properties. Partly because this level of effort is inconsistent
of thermophysical properties, are now being reevaluated with the vision of plant-scale simulation on every engineer’s
in light of the developments in process simulation. The desktop, all major process simulators now make it possible
purpose of this paper is to highlight some of the needs and to get a simulation result with very little or no consider-
concerns in the area of thermophysical properties related ation of the system’s thermodynamic or physical properties.
to the process simulation revolution as viewed from an While increased user-friendliness is clearly a step in the
evolving thermophysical property skill center in the chemi- right direction, it is a two-edged sword which could
cal industry. potentially be combined with clever (or inept) marketing
Another skill area whose evolution has impacted the and superficial engineering knowledge to completely dis-
application of thermophysical properties in process simula- count the truly critical impact of an understanding of
tors is the collection, evaluation, and compilation of ther- thermophysical properties on process simulation and de-
mophysical property data and prediction methods. Much sign. No quantity of user-friendly forms and menus can
of this work has been determined to be of common interest adequately substitute for expertise in the application of
to the chemical industry but has been administered thermophysical properties and an understanding of the
through industrial consortia and university-based research chemical process being simulated. The questions which
efforts, often with some governmental support rather than users of process simulators must consider include what
through direct commercial competition, as in the case of level of centralized expertise in thermophysical properties
the process simulators. These developments would, them- is optimal and how is this expertise best used to optimize
selves, be best addressed as a separate issue, but their the impact on process simulation and design.
impact is coincident with that of the process simulation
Process Simulation Tools
revolution on thermophysical properties. The product
databases of many of these efforts are available directly Competition between the major vendors of chemical
or as an optional add-on feature of most process simulators. process simulators has resulted in some truly extraordinary
The combined result of these and other effects on thermo- tools which are remarkably similar in many respects. This
physical property skill centers in the chemical industry has section is devoted to acknowledging the most significant
been a significant decrease in the number of corporations advances in process simulation directly related to thermo-

S0021-9568(96)00029-5 CCC: $12.00 © 1996 American Chemical Society

948 Journal of Chemical and Engineering Data, Vol. 41, No. 5, 1996

physical properties and their application in the chemical such as residue curve maps make it possible to understand
industry. The emphasis here is more on trends than on the thermodynamic implications of azeotropic systems.
specific features of any of the individual simulators, since ‚Incorporation of alternate property options. For those
they are similar in many ways and appear to be moving in corporations with thermodynamic models from older simu-
the same direction. lators or who wish to use methods not yet incorporated in
‚Pure component databases. The major simulators now the simulator, the ability to incorporate alternate methods
have pure component databases with critically reviewed to calculate individual thermophysical properties is an
data and parameters for well over 1000 compounds of invaluable tool.
industrial interest.
‚Binary interaction databases. Direct access to binary Development Needs
and multicomponent phase equilibrium data collections is Here are some areas in which additional development
a common add-on option and databases of binary interac- work could significantly aid users in the application of
tion parameters derived from such data are included in thermophysical properties in process simulators:
some cases. In some cases, separate activity coefficient ‚Incorporate new thermodynamic model developments.
parameters are provided for each vapor phase model, As new methods are developed, tested, and accepted, they
enhancing the applicability of these models. must continue to be incorporated into the simulators.
‚Data regression. Regression of pure component param- ‚Make prediction more flexible (controllable). In cases
eters and binary interactions to fit experimental data where additional knowledge of the compounds or conditions
allows easier incorporation of additional components and of interest could be used to guide the estimation process, a
binary systems. flexible framework allowing easier user-guidance would be
‚Expanded suite of thermodynamic options. Inclusion helpful. One possible step would be an optional “expert
of additional and more flexible thermodynamic options has prediction mode” which interacts with the user in the
made it easier to adapt thermodynamic models developed selection and application of prediction methods. This
for other simulators. Inclusion of the latest equation of would make the options more obvious and allow for user
state options including extended R functions, asymmetric override where appropriate.
interactions, etc. has made it easier to model many ‚Ease access and archival of property parameters. Pa-
systems with greater accuracy. rameters estimated or extracted from databases should be
‚Flexible combination of thermodynamic options. The available for inspection (optionally) along with details of
ability to mix and match parts of various thermodynamic the estimation procedure where appropriate. In addition,
options into a customized model allows the design of a the ability to archive all thermophysical property param-
thermodynamic model based on the data available and eters used in a simulation for future use ensures reproduc-
calculations desired for a given system. ibility (especially when databases change from version to
version of the simulator).
‚Pure component property estimation. In cases where
‚Simulate polymer systems. There is currently a lot of
only rough estimates of certain properties are required, the
development work in this area, but little consensus on the
ability to easily obtain estimated parameters from struc-
best models. As the best models are identified, thermo-
ture is a useful tool.
dynamic methods and unit operations for polymer systems
‚Binary interactions from structure. Inclusion of predic- should become more readily available in process simulators.
tion methods such as UNIFAC have made it easier to do
‚Do more data visualization. These tools are useful for
preliminary screening studies for some systems. In some
analysis by thermophysical property experts and instruc-
cases, UNIFAC estimates can be used to fill in gaps in the
tion of other simulator users. Development and inclusion
interaction matrices of other activity coefficient models
of more such tools is an important goal.
automatically!
‚Help gauge the impact of thermophysical property
‚Electrolyte equilibria. All of the major simulators now choices. The impact of the thermophysical property options
include methods to model the complex interactions in and parameters needs to be made more visible to users.
electrolyte systems. Additional automated features and Perhaps this could be done through multiple simulations
inclusion of interaction databases are making these sys- with different options and/or some random perturbation
tems easier to model. of property parameters to indicate the true range of
‚Reactive distillation. There is no way to adjust a variability.
traditional thermodynamic model to properly account for
a column which does not maintain a component mass Concerns
balance. The ability to model reactions and phase equi-
Many of the drawbacks of the process simulators with
libria simultaneously has extended the use of thermody-
respect to thermophysical properties are direct results of
namic models to new types of systems.
the advances. Some could be addressed (at least partially)
‚Mass-transfer limited distillation. A more rigorous through modified implementation, while others can only
approach to tray efficiencies makes the underlying ther- be addressed through education of the user community. It
modynamic model more useful and extends the under- is much too easy to blame the tool rather than the
standing of how many systems work. It also requires a uninformed or naive user, but increasing the level of user
better understanding of some other physical properties, sophistication should be a goal of the vendor and the
however, placing new emphasis on the estimation and licensee. Here are some concerns about the application of
correlation of transport properties. thermophysical properties in current and future process
‚Property table and plot generation. The ability to simulators. Efforts to alleviate and/or avoid these problems
generate tables and plots of thermophysical property data would do a service to the process simulation community
makes it easier to understand and to debug as well as to and the chemical process industry.
take advantage of unique features in some systems. ‚Can simulation be too easy? When a process can be
‚Data visualization. Binary and ternary equilibrium simulated by graphically placing and connecting process
plots make it easier to prepare, analyze, and evaluate units and responding to a small number of forms windows,
thermodynamic models. Newer data visualization tools the number of assumptions and simplifications made along
 Journal of Chemical and Engineering Data, Vol. 41, No. 5, 1996 949
the way is less obvious than when the user was forced to In order to maximize the value of the thermophysical
think about each of these individually. This can easily property skill area as a corporate asset, application ther-
result in overconfidence in the validity of the default modynamicists will have to spend a higher percentage of
assumptions and in the quality of the simulation results. time on
‚A bad answer can be worse than no answer at all! There ‚educating the process simulator user community on the
seems to have been a lot of effort put into maximizing the usage and limitations of the thermophysical options present
likelihood of arriving at a converged simulation result while in the simulators,
minimizing the user interaction. Simulator users and ‚working with the skeptical to resolve questions about
developers alike need to be more aware that it is not simulation results,
obtaining an answer which is important but obtaining the
‚developing thermodynamic models which take advan-
correct answer (or a close approximation). For example,
tage of the built-in options of the simulators and incorpo-
no simulator should give a converged solution for a liquid/
rate additional knowledge required for specific applications,
liquid extractor modeled with the Wilson activity coefficient
model (which does not allow the coexistence of two liquid ‚evaluating new thermodynamic models, correlations,
phases). False solutions and misidentified phases in high- and prediction methods in the role of corporate technology
temperature, high-pressure flashes using the equation of steward,
state are also problematic. ‚working with experimental thermodynamicists to en-
‚Beware of limitations. Commercial chemical process sure that experiments are designed to yield the optimal
simulators are powerful tools with many capabilities, but amount of additional information required for process
there are limitations. The developers at all the major simulation applications,
simulator companies agree that neither their product or ‚cooperating with universities, government agencies, and
their help line is an adequate substitute for expertise in industrial partners in consortia organized to pursue the
thermophysical properties or a number of other skill areas. more general common interests which are most cost-
Somewhere in marketing-management interchange and/ effectively addressed through mutual effort,
or user training, this is too often de-emphasized. We must ‚determining the level of complexity and effort required
all work to make sure expectations are more in line with for each model based on the intended application, and
the capabilities of the tools. ‚consulting directly with users to solve problems and
‚Application must remain a responsibility of the user. answer questions which require detailed understanding of
Keep in mind that the goal of the simulator vendor is to thermophysical properties and their application.
develop and market simulation tools, while the goal of a Correspondingly, less time will be spent on
chemical company is to develop and market chemical
‚searching literature for experimental data of all types,
products. Since the ultimate responsibility for operation
of the chemical plant lies with the chemical company, it ‚regressing pure component correlation parameters and
must ensure that both thermophysical properties and the binary interactions,
simulation tool are properly applied toward this end. In ‚estimating thermophysical property data and param-
addition, experience gained through application of ther- eters,
mophysical properties and simulation tools becomes a ‚compiling data and correlation parameters,
corporate asset which can enhance the quality and ef- ‚preparing tables and plots of thermophysical properties
ficiency of future applications. for users, and
‚Think about it! There is no substitute for a skeptical ‚programming new thermodynamic models for use in
user. A user who spent days or weeks putting together a process simulators.
model of a process perhaps felt obligated to spend at least While all of the items on the latter list remain important,
a few minutes thinking about the results to see if they more responsibility for some must be transferred to users,
made sense, but this effort may seem less justifiable when simulator vendors, and consortia, while others will be made
the model is put together in minutes or hours. If anything, more efficient through more advanced tools. The net result
the opposite should be true, since the user has spent less is that the thermophysical property skill area will be much
time developing an understanding of the system in the more interactive with process engineers and others inside
latter case. Users should be encouraged to review all and outside the corporation. Because of this, the duties
simulation results with a skeptical eye and get a second will be more diverse, less routine or predictable, and even
opinion when there is any doubt. more challenging. The process simulation revolution has
The Role of the Thermodynamicist helped to make thermophysical properties more accessible
to everyone and irreversibly changed the role of the
Until there are no new chemical processes and products thermophysical property specialist.
or all pure component and mixture properties can be
accurately determined from chemical structure alone, there Conclusions
will be some need for engineers skilled in the thermophysi-
cal property area. The availability of pure component and Many developments in chemical process simulators have
binary phase equilibrium databases does not invalidate the made it easier to apply thermophysical properties to
role of the experimental thermodynamicist. In some process design, development, and operation. There are
respects, this job becomes more difficult as the “easy” still, however, a number of development needs and areas
measurements are made and those which were previously of concern which have been identified. One of the continu-
difficult or impossible remain. Similarly, the role of the ing roles of thermodynamicists will be to address these
thermophysical property specialist in process simulation needs and concerns within the process simulator vendors.
applications is becoming less routine. While it may have Collection and evaluation of thermophysical property
been common in the past to spend weeks or months data are increasingly becoming centralized as part of large
collecting data and regressing parameters for each process cooperative efforts with industrial sponsorship. Thermo-
thermodynamic model, this is now rarely necessary and dynamicists will continue to play primary roles in these
justifiable. In the future, the primary role will be that of important efforts as well as experimental measurement
a problem solver/consultant/skill resource. and the development and evaluation of prediction methods.
950 Journal of Chemical and Engineering Data, Vol. 41, No. 5, 1996

With better thermophysical property data and methods oversight of consortia and communication with simulator
more accessible to process engineers through chemical vendors are increasingly important.
process simulators, the role of thermophysical property
specialists in the chemical industry will be more focused Received for review January 27, 1996. Accepted June 26, 1996.X
on experimental measurement and applications specific to
JE960029B
the needs of the individual chemical company. Accordingly,
support and education of the user community as well as X Abstract published in Advance ACS Abstracts, August 1, 1996.