As discussed in PEP Review 2001-2, the engineering aspects of process design
attempt to maximize the economic impact and define the best possible process.
However, the most underrated aspect of such efforts is the determination of
the underlying physical properties used to define the process. Where the previous
review focused on gas solubility, this review examines the impact of vapor-liquid
equilibria (VLE) on process design.
As with gas solubility, early VLE assumptions tend to be defined by a favored
property system with ideal gas behavior unless there is a priori knowledge
of the system. When pressed for answers from simulation results, an engineer
rarely has the luxury of researching the strengths and weaknesses of alternate
property models, nor to evaluate the depth and breadth of parameters available
for the chosen property model.
For an existing process, that does not necessarily pose a problem, since
some effort will typically be expended on such issues during the development
and commercialization of the process. However, inadequate property studies
for existing processes and the potential complete lack of appropriate property
information for new processes do present difficulties to the engineer. The
consequences are illustrated using same ammoximation process from the previous
review employing ammonia gas and aqueous hydrogen peroxide to convert cyclohexanone
to cyclohexanone oxime.
In the previous review, ammonia was treated as a Henry's Law component in
one case and as a normal component for VLE in the next case. The Henry's Law
case handles the ammonia easily, while the VLE case struggles with unusual
behavior from the ammonia. The cases are repeated in this review to emphasize
implications on the t-butanol distillation column. Since ammonia is not supercritical
at the process conditions, the VLE route should technically be favored. In
both cases, too much water is sent to the distillate, exposing the cyclohexanone
oxime to excessive heat. A third case explores the t-butanol / water binary
and finds better parameters for simulation and correcting the azeotrope position,
but the case fails to solve the ammonia problem. The final case suppresses
ammonia-containing binaries to circumvent erroneous parameters caused by simplistic
assumptions in group contribution estimation methods.
This review is intended to show the problems and limitations with physical
property data and the impact on simulation results, as well as the subsequent
process design.
By Peter D. Pavlechko
