<p>Molecular
separations are essential in the production of many chemicals and purified
products. Of all the available separation technologies, distillation, which is
a thermally driven process, has been and continues to be one of the most
utilized separation methods in chemical and petrochemical plants. Although
distillation and other commercial technologies fulfilled most of the current
separation needs, the energy-intensive nature of many molecular separations and
the growing concern of reducing CO<sub>2</sub> emissions has led to intense research to
seek for more energy-efficient separation processes.<br></p><p><br></p><p></p>
<p>Among the
emerging separation technologies alternative to distillation, there is special
attention on non-thermally driven methods, such as membranes. The growing
interest in non-thermal methods, and particularly in the use of membranes, has
been influenced significantly from the widespread perception that they have a
potential to be markedly less energy-intensive than thermal methods such as
distillation. Even though many publications claim that membranes are more
energy-efficient than distillation, except for water desalination, the relative
energy intensity between these processes in the separation of chemical mixtures
has not been deeply studied in the literature. One of the objectives of this
work focuses on introducing a framework for comparative analysis of the energy
intensity of membranes and distillation. </p><p><br></p>
<p>A complication
generally encountered when comparing the energy consumption of membranes
against an alternative process is that often the purity and recovery that can
be achieved through a single membrane stage is limited. While using a
multi-stage membrane process is a plausible solution to achieve both high
purity and recovery, even for a simple binary separation, finding the most
suitable multistage membrane process is a difficult task. This is because, for
a given separation, there exists multiple cascades that fulfill the separation
requirements but consume different amounts of energy. Moreover, the energy
requirement of each cascade depends on the operating conditions. The first part
of this work is dedicated to the development of a Mixed Integer Non-linear
Program (MINLP) which allows for a given gaseous or liquid binary separation,
finding the most energy-efficient membrane cascade. The permeator model, which
is derived from a combination of the cross-flow model and the solution
diffusion theory, and is originally expressed as a differential-algebraic
equation (DAE) system, was integrated analytically before being incorporated in
the optimization framework. This is in contrast to the common practice in the
literature, where the DAE system is solved using various discretization
techniques. Since many of the constraints have a non-convex nature, local
solvers could get trapped in higher energy suboptimal solutions. While an
option to overcome this limitation is to use a global solver such as BARON, it
fails to solve the MINLP to the desired optimality in a reasonable amount of
time for most of the cases. For this reason, we derive additional cuts to the
problem by exploiting the mathematical properties of the governing equations
and from physical insights. Through numerical examples, we demonstrate that the
additional cuts aid BARON in expediting the convergence of branch-and-bound and
solve the MINLP within 5%-optimality in all the cases tested in this work.</p><p><br></p>
<p>The proposed
optimization model allows identifying membrane cascades with enhanced energy
efficiency that could be potentially used for existing or new separations. In
addition, it allows to compare the optimum energy consumption of a multistage
membrane process against alternative separations methods and aid in the
decision of whether or not to use a membrane system. Nevertheless, it should be
noted that when a membrane process or any other non-thermal separation process
is compared with a thermal process such as distillation, an additional
complication often arises because these processes usually use different types
of energies. Non-thermal processes, such as membranes, consume electrical
energy as work, whereas thermal processes, such as distillations, usually
consume heat, which is available in a wide range of temperatures. Furthermore,
the amount of fuel consumed by a separation process strongly depends on how its
supplied energy is produced, and how it is energy integrated with the rest of
the plant. Unfortunately, common approaches employed to compare the energy
required by thermal and non-thermal methods often lead to incorrect conclusions
and have driven to the flawed perception that thermal methods are inherently
more energy-intensive than non-thermal counterparts. In the second part of this
work, we develop a consistent framework that enables a proper comparison of the
energy consumption between processes that are driven by thermal and non-thermal
energy (electrical energy). Using this framework, we refute the general
perception that thermal separation processes are necessarily the most
energy-intensive and conclusively show that in several industrially important
separations, distillation processes consume remarkably lower fuel than non-thermal
membrane alternatives, which have often been touted as more energy efficient.</p><p><br></p>
<p>In order to
gain more understanding of the conditions where membranes or distillation are
more energy-efficient, we carried out a comprehensive analysis of the energy
consumed by these two processes under different operating conditions. The
introduced energy comparison analysis was applied to two important separation
examples; the separation of p-xylene/o-xylene, and propylene/propane. Our
results showed that distillation is more energy favored than membranes when the
target purity and recovery of the most volatile (resp. most permeable)
component in the distillate (resp. permeate) are high, and particularly when
the feed is not too concentrated in the most volatile (resp. most permeable)
component. On the other hand, when both the recovery and purity of the most
volatile (resp. most permeable) component are required at moderate levels, and
particularly when the feed is highly enriched in the most volatile (resp. most
permeable) component, membranes show potential to save energy as compared to
distillation.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/13072898 |
| Date | 16 December 2020 |
| Creators | Jose Adrian Chavez Velasco (9503810) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/COMPREHENSIVE_STUDY_OF_THE_ENERGY_CONSUMPTION_OF_MEMBRANES_AND_DISTILLATION/13072898 |
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