Estimation of xylenes adsorption selectivities
Impact on the p-xylene Simulated Moving Bed separation process
Vasco Barata Monteiro
Dissertação para obtenção do grau de Mestre em
Engenharia Química
Júri:
Presidente: Professor Sebastião Manuel Alves
Orientadores: Professor Carlos Manuel Henriques (Instituto Superior Técnico)
Doutora Catherine Laroche (Institut Français du Pétrole)
Vogais: Professora Maria Filipa Ribeiro
Outubro de 2010
Acknowledgments
The first acknowledgment must go to Prof. Ramôa Ribeiro, without whom this journey at IFP
would not be possible.
This work was also made possible by the support and assistance of a number of people of the
Catalysis and Separation Division, particularly from the Separation section whom I would like to
express my deepest gratitude.
To Denis Guillaume, director of the Catalysis and Separation Division, and to Alain Methivier,
responsible for the Separation department, for receiving me in this Institution.
To the people with whom I worked most closely. Aurelie Marsallon and Morgane Josserand,
the Technicians who helped me the most when the equipment would not let me work during my
laboratory adventures. My special gratitude to my IFP supervisor Catherine Laroche, for all the
support and guidance during the internship.
To Marco Aleixo and Anthony Tanguy for their priceless help in my integration within the
separation department during all the six months as well as to my office colleagues, thank you all.
I would also like to thank to all the Portuguese people from IFP, the "Thesards" and Victor
Costa, for all their help throughout this French adventure. To my felon companions from IST who
shared with me this great life experience. Mouro, Ferrão, Maria, Tiago, Saturnino, Bruno Santos,
without you this would never have been possible.
Finally, I dedicate some special words to my family and my dear girlfriend. You are the
foundations that provide balance in everything I do in my life. Thank you for all the support.
II
Resumo
A maioria dos xilenos com aplicação na indústria petroquímica, são utilizados na produção de
para-xileno o qual, considerando os diferentes isómeros da mistura, é o mais produzido para ser
consumido na cadeia de poliéster, principalmente na produção de fibras.
No âmbito da separação de para-xileno por tecnologia de leite móvel simulado (SMB), tornase importante melhorar as ferramentas existentes utilizadas no estudo dos diferentes aspectos que
afectam o seu desempenho global, a fim de alcançar uma melhor compreensão do processo.
Com este trabalho pretende-se estudar o impacto de um novo modelo de adsorção na
performance do processo SMB. Para isso, foi desenvolvida uma metodologia baseada num esquema
iterativo entre trabalho laboratorial (regressão a partir dos dados experimentais) e simulações
computacionais (implementar o modelo no simulador do processo) com o fim de descrever a variação
real das diferentes selectividades ao longo da coluna de separação utilizando dois tipos de
adsorbantes (FAU1 e FAU2). Os modelos desenvolvidos são válidos unicamente para os sólidos
estudados e na gama de concentrações do processo SMB.
As regressões a partir dos dados experimentais foram obtidas com o auxílio de um software
estatístico (DESIGN EXPERT), calculando e fornecendo como dados de saída vários coeficientes
associados às concentrações dos diferentes isómeros para serem utilizados nas equações das
selectividades.
Para proceder à validação dos modelos, foi feita uma comparação entre os perfis de
concentração das espécies implicadas no processo utilizando o simulador e os obtidos a partir da
planta piloto existente no IFP. Foi observado que utilizando os novos modelos obteve-se uma boa
previsão do comportamento da planta piloto. Contudo, os resultados mostraram que podem ser feitos
alguns melhoramentos no modelo baseados na escolha das diferentes selectividades que são
directamente calculadas resolvendo as equações implicadas.
Palavras-chave: PX, SMB, selectivities, model, simulation, FAU1, FAU2
III
Abstract
The majority of xylenes dedicated to petrochemicals use are used to produce PX which is,
among the different isomers of the mixed xylenes, the most produced one to be consumed in the
polyester chain, mainly in the production of fiber.
In the scope of the p-xylene separation by Simulated Moving Bed Technology (SMB) it is
important to improve the existing tools to study the different aspects that will affect its overall
performance in order to achieve a better understanding of the process.
This work intended to study the impact of a new thermodynamic model for the xylenes
adsorption on the SMB process. For that, was developed a new methodology based on an iteration
scheme between the laboratory (using experimental data regression) and simulation work to be able
of describing the real xylenes selectivities variation along the separation column for two types of
adsorbents ( FAU1 and FAU2). Those models are only valid for the specific solids studied and in the
concentrations range of the SMB process.
For the data regression was used an auxiliary software ( DESIGN EXPERT) that gave as an
output a series of coefficients associated to the xylenes concentration and to be used in the different
selectivities equations.
To validate the models, a comparison between the SMB simulator and the pilot plant
concentration profiles was established and a good prediction of the process behavior was observed.
However, the results have shown that significant model improvements can be made based on the
choice of the different selectivities that are directly calculated by solving the model.
Keywords: PX, SMB, selectivities, model, simulation, FAU1, FAU2
IV
TABLE OF CONTENTS
ACKNOWLEDGMENTS....................................................................................................................II
RESUMO............................................................................................................................................. III
ABSTRACT ........................................................................................................................................ IV
NOMENCLATURE AND ABBREVIATIONS ............................................................................ VIII
INTRODUCTION .................................................................................................................................1
BIBLIOGRAPHIC STUDY..................................................................................................................2
2.1
2.2
2.2.1
2.2.2
2.2.3
2.3
2.3.1
2.3.2
2.3.3
2.4
INTRODUCTION .............................................................................................................................2
P-XYLENE BACKGROUND .............................................................................................................2
CHEMICAL AND PHYSICAL PROFILE ............................................................................................2
USES & GLOBAL MARKET ...........................................................................................................3
PRODUCTION TECHNOLOGY ........................................................................................................7
FAU ZEOLITES IN P-XYLENE PRODUCTION ..............................................................................12
STRUCTURE ...............................................................................................................................12
NONFRAMEWORK CATIONS .......................................................................................................14
SELECTIVITY ..............................................................................................................................16
TRUE MOVING BED ( TMB) & SIMULATED MOVING BED ( SMB) ............................................17
EXPERIMENTAL PART...................................................................................................................17
WORK METHODOLOGY ................................................................................................................17
4.1.
4.2.
4.3.
MODEL PARAMETERS ................................................................................................................18
GENERAL MODELLING PROCEDURE ........................................................................................18
OPTIMIZATION STRATEGY ........................................................................................................18
RESULTS AND DISCUSSION ..........................................................................................................19
V
5.1.
5.2.
5.3.
5.4.
5.5.
THREE COMPONENTS APPROACH .............................................................................................19
FOUR COMPONENTS APPROACH ................................................................................................19
IMPACT OF EB IN THE ADSORPTION OF XYLENES ON FAU1...................................................19
NEW ADSORPTION MODELS ANALYSIS - FAU1 AND FAU2......................................................19
COMPARISON OF THE NEW MODEL AND PILOT PLANT CONCENTRATION PROFILES .............20
CONCLUSIONS AND PERSPECTIVES .........................................................................................20
REFERENCES ....................................................................................................................................21
VI
List of Tables
Table 1. Physical properties of xylenes and EB. ........................................................................3
Table 2. Anual average growth rate between 2002 and 2008. ...................................................6
Table 3. Parex, Aromax and Eluxyl 2004 commercialisation data[11] [12].................................11
List of Figures
Figure 1. Molecular structure of the different Xylenes [1].........................................................2
Figure 2. Production of PET with PX as raw material. ..............................................................4
Figure 3. Aromatics and derivative flow scheme[8]....................................................................4
Figure 4 Major applications for PX in 2002[7]............................................................................5
Figure 5 Evolution of world capacity in different regions of the world since 2002[4] [5] [6] [7]. ...5
figure 6. Total Capacity for production PX in 2008 and the Regions share[4]. ..........................6
Figure 7 Capacity and demand for different regions of the world in 2008[4]. ............................6
Figure 8 Aromatics complex highlighting the xylenes separation unit ( adaptation from [8])..............8
Figure 9 Continuous Countercurrent adsorption (Principle of true moving bed process, TMB).
....................................................................................................................................................9
Figure 10 principle of the simulated moving bed process (SMB)............................................10
Figure 11 Simplified scheme of the PX production loop[13].....................................................11
Figure 12 Idealized presentation of zeolite pellet as used in PX separation. ...........................13
Figure 13 FAU framework showing the building units of the structure. .................................14
Figure 14 FAU framework with the principal cationic sites indicated.....................................15
VII
Nomenclature and abbreviations
Abbreviations
PX - P-Xylene
EB – Ehtylbenzene
OX – O-Xylene
MX – M-Xylene
PET- Polyethylene terephthalate
TA- Terephthalic Acid
m.t.- Metric Tons
TMB- True Moving Bed
SMB- Simulated Moving Bed
BTX- Benzene, Toluene, Xylene
FAU- Faujasite
Nomenclature
α A / B - Selectivity between the component A and component B
X A - Mole fraction of component A in the adsorbed phase at equilibrium.
YA - Mole fraction of component A in the fluid phase at equilibrium.
VIII
Introduction
The petrochemical industry of BTX (benzene, toluene and xylenes) has experienced an important
economic development as a result of the constant increase of the uses of p-xylene. P-xylene is the largest
volume isomer of the mixed xylenes, being also the most produced one, mainly to be consumed in the
polyester chain as a starting material for the synthesis of poly(ethylene terephthalate) ( PET).
The simulated moving bed has been extensively used for forty years as the preferred technology
for liquid adsorptive industrial separations. Among the different applications of this technology the pxylene recovery from C8-aromatic cut is the far most known. It is based on continuous countercurrent
adsorption principle where adsorbents are used to obtain pure p-xylene in the extract port by preferentially
removing it from the rest of the isomers, obtained in the raffinate port. This process exploits essentially the
differences in affinity of a molecular sieve for p-xylene, relative to the other isomers. The most frequently
used adsorbents are faujasite-type zeolites, among which prehydrated FAU1 zeolite holds an important
place.
Due to its complex physical configuration, the modelling of an SMB unit is a necessary but not
straightforward task. To achieve a simplified solution, it can be done by solving the equivalent TMB model,
resulting in a small required time to run the simulations.
In this work we propose to study the impact of the adsorption properties of two types of zeolites,
the FAU1 and FAU2, on the performance of a SMB unit by solving the true moving bed model using a
FORTRAN code in the steady state. Several experiments were performed with the aim of obtaining
different xylenes selectivities under conditions representative of the industrial SMB process to be able to
design a more realistic thermodynamic model to be implemented in the simulator of the process. Using an
iterative procedure involving the experimental data and the subsequent simulations, we are able to
progressively get closer to the real concentration profiles of the different species along the separation
column.
1
Bibliographic Study
2.1
Introduction
In the present chapter, the p-xylene background is discussed. The product chemical properties as
well as the uses and the market overview are presented. Particular importance is given to the production
processes of PX from the mixed xylenes. The industrial processes used for PX separation are described
with a particular emphasis on the studied True Moving Bed (TMB) and Simulated Moving Bed (SMB)
processes. The modelling of such process will also be discussed. Finally, some theoretical aspects
concerning adsorption and selectivity framed in the context of this work are reviewed.
2.2
P-Xylene Background
2.2.1 Chemical and physical Profile
Xylenes and ethylbenzene are C8 aromatic isomers having the molecular formula C8H10. The
xylenes exist under three isomer forms; o-xylene, m-xylene, and p-xylene , which differ in the positions of
the two methyl groups on the benzene ring. The molecular structures are shown below [1].
Figure 1. Molecular structure of the different Xylenes [1].
The only natural source of xylenes is the petroleum. The concentration of xylenes in the crude oil
varies depending on the location and geological age of the oil, varying between 0.50 and 1.47 wt% [2].
Some chemical properties of the xylenes are of great importance for their industrial use.
Oxidation of the xylene isomers gives the corresponding aromatic dicarboxylic acids, which are important
2
intermediate compounds to achieve final products in the polyester chain. The ability of the xylene to
undergo isomerization and disproportionation reactions is also exploited industrially [2] [1].
Because of their similar molecular structure, the three xylenes and EB exhibit many analogous
physical properties.
Table 1. Physical properties of xylenes and EB.
Property
PX
MX
OX
EB
Molecular weight
106.167
106.167
106.167
106.167
Density at 25°c (g/cm3)
0.861
0.8642
0.8802
0.8671
Boiling point (°C)
138.37
139.12
144.41
136.19
Freezing point (°C)
13.263
-47.872
-25.182
-94.975
As can be seen in Table 1, the very close boiling points make it difficult to separate the isomers by
conventional distillation, except for OX which can be separated from the other compounds by distillation,
as its boiling point differs by at least 5 °C from the others. PX is not separated by distillation because its
boiling point is too close to that of MX. Instead, the differences in freezing points and adsorption
characteristics are exploited commercially, as it will be discussed further [1].
Since xylenes are important components of gasoline, there are also of interest for their
combustion and octane characteristics.
2.2.2 Uses & Global Market
The majority of xylenes dedicated to petrochemicals use is used to produce PX and, to a lesser
extent, MX. Mixed Xylenes are also used as a component in aviation fuel and gasoline. High purity mixed
xylenes are also used as solvent in chemical manufacture, agricultural sprays, paintings, coatings, etc.
They are the second most important aromatic product in terms of world chemical manufacture, ranking
behind benzene and ahead of toluene[1] .
PX is the largest volume isomer of the mixed xylenes, being also the most produced one (68,7%).
Around 98% of PX is consumed in the polyester chain, mainly in the production of fibre, film and PET
bottle resins through a polymerization reaction, as shown in Figure 2. A small amount of PX is used as a
solvent or to produce di-paraxylene and herbicides [3]. In the Figure 3 we can see a general scheme
showing the chain starting with the BTX arriving to the complex up to the derivatives final products (Figure
4).
3
Figure 2. Production of PET with PX as raw material.
Figure 3. Aromatics and derivative flow scheme[8].
4
% of total applications
100%
15%
80%
25%
Others
60%
Polyester fibres
PET
60%
40%
20%
0%
2002
Figure 4 Major applications for PX in 2002[7].
The Global capacity for PX production in 2008 was 32 million m.t. (figure 6) while the global
demand was about 30 million m.t and is forecast to grow 6.9%/year to about 44.7 million m.t. through
2015 [4].
China has emerged as the dominant world consumer of PX, using in 2008 about 9 million m.t,
compared with the 4 and 2.5 million m.t. of respectively USA and Europe (Figure 7) [4]. China has also
improved the capacity of production of PX since 2002 in a annual growth rate much higher than Europe
and USA (Figure 5 and Table 2) In Asia, much of the growth in polyester is for fibre which represents
nearly two thirds of global demand. In USA and Europe, polyester fibre production has been stagnant or
even declining as the textile industry has migrated to Asia. However, this decline has been compensated
by the strong growth in PET bottle resin market [3].
World Capacity
20000
18000
Quantity PX (tons)
16000
14000
USA
12000
China
10000
Europe
8000
Others
6000
4000
2000
0
2002
2004
2006
2008
years
Figure 5 Evolution of world capacity in different regions of the world since 2002[4] [5] [6] [7].
5
Table 2. Anual average growth rate between 2002 and 2008.
Countries
Anual Average Growth Rate
USA
1.6%
Europe
China
5.8%
12.7%
Others
4.1%
Total Capacity 2008 - 32 million m.t.
USA
12%
15%
Europe
10%
China
14%
49%
Rest of Asia
Others
figure 6. Total Capacity for production PX in 2008 and the Regions share[4].
Capacity vs Demand 2008
20000
18000
Quantity PX ( tons)
16000
14000
12000
Capacity 2008
10000
Demand 2008
8000
6000
4000
2000
0
Others
EUA
Europe
china
Countries
Figure 7 Capacity and demand for different regions of the world in 2008[4].
6
2.2.3 Production Technology
Mixed xylenes, as said before, came in a few quantity from the lighter fraction in the top of the
atmospheric column containing C8 aromatics. Therefore, it is not economically viable to separate them
directly.
The raw materials to produce xylenes are the crude oil and coal. They are subjected to thermal or
catalytic treatment in which aromatics and fractions containing xylenes are obtained. There are other
methods of xylenes production, i.e. disproportionation and transalkylation of toluene.
•
From Catalytic reforming – Low octane naphta cut is converted into high octane Aromatics –
80%
– Rich in aromatics, is by-product of ethylene and propylene
production by cracking naphtha – 11.1%
•
From Pyrolysis gasoline
•
From Toluene Disproportination
•
From coal - 1.3%
– 7.6%
The production of xylenes, and specifically PX, is integrated into refinery and petrochemical
complexes (Figure 8), benefiting from several logistical advantages.
First, naphtha is fed to a reforming unit where paraffins and naphthenes are converted to
aromatics. This is the only unit in the complex that produces aromatic rings from non aromatics. After
that, the resulting reformate is distilled by solvent extraction in order to separate the BTX products. The
-
overhead product C7 , containing toluene and benzene, is sent to an extraction unit where toluene is
+
further transformed in the xylenes isomers via dispropornation/transalkylation. The bottom C8 containing
xylenes, is sent to the xylenes recovery section, highlighted in the Figure 8.
Axens Paramax and UOP aromatics complex are typical aromatics production units that are
commercialized.
7
Figure 8 Aromatics complex highlighting the xylenes separation unit ( adaptation from [8]).
As was said before, it is very difficult to separate the xylene isomers by distillation. The MX is
separated industrially in one or two columns with an amount of 150 trays and a high reflux ratio. EB can
also be separated from the mixture by distillation but it would be necessary to have several columns and a
total amount of 300 trays. According to that, for obtaining a high purity PX, it is necessary to use
alternative separation processes.
To perform this separation there are essentially three methods that are exploited commercially:
-
Crystallization;
-
Adsorption processes;
-
Hybrid Crystallization & adsorption process
Crystallization was the first and for many years the only commercial technique for separating PX
from mixed xylenes. As we can see in Table 1, PX has a much higher freezing point than the other
xylenes, which make it easy to separate by cooling down to a certain temperature is reached, just above
the eutectic point where is still possible to separate PX without precipitation of the others xylenes [1].
8
At all temperatures above the eutectic point, PX is still soluble in the remaining C8 aromatics. This
fact limits the efficiency of crystallization processes to a PX recovery of 60-65%, achieving a purity in the
final stage of crystallization in most cases higher than 99%. The pair purity/recovery it is not satisfactory
by comparison with other kind of processes mainly because of the recovery values, nevertheless it can
still be counted nowadays as a major process since about 40% of the PX produced worldwide is by
crystallization [1] [9].
The more common crystallization processes are those developed by Chevron, Krupp, Amoco,
ARCO ( Lyondell), and Philips [1].
Adsorption constitutes the second and the most recent method for separating and producing highpurity PX. Currently about 60% of the PX produced worldwide is by adsorption technology. The
technology is based on continuous countercurrrent adsorption principle (Figure 9) and adsorbents are
used to obtain pure PX in the extract port by preferentially removing it from the rest of the isomers
obtained in the raffinate port. This process exploits essentially the differences in affinity of a molecular
sieve for PX, relative to the other C8 isomers. The higher the selectivity for PX, the better the overall
performances are. The current molecular sieve permits to achieve a 99.9% PX purity with a recovery over
95% compared to the 60-65% of crystallization [8].
Figure 9 Continuous Countercurrent adsorption (Principle of true moving bed
process, TMB).
9
In 1971, UOP first commercialized their PX adsorbent process, Parex in which by holding the bed
stationary it is able to overcome the main problems of a real countercurrent movement associated with the
true moving bed process: moving the solid causes difficulty in obtaining uniform flow of both solids and
liquids in large-diameter bed and also attrition problems. The principle (Figure 10) of this new technology
is to simulate the countercurrent movement of solid by shifting the positions of feed-injection and product
withdrawal streams in the direction of flow through the bed using a rotary valve [1] [10]. This type of
Technology is called Simulated moving bed (SMB).
Figure 10 principle of the simulated moving bed process (SMB).
After the UOP Parex, other SMB process technologies reached the market. In 1973 Toray
Industries developed the Aromax process which can achieve a typical purity of 99.5% and a recovery
higher than 90%. In 1994, IFP commercialized the Eluxyl process, for which the rotary valve used to
simulate the movement of the adsorbent is replaced by individual on-off valves controlled by a
microprocessor. Production processes based on Eluxyl technology can achieve purities up to 99.9% and
recoveries of 97%. [11] [12].
10
In 1994, IFP and Chevron announced the development of a hybrid process based on Eluxyl that
combines the best features of adsorption and crystallization. The simulated moving bed is used to
produce PX with a purity of 90-95%. The PX outlet from adsorption section is then further purified in a
small single-stage crystallizer and the filtrate is recycled back to the adsorption section. It is reported that
ultrahigh ( 99.9%) purity PX can be produced easily and economically with this scheme for both retrofits of
existing crystallization units as well as grass-roots units [1] [11].
Table 3. Parex, Aromax and Eluxyl 2004 commercialisation data[11] [12].
Year of commercialization
Number of units
Unit capacity
( m.t./year)
Total capacity
(m.t./year)
8.2+5.5(projected)
UOP Parex
1971
71
24000-1200000
Toray Aromax
1973
7+10(projected)
180000-750000
2.7
IFP Eluxyl
1994
2
200000
~0.4
Between 2005 and 2008, Axens has been awarded 8 ParamaX complexes for a cumulative
paraxylene plus benzene production capacity of 9.3 million tpa. There are currently 7 Eluxyl units in
operation and 7 under various stages of design and construction [12].
To maximize the production of PX a Xylene isomerization unit is used in a loop scheme with the
adsorption or crystallization processes (Figure 11 ).
Figure 11 Simplified scheme of the PX production loop[13].
11
The mechanism of xylenes isomerization involves rapid and reversible addition of a proton to the
aromatic ring, followed by 1,2-intramolecular methyl shifts. Isomerization of xylenes requires an acidic
catalyst, whereas isomerization of ethylbenzene additionally requires a metallic site to perform the
hydrogenation being a dual function catalytic process.
After separation of PX in a xylene-splitter, the raffinate containing the remaining isomers is sent to
an isomerization unit where additional PX is produced by re-establishing an equilibrium distribution of
xylene isomers. The effluent from isomerization is sent to a deheptanizer column. The bottoms from this
column are recycled back to the xylenes column. In this way, all the C8 aromatics are continually recycled
within the xylenes recovery section until they exit the complex as PX, OX, or benzene.
Examples of processes which use dual-functional catalysts are Toray's Isolene, UOP's Isomar and
Engelhard's Octafining with EB conversions ranging between 25 and 65% [11]. The zeolite and molecular
sieve-based processes like Xymax from Mobil and Oparis, developed by IFP and commercialized by
Axens, use recent developed catalysts that provide greater versatility to the isomerization process. The
first one can achieve ethylbenzene conversions from 60 to 85% [14] and the second one optimizes the
production of PX and presents paraxylene yields up to 93% out of C8 aromatics. Xymax is a EB
dealkylation and xylenes isomerization unit which can process feed with higher EB content converting it
into benzene, like those who come from steam cracking [15] [16] .
2.3
FAU Zeolites in P-xylene Production
The requirement for adequate adsorptive capacity restricts the choice of adsorbents for industrial
processes to microporous adsorbents with pore diameters ranging from few Ǻngstroms to a few tens of
Ǻngstroms, capable of achieving high specific surface areas [18]. This includes both the traditional
adsorbents as silica gel, activated aluminia, and activated carbon as well as the more recently developed
zeolites.
The separation processes of xylenes isomers by selective adsorption as Parex, Eluxyl and
Aromax use Faujasite X or Y type zeolites modified by mobile alkaline or alkaline-earth exchange cations.
2.3.1 Structure
Zeolites are natural or synthetic crystalline, hydrated aluminosilicates with a framework structure.
As shown in figure 13 for a FAU type zeolite, their three dimension polyanionic networks are built of SiO4
12
and AlO4 tetrahedra linked through oxygen atoms [19]. The most important characteristic of all the
structures is the well-defined system of regular cavities or channels which is essential for application as
molecular sieves. Indeed, contrasting with other adsorbents the micropore size of a zeolitic adsorbent is
controlled by the crystal structure and there is virtually no distribution of pore size. This is the outstanding
property of zeolites that gives them their value as selective adsorbents for separating substances and as
shape-selective catalysts. The different zeolite structures are characterized by the type and dimensionality
of their pore system and the size of the pore apertures.
As shown in Figure 12, a typical commercial zeolite pellet presents two kinds of porosity. It
contains several microporous crystals of zeolite with a given pore opening (0.74nm for FAU type) bound
with a binder such as clay. The microporosity within the crystals, due to its great surface area, is where
most of the adsorption occurs. With a negligible surface area the pellet presents greater spaces in the
voids between the crystals called the macropores (100nm<pore diameter<1000nm) which act as passage
for the molecules to diffuse from the surrounding fluid to the interior of the pellet.
Figure 12 Idealized presentation of zeolite pellet as used in PX separation.
The faujasite unit cell is built up from the assembly of β-cages (sodalite cages), joined by double
six membered rings (D6R) which are called the secondary building units leading to the formation of super
α-cages within the framework (Figure 13).
13
Figure 13 FAU framework showing the building units of the structure.
2.3.2 Nonframework cations
The zeolite framework is not neutral and therefore, to compensate the negative charge of AlO4 the
anionic structure is counter-balanced with exchangeable cations as, e.g., Barium or Calcium. It is well
known that the separation of PX is not a result of steric selectivity due to the size of the micropore, it is
rather a consequence of an energetic selectivity with both enthalpic and entropic contribution, which
depends on the adsorption affinity of the solid for each isomer [10]. As a result, the cations nature, their
size and their location are strongly involved in the interaction between the solid and the molecules, being
all key aspects to the understanding of the adsorption mechanisms and selectivities.
There are mainly three types of sites were cations are normally localized (Figure 14); the site I
situated in the centre of the building unit D6R, the sites II situated in the centre of the 6-ring window and
sites III situated on the cage wall in close proximity to a four-ring window between the supercage and the
sodalite cage. It is important to note that some variations of this specific positions are encountered in the
structure, upon adsorption. On BaX zeolite it is possible to encounter some changes in the cations
positions with the variation of the solid filling with water and xylene molecules. Indeed, cations move from
site I to site I' in the direction of the sodalite cage during adsorption of water molecules in an empty BaX
[C. Mellot PhD thesis]. The complete filling of pre-hydrated BaX by xylenes then leads to a decrease of
the number of cations in site I' and to an increase of the number of cations in sites I and II [29].
Aromatic molecules are too large to be adsorbed in β-cages. Therefore, the selective adsorption
sites accessible to xylene isomers are near the exchangeable cations located in the α-cages or on the
external surface of zeolite crystallites. Actually, at low loading, PX and MX molecules pass through a
similar adsorption process in the α-cages, presenting adsorption capacities that are nearly the same.
14
These observations were made with Faujasite exchanged with potassium or barium [28]. At high loading,
using BaX zeolite, it is known that the adsorption sites for PX and MX are located in the supercage, close
to the Ba
2+
cations in site II. It is also known that for the PX adsorption there is another less favourable site
available in the structure, where no interaction between the xylene and the cation occurs but where the
adsorption seems to be stabilized by electrostatic interactions. Yet, it is not possible to define the precise
location of this site. These two sites seem to allow a more efficient packing of the PX molecules, resulting
in a higher capacity of the solid to retain this specie [29].
Figure 14 FAU framework with the principal cationic sites indicated.
Another important feature of zeolite which has a great impact on the adsorption capacity of the
solid is the relative proportion of Si and Al measured by the Si/Al ratio. By changing the Si/Al ratio the
properties of the zeolite are modified because of the cation content variation. For the Faujasite, there is a
specific name depending on the Si/Al ratio ; X-type : 1<Si/Al<1,5 ; for the Y type: 1,5<Si/Al and for the
more recent LSX ( low silica X): Si/Al=1.
The exchangeable cations within a specific zeolite are mobile and can be replaced by other
cations by ion exchange. This confers great versatility to the structure leading to different adsorption
behaviours.
15
2.3.3 Selectivity
The high selectivity of an adsorbent is a parameter of major importance in an industrial process. It
is the primary requirement for an economic separation combined with a good capacity and resistance to
ageing.
One can define a separation factor as follows:
(1)
α AB =
X AY B
X BYA
This definition can be compared with the relative volatility of two components in a distillation
process, as the ease that two components can be separated by having different affinities with the
adsorbent.
In the study performed by Tournier et al. [30] it was concluded that the selective adsorption
process is complex, and the quality of separation depends on many chemical and physical parameters.
Several studies on co-adsorption of xylene isomers in the liquid phase have shown that the FAU zeolite
may be selective for one isomer or the other, depending on the nature of the exchange cations, the
loading of the adsorbent, the composition of the mixture, and the presence and amount of preadsorbed
water. The temperature, through its impact on the adsorption properties of the adsorbent and mainly on
the transport properties, also plays an important role in the separation process. According to what was
said, we can easily anticipate that the relative selectivities of the different xylene isomers will vary along
the separation column.
Considering the BaX zeolite, the selectivity between PX and MX can be explained by the extra
available site for PX in the zeolite structure. Actually, because of the orientation of the methyl groups, the
adsorption in this noncationic site is not possible for MX. This fact, despite a higher population of MX in
the cationic site, results in a higher adsorption of PX, which can at least partially explain the adsorption
selectivity in favour of the PX in the prehydrated zeolite BaX [29].
16
2.4
( SMB)
True moving bed ( TMB) & simulated moving bed
Experimental part
Work Methodology
17
4.1. Model Parameters
4.2. General Modelling procedure
4.3. Optimization strategy
18
Results and discussion
5.1. Three components approach
5.2. Four components approach
5.3. Impact of EB in the adsorption of xylenes on FAU1
5.4. New adsorption models analysis - FAU1 and FAU2
19
5.5.
Comparison of the new model and pilot plant
concentration profiles
Conclusions and perspectives
20
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