Philippe Gall

Thierry Guizouarn

Michel Potel

Patrick Gougeon
email: patrick.gougeon@univ-rennes1.fr

Synthesis, crystal structure, and electrical and magnetic properties of BaMo 6 Te 6 : a novel reduced molybdenum telluride containing infinite chains of trans-face shared Mo 6 octahedra

Keywords: Reduced molybdenum chalcogenide, infinite molybdenum chains, magnetic susceptibility, resistivity measurement

published or not. The documents may come

Synthesis, crystal structure, and electrical and magnetic properties of BaMo6Te6: A novel reduced molybdenum telluride containing infinite chains of trans-face shared Mo6 octahedra

INTRODUCTION

Since the discovery of the Chevrel phases MMo 6 X 8 (M = cation; X = S, Se, Te) [START_REF] Chevrel | Superconducting in Ternary Compounds , part I[END_REF] containing octahedral Mo 6 clusters about forty years ago, a plethora of ternary and quaternary reduced molybdenum chalcogenides containing larger clusters have been synthesized. The latter compounds contain clusters such as Mo 9 , Mo 12 , Mo 15 , Mo 18 , Mo 21 , Mo 24 , Mo 30 and Mo 36 . All of these clusters result from the uniaxial trans-face-condensation of octahedral Mo 6 clusters and are present, for example, in the series of compounds M n-2 Mo 3n X 3n+2 (M = Rb, Cs; X = S, Se or Te; n = 3, 4, 5, 6, 7, 8, 10 and 12) [START_REF] Gougeon | [END_REF][3][4][5][6][7][8][9][10][11][12]. The members with even n crystallize in the rhombohedral trigonal space group R-3 with a structure closely related to the ternary chalcogenides MMo 6 X 8 and the members with odd n in the hexagonal space group P6 3 /m. The final step of the condensation corresponds to the infinite chain of trans-facesharing Mo 6 octahedra Mo 6/2  ∞ 1 [13,14] found in the hexagonal compounds M 2 Mo 6 X 6 (M = Na, K, Rb, Cs, In, Tl; X = S, Se, Te) and monoclinic one, AgMo 6 Te 6 [15]. The interest for these compounds lies not only in their fascinating structural aspect but also in their interesting physical properties. Indeed, the sulfides and selenides generally present superconducting or metal-insulator transitions at low temperature. Thus, studies of the normal and superconducting states of both Cs 2 Mo 12 Se 14 (n = 4) and Rb 4 Mo 18 Se 20 (n = 6) by measuring the conductivity and magnetization of single-crystals and powder samples have shown that these compounds can be classified among the "exotic" superconductors [16]. The quasi 1-D superconductor Tl 2 Mo 6 Se 6 presents extreme type II and non-B.C.S. behaviors [17][18][19][20]. In addition, the anisotropy of the electronic properties in the latter compound is one of the largest ever observed in a superconductor with the ratio of the conductivities parallel and perpendicular to the infinite chains (σ // /σ ⊥ ) of about 1000, and the ratio of the upper critical fields (Hc 2// /Hc 2⊥ ) of about 26. In contrast, the M 2 Mo 6 X 6 compounds with M being an element of the group IA present a continuous metal-nonmetal transition between 100 and 150 K [21]. Recent local density approximation calculations show that in all members of the M 2 Mo 6 Se 6 series, a single 1D helix band crosses the Fermi level. Its in-plane dispersion is reduced by a factor of ten in Rb 2 Mo 6 Se 6 compared with Tl 2 Mo 6 Se 6 and In 2 Mo 6 Se 6 while the out-of-plane dispersion is practically unchanged [22]. This reduction is sufficient to explain the trend from metallic conductivity followed by strongly-coupled superconductivity in Tl 2 Mo 6 Se 6 to a high-temperature metal-insulation transition in Rb 2 Mo 6 Se 6 . This insulating behaviour is consequence of either a Peierls distortion (Charge Density Wave or CDW), a Spin Density Wave or an Anderson localization. In 1985, Chevrel et al. [23] mentioned the existence of an isomorphous compound, Ba 2 Mo 6 Te 6 . However, such a stoichiometry would imply 14 electrons per Mo 3 Te 3 fragment and thus, which seems improbable, the occupation of antibonding states since the optimal count per Mo 3 Te 3 is 13 [24]. Thus, we decided to reinvestigate this compound by growing single crystals, in particular. This study has led that the right stoichiometry is in fact BaMo 6 Te 6 . We describe the synthesis, crystal structure, and electrical and magnetic properties of this compound here.

EXPERIMENTAL,

Synthesis and Crystal Growth X-ray diffractometrically pure powder of BaMo 6 Te 6 was prepared from a stoichiometric mixture of BaTe, MoTe 2 and Mo. BaTe and MoTe 2 were prepared, respectively, by heating the elements together in silica tubes at 800 °C during 48h. Before use, the Mo powder was heated under a hydrogen flow at 1000°C for 6 hours. The starting reactants were then pressed into pellets (ca. 5g) and loaded into a molybdenum crucible which was previously outgassed at 1500 °C for 15 min under a dynamic vacuum of about 10 -5 Torr. The Mo crucible was subsequently sealed under a low argon pressure using an arc-welding system. The sample was heated at a rate of 300°C/hour to 1400 C, kept at the temperature for 48 hours and then cooled at 100°C/hour down to 1100°C at which point the furnace was shut down and allowed to cool to room temperature. The resulting product was found to be single-phase on the basis of its Xray powder diffraction pattern carried out on a D8 Bruker Advance diffractometer equipped with a LynxEye detector (CuKα 1 radiation) and operating at 40 kV and 40 mA. (Figure 1). Crystals were grown by heating a mixture of composition "Ba 2 Mo 6 Te 6 " in a sealed molybdenum crucible at about 1500 °C for 48h. The crucible was then cooled at a rate of 100 °C/hour down to 1000 °C, and finally furnace-cooled to room temperature. Crystals thus obtained have the shape of needles with hexagonal cross section.

Single Crystal X-ray Study

The X-ray diffraction data were collected on a Nonius Kappa CCD diffractometer using graphite-monochromated Mo-Kα radiation (λ• = 0.71073 Å) at room temperature. The COLLECT program package [START_REF] Nonius | Data Collection Software[END_REF] was employed to establish the angular scan conditions ( and scans) used in the data collection. The data set was processed using EvalCCD [START_REF] Duisenberg | Reflections on area detectors[END_REF] for the integration procedure. An absorption correction was applied using the description of the crystal faces and the analytical method described by de Meulenaar and Tompa [START_REF] De Meulenaar | [END_REF]. Analysis of the data revealed that the systematic absence (000l) l = 2n+1 was only consistent with the hexagonal space group P6 3 /m.The structure was solved with the direct methods program SIR97 [28] and refined using SHELXL97 [START_REF] Sheldrick | SHELXL97, Program for the Refinement of Crystal Structures[END_REF] in the P6 3 /m space group. Refinement of the occupancy factor of the Ba site led to a value of 0.492(3) indicated a half occupation of the cationic site compared to the other hexagonal compounds M 2 Mo 6 X 6 (M = Na, K, Rb, Cs, In, Tl; X = S, Se, Te). An attempt to refine the structure in the acentric space group P6 3 was unsuccessful and led to a higher R factor as well as larger residual electron density peaks.

Because of the disordering of the Ba atom, we made long-exposure rotation photographs along the a and c axes on a single crystal of BaMo 6 Te 6 . The latter did not reveal any superlattice reflection. Crystallographic data and X-ray structural analysis for the BaMo 6 Te 6 compound are summarized in Table 1. The final atomic coordinates, and the equivalent isotropic displacement parameters are gathered in Table 2, and selected interatomic distances are listed in Table 3. Further details of the crystal structure investigation can be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany, (fax:

(49) 7247-808-666; e-mail: crysdata@fiz.karlsruhe.de) on quoting the depository number CSD-427614.

Electrical Resistivity Measurements

The study of the temperature dependence of the electrical resistivity was carried out on a single crystal of BaMo 6 Te 6 using a conventional ac four-probe method with a current amplitude of 0.1 mΑ. Contacts were ultrasonically made with molten indium on the single crystal previously characterized on a Nonius Kappa CCD diffractometer. The ohmic behavior and the invariance of the phase were checked during the different measurements at low and room temperature.

Magnetic susceptibility measurements.

Susceptibility data were collected on cold pressed powder samples (ca. 100 mg) using a Quantum Design SQUID magnetometer between 4.2 K and 400 K and at an applied field of 0.1 T.

RESULTS AND DISCUSSION

Crystal Structure

The comparison of the unit cell parameters of the previous compound reported as "Ba 2 Mo 6 Te 6 " (a = 9.41 Å, c = 4.57 Å, V = 350 Å 3 ) with those of the title compound (a = 9.3941 (2) Å, c = 4.5848 (1) Å, V =350.40(1) Å 3 ) clearly shows that we are in the presence of the same phase i.e. BaMo 6 Te 6 .

A view of the unit cell of BaMo 6 Te 6 is given in Figure 1 [5] in which the average intratriangle distances range from 2.663 to 2.688 Å and the average intertriangle ones from 2.765 to 2.785 Å. However, they differ significantly from those found in In 2 Mo 6 Te 6 (2.639 and 2.755 Å) and Tl 2 Mo 6 Te 6 (2.619 and 2.746 Å), the structures of which have been solved from RX powder data [15]. The Mo-Te distances range from 2.8015(4) to 2.8560(2) Å as usually observed. The Ba-centered tricapped trigonal prisms share triangular faces along [001]. The distance between the Ba and the Te atoms forming the prism is 3.4799(2) Å while that with the Te atoms capping the prism is longer, 3.6139 (3) Å.

These distances are in agreement with that found in BaTe (3.502 Å), for example.

Electrical and Magnetic Properties.

Electrical resistivity measurements were performed on a single-crystal of BaMo 6 Te 6 .

The temperature dependence of the electrical resistivity of BaMo 6 Te 6 measured along the chain growth is similar to that reported for the M 2 Mo 6 X 6 compounds (M = Na, K, Rb and Cs; X = Se, Te) (Fig. 5). Thus, BaMo 6 Te 6 is metallic in the temperature range 160-290 K with a resistivity of about 0.038 mΩ.cm at room temperature and 0.022 a t 160 K. Below 160 K, a semiconductor behaviour is observed since the electrical resistivity increases with decreasing temperature. The activation energy deduced from the linear part of the 1/T vs. logρ plot is 220K that is close to that reported for the Cs 2 Mo 6 Te 6 compound (ie 225 K) [21].

The temperature dependence of the magnetic susceptibility of BaMo 6 Te 6 is shown in Figure 5. The susceptibility of the BaMo 6 Te 6 compound is nearly temperature-independent and weakly diamagnetic in the range 100-300 K with a value of about -7 10 -8 emu/g closed to that reported for some M 2 Mo 6 X 6 compounds with M being an element of the group IA [21].

This behavior is consistent with the absence of localized moments on the Mo atom network.

The low-temperature upturn could be attributed to small amounts of paramagnetic impurities often present in the starting reactants. It is interesting to note that no anomaly was observed at the metal-nonmetal transition as previously observed for the M 2 Mo 6 X 6 compounds [21]. This is reminiscent of the well know Anderson localisation that arises from defects [START_REF] Anderson | [END_REF]. Indeed, unlike the CDW transitions [START_REF] Peierls | Quantum Theory of Solids[END_REF][START_REF] Grüner | [END_REF] that require an opening of gap near the Fermi level and associated changes in the structural and variable temperature magnetization studies, the present compound does not show any reduction in the magnetization value near the metalnonmetal transition. In addition, the broad nature of the transition in the electrical resistivity also supports defect-induced localization of the carriers. A similar observation has also been reported for KMo 4 O 6 that contains infinite chains of trans-edge shared Mo 6 octahedral clusters [33].

CONCLUSION

In summary, the new compound BaMo 6 Te 6 has been synthesized by solid state reaction. This compound is isomorphous with the M 2 Mo 6 X 6 (M = Na, K, Rb, Cs, In, Tl; X = S, Se, Te) compounds and, thus its crystal structure contains infinite chains of trans-facesharing Mo 6 octahedra Mo 6/2  ∞ 1 capped by Te atoms. The Ba 2+ cations occupy statically half of the tricapped trigonal prisms formed by the Te atoms between the chains while, in the M 2 Mo 6 X 6 compounds, all the tricapped trigonal prismatic sites are occupied. The temperature dependences of the electrical resistivity and magnetic susceptibility are similar to those observed for the M 2 Mo 6 X 6 compounds with M being an element of the group IA. In We present here the synthesis, the crystal structure, and the electrical and magnetic properties . Synthesis by solid state reaction.

. Single-crystal X-ray study.

. Continuous metal-nonmetal transition.

. Anderson localization.

  . The structure is composed of infinite chains of trans-face-sharing Mo 6 octahedra Mo 6/2  ∞ 1 capped by Te atoms. These Mo-Te chains run along the c axis and have crystallographically imposed symmetry -3. The Ba 2+ cations separate the Mo 6/2 Te 6/2 ∞ 1 chains and are each coordinated to nine Te atoms in a tricapped trigonal-prismatic arrangement with imposed -6 symmetry. The Mo-Mo distances within and between the Mo 3 triangles perpendicular to the c axis are 2.6602(6) and 2.7593(2) Å, respectively. The latter values are of the same order than those observed in other reduced molybdenum tellurides such as Mo 6 Te 8 , Cs 2 Mo 12 Te 14 , Cs 3 Mo 15 Te 17 , Cs 4 Mo 18 Te 20 and Cs 6 Mo 24 Te 26

Figure 1 .

 1 Figure 1. Observed (crossed), calculated (black line) and difference profiles for the refinement of BaMo 6 Te 6 in profile-matching mode (λ = 1.5406 Å).

Figure 2 .

 2 Figure 2. The crystal structure of BaMo 6 Te 6 as viewed down the c axis, parallel to the direction of the Mo 6/2 chain growth. Ellipsoids are drawn at the 97 % probability level.

Figure 3 .

 3 Figure 3. The crystal structure of BaMo 6 Te 6 as viewed perpendicularly to the Mo 6/2 chain growth (ellipsoids at the 97 % probability level).

Figure 4 .

 4 Figure 4. Tellurium environment for the Ba atom (ellipsoids at the 97 % probability level).

Figure 5 .

 5 Figure 5. Temperature dependence of the resistivity for BaMo 6 Te 6 . The inset shows the Arrhenius plots.

Figure 6 .

 6 Figure 6. Magnetic susceptibility of BaMo 6 Te 6 as a function of temperature. Data were taken under an applied field of 0.1 T.

Figure 1 .

 1 Figure 1. Observed (crossed), calculated (black line) and difference profiles for the refinement of BaMo 6 Te 6 in profile-matching mode (λ = 1.5406 Å).

Figure 2 .

 2 Figure 2. The crystal structure of BaMo 6 Te 6 as viewed down the c axis, parallel to the direction of the Mo 6/2 chain growth. Ellipsoids are drawn at the 97 % probability level.

Figure 3 .

 3 Figure 3. The crystal structure of BaMo 6 Te 6 as viewed perpendicularly to the Mo 6/2 chain growth (ellipsoids at the 97 % probability level).

Figure 4 .Figure 5 .Figure 6 .

 456 Figure 4. Tellurium environment for the Ba atom (ellipsoids at the 97 % probability level).

of the new compound BaMo 6

 6 Te 6 containing infinite chains of trans-face shared Mo 6 octahedra Highlights . BaMo 6 Te 6 contains infinite chains of trans-face-sharing Mo 6 octahedra Mo 6/2  ∞ 1 .

Table 1 .

 1 Crystal data and structure refinements of BaMo 6 Te 6

	Empirical formula	BaMo 6 Te 6
	Formula weight (gmol -1 )	1478.58
	Crystal system, space group	hexagonal, P6 3 /m
	Unit cell dimensions (Å, deg)	a = 9.3941 (2), c = 4.5848 (1)
	Volume (Å 3 )	350.40 (1)
	Z, Calculated density (g/cm 3 )	1, 7.007
	Absorption coefficient (mm -1 )	20.151
	Crystal color and habit	Black, needle like
	Crystal size (mm 3 )	0.152 × 0.063 × 0.057
	Theta range for data collection (deg) 2.50-32
	Limiting indices	-14 h 14, -14 k 14, -5 l 6
	Reflections collected/unique	8209/452
	R (int)	0.0444
	Absorption correction	Analytical
	Max./min. transmission	0.4142/0.1328
	Data/restraints/parameters	3903/0/101
	Goodness-of-fit on F 2	1.081
	R indices [I>2 (I)]	R1=0.0208, wR2=0.0539
	Extinction coefficient	0.00418(11)
	Largest diff. peak and hole (eÅ -3 )	1.437 and -1.177

Table 2 .

 2 Atomic coordinates and equivalent isotropic displacement parameters (Å 2 ) for the compound BaMo 6 Te 6 .

	Atom Wyckoff	x	y	z	Ueq	s.o.f
		position
	Mo	6h	-0.14384(4) 0.03396(4) 0.7500 0.00504(14) 1
	Te	6h	0.07032(3) 0.37457(3) 0.7500 0.00659(13) 1
	Ba	2c	0.3333	0.6667	0.2500 0.0075(3)	0.492(3)

Table 3 .

 3 Selected Interatomic Distances (Å) for BaMo 6 Te 6 .

	Mo-Mo (X2)	2.6602(6)	Ba-Te (X6)	3.4799(2)
	Mo-Mo (X4)	2.7593(2)	Ba-Te (X3)	3.6139(3)
	Mo-Te	2.8015(4)
	Mo-Te	2.8117(4)
	Mo-Te (X2)	2.8560(2)

particular, the broad metal-nonmetal transition around 160 K results probably from an Anderson type localization due to the absence of anomaly in the magnetization curve. Finally, it would be interesting to study the thermoelectric properties of BaMo 6 Te 6 and M 2 Mo 6 X 6 (M = Na, K, Rb, Cs; X = S, Se, Te). Indeed, recent works on the finite chain cluster compounds Ag x Mo 9 Se 11 (3.4 x 3.9) and Ag 3 In 2 Mo 15 Se 19 containing Mo 9 clusters [34][35][36] have shown that Mo cluster chalcogenides are good candidates for thermoelectricity.


Yusmel González Hernández

Ulises Javier

Jáuregui Haza

Claire Albasi

Marion Alliet
email: marion.alliet@ensiacet.fr

Development of a Submerged Membrane Bioreactor simulator: a useful tool for teaching its functioning

Keywords: Improving classroom teaching, Interactive learning environments, Simulations, Submerged Membrane Bioreactor

Among the technologies used to treat wastewater, the Submerged Membrane Bioreactor (SMBR) has excellent prospects because of the possibility it provides for water reuse. In this work, an SMBR computer simulator is developed. A mathematical model was implemented, which integrated the biological degradation process using activated sludges with the physical separation process using membranes. The simulator functioning was validated with experimental results and its use in teaching was evaluated through the development of a simulated laboratory running for three and a half hours. This gave access to trends and orders of magnitude that would take more than fifteen months to obtain with real experiments. It was successfully used and accepted by the students.

Introduction

Fresh water is becoming known as the "blue gold" of the 21st century. It is a natural resource already in short supply and it will become even scarcer with increased urbanization and population, climate change, and industrial pollution, making it humanity's most precious resource and one of the major environmental issues of this century [START_REF] Buzatu | Optimal operating strategies of a Submerged Membrane Bioreactor for wastewater treatment[END_REF]. For this reason, many governments today are devoting considerable resources and efforts to the development of new technologies for wastewater treatment and the decontamination of contaminated sources. An example of these technologies is the Submerged Membrane Bioreactor (SMBR).

The SMBR can be defined as a system that combines biological degradation of wastewater effluents with membrane filtration [START_REF] Cicek | Characterization and comparison of a membrane bioreactor and a conventional activated-sludge system in the treatment of wastewater containing high molecular weight compounds[END_REF]. For many years, these systems have shown their effectiveness in the treatment of municipal and industrial wastewater [START_REF] Jimenez | Biological modelling of MBR and impact of primary sedimentation[END_REF][START_REF] Santos | Membrane bioreactors: two decades of research and implementation[END_REF]. In the last two decades, SMBR technology has grown exponentially due to its advantages over conventional wastewater treatment processes, such as reduced environmental impact, improved effluent quality and better process control [START_REF] Buer | MBR module design and operation[END_REF][START_REF] Drews | Membrane fouling in membrane bioreactorscharacterisation, contradictions, cause and cures[END_REF]. The major potential advantage of this technology is found in the field of water reuse. This is because the SMBR can use ultrafiltration membranes and thus retain bacteria, some viruses and many organic and inorganic components that are often found in the effluent from conventional biological treatments [START_REF] Lobos | Membrane bioreactor performances: effluent quality of continuous and sequencing systems for water reuse[END_REF][START_REF] De Luca | Removal of indicator bacteriophages from municipal wastewater by a full-scale membrane bioreactor and a conventional activated sludge process: implications to water reuse[END_REF].

Therefore, the effluent of an SMBR may be suitable for direct reuse or water supply for a reverse osmosis process. That is one of the reasons why research in the SMBR field is increasing continuously at present, due the commercial and scientific interest that it has aroused [START_REF] Stephenson | Membrane Bioreactors for Wastewater Treatment[END_REF][START_REF] Van Nieuwenhuijzen | Review on the state of science on membrane bioreactors for municipal wastewater treatment[END_REF]. Nevertheless, the effective application of membrane bioreactors (MBRs) apparent shear intensity of the fluid turbulence on the ith section of the membrane surface (s -1 ) J overall flux (m 3 /m 2 s) J i local filtration flux through the ith membrane section (m 3 /(m 2 day)) M td mass of sludge in the dynamic sludge film cake adhering to the membrane surface (kg/m 2 ) M tf (i) mass of sludge in the stable sludge cake attached to the ith membrane section (kg/m 2 ) M td (i) mass of sludge in the dynamic sludge film cake in the ith membrane section (kg/m 2 ) n total number of sections in the membrane surface area q a aeration intensity (L m -2 s -1 ) Q BG coarse bubble flow (L/s) R m intrinsic resistance of the membrane (m -1 ) R p pore fouling resistance (m -1 ) r p specific pore fouling resistance in terms of filtrate volume (m -2 ) R T overall filtration resistance (m -1 ) r td specific filtration resistance of dynamic sludge film (m/kg) R td resistance of dynamic sludge film (m -1 ) R tf resistance of stable sludge cake layer (m -1 ) r tf specific filtration resistance of sludge cake layer (m/kg) R TS (i) filtration resistance for the ith membrane section (m membrane fouling and the associated cost and energy burdens [START_REF] Menniti | Mechanisms of SMP production in membrane bioreactors: choosing an appropriate mathematical model structure[END_REF]. At the same time, experimentation in these types of installations is very expensive and time consuming.

On the other hand, it is necessary to take all the elements mentioned above into account in the training of engineers and of the staff that will operate the SMBR. It is essential to develop tools that can help in the learning process, both at universities and at operator training centres. The development of simulators is a necessity since they constitute a platform to enhance virtual laboratories [START_REF] Corter | Process and learning outcomes from remotely operated, simulated, and hands-on student laboratories[END_REF]. Virtual laboratories can provide a dynamic Problem-Based Learning experience where students engage in an authentic, industrially situated task. They simulate what expert engineers do in practice, and are very different in character from the physical laboratory at university [START_REF] Koretsky | Fundamental Research in Engineering Education. Student Learning in Industrially Situated Virtual Laboratories[END_REF]. Another advantage of a simulator is its value in the training process from the research point of view: to help to solve problems that are as yet unsolved. Simulators are also an important support for the study of process optimization.

The use of simulated experiments can considerably reduce the cost of a laboratory course, increase the number of experiments in the learning process and enable experiments to be carried out that would otherwise involve working with dangerous materials and/or in dangerous conditions [START_REF] Skorzinski | A simulation program for modelling pollutant dispersion for educational applications[END_REF]. For all these reasons, the mathematical modelling of an SMBR and the development of a simulator of this process provides an alternative that can solve many problems. The objective of this work is to develop a computer simulator of an SMBR and to show its potential in teaching how such processes work.

Materials and methods

A computer simulator consists of three main parts: the mathematical model, the numerical solution method and the graphical interface. The integrated model proposed by The model was implemented using the Pascal programming language. The differential equations that appear in the model were solved using the fourth-order Runge Kutta numerical method.

Finally, the simulator graphical interface was developed. Designing educational software interfaces is a complex task, given its strong domain dependency and multidisciplinary nature. It requires the teachers' knowledge and pedagogical beliefs to be incorporated into the interface, posing a challenge to both teachers and designers as they have to act as partners from the earliest phases of the process, sharing their knowledge [START_REF] Perry | A case study on the design of learning interfaces[END_REF]. In the present case, the simulator graphical interface was built using the facilities provided by Delphi 2009 for object-oriented programming, in order to achieve a friendly graphical interface enabling the assignation and manipulation of different operating parameters, as well as observation of how the variables of interest behave over time.

Mathematical model and its implementation

The mathematical model proposed by [START_REF] Zarragoitia | Modelling of submerged membrane bioreactor: conceptual study about link between activated sludge biokinetics, aeration and fouling process[END_REF] integrates the biological degradation process by activated sludges with the physical separation process using membranes.

In order to facilitate the evaluation of the model, the selection of equations and biological processes considered during modelling was linked to the characteristics of the experimental reactor and its operating conditions. However, the final structure of the model offers the possibility of adding other process rates and stoichiometries. The conceptual scheme of the model developed is shown in Fig. 1. It gives the main relations operating during simulation and also the information flow established among the different parts of the model during calculation. The model is divided into three sections, the first considers the biological behaviour (stoichiometry and kinetics), the second is related to membrane fouling evolution and the behaviour of all filtration resistances, and the last consists of a set of periodic equations that represent the process associated with coarse bubble aeration, feeding and discontinuous filtration.

To simulate the activated sludge process, a modified model was established considering the formation-degradation kinetics of soluble microbial products proposed in the modification of ASMl developed by [START_REF] Lu | A model for membrane bioreactor process based on the concept of formation and degradation of soluble microbial products[END_REF], but adapting these equations to a strictly aerobic SMBR. The biological model consists of a system of differential equations obtained from the Peterson matrix [START_REF] Zarragoitia | Modelling of submerged membrane bioreactor: conceptual study about link between activated sludge biokinetics, aeration and fouling process[END_REF].

On the other hand, in the physical separation model, the main process is the mass attached to the membrane surface. This process is described by Eq. ( 1):

dM td dt = 24CJ 2 24J + C d d p G - ˇ(1 -˛)GM 2 td V f t + M td (1)
The first term of Eq. ( 1) represents the net mass deposited on the membrane surface due to the equilibrium between the suction and lifting forces (Fig. 2), while the second term represents the mass removed by the shear forces caused by the coarse bubble aeration [START_REF] Li | Modelling of membrane fouling in a submerged membrane bioreactor[END_REF]. Shear forces are not uniformly distributed over the membrane surface. When modelling the deposition of sludge on the membrane surface, in the case where the membrane module resembles to a cylindrical object, the variation of G values in the vicinity of the membrane module can be estimated as follows:

-at the bottom of the membrane module, G value is assumed the smallest possible value, which is one-tenth of its maximum value, -for distance ratios to the bottom higher than ε a , G value is assumed to take its maximum value, -between the two, sinusoidal growth is assumed (Fig. 2). Therefore, Eqs. ( 2) and (3) are used to calculate the G i values. These equations must be modified for the case of modules with different geometry or different aeration systems, e.g. with air nozzles located at multiple heights of the membrane module, which are not frequent due the structural complexities and changes that the hydrodynamics of the system introduces [START_REF] Zarragoitia | Modelling of submerged membrane bioreactor: conceptual study about link between activated sludge biokinetics, aeration and fouling process[END_REF].

G(ε) =        0.1 + 0.45 1 + sin (2ε -ε a ) 2ε a s gq a s , ε < ε a s gq a s , ε≥ε a        (2)
where

q a = Q BG A (3)
For the modelling of membrane fouling, the membrane surface was divided into sections of equal area (S i ). It was considered that each area S i was traversed by a flow of equal magnitude, which was calculated by the following equation:

J i = J n (4)
Eq. ( 4) was adapted so as to calculate the mass of sludge deposited in the ith section of the membrane surface. [START_REF] Li | Modelling of membrane fouling in a submerged membrane bioreactor[END_REF] found that 128 sections of equal area (S i ), guaranteed a good approximation to system behaviour. Also, the overall flux (J) in Eq. ( 1) is replaced by the local filtration flux through the ith membrane section (J i ) and the apparent shear intensity of the fluid turbulence (G) is replaced by the actual shear intensity on the ith section of the membrane surface (G i ). Hence, with these transformations Eq. ( 1) can be expressed as:

dM td(i) dt = 24CJ 2 i 24J i + C d d p G i - ˇ(1 -˛)G i M 2 td(i) V f (i) t + M td(i) (5)
This type of installation works in periodic cycles of filtration and cleaning by coarse bubble aeration so, when the system is filtering, the mass attached to the membrane surface is calculated using Eq. ( 5), but when the system is not filtering, the only process that can occur is removal of the sludge from the membrane surface. In this case Eq. ( 6) is used to calculate the mass attached to the membrane surface, which is a modification of Eq. (5).

dM td(i) dt = - ˇ(1 -˛)G i M 2 td(i) V f (i) f + M td(i) (6)
For each new time step:

M tf(i) (t + t) = M tf(i) (t) + M td(i) (t) .
Another parameter to be taken into account is the pore fouling resistance. Pore fouling occurs as a result of the adhesion of some sludge particles having a diameter less than or of the order of the diameter of the pores on the interior walls [START_REF] Meng | Recent advances in membrane bioreactors (MBRs): membrane fouling and membrane material[END_REF]. Then, the filtration resistance for the ith membrane section is determined by the following expression:

R TS(i) = R m(i) + R p(i) + R td(i) + R tf (i) (7)
where Once the value of the filtration resistance for the ith membrane section has been determined, the value of the overall resistance can be calculated by means of the expression (12).

R m(i) = const. (8) R p(i) = r p m k=1 J i f (k) (9) R td(i) = r td M td(i) (10) R tf (i) = r tf M tf (i) (11
1 R T = n i=1 S i R TS(i) (12)
Finally, the value of TMP can be determined using Darcy's law (13):

TMP = s JR T (13)
The general scheme of the algorithm for implementing the mathematical model is shown in Fig. 3. For the calculation, the sludge concentration is determined by solving the system of differential equations for the biological system. Then, the mass of sludge attached to the membrane surface and its resistance to filtration are determined. Later, the overall resistance value is evaluated in the expression of Darcy's law so that the TMP value can be obtained. The output parameters of the simulator are shown at each 1 s time interval, which is the step used by the fourthorder Runge Kutta numerical method during the calculation.

Nowadays, many unit process models are available in the field of wastewater treatment. All of these models use their own notation, which causes problems for documentation, implementation and connection of different models (using different sets of state variables) [START_REF] Corominas | New framework for standardized notation in wastewater treatment modelling[END_REF]. For this reason, in this work, the universal notation proposed by [START_REF] Corominas | New framework for standardized notation in wastewater treatment modelling[END_REF] to describe mathematical models is used.

Evaluation of the simulator

To demonstrate the simulator capacity to describe the performance of a real membrane bioreactor, the experimental data reported by [START_REF] Zarragoitia | Modelling of submerged membrane bioreactor: conceptual study about link between activated sludge biokinetics, aeration and fouling process[END_REF] were used. Tables 123show the working conditions in which the experiments were conducted, the characteristics of the wastewater used and the characteristics of the activated sludge at the beginning of the experiments, respectively.

Practical application of the simulator in chemical engineering education

The simulator was used in chemical engineering education in December 2012 and in Decembre 2013 at the Ecole Nationale Supérieure des Ingénieurs en Arts Chimiques Et Technologiques (ENSIACET), France. The use of this software was incorporated into the syllabus of the "Water treatment" course in the fifth year of higher education corresponding to the Chemical Engineering Careers unit. The objectives of the practical activity are that the students become familiarized with a water treatment process, that they understand how the SMBR process works and that they analyze the influence of operating parameters on the functioning of the process. For this purpose practical work lasting three and a half hours in a simulated laboratory was developed. It was structured in three steps:

1. Analysis of the mathematical model implemented in the simulator in order to better understand the results obtained with the simulator. 2. Resolution of a practical problem using the simulator. 3. Writing of the report by the students, with the results and discussion of all the simulator predictions.

Practical activity

A practical exercise was developed using experimental data from a real plant [START_REF] Zarragoitia | Modelling of submerged membrane bioreactor: conceptual study about link between activated sludge biokinetics, aeration and fouling process[END_REF]. The exercise is described below.

An MBR pilot plant with a submerged membrane configuration is located in the town of Brax, France. This plant is fed with a volumetric flow of 0.09 m 3 day -1 of real wastewater, the characteristics of which are shown in Table 1. Specifically, a polysulfone hollow-fibre membrane module supplied by Polymen (pore size = 0.2 m, surface area = 0.3 m 2 ) is immersed directly in an aerobic tank with a volume of 10 L. The sludge retention time is around 30 days. The biological reactor is aerated and stirred by a fine bubble air diffuser located at the bottom of the reactor. A second air blower, located directly at the bottom of the membrane module, produces an air flow rate of 6 L m -2 s -1 to generate coarse bubbles that cause strong turbulence so as to clean the surface of the membrane and thus limit membrane fouling. Membrane filtration is carried out in conventional sequential cycle mode: 9 min with filtration and 1 min without filtration (relaxation time). During the relaxation time, the coarse bubble aeration is on, so the membrane cleaning is carried out in a cycle mode: 1 min of coarse bubble aeration and 9 min without such aeration.

The maintenance of the system is performed when transmembrane pressure reaches its critical value, which in this case is fixed at 60 kPa. The mean temperature inside of MBR is 20 • C during the entire campaign. Tables 234show the data necessary for the MBR operation: the supply characteristics, the characteristics of the mixture inside the MBR and the properties of the activated sludge and cake formed on the membrane surface, respectively.

Activities.

Using the software 1. Simulate the operation of the MBR over 8 days. Describe the behaviour over time of the following parameters:

• Trans-membrane pressure.

• Total suspended solids.

• Chemical oxygen demand.

• Filtered water volume.

• Soluble microbial products.

• Overall resistance to filtration.

• Dissolved oxygen. 2. Determine the time necessary for the system to reach the critical trans-membrane pressure working in the operating conditions described above. 3. What needs to be done if the system reaches the critical trans-membrane pressure? 4. In order to understand the influence of the following variables on the trans-membrane pressure, perform a sensitivity analysis in the indicated ranges:

• Filtration flux (±5% of established value).

• Specific filtration resistance of the sludge cake layer (±5% of established value). Discuss the results of the sensitivity analysis. Compression coefficient for the dynamic sludge film (kg m -3 s -1 ) 2.5 × 10 -5

Coefficient of the drag and lifting force 0.4 Reduction index of the cake compression coefficient 0.1 5. For the purpose of determining the best operating conditions, plot the time necessary for the system to reach the critical trans-membrane pressure versus the following variables (maintaining the rest of the operating conditions constant):

• Filtration frequency (Filtration time/Relaxation time = 9:1, 7:3, 5:5, 3:7 and 1:9) • Aeration intensity (3, 6, 9, 12 and 15 L m -2 s -1 ) Discuss the results obtained.

Anonymous questionnaire

At the end of the practical activity, the students were asked to fill in an anonymous questionnaire giving their opinion about the simulated laboratory (Table 5).

Results and discussion

Description and operation of the simulator

For teaching use, the SBRM computer simulator developed should be user friendly and provide an easily accessible introduction to the subject. Since other uses are advanced training and research, many parameters should be easily modifiable.

The simulator shows a general standard scheme of the SMBR, which allows the main structural components of the system to be apprehended, so that the user can gain a better understanding of the installation performance and thus a better understanding of the processes that are involved in these types of installations (Fig. 4).

The simulator allows the user to study of the influence of the 35 model input variables (those presented in Tables 1234, bioreactor volume, membrane surface and time to be simulated) on 16 output parameters, which can be displayed graphically or numerically. These output parameters are:

1. Trans-membrane pressure. 2. Resistance of the stable sludge cake layer. 3. Resistance of the dynamic sludge film. 4. Pore fouling resistance. 5. Overall resistance. 6. Chemical oxygen demand. 7. Dissolved oxygen. 8. Easily biodegradable substrates. The simulator enables the user to select, before calculating, the time-scale over which results will be presented. This time-scale can be seconds, minutes, hours or days. The precision of calculation does not change with the time scale selected because it is always performed in seconds.

Also, the simulator allows the numerical results to be exported to a txt file. This option may permit the user to process these results using other computational tools. Similarly, the graphical results can be saved in bmp image format. Another advantage is that each calculation can be saved in a file using bms format, an extension created for this software. Finally the simulator opens the possibility of a sensitivity analysis with respect to various parameters.

Comparison of simulator performance with experimental data

To substantiate and justify the use of the computer simulator to study an SMBR, it is essential to know the level of approximation to which the mathematical model can reproduce SMBR operation. For this reason, the simulation results were compared with experimental data.

The parameter chosen was the trans-membrane pressure because of its importance in the operation of the SMBR [START_REF] Meng | Recent advances in membrane bioreactors (MBRs): membrane fouling and membrane material[END_REF][START_REF] Fenu | Activated sludge model (ASM) based modelling of membrane bioreactor (MBR) processes: a critical review with special regard to MBR specificities[END_REF]. Fig. 5 shows the experimental and calculated values of TMP. A mean relative error of estimation of 15% was obtained.

This result can be considered acceptable for predicting the behaviour of a system of such complexity because, as explained above, the simulator was built by implementing a model that combines biological degradation with the filtration process. The biological system modelling introduces a high percentage of error as the input values of biological variables correspond to the mean values measured during the experiment [START_REF] Zarragoitia | Modelling of submerged membrane bioreactor: conceptual study about link between activated sludge biokinetics, aeration and fouling process[END_REF].

Even with 15% of error, the simulator already gives the trends in the evolution of physical quantities and the order of magnitude of their values, which is the information sought.

Practical application of the simulator in chemical engineering education

Before beginning the practical activity, the teacher gave the students a brief overview of the model implemented in the simulator as a "black box". Once the students had received the orientation information and the necessary materials, they were ready to start the proposed exercise. The instructor was at hand during the entire practical activity, and was available to clarify any points the students had doubts about. Finally, the students prepared a report with the exercise solution. When they had finished the work in the simulated laboratory, the students answered the anonymous questionnaire, which was drawn up in the aim of knowing the students' opinions about the simulated laboratory.

The evaluations of how well the objectives of this practical activity were attained were made:

during the activity itself, by the discussion between the students and the teaching staff, -from the reports given by the students, -by the students' answers to the questionnaire.

Considerations about the practical activity

The practical activity was carried out successfully by the students. They showed their abilities in the use of computer programs and, in general, they managed the simulator with success. Nevertheless, there were some students who had difficulty solving this task because they did not understand the functioning of an SMBR correctly and others who had problems with the simulator language. However, with the instructor's help, they finished the proposed exercise correctly. The students' correct use of the different simulator tools and their understanding of the SMBR operation was evaluated from their analysis of the results they reported.

The reports were corrected and graded according to the French norm, which gives points out of 20, with the following appreciation: 10 = pass, 12 = quite good, 14 = good, 16 = very good, 18 = excellent, and 20 = congratulations. The average was 13.6/20 with a minimum of 12/20 and a maximum of 16/20, which is a rather good result. Parts 1-4 were achieved very well, with only minor mistakes. Part 5 was completed in a more variable way, mainly due to a lack of time (and to the French way of teaching, which discriminates using time).

Students' opinions

The students' responses to the questionnaire are presented in Fig. 6. A grading scale obtained by using numerical equivalents for the opinions: "Strongly agree" = 20, "Agree" = 13.33, "Disagree" = 6.67, "Strongly disagree" = 0 (in order to correspond to the French grading system, which is out of 20) has been added. For each of the statements proposed in the questionnaire, a "grade" is indicated, which was obtained by averaging the answers. To analyze these responses, the questions with the most numerous answers "Disagree" were considered as well as the ones with less good grades. The students' evaluations were very positive. This simulated laboratory aroused great interest in more than 95% of the students.

As noticed by the teaching staff during the practical activity, although some students had some difficulties in understanding the functioning of the simulator by themselves (Q4), the participation of the teacher helped them in this task (Q5).

Slightly more than 10% of the students did not agree that this laboratory was relevant to their programme and the lowest evaluation concerned the situation of this laboratory in the education programme (Q8). A discussion with the students showed that an additional experimental activity may help to improve this impression. The teaching staff is thinking about a convenient and not too expensive way to include it (visit to a Fig. 6 -Results of the anonymous questionnaire (see Table 5).

water treatment plant, visit to a research experimental device, short experimental practical activity, etc.)

Advantages of simulator in teaching

Laboratory exercises, field observations and field trips are a fundamental part of many earth science and environmental science courses. Field observations and field trips can suffer from constraints of distance, time, expense, and the scale, safety, or complexity of real-world environments [START_REF] Ramasundaram | Development of an environmental virtual field laboratory[END_REF]. [START_REF] Corter | Process and learning outcomes from remotely operated, simulated, and hands-on student laboratories[END_REF] state that the majority of comparative studies have concluded that simulation is a good substitute for hands-on labs in teaching course concepts and their application but some researchers have proposed that simulation might be most effective when it is integrated as a complementary part of a course involving hands-on laboratory activity.

In comparison with textbooks and lectures, a learning environment with a computer simulation has the advantages that students can systematically explore hypothetical situations, interact with a simplified version of a process or system, change the time-scale of events, and practice tasks and solve problems in a realistic environment without stress (van Berkum and de Jong, 1991).

El-Naas ( 2011) developed a course following the active learning approach, where students are heavily involved in class activities and they can directly assess the effect of input variables on the design parameters, allowing them to carry out "What If" or parameter sensitivity analysis. For this he used Excel and Ez-Solve in designing and analysing desalination processes. Simulations have earned a place in the classroom as robust additions to the teachers' repertoire, either as an addition to the traditional teaching methods available or as a replacement of parts of the curriculum [START_REF] Rutten | The learning effects of computer simulations in science education[END_REF]. As shown here, the SMBR computer simulator developed is able to predict the behaviour of various output parameters in the short-and long-term. It allows users to study the influence of the bioreactor initial working conditions on the behaviour of the main parameters that describe the plant performance.

It also allows the optimum SMBR operating conditions to be determined so as to increase the membrane lifespan. At the same time, it facilitates studies related to the search for the optimal design parameters of the plant and the influence of sludge properties, which will lead to better efficiency in the process of wastewater treatment. It also facilitates sensitivity studies on the most important parameters in the system, which is a fundamental aspect to be considered in future modelling work so that the major contributors to the estimation error of the model can be determined. Last but not least, it gives access to intermediate parameters that cannot be regularly measured in real conditions, such as the resistances of the stable sludge cake layer and the dynamic sludge film, or the concentration of soluble microbial products, but which contribute to a better understanding of the way the process functions (membrane fouling, COD, etc.). It is important to note that the simulator was built in a manner that makes it an invaluable tool for teaching how an SMBR works, since it allows the user to interact with the operating conditions of the bioreactor and observe the influence of these parameters on the behaviour of the main control variables of the system over time.

Computer modelling has become a helpful tool in the analysis of the performance and effectiveness of wastewater treatment systems [START_REF] Korniluk | Simulation of wastewater treatment systems with membrane separation[END_REF]. Nowadays, industrial firms have become very interested because computer simulations significantly lower costs compared to experimental studies [START_REF] Kraft | Using a web module to teach stochastic modeling[END_REF]. The use of the SMBR simulator allows considerable saving of resources and time since, in general, performing experiments in real time on an SMBR is very expensive and time consuming. For the practical activity, we were able to place 24 students in front of their own 12 "devices". This allowed them to virtually implement several operating conditions and to analyze the consequences of this control. The user friendliness of the simulator helped in this, in a time much shorter than in the real world and at a cost compatible with the university's resources. The role of the teachers was to supply the students with all the practical procedures that are not included in the simulator, such as chemical cleaning or membrane replacement, and to keep the students aware of the real context of in situ experiments compared to in silico ones. In the case of the treatment plant under study, the plant needs a period of almost 20 days for the sludge mixture to become stabilized and, each time the critical value of trans-membrane pressure is reached, the system must be stopped to carry out expensive chemical cleaning or replacement of the membrane, often before the end of its lifespan [START_REF] Zarragoitia | Modelling of submerged membrane bioreactor: conceptual study about link between activated sludge biokinetics, aeration and fouling process[END_REF][START_REF] Kim | Determination of MBR fouling and chemical cleaning interval using statistical methods applied on dynamic index data[END_REF]. However, this study can be performed in the simulator in a short time and with considerable saving of resources.

Conclusions

An SMBR computer simulator was built with recent modelling knowledge and a friendly interface. The results given by the simulator are accurate enough to provide the trends and orders of magnitudes of physical quantities needed for the teaching application of the simulated MBR. The practical use of the simulator was evaluated with the development of simulated laboratory work lasting three and a half hours, which gave results that would take more than fifteen months of real-world experiments. It was successfully applied, and achieved the most difficult objectives of enabling the students to analyze the influence of operating parameters on the SMBR functioning and being largely accepted by the students. While this has not been tested, it appears clear that the dynamic model used would permit the training of professionals.

Fig. 1 -

 1 Fig. 1 -Conceptual scheme of the integrated model proposed by Zarragoitia et al. (2008).

  [START_REF] Zarragoitia | Modelling of submerged membrane bioreactor: conceptual study about link between activated sludge biokinetics, aeration and fouling process[END_REF] was used to build the SMBR simulator.

Fig. 2 -

 2 Fig. 2 -Modelling the membrane fouling process.

Fig. 3 -

 3 Fig. 3 -General algorithm for the implementation of the mathematical model.

Fig. 4 -

 4 Fig. 4 -Main graphical interface of the SMBR simulator and general scheme of the installation.

Fig. 5 -

 5 Fig. 5 -Comparison between the experimental data (points) and simulation results (line). The left-hand diagram shows the working conditions of the experiments.

  is limited by http://dx.doi.org/10.1016/j.ece.2014.03.001

	Nomenclature
	A	membrane area (m 2 )
	C	sludge concentration (kg/m 3 )
	C d	coefficient of drag and lifting forces
	d p	particle size (m)
	F l	lifting force
	F a	suction force
	G	apparent shear intensity of the fluid turbulence
		(s -1 )
	g	gravitational constant (m/s 2 )
	G i

Table 1 -Working conditions in which the experiments were performed in the SBRM.

 1

	Temperature ( • C)	q a (L m -2 s -1 )	t f (min)	t pf (min)	t aBG (min)	t paBG (min)	SRT (days)	J (m 3 /(m 2 day))
	7.3	11	10	4	2	8	40	0.25
	Source: Zarragoitia et al. (2008).

Table 2 -Characteristics of the wastewater to be treated.

 2

	X SST (mg/L)	X S (mg/L)	X I (mg/L)	X H (mg/L)	S S (mg/L)	S I (mg/L)	S PMS (mg/L)	S O 2 (mg/L)
	50	70	40	10	220	10	60	0.2
	Source: Zarragoitia et al. (2008).

Table 3 -Characteristics of the activated of sludge at the beginning of the experiments.

 3

	X SST (mg/L)	X S (mg/L)	X I (mg/L)	X H (mg/L)	S S (mg/L)	S I (mg/L)	S PMS (mg/L)	S O 2 (mg/L)
	5500	10	10	550	130	10	90	10
	Source: Zarragoitia et al. (2008).

Table 4 -Properties of the activated sludge and cake formed on the membrane surface.

 4

	Parameters	Value
	Erosion rate coefficient of the dynamic sludge film	3.5 × 10 -4
	Stickiness of the biomass particles	0.7
	Particle size (m)	1 × 10 -4
	Specific filtration resistance of the dynamic sludge	1 × 10 15
	film (m/kg)
	Specific filtration resistance of the sludge cake layer	1 × 10 15
	(m/kg)

Table 5 -Questions presented to the students.

 5

	No	Questions	Strongly agree	Agree	Disagree	Strongly disagree	I don't know
	1	This laboratory is interesting
	2	This laboratory helped me to represent
		the real functioning of a membrane
		bioreactor
	3	With this exercise, I have a better
		understanding of the influence of the
		membrane bioreactor functioning
		parameters on the process performance
	4	I was able to understand the functioning
		of the simulator by myself
	5	The participation of the teacher helped
		me understand the functioning of the
		simulator better
	6	The problem presented to me was clearly
		defined
	7	This activity enabled me to understand
		why a computer simulators is an useful
		tool in chemical engineering
	8	This laboratory is well situated in my
		education programme
	9	This laboratory is relevant to my
		programme
	10	The activity helped me to consolidate the
		concepts exposed in the classroom

9. Soluble undegradable organics. 10. Soluble microbial products. 11. Particulate undegradable organics. 12. Slowly biodegradable substrates. 13. Ordinary heterotrophic organisms. 14. Total suspended solids. 15. Mass attached to the membrane. 16. Filtered water volume.


César Ducruet

Prof W Rhoads Murphey

Asian cities in the global maritime network since the late nineteenth century

Introduction

One of the most striking features of contemporary Asia is the dominant concentration of population and economic activities along its shores, with a limited inland penetration of hinterlands [START_REF] Arasaratnam | ʻPre-colonial and early-colonial port towns'[END_REF]. Many scholars have already acknowledged the importance of port activities throughout Asian history (McGee, 1967;[START_REF] Basu | The Rise and Growth of the Colonial Port Cities in Asia, Berkeley: Center for South and South East Asian Studies[END_REF][START_REF] Murphey | ʻOn the evolution of the port city'[END_REF][START_REF] Broeze | Gateways of Asia: Port Cities of Asia in the 13th-20th Centuries[END_REF][START_REF] Gipouloux | La Méditerranée Asiatique[END_REF], Asian cities being best understood in their form and function as "brides of the sea" [START_REF] Broeze | Brides of the Sea: Port Cities of Asia From the 16th-20th Centuries[END_REF]. Despite many physical difficulties inherent to rapid urban and port growth [START_REF] Ness | Population Dynamics and Port City Development: Comparative Analysis of Ten Asian Port Cities[END_REF], maritime networks remain more than ever essential to the development of Asian cities and their host nations [START_REF] Taillard | Intégrations régionales en Asie orientale[END_REF]. Port traffics have long been closely tied to their adjacent urban economies although like in other regions, Asian port cities have developed a preference for other, more lucrative transport functions such as airlines [START_REF] Okuno | ʻUrban policies of some port cities in the Asia-Pacific corridor'[END_REF] and value-added logistics [START_REF] Cheung | ʻThe transition from freight consolidation to logistics: the case of Hong Kong'[END_REF]Wang and Chen, 2010). Nevertheless, certain aspects such as urban morphology have remained rather stable over time, as in the case of main Indian cities [START_REF] Kosambi | ʻThree colonial port cities in India'[END_REF].

While such aspects have been abundantly documented through case studies of particular places at specific time periods, this chapter innovates by mapping for the first time the changing distribution of port activities in Asia over the last 120 years. A snapshot of maritime activity every 40 years or so was chosen to best reflect major evolutions in the port hierarchy and the functions of Asian cities. How path-dependent have been port activities in Asia since the late nineteenth century? How can a study of maritime flows reveal some of the factors explaining the emergence of the current port city hierarchy? In this chapter, Asia is defined by its most extended definition ranging geographically from Greece to Siberia, but excluding Oceania. Whenever possible, reference is made to existing works on Asian port cities but without pretending to be exhaustive. One way to complement historical research has been to cover a number of papers written by geographers about Asian ports since the 1950s based on a review by [START_REF] Ng | The changing tides of port geography (1950-2012)'[END_REF] 1 on port geography in general. All in all, the main goal of this research is to provide a solid comparative overview serving as a reference for more detailed studies.

Methodology and preliminary results

An untapped resource was used in this study to measure the level and nature of port activities worldwide, namely merchant vessel movements published by Lloyd's List, the world's leader 1 See Appendix 1 for an overview of port geography papers about Asian port cities consulted as secondary sources in this chapter in maritime insurance and intelligence, on a daily or weekly basis. Such publications have never been used in a systematic manner but as a secondary source to identify specific vessels for detailed uses. This is rather surprising because it contains most of world trade activities through a very detailed description of vessels and their circulation, thereby allowing an analysis of shipping routes and port hierarchies as well as traffic concentration levels. Such documents provide new possibilities to understand, among other possible research fields, the precise position of Asian port cities in light of global maritime flows. The number of vessels, vessel calls was calculated in order to establish a ranking of Asian port cities according to their level of overall maritime and port activity.

The study period chosen in this chapter starts in 1890 and ends in 2008. This choice is partly explained by data availability but mostly by the fact that Lloyd's List started to include information from its direct competitors since 1890 onwards, i.e. vessel movement information from fleets insured by other companies, such as Nordske Veritas (Denmark) and Bureau Veritas (Belgium-France), but also Russian and Japanese fleets. This makes the spatial coverage of the analysis more relevant and complete than in the pre-1890 period when only the movements of British vessels were reported. More concretely, Lloyd's List reports in its Shipping Index the origin-destination of vessel voyages among ports of the world, including the dates of departures and arrivals, the type of vessels (e.g. steamer, tanker, containership) as well as their flag, registry, and tonnage capacity. Not all such information is used in this chapter, which focuses dominantly on counting the number of port calls per continent, region, and port city. A snapshot of global maritime activity has thus been extracted manually from paper sources in 1890, 1925, 1951, 1960, 1965, 1970, 1975, 1980, 1985, 1990, 1995, 2000, and 2008, around the months of April-May, i.e. one publication only among the 52 weekly publications each year. Figure 1 provides an example of such records for sailing vessels in 1890: each vessel has one voyage reported between two or more ports, depending on the mention or not of intermediary calls along the route. The method has been to calculate the sum of movements for each port on the level of the global maritime network formed by the links among the ports (see also [START_REF] Ducruet | ʻMapping global urban interactions: maritime flows and port hierarchies since the late nineteenth century '[END_REF].

[Insert Figure 1 about here] Lastly and although it is strictly impossible to systematically confront the global data from Lloyd's to more local information on port activities due to obvious constraints, a focus on China is proposed in Figure 2. The data collected by [START_REF] Wang | ʻRegional resilience and spatial cycles: long-term evolution of the Chinese port system (221BC-2010AD)'[END_REF] about Chinese port traffics over the 1868-2009 period 2 confirms the accuracy of the global source, despite, for instance, the non-inclusion of river traffics by Lloyd's, which is more specialized in tracking ocean-going vessels embarked on international trades. For China's totals, the powerlaw relation between tonnage (local) and vessel calls (global) accounts for no less than 95%, while for Shanghai, it amounts to 92% using the same statistical method. It is also satisfactory to observe identical traffic variations in times of drastic political changes, as seen with the sharp decline of all traffics between 1925 and 1951. The rest of the evolution is more linear for tonnage and more fluctuating for vessel calls, but this is probably due to the fact that vessel calls do not take into account the size of vessels, in terms of their carrying capacity.

Nevertheless, such an exercise, especially based on such a complex case like China, demonstrates the accuracy of Lloyd's information and its ability to translate not only global but also local dynamics, thereby motivating its application to the rest of Asia.

2 Chinese data consists in port traffics provided by the Chinese Customs measured in Hong Kong Taels for the period 1868-1938), and by the Ministry of Transportation measured in metric tons for the period 1932-2009. The two time series were adjusted based on one single unit, the metric ton, although the first does only report international traffics, thus excluding domestic activities [START_REF] Wang | ʻRegional resilience and spatial cycles: long-term evolution of the Chinese port system (221BC-2010AD)'[END_REF].

[Insert Figure 2 about here] It is a fact that Asia as a whole had concentrated a growing proportion of global port activity, from about 10% in 1890 to nearly 45% in 2008. This evolution is by no means illustrative of the well-known global shift of production and manufacturing that occurred since the 1960s, notably promoting East Asia as one of the three major economic poles of the world economy.

As seen in Figure 3, the share of Asia as a whole has grown at the expense of the two other main economic poles, namely Europe and North America, while other regions have seen their relative weight rather stagnant over the period. One notable exception is the initial and overwhelming importance of Latin America, mostly backed at the time by the economic boom of Argentina around Buenos Aires. In fact, Asia's growth has been rapid in the first phase in a context of increased colonial interests, then stabilized in the 1970s and 1980s following the independence of former colonies, and went through a rapid growth again from the 1990s onwards, based on rapid port modernization and the "China effect".

[Insert Figure 3 about here] Internal trends are more visible in Figure 4, where one can observe very huge traffic shifts across Asia over time. Northeast Asia has become Asia's largest concentration of port activity as it comprises Japan, China, and three of the four so-called "Asian Dragons" (South Korea, Taiwan, and Hong Kong) where port growth has been the fastest. At the start of the study period, the Indian subcontinent was by far the largest port region, but its share rapidly shrunk up to the 1970s, leaning towards a mere 10% nowadays. This evolution has a lot to do with the demise of the British Empire and, of course, the rapid industrialization of Japan, largely dependent upon maritime trade. Southeast Asia occupies the second rank nowadays, which was also true in 1890, but it is only from the 1990s that it regained this position. One main reason has been the reinforced connectivity of the Singapore hub to cope with the rise of the so-called "Asian Tigers" in that region. The fast growth of port activity in the Middle East in the early period is largely due to the boom of oil trade and shipping. East and South Asia altogether occupy no less than 70% of Asia's total port activity nowadays. The role of individual port cities is explored in the next section.

[Insert Figure 4 about here]

Changing port hierarchies of Asian cities

Overall traffic distribution

Asian ports have become organized around a trunk line or corridor along which major hubs concentrate and redistribute cargo flows to/from secondary ports [START_REF] Ducruet | ʻPort competition and network polarization at the East Asian maritime corridor'[END_REF].

Recent structures are, of course, inherited from the past and notably colonial times when East-West routes were based on strategically located comptoirs for entrepot trade established by European powers [START_REF] Murphey | ʻUrbanization and the Western impact in East Asia: the treaty-port phenomenon'[END_REF] and even on earlier trade patterns taking place prior colonialism [START_REF] Gipouloux | La Méditerranée Asiatique[END_REF]. Figure 5 serves to situate the main port cities of Asia and their changing level of activity from one period to the other.

[Insert Figure 5 about here] The most active cities in 1890 belong to European empires (e.g. India, Hong Kong, Singapore, Myanmar, Indonesia, Macau, Philippines) with Calcutta being the largest port at the time [START_REF] Dutt | ʻDelineation of the hinterland of Calcutta port'[END_REF][START_REF] Tan | ʻPort cities and hinterlands: a comparative study of Singapore and Calcutta'[END_REF] but also Japan, China, and the Black Sea. Overall, Asia has a high proportion of steamer traffic compared with the rest of the world outside the North Atlantic in the late nineteenth century [START_REF] Ducruet | ʻPorts et routes maritimes dans le monde (1890-1925)'[END_REF]. Besides the stability of the port system in the second period, certain cities have witnessed tremendous growth, as reflected by Yokohama and Kobe in a context of industrial and colonial expansion since the Meiji era, with direct impacts on Dalian in China [START_REF] Todd | ʻPolitical and technical factors impinging on port operations: the case of Manchuria'[END_REF]. Other cases such as Karachi in Pakistan [START_REF] Maxwell | ʻThe development of the ports of East Pakistan'[END_REF][START_REF] De Nie | ʻThe Indus, Karachi and its hinterland'[END_REF], Mumbai, Galatz, Constantza, and Piraeus better depict the reinforced importance of the Europe-Asia link through the Suez Canal built in 1869. In comparison, Hong Kong and Shanghai have seen their activity stabilizing or even declining due to political instabilities in mainland China. Yet, Shanghai remains China's main gateway with the outside world due to its sea-river connection through the Yangtze [START_REF] Reischauer | ʻNotes on Tang Dynasty sea routes'[END_REF][START_REF] Wiens | ʻRiverine and coastal junks in China's commerce'[END_REF][START_REF] Tang | ʻInter-port transshipment between Shanghai and Hankou in foreign trade: 1864-1930'[END_REF], far beyond Tianjin for instance [START_REF] Hitch | ʻThe port of Tientsin and its problems'[END_REF].

In the early 1960s, Japan keeps its prominence as five of the top twenty cities are Japanese [START_REF] Hanrath | ʻPort development in Japan'[END_REF]. There is a rapid growth of Hong Kong [START_REF] Boxer | ʻShipping movement and economic growth in Hong Kong, 1957-1958'[END_REF][START_REF] Dwyer | ʻThe recent development of the port of Hong Kong'[END_REF] and Singapore as the new Asian Tigers promoting export-led growth through free-zone development as well as elsewhere in Asia [START_REF] Robinson | ʻIndustrial strategies and port development in developing countries: the Asian case'[END_REF], compared with the relative decline of Indian ports where new ports have developed outside traditional port cities for the bulk handling of raw materials [START_REF] Van Suylichem | ʻCalcutta's new harbour as a magnet in the urbanisation process in Eastern India'[END_REF][START_REF] Kidwai | ʻPort cities in a national system of ports and cities: a geographical analysis of India in the 20th century'[END_REF]. Another specificity of the 1960s is the fast growth of Gulf ports (Hugues, 1979;[START_REF] Walker | ʻRecessional and Gulf War impacts on port development and shipping in the Gulf States in the 1980's'[END_REF][START_REF] Jacobs | ʻWhat conditions supply chain strategies of ports? The case of Dubai'[END_REF] of which Dubai and specialized oil ports (e.g. Mena al Ahmadi, Ras Tanura), transit hubs (Aden), but also of Black Sea ports such as Odessa in a context of Soviet control [START_REF] Krug | ʻThe development of Odessa as a port and trading centre'[END_REF]. Aden was still at the time part of the British Empire with facilities dedicated to British Petroleum, but also acting as an interregional hub port for mail. Other Asian ports received far less attention from geographers except from specific papers on Keelung [START_REF] Chen | ʻThe port of Keelung'[END_REF], Borneo [START_REF] Lee | ʻThe port towns of British Borneo'[END_REF], Cebu [START_REF] Uernstedt | ʻCebu: focus of Philippine inter-island trade'[END_REF], and Malaysia [START_REF] Ward | ʻMalayan fishing ports and their inland connections'[END_REF][START_REF] Ward | ʻMajor port hinterlands in Malaya'[END_REF].

The last period confirms the overwhelming dominance of the Asian Tigers and Japan concentrating twelve of the top twenty ports in 2008, followed by other major ports in China (Shanghai), East Mediterranean and Black Sea (Alexandria, Piraeus, Constantza), Southeast Asia (Port Klang, Jakarta), and the Middle East (Fujairah, Jebel Ali). Across the entire study period, only Japan, Hong Kong, Singapore, and Jakarta have remained among the top twenty Asian ports notwithstanding certain fluctuations in their rank order. This relative success is well reflected by a voluminous literature on the evolution of containerization in such Asian global port cities [START_REF] Robinson | ʻContainerization and the port of Hong Kong in the 1970s'[END_REF][START_REF] Bristow | ʻSome consequences and impacts of port development -the Hong Kong case'[END_REF]Airriess, 2001a) but also urban and regional issues in port development [START_REF] Todd | ʻNew port developments and balanced regional growth: a Taiwan example'[END_REF][START_REF] Chang | ʻOn the waterfront: globalization and urbanization in Singapore'[END_REF][START_REF] Lee | ʻSpatial globalization in Asia-Pacific hub port cities: a comparison of Hong Kong and Singapore'[END_REF] as well as competition and technological diffusion in port systems [START_REF] Airriess | ʻThe spatial spread of container transport in a developing regional economy: North Sumatra, Indonesia'[END_REF][START_REF] Airriess | ʻGlobal economy and port morphology in Belawan, Indonesia'[END_REF][START_REF] Slack | ʻThe challenge of peripheral ports: an Asian perspective'[END_REF][START_REF] Song | ʻRegional container port competition and co-operation: the case of Hong Kong and South China'[END_REF]. South Asian ports have clearly lost their initial importance in recent times (India, Pakistan, Sri Lanka, Myanmar) compared with the rapid growth of Korean and Taiwanese ports such as Busan [START_REF] Frémont | ʻThe emergence of a mega-port -from the global to the local: the case of Busan'[END_REF], Incheon [START_REF] Ducruet | ʻThe flight of Icarus? Incheon's transformation from port gateway to global city'[END_REF], and Kaohsiung [START_REF] Todd | ʻThe interplay of trade, regional and technical factors in the evolution of a port system: the case of Taiwan'[END_REF][START_REF] Haynes | ʻRegional port dynamics in the global economy: the case of Kaohsiung, Taiwan'[END_REF].

Chinese ports received attention only recently after a period of maturation in the container business, on the level of the whole country [START_REF] Todd | ʻChina's energy needs, coal transfers and the ports sector'[END_REF][START_REF] Wang | ʻThe geographical connectedness of Chinese seaports with foreland markets: a new trend?'[END_REF][START_REF] Wang | ʻRegional resilience and spatial cycles: long-term evolution of the Chinese port system (221BC-2010AD)'[END_REF], large regions such as the Pearl River Delta [START_REF] Wang | ʻThe evolution of a regional container port system: the Pearl River Delta'[END_REF], or individual cities and corridors [START_REF] Todd | ʻChanging technology, economic growth and port development: the transformation of Tianjin'[END_REF][START_REF] Wang | ʻPort-FEZ bundles as spaces of global articulation: the case of Tianjin, China'[END_REF][START_REF] Wang | ʻNew port development and global city making: emergence of the Shanghai-Yangshan multiayered gateway hub'[END_REF], except from very few case studies in the past (see [START_REF] Tregear | A Chinese river port with a future'[END_REF]. Other abundant works have been published in more transport-oriented journals dealing with aspects of port governance, operation, and management, thus reaching beyond the scope of this chapter despite their relevance. Elsewhere in Asia, Piraeus also had a relatively stable position among major ports, while the Black Sea was always represented but by different ports over time.

Individual port trajectories

Mapping the more detailed evolution of individual Asian port cities is addressed here based on a selection of the three largest ports of each sub-region of Asia, in terms of their average number of vessel calls all along the period (Figure 6). Hong Kong and Singapore are kept apart for the sake of comparability as their transshipment hub activity gives them a too high number of calls to be placed in the same figure with other Asian ports. The most striking fact is the vast heterogeneity of trajectories among Asian ports, even within the same sub-region.

Still, in Japan, the top ports went through growth and decline phases rather accordingly, while Nagoya could enjoy a more stable profile probably due to the prolonged vitality of its automobile industry, much relying on the port for coastal and overseas trade. The relative decline of Japanese ports in recent decades is explained by several factors: an environmentfriendly transport policy in favor of smaller ports and coastal shipping to prevent further expansion of already gigantic ports, the shift of transshipment activities to South Korea and notably Busan, and an overall reduction of demand in an ageing country where cargo value takes over cargo volume in the priorities. In the rest of Northeast Asia, Busan and Kaohsiung show a close trajectory; with an earlier take-off of Kaohsiung notably since the development of its free-trade zone in 1966. Shanghai has a distinct profile although it also has grown fast in recent decades and, interestingly, it is only in the 1990s that its number of calls had been lesser than Busan. From 2008 however, Shanghai has become the world's largest port in terms of tonnage, and the second in Asia after Singapore for the number of vessel calls. As already investigated earlier, the rise of traffic volumes did not necessarily mean a shift of centrality towards Shanghai from other established hub ports, which, at the contrary, reinforced their transshipment activities during that period, for technologiczal but also geopolitical reasons of logistical and information control, and also due to the fact that China still nowadays does not offer a transparent business environment to foreign investors, of which global shipping lines and terminal operators (Ducruet et al., 2010).

[Insert Figure 6 about here] Between Hong Kong and Singapore, the huge difference observed by the number of vessel calls overestimates Singapore's true importance in the region. The simple fact that more than 80% of Singapore's total port activity is dedicated to transit trade (transshipment) largely explains those numbers, bearing in mind that most of containers are counted twice during the transit over Singapore. Also, such a transshipment activity has the direct effect of increasing the call frequency of smaller vessels (feeder vessels) that redistribute the cargo from larger (mother) vessels within and between regions. Another differentiating factor is the lack of landside access from Singapore to the continent, whereas for Hong Kong, its gateway function towards Mainland China has always been a decisive aspect of its activity, even prior China's official opening to the world in the late 1970s. In the rest of Southeast Asia, Manila is rather stagnating over the period, while Bangkok went through a rapid decline after reaching a peak of activity in the early 1990s. Like in other South Asian cities, and as seen in Figure 6 with other cases such as Calcutta, Mumbai and Karachi, modern port activities have been shifted outside the original urban core. Yet, such spatial dislocations of port areas to cope with lack of space and congestion in rapidly growing cities are not counted here because the unit of analysis if the compact port area, not the larger metropolitan city or agglomeration. New ports such as Bin Qasim for Karachi, Haldia for Calcutta, Neva Sheva for Mumbai, Laem Chabang for Bangkok are clear example of renewed port functions outside traditional Asian port cities.

Further research on those shall, perhaps, reintegrate the activity of new ports in the data analysis in order not to underestimate the current level of their total activity. Still, local factors and planning problems remain secondary compared with the wider geopolitical shifts that occurred, for instance, in India when the British Empire was dismantled, with direct effects on port activity in Mumbai and Calcutta, but also Karachi and Yangon (see also Figure 5).

Competition for oil resources extraction and shipping better explains the decline of Ras

Tanura and Mina al Ahmadi in the Middle East, but again, the activity of Dubai (Port Rashid) is by no means underestimated given the rapid growth of the recently developed Jebel Ali container port, which has become the main hub of the region in recent years.

Conclusion

This chapter has tackled a difficult exercise that was to map and analyze the changing spatial pattern of the Asian port system since the late nineteenth century. This posed a number of conceptual and methodological challenges, from the definition of "Asia" to the technical feasibility of measuring port activity across such a large land mass and over such a long period of time. Due to the existence of a largely untapped resource, namely vessel movement information published by the maritime insurer Lloyd's List, the goal has been achieved and the main results provide novel evidence about one of the major economic functions exerted by Asian cities, "brides of the sea" [START_REF] Broeze | Brides of the Sea: Port Cities of Asia From the 16th-20th Centuries[END_REF]. Although such a large-scale approach has blurred the local situations and dynamics, it made possible to evaluate and compare the unevenness of port activity throughout the Asian port system at selected key periods based on harmonized measures. The main lesson to be drawn from this study is, perhaps, the confirmation of Asia's early and reinforced importance in global trade during the contemporary era. The current pattern of port activity has witnessed important shifts in the region, but it still rests on inherited patterns that exhibit strong path-dependency. Yet, "colonial Asia" remains very different from "global Asia" in many ways. Like other main economic poles of the world system, it faces important challenges of port-city planning due to the unprecedented levels of traffic concentration at its gateways and hubs.

The analysis of the changing distribution of vessel movements is only a first step towards a better understanding of Asian port cities as a whole. Further research shall concentrate on better classifying port cities based on more advanced statistical methods that would highlight, for instance, specific clusters of port cities sharing comparable evolutionary pathways.

Another research direction would be to better link port dynamics with urban dynamics over the period under study, perhaps through immense efforts put on collecting urban indicators such as demographic size for cities. This would better highlight the relative importance of port activities compared with the rest of the urban economy, as port functions grow but also decline depending on local policies and strategies in various domains. Lastly, it was not assessed how Asian port cities have been connected through maritime flows and what is the architecture of such linkages, based on a more detailed view of the network designed by the overlap of vessel flows. Such findings would prove even more fruitful when put into a comparative perspective with other large regions of the world such as Europe and North America for instance, where larger cities and larger ports continue, more than ever, to concentrate the bulk of economic life. 1925 1951 1960 1965 1970 1975 1980 1985 1990 1995 1890 1925 1951 1960 1965 1970 1975 1980 1985 1990 1995 1890 1925 1951 1960 1965 1970 1975 1980 1985 1990 1995

Figure 1 .Figure 2 .

 12 Figure 1. The Lloyd's Shipping Index in 1890

Figure 4 .

 4 Figure 3. Regional distribution of world vessel movements, 1890-2008

Figure 5 .Figure 6 .

 56 Figure 5. Traffic hierarchy of Asian port cities, 1890-2008

  2000 2008

	(%)
	movements	Africa Latin America
	vessel world	North America Europe Oceania
	Share of	Asia
	100%
	90%
	80%
	70%
	60%
	50%
	40%
	30%
	20%
	10%
	0%

Acknowledgements

The author wishes to thank Mrs. Liliane Lizzi at Géographie-Cités for her help with cartography as well as Prof. François Durand-Dastès at Paris I University for his useful comments on the results of this work. The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. [313847] "World Seastems" 3 .