6,196 research outputs found
Tot hier en verder: over de grenzen van een vakgebied
No full textOratie uitgesproken door Prof.dr. J.C. Jansen bij de aanvaarding van het ambt van Hoogleraar KNO in het bijzonder Hoofd- Hals- en Schedelbasisoncologie aan de Universiteit Leiden op vrijdag 18 oktober 2019LUMC / Geneeskund
Flora of the Guianas. Series A: Phanerogams. Fascicle 27
No full text71. Cyrillaceae / J.C. Yesilyurt. 79. Theophrastaceae / B. Ståhl. 86. Rhabdodendraceae / G.T. Prance. 90. Proteaceae / G.T. Prance. 100. Combretaceae / G.A. Stace. 113. Dichapetalaceae / G.T. Prance. 167. Limnicharitaceae / R.R. Haynes & L.B. Holm-Nielsen. 168. Alismataceae
/ R.R. Haynes & L.B. Holm-Nielsen. Including Wood and Timber / I. Poole & J. Koek-Noorman, F. Lens & S. Jansen, J. Koek-Noorman, I. Poole, L.Y.T. Westra & J.W. Maas
De eerste minuten. Attentum, benevolum en docilem parare in de inleiding van toespraken
No full textContains fulltext :
62004.pdf (Publisher’s version ) (Open Access)Katholieke Universiteit Nijmegen, 17 mei 2004Promotores : Hoeken, H., Jansen, C.J.M.565 p
J.C. Steyn Collection index
No full textThis index describes the J. C. Steyn collection which includes background material for 3 biographies by J.C. (Jaap) Steyn namely N.P. van Wyk Louw, P.J. Cillié and MER (M.E. Rothman). Prof. J.C. Steyn (1939-) is an educationist, linguist and author. Correspondence ; clippings ; photographs ; book reviews ; articles ; speeches ; varia compiled in 23 pamphlet boxes
The potential of zeolite membranes in hydroisomerization processes
No full textThe oil we are burning in two centuries took hundreds of millions of year to form from primeval plants. The average light vehicle burns 100 x the car weight in prehistoric plants in the form of gasoline. Only 12.5 % of the fuel energy reaches the wheels, 7% is used in accelerating the car and less than 1% moves the driver. The draconically inefficient use of our energy sources in car transportation is alerting the community in other areas as well. Gradually, a mind setting turns to focus on higher efficiencies. A typical example of the size of changes that should be made is given in the Innovative Roadmap Separation Technology, 2004 . The authors recommend to reduce energy consumption in separation processes up to 75%. Such numbers cannot be realised by improving state-of-the-art systems but only by developing revolutionary, new technologies, based on new materials. The content of this thesis pertains to a specific case in energy efficiency, the separation of linear and branched alkanes using membranes as an alternative for distillation columns and adsorption towers. Production processes of branched alkanes applying membranes are suggested. The membranes used for the study were zeolite-based materials as they have extremely interesting properties for application in this area. In the following paragraphs the zeolite properties and the zeolite membrane synthesis and performance in general are given.Applied Science
Separation of Process Water using Hydroxy Sodalite Membranes
No full textThis thesis describes the synthesis, characterization, and application of Hydroxy Sodalite (H-SOD) membranes in selective separation of water from aqueous solutions and reaction media. The emphasis has been put on the development of a tight membrane film that could be primarily used for water separation only, followed by an explorative study on the performance of the membrane under elevated temperatures and pressures. Zeolite membranes are often composed of (an) intergrown layer(s) of crystals bonded to the surface of a support material. The crystal size and morphology, affinity of crystals with the support surface, and synthesis conditions determine the closure of the film. An interplay between these factors contributes to the formation of a film with no or very few defects. As described in Chapter 2, prior to preparation of the membrane, the conditions needed for synthesizing the powder were optimized through a series of experimentation studies. The synthesis time and temperature were varied accordingly to favour formation of hydroxy sodalite and inhibit the co-formation of other types of zeolites, which are often co-found as impurity phases (e.g., zeolites A, -X, and -P). The as-synthesized powder was tested for the amount of water it was capable of adsorbing and releasing. Similar to other members of the sodalite family, hydroxy sodalite consists of ?-cages, which can be occupied by water. As the cages are very small, the presence of this water contributes to the occupation of most of the available space. From TG/DT studies, it was found that hydroxy sodalite can release its entire crystalline water, but after this it is no longer capable of adsorbing back the lost water. Subsequently, if the material is only partially dehydrated, the lost water can be re-adsorbed. Therefore, provided that (part of) the crystalline water in the cages is preserved, sodalite has potential for water uptake. Following this, the thermostability of the crystals was studied during high temperature XRD measurements. The material maintained its structure up to 650oC after which it started transforming to denser phases. Interestingly, the supported membrane showed a higher thermostability than the powder material. Attempts to increase the Si/Al ratio of the SOD were not successful, with the followed template-free synthesis procedure always the basic ratio of 1 was obtained. Based on the conditions optimized for synthesis of H-SOD powder, the membrane was developed. As highlighted in Chapter 3, unlike powder synthesis, which was successful over a range of different conditions (90-140oC, 3-20 hrs), membrane synthesis only prevailed in a narrow range (~140oC, 3.5 hours). In contrast to powder formation, in membrane synthesis the rotation of the autoclave containing the sol and the support was essential, as was pre-treatment of the support. Rotation during synthesis ensures better mixing of the solution whereby concentration and temperature gradients in the autoclave are minimized. Support pre-treatment was found as another prerequisite. Supports were degassed prior each synthesis, this was done to prevent release of trapped air from the support pores during synthesis in the form of bubbles on the surface where crystals cannot (inter)grow. This combination of rotation during synthesis and support degassing resulted in the formation of closed membrane films which showed no N2 permeability and very little He permeation (beyond detection limit of the set-up). The membrane permeated water at reasonable fluxes (up to 2.1 kg.m-2.h-1) at elevated temperatures and pressures (200oC, 22 bar), and was further used to separate water from a series of organic alcohols-water mixtures as demonstrated in Chapter 4. Two different types of alcohol homologues were dewatered, primary and secondary, where effect of operating conditions on the water flux were studied. In case of primary alcohols (methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol) the water flux through the membrane increased with increasing water concentration, whereas the flux in the secondary alcohol systems showed a different dependency. In these, the membrane showed almost no permeability towards water until a certain water concentration above which the flux started increasing rapidly, although it generally remained very low compared to the corresponding case as in dewatering the primary alcohol. The behavior, although largely speculative, is attributed to the blocking of surface voids in the membrane by the secondary alcohols. The membranes showed good stability under the high temperatures and pressures (up to 200oC at 22 bar) applied and were highly water selective with no traces of any of the tested alcohols in the permeate. Establishing the thermal and mechanical stability of the membrane and building on its excellent selectivity towards water, the long-term chemical stability of the membrane was evaluated under exposure to strong acidic and basic media as shown in Chapter 5. The membrane preserved its performance in a range of 2.9 ? pH ? 13.7. This characteristic was used in employing a H-SOD membrane later under reaction conditions (Chapters 7 and 8). Due to its very high water selectivity over other components, the H-SOD membrane was employed in desalinating seawater and other salt solutions (Chapter 6). The resultant permeate water was found to be ultra pure, i.e., (almost) no traces of original ions or salts were detected in the water. The findings open different windows for application of H-SOD membranes in industry or laboratories. Also here some unexplained phenomena were observed, namely the addition of NaCl increased the water flux through the membrane, like for seawater, whereas NaNO3 addition lowered the flux. The applicability of the membrane in a catalytic membrane reactor was tested in esterification of acetic acid with ethanol and butanol. As illustrated in Chapter 7, the membrane was capable of selectively removing water under the reaction conditions, whereby the conversion was increased by 30% leading to completion of the reaction. Chapter 8 finalises the use of H-SOD in relation to the Fischer-Tropsch synthesis. The performance of H-SOD is compared with that of an amorphous silica membrane and a ceramic supported polymer membrane. A previous study on the application of the latter two membranes under FT conditions had showed the disintegration of the membranes at the high reaction temperature and pressure and abundant presence of water. In contrast, a hydroxy sodalite membrane which was employed in separating water from water/hydrogen streams at elevated temperatures and pressures showed absolute selectivity towards water while maintaining its stability. Hydroxy sodalite is an attractive material in terms of water separation applications at elevated temperatures and pressures. Fluxes remain low compared to other reported hydrophilic membranes. However, depending on the application and where costs of product recovery or recycling are detrimental to the process, sodalite is a clear candidate in that it is absolutely selective towards water, withstands thermal and mechanical stresses, and is chemically stable under not-too severe acidic and basic conditions. The future challenges in this respect comes down to the production of defect free membranes that can be scaled out in form of a membrane module or otherwise to drive flux to higher values worthy of industrial appreciation. Following the experimental study demonstrated in this thesis, utilization of hydroxy sodalite membranes under different reactive conditions will be a field for future exploration.Catalysis EngineeringApplied Science
Vormen van accessoriëteit: een romanistische studie over het verschijnsel accessoriëteit bij het goederenrechtelijke zekerheidsrecht
No full textItem does not contain fulltextUniversiteit Leiden, 26 mei 2005Promotores : Zwalve, W.J., Jansen, C.J.H. Co-promotores : Hijma, J., Kortmann, S.C.J.J., Sirks, A.J.B., Snijders, H.J., Wacke, A.XXVI, 285 p
Understanding ordered silica: Linking topology and energetics
No full textTraditionally compounds in crystal chemistry are described in terms of their composition and geometry (i.e. bond lengths and angles). While this geometrical description is powerful, it has an intrinsic weakness in that it is hard to describe structures in terms of features (e.g. pores) important to their application in the chemical industry (e.g. catalysis, adsorption). An alternative method of describing structures in terms of their underlying topology (i.e. atomic connectivity) tries to overcome this weakness. The work described in this thesis mainly focuses on expanding this topological description to a set of calculable descriptors that allow for facile comparison between structures. The first part of the thesis describes the development of topological descriptors, based on the face-size distribution, and its application to rationalise the energetics of the all-siliceous zeolites. It is shown that decomposing framework materials into a space-filling set of face-sharing polyhedral cages, and analysing the face-size distribution obtained, leads to powerful insights into the dependence of the thermodynamic viability of zeolite- and more general framework material synthesis upon changes in structural properties such as pore-size and framework density. The second part of the thesis focuses on silica nanoclusters and a description of their possible topologies. It is shown that silica and related oxides can in principle be synthesized in the form of fully coordinated nanoclusters with structures reminiscent of those of carbon buckyballs (fullerenes). Furthermore, it is demonstrated that such fully coordinated nanoclusters are interesting computational model systems (surface defects, infrared spectra) for dehydrated silica surfaces.Applied Science
Opportunities and limitations of hydrogen storage in zeolitic clathrates
No full textThe feasibility of using zeolites, and more specifically the clathrasil subgroup, for hydrogen storage has been investigated by comparing their H2 loading rate and storage capacity to the technically required values. The uptake rate and capacity are determined by means of computational modelling for a large number of clathrasil structures. It appeared that the loading rate is sufficient for most structures, while the capacity at reasonable operating pressures and temperatures is far too low. This implies that clathrasils should not be used for H2 storage, however, thanks to the high diffusion rates in these materials, they might be interesting as size-selective membranes. Additionally, the encapsulation of hydrogen during the hydrothermal synthesis of the clathrasil DD3R is explored experimentally and found to be possible.Applied Science
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