Semali Perera

Use of the linear driving force approximation to guide the design of monolithic adsorbents

The objective of this paper is to provide the basic tools necessary to guide the optimal design of monolithic adsorbents. Previous work has concentrated on optimizing monolith manufacturing processes and experimental studies have suggested that the mass transfer performance of the monolithic form might be inferior to that of the more traditional packed bed form. In this paper, the classical linear driving force approximation, along with the parabolic concentration gradient assumption, is applied to a number of simple geometries. Transformation of square, rectangular, triangular and hexagonal geometries to an equivalent hollow cylinder on the basis of equal volume and equal internal surface area, facilitates use of the linear driving force analytical solution for a cylinder in order to guide the design of the more complex monolith geometries. Taking channel mass transfer performance and pressure gradients into consideration as well, the analyses indicate that regular hexagonal channels offer the best compromise on overall performance, with minimization of the wall thickness being the key design objective. Use of the algebraic design equations for the circular channel provides an excellent approximation for the regular hexagon and thus design work can be carried out with the former and simpler geometry. The engineering challenge now becomes one of manufacturing monoliths with appropriately thin walls. A challenge for the future is to obtain the full numerical solutions for the square, rectangular, triangular and hexagonal geometries.

The effect of the binder on the manufacture of a 5A zeolite monolith

Abstract The effect of the binder on the manufacture, by a paste extrusion process, of a 2 mm square lattice channel zeolite monolith with a 0.98 mm wall thickness and an overall diameter of 20 mm has been studied using a variety of visual and analytical techniques. Crucial factors for manufacturing defect-free 5A zeolite monoliths have been found to be the use of a binder with good plasticity properties, such as Na–bentonite, extrusion conditions, and a well-controlled drying process. Pastes containing different amounts of water and binder were characterised from the relationship between pressure drop and extrudate velocity during flow from a circular barrel into a circular die-land. From the relationship between the extrusion pressure and the extrudate velocity, six extrusion parameters were derived for each paste. A higher extrusion pressure is required when there is either a decreased water content or an increased binder content.

Boronate affinity saccharide electrophoresis: A novel carbohydrate analysis tool

The incorporation of specialised carbohydrate affinity ligand methacrylamido phenylboronic acid in polyacrylamide gels for fluorophore‐assisted carbohydrate electrophoresis greatly improved the effective separation of saccharides that show similar mobilities in standard electrophoresis. Polyacrylamide gel electrophoresis using methacrylamido phenylboronic acid in low loading (typically 0.5–1% dry weight) was unequivocally shown to alter retention of labelled saccharides depending on their boronate affinity. While conventional fluorophore‐assisted carbohydrate electrophoresis of 2‐aminoacridone labelled glucose oligomers showed an inverted parabolic migration, an undesired trait of small oligosaccharides labelled with this neutral fluorophore, boron affinity saccharide electrophoresis separation of these carbohydrates completely restored their predicted running order, based on their charge/mass ratio, and resulted in improved separation of the analyte saccharides. These results exemplify boron affinity saccharide electrophoresis as an important new technique for analysing carbohydrates and sugar‐containing molecules.

Zeolite Monoliths for Air Separation: Part 1: Manufacture and Characterization

A 2 mm square lattice channel monolith with a 0.98 mm wall thickness and an overall diameter of 20 mm has been prepared from 5A zeolite powder and Na-bentonite (as a binder) by the unit operations of paste preparation, extrusion, drying and firing. The manufactured monolith, which shows good strength, retains the crystal structure and micropore size of the source 5A zeolite powder, and adsorption measurements made by the constant volume method confirm that the monolith exhibits an equilibrium performance equal to that of commercial 5A zeolite pellets. In addition, the monolith possesses a higher macroporosity than the commercial pellets, a feature which is expected to be of benefit in pressure swing adsorption air separation processes.

Zeolite Monoliths for Air Separation: Part 2: Oxygen Enrichment, Pressure Drop and Pressurization

Experiments have been carried out on two individual idealized steps in a pressure swing cycle in order to compare the performance of a novel adsorbent monolith with that of a packed bed of commercial pellets containing the same weight of adsorbent. The application is the production of oxygen-enriched air using 5A zeolite. For feed pressures up to 3.8 bar, maximum oxygen compositions in the constant pressure production step up to 100% and 52%were achieved for the monolith with the previous step being carried out (1) by purging the column with pure oxygen at 1 bar, and (2) by evacuating the column, respectively. For virtually all experimental conditions studied, the separative performance of the monolith was found to be somewhat inferior to that of the bed of pellets, this being due largely to the former’ s poorer film mass transfer coefficient together with the virtual independence of this coefficient on velocity. To its advantage, however, the pressure drop through the monolith was found to be 3-5 times lower than that through the equivalent packed bed and consequently the time to pressurize the monolith was found to be 3-5 times faster than for the bed of pellets. Thisexperimental feasibility study has demonstrated that the novel zeolite monolith configuration shows good potential for the production of oxygen-enriched air of low-to-medium purity, in a low energy, short cycle time, pressure swing process. The challenges associated with both improving and modelling the performance of the monolith are described.

An experimental and theoretical investigation of a catalytic membrane reactor for the oxidative dehydrogenation of methanol

Abstract The catalytic oxidation of methanol was investigated employing a tubular wall reactor to elucidate reaction kinetics and a membrane reactor to record performance. The membrane reactor consisted of a composite multilayered ceramic tube impregnated with platinum catalyst and housed within a shell of stainless steel construction. Thermodynamic calculations and catalyst activity experiments revealed that hydrogen is a main product of reaction for mixtures rich in methanol and lean in oxygen for temperatures up to 300°C and 1 bar pressure. Kinetic experiments indicated that two separate pathways yielding hydrogen were prevalent: a catalytic dehydrogenative oxidation giving H 2 and CO 2 as products and complete catalytic combustion giving CO 2 and H 2 O. Further experimental measurements using the catalytic membrane reactor showed that hydrogen as product could be partially separated from the reaction products by the action of the ceramic membrane. A comprehensive theoretical model of the membrane reactor was constructed using Maxwell–Stefan equations, the dusty gas model and differential energy balances. Results of the theoretical investigation utilising the kinetic parameters found by experiment indicated reasonably good agreement between theory and experiment. However, it was also clear that using a ceramic membrane impregnated with catalyst is not an efficient way to achieve H 2 separation during reaction on account of the ability of H 2 under the prevailing reaction conditions to diffuse in opposite directions simultaneously.

Manufacture and Characterisation of Silicalite Monoliths

Multichannel monoliths containing up to 90% silicalite by weight and with cell densities up to 28 cells/cm2, wall thicknesses down to 0.6 mm and an overall diameter of 40 mm have been prepared from silicalite powder and sodium bentonite (as a binder) by the unit operations of paste preparation, extrusion, drying and firing. The manufactured monoliths, which show good strength, retain the crystal structure and micropore size of the source silicalite powder, and adsorption measurements made by using a dynamic flow system confirm that the monoliths exhibit an equilibrium performance broadly similar to that of commercial silicalite pellets. In addition, the manufactured monoliths possess a higher macroporosity than the commercial pellets. Regeneration of the monoliths was found to be possible at mildly increased temperature. These features augur well for the recovery and/or separation of organic compounds in simple pressure swing and thermal swing processes.

Comparison of drug release from poly(lactide-co-glycolide) microspheres and novel fibre formulations.

Intraperitoneal cisplatin delivery has recently been shown to benefit ovarian cancer patients. Cisplatin-containing poly(lactide-co-glycolide) (PLGA) microspheres have been proposed for cisplatin delivery. The drug loading of cisplatin containing microspheres produced elsewhere is 3–10%w. Similar microspheres are reported here with a mean diameter of 38.8 µm, and a drug loading of 11.7%w, but using ethyl acetate as a safer solvent. In addition, novel formulations of cisplatin-containing solid and hollow PLGA 65:35 (lactide:glycolide) fibres were prepared and are reported here for the first time. PLGA hollow fibres were produced by phase inversion with a high drug loading of 27%w. Mechanistic mathematical models were applied to the cisplatin release profiles to allow quantitative comparison of microsphere, solid fibre and hollow fibre formulations. The diffusion coefficient of cisplatin eluting from a typical batch of PLGA microspheres was 4.8 × 10−13 cm2 s−1; this low diffusivity of cisplatin in microspheres was caused by the low porosity of the polymer matrix. The diffusion coefficients of cisplatin eluting from a batch of PLGA solid fibres and hollow fibres were 6.1 × 10−10 and 3.3 × 10−10 cm2 s−1, respectively. These fibres allowed the controlled release of high doses of cisplatin over four days and may represent an improvement in slow release technology for treatment of ovarian cancer.

Hybrid Membrane Adsorbents for Fluidized Bed Separations

Membrane coatings of various polymers have been applied successfully to the external surface of commercial spheres of zeolite 5A in order to render them more resistant to attrition. Starting materials have been phenol-formaldehyde (PF) resins, styrene-divinyl benzene (SDVB), and polyvinylidene chloride (PVDC). Once laid down on the surface as polymers, the PF and PVDC membranes have been tested in carbonized form as well. All the membranes tested showed improved resistance to attrition over the virgin zeolite spheres. Membrane coatings made from the PF precursor show improved adsorption properties as well and therefore this type of membrane coating augurs well for fluidized bed applications in which a further advantage, namely that of reduced external fluid film resistance, can be gained. Some loss of adsorptive capacity was observed with coatings made from SDVB and PVDC and their carbonized forms.