R.G. Holdich

Hydrothermal carbonisation of sewage sludge: effect of process conditions on product characteristics and methane production.

Hydrothermal carbonisation of primary sewage sludge was carried out using a batch reactor. The effect of temperature and reaction time on the characteristics of solid (hydrochar), liquid and gas products, and the conditions leading to optimal hydrochar characteristics were investigated. The amount of carbon retained in hydrochars decreased as temperature and time increased with carbon retentions of 64-77% at 140 and 160°C, and 50-62% at 180 and 200°C. Increasing temperature and treatment time increased the energy content of the hydrochar from 17 to 19 MJ/kg but reduced its energy yield from 88% to 68%. Maillard reaction products were identified in the liquid fractions following carbonisations at 180 and 200°C. Theoretical estimates of the methane yields resulting from the anaerobic digestion of the liquid by-products are also presented and optimal reaction conditions to maximise these identified.

Pore design and engineering for filters and membranes

In filtration, the concept of pore size is not easy to define. In microfiltration, there are numerous advantages in employing a surface filtering membrane, rather than one relying on depth filtration mechanisms from a tortuous pore flow channel. Modern manufacturing techniques provide means to produce surface filtering membranes. For filtration, it is shown that a suitable pore design is an array of long thin slots. An analysis of fluid flow through the slots suggests that a short slot is adequate, but experimental data with suspended material indicates that slot length is important. Using long slots and careful control of the flow through the membrane it is possible to filter deforming particles such as oil drops from water.

Particulate fouling of surface microfilters with slotted and circular pore geometry

During microfiltration it is possible to obtain a permeate rate equal to that of the clean liquid permeation rate under certain critical conditions. This occurs when a deposit does not foul the surface of the membrane and internal deposition of material within the filter does not occur. Surface filters do not possess an internal structure, therefore, the only particulate fouling possible is that on the surface. Experiments in a stirred cell compared the surface fouling of two types of true surface filters: a commercially available track-etched filter with circular pores 10 μm in diameter and a filter with slotted pores 10μm×420 μm in size. When using a challenge suspension containing dilute latex particles of diameters 15 μm, and less, the circular pore membrane exhibited a critical flux at 260 l m−2 h−1. Under the same conditions, the critical flux for the slotted membrane was in excess of 1500 l m−2 h−1. Surface fouling was removed by back-flushing for both filters. These results indicate that slotted pores are less likely to suffer from particles bridging the pores leading to cake deposition, or secondary membrane formation, and that further development of filters with slotted pores of smaller slot widths would be worthwhile.

Measurement of slurry solids content by electrical conductivity

Abstract Electrodes positioned within a sedimenting and flowing suspension have been used to measure the conductivity of a dispersion and the supporting liquid. Variation in the ratio between these conductivities with solid concentration has been investigated and shown to conform to known equations for isotropic solids. Anisotropic and conducting materials had to be correlated using an empirically determined parameter. This was used to deduce solid concentration during solid/liquid separations, in situ , under carefully controlled experimental conditions, and in a system where temperature and electrolyte concentration were allowed to vary at random. Using this correlation solid concentrations up to 56% by volume were measured, during ballotini sedimentation. Sedimentation of minerals other than ballotini did not exhibit simple zone settling alone. The particle size distribution of the suspended non-conducting solids did not influence the correlation.

Preparation of liposomes: A novel application of microengineered membranes-From laboratory scale to large scale

A novel ethanol injection method using microengineered nickel membrane was employed to produce POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) and Lipoid(®) E80 liposomes at different production scales. A stirred cell device was used to produce 73ml of the liposomal suspension and the product volume was then increased by a factor of 8 at the same transmembrane flux (140lm(-2)h(-1)), volume ratio of the aqueous to organic phase (4.5) and peak shear stress on the membrane surface (2.7Pa). Two different strategies for shear control on the membrane surface have been used in the scaled-up versions of the process: a cross flow recirculation of the aqueous phase across the membrane surface and low frequency oscillation of the membrane surface (∼40Hz) in a direction normal to the flow of the injected organic phase. Using the same membrane with a pore size of 5μm and pore spacing of 200μm in all devices, the size of the POPC liposomes produced in all three membrane systems was highly consistent (80-86nm) and the coefficient of variation ranged between 26 and 36%. The smallest and most uniform liposomal nanoparticles were produced in a novel oscillating membrane system. The mean vesicle size increased with increasing the pore size of the membrane and the injection time. An increase in the vesicle size over time was caused by deposition of newly formed phospholipid fragments onto the surface of the vesicles already formed in the suspension and this increase was most pronounced for the cross flow system, due to long recirculation time. The final vesicle size in all membrane systems was suitable for their use as drug carriers in pharmaceutical formulations.

pH-sensitive micelles for targeted drug delivery prepared using a novel membrane contactor method

A novel membrane contactor method was used to produce size-controlled poly(ethylene glycol)-b-polycaprolactone (PEG-PCL) copolymer micelles composed of diblock copolymers with different average molecular weights, Mn (9200 or 10,400 Da) and hydrophilic fractions, f (0.67 or 0.59). By injecting 570 L m(-2) h(-1) of the organic phase (a 1 mg mL(-1) solution of PEG-PCL in tetrahydrofuran) through a microengineered nickel membrane with a hexagonal pore array and 200 μm pore spacing into deionized water agitated at 700 rpm, the micelle size linearly increased from 92 nm for a 5-μm pore size to 165 nm for a 40-μm pore size. The micelle size was finely tuned by the agitation rate, transmembrane flux and aqueous to organic phase ratio. An encapsulation efficiency of 89% and a drug loading of ~75% (w/w) were achieved when a hydrophobic drug (vitamin E) was entrapped within the micelles, as determined by ultracentrifugation method. The drug-loaded micelles had a mean size of 146 ± 7 nm, a polydispersity index of 0.09 ± 0.01, and a ζ potential of -19.5 ± 0.2 mV. When drug-loaded micelles where stored for 50 h, a pH sensitive drug release was achieved and a maximum amount of vitamin E (23%) was released at the pH of 1.9. When a pH-sensitive hydrazone bond was incorporated between PEG and PCL blocks, no significant change in micelle size was observed at the same micellization conditions.

Effect of Ionic Strength of Solution on Boron Mass Transfer by Ion Exchange Separation

Abstract In the present study, batch kinetic tests have been performed for boron removal from model solutions using boron selective ion exchange resins Diaion CRB 02, Dowex (XUS 43594.00) and Purolite S 108. Several kinetic models have been used to evaluate the sorption kinetics of boron by means of a well mixed stirred system, diffusional models, pseudo‐first‐order, and pseudo‐second‐order kinetic models. The mass transfer model, based on a well stirred system including maximum capacity (Qm, mg/g) and Langmuir constant (b, L/mg) values obtained from Langmuir isotherms, has been used to obtain predictive concentration changes against time. The experimental results have been used to compare with the modelling data for different ionic strength media.

Preparation of liposomes: a novel application of microengineered membranes - investigation of the process parameters and application to the encapsulation of vitamin E

Liposomes with a mean size of 59–308 nm suitable for pulmonary drug delivery were prepared by the ethanol injection method using nickel microengineered flat disc membranes with a uniform pore size of 5–40 μm and a pore spacing of 80 or 200 μm. An ethanolic phase containing 20–50 mg ml−1 phospholipid (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) or Lipoid® E80), 5–12.5 mg ml−1 stabilizer (cholesterol, stearic acid or cocoa butter), and 0 or 5 mg ml−1 vitamin E was injected through the membrane into an agitated aqueous phase at a controlled flux of 142–355 l m−2 h−1 and a shear stress on the membrane surface of 0.80–16 Pa. The mean particle size obtained under optimal conditions was 84 and 59 nm for Lipoid E80 and POPC liposomes, respectively. The particle size of the prepared liposomes increased with an increase in the pore size of the membrane and decreased with an increase in the pore spacing. Lipoid E80 liposomes stabilized by cholesterol or stearic acid maintained their initial size within 3 months. A high entrapment efficiency of 99.87% was achieved when Lipoid E80 liposomes were loaded with vitamin E. Transmission electron microscopy images revealed spherical multi-lamellar structure of vesicles. The reproducibility of the developed fabrication method was high.

Rotary vacuum filter scale-up calculations — and the use of computer spreadsheets

Abstract In laboratory studies of the filterability of a slurry it is usual to determine the cake permeability (or specific resistance) and cloth resistance. This data is then used to design the commercial filter. The data obtained in the laboratory can, however, provide a considerable amount of information about the performance of a filter, with operating conditions (ie design variables), which are not usually thoroughly investigated. This is due to the complex computations or computer program which would be needed for such a numerical investigation. This can easily be performed on a computer spreadsheet, and the result can be visually presented for immediate selection of preferred operating conditions. This has been achieved for a slurry formed by wet scrubbing a flue gas from a metallurgical smelter as an example. Rotary vacuum filter (RVF) performance was calculated incorporating the resistance due to the filter medium, by solving the resulting quadratic equation. It is usual to simplify the performance calculations by neglecting medium resistance, the error in making this assumption is shown. The performance of a pilot scale RVF on this slurry was experimentally investigated and compared with the spreadsheet prediction. The resulting scale-up calculations show the options open to the design engineer; to scale-up on dry solids throughput, filter area, cake depth or slurry throughput. Clearly this varies from one filter application to another, but the use of the spreadsheet makes these scale-up calculations easy and visible. It is also possible to test the affect on filter throughput of concentrating the feed slurry. This being one option to increase the capacity of an existing filter installation.

The solubility of aqueous lead chloride solutions

Abstract Data are presented for the solubility of lead in acidic solutions of between 5 and 10 M chloride concentration, present as the salts of copper (II) and either sodium or calcium. Cooling these solutions from boiling point to 20°C precipitates approximately two thirds of the lead as PbCl2 crystals of >99% purity. The solubility of PbCl2 is shown to depend upon the activity of the lead and chloride ions in solution, and can be predicted from known thermodynamic constants. This has been tested for solutions containing only one cation, other than lead, at 25°C. This approach accurately predicts the lead solubility in solutions of calcium chloride but needs improving before it can be applied to sodium chloride solutions and hydrochloric acid.