Modelling of artefacts in estimations of particle size of needle-like particles from laser diffraction measurements

Manufacturing of particulate products across many industries relies on accurate measurements of particle size distributions in dispersions or powders. Laser diffraction (or small angle light scattering) is commonly used, usually off-line, for particle size measurements. The estimation of particle sizes by this method requires the solution of an inverse problem using a suitable scattering model that takes into account size, shape and optical properties of the particles. However, laser diffraction instruments are usually accompanied by software that employs a default scattering model for spherical particles, which is then used to solve the inverse problem even though a significant number of particulate products occur in strongly non-spherical shapes such as needles. In this work, we demonstrate that using the spherical model for the estimation of sizes of needle-like particles can lead to the appearance of artefacts in the form of multimodal populations of particles with size modes much smaller than those actually present in the sample. This effect can result in a significant under-estimation of the mean particle size and in false modes in estimated particles size distributions.Comment: 28 pages 8 figures accepted in the journal of Chemical Engineering Scienc

Dynamic response studies on aggregation and breakage dynamics of colloidal dispersions in stirred tanks

Aggregation and breakage of aggregates of fully destabilized polystyrene latex particles in turbulent flow was studied experimentally in both batch and continuous stirred tanks using small-angle static light scattering. It was found that the steady-state values of the root-mean-square radius of gyration are fully reversible upon changes of stirring speed as well as solid volume fraction. Steady-state values of the root-mean-square radius of gyration were decreasing with decreasing solid volume fraction as well as with increasing stirring speed. Moreover, it was found that the steady-state structure and shape of the aggregates is not influenced by the applied stirring speed

Closely Held Firms As Going Concerns

This current GAAP determination of a going concern is shortsighted for two important reasons. The most important deals with creditors and other stakeholders involved with the business. Do they enter into contracts with the business or with the individual owner/manager? Currently, they contract with both since, in reality, they make no determination whether a separate firm (entity) exists. The second deals with valuing a business. If the business is not really a separate going concern, it would typically be valued as the sum of its individual assets instead of the present value of its future cash flows. Many times when buying a business, the acquirer is really just buying the assets to start his own business. This is particularly true in most service businesses. The purpose of this paper is to advocate reintroducing a qualification to the going concern audit opinion when an entity separate from its owner/manager does not exist. Criteria for determination are also proposed. Arguably, this will make audited accounting statements more meaningful for closely-held firms. More important, this should produce information useful for potential creditors and outside owners. Traditionally, banks have extended loans to small, closely-held firms with only compiled statements; there was no need to provide audited statements. However, the process of lending is changing from a direct, face-to-face process between borrower and lender to an indirect one where credit scoring systems are used. Audited statements can provide better, higher quality information to lenders extending credit

Scaling of glycine nucleation kinetics with shear rate and glass-liquid interfacial area

The scaling of the nucleation kinetics of glycine was investigated in supersaturated aqueous solutions under isothermal conditions. Induction times were measured in a Couette cell with a wide range of average shear rates γ_avg (25-250 s^-1) and a range of glass-liquid interfacial areas A (2.5-10 cm^2 per ml solution). The probability distributions of induction times were found to scale with shear rate and glass-liquid interfacial area, with the characteristic timescale (γ_avg.A)^-1. Primary nucleation rates and growth times to reach detection (estimated from the probability distributions) were both dependent on this timescale. In-situ dynamic light scattering revealed mesoscale clusters in the solutions that increased in size over time at rates which also depended on this timescale. The increase in size was thought to be due to the shear-enhanced aggregation or coalescence of mesoscale clusters leading to a higher number of larger mesoscale clusters, resulting in higher rates of primary nucleation

An Accelerated Development, Reduced Cost Approach to Lunar/Mars Exploration Using a Modular NTR-Based Space Transportation System

The results of integrated systems and mission studies are presented which quantify the benefits and rationale for developing a common, modular lunar/Mars space transportation system (STS) based on nuclear thermal rocket (NTR) technology. At present NASA's Exploration Program Office (ExPO) is considering chemical propulsion for an 'early return to the Moon' and NTR propulsion for the more demanding Mars missions to follow. The time and cost to develop these multiple systems are expected to be significant. The Nuclear Propulsion Office (NPO) has examined a variety of lunar and Mars missions and heavy lift launch vehicle (HLLV) options in an effort to determine a 'standardized' set of engine and stage components capable of satisfying a wide range of Space Exploration Initiative (SEI) missions. By using these components in a 'building block' fashion, a variety of single and multi-engine lunar and Mars vehicles can be configured. For NASA's 'First Lunar Outpost' (FLO) mission, an expendable NTR stage powered by two 50 klbf engines can deliver approximately 96 metric tons (t) to translunar injection (TLI) conditions for an initial mass in low earth orbit (IMLEO) of approximately 198 t compared to 250 t for a cryogenic chemical TLI stage. The NTR stage liquid hydrogen (LH2) tank has a 10 m diameter, 14.5 m length, and 66 t LH2 capacity. The NTR utilizes a UC-ZrC-graphite 'composite' fuel with a specific impulse (Isp) capability of approximately 900 s and an engine thrust-to-weight ratio of approximately 4.3. By extending the size and LH2 capacity of the lunar NTR stage to approximately 20 m and 96 t, respectively, a single launch Mars cargo vehicle capable of delivering approximately 50 t of surface payload is possible. Three 50 klbf NTR engines and the two standardized LH2 tank sizes developed for lunar and Mars cargo vehicle applications would be used to configure the Mars piloted vehicle for a mission as early as 2010. The paper describes the features of the 'common' NTR-based moon/Mars STS, examines performance sensitivities resulting from different 'mission mode' assumptions, and quantifies potential schedule and cost benefits resulting from this modular moon/Mars NTR vehicle approach

Influence of controlled fluid shear on nucleation rates in glycine aqueous solutions

The nucleation of glycine was investigated in supersaturated aqueous solutions exposed to well-controlled fluid shear under isothermal conditions. Shear rates between 25 s-1 and 1000 s-1 were studied using Couette and capillary flow devices. Induction times were obtained from imaging, transmission and scattering measurements, or visual monitoring. Great care was taken to eliminate any seeding in order to avoid secondary nucleation preceding formation of first crystals through primary nucleation. The observed induction times of sheared solutions were considerably lower than those of unsheared solutions. Increasing the average shear rate was found to reduce the mean induction time through a power law relationship. A detailed statistical analysis showed that the number of experimental repetitions used was sufficient to obtain statistically significant trends for the system studied. Induction times appeared to closely follow a probability distribution based on a previously published model of Jiang and ter Horst. Using their model, where the induction time is related to the rate of formation of primary nuclei and the time it takes them to grow to the size where the secondary nucleation becomes significant, it was found that both the primary nucleation rate and the growth time were strongly dependent on the shear rate imposed

Effect of process conditions on particle size and shape in continuous antisolvent crystallisation of lovastatin

Lovastatin crystals often exhibit an undesirable needle-like morphology. Several studies have shown how a needle-like morphology can be modified in antisolvent crystallisation with the use of additives, but there is much less experimental work demonstrating crystal shape modification without the use of additives. In this study, a series of unseeded continuous antisolvent crystallisation experiments were conducted with the process conditions of supersaturation, total flow rate, and ultrasound level being varied to determine their effects on crystal size and shape. This experimental work involved identifying acetone/water as the most suitable solvent/antisolvent system, assessing lovastatin nucleation behaviour by means of induction time measurements, and then designing and implementing the continuous antisolvent crystallisation experiments. It was found that in order to produce the smallest and least needle-like particles, the maximum total flow rate and supersaturation had to be combined with the application of ultrasound. These results should aid development of pharmaceutical manufacturing processes where the ability to control particle size and shape would allow for optimisation of crystal isolation and more efficient downstream processing

Nuclear electric propulsion: A better, safer, cheaper transportation system for human exploration of Mars

NASA has completed a preliminary mission and systems study of nuclear electric propulsion (NEP) systems for 'split-sprint' human exploration and related robotic cargo missions to Mars. This paper describes the study, the mission architecture selected, the NEP system and technology development needs, proposed development schedules, and estimated development costs. Since current administration policy makers have delayed funding for key technology development activities that could make Mars exploration missions a reality in the near future, NASA will have time to evaluate various alternate mission options, and it appears prudent to ensure that Mars mission plans focus on astronaut and mission safety, while reducing costs to acceptable levels. The split-sprint nuclear electric propulsion system offers trip times comparable to nuclear thermal propulsion (NTP) systems, while providing mission abort opportunities that are not possible with 'reference' mission architectures. Thus, NEP systems offer short transit times for the astronauts, reducing the exposure of the crew to intergalactic cosmic radiation. The high specific impulse of the NEP system, which leads to very low propellant requirements, results in significantly lower 'initial mass in low earth orbit' (IMLEO). Launch vehicle packaging studies show that the NEP system can be launched, assembled, and deployed, with about one less 240-metric-ton heavy lift launch vehicle (HLLV) per mission opportunity - a very Technology development cost of the nuclear reactor for an NEP system would be shared with the proposed nuclear surface power systems, since nuclear systems will be required to provide substantial electrical power on the surface of Mars. The NEP development project plan proposed includes evolutionary technology development for nuclear electric propulsion systems that expands upon SP-100 (Space Power - 100 kw(e)) technology that has been developed for lunar and Mars surface nuclear power, and small NEP systems for interplanetary probes. System upgrades are expected to evolve that will result in even shorter trip times, improved payload capabilities, and enhanced safety and reliability