Effects of congestion and confining walls on turbulent deflagrations in a hydrogen storage facility-part 2: Numerical study

Safety studies for hydrogen retail stations involve identification of possible accidental scenarios, modelling of consequences and measures to mitigate associated hazards with it. Accidental release of hydrogen during its handling and storage can lead to formation of ignitable mixture in a very short time. Ignition of such a mixture can lead to generation of overpressure affecting structure and people. Understanding of the possible overpressures generated is critical in designing the system safe from explosion hazards. In the present study, the worst-case scenario where high-pressure hydrogen storage cylinders are enveloped by a premixed hydrogen-air cloud is numerically simulated. The computational domain mimics the setup for premixed hydrogen cloud in a mock hydrogen cylinder storage congestion environment experimentally studied by Shirvill et al. [1]. Large Eddy Simulations (LES) are performed using OpenFOAM CFD toolbox solver. The Flame Surface Wrinkling Model in LES context is used for modelling deflagrations [2]. Numerical simulation results are compared against experiments. Simulations are able to predict experimental flame arrival and overpressure reasonably well. The effects of ignition location, congestion and confining walls on the turbulent deflagrations in particular on explosion overpressure are discussed. It was concluded that explosion overpressure increases with increase in confinement.     

RF modeling approach to determining end-of-life reliability for InP-based HBTs

Reliability testing for high-speed, compound semiconductor, heterojunction bipolar transistors (HBTs) has focused on determining device lifetimes based on changes in the DC characteristics. The work reported here considers aging effects on the RF characteristics of InP-based HBTs. In addition, an intended system life of 15 years has been assumed and we have attempted to stress the devices to approximate 15 years of circuit operation. As such, SPICE models were generated for the devices before and after the stress. The basic differences between these models are that the aged model shows increases in the emitter resistance (R_e) and in the base–collector capacitance (C_jc). The-end-of life model can be obtained from the standard model by increasing R_e from 3.6 to 5.7 Ω and C_jc from 20.1 to 23.4 fF. These changes lead to decreases in: transistor gain, F_t, and F_max, and an increase in the base–emitter voltage at typical operating conditions. No increase in low current base–emitter voltage occurs, indicating the lack of Be diffusion into the emitter. The changes in the device parameters generally fall within the normal process variations.     

Adolescents in the City: Material and Social Living Conditions in Johannesburg–Soweto, South Africa

Urban populations are growing rapidly in developing countries. As a result, challenges to the well-being of adolescents are increasing in priority. This paper describes the material and social living conditions of 5,367 young adolescents, in Johannesburg–Soweto, South Africa in 2002–2003. The majority of children in South Africa’s urban hub have adequate access to basic services. However, social conditions are a key concern—lack of financial support by parents; defaulting caregiver roles to grandparents; minimal or no contact with fathers; and poor school performance. Weakened levels of family support in urban environments may negatively impact on adolescent development. In the long term, family systems as well as other social networks and institutions need to be strengthened to improve adolescent outcomes.

Selective reverse micellar extraction of penicillin acylase from E coli

The recovery of penicillin acylase from E coli by a new reverse micellar treatment is described. The results are compared with the cell disruption by ultrasound followed by reverse micellar extraction. The process gave selective extraction of penicillin acylase directly from the periplasmic space of E coli without disrupting the cells. Unlike ultrasonication which breaks open the cells entirely, making subsequent processing difficult and expensive, reverse micellar treatment of cells gave a moderate recovery of 60% of enzyme activity in a highly pure form. © 2001 Society of Chemical Industry     

Synthesis of optimal thermal membrane distillation networks

Thermal membrane distillation (TMD) is an emerging separation method which involves simultaneous heat and mass transfer through a hydrophobic semipermeable membrane. Traditionally, studies of this technology have focused on the performance of individual modules. Because of purity and recovery requirements, multiple TMD modules may be used in various configurations including series, parallel, and combinations. Furthermore, there may be a need to reroute streams from one module to another or to recycle a stream to the same unit. The objective is to develop a systematic approach to synthesize an optimal TMD network. A structural representation is developed to embed potential configurations of interest. A mathematical formulation is developed to transform the design problem into an optimization task that seeks to minimize the cost of the system. Two case studies are presented to illustrate the applicability of the developed approach and its merit over conventional design scenarios. © 2014 American Institute of Chemical Engineers AIChE J, 61: 448–463, 2015     

Study on the enzymatic hydrolysis of racemic methyl ibuprofen ester

The hydrolysis of racemic methyl ibuprofen ester in the presence of lipase from Candida rugosa was investigated in shake flasks. Experiments were performed to study the effect of temperature, pH and shaking speed on the reaction rate. Different hydrophobic co‐solvents were screened for the highest reaction rate and the presence of enzyme inhibition by substrate and products was examined. A kinetic expression was then proposed to describe the reaction. Kinetic parameters were determined for the optimum operating conditions and the proposed model was verified with the experimental results. Next, this reaction was scaled up to a fed batch stirred tank reactor. Batch reactor and fed batch reactor configurations were compared for better conversions. The effects of aqueous phase hold‐up, substrate concentration and feed flow rate on the conversion of the reaction were also studied. Higher conversions were obtained in a fed batch reactor when compared with the batch reactor. In the fed batch reactor, increased conversions were observed with lower feed flowrates and high aqueous phase hold‐up. © 2001 Society of Chemical Industry