Control system design for recycle systems

This paper considers the control system design for recycle systems. We derive a condition which quantifies the effect of recycling on closed-loop performance. The condition is independent of control/controller structure. With insight from this condition, we develop a set of heuristics for properly designing a controller without detailed knowledge of recycle dynamics for the important special case where the system is represented by a first-order-plus-time-delay model and where the IMC tuning rules are used. The heuristics require only a minimal amount of information on the recycle dynamics. Their effectiveness is illustrated on a reactor/feed-effluent heat exchanger/furnace process under various conditions.     

Biomimetic Multienzyme Complexes Based on Nanoscale Platforms

Inspired by the widely present multienzyme complexes in nature that enable highly cooperative catalytic mechanisms, we designed a biomimetic dual‐functionalized nanoparticle‐based platform for colocalizing multiple enzymes. The use of nanoscale materials together with a novel sequential colocalization approach with two model enzymes [glucose oxidase (GOX) and horseradish peroxidase] resulted in a 100% increase in the overall conversion rate compared to the equivalent amount of free enzymes in solution and a physical mixture of individual immobilized enzymes on nanoparticles. GOX is an important enzyme used in glucose biosensors for diagnostics. Colocalizing GOX with peroxidase allows for colorimetric visualization of the peroxide formed that enables monitoring glucose levels in solution. This platform can be readily applicable to other multienzyme systems as well. © 2012 American Institute of Chemical Engineers AIChE J, 59: 355–360, 2013     

First principles study on desorption of chemisorbed hydrogen atoms from single-walled carbon nanotubes under external electric field

Based on first principles calculations, we suggest a new way to release chemisorbed hydrogen atoms from the single-walled carbon nanotubes. The transverse external electric field serves as the key source for this mechanism. As the electric field strength increases, there is a notable increase in the C–H bond lengths at the sharp highly curved sites of the carbon nanotube. The compression of the tube along the direction of electric field results in the release of hydrogen atoms from the sharp sites. The synergetic action of electric field and compression turns out the desorption process easier at favorable field strengths. This present investigation provides a new alternative way to release hydrogen atoms from the hydrogenated carbon nanotubes under the influence of external electric field that requires comparatively lower temperature suitable for hydrogen based fuel cells used in mobile applications.     

Simultaneous optimal integration of water utilization and heat exchange networks using holistic mathematical programming

A systematic holistic mathematical programming (HMP) is proposed to formulate a mixed integer nonlinear programming (MINLP) model for one-step optimization of water-allocation and heat exchange network (WAHEN) designs with single- or multi-contaminant water streams. The proposed model formulation and solution strategy are believed to be superior to the available ones in the following aspects. First, a comprehensive representation combining two separate superstructures is adopted to capture the structural characteristics of the integrated WAHEN. Then, a hybrid optimization strategy integrating stochastic and deterministic components is developed for the resulting MINLP model and, also, an interactive iteration method is adopted based on sensitivity analysis to guide the search toward a potential global optimum. Finally, evolutionary strategies and manipulations are executed to enhance WAHEN configurations. Two illustrative examples are presented to demonstrate the validity and advantages of the proposed approach.     

Characterizations of GaN films grown with indium surfactant by RF-plasma assisted molecular beam epitaxy

A small indium flux was used as a surfactant during the growth of gallium nitride by rf-plasma assisted molecular beam epitaxy. The effects of the In surfactant on the optical and structural properties of undoped GaN were studied by photoluminescence (PL), X-ray diffraction, atomic force microscopy (AFM), and Rutherford backscattering spectrometry (RBS). PL studies show that the use of In surfactant is beneficial to the reduction of deep-level defects. The X-ray rocking curves demonstrate a 20% decrease in the full width at half maximum value for the films grown with In surfactant. AFM studies show that the root mean squared surface roughness for films grown with and without In surfactant are 5.86 and 6.99 nm respectively indicating significant improvement in surface morphology. The improved surface morphology is attributed to the enhanced 2-dimensional growth promoted by the application of In surfactant. RBS studies show that the χ_min values along [0 0 0 1] direction are 2.06% and 2.16% for the samples grown with and without In surfactant respectively. Off-normal ion channeling studies were performed to further investigate the effects of In surfactant on the crystallinity. It is found that the number density of stacking faults is smaller for the sample grown with In surfactant compared to the one grown without In surfactant. However, defect analysis shows that dislocations are found in the sample grown with In surfactant in contrary to the one grown without In surfactant. We speculate that there is a thickness limit of GaN grown with In surfactant and the thickness of our samples exceed this limit, leading to the presence of dislocation.     

Single operation earliness–tardiness scheduling with machine activation costs

We consider a static, single operation, non-pre-emptive, deterministic scheduling problem in which a set of n jobs is to be processed on k identical machines. Jobs assigned to each machine have a common due date. The number of machines (k) is unknown. Activating a machine will require additional costs to be incurred. The objective is to find an optimal sequence, the optimal number of machines (k), and the respective due dates to minimize the weighted sum of earliness, tardiness, and machine activation costs. We propose a polynomial time algorithm to solve the problem.     

Tribological studies of unpolished laser surface textures under starved lubrication conditions for use in air-conditioning and refrigeration compressors

An experimental study was carried out to investigate the tribological performance of different laser surface texture patterns, with unpolished material bulges around the dimples, under realistic operating conditions of starved lubrication, for use in air-conditioning and refrigeration compressors. Compared to untextured gray cast iron surfaces, the texture patterns showed significant tribological improvements. Long durability tests also highlighted the long term usefulness of surface texturing. The dominant wear mode of the texture patterns was found to be mechanical polishing and the tribological behavior was found to be largely independent of the type of lubricant or refrigerant.     

Residential fuelwood consumption in the Southeastern U.S.

A telephone survey of households in 13 states in the southeastern U.S. determined residential fuelwood acquisition and use for heating during the 1991 heating season. Although wood burning accounted for only 10% of the total household heat requirement of the region, it accounted for 51% of the total heat requirements of wood-burning households. One-quarter (25%) of the households burned wood, consuming almost 9 million standard cords. Three-quarters (77%) of wood-burning households reported that wood burning contributed to household heating requirements. Wood as a source of home heating was particularly important to low income households, both rural and urban. Wood is the sole source of home heating for 17% of the wood-burning households in the region.     

Growing trade of bioenergy in the EU: Public acceptability, policy harmonization, European standards and certification needs

Since 2000, the consumption of bioenergy in the European Union has grown, along with a concurrent growth in the trade of biomass for energy purposes (though traded volumes still remain small). Bioenergy production and trade will likely continue to increase into the future, driven by climate change concerns, emissions reduction targets, increasing concerns about domestic energy security and favourable policies. The harmonization of European standards and the development of certification systems are key issues to resolving potential negative effects of increased biomass trade. Certification systems not only address the issue of environmental sustainability from production to end-use, but also allow for product differentiation while adding value to sustainably produced products, which can ultimately enhance a competitive and sustainable bioenergy market. In addition to analyzing bioenergy trade growth in the European Union, a questionnaire survey of 92 bioenergy experts from eight member states within the European Union was conducted. Survey results show that bioenergy is highly accepted in the European Union but that there is a lack of European standards and policy harmonization, along with the absence of a competitive market or a certification system, all of which are necessary for sustainable production and trade of bioenergy. A large majority (63 percent) of the total respondents agreed that the certification of bioenergy is necessary to promote the sustainable use of biomass.