Etherification on Zeolites: MTBE synthesis

Although the use of methyl-tert butyl ether (MTBE) in reformulated gasoline has raised concerns due to its detection in ground water and has led to its gradual phase out in parts of the United States, the use of heavier ethers in gasoline in the future is possible. The synthesis of MTBE provides us with an insight into etherification reactions in general. This article reviews the extensive findings over the past 15 years on the application of acidic zeolites as alternative catalysts for etherification reactions and, in particular, MTBE synthesis, and compares the results with those of the commercially used ion-exchange resins. Although the resin catalysts are very active, they have some significant drawbacks (i.e., thermal fragility, sensitivity to methanol/isobutene ratios, and corrosive/disposal problems). Zeolites have been considered to be potential alternative catalysts for MTBE synthesis due to their excellent properties such as high thermal stability and modifiable acidity. The impact of various zeolite parameters, such as Si/Al ratio, type of zeolite, and the presence of extra-lattice Al, on activity is explored. Although the specific activities of most zeolites have been found to be too low to compete at low temperatures required to avoid thermodynamic limitations, H-beta zeolite has been found to be as active as the current commercial resin catalysts being used for MTBE synthesis. In addition, all zeolites studied have been found to have high selectivities to MTBE and low sensitivities to methanol/isobutene molar ratio, permitting the use of the stoichiometric reactant ratio. Application of zeolites for the synthesis of higher ethers is suggested.     

Tables of a Truncated Standard Normal Distribution: A Doubly Truncated Case

As an extension to the singly truncated case, this article further examines a doubly truncated normal distribution. In this article, we consider both symmetric and asymmetric cases with two truncation points on the left and the right, and develop the tables of a truncated mean, a truncated variance, and a cumulative probability.     

Balancing mixed-model assembly lines to reduce work overload

We propose a new line balancing approach for mixed-model assembly lines with an emphasis on how the assignment of tasks to stations affects the ability to construct daily sequences of jobs (customer orders) that provide stable workloads (in a minute-to-minute sense) on the assembly line, while also achieving reasonable workload balance among the stations. The issue of short-term workload stability has received little attention in the assembly line balancing literature. Such stability allows assembly workers to complete their tasks without being rushed and thereby contributes to product quality. We propose a new objective for assembly line balancing that helps to achieve better short-term workload stability and develop a heuristic solution procedure based on filtered beam search for this new objective. Computational results show that for small problems (which can be solved optimally), this approach provides near optimal solutions, and for larger problems, it provides significantly better results than traditional assembly line balancing methods.     

Tables of a Truncated Standard Normal Distribution: A Singly Truncated Case

A normal distribution has a unique position in many engineering fields, and the standard normal distribution table has been widely used for more than a century. There are many situations, however, in which a truncated normal distribution needs to be considered. Although the theoretical foundations of the truncated normal distribution are well established, there has been little work on tabulating the characteristics associated with the truncated normal distribution, such as a cumulative probability, a truncated mean, and a truncated variance. In this article, we provide tables for a singly truncated normal distribution, which may be useful for quality practitioners.     

Adsorption and Deactivation Characteristics of Cu/ZnO-Based Catalysts for Methanol Synthesis from Carbon Dioxide

The adsorption and deactivation characteristics of coprecipitated Cu/ZnO-based catalysts were examined and correlated to their performance in methanol synthesis from CO₂ hydrogenation. The addition of Ga₂O₃ and Y₂O₃ promoters is shown to increase the Cu surface area and CO₂/H₂ adsorption capacities of the catalysts and enhance methanol synthesis activity. Infrared studies showed that CO₂ adsorbs spontaneously on these catalysts at room temperature as both mono- and bi-dentate carbonate species. These weakly bound species desorb completely from the catalyst surface by 200 °C while other carbonate species persist up to 500 °C. Characterization using N₂O decomposition, X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy with energy-dispersive X-ray spectroscopy analysis clearly indicated that Cu sintering is the main cause of catalyst deactivation. Ga and Y promotion improves the catalyst stability by suppressing the agglomeration of Cu and ZnO particles under pretreatment and reaction conditions.     

Porous polymers prepared via high internal phase emulsion polymerization for reversible CO2 capture

A series of porous polymers with different pore volumes, pore sizes, and crosslinking densities were synthesized by high internal phase emulsion (HIPE) polymerization. The crosslinked polymerized HIPEs (polyHIPEs) were formed by the copolymerization of 4-vinylbenzyl chloride and divinylbenzene using water droplets in conventional or Pickering HIPEs as the templates. These porous materials were further modified by quaternization and ion exchange to introduce quaternary ammonium hydroxide groups. The resulting polyHIPEs were utilized as sorbents for reversible CO₂ capture from air using the humidity swing. The effect of pore structure on the CO₂ adsorption and desorption processes was studied. The polyHIPEs containing large pores and interconnected porous structures showed improved swing sizes and faster adsorption/desorption kinetics of CO₂ compared to a commercial Excellion membrane with similar functional groups.     

Thermo-mechanical finite element simulation and validation of rubber curing process

Journal of Mechanical Science and Technology - Vulcanization, or curing, is a very important process in producing useful rubber products. The curing process takes place when heat is transferred to...