Silver has been used in medicine for centuries because of its antimicrobial properties. More recently, silver nanoparticles have been synthesized and incorporated into several biomaterials, since their small size provides great antimicrobial effect, at low filler level. Hence, these nanoparticles have been applied in dentistry, in order to prevent or reduce biofilm formation over dental materials surfaces. This review aims to discuss the current progress in this field, highlighting aspects regarding silver nanoparticles incorporation, such as antimicrobial potential, mechanical properties, cytotoxicity, and long-term effectiveness. We also emphasize the need for more studies to determine the optimal concentration of silver nanoparticle and its release over time.

Analysis of the mass balance equations that describe a reaction system may be useful to provide information about its dynamics, such as the restricted set of compositions that can be achieved from a given set of initial compositions and the effect of feeding reactants to the reaction environment along the reaction course. Since these results may be important for the formulation of reaction policies, this work presents the properties of a matrix polymerization model previously developed and extended to describe transient conditions. This model is based on the definitions of two matrices: the consumption matrix (A À Kt), which contains information about chemical transformations among the many active polymer species in the system, and the propagation matrix Kp, which contains information about chain growth. It is shown that the set of mass balance equations that describes the dynamics of active chemical species in polymerization reactions has a stable and unique solution, which is bounded if feed rates are also bounded. It is also shown that the set of compositions that may be reached through manipulation of the feed rates is restricted and may not include all possible chemical compositions. Finally, it is shown that the obtained molecular weight distributions are special multiple time convolutions of the initiation rates.

Cooling water production is very important to oil refineries, and cooling towers are associated with high costs. An analysis of blowdown and make-up streams, cycles of concentration, use of biocides, and corrosion and scale inhibitors, can be performed to find alternatives to reduce process costs. Furthermore, a cooling system with chemical additives is advantageous because it prolongs the lifespan of the cooling tower. It also permits the elevation of the concentration cycle, consequently, reducing the cost of makeup water and minimizing inhibitors. This work seeks the optimization of cooling tower in terms of make-up water and suggests the use of chemical additives for reducing costs. The optimization was performed for scenarios with different make-up water compositions, and the Puckorius Scaling Index (PI) was evaluated to assure the quality of the water. The results showed that the lowest cost is obtained with the highest allowed cycle of concentration, according to the Puckorius Scaling Index interval.

A mixed-integer multiperiod model is used to propose a long-range planning for the Brazilian petrochemical industry. The specific aims of this work are to present a model to select the petrochemical processes capable of increasing over the next 20 years and carry out a sensitivity analysis of this selection if there are changes in demands for products or in the purchase prices of chemicals. The optimization model seeks to minimize the cost and the constraints related to demand, supply, and plant capacities in the petrochemical industry. The long-range planning was suggested for six scenarios, which differ according to the investment constraints, demands for products, and chemicals purchase prices imposed. Among the 295 processes considered, 26 were selected for investment in new process units through the optimization procedure in all scenarios, such as the processes for the production of polyurethane and acrylic fibers. These results are consistent with previous studies of investment assessment of the Brazilian petrochemical industry. KEYWORDSlong-range planning; mixed-integer programming; multiperiod optimization model; petrochemical industry; investment assessment

The mass balance equations used to describe different polymerization systems are represented and analyzed with the help of a general matrix framework previously developed. Some of its properties were presented in previous works and are illustrated here in three examples. The first example regards the dynamics of a copolymerization reactor. It is shown that terminal and penultimate models may predict qualitatively different responses in batch reactions. The second example also regards the dynamics of copolymerization reactions. It is shown how manipulation of initial conditions and monomer feed rates can be used to keep copolymer composition constant throughout the batch. The third example concerns the existence of oscillating chain size distributions when there are multiple monomer insertion steps.magnified image