Dynamic modeling and Molecular Weight Distribution of ethylene copolymerization in an industrial gas-phase Fluidized-Bed Reactor

A dynamic model for ethylene copolymerization in an industrial Fluidized-Bed Reactor (FBR) is developed to describe its behavior and calculate the properties of polyethylene. The presented model considers particle entrainment and polymerization reaction in two phases. Two-site kinetic and hydrodynamic models in combination, provide a comprehensive model for the gas phase fluidized-bed polyethylene production reactor. The governing moment and hydrodynamic differential equations were solved simultaneously and the results compared with a similar work, as well as industrial data. The dynamic model showed accurate results for predicting Polydispersity Index (PDI), Molecular Weight Distribution (MWD), reactor temperature and polymer production rate

Dynamic modeling and Molecular Weight Distribution of ethylene copolymerization in an industrial gas-phase Fluidized-Bed Reactor

A dynamic model for ethylene copolymerization in an industrial Fluidized-Bed Reactor (FBR) is developed to describe its behavior and calculate the properties of polyethylene. The presented model considers particle entrainment and polymerization reaction in two phases. Two-site kinetic and hydrodynamic models in combination, provide a comprehensive model for the gas phase fluidized-bed polyethylene production reactor. The governing moment and hydrodynamic differential equations were solved simultaneously and the results compared with a similar work, as well as industrial data. The dynamic model showed accurate results for predicting Polydispersity Index (PDI), Molecular Weight Distribution (MWD), reactor temperature and polymer production rate

Dynamic process modeling and hybrid intelligent control of ethylene copolymerization in gas phase catalytic fluidized bed reactors

BACKGROUND: Polyethylene (PE) is the most extensively consumed plastic in the world, and gas phase‐based processes are widely used for its production owing to their flexibility. The sole type of reactor that can produce PE in the gas phase is the fluidized bed reactor (FBR), and effective modeling and control of FBRs are of great importance for design, scale‐up and simulation studies. This paper discusses these issues and suggests a novel advanced control structure for these systems. RESULTS: A unified process modeling and control approach is introduced for ethylene copolymerization in FBRs. The results show that our previously developed two‐phase model is well confirmed using real industrial data and is exact enough to further develop different control strategies. It is also shown that, owing to high system nonlinearities, conventional controllers are not suitable for this system, so advanced controllers are needed. Melt flow index (MFI) and reactor temperature are chosen as vital variables, and intelligent controllers were able to sufficiently control them. Performance indicators show that advanced controllers have a superior performance in comparison with conventional controllers. CONCLUSION: Based on control performance indicators, the adaptive neuro‐fuzzy inference system (ANFIS) controller for MFI control and the hybrid ANFIS–proportional‐integral‐differential (PID) controller for temperature control perform better regarding disturbance rejection and setpoint tracking in comparison with conventional controllers. © 2019 Society of Chemical Industr

The effect of radiotherapy on survival of dental implants in head and neck cancer patients

Objectives: To explore the current literature of the survival of dental implants in irradiated head and neck cancer patients considering the role of implant location, bone augmentation, dose of radiation and timing of implant placement. Study Design: Pubmed search was conducted to identify articles published between January 2000 and December 2014 and presenting data of dental implant survival with radiotherapy in head and neck cancer patients. Studies on animal subjects and craniofacial implants were excluded. Results: 18 articles out of 27 were eligible for inclusion in this systematic review. 12 out of 18 studies reported favorable outcome of dental implants and radiotherapy with survival rates between 74.4% and 97%. Seven out of ten studies comparing the survival rates according to site of implant placement reported that implants were found to osseointegrate with greater success in the irradiated mandible than irradiated maxilla. 5 studies which compared implant survival in irradiated native bone versus irradiated grafted bone reported that irradiated grafted bone showed a significantly reduced dental implant survival rate in comparison to irradiated native bone. 6 out of 18studies in which radiation doses exceeded 70 Gy reported lower survival rates of dental implants in comparison to the studies in which radiation doses were ≤70Gy. Higher survival rates were reported in 2 studies in which implants placement was before radiotherapy in comparison to the remaining 16 studies in which implants placement was after radiotherapy. Conclusions: Dental implants may be affected by radiotherapy especially when they are placed in maxilla, in grafted bone, or after radiation, however, they remain a functional option for rehabilitation of head and cancer patients. More prospective cohort studies and randomized controlled trails are still needed to draw more evidence based conclusion

Co2absorption/desorption in aqueous single and novel hybrid solvents of glycerol and monoethanolamine in a pilot-scale packed bed column

CO2 removal from mixed CO2-N2 gas was investigated by using aqueous solutions of monoethanolamine (MEA) (10 wt %), glycerol (10 wt %), and a mixture of MEA (10 wt %)-glycerol (10 wt %) in a pilot-scale packed column. An Aspen Plus simulator was employed to simulate the CO2-MEA-glycerol process using a rate-based model. Then, the experimental data of the pilot-scale columns were applied to validate the simulation results. The lowest and highest rich CO2 loadings for the MEA solvent were measured in 3.65 and 13.9% mol CO2/mol MEA with 1.4 and 3.9 L/min gas flow rates, respectively. In comparison to the CO2-MEA system, the lowest and highest rich CO2 loadings for the CO2-MEA-glycerol system increased by 42.2 and 14.8%, respectively, under the same conditions. The values of CO2 loadings predicted by the simulation were in concordance with the experimental values. The results showed that the hybrid MEA-glycerol solution had a better CO2 absorption performance than the aqueous MEA solution

Lip and oral lesions in children with Down syndrome: a controlled study

Background: Down syndrome (DS) is the most common chromosomal abnormality affecting numerous organs, including the orofacial region. The objective of the present study was to assess the prevalence of lip and oral soft tissue lesions, with particular emphasize on the incidence of fissured tongue, lip fissures and angular cheilitis, among individuals with DS in Yemen. Material and Methods: This controlled cross-sectional study included 50 children with DS (6-18 years), and 50 age- and gender-matched healthy controls. The prevalence of orofacial soft tissue lesions was evaluated in both groups. Data were analyzed by Chi-square and Fisher tests, and p <0.05 was considered to be statistically significant. Results: Ten orolabial lesions were identified among the subjects. The most frequently seen lesions were: Fissured tongue (78.0%), lip fissures (64.0%), angular cheilitis (38.0%) and Cheilitis (14.0%). The frequencies of these lesions were significantly higher in children with DS than healthy controls ( P < 0.001). Most of lip fissures were in the lower lip, and 80% of the fissures were in the midline. Conclusions: The prevalence of lip and oral lesions among individuals with DS is remarkably high. Hence, oral physicians should be more aware of the orofacial findings seen more frequently in this genetic disorder

The Effect of surface topographical changes of two different surface treatments rotary instrument

One of the major innovations in endodontics has been the introduction of nickel-titanium (NiTi) alloy. This study evaluated the surface topographical changes of two different surface treatments rotary instrument after instrumentation and sterilization. 240 Extracted teeth were included in this study. 90 new AlphaKite and Revo-S NiTi rotary instruments were selected and divided into two groups (Group A 45 AlphaKite and group B 45 Revo-S). Each group were divided into three subgroups: (A1, B1) n=5 files were used as a control, (A2,B2) n=20 files were used to prepare three root canals using endodontic rotary motor then sterilized by autoclave for one cycle under 121°C at 15 psi for 30 minutes and (A3,B3) n=20 files were used to prepare nine root canals using the same rotary system then sterilized by autoclave for three cycles under 121°C at 15 psi for 30 minutes. Files were examined under scanning electron microscopy. On examining the AlphaKite, A1 revealed gross machining grooves on their surface with no pits, A2 showed disruption of cutting and A3 showed microcracks and deepening of the machining grooves. B1 showed a smoother surface with few machining grooves, B2 showed dulling and blunting of the cutting edges was predominant and B3 files showed plastic deformation in the form of unwinding of the flutes. The defects were less distributed along the electropolished Revo-S files than the physical vapor deposition AlphaKite

Different hydrodynamic model for gas-phase propylene polymemation in a catalytic fluidized bed reactor

A comparative simulation study was carried out using the improved well-mixed, constant bubble size and well mixed models. These fluidized bed reactor models, combined with comprehensive kinetics for propylene homo-polymerization in the presence of a multiple active site Ziegler-Natta catalyst. In the improved model, the effect of the presence of particles in the bubbles and the excess gas in the emulsion phase was taken into account to improve the quantitative understanding of the actual fluidized bed process. The superficial gas velocity and catalyst feed rate have a strong effect on the hydrodynamics and reaction rate, which results in a greater variation in the polymer production rate and reactor temperature. At typical operating conditions the improved well mixed and well mixed models were in good agreement. While the COO!ICU bubble size model was found to over-predict the emulsion phase temperature and underpredict propylene concentration

Is Khat (Catha edulis) chewing a risk factor for periodontal diseases? : a systematic review

Background: Khat (Catha edulis) chewing is a highly prevalent habit in the Arabian Peninsula and East Africa, and has recently spread to Western countries. The association between khat chewing and oral mucosal lesions is well documented in the literature. However, there is no concrete evidence on the association between khat chewing and periodontal disease. The purpose of this systematic review was to analyze the influence of khat chewing on periodontal health. Material and methods: A literature search of PubMed, Scopus and Web of Sciences databases was carried out to identify relevant articles published from 1990 to May 2017. The inclusion criteria were all clinical studies that assessed the relationship between khat chewing and periodontal disease. Results: The search yielded 122 articles, of which 10 were included in this systematic review. Most of the studies exhibited a positive correlation between khat chewing and periodontal disease. Conclusions: Altogether, the analysis of the current evidence reveals that khat chewing is destructive to the periodontium and enhances the risk of periodontal disease progression. However, due to variability of studies, more longitudinal case-controlled studies are highly warranted to establish a causal relation between khat chewing and periodontal disease

Мишени SiO2 : CuO (Cu°) для нанесения тонких пленок ионнолучевым распылением, полученные золь-гель методом

Studies of the structure formation features of composite inorganic materials formed on the basis of silicon dioxide, namely, pyrogenic silica of the A-380 brand (technical name – aerosil). The main purpose of the research was to study the possibility of improving the uniformity of the distribution of injected dopants on the surface of SiO2 globules forming the xerogel framework. This goal was achieved due to the uniformity of the distribution of alloying salts (for example, copper nitrate) at the stage of sol formation, which was converted to the state of xerogel as a result of sequential heat treatment in a controlled gas environment (air or dried hydrogen). The final shape of the samples was either micro-powders or tabletshaped blanks obtained on their basis – with a diameter of about 12.5; 2.50; 30; 40 and 80 mm and a phase composition of SiO2 : CuO and SiO2 : Cu°.Проведены исследования особенностей структурообразования композиционных неорганических материалов, формируемых на основе диоксида кремния, а именно – пирогенного кремнезема марки А-380 (техническое название – аэросил). Изучена возможность улучшения однородности распределения вводимых веществдопантов по поверхности SiO2-глобул, образующих каркас ксерогеля. Указанная цель достигалась за счет однородности распределения легирующих солей (на примере нитрата меди) еще на стадии формирования золя, который переводился в состояние ксерогеля в результате последовательной термообработки в контролируемой газовой среде (на воздухе или осушенном водороде). Полученные материалы затем размалывали до состояния высокодисперсных микропорошков. Из порошков формовали мишени в виде таблеток требуемого размера методом одноосного прессования. Конечная форма образцов представляла собой или микропорошки, или таблетированные заготовки, полученные на их основе – диаметром порядка 12,5; 2,50; 30; 40 и 80 мм и фазового состава SiO2:CuO и SiO2:Cu°