Search results for "Crack"
showing 10 items of 209 documents
A functional study of the esophagus in patients with non-cardiac chest pain and dysphagia.
2015
Background/Aims: Nutcracker esophagus and non-specific motility disorders are the main causes of non-cardiac chest pain (NCCP), with gastroesophageal reflux in 60% of cases. Achalasia and diffuse esophageal spasm are the most frequent anomalies described in patients with dysphagia. The goal of this study was to evaluate the occurrence of esophageal body and lower esophageal sphincter motor abnormalities in patients with dysphagia, NCCP, or both. Materials and Methods: This study is a retrospective analysis of 716 patients with NCCP and/or dysphagia tested between January 1994 and December 2010. 1023 functional studies were performed, 707 of which were esophageal manometries, 225 esophageal …
Transposition of the left renal vein for treatment of the nutcracker phenomenon: long-term follow-up
2002
Abstract Objectives To assess the therapeutic value of left renal vein transposition for treatment of the nutcracker phenomenon in long-term follow-up. Methods Eight patients (4 women and 4 men) between 23 and 58 years old (mean 39.1) underwent transposition of the left renal vein for treatment of the nutcracker phenomenon associated with recurrent gross hematuria and flank pain. The postoperative follow-up was 41 to 136 months (mean 66.4). Results No perioperative complications were encountered. The postoperative complications comprised deep vein thrombosis (n = 1), retroperitoneal hematoma necessitating surgical revision (n = 1), and paralytic ileus that resolved with conservative managem…
The Nutcracker Syndrome: New Aspects of Pathophysiology, Diagnosis and Treatment
1991
Magnetic resonance imaging (MRI) was used to study vascular anatomy in 3 patients with the nutcracker syndrome and in 10 healthy volunteers. From these studies an abnormal branching of the superior mesenteric artery from the aorta was identified as being the cause of the nutcracker syndrome. Consequently, surgical transposition of the left renal vein to achieve an unobstructed renal venous backflow was performed successfully in 2 patients, while 1 underwent nephrectomy. In 1 patient adjuvant ureteral instrumentation became necessary to aid occlusion of persisting shunts between peripelvic venous varicosities and the urinary tract. Awareness of the pathophysiology of the nutcracker syndrome …
Selective Introduction of Acid Sites in Different Confined Positions in ZSM-5 and Its Catalytic Implications
2018
Controlling the location of acid sites in zeolites can have a great effect on catalysis. In this work we face the objective of directing the location of Al into the 10R channels of ZSM-5 by taking advantage of the structural preference of B to occupy certain positions at the channel intersections, as suggested by theoretical calculations. The synthesis of B-Al-ZSM-5 zeolites with variable Si/Al and Si/B ratios, followed by B removal in a postsynthesis treatment, produces ZSM-5 samples enriched in Al occupying positions at 10R channels. The location of the acid sites is determined on the basis of the product distribution of 1-hexene cracking as a test reaction. The higher selectivity to prop…
Comparison of the activity, selectivity and decay properties of lay and hyultrastable zeolites during the cracking of alkanes
1984
Abstract The cracking of n-heptane on LaY ultrastable zeolite has been studied in a continuous glass flow reactor, at atmospheric pressure, up to 470°C. The initial selectivity to cracking, isomerization and disproportionation, kinetic rate constants, activation energies and decay parameters have been calculated and compared with those obtained using a HY ultrastable zeolite as a catalyst. It has been found that the HY ultrastable zeolite is about 7 times more active for cracking, and about 10 times more active for isomerization and disproportionation than the LaY ultrastable zeolite. The protolytic to β-cracking ratio is higher for the HY ultrastable zeolite. The deactivation takes place b…
Catalytic cracking of alkanes on MCM-22 zeolite. Comparison with ZSM-5 and beta zeolite and its possibility as an FCC cracking additive
1995
Abstract N-heptane cracking has been carried out on MCM-22, and its kinetic and decay behaviour is compared with that of ZSM-5 and Beta zeolites. In the case of MCM-22 cracking occurs in the 10-member ring channel system, as well as in the large cavities formed by 12-member rings. Product selectivities show that MCM-22, while presenting features which indicate structural similarities with ZSM-5, it also presents characteristics corresponding to pores or cavities larger than ZSM-5. The selectivities to the individual products together with C1 + C2/iC4, C3/C4, C5/C2, iC4/total C4, and alkane/alkene ratios indicate that MCM-22 has good properties to be used as an FCC octane booster additive sp…
Formation of products responsible for motor and research octane of gasolines produced by cracking The implication of framework Si/Al ratio and operat…
1989
Abstract An alkane in the range of gasoline fraction ( n -heptane) has been used as a reactant to study the influence of zeolite Y catalyst and process variables (i.e., framework Si Al ratio and procedure of dealumination, time on stream, and contact time) on the formation of products responsible for motor and research octane of gasoline during cracking, namely branched, aromatics, and olefins. It is found that the branched isomers in the C 5 and C 6 fractions appear as primary products and are partly produced by disproportionation, since the ratio of iso to normal compounds is above equilibrium. The ratio of branched to linear products strongly decreases when the number of carbon atoms of …
Catalytic cracking of n-alkane naphtha: The impact of olefin addition and active sites differentiation
2015
An extended dual kinetic model allows to fit the n-heptane cracking results working in a wide range of reaction conditions. The duality of the model is provided by the contribution of monomolecular and bimolecular cracking mechanisms. It takes into account the role played by the olefins formed on the global cracking or added within the feed. Furthermore by means of this model and the kinetic parameters obtained when cracking n-heptane on ZSM-5, it has been observed that, while some characterization techniques show a homogeneous zeolite surface from the point of view of the active sites, rigorous kinetic experiments point to the possibility that the reactant sees a heterogeneous surface with…
Cracking of n-heptane on a hzsm-5 zeolite. The influence of acidity and pore structure
1985
Abstract The crackino of n-heptane on a HZSM-5 zeolite has been studied in a continuous qlass flow reactor, at atmospheric pressure, up to 470°C. The initial selectivities to the different reaction products, kinetic rate constants, activation energies and decay parameters have been calculated and compared with those obtained using a large pore zeolite (HYUS). A different product distribution is obtained with the two zeolites. In that way, and considering initial selectivities, different C 6 /C 1 , C 5 /C 2 , C 4 /C3, i-C 4 /n-C 4 , ethylene/ethane, propylene/propane ratios have been found on the two zeolite catalysts. The results have been quantitatively explained by considerinq a direct cr…
Localized hydrogen cracking in the austenitic phase of a duplex stainless steel
1996
The aim of this study is to investigate the role of hydrogen on the mechanical behavior of an austenitic phase, in the particular situation of duplex stainless steels. In these duplex alloys, in presence of hydrogen, the ferritic phase is embrittled by hydrogen and the resistance to cracking is mainly related to the behavior of the austenitic phase. Thus, a discussion of the role of hydrogen at the crack tip of a duplex alloy (as function of the microstructure) has been proposed by T. Perng and C.J. Altester after experiments conducted in gaseous environment. A similar experimental approach has been followed in this study; slow strain rate tests (SSRT) have been performed on duplex stainles…