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The use of heavier feedstocks, or even the possibility of direct conversion of crude oil into chemicals has created new challenges for the steam cracking process, as severe fouling in the different sections of steam crackers is observed then. In order to obtain a more fundamental understanding about the fouling phenomenon and its relation to feedstock composition and process conditions the "Fouling Assessment SeTup (FAST)" was developed. This experimental unit was used to determine the fouling tendency of a heavy hydrocarbon mixture (HHM) in the different sections of a steam cracker, i.e. convection, radiant, and transfer line heat exchanger (TLE) sections via a post cracking decoking procedure and the continuous weighting of a suspended coupon in TLE with a Magnetic Suspension Balance (MSB). Additionally, a fouling mitigation concept of wetting fluid injection in TLE via an in-house apparatus was tested. The injection of a paraffinic fluid substantially reduced fouling in this section. [ABSTRACT FROM AUTHOR]

It is shown analytically, based on pulse-response experiments, that under special conditions the activity profile of a prepared catalytic system depends only on the total amount of admitted substance. This property, previously found computationally, is here established mathematically for porous and nonporous catalysts in different pulse reactors. [ABSTRACT FROM AUTHOR]

New synthetic pathways are proposed for the production of a broad range of innovative sophorolipid amines and sophorolipid quaternary ammonium salts starting from microbially produced sophorolipids. The selective formation of an intermediate sophorolipid aldehyde proved to be a key synthetic step of the new derivatives. The sophorolipid quaternary ammonium salts were evaluated for their antimicrobial activity against Gram-negative and Gram-positive test strains. Minimum inhibitory concentration (MIC) values were determined for the active compounds. Derivatives with an octadecyl group on the nitrogen atom proved to be more active than the antibiotic gentamicin sulfate against all tested Gram-positive strains. The results show great promise for modified sophorolipids in the medical sector. [ABSTRACT FROM AUTHOR]

Addition of di-nitrogen to the feed gas has been shown to greatly influence growth rates and morphology of the deposited layer in various diamond chemical vapor deposition (CVD) techniques. In this article, several hypotheses for these phenomena, as presented in literature, are tested for the case of diamond combustion CVD with the aid of an atmospheric pressure oxyacetylene flame. For this purpose, one-dimensional and two-dimensional simulations are performed of the hydrodynamics, the combustion and deposition chemistry, and the nitrogen chemistry. Based on the simulation results, several proposed hypotheses can be ruled out as possible explanations for the observed phenomena. It is concluded, that the most likely hypotheses are: (i) the presence of nitrogen atoms in the diamond lattice, enhancing diamond growth by acting on the electron structure of surface dimer bonds, and (ii) selective adsorption of nitrogen-containing species on the surface, selectively increasing growth in the (100) direction. It is found that possible gas phase candidates for affecting diamond growth are NH, NH[sub 2], NH[sub 3], CN, HCN, H[sub 2]CN, and NCO. [ABSTRACT FROM AUTHOR]

Density functional theory results are presented for elementary steps leading to coke growth within a steam cracking unit. The discussed pathway starts from toluene and ultimately, 1-methylnaphthalene is formed. In order to find the rate determining step for coke formation, the pseudo first-order rate coefficients of the various steps are compared taking into account the concentrations of diverse coke precursors. The influence of the polyaromatic environment is studied for a large set of methylated polycyclic aromatic molecules, by means of carbon-hydrogen bond dissociation enthalpy values. Subsequent hydrogen abstraction reactions at the methylated polyaromatics, by a methyl radical, are also examined. The abstraction is found to preferentially occur at the larger systems and is in general faster compared to abstractions at the analogous non-methylated species. [ABSTRACT FROM AUTHOR]

Theoretical calculations are presented on elementary reactions which are important during coke formation in a thermal cracking unit. This process is known to proceed through a free radical chain mechanism. The elementary reaction steps that lead to the growth of the coke surface can be divided into five classes of reversible reactions: hydrogen abstraction, substitution, gas phase olefin addition to radical surface species, gas phase radical addition to olefinic bonds and cyclization. To identify the elementary reaction classes that determine the coking rate, all microscopic routes that start from benzene and lead to naphthalene have been investigated. It is found that initial creation of surface radicals, either by hydrogen abstraction or substitution and subsequent hydrogen abstractions, determines the global coking rate. The influence of the local polyaromatic structure on the kinetics of the hydrogen abstraction reactions is determined by performing calculations on a large set of polyaromatic hydrocarbons (PAHs). On basis of the BDE values six types of possible reactive sites at the coke surface can be distinguished. For the initial hydrogen abstraction the local polyaromatic structure strongly influences the reaction kinetics and abstraction is preferred from less congested sites of the polyaromatic. [ABSTRACT FROM AUTHOR]

Theoretical ab initio calculations are presented on cyclization reactions of polyaromatics that occur by a radical mechanism. Such processes are one of the elementary steps for polyaromatic hydrocarbon formation in thermal cracking units and during soot formation. Ring closure can take place at various sites of the polyaromatic surface. It is the aim of this study to obtain insight into the influence of the local structure on the reactivity of the polyaromatics. Aromaticity is a determining factor for the reactivity and can be probed by various magnetic properties such as the diamagnetic susceptibilities, proton chemical shifts, and nucear-independent chemical shifts. A correlation is established between the magnetic properties and activation energy of the studied reactions. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004 [ABSTRACT FROM AUTHOR]

Ab initio calculations are presented on radical reactions that occur during the formation of coke in a thermal cracking unit. Kinetic parameters for the addition reaction of the ethylbenzene radical to ethene and the subsequent cyclization of the butylbenzene radical are calculated by means of Transition State Theory and Density Functional Theory. Special care is taken to correctly treat the internal rotations to predict accurate values of the preexponential factor. The influence of the local structure of the coke matrix on the kinetic parameters is tested by calculating kinetic parameters of clusters consisting of more than one benzene ring. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003 [ABSTRACT FROM AUTHOR]