Structure and thermal stability of polyethylene nanolayers

Confinement of the crystallizable polymer chain to the lamellar size scale is expected to affect nucleation and growth habit to the extent that new crystalline structures might be created. In this study, films with hundreds of extremely thin layers of high density polyethylene (HDPE) sandwiched between thicker polystyrene (PS) layers were fabricated by ‘forced assembly’ using layer multiplying coextrusion. Thermal analysis showed that as the HDPE layers became thinner, the crystallinity decreased from about 60% to almost 30%. Decreased crystallinity was accompanied by a change in morphology from banded discoids in HDPE microlayers (>100 nm) to long bundles of edge-on lamellae in HDPE nanolayers (<100 nm) as shown by atomic force microscopy and wide angle X-ray diffraction. Changes in crystallinity and crystalline morphology were responsible for an increase in oxygen permeability of the HDPE layer by a factor of 3 as the layer thickness decreased from 1.1 μm to 20 nm. It is inherent to the concept of forced assembly that nanolayers may not be stable when they are heated into the melt state. Heating films above the melting temperature of HDPE resulted in fractionated crystallization as indicated by two crystallization exotherms in thermograms. The lower temperature exotherm at 80 °C was identified with homogeneous nucleation. The droplets responsible for fractionated crystallization resulted from instability and breakup of the layers when they were taken into the melt. The number of nanodroplets formed by breakup of nanolayers was large enough that the majority did not contain an active heterogeneity and crystallization occurred primarily by homogeneous nucleation.     

Structure and thermodynamic properties from molecular dynamics simulations of the polyethylene crystal

This paper describes the results of molecular dynamics simulations of polyethylene crystal at two temperatures in the absence of periodic boundary conditions. We report the profiles of the temperature and pressure tensor along two directions in the crystal. We check that the two different methods (IK) and (MOP) for calculating the pressure tensor give similar profiles, which are constant in the middle of the crystal. In addition, we compare the data resulted from the pressure profiles with the values obtained from the virial route. The internal structure of the polyethylene is visualized through the intramolecular and intermolecular radial distribution functions.     

Effects of HZSM-5 addition during catalytic cracking ofn-hexadecane on HY

Additions of HZSM-5 to HY during cracking ofn-hexadecane enhances formation of olefins in the range C₃–C₅, with concurrent suppression of hydrogen transfer processes. Ratios of branched to linear isomers are decreased for paraffins by addition of the pentasil, while the reverse is observed for olefinic products.     

提高裂解炉热效率的有效途径

从完善设备结构、提高管束清洁度、减少辐射段热量损失、改善燃烧环境以及生产调优等五方面论述了提高裂解炉热效率的有效途径。旨在为同行改善裂解炉效能,节能降耗提供借鉴。     

Systematics and modelling representations of LPG thermal cracking for olefin production

Pyrolysis of hydrocarbons is an important commercial process for the production of ethylene, propylene and 1,3 butadiene. These low molecular weight olefins are among the most important base chemicals for the petrochemical industries for polymer production. A simulation program of the reaction kinetics and coke formation inside the coils of a thermal cracking unit can provide information on the effects of operating conditions on the product distribution. The aim of this study was to develop a mechanistic reaction model for the pyrolysis of LPG that can be used to predict the yields of the major products from a given LPG sample with commercial indices. A complete reaction network, using a rigorous kinetic model, for the decomposition of the LPG feed has been developed, which is used for the simulation of industrial LPG crackers. This model has been adapted using industrial data for the pyrolysis yields of LPG. The present paper attends on the asymptotic coking mechanism and describes the development of a kinetic coking model in the pyrolysis of LPG. Detailed and accurate information about the product distribution, growth of coke layer, the evolution of the tube skin temperatures can be obtained from this simulation. Simulations of this kind can be used to optimize the furnace operation. They can be used as a guide for the adaptation of the operating variables aiming at prolonging the run length of the furnace. The reactor model, as well as kinetic scheme, is tested in an industrial cracking furnace.     

Kinetics of asphaltene thermal cracking and catalytic hydrocracking

A pentane-insoluble asphaltene was processed by thermal cracking and catalytic hydrocracking over NiMo/γ-Al₂O₃ in a microbatch reactor at 430 °C. Kinetic analysis shows that the first-order kinetics fits the data of conversion in reaction times ≤ 30 min approximately, but deviates from the data of times over 30 min significantly; whereas the second-order kinetics fits the data of the reaction times up to 60 min adequately, to give the apparent rate constants of 1.704 × 10⁻² and 9.360 × 10⁻² wt frac⁻¹min⁻¹ for the two cracking processes. Furthermore, a three-lump kinetic model is proposed to include parallel reactions of asphaltenes to produce liquid oil (k₁) and gas + coke (k₃), and consecutive reaction from liquid to gas + coke (k₂). The evaluated value of k₁ is 1.697 × 10⁻² and 9.355 × 10⁻² wt frac⁻¹min⁻¹, k₂ is 3.605 × 10⁻² and 6.347 × 10⁻³ min⁻¹ , and k₃ is 6.934 × 10⁻⁵ and 4.803 × 10⁻⁵ wt frac⁻¹min⁻¹ for asphaltenes thermal cracking and catalytic hydrocracking, respectively. Selectivity analysis shows that the catalytic hydrocracking process promotes liquid production and inhibits coke formation effectively.     

Development of catalysts for the fluid catalytic cracking process: An example of CYTED-D program

The present paper describes the development of a catalyst for the fluid catalytic cracking process of gasoils. The catalyst produced is to maximize the high octane components in the gasolines and the liquefied gases for petrochemical use. The results obtained in the riser type pilot plants of the Compañia Española de Petróleo, S.A. (CEPSA), and INTEVEP, S.A., R&D Center of Petróleos de Venezuela, S.A. are also reported. Finally, information is given on the technical-economic feasibility of a prototype plant for manufacturing this type of catalyst with an annual capacity often thousand tons.This project has been performed under the Ibero-American Science and Technology Program for Development, CYTED-D, in which 25 researchers from five Ibero-American countries have participated: Argentina, Brazil, Mexico, Spain, and Venezuela, offering an example of multinational cooperation.The present work is an abstract of the final project report. The results of the different phases and activities described will be published by the authors separately.     

高压副产蜡做色母分散剂的研究

研究了高压装置副产蜡做色母分散剂的性能。从微观角度分析了高压副产蜡的性质，探索加工工艺。试验表明，高压副产蜡做色母分散剂性能优越，工艺路线可行。     

水平井压裂起裂规律研究现状

介绍了国内外在水平井压裂裂缝起裂规律的研究进展,主要包括运用解析法和有限元法两种不同方法,研究了水平井井筒周围地应力分布对裂缝起裂的影响,破裂压力及起裂方位,指出了目前研究中的不足,并对未来的研究方向进行了分析和展望。     

裂解炉出口温度控制技术的优化

通过对大庆乙烯装置80-U型裂解炉出口温度(COT)测量方式进行优化及改进,提高了对裂解深度的控制程度,进而改善了裂解炉日常的运行状况,达到了提高乙烯产品收率的目的。     

Effect of thermal history on free volume and transport properties of high molar mass polyethylene

The desorption of n-hexane from high molar mass polyethylene at 298 K has been studied. The polymer was given different thermal treatments to obtain differences in crystallinity, morphology and composition of the non-crystalline fraction. Crystal contents determined by Raman spectroscopy were always lower than those determined by density measurements. The systems with high crystallinity showed in relative terms a low content of interfacial component. The n-hexane solubilities of the samples were strictly proportional to the volume fraction of penetrable polymer (liquid-like and interfacial components) and the solubility was low in comparison with that of branched polyethylene of the same crystallinity. The Cohen-Turnbull-Fujita free volume theory was numerically capable of describing the desorption process in the polyethylenes studied. One of the methods used assumed that all three non-crystalline components—liquid-like, interfacial liquid-like and interfacial crystal core components—are penetrable by n-hexane and this method yielded data for the geometrical impedance factor and the fractional free volume with physically realistic trends, but the changes in the geometrical impedance factor was not in quantitative agreement with the Fricke model applied to the morphological data. This lack of numerical agreement is tentatively due to the fact that the assessment of crystal shape by transmission electron microscopy of stained sections systematically underestimates the crystal widths, particularly for the low-crystallinity samples with curved and ‘twisting’ crystal lamellae.     

Determination of yield distribution in olefin production by thermal cracking of atmospheric gasoil

A pilot plant was designed and set up to study the thermal cracking of atmospheric gasoil. Based on the CCD (central composite design) method, a set of systematic experiments were designed and carried out. The designed variables were COT (coil outlet temperature), steam ratio and feed flow rate. The ranges of these variables were, respectively, equal to 716–884 °C, 0.46–1.136 and 0.977–6.02 g/min. The obtained minimum and maximum yield of ethylene was, respectively, equal to 1.7% and 30.9%, as well as the maximum yield of propylene was 12.2%. To predict the yield distribution of products and the coke formation in the range of operating conditions, a mechanistic model was developed based on experimental results. To analyze and characterize the atmospheric gasoil, a novel algorithm was applied. This algorithm utilized density, ASTM distillation curve, H/C ratio and the total aromatic fraction and generates the detail analysis of feedstock including paraffinic, naphthenic, aromatics and poly aromatic compounds.     

Pressure fluctuation characteristics of polyethylene particles in a gas-solid fluidized bed with different distributors

The effects of gas velocity (0.10–0.68 m/s), distributor geometry (opening area ratio, number of orifices and hole sizes) and the particle size distribution (wide and narrow) of polyethylene on the pressure fluctuation of the distributor and the local pressure drop in the beds have been determined in a gas-solid fluidized bed of 0.07-m I.D. and 0.7 m in height. The mean amplitude and the normalized standard deviation of the local pressure drop were determined by analyzing pressure fluctuation in the bed. The variation of the mean amplitude of the local pressure drop with the gas velocity is independent on the different distributors. The normalized standard deviation of the local pressure drop increased with increasing the ratio of the excess gas velocity to the minimum fluidization velocity and the lumped parameter of the distributor, hD. Also, the normalized standard deviation of the local pressure drop of polyethylene polymerized by Metallocene catalyst (narrow) is higher than that of polyethylene polymerized by Ziegler-Natta catalyst (wide). This paper is dedicated to Professor Dong Sup Doh on the occasion of his retirement from Korea University.     

Surface modification of low density polyethylene films by homogeneous catalytic ozonation

Low density polyethylene films were treated by ozone to generate peroxides on the surfaces. The peroxides generated are capable of initiating radical graft polymerization of hydrophilic vinyl monomers onto the polymers, resulting in hydrophilic surfaces. Results of ozonation revealed that molecular ozone instead of hydroxyl radicals was the main oxidant for peroxide generation. A novel approach, aqueous ozonation with the addition of a soluble transitional metal salt, FeCl₃, as a homogeneous catalyst, was proposed and proved to be successful in this study. The addition of FeCl₃ could increase peroxide generation by 22.7%, compared to its non-catalyzed counterpart. An optimum catalyst concentration, 0.04 g/L, was determined. Also, the effects of pH, ozonation time and applied ozone dose on peroxide generation were investigated. The loss in tensile strength of the films would be 15% or less if the applied ozone dose was not over 2 wt.%. The functional groups generated on the film surfaces were characterized by FTIR, the contact angle and surface roughness of the film were also examined before and after ozonation.     

In situ analysis of volatiles obtained from the catalytic cracking of polyethylene

The effects of the solid‐acid‐catalyst pore size and acidity on polyethylene catalytic cracking were examined with a comparison of the temperature‐dependent volatile‐product‐slate changes when the polymer was cracked with HZSM‐5 and HY zeolites and the protonated form of MCM‐41. Volatile‐product distributions depended on the catalyst acidity and pore size. With HZSM‐5, paraffins were detected initially, and olefins were produced at somewhat higher temperatures. Aromatics were formed at temperatures 30–40°C higher than those required for olefin production. Small olefins (C₃–C₅) were the most abundant products when HZSM‐5 and MCM‐41 catalysts were employed for cracking polyethylene. In contrast, cracking with HY produced primarily paraffin volatile products (C₄–C₈). HY pores were large enough and the acid sites were strong enough to promote disproportionation reactions, which led to the formation of volatile paraffins. Compared with the other catalysts, HZSM‐5 with its smaller pores inhibited residue formation and facilitated the production of small alkyl aromatics. Volatile‐product variations could be rationalized by a consideration of the combined effects of catalyst acidity and pore size on carbenium ion reaction pathways. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3118–3125, 2001     

Safety assessment of high density polyethylene pipe with thermal damages

High density polyethylene pipes exposed in fire spot need to be assessed properly to assure safety operation. In this essay, thermal aging experiments were conducted to gain specimen containing thermal damage. Changes in mechanical properties and microstructure have been investigated by means of various characterisation analyses. The effect of exposure temperature, heat duration time, and cooling method on the mechanical properties were studied, respectively. Analysis results revealed that the effect on the mechanical properties is the consequence of interaction between annealing and heat damage. The results of oxidative induction time experiments, in accordance with attenuated total reflectance Fourier transform infrared spectroscopy analysis, indicated the consumption and migration of antioxidants. On the basis of practice and research, safety assessment procedure and method of high density polyethylene pipe after fire was proposed.     

Development of membranes by radiation grafting of acrylamide into polyethylene films: Characterization and thermal investigations

Polyethylene-g-polyacrylamide membranes were prepared by graft polymerization of acrylamide into polyethylene films by preirradiation technique. The characterization and thermal behavior of membranes with different degrees of grafting were evaluated by density, X-ray diffraction, thermogravimetric analysis, and differential scanning calorimetry measurements. Grafting led to considerable changes in the structure of polyethylene membranes. The density of the polyethylene film increased wtih the increase in the degree of grafting, although the increase beyond 100% grafting was less pronounced than at lower graft levels. The heat of fusion and the crystallinity of polyethylene decreased with the increase in the degree of grafting. The decrease in crystallinity is because of the cumulative effect of the dilution of inherent crystallinity by the incorporation of amorphous polyacrylamide grafts within the noncrystalline region of polyethylene (dilution effect) and partial disruption of the crystallites (crystal defects). X-ray diffraction measurements also revealed a decrease in the crystallinity in grafted films. Membranes behaved as a two-component system where polyethylene and polyacrylamide components underwent independent degradation irrespective of the graft levels. In general, the thermal stability of polyethylene in membranes was markedly improved by the grafting of acrylamide monomer as evident from the initial decomposition temperature increasing from 311°C for virgin PE to 390°C in grafted membranes. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2629–2635, 2001     

采用环境应力开裂速度测定仪对聚乙烯燃气管道料蠕变开裂性能的研究

采用ＰＥＳＣ—１型高分子材料环境应力开裂速度测定仪研究了７种聚乙烯燃气管道料的蠕变开裂性能。Ｇａｓ－１是实用中的燃气管道料。Ｐｒｅ－１因其Ｂｅｌ法开裂时间大于１０００ｈ成为有待进一步评价的基础树脂。由Ｐｒｅ－１与各种色母料或改性剂配混成５种配混料：Ｐｒｅ－１０１、Ｐｒｅ－１０２、Ｐｒｅ－１０３、Ｐｒｅ－１０４和Ｐｒｅ－１０５。通过实验得到了开裂过程（即裂纹的增长量△ａ）与时间ｔ的关系。开裂过程含有一诱导期ｔｉ,然后是裂纹增长期,裂纹增长速度为ａ。ｔｉ和ａ可以表征材料的蠕变开裂性能。Ｋ１０＝０２ＭＰａｍ１／２时,Ｇａｓ－１的ｔｉ大于３４０ｈ,Ｋ１０＝０．３ＭＰａｍ１／２时,Ｇａｓ－１的ｔｉ和ａ分别为８５ｈ和００１１５ｍｍｈ－１。Ｐｒｅ－１的蠕变开裂性能远不如Ｇａｓ－１,在Ｋ１０＝０．２ＭＰａｍ１／２时的ｔｉ和ａ分别为７４ｈ和０．００７６ｍｍｈ－１,在Ｋ１０＝０．３ＭＰａｍ１／２时的ｔｉ和ａ分别为２４ｈ和０．０３２３ｍｍｈ－１。配混料中的色母料和改性剂使基础树脂Ｐｒｅ－１的蠕变开裂性能进一步降低。     

Studies on thermal cracking behavior of residual feedstocks in a batch reactor

Thermal cracking of residual fractions has gained interest of refiners due to increasing demand of middle distillates and at the same time decline in demand of fuel oils. The present study is an attempt to gain deeper insight into the thermal cracking behavior of residual feedstocks in terms of certain key characteristics. Laboratory scale experiments on a 400 ml capacity stainless steel batch reactor were conducted with four residual feedstocks of Indian and Middle East origin—North Gujarat short residue (NGSR), Visbreaker feed from Mathura refinery (MVBF), Bombay High short residue (BHSR) and Asphalt from Haldia refinery (HRA), with asphaltene content varying in the range 1.85-10.15 wt%. The cracked products were separated by distillation up to 500 ^°C. The distillate (500 ^°C-) was analyzed by ASTM D2887 (SIMDIST) method and obtained data were classified into lumps, namely Gas (C₅-), Gasoline (IBP-150 ^°C), Light Gas Oil (150-350 ^°C) and Vacuum Gas Oil (350-500 ^°C) prior to detailed data analysis. The analysis of results reveals that the thermal cracking of petroleum residues follows first order kinetics. The rate constants and activation energies have also been estimated.