مسیرهای غیرشیمیایی برای تنظیم پراکندگی پلیمر، فعالیت و سازوکار شکست سامانه‌های کاتالیزی زیگر-ناتا در پلیمرشدن ناهمگن پروپیلن

نوع مقاله : پژوهشی

نویسندگان

تهران، دانشگاه صنعتی امیرکبیر، دانشکده‌ مهندسی پلیمر، صندوق پستی ۴۴۱۳-۱۵۸۷۵

چکیده

فرضیه‌: اصلاح فرمول‌بندی‌های شیمیایی کاتالیزگرهای پلیمری تجاری موجود، ممکن است سایر خواص کاتالیزگر به‌ویژه فضاگزینی و جهت‌گزینی را کاهش دهد. بنابراین، نیاز ضروری در مجتمع‌های صنعتی پتروشیمیایی به‌منظور اصلاح سامانه‌های کاتالیزی موجود، یافتن شیوه‌های عملی غیرشیمیایی برای تنظیم کمیت‌های اساسی شامل پهنای توزیع وزن مولکولی (MWD)، فعالیت و سازوکار شکست است. 
روش‌ها: برای دستیابی به این هدف، از مدل تک‌ذره‌ای تازه توسعه‌یافته، چندمنفذی (MPM) استفاده شده است که فرایندهای واکنشی-نفوذی دخیل در پلیمرشدن اولفینی ناهمگن را توصیف می‌کند. این کار با هدف بررسی اثر تخلخل اولیه کاتالیزگر، اندازه ذره اولیه آن، غلظت توده مونومر و آرایش اندازه منفذها بر کمیت‌های نام‌برده انجام شده است. 
یافته‌ها: مدل‌سازی سامانه کاتالیزی پایه‌دار زیگلر-ناتا (ZN) نشان داد، افزایش تخلخل اولیه کاتالیزگر یا اندازه ذره اولیه یا کاهش غلظت توده مونومر، پهنای توزیع سرعت واکنش موضعی را کاهش داد که به توزیع وزن مولکولی‌های باریک‌تر منجر شد. اگر چه شاخص چندپراکندگی (PDI) معمولاً به‌طور معکوس تغییر کرد که به‌دلیل وابستگی این کمیت به موضع توزیع وزن مولکولی (MWD) افزون بر پهنای آن است. مدل MPM همچنین دو مشاهده تجربی غیرقابل توضیح را شفاف و دلیل وقوع آن‌ها را روشن کرده که عبارت است از کاهش فعالیت کاتالیزگر در صورت افزایش تخلخل اولیه که در بعضی از مطالعات تجربی مشاهده شده است و تغییرات نامنظم و غیرقابل پیش‌بینی شاخص چندپراکندگی با تغییرات غلظت توده مونومر. برای کمیت‌های فیزیکی مطالعه‌شده در این پژوهش، پهنای MWD به‌طور مستقیم به سرعت واکنش وابسته است که نشان می‌دهد، برای کاربردهایی که در آن‌ها مقاومت بیشتر در برابر پدیده‌های شکست مذاب، موج‌داری لبه‌ و تشدید کشش مطلوب است، باید تعادلی بین پهنای MWD و مقدار محصول را جست‌و‌جو کرد. با اینکه سرعت واکنش، پهنای MWD و شاخص چندپراکندگی، هیچ وابستگی به نسبت مشارکت دو سازوکار شکست نشان ندادند، افزایش تخلخل اولیه یا شعاع اولیه کاتالیزگر، سازوکار ترجیح‌داده‌شده دو نیم‌شدن پیوسته را تشدید کرد که در نتیجه احتمال وقوع پدیده جرم‌گرفتگی را کاهش داد.

کلیدواژه‌ها

عنوان مقاله [English]

Non-Chemical Routes to Tailor Polymer Polydispersity, Activity and Fragmentation Mechanism of ZN Catalytic Systems in Heterogeneous Propylene Polymerization

نویسندگان [English]

  • mansoor sheikhzadeh
  • Saeed Pourmahdian
Department of Polymer Engineering, Amirkabir University of Technology, P.O. Box: 15875-4413, Tehran, Iran
چکیده [English]

Hypothesis: Modifications in chemical formulations of existing commercial polymerization catalysts may deteriorate other catalyst properties, especially stereo- and region-selectivity. Therefore, an absolute necessity for petrochemical polymerization facilities is to find feasible non-chemical routes for tailoring essential parameters, including molecular weight distribution (MWD) width, activity and fragmentation mechanism in order to modify existing catalytic systems. 
Methods: To this goal, use is made of a recently developed single-particle multipore model (MPM), which describes the reaction-diffusion processes involved in the heterogeneous olefin polymerization to investigate the impacts of initial catalyst porosity, initial catalyst particle size, bulk monomer concentration and pore size arrangement on the above-mentioned parameters. 
Findings: Modeling a supported Ziegler-Natta catalyst system showed that increasing the initial catalyst porosity or initial particle size or decreasing the bulk monomer concentration decreased the local reaction rate distribution width, resulting in narrower MWDs. Although, the polydispersity index generally changed oppositely due to its dependence on the location of the MWD in addition to its width. The model has elucidated and rationalized two unexplained experimental observations, i.e., increasing initial porosity reduces the catalyst activity in some studies and that polydispersity index generally changes irregularly and unpredictably with bulk monomer concentration. For the physical quantities studied in this work, the reaction rate is directly related to the MWD width, revealing that a trade-off between MWD width and yield should be sought for applications that require higher resistance to melt fracture phenomena, edge waviness and draw resonance. While, the reaction rate, MWD width and polydispersity index did not show any relationship with the participation ratio of the two fragmentation mechanisms. Increasing the initial catalyst porosity or the initial particle radius intensified the more preferred continuous bisection mechanism, thereby dropping the probability in fouling.

کلیدواژه‌ها [English]

  • Modeling
  • catalyst modification
  • heterogeneous catalyst
  • catalyst fragmentation
  • polydispersity index

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مجله علوم و تکنولوژی پلیمر
دوره 35، شماره 3 - شماره پیاپی 179
مرداد و شهریور 1401
صفحه 233-255

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