Bead Size Distribution Dependency on Reactor Geometry and Agitation Conditions of Polystyrene Production with Suspension Polymerization

The relation between the reactor geometry and agitation conditions on particle size distribution for suspension polymerization of polystyrene was analyzed. The reactor geometries were selected as cylindrical and spherical, respectively. Mechanical agitation was provided via double-impeller Marine type propeller at 1600 rpm and 2000 rpm. The diameters of the reactors were selected approximately the same for cylindrical and spherical reactors in order to keep effective mixing area similar in both geometries. All experiments were conducted around 8 hours to complete the polymerization reaction. Particle size distribution analyses showed that at faster agitation conditions in cylindrical reactor narrower and smaller particles are obtained as expected. On the other hand, in spherical reactor, vortex formation and non-uniform particle size distribution were observed at faster agitation. Interestingly, when agitation speed was decreased in spherical reactor, quite narrower and smaller particle size distributions with respect to cylindrical reactor were obtained. However, the number and average molecular weight analyses suggested that the particles obtained from spherical reactor has a lower molecular weight distribution than particles from cylindrical reactor. This was attributed to the decrease in effective mixing area due to the non-homogenous changes of the distance between reactor walls and impellers throughout the spherical reactor. The geometry change to spherical geometry obviously causes diminishing in mass transfer of initiators and suspension stabilizers so that a shortening of polymer chains lead to a slight increase in polydispersity index.

Polistirenin Süspansiyon Polimerizasyonuyla Üretiminde Tanecik Boyutu Dağılımının Reaktör Geometrisi ve Karıştırma Koşulları ile İlişkisi

İlgili çalışmada, süspansiyon polimerizasyonu yoluyla elde edilen polistirenin reaktör geometrisi ve karıştırma koşullarına bağlı olarak parçacık büyüklüğü ve ortalama moleküler ağırlık dağılımları incelenmiştir. Karıştırma işlemi, ikili marin tipi pervaneli bir karıştırıcı ile 1600 dev/dak ve 2000 dev/dak olmak üzere iki farklı karıştırma hızında gerçekleştirilmiştir. Etkin karıştırma alanını kaybetmemek için her iki reaktörün de çaplarının birbiriyle uyumlu olmasına dikkat edilmiş, ayrıca polimerizasyonun tümüyle bittiğinden emin olmak için tüm deneyler yaklaşık 8 saat sonunda kesilmiştir. Silindirik reaktörde beklenildiği üzere artan karıştırıcı hızıyla küçük çaplı taneciklerin arttığı ve dağılımın bu aralıkta yoğunlaştığı gözlenirken, küresel reaktörde ise yüksek karıştırıcı hızının vorteks oluşumuna ve tanecik boyutlarının kontrolsüze gelişmesine neden olduğu ancak düşük hızlarda ise çok daha etkin bir yoğunlukta küçük tanecik aralığının elde edildiği görülmüştür. Öte yandan, ortalama moleküler ağırlığı dağılımı tayinleri küresel reaktörde elde edilen polistirenin silindirik reaktörde elde edilenlere göre daha düşük moleküler ağırlık dağılımına sahip olduklarını ortaya koymuştur. Bunun nedeninin küresel reaktörde, karıştırıcı pervanelerinin reaktör duvarlarına göre yukarıdan aşağıya sabit mesafede olmayışı ve dolayısıyla silindirik reaktöre nazaran etkin bir kütle ve ısı transferinin sağlanamayarak polimer zincirlerinin kısalmasına ve polidispertiste endeksinin yükselmesine yol açtığı düşünülmektedir.

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