Küresel olmayan malzemelerin akışkanlaşma sırasındaki davranışlarının incelenmesi

• Published in: İTÜ dergisi. D, Mühendislik Vol. 8; no. 6; pp. 3 - 12
• Main Authors: AKGİRAY, Ömer, SOYER, Elif, SARIKAYA, Hasan Z
• Format: Journal Article
• Language: Turkish
• Published: İstanbul Teknik Üniversitesi 2009
• ISSN: 1303-703X

Akışkan yatakların genişlemesi sırasındaki hız-gözeneklilik ilişkileri üzerine önerilen pek çok denklem mevcuttur. Bununla birlikte bu denklemlerin tamamına yakını sadece küresel malzemelere uygulanabilir nitelikte olup küresel olmayan tanecikler için önerilen az sayıda modelin büyük çoğunluğu da ampiriktir ve şekil faktörü etkisini açık bir şekilde ortaya koymamaktadırlar. Ayrıca küresel olmayan tanecikler için geçerli genişleme modellerinin doğrulukları sistematik bir şekilde değerlendirilmemiş ve karşılaştırılmamıştır. Bu çalışmada küresel olmayan malzemeler ile ilgili akışkanlaşma deneyleri yürütülmüştür. Bu amaçla 10 farklı kum, 7 farklı perlit ve 11 farklı kırılarak oluşturulmuş cam fraksiyonu hazırlanmış, malzemelerin şeklinin akışkanlaşma sırasındaki davranışları üzerine etkisini belirlemek amacıyla sabit yatak yük kayıpları ve Ergun denklemi kullanılarak elde edilen küresellik katsayıları tespit edilmiştir. Bu şekilde her malzeme için ayrı ayrı hesaplanan kü-resellik parametresi akışkanlaşma çalışmalarının modellenmesinde oldukça iyi neticeler vermiştir. Çalışılan malzemelerin küresellik katsayıları kum, perlit, tek seferde kırılarak oluşturulmuş kırık cam ve tekrarlı kırma suretiyle oluşturulmuş kırık cam fraksiyonları için sırasıyla yaklaşık 0.74, 0.66, 0.42 ve 0.55 olarak bulunmuştur. Literatürde sıklıkla atıf alan küresel olmayan malzemelerin akışkanlaşma sırasındaki hız-gözeneklilik ilişkisini veren Dharmarajah-Cleasby (1986) denklemi bu çalışmada elde edilen deneysel veriler kullanılarak irdelenmiş ve küresel ve küresel olmayan malzemelerin genişlemiş yatak yükseklikleri ve gözenekliliklerinin tahmininde kullanılacak alternatif yeni bir denklem ile karşılaştırılmıştır. Geliştirilen yeni modelin Dharmarajah-Cleasby (1986) denklemine göre çok daha tutarlı sonuçlar verdiği görülmüştür.
Liquid-solid fluidization has a number of applications in engineering. The expansion of granular filter media during backwashing is of particular interest. Another area of application that is of growing importance is fluidized-bed reactors used in -waste-water treatment. It is important to have an understanding of fluidization principles and an ability to predict bed expansion as a function of liquid velocity to design such systems properly. More often than not, the media involved are not spherical and it is necessary to have an expansion model that can be applied to beds of non-spherical particles.
Numerous equations have been proposed to predict the expansion of liquid fluidized beds of spherical particles. Very few general equations exist, however, for non-spherical media. Furthermore, the accuracies of the expansion models for non-spherical media have not been evaluated or compared in a conclusive manner to this date.
This study considers the expansion of beds of possibly non-spherical particles during paniculate fluidization. New experimental data with both spherical and non-spherical media are presented.
Fluidization experiments have been carried out with glass balls of eight different sizes (1.11, 1.19, 2.03, 2.99, 3.18, 4.03, 4.98 and 6.01 mm), plastic balls of three different sizes (1.97, 2.48 and 2.87 mm), ten sieved fractions of silica sand, eleven sieved fractions of crushed glass, and seven sieved fractions of perlite. Perlite and crushed glass were included in this study because their properties (densities and sphericities) are different than those of silica sand, and as such they can provide additional fluidization data. It may also be noted that both materials have been considered as substitutes for silica sand in rapid filters. To obtain additional fractions of crushed glass material, particles retained in the topmost sieve tray were crushed again and sieved. In this manner sufficient quantities of additional fractions of crushed glass were obtained. Glass fractions obtained by repeated crushing and sieving were observed to have higher sphericity values. Using this procedure, crushed glass fractions with approximately the same size and density but different sphericities were produced.
This allowed the collection of additional fluidization data to investigate the effect of shape on expansion behavior.
The sand, perlite, and crushed glass fractions were obtained by a manual sieving procedure followed by an additional 1 minute of manual sieving such that the change in weight during the latter was less than l%for each fraction. Densities were measured by a water-displacement technique. Equivalent diameters have been measured by counting and weighing 200 grains of each fraction. Porosities were calculated from bed weight, bed height, and density values. Sphericity of each material was determined using fixed-bed head loss data in conjunction with the Ergun equation. For all the materials studied in this work, sphericity values calculated using fixed-bed head loss measurements and the Ergun equation allowed successful prediction of the effect of particle shape on bed expansion during fluidization. Sphericity values of the materials studied was found to be 0.74, 0.66, 0.42 and 0.55 for sand, perlite, crushed glass and crushed glass produced by repeated crushing, respectively. For the eight different sizes of glass and plastic balls, the calculated sphericities by using the Ergun equation were always close to 1.0.
A new equation is developed by analyzing fluidization data from the literature and the data collected in this work. The proposed equation represents the mentioned data very accurately and can be used to predict the expansion of both spherical and non-spherical media.
The non-spherical particle data fall below the curve for spheres on the friction factor versus the modified Reynolds number diagram. For the materials studied, it has been observed that this shape effect depends on the Reynolds number and is considerably stronger than documented previously in the literature. The proposed equation can be used to predict the expansion of both spherical and non-spherical media. When applied to the non-spherical particle data obtained in this work (bed expansions from 20% to about 100%, sphericities between 0.410 and 0.757), the mean error in the predicted porosity values is 2.45%. The corresponding mean error that results from Dharmarajah-Cleasby (1986) equation is 4.4%.