Effect of particle size, density, and concentration on granular mixing in a double screw pyrolyzer Breanna L. Marmur ⁎, Theodore J. Heindel Department of Mechanical Engineering, Iowa State University, Ames, IA 50011, United States abstract article info Article history: Received 7 March 2016 Received in revised form 29 June 2016 Accepted 14 August 2016 Available online 16 August 2016 Double screw pyrolyzers can be used to convert cellulosic biomass into bio-oil. Bio-oil can then be converted into synthetic gasoline, diesel, and other transportation fuels, or can be converted into bio-based chemicals for a wide range of purposes. One method of industrial bio-oil production is called fast pyrolysis, the fast thermal decompo- sition of organic material in the absence of oxygen. One type of pyrolyzer, a double screw pyrolyzer, features two intermeshing screws encased in a reactor which mechanically conveys and mixes the biomass and heat carrier media. The mixing effectiveness of the two materials in the pyrolyzer is directly correlated to the bio-oil yield—the better the mixing, the higher the yields. This study investigates the effects of particle size, density, and concentration on mixing effectiveness in a double screw pyrolyzer. Using glass beads as simulated heat car- rier media and various organic particles as biomass, a cold-flow double screw mixer with 360° of optical access and full sampling capabilities was used to collect mixing data. Unique optical visualization and composition anal- ysis paired with statistical methods were used to evaluate the effects of varying the biomass particle size and den- sity, the heat carrier particle size, and the biomass particle concentration. Both qualitative and quantitative analysis indicated that reducing the biomass particle size, for counter-rotating down pumping screw rotation ori- entations, noticeably increased mixing effectiveness. Increasing the heat carrier media particle size showed both increases and decreases in mixing effectiveness depending on operating condition. For all screw rotation orien- tations, a change in biomass particle density resulted in little change in mixing effectiveness, while reducing the biomass particle concentration reduced the overall mixing effectiveness. © 2016 Elsevier B.V. All rights reserved. Keywords: Granular materials Mixing Multiphase flow Particulate process Powder technology Renewable energy 1. Introduction Granular mixing processes are very important across many indus- tries including, but not limited to, the pharmaceutical, agricultural, and biotechnology industries [1–3]. These processes often require both a high degree of homogeneity and a high degree of customizability [4]. As granular mixing processes are so widely employed, a thorough understanding of the mixing dynamics is necessary to understand and control the resulting products [5]. Granular mixing has been widely studied since the 1950′s when the science began to gain attention [6]. Considerable experimental research has since been done on a wide variety of mixers, including: bladed mixers [7,8], vertical shakers [9,10], single screw mixers [11–13], and rotating cylinders [14]. Granular mixing processes have also been stud- ied using simulations [10,15,16]. Several literature reviews are available on the subject [1,2,17,18]. Although many studies have investigated granular mixing, most work has focused on mixers with relatively simple geometries. While these studies have done much to improve the theoretical understanding of granular flows, these theoretical understandings are often difficult to apply to broader industrial uses. Moreover, granular mixing processes are often very sensitive to changes in operating conditions, and any so- lutions provided to deal with specific mixing problems are highly sys- tem-sensitive. These sensitivities mean it is necessary to study more complicated mixer geometries and more complicated operating condi- tion interactions if we hope to apply knowledge of granular mixing to industrial processes [19]. One example of a complicated industrial mixer is a double screw py- rolyzer used in the bioenergy industry to produce bio-oil via fast pyrol- ysis [3,20]. Fast pyrolysis is the thermochemical conversion of biomass into bio-oil in the absence of oxygen [3]. In a double screw pyrolyzer, a biomass material is mixed with a heat carrier media at high tempera- tures to produce vapors that are collected and condensed into bio-oil [21]. Bio-oil has the potential to be converted and upgraded into trans- port fuels, such as synthetic gasoline or diesel, to be used as a source of fuel via direct combustion, or to be converted into a wide variety of bio- based chemicals [22,23]. Fast pyrolysis is still, however, a relatively new technology [21] and much of the research that has been done with dou- ble screw pyrolyzers has focused on the products [20,24,25] and not on the mixing dynamics of the mixer. Recent work by Kingston and Powder Technology 302 (2016) 222–235 ⁎ Corresponding author. E-mail addresses: bmarmur@gmail.com (B.L. Marmur), theindel@iastate.edu (T.J. Heindel). http://dx.doi.org/10.1016/j.powtec.2016.08.040 0032-5910/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec