Review A review of the Residence Time Distribution (RTD) applications in solid unit operations Yijie Gao, Fernando J. Muzzio, Marianthi G. Ierapetritou ⁎ Department of Chemical and Biochemical Engineering, Rutgers, The State University of New Jersey, 98 Brett Road, Piscataway, NJ 08854, USA abstract article info Article history: Received 30 January 2012 Received in revised form 21 May 2012 Accepted 26 May 2012 Available online 9 June 2012 Keywords: Solids RTD Unit operation Modeling Measurement Performance This review traces current applications of the residence time theory in various solid unit operations. Besides reviewing recent experimental and simulation studies in the literature, some common modeling and tracer detection techniques applied in continuous flow systems are also considered. We attempt to clarify and em- phasize the influence of the residence time profile on the unit performance, which is the key in system design and performance improvement of practical unit operations. The development of predictive modeling is also an important goal in the long-term development of the residence time theory. © 2012 Elsevier B.V. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416 2. RTD modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417 2.1. CSTR and PFR series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417 2.2. Axial dispersion model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417 2.3. Stochastic model and Markov chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418 2.4. Bimodal RTD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418 2.5. Convolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418 2.6. RTD constructed by velocity profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 3. RTD measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 4. RTD applications in solid process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 4.1. Continuous blender . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 4.2. Extruder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420 4.3. Rotary drum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420 4.4. Fluidized bed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421 5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421 1. Introduction In chemical engineering and related fields, the Residence Time Distribution (RTD) is defined as the probability distribution of time that solid or fluid materials stay inside one or more unit operations in a continuous flow system. It is a crucial index in understanding the material flow profile, and is widely used in many industrial pro- cesses, such as the continuous manufacturing of chemicals, plastics, polymers, food, catalysts, and pharmaceutical products. In order to achieve satisfactory output from a specific unit operation, raw mate- rials are designed to stay inside the unit under specific operating con- ditions for a specified period of time. This residence time information is usually compared with the time necessary to complete the reaction Powder Technology 228 (2012) 416–423 ⁎ Corresponding author. Tel.: + 1 732 445 2971; fax: + 1 732 445 2581. E-mail address: marianth@soemail.rutgers.edu (M.G. Ierapetritou). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2012.05.060 Contents lists available at SciVerse ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec