Chemical Engineering and Processing 41 (2002) 267 – 279 An experimental investigation into vapor dispersion and solid suspension in boiling stirred tank reactors N. Dohi a , Y. Matsuda a , K. Shimizu b , K. Minekawa b , Y. Kawase b, * a Deelopment and Engineering Research Center, Mizushima Plant, Mitsubishi Chemical Corporation, Kurashiki, Okayama712, Japan b Department of Applied Chemistry, Biochemical Engineering Research Center, Toyo Uniersity, Kawagoe, Saitama 350 -8585, Japan Received 9 February 2001; accepted 25 April 2001 Abstract Power consumption, gas hold-up, critical impeller speed for just complete off-bottom solid suspension and minimum impeller speed for ultimately homogeneous solid suspension were measured in boiling water using a 0.2 m i.d. stirred tank reactor with three four-pitched blade downflow turbines. Water and three different size glass beads were used as the liquid and solid phases, respectively. At higher vapor generation rates nucleation occurred at the heater, whereas at low vapor generation rates vapor was mainly generated from the impellers instead of the heater. At higher vapor generation rates, the boiling-to-nonboiling mechanical power ratio and the gas hold-up decreased and increased uniformly with increasing impeller rotational speed, respectively. At lower vapor generation rates, however, boiling-to-nonboiling mechanical power ratio exhibited a minimum and gas hold-up went through a maximum with varying impeller rotational speed. The changes in the nucleation site and solid suspension with impeller speed were responsible for these hydrodynamic behaviors. An empirical correlation for the Reynolds number corresponding to the minimum power consumption ratio or corresponding to the maximum gas hold-up in boiling liquids was developed using the present experimental data. The critical impeller speed for just complete off-bottom solid suspension and minimum impeller speed for ultimately homogeneous solid suspension in boiling systems were higher than those in gas-sparging systems. On the whole, the solid suspension in the boiling systems was poor as compared with that in the gas-sparging systems. Empirical correlations for critical impeller speed for just complete off-bottom solid suspension and minimum impeller speed for ultimately homogeneous solid suspension in boiling slurry stirred tank reactors were proposed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Boiling slurry stirred tank; Solid suspension; Gas hold-up; Power consumption www.elsevier.com/locate/cep 1. Introduction In the chemical industries, there are many processes in which considerable vapor evolution occurs in stirred tank reactors with solid suspensions [1 – 3]. However, very little information is available on the boil-off stirred tank reactors in open literature. In particular, there is little knowledge regarding the hydrodynamic behaviors of vapor and solid phases in boiling systems. For efficient and rational design of the boiling slurry stirred tank reactors, it is essential to understand the disper- sions of vapor and solid particle in the boil-off reactors. A significant amount of data can be found in the literature on hydrodynamics in cold gas-sparging stirred tanks. However, they can not be used to esti- mate mixing in boiling slurry stirred tank reactors. It should be emphasized that the phenomena in boil-off stirred tank reactors are not similar to those in gas- sparged stirred tank reactors and knowledge lack exists in our understanding of hydrodynamics in boiling sys- tems with solid particles. Few investigators have studied the subject of hydro- dynamics in boil-off stirred tank reactors. Although Smith and co-workers [4 – 6] measured power consump- tions in stirred tank reactors with boiling liquids, their study is exclusively limited to power consumption in a stirred tank with a single impeller. When a configura- tion employing two or more impeller is adopted, the flow complexity is greatly increased. Recently, Dohi et al. [3] presented data of power consumption and gas * Corresponding author. Tel.: +81-492-391377; fax: +81-492- 311031. E-mail address: bckawase@eng.toyo.ac.jp (Y. Kawase). 0255-2701/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0255-2701(01)00142-8