a b  s t r a c   t Keywords:Mass transfer Stirred tank reactor Catalytic reactor Electrochemical reactor Diffusion controlled reactions Cylindrical blade impeller

The liquid–solid mass transfer behavior of 8 cylindrical blade impeller it can act as electrode or catalyst support in a new stirred tank reactor was studied in relation to catalytic and electrochemical reactor design by the electrochemical technique which involved measuring the limiting current of the cathodic reduction of K3Fe(CN)6. Variables studied were blade length, impeller rotation speed, physical properties of the solution, effect of baffles, effect of immiscible liquids, effect of superimposed solution flow (continuous operation), effect of the distance between successive impellers in case of multi-impeller reactor and the effect of drag reducing polymers. The mass transfer data for single rotating impeller were found to fit the following equation:

Superimposed solution flow and impeller separation were found to have little effect on the rate of mass transfer, baffles were found to increase the rate of mass transfer while the presence of immiscible liquid decreases the rate of mass transfer. Addition of drag reducing polymers (Polyox WSR-301) was found to reduce the rate of mass transfer at single impeller by a maximum of 15.8%. Examination of heavy metal removal from dilute solutions revealed that the rate of metal deposition agrees fairly with the prediction of the equation despite the presence of surface roughness and H2 evolution. The advantages of this reactor in conduct- ing diffusion controlled catalytic and electrochemical liquid–solid reactions and liquid–solid reactions involving sparingly soluble reactants that to be dispersed such as gases, solid particles and immiscible liquids were highlighted.70598

© 2016 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction

Heterogeneous stirred tank reactors with fluidized bed of catalyst particles (slurry  reactors)  have  been  used  widely to conduct diffusion controlled liquid–solid, liquid–gas–solid reactions. Despite the high reaction area of these reactors they

suffer from the following drawbacks: (i) the low relative veloc- ity between the solution and the particles gives rises to low rate of mass transfer and low reaction rates, (ii) catalyst attri- tion due to collision with themselves or with blades of the rotating impeller. (iii) Separation of the final product from the catalyst is expensive and time consuming. (iv) Overflow of

∗   Corresponding author. Tel.:  +20 1223999431.

E-mail address: Dina elgayar83@yahoo。com (D.A. El-Gayar). http://dx.doi.org/10.1016/j.cherd.2015.12.019

0263-8762/© 2016 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

of mass transfer at the impeller blades but also assists in dis- persing sparingly soluble reactants (gases, solid particles and immiscible liquids) by virtue of the flow induced by impeller rotation. (ii) Rapid removal of heat evolved from exothermic reactions from the impeller zone to a cooling jacket surround- ing the cylindrical reactor, this is highly desirable in case of heat sensitive catalysts or heat sensitive products. (iii) The reactor can be extended vertically to increase its capacity by increasing the number of impellers mounted  on  the rotat- ing shaft; this would lead to saving floor spare and capital costs. The present reactor can be used to conduct diffusion control electrochemical reactions such as electro-organic syn- thesis and removal of organic pollutants and heavy metals from wastewater. The reactor can be used also for conduct- ing diffusion controlled liquid–solid catalytic reactions such as photo-catalytic reactions and immobilized enzyme biochem- ical reactions where the catalyst is fixed on the blades of the rotating impeller.