1. Study of the Performance Characteristics of a Stirred Tank Reactor Suitable for Diffusion Controlled Liquid-Solid Catalytic Reactions
M.M. Taha, M.H. Abdel-Aziz, Y.O. Fouad, A. Konsowa, and G.H. Sedahmed
Chemical Engineering Department, Alexandria University, Egypt
Objectives
This work aims to developing a high space-time stirred tank reactor suitable for diffusion controlled liquid-solid reactions. The work is divided into two parts:
Evaluation of the reactor performance by determining the mass transfer coefficient.
Obtaining a correlation the describe the performance of the reactor.
Examination of the reactor performance in such a diffusion controlled process.
Background
Liquid-solid diffusion reactions such as; catalytic, electrochemical, biochemical, and photocatalytic reactions are widely applied . These reactions kinds are conducted in many reactors. Slurry reactors are widely used for such reactions, but there are many shortcomings accompanied with such reactors such as;
Catalyst attrition
Erosion of impeller blades
Low rate of mass transfer
Separation of the final product is a troublesome process
There are several attempts carried out to study such reactions performance by using flat plate, raschig rings, and expanded metals and screens. Previous work investigate the effect of using fixed screens and screens in motion.
The electrochemical technique was used because of its simplicity and accuracy. The value of the mass transfer coefficient (k) is obtained using the limiting current (IL) values determined from potential-current curves according to the following mass transfer equation.
I L Z F =k A C
Results and Discussion
The effect of impeller rotational speed is shown in the following figure.
The screen geometry is expressed by the dimensionless group (rh/dw) where; rh is screen hydraulic radius and calculated by (rh = ε/a) and dw is the screen wire diameter. The results have shown that
Sh = f (rh/dw)
A comparison between both impeller used is shown in the following figure.
The correlations obtained for each impeller are as follows:
For axial flow (45o pitched blade turbine)
Sh= Sc Re r h d w
For radial flow (90o flat blade turbine)
Sh= Sc Re r h d w
Increasing number of closely packed screens causes a reduction is the mass transfer coefficient because of:
Turbulence damping inside screen bed
Active area decreases at points of contact between screen
Concluding Remarks
The present dimensionless mass transfer correlation not only can be used to design batch reactors but also continuous reactors.
By analogy, the internal heat transfer coefficient can be calculated using the overall correlations obtained.
This reactor offers many advantages:
Enhancing rate of mas transfer
Enhancing rate of heat transfer
Provide high surface area per unit volume
Dispersion of sparingly soluble components
Experimental conditions
0.025 M K3Fe(CN) M K4Fe(CN)6 dissolved in NaOH solution.
Room temperature at 25±1℃
High ration of anodic to cathodic area
The solution must kept away from light
Variables studied
Impeller rotational speed (50:400 rpm)
Impeller geometry (45o pitched blade turbine and 90o flat blade turbine)
Screen geometrical parameters (mesh number and wire diameter i.e. 5, 10, and 14 mesh no.)
Solution physical properties (ρ, μ, and D)
Number of closely packed screens forming the reaction surface.
The mass transfer coefficient (k) change is investigated and expressed in terms of dimensionless group (Sh).
Experimental Apparatus