Indirect Dryer for the Next Generation

Abstract

Introduction: In industrial settings, drying slurry and clay-like materials can be achieved through direct or indirect heating methods. The Solidaire^TM dryer, an indirect heating dryer, stands out due to its efficient combination of jacket heating and high-speed paddle agitation. This study explores new technological advancements in Solidaire to enhance its drying capabilities.

About Solidaire^TM: The Solidaire dryer indirectly transfers heat to the material via a heated jacket while agitating the material with high-speed rotating paddles, ensuring efficient and rapid drying. The internal structure includes a cylindrical casing with a surrounding heating jacket and a rotating paddle assembly. The paddles’ high rotating speed helps to break down agglomerates, reducing the residence time and minimizing heat damage to the material.

Experimental methods: A series of tests was conducted to evaluate the effects of increased paddle rotating speed and reduced clearance between the paddles and the casing. These tests used a prototype designed to achieve higher paddle speeds and adjustable clearance. Experiments were conducted using a lightweight calcium carbonate slurry (primary particle size 150 nm, 67% W.B.), which is prone to adhesion. The overall heat transfer coefficient and moisture content of the dried product were measured under various conditions.

Results and Discussion:

1. Clearance Effect: Reducing the clearance between the agitator paddles and the inner wall increased the overall heat transfer coefficient, likely because of a decrease in the thickness of the boundary layer (L_p in Fig. 3).

2. Paddle Speed Effect: Increasing the paddle peripheral speed from the standard 11 m/s to 22 and 33 m/s improved the overall heat transfer coefficient (Figs. 7–9). Higher speeds facilitated particle dispersion, increased contact between particles and the heated wall, and reduced adhesion.

3. Jacket Temperature Effect: Higher jacket temperatures (steam pressures) increased the overall heat transfer coefficient and reduced the moisture content of the dried product, as expected (Figs. 11–14).

4. Adhesion Reduction: Higher paddle speeds effectively reduced adhesion inside the dryer, which is a common issue in drying operations (Fig. 15).

Conclusions: The study demonstrated that increasing the paddle rotating speed and optimizing the clearance in the Solidaire^TM indirect heating dryer significantly improved the drying capacity by enhancing the overall heat transfer coefficient and reducing adhesion. These findings provide insights into the development of next-generation dryers with improved performance for various industrial applications.