Particle Size Control in Crystallization

1. Why Is Mixing Important in Crystallization?

The success of a crystallization process is directly related to the mixing conditions inside the tank. Mixing keeps the concentration and temperature distribution in the solution homogeneous, allows crystal nuclei to grow under uniform conditions, and prevents the formation of unwanted coarse crystals.

When mixing is insufficient, dead zones form in the tank. Local supersaturation occurs in these zones and uncontrolled crystal growth begins. The result: a wide particle size distribution, low product purity and a non-reproducible process. Excessive mixing, on the other hand, mechanically breaks crystals and produces unwanted fine fractions.

The right mixing intensity strikes the balance between these two extremes, providing a narrow particle size distribution, high yield and reproducible product quality. This balance is determined by the process chemistry, tank geometry and target crystal size.

2. Particle Size and Homogeneity

The goal in crystallization processes is to achieve the narrowest possible particle size distribution (PSD). A wide PSD leads to efficiency losses and product quality issues in subsequent process steps (filtration, drying, packaging).

The Effect of Mixing on Particle Size

Mixing intensity and flow pattern affect crystallization outcomes through three fundamental mechanisms:

• Concentration homogeneity: Homogeneous mixing provides uniform supersaturation throughout the tank. This allows all crystal nuclei to grow at similar rates.
• Temperature distribution: In cooling crystallization, temperature gradients can develop with distance from the jacketed tank. Effective mixing minimizes these gradients, providing a homogeneous cooling profile.
• Crystal suspension: Sufficient suspension capacity is required to prevent formed crystals from settling at the tank bottom. Otherwise, accumulated crystals exhibit uncontrolled growth, leading to coarse particles.

Shear Stress Balance

Shear stress balance is critically important in crystallization processes. Low shear means insufficient mixing and dead zones, while excessive shear leads to crystal breakage. The Mechanimix engineering team determines the optimal shear level for each process via CFD analysis and recommends the appropriate impeller type.

3. Mechanimix Solutions

Mechanimix offers mixer series and impeller types that meet the special requirements of crystallization processes. Each solution is configured by the engineering team according to the chemical and physical conditions of the process.

4. Impeller Selection

In crystallization processes, the impeller type directly determines the flow pattern and shear intensity. The wrong impeller selection leads to deviation from the target particle size, low yield and product quality issues.

The HM impeller is recommended as the first choice for most crystallization applications. It minimizes concentration and temperature gradients by providing homogeneous axial circulation throughout the tank. In large-volume tanks or applications where sensitive crystal structures must be preserved, the HWM impeller, with its wide blade geometry, provides high flow at low speed and reduces the risk of crystal breakage.

The HVM impeller is preferred in high-solids processes (30 wt% and above). It provides effective circulation in dense suspensions, preventing crystals from accumulating at the tank bottom. In gas-fed crystallization applications, the GDM impeller optimizes gas bubble distribution, creating a homogeneous crystallization environment.

5. Crystallization Optimization with CFD

CFD (Computational Fluid Dynamics) analysis visualizes and optimizes the flow conditions inside the crystallization reactor before production. This approach replaces trial-and-error engineering practices with data-driven decisions.

What Does CFD Reveal?

• Flow pattern map: Flow directions and velocities inside the tank are visualized. Dead zones are identified and impeller position is optimized.
• Mixing homogeneity: The degree of homogeneity in concentration and temperature distribution is evaluated numerically.
• Shear stress distribution: The shear profile in the impeller region and across the tank is extracted and crystal breakage risk is evaluated.
• Suspension capacity: Whether crystals can be kept suspended without settling at the tank bottom is checked.
• Energy consumption: Operating conditions that achieve the target mixing performance with minimum energy consumption are determined.

Mechanimix CFD Service

Mechanimix offers CFD analysis for every crystallization project. The analysis results are presented to the customer with an engineering report included in the technical proposal. This report contains flow contour graphs, velocity vector distributions, dead zone analysis and recommended operating parameters.

The most important contribution of CFD analysis is identifying potential issues before producing a physical prototype. Wrong impeller selection or improper speed setting leads to particle size deviation and low yield in actual production. With CFD, these risks are eliminated before production.

6. Industry Applications

Crystallization processes are applied with different requirements across industrial sectors. Below is a summary of leading application areas and the solutions offered by Mechanimix.