Stirred Reactors

The software input supports a convenient format for entering chemical kinetics into simulations and allows the user to analyze the effect of inlet point positions, chemical composition of the inlet flow, mixing system design, etc.

VisiMix Chem is developed for the simulation of dynamics of stirred chemical reactors operated in the turbulent flow regime and utilizes the sophisticated VisiMix Turbulent models for simulation of all hydrodynamic mixing processes.

Using tank design, agitator or rotor speed, fluid rheology and feed flow rates as input, these models support the analysis of fluid dynamics variables—i.e., pressure, velocity, circulation flow rate and local turbulence characteristics (such as local energy dissipation and the smallest scale of turbulence fluctuations) —which determine local macromixing and micromixing characteristics.

VisiMix Chem calculations are conducted for actual chemical reactors where neither macromixing nor micromixing is ideal. Results of these calculations are compared in all output tables and charts against respective calculations conducted for a respective reactor with perfect macromixing.

Rational

Using design characteristics of a reactor, we can calculate primary hydrodynamic characteristics, such as:

(i) Rotational, radial and axial velocities in the tank

(ii) Turbulent energy dissipation and turbulent diffusivity in individual large-scale zones of the reactor

Based on the primary characteristics of the turbulent flow generated at Step A, we can obtain the secondary flow characteristics, which govern macroscale and microscale mixing in the tank.

Finally, we can obtain the local concentration of reactants based on both primary (hydrodynamics) and secondary (mixing) characteristics of turbulence in the tank.

We can scale up to the industrial scale (where significant non-uniformities of local concentration occur and their impact on product purity is high) a process successfully implemented at the laboratory scale (where mixing time is very small, mixing is almost perfect, and its impact on product purity is negligible).

Brief Description of Mixing Phenomena

The goal of mixing is to minimize non-uniformities in chemical composition over the volume of fluid in a chemical reactor.

These non-uniformities occur in a continuous range of scales that can be conditionally divided into two levels: macroscopic and microscopic.

The smallest scale of turbulence fluctuations (so-called Kolmogorov’s scale) is adopted as a conditional border between macroscopic and microscopic scales.

The Kolmogorov’s scale is estimated as 𝛿=𝑣^0.75/𝜀^0.25.

Brief Description of Mixing Phenomena

Macromixing Time

Macromixing is a process of mixing at a macroscopic level exceeding 𝜹. It is achieved by creating turbulence of necessary intensity in a reactor.

Perfect macromixing is a state where chemical composition of any macroscopic volume (i.e., a volume of a size exceeding 𝜹) is uniform over the entire volume of fluid in a mixing tank at any given moment.

A chemical reactor with perfect macromixing is an ideal reactor where perfect macromixing is achieved.

However, local non-uniformities can exist in a reactor with perfect macromixing at a microscopic level, and such a reactor is different from a perfectly mixed reactor where no non-uniformities exist at both macroscopic and microscopic levels.

Characteristic Time of Mmicromixing

Micromixing is a process of mixing at a microscopic scale, which is below 𝜹. It occurs via molecular diffusion as no turbulent fluctuations exist at such a small scale.

The characteristic time, 𝝉, of micromixing (i.e., mixing inside a volume of size 𝜹) can be estimated as 𝜏=^2/𝐷_𝑚, where D_m is an effective molecular diffusivity of reactants in liquid.

Substituting equations, we obtain the following correlation between the characteristic time of micromixing, 𝝉 and local energy dissipation, 𝜺: 𝜏=𝑣^1.5/(𝜀^0.5 𝐷_𝑚).