C. Mass Transfer – The importance of Mixing in Scale-Up
This is the second installment of my series of articles on aspects of Scale-up.
A fundamental understanding of the mixing process is essential for scale-up in chemical development. Mixing is the reduction or elimination of inhomogeneity of phases that are either miscible or immiscible. Laboratory reactors must be operated under conditions that will allow meaningful mixing characterization and scale-up.
1. Laboratory Mixing Studies
Development and scale-up of a chemical process can be challenging because exact conditions are unknown. Matching performance and mixing between small and large-scale reactors is difficult when relying only upon agitation rate, geometry, agitator design, or other mechanical measurements. A better approach is to measure mass transfer coefficients directly in the actual reaction system.
A poorly designed mixing process can play havoc with the operations of a chemical plant. One of the plants in which I worked made what are classified as formulated products: products made by simple mixing of reactants.
One persistent mystery was why a relatively viscous, dense raw material took hours to adequately mix. This was in spite of the fact that studies done in large glass beakers showed rapid mixing. No matter how fast the unit mixing agitators turned, the mixing continued at a seemingly glacial pace.
We took a very close look at how the component mixing was performed. In any mixing process, the most intense mixing takes place in the mixing vortex at the center of the tank. The least intense mixing takes place at the tank inner surface, where viscous drag is pronounced. It turned out that plant operators were introducing the viscous component at the tank edge; and because it was a dense liquid, the material accumulated at the bottom of the tank where it was difficult to mix.
Once the liquid was introduced into the vortex region in the tank middle, the problem was eliminated and process time substantially improved.
D. Mass Transfer vs. Kinetic Control
1. Mass transfer is the net movement of mass from one location to another, and occurs in many processes. A common example is the evaporation of water from a pond to the atmosphere. Mass transfer can be applied to a number of chemical engineering problems.
2. Mass Transfer (diffusion)-controlled Reactions are those which occur so quickly that the reaction rate is the rate of transport of the raw materials to the reaction site, such as a catalytic reaction.
Catalytic reactions involve both mass transfer and mass reaction. The overall reaction rate typically relies on the mass transfer or diffusion between liquids or gas to the catalytic structure.
The catalytic reaction takes place after the reactants diffuse through the fluid layer surrounding the catalyst particles, into the pores within the particle (internal diffusion). The internal diffusion of the molecules competes with the reaction.
The kinetic data obtained from catalytic reactors under these conditions cannot be used for formulating meaningful kinetic expressions because of distorting mass transfer effects.
3. Kinetic Control determines the final composition of the product when competing reaction pathways are slower than mass transfer. Potentially, competing reaction pathways influence the selectivity of the reaction – i.e., which pathway is taken.
One of my first assignments as a Research Engineer was to optimize a series of cumene oxidizers located at a petrochemical plant. In these types of units, cumene is oxidized to cumene hydroperoxide (CHP), which is decomposed to form phenol and acetone.
The technical problem which I was asked to tackle was development of an optimum temperature profile for the cumene oxidizer system. These were five isothermal 10,000-gallon tanks in series, with air bubbled up at the bottom.
Each tank’s temperature needed to be kept in the kinetic regime – where the chemical kinetic reaction between the cumene and oxygen predominated. The key for process optimization was determining that temperature for each oxidizer where kinetic control just came into equivalence with mass transfer control.
I wrote a computer optimization program that balanced chemical oxidation kinetics (temperature) with oxygen mass transfer (bubble size) to balance the two processes. The resulting optimum profiles boosted efficiency by 5% – saving the company $100,000 per year.