8 Key Challenges To Pilot Plant Scale-Up

Non-linear scale-up is the number one challenge to pilot plant scale-up and is the cause behind the other seven challenges. Non-linear scale-up basically means you cannot take a chemical process in the lab and drop it into a pilot plant by simply increasing the chemicals and equipment involved in a proportionate manner.

When a system is increased in size, surface area to mass change in proportion. Many other properties related to system size also change, such as laminar and turbulent flow regimes based on changes caused by the surface are to mass proportion. These physical changes cause reaction kinetics, fluid mechanics and thermodynamics change in a non-linear fashion.

For example – a chemical reaction in a beaker requires a certain amount of chemical A and a certain amount of chemical B. The beaker is a certain size. The reaction between these two chemicals results in a certain amount of chemical C and a specific amount of heat being released. If you linearly scaled this system – increased the beaker size 500 times to a tank and increased the amounts of chemical’s A and B by 500 times, then you should have 500 times the amount of chemical C and 500 times more heat released.

But you won’t. Several things could be affecting your reaction. First of all, a tank that is 500 times larger than a beaker has a completely different relationship with the mass of chemicals inside. A beaker will have a lot more contact with the fluid contained it than a large tank would, because the majority of the chemicals in a tank do not touch the walls of the tank. This is the surface area to mass relationship that affects the physical properties of a reaction.

Several other things can change. Flow regimes change and affect the way the chemicals are mixing and reacting with each other. Thermodynamics, fluid dynamics and reaction kinetics will all be different in the larger sized system. All of these factors affect the amounts of Chemical A and Chemical B required and the amounts of Chemical C produced.

Below are seven specific things that change during scale-up. These are all based on the idea above, that scale-up is non-linear.

Molecules in a reaction need to mix well to reach the state of equilibrium as quickly as possible. There are various physical and chemical factors that can affect the mixing and colliding from happening efficiently and result in bad reaction kinetics.

The time to reach chemical equilibrium increases as larger quantities of chemicals are mixed. A reaction is not productive until chemical equilibrium is reached.

The physical and chemical properties of the materials in contact with the reaction can influence the reaction, erode over time or drive the cost of the system up unnecessarily. Material selection is therefore extremely important.

Fluid dynamics change at a non-linear rate as systems increase in size, and keeping flow at the correct Reynolds number is important for thermal transfer and mixing efficiency. Changes between laminar and turbulent flow are hard to predict because of the non-linear nature of fluid dynamics.

A thorough thermodynamic analysis is necessary for successful scale-up due to the sensitivity of chemical reactions to heat gain and heat loss.

Equipment physical limitations can seriously impact chemical reactions. Incorrectly sized equipment can make it hard to control reactions, affect thermodynamics, fluid dynamics, and other aspects of reactions. System longevity also relies heavily on correct equipment selection. Further, materials of construction easily available in bench scale may not be available in the quantities required or may be too expensive in a large-scale systems.

The correct amount of turbulence within a system is critical to creating good reaction kinetics. Angled agitators and baffles can be used to increase turbulence, and the issue of stirring must be addressed. This video further illustrates this issue