Optimizing energy efficiency during high-temperature materials processing seems a worthwhile goal to strive for in developing a new production line. But what does it mean? And where do you start? To consider the scope of opportunity for energy efficiency to reduce cost and improve operations, it's important to pinpoint a few baseline principles.
First, the initial development of an advanced material process usually focuses on the technical value of the product, not the elegance or efficiency of the process. The material's end physical properties drive development. Energy efficiency gets considered later, when product cost begins to become a significant concern. In a worst-case scenario, the production route developed in the laboratory can hinder true efficiency as the process is scaled up to industrial levels.
Second, high-temperature processes are notorious wasters of energy — as exemplified by the hot working environment around steel forges and aluminum smelting pots. Those processes are modest compared to ones for some cutting-edge materials that operate at 2,000–3,000°C. The opportunity for improvements in efficiency is great but these come at a cost or a sacrifice.
THIMBLES TO TONS
The road from process inception to commercial production is arduous. Scaling of thermal processes rarely is a simple matter of linear extrapolation.
At experimental scale, the conversion rate of many solid/solid and solid/gas reactions primarily depends upon the setpoint temperature, overall atmospheric chemistry, size of the reactants and the quality of the intermixing. In small research test furnaces, the furnace temperature can track the control profile very well. The sample load, if relatively small, also may track the desired profile well. At the same time, the small internal volume of the furnace and ratio of sample to furnace volume simplify removing product gases and replenishing with fresh gas. Under these experimental conditions, product uniformity is rarely a significant concern.
As the reaction is scaled to larger sizes, the ability to heat or cool the mass of material and introduce or remove gases from the solids plays an increasingly important role in reaction efficiency. Often these factors become the primary variables that control the conversion rate. They can create limitations that extend the total processing time, impacting both throughput and total energy utilization.
Moreover, batch processing, if chosen for production, takes a further, significant toll on energy efficiency. The requirement to heat and then cool the entire product load, the reaction containers, structural components and all the refractories is especially inefficient. In cases demanding higher temperatures and faster heating/cooling rates, water cooling may improve the functionality of the equipment — but at a substantial sacrifice in energy efficiency.
MOVING BEYOND BATCH
The most efficient thermal processes are engineered to apply heating and cooling to the smallest load possible. In a perfect situation, this means processing the reactants in a continuous manner without need for material containment. The three most critical design criteria that affect efficiency are reactor type, refractory and atmosphere management. So, let's look at each of these.
Reactor type. During scaleup, it's crucial to select the most appropriate reactor. Table 1 lists some characteristics of various types of reactors. Each design brings process advantages and challenges. Often a material's handling characteristics and behavior during processing will dictate choice of a certain type of reactor. Additionally, the need to mix material while it's heated can preclude the use of a reactor where the material is constrained or stagnant.